DOCTORA L T H E S I S

Department of Civil, Environmental and Natural Resources EngineeringDivision of Sustainable Process Engineering Adsorption of Surfactants and Polymers on

Iron Oxides: Implications For Flotation and Agglomeration of Iron Ore

Elisaveta Potapova

ISSN: 1402-1544 ISBN 978-91-7439-309-5

Luleå University of Technology 2011

Elisaveta Potapova A

dsorption of Surfactants and Polymers on Iron O

xides: Implications For Flotation and A

gglomeration of Iron O

re

ISSN: 1402-1544 ISBN 978-91-7439-XXX-X Se i listan och fyll i siffror där kryssen är

Adsorption of surfactants and polymers on iron

oxides: implications for flotation and

agglomeration of iron ore

Elisaveta Potapova

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering

Division of Sustainable Process Engineering

September 2011

Printed by Universitetstryckeriet, Luleå 2011

ISSN: 1402-1544 ISBN 978-91-7439-309-5

Luleå 2011

www.ltu.se

Cover illustration: schematic illustrations of surfactan adsorption on a mineral particle (top), mineral flotation (bottom left), and a wet agglomerate (bottom right).

ABSTRACT

Iron ore pellets are an important refined product used as a raw material in the production of

steel. In order to meet the requirements of the processes for iron production, the iron ore is

upgraded in a number of steps including, among others, reverse flotation. Under certain

circumstances the flotation collector may inadvertently adsorb on the iron ore particles

increasing the hydrophobicity of the iron ore concentrate, which in turn has been shown to

have an adverse effect on pellet strength. To minimize the influence of the collector on pellet

properties, it is important to understand the mechanism of collector adsorption on iron oxides

and how different factors may affect the extent of adsorption.

In Papers I-III, the adsorption of a commercial anionic carboxylate collector Atrac 1563 and

a number of model compounds on synthetic iron oxides was studied in-situ using attenuated

total reflectance Fourier transforms infrared (ATR-FTIR) spectroscopy. The effect of

surfactant concentration, pH, ionic strength, calcium ions and sodium silicate on surfactant

adsorption was investigated. The adsorption mechanism of anionic surfactants on iron oxides at

pH 8.5 in the absence and presence of other ions was elucidated. Whereas silicate species were

shown to reduce surfactant adsorption, calcium ions were found to facilitate the adsorption and

precipitation of the surfactant on magnetite even in the presence of sodium silicate. This

implies that a high concentration of calcium in the process water could possibly enhance the

contamination of the iron ore with the flotation collector.

In Paper III, the effect of calcium, silicate and a carboxylate surfactant on the zeta-potential

and wetting properties of magnetite was investigated. It was concluded that a high content of

calcium ions in the process water could reduce the dispersing effect of silicate in flotation of

apatite from magnetite. Whereas treatment with calcium chloride and sodium silicate made

magnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the

water contact angle of magnetite. The hydrophobic areas on the magnetite surface could result

in incorporation of air bubbles inside the iron ore pellets produced by wet agglomeration,

lowering pellet strength.

Based on the adsorption studies, it was concluded that calcium ions could be detrimental for

both flotation and agglomeration. Since water softening could result in further dissolution of

calcium-containing minerals, an alternative method of handling surfactant coatings on

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magnetite surfaces was proposed in Paper IV. It was shown that the wettability of the

magnetite surface after surfactant adsorption could be restored by modifying the surface with

polyacrylate or sodium silicate.

In Paper V, the results obtained using synthetic magnetite were verified for natural

magnetite. It was illustrated that the conclusions made for the model system regarding the

detrimental effect of calcium ions were applicable to the natural magnetite particles and

commercial flotation reagents. It was confirmed that polyacrylate and soluble silicate could be

successfully used to improve the wettability of the flotated magnetite concentrate. The fact that

polyacrylate improved the wettability of magnetite more efficiently at the increased

concentration of calcium ions indicates that this polymer is a good candidate for applications in

hard water.

Finally, it was concluded that in-situ ATR-FTIR spectroscopy in combination with zeta-

potential and contact angle measurements could be successfully applied for studying surface

phenomena related to mineral processing.

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ACKNOWLEDGEMENTS

First, I would like to acknowledge the financial support of this work provided by the

Hjalmar Lundbohm Research Centre (HLRC).

Secondly, I would like to express my gratitude to the people who made these four years an

exciting and fruitful journey: my supervisor, Prof. Jonas Hedlund, for his guidance and trust in

me; my assistant supervisor, Dr. Mattias Grahn, for all his help and encouragement, no matter

what; and Assoc. Prof. Allan Holmgren for being able to solve any problem and answer any

question.

Further, I am grateful to Dr. Seija Forsmo and Dr. Andreas Fredriksson for valuable advice

and feedback about my work from an industrial perspective.

The assistance of Dr. Johanne Mouzon, Lic. Eng. Ivan Carabante, and Lic. Eng. Iftekhar

Uddin Bhuiyan in working with the new SEM and of Dr. Annamaria Vilinska in zeta-

potential and contact angle measurements is highly appreciated.

I would like to thank my close colleagues Lic. Eng. Ivan Carabante, Dr. Xiaofang Yang, Lic.

Eng. Magnus Westerstrand, and Lic. Eng. Richard Jolsterå for their co-operation, sharing ideas

and experiences.

Ulf Mattila and Oniel Albino, thank you for saving me from the scariest PhD nightmare – a

broken computer during the writing of the thesis.

Dear administrators, thank you for being helpful and patient when settling all the

administrative issues and for not talking work and football during coffee breaks.

I would like to thank my colleagues at the former Department of Chemical Engineering and

Geosciences and especially my colleagues at the Division of Sustainable Process Engineering

for being such great people. You are the best colleagues I could ever have!

A big hug goes to all my friends outside the department, outside the university and outside

Sweden for being there when I needed you and for making my life a fantastic, unforgettable

adventure.

A final and very special thank you goes to my family, who have always supported me from a

distance, and especially to my mom for her guidance through the moments of confusion. I

love you!

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LIST OF PAPERS

This thesis is based on the following papers:

Paper I: Studies of collector adsorption on iron oxides by in-situ ATR-FTIR

spectroscopy

E. Potapova, I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund

Industrial and Engineering Chemistry Research 49 (2010) 1493-1502

Paper II: The effect of calcium ions and sodium silicate on the adsorption of

anionic flotation collector on magnetite studied by ATR-FTIR spectroscopy

E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund

Journal of Colloid and Interface Science 345 (2010) 96-102

Paper III: The effect of calcium ions, sodium silicate and surfactant on charge

and wettability of magnetite

E. Potapova, X. Yang, M. Grahn, A. Holmgren, S. P. E. Forsmo, A. Fredriksson, and J.

Hedlund

Colloids and Surfaces A: Physicochemical and Engineering Aspects 386 (2011) 79-86

Paper IV: The effect of polymer adsorption on the wetting properties of partially

hydrophobized magnetite

E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund

Submitted to Journal of Colloid and Interface Science

vv

Paper V: Interfacial properties of natural magnetite particles compared with

their synthetic analogue

E. Potapova, X. Yang, M. Westerstrand, M. Grahn, A. Holmgren, and J. Hedlund

Full-length paper to be submitted to Minerals Engineering and accepted for presentation at the Flotation

2011 Conference in Cape Town, South Africa

Author’s contribution to the appended papers

Papers I, II, and IV: All experimental work and evaluation, and almost all writing.

Paper III: Approximately one-third of experimental work, two-thirds of evaluation, and

almost all writing.

Paper V: Approximately half of experimental work and evaluation and almost all writing.

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CONTENTS

INTRODUCTION............................................................................................................... 1

SCOPE OF THE PRESENT WORK .................................................................................. 3

BACKGROUND .................................................................................................................. 5

Upgrading of iron ore........................................................................................................ 5

Froth flotation.................................................................................................................... 6

Flotation of iron oxides...................................................................................................... 8

Flotation effect on wet agglomeration of iron ore.............................................................. 9

Surface wettability and contact angle measurements ........................................................ 10

Adsorption at the solid/liquid interface............................................................................ 12

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) ........ 16

EXPERIMENTAL PART................................................................................................... 19

Materials .......................................................................................................................... 19

Methods........................................................................................................................... 21

Film preparation............................................................................................................ 21

ATR-FTIR spectroscopy ............................................................................................... 21

Contact angle................................................................................................................ 22

Zeta-potential ............................................................................................................... 22

RESULTS AND DISCUSSION.......................................................................................... 23

Characterization of iron oxides (Papers I, II, V)............................................................... 23

Surfactant adsorption and factors affecting the adsorption (Papers I-III)........................... 26

Adsorption mechanism ................................................................................................... 26

Factors affecting surfactant adsorption on iron oxides........................................................... 29

The effect of surfactant adsorption on the properties of the magnetite surface (Paper III) 32

Zeta-potential ............................................................................................................... 32

Contact angle................................................................................................................ 34

Verification for natural magnetite (Paper V) .................................................................... 35

Summary and implications for flotation and agglomeration of iron ore ........................... 36

Restoring magnetite wettability after surfactant adsorption (Papers IV, V) ...................... 37

Modification with sodium silicate ..................................................................................... 37

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Modification with hydrophilic polymers............................................................................. 38

Verification for the flotated magnetite concentrate (Paper V) .......................................... 40

Summary and implications for agglomeration of iron ore ................................................ 41

CONCLUSIONS................................................................................................................. 43

FUTURE WORK............................................................................................................... 45

BIBLIOGRAPHY................................................................................................................ 47

INTRODUCTION

Adsorption of surfactants and polymers on mineral surfaces is important for many industrial

applications such as detergency, coatings, flocculation of fibres and fine mineral particles,

dispersion of pigments, stabilization of colloidal suspensions in cosmetics and pharmaceuticals,

flotation, and agglomeration.

Undesired adsorption of surfactants and polymers can be of interest, too, in cases where it has

an adverse effect on the performance of a certain process. For instance, interaction of fulvic and

humic acids with iron oxides is a subject of many research publications, since the adsorption of

these natural polymers has been shown to impair the remediation of arsenic-contaminated soils

using iron oxides [1]. Another example of surface contamination that has received much

attention in the literature is the surfactant coating on iron ore concentrates upon flotation.

Flotation of iron ore is performed in order to reduce the amount of certain elements, such as

phosphorous, in the concentrate to an acceptable level for iron production. Phosphorous-

containing minerals, like apatite, associated with the iron ore are separated by reverse flotation

using anionic carboxylate surfactants [2]. Any contamination of the iron ore concentrate with a

surfactant decreases wettability of the concentrate and thus has an adverse effect on the

subsequent pelletizing process and strength of the iron ore pellets produced [3-5].

In order to minimize any negative effects of surfactant adsorption on the surface properties of

the iron ore concentrate after flotation, it is important to elucidate the mechanism of

interactions between anionic carboxylate surfactants and iron oxides and to identify the factors

that may affect these interactions. This information can further provide an idea about the

possibilities of reducing surfactant adsorption on iron oxides or to restore the wettability of the

iron ore concentrate after flotation.

Several different natural polymers and their derivatives have been proposed as depressants of

iron oxides in reverse flotation of iron ore. The depression phenomenon is complex and not

fully understood but the major mechanisms are believed to be by blocking surface sites for

collector adsorption and by co-adsorption resulting in a hydrophilic surface. Additionally, a

number of organic polymeric binders for agglomeration of iron ore have been developed in

the last decades (see references [3-14] in [6]). The main advantage of organic binders compared

to traditional inorganic binders, like bentonite, is that the former are completely eliminated

1

during the heat treatment of iron ore pellets and thus do not introduce any contaminants to

the final product. Therefore, studies on interactions of different types of polymers with iron

oxides, especially in combination with anionic surfactants, could further extend the possibilities

of using polymers as depressants and binders in iron ore beneficiation and agglomeration.

Studies on the interactions between anionic carboxylate surfactants and iron oxides are rather

sparse. Even less common are investigations involving co-adsorption of anionic surfactants and

polymers. The existing studies primarily involve ex-situ methods, batch adsorption and

flotation experiments. Application of in-situ techniques like attenuated total reflectance Fourier

transform infrared spectroscopy (ATR-FTIR) could provide insight into the mechanism of

interaction between surfactants, polymers and iron oxides at the solid-liquid interface. Further,

with this technique, the adsorption and desorption kinetics may be monitored in-situ, as may

any possible changes in the adsorption mode at different experimental conditions.

ATR elements coated with thin films of synthetic analogues of natural mineral particles are

commonly applied in the adsorption studies by ATR-FTIR spectroscopy [7] to achieve a high

signal-to-noise ratio and to simplify interpretation of the spectroscopic results. However, a

possible difference in interfacial properties of synthetic and natural materials is an important

issue to consider and might require verification of the results, obtained using synthetic particles,

for their natural analogue.

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SCOPE OF THE PRESENT WORK

The scope of the present work may be divided into two main parts:

� Studying the adsorption of surfactants on iron oxides and different factors that can

affect the adsorption;

� Investigating the possibilities of restoring surface wettability after surfactant adsorption.

To achieve the first goal, a method based on ATR-FTIR spectroscopy for in-situ studies of

the adsorption of organic and inorganic species from aqueous solutions on thin films of

synthetic iron oxides was developed. Adsorption and desorption of different surfactants on iron

oxides were investigated in order to elucidate the mechanism of interaction and to study the

stability of the surface complexes formed. The effect of pH, surfactant concentration, ionic

strength, presence of calcium ions and sodium silicate on the adsorption of surfactants on iron

oxides was also studied. In the next step, the change of the charge and wettability of the iron

oxide surface upon adsorption of calcium ions, sodium silicate, and an anionic carboxylate

surfactant was investigated.

Based on the information collected in the first part of the work, several means to reduce the

effect of surfactant adsorption on the wettability of the iron oxide surface were evaluated,

including treatment with sodium silicate and hydrophilic polymers. Finally, the results obtained

using synthetic iron oxide particles were verified for mineral magnetite concentrate.

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4

BACKGROUND

Upgrading of iron ore

Being the most widely used metal in the world, iron is found in nature mainly in the form of

oxide and sulphide ores. Due to their wide occurrence in nature and their high iron content

[2], the most industrially important ores are: hematite (�-Fe2O3), magnetite (Fe3O4) and

goethite (�-FeOOH).

After extraction from the deposit, the iron ore is subjected to grinding and enrichment to

produce an iron ore concentrate with a required chemical composition and particle size

distribution. The main purpose of the ore enrichment is to separate the valuable iron-

containing mineral from the waste minerals (gangue) and to reduce the amount of certain

elements (like silicon, phosphorus, aluminium and sulphur) in the concentrate to an acceptable

level for the iron production. Concentration of the iron ore can be achieved by gravity

separation and/or magnetic separation, sometimes followed by froth flotation in order to

further reduce the silica and phosphorous content of the ore [2].

In order to make iron ore concentrates suitable for the blast furnace, fine iron ore particles

have to be agglomerated. Two commercial agglomeration processes exist today: sintering and

pelletizing. Pelletizing is a more energy-efficient process than sintering and requires less than

half the amount of fuel [8].

The pelletizing process starts with balling of wet, so-called green pellets from the iron ore

concentrate. This is done in balling drums using bentonite as a binder. Different additives can

be introduced to the pellet feed to produce pellets with required properties.

Balling of the green pellets is followed by screening where the desired size fraction is

separated from the under-size fraction, which is returned to the balling drum, and the over-size

fraction, which is first crushed and then returned to the balling drum.

Finally, the green iron ore pellets are dried, oxidized and sintered to give the final product.

Here, depending on the type of iron-containing mineral, fuel consumption can vary

significantly. When magnetite ore is used, a highly exothermic oxidation reaction takes place.

In this reaction, magnetite is converted to hematite, accompanied by a heat release that

5

accounts for more than two thirds of the total energy required for the subsequent sintering of

pellets [9].

The quality of the final pellets produced is highly dependent on the green pellets’ strength

and the pellet size distribution. Breakage of the green pellets results in creation of crumbs and

fines that increase the packing density of the pellet bed during drying, oxidation and sintering,

thus reducing the pellet bed permeability to air, which is undesirable since it negatively affects

both the production capacity and pellet quality [3].

Of all the process steps, froth flotation has probably the largest impact on the surface

properties of iron ore concentrate, which also affects pelletization.

Froth flotation

Froth flotation is based on the difference in surface properties of minerals, namely, their

affinity to air and water. Separation of two minerals by flotation can occur if the surface of one

of the minerals is hydrophobic and the surface of the other mineral is hydrophilic. Upon

introduction of air to the flotation cell, hydrophobic particles will be floated by the air bubbles

attached to the particle surface while hydrophilic particles will remain in the water. Fig. 1

illustrates the principle of froth flotation.

Figure 1. The principle of froth flotation.

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Two types of flotation processes can be distinguished based on the floated fraction: direct

flotation refers to the process in which the valuable mineral is transferred to the floated fraction,

leaving the gangue in the slurry while in reverse flotation, the gangue is floated and the valuable

mineral remains in the slurry [10].

Most minerals are not hydrophobic by nature so their surface has to be modified by a flotation

collector, which selectively adsorbs on the surface of a mineral to be floated, rendering it

hydrophobic and thus easily attached to the hydrophobic air bubbles. Flotation collectors are

heteropolar organic molecules containing both a non-polar hydrophobic hydrocarbon group

and a polar head group. Depending on the properties of the head group, flotation collectors

can be classified as ionic or non-ionic. Ionic collectors become ionized upon dissolution in water

and are further subdivided into anionic (e.g. carboxylates, sulphonates, xanthates [11]), cationic

(e.g. amines, quaternary ammonium salts, pyridinium salts [12]) and amphoteric or zwitterionic

(e.g. amino acids, glycines, quaternary ammonium sulphonates [12]) based on the charge of the

head group after dissociation. Non-ionic collectors contain a head group that does not

dissociate in water, e.g. a polyoxyethylene glycol group [13]. Among all the collectors, anionic

collectors are most widely used in mineral flotation [10]. For instance, fatty acids and

petroleum sulphonates are applied in non-sulphide mineral flotation while xanthates are

commonly used for most sulphide ores [14].

In order to prevent the air bubbles holding mineral particles from bursting when they reach

the air-water interface, a frother is added to the flotation cell to facilitate the formation of a

stable froth, which is further transferred from the flotation cell surface to the collecting launder.

Additionally, different modifiers are typically used in order to increase flotation selectivity [10].

Modifiers can either enhance or reduce the effect of a collector on a certain mineral and are

therefore referred to as activators and depressants. Activators are usually soluble salts that become

ionized in solution and interact with the mineral surface altering its chemical nature and

making it more favourable for collector adsorption [10]. The action of depressants is more

complex and not always fully understood. However, one of the main mechanisms is blocking

of the surface sites by adsorbing the depressant to prevent collector adsorption [14]. Dispersants

may also be added to the flotation system to facilitate liberation of different small-size mineral

fractions (slime) from the surface of larger ore particles, thus facilitating increased floatability of

the larger particles, which in turn improves the recovery. Finally, pH regulators are added to

control the pH – one of the key variables in the flotation process that affects the surface

7

properties of the minerals and the speciation of both the flotation chemicals and naturally

occurring inorganic ions (e.g. carbonates, sulphates) in the process water.

Apart from modifiers added deliberately, different inorganic ions naturally present in the

process water may affect the flotation performance [15] by activating or depressing the flotation

of a certain mineral, changing collector solubility and zeta-potential of the mineral surface. For

instance, pyrite can be activated in the presence of copper ions [16] and activation of magnetite

for flotation with fatty acids can occur in the presence of calcium ions [17].

Flotation of iron oxides

The choice of flotation process in iron ore beneficiation depends on the nature of the gangue

associated with the iron-containing mineral, which can be siliceous or acidic (rich in silica) and

calcareous or basic (rich in calcium oxide) [10].

When iron oxide is to be separated from the siliceous gangue, either direct or reverse

flotation can be applied. Anionic collectors such as fatty acids and alkyl sulphates and

sulphonates [18] are most commonly used for flotation of iron oxides from siliceous gangue

minerals at pH values where the surface of iron oxide is positively charged. An example of a

process utilizing fatty acid flotation for the concentration of hematite is the Republic mine

process in the state of Michigan in the USA [18].

Quartz and silicate minerals can be floated from iron oxides with cationic collectors,

primarily amines, when their surface is negatively charged. In order to increase flotation

selectivity, iron oxides can be successfully depressed by starch or dextrin [18]. For instance, the

Empire and Tilden mines (Michigan, USA) and the Griffith Mine (Ontario, Canada) have

been using ether amines to float the siliceous gangue from the iron oxides [18]. This type of

flotation is also utilized in Brazil, Chile, India, Mexico, Russia, and South Africa [19].

Calcareous phosphate gangue minerals can be floated from iron oxides with modified fatty

acids. Selectivity is improved when sodium silicate or starch is used as a depressant [2]. The

Swedish mining company LKAB has been using reverse froth flotation with an anionic fatty

acid based collector for dephosphorization of magnetite concentrate. In order to improve

flotation selectivity and phosphorous recovery, sodium silicate is added as a

dispersant/depressant. A distinctive feature of the flotation of calcareous ores is the presence of

calcium ions in the process water [20]. Calcium is known to facilitate precipitation of fatty

acids [21] which may result in unnecessary increases in fatty acid collector consumption.

8

Moreover, high concentrations of calcium ions have been shown to activate magnetite for

flotation with a fatty acid collector by adsorbing on the magnetite surface and changing its

charge [17].

Flotation effect on wet agglomeration of iron ore

Wet agglomeration implies that fine particles in agglomerates are held together by a liquid,

which acts as a binder. The amount of liquid in the structure of agglomerates determines

agglomerate strength and is characterized by the liquid saturation (S) (Eq. 1):

L

P

FFS

��

���

��

��

�1

100100

, (1)

where F – liquid content in the agglomerate; � – fractional porosity; �P – density of particles;

�L – density of liquid.

Wet agglomerates can be in different liquid saturation states (see Fig. 2).

Figure 2. States of liquid saturation in wet agglomerates [22-24].

According to the capillary theory [25] developed for wet agglomerates with a freely movable

binder (like water), the tensile strength of agglomerates increases with the increase in liquid

saturation due to the development of the capillary forces. The tensile strength reaches

maximum in the capillary state (liquid saturation 80-90%) when all the pores inside the

agglomerate are filled with liquid and concave menisci are formed at the pore openings (see

Fig. 2). Complete wetting of the surface is required for full development of the capillary forces.

The relation between the tensile strength (�c) of wet agglomerates in the capillary state and

surface wettability is described by the Rumpf equation [25]:

LSc da �

�� cos11

����

�� , (2)

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where a – constant; � – fractional porosity; � – liquid surface tension; d – average particle size;

�LS – liquid-solid contact angle.

Strictly speaking, the Rumpf equation is only valid for agglomerates produced using a freely

movable binder and is not applicable to iron ore pellets balled with water and bentonite clay

[26, 27]. However, as illustrated below, similar trends as those described by the Rumpf

equation are observed for the wet strength of iron ore pellets.

Flotation of the iron ore prior to agglomeration may affect several parameters in the Rumpf

equation. The presence of a flotation collector in the water reduces the surface tension of the

water, which has been shown to decrease the wet strength of iron ore pellets [3, 22, 28].

Adsorption of flotation collector on the surface of the concentrate makes the surface more

hydrophobic and could be expected to further reduce pellet wet strength. Although no

experimental studies investigating the dependency of the agglomerate strength on the contact

angle of the feed have been found, an adverse effect of flotation collector adsorption on the

wet strength of iron ore pellets is commonly reported [3-5]. Additionally, collector adsorption

increases the affinity of the concentrate surface for air, which results in attachment of air

bubbles to the surface of the concentrate, followed by incorporation of bubbles inside green

pellets, increasing pellet porosity and decreasing the liquid saturation [3]. However, the authors

conclude that the decrease in pellet wet strength upon adding flotation collector was not due

to the decreased liquid saturation, but was due to the fact that air bubbles inside the green

pellets behaved like large plastic particles, increasing plastic deformations in pellets and

weakening the pellet structure.

To reduce the disturbances in balling circuits, variation in the properties of the pellet feed

should be minimized. Together with moisture content and fineness, wettability of the iron ore

concentrate should be monitored, so that necessary process adjustments could be made in both

flotation and pelletization.

Surface wettability and contact angle measurements

Wetting of a solid surface occurs due to adhesion forces between the surface and the wetting

liquid, which act against the cohesive forces within the liquid and make the liquid spread over

the surface at a certain contact angle (see Fig. 3).

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Figure 3. Schematic illustration of the contact angle at the solid-liquid-gas contact line.

The solid-liquid contact angle (�LS) of a liquid drop on a polished, flat, solid surface is defined

by the Young equation [29]:

LSLGLSSG � cos��� , (3)

where �SG, �LS, and �LG are the solid-gas, solid-liquid and liquid-gas interface tension,

respectively.

For each pair of liquid and solid characterized by certain �SG and �LG, the solid-liquid contact

angle is determined by the liquid-solid interfacial free energy (�LS). According to the van Oss

theory [30], the liquid-solid interfacial free energy can be divided further into the apolar

Lifshitz-van der Waals part (�LW) and the polar part, with the latter comprising Lewis acid (�+)

and Lewis base (�-) components:

� ���� ����� LGSGLGSGLWLG

LWSGLGSGLS 2 . (4)

Iron oxides have a large amount of acid and base sites [31], contributing to the polar

component of the surface free energy, and are consequently hydrophilic. For instance, a

contact angle of 25° ± 5° was reported [32] for water on the polished surface of natural

magnetite, measured using a static sessile drop method. However, it is not always possible to

obtain a completely smooth surface for contact angle measurements, which leads to the

problem of high variation in the results reported for the same iron oxide. Additionally,

chemical heterogeneity, introduced by impurities present in the natural mineral samples, for

example, may have a significant effect on the measured contact angle. Iveson et al. have shown

that the contact angle of the mixed hematite-goethite ores varied from 0° to 74° depending on

the relative content of these two minerals [33].

Depending on the particle size and morphology, different techniques are used for contact

angle measurements [34, 35]. Optical tensiometry methods (e.g. a static sessile drop method)

are based on capturing and analyzing images of a liquid drop placed on a surface and are

suitable mainly for measuring the contact angle on flat surfaces. However, the static sessile drop

11

method can also be used to assess the wettability of colloid particles, providing that a closely

packed layer of particles can be formed [36]. The contact angle of natural mineral powders is

commonly estimated by the Washburn method, which is an example of force tensiometry

methods and is based on measuring the sorption of a wetting liquid by a powder material upon

immersion. In this method, the packing of particles is also important since it may affect the

penetration rate of the wetting liquid and thus the measured value of the contact angle [37].

Although it might be a challenge to obtain a true value of contact angle for non-ideal

systems such as porous films and mineral powders, contact angle measurements can be

successfully used to characterize the changes in the wettability of these materials upon

adsorption of reagents related to flotation and pelletization.

Adsorption at the solid/liquid interface

Adsorption is a process of accumulation of adsorbate species from a bulk gas or liquid on the

surface of an adsorbent. In the case of interactions of surfactants and polymers with mineral

surfaces, it is the adsorption at the liquid/solid interface that is of interest. When a solid surface

is placed in contact with a polar liquid (like water), the surface may acquire a net surface charge

due to ionization of the surface groups, adsorption of ions from solution or dissolution of ions

comprising the surface [38]. Consequently, an electrical double layer may be formed, due to

the concentration of oppositely charged counter-ions at the charged surface to maintain

charge-neutrality (see Fig. 4).

Figure 4. Schematic figure of the electrical double layer at a liquid/solid interface.

12

In Fig. 4, � represents the so-called Stern layer where the counter-ions have the highest

concentration and are held close to the surface. Beyond the Stern layer, the concentration of

counter-ions decreases until it reaches the bulk concentration. The charge of the surface in the

slip plane just outside the Stern layer is referred to as zeta-potential and can be estimated from

electrokinetic measurements.

Considering an iron oxide surface in contact with water, a fully hydroxylated surface should

be expected. The net charge of the iron oxide surface is dependent on

protonation/deprotonation of the hydroxyl groups when the pH of the solution changes (see

Eq. 5).

���� �������� �����

FeOFeOHFeOH HH2 (5)

The pH at which the net charge of the surface is zero is referred to as the point of zero charge

(PZC). For iron oxides, the PZC is usually observed at pH 7-8 [39]. Above this pH, the

surface is charged negatively, whereas below the PZC the surface bears a positive charge. The

PZC of a surface can be determined by a potentiometric titration in an indifferent electrolyte.

When the charge of a surface is measured by electrophoresis, the pH at which the zeta-

potential is equal to zero is termed the isoelectric point (IEP). In the absence of specific adsorption

of non-potential-determining ions, the values of the PZC and the IEP should be the same.

The zeta-potential plays an important role in the adsorption of ionic species at mineral-water

interfaces. The change in the zeta-potential upon adsorption can be used as an indication of the

type of forces involved in adsorption [40]. If adsorption takes place only through electrostatic

interaction, the absolute value of the zeta-potential will decrease upon adsorption and will

eventually reach zero when the surface is fully saturated with the adsorbate. However, if apart

from electrostatic interaction, specific adsorption occurs due to affinity of certain species to the

surface, the zeta-potential of the surface can go through zero and then become reversed.

Another indication of specific adsorption is the shift of the PZC and the IEP of a surface in the

opposite directions upon adsorption.

Depending on the forces contributing to adsorption, specific adsorption can be either

physical or chemical. Chemical adsorption refers to when an adsorbate forms a covalent bond

with the surface of the adsorbent while physical adsorption implies contribution of weaker

forces such as hydrogen bonding and van der Waals interactions.

13

Quantitatively, adsorption of a certain compound on a solid surface is described by an

adsorption isotherm. It is obtained by plotting the measured amount of the adsorbate on the

surface against the equilibrium concentration of adsorbate in solution. Different adsorption

models have been developed to describe experimental adsorption data; the most common

models used for describing adsorption at the solid-liquid interface are the Langmuir and the

Freundlich models [38].

The Langmuir adsorption isotherm (Eq. 6) is based on the assumption of localised monolayer

adsorption and that the heat of adsorption is independent of surface coverage.

aCaC

xx

��

1max

(6)

In this equation, maxxx

is the fraction of the surface covered with the adsorbate; C is the

equilibrium concentration of the adsorbate in solution and a is the adsorption constant.

The Freundlich adsorption isotherm (Eq. 7) can be derived from the Langmuir isotherm by

introducing an exponential change to the heat of adsorption with surface coverage. Thus, this

model implies adsorption on an energetically heterogeneous surface. The different adsorption

sites may be grouped patchwise, with sites having the same heat of adsorption grouped

together.

nkCmx /1� (7)

In this equation, x is the amount of the adsorbate adsorbed on a specific mass m of the

adsorbent; k and n are empirical constants.

Both the Langmuir and the Freundlich isotherms are applicable to the adsorption of

surfactants on mineral surfaces. However, due to specific properties of surfactant molecules

(e.g. their ability to form micelles or adsorbed multi layers) the adsorption of these molecules

can be characterized by other types of isotherms. For instance, adsorption of ionic surfactants

on oppositely charged surfaces is frequently described by an S-shaped isotherm when plotted

using a logarithmic scale and referred to as a “Somasundaran-Fuerstenau” isotherm [41]. This

isotherm has four characteristic regions as illustrated in Fig. 5 [42].

Region I represents adsorption at low surfactant concentrations due to electrostatic forces

between the surfactant species and oppositely charged surface sites. In Region II, surfactant

14

species on the surfaces begin to form two-dimensional surface aggregates due to hydrophobic

interactions between the hydrocarbon chains in surfactant molecules. Since the electrostatic

interactions are still active in this region, the adsorption density shows a sharp increase. In

Region III, the surface charge is fully neutralized by the adsorbed surfactant species and

electrostatic forces do not contribute to adsorption any longer. However, interaction between

hydrophobic chains in surfactant species still occurs, further increasing adsorption density,

though at a lower rate. In region IV, the surfactant concentration in solution reaches the

critical micelle concentration (CMC) and any increase in concentration contributes primarily

to formation of micelles in solution without changing the adsorption density on the surface

much.

Figure 5. Somasundaran-Fuerstenau isotherm [42].

Polymer adsorption on solid surfaces is commonly characterized by a so-called high-affinity

adsorption isotherm, exhibiting a sharp increase in surface loading at a very low polymer

concentration, which is followed by a plateau at higher concentrations [43]. This type of

adsorption isotherm has been reported for polyelectrolytes adsorbed on the surfaces of the same

[44] and opposite charge [45] as well as for the adsorption of non-charged polymers [46]. A

specific feature of polymer adsorption is that it can hardly be reversed by dilution [43] due to

the fact that a polymer molecule is bound to the surface through a number of segments, which

have to be detached from the surface in order to desorb the polymer molecule. The ability of

a polymer molecule to adopt a large number of configurations both in solution and at the

solid/liquid interface makes polymer adsorption rather complex as compared to adsorption of

small molecules and ions. Numerous theoretical models describing polymer adsorption have

been proposed and can be found elsewhere [43, 47].

15

Attenuated total reflectance Fourier transform infrared

spectroscopy (ATR-FTIR)

Fourier transform infrared (FTIR) spectroscopy is based on the ability of molecules to

undergo transitions from one vibrational energy state to another by absorbing infrared radiation

[48]. In order for absorption to occur, the transition must involve a change in the dipole

moment of a vibrational mode. Each molecule can only absorb radiation of certain frequencies,

that is, the natural vibrational frequencies of the molecule, resulting in a number of absorption

bands located at different frequencies in the spectrum. In infrared spectroscopy, the frequency

is traditionally expressed in wavenumbers (cm-1).

The most commonly used types of vibrational modes are stretching and bending. A stretching

vibration is characterized by a change in the length of a bond between atoms while a bending

vibration involves the change in the angle between bonds.

The amount of infrared radiation absorbed (A, Absorbance) by a certain species is described by

the Lambert-Beer law (Eq. 8).

Cl�IIA ���� 0log (8)

In this equation, I0 is the initial intensity of the radiation and I is the intensity of the radiation

after interaction with the sample; is the molar absorptivity of the species at a certain

wavelength; l is the path length of the radiation in the sample; C is the concentration of the

species of interest in the sample.

�

Thereby, the Lambert-Beer law illustrates that the intensity of a specific absorption band is

proportional to the amount of the corresponding group in the sample and can thus be used for

quantitative studies.

FTIR spectroscopy enables both ex-situ and in-situ studies. Attenuated total reflectance FTIR

spectroscopy (ATR-FTIR) is a technique well suited to in-situ studies, providing an

opportunity to study the interactions at the solid-liquid interface without changing the surface

characteristics of the sample [49].

The ATR technique is based on the phenomenon of attenuated total reflectance, which is

schematically illustrated in Fig. 6.

16

Figure 6. Schematic figure of an ATR waveguide illustrating the ATR phenomenon.

In this technique, the IR beam with the initial intensity I0 passes through a waveguide,

having a high refractive index n2 and being surrounded by a medium with lower refractive

index n1. The difference in the refractive indices results in attenuated total reflection of the

beam inside the waveguide, provided that the incident angle � fulfils the relation shown in

Eq. 9.

2

1sinnn

�� (9)

At each point of reflection, an evanescent wave of the IR radiation is formed perpendicular

to the waveguide. The wave can interact with the surrounding medium in the vicinity of the

waveguide resulting in attenuation in the intensity of the totally reflected beam. The amount

of radiation of a certain wavelength (� ) absorbed by the surrounding medium depends on the

penetration depth dp (see Fig. 6), which is defined by Eq. 10.

2/12

2

122 sin2

��

�

�

��

�

����

����

��

�

nnn

d p

��

� (10)

The penetration depth is, by definition, the distance from the interface where the intensity of

the electric field (E) of the wave has declined to a value equal to:

E = E0·e-1 (11)

In this equation, E0 is the intensity of the electric field at the surface of the waveguide.

The values of the penetration depth typically vary in the range from some hundred

nanometres to a few micrometres, making the ATR-FTIR spectroscopy a surface-sensitive

17

technique. Furthermore, the short penetration depth significantly reduces the absorption of IR

radiation by water, facilitating studies of aqueous systems. In-situ spectroscopic measurements

open up possibilities for following the adsorption process in real time and to obtain

information about adsorption and desorption kinetics and equilibria, surface complexes formed

at the solid/liquid interface and the orientation of adsorbed species.

ATR-FTIR spectroscopy has been extensively used to study the adsorption of surfactants and

polymers on mineral surfaces [7, 49-51]. The fact that adsorption can be performed either

directly on a bare waveguide or on a waveguide coated by a thin layer of adsorbent makes the

technique applicable to a wide variety of systems [52].

18

EXPERIMENTAL PART

Materials

Three different iron oxide materials were used in the experimental work (see Table 1).

Table 1. Iron oxide materials used in the experimental work.

Iron oxide Synthesis method/Origin Paper(s)

Synthetic hematite Matijevic [53] I

Synthetic magnetite Massart and Cabuil [54] II-IV

Mineral magnetite* LKAB, Kiruna, Sweden V

*Cleaned by magnetic separation and flotation, stored at the ambient conditions for two years.

The iron oxides were characterized using X-ray diffraction (XRD), scanning electron

microscopy (SEM), electrophoresis, gas adsorption, and contact angle measurements.

Adsorption of a commercial flotation collector, Atrac 1563 from Akzo Nobel, and four

model compounds was investigated in this work (see Fig. 7).

Figure 7. Chemical structures of Atrac 1563 (a), ethyl oleate (b), maleic acid (c), poly

(ethylene glycol) monooleate (PEGMO) (d), and dodecyloxyethoxyethoxyethoxyethyl maleate

19

(e). R represents a linear alkyl chain in fatty acids or a C19H29 chain in resin acids, R’ –

CH3(CH2)7CH=CH(CH2)7, R’’ – CH3(CH2)11.

Commercial flotation collector Atrac 1563 has a complex chemical composition: 50-100 %

ethoxylated tall oil ester of maleic acid, and 1-5 % maleic anhydride (Akzo Nobel material

safety data sheet). Since the exact composition and chemical structure of Atrac 1563 are

unknown, four different reagents, as shown in Fig. 7b-e, were evaluated as model compounds

to be used in the experimental work instead of Atrac 1563.

Two types of soluble silicate were used in the experiments. Water glass, i.e. an aqueous

solution of sodium silicate, in this case with a SiO2:Na2O weight ratio of 3.25, is used as a

dispersant/depressant in the flotation of iron ore. Sodium metasilicate (Na2SiO3·9H2O) was

used as an analytical grade alternative of water glass.

In the experiments described in Papers IV and V, adsorption of four different polymers was

investigated (see Table 2).

Table 2. Polymers used for surface modification of magnetite.

Polymer name Structural formula Average

molecular weight Supplier

Dispex A40 (ammonium

polyacrylate)

4000 BASF

Dispex N40 (sodium

polyacrylate)

4000 BASF

ATC 4150

(aliphatic quaternary

polyamine)

50000 Eka

chemicals

Soluble starch*

N/A Merck

*1 wt % aqueous starch solution containing 0.5 wt % NaOH was heated to 84°C for 10

minutes and then cooled to room temperature [55].

20

Methods

The main instrumental techniques used in the present work were ATR-FTIR spectroscopy,

contact angle and zeta-potential measurements.

Film preparation

For the spectroscopic and contact angle measurements, the appropriate substrate was coated

with a film of synthetic iron oxide. For the experiments described in Paper I, both sides of the

waveguide were coated with a hematite film by means of dip-coating. In the experiments

described in Papers II-V where synthetic magnetite was used, only one side of the waveguide

was coated with a film by spreading a certain amount of magnetite dispersion and air-drying it

at room temperature. The reason for this was to prevent the magnetite film from absorbing too

much IR radiation in the spectroscopic measurements.

ATR-FTIR spectroscopy

Spectral data were collected using a Bruker IFS 66v/S spectrometer equipped with a liquid

nitrogen cooled mercury-cadmium-telluride (MCT) detector and a deuterated triglycine

sulphate (DTGS) detector, a vertical ATR accessory and a stainless steel sample cell (see Fig. 8).

Trapezoidal ZnSe crystals (Crystran Ltd) with 45° cut edges and dimensions of 50x20x2 mm

were used as ATR waveguides.

Figure 8. Schematic illustration of the experimental setup. Thick solid lines represent liquid

flow whereas the dashed arrows indicate the IR beam.

Adsorption measurements were performed in-situ at room temperature with a continuous

flow of working solution pumped through the cell with recirculation, except for the

desorption experiments in which the solution was not recirculated. The pH during the

adsorption experiments was kept constant by a Mettler Toledo T70 titrator.

21

Contact angle

The static sessile drop method was used to determine the contact angle of the synthetic

magnetite nanoparticles (Papers III-V). Contact angle measurements were performed using a

Fibro 1121/1122 DAT-Dynamic Absorption and Contact Angle Tester equipped with a CCD

camera. The measurement was performed by placing a 4 �L water droplet onto the magnetite-

coated substrate using a microsyringe. A series of images were taken and analysed using the

DAT 3.6 software. To investigate the effect of different reagents on the wettability of the

synthetic magnetite particles, consecutive adsorption of the reagents was performed on the

magnetite film in the same way as in the spectroscopic measurements. Between the adsorption

steps, the contact angle of the magnetite film was measured.

The contact angle of the natural magnetite particles (Paper V) was determined by the

Washburn method using a Krüss K100 force tensiometer. Liquid sorption by the magnetite

powder was recorded as a function of immersion time, and Krüss LabDesk 3.1 software was

used to calculate the contact angle applying the Washburn equation. First, the capillary

constant of the Washburn equation was estimated for each sample using n-hexane. Thereafter,

the contact angle of the magnetite powder was measured using deionized water. The values of

the capillary constant and the contact angle were calculated as an average of three replicates.

To investigate the effect of different reagents on the wettability of the natural magnetite

particles (Paper V), batch adsorption was performed using suspensions containing 10 g

magnetite per ca 40 mL solution at pH 9 and room temperature. After adsorption, the solution

was decanted and magnetite was dried in an oven overnight at 50°C.

Zeta-potential

The zeta-potential of both synthetic and natural iron oxides as a function of pH was

determined by electrophoresis using a ZetaCompact instrument equipped with a charge-

coupled device (CCD) tracking camera. The electrophoretic mobility data was further

processed by the Zeta4 software applying the Smoluchowski equation. For the case of

magnetite concentrate, the measurements were performed using the 0.22-8 �m fraction of the

magnetite slurry collected at the LKAB concentrating plant in Kiruna, Sweden, after flotation.

The required size fraction of the particles was separated by vacuum filtration.

Further experimental details are available in the appended papers.

22

RESULTS AND DISCUSSION

Characterization of iron oxides (Papers I, II, V)

The X-ray diffraction data of the iron oxides used in the present study (Fig. 9) confirmed

pure crystalline phases of hematite (a) and magnetite (b, c), without any other phases present in

amounts detectable by XRD. The peak width decreases in the sequence synthetic magnetite >

synthetic hematite > natural magnetite, reflecting the increasing particle size of the iron oxide

materials (10 nm and 130 nm, as determined by SEM, see below, and < 45 �m [56],

respectively).

Figure 9. XRD patterns of a synthetic hematite (a), a synthetic magnetite (b), and a natural

magnetite (c). The reflections originating from the corresponding iron oxides are indexed with

the appropriate Miller indices.

23

SEM images in Fig. 10 show examples of cross-sections of the films of synthetic hematite (a)

and magnetite (b) on a ZnSe substrate, which were used in the spectroscopic and contact angle

measurements. In both cases, porous films were formed with a thickness of ca 1 �m and 250-

300 nm, respectively. Fig. 10 illustrates that the synthetic iron oxide crystals had a uniform

spherical habit and were slightly aggregated. The particles in the magnetite concentrate in

Fig. 10c exhibited high variation in both size and shape. The figure illustrates that the coarse

magnetite particles were covered by very fine particles (less than 1 �m in size), some of which,

according to the EDS results, had a high content of silicon and aluminium and could be the

remains of aluminosilicate minerals, which are present in the iron ore before concentration.

Figure 10. Side view SEM images of a hematite film (a) and a magnetite film (b) on a ZnSe

crystal, a top view SEM image of the mineral magnetite particles on a carbon tape (c), and a

close-up SEM image of a magnetite particle shown in Fig. 10c (d).

24

Fig. 11 shows the zeta-potential of the iron oxides as a function of pH. The fraction of the

mineral magnetite concentrate used for the zeta-potential measurements was found to be

mainly comprised of other minerals that magnetite and will not be discussed here. The IEP for

the synthetic magnetite (empty triangles) was observed at pH 7, as per the literature [39],

whereas the IEP for the hematite particles (filled diamonds) was observed around pH 5, which

is lower than expected and could be caused by the adsorption of chloride [57] or carbonate

[58] ions on the surface.

Figure 11. Zeta-potential as a function of pH of the synthetic hematite in 10 mM KNO3 (�)

and of the synthetic magnetite in 10 mM NaCl (�).

Regarding the wetting properties of the iron oxides used in this work, the contact angle of

the synthetic magnetite was 15-25°, whereas the magnetite concentrate had a contact angle of

50-60°. The lower wettability of the magnetite concentrate was likely due to the hydrophobic

flotation collector species that have been reported to be present on the surface of the

concentrate after flotation [3]. However, the inconsistency in the obtained values could also be

due to the difference in particle size as well as the measuring techniques used.

Table 3 summarizes the morphological properties of the iron oxides used in this study.

Table 3. Morphological properties of the iron oxide materials.

Property Synthetic hematite Synthetic magnetite Mineral magnetite

Particle size 130 nm 5-15 nm 85% -45 �m [56]

Particle shape Spherical Spherical Irregular

BET (N2) surface area, m2 g-1 13 90 0.5 [56]

25

Surfactant adsorption and factors affecting the adsorption

(Papers I-III)

In the present work, the adsorption of one commercial flotation collector (Atrac 1563) and

four model compounds (PEGMO, maleic acid ester, ethyl oleate, and maleic acid) on synthetic

iron oxides was investigated. However, only three of the compounds were found to show

similar adsorption behaviour: Atrac 1563, PEGMO, and the maleic acid ester. The adsorption

of these three compounds will be discussed below.

Adsorption mechanism

Fig. 12 shows the spectra of Atrac 1563, PEGMO, and the maleic acid ester, as-received and

adsorbed on synthetic hematite and magnetite at pH 8.5.

Figure 12. ATR-FTIR spectra of Atrac 1563 (1), PEGMO (2) and the maleic acid ester (3)

as-received and spread over an uncoated ZnSe crystal (a); of Atrac 1563 (1) and PEGMO (2)

adsorbed on hematite from a 10 mg L-1 solution at pH 8.5, and maleic acid ester (3) adsorbed

on magnetite from a 25 mg L-1 solution containing 0.01 M NaCl at pH 8.5 (b).

As illustrated in Fig. 12, similar absorption bands are observed in the spectra of the

surfactants, confirming structural resemblance of the head groups in these molecules.

Assignment of the main absorption bands in the spectra of the surfactants is presented in

Table 4. More detailed discussions of the spectral features displayed by the surfactants used in

the present study are given in Papers I, II, and V.

26

Adsorption of the surfactants on synthetic iron oxides was performed from aqueous solutions

at pH 8.5, i.e. at an optimum pH for the flotation of apatite from iron oxide [2]. At this pH,

the free carboxylic groups in Atrac 1563 and maleic acid ester become deprotonated, forming a

negatively charged carboxylate ion as indicated by two new bands originating from the

symmetric and asymmetric stretching vibrations of the carboxylate ion (vs(COO-) and

vas(COO-), respectively) in the spectra of these compounds adsorbed on the iron oxides

(spectra (1) and (3) in Fig. 12b). No bands associated with the carboxylate ion were found in

the spectrum of PEGMO (spectrum (2) in Fig. 12b) suggesting that the ester bond in PEGMO

does not break upon adsorption on hematite at the conditions studied.

Table 4. Assignment of absorption bands originating from Atrac 1563, PEGMO, and maleic

acid ester adsorbed on synthetic hematite and magnetite in-situ at pH 8.5. The numbers in

parentheses represent the position of the corresponding absorption bands in the same

compounds as-received, spread over a ZnSe substrate.

Peak position, cm-1

Atrac 1563 PEGMO Maleic acid ester Peak assignment

1724 (1736) 1740 (1736) 1724 (1728) �(C=O) in ester [59]

(1709) - (1715) �(C=O) in acid [48]

(1645) - (1643) �(C=C) [60]

1564

1424

-

-

1571

1402

�as(COO-) [60]

�s(COO-) [60]

1171 (1159) 1175 (1173) 1178 (1161) �(C-O) in esters [61]

- 1095 (1115) 1104 (1105) �(C-O-C) [62]

- 1047 (1070) - �(C-OH) [63]

At pH 8.5 the surface of the iron oxides was characterized by a negative zeta-potential (see

Fig. 11) and, consequently, no considerable adsorption of anionic carboxylate surfactants on

iron oxides would be expected at this pH due to electrostatic repulsion between the negatively

charged carboxylate ions and the surface bearing the same charge. In the present work, no

adsorption of maleic acid on hematite took place at pH 8.5, which agrees with the results

reported by Hwang and Lenhart [64]. However, both in the previous studies on oleate-

hematite systems [65, 66] and in this work (spectra (1) and (3) in Fig. 12), the anionic

carboxylate surfactants exhibited considerable adsorption on iron oxides even at pH values

27

above the IEP suggesting that the adsorption of anionic carboxylate surfactants on iron oxides

is not exclusively determined by the electrostatic forces.

Based on the results from adsorption of maleic acid, it may be concluded that the carboxylate

function is not likely to be responsible for the adsorption of the carboxylate surfactants onto

the iron oxides above their IEP. Similar to the ability of non-ionic surfactants (like PEGMO)

to adsorb on solid surfaces via the polar head group [42], the adsorption of Atrac 1563 and

maleic acid ester on iron oxides above their IEP could be determined by the presence of polar,

but not charged, groups such as ester carbonyl, hydroxyl, and ethoxy-groups. The suggested

mechanism of surfactant adsorption on iron oxides at pH values above the IEP is illustrated in

Fig. 13.

Figure 13. Proposed adsorption mechanism of Atrac 1563 (a), PEGMO (b), and maleic acid

ester (c) on iron oxides from aqueous solutions at pH 8.5. R represents a linear alkyl chain in

fatty acids or a C19H29 chain in resin acids, R’ – CH3(CH2)7CH=CH(CH2)7, R’’ –

CH3(CH2)11. Dashed ovals indicate the moieties interacting with the surface.

28

Additionally, hydrophobic interaction between the hydrocarbon chains of the surfactants

could possibly contribute to the adsorption, as indicated by the shift of the CH2 asymmetric

stretching vibration band in the spectra of Atrac 1563, PEGMO, and maleic acid ester with the

increase of surfactant loading on the surface (not shown) [67].

Thus, a conclusion can be made that both the hydrophobic tail and the polar head group

determine the ability of a surfactant to adsorb on iron oxides.

The desorption experiments (Fig. 13 in Paper I) showed that the adsorbed species of Atrac

1563 could be removed from the hematite surface only partially, even at increased pH,

implying rather strong interaction between the surfactant and the iron oxide.

It is important to mention here that carboxylate ions can be expected to facilitate the

adsorption of the surfactants on iron oxides below the IEP when the net charge of the surface

is positive. The contribution of electrostatic forces to the adsorption of surfactants containing

free carboxylic groups explains their strong adsorption dependency on the surface charge of the

iron oxide and consequently on pH and ionic strength [66], as will be discussed later.

Factors affecting surfactant adsorption on iron oxides

Surfactant adsorption on a solid surface can be affected by many factors, including surfactant

concentration, pH, temperature and presence of inorganic ions. In this chapter, the effect of

surfactant concentration, pH, and total concentration of ions (ionic strength) on surfactant

adsorption onto iron oxides is discussed. The results of adsorption of an anionic carboxylate

surfactant on magnetite in the presence of calcium ions and sodium silicate are also presented.

Surfactant concentration. Due to the fact that the absorbance of infrared radiation is proportional

to the concentration of the absorbing species according to the Lambert-Beer law (Equation 8),

the intensity of the bands in a spectrum of a surfactant adsorbed on iron oxide can be assumed

to be proportional to the amount of surfactant on the surface. This assumption is reasonable as

long as all the adsorbed species have transition dipole moments of similar value.

The absorbance of the C-H symmetric stretching vibration band in the spectra of PEGMO

and Atrac 1563 adsorbed on hematite at pH 8.5 plotted as a function of surfactant

concentration in solution (see Fig. 8 and 12 in Paper I, respectively) was in good agreement

with the Freundlich adsorption model (Equation 7). This type of adsorption implies that the

heat of adsorption changes depending on the surface coverage [68], as discussed above.

29

Ionic strength. For the adsorption of the maleic acid ester on magnetite at pH 8.5, a ten-fold

increase in ionic strength (from 10-2 to 10-1 M NaCl) resulted in a 20-25% increase in the

intensity of the bands originating from the surfactant adsorbed on magnetite (see Fig. 7 in

Paper II), indicating the contribution of electrostatic forces to adsorption. This can be regarded

as further evidence for the formation of outer-sphere surface complexes.

Calcium chloride and sodium silicate. Fig. 14 illustrates the effect of calcium chloride and

sodium silicate on the adsorption of maleic acid ester on magnetite.

Figure 14. Intensity of the ester C=O stretching vibrations band as a function of time during

in-situ adsorption of maleic acid ester on magnetite at pH 8.5 from a 25 mg L-1 aqueous

solution without Ca2+ and Na2SiO3 added (�), with 4 mM Ca2+ (�), 0.4 mM Na2SiO3 (�),

and with 4 mM Ca2+ and 0.4 mM Na2SiO3 (�). Background electrolyte: 10 mM NaCl.

The adsorption of maleic acid ester on magnetite in the presence of calcium ions (open

triangles in Fig. 14) increased dramatically compared to when no calcium ions were added

(open circles in Fig. 14). This result agrees with the findings reported by Rao et al. [17] that

activation of magnetite for flotation with anionic collector occurred in the presence of calcium

ions. Calcium ions are also known to facilitate precipitation of fatty acids by forming calcium

soaps [21], which may also adsorb on the surface of magnetite [4, 17].

Considering the effect of sodium silicate, competitive adsorption of silicate and surfactant

species on magnetite was observed resulting in a three-fold decrease in surfactant adsorption

(filled circles in Fig. 14) as compared to when no silicate was added (open circles in Fig. 14).

However, desorption experiments (see Fig. 10 in Paper II) revealed higher stability of the

30

surfactant-magnetite complex as compared to the silicate-magnetite complex, suggesting that

silicate species in solution are not likely to replace the surfactant molecules already adsorbed on

magnetite. Similar results were recently reported by Roonasi et al. for silicate-oleate adsorption

on magnetite [69].

The adsorption behaviour in the silicate-surfactant-magnetite system changed significantly

with the introduction of calcium ions. Despite the fact that silicate adsorption slightly increased

in the presence of calcium ions (see Fig. 5 in Paper II), almost no silicate adsorption was

observed when the surfactant was added to the system, resulting in nearly as high adsorption of

the surfactant (filled triangles in Fig. 14) as with calcium ions only (open triangles in Fig. 14).

Thus, the depressing activity of sodium silicate on surfactant adsorption was almost completely

suppressed in the presence of calcium ions. One explanation for such behaviour could be a

much higher affinity of the surfactant for the calcium ions as compared to that of sodium

silicate, which is not surprising since carboxylate surfactants are known to adsorb on calcium

sites on apatite and other calcareous minerals [70].

pH change. Fig. 15 illustrates the adsorption of maleic acid ester on magnetite as a function of

pH.

Figure 15. Intensity of the ester C=O stretching vibration band originating from the maleic

acid ester adsorbed on magnetite in-situ from a 25 mg L-1 solution at different pH. The pH

was gradually decreased from pH 10. The surfactant was allowed to adsorb for 5 hours at each

pH. The background electrolyte was 10 mM NaCl.

The spectral data indicates that the amount of surfactant on the magnetite surface decreased

with increasing pH, as typically observed for the adsorption of anionic surfactants on the

31

surfaces bearing the same charge. As the pH decreases, the surface charge first becomes less

negative and then turns positive (see Fig. 11) thus making the surface more electrostatically

favourable for adsorption of the negatively charged deprotonated surfactant species. An

increased precipitation of the surfactant on the magnetite surface at acidic pH could further

contribute to the surfactant loading on the surface.

When the magnetite surface was pretreated with calcium ions and sodium silicate prior to

surfactant adsorption, the adsorption of the surfactant in the pH range 7.5-9.5 went through a

maximum at pH 8.5, in concert with the results reported by Morgan [71] for oleate adsorption

on hematite and explained by the formation of an acid-soap complex [(RCOO)2H]- [66, 72].

Fig. 6b in Paper III further illustrates that surfactant adsorption was denser at pH 9.5 than at

pH 7.5, which opposes the trend in Fig. 15. Such behaviour could be explained by the affinity

of the surfactant towards calcium ions, which are expected to be present on the magnetite

surface in a larger amount at higher pH, as becomes evident from the zeta-potential results

presented in Fig. 16a. An increased calcium-surfactant precipitation at higher pH would

contribute to this behaviour.

The effect of surfactant adsorption on the properties of the

magnetite surface (Paper III)

In this chapter, the effect of adsorption of an anionic carboxylate surfactant (maleic acid ester)

onto synthetic magnetite in the presence of calcium ions and sodium silicate is discussed.

Zeta-potential

Fig. 16 shows the zeta-potential of synthetic magnetite as a function of pH in the presence of

calcium chloride, sodium silicate, and maleic acid ester. Whereas calcium ions were capable of

reversing the zeta-potential of magnetite at pH values above the IEP (empty squares in

Fig. 16a), sodium silicate exhibited the opposite effect, making the magnetite surface more

negatively charged and shifting the IEP to lower pH (empty triangles in Fig. 16a). Considering

the combined effect of calcium ions and silicate species, the resulting zeta-potential of the

magnetite particles was determined by the ratio of these compounds in solution (filled

diamonds and empty squares in Fig. 16b). As the calcium-to-silicate ratio increased, the IEP of

32

the magnetite particles shifted to higher pH values and the zeta-potential above the IEP

became less negative.

When maleic acid ester was added to the solution containing calcium chloride and sodium

silicate (filled triangles in Fig. 16b), the zeta-potential of the magnetite particles became slightly

more negative as compared to that with only calcium and silicate, probably due to the

adsorption of the surfactant on the magnetite surface via positively charged calcium ions. The

adsorption of the surfactant in a bi-layer structure due to hydrophobic chain-chain interactions

could also result in an additional negative charge introduced by the deprotonated surfactant

head groups oriented towards the solution in the second adsorbed layer [73, 74].

Figure 16. Zeta-potential as a function of pH: (a) of the magnetite crystals in 10 mM NaCl

(�), 3.3 mM CaCl2 (�), and 1 mM Na2SiO3 (�); (b) of the magnetite crystals in 3.3 mM CaCl2

and 0.4 mM Na2SiO3 (�), in 3.3 mM CaCl2 and 1 mM Na2SiO3 (�), 3.3 mM CaCl2, 0.4 mM

Na2SiO3, and 25 mg L-1 maleic acid ester (�), of the maleic acid ester (no magnetite crystals)

in a 15 mg L-1 aqueous solution containing 10 mM NaCl and 2.4 mM CaCl2 (�).

The zeta-potential of the magnetite particles in the presence of surfactant, calcium, and

silicate was nearly constant in the entire pH range studied, with the IEP expected to be below

pH 5, suggesting specific interaction between the surfactant and magnetite. Similar results were

reported by Rao et al. [73] for oleate adsorption on fluorite and were explained by the

adsorption of calcium oleate precipitate, characterized by a strongly negative and nearly

constant zeta-potential at pH 5-10.

Regarding the maleic acid ester, it is difficult to say whether calcium-surfactant complexes

were formed at the surface or already in solution, followed by the adsorption of the calcium-

surfactant complexes onto magnetite. The zeta-potential of the surfactant in solution

33

containing calcium chloride (without magnetite particles, empty triangles in Fig. 16b) showed

similar dependency on pH as the zeta-potential of magnetite in solution containing calcium

chloride, sodium silicate, and maleic acid ester (filled triangles in Fig. 16b). However, the

values of the zeta-potential in the latter case were significantly less negative, confirming the

proposed mechanism of surfactant adsorption on magnetite in the form of ternary complexes

with calcium ions.

Contact angle

Table 5 illustrates the effect of calcium chloride, sodium silicate, and surfactants on the

wettability of synthetic magnetite.

Table 5. Water contact angle of the as-synthesized synthetic magnetite and magnetite after

consecutive conditioning with calcium ions, sodium silicate and a surfactant. The background

electrolyte was 10 mM NaCl. The values reported were measured 1 second after a drop of

water was deposited on the surface and are presented as an average value ± one standard

deviation.

Treatment As-synthesized

magnetite 4 mM CaCl2 0.4 mM Na2SiO3 25 mg L-1 surfactant

20 ± 3 15 ± 4 �10* 43 ± 8 (Atrac 1563)

Contact angle, ° 22 ± 3 19 ± 2 �10*

44 ± 3 (maleic acid

ester)

*The exact value of the contact angle could not be estimated since the contact angle after

silicate adsorption was below the detection limit of the instrument (10°).

Whereas treatment with sodium silicate improved magnetite wettability as discussed in detail

in Paper III, adsorption of the surfactants resulted in an increased hydrophobicity of synthetic

magnetite. Nearly the same contact angle was obtained after treatment with Atrac 1563 or

maleic acid ester, suggesting that these compounds had a similar effect on magnetite wettability

at the conditions studied. A higher variation of the contact angle for the case of Atrac 1563

could possibly be the result of a complex chemical composition of the surfactant. The results in

Table 5 further indicate that sodium silicate did not prevent the adsorption of the surfactants

on magnetite in the presence of calcium ions, in agreement with the spectroscopic result

discussed above.

34

Verification for natural magnetite (Paper V)

In order to test whether the conclusion regarding surfactant adsorption on synthetic

magnetite in the presence of calcium ions and sodium silicate were applicable to natural

magnetite particles, adsorption of Atrac 1563 and water glass in the presence and absence of

calcium ions was performed on the mineral magnetite (see Fig. 9, 10 and Table 3 for material

characterization). Fig. 17 shows the results of the contact angle measurements performed after

batch adsorption experiments.

Figure 17. Water contact angle of the natural magnetite particles after adsorption of 1 mg g-1

water glass at pH 9 for 1 h, followed by the adsorption of Atrac 1563 for 20 minutes at the

same pH. Adsorption of both compounds was performed in the presence of either 10 mM

NaCl (�) or 4 mM CaCl2 (�). The points at 0 mg g-1 represent the contact angle of the

magnetite concentrate after adsorption of 1 mg g-1 water glass at pH 9 for 1 h.

In the absence of calcium ions, the contact angle of the natural magnetite did not change

upon the adsorption of Atrac 1563 indicating that no or very little adsorption took place on

magnetite pretreated with water glass. However, the adsorption of Atrac 1563 in the presence

of calcium ions resulted in an increased contact angle of the magnetite particles, despite the

pretreatment with water glass, due to the activation of the magnetite surface for surfactant

adsorption by calcium ions. These findings agree with the spectroscopic and contact angle

results obtained using synthetic magnetite.

35

Summary and implications for flotation and agglomeration of

iron ore

Based on the results discussed above, the following conclusions regarding surfactant

adsorption on iron oxides and its effect on the surface properties of iron oxides can be made:

1. Anionic carboxylate surfactants were capable of adsorbing on iron oxides at pH values

above the IEP of iron oxides. The adsorption increased in the presence of cations,

especially calcium, and could be reduced by preconditioning with sodium silicate, but

only in the absence of calcium ions. Desorption of the surfactants from the surface was

only partial, even at elevated pH (up to pH 10).

2. The zeta-potential and the IEP of magnetite particles in the presence of calcium

chloride and sodium silicate was determined by the relative content of these

compounds. Adsorption of an anionic carboxylate surfactant did not have any drastic

effect on the zeta-potential of magnetite.

3. Magnetite wettability improved after treatment with calcium chloride and sodium

silicate, whereas subsequent surfactant adsorption made the magnetite surface more

hydrophobic.

For the flotation of iron ore, these results imply that a certain amount of the flotation

collector would likely adsorb on magnetite increasing the required collector dosage, especially

at high concentrations of calcium ions in the process water. Calcium ions may also have an

adverse effect on the dispersing performance of water glass. The adsorbed flotation collector on

the magnetite surface could facilitate flotation of magnetite to a certain extent, resulting in

reduced flotation selectivity.

The fact that adsorbed collector species can hardly be removed from the magnetite surface by

rinsing with water suggests that a certain amount of flotation collector would remain on the

surface of the magnetite concentrate after flotation and would be carried over to the balling

drums. This would affect the pelletizing process negatively and would reduce the strength of

the pellets produced, as discussed above.

Contamination of magnetite with flotation collector may be expected to increase with

increased concentration of calcium ions in the process water. Consequently, the second part of

the present work was focused on finding the means to minimize the effect of adsorbed

flotation collector on magnetite wettability.

36

Restoring magnetite wettability after surfactant adsorption

(Papers IV, V)

Based on the discussion above, the best way to reduce the adsorption of flotation collector on

magnetite would probably be by decreasing the concentration of free calcium (and

magnesium) ions in the process water. Application of different chelating agents [17, 75-78] and

ion exchangers [79] has been proposed for that purpose. However, the removal of calcium ions

from the process water would result in further dissolution of sparingly soluble calcareous

minerals, again increasing the calcium concentration.

Alternatively, modification of the magnetite surface after flotation could be performed in

order to increase surface wettability. In the present work, two types of hydrophilizing agents

were investigated, namely, hydrophilic polymers and sodium silicate. Their effect on the

wettability of synthetic magnetite after surfactant adsorption is presented in this chapter.

Modification with sodium silicate

Since sodium silicate is known to have a depressing effect on iron oxides in flotation with

anionic carboxylate collectors, the ability of sodium silicate to improve magnetite wettability

after surfactant adsorption was investigated (see Table 6).

Table 6. Water contact angle of synthetic magnetite after consecutive adsorption of calcium

chloride, sodium silicate, and a surfactant, followed by treatment with sodium silicate in the

presence of calcium chloride for 24 hours. Concentrations of the reagents were the same as in

the experiments described in Table 5. Adsorption was performed at pH 8.5.

Treatment CaCl2, Na2SiO3, and surfactant CaCl2 and Na2SiO3

42 ± 2 (maleic acid ester) 21 ± 1 Contact angle, °

49 ± 3 (Atrac 1563) 16 ± 1

A considerable decrease in the magnetite contact angle was achieved after 24 hours of

conditioning with sodium silicate in the presence of calcium ions. The observed effect could

be caused by desorption of surfactant due to the difference in concentration at the surface and

in solution or by substitution of the surfactant species for silicate. However, the latter would

contradict the results reported by Roonasi et al. [69] for competitive adsorption of sodium

37

oleate and sodium silicate on magnetite, stating that silicate in solution could not easily replace

oleate adsorbed on the magnetite surface.

Modification with hydrophilic polymers

Three types of polymers, viz. cationic, anionic, and non-ionic (Table 2), were investigated

regarding their ability to adsorb on surfactant-coated magnetite and improve the wettability of

the magnetite surface. Although all the polymers tested adsorbed on magnetite (Fig. 2 in

Paper IV) independent of their charge and functionality, only anionic ammonium polyacrylate

could increase magnetite wettability after surfactant adsorption (see Table 7).

Table 7. Water contact angle of synthetic magnetite after consecutive adsorption of calcium

chloride, sodium silicate, and a surfactant, followed by treatment with a polymer, and storage

in air for 24 h. Concentrations of the reagents were the same as in the experiments described

in Table 5. Polymer concentration was 12.5 mg L-1. Adsorption was performed at pH 8.5.

Treatment Contact angle, °

Maleic acid ester Atrac 1563 CaCl2, Na2SiO3,

and surfactant 46 ± 5 43 ± 8

Polymer Cationic aliphatic

polyamine Starch Anionic ammonium polyacrylate

68 ± 2 40 ± 4 24 ± 6 20 ± 6

24 h in air 49 ± 11 46 ± 3 20 ± 4 24 ± 8

Spectroscopic results (Fig. 2 in Paper IV) did not provide any evidence for the detachment of

the surfactant from the magnetite surface upon polyacrylate adsorption, since no negative

absorption bands originating from the surfactant were present in the spectra of polyacrylate

adsorbed on magnetite. Accordingly, the decrease in the contact angle of synthetic magnetite

upon polyacrylate adsorption was most likely due to shielding of the hydrophobic surfactant

moieties from the water phase by long, flexible polymer chains able to form loops on the

surface, especially in the presence of calcium ions [45], as illustrated in Fig. 18. The high

density of the carboxylic groups in the polyacrylate chain makes it highly hydrophilic, resulting

in an improved wettability of the magnetite surface.

A similar phenomenon was reported by Somasundaran and Cleverdon [80] for

amine/cationic PAM adsorption on quartz. The authors concluded that the polymer interacted

38

with the surface without affecting surfactant adsorption and that depression of quartz was

achieved due to masking of the surfactant by the polymer.

Figure 18. Schematic illustration of polyacrylate adsorption on magnetite pretreated with

surfactant. For the sake of simplicity, iron oxide surface sites and other adsorbed species are not

shown.

Considering the mechanism of interaction of polyacrylate with the magnetite surface,

polymer adsorption likely took place via calcium ions [81], similar to the adsorption of

carboxylate surfactants. Calcium ions have been shown to facilitate polyacrylate adsorption on

oxides [45] due to reduced electrostatic repulsion both between the carboxylate ions in the

polymer chain and the negatively charged oxide surface, and between the carboxylate ions

within the molecule. Such intramolecular bridging results in a more coiled conformation of

polyacrylate chains both in solution and on the surface, increasing packing efficiency of the

polymer species on magnetite.

The conclusion regarding the mode of adsorption of polyacrylate on magnetite was further

confirmed by the zeta-potential measurements (Fig. 5 in Paper IV), which showed that the

zeta-potential of calcium-polyacrylate complex in solution was nearly the same as the zeta-

potential of the magnetite particles treated with calcium chloride, sodium silicate, anionic

surfactant, and polyacrylate, suggesting that polyacrylate was adsorbed on magnetite as a ternary

complex with calcium ions.

39

Verification for the flotated magnetite concentrate (Paper V)

To investigate whether soluble silicate and polyacrylate were efficient in improving the

wettability of magnetite concentrate after flotation, the contact angle of the concentrate was

measured by the Washburn method before and after adsorption of water glass and sodium

polyacrylate.

Fig. 19 illustrates the effect of treatment with water glass on the wettability of the magnetite

concentrate.

Figure 19. Water contact angle of the magnetite concentrate upon modification of the

surface with water glass in 10 mM NaCl at pH 9 for 9 h. Prior to water glass adsorption, the

concentrate was preconditioned with 10 mM NaCl at pH 9 for 1 hour. The points at

0 mg g-1 represent the contact angle of the magnetite concentrate after conditioning with

10 mM NaCl at pH 9 for 1 hour.

In concert with the results obtained for the synthetic magnetite, wettability of magnetite

concentrate after flotation was significantly improved by water glass adsorption, with a contact

angle of 28° ± 3° obtained at the highest water glass dosage. As could be expected, the

hydrophilizing effect increased with increased water glass concentration.

The adsorption of sodium polyacrylate on magnetite concentrate was performed in the

presence of calcium ions, based on the conclusion about the adsorption mode of polyacrylate

on magnetite discussed above. The wettability of the concentrate slightly improved upon

polymer adsorption at the concentration of 0.04 mg g-1 (Fig. 9 in Paper V). However, the

increase in polymer concentration at constant concentration of calcium ions did not lead to a

further decrease in the contact angle of the magnetite concentrate. Since calcium ions are

40

known to facilitate interaction of polyacrylate with metal oxides [45], polyacrylate adsorption

in the presence of calcium ions could be determined not only by polymer concentration in

solution but also by the calcium-to-polymer ratio. Fig. 20 illustrates the effect of calcium

chloride concentration on the wettability of magnetite concentrate at constant concentration of

sodium polyacrylate.

Figure 20. Water contact angle of the magnetite concentrate upon modification of the surface

with 0.04 mg g-1 sodium polyacrylate at pH 9 for 1 hour measured with the Washburn

technique. The point at 0 mM represents the contact angle of the magnetite concentrate

treated with sodium polyacrylate in the presence of 10 mM NaCl and without calcium ions.

Without calcium ions, treatment with the polymer did not have any effect on the wettability

of the magnetite concentrate. On adding 4 mM CaCl2 at the same concentration of

polyacrylate, the contact angle of the magnetite concentrate decreased as discussed above. Even

better results were achieved when the CaCl2 concentration was increased to 6 mM, indicating

that the efficiency of polyacrylate in improving magnetite wettability was affected by the

concentration of calcium ions.

41

Summary and implications for agglomeration of iron ore

To summarize the results of surface modification of magnetite after surfactant adsorption, the

following conclusions can be drawn:

1. Soluble silicate and polyacrylate were shown to improve wettability of synthetic

magnetite after surfactant adsorption and of mineral magnetite concentrate cleaned by

flotation.

2. The effect of sodium polyacrylate on magnetite wettability improved in the presence

of calcium ions.

3. The adsorption of sodium polyacrylate did not have any major effect on the zeta-

potential of synthetic magnetite particles.

Consequently, both water glass and sodium polyacrylate may be used to improve the

wettability of magnetite concentrate after flotation and prior to agglomeration. Improved

wetting of magnetite concentrate could be expected to facilitate agglomeration and increase

the strength of the pellets produced. Compared to water glass, treatment with polyacrylate

would require less time and would not introduce any impurities to the final product after

sintering. The fact that calcium ions facilitate adsorption of polyacrylate on magnetite makes

the polymer suitable for the application in processes utilizing process water rich in calcium.

Since the zeta-potential of magnetite was not affected by polyacrylate adsorption to any

considerable extent, treatment with the polymer would not impair the electrostatic interaction

between the magnetite concentrate and bentonite binder in agglomeration.

Considering environmental issues, polyacrylate with a molecular weight similar to the one

used in this study (Mw 4500 and 4000, respectively) has not been found to have any adverse

effect on the environment [82]. Despite low biodegradability [83], polyacrylate and its products

of degradation are not toxic to aquatic, terrestrial, and mammalian species [82, 84]. In hard

water, polyacrylate can precipitate in the form of calcium polyacrylate when all the carboxylic

groups in the polymer become neutralized by calcium ions [82], resulting in polymer removal

from the aqueous phase.

Accordingly, polyacrylate seems to be a good candidate for use in improving wettability of

flotated magnetite concentrate prior to agglomeration.

42

CONCLUSIONS

A versatile method based on ATR-FTIR spectroscopy was developed and successfully used

for in-situ studies of the adsorption of surfactants, polymers, and inorganic compounds on thin

films of synthetic iron oxides.

Using the developed method, the adsorption mechanism of several surfactants on iron oxides

was elucidated.

The dramatic effect of calcium ions on the adsorption of carboxylate surfactants on magnetite

was for the first time confirmed in-situ. It was also illustrated that soluble silicate could reduce

surfactant adsorption on magnetite but only in the absence of calcium ions.

Among other factors that affected surfactant adsorption on iron oxides were surfactant

concentration, pH, and ionic strength. Variation of conditioning time with sodium silicate was

not found to have any considerable effect on surfactant adsorption on magnetite in the

presence of calcium ions.

Surfactant adsorption considerably decreased magnetite wettability. Once adsorbed, surfactant

species could not be completely removed from the surface by rinsing with water, even at

elevated pH.

Treatment with soluble silicate and polyacrylate proved to be a feasible means for restoring

wettability of magnetite after surfactant adsorption. The effectiveness of polyacrylate improved

in the presence of calcium ions, making this polymer a good candidate for applications in hard

water.

The results obtained using synthetic iron oxides were verified for natural magnetite particles,

suggesting that in-situ ATR-FTIR spectroscopy in combination with zeta-potential and

contact angle measurements on synthetic materials could be successfully applied to studying

surface phenomena related to mineral processing.

43

44

FUTURE WORK

Since it has previously been shown that storage of the magnetite concentrate prior to

agglomeration helps to improve green pellet quality, it would be interesting to investigate the

aging of flotation collector and other species (e.g. silicate) present on the magnetite surface.

To confirm or disprove the proposed adsorption mechanism of silicate, surfactant, and

polymer on magnetite in the presence of calcium ions, it would be useful to study the

distribution of these species on the magnetite surface using ATR-FTIR microscopy.

To test whether treatment of the flotated magnetite concentrate with water glass or

polyacrylate could improve green pellet quality, small-scale balling of the concentrate could be

performed, followed by porosity, strength, and plasticity studies of the green pellets.

Interactions between bentonite binder and magnetite modified with surfactant and/or

polyacrylate could also be studied in order to obtain information about possible effects of these

species adsorbed on magnetite concentrate on bentonite performance as a binder.

45

46

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53

54

PAPER I

Studies of collector adsorption on iron oxides by in-situ

ATR-FTIR spectroscopy

E. Potapova, I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund

Industrial and Engineering Chemistry Research 49 (2010) 1493-1502

Studies of Collector Adsorption on Iron Oxides by in Situ ATR-FTIRSpectroscopy

E. Potapova,* I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund

DiVision of Chemical Engineering, Luleå UniVersity of Technology, SE-971 87 Luleå, Sweden

In this work, the adsorption of three model collectors, viz., poly(ethylene glycol) monooleate (PEGMO),ethyl oleate, and maleic acid, as well as the commercial fatty-acid-type collector Atrac 1563, was studied insitu on synthetic hematite using attenuated total reflectance Fourier transform infrared (ATR-FTIR)spectroscopy. The adsorption behavior of the studied compounds on hematite was determined to a largeextent by the polar headgroup. Adsorption of Atrac and PEGMO as a function of concentration showed goodagreement with the Freundlich adsorption model, suggesting energetically heterogeneous adsorption. In situdesorption experiments revealed that a large fraction of the Atrac was weakly attached to the hematite surface,as it was partially removed by flushing with water at pH 8.5 and 10. These results suggest that a separatewashing unit after the flotation step could be beneficial in reducing the contamination of iron ore by flotationchemicals.

Introduction

Iron ore pellets are an important refined product used as araw material in the manufacturing of steel. The production ofiron ore pellets comprises several stages: grinding and upgradingof the iron ore; balling of wet, so-called, green pellets; anddrying, sintering, and oxidation of the green pellets to the finalproduct, to be transported to iron or steel plants.

LKAB is a Swedish mining company whose pelletizing plantsutilize magnetite iron ore from two deposits located in northernSweden: Kiruna and Malmberget. The Kiruna ore is a mixtureof magnetite and apatite having a phosphorus content of ca. 1wt %. To reduce the phosphorus content to an acceptable levelfor the blast furnace process1 (i.e., to less than 0.025%), theore is subjected to reverse flotation with an anionic fatty-acid-type collector reagent (Atrac 1563) with methyl isobutyl carbinol(MIBC) used as a frother. To increase flotation selectivity andphosphorus recovery, sodium silicate is added to the system.The sodium silicate is used in fairly small amounts (300-500g t-1) and thus acts primarily as a dispersant, and it has notbeen found to prevent collector adsorption on the magnetitesurface to any great extent.2,3

Ideally, the collector should adsorb only on the apatite gangue,rendering it hydrophobic and thus easily floated from themagnetite. However, unwanted adsorption of the flotationreagents on magnetite also occurs. It has been estimated thatthe amount of Atrac adsorbed on the magnetite fed to ballingcircuits is 10-30 g t-1.4 Once the collector is adsorbed on thesurface of magnetite, it is difficult to eliminate.5 Therefore, thecollector should be added in sufficiently small amounts so thatthe apatite surface is rendered hydrophobic whereas the adsorp-tion on magnetite is minimized. However, in the real process,the Atrac dosage is adjusted based on the phosphorus contentin the pellet feed, and there is a slight degree of overdosage toensure that the desired phosphate levels are achieved. Typically,the dosage varies in the range of 30-70 g per tonne of magnetiteconcentrate.6

After the flotation step, the pulp passes through magneticseparation and filtration steps where it is subjected to repeated

dilutions and thickenings that can have a mild washing effecton the ore, partly removing the flotation reagents from themagnetite surface. However, no separate washing unit is usedfor the purification of magnetite from the flotation chemicals atLKAB.

It has been found that adsorption of the flotation collectoragent on the iron ore has a negative effect on the balling process,as the collector adsorbed on the surface of magnetite makesthe particles more hydrophobic, which can lead to the attachmentof air bubbles onto the surface of the iron ore particles.6 Theair bubbles incorporated into green pellets decrease the greenpellet strength in both the wet and dry states. A low wet strengthtends to cause a wide size distribution of the green pelletsleaving the balling drums. The undersize fraction is recirculatedback to the balling drum, thus leading to increased energyconsumption and decreased capacity of the pelletizing plant.Breakage of the pellets at the stage of drying and indurationcauses dust formation and decreased pellet bed permeability,resulting in lower production volumes and aggravated pelletquality.7

To minimize the influence of the collector on the pelletizingprocess, it is important to understand the mechanism by whichthe collector interacts with the iron oxide. For instance, it hasbeen shown that the presence of Ca2+ ions in the process waterincreases the adsorption of flotation collector reagent onmagnetite.8 In that work, the adsorption of the collector OS 130on magnetite was studied in the batch experiments by measuringthe residual concentration of the collector in solution afteradsorption using the method of Gregory.9 According to thesuggested8 mechanism, positively charged calcium ions adsorbat negatively charged surface sites on the magnetite surface,which results in a more positively charged surface, rendering itmore favorable for the adsorption of negatively charged collectorspecies.

Spectroscopic techniques have been used extensively forstudying the adsorption of fatty-acid-based collectors ontomineral surfaces.10-12 Fourier transform infrared (FTIR) spec-troscopy is widely applied because it provides the possibilityof identifying complexes formed at the surface.13 Ex situ FTIRtechniques, such as diffuse reflectance infrared Fourier transform(DRIFT) spectroscopy and reflection absorption infrared spec-

* To whom correspondence should be addressed. E-mail:elisaveta.potapova@ltu.se. Tel.: +46 920 491 776.

Ind. Eng. Chem. Res. 2010, 49, 1493–1502 1493

10.1021/ie901343f © 2010 American Chemical SocietyPublished on Web 01/07/2010

troscopy (RAIRS), imply drying of the sample after adsorption,which can affect surface complexes and, hence, the validity ofthe results. On the contrary, FTIR attenuated total reflectance(FTIR-ATR) spectroscopy facilitates in situ studies of bothadsorption kinetics14 and complexes formed on the surface inthe presence of water.13,15 In the ATR technique (Figure 1), asample with a certain refractive index, n1, is placed in a closecontact with a waveguide characterized by a high refractiveindex, n2. The IR beam is passed through the waveguide at acertain incident angle θ. In the case when sin θ is larger thanthe ratio n1/n2, the beam is totally reflected inside the waveguide,forming a perpendicular evanescent wave of the IR radiation ateach point of reflection. This wave propagates through thesample in the vicinity of the waveguide and interacts with thesample, causing partial absorption of the radiation by the samplematerial and thus a reduction of the intensity of the totallyreflected beam. The penetration depth, dp, is defined as thedistance from the interface at which the electric field of the waveis equal to E0e-1 (see Figure 1), where E0 is the electric field ofthe wave at the interface. Typical values of the penetration depthare from some hundred nanometers to a few micrometers,depending on the refractive indices of both the waveguide andthe sample, as well as on the wavelength and incident angle ofthe radiation. Such a range of penetration depths makes FTIR-ATR spectroscopy a very surface-sensitive technique, providingthe possibility for in situ studies of surfaces in contact withaqueous solutions.

During the past decade, the FTIR-ATR technique has beendeveloping, and new applications in surface chemistry haveevolved, for example, as a tool for studying adsorption13,16-19

and diffusion20-24 in thin films or at interfaces. This techniquehas also been used in studies of catalytic reactions,25,26 aswell as in sensor applications.27,28 FTIR-ATR spectroscopyhas also been applied for studying the adsorption of surfac-tants at mineral surfaces, both qualitatively29,30 and quantita-tively.14,31-33 Our group has developed the ATR technique forstudies of adsorption in zeolite films17,27,34-37 and on mineralsurfaces.14,18,38-41 The studies have been both qualitative35-38,40,41

and quantitative,14,17,18,27,34,39 and even the molecular orientationsof adsorbates have been determined.17,18,35,38,41

Although magnetite (Fe3O4) is the main iron-containingmineral in the ore utilized by LKAB, it was found to becomepartly oxidized during storage in air, forming first maghemite(γ-Fe2O3), which has the same crystal structure but mostly Fe3+

on the surface, and then hematite (R-Fe2O3), which has adifferent crystal structure than magnetite and maghemite.42 Thisphenomenon was described earlier for submicrometer-sizedmagnetite particles by both Haneda and Morrish43 and Gediko-

glu.44 In solution, the oxidation of magnetite was also observedand explained by the leaching of Fe2+ ions from the surface.45

Thus, when magnetite ore is subjected to grinding and flotation,apparent oxidation of the surface can occur both in water andin air.

In this work, the adsorption of flotation collector reagentson iron oxide was studied in situ using FTIR-ATR spectroscopyfor the purpose of obtaining essential information about theinteraction between the flotation collector reagents and the ironoxide surface in the presence of water. This information iscrucial for the improvement of the pelletizing process, as it couldsuggest possibilities to reduce the unwanted collector adsorptionon the iron ore surface and improve green-pellet strength.Hematite was chosen as the adsorbent because it was reportedto be the final product of the surface oxidation of magnetite. Inaddition, reports on the adsorption properties of collector-typemolecules on hematite were found to be very scant in theliterature.

Materials and Methods

Materials. Hematite crystals were synthesized from an FeCl3

solution according to the method described by Matijevic.46 Theobtained hematite crystals were purified by repeated (five times)centrifugation at 20000 rpm for 30 min and redispersion in a0.06 M aqueous solution of acetic acid (glacial, >99.7%, AlfaAesar). The crystals were stored as a 2 wt % suspension in a0.06 M aqueous solution of acetic acid at pH 3. For powderX-ray diffraction analysis, the suspension was freeze-dried,yielding a fine hematite powder.

The flotation collector reagent, Atrac 1563 (Akzo Nobel,Sweden), was provided by LKAB. Atrac 1563 is a yellowviscous liquid with a complex chemical composition: 50-100%ethoxylated tall oil ester of maleic acid and 1-5% maleicanhydride (Akzo Nobel material safety data sheet). Tall oil is abyproduct of the Kraft pulp manufacturing process and is amixture of mainly fatty acids (e.g., oleic acid) and resin acids(e.g., abietic acid). A similar collector for the froth flotation ofoxide and salt-type minerals that is a combination of a monoesterof a dicarboxylic acid and a monocarboxylic acid is describedin a patent.47 As described in the patent, the first component isan aliphatic monocarboxylic acid containing 8-22 carbon atomsbonded to a dicarboxylic acid containing 4-8 carbon atomsthrough an alkylene oxide group with 2-4 carbon atoms, thusresulting in a molecule with two ester carbonyls and one freecarboxylic group at the end of the molecule. Monocarboxylicacid with 6-24 carbon atoms is added to increase the selectivityand/or yield of the monoester.

Poly(ethylene glycol) monooleate (PEGMO) with a typicalnumber-average molecular weight (Mn) of 460 (Aldrich), maleicacid (Fluka, g 99%), and ethyl oleate (Aldrich, 98%) were usedas model collector reagents. From the average molar mass, thelength of the poly(ethylene glycol) chain in the PEGMO modelcollector reagent was estimated to be about four ethylene glycolunits. Oleic acid esters were chosen because oleic acid is oneof the main components of tall oil. Ethoxylated tall oil couldthus be modeled by using PEGMO, which has the same structureas ethoxylated oleic acid. Maleic acid was chosen as one of themodel compounds because it is the tail group of the moleculesin Atrac. Ethyl oleate was used as the third model compoundto study the effect of the poly(ethylene glycol) chain on theadsorption properties of oleate.

Working solutions of Atrac and the model collector reagentswere prepared in the following way: First, a 0.1 g L-1 stocksolution of the compound in distilled water was prepared. In

Figure 1. Schematic image of an ATR waveguide illustrating the ATRphenomenon. For the sake of clarity, the thickness of the sample (in thiscase, iron oxide film) and the penetration depth dp are enlarged.

1494 Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

the next step, the required amount of the stock solution wasmixed in distilled water to give final solutions of the desiredconcentration (1-25 mg L-1).

All aqueous solutions were prepared using distilled water.The distilled water used for the spectroscopic measurementswas first boiled for 1 h, and then argon was bubbled throughthe water to minimize the amount of dissolved carbon dioxide.

The pH was controlled during the experiments by a MettlerToledo T70 titrator using a 0.05 M aqueous solution of sodiumhydroxide (per analysis, Merck).

Dip-Coating. Hematite films were deposited on ZnSe sub-strates using a Nima DC-multi 8 dip-coater. Prior to deposition,the substrates were washed in acetone (g99.5%, VWR), ethanol(99.7%, Solveco Chemicals AB), and distilled water (10 minin each). A 2 wt % hematite suspension was prepared bydispersing the hematite crystals in a 6.27 M aqueous solutionof acetic acid. The substrates were immersed in the suspensionand withdrawn at a speed of 5 mm min-1, and the film thicknesswas controlled by the number of dips. To prepare a film with athickness of ca. 1 μm, eight dips were needed.

Scanning Electron Microscopy (SEM). SEM images of thehematite film on a ZnSe substrate were obtained using a PhilipsXL 30 microscope with a LaB6 filament. The samples weremounted on alumina stubs using carbon glue and subsequentlysputtered with a thin layer (ca. 10 nm) of gold to provideconductivity.

X-ray Diffraction. X-ray diffraction patterns of both hematitepowder and film were collected with a Siemens D5000diffractometer running in Bragg-Brentano geometry using CuKR radiation. To analyze the film, the hematite-covered ZnSesubstrate was mounted with carbon glue onto a custom-madealumina holder.

Zeta-Potential Measurements. The point of zero charge(PZC) of the hematite crystals used in this work was determinedby electrophoresis using a ZetaCompact instrument equippedwith a charge-coupled device (CCD) tracking camera. Theobtained electrophoretic mobility data were further processedby the Zeta4 software applying the Smoluchowski equation. Thesamples were prepared in the following way: One drop of thehematite suspension was dispersed in 1 L of 0.01 M potassiumnitrate. The pH of the samples (10 samples, 100 mL each) wasadjusted using potassium hydroxide and nitric acid. The samplesspanned the pH range from 2 to 11. For each sample, themeasurement was repeated three times, and the final PZC wascalculated as an average of the obtained values.

FTIR-ATR Spectroscopy. Infrared spectra were recordedusing a Bruker IFS 66v/S spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector.ZnSe ATR crystals (Crystran Ltd.) in the form of a trapeze with45° cut edges and dimensions of 50 × 20 × 2 mm were usedin this study. Measurements of adsorption on the hematite-coatedATR crystals were performed in situ in a cell with a flowpumped through on both sides of the ATR crystal; see Figure2. The incidence angle of the infrared beam was set to 45°.

All adsorption experiments on hematite were performed atroom temperature using water solutions of model collectors orAtrac at pH 8.5 (the pH used in the flotation process at LKAB)pumped continuously through the cell at a flow rate of 10 mLmin-1 with recirculation. Prior to the adsorption measurements,the hematite film was flushed with a weakly alkaline solution(pH 8.5) for 2 h to remove the residues of acetic acid andcarbonate species from the surface. A background spectrum wasrecorded afterward with water at pH 8.5 in contact with ahematite-coated ZnSe substrate. Spectra of the model collectors

and Atrac in pure form were recorded in argon atmosphere usinga bare ZnSe substrate with a droplet of a collector spread overits surface. ZnSe in argon atmosphere was recorded as a single-beam background spectrum. All background and sample spectrawere obtained by averaging 500 scans at a resolution of 4 cm-1.Data processing was performed using Bruker Opus 4.2 software.

Results and Discussion

Film and Powder Characterization. Figure 3 shows aphotograph of ZnSe crystals before and after being coated witha hematite film. As is evident from the uniform red color of thecoated ATR crystal, the obtained film appeared to be quiteuniform, continuous, and even along the crystal surface andstable under the conditions used for the in situ experiments,because the visual appearance of the coated crystal did notchange after the experiments.

SEM images (Figure 4) showed that the hematite crystalshad a uniform spherical habit with a diameter of ca. 130 nmand that the crystals were distributed evenly over the ATRcrystal surface, forming a porous film with an average thicknessof ca. 1 μm.

Figure 5 shows XRD patterns of freeze-dried synthetichematite powder and a hematite film on a ZnSe crystal. TheXRD pattern of the powder shows that the synthesized materialcontained pure, randomly oriented hematite crystals without anyother iron oxide phase present in amounts detectable by XRD.By comparing the relative reflection intensities in the powderand in the film, it can be concluded that the crystals in the filmwere also randomly oriented.

Figure 6 shows the �-potential of the synthetic hematitecrystals used in this work as a function of pH. The point of

Figure 2. Schematic image of the FTIR-ATR flow cell. For the sake ofclarity, the thickness of the iron oxide film is enlarged.

Figure 3. Images of (a) uncoated and (b) hematite-coated ZnSe ATRcrystals.

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010 1495

zero charge (PZC) is at about pH 4.8. According to datacompiled by Fuerstenau48 and Cromieres,49 the PZC of hematitehas been reported to occur within a broad range of pH values,viz., from 4.8 to 10.3. Factors that can influence the value ofthe PZC are1 the technique used to determine the �-potential,impurities (especially in the case of minerals), sample prepara-tion procedure, and species adsorbed on the surface (e.g.,carbonates).50,51

In the present study, the PZC of hematite was probablyaffected by the carbonate species adsorbed on the surface, whichare known to lower the PZCs of iron oxides by ca. 1 pH unit.52

The presence of the carbonate species was confirmed byspectra (not shown) recorded during flushing of the hematitefilm with distilled water at pH 8.5 prior to the adsorptionexperiments. Weak negative bands at ca. 1490 and 1340 cm-1

originating from the outer-sphere carbonate species52 wereobserved in the spectra, indicating the desorption of carbonatesfrom the hematite surface. Desorption of carbonates had ceasedalready after 30 min of flushing, suggesting that carbonatespecies in solution and on the surface were in equilibrium.

Adsorption of Model Compounds. To better understand theadsorption mechanism of Atrac on hematite, three modelcompounds of Atrac were studied. As it is known that Atraccontains maleic acid esterified with an ethoxylated tall oil (AkzoNobel material safety data sheet), poly(ethylene glycol) mo-nooleate (PEGMO) and maleic acid were chosen as modelcompounds. From the number-average molecular weight, Mn

) 460, it was estimated that an average poly(ethylene glycol)chain in PEGMO contained four repeated ethoxy units(-CH2-O-CH2-). Ethyl oleate was used as an additionalmodel compound to study the effect of the poly(ethylene glycol)chain on the adsorption properties of oleate.

PEGMO is a nonionic surfactant53 characterized by a highersolubility in water than the corresponding fatty acid because ofthe poly(ethylene glycol) chain. PEGMO can be expected tointeract with the iron oxide surface in three different ways, viz.,through the ester carbonyl, the ether oxygen linkages, or thetail hydroxyl group. Figure 7 shows spectra of a droplet of purePEGMO on ZnSe and PEGMO adsorbed on hematite fromaqueous solution.

Strong absorption bands at 2922 and 2854 cm-1 in Figure7a originate from asymmetric and symmetric stretching vibra-tions of the C-H bonds (νas and νs).

54 Long-chain carboxylicacids (e.g., oleic acid) contain significantly more methylenegroups than methyl groups and are thus characterized by muchstronger absorption bands corresponding to the CH2 groupscompared to the CH3 groups. The latter can be observed onlyas shoulders. C-H deformation in methyl and methylene groupsis found around 1458 cm-1.55 The absorption band observed inpure PEGMO at 1736 cm-1 (Figure 7a) is associated withstretching vibrations of the CdO ester bond.56 It is only slightlyshifted upon adsorption on hematite (Figure 7b), indicating thatno significant interaction occurs between the ester carbonyl andthe surface. The absence of bands around 1570 and 1430 cm-1,corresponding, respectively, to asymmetric and symmetric

Figure 4. (a) Top- and (b) side-view SEM images of a hematite film on aZnSe crystal.

Figure 5. XRD patterns of hematite powder (top) and hematite film on aZnSe crystal (bottom). The peak labeled with an asterisk (*) emanates fromthe ZnSe substrate.

Figure 6. �-potential of the hematite crystals as a function of pH.

1496 Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

stretching of a carboxylate group,12,57 suggests that the esterbond in PEGMO does not break upon adsorption on hematite.

The intense band at 1115 cm-1 (Figure 7a) emanates fromthe C-O-C stretching vibration in the poly(ethylene glycol)chain.58 It has a shoulder on its low-frequency side at ca. 1070cm-1 probably emanating from the stretching of the C-O bondbetween the hydroxyl group and the CH2 group at the end ofthe poly(ethylene glycol) chain.59 Both the peak frequency andthe shoulder frequency are shifted to lower wavenumbers whenPEGMO is adsorbed on hematite (see Figure 7b; 1115f 1095cm-1 and 1070f 1047 cm-1, respectively), suggesting that thepoly(ethylene glycol) chain is involved in the bonding to thesurface.

Figure 8 shows the change in absorbance (measured as peakheight) of the 2854 cm-1 band during the adsorption of PEGMOon hematite at different concentrations after 1 h of adsorptionat each concentration. The experiment was started at the lowestconcentration of 1 mg L-1; thereafter, the concentration insolution was increased, and the measurement was continued.

Figure 8 shows that the intensity of the band originating fromthe symmetric stretching vibration (νs) of the C-H bondincreased with increasing concentration in solution, suggestingan increase of the adsorption of PEGMO on hematite. Theexperimental data exhibited a poor fit with the Langmuir modelof adsorption, with a coefficient of determination (R2) of 0.87.Plotting the data on a logarithmic scale yielded a straight linewith R2 ) 0.99, indicating that the obtained data were in goodagreement with the Freundlich model of adsorption, whichimplies that the surface of adsorption is heterogeneous, for

instance, with different adsorption sites grouped patchwise basedon their adsorption energies, as suggested earlier.60 A desorptionexperiment (not shown) showed that the intensity of the bandoriginating from the symmetric stretching vibration (νs) of theC-H bond in PEGMO was reduced only by 5% upon flushingthe cell with water at pH 8.5 for 1 h, indicating that most of thePEGMO was strongly adsorbed to the hematite surface.

Maleic acid was the second model compound studied. Theability of maleic acid to form intramolecular hydrogen bondsmakes it easily soluble in water. In aqueous solution at pH 8.5,it is expected to be fully deprotonated.61

From the spectroscopic measurements it was concluded thatmaleic acid did not adsorb on hematite to any considerableextent at pH 8.5 in the concentration range studied, viz., 1-25mg L-1. This could be explained by the fact that, at pH 8.5, thehematite surface is negatively charged and thus repels maleicacid, which, at this pH, contains two negatively chargedcarboxylate ions. Hwang and Lenhart60 studied the adsorptionof maleic acid on hematite at various pH values and concludedthat the adsorption was controlled by the surface charge ofhematite, suggesting that electrostatic interaction is the pre-dominant force of adsorption of maleic acid on hematite, whichagrees well with our observations.

Ethyl oleate was the third model compound of Atrac used inthe experiments. It is almost insoluble in water due to the factthat, instead of a hydrophilic poly(ethylene glycol) chain, it hasa hydrophobic ethyl group bonded to the carboxylate. Thischange in chemical composition, of course, also affects theadsorption properties of the molecule. The change in absorbanceof the 2852 cm-1 band during the adsorption of ethyl oleate onhematite at different concentrations is shown in Figure 9. Themeasurements were performed in the same way as for PEGMO.Equilibrium was achieved at each concentration within 2 h.

Figure 9 shows that the absorbance of the band originatingfrom the symmetric stretching vibration (νs) of the C-H bondis about a factor of 6 lower at all concentrations of ethyl oleatethan the absorbance of the same band at correspondingconcentrations of PEGMO. This indicates that about 6 timesless ethyl oleate is adsorbed compared to PEGMO at thecorresponding concentrations. The interaction between ethyloleate and the surface was probably very weak, as no significantband shifts were observed in the spectrum of ethyl oleate onhematite (not shown) compared to the spectrum of pure ethyloleate.

Figure 7. Infrared spectra of (a) a droplet of pure PEGMO on ZnSe and(b) PEGMO adsorbed on a hematite film in situ from a 10 mg L-1 aqueoussolution. Note that a.u. represents arbitrary units here and elsewhere.

Figure 8. Intensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond as a function of concentration of PEGMOin solution (0). The solid line represents the fitted Freundlich adsorptionmodel.

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Atrac Adsorption. Anionic fatty-acid surfactants are believedto interact with iron oxides electrostatically, that is, to adsorbon the positively charged iron oxide surface below its point ofzero charge (PZC).1 Nevertheless, oleate species are also knownto chemisorb on hematite at several pH units above the PZC,forming iron oleate.12,62 Despite the fact that, at pH 8.5, thehematite surface is charged negatively (see Figure 6), adsorptionof Atrac on hematite at this pH was still observed. Figure 10shows the infrared spectra of a droplet of pure Atrac on ZnSeand Atrac adsorbed on hematite from an aqueous solution.

Being a multicomponent system, Atrac presents a rathercomplicated infrared spectrum with several absorption bandsin the 1000 and 3000 cm-1 regions (Figure 10). Several bandsin the spectrum of pure Atrac (see Figure 10a) are similar tothose observed in the spectrum of pure PEGMO (see Figure7a), including CH2 stretching vibrations at 2924 and 2855 cm-1

and CH2 deformation at 1456 cm-1. The absorption bandsobserved in pure Atrac at 1736 and 1159 cm-1 (see Figure 10a)are associated with stretching vibrations of the CdO55 andC-O63 bonds in esters, respectively. Upon complexation of theester group with a metal ion, ν(CdO) shifts to lower frequency,and ν(C-O) shifts to higher frequency.64 These shifts areobserved in the case of adsorption of Atrac on hematite, withν(CdO) and ν(C-O) shifting by 14 and 16 cm-1, respectively(see Figure 10b), suggesting rather strong interaction betweenthe ester carbonyls in Atrac and the hematite surface.

The stretching vibration of the CdO bond of free carboxylicacids is observed in pure Atrac at 1709 cm-1 (see Figure 10a).65

Upon deprotonation of the carboxylic group in solution this banddisappears, and two new bands are observed at 1568 and 1427cm-1 in the spectra of Atrac adsorbed on hematite (νas and νs,respectively; see Figure 10b).66 As discussed above, adsorptionof maleic acid on hematite was not observed under theexperimental conditions used because of the electrostaticrepulsion of the carboxylate anion and negatively chargedhematite surface, suggesting that the adsorption of Atrac onhematite through carboxylate ions is unlikely and that the mostprobable interaction is through ester carbonyls connected byan ethoxy group. However, the carboxylate ions in the adsorbedmolecules of Atrac are situated in the vicinity of the surface,and the bands originating from them can thus be found in thespectra of Atrac adsorbed on hematite. For band assignments,see Table 1.

As one of the main components of Atrac is known to beethoxylated tall oil, stretching vibrations of the C-O-C groupin the ethoxy chain were expected to be observed around 1100cm-1.58 However, rather weak absorption bands were found inthat wavenumber range in the spectra of both pure Atrac andAtrac adsorbed on hematite (see Figure 10), indicating that thedegree of ethoxylation of the tall oil is quite low. This is furthersupported by the information found in the patent47 indicatingthat only one ethoxy group derived from an alkylene oxide withtwo to four carbon atoms is found in the collector. A shorterpoly(ethylene glycol) chain and the possible presence of highlynonpolar resin acids in Atrac result in a lower solubility of Atracas compared to that of PEGMO.

Prior to studying the adsorption of Atrac as a function ofconcentration on hematite, the adsorption as a function ofconcentration on an uncoated ZnSe crystal was investigated. InFigure 11, the intensity of the 2855 cm-1 band is plotted as afunction of Atrac concentration in solution. The measurementswere carried out in a similar way as for PEGMO. At eachconcentration, steady state was reached within 1 h.

Figure 9. Intensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond as a function of the concentration of ethyloleate in solution.

Figure 10. Infrared spectra of (a) a droplet of pure Atrac on ZnSe and (b)Atrac adsorbed on a hematite film in situ from a 10 mg L-1 aqueous solution.

Table 1. Assignment of Absorption Bands Originating from PureAtrac Spread over ZnSe and Atrac Adsorbed on Hematite in Situfrom a 10 mg L-1 Aqueous Solution

pure Atrac on ZnSe Atrac adsorbed on hematite peak assignment

2924 2926 νas (CH2)54

2855 2855 νs (CH2)54

1736 1722 ν(CdO) in ester56

1709 ν(CdO) in acid65

1568 νas (COO-)66

1427 νs (COO-)66

1456 1456 δ (CH2 and CH3)55

1159 1175 ν (C-O) in esters63

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Figure 11 illustrates that the absorbance increases withincreasing concentration, but not linearly, suggesting that therecorded signal corresponds not to the bulk concentration butto the amount of Atrac adsorbed on ZnSe. The measuredabsorption intensities are quite low, as expected for a polishedZnSe surface with a very low surface area. Atrac was detectedon the crystal even after the cell had been flushed with waterfor 1 h and the crystal had been dried in air, indicating a ratherstrong affinity for the ZnSe surface.

Figure 12 shows the change in the intensity of the bandoriginating from the symmetric stretching vibration (νs) of theC-H bond during the adsorption of Atrac on hematite as afunction of concentration. The measurements were performedin the same way as for PEGMO. Adsorption equilibrium wasachieved at each concentration within 3-5 h.

Figure 12 demonstrates that the adsorption of Atrac on thehematite surface increased with increasing concentration insolution. A poor fit of the experimental data was observed forthe Langmuir model of adsorption with a coefficient ofdetermination (R2) of 0.66. When plotted on a logarithmic scale,the experimental data resulted in a straight line with R2 ) 0.99,suggesting that the Freundlich adsorption model fitted theexperimental data quite well.

Desorption of Atrac from hematite with time was studied insitu by flushing the cell with distilled water at two different pHvalues; see Figure 13. Prior to desorption, adsorption of Atrac

on hematite was performed in situ from a 25 mg L-1 solutionat room temperature and pH 8.5 until adsorption equilibriumwas reached. After the first desorption experiment at pH 8.5,Atrac was adsorbed again on the same hematite film from thesame solution until adsorption equilibrium was reached. Afterthat, desorption at pH 10 was performed. As the two adsorptioncurves leveled out at approximately the same absorbance values,the second adsorption curve is not shown.

As illustrated by Figure 13, the absorbance was reduced ratherrapidly, even when the sample was flushed with distilled waterat pH 8.5 (i.e., the same pH as used during the adsorption),indicating that Atrac desorbed rather rapidly. However, the dataindicate that, after 1 h of flushing, more than 50% of the Atracstill remained on the surface, suggesting that some amount ofAtrac was strongly attached to the hematite surface. When thesample was flushed with distilled water at pH 10, the dataindicate that Atrac desorbed more rapidly, probably because ofthe electrostatic repulsion between the carboxylic groups andthe, at that pH, highly negatively charged hematite surface.Nevertheless, after 1 h of flushing, the data indicate that stillabout 40% of the originally adsorbed Atrac remained on thehematite surface. These results suggest that mild washing ofthe iron ore during magnetic separation and filtration in LKAB’sprocess is not sufficient to completely remove the adsorbedflotation collector from the iron ore, especially if the pH of thewater at these steps is below the flotation pH. At LKAB, theyearly average pH of water in the clarifying pond, whichprovides 80% of the process water, is reported to have variedin the range from 7.8 to 8.1 during the period of time from1992 to 2004.67 However, the seasonal variation of pH is muchhigher and covers the pH range from 7.2 to 9.2, with lowervalues during the period of snowmelt. A decrease of pH duringwashing as compared to that during flotation (8.5) could leadto further adsorption of the flotation reagent on the surface ofiron ore. A separate washing unit operating at a pH higher thanthe flotation pH before the pelletization plant could be helpfulin reducing the amount of Atrac adsorbed on the magnetitesurface and possibly improving green-pellet strength.

Adsorption Mechanisms of Atrac and Model Compounds.As shown above, the mode of adsorption of carboxylic acidsand their derivatives is determined by the tail group of themolecule and its polarity. Maleic acid did not adsorb on hematiteat the pH of the experiments likely because of the electrostaticrepulsion between the carboxylate anion and the negativelycharged surface. Ethyl oleate showed a very weak interaction

Figure 11. Intensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond as a function of the concentration of Atracin solution.

Figure 12. Intensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond as a function of the concentration of Atracin solution (0). The solid line represents the Freundlich adsorption modelfitted to the experimental data.

Figure 13. Intensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond in Atrac as a function of time during the insitu adsorption of Atrac onto hematite from a 25 mg L-1 solution at pH 8.5(Δ) and desorption by flushing with water at pH 8.5 (() and pH 10 (0).

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with hematite because of the nonpolar ester group. BothPEGMO and Atrac were found to adsorb on hematite at theexperimental conditions. Based on the interpretation of thespectroscopic data, PEGMO and Atrac are proposed to adsorbas illustrated in Figure 14.

PEGMO showed rather strong absorption intensities in theIR spectra when adsorbed on hematite, with significant shiftsof the bands originating from the ethoxy and hydroxyl groups,which indicate that the adsorption of PEGMO probably occursthrough the long ethoxy chain and the tail hydroxyl group(Figure 14a), where the latter is not likely to be deprotonatedat the pH used in this work.

Being a combination of ethoxylated fatty acids and maleicacid, Atrac contains ester carbonyls, ethoxy groups, and a freecarboxylic group. The free carboxylic group is deprotonated insolution at pH 8.5 and is not likely to adsorb on the negativelycharged hematite surface, whereas the ester carbonyls exhibitedrather strong interactions with the hematite surface, as indicatedby considerable shifts of the band originating from estercarbonyls in the IR spectra. Thus, the suggested mode ofadsorption of Atrac on hematite is probably through estercarbonyls and the short ethoxy chain, as illustrated in Figure14b.

Conclusions

Continuous and evenly distributed hematite films of control-lable thickness were deposited on ATR crystals by dip-coating.It was shown that the adsorptions of both a flotation agent andthe selected model compounds on such films could be monitoredin situ by FTIR spectroscopy.

Maleic acid does not adsorb on hematite at pH 8.5 becauseof the repulsion between the negatively charged hematite surfaceand two carboxylate anions present in maleic acid at this pH,suggesting that electrostatic interactions affect the adsorptionof maleic acid on hematite. No breakage of the ester bond wasobserved in either the model compounds or Atrac at theexperimental conditions used in this work. Ethyl oleate showedvery low adsorption on hematite, suggesting that the estercarbonyl does not form strong complexes with the hematitesurface. For PEGMO, the adsorption on hematite likely took

place through the tail hydroxyl group accompanied by theinteraction of the poly(ethylene glycol) chain with the surface.Based on the adsorption behavior of maleic acid, it wasconcluded that Atrac could not adsorb on hematite throughdeprotonated carboxylic group at the chosen experimentalconditions. The most probable mode of adsorption of Atrac onthe hematite surface is through the ester carbonyls and theethoxy group. Adsorption isotherms for both Atrac and PEGMOwere in good agreement with the Freundlich model of adsorptiondescribing adsorption on energetically heterogeneous surfaces.

Based on the desorption experiments, it was concluded thatthe strength of adsorption of Atrac on the surface of hematitevaried for different species. Some of them, probably thoseattached directly to the surface, were strongly adsorbed andremained on the surface even when the sample was flushed withwater at increased pH compared to the pH of adsorption. OtherAtrac species showed rather weak interaction and could beremoved from the surface by flushing with water at pH 8.5 (i.e.,the same pH as during the adsorption). For PEGMO, theintensity of the band originating from the symmetric stretchingvibration (νs) of the C-H bond in PEGMO was reduced byonly 5% upon flushing of the cell with water at pH 8.5 for 1 h,suggesting a stronger interaction with hematite as compared withAtrac, probably because of the longer ethoxy chain and thepresence of the tail hydroxyl group, which is not likely to bedeprotonated at pH 8.5 and is thus not causing electrostaticrepulsion of the molecule from the surface.

Further, the results of the desorption experiments suggestedthat a separate washing unit after the flotation step could bebeneficial in reducing the contamination of iron ore by theflotation collector and possibly improving green-pellet strength.

The method developed here will be used in future works inwhich the effects of ionic strength, calcium ions, and silicateson the adsorption of Atrac and selected model compounds willbe studied in detail.

Acknowledgment

The Hjalmar Lundbohm Research Center (HLRC) is grate-fully acknowledged for financial support of this work.

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Figure 14. Proposed adsorption geometries of (a) PEGMO and (b) Atracon hematite from aqueous solutions at pH 8.5. R represents alkyl radical inoleic acid [CH3(CH2)7CHdCH(CH2)7], and R′ represents alkyl radical infatty acids (including oleic acid). Dashed ovals indicate the groups involvedin adsorption. These groups are polar without negative ionic entities andshould interact with the polar and negatively charged Fe-O surface.

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(67) Westerstrand, M. Process water geochemistry at the Kiirunavaarairon mine, northern Sweden. Ph.D. Thesis, Luleå University of Technology,Luleå, Sweden, 2009.

ReceiVed for reView August 27, 2009ReVised manuscript receiVed December 3, 2009

Accepted December 19, 2009

IE901343F

1502 Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

PAPER II

The effect of calcium ions and sodium silicate on the

adsorption of anionic flotation collector on magnetite studied

by ATR-FTIR spectroscopy

E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund

Journal of Colloid and Interface Science 345 (2010) 96-102

Author's personal copy

The effect of calcium ions and sodium silicate on the adsorption of a modelanionic flotation collector on magnetite studied by ATR-FTIR spectroscopy

E. Potapova *, M. Grahn, A. Holmgren, J. HedlundDivision of Chemical Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden

a r t i c l e i n f o

Article history:Received 25 November 2009Accepted 14 January 2010Available online 28 January 2010

Keywords:MagnetiteFlotation collectorCalciumSilicateAdsorptionATR-FTIR

a b s t r a c t

Previous studies have shown that agglomeration of the magnetite concentrate after reverse flotation ofapatite is negatively affected by the collector species adsorbed on the surface of magnetite. In this work,the effect of ionic strength, calcium ions and sodium silicate on the unwanted adsorption of a model anio-nic flotation collector on synthetic magnetite was studied in situ using attenuated total reflectanceFourier transform infrared spectroscopy (ATR-FTIR). The amount of collector adsorbed was found toincrease with increasing ionic strength at pH 8.5 providing evidence to the contribution of electrostaticforces to the adsorption of the collector. Adding sodium silicate to the system resulted in a threefolddecrease in the amount of collector adsorbed compared to when no sodium silicate was added, confirm-ing the depressing activity of sodium silicate on magnetite. Calcium ions were shown to increase theadsorption of both the collector and sodium silicate on magnetite. The depressing effect of sodium silicateon collector adsorption was completely suppressed in the presence of calcium ions under the conditionsstudied. Furthermore, the amount of collector adsorbed on magnetite from the silicate-collector solutionincreased 14 times upon addition of calcium ions suggesting that calcium ions in the process water mayincrease undesired adsorption of the collector on the iron oxide.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Being themost commonlyusedmetal in theworld, iron is usuallyfound as oxide minerals in nature and is extracted from iron-con-taining ores, of which the most common are magnetite (Fe3O4),hematite (a-Fe2O3) andgoethite (a-FeO(OH))ores.Due to theirmag-netic properties, magnetite and hematite can be separated from thegangueminerals usingmagnetic separators [1]. However, additionalupgrading steps can be required in order to reduce the amount oftrace elements, such as phosphorous, to an acceptable level for theblast furnace process. Dephosphorization of the iron ore is carriedout by reverse flotation with modified fatty acid based collectors[2]. Ideally, the collector should selectively adsorb on the surfaceof phosphorous-containing mineral rendering it hydrophobic andthus easily floated and not affecting the surface of iron oxide.

In order to achieve a high recovery of phosphorous in the flota-tion of calcareous gangue containing minerals, such as apatite(Ca5(PO4)3(F, Cl, OH)), fluorite (CaF2) and calcite (CaCO3), sodiumsilicate is widely used as a dispersant. Depending on the dosage, so-dium silicate can also have a depressing effect on iron oxides. Gonget al. [3] showed that polymeric silicate species formed in concen-trated solutions provided a depressing effect on iron oxide due to

the ability of the polymeric silicate species to adsorb strongly onthe surface of the iron oxide, thus blocking it from collector adsorp-tion. In another study, it was found that too high amounts of sodiumsilicate resulted in decreased flotation selectivity as both iron oxideand gangue minerals lost their floatability [4]. At moderate concen-trations, silicate species adsorbed on the gangue mineral are re-placed by the collector while the iron oxide surface is stillprotected from the collector adsorption, thus a selective depressingeffect is achieved [5]. Low concentrations of sodium silicate(<500 g t�1) have not been found to prevent collector adsorptionon the iron oxide surface to any greater extent [4,6,7].

In our previous study [8] it was shown that a commercial anio-nic fatty acid based collector readily adsorbs on hematite fromaqueous solutions at pH 8.5. Furthermore, from desorption exper-iments, it was proven to be difficult to completely eliminate thecollector adsorbed on the surface by flushing with water even atpH 10, which was higher than the adsorption pH.

Besides the obvious fact that undesired adsorption of the flota-tion collector on the iron ore increases collector consumption inthe process, it may have a negative effect on the subsequentagglomeration process where the iron ore is balled into pellets[9]. Forsmo et al. [10] showed that even small amounts of the col-lector added to the balling feed caused a significant decrease in thegreen (before thermal treatment) pellet strength. As suggested bythe authors, the collector adsorbed on magnetite decreases its

0021-9797/$ - see front matter � 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2010.01.056

* Corresponding author. Fax: +46 920 491199.E-mail address: elisaveta.potapova@ltu.se (E. Potapova).

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wettability, resulting in the formation of stable air bubbles insidethe pellet, which in turn compromises green pellet strength. Lowgreen pellet strength, in turn, causes increased recirculation inthe pelletizing plant and breakage of the pellets during dryingand induration [9,11].

Apart from the flotation collector anddispersant, another param-eter that may affect the flotation performance is the process waterchemistry. Thepresenceof inorganic speciesmayactivate or depressthe flotation of a certainmineral, change collector solubility and thezeta-potential of the mineral surface [12]. Monovalent ions with anopposite charge compared to the charge of the surface have beenshown to reduce the zeta-potential of a mineral while polyvalentions are even capable of reversing the surface charge [13].

Previous studies indicate that high concentrations of Ca ions inthe process water may affect both the flotation step [14,15] and thestrength of the iron ore pellet [16,17]. During flotation, Ca ions inthe process water may form precipitates with the collector and/or adsorb on the iron oxide surface reversing its charge and thusmaking it more favourable for collector adsorption [14,15].

Infrared spectroscopy has been applied widely for studies ofinteractions in flotation systems as revealed by several compre-hensive reviews [18–20]. The mechanism of fatty acid adsorptionon calcareous and iron oxide minerals has been studied extensively[21–26] since 1965, when Peck and Wadsworth published their re-sults on the adsorption of oleate on fluorite and barite [27]. Thedevelopment of infrared external and internal reflection tech-niques provided the possibility to carry out in situ studies of collec-tor/mineral systems, which are important since such studiesprovide real-time information about the complexes formed at thesolid–liquid interface. Our research group has been utilizingin situ ATR-FTIR spectroscopy for studying interactions of flotationcollectors [28–30], sodium silicate [31,32], bentonite [33] and inor-ganic ions [34,35] with iron oxides as well as adsorption of hydro-carbons in zeolite films [36–39].

In our previous work on the interactions between fatty acidbased collectors and iron oxides, the application of ATR-FTIR spec-troscopy provided the possibility to elucidate the mechanism bywhich different collectors and model compounds were adsorbedon the hematite surface in the presence of water, to follow adsorp-tion and desorption kinetics in situ and to make a conclusion aboutthe stability of the complexes formed [8].

The objective of the present work is to study the effect of ionicstrength, calcium ions and sodium silicate on the adsorption of amodel flotation collector reagent on magnetite. A better under-standing on how sodium silicate and the process water chemistryaffect the unwanted adsorption of the collector on magnetite maylead to insights on how to minimize the adsorption of the collectoron magnetite.

2. Materials and methods

2.1. Materials

Magnetite nanoparticles were synthesized by co-precipitationof Fe(II) and Fe(III) according to the method described by Massartand Cabuil [40]. In short, 50 mL of an aqueous solution containing0.33 M FeCl2�4H2O (pro analysi, KEBO) and 0.66 M FeCl3�6H2O (proanalysi, Riedel-de Haën) was added dropwise under continuousstirring to 450 mL of a 1 M solution of NH3 (25%, Suprapur, Merck)in degassed MilliQ water. The black precipitate formed was puri-fied by repeated sedimentation and redispersion in degassed Mil-liQ water until the supernatant remained turbid. The suspensionwas further dialyzed until the conductivity of the water outsidethe dialysis tube reached 1.6 lS cm�1 and remained constant uponreplacing it with the fresh water. The obtained suspension of finemagnetite crystals was then diluted with methanol resulting in a

water-to-methanol ratio of 3:1 by volume. The dry solid contentof the suspension was estimated to ca 1.2 mg mL�1 by weighinga known volume of the suspension after drying in an oven at100 �C. The suspension was stored in a refrigerator in order to min-imize oxidation of magnetite.

Dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma–Aldrich),for the sake of clarity and simplicity hereafter referred to as ‘collec-tor’ was chosen as a model flotation collector reagent. This collectorwas chosen based on the information given in a patent [41] describ-ing a similar collector for froth flotation of oxide and salt type min-erals, which consists of a long aliphatic hydrocarbon groupconnected by an alkylene oxide group to a dicarboxylic acid. Thechemical structure of the collector used in this work is shown inFig. 1.

Stock solutions of the collector, sodium silicate (Na2SiO3�9H2O,�98%, Sigma) and calcium chloride (CaCl2�2H2O, 95%, Riedel-deHaën) were prepared by dissolving appropriate amounts of the cor-responding chemicals in 0.01 M aqueous solutions of NaCl (proanalysi, Riedel-de Haën), which was used as a background electro-lyte. Further, working solutions were prepared by diluting appro-priate amounts of the stock solutions with the 0.01 M aqueoussolution of NaCl to give the required final concentrations.

All aqueous solutions were prepared using distilled water de-gassed by applying vacuum. The distilled water used for the spec-troscopic measurements was, after degassing, saturated with argonin order to minimize the amount of dissolved carbon dioxide.

2.2. Film deposition and general characterization of the film

Prior to film deposition, the trapezoidal ZnSe crystals (CrystranLtd.), with the dimensions 50 � 20 � 2 mm and 45 cut edges, werefirst rinsed in ethanol (99.7%, Solveco chemicals AB) and distilledwater. Thereafter, 0.3 mL of the magnetite suspension describedabove was spread evenly over one side of the ZnSe crystals anddried producing a thin film. The other side of the crystal was leftuncoated in order to reduce the attenuation of the IR radiationby magnetite and thus obtain a higher throughput to the detectorin the spectroscopic measurements.

An X-ray diffraction pattern of the magnetite film was recordedwith a Siemens D5000 powder diffractometer operating in Bragg–Brentano geometry and utilizing Cu Ka radiation. In order to ana-lyse the film, a magnetite-coated silicon wafer was mounted in acustom-made aluminium holder. The pattern of the assemblywithout magnetite film was also recorded in order to identify thepeaks belonging to the substrate and the holder.

The magnetite film on a ZnSe crystal was investigated with anFEI Magellan 400 field emission high resolution scanning electronmicroscope (HR-SEM) using an accelerating voltage of 1 kV. Nogold coating or similar was deposited on the film to render it elec-trically conductive. The magnetite coated ZnSe crystal was cut inthe middle and mounted vertically in the sample holder in orderto measure the film thickness and investigate the morphology ofthe magnetite particles.

2.3. ATR-FTIR spectroscopy

Spectral data were collected using a Bruker IFS 66v/S spectrom-eter equipped with a liquid nitrogen cooled mercury–cadmium–

Fig. 1. Chemical structure of the collector. R represents the linear alkyl chainCH3(CH2)11.

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telluride (MCT) detector. A ZnSe crystal coated with magnetite wasplaced in a stainless steel flow cell which was further mounted onthe ATR accessory in the spectrometer (see Fig. 2). The IR beam,guided by the mirrors, enters the ZnSe through the cut edge atan angle of incidence of 45� and passes through the crystal via anumber (ca 25) of total reflections. At each point of reflection, anevanescent wave of IR radiation is formed, which penetrates intothe sample perpendicular to the surface of reflection and interactswith the sample material, simultaneously losing a part of itsintensity.

Adsorption measurements were performed in situwith a flow ofthe solution pumped continuously through the cell on one side ofthe ATR crystal at a rate of 10 mL min�1 with recirculation of thesolution. The other half of the cell was evacuated. Both single beambackground and sample spectra were obtained by averaging 500scans at a resolution of 4 cm�1. Data evaluation was performedusing the Bruker Opus 4.2 software.

A spectrum of the pure collector was recorded in argon atmo-sphere by spreading a droplet of the collector over a bare ZnSe crys-tal; as a background, a spectrum of the bare crystal in argon wasused. All adsorption and desorption experiments were performedat room temperature and pH 8.5. The pHwas controlled by aMettlerToledo T70 titrator using a 0.05 M aqueous solution of sodiumhydroxide (proanalysi,Merck). Theconcentrationof thebackgroundelectrolyte was 0.01 M NaCl in all the experiments if not statedotherwise. Prior to adsorption, the magnetite film was equilibratedwith 75 mL of a 0.01 M aqueous solution of NaCl at pH 8.5 for30 min and at this point a background spectrum of the solution incontact with the magnetite coated ZnSe substrate was recorded.

In the experiments where calcium ions or sodium silicate werepre-adsorbed on magnetite, a 4 mM solution of CaCl2 or a 0.4 mMsolution of Na2SiO3, both at pH 8.5, was pumped through the cellfor 1 h. Infrared spectra were recorded every 5 min and a newbackground spectrum was recorded when the pre-adsorptionwas completed. After that, an appropriate amount of the collectorwas added to the solution to give a final concentration of the col-lector of 25 mg L�1 whereas the concentration of calcium or silicateions was kept constant. Adsorption of the collector on magnetitewas followed by recording infrared spectra every 5, 10 or 30 min.

In the experiment where the influence of calcium ions on thedepressing effect of sodium silicate was studied, calcium ions werefirst pre-adsorbed, thereafter a new background spectrum was re-corded and then sodium silicate was added to the solution and theadsorption was monitored with time. After silicate adsorption, an-other background spectrum was recorded and finally the adsorp-tion of the collector from a 25 mg L�1 solution was performed.

Desorption experiments were carried out by flushing the cellwith a 0.01 M NaCl solution at pH 8.5 without recirculation ofthe solution.

3. Results and discussion

3.1. Characterization of the synthetic magnetite

Fig. 3 shows an HR-SEM image of a cross-section of a magnetitefilm on a ZnSe crystal.

The image illustrates that the film thickness is about 250–300 nm and the size of the individual particles vary between about5–15 nm.

Fig. 4 shows an X-ray diffraction pattern of magnetite crystalsdeposited on a silicon wafer.

Except for the narrow diffraction peaks emanating from the sil-icon wafer, the XRD pattern is characteristic for randomly orientedmagnetite crystals. The diffraction peaks from magnetite are quitebroad, which shows that the magnetite crystals are quite small inaccordance with the HR-SEM observations.

3.2. Effect of calcium ions on the adsorption of sodium silicate

Prior to studying the combined effect of calcium ions and so-dium silicate on the collector adsorption on magnetite, the influ-ence of calcium ions on the adsorption of sodium silicate onmagnetite was investigated in order to assess if calcium ions in-creased the polymerization of silicate species as proposed by Gonget al. [5].

Fig. 2. Schematic image of the ATR-FTIR setup.

Fig. 3. Side view SEM image of a magnetite film on a ZnSe crystal.

Fig. 4. XRD pattern of synthetic magnetite crystals deposited on a silicon wafer. Thereflections originating from magnetite are indexed with the appropriate Millerindices. The peaks labelled with (�) emanate from the silicon wafer.

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Silicate species adsorbed on magnetite at pH 8.5 (see spectrum(a) in Fig. 5) are characterized by a broad combination of bands be-tween 800 and 1300 cm�1. At this pH, the most pronounced bandis located at 1014 cm�1 with two shoulders at ca 1120 cm�1 and ca950 cm�1.

The band at 950 cm�1 has been assigned to the monomeric sur-face bidentate complex while the band at 1014 cm�1 has been as-signed to the oligomeric silicate species on the surface [31].Further, the same authors attributed the band at 1120 cm�1 tothe 3-dimensional silica framework structure. With the increaseof surface polymerization of the silicate species the band at1014 cm�1 is expected to shift to higher wavenumbers [31].

Spectra recorded when calcium ions were pre-adsorbed onmagnetite showed rather intense bands at 1490 and 1350 cm�1 as-signed to the asymmetric and symmetric stretching vibrations,respectively, of the carbonate species [42] (see spectrum (b) inFig. 5). This result indicates that some carbonate species were pres-ent in the water despite degassing, and further, these carbonatespecies adsorbed on the surface of magnetite when calcium ionswere added to the solution.

As sodium silicate was added to the system after pre-adsorptionof calcium ions, negative bands associated with the carbonate spe-cies were observed in the spectra simultaneously as the positivebands originating from adsorbed silicate species appeared, sug-gesting that silicate species were replacing the carbonates on themagnetite surface (see spectrum (c) in Fig. 5).

The shape of the absorption bands emanating from the silicatespecies was not particularly affected by the presence of calciumions; however, the band originating from the oligomeric silicatespecies was slightly shifted (�2 cm�1) to higher wavenumbersindicating that the silicate species adsorbed on the magnetite sur-face in the presence of calcium ions were possibly polymerized to agreater extent as compared with no calcium ions present [5].Moreover, from the intensities of the bands it may be concludedthat the amount of sodium silicate adsorbed on magnetite in-creased by ca 35% when calcium was pre-adsorbed suggesting thatelectrostatic forces contributed to the interaction between silicatesand magnetite.

Similar results were reported by Roonasi et al. for the systemcalcium–sulfate–magnetite [34]. Calcium ions were found toincrease the adsorption of sulfate on magnetite at pH 8.5 while

no effect on the adsorption was observed at pH 4 indicating theimportance of electrostatic interaction for the calcium/sulfate/magnetite system and a similar behaviour could be expected forthe calcium/silicate/magnetite system.

3.3. Collector adsorption on magnetite

The collector studied in this work contains a carboxylic headgroup (see Fig. 1), which, depending on pH, can be deprotonatedwhen dissolved in water. Fig. 6 shows a spectrum of the collectorin pure form (non-dissolved).

The strong absorption bands at 2922 and 2854 cm�1 in thespectrum are characteristic for the molecules containing long alkylchain as these bands emanate from symmetric and asymmetricstretching vibrations (ms and mas) of the CH2 group [22]. Stretchingvibrations of the CAOAC group in the polyethylene glycol chaingive rise to a characteristic intense band at 1105 cm�1 [43]. Theband at 1728 cm�1 is associated with the stretching vibrations ofthe C@O bond [43]. As the collector molecule contains two carbox-ylic groups, one free and one esterified, two separate bands in thecarbonyl stretching region are expected to be observed. However,when an ester carbonyl is conjugated with a C@C group (like inmaleate), the band from the ester carbonyl, typically observedaround 1740 cm�1, is shifted to lower wavenumbers viz. around1725 cm�1 [44]. At the same time, intramolecular hydrogen bond-ing between the carboxylic groups in maleic acid may cause a shiftof the C@O stretching vibration to higher wavenumbers (1730–1705 cm�1) [44] resulting in possible overlapping with the bandoriginating from the ester carbonyl.

In our previous work, it was shown that the adsorption of anio-nic flotation collectors on iron oxides is not fully governed by elec-trostatic forces. Anionic collectors can adsorb on iron oxidesdespite repulsion between the negatively charged head groupand the surface bearing the same net charge. The influence of elec-trostatic forces on the adsorption can be revealed by studying theeffect of ionic strength on adsorption. Fig. 7 illustrates the effect ofincreased ionic strength on the intensity of one of the bands orig-inating from the collector adsorbed on magnetite.

Assuming that the band intensity is proportional to the amountof the collector adsorbed on magnetite, the data presented in Fig. 7suggests that higher ionic strength results in a higher adsorption ofthe collector on magnetite due to the fact that the increasedamount of sodium ions reduces the negative effective surfacecharge of the magnetite surface and thereby facilitating a greatercollector adsorption. These results indicate that electrostatic forcescontribute to the adsorption of the collector on magnetite.

Fig. 5. Infrared spectrum of silicate species adsorbed on magnetite without anycalcium ions present (a), spectrum of the magnetite film recorded after the pre-adsorption of calcium ions on magnetite (b), and spectrum of silicate speciesadsorbed on magnetite after pre-adsorption of calcium ions (c).

Fig. 6. Infrared spectrum of a droplet of the pure collector on a ZnSe crystal in argonatmosphere.

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Moreover, as the ionic strength was changed from 0.01 M NaClto 0.1 M NaCl it was also observed (not shown) that the band orig-inating from the asymmetric stretching vibration of the CH2 groupin the spectra shifted from 2924 to 2922 cm�1 indicating that theinteraction of the alkyl chains in the collector increased, probablydue to a higher packing density of the molecules in the adsorbedlayer [45].

Fig. 8 shows spectra of the collector adsorbed on the magnetitesurface from aqueous solutions at pH 8.5. Compared to the spec-trum of the pure collector, two new bands are observed in thespectra of the collector (spectra (a–d)), viz. at ca 1570 and1400 cm�1. These bands are assigned to the asymmetric and sym-metric stretching vibrations of the carboxylate ion respectively[46] indicating the deprotonation of the head carboxylic group.However, the band originating from the stretching vibrations ofthe C@O bond is still present in the spectra at 1724–1726 cm�1

suggesting that adsorption likely takes place without breakingthe ester bond.

When pre-adsorption of calcium ions was performed (see spec-tra (b) and (d) in Fig. 8), an additional band at 1425 cm�1 appearedin the spectra of the collector adsorbed on magnetite suggestingthat calcium ions affected the adsorption mode of the collectoron magnetite.

When the effect of sodium silicate on collector adsorption wasstudied, the magnetite film was pre-treated with sodium silicatefor 1 h before the collector was added to the system. The spectrarecorded during collector adsorption (spectrum (c) in Fig. 8), inaddition to the bands originating from the collector, contained anintense band at 1030 cm�1 with a shoulder at 1120 cm�1 emanat-ing from the silicate species adsorbed on the surface of magnetite.The shoulder became evident when subtracting a spectrum of thecollector adsorbed on magnetite without sodium silicate (spec-trum (a) in Fig. 8) from the spectrum (c) in Fig. 8. The position ofthe band at 1030 cm�1 indicates higher degree of polymerizationof the silicate species adsorbed on magnetite as compared to thoseafter 1 h of adsorption prior to addition of the collector (see spec-trum (a), the band at 1014 cm�1). These results suggest that the sil-icate species continued to adsorb on magnetite even in thepresence of the collector implying competitive adsorption betweenthe collector and sodium silicate for the magnetite surface sites.

In the case when the magnetite film was pre-treated with bothcalcium ions and sodium silicate, which would be the most realis-tic conditions in a flotation process, no bands associated with sili-cate species were observed in the spectrum of the collector

adsorbed on magnetite suggesting that the adsorption of sodiumsilicate was terminated by the collector when calcium ions werealso present. It should be noted that a new background spectrumwas recorded after the pre-adsorption of silicate and calcium ions.However, silicate species already adsorbed on magnetite were notsubstituted with the collector since no negative bands associatedwith the silicate species were observed in the spectra. Thereby, itmay be concluded that adsorption and desorption of the silicatespecies were equilibrated by the addition of the collector to thesystem.

After 13 h of adsorption from a solution of 4 mM calcium chlo-ride and 25 mg L�1 collector at pH 8.5, an in situ desorption exper-iment was performed. Fig. 9 shows spectra recorded duringflushing the cell with water containing 0.01 M NaCl at pH 8.5 for2 h. As has been mentioned above, an additional band at1425 cm�1 appeared in the spectra of the collector adsorbed onmagnetite when pre-adsorption of calcium ions was performed.Upon desorption, the intensity of this band decreased much fasterthan the band at 1402 cm�1 suggesting the presence of two kindsof carboxylate complexes on the surface of magnetite: an inner-sphere complex characterized by the band at 1402 cm�1 [47] andan outer-sphere complex characterized by the band at 1425 cm�1

[47] as was previously reported by Hwang and Lenhart for maleateadsorption on hematite [47].

After13 hof adsorption fromasolutionof0.4 mMsodiumsilicateand 25 mg L�1 collector at pH 8.5, an in situ desorption experimentwas performed. Fig. 10 shows spectra recorded during flushing thecell with water containing 0.01 M NaCl at pH 8.5 for 2 h.

Fig. 7. Intensity of the ester C@O stretching vibrations band as a function of timeduring in situ adsorption of the collector on magnetite at pH 8.5 from a 25 mg L�1

solution containing 0.01 M NaCl (D) and 0.1 M NaCl (h). The ionic strength in theworking solution was increased after 13 h of adsorption by adding the appropriateamount of NaCl.

Fig. 8. Infrared spectra of the collector adsorbed on magnetite at pH 8.5 from a25 mg L�1 aqueous solution containing no added Ca2+ or Na2SiO3 (a), 4 mM Ca2+ (b),0.4 mM Na2SiO3 (c), and 4 mM Ca2+ and 0.4 mM Na2SiO3 (d). All spectra wererecorded after 13 h of collector adsorption. For the sake of clarity, the absorbance ofthe spectra (a) and (c) was multiplied with a factor 5.

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The intensity of the bands emanating from the silicate specieswas decreasing upon flushing with water at pH 8.5. After the first10 min of flushing, the intensity of the band at 1030 cm�1 emanat-ing from the oligomeric silicate species was reduced as much as40%. The intensity of this band was decreasing more slowly asthe flushing continued. Furthermore, the peaks associated withthe collector adsorbed on magnetite were slightly increasing inintensity at the beginning of desorption of silicate species likelydue to the increasing amount of vacant surface sites available foradsorption of the collector. However, after 1.5 h of flushing withwater, the bands originating from the collector also started to de-crease slowly, indicating that no more empty surface sites wereavailable for collector adsorption.

Thereby, the in situ desorption experiment revealed that thecollector was adsorbed on the magnetite surface much strongerthan the silicate species since the absorption bands originatingfrom the collector decreased very slowly or even increased uponflushing with water.

Fig. 11 illustrates the effect of calcium ions and sodium silicateon the intensity of the band originating from the collector adsorbedon magnetite as a function of time.

The data presented in Fig. 11 shows that the intensity of the es-ter C@O stretching vibrations band increased slowly during in situadsorption of the collector on magnetite at pH 8.5 when no calciumor silicate ions were present in the solution (open triangles).Assuming that the band intensity is proportional to the amountof the collector adsorbed on magnetite, the data presented inFig. 11 indicates an almost fivefold increase in the amount of thecollector adsorbed on magnetite in the presence of calcium ions

(open circles) as compared to when no calcium ions were added(open triangles), supporting the results reported earlier [14,15]and suggesting that calcium ions interact with the magnetite sur-face reducing the negative net charge of the surface thus making itmore favourable for collector adsorption. Similarly to collectoradsorption on semisoluble calcium-containing minerals like apa-tite, fluorite, and calcite [48], collector species can then interactspecifically with calcium ions adsorbed on magnetite formingmagnetite–calcium-collector complexes. Additionally, calciumions can facilitate the formation of calcium-collector precipitate,which may subsequently adsorb on the magnetite surface [9,15].

A threefold decrease in the intensity of the band originatingfrom the collector adsorbed on magnetite was observed in thepresence of sodium silicate (filled triangles) as compared to whenno silicate was present (open triangles). The observed decrease isan effect of competitive adsorption between the silicate speciesand the collector on the magnetite surface sites as was also shownin Fig. 8 by comparing spectra (a) and (c). The results confirm thatsodium silicate depresses the adsorption of the collector on mag-netite in concert with previous findings [3,5].

In the experiment where both silicate and calcium ions werepre-adsorbed, the intensity of the bands originating from the col-lector adsorbed on magnetite was reduced by less than 8% (filledcircles) as compared to the case when only calcium ions were pres-ent in the system (open circles) and was increased almost 12 timesas compared to when only sodium silicate was present (filled trian-gles). Thereby, the depressing activity of sodium silicate was lesspronounced in the presence of calcium ions, in contradiction tothe results previously reported by Gong et al. [5], who observedmuch stronger depressing activity of the silicate–calcium ion mix-tures on hematite in the flotation of apatite with tall oil fatty acidas compared to that of pure sodium silicate. The reason for thatwas probably much higher calcium/Si ratio used in the presentwork (10) as compared to those in the study by Gong et al. (0.2–0.6). In the present study, calcium ions were found to only slightlyincrease the amount and polymerization degree of the silicate spe-cies adsorbed on magnetite (see Fig. 5), while they made muchmore significant contribution to the increase of the collectoradsorption on magnetite (see Fig. 11).

The results presented in this work indicate that calcium ionspresent in the water significantly increase the adsorption, and pos-sibly precipitation, of the collector on synthetic magnetite. A sim-ilar effect may be expected in the industrial flotation process:contamination of the iron ore with the flotation collector may beenhanced upon high concentrations of calcium in the processwater increasing collector consumption and affecting the apatite

Fig. 9. Spectra recorded during flushing the cell with water at pH 8.5 after collectoradsorption in the presence of calcium ions. The time interval between the spectra is10 min.

Fig. 10. Spectra recorded during flushing the cell with distilled water at pH 8.5 and0.01 M NaCl after collector adsorption in the presence of silicates. The time intervalbetween the spectra is 10 min.

Fig. 11. Intensity of the ester C@O stretching vibrations band as a function of timeduring in situ adsorption of the collector on magnetite at pH 8.5 from a 25 mg L�1

aqueous solution containing no Ca2+ and Na2SiO3 added (D), 4 mM Ca2+ (s), 0.4 mMNa2SiO3 (N), 4 mM Ca2+ and 0.4 mM Na2SiO3 (d).

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flotation as has been previously reported by Rao et al. [15]. Further,hydrophobic collector coating on the iron ore concentrate has beenshown to reduce the strength of iron ore green pellets [9,11] sug-gesting that high concentration of calcium ions in the processwater may have a negative impact on the green pellet strengthby increasing adsorption and/or precipitation of the collector onthe iron oxide surface.

4. Conclusions

The effect of ionic strength, calcium ions and sodium silicate onthe adsorption of a model flotation collector on magnetite wasinvestigated by in situ ATR-FTIR spectroscopy.

Monovalent cations were found to slightly increase the adsorp-tion of the collector on magnetite at the studied pH by partly com-pensating the negative charge of the magnetite surface and thusreducing the electrostatic repulsion between the surface and car-boxylate ions suggesting that electrostatic forces contribute tothe adsorption of the anionic flotation collector on magnetite.

Divalent calcium ions were found to have a significant effect onthe adsorption of the flotation collector on magnetite. An almostfivefold increase in the amount of the collector adsorbed on mag-netite was observed when calcium ions were pre-adsorbed, in con-cert with previous findings for anionic fatty acid based collectors.From the desorption experiments it became evident that in thepresence of calcium ions, collector adsorption took place via bothinner-sphere and outer-sphere complexes, the latter could berather easily removed by flushing with water.

When the magnetite film was pre-treated with sodium silicate,a competitive adsorption of the collector and sodium silicate tookplace on the surface of magnetite resulting in a threefold decreasein the amount of the collector adsorbed on magnetite as comparedto the case when no pre-adsorption of silicates was performed con-firming that sodium silicate can depress collector adsorption onmagnetite. However, the stability of the magnetite-collector com-plexes was greater as compared to the magnetite–silicatecomplexes.

Furthermore, the depressing activity of sodium silicate on thecollector adsorption was almost completely suppressed in thepresence of calcium ions. Moreover, silicate adsorption on magne-tite was terminated when the collector was added to the systemand even though the amount and the polymerization degree ofthe silicate species adsorbed on magnetite in the presence of cal-cium ions were higher than without calcium, it was not sufficientto prevent the collector adsorption to any greater extent.

The results presented in this work suggest that high concentra-tions of calcium in the process water may enhance collectoradsorption and precipitation on iron oxides, resulting in increasedcollector consumption and a more hydrophobic surface. The latterhas been previously shown to decrease the green pellet strength.

Acknowledgments

The authors acknowledge the financial support from the Hjal-mar Lundbohm Research Centre (HLRC) and the Knut and AliceWallenberg Foundation.

References

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102 E. Potapova et al. / Journal of Colloid and Interface Science 345 (2010) 96–102

PAPER III

The effect of calcium ions, sodium silicate and surfactant on

charge and wettability of magnetite

E. Potapova, X. Yang, M. Grahn, A. Holmgren, S. P. E. Forsmo, A. Fredriksson, and J.

Hedlund

Colloids and Surfaces A: Physicochemical and Engineering Aspects 386 (2011) 79-86

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Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79– 86

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

The effect of calcium ions, sodium silicate and surfactant on charge andwettability of magnetite

E. Potapovaa,∗, X. Yanga,b, M. Grahna, A. Holmgrena, S.P.E. Forsmoc, A. Fredrikssond, J. Hedlunda

a Division of Sustainable Process Engineering, Luleå University of Technology, SE-971 87 Luleå, Swedenb Research Centre for Eco-Environmental Sciences, Chinese Academy of Science, Beijing 100085, Chinac LKAB, SE-983 81 Malmberget, Swedend LKAB, SE-981 86 Kiruna, Sweden

a r t i c l e i n f o

Article history:Received 31 March 2011Received in revised form 23 June 2011Accepted 30 June 2011Available online 7 July 2011

Keywords:AdsorptionATR-FTIRContact angleSilicateSurfactantZeta-potential

a b s t r a c t

Anionic carboxylate surfactants and sodium silicate are used in the reverse flotation of iron ore to separatemagnetite from apatite. In this work, consecutive adsorption of sodium silicate and an anionic surfactanton synthetic magnetite modified with calcium ions was studied in the pH range 7.5–9.5 using in situ ATR-FTIR spectroscopy. The effect of these chemicals on the zeta-potential and wetting properties of magnetitewas also investigated. While adsorption of silicate increased with increasing pH, subsequent surfactantadsorption went through a maximum at pH 8.5. Surfactant adsorption in the presence of calcium ionswas not affected by the amount of silicate adsorbed on magnetite. Calcium ions were found to renderthe magnetite surface positive in the pH range 3–10 and could reduce the dispersing effect of silicatein flotation of apatite from magnetite. While treatment with calcium chloride and sodium silicate mademagnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the water contactangle on the magnetite surface from about 10◦ to 40–50◦. Although the latter values are not high enoughto make magnetite float, the hydrophobic areas on the magnetite surface could result in the incorporationof air bubbles inside the iron ore pellets produced by wet agglomeration, lowering the pellet strength.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Surfactant adsorption on mineral surfaces is important formany industrial applications including detergency, dispersion ofpigments, stabilization of colloidal suspensions in cosmetics andpharmaceuticals, flocculation of fine mineral particles, and oreflotation.

Separation of minerals by flotation can occur if the minerals havedifferent affinities for air and water. A mineral can be flotated onlyif the work of adhesion between a mineral particle and an air bub-ble is high enough to prevent the disruption of the particle–bubbleinterface. The work of adhesion between a particle and an air bubbleincreases with increasing contact angle at the surface–air interfaceimplying that the floatability of a mineral improves as the min-eral surface becomes more hydrophobic. Most mineral surfaces arehighly polar and have a high Gibbs energy, which makes themhydrophilic. To make the mineral float with the air bubbles, thesurface of such minerals has to be modified by a suitable surfac-tant in order to reduce the Gibbs energy. Providing that surfactant

∗ Corresponding author. Tel.: +46 920 491776; fax: +46 920 491199.E-mail address: elisaveta.potapova@ltu.se (E. Potapova).

adsorption in a flotation system occurs selectively, good separationof the minerals can be achieved.

Interactions between ionic surfactants and minerals are to alarge extent controlled by the charge density of the mineral sur-face [1]. Adsorption of different species as well as pH of the processwater can alter the charge density enhancing or reducing the inter-actions between the mineral and the surfactant and thereforeaffect flotation performance. It is thus very important to know theeffects that various species in the process water may have on thecharge density of a mineral surface. The effect of adsorption on thecharge density of mineral particles can be determined from elec-trophoretic measurements and then expressed in terms of changesin the zeta-potential.

Another phenomenon that is highly dependent on the chargedensity of the mineral surface is the dispersion of the mineral parti-cles. High dispersion is required to maximize mineral recovery andflotation selectivity [2]. Dispersion is facilitated by increasing thenet surface charge density of the mineral particles, which results inincreased electrostatic repulsion. Increased surface charge can beachieved by the adsorption of compounds forming charged com-plexes on the mineral surface.

One of the important parameters in flotation that affects boththe surface properties of the minerals and the speciation of theflotation chemicals as well as species naturally occurring in the pro-

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cess water is pH. The net charge of a mineral surface and adsorptionof ionic collectors on the surface are dependent on pH, which hasalso been shown to have a certain effect on natural wettability ofminerals [3,4]. In the recent paper by Puah et al. [5], the variationof the contact angle of titania as a function of pH was explained bythe ionization of the surface hydroxyl groups above and below thepoint of zero charge of the surface resulting in increased surfacecharge and, consequently, surface wettability. Together with thereagent dosing, pH determines to a large extent the selectivity inseparation of minerals by flotation [4].

In the reverse flotation of the iron ore rich in apatite, fatty acidbased surfactants are used as flotation collectors and sodium sil-icate as a dispersing agent [6]. Sufficiently high concentrations ofsodium silicate have been shown to have a depressing effect on theiron oxides [7]. Flotation of apatite from the iron oxides is typicallyconducted at pH > 7 whereas the optimum pH in any specific pro-cess depends on the type of apatite mineral which is to be flotated[8].

If the iron ore concentrate is pelletized by wet agglomeration, itis important that the surface of the iron oxide after flotation is suf-ficiently hydrophilic to ensure efficient balling and high strength ofthe iron ore pellets in both wet and dry states [9]. Contaminationof magnetite with flotation collector renders magnetite hydropho-bic and reduces pellet strength [10–12]. While sodium silicate canto some extent protect magnetite surface from collector adsorp-tion, calcium ions present in the process water enhance undesiredadsorption and precipitation of the collector on magnetite [13,14].In our previous work [15] we have also shown that calcium ionsincrease dramatically the adsorption of an anionic surfactant onmagnetite even in the presence of sodium silicate. The results fur-ther emphasize the importance of taking into account the influenceof additives and ions in the process water on the adsorption of flota-tion collectors on mineral surfaces in order to better understand thephenomena in real flotation systems.

The scope of the present work is to show the effect of pH andsodium silicate on the adsorption of an anionic surfactant onto mag-netite in the presence of calcium ions and to investigate how theadsorption of the different constituents in this system affects mag-netite surface properties relevant to flotation and agglomeration,namely, surface charge and wettability.

2. Experimental

2.1. Materials

Magnetite nanocrystals were synthesized and purified asdescribed earlier [15]. The magnetite crystals had a spherical habit,the size of 5–15 nm and the surface area of 80–100 m2 g−1. Theobtained suspension of magnetite in distilled water was furtherdiluted with methanol and degassed distilled water to give a work-ing suspension containing 25 vol.% methanol and ca. 1.1 mg mL−1

magnetite. To minimize oxidation of magnetite, the suspension wasstored in a refrigerator at 6 ◦C.

Stock solutions of dodecyloxyethoxyethoxyethoxyethylmaleate (Sigma–Aldrich) used as a model flotation collector,sodium silicate (Na2SiO3·9H2O, ≥98%, Sigma) and calcium chloride(CaCl2·2H2O, 95%, Riedel-de Haën) were prepared by dissolvingrequired amounts of the corresponding chemicals in a 0.01 Maqueous solutions of NaCl (per analysis, Riedel-de Haën). Fig. 1illustrates the structure of the model flotation collector used inthis work.

All solutions were prepared using distilled water degassedunder vacuum in order to minimize the amount of dissolved gases.The distilled water used for the spectroscopic measurements was,after degassing, bubbled with argon.

Fig. 1. Chemical structure of dodecyloxyethoxyethoxyethoxyethyl maleate. R rep-resents the linear alkyl chain CH3(CH2)11.

The pH of working solutions was adjusted using aqueoussolutions of sodium hydroxide (NaOH, per analysis, Merck) andhydrochloric acid (HCl, 37%, per analysis, Merck).

2.2. Methods

2.2.1. Zeta-potential measurementsThe zeta-potential measurements were performed using a Zeta-

Compact instrument equipped with a charge-coupled device (CCD)tracking camera. The collected electrophoretic mobility data wereprocessed by the Zeta4 software applying the Smoluchowski equa-tion. Six different aqueous dispersions of synthetic magnetitecrystals were prepared in: (a) 10 mM NaCl as ionic medium; (b)3.3 mM CaCl2; (c) 10 mM NaCl and 1 mM Na2SiO3; (d) 3.3 mMCaCl2 and 1 mM Na2SiO3; (e) 3.3 mM CaCl2 and 0.4 mM Na2SiO3;and (f) 3.3 mM CaCl2, 0.4 mM Na2SiO3 and 25 mg L−1 (0.06 mM)maleic acid ester. The concentration of magnetite particles in thedispersion was ca. 5 mg L−1. The ionic strength of different sam-ples was constant at 10 mM since the contribution of Na2SiO3 andthe surfactant to the ionic strength is insignificant. Additionally,the zeta-potential of 15 mg L−1 (0.04 mM) maleic acid ester wasmeasured in aqueous solutions containing (a) 10 mM NaCl; and (b)10 mM NaCl and 2.4 mM CaCl2. The pH of the samples was adjustedusing aqueous solutions of NaOH and HCl and the samples spannedthe pH range from 3 to 11. For each sample, the zeta-potential wasdetermined as an average of the values obtained in three replicatemeasurements.

2.2.2. Contact angle measurementsContact angle measurements were performed using a Fibro

1121/1122 DAT-Dynamic Absorption and Contact Angle Testerequipped with a CCD camera. A magnetite film was prepared byspreading 0.5 mL of the magnetite dispersion on a ZnSe crystal andletting it dry in air. Thereafter, the film was rinsed with distilledwater and dried in a vacuum desiccator at ca. 1 kPa for 30 min.The contact angle was measured by applying a drop of distilledwater, 4 �L in volume, onto the film using a microsyringe. A seriesof images were taken at different time points and analyzed usingthe DAT 3.6 software. Eight fresh drops placed at different sam-ple locations were measured, and the average contact angle valuewas calculated. After the measurement, the film was immersed for60 min in an aqueous solution containing 4 mM of CaCl2 and 10 mMof NaCl, then rinsed with distilled water and dried in a vacuumdesiccator for 30 min. Thereafter, the contact angle was measuredagain. The measurement was repeated in the same manner twiceafter subsequently adding 0.4 mM of sodium silicate and 25 mg L−1

of the maleic acid ester to the solution already containing CaCl2 andNaCl. The pH of the solutions was kept constant at 8.5.

2.2.3. ATR-FTIR spectroscopyInfrared spectra were recorded on a Bruker IFS 66v/S

spectrometer equipped with a liquid nitrogen cooledmercury–cadmium–telluride (MCT) detector and a verticalATR accessory. A trapezoidal ZnSe crystal (Crystran Ltd.), with thedimensions 50 mm × 20 mm × 2 mm and 45◦ cut edges, was coatedwith a magnetite film as described earlier [15] and mounted ina stainless steel flow cell. In situ adsorption measurements were

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performed by pumping the working solution continuously throughthe cell at a rate of 10 mL min−1 with recirculation and recordinginfrared spectra every 5 min.

All adsorption experiments were performed at room tempera-ture. The pH was adjusted and controlled either by a Mettler ToledoT70 titrator or manually using a conventional pH-meter. The con-centration of the background electrolyte was 10 mM NaCl in all theexperiments. Prior to adsorption, the magnetite film was rinsed for30 min by pumping a 10 mM aqueous solution of NaCl at the pHof adsorption through the flow cell and thereafter a single beambackground spectrum of the solution in contact with the magnetitecoated ZnSe substrate was recorded. After that, calcium chloridewas added to give a 4 mM aqueous solution that was pumpedthrough the cell for 1 h. Thereafter, a new single beam backgroundspectrum was recorded and then sodium silicate was added to thesolution to give a 0.4 mM aqueous solution. The adsorption of sili-cate was monitored during 20 min by recording a spectrum every5 min. After silicate adsorption, another single beam backgroundspectrum was recorded and finally the adsorption of the maleicacid ester from a 25 mg L−1 solution was monitored for 1 h.

All spectra were acquired by averaging 500 scans at a resolutionof 4 cm−1. Spectra evaluation was performed using the Bruker Opus4.2 software.

3. Results and discussion

3.1. Zeta-potential measurements

The results of the zeta-potential measurements presented inFig. 2 illustrate that the isoelectric point (IEP) of the magnetite crys-tals in 10 mM aqueous NaCl (filled squares) was around pH 7, whichis similar to the values typically reported [16] for magnetite.

Addition of 3.3 mM calcium chloride made the magnetite sur-face positively charged in the whole pH range studied (opensquares). Furthermore, in the pH range between 4 and 7 the zeta-potential of magnetite in 3.3 mM calcium chloride (open squares)became more positive than the zeta-potential of magnetite in10 mM NaCl (filled squares) indicating that the charge of the mag-netite surface was affected by calcium ions even at pH below theIEP for magnetite crystals in NaCl. At pH above the IEP, the acquiredpositive net surface charge increased with increasing pH in therange between pH 7.5 and 10 indicating that the affinity for calcium

Fig. 2. Zeta-potential of the magnetite crystals as a function of pH in 10 mM NaCl(�), 3.3 mM CaCl2 (�), 10 mM NaCl and 1 mM Na2SiO3 (�), and 3.3 mM CaCl2 and1 mM Na2SiO3 (�).

ions increased as the magnetite surface became more negativelycharged. Su [8] as well as Dixon [17] reported similar results for themagnetite–calcium system and suggested that calcium ions reactedwith surface hydroxyls by the substitution of hydrogen ions forCa2+, which increased the surface charge.

When 1 mM sodium metasilicate was added to the magnetitedispersed in 10 mM NaCl (open triangles), the zeta potential shiftedto lower values in the whole investigated pH range, with the IEPobserved at pH 5.5. These observations are in agreement withthe recent study reported by Jolsterå et al. [18]. Based on resultsobtained by potentiometric titrations, the authors suggested thatthe negatively charged silicate surface complex FeOSiO(OH)2

−

starts to form by deprotonation already at pH below 5 and domi-nates at pH 7.0–9.8 giving the surface increased negative charge.

Pre-treatment of magnetite with both calcium ions and sodiumsilicate (filled triangles) reduced the negative net surface charge inthe pH range between 6 and 10 as compared to silicate-modifiedmagnetite (open triangles in Fig. 2). This suggests that high concen-trations of calcium in the process water (pH ∼ 8.5) can reduce theefficiency of sodium silicate as dispersing agent due to decreasedelectrostatic repulsion between the ore particles. However, in thepH range between 3 and 6, calcium ions do not seem to have anysignificant effect on the magnetite net surface charge since the val-ues of the zeta-potential remained almost the same as in the casewhen only sodium silicate was added to magnetite. Similar resultswere reported by Dixon [17] for calcium adsorption on magnetitecoated with silica and were explained by the effect of solvationenergy. Adsorption from aqueous solutions is facilitated on surfaceswith dielectric constants close to that of water. Magnetite has muchhigher dielectric constant than silica [19], consequently, less cal-cium is expected to adsorb on silica modified magnetite. However,in the present work, magnetite particles were brought in contactwith calcium and silicate ions simultaneously and similar adsorp-tion behaviour as described by Dixon was observed suggesting thatadsorption of silicate species on magnetite prevails over adsorptionof calcium at pH 3–6, most likely due to electrostatic interactions,and that calcium ions do not adsorb on silicate species attached tothe magnetite surface.

Upon the decrease of silicate concentration from 1 mM to0.4 mM while keeping the concentration of calcium ions (open dia-monds in Fig. 3a) constant, the IEP of the magnetite surface shiftedto slightly higher pH and the net surface charge above the IEPbecame less negative as compared to the case with 1 mM of sili-cate (filled diamonds in Fig. 3a) suggesting that the effect of silicateon the surface charge was partly suppressed by calcium.

Another observation was that the zeta-potential of the mag-netite surface in calcium–silicate solutions started to get lessnegative above pH 9 (filled and open diamonds in Fig. 3a). The rea-son for that could be decreased silicate adsorption which is knownto go through a maximum around pH 9 [18,20,21] and at the sametime an increased positive contribution from calcium ions.

When the maleic acid ester was added to the calcium–silicatemixture (open squares in Fig. 3a), the zeta-potential became morenegative, probably due to adsorption of the surfactant anionsonto calcium attached to the magnetite surface (ternary adsorp-tion) partly compensating for its positive charge. Additionally,the hydrophobic chain–chain interaction between the surfactantmolecules could result in formation of a bi-layer on the surfacewith the head groups of the surfactant species in the second layeroriented towards the solution and introducing additional negativecharge to the surface [22].

It is important to mention here that the magnetite dispersioncontaining surfactant could not be analyzed at pH below 5.5 dueto the fact that the amount of particles in the sample was higherthan the tracking limit of the CCD camera. The observed particleswere smaller in size than the magnetite particles and most likely

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Fig. 3. Zeta-potential of the magnetite crystals as a function of pH in 3.3 mM CaCl2and 1 mM Na2SiO3 (�), 3.3 mM CaCl2 and 0.4 mM Na2SiO3 (♦), and 3.3 mM CaCl2,0.4 mM Na2SiO3 and 25 mg L−1 maleic acid ester (�) (a); zeta-potential of the maleicacid ester (no magnetite crystals) in a 15 mg L−1 aqueous solution containing 10 mMNaCl (�), and 10 mM NaCl and 2.4 mM CaCl2 (�) (b).

were the result of precipitation of the surfactant in solution as pHdecreased.

Fig. 3b shows the zeta-potential of the maleic acid ester in aque-ous solution not containing magnetite particles. The fact that it waspossible to measure the zeta-potential of the surfactant dissolvedin water indicates that it was significantly aggregated. Surfactantconcentration used in this work was above the critical micelle con-centration so the presence of micelles and possibly even largeraggregates in solution could be expected. Surfactant aggregatesin a 10 mM aqueous solution of NaCl were found to be highlynegatively charged at pH 4–11 (empty triangles) indicating thatthe head groups of surfactant species in aggregates were depro-tonated and oriented towards the solution. Less negative chargewas observed below pH 6, probably due to protonation of surfac-tant anions since the dissociation constant (pKa) for this type ofmolecules could be expected to be about 3.5 [23]. When calcium

ions were added to the surfactant solution (filled triangles) thecharge became much less negative and independent of pH indi-cating the interaction between calcium and surfactant in solution.Furthermore, the charge of surfactant–calcium species in solutionwas rather similar to the charge of the magnetite particles treatedwith calcium, silicate and surfactant (filled triangles in Fig. 3b andempty squares in Fig. 3a, respectively) supporting the proposedmechanism of surfactant adsorption on magnetite via calcium ions.

3.2. Contact angle measurements

Table 1 illustrates how consecutive conditioning with calciumions, sodium silicate and the maleic acid ester at pH 8.5 affected thehydrophilic properties of the magnetite film.

The water contact angle of the as-prepared magnetite film wasdetermined to be slightly above 20◦, which is rather close to thevalue of 25 ± 5◦ previously reported by Wang and Ren [3] for mag-netite in distilled water. Having a high concentration of acid andbase sites [24] contributing to the polar component of the Gibbsenergy according to the van Oss theory [25], iron oxides are com-monly considered to be hydrophilic and thus not floatable withoutcollector.

In the present work, treatment of the magnetite film with cal-cium chloride lowered the water contact angle by ca. 3◦. Accordingto a proposed mechanism [8,17], calcium ions can react specifi-cally with surface hydroxyls releasing protons and adding positivecharge to the surface. In other words, adsorption of calcium ionsreduces the amount of surface hydroxyl groups and at the sametime increases the amount of unsaturated Lewis acid sites on thesurface. Gentleman and Ruud [26] recently showed that dehydra-tion of the metal oxide surface decreases the water contact angleof the surface due to the fact that metal–water interactions arestronger than hydrogen bonding between the surface hydroxylsand water. Similarly, when hydrogen in the hydroxyl groups is sub-stituted with calcium, the polar contribution to the Gibbs energyis expected to increase, interaction with water becomes strongerthus increasing surface hydrophilicity and decreasing the contactangle.

Subsequent adsorption of sodium silicate in the present worklowered the water contact angle on the magnetite film even further,to at least 10◦. Decreased contact angle could possibly be a resultof the increased amount of hydroxyl groups [27] on the magnetitesurface introduced by the silicate species adsorbed. Deprotonationof the silicate surface complexes as proposed by Jolsterå et al. [18]would further increase surface polarity and consequently enhancesurface interaction with water reducing the water contact angleand making the magnetite surface more hydrophilic.

Adsorption of the maleic acid ester increased the contact anglebetween water and the magnetite surface to 40–50◦, see Table 1.For alkyl sulfonate with a hydrocarbon chain containing 12 car-bon atoms (as the surfactant used in this work) the reportedcontact angle of water on alumina varied from less than 20◦ toca. 80◦ depending on the concentration of surfactant in solution[22]. According to the results presented by the authors, a contactangle of 40–50◦ corresponds to the beginning of the formation ofhemimicelles on the surface. This value of the contact angle wouldprobably not be sufficient to facilitate flotation of magnetite since

Table 1Contact angle of the as-synthesized magnetite film and magnetite film after consequent conditioning with calcium ions, sodium silicate and maleic acid ester. The backgroundelectrolyte was 10 mM NaCl. The given values were measured 1 s after a drop of water was deposited on the surface and are presented as an average value ± one standarddeviation.

Treatment As-synthesized magnetite 4 mM CaCl2 0.4 mM Na2SiO3 25 mg L−1 maleic acid ester

Contact angle,◦ 22 ± 3◦ 19 ± 2◦ ≤10◦a 44 ± 3◦

a The exact value of the contact angle could not be estimated since 5 out of 8 measurements after silicate adsorption were below detection limit of the instrument (10◦).

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in order to have natural floatability the mineral should have a con-tact angle above 60◦ [4]. However, the hydrophobic areas formedby surfactant adsorbed on the magnetite surface may facilitate theattachment of air bubbles and incorporation of these bubbles insidethe green pellets produced from the flotated magnetite concentrateby wet agglomeration. Inclusions of air in the pellets have beenshown to lower pellet strength both in wet and dry state [10–12]as discussed above.

It is important to mention that the variation of the contact angleobserved in the present work within the same sample was likely aresult of surface heterogeneity [28] including geometrical hetero-geneity or surface roughness and chemical heterogeneity which isenhanced by uneven distribution of adsorbate on the surface, e.g.,by patchy adsorption of the surfactant [29].

3.3. ATR-FTIR spectroscopy

3.3.1. Adsorption of sodium silicateThe spectra of silicate species after 20 min of adsorption onto

magnetite at pH 7.5–9.5 in the presence of calcium ions are shownin Fig. 4a. The broad absorption band observed in the spectra at1200–800 cm−1 is associated with the Si–O stretching vibrationsand is commonly reported [30] for silicate species adsorbed on

Fig. 4. ATR-FTIR spectra of the silicate species on magnetite after 20 min of adsorp-tion at pH 7.5–9.5 after pre-adsorption of calcium at the same pH (a), and integratedabsorbance of the silicate band between 1250 and 800 cm−1 as a function of adsorp-tion time at pH 7.5 (♦), 8.5 (�) and 9.5 (�) (b). The background electrolyte was 10 mMNaCl. The dotted lines in Fig. 5a indicate the shift of the absorption band upon theincrease of pH.

iron oxides. The band observed at ca. 950 cm−1 is assigned to themonomeric surface silicate species whereas the bands observedat 1200–1000 cm−1 are assigned to the oligomeric and polymericsilicate species with increasing degree of polymerization as thebands are shifted to higher wavenumbers [30]. The intensity of theobserved bands increases with increasing pH, see Fig. 4a, whichindicates that more silicate is adsorbed on the magnetite surfaceat higher pH. The band assigned to the oligomeric silicate speciesin the spectra recorded at pH 8.5 and 9.5 had the highest inten-sity and was shifted to higher wavenumbers as compared to thesame band observed in the spectrum recorded at pH 7.5 (the shiftis indicated by the dotted lines in Fig. 4a). This implies that moreoligomerized species were present on the magnetite surface at pH8.5–9.5 than at pH 7.5 probably due to the higher surface loadingof silicate species at higher pH facilitating surface polymeriza-tion.

The adsorption of inorganic anions from aqueous solutions rep-resents usually a two-step process with a fast and a slow stage [16];the latter is often described by the Elovich equation [31]:

� = 1ˇ

ln(˛ˇ) + 1ˇ

ln(t) (1)

where and are constants, t is time, and � is the surface coverageat the time t.

Upon the reasonable assumptions that the amount of sili-cate species adsorbed on the magnetite surface is proportional tothe integrated absorbance of the Si–O stretching vibration bandbetween 1250 and 800 cm−1 in the spectra (Fig. 4a), the surface cov-erage � in the Elovich equation can be replaced by the integratedabsorbance and plotted as a function of ln(t) (Fig. 4b).

The obtained linear dependences of the integrated absorbancevs. ln(t) in Fig. 4b indicate that the experimental data were in goodagreement with the Elovich equation (with a regression coefficientR2 > 0.995 for all the pH studied).

Considering the amount of silicate adsorbed after 5 min (thedata points in the plot at ln(t) = 1.6), Fig. 4b shows that the adsorp-tion increased with increasing pH, in concert with the previouslyreported findings for silicate adsorption on maghemite [18] andgoethite [20,21] without pre-treatment of iron oxides with cal-cium chloride. The results obtained in the present work indicatethe same tendency of increased silicate adsorption onto magnetiteand that calcium ions did not change this tendency. As a result ofcalcium ions adsorbing on the magnetite surface, the positive netsurface charge increased with increasing pH in the range 7.5–9.5(see Fig. 2) resulting in an increased attraction between the mag-netite surface and the negatively charged silicate species that startto form in solution above pH 7 [32]. Additionally, surface precipi-tation of calcium silicate which increases with increasing pH [33],may contribute to silicate loading on the magnetite surface.

The fact that the plots for pH 8.5 and 9.5 shown in Fig. 4b arealmost parallel (the slopes are 2.44 and 2.64, respectively) suggeststhat pH did not have significant effect on the slow stage of silicateadsorption onto magnetite in this pH range. However, the slope ofthe plot at pH 7.5 is considerably lower (viz. 1.46) indicating thatadsorption at this pH proceeds with a lower rate. At this pH, thesurface charge density approaches zero, likely facilitating coagula-tion of the particles in the magnetite film. Coagulation would resultin decreased surface area available for adsorption, i.e. decreasednumber of available surface sites, and consequently, would affectthe amount adsorbed.

3.3.2. Adsorption of the anionic surfactantFig. 5 shows the results from the adsorption of maleic acid ester

onto a magnetite film at different pH values followed by ATR-FTIRspectroscopy. The pH was gradually decreased starting at pH 10.The surfactant was allowed to adsorb for 5 h at each pH to allow the

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Fig. 5. ATR-FTIR spectra of the maleic acid ester on magnetite after 5 h of adsorptionfrom a 25 mg L−1 solution at pH 4, 6, 8, 8.5 and 10 (top to bottom) (a), and absorbanceof the carbonyl stretching vibration band originating from the maleic acid esteradsorbed on magnetite at different pH (b). The background electrolyte was 10 mMNaCl.

adsorption approach equilibrium. Assignment of the main absorp-tion bands in the spectra in Fig. 5a is presented in Table 2 anddiscussed in detail in our previous work [15].

The data in Fig. 5 suggest that the amount of surfactant on themagnetite surface increased with decreasing pH, likely since thesurface charge first becomes less negative and then turns posi-tive with decreasing pH (see Fig. 2) thus making the surface moreelectrostatically favourable for the adsorption of the negativelycharged deprotonated surfactant species. Another possible expla-nation could be an increased precipitation of the surfactant on themagnetite surface with decreased pH since the solubility of thesurfactant is reduced as pH gets more acidic.

The increase in intensity of the bands originating from the sur-factant adsorbed on magnetite is accompanied by the increase ofthe negative absorption bands at 1630 cm−1 and 1487 cm−1 origi-nating from the bending vibrations of water [34] and asymmetricstretching vibrations of the carbonate species [35], respectively.

Table 2Assignments of the main absorption bands originating from the maleic acid esteradsorbed on magnetite. The background electrolyte was 10 mM NaCl.

Peak position, cm−1 Assignment

1566, 1394 �as(COO−), �s(COO−) in carboxylic acid [36]1721 �(C O) in ester [37]1098 �(C–O–C) in the polyethylene glycol chain [38]

Fig. 6. ATR-FTIR spectra of the maleic acid ester on magnetite after 1 h of adsorptionfrom a 25 mg L−1 solution at pH 7.5–9.5 after pre-adsorption of calcium and silicateat the same pH (a), and absorbance of the carbonyl stretching vibration band orig-inating from the maleic acid ester adsorbed at pH 7.5 (♦), 8.5 (�) and 9.5 (�) onmagnetite pre-treated with calcium ions and sodium silicate at the same pH (b).The background electrolyte was 10 mM NaCl.

The presence of these bands in the spectra suggests that both waterand the carbonate species are partly removed from the surfaceas surfactant loading increases. The displacement of water furtherindicates that the surface gets more hydrophobic upon surfactantadsorption, in concert with the contact angle results presented inTable 1.

The point at about pH 8 in Fig. 5b diverges slightly from the trendmarked by the dotted line. Similar results illustrating an increasedsurfactant adsorption in the neutral pH region were previouslyreported by Morgan et al. [39] for oleate adsorption on hematiteand were explained by the formation of an acid soap complex[(RCOO)2H]− [40]. However, no evidence of the presence of thistype of aggregate has been found for the surfactant used in this worksuggesting that further experiments should be performed before aplausible explanation for such behaviour can be given.

The spectra recorded after 1 h of surfactant adsorption at dif-ferent pH values onto magnetite pre-treated with calcium ions andsodium silicate are shown in Fig. 6a. The spectra in Fig. 6a are rathersimilar to the ones shown in Fig. 5a, except for an additional band atabout 1030 cm−1 arising from the silicate species which continuedto adsorb on magnetite even after surfactant addition.

Fig. 6b shows the intensity of the carbonyl stretching vibrationband in the spectra of surfactant adsorbed on magnetite at differ-ent pH values as a function of time. The band intensity after 1 h

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of adsorption at pH 8.5 was higher than after adsorption at pH7.5 or pH 9.5 (Fig. 6b) for several repeated measurements indi-cating that there would be a maximum in surfactant adsorptionon magnetite around pH 8 also when the magnetite surface hadbeen modified with calcium and silicate solutions. Fig. 6b suggeststhat more surfactant was adsorbed at pH 9.5 than at pH 7.5, whichis opposite to the results obtained for surfactant adsorption on as-synthesized magnetite and shown in Fig. 5b. Such behaviour cannotbe explained by the zeta-potential of the magnetite particles sincethe charge of the magnetite surface with calcium and silicate israther constant and increases only slightly above pH 9 (diamonds inFig. 3a). It is more likely that adsorption increases due to high affin-ity of the surfactant towards calcium ions, which were expectedto be present on the magnetite surface in larger amount at pH9.5 than at pH 7.5 in accordance with the zeta-potential results inFig. 2 (open squares) for calcium-treated magnetite. Another reasoncould be the increased calcium-collector precipitation at higher pH.

Considering the fact that sodium silicate has previously beenshown to reduce flotation collector adsorption on iron oxides[7,41,42], adsorption of the maleic acid ester could be anticipated todecrease with increasing pH due to higher silicate adsorption. Fur-thermore, conditioning time with silicate could be expected to havean impact on the amount of surfactant adsorbed afterwards. How-ever, variation of the conditioning time with silicate in the range of5–20 min at pH 7.5, 8.5 and 9.5 did not have any considerable effecton the amount of the maleic acid ester adsorbed on magnetite atany of the pH values (not shown), which accentuates the impor-tance of the initial fast stage of the silicate adsorption. Togetherwith the observed increase in surfactant adsorption with increas-ing pH, these findings suggest that the maleic acid ester and silicateprobably adsorbed independently on different surface sites: silicatespecies mainly on magnetite surface hydroxyls and the surfactantmainly on the calcium ions adsorbed on magnetite. The presentedresults also support the conclusion made in our previous work [15]that in the presence of calcium ions adsorption of sodium silicatedoes not affect the amount of the maleic acid ester adsorbed onmagnetite to any considerable extent at pH 8.5 used in the flotationof apatite from magnetite.

4. Conclusions

ATR-FTIR spectroscopy in combination with zeta-potential andcontact angle measurements proved to be a powerful tool for study-ing simultaneous adsorption of several species from solution ontomineral surfaces. In the present work, adsorption of sodium silicateand maleic acid ester on magnetite in the presence of calcium ionswas investigated at different pH values.

Whereas calcium ions cannot be directly detected on the mag-netite surface by ATR-FTIR spectroscopy, the zeta-potential resultsprovided evidence for specific adsorption of calcium on magnetite.Adsorption of calcium ions reduced the negative charge of themagnetite surface treated with silicate, suggesting that high con-centrations of calcium in process water could have an adverse effecton the dispersing performance of sodium silicate in flotation.

Adsorption of sodium silicate on magnetite pre-treated with cal-cium chloride increased with increasing pH in the range 7.5–9.5.Subsequent surfactant adsorption in the same pH range was thehighest at pH 8.5 and was not very much affected by the amount ofsilicate adsorbed on magnetite under the conditions studied sug-gesting that in the presence of calcium ions, silicate and collectoradsorbed independently on different surface sites. According to thezeta-potential results, surfactant was likely adsorbed on magnetitethrough calcium ions.

Whereas treatment with calcium chloride and sodium silicatedecreased the contact angle of the magnetite surface, subsequently

adsorbed surfactant species made the magnetite surface partlyhydrophobic. Although this degree of hydrophobicity is unlikely tohave a negative effect on the reverse flotation of iron ore, flotationcollector species adsorbed on the magnetite surface could causeinclusion of air bubbles inside the green pellets produced by wetagglomeration of iron ore thereby lowering the strength of the pel-lets. In forthcoming work, methods to improve the wettability ofmagnetite surfaces contaminated with a flotation collector will beinvestigated.

Acknowledgements

This is a contribution by Centre of Advanced Mining and Met-allurgy. The financial support by the Hjalmar Lundbohm ResearchCentre and Luossavaara-Kiirunavaara Aktiebolag (LKAB) is grate-fully acknowledged. The authors would like to thank M.Sc. D.Sammelin from Umeå University for performing the ATR-FTIR mea-surements as a part of her Master thesis project.

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PAPER IV

The effect of polymer adsorption on the wetting properties

of partially hydrophobized magnetite

E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund

Submitted to Journal of Colloid and Interface Science

The effect of polymer adsorption on the wetting properties of partially hydrophobized

magnetite E. Potapova,* M. Grahn, A. Holmgren, and J. Hedlund

Division of Sustainable Process Engineering, Luleå University of Technology, SE-971 87 Luleå,

Sweden

* To whom correspondence should be addressed: elisaveta.potapova@ltu.se; tel.: +46 920 491776;

fax: +46 920 491199.

Abstract

Upon reverse flotation of iron ore, the surface of the iron ore concentrate may become partially

hydrophobized due to adsorption of flotation collector, which is facilitated by the calcium ions

present in the process water. Hydrophobic areas on the concentrate surface may introduce problems

in subsequent pelletization of the concentrate. A possible way to restore the wettability of the

surface could be by modifying the surface with a hydrophilic polymer. The effect of hydrophilic

polymers of different types, viz. cationic, anionic, and non-ionic, on the wettability of the magnetite

surface after adsorption of a surfactant was investigated. Although all the polymers could adsorb on

magnetite at pH 8.5, the contact angle measurements revealed that only anionic ammonium

polyacrylate could decrease the contact angle of synthetic magnetite after surfactant adsorption to a

level close to that of as-synthesized magnetite. Such effect was probably achieved due to shielding

of the hydrophobic surfactant chains from the aqueous phase by hydrophilic polyacrylate

molecules. The fact that polyacrylate adsorption on magnetite occurred via calcium ions makes

polyacrylate suitable for application in calcium-rich process water. The results presented in this

work illustrate that ammonium polyacrylate could be successfully used to improve the wettability of

magnetite after adsorption of surfactants.

Keywords

Adsorption, ATR-FTIR, contact angle, magnetite, polymer, wettability

1

1. Introduction

Hydrophilic properties of iron oxides are important for several industrial applications including

production of pigments, ferrofluids, and iron ore pellets. Pelletization of iron ore concentrates

cleaned by flotation has always been recognized as a potential problem [1-3] due to a combination

of decreased surface tension in the process water and increased hydrophobicity of the iron oxide

surface upon adsorption of flotation collector.

Numerous efforts have been made to minimize collector coating on iron oxides by either reducing

collector adsorption during reverse flotation or desorbing collector species from the iron oxide

surface after flotation. It has been suggested that collector adsorption could be reduced by removal

of calcium and magnesium ions [4-10] naturally present in the process water and known to increase

contamination of iron oxides with collector. However, water softening becomes a challenge in the

case of sparingly soluble calcareous gangue minerals that release calcium to the process water upon

partial dissolution.

Another way to reduce collector adsorption on iron oxide is to use depressants [5, 7, 11-16].

Whereas depressants make the mineral surface hydrophilic enough to prevent the mineral from

floating, the surface is not necessarily completely free from surfactant.

Once a flotation collector is adsorbed on the iron oxide surface, it has been proven difficult to

eliminate it by simple acidic or basic treatment [1]. To remove hydrophobic collector coatings from

the iron oxide surface various methods were proposed [1, 17].

In the present study, we propose an alternative way of improving wettability of the iron ore

concentrate after flotation by modifying the surface with a hydrophilic organic polymer.

Organic polymers have previously been tested and used as binders in agglomeration [18, 19].

However, rather high dosage of a polymer may be required to completely substitute the binder (e.g.

bentonite), which might not be economically beneficial because of the normally high cost of

synthetic polymers. Accordingly, the efficiency of using low dosages of polymers to improve

wettability of the iron oxide surface prior to binder addition was investigated in the present work.

A polymer, which could be used for increasing magnetite wettability prior to agglomeration, should

satisfy the following requirements:

� The polymer should be hydrophilic by nature and should contain functional groups that can

facilitate adsorption of the polymer on magnetite.

� Polymer performance should not be impaired by calcium and magnesium ions.

� The polymer should not contain environmentally and metallurgically harmful elements (such

as sulphur and phosphorus) and should not introduce other impurities to the final product.

� The polymer should not impair interaction between magnetite and binder.

� The polymer should be easy to handle implying, first of all, good solubility in water.

2

� The price of the polymer should be reasonable in relation to the required dosage.

Based on the above mentioned requirements, three different types of polymers were chosen for the

study, viz. anionic polyacrylate salt, cationic aliphatic quaternary amine, and non-ionic water

soluble starch. Adsorption of the polymers on magnetite was studied in-situ using ATR-FTIR

spectroscopy and the effect of polymer adsorption on the wettability and zeta-potential of magnetite

was determined using contact angle and electrophoretic mobility measurements, respectively.

2. Materials and Methods

2.1. Materials

Magnetite nanocrystals were synthesized and purified according to the procedure described

previously [20]. The crystals had a spherical habit and the size of 5-15 nm [20]. A dispersion of

magnetite in methanol and degassed distilled water containing 25 vol. % methanol and ca

1.1 mg mL-1 magnetite was prepared and stored in a refrigerator in order to minimize oxidation of

magnetite.

Magnetite films for the spectroscopic and contact angle measurements were prepared by spreading

0.3 and 0.5 mL of the dispersion, respectively, over a ZnSe substrate. The dispersion medium was

then allowed to evaporate in air at room temperature. We have previously reported [20] that films

prepared by spreading 0.3 mL dispersion on a ZnSe substrate are even, 250-300 nm thick layers of

particles. The layers are porous and the pore size is comparable with the crystal size.

Stock solutions of calcium chloride (CaCl2·2H2O, 95 %, Riedel-de Ha�n), sodium silicate

(Na2SiO3·9H2O, � 98 %, Sigma) and dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma-Aldrich)

used as a model flotation collector were prepared by dissolving the required amounts of the

corresponding chemicals in 10 mM aqueous solutions of NaCl (pro analysi, Riedel-de Ha�n). Fig. 1

shows the chemical structure of the surfactant used as a model flotation collector.

Figure 1. Chemical structure of dodecyloxyethoxyethoxyethoxyethyl maleate. R represents the

linear alkyl chain CH3(CH2)11.

The polymers used in this work are presented in Table 1. Stock solutions of the polymers were

prepared using distilled water.

Distilled water used in the experiments was degassed under vacuum to minimize the amount of

dissolved gases. The pH of the working solutions was adjusted using aqueous solutions of sodium

hydroxide (NaOH, pro analysi, Merck) and hydrochloric acid (HCl, 37 %, pro analysi, Merck).

3

Table 1. Polymers used for magnetite surface modification.

Polymer name Structural formula Average

molecular weight

Supplier

Dispex A40 (ammonium

polyacrylate)

4000 BASF

ATC 4150

(aliphatic quaternary

polyamine)

50000 Eka chemicals

Soluble starcha N/A Merck

a1 wt % starch solution in distilled water containing 0.5 wt % NaOH was heated to 84°C during

10 min and then cooled to room temperature [21].

2.2. Methods

2.2.1. ATR-FTIR spectroscopy

A Bruker IFS 66v/S spectrometer equipped with a liquid nitrogen cooled mercury-cadmium-

telluride (MCT) detector and a vertical ATR accessory was used for collecting infrared data. Both

single beam background and sample spectra were acquired by averaging 500 scans at a resolution of

4 cm-1. Spectra evaluation was performed using the Bruker Opus 4.2 software. A trapezoidal ZnSe

crystal (Crystran Ltd.), with the dimensions of 50x20x2 mm and 45° cut edges, was coated with a

magnetite film as described above and mounted in a stainless steel flow cell. In-situ adsorption

measurements were performed by pumping the working solution continuously through the cell at a

rate of 10 mL min-1 with recirculation. All the spectroscopic experiments were performed at pH 8.5

and room temperature, with 10 mM NaCl as a background electrolyte. The working solution was

continuously bubbled with argon during the experiment to minimize the amount of dissolved carbon

dioxide. The pH of the solution was controlled by a Mettler Toledo T70 titrator. Prior to adsorption,

the magnetite film was rinsed with a 10 mM NaCl solution at pH 8.5 for 30 min. The chemicals

were added to the working solution in the following sequence:

4 mM CaCl2 � 0.4 mM sodium silicate � 25 mg L-1 surfactant � 12.5 mg L-1 polymer.

Prior to addition of each solute, a new background spectrum was recorded. Adsorption of each

component was monitored for 2 h by recording infrared spectra with 5 min interval. Thereafter,

4

desorption of the adsorbed species from the magnetite surface was attempted by flushing the cell

with a 10 mM aqueous solution of NaCl at pH 8.5 for 25 min.

2.2.2. Contact angle

The static sessile drop method was used to determine the contact angle of the synthetic magnetite

nanoparticles. Contact angle measurements were done using a Fibro 1121/1122 DAT-Dynamic

Absorption and Contact Angle Tester equipped with a CCD camera. Magnetite films for the contact

angle measurements were prepared as described above. Prior to the contact angle measurement, the

magnetite film was rinsed with distilled water and dried in a vacuum desiccator for 30 min. The

measurement was performed by placing a water droplet with a volume of 4 μL onto the magnetite

film using a microsyringe. A series of images were captured and analysed using the DAT 3.6

software. The value of the contact angle was determined as an average of the values measured for 8

fresh droplets placed on the same film. Thereafter, consecutive adsorption of CaCl2, Na2SiO3,

anionic surfactant, and a polymer was performed on the magnetite film at the same concentrations

and conditions as in the spectroscopic measurements. Between the adsorption steps, the film was

rinsed with distilled water and dried in a vacuum desiccator before the contact angle was measured.

After the last measurement (polymer solution), the film was left in air for 24 h and then a new

measurement was performed.

2.2.3. Electrophoretic mobility

The zeta-potential of the magnetite nanoparticles and a polymer in solution was determined by

electrophoresis using a ZetaCompact instrument equipped with a charge-coupled device (CCD)

tracking camera. The electrophoretic mobility data was further processed by the Zeta4 software

applying the Smoluchowski equation. The samples containing magnetite were prepared in the

following way: one drop of the magnetite suspension was dispersed in 1 L of distilled water

containing 10 mM NaCl, 4 mM CaCl2, 0.4 mM Na2SiO3, 25 mg L-1 maleic acid ester and 25 mg L-1

ammonium polyacrylate. The zeta-potential of the polymer in aqueous solution was determined

using 12.5 mg L-1 ammonium polyacrylate dissolved in 10 mM NaCl. In order to investigate the

effect of calcium ions on the charge of the polymer in solution, the measurement was performed on

a solution containing 10 mM NaCl, 25 mg L-1 ammonium polyacrylate, and 4 mM CaCl2. The

samples spanned the pH range from 4 to 10. For each sample, the measurement was repeated three

times and the final zeta-potential was calculated as an average of the obtained values.

5

6

3. Results and Discussion

3.1. ATR-FTIR spectroscopy

To determine whether the polymers adsorbed on magnetite, adsorption experiments were performed

on films of synthetic magnetite and monitored by in-situ ATR-FTIR spectroscopy. Infrared spectra

of the polymers adsorbed on magnetite at pH 8.5 after pre-adsorption of CaCl2, Na2SiO3, and the

anionic surfactant are shown in Fig. 2.

We have previously reported that in the presence of calcium ions adsorption of carboxylate

surfactant and sodium silicate on magnetite at pH 8.5 occurred independently on different surface

sites [22]. Silicate could be expected to interact mainly with the hydroxyl groups on the magnetite

surface whereas the surfactant likely adsorbed on magnetite in the form of a ternary complex with

calcium ions. The zeta-potential of the magnetite particles in the presence of calcium chloride,

sodium silicate, and anionic carboxylate surfactant was negative and nearly constant in the pH

range 5-10.

The increase in intensity of the absorption bands in Fig. 2 with time indicates that the three

polymers, independent of their charge and functionality, adsorbed on magnetite modified with the

anionic surfactant.

Assignment of the major absorption bands originating from the vibrations of different groups in the

polymers adsorbed on magnetite is presented in Table 2. Apart from the bands attributed to the

polymers, absorption bands originating from the anionic surfactant (at 1728 and 1582 cm-1 in

Fig. 2a and 2c) and silicate (at 1026 cm-1 in Fig. 2a) are present in the spectra of cationic polyamine

and starch, implying that both silicate and surfactant continued to adsorb on magnetite after addition

of the polymers to the working solutions.

No pronounced bands associated with the surfactant species are found in the spectra of polyacrylate

in Fig. 2b suggesting that adsorption of the polymer in that case prevailed over surfactant

adsorption. Furthermore, negative intensity between 1100 and 900 cm-1 in Fig. 2b indicates that

polyacrylate caused desorption of silicate from the magnetite surface.

Normally, polymers carrying multiple charged groups (polyelectrolytes) adsorb on oppositely

charged surfaces but can also adsorb on the surfaces bearing the same type of charge providing that

the charge is not too high to hinder adsorption [23].

Figure 2. ATR-FTIR spectra of the polymers adsorbed onto magnetite at pH 8.5 for 0, 15, 30,

45, and 60 min: cationic aliphatic polyamine (a), anionic ammonium polyacrylate (b), and

7

starch (c). The single beam spectrum of pre-adsorbed CaCl2, Na2SiO3, and the anionic

surfactant served as background. Ionic medium: 10 mM NaCl.

In the cationic polymer used in the present work, quaternary ammonium cations bear a

permanent positive charge which is independent of pH. However, primary amine

functionalities in the ethylene diamine monomer can become protonated forming –NH3+

groups. The protonation constants (log KH1 and log KH

2) for ethylene diamine at 25°C and

zero ionic strength are 9.91 and 6.86 [24], respectively, so assuming that ethylene diamine

monomers in the polymer exhibit similar deprotonation behaviour as pure ethylene diamine,

one could expect the monoprotonated ethylene diamine [NH2(CH2)2NH3+] to be the

predominant specie at pH 8.5.

Apart from the fact that adsorption of the cationic polymer on the negatively charged

magnetite is favoured by electrostatic forces [25-27], specific adsorption through -OH and

-NH2 groups could also occur. Hydrophobic interaction between the polymer and the

hydrocarbon chains of the surfactant species adsorbed on magnetite may contribute to

adsorption as well.

Table 2. Assignments of the main absorption bands originating from the polymers adsorbed

on magnetite in-situ at pH 8.5 in the presence of CaCl2, Na2SiO3 and the anionic surfactant.

Ionic medium: 10 mM NaCl.

Peak position, cm-1 Assignment

Cationic aliphatic polyamine [25, 28]

1481, 1470 (CH2), (CH3)

1200-950 �(CH-OH), �(CH-NH2)

Anionic ammonium polyacrylate [29]

1555, 1410 �as(COO-), �s(COO-)

1731 �(C=O) hydrogen bonded

1678 �(C=O) in monodentate configuration

1457 (CH2)

Starch [30]

1153 �(C-O-C) in glucosidic linkage

1081, 1026 �(C-O) coupled with �(C-C) and (O-H)

8

For ammonium polyacrylate, the pKa is strongly dependent of the overall dissociation degree

(), especially at low ionic strength, since removal of a proton is hampered by the negative

potential of the polyanion [23]. The pKa of polyacrylic acid (MW = 2000 Da) at 25°C and

zero ionic strength when �1 was reported to be 6.95 ± 0.01 [31]. Accordingly, polyacrylate

at pH 8.5 could be expected to be nearly fully deprotonated.

In concert with previous studies [32, 33], the negatively charged polyacrylate was found to

adsorb on negatively charged magnetite surface indicating that forces other than Coulomb

interaction caused the adsorption. The intense bands at 1555 and 1410 cm-1 in the spectra of

polyacrylate adsorbed on magnetite (Fig. 2b) emanate from the asymmetric and symmetric

stretching vibration of the deprotonated carboxylic group, respectively, with �� = 145 cm-1

which is comparable to �� = 141 ± 4 cm-1 for calcium polyacrylate in solution [29]. It

indicates that polyacrylate was adsorbed on magnetite mainly via calcium ions as suggested

by Jones et al. [29]. According to Vermöhlen [34], the presence of calcium ions greatly

increases polyacrylate adsorption on oxide surfaces and makes the structure of the adsorbed

molecules more coiled due to the ability of calcium ions to stabilize coils through

intramolecular bridging. The presence of two weak bands at 1731 and 1678 cm-1 in the

spectra of polyacrylate adsorbed on magnetite (Fig. 2b), originating from the stretching

vibrations of the C=O bond, implies that some of the polyacrylate species were adsorbed via

hydrogen bonding and as a monodentate complex, respectively [29]. The band at 1731 cm-1

could also indicate that the carboxylic groups in the polymer were not fully deprotonated [35].

Considering adsorption of starch, no significant shifts of the absorption bands were observed

as compared to their position in the spectrum of non-adsorbed starch reported in the literature

[30]. Two mechanisms are commonly suggested for starch adsorption on metal oxides:

surface complexation with metal sites or hydrogen bonding with surface hydroxyls [30, 36].

In the present study, both mechanisms would be possible due to the presence of different

surface sites on the magnetite surface after the adsorption of calcium chloride, sodium silicate,

and the surfactant.

A negative absorption band at around 1640 cm-1 present in the spectra in Fig. 2 (more clearly

observed in the spectra in Fig. 2a and 2c) originates from the bending vibration of water [25].

The fact that the negative intensity of the band increased with time suggests that water was

removed from the surface upon adsorption of the polymers. Considering the intensity of this

band for different polymers, the intensity decreased in the sequence cationic polyamine >

starch > anionic polyacrylate, suggesting that the cationic polyamine caused the largest

9

displacement of water from the magnetite surface. Displacement of water implies an increased

surface hydrophobicity upon polymer adsorption.

Desorption of the polymers from the magnetite surface was performed using a 10 mM

aqueous solution of NaCl at pH 8.5, pumped through the cell without recirculation. No new

single beam background spectra were recorded prior to desorption. Fig. 3 shows the spectra

obtained by subtraction of the spectra recorded after 25 min of desorption from the spectra

recorded after 2 h of adsorption. The resulting spectra in all the three cases contained similar

absorption bands, independent of the polymer used, indicating that the desorbed species were

mainly those adsorbed prior to polymer addition, viz. surfactant (the bands at 1718, 1562,

1466, 1426, 1349, and 1102 cm-1) and silicate (the bands at 1022 and 953 cm-1). This

conclusion was supported by the fact that a similar spectrum was previously obtained by

spectral subtraction for the desorption of surfactant adsorbed in the presence of calcium

chloride and sodium silicate [20]. Thereby, the desorption results suggest that the adsorbed

polymers had high affinity to the magnetite surface.

Figure 3. ATR-FTIR spectra obtained by subtraction of the spectra recorded after 25 min of

desorption from the spectra recorded after 2 h of adsorption of polyacrylate (a), starch (b) and

cationic polymer (c). Desorption was performed by flushing with 10 mM NaCl at pH 8.5.

10

The shape and the position of the bands at 1800-1050 cm-1 in spectrum (a) in Fig. 3 are

slightly different from those in spectra (b) and (c) indicating that ammonium polyacrylate was

probably partially removed from the magnetite surface upon flushing with a 10 mM aqueous

NaCl solution. The desorption could be caused by depletion of calcium ions in solution upon

flushing with aqueous NaCl that resulted in repulsion of polymer molecules from each other.

However, the intensity of the bands in spectrum (a) in Fig. 3 is much lower than the intensity

of the bands in the last spectrum in Fig. 2b, suggesting that polyacrylate desorbed only to a

little extent.

The intensity of the bands originating from the silicate species in spectrum (a) is considerably

lower than the intensity of these bands in spectra (b) and (c), implying that the silicate species

were desorbed from the surface already during polymer adsorption and, accordingly, less

silicate left the surface when flushing with aqueous NaCl.

3.2. Contact angle of magnetite

Since all the three polymers were found to adsorb on magnetite, the effect of the polymers on

the wettability of the surfactant-coated synthetic magnetite particles was further investigated

using the static sessile drop method. The results are presented in Table 3 as an average of 8

replicates ± one standard deviation. The value of the contact angle for each water droplet was

collected one second after the droplet was placed on the surface. Since the flotated pellet feed

is normally stored for a certain period of time prior to agglomeration [37], the contact angle of

the magnetite film treated with a polymer was measured twice – directly after polymer

adsorption and after storage in air for 24 h.

Table 3 illustrates that the wettability of the magnetite particles was significantly reduced by

surfactant adsorption as compared to the wettability of as-synthesized magnetite and

magnetite after adsorption of calcium ions and sodium silicate, in agreement with the results

reported previously [22]. Whereas the resulting contact angle after surfactant adsorption

would probably not be high enough to facilitate flotation of magnetite, the adsorbed surfactant

species could facilitate attachment of air bubbles to the magnetite particles and incorporation

of air bubbles inside the iron ore pellets during agglomeration.

Adsorption of the polymers affected the contact angle very differently, most likely due to the

nature of the polymers. Treatment with cationic polyamine significantly increased

hydrophobicity of the magnetite surface. Such behaviour could possibly be explained by

orientation of the hydrophilic –OH and –NH3+ groups in the adsorbed polymer towards the

magnetite surface. In that case, the hydrophobic hydrocarbon chain of the polymer would be

11

exposed to water resulting in a high contact angle. The drop of the contact angle upon storage

in air could be caused by the alteration of the polymer conformation on the surface.

Table 3. Water contact angle of the as-synthesized magnetite and magnetite after sequential

adsorption of calcium chloride, sodium silicate, and surfactant, followed by treatment with a

polymer and storage in air for 24 h. The table shows three independent sets of measurements.

Treatment Contact angle (degrees)

As-synthesized magnetite 14 ± 3 22 ± 3 14 ± 2

Ca2+, Na2SiO3 10a

Surfactant 44 ± 6 44 ± 3 49 ± 4

Polymer Cationic aliphatic

polyamine

Anionic ammonium

polyacrylate

Starch

68 ± 2 24 ± 6 40 ± 4

24 h in air 49 ± 11 20 ± 4 46 ± 3 aThe contact angle could not be measured since it was below the detection limit of the

instrument (10°).

Treatment with starch decreased the contact angle of the surfactant-coated magnetite film to

ca 40°. Similar value was reported by dos Santos and Oliveira [15] for hematite after starch

adsorption. However, after 24 h in air the contact angle increased again, probably due to re-

arrangement of surfactant and starch in contact with the hydrophobic environment.

Adsorption of ammonium polyacrylate decreased the contact angle of the magnetite film

almost to the value of pure magnetite. Furthermore, the low contact angle was preserved even

after the film was kept in air for 24 h. Since no negative bands originating from surfactant

species were observed in the spectra of polyacrylate on magnetite (Fig. 2b), the decrease of

the contact angle was likely not due to surfactant desorption. Similar phenomenon was

observed by Somasundaran and Cleverdon [26] for amine/cationic PAM adsorption on quartz.

The authors reported that flotation of quartz was depressed by the adsorption of the polymer

whereas adsorption of the amine collector remained unchanged. The authors attributed such

behaviour to the masking of the adsorbed collector molecules by the massive polymer chains.

Similarly, the ability of polyacrylate to form loops, especially in the presence of calcium ions

[34], could facilitate adsorption of the polymer on the magnetite surface sites free from

surfactant and at the same time shield hydrophobic surfactant moieties from the water phase.

12

The high density of the carboxylic groups in the polyacrylate chain renders it highly

hydrophilic, resulting in an improved wettability of the magnetite surface.

It is important to point out that the effect of the polymers on the wettability of magnetite was

in agreement with the spectroscopic results illustrating water displacement from the magnetite

surface upon polymer adsorption. As discussed above, the negative absorption band of water

had the highest intensity in the spectra of the cationic polyamine. Accordingly, adsorption of

this polymer resulted in the highest contact angle of the magnetite surface. As the spectra of

ammonium polyacrylate on magnetite had the least intense negative water bands, treatment

with polyacrylate resulted in the most hydrophilic magnetite surface. These observations

suggest that the signal from water in the ATR-FTIR spectra recorded in-situ can be used to

predict the change of surface wettability.

Since ammonium polyacrylate was the only polymer that could restore magnetite wettability

after surfactant adsorption, this polymer was investigated further. In order to find out whether

the polymer could penetrate inside the magnetite particle film, contact angle, drop area, and

drop volume were analyzed as a function of time, see Fig. 4.

The contact angle of the magnetite surface prior to polymer adsorption decreased fast during

the first second and thereafter continued to decrease slowly throughout the measurement. On

the contrary, the drop area increased fast initially and then almost levelled off indicating that

the drop was slightly spreading on the surface at the beginning of the measurement. In

addition, the drop volume was almost constant suggesting that no penetration of water into

pores occurred and the decrease of the contact angle was solely due to water spreading on the

magnetite surface. According to Shang et al. [38], decreasing contact angle of polar liquids

like water could be a result of hydration and polar acid-base interactions between the wetting

liquid and different functional groups present on the surface. In the present work, apart from

the hydroxyl groups on the magnetite surface, water could interact with adsorbed species such

as calcium ions and silicate since these species enhance surface wettability [22]. Additionally,

surfactant head groups oriented towards the water phase could contribute to surface wetting.

After polyacrylate adsorption, the decrease of the contact angle during the first second

became much larger (� = 29° and 11° for polymer and surfactant, respectively), accompanied

by a significant increase in the drop area, which continuously expanded during the first 4 s

and then stabilized. The decrease in the contact angle during the first 4 s was probably due to

ionization of carboxylic groups in the polymer in the presence of water enhancing interaction

of the surface with water. No penetration of water into the pores took place since the drop

13

volume remained almost constant. However, after 4 s, the drop volume started to decrease

rather fast and dropped down even faster after 8 s.

Figure 4. Contact angle, drop area, and drop volume measured on the surface of the

magnetite film after consecutive adsorption of calcium ions, sodium silicate, and maleic acid

ester (�), and ammonium polyacrylate (�).

14

At the same time, the drop area decreased, implying that water at the droplet boundary

penetrated into the pores. Decreasing droplet volume indicates that water could penetrate into

the film, implying that polymer adsorption occurred even in the pores. In summary, the results

presented in Fig. 4 indicate that the wetting of the magnetite surface was significantly

improved by adsorption of the polymer.

The effect of pH on the contact angle of the magnetite surface was investigated using

DISPEX N40 – the sodium form of the polyacrylate used in the experiments discussed above.

The effect of sodium polyacrylate on the wettability of magnetite was similar to that of

ammonium polyacrylate, namely, the contact angle was decreased by ca 20° as compared to

the contact angle after surfactant adsorption.

The stepwise increase of pH from 8.5 to 9.5 and then to 10.5 during polymer adsorption did

not have any considerable effect on the contact angle measured after adsorption at each pH,

indicating that the hydrophilizing effect of the polymer was present also at alkaline

conditions.

Finally, as a control experiment, to illustrate the importance of using a polymer to render

magnetite hydrophilic after surfactant adsorption, the effect of a short dicarboxylic acid

(maleic acid, HOOC-CH=CH-COOH) on the contact angle of water on magnetite was

investigated. This molecule was chosen since it is a part of the surfactant head group and

could thus be expected to have a similar affinity to magnetite as the surfactant. Whereas we

have previously shown that maleic acid did not adsorb on hematite at pH 8.5 [39], the

presence of calcium ions could possibly facilitate the adsorption of maleic acid on magnetite.

Adsorption of maleic acid would illustrate if it is possible to render magnetite hydrophilic by

replacing surfactant molecules adsorbed on the surface by molecules without a hydrophobic

chain.

By means of the contact angle measurements it was concluded that treatment with maleic acid

at the same conditions as for the case of ammonium polyacrylate did not have any effect on

the wettability of magnetite. Furthermore, according to the spectroscopic adsorption data (not

shown), maleic acid did not adsorb on magnetite modified with calcium, silicate and anionic

surfactant. These findings indicate that both the length and the functionalities of a molecule

are important for its performance as a surface hydrophilizer.

3.3. Zeta-potential

One of the requirements set for the polymer that could be used to improve wettability of the

magnetite concentrate prior to agglomeration is that it should not impair the interaction

15

between the magnetite surface and the binder. Among the parameters that could affect the

interaction is surface charge. When bentonite is used as a binder, the magnetite surface should

not have too high negative charge not to cause electrostatic repulsion between the bentonite

platelets and the magnetite surface. In order to investigate the effect of polyacrylate

adsorption on the charge of the magnetite surface at different pH values, zeta-potential

measurements were performed, and the results are shown in Fig. 5.

The charge of the magnetite particles after polyacrylate adsorption (filled diamonds) was

negative and constant with pH, similarly to the charge of the magnetite particles after

adsorption of an anionic carboxylate surfactant [22]. The charge decreased by ca 5 mV, as

compared to the charge prior to polymer adsorption suggesting that polyacrylate adsorption

would not have any significant electrostatic effect on the interaction between the binder and

magnetite.

Similar results were reported by Pettersson et al. [33] for polyacrylate adsorption on alumina

and zirconia. The authors observed a shift in IEP to lower pH upon adsorption of the polymer;

however, in the present work, measurements below pH 5 could not be performed due to

precipitation of the surfactant at low pH.

Figure 5. Zeta-potential as a function of pH; of the magnetite particles after adsorption of

calcium ions, sodium silicate, maleic acid ester, and ammonium polyacrylate (�); of

ammonium polyacrylate in aqueous solution (no magnetite particles) in the presence (�) and

absence (�) of calcium ions. Ionic medium: 10 mM NaCl.

The fact that polymer adsorption did not decrease the charge of the magnetite particles to any

considerable extent suggests that the charge of the ionized groups in the polymer was

16

compensated by counter-ions, both at the surface and in solution. Fig. 5 illustrates that the

charge of the polyacrylate species in aqueous solution in the presence of 10 mM NaCl, but

without any calcium ions present (empty triangles), decreased with increasing pH, probably

due to the gradual deprotonation of carboxylic groups in the polymer.

The charge of the polyacrylate species in solution in the presence of calcium ions (empty

squares) was nearly constant with pH, probably due to complexation of polyacrylate with

calcium in solution. The presence of divalent calcium ions in solution screens neighbouring

negative charges better than monovalent sodium ions. Additionally, as has been discussed

earlier, calcium ions can facilitate intramolecular interactions stabilizing polyacrylate.

Furthermore, the charge of calcium polyacrylate in solution was rather similar to the charge of

the magnetite particles after polymer adsorption, suggesting that polyacrylate was adsorbed

on magnetite by forming a ternary complex with calcium ions.

4. Conclusions

Three types of polymers (cationic, anionic, and non-ionic) were tested for their ability to

restore wetting of synthetic magnetite pre-treated with a surfactant. All the three polymers

could adsorb on magnetite although they were differently charged. However, only the anionic

polyacrylate could improve magnetite wettability to the level of pure magnetite under the

conditions studied. No desorption of the surfactant was observed upon polymer adsorption

suggesting that the improved surface hydrophilicity was achieved due to shielding of the

hydrophobic surfactant tails by hydrophilic polymer chains. From the results of the

ATR-FTIR and zeta-potential measurements, it was concluded that polyacrylate was adsorbed

on the surface of magnetite via calcium ions. Polyacrylate adsorption only slightly increased

the negative zeta-potential of the magnetite surface and is not likely to have any significant

electrostatic effect on the interaction between bentonite and magnetite in agglomeration.

Magnetite treated with polyacrylate remained hydrophilic at alkaline pH and during storage in

air, suggesting that treatment with polyacrylate could be a feasible means of improving

wettability of the flotated magnetite concentrate prior to agglomeration.

Acknowledgements

This is a contribution by the Centre of Advanced Mining and Metallurgy (CAMM). The

financial support by the Hjalmar Lundbohm Research Centre (HLRC) is gratefully

acknowledged.

17

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[27] E.K. Kim, H.W. Walker, Colloids and Surfaces A: Physicochemical and Engineering

Aspects 194 (2001) 123.

[28] J.E. Stewart, The Journal of Chemical Physics 30 (1959) 1259.

[29] F. Jones, J.B. Farrow, W. Van Bronswijk, Langmuir 14 (1998) 6512.

[30] P.K. Weissenborn, L.J. Warren, J.G. Dunn, Colloids and Surfaces A: Physicochemical

and Engineering Aspects 99 (1995) 11.

[31] C. De Stefano, A. Gianguzza, D. Piazzese, S. Sammartano, J. Chem. Eng. Data 45 (2000)

876.

[32] L.J. Kirwan, P.D. Fawell, W. Van Bronswijk, Langmuir 20 (2004) 4093.

[33] A. Pettersson, G. Marino, A. Pursiheimo, J.B. Rosenholm, J. Colloid Interface Sci. 228

(2000) 73.

[34] K. Vermöhlen, H. Lewandowski, H.D. Narres, M.J. Schwuger, Colloids and Surfaces A:

Physicochemical and Engineering Aspects 163 (2000) 45.

[35] M.A. Romero, B. Chabert, A. Domard, J. Appl. Polym. Sci. 47 (1993) 543.

[36] Q. Liu, Y. Zhang, J.S. Laskowski, Int. J. Miner. Process. 60 (2000) 229.

[37] S.P.E. Forsmo, Influence of Green Pellet Properties on Pelletizing of Magnetite Iron Ore,

Doctoral thesis, Luleå University of Technology, Luleå, 2007.

[38] J. Shang, M. Flury, J.B. Harsh, R.L. Zollars, J. Colloid Interface Sci. 328 (2008) 299.

[39] E. Potapova, I. Carabante, M. Grahn, A. Holmgren, J. Hedlund, Ind. Eng. Chem. Res. 49

(2010) 1493.

19

20

PAPER V

Interfacial properties of natural magnetite particles compared

with their synthetic analogue

E. Potapova, X. Yang, M. Westerstrand, M. Grahn, A. Holmgren, and J. Hedlund

Full-length paper to be submitted to Minerals Engineering Abstract and accepted for presentation at the

Flotation 2011 Conference in Cape Town, South Africa

INTERFACIAL PROPERTIES OF NATURAL MAGNETITE PARTICLES

COMPARED WITH THEIR SYNTHETIC ANALOGUE E. Potapova,*, a X. Yang, b M. Westerstrand,c M. Grahn,a A. Holmgren,a and J. Hedlunda

aChemical Technology, Luleå University of Technology, SE-971 87 Luleå, Sweden bResearch Centre for Eco-Environmental Sciences, Chinese Academy of Science, Beijing 100085,

China cApplied Geology, Luleå University of Technology, SE-971 87 Luleå, Sweden

*To whom correspondence should be addressed: elisaveta.potapova@ltu.se; tel.: +46 920 491776;

fax: +46 920 491199.

ABSTRACT

Understanding of the interactions between iron oxides and flotation reagents is important both for

flotation and agglomeration of iron ore. Model systems comprising synthetic iron oxides and pure

chemical reagents are commonly applied in experimental work in order to obtain high quality data

and to ease the interpretation of the empirical data. Whether the results obtained using model

systems are valid for iron ore minerals and commercial reagents is a question seldom addressed in

the literature. It is shown in this work that previously reported results obtained from a model

system, concerning adsorption of a carboxylate surfactant and sodium metasilicate onto synthetic

magnetite nanoparticles, as obtained by in-situ ATR-FTIR spectroscopy and contact angle

measurements, are applicable to adsorption of flotation reagents on magnetite concentrate.

Additionally, the problem of restoring magnetite wetting after flotation is addressed since good

wetting of a magnetite concentrate is required to produce iron ore pellets by wet agglomeration.

The results from the present work indicate that the wettability of both synthetic magnetite coated

with surfactant and magnetite concentrate after flotation can be improved by adsorbing a

hydrophilizing agent such as silicate or polyacrylate.

KEYWORDS

Flotation reagents, Iron ores, Particle size, Surface modification

1

1 INTRODUCTION

Interfacial phenomena such as adsorption at the solid/liquid interface and wetting are important for

several mineral processing operations including desliming, flocculation, flotation and

agglomeration. In-situ infrared spectroscopy and contact angle measurements have proven to be a

powerful combination of tools for studying adsorption on mineral surfaces and its effect on surface

wettability (Fuerstenau (2007)). ATR-FTIR spectroscopy allows monitoring interactions in-situ in

the presence of water and provides information about adsorption kinetics and equilibrium as well as

the surface complexes formed (Smart et al. (2007)).

Since most of the minerals are not transparent for infrared radiation, ATR elements coated with thin

films of mineral particles are commonly applied in the adsorption studies by ATR-FTIR

spectroscopy (Smart et al. (2007)). Natural mineral particles are not always appropriate for this type

of measurements since they may have relatively low surface area resulting in a low signal-to-noise

ratio and may contain impurities that would complicate the interpretation of the spectroscopic

results. For that reason, synthetic mineral particles with high surface area are commonly applied in

the adsorption studies by ATR-FTIR spectroscopy.

In our previous work (Potapova et al. (2010a), Potapova et al. (2010b)), a method based on in-situ

ATR-FTIR spectroscopy was developed and successfully used for studying adsorption of flotation

related chemicals from aqueous solutions on thin films of synthetic iron oxides. The spectroscopic

data were later complemented by contact angle and zeta-potential measurements (Potapova et al.

(2011b)).

The interactions between iron oxides and flotation reagents are important, not only for the

performance in flotation of apatite from magnetite, but also for the subsequent agglomeration of

magnetite concentrate to produce iron ore pellets. Reduced wetting of the magnetite concentrate due

to adsorption of a flotation collector results in lower pellet strength (Forsmo et al. (2008),

Gustafsson and Adolfsson (1997), Iwasaki et al. (1967)) and may hamper the production capacity of

a pelletizing plant.

The scope of the present work was to characterize the interfacial properties of natural magnetite

particles cleaned by magnetic separation and flotation and to compare with the properties of the

synthetic magnetite nanoparticles used in our previous experimental work, especially to test

whether the conclusions drawn regarding adsorption behaviour and wettability of synthetic

magnetite are valid for mineral particles. Whereas this aspect is seldom addressed in the literature,

the difference in interfacial properties of synthetic and natural materials is an important issue to

consider when substituting synthetic particles for their mineral analogue in experimental work.

2

2 MATERIALS AND METHODS

2.1 Materials

Magnetite nanocrystals were synthesized and purified according to the procedure described

previously (Potapova et al. (2010b)). A dispersion of magnetite in distilled water was further diluted

with methanol and degassed distilled water resulting in a working dispersion containing 25 vol. %

methanol and ca 1.1 mg mL-1 magnetite. The dispersion was stored in a refrigerator in order to

minimize oxidation of magnetite.

Magnetite pellet concentrate after flotation and magnetic separation from the pelletizing plant in

Kiruna, Sweden was provided by LKAB. The concentrate had been stored at ambient conditions for

more than two years. Moist when received, the concentrate was stored in a refrigerator in a sealed

plastic bag. Prior to usage, the concentrate was dried in an oven at 50°C.

Flotation collector Atrac 1563 (Akzo Nobel) and dispersant/depressant water glass were provided

by LKAB, Sweden.

Water glass is an aqueous solution of sodium silicate, in this case with a SiO2:Na2O weight ratio of

3.25. Sodium metasilicate (Na2SiO3·9H2O, � 98 %, Sigma) was used in the experiments with

synthetic magnetite as an analytical grade alternative of water glass.

Flotation collector Atrac 1563 has a complex chemical composition: 50-100 % ethoxylated tall oil

ester of maleic acid, and 1-5 % maleic anhydride (Akzo Nobel material safety data sheet). Since

exact composition and chemical structure of Atrac 1563 were not specified by the supplier,

dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma-Aldrich) was used as a model flotation

collector to ease the interpretation of the spectroscopic data. Fig. 1 shows the supposed chemical

structure of flotation collector Atrac 1563 and the structure of the surfactant used as a model

flotation collector.

Figure 1. Supposed chemical structure of Atrac 1563 (a) and the structure of

dodecyloxyethoxyethoxyethoxyethyl maleate (b). R represents the alkyl chain in fatty acids, R’

represents the linear alkyl chain CH3(CH2)11.

Calcium chloride (CaCl2·2H2O, 95 %, Riedel-de Ha�n) was used to provide a solution with calcium

ions. Sodium chloride (NaCl, pro analysi, Riedel-de Ha�n) at a concentration of 10 mM was used as

3

ionic medium in spectroscopic, zeta-potential and contact angle measurements, if not stated

otherwise.

Dispex A40 and Dispex N40 (ammonium and sodium polyacrylate, respectively, BASF) with the

average molecular weight of 4000 were used for surface modification of magnetite after surfactant

adsorption.

All aqueous solutions were prepared using distilled water. Distilled water for the spectroscopic and

contact angle experiments was degassed under vacuum to minimize the amount of dissolved gases.

The pH of the working solutions was adjusted using aqueous solutions of sodium hydroxide

(NaOH, pro analysi, Merck) and hydrochloric acid (HCl, 37 %, pro analysi, Merck).

2.2 Methods

2.2.1 Characterization of the synthetic and natural magnetite particles

X-ray diffraction (XRD). XRD patterns of both synthetic and natural magnetite particles were

collected with a Siemens D5000 diffractometer running in Bragg-Brentano geometry using Cu-K

radiation.

High-resolution scanning electron microscopy (HR-SEM). The morphology of synthetic and

natural magnetite particles without any coating was investigated with an FEI Magellan 400 field

emission extreme high resolution scanning electron microscope (XHR-SEM) using an accelerating

voltage of 1 kV.

Energy dispersive X-ray spectroscopy (EDS). Chemical analysis of the natural magnetite

particles in the size range from 0.22 to 8 μm was carried out by SEM-EDS using an Oxford

instruments X-Max50 SDD detector, combined with a Zeiss Merlin field emission SEM. In the

EDS measurements, the accelerating voltage of 20 kV was applied.

BET surface area. Specific surface area of synthetic magnetite particles was estimated using the

BET method from nitrogen adsorption data recorded at liquid nitrogen temperature using a

Micrometrics ASAP 2010 gas adsorption analyzer. Degassing was performed by evacuating the

sample at 130°C overnight.

Zeta-potential. The zeta-potential of both synthetic and natural magnetite at different pH was

determined by electrophoresis using a ZetaCompact instrument equipped with a charge-coupled

device (CCD) tracking camera. The electrophoretic mobility data was further processed by the

Zeta4 software applying the Smoluchowski equation. The zeta-potential was calculated as an

average of three replicates.

The natural magnetite sample for the zeta-potential measurements was collected in the form of a

slurry after flotation at the LKAB concentration plant in Kiruna, Sweden. Due to the particle size

limitations of the measuring technique, the zeta-potential was measured using the particles with a

4

size of 0.2-8 μm, obtained by vacuum filtration of the slurry using cellulose filters. Prior to

filtration, the filters were washed with a 5% aqueous solution of acetic acid (Odman et al. (1999)).

Process water used in the zeta-potential measurements on the synthetic magnetite was obtained

from the filtration step at the LKAB concentration plant in Kiruna, Sweden. The process water was

filtered through a cellulose filter (Millipore, 0.22 μm pore size), washed with a 5% aqueous solution

of acetic acid (Odman et al. (1999)). The concentration of dissolved species in the process water

was analyzed at ALS Scandinavia in Luleå, Sweden, accredited according to the international

standards ISO 17025, ISO 9001:2000, SS EN 1484 and ISO/IEC Guide 25. The samples were

analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and

inductively coupled plasma sector field mass spectrometry (ICP-SFMS). Total organic carbon

(TOC) was analyzed using a Shimadzu TOC-5000 high-temperature combustion instrument.

Contact angle. The static sessile drop method was used to determine the contact angle of the

synthetic magnetite nanoparticles. A thin film of synthetic magnetite was produced on a substrate

by spreading 0.5 mL of the magnetite dispersion described in Section 2.1 and drying it in air at

room temperature. Contact angle measurements were performed using a Fibro 1121/1122

DAT-Dynamic Absorption and Contact Angle Tester equipped with a CCD camera. The

measurement was performed by placing a 4 μL water droplet onto the magnetite coated substrate

using a microsyringe. A series of images of the droplet were recorded and analysed using the DAT

3.6 software. For each film, the measurement was repeated 8-10 times by applying fresh droplets.

The value of the contact angle was determined as an average of the replicates. Consecutive

adsorption of CaCl2, Na2SiO3, collector, and a hydrophilizing agent (either ammonium polyacrylate

for 1 h or sodium metasilicate for 24 h) was performed on the magnetite film at the same

concentrations and pH as in the spectroscopic measurements. After each adsorption step, the contact

angle of the magnetite film was measured. Further experimental details are described elsewhere

(Potapova et al. (2011a)).

The contact angle of the natural magnetite particles was determined by the Washburn method using

a Krüss K100 force tensiometer. Liquid sorption by the magnetite powder was recorded as a

function of immersion time and Krüss LabDesk 3.1 software was used to calculate the contact angle

applying the Washburn equation. First, the capillary constant of the Washburn equation was

estimated for each sample using n-hexane. Thereafter, the contact angle of the magnetite powder

was measured using deionized water. The values of the capillary constant and the contact angle

were calculated as an average of three replicates. For a single measurement, ca 1 g of the magnetite

powder was used.

To investigate the effect of different reagents on the wettability of the natural magnetite particles,

batch adsorption was performed using suspensions containing 10 g magnetite per ca 40 mL solution

5

at pH 9 and room temperature. After adsorption, the solution was decanted and the magnetite was

dried in an oven overnight at 50°C.

To investigate the effect of calcium ions and water glass on collector adsorption onto the natural

magnetite particles, magnetite powder was preconditioned for 1 h in an aqueous solution containing

water glass at a concentration of 1.0 mg g-1 magnetite (or ca 1.1 mM [Si]), together with 10 mM

NaCl or 4 mM CaCl2. Thereafter, the flotation collector Atrac 1563 was added to the suspension

and was allowed to adsorb for 20 min.

To investigate the effect of sodium polyacrylate and water glass on the wettability of flotated

magnetite concentrate, as-received magnetite concentrate was treated with these reagents for 1 h

and 9 h, respectively. The effect of sodium polyacrylate was examined in the aqueous solutions of

either calcium chloride or sodium chloride.

2.2.2 In-situ ATR-FTIR spectroscopy

Infrared spectra were recorded using a Bruker IFS 66v/S spectrometer equipped with a deuterated

triglycine sulphate (DTGS) detector and a liquid nitrogen cooled mercury-cadmium-telluride

(MCT) detector. Magnetite coated ZnSe ATR crystals (Crystran Ltd) in the form of a trapeze with

45° cut edges and dimensions of 50x20x2 mm were used in this study.

The incidence angle of the infrared beam was set to 45°. Both single beam background and sample

spectra were obtained by averaging 500 scans at a resolution of 4 cm-1. Data processing was

performed using the Bruker Opus 4.2 software. All the spectroscopic experiments were performed

on solutions at pH 8.5 and at room temperature. The pH of the solution was controlled by a Mettler

Toledo T70 titrator. Further experimental details are described elsewhere (Potapova et al. (2010b),

Yang et al. (2008)).

3 RESULTS AND DISCUSSION

3.1 Characterization of the synthetic and natural magnetite particles

XRD patterns presented in Fig. 2 indicate pure crystalline magnetite without any other phases

present in amounts detectable by XRD. Broader reflections in Fig. 2a compared to the ones in

Fig. 2b indicate that the crystal size was much smaller for the case of the synthetic magnetite than

for the magnetite mineral, in concert with SEM observations, see below.

6

Figure 2. XRD patterns of the synthetic (a) and natural (b) magnetite crystals. The reflections

originating from magnetite are indexed by the appropriate Miller indices. For the sake of clarity, the

intensity of pattern (a) was multiplied by a factor 2 and shifted.

Fig. 3 shows HR-SEM images of the cross-section of a thin layer of synthetic magnetite

nanoparticles deposited on a ZnSe substrate and natural magnetite particles spread over a carbon

tape. Synthetic magnetite particles (Fig. 3a) showed a spherical habit with a diameter of 5-15 nm.

The particles were partially aggregated and formed a continuous porous film on the ZnSe substrate.

The particles in the magnetite concentrate (Fig. 3b) had irregular shape and varied in size. The

coarse magnetite particles in Fig 3b were covered by very fine particles (less than 1 μm in size),

some of which, according to the EDS results, had a high content of silicon and aluminium and could

be the remains of aluminosilicates present in the iron ore before concentration.

Figure 3. HR-SEM images of synthetic magnetite particles on a ZnSe substrate (a) and natural

magnetite particles on a carbon tape (b).

7

The specific surface area of synthetic magnetite determined from nitrogen adsorption data was

about 90 m2 g-1, which is much higher than the specific surface area of natural magnetite that is only

about 0.5 m2 g-1 (Forsmo (2005)). The synthetic magnetite thus had much more surface sites per

gram material available for adsorption, which motivates the use of synthetic magnetite in model

systems.

Regarding the contact angle of synthetic and natural magnetite particles, different measuring

techniques had to be used due to the difference in the particles’ size. The static sessile drop method

is suitable for measuring the contact angle of colloid particles (like synthetic magnetite

nanoparticles used in this work), providing that a closely packed layer of particles can be formed

(Shang et al. (2008)). Using this method, the contact angle of synthetic magnetite in the present

work was determined to be 15-25°. A value of 12 ± 1° was previously reported (Galindo-González

et al. (2005)) for spherical magnetite nanoparticles with a mean size of 11 ± 2 nm. Wang and Ren

(Wang and Ren (2005)) measured the contact angle of water on a polished surface of natural

magnetite using the static sessile drop method and reported a value of 25 ± 5°. The contact angle

determined in the present work is thus comparable to previously reported contact angles.

The contact angle of natural mineral powders is commonly estimated by the Washburn method. In

this method, packing of particles is also important since it may affect the penetration rate of the

wetting liquid and thus the measured value of the contact angle (Kirchberg et al. (2011)).

For the natural magnetite particles used in this work, the contact angle was estimated to be 50-60°,

which is similar to previously reported values. A contact angle of 72-75° (Kirchberg et al. (2011))

was reported for unsieved magnetite powder with a particle size of � 146 μm and irregular particle

shape. For the magnetite with a size of 86 % -74 μm, the contact angle was determined to be 46°

(Qiu et al. (2004)). Thereby, the contact angle of natural magnetite measured by the Washburn

method is normally reported to be higher than the contact angle of synthetic magnetite

nanoparticles. This difference could be partly due to the measuring technique but also the particle

size and impurities present on the surface of natural magnetite particles. Apart from that, the contact

angle measured by the sessile drop method may vary depending on the surface roughness, resulting

in an underestimated contact angle for hydrophilic surfaces and an overestimated contact angle for

hydrophobic surfaces. For the case of natural magnetite used in this work, the high contact angle

could also be caused by adsorbed flotation collector since the iron ore was concentrated both by

magnetic separation and flotation. The amount of flotation collector adsorbed on the magnetite

surface after flotation was previously estimated to be 10-30 g t-1 (Forsmo et al. (2008)). Already a

partial coverage by a surfactant could be enough to obtain a hydrophobic surface (Holmberg et al.

(2003)).

8

Fig. 4 illustrates the zeta-potential of the synthetic and natural magnetite particles as a function of

pH. The zeta-potential curve for the synthetic magnetite in a 10 mM NaCl solution (empty

triangles) exhibits the shape typically observed for iron oxides, with the IEP at pH 7 (Potapova et al.

(2011b)).

Figure 4. Zeta-potential as a function of pH; of the synthetic magnetite particles in 10 mM NaCl

(�) and in the process water from the LKAB concentrating plant in Kiruna, Sweden (�); of the

0.22-8 μm fraction of the iron ore concentrate in the process water (�).

However, the zeta potential of the 0.22–8 μm particle fraction of the magnetite slurry in the process

water after flotation (filled squares) showed a completely different dependency on pH, with two IEP

observed at about pH 11 and at pH 2. Such behaviour could be due to the specific adsorption of

soluble species present in the process water onto the surface of the particles and due to the presence

of mineral impurities in the slurry.

To test the first hypothesis, the zeta-potential was measured on the synthetic magnetite particles

dispersed in the process water from the LKAB concentrating plant in Kiruna, Sweden (open

diamonds in Fig. 4).

The curves for the iron ore concentrate after flotation and the synthetic magnetite in the process

water are fairly similar and characterized by a weakly negative zeta-potential almost in the whole

pH range with an increase in the zeta-potential at highly alkaline pH. These results suggest that the

dissolved species present in the process water can have a significant effect on the zeta-potential of

magnetite, in concert with the results reported previously (Potapova et al. (2011b)). The measured

concentration of dissolved species present in the process water is shown in Table 1.

The decrease of the zeta-potential of the magnetite particles in the process water at pH < 8 and the

shift of the IEP to a lower pH as compared to the zeta-potential of the synthetic magnetite in 10 mM

NaCl could be explained by the specific adsorption of anions such as sulphate, bicarbonate, and

9

silicate on the magnetite surface. The specific adsorption of cations such as calcium and magnesium

on the magnetite surface could result in the increase of the zeta-potential of the magnetite particles

in the process water at pH > 8 and could cause a shift of the IEP to a higher pH as compared to the

zeta-potential of the synthetic magnetite in 10 mM NaCl.

Table 1. Concentration of solutes in the process water.

Specie Ca S Na Cl K NO3 Mg HCO3 Si TOC

Concentration, mM 8.8 10.6 9.5 9.0 2.4 2.0 1.6 1.1 0.5 4.3a aConcentration in mg L-1.

To determine whether the natural magnetite particles used in the zeta-potential measurements

contained any mineral impurities, the particles of the 0.22-8 μm fraction of the magnetite slurry

obtained after flotation were investigated by SEM-EDS, which showed that the measured particle

fraction contained mainly silicon, aluminium, and oxygen. Thereby, it was concluded that the

mineral particles used in the zeta-potential measurements were to a large extent aluminosilicate.

Whereas aluminosilicate minerals at the LKAB concentrating plant in Kiruna, Sweden, are removed

from the iron ore by magnetic separation, small aluminosilicate particles still remain in the iron ore

concentrate after magnetic separation.

3.2 Adsorption behaviour of model compounds and commercial flotation reagents

Careful analysis of the ATR-FTIR spectra of model compounds and commercial flotation reagents

can provide important information about their chemical structure and adsorption mechanisms.

Fig. 5 shows spectra of the flotation collector Atrac 1563 and the model collector adsorbed on

synthetic magnetite at pH 8.5. Atrac 1563 and the model collector display rather similar spectral

features, in agreement with the structural resemblance of the head groups in these compounds

(Fig. 2). A significant difference that should be pointed out is the absence of the band at 1104 cm-1

in the spectrum of Atrac 1563, associated with the stretching vibrations of the C-O-C groups

(v(C-O-C)) in the ethoxy-chains (Beentjes et al. (2006)). This implies a low degree of ethoxylation

of the molecules in Atrac 1563 as compared to the model collector, in concert with the chemical

structures shown in Fig. 2.

10

Figure 5. ATR-FTIR spectra of maleic acid ester (a) and Atrac 1563 (b) adsorbed on magnetite for

6 h from 25 mg L-1 aqueous solutions at pH 8.5 with 10 mM NaCl as background electrolyte.

In aqueous solutions at pH 8.5, the carboxylic acid groups in Atrac 1563 and the maleic acid ester

become deprotonated forming a negatively charged carboxylate ion as indicated by the presence of

the symmetric (vs(COO-)) and asymmetric (vas(COO-)) stretching vibrations bands (Dobson and

McQuillan (1999)) in the spectra of these compounds. Previously, it was concluded that carboxylate

ions were not likely to be responsible for collector adsorption on iron oxides at pH 8.5 in the

absence of a background electrolyte due to electrostatic repulsion (Potapova et al. (2010a)) and that

the adsorption took place via the non-charged polar ester carbonyl and ethoxy groups. However, the

presence of a background electrolyte in the present work could reduce the repulsion between the

ions and the magnetite surface further facilitating collector adsorption (Potapova et al. (2010b)).

For the adsorption of maleic acid on hematite, two co-existing surface complexes were reported

(Hwang and Lenhart (2008)) – an inner-sphere complex characterized by the vs(COO-) band at ca

1407 cm-1, and an outer-sphere complex characterized by the vs(COO-) band at ca 1430 cm-1.

Similarly, Atrac 1563 and the model collector both containing maleic acid in their head groups

could form an outer-sphere complex and an inner-sphere complex on magnetite.

Additionally, ester carbonyls present in the collectors could contribute to their adsorption on the

magnetite surface, as discussed elsewhere (Potapova et al. (2010a), Potapova et al. (2010b)).

Thereby, despite the difference in the structure of the head groups, similar adsorption mechanism

was observed for both collectors justifying the usage of the maleic acid ester as a model compound

for studies of the adsorption behaviour of commercial collectors such as Atrac 1563.

Fig. 6 shows infrared spectra of sodium metasilicate and water glass adsorbed on magnetite at pH

8.5. Despite the fact that these soluble silicates have different SiO2:Na2O weight ratio, the spectral

line shapes in Fig. 6 are rather similar.

11

Figure 6. ATR-FTIR spectra of silicate adsorbed on magnetite at pH 8.5 from a 1 mM aqueous

water glass solution for 110 min (a) and from a 1 mM aqueous sodium metasilicate solution for

150 min (b). For the sake of clarity, the absorbance of spectrum (b) was multiplied by a factor 2 and

spectrum (a) was shifted.

At this pH, the most pronounced band in the infrared spectrum is located at 1020 cm-1 with two

shoulders at ca 1120 cm-1 and ca 950 cm-1. The bands at 950 cm-1 and 1020 cm-1 originate from

adsorbed monomeric and oligomeric silicate species, respectively, whereas the band at 1120 cm-1 is

associated with the 3-dimentional silica framework structure (Yang et al. (2009)).

Although the time of adsorption was higher for sodium metasilicate, greater absorption intensity is

observed for water glass due to a larger amount of magnetite used in the adsorption experiment with

water glass.

According to previous results (Yang et al. (2008)), the speciation of the adsorbed silicate on the

magnetite surface is determined by pH and silicate concentration rather than by the SiO2:Na2O ratio

suggesting that silicate sources with different composition could be expected to show similar

adsorption behaviour.

3.3 Adsorption of flotation reagents and effect on magnetite surface properties

The effect of calcium ions and sodium metasilicate on the adsorption of the model collector on

synthetic magnetite was investigated in our previous work (Potapova et al. (2010b)). It was

concluded that sodium metasilicate could suppress the adsorption of the model collector on

magnetite but only in the absence of calcium ions. When calcium ions were present in the system,

the adsorption of the collector was dramatically increased both with and without sodium

metasilicate present in solution.

12

This conclusion was confirmed by contact angle measurements (see Table 2) showing that whereas

the contact angle decreased upon treatment with calcium and metasilicate, addition of the model

collector resulted in a considerable increase in the contact angle. These results imply that sodium

metasilicate could not prevent the adsorption of the model collector in the presence of calcium ions.

Approximately the same increase in contact angle was observed for the model collector and Atrac

1563, confirming that these compounds had similar effect on the wettability of magnetite.

Table 2. Contact angle of surface modified synthetic magnetite measured by the static sessile drop

method.

Contact angle (degrees) Treatment

Test 1 (Potapova et al. (2011b)) Test 2

As-synthesized magnetite 22 ± 3 20 ± 3

4 mM calcium, 1 h 19 ± 2 15 ± 4

0.04 mM silicate, 1 h 10a 10a

25 mg L-1 maleic acid ester, 1 h 44 ± 3 -

25 mg L-1 Atrac, 1 h - 43 ± 8 aThe exact value of the contact angle could not be estimated since most of the measurements after

silicate adsorption were below detection limit of the instrument (10°).

Further, it was investigated whether the conclusions regarding the effect of calcium and silicate on

collector adsorption onto synthetic magnetite were applicable to natural magnetite particles.

Adsorption of Atrac 1563 and water glass on magnetite concentrate was performed in the presence

and absence of calcium ions and the contact angle of the concentrate was measured by the

Washburn method, see Fig. 7.

In the absence of calcium ions, the contact angle did not change upon collector adsorption

indicating that no or very little adsorption took place on magnetite concentrate pre-treated with

water glass. However, collector adsorption in the presence of calcium ions resulted in an increased

contact angle of magnetite concentrate, despite the pretreatment with water glass, due to the

activation of the magnetite surface for collector adsorption by calcium ions, in accordance with

conclusions drawn from spectroscopy data and reported previously (Potapova et al. (2010b)).

Adsorption of water glass is expected to result in a better wetting and protect magnetite from

collector adsorption, whereas collector adsorption (before bi-layer structure formation) has an

opposite effect on wettability. With a contact angle of 60°, the surface of the magnetite concentrate

still remains in the hydrophilic domain (� < 90º) implying that the particles would not necessarily

float with the air bubbles. However, hydrophobic areas on the magnetite surface impair wetting of

13

the concentrate and may result in air inclusions inside the green pellets produced by the

agglomeration of the magnetite concentrate, which has previously been shown to reduce pellet

strength in both wet and dry state (Forsmo et al. (2008)).

Figure 7. Water contact angle of magnetite concentrate upon modification of the surface with 1 mg

g-1 water glass and Atrac 1563 in 10 mM NaCl (�) and 4 mM CaCl2 (�) solution at pH 9 measured

with the Washburn technique.

3.4 Improvement of magnetite wettability after flotation

In order to improve the wettability of the flotated magnetite concentrate prior to agglomeration, the

effect of two hydrophilizing agents on the contact angle of synthetic magnetite coated with the

model collector or Atrac 1563 was investigated. Table 3 shows the contact angle of synthetic

magnetite nanoparticles measured by the static sessile drop method before and after adsorption of a

hydrophilizing agent.

When a magnetite film treated with a collector was subjected to conditioning with 4 mM CaCl2 and

0.4 mM sodium metasilicate at pH 8.5 for 1 day, the contact angle was lowered to 21 ± 1° for the

model collector and to 16 ± 1° for Atrac 1563. The increase in surface hydrophilicity was caused by

either desorption of the collector due to concentration gradient or replacement of the collector by

silicate species.

Conditioning with ammonium polyacrylate was performed by adding the polymer to a solution

already containing calcium chloride, sodium metasilicate and a collector (either Atrac 1563 or the

model collector) implying that the desorption of the collector due to the change of concentration

could not take place. As a result of the treatment, the contact angle of the magnetite film decreased

from 44 ± 3° to 24 ± 6° for the model collector and from 43 ± 8° to 20 ± 6° for Atrac 1563. The

observed increase in surface hydrophilicity was probably due to masking of the hydrophobic

collector species adsorbed on the magnetite surface by hydrophilic polymer molecules (Potapova et

al. (2011a), Somasundaran and Cleverdon (1985)).

14

Table 3. Change in the contact angle of synthetic magnetite pretreated with calcium chloride,

sodium metasilicate, and a collector upon adsorption of sodium metasilicate and ammonium

polyacrylate.

Contact angle of synthetic magnetite (degrees)

25 mg L-1 model collector 25 mg L-1 Atrac 1563

42 ± 2 44 ± 3 49 ± 3 43 ± 8

0.4 mM sodium

metasilicatea

12.5 mg L-1

polyacrylateb

0.4 mM sodium

metasilicatea

12.5 mg L-1

polyacrylateb

21 ± 1 24 ± 6 16 ± 1 20 ± 6 aIn the presence of 4 mM CaCl2. bPolyacrylate was added to the solution already containing 4 mM CaCl2, 0.4 mM Na2SiO3,

25 mg L-1 collector, and 10 mM NaCl.

To verify that silicate and polyacrylate could be used for improving magnetite wettability after

flotation, adsorption of these compounds was performed on flotated magnetite concentrate at pH 9

and the contact angle of the magnetite concentrate was measured using the Washburn method.

Fig. 8 shows the effect of water glass adsorption on the wettability of the magnetite concentrate.

Figure 8. Water contact angle of the magnetite concentrate upon modification of the surface with

water glass in 10 mM NaCl at pH 9 for 9 h measured with the Washburn technique. Prior to water

glass adsorption, the concentrate was preconditioned with 10 mM NaCl at pH 9 for 1 h.

A contact angle of 57 ± 5º was obtained for the magnetite concentrate before water glass adsorption.

Such a high value could be due to residues of the collector adsorbed on the surface after flotation, as

has been discussed in Section 3.1.

15

Treatment with water glass clearly decreased the contact angle of the magnetite concentrate, and the

hydrophilizing effect improved with increased concentration of water glass. At the highest water

glass dosage (3 mg per g magnetite), the resulting contact angle was 28 ± 3º, which is rather close to

the values obtained for synthetic magnetite and sodium metasilicate presented in Table 3.

Sodium polyacrylate was adsorbed on the magnetite concentrate in the presence of calcium ions

(4 mM CaCl2). Fig. 9 shows the effect of polymer adsorption on the wettability of the magnetite

concentrate. The contact angle of the magnetite after flotation was 52 ± 1º, which, again, could

indicate that the magnetite surface was partly coated by the flotation collector. Upon polymer

adsorption from a 0.04 mg g-1 solution, the contact angle of magnetite concentrate decreased;

however, the variation of the contact angle within the replicates was in the range 34-54° indicating

an uneven distribution of the polymer on the magnetite surface. Increasing the polymer

concentration at a constant concentration of calcium ions did not result in further decrease of the

contact angle. A possible explanation could be the depletion of the magnetite surface sites available

for polymer adsorption already at lower concentration of the polymer, or the dependence of

polymer adsorption on the concentration of calcium ions, i.e. on the calcium-to-polymer ratio.

Figure 9. Water contact angle of the flotated magnetite concentrate upon modification of the

surface with sodium polyacrylate in 4 mM CaCl2 at pH 9 for 1 h measured with the Washburn

technique. The point at 0 mg g-1 represents the contact angle of as-received magnetite.

To test the latter hypothesis, polymer adsorption on magnetite was performed at different

concentrations of calcium ions keeping the concentration of the polymer constant at 0.04 mg g-1

magnetite, see Fig. 10. When polymer adsorption was performed without calcium ions, the contact

angle of the magnetite concentrate was virtually the same as that of the as-received concentrate

suggesting that in the absence of calcium, the polymer was not effective in improving the

wettability of the magnetite surface. The slight increase in the measured contact angle (to 55 ± 1°)

16

was possibly caused by desorption of hydrophilic silicate species, which are expected to be present

on the magnetite surface after flotation.

When 4 mM calcium chloride was added to the polymer solution, the contact angle of the magnetite

concentrate decreased to 41 ± 8° as discussed above. Further increase in calcium concentration to

6 mM resulted in an even lower contact angle (25 ± 11°) confirming the importance of the calcium-

to-polymer ratio for polymer adsorption onto magnetite. Since the concentration of calcium in the

process water at the LKAB concentrating plant in Kiruna, Sweden, is 8.8 mM (see Table 1), even

better effect of polyacrylate adsorption on the wettability of the magnetite concentrate could be

expected.

Figure 10. Water contact angle of the magnetite concentrate upon modification of the surface with

0.04 mg g-1 sodium polyacrylate at pH 9 for 1 h measured with the Washburn technique. The point

at 0 mM represents the contact angle of the magnetite concentrate treated with sodium polyacrylate

in the presence of 10 mM NaCl and no calcium ions.

An increased adsorption of polyacrylic acid on aluminium oxide in the presence of calcium ions

was reported (Vermöhlen et al. (2000)). The authors explained such behaviour by the ability of

calcium ions to screen the negative charge of the carboxylic groups in the polymer more efficiently

as compared to monovalent sodium ions, allowing the polymer to adopt a more coiled conformation

on the surface thus increasing surface loading. Hence, when the polymer concentration was

increased without increasing the concentration of calcium ions, the amount of calcium ions was not

sufficient to facilitate the adsorption of the highly negatively charged polyacrylate onto the

magnetite surface also being negatively charged at this pH.

It is important to mention here that the decrease in the contact angle in Fig. 10 is not likely to be due

to increased ionic strength upon increasing calcium concentration. According to Vermöhlen et al.,

polyacrylate adsorption on alumina in the presence of 3.3 mM calcium chloride was twice higher

17

than adsorption from solution containing 10 mM NaCl, having the same ionic strength (Vermöhlen

et al. (2000)).

The fact that polyacrylate improves the wettability of magnetite more efficiently in the presence of

calcium ions makes it suitable for application in processes having a process water rich in calcium

ions. Thereby, polyacrylate could be a good candidate for improving the wettability of magnetite

after reverse flotation from calcareous gangue minerals.

4 CONCLUSIONS

In spite of differences in surface properties and morphology of synthetic and natural magnetite

particles, similar tendencies were observed for adsorption of calcium ions, soluble silicates, anionic

carboxylate surfactants, and polyacrylate polymers on these materials, as illustrated by contact

angle measurements. It was confirmed that the wettability of magnetite was reduced by collector

adsorption when calcium ions were present in the system, despite pre-conditioning with water glass.

Wettability of the flotated magnetite concentrate could be significantly improved by prolonged

treatment with water glass or rather short conditioning with sodium polyacrylate, in agreement with

the results obtained for the synthetic magnetite nanoparticles. Better wetting of the concentrate

would facilitate wet agglomeration and could possibly increase the strength of iron ore pellets

produced.

ACKNOWLEDGEMENTS

This is a contribution by the Centre of Advanced Mining and Metallurgy (CAMM) at Luleå

University of Technology, Sweden. The authors acknowledge the financial support from the

Hjalmar Lundbohm Research Centre (HLRC). The Knut and Alice Wallenberg Foundation is

acknowledged for financial support of the Magellan SEM instrument.

18

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