FERRIC SULPHATE LEACHING OF PYRRHOTITE

TAILINGS

by

Nazanin Samadifard

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Chemical Engineering and Applied Chemistry

University of Toronto

© Copyright by Nazanin Samadifard 2015

ii

Ferric Sulfate Leaching of Pyrrhotite Tailings

Nazanin Samadifard

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2015

Abstract

The present study investigates the potential to recover nickel from pyrrhotite tailings produced

at the smelting operations of Vale in Sudbury, Ontario. Leaching tests were performed in acidic

ferric sulphate media with 0.14 wt.% solids. The temperature was varied between 30 °C and

55 °C, and the ferric concentration was in a range 0.02- 0.3 M. The shrinking core model (SCM)

was applied on the nickel extraction data. The dominant reaction mechanism was found to be

diffusion control. The Arrhenius plot yielded an activation energy of Ea=62.12 kJ/mol based on

apparent reaction rates obtained by the SCM. The reaction order with respect to ferric ion was

found to be 1.0 at the high concentration range. SEM images of partially leached tailings

confirmed the presence of elemental sulphur around the pyrrhotite particles, which was

responsible for the observed non-linear leaching kinetics (diffusion control).

iii

Acknowledgments

I would like to express my sincere appreciation to my supervisor Dr. Vladimiros Papangelakis

for his constant support, guidance and patience throughout the course of my study.

Financial support of Vale Canada, Glencore, the Ontario Centers of Excellence (OCE), the

Centre for Excellence in Mining Innovation (CEMI), and the Natural Sciences and Engineering

Research Council of Canada (NSERC) are gratefully acknowledged.

I would extend my grateful thanks to Dr. Cheryl Washer for being a great consultant in the later

stages of my project, and other members of APEC laboratory for their friendship, guidance and

generosity.

The most sincere gratitude goes to my parents and my lovely family for their support and

encouragement in all aspects of my life.

Finally, I dedicate this thesis to my dear husband Mohammadreza Dadkhah for his

unconditional love and support during the completion of my master’s degree.

iv

Table of Contents

Acknowledgments ..................................................................................................................... iii

List of Tables .............................................................................................................................vii

List of Figures .......................................................................................................................... viii

Abbreviations ............................................................................................................................... x

Chapter 1: Introduction ................................................................................................................ 1

1.1 Overview ....................................................................................................................... 1

1.2 Research scope .............................................................................................................. 2

1.3 Objectives ..................................................................................................................... 2

1.4 Thesis organization ....................................................................................................... 2

Chapter 2: Literature review ........................................................................................................ 4

2.1 Nickeliferous pyrrhotite tailings ................................................................................... 4

2.2 Pyrrhotite ....................................................................................................................... 6

2.3 Dissolution pathways of pyrrhotite ............................................................................... 6

2.3.1 Oxidative dissolution ............................................................................................. 6

2.3.2 Non-oxidative dissolution ...................................................................................... 7

2.4 Factors affecting pyrrhotite oxidation ........................................................................... 8

2.4.1 Crystal structure ..................................................................................................... 8

2.4.2 Oxygen ................................................................................................................... 9

v

2.4.3 Ferric ion concentration ......................................................................................... 9

2.4.4 Temperature ........................................................................................................... 9

2.5 Oxidation-reduction potential ..................................................................................... 10

2.6 Shrinking core model .................................................................................................. 11

2.7 Passivation .................................................................................................................. 13

Chapter 3: Materials and experimental procedures ................................................................... 15

3.1 Materials ..................................................................................................................... 15

3.2 Characterization of pyrrhotite tailings ........................................................................ 15

3.2.1 Particle size distribution ...................................................................................... 15

3.2.2 Chemical characterization ................................................................................... 16

3.2.3 Mineralogical characterization ............................................................................ 17

3.3 Leaching tests .............................................................................................................. 18

3.4 Analytical techniques .................................................................................................. 19

3.4.1 ICP analysis ......................................................................................................... 19

3.4.2 Potassium dichromate titration ............................................................................ 20

3.4.3 HPLC analysis ..................................................................................................... 20

3.4.4 SEM analysis ....................................................................................................... 21

3.5 Elemental sulfur determination ................................................................................... 21

Chapter 4: Results and discussions ............................................................................................ 23

4.1 Effect of temperature on nickel extraction .................................................................. 23

4.2 Effect of ferric sulphate concentration on nickel extraction ....................................... 27

vi

4.3 Elemental sulphur generation during leaching............................................................ 30

4.4 Morphology of elemental sulphur ............................................................................... 36

Chapter 5: Conclusions .............................................................................................................. 39

References.................................................................................................................................. 41

Appendices ................................................................................................................................ 45

Appendix A. Nickel extraction kinetics at different temperatures......................................... 45

Appendix B. Ferric sulphate concentration effect on nickel extraction ................................. 53

Appendix C. Elemental sulphur determination ...................................................................... 63

Appendix D. SEM images and EDX Spectra ........................................................................ 72

vii

List of Tables

Table 2-1. Activation energies obtained for oxidation of pyrrhotite and pentlandite. .............. 10

Table 2-2. ORP correction factors for different reference electrodes. ...................................... 11

Table 2-3. Shrinking core model mechanisms. ......................................................................... 13

Table 3-1. Elemental composition of pyrrhotite tailings. .......................................................... 17

Table 3-2. Mineralogical composition of pyrrhotite tailings. .................................................... 17

Table 4-1. Diffusion-control apparent rate constant values at various temperatures. ............... 26

Table 4-2. Chemical reaction-control apparent rate constant values at various temperatures. . 26

Table 4-3. Diffusion-control apparent rate constants at various ferric sulphate concentrations.

................................................................................................................................................... 29

viii

List of Figures

Figure 2-1. Schematic diagram of the SCM. ............................................................................. 12

Figure 3-1. Particle size distribution of pyrrhotite tailings. ....................................................... 16

Figure 3-2 Experimental setup for the leaching experiments. ................................................... 19

Figure 3-3. Leached residues for elemental sulphur analysis. ................................................... 22

Figure 4-1. Effect of temperature on nickel extraction kinetics. Conditions: 0.14 wt.% solids,

0.12 M Fe2(SO4)3, and 0.2 M H2SO4. The error bars represent standard deviation from triplicate

tests. ........................................................................................................................................... 23

Figure 4-2. Plot of the diffusion-control SCM vs. time at different temperatures. ................... 25

Figure 4-3. Plot of the chemical reaction-control SCM vs. time at different temperatures. ..... 24

Figure 4-4. Arrhenius plot for the diffusion-control process. .................................................... 26

Figure 4-5. Effect of Fe2(SO4)3 concentration on nickel extraction kinetics. Conditions: 0.14

wt.% solids, 0.2 M H2SO4, and 55 °C. The error bars represent standard deviation from triplicate

tests. ........................................................................................................................................... 29

Figure 4-6. Determination of reaction order with respect to ferric ion. .................................... 30

Figure 4-7. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions:

55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4. .................................................. 31

Figure 4-8. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings.

Conditions: 55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3 , 0.2 M H2SO4. The lines are least

square linear fits to the data excluding the first time point (1 h). .............................................. 32

ix

Figure 4-9. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions:

40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 ................................................... 34

Figure 4-10. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings.

Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3 , 0.2 M H2SO4. The lines are least

square linear fits to the data. ..................................................................................................... 34

Figure 4-11. ORP and pH values of the leach solution measured at different times for 55 °C and

40 °C. Conditions: 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 ............................... 36

Figure 4-12. SEM-BSE images of partially leached pyrrhotite particles after (a) time 0 (0%), (b)

19h (50%), (c) 72 h (80%) and (d) 144 h (90%) of leaching. Conditions: 55 °C, 0.56 wt.% solids,

0.2 M Fe2(SO4)3, and 0.2 M H2SO4. .......................................................................................... 37

Figure 4-13. Elemental maps of a pyrrhotite particle after 90% nickel extraction. ................... 38

x

Abbreviations

AES Auger Electron Spectroscopy

AMD Acid Mine Drainage

CV Coefficient of Variation

HPLC High Pressure Liquid Chromatography

ICP-OES Inductively Coupled Plasma- Optical Emission Spectrometry

PCE Perchloroethylene

PSD Particle Size Distribution

QEMSCAN Quantitative Evaluation of Minerals by Scanning Electron

Microscopy

SCM Shrinking Core Model

SEM Scanning Electron Microscopy

SHE Standard Hydrogen Electrode

SIMS Secondary Ion Mass Spectrometry

UV

Ultraviolet

XPS X-ray Photoelectron Spectroscopy

XRD X-ray Diffraction

1

Chapter 1: Introduction

1.1 Overview

Due to continuous exploitation, high-grade sulphide ores are gradually being exhausted. Low-

grade sulphide minerals, including mining wastes are being considered as alternative metal

resources for the future. Over the past 50 years, the Sudbury region in Ontario, Canada, has

accumulated 50-100 million tonnes of pyrrhotite tailings as a consequence of the local smelting

operations [1]. These tailings would be a significant low-grade nickel resource if a low-cost

process for nickel recovery could be developed. In addition, environmental hazards like acid

mine drainage (AMD) could be reduced by treating the pyrrhotite tailings. Previously, several

pyrometallurgical processes were investigated for the treatment of the pyrrhotite tailings [1, 2,

3, 4]; however, most were abandoned due to the production of SO2 gas during the roasting

process and the introduction of additional Fe in the slag. On the other hand, limited laboratory

studies have been performed using hydrometallurgical techniques such as acid leaching by

hydrochloric and nitric acid, or by high-pressure oxidation [5, 6, 7].

Although hydrometallurgical methods offer the advantage of avoiding SO2 emissions, none of

these lab-scale processes has ever been commercialized. Recently, a bioleaching process was

investigated as an alternative hydrometallurgical process in our lab [8]. In this process concept,

pyrrhotite and pentlandite, the two principal nickel-bearing minerals in the tailings, are leached

by ferric sulphate which is regenerated by microbial assistance.

Ferric ion is a strong oxidizing agent that plays an important role in oxidation of sulphide ores.

Only few studies have been conducted on the leaching of pyrrhotite in ferric sulphate solution

2

[9, 10, 11], whereas the direct effect of ferric ion on the oxidation of the pyrrhotite tailings has

not been systematically studied. Earlier studies have shown that elemental sulphur is the

dominant product of the ferric sulphate leaching of pyrrhotite [12, 13]. This is advantageous

over the production of sulphuric acid as a by-product that would require recovery, storage, or

neutralization, and impoundment of vast amounts of gypsum, which would make the process

uneconomical.

1.2 Research scope

The aim of this work is to study the leaching kinetics of the nickeliferous pyrrhotite tailings in

concentrated ferric sulphate media under abiotic conditions and develop a kinetics model that

can be used for process design. The results of this study would be very useful in any follow-up

bioleaching study, as it would set a baseline for the chemical performance of the system.

1.3 Objectives

The main objectives of this study are the following:

1. Characterize the mineralogy and chemistry of pyrrhotite tailings

2. Evaluate and explain the effects of ferric ion concentration and temperature on the

kinetics of nickel extraction

3. Determine the dissolution mechanism of pyrrhotite tailings

4. Investigate the formation and morphology of elemental sulphur produced during ferric

sulphate leaching of pyrrhotite tailings

1.4 Thesis organization

Chapter 2 provides a literature review on nickeliferous pyrrhotite tailings and pyrrhotite

3

oxidation. Chapter 3 introduces the experimental procedures and analytical techniques. Chapter

4 discusses the experimental results, and Chapter 5 summarizes the key findings of this study.

The complete set of results obtained from the experiments is given in the appendices.

4

2 Chapter 2: Literature review

2.1 Nickeliferous pyrrhotite tailings

The Sudbury basin in Ontario contains ore reserves from which, currently, both nickel and

copper concentrates are produced. Both Vale and Glencore have been mining this ore type for

more than 100 years. The ore contains the sulphide minerals pentlandite, chalcopyrite, pyrite,

and large amounts of nickeliferous pyrrhotite. After grinding the ore, the minerals are

concentrated into various fractions using flotation processes. Chalcopyrite is recovered as a

copper concentrate for smelting, while pentlandite is recovered as a nickel concentrate. Since

the nickel content of nickeliferous pyrrhotite is low (~0.8%) compare to pentlandite (~32%), it

is removed from the nickel concentrate using pyrrhotite rejection circuits at the mills and

disposed of in tailing ponds. The tailings are predominantly pyrrhotite with trace amounts of

other sulphide minerals, as well as some gangue minerals (silicates). The majority of nickel is

found within the pyrrhotite matrix, and the remainder is present as an interlocked fine-grained

pentlandite fraction [1].

As mentioned earlier, pyrometallurgical processes for the treatment of the pyrrhotite tailings

were abandoned due to the production of SO2 gas. In 1947-1954 Inco (Vale) developed two

processes based on the dead roasting of the pyrrhotite tailings to produce SO2 gas and iron-

nickel oxide. The SO2 gas was processed in an acid plant to produce saleable high-grade

sulphuric acid. The iron-nickel oxide was selectively reduced to an iron-nickel alloy (FeNi) in

a rotary kiln and then leached in ammonia-ammonium carbonate solution. The nickel was

recovered as nickel oxide (NiO) by precipitation, then the iron oxide was indurated in a sintering

machine. From this process the overall nickel recovery and the sulphur content of the iron ore

5

were about 85% and 0.25%, respectively. Thomson et al. has provided some details about the

Inco roasting process [2].

In 1960, Falconbridge (Glencore) began the operation of a pilot plant using a selective

sulphation roast. The nickel from leaching was precipitated with fresh pyrrhotite, iron, and

sulphur to form a matte that was treated in a smelter for nickel recovery. The iron ore and nickel

produced from this process represented 97.2% and 87.5% recovery, respectively. Despite its low

sulphur content (0.47%), the iron ore did not meet the quality standards as a salable product.

The sulphation conditions in the process produced high concentrations of SO3 and caused

extensive aerial pollution [1, 3, 4].

Furthermore, limited lab-scale studies have been performed using hydrometallurgical

techniques. Van Weert et al. [5] conducted an investigation on leaching of nickeliferous

pyrrhotite in 6-8 N hydrochloric acid at 60-90 °C. Relatively fast leaching rates were obtained.

The main drawback of this method was that pentlandite, the other nickel-bearing mineral, could

not be leached under these conditions; also part of the solubilized nickel reprecipitated with iron

as nickeliferous marcasite (FeS2). In general, acid leaching is the least known practiced

application since it involves the formation and handling of hydrogen sulphide (H2S).

Falconbridge performed a preliminary study on oxidation of pyrrhotite tailings using an

autoclave at 100 psi air pressure and 110 °C. Although, there were no mineralogical restrictions

in this route, the process never progressed beyond the experimental stage [7].

In order to design and optimize a hydrometallurgical treatment flowsheet for nickeliferous

pyrrhotite tailings, a good understanding of chemistry and dissolution pathways of pyrrhotite is

necessary.

6

2.2 Pyrrhotite

Pyrrhotite is one of the most abundant sulphide minerals in mining environments. It has the

generic formula Fe1-xS, where x varies between 0 and 0.125 based on the vacancy of iron atoms

in the crystal structure. Pyrrhotite with the stoichiometric formula FeS is called troilite, and it

has a hexagonal structure. The monoclinic and more common structure of pyrrhotite (Fe7S8) is

magnetic due to the deficiency of iron atoms in the structure.

2.3 Dissolution pathways of pyrrhotite

For most sulfide minerals, two main pathways of dissolution were suggested, namely oxidative

and non-oxidative dissolution [10, 12, 14].

2.3.1 Oxidative dissolution

Both oxygen and ferric ion are important oxidants of sulphide minerals. When oxygen is the

primary oxidant of pyrrhotite, the oxidation reaction can proceed by reaction 2-1, where x varies

between 0 and 0.125.

Fe1−xS + (2 − 0.5x)O2 + xH2O → (1 − x)Fe2+ + SO42− + 2xH+ 2-1

2Fe2+ + 0.5O2 + 2H+ → 2Fe3+ + H2O 2-2

Fe3+ + 3H2O → Fe(OH)3 + 3H+ 2-3

Fe1−xS + (8 − 2x)Fe3+ + 4H2O → (9 − 3x)Fe2+ + SO42− + 8H+ 2-4

In reaction 2-2, ferrous ion (Fe2+) is oxidized to ferric ion (Fe3+) by oxygen. At pH > 3, ferric

ion precipitates as ferric hydroxide (Fe(OH)3), shown in reaction 2-3, but if the reaction occurs

7

under acidic conditions, a significant quantity of ferric ion will remain in the solution and can

act as the oxidant of pyrrhotite (reaction 2-4). However, the oxidation reactions may not proceed

to completion, and partial oxidation may occur instead [10, 13, 15]. In this case, a small

proportion of sulphide in the mineral is transformed to sulphate ions, and the remainder is

accumulated as elemental sulfur, shown in reactions 2-5 and 2-7. Reactions 2-6 and 2-8 show

that elemental sulphur from the partial oxidation of pyrrhotite can further oxidize to sulphate

ions; however, this can occur only at temperatures above the melting point of sulphur (~120 °C)

[16, 17].

Fe1−xS + (0.5 − 0.5x)O2 + (2 − 2x)H+ → (1 − x)Fe2+ + S0 + (1 − x)H2O 2-5

S0 + 1.5O2 + H2O → SO42− + 2H+ 2-6

Fe1−xS + (2 − 2x)Fe3+ → (3 − 3x)Fe2+ + S0 2-7

S0 + 6 Fe3+ + 4H2O → SO42− + 8H+ + 6Fe2+ 2-8

2.3.2 Non-oxidative dissolution

Non-oxidative dissolution of sulphide minerals is another pathway of dissolution, which is an

acid consuming reaction and produces a significant amount of hydrogen sulphide (H2S)

according to reaction 2-9.

Fe1−xS + 2H+ → H2S + (1 − x)Fe2+ 2-9

Tewari and Campbell [18] studied the dissolution of commercial troilite in dilute sulfuric acid

and reported that the dissolution rate constant of troilite in pH values between 2 and 5.3 at 25

°C is 2.7 ± 0.2 ×10 -6 m/s. Van Weert et al. [5] investigated the kinetics of pyrrhotite dissolution

under various conditions in hydrochloric acid, and they found that relatively rapid dissolution is

8

preceded by an induction period, during which no hydrogen sulphide is evolved. Thomas et al.

[19] studied the oxidation of synthesized pyrrhotite by 1 M perchloric acid and observed an

induction period followed by hydrogen sulphide production at different temperature ranges.

Temperature, acid strength, and the amount of surface oxidation products were found to be the

controlling factors of the length of the induction period.

2.4 Factors affecting pyrrhotite oxidation

Several variables have been identified as the main controlling factors of the rate of pyrrhotite

oxidation.

2.4.1 Crystal structure

There is a lack of consensus in the literature about whether monoclinic or hexagonal pyrrhotite

is more reactive towards oxidation. Conventionally, it is thought that the monoclinic structure

is more reactive phase of pyrrhotite; this can be attributed to the vacancy of iron atoms in the

crystal structure, which promotes electron transfer between the oxidant and the crystal lattice.

Lehmann et al. [20] showed that in a cyanide solution, monoclinic pyrrhotite has a higher

dissolution rate than the hexagonal structure under various experimental conditions. Orlova et

al. [21] reported the opposite result, finding that hexagonal pyrrhotite is more reactive than

monoclinic pyrrhotite. Janzen et al. [10] studied the oxidation behaviour of twelve pyrrhotite

samples, ranging from pure hexagonal to pure monoclinic. They measured the oxidation rates

in terms of sulphate production and iron release and found no consistent correlation between the

oxidation rate of pyrrhotite and the pyrrhotite structure.

9

2.4.2 Oxygen

Oxygen is the primary oxidant of pyrrhotite in basic environments. Knipe et al. [22] investigated

the interaction of water and the surface of pyrrhotite in the absence of oxygen, concluding that

no oxidation is observed upon exposure of pyrrhotite to deoxygenated water. There are relatively

few studies that relate the oxidation rate of pyrrhotite to the amount of dissolved oxygen. Many

studies have related the oxidation rates of sulphide ores to the partial pressure of oxygen [23,

10, 24]. Filippou et al. [24] studied the oxidative pressure leaching of monoclinic pyrrhotite at

two different temperature regimes: below and above the melting point of sulphur (119 °C). They

found that for temperatures above 119 °C, the rate of pyrrhotite oxidation shows first order

dependency on oxygen partial pressure, whereas in the low temperature regime, the reaction rate

is proportional to the square root of oxygen partial pressure.

2.4.3 Ferric ion concentration

At low pH values (pH <3) and atmospheric pressure, sulfides are mainly oxidized by ferric ion,

and the kinetics of pyrrhotite oxidation by oxygen are relatively slow [10]. Accordingly, oxygen

is considered to be the oxidizing agent for oxidation of ferrous ion to ferric ion as shown in

reaction 2-2.

2.4.4 Temperature

The oxidation kinetics of pyrrhotite increases with increasing temperature. Generally, chemical

leaching follows the Arrhenius law that describes the reaction rate constant (k) as a function of

temperature (T) and activation energy (Ea) according to Equation 2-10. Several studies have

reported activation energies for oxidation of pyrrhotite and pentlandite under different leaching

conditions. Some of these values are listed in Table 2-1.

10

k = A e−EaRT

2-10

Table 2-1. Activation energies obtained for oxidation of pyrrhotite and pentlandite.

Sample Condition Oxidant Activation energy

(kJ/mol) Reference

Nickeliferous

Pyrrhotite

4-37 °C

pH=2.5

Bacteria 40 [15]

12 Museum-grade

pyrrhotite samples

25-45 °C

pH=2.5

Oxygen 47.7-62.5 a

79.1-106 b

[10]

Museum-grade

pyrrhotite samples

7.5-35 °C

pH=2.5

Ferric 22.8-63

[11]

Museum-grade

pyrrhotite samples

10-33 °C

pH=2

pH=4

pH=6

Oxygen 58.1

52.4

100.4

[11]

Pyrrhotite ore

from Bolivia

30-45 °C

pH=2

Ferric 99 [9]

Nickeliferous

pyrrhotite from

Sudbury

80-180 °C

0.5 M H2SO4

Oxygen

68.5 [24]

Pentlandite 30, 80 °C

PO2 1,100%

Ferric

61 [25]

a. Based on iron release

b. Based on sulphate release

2.5 Oxidation-reduction potential

Ferric ion is a strong oxidizing agent and the relative abundance of ferric and ferrous ions in

solution is the principal factor that determines oxidation-reduction potential (ORP) of a leach

solution (also known as redox potential). Equations 2-11 and 2-12 are used to determine the

11

ORP at 25 °C, where the activities of ferrous and ferric irons are replaced by their concentrations

in dilute solutions.

Fe3+ + �̅� → Fe2+ 𝜀0 = 0.771 2-11

Eh = 0.771 + 0.059 log[Fe3+]

[Fe2+]

2-12

ORP is defined by Equation 2-12 when the standard hydrogen electrode (SHE) is the reference

electrode. Correction factors should be used to convert the ORP measurements with different

reference electrodes to Eh, shown in Table 2-2.

Table 2-2. ORP correction factors for different reference electrodes.

Reference electrode Electrode potential with respect to SHE

(mV)

Standard hydrogen electrode (SHE) 0

Saturated calomel electrode (SCE) +241

Ag/AgCl , 1 M KCl +192

Ag/AgCl , 4 M KCl + 228

Ag/AgCl , sat. KCl +236

2.6 Shrinking core model

Shrinking core model (SCM) is the most relevant kinetic model applied to the leaching kinetics

of low-grade ores. The model is based on the reaction zone on a particle, which shrinks inwards

as the reaction proceeds and leaves a solid product layer behind. The SCM assumes that the

12

reaction is first order with respect to the reactant, and particles are of uniform size [26].

Considering this model, five steps contribute to the overall rate of reaction as shown in Figure

2-1.

1) Diffusion of the reactant through the liquid film to the surface of the particle.

2) Diffusion of the reactant through the solid product layer to the surface of the unreacted

core.

3) Reaction of the reactant with the surface of the unreacted core.

4) Diffusion of the liquid products through the layer to the exterior surface of the particle.

5) Diffusion of the liquid products through the liquid film back into the main fluid.

Figure 2-1. Schematic diagram of the SCM.

In cases where the reaction is irreversible and no liquid product is formed, steps 4 and 5 do not

contribute to the overall reaction rate, therefore the reaction would be controlled by one or a

combination of the following mechanisms:

1) Liquid film diffusion

Liquid Film

Solid Product

Unreacted Core

Surface of the particle

rc

R

13

2) Chemical reaction

3) Diffusion through the product layer

Table 2-3 shows the mathematical description of the shrinking core kinetic models, where

XB is the fraction of ore that is converted.

Table 2-3. Shrinking core model mechanisms.

Rate controlling

mechanism

Reaction equation

The time of

complete reaction

(τ)

Parameters

Film diffusion t

τ= XB 𝜏 =

𝑅𝜌𝐵

3𝑏𝑘𝑔𝐶𝐴

kg: mass transfer coefficient

between fluid and particle

b: stoichiometric reaction

coefficient

τ: the time of complete

reaction

ks: first order rate constant for

the surface reaction

De: effective diffusion

coefficient of A in the

product layer

CA: concentration of A in the

fluid

ρB: molar density of B in the

solid

R: initial particle radius

Chemical reaction t

τ= 1 − 3(1 − XB)

13 𝜏 =

𝑅𝜌𝐵

𝑏𝑘𝑠𝐶𝐴

Diffusion through

the product layer

t

τ= 1 − 3(1 − XB)

23 + 2(1 − XB)

𝜏 =𝑅2𝜌𝐵

6𝑏𝐷𝑒𝐶𝐴

2.7 Passivation

Passivation is one of the most common problems of leaching of sulphide minerals. It describes

the situation in which the mineral surface is covered with a layer of by-product like an oxide or

other insoluble species. The product layer hinders the diffusion of reactants and products in/out

of the sulphide particles and slows down the leaching kinetics. The existence of a passivation

layer may not be proven based on only the chemical and thermodynamic properties of the leach

solution, and it would only be directly detectable by the sensitive surface analysis methods such

14

as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES or SAM) or

secondary ion mass spectrometry (SIMS).

The formation of a product layer on pyrrhotite and pentlandite has been investigated by Legrand

et al. [27]. They examined the oxidation of pyrrhotite and pentlandite in an aqueous solution

with pH=9.3 and showed that a thin layer of Fe (III)-hydroxide (probably FeOOH) is formed on

the surface of both minerals. Lu et al. [28] studied the kinetics and mechanism of pentlandite

leaching in aerated acidic chloride/sulphate solutions and observed that pentlandite experiences

passivation by elemental sulphur at low potentials. Ahonen and Tuovinen [15] performed a

number of column bioleaching experiments with a complex sulphide ore and observed a

compact ferric oxide/oxy-hydroxide, elemental sulphur, and covelite layer on pyrrhotite

surfaces.

15

3 Chapter 3: Materials and experimental procedures

This chapter describes material characterization, experimental procedure, and analytical

techniques used in this work.

3.1 Materials

Upgraded pyrrhotite tailings were obtained from Vale’s nickel smelting operations in Sudbury,

Ontario. The slurry was delivered in a large bucket, containing deoxygenated water due to the

reducing and reactive nature of pyrrhotite. A core sample was taken and homogenized using a

heavy-duty impeller. It was subsequently vacuum-filtered through Whatman quantitative filter

paper and collected as several small cakes. The cakes were kept in double-sealed plastic bags in

a freezer at -20 °C to minimize surface oxidation by oxygen in air. One day prior to any

experiment, samples were thawed and their moisture content was determined. All chemicals

used for preparing the solutions were reagent grade, and MilliQ deionized water was used.

3.2 Characterization of pyrrhotite tailings

3.2.1 Particle size distribution

The average particle size distribution of the pyrrhotite taillings were determined using a light

scattering technique with Malvern Mastersizer 2000. Figure 3-1 shows the particle size

distribution (PSD) of the pyrrhotite tailings. The distribution shows a tail on the left side

indicating the presence of fine particles at about 20 Vol% below 10 μm. The average particle

size is about 32 μm with a d50 of 28 μm and a coefficient of variation (CV) of 0.82.

16

Figure 3-1. Particle size distribution of pyrrhotite tailings.

3.2.2 Chemical characterization

Elemental composition of the pyrrhotite tailings was determined by digestion in aqua regia (1:3

HNO3: HCl) at high temperature. Approximately 0.4 g of the tailings was dissolved in a 20 mL

aqua regia solution, transferred to a sealed vessel, and heated to 200 °C in a microwave digester

(Ethos EZ Microwave Digestion System). The mixture was diluted with 5% trace metal nitric

acid in a volumetric flask. Multiple sets of dilutions were made based on the expected

concentrations of base metals in the solution and were analyzed using Inductively Coupled

Plasma-Optical Emission Spectrometry (ICP-OES), Agilent Technologies 700. Although the

exact composition of the pyrrhotite tailings produced at the mill varies with variations inside the

ore body, the general composition is shown in Table 3-1. Fe (52.8%), S (32.0%), and Ni (1.0%),

were found to be the most abundant elements in the pyrrhotite tailings.

17

Table 3-1. Elemental composition of pyrrhotite tailings.

3.2.3 Mineralogical characterization

Mineralogical characterization of the tailings was determined by Quantitative Evaluation of

Materials by Scanning Electron Microscopy (QEMSCAN) at SGS Canada Inc., Lakefield, ON,

and summarized in Table 3-2. Monoclinic Pyrrhotite (Fe7S8) is dominant at 86.2 wt.%, whereas

pentlandite accounts for only 1.2 wt.% of the tailings although it holds 40% of the total nickel.

The balance of 60% is associated with pyrrhotite.

Table 3-2. Mineralogical composition of pyrrhotite tailings.

Mineral Chemical Formula wt. %

Pyrrhotite Fe7S8 86.2

Pentlandite (Fe,Ni)9S8 1.2

Chalcopyrite CuFeS2 0.7

Pyrite FeS2 0.4

Silicates Mg2SiO4 3.8

Fe-(Ti) Oxides Fe3O4, FeTiO3 4.6

Quartz SiO2 3.1

Element Concentration (wt.%)

Fe 52.81

S 32.00

Ni 1.00

Al 0.38

Cu 0.28

Ca 0.28

Mg 0.18

Co 0.02

Mn 0.02

Cr 0.01

18

3.3 Leaching tests

Leaching experiments were conducted in baffle-based Erlenmeyer flasks. Each flask was filled

with 500 mL solution containing 0.2 M sulphuric acid and different ferric sulphate

concentrations. The ferric sulphate concentrations tested were 0.02, 0.1, 0.2, and 0.3 M,

corresponding to 1.3, 6.4, 12.7, and 19.1 excess stoichiometric levels with respect to monoclinic

pyrrhotite based on the following overall reaction.

Fe7S8 + 14Fe3+ → 21Fe2+ + 8S0 3-1

Before adding the pyrrhotite sample, the solution was heated to the desired temperature (30, 40,

or 55 °C) on an orbital shaker set at 250 rpm. 250 rpm was enough to ensure solid suspension.

The reaction was initiated by adding about 0.7 g of the pyrrhotite tailings to the solution, unless

otherwise stated. The small solids content ensured that leaching was performed under constant

ferric concentration, which is a prerequisite to test the SCM on the extraction values. The flasks

were then capped with aluminum foil to prevent cross-contamination. At regular time-intervals,

the shaker was stopped, and the flasks were weighed to determine the mass of water that had

evaporated. The evaporated water loss was made up by adding an equal volume of deionized

water. The flasks were mixed thoroughly by shaking to keep the pulp density uniform, and 5

mL of slurry samples were taken for analysis. The samples were then immediately filtered using

0.45 μm syringe filters and analyzed for ORP, pH, nickel and ferrous ion concentration. Figure

3-2 shows the experimental setup for the leaching experiments.

19

Figure 3-2 Experimental setup for the leaching experiments.

3.4 Analytical techniques

Liquid samples taken during the leaching experiments were analyzed for the dissolved ferrous

ion (Fe2+), dissolved nickel (Ni2+) and other trace metal elements. Selected solid samples after

filtration were used for acid digestion or elemental sulphur analysis.

3.4.1 ICP analysis

The concentrations of trace metals in the liquid samples were analyzed via ICP-OES.

Wavelength calibration and torch alignment were routinely applied to ensure the normal

operation of the ICP system. To prepare the calibration standards, a multi-element stock solution

containing 250 mg/L Fe, Ni, S, Mg, and 100 mg/L Al, Cu, Co, Mn, Zn, and Cr was diluted with

5% trace metal nitric acid. The dilution factors were selected according to the expected

concentrations of the elements of interest and the detection limit of the ICP.

20

3.4.2 Potassium dichromate titration

Ferrous ion concentration in the liquid samples was determined potentiometrically with 0.01 M

potassium dichromate solution (K2Cr2O7). In the redox potential between potassium dichromate

and ferrous ion, dichromate ion (Cr2O72-) gets reduced to two chromium III (Cr3+) ions in an

acidic environment with enough hydrogen ions (H+), shown in reactions 3-2 and 3-3.

Cr2O72− + 14H+ + 6�̅� → 2Cr3+ + 7H2O 3-2

Fe3+ + �̅� → Fe2+ 3-3

Cr2O72− + 6Fe2+ + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O 3-4

Reaction 3-4 represents the net ionic reaction. The 1:6 mole ratio of Cr2O72- : Fe2+ provides the

stoichiometric basis for all of the titration calculations.

3.4.3 HPLC analysis

All HPLC analyses were performed by High Performance Liquid Chromatography (HPLC)

analysis with a DIONEX 3000 HPLC system having a 20 μL injection volume and a DIONEX

UV detector operating at 254 nm with a 5 nm bandwidth. A Dionex Acclaim 120 C18 reverse-

phase column (4.6 x 250 mm; 5 μm) was used with a mobile phase composed of 95% methanol

(HPLC Grade, min. purity 99.9%) and 5% water (0.2 μm filtered) at a flow rate of 1 mL/min.

Because the presence of air bubbles in the mobile phase accounts for a variety of interferences,

it was degassed using helium sparging for about 30 min before passing it through the column.

Each measurement lasted for 25 min, and the peak corresponding to the elemental sulphur

appeared somewhere between 15 and 20 min, due to small changes in the eluent composition.

21

3.4.4 SEM analysis

The presence of elemental sulphur on the reacted pyrrhotite tailings was visualized on polished

epoxy mounts using JEOL JSM-840 Scanning Electron Microscope (SEM) utilizing back-

scattered electrons (BSE) at 15 keV accelerating potential difference. Energy-dispersive X-ray

(EDX) spectra were obtained with the same instrument. About 1 g of the solid sample was gently

dispersed in epoxy and adequate time was allowed to ensure that the material is fully cured

(about 24 hours). The sample mount was gently ground and polished with a 1200 grit silicon

carbide paper under running water and then subjected to SEM analysis.

3.5 Elemental sulphur determination

To extract elemental sulphur from the leach slurry, a combination of the techniques described

by Dutrizac, and McGuire and Hamers [29, 30] was used. At the end of the leaching test the

contents of the flask were vacuum filtered with a 10 cm Buchner filter using 15 cm Whatman

filter paper. To remove any sulphur from the walls of the flask, it was rinsed several times with

water and re-filtered. The residue was then washed twice with a weak sulphuric acid solution

(pH=3) to remove metal ions adsorbed by the residue. The filtrates were analyzed for nickel

content using ICP. The filter paper with the leach residue was dried in a vacuum oven at 40 °C

(overnight) as shown in Figure 3-3. It was then folded several times and placed in a beaker

containing 50 mL of PCE for 24 h. After sulphur extraction, the PCE solution was filtered using

a 0.2 µm nylon syringe filter, and, if necessary, sequential dilutions were performed with PCE

to bring the concentration of the sample to within the linear range of the standard solutions (6 -

240 mg/L of sulphur). The samples and the standards were then analyzed for elemental sulphur

using HPLC. To measure any elemental sulfur that might exist on the pyrrhotite surface prior to

22

leaching, a known amount of sample (~0.25 g) was dissolved in 25 mL PCE and subjected to

the same HPLC analysis. Standard solutions for elemental sulphur analysis were prepared within

the range of 6-240 mg/L by a procedure similar to that employed by McGuire and Hamers [30].

Figure 3-3. Leached residues for elemental sulphur analysis.

23

4 Chapter 4: Results and discussions

4.1 Effect of temperature on nickel extraction

Figure 4-1 shows the effect of temperature on the kinetics of nickel extraction from the

pyrrhotite tailings. These results were obtained by leaching about 0.14 wt.% solids in acidic

ferric sulphate solution at 30, 40, and 55 °C, 0.12 M Fe2(SO4)3, and 0.2 M H2SO4. At 55 °C the

nickel extraction is about 90% within 50 h, whereas at 30 and 40 °C the nickel extraction rate is

slower, indicating a possible leveling off before complete dissolution. The curve fitting in Figure

4-1 was made with the diffusion-control SCM which is discussed below. The raw extraction

data is reported in Appendix A, Tables A1.2, A2.2, and A3.2.

Figure 4-1. Effect of temperature on nickel extraction kinetics. Conditions: 0.14 wt.% solids,

0.12 M Fe2(SO4)3 and 0.2 M H2SO4. The error bars represent standard deviation from

triplicate tests.

24

The extraction of nickel from the pyrrhotite tailings involves a reaction between Fe2(SO4)3 and

the nickel-containing sulfide minerals of pyrrhotite and pentlandite, which exist as a mixture.

Although tailings are non-uniform particulate material in terms of composition and size, the

SCM was tested as a semi-empirical kinetics model for the overall nickel dissolution kinetics

[26]. The nickel extraction vs. time data at different temperatures were plotted for three rate-

controlling forms of the SCM: liquid film diffusion (Equation 4-1); diffusion through the

product layer (Equation 4-2), and chemical reaction (Equation 4-3); where km, kd and kr are the

apparent reaction rate constants, and x is the fraction of the solid converted. The data is reported

in Appendix A, Tables A1.4, A2.4, and A3.4.

x = kmt 4-1

1 − 3(1 − x)2

3⁄ + 2(1 – x) = kdt 4-2

1 − (1 − x)1

3⁄ = krt 4-3

4-3

Figure 4-2. Plot of the diffusion-control SCM vs. time at different temperatures.

25

Figure 4-3. Plot of the chemical reaction-control SCM vs. time at different temperatures.

Equation 4-1 was ruled out because the curves in Figure 4-1 are not linear. The straight-line

plots in Figure 4-2 produce higher correlation coefficients for all the temperatures in comparison

to the chemical reaction-control SCM (Figure 4-3). Moreover, in Figure 4-2, the fitted lines at

different temperatures extrapolate through the origin or are very close to the origin, whereas in

Figure 4-3 they do not. Apparent diffusion-control and chemical reaction-control rate constants

(kd and kr) at various temperatures were obtained from the slopes of the linear fits in Figures 4-

2 and 4-3 as shown in Tables 4-1 and 4-2, respectively. These values were used in an Arrhenius

plot shown in Figure 4-4. The linear fit to the data in Figure 4-4 gave an activation energy value

of Ea=62.12 kJ/mol with a correlation coefficient close to unity (R2=0.99). This is a high

activation energy value commonly attributed to chemical reaction-control processes rather than

diffusion-control processes since the former are more sensitive to temperature than the latter

[26].

26

Table 4-1. Diffusion-control apparent rate constant values at various temperatures.

Temperature (°C) kd (h-1) R2

55 1.05×10-2 0.99

40 0.35×10-2 0.99

30 0.16×10-2 0.94

Table 4-2. Chemical reaction-control apparent rate constant values at various temperatures.

Temperature (°C) kr (h-1) R2

55 0.96×10-2 0.96

40 0.45×10-2 0.92

30 0.25×10-2 0.83

Figure 4-4. Arrhenius plot for the diffusion-control process.

However, a number of kinetic studies on different systems have reported high values of

activation energy for diffusion-control dissolution of sulphide minerals [25, 31]. Gbor and Jia

27

[32] proposed that this behaviour is due to the wide particle size distribution (PSD) of the

particles. They showed mathematically that when the coefficient of variation (CV) of the PSD

is large (0.7 <CV< 1.2), it could affect the interpretation of the leaching data; for example, a

chemical reaction-control process can be mistakenly interpreted as a diffusion-control process.

The CV of the PSD of the pyrrhotite tailings was found to be 0.82, which is within the range

indicated above.

The fact that the tailings are composed of two individual nickel-bearing minerals, each

containing almost half of the total nickel, may complicate things. The present study was set to

develop an overall model accounting for the overall kinetics behaviour of nickel in both

pyrrhotite and pentlandite in the tailings. To obtain an accurate kinetic model for nickel

extraction, the oxidation kinetics of pyrrhotite and pentlandite need to be known separately. This

information needs to become available by tracking the conversion of each individual mineral in

the mixture. A method proposed by Ingraham et al. [33] based on the idea that pyrrhotite is an

acid soluble mineral, while pentlandite is essentially not acid soluble could be applied to the

pyrrhotite tailings. Given the very small amount of pentlandite (1.2 wt.%) in the tailings and the

experimentation under very low percent solids (~1.4 wt.%), it was not possible to achieve this

in the present study, and therefore it was decided to treat the tailings as a uniform mixture until

we develop a mineral separation (physical or chemical) protocol.

4.2 Effect of ferric sulphate concentration on nickel extraction

Figure 4-5 shows the effect of ferric sulphate concentration on the percent extraction of nickel

at 55 °C. The results show that the rate of nickel extraction increases with increasing ferric

sulphate concentration from 0.02 to 0.3 M. At 0.3 M ferric sulphate about 30 h of retention time

is needed to achieve 96% extraction. At lower concentrations of ferric sulphate, the kinetics are

28

slower and the extraction curves tend to reach a plateau before complete dissolution of the

tailings was achieved. The raw extraction data is reported in Appendix B, Tables B1.2, B2.2,

B3.2, and B4.2. The curve fitting in Figure 4-5 was made with the diffusion-control SCM. The

plot of the diffusion-control SCM vs. time produced linear fits with higher correlation

coefficients for all the ferric sulphate concentrations as to the chemical reaction-control SCM.

Apparent diffusion-control rate constants (kd) were obtained from the slopes of the linear fits

and are shown in Table 4-3. (More details of the SCM data and the corresponding plots are

provided in Appendix B, Tables B1.4, B2.4, B3.4, and B4.4 and Figures B-2 and B-3).

Using the mixed solvent-electrolyte (MSE) chemical model of the OLI software (OLI Analyzer

Studio 3.2), the concentrations of different iron species were calculated at 55 °C, 0.2 M H2SO4,

and different ferric sulphate concentrations (0.02-0.3 M). It can be seen in Table 4-3 that ferric

ion was found to be the dominant species, while Fe(OH)2+ concentration was low and did not

vary significantly over the range of ferric sulphate concentrations tested. Therefore, the order of

reaction was determined with respect to ferric ion concentration.

29

Figure 4-5. Effect of Fe2(SO4)3 concentration on nickel extraction kinetics. Conditions: 55 °C, 0.14

wt.% solids, and 0.2 M H2SO4, and. The error bars represent standard deviation from

triplicate tests.

Table 4-3. Diffusion-control apparent rate constants at various concentrations of ferric sulphate and

other iron species.

Fe2(SO4)3

(M)

Fe3+ Fe(OH)2+ kd (h-1) R2

0.3 0.502 0.0021 2.50×10-2 0.99

0.2 0.311 0.0025 1.00×10-2 0.97

0.1 0.153

0.0024

0.

0.71×10-2

0.98

0.02 0.039 0.0011 0.39×10-2 0.99

Figure 4-6 7 shows a plot of the log10 of kd against the log10 of ferric concentration, and the

linear fit to the data indicates that the order of reaction with respect to ferric ion is about 0.6.

However, the order of reaction with respect to ferric ion within the range of 0.1 to 0.3 M ferric

sulphate gives a value of 1.0 which is consistent with the expected value for a diffusion-

control process, since Fick’s law is first order with respect to diffusing species. This

discrepancy is due to the fact that at 0.02 M ferric sulphate the stoichiometric ratio of ferric

30

sulphate: pyrrhotite is 1.2:1 and does not meet the requirements for the use of the SCM, where

the concentration of the reactant is required to remain constant during the reaction. Therefore,

the lowest ferric sulphate concentration (0.02 M) was ruled out for determination of the

reaction order with respect to ferric ion.

Figure 4-6. Determination of reaction order with respect to ferric ion.

4.3 Elemental sulphur generation during leaching

Figure 4-7 shows the extraction of nickel and elemental sulphur formation at 55 °C, 0.2 M

Fe2(SO4)3 and 0.2 M H2SO4. Each pair of points represents a separate experiment since the low

percent solids employed did not permit sufficient slurry sampling for sulphur analysis. It can be

seen in Figure 4-7 that the pyrrhotite tailings dissolve relatively fast, and after about 50 h 85%

extraction of nickel is achieved, and about 75% of the sulphide in the pyrrhotite tailings has

been converted into elemental sulphur. Although the extraction curves follow a similar trend,

the elemental sulphur extraction is consistently lower. The difference should be the amount of

31

sulphide oxidized to sulphate ions, although the amount of sulphate could not be determined due

to the high sulphate background in the ferric sulphate solution. The data of nickel extraction and

elemental sulphur conversion is reported in Appendix C, Tables C1.2, C1.3, and C1.4.

Figure 4-7. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions: 55 °C,

0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

Figure 4-8 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in Figure 4-7.

Initially (at ~ 1 h), the molar ratios of Fe2+/S0 and Ni2+/S0 are high suggesting that the dissolution

of the pyrrhotite tailings at the beginning of leaching does not produce as much elemental

sulphur as at later stages of reaction. This is likely due to the direct acid attack of pyrrhotite with

the formation of dissolved hydrogen sulphide, shown in reaction 4-4 (two electrons are added

to account for the non-stoichiometric nature of pyrrhotite). However, no noticeable gaseous

hydrogen sulphide was detected, it is possible that subsequent oxidation of hydrogen sulphide

by ferric ions occurs at a slower rate according to reaction 4-5. In any case, initial surface

32

oxidation of sulphides in the tailings could not account for the initial high ratios. These values

were determined to be about 2% of total nickel in the tailings samples. The lines in Figure 4-8

are least-square linear fits to the data points (excluding 1 h), where the average Fe2+/S0 ratio is

2.26 and the Ni2+/S0 ratio is 0.022. The data is provided in Appendix C, Table C1-6.

Figure 4-8. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings.

Conditions: 55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4. The lines are least

square linear fits to the data excluding the first time point (1 h).

Fe7S8 + 14H+ → 7H2S + 7Fe2+ + S0 4-4

H2S + 2Fe3+ → 2Fe2+ + 2H+ + S0 4-5

Fe7S8 + 14Fe3+ → 21Fe2+ + 8S0 4-6

It can be seen in reaction 4-6 that if monoclinic pyrrhotite were completely dissolved, the molar

ratio of Fe2+/S0 would be 2.6. The fact that the pyrrhotite tailings are not pure and contain small

quantities of other sulphide minerals (pentlandite, pyrite and chalcopyrite) may explain the

difference between the expected and observed values.

33

Pyrrhotite is the dominant mineral of the tailings (86.2 wt.%), and therefore most of the

elemental sulphur produced is the by-product of pyrrhotite dissolution. However, nickel in the

tailings is attributed to both pentlandite and pyrrhotite, accounting for 40 and 60% of the total

nickel, respectively. Accordingly, if pyrrhotite and pentlandite react at different rates, the

Ni2+/S0 ratio is expected to change during the reaction, but it can be seen in Figure 4-8 that the

Ni2+/S0 ratio remains relatively constant over time. This shows that there is a similarity

between the dissolution rates of both pyrrhotite and pentlandite, indicating that no galvanic

interaction has occurred between the two minerals. This supports the use of the SCM for the

analysis of the dissolution kinetics. The theoretical value of Ni2+/S0 obtained based on the

assumption that pyrrhotite and pentlandite react at a similar rate is 0.02 which is comparable

with the molar ratios obtained for both 55 and 40 °C (Ni2+/S0 = 0.022).

To examine whether ferrous ion is re-oxidized to ferric ion due to oxygen entrainment, a control

experiment was conducted in which 0.02 M ferrous ion was added to 0.2 M ferric sulphate

solution at 55 °C and 250 rpm. After one week, the concentration of ferrous ion remained

reasonably constant over time (1% overall reduction). This suggests that there was no oxygen

entrainment in these tests, and all oxidative reactions are due to ferric attack.

Figure 4-9 shows the extraction of nickel and elemental sulphur formation from the leaching of

the pyrrhotite tailings at a lower temperature of 40 °C, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4, where

each pair of points represents a separate experiment. It can be seen in Figure 4-9 that the

pyrrhotite tailings dissolve relatively fast, and after about 100 h 80% of nickel is extracted, while

only about 70% of sulphide is extracted as elemental sulphur. Although the extraction curve for

elemental sulphur follows a similar trend to that of nickel, the elemental sulphur extraction is

34

consistently lower due to reasons explained above. The data of nickel extraction and elemental

sulphur conversion is reported in Appendix C, Tables C2.2, C2.3, and C2.4.

Figure 4-9. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings.

Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

Figure 4-10. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite

Tailings. Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 . The lines are least-

square linear fits to the data.

35

Figure 4-10 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in Figure 4-9.

The lines are least-square linear fits to the data points, where the average ratios of Ni2+/S0 and

Fe2+/S0 are 2.62 and 0.022, respectively. Here, the relatively constant Ni2+/S0 ratio shows that

the rates of nickel extraction from both pyrrhotite (60% of total Ni) and pentlandite (40% of

total Ni) must be very similar. The constant molar ratios of Ni2+/S0 and Fe2+/S0 are similar at

both temperatures of 40 and 55 °C indicating that the measured elemental sulphur yield was

essentially independent of the temperature of the leach solution. Figure 4-11 shows that the ORP

and pH values of the leach solution remain relatively constant at both temperatures. The data is

provided in Appendix C, Tables C2-5 and C2-6.

36

Figure 4-11. ORP and pH values of the leach solution measured at different times for 55 °C and

40 °C. Conditions: 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

4.4 Morphology of elemental sulphur

To observe the morphology of elemental sulphur on partially leached pyrrhotite particles at

different percent extractions of nickel, three additional experiments were performed by adding

about 2.8 g of the pyrrhotite tailings into 500 mL leach solution containing 0.2 M ferric sulphate

and 0.2 M sulphuric acid at 55 °C. After 19, 72, and 144 h of leaching, the contents of the flasks

were vacuum filtered, and the residues were dried, epoxy mounted, polished, and subjected to

SEM analysis. The ICP analysis of the leach solutions showed that the 19, 72, and 144 h of

leaching correspond to 50, 80 and 90% nickel extractions, respectively. These values are lower

than those obtained with 1.4 g/L of initial solids loading due to different stoichiometric excess

of ferric.

Figure 4-12 shows the SEM-BSE images of the leached pyrrhotite particles at 0% (a), 50% (b),

80% (c), and 90% (d) nickel extractions. Clearly, a distinct and relatively porous sulphur layer

forms on the pyrrhotite particles that allows the diffusion of the reactants to the surface of the

37

particles. This is consistent with the postulated SCM for diffusion through the product layer-

control processes. It seems that as the layer grows, it reaches a point that halts the reaction. This

phenomenon is less dominant as the temperature increases and accelerates the rate of allowing

sulphur to remain amorphous and porous for a longer time. Elemental mapping of a pyrrhotite

particle after 90% nickel extraction was obtained with the help of EDX and are shown in Figure

4-13.

Figure 4-12. SEM-BSE images of partially leached pyrrhotite particles after (a) time 0 (0%), (b) 19h

(50%), (c) 72 h (80%) and (d) 144 h (90%) of leaching. Conditions: 55 °C, 0.56 wt.% solids, 0.2 M

Fe2(SO4)3, and 0.2 M H2SO4.

38

Figure 4-13. Elemental maps of a pyrrhotite particle after 90% nickel extraction.

20 20 µm

39

5 Chapter 5: Conclusions

The present work investigated the dissolution kinetics of pyrrhotite tailings from Sudbury,

Ontario, Canada, in concentrated ferric sulphate media. The effect of temperature on the nickel

extraction kinetics was investigated, and the dissolution mechanism of the tailings was

determined using the SCM. The diffusion-control kinetic model best fit the nickel extraction

data within the temperature range studied (30- 55 °C) with an activation energy value of 62.12

kJ/mol. The high activation energy was attributed to the wide PSD and the non-homogeneity of

the tailings (two nickel-bearing minerals). The reaction order with respect to ferric ion was found

to be 0.65 for the whole range of concentrations studied. However, the order of reaction with

respect to ferric ion was found to be 1.0 for ferric sulphate concentrations ranging from 0.1 to

0.3 M.

The results of this study suggested that the overall dissolution process of the pyrrhotite tailings

was governed by diffusion through the sulphur product layer. The molar ratios of Fe2+/S0 and

Ni2+/S0 at 40 °C were about 2.62 and 0.022, respectively, regardless of time. Similar values of

the molar ratios were obtained at 55 °C, where Fe2+/S0 ratio was 2.26 and Ni2+/S0 ratio was

0.022. The Fe2+/S0 ratios compare well with the theoretical value of 2.6 based on reaction

stoichiometry, whereas the constant Ni2+/S0 ratio is indicative of a similarity between the

dissolution rates of both pyrrhotite and pentlandite.

SEM-BSE images showed a distinct and relatively porous layer of elemental sulphur around the

leached pyrrhotite particles. Continuation of the leaching process resulted in a progressive

thickening of the sulphur layer to the point of reaction stoppage at certain extraction levels

40

depending on temperature. This is consistent with the observed non-linear leaching kinetics

(diffusion control).

41

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45

Appendices

Appendix A. Nickel extraction kinetics at different temperatures

Figure 4-1 in chapter 4 shows the effect of temperature on the kinetics of nickel extraction from

the pyrrhotite tailings. The experimental conditions performed at 55, 40, and 30 °C are shown

in Tables A1.1, A2.1, and A3.1, respectively. The raw data corresponding to the extraction

curves in Figure 4.1 are shown in tables A1.2, A2.2, and A3.2.

Table A1.1 Experimental conditions for nickel extraction at 55 °C.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 28.7

Fe2(SO4)3.5H2O (M) 0.12

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2, Exp-3

Wet weight (g) Exp-1. 0.72

Exp-2. 0.72

Exp-3. 0.72

Sample moisture content (wt%) 8.26

Dry weight (g) Exp-1. 0.67

Exp-2. 0.67

Exp-3. 0.67

Ni weight in the dry sample (mg)

(1 wt.%)

Exp-1. 6.7

Exp-2. 6.7

Exp-3. 6.7

46

Table A1.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Table A2.1 Experimental conditions for nickel extraction at 40 °C.

Time (h) Ni (mg) Ni extraction (%)

0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average

1 1.37 1.37 1.36 20.44 20.58 20.38 20.45±0.10

3 2.01 2.01 2.14 29.94 30.12 31.98 30.68±1.13

6 2.79 2.68 2.72 41.59 40.14 40.70 40.81±0.73

9 3.08 3.41 2.99 46.00 51.07 44.66 47.24±3.38

24 4.69 5.06 4.49 69.95 75.72 67.11 70.92±4.39

31 5.10 5.01 4.83 76.30 75.00 72.16 74.39±2.06

49 6.44 - 5.50 95.96 - 82.14 89.05±9.77

59 5.88 5.99 - 87.66 89.69 - 88.67±1.43

76 - 6.07 6.02 - 91.30 90.28 90.79±0.72

97 5.99 6.05 - 88.66 90.77 - 89.71±1.49

Temperature 40 ° C

Fe2(SO4)3.5H2O (g) 28.7

Fe2(SO4)3.5H2O (M) 0.12

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2

Wet weight (g) Exp-1. 0.72

Exp-2. 0.73

Sample moisture content (wt%) 8.26

Dry weight (g) Exp-1. 0.67

Exp-2. 0.68

Ni weight in the dry sample (mg)

(1 wt.%)

Exp-1. 6.7

Exp-2. 6.8

47

Table A2.2 Nickel extraction data at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Table A3.1 Experimental conditions for nickel extraction at 30 °C.

Time (h) Ni (mg) Ni extraction (%)

0 Exp-1 Exp-2 Exp-1 Exp-2 Average

1 1.36 1.32 20.30 19.75 20.02±0.39

3 1.59 1.78 23.76 26.70 25.23±2.08

7 1.95 - 29.05 - 29.05

24 3.27 3.01 48.85 45.15 47.00±2.61

31 3.35 3.20 50.07 48.01 49.04±1.46

48 3.96 3.92 59.17 58.82 58.99±0.25

61 4.40 - 65.62 - 65.62

73 4.46 4.50 66.57 67.45 67.01±0.62

96 5.01 4.68 74.79 72.10 72.44±3.32

120 5.44 5.39 81.13 80.75 80.94±0.27

Temperature 30 °C

Fe2(SO4)3.5H2O (g) 28.7

Fe2(SO4)3.5H2O (M) 0.12

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2, Exp-3

Wet weight (g) Exp-1. 0.72

Exp-2. 0.72

Exp-3. 0.72

Moisture content (wt%) 8.26

Dry weight (g) Exp-1. 0.67

Exp-2. 0.67

Exp-3. 0.67

Ni weight in the dry sample (mg)

(1 wt.%)

Exp-1. 6.7

Exp-2. 6.7

Exp-3. 6.7

48

Table A3.2 Nickel extraction data at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Ferrous ion concentration and pH values of the leach solutions, corresponding to Figure 4-1 in

chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure A-1 shows

the ferrous ion concentration curves at 55, 40, and 30 °C, and the raw data are shown in tables

A1.3, A2.3, and A3.3, respectively.

Figure A-1. Ferrous ion concentration at different temperatures and 0.12 M ferric sulphate-0.2 M

H2SO4 media.

Time(h) Ni (mg) Ni extraction (%)

0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average

3 1.37 1.32 1.37 20.44 19.74 20.33 20.17±0.37

7 1.55 - - 23.12 - - 23.12

10 1.63 1.66 1.70 24.30 24.86 25.23 24.80±0.47

24 2.15 2.02 2.15 31.96 30.32 31.91 31.40±0.93

31 2.69 2.24 2.89 39.98 33.55 42.94 38.83±4.80

49 2.75 2.53 3.31 40.95 37.92 49.05 42.64±5.75

59 3.03 3.10 - 45.08 46.47 - 45.78±0.98

76 3.01 2.72 2.74 44.78 40.68 40.67 42.04±2.37

97 3.21 3.10 3.13 47.72 46.44 46.49 46.88±0.72

120 3.50 3.29 3.41 52.08 49.28 50.64 50.67±1.40

49

Table A1.3 Ferrous ion concentrations and pH values at 55 °C, 0.2 M H2SO4, 0.12M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH

Exp.1

Exp.2

Exp.3

1

0.0014

0.0017

0.0014

0.87

0.81

0.84

0.0015±0.0001 0.84±0.030

Exp.1

Exp.2

Exp.3

3 0.0057

0.006

0.0055

0.79

0.83

0.83

0.0057±0.0002

0.82±0.028

Exp.1

Exp.2

Exp.3

6 0.0091

0.0096

0.0091

0.78

0.80

0.80

0.0093±0.0003

0.79±0.013

Exp.1

Exp.2

Exp.3

9 0.0110

0.0115

0.0115

0.76

0.79

0.82

0.0114±0.0003

0.79±0.028

Exp.1

Exp.2

Exp.3

24 0.0168

0.0173

0.0173

0.81

0.87

0.79

0.0171±0.0003

0.82±0.045

Exp.1

Exp.2

Exp.3

31 0.018

0.0195

0.018

0.78

0.83

0.87

0.0185±0.0008

0.82±0.045

Exp.1

Exp.2

Exp.3

49 0.021

0.021

0.021

0.84

0.85

0.84

0.021

0.84±0.002

Exp.1

Exp.2

Exp.3

59 0.021

0.021

0.021

0.79

0.79

0.83

0.021

0.80±0.022

Exp.1

Exp.2

Exp.3

76 0.021

0.021

0.021

0.80

0.85

0.83

0.021

0.83±0.022

Table A2.3 Ferrous ion concentrations and pH values at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH

Exp.1

Exp.2

3 0.0031

0.0035

0.88

0.86

0.0033±0.0003

0.87±0.009

Exp.1

Exp.2

7 0.0058

-

0.891

-

0.0058

0.89

Exp.1

Exp.2

24 0.0108

0.0117

0.81

0.82

0.0112±0.0006

0.81±0.009

Exp.1

Exp.2

31 0.012

-

0.79

-

0.012

0.79

Exp.1 48 0.015 0.85

50

Exp.2 0.0154 0.79 0.0152±0.0003 0.82±0.042

Exp.1

Exp.2

61 0.0171

-

0.81

-

0.0171

0.81

Exp.1

Exp.2

73 0.0182

0.0187

0.78

0.74

0.0185±0.0004

0.76±0.029

Exp.1

Exp.2

96 0.02

0.0199

0.82

0.80

0.0199±0.0001

0.81±0.018

Table A3.3 Ferrous ion concentrations and pH values at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH

Exp.1

Exp.2

Exp.3

3

0.0019

0.0019

0.0019

0.90

0.87

0.90

0.00192

0.89±0.004

Exp.1

Exp.2

Exp.3

7 0.0041

0.0034

0.0041

0.92

0.82

0.83

0.0038±0.000

42

0.86±0.011

Exp.1

Exp.2

Exp.3

10 0.0055

0.0048

0.0058

0.77

0.78

0.76

0.0054±0.000

5

0.78±0.047

Exp.1

Exp.2

Exp.3

24 0.0096

0.0084

0.0091

0.86

0.87

0.84

0.009±0.0006

0.86±0.015

Exp.1

Exp.2

Exp.3

31 0.0096

0.009

0.0096

0.89

0.86

0.76

0.0094±0.000

35

0.84±0.069

Exp.1

Exp.2

Exp.3

49 0.012

0.0114

0.0126

0.80

0.73

0.70

0.012±0.0006

0.74±0.054

Exp.1

Exp.2

Exp.3

59 0.0132

0.0126

0.0126

0.80

0.73

0.84

0.0128±0.000

35

0.79±0.536

Exp.1

Exp.2

Exp.3

76 0.0144

0.0132

0.0144

0.81

0.79

0.89

0.014±0.0007

0.83±0.051

Exp.1

Exp.2

Exp.3

97 0.0156

0.0144

0.0156

0.85

0.79

0.78

0.0152±0.000

7

0.81±0.037

51

The shrinking core model (SCM) was applied to the nickel extraction data at different

temperatures (30, 40, and 55 °C), and the results were plotted in Figures 4-2 and 4-3 in Chapter

4 for diffusion-control and chemical reaction-control kinetic models. The raw data of the figures

is summarized in Tables A1-4, A2-4, and A3-4.

Table A1.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.205 0.073 0.015

3 0.307 0.115 0.037

6 0.408 0.160 0.069

9 0.453 0.182 0.087

24 0.685 0.320 0.241

31 0.744 0.364 0.302

49 0.890 0.521 0.532

Table A2.4 SCM fitted to nickel extraction data at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Time (h) Conversion (x) Chemical-control

model

1-(1-x)1/3

Diffusion-control

model

1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.200 0.072 0.015

3 0.252 0.092 0.024

7 0.290 0.108 0.032

24 0.470 0.191 0.095

31 0.490 0.201 0.105

48 0.590 0.257 0.164

61 0.656 0.299 0.215

52

Table A3.4 SCM fitted to nickel extraction data at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Time (h) Conversion (x) Chemical-control

model

1-(1-x)1/3

Diffusion-control

model

1-3(1-x)2/3+2(1-x)

0 0 0 0

3 0.201 0.072 0.015

7 0.231 0.088 0.022

10 0.247 0.091 0.023

24 0.314 0.118 0.038

31 0.389 0.151 0.061

49 0.426 0.169 0.076

59 0.458 0.185 0.090

53

Appendix B. Ferric sulphate concentration effect on nickel extraction

Figure 4-1 in chapter 4 shows the effect of ferric sulphate concentration on the kinetics of nickel

extraction from the pyrrhotite tailings. The experimental conditions performed at 0.3, 0.2, 0.1,

and 0.02 M ferric sulphate are shown in Tables B1.1, B2.1, B3.1, and B4.1, respectively. The

raw data corresponding to the extraction curves in Figure 4.5 is shown in tables B1.2, B2.2,

B3.2, and B4.2.

Table B1.1 Experimental conditions.

.

Table B1.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 72.8

Fe2(SO4)3.5H2O (M) 0.3

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2, Exp-3

Wet weight (g) Exp-1. 0.681

Exp-2. 0.680

Exp-3. 0.682

Sample moisture content (wt.%) 8.26

Dry weight (g) Exp-1. 0.625

Exp-2. 0.621

Exp-3. 0.626

Nickel content in the dry samples

(mg) (1 wt.%)

Exp-1. 6.25

Exp-2. 6.21

Exp-3. 6.26

Time (h) Ni (mg) Ni Extraction (%)

0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average

1 2.56 2.61 2.63 40.90 42.09 42.08 41.69±0.69

3 3.13 3.29 3.32 50.06 52.97 53.09 52.04±1.72

6 3.66 3.61 3.67 58.50 58.06 58.73 58.43±0.34

9 4.09 4.05 3.95 65.36 65.17 63.11 64.54±1.25

24 5.57 5.76 5.82 89.12 92.69 93.14 91.65±2.20

30 5.91 6.08 6.06 94.62 97.88 97.05 96.52±1.69

54

Table B2.1 Experimental conditions.

Table B2.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 49

Fe2(SO4)3.5H2O (M) 0.2

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2

Wet weight (g) Exp-1. 0.6821

Exp-2. 0.6823

Sample moisture content (wt.%) 7.00

Dry weight (g) Exp-1. 0.634

Exp-2. 0.634

Nickel content in the dry samples

(mg) (1 wt.%)

Exp-1. 6.34

Exp-2. 6.34

Time (h) Ni (mg) Ni extraction (%)

0 Exp-1 Exp-2 Exp-1 Exp-2 Average

1 1.88 1.87 29.63 29.47 29.55±0.11

3 2.39 2.40 37.61 37.78 37.69±0.12

6 2.90 3.00 45.74 47.33 46.54±1.12

8.5 3.14 3.25 49.53 51.20 50.37±1.18

23 4.49 4.53 70.70 71.37 71.04±0.48

30 4.83 4.87 76.13 76.72 76.43±0.42

48 5.30 5.40 83.55 85.18 84.36±1.15

72 5.69 5.73 87.82 88.43 88.13±0.43

96 5.70 5.66 89.87 89.23 89.55±0.45

55

Table B3.1 Experimental conditions.

Table B3.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 24.5

Fe2(SO4)3.5H2O (M) 0.1

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2

Wet weight (g) Exp-1. 0.681

Exp-2. 0.682

Sample moisture content (wt.%) 7.00

Dry weight (g) Exp-1. 0.634

Exp-2. 0.634

Nickel content in the dry samples

(mg) (1 wt.%)

Exp-1. 6.34

Exp-2. 6.34

Time (h) Ni (mg) Ni extraction (%)

Exp-1 Exp-2 Exp-1 Exp-2 Average

1 1.18 1.18 18.55 18.55 18.55±0.0016

3 1.69 1.70 26.63 26.83 26.73±0.13

6 2.27 2.26 35.86 35.60 35.73±0.18

8.5 2.57 2.68 40.56 42.19 41.38±1.15

23 3.83 3.89 60.51 61.29 60.90±0.55

30 4.41 4.22 69.63 66.54 68.09±2.19

48 4.78 4.77 75.45 75.22 75.34±0.17

56.5 4.79 4.86 75.65 76.66 76.15±0.72

72 4.93 5.04 77.81 79.51 78.66±1.21

96 5.70 5.16 80.40 81.31 80.85±0.64

56

Table B4.1 Experimental conditions.

Table B4.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate.

Ferrous ion concentration and pH values of the leach solutions, corresponding to Figure 4-5 in

chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure B-1 shows

the ferrous ion concentration curves at 0.3, 0.2, 0.1, and 0.02 M and the raw data are shown in

tables B1.3, B2.3, B3.3, and B4.3, respectively.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 4.9

Fe2(SO4)3.5H2O (M) 0.02

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2

Wet weight (g) Exp-1. 0.6826

Exp-2. 0.6825

Sample moisture content (wt%) 7

Dry weight (g) Exp-1. 0.6348

Exp-2. 0.6347

Ni weight in the dry sample (mg) (1

wt.%)

Exp-1. 6.3482

Exp-2. 6.3472

Time (h) Ni (mg) Ni Extraction (%)

0 Exp-1 Exp-2 Exp-1 Exp-2 Average

1 0.69 0.63 10.90 9.93 10.42±0.68

3 1.13 1.07 17.82 16.92 17.36±0.64

6 1.66 1.57 26.25 24.74 25.49±1.07

8.5 1.93 1.81 30.33 28.59 29.47±1.23

23 3.12 3.04 49.10 47.89 48.49±0.86

30 3.44 3.36 54.21 52.96 53.59±0.89

48 4.10 4.06 64.65 64.00 64.32±0.46

72 4.46 4.42 70.27 69.61 69.94±0.47

96 4.63 4.54 72.98 71.50 72.24±1.05

57

Figure B-1. Ferrous ion concentration at different ferric sulphate concentrations, 55 °C and 0.2 M

H2SO4.

Table B1.3 Ferrous ion concentrations and pH values at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH

Exp.1

Exp.2

Exp.3

1

0.0018

0.0018

0.0018

0.649

0.739

0.723

0.0018

0.7037±0.048

Exp.1

Exp.2

Exp.3

3 0.0054

0.0054

0.006

0.625

0.706

0.692

0.0056±0.000

3

0.6743±0.043

2

Exp.1

Exp.2

Exp.3

6 0.0072

0.009

0.009

0.769

0.697

0.785

0.0084±0.001

0

0.7503±0.046

9

Exp.1

Exp.2

Exp.3

9 0.0096

0.0102

0.0108

0.583

0.667

0.664

0.0102±0.000

6

0.638±0.0476

Exp.1

Exp.2

Exp.3

24 0.015

0.0156

-

0.611

0.706

0.599

0.0153±0.000

4

0.6386±0.058

6

Exp.1

Exp.2

Exp.3

30 0.0162

0.0168

0.0168

0.689

0.716

0.699

0.0166±0.000

5

0.7013±0.013

6

58

Table B2.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.2 M ferric sulphate.

Samples Time (h) Fe2+ (M) Average Fe2+ (M)

Exp.1

Exp.2

1

0.0012

0.0012

0.0012

Exp.1

Exp.2

3 0.0039

0.0042

0.00405±0.0002

Exp.1

Exp.2

8.5 0.0084

0.009

0.0087±0.0004

Exp.1

Exp.2

23 0.0132

0.0138

0.0135±0.0004

Exp.1

Exp.2

30 0.0144

0.0144

0.0144

Exp.1

Exp.2

48 0.0156

0.0162

0.0159±0.0004

Exp.1

Exp.2

56.5 0.0162

0.0162

0.0162

Exp.1

Exp.2

72 0.0168

0.0168

0.0168

Exp.1

Exp.2

96 0.0174

0.0174

0.0174

Table B3.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Samples Time (h) Fe2+ (M) Average Fe2+ (M)

Exp.1

Exp.2

1 0.0009

0.0012

0.00105±0.0002

Exp.1

Exp.2

3 0.0036

0.0036

0.0036

Exp.1

Exp.2

8.5 0.0078

0.0078

0.0078

Exp.1

Exp.2

23 0.0132

0.0126

0.0129±0.0004

Exp.1

Exp.2

30 0.0132

0.0138

0.0135±0.0004

Exp.1

Exp.2

48 0.0156

0.0150

0.0153±0.0004

Exp.1

Exp.2

56.5 0.0156

0.0156

0.0156

Exp.1

Exp.2

76 0.0162

0.0162

0.0162

Exp.1

Exp.2

96 0.0168

0.0168

0.0168

59

Table B4.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate.

Samples Time (h) Fe 2+ (M) Average Fe2+

Exp.1

Exp.2

3 0.0027

0.0024

0.00255±0.0002

Exp.1

Exp.2

8.5 0.0054

0.0054

0.0054±0

Exp.1

Exp.2

23 0.009

0.0096

0.0093±

Exp.1

Exp.2

30 0.0102

0.0102

0.0102±0

Exp.1

Exp.2

48 0.012

0.012

0.012±0

Exp.1

Exp.2

56.5 0.0123

0.012

0.01215±0.0002

Exp.1

Exp.2

79 0.0132

0.0138

0.0135±0.0004

Exp.1

Exp.2

96 0.0138

0.0138

0.0138±0

The shrinking core model (SCM) was applied to the nickel extraction data at different ferric

sulphate concentrations (0.3, 0.2, 0.1, and 0.02 M), and the results are plotted in Figures B-2

and B-3 for diffusion-control and chemical reaction-control kinetic models. The raw data of

these figures is summarized in Tables B1-4, B2-4, B3-4, and B4-4. The straight-line plots in

Figure 4-2 produce higher correlation coefficients for all the temperatures in comparison to the

chemical reaction-control SCM (Figure 4-3). Moreover, in Figure 4-2, the fitted lines at

different temperatures extrapolate through the origin or are very close to the origin, whereas in

Figure 4-3 they do not. Therefore, the process is diffusion control.

Table B1.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate.

Time (h) 0.3M Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.42 0.07 0.16

3 0.52 0.12 0.22

6 0.58 0.16 0.25

9 0.65 0.21 0.29

24 0.92 0.59 0.56

30 0.97 0.75 0.67

60

Table B2.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.2 M ferric sulphate.

Time (h) 0.2M Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.30 0.03 0.11

3 0.38 0.06 0.15

6 0.47 0.09 0.19

8.5 0.50 0.11 0.21

24 0.71 0.27 0.34

30 0.76 0.33 0.38

48 0.84 0.44 0.46

Table B3.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.19 0.01 0.07

3 0.27 0.03 0.10

6 0.36 0.05 0.14

8.5 0.41 0.07 0.16

23 0.61 0.18 0.27

30 0.68 0.24 0.32

48 0.75 0.31 0.37

Table B4.4 SCM data fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x)

0 0 0 0

1 0.1 0.003 0.04

3 0.17 0.01 0.06

6 0.25 0.02 0.09

8.5 0.29 0.03 0.11

23 0.48 0.10 0.20

30 0.53 0.13 0.23

48 0.64 0.20 0.29

72 0.69 0.25 0.33

61

Figure B-2. Plot of chemical reaction-control SCM at various ferric sulphate concentrations.

Figure B-3. Plot of diffusion-control SCM at various ferric sulphate concentrations.

62

Table B5.1 Surface reaction-control apparent rate constant values at various ferric sulphate

concentrations obtained from the slopes of linear fits in Figure B-2.

Fe2(SO4)3 (M) kr (h-1) R2

0.3 2.0×10-2 0.90

0.2 0.84×10-2 0.90

0.1 0.72×10-2

0.91

0.02 0.45×10-2 0.91

Table B5.2 Diffusion-control apparent rate constants at various ferric sulphate

concentrations obtained from the slopes of linear fits in Figure B-3.

Fe2(SO4)3 (M) kd (h-1) R2

0.3 2.50×10-2 0.99

0.2 1.00×10-2 0.97

0.1 0.71×10-2

0.98

0.02 0.39×10-2 0.98

63

Appendix C. Elemental sulphur determination

Figure 4-7 in Chapter 4 shows the extraction of nickel and elemental sulphur formation at 55

°C, 0.2 M Fe2(SO4)3 and 0.2 M H2SO4. Table C1.1 provides the experimental conditions of the

tests performed. The raw data corresponding to the nickel extraction curve in Figure 4.7 is

provided in Table C1.2.

Table C1.1 Experimental conditions

Table C1.2 Nickel extraction data at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 49

Fe2(SO4)3.5H2O (M) 0.2

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1- Exp.5

Moisture content (wt.%) 9

Ni (wt.%) 1.0

Sulphur (wt.%) 32.1

Dry weight 6.72

6.75

6.72

6.67

6.72

Time (h) Measured Ni (mg)

Ni Extraction (%) Ni2+ (M) MW:58.7

1 1.97 29.26 0.07×10-3

4 2.92 43.26 0.1×10-3

9 3.76 55.89 0.13×10-3

24 4.95 74.21 0.17×10-3

49 5.78 86.10 0.20×10-3

64

As explained in section 3.5 of chapter 3, the concentration of elemental sulphur present in the

leached residue was measured using HPLC. Standard solutions for elemental sulphur analysis

were prepared within the range of 6-240 mg/L by a procedure similar to that employed by

McGuire and Hamers [30]. The standard HPLC calibration curve is shown in Figure C-1, where

the equation of the linear fit is y= 2.35 x – 1.36. In order to calculate the sulphur concentration

of the leached sample, the peak area (from HPLC) is inserted in the linear equation of the

following calibration curve.

Figure C-1. Standard HPLC calibration curve for sulphur analysis at 55 °C.

Each measurement lasted for 25 min, and the peak corresponding to the elemental sulphur

appeared somewhere between 15 and 20 min, due to small changes in the eluent composition.

Figure C-2 shows the HPLC chromatographs of elemental sulphur having a retention time of

15.5 min. The data obtained from the HPLC analysis of the samples leached at 55 °C, 0.2 M

ferric sulphate, and 0.2M H2SO is provided in Table C1.3.

65

Figure C-2. HPLC Chromatograms of elemental sulphur. The elemental sulphur peak appears at 15.5

min.

Table C1.3 HPLC data for the analysis of elemental sulphur kinetics at 55 °C, 0.2 M ferric sulphate,

and 0.2M H2SO4.

Peak area

HPLC

Dilution

factor

Peak Area Peak area

x dilution

factor

Peak are

with filter

paper

subtracted

Sulphur

(mg/L)

Volume

(mL) Sulphur

(mg)

Filter

paper 1

5.54

5.54

0

0

25

0

1 h 50 3.77 188.56 183.02 427.85 25 10.70

4 h 50 22.47 1123.34 1117.80 2620.11 25 65.50

9 h 50 29.00 1449.97 1444.43 3386.11 25 84.65

24 h 50 45.00 2250.12 2244.58 5262.62 25 131.57

49 h 50 52.48 2624.01 2618.47 6139.47 25 153.49

66

Table C1.4 Elemental sulphur conversion data at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Ferrous ion concentration, pH, and ORP values of the leach solutions, corresponding to Figure

4-7 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure C-3

shows the ferrous ion concentration curve and the raw data is shown in Table C1.5.

Figure C-3. Ferrous ion concentration at 55 °C, 0.2 M ferric sulphate and 0.2 M H2SO4 media.

Time (h) Measured S0 (mg)

S conversion (%) S0 (M) MW:32

1 0 0 0.66×10-3

4 10.70 5.13474 4.1×10-3

9 65.50 31.2611 5.3×10-3

24 84.65 40.61268 8.22×10-3

49 131.57 63.5763 9.6×10-3

67

Table C1.5 Ferrous ion concentrations, pH and ORP values at 55 °C, 0.2 M ferric sulphate, and 0.2M

H2SO4.

Sample Time (h) Fe2+ (M) Eh

SCE (mV)

pH ORP

SHE (V)

Exp-1 1 0.0024 550 0.64 791

Exp-2 4 0.0096 523 0.67 764

Exp-3 9 0.0141 510 0.67 751

Exp-4 24 0.0165 505 0.67 746

Exp-5 49 0.0195 501 0.67 742

Figure 4-8 in chapter 4 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in

Figure 4-7. The calculation of these molar ratios is provided in Table C1.6, where the molarities

of, Ni2+, S0, and Fe2+ are from Tables C1.2, C1.4, and C1.5, respectively.

Table C1.6 Analysis of molar ratios of Fe2+/S0 and Ni2+/S0 at 55 °C.

Time (h) Ni2+/S0 Fe2+/S0

1 0.100 3.59

4 0.024 2.34

9 0.024 2.84

24 0.021 2.19

49 0.021 2.03

68

Figure 4-9 in Chapter 4 shows the extraction of nickel and elemental sulphur formation at 40

°C, 0.2 M Fe2(SO4)3 and 0.2 M H2SO4. Table C2.1 provides the experimental conditions of the

tests performed. The raw data corresponding to the nickel extraction curve in Figure 4.9 is

provided in Table C2.2.

Table C2.1 Experimental conditions.

Table C2.2 Nickel extraction data at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Temperature 40 °C

Fe2(SO4)3.5H2O (g) 49

Fe2(SO4)3.5H2O (M) 0.2

Shaking speed (rpm) 250

H2SO4 (M) 0.2

Reaction volume (mL) 500

Sample IDs Exp-1- Exp.5

Moisture content (wt.%) 9

Ni (wt.%) 1.0

Sulphur (wt.%) 32.1

Dry weight (g) Exp-1 0.46

Exp-2 0.65

Exp-3 0.65

Exp-4 0.65

Exp-5 0.65

Time (h) Measured Ni (mg)

Ni Extraction (%) Ni2+ (M)

4 1.967179 29.3 0.06×10-3

24 2.924076 43.3 0.13×10-3

48 3.758084 55.9 0.16×10-3

72 4.954033 74.2 0.17×10-3

97 5.778863 86.0 0.19×10-3

69

As explained in section 3.5 of chapter 3, the concentration of elemental sulphur present in the

leached residue was measured using HPLC. Standard solutions for elemental sulphur analysis

were prepared within the range of 6-240 mg/L by a procedure similar to that employed by

McGuire and Hamers [30]. The standard HPLC calibration curve for elemental sulphur analysis

at 40 °C is shown in Figure C-4, where the equation of the linear fit is y= 1.84 x – 6.002. In

order to calculate the sulphur concentration of the leached sample, the peak area (from HPLC

analysis) is inserted in the linear equation of the following calibration curve. The data obtained

from the HPLC analysis of the samples leached at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4

is provided in Table C2.3.

Figure C-4. Standard HPLC Calibration curve for sulphur analysis at 40 °C.

70

Table C2.3 HPLC data for the analysis of elemental sulphur kinetics at 40 °C, 0.2 M ferric sulphate,

and 0.2M H2SO4.

Peak area

HPLC

Dilution

factor

Peak Area Peak area

x dilution

factor

Peak are

with filter

paper

subtracted

Sulphur

(mg/L)

Volume

(mL)

Elemental

sulphur

(mg)

Filter

paper 1

4.54 4.54 0 0

20

0

4 h 50 16.42 821.22 816.68 1501.17 20 30.02

24 h 50 46.24 2311.93 2307.39 4252.28 25 106.31

48 h 50 25.73 1286.58 1282.02 2360.01 50 118.01

72 h 50 28.60 1430.07 1425.53 2624.80 50 131.24

97 h 50 33.83 1691.43 1686.89 3107.14 50 155.35

Table C2.4 Elemental sulphur conversion data at 40°C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Time (h) Measured S0 (mg)

S conversion (%) S0 (M) MW:32

4 30.02 20.16 1.87×10-3

24 106.31 50.85 6.64×10-3

48 118.01 57.73 7.38×10-3

72 131.24 62.95 5.20×10-3

97 155.35 74.33 9.71×10-3

Ferrous ion concentration, pH, and ORP values of the leach solutions, corresponding to Figure

4-9 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure C-5

shows the ferrous ion concentration curve and the raw data is shown in Table C2.5

Table C2.5 Ferrous ion concentrations and pH values at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4

.Time (h) Fe2+ (M) ORP

SCE (mV)

pH ORP

SHE (V)

4 0.0054 527 0.72 768

24 0.0156 498 0.72 739

48 0.0204 492 0.76 733

72 0.0216 488 0.73 729

97 0.024 480 0.73 721

71

Figure C-5. Ferrous ion concentration at 40 °C, 0.2 M ferric sulphate and 0.2 M H2SO4 media.

Figure 4-8 in chapter 4 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in

Figure 4-7. The calculation of these molar ratios is provided in Table C2.6, where the molarities

of, Ni2+, S0, and Fe2+ are from Tables C2.2, C2.4, and C2.5, respectively.

Table C2.6 Analysis of molar ratios of Fe2+/S0 and Ni2+/S0 at 40 °C.

Time (h) Ni2+/S0 Fe2+/S0

4 0.032 2.88

24 0.019 2.35

48 0.021 2.77

72 0.021 2.63

97 0.019 2.47

72

Appendix D. SEM images and EDX Spectra

D.1 Additional SEM image, elemental maps, and EDX spectra of the leached pyrrhotite particle after

80% nickel extraction.

SEM image Fe

S Ni

1 +

2

+

73

Element Weight %

S 27.20

Fe 46.75

Ni 0.7

C and O 25.35

Total 100

Element Weight %

S 29.15

Fe 2.16

Ni 0.09

C and O 68.60

Total 100

1

.

2

.

74

D.2 Additional SEM image, elemental maps, and EDX spectra of the leached pyrrhotite particle after

50% nickel extraction

SEM image S

Fe Ni

1 +

2 +

75

D3. Additional SEM-BSE images

Element Weight %

S 49.15

Fe 6.39

Ni 0.22

C and O 44.24

Total 100

Element Weight %

S 23.67

Fe 38.54

Ni 0.9

C and O 36.89

Total 100

1 2

.

80% reacted

50% reacted

80% reacted

90% reacted