Minerals Engineering 32 (2012) 68–73

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Minerals Engineering

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Talc–serpentine interactions and implications for talc depression

Bo Feng a, Yiping Lu a,⇑, Qiming Feng a, Mingyang Zhang a,b, Yanling Gu a

a School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, Chinab Changchun Gold Design Institute, Changchun 130021, China

a r t i c l e i n f o

Article history:Received 4 December 2011Accepted 5 March 2012Available online 1 May 2012

Keywords:SerpentineTalcSlime coatingsSHMPCMC

0892-6875/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.mineng.2012.03.004

⇑ Corresponding author. Tel./fax: +86 731 8883681E-mail addresses: luyp309@sohu.com (Y. Lu), feng

a b s t r a c t

The interactions of serpentine with talc and implications for talc depression by carboxymethyl cellulose(CMC) have been investigated through flotation tests, sedimentation tests and zeta potential measure-ments. Talc is a hydrophobic mineral and CMC is effective for talc depression. At pH value 9, where flo-tation of nickel sulfide ores is routinely performed, the two magnesium silicate bearing (MgO) gangueminerals are oppositely charged and therefore attract through an electrostatic mechanism. Slime coatingsof serpentine adhere to the surface of talc, reducing talc flotation recovery to some extent, but also caus-ing problem to its depression by CMC as serpentine is a hydrophylic gangue mineral which cannot bedepressed by CMC depressant. Pretreatment of serpentine with acid leaching results in a shift of the zerocharge points, from pH value 10.2 to 6.8 and the leached serpentine is negatively charged at pH 9. Thenegatively charged serpentine does not interfere with the talc depression by CMC. Adsorption of sodiumhexametaphosphate (SHMP) at the serpentine/solution interface also compensates the positive charge onthe serpentine particle and its zeta potential is rendered negative. When the serpentine surface is nega-tively charged, a repulsive interaction energy generates and serpentine slimes drop off from the surface oftalc, allowing talc to once again be depressed by CMC.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction slime particles to reverse their potential and prevent electrostatic

In nickel sulfide processing, magnesium silicate bearing gangueminerals often report to the concentrate causing downstream pro-cessing problems as well as increased smelting costs. Talc andserpentine are gangue minerals commonly encountered in complexsulfide ores (Bremmell et al., 2005; Chen et al., 1999a,b; Engel et al.,1997; Pietrobon et al., 1997; Senior and Thomas, 1997; Wang et al.,2005; Wiese et al., 2007; Witney and Yan, 1997). Being naturallyhydrophobic, talc easily reports to flotation concentrates, thusreducing concentrate grade (Beattie et al., 2006a,b; Steenberg andHarris, 1984; Shortridge et al., 2000). Different from talc, serpentinedoes not have natural flotability (Fornasiero and Ralston, 2005), flo-tation of the serpentine particles may be via composite particles orthrough attachment to the valuable minerals as ‘‘slime coatings’’(Bremmell et al., 2005; Edwards et al., 1980; Wellham et al.,1992). It has been deduced from flotation studies that the formationof slime coatings is directly related to the surface potentials of themineral particles (DiFeo et al., 2001; Edwards et al., 1980). As ser-pentine has a positive potential at alkaling condition, it is thereforelikely that it will attach through electrostatic attraction to the neg-atively charged mineral surface and reduce both the flotation rateand recovery of target minerals (Li, 1993; Lu et al., 2011). It has beendiscussed previously that dispersants will act by adsorbing onto the

ll rights reserved.

7._309@csu.edu.cn (Q. Feng).

attraction (Huynh et al., 2000; Lu et al., 2011; Pugh, 1989a,b).Depressants play an important role in effecting selective separa-

tion of minerals from ores by flotation. Polysaccharides and theirderivatives are effective depressants for talc and other magnesia-bearing minerals. There are numerous reports (Hicyilmaz et al.,2004; Khraisheh et al., 2005; Liu et al., 1994; Rath et al., 1997; Morriset al., 2002; Shortridge et al., 2000) on the application of polymer as adepressant for talc and other magnesia-bearing minerals present asimpurities in various sulfide ores, but these studies only consider thecondition that one kind of magnesium silicate bearing gangue min-eral presents in the ores. However, in the processing of nickel sulfideores, many kinds of gangue minerals usually coexist, and the inter-actions of different kinds of gangue minerals and implications fortheir depression by polysaccharides have not been studied.

In the present study, the interactions of serpentine with talc andtheir influences on the depression of talc using CMC as depressantwere studied. Moreover, the method that uses SHMP to eliminatethe interactions is also discussed, providing a reference for furtherstudy.

2. Experimental

2.1. Samples and reagents

The serpentine used for all experiments was obtained fromDonghai, Jiangsu Province, China. Mineralogical and X-ray powder

Table 2Grain size distribution of talc sample.

D50 (lm) D90 (lm) Averagediameter (lm)

Content(below 38 lm) (%)

10.25 18.4 10.73 100

Table 3Concentration of ions dissolved from serpentine by acid leaching.

Elements Mg Si Fe Ca Ni Cu P Mn

Concentration(mg/L)

14.63 2.01 1.10 1.39 0.18 0.13 0.07 0.04

B. Feng et al. / Minerals Engineering 32 (2012) 68–73 69

diffraction data confirmed that the serpentine sample was of highpurity with trace amounts of chlorite. The sample was dry groundand screened. The grain size distribution is shown in Table 1. Thepure talc sample was obtained from Haicheng, Liaoning, China.And the mineral composition of the talc selected for experimentsand tested by XRD was as follows: talc 95%, chlorite 5% and thegrain size distribution is shown in Table 2.

Leached serpentine was prepared by leaching pure serpentinesample in solution at pH 3 for 24 h. As a result of leaching, the sur-face characteristic of serpentine changed, as many ions dissolvedfrom the sample (Table 3).

The sample of CMC used in the study was obtained from Tianj-ing Kermil Chemical Reagents Development Centre, Tianjing,China. The molecular weight of CMC is 700,000 and degree of sub-stituent (DS) is 0.9. The CMC solution was prepared by dispersing aknown weight of sample in cold distilled water and then dissolvingit in boiling distilled water. The solutions were prepared fresh eachday.

Potassium nitrate was used to maintain the ionic strength andhydrochloric acid (HCl) and potassium hydroxide (KOH) were usedas pH regulators. All the reagents used in this study were of analyt-ical grade. Deionized double distilled water was used for all tests.

2.2. Experiments

2.2.1. Flotation testsSingle mineral flotation tests were carried out in a mechanical

agitation flotation machine at a constant rate. The mineral suspen-sion was prepared by adding 2.0 g of minerals to 40 ml of solutions.The pH of the mineral suspension was adjusted to a desired valueby adding KOH or HCl stock solutions. The prepared CMC andSHMP solution were added at a desired concentration and condi-tioned for 5 min. The frother Methyl Isobutyl Carbinol (MIBC)was then added to the slurry and flotation was carried out for a to-tal of 4 min. The floated and unfloated particles were collected, fil-tered and dried. The flotation recovery was calculated based onsolid weight distributions between the two products.

2.2.2. Sedimentation testsCoagulation and dispersion between serpentine and talc were

studied using the sedimentation tests. For the sedimentation tests,0.1 g of sample powder was taken and made up to 100 ml afteraddition of desired mounts of 10�2 mol/l KNO3. The suspensionswere then agitated for half an hour using a magnetic stirrer at25 �C and transferred to 100 ml graduated flasks. The solutionwas then settled at a fixed time (3 min), and the supernatant liquorof fixed height (25 ml) was pipetted out and measured by Scatter-ing turbidimeter. The dispersion of the supernatant liquor wascharacterized by its turbidity. The higher the turbidity value is,the better dispersed the sample is.

2.2.3. Zeta potential measurementsZeta potential measurements on talc and serpentine were car-

ried out using a zeta plus zeta potentialmeter. Potassium nitratewas used to maintain the ionic strength at 10�3mol/l. Smallamounts of sample were added to desired amounts of solutionand ultrasonicated for 3 min, magnetically stirred for 10 min andthe pH adjusted using HCl or KOH. The zeta potential of samples

Table 1Grain size distribution of serpentine sample.

D50 (lm) D90 (lm) Average diameter(lm)

Content (below 38 lm) (%)

3.94 13.62 6.17 100

was then measured alone or in the presence of SHMP using a zetaplus potentialmeter.

3. Theoretical background

Heterocoagulation is usually described by the DLVO theory(Adamczyk and Weronski, 1999; Missnan and Adell, 2000). Thecolloidal forces considered include the electrostatic double-layerforce and the van der Waals force.

3.1. Electrostatic double-layer interaction

The model used to describe the EDL interaction energy is basedon the Poisson–Boltzmann equation, which describes the electro-static potential in an ionic solution as a function of position relativeto the particle surface and has been found to be accurate down toseparations of a few nanometres (Mitchell et al., 2005). The inter-action energy at constant surface potentials is often used and canbe described by:

VE ¼pe0erR1R2

ðR1 þ R2Þðw2

1 þ w22Þ

� 2w1w2

ðw21 þ w2

2Þ� ln 1þ expð�jHÞ

1� expð�jHÞ

� �þ ln½1� exp ð � 2jHÞ�

( )

ð1Þ

where the radius of serpentine particle R1 is 3.085 lm (Table 1), theradius of talc particle R2 is 5.365 lm (Table 2); j is the thickness ofthe electric double-layer, j = 0.180 nm�1; e0, er represents the vac-uum dielectric constant and the relative dielectric constant of thecontinuous phase, with given value of 6.95 � 10�10 C2/(J m), and Hrepresents the distance between particles; w1 and w2 are the sur-face potential, when contact time between the particles is short,the assumption of constant surface charge is appropriate (Nguyenet al., 2002).

3.2. van der Waals interaction

The van der Waals interaction energy is

VW ¼ �A

6HR1R2

R1 þ R2ð2Þ

The Hamaker constant for talc/water/serpentine, is not avail-able in literature. For talc (Chen, 2005) lists a value of the Hamakerconstant acting through vacuum as A11 = 12.9 � 10�20 J. As serpen-tine is a magnesium silicate mineral, an approximation using theHamaker constant for mica in vacuum could be usedA22 = 9.7 � 10�20 J (Bremmell et al., 2005). The Hamaker constant(A123) for two different materials (1 and 3) interacting throughmedia (3) is

60

80

100

Talc+SHMP Serpentine+SHMP(%

)

70 B. Feng et al. / Minerals Engineering 32 (2012) 68–73

A132 ¼ ðffiffiffiffiffiffiffiA11

p�

ffiffiffiffiffiffiffiA33

pÞð

ffiffiffiffiffiffiffiA22

p�

ffiffiffiffiffiffiffiA33

pÞ ð3Þ

When medium 3 is water (A33 = 3.7 � 10�20 J), a value of theHamaker constant of 0.56 � 10�20 J was calculated for the talc/water/serpentine system on the basis of Eq. (3) and is used in thecurrent study.

0 40 80 120 1600

20

40

Leached serpentine+CMC

Rec

over

y

Concentration(mg/L)

ig. 2. Recovery of talc and serpentine as a function of reagent concentrationIBC = 1 � 10�4 M).

0 50 100 150 2000

20

40

60

80

100 Talc+serpentine+SHMP Talc+serpentine+CMC Talc+serpentine+SHMP+CMC Talc+leached serpentine+CMC

Rec

over

y (%

)

Concentration(mg/L)

ig. 3. Recovery of mixed ores as a function of depressant concentrationIBC = 1 � 10�4 M).

4. Results and discussion

The effect of pH on the floatability of talc and serpentine in theabsence and presence of CMC is shown in Fig. 1. It is evident fromthe picture that the flotation recovery of talc is independent of pH.The flotation recovery of talc is very high (>90%) over the entire pHrange tested in the absence of CMC depressant. With the additionof CMC, talc is depressed and this depression effect depends on pH.The depression effect of CMC steadily decreased with increasingpH from 3 to about 11, which is in agreement with earlier observa-tion (Jenkins and Ralston, 1998; Khraisheh et al., 2005). Differentfrom talc, serpentine does not have natural floatability, and CMChas little effect on serpentine flotation in the acidic condition whileactivates serpentine flotation in the alkaline condition. The leachedserpentine has the same flotation characteristic as original serpen-tine (presented in Fig. 2).

Sodium hexametaphosphate, also known as Calgon, has wide-spread use as a dispersant in the minerals industry. The effect ofsodium hexametaphosphate on the flotation of talc and serpentinewas also studied and the results were presented in Fig. 2. Results inFig. 2 also show that the SHMP does not affect talc and serpentineflotation at the used concentration.

The experimental results described above show that serpentineis a hydrophilic gangue mineral and its flotation is not depressedby CMC, which is effective for talc depression. In order to investi-gate whether or not serpentine interferes with talc depression byCMC, talc and serpentine were mixed by 1:1 (mass ratio) andfloated at pH 9 using CMC and SHMP as depressants, and the resultis presented in Fig. 3. The actual flotation recovery of talc and ser-pentine mixed ores is 33%, lower than the theory recovery (55%) ofthis kind of mixed ores, illustrating that serpentine decreases talcflotation recovery to some extent. Edwards et al. (1980) have alsofound that serpentine slime particles decreased the hydrophobicityof the sulfide particle as a result of slime coating. Serpentine mayreduce talc flotation recovery by the same manner. However, theflotation recovery of talc and leached serpentine mixed ores is54.7%, nearly to the theory recovery of mixed ores, indicating thatleached serpentine does not interfere with the talc flotation. Theeffective talc depressant, CMC, reduces the flotation recovery ofmixed ores consisting of talc and leached serpentine. However,

2 4 6 8 10 120

20

40

60

80

100

Serpentine Talc Talc+100mg/L CMC Serpentine+100mg/L CMC

Rev

over

y (%

)

pH

Fig. 1. Recovery of talc and serpentine as a function of pH (MIBC = 1 � 10�4 M).

F(M

F(M

both CMC and SHMP do not show depression effect on mixed oresconsisting of talc and original serpentine and mixed ore flotationrecovery increases to some extent with the increase of reagent con-centration. With the addition of 100 mg/L SHMP first, the mixedore is depressed by CMC.

These results illustrate that some interactions exist betweenserpentine and talc, which lowers the flotation recovery of talcand interferes with talc depression by CMC. SHMP can eliminatethese interactions and allow talc to once again be depressed byCMC.

In order to investigate the reason for the decreased CMC depres-sion effect on talc in the presence of serpentine, one needs to eval-uate how the serpentine interacts with the talc particles. Toascertain this, the zeta potential of talc and serpentine particleswere analyzed. It can be seen from Fig. 4 that serpentine has apoint-of-zero charge (PZC) of pH 10.2. The zeta potential of serpen-tine is positive in the pH value range of 2–10.2. The PZC of talc ispH 3, and the surface of talc is negatively charged in the pH valuerange of 3–12. At pH value 9, where flotation of nickel sulfide oresis routinely performed, surface potential of serpentine and talc areopposite, and the positively charged fine serpentine particles willattach to the negatively charged talc particle surface through elec-trostatic attraction.

The acid leaching changes the surface characteristics of serpen-tine and shifts the PZC of serpentine from pH 10.2 to 6.2. At pH 9,

2 4 6 8 10 12-80

-60

-40

-20

0

20

40

60

80 Talc Serpentine Leached serpentine

Zeta

pot

entia

l (m

v)

pH

Fig. 4. Zeta potential of serpentine and talc particles as a function of pH.

2 4 6 8 10 12-80

-60

-40

-20

0

20

40

60

80 serpentine talc serpentine+SHMP talc+SHMP

Zeta

pot

entia

l (m

v)

pH value

Fig. 6. Zeta potential of serpentine and talc as a function of pH in the absence andpresence of SHMP.

80

B. Feng et al. / Minerals Engineering 32 (2012) 68–73 71

both talc and leached serpentine are negatively charged, so theinteraction between talc and serpentine will be negligible.

Turbidity technique, due to its non-invasive, non-contact prop-erties, is well suited to studies of colloidal suspensions. The turbid-ity value of suspensions reflects the particle numbers of colloidalsuspensions. A decrease of turbidity value indicates a decrease inparticle number, which is the result of particle aggregation. Thesedimentation tests results in Fig. 5 confirm the result that serpen-tine particles attach to the surface of talc, as the actual turbidity ofmixed ores is lower than the theory turbidity.

As a result of serpentine adsorption, the surface characters oftalc become similar to that of serpentine, which is less hydropho-bic and therefore its flotation recovery decreases (Fig. 3). Theadsorption of serpentine slimes onto talc surface also interfereswith CMC depression effect on talc flotation. In order for talc tobe susceptible to depression by CMC, the serpentine slimesadhered to the surface of talc must be removed.

Addition of a negatively charged regulator, such as SHMP, hasbeen found to disperse fine particles from the target mineral sur-face (Huynh et al., 2000). In the presence of SHMP, the zeta poten-tial of serpentine turns negative at all pH values measured (Fig. 6),as SHMP adsorbs on the surface of serpentine and makes magne-sium ions dissolve from the surface of serpentine (Lu et al.,2011). SHMP has a negligible effect on the zeta potential of talcand talc surface is also negatively charged. With the addition ofSHMP, both talc and serpentine are negatively charged and there-fore the electric double layer attraction may also be reduced.

Particle interaction energys in aqueous solution are commonlydescribed through application of DLVO theory, which allows quan-titative prediction of the interaction energy. The zeta potential val-

2 4 6 8 10 12300

400

500

600

700

Turb

idity

pH

serpentinetalcactual turbidity (1:1)theory turbidity (1:1)

Fig. 5. Turbidity of serpentine and talc as a function of pH value.

ues for the serpentine and talc surface can be obtained from theelectro-kinetic results in Figs. 4 and 6. In the absence of SHMP,the zeta potential value of talc at pH 9 is �47 mV, and that valueof the original and leached serpentine are 9.58 mV and �24.2 mVrespectively. With the addition of SHMP, the zeta potential of ori-ginal serpentine and talc changed to �59.7 mV and �53 mVrespectively. The total interaction energy E was calculated byreplacing the relative data into Eqs. (1) and (2), and the result isshown in Fig. 7. The total interaction energy between serpentineand talc of the samples without SHMP is negative, as shown incurve 1 of Fig. 7. They attract each other and easily form aggre-gates. In the presence of 100 mg/L sodium hexametaphosphate(pH 9), attraction between the two mineral particles is no longerobserved and a repulsive force exists between the talc and serpen-tine particles. The strong adhesion between the native talc andserpentine surfaces is removed in the presence of sodium hexa-metaphosphate. The total interaction energy between leached ser-pentine and talc is positive and complement the result that leachedserpentine does not interfere with talc flotation and depression.

Evidence of the attachment and removal of serpentine particlesonto talc are shown in Fig. 8 in the scanning electronmicroscopyimage together with the energy dispersive X-ray spectrum of talcparticles at different conditions. The spectrum confirms theadsorption of serpentine slime particles onto the talc surface inthe absence of SHMP. This image also shows that only serpentine

0 5 10 15 20 25 30-60

-40

-20

0

20

40

601-talc and serpentine2-talc and leached serpentine3--talc and serpentine, 100mg/L SHMP

Parti

cles

inte

ract

ion

ener

gy (1

0-18 J

)

particle distance/nm

Fig. 7. Interaction energy between a talc and serpentine particle in the absence andpresence of SHMP.

Serpentine Talc

(a) (b)

(c) (d)

talc

Fig. 8. SEM and EDS of talc and serpentine mix ores: (a) in the absence of SHMP, (b) in the presence of SHMP, (c) EDS spectrum of serpentine and (d) EDS spectrum of talc.

72 B. Feng et al. / Minerals Engineering 32 (2012) 68–73

particles of less than 5um in diameter are covering the surface ofthe talc particles. Previous studies have also shown that fineparticles have a higher probability of remaining attached thancoarse ones during agitation in the conditioning stage or flotationChen et al. (1999a,b).

Addition of a negatively charged reagent, SHMP, has been foundto render the zeta potential of serpentine negative and change theinteraction forces between a talc and serpentine particle fromattraction force to repulsive force. From Fig. 8b, we can see thatthe addition of SHMP helps remove these fine serpentine particlesfrom the talc surface.

5. Conclusions

From the results of this investigation, the following conclusionscan be drawn:

(1) At pH 9, where flotation of nickel sulfide ores is routinelyperformed, the two magnesium silicate bearing (MgO) gan-gue minerals, talc and serpentine, are oppositely chargedand therefore attract through an electrostatic mechanism.

(2) As serpentine is naturally hydrophilic and cannot bedepressed by CMC, the formation of serpentine slime coatingson talc surface reduces the flotation recovery of talc to someextent, but also interferes with talc depression by CMC.

(3) The attraction between the two minerals is changed to anelectrostatic repulsion as the result of serpentine becomingnegatively charged as SHMP (a negatively charged modifier)adsorbs preferentially on its surface. SHMP therefore acts asa dispersant of the hydrophilic serpentine slime particlesfrom the talc surface, which results in the improved talcdepression. Acidic leaching has the same effect as SHMP does.

Acknowledgement

The authors acknowledge the support of the National BasicResearch Program of China (2007CB613602).

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