Keywords:FlotationCopper activationCollectorSphaleritePyrite

Advances in Colloid and Interface Science 145 (2009) 97110

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Advances in Colloid and Interface Science1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 982. Activation of sphalerite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2.1. Mechanisms of Cu(II) activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 982.2. Reaction kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002.3. Mechanisms of Cu(OH)2 activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002.4. Zeta potential and isoelectric point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002.5. Copper concentration and activation duration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1012.6. The effect of sphalerite iron content on copper activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1022.7. Sphalerite surface oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

2.8. Lead and iron sphalerite activation

3. Copper activation of pyrite . . . . . .3.1. Unactivated otation . . . . .3.2. Mechanisms of Cu(II) and Cu(OH)23.3. Effectiveness of activation . . .

Corresponding author. Tel.: +61 8 8302 3044; fax: +E-mail address: andrea.gerson@unisa.edu.au (A.R. Ge

0001-8686/$ see front matter 2008 Elsevier B.V. Aldoi:10.1016/j.cis.2008.09.001Contentscharacterised by varying amthan one coppersulde strinadvertent activation and otation of pyrite into zinc concentrates. Selectivity can further be increased andreagent use minimised by opting for inert grinding and by carefully choosing selective pyrite depressantssuch as sulfoxy or cyanide reagents. Studies that approximate plant conditions are essential for thedevelopment of better separation techniques and methodologies.Improved experimental approaches and surface sensitive techniques with high spatial resolution are neededto precisely verify surface structures formed after copper activation. Sphalerite and pyrite surfaces are

ounts of steps and defects, and this heterogeneity suggests co-existence of moreA review of the considerable, but often contradictory, literature examining the specic surface reactionsassociated with copper adsorption onto the common metal sulde minerals sphalerite, (Zn,Fe)S, and pyrite(FeS2), and the effect of the co-location of the two minerals is presented. Copper activation, involving thesurface adsorption of copper species from solution onto mineral surfaces to activate the surface forhydrophobic collector attachment, is an important step in the otation and separation of minerals in an ore.Due to the complexity of metal sulde mineral containing systems this activation process and the emergenceof activation products on the mineral surfaces are not fully understood for most sulde minerals even afterdecades of research.Factors such as copper concentration, activation time, pH, surface charge, extent of pre-oxidation, water andsurface contaminants, pulp potential and galvanic interactions are important factors affecting copperactivation of sphalerite and pyrite. A high pH, the correct reagent concentration and activation time and ashort time delay between reagent additions is favourable for separation of sphalerite from pyrite. Sufcientoxidation potential is also needed (through O2 conditioning) to maintain effective galvanic interactionsbetween sphalerite and pyrite. This ensures pyrite is sufciently depressed while sphalerite oats. Goodwater quality with low concentrations of contaminant ions, such as Pb2+and Fe2+, is also needed to limit

ucture after activation. 2008 Elsevier B.V. All rights reserved.Available online 9 September 2008a b s t r a c ta r t i c l e i n f oA review of the fundamental studies of the copper activation mechanisms forselective otation of the sulde minerals, sphalerite and pyrite

A.P. Chandra, A.R. Gerson Applied Centre for Structural and Synchrotron Studies, University of South Australia, Adelaide, South Australia 5095, Australia

j ourna l homepage: www.e lsev ie r.com/ locate /c is. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

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98 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 971102. Activation of sphalerite

The activation of sphalerite has been studied extensively over severaldecades [5,1214]. While there is general agreement on the overallprocess of copper activation, the actualmechanism and surface reaction

copper-activated sphalerite which was about 0.3 eV lower than thatfor covellite and hence suggest that even if a CuS-like phase forms it isnot same as covellite. They also did not nd any evidence of elementalsulfur formation. Buckley et al. [20] describe the activated sphalerite

1 The nomenclature (I), (II) or (III) is used throughout to denote surface or bulk4. Mixed pyrite and sphalerite otation . . . . . . . . . . . . .5. Summary . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgments . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Flotation is an important and versatile mineral processing stepused to achieve selective separation of minerals and gangue. It utilisesthe hydrophobic (aerophilic) nature of mineral surfaces and theirpropensity to attach to rising air bubbles in a waterore pulp as thebasis for separation [1]. Metal sulde minerals, for which this processwas originally developed, are generally weakly polar in nature andconsequently most have a hydrophilic surface [2]. Hence, collectormolecules such as xanthates and dithiophosphates are normally usedto increase hydrophobicity [3]. Certain sulde minerals such assphalerite (ZnS) do not respond well to short chain thiol collectors,due to the relative instability of zincxanthate and hence require theuse of activators to enhance the adsorption between collectormolecules and the sphalerite surface [2,4]. The cupric ion (Cu2+),generally in the form of sulfate or nitrate, is the most widely usedactivator. Other heavy metal ions such as lead, silver, cadmium,mercury and Fe2+/Fe3+ can also activate the sphalerite surface, but areeither not used commercially or are present as impurities within thesphalerite lattice or in process water [5,6].

Separation of sphalerite through copper activation becomesproblematical when other minerals within the pulp are inadvertentlyactivated along with the sphalerite. Pyrite (FeS2) is one such mineralthat responds to copper activation and can be oated together withsphalerite [2]. Being the most abundant sulde mineral pyrite isundesirably associated, and in most cases ne grained and intimatelyintergrown, with minerals of economic value [7]. This gangue pyrite isa cause of reduced concentrate grade and increased smelting costs, formost minerals such as sphalerite, chalcopyrite and galena. Pyrite isalso a primary contributor towards the substantial environmentalproblem of acid mine drainage resulting in acidication of naturalwater systems. Mining industry treatment costs, in the US alone, areover $1 million/day [8].

Sphalerite and pyrite frequently occur together in ore depositsalong with galena and copper containing ores such as chalcopyrite.The common practice in mine otation is to rst oat the coppercontaining minerals (if present) followed by galena [2,9]. The tailsfrom the galena otation are then used to oat sphalerite away frompyrite primarily using copper activation. Effective separation isneeded to minimise iron in the nal zinc concentrate that mayoccur through pyrite copper activation and otation. Loss of selectivityand unwanted activation can also occur due to contaminants presentin the mine water used for separation of these minerals. Despitecontinuous process improvements the problem of pyrite misreportingto sphalerite concentrates still remains [10,11].

A review of the key factors affecting copper activation of sphaleriteand pyrite is presented herein. Special attention is given to the role ofsphalerite iron content. A discussion of the proposed activationproducts as identied by various fundamental studies is provided as isan examination of the literature regarding the activation and otationresponse of the mixed sphalerite/pyrite system. While only sphaleriteand pyrite activation is discussed, the issues raised may also apply toother sulde minerals.products controlling activation/otation still remains controversial.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

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It has been well established that copper activation of sphaleritefollows an ion exchange mechanism where the uptake of Cu(II)1

results in approximately 1:1 release of Zn2+ into the solution [5,13,15]and is generally represented by Eq. (1) [16].

ZnSs Cu2aqCuSs Zn2aq 1

Cu(II) on the sphalerite surface is subsequently reduced to Cu(I) withthe resulting oxidation of the surface sulde. Collector molecules, suchas xanthates, then reactwith the surface copper sulde species formed,thus increasing the otation response [17]. Cu(I)xanthate is the mainsurface product formed, especially at lowpH [15]. The uptake of copperand subsequent otation of sphalerite is seen to depend on impurities,such as iron, present within natural sphalerite, surface oxidation,copper and xanthate concentration, activation time, solution O2concentration (pulp potential) and most importantly on pH.

Under contrasting pH conditions either an abundance of hydro-phobic (collector absent) or hydrophilic species maybe present on thesphalerite surfaces [14]. Hydrophobic species such as polysuldes (Sn2)and elemental sulfur (Sn0) appear to predominate at mildly acidicconditions while hydrophilic species such as zinc hydroxide andcopper hydroxide, along with some sulte/sulfate, occurs at higher pH[14,15,18]. Polysuldes or elemental sulfur forms as a result ofoxidation of metal-decient sulde on sphalerite surfaces, howeverit is still unclear if one or both of these products form and their relativeabundance on the surface [14,18,19]. The presence of such hydrophobicspecies tends to promote collectorless otation of sphalerite which ismore prevalent at low pH [5,15]. Collectorless otation of sphaleritemay also be seen when impurities such as copper and iron, diffusesfrom the bulk to the surface under acidic conditions [20]. This happensafter zinc dissolution when bulk cationic impurities migrate to themetal-decient (sulfur-rich) sphalerite surface resulting in a self-activatingmechanism. Surface hydrophilic species are seen to reducerecovery by otation with xanthate collectors, only within the pHrange where such species are stable [14].

Fig. 1 shows the various processes that may take place simulta-neously during copper activation of sphalerite that result in theproduction of hydrophobic and hydrophilic species. These processesmay also occur during activation of other suldes. Selective adsorptionof Cu(II) is the desired process. However, depending on activationconditions, precipitation of hydrophilic copper containing hydroxidespecies onto sphalerite surface takes place. In addition, aqueous Cu2+

and/or Cu(OH)2 also reacts with collector molecules leading to non-selective adsorption of hydrophobic CuX or dixanthogen (X2) onsphalerite surfaces.

2.1. Mechanisms of Cu(II) activation

It has been noted that out of the many stable and metastableintermediates present in a copper/sulfur system, researchers havemainly considered CuS (covellite) and Cu2S (chalcocite) as possibleend products of copper activation of sphalerite (Buckley et al. [20]).Buckley et al. [21] found a Cu 2p3/2 Auger parameter of 1850.0 eV onspecies while superscript oxidation state numbers are used to denote aqueous species.

99A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110surface as a copper-substituted sphalerite latticewith the formation ofa metal-decient sulde (sulfur-rich) surface layer in both acidic andalkaline media. This, according to them, is better represented byEq. (2), than by Eq. (1), where the product is metastable.

ZnSs xCu2aqZn1xCuxSs xZn2aq 2

Using conventional X-ray diffraction (XRD), scanning electron micro-scopy (SEM) coupled with energy dispersive spectrometry (EDS) andelectron microprobe analysis (EMPA), Vinals et al. [22] found digenite(Cu1.8S) to be the main activation product at pH 1.11.3 andtemperatures of 180212 C while chalcocite appears to predominateat 225 C. These temperatures are well above the temperatures foundwithin mineral processing plants. Chen and Yoon [23] conducted restpotential measurements and voltammetry at pH 9.2 using a carbonmatrix composite (CMC) electrode containing sphalerite particles, toshow that when copper activation is carried out at open circuitconditions a CuS-like activation product forms while activationconducted at lower potentials produces a Cu2S-like product. They

Fig. 1. Schematics of sphalerite copper activation showing the various simultaneousprocesses likely to occur under different activation conditions.further showed that activation conducted under slightly oxidisingconditions produced hydrophobic species, such as copper polysul-des, on the surface.

Pattrick et al. [17] used Cu K edge and S K edge X-ray absorptionspectroscopy (XAS, consisting of both extended X-ray absorption nestructure, EXAFS, spectroscopy and X-ray absorption near edgespectroscopy, XANES) data to show that copper on activatedsphalerite, at pH 1012, exists in a tetrahedrally coordinated form,bonded to three sulfur atoms and one oxygen atom. However, thesemeasurements were carried out on dry samples and hence chemad-sorbed water may have been present that is not localised on theadsorbed copper atoms within wet slurry. Upon addition of xanthatecollector the oxygen of the CuO (2.07 ) bond is replaced by sulfurfrom the xanthate and a primitive covellite species forms.

However, also using XAS, it was shown by Gerson et al. [13] thatunder mildly acidic conditions both bulk and surface copper iscoordinated only to three sulfur atoms in a distorted trigonal planargeometry with a CuS average bond length of 2.270.02 . Thesemeasurements were carried out using wet slurry. It was proposed thatthis geometry cannot be attributed to the formation of a distinctcrystallographic copper sulde phase and the data could not beadequately tted to a structural model encompassing CuO bonds. Ithas been further shown in Gerson et al. [24] that this distorted trigonalplanar structure (CuS3) on the sphalerite surface is slightly pushed upand outwards resulting from the shorter CuS bonds as compared tobulk ZnS bonds, and elongation of the attaching ZnS bonds. It wasfound that the copper-activated sphalerite surface has CuS bonds0.0090.001 nm shorter than bulk ZnS bonds with a copper to zincdistance 0.0130.006 nm longer than the zinc to zinc distance.

Buckley et al. [21] recently examined a relatively pure copper-activated sphalerite surface (from Santander, Spain) using synchrotronX-ray photoelectron spectroscopy (XPS, Cu, Zn and S 2p3/2 bindingenergies) and XANES (Cu L2,3 edge). This study reconrmed thatcopper on activated sphalerite surface exists predominantly as Cu(I),although a higher than expected d9 character was found. Moreover itwas also found that some Cu(II) ions were present on the surface,associated with oxygen (Cu(II)O species) possibly due to chemi-sorbed water on the sphalerite surface. Buckley et al. [21] suggestedthe formation of a metastable phase (Zn1 xCuxS) with formaloxidation state of sulfur being more positive than 2. This notion ofS being more positive is consistent with the model proposed byGerson et al. [13]. Furthermore, on the basis of the poor correlationseen between the concentration of high binding energy sulfur andsurface copper concentrations, Buckley et al. [21] suggest unsubsti-tuted (by copper) oxidative losses of zinc (Eq. (3)) from the sphaleritelattice which leads to the development of enhanced sulfur regionswith oligosulde-like electronic environments.

Zn1xCuxSsZn1xyCuxSs yZn2aq 2ye 3

Kartio et al. [19] suggested that there may be difculties in assigningCu 2p3/2 binding energies (even from synchrotron XPS) to either CuSor Cu2S. The variable XPS S 2p binding energies observed have led tothe suggestion that sulfur was present in different oxidation states(non-integer) depending on the extent of copper activation and mostproposed mechanisms fail to account for this [13]. On this basis it wasproposed that increased copper activation resulted in less negativesulfur oxidation states. The reaction mechanism provided for surfacesubstitution (Eqs. (4) and (5) for an initial and then a second coppersubstitution respectively) and bulk substitution (Eq. (6) for a singlesubstitution) of zinc with copper result in the formation of a similardistorted trigonal planar geometry with copper bonded to three sulfuratoms but variable sulfur oxidation states. Note in Eq. (5) the lessnegative oxidation state of the bridging sulfur atom.

Cu2aq Zn2S23 4surfaceCu0:9S1:633 4surface Zn2aq 4

Cu2aq S1:632 Cu0:9S1:63Zn2S22 6surfaceS1:632 Cu0:9S1:28Cu0:9S1:632 6surface Zn2aq

5

S23 Zn2S2Zn23 0bulk Cu2aqS1:633 Cu0:84:09bulk S0:8Zn1:633 6bulk Zn2aq 6

Activation by Cu(II) results in the formation of a conducting layer(with the band gap as low as 1.1 eV) on sphalerite surfaces [14,19].Sphalerite is naturally an insulator with a band gap of 3.5 eV. Thisreduced band gap aids in electron transfer reactions and allows thethiol collectors to form an insoluble collector complex on thesphalerite surface through mixed potential electrochemical reactions[19,20]. Ab initio cluster model calculations have also shown thatcopper atoms incorporated into the sphalerite lattice enhanced theelectron acceptor ability of sphalerite [25]. Cu(I)xanthate anddixanthogen formation in solution in acidic medium is proposed tooccur via Eq. (7) [26]. Dixanthogen adsorption is more prevalent onunactivated sphalerite [17] or when the Cu(II) concentration is low [4].

2Cu2aq4EXaq2CuEX2aq2CuEXaq X2aq 7

100 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 971102.2. Reaction kinetics

The initial stage of the activation process, generally, is very fastwith a high rate of copper uptake within rst 10 to 15 min [15,19,27].This initial copper migration into the sphalerite surface can have adiffusion coefcient of 1.01011 to 1.51010 cm2s1 in thetemperature range of 80 to 400 C [28]. It has been proposed thatthe rate of reaction is rst order with respect to copper concentration,with the rate controlled by diffusion through the solution phase [5].Thereafter the rate of copper uptake generally slows and showslogarithmic time dependency [19] which is associated with copperdiffusion into the bulk of sphalerite lattice [5,29]. Vinals et al. [22]found that this latter reaction rate followed a parabolic kinetic law athigh temperatures (160225 C) and had a high activation energy of147 kJmol1. This indicated a kinetic control of the reaction by solid-state counter diffusion of Cu(I) and Zn(II) ion through the coppersulde layer.

The two step activation kinetics observed may be due to adifference in mechanism for the adsorption/absorption of surfaceand bulk copper. It has been proposed, based on EXAFS data, thatcopper is incorporated onto the surface via replacement of three ZnSbonds with commensurate low activation energy [13]. However, thebulk copper results from replacement of four ZnS bonds to form onlythree CuS bonds which requires greater activation energy.

XPS depth proling showed signicant Cu 2p signal even after15 nm of argon ion etching [14,18]. Such copper diffusion processesmay however reduce the formation of surface Cu(I)xanthateespecially in dilute acidic solutions [26]. Popov and Vucinic [26]showed, through IR internal reection measurements, that when theactivation time was long and copper was allowed to diffuse into thebulk sphalerite structure, the concentration of Cu(I) xanthate on thesurface was reduced. Flotation studies however do not show anysignicant decrease in sphalerite recoveries as a result of this decreasein surface xanthate (refer Section 2.5 for further details) [30].

2.3. Mechanisms of Cu(OH)2 activation

Prestidge et al. [18] studied the activation of synthetic sphalerite,conditioned for 30 min at pH 9, using different concentrations ofcopper nitrate. Using conventional XPS, they showed that at highcopper concentrations (10 or more monolayer coverage) the sphaler-ite surface becomes heavily coated with Cu(OH)2. SIMS results byGerson et al. [13] conrmed the occurrence of colloidal Cu(OH)2 onsphalerite particles at high pH and high nominal copper coverage.Similar results were also found by Fornasiero and Ralston [14], whilePopov and Vucinic [15] also noted an apparent depressing effect onsphalerite otation at high copper concentration and alkaline pH.

Prestidge et al. [18] explain that Eq. (2) is more representative ofcopper activation in an acidic mediumwhere the activating species isonly Cu2+. According to them, activation in alkaline conditions wheresurface Cu(OH)2 precipitate is found to be the predominant activatingspecies, is better represented by Eqs. (8), (9).

nZns xCuOH2pptZnSnd xCuOH2surface 8

The Cu(II) from the hydroxide may then exchange with the Zn(II)from the sulde.

ZnSnd xCuOH2surfaceZnnxCuxd xZnOH2surface 9

The zinc hydroxide formed undergoes dissolution and/or dispersion,the extent of which controls the surface hydrophobicity [14,18]. Theresulting Cu(II) sulde then undergoes redox disproportionation toform Cu(I) sulfur products. These products may then form Cu(I)xanthate upon collector addition by combining with xanthate [17]. Atincreased copper concentration the copper-substituted zinc sulde

layer becomes coated with an inhibiting copper hydroxide over-layer[18]. A similar process has also been observed for pyrite [31] howeverCu(I) was observed on the pyrite surface prior to Cu(OH)2 deposition.

There has been suggestions that X-ray based techniques (especiallyin ultrahigh vacuum, e.g. XPS)may cause an increase in the Cu(I) signalfrom activated sphalerite, through photoreduction of Cu(II) to Cu(I)[5,18]. Such a reduction process has now been shown to be connedonly to Cu(OH)2 over-layers, at extended X-ray irradiation time, anddoes not affect Cu(II) involved with activation [32]. An associatedreduction in the surface concentration of oxygen was howeverobserved, and authors have suggested the use of a N2-cooled sampleholder to minimise this loss. An additional experimental uncertaintymay result from the necessity to conduct measurements on drysamples where the surface species may not necessarily be represen-tative of those in mineral slurry. The use of in situ techniques such asIR will be ideal as it eliminates the need to house samples in ultrahighvacuum.

Some researchers believe that for sphalerite in alkaline media thesurface Cu(OH)2 directly interacts with the xanthate, where the OH

ion is exchanged with the xanthate ion [4,15]. The resulting productthen decomposes to form Cu(I)xanthate and dixanthogen on thesurface (Eqs. (10) and (11)).

CuOH2surface 2EXaqCuEX2surface 2OHaq 10

2CuEX2surface2CuEXsurface EX2surface 11

2.4. Zeta potential and isoelectric point

The zeta potential and the isoelectric point (iep) of sphalerite arealso inuenced by the surface speciation resulting from variations inpH, conditioning time, reagent, and reagent concentrations[14,15,33,34]. Popov and Vucinic [15] used microelectrophoresis andperformed electrokinetic measurements on sphalerite particles,conditioned with different reagents of varying concentration, as afunction of pH. It was found that sphalerite on its own had positivezeta potential in acidic conditions and negative zeta potential inalkaline conditions with the iep at pH 6.5. This remained the samewhen sphalerite was conditioned with xanthate.

However conditioning sphalerite with copper sulfate solutionyielded a different zeta potentialpH curve with copper solutions ofdifferent concentrations resulting in different sphalerite zeta potentialpH dependencies. Zeta potential for sphalerite was negative below pH6 for all copper concentrations used, which according to Popov andVucinic [15] showed the exchange of Cu(II) with Zn(II) fromthe sphalerite lattice. The negative zeta potential value suggests thatthe surface was stabilised and no H+ adsorption took place. Zetapotential of sphalerite conditioned with higher copper concentration(8.0104 moldm3) showed two charge reversals with increasingpH. According to Popov and Vucinic [15], the rst charge reversal fromnegative to positive at pH 6was due to adsorption and precipitation ofpositive hydrolysed copper ion species (Cu2(OH)22+, Cu(OH)+) pre-dominantly on the sphalerite surface while the second charge reversalfrom positive to negative at pH 7.6 resulted from deprotonation of thecopper hydroxide that predominantly covered the sphalerite surface.This behaviour is in contrast to the less concentrated copper solution(1.56104 moldm3) which showed a signicant decrease to morenegative zeta potential values from pH 6 to 8 and no charge reversals.Sphalerite conditioned with copper and xanthate showed negativezeta potential throughout the pH range (5.8 to 9.2) due most likely tothe adsorption of xanthate (X) onto the sphalerite surface. Chargereversals are usually common in mineral oxides, silicates and suldesin the presence of adsorbing metal ions and this behaviour can varydepending on type of metal ion, its salt and its concentration [35].

Zhang et al. [34] used a similar technique to study the zetapotential of sphalerite alone and conditioned with various concentra-

tions of ferrous ion and xanthate as a function of pH. While basic zeta

101A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110potentialpH trends obtained by Zhang et al. [34] were similar tothose from Popov and Vucinic [15], the sphalerite zeta potential valuesof Zhang et al. [34] were predominantly negative for all conditionsstudied throughout the pH range (212) and the iep of sphalerite waslocated at pH 2.5. A signicant difference in these two studies was theiron content of sphalerite samples used. Popov and Vucinic [15] used anatural sphalerite sample containing approximately 13 wt.% ironwhile Zhang et al. [34] also used a natural sphalerite sample butcontaining only 2.8 wt.% iron.

Sphalerite samples with different iron contents are known toexhibit different zeta potential values [33]. Gigowski et al. [33]demonstrated that the zeta potential and iep of sphalerite varies non-linearly with the iron content of sphalerite. Mirnezami et al. [36] alsosuggests the likelihood of iron content inuencing iep (and hence zetapotential) of sphalerite. While investigating the aggregation mechan-ism of sphalerite particles Mirnezami et al. [36] measured theelectrophoretic mobilities of sphalerite of different iron contentsand found that the iep of the sample with low iron (mineral sample)was at a lower pH than the samples with higher iron content (plantconcentrates). This was attributed to the presence of increasedconcentrations of zinc oxidation products on the plant concentrates,however the added inuence of iron was also suggested. Sphaleritesurface charge thus appears to be an important factor to be consideredin the differential activation/otation response of sphalerite withdifferent iron contents (refer Section 2.6), normally observed infundamental studies.

In a recent study, Fornasiero and Ralston, [14] measured electro-phoretic mobilities of sphalerite, conditioned at either pH 6.0 or 8.5,with increasing copper concentrations and used XPSmeasurements toidentify the surface species causing the observed change in zetapotential with pH. The zeta potential trend with pH was similar tothose found by Popov and Vucinic [15] and Zhang et al. [34]. At pH 6.0,polysuldes were one of the species found on the surface of sphaleriteresponsible for decreasing the zeta potential to negative values. Thesepolysuldes may have appeared as a result of the formation of metal-decient sulfur-rich phases formed on metal dissolution and maypossibly exist as (Cu+)2S6 [19] or as (Cu+)2S2 [18]. In addition, at thispH, Fornasiero and Ralston, [14] also identied zinc and cupric suldesas important species which were also responsible for the low zetapotential. The sphalerite surface conditioned at pH 8.5 was dominatedby zinc and copper hydroxide/oxide and sulte/sulfate species. Theabundance of these species at pH 8.5 increases further with increasingconcentration of conditioning copper. These species are responsiblefor making zeta potential values less negative above pH 5 and positiveabove pH 6.5.

2.5. Copper concentration and activation duration

Fornasiero and Ralston [14] showed (under their reactionconditions) that maximum collectorless otation recovery can beobtained at copper concentrations of 2106 moldm3/g of38bxb75 m sphalerite while Popov and Vucinic [15] found 3105

to 2.5105 moldm3 Cu2+ concentration better for 104bxb208 msphalerite otation using xanthate. Typically copper concentrationsranging from 1104 to 1106 M have been used to conductactivation and otation studies of sphalerite [26,30,31,3744]. Section3.3 provides typical plant concentrations used for copper activation.The copper concentration needed to givemaximum otation responsewill differ dependingon the activation conditions, origin of themineralsample and available surface area for activation. This ideal copperconcentration needs to be established through experimentation atxed activating conditions. Fornasiero and Ralston [14] found thatincreased Cu2+ concentration had the effect of continuously decreasingthe otation of sphalerite (in the absence of collector), with maximumreduction seen at pH 8.510, probably due to the formation of Cu(OH)2

precipitates. The thickness of this Cu(OH)2 layer generally increaseswith increasing copper concentration at mildly acidic conditions [18].The effect of increasing copper concentration on otation is not evidentabove pH 12 where Cu(OH)3 is the stable species (not Cu(OH)2 ppt)and below pH 5 where only Cu2+ is the stable copper species [14].High copper concentrations may also interact with xanthatecollectors in the pulp preventing adsorption onto the activatedsphalerite surfaces [15]. To minimise this side reaction, effectivelycausing reagent loss, sequential addition (rst Cu2+, then collectorfollowed by lime) is now the preferred technique at Noranda Groupoperations (Section 4.0) [11].

Prestidge et al. [18] found that an increase in activation time oraging had the effect of reducing the Cu(OH)2 over-layer, evident fromthe absence of high binding energy components and satellites fromthe Cu(II) 2p XPS signal. They proposed that happens due tomigrationof copper into the bulk sphalerite as surface Cu(OH)2 continues totransform into the copper-substituted zinc sulde structure withprolonged activation. Increasing the activation time thereforeincreases the copper uptake with subsequent increase in the releaseof Zn2+ [19] while increasing collector concentrations increasescollector adsorption onto the surface [4,5].

However, increased copper uptake may not be equally benecialfor surface xanthate adsorption Popov and Vucinic [26] conductedoatability and infrared internal reection studies on a high iron(13 wt.%) containing sphalerite (104bxb208 m) in weakly acidicmedium and investigated the effects of prolonged activation time andcopper concentration. For the short activation time (2 min) it wasfound that Cu(I)xanthate was the dominant species on the surfacewith a small concentration of Cu(II)xanthate. However, the amountof Cu(I) xanthate decreased with increasing activation time, evidentfrom reduced peak intensity (at 1197 cm1) in the IR spectra.Prolonged activation time also resulted in a slow diffusion of Cu(I)into the bulk sphalerite structure. Increased copper adsorption wasobserved when the copper concentration was increased in solutionwith long activation times, however Cu(I) xanthate formationwas stilllower at than at short activation times. Interestingly, the otationrecovery of sphalerite was greater for samples activated for prolongedperiods and/or with increased activating copper concentration.Lascelles et al. [30] demonstrated that an increased time delaybetween Cu2+ addition and xanthate addition decreased the amountof collector adsorbed on to the sphalerite surface at high pH (9.2) butnot at neutral or mildly acidic pH. At high pH it is known that Cu2+ canalso adsorb as Cu(OH)2 colloids [18]. On the basis of XPS measure-ments, Lascelles et al. [30] suggested that on delay between Cu2+ andxanthate addition, colloidal Cu(OH)2 adsorbed onto the sphaleritesurface is lost to the solution resulting in decreased surface xanthateadsorption. There was no (as was expected) decrease in the otationresponse of sphalerite with reduced surface xanthate probably due tocollectorless otation, however the authors suggest early addition ofxanthate.

The presence of both copper and xanthate (and their quantity) mayalso inuence interactions between sphalerite and gangue particles.Duarte and Grano [39] conducted batch otation, zeta potentialmeasurements and rheological studies on silicate gangue minerals(d85 of 1.0 m) and ultrane sphalerite (d85 of 7.9 m) to show that atpH 9 silica can misreport to sphalerite concentrates through acombination of entrainment and aggregation (with sphalerite parti-cles) but not through true otation via surface hydrophobisation.Under their reaction conditions, Duarte and Grano [39] found thatwhen no reagents were added in a mixed mineral system (silica andsphalerite), silicate gangue minerals misreport to sphalerite concen-trate primarily through entrainment. However, when 1800 g/t coppersulfate is present with 1500 g/t isopropyl xanthate, silicate gangueminerals misreport via a combination of entrainment and aggregationas determined from batch otation and cryogenic SEM analysis. Inaddition, rheological studies were used to show that particle

interactions increased upon copper sulfate and xanthate addition.

They also found that the zeta potential of silica became less negativeupon addition of copper sulfate and xanthate. They theorised thatreagents modied the surface characteristics of silica causing thesurface charge to become less negative to values near the iep of silica,where electrostatic repulsive forces are relatively low. This allowedsphalerite and silica particles to interact and form aggregates thatreport to the concentrates along with entrainment.

It has previously been shown that aggregation of sphaleriteparticles can occur through occulation [36]. Mirnezami et al. [36],on the basis of settling velocity, suspension analysis and opticalmicroscopy, showed the presence of aggregates in sphalerite particlesat pH 79 as a mechanism of sphalerite misreporting to lead andcopper concentrates in the processing of PbCuZn sulde ores. Theyfound that sphalerite releases sufcient zinc ions in solution (whichforms hydroxide in the pH range 79) and causes aggregation. Aocculating mechanism of aggregation was suggested involvingpolymeric zero-charge Zn2+ species, [Zn(OH)2(H2O)2]n0.

2.6. The effect of sphalerite iron content on copper activation

content of the two samples, where the sample with higher ironcontent also had higher lead content, could also be exerting an effect.

Recent studies by Boulton et al. [38] also support the notion of ironcontent of sphalerite reducing the rate of copper-activated sphaleriteotation. By carrying out otation studies (at pH 11) on two naturalsphalerite samples, with high iron (12.5 wt.%) and low iron (0.3 wt.%),Boulton et al. [38] concluded that the presence of iron in the sphaleritelattice reduces the exchange sites (zinc) for Cu2+, with this effect beingmore pronounced for coarser particles presumably due to the lowersurface area to volume ratio. They however found that iron contenthad no inuence on maximum recovery and otation rate constant atlow copper concentrations.

However, Gigowski et al. [33] and Harmer et al. [48] have reportedcontrasting trends to those found by Solecki et al. [27], Szczypa et al.[49], Buckley et al. [20] and Boulton et al. [38]. Using natural sphaleritewith varying iron content (up to 12 wt.%) they showed that copper-activated iron-rich sphalerite preferentially adsorbs xanthate. How-ever, despite this no direct relationship between oatability, ironcontent and copper concentration was found [33]. Recent studies by

sio

ecto

yl xa

iumthatiumthatiumthat

102 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110Pure sphalerite (cubic ZnS) contains nearly 67wt.% Zn and 33wt.% S[45]. However, natural ZnS normally contains iron (along with otherminor impurities) substituted for zinc atoms [46], amounts of whichdepend on the temperature and chemistry of the crystallisationenvironment [45,47]. The presence of iron decreases the band gap ofsphalerite,which is naturallyan insulator, and affects its reactivity [48].The iron content of sphalerite has been seen to inuence the activationand subsequent otation behaviour of sphalerite during fundamentalstudies; however contradictory results have been reported[20,27,33,38,48,49]. Table 1 provides a summary of copper activationand otation studies conducted over the last two decades withsphalerite samples of various iron and lead (where this data is availableor lead is present) contents. This table also lists the various conditions(where available) used for these studies. The pH, activator and collectorused are fairly consistent. However, other important factors such asactivation time and particle size are quite different.

Using synthetic sphalerite containing various iron contents (up to40 wt.%) and 64Cu labelled CuSO4, Solecki et al. [27] showed thatadsorption of Cu2+decreased with increasing concentration of iron insphalerite. In another study, Szczypa et al. [49] further demonstrated,with synthesised sphalerite, that increasing the iron content alsoresults in decreasing attachment of xanthate to copper-activatedsphalerite, primarily due to reduced copper on the sphalerite surface.XPS studies by Buckley et al. [20] on two natural sphalerite sampleswith different (high and low) iron contents also appear to supportthese ndings. However, they acknowledged the gradient in lead

Table 1Copper activation and otation studies conducted over the past two decades and conclu

Sample (size) Fe content Pb content Activator(time)

Coll

Synthetic (not given) 0% Notpresent

64CuSO4(not given)

5%40%

Synthetic (not given) 0% Notpresent

CuSO4(20 min)

Eth5%40%

Natural (not given) 33a 0.2a CuSO4(1 h)

115a 24a

Natural (125200 m) 0.38% 0.04% CuSO4(not given)

Sodxan12% 0.20%

Synthetic: ZnSe and ZnS Natural:Fe rich (b10 m)

High (exactvalues not given)

(Notgiven)

CuSO4(1 min)

Sodxan

Natural (range; 45 m) 0.3% 0.07% CuSO4(2 min)

Sodxan12.5% 0.24%

Natural fresh (110)surface 0.02% to 14.79% (Notgiven)

CuSO4(1 h)

a 3Values reported as atomic ratios (10 ) of metallic impurity elements relative to zinc.Harmer et al. [48] demonstrated that as the iron content of sphaleriteincreased the amount of Cu2+ adsorbing onto the sphalerite surfacealso increased. Using a combination of electron microprobe analysis(EPA), atomic force microscopy (AFM) and XPS on ve differentsphalerite samples with varying iron content (Table 1), they showedthat as the iron content of the sample increased the number of surfacedefects and steps along with the size of surface oxidation productsalso increased. The increased surface defect sites allow more Cu2+ tobe adsorbed compared to samples with low iron content (less defectsites). In addition samples with higher iron content undergo a morerapid oxidation than those with lower iron content, hence iron furtheraids in Cu2+ adsorption. A decrease in the Fe 2p3/2 doublet intensitywas also noted compared that of the zinc, suggesting copper replacediron in the sphalerite preferentially over zinc.

2.7. Sphalerite surface oxidation

The sphalerite surface is characterised by steps and defects, sizeand frequencies of which tends to increase with impurities such asiron [48]. Therefore, sphalerite surface preparation for activation andthe degree of pre-oxidation of the surface may affect the availablesurface area for the uptake of copper and xanthate, and are thus alsoimportant factors to be considered. Harmer et al. [46] studied a highiron sphalerite surface (110) using electron probe microanalysis(EMPA), Rutherford backscattering (RBS), PIXE, XPS, and mediumenergy ion scattering (MEIS) and found that vacuum fractured and airfractured samples undergo varying degrees of relaxations and

ns reached using sphalerite samples with different iron and lead contents

r used pH Conclusion Reference

8 Increase in Fe causes decrease inCu adsorption

[27]

nthate 6, 8, 10 Increase in Fe reduces xanthateadsorption due to lower Cu adsorption

[49]

9.2 Cu uptake by high Fe sample was lessthan for the low Fe sample

[20]

isopropyle

Fe rich sphalerite adsorbs more Cu andpreferentially binds to xanthate

[33]

isopropyle

10 and12

Fe inhibits initial Cu activation but catalysescovellite formation upon xanthate addition

[17]

isopropyle

11 Fe is detrimental to copper activation whichreduces collector adsorption

[38]

5 Fe enhances Cu adsorption [48]

(without exchange) of Pb2+ followed by development and precipita-tion of lead hydroxy species such as PbOH+ and Pb(OH)2 [51,53]. Inaddition, carbonate species such as Pb3(CO3)2(OH)2 precipitates canalso occur at high pHs and under prolonged exposure to air, which istypical in processing circuits. This is evident from solution speciationcalculations of O'Dea et al. [57]. The Pb(OH)2 may then react withxanthate to form lead xanthate through an ion exchange mechanism[51]. Rashchi et al. [52] however found through micro-otation teststhat lead activation of sphalerite was signicant below pH 7 andprogressively decreased to zero at pH 11. Based on their results and

103A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110reconstructions resulting in possible SS (dimer) type bonding.However these surfaces are likely to be considerably different tothose exposed to an aqueous environment.

Leaching studies conducted by Weisener et al. [50] have showniron-rich sphalerite to be more oxidised and to leach more rapidlythan low iron containing sphalerite, suggesting greater reactivity ofiron-rich sphalerite surfaces. Recent AFM images have shown theoccurrence of larger and more frequent oxidation features on airexposed high iron sphalerites (compared to low iron sphalerite),conrming higher reactivity of high iron sphalerite [48].

Solecki et al. [27] established that surface oxidation prior toactivation had a greater effect on copper adsorption onto the low ironsphalerite in comparison to the high iron sphalerite. Szczypa et al. [49]demonstrated that the copper/xanthate uptake was much greateronto unoxidised than oxidised sphalerite surfaces for both high andlow iron sphalerite and that the effect of iron content was morepronounced for unoxidised as compared to oxidised surfaces.Gigowski et al. [33] however found that copper activation of sphaleritesurfaces was more inuenced by iron content than by the degree ofoxidation. These results suggest that while both increased oxidationand increased iron content are detrimental to otation it is the latterwhich has the more profound effect.

Using XAS Pattrick et al. [17] showed that activated natural (Zn,Fe)Swas more oxidised than ZnS, and that surface iron in (Zn,Fe)S existedas FeO. Similar FeO structures were observed by Buckley et al. [20] onfreshly fractured sphalerite samples, which were believed to haveexisted prior to the fracture. According to Pattrick et al. [17], theexistence of FeO had the effect of accelerating the formation of CuOand covellite (CuS), which is proposed by some authors to be amongthe major products of activation and xanthate addition. Additionally,the EXAFS Fe K edge data revealed that iron also reacts with the sulfurof xanthates forming FeS, thus adding further evidence to the notionof preferential xanthate adsorption on high iron sphalerite. Thesendings also provide a possible explanation for the observation madeby Gigowski et al. [33].

2.8. Lead and iron sphalerite activation

Sphalerite containing a high iron concentration also generally hashigher lead content. Addition of, or the presence of, lead ions (Pb2+) inthe otation slurry can directly activate sphalerite and promoteotation [5153]. Pb2+ can be present due to dissolution from galenaand also through recycling of plant water, which can lead toinadvertent lead activation and misreporting of sphalerite to copperand lead concentrates [11,54]. The presence of greater lead content inhigh iron sphalerite (compared to low iron sphalerite) is likely tocontribute towards higher otation response seen in studies with highiron sphalerite (Section 2.6).

Pb2+ on the sphalerite surface (either present naturally as part ofsphalerite lattice or adsorbed from the solution) promotes otation byreacting with xanthate forming stable xanthate and/or dixanthogen[55]. Basilio et al. [55] using kinetic studies and spectroscopic analysis(XPS, XRFand FTIR) suggested that lead activation of sphalerite occurredthrough an exchange mechanism involving exchange of Pb2+ with Zn2+

(similar to copper activation) on the sphalerite surface. This exchangemechanism is however not supported by other studies as accommodat-ing lead in place of zinc sites would require considerable relaxations oflocal structure as lead is much larger than zinc (or copper) [53,56].

Computer (atomistic) simulations by Pattrick et al. [56] showedthat lead is incompatible in zinc sites in a sphalerite lattice (bothsurface and bulk) and the exchange process is not energeticallyfavourable. Further uorescence REFLEXAFS study by Pattrick et al.[56] favoured a mechanism involving the formation of Pb-oxidespecies which become a point of attachment of xanthate sulfur to thelead. A high pH (9.2) normally favours more lead adsorption on

sphalerite surface and it may occur through direct adsorptionthose from the literature, Rashchi et al. [52] suggested a mechanismwhere lead exchanges with zinc in the sphalerite lattice and reactswith xanthate to form PbX only in the acidic pH range. From pH 710lead forms a ZnOPb+ species on the surface through adsorption ofPb(OH)+ but also forms the same PbX species with xanthate. BeyondpH 10 the Pb(OH)2 precipitates dominateswhich can render sphaleritesurface hydrophilic thus depressing otation.

The mechanism of lead activation of sphalerite remains poorlyunderstood and further surface studies involving both spectroscopic(XPS, ToF-SIMS, NEXAFS, EXAFS) and electrochemical kinetic studiesare needed for clarication. Synchrotron based measurements(spatially resolved) may be better suited as the surface leadconcentrations are relatively low and may involve localised hetero-geneous adsorption.

Additions of Fe2+ in the presence of oxygen can also activate thesphalerite surfaces (through adsorption of Fe2+ as Fe(OH)+ followed byanodic oxidation to Fe(OH)2+) and aid sphalerite oatation by forminga ferric hydroxy complex with collector molecules at moderatelyalkaline pH [34,58]. Such results with ferrous activation via aqueousaddition may provide insight regarding the possible inuence of ironin the sphalerite structure (bulk and surface) with evidence of FeO [17]type structures on the sphalerite surface and also the possibleinuence of the presence of iron containing minerals such as pyrite,which can contribute to the pulp iron concentration throughsolubilisation. The use of steel grinding media however contributesto a larger portion of pulp iron content through oxidation of steel(corrosion) during grinding (Section 4.0).

However, iron or lead within the sphalerite lattice do not appear tosignicantly inuence collector adsorption (through solubilisationduring conditioning) under laboratory conditions. Tong et al. [59]conducted unactivated (without copper) otation tests (2 min collec-tion time) on high iron marmatite with varying concentrations ofbutyl xanthate. The recoveries obtained for all xanthate concentra-tions used were extremely low with percent recoveries less than 5%.Similar results were also found by Boulton et al. [38] for ZnS and (Zn,Fe)S using the collector SIPX. Zhang et al. [34] found that activationeffect of Fe2+ in the pulp solution was more pronounced between 1and 2 ppm and that their effect continuously decreased above 2 ppm.They also did not nd any activation effect by Fe3+. Under laboratoryconditions of activation, iron or lead within the sphalerite lattice maynot be solubilising sufciently to produce similar effects to those seenin studies with aqueous additions of these ions into themineral slurry.

Within a otation circuit these ions (lead and iron) possibly occuras a result of dissolution from minerals that predominantly containthese elements. Table 2 shows rest potential values of some common

Table 2Rest potential of some common sulde minerals

Sulde mineral Rest potential (SHE) V

Pyrite 0.66Chalcopyrite 0.56Sphalerite 0.46Covellite 0.45Bornite 0.42Galena 0.40Obtained from [60].

suldeminerals [60]. In a mixedmineral system (as in plants) galvanicinteractions occur, with minerals of higher rest potential beingcathodic and those with lower rest potential being anodic. Anodicmineral are prone to oxidation unlike cathodic minerals. Galena, withthe lowest rest potential, will undergo oxidative dissolution in amixedmineral system and may be a signicant source of Pb2+ in otationcircuits. Pyrite will be cathodic and will not contribute as much iron tothe system. The source of iron in mineral circuits may be from othersources such steel grinding media (Section 4.0).

Zielinski et al. [61], using SEM-EDS analysis, found evidence ofpreferential misreporting only of low-iron sphalerite into leadconcentrates within three Cominco lead otation circuits, most likelythrough activated otation with collector adsorption. Its wasenvisaged that the low-iron particles (both coarse and ne sizefractions) were activated possibly by lead solubilised from the orewithin the slurry and that preferential activation of only low ironsphalerite particles could have been due to greater oxidation of high

pyrite surface without activation reaches a maximum at pH 5 and aminimum at pH 7. Flotation results showed that as much as 80 to 90%of pyrite can be recovered through otation (without activation) at pH4 to 5, while recovery is much lower at other pH values [4,6]. Surfaceanalysis by Leppinen [4], of unactivated pyrite samples conditionedwith ethyl xanthate conrmed the occurrence of ironxanthate withdiethyl dixanthogen at monolayer coverage on the pyrite surface.

A clear distinction as to whether the ironxanthate is in the Fe(II)or Fe(III) form was not made by Leppinen [4] however some Fe(III)character was evident. Studies by Valdivieso et al. [66] have shownthat adsorption of xanthate onto unactivated pyrite surface increasesaqueous Fe2+. The surface oxidation of xanthate to dixanthogen resultsin a corresponding reduction of the surface Fe(III) hydroxide withconversion to Fe2+. This mechanism proposed by Valdivieso et al. [66]is shown in Fig. 2. According to this mechanism as the hydrophobicdixanthogen develops on the pyrite surface there is a subsequentreduction in hydrophilic surface hydroxide.

104 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110iron sphalerite particles due to their higher reactivity. Such studieshighlight the complexity of the activation and otation process usedfor mineral separation in actual plant environment which may bemissed under controlled laboratory conditions.

3. Copper activation of pyrite

Unlike sphalerite, pyrite responds well to thiol collector moleculesin the absence of activation [4,6], however pyrite can be activatedwhen copper is present in the slurry. In either case, inadvertent pyriteotation can result, either during otation of copper bearingminerals/ores due to partial dissolution [62] or during activatedotation of sphalerite especially where the concentration of sphaler-ite in the slurry is low [6]. Secondary copper bearing ores such aschalcocite, covellite and bornite pose greater risk of inadvertent pyriteactivation and otation compared to chalcopyrite, as evident fromrest potential values in Table 2 [60,62,63]. It is therefore important tounderstand the controlling mechanisms of pyrite activation and theotation of pyrite into copper or zinc concentrates. In addition to Cu2+,pyrite can also be activated by Pb2+, Fe2+ and Ca2+ which can lead toinadvertent pyrite otation causing it to misreport to copper, zinc orlead concentrates [6,64,65]. As for sphalerite, pyrite activation andotation is also inuenced by pH, copper concentration and activationtime.

3.1. Unactivated otation

Pyrite has a natural otability and a small percentage of it canusually be recovered without any use of activators or collectors[40,65]. Much higher recoveries can be achieved with collectors. It hasbeen shown by Leppinen [4] that adsorption of xanthate onto theFig. 2. Adsorption and dixanthogen formation on u3.2. Mechanisms of Cu(II) and Cu(OH)2 activation

There are important structural and electronic differences betweenpyrite and sphalerite and as such it is no surprise that the copperactivation of pyrite follows a different mechanism from that ofsphalerite. It was shown by Weisener and Gerson [67] that copperuptake during pyrite activation does not results in a related 1:1 (orany) iron release from pyrite, thus ruling out an ion exchangeactivation mechanism. Pyrite activation follows a single fast stepinvolving Cu(II) adsorption onto the reactive sulfur sites only on thesurface with no migration into the bulk pyrite [67]. During adsorptionCu(II) is reduced to Cu(I) with subsequent oxidation of surface sulde.Boulton et al. [37], conrmed that copper adsorbs only to the surfaceand does not migrate to the bulk, as is case for sphalerite.

Using Fourier transform infrared spectroscopy employing attenu-ated total reection (FTIR-ATR), Leppinen [4] suggested that the pyritesurface upon activation is fully covered with coppersulde likeproducts which resemble Cu2S more than CuS. However, subsequentEXAFS data analysis has indicated that copper on the activated pyritesurface has a distorted trigonal planar position between three sulfuratoms [67] with an average CuS bond length of 2.270.02 .

Weisener and Gerson [31,67] studied pyrite activation usingEXAFS, XPS, angle resolved XPS and time of ight secondary ionmass spectrometry (ToF-SIMS) and found Cu(I) to be present for all pHand all copper concentrations studied, with Cu(II) occurring (ashydroxide) and overlaying the Cu(I) activated surface only at alkalinepH. Moreover, no further copper uptake was observed after 5 min ofactivation. At alkaline pH, along with the distorted trigonal planarcopper moiety, Cu(OH)2 precipitates were also found with a CuObond length of 2.00 .nactivated pyrite surface. Redrawn from [66].

nearly 2% at pH 11. Pyrite recovery at pH 4 is seen to be much higherthan recoveries at all other pH even when conducted in the presenceof sphalerite. Flotation without copper activation generally has noeffect on pyrite recovery as sphalerite does not combinewith xanthatewithout copper [6].

Using micro-otation, laboratory batch, and in-plant minicellexperiments, Dichmann and Finch [70] observed similar behaviourduring combined otation of activated pyrite and sphalerite. Fromthese studies it appeared that during mixed activation/otationsphalerite preferentially consumed copper and xanthate while pyritebecomes depressed when in the presence of sphalerite. According toDichmann and Finch [70], addition of copper increases galvaniccoupling between sphalerite and pyrite particles, and this favoursxanthate adsorption on sphalerite while pyrite becomes coated with

105A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110Zhang et al. [6] suggested through zeta potential and FTIR (ATR)measurements that copper is chemisorbed to the surface. It washowever shown by Hicyilmaz et al. [68] that the interaction betweencopper and pyrite is solely an electrochemical process while theinteraction between activated copper and collector, and the pyritesurface and collector is primarily chemical in nature. Results fromPecina et al. [65] have also conrmed the chemical interaction ofcollectors with activated and unactivated pyrite surfaces.

Leppinen [4], through IR internal reection surface analysis ofactivated pyrite surfaces conditioned with ethyl xanthate collector,showed Cu(I)ethyl xanthate to be the dominant activation productwith monolayer coverage. Shen et al. [69] also reported cuprousxanthate as the only xanthate product formed under their reactionsconditions. Leppinen [4] found copper concentration and pHdependency of xanthate adsorption and subsequent otation ofpyrite. They showed at pH 7 that when equal amounts of copperand xanthate are used, copper xanthate is the only product formed,however when either relatively higher or lower concentrations ofcopper is used, signicant amounts of dixanthogen also forms. SomeCu(II)xanthate like compounds were also observed at higher copperconcentrations. Oxidation of xanthate to dixanthogen has also beenreported by [6].

3.3. Effectiveness of activation

The amount of xanthate adsorbed on activated pyrite surfaces wasseen to increase from aminimum at pH 45 to a maximum at pH 8 [4].The adsorption decreased drastically above pH 8. This can also beobserved from the data of Zhang et al. [6] and Dichmann and Finch[70] where percentage recovery of activated and unactivated pyrite,with andwithout xanthate, undergoes a similar decrease after pH 89.They however obtained a signicantly higher recovery than Leppinen[4] at pH 4.

Generally, activation by copper results in signicant increases inrecovery only within the pH range of 610 [4,6,70]. At mildly acidic pHa higher than expected recovery of unactivated pyrite is observedowing, most probably, to the emergence of sulfur-rich products (dueto the dissolution of iron). Hence, in these studies, the maximumotation, using xanthate collector, of unactivated pyrite at pH 4 is seento exceed otation of activated pyrite at pH 8. This may have verylimited practical implications as most plants don't operate at a low pHof 4. It is generally seen that hydrophobicity of pyrite surfacesincreases at low pH (compared to higher pH under similar conditions)with either activation or collector addition or both [68], thus resultingin higher recovery. Flotation of both activated and unactivated pyritegenerally follows rst-order kinetics [63].

He et al. [40] have shown pulp oxidation potential (Eh) to be animportant factor in determining recoveries and speciation on pyritesurfaces, with maximum recoveries obtainable at the conditioningoxidation potential of 35 mV (SHE) at pH 9. EDTA extraction andsurface studies (XPS) revealed that Eh inuences the production ofhydrophilic (iron oxide/hydroxide) and hydrophobic (Cu(I)S) species,and also promotes formation of Cu(I)xanthate species on pyritesurface The presence and relative abundance of such surface specieshas a corresponding effect on pyrite recovery.

Typically copper and xanthate concentrations from 1104 to1106 M are used to conduct fundamental lab-based studies[14,26,30,31,3744]. A few studies have also reported using 1001500 g/t collector [3739,4143] and 2503000 g/t copper containingactivator (such as nitrate or sulphate) [3739,42,70]. Table 3 givesnames of some collectors and their typical addition rate alongwith thedose of copper sulfate used in otation of sulde ores in processingplants. A range of collector and copper concentrations is useddepending on the exact mineralogy of the ore being processed andthe target minerals. It is however difcult to compare the values given

in literature, which are normally expressed in molar concentrations,and plant values. Most of these studies do not state the conditioningvolume, hence it becomes difcult to convert molar concentrationsinto g/t. From the studies that do express reagent concentration in g/t,the collector concentrations used appear to be an order of magnitudehigher than normal plant practice. Studies beyond the normalpractical range are however needed to provide new perspectives forbetter separation. With exception of a few studies, most referencesquoted have conducted tests on single mineral system whilemaintaining concentrations similar to that used in plant. Needless tosay that processing plants have mixed mineral systems with galvanicprocesses which does not occur in single mineral system.

4. Mixed pyrite and sphalerite otation

Separation of pyrite and sphalerite through activated otation isnormally carried out at high pH [3,71] although some plants such asTeck Cominco also does this at lower pH [48]. Pyrite has a higher restpotential than sphalerite and due to this pyrite surfaces becomecoated with OH products (resulting from reduction of O2) due togalvanic coupling, making pyrite surface less hydrophobic, thusincreasing sphalerite selectivity [72]. This galvanic coupling canhowever be suppressed by saturating the ore slurry with N2 gas andusing N2 gas for otation instead of air. This causes the pulp potentialto be reduced due to reduced oxygen activity [72]. The use of N2 gas toreduce galvanic interactions can be used in reverse otation of pyritefrom sphalerite [11]. It is also used effectively in the N2TEC otationtechnology for otation of auriferous pyrite by the Newmont MiningCorporation at their Lone Tree Plant in Nevada [64]. This process wasdeveloped initially to improve recovery of gold from low grade suldeores, the N2TEC process involves processing of ores (from grinding tootation) in an inert N2 gas environment where operating potentialranges from 100mV to 300mV vs. SHE (0.1 to 0.5 V vs. Ag/AgCl) anduses potassium ethyl xanthate [64,73].

Zhang et al. [6] conducted micro-otation studies of activatedpyrite in the absence and presence of sphalerite. Activated pyriteotation is observed to be depressed signicantly in the presence ofsphalerite at all pH values with recovery continuously decreasing to

Table 3Typical amounts of collector and activator used in otation and separation of suldeores at processing plants

Reagent Name Amount used (g/t)

Collector Xanthate 5350Dithiophosphate 10250Thionocarbamate 1015Xanthogen formate 225Xanthic ester 225Mercaptobenzothiazole 25250Thiocarbanilide 2575

Activator Copper sulfate 1002500

Data obtained from [9].hydrophilic hydroxide ions. The effectiveness of this electrochemical

separation of copper-activated sphalerite from pyrite in the presenceof sodium sulte at pH 8.5 and separations were much better when O2was used as the conditioning gas and when the time interval betweenadditions of sulte and xanthate were relatively short. Fig. 3 showscomparative results from this study with varying intervals betweenxanthate and sodium sulte addition (20 s and 10 min) and the use ofO2 or N2 as conditioning gas. Sodium sulte and O2 conditioning is alsoseen to increase separation efciencies through pyrite depressionrather than through an increase in sphalerite otation. A 70 mVdecrease in solution Eh was also observed upon sodium sulteaddition. Based on these results and combined UVvisible, IR and XPSstudies, Shen et al. [69] proposed an electrochemical mechanism ofpyrite depression with the formation of copper hydroxide on copper-activated pyrite surfaces through consumption of O2 which causes thedecrease seen in solution Eh. The use of sodium sulte and O2conditioning helps depress copper-activated pyrite surfaces through

106 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110reaction in the separation of sphalerite from pyrite is still not fullyunderstood and remains an area of continued research [74]. However,according to Finch et al. [11], the notion of pyrite depression uponcopper addition, by increased galvanic coupling with sphalerite, hassuccessfully been tested at two plant operations in Canada, AgnicoEagle's Laronde and Noranda's Matagami-Bell Allard. In these testscopper and lime were added sequentially to initiate selective copperactivation of sphalerite rather than using traditional method ofsimultaneous addition of copper and lime or lime addition prior tocopper addition. This has now become part of Noranda operationsafter a signicant reduction of iron from zinc concentrates wasachieved.

Recent surface analytical studies, XPS and ToF-SIMS, with mixedpyrite and sphalerite otation have shown evidence of hydrophilicspecies such as ferric hydroxide/sulfate obscuring the pyrite surface[37]. This iron hydroxide layer appeared to have inhibited copper andcollector adsorption onto the pyrite, thus resulting in a decrease inpyrite recovery. Hydrophobic species like cuprous sulde andcollector were on-the-other-hand dominate on the sphalerite surface[37]. He et al. [40] also observed the inhibiting iron oxidation specieson pyrite, leading to a decreased recovery. Boulton et al. [37] furtherestablished that separation of these minerals depends on theconcentration of activating copper and concentration of collectorused. Increasing copper concentration was found to increase sphaler-ite recovery while increasing collector concentration increased pyriterecovery. The concentrations of copper and xanthate should be aimedto be kept at a level which gives maximum otation response fromsphalerite while still retarding pyrite otation into zinc concentrates.Particle size may also have an effect on the otation response of thesphalerite when mixed with pyrite. Boulton et al. [37] further foundthat ner sphalerite particles report to the tails due to the lowerprobability of such particles combining with gas particles duringotation.

Further selectivity can be achieved by carefully choosing and usingdifferent collectors and otation conditions. Shen et al. [71] foundethoxycarbonyl thionocarbamate and thiourea collectors to be moreselective for separation of copper-activated sphalerite from pyritethan xanthate in the neutral to alkaline pH range. Moreover, they alsofound that the use of O2 gas, for conditioning, improved selectivitytowards sphalerite separation from pyrite especially at short mineralconditioning times of less than 20 min. Galvanic interactions betweenmineral particles are reduced to some degree at high pH as the pulppotential is lowered [72]. Pulp potentials and hence galvanic reactionscan be restored by conditioning with air or O2, thus increasingsphalerite selectivity. Excessive oxygenation should however beavoided as otability by thiol collectors can be affected [72]. Grindingwith the use of mild steel also reduces pulp potential as corrosion ofsteel consumes O2. The corrosion of the grinding media is acceleratedby presence of galvanic interactions between the steel media (lowestrest potential) and mineral particles, with cathodic reduction of O2occurring on mineral surfaces [9,42]. This not only leaves mineralsurfaces rich in hydrophilic hydroxyl products which affect selectivitybut is also a signicant source of iron contamination [11]. Grindingwith inert media (sand, slag, ore or ceramic) is preferred over steel[11]. Modications were made in the otation circuit at Mt. Isa MineLtd., Australia, to obtain better zinc concentrate grades through inertregrinding of leadzinc ore at key streams to liberate more sphaleritethrough ner grinding [9,75].

Separation efciencies can also be increased by the use of sulfoxyspecies such as sodium sulte [69], SO2 or sodium metabisulte [72].Sulfoxy species acts as depressants and they depress minerals likepyrite by preventing collectors from adsorbing onto the mineralsurface. Rao [72] suggests that since no sulfuroxygen products areobserved on pyrite surfaces, depression by sulfoxy species tends tofollow an electrochemical mechanism. Using a modied Smith and

Partridge micro-otation column, Shen et al. [69] obtained maximumthe following mechanism:

SO23 aq 12O2 aq YSO24 aq 2e 12

H2O aq 2e 12O2 aq Y2OHaq 13

Cu2aq 2OHaqCuOH2surface 14

In addition to this mechanism Shen et al. [69] propose that galvanicinteractions occur between the cuprous sulde layer on the pyritesurface and the pyrite mineral surface. The cuprous sulde layer beingless cathodic is oxidised to produce cupric ions while O2 is reduced atthe pyrite surface producing hydroxide ions. A similar process alsooccurs on the sphalerite surface however it occurs predominantly onpyrite as it is the more cathodic of the two minerals. Hence, sodiumsulte with O2 induces more hydroxide formation on pyrite surfacesthan on sphalerite.

A range of sulfoxy species from S2 to SO32 can be used asdepressants, and the use of metabisulte in addition to controllingelectrochemical conditions (Eh and pH), has resulted in signicantpyrite depression in-plant trials at Heath Steele for oating galenaaway from pyrite [11]. Sulfoxy species sodium sulde is also used inthe suldisation of oxidised sulde mineral surfaces to restoreweathered surfaces and to retard metal ion dissolution (such aslead) that can inadvertently activate pyrite [11,72]. The sulfoxy speciesalso sequesters metal ions already released into the solution. Thisreduces the concentration of xanthate needed to oat oxidisedminerals as side precipitation of metalxanthate is reduced.

Polyphosphates has also been found to prevent accidentalactivation of unwanted minerals by metal ions such as lead presentin the mineral slurry [76]. Micro-otation tests by Rashchi and Finch[76] showed that sodium polyphosphates effectively deactivated

Fig. 3. Sphalerite and pyrite recoveries obtained at pH 8.5 with [Cu2+] and [KEX]=5 3 4 3210 moldm and [Na2SO3]=210 moldm . Adopted from [69].

107A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110sphalerite in a sphaleritelead otation at near neutral conditions.Based on SEM and XPS studies a cleaning mechanism of polypho-sphate action was suggested which involved removal of activatinglead ions from the sphalerite surface by formation of a solublecomplex. The use of phosphate as a lead selective deactivator greatlyimproved copper concentrates at NVIMining's Myra Falls operation byreducing the amount of zinc and lead in the concentrates [54].

Sodium cyanide can also effectively depress pyrite by inhibitingxanthate adsorption and its subsequent oxidation [72,77]. Thishappens through the formation of an insoluble ironcyanide complexthrough an electrochemical mechanism [44,78]. Cyanide also dis-places xanthate already adsorbed on to pyrite surfaces by an exchangemechanism [44]. Organic depressants such as dextrin, starch, di-ethylene-triamine and tri-ethylene-triamine (DETA/TETA) are beingconsidered as a more environmentally friendly option to cyanide asdepressant [11,79,80]. In addition cyanide has been found to beresponsible for inadvertent activation of sphalerite (by Cu) duringotation of chalcopyrite, which had cyanide added to depress pyrite[81]. Mining plant in Quebec, Canada, suffers from this inadvertentactivation and otation of sphalerite into chalcopyrite concentrateswhen using cyanide as a depressant. Studies by Rao et al. [81]concluded that Cu is leached out of chalcopyrite by the cyanide andunder sufciently anodic pulp potentials inadvertent activationof sphalerite takes place. However when the pulp potential is cathodic(b 155 mV SHE) this inadvertent activation is suppressed as thesolution Cu exists in Cu+ oxidation state and is not able to exchangewith divalent zinc from sphalerite.

Another important factor that can impact the selectivity forsphalerite and pyrite otation is the quality of water used in-plantotation. The primary water source for mine usage can be relativelyimpure. Treated sewerage and hard water are regularly sourced whichcan have varying organic and inorganic content [82]. In addition theprocess water in-plant otation is increasingly recycled from tailingsdams, thickener overows, dewatered and lter products, which canlead to a build up of residual reagents and reaction products (such asxanthates and dixanthogen), mineral dissolution and oxidationproducts (metal ions such as Fe2+, Cu2+, Pb2+, Na+, K+, Ca2+ and Mg2+),colloidal precipitates, residual suldes, increased microbiologicalactivity and highly variable pH and Eh [8284]. Such degradation ofwater quality tends to affect otation and selectivity of both suldeand non-sulde mineral systems and increases reagent consumption[8388]. Depending on water quality otation and selectivity ofsphalerite and pyrite (and other sulde systems) can be affected bynon-selective adsorption of xanthate and dixanthogen, depressionfrom accumulated suldes/cyanides and inadvertent activation frommetallic ions such like Fe2+, Cu2+, Pb2+ and their colloidal precipitates.Water quality in otation circuits can be improved by employingwaste water treatment techniques such as dissolved air otation(DAF), organic removal through biological oxidation, occulation andaggregation, precipitation, ion exchange, UV exposure and reverseosmosis [82,83,89]. Water quality is not a problem in lab-basedfundamental studies as distilled, de-ionised or de-mineralised waterare frequently used, which may not approximate in-plant (electro-chemical) conditions. As such outcomes from fundamental studiesmay not equate to similar outcomes in an actual processingenvironment. Fundamental studies however provide the basicchemical and kinetic information that is invaluable and plant trialswhere possible can add further value.

5. Summary

Awide range of studies on sphalerite and pyrite otation has beenconducted over several decades, which range from the investigation ofbasic mineralogy to elucidating complex surface structures, with theoverriding aim of providing fundamental knowledge to enable

increased economic benets. The information from such studiesneeds to be consolidated as a feed back system to realise the extent ofprogress made and the milestones that are yet to be achieved. Asevident from the literature reviewed there are three general areaswhich require further attention. The rst is to recognise and be able topredict the optimum conditions needed to activate, oat andeffectively separate pyrite and sphalerite. The second area is todetermine the structures formed after activation and collectoraddition and the mechanism leading to these species. And nally aneffective way to reconcile these lab-based studies with plant practicesis required to ensure maximised process throughput, efciency andeconomy and minimised environmental impact.

Both sphalerite andpyrite can be activated by ions such as Cu2+, Fe2+

and Pb2+ in solution; however it is only copper that is usedcommercially while the other ions are considered to be contaminantswhich can cause inadvertent activation and loss of selectivity. Copperactivation of sphalerite proceedswith a 1:1 exchange of Cu2+ with Zn2+

from the rst couple of atomic layers of the sphalerite surface afterwhich the related sulde is oxidised to a Cu(I)S species. This step isrelatively rapid (completed within rst 1520 min) with the rate mostlikely controlled by solution mass transfer, thus requiring sufcientstirring during conditioning to maximise the rate of activation.After the initial step, there is a slow steady state counter diffusion ofcopper (most likely in the Cu(I) oxidation state) into the bulk structureand Zn2+ out of the bulk structure. The surface copper sulde speciesformed is hydrophobic and can induce collectorlessotation especiallyunder low pH conditions. Pyrite activation on-the-other-hand followsa single fast step involving copper adsorption to the surface withoutany exchange with iron in the pyrite lattice. Copper adsorbed onto thepyrite surface does not migrate into the bulk pyrite lattice.

Unlike sphalerite pyrite responds to thiol collectors and tends tooat well evenwithout copper activation. This behaviour is due to theactivating nature of Fe2+ which is naturally present on pyrite surfaces,where ironxanthate structures form along with dixanthogen. Thislargely happens under low pH conditions as the surface Fe2+ sitesform iron hydroxide species at higher pH which retards collectoradsorption.

Cu(I)xanthate (or any other collector) is the dominant speciesformed when sphalerite and pyrite are individually activated andconditioned with xanthate collector. Under conditions (such as longactivation time or increased delay between copper activator andcollector addition) which allow copper to migrate into the sphaleritelattice, the amounts of Cu(I)xanthate formed on the surface will berelatively low, which depending upon other slurry conditions may ormay not affect otation. Xanthate on the mineral (sphalerite andpyrite) surface or in the solution may also dimerize to formhydrophobic dixanthogen especially at low pH and relatively lowcopper concentrations. Dixanthogen formed in solution tends toadsorb non-selectively and can cause unwanted otation of mineralsespecially gangue. Xanthate may also react with Cu2+ in theconditioning solution causing precipitation of Cuxanthate andagain non-specic adsorption. This limits the amount of xanthateavailable for oating the desired mineral thus increasing reagentconsumption. Sequential addition of copper and xanthate is conse-quently preferred over simultaneous additions of these reagents.

At high pH conditions colloidal Cu(OH)2 will also precipitate ontomineral surfaces and a thick obscuring layer of Cu(OH)2 may form ifthe conditioning copper concentration is relatively high. The presenceof Cu(OH)2 on the surface can cause loss of selectivity as Cu(OH)2precipitates non-selectively, hence has the potential to cause otationof unwanted mineral or gangue, through reaction with xanthate. TheCu(OH)2 layer also obscures the Cu(I)sulde layer thus furtherreducing surface hydrophobicity. Cu(OH)2 precipitated on sphaleritesurface may undergo ion exchange with surface Zn2+ followed byreduction to Cu(I). With time (or activation time) this Cu(OH)2 willcontinue substituting with Zn and will slowly migrate into the bulk.

This exchange does not take place between iron on the pyrite surface

108 A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110and adsorbed Cu(OH)2. Some Cu(OH)2 precipitates will also be lostback into the solution especially if the solution copper concentrationfalls below Cu(OH)2 saturation. The effect of Cu(OH)2 will howeverdepend on conditioning copper and xanthate concentrations, con-ditioning time and pH. In the case of pyrite higher pH tends topromote formation of iron hydroxide and copper hydroxide specieswhich not only block collector adsorption sites but also makes thesurface more hydrophilic thus reducing the otation response. HigherpH is ideal for depressing pyrite while lower pH can be used foroating auriferous pyrite.

While copper concentration, activation time and pH have clearlybeen shown to affect sphalerite activation and otation, the ironcontent of sphalerite has been a point of controversy, connedhowever only to fundamental studies. Iron content of sphalerite iscurrently not used as a basis for sphalerite processing in plants. It islikely that the presence of iron in the sphalerite lattice reduces itsband gap and increases its reactivity by aiding in electron transferreactions. However, the presence of a high lattice iron concentrationhas been shown to give rise to increased adsorbed copper andsubsequently higher xanthate adsorption but also lower adsorbedcopper and xanthate adsorption. The reason for this discrepancy liesentirely on the higher reactivity imparted to sphalerite by presence ofhigh iron. Sphalerite containing high iron oxidises much faster thanlow iron sphalerite and at high pH is likely to be covered byhydroxides possibly at iron sites which may be located near steps ordefects. Surface pre-oxidation and pH is therefore critical. The effect ofhigh iron content on sphalerite copper adsorption, in the absence ofsurface oxidation, can only be clearly demonstrated at low pH usingsamples cleaved (or ground) in situ or in an inert environment. LowpH ensures that there are no obscuring hydroxides on the surfacewhile in situ cleaving or cleaving under an inert environment ensuresno atmospheric oxidation of the surface.

Apart from enhancing electrochemical reactions, iron may aid incopper adsorption by preferentially (over zinc) exchanging withcopper and may also combine directly with sulfur in xanthate. Underconditions of high pH or conditions which do not limit atmosphericsurface oxidation, surface reactive sites become obscured by oxidationproducts including hydroxide species. Under such condition low ironsphalerite adsorbs more copper or xanthate due entirely to its lessreactive nature. This can cause sphalerite with low iron content tomisreport to copper or lead concentrates. While the notion ofsphalerite reactivity affecting its activation by copper is plausible, itdoes not however rule out inuence from other factors such asactivating copper concentration or presence of contaminants such aslead. The concentration of lead in sphalerite tends to increase withincreasing iron concentration however this factor is frequently nottaken into account. While the presence of contaminants such as leadin process water is usually not an issue for lab-based single mineralstudies, long activation times may however be sufcient to solubiliselead or even iron (Table 1).

It is also important to consider the practical implications of theeffect of iron content on sphalerite activation. Under typical plantconditions, minerals are usually not under an inert environment aftercomminution and hence may undergo oxidation. Sphalerite with ahigh iron content will oxidise more than sphalerite with low ironcontent. In addition minerals in plants are mixed with other mineralshence galvanic interactions will play a signicant role as these governthe relative degrees of surface oxidation. It is worth investigating theeffect galvanic interactions (with minerals such as pyrite, galena orchalcopyrite) have on copper adsorption of high iron sphalerite andlow iron sphalerite. It is envisaged that with a high rest potentialmineral such as pyrite, the reactive nature of high iron sphaleriteshould promote more copper adsorption. However, this will be afunction of the degree of surface pre-oxidation.

Pb2+ and Fe2+ present in themineral slurry as a result of dissolution

from lead and iron containing minerals such as galena and pyrite, andalso due to water recycling, tend to inadvertently activate sphaleriteand cause it to misreport to copper or lead concentrates. Pyrite (if notsufciently depressed) can also be activated by such contaminant ions.The exact mechanism by which activation of pyrite by the adsorptionof aqueous lead or iron species occurs is still poorly understood. Ionexchange and colloidal adsorption mechanisms as a function of pHhave been suggested however the literature lacks sufcient denitiveevidence. Further studies need to be done using mixed mineralsystems and with lead and iron solution concentrations similar tothose encountered in otation circuits. The surface should also becharacterised using the methods suggested further-on in this section.Determining the exact mechanism of lead and iron activation shouldallow the development of techniques and reagents for minimising thisunwanted activation and otation of sphalerite and pyrite into otherconcentrates. Investigations should also be made into minimising themisreporting of sphalerite or pyrite and other gangue materials byway of entrainment and aggregation through occulation. Reagentssuch as sulfoxy species can be used to restore mineral surfaces tominimise contaminant ion release. Polyphosphates can also beemployed to selectively remove contaminant ions (that have adsorbedinadvertently) from mineral surfaces. Inert grinding should also beconsidered to minimise pulp iron content, in addition to improvingmine water quality.

The study of mixed mineral system is more practical than singlemineral studies and provides a better approximation to actual plantprocessing circuits where galvanic interactions between mineralparticles plays a signicant role. From studies conducted with mixedsphalerite and pyrite it is obvious that a high pH gives betterseparation of these two minerals. Pyrite has one of the highest restpotentials and consequently is usually the cathodic part of anygalvanic couple. In the presence of sphalerite, pyrite becomes coveredwith hydroxides (both ferrous and ferric) due to cathodic reduction ofO2 on pyrite surfaces. Pyrite otation is restricted as the hydroxidespecies not only obscures sites for copper and xanthate adsorption butalso renders the pyrite surface hydrophilic. In addition sphaleritebeing anodic preferentially consumes aqueous copper and hydro-phobic cuproussulde and xanthate dominate its surface. Pulpoxidation potential control is critical for effective galvanic interactionsbetween pyrite and sphalerite, and O2 conditioning is thereforeneeded to maintain sufcient O2 activity. This galvanic interaction canhowever be reversed by purging and oating the minerals with aninert gas like N2. N2 purging reduces pulp potential by decreasing O2activity, and can be used for reverse otation of pyrite away fromsphalerite (and other anodic minerals) for maximising auriferouspyrite recovery. The use of an inert gas to reverse the otationprovides conclusive evidence for the existence of galvanic interactionsand associated electrochemical processes. In addition to having theright potential, using an appropriate copper and xanthate concentra-tion, activation time and particle size should further improveselectivity. Limits for these parameters will however depend on theactual oremineralogy (which changeswith time and extent ofmining)and plant operating conditions. New collectors are also emerging thatcan offer better selectivity depending on the minerals that need to berecovered.

Grinding approaches should also be altered to enhance selectivityand to minimise reagent use. Grinding using steel media uses up O2 inthe pulp causing the pulp oxidation potential to decrease. O2 isconsumed as a result of steel corrosion, which increases when there isgalvanic interaction between the steel media and mineral particles.This promotes the development of hydrophilic hydroxides on mineralsurfaces (through O2 reduction) while the steel media is oxidised. Thisoxidation of steel media is a signicant source of excess iron inotation pulp. The decreased pulp potential and hydrophilic hydro-xides reduce mineral selectivity as galvanic interactions betweenmineral particles are reduced. This requires the use of additional O2

and copper/xanthate to restore selectivity. Inert grinding should be

109A.P. Chandra, A.R. Gerson / Advances in Colloid and Interface Science 145 (2009) 97110preferred in practices which require selective otation of an anodicmineral such as sphalerite.

Selectivity can also be improved through the choice of anappropriate pyrite depressant such as sulfoxy species (S2 to SO32)and cyanide. Common sulfoxy reagents are sodium sulte, sulfurdioxide and sodiummetabisulte. They work by enhancing hydroxideformation on pyrite surfaces through an electroche