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Sulfuric acidoxygen leachingChloride effectKineticsMechanism

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sis forsulde

Hydrometallurgy 98 (2009) 2132

Contents lists available at ScienceDirect

Hydrome

l seFlett, 2002; Senanayake and Muir, 1988, 2003). The HydroCopperprocess uses copper(II) chloride in strong brine (4.3 mol dm3 NaCl)to leach chalcopyrite concentrates at pH 1.52.5 and near boiling pointof water (Lundstrom et al., 2005). New process owsheets have beendeveloped for chalcopyrite concentrates which include leaching inacidic calcium chloride solutions (~2 mol dm3 Cl) followed by con-ventional solvent extraction, stripping as sulfate, electrowinning, andiron removal as goethite or hematite (Liddicoat and Dreisinger, 2007).

The benecial effect of low chloride addition for the leaching

Watling, 2006).Despite some detailed studies, reasons for the benecial role of low

chloride in sulde leaching in sulfate systems remain unresolved(Cheng and Lawson,1991a,b; Fisher et al.,1992; Kinnunen andPuhakka,2004; Brown and Papangelakis, 2005; Carneiro and Leao, 2007). Thegeneral view is that during atmospheric leaching in the presence ofsodium chloride in sulfuric acid, a porous and somewhat crystallinesulfur layer is formed on the sulde surface, instead of a tight pas-sivating sulfur layer which blocks the surface (Cheng and Lawson,of coppernickel suldes in sulfate systemby Subramanian and Ferrajuolo (1976), hasinterest in atmospheric and pressure oxidatiosuldes (Lu et al., 2000a,b; Deng et al., 2001;2002; Peacey et al., 2003; Ferron et al., 2002;

Tel.: +61 8 93602833; fax: +61 8 93606343.E-mail address: G.Senanayake@murdoch.edu.au.

0304-386X/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.hydromet.2009.02.010leaching in non-sulfateidely reported (Peters,izac, 1992; Muir, 2002;

in the ACTIVOX process (Johnson and Streltsova, 1999). Low chloridealso shows a benecial role in PLASTOL, BHAS, CESL, and bioleachingprocesses (Ferron et al., 2002; Peacey et al., 2003; Defreyne et al., 2006;systems such as strong brines has been w1977a,b; Winand, 1991; Majima, 1995; Dutr1. Introduction

The thermodynamic and kinetic babenecial effect of chloride ions onof chloride, dissolved oxygen, and hydrogen ions. The rst stage leaching of chalcocite to an intermediate CuSappears to be controlled by the mass transport of oxygen to the sulde surface. A comparison has been madebetween the second stage leaching of chalcocite and the initial leaching of pure covellite to produce elementalsulfur by considering the effect of temperature on dissolved oxygen concentration and apparent rate constants.The Arrhenius plots gave comparable values for activation energy for covellite (101 kJ mol1) and second stageleaching of chalcocite (96 kJmol1). A linear correlation of stability constants was used to determine equilibriumconstants for the formation of CuS2 and Cu(OH)(Cl). The peak potentials of voltammograms of covellite arein reasonable agreement with the predicted potentials based on thermodynamics of a range of solid phasesincludingCuS, CuS2, Cu(OH)Cl, and Cu2Cl(OH)3. These observations are used to propose a reactionmechanismviamixed-ligand complex species.

2009 Elsevier B.V. All rights reserved.

the rationalisation of the

2004; McDonald and Muir, 2007a,b; Carneiro and Leao, 2007). Forexample, low chloride has a benecial effect upon the acid-pressureleaching of copper and nickel from nely ground sulde concentratesCovelliteChalcocite

model to determine the apparent rate constants and reaction orders with respect to the concentrations

Keywords: published results for initial copper leaching from covellite are analysed on the basis of a shrinking particle kineticAccepted 20 February 2009Available online 6 March 2009

on solid surfaces or in solutisulfuric acid in the presence of sodium chloride and comparison with the leaching data of chalcocite. TheA review of chloride assisted copper suldmechanistic considerations

G. Senanayake Parker Centre, Faculty of Minerals and Energy, Murdoch University, Perth, WA 6150, Austra

a b s t r a c ta r t i c l e i n f o

Article history:Received 27 June 2008Received in revised form 19 February 2009

The benecial effect of chlo34 decades but the reasoleaching is complex due to

j ourna l homepage: www.es, previously reportedalso attracted renewednprocesses for a range ofBerezowsky and Trytten,Kinnunen and Puhakka,

ll rights reserved.leaching by oxygenated sulfuric acid and

e on sulde leaching in sulfuric acid has been widely reported over the lasthave not been resolved. A review of recent literature shows that suldernative reaction paths, competitive reactions and interim compounds formedThis study focuses on analysis of rate data for covellite leaching by oxygenated

tallurgy

v ie r.com/ locate /hydromet1991a, Muir and Ho, 2006; Carneiro and Leao, 2007). Elemental sulfurformed during the leaching of chalcopyrite with ferric chloride is alsomore porous than that formedwith ferric sulfate, thus the leaching rateis an order of magnitude larger in the former (Majima et al., 1985).

Peters (1976a) reported that chalcopyrite, the primary and mostimportant sulde mineral of copper, is most difcult of the coppersuldes for hydrometallurgical treatment due to slow leaching kinetics.Recent reviews have highlighted the growing interest and conicting

views amongst researchers to understand the mechanistic details ofleaching of chalcopyrite and secondary copper minerals in order topromote hydrometallurgical routes to extract copper (Dreisinger, 2006;Watling, 2006; Nicol, in press; Klauber, 2008; Cordoba et al., 2008a).Chalcocite and covellite are two of the common copper-bearingminerals also formed as intermediate products during the leaching ofchalcopyrite, copper matte/residue and coppernickel suldes; asshown by some of the reactions relevant to the present study listed inTable 1 (Peters, 1976a,b; Grizo et al., 1982; McDonald and Langer, 1983;Cheng and Lawson, 1991a,b; Fisher et al., 1992; Grewal et al., 1992;Rademan et al.,1999;Muir and Ho, 2006; Deng et al., 2001; Provis et al.,2003; Nicol and Lazaro, 2003; Ruiz et al., 1998, 2007; Dreisinger, 2006;Herreros et al., 2006; Herreros and Vinals, 2007, Cordoba et al., 2008a,b). Thedissolutionofnickel suldes via thedisplacement reactions suchas reactions R6R8 inTable 1 (Rademan et al.,1999;Muir andHo, 2006)is thermodynamicallymore favourablewith large values of equilibriumconstants, compared to the acid dissolution via reactions R4R5. More-

Table 1Chemical reactions related to formation or oxygenated leaching of copper suldes(Some of these reactions were reported by Peters (1976a,b), Muir and Ho (2006);Rademan et al. (1999); Provis et al. (2003); Nicol and Lazaro (2003) (also see text),equilibrium constants based on Outokumpu HSC 6.1 software).

Reaction Equilibrium constant (K)

25 C 90 C

Leaching reactions which involve Cu2S and CuSR1. CuFeS2+2H+=CuS+Fe2++H2S 102.6 102.4

R2. CuFeS2+2H+=0.5Cu2S+Fe2++H2S+0.5S 109.9 108.5

R3. Cu2S+2H++0.5O2=CuS+Cu2++H2O 1026 1020

R4. Ni3S2+2H+=2NiS+Ni2++H2 100.99 100.54

R5. NiS+2H+=Ni2++H2S 102.08 102.16

R6. NiS+Cu2+=CuS+Ni2+ 1014 1011

R7. Ni3S2+2Cu2+=Cu2S+NiS+2Ni2+ 1033 1026

R8. Ni3S4+3Cu2++H2O+1.5O2=3CuS+3Ni2++2H++SO42 10130 10130

Non-oxidative leaching of CuSR9. CuS+2H+=Cu2++H2S 1016.5 1013.8

R10. CuS+H++HSO4=Cu(SO4)0+H2S

Leaching of Cu2S by oxygen (rst stage)R11. Cu2S+0.5O2+H2O=Cu(OH)2+CuS 1020 1015.6

R12. Cu2S+0.5O2+2H+=Cu2++CuS+H2O 1026.4 1020.3

Leaching of CuS by oxygenR13. CuS+0.5O2+H2O=Cu(OH)2+S 1015.5 1011.8

R14. CuS+0.5O2+2H+=Cu2++S+H2O 1020.3 1015.1

R15. CuS+0.5O2+2H++Cl=CuCl++H2O+SR16. CuS+0.5O2+H++HSO4=Cu(SO4)0+S+H2O

Side reactions of CuSR17. 2CuS+2H++2Cl+1.5O2=2CuCl+H2S2O3 1037.2 1030.2

R18. 2CuS+2Cl+2O2=2CuCl+S+SO42 1056.2 1044.5

22 G. Senanayake / Hydrometallurgy 98 (2009) 2132Anion inuenced non-oxidative leaching of CuSR19. CuS+H+ X=Cu(SH)(X)ads/aqR20. Cu(HS)(X)ads/aq+H+ X=CuX2+H2S

Anion inuenced oxidative leaching of CuSR21. CuS+H+ X=Cu(SH)(X)ads/aqR22. Cu(HS)(X)ads/aq+0.5O2=Cu(OH)(X)ads/aq+SR23. Cu(OH)(X)ads/aq+H+ X=CuX2+H2O

Chloride assisted leaching of CuS in sulfate mediumR24. CuS+H+=Cu(SH)+(ads)R25. Cu(SH)+(ads)+Cl=Cu(SH)(Cl)0(ads/aq)R26. Cu(SH)(Cl)0(ads/aq)+0.5O2(ads/aq)=Cu(OH)(Cl)0(ads/aq)+SR27. 2Cu(SH)(Cl)0(ads/aq)=2CuCl(ads/aq)+H2S+SR28. 2CuCl(ads/aq)+0.5O2+H2O=2Cu(OH)Cl(ads/aq)R29. Cu(OH)(Cl)0(ads/aq)+HSO4=Cu(SO4)0+Cl+H2O

Possible side reactionsR30. 2Cu(SH)(Cl)0(ads/aq)+O2=2CuCl(ads/aq)+H2S2O2R31. H2S2O2+O2=H2S2O4R32. H2S2O4=H2SO4+Sover, the copper sulde formed on the surface of millerite via reactionR6 is oxidized more rapidly than the base mineral (Miki and Nicol,2008a). The oxidative dissolution of covellite by oxygen according toreaction R14 is also thermodynamically more favourable (K298 K=1020), compared to non-oxidative acid dissolution according to re-action R9 (K298 K=1016). However, a range of stable or interimproducts can be predicted from the published thermodynamic dataas noted in reactions R10R18, all with favourable equilibriumconstants. For example, the benecial effect of chloride ions on theoxygenated-acid dissolution of copper from synthetic Cu2S parti-cles (Cheng and Lawson, 1991a) and acid dissolution of copperfrom synthetic CuO discs (Majima et al., 1980) has been rationa-lised by considering the formation of an interim adsorbed complexspecies of the formula: Cu(OH)(Cl)0 (Senanayake, 2007b,c, 2008b).

The objectives of this investigation are: (i) to briey review thecomplexity of leaching of copper sulde ores/concentrates andmechanistic considerations, with special attention to the formationof interim compounds and competitive reactions in solid state orsolution; (ii) to present a quantitative analysis of published data for thedissolution of similar sized covellite particles in the presence of sodiumchloride (Cheng and Lawson, 1991b); and (iii) to highlight theimportance of considering a surface reaction which involves mixed-ligand complexes of copper(II) with chloride and hydroxide ions. Theaim is to rationalise the leaching behaviour of complex or mixedsuldes and the selection of conditions for accelerated leaching toproduce sulfur and iron oxide products.

2. Complexity of mechanistic studies

Results based on the dissolution of sulphide particles in batch leach-ing experiments can be successfully used to investigate the rate con-trolling steps and mechanistic details of leaching of metals/minerals,especially if a comparison can be made between the rates based onat surfaces and particles (Parker et al., 1981). However, a quantitativeanalysis of leaching kinetics of complex/mixed suldes and nickelcopper mattes presents a number of challenges due to the complexityof the dissolution mechanism caused by a range of competing factorsdepending on mineralogy:

(i) alternative pathways of non-oxidative and/or oxidative leach-ing behaviour of different suldes in acid media,

(ii) possible involvement of more than one redox couple as oxidants,such as Fe(III)/Fe(II), Cu(II)/Cu(I), O2/H2O, O2/H2O2, and H2O2/H2O,

(iii) formation of elemental sulfur, thiosulfate, sulte, and sulfate asthe interim/nal oxidation product(s) of sulde depending onthe conditions,

(iv) difculty in detection/characterisation of some solution/sur-face species or interim solid metal-sulfur phases,

(v) galvanic interactions caused by host minerals,(vi) passivation or blockage of mineral surface by insoluble products,(vii) benecial or detrimental effects caused by background reagents.

Thus, the rate analysis based on individual suldes allows a betterunderstanding of the mechanism of surface reactions correspondingto dissolution, passivation, and de-passivation. Some examples follow.

Hirato et al. (1989) conducted a comparative study of chemicaland electrochemical dissolution of copper from sintered CuS discsin acidied iron(III) chloride solutions. The half order reaction withrespect to Fe(III) concentration supported an electrochemical mecha-nism for the dissolution. Nicol (in press) noted the importance ofconsidering the formation of sulfur during leaching via two differentmechanisms for a range of base metal suldes. Firstly, direct oxidationby an oxidant such as Fe(III) leads to the precipitation of sulfur onthe mineral surface which may block the surface. Secondly, non-oxidative acid dissolution of sulde and indirect oxidation of hydro-

gen sulde by Fe(III) in solution leads to the precipitation of sulfur

in the bulk. The rate of dissolution of a sulde mineral MS underacidic conditions can be estimated by considering the equilibriumconstants for the dissolution of MS to M(II) and H2S. Here, the ratecontrolling step is the rate of mass transfer of products such as M(II)and H2S away from surface, shifting the dissolution equilibrium in theforward direction with time (Nicol and Lazaro, 2003; Nicol, in press).

3. Effect of chloride on redox potentials and leach results

The possibility of involvement of more than one redox couple asoxidant(s) is evident from Fig. 1a where the potentials in oxygenated

chloride systems follow the descending order O2/H2ONFe(III)/Fe(II)NCu(II)/Cu(I). It is widely accepted that in such systems with chloro-complexes the redox couples Cu(II)/Cu(I) (fast) or Fe(III)/Fe(II)(slow) can oxidize the sulde mineral or hydrogen sulde, whilst theoxidation of Cu(I) to Cu(II) by Fe(III) or dissolved oxygen reproducesthe oxidant for leaching (Parker et al., 1981; Nicol and Lazaro, 2003;Nicol, in press; Miki and Nicol, 2008a). Lee et al. (2008) showed that inchloride medium the cathodic current for the reduction of Cu(II) ona covellite surface is larger than that for the reduction of dissolvedoxygen, which results in faster dissolution of covellite in the presenceof copper(II). The rapid equilibration Cu(II)+Fe(II)=Cu(I)+Fe(III) and

and

23G. Senanayake / Hydrometallurgy 98 (2009) 2132Fig. 1. (a) Potential-log[Cl] diagrams for ironcopperchloridewater system at 25 C

chalcocite and covellite leaching by oxygen at 25 C and 90 C and unit activities based on0.1 mol dm3 metal ions (Senanayake and Muir, 2003); (b) potential-pH diagram for+Outokumpu HSC 6.1 software (dashed lines represents O2/H2O and H2/H couples).

the rapid oxidation of Cu(I) to Cu(II) by dissolved oxygen rationalises thecatalytic effect of Cu(II) on the oxidation of Fe(II) (Miki and Nicol,2008b).

The leaching of chalcocite/covellite by oxygen represented by thepotential-pH diagram in Fig. 1b shows the oxidation of Cu2S via CuS toproduce elemental sulfur and CuCl+, Cu2Cl(OH)3, or Cu(OH)2/CuOdepending on pH at 2590 C. Fig. 1b does not show the thermo-dynamically unstable copper(I) species. It also does not consider thespecies such as Cu(OH)2 and Cu(OH)n(n-2) (n=14) which areformed in solutions of high pH in the absence of other complexingmedia (Beverskog and Puigdomenech, 1997). Nevertheless, the rapidoxidation of Cu(I) by dissolved oxygen produces CuCl+ which isconverted to Cu(SO4)0 in a predominantly sulfate system. Thus, therationalisation of initial leaching of synthetic covellite by oxygen inironcopper-free sulfate solutions would be less challenging than thatof complex/mixed suldes and would produce more comprehensiveinformation on the role of chloride ions during the oxidation ofcovellite by oxygen in a sulfate background.

Fig. 2a shows the results of atmospheric leaching of copper fromsynthetic covellite by oxygen after 1 or 6 h (Cheng and Lawson,1991b).The benecial effect of chloride addition increases up to a concentra-tion of 0.5 mol dm3 NaCl where the copper extraction exceeds 80%after 6 h. According to Fig. 2b, the copper leaching from synthetic

24 G. Senanayake / Hydrometallurgy 98 (2009) 2132chalcocite by oxygen is faster than that of covellite and reaches about70% after 20 min (Cheng and Lawson, 1991a). The reaction orderwith respect to oxygen pressure was found to be 0.3 in the case ofcovellite in the presence of sodium chloride (Cheng and Lawson,1991b). Although no physical signicance for this order was given, thechloride ions were considered to promote the porosity of the sulfurlayer formed on the covellite surface. However, the rationalisation ofkinetics of chalcopyrite poses more challenges with regard to thedifferent reaction pathways that may produce different product layersresponsible for slow leaching. Further studies are essential to compare,contrast, rationalise, and quantify the benecial effect of chloride ionsat a low concentration in a sulfate system.

4. Interim copper compounds

An understanding of the nature of interim compounds and com-petitive reactions at the surface of sulde minerals or in solutionplays an important role in successful elucidation of reaction mecha-nisms. Different types of interim compounds in the solid state have

Fig. 2. Effect of sodium chloride on copper leaching from (a) covellite by oxygen, 13 m,90 C, 0.5 mol dm3 H2SO4; (b) chalcocite by oxygen, 31 m, 85 C, 0.5 mol dm3 H2SO4

(data from Cheng and Lawson, 1991a,b).been considered in the mechanistic interpretation of copper suldeleaching. They are suldes, oxides, and chlorides or mixed chloro-hydroxides. In addition, copper(I) chlorides and a number of interimsulfur species in solution have also been reported.

4.1. Copper sulde reactions

According to the potential-pH diagram of CuFeSOH2O system theacid dissolution of CuFeS2 takes place via intermediate suldes such asCu5FeS4, CuS, and Cu2S (Peters,1976b; Cordoba et al., 2008a). The reactionmechanisms of dissolution/passivation of secondary copper suldes arerelatively simple compared to that of chalcopyrite, but involves a numberof interim species. Despite the ionic formula of Cu2S2 reported in theliterature, it has been general practice, for simplicity, to use the formulaCuS in chemical reactions to represent covellite (Fisher et al., 1992; Gohet al., 2006). Koch and McIntyre (1976) identied four intermediatephases: Cu1.951.91S, Cu1.861.80S, Cu1.681.65S, and Cu1.401.36S formed duringelectrochemical oxidation of covellite. These phases have been charac-terised by XRD and in-situ treatment of reectance spectra. The oxidativedissolution of chalcocite by Fe(III), Cu(II), or oxygen is amultistageprocesswhich involves the initial fast oxidation via djurleite (Cu1.93S) and digenite(Cu1.8S) to blue-remaining covellite (Cu1.01.2S), followed by the slowoxidation of Cu1.01.2S (Sullivan, 1933; Mao and Peters, 1983; McDonaldandLanger,1983;Rademanet al.,1999, ChengandLawson,1991a,b; Fisheret al., 1992). The non-oxidative acid decomposition of chalcopyriteproduces chalcocite and covellite (Peters, 1976a,b), while the chemicalmetatheses of chalcopyrite in the presence of Cu(II) ions producescovellite and digenite (Sequeira et al., 2008).

Gerlach and Kuzeci (1983) conducted an electrochemical studyof covellite and the results were rationalised on the basis of the for-mation of non-stoichiometric compounds, solid state diffusion ofcopper ions, and chemical dissolution. Bertram and Hillrichs (1981),and Hillrichs and Bertram (1983a,b) interpreted currentvoltagecurves for the anodic dissolution of synthetic/natural chalcocite andcovellite discs in sulfuric acid solutions on the basis of the formationof metastable non-stoichiometric copper oxide or hydroxides. Basedon cyclic voltammograms of CuS in sulfuric acid medium, Hillrichsand Bertram (1983a) considered the formation of intermediate oxides:CuSCuO0.67(brown)CuO(black). Although the formation of theseoxides stopped the total anodic decomposition of CuS, subsequentdecomposition was evident from hindrance of Cu2+ ux by adherentsulfur on the anode surface at higher potentials. The addition of surfaceactive anions such as Cl, ClO4, and NO3 caused a further activationof CuS, indicating the importance of considering the role of anions.Miki and Nicol (2008a) reported linear sweep voltammograms ofsynthetic covellite in a mixed H2SO4/NaCl systemwhich showed thatthe active dissolution at low potentials was followed by passivation atabout 0.6 V (SHE), whilst the anodic currents increased as the chlorideconcentration was increased from 0.2 mol dm3 to 2 mol dm3.The potential-pH diagram in Fig. 1b predicts the dissolution of CuS toform CuCl+ aswell as the passivation due to the formation of insolubleS or Cu2Cl(OH)3 at a potential close to 0.6 V at 25 C and 90 C.

4.2. Interim chlorides and mixed chloro-hydroxides

Fisher et al. (1992) considered the formation of copper(I) chlorideand sulfate ions in the anodic half reaction of covellite. In oxygenatedchloride solutions, the rate of copper extraction from a chalcociteconcentrate (at 70 C) was 70 times faster than that in oxygenatedsulfate systems. This was related to the formation of CuCl32 in theanodic reaction for dissolution at both stages (Cu2S and CuS). It wasnoted that dissolved oxygen was reduced to water via hydrogen per-oxide but sulde was converted to sulfate in the second stage (Fisheret al., 1992). However, equations R34 and R35 in Table 2 representthe reaction steps and widely reported anodic and cathodic half cell

reactions. These reactions lead to the formation of Cu(II) ions and

25G. Senanayake / Hydrometallurgy 98 (2009) 2132elemental sulfur as major products according to the thermodynami-cally favourable overall reaction R14 in Table 1.

Cheng and Lawson (1991b) noted that the formation of copper(I)chlorides such as CuCl in sulfuric acid media in the presence of

Table 2Electrochemical reactions and standard reduction potentials at 25 C.

Reaction Eo (V) at 25 Ca

Stepwise anodic reactionsR33. Cu2S=CuS+Cu2++2e 0.491R34. CuS=Cu2++S+2e 0.631

Cathodic reactionsR35. 0.5O2+2H++2e=H2O 1.230

Surface blockage by hydroxideR36. Cu2S+2H2O=Cu(OH)2(ads)+2H++CuS+2e 0.680R37. CuS+2H2O=Cu(OH)2(ads)+S+2H++2e 0.820

Surface activation by anionsR38. Cu2S+X=(CuX)2+(ads)+CuS+2e

(X=H2O, SO42, Cl, ClO4, NO3)R39. CuS+Cl=CuCl++S+2e 0.619

Anodic reactions involving disulde and chloride speciesR40. 2H2S=4H++S22+2e 0.700R41. 2CuS=2Cu2++S22+2e 1.675R42. CuS+2Cl=CuCl2+S+2e 0.792R43. CuS2=Cu2++2S+2e 0.715 (0.410 for NiS2)b

R44. 2CuS+3H2O+Cl=Cu2Cl(OH)3+2S+3H++4e 0.742R45. CuS+Cl+H2O=Cu(OH)Cl+S+H++2e 0.622R46. 2CuS=CuS2+Cu2++2e 0.525 (0 V for NiS2)b

R47. CuFeS2=CuS2+Fe2++2e 0.134

Mechanism for chloride assisted dissolution of CuSR48. CuS+H++Cl=Cu(SH)ClR49. Cu(SH)Cl+H2O=Cu(OH)Cl+S+2H++2e

R50. Cu(OH)Cl+HSO4=Cu(SO4)0+H2O+Cl

a Data from HSC 6.1 data base or calculated in the present study (see text).b Corresponding values for couples involving NiS, NiS2, and Ni2+.2

sodium chloride is only possible at higher pulp densities, where thereis inadequate mass transfer into the liquid phase, causing oxygendeciency during leaching. Nicol (in press) proposed that the forma-tion of Cu(I) and elemental sulfur in oxygenated chloride solutionsis a result of the reaction between Cu(II) and hydrogen sulde.Peters (1976b), McDonald and Langer (1983), Cheng and Lawson(1991a), and Muir (2002) reported the formation of basic copperchloride Cu2(OH)3Cl, i.e. Cu(OH)(Cl).Cu(OH)2. A potential-pH dia-gram which shows the predominant areas of CuCl+ and Cu(OH)Clspecies in solutions of pHb7 was published by Sabba and Akretche(2006). The precipitation of Cu2(OH)3Cl has been observed at pH 4in leaching experiments with covellite conducted with small mono-sized particles (13 m) at low initial sulfuric acid concentrations of5.103 mol dm3. The copper extraction increased in the rst 3 h,but then decreased as a result of an increase in pH to 4. Thus, Chengand Lawson (1991b) proposed the overall reactionwhich involved theformation of CuCl+ (R15) in acid solutions at low pH.

5. Competitive reactions and sulfur products

Base metal sulde minerals are likely to undergo oxidation withoxygen by two different reaction pathways via elemental sulfur orthiosulfate depending on the conditions such as ammoniacal leaching,or acid leaching at atmospheric or elevated pressures (Muir et al.,1981; McDonald and Muir, 2007a). Rotating ring-disc electrochemicalstudies with nickel matte (Ho, 1996) and chalcopyrite (Lazaro andNicol, 2006) have revealed the formation of interim sulfur-oxy speciessuch as thiosulfate. Under some leaching conditions, NiS2 and FeS2produce sulfate ions rather than elemental sulfur, while chloride ionsappear tomodify the passivating sulfur or inhibit the formation of NiS2layers (Muir and Ho, 2006).Quantitative analysis of sulfur products of covellite oxidation byoxygen in mixed systems such as H2SO4NaCl with a low concen-tration of NaCl revealed that the majority of sulde was oxidized toelemental sulfur by oxygen (Cheng and Lawson, 1991b). A reactionscheme based on the formation of intermediates H2S2O2 and H2SO3has been proposed to account for the quantitative yields of sulfateions and elemental sulfur during sulde leaching (Lotens andWesker, 1987). The oxidation products of suldes would dependupon whether: a one-electron transfer takes place by oxidants suchas Cu(II) or Fe(III), or a two-electron transfer takes place by oxi-dants such as chlorine or hydrogen peroxide at the solid/liquidinterface. The reaction via 2H2S2O2=3S+H2SO3+H2O would yield75% elemental sulfur whilst the reaction 2H2SO2=S+H2SO3+H2Owould yield 50% elemental sulfur. Moreover, the increase in yield ofelemental sulfur in acidic solutions of low pH is a result of theformation of H2S due to acid dissolution of a metal sulde, followedby the reaction H2S+H2S2O2=3S+2H2O (Lotens and Wesker,1987). Thus, sulfate formation has been ignored in the kineticanalysis of sulde oxidation. Nevertheless, the nal copper(II)product of chalcocite/covellite oxidation in oxygenated sulfuric acidsolutions with low sodium chloride is the thermodynamically stableion-pair Cu(SO4)0 shown in reaction R16 (Table 1).

Miki and Nicol (2008a) reported the similarities of voltammetricpeak potentials and the potentiometric current transients of covelliteand chalcopyrite. The onset of the transpassive region during chalco-pyrite oxidation has been considered as an indication of the formationof covellite (Viramontes-Gamboa et al., 2007). The leaching results havealso demonstrated that chalcopyrite dissolves through the formation ofcovellite and subsequent oxidation of covellite in iron(III) sulfate solu-tions (Cordoba et al., 2008b). These facts support the dissolutionmechanismof chalcopyrite via reaction R1 in Table 1 proposed by Peters(1976b) and Nicol (in press) and justify the extension of the presentdiscussion on the basis of electrochemical activepassive behaviour ofcovellite to discuss reasons for passivation in sulfate media, and non-passivating role of chloride in sulfate systems.

Muir and Ho (2006) described the benecial effect of chloride innickel sulde leaching as a result of inhibition of the formation of apassivating layer of NiS2. By analogywith reactions of NiS, it would seemthat passivation of CuS due to formation of CuS2 is inhibited by chloridevia an electrochemical mechanism (Muir, 2009). The Raman spectro-scopic studies by Parker et al. (2003) also provide some evidence forthe disruption of the formation of disulde type layer on chalcopyritesurface in the presence of chloride. However, subsequent studies byParker et al. (2008a,b) using surface enhanced Raman scattering ofchalcopyrite surface showed similar oxidation products in both hydro-chloric and sulfuric acid solutions. The surface specieswith S\S bondingin transpassive region were not characteristic of polysulde or poly-thionates. The formation of oxidationproducts with S\S bonds at lowerpotentials in sulfuric acid, than that in hydrochloric acid, appeared to bea result of themechanistic or ionic strength differences in the sulfate andchloride systems (Parker et al., 2008b).

Based on electrochemical and XPS studies, the dissolution/passiva-tion products formed during electrochemical oxidation of chalcopyritein 1 mol dm3 perchloric acid were identied as Fe2+ and metastableCuS2 at lower potentials (~0.74 V) and elemental sulfur at higher po-tentials (N1.24 V) (Yin et al., 1995). Buckley and Woods (1984)considered the depletion of iron and conversion of chalcopyrite to(i) CuS2 after exposure to air for several weeks, or (ii) Cu0.8S2 afterconditioning in acid solutions. The structure of the metastable phaseCuS2 formed on the surface (passive lm) has not been identied, butreported to be different from that of pyrite. A wide variety ofexperimental techniques have shown that CuS2 has a monovalent(d10) rather than a divalent (d9) oxidation state of copper (Ueda et al.,2002). If the chalcopyrite structure is represented by an ionic bondingmodel (Cu+)(Fe3+)(S2)2 (Crundwell, 1988; Mikhlin et al., 2004), the

+ 3+formationof a passivelmaccording to the anodic reaction (Cu )(Fe )

(S2)2=CuS2+Fe2++2e can be considered with a possible changein oxidation states (Cu+)(S2) proposed by Yin et al. (1995). Anotherpossibility is the redox or precipitation reaction to produce CuS2 on thesurface, from ions in solution: Cu2++S22=(Cu+)(S2), where thedisulde (S22) can be formed by the oxidation of H2S formed by non-oxidative dissolution (reaction R40 in Table 2).

Hackl (1995) described chalcopyrite dissolution as a two stagereaction: (i) formation of intermediate disulde Cu1-xFe1-yS2 (y xand x+y ~1) during pressure oxidation due to preferential initialleaching of iron, and (ii) slow dissolution of copper from thedisulde producing a copper polysulde (Cu1-x-zS2) causing passiva-tion. Klauber et al. (2001) ruled out the formation of polysulde CuSn(nN2) and noted the possibility of formation of only a few atomiclayers of the non-stoichiometric Cu0.8S2. Thus, it is important toestimate the formation constants of CuS2 to improve the under-standing of passivation reaction(s) of copper suldes.

6. Formation constants of interim copper species

6.1. CuS2 Species

Table 3 lists equilibrium constants for the formation of various ionicor neutral complex species and insoluble salts of Cu(II) and Fe(II) forcomparison. The approximate linear correlation of formation constantsof Cu(II) and Fe(II) complexes in Fig. 3b can be used to predict theformation constants at 25 C: Cu(HS)20 (~108), CuS2 (~1039). The valuesshown inbracketswere calculatedby substituting the reportedvalues forFe(HS)2 (102.2) and FeS2 (1026) fromYounget al. (2003) andHSC6.1 data

6.2. Cu(OH)(Cl) species

If oxygen is adsorbed on the surface, as noted by Fisher et al.(1992), the direct oxidation represented by reaction R14 willproduce sulfur on the sulde mineral surface. Other reactions inTable 1 represent the non-oxidative or oxidative dissolution ofcovellite and the proposed role of the common anion X. Forexample, the formation of ion-pairs HSO4 and Cu(SO4)0 is taken intoaccount in reactions R10 and R16, while the formation and reactionof interim mixed complexes Cu(SH)(X) and Cu(OH)(X) is consideredin reactions R21R27. McDonald and Langer (1983) considered theformation of Cu(OH)(Cl)x(1-x) during the leaching of copper suldesand estimated the formation constants 1010.5 (25 C) and 109.1

(105 C). Fig. 3a shows a linear correlation of the logarithmic values ofequilibrium constants listed in Table 3 for Cu(II) and Pb(II). Anequilibrium constant of 1013.7 has been reported for the formation ofPb(OH)(Cl)s (Wagman et al., 1982). This value can be used in theequation for the linear correlation in Fig. 3a to predict the relevantequilibrium constant (1014.2) for the formation of Cu(OH)(Cl)s at 25 C.The calculated concentration of Cu2+ in 0.5 mol dm3 NaCl solution atpH 4 is 9.5.104 mol dm3. This explains the precipitation of copper atlow acidities as observed by Cheng and Lawson (1991a).

It can be seen from Table 3 and Fig. 3a that the magnitudes of2+

26 G. Senanayake / Hydrometallurgy 98 (2009) 2132base in the mathematical expression representing an approximatelylinear correlation in Fig. 3b. This allows the calculation of approximatevalues of standard reductionpotentials for reactions involvingCuS2 listedin Table 2 (R43, R46, R47). The lower values of E for the correspondingnickel couples listed inTable 2 indicate the passivation of NiS byNiS2 at alower potential. Potential-pH diagrams for NiSH2O system whichshow large stability region of NiS2 in the potential range 00.25 V inacidic media, and that of Ni2+ at higher potentials were published byMuir and Ho (2006) and Senaputra et al. (2008).

Table 3Equilibrium constants.

Reaction a a b b

Pb(II) Cu(II) Fe(II) Cu(II)

M2++S2O32=M(S2O3)0 102.42 102.4 102.00 102.4

M2++2S2O32=M(S2O3)22 104.86 105.4 M2++CO32=M(CO3)0 107 106.77 105.69 106.5

M2++2CO32=M(CO3)22 109 1010 M2++OH=M(OH)+ 106.9 106.27 104.67 (104.70) 105.98 (105.86)M2++2OH=M(OH)20 1010.8 1012.5 M2++Cl=M(Cl)+ 101.6 100.6 M2++2Cl=M(Cl)20 101.8 100.7 M2++SO42=M(SO4)0 102.75 102.36 M2++NO3=M(NO3)+ 101.2 100.5 M2++2NO3=M(NO3)20 101.4 100.4 M2++2OH=M(OH)2(s) 1019.9 1019.8 1015.0 (1014.0) 1021.6 (1020.0)M2++C2O42=M(C2O4)(s) 109.1 109.6 M2++CO32=M(CO3)(s) 107 106.77 108.27 109.85

M2++H2O=MO(s)+2H+ 1014.5 (1011.1) 107.71 (105.70)M2++SO42=M(SO4)(s) 101.93 (100.70) 103.0 (100.38)M2++S2=MS(s) 1016.9 (1015.8) 1036.4 (1031.8)M2++3HS=M(HS)3 102.71c 106.23c

M2++2HS=M(HS)20 102.2c 108d

M2++S22=MS2(s) 1026 1039d

aAt 25 C, from Sillen and Martell (1964), Hogfeldt (1982).bAt 25 C, from HSC 6.1 database, values in parentheses are for 90 C; inconsistency ofvalues in columns a and b for Cu(II) from different references is due to differences inionic strength and/or methods of measurements.cFrom Young et al. (2003).dPredicted values in this work based on Fig. 3b.Cyclic voltammetric investigations of the dissolution of covellitein 1 mol dm3 H2SO4 at 20 C have shown that the decomposition ofCuS starts at 0.56 V (Hillrichs and Bertram, 1983a,b). This isconsistent with E=0.53 V for reaction R46 in Table 2. The peakpotential of 0.72 V of the cyclic voltammogram of CuS in sulfuric acidmedia is also in excellent agreement with the calculated E=0.72 Vfor reaction R43. However, visual evidence for sulfur formation wasfound after anodization at amuch higher potential of 2.35 V (Hillrichsand Bertram, 1983a,b). In contrast, the current-potential curve of NiSin 0.5 mol dm3 H2SO4 showed a peak at a lower potential of ~1.34 V(Muir and Ho, 2006). Moreover, anodic oxidation of NiS produced S,NiS2 and S2O32 at a potential of ~1 V inmixed sulfate/chloridemedia.These differences warrant further electrochemical studies anddiscussion beyond the scope of the present study. Here, the mainfocus is the benecial role and the de-passivation effect of chlorideions during oxidation of covellite by oxygen.

Fig. 3. Linear correlation between equilibrium constants for complexation andprecipitation of M(II): (a) Cu(II)Pb(II) at 25 C; (b) Cu(II)Fe(II) at 25 C and 90 C(data from Table 3, Young et al., 2003 and HSC 6.1 database).formation constants of complex species or solids from Cu ions are

surface area due to the decrease in initial particle radius ro (m) withtime t (s) (Levenspiel, 1972). The terms (mol m3) and kss (s1)represent the molar concentration of reacting metal in solid B and theapparent rate constant, respectively, while X represents the fractionof metal leached after time t. The term k (m s1) in Eq. (1) representsthe intrinsic rate constant of the surface reaction, which has the sameunits as the mass transfer coefcient.

Fig. 5 shows the applicability of Eq. (1) for the dissolution ofcovellite during initial leaching over 2 h at different oxygen partialpressures. Similar plots were used to determine the apparent rateconstants listed in Table 4 from the slopes of linear relations, such as inFig. 5. The increase in rate constant with increase in temperature from75 C to 95 C obeyed the Arrhenius equation k=A e(E/RT) with an

Fig. 5. Effect of oxygen mol% in the bubbling gas on 1-(1-X)1/3 during covellite leaching(D1D4 described in Table 4, data from Cheng and Lawson, 1991b).

F1 90 13 0.005 0.5 100 2F2 0.02 3F3 0.50 4

27G. Senanayake / Hydrometallurgy 98 (2009) 2132comparable to those from Pb2+ ions. This may be related to the com-parable softness of Cu2+ and Pb2+ (Fig. 4). Softness is a property relatedto the polarizability of ions (Marcus, 1997). According to the conceptof soft and hard acids and bases (Pearson, 1973), soft ions preferforming complexes with soft ligands. It is reasonable to suggest, on thebasis of the apparent descendingorder of softness S2NHSNOHNCl

in Fig. 4, that the thermodynamically favourable oxidative dissolutionprocess of CuS to Cu(SO4)0 by oxygen in acidic sulfate media wouldinvolve the three step transformation:

CuSYCu SH Cl YCu OH Cl = SYCu SO4 0 = Si ii iii

as described in reactions R24, R25, R26, and R29, as a result of (i)protonation and adsorption of Cl, (ii) reaction with O2, and (iii)reaction with HSO4.

A comparison of rate data for the dissolution of covellite in theabsence or presence of chloride ions also supports the formation ofCu(OH)(Cl) as an interim species. A low rate was observed forchloride-free leaching of covellite in 0.5 mol dm3 HNO3 and0.25 mol dm3 H2SO4 solutions at 90 C under an oxygen pressureof 101.3 kPa, where b10% copper was leached after 6 h. However, theleaching was enhanced by the addition 0.5 mol dm3 NaCl to bothacids. Leaching at 101.3 kPa oxygen pressure in the two cases: (i)0.5 mol dm3 HNO3+0.5 mol dm3 NaCl, and (ii) 0.25 mol dm3

H2SO4+0.5 mol dm3 NaCl showed identical leaching curves andN80% copper extraction after 6 h (Cheng and Lawson, 1991b). Theseresults were also identical to those obtained with 0.5 mol dm3 HCl,indicating that the benecial role of chloride ions is due to theinvolvement of the anion X as in reactions R21R23, and thestronger complexation when X=Cl, compared to X=HSO4.Further support for the benecial role of chloride ions in the surfacereaction is evident from rate analyses based on heterogeneous kineticmodels described in Section 7.

7. Rate data and kinetic models for covellite

Cheng and Lawson (1991b) reported the effect of temperature (75

Fig. 4. Softness of cations and anions (Marcus, 1997).95 C), oxygen partial pressure (5.1101.3 kPa), particle size (1356 m), and concentration of sulfuric acid (0.0052 mol dm3) andsodium chloride (0.012 mol dm3) on the leaching of mono-sizedcovellite particles at a solid/liquid ratio of 1 g dm3, and an agitationspeed of 1000 rpm in batch leaching. The rate data for dissolutionduring rst 2 h obeyed a non-porous shrinking particle (sphere)kinetic model described by Eq. (1).

1 1XB 1=3 =bkBCAro

t = ksst 1

Thismodel considers: (i) a surface reaction of a solidBwith a reactantA of concentration CA (mol m3) in solution at a stoichiometric molarratio of B/A=b/1, A(aq)+bB(s) products, and (ii) a decrease inactivation energy of E=77 kJ mol1 (Cheng and Lawson, 1991b).According to Eq. (1), a decrease in particle size from 58 m to 28 mis expected to increase the value of kss by a factor of 58/28=2.1/1. Thisis consistent with the observed ratio 2/0.9=2.2/1 of kss values of testsG1 and G2 in Table 4. Thus, all the results indicate surface reactioncontrolled dissolution kinetics of covellite over the initial period of 1or 2 h.

Table 4Test conditions used by Cheng and Lawson (1991a,b).

Test T Size [H2SO4] NaCl [O2] [O2] 105 kss(C) (m) (mol dm3) (mol dm3) (%) (mol m3) (s1)

Data for covelliteD1 90 13 0.5 0.5 5 0.039 1D2 21 0.164 2D3 59 0.461 3D4 100 0.782 4E1 90 13 0.5 0.1 100 0.7E2 0.25 2E3 0.5 3E4 2.0 4F4 2.0 4G1 85 58 0.5 0.5 100 0.9G2 28 2

Data for chalcociteA1 85 23 0.5 0.5 5 2.5A2 21 9.5A3 39 18.7A4 59 31.8A5 81 30.7A6 100 42.7B1 85 31 0.5 0.02 100 3.7B2 0.10 16.2B3 0.25 32.3B4 0.50 36.5B5 2.0 39.0

1000 rpm, kss values are based on slopes of linear relationships as in Fig. 5 (see text).

Other forms of Eq. (1) have also been employed by researchersdepending on the minerals and reagents that affected the surfacereaction. For example, copper leaching from chalcopyrite particles byoxygenated acid solutions has been rationalised on the basis of Eq. (2).In such cases empirical values of K, bBkB have been useful in predictingleaching performance under different conditions (Lin and Sohn,1987).The half order reaction with respect to H+ and O2 shows the elec-trochemical nature of the surface reaction. Similar analysis of apparentrate constants of covellite leaching is essential for the comparison ofmeasured leaching rates with mass transport calculations.

1=3 +h i0:5 O2 0:5

ferric sulfate in non-chloride solutions. Bertram and Hillrichs (1981)noted that the adsorption of metastable copper oxides (hydroxides) aswell as a product layer of S/CuO (Fig. 1b) on the sulde surface willhinder the copper ion ux during anodic oxidation. It is expected thatthe benecial effect of chloride ion is a result of avoiding such productlayers or passivating layers of CuS2 being formed on the sulde surface.Thus, it is important to consider the effect of the actual concentration ofdissolved oxygen, hydrogen ions, and chloride ions on the apparent rateconstants, which would lead to reaction orders and activation energyand shed more light on reaction mechanism.

9. Reaction orders and activation energy of covellite leaching

Fig. 8a shows a logarithmic plot of kss as a function of the con-centration of variables such as hydrogen ions, chloride ions, or dissolved

Fig. 7. Effect of temperature on concentration of dissolved oxygen in water (Tromans,1998).

Fig. 8. Loglog plots of kss versus reagent concentration (a) covellite: Y=O2 (D1D4), +

28 G. Senanayake / Hydrometallurgy 98 (2009) 21321 1XB = bBkB H 1 + K O2 t 2

8. Factors affecting apparent rate constants

A proper analysis of kss values for covellite leaching as a functionof different variables can be complicated due to a number of reasons.Firstly, the change in hydrogen ion concentration in bulk according tothe sulfate/bisulfate equilibrium and the sodium ion concentrationhas to be taken into account by considering the relevant equilibriumconstants at elevated temperatures. Fig. 6 shows the change in actualconcentration of hydrogen ions at 90 C with increase in acid con-centration, while the concentration of sulfate ions remains negligiblysmall.

Secondly, the dependence of dissolved oxygen concentration ontemperature and background electrolytes has to be considered on thebasis of empirical relationships proposed by Tromans (1998). Theconcentration of dissolved oxygen increases with increasing oxygenmole fraction (Table 4), but decreases with increasing concentrationof sodium chloride and sulfuric acid. Fig. 7 shows the effect of tem-perature on oxygen solubility in water based on the equations pro-posed by Tromans (1998). The effect of temperature on the intrinsicrate constant for the dissolution of B based on the Arrhenius equationcan be combined with Eq. (1) to derive Eq. (3). The superscript a inEq. (3) represents the reaction order with respect to the concentrationof reagent A. Thus, the activation energy, E (kJmol1), based on Eq. (4)is more reliable and suitable for comparison.

1 1XB 1=3 =b AVeE =RT

kB CA a

ro

0@

1At = ksst 3

lnkssCA a

= ln

bAVro

E 10

3

RT

!4

Thirdly, the formation of elemental sulfur or other products mayblock the surface/pores. This would change the kinetic behaviour to thatdescribedbyashrinkingcoremodel:13(1-X)2/3+2(1-X)=6CDro21t,where D=diffusion coefcient of reactant or product (m2 s1) through aporous solid on the surface. For example, Carneiro and Leao (2007)showed the validity of this model for the leaching of chalcopyrite by

Fig. 6. Effect of sulfuric acid concentration on species distribution in 0.5 mol dm3NaCl+H2SO4 at 90 C (Thermodynamic data from Hogfeldt, 1982).oxygen, when the concentration of acid, salt, or oxygen partial pressurewas changed in the case of covellite leaching. The slopes of the linearrelationships conrm a reaction order of ~0.5 and ~1with respect to theconcentration of dissolved oxygen and chloride ions, respectively. Thereaction orders with respect to hydrogen and chloride ions areapproximately zero at higher concentrations. Although the presence ofchloride ions is essential in order to increase the rate of dissolution ofcovellite by dissolved oxygen in sulfuric acid media at 90 C, concentra-tions of sodium chloride above 0.5 mol dm3 do not signicantlyY=Cl (E1E4), Y=H (F1F4); (b) chalcocite; data from Table 4.

The rst order dependence of kss on chloride ion concentration (at[Cl] b0.5mol dm3) canbeused todetermine the value of kB in Eq. (1).The y-intercept of line (e) corresponding toY=Cl in Fig. 8a represents:

logbkBro

= 6:96 9

Table 5Calculated mass transfer coefcients of Y to particles suspended in water.

Particle Size(m)

D(m2s1)a

(m2s1)b

Gr Sc Sh km(m s1)c

CuS 13 6109 2.5107 0.27 42 3.47 2103(at 85 C)

a Based on StokesEinstein equation (Cussler, 1997) at 85 C.b Haywood, 1990.c Eqs. (5)(8).

29G. Senanayake / Hydrometallurgy 98 (2009) 2132inuence the rate. This is consistent with many examples reported inthe literature, where an increase in background chloride beyond 520 gdm3 in sulfate systems did not show a signicant inuence on theleaching kinetics and the nal copper extraction (Subramanian andFerrajuolo,1976; Lu et al., 2000a,b; Deng et al., 2001; Ferron et al., 2002,Kinnunen and Puhakka, 2004; Carneiro and Leao, 2007; Herreros andVinals, 2007). The results for the rst stage leaching of chalcocitedescribed previously (Cheng and Lawson, 1991a, Senanayake, 2007c,2008b) are shown in Fig. 8b for later comparison in Section 10.

Cheng and Lawson (1991a) showed that the second stage leachingof chalcocite also obeys a shrinking sphere kinetic model. Fig. 9 plotsln{kss/(CO2)0.5} as a function of (103/RT) according to Eq. (4). Thesecond stage leaching of chalcocite and the rst stage (initial) leachingof covellite show good linear relationships with y-intercepts close to29 and slopes corresponding to activation energies of 96 and 101 kJmol1, respectively. These activation energies are higher than thevalues of 69 and 77 kJmol1 reported by Cheng and Lawson (1991a,b),but they support the two stage leaching mechanism of chalcocite, viacovellite, and conrm that covellite leaching is controlled by a surfacechemical reaction. Further support for the benecial role of chlorideions on the surface reaction is evident from the mass transfer cal-culations described below.

10. Intrinsic rate constant and mass transfer coefcient

Previous researchers showed the importance of comparing ex-perimental intrinsic rate constants or rates per unit surface area withthe estimated values based on mass transfer coefcients, in orderto examine the chemical or mass transport controlled nature of thesurface reaction (McCarthy et al., 1998; Nicol and Lazaro, 2003; Nicol,in press). Following previous work by Raschman and Fedorockova(2006), the experimental value of kB in Eq. (1) can be compared withthemass transfer coefcient (km) based on Eqs. (5)(8) (Cussler,1997):

Fig. 9. Arrhenius plots for the second stage leaching of chalcocite and initial leaching ofcovellite based on Eq. (4) (kss from Cheng and Lawson 1991a,b; dissolved oxygenconcentration from Fig. 7).km =ShDL

5

Sh = 2 + 0:6Gr1=4Sc1=3 6

Gr =L3g =

m27

Sc =mD

8

where Sh=Sherwood number, Gr=Grashof number, Sc=Schmidtnumber, D=diffusion coefcient, L=characteristic length (diameterof particle), /=fractional density change based on density ofuid and solid particle, =kinematic viscosity of uid. Here, kmrepresents the mass transfer coefcient of the reactant due to naturalconvection.For pure covellite of density=4.6 g cm3 (Kelly and Spottiswood,1997), particle diameter=13106mandmolarmasses of Cu=63.6 gmol1, S=32.1 g mol1, the value of roCu is 0.31 mol m2. The sub-stitution of these values, and the value of b=1 (for Cl) based onreaction R15 in Table 1 in Eq. (9), gives an experimental value ofkB=3.3108 m s1 for the intrinsic rate constant. The diffusioncoefcients of H3O+, Cl and O2 at 25 C are all close to 2.109 m2 s1

(Marcus, 1997). The use of D=6. 109 m2 s1 at 85 C leads to acalculated value of km=2103m s1 based on Eqs. (5)(8) (Table 5).The experimental value of kB (~106 m s1) for the surface reaction(Eq. (9)) is several orders of magnitude lower than the estimated valueof km from Eqs. (5)(8) (~108 m s1). This shows that theapproximately rst order dependence of kss of covellite on the con-centration of chloride ions is due to the involvement of chloride in thesurface reaction, rather thanmass transport to the surface. This situationis similar to the previously reported results (Raschman and Fedorock-ova, 2006) in which the intrinsic rate constant for the surface reactionfor the dissolution of periclase in hydrochloric acid was three orders ofmagnitude lower than the calculated mass transfer coefcient of acid.

The initial rate of the surface reaction per unit surface area can beexpressed by Eq. (10) (Senanayake, 2007a). According to Fick's rstlaw, the ux of A is expressed by Eq. (11). At steady state, reagent Areacts with solid B at the same rate as it arrives at the surface. Thisleads to Eq. (12) for the rate of surface reaction at steady state (Cs=0).The use of km calculated from Eqs. (5)(8) allows the comparison ofestimated rate (Eq. (12)) with measured rate (Eq. (10)).

R = kssro 10

Flux = km CA Cs 11

R = bkmCA R and flux in mol m2s1

12

Line (b) in Fig. 10 shows the effect of concentration of dissolvedoxygen on the experimental rate of dissolution of covellite. The resultsFig.10. Correlation between dissolved oxygen concentration andmeasured (Eq. (10)) orestimated (Eq. (12)) rate data for chalcocite (85 C) and covellite (90 C) (see text).

for chalcocite from previous studies (Table 4, Fig. 8b) are alsoshown by line (a) in Fig. 10 for comparison. In the case ofchalcocite, the estimated and measured rates closely agree,especially if a value of b=1 was used, indicating that at steadystate the surface reaction is controlled by the oxygen mass transfer.However, in the case of covellite, the measured rates are one or twoorders of magnitude lower than the estimated rates. The fact thatthe rate is proportional to (CO2)0.45 and (CCl)1 based on the slopesof lines d and e in Fig. 8a, respectively, indicates the importance ofconsidering a mechanism which involves both these species in thesurface reaction.

11. Electrochemical evidence for chloride assisted surface reaction

Previous studies have shown that a comparison of the predic-ted potentials of various redox couples with the peak potentials ofvoltammograms of relevant minerals/metals can be used to ex-amine the nature of interim species formed on the surface duringleaching reactions (Hillrichs and Bertram, 1983a,b; Wadsworth andZhu, 2005, Senanayake, 2008a). The lines in Fig. 11 show the cal-

30 G. Senanayake / Hydrometallurgy 98 (2009) 2132culated Eo values for the formation of elemental sulfur and dif-ferent copper species including chlorides, oxides and hydroxidesand selected sulfur species during covellite oxidation as a functionof temperature.

Hillrichs and Bertram (1983a,b) noted that the anodic dissolu-tion/decomposition of CuS will proceed only when the currentdensities are low and the rate of oxidation reaction CuSCuO/Sbecomes low compared to that of CuSCu2+/S. This was consistentwith the non-passivated CuS decomposition at very low currentdensities reported by Peters (1977b). In the reported voltammogramfor the anodic oxidation of covellite in 0.2 mol dm3 H2SO4/NaCl at25 C (Miki and Nicol, 2008a), the two peak potentials at 0.62 V and0.74 V are consistentwith the calculated potentials (reactions R39 andR44 inTable 2 and a in Fig.11). The two couples represented by thesepotentials are CuS/CuCl+, S and CuS/Cu2(OH)3Cl, S, respectively. Thelower peak potential of 0.62 V is also in excellent agreement withthe Eo value of 0.62 V relevant to Cu(OH)Cl calculated in the presentstudy (reaction R39 in Table 2 and b in Fig. 11). Moreover, thecalculated values of Eo for these two redox couples at 25 C basedon the Nernst equation, Eo in Table 2, and the two concentrations0.2 mol dm3 Cl, and 0.2 mol dm3 H+; assuming the dissociationH2SO4=H++HSO4 (Fig. 6) and unit activity coefcients are 0.721 Vand 0.601 V respectively. These values are close to the measured peakpotentials reported by Miki and Nicol (2008a) indicating the pos-sibility of precipitation of these solids under the specied condi-

Fig. 11. Effect of temperature on standard reduction potentials of covellite leachingproducts based on Outukumpu HSC 6.1 software. (a) Peak potentials of anodic responseof rotated electrodes of covellite at ambient temperature in H2SO4+NaCl(0.2 mol dm-3

each) (Miki andNicol, 2008a)marked byasterisks; (b) Calculated value for CuS+H2O+- -Cl =Cu(OH)Cl(s)+S+2e (this work).tions. In contrast, the Eo values of other redox couples of interest: CuS/CuCl2,S (0.792 V), CuS/CuCl,S (0.704 V) and CuS/CuCl, H2S2O3(0.539 V) differ from the measured values noted above and in Fig. 11.The observed increase in anodic peak currents with increasingchloride concentration also supports the involvement of chlorideions in the rate controlling step. The benecial effect of chloride ionsupon anodic oxidation of covellite in a sulfate system can berationalised on the basis of reactions R48R50 in Table 2, where thesum of reactions R48R50 and R35 gives the overall oxidationreaction R16. Thus, the formation of Cu(OH)Cl (reaction R26) can beconsidered as the rate controlling step for covellite leaching inoxygenated sulfuric acidchloride solutions.

12. Surface reaction mechanism of covellite leaching

The thermodynamically favoured reactions between dissolvedoxygen and covellite expressed by R13R14 in Table 1 at zero chlo-ride concentration are expected to be very slow to yield very lowcopper extraction from CuS even in 0.5 mol dm3 H2SO4 at 90 C, asobserved in Fig. 2a. Thus, the benecial role of chloride ions duringthe leaching of covellite by oxygen can be rationalised accordingto the anion-inuenced adsorption-reactiondesorption mechan-ism via Cu(SH)(Cl) and Cu(OH)(Cl), described by reactions R24R29in Table 1. Relevant mathematical expressions are described inAppendix A.

Reactions R26 and R27 would lead to the formation of a moreporous surface, compared to the tight passivating surface formedin the absence of chloride. The formation of Cu(I) chlorocomplexes in oxygen decient chloride slurry systems, reportedby Cheng and Lawson (1991b) is possible via CuCl producedaccording to reaction R27. Likewise, the formation of both sulfurand sulfate can be related to the interim species H2S2O2 in sidereactions R3032 as well as other species such as thiosulfate (Muirand Ho, 2006). The interim product CuCl can be dissolved asCuCln(n-1) in excess chloride and/or be oxidized to Cu(II) salts byoxygen (R28). The non-existence of thiosulfurous acid (H2S2O2)and dithionous acid (H2S2O4) in the free molecular state (Green-wood and Earnshow, 1984) indicates the possible conversion ofonly a small percentage of sulde to sulfate via these unstableintermediates and side reactions, or other species such asthiosulfate, as observed by Cheng and Lawson (1991b). Furtherwork is essential to compare and contrast the leaching behaviourof CuS and CuFeS2 and the different reaction mechanisms proposedin Appendix A.

13. Summary and concluding remarks

The leaching results of synthetic covellite in the initial 1530 min follow a shrinking sphere kinetic model, indicating that therate is controlled by a chemical reaction at the surface. Rateconstants correspond to a half order reaction with respect to dis-solved oxygen concentration, but zero order with respect tohydrogen ions at high acid concentrations. Chloride ions areinvolved in the surface reaction as revealed by the rst orderdependence of the apparent rate constant on chloride ion con-centration in the range 0.020.25 mol dm3. However, thecalculated mass transfer coefcient of species such as oxygen andchloride ions (km ~103 m s1) is three orders of magnitude largerthan the intrinsic rate constants based on a shrinking sphere kineticmodel (kB ~106 m s1). This also supports the view that chloride ionsare involved in the rate controlling surface chemical reaction. Thecalculated standard reduction potentials compare reasonably well withthe reported peak potentials of linear sweep voltammograms ofcovellite. This supports the proposed surface reaction mechanism viathe formation of mixed-ligand complexes, in the form of adsorbed or

dissolved species.

31G. Senanayake / Hydrometallurgy 98 (2009) 2132Appendix A

Reactions of CuS in oxygenated acid in the presence of chlorideions

CuSH + + ClCu SH Cl ads adsorption 1 1

Cu SH Cl + 0:5O2k S + Cu OH Cl ads or 0:5H2S2O2 + CuCl rate controlling

2

The involvement of H2O in the surface reaction may lead to thepartial oxidation: Cu(SH)(Cl)+0.5H2O+0.5O2=Cu(OH)(Cl)0+HS+0.5H2O2 and the formation of sulfur: HS+0.5H2O2=S+H2O.

CuOHClads HSO4 CuSO40 H2O Cl 3

CuS H2SO4 0:5O2 CuSO40 H2Ooverall 4

K =

1 Cl

=K Cl

1 + K Cl

R = k O2 a =kK Cl O2 a1 + K Cl

=fraction of CuS surface that undergoes reaction with O2K=equilibrium constant for the adsorption equilibrium in reaction

(1)k=rate constanta=reaction order with respect to O2R=reaction rate

At high concentrations of Cl it may be assumed that 1K[Cl].This leads to the expression R=k[O2]a, which explains the chlorideindependent rates at higher chloride concentrations.

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A review of chloride assisted copper sulfide leaching by oxygenated sulfuric acid and mechanist.....IntroductionComplexity of mechanistic studiesEffect of chloride on redox potentials and leach resultsInterim copper compoundsCopper sulfide reactionsInterim chlorides and mixed chloro-hydroxides

Competitive reactions and sulfur productsFormation constants of interim copper speciesCuS2 SpeciesCu(OH)(Cl) species

Rate data and kinetic models for covelliteFactors affecting apparent rate constantsReaction orders and activation energy of covellite leachingIntrinsic rate constant and mass transfer coefficientElectrochemical evidence for chloride assisted surface reactionSurface reaction mechanism of covellite leachingSummary and concluding remarksAppendix AReferences