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Comparative kinetic study of the silver-catalyzed chalcopyrite leachingat 35 and 68 °C

E.M. Córdoba a, J.A. Muñoz b, M.L. Blázquez b, F. González b, A. Ballester b,⁎a Escuela de Ingeniería Metalúrgica y Ciencia de los Materiales, Facultad de Ingenierías Físico-Químicas, Universidad Industrial de Santander, Bucaramanga, Colombiab Departamento de Ciencia de Materiales e Ingeniería Metalúrgica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

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

Article history:Received 25 June 2008Received in revised form 18 March 2009Accepted 22 March 2009Available online 5 April 2009

Keywords:Silver catalysisChalcopyriteFerric leachingTemperatureOxygen

The influence of silver and iron concentration and the presence or absence of oxygen was investigated in thesilver-catalyzed chalcopyrite leaching. The leaching tests were performed at two different temperatures(35 and 68 °C) in stirred flasks (180 rpm) containing 0.5 g of mineral and 100 mL of Fe3+/Fe2+ sulphatesolutions at pH 1.8 and at an initial redox potential of 500 mV vs. Ag/AgCl. The addition of a great excessof silver favoured the transformation of chalcopyrite into copper-rich sulphides, such as: covellite, CuS, andgeerite, Cu8S5. These sulphides prevented the formation of CuFeS2/Ag2S galvanic couple and, thus, theregeneration of silver. In addition, oxygen in solution plays a key role in the regeneration of silver ions actingas the main oxidizing agent for Ag2S.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Chalcopyrite, the main copper mineral at present, is veryrecalcitrant to oxidation which is probably related to both itscrystalline structure and electrochemical characteristics (Forwardand Warren, 1960). The abundance of this sulphide and the shortageof other mineral resources more easily leachable have pusheddevelopment forward kinetically profitable processes.

The addition of silver ion has been considered for the catalytichydrometallurgical treatment of chalcopyrite. The reaction modelproposed byMiller and Portillo (1979) is still accepted to interpret thecatalytic effect of silver ions. In the absence of silver ion, the oxidativechalcopyrite leaching with ferric ion is delayed by a dense layer ofelemental sulphur formed on its mineral surface, which behaves as adiffusion barrier. However, in the presence of silver ion, thechalcopyrite leaching takes place according to the following schemeof reactions:

CuFeS2 þ 4 Agþ→Cu2þ þ Fe2þ þ 2 Ag2S ð1Þ

Ag2S þ 2 Fe3þ→2 Agþ þ 2 Fe2þ þ So ð2Þ

The copper extraction is higher with silver ions because of theproduct formed, amixture of S° and Ag2Swhich is porous and does not

passivate the chalcopyrite surface. Price andWarren (1986) concludedthat the elemental sulphur produced in this catalytic process, besidesbeing more porous, has a higher electrical conductivity facilitating thetransport of electrons through the chalcopyrite surface.

Previously, the authors reported the beneficial effect of silver ionson the chalcopyrite dissolution with ferric sulphate and on the Fe3+/Fe2+ ratio in solution by delaying the nucleation of ferric hydrolysisproducts through an increase of ferrous concentration (Córdoba et al.,2007). In addition, unlike in the uncatalyzed chalcopyrite leaching,the copper dissolution rate increased with the redox potential sincethe regeneration of Ag+ requires of a high ferric concentration. In thepresent study, the effect of silver concentration on the catalyzedchalcopyrite leaching has been investigated using different oxidizingagents (Fe3+ and O2).

2. Experimental procedure

2.1. Materials

A chalcopyrite mineral (approximately 80% of CuFeS2) fromMessina, Transvaal (South Africa) was used in the leaching testswith the following chemical composition: 34.30% S, 29.65% Fe, 27.36%Cu, 0.31% Zn and 0.02% Pb. XRD analysis of powder samples showedthe presence of pyrite (FeS2), siderite (FeCO3) and quartz (SiO2) as themain impurities in the mineral.

Themineral was dry ground using a ball mill and a BET surface areaof 0.07 m2/g was determined. An average particle size of 70 μm wasdetermined by laser pulse.

Int. J. Miner. Process. 92 (2009) 137–143

⁎ Corresponding author. Departamento de Ciencia de Materiales e IngenieríaMetalúrgica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Av.Complutense s/n, 28040 Madrid, Spain. Tel.: +34 91 394 4339; fax: +34 91 394 4357.

E-mail address: [email protected] (A. Ballester).

0301-7516/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.minpro.2009.03.007

Contents lists available at ScienceDirect

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2.2. Leaching solutions

The leaching solutions, with an initial redox potential of 500mV vs.Ag/AgCl, were prepared bymixing stock solutions of ferric and ferroussulphate, keeping constant a total iron concentration of 5 or 0.5 g/L atpH 1.8. The stock solutions were prepared with 0 K (modified 9 Kmedium of Silverman and Lundgren, 1959) and Norris nutrientmedium (Norris and Barr, 1985) for tests at 35 and 68 °C respectively,which are normally used in bioleaching tests at low (35 °C) and high(68 °C) temperature.

2.3. Leaching tests

All leaching tests were performed in an orbital shaker at 180 rpmand at constant temperature (35 or 68 °C). Aerobic tests were carriedout in 250 mL Erlenmeyers covered with hydrophobic cotton thatallowed the access of oxygen but reduced water loss evaporation.100 mL hermetically closed flasks were used in anaerobic tests. A lowpulp density of 0.5% (100mL of leaching solution and 0.5 g of mineral)was chosen to prevent sharp changes of the redox potential of theliquid medium at the start of leaching. Silver was added in solution assilver sulphate before starting each test.

Periodically, water evaporation was restored, pH adjusted whenabove the initial value, redox potential recorded and 1 mL of liquidsample removed for analysis of metals in solution by atomicabsorption spectrophotometry (Cu and FeTotal) and by photocolori-metry (Fe2+). Finally, solid residues were characterized by XRD andSEM-EDS.

3. Results and discussion

3.1. Influence of silver concentration

The silver product formed in the silver-catalyzed chalcopyriteleaching, reaction (1), would cover the chalcopyrite surface protectingit from passivation of jarosites and other ferric hydrolysis products. Ina previous study using 1 g Ag/kg Cu, the authors observed thatbehaviour during the whole experiment except for the last stage, inwhich the lack of catalysis was attributed to silver loss (Córdoba et al.,2007).

Higher amounts of silver could also prevent chalcopyrite frompassivation. Three different silver concentrations were tested: 1, 10and 50 g/kg Cu, at both temperatures (35 and 68 °C) and using a Fe3+/Fe2+ sulphate solution of 5 g/L of FeTotal (500 mV vs. Ag/AgCl).

At 35 °C, the copper dissolution from chalcopyrite was suppressedwith a large excess of Ag+ (50 g/kg Cu) (Fig. 1a) while the redoxpotential remained constant around 500 mV (Fig. 1b). Hence, theunreactivity of the chalcopyrite surface seems to be related to itspassivation. Fig. 1c shows an intense iron precipitation for that test inthe form of jarosites (Fig. 2a) according to the EDSmicroanalysis of theresidues (12.17% S; 43.45% Fe; 2.47% Cu; 1.39% Ag; 34.78% O; 5.47% K;0.28% P). The natural instability of ferric solutions and the newunreactive surface formed seems to have promoted ferric hydrolysisand nucleation of jarosites on the particles.

Miller et al. (1981) were pioneers in the silver-catalyzedchalcopyrite leaching. They found that, at 90 °C, silver concentrationsbetween 1.18⁎10−4 M and 1⁎10−3 M had little effect on the process.These authors assumed that the excess of silver increased notably thethickness of the silver sulphide layer formed on the chalcopyritesurface, slowing down the transport of ions from and towards thesulphide surface. That could be likely the case for massive samples,with a small surface area, but unlikely for ground samples with amuch greater surface area. Nevertheless, the initial deposition of silveron chalcopyrite takes place preferentially over certain regions and thesilver homogenization process requires time (Córdoba et al., 2007).

Therefore, that hypothesis could hardly explain the results obtained inthe present study.

As mentioned, chalcopyrite particles were totally covered by a filmof jarosites after 11 days (Fig. 2a). For that reason, it was necessary toevaluate the surface changes at shorter times. After 1 day of attack, anew phase was formed over most of chalcopyrite particles in contactwith 50 g Ag/kg Cu. The residue with a purple colour contrasts withthe blue-blackish appearance of the silver sulphide. That new purplephase could be responsible for the quick passivation of the mineral.

Small amounts of a scattered precipitate, probably Ag2SO4, can beobserved in the SEMmicrograph of that residue after 1 day of leaching(Fig. 2). Price and Warren (1986) also detected the presence of silversulphate in the catalyzed chalcopyrite process when silver was addedin excess. They concluded that that precipitate could be responsiblefor the slow down in the copper dissolution rate.

The X-ray diffractogram of that residue (Fig. 3) shows thatchalcopyrite was transformed into copper-rich sulphides (covellite,CuS, and geerite, Cu8S5). The height of the peaks corroborates that thechalcopyrite transformation into new sulphides took place in most ofthe mineral particles, which would explain the change in colourpreviously mentioned. Covellite with an indigo blue colour turns intopurple (Betejtin, 1977). Silver sulphide was also detected in thediffractogram, but in less proportion than the copper-rich sulphides.

Fig. 1. Influence of silver concentration on the chalcopyrite chemical leaching at 35 °C.

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These new copper sulphides would be the result of solid statetransformations from chalcopyrite and would explain the appearanceof particles free of passivating products in the SEMmicrograph shownin Fig. 2b.

The solid transformation from chalcopyrite into other copper-richsulphides can be described as follows:

CuFeS2 þ 2 Agþ→CuS þ Ag2S þ Fe2þ ð3Þ

3 CuFeS2 þ 6 Agþ þ 3=2 O2 þ H2O→CuS þ Cu2S þ 3 Ag2S

þ 3 Fe2þ þ H2SO4

ð4Þ

Reactions (3) and (4) are thermodynamically favoured accordingto their standard free energies, ΔG°298 K: −152.4 and −302.6 kJ/molof CuFeS2, respectively (Roine, 2002). In the case of reaction (1),proposed by Miller and Portillo (1979), ΔG°298 K=−224.9 kJ/mol ofCuFeS2. Therefore, in the presence of silver, the solid state transforma-tion from chalcopyrite is thermodynamically more favoured tocopper-rich sulphides (reaction (4)) than its dissolution to cupricand ferrous ions (reaction (1)).

In addition, that transformation, following reaction (4), supportsthe hypothesis of Duyvesteyn and Saback (1993), among others, thatthe actual formula of chalcopyrite is Cu2S.2CuS.3FeS.FeS2, and thatduring its leaching decomposes in those copper sulphides.

The sulphides formed could be dissolved by ferric ion, according tothe following reactions:

Cu2S þ 4 Fe3þ→2 Cu2þ þ 4 Fe2þ þ So ð5Þ

CuS þ 2 Fe3þ→Cu2þ þ 2 Fe2þ þ So ð6Þ

However, the copper dissolution rate of these copper-richsulphides was very slow at 35 °C (Fig. 1). These results are coincidentwith the slow covelline dissolution rate observed by Acar et al. (2005)at 20 °C. Dutrizac et al. (1985) found that the ferric leaching of bornite(Cu5FeS4) over the range 5–94 °C renders copper-rich sulphides,covelline being the most recalcitrant.

The model derived from reactions (3) and (4), unlike the modelproposed by Miller and Portillo (1979), (reactions (1) and (2)),assumes that copper in chalcopyrite, instead of being release tosolution, forms new mineral phases, covelline and geerite (theoreti-cally chalcocite). According to this model, some iron in chalcopyrite isdissolved. Nevertheless, the drop in the amount of iron in solutionshown in Fig. 1c (50 g Ag/kg Cu) would be due to its precipitation.

Fig. 3. XRD diffractogram of the leaching residue after 1 day of attack (35 °C and 50 g Ag/kg Cu).

Fig. 2. SEM micrographs of leaching residues at 35 °C with addition of 50 g Ag/kg Cu:(a) 11 days and (b) 1 day of attack.

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The addition of silver in excess, but in less proportion, 10 g Ag/kgCu, was insufficient to provoke the complete solid state transforma-tion from chalcopyrite into other copper-rich sulphides. Unlike at 50 gAg/kg Cu, the copper dissolution rate was slightly slower than for thetest with 1 g Ag/kg Cu and silver was an effective catalyst.

Thus, a large excess of silver added favours the solid statetransformations from chalcopyrite into copper-rich sulphides,which, unlike the former sulphide, are not preferentially dissolvedafter contact with silver sulphide.

The dissolution of silver sulphide can be depicted by reaction (2)(Miller and Portillo, 1979) or by reaction (7) (Hu et al., 2002):

Ag2S þ 2 Fe3þ→2 Agþ þ 2 Fe2þ þ So ð2Þ

2 Ag2S þ O2 þ 4 Hþ→4 Agþ þ 2 So þ 2 H2O ð7Þ

Therefore, the coupled semireactions would be:

Anode : Ag2S→2 Agþ þ So þ 2 e− ð8Þ

Cathode : 2 Fe3þ þ 2 e−→2 Fe2þ ð9Þ

1=2 O2 þ 2 Hþ þ 2 e−→H2O ð10Þ

This sequence of reactions assumes that, for the CuFeS2–Ag2Sgalvanic couple, the latter acts as anode and the former as cathode

with reactions (9) and/or (10) taking place on it. In that case,chalcopyrite should be nobler than silver sulphide, as mentioned byMajima (1969) and Hu et al. (2002).

Different authors have reported that the rest potential of silversulphide in sulphuric acid at pH 2 is around 0.50 V (SHE) (Warrenet al., 1984) and of 0.53 (SHE) (Crundwell, 1988) or 0.66 V (SHE) (Huet al., 2002) for chalcopyrite. Then, the electrical contact between bothsulphides would accelerate the galvanic dissolution of Ag2S and, later,the regeneration of silver ions. In addition, the rest potentials forcovelline and chalcocite, 0.42 and 0.44 (SHE) (Holmes and Crundwell,1995) respectively, are lower than for silver sulphide. Thus, thedissolution of Ag2S is not favoured in contact with those coppersulphides.

At 68 °C and an initial redox potential of 500mV, the effect of silverconcentration was different from that at 35 °C (Fig. 4). An excess of

Fig. 4. Influence of silver concentration on the chalcopyrite chemical leaching at 68 °C.

Fig. 5. Backscattered electron SEM micrographs at 100× (a) and 1000× (b), of thecircled particle at (a), and XRD diffractogram (c) of the leaching residue at 68 °C withaddition of 50 g Ag/kg Cu.

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silver did not improve the copper dissolution rate from chalcopyrite,but, unlike at 35 °C (Fig. 1a), neither inhibited the process (Fig. 4a).That suggests a temperature-dependent process. At 68 °C, the copper-rich sulphides, formed through reactions (3) and (4), would beoxidized by ferric ion without the participation of silver. Silver, asmentioned previously, does not catalyze the dissolution of suchsulphides andwould be lost with the ferric hydrolysis products. Again,after 1 day of leaching (Fig. 4b and c), the lower reactivity ofchalcopyrite with an excess of silver facilitates the nucleation andprecipitation of iron compounds on mineral particles, which is relatedto high redox potentials.

SEM backscattered electron micrographs of the leaching residue at68 °C after 11 days of attack are shown in Fig. 5 for the higher doseaddition of silver (50 g Ag/kg Cu). Some chalcopyrite particles arecovered by a sandwich compound which contains silver (Fig. 5b). TheXRD diffractogram suggests that silver precipitated as argentojarosite(Fig. 5c). Therefore, a new layer of products was formed over the layerrich in silver. The new layer is composed by ferric hydrolysis products,mainly potassium jarosite and ferric hydroxide phosphate. Both filmsformed on the chalcopyrite surfaces were detected by backscatteredSEM by the difference in atomic weight.

In this way, Carranza et al. (1997) and Bolorunduro et al. (2003)indicated that silver ions could be removed from solution asargentojarosite as follows:

Agþ þ 3 Fe3þ þ 2 SO2−4 þ 6 H2O→AgFe3ðSO4Þ2ðOHÞ6 þ 6 Hþ ð11Þ

3.2. Influence of iron concentration

The effect of iron concentration on the catalyzed chalcopyritechemical leaching was investigated in a series of tests usingsolutions of low (0.5 g/l) and high (5 g/l) total iron concentration,with an initial redox potential of 500 mV vs. Ag/AgCl, both at 35 °Cand 68 °C.

The experimental results (Fig. 6) seem to indicate that thecatalytic effect of silver in the chalcopyrite dissolution is related tothe concentration of the oxidizing agent, Fe3+. In fact, unlike theuncatalyzed chalcopyrite leaching (Córdoba et al., 2008), thisvariable becomes more relevant than temperature in the catalyzedprocess.

These results are coincident with the observations of Miller et al.(1981) on the silver-catalyzed chalcopyrite leaching, that an increaseof the ferric iron concentration has a positive effect on the copperdissolution rate.

The favourable effect of ferric ion in the catalyzed chalcopyritechemical leaching, at low and high temperature, could be explained interms of silver ion regeneration (reaction (2)), which is only favouredat high Fe3+/Fe2+ ratios. The standard free energy at 25 °C for thatreaction is +46.0 kJ/mol of Ag2S (Roine, 2002). Although thatreaction is thermodynamically unfavourable, its value of free energy

Fig. 6. Influence of initial iron concentration on the chalcopyrite chemical leaching at 35and 68 °C (Einitial=500 mV and 1 g Ag/kg Cu).

Fig. 7. SEM micrographs of leaching residues at 35 °C, Einitial=500 mV and addition of1 g Ag/kg Cu: a) 0.5 and b) 5 g Fe/L.

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becomes negative when the Fe3+/Fe2+ molar ratio is high enough(Eqs. (12) and (13)).

ΔG = ΔG- + RT ln αAg+αFe2+

αFe3+

� �� �2ð12Þ

ΔG = ΔG- + 2RT lnαAg+ − 2RT lnαFe3+

αFe2+ð13Þ

The SEM study of the residues obtained in the catalyzed leaching at35 °C, with low (Fig. 7a) and high (Fig. 7b) iron concentration,indicates that the main difference resides in the amount of elementalsulphur formed, which is larger for the higher iron concentration andis directly related to the chalcopyrite dissolution. Furthermore, theelemental sulphur formed is porous and does not prevent mass

transport phenomena. Thus, the slower copper dissolution rate at lowiron concentrationwould be the result of a shortage of oxidizing agent(Fe3+).

3.3. Influence of oxygen

The effect of oxygen in the silver-catalyzed chalcopyrite chemicalleaching at 35 and 68°C was investigated under aerobic and anaerobicconditions (Fig. 8).

At both temperatures, the absence of oxygen delayed remarkablythe copper extraction from chalcopyrite (Fig. 8a). That delay wasespecially marked at low temperature during the first leaching days,with a lag phase characterized by a slow dissolution rate. The lowreactivity of chalcopyrite during the initial phase is accompanied by a

Fig. 8. Influence of oxygen on the chalcopyrite chemical leaching at 35 and 68 °C (5 g Fe/L, Einitial=500 mV vs. Ag/AgCl and 1 g Ag/kg Cu).

Fig. 9. Schematic representation steps taking place during the silver-catalyzed chalcopyrite chemical leaching.

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sharp decrease of pH (Fig. 8d). That phenomenon could be related tothe start of ferric hydrolysis and to a decrease of the initial ironconcentration (Fig. 8c):

Mþ þ 3 Fe3þ þ 2 SO2−4 þ 6 H2O→MFe3ðSO4Þ2ðOHÞ6 þ 6 Hþ ð14Þ

where

Mþ ¼ Naþ;Kþ;NHþ

4 or H3Oþ

After 5 days of leaching, the copper dissolution rate becomes fasteras shown by an increase of metals in solution, and by a drop of theredox potential and an increase of pH. Later, the copper rate slowsdown probably due to the passivation of chalcopyrite by ferrichydrolysis products.

Conversely, the copper dissolution rate in aerobic conditions isfaster from the start of leaching and also the subsequent sharp drop ofpotential and the increase of pH.

The effect of oxygen in the catalyzed chalcopyrite chemicalleaching was more pronounced at 68 °C than at 35 °C. Chalcopyritewas almost completely dissolved after 3 days of leaching, comparedwith the low copper extraction in anaerobic conditions at 68 °C (65%Cu, after 24 days). Since the redox potential remains at relatively highvalues (N450 mV), the slow down observed in the copper dissolutionrate in the absence of oxygenwould be related to the shortage of ferricion.

The pronounced effect of oxygen on the catalyzed chalcopyritechemical leaching at both temperatures tested, suggests that oxygenparticipates in the rate-controlling step. A scheme of the steps takingplace in the chalcopyrite leaching with Fe3+ and Ag+ is shown inFig. 9.

According to this Fig. 9, after dissolution of oxygen in the air-bulksolution interface (step 1), oxygenwould oxidize ferrous to ferric ironin the bulk solution (step 2). Assuming that all oxygen is consumed instep 2, step 3 would account for the mass transfer of Fe3+ and Ag+

through the Nernst's boundary layer in the bulk solution and step 4 forthe mass transfer of Fe3+ and Ag+ through the porous elementalsulphur layer (Fig. 7) formed which would be unreactive. Then, anelectrochemical reaction between Fe3+ and silver sulphidewould takeplace at the Ag2S–S° interface (step 5) followed by themass transfer ofAg+ through the silver sulphide layer (step 6) and the chemicalreaction between Ag+ and chalcopyrite at the CuFeS2–Ag2S interface(step 7). Finally, the soluble products, Fe2+ and Cu2+, would diffusethrough the different solid layers: silver sulphide (step 8), elementalsulphur (step 9) and Nernst's boundary layer in the bulk solution(step 10).

4. Conclusions

• The silver-catalyzed chalcopyrite chemical leaching can bedescribed by the following reactions:

CuFeS2 þ 4 Agþ→Cu2þ þ Fe2þ þ 2 Ag2S

Ag2S þ 2 Fe3þ→2 Agþ þ 2 Fe2þ þ So

2 Ag2S þ O2 þ 4 Hþ→4 Agþ þ 2 So þ 2 H2O

Initially silver ions are deposited on the chalcopyrite surface as silversulphide, releasing cupric and ferrous ions. Then, the silver sulphideis dissolved by ferric ion and oxygen, regenerating silver in solutionthrough the CuFeS2/Ag2S galvanic couple formed and restarting anew cycle of reactions.

• A large excess of silver favoured the transformation of chalcopyriteinto copper-rich sulphides such as covelline, CuS, and geerite, Cu8S5

(theoretically chalcocite, Cu2S). That transformation prevents theelectrical galvanic contact between CuFeS2 and Ag2S and, in turn, theregeneration of silver ions. Under such conditions, silver does notcatalyze the chalcopyrite dissolution.

• An increase of Fe3+concentration increases the copper extraction inthe catalyzed process because the silver regeneration is morefavoured at a high Fe3+/Fe2+ ratio.

• The importance of oxygen in the catalyzed chalcopyrite dissolutionresides in that the solubilization of the silver sulphide layer isthermodynamically more favoured by oxygen than by ferric ion.

• The chemical control observed in the uncatalyzed chalcopyriteleaching with Fe3+ is less evident in the presence of Ag+.Conversely, the transport of oxidizing agents, ferric ion and oxygen,acquires more relevance in the catalyzed process since they arenecessary to regenerate the silver ions from the CuFeS2/Ag2Sgalvanic couple.

Acknowledgement

The authors wish to express their gratitude to the SpanishMinistryof Science and Innovation for funding this work.

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