Effects of coexisting metal ions on the redox potential dependence of chalcopyrite leaching in sulfuric acid solutions

Hydrometallurgy 87 (2007) 1 – 10 www.elsevier.com/locate/hydromet

Effects of coexisting metal ions on the redox potential dependence of chalcopyrite leaching in sulfuric acid solutions N. Hiroyoshi a,⁎, S. Kuroiwa b , H. Miki c , M. Tsunekawa a , T. Hirajima d b

d

a Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Niihama Research Laboratories, Sumitomo Metal Mining Co., Ltd., 1-5 Oji-Cho, Niihama, Ehime 792-0008, Japan c A.J. Parker Cooperative Research Center for Hydrometallurgy, Murdoch University, Perth, WA, Australia Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan

Received 12 December 2005; received in revised form 28 June 2006; accepted 23 July 2006 Available online 8 March 2007

Abstract The leaching rate of chalcopyrite (CuFeS2) in H2SO4 solutions depends on the redox potential determined by the concentration ratio of Fe3+ to Fe2+, and the rate is higher at redox potentials below a critical value (critical potential). In actual leaching systems, different metal ions are released from coexisting minerals to the aqueous phase. The present study investigated the effects of coexisting metal ions on the critical potential of chalcopyrite leaching. Shaking-flask leaching experiments were carried out with 0.1 g of ground chalcopyrite and 10 cm3 of 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and 0.001 kmol m− 3 of the metal ions at 298 K in air. The initial redox potential was adjusted by adding Fe3+, and the amount of Cu extracted after 24 h was investigated as a function of the potential. The results indicate that the critical potential increases by the addition of Ag+ or Bi3+ but is not affected by Pd2+, Hg2+, Cd2+, Zn2+, Ni2+, Co2+, or Mn2+. The results were interpreted by a reaction model assuming the formation of intermediate Cu2S due to the reduction of chalcopyrite and subsequent oxidation of the Cu2S at potentials below the critical potential. Catalytic effects of metal ions on chalcopyrite leaching are also discussed based on the experimental results and the proposed model. © 2007 Published by Elsevier B.V. Keywords: Chalcopyrite; Leaching; Oxidation; Reduction; Redox potential; Catalyst; Metal ions

1. Introduction In the past decades, dump and heap leaching of lowgrade copper ore, combined with solvent-extraction/ electro-winning methods, have become important in copper production, because of low capital requirements and operation costs. However, this hydrometallurgical process has not been fully successful, due to the extremely slow leaching kinetics of a common copper ⁎ Corresponding author. Tel./fax: +81 11 706 6313. E-mail address: [email protected] (N. Hiroyoshi). 0304-386X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.hydromet.2006.07.006

source, chalcopyrite (CuFeS2), which causes less than optimal copper recovery when ores contain chalcopyrite. In dump and heap leaching, sulfuric acid solution is used as the leaching solution and chalcopyrite is oxidized by Fe3+ or O2 to release Cu2+ and Fe2+. An important factor affecting the leaching rate of chalcopyrite is the solution redox potential, E, given by the following Nernst equation.    3þ  RT aFe3þ RT ½Fe  o o ln ln E¼E þ ¼ E Vþ ð1Þ F F aFe2þ ½Fe2þ  where, ai indicates activity of the chemical species i, [i] the molarity of chemical species i, R the gas constant, T the

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absolute temperature, and F the Faraday constant. The Eo and Eo′ terms indicate the standard redox potential and the formal potential of a Fe3+/Fe2+ redox pair, respectively. The value of Eo is 0.771 Vand the value of Eo′ in 0.1 kmol m− 3 H2SO4 is about 0.67 V at 298 K at 1 atm against a standard hydrogen electrode (SHE). The relation between the redox potential and chalcopyrite leaching rate is complex and similar to the active–passive behavior observed in the oxidation process of metals like iron. With increasing potential, the leaching rate increases to reach a maximum and then suddenly decreases, and the leaching rate is little dependent on the potential above a critical value (critical potential) (Ahonen and Tuovinen, 1993; Kametani and Aoki, 1985; Okamoto et al., 2003, 2004a, 2004b, 2005; Hiroyoshi et al., 1997, 1998, 1999a, 1999b, 1999c, 2000, 2001, 2002, 2003, 2004; Miki et al., 2003, 2001; Pinches, 1997; Third et al., 2000, 2002). The active– passive behavior is a common phenomenon in chalcopyrite leaching and has been observed under various conditions. For example, Hiroyoshi et al. (2000) reported that the active–passive behavior is observed in leaching experiments at 25 °C under aerobic conditions with a ground chalcopyrite sample for 1 day. The active–passive behavior has also been observed in experiments at 90 °C (Kametani and Aoki, 1985), under anaerobic conditions (Hiroyoshi et al., 2001), with a large chalcopyrite electrode (Hiroyoshi et al., 2004), and in long-time column leaching over 200 days (Okamoto et al., 2005). To achieve higher chalcopyrite leaching rates, the solution redox potential must be controlled to be lower than the critical potential, and the factors affecting the critical potential must be well understood. In previous studies (Hiroyoshi et al., 2000, 2001, 2002, 2003, 2004; Okamoto et al., 2003, 2004a, 2004b, 2005; Miki et al., 2001, 2003), the authors proposed a reaction model for the redox-potential dependence of chalcopyrite leaching and reported that the critical potential is a function of the Cu2+ and Fe2+ concentrations. It was also reported that Ag+, a catalyst for chalcopyrite leaching, causes an increase in the critical potential and a broadening of the potential range where rapid copper extraction occurs (Hiroyoshi et al., 2002). In actual leaching systems, different metal ions are released from the gangue minerals in the copper ore, and coexist in the aqueous phase. It may be that other metal ions affect the critical potential of chalcopyrite leaching similar to the effect of Ag+. To elucidate this, the present study investigated the effects of several metal ions on the critical potential of chalcopyrite leaching by shaking-flask experiments. Based on the results, the catalytic effect of coexisting metal ions is also discussed.

2. Materials and methods 2.1. Chalcopyrite sample A ground chalcopyrite (median stokes diameter on weight basis, 5.5 μm) was used in the leaching experiments. Details of the sample preparation were provided elsewhere (Hiroyoshi et al., 2002), and the chemical analysis of the chalcopyrite sample showed 28.0 wt.% Cu, 29.7 wt.% Fe, 32.9 wt.% S, and 1.08 wt.% Si in the sample. Fig. 1 shows an X-ray diffraction (XRD) pattern of the sample with strong peaks corresponding to chalcopyrite, and weak peaks corresponding to quartz (SiO2) and pyrite (FeS2). 2.2. Solution preparation Except where otherwise mentioned, leaching solutions were 0.1 kmol m− 3 H2SO4 containing 0.001 kmol m− 3 of specific metal ions, 0.1 kmol m− 3 Fe2+ and various concentrations of Fe3+. For the preparation of the solutions, pure water (distilled-ion exchanged water) and reagent grade chemicals of H2SO4, Fe(SO4)·7H2O, Fe2 (SO4)3.nH2O, Ag2SO4, Bi(NO3)3.5H2O, CdSO4.7H2O, CoSO4.7H2O, MnSO4.5H2O, NiSO4.6H2O, ZnSO4.7H2O, PdSO4, and HgSO4 (Wako Pure Chemicals Industries, Ltd., Japan) were used. The leaching solutions were prepared by mixing stock solutions of metal salts and sulfuric acid to obtain the desired concentrations of the components. In most cases, the stock solutions of metal salts were obtained by dissolving the metal salts in diluted sulfuric acid at room temperature, but PdSO4 stock solution was prepared by dissolving the metal salt in 96% sulfuric acid at elevated temperatures (about 353 K) because PdSO4 is not dissolved in diluted H2SO4 at room temperature.

Fig. 1. XRD pattern of the chalcopyrite sample.

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During the preparation of the leaching solutions containing Ag+, Hg2+, and Pd2+, precipitates formed as detailed in the following sections. The precipitates were not removed and were added to the flasks for leaching experiments together with the solutions. 2.3. Leaching experiments Leaching experiments were performed with 50 cm3 Erlenmeyer flasks containing 10 cm3 of leaching solutions and 0.1 g of chalcopyrite. Before adding the chalcopyrite to the flasks, the initial redox potentials of the solutions were measured with a high-resistance potentiometer using a platinum electrode and a KCl saturated Ag–AgCl reference electrode at room temperature, and the values were converted to values against a standard hydrogen electrode. Then the chalcopyrite was added and the flasks were capped with gas permeable plugs, and shaken reciprocally (amplitude, 40 mm; shaking

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rate, 120 strokes min− 1) in a water bath at 298 K in air. After 24 h, the suspension was filtered with a membrane filter (pore size: 0.2 μm) and the metal concentrations in the filtrate were determined by a SEIKO SPS7800 inductively coupled plasma atomic emission spectroscope (ICP-AES). The redox potential of the filtrate (final redox potential) was measured in the same manner as the initial redox potential. 2.4. X-ray diffraction analysis Leach residues for X-ray diffraction (XRD) analysis were prepared with experiments using 500 cm3 flasks containing 2.5 g chalcopyrite and 250 cm3 of leaching solution, with shaking conditions and leaching times as detailed above. The residue was recovered by filtration using No. 5A filter paper and the residue was washed first with 0.1 kmol m− 3 H2SO4 then distilled water. Next the residue was air-dried at room temperature and the XRD

Fig. 2. Effects of the addition of 0.001 kmol m− 3 of Cd2+, Zn2+, Ni2+, Co2+, Mn2+, Pd2+, and Hg2+ on chalcopyrite leaching in 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ in air at 298 K for 24 h. The initial solution potential was adjusted by adding Fe3+. Panels (A) and (B) show the Cu extraction from chalcopyrite; panels (C) and (D) are the concentration of metal ions after 24 h. The dotted lines in panels (C) and (D) indicate the initial concentrations of the metal ions added.

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3.2. Effects of Pd2+ and Hg2+

Fig. 3. Effects of addition of 0.001 kmol m− 3 Hg2+ and Pd2+ on Cu extraction from chalcopyrite leaching in 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ in air at 298 K for 24 h. The initial solution potential was adjusted by adding Fe3+. Concentrations of Hg2+ and Pd2+ after 24 h were below the detection limits of ICP-AES.

analysis made with a JEOL JDX-3500 X-ray diffractometer with Cu-Kα radiation. 3. Results and discussion

Fig. 3 shows the results of the leaching experiments with Pd2+ and Hg2+. As will be discussed in Section 3.4, Pd2+ and Hg2+ are very easily reduced to the elemental metals with Fe2+ as the electron donor (reductant). Therefore, the Fe2+ in leaching solutions with Pd2+ or Hg2+ is rapidly oxidized to Fe3+, causing an increase in the solution redox potential during the solution preparation. As a result, leaching experiments at initial redox potentials below 0.57 V could not be performed with Pd2+ or Hg2+, and the effects of these ions on the redox potential dependence of chalcopyrite leaching cannot be estimated at potentials below 0.57 V. As can be seen in Fig. 3, however, it is still possible to conclude that Pd2+ and Hg2+ did not affect Cu extraction above 0.57 V, and that these metal ions do not increase the critical potential of chalcopyrite leaching. The initial and final concentrations of Pd2+ and Hg2+ in the solutions were below the detection limit of the ICPAES, indicating that these ions are removed from the aqueous phase into the leaching residue. Fig. 4 shows the XRD patterns of the residues of chalcopyrite leached in 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and 0.001 kmol m− 3 PdSO4. The XRD pattern of the residue leached with PdSO4 shows peaks corresponding to elemental Pd, confirming the reduction of Pd2+ to elemental Pd. In the XRD patterns of the residue leached with HgSO4, peaks corresponding to solid Hg compounds like sulfide and sulfate were not detected. This implies that Hg2+ is also reduced to elemental Hg, which is liquid at ambient temperatures and is not detectable by XRD analysis. 3.3. Effects of Ag+ and Bi3+ Fig. 5 shows the results of the leaching experiments with Bi3+, and the results with Ag+, from a previous

3.1. Effects of Cd2+, Zn2+ , Ni2+ , Co2+ and Mn2+ Fig. 2 shows the results of the leaching experiments with and without Cd2+, Zn2+, Ni2+, Co2+or Mn2+. Without the metal ions, the amounts of Cu extracted after 24 h were higher at the initial and final potentials above 0.60 V, which corresponds to the critical potential without the metal ions. The Cu extraction is almost equal to 10% of the Cu in the chalcopyrite sample at the low redox potential, but was less than 3% at higher potentials. The amounts of Cu extracted and the critical potentials were little affected by the addition of Cd2+, Zn2+, Ni2+, Co2+or Mn2+. Metal ion concentrations after 24 h were about 10− 3 kmol m− 3 at any potential and were almost the same as the initial concentrations of the metal ions added.

Fig. 4. XRD patterns for chalcopyrite residue leached in 0.1 mol dm− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and 0.001 kmol m− 3 of Pd2+ in air at 298 K for 24 h.

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Fig. 5. Effects of addition of 0.001 kmol m− 3 of Bi3+ and Ag+ on chalcopyrite leaching in 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ in air at 298 K for 24 h. The initial solution potential was adjusted by adding Fe3+. Panels (A) and (B) show the Cu extraction from chalcopyrite; panels (C) and (D) are the concentration of metal ions after 24 h. The dotted lines in panels (C) and (D) indicate initial concentrations of metal ions added.

paper (Hiroyoshi et al., 2002), are also shown in Fig. 5. As detailed in the previous paper, the critical potential increased with increasing concentrations of Ag+ added and the amount of extracted Cu increased with increasing potentials when 0.001 kmol m− 3 Ag+ was added. It was also reported that a part of the Ag+ added was reduced by Fe2+ to form elemental Ag before the chalcopyrite addition, and that Ag2S forms during the leaching (Hiroyoshi et al., 2002). Similar to Ag+, Bi3+ increases the critical potential of chalcopyrite leaching; with 0.001 kmol m− 3 Bi3+, the critical potential shifted to 0.63 V from the critical potential without Bi3+, 0.60 V. The Bi3+ concentrations after 24 h were almost the same as those added at potentials over the critical potential (0.63 V) while they decreased at potentials below the critical potential. Fig. 6 shows the XRD pattern for the residue leached with 0.1 kmol m− 3 H2SO4 containing 0.1 kmol Fe2+ and 0.001 kmol m− 3 Bi3+ for 24 h. The XRD patterns showed small peaks corresponding to Bi2S3.

3.4. Reaction model and thermodynamics As mentioned in the previous sections, chalcopyrite leaching in sulfuric acid solutions depends on the

Fig. 6. XRD patterns for chalcopyrite residue leached in 0.1 mol dm− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and 0.001 kmol m− 3 of Bi3+ in air at 298 K for 24 h.

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redox potential and the leaching rate is higher at redox potentials below a critical value. It was also found that the critical potential increases with coexisting Ag+ or Bi3+ but it is not affected by Cd2+, Zn2+, Ni2+, Co2+, Mn2+, Pd2+, or Hg2+. Here, these results are discussed based on a reaction model proposed previously by the authors (Hiroyoshi et al., 2000, 2001, 2002, 2003, 2004; Okamoto et al., 2003, 2004a, 2004b, 2005; Miki et al., 2001, 2003). The model assumes that the anodic dissolution of chalcopyrite at the redox potentials above the critical value is according to

that the intermediate Cu2S is formed according to a combination of the following two reactions.

CuFeS2 ¼ Cu2þ þ Fe2þ þ 2S þ 4e−

2CuFeS2 þ 6Agþ þ 2e− ¼ Cu2 S þ 2Fe2þ þ 3Ag2 S

ð2Þ

The oxidant (electron acceptor) for this reaction is Fe3+ or O2. At redox potentials below the critical potential, chalcopyrite leaching is assumed to be due to a two-step reaction: the formation of intermediate Cu2S by the reduction of chalcopyrite with Fe2+ as a reductant and subsequent oxidation of the Cu2S with Fe3+ or O2 as an oxidant. The critical potential, Ec, corresponds to the equilibrium redox potential of the Cu2S formation (chalcopyrite reduction). The oxidation rate of the intermediate Cu2S is higher than that of chalcopyrite and this causes the rapid Cu extraction at potentials below the critical potential. In the leaching system composed of a pure chalcopyrite and H2SO4 solution, Cu2+, Fe2+, and Fe3+ are present in the aqueous phase. In this case, the half-cell reactions of the chalcopyrite reduction (Cu2S formation) and the Cu2S oxidation are CuFeS2 þ 3Cu2þ þ 4e− ¼ 2Cu2 S þ Fe2þ

ð3Þ

Cu2 S ¼ 2Cu2þ þ S þ 4e−

ð4Þ

2CuFeS2 þ 6Hþ þ 2e− ¼ Cu2 S þ 2Fe2þ þ 3H2 S

ð6Þ

2Agþ þ H2 S ¼ Ag2 S þ 2Hþ

ð7Þ

In the first reaction (Eq. (6)), Cu2S and H2S are formed as a product, and the H2S is removed from the aqueous phase by formation of silver sulfide precipitate in the second reaction (Eq. (7)). Summing up Eqs. (6) and (7) gives Eq. (8) the overall reaction for the Cu2S formation from chalcopyrite. ð8Þ The critical potential (equilibrium potential) for this reaction is Ec ðAgþ Þ ¼ Eco þ

3 RT ðaAgþ Þ ln : F ðaFe2þ Þ

Eco ¼ 2:361V

The value of the standard redox potential Eco in Eq. (9) is much higher than that in Eq. (5), and this is a reason why the critical potential with Ag+ is higher than the intrinsic critical potential. A similar approach is used to evaluate the effects of the other metal ions on the critical potential of chalcopyrite leaching, i.e. it is assumed that the H2S generated due to the chalcopyrite reduction in Eq. (6) is removed from the liquid phase to form sulfide precipitate with coexisting metal ions, causing an enhancement in the Cu2S formation according to Eq. (6). For divalent metal ions, M2+, the following chemical equation is assumed to be the overall reaction of the Cu2S formation. 2CuFeS2 þ 3M2þ þ 2e− ¼ Cu2 S þ 2Fe2þ þ 3MS ð10Þ

and the critical potential, Ec, is given by Ec ¼

Eco

RT ðaCu2þ Þ0:75 ln þ : F ðaFe2þ Þ0:25

Eco

¼ 0:681 V

ð9Þ

ð5Þ

The critical potential for this reaction and the standard redox potential are given by Ec ðM 2þ Þ ¼ Eco þ

To avoid confusion, the critical potential defined by Eq. (5) will be termed the “intrinsic critical potential” in the following. A previous paper (Hiroyoshi et al., 2002) investigated the effects of Ag+ addition on chalcopyrite leaching and found that the critical potential for chalcopyrite leaching in the presence of Ag+ is higher than the intrinsic critical potential. To explain this, it was assumed

Eco ¼ −

RT ðaM 2þ Þ3 ln 2F ðaFe2þ Þ2

1 fðDGoCu2 S þ 2DGoFe2þ þ 3DGoMS Þ 2F −ð2DGoCuFeS2 þ 3DGoM 2þ Þg

ð11Þ ð12Þ

where ΔGio is the standard Gibbs free energy change of formation for the chemical species i.

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For the trivalent Bi3+, the following equations are assumed. 2CuFeS2 þ 2Bi3þ þ 2e− ¼ Cu2 S þ 2Fe2þ þ Bi2 S3 ð13Þ 2

Ec ðBi3þ Þ ¼ Eco þ Ec0 ¼ −

RT ðaBi3þ Þ ln 2F ðaFe2þ Þ2

1 fðDG0Cu2 S þ 2DG0Fe2þ þ DG0Bi2 S3 Þ 2F −ð2DG0CuFeS2 þ 2DG0Bi3þ Þg

ð14Þ ð15Þ

The calculated values of Eco for various metal ions and the standard Gibbs free energy used in the calculations are summarized in Table 1. The critical potentials for various metal ions were calculated with Eqs. (9) (11) and (13), and assuming that the Fe2+ activity is 0.1. Fig. 7 shows the calculated values of the critical potentials as a function of the metal ion activities. For comparison, the intrinsic critical potential, Ec, defined by Eq. (5) is also shown in Fig. 7 as a function of Cu2+ activity. The critical potentials with Cd2+, Zn2+, Ni2+, Co2+, and Mn2+ are lower than the intrinsic critical potential, indicating that these metal ions do not increase the critical potential of chalcopyrite leaching. This agrees with the experimental results shown in Fig. 2. The critical potentials with Ag+ and Bi3+ are higher than the intrinsic critical potential (Fig. 7). This suggests the increases in the critical potential in the presence of these metal ions, and agrees well with the results in Fig. 5. The results of the XRD analysis showing the formation or Ag2S (Hiroyoshi et al., 2002) and Bi2S3 (Fig. 6) in the leaching with Ag+ and Bi3+ below the critical potential also support the proposed reaction model in which these sulfides are formed (Eqs. (8) and (13)). As above, the observed effects of Cd2+, Zn2+, Ni2+, 2+ Co , Mn2+, Ag+, and Bi3+ on the critical potential are well explained by the proposed model. For the effects of Pd2+ and Hg2+, however, there is an apparent inconsistency

Fig. 7. Calculated values of the critical potentials of chalcopyrite leaching in the presence of various metal ions at 298 K. The activity of Fe2+ was assumed to be 0.1.

between the model and the experimental results: the model predicts a higher critical potential with Pd2+ and Hg2+ (Fig. 7), but the experimental results (Fig. 3) showed no increase in the critical potential by these ions. As described in Section 3.2, the ICP-AES analysis of the solutions and XRD analysis of the leached residue suggests thatthePd2+ andHg2+ addedarereducedtoelementalPdand Hg. This implies that the Pd2+ and Hg2+ concentrations are too low to form Cu2S according to Eq. (10), as follows: the reductant (electron donor) for the reduction of these metal ions would be the Fe2+ in the leaching solutions, and the halfcell reactions for the metal reduction are expressed by Pd2þ þ 2e− ¼ Pd

ð16Þ

Hg2þ þ 2e− ¼ Hg

ð17Þ

In the equilibrium state, the relation between metal ion activity (aM 2+) and redox potential (E ) is given by E ¼ Eo þ

RT lnaM 2þ : 2F

Eo ¼ −

DGoM 2þ 2F

ð18Þ

Fig. 8 shows the equilibrium activities of Pd2+ and Hg2+ as a function of the redox potential, and the equilibrium

Table 1 Calculated values of Eco for various metal ions, and standard Gibbs energy changes of formation for metal ions, ΔGo (ion) and for metal sulfide, ΔGo (sulfide) at 298 K under 1 atm (referred from Yazawa and Eguchi, 1975) Metal ion

ΔGo (ion) / kJ mol− 1

ΔGo (sulfide) / kJ mol− 1

Eco / V

Ag+ Pd2+ Hg2+ Mn2+ Co2+ Cd2+ Ni2+ Zn2+ Bi3+

77.11 177.8 164.4 − 228.1 − 54.39 − 77.61 − 45.61 − 147.1 82.80

− 40.67 − 66.99 − 50.58 − 218.4 − 82.84 − 156.5 − 79.50 − 201.3 − 140.6

2.384 3.160 2.696 − 0.797 − 0.203 0.580 − 0.119 0.197 0.941

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tential. The presence of Ag+ and Bi3+ causes an increase in the critical potential, but the effect of Bi3+ is not significant (Fig. 4). These results may allow the assumption that the critical potential of chalcopyrite leaching is determined by the Cu2+ and Fe2+ concentrations according to Eq. (5), and that the effect of other coexisting metal ions, except Ag+, on the critical potential can be ignored in the design of a redox-potential controlled leaching of chalcopyrite. 3.5. Catalytic effects of metal ions on chalcopyrite leaching Fig. 8. Equilibrium activities of Pd2+ and Hg2+ coexisting with Pd and Hg as a function of redox potential. The equilibrium activities of Ag+ and Bi3+ coexisting with Ag and Bi are also shown. The vertical dotted lines express the redox potential range observed in the leaching experiments in the present study.

activities of Ag+ and Bi3+ are also shown in this figure. The redox potential range observed in the leaching experiments in the present study is expressed as two vertical dotted lines in the figure. The activities of Pd2+ and Hg2+ are much lower than those of Ag+ and Bi3+. If the activity coefficients of the metal ions are assumed to be one, the concentrations of Pd2+ and Hg2+ are less than 10− 6 kmol m− 3 in the observed redox potential range. From a thermodynamic viewpoint, the critical potential with Pd2+ or Hg2+ should be higher than the intrinsic critical potential even with such low activities or concentrations (Fig. 7). However, very low concentrations of Pd2+ or Hg2+ would cause a very slow reaction rate for the Cu2S formation according to Eq. (10). This may be the reason why Pd2+ and Hg2+ do not affect the critical potential of chalcopyrite leaching. In the model, Cu2S is assumed to be an intermediate, but clear peaks corresponding to Cu2S were not observed in the XRD patterns of the leaching residue. This may be due to rapid oxidation of Cu2S. Although for simplicity elemental sulfur S is assumed as the solid product of the Cu2S oxidation in Eq. (4), it is known that the oxidation product of Cu2S is non-stoichiometric copper sulfide Cu2−xS (1 N x N 0) (Dutrizac and MacDonald, 1974; Arce and González, 2002), which may be amorphous. As above, there is no direct evidence of the presence of the intermediate Cu2S, however, the proposed model interprets the effect of various coexisting metal ions on the critical potential well. Both the model and the experimental results show that most coexisting metals (except Ag+ and Bi3+) affect neither the redox potential dependence of chalcopyrite leaching nor the critical po-

It has been reported that metal ions such as Ag+, Bi3+, and Hg2+ act as promoters or catalyst for chalcopyrite leaching (Ahonen and Touvinen, 1990a, 1990b; Ballester et al., 1990, 1992; Bruynesteyn et al., 1983; Carranza et al., 1997, 2004; Escudero et al., 1993; Gómez et al., 1997, 1999; Hu et al., 2002; Mier et al., 1994; Miller and Portillo, 1979; Miller et al., 1981; Palencia et al., 1998; Price and Warren, 1986; Romero et al., 1998, 2003; Sukla et al., 1990). A previous paper (Hiroyoshi et al., 2002) discussed the catalytic effect of Ag+ on chalcopyrite leaching as due to an increase in the critical potential and a broadening of the potential range where rapid copper extraction occurs. In this section, the catalytic effects of other metal ions (Bi3+ and Hg2+) are discussed based on the results of the present study and the proposed model. Mier et al. (1994) found that Bi3+ enhances the bioleaching of chalcopyrite and interpreted this as follows: PO43− added in solutions as nutrient for bacteria reacts with Fe3+ to form a complex such as FeHPO4+ and FeH2PO42+, lowering the concentration of free Fe3+, an oxidant for chalcopyrite, and suppressing chalcopyrite oxidation. The Bi3+ reacts with PO43− to form BiPO4, a precipitate, causing an increase in the concentrations of oxidant Fe3+ and an enhancement in the chalcopyrite oxidation. The above interpretation may be a possible explanation for the bioleaching of chalcopyrite. As shown in the present study, however, even in the absence of PO43−, Bi3+ increases the critical potential of chalcopyrite leaching and a broadening of the potential range where rapid copper extraction occurs. This may be a second reason why Bi3+ enhances the bioleaching of chalcopyrite. It has also been reported that Hg2+ enhances bioleaching rates of chalcopyrite (Ballester et al., 1990, 1992; Escudero et al., 1993; Mier et al., 1994), but the mechanism of this catalytic effect is unclear. As shown in Fig. 7, the proposed model suggests that Hg2+ has the potential to increase the critical potential of chalcopyrite leaching, although the increase in the critical potential was not confirmed in the experiments, possibly due to the precipitation of Hg as

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mentioned above. Therefore, the explanation for the catalytic effect of Hg2+ on bioleaching of chalcopyrite may be similar to that of Ag+ and Bi3+, i.e. in the presence of bacteria significant concentrations of Hg2+ remains in the liquid phase and causes the broadening of the potential range where rapid chalcopyrite leaching occurs. This explanation is not necessarily correct, however, because Hg2+ is very easily reduced to Hg as noted above. It is known that Hg2+ strongly suppresses the ability of bacteria to oxidize Fe2+. Considering this, a further explanation for the catalytic effect of Hg2+ is possible: Hg2+ suppresses the bacterial Fe2+ oxidation and this causes suppression of Fe3+ formation and the delay in the increase in the redox potential. This results in a low redox potential suitable for chalcopyrite leaching and rapid Cu extraction over long periods in the presence of iron oxidizing bacteria. If this is the case, the effect of Hg2+ on bioleaching of chalcopyrite is not a true catalytic effect and only an apparent effect due to the low redox potentials. The above interpretations of the catalytic effects of metal ions on bioleaching of chalcopyrite are not supported by experimental evidence, and must be substantiated by future investigations. However, the model proposed in the present study does explain the effect of various coexisting metal ions on chalcopyrite leaching and would be useful to predict the redox potential range suitable for chalcopyrite oxidation in actual heap or dump leaching situations. 4. Conclusions The effects of coexisting metal ions on the critical potential of chalcopyrite leaching were investigated by shaking-flask leaching experiments with 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and various concentrations of Fe3+ at 298 K. Addition of 0.001 kmol m− 3 Ag+ or Bi3+ caused an increase in the critical potential, but 0.001 kmol m− 3 of Cd2+, Zn2+, Ni2+, Co2+, Mn2+, Pd2+, or Hg2+did not affect the critical potential. The results are explained by a reaction model assuming the formation of intermediate Cu2S during chalcopyrite leaching. References Ahonen, L., Touvinen, O.H., 1990a. Catalitic effects of silver in the microbiological leaching of finely ground chalcopyrite-containing ore minerals in shake flask. Hydrometallurgy 24, 219–236. Ahonen, L., Tuovinen, O.H., 1990b. Silver catalysis of the bacterial leaching of chalcopyrite-containing ore material in column reactors. Minerals Engineering 3, 437–445. Ahonen, L., Tuovinen, O.H., 1993. Redox potential-controlled bacterial leaching of chalcopyrite ores. Proceedings of International Biohydrometallurgy Symposium, vol. 1, pp. 571–578. Arce, E.M., González, I., 2002. A comparative study of electrochemical behavior of chalcopyrite, chalcocite and bornite in sulfuric acid

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