Co-Bioleaching of Chalcopyrite and Silver-Bearing Bornite in a Mixed Moderately Thermophilic Culture

Abstract: Chalcopyrite and bornite are two important copper minerals, and they often coexist. In this study, the co-bioleaching of chalcopyrite and silver-bearing bornite by mixed moderately thermophilic culture at 50 ◦C was investigated. The bioleaching results show that the extraction percentage of Cu for co-bioleaching of chalcopyrite (Ccp) and silver-bearing bornite (Bn) (Ccp/Bn = 3:1) was 94.6%. Compared to bioleaching of chalcopyrite or silver-bearing bornite alone, the Cu extraction percentage was greatly enhanced when they were bioleached together. The leaching residues were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Silver-bearing bornite dissolved preferentially compared to chalcopyrite, due to galvanic interactions. Simultaneously, Ag+ was released from the silver-bearing bornite into solution. Ag2S formed on the surface because Cu and Fe in the chalcopyrite were replaced by Ag+, accelerating chalcopyrite dissolution and enrichment of Ag on the surface of the chalcopyrite.

A number of studies have been conducted on the bioleaching of bornite [39][40][41][42] because bornite (Cu 5 FeS 4 ) is another important Cu-Fe-S mineral [1].The bioleaching of bornite by Acidithiobacillus ferrooxidans was evaluated in oxygen uptake and shake flask experiments.Covellite was detected as a secondary phase [43], and cell action on the surface of the bornite accelerated the leaching rate via both direct and indirect mechanisms [44].Recently, the combined effects of chalcopyrite and bornite during bioleaching have been investigated, and it was found that the bioleaching of chalcopyrite was enhanced in the presence of bornite [41,45,46].

Bioleaching Experiments
The mixed moderately thermophilic culture consisted of Sulfobacillus thermosulfidooxidans, together with a small amount of Acidithiobacillus caldus and Ferroplasma sp; this culture was an enrichment from a leaching solution sample at 50 • C, and was obtained from Inner Mongolia, China [7].In each experiment, 2 g of sample was added into 250 mL flasks containing 100 mL of medium (components: 3.0 g/L (NH 4 ) 2 SO 4 , 0.5 g/L K 2 HPO 4 , 0.5 g/L MgSO 4 •7H 2 O, 0.1 g/L KCl, 0.01 g/L Ca(NO 3 ) 2 , and 0.02% yeast extract, with a pH of 1.6).The experimental sets are presented in Table 1.The initial concentration of cells was 1.0 × 10 7 cells/mL.The flasks were placed in an orbital shaker at 160 rpm and 50 • C. The solution pH was adjusted periodically to 1.6 with diluted sulfuric acid.In addition, 1.0 mL of leaching solution was sampled and analyzed for Cu and Fe using inductively coupled plasma-atomic emission spectrometer (ICP−AES) (PS−6, Baird, Bedford, MA, USA).The sample volume was replaced with an equal volume of medium.Water lost to evaporation was supplemented periodically by adding distilled water until the mass of the flask equaled its initial mass.

Electrochemical Test
The electrodes were prepared from high-quality natural chalcopyrite and bornite.The effective area of the electrode exposed to the solution was 1 cm 2 .Before each electrochemical test, the surface of the electrode was polished with 600-grit and 2500-grit metallographic abrasive papers, sequentially, and then rinsed with deionized water.Electrochemical tests were performed using a conventional three-electrode electrolytic cell: a counter electrode (graphite rod), a reference electrode (saturated Ag/AgCl electrode), and a working electrode (chalcopyrite or bornite electrode).The electrolyte solution consisted of 3.0 g/L (NH 4 ) 2 SO 4 , 0.5 g/L K 2 HPO 4 , 0.5 g/L MgSO 4 •7H 2 O, 0.1 g/L KCl, and 0.01 g/L Ca(NO 3 ) 2 , pH 1.6.The scan rate of the Tafel polarization was 0.5 mV/s.All tests were conducted at 50

Analytical Techniques
The solution pH was measured with a pH meter (BPP-922, BELL Analytical Instruments (DaLian) Co., Ltd., Dalian, China) and the solution potentials were measured with a Pt electrode with reference to a saturated Ag/AgCl electrode.The leaching residues were filtered and rinsed with dilute sulfuric acid (pH 1.6) at room temperature and atmosphere, and then the samples were transferred into a vacuum box, and dried at 40 • C; afterwards, all of these properly dried samples were used for analysis with XRD (Advance D8, Bruker AXS, Karlsruhe, Germany) and XPS (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).XPS was conducted with a monochromatic Al excitation (1486.6 eV) operating at 200 W, and the chamber pressure was 1 × 10 −9 mbar.High-resolution spectra were collected using a pass energy of 20 eV and an energy step of 0.1 eV.Binding energy calibration was based on C 1s at 284.6 eV.A summed Gaussian (70%)-Lorentzian (30%) function was used for peaks fitted to the high-resolution spectra.

Leaching Characteristics
The leaching characteristics of the minerals, in terms of changes in the extraction percentages of Cu and Fe, pH and redox potentials, by sterile controls and microorganisms, are shown in Figure 1.

Analytical Techniques
The solution pH was measured with a pH meter (BPP-922, BELL Analytical Instruments (DaLian) Co., Ltd., Dalian, China) and the solution potentials were measured with a Pt electrode with reference to a saturated Ag/AgCl electrode.The leaching residues were filtered and rinsed with dilute sulfuric acid (pH 1.6) at room temperature and atmosphere, and then the samples were transferred into a vacuum box, and dried at 40 °C; afterwards, all of these properly dried samples were used for analysis with XRD (Advance D8, Bruker AXS, Karlsruhe, Germany) and XPS (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).XPS was conducted with a monochromatic Al excitation (1486.6 eV) operating at 200 W, and the chamber pressure was 1 × 10 −9 mbar.Highresolution spectra were collected using a pass energy of 20 eV and an energy step of 0.1 eV.Binding energy calibration was based on C 1s at 284.6 eV.A summed Gaussian (70%)-Lorentzian (30%) function was used for peaks fitted to the high-resolution spectra.

Leaching Characteristics
The leaching characteristics of the minerals, in terms of changes in the extraction percentages of Cu and Fe, pH and redox potentials, by sterile controls and microorganisms, are shown in Figure 1.Under the sterile control, 4.8% of the Cu and 5.1% of the Fe were leached from the chalcopyrite after 30 days (Figure 1a,b), and compared to chemical leaching, the extraction percentages of Cu and Fe for bioleaching of chalcopyrite were 21.5% and 17.0%, respectively.Simultaneously, the Fe extraction percentage decreased owing to the precipitation of Fe 3+ as jarosite after 24 days of bioleaching [7,50].The pH of the solution changed slightly because only a small amount of chalcopyrite was leached either by chemically or microorganisms (Figure 1c).During the bioleaching of chalcopyrite, Fe 2+ in the solution was oxidized to Fe 3+ by ferrous oxidizing microbials, and thus redox potentials rose and stabilized at about 630 mV, while the redox potentials of chemical leaching stabilized at about 370 mV (Figure 1d).
The extraction percentages of Cu and Fe for the chemical leaching of bornite after 30 days were 36.7% and 18.5%, respectively, whereas 59.4% of Cu and 18.2% of Fe were bioleached from the bornite.Figure 1c shows that a large amount of H + was consumed during both chemical and bio-leaching of bornite.In the first 10 days of bornite bioleaching, the Fe 2+ was oxidized into Fe 3+ by ferrous oxidizing microbials, resulting in a redox potential increase to 460 mV.The potential slowly decreased from 460 to 380 mV over the following 20 days because the rate of Fe 2+ oxidation to Fe 3+ by the microorganisms was less than the consumption rate of Fe 3+ via bornite oxidation (Figure 1d).In contrast, the potentials increased slowly and reached 370 mV after 30 days of leaching in the sterile control.
Compared to bioleaching of chalcopyrite (Ccp) or bornite (Bn) alone, the Cu extraction percentage was greatly enhanced in co-bioleaching (Figure 1a).When the ratio of Ccp/Bn was 1:3, the extraction percentages of Cu and Fe for co-bioleaching of chalcopyrite and bornite were 91.1% and 16.4%, respectively; and when the ratio of Ccp/Bn was 1:1, 89.0% of the Cu and 11.9% of the Fe were synergistically bioleached after 30 days.Interestingly, when the ratio of Ccp/Bn was 3:1, the extraction percentage of the Cu and Fe for co-bioleaching of chalcopyrite and bornite were 94.6% and 48.0%, respectively.
A large amount of H + was consumed during the co-bioleaching of chalcopyrite and bornite (Figure 1c).When the ratio of Ccp/Bn was 1:3 or 1:1, the change in redox potentials was similar to that when leaching bornite alone.In the first five days of co-bioleaching chalcopyrite and bornite, the redox potentials went up to about 540 mV, and then the potential slowly decreased and finally stabilized in the range of 450 to 480 mV.However, the redox potentials were still higher than those from leaching bornite alone (Figure 1d).When the ratio of Ccp/Bn was 3:1, the change in redox potentials was similar to that of leaching chalcopyrite alone.The redox potential increased with time and finally reached 630 mV (Figure 1d).
In summary, 21.5% of Cu was bioleached from the chalcopyrite; compared to bioleaching of chalcopyrite, bornite was more easily bioleached, but 40.6% of the Cu was still locked in the residues.When chalcopyrite and silver-bearing bornite were co-leached in a mixed moderately thermophilic culture, both Cu extraction percentages were greatly enhanced.

Characterization of the Leaching Residues
In order to better understand the co-bioleaching of chalcopyrite and bornite (when the ratio of Ccp/Bn was 3:1) in a mixed moderately thermophilic culture, the leaching residues were characterized by XRD and XPS.
As shown in Figure 2, the XRD patterns of co-bioleaching chalcopyrite and bornite (when the ratio of Ccp/Bn was 3:1) show that only the characteristic peaks of chalcopyrite were detected after 5 and 10 days of bioleaching (Figure 2D,E).Simultaneously, the mineralogical analysis of the leaching residues shows that 81% of the bornite and 15% of the chalcopyrite were bioleached after 5 days.Thus, the bornite preferentially dissolved.When the mixed sample was bioleached for 15 days, the main composition of the leaching product was still chalcopyrite, together with low-intensity peaks of jarosite (Figure 2F).Jarosite and elemental sulfur were detected after 30 days of bioleaching, and, at the same time, low-intensity peaks of chalcopyrite were detected (Figure 2G).
The chalcopyrite, bornite, and leaching residues were also analyzed using XPS.The survey (full range) XPS spectra of the samples are provided in Figure 3. Prior to the bioleaching of chalcopyrite, C, O, Cu, Fe and S peaks were identified (Figure 3A).For the bornite, in addition to C, O, Cu, Fe, and S peaks, a low-intensity Ag peak was also detected (Figure 3B).The peak intensity of Cu 2p decreased Minerals 2018, 8, 4 5 of 14 with leaching time and eventually disappeared.A low-intensity silver peak was detected between 5 and 23 days (Figure 3C-F), but disappeared after 30 days of bioleaching (Figure 3G).A low-intensity nitrogen peak was detected throughout the bioleaching process; this could have originated from ammoniojarosite (Figure 3C-G).The changes in atomic percentages on the surface of residues during the bioleaching process are shown in Figure 3H.C, O, Cu, Fe and S peaks were identified (Figure 3A).For the bornite, in addition to C, O, Cu, Fe, and S peaks, a low-intensity Ag peak was also detected (Figure 3B).The peak intensity of Cu 2p decreased with leaching time and eventually disappeared.A low-intensity silver peak was detected between 5 and 23 days (Figure 3C-F), but disappeared after 30 days of bioleaching (Figure 3G).A low-intensity nitrogen peak was detected throughout the bioleaching process; this could have originated from ammoniojarosite (Figure 3C-G).The changes in atomic percentages on the surface of residues during the bioleaching process are shown in Figure 3H.
Intensity (a.u.)  S peaks, a low-intensity Ag peak was also detected (Figure 3B).The peak intensity of Cu 2p decreased with leaching time and eventually disappeared.A low-intensity silver peak was detected between 5 and 23 days (Figure 3C-F), but disappeared after 30 days of bioleaching (Figure 3G).A low-intensity nitrogen peak was detected throughout the bioleaching process; this could have originated from ammoniojarosite (Figure 3C-G).The changes in atomic percentages on the surface of residues during the bioleaching process are shown in Figure 3H.
Intensity (a.u.)   High-resolution spectra of Cu 2p for bornite, chalcopyrite, and leaching residues are shown in Figure 4.It is well established in literature that the high binding energy (in the range of 933.0 to 933.8 eV) and the presence of a satellite peaks (around 942 eV) are two main XPS characteristics of Cu(II), while a lower binding energy (in the range of 931.8 to 933.1 eV) and the absence of satellite peaks are characteristics of Cu(I) and metal Cu [14,[51][52][53].From Figure 4, it can be seen that the oxidation state for Cu after bioleaching was still +1.Although low-intensity peaks of chalcopyrite were detected by XRD after 30 days of bioleaching, no Cu peak was detected by XPS.This may be because the chalcopyrite was covered by jarosite and elemental sulfur (Figure 2F).High-resolution spectra of Cu 2p for bornite, chalcopyrite, and leaching residues are shown in Figure 4.It is well established in literature that the high binding energy (in the range of 933.0 to 933.8 eV) and the presence of a satellite peaks (around 942 eV) are two main XPS characteristics of Cu(II), while a lower binding energy (in the range of 931.8 to 933.1 eV) and the absence of satellite peaks are characteristics of Cu(I) and metal Cu [14,[51][52][53].From Figure 4, it can be seen that the oxidation state for Cu after bioleaching was still +1.Although low-intensity peaks of chalcopyrite were detected by XRD after 30 days of bioleaching, no Cu peak was detected by XPS.This may be because the chalcopyrite was covered by jarosite and elemental sulfur (Figure 2F).The copper Auger spectra (Cu LMM) excited with X-rays and the atomic percentages of Cu 2p and Ag 3d on the surface of bioleaching residues are shown in Figure 5.It was very interesting that the Cu LMM, Ag 3p3/2 peak was detected at about 572.5 eV [54] (Figure 5C-F), this was in agreement with the survey XPS spectra (Figure 3C-F).Figure 5C-F show that the peak area ratio of Ag/Cu increased as leaching time increased from 5 to 23 days.Simultaneously, the ratio of Ag3d/Cu2p also increased with leaching time (Figure 5H).These results indicated that silver was enriched on the surface of chalcopyrite and/or bornite.The Ag 3p3/2 peak disappeared after 30 days of bioleaching (Figure 5G), indicating that silver compounds had been leached and/or covered by jarosite.The copper Auger spectra (Cu LMM) excited with X-rays and the atomic percentages of Cu 2p and Ag 3d on the surface of bioleaching residues are shown in Figure 5.It was very interesting that the Cu LMM, Ag 3p 3/2 peak was detected at about 572.5 eV [54] (Figure 5C-F), this was in agreement with the survey XPS spectra (Figure 3C-F).Figure 5C-F show that the peak area ratio of Ag/Cu increased as leaching time increased from 5 to 23 days.Simultaneously, the ratio of Ag 3d /Cu 2p also increased with leaching time (Figure 5H).These results indicated that silver was enriched on the surface of chalcopyrite and/or bornite.The Ag 3p 3/2 peak disappeared after 30 days of bioleaching (Figure 5G), indicating that silver compounds had been leached and/or covered by jarosite.The copper Auger spectra (Cu LMM) excited with X-rays and the atomic percentages of Cu 2p and Ag 3d on the surface of bioleaching residues are shown in Figure 5.It was very interesting that the Cu LMM, Ag 3p3/2 peak was detected at about 572.5 eV [54] (Figure 5C-F), this was in agreement with the survey XPS spectra (Figure 3C-F).Figure 5C-F show that the peak area ratio of Ag/Cu increased as leaching time increased from 5 to 23 days.Simultaneously, the ratio of Ag3d/Cu2p also increased with leaching time (Figure 5H).These results indicated that silver was enriched on the surface of chalcopyrite and/or bornite.The Ag 3p3/2 peak disappeared after 30 days of bioleaching (Figure 5G), indicating that silver compounds had been leached and/or covered by jarosite.The high-resolution spectra of S 2p for bornite, chalcopyrite and leaching residues are shown in Figure 6, and the fitted photoelectron spectra of S 2p peaks of Figure 6 are presented in Table 2.In order to obtain the data in Figure 6H, it was assumed that the percentages of S 2− , S 2 2− , S n 2− , S 0 , and SO 4 2− were directly proportional to the area under the S 2p peak presented in Figure 6C-G.
order to obtain the data in Figure 6H, it was assumed that the percentages of S , S2 , Sn , S , and SO4 2− were directly proportional to the area under the S 2p peak presented in Figure 6C-G.
As can be seen from Figure 6C-G, except for the S 2− that originated from chalcopyrite, the S2 2− , Sn 2− , S 0 , and SO4 2− were detected on surface leaching residues during the bioleaching time from 5 to 23 days.Furthermore, the percentage of S 2− , S2 2− , Sn 2− , and S 0 decreased with increasing time from 5 to 23 days, while the percentage of SO4 2− increased with increasing bioleaching time (Figure 6H).Only SO4 2− was detected on the on surface leaching residues after 30 days of bioleaching.This result is not consistent with the XRD results.One possible explanation is that chalcopyrite and elemental sulfur were covered by jarosite, and only surface substances (<10 nm) can be detected by XPS.
Previous studies have demonstrated that the Ag-enhanced leaching of chalcopyrite was due to the formation of Ag2S [31,36].Table 2 shows that when bioleaching time increased from 5 to 23 days, the binding energy of S 2− shifted to the binding energy of Ag2S [55], indicating that Ag-S bonds formed on the surface.As can be seen from Figure 6C-G, except for the S 2− that originated from chalcopyrite, the S 2 2− , S n 2− , S 0 , and SO 4 2− were detected on surface leaching residues during the bioleaching time from 5 to 23 days.Furthermore, the percentage of S 2− , S 2 2− , S n 2− , and S 0 decreased with increasing time from 5 to 23 days, while the percentage of SO 4 2− increased with increasing bioleaching time (Figure 6H).
Only SO 4 2− was detected on the on surface leaching residues after 30 days of bioleaching.This result is not consistent with the XRD results.One possible explanation is that chalcopyrite and elemental sulfur were covered by jarosite, and only surface substances (<10 nm) can be detected by XPS.
Previous studies have demonstrated that the Ag-enhanced leaching of chalcopyrite was due to the formation of Ag 2 S [31,36].Table 2 shows that when bioleaching time increased from 5 to 23 days, the binding energy of S 2− shifted to the binding energy of Ag 2 S [55], indicating that Ag-S bonds formed on the surface.
High-resolution spectra of Fe 2p are provided in Figure 7. Compared with chalcopyrite, the Fe 2p spectra show that the surface of bornite was heavily oxidized.The peak near 708 eV can be assigned to Fe−S [14].The component near the binding energy of 712 eV can be assigned to jarosite.When bioleaching time increased from 5 to 30 days, the peak intensity of Fe−S decreased with increasing leaching time, while the peak intensity of jarosite increased with increasing leaching time (Figure 7).This is in agreement with Figure 6H.In other words, the mixed sample was bioleached when a mixed moderately thermophilic culture was employed.High-resolution spectra of Fe 2p are provided in Figure 7. Compared with chalcopyrite, the Fe 2p spectra show that the surface of bornite was heavily oxidized.The peak near 708 eV can be assigned to Fe−S [14].The component near the binding energy of 712 eV can be assigned to jarosite.When bioleaching time increased from 5 to 30 days, the peak intensity of Fe−S decreased with increasing leaching time, while the peak intensity of jarosite increased with increasing leaching time (Figure 7).This is in agreement with Figure 6H.In other words, the mixed sample was bioleached when a mixed moderately thermophilic culture was employed.

Electrochemical Behavior of Chalcopyrite and Bornite
The corrosion current density, corrosion potential and polarization resistance of chalcopyrite were 0.14 µA•cm −2 , 406.5 mV, and 165.2 kΩ•cm 2 , respectively (Figure 8).Compared to chalcopyrite, the corrosion current density of bornite was greater, and the corrosion potential and polarization resistance of bornite were smaller.These values were 1.55 µA•cm −2 , 306.5 mV, and 23.9 kΩ•cm 2 , respectively (Figure 8).The higher value of the corrosion current density or lower value of the polarization resistance means that the oxidation kinetics were higher during the dissolution process.On the other hand, when chalcopyrite and bornite were bioleached together, bornite dissolved preferentially compared to chalcopyrite due to the galvanic interactions because the open circuit potential (OCP) of bornite (340 mV) was lower than that of chalcopyrite (475 mV) (Figure 8).This is well in agreement with the leaching results.

Electrochemical Behavior of Chalcopyrite and Bornite
The corrosion current density, corrosion potential and polarization resistance of chalcopyrite were 0.14 μA•cm −2 , 406.5 mV, and 165.2 kΩ•cm 2 , respectively (Figure 8).Compared to chalcopyrite, the corrosion current density of bornite was greater, and the corrosion potential and polarization resistance of bornite were smaller.These values were 1.55 μA•cm −2 , 306.5 mV, and 23.9 kΩ•cm 2 , respectively (Figure 8).The higher value of the corrosion current density or lower value of the polarization resistance means that the oxidation kinetics were higher during the dissolution process.On the other hand, when chalcopyrite and bornite were bioleached together, bornite dissolved preferentially compared to chalcopyrite due to the galvanic interactions because the open circuit potential (OCP) of bornite (340 mV) was lower than that of chalcopyrite (475 mV) (Figure 8).This is well in agreement with the leaching results.

Discussion
When silver-bearing bornite was dissolved preferentially due to the galvanic interactions, Ag + was also released into the solution.Ag + then diffused to the surface of chalcopyrite, and replaced Cu and Fe (Equation ( 1)) [36].This resulted in the leaching of copper and iron from the chalcopyrite.Figures 3 and 5 show that silver accumulated onto the surface of the chalcopyrite, and Table 2 shows that the binding energy of S 2− shifted to the binding energy of Ag2S.These findings agreed with previous studies that Ag2S formed on the surface [31,36].Ag + can be regenerated by oxidation of Ag2S with Fe 3+ (Equation ( 2)) [36].Therefore, chalcopyrite can be continuously leached because it is silvercatalyzed.This result is well in agreement with previous studies [28,56].On the other hand, the OCP of Ag2S was about 500 mV (vs.SHE) [57], and the contact between CuFeS2 and Ag2S could accelerate the dissolution of CuFeS2 owing to the galvanic interactions: (1)

Discussion
When silver-bearing bornite was dissolved preferentially due to the galvanic interactions, Ag + was also released into the solution.Ag + then diffused to the surface of chalcopyrite, and replaced Cu and Fe (Equation ( 1)) [36].This resulted in the leaching of copper and iron from the chalcopyrite.Figures 3 and 5 show that silver accumulated onto the surface of the chalcopyrite, and Table 2 shows that the binding energy of S 2− shifted to the binding energy of Ag 2 S.These findings agreed with previous studies that Ag 2 S formed on the surface [31,36].Ag + can be regenerated by oxidation of Ag 2 S with Fe 3+ (Equation ( 2)) [36].Therefore, chalcopyrite can be continuously leached because it is silver-catalyzed.This result is well in agreement with previous studies [28,56].On the other hand, the OCP of Ag 2 S was about 500 mV (vs.SHE) [57], and the contact between CuFeS 2 and Ag 2 S could accelerate the dissolution of CuFeS 2 owing to the galvanic interactions: Ag 2 S + 2Fe 3+ → 2Ag + + 2Fe 2+ + S 0 (2) S 2 2− and S n 2− were detected on surface leaching residues during the bioleaching time from 5 to 23 days, but the Cu extraction percentage still increased from 46% to 84% (Figure 6H).In previous study, S 2 2− and S n 2− were considered as chalcopyrite passive film [7,11,12].However, higher Cu extraction has been obtained in the presence of Ag.Ghahremaninezhad et al. [37] proposed a new model for silver-catalyzed chalcopyrite leaching: when Ag + was present in solution, Ag + adsorbed onto the surface of the chalcopyrite passive film and formed Ag 2 S; and the sulfur in Ag 2 S was a sulfur vacancy.The sulfur vacancy caused a faster diffusion rate in passive film and the formation of a porous sulfur layer; thus, high Cu extraction has been obtained in the presence of Ag although S 2 2− and S n 2− were detected on surface leaching residues during the bioleaching [37].On the other hand, jarosite (SO 4 2− ) and S 0 were detected during the leaching process, but the Cu extraction percentage increased with increased leaching time (Figure 6H).Many studies have indicated that neither jarosite nor S 0 hindered the dissolution of chalcopyrite [13,20,58,59].

Conclusions
The achieved results show that only 21.5% and 59.4% of Cu can be leached from chalcopyrite and silver-bearing bornite, respectively, using a mixed moderately thermophilic culture, while 94.6% of the Cu was successfully bioleached from the mixed sample (ratio of chalcopyrite/bornite was 3:1).Compared to bioleaching chalcopyrite or silver-bearing bornite alone, the Cu extraction percentage was greatly enhanced when they were co-bioleached.The results of XRD show that silver-bearing bornite dissolved preferentially compared to chalcopyrite, owing to the galvanic interactions.The Ag + released from silver-bearing bornite was enriched on the surface of the mineral, owing to the formation of Ag 2 S.During the bioleaching process, Ag + can be regenerated by the oxidation of Ag 2 S with Fe 3+ ; thus, chalcopyrite can be continuously leached when it is silver-catalyzed.

Figure 1 .
Figure 1.Copper extraction (a); iron extraction (b); pH (c) and redox potential (d) in leach liquors during the leaching process.

Figure 1 .
Figure 1.Copper extraction (a); iron extraction (b); pH (c) and redox potential (d) in leach liquors during the leaching process.