Bioleaching of Chalcopyrite by Moderately Thermophilic Microorganisms ⇑ ⇑ Wenqing Qin A, , Congren Yang A, , Shaoshi Lai B, Jun Wang A, Kai Liu A, Bo Zhang A

Bioresource Technology 129 (2013) 200–208

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Bioresource Technology

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Bioleaching of chalcopyrite by moderately thermophilic microorganisms ⇑ ⇑ Wenqing Qin a, , Congren Yang a, , Shaoshi Lai b, Jun Wang a, Kai Liu a, Bo Zhang a

a School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, PR China b Zhongye Changtian International Engineering Co. Ltd., No. 1 Laodong Middle Road, Changsha 410007, Hunan, PR China

highlights

" Leaching results showed the existence of an optimum pH in leaching of chalcopyrite. " There was clear beneﬁt in leaching chalcopyrite within the low solution potential. " Maintenance of an appropriate concentration of total dissolved iron was necessary. " Chalcopyrite was reduced to a series of intermediate products at the low potential. " The reduction of chalcopyrite to talnakhite and/or bornite is the rate-limiting step.

article info abstract

Article history: The leaching of chalcopyrite by moderately thermophilic microorganisms was investigated by employing Received 4 September 2012 cyclic voltammetry (CV), accompanying with the leaching behavior elucidation. Leaching experiment Received in revised form 9 November 2012 showed that there was clear beneﬁt in leaching chalcopyrite within the low solution potential (below Accepted 11 November 2012 400 mV vs. SCE), compared to the high potential leach (above 550 mV vs. SCE). Simultaneous mainte- Available online 23 November 2012 nance of an appropriate concentration of total dissolved iron was necessary and also beneﬁcial to leach chalcopyrite. The leaching results showed the existence of an optimum pH in the leaching of chalcopyrite Keywords: by the moderately thermophilic microorganisms. The analysis of CV results revealed that the chalcopyrite Bioleaching was reduced to a series of intermediate products (such as talnakhite, bornite and chalcocite) in the catho- Chalcopyrite Moderately thermophilic microorganisms dic, and then the intermediate product (chalcocite) was oxidized in the anodic. Cyclic voltammetry Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction et al., 2001; Sandström et al., 2005; Sasaki et al., 2009; Xia et al., 2010; Zhu et al., 2011). Bioleaching low-grade copper ores with microbes offers attrac- As many researchers reported, the passivation layer and the tive alternative methods to conventional leaching methods which intermediate compounds would vary at different conditions (pH, has been successfully applied for leaching secondary copper sul- Eh, etc.) (Dutrizac, 2008; Sandström et al., 2005; Todd et al., ﬁdes (such as bornite, chalcocite and covellite) in many countries. 2003; Vilcáez et al., 2008; Vilcáez et al., 2009). In order to ﬁnd

However, the chalcopyrite (CuFeS2) is recalcitrant to both chemical out the optimum pH conditions for chalcopyrite bioleaching by and biological leaching, due to the passivation of the mineral moderately thermophilic bacteria Sulfobacillus thermosulfidooxi- surface by sulfur (Hackl et al., 1995; Harmer et al., 2006; Klauber dans and archaea Ferroplasma sp., taking the minimization of pas- et al., 2001) and/or ferric iron compounds, such as jarosite sivation effect as well as maximization of the extraction (He et al., 2009; Kinnunen et al., 2006; Parker et al., 2003; percentage of chalcopyrite into consideration, the surface specia- Sandström et al., 2005; Xia et al., 2010; Zhu et al., 2011). tion of chalcopyrite during the leaching process under different In addition, chalcopyrite leaching may involve the formation of pH conditions along with S. thermosulfidooxidans and Ferroplasma iron deficient secondary minerals and intermediates (Sasaki et al., sp. were investigated in this paper. 2009). It is reported that chalcocite and/or covellite might be formed during the leaching process (He et al., 2009; Hiroyoshi 2. Methods

2.1. Chalcopyrite and copper-bearing sulﬁde ore

⇑ Corresponding authors. Tel.: +86 731 8830884; fax: +86 731 8710804. E-mail addresses: [email protected] (W. Qin), [email protected] The high grade chalcopyrite used in the test was obtained from (C. Yang). Meizhou, Guangdong Province, China. The chemical analysis of the

0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.11.050 W. Qin et al. / Bioresource Technology 129 (2013) 200–208 201 ore sample showed Cu 31.12%, Fe 25.50%, S 32.79%, Pb 1.93%, Zn 2.4. Electrochemical measurements

0.36%, MgO 0.10%, CaO 0.12% and Al2O3 0.14%. The X-ray diffraction analysis of the sample showed the main mineral was chalcopyrite. A high quality natural chalcopyrite sample originating from the The chalcopyrite samples used for flask leaching were ground and Meizhou, Guangdong Province, China, was used for electrode fabri- screened to 0.038 mm. cation. Natural chalcopyrite sample was cut into a cylinder with a The copper-bearing sulfide ore used in the test was mixed with diameter of about 1.5 cm and a thickness of approximately 5 mm, 4.5 kg10 mm low grade copper-bearing sulfide ore (Cu 0.51%) areas of 1 cm2 exposed to the solution. Before every electrochem- and 1.1 kg3 mm high grade copper-bearing sulfide ore. The ical experiment, the working electrode surface was polished with chemical analysis of the artificial mixture of copper-bearing sulfide 600-grit and 2500-grit metallographic abrasive papers, sequen- ore sample showed Cu 5.96%, Fe 14.06% and S 8.1%. Mineralogical tially, and then rinsing with deionized water. analysis of the ore sample showed that 53.36% and 45.97% Cu Electrochemical experiments were carried out using a conven- was present in primary copper sulfide and secondary copper sul- tional three-electrode electrolytic cell with graphite rod as a coun- fide respectively. The X-ray diffraction analysis of the sample ter electrode and the prepared chalcopyrite electrode as the showed the main copper minerals were chalcopyrite and bornite. working electrode. A saturated calomel electrode (SCE) was used as a reference electrode for all of the electrochemical tests. 2.2. Enrichment culture Electrochemical response was measured on a Princeton Model 283 potentiostat (EG&G of Princeton Applied Research) coupled The moderately thermophilic bacteria S. thermosulfidooxidans to a personal computer with the M270 software. The electrolyte and archaea Ferroplasma sp. were enrichment cultured from a solution was prepared as follows: 3.0 g/L (NH4)2SO4, 0.1 g/L KCl, leaching solution sample, which was obtained from Inner Mongolia 0.5 g/L MgSO47H2O, 0.5 g/L K2HPO4 and 0.01 g/L Ca(NO3)2. All Province, China, in 250 mL shake flasks using an orbital incubator the electrochemical experiments were performed at 50 °C in elec- with a stirring speed of 160 rpm at 50 °C. Acidithiobacillus ferroox- trolyte solution, and the medium was adjusted to target pH values idans was obtained from the Key Laboratory of Biometallurgy of with diluted sulfuric acid before each experiment. Cyclic voltam- Ministry of Education, Central South University, Changsha, China, metry experiment of different pH values was performed at a poten- which was cultured in 250 mL shake flasks using an orbital incuba- tial sweep rate of 20 mV/s, with the scan initiated from +570 (vs. tor with a stirring speed of 160 rpm at 30 °C. The medium used for SHE) in the negative directions, without stirring the electrolytic cell cultivation consisted of the following components: 3.0 g/L solution. Prior to the scans, the electrolyte was bioleached by mod- 7 (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L MgSO47H2O, 0.5 g/L K2HPO4 and erately thermophilic microorganisms (1.0 10 cells/mL) for 2+ 0.01 g/L Ca(NO3)2. The moderately thermophilic were supple- 15 min. Cyclic voltammetry experiment of different ratio of Fe / mented with yeast extract 0.02%. All the bacterial cultures were Fe3+ in electrolyte solution (pH1.6) was performed at a potential sub-cultured into basal salts medium supplemented with 2% ore sweep rate of 20 mV/s, with the scan initiated from +800 mV (vs. powder as the energy source. The resulting mixed culture was used SHE) in the negative directions, without stirring the electrolytic as inoculums for the all experiments. solution.

2.3. Bioleaching experiments 2.5. Analytical techniques

2.3.1. Bioleaching in shake flasks The mineralogical compositions of solid samples were exam- For leaching experiments, cells were inoculated into 250-mL ined by X-ray diffraction (XRD) (Rigaku Dmax-2000). Soluble Cu flasks containing 100 mL of sterilized culture medium and 2 g of and Fe were determined by inductively coupled plasma-atomic chalcopyrite. The flasks were placed in an orbital shaker at emission spectrometer (ICP-AES) (America Baird Co. PS-6), and 160 rpm and 50 °C (moderately thermophilic microorganisms) or the ferrous concentration was determined by standard potassium 30 °C(A. ferrooxidans). The initial cells concentration was dichromate titration (Peng et al., 2006). The pH of the supernatant 1.0 107 cells/mL. The parallel experiments without cells, but with was measured with a pH meter (PHSJ-4A) and the redox potentials the same culture medium and chalcopyrite, were run as sterile in the leaching solution were measured with a Pt electrode with controls. Many sets at different target pH values were set up. The reference to a saturated calomel electrode. solution pH was adjusted periodically to the target pH values with diluted sulfuric acid. Water lost by evaporation was supplemented periodically by adding sterile water until the mass of the flask 3. Results and discussion equaled its initial mass. The bioleaching of chalcopyrite by moderately thermophilic microorganisms was supplemented with yeast 3.1. The leaching characteristics of moderately thermophilic extract 0.02%. microorganisms and A. ferrooxidans

2.3.2. Column bioleaching The leaching characteristics of chalcopyrite by different cells Representative artificial mixture of copper-bearing sulfide ore and sterile controls, in terms of changes in Cu extraction percent- sample was loaded into a column (0.5 m tall, 15 cm diameter) age, the concentrations of total dissolved iron and redox potentials, equipped with a 3 L solution reservoir, and the column leaching are shown in Fig. 1. experiment was carried out at 50 °C. Before inoculation, the charge Fig. 1a shows that, in the bioleaching and sterile controls leach- was ideally pre-leached by 2.5 L diluted sulfuric acid solution (the ing system, the Cu extraction percentage increased with time and solution pH value was about 1.2) to control the discharged solution reached a maximum value after 30 days. The final extraction per- pH at a level for good bacterial activity. When the pH of the dis- centage of Cu for the leaching by the moderately thermophilic charged solution was less than 2.0, 5% (v/v) inoculums (moderately microorganisms and the sterile control at 50 °C were 92.5% and thermophilic microorganisms) with a cell density of about 107 cell/ 19.4%, respectively; in comparison, 52.4% Cu was bioleached by mL and nutrients of 3.0 g/L (NH4)2SO4, 0.5 g/L K2HPO4 and 0.02% A. ferrooxidans, while 26.6% Cu was leached by sterile control at yeast extract were added into the discharged solution. The dis- 30 °C. charged solution pH value was adjusted periodically to about 1.2 During the first 9 days of the bioleaching by the moderately during the bioleaching process. thermophilic microorganisms, the concentrations of total dissolved 202 W. Qin et al. / Bioresource Technology 129 (2013) 200–208

100 2000

90 a 1800 b

80 1600

70 1400

60 1200

50 1000

40 800

30 600 Copper extraction (%) 20 400 TFe concentration (mg/L) 10 200

0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Time / day Time / day

700 c 600

500

400 Sterile control (50 °C, pH 1.6) Moderately thermophilic microorganisms (50 °C, pH 1.6) 300 Sterile control (30 °C, pH 1.8) Acidithiobacillus ferrooxidans (30 °C, pH 1.8) 200

Redox potential (mV vs. SCE) 100

0 0 5 10 15 20 25 30 35 Time / day

Fig. 1. Copper extraction (a), the concentrations of total dissolved iron (b) and redox potentials (c) in leach liquors during the bioleaching of chalcopyrite.

iron increased to 1.4 g/L, then slightly decreased and stabilized at 2þ þ bacteria 3þ 4Fe þ O2 þ 4H ! 4Fe þ 2H2O ð2Þ about 1.2 g/L due to the precipitation of the soluble ferric iron as jarosite (Eq. 1), and finally reached 1.9 g/L after 30 days of biole- CuFeS 4Fe3þ Cu2þ 5Fe2þ 2S0 3 aching (Fig. 1b). The concentrations of total dissolved iron in- 2 þ ! þ þ ð Þ creased steadily and reached 1.4 g/L, 0.5 g/L and approximately 0.8 g/L after 30 days of leaching by A. ferrooxidans and sterile con- 0 bacteria 2 þ 2S þ 3O2 þ 2H2O ! 2SO4 þ 4H ð4Þ trol at 50 and 30 °C, respectively (Fig. 1b). There was clear benefit in leaching chalcopyrite within the low þ 3þ 2 þ solution potential, compared to the high potential leach. Neverthe- M þ 3Fe þ 2SO4 þ 6H2O ! MFe3ðSO4Þ2ðOHÞ6 þ 6H ð1Þ less, the solution potential of the sterile control at 50 and 30 °C was þ + + þ no more than 340 mV, with 80.6% and 73.4% copper still locked in where M is a monovalent cation, e.g., H3O ,Na,K and NH4 . In the first 3 days, the redox potentials of bioleaching with the the chalcopyrite. From these results, it was apparent that the solu- moderately thermophilic microorganisms reached 550 mV due to tion potential of the bioleaching system needed to be controlled at ferrous ions being oxidized to ferric by the microorganisms (Eq. an appropriate value (Hiroyoshi et al., 1997, 2001; Sandström et al., 2), while the potential slowly decreased from 550 to 390 mV over 2005; Vilcáez et al., 2008). the following 9 days due to the rate of ferrous oxidation to ferric by the microorganisms being less than the consumption rate of ferric 3.2. Effect of pH on copper extraction from the chalcopyrite via chalcopyrite oxidation (Eqs. 3 and 4) or the precipitation of the soluble ferric iron as jarosite (Eq. 1), and then the solution potential The leaching characteristics of chalcopyrite by cells at different was stabilized at about 400 mV. In contrast, the redox potentials pH, in terms of changes in Cu extraction percentage, the concentra- increased steadily and reached 640 mV after 30 days of bioleaching tions of total dissolved iron, ratio of [Fe2+]/[TFe], and redox poten- by A. ferrooxidans. While the solution potential of leaching by ster- tials are shown in Fig. 2. At pH 1.6, 92.5% Cu was bioleached after ile control at 50 and 30 °C was maintained at about 320 mV. The 30 days; in comparison, at pH 1.0 and 2.5 only 56.5% and 44.6% Cu copper extraction continued until 92.5% of the copper was recov- were bioleached after 30 days, respectively; while at pH 2.0, 77.7% ered from the chalcopyrite even though the solution potential Cu was bioleached after 30 days (Fig. 2a). was below 400 mV after 12 days leaching by the moderately ther- The work conducted by Hiroyoshi et al. (2001), demonstrated mophilic microorganisms. The copper extraction rate was superior that when the redox potential was under a critical value (310– to the rate obtained with the A. ferrooxidans mediated high poten- 400 mV vs. Ag/AgCl), the oxidative acid leach mechanism (Eq. 5) tial leach condition (above 400 mV after 12 days leaching). The was favored. Vilcáez et al. (2009) studied the effect of pH on chal- copper dissolution with the A. ferrooxidans almost stopped when copyrite leaching, and reported a lower pH notably promoted cop- the solution potential reached 550 mV after 24 days leaching. per leaching rate. Comparisons between the copper extractions at W. Qin et al. / Bioresource Technology 129 (2013) 200–208 203

100 600 pH1.0 90 a 500 b pH1.6 80 pH2.0 pH2.5 70 400 60

50 300

40 200 30 pH=1.0

Copper extraction (%) pH=1.6 20 100

pH=2.0 Redox potential (mV vs. SCE) 10 pH=2.5 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Time / day Time / day

2000 100

1800 pH1.0 90 c pH1.6 d 1600 pH2.0 80 1400 pH2.5 70 pH1.0 pH1.6 1200 60 pH2.0 1000 50 pH2.5

800 (%) /TFe 40 + 2

600 Fe 30

400 20 TFe concentration (mg/L) 200 10

0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Time / day Time / day

Fig. 2. Copper extraction (a), redox potentials (b), the concentrations of total dissolved iron (c) and ratio of Fe2+/TFe (d) in leach liquors during the bioleaching of chalcopyrite by moderately thermophilic microorganisms at different pH. different pH values (Fig. 2a) in bioleaching with moderately ther- 380 mV. While at pH 2.5, total iron remained below 200 mg/L mophilic microorganisms reveal that the highest Cu extraction (Fig. 2c) throughout the experiment due to brown iron (III) oxide percentage was obtained at pH 1.6, because the moderately ther- precipitates on the surfaces of the ore material. mophilic microorganisms has better oxidation ferrous ions and/ The most noteworthy observation from the results was that the or elemental sulfur activity at pH 1.6 than others (pH 1.0, 2.0 and copper dissolution stopped at pH 2.0 after 12 days. This phenome- 2.5). non also reflected on the concentrations of total dissolved iron, which started to decrease due to the ferric formation jarosite precipitation after 6 days. A second observation was the enhanced CuFeS þ O þ 4Hþ ! Cu2þ þ Fe2þ þ 2H O þ 2S0 ð5Þ 2 2 2 extraction percentage obtained during the high total iron concen- The high solution potential (above 400 mV) reflected negatively tration (pH 1.6, more than 1.0 g/L), compared with the low total on the copper recovery, achieving only 57.3% in 9 days at pH 1.6, iron concentration (pH 2.0, lower than 0.5 g/L), although the solu- compared to 71.8% with the low solution potential (about tion potential maintained at about 390 and 380 mV, respectively. 380 mV) at pH 2.0. There was a great advantage (14.5% higher cop- According to Eq. 2, increasing concentration of the ferric ions in per recovery) in leaching chalcopyrite within the low solution po- solution is supposed to result in higher amounts of copper release. tential, compared to the high potential leach. Whereas, at pH 1.0 However, it is reported that dissolution of chalcopyrite would be and 2.5, the solution potential was no more than 360 mV, with favored when redox potential under a critical value which was 44% and 56% copper still locked in the chalcopyrite. From the re- identified between approximately 310 and 400 mV (Ag/AgCl), or sults, it was evident that the solution potential and pH needed to at an appropriate ratio of [Fe3+]/[Fe2+](Hiroyoshi et al., 2001; be controlled at appropriate values. Sandström et al., 2005; Vilcáez et al., 2008). Vilcáez et al. (2009) The concentrations of total dissolved iron increased steadily and suggested that the leaching of chalcopyrite occurs through a direct reached 1.7 g/L after 30 days of bioleaching at pH 1.0 (Fig. 2c). At oxidation by ferric ion (Eq. 2), favored at high redox potential val- pH 1.6, during the first 9 days of the bioleaching, the concentra- ues (>450 mV vs. Ag/AgCl), and indirectly through the formation of tions of total dissolved iron increased to 1.4 g/L, then slightly de- intermediates, favored at low redox potential values (<450 mV vs. creased and stabilized at about 1.2 g/L and finally reached 1.9 g/L Ag/AgCl). Electrochemical studies have shown that chalcocite after 30 days of bioleaching (Fig. 2c). The rational was to control might be a possible intermediate compound during chalcopyrite the solution potential below 400 mV via the precipitation of the oxidation process whether in acid or alkaline environments (Arce soluble ferric iron as jarosite. At pH 2.0, the total iron concentration and González, 2002; Velásquez et al., 2005). Vilcáez and Inoue increased to approximately 0.8 g/L after 6 days of bioleaching, but (2009) indicated that chalcopyrite dissolution is regarded as a then, it started to decrease (Fig. 2c) due to jarosite precipitation. cooperation process of cathode reduction and anode oxidation, During all time, the solution potential also stabilized at about which is controlled at beginning by the concentrations of Fe2+ 204 W. Qin et al. / Bioresource Technology 129 (2013) 200–208 and Fe3+, respectively. At high concentrations of Fe3+, the rate of chalcopyrite oxidation tends to prevail over the rate of chalcopyrite reduction. On the contrary, at high concentrations of Fe2+, the rate of chalcopyrite reduction may prevail over the rate of chalcopyrite oxidation. Therefore, it seems at low redox potential, the cooper release rate is not controlled by concentration of Fe3+ but by Fe2+, which is extraordinary important for the formation of Cu2S (Vilcáez and Inoue, 2009). With increasing ratio of [Fe3+]/[Fe2+], redox potential would rise continuously over a critical value for chalcocite formation, and only direct oxidation of chalcopyrite (Eq. 1) exists, which would be more insoluble, and would drag the copper releasing rate (Vilcáez and Inoue, 2009; Vilcáez et al., 2008). Hiroyoshi et al. (2001) reported that at low redox potential, chalcopyrite is reduced in the first step by ferrous ions to form chalcocite which is more soluble at cathode (Eq. 6), and the formed chalcocite is oxidized in the second step by ferric ions at anode (Eq. 7).

2þ 2þ 3þ CuFeS2 þ 3Cu þ 3Fe ! 2Cu2S þ 4Fe ð6Þ Fig. 4. Cyclic voltammetry spectra for the bioleaching with moderately thermo- 3þ 2þ 2þ 0 philic microorganisms at different pH. 2Cu2S þ 8Fe ! 4Cu þ 8Fe þ 2S ð7Þ Apparently, the reactions above could happen synchronously only when a particular range of the redox potential is maintained, jarosite, but at pH 2.0, the main leaching product was jarosite, to- i.e., the redox potential must be low enough for chalcocite forma- gether with lesser amounts of elemental sulfur; the leaching prod- tion and high enough for the subsequent chalcocite oxidation. This ucts of the bioleaching at pH 1.6 were elemental sulfur and model suggests that the leaching of chalcopyrite should be en- jarosite; when the solution pH was 2.5, only lesser amounts of hanced by increasing not only the ferric ion concentration but also jarosite was observed on chalcopyrite surface. Klauber (2008) re- the ferrous and cupric ion concentrations. In the absence of suffi- ported that rate of jarosite formation increases dramatically with cient ferrous and cupric ions to accelerate the formation of chalco- temperature but the rate is very slow below 60 °C, and the rate cite, the boosting effect of ferric ion appears to be attenuated or is at a maximum at pH 2; jarosite formation is increasingly de- suppressed. pressed at pH < 1 or pH > 3. Due to the relationship between the ferrous/ferric iron ratio and the iron based solution potential, at a higher total iron concentra- 3.3. Cyclic voltammetry studies tion would therefore contribute to a higher ferrous and ferric iron concentration when the solution potential maintained at about Fig. 4 shows the influence of pH on the cyclic voltammograms of 390 mV (Fig. 2d). Thus, when the solution potential maintained massive chalcopyrite electrodes. Two cathodic peaks (C1 and C2) at about 390 mV, increasing the concentration of total dissolved were detected at all pH. Two anodic peaks (A1 and A4) were de- iron, which represents an accumulating Fe3+ and Fe2+, should favor tected at pH 2.5, two anodic peaks (A1 and A5) were detected at the chalcopyrite leaching. pH 2.0 and 1.6, and four anodic peaks (A1, A2, A3 and A4) were de- XRD analysis indicated that a significant amount of elemental tected at pH 1.0. sulfur and jarosite was formed on the chalcopyrite surface after A catholic peak C1 was detected at about 400–200 mV. Peak C1 bioleaching (Fig. 3). The main leaching product of the bioleaching should represent the reduction of copper ions and elemental sulfur at pH 1.0 was elemental sulfur, together with lesser amounts of to covellite (Eq. 8), and/or the reduction of copper ions and

7500

pH=2.5

5000 pH=2.0

pH=1.6 Intensity(Counts) 2500

pH=1.0

0 Chalcopyrite - CuFeS2

Jarosite - K(Fe3(SO4)2(OH)6)

Sulfur - S

10 20 30 40 50 60 70 2-Theta(°)

Fig. 3. The mineralogical compositions of residuals in bioleaching experiment with moderately thermophilic microorganisms at different pH. W. Qin et al. / Bioresource Technology 129 (2013) 200–208 205

Fig. 5. Cyclic voltammograms of chalcopyrite in the presence of different ratio of Fe2+/Fe3+ (a) and cyclic voltammogram focusing on peak C10 and C20 (b). 206 W. Qin et al. / Bioresource Technology 129 (2013) 200–208

2þ 2þ covellite to chalcocite (Eq. 9) (Gómez et al., 1996; López-Juárez 2CuFeS2 þ 3Cu þ 4e ! Cu5FeS4 þ Fe ð11Þ et al., 2006; Lee et al., 2008; Liang et al., 2011; Mikhlin et al., 2004). We considered the cupric ions during the bioleaching of 2þ 2þ CuFeS2 þ 3Cu þ 4e ! 2Cu2S þ Fe ð12Þ chalcopyrite were reduced through the same reactions taking place under this potential. þ 2þ 2Cu5FeS4 þ 6H þ 2e ! 5Cu2S þ 3H2S þ 2Fe ð13Þ Cu2þ þ S0 þ 2e ! CuS ð8Þ þ 2þ 2CuFeS2 þ 6H þ 2e ! Cu2S þ 3H2S þ 2Fe ð14Þ 2þ Cu þ CuS þ 2e ! Cu2S ð9Þ þ 2þ 9CuFeS2 þ 4H þ 2e ! Cu9Fe8S16 þ 2H2S þ Fe ð15Þ Another catholic peak C2 was observed at about 0 to 300 mV, simultaneity, when the pH reduced from 2.0 to 1.0, peak C2 was When the potential scan was switched to a positive direction at obviously negative shifted and an increase in current density is ob- 600 V, a series of anodic peak (A1, A2, A3, A4 and A5) was de- served. In the potential range of 100 to 100 mV vs. SHE, peak C2 tected. Many authors reported that chalcocite was oxidized to a should represent the initial reduction of chalcopyrite as shown in series of non-stoichiometric sulﬁdes Cu2xS in the potential range Eqs. (10)–(12) (Eghbalnia and Dixon, 2011; Liang et al., 2011; of 0–800 mV (Eq. 16) (Eghbalnia and Dixon, 2011; Elsherief, 2002; Mikhlin et al., 2004). When the scan continued moving to the neg- Liang et al., 2011; Mikhlin et al., 2004; Nava and González, 2006; ative direction, the reduction of bornite and chalcopyrite to chalco- Nava et al., 2008; Zeng et al., 2011). cite took place in the potential range of 100 to 560 mV vs. SHE 2þ Cu2S ! Cu2 xS þ xCu þ 2xe ð16Þ Eqs. 13 and 14 (Eghbalnia and Dixon, 2011; Liang et al., 2011; Mikhlin et al., 2004). However, Nava et al. (2008), Eghbalnia and Especially, peak A1 might represent the oxidation of hydrogen Dixon (2011) assigned the cathodic peak that appeared in the po- sulﬁde to elemental sulfur (Eghbalnia and Dixon, 2011; Elsherief, tential domain of 115 to 85 mV vs. SHE to chalcopyrite reduction 2002; Liang et al., 2011), as illustrated in Eq. 17. Peak A5 can be to talnakhite type compounds, as shown in Eq. 15, in the potential seen a pre-wave, which has been reported by others authors domain of 85 to 285 mV vs. SHE, the chalcopyrite is reduced to (Eghbalnia and Dixon, 2011; Elsherief, 2002; Liang et al., 2011; bornite (Eq. 10), as well as the reduction of bornite to chalcocite Mikhlin et al., 2004; Zeng et al., 2011). There is a selective disso- (Eq. 13), and in the potential domain of 135 to 385 mV vs. lution of iron from the crystal lattice of chalcopyrite according to SHE, the chalcopyrite is reduced to chalcocite (Eq. 14). Eq. 18.

þ 2þ 0 þ 5CuFeS2 þ 12H þ 4e ! Cu5FeS4 þ 6H2S þ 4Fe ð10Þ H2S ! S þ 2H þ 2e ð17Þ

60 500 a Cu b 50 Fe 450

40 400 III

30 350

20 300 Metal leached (%)

10 250 Redox potential (mV vs. SCE)

0 200 0 10203040506070 0 10203040506070 Leaching time / day Leaching time / day

3500 7 c TFe d 3000 Fe2+ 6

2500 5

2000 4

1500 pH 3

1000 2 Concentration (mg/L) 500 1

0 0 0 10203040506070 0 10203040506070 Leaching time / day Leaching time / day

Fig. 6. Metal leached (a), redox potentials (b), the concentrations of total dissolved iron and Fe2+ (c) and pH (d) in leach liquors during the bioleaching of copper-bearing sulﬁde ore by moderately thermophilic microorganisms. W. Qin et al. / Bioresource Technology 129 (2013) 200–208 207

2þ 2þ 0 CuFeS2 ! Cuð1xÞFeð1yÞSð2ZÞ þ xCu þ yFe zS þ 2ðx þ yÞe 5. Conclusions ð18Þ The leaching characteristic study showed the existence of an 2+ 3+ Fig. 5 shows the influence of ratio of Fe /Fe on the cyclic vol- optimum domain of the pH (around 1.6) and solution potential tammograms of massive chalcopyrite electrodes. A cathodic peaks (below 400 mV) in the leaching of chalcopyrite by the moderately (C20) and two anodic peaks (A10 and A30) were detected without thermophilic microorganisms, with simultaneous maintenance of the addition of Fe ions, a cathodic peaks (C20) and two anodic peaks an appropriate concentration of total dissolved iron. 2+ (A10 and A20) were detected with the addition of 4 g/L Fe ions, The analysis of CV results revealed that the chalcopyrite is re- two cathodic peaks (C10 and C20) and two anodic peaks (A10 and duced to a series of intermediate products (such as talnakhite, 3+ A20) were detected with the addition of Fe ions. bornite and chalcocite) at the low redox potential. The leaching A cathodic peak C10 was observed at about 400 mV. This peak is of the copper-bearing sulfide ore by the moderately thermophilic attributed to ferric reduction according to reaction (Eq. 19) microorganisms indicated that the reduction of chalcopyrite to tal- (Eghbalnia and Dixon, 2011; Gómez et al., 1996; Zeng et al., nakhite and/or bornite is the rate-limiting step. 2011). Current densities increased with decreasing ratio of Fe2+/ Fe3+ in the electrolyte, thus proving that this peak is caused by Acknowledgements the reduction of ferric. C20 and C2 are similar, so, we did not discuss here. By comparing the voltammograms of Fig. 5, it can be seen This work was supported by the National Basic Research Pro- that peak C20 was obviously negative shifted with decreasing ratio gram of China (No. 2010CB630905) and the High-tech Research of Fe2+/Fe3+ in the electrolyte. Ferrous ions act as a reductant, and Development Program of China (No. 2012AA061501). which enhance the formation of intermediate CuS2 (Elsherief, 2002). References Fe3þ þ e ! Fe2þ ð19Þ Arce, E.M., González, I., 2002. A comparative study of electrochemical behavior of Peak A10 can be seen a pre-wave (Eghbalnia and Dixon, 2011; chalcopyrite, chalcocite and bornite in sulfuric acid solution. Int. J. Miner. Elsherief, 2002; Liang et al., 2011; Mikhlin et al., 2004; Zeng Process. 67, 17–28. Dutrizac, J.E., 2008. Factors affecting the precipitation of potassium jarosite in et al., 2011), which is a selective dissolution of iron from the crystal sulfate and chloride media. Metall. Mater. Trans. B 39, 771–783. lattice of chalcopyrite according to Eq. 21. Peak A20 represents oxi- Eghbalnia, M., Dixon, D.G., 2011. Electrochemical study of leached chalcopyrite dation of Fe2+ to Fe3+ (Eq. 20)(Eghbalnia and Dixon, 2011). Peak A30 using solid paraffin-based carbon paste electrodes. Hydrometallurgy 110, 1–12. Elsherief, A.E., 2002. 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