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Spectroscopic analysis of the bioleaching of chalcopyrite by Acidithiobacillus caldus
Hydrometallurgy 127-128 (2012) 116–120
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Spectroscopic analysis of the bioleaching of chalcopyrite by Acidithiobacillus caldus☆ Keiko Sasaki a,⁎, Koichiro Takatsugi a, Olli H. Tuovinen a, b a b
Department of Earth Resources Engineering, Kyushu University, Fukuoka 819‐0395, Japan Department of Microbiology, Ohio State University, Columbus, OH 43210‐1292, USA
a r t i c l e
i n f o
Available online 3 August 2012 Keywords: Acidithiobacillus caldus Bioleaching Chalcopyrite Raman spectroscopy X-ray photoelectron spectroscopy
a b s t r a c t The formation and consumption of elemental S were investigated in the bioleaching of chalcopyrite within and without a CE dialysis membrane in Acidithiobacillus caldus cultures. The bacteria were supplemented with tetrathionate outside the membrane. XPS analysis showed that monosulfide species and elemental S were formed on CuFeS2 surface regardless of the enclosure in the dialysis membrane. They diminished over time in the inoculated systems. In the membrane enclosure, colloidal S partially dissolves and can pass the dialysis membrane for bacterial oxidation to sulfuric acid. Anaerobic pre-oxidation of CuFeS2 by Fe 3+ produced S-rich deposition on mineral surface. The solubilization of Cu from membrane-enclosed pre-oxidized chalcopyrite in the A. caldus culture was much slower as compared to direct contact with bacteria. The difference is attributed to slow diffusion through the dialysis membrane as well as clogging caused by polymerized S on the inside of the membrane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite is a worldwide Cu mineral resource but its recovery by conventional mining technology poses a major technical and economic predicament if the grade is low. It is possible to exploit bioleaching techniques for processing of low-grade chalcopyrite ores but in practice this route is hampered by recalcitrance of CuFeS2. This is largely due to surface passivation with secondary minerals formed during oxidation of CuFeS2 (Klauber, 2008; Watling, 2006). Passivation can gradually reduce the leaching to negligible rates with time. Bioleaching at thermophilic temperatures can partially overcome the passivation effect due to enhanced rates of microbial activity (Vilcáez et al., 2008). Re-grinding of passivated CuFeS2 can also eliminate the passivated surface layers. In the bioleaching, chalcopyrite is subject to proton attack and oxidation by Fe3+ (Watling, 2006). In partial chalcopyrite oxidation the S-entity is converted to elemental S which may deposit on mineral surface. In addition, Fe3+ forms insoluble hydroxysulfate complexes in leach solutions at pH >1.5, and their precipitation on CuFeS2 surfaces is also a major cause of passivation (Klauber, 2008; Pradhan et al., 2008; Sasaki et al., 2009; Watling, 2006). Bioleaching of CuFeS2 while preventing or suppressing iron oxidation has been tested with promising results (Gericke et al., 2010). Such low redox bioleaching may be feasible in tank leaching as it would involve the exclusion of iron-oxidizing bacteria and archaea from the leaching systems. ☆ This paper was originally presented at the International Biohydrometallurgy Symposium (IBS), Changsha, China, 18-22 September 2011. ⁎ Corresponding author. Tel./fax: +81 92 802 3338. E-mail address: [email protected] (K. Sasaki). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.07.013
For assessing the passivation and mechanism of bioleaching of chalcopyrite, bacteria that are only capable of sulfur compound oxidation offer a research tool to investigate sulfur transformations without the Fe 3+/Fe 2+ redox shuttle. Acidithiobacillus caldus, a sulfur-oxidizing moderate thermophile (Mangold et al., 2011), is relatively common in bioleaching systems together with iron-oxidizing bacteria (Fouchera et al., 2003; Mutch et al., 2010; Zeng et al., 2009; Zhou et al. 2009). It produces sulfuric acid from sulfur and may, therefore, help delay the onset of passivation caused by elemental S formation. The lack of iron oxidation by A. caldus makes it possible to investigate sulfur formation and oxidation during CuFeS2 bioleaching as a discreetly separate system from the usually associated iron oxidation and jarosite precipitation. The purpose of this work was to elucidate the role of A. caldus in the bioleaching of CuFeS2 and discern between sulfur transformations in direct contact and indirect contact mechanisms. 2. Experimental 2.1. Mineral The museum-grade specimen of chalcopyrite was obtained from Sayama Mines, Japan. The sample was ground to − 77 to + 38 μm size fraction under N2 headspace. The sample contained 35.41% Cu, 29.56% Fe, 30.27% S, and 0.82% Si and had a molar ratio Cu:Fe:S = 1:0.94:1.96. The sample was estimated to be about 95% CuFeS2 assuming all Cu was associated with chalcopyrite. Thus chalcopyrite was the predominant sulfide mineral in the sample, as confirmed by X-ray diffraction (XRD Multi-Flex, Rigaku, Japan), with quartz as a minor impurity.
K. Sasaki et al. / Hydrometallurgy 127-128 (2012) 116–120
10
2.8
pH
2.6 2.4
sterile sterile, enclosed in membrane
2.2 2.0
inoculated inoculated, enclosed in membrane
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4 5 2 0
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c)
8
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b) Fe concentration / mM
Cell number / cells ml-1
1010
Cu recovery / %
Cu concentration / mM
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2
109 6 4 2
108 6 4
d) 15
8 6
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4 5
Fe recovery / %
3.2
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2
107
0
20
40
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80
100
120
0
0
Time / day
20
40
60
80
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0 120
Time / day
Fig. 1. Changes in (a) pH, (b) cell counts and dissolved (c) Cu and (d) Fe concentrations in A. caldus cultures (closed symbols) and sterile controls (open symbols). ●&○: CuFeS2 suspended in the medium; ▲&△: CuFeS2 enclosed in dialysis membrane.
0.002 g H3BO3 , 0.002 g MnSO4·H2O, 0.0006 g CoCl2·6H2O, 0.0009 g ZnSO4·7H2O, and 0.0008 g Na2MoO4·2H2O; pH 1.5.
2.2. Culture conditions A. caldus (ATCC 51756), a moderately thermophilic acidophile, was procured from the American Type Culture Collection (ATCC). This bacterium oxidizes inorganic S-compounds including elemental S and tetrathionate, but not ferrous iron (Hassan et al., 2010; Shiers et al., 2011). Sulfide oxidation by A. caldus is unclear at this time. It was subcultured with tetrathionate at 45 °C in modified ATCC 1995 medium containing (per L): 3.85 g Na2S4O6, 3.0 g (NH4)2SO4, 3.2 g Na2SO4·10H2O, 0.1 g KCl, 0.05 g K2HPO4, 0.5 g MgSO4·7H2O, 0.01 g Ca (NO3)2·4H2O, 0.011 g FeCl3·6H2O, 0.0005 g CaSO4·5H2O,
a) S 2p 1 kcps
2.3. Bioleaching of chalcopyrite In the first bioleaching experiment, finely ground chalcopyrite (0.5 g, 38 μm–74 μm) was suspended in 50 mL tetrathionate medium in 200 mL conical flasks, and A. caldus was inoculated to 1×107 cells/mL. To investigate whether a direct contact of A. caldus with chalcopyrite is necessary, finely ground CuFeS2 (0.5 g) was also enclosed in a Spectra/ Por CE dialysis membrane MWCO 5000 (Spectrum Japan, Otsu,
b) S 2p 110 days
c) S 2p
after 80 days (strile) 1 kcps
1 kcps
110 days
85 days
25 days
after 50 days
Intensity / cps
50 days
Intensity / cps
Intensity / cps
after 80 days
50 days
25 days 4 days
0 day
0 day
0 day
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
Binding energy / eV
Binding energy / eV
Binding energy / eV
Fig. 2. Changes in XPS S 2p spectra for the solid residues after leaching of CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria; (b) CuFeS2 was enclosed in dialysis membrane; (c) sterile control (no dialysis membrane).
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K. Sasaki et al. / Hydrometallurgy 127-128 (2012) 116–120
a)
b)
after 110 days
Intensity/ arbitrary unit
after 50 days
after 25 days
after 4 days
sulfur
Intensity/ arbitrary unit
after 80 days
after 85 days
Intensity/ arbitrary unit
c)
after 80 days (sterile)
after 110 days
after 50 days
sulfur
after 50 days
after 25 days
sulfur
covellite
covellite
covellite
500
450
potassium jarosite
potassium jarosite
potassium jarosite
chalcopyrite
chalcopyrite
chalcopyrite
400
350
300
250
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500
450
Raman shift/ cm-1
400
350
300
250
200
500
450
Raman shift/ cm-1
400
350
300
250
200
Raman shift/ cm-1
Fig. 3. Changes in Raman spectra for solid residues after leaching of CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria; (b) CuFeS2 was enclosed in dialysis membrane; (c) sterile control (no dialysis membrane).
Shiga) and immersed in 50 mL A. caldus cultures in conical flasks. Special care was taken to ascertain that bacteria were not introduced into the dialysis membrane. Sterile controls were included in parallel experiments. The flasks were incubated in a water bath at 100 strokes/min and at 45 °C. In the second bioleaching experiment, chalcopyrite was first pre-oxidized with 50 mM FeCl3 at 22± 2 °C under anaerobic conditions to produce a sulfur layer on chalcopyrite, as already previously confirmed by Raman spectroscopy (Sasaki et al., 1995). The pre-oxidized, passivated chalcopyrite was collected by filtration and freeze-dried.
2.8
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Fe concentration / mM
Cell number / cells ml-1
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Time / day
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60
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8 6
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4 5
Fe recovery / %
3.0
10
Cu recovery / %
inoculated inoculated, enclosed in membrane sterile sterile, enclosed in membrane
a)
Cu concentration / mM
3.2
The conditions for the subsequent aerobic oxidation experiments were the same as in the first experiment. At intervals, samples of leach solution were removed for pH and Eh measurements and microscopic (×600) cell counts. Samples were filtered (0.2 μm) for the determination of Cu and Fe concentrations with ICP-AES (Vista-MPX, Chiba, Japan). The concentrations of dissolved Fetotal and Fe2+ were measured using the 1,10-phenanthroline method with ascorbic acid as the reducing agent (Tamura et al., 1974). At the end of each batch experiment the solid residues were recovered by filtration and freeze-dried before analysis by Raman spectroscopy
2
2
107
0
20
40
60
Time / day
80
100
120
0
0
20
40
60
80
100
0 120
Time / day
Fig. 4. Changes in (a) pH, (b) cell counts and dissolved (c) Cu and (d) Fe concentrations in A. caldus cultures (closed symbols) and sterile controls (open symbols). ●&○: pre-oxidized CuFeS2 suspended in the medium; ▲&△: pre-oxidized CuFeS2 enclosed in dialysis membrane.
K. Sasaki et al. / Hydrometallurgy 127-128 (2012) 116–120
a) S 2p
b) S 2p
1 kcps after 85 days (sterile)
1 kcps after 85 days (sterile)
after 85 days
after 85 days
Intensity / cps
Intensity / cps
119
after 43 days
after 43 days
after 11 days
after 11days
0 day
0 day
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
Binding energy / eV
Binding energy / eV
Fig. 5. Changes in XPS S 2p spectra for solid residues after leaching of pre-oxidized CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria and 85-d sterile control (no dialysis membrane); (b) pre-oxidized CuFeS2 and 85-d sterile control, both in dialysis membrane.
(NRS 2000, JASCO International, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI 5800 ESCA system (ULVAC-PHI, Chigasaki, Kanagawa, Japan). For Raman spectroscopy, excitation was accomplished with an Ar ion laser (514.5 nm, incident power 50 mW) and detected by a laser Raman spectrometer. The laser light was standardized with silicon, using a band maximum at 520.2 cm−1. Samples were diluted to 5 wt.% with KBr powder and compressed to a 10 mm disk. The Raman spectra were obtained after four accumulations using 60 s integration times. The XP spectra were analyzed with Casa XPS software (Version 2.3.12). Background corrections were made using the Shirley method (Hassan et al., 2010; Shiers et al., 2011). For O 1s, C 1s, Cu 2p, and S 2p spectra. Peak shapes were defined using a Gaussian– Lorentzian mixed function. The C–C binding energy, EB[C 1s], from the vacuum pump oil in the apparatus has a well-defined value at 284.6 eV, which was used as a reference peak to correct for differential charging effects.
3. Results and discussion 3.1. Experiments with CuFeS2 Fig. 1(a), (b) shows similar decreases in pH and increases in microscopic cell counts within a few days in A. caldus culture systems regardless of the enclosure of chalcopyrite inside the membrane. This initial phase represented tetrathionate oxidation to sulfuric acid. The dissolution of Cu and Fe over time was noticeably suppressed when chalcopyrite was enclosed in the dialysis tubing out of direct contact with A. caldus (Fig. 1(c), (d)). Because CuFeS2 was not in direct contact with bacteria, the suppressed Cu and Fe solubilization is attributed to slow leaching due to proton attack and diffusion-limitation of fluxes of reactants (mainly sulfuric acid and dissolved O2) and products (dissolved metals) across the dialysis membrane.
a)
b)
after 85 days (sterile)
after 85 days (sterile)
after 85 days after 43 days
Intensity/ arbitrary unit
Intensity/ arbitrary unit
after 85 days
after 43 days after 11 days oxidized chalcopyrite
after 11 days
oxidized chalcopyrite sulfur
sulfur
covellite
covellite potassium jarosite potassium jarosite chalcopyrite
500
450
400
350
chalcopyrite
300
Raman shift/ cm-1
250
200
500
450
400
350
300
250
200
Raman shift/ cm-1
Fig. 6. Changes in Raman spectra for solid residues after leaching of pre-oxidized CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria and 85-d sterile control (no dialysis membrane); (b) pre-oxidized CuFeS2 and 85-d sterile control, both in dialysis membrane.
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Fig. 2(a), (b) shows changes in S 2p spectra for solid residues after the bioleaching of CuFeS2 in A. caldus cultures with direct contact or separated with dialysis membrane. These XPS data showed the presence of monosulfide and elemental S species under both conditions. The predominant species was still CuFeS2 after 110 days of contact. In the sterile control elemental S accumulated on the surface of chalcopyrite (Fig. 2(c)). Thus a comparison of these S 2p spectra indicates that A. caldus oxidized elemental S, which was formed as a secondary phase during the bioleaching of CuFeS2. Raman spectra for the solids were consistent with the XPS results. Elemental S was oxidized in the inoculated systems but remained in the bulk phase of the sterile controls regardless of the membrane (Fig. 3(a), (b), (c)). The results did not show a time-dependent sequence of formation and oxidation of elemental S on chalcopyrite surface in the inoculated system. 3.2. Experiments with pre-oxidized CuFeS2 In the second experiment, a S-rich layer was chemically produced on chalcopyrite surface by anaerobic pre-oxidation with Fe 3+. During this chemical pre-oxidation phase incongruent dissolution was observed with the preferential dissolution of Cu over S at pH 2 (data not shown). Fig. 4(a), (b) shows changes in pH and microscopic cell counts in the aerobic bioleaching of pre-oxidized chalcopyrite in inoculated systems with and without the dialysis membrane. The pH decreased and cell numbers increased within a few days in both inoculated systems. In this case, the decrease in pH was more extensive than shown in the first experiment. Again, the dissolution of Cu and Fe was suppressed when chalcopyrite was enclosed in dialysis membrane (Fig. 4(c), (d)). Fig. 5(a), (b) shows changes in the S 2p spectra for solid residues after the bioleaching of pre-oxidized chalcopyrite ± membrane and a comparison with the sterile control after 85 days. Elemental S was oxidized in all cases. Raman spectra for these samples indicated the lack of elemental S in bulk phases of all samples (Fig. 6). Thus the data indicated that elemental S layers on CuFeS2 were oxidized by A. caldus. Both tetrathionate and elemental S oxidation produce sulfuric acid. The bioleaching of Cu from CuFeS2 by A. caldus involves proton attack by sulfuric acid because this bacterium is not an iron oxidizer. While the complete oxidation of CuFeS2 is an acid-producing reaction, dissolution by proton attack and dissolved O2 consumes acid, involves a low Eh and forms elemental S on mineral surface. þ
2þ
CuFeS2 þ O2 þ 4H →Cu
2þ
þ Fe
0
þ 2S þ 2H2 O
Precursors of colloidal S and the rhombic elemental S ring structure (S8) can be ionized as polysulfide complexes such as H–S–S–S–S–S–S– S–S–H=–S–S–S–S–S–S–S–S– (S82−) + 2 H+ (Harmer et al., 2006; Steudel, 1996). The ionized precursor form can diffuse through the dialysis membrane. However, with increasing formation of elemental S, polymerized S accumulated inside the dialysis membrane, which resulted in the clogging and impeded the leaching and diffusion of Cu and Fe to the bulk solution. 4. Conclusions The oxidation of sulfur to sulfuric acid was the primary role of A. caldus in CuFeS2 bioleaching. Separation of CuFeS2 with a dialysis
membrane from A. caldus resulted in slow copper and iron dissolution. Iron precipitation as jarosite on CuFeS2 surface was prevented by virtue of using bacteria that only oxidized sulfur compounds. The oxidation of S formed from CuFeS2 in the membrane enclosure was also slow and caused colloidal S accumulation inside the membrane. This accumulation suggested that S-rich layers are not all tenacious associated with CuFeS2 surface. Acknowledgements Financial support was provided to KS by the Japan Society for the Promotion of Science (JSPS Grant-in-Aid for Scientific Research No. 22246117), the JSPS Funding Program for Next Generation WorldLeading Researchers (GR078) and JSPS Invitation Fellowship Program for Research in Japan (Short Term) FY2011 (S11078). Raman spectroscopy and XPS measurements were performed at the Center of Advanced Instrumental Analysis, Kyushu University. References Fouchera, S., Battaglia-Bruneta, F., d'Huguesa, P., Clarens, M., Godonc, J.J., Morin, D., 2003. Evolution of the bacterial population during the batch bioleaching of a cobaltiferous pyrite in a suspended-solids bubble column and comparison with a mechanically agitated reactor. Hydrometallurgy 71, 5–12. Gericke, M., Govender, Y., Pinches, A., 2010. Tank bioleaching of low-grade chalcopyrite concentrates using redox control. Hydrometallurgy 104, 414–419. Harmer, S.L., Thomas, J.E., Fornasiero, D., Gerson, A.R., 2006. The evolution of surface layers formed during chalcopyrite leaching. Geochim. Cosmochim. Acta 70, 4392–4402. Hassan, S.H.A., Van Ginkel, S.W., Kim, S.-M., Yoon, S.-H., Joo, J.-H., Shin, B.-S., Jeon, B.-H., Bae, W., Oh, S.-E., 2010. Isolation and characterization of Acidithiobacillus caldus from a sulfur-oxidizing bacterial biosensor and its role in detection of toxic chemicals. J. Microbiol. Meth. 82, 151–155. Klauber, C., 2008. A critical review of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite with regards to hindered dissolution. Int. J. Miner. Process. 86, 1–17. Mangold, S., Valdés, J., Holmes, D.S., Dopson, M., 2011. Sulfur metabolism in the extreme acidophile Acidithiobacillus caldus. Front. Microbiol. 2, 17 http://dx.doi.org/10.3389/ fmicb.2011.00017. Mutch, L.A., Watling, H.R., Watkin, E.L.J., 2010. Microbial population dynamics of inoculated low-grade chalcopyrite bioleaching columns. Hydrometallurgy 104, 391–398. Pradhan, N., Nathsarma, K.C., Rao, Sinivasa, Sukla, L.B., Mishra, B.K., 2008. Heap bioleaching of chalcopyrite: a review. Miner. Eng. 21, 355–365. Sasaki, K., Tsunekawa, M., Ohtsuka, T., Konno, H., 1995. Confirmation of sulfur-rich layer formed on pyrite after dissolution by Fe(III) ions around pH 2. Geochim. Cosmochim. Acta 59, 3155–3158. Sasaki, K., Nakamuta, Y., Hirajima, T., Tuovinen, O.H., 2009. Raman characterization of secondary minerals formed during chalcopyrite leaching with Acidithiobacillus ferrooxidans. Hydrometallurgy 95, 153–158. Shiers, D.W., Ralph, D.E., Watling, H.R., 2011. Batch culture of Acidithiobacillus caldus on tetrathionate. Biochem. Eng. J. 54, 185–191. Steudel, R., 1996. Mechanism for the formation of elemental sulfur from aqueous sulfide in chemical and microbiological desulfurization processes. Ind. Eng. Chem. Res. 35, 1417–1423. Tamura, H., Goto, K., Yotsuyanagi, T., Nagayama, M., 1974. Spectroscopic determination of iron (II) with 1, 10-phenanthroline in the presence of large amounts of iron (II). Talanta 21, 314–318. Vilcáez, J., Suto, K., Inoue, C., 2008. Bioleaching of chalcopyrite with thermophiles: temperature–pH–ORP dependence. Int. J. Miner. Process. 88, 37–44. Watling, H.R., 2006. The bioleaching of sulphide minerals with emphasis on copper sulphides — a review. Hydrometallurgy 84, 81–108. Zeng, W.-M., Zhou, H.-B., Wan, M.X., Chao, W.-L., Xu, A.-L., Liu, X.-D., Qiu, G.-Z., 2009. Preservation of Acidithiobacillus caldus: a moderately thermophilic bacterium and the effect on subsequent bioleaching of chalcopyrite. Hydrometallurgy 96, 333–336. Zhou, H.-B., Zeng, W.-M., Yang, Z.-F., Xie, Y.-J., Qiu, G.-Z., 2009. Bioleaching of chalcopyrite concentrate by a moderately thermophilic culture in a stirred tank reactor. Bioresour. Technol. 100, 515–520.
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Spectroscopic analysis of the bioleaching of chalcopyrite by Acidithiobacillus caldus☆ Keiko Sasaki a,⁎, Koichiro Takatsugi a, Olli H. Tuovinen a, b a b
Department of Earth Resources Engineering, Kyushu University, Fukuoka 819‐0395, Japan Department of Microbiology, Ohio State University, Columbus, OH 43210‐1292, USA
a r t i c l e
i n f o
Available online 3 August 2012 Keywords: Acidithiobacillus caldus Bioleaching Chalcopyrite Raman spectroscopy X-ray photoelectron spectroscopy
a b s t r a c t The formation and consumption of elemental S were investigated in the bioleaching of chalcopyrite within and without a CE dialysis membrane in Acidithiobacillus caldus cultures. The bacteria were supplemented with tetrathionate outside the membrane. XPS analysis showed that monosulfide species and elemental S were formed on CuFeS2 surface regardless of the enclosure in the dialysis membrane. They diminished over time in the inoculated systems. In the membrane enclosure, colloidal S partially dissolves and can pass the dialysis membrane for bacterial oxidation to sulfuric acid. Anaerobic pre-oxidation of CuFeS2 by Fe 3+ produced S-rich deposition on mineral surface. The solubilization of Cu from membrane-enclosed pre-oxidized chalcopyrite in the A. caldus culture was much slower as compared to direct contact with bacteria. The difference is attributed to slow diffusion through the dialysis membrane as well as clogging caused by polymerized S on the inside of the membrane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite is a worldwide Cu mineral resource but its recovery by conventional mining technology poses a major technical and economic predicament if the grade is low. It is possible to exploit bioleaching techniques for processing of low-grade chalcopyrite ores but in practice this route is hampered by recalcitrance of CuFeS2. This is largely due to surface passivation with secondary minerals formed during oxidation of CuFeS2 (Klauber, 2008; Watling, 2006). Passivation can gradually reduce the leaching to negligible rates with time. Bioleaching at thermophilic temperatures can partially overcome the passivation effect due to enhanced rates of microbial activity (Vilcáez et al., 2008). Re-grinding of passivated CuFeS2 can also eliminate the passivated surface layers. In the bioleaching, chalcopyrite is subject to proton attack and oxidation by Fe3+ (Watling, 2006). In partial chalcopyrite oxidation the S-entity is converted to elemental S which may deposit on mineral surface. In addition, Fe3+ forms insoluble hydroxysulfate complexes in leach solutions at pH >1.5, and their precipitation on CuFeS2 surfaces is also a major cause of passivation (Klauber, 2008; Pradhan et al., 2008; Sasaki et al., 2009; Watling, 2006). Bioleaching of CuFeS2 while preventing or suppressing iron oxidation has been tested with promising results (Gericke et al., 2010). Such low redox bioleaching may be feasible in tank leaching as it would involve the exclusion of iron-oxidizing bacteria and archaea from the leaching systems. ☆ This paper was originally presented at the International Biohydrometallurgy Symposium (IBS), Changsha, China, 18-22 September 2011. ⁎ Corresponding author. Tel./fax: +81 92 802 3338. E-mail address: [email protected] (K. Sasaki). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.07.013
For assessing the passivation and mechanism of bioleaching of chalcopyrite, bacteria that are only capable of sulfur compound oxidation offer a research tool to investigate sulfur transformations without the Fe 3+/Fe 2+ redox shuttle. Acidithiobacillus caldus, a sulfur-oxidizing moderate thermophile (Mangold et al., 2011), is relatively common in bioleaching systems together with iron-oxidizing bacteria (Fouchera et al., 2003; Mutch et al., 2010; Zeng et al., 2009; Zhou et al. 2009). It produces sulfuric acid from sulfur and may, therefore, help delay the onset of passivation caused by elemental S formation. The lack of iron oxidation by A. caldus makes it possible to investigate sulfur formation and oxidation during CuFeS2 bioleaching as a discreetly separate system from the usually associated iron oxidation and jarosite precipitation. The purpose of this work was to elucidate the role of A. caldus in the bioleaching of CuFeS2 and discern between sulfur transformations in direct contact and indirect contact mechanisms. 2. Experimental 2.1. Mineral The museum-grade specimen of chalcopyrite was obtained from Sayama Mines, Japan. The sample was ground to − 77 to + 38 μm size fraction under N2 headspace. The sample contained 35.41% Cu, 29.56% Fe, 30.27% S, and 0.82% Si and had a molar ratio Cu:Fe:S = 1:0.94:1.96. The sample was estimated to be about 95% CuFeS2 assuming all Cu was associated with chalcopyrite. Thus chalcopyrite was the predominant sulfide mineral in the sample, as confirmed by X-ray diffraction (XRD Multi-Flex, Rigaku, Japan), with quartz as a minor impurity.
K. Sasaki et al. / Hydrometallurgy 127-128 (2012) 116–120
10
2.8
pH
2.6 2.4
sterile sterile, enclosed in membrane
2.2 2.0
inoculated inoculated, enclosed in membrane
1.8 1.6
0
20
40
60
80
100
15
6
10
4 5 2 0
120
c)
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Time / day 6 4
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Time / day 10
b) Fe concentration / mM
Cell number / cells ml-1
1010
Cu recovery / %
Cu concentration / mM
a)
3.0
2
109 6 4 2
108 6 4
d) 15
8 6
10
4 5
Fe recovery / %
3.2
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2
2
107
0
20
40
60
80
100
120
0
0
Time / day
20
40
60
80
100
0 120
Time / day
Fig. 1. Changes in (a) pH, (b) cell counts and dissolved (c) Cu and (d) Fe concentrations in A. caldus cultures (closed symbols) and sterile controls (open symbols). ●&○: CuFeS2 suspended in the medium; ▲&△: CuFeS2 enclosed in dialysis membrane.
0.002 g H3BO3 , 0.002 g MnSO4·H2O, 0.0006 g CoCl2·6H2O, 0.0009 g ZnSO4·7H2O, and 0.0008 g Na2MoO4·2H2O; pH 1.5.
2.2. Culture conditions A. caldus (ATCC 51756), a moderately thermophilic acidophile, was procured from the American Type Culture Collection (ATCC). This bacterium oxidizes inorganic S-compounds including elemental S and tetrathionate, but not ferrous iron (Hassan et al., 2010; Shiers et al., 2011). Sulfide oxidation by A. caldus is unclear at this time. It was subcultured with tetrathionate at 45 °C in modified ATCC 1995 medium containing (per L): 3.85 g Na2S4O6, 3.0 g (NH4)2SO4, 3.2 g Na2SO4·10H2O, 0.1 g KCl, 0.05 g K2HPO4, 0.5 g MgSO4·7H2O, 0.01 g Ca (NO3)2·4H2O, 0.011 g FeCl3·6H2O, 0.0005 g CaSO4·5H2O,
a) S 2p 1 kcps
2.3. Bioleaching of chalcopyrite In the first bioleaching experiment, finely ground chalcopyrite (0.5 g, 38 μm–74 μm) was suspended in 50 mL tetrathionate medium in 200 mL conical flasks, and A. caldus was inoculated to 1×107 cells/mL. To investigate whether a direct contact of A. caldus with chalcopyrite is necessary, finely ground CuFeS2 (0.5 g) was also enclosed in a Spectra/ Por CE dialysis membrane MWCO 5000 (Spectrum Japan, Otsu,
b) S 2p 110 days
c) S 2p
after 80 days (strile) 1 kcps
1 kcps
110 days
85 days
25 days
after 50 days
Intensity / cps
50 days
Intensity / cps
Intensity / cps
after 80 days
50 days
25 days 4 days
0 day
0 day
0 day
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
Binding energy / eV
Binding energy / eV
Binding energy / eV
Fig. 2. Changes in XPS S 2p spectra for the solid residues after leaching of CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria; (b) CuFeS2 was enclosed in dialysis membrane; (c) sterile control (no dialysis membrane).
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a)
b)
after 110 days
Intensity/ arbitrary unit
after 50 days
after 25 days
after 4 days
sulfur
Intensity/ arbitrary unit
after 80 days
after 85 days
Intensity/ arbitrary unit
c)
after 80 days (sterile)
after 110 days
after 50 days
sulfur
after 50 days
after 25 days
sulfur
covellite
covellite
covellite
500
450
potassium jarosite
potassium jarosite
potassium jarosite
chalcopyrite
chalcopyrite
chalcopyrite
400
350
300
250
200
500
450
Raman shift/ cm-1
400
350
300
250
200
500
450
Raman shift/ cm-1
400
350
300
250
200
Raman shift/ cm-1
Fig. 3. Changes in Raman spectra for solid residues after leaching of CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria; (b) CuFeS2 was enclosed in dialysis membrane; (c) sterile control (no dialysis membrane).
Shiga) and immersed in 50 mL A. caldus cultures in conical flasks. Special care was taken to ascertain that bacteria were not introduced into the dialysis membrane. Sterile controls were included in parallel experiments. The flasks were incubated in a water bath at 100 strokes/min and at 45 °C. In the second bioleaching experiment, chalcopyrite was first pre-oxidized with 50 mM FeCl3 at 22± 2 °C under anaerobic conditions to produce a sulfur layer on chalcopyrite, as already previously confirmed by Raman spectroscopy (Sasaki et al., 1995). The pre-oxidized, passivated chalcopyrite was collected by filtration and freeze-dried.
2.8
pH
2.6 2.4 2.2 2.0 1.8 1.6
0
20
40
60
80
100
15
8 6
10
4 5 2 0
120
c)
0
20
40
10
b)
Fe concentration / mM
Cell number / cells ml-1
6 4
80
100
0 120
Time / day
Time / day 1010
60
2
109 6 4 2
108 6 4
d) 15
8 6
10
4 5
Fe recovery / %
3.0
10
Cu recovery / %
inoculated inoculated, enclosed in membrane sterile sterile, enclosed in membrane
a)
Cu concentration / mM
3.2
The conditions for the subsequent aerobic oxidation experiments were the same as in the first experiment. At intervals, samples of leach solution were removed for pH and Eh measurements and microscopic (×600) cell counts. Samples were filtered (0.2 μm) for the determination of Cu and Fe concentrations with ICP-AES (Vista-MPX, Chiba, Japan). The concentrations of dissolved Fetotal and Fe2+ were measured using the 1,10-phenanthroline method with ascorbic acid as the reducing agent (Tamura et al., 1974). At the end of each batch experiment the solid residues were recovered by filtration and freeze-dried before analysis by Raman spectroscopy
2
2
107
0
20
40
60
Time / day
80
100
120
0
0
20
40
60
80
100
0 120
Time / day
Fig. 4. Changes in (a) pH, (b) cell counts and dissolved (c) Cu and (d) Fe concentrations in A. caldus cultures (closed symbols) and sterile controls (open symbols). ●&○: pre-oxidized CuFeS2 suspended in the medium; ▲&△: pre-oxidized CuFeS2 enclosed in dialysis membrane.
K. Sasaki et al. / Hydrometallurgy 127-128 (2012) 116–120
a) S 2p
b) S 2p
1 kcps after 85 days (sterile)
1 kcps after 85 days (sterile)
after 85 days
after 85 days
Intensity / cps
Intensity / cps
119
after 43 days
after 43 days
after 11 days
after 11days
0 day
0 day
174 172 170 168 166 164 162 160 158
174 172 170 168 166 164 162 160 158
Binding energy / eV
Binding energy / eV
Fig. 5. Changes in XPS S 2p spectra for solid residues after leaching of pre-oxidized CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria and 85-d sterile control (no dialysis membrane); (b) pre-oxidized CuFeS2 and 85-d sterile control, both in dialysis membrane.
(NRS 2000, JASCO International, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI 5800 ESCA system (ULVAC-PHI, Chigasaki, Kanagawa, Japan). For Raman spectroscopy, excitation was accomplished with an Ar ion laser (514.5 nm, incident power 50 mW) and detected by a laser Raman spectrometer. The laser light was standardized with silicon, using a band maximum at 520.2 cm−1. Samples were diluted to 5 wt.% with KBr powder and compressed to a 10 mm disk. The Raman spectra were obtained after four accumulations using 60 s integration times. The XP spectra were analyzed with Casa XPS software (Version 2.3.12). Background corrections were made using the Shirley method (Hassan et al., 2010; Shiers et al., 2011). For O 1s, C 1s, Cu 2p, and S 2p spectra. Peak shapes were defined using a Gaussian– Lorentzian mixed function. The C–C binding energy, EB[C 1s], from the vacuum pump oil in the apparatus has a well-defined value at 284.6 eV, which was used as a reference peak to correct for differential charging effects.
3. Results and discussion 3.1. Experiments with CuFeS2 Fig. 1(a), (b) shows similar decreases in pH and increases in microscopic cell counts within a few days in A. caldus culture systems regardless of the enclosure of chalcopyrite inside the membrane. This initial phase represented tetrathionate oxidation to sulfuric acid. The dissolution of Cu and Fe over time was noticeably suppressed when chalcopyrite was enclosed in the dialysis tubing out of direct contact with A. caldus (Fig. 1(c), (d)). Because CuFeS2 was not in direct contact with bacteria, the suppressed Cu and Fe solubilization is attributed to slow leaching due to proton attack and diffusion-limitation of fluxes of reactants (mainly sulfuric acid and dissolved O2) and products (dissolved metals) across the dialysis membrane.
a)
b)
after 85 days (sterile)
after 85 days (sterile)
after 85 days after 43 days
Intensity/ arbitrary unit
Intensity/ arbitrary unit
after 85 days
after 43 days after 11 days oxidized chalcopyrite
after 11 days
oxidized chalcopyrite sulfur
sulfur
covellite
covellite potassium jarosite potassium jarosite chalcopyrite
500
450
400
350
chalcopyrite
300
Raman shift/ cm-1
250
200
500
450
400
350
300
250
200
Raman shift/ cm-1
Fig. 6. Changes in Raman spectra for solid residues after leaching of pre-oxidized CuFeS2 in A. caldus cultures and sterile control. (a) Direct contact with bacteria and 85-d sterile control (no dialysis membrane); (b) pre-oxidized CuFeS2 and 85-d sterile control, both in dialysis membrane.
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Fig. 2(a), (b) shows changes in S 2p spectra for solid residues after the bioleaching of CuFeS2 in A. caldus cultures with direct contact or separated with dialysis membrane. These XPS data showed the presence of monosulfide and elemental S species under both conditions. The predominant species was still CuFeS2 after 110 days of contact. In the sterile control elemental S accumulated on the surface of chalcopyrite (Fig. 2(c)). Thus a comparison of these S 2p spectra indicates that A. caldus oxidized elemental S, which was formed as a secondary phase during the bioleaching of CuFeS2. Raman spectra for the solids were consistent with the XPS results. Elemental S was oxidized in the inoculated systems but remained in the bulk phase of the sterile controls regardless of the membrane (Fig. 3(a), (b), (c)). The results did not show a time-dependent sequence of formation and oxidation of elemental S on chalcopyrite surface in the inoculated system. 3.2. Experiments with pre-oxidized CuFeS2 In the second experiment, a S-rich layer was chemically produced on chalcopyrite surface by anaerobic pre-oxidation with Fe 3+. During this chemical pre-oxidation phase incongruent dissolution was observed with the preferential dissolution of Cu over S at pH 2 (data not shown). Fig. 4(a), (b) shows changes in pH and microscopic cell counts in the aerobic bioleaching of pre-oxidized chalcopyrite in inoculated systems with and without the dialysis membrane. The pH decreased and cell numbers increased within a few days in both inoculated systems. In this case, the decrease in pH was more extensive than shown in the first experiment. Again, the dissolution of Cu and Fe was suppressed when chalcopyrite was enclosed in dialysis membrane (Fig. 4(c), (d)). Fig. 5(a), (b) shows changes in the S 2p spectra for solid residues after the bioleaching of pre-oxidized chalcopyrite ± membrane and a comparison with the sterile control after 85 days. Elemental S was oxidized in all cases. Raman spectra for these samples indicated the lack of elemental S in bulk phases of all samples (Fig. 6). Thus the data indicated that elemental S layers on CuFeS2 were oxidized by A. caldus. Both tetrathionate and elemental S oxidation produce sulfuric acid. The bioleaching of Cu from CuFeS2 by A. caldus involves proton attack by sulfuric acid because this bacterium is not an iron oxidizer. While the complete oxidation of CuFeS2 is an acid-producing reaction, dissolution by proton attack and dissolved O2 consumes acid, involves a low Eh and forms elemental S on mineral surface. þ
2þ
CuFeS2 þ O2 þ 4H →Cu
2þ
þ Fe
0
þ 2S þ 2H2 O
Precursors of colloidal S and the rhombic elemental S ring structure (S8) can be ionized as polysulfide complexes such as H–S–S–S–S–S–S– S–S–H=–S–S–S–S–S–S–S–S– (S82−) + 2 H+ (Harmer et al., 2006; Steudel, 1996). The ionized precursor form can diffuse through the dialysis membrane. However, with increasing formation of elemental S, polymerized S accumulated inside the dialysis membrane, which resulted in the clogging and impeded the leaching and diffusion of Cu and Fe to the bulk solution. 4. Conclusions The oxidation of sulfur to sulfuric acid was the primary role of A. caldus in CuFeS2 bioleaching. Separation of CuFeS2 with a dialysis
membrane from A. caldus resulted in slow copper and iron dissolution. Iron precipitation as jarosite on CuFeS2 surface was prevented by virtue of using bacteria that only oxidized sulfur compounds. The oxidation of S formed from CuFeS2 in the membrane enclosure was also slow and caused colloidal S accumulation inside the membrane. This accumulation suggested that S-rich layers are not all tenacious associated with CuFeS2 surface. Acknowledgements Financial support was provided to KS by the Japan Society for the Promotion of Science (JSPS Grant-in-Aid for Scientific Research No. 22246117), the JSPS Funding Program for Next Generation WorldLeading Researchers (GR078) and JSPS Invitation Fellowship Program for Research in Japan (Short Term) FY2011 (S11078). Raman spectroscopy and XPS measurements were performed at the Center of Advanced Instrumental Analysis, Kyushu University. References Fouchera, S., Battaglia-Bruneta, F., d'Huguesa, P., Clarens, M., Godonc, J.J., Morin, D., 2003. Evolution of the bacterial population during the batch bioleaching of a cobaltiferous pyrite in a suspended-solids bubble column and comparison with a mechanically agitated reactor. Hydrometallurgy 71, 5–12. Gericke, M., Govender, Y., Pinches, A., 2010. Tank bioleaching of low-grade chalcopyrite concentrates using redox control. Hydrometallurgy 104, 414–419. Harmer, S.L., Thomas, J.E., Fornasiero, D., Gerson, A.R., 2006. The evolution of surface layers formed during chalcopyrite leaching. Geochim. Cosmochim. Acta 70, 4392–4402. Hassan, S.H.A., Van Ginkel, S.W., Kim, S.-M., Yoon, S.-H., Joo, J.-H., Shin, B.-S., Jeon, B.-H., Bae, W., Oh, S.-E., 2010. Isolation and characterization of Acidithiobacillus caldus from a sulfur-oxidizing bacterial biosensor and its role in detection of toxic chemicals. J. Microbiol. Meth. 82, 151–155. Klauber, C., 2008. A critical review of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite with regards to hindered dissolution. Int. J. Miner. Process. 86, 1–17. Mangold, S., Valdés, J., Holmes, D.S., Dopson, M., 2011. Sulfur metabolism in the extreme acidophile Acidithiobacillus caldus. Front. Microbiol. 2, 17 http://dx.doi.org/10.3389/ fmicb.2011.00017. Mutch, L.A., Watling, H.R., Watkin, E.L.J., 2010. Microbial population dynamics of inoculated low-grade chalcopyrite bioleaching columns. Hydrometallurgy 104, 391–398. Pradhan, N., Nathsarma, K.C., Rao, Sinivasa, Sukla, L.B., Mishra, B.K., 2008. Heap bioleaching of chalcopyrite: a review. Miner. Eng. 21, 355–365. Sasaki, K., Tsunekawa, M., Ohtsuka, T., Konno, H., 1995. Confirmation of sulfur-rich layer formed on pyrite after dissolution by Fe(III) ions around pH 2. Geochim. Cosmochim. Acta 59, 3155–3158. Sasaki, K., Nakamuta, Y., Hirajima, T., Tuovinen, O.H., 2009. Raman characterization of secondary minerals formed during chalcopyrite leaching with Acidithiobacillus ferrooxidans. Hydrometallurgy 95, 153–158. Shiers, D.W., Ralph, D.E., Watling, H.R., 2011. Batch culture of Acidithiobacillus caldus on tetrathionate. Biochem. Eng. J. 54, 185–191. Steudel, R., 1996. Mechanism for the formation of elemental sulfur from aqueous sulfide in chemical and microbiological desulfurization processes. Ind. Eng. Chem. Res. 35, 1417–1423. Tamura, H., Goto, K., Yotsuyanagi, T., Nagayama, M., 1974. Spectroscopic determination of iron (II) with 1, 10-phenanthroline in the presence of large amounts of iron (II). Talanta 21, 314–318. Vilcáez, J., Suto, K., Inoue, C., 2008. Bioleaching of chalcopyrite with thermophiles: temperature–pH–ORP dependence. Int. J. Miner. Process. 88, 37–44. Watling, H.R., 2006. The bioleaching of sulphide minerals with emphasis on copper sulphides — a review. Hydrometallurgy 84, 81–108. Zeng, W.-M., Zhou, H.-B., Wan, M.X., Chao, W.-L., Xu, A.-L., Liu, X.-D., Qiu, G.-Z., 2009. Preservation of Acidithiobacillus caldus: a moderately thermophilic bacterium and the effect on subsequent bioleaching of chalcopyrite. Hydrometallurgy 96, 333–336. Zhou, H.-B., Zeng, W.-M., Yang, Z.-F., Xie, Y.-J., Qiu, G.-Z., 2009. Bioleaching of chalcopyrite concentrate by a moderately thermophilic culture in a stirred tank reactor. Bioresour. Technol. 100, 515–520.