Bioleaching of chalcopyrite by Acidianus manzaensis under different constant pH

Minerals Engineering 98 (2016) 80–89

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Bioleaching of chalcopyrite by Acidianus manzaensis under different constant pH Hongchang Liu a,b, Jinlan Xia a,b,⇑, Zhenyuan Nie a,b, Chenyan Ma c, Lei Zheng c, Caihao Hong c, Yidong Zhao c, Wen Wen d a

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Lab of Biometallurgy of Ministry of Education of China, Central South University, Changsha 410083, China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China d Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China b c

a r t i c l e

i n f o

Article history: Received 21 December 2015 Revised 23 July 2016 Accepted 28 July 2016 Available online 3 August 2016 Keywords: Bioleaching Chalcopyrite Acidianus manzaensis Constant pH Synchrotron radiation XANES

a b s t r a c t The bioleaching of pure chalcopyrite by thermophilic Archaea strain Acidianus manzaensis YN-25 under different constant pH was first comparatively investigated. Then the relevant sulfur speciation was analyzed by synchrotron radiation based X-ray diffraction (SR-XRD) and S K-edge X-ray absorption near edge structure (XANES) spectroscopy. The acidity of the leaching solution was monitored at 3-h intervals to make it steady at pH 1.25, 1.50, 1.75, 2.00, 2.25 and 2.50, respectively. Leaching results showed that the copper ion extraction increased during chemical leaching but decreased during bioleaching when pH value decreased from 2.50 to 1.25. SR-XRD analysis showed that, during bioleaching, new elemental sulfur (S0) phase was detected at all tested pH cases; new jarosite phase was detected at cases of pH 1.50 to 2.50; and jarosite gradually became a major phase when pH value increased. XANES analysis further showed that covellite was detected during bioleaching at cases of pH 1.25 to 2.00 at higher redox potential (ORP) value, while chalcocite and bornite were detected at cases of pH 2.25 and 2.50 at lower ORP value. These results suggested that the formation of S0 was mainly accounting for hindering the dissolution of chalcopyrite while the formation of bornite could accelerate the dissolution of chalcopyrite by A. manzaensis. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Biohydrometallurgy is an emerging technology and plays an important role in copper recovery from copper sulfides with economic, environmental and social benefits (Li et al., 2013; Pradhan et al., 2008). Among these sulfides, chalcopyrite is the most abundant and widespread. Up to date, plenty of publications have been focusing on bioleaching of chalcopyrite (Lotfalian et al., 2015; Panda et al., 2015; Vilcáez and Inoue, 2009), during which the forming and evolution of secondary minerals and other intermediates get increasing attention (Klauber, 2008; Liu et al., 2015a). Chalcopyrite is an acid soluble metal sulfide, which can be dissolved by H+ as well as Fe3+. The dissolved mechanism for chalcopyrite and other acid-soluble metal sulfides is named polysulfide mechanism due to the formation of polysulfide (Eq. (1)), which is an intermediate to form elemental sulfur (S0) (Eq. (2)) ⇑ Corresponding author at: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (J. Xia). http://dx.doi.org/10.1016/j.mineng.2016.07.019 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.

(Vera et al., 2013). The dissolution of chalcopyrite by microorganisms can be affected by many factors, such as microorganism species, temperature, solution acidity and redox potential (ORP).

CuFeS2 þ Fe3þ þ Hþ ! Cu2þ þ Fe2þ þ 0:5H2 Sn ðn P 2Þ 3þ

0:5H2 Sn þ Fe

! S8 þ Fe

2þ

þ

þ H ðn P 2Þ

ð1Þ ð2Þ

The most studied bioleaching microorganisms include Acidithiobacillus, Leptospirillum, Sulfobacillus and Acidianus, etc. These acidophilic microbes can oxidize ferrous iron and/or S0 to ferric iron and/or finally sulfuric acid, respectively, through indirect contact mechanism (Li et al., 2013). The recycles of ferric iron and protons during bioleaching can significantly promote the dissolution of chalcopyrite (Vilcáez and Inoue, 2009; Vilcáez et al., 2009). Due to the tolerance to higher temperature and the higher metal recovery than mesophiles, thermophiles take the peculiar role in bioleaching of copper sulfides and have been a research hotspot in recent years (du Plessis et al., 2007; Pradhan et al., 2008; Zhu et al., 2013).

H. Liu et al. / Minerals Engineering 98 (2016) 80–89

Due to the consumption and regeneration of protons (Eqs. (1) and (2), the acidity of the leaching solution changes and further affects the dissolution of chalcopyrite. According to Vilcáez et al. (2009), chemical leaching of chalcopyrite at initial pH 1.0 can have a higher copper extraction than that at initial pH 1.5 at thermophilic temperature, suggesting that copper extraction increases when the concentration of protons increases. However, when introducing leaching microorganisms, the influence of initial pH on bioleaching is opposite (Liang et al., 2014), probably due to the effect of acidity on sulfur- and/or iron-oxidizing activity of bioleaching microorganisms (Plumb et al., 2008). On the other hand, recent studies reveal that bioleaching of chalcopyrite can be enhanced at low ORP (<450 mV, Pt vs. Ag-AgCl) rather than at high ORP (>600 mV) because of the accumulation and precipitation of jarosite onto chalcopyrite surface at high ORP (Petersen and Dixon, 2006; Sandström et al., 2005; Vilcáez et al., 2009). Bioleaching of chalcopyrite can be also affected by ORP, and the passivation potential increases when the pH value decreases (ViramontesGamboa et al., 2007). Besides, during bioleaching, some leaching products may precipitate on mineral surface to hinder the dissolution (Klauber, 2008). The most studied intermediates are S0 and jarosite (Liang et al., 2010, 2012). The formation of S0 on the mineral surface is supposed to be derived from the mono-sulfide (S2) via polysulfide 2 (S2 and S2 are n ) (Harmer et al., 2006). The intermediates S n unstable and they can also lead to the formation of CuSn-like species (Li et al., 2013). The hindering mechanism of jarosite summarized by Klauber (2008) suggests that the accumulation of ferric iron ion and the increase of pH allow the spontaneous formation of jarosite (Eq. (3)), which may coat on the remaining chalcopyrite and reduce the copper extraction by reducing the chalcopyrite surface area or the mass transport of the mineral. However, not all researches have observed the passivation of jarosite during bioleaching (Khoshkhoo et al., 2014; Liu et al., 2015b).

þ Mþ þ 3Fe3þ þ 2SO2 4 þ 6H2 O ! MFe3 ðSO4 Þ2 ðOHÞ6 þ 6H

ð3Þ

where M+ is a monovalent cation, such as H3 Oþ , Kþ , Naþ or NHþ 4. Though pH can determine the bioleaching process and it is usually monitored and maintained in a suitable value in industrial bioleaching practice, the effect of pH on bioleaching of chalcopyrite is not clear, because of the limitation in the study by the following reasons: (1) the pH always changes with (bio)leaching; (2) the pH consumption or accumulation is always accompanied with change of ORP and precipitation of leaching products, such as S0 and jarosite; and (3) the sulfur- and/or iron-oxidizing activity of bioleaching microorganisms can be influenced by pH conditions. Comparatively studying the leaching process and the evolution of intermediate products under different constant pH values will provide useful idea for promoting leaching efficiency by controlling of leaching conditions. Up to now, synchrotron radiation-based X-ray diffraction (SRXRD) and S K-edge X-ray absorption near edge structure (XANES) spectroscopy have been efficient for analysis of the composition on mineral surface with high spatial resolution and high sensitivity. By linear combination fitting of ‘‘unknown spectra” with ‘‘reference spectra”, S K-edge XANES can determine the possible species and speciation transformation on the mineral surface (He et al., 2009; Xia et al., 2010). In the present study, by using of SR-XRD and S K-edge XANES spectroscopy, the effect of constant pH on copper extraction and the evolution of relevant intermediate products during bioleaching of chalcopyrite with the extremely thermophilic Acidianus manzaensis were studied. The results will be helpful to comprehend bioleaching operation.

81

2. Material and methods 2.1. Strain and culture medium The extremely thermoacidophilic Archaea strain A. manzaensis YN-25 (Accession number of 16S rDNA in GeneBank: EF522787) was preserved at the Key Laboratory of Biometallurgy of Ministry of Education of China, Changsha, China. The basal medium for A. manzaensis cultivation comprised the following components: (NH4)2SO4, 3.0 g/L; MgSO47H2O, 0.5 g/L; K2HPO4, 0.5 g/L; KCl, 0.1 g/L; Ca(NO3)2, 0.01 g/L; and yeast extracts 0.2 g/L. 2.2. Mineral samples The original chalcopyrite used in this study was provided by the School of Minerals Processing and Bioengineering, Central South University, Changsha, China. X-ray fluorescence spectroscopic analysis showed the original chalcopyrite contained: 33.69% of Cu, 29.62% of Fe, 34.34% of S; 1.91% of O, 0.31% of Si; 0.04% of Al, 0.02% of Ca, 0.01% of Se and 0.02% of P. The mineralogical composition tests (by XRD) indicated that the original chalcopyrite mineral is basically pure. The mineral was ground to fine powder guaranteeing that the particle size was 37–75 lm. 2.3. Bioleaching experiment Before the bioleaching experiment, A. manzaensis was initially activated according to Liang et al. (2012). Then the strain was incubated in a 500 mL Erlenmeyer flask containing 200 mL sterilized basal medium and 2 g of chalcopyrite. The initial cell density was 4  107 cells/mL. The pH of the culture medium was initially adjusted to 1.25, 1.5, 1.75, 2.0, 2.25 and 2.5, respectively. The cultivation was performed in a high-temperature bath rotary shaker (SHZ-GW) at 170 r/min and 65 °C. The leaching experiments were performed in triplicate. The abiotic experiments (without A. manzaensis) were carried out as controls. During cultivation, the evaporated water was compensated with sterilized ultra-pure water based on weight loss at 12-h intervals. The acidity of the culture medium was monitored at 3-h intervals and adjusted with 2.5 M sulfuric acid (A.R.) or 5 M ammonia hydroxide (A.R.) to make sure the constant acidity of the leaching solution. 2.4. Analysis methods During bioleaching, solution samples were taken out at 1-day intervals to monitor cell densities, pH value, ORP value, ferric iron ion concentration ([Fe3+]), total iron concentration (total [Fe]), and copper ion concentration ([Cu2+]). The cell density was determined with a blood corpuscle counter (XB-K-25). The pH value was measured with a pH meter (PHS-3C). The ORP was measured with a Pt electrode, using a Hg/Hg2Cl2 electrode as reference. The [Fe3+] and total [Fe] were determined by 5-sulfosalicylic acid spectrophotometry. The [Cu2+] was determined by bis-(cyclohexanone)oxalyldihy drazone spectrophotometry. The [Fe3+]/{total [Fe]} ratio was calculated to determine the iron-oxidizing activity of A. manzaensis. During bioleaching, leaching residues were characterized by SRXRD and S K-edge XANES spectroscopy. Before these analyses, the residue samples were washed three times with diluted sulfuric acid and diluted hydrochloric acid, respectively, whose pH value was corresponding to the acidity of each culture medium (i.e. 1.25, 1.5, 1.75, 2.0, 2.25 and 2.5). Then the residue samples were dried in a vacuum drying oven (DZF-6020) and stored in nitrogen atmosphere at 20 °C until analyses. The SR-XRD patterns were recorded at beamline BL14B1 of Shanghai Synchrotron Radiation

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Facility (SSRF), Shanghai, China with a step of 0.01° and a dwell time of 0.5 s at each step. The energy and the spot size for SRXRD were 10 keV and 0.5  0.5 mm2, respectively. The S K-edge XANES analysis was performed at beamline 4B7A of Beijing Synchrotron Radiation Facility, Beijing, China. The XANES spectra data were recorded in fluorescence mode at ambient temperature and scanned at step width of 0.2 eV between 2450 eV and 2520 eV. The XANES spectra were normalized and fitted for their linear combinations using the standard spectra with IFEFFIT program according to previous descriptions (He et al., 2009; Ravel and Newville, 2005).

1.0

3. Results 3.1. Leaching parameters The parameters of chalcopyrite leached by A. manzaensis and in sterile control were characterized by [Cu2+], total [Fe], [Fe3+]/{total [Fe]} ratio, ORP and cell density of the leaching solution (Figs. 1 and 2). For the bioleaching experiment, the results showed that at day 1 the [Cu2+] (Fig. 1a) at cases of pH 1.25, 1.50, 1.75 and 2.00 were similar, and higher than that at cases of pH 2.25 and 2.5. While

(a)

0.9

(b)

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

0.8 0.7

total[Fe] (g/L)

2+

[Cu ] (g/L)

0.8 0.6 0.4

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

0.2 0.0

0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

2

4

6

8

10

0

2

4

Time (d)

8

10

8

10

Time (d)

(c)

650

1.0

600

0.8

550

ORP (mv)

3+

[Fe ]/{total [Fe]}

1.2

6

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

0.6 0.4

(d)

500 450 pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

400 350

0.2

300

0.0

250 0

2

4

6

8

10

Time (d) 28

0

2

4

6

Time (d)

(e)

20 16

7

Cell density (10 cells/mL)

24

12 pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

8 4 0 0

2

4

6

8

10

Time (d) Fig. 1. Leaching parameters of chalcopyrite by A. manzaensis at constant pH 1.25, 1.50, 1.75, 2.00, 2.25 and 2.50, respectively. Where (a) [Cu2+] vs. time curves; (b) total [Fe] vs. time curves; (c) [Fe3+]/total [Fe] vs. time curves; (d) ORP value vs. time curves; (e) cell density vs. time curves.

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(a)

0.4

2+

[Cu ] (g/L)

0.5

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

0.3

0.2

0.2 0.1

0.0

0.0 2

4

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

0.3

0.1

0

(b)

0.4

total[Fe] (g/L)

0.5

6

8

10

0

2

4

(c)

0.06 0.04

8

10

(d)

350 340

0.02

3+

[Fe ]/{total [Fe]}

360

pH 1.25 pH 1.50 pH 1.75 pH 2.00 pH 2.25 pH 2.50

ORP (mv)

0.08

6

Time (d)

Time (d)

pH 1.25 sterile pH 1.50 sterile pH 1.75 sterile

pH 2.00 sterile pH 2.25 sterile pH 2.50 sterile

2

6

330 320 310 300

0.00

290 -0.02

280 270

-0.04 0

2

4

6

8

10

0

4

8

10

Time (d)

Time (d)

Fig. 2. Leaching parameters of chalcopyrite in the sterile control at constant pH 1.25, 1.50, 1.75, 2.00, 2.25 and 2.50, respectively. Where (a) [Cu2+] vs. time curves; (b) total [Fe] vs. time curves; (c) [Fe3+]/{total [Fe]} vs. time curves; (d) ORP value vs. time curves.

after 10 d the [Cu2+] at different constant pH cases followed the order: pH 2.50 > pH 2.25 > pH 2.00 > pH 1.75 > pH 1.50 > pH 1.25. The total [Fe] (Fig. 1b) at cases of pH 1.25, 1.50 and 1.75 showed similar trend, which increased gradually with time from day 0 to day 8 and then basically unchanged. While the total [Fe] at cases of pH 2.00, 2.25 and 2.50 first increased and then decreased. Generally, the total [Fe] at different constant pH cases followed the order: pH 1.25 > pH 1.50 > pH 1.75 > pH 2.00 > pH 2.25 > pH 2.50. Both [Fe3+]/{total [Fe]} ratio (Fig. 1c) and ORP (Fig. 1d) increased gradually and then kept steady, but these steady ORP followed the order: pH 1.25 > pH 1.50 > pH 1.75 > pH 2.00 > pH 2.25 > pH 2.50. The growth of A. manzaensis at different constant pH cases was significantly different, where the cell density of the solution at the stationary stage increased with the increase of the acidity of the leaching solution (Fig. 1e). By contrast, for the sterile control, after 10 d the order of the [Cu2+] at different constant pH cases (Fig. 2a) was opposite to that in the bioleaching experiment, i.e. the lower was the pH of leaching solution, the higher [Cu2+] obtained. The total [Fe] (Fig. 2b) and ORP (Fig. 2d) showed the same trend to [Cu2+]. For the [Fe3+]/{total [Fe]} ratio (Fig. 2c), the values at different constant pH cases were basically zero because of lack of efficient iron-oxidizer.

3.2. SR-XRD analysis The SR-XRD patterns of chalcopyrite residues leached by A. manzaensis (Fig. 3) showed that the residues at cases of pH 1.25, 1.50 and 1.75 were mainly chalcopyrite. Meanwhile little S0 was

detected at case of pH 1.25, and little S0 and jarosite were detected at cases of pH 1.50 and 1.75. For the residues at cases of pH 2.00, 2.25 and 2.50, besides chalcopyrite, jarosite was detected as one major phase, meanwhile little S0 was found. By contrast, in the sterile control, except chalcopyrite as the major phase, only little S0 and/or jarosite were found (see Fig. 4).

3.3. S K-edge XANES analysis The S K-edge XANES spectra of chalcopyrite during bioleaching (Fig. 5) showed that the intensity of the peak at 2470.4 eV gradually decreased and the intensity of the peak at 2483.0 eV gradually increased at all tested pH conditions. Meanwhile, these changes of the peaks intensities were more and more obvious with the increase of pH value. It should be noted that the new peak at 2472.8 eV appeared for chalcopyrite residues at day 10 in the sterile controls at cases of pH 1.25, 1.50, 1.75 and 2.00. The S K-edge XANES spectra were then fitted for their composition based on linear combinations fitting with the reference spectra of chalcopyrite, bornite, chalcocite, covellite and S0 (Fig. 6a). The fitted spectra of chalcopyrite at day 10 in bioleaching and sterile control experiment were taken as examples to show the quality of the fitting results (Fig. 6b). The fitting compositions are shown in Fig. 7. The results showed the content of chalcopyrite gradually decreased and the content of jarosite gradually increased at all tested pH conditions. However, the decrease of chalcopyrite and the increase of jarosite during bioleaching were sped up with the increase of solution pH, which

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(a)

(b)

Chalcopyrite Elemental sulfur Jarosite

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur

10

20

30

40

50

10

60

20

30

50

60

2θ (°)

2θ (°)

(c)

40

(d)

Chalcopyrite Elemental sulfur Jarosite

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur Jarosite

10

20

30

40

50

10

60

20

30

50

60

2θ (°)

2θ (°)

(e)

40

(f)

Chalcopyrite Elemental sulfur Jarosite

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur Jarosite

10

20

30

40

50

60

10

20

30

40

50

60

2θ (°)

2θ (°)

Fig. 3. SR-XRD patterns of chalcopyrite residues bioleached by A. manzaensis at constant pH 1.25 (a), 1.50 (b), 1.75 (c), 2.00 (d), 2.25 (e) and 2.50 (f) (j chalcopyrite; 4 elemental sulfur; } jarosite).

was just opposite to the accumulation of S0. It should be noted that covellite was detected during bioleaching at cases of pH 1.25, 1.50, 1.75 and 2.00 (except at day 2 at case of pH 1.25), while chalcocite and bornite were detected at cases of pH 2.25 and 2.50. By contrast, in the sterile control, new chalcocite and bornite phases were detected at all tested pH cases; new S0 was detected at all tested pH cases except at case of pH 2.50, while little jarosite was detected at cases of pH 2.25 and 2.5.

4. Discussion This work compared the leaching process of chalcopyrite by A. manzaensis and in sterile control under different constant pH conditions. The chemical leaching experiment showed that the copper extraction increased with the decrease of solution pH. In case of lacking ferric ion in chemical leaching solution, the dissolution of chalcopyrite was suggested to be mainly due to the action of

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(a)

(b)

Chalcopyrite Elemental sulfur

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur

10

20

30

40

50

10

60

20

30

2θ (°)

(c)

40

50

60

2θ (°)

(d)

Chalcopyrite Elemental sulfur Jarosite

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur

10

20

30

40

50

10

60

20

30

50

60

2θ (°)

2θ (°)

(e)

40

(f)

Chalcopyrite Jarosite

Normalized intensity

Normalized intensity

Chalcopyrite Elemental sulfur Jarosite

10

20

30

40

50

60

2θ (°)

10

20

30

40

50

60

2θ (°)

Fig. 4. SR-XRD patterns of chalcopyrite residues in the sterile control at constant pH 1.25 (a), 1.50 (b), 1.75 (c), 2.00 (d), 2.25 (e) and 2.50 (f) (j chalcopyrite; 4 elemental sulfur; } jarosite).

oxygen and protons (Vilcáez et al., 2009) (Eq. (4)). It might be also along with another non-oxidative dissolution process (Liang et al., 2014) (Eq. (5)). Based on Eqs. (4) and (5), the increase of [H+] in the leaching solution resulted in the enhancement of copper extraction. Considering the adding of protons to leaching solution to make the pH value steady, the copper extraction should be enhanced with time. However, the leaching rate of copper ion gradually decreased from day 6. The decrease of copper extraction rate under each pH condition was probably caused by the precipitation of S0.

CuFeS2 þ O2 þ 4Hþ ! Cu2þ þ Fe2þ þ 2S0 þ 2H2 O

ð4Þ

CuFeS2 þ 4Hþ ! Cu2þ þ Fe2þ þ 2H2 S

ð5Þ

Compared with chemical leaching, the copper recovery during bioleaching was significantly promoted by thermophilic A. manzaensis. The promotion of copper recovery in bioleaching was mainly due to the recycle of ferrous ion to ferric ion and the biooxidation of S0 to sulfuric acid by A. manzaensis. The growth curve indicated that A. manzaensis grew better when pH was lower than 2.0, because its optimal pH for growth was always lower than pH 2.0,

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(a)

2483.0

2470.4

4

Normalized absorption

Normalized absorption

4

3 day 10

2

day 8 day 6

1

day 4 day 2

0

sterile

2450

2460

2472.8

2470

2480

2500

2510

2520

day 10

2

day 8 day 6

1

day 4 day 2 sterile

2450

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2472.8

2470

Energy (eV)

(c)

day 10 day 8 day 6

1

day 4 day 2

0

2460

2472.8

2470

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2500

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2

2520

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1

day 4

sterile

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2472.8

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2483.0

(f) 5

Normalized absorption

Normalized absorption

2510

day 10

2483.0

(e)

2470.4 day 10 day 8 day 4

sterile

2450

2460

4 3 2 1

day 2

0

2500

2470.4

3

Energy (eV)

day 6

1

2520

4

0

4

2

2510

2483.0

(d)

Energy (eV)

3

2500

day 2

sterile

2450

5

2490

2470.4

Normalized absorption

Normalized absorption

5

3

2

2480

Energy (eV)

2483.0

4

2483.0

2470.4

3

0 2490

(b)

2470.4 day 10 day 8 day 6 day 4 day 2

2472.8

2470

2480

0 2490

2500

2510

2520

Energy (eV)

sterile

2450

2460

2472.8

2470

2480

2490

Energy (eV)

Fig. 5. Normalized S K-edge XANES spectra of chalcopyrite bioleached by A. manzaensis from 2nd to 10th d and in the sterile control at 10th d at constant pH 1.25 (a), 1.50 (b), 1.75 (c), 2.00 (d), 2.25 (e) and 2.50 (f).

which was agreed with He et al. (2008). However, the copper extraction at day 10 in bioleaching was just opposite to that in the chemical leaching, i.e. the copper extraction increased with the increasing of solution pH, indicating the copper extraction during bioleaching was significantly affected by different pH conditions, which was consistent with Liang et al. (2014). The variation tendency of total [Fe] showed similarity to that of [Cu2+] during chemical leaching. However, the variation tendency of total [Fe] during bioleaching at different constant pH cases seemed opposite to that of [Cu2+]. This result was probably

occurred due to the precipitation of jarosite (Eq. (3)) at higher pH value (Klauber, 2008), resulting in the decrease of total [Fe]. The results of SR-XRD and XANES analyses provided direct evidence for the jarosite precipitation under higher pH conditions. The SR-XRD patterns showed the bioleaching residues at lower pH (1.25, 1.50 and 1.75) were mainly chalcopyrite with little jarosite and/or S0, while at higher pH (2.00, 2.25 and 2.50) jarosite became one major phase besides chalcopyrite. The fitted results of XANES spectra also showed the increase of jarosite was enhanced when the pH value increased (Fig. 7). It should be noted that the constant

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7

(a) Chalcopyrite

Normalized absorption

Normalized absorption

4

Bornite Chalcocite

2

(b)

6

Covellite Elemental sulfur

5 4

pH 2.50

3

pH 2.25

2

pH 2.00

1

pH 1.75

0

pH 1.25

pH 1.50

Jarosite

0 2450

2460

2470

2480

2490

2500

2510

2520

2450

2460

Energy (KeV)

2470

2480

2490

2500

2510

2520

Energy (eV)

Fig. 6. Normalized S K-edge XANES spectra (a) of reference compounds chalcopyrite, bornite, covellite, chalcocite, jarosite and S0, and the spectra (b) of fitted and measured spectra at 10th d at different constant pH conditions.

pH was controlled by adding sulfuric acid or ammonia, therefore, the amount of sulfuric acid and ammonia added to the experiment might affect the forming of jarosite according to Eq. (3). However, the amount of ammonia added to the experiment at pH 2.25 and 2.5 was zero (Table 1), while the amount of jarosite precipitation under higher pH conditions was higher than that at other cases, indicating that the formation of jarosite was mainly resulted from the higher pH value of the solution, because the iron precipitation could be accelerated at higher pH condition (Li et al., 2013). The results also suggested the lower pH value, the higher ORP value for both chemical leaching and bioleaching (Figs. 1d and 2d), indicating that the concentration of protons could significantly influence the ORP of leaching solution. By now, the effect of ORP and pH conditions on the dissolution of chalcopyrite had been widely studied (Liang et al., 2014; Liu et al., 2015a; Vilcáez et al., 2009). These studies suggested that the sufficient protons at low pH cases were useful to reduce the precipitation of ferric ion (such as jarosite), and the leaching rate was controlled mainly by the ORP of the leaching solution. For bioleaching, the low ORP value (<450 mV) can enhance the leaching rate with high amount of ferric ion, while the high ORP value (>600 mV) can promote the jarosite precipitation to hinder dissolution (Petersen and Dixon, 2006; Sandström et al., 2005; Vilcáez et al., 2009). In the present study, the copper extraction was promoted at low ORP conditions. However, it should be noted that although plenty of publications suggested jarosite precipitation would hinder the dissolution (Dong et al., 2013; Klauber, 2008), it was not observed in the present study. Since the different pH values of the leaching solution resulted in the different ORP values in the present study, the corresponding ORP value decreased when pH value increased. Meanwhile, when pH value increased, the jarosite formed easily, and the [Cu2+] increased during bioleaching. It indicated that the precipitation of jarosite at low ORP conditions could not hinder dissolution. On the other hand, the dissolution of chalcopyrite at low ORP value was suggested due to the oxidation of the formed intermediates (iron deficient second minerals), while chalcopyrite was directly oxidized by ferric ion at high ORP value (Vilcáez et al., 2008, 2009). The iron deficient second minerals could be also formed under proper conditions. A two-step mode (Hiroyoshi et al., 2002, 2008) was proposed to explain the formation of iron deficient chalcocite at low ORP value in the acid solution in the first step (Eq. (6)) and then chalcocite was oxidized by ferric ions or oxygen. Our previous studies (Liang et al., 2010; Liu et al., 2015a,

2015b) also found that the Cu2S-like species could be formed at low ORP value and then transformed into CuS-like species with ORP value increasing (Eq. (7)). In the present study, the results of S K-edge XANES analyses showed that new phase of covellite was detected during bioleaching at cases of pH 1.25, 1.50, 1.75 and 2.00. In addition, the covellite species was gradually increased with the decrease of solution pH at pH 1.25, 1.50 and 1.75 conditions during bioleaching, corresponding to higher ORP value of the solution. The accumulation of covellite species at lower solution pH was probably occurred by Eqs. (6) and (7), which was probably relevant to the higher ORP value resulted from the lower pH of the bioleaching solution.

CuFeS2 þ 3Cu2þ þ 3Fe2þ ! 2Cu2 S þ 4Fe3þ

ð6Þ

Cu2 S þ 2Fe3þ ! Cu2þ þ 2Fe2þ þ CuS

ð7Þ

It should be noted S0 was detected at all tested pH conditions except at case of pH 2.50, indicating S0 was an important intermediate during dissolution of chalcopyrite. The observed S0 probably formed by a chemical polymerization process from mono-sulfide (S2) of bulk chalcopyrite via surface S2 species to S8 (Eq. (8)), n which was resulted from the preferential dissolution of metal ions (Harmer et al., 2006). However, the amount of S0 was apparently higher at low pH conditions, which suggested that S0 might slow down the dissolution (Yang et al., 2013). On the other hand, considering the chemical leaching, more S0 was also detected when pH value was lower. Since the copper extraction during chemical leaching was mostly determined by the proton concentration (Eqs. (4) and (5)), the copper extraction increased with the decrease of pH value. However, though the copper extraction for chemical leaching decreased when pH value increased, the leaching rate of chalcopyrite almost became zero at days 9–10 at pH 1.25–2.00. The results further indicated the hindering of S0 at low pH conditions.

S2 ! S2 n ! S8

ð8Þ

For the XANES results at cases of pH 2.25 and 2.50 during bioleaching, except plenty of jarosite, new phase of bornite was also detected as the main phase. The formation of bornite was probably caused by the faster release of iron into electrolyte than copper (Majuste et al., 2012; Richardson et al., 1984). The dissolution of bornite was suggested to be easier than chalcopyrite (Zhao et al., 2015), indicating the formation of bornite at higher pH and lower ORP conditions might accelerate the copper extraction.

H. Liu et al. / Minerals Engineering 98 (2016) 80–89

Percentage of contribution of standard spectra (%)

(a)

100 S J H V B C

90

80 20

(b) Percentage of contribution of standard spectra (%)

88

Percentage of contribution of standard spectra (%)

day 4

day 6

day 8 day 10

80 20

day 2

100 S J H V B C

90

80

20

(d)

day 4

day 6

day 8 day 10

sterile

100 S J H V B C

90

80

70 20 0

0 day 4

day 6

day 8 day 10

day 2

sterile

100 S J H B C

80

60

40

20

(f) Percentage of contribution of standard spectra (%)

day 2

Percentage of contribution of standard spectra (%)

90

sterile

Percentage of contribution of standard spectra (%)

day 2

(e)

S J H V B C

0

0

(c)

100

day 4

day 6

day 8 day 10

sterile

100 S J H B C

80

60

40

20

0

0 day 2

day 4

day 6

day 8 day 10

day 2

sterile

day 4

day 6

day 8 day 10

sterile

Fig. 7. Sulfur speciation compositions on the surface of chalcopyrite bioleached by A. manzaensis from 2nd to 10th d and in the sterile control at 10th d at constant pH 1.25 (a), 1.50 (b), 1.75 (c), 2.00 (d), 2.25 (e) and 2.50 (f) by the linear combinations fitting of unknown spectra (from Fig. 5) with reference spectra (from Fig. 6a). The letters S, J, H, V, B and C represent S0, jarosite, chalcocite, covellite, bornite and chalcopyrite, respectively.

Table 1 The total amount of acid and ammonia added to each experiment. Acid/ ammonia

Total amount added to each experiment (l mol) pH 1.25

pH 1.50

pH 1.75

pH 2.00

pH 2.25

pH 2.50

Bioleaching

Acid Ammonia

1100 210

620 280

80 350

200 630

1060 0

920 0

Sterile control

Acid Ammonia

1700 0

960 0

540 0

240 0

80 0

60 0

Experiment

5. Conclusions Leaching results showed that the copper extraction increased when pH value decreased during chemical leaching, while copper

ion extraction decreased when pH value decreased during bioleaching. Since the higher pH values could result in the lower ORP value, the leaching results also indicated that the bioleaching of chalcopyrite could be promoted at lower ORP value by A. manzaensis. SR-XRD and XANES analyses showed new S0 phase was detected at all tested pH cases, however, the content of S0 was higher at lower pH case. Jarosite increased and ORP value decreased when pH increased during bioleaching. New covellite was detected during bioleaching at cases of pH 1.25, 1.50, 1.75 and 2.00, while chalcocite and bornite were detected at cases of pH 2.25 and 2.50. These results suggested that (1) the accumulation of S0 was mainly accounting for slowing down the dissolution of chalcopyrite at the lower pH cases, (2) the precipitation of jarosite at the higher pH and lower ORP conditions cannot hinder dissolution, (3) while the forming of bornite at the higher pH and

H. Liu et al. / Minerals Engineering 98 (2016) 80–89

lower ORP conditions could accelerate the dissolution of chalcopyrite by A. manzaensis. Acknowledgement We are grateful to the staff at beamline BL14B Shanghai Synchrotron Radiation Facility (SSRF) and at beamlines 4B7 Beijing Synchrotron Radiation Facility for their help in beamlines operation and data collection. This work is supported by the Joint Funds of National Natural Science Foundation of China and Large Scientific Facility Foundation of Chinese Academy of Sciences (Grant No. U1232103), the National Natural Science Foundation of China (Grant No. 51274257), and the Open Funds of SSRF, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, China (Grant No. 14SRBL14B17938). References Dong, Y.B., Lin, H., Xu, X.F., Zhang, Y., Gao, Y.J., Zhou, S.S., 2013. Comparative study on the bioleaching, biosorption and passivation of copper sulfide minerals. Int. Biodeter. Biodegr. 84, 29–34. du Plessis, C.A., Batty, J., Dew, D., 2007. Commercial applications of thermophile bioleaching. In: Rawlings, D., Johnson, D.B. (Eds.), Biomining. Springer, Berlin Heidelberg, pp. 57–80. 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. He, H., Xia, J.L., Yang, Y., Jiang, H., Xiao, C.Q., Zheng, L., Ma, C.Y., Zhao, Y.D., Qiu, G.Z., 2009. Sulfur speciation on the surface of chalcopyrite leached by Acidianus manzaensis. Hydrometallurgy 99, 45–50. He, H., Yang, Y., Xia, J.L., Ding, J.N., Zhao, X.J., Nie, Z.Y., 2008. Growth and surface properties of new thermoacidophilic Archaea strain Acidianus manzaensis YN-25 grown on different substrates. Trans. Nonferrous Met. Soc. China 18, 1374– 1378. Hiroyoshi, N., Arai, M., Miki, H., Tsunekawa, M., Hirajima, T., 2002. A new reaction model for the catalytic effect of silver ions on chalcopyrite leaching in sulfuric acid solutions. Hydrometallurgy 63, 257–267. Hiroyoshi, N., Tsunekawa, M., Okamoto, H., Nakayama, R., Kuroiwa, S., 2008. Improved chalcopyrite leaching through optimization of redox potential. Can. Metall. Quart. 47, 253–258. Khoshkhoo, M., Dopson, M., Shchukarev, A., Sandström, Å., 2014. Electrochemical simulation of redox potential development in bioleaching of a pyritic chalcopyrite concentrate. Hydrometallurgy 144–145, 7–14. 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. Li, Y., Kawashima, N., Li, J., Chandra, A.P., Gerson, A.R., 2013. A review of the structure, and fundamental mechanisms and kinetics of the leaching of chalcopyrite. Adv. Colloid Interface Sci. 197–198, 1–32. Liang, C.L., Xia, J.L., Nie, Z.Y., Yu, S.J., Xu, B.Q., 2014. Effect of initial pH on chalcopyrite oxidation dissolution in the presence of extreme thermophile Acidianus manzaensis. Trans. Nonferrous Met. Soc. China 24, 1890–1897. Liang, C.L., Xia, J.L., Zhao, X.J., Yang, Y., Gong, S.Q., Nie, Z.Y., Ma, C.Y., Zheng, L., Zhao, Y.D., Qiu, G.Z., 2010. Effect of activated carbon on chalcopyrite bioleaching with extreme thermophile Acidianus manzaensis. Hydrometallurgy 105, 179–185.

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