Mixed Potential Plays a Key Role in Leaching of Chalcopyrite

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The mixed potential plays a key role in leaching of chalcopyrite: experimental and theoretical analysis Congren Yang, Wenqing Qin, Hongbo Zhao, Jun Wang, and Xingjie Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02051 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Industrial & Engineering Chemistry Research

The mixed potential plays a key role in leaching of chalcopyrite: experimental and theoretical analysis Congren Yang 1, 2, Wenqing Qin 1, *, Hongbo Zhao 1, Jun Wang 1, Xingjie Wang 1

1

School of Minerals Processing and Bioengineering, Central South University, Changsha

410083, Hunan, China 2

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing 100084, China

*

Corresponding Author

Address: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China E-mail address: [email protected] Tel.: +86 731 88830884; fax: +86 731 88710804.

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The mixed potential plays a key role in leaching of chalcopyrite. Therefore, the impact of Fe2+ and Fe3+ on chalcopyrite leaching was investigated in this work. Simultaneously, the chalcopyrite passive film was studied by applying cyclic voltammetry (CV), potentiodynamic, potentiostatic and Tafel polarization. X-ray photoelectron spectroscopy (XPS) was used to analyze the products formed during the electrochemical treatment of chalcopyrite. Furthermore, the band theory was used to analyze the oxidation and reduction of chalcopyrite. High copper extraction percentage was obtained at low mixedpotential or ratio of Fe3+/Fe2+. The empty states of chalcopyrite overlapped with filled states of Fe2+, chalcopyrite captured electrons form Fe2+ and was reduced to chalcocite which was very easily oxidized by Fe3+. The Fe dissolves preferentially from the chalcopyrite surface in potentials range of 475 to 700 mV, and leave behind a S22− and Sn2− passive film. The chalcopyrite transpassive dissolution occurs above 700 V.

Keywords: Chalcopyrite; Leaching; Electrochemical behavior; Mixed potential; Band theory

2 ACS Paragon Plus Environment

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1. INTRODUCTION A great deal of research has been carried out on the chemical leaching or bioleaching of chalcopyrite, such as structure, dissolution mechanism, surface species 1-3

and passivation, leaching kinetics, etc.

. Especially, several researchers have

investigated the passivation film formed in the chalcopyrite dissolution process; however, there remains uncertainty regarding the composition of the passive film that forms during chemically and bio-leaching. Nava and González

4

found that, the initial chalcopyrite

dissolution occurred at 0.615 V ≤ Eanod < 1.015V and formed Cu1−xFe1−yS2−z that passivated the electrode surface. Majuste et al. 5 stated that, Cu1−xFe1−yS2−z, Cu5FeS4 and CuS hinder chalcopyrite dissolution, but S0 seemed to have no impact on the electrochemical dissolution of chalcopyrite. Yin et al.

6

reported that, CuS2 passivates

chalcopyrite surfaces at lower potentials and S0 passivates chalcopyrite surfaces at lower potentials in a strongly acidic electrolyte. S32− and S42− was identified by Mikhlin et al. 7 on the surface of chalcopyrite after leaching with H2SO4−Fe2(SO4)3 and HCl−FeCl3, respectively. S22− and Sn2− reported by Acres et al. 8 and Harmer et al. 9 on the surfaces of chalcopyrite in chemical leaching, both are also detected by Zhao et al. bioleaching of chalcopyrite. Klauber et al.

12

10, 11

during the

interpreted, S0 and S22− are two main

passivation candidates for ferric leaching inhibition. He et al.

13

and Zhu et al.

14

stated

that, the jarosite was the major passivation composition hindering the bioleaching of chalcopyrite by thermophilic archaea, whilst S0 seemed to have no impact on the bioleaching of chalcopyrite 14. The mixed potential of a mineral is that potential which results from the mineral being placed in a corroding environment 15, 16. Several researchers reported that the mixed



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potential or ratio of Fe3+/Fe2+ was considered to play a key role in affecting chalcopyrite leaching 17-19. Fe3+ is the most important oxidant in chemical leaching or bioleaching, and chalcopyrite was oxidized by Fe3+ according to Eq. 1, and then Fe2+ was oxidized to Fe3+ by dissolved oxygen and/or microorganisms (Eq. 2). In ferric/ferrous sulfate leaching solution, the mixed potential is mainly decided by the ratio of Fe3+/Fe2+ 15, 16, 20.

CuFeS2 +4Fe3+ → Cu 2+ +5Fe2+ + 2S0

(1)

4Fe2+ +O2 +4H + → 4Fe3+ + 2H 2 O

(2)

Hiroyoshi et al.

21

found that chalcopyrite was more effectively leached in FeSO4

solution, the Cu extraction increased with increasing concentration of Fe2+ and decreasing pH, and further found that coexisting Cu2+ and Fe2+ also accelerated chalcopyrite leaching at low mixed potential. In order to explain this phenomenon, Hiroyoshi et al. 2225

propose a two−step dissolution model for chalcopyrite leaching at low mixed potential,

chalcopyrite is firstly reduced to chalcocite by ferrous ions (Eq. 3), and then the chalcocite is oxidized by ferric ions (Eq. 4). Compared with chalcopyrite (0.52 V vs. SHE), chalcocite (0.44 V vs. SHE) is more easily oxidized due to lower rest potential 26. Similarly, Zhao et al. 17 suggested that in an optimum range of mixed potential (EH−EL), chalcopyrite dissolution can be enhanced due to reduction to chalcocite, where the EH depended on the concentration of Cu2+ and Fe2+, the EL depended on the Cu2+ concentration. Furthermore, they stated that the reason of pyrite enhanced chalcopyrite leaching was due to controlling the mixed potential in an appropriate range, not due to the galvanic assisted leaching

27

. High Cu extraction from chalcopyrite was obtained by

Yang et al. 19 in the potential range of 350 to 480 mV vs. Ag/AgCl. The impact of ratio of Fe2+/Fe3+ on cyclic voltammograms of chalcopyrite was investigated by Qin et al. 4 ACS Paragon Plus Environment

18

, it

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can be seen that peak potential of reduction chalcopyrite to bornite or chalcocite was negative shifted with decreasing ratio of Fe2+/Fe3+. CuFeS2 + 3Cu 2 + + 3Fe 2 + → 2Cu 2S + 4Fe3+

(3)

2Cu 2S + 8Fe3+ → 4Cu 2+ + 8Fe 2 + + 2S0

(4)

In this work, the impact of Fe2+ and Fe3+ on chalcopyrite leaching was investigated. Leaching residuals were examined by X−ray diffraction (XRD) and Raman spectroscopy. Furthermore, the chalcopyrite passive film was further studied by applying cyclic voltammetry (CV), potentiodynamic, potentiostatic and Tafel polarization in pH 1.6 electrolyte solutions at 50 °C. The products formed during the electrochemical treatment of chalcopyrite were analyzed by X-ray photoelectron spectroscopy (XPS). In order to better understand the leaching of chalcopyrite, the band theory was used to analysis oxidation and reduction of chalcopyrite.

2. EXPERIMENTAL DETAILS 2.1. Chalcopyrite sample. The high grade natural chalcopyrite used in this study was obtained from Daye, Hubei Province, China. Powder samples and massive electrodes were prepared as the description of Wu et al.

28

. Figure S1 presents X−ray diffraction

(XRD) (Germany Bruker−axs D8 Advance) patterns of chalcopyrite, and only the characteristic diffraction peaks of tetragonal chalcopyrite was identified. The chalcopyrite sample consists of 33.91% Cu, 30.62% Fe, 32.90% S, 0.039% Pb, 0.018% Zn and 6.91 g/t Ag.



2.2. Leaching experiments. For leaching experiments, 2 g of chalcopyrite with particle size of 100% passing −0.074 mm was added into 250−mL flasks containing 100 mL of sulfuric acid solutions with a pH of 1.6, and target concentrations of Fe2+ and Fe3+ was added into solution. Then the flasks were placed in an orbital shaker at 160 rpm and 50°C. The solution pH was adjusted periodically to 1.6 with diluted sulfuric acid. 1.0 mL of leaching solution was sampled and analyzed for Cu and Fe using inductively−coupled plasma−atomic emission spectrometry (ICP−AES) (America Baird Co. PS−6). The sample volume was replaced with an equal volume of sulfuric acid solutions with a pH of 1.6. Water lost by evaporation was supplemented periodically by adding distilled water until the mass of the flask equaled its initial mass. After the leaching, the leaching residuals were washed with sulfuric acid solutions of pH 1.6 and dried in a vacuum box at 50 °C. The dried leaching residuals were examined by XRD and Raman spectroscopy with 633 nm He−Ne laser (Horiba Jobin Yvon Labram HR800). Similarly, 2 g of chalcopyrite with particle size of −0.074 mm was added into 250−mL flasks containing 100 mL of 1.0 mol/L sulfuric acid solutions, simultaneously, 1 mol/L Fe2(SO4)3, 3 mol/L HNO3 and 0.3 mol/L K2Cr2O7 was added respectively. Then the flasks were placed in an orbital shaker at 160 rpm and 50 °C. After 24 hours of leaching time, leaching solution was sampled and analyzed for Cu using ICP−AES. The leaching residuals were washed with sulfuric acid solutions of pH 1.6 and dried in a vacuum box at 50 °C. The dried leaching residuals were examined by XRD and XPS (ESCALAB 250Xi). The dried powder sample is first pressed into a sheet and then placed onto the sample holder for XPS analysis. XPS was conducted with a monochromatic Al excitation


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(1486.6 eV) operating at 200 W. Simultaneously, a flood-gun was used for charge compensation. High resolution spectra were collected with a pass energy of 20 eV and 0.1 eV steps. All samples were run at ambient temperature and a chamber pressure of 1×10−9 mbar. Binding energy calibration was based on C 1s at 284.6 eV. The High resolution S 2p spectra were fitted with a summed Gaussian (70%) – Lorentzian (30%) function and Shirley background.

2.3. Electrochemical test. All electrochemical experiments were conducted with a Princeton Model 283 potentiostat (EG&G of Princeton Applied Research) controlled by PowerSuite under ambient atmosphere at 50 °C in electrolyte solution. A conventional three-electrode electrolytic cell with graphite rod as a counter electrode, saturated Ag/AgCl electrode as a reference electrode and the chalcopyrite electrode with areas of 1 cm2 exposed to the solution as the working electrode was used for electrochemical analysis. The electrolyte solution was prepared as follows: 3.0 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L MgSO4·7H2O, 0.5 g/L K2HPO4, 0.01 g/L Ca(NO3)2 with a pH of 1.6. Before every electrochemical experiment, the chalcopyrite electrode surface was polished with carbide papers, from no. 800 to no. 3000, and then rinsing with deionized water. Potentiodynamic polarization was scanned from 200 mV to 1000 mV with scan rate of 20 mV/s. Potentiostatic polarization was conducted at 475, 600, 700, 800, 900 and 1000 mV for 1800 second, respectively. The cyclic voltammograms and Tafel polarization curves were obtained after electrochemical treatment of a chalcopyrite electrode at 475, 600, 700, 800, 900 and 1000 mV for 1800 second. CV was scanned from 475 mV in the positive directions with scan rate of 20 mV/s. The scan rate of the Tafel polarization is 0.5 mV/s.



Icorr (corrosion current density) was obtained with extension of the cathodic Tafel slope to the Ecorr (corrosion potential), and Rp (polarization resistance) was obtained with fitting the Tafel polarization curves at Ecorr ± 20 mV.

3. RESULTS AND DISCUSSION 3.1. Fe2+ accelerating dissolution of chalcopyrite. Figure 1 shows the copper extraction percentage, total iron concentration and mixed potential during the leaching of chalcopyrite with different Fe2+ and Fe3+ concentrations. In blank solutions with a pH of 1.6, the copper extraction percentage increased with time and reached 12% after 14 days. Similar to copper, the total iron concentration increased with time and reached 0.66 g/L after 14 days. While during the whole leaching process, the mixed potential stabilized at about 330 mV, meaning that the iron in leaching solution presented as Fe2+ because the mixed potential of sulfate leaching solution decided by the ratio of Fe3+/Fe2+

15, 16

. The

chalcopyrite dissolution in acid solutions was described as Eq. 5.

CuFeS2 +O2 +4H + → Cu 2+ +Fe2+ + 2S0 + 2H 2O

50

(5)

550

b Mixed potential (mV vs. Ag/ACl)

a 40

Copper extraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

c

7

TFe concentration (g/L)

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6 5

Blank 3+ Fe =0.1 mol/L 2+ 3+ Fe :Fe =0.05 mol/L:0.05 mol/L 2+ Fe =0.1 mol/L

4 3 2 1 0 0

2

4

6

8

10

12

14

16

Time / day

Figure 1 Effect of Fe2+ and Fe3+ ions on the chemical leaching of chalcopyrite. a−copper extraction, b−mixed potentials, c−concentrations of total dissolved iron

Figure 1a shows that the copper extraction percentage decreased with adding Fe3+ into solutions; but the copper extraction percentage increased with adding Fe2+. After 14 days of leaching time, 10%, 26% and 40% of copper was leached from chalcopyrite with 0.1 mol/L Fe3+, 0.05 mol/L Fe3+ + 0.05 mol/L Fe2+, and 0.1 mol/L Fe2+, respectively. Moreover, the leaching of chalcopyrite in Fe3+ solutions was lower than it leaching in blank solutions. The initial mixed potentials in three kinds of leaching systems were 510, 420 and 270 mV, respectively (Figure 1b). In 0.1 mol/L Fe3+, and 0.05 mol/L Fe3+ + 0.05 mol/L Fe2+ solutions, the mixed potential decreased with leaching time due to the precipitation of Fe3+ (Eq. 6) and/or reduction Fe3+ to Fe2+ (Eq. 1) 18. But in 0.1 mol/L Fe2+ solutions, the mixed potential increased from 270 mV to 370 mV in the first 3 days, and then it slowly increased from 340 mV to 370 mV. Although the mixed potential was less than 340 mV in first 3 days, but 11% of copper was leached from chalcopyrite. It was higher than copper extracted from chalcopyrite in 0.1 mol/L Fe3+ solutions after 14 days. Therefore, there was clear benefit in leaching chalcopyrite within the low mixed potential



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or ratio of Fe3+/Fe2+. The total iron concentrations in leaching solution are shown in Figure 1c. During the leaching process, precipitation of Fe3+ as shown in Eq. 6 caused the decrease of total iron concentration. M + + 3Fe3+ + 2SO 24 − + 6H 2 O → MFe3 (SO 4 ) 2 (OH) 6 + 6H +

(6)

Figure 2 and 3 show XRD and Raman patterns of chalcopyrite leached after 14 days. Raman spectra of CuFeS2, S0 and jarosite shown in Figure S2 are used as reference. Where, Raman spectrum of the original CuFeS2 indicates an intense band at 290 cm−1 and additional low intensity bands at 319, 351 and 375 cm−1; Raman spectrum for S0 has three bands at 153, 219 and 472 cm−1; jarosite has five intense bands at 222, 433, 624, 1006, 1100 cm−1, and additional three low intensity bands was at 298, 572, 1151 cm−1, respectively. Smaller amounts of elemental sulfur were detected by XRD when chalcopyrite was leached in solutions with a pH of 1.6 (Figure 2a), and the same result was obtained by Raman (Figure 3a). The main leaching product of the chalcopyrite leached with 0.1 mol/L Fe2+, 0.05 mol/L Fe2+ + 0.05 mol/L Fe3+ was elemental sulfur and jarosite (Figure 2b−c), but leached with 0.1 mol/L Fe3+, the main leaching product was jarosite, together with lesser amounts of elemental sulfur (Figure 2d). Raman analysis indicated that a significant amount of elemental sulfur and jarosite covered on chalcopyrite surface after chalcopyrite leaching with 0.1 mol/L Fe2+ (Figure 3b). Only a low intensity band of chalcopyrite was detected at 288 cm−1. When chalcopyrite was leached with 0.1 mol/L Fe2+ or 0.05 mol/L Fe2+ + 0.05 mol/L Fe3+, excepted chalcopyrite bands centered at about 290, 317 and 350 cm−1, only bands of jarosite was detected on the chalcopyrite surface (Figure 3c−d).


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4000

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Figure 2 XRD patterns of chalcopyrite leached in solutions with a pH of 1.6 after 14 days. a−blank, b−0.1 mol/L FeSO4, c−0.05 mol/L FeSO4+0.025 mol/L Fe2(SO4)3, d−0.05 mol/L Fe2(SO4)3 280

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Figure 3 Raman patterns of chalcopyrite leached in solutions with a pH of 1.6 after 14 days. a−blank, b−0.1 mol/L FeSO4, c−0.05 mol/L FeSO4+0.025 mol/L Fe2(SO4)3, d−0.05 mol/L Fe2(SO4)3

Jarosite was detected by XRD and/or Raman spectroscopy during leaching of chalcopyrite. But D'Hugues et al.

29

and Gu et al. 30 did not find any evidence of jarosite

hindering chalcopyrite chemical or bio−leaching. Qin et al. 18 reported that 92.5% Cu was extracted from chalcopyrite by moderately thermophilic microorganisms even though a significant amount of S0 and jarosite formed on the surface of chalcopyrite, and the 19, 31, 32

similar results were obtained by Yang et al.

. In our viewpoint, elemental sulfur

and jarosite was not candidates for passivation chalcopyrite surface. It can be concluded from leaching results, leaching of chalcopyrite was accelerated by Fe2+. This was similar with previous study

17, 21-24

, they suggested that at low mixed

potential, chalcopyrite was firstly reduced to chalcocite (Eq. 3), and then the formed chalcocite was oxidized by Fe3+ (Eq. 4). Chalcopyrite was reduced to bornite and/or chalcocite in the cathodic (potential less than 100 mV vs. SHE) according to Eq. 7−9, and then

chalcocite

was

oxidized

in

the

anodic

as

following

sequence:

Cu2S→Cu1.92S→Cu1.60S→CuS 18, 30, 33. Bornite and/or chalcocite were detected by Liu et 12 ACS Paragon Plus Environment

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al.

34, 35

when the mixed potential was less than 500 mV vs. SCE, the covellite was

detected when mixed potential was at about 550 mV vs. SCE. The covellite was also detected by Yang et al. 19 during the bioleaching of chalcopyrite with different Fe2+/Fe3+ ratio at 30 and 48 °C. The chalcocite was not detected by XRD and Raman in our experiments because the chalcocite was very easily oxidized by Fe3+. 5CuFeS2 + 12H + + 4e − → Cu 5 FeS4 + 6H 2S + 4Fe 2 +

(7)

2Cu 5 FeS4 + 6H + + 2e − → 5Cu 2S + 3H 2S + 2Fe 2+

(8)

2CuFeS2 + 6H + + 2e − → Cu 2S + 3H 2S + 2Fe 2+

(9)

3.2. Electrochemical behavior of chalcopyrite. In order to understand the dissolution and passivation of chalcopyrite, electrochemical behavior of chalcopyrite was investigated. Figure 4a shows the potentiodynamic polarization of chalcopyrite. The initial oxidation of chalcopyrite occurs in region A (potentials between OCP and 600 mV), in which the small quantity of chalcopyrite was oxidized. The change in slope in region B (potentials between 600 and 700 mV) illustrates that the product of chalcopyrite dissolution in the potential window of OCP to 600 mV accumulated on the surface, and passivated the electrode surface. The electrode shows a significant dissolution in region C (potentials between 700 and 830 mV) and E (above 920 mV). In region D (potentials between 830 and 920 mV), the current density decreases within scan potential increasing, this indicates that the chalcopyrite surface appears to be in the passive state.



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Figure 4 Potentiodynamic polarization of chalcopyrite electrode (a). Potentiostatic polarization of chalcopyrite electrode at 475, 600, 700, 800, 900 and 1000 mV for 1800 second (b−d) Figure 4b-d presents i-t behavior of chalcopyrite for applied potentials at 475, 600, 700, 800, 900 and 1000 mV. The i-t curves of chalcopyrite for applied potentials in the potential window of 475 to 900 mV show that the current density decreases sharply owing to the formation of a stable passive film during the first 300 second. However, potentiostatic polarization of chalcopyrite at 1000 mV shows that the current density decreases sharply during the first 50 s, and then the current density decreases slowly owing to accumulation of the product of oxidized chalcopyrite on the surface. The electrode surface reaches the steady state current after 600 s. The passive layer formed on


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the chalcopyrite surface was further confirmed by using the cyclic voltammograms obtained after dissolution of chalcopyrite at 475, 600, 700, 800, 900 and 1000 mV for 1800 second (Figure 5a-b). Figure 5a-b shows that the passive film formed in the potential window of 475 to 700 mV would be oxidized when the sweeping potential is above 800 mV. The Tafel polarization curves obtained after electrochemical treatment of a chalcopyrite electrode at 475, 600, 700, 800, 900 and 1000 mV for 1800 second are shown in Figure 5c. From the Tafel polarization curves, the Icorr and Rp can be obtained (as presented in Figure 5d). It can be seen that increasing the polarization potential leads to a significant increase in the corrosion potential (Figure 5c). The Rp increased with an increase of polarization potential from 475 to 700 mV suggests the formation of passivation films on the chalcopyrite surface. In general, a passivation layer on the material surface hinders dissolution rate owing either to its low electron or ionic conduction ,and Ghahremaninezhad et al.

36

reported that the passive film hinders the

dissolution of chalcopyrite due to its low ionic conduction. When the polarization potential increased from 700 to 1000 mV, the accumulation of product on the surface of the chalcopyrite resulted in the current decreases (Figure 4c-d), but the Rp decreased with increasing polarization potential, it suggests the chalcopyrite transpassive dissolution occurs. Figure 5d shows that a decrease in the current density from 0.23 to 0.10 µA/cm2 with polarization potential increasing from 475 to 700 mV, and then the current density increases from 0.10 to 0.90 µA/cm2 within polarization potential increasing from 700 to 1000 mV. Especially, the current density increases quickly when the polarization potential is above 800 mV. The higher value of the corrosion current density or lower



value of the polarization resistance means that the oxidation kinetics is higher during the dissolution process. From potentiostatic polarization and Tafel polarization, it was concluded that the passive film-covered chalcopyrite obtained after the electrochemical treatment in the potential window of 800 to 1000 mV is dissolved faster. However, the oxidation of the passive film-covered chalcopyrite obtained after the electrochemical treatment in the potential window of 475 to 700 mV is slow. Thus, the passive film which formed in the potential window of 475 to 700 mV will hinder further dissolution of chalcopyrite. In the potential region of 800 to 1000 mV, accumulation of the product of oxidized chalcopyrite on the surface also has a significant effect on chalcopyrite dissolution. 3

2

a

2

0

475 mV

-1

0

-2

-1 -2

700 mV

800 mV

-3

I (mA)

I (mA)

b

1

600 mV

1

-4

-3

900 mV

-5

-4

-6

-5

-7

-6

-8

-7

-600

-400

-200

0

200

400

600

800

-9

1000

1000 mV

-600

-400

-200

0

E vs. SHE (mV)

200

400

600

800

1000

E vs. SHE (mV) -7 X 10 240

10

900

800 mV

c

9

800

d

Icorr

220

Rp

200

8

180 7

500 400

700 mV 475 mV

140

5

120

4

100 80

3

60 2

300

1

200 1E-10

40

600 mV

20 0

0 1E-9

1E-8

1E-7

1E-6

1E-5

400

1E-4

500

I/area (A/cm^2)

600

700

800

E vs. SHE (mV)


900

1000

2

2

1000 mV

600

160

6

Rp (kΩ /cm )

900 mV

700

Icorr (A/cm )

E vs. SHE (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 Cyclic voltammograms (a and b) and Tafel plot (c) for chalcopyrite electrode after dissolution at 475, 600, 700, 800, 900 and 1000 mV for 1800 second. Corrosion current density and polarization resistance obtained from c (d)

3.3. XPS study of the surface composition of chalcopyrite. XRD and Raman spectroscopy would analyse bulk of material, but XPS would analyse approximately 1-5 nm surface of material. Thus XPS is employed to investigate the surface component of chalcopyrite electrodes after potentiostatic polarization in electrolyte solution at 475, 600, 700, 800, 900 and 1000 mV for 1800 second. The XPS of Cu 2p, Cu LMM, S 2p, and Fe 2p for chalcopyrite was reported in our previous publish

28

. Figure 6a-b indicate the Cu 2p and LMM photoelectron spectra

obtained for chalcopyrite electrode after electrochemical treatment at 475, 600, 700, 800, 900 and 1000 mV for 1800 second respectively. The binding energies for Cu 2p3/2 and kinetic energies for Cu LMM are summarized in Table S1. It is well established in literature that there are two main XPS characteristics of Cu(II): the Cu 2p3/2 peak with binding energy of 933.0–933.8 eV and the presence of a satellite features around the binding energy of 942 eV, while a lower Cu 2p3/2 binding energy of 931.8–933.1 eV and the absence of the satellite features are characteristics of Cu(I) and metal Cu 37-40. Simultaneously, there is no known report of an Auger parameter less than 1850.5 eV for a Cu(II) compound 40. From Figure 6a-b and Table S1, it can be seen that the Cu on the surface of chalcopyrite after electrochemical treatment at different potentials is still +1. Cu5FeS4 5, CuS 5, 39, CuS2 6 and Cu1−xFe1−yS2−z

4, 5, 7, 39

are suggested

to be the products of chalcopyrite electrochemical dissolution. While the 2p3/2 binding



energies for Cu in CuFeS2

39, 41-43

, Cu5FeS4

40, 44

and CuS

42, 45-47

are very close, it is

difficult to use the Cu 2p binding energy and Cu LMM to identify which compounds are present.

b Intensity (a.u.)

Intensity (a.u.)

a

475 mV 600 mV 700 mV

475 mV 600 mV 700 mV

800 mV 900 mV

800 mV 900 mV

1000 mV

1000 mV

970

965

960

955

950

945

940

935

930

906

925

908

910

912

c

475 mV

Intensity (a.u.)

Intensity (a.u.)

918

920

922

924

926

928

d

900 mV

700 mV

735

916

800 mV

600 mV

740

914

Kinetic Energy (eV)

Binding Energy (eV)

730

725

720

715

710

705

700

1000 mV

740

735

730

Binding Energy (eV)

725

720

715

710

705

700

Binding Energy (eV)

f

e

Original chalcopyrite

475 mV

Intensity (a.u.)

600 mV Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 39

700 mV

800 mV

475 mV 600 mV 700 mV 800 mV

900 mV

900 mV

1000 mV 174

172

1000 mV 170

168

166

164

162

160

158

232

230

228

Binding Energy (eV)

226

224

222

220

Binding Energy (eV)


218

216

214

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6 Comparison of Cu 2p (a), Cu LMM (b), Fe 2p (c and d), S 2p (e) and S 2s (f) and peaks of chalcopyrite after dissolution at 475, 600, 700, 800, 900 and 1000 mV for 1800 second Figure 6c-d present the Fe 2p photoelectron spectrum obtained for chalcopyrite electrode after electrochemical treatment at 475, 600, 700, 800, 900 and 1000 mV for 1800 second. When the polarization potential is less than 800 mV, Fe 2p peaks are similar. However, when the polarization potential is above than 800 mV, the value of Fe 2p peak shifted to a higher binding energy. The peak near 708 eV was assigned to Fe−S 39

. From Figure 6c-d, the peak in the binding energy range of 710 to 712 eV was assigned

to Fe−OH Fe−OH

48

48

when the polarization potential is below 700 mV, and was assigned to

and/or MFe3(SO4)2(OH)6

28

when the polarization potential is above 800 mV.

The binding energy near 712.5 eV would most probably be a Cu LMM peak 39, 44. Figure 6e-f illustrate a series of S 2p and S 2s photoelectron spectrum obtained for chalcopyrite electrode after electrochemical treatment at 475, 600, 700, 800, 900 and 1000 mV for 1800 second. The fitted photoelectron spectra of S 2p peaks of Figure 6e are presented in Figure S3 and Table 1. The S 2p2/3 binding energies centered in the range of 161.2-161.5 eV is in agreement with S2- 49, 50. The S 2p2/3 peak with the binding energy of 161.9-162.2 eV is in agreement with the S 2p3/2 peak position of S22-

8, 39, 51

, and S 2p2/3

peak with the binding energy of 162.8-163.2 eV agrees with the S 2p3/2 peak position of Sn2-8, 39, 52. The S 2p2/3 peak with the binding energy of 163.5-163.8 eV is in agreement with the S 2p3/2 peak position of S0

50, 53, 54

. It has been well established in the literature

that the S 2s peaks of the S atoms in chalcopyrite, covellite and chalcocite are 225.6, 225.6 and 225.3 eV , respectively; while the S 2s peak of elemental sulfur occurs at a



binding energy of 228.4 eV

39

Page 20 of 39

. Therefore, the S 2s peak can be used for detection of

elemental sulfur formation. From Figure 6f, it can be seen that the S 2s binding energy shifts to higher energies within increasing potential from 475 to 1000 mV, this indicates the elemental sulfur formation on the chalcopyrite surface. When the applied potential is above 800 mV, the S 2p2/3 binding energy centered at about 167.9 eV is related to SO42− 39

and most probably due to jarosites formation 28. The SO42− did not be detected on the

surface of chalcopyrite polarized at 475, 600 and 700 mV (Table 1).

Table 1 Binding energy values for XPS spectra of S 2p3/2 peaks (eV) Condition Monosulfide Disulfide Polysulfide

Elemental sulfur

Sulfate

CuFeS2 28

161.3

161.9

162.8

475 mV

161.2

162.0

163.0

163.8

600 mV

161.3

162.2

163.2

163.7

700 mV

161.3

162.2

163.2

163.6

800 mV

161.4

162.2

163.5

167.8

900 mV

161.5

162.2

163.6

167.9

1000 mV

161.5

163.5

167.9

Energy loss 164.3

3.4. Disulfide and polysulfide passivate the chalcopyrite surface. The low leaching rate of chalcopyrite is due to the passivation of the mineral surface. However, the composition of the passive film is still on argument. Therefore, the understanding of chalcopyrite dissolution and surface composition are necessary to improve Cu extraction.


Page 21 of 39

Figure 7 shows the Cu, Fe and S fraction and sulfur species on the surface of chalcopyrite after 1800 second of the electrochemical treatment at 475, 600, 700, 800, 900 and 1000 mV. In order to obtain the data of Figure 7b, it was assumed that the percentage of each species (S2−, S22−, Sn2−, S0 and SO42−) is directly proportional to the area under its S 2p peak presented in Figure S3. When the chalcopyrite electrode was polarized at 475 mV, Fe atoms dissolve preferentially than Cu atoms from the chalcopyrite lattice which leads to the increase in S and Cu fraction at the surface (Figure 7a). Simultaneously, it leaves behind a S22− and Sn2− film (Figure 7b).

b

a

100

100

80 70 60 50 40 30

90 80 70

Sulfur species (%)

Chalcopyrite 475 mV 600 mV 700 mV 800 mV 900 mV 1000 mV

90

Atomic (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30

20

20

10

10 0 400

0 S

Fe

Cu

Monosulfide Disulfide Polysulfide Elemental sulfur Sulfate

500

600

700

800

900

1000

E vs. SHE (mV)

Elements

Figure 7 Composition of Cu, Fe and S fraction (a) and sulfur species (b) on the surface of chalcopyrite after dissolution at 475, 600, 700, 800, 900 and 1000 mV for 1800 second

When the polarization potential increases from 475 mV to 600 mV, S fraction increases due to the dissolution of Cu from the chalcopyrite surface, but the Fe fraction changes little (Figure 7a). When the polarization potential further increases from 600 mV to 700 mV, the S, Fe and Cu fraction on the chalcopyrite surface is almost a constant



Page 22 of 39

(Figure 7a). Furthermore, the S2− fraction on the chalcopyrite surface slightly decrease from 60% to 53% with an increase of the polarization potential from 475 to 600 mV (Figure 7b); but the S2− fraction on the chalcopyrite surface is a constant (about 53%) in potentials range of 600 to 700 mV (Figure 7b), which indicates that there is no more chalcopyrite dissolved with increasing potential. In other words, a passivation film is formed on the chalcopyrite surface in potential range of 475 to 700 mV. In this potentials range, in addition to S2−, the most sulfur species on the chalcopyrite surface is Sn2−, followed by S22−. Therefore, the S22− and Sn2− are considered as composition of chalcopyrite passivation film. While, Ghahremaninezhad et al.

36, 39

suggested that

Cu1−xFe1−yS2−z which consists of Cu-S and Fe-S bonds passivates the chalcopyrite surface due to its low ionic conductivity when the potential is below 0.90 V, but above 0.90 V, transpassive dissolution occurs. Simultaneously, S0 and Sn2− were detected on the surface. Yang et al.

55

stated that the metal-deficient Sn2−/S0 layer passivates chalcopyrite anodic

dissolution when the potential is lower than 530 mV (vs. Ag/AgCl), and they concluded the same results during the chalcopyrite bioleaching 56. However, Klauber

57

argued that

the physical reality of Cu1−xFe1−yS2−z is also questioned, and Sn2− could not be considered as passivation candidates because they were easily oxidized to S0 on exposure to air and water, but more and more researchers detected Sn2− on the chalcopyrite surface by XPS 7, 8, 10, 11, 28, 58

. It is also detected in our study. Furthermore, according to density functional

theory calculations, S42− was formed on the chalcopyrite surface and Crundwell

61

58, 59

. Holmes et al.

60

suggested that the Sn2− and Cu1−xFe1−yS2−z are essentially the same, the

only difference is the focus on either the sulfur or on the metal removal. But Crundwell et al. 62, 63 argued that the rate of chalcopyrite dissolution is not limited by passivation films


Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or layers, but is intrinsically slow due to the semiconducting properties of chalcopyrite. Furthermore, they identified the chalcopyrite anodic dissolution as depletion region (0.3 to 0.6 V vs. SCE) and inversion region (> 0.7 V vs. SCE) for ideal n-type semiconductor. In the depletion region, the current is limited by the depletion of electrons in the spacecharge layer, and no reactions take place at the surface and supply electrons in this region. While, Figure 7b shows that, within increasing potential from 475 to 600 mV, the fraction of S2− and S22− on the chalcopyrite surface decreases ~7% and ~6%, respectively, but the fraction of Sn2− and S0 increases ~7% and ~6%, respectively; further increasing potential from 600 to 700 mV, the fraction of S2− and S22− on the chalcopyrite surface is almost a constant, but the Sn2− is oxidized to S0. Therefore, the low current is due to the chalcopyrite surface passivation. When the applied potential is above 700 V, the fraction of Cu and Fe on the chalcopyrite surface decrease with increasing potential, while the fraction of S on the chalcopyrite surface increase with increasing potential (Figure 7a). It can be noticed from Figure 7b that when the applied potential is above 700 V, the fraction of S2−, S22− and Sn2− show an obvious decrease (S2− decreases from 53% to 3%, S22− decreases from 10% to 0%, and Sn2− decreases from 20% to 0%). Simultaneously, S0 increases from 17% to 92%. Additionally, the concentration of SO42− increases from 0% to 5%. Especially, when the applied potential is increased from 700 to 800 mV, the fraction of Sn2− decreases from 20% to 0%. Simultaneously, the fraction of S2− decreases from 53% to 25%, while the S0 shows a sudden increase (S0 increases from 17% to 69%) which indicated that the chalcopyrite was dissolved. Elemental sulfur is generally considered as a product of CuFeS2 dissolution (Eq. 10-11)

5, 39, 64

, and/or the product of CuS (Eq. 12)



Page 24 of 39

dissolution at high potentials (E ≥ 0.8 V) 4, 5, 39, which was detected by Majuste et al. 5 on surface of chalcopyrite after electrochemical dissolution at 0.80 V (Eq. 11). In our currently study, when the polarization potential increases from 900 mV to 1000 mV, the increase in S fraction and the decrease in Cu fraction are equivalent, and the Fe fraction does not change (Figure 7a). This indicates the CuS presence on the chalcopyrite surface. SO42− is originated from jarosites which formed on the chalcopyrite surface according to Eq. 6 28.

CuFeS2 → Cu 2+ + Fe3+ + 2S0 + 5e−

(10)

CuFeS2 → CuS + Fe3+ + S0 + 3e−

(11)

CuS → Cu 2+ + 2S0 + 2e −

(12)

In order to further certify that the S22− and Sn2− passivate the chalcopyrite surface, and the passive film would be oxidized when the potential is above 800 mV. Chalcopyrite was leached with 1 mol/L Fe2(SO4)3, 3 mol/L HNO3 and 0.3 mol/L K2Cr2O7, respectively. The redox potential of Fe3+/Fe2+, NO3−/NO and Cr2O72−/Cr3+ was 0.771, 0.957 and 1.232 V, respectively. 3%, 83% and 96% Cu was extracted after 24 hours of leaching with Fe2(SO4)3, HNO3 and K2Cr2O7, respectively (Figure 8). XRD patterns of leaching residuals are shown in Figure S4. Only the characteristic peaks of chalcopyrite were detected by XRD when chalcopyrite was leached with Fe2(SO4)3. The S2−, S22−, Sn2− and S0 were observed by XPS on the surface of chalcopyrite (Figure 8). When chalcopyrite was leached with HNO3, the characteristic peaks of chalcopyrite were detected by XRD, together with low intensity characteristic peaks of elemental sulfur. But the XPS analysis shows that main sulfur species on the surface of chalcopyrite was S0, together with a low intensity SO42− (Figure 8). Only the characteristic peaks of 24 ACS Paragon Plus Environment

Page 25 of 39

elemental sulfur were detected by XRD when chalcopyrite was leached with K2Cr2O7. At the same time, S0and SO42− was observed by XPS (Figure 8). It can be concluded from above, the S22− and Sn2− passivate the chalcopyrite surface, which cannot be oxidized by Fe3+. But the passive film can be oxidized to S0 by NO3− and Cr2O72− due to their higher redox potential, therefore higher copper extraction was obtained (Figure 8). Furthermore, the presence of S0 on the surface of the chalcopyrite does not hinder chalcopyrite leaching (Figure 8). These are in agreement with electrochemical results that the S22− and Sn2− passivate the chalcopyrite surface.

90 80 70

2S

100

2S2 0 S

90

2Sn 2SO4 Energy-loss Copper extraction

80 70

60

60

50

50

40

40

30

30

20

20

10

10

0


100

S composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Fe2(SO4)3

K2Cr2O7

HNO3 Oxidant

Figure 8 Sulfur species on the surface of chalcopyrite and copper extraction In summary, during the leaching process, Fe atoms dissolve preferentially than Cu atoms from the chalcopyrite lattice, leaving behind a S-rich layer consisted with S22− and Sn2−. The S-rich layer is considered as Cu1−xFe1−yS2 (y>>x), and passivates the chalcopyrite surface. When the metal atom released entirely from the Cu1−xFe1−yS2



Page 26 of 39

(y>>x), and this fully-depleted layer becomes thicker than several atomic layers, restructuring could occur to form S8-like S0.

3.5. The electronic structure impact on the leaching of chalcopyrite. In order to better understand the leaching of chalcopyrite, the band theory was used to analyze oxidation and reduction of chalcopyrite. The band gap of chalcopyrite was in the range of 0.53 to 0.6 eV 1, and the Fermi energy was calculated to −5.4 eV

65

. The valence band

consisted of S3p and Cu 3d together with less amount of Fe 3d, and the conduction band consisted of Fe 3d together with less amount of S3p 1. The initial oxidation of chalcopyrite should be related to the electrons in occupied states near Fermi energy; the initial reduction of chalcopyrite should be related to the unoccupied states near Fermi energy. Therefore, empty states of Fe 3d captured electrons from filled states of reducing agent during the reduction of chalcopyrite; but unoccupied states of oxidizing agent captured electrons from occupied states of S3p and Cu 3d during the oxidation of chalcopyrite. The energy median between the occupied and unoccupied states (Eredox) is related directly to the oxidation potential of the redox couple couple of H+/H2 was −4.5 eV

67, 68

66

, and the Eredox of redox

. Thus the Eredox of Cu2+/Cu+, Fe3+/Fe2+, NO3−/NO,

Cr2O72−/Cr3+ was calculated to −4.7, −5.3, −5.5 and −5.9 eV, respectively

67

. Electron

transfer can take place from any occupied state that is matched in energy with an unoccupied state. Figure 9A shows that the Eredox of Cu2+/Cu+ was higher than the bottom of conduction band, the empty states of chalcopyrite overlap with filled states of Cu+, therefore chalcopyrite captured electrons from Cu+ and was reduced. Zies


69

proved that

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the chalcopyrite converted into chalcocite was faster in the presence of Cu+ than in the presence of Cu2+. The Eredox of Fe3+/Fe2+ located in the band gap and was higher than the Fermi Energy of chalcopyrite, although increasing the ratio of Fe3+/Fe2+ would cause the Eredox lower than the Fermi level, but still higher than the top of valence band (Figure 9). Thus Fe3+ is difficult to capture electrons from the surface of chalcopyrite, in other words, chalcopyrite is difficult to be leached by Fe3+. When chalcopyrite was leached with 1 mol/L H2SO4 and 1 mol/L Fe2(SO4)3 at 50 °C, only 3% of Cu was extracted after 24 hours. Similarly, in 0.1 mol/L Fe3+ solutions with a pH of 1.6, the copper extraction percentage was only 10% after 14days. Figure 9B also shows that decreasing the ratio of Fe3+/Fe2+ would result in the Eredox close to the bottom of conduction band, even the empty states of chalcopyrite overlaps with filled states of Fe2+, which correspond to strongly enabled reduction of chalcopyrite and oxidation of Fe2+. Many researchers suggested that chalcopyrite was reduced to chalcocite at low mixed potential or ratio of Fe3+/Fe2+, then the chalcocite was very easily oxidized by Fe3+, thus high copper extraction percentage was reached

17, 18, 21, 23, 30, 33-35

. In 0.1 mol/L Fe2+ solutions with a

pH of 1.6, the copper extraction percentage finally reached 40% after 14 days of leaching time. Furthermore, chalcopyrite was leached with 1 mol/L H2SO4 and 3 mol/L HNO3 or 1 mol/L H2SO4 and 0.3 mol/L K2Cr2O7 at 50 °C, 83% and 96% Cu was extracted after 24 hours, respectively. Figure 9A shows that the Eredox of NO3−/NO was lower than the Fermi level, therefore NO3− captured electrons form chalcopyrite and was reduced, in other words, chalcopyrite was oxidized by NO3−. The empty states of Cr2O72− overlapped with filled states of chalcopyrite, which corresponded to strongly enabled reduction of



Cr2O72− and oxidation of chalcopyrite, therefore higher the copper extraction percentage was obtained. A

Energy (eV)

B

-5.0

H+/H2 −4.5 -5.1

Cu2+/Cu+ −4.7 Conduction band Fe 3d Fe3+/Fe2+ NO3+/NO

−5.3 −5.5

Cr2O72-/ Cr3+ −5.9

Eox

Fermi Energy

E0

−5.4

Ered Valence band S 3p Cu 3d

Eredox (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

-5.2

-5.3

-5.4

-5.5 10

20

30

40

50

3+

60

70

80

90

100

2+

[Fe ]/[Fe ]

Figure 9 Schematic illustration for band structure of chalcopyrite (A), and relation between Eredox and ratio of Fe3+/Fe2+ (B) In summary, the electronic structure plays a key role for oxidation or reduction of chalcopyrite, empty states of oxidizing agent overlap with filled states of chalcopyrite or filled states of reducing agent overlap with empty states of chalcopyrite, which correspond to strongly enabled oxidation and reduction of chalcopyrite, Thus higher copper extraction was obtained.

4 CONCLUSION There was clear benefit in leaching chalcopyrite within the low mixed potential or ratio of Fe3+/Fe2+. 10%, 26% and 40% of copper was leached from chalcopyrite with 0.1 mol/L Fe3+, 0.05 mol/L Fe3+ + 0.05 mol/L Fe2+, and 0.1 mol/L Fe2+, respectively. Chalcopyrite converted into chalcocite which was very easily oxidized by Fe3+ at low mixed potential or ratio of Fe3+/Fe2+.


Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe dissolves preferentially from the chalcopyrite surface in potentials range of 475 to 700 mV, therefore leaving behind a S22− and Sn2− passive film. The passive film consisted of S22− and Sn2− was oxidized when the applied potential is higher than 700 V, and transpassive dissolution of chalcopyrite occurred. The similar results were obtained by leaching chalcopyrite with Fe2(SO4)3, HNO3 and K2Cr2O7. The electronic structure plays a key role for oxidation or reduction of chalcopyrite, empty states of oxidizing agent overlap with filled states of chalcopyrite or filled states of reducing agent overlap with empty states of chalcopyrite, which correspond to strongly enabled oxidation and reduction of chalcopyrite, so higher copper extraction was obtained.

ASSOCIATED CONTENT Supporting Information XRD patterns of chalcopyrite; Raman spectra of CuFeS2, S0 and jarosite; binding energy values for XPS spectra of Cu 2p3/2 and kinetic energy values for Cu LMM peaks; the fitted S 2p XPS spectra for chalcopyrite electrode surface obtained after dissolution at different time; XRD patterns of chalcopyrite after dissolution with different oxidants; the fitted S 2p peaks of chalcopyrite dissolution with different oxidants.

AUTHOR INFORMATION Corresponding Author *

E-mail address: [email protected]; Tel.: +86 731 88830884, Fax: +86 731

88710804.



Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Provincial Science and technology leader (Innovation team of interface chemistry of efficient and clean utilization of complex mineral resources, Grant No. 2016RS2016); the Co-Innovation Centre for Clean and Efficient Utilization of Strategic Metal Mineral Resources; and the Innovation Driven Plan of Central South University (Grant No. 2015CX005).

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Page 39 of 39

For Table of Contents Use Only

Energy (eV) 90 2

−4.5

80

Cu2+/Cu+ −4.7

70

Conduction band Fe 3d Fe3+/Fe2+ −5.3 NO3+/NO −5.5 Cr2O7

2-/

Cr3+

−5.9

Fermi Energy −5.4

Valence band S 3p Cu 3d

100

2S

2S2 20 Sn S 2SO4 Energy-loss Copper extraction

90 80 70

60

60

50

50

40

40

30

30

20

20

10

10

0

0

Fe2(SO4)3

HNO3 Oxidant


K2Cr2O7


100

H+/H

S composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mixed Potential Plays a Key Role in Leaching of Chalcopyrite

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