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Investigation of Cu-S intermediate species during electrochemical dissolutionand bioleaching of chalcopyrite concentrate

Weimin Zeng, Guanzhou Qiu, Miao Chen

PII: S0304-386X(13)00046-7DOI: doi: 10.1016/j.hydromet.2013.02.009Reference: HYDROM 3683

To appear in: Hydrometallurgy

Received date: 21 November 2012Revised date: 31 January 2013Accepted date: 11 February 2013

Please cite this article as: Zeng, Weimin, Qiu, Guanzhou, Chen, Miao, Investigationof Cu-S intermediate species during electrochemical dissolution and bioleaching of chal-copyrite concentrate, Hydrometallurgy (2013), doi: 10.1016/j.hydromet.2013.02.009

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Investigation of Cu-S intermediate species during electrochemical

dissolution and bioleaching of chalcopyrite concentrate

Weimin Zeng a, b, c

, Guanzhou Qiu a, b*

, Miao Chen c*

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

b Key Laboratory of Biometallurgy, Ministry of Education, Changsha, China

c CSIRO Process Science and Engineering, Clayton, Victoria, 3169, Australia

Abstract: the electrochemistry behaviour of chalcopyrite electrodes was investigated by

cyclic voltammetry. The results showed that the Cu-S intermediate species during

electrochemical dissolution of chalcopyrite was mainly as Cu2S, CuxS (1<x<2) and CuS. The

formation process of these species was analysed. It was shown that the oxidation of

chalcopyrite can only produce CuS, but CuxS and Cu2S were mainly formed due to the

reduction reaction of copper ion and sulphur. Furthermore, it was found that the addition of

copper ion could greatly affect the formation of Cu-S intermediate species. Therefore, during

bioleaching of chalcopyrite, the effect of different concentration of copper ion on the

bioleaching process was investigated. The results revealed that when the copper concentration

was low, it was hard to form Cu-S species in the ore residue. However, as the increase of

copper concentration, the formation of Cu-S intermediate species also increase and could be

detected by X-ray diffraction.

Keywords: Bioleaching, chalcopyrite; Cu-S intermediate species; moderate thermophiles

1. Introduction

Chalcopyrite is the most important copper sulphide due to it takes about 70% of

copper resource in the world (Dutrizac 1978). As it has higher lattice energy than

other copper sulfides, the bioleaching of chalcopyrite by mesophilic microorganisms

would lead to slow kinetics and poor extraction and its industrial application is still a

major challenge in the biohydrometallurgy field (Rawlings 2002). Since the end of

last century, more and more researchers in hydrometallurgy field focus on the use of

moderate thermophiles during bioleaching of chalcopyrite. This is mainly due to that

* Co-corresponding authors. Tel.: +86 731 8879212, +61 03 95458749.

E-mail address: qgzhoucsu@163.com (Guanzhou Qiu); miao.chen@csiro.au (Miao Chen).

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using moderate thermophilies compared with mesophiles to bioleach chalcopyrite can

greatly improve the leaching reaction kinetics, avoids excessive chalcopyrite

passivation and finally improve the total copper extraction percentage (Cancho et al.,

2007; Wu et al., 2007; Zhou et al., 2009).

Bioleaching of chalcopyrite always involves the oxidation of elemental sulphur, iron

and copper depending on the leaching parameters (Nicol et al., 1984). The oxidation

of iron ion was simple and its valence was from Fe2+

to Fe3+

. The oxidation of copper

was mainly performed among Cu0, Cu

+ and Cu

2+, while the oxidation of sulphur was

very complex (from S2-

to S6+

). Due to these, there were several potential intermediate

species during dissolution of chalcopyrite, such as Cu2S, Cu1.92S, Cu1.6S, Cu1.4S and

CuS (Koch et al., 1971; Crundwell 1988).

In the electrochemistry researches, cyclic voltammetry was often used to detect the

intermediate species during dissolution of chalcopyrite (Li et al., 2006; Lopez -Juarez

et al., 2006). Nava and Gonzalez (2006) investigated the oxidation of chalcopyrite by

cyclic voltammetry and found that Cu1-rFe1-sS2-t, Cu1-xFe1-yS2-z, CuS and Fe2(SO4)3

(solid) were the oxidation products of chalcopyrite when the applied potential of

cyclic voltammetry was from 0.411 to 0.961V (vs Ag/AgCl). Mikhlin et al. (2004)

found that S0, Cu1-xFe1-yS2-z and CuS were the intermediate species during oxidation

of chalcopyrite when the applied potential was from 0.4 to 0.8V (vs Ag/AgCl). But

Yin et al. (1995) did not find the evidence of S0 on the chalcopyrite surface and they

proposed the formation of a copper sulphide CuS2 when the potential was from 0.4 to

0.5V (vs Ag/AgCl). Furthermore, Hiroyoshi et al. (2002) found that the addition of

copper ion would promote the dissolution of chalcopyrite, so they considered Cu2S

would formed due to the reaction of chalcopyrite with copper ion when the potential

was from 0.4 to 0.5V (vs Ag/AgCl). It can be seen that although there were many

researchers attempted to identify the process involved in the formation of intermediate

species by cyclic voltammetry, many inconsistencies existed compared with their

reports. These may be due to that the reactions of chalcopyrite would change mainly

according to the type of impurities or because of variations in stoichiometry or

depending on the leaching parameters (Shuey 1975; Prosser 1970).

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Although the Cu-S intermediate species could be analysed successfully by cyclic

voltammetry in this study, it was difficult to use normal XRD or EDX to detect these

species during bioleaching process, which had two possible reasons: 1) the amounts

of these species produced by bioleaching were small; 2) these species were very easy

to be oxidized during bioleaching. However, in the electrochemistry experiment,

Woods et al. (1987) found that the high concentration of copper ion would promote

the production of Cu-S intermediate species during chemical leaching. Therefore, in

the bioleaching experiment, when the high concentration of copper ions was added or

produced, the amounts of these intermediate species maybe also increase. This paper

investigated the potential Cu-S intermediate species during electrochemical

dissolution of chalcopyrite, and then these species were identified to be either the

oxidation production of chalcopyrite or the reduction production of copper ion and

sulphur. Finally, the chalcopyrite concentrate was bioleached by moderate

thermophiles and the effect of copper ion on the formation of Cu-S intermediate

species was analysed.

2. Materials and methods

2.1 Chalcopyrite sample

The chalcopyrite used in the electrochemistry experiments was obtained from Yushui

Copper Sulphide Mine in Meizhou, Guangdong, China. XRD analysis indicates that

the ore sample contains about 98% chalcopyrite and 2% silicate.

The chalcopyrite used in the bioleaching experiment was also obtained form Yushui

Copper Sulphide Mine. This ore sample was a chalcopyrite concentrate, and the XRD

analysis indicates that the sample mainly includes chalcopyrite (64%), pyrite (17%),

galena (15%), and gangue (4%). This chalcopyrite concentrate had been used for

culture moderate thermophiles for more than 2 years and thus the bioleaching

microorganisms were very adaptive to this concentrate (Zhou et al., 2009).

2.2 The Cu-S intermediate species during electrochemistry experiments

The chalcopyrite electrodes were made by the pure chalcopyrite. The pure

chalcopyrite was cut with a work surface of approximately 0.2 cm2 and, as far it was

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possible, with no visible imperfections. The specimens were placed on an epoxy resin

and were connected to a copper wire by silver paint on the back face. Preparation of

electrode surface after coarse grinding was completed on 1200-grit silicon carbide

paper. Furthermore, after each experiment, the electrode was taken out from the

electrolyte and then repeats to grind on 1200-grit silicon carbide paper to get a fresh

surface.

For the electrochemistry experiment, a three-electrode system was used. The cell

consisted of the chalcopyrite working electrode, a platinum counter electrode and an

Ag/AgCl reference electrode. The electrolyte used, modified 9K medium (Shown as

2.3), was prepared using analytical grade reagents. The electrochemical experiments

were carried out at 48 °C and pH 2.0, using a PARSTAT 2273 Potentiostat with

Power-Suite Software of the same company. Cycles were performed from 0 to 800

mV (vs Ag/AgCl), then to -800 mV (vs Ag/AgCl), and back to 0 mV (vs Ag/AgCl).

All tests were carried out at a scan rate of 30 mV/s (vs Ag/AgCl). In the paper, all

potential values are expressed vs. the Ag/AgCl electrode (3M KCl).

The mineral surface was characterised by SEM and EDX, which were performed on

an FEI Quanta 400 field emission, environmental scanning electron microscope

(ESEM) under high vacuum conditions. Secondary electron imaging was performed

using beam energies of 15 kV and probe currents of approximately 140 to 145 pA.

EDS was performed using a beam energy of 15 kV and a probe current of

approximately 800 pA.

2.3 Moderate thermophiles used in bioleaching experiments

Acid Mine Drainages (AMD) samples from several chalcopyrite mines in China were

collected. The samples were mixed and then inoculated into the culture medium for

enrichment of moderate thermophiles. This medium was modified 9K consisted of the

following compounds: (NH4)2SO4 3.0 g/L, Na2SO4 2.1 g/L, MgSO4·7H2O 0.5 g/L,

K2HPO4 0.05 g/L, KCl 0.1 g/L, Ca(NO3)2 0.01 g/L. And 10 g/L chalcopyrite

concentrate (with diameter of the particles less than 75 μm) was added as the energy

source. The moderate thermophiles were enriched at 48°C and initial pH of 2.0 in a

stirred tank reactor (shown as 2.4.).

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2.4 The Cu-S intermediate species during bioleaching of chalcopyrite concentrate

Bioleaching of chalcopyrite concentrate experiments were carried out in a 3 L glass

cylindrical reactor with a mechanic stirrer operating at 500 r/min. About 1950 mL

modified 9K medium was added into the reactor; 50 mL seed culture (no soluble

copper) was inoculated to get a cell density of 107 cells/mL. In addition, 80 g

chalcopyrite concentrate (with diameter of the particles less than 75 μm) was added

into the reactor to get a pulp density of 4%. After these, different concentration of

copper sulphate was added into the bioleaching solution to obtain the different

concentration of copper ion (0, 6, 12, 24, 36 g/L).

The reactor was placed in a thermostatic bath to keep the constant temperature at 48 ±

0.2°C. Sterile air was introduced into the base of the reactor at an approximate rate of

360 mL/min. The experiments were performed at initial pH 2.0. The acid

consumption was compensated by addition of 10 mol/L sulfuric acid to keep pH value

around 2. Distilled water was added to the reactor through a peristaltic pump in order

to compensate for evaporation losses. The levels of Cu2+

, Fe2+

, total iron in solution

were analysed at the end day of bioleaching experiment, while the cell density, pH

value and redox potential was analysed every day. The ore residue bioleached after 10

days was filtered to remove some of the water and then dried under vacuum (for 24

hours) for X-ray diffraction (XRD) analysis.

The components of the mineral sample and ore residue were analysed by XRD.

Copper and total iron concentrations in solution were determined by ICP-AES.

Ferrous iron concentration in solution was assayed by titration with potassium

dichromate. Ferric iron concentration is the concentration of total iron minus the

concentration of ferrous irons. The redox potential (or Eh) was measured using a

platinum electrode with an Ag/AgCl reference electrode. Free cells in solution were

observed and counted under an optical microscope.

3. Results and discussion

3.1 Cyclic Voltammetric study of chalcopyrite

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Cyclic voltammetry tests were performed to characterize the oxidation and reduction

reactions during dissolution of chalcopyrite. Fig.1 shows the results of cyclic

voltamperometry of chalcopyrite electrodes in the modified 9K electrolyte. There are

five anodic peaks (A1, A2, A3, A4 and A5) and four cathodic peaks (C1, C2, C3 and

C4). A4 and A5 are the very common anodic peaks during the study of chalcopyrite

electrochemical behaviour, which has been reported by many authors (Biegler and

Swift, 1979; Biegler and Horne, 1985; Gomez et al., 1996). There was a selective

dissolution of iron from the crystal lattice of chalcopyrite (peak A4) according to Eq.

(1) and Eq. (2). They reported that a blue layer was observed obviously on the

chalcopyrite surface after this reaction. This was associated with the formation of

covllite shown in Eq. (2). When at potential values more than 0.6 V (vs Ag/AgCl), the

blue layer of covllite could be destroyed according to Eq. (3).

CuFeS2 ↔ Cu1-xFe1-yS2-z + xCu2+

+ yFe2+

+ 2S + 2(x+y) e-

(1)

CuFeS2 ↔ 0.75CuS +1.25S0

+ 0.25Cu2+

+ Fe2+

+ 2.5e- (2)

CuS ↔ S0

+ Cu2+

+ 2e-

(3)

However, Hiroyoshi et al. (2001, 2002) considered that the dissolution of chalcopyrite

during potential 0.4—0.5 V (vs Ag/AgCl) was related to the production of Cu2S.

Because they found that after addition of 0.1 mol/L copper ion into the electrolyte, the

peak A4 would increase in a large scale and they proposed the reaction model as Eq.

(4):

CuFeS2 + 3Cu2+

+ 3Fe2+

↔ 2Cu2S + 4Fe3+

(4)

Furthermore, Yin et al. (1995) also did not agree with Eq(2), because they did not find

the evidence of S0 on the chalcopyrite surface and they proposed the formation of a

copper sulphide according to Eq(5). It can be seen from these that the oxidation

reaction of chalcopyrite at 0.4—0.5 V (vs Ag/AgCl) was different according to the

different authors’ reports. Therefore, it is necessary to investigate which of the Cu-S

intermediate species such as CuS, Cu2S and CuS2 was the oxidation product of

chalcopyrite in the experiment.

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CuFeS2 ↔ CuS2 + Fe2+

+ 2e-

(5)

At potential values more than 0.7 V (vs Ag/AgCl), it is reported as the oxidation of

ferrous iron to ferric iron and the oxidation of sulphur to sulphuric acid (Biegler and

Horne, 1985; Nava et al., 2002). But Lopez-Juarez et al. (2006) considered that the

oxidation of ferrous iron should be favoured (Eq. (6)), because a large quantity of S0

was still observed on the chalcopyrite surface with scanning electron microscope

(SEM) and energy dispersive x-ray analysis (EDX).

CuFeS2 ↔ 2S0

+ Cu2+

+ Fe3+

+ 5e- (6)

In the inverse scan, there was series reduction peaks appeared especially from -0.2 to

0.2 V (vs Ag/AgCl) of potential value. These peaks could be attributed to the

reduction of some species produced during the anodic scan like Cu2+

and Fe3+

(Eq. (7),

Eq. (8) and Eq. (9)), according to Holliday and Richmond (1990).

Fe3+

+ e- ↔ Fe

2+ (7)

Cu2+

+ S0 + 2e

- ↔ CuS (8)

Cu2+

+ 2e- ↔ Cu

0 (9)

When the potential value was lower than -0.3 V (vs Ag/AgCl), peak C3 and C4 were

observed. Arce and Gonzalez (2002) and Biegler and Horne (1985) attributed these

peaks to reduction of covellite and chalcopyrite, respectively (Eq. (10) and Eq. (11)).

Furthermore, Wood et al. (1987) considered that the reduction of covellite would

produce several kinds of Cu-S intermediate species such as Cu1.04S, Cu1.38S, Cu1.67S,

Cu1.83S, Cu1.93S and Cu1.96S.

2CuS + 2H+

+ 2e- ↔ Cu2S + H2S (10)

2CuFeS2 + 6H+

+ 2e- ↔ Cu2S + 2Fe

2+ + 3H2S

(11)

While Velasquez et al. (2001) recognized there was the reduction of chalcocite in

peak C4 according to Eq. (12). The reverse of this reaction accounts for the process

associated with anodic peak A1 (from -0.4 to -0.3 V, vs Ag/AgCl).

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Cu2S + H2O + 2e- ↔ 2Cu + HS

- + OH

- (12)

In the anodic scanning, peaks A1, A2 and A3 were reported not much during the

dissolution of chalcopyrite, but always existed in the electrochemitry study of

chalcocite and bornite (Velasquez et al., 2001; Arce and Gonzalez, 2002; Lu et al.,

2000). Furthermore, according to cathodic reaction shown as above, peaks A1 and A2

were considered as the oxidation of copper (Eq. (13)) and chalcocite (Eq. (14)).

2Cu + HS- + OH

- ↔ Cu2S + H2O + 2e

- (13)

Cu2S ↔ Cu1.92S + 0.08Cu2+

+ 0.16e- (14)

While the reaction about peak A3 was relatively complicated, Arce and Gonzalez

(2002) reported that peak A3 maybe associate with the oxidation of CuxS (1<x<2)

mainly included djurleite and geerite (Eq. (15) and (16)), which were the intermediate

species during dissolution of most copper sulphides.

Cu1.92S ↔ Cu1.60S + 0.32Cu2+

+ 0.64e-

(15)

Cu1.60S ↔ CuS + 0.60Cu2+

+ 1.20 e-

(16)

The difference of the electrochemical reactions associated to chalcopyrite dissolution

process could be associated to the different oxidation states of copper (i.e. Cu+, Cu

2+)

and sulphur (i.e. S2-

, S0) in the chalcopyrite structure. In the anodic scanning (from -

0.8 V to 0.8 V, vs Ag/AgCl), the states of copper and sulphur are from Cu0 to Cu

2+,

and S2-

to S0, respectively, while in the cathodic scanning (from 0.8 V to -0.8 V, vs

Ag/AgCl), the states of copper and sulphur are from Cu2+

to Cu0, and S

0 to S

2-,

respectively. The potential intermediate species during dissolution of chalcopyrite

mainly include Cu2S, Cu1-xS (1<x<2) and CuS.

3.2 The Cu-S intermediate species during electrochemistry experiments

The detailed oxidation process of chalcopyrite was not clear till now. But there were

some reports recognized that the oxidation of chalcopyrite would remove iron and

then became CuxS (1≤x≤2), which were the Cu-S intermediate species (Nava and

Gonzalez, 2006; Mikhlin et al., 2004; Yin et al., 1995; Hiroyoshi et al., 2002). There

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are many kinds of Cu-S intermediate species, such as CuS, Cu2S and Cu1.96S (Lee et

al., 2008). The investigation about their formation process and relations is very

important to understand the dissolution mechanism of chalcopyrite.

3.2.1 CuS

The Fig.1 showed that CuS could be produced by the oxidation of chalcopyrite when

the potential was from 0.4 to 0.5V (vs Ag/AgCl), but it was difficult to detect the

existence of CuS on the electrode surface by XRD or EDX, which was due to that the

production amount was very small and easy to be oxidized at higher potential than

0.5V (vs Ag/AgCl). For confirming the formation of CuS during the potential 0.4—

05V (vs Ag/AgCl), potentiostatic experiment was applied to the chalcopyrite

electrode in order to induce a higher amount of solid products formation on the

electrode surface.

The potentiostatic experiment (0.45V, vs Ag/AgCl) was applied to chalcopyrite

electrode surface for 60 s and then the surface was detected and analysed by SEM-

EDX. After the 60s anodic potentiostatic experiment, a blue layer on the electrode

surface could be observed (Fig.2). The EDX result showed that the amount of S and

Cu produced by the oxidation of chalcopyrite were large, but the amount of Fe was

very small. This indicated that the oxidation of chalcopyrite firstly removed Fe and

formed a blue layer which should be the complex substances of CuS and S0.

The copper concentration in the electrolyte after the potentiostatic experiment

performed for 60s on the electrode was analysed by ICP-OES, and showed a very low

value (data not shown). In this case when the applied potential was 0.45V (vs

Ag/AgCl), the reduction reaction of chalcopyrite or copper ion was not easy. As a

result, CuS on the electrode surface here should be the oxidation production of

chalcopyrite but not the reduction production, and this prove the correction of Eq(2).

3.2.2 Cu2S

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It can be seen from Fig.1 that the oxidation potential of chalcopyrite was much higher

than Cu2S, and this suggested that it is difficult to form Cu2S during the oxidation of

chalcopyrite. For further proving this suggestion, linear votalmmetry of chalcopyrite

electrode was carried out from the different start potential (-0.2V, -0.2V, -0.3V, -0.5V

and -0.75V, vs Ag/AgCl) to the end potential 0.65V (vs Ag/AgCl, the electrodes were

not taken out from the electrolyte after the 1st scan, but continue to be used for the

next scan). The experiment principle was that if Cu2S was the product of chalcopyrite

oxidation at potential 0.4—0.5V (vs Ag/AgCl) (Fig.1), the oxidation peak of Cu2S

would be detected at the next linear scan at potential -0.2—0V (vs Ag/AgCl).

Otherwise, Cu2S should be the reduction products of copper ion or CuS (at potential

lower than -0.3V, vs Ag/AgCl), or the oxidation production of Cu0 (at potential -0.4—

0.3V, vs Ag/AgCl).

It can be seen from Fig.3 that in the 1st

scan, the potential range was from -0.2V to

0.65V (vs Ag/AgCl) and the chalcopyrite was oxidized during this scan. After this,

the second scan was carried out and the potential range was same as the 1st one. In the

2nd

scan, the scan plot was rather similar with the 1st one and the oxidation peak of

Cu2S still did not appear during the potential -0.2—0 V (vs Ag/AgCl). This indicated

that Cu2S did not form by the chalcopyrite oxidation at the potential 0.4—0.5 V (vs

Ag/AgCl). When the start potential decreased to -0.3 V (vs Ag/AgCl, in the 3rd

scan),

a slightly oxidation peak was observed at potential -0.2—0 V (vs Ag/AgCl) and this

was due to that Cu2S began to form by the reduction of CuS (which formed as Eq(7)

from -0.2 to 0.2 V, vs Ag/AgCl). In the 4th

scan, the decrease of start potential to -0.5

V (vs Ag/AgCl) led the increase of reduction of CuS, and thus the oxidation peak of

Cu2S became more obvious. In the 5th

scan, when the start potential decreased to -0.75

V (vs Ag/AgCl), the oxidation peak of Cu2S increased due to another reduction

reaction (Eq. (10)). These concluded that Cu2S here was the reduction production of

CuS or chalcopyrite when the potential was lower than -0.3V (vs Ag/AgCl), but not

the oxidation product of chalcopyrite at potential 0.4—0.5V (vs Ag/AgCl).

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3.2.3 CuxS (1<x<2)

It is reported that CuxS (1<x<2) includes several intermediate species during

dissolution of chalcopyrite, such as Cu1.38S, Cu1.67S, Cu1.83S, Cu1.75S and Cu1.96S

(Nava and Gonzalez, 2006; Lee et al., 2008). Their oxidation peaks in cyclic

voltammetry are between the oxidation peak of Cu2S and CuFeS2, generally from

potential 0—0.4 V (vs Ag/AgCl). However, in this cyclic voltammetry experiment,

there was only one obvious oxidation peak of CuxS (1<x<2) in Fig.1 (peak A3). The

production of CuxS (1<x<2) was reported to have two potential ways: the oxidation of

Cu2S and the reduction of CuS. Therefore, according to Eq.(8) , the addition of copper

ion would promote the production of CuS and thus would benefit the formation of

CuxS (1<x<2) during the reduction reaction (Woods et al., 1987).

In the experiment, in order to investigate the formation of intermediate species CuxS

(1<x<2) during the dissolution of chalcopyrite electrode, copper ion was added into

the electrolyte to promote the formation of CuxS (1<x<2) and the linear voltammetry

of chalcopyrite electrode was carried out. It can be seen from Fig.4 that the addition

of copper ion greatly affected the anodic peak current and potential. As the increase of

addition of copper ion concentration, oxidation peak current increased as well and the

peak potential moved positively.

When the addition of copper ion was 1 g/L, three oxidation peaks was obtained. One

peak was at about potential 0.1V and considered as the oxidation of CuxS (B1)

(1<x<2) (Eq.(16)). One peak was at potential 0.4—0.5V (vs Ag/AgCl) and considered

as the oxidation of chalcopyrite (Eq.(2)). The other one peak was at potential 0.7V

(vs Ag/AgCl) and considered as the oxidation of CuS (Eq.(3)). Furthermore, a tight

blue layer could be observed as CuS when the linear voltammetry potential was lower

than 0.6 V (vs Ag/AgCl). The formation of CuS was due to the oxidation of

chalcopyrite and reduction of Cu2+

and S0 (Eq(8)). When the concentration of copper

addition increased to 3 g/L, the oxidation peak of chalcopyrite disappeared, which

maybe due to the CuS layer covered on the chalcopyrite electrode surface and block

the oxidation of chalcopyrite. But then one new oxidation peak was obtained at

potential -0.05V (vs Ag/AgCl). According to Fig.1 and Fig.3 this peak was related

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with the oxidation of Cu2S. When the concentration of addition copper increased to 6

g/L, another new oxidation peak at about potential 0.3V (vs Ag/AgCl) was observed

slightly, this should be the oxidation of another kind of CuxS (B5) (1<x<2). In this

case, four Cu-S intermediate species were obtained, and they were Cu2S (B4), CuxS-1

(B5), CuxS-2 (B1) and CuS (B3). When the concentration of addition copper

increased to 12 g/L, the reduction products increased in a large scale, covered on the

chalcopyrite electrode surface, and the anodic peak potential moved positively, thus

the oxidation peak of CuS disappeared below potential 1.0 V (vs Ag/AgCl).

3.3 The intermediate species during bioleaching of chalcopyrite

The results of electrochemistry experiment (section 3.2.3) showed that the high

concentration of copper ion would promote the reduction reaction and thus the

amount of reduction products increased, such as Cu2S and CuS. Therefore, during

bioleaching of chalcopyrite, when the concentration of copper was very high, the

intermediate species may be promoted and increase in a large scale. In the experiment,

different concentration of copper ion was added into the bioleaching solution and the

ore residue after bioleached for 10 days was analysed by XRD.

Table 1 showed the bioleaching parameters during bioleaching of chalcopyrite

concentrate after addition of different concentration of copper ion. When the addition

of copper ion was 0 g/L (A), copper extraction from chalcopyrite concentrate could

reach 4.32 g/L and the ferric iron concentration was 1.83 g/L. As the increase of

copper addition, the concentration of both ferric iron and ferrous iron decreased. In

addition, the redox potential value and cell density also decreased. Especially when

the addition of copper ion was 36 g/L (E), the ferric iron concentration decreased

remarkably to about 0.94 g/L, which accompanied with a very low redox potential

value (477 mV, vs Ag/AgCl). At high concentration of copper ion, the cell density of

moderate thermophiles for bioleaching of chalcopyrite concentrate was relatively low

and this would lead to the low copper and iron extraction.

The ore residues after bioleached for 10 days were analysed by XRD. It can be seen

from Fig.5 that the high concentration of copper ion in the bioleaching system greatly

affected the component of ore residue. The results showed that during bioleaching of

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chalcopyrite concentrate, the major components of ore residue were chalcopyrite,

pyrite, lead sulphate, jarosite and chalcanthite (the formation of chalcanthite was due

to that copper sulphate crystallized with water when the ore residue was drying under

vacuum). In the experiment A and B, when the addition of copper ion was only 0 and

6 g/L (Table 1), it was hard to find any Cu-S intermediate species by XRD analysis.

But when the addition of copper ion increased to 12 g/L (C), the XRD result showed

that Cu2S and CuS were produced in the ore residue. When the addition of copper ion

increased to 24 g/L (D), the other intermediate species like Cu34S32 and Cu1.96S could

be found. However, when the concentration of total copper ion increased to about 36

g/L (E), only Cu2S was detected, but its amount increased in a large scale (data not

shown). This may be due to that when the copper concentration was the highest (E),

the redox potential was only 477mV (vs Ag/AgCl), and then the formation of Cu2S

would be easier than any other intermediate species (Lee et al., 2008; Woods et al.,

1987).

As a result, it can be seen that during bioleaching of chalcopyrite concentrate the

different concentration of copper ion could affect the bioleaching process. When the

copper concentration was relatively low, the reduction reaction in the bioleaching

system was not obvious and Cu-S intermediate species were very difficult to be

detected by XRD. However, when the copper concentration increased, the reduction

reaction would be accelerated and the amount of reduction products increased. There

were several intermediate species formed, but CuS and Cu2S were the majority. When

the copper concentration was very high, such as in the experiment E, the Cu-S species

became simple and only Cu2S was the reduction product, but its amount increased in a

large scale than before.

However, in our previous studies and other author’s reports, it was hard to detect Cu-

S intermediate species in the ore residue by XRD during bioleaching of chalcopyrite.

There were two possible reasons: 1) in the previous studies, the ore residue sample

after bioleached was dried in the oven at 50-75°C, and in this case the Cu-S

intermediate species were very easy to be oxidized; 2) when the copper concentration

in the solution was low, the amount of Cu-S intermediate species was small and easy

to be dissolved by acid or bioleaching microorganisms, and thus it was hard to detect

them by XRD. These results indicate that during dissolution of chalcopyrite, the

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oxidation and reduction reactions were performed simultaneously, but at many cases,

such as when the copper concentration was low, the oxidation reaction was the

dominant reaction.

4. Conclusions

The analysis of the formation process of Cu-S intermediate species indicated that the

oxidation of chalcopyrite can only produce CuS, but not CuxS (1<x<2) and Cu2S, and

the later two species were mainly produced due to the reduction reaction of copper ion

and sulphur. During bioleaching of chalcopyrite, when the copper concentration was

high, the reduction reaction of copper ion and sulphur could be promoted and the Cu-

S intermediate species such as CuS and Cu2S formed largely. However, when the

copper concentration was relatively low (lower than 10 g/L); these intermediate

species amounts were very low and easy to be oxidized, and thus hard to be detected

by XRD.

Acknowledgements

This work was supported by the China National Basic Research Program (No.

2010CB630901), the National Natural Science Foundation of China (No. 31200382),

Postdoctoral Science Foundation of Central South University (No. 1332-

74141000076), and Australia CSIRO OCE Science Leader Grant.

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Table 1 The bioleaching parameters during bioleaching of chalcopyrite concentrate

after addition of different concentration of copper ion in the stirred tank reactor after

10 days. A: without addition of copper ion, B: addition of 6 g/L copper ion, C:

addition of 12 g/L copper ion, D: addition of 24 g/L copper ion, E: addition of 36 g/L

copper ion.

Experiments Total copper

(g/L)

Ferric iron

(g/L)

Ferrous iron

(mg /L)

pH

value

Redox

potential

(mV)

Cell density

(108cells/mL)

A 4.32 1.83 151 1.66 584 7.9

B 10.28 1.82 144 1.67 582 7.8

C 16.04 1.79 148 1.67 579 7.2

D 27.73 1.72 143 1.7 562 5.6

E 37.58 0.94 84 1.84 477 0.4

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Fig.1. Cyclic voltammetry of chalcopyrite without bioleaching process in the modified 9K

medium at sweep rate = 30 mV/s. Temperature: 48°C, pH 2.0. Reference electrode: Ag/AgCl

electrode.

A1

A2

A3

A4

C1 C2

C3

C4

A5

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

I / m

A

E / V vs Ag/AgCl

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Fig.2. the formation of CuS and S0 on the electrode surface during dissolution of chalcopyrite

when the constant potential pulse was 0.45 V in the electrochemistry experiments. A: the

electrode surface observed by eye, B: SEM investigation of electrode surface, C: EDX

analysis of electrode surface.

C

B A

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Fig.3. The linear voltammetry of chalcopyrite from different start potential (-0.2V, -0.2V, -

0.3V, -0.5V and -0.75V) to the end potential 0.65V in the modified 9K medium at sweep rate

= 30 mV/s. Temperature: 48°C, pH 2.0. Reference electrode: Ag/AgCl electrode.

E/V vs Ag/AgCl

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Fig.4. the effect of different concentration of Cu2+

(1, 3, 6, 12 g/L) on the electrochemistry

behaviour of chalcopyrite electrode in the modified 9K medium at sweep rate = 30 mV/s.

Temperature: 48°C, pH 2.0. Reference electrode: Ag/AgCl electrode. A is the amplified

figure of linear voltammetry when the addition of Cu2+

was 1 g/L.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.5

1.0

1.5

2.0

-0.2 0.0 0.2 0.4 0.6 0.8

0.00

0.05

0.10

0.15

0.20

0.25

Cur

rent

/ A

E / V

1 g/L

Cu

rre

nt / A

E / V vs Ag/AgCl

1 g/L

3 g/L

6 g/L

12 g/L

B1 B2

B3

B1 B3

B4

B4

B5

B1

B3 B4

B1

B5 A

B1 B2

B3

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Fig.5. the XRD analysis of ore residue during bioleaching of chalcopyrite

concentration after addition of different concentration of Cu2+

in the stirred tank

reactor after 10 days. A: without addition of copper ion, B: addition of 6 g/L copper

ion, C: addition of 12 g/L copper ion, D: addition of 24 g/L copper ion, E: addition of

36 g/L copper ion.

A

C B

E D