J. Cent. South Univ. (2014) 21: 728−734 DOI: 10.1007/s11771-014-1995-3

Bioleaching of low-grade copper sulfide ores by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans

WANG Jun(王军)1, 2, ZHU Shan(朱珊)1, 2, ZHANG Yan-sheng(张雁生)1, 2, ZHAO Hong-bo(赵红波)1, 2,

HU Ming-hao(胡明皓)1, 2, YANG Cong-ren(杨聪仁)1, 2, QIN Wen-qing(覃文庆)1, 2, QIU Guan-zhou(邱冠周)1, 2

1. Key Laboratory of Biohydrometallurgy of Ministry of Education (Central South University), Changsha 410083, China;

2. School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: The grown conditions of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans were investigated, and then experiments were conducted to research the bioleaching behaviors of crude ore of copper sulfide and hand-picked concentrates of chalcopyrite by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The experimental results show that the bacteria grow best when the temperature is (30±1) °C and the pH value is 2.0. The bacteria concentration is 2.24×107 mL−1 in this condition. It is found that the copper extraction yield is affected by the inoculum size and the pulp density and the extraction yield increases as the inoculum size grows. The bioleaching rates reach their highest point in sulfide copper and chalcopyrite with a pulp density of 5% and 10%, respectively. Column flotation experiments of low-grade copper sulfide ores show that the bioleaching recovery reaches nearly 45% after 75 days. Key words: biohydrometallurgy; chalcopyrite; Acidithiobacillus ferrooxidans; Acidithiobacillus thiooxidans; copper sulfide ore

1 Introduction

As an environmentally benign technology with wide applications, biohydrometallurgical process is characterized by low cost for recovering metals from low-grade refractory ores. Biohydrometallurgy has received growing attentions due to the increasingly stringent environmental protection regulations [1−3]. As the application of the bioleaching process for copper sulfide ores has been industrialized for two decades, the combined flow of biooxidation−bioleaching− electrodeposition has been successfully used in the extraction of uranium, gold, zinc and other metals [4−7]. Furthermore, in the past ten years, growing interest has been shown in biooxidation for copper recovery from refractory ores [8−9].

Bacterial leaching of copper sulfide ore is a complex process. Optimum temperature, nutrients, oxygen supply, pulp density and residence time affect the extraction of copper and the best condition needs to be determined before bioleaching [10−12]. Bioleaching of copper concentrates with mesophilic bacteria and

extreme thermophilic bacteria has been investigated [7, 13−15]. By the analysis of the pH value, electric potential, absorption mechanism of oxygen and carbon dioxide and pulp density, the leaching kinetics was optimized and the leaching rate was increased [16−18]. The flotation concentrate of mesophilic bacteria was investigated [19]. The results have demonstrated that the high copper extraction yield was achieved in the presence of ferrous ion (2K medium) or pyrite (3%). The changes in the surface area of the leaching solid phase may be effective as an indicator of the bioleaching process. The effects of three bacteria cultures and the absorption mechanism of oxygen and carbon dioxide were demonstrated and they showed the positive role of oxygen [20]. The leaching rate increased with the dissolved O2 increasing, O2 demand increased with increasing pulp density, and the average dissolved O2 concentrations were different in different cultures, which suggested that O2 and CO2 are not limited [21−24]. The analysis of ferric ion requirement for mineral oxidation versus ferric ion supply by microbial oxidation suggests that chalcopyrite leaching is promoted by extreme thermophiles due to a favorable interactions among

Foundation item: Project(2012AA061501) supported by the National High-tech Research and Development Program of China; Project(20120162120010)

supported by the Research Fund for the Doctoral Program of Higher Education of China; Project(NCET-13-0595) supported by the program for New Century Excellent Talents in University of China; Project(51374248) supported by the National Natural Science Foundation of China; Project(2010CB630905) supported by the National Key Basic Research Program of China

Received date: 2013−01−21; Accepted date: 2013−05−02 Corresponding author: WANG Jun, Associate Professor, PhD; Tel: +86−731−88876557; E-mail: wjwq2000@126.com

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reaction kinetics, solution potential and chalcopyrite ‘passivation’ phenomenon, whereas pyrite leaching is favored at lower temperatures [25]. A comparative study on the bioleaching of copper sulfide ores with pure and mixed cultures of mesophilic bacteria was performed by POGLIANI et al [26]. It has also been reported that copper extractions by the pure and mixed cultures achieved 14.87% and 20.11%, respectively, over a period of 117 days, including 15 days of acid pre-leaching and 102 days of bioleaching [27]. Copper solubilization was at its maximum (more than 90%) when A ferrooxidans was present (in pure or mixed cultures) and only the mixed cultures composed of A thiooxidans and L ferrooxidans reached similar solubilization. The copper extraction by L ferrooxidans was very slow and that by A thiooxidans was similar but both were increased after the second inoculation (with other species), reaching similar percentage of copper extraction with that obtained in the cultures with A ferrooxidans. At the same time, the results showed that the bioleaching kinetics of copper extraction was correlated with iron(III) concentration (almost equal to soluble iron concentration) in pure and mixed cultures [28−29].

In this work, chalcopyrite and copper sulfide mineral were bioleached by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The present work is to examine the effect of pulp density, inoculum size and pulp pH value on the copper extraction in the bioleaching process to improve the understanding of influence of factors in the bioleaching process. The bioleaching of copper sulfide ore was carried out in the column bioleaching controlling system. 2 Materials and methods 2.1 Experiment materials 2.1.1 Bacteria

Bacteria used in the experiments were isolated from the acid mine drainage in Yushui Copper Mine of Guangdong province in China. The microbial identification analyses of cultures showed that they consisted mainly of Acidithiobacillus ferrooxidans and

Acidithiobacillus thiooxidans. 2.1.2 Mineral

The chalcopyrite and copper sulfide mineral used were obtained from Yushui Copper Mine. Chalcopyrite and copper sulfide mineral samples used in the leaching experiments which were carried out in Erlenmeyer flasks were ground to the particle size of <0.074 mm over 90%. The copper sulfide mineral samples used in the bioleaching experiments which were carried out in the bioleaching controlling system were ground to the particle size of <25 mm over 90%. The chemical analyses of the chalcopyrite and copper sulfide mineral are shown in Table 1 and Table 2. 2.2 Methods 2.2.1 Strain-culturing experiments

Bacteria were cultured in 9K culture medium which contains ferrous sulphate (10 g/L, as the energy source), (NH4)2SO4 (3.0 g/L), MgSO4 (0.5 g/L), K2HPO4 (0.5 g/L), KCl (0.1 g/L) and Ca (NO3)2 (0.01 g/L). 100 mL of 9K culture medium was transferred into a 250 mL Erlenmeyer flask and sulfuric acid was added to adjust the pH value to 2.0. The mixed bacteria (5 mL) were transferred to the culture medium and the flask was then put on the constant-temperature shaker with the temperature at (30±1) °C and the rotatory speed of the shaker at 170 r/min. The bacteria strains used in the leaching experiments were adapted to the concentrate through continuous subculturing for several times in the way described above [30−31]. 2.2.2 Bioleaching experiments

The Erlenmeyer flask bioleaching experiments were carried out on the constant-temperature shaker at a required pulp density and inoculum size. Incubation was performed at (30±1) °C and in an initial pH range of 1.8−2.0 on the shaking table at 170 r/min. The process was monitored through measurements of [Cu2+] and pH value. The concentration of Cu2+ and the pH value in the solution were measured by atomic adsorption spectophometer and a pH-meter (PHSJ-4A), respectively. The leaching residue was also collected, filtered, dried in air and analyzed by energy dispersive X-ray analysis

Table 1 Mineral compositions of two samples (Mass fraction, %)

Sample Primary cupric sulphide Secondary cupric sulphide Free cupric oxide Associated cupric cupric oxide

Copper sulfide ore 0.41 0.54 0.024 0.016

Chalcopyrite 29.41 3.45 0.01 0.094

Table 2 Chemical compositions of two samples (Mass fraction, %)

Sample Cu Pb Zn Fe S SiO2 Al2O3 CaO MgO As

Copper sulfide ore 0.99 0.13 0.032 2.05 1.10 86.95 4.33 0.27 0.22 0.0062

Chalcopyrite 32.96 7.28 1.16 22.33 29.85 1.33 0.35 1.65 0.32 0.002

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(EDXA).

The column bioleaching of copper sulfide ore was carried out in the bioleaching controlling system. The experimental sample was put into the bioleaching column and washed at a running speed of 2−4 mL/min for several times with 2500 mL dilute sulfuric acid of pH 2.0 to bioleach a small amount of cupric oxide and consume the acid-consuming compositions. The 1:1 (volume ratio) sulfuric acid was added to adjust and maintain the pH of the lixivium at 2.0. The 50 mL inoculum of a five-day-old culture of bacteria was inoculated in the lixivium when the pH value of the solution was maintained at 2 for 24 h and was added to keep the lixivium at 2500 mL. The process was monitored through measurements of cupric ion concentration and pH value with atomic adsorption spectophometer and a pH-meter (PHSJ-4A), respectively. Volatilization loss was regained from tap water. 3 Results and discussion 3.1 Bacteria-culturing 3.1.1 Influence of temperature on bacteria growth

The growth and activity of bacteria were influenced by the ambient temperature. To determine the effect of experimental temperature on the growth of bacteria, the bacteria growth was investigated in the temperature range of 25−45 °C [32−34] due to the characteristics of mesophilic bacteria culture. The effect of temperature on the activity of bacteria is shown in Fig. 1. In the temperature range of 25−30 °C, the bacteria could grow well and the concentration of bacteria cell is 2.24× 107 mL−1 at 30 °C.

Fig. 1 Effect of temperature on activity of bacteria 3.1.2 Influence of pH value on bacteria growth

The results of bacteria growth experiment at different pH values shown in Fig. 2 indicate that the pH value has an effect upon the growth and activity of bacteria. The appropriate pH value is in the range of 1.8−2.5 and the concentration of bacteria cell is the highest when the pH value is 2.0. The activity of bacteria

Fig. 2 Effect of pH on activity of bacteria

is inhibited out of pH value range of 1.8−2.5. 3.2 Bioleaching 3.2.1 Effect of inoculum size

To determine the influence of inoculum size on the dissolution rate of chalcopyrite and copper sulfide ore, the leaching behaviors of the chalcopyrite and copper sulfide ore were investigated by comparing three parallel leaching runs of inoculation with inoculum size of 0.03 mL, 0.3 mL and 3 mL bacterium suspension, as shown in Fig. 3 and Fig. 4, respectively.

Fig. 3 Effect of inoculum size on bioleaching of chalcopyrite

Fig. 4 Effect of inoculum size on bioleaching of copper sulfide

ore

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It is demonstrated that the copper extraction yield increases as the inoculum size increases. Over a leaching period of 15 days, the leaching rate of Cu is around 30% under the inoculum size of 3 mL, compared with 14% with size of 0.3 mL and about 12% with size of 0.03 mL. This suggests that the large inoculum size could promote markedly the dissolution rate of chalcopyrite and copper sulfide ore. MCSWEENEY et al [35] indicated that microbial attachment to the surface of the copper ore was related to the dissolution rate of the mineral, and the extraction yield of copper increased with the microbial adsorption quantity enhancements due to the large inoculum size. 3.2.2 Pulp density

Figures 5 and 6 show the effects of different pulp densities on the dissolution rate of copper during the bioleaching of chalcopyrite and copper sulfide ore, respectively. The result indicates that the extraction of copper rapidly increases under the pulp density of 5% and the extraction yield of copper reaches 47% after 75 days. Comparatively, under the pulp density of 15%, the extraction of copper is only 10% after 50 days and that of copper keeps at the low level in the bioleaching of chalcopyrite. Under the pulp density of 1% and 3%, the

Fig. 5 Effect of pulp density on bioleaching of chalcopyrite

Fig. 6 Effect of pulp density on bioleaching of copper sulfide

ore

leaching rates of Cu are about 35% and 30% after 75 days, respectively. It is suggested that the pulp density has an important influence on the dissolution rate of chalcopyrite because the higher pulp density leads to the higher shearing force while the lower pulp density can not provide enough energy for the bacteria growth. In the bioleaching of copper sulfide ore, the extraction rate of copper is the highest at 68% under the pulp density of 10% compared with the other pulp densities. 3.2.3 Variation of pH in lixivium

The variations of pH in the lixivium during the bioleaching process are shown in Figs. 7 and 8. The pH value decreases when the leaching goes on, and its value drops to less than 2.0 and remains almost constant in the range of 1.6−1.8 for the duration of the experiment, except for the pH value under the pulp density of 15% which increases rapidly after 15 days of bioleaching, and subsequently decreases with the leaching time. The chalcopyrite bioleaching can be described briefly as follows [36−38]:

).( Bacteria

22 HO25.4CuFeS fA OH5.0FeSO2Cu 2

324

2 (1)

20232 Fe5S2CuFe4CuFeS (2)

Fig. 7 Variation of pH in lixivium during bioleaching of

chalcopyrite

Fig. 8 Variation of pH in lixivium during bioleaching of copper

sulfide ore

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CuFeS2+16Fe3++8H2O→Cu2++ 242SO +17Fe2++16H+

(3)

The elemental sulfur and Fe2+ ions are oxidized to SO4

2− and Fe3+ ions in the presence of bacteria:

Fe2++1/4O2+H+ ).( Bacteria fA Fe3++1/2H2O (4)

S0+3/2O2+H2O ).( Bacteria fA 24SO +2H+ (5)

Fe2++1/4O2+H+ ).( Bacteria tA Fe3++1/2H2O (6)

S0+3/2O2+H2O ).( Bacteria tA 24SO +2H+ (7)

The consumption and formation of H+ ions lead to

the change of pH in the lixivium. According to Eqs. (3), (5) and (7), the pH value decreases due to the formation of H+. But the depletion of H+ described in Eqs. (1), (4) and (6) is subsistent in the chalcopyrite bioleaching and the regeneration of Fe3+ ions. The pH value remains almost constant in the range of 1.6−1.8 when the depletion and formation of H+ ions keep in dynamic balance. 3.2.4 EDXA analysis of chalcopyrite and leaching

residues The contents of three elements, Cu, S and Fe, in

mineral sample and leach residues were examined by area analysis of EDXA. The EDXA patterns of the chalcopyrite and leach residues are shown in Fig. 9 and Fig. 10, respectively. And the contents of these elements are presented in Table 3. The results indicate that S constitutes the main portion of the leach residues and the spectral intensity of Cu is weakened gradually. The mass fraction of copper decreases from 34.13% to 5.02% and molar fraction of Cu decreases from 24.57% to 2.42%, which indicates that copper is dissolved intensively. Also the mass and molar fractions of S decrease due to the fact that the elemental sulfur is oxidized to 2

4SO . The formation of H+ ions leads to the descend of pH value in the lixivium, which is consistent with the experiment results. In the leaching process, elemental sulfur and Fe2+ are oxidized without formation of precipitation on the

Fig. 9 EDXA pattern of chalcopyrite

Fig. 10 EDXA pattern of leaching residues

Table 3 Element contents of chalcopyrite and leach residues

Sample Mass fraction/% Molar fraction/%

S Fe Cu S Fe Cu

Chalcopyrite 35.32 30.55 34.13 50.40 25.03 24.57

Leaching residues

22.13 34.56 5.02 18.49 16.58 2.12

surface of the mineral. So, the dissolution reaction of copper goes smoothly. The oxidation of elemental sulfur and Fe2+ offers enough energy for bacteria growth. 3.2.5 Column bioleaching of copper sulfide ore

To determine the leaching behavior of lump copper sulfide ore, the bioleaching experiments of lump ore of <25 mm were carried out in the bioleaching controlling system. The extraction rate of copper and variation of pH in the solution are shown in Fig. 11 and Fig. 12, respectively. The results indicate that the copper extraction rate increases gradually with the leaching time and reaches nearly 45% after 75 days. During the leaching process, sulfuric acid was added into the solution to deplete alkaline ingredients and keep the pH value in the range of 1.8−2.5 till inoculating the bacteria. The pH value of the lixivium reduces from 2.7 to 2.3 and

Fig. 11 Copper extraction on bioleaching of <25 mm lump

copper sulfide ore

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Fig. 12 Variation of pH in solution during bioleaching of lump

copper sulfide ore

remains in the range of 2.3−2.1 to the end of the leaching run after inoculating the bacteria. 4 Conclusions

1) The bacteria grow best when temperature is (30±1) °C and the pH value is 2.0. Under this condition, the concentration of bacteria is 2.24×107 mL−1. Inoculum size and pulp density have intense influences on the copper extraction rate during the bioleaching process. The copper leaching rate is the highest under the pulp density of 5% in chalcopyrite bioleaching, while under the pulp density of 10%, the copper leaching rate is the highest in the bioleaching of sulfide copper ore.

2) EDXA observations of chalcopyrite and leaching residues reveal that copper is dissolved intensively and the mass and molar fractions of S decrease due to the fact that the elemental sulfur is oxidized to

24SO . The

oxidation of elemental sulfur and Fe2+ offers adequate energy for bacteria growth.

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(Edited by YANG Bing)