J. Cent. South Univ. Technol. (2008) 15: 650-655

DOI: 10.1007/s11771-008-0121-9

Bioleaching of marmatite using moderately thermophilic bacteria

ZHOU Hong-bo(周洪波)^{1, 2}, LIU Fei-fei(刘飞飞)¹, ZOU Ying-qin(邹颖钦)¹,

ZENG Xiao-xi(曾晓希)¹, QIU Guan-zhou(邱冠周)^{1, 2}

(1. School of Resources Processing and Bioengineering, Central South University,

Changsha 410083, China;

2. Key Laboratory of Biometallurgy, Ministry of Education, Central South University,

Changsha 410083, China)

Key words:

marmatite; moderate thermophilies; zinc; bioleaching；

1 Introduction

China has zinc storage capacity of 75×10⁶ t, which accounts for 25% of the world gross storage and ranks second in the world. Among available zinc storage capacity of 33×10⁶ t, a high percentage is rich in marmatite. Due to the concomitance of zinc ore and other metals like Pb, Sn, Ag, Co, etc, in the process of exploiting the concomitant metals, by-products rich in marmatite are yielded^[1]. It proves difficult to treat marmatite by conventional extraction processes due to the high content of iron as well as the little economic benefit. Comparatively, bioleaching has received increasing attention due to the advantages of maneuverability, low cost, low energy consumption and, most important, high utilization of resources and environment friendliness. With further research, bioleaching will be increasingly used in extraction of ore concentrate, and applied extensively in the pre-process of difficult-to-process gold ores and processing of Zn, Ni, Co and other types of sulfide minerals^[2].

Dozens of bacterial species have been identified as having bioleaching capabilities, including chemoautotroph bacteria, heterotrophic bacteria, fungi, etc. According to the optimal growth temperature, those bacteria can be categorized into mesophiles (30 ℃), moderate thermophiles (50 ℃), and extreme thermo- philes (70 ℃). Traditionally, close attention has been paid to the mesophilic bacteria, especially Acidithioba- cillus ferrooxidans (At. ferrooxidans). It has been proved that At. ferrooxidans has the capacity of bioleaching of sulphide minerals like Zn^[3-4], Co^[5-6], Ni^[7-8], etc. However, At. ferrooxidans also has the disadvantage of low efficiency of dynamics in bioleaching, which restricts its industry application. Recent researches show that moderate thermophilic bacteria have superiority on leaching dynamics in the bioleaching of complex of lead and zinc sulphide, which attracts our attention^[9-10]. SHI and FANG^[11] reported that moderate thermophilic bacteria could work at a higher temperature and lower pH compared to At. ferrooxidans and achieved higher zinc extraction efficiency. Moreover, using the mixture of several species can improve the extraction rate of valuable minerals^[12].

In this work, moderate thermophiles were used to leach marmatite and the bioleaching capabilities were compared between moderately thermophilic bacteria and mesophilic bacteria. The effects of venting and ore concentration were investigated in the bioleaching of marmatite with moderate thermophiles in order to provide knowledge for further research and industrial application.

2 Experimental

2.1 Mineral

The marmatite used in the experiments was not processed by flotation. The sample contained 48.38% Zn, 1.10% Fe and 20.2% S (mass fraction), and was ground to particle size less than 75 μm. The 9k inorganic salt medium was used as leaching solution with initial pH value of 1.9^[13].

2.2 Microorganisms

The moderate thermophiles used in this work were collected in the author’s laboratory from hot spring, ore, acid mine drainage in provinces of Yunnan, Jiangxi, Hunan, Guangdong and Gansu, etc. Community analysis by PCR-RFLP showed that it mainly consisted of Sulfobacillus thermotolerans (S. thermotolerans), Sulfo- bacillus thermosulfidooxidans (S. thermosulfidooxidans), At. caldus, Leptospirillum ferriphilum (L. ferriphilum). Mesophilic bacteria used in this research were the product of mixing after isolating from acidophilic mesophilic bacteria, which mainly contained At. ferrooxidans and At. thiooxidans.

2.3 Experimental process

In 250 mL Erlenmeyer flasks, the marmatite sample was added into 120 mL 9k medium to concentration of 50 g/L. Each flask was inoculated with 20 mL (1×10⁸ /mL) moderate thermophilic culture and mesophilic culture respectively. By adjusting the pH value to 1.9, the flasks were shaken on the orbital shakers at 160 r/min and growth temperatures of 45 and 30 ℃, respectively. The volume reduced by evaporation was compensated by sterilized water, and pH, redox potential E_h (vs SCE) and bacterial density were measured everyday. Zn concentration in the supernatant was analyzed every other day.

In 250 mL Erlenmeyer flasks, the marmatite sample was added into 180 mL 9k medium to concentration of 50 g/L. Each flask was inoculated with 20 mL moderately thermophilic bacteria. The pH value was adjusted to 1.9, and the flasks were controlled at 45 ℃ in thermostatic waterbath. To study the effect of venting capacity on marmatite leaching performance, the venting flowrates were adjusted to 50, 200 and 800 mL/min, respectively.

In 250 mL Erlenmeyer flasks, the marmatite sample was added into 120 mL 9k medium to concentrations of 50 g/L, 100 g/L and 200 g/L, respectively. Each flask was inoculated with 20 mL moderately thermophilic bacteria. The pH value was adjusted to 1.9, and the flasks were shaken on the orbital shakers at 160 r/min and 45 ℃.

2.4 Analysis methods

Elemental analysis: the concentrations of Zn and Fe in the leaching solution were measured by atomic absorption spectrometry (AAS). The pH value and E_h value in the leaching solution were measured by a pH S-3C acid meter and a platinum electrode with an Ag/AgCl reference electrode, respectively. Free cells in solution were observed and counted under an optical microscope.

3 Results and discussion

3.1 Comparison of marmatite leaching with mesophilic bacteria and moderately thermophilic bacteria

Under the condition of 160 r/min and 50 g/L pulp density, the moderately thermophilic bacteria show higher leaching efficiency than mesophilic bacteria in a leaching period of 20 d (Fig.1). At 45 ℃, compared with the zinc extraction efficiency of 3.1% in the sterile control, the zinc extraction efficiency of moderately thermophilic bacteria is 93.1%. At 30 ℃, the zinc extraction of mesophilic bacteria is 57.0%, while the value is 2.6% in the sterile control.

Fig.1 Zinc extraction efficiency of marmatite with different bacteria groups

At the beginning, the cell densities of mesophilic culture and thermophilic culture increase fast until 10th day when the densities become stable (Fig.2). The cell density (shown as logarithm of the cell number, ρ_c) of moderately thermophilic bacteria is greater than that of mesophilic bacteria at 20th day, and the maximum cell densities are 3.64×10⁸ /mL and 2.88×10⁸ /mL, respectively.

The variations of pH value and E_h under different conditions (Fig.3) indicate that pH value is generally increasing both in thermophilic and mesophilic sterile conditions, which may be due to the dissolution of marmatite in the acid leaching solution, thus leading to the release of the alkaline substances. While in the condition with bacteria added, pH values in leaching solution have a transient increase then decrease quickly, and the decrease in moderately thermophilic bacteria solution is more significant, and the final pH value is only 0.8. E_h values also increase after being inoculated with bacteria, and the redox potentials increase from 278 (vs SCE) to 552 mV (vs SCE), and from 278 (vs SCE) to 500 mV (vs SCE) for thermophilic and mesophilic cultures, respectively. However, E_h values in both sterile systems decrease from 320 (vs SCE) to around 220 mV (vs SCE).

Fig.2 Variations of cell densities in leachates during processes of marmatite leaching with different bacteria groups

Fig.3 Changes of pH (a) and E_h (b) values of leachate vs time during processes of marmatite leaching with different bacteria groups

Similar to the results reported by DEVECI et al^[10], under the condition of shaking, moderately thermophilic bacteria have a superior quality over mesophilic bacteria in bioleaching. At 45 ℃, the higher cell density and stronger metabolic ability cause lower pH value that increases the dissolution of marmatite and improves the direct and indirect effects of mineral leaching and elevates the zinc extraction.

3.2 Effect of venting capacity on marmatite leaching performance of moderately thermophilic bacteria

The influence of venting level on the marmatite leaching is obvious. Venting levels of 200 and 800 mL/min result in good agitation, venting level of 50 mL/min, however, results in low pulp density because the low venting level fails to promote all the slurry with part of ore powder deposited at the bottom. At pulp density of 50 g/L and venting levels of 50, 200 and 800 mL/min, the zinc extraction efficiencies by moderately thermophilic bacteria in 20 d are 57.8%, 92.5% and 96.0%, respectively (Fig.4). The cell densities are also influenced by aeration, the highest values for 800 and 50 mL/min venting levels are 1.1×10⁹ and 8.5×10⁸ /mL (Fig.5), respectively.

Fig.4 Zinc extraction efficiency of marmatite at different venting levels

Fig.5 Cell densities of leachate during leaching process of marmatite with different venting levels

Variations of pH and E_h values at different venting levels are shown in Fig.6. The final pH values are 1.11, 0.75 and 0.72 for aeration levels of 50, 200 and 800 mL/min, respectively. E_h value increases in general. It shows higher E_h values at aeration level of 800 mL/min, and the final E_h value is up to 625 mV (vs SCE).

Fig.6 Changes of pH (a) and E_h (b) values of leachate vs time during processes of marmatite leaching with different venting levels

Moderately thermophilic culture mainly consists of S. thermotolerans, S. thermosulfidooxidans, A. caldus and L. ferriphilum that are all aerobic bacteria, therefore, aeration has significant influence on the growth of bacteria and thus plays an important role in dissolution of ores. Strong aeration can improve the biomass, lower the pH value and increase the redox potential of solution, which results in better bioleaching performance. However, too much aeration will cause strong shear force that may damage the cells, interfere with adhesion of bacteria to mineral surface and improve the cost as well.

3.3 Effect of pulp density on marmatite leaching performance of moderately thermophilic bacteria

The results of zinc dissolution at different pulp densities are shown in Fig.7. After 20 d, the zinc extraction efficiencies at pulp densities of 50, 100 and 200 g/L are 93.1%, 81.9% and 58.9%, respectively. With the increase of pulp density, the total amount of dissolved valuable metals increase, however, the extraction efficiency decreases, which conforms with the results made by BAILEY and HANSFORD^[14] in a pyrite bioleaching experiment. This is mainly because the dissolution of minerals is primarily limited to the specific surface area of mineral in per unit volume of solution; also, it is limited to CO₂ concentration^[15]. At higher pulp density, the system has larger specific surface area that increases the time of mineral dissolution and decreases the extraction efficiency. Meanwhile, with the increase of pulp density, the transfer of O₂ and CO₂ is restricted and the damage of shear force and metal ions to cells increases^[16].

Fig.7 Variations of zinc concentration (a) and extraction efficiency (b) vs time at different pulp densities

The changes of pH and E_h values of leachate at different pulp densities are shown in Fig.8. The pH values in leaching system with pulp density of 50 g/L drop down most, the final pH is 0.81, while E_h values in leaching system with pulp density of 50 g/L rise most, the final E_h values in leaching systems with 50, 100 and 200 g/L pulp densities are 561, 538 and 474 mV (vs SCE), respectively. These are consistent with the above results, which may indicate that lower pH value and higher E_h value are beneficial to marmatite dissolution.

The cell densities are also influenced by pulp density. And the results are shown in Fig.9. It can be seen that the maximum of cell densities at pulp densities of 50, 100 and 200 g/L are 8.6×10⁸, 8.3×10⁸ and 8.0×10⁸ /mL, respectively.

Fig.8 Changes of pH (a) and E_h (b) values of leachate vs time at different pulp densities

Fig.9 Cell densities of leachate vs time at different pulp densities

4 Conclusions

1) Moderately thermophilic bacteria have more advantages over mesophilic bacteria in bioleaching of marmatite.

2) Stronger aeration is more beneficial to dissolve the marmatite, improve the transfer of O₂ and CO₂ in solution, promote the growth of bacteria and therefore, enhance the leaching performance.

3) Pulp density has a significant effect on zinc extraction: higher zinc extraction can be obtained at low pulp density, while improving the pulp density can increase the amount of leached zinc, whereas, it may cut down the transfer of O₂ and CO₂ and increase the shear force, and therefore, decrease the recovery of Zn.

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(Edited by ZHAO Jun)

Foundation item: Projects(50621063, 40646029) supported by the National Natural Science Foundation of China; Project(2004CB619204) supported by the Major State Basic Research Development Program of China; Project(NCET-06-0691) supported by the Program for New Century Excellent Talents in University

Received date: 2008-03-15; Accepted date: 2008-05-20

Corresponding author: ZHOU Hong-bo, Professor, PhD; Tel: +86-731-8877216; E-mail: zhouhb@mail.csu.edu.cn