Bioleaching of Sphalerite by Acidithiobacillus Ferrooxidans and Acidithiobacillus Thiooxidans Cultured in 9K Medium Modified with Pyrrhotite

J. Cent. South Univ. Technol. (2008) 15: 503−507 DOI: 10.1007/s11771−008−0095−7 Bioleaching of sphalerite by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans cultured in 9K medium modified with pyrrhotite CHEN Song(陈松), QIU Guan-zhou(邱冠周), QIN Wen-qing(覃文庆), LAN Zhuo-yue(蓝卓越) (School of Resource Processing and Bioengineering, Central South University, Changsha 410083, China) Abstract: Elective culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans in 9K medium modified with pyrrhotite was studied. Bioleaching of flotation concentrate of sphalerite by the selected bacteria was carried out. The results show that the microorganisms cultured by pyrrhotite are a mixture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, of which the capability to oxidize ferrous to ferric irons is enhanced by the high mass ratio of Fe to S in pyrrhotite. Three pyrrhotite samples were separated into various parts with corresponding S/Fe ratios by magnetic separation and were used to culture the elective bacteria as the substrate. The association of the cultures could provide a more rapid and complete oxidation of sphalerite than that of bacteria cultivated by conventional methods. Key words: bioleaching; pyrrhotite; elective culture; sphalerite the natural environment, the microorganisms with the 1 Introduction capacity to dissolve minerals are a mixed culture of different bacterial strains. Being cultivated with the Bioleaching has been widely used in the conventional methods of domestication, mutation, commercial extraction of uranium, copper and gold from crossbreed, cell syncretizing and gene engineering, some ore, and it is being exploited in the extraction of other capabilities of the microorganism such as the activity, the base metals and rare noble metals, such as zinc, cobalt, tolerance of heavy metal ions and the adaptability to nickel, molybdenum, gallium, germanium etc. In some sulfide ore were improved into some extent. However, cases it has been in the phase of pilot scale the microorganisms derived from the above methods experiment[1−2]. It presents more advantages over the usually are a single strain, and the improved capability conventional metallurgical methods in treating low-grade shown in bench scale test may degenerate in industrial and complicated ore. The metal sulfides are oxidized by cases, so their effect on bioleaching is limited. The some special bacteria, such as Thiobacillus ferrooxidans, bacteria which inhabit sulfide deposits can use sulfide Thiobacillus thiooxidans and Leptospirillum minerals as energy source by dissolving the minerals and ferrooxidans, to form soluble metal sulfates, elemental oxidizing ferrous iron and elemental sulfur. According to sulfur and sulfuric acid[3−4]. Although some special literature reports, the role of bacteria in bioleaching bacteria have the potential to dissolve a number of includes the direct attack on minerals, and the oxidation sulfide minerals containing copper, their widespread of Fe2+ and S[1−5]. Therefore, we supposed to cultivate commercial acceptance still remains some restrictions bacteria with an appropriate mineral as a selective resulted from slower growth rate and lower cell density, culture, which can enrich strains of microorganisms and both leading to poor leaching kinetics compared with the improve their integral capability on bioleaching of the hydrometallurgy alternatives. Hence, to enhance the similar mineral. Pyrite (FeS2) and pyrrhotite (Fe1−xS) leaching rate, many studies have been done in the aspects are the familiar components of sulfide deposit, of which of microbiology, electro- chemistry, metallurgy etc. One Fe and S are the essential elements to cultivate bacteria, of the solutions proposed by researchers is to cultivate so both are the perfect substrates for selective culture of effective microorganisms used in bioleaching[5]. There bacterial used in bioleaching. Considering that pyrrhotite have been more than 20 strains of microorganisms used is dissolved more easily than pyrite, and has various mass in the bioleaching of sulfide ore; the most familiar and ratios of Fe to S, experiments were carried out to explore widely used are Thiobacillus ferrooxidans, Thiobacillus the elective culture of bacteria using pyrrhotite as the thiooxidans and Thermoacido philic achaebacterium. In substrate. Foundation item: Project(50621063) supported by the National Natural Science Foundation of China; Project(2004CD619205) supported by the Major State Basic Research Development Program of China Received date: 2008−10−15; Accepted date: 2008−03−25 Corresponding author: QIN Wen-qing, Professor, PhD; Tel/Fax: +86−731−8879815; E-mail: [email protected] 504 J. Cent. South Univ. Technol. (2008) 15: 503−507 Sphalerite is the most important resource of zinc ore the zinc minerals are mainly sphalerite. in China and it comes from different regions and has The chemical compositions of raw ore are given in different physicochemical futures due to the difference Table 3. Microscopic examination in thin slides of the metallogeny, the content of iron and other impurities. groundmass shows that the zinc minerals are mainly Although some investigations on bioleaching of zinc sphalerite (ZnS), pyrrhotite (FeS1.17) and pyrite (FeS2), sulfide have been carried out, the mechanism for the and gangue minerals are quartz, gypsum, dickite and biooxidation of zinc sulfides is not completely sericite. recognized[6−10]. In this paper, the elective culture of Acidithiobacillus ferrooxidans and Acidithiobacillus 2.2 Elective culture of bacteria on pyrrhotite thiooxidans in 9K medium modified with pyrrhotite was The original microorganisms were collected from studied, and bioleaching of flotation concentrate of Guangxi Dachang Mine, and they were grown in the sphalerite by the selected bacteria was carried out. modified 9K nutrient medium, which was adopted with adding 6% (the ratio of mass (g) to volume (mL)) the 2 Materials and methods pyrrhotite samples No.1−No.3, respectively, replacing 10% iron (Fe2+) as the only energy source. The flasks 2.1 Mineral and ore were placed on an orbital shaker (170 r/min) and The chemical components of pyrrhotite are not incubated at 30 ℃. The pH value was measured fixed and the structural formula could be illustrated as periodically, and when it dropped below 1.80, it was Fe1−xS, where x=0−0.223. And when x=0, the iron atoms adjusted to 1.80 with 5 mol/L sulfuric acid. The number are perfect in the crystal[11−12]. With the increase of the of bacterial cells in solution was counted with a mass ratio of S to Fe, the magnetism of pyrrhotite hemacytometer under a biomicroscope with a increases. Therefore, pyrrhotite can be separated into magnification of 1 000. Repeatedly, when the bacteria various parts by magnetic separation. reached an exponential growth phase, one-fourth of the Table 1 lists the results of magnetic separation of a culture volume was transferred to the next incubation. pyrrhotite collected from Dachang Mine, Guangxi After three times of transfer, the suspended solution Province, China. The crystal form of the pyrrhotite is of of the culture was filtrated through a millipore filter, the monocline. bacteria enriched on the membrane were washed with The sphalerite concentrate and zinc ore containing sulfuric acid solution (pH=1.80) to reduce iron sphalerite were obtained from Gaofeng Mine in Guangxi contamination. The concentration of bacterial cells was Province, China. The chemical compositions of diluted with iron-free 9K nutrient solution to the density sphalerite concentrate are listed in Table 2. Microscopic of 107 /mL, which was used as the inoculum of the next examination in thin slides of the groundmass shows that experiments. Table 1 Pyrrhotite samples with various mass ratios of S to Fe Mass fraction/% Sample No. Mass ratio of S to Fe S Fe Si Zn Mg 1 29.64 65.43 1.75 1.23 1.76 0.453 2 28.59 45.75 11.58 4.09 2.22 0.625 3 30.07 35.91 22.31 3.50 3.94 0.837 Table 2 Chemical compositions of sphalerite concentrate (mass fraction, %) Zn Pb Cd Cu Fe MgO CaO SiO2 44.240 0.200 0.030 0.010 13.100 1.000 0.640 1.100 Mn Al2O3 Co Ni Ag S As 0.420 2.520 Mim. 0.030 0.002 32.550 0.110 Table 3 Chemical compositions of zinc ore (mass fraction, %) Zn Pb Cd Cu Fe MgO CaO SiO2 6.020 0.280 0.020 0.020 28.100 2.340 3.540 30.340 Mn Al2O3 Co Ni Ag S As 2.670 8.520 Mim. 0.022 0.001 12.150 0.320 J. Cent. South Univ. Technol. (2008) 15: 503−507 505 2.3 Bacterial leaching experiments improved, which can accelerate the bioleaching rate of Leaching experiments were performed in 250 mL some sulfides. shake flasks at a pulp density of 6% pyrrhotite sample + From Fig.1 the acid consumption of the three 90 mL 9K nutrient medium without iron + 10 mL samples of pyrrhotite from high to low is: No.1＞No.2＞ inoculums of cultures. The initial density of bacterial No.3. With increasing time, the pH value has a cells in the liquor phase was about 107 /mL. Incubation decreasing tendency. The bacterial density is above 2× 7 was performed at 30 ℃ and an initial pH of 2.0 on an 10 cell/mL in the third day. In sample No.1, the high orbital shaker at 160 r/min to determine the contribution concentration of iron ion leads to precipitation, so the from chemical leaching of zinc. bacteria are transferred to the next incubation with fresh The solution samples were withdrawn at the same pyrrhotite. intervals and the concentration of Zn2+ was determined by EDTA titration[13], and the concentration of ferrous iron and total iron were determined by colourimetry using the phenanthroline method[14]. The pH value in leaching solution was measured with a pH meter. The pH value was monitored and a solution of 4.0 mol/L H2SO4 was used to maintain it at the initial value during the leaching process.

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