sign in sign up
<<
Home , Bioleaching

J. Cent. South Univ. Technol. (2008) 15: 503−507 DOI: 10.1007/s11771−008−0095−7

Bioleaching of sphalerite by 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 cultured by pyrrhotite are a mixture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, of which the capability to oxidize ferrous to ferric 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, and from crossbreed, syncretizing and gene engineering, some , and it is being exploited in the extraction of other capabilities of the such as the activity, the base and rare noble metals, such as , , tolerance of heavy and the adaptability to , , gallium, germanium etc. In some 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 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 source by dissolving the minerals and ferrooxidans, to form soluble metal sulfates, elemental oxidizing ferrous and elemental . According to sulfur and [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 kinetics compared with the improve their integral capability on bioleaching of the hydrometallurgy alternatives. Hence, to enhance the similar mineral. (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 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 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 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 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.

3 Results and discussion

3.1 Mechanism of elective culture on pyrrhotite Bioleaching of pyrrhotite bearing nickel or gold to [15] extract nickel or gold was reported . Oxidation of Fig.1 Variation of pH in cultivation of bacteria on pyrrhotite [16] pyrrhotite can be illustrated as follows . with various mass ratios of S to Fe 1) Acid leaching

+ + ⎯⎯→ − 2+ + 3+ + 3.2 Characteristics of selected bacteria Fe1−xS 2H (1 3x)Fe 2xFe H 2S (1) With the microscopic examination and observation 2) Bacterial catalyzing of solid culture, the bacteria cultured in 9K medium + + + ⎯Bacteria⎯→⎯⎯ modified with pyrrhotite were characterized; the main Fe1−xS O 2 4H strains were Acidithiobacillus ferrooxidans and − 2+ + 3+ + + (2 6x)Fe 4xFe 2S 2H 2O (2) Acidithiobacillus thiooxidans. In sample No.1, + + ⎯Bacteria⎯→⎯⎯ 2− + + Acidithiobacillus ferrooxidan is dominant; S 1.5O 2 H 2O SO 4 2H (3) 2+ + + + ⎯Bacteria⎯→⎯⎯ 3+ + Acidithiobacillus thiooxidans is in the majority in sample 4Fe O2 4H 4Fe 2H 2O (4) No.2; and in sample No.3, the two strains of 3) Oxidizing action of Fe3+ Acidithiobacillus ferrooxidans and Acidithiobacillus 3+ thiooxidans are approximately equiponderant. It can be Fe − S + (8 − 2x)Fe + 4H O ⎯⎯→ 1 x 2 seen here that the bacteria obtained from pyrrhotite − 2+ + 2− + + (9 3x)Fe SO4 8H (5) cultivation are a mixed culture, in which some strains are + − 3+ ⎯⎯→ − 2+ + Fe1−xS (2 2x)Fe (3 3x)Fe S (6) dominant in numbers and in oxidizing capacity corresponding to the variation of the S/Fe ratio. 4) Hydrolyzation of Fe3+ Fig.2 shows the oxidation of ferrous ion by bacteria 3+ + ⎯⎯→ + + Fe 2H 2O FeOOH 3H (7) in 9K medium (the original ferrous concentration was 9 g/L, the inoculation was 3 mL bacterial medium If the bacteria are cultivated purposefully to respectively). It can be seen that bacterium No.1 exhibits accelerate the chemical oxidation of sulfide minerals by a fast oxidizing rate, and ferrous ions are completely ferric ion or the biooxidation of sulfides and elemental oxidized to ferric ions after 40 h. The time for oxidizing sulfur to sulfate, the rate of bioleaching will be increased. the ferrous ion for bacteria No.3 and No.2 is 80 and 100 A mixed culture can be obtained from the cultivation of h, respectively. The results are consistent with the bacteria on pyrrhotite with various S/Fe ratios; bacterial characterization above, in which accordingly, the mixed culture has a strengthened Acidithiobacillus ferrooxidans is dominant and has the capability to oxidize ferrous ion or sulfides and highest capacity of ferrous oxidation. Correspondingly, a elemental sulfur, and the adaptability to minerals is also higher capacity of elemental sulfur oxidation was

506 J. Cent. South Univ. Technol. (2008) 15: 503−507 observed when the Acidithiobacillus thiooxidans dominate. Comparing the bacterial culture with the different S/Fe ratio, it can be concluded that ferrous oxidizing bacteria are dominant in the pyrrhotite culture with low S and high Fe mass ratio. On the contrary, the sulfur oxidizing bacteria are dominant in the culture with high S and low Fe mass ratio.

Fig.3 Effect of three bacteria samples obtained from pyrrhotite culture on bioleaching of sphalerite flotation concentrate

and reaction product is suppressed in some extent if the elemental sulfur is not eliminated in time, which decreases the leaching rate. The results of bioleaching of sphalerite concentrate Fig.2 Ferrous ion oxidation of three bacterial cultures with a mixed culture obtained from conventional induction mutation are shown in Fig.4. Compared with a 3.3 Bioleaching of flotation concentrate of sphalerite mixed culture of Acidithiobacillus ferrooxidans and Bioleaching of sphalerite can be represented as Acidithiobacillus thiooxidans obtained from follows. conventional induction mutation, all the three bacteria 1) Biochemical reactions samples have more positive effects on bioleaching of sphalerite flotation concentrate. Though the bacteria also Direct mechanism: + ⎯Bacteria⎯→⎯⎯ 2+ + 2− have the high capacity of ferrous ion oxidation and sulfur ZnS 2O2 Zn SO 4 (8) oxidation respectively, the leaching rate of zinc Biooxidation: extraction of the mixed culture is 85% after 25 d. 0 + + ⎯Bacteria⎯→⎯⎯ + + 2− 2S 3O2 2H 2O 4H 2SO 4 (9) 2+ + + + ⎯Bacteria⎯→⎯⎯ 3+ + 4Fe O 2 4H 4Fe 2H 2O (10)

2) Chemical reactions

+ + + ZnS + 2Fe3 ⎯⎯→ Zn 2 + 2Fe 2 + S0 (11) + 3+ + ⎯⎯→ 2+ + 2− + + FeS 8Fe 4H 2O 9Fe SO 4 8H (12)

The results of bioleaching of sphalerite concentrate are shown in Fig.3. It can be seen that bacterium No.1 has the highest leaching rate. The leaching rate of zinc used with bacterium No.1 is about 84.3%. The leaching rate of zinc used with bacteria No.2 and No.3 are 82.0% and 80.3%, respectively. Thus, the bacteria with the high capacity of ferrous oxidation or sulfur oxidation have Fig.4 Bioleaching of flotation concentrate of sphalerite with better effect on bioleaching of sphalerite concentrate. mixed culture obtained from conventional induction mutation These results are consistent with the literature report, that the indirect mechanism is dominant in bacterial leaching The results of bioleaching of raw ore containing of sphalerite[17−18]. sphalerite are quite different from those of sphalerite Bacterium No.1 has a high capacity of oxidizing concentrate (Fig.5). The bacteria No.2 and No.3, which ferrous ion to ferric ion, and the generation of ferric ion show the lowest leaching rate of zinc in bioleaching of accelerates sphalerite dissolving, and the products of the sphalerite concentrate, have the highest leaching rate of dissolving reaction are ferrous ion and elemental sulfur. zinc of 90% and 82% respectively in 9 d, while the With increasing elemental sulfur, the transfer of reactant leaching rate of zinc of bacterium No.1 is 70%. The

J. Cent. South Univ. Technol. (2008) 15: 503−507 507 Acidithiobacillus thiooxidans adapt to bioleaching of zinc sulfide ore. The role of bacteria in bioleaching of sphalerite is primarily to oxidize the products of ferrous ion and elemental sulfur, thus the dominant mechanism is the indirect one.

References

[1] MILLER P C, RHODES M K, WINBY R. Commercialization of bioleaching for base-metal extraction [J]. Minerals & Metallurgical Processing, 1999, 16(4): 42−50. [2] EHRLICH H L. Past, present and future of bioleaching [J]. Hydrometallurgy, 2001, 59(2/3): 127−134. [3] BOON M, BRASSER H J, HANSFORD G. Comparison of the oxidation kinetics of different pyrite in the presence of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans [J]. Hydrometallurgy, Fig.5 Effect of three bacteria samples obtained from pyrrhotite 1999, 53(1): 57−72. culture on bioleaching of raw ore of zinc sulfide [4] SAND W, RHODE K, SOBOTKE B. Evaluation of Leptospirillum ferrooxidants for leaching [J]. Appl Environ Microbiol, 1992, 58(1): 85−92. curves do not pass through the original point because [5] LUO Lian-ming, WANG Jun, XU Jing. Study on methods of portions of zinc are released during pH adjustment improving bacterial leaching velocity of gold [J]. Conservation before inoculation. and Utilization of Mineral Resources, 1994(4): 40−43. (in Chinese) [6] GARCIA J O, MIGHAM B J, TUOVIMEN O. Sphalerite oxidation It can be seen that a mixed culture of by Thiobacillus ferrooxidans and thiobacillus thiooxidans [J]. Can J Acidithiobacillus ferrooxidans and Acidithiobacillus Microbiol, 1995, 41: 578−584. thiooxidans being approximately equiponderant has a [7] KONISHI Y, KUBO H, ASIA S. Bioleaching of zinc sulfide concentration by Thiobacillus ferrooxidants [J]. Biotechnol Bioeng, better effect on bioleaching of raw ore of zinc sulfide. 1992, 39: 66−74. The iron in the ore is more likely to be released than that [8] CHOI W K, TORMA A E, OHILINE R. Electrochemical aspects of in the concentrate, because portion of the pyrrhotite and zinc sulphide leaching by Thiobacillus ferrooxidans [J]. Hydrometallurgy, 1993, 33(1/2): 137−152. pyrite are dissolved by chemical or biochemical process. [9] LAN Zhuo-yue, HU Yue-hua, LIU Jian-she, WANG Jun. Solvent The iron concentration reaches about 5.0 g/L after the 9th extraction of copper and zinc from bioleaching solutions with day in bioleaching of zinc sulfide ore, and in the LIX984 and D2E [J]. Journal of Central South University of Technology, 2005, 12(1): 45−49. bioleaching of sphalerite concentrate that is about 1.8 g/L [10] QIN Wen-qing, LAN Zhuo-yue, LI Wei-zhong. Recovery of zinc after 25 d. With the galvanic reaction and the high from low-grade zinc oxide ores by solvent extraction [J]. Journal of concentration of iron in the leached solution, the leaching Central South University of Technology, 2003, 10(2): 98−102. [11] LIANG Dong-yun, HE Guo-wei, ZOU Ni. The isomeromorphism of rate of sulfide ore is faster than that of the concentrate. pyrrhotite and its treatment feature differentia [J]. Journal of The sulfur product does not become the baffle of Guangdong Nonferrous Metals, 1997, 7(1): 1−5. (in Chinese) leaching because of low sulfur content in zinc sulfide [12] XIA Xue-hui. A kind of pyrrhotite from pyrite deposits in east Liaoning rift mineralogical implications [J]. Geology of Chemical ores. Minerals, 1996, 18(4): 263−270. (in Chinese) [13] JI S, SHANG Z C. The determination of Zn2+ in compound of Zn2+ 4 Conclusions and Fe2+ by EDTA titration [J]. Shandong Chem Ind, 2000, 29: 40−41. (in Chinese) [14] KARAMANEV D G, NIKOLOV L N, MAMATARKOVA V. Rapid 1) The elective bacteria culture on pyrrhotite is a simultaneous quantitative determination of ferric and ferrous ions in mixed culture of mainly Thiobacillus ferrooxidans, drainage and similar solutions [J]. Mineral Engineering, 2002, 15(5): 341−346. Thiobacillus thiooxidans and Leptospirillum [15] VEGLIÒ F, BEOLCHINI F, NARDINI A, TORO L. Bioleaching of a ferrooxidans etc. The leaching capacity of the culture is pyrrhotite ore by a sulfooxidans strains: Kinetic analysis [J]. improved remarkably. Chemical Engineering Science, 2000, 55(4): 783−795. [16] LI Hong-mei. Bioleaching of pyrrhotite bearing nickel—A review [J]. 2) The capability of bacteria cultured on pyrrhotite Hydrometallurgy, 1999(3): 8−12. (in Chinese) varies with the mass ratio of S to Fe. With the low S and [17] FOWLER T A, CRUNDWELL F K. Leaching of zinc sulfide by high Fe content, the ferrous oxidizing bacteria are thiobacillus ferrooxidans: Experiments with a controlled redox potential indicate no direct bacterial mechanism [J]. Appl Environ dominant; on the contrary, the sulfur oxidizing bacteria Microbiol, 1998, 64(10): 3570−3575. are dominant. [18] FOWLER T A, CRUNDWELL F K. Leaching of zinc sulfide by 3) Bacteria with a high capacity of ferrous thiobacillus ferrooxidans: Bacterial oxidation of the sulfur product layer increases the rate of zinc sulfide dissolution at high oxidization or sulfur oxidization adapt to bioleaching concentration of ferrous ions [J]. Appl Environ Microbiol, 1999, flotation concentrate of sphalerite. A mixed culture of 65(12): 5285−5292. (Edited by YANG Hua) equivalent Acidithiobacillus ferrooxidans and