Indian Journal of Biotechnology Vol 3, January 2004, pp. 86-91 Medium optimization for bioleaching of metals from Indian bulk polymetallic concentrate D R Tipre 1 , S B Vora 2 and S R Dave 1 * 1 Department of Microbiology, School of Sciences, Gujarat University, Ahmedabad 380 009, India 2 Gujarat Mineral Development Corporation, Ahmedabad 380 052, India Received 1 July 2002; accepted 9 April 2003 Extensive bioleaching work is mainly done only for the Cu and gold extraction from concentrates. Bioleaching of bulk concentrate is at the developing stage. The present work deals with the formulation of an economic medium for the bioextraction of metals from GMDC polymetallic bulk concentrate. For the medium formulation, a 2 4 factorial statistically designed experiment was adapted. Effect of concentration at two levels each of K 2 HPO 4 , (NH 4 ) 2 SO 4, KCl and inoculum was studied. For Cu and Zn extraction, inoculum was the most significant factor with the nutrients checked. KCl and its interaction were found negatively significant. The formulated medium was compared with the 9K − and Jordan's medium. The highest Cu (77.8%) and Zn (84%) extraction achieved with the formulated medium was 1.25 and 1.32 folds higher than the universally used 9K − medium. The final metal extraction rates obtained were 0.65 and 4.2 g/l/d for Cu and Zn respectively with the designed medium. The composed medium was at least 4 times cheaper, less acid consuming and more suitable for the metal extraction from GMDC polymetallic concentrate compared to 9K − medium. Keywords: bioleaching, polymetallic concentrate, Cu, Zn, Thiobacillus ferrooxidans, factorial Introduction In bioleaching, microorganisms play a major role in leaching of base and precious metals from mineral resources. Bioextraction processes use the ability of acidophilic chemolithoautotrophic bacteria 1,2 , such as Thiobacillus and Leptospirillum. Extensive bio- leaching studies have been made on Cu and gold ores and concentrates 3-5 . However, little work is available on bioleaching of complex sulphides. In microbiological leaching of sulphidic ore or concentrate, the role of nutrients is chemically, biologically and economically important. Micro- organisms need an energy source as well as elements (C, N, P, Mg, S etc.), which must be present in the form suitable for the absorption by cell 6 . Ferrous deficient 9K medium (9K − ) is generally used in the bioextraction of sulphidic minerals. Bioleaching bacteria (Acidithiobacillus ferrooxidans, A. thiooxidans and Leptospirillum ferrooxidans), relatively undemanding, grow in media consisting only of inorganic compounds utilising CO 2 as carbon source and obtaining energy from the oxidation of reduced sulphur compounds, elemental sulphur and/or ferrous 5-8 . These organisms grow well in simple, defined media, but the condition of growth becomes complex when ore or concentrate is added to the medium. In such cases several inorganic nutrient requirements of cells can be satisfied by components of the ore or concentrate added in the media. So, there is no reason to add 9K medium for bioleaching experiments. The present study aims to assess the significance of the major nutrient factors and their possible interactions for metal extraction using 2 4 factorial statistically designed experiment and formulation of suitable economically viable medium with special reference to bioleaching of Gujarat Mineral Development Corporation (GMDC) polymetallic bulk concentrate for the bioextraction of Cu and Zn. Materials and Methods Mineral The polymetallic bulk concentrate, obtained from GMDC, Ambamata Multimetal Project, Gujarat, India, was a mixer of particles (size, +150 to −325 # B.S.S.) available in the form of sphalerite, chalcopyrite, galena and pyrite 9 . The major constituents of the concentrate are: Zn, 30.0; Cu, 2.5; Pb, 13.2; Fe, 9.2; and S, 27.1 wt%. __________ *Author for correspondence: Tel: 079-26303225; Fax: 079-26302654 E-mail: shaileshrdave@hotmail.com