European J. Appl. Microbiol. Biotechnol. 5,301-304 (1978) European A r ~ l i , ' , r l Journal of D"L~,JpI II~tU Microbiolooyand Biotechnology 9 by Springer-Verlag 91978 Inhibition of Thiobacillus ferrooxidans by Mineral Flotation Reagents Olli H. Tuovinen* Department of General Microbiology, University of Helsinki, Malminkatu 20, SF-00100 Helsinki 10, Finland Summary. Ferrous-iron oxidation by Tbiobacillus ferrooxidans was inhibited by a number of mineral flotation reagents. Dowfroth 250 and sodium butyl- xanthate were the least toxic reagents studied. Iron-oxidizing thiobacilli provide a rapid way of producing acidic ferric-sulfate solutions for the leaching of metals from ores. In microbiological leaching operations the bacterial regeneration of acidic ferric iron can be carried out as a separate step of the overall cyclic process although undoubtedly bacteria are present in the leaching liquor and are attached to ore throughout all stages. The direct oxidation of metal sulfides by the bacteria is a relatively slow process as the oxidation rates are limited by physical factors such as the pulp density and specific surface area (Kelly, 1977). Hydrometallurgical process involving bacteria have been developed for the revovery of metals from sulfide ore concentrates which are in general leached at faster rates than, for example, railings or low-grade ores (Tuovinen and Kelly, 1974a; Torma, 1977). The use of concentrates for leaching involves a prior treatment by flotation for most types of ores. Depending on the operational conditions and choice of chemicals, residual flotation reagents may remain in the slurry and ore material and be carried over to the leaching reactions. The present study highlights the toxicity of mineral flotation reagents to iron-oxidizing thiobacilli which may be employed in the leaching processes. These bacteria are gener- ally sensitive to organic compounds in their environment. Previously the effects of other mineral processing reagents have been investigated in some detail; the compounds include surface active agents and solvents used in the extraction of uranium and other metals (Torma et al., 1976; Torma and Itzkovich, 1976; 1977). Materials and Methods The floation reagents were provided by the Research Laboratory of Outokumpu Ltd. and are listed in Table 1. The strain of Tbiobacillusferrooxidans and the culture media *Present address: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA 0340-2118/78/0005/0301/t~ 01.00