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Bioleaching has been heavily focused attracted by researchers due to its many advantages such as simple process, environmental-friendly and suitable to various minerals. Zinc smelting slag contains large amount of heavy metals, such as As, Cu, Zn, Pb and Fe. Bioleaching is available method for reuse and reducing toxicity of the slag. In this study, the influences of strains, pH, solids concentration, inoculum and nutrition conditions on the basic characteristics of moderate thermophile in bioleahing of a zinc smelting slag were studied through a series of erlenmeyer flasks experiments while the positive conditions for microorganism growth were elucidated. At the same time, the bioleaching parameters for metal values from the zinc smelting slag were optimized. Based on the study, a rotary-kiln bioreactor was designed. The results were as follows:The shape of moderate thermophile was rod or ball with size of (1.5-2.2)×(0.2-0.5)/μm, and could use sulfur powder (S0) and yeast extract as energy materials. Moderate thermophile grew well under the conditions as follows:initial pH0.5, control pH1.0 in bioleaching process, solids concentration 5%, the amount of inoculum 10%, Ag+, S0and yeast extract concentration was 1 mg L-1,0.5% and 0.5%, respectively, and the biomass reached 1.7×108 cells-mL"1 in bioleaching system. Especially, pH and the concentration of yeast extract played a significant effect on iron concentration in bioleaching system while the amount of inoculum was slight. ORP value in bioleaching was closely related with the ratio of c(Fe3+) and c(Fe2+), which might be determined by the potential of Fe3+/Fe2+.The strains, the amount of inoculum, pH, concentration of solids and yeast extract had significant influence on the metal solubilization in bioleaching for the zinc smelting slag (P< 0.05), while that of concentration of Ag+ and S0 was slight. The optimal parameters of bioleaching for valuable metals from the zinc smelting slag using moderate thermophile were initial pH0.5, control pH1.0 in the bioleaching process, conncentration of solids, inoculum and yeast extract was 5%,10% and 0.5%, respectively. Based on the behalf of the optimized paratmeters, the bioleaching precentages of As, Cu, Zn, Pb reached 94-97%,94-99%,96-100%,18-25%, respectively.In the stage of bioleaching for the zinc smelting slag, moderate thermophile adsorbed tightly on the surface of the slag. The surface of the slag was with micropore, sunken hole and fault and indicated that the microorganism produced a series of biological chemical reaction with the slag and valuable metal in sulfide ore from the slag were dissoluted due to the amorphous structure of zinc smelting slag breaken.
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