The effects of carbon dioxide‐enriched air on the rate of zinc extraction during the microbiological leaching of a, high‐grade zinc sulfide concentrate by Thiobacillns ferrooxidans have been studied. Under normal air‐aeration conditions, the leach rate is limited initially by the availability of solid substrate surface area per unit volume of leach liquor, then by availability of carbon dioxide. If carbon dioxide‐enriched air is supplied, along with excess substrate, the zinc extraction rate increases as the carbon dioxide content increases until some other, as yet unknown, factor becomes limiting.

The effects of suspended, inert solids concentration, ferrous iron concentration and dissolved oxygen concentration on the kinetics of iron oxidation by Thiobucillus ferrooxidans are reported. It is shown that the maximum specific growth rate for this organism, oxidizing ferrous iron, is of the order of 0.1 h−1. Competitive inhibition by femc iron is demonstrated. The dissolved oxygen concentration below which the bacteria will not grow is 0.20 mg/L. The dissolved oxygen concentration below which O2 availability is limiting is around 0.29‐0.7 mg/L. 10.4 millimols of CO2 are fixed by the bacteria per mol of ferrous iron oxidized. 0.0185 mg of bacterial carbon are generated per mg of O2 consumed. Comparative mass transfer rates for O2 and CO2 are discussed. Oxidation rates decreased significantly in shake flasks as suspended solids concentrations rose above 0.5%, whereas in stirred tanks solids concentrations up to 15% had little effect on oxidation rate.

Oxidation of various inorganic sulfur compounds by Thiobacillus ferrooxidans was studied, and conditions necessary for maximum respiration rates were established. Optimum oxidation of elemental sulfur occurred at pH 5.0 and gave a Qo2(N) of 726; oxidation of thiosulfate gave a maximum Qo2(N) of 514 at pH 4.0; tetra- and tri-thionate, when oxidized at pH 6.0, gave a maximum Qo2(N) of 103 and 113, respectively. Polythionates accumulated during thiosulfate oxidation, but did not during oxidation of elemental sulfur. Metallic sulfide minerals were oxidized optimally as follows: chalcopyrite, pH 2.0, maximum Qo2(N) 3200; bornite, pH 3.0, maximum Qo2(N) 450; pyrite, pH 2.0, maximum Qo2(N) 1600. Maximum temperature for oxidation of all inorganic sulfur compounds tested was 40 C.The effect of a variety of organic compounds on sulfur oxidation is presented.T. ferrooxidans requires growth adaptation on iron for maximum respiration on that substrate; however, sulfur oxidation is not inducible. Iron and sulfur can be oxidized simultaneously, giving a rate equal to the sum of the maximum rates of oxidation of the two substrates individually.

When Thiobacillus ferrooxidans was grown on ferrous iron and chalcopyrite (CuFeS2) in excess of 96% of the bacterial population was associated with the insoluble material. When sulfur was the substrate 77% of the bacteria were so associated. This necessitated consideration of the complete growth system to obtain accurate growth curves. By using total bacterial nitrogen as the measure of growth, it was shown that T. ferrooxidans had a minimum generation time of 6.5 to 10 hours on a ferrous iron substrate, 7 to 8 days on a sulfur substrate, and 14 to 17 hours on a chalcopyrite substrate. The pH range for growth was dependent on the substrate used.