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Kinetics of the Removal of Iron Pyrite from Coal by Microbial Catalysis

Different strains of Thiobacillus ferrooxidans and Thiobacillus thiooxidans were used to catalyze the oxidative dissolution of iron pyrite, FeS2, in nine different coal samples. Kinetic variables and parametric factors that were determined to have a pronounced effect on the rate and extent of oxidative dissolution at a fixed Po2 were: the bacterial strain, the nitrogen/phosphorus molar ratio, the partial pressure of CO2, the coal source, and the total reactive surface area of FeS2. The overall rate of leaching, which exhibited a first-order dependence on the total surface area of FeS2, was analyzed mathematically in terms of the sum of a biochemical rate, ν1, and a chemical rate, ν2. Results of this study show that bacterial desulfurization (90 to 98%) of coal samples which are relatively high in pyritic sulfur can be achieved within a time-frame of 8 to 12 days when pulp densities are ≤20% and particle sizes are ≤74 μm. The most effective strains of T. ferrooxidans were those that were isolated from natural systems, and T. ferrooxidans ATCC 19859 was the most effective pure strain. The most effective nutrient media contained relatively low phosphate concentrations, with an optimal N/P molar ratio of 90:1. These results suggest that minimal nutrient additions may be required for a commercial desulfurization process.