OXIDATION RATE OF IRON SULFIDES AS AFFECTED BY SURFACE-AREA, MORPHOLOGY, OXYGEN CONCENTRATION, AND AUTOTROPHIC BACTERIA

A study was made to determine the relationship between surface and rate of oxidation of Fe sulfides (pyrite and marcasite) separated from Texas [USA] lignite. The reaction kinetics was evaluated with respect to Fe sulfide morphology and particle size. The O2 concentration and the presence of autotrophic Fe- and S-oxidizing bacteria (Thiobacillus ferrooxidans) on the rate of oxidation were also evaluated. The formation of sulfate from Fe sulfide was selected to measure the rate of oxidation. Relative reaction rates for different morphological forms of Fe sulfide were: marcasite > framboidal pyrite > massive pyrite. The rate of oxidation of marcasite was much higher than for pyrite at an equivalent surface area. As the surface area of pyrite doubled, reaction rate increased by a factor of 1.5. Sulfate production at optimum reaction conditions in this experiment for the 5-2 .mu.m fraction was twice that of the 50-20 .mu.m fraction. Reaction rate was .apprx. 5-fold greater for noninoculated treatments at 20% O2, compared with 0% O2, and was 9-fold greater for the same treatments inoculated with T. ferrooxidans.