Rates of iron sulfide oxidation in coal spoil suspensions

Rates of pyritic coal spoil oxidation were studied in water suspensions of each of two particle-size fractions (< 0.5 mm and 0.5-2 mm). The treatments on each of the size fractions included liming with reagent grade CaCO₃ at rates of 0, 14.4, and 28.8 g kg^-1. In another treatment, toluene was added to unlimed suspensions to inhibit microbial growth. The greatest rate of salt formation (taken as a measure of pyrite oxidation) was associated with the larger size fraction. Toluene suppressed the oxidation-dissolution rate, suggesting a microbial role in pyrite oxidation in these systems. The application of CaCO₃ at 14.4 g kg^-1 spoil to the 0.5- to 2.0-mm size fraction enhanced the oxidation-dissolution process during the initial phase of the reaction (0-100 min). When the larger size fraction was amended with CaCO₃ at a rate of 28.8 g kg^-1, three rates of oxidation-dissolution were apparent. The first rate was attributed to dissolution of indigenous sulfate salts. The second rate was attributed to oxidation of a more reactive pyrite fraction than that described by the third rate (slowest rate). This last rate was attributed to a nonmicrobial oxidation pathway due to the higher pH values observed in the suspension.