The kinetics of iron sulfide oxidation in lignite overburden as influenced by calcium carbonate

Abstract

Surface mine reclamation in Texas, is often hindered by the formation of acid minesoil. The acidity is a product of FeS2 oxidation. Mixed overburden topsoil substitutes containing FeS2, are often limed to prevent acid minesoil formation. The main objective of this study was to measure the effects of liming rates on the kinetics of FeS2 oxidation in overburden. To accomplish this objective, two overburden materials with different FeS2 content (1.9 and 4.1%) and low acid neutralization potential were limed with CaCO3. Lime rates of 0, 25, 50, 75, 100 and 125% were based on the amount of CaCO3 needed to provide an acid/base account (A/B[a]) of zero (A/B[a] = acid neutralization potential--Potential acidity--exchangeable acidity). The limed overburdens were inoculated with Thiobacillus ferrooxidans and leached weekly with deionized water for 53 weeks. Iron sulfide oxidation followed two different rate laws depending on the pH of the system. The oxidation followed zero-order kinetics with respect to FeS2 concentration at pH value above 4 and first-order kinetics below 4. The rate of oxidation was also found to differ with FeS2 source. The zero-order oxidation rate ranged from 0.01 to 0.46 μmol g^-1d^-1 in overburden 2 and from 0.01 to 0.22 μmol g^-1d^-1 in overburden 4. Oxidation following the first-order rate law had a first-order rate constant of 0.03 d^-1 in overburden 2 and 0.01 d^-1 in overburden 4. The calculated half-life is 23 and 69 d, respectively. Additions of CaCO3 affected FeS2 oxidation by controlling the pH of the system. The higher the pH the slower the oxidation. Liming to 25% of the A/B[a] deficit maintained the pH above 4 for approximately 100 d in overburden 4. At that time, oxidation changed from zero- to first-order. The addition of lime did not affect the subsequent half-life of FeS2 after the pH decreased below 4. Liming to greater than 50% A/B[a] deficit did not significantly affect the zero-order oxidation rate. The dissolution of the applied CaCO3 was found to be faster than the release of potential acidity. It is projected that the lime would dissolve out of the system before all the FeS2 would oxidize leaving the potential for acid minesoil formation.