Pyrrhotite reaction kinetics: reaction rates for oxidation by oxygen, ferric iron, and for nonoxidative dissolution

Abstract

The oxidation kinetics of 12 well-characterised pyrrhotite samples by oxygen and ferric iron and the nonoxidative dissolution in acidic solution were evaluated. The samples, obtained from various North American localities, were characterised for crystal structure, specific surface area, trace metal, and accessory mineral content. The crystal structure varied from pure hexagonal to pure monoclinic. The specific surface area of the samples also varied considerably and was unrelated to crystal structure. Specific surface area values calculated from a regular geometry provided poor estimates for BET measured specific surface areas. Chalcopyrite was the most dominant accessory mineral. The mean oxidation rate of pyrrhotite samples by oxygen was 4 × 10 −9 ± 6 × 10 −10 mol · m −2 · s −1. Oxidation by ferric iron was faster; at an initial Fe +3 concentration of 2 × 10 −4 mol/L and pH 2.75, the mean oxidation rate of the samples by ferric iron was 3.5 × 10 −8 ± 1.5 × 10 −9 mol · m −2 · s −1. The mean nonoxidative dissolution rate was 5 × 10 −10 ± 9 × 10 −11 mol · m −2 · s −1 in solution with pH 2.75. Oxidation reactions of pyrrhotite by either ferric iron or oxygen resulted in incomplete oxidation of the sulfide in pyrrhotite. On the basis of iron release, the activation energies for pyrrhotite oxidation by oxygen and ferric iron ranged from 47 to 63 kJ/mol. Neither pyrrhotite crystal structure nor trace metal content had a consistent or systematic effect on pyrrhotite oxidation rates.