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

The oxidation kinetics of 12 well-characterised pyrrhotite samples by oxyge n and ferric iron and the nonoxidative dissolution in acidic solution were evaluated. The samples, obtained from various North American localities, we re characterised for crystal structure, specific surface area, trace metal, and accessory mineral content. The crystal structure varied from pure hexa gonal to pure monoclinic. The specific surface area of the samples also var ied considerably and was unrelated to crystal structure. Specific surface a rea values calculated from a regular geometry provided poor estimates for B ET measured specific surface areas. Chalcopyrite was the most dominant acce ssory mineral. The mean oxidation rate of pyrrhotite samples by oxygen was 4 x 10(-9) +/- 6 x 10(-10) mol . m(-2) . s(-1). Oxidation by ferric iron wa s faster; at an initial Fe+3 concentration of 2 x 10(-4) mol/L and pH 2.75, the mean oxidation rate of the samples by ferric iron was 3.5 x 10(-8) +/- 1.5 x 10(-9) mol . m(-2) . s(-1). The mean nonoxidative dissolution rate w as 5 x 10(-10) +/- 9 x 10(-11) mol . m(-2) . s(-1) in solution with pH 2.75 . Oxidation reactions of pyrrhotite by either ferric iron or oxygen resulte d in incomplete oxidation of the sulfide in pyrrhotite. On the basis of iro n release, the activation energies for pyrrhotite oxidation by oxygen and f erric iron ranged from 47 to 63 kJ/mol. Neither pyrrhotite crystal structur e nor trace metal content had a consistent or systematic effect on pyrrhoti te oxidation rates. Copyright (C) 2000 Elsevier Science Ltd.