Pyrite oxidation in alkaline solutions: The carbonate effect

An investigation of pyrite oxidation in alkaline solutions was carried out with the aid of thermodynamic modeling, kinetic experiments and infrared spectroscopy. In the presence of carbonate, Eh-pH diagrams show that soluble iron-carbonate complexes (FeCO₃⁰, FeOHCO₃ ^- and FeCO₃^2-) coexisting with pyrite may be formed at a pH range of 6-12. Above pH 11 and under oxidizing conditions, only Fe(OH)₄^- is stable, even in the presence of carbonate/bicarbonate ions. In addition to iron oxyhydroxides and soluble sulfate species, iron carbonate compounds were identified as one of the products of pyrite oxidation with the help of diffuse reflectance infrared spectroscopy. The conditions under which thermodynamic and IR analyses indicate the presence of carbonate compounds also conespond to those in which the fastest rate of pyrite oxidation in carbonate solutions was found. By expanding the Singer-Stumm model for pyrite oxidation to alkaline solutions, the oxidation of Fe(II) may be considered the rate-limiting step. We suggest that the formation of Fe(II)-CO₃ soluble complexes favors the oxidation of iron, which in turn favors the overall kinetics of pyrite oxidation in carbonate solutions. The electron transfer to O₂ is facilitated by the formation of a Fe(II)-CO₃ complex at the pyrite surface or in solution.