The kinetics of the oxidation of pyrite by ferric ions and dissolved oxygen: An electrochemical study

The dissolution of pyrite is important in the geochemical cycling of iron a nd sulphur, in the formation of acid mine drainage, and in the extraction o f metals by bacterial leaching. Many researchers have studied the kinetics of dissolution, and the rate of dissolution has often been found to be half -order in ferric ions or oxygen. Previous work has not adequately explained the kinetics of dissolution of pyrite. The dissolution of pyrite is an oxi dation-reduction reaction. The kinetics of the oxidation and reduction half -reactions was studied independently using electrochemical techniques of vo ltammetry. The kinetics of the overall reaction was studied by the electroc hemical technique of potentiometry, which consisted of measuring the mixed potential of a sample of corroding pyrite in solutions of different composi tions. The kinetics of the half reactions are related to the kinetics of th e overall dissolution reaction by the condition that there is no accumulati on of charge. This principle is used to derive expressions for the mixed po tential and the rate of dissolution, which successfully describe the mixed potential measurements and the kinetics of dissolution reported in the lite rature. It is shown that the observations of half-order kinetics and that t he oxygen in the sulphate product arises from water are both a direct conse quence of the electrochemical mechanism. Thus it is concluded that the elec trochemical reaction steps occurring at the mineral-solution interface cont rol the rate of dissolution. Raman spectroscopy was used to analyze reactio n products formed on the pyrite surface. The results indicated that small a mounts of polysulphides form on the surface of the pyrite. However, it was also found that the mixed (corrosion) potential does not change over a 14-d ay leaching period. This indicates that even though polysulphide material i s present on the surface, it does not influence the rate of the reactions o ccurring at the surface. Measurements of the sulphur yields as a function o f electrode potential indicate that thiosulphate is not the only source of the sulphur product. Copyright (C) 2000 Elsevier Science Ltd.