Iron(II) oxidation by SO2/O2 in acidic media: Part I. Kinetics and mechanism

The kinetics of iron(II) oxidation by SO2/O2 has been studied at 80°C as a means of generating Fe(III) and H2SO4 for subsequent leaching reactions. It is shown that both Fe(III) and S(IV) species are involved in the initial step of the oxidation. The rate of Fe(II) oxidation can be expressed by the following equation: r=kobs[Fe(III)][S(IV)]/fobs([H+]) for 0–0.02 M Fe(III) at the optimum SO2/O2 ratio, where fobs([H+]) is a function of pH. At 80°C, the optimum gas composition of SO2 is around 2% SO2 and both Fe(II) and SO2 were oxidised with the Fe(III)/H2SO4 ratio of about 2. At a SO2/O2 ratio above 9% SO2 and pH 1, dithionate was detected, and the proportion of dithionate to sulphate increased with higher SO2/O2 ratio. A radical chain reaction mechanism is proposed, involving a slow rate of formation of the ferric sulphite complex FeSO3+and decomposition to produce the sulphite radical SO3⋅−. This is followed by a fast reaction with O2 to form the peroxo-monosulphate radical SO5⋅−, which is responsible for the autoxidation of Fe(II). The form of the derived rate expression and the predicted ratio of Fe(III)/H2SO4 from the proposed mechanism essentially agree with the experimental results