Iron(II) oxidation by SO 2/O 2 in acidic media: : Part I. Kinetics and mechanism

Abstract

The kinetics of iron(II) oxidation by SO 2/O 2 has been studied at 80°C as a means of generating Fe(III) and H 2SO 4 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= k obs[Fe(III)][S(IV)]/ f obs([H +]) for 0–0.02 M Fe(III) at the optimum SO 2/O 2 ratio, where f obs([H +]) is a function of pH. At 80°C, the optimum gas composition of SO 2 is around 2% SO 2 and both Fe(II) and SO 2 were oxidised with the Fe(III)/H 2SO 4 ratio of about 2. At a SO 2/O 2 ratio above 9% SO 2 and pH 1, dithionate was detected, and the proportion of dithionate to sulphate increased with higher SO 2/O 2 ratio. A radical chain reaction mechanism is proposed, involving a slow rate of formation of the ferric sulphite complex FeSO 3 +and decomposition to produce the sulphite radical SO 3 ⋅−. This is followed by a fast reaction with O 2 to form the peroxo-monosulphate radical SO 5 ⋅−, which is responsible for the autoxidation of Fe(II). The form of the derived rate expression and the predicted ratio of Fe(III)/H 2SO 4 from the proposed mechanism essentially agree with the experimental results.