The role of non-oxidative processes in the leaching of chalcopyrite

It has previously been assumed that the dissolution of chalcopyrite in acidic ferric solutions occurs by a typical oxidative process which can be described by the mixed potential electrochemical model. However, it has been demonstrated in this study that a parallel non-oxidative dissolution process must also be taken into account in the description of the kinetics. This has been based on both detailed ring-disk electrochemical studies and also by direct measurement of the rates of dissolution as measured by the iron and copper in solution. Thus, the direct oxidation path is characterized by the well known rapid initial dissolution rate which rapidly decays to a slow rate at longer times. This has been shown to be the case for potentials within the so-called passive region in the anodic behaviour of chalcopyrite and is typical of the situation with ferric ions as the oxidant under ambient conditions. The effect of potential on the rate of the oxidative process has been found to be relatively small in the potential region 0.5 to 0.75V with significantly increased rates only in the transpassive region above 0.75V. The effect of potential on the rate of the non-oxidative process has also been studied at potentials below those in the oxidative region and there appears to be no noticeable effect of potential on the rate in this region. Rotating ring-disk experiments have been used to confirm the simultaneous production of copper ions, ferrous ions and hydrogen sulfide during the dissolution in sulfuric acid solutions. Furthermore, it is suggested, without direct experimental evidence, that the non-oxidative reaction can be sustained by oxidation of the hydrogen sulfide by ferric ions. This model for the oxidative process will be described and quantitative simulation of this scheme has produced results which are remarkably consistent with the observed rates of dissolution. The practical implications of these findings to the leaching of chalcopyrite will also be discussed