The ferric leaching kinetics of arsenopyrite

In this investigation batch, ferric leaching experiments were carried out i n a 100 ml jacketed vessel maintained at 25 degrees C. The parameters varie d during the course of the experimental program included the initial redox potential, the total iron concentration, the solids concentration and the p H of the leaching solution. The initial redox potential used ranged from 62 5 to 470 mV, the overall iron concentration ranged from 8 to 32 g.l(-1), th e mineral concentration ranged from 5 to 20 g.l(-1) and the initial pH used ranged from 1.10 to 1.45. The redox potential of the leach solution was mo nitored continuously using a redox probe connected to a computer. The leach rates were calculated from the measured change in the redox potential of t he leaching solution. The variation in the ferric leaching rate of the arse nopyrite as a function of the solution redox potential displayed similar tr ends, irrespective of the conditions employed. The ferric leaching rate of the arsenopyrite decreased with decreasing redox potential of the leaching solution and could be accurately described using a modified Butler-Volmer e quation; -r(Fe2+) = r(0)(e(alpha beta(E-E'))-e((1-alpha(beta(E-E'))). High concentrations of ferric iron and protons, and a reduction in the solids co ncentration were found to impede the leach rate. The 'rest potential' (i.e. , the redox potential at which the dissolution of arsenopyrite stops) of th e arsenopyrite was found to be higher under these conditions. However, no o ccluding sulphur layer could be detected on the surface of mineral particle s, hence the results suggest that the reactivity of the mineral decreases w ith an increase in the effective concentration of the ferric iron species. Therefore, although the results suggest the likelihood of an electrochemica l mechanism being operative, it is necessary to modify the Butler-Volmer-ba sed model to account for the above observations in order to obtain a model capable of predicting the ferric leaching rate of arsenopyrite across a bro ad range of operating conditions. (C) 1999 Elsevier Science B.V. All rights reserved.