The ferric leaching kinetics of arsenopyrite

Abstract

In this investigation batch, ferric leaching experiments were carried out in a 100 mℓ jacketed vessel maintained at 25°C. The parameters varied during the course of the experimental program included the initial redox potential, the total iron concentration, the solids concentration and the pH of the leaching solution. The initial redox potential used ranged from 625 to 470 mV, the overall iron concentration ranged from 8 to 32 g.ℓ −1, the 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 monitored continuously using a redox probe connected to a computer. The leach rates were calculated from the measured change in the redox potential of the leaching solution. The variation in the ferric leaching rate of the arsenopyrite as a function of the solution redox potential displayed similar trends, 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 equation; − r Fe 2+ = r 0(e αβ( E− E′) −e (1− α( β( E− E′) ). High concentrations of ferric iron and protons, and a reduction in the solids concentration were found to impede the leach rate. The `rest potential' ( i.e., the redox potential at which the dissolution of arsenopyrite stops) of the arsenopyrite was found to be higher under these conditions. However, no occluding sulphur layer could be detected on the surface of mineral particles, hence the results suggest that the reactivity of the mineral decreases with an increase in the effective concentration of the ferric iron species. Therefore, although the results suggest the likelihood of an electrochemical mechanism being operative, it is necessary to modify the Butler–Volmer-based model to account for the above observations in order to obtain a model capable of predicting the ferric leaching rate of arsenopyrite across a broad range of operating conditions.