A review of the structure, and fundamental mechanisms and kinetics of the leaching of chalcopyrite

Most investigators regard CuFeS2 as having the formal oxidation states of Cu+Fe3+(S2−)2. However, the spectroscopic characterisation of chalcopyrite is clearly influenced by the considerable degree of covalency between S and both Fe and Cu. The poor cleavage of CuFeS2 results in conchoidal surfaces. Reconstruction of the fractured surfaces to form, from what was previously bulk S2−, a mixture of surface S2−, S22 and Sn2− (or metal deficient sulfide) takes place. Oxidation of chalcopyrite in air (i.e. 0.2atm of O2 equilibrated with atmospheric water vapour) results in a Fe(III)–O–OH surface layer on top of a Cu rich sulfide layer overlying the bulk chalcopyrite with the formation of Cu(II) and Fe(III) sulfate, and Cu(I)–O on prolonged oxidation. Cu2O and Cu2S-like species have also been proposed to form on exposure of chalcopyrite to air. S22−, Sn2− and S0 form on the chalcopyrite surface upon aqueous leaching. The latter two of these species along with a jarosite-like species are frequently proposed to result in surface leaching passivation. However, some investigators have reported the formation of S0 sufficiently porous to allow ion transportation to and from the chalcopyrite surface. Moreover, under some conditions both Sn2− and S0 were observed to increase in surface concentration for the duration of the leach with no resulting passivation. The effect of a number of oxidants, e.g. O2, H2O2, Cu2+, Cr6+ and Fe3+, has been examined. However, this is often accompanied by poor control of leach parameters, principally pH and Eh. Nevertheless, there is general agreement in the literature that chalcopyrite leaching is significantly affected by solution redox potential with an optimum Eh range suggesting the participation of leach steps that involve both oxidation and reduction. Three kinetic models have generally been suggested by researchers to be applicable: diffusion, chemical reaction and a mixed model containing diffusion and chemical components which occur at different stages of leaching. Passivation effects, due to surface diffusion rate control, may be affected by leach conditions such as pH or Eh. However, only initial conditions are generally described and these parameters are not controlled in most studies. However, at fixed pH, Eh and temperature, it appears most likely that leaching in sulfuric acid media in the presence of added Fe3+ is surface reaction rate controlled with some initial period, depending on leach conditions, where the leach rate