Generally in the sulphate system, oxidative leaching of metal sulphides is not conducive to selectivity, due to the unselective oxidation of sulphide to elemental sulphur or to sulphate. However, in the chloride system, the ability to control the redox potential at a lower level on account of the stability of the cupric / cuprous ion species, permits the selective dissolution with respect to copper, of metals such as Pb, Ni, Zn, Fe by means of cupric chloride, according to the metathesis reaction, provided that the sulphide mineral is thermodynamically unstable under the conditions of leaching.
In the case of easily oxidisable sulphides, such as the components of copper matte or leadsulphide, under the oxidation potential conditions, at which cuprous sulphide is converted to cupric sulphide, according to the reaction:CUS + CuCl2 → CuS + 2 CuCl, selectivity of leaching of metals (Me) in respect to copper can be achieved, according to the simplified metathesis reaction MeS + CuCl2 → MeCl2 + CuS
Thereby, the cupric/cuprous chloride leach solution can be regenerated from the covellite formed by oxidation with CI2 or HC1 and O2.
In the case of a matte, the selective leaching and regeneration occur simultaneously, according to the overall reaction: Cu2S.MeS + Cl2 → 2CuS + MeCl2
In the case of less easily oxidisable sulphide minerals, selectivity of leaching of metals with regard to copper can still be achieved by the metathesis reaction at elevated temperature. The oxidation of sulphide to sulphate is prevented by maintaining a high cuprous to cupric ratio in the chloride solution. An industrial application is the Falconbridge chlorine leach process for copper-nickel matte. Based on investigations, potential applications were identified in the field of complex or bulk metal sulphide concentrate treatment, copper-lead matte processing and also chemical converting.
The application of selectivitiy principle in sulphide leaching provides environmental benefits, since the sulphide itself is used for separation and the sulphur is brought in an environmental compatible form.