Leaching behaviour of sulphides in ammoniacal thiosulphate systems

Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite>pyrrhotite>arsenopyrite>pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite dissolution.