Base metals recovery from copper smelter slag by oxidising leaching and solvent extraction

Abstract

Due to its amorphous structure, smelter slag cannot efficiently be leached with sulphuric acid; the formation of silica gel induces an increase of leach liquor viscosity, difficult pulp filtration and crud formation during solvent extraction. The problem was solved by leaching with sulphuric acid under hydrogen peroxide, which also performs simultaneous iron oxidation and removal. A copper smelter slag from Lubumbashi, Democratic Republic of Congo, containing 1.4% Cu, 0.7% Co, 8.9% Zn and 20.9% Fe(II), has been used in this study. The leaching tests have been carried out at normal pressure on ground slag samples (<100 μm). Silica gel-free solutions containing copper, cobalt and zinc were produced, so that pulp filtration could be easily carried out. The dissolved base metals were successfully extracted from solution by solvent extraction using kerosene Shellsol D70 as diluent. Copper was extracted with LIX 984 and stripped with sulphuric acid solution. Thereafter, cobalt and zinc were collectively extracted with D2EHPA and then separated by selective scrubbing with sulphuric acid solutions of different dilutions. This method provided an overall recovery of 80% Cu, 90% Co and 90% Zn in separated solutions which could be further treated by electrowinning or salt precipitation.