Agitation and column leaching studies of oxidised copper-cobalt ores under reducing conditions

Sulphuric acid agitation and column leaching tests of an oxidised copper-cobalt-bearing ore (1.44% Cu and 1.04% Co) were performed to investigate its amenability to copper and cobalt metals extraction under reducing conditions. Oxidised copper-cobalt-bearing ores from the Congolese Copperbelt contain a large proportion of cobalt in trivalent form, which is readily soluble in solutions of sulphuric acid, provided a reducing agent is also present. Sodium metabisulphite (Na2S2O5) is one of the most commonly recommended reducing agents, but its addition during the leach phase could cause significant environmental problems and could affect copper recovery if present in large amounts. During leaching, sodium metabisulphite reacts with sulphuric acid and dissociates to form SO2 which reduces Co(III). However, SO2 is only partially utilised, and much is entrained unreacted in the leach liquor. The paper presents and discusses the effect of recirculation of the leach liquor after pH readjustment to 1.5 on the minimisation of unreacted SO2, sulphuric acid consumption, and the overall improvement of copper and cobalt extraction yields. The most important information found is that recirculation of the leach liquor proved to be very useful in reducing the amount of unreacted SO2 and significantly increasing recovered metals yields with little acid consumption.