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 in fact reduces Co(III). However, SO2 is only partially utilised, and much of it is entrained unreacted in the leach liquor. A large accumulation of SO2 can become uncontrollable and therefore escape to the environment if not consumed by the ore. 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 entrained in the leach liquor, 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 unreacted SO2 and significantly increasing recovered metals yields with little acid consumption. Moreover, the injection of Na2S2O5 solution at 75% of the column bed-height considerably reduced the potential risks of SO2 emanation.