Leaching of Cobalt from Gypsum Using Nickel Eluate

Gypsum is a waste product of the iron precipitation stage of cobalt purification in many cobalt extraction plants. Most cobalt producers either landfill the gypsum or sell it cheaply to cement manufacturers. The gypsum contains residual cobalt which can be extracted through sulphuric acid leaching but the cost of the sulphuric acid is prohibitive. Leaching using recycled acidic liquors like nickel eluate offers an economical alternative. This study investigated the technical feasibility of leaching gypsum with nickel eluate, which is an acidic waste stream from the nickel ion exchange plant at the specific cobalt producer. This study also established the optimum pH and solids concentration for nickel eluate leaching. Nickel eluate leaching tests and sulphuric acid leaching tests were conducted on a one-factor-at-a-time experimental design basis. pH and pulp densities were varied while temperature, time and agitation speed were kept constant. Cobalt recoveries from the two sets of tests were compared. In both cases the mean recoveries were ≥ 90%. Thus, on the basis of high cobalt recoveries, nickel eluate leaching of cobalt from gypsum is highly feasible despite a significant difference at the 5% level in cobalt recoveries from leaching with sulphuric acid. Optimum cobalt recoveries for both tests occurred at a pH of 0.5 and 50% solids. Nevertheless, it is advisable to leach at a pH of 1 and 30% solids to limit costs, corrosion of equipment and the toll on the agitators in the leaching tanks due to high pulp viscosity.