Aqueous reduction of cobalto-cobaltic oxides in ammoniacal medium using ammonium sulphite as the reductant

摘要

Studies have been carried out on aqueous reduction of cobalto-cobaltic oxide in ammoniacal medium using ammonium sulphite as the reductant. The various parameters studied include agitation speed, leaching time, ammonia and ammonium sulphite concentrations. The kinetics of dissolution follow a surface-controlled model. The order of reactions with respect to both ammonia and ammonium sulphite were found to be first order. The XRD patterns and TG-DTA traces of the original cobalto-cobaltic oxide and a typical leach residue were identical, showing that the reaction is surface-controlled and no preferential dissolution of Co(II) or Co(III) takes place. It was observed that on cooling and storing leach solutions obtained at 391 K using ammonia more concentrated than 4.5 and 0.23 M ammonium sulphite resulted in precipitation of a crystalline compound which showed distinct lines in the XRD pattern but did not correspond to any known compound of Co. The chemical analysis, TG-DTA and FTIR analysis suggest the compound to be Co(NH_3)_4S_2O_6 centre dot 2H_2O.