Behaviour of Co(II) in solutions obtained by dissolution of cobalto¯cobaltic oxide in NH3_SO2_H2O medium

In an earlier study [Hydrometallurgy 58 (2000) 193], detailed kinetic work on dissolution of Co3O4 in an ammoniacal system using SO2 as the reductant was carried out. It was observed that under certain experimental conditions, Co(II) becomes unstable and precipitates as Co(NH3)4S2O6·2H2O. With a view to study the precipitation behavior of Co in the Co3O4¯NH3¯SO2 system, the present systematic work was done. A 23 factorial design was used to study the effects of three leaching parameters, i.e., NH3, SO2 and temperature on the dissolution and the precipitation of Co(II). The conditions for maximum Co precipitation were then chosen to study the effect of various parameters such as time and temperature of storage and initial cobalt concentration on extent of precipitation. It was observed that an increase in storage time and initial cobalt concentration increased the precipitation of Co(II). With a decrease in storage temperature, an increase in Co(II) precipitation was observed. In order to minimize the Co precipitation, the effect of addition of ammonium sulphate was studied. The increase in ammonium sulphate concentration from nil to 280 g L-1 resulted in a decrease of cobalt precipitation from 58% to 5%.