Selective precipitation of mixed nickel–cobalt hydroxide

The precipitation parameters of mixed hydroxide precipitate (MHP) using caustic calcined magnesia were evaluated at simulated industrial conditions using a mixed sulphate–chloride feed solution and fine magnesia powder. Controlling the rate of magnesia addition was found to decrease the manganese content of the precipitate but the benefit was minimal. Thermodynamic analysis combined with oxidation–reduction potential measurements suggests that some of the manganese is reporting to the solids by an oxidative precipitation reaction. The oxidative precipitation of manganese in air was confirmed experimentally. Contacting MHP with feed solution was found to substantially improve the solids' nickel and cobalt grade while decreasing the manganese and magnesium content. Precipitate contacting with the feed solution also resulted in a higher concentration of sulphate and chloride in the precipitate.

Research Highlights
► Cobalt precipitates prior to nickel contrary to thermodynamic predictions.
► Manganese precipitates prior to thermodynamic prediction for manganous hydroxide.
► De-aerated manganese precipitation occurred at significantly higher pH than aerated experiments.
► Precipitate feed contacting results in higher purity MHP.