Purification of Nickel Sulfate by Batch Cooling Crystallization

Cooling crystallization was successfully carried out in a batch cooling crystallizer from impure acidic solutions to recover nickel as nickel sulfate salt with three different cooling rates. The compositions of the solutions included impurities of sodium, magnesium, chloride, and sulfuric acid to mimic industrial nickel electrowinning process solutions. The results show that NiSO4·6H2O mainly crystallized, and its solubility and purity decreased with the increase in the mass ratio of chloride to sulfate. Cooling rates did not have a significant influence on crystal purity; however, it did affect the filter cake moisture; lower moisture can be obtained with a lower cooling rate. The sequence of impurity removal efficiency from high to low was Cl > Na > Mg. Batch cooling crystallization is successfully applied to recover nickel as NiSO4 crystals from acidic multicomponent solutions that mimic electrowinning downstream process solutions. A high concentration of chloride reduces NiSO4 solubility and decreases the crystal purity. Magnesium persistently remains in the crystals and is harder to remove than the other two impurities: Na and Cl.