Enhanced selectivity of hydrolytic precipitation of Zn from Zn-Ni sulfate solution via chelation of Ni

The selective precipitation of zinc from zinc–nickel sulfate solution with the Zn/Ni molar ratio of 20:1 was studied. Dropwise addition of 0.5 mol/L NaOH solution into the zinc–nickel sulfate solution containing 0, 0.01, 0.02, 0.03 and 0.04 mol/L ethylene diamine tetraacetate (EDTA) as a chelating agent was done. The equilibrium analysis of precipitation pathway was performed using Visual MINTEQ program. The equilibrium analysis showed that the presence of small amounts of EDTA can prevent nickel precipitation in alkaline conditions without any negative effect on zinc precipitation. On this basis, more than 90% of zinc could be precipitated as a product with about 50% Zn and only 0.11% Ni at pH=9.0 merely as a result of the presence of 0.03 mol/L EDTA in the solution. The stirring time of 120 min after precipitation was found to be essential for more complete separation. The X-ray diffraction studies on the precipitate revealed that the precipitated phase was Zn₄(OH)₆SO₄.4H₂O.