Selective leaching of nickel from mixed nickel cobalt hydroxide precipitate

A new method for acid leaching mixed nickel-cobalt hydroxide precipitate (MHP) is proposed; whereby nickel is largely dissolved while cobalt and manganese are stabilised in the solid phase by preferential oxidation. The relevant oxidation thermodynamics are presented. Metal extraction and leaching kinetics were not significantly affected by temperature over the range tested (30-80 C), or by the length of MHP ageing prior to leaching (three months vs. four years). Calcium and magnesium are the major solution impurities (0.1 g-Ca/L, 3 g-Mg/L). A portion of the calcium and magnesium can be selectively leached from the MHP at pH 7. Thermodynamic analysis indicates that the leaching of nickel and the onset of copper and zinc leaching can be predicted assuming MHP is an ideal solid solution. Both thermodynamic analysis and chemical assay confirm that the nickel hydroxyl-sulphate stoichiometry is approximately Ni 7(OH)12SO4. The Gibbs energy of formation of metastable Ni7(OH)12SO4 at 80 C (353 K) was estimated to be - 3356 kJ/mole