Magnesium removal from pregnant solution of acidic leaching of zinc oxidised ore

In the process of zinc hydrometallurgy, many impurities such as nickel, cobalt, and cadmium, are removed, but magnesium remains with zinc in the electrowinning stage and its concentration increases during the process. The aim of this study is to reduce the concentration of magnesium in zinc leaching solution. XRD analysis showed that the prepared zinc oxidised sample contains smithsonite and hemimorphite as valuable minerals and dolomite and calcite as tailings. The sample contains 8.27% zinc and 8.15% magnesium. Here, alkaline zinc sulfate precipitation was carried out to bring magnesium to the permissible level in electrowinning solution. In acidic leaching, effective factors such as pH, temperature, solid percent and leaching time optimized to achieve maximum zinc recovery. In alkaline zinc sulfate precipitation, effective factors such as pH, temperature and retention time examined for selective precipitation of alkaline zinc sulfate. Recovery of zinc in one leaching step reached to 70.81% at pH=2, temperature of 60°C, solid percent of 10% and leaching time of 120 minutes. The leach residue was then dissolved at pH=3 for 60 minutes, in which the total recovery of zinc increased to 86.32%. Under optimal conditions of 90°C, pH =6.5-7, zinc recovery was found to be 99.90% in the precipitate, while the amount of magnesium precipitation was 3.6%. After re-dissolution of alkaline zinc sulfate, the amount of magnesium in the solution found to be 1.3 g/l, which was much less than the allowable magnesium level in solution used for electrowinning (10-15 g/l).