Fluoride based control of Ca and Mg concentrations in high ionic strength base metal sulphate solutions in hydrometallurgical circuits

Abstract

The presence of calcium (Ca) and magnesium (Mg) in the hydrometallurgical processing of base metals results in a number of difficulties. These problems range from the contamination of the final product, to high energy consumption and large bleed streams during electrowinning. Calcium poses a greater problem in sulphate solutions due to the low solubility of its sulphate salts. In the first part of this study, fluoride precipitation of calcium and magnesium is considered for investigation, using a batch precipitation reactor. The batch precipitation reaction investigated the effects of the quantity of fluoride, molar ratio of Ca to Mg, identity of the soluble fluoride and temperature. The possibility of controlling Ca and Mg through the precipitation of their respective fluorides, without the co-precipitation of nickel and cobalt was shown. At Mg to Ca molar ratio above 3, the precipitation of Mg dominated over that of Ca due to the former’s high concentration in the reaction vessel and therefore, its high activity. For a 10% stoichiometric excess amount of fluoride, 96.5% Ca and 98.5% Mg were removed. When the identity of soluble fluoride was considered, results demonstrated that NH4F solution caused a higher Ca and Mg precipitation compared with HF. This was found to be due to the high stability of the HF complex in solution when using HF as the fluoride source, resulting in a lower free fluoride activity in the solution. For the range investigated (45–65 C) temperature was found to have a negligible effect on the precipitation of the alkaline earth metals. The supersaturation ratio for CaF2 was 73 and 27 for MgF2 and it was found that while primary nucleation did not dominate, it could not be ruled out completely. A conceptual process is proposed whereby only a portion of the leach stream is subjected to the fluoride precipitation process, after which it is returned to lower the overall Ca and Mg concentrations. This method is expected to reduce the effect of the observed dominance of Mg precipitation in processes not requiring the maximum removal of both Ca and Mg. This study proposes the integration into the production line of a hydrometallurgical industry the three main steps of fluoride precipitation process and recovery; precipitation, solid–liquid separation and adsorption–desorption cycle of the unreacted fluoride.