Crystallization and solubility of KSc(SO) for improving the efficiency of scandium extraction

In the development of methods for the efficient extraction and concentration of rare earth elements (REE), as well as for the removal of impurities of related metals, the solubility of salts, in particular sulfates, often used in hydrometallurgy as intermediates, is of great importance. In this work, the precipitation of complex potassium scandium sulfate KSc(SO), whose crystals are elongated hexagonal prisms 5-10 µm wide and 20-50 µm long, was proposed for the extraction of scandium from sulfate solutions. The absence of crystallization water and the presence of a reversible phase transition around 447°C are shown by DTA method. The solubility of KSc(SO) in water at 25°C was 0.28±0.01 wt.% Sc. A decrease in the solubility of KSc(SO) was achieved by increasing the concentration of sulfuric acid (HSO) over 3-4 mol/L. Additional introduction of 0.5 mol/L KSO reduces the solubility of scandium as a complex compound by an order of magnitude and increases the efficiency of scandium extraction from sulfate solutions. Experimental results on the solubility of KSc(SO) are described by the change in the ionic strength of the solution in the presence of homonymous ions (K and HSO). The degree of scandium extraction from sulfate solutions with the addition of 0.5 mol/L KSO is more than 99%. The principal technological scheme of scandium extraction from red slimes with crystallization of KSc(SO) is proposed. The results will be useful for the development of methods of metal separation at sulfuric acid processing of raw materials and expansion of methods for obtaining concentrates and pure metal oxides, as well as for studying the behavior of REE compounds close in properties.