Solubility of Rare Earth Salts in Sulphate-Phosphate Solutions of Hydrometallurgical Relevance

Abstract

The solubility of rare earth (RE) salts in aqueous solutions depends on many factors including the type of cations and anions in solution, their concentrations , temperature and the type of solid precipitated. Experiments were conducted using anhydrous La 2 (SO 4) 3 and Ce 2 (SO 4) 3 ⋅8H 2 O as solids and lanthanum(III) perchlorate prepared by dissolving La 2 O 3 in HClO 4 solutions as REsources. Solutions of H 2 SO 4 , H 3 PO 4 and their mixtures in the presence or absence of Na + , Ca 2+ , Fe(III) or Al(III) as perchlorates served as test solutions. Saturated solutions and precipitated solids were assayed and characterized using standard techniques to show that the La(III) and Ce(III) ions precipitate as hydrated sulphates, phosphates or double salts with sodium, depending upon the solution composition. The change in solubility of RE-salts with solution composition and temperature is the result of the composite effect of changes in solubility products (pK SP) of salts, pK a of acids and other equilibrium constants for ion-association.