Influence of anion nature on acid leaching of silicate minerals and solvent extraction of rare and rare-earth elements

Abstract

The «hardness» (η) of an inorganic ligand that behaves as a Pearson base depends on the electronegativity of the central atom of a ligand and increases in the following order: NO3- (η = 4.7) < Cl- (η = 4.9) < SO42- (η = 5.5) < F- (η = 7.0). As the «hardness» of an inorganic anion-ligand increases, the mole fraction of the metal anion complex [MeAnx](z−nx)- – a Pearson acid – increases. In the presence of «hard» Pearson bases, the solubility of silicate minerals in acidic media increases due to the formation of soluble complexes and the breakdown of the gel structure of silicon oxide. Due to the increasing «hardness» of the anion, which behaves as a Pearson base, the effective Gibbs energy of the formation of complexes with oxygen-containing organic reagents increases, as shown through the example of extraction equilibria. On the one hand, this behaviour decreases the extraction of metals from aqueous solutions; on the other hand, it contributes to the growth of the separation factor.