Kinetics and Interfacial Phenomena in Solvent Extraction of Metals

Abstract

Kinetic and interfacial phenomena in classical metal extraction systems and in micellar media are discussed. The work is limited to the processes controlled by kinetics. Phenomena affecting the rate of extraction are examined and appropriate examples of acceleration or inhibition of metal extraction with hydrophobic chelating extractants are given. The effects of surfactants on the rate of extraction and complexation in micellar media are reviewed. The effect of extractant hydrophobicity upon their interfacial behaviour and the rate of extraction in classical and non classical extraction systems is considered. Appropriate kinetic models for interfacial and bulk volume reactions are presented. The results suggest that the true mechanism of classical and non classical solvent extraction may include both of these pathways. Although important differences exist between classical extraction systems and micellar systems, their kinetic behaviour indicates important similarities. It is concluded that further studies of the micellar systems, with the use of a broad range of extractants, may yield useful information for prediction of the structural effects of chelates upon the kinetics of metal extraction. The aqueous micellar media appear to be a useful tool for obtaining equilibrium and mechanistic information for reaction in water in cases where direct determinations are precluded by experimental difficulties, such as those caused by a low solubility of reactants or a very high reaction rate Micellar systems alone can also have important applications in the extraction of slowly extractable species and for separation of toxic low level metal contaminants. Appropriate examples are given for the recovery and separation of metals with the use of hydrophobic complexing agents in micellar media and for separation by ultrafiltration.