CHAPTER 4 - SOLVENT EXTRACTION

Abstract

This chapter discusses the process of solvent extraction for purification of metals. Most of the rare metals are appreciably soluble in both aqueous media and organic solvents. The conditions for high solubility in an organic solvent are often fairly specific to an element, compared with the metal ion impurities with which it may be associated. Consequently, the process of solvent extraction, involving a transfer of a rare metal species from an aqueous to an organic solvent phase, can be used for purification in a relatively simple manner. Multi-stage processes are generally necessary if it is desired to extract a high proportion of the solute into the solvent phase, and further complications arise when impurities also have appreciable solubilities in the solvent. The property of solubility of an inorganic salt in an organic solvent is usually associated with a low degree of ionization in the solvent phase, and the non-ionized species is capable of forming addition compounds with the solvent. These are electrically neutral co-ordination compounds and they are soluble in the solvent. The compounds formed with the complexing agent must also be soluble in the diluent.