Extractants Used in Solvent Extraction-Separation of Rare Earths: Extraction Mechanism, Properties, and Features

Solvent extraction-separation is the main method to separate rare earths. The main extractants used in extraction-separation of rare earths are HEH(EHP), HDEHP, Cyanex 272, N235, N1923, and so on. This chapter systematically discusses their extraction mechanism, properties, and features of rare earths, including the effects of solvents, type of acids, acidities, concentrations of rare earths, and saponification on extraction-separation. Since the separation factors of adjacent rare-earth element are very small and change with the content changes of adjacent rare-earth element and other rare-earth elements, it is important to understand this feature and the basic knowledge of these extractants in extraction of rare earths for correctly designing the test works or processes of extraction-separation of rare earths. The stripping of heavy rare earths from the loaded organic phase, including selecting acid concentration and the stripping methods, is also discussed. The saponification of extractants, one of the important steps of extraction-separation of rare earths, including saponification mechanism and principle, is also discussed in detail. The last section discusses the extraction-separation of scandium from different solutions or leachates.