Advanced liquid–liquid extraction systems for the separation of rare earth ions by combination of conversion of the metal species with chemical reaction

Abstract

Advanced liquid–liquid extraction combined with the conversion of the metal species with chemical reactions has been investigated. The complex formation reaction with water-soluble complexing reagent and the photochemical redox reaction for metal ions are focused. In the case of complex formation reaction, ethylenediaminetetraacetic acid (EDTA) was added to the aqueous phase of the extraction system. The separation of adjacent rare earth metals improves, due to the difference of the complex formation ability between metals and EDTA. The design of separation process of the metals with countercurrent mixer-settler cascade was also investigated with simulation based on the extraction equilibrium, determined up to high loadings of the extractant, and the material balance equations. In the case of photochemical redox reaction, the photoreductive stripping of Eu from Sm/Eu/Gd mixture and the photooxidative extraction of Ce from La/Ce/Pr mixture were carried out. The extractability of the target metals changes dramatically when their valences are changed, and then the effective separation can be achieved.