Development of new high-selective solvent extraction method for separation of rare earth metals by using aqueous complexing agent

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In this work, the selective separations of erbium(Er) and yttrium(Y) by both the solvent extraction and the supported liquid membrane were carried out with a complexing agent, diethylenetriaminepentaacetic acid(DTPA) in the aqueous phase. The purpose of this work is to elucidate the separation mechanism and to establish the experimental conditions in which the highest selectivity can be obtained.First, kinetics of the exchange reactions between rare earth metal ions and their DTPA complexes were studied in order to elucidate the dissociation mechanism of the DTPA complexes. The dissociation rate constants of the DTPA complexes are very important for the clarification of the separation mechanism of the solvent extraction and the liquid membrane containing DTPA in the aqueous phase. It was found that the obtained dissociation rate constant of Y-DTPA complex was about 5.8 times that of Er-DTPA complex.Further, the extractions of Er^<3+> and Y^<3+> from an aqueous phase containing DTPA were carried out. The selectivity obtained was about 5.5, which was much larger than that by the usual solvent extraction, 1.43. The selectivities for the various cases were studied in detail. The extraction rates and selectivities were quantitatively analyzed by taking account of the dissociation reaction of DTPA complexes in the aquous stagnant layer.Furthermore, by using the supported liquid membrane system with, DTPA in the aquous feed phase, the separation of Er^<3+> and Y^<3+> was investigated. This separation method has the feature that the highest selectivity can be obtained when the mass transfer accompaning the dissociation reaction of DTPA complexes in the aqueous stagnant layer is rate-determining. The experimental results were satisfactorily simulated.