EQUILIBRIUM AND NONEQUILIBRIUM EXTRACTION SEPARATION OF RARE-EARTH-METALS IN PRESENCE OF DIETHYLENETRIAMINEPENTAACETIC ACID IN AQUEOUS-PHASE

Equilibrium and non-equilibrium extraction separations of rare earth m etals were carried out in the presence of chelating agent in the aqueo us phase. The separation systems of the rare earth metal mixtures used were Y/Dy, Y/Ho, Y/Er and Y/Tm, and the chelating agent and the extra ctant were diethylenetriaminepentaacetic acid (DTPA) and bis (2,4,4-tr imethylpentyl) phosphinic acid (CYANEXR(R) 272), respectively. For Y/D y and Y/Ho systems, higher selectivities were obtained in equilibrium separation compared with those in non-equilibrium separation. On the o ther hand, the selectivities in non-equilibrium separation were higher for Y/Er and Y/Tm systems. In the separation condition suitable to ea ch system, the addition of DTPA to the aqueous phase was found to be v ery effective for obtaining higher selectivities. The distribution rat ios of the rare earth metals and the selectivities in the equilibrium separations obtained experimentally were thoroughly analyzed by consid ering various equilibria such as the extraction equilibrium and the co mplex formation equilibrium between rare earth metals and DTPA in the aqueous phase. Moreover, the extraction rates and the selectivities in the non-equilibrium separations were also analyzed by the extraction model considering the dissociation reactions of the rare earth metal-D TPA complexes in the aqueous stagnant layer. Based on these analyses, we presented an index which is useful for selecting the optimum operat ion mode, that is, for determining whether equilibrium or non-equilibr ium separation should be employed. Using this index, we can predict th at the selectivities under equilibrium conditions are higher than thos e under non-equilibrium conditions for Y/Dy and Y/Ho systems, while fo r Y/Er and Y/Tm systems, higher selectivities are obtained under non-e quilibrium conditions. The experimental results were in agreement with predictions by this index. Further, the selectivities in various syst ems including other chelating agents and extractants were discussed ba sed on this index.