Separation of rare earth metal elements by electromigration in melts

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In order to clucidate the mechanism of electric conduction in molten salts, the internal mobilities of rare carth chloride melts have been measured by a countercurrent electromigration method. If there is an appreciable difference in the mobilities among the rare earth elements, countercurrent electromigration may be used for separation of these elements. Then, this method is expected to be useful particularly for reprocessing of the nuclear fuels. We have performed the following experiments.In the molten mixture system (Y, La)Cl_3 relative differences in internal cation mobilities have been measured at 1073 K by a countercurrent electromigration method. The electric conductivities of the mixture system have been measured. From these data the internal cation mobilities b of Y^<3+> and La^<3+> have been calculated. At all the concentrations La^<3+> is more mobile than Y^<3+> by 20-30 %. This is presumably because the interaction for Y-Cl pair is stronger than that for La-Cl pair. Thus, … More this method is applicable for separation of these ions. while the ionic radius of Y^<3+> (0.900 A) is distinctively smaller than that of La^<3+> (1.032 A), the molar volume of pure LaCl_3 is slightly smaller than that of pure molten YCl_3. This suggests that the structure of these melts is considerably different. In the mixture system, both b_Y and b_<La> decrease with increasing molar volume, the trend of which is the same as that in other systems such as monovalent and divalent cation systems.By a similar method b's in the mixture system (Y, Dy)Cl_3 have been measured. In this case the ionic radii of the two cations are nearly the same, that is 0.900 A for Y^<3+> and 0.912 A for Dy^<3+>. b_Y is appreciably greater than b_<Dy>. Thus, it can be conjectured that, when the ionic radii are nearly the same, the difference in the masses (Y : 88.9059 and Dy : 162.50) plays a role.The isotope effect in b of Dy^<3+> has veen studied in pure DyCl_3 also by countercurrent electromigration. The mass numbers of the main stable Dy isotopes are 160(2.34 %), 161(18.9 %), 162(25.5 %), 163(24.9 %) and 164(28.2 %). The value of the mass effect defined as mu=( b/b)/( M/M) is about -0.030 nearly independent of temperature in the range 973 K-1043 K, whose absolute value is relatively large.In conclusion, electrically-conducting species in rare earth chlorides are conjectured to be monoatomic ions and countercurrent electromigration method in molten systems can be used for separation of rare earth metals. Less