Electrochemical behavior and separation of dysprosium from molten NaCl-KCl eutectic by co-reduction with aluminum

Abstract

To electrolytically separate dysprosium from spent fuel, the electrochemical behavior of Dy3+ and its co-reduction process with Al3+ on inert Mo electrode in molten NaCl-KCl eutectic was systematically investigated employing a series of electrochemical methods. Dy3+ ions were directly reduced to dysprosium metal on inert Mo electrode through one step with three electrons exchanged. The reduction of Dy3+ was considered to be a quasi-reversible process, and the deposition of dysprosium on Mo follows the progressive nucleation mode. However, the co-reduction of Dy3+ with Al3+ on the Mo electrode allows the reduction of Dy3+ at more positive potentials because of the formation of Dy-Al intermetallics. The electromotive force method was then adopted to determine the thermodynamic properties of the Dy-Al intermetallics, such as the relative partial molar Gibbs free energy and the activity of dysprosium, and the standard formation Gibbs energy for Dy-Al intermetallics. Finally, potentiostatic electrolysis was conducted to extract Dy-Al alloys from the NaCl-KCl-0.40mol%DyCl3-0.56mol%AlCl3 melts. The XRD and FE-SEM/EDS analyses indicated that DyAl3, DyAl2 and DyAl intermetallics and metallic Al were generated in the products by electrolysis at −1.8 V, while the dysprosium-rich phases of DyAl2 and DyAl were produced as electrolysis at −2.0 V. The electrochemical separation of dysprosium from molten NaCl-KCl eutectic on inert Mo electrode was proved to be practically feasible by co-reduction with aluminum.