An investigation into the electrochemical recovery of rare earth ions in a CsCl-based molten salt

A CsCl-based melt, was used as a supporting electrolyte for a fuel cycle in pyrochemical separation, as it has a high solubility for lanthanide oxide. Cyclic voltammetry and square wave voltammetry were carried out to investigate the cathodic reduction of those rare earth ions. The results prove that the cathodic process of La(III) ions dissolved in a CsCl-based melt, with a one-step reduction La³⁺+3e^-=La, and is similar to those of other reports which have utilised LiCl-KCl or CaCl₂-KCl molten salt systems. However, for the Ce(III) ions that dissolved in a CsCl-based melt, there is a significant difference when compared with published literature as there are two reduction steps instead of the reported single step Ce³⁺+e^-=Ce²⁺ and Ce²⁺+2e^-=Ce. In order to explain the novel result, a detailed investigation was focused on the cathodic process of Ce(III) in a CsCl-based melt. The identification of the M-O (M=La, Ce) compounds that are stable in the electrolyte, as well as the determination of their solubility products, were carried out by potentiometric titration using an oxide ion sensor. Furthermore, the E-pO^2- (potential-oxide ion) diagram for the M-O stable compound was constructed by combining both theoretical and experimental data.