Influence of Fluoride Ions on Electrochemical Behavior of Lanthanum and Neodymium Ions in Molten LiCl

Abstract

The influence of fluoride ions on electrochemical behavior of lanthanum and neodymium ions in molten LiCl at 1073 K was experimentally investigated. Cyclic voltammetry and square wave voltammetry were conducted using a molybdenum working electrode. Galvanostatic electrolysis using a molybdenum electrode as cathode in molten LiCl-5 mol%LaCl3, LiCl-5 mol%NdCl3, and LiCl-30 mol%LiF-5 mol%NdCl3 were conducted to investigate the influence of fluoride ions. The result indicates that the cathodic process for La3+ in molten LiCl is a one-step reaction, i.e., La3+ + 3e = La. The cathodic process of Nd3+ in molten LiCl is a two-step reaction, including Nd3+ + e = Nd2+ and Nd2+ + 2e = Nd. With the introduction of fluoride ions, however, the current of the first cathodic reaction decreases and when the molar ratio of F− to Nd3+ reaches to 6, the cathodic process of Nd3+ turns to a one-step process: Nd3+ + 3e = Nd. The results indicate that the addition of fluoride ions makes Nd3+ stabler and thus inhibits the reaction to Nd2+ in molten chloride. The deposited neodymium metal was dispersed in the electrolyte in pure chloride melt due to the shuttle of disproportionation and proportionation reactions of neodymium ions. With the addition of enough LiF to melt, the deposited neodymium metal adhered on the cathode. The current efficiency of neodymium electrolysis was remarkably improved and reached the similar value as that for lanthanum electrolysis.

Keywords

• molten salt
• influence of fluoride ion
• electrochemical behavior
• neodymium electrolysis