Fundamental electrochemical study on neodymium molten salt electrolysis in fluoride bath

Abstract

In the recent decades, the clean energy economy has been driving a rapidly increasing demand for rare earth materials with their applications in essential high technologies such as electric vehicles and wind turbines. The technologies for the winning of rare earth metals are developing. As the predominant winning technique, electrolysis of rare earth oxides in molten fluoride systems has been faced with two major problems: one is low energy efficiency and the other is high emissions of perfluorocarbons (PFCs). Therefore, there exists a need for the metallurgy community to address those problems both theoretically and practically, and to develop improved processes for reducing rare earth oxides. This thesis uses the example of neodymium winning to elucidate the fundamental electrochemical properties of the molten fluoride electrolytes and the mechanism of the electrolysis process, and provides a guide to economically win rare earth metals with elevated energy efficiency and decreased emissions for the process. As for the property investigation, this research carried out measurements to determine the liquidus temperatures of the NdF3-PrF3-LiF ternary salt system, the solubilities of Nd2O3 in the electrolytes, and the electrical conductivity of the NdF3-LiF salt. A conductance cell system was developed to investigate electrical conductivity and produce reliable data. The experimental results indicate that the electrical conductivities of the molten NdF3-LiF system between 70 wt% to 85 wt% NdF3 within the range of temperature from 950 °C to 1050 °C range from 4.38 ohm-1 cm-1 to 6.08 ohm-1 cm-1. Furthermore, an empirical equation to estimate the value of the electrical conductivity for a specific molten salt is proposed. A mathematical model regarding the voltage change against current in the molten Nd2O3-NdF3-LiF electrolysis is proposed based on the thermodynamics and kinetics study and validated through experiments and literature observations. The model and the experimental results illustrate that the limiting current of the electrolysis cell increases with the increase of anode surface area, higher energy efficiency can be achieved with reduced electrode distance and more effectively, reduce the submerged depth ratio of anode to cathode. An effective technique to prevent the generation of PFCs is to design the cell conditions which allow the limiting current to be smaller than the critical current.