Development of a rare earth electrolysis process in molten salt medium

This work deals with the influence of the solvent composition in the elaboration of neodymium and dysprosium metals by molten salt electrolysis on inert electrode. Three different media have been studied: chloride, fluoride and mixed chloro-fluoride salts. In order to improve the understanding of the solvent composition's role, a comparative study of the electrochemical behaviour of these two elements in different media was carried out. The melt composition modifies the stability of the different oxidation states in solution. This study highlighted the electrochemical behaviour variations of these rare earths with respect to both the solvent anion (chloride and/or fluoride) and the solvent cation determining the chloro(fluoro)-acidity of the bath. The reduction mechanism of both neodymium and dysprosium, initially a two steps process in chloride media, is simplified as a single reduction step through addition of sufficient amount of fluoride ions. Electrochemical techniques have also demonstrated the formation of an intermetallic compound by co-deposition of the alkalis and alkaline earths with rare earths. The chemical composition was assessed during dysprosium electrolysis in the LiCl-KCl medium. A methodology based on reverse chrono-potentiometry was developed in order to optimize the electrolytic operating parameters along with the study of the kinetics of the comproportionation reaction between rare earth metal and its chloride salt. Results show that temperature and composition of the molten salt play an important role on the kinetics of the comproportionation reaction. The faradaic electrolysis yields increase from LiCl to LiCl-KCl to LiCl-LiF media and decrease with increasing temperature. Moreover, at a given bath composition, the faradaic yields for metal deposition are higher for dysprosium than for neodymium. This difference is attributed to the instability of Nd(II) species in solution, and also to a higher kinetics of the comproportionation reaction for neodymium. Neodymium and dysprosium electrolysis trials at a gram scale are in good agreement with the electro-analytical approach developed in this work. The combination of a set of electro-analytical techniques developed in this study allowed us to propose new processes regarding the preparation of neodymium and dysprosium metal in unalloyed form by electrolysis. Faradaic yields are close to 70% for neodymium and 95% for dysprosium in LiCl-LiF at 550 C. Unlike neodymium, dysprosium electrolysis can be carried out directly in low temperature chloride media with high yields, above 70%. (author)