Solubilization of rare earth oxides in the eutectic LiCl–KCl mixture at 450 °C and in the equimolar CaCl2–NaCl melt at 550 °C

Solubilization studies of rare earth oxides and oxohalides have been carried out in the eutectic LiCl–KCl melt at 450 °C and the equimolar CaCl2–NaCl mixture at 550 °C. First, chlorinating conditions were predicted by comparing the E–pO2− diagram of the different rare earth-O compounds to that of several chlorinating gaseous mixtures. Then, experimental solubilization tests were performed by using pure HCl and Cl2 gases and the chlorinating Cl2(g)+C(s) mixture. In order to build up the potential–acidity diagrams, stability of the oxidation states of rare earths (RE being La, Ce, Nd, Pr and Y), standard potentials of the different redox couples as well as solubility products of oxides and oxychlorides were determined in both molten chlorides at the temperatures studied. Oxidation states (III) and (0) were found to be stable in both molten chlorides for all REs and (II) in the case of neodymium. The standard potential values of the different redox couples were also determined by combining both potentiometry and cyclic voltammetry and then allowed us to obtain the activity coefficients of RECl3 compounds in the molten media. It was found that RE(III) cations were less solvated by the chloride ions in the calcium melt, which could be explained by formation of CaCl42− ions. Moreover it was demonstrated that at a given working temperature, the rare earth cations with higher polarizability (higher charge to ionic radius values) are also more solvated by the melt. Solubilization tests of RE–O samples showed that in the case using gaseous Cl2 as chlorinating agent, the kinetics of the chlorinating reaction were much slower than those for gaseous HCl.