Solubility of Rare Earth Oxides in Fused Alkali and Alkaline Earth Halides

Melts based on mixtures of alkali and/or alkaline earth halides are considered as prospective media for electrowinning rare earth metals as well as for pyrochemical reprocessing spent nuclear fuels. Fluoride or mixed fluoride-chloride baths can be operated at high temperatures yielding molten rare earth metals (REMs) thus simplifying separation of the metal and salt. One of the problems in using fluoride melts is possible formation of fluorine at the anode. This can be avoided by adding a rare earth oxide to the melt both as the source of REM and oxide ions. The latter will be oxidized to oxygen producing carbon mono- or dioxide at the anode. The limiting factor for feeding the electrolysis bath with REM oxides is their solubility in the halide melt. The aim of the present study was determining the effect of temperature and melt composition on solubility of REM oxides in fused halides. The experiments were performed in CaCl 2 –CaF 2 mixtures containing 20 or 75 mol. % CaF 2 ; BaCl 2 –BaF 2 mixtures containing 15 or 73 mol. % BaF 2 ; equimolar CaF 2 –BaF 2 mixture and NaCl–NaF eutectic mixture (34 mol. % NaF). Solubility of REM oxides was determined by the method of isothermal saturation. Time required for reaching the equilibrium between solid REM oxides and fused salts was determined in a preliminary set of experiments. To compare the behavior of 4 f - and 5 f -elements, solubility of uranium dioxide was also measured. The measurements were performed at the temperatures up to 1400 o C under argon atmosphere. The lower limit of the temperature range varied from 700 to 1100 o C depending on the melting temperatures of the salt mixtures used. Oxides of yttrium, lanthanum, cerium, praseodymium, neodymium and samarium were selected for the study. To assess a possible mutual influence of rare earth elements on solubility of their oxides in fused salts the solubility of a mixture of REM oxides was determined in a separate series of experiments and concentrations of individual REMs in the melt was determined. Solubility of REM oxides increased with increasing temperature and an example of effect of temperature on solubility of neodymium oxide in various melts is shown in Fig. Fig. Concentration of neodymium in alkali and alkaline halide based melts saturated with Nd 2 O 3 . Melt: CaCl 2 –CaF 2 20 mol. % (1); CaCl 2 –CaF 2 75 mol. % (2); BaCl 2 –BaF 2 73 mol. % (3); BaCl 2 –BaF 2 15 mol. % (4); CaF 2 –BaF 2 50 mol. % (5; and NaCl–NaF 34 mol. % (6). Figure 1