Research On Physicochemical Properties Of LaF₃-LiF-La₂O₃ Molten Salt

At present, the rare earth lanthanum metal is mainly producted by fluoride-oxide molten salt electrolysis process. In this process, LaF3-LiF molten salt is used as the electrolyte, and La2O3 is used as the raw material. However, the physicochemical properties of molten salt determine the process parameters of feeding system, composition of electrolyte and electrolysis temperature and so on. Moreover, they are the basis of theory of melt structure and mechanism of electrolysis. Therefore, it is very important to have a comprehensive and systematic research on the physicochemical properties of LaF3-LiF-La2O3 molten salt.In this paper, firstly, by X-ray Diffraction techniques, the main chemical reaction was determined when La2O3 added to the LaF3-LiF molten salt:La2O3+LaF3=3LaOF. Moreover, by Differential Thermal Analysis, the reaction was determined endothermic and the initial reaction temperature was 451℃. And then, Isothermal Saturation method and EDTA Titration method was used to measure the solubility of La2O3 in LaF3-LiF molten salt. When less than 60 ℃, the superheat of the molten salt, the time of La2O3 reaching dissolution saturation was about 1.5 hours. The solubility of La2O3 in 25LaF3-75LiF molten salt was 1.9wt%-2.2wt%; in 30LaF3-70LiF molten salt was 2.1wt%-2.4wt%; in 35LaF3-65LiF molten salt was 2.1wt%-2.6wt%. Under certain other conditions, as temperature and concentration of LaF3 increasing, the solubility of La2O3 in LaF3-LiF molten salt was presented in an increasing trend.In this paper, Archimedes method was used to study the density of LaF3-LiF molten salt, and the fitting equation of LaF3-LiF molten salt density with the changes of temperature and molar amount of LaF3 was obtained. When the composition of LaF3-LiF molten salt was invariant, its density decreased linearly with the increase of temperature; when the temperature of LaF3-LiF molten salt was invariant, the density of it increased linearly with the increase of concentration of LaF3. With increasing amount of La2O3, the density of LaF3-LiF molten salt had initial increase, and next decrease, then reached the maximum close to solubility.In this paper, Pull Cylinder method was used to study the surface tension of LaF3-LiF molten salt, and the fitting equation of LaF3-LiF molten salt surface tension with the changes of temperature and molar amount of LaF3 was obtained. When the composition of LaF3-LiF molten salt was invariant, its surface tension decreased linearly with the increase of temperature; when the temperature of LaF3-LiF molten salt was invariant, the surface tension of it increased linearly with the increase of concentration of LaF3. With increasing amount of La2O3, the surface tension of LaF3-LiF molten salt had initial increase and next decreased, then reached the maximum close to solubility.