Experimental determination and thermodynamic optimization of the LiF-NdF3 system

Abstract

Neodymium is mainly obtained by electrolysis of a molten LiF-NdF₃-Nd₂O₃ system. LiF-NdF₃ is a basic system, and the phase diagram of this system provides important information in the production of electrolytic neodymium. An accurate LiF-NdF₃ binary phase diagram helps in the selection of the appropriate molten salt component in production and optimizing the production process, which is of great significance to improve the electrolysis efficiency and reduce the production cost. To obtain an accurate phase diagram of the LiF-NdF₃ binary system, liquidus and solidus temperatures were experimentally determined in the LiF-NdF₃ binary system by differential scanning calorimetry. The experimental results were used to construct the phase diagram and develop a new database for the LiF-NdF₃ system using the FactSage software. The sub-regular solution model was used to describe the excess Gibbs free energy of the liquid phase, and the thermodynamic optimization calculation was carried out for the binary system. The binary interaction coefficients ⁰L = −39 966 + 17.68 T, ¹L = −7667 + 26.1 T, and ²L = −6000 were used to describe the system's excess Gibbs free energy. The results show that the eutectic point of the system is 68.4% LiF-31.6% NdF₃ at 731.5 °C. The effects of industrial and high purity NdF₃ and the presence of Nd₂O₃ on the liquidus temperature of the LiF-NdF₃ system were also investigated, high liquidus temperatures have been observed in tests using industrial NdF₃ and NdF₃ feedstock that contains a specific quantity of Nd₂O₃.