Research On Physical And Chemical Properties Of LiF-NdF₃-Nd₂O₃ Molten Salt System

What the important method to produce metal neodymium and neodymium iron alloy is to use of LiF-NdF3-Nd2O3 molten salt system. It has the important theoretical and practical significance for studying of the physical and chemical properties of the electrolyte system. The paper studies the electrical conductivity of LiF-NdF3 and LiF-NdF3-Nd2O3 molten salt system and influence on electrical conductivity of LiF-NdF3 system and LiF-NdF3-Nd2O3 molten salt system when adding Fe2O3.At the same time, It has researched Nd2O3 solubility and dissolved rate in LiF-NdF3molten salt system by using square wave voltammetry.In this paper, the work carried out in three parts:(1) Alternating current (AC) impedance spectroscopy technique are used to measure the electrical conductivity for LiF-NdF3and LiF-NdF3-Nd2O3 molten salts in a boron nitride tube-type capillary cell by continuously vary cell constant (CVCC).The effects of temperature and molten salt composition on the conductivity change and conductivity activation energy are investigated. Obtained the preferred electrolyte composition and the electrolysis temperature:the temperature is in the range of 1100℃～1200℃,In the LiF-NdF3(20wt%-30wt%)-Nd2O3(lwt%)melt. The conductivity fitting formula for LiF-NdF3and LiF-NdF3-Nd2Ｏ3 melts are described as follows: K=-3.44495+0.00633T+0.04703 xLｉF(LiF-NdF3 molten salt system, T:950-1200℃, xLiF: 10wt%-30wt%) K=-3.33306+0.00616T+0.05133xLiF-0.15603 xNd2O3(LiF-NdF3-Nd2O3 molten salt system T:950～1200℃,xLiF:10wt%-30wt%, xNd2O3:0-3wt%)(2)Using the same conductivity test method to test conductivity of LiF-NdF3-Fe2O3 and LiF-NdF3-Nd2O3-Fe2O3 molten salt system.As Fe2O3 concentration adding in LiF-NdF3 molten salt system, temperature can increase the conductivity, Fe2O3 concentration can decrease the conductivity. Adding the same weight percent of Fe2O3 or Nd2O3, the electrical conductivity of LiF-NdF3-Fe2O3 is bigger than the electrical conductivity of LiF-NdF3-Nd2O3.With the increase of concentration of Fe2O3, conductivity activation energy increase.adding the same weight percent of Fe2O3 or Nd2O3, conductivity activation energy of LiF-NdF3-Nd2O3 is bigger than the conductivity activation energy of LiF-NdF3-Fe2O3.In the LiF-NdF3-Nd2O3-Fe2O3 molten salt system, temperature can increase the conductivity, Fe2O3 concentration can decrease the conductivity. Conductivity activation energy increases with the increase of Fe2O3 concentration.(3) By using square-wave voltammograms (SWV), the oxidation processes of oxygen ions in LiF-NdF3-Nd2O3 melts on a graphite electrode were measured. The relationship between the peak current density Ip of the SWV and the Nd2O3 concentration in the LiF-NdF3-Nd2O3 melts has been build, and can be expressed as the following formula by the least square method, showing average error of 5.405% from the XRF analysis results. Ip=0.09594[Nd2O3](wt%)+0.00497Then the square wave voltammetry curves and the equation were used to characterize the Nd2O3 solubility and dissolution rate in LiF-NdF3molten salts. The dissolution of Nd2O3 in LiF-NdF3molten salts is an endothermic process and the solubility of Nd2O3 has increased with increase either temperature or NdF3 concentration. As the NdF3 concentration increases from 70wt% to 90wt% at 1200℃,the solubility of Nd2O3 has increased by 1.3wt%. The dissolution rate of Nd2O3 in the LiF-NdF3molten melts has reached the maximum value at about the first 10 min, which is independent of the electrolyte composition. The dissolution rate of Nd2O3 in NdF3-LiF=90(wt%):10(wt%) has maintained the largest value of 0.057wt%/min in the first 10-20minutes at 1200℃.