Study On Ionic Structure And Its Application Of NdF₃-LiF-Nd₂O₃ System Melts

Electrolysis of Nd2O3 in fluoride melts is an important process for production of neodymium metal in modern industry. It is of most importance to study the ionic structure of fluoride-Nd2O3 electrolysis melts system from both theoretical and practical views. However, there is little research on the ionic structure of fluoride-Nd2O3 electrolysis melts at home and abroad, which makes it is difficult to meet the need of guiding practice. It is partly because that there is little use of some modern intuitive methods for matter structure research in this field for the volatile and corrosion of fluoride melts. Furthermore, further accuracy requirements for melts physicochemical properties research were put forward for development and technical innovation of Nd2O3 electrolysis industry. But it is difficult to make it clear that the changing law of the melts physicochemical properties for the lack of related structure theory. Based on the analysis above, in this paper, ionic structure of NdF3-LiF and NdF3-LiF-Nd2O3 melts was studied by Raman spectroscopy, cryoscopic method and density method. Density and electrical conductivity of the melts, and solubility and dissolution kinetics behavior of Nd2O3 in the melts were invegisted, then, reason for effects and changing law of researching factors on the physicochemical properties mentioned was analyzed from view of the melts structure.Liquidus temperature of LiF-NdF3 and LiF-NdF3-LiCO3 melts system was determined by differential thermal analysis(DTA). On this basis, cryoscopic research was carried on. It was found that the general formula of Nd-F species was Nd2Fx(x-6)- with the most possible form of Nd2F7- in LiF-NdF3 melts with very small weighted-in mole fraction of NdF3. When O2- was added to the melts mentioned above, general formula of Nd-F-O species was Nd2O2Fx(x-2)-, and the most possible form of Nd-F-O species was Nd2O2F42-. For the LiF-NdF3 in which weighted-in mole fraction of NdF3 was less than 20%, the most possible dissolution mode of NdF3 was NdF4- with Temkin or Flood model or NdF63- with Flood model. The mode of both NdF4- and NdF63- with Temkin model was also a reasonable one. When O2- was added to the melts, the most possible dissolution mode was of NdOF54- with Flood model.A new type of sample cell-sealed sample cell for high-temperature melts Raman spectra research was developed on the basis of analysis of advantages and disadvantages of previous sample cell and structural characteristics of micro Raman spectrameter. The most obvious advantage of the sealed sample cell lied in the stable composition of the melts during the Raman spectra measurement. High-quality Raman spectrum with low signal-to-background-ratio could be obtained by measuring spectrum with UV confocal Raman spectrameter equipped with sealed cell. On this basis, Raman spectra research on NdF3-LiF melts was carried on. The results showed that NdF63- with octahedral structure and NdF4- with tetrahedral structure were existed in the melts. The Raman shift corresponded to main vibration peak for NdF63- and NdF4- was located in 410cm-1 and 470cm-1, respectively, and the two wavenumbers were basically independent on temperature and weighted-in composition of the melts. NdF63- was more unstable for higher temperature and its octahedral structure was distorted by the attraction of Li+to F" in the structure. Temperature and weighted-in composition of the melts had little effect on "Lifetime" of NdF4-. When Nd2O3 was added to NdF3-LiF melts, Nd-F-O species with octahedral structure was formed by the reaction of Nd2O3 and Nd-F species which had already been existed in the melts. The two possible forms of this kind of Nd-F-O species were NdOF54- and Nd20F106-. "Lifetime" of the Nd-F-O species was shorter and its structural distortion was severer than NdF63-. Increase of Nd-F-O species instability was more obvious with temperature increase.Anion quantitative analysis of 25mol%NdF3-75mol%LiF melts was carried on on the basis of density data which was determined by Archimedean method. Then, ratio of scattering coefficient of NdF63- and NdF4- was obtained which was then used for quantitative calculation of the melts with other compositions. The results showed that relative content of NdF63- and free F- was decreased with increasing weighted-in mole fraction of NdF3, meanwhile, that of NdF4- was increased. The chaning range of free F- and NdF4- was large. When weighted-in mole fraction of NdF3 was increased from 25% to 40%, mole fraction of free F- in all kinds of anion was decreased from about 50% to 3-4%, and that of NdF4- was increased from 25% to 74%. Influence of temperature to relative content of different kinds of anion was small for a fixed weighted-in composition.It was thought that the AC impedance course of the reasearched circuit for measuring electrical conductivity by continuously varying cell constant(CVCC) technique was controlled by both of electrochemical polarization and concentration polarization with melts'concentration polarization impedance assumed to be Gerischer impedance. When electrical conductivity was calculated by CVCC equation, and AC impedance method was used for reading data, the best option of the circuit resistance was the sum of melts resistance and electrode & line resistance gained by fitting Nyquist curves. If fitting error was considered, the circuit high frequency resistance was also a good option.Nd2O3 solubility in NdF3-LiF-Nd2O3 melts was studied by isothermal saturation method. It was found that Nd2O3 solubility was linear increased with temperature and relative content of NdF3 of the melts, and the increase effect was more obvious for the NdF3-LiF-Nd2O3 melts with lower NdF3 relative content. The behaviors mentioned above could be explained from the view of melts micro-structure. Dissolution reaction of Nd2O3 was an endothermic reaction.Dissolution rate of Nd2O3 in NdF3-LiF-Nd2O3 melts was studied. Rate controlling step and dissolution model of Nd2O3 dissolution was studied through comparison of measured value and calculated value of the three models researched. Nd2O3 dissolution behavior could be described by shrinking sphere model or mass transport control model and rate controlling step was mass transort or heat transport. Dissolution enthalpy of Nd2O3 was high for low Nd2O3 concentration melts and mass transort control was more important, meanwhile for high Nd2O3 concentration melts, dissolution enthalpy of Nd2O3 was low and heat transport control was more obvious.