Study On The Dissolution Process Of La₂O₃ In The Molten Salts Of LaF₃-LiF

At present,the preparation methods of rare earth metals and their alloys are mainly molten salt electrolysis,and the concentration of rare earth oxides in fluorine salt electrolytes is one of the most important parameters to maintain the normal production of electrolysis,during the production process,due to the continuous consumption of oxides in electrolytes,continuous feeding is needed to supplement the concentration of rare earth oxides in electrolytes,resulting in dynamic changes in the concentration of rare earth oxides in electrolytes.The variation and fluctuation of oxide concentration directly affect the technical and economic indexes of electrolytic production.Therefore,the real-time accurate measurement of oxide concentration in electrolytes was carried out in order to realize the accurate cutting and automatic control of oxides in the electrolytic production of rare earth metals and their alloys.In this paper,the representative light rare earth metal lanthanum in rare earth metal prepared by fluoride molten salt electrolysis was selected as the research object.Taking LaF3-LiF-La2O3 molten salt system as the basic electrolyte system,the on-line measurement of lanthanum oxide concentration in molten salt electrolyte was designed and realized,and the dissolution mechanism of oxide in fluorine salt was explored.The main results are as follows:The relationship between the electromotive force of the primary cell and the concentration of La2O3 is obtained by means of the cell electromotive force method.The relation between the electromotive force of the primary cell and the concentration of La2O3 is obtained by fitting.C=e25.51-20.12E(the gas is CO2),C=e5.32-7.94E(the gas is CO),The application range is at 1000?,and the addition amount of La2O3 in the 65%LaF3-35%LiF molten salt electrolyte is 0?1.79 mass%.The concentration of lanthanum oxide in LaF3-LiF-La2O3 molten salt electrolyte was studied by chemical element analysis.Compared with the calculated results by the electromotive force method of the original battery,the average error is about 5.3%.Combined with the phase composition of the XRD analysis,the results show that the concentration of lanthanum oxide can be measured online by the electromotive force method of the original battery.The effects of temperature and electrolyte ratio on the EMF of the original battery were studied.The results show that the electromotive force of the original battery decreases with the increase of experimental temperature and LaF3 content when the concentration of La2O3 in molten salt electrolyte is certain,which further indicates that the solubility of lanthanum oxide increases.Through XRD,differential thermal analysis and thermodynamic calculation,the dissolution process of La2O3 is a complex physical and chemical process with chemical reaction,and when LaF3 and LiF are present in the electrolyte,La2O3 will preferentially react with LaF3,and the initial temperature of the reaction is 388?.The generated new material LaOF is mainly in the form of LaO54-,and the LaO54-ionic cluster has three different crystal structure types,and when the temperature is low,the hexahedral crystal structure is present,and as the temperature increases,the regular octahedral cubic crystal form structure with more stable structure is gradually changed.The structure of molten salt ions added to LaF3-LiF-La2O3 system with different La2O3 was studied by Raman spectrum analysis.The results show that the ions in binary LaF3-LiF molten salt mainly exist in the form of Lax63-,in addition,there are a small part of LaF4-.With the increase of LaF3 content in electrolyte ratio,the contents of LaF4-and LaF63-ions in molten salt increase.When La2O3 is added to LaF3-LiF molten salt,the[O]produced by the dissolution of La2O3 will partially replace the F-in the regular octahedral structure LaF63-ion group,forming a new octahedral La-F-O ion cluster.The characteristic peak of La-O-F ion group increases with the increase of La2O3 content.The dissolution rate of La2O3 in LaF3-LiF based molten salt electrolyte was measured by the primary cell electromotive force method.the results show that the dissolution rate of lanthanum oxide decreases with the dissolution of La2O3 in molten salt electrolyte under certain experimental temperature and composition of molten salt electrolysis.the dissolution rate of La2O3 reaches the maximum at about 10 min,and the maximum dissolution rate is about 1.63 mass%/h.