| At present,electrolysis of rare earth oxide in fluoride melts is the main process for production of single rare earth metal and its alloys in modern industry.The new progresses of process on electrolysis of rare earth oxide in fluoride melts have been achieved in recent years.On the one hand,the cell capacity has increased from initial 3 kA to 60 kA.On the other hand,the revolutionary changes were happening on electrolytic equipment that the electrode changed from traditional type with upper cathode and anode to the novel type with liquid cathode at bottom.These new developments thoroughly have changed the cell structure,electrolytic process conditions and parameters.In order to develop a new electrolytic cell and find reasonable electrolytic process conditions and parameters,the systematical study on physicochemical properties of fundamental electrolyte is necessary.It is worthly mentioned that whether the electrolytic technology development on preparation of rare earth metal and its alloys or extraction rare earth elements from nuclear waste is restricted by the lack of study on physicochemical properties of oxide-fluoride melts for electrolysis.Based on the above discussions,the systematical investigation on physicochemical properties of fundamental electrolyte REF3-LiF-RE2O3（RE=La,Nd）in the present study is not only of theoretical significance but also important practical significance.The dynamic thermal analysis method is used to measure liquidus temperature of REF3-LiF（RE=La,Nd）melts.By analyzing step cooling curves,the liquidus temperatures of the melts with different component content are obtained.The structural ions of LaF3-LiF molten salts in which the mole fraction of LaF3 was less than 15%were studied by cryoscopic method.It gets to eutectic point when the mole fraction of LaF3 and NdF3 are about 15%and 20%in the corresponding melt,and the eutectic temperatures are about 771℃ and 724℃ for the two systems,respectively.The most reasonable dissociation reaction mechanism of LaF3 in LaF3-LiF melts in which LaF3 mole fraction is less than 15%is LaF3+F-=LaF4-.The X-ray diffraction technique（XRD）,differential thermal analysis（DTA）and thermodynamic calculation were used to investigate the chemical reaction of RE2O3 in REF3-LiF（RE=La,Nd）melts.Results show that the main reaction of RE2O3 in REF3-LiF（RE=La,Nd）melts is RE2O3+REF3=3REOF,and the initial reaction temperature of La2O3 and LaF3,Nd2O3 and NdF3 are 437℃ and 395℃ respectively.The solubility of RE2O3 in REF3-LiF（RE=La,Nd）melts was determined by the chemical titration method.The significant knowledge of RE2O3 solubility in REF3-LiF melts electrolyte with different compositions x（REF3）=20%to 40%at different temperature（950℃ to 1150℃）was obtained.It was about 1.79 to 3.05 g per 100 g electrolyte and the corresponding mole concentration was about 0.33%to 0.87%.When the traditional cell voltage was about 10.5 V and the current was about 4.2 kA,through analyzing the changing law of RE2O3 concentration with electrolytic time,the reasonable time interval for RE2O3 addition was recommended defining as 1.75 h.RE2O3 solubility increases with increase in temperature and the REF3 concentration in the melts.The natural logarithm of the RE2O3 solubility in REF3-LiF melts is plotted against the reciprocal of the absolute temperature（1/T）and can be interpreted by a linear relationship.The fitting parameters show the good linearity and can be used to estimate the solubility.The solubility RE2O3 increases with an increase REF3 concentration in the molten salts.The surface tension,density and electrical conductivity of REF3-LiF（RE=La,Nd）melts were measured using the slide method,Archimedean method and continuously varying cell constant（CVCC）method,respectively.Over the temperature ranged from 950℃ to 1150℃,the surface tension,density and electrical conductivity of LaF3-LiF and NdF3-LiF melts were 236.9 to 281.7 mN·m-1 and 230.8 to 284.1 mN·m-1,2.925 to 3.446 g·cm-3 and 3.134 to 3.698 g·cm-3,4.11 to 5.96 S·cm-1 and 3.62 to 6.13 S·cm-1,respectively.The surface tension,density and electrical conductivity were in good linear relationship with temperature.The surface tension and density decreased with increasing temperature,and on the contrary,the electrical conductivity increased with increasing temperature.For a certain temperature,the surface tension and density increased with increasing content of REF3 in the melts,and the electrical conductivity decreased with increasing REF3.There are effects of RE2O3 content on surface tension and density that the surface tension and density firstly increased with the increasing RE2O3 and reached the maximum value near the solubility of RE2O3,and then decreased with the continuous adding RE2O3.The densities of REF3-LiF-RE2O3 melts studied in this paper are much smaller than the density of liquid rare earth metals.So,it can satisfy the requirements of production application.The addition of RE2O3 in REF3-LiF melts can decrease the electrical conductivity,to a certain extent.Because the addition of RE2O3 led to the generation of ion-cluster REOF54-and RE2OF106-which are in bigger volume,this process did not only reduce the concentration of conductive carrier F-anion but also increase the obstruction of directional moving of another conductive carrier Li+ ion.Moreover,although the influence of RE2O3 on electrical conductivity of melts is limited for the extremely limited solubility in the melts,it is important to control the addition of RE2O3 within its solubility in the practical production.This is crucial for satisfaction of requirement in electrical conductivity of electrolyte. |
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