Study On Purification Of La By Electron Beam Melting

Because of its unique physical and chemical properties,lanthanum metal has broad application prospects in high-tech materials such as storage materials,hydrogen storage materials,filter materials and high-K gate dielectric materials.However,because of its low melting point,very low vapor pressure and high activity,lanthanum metal has become one of the most difficult elements to purify in 17 rare earth elements.Electron beam melting(EBM)technology has the characteristics of high vacuum and ultra-high temperature,which is widely used to purify high melting point metals such as tantalum,niobium,tungsten and molybdenum and has good effect on removing high vapor pressure metal impurities and gas impurities.At present,electron beam smelting technology has rarely been reported and is still a lack of systematic and in-depth study in the field of rare earth metal purification.In this paper,the thermodynamic theoretical analysis,kinetic behavior and purification technology of lanthanum metal by electron beam melting were studied,in order to provide useful reference for further application of electron beam melting technology in the field of rare earth metal purification.The main conclusions are as follows:(1)In the temperature range of 1800-2400K,the activity coefficients and evaporation coefficients of the main metallic impurities of Cu,Cr,Fe,Ni,Si and Ti were calculated by using sub-regular melt model Redlich-Kister equation and Miedema model based on physical parameters.All the evaporation coefficients of Cr,Cu,Fe and Ti are greater than 10,and display a decreasing trend with the increase of temperature,which can be effectively removed theoretically,while the evaporation coefficients of Ni and Si are less than 4 and display an increasing trend with the increase of temperature,which makes it difficult to remove them.(2)The experimental study and theoretical calculation of the weight loss of lanthanum in the melting process show that the weight loss rate of lanthanum is only related to the average temperature of the melt surface and the melting time when the initial mass of lanthanum is fixed.When the melting power is constant,the weight loss rate of lanthanum is proportional to the melting time;when the melting time is constant,the weight loss rate of lanthanum is exponential to the melting power(PLa*is exponential to T).When the melting power is 30 kW,40 kW and 50 kW,the average surface temperatures of the melt are 2108 K,2187 K and 2218 K,respectively.(3)The result of purification mechanism of lanthanum by electron beam smelting show:the volatilization removal of key impurity Fe follows the first-order elementary reaction,and Fe is evaporated from lanthanum melt in the form of single atom.The kinetic control step of Fe removal is liquid phase diffusion.When the melting power is 30 kW,40 kW and 50 kW,the diffusion coefficients are 5.39×10-5 m/s,5.65×10-5 m/s and 5.75×10-5 m/s,respectively.Prolonging the melting time is beneficial to the removal of impurity Fe.The dynamic limiting step of evaporation loss of lanthanum is the free interface evaporation step.At the melting power of 30 kW,40 kW and 50 kW,the evaporation and mass transfer coefficients of lanthanum in the free interface layer are 1382.17 m/s,3390.76 m/s and 5178.39 m/s,respectively.Increasing the melting power will aggravate the evaporation loss of lanthanum.(4)The removal effects and rules of different impurities in the purification of lanthanum by electron beam smelting are as follows:electron beam smelting can effectively remove Mg,Li,Mn,Cu,Cr,Fe,Ti et al..After smelting,all the above impurities in lanthanum are less than 1 ppmw;electron beam smelting can not remove Ni,Si et al..After smelting,the content of Ni impurities in lanthanum is basically unchanged,while the content of Si impurities is greater than initial content.The rule of removal of impurities in lanthanum by electron beam melting is consistent with the predicted results of thermodynamic calculation.The evaporation coefficient of impurities in lanthanum can be used to determine whether the impurities can be removed by electron beam melting.(5)Optimum process conditions and effects of purification of lanthanum by electron beam melting:at the experimental temperature of electron beam melting,the copper in water-cooled copper crucible will diffuse into the molten lanthanum,resulting in the content of copper in the molten lanthanum more than 200 ppmw.The evaporation coefficient of copper in lanthanum is much greater than 10,which can be theoretically removed by electron beam melting.Therefore,on the basis of theoretical analysis and experimental research,two-step smelting of industrial grade lanthanum metal is carried out,which can be described that first using the melting power which is greater than 30 kW,after reaching the prescribed smelting time,the melting power is reduced to 12 kW,and then to the prescribed time.The purity of lanthanum melted by two-step smelting process increases from 2N8 to 3N8(except for rare earth impurities and gas impurities).