Distribution Models Of Impurities In Terbium Metal Purified By Vacuum Distillation

Vacuum distillation is an important purification method of rare earth metals and is widely used in the large-scale preparation of high-purity rare earth metals.Aimed at the research status that,currently,the impurity separation coefficient is used to qualitatively judge the separation degree of impurity during the vacuum distillation purification of rare earth metals,while the lack of quantitative and systematic research on the impurity distribution in the distillate and the efficiency of distillation purification,the study on distribution models of impurities in the terbium(Tb)metal purified by vacuum distillation was carried out.Firstly,according to the mass-transfer rate equation in the liquid boundary layer,the distribution model of the highly volatile impurity in the distillate was established,and according to the law of conservation of mass,the distribution models of the medially and lowly volatile impurities in the distillate were perfected.Secondly,the software Fluent was used to simulate the distributions of the thermal field and flow field of the liquid metal of Tb during the distillation process,and the average temperature and flow velocity at the evaporating surface were calculated for providing the calculated parameters of the models.Finally,the vacuum distillation purification experiments were carried out to verify and modify the impurity distribution models.The research results showed that:(1)As the distillation continues,the average temperature at the evaporating surface gradually decreases,which the temperature difference between the former and latter stage of distillation reaches more than 30?;as the distillation proceeds,the average flow velocity at the evaporating surface increases firstly and then decreases sharply,which the flow velocity is in the range of mm/s level and the maximum flow velocity can reach more than 10mm/s.(2)The mass-transfer control steps of the highly volatile impurities of Mn and Cr are the mass-transfer process in the liquid boundary layer;as the distillation proceeds,the contents of Mn and Cr impurities present exponentially decreasing tendency in the distillate;the calculated results of Cr impurity are consistent with the experimental results,and the average standard deviation is 10.068;for Mn impurity,there are the similar distribution rules between the calculated and experimental results;Mn and Cr impurities in the Tb metal can be effectively removed by vacuum distillation,which the removal rates are 97.14%and 80.64%respectively.(3)The mass-transfer control steps of the medially volatile impurities of Fe and Cu are the hybrid mass-transfer process;as the distillation proceeds,the contents of Fe and Cu impurities present linearly decreasing tendency in the distillate;for Fe and Cu impurities,there are the similar distribution rules between the calculated and experimental results;Fe and Cu impurities in the Tb metal can be hardly removed by vacuum distillation.(4)The mass-transfer control steps of the lowly volatile impurities of Al,Ti and Ni are the hybrid mass-transfer process or the evaporation mass-transfer process;as the distillation proceeds,the contents of Al,Ti and Ni impurities present exponentially increasing tendency in the distillate;the calculated results of A1 impurity are consistent with the experimental results,and the average standard deviation is±0.048;for Ti and Ni impurities,there are the similar distribution rules between the calculated and the experimental results;Ni impurity in the Tb metal can be effectively removed by vacuum distillation,while the removal rates of A1 and Ti impurities are lower,which the removal rates of Al,Ti and Ni impurities are 17.44%,32.32%and 75.14%respectively.(5)As the distillation proceeds,the contents of the gaseous impurities of O and N present exponentially increasing tendency in the distillate;O and N impurities may be volatilized in the form of suboxide and nitride during the distillation process respectively,resulting in the distributions of gaseous impurities in the distillate not to be described by the models;O and N impurities in the Tb metal can be effectively removed by vacuum distillation,which the removal rates are more than 70%.(6)At the distillation temperature of 1820?,and when the infinite dilute activity coefficients of Ti and Ni impurities calculated by Miedema model are modified to 3.7 and 0.045 respectively,and the modified coefficients of activity coefficients are 0.39 and 5.63 respectively,the calculated results are consistent with the experimental results,which the average standard deviations are±0.096 and±0.105 respectively.