Exploration of impurity migration behavior in the process of lanthanum purification by Czochralski method

Abstract

In order to improve the purification efficiency of La, the Czochralski (Cz) method was introduced into the purification of rare earth metal La for the first time in this work. And the migration behavior of Al, Fe and Ni impurity in the process of La purification by Czochralski method was obtained. The equilibrium separation coefficients k0 of Al, Fe and Ni were first calculated by the slope of liquidus. As k0 < 1, Al, Fe and Ni impurities in La will be concentrated in the melt and removed by the Cz method. Then, the effect of pulling speed on the effective separation coefficient keff of Al, Fe and Ni impurities in La was calculated by BPS model, which indicates that with the reduction of the pulling speed, keff of Al, Fe and Ni impurities gradually decreases, and the impurity removal efficiency increases. And by the purification experiments, when the pulling speed is 0.3 mm/min, the residual rates of Al and Fe impurities in La are the lowest, 40 % and 45.65 % respectively; Ni and Ca impurities are reduced below 10 ppm.wt. The purity of La can be increased from 99.97 wt% to more than 99.995 wt% after purification by the Cz method, indicating that the Cz method is an effective method for purification of La.