Medium-volatile impurity distribution in purification of terbium metal by vacuum distillation

•Theoretical and experimental distributions of medium-volatile impurity in distillates were studied.•Theoretical distribution of this impurity in distillate is successfully verified by the vacuum distillation experiments.•This impurity distribution in distillate presents linearly decreasing tendency as the distillation proceeds.•There are the similar distribution rules between the theoretically calculated results and experimental results.•Impurity activity coefficient in the infinitely dilute melt is calculated by Calphad approach. Theoretical distributions of medium-volatile impurities in distillates of terbium(Tb) metal were discussed and analyzed, and theoretical distributions were verified by vacuum distillation purification experiments with the different distillation temperatures. The research results indicated that, Fe and Cu in Tb metal are medium-volatile impurities, the mass-transfer control steps of Fe and Cu impurities are the hybrid mass-transfer process, the effective separation coefficients of above impurities are in the range of 1～2, and distributions of above impurities in distillates present linearly decreasing tendency as the distillation proceeds; there are the similar distribution rules between the theoretically calculated results and experimental results, and the experimental results are less than the theoretically calculated results, which the average standard deviations of Fe impurity at 1790 °C, 1820 °C and 1850 °C are ±0.352, ±0.256 and ±0.284 respectively, and that of Cu impurity are ±0.327, ±0.356 and ±0.290 respectively; the experimental removal rates of Fe and Cu impurities are only in the range of 20～35%, indicating that the medium-volatile impurities of Fe and Cu in Tb metal can be hardly removed by vacuum distillation.