Impurity Distribution in Distillate of Metallic Terbium during Distillation Purification
Xiaowei Zhang*, Daogao Wu, Zhiqiang Wang, Dehong Chen, Shihong Yan, Zongan Li
National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem
Advanced Materials Co.LTD.,Beijing, 100088, PR China
(*Corresponding author: 0420295@163.com)
ABSTRACT
Metallic terbium has been purified by vacuum distillation at 1650℃ for 30h, and the distribution rules of
impurities concentration in distillate of metallic terbium have been researched, and a theoretical analysis
has been carried out. The research results indicate that, the concentration of impurity Ti in the distillate, is
925 ppmw in the initial stage, increases slowly in the middle stage, and increases rapidly in the last stage
during the process of distillation purification, and it reaches to 1756 ppmw; while the distribution profile of
impurity Cu is opposite, that is the concentration is 441 ppmw in the initial stage, and decrease gradually,
and it decrease to 341 ppmw in the last stage. In the vacuum distillation field, separation coefficient of
impurity, calculated by impurity concentration in the distillate, is used to judge that the impurity can be
separated from the matrix metal or not; however, the concentration of impurity in the distillate is changed
throughout the distillation process in present experiment, the traditional calculated method is not suitable,
and the modified separation coefficient (β′) has been introduced, defined as a dimensionless ratio of
distillation velocity of the impurity element to the matrix metal, in present study, the modified separation
coefficients equal 1/32.3 and 22.0 for impurity Ti and Cu respectively, it is predicted that the impurity Ti
can be removed and Cu is not, and the predicted results well agree with the experimental results as seen in
Fig.1 and Fig.2. Compared to the solidification process, the modified separation coefficient is similar
to the equilibrium partition coefficient of solute, and solute re-distribution theory in solidifying front can be
used to describe the impurity distribution near evaporate surface, it is found that in the former stage of
distillation purification, the diffusion of impurity in liquid metal can reach a quasi-equilibrium state, and in
the latter stage of distillation process, the diffusion rate of impurity in liquid metal decrease, and a
concentration gradient appears near the evaporate surface.
440
Concentration (ppmw)
Concentration (ppmw)
1800
1600
1400
1200
1000
420
400
380
360
340
800
20
40
60
80
100
Finish ratio of sublimation purification (%)
Figure 1 – Impurity Ti in distillate of metallic terbium
100
80
60
40
20
Finish ratio of sublimation purification (%)
Figure 2 – Impurity Cu in distillate of metallic terbium
KEYWORDS
Vacuum distillation purification, Metallic terbium, Distillate, Impurity distribution