Purification of rare earth metals by vacuum distillation - application examples

(1) intermediate alloy - Preparation of high purity metal Dy vacuum distillation process SYSTEM ^[5] Firstly, the intermediate alloy sponge method were dysprosium, dysprosium and the sponge loaded into crucible of tungsten, tantalum sheet in high temperature and high vacuum oven at 1450 The distillation was carried out at ° C, 4 × 10 ^-5 Pa, and collected by a molybdenum condenser, and the obtained distillation crucible was further subjected to secondary distillation at 155 ° C to obtain a final high-purity antimony product. The results show that during the distillation purification process, most of the rare earth metal impurities (Fe, Si, Ca, Al, Cu, Zn, Mn, Ti, etc.) with different vapor pressure and enthalpy difference have different degrees of reduction, steam pressure and dysprosium similar holmium dare, erbium not substantially removed; removal C, N, O is obvious, since C, N, O mainly a high melting point compound is present in dysprosium, difficult to evaporate and remains in the bottom of the crucible. Table 1 shows the analysis results of the intermediate product and the final product, wherein: A is a sponge mash; B is a primary distillation enthalpy; and C is a secondary distillation enthalpy.

Table 1 Analysis results of non-rare earth impurities in metal bismuth Unit: μg/g

(2) Process for preparing high-purity metal ruthenium by vacuum distillation method ^[6] Metal ruthenium obtained by calcium heat reduction method removes impurities such as Ca, Al, Fe by vacuum distillation. During the distillation process, the distillation temperature is a major factor affecting the yield of metal ruthenium and the quality of the product. Practice has shown that when the distillation temperature is low, the metal yield is low; when the distillation temperature is higher, the metal yield is improved, but the non-rare earth impurity aluminum and iron content will increase. For a comprehensive analysis, the optimum distillation temperature was chosen to be 1700 ° C (see Figure 1). The distillation process was carried out in a SL63-7B vacuum electric furnace. The main operating conditions were: the casting vacuum was greater than 75×10 ^-7 Pa. The mass analysis results of the metal ruthenium purified by distillation are shown in Table 2.

Table 2 Results of mass analysis of metal ruthenum Unit: μg/g

(3) Example of vacuum smelting and purifying rare earth metal The crude rare earth metal obtained by the calcium thermal reduction method contains 0.1% to 0.3% of calcium and impurities such as carbon, magnesium and oxygen, and remelting the rare earth metal under vacuum, and the impurity removal is obvious. In the process of producing metal content by calcium calorimetry, remelting and decalcifying and adding 3% to 5% barium fluoride at the same time have a good effect on reducing the content of impurities such as calcium and magnesium. The results are shown in Table 3. The casting and casting process in the process of preparing rare earth metal by the intermediate alloy method is carried out in an electric arc furnace filled with argon gas, and has a good purification effect on calcium, magnesium, chloride, etc. ^[8] . In actual production, when the rare earth metal is prepared by using calcium heat reduction rare earth fluoride, the calcium content of the product is often reduced by vacuum melting method due to the excess of the reducing agent calcium. Calcium in metal strontium exists in the form of free and dissolved calcium fluoride ^[9] . At present, the conditions of vacuum smelting in China can remove free or dissolved calcium, and partially remove calcium fluoride, but the removal of calcium fluoride is removed. difficult. In the process of vacuum melting metal crucible, in order to overcome the disadvantage that the metal crucible is in a semi-melt state, the calcium vapor is volatilized in a large amount to cause splashing, so that the melting yield is reduced, solid state sublimation (700 to 800 ° C, 45 min) can be used to remove a large amount of After the calcium is volatilized, liquid remelting (1400 ~ 1500 ° C) is carried out ^[6] . When the vacuum remelting time is 8 min, the volatile impurities such as calcium are basically removed. If the time is too long, the metal volatilization loss is large, and the yield is decreased; the time is short, the metal is not melted, and the calcium removal effect is poor. Figure 2 is a graph showing the relationship between vacuum melting time and calcium removal rate and metal yield.

Table 3 Impurities in metal ruthenium before and after remelting decalcification ^[7] Unit: %

Figure 1 Relationship between distillation temperature and yield

Figure 2 Relationship between smelting time and calcium removal rate and yield

The metal ruthenium obtained by electrolysis is vacuum smelted in a vacuum induction furnace to remove electrolytes and other volatile impurities, thereby purifying the metal ruthenium ^[10] . After the metal ruthenium , osmium , iridium , osmium and CaF ₂ slag obtained by the reduction of fluoride are separated, impurities such as Ca, CaF ₂ and H are still contained. These impurities are more volatile than rare earth metals and can be purified by vacuum melting. Ca and H can be removed near the melting point, but the quantitative removal of F needs to be maintained at 1800 ° C for 30 min to complete ^[11] . Such high temperature is also very effective in removing C, O, and N. However, in the vacuum melting process using hydrazine, part of Ta will melt into the rare earth metal. When the temperature drops to the melting point, Ta will precipitate from the metal solution. The bottom of the crucible is removed by mechanical means.

references

5, Xu Jing, Zhang Wei, Xiao Feng, etc., intermediate alloy - vacuum distillation method for the preparation of high purity metal bismuth process [J], rare earth, 2003, 24 (4): 36 ~ 38
6. Wei Xuwei, Xu Xiuzhi, Rare Earth, 1997, 17(4): 51~58
7, Yuan Ping, Yan Haowei, etc., trial production of high purity metal bismuth [J], Jiangxi Metallurgy, 1999, 19 (5): 20
8. Chang Ke et al. Study on the process of producing metal ruthenium by calcium calorimetry [J], Rare Metals, 1994, 19(1): 79
9. Zhang Shirong, Preparation and Purification of Metal Cerium [J], Rare Metals and Cemented Carbides, 2000, (3): 56
10, Guo Feng, calcium and oxygen in metal bismuth [J], rare earth, 1995, 16 (6): 61 ~ 63
11. Li Zuoshun, High Purity Development Trend of Rare Earth Metals, Paper of the Third Annual Conference of China Rare Earth Society, Beijing, 1995, 3

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