December 09, 2019

Vacuum distillation process and influencing factors

Rare earth metals purified by vacuum distillation device comprises a main frequency induction furnace, vacuum and high temperature carbon nanotubes pure tantalum sheet fire furnace. In order to prevent the contamination of the heating material, the former is mainly in production, and the distillation device for induction heating is shown in Fig. 1.
The energy and process conditions for purifying rare earth metals by vacuum distillation are mainly determined by the melting points of impurities in the base metals and metals and their vapor pressure and other properties at a certain temperature. The distillation apparatus should also be based on the properties of the base metals and impurities. Come choose. According to the melting point of the rare earth metal, the vapor pressure and the process conditions of distillation purification, the distillation purification of the rare earth metal can be roughly divided into four groups, as shown in Table 1.
There are many factors affecting the purification of distillation. Among them, the distillation temperature, the nature and content of impurities, the residual pressure and composition of the gas in the furnace, the surface area and agitation of the volatilization, the temperature and structure of the condenser, the structure and characteristics of the distillation unit, etc. [ 2] . For rare earth metals with a high vapor pressure, distillation and purification at a lower temperature of less than 1500 ° C can effectively remove impurities such as refractory metals such as titanium , tungsten, molybdenum and oxygen, nitrogen, carbon and other gaseous impurities in rare earth metals. However, if the distillation temperature exceeds 1650 ° C, the volatile low-cost rare earth oxide will vaporize the product with the metal and control the direction of the metal vapor pressure. However, if the baffle is removed and the distillation temperature is lowered, the purification effect can be improved. The crucibles and condensers used in the vacuum distillation of rare earth metals are refractory metal materials such as tungsten, molybdenum and niobium. However, due to the high temperature corrosion of rare earth metals, the crucible has a short service life and accounts for a relatively high proportion of production costs.

Figure 1 Schematic diagram of the distillation equipment
1- quartz glass sheet; 2-sealing ring; 3-cooling water pipe; 4- copper cover;
5-distillation crucible ; 6-quartz tube; 7-distillation tower; 8-rough crucible; 9-induction coil;
10- graphite protective layer; 11-vacuum joint; 12-seal; 13-insulating breeze; 14-é’½ heating element
Table 1 Classification of rare earth metal distillation purification [2]
Rare earth metal
Melting point / °C
Boiling point / °C
Vapor Pressure
Distillation process conditions
Sc, Dy, Ho, Er
Higher (1400~1540)
Lower (2560 ~ 2870)
Higher
Distillation at a temperature close to the melting point (about 1600 ° C)
Y, Gd, Tb, Lu
Higher (1310 ~ 1660)
High (3200 ~ 3400)
low
High distillation temperature (2000 ° C)
Sm, Eu, Tm, Yb
Lower (820~1070)
Low (1200 ~ 1950)
high
Purify below the melting point or distill at slightly above the melting point
La, Ce, Pr, Nd
Low (800 to 1000)
High (3070 ~ 3460)
low
Distilled at about 2200 ° C, the condensate is liquid
The precondition for vacuum distillation to proceed smoothly is that the distillation rate is less than or equal to the condensation rate. If the distillation rate is greater than the condensation rate, the metal will overflow or flow back from the condensation system. This phenomenon is called reflux.
The temperature and surface area of ​​the condenser are two key parameters that determine the rate of condensation. At a certain condensation rate, the surface area of ​​the condenser should be selected according to the temperature of the condenser. The two are generally inversely proportional. The condensing temperature of the rare earth metal is generally lower than the melting point of the metal by more than 200 ° C [2 , 4] . In the process of purifying rare earth Gd, Tb, Dy, Ho, and Er by vacuum distillation, the condensation surface area is selected to be 3 to 4 times the evaporation surface area (the area of ​​the bottom surface), and the condenser temperature is lower than the melting point of the metal 200 to 300 ° C. The distillation temperature at which no reflux occurred in the early stage of distillation is shown in Table 2. Distillation at this temperature gives a large distillation rate and enables scale production. As the distillation process progresses, the metal distillation rate is gradually reduced, the amount of metal deposition in the condenser is continuously increased, the condensation condition is improved, and the distillation temperature can be gradually increased by 100 ° C [3] .
Table 2 Rare earth metals with higher distillation temperature
element
Gd
Tb
Dy
Ho
Er
Distillation temperature / ° C
1850
1800
1550
1650
1700

references

2. Editorial Board of the Handbook of Rare Metals, Handbook of Rare Metals (Volume 2) [M], Beijing: Metallurgical Industry Press, 1995, 930
4. Xu Guangxian, Rare Earth (2) [M], Beijing: Metallurgical Industry Press, 1995, 64-69


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