Regional melting method
There are two types of regional smelting purification.
(1) horizontal zone melting purification metal ingot in a horizontally elongated slot vessel, through a horizontal melt zone ingot.
(2) The molten metal ingot in the suspension zone is placed vertically, and the ingot is fixed at both ends without the container. When the ingot is heated, the metal surface tension is used to maintain a narrow melting zone. The metal is suspended in the middle of the metal rod in the liquid state, and the melting zone is along the ingot. The long moving from bottom to top passes through the ingot to purify the metal. The melting zone can be obtained by induction heating or electron beam bombardment heating. Electron beam heating is used, which is called electron beam suspension zone melting method. The electron beam suspension zone melting equipment is shown in Figure 1.
Figure 1 Schematic diagram of electron beam heating suspension zone melting equipment
1-sample; 2-tungsten wire; 3-focus plate; 4-sealing
Factors affecting the melting of the suspension zone
When the rare earth metal is purified by the electron beam suspension zone melting method, the rare earth metal acts as a fixed anode, and the cathode moves from bottom to top along a certain speed of the metal bar. This purification process is mainly affected by the following factors.
(1) The temperature of the melting zone of the melting zone should not be too low, so as to avoid incomplete melting and affect the diffusion speed of the impurities; but it should not be too high, otherwise the middle of the melting zone will be thinned, resulting in poor coupling of the coils to the fine parts. The molten metal particles fall on the lower interface and become a new crystal nucleus. The temperature should be kept stable during the operation, otherwise the crystal interface may be polycrystalline.
(2) The number of times of purification in the melting zone is the same as the melting rate of the zone. Generally, the purification is mostly increased, and the purity of the metal is increased. When the impurity reaches the limit distribution, the number of additional increases is meaningless, and some impurities may increase. Trends must therefore be based on specific circumstances.
Under vacuum, has been determined after purification of metal impurities in the final concentration c, the number n may be estimated by purification.
n | = | Ln | Co | / | Ln | ( | lE | + 1 | ) | [ 1] |
 c end | Kf β |
Where co is the impurity content of the original metal;
L——the length of the melting zone;
E——the evaporation constant of the impurity, when the impurity concentration is 1, the amount of impurity evaporation per unit area and time;
F——the moving speed of the melting zone;
β - volume to area ratio of the melt zone, V / A.
(3) Width of the melt zone When melting in the suspension zone, the width of the melt zone is limited, which has little effect on the process. Generally, the width of the melt zone is preferably 1/2 to 1/3 of the diameter of the bar.
(4) The moving speed of the molten zone Reduces the moving speed of the molten zone, which is conducive to the diffusion of impurities and the improvement of the purity of the metal; however, if the moving speed is too slow, the metal evaporation loss will increase.
(5) Vacuum degree Maintaining a high degree of vacuum is conducive to the discharge of gaseous impurities, but too high a vacuum will cause an increase in the volatilization loss of the metal.
It can be seen from the above analysis that when the rare earth metal is purified by the regional melting method, the applicable process conditions should be specifically determined according to the nature of the rare earth metal itself, the characteristics of the equipment, and the operating environment.
It should be noted that the regional melting has not been applied to the purification of rare earth metals for a long time because the rare earth metals are active in nature and lack proper containers for metal ingots in the horizontal zone. In addition, the rare earth metal is easy to absorb the gas, and the operating atmosphere is unsuitable, so that the metal is easily contaminated, and some interstitial metal impurities cannot be completely removed, and the purification effect is not good. In the near view, the water-cooled copper container and the suspension zone are purified and purified in the horizontal zone, and the ultra-high vacuum technology is applied to create favorable conditions for the zone melt purification to be applied to the rare earth metal. Jones [15] and others have discussed in detail the refining of rare earth metals. Regional smelting is a very attractive purification technology in the preparation of ultra-pure metals. The content of treated metal impurities tends to drop hundreds of times, but the gap-like impurities only decrease by 2 to 3 times. At this time, electromigration technology can be used to make up for the deficiency.
References
15 , Huffine CL, Williams J M. in ref 8,145 ~ 162
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