Industrial pure rare earth metals are widely used in industry, and higher purity rare earth metals are mainly used for determining physical and chemical properties. There are four main purification methods used in the laboratory, namely vacuum melting, vacuum distillation or sublimation, electromigration, and zone melting.
Vacuum melting method
Rare earth metals with low vapor pressure, such as scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, and lutetium, are melted and purified at a vacuum degree greater than 10-6 Torr at a temperature 200-1000 ℃ higher than the melting point of the metal. In this case, impurities with high vapor pressure such as alkali metals, alkaline earth metals, fluorides, and low valent oxides (RO) can be distilled out, but the removal effect on high boiling point impurities such as tantalum, iron, vanadium, and chromium is poor.
Vacuum distillation method
Also known as vacuum sublimation method. Distillation purification of yttrium, gadolinium, terbium, and lutetium is carried out at a vacuum degree of 10-6-10-9 ° C and a temperature of 1600-1725 ° C, while sublimation purification of scandium, dysprosium, holmium, erbium, thulium, samarium, europium, and ytterbium is carried out at 1550-1650 ° C. Under these conditions, metal impurities with low vapor pressure such as tantalum and tungsten, as well as compounds containing carbon, nitrogen, and oxygen, will remain in the crucible. This method is often used in conjunction with vacuum melting.
Electromigration method
Apply direct current to the rare earth metal rod in ultra-high vacuum or inert atmosphere, and maintain it at 100-200 ℃ lower than the melting point of the metal for 1-3 weeks. Under the influence of high temperature and direct current electric field, various impurity elements accumulate along the test rod towards both ends due to differences in effective charge, diffusion coefficient, and mobility. Cut off both ends of the test rod, and perform electromigration purification again in the middle section. The purification of lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, yttrium, and lutetium using electromigration method in the laboratory has a significant effect on removing impurities such as carbon, oxygen, and nitrogen.
Regional melting method
Rare earth metal rods are melted multiple times in a zone melting furnace at a very slow speed (such as 0.4 mm/min for yttrium purification), which has a significant effect on removing metal impurities such as iron, aluminum, magnesium, copper, and nickel, but is ineffective for oxygen, nitrogen, carbon, and hydrogen. In addition, electrolytic refining and zone melting electric migration combined method also have certain effects on purifying rare earths.
