Research progress on novel technology and mechanisms for the removal of oxygen impurities in rare-earth metals

Rare-earth metals play a very important role in the development of high-tech industries and the research of functional materials. The demand for high-purity rare-earth metals has rapidly increased because of the development of modern society and the rare-earth industry. However, the purity of commercial rare-earth metal materials is the key factor limiting the rapid development of rare-earth industry. The major impurities in commercial rare-earth metal materials are gas impurities, especially oxygen. The removal of oxygen impurities is extremely challenging because of their strong affinity, which causes the problem of high oxygen concentrations of raw rare-earth metal materials. Therefore, effectively removing oxygen purities is the key to achieve ultra-high purity rare-earth metals. In recent years, several new and innovative strategies to remove oxygen impurities from rare earth metals have been proposed and investigated. To overcome the kinetic and thermodynamic barriers, various driving forces have been creatively introduced into the refining process. This article presented some novel purification methods to remove oxygen impurities, including active metal external getting method, hydrogen plasma arc melting method and hydrogen in-situ refining method. Experimental results show that the oxygen concentrations in rare-earth metals can be reduced effectively by introducing various driving forces. The optimum process conditions of these techniques were introduced in detail, with the final concentrations of oxygen being reduced to below 5×10^-5. The design concept and refining mechanism of the driving forces, including active metal, hydrogen plasma and dissolved active hydrogen atoms in rare-earth metal, were discussed systematically in this paper. The refining principle and migration mechanism of oxygen impurities were also discussed by combined experimental and calculated studies. The FAST-2D combined Stefan simulating method, ¹⁸O₂ isotope tracking method and CALPHAD method were used to study the purification mechanism. The study results provide theoretical basis for the removal of oxygen impurities in rare-earth metals. The methods demonstrated here may provide insight on the future research in rare-earth purification.