액상 마그네슘을 이용한 Nd-Fe-B 영구자석내의 네오디뮴 추출에 관한 연구

Abstract

More than 90 % of rare earth elements (REEs) are currently produced from China. However, due to the increasing usage of REEs in high-valuable electronic devices and important components for electronic, communication, automotive, etc., shortage of supply is expected in the near future [1-3]. Moreover, the Chinese government limits the export of REEs and may use it as a political weapon. In particular, Nd is one of the most critical REEs that could face a supply constraint soon. The usage of Nd shall be increased drastically due to the application to the electrical motors in current and future hybrid and electric vehicles [4-6]. In addition, Nd is a quite promising alloying element for Mg alloys for high temperature and sheet applications. Thus, recycling of Nd from Nd-Fe-B magnet scraps is inevitable to keep the balance between supply and demand. Currently, more than 50,000 tons of Nd-Fe-B magnets are produced each year [7]. During the processing of these magnet materials, a large amount of scrap material (eg. machining swarf, broken components, etc.) is generated. Moreover, scrap Nd-Fe-B material is created from end of life products. Together, these account for a potentially large source of Nd that can be recovered. The objective of this research was to develop a metallurgical process by which high-purity Nd can be recovered from magnetic scrap material while minimizing contaminant elements. To accomplish this we focused on utilizing liquid-Mg extraction, which is a technology that was previously developed at Ames Lab [8, 9].

In this study, in order to identify the specific reaction mechanism, a diffusion couple consisting of an Nd magnet and Mg was prepared. Pure Mg was melted with Nd-Fe-B magnet and held at temperatures in the range 993- 1073 K. The Mg was allowed to solidify, and the castings were then sectioned and characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analysis. Nd was found to have diffused rapidly out of the solid Nd-Fe-B magnet into the molten Mg. The thickness of the diffusion layer was measured, the diffusion of Nd through the Nd-Fe-B magnet into liquid Mg was described, and the diffusion coefficient of Nd in liquid Mg was estimated. In addition, the Nd-Fe-B scrap material, which was non-magnetic, was broken up in a jaw crusher and sieved out to -10/+30 mesh corresponding to particles sizes between 0.6– 4.0 mm. The particles were encapsulated in a box of 30 mesh screen that was sealed on all sides. The Mg-(Nd-Fe-B) alloys fabricated according to induction melting process. Lastly, we discuss to decrease the amount of liquid frozen in the mesh box containing the scrap pieces and reduce the work required to crush and sieve fine particle sizes, we examine the effects of larger particle sizes on the diffusivity of RE metals into the liquid Mg. In addition, obtaining high-purity RE metal from magnetic scrap material, we utilized vacuum distillation of the Mg-RE alloy that had undergone the two-step enrichment process. This study shows that liquid metal extraction of Nd may be a viable and inexpensive method for recovering the expensive rare earth element Nd for use in Mg castings.