(Invited) A Novel Electrochemical Recycling Method for Rare Earth Metals from Scrap Magnets Using Molten Salt Electrolysis and Alloy Diaphragms

Abstract

Today, rare earths (RE) are indispensable as industrial materials all over the world. At the same time, the world is facing a shortage of rare earth resources and a sharp rise in rare earth prices. In order to alleviate this situation and establish a more sustainable use of RE metals, it is necessary to develop a simple, inexpensive, and environmentally friendly process for the recovery and separation of RE metals. In particular, the recovery of RE from Nd-Fe-B magnet scraps is very important because a large amount of magnet scraps are expected to be disposed of from EVs and HEVs in 10 years. For use in EVs and HEVs, Dy must be added to Nd-Fe-B magnets in order to maintain strong magnetic properties even at temperatures above 473 K. Therefore, the recovery and separation of Dy and Nd, which is generally difficult, is required for the new recycling process.With this background, we proposed a process for the separation and recovery of RE metals from magnet scraps using molten salt and alloy diaphragms (Figure 1) [1-4, 6-11]. The proposal was based on our previous finding that certain RE metals can be rapidly alloyed and de-alloyed with iron group (IG) metals in molten salt by electrochemical methods [5].As a candidate molten salt, we used LiCl-KCl eutectic melt at 723 K. We first investigated the electrochemical formation of Dy-Ni, Nd-Ni, and Dy-Nd-Ni alloys in the molten salts containing DyCl3, NdCl3, and DyCl3-NdCl3, respectively, and achieved a high separation mass ratio of Dy/Nd = 72 in the alloy samples [6]. As a next step, we demonstrated the selective permeation of Dy through the alloy diaphragm using a primitive-type electrolytic cell [7]. Recently, we further investigated the selective permeation of Dy through an alloy diaphragm made of RENi2 [8].Considering that molten fluorides such as LiF-REF3 and LiF-CaF2-REF3 are used for smelting RE metals, we also investigated the electrochemical formation of Dy-Ni, Nd-Ni, and Dy-Nd-Ni alloys in LiF-CaF2 eutectic melts containing DyF3, NdF3, and DyF3-NdF3, respectively, at 1123 K [2-4]. The results showed that the Dy/Nd ratio in Dy-Nd-Ni alloys has a potential dependence. The highest value of Dy/Nd was 5.6, which is lower than the value obtained for the LiCl-KCl melt [2].We originally focused on RE-Ni alloy, which has both high alloy formation rate and high Dy/Nd separation rate, and started to investigate it as an alloy diaphragm. However, RE-Ni alloy is not suitable for long-term use because of its brittleness and low durability. In order to develop a more durable alloy diaphragm, iron-group alloys such as Ni-Cr, Ni-Mo, and Ni-Cr-Mo alloys were selected as candidates for the base material [9,10]. Since Cr and Mo are difficult to alloy with RE elements, these components are expected to reinforce the mechanical strength of the alloy diaphragm. The shear stress measurements of the Dy-alloyed samples showed that the Ni-Cr-Mo alloy is the most suitable base material to improve mechanical strength [10]. We are also investigating the use of liquid alloys as another candidate for the diaphragm material [11].AcknowledgementA part of this study was conducted as commission research with the New Energy and Industrial Technology Development Organization (NEDO) of Japan.