Selective Recovery of Rare Earth Elements from Waste Magnets by Molten Salt Electrochemical Process

Abstract

Neodymium magnets are the strongest permanent magnets with Nd2Fe14B as the main phase. In applications where high temperature operation is expected, such as motors for electric vehicles (EVs) and hybrid electric vehicles (HEVs), Dy is added to prevent degradation of magnetic properties. Currently, the demand for EVs and HEVs is growing rapidly, and Dy-doped NdFeB magnets are used in these vehicles. Thus, a large amount of waste magnets will be generated in the future. Moreover, for heavy rare earths such as Dy, high-quality resources are extremely unevenly distributed in the world, and there are already concerns about their stable supply. Given the above background, there is a need to develop an efficient recycling process that can replace the conventional wet recycling process, which has a large environmental impact and energy consumption.Our research group has proposed a process for the separation and recovery of rare earth elements (REEs) from the waste of neodymium magnet using molten salt electrolysis and alloy diaphragms [1,2]. The principle of this process is shown in Fig. 1. First, the Nd and Dy ions in the waste magnet used as the anode are dissolved into the molten salt by anodic dissolution. Next, the dissolved Nd or Dy ions are reduced on the surface of the diaphragm, which functions as a bipolar electrode, and only one of the ions is reduced to form an alloy. Furthermore, Nd or Dy diffuses through the alloy diaphragm and is dissolved into the molten salt by anodic dissolution on the cathode chamber side surface of the diaphragm. Finally, Nd and Dy are separated and recovered on the cathode.So far, we have reported that selective permeation of Dy [2] and Nd [3] is possible using LiCl-KCl as the molten salt and a solid Ni-RE alloy as the diaphragm. In the present study, we selected LiF-CaF2 as a more practical fluoride-based molten salt and Hastelloy as a more durable diaphragm, and investigated the electrochemical permeation of RE. We also considered that liquid Fe-RE alloy is expected to be a more desirable diaphragm with higher durability and faster permeation. As a fundamental study, the formation behavior of liquid Fe-Nd and Fe-Dy alloys was investigated, and the results will also be reported.[1] T. Oishi, H. Konishi, T. Nohira, M. Tanaka, and T. Usui, Kagaku Kogaku Ronbunshu, 36, 299 (2010). [in Japanese][2] T. Oishi, M. Yaguchi, Y. Katasho, H. Konishi, and T. Nohira, J. Electrochem. Soc., 167, 163505 (2020).[3] T. Oishi, M. Yaguchi, Y. Katasho, and T. Nohira, J. Electrochem. Soc., 168, 103504 (2021). Figure 1