Electrolytic Refining of Rare Earth Element from Neodymium Magnet Using Molten Salt
Authors: Kamimoto Yuki , Wakita Masaki , Ichino Ryoichi 
Published: February 2020

Introduction

 Neodymium magnets contain large amounts of neodymium, praseodymium, and dysprosium. Dysprosium, which is more expensive than neodymium and praseodymium, is added to neodymium magnets to increase their heat resistance. Neodymium magnets are used in numerous consumer electronics and electric- and hybrid-powered vehicles. Large amounts of neodymium are wasted each year when products that contain these magnets are discarded. Processes for the recovery of rare earth elements from neodymium magnets have been developed. Rare earth oxides are recovered from these processes, and these oxides are subsequently reduced to rare earth metals using molten salt electrolysis. We have reported a recovery process for rare earth elements from neodymium magnets using molten salt electrolysis, where these elements were recovered as alloys.

In this study, we focus on the electrical behavior of rare earth elements in neodymium magnets during molten salt electrolysis. We use anodic polarization experiments to determine the optimal electric leaching conditions and observe the leaching behavior of rare earth elements.

Materials and Methods

 The electrolysis potential was controlled using a potentiostat. The reactor was made of Pyrex glass and purged with Ar gas. A eutectic salt mixture of 59-mol% LiCl and 41-mol% KCl (melting point: 626 K), which melted at 723 K, was used in the electrolysis bath. The cathode electrode was a glassy carbon rod. The anode electrode was a neodymium magnet, a neodymium rod, a dysprosium rod, or an iron wire. The reference electrode was Ag/AgCl (0.1 N) in a eutectic composition of LiCl–KCl; this electrode was placed in a mullite tube. The electrolysis bath was maintained at 473 K for 24 h under a vacuum to eliminate water.

Results and Discussion

 The scan rate of anodic polarization was 5 mV s−1. The oxidation current of neodymium and dysprosium is generated at approximately −2.2 V, and oxidation potential is the lowest among the elements in the neodymium magnet. The iron oxidation potential was the highest at −0.7 V. Iron is the main component of the neodymium magnet, and the leaching of iron must be avoided in the recovery process. The use of potentiostatic electrolysis enables the selective leaching of rare earth elements from the neodymium magnet, as reported in our previous research. Rare earth elements were leached from the neodymium magnets using potentiostatic electrolysis. The neodymium magnets were used as the anode, and a carbon rod served as the cathode. The electrolysis potential was −1.0 V, and the quantity of electricity was approximately 1200 C. the residual magnet and molten salt compositions were similar for all three neodymium magnets used in the experiments. The residual iron content increased, and the concentrations of the rare earth elements decreased. The rare earth elements were leached from the neodymium magnet into the molten salt, and the total rare earth content in the molten salt was greater than 99.0 mass%.

Conclusion

Rare earth elements were leached from neodymium magnets using electrolysis in a molten eutectic mixture of LiCl and KCl. The oxidation potential of all neodymium magnet was −1.0 V. The oxidation potential of dysprosium was similar to those for neodymium and praseodymium. The content of rare earth elements in the leaching component was greater than 99.0 mass%.

Acknowledgement

This work was supported by Environment Research and Technology Development Fund of the Ministry of the Environment, Japan 3K143005.