High Temperature Phase Relations of the Nd₂O₃-Na₂B₄O₇ Pseudo Binary System and Their Application to Recycling of Neodymium Magnets

Abstract

A high temperature recycling process of neodymium magnets using B 2 O 3 flux has been reported. However, the B 2 O 3 flux shows a poor fluidity of the slag. For this purpose of solving the problem, present work considers the use of Na 2 B 4 O 7 with lower viscosity than the B 2 O 3 flux.The phase equilibria for the Nd 2 O 3 -Na 2 B 4 O 7 pseudo binary system was investigated in the temperature range from 1460 K to 1780 K. We found that this pseudo-binary system forms the homogeneous melt in the concentration range of from 16 mass％Na 2 B 4 O 7 to 20 mass％Na 2 B 4 O 7 and over 55 mass％ Na 2 B 4 O 7 at 1673 K. Based on the determined this diagram, the feasibility study of the recycling process of the neodymium magnets using Na 2 B 4 O 7 flux was carried out 20 kg and 100 kg high frequency induction furnace. When neodymium magnets and an EV rotor were melted with Na 2 B 4 O 7 in a carbon crucible, the rare earth elements of Nd, Pr, Dy and Tb in the magnets and rotor were enriched to the Na 2 B 4 O 7 slag phase and the iron in the magnets and the rotor were contributed to the Fe-C alloy phase. The oxalate precipitation method was used for the recovered Nd 2 O 3 -Na 2 B 4 O 7 slag, more 99 mass％ purity ratio of RE x O y was recovered.