Recycling of Rare Earth Magnet Scraps: Part I Carbon Removal by High Temperature Oxidation

In the recycling of the permanent Nd–Fe–B magnet, the carbon such as free carbon and carbides at the grain boundary segregation in the magnet scrap are elements that are harmful and which obstruct the magnet performance. The removal of the carbon is currently a major problem. Remelting of scraps should, therefore, be done on condition that the carbon content is lowered to an appropriate level prior to melting. The aim of this work was to open the route for decarburization of the Nd magnet powder scraps with high carbon, as well as of the sintered body scraps with moderate carbon concentration. Decarburization via oxidation in air allowed the removal of not only free carbon but also grain boundary carbides at temperatures higher than 1273 K down to 300 mass ppm. However, an increase in the oxygen content is inevitable with the high temperature oxidation. The next step was then to reduce the iron oxide by heating it in a hydrogen atmosphere at 1273 K and subsequently to eliminate the oxygen combined with the rare-earth constituent using calcium which has a strong affinity with oxygen. The basic technology for the recycling process of the permanent Nd–Fe–B magnet scraps was discussed.