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.ˇˇRemelting 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.