Hydrogen decrepitation and recycling of NdFeB-type sintered magnets

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000 degrees C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700 degrees C, giving powder with a remanence (B-r) of similar to 1350mT (+/- 10 mT) and an intrinsic coercivity (H-cj) of similar to 750kAm(-1) (+/- 10 kAm(-1)). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080 degrees C for 1 hour. The magnetic properties of this magnet were: (BH)(max) = 290 kJm(-3) (+/- 5 kJm(-3)), B-r = 1240mT (+/- 5 mT) and H-cj = 830 kAm(-1) (+/- 5 kAm(-1)); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.