2. Magnet-to-magnet recycling route
We used the commercial-scale, recycling process presented by Zakotnik and Tudor (2014) and Zakotnik et al. (0000),
which is highlighted with orange and gray in Fig. 1. Using this method, the bulk of the waste Nd–Fe–B magnets are directly
subjected to the magnet-to-magnet recycling process in an effort to obtain sintered Nd–Fe–B magnets with the magnetic
performance required by the market.
Harvesting: The recycling process started with a custom-built, semi-automated mechanical apparatus that was used
to remove the magnetic assembly holding the sintered Nd–Fe–B magnet from the end product (e.g., hard-disk drives).
Essentially, this was achieved by a single cut across the correct corner of each hard-drive or automatic milling to remove
screw heads such that the Nd–Fe–B magnet was separated away from the rest of the scrap material.
Demagnetization: The magnetic assembly was loaded into a 1-m3 muffle furnace and heated up to 400 °C, above the
Curie point of 325 °C, in order to demagnetize the Nd–Fe–B magnet and weaken any adhesive used to hold the magnet to
the assembly. The temperature measurement during the demagnetization step was made using a K-type thermocouple. A
hold time of 30 min was sufficient to weaken the adhesive for a 120 kg batch size. The heating can be carried out in air or
an inert atmosphere. As there was a risk of toxic emissions, generated while heating these magnet assemblies, the gaseous
wastes were passed through a regenerative thermal oxidizer, designed to burn off the volatile organic compounds (Rotival
et al., 1994). Finally, the waste magnets were separated and collected from the assembly.
Cleaning: Once the sintered magnets were removed from the assembly, the surfaces of the magnets were then cleaned
prior to further processing. For a 120 kg batch of magnets, 15 min of abrasive jet was sufficient to remove the corrosionprotection
layer. Mechanical grinding was then used to treat the surface, followed by a 3-min immersion in a hot chemical bath containing dilute nitric acid. The loss of waste sintered Nd–Fe–B material observed during the surface treatment was
1–2 wt%.
Hydrogen decrepitation: The surface-cleaned waste sintered Nd–Fe–B magnets were then exposed in the hydrogendecrepitation
reactor, together with the additives necessary for restoring magnetic properties. The hydrogenation process
was carried out at 80 °C using 1 bar of hydrogen pressure. After that, the material was heated in situ to 550 °C to
partially degas the material. The wide particle size range generated by the hydrogenation step was typically in the range of
10–2000 µm.
Jet milling: Upon completion of Jet milling, the powders were transferred into to light roller mill while 1 wt% of lubricant
was added to ensure better magnetic alignment. These powders were further subjected to jet milling under nitrogen, to
decrease the powder size to approximately 4µm, at a rate of 28.6 kg/h. Typically 1 wt% of material loss was observed. During
this process, the recycled powders were blended and pulverized into fine powder, together with in-situ added Nd–Pr hydride
using jet milling. The reason for adding the Nd–Pr is to achieve total recovery or a temperature-stability improvement of
the recycled magnets.
The subsequent processing; e.g. aligning and pressing in a magnetic field, isostatic pressing, sintering and annealing, was
very similar to the traditional magnet-manufacturing route.