Regarding the restrictions
recently imposed by China
on the export
of rare-earth elements (REEs), the world may face a serious challenge
in supplying some REEs such as neodymium and dysprosium soon. Recycling
secondary sources is strongly recommended to mitigate the supply risk
of REEs. Hydrogen processing of magnetic scrap (HPMS) as one of the
best approaches for magnet-to-magnet recycling is thoroughly reviewed
in this study in terms of parameters and properties. The processes
of hydrogen decrepitation (HD) and hydrogenation–disproportionation–desorption–recombination
(HDDR) are two common methods for HPMS. Employing a hydrogenation
process can shorten the production route of new magnets from the discarded
magnets compared to other recycling routes such as the hydrometallurgical
route. However, determining the optimal pressure and temperature for
the process is challenging due to the sensitivity to the initial chemical
composition and the interaction of temperature and pressure. Pressure,
temperature, initial chemical composition, gas flow rate, particle
size distribution, grain size, and oxygen content are the effective
parameters for the final magnetic properties. All these influencing
parameters are discussed in detail in this review. The recovery rate
of magnetic properties has been the concern of most research in this
field and can be achieved up to 90% by employing a low hydrogenation
temperature and pressure and using additives such as REE hydrides
after hydrogenation and before sintering.