**1. Introduction**

Disquiet around the sustainability of rare-earth elements (REE) provisions has stimulated determination not only to recycle but also to improve the proficiency of the materials they are used to make [1]. With respect to the amount of REEs produced, neodymium (Nd) usage in the production of neodymium-Iron-Boron (NdFeB) magnets from mine output is about 13%. Out of that, 34% of the magnets produced are used in the manufacturing of actuator hard disk drives [1–3]. With their life span centered on their application, rare earth materials in hard disk drives are also applied in parts such as printed circuit boards (PCB), spindles, and so on [4].

In the production of these magnets, specific additions of elements are also used to adjust their properties [5,6]. For example, cobalt (Co) is used to substitute REE and iron (Fe) materials (up to over 5%) to increase the Curie temperature [7,8]. Dysprosium (Dy) addition increases the temperature characteristics such that the compound has a better stability against demagnetization. It also decreases the residual induction of the magnet, which leads to lower magnetic field properties [6,9]. To lower the production costs, Pr is now used as a substitute of Nd (up to 20–25%) in the production of magnets. Additionally, the magne<sup>t</sup> material is coated with a protective layer, such as copper (Cu), aluminum (Al), or nickel (Ni), as well as polymeric material to hold the magne<sup>t</sup> on the steel plate [4]. Although this shows that recycling of end-of-life magnets can help reduce the criticality of these REE in the near future, commercial recycling of REE is low, at less than 1%. This is mainly due to inefficient collection, technological difficulties, and high cost of processing [10–15].

Separation stage(s) have always been an important step in recycling. For rare earth magnets, since the element of interest is found with two or more others, they may require di fferent extraction technology. Hydrometallurgical approaches have been shown to be a very e fficient way to separate the REEs in which chemical separation by leaching is performed [16,17]. For REE leaching, lixiviants are directly added with or without heat treatment to dissolve the solid materials. Once the materials are in solution, various processes such as precipitation, solvent extraction, and ion exchange can be used to economically produce individual REE in the required form.

As a part of the impurities encountered in REE recovery, some of the leached Fe is also recovered as ammonium jarosite in the final REF product. To remove this impurity, the final product form is subjected to high temperature (250–500 ◦C) to decompose the ammonium jarosite [18–21]

For solvent extraction and ion exchange, di fferent cationic, anionic and solvating extractants such as di (2-ethyl-hexyl) phosphoric acid (D2EHPA), dialkyl phosphonic acid (Cyanex 272), 2-ethyl-hexyl phosphonic acid mono-2-ethyl-hexyl ester (PC 88A), neodecanoic acid (Versatic 10), tributyl phosphate (TBP), and tricaprylylmethylammonium chloride (Aliquat 336) have been reported for the separation of REEs from solution with D2EHPA being more commonly used with nitrate, sulfate, chloride and perchlorate solutions, PC 88A with chloride solutions, and TBP with nitrate solutions [22–35]. Interestingly, many of the same chemical types used as solvent extractants are also used in solid form as ion-exchange resins from the same type of leaching solutions [23,24,28,32]. For both solvent extraction and ion exchange, the REE-loaded material must then be selectively stripped. The resulting solutions are then predominantly processed to precipitate the individual REEs, often as REOs, but not always [36–39].

This paper presents the recycling Nd magne<sup>t</sup> scrap using a novel hydrometallurgical process involving H2SO4 leach, NH4OH precipitation, and NH4F·HF reaction. The latter step transforms the precipitate into rare earth fluorides (REF) which should be appropriate feedstock for subsequent pyrometallurgical processing into metal in molten fluoride electrolysis [39]. The application of hydrofluoric acid (HF) was completely avoided and the process was optimized through statistical analysis and modelling.
