Alkali baking and solvometallurgical leaching of NdFeB magnets

Abstract

End-of-life NdFeB magnets are an important secondary source of rare-earth elements (REEs) and cobalt. Recycling of these magnets can also mitigate the supply problems of its constituent critical REEs (mainly neodymium and dysprosium). The recycling of bonded NdFeB magnets has received much less attention than that of sintered NdFeB magnets. In this study, a novel flow sheet is presented for recycling of bonded NdFeB magnets that is applicable to sintered magnets as well. Demagnetized magnet powder was mixed with 25 or 40 wt./vol% NaOH solution and baked at 150 to 200 °C for 30 to 540 min. In this way, REE metals were transformed into their corresponding hydroxides, whereas iron metal formed NaFeO2. By washing the reaction mixture with water, 96.5% of Na was recovered as NaOH and Na2CO3, whereas 90.3% of B was recovered as borax. The calcine containing REE hydroxides and iron oxide was then leached at 60 or 90 °C with 20 vol% Versatic Acid 10 diluted in an aliphatic diluent. >95% of the REEs were dissolved, with <1% co-dissolution of iron and <10% co-dissolution of cobalt. Precipitation stripping with an oxalic acid solution quantitatively regenerated the organic solvent with virtually the same composition of fresh solvent. After calcination of the oxalate precipitate, a mixed rare-earth oxide with 98.4 wt% purity was produced. The waste oxalic acid solution (pH ≤1) containing co-dissolved sodium and minor amounts of iron and cobalt could be used as a leaching agent for Co in the maghemite-dominated residue.