A Green Process to Acquire a High Purity Rare Earth Elements Leach Liquor from Nd–Fe–B Magnets by Caustic Digestion and Roasting Processes

To recover rare earth elements (REEs) from Nd–Fe–B magnets, a variety of hydrometallurgical processes have been developed, and among them, caustic digestion-acid leaching is the most promising. Through the caustic digestion, Nd and Fe in the magnet alloy could be converted into Nd(OH)₃ and Fe₃O₄, respectively, and they were easily recovered in the following acid leaching step. To remove iron from REE leach liquor, it was essential to precipitate iron after the acid leaching, and it consumed an amount of chemical like lime. So, to solve the aforementioned problem, a new green process was developed, lowering iron content in leach liquor; through an oxidative roasting of the digested product prior to the acid leaching, iron dissolution was considerably decreased. The digested powder was roasted at 350–450 °C, and the product was leached in 0.5 M HCl. The final leach liquor contained ca. 17 g/L Nd and ca. 330 mg/L Fe.