Al(NO3)3·9H2O solution: a green Brønsted solvent for the recycling of NdFeB magnet waste

Spent neodymium–iron–boron (NdFeB) magnets represent a valuable secondary source of rare earth elements (REEs). This study presents an innovative process for recovering Neodymium (Nd) from spent NdFeB magnets using inorganic Al(NO 3 ) 3 ·9H 2 O solution composed of aluminium nitrate nonahydrate (Al(NO 3 ) 3 ·9H 2 O) and water referred to as a “green Brønsted solvent.” Hammett acidity measurements confirm that concentrated solutions of Al(NO 3 ) 3 ·9H 2 O and H 2 O exhibit strong Brønsted acidity, enabling the selective leaching of Nd from NdFeB magnet sample roasted at 660 °C. A high separation factor of 4150 for Nd over iron (Fe) was achieved. This Al(NO 3 ) 3 ·9H 2 O solution offers significant advantages, including non-volatility, safety, high solubility, and excellent selectivity for REEs. Under optimal conditions (80 °C, solid-to-liquid ratio of 1:20, and leaching time of 150 min), 96.9% of Nd was leached, while Fe leaching was limited to just 0.9%. The dissolved Nd 3+ was subsequently recovered from the leachate by precipitation with oxalic acid, followed by oxidative roasting, yielding Nd 2 O 3 with a purity greater than 95%. This high selectivity and product purity demonstrate the feasibility of this process for producing high-quality Nd 2 O 3 suitable for reuse in NdFeB magnet manufacturing. Moreover, the approach minimizes impurity incorporation and downstream processing requirements, offering an environmentally friendly strategy for REEs recovery from secondary resources. • Green Brønsted solvent enables selective Nd leaching from spent NdFeB magnets. • Roasting of NdFeB at 660 °C significantly enhances the selective recovery of Nd. • High Nd/Fe separation factor of 4150 achieved with Al(NO 3 ) 3 ·9H 2 O and H 2 O solution. • Optimised leaching yielded 96.9% Nd recovery with only 0.9% Fe co-leached. • Nd 3+ recovered as >95% pure Nd 2 O 3 via oxalic acid precipitation and roasting.