Evaluation of a hybrid pyro–hydrometallurgical process for the selective leaching of rare earth elements from spent NdFeB magnets
Soroush Rahmati, Hossein Shalchian, Roshanak Adavodi, Ionela Birloaga, Pietro Romano
Abstract
The rising global demand for rare earth elements (REEs) has made the recovery of Nd, Pr, and Dy from end-of-life NdFeB magnets a strategic priority for sustainable resource management. Coventional hydrometallurgical processes often face challenges related to poor selectivity and complicated purification. This study proposes a hybrid pyro-hydrometallurgical approach that enhances REE recovery by promoting the selective formation of stable oxide phases during thermal pretreatment, resulting in a ∼ 34 % increase in sample mass. Subsequent selective leaching experiments using sulfuric, citric, and acetic acids at temperatures up to 90 °C revealed that nearly complete REE dissolution (∼100 %) with limited iron solubilization (∼15 %) after 4 h can be achieved using sulfuric acid, demonstrating outstanding selectivity that redices downstream purification requirements. Kinetic modeling based on the shrinking core model indicated that leaching is governed by multiple rate-controlling steps, including chemical reaction, product layer diffusion, and liquid film diffusion, with product layer diffusion exhibiting the highest activation energy, offering valuable insight for process optimization. Thermodynamic analysis confirmed that REE dissolution in sulfuric acid is spontaneous (negative ΔG), while iron dissolution remains non-spontaneous (positive ΔG), reinforcing the observed selectivity. This integrated pyro-hydrometallurgical process demonstrates a viable, scalable, and sustainable alternative to primary mining for critical REEs, improving extraction efficiency, minimizing purification efforts, and reducing operational costs. • 950 °C oxidative roasting converts magnet metals to oxides, boosting REEs dissolution. • Sulfuric acid leach yields ∼100 % REEs and dissolves only 15 % iron. • Hybrid thermal-acid method cuts reagents and downstream purification. • Kinetic modeling reveals product-layer diffusion as the rate determining leaching step.