Development of a hydro-pyrometallurgical process for the recycling of spent NdFeB magnet

Jury members
Reviewer - Sylvie Delpech, IJCLab, Université de Paris-Saclay, France
Reviewer - Jean-Christophe Gabriel, LICSEN - Commissariat à l'Énergie Atomique, Université de Paris-Saclay, France - Nanyang Technological University, Singapore
Examiner - Sophie Rivoirard, CTO & Co-founder of MagREEsource, Noyarey, France - Institut Néel, Université de Grenoble, France
Examiner - Hervé Muhr, LRGP, Université de Nancy, France
Examiner - Gilles Philippot, ICMCB, Université de Bordeaux, France 
PhD Supervisor - Alexandre Chagnes, GeoRessources, Nancy, France
PhD Co-Supervisor - Thibault Quatravaux, IJL, Nancy, France

Abstract
Human activities cause severe environmental challenges like climate change and pollution. Transitioning to renewable energy sources and electrifying transportation are essential to reduce carbon emissions. Wind energy and EVs play crucial roles in this transition, and they require permanent magnets made of rare-earth elements such as neodymium and dysprosium. However, the supply of these metals is limited by geopolitical and environmental issues. Developing a recycling chain for used magnets in Europe is essential to secure a sustainable supply, reduce external dependencies, and support green technologies and economic stability. This report investigates two recycling routes: liquid metal extraction and a hydrometallurgical process focused on leaching and purification steps. In the liquid metal extraction process, NdFeB magnets are immersed in liquid magnesium, allowing rare-earth elements to diffuse into the liquid metal. An experimental setup includes a heating system and a sealed capsule reactor to ensure optimal temperature control. The diffusion zone, where rare-earth elements diffused into the liquid metal, is analyzed using SEM-EDS. Elemental composition analysis is conducted using EPMA to determine the rare-earth element content in the depleted zones of the magnets. The study resulted in the extraction of 88.07% of the neodymium after 50 minutes of treatment.
The hydrometallurgical route involves a detailed leaching process. Spent NdFeB magnets undergo thermal oxidation to prepare the material for leaching, enhancing selectivity by altering the chemical composition. The oxidized powder is then immersed in acid under controlled conditions. Characterization is conducted using SEM-EDS and XRD, and results are analyzed by MP-AES to determine leaching efficiency. The leaching process produced a pregnant liquor solution with 18.51 g/L neodymium. The purification stage uses liquid-liquid extraction, transferring metal ions from the aqueous to the organic phase. The study investigated various extractants and conditions. Single extractants and synergistic solvents were tested, resulting in low neodymium extraction efficiency (~10%) but high efficiencies for dysprosium and gadolinium (>80%). The final strategy aimed to retain neodymium in the raffinate, yielding 16.03 g/L neodymium with minimal impurities.
The report is completed by a screening life cycle assessment comparing the environmental impacts of the hydrometallurgical and the pyrometallurgical recycling processes for NdFeB magnets with the conventional route. The pyrometallurgical route emerged as the best option for producing metallic neodymium from recycled magnets, with impacts 6 times lower than the conventional route and 3.4 times lower than the hydrometallurgical route. However, this study provided preliminary insights, and a complete life cycle assessment is necessary for more accurate results.