A comprehensive review on recycling of critical raw materials from spent neodymium iron boron (NdFeB) magnet

•Supply risk of critical raw materials of green energy technologies are examined.•The rare earth elements (REEs) present in NdFeB magnet fall under critical category.•End-of-life NdFeB magnet is presented as potential secondary resource of REEs.•Recycling of critical raw materials from spent NdFeB magnet has been reviewed.•Advantages and limitations of various recycling methods discussed. The present paper focuses on the importance of critical raw materials (CRMs) of green energy technologies especially, wind mill and electric vehicle (EVs) sectors. The Neodymium-Iron-Boron (NdFeB) magnet is one of the essential component of these green energy technologies. The rare earth elements (neodymium, praseodymium and dysprosium) present in NdFeB magnet, due to their increasing demand, high tech application and limited availability, fall under critical category. In the present review paper, the supply risk associated with these critical raw materials has been examined. To meet the demand of these critical rare earth metals, the end-of-life NdFeB magnets have emanated as a potential secondary resource of rare earths. Therefore, the recycling aspects of critical rare earth metals from spent NdFeB magnet have been comprehensively reviewed in the present paper under (i) Direct recycling of spent NdFeB magnet (ii) Extraction of critical rare earth metals using pyrometallurgical, hydrometallurgical, electrometallurgical methods. The advantages and limitations of different methods have been critically examined. The review indicated that effectiveness of a recycling method depends on the type of spent NdFeB magnet (scrap, sludge or spent magnet) and their level of contamination. Particularly, direct recycling method suitable only for non-oxidized magnets (low contamination). Pyrometallurgical methods use high temperature processes recovering rare earths in the form of alloy or mixed compound, which require subsequent processing for purification. The yield and purity are in trade-off situation. Consumption of large amount of chemicals and loss of rare earths during purification of leach solution is one of the major drawbacks for hydrometallurgical processing of waste NdFeB magnet. Very few processes have been studied at large scale from commercialization aspect. Thus, there is a scope exist in the coming future to develop a robust, eco-friendly and energy efficient process for selective extraction and separation of critical rare earths from spent NdFeB magnets. The efficien