Recovery of rare earth metals from Ni-MH batteries: A comprehensive review

The recycling of nickel-metal hydride batteries (NiMHBs) has garnered significant attention in recent years due to the growing demand for critical metals and the implementation of national and international legislation aimed at achieving zero carbon emissions and reducing environmental impact. Typically, NiMHBs contain 10 wt% of rare earth elements (REEs) including La, Ce, Nd, and Pr. However, the majority of these REEs (>90%) are being discarded in landfills each year. The scarcity of these metals and the concentrated distribution of their ore deposits in only a few countries have prompted significant concern globally. One of the existing strategies to address this issue is extraction of REEs through urban mining. This study provides an in-depth fundamental and systematic review on the existing strategies and technologies for the recovery of REEs from spent NiMHBs. Further, the state-of-the-art approaches for the individual separation of La, Ce, Nd, and Pr from aqueous media are discussed, along with their corresponding challenges and shortcomings as well as the potential future directions. The research aims to provide a transformative understanding of various methods for the recovery of REEs from NiMHBs, the available techniques for the individual separation of REEs from different secondary resources, and potential improvements in the recycling process of spent NiMHBs. The outcome of this work will contribute to the development of more efficient and effective REEs recovery methods and help address the growing concern of REEs scarcity and extraction environmental impact.