Review of rare earth elements recovery from secondary resources for clean energy technologies: Grand opportunities to create wealth from waste

Rare earth elements (REEs) are vital components of high-tech electrical and electronic based materials. In the global scenario, the deposits of natural rare earth elements (REEs) are limited except for countries such as China. This has led to a dependence on major secondary rare earth-bearing sources such as battery waste, scrap alloy, spent magnets, spent catalysts, waste light-emitting diodes (LEDs), and fly ash for a substantial recovery of REEs for their use. Recycling REEs from these secondary waste sources by hydrometallurgical routes appears to be a sustainable approach with low waste generation, few emissions, low energy consumption, economically feasible, and environmentally friendly. In most of the reported studies, secondary waste is subjected to chemical and or bioleaching followed by solvent extraction processes for clean separation of REEs. Subsequently, rare earth compound(s) are recovered through precipitation. Solvent extraction (liquid-liquid extraction) is one of the most vital stages in the overall process as REEs (light and heavy rare earths) exhibit a low separation tendency while being extracted with various solvent reagents from corresponding aqueous media. This review provides an in-depth discussion on the recycling strategies implemented to obtain REEs from numerous secondary wastes; extraction behavior of REEs; challenges, advantages, and disadvantages of the proposed methodologies; as well as proposed flowsheets for clean separation and recovery of REEs. In addition, the role of REEs in renewable and green energy technologies and the future aspects of the reprocessing technology of secondary REE sources are summarized and reported.