Methods for the Extraction of Rare Earth Elements from Coal Fly Ash: A Review

Rare earth elements (REEs) have become increasingly important due to their crucial roles in high-performance magnets, batteries, catalysts, and other advanced materials. However, the limited availability of primary rare earth element deposits and their uneven global distribution have increased the need to identify alternative secondary sources. Coal fly ash (CFA), a major by-product generated from coal combustion in thermal power plants, is considered a promising secondary source of rare earth elements. Utilization of fly ash is favorable not only due to its abundance, but also because of its potential to address waste management challenges. Therefore, the efficient recovery of rare earth elements from CFA is essential to meet the increasing global demand while supporting the principles of a circular economy. At present scenario, there is a need to gain insight into the status of global REE availability, applications, primary and secondary sources, and extraction processes. Several conventional methods, such as physical separation, leaching, precipitation, and solvent extraction have been adopted for the extraction of REEs from coal fly ash. However, to overcome the limitations associated with these methods, environmentally friendly electrochemical techniques have been explored. In recent years, ionic liquids is identified as green solvents for the extraction of REEs from fly ash in both chemical and electrochemical processes, thereby reducing the use of hazardous chemicals. Consequently, the combination of environmentally friendly electrochemical methods and green ionic liquid solvents has the potential to create a new trend in extractive metallurgy. Therefore, this review focuses on an overview of conventional methods and solvents used for REE extraction, in addition to advancements in electrochemical techniques and green electrolytes/solvents used.