Coal and Coal Combustion Byproducts as Promising Resources for the Extraction of Rare Earth Elements for the Current Energy Transition

ABSTRACT There is a growing demand for rare earth elements (REE) due to their extensive applications in carbon‐neutral technologies, which are crucial to mitigating the impacts of human‐induced climate change. Due to insufficient conventional REE resources, a global search is underway to extract them from alternative resources in addition to recycling efforts. In recent times, coal and coal combustion byproducts such as coal fly ash (CFA) have emerged as low‐grade sources of REE, as significant REE concentrations characterise coal and coal combustion byproducts worldwide. Multiple studies demonstrated that the REE concentrations in CFA resources are equal to or higher than those in some conventional REE ores. The continuous coal combustion in coal‐fired thermal power plants worldwide can lead to further accumulation of CFA, which can serve as a potential new source for REE. Several CFA resources from different coal basins and coal‐fired thermal power plants across the world reported a concentration of ~2000 μg/g of ∑REE, which is significantly higher than the world average value of 404 μg/g. This comprehensive article aims to evaluate the potential of the world CFA to serve as an alternative resource for REE. This review also discusses the geochemical distribution of REE in coal, CFA, bottom ash and other coal combustion byproducts; mineralogical variations in them, mineral associations of REE within different phases, various extraction methods, different chemical characterisation methods and related environmental aspects. The usefulness of machine learning (ML) algorithms for an easy understanding of the REE potential of coal and CFA, and various methods to prevent environmental REE pollution, is also covered.