Rare Earth Elements Occurrences in Coal Fly Ash and Methods of their Extraction: A Review

ABSTRACT A significant portion of global electrical power is derived through coal-based power plants especially in Asian countries. Burning of pulverized coal in these power plants, produces a powdered residue known as coal fly ash (CFA) which is partially used for manufacturing of material used in construction like cement, bricks, etc. Majority of the CFA amounting to a quantity of around 500-600 million tonnes is dumped in landfills posing a serious risk to environment and human health. Several studies have identified CFA as the considerable source of Rare Earth Elements (REEs) and it has also been confirmed that REEs concentration in fly ashes can be several times richer as compared to concentration in coal because of loss of organic matter during combustion of coal. This insight gave rise to research in this domain and several methods for extraction of REEs from coal fly ash were established over last one and half decade, the most prominent being acid or alkali leaching methods. Several other methods that were used for extraction are bioleaching and ionic leaching methods. All these methods recover 30-90% of REEs present in the coal fly ash based on the chemical composition of REEs, coal properties, and additional physical processes to enhance leaching. The recovery of REEs from CFA has been gaining momentum recently in the research domain with the scope of offering an alternative source of REEs to the global market where the supply has been unstable and skewed. The challenge, however, remains to design and develop effective processes for economically extracting and recovering REEs. This review focusses on the recent and significant studies conducted on the extraction of REEs from coal fly ash to highlight the advancements and showcasing the gap in working towards the scalability and commerciality of the technology at industrial level.