A review on mineralogical speciation, global occurrence and distribution of rare earths and Yttrium (REY) in coal ash

ABSTRACT: Rare earths and Yttrium (REY) are a group of critical metals essential for this electronic and digital era. China is the leading producer of REY with more than 90% of global export. Mines of REY are limited and the need for green and efficient energies have augmented the demand of REY and it is putting enormous pressure on global production. REY market is predicted to grow from USD 5.3 billion in 2021 to 9.6 billion by 2026, at a CAGR (Compound Annual Growth Rate) of 12.3%. The need for permanent magnets is propelling the demand of the critical group REY and is expected to rise gradually in the coming years. In the present review, we have summarized the minable REY resources and their applications. The requirement for alternative resource is pivotal to meet our future needs. We have extensively reviewed the studies of REY in coal fly ash (CFA). A comprehensive analysis has been done for the REY resources worldwide for the last several decades in coal ash (CFA and bottom ash) and divulged into the application, speciation and distribution for major coal-consuming countries like China, India, USA, Russia, UK, Poland, etc., individually. We have also made a comparative global study and inferred potential extractable coal ash resources using various parameters such as global average, critical percentage (C(p)), outlook coefficient (C(out)), etc., for a better understanding of economic exploitation. RESEARCH HIGHLIGHTS: We have put up enormous effort to synthesize rare earth elemental data of coal ash from different coal-consuming countries. Following are the major highlights of this review article. We have compiled data on occurrence of Rare Earths and Yttrium (REY) in coal ash from 13 countries such as China, India, USA, UK, Poland, etc. Up-to-date global data of mined REY resources and reserves have been compiled. Broad characterization and classifications of REY have been introduced. Comprehensive analysis of application, speciation and environmental impact of REY in coal ash have also been compiled. Comparative study has been done using parameters such as global average, critical percentage, outlook coefficient, etc. These parameters would help in determining ideal candidates for beneficial extraction of REY. This study would serve as a knowledge resource centre for new research related to REY.