A study on speciation and enrichment of rare earth elements (REE) by sequential extraction from a potential coal fly ash resource and its role in REE extractability

Coal fly ash (CFA) is a potential secondary Rare Earth Element (REE) source that needs an efficient enrichment and extraction process. Annually, coal-fired power plants generate nearly 1 billion tonnes of coal ash. India alone produced 270.82 million tonnes of CFA in 2021–22. The average global concentration of REE in coal ash is 403.5 mg/kg. The present authors have already studied some promising CFAs from Indian thermal power stations and one such CFA is used in the present study for the sequential extraction (SE), speciation, and enrichment of REEs. Organic acids are used for SE to determine different fractions of REEs and it is found that REEs are mostly associated with amorphous Mn-oxide and organic and sulfide fractions. About 84% of REEs are in the glassy phase that SE cannot dissolve. The main components of CFA and SE residues are quartz, mullite, and glassy phase (52.0–59.2%). The XRF analysis showed that CFA is ‘Class F' (ASTM C618–19). The ESEM – EDS and backscatter imaging revealed monazite-(La) mineral as the REE carrier in CFA. Furthermore, enrichment studies using acid and alkali media have been performed. Alkali treatment with NaOH and KOH increased total REE by 1.61 and 1.32 times, respectively. Acidic treatment with HCl and oxalic acid increased total REE by 1.27 and 1.09 times, respectively. Therefore, NaOH followed by HCl treatment could be a cost-effective and sustainable REE enrichment and extraction method from CFA.