Geochemistry, phase association, and economic potential of rare earth elements and yttrium in lignite fly ash from Rajasthan, India

Rare earth elements plus yttrium (REY) are critical for high-tech applications, with demand rising amid conventional ore scarcity. This study evaluates REY content, distribution, and occurrence modes in lignite fly ash from three Indian thermal power plants – Barsingsar (BTPS), VS Lignite (VSPP), and Ambuja Cement (ACPP) – as potential secondary REY resources. Total REY concentrations (788–1910 ppm) substantially exceeded upper continental crust (168 ppm) and global hard coal ash averages (445 ppm). VSPP samples showed highest enrichment (1790–1910 ppm), while BTPS and ACPP exhibited moderate-to-lower levels. Economic assessment using outlook coefficient (Coutl) and critical element content classified VSPP and ACPP as promising Category II resources (Coutl: 0.89–1.14; critical elements: 34.2–40.5%), representing threefold enrichment versus conventional deposits. BTPS samples were unpromising (Category I). Sequential extraction revealed 86–97% of REY within refractory aluminosilicate glassy phase, confirmed by FE-SEM-EDS mapping showing REY-Al-Si spatial correlation. XRD identified mullite, magnetite, hematite, quartz, anhydrite, and rutile; XRF showed SiO2-Al2O3-Fe2O3 dominance. The refractory REY encapsulation necessitates advanced extraction strategies beyond conventional acid leaching, such as alkaline roasting pretreatment. This study establishes that selected Indian lignite fly ashes represent viable alternative REY sources and provides foundational technical data for the development of economically feasible extraction processes.