Temperature-Dependent Enrichment, occurrence modes and speciation evolution of critical metals (In, Li, Ga, REY) during Ningxia coal combustion

This study systematically investigated the enrichment behavior, occurrence modes, and speciation evolution of critical metals—indium (In), lithium (Li), gallium (Ga), and rare earth elements with yttrium (REY)—during the combustion of Ningxia coal. Simulated combustion experiments were conducted over a temperature range of 500–900 °C, coupled with sequential chemical extraction procedure (SCEP), X-ray diffraction (XRD), and thermodynamic modeling (FactSage). Results indicate that these metals are significantly enriched in the ash, with In exhibiting the highest enrichment ratio (up to 23.64). Mineral phase transformations—especially the formation of hematite, anhydrite, quartz, and amorphous glass—govern the redistribution of the metals. For In, the primary host shifts from metal oxides at lower temperatures to sulfate and quartz phases at higher temperatures, while Li and Ga remain predominantly associated with aluminosilicate glass and sulfate phases. REY migrate from metal oxide carriers toward sulfate and quartz phases as temperature increases. Thermodynamic predictions further confirm the predominance of solid phases such as In 2 O 3 , LiAlSi 4 O 10 , and Ga 2 O 3 under the investigated combustion conditions. These findings provide critical insights into optimizing the recovery of critical metals from coal fly ash, supporting its high-value utilization and sustainable resource strategies.