Revisiting sustainable resources in the combustion products of alumina-rich coal: Critical metal (Li, Ga, Nb, and REY) potential of ash from the Togtoh Power Plant, Inner Mongolia, China.

Coal fly ash has gained much attention as a potential alternative source for extracting critical metals such as Li, Ga, Nb, and lanthanides and yttrium (REY). This study investigates their distribution characteristics and modes of occurrence in alumina-rich fly ashes from the Togtoh Power Plant in Inner Mongolia, using various analytical methods. The objective was to provide a reference for the pre-enrichment of critical metals in fly ash. Lithium is primarily present in the glass phase, and its concentration is extremely low in the crystalline phases. Lithium is mainly concentrated in "pure" aluminosilicate glass, and is present but at a low level in Ca-rich aluminosilicate glass. Gallium is primarily present in the glass phase and in corundum, while Nb mainly exists in submicron zircon particles surrounded by Si-Al-Ca glass. Lanthanides and yttrium primarily occur in the glass phase and in crystalline phases, including an intermediate phase composed of the three end-member minerals of the gorceixite-crandallite-florencite series, as well as in monazite, crystalline forms of iron oxides and REY oxides. The Li concentrations in the alumina-rich fly ashes range from 562 to 894 μg/g for LiO, from 43.9 to 81.9 μg/g for Ga, from 58.7 to 70.6 μg/g for NbO, and from 258 to 450 μg/g for REY oxides, respectively, indicating their substantial potential for resource recovery. Especially, the 2nd row fly ash has the highest contents of these metals, allowing for direct extraction without the necessity for complex pre-processing. Physical separation can further enrich Li, Ga, Nb, and REY in the fly ash. In particular, particle size separation enriches these elements in the < 20 μm size range and magnetic separation enriches Li, Ga, Nb, and REY (except Ce) in the non-magnetic fraction. However, Ce is significantly enriched in the magnetic fraction compared to the original fly ash.