The Migration and Mineral Host Changes of Lithium During Coal Combustion: Experimental and Thermodynamic Calculation Study

The recovery of lithium (Li) from coal ash has gained growing attention as a promising Li resource. Compared with extensive studies on the modes of occurrence of Li in coal and coal ash, few studies have investigated the migration and mineral host changes of Li during coal combustion. This study investigates the modes of occurrence of Li in feed coal, the conversion of minerals, the Li-host changes, and the influencing factors during coal combustion. The results show that Li is enriched in the feed coal, possibly mainly in the octahedral sites of kaolinite and illite/smectite (I/S). Kaolinite and I/S are dehydrated and dehydroxylated at high temperatures to form aluminosilicate glass, which then partially transforms into pseudomullite at 826 °C and further converts to mullite at 1282 °C. Temperature is the main factor controlling the Li-host changes during coal combustion. The proportion of easily leachable Li decreases with increasing in temperature (400–1000 °C), and the leachability of Li is optimal at 500 °C regarding possible extraction prospects. Oxy-fuel combustion reduces the temperature of coal particles, resulting in a relatively higher proportion of easily leachable Li from oxy-fuel combustion compared with air combustion at 700 °C. The Li-host phase changes are discussed in detail using sequential chemical extraction assisted with thermodynamic calculation (HSC Chemistry 10). The Li related to kaolinite and I/S in coal mainly migrates into the octahedral positions of clay minerals to form lithium aluminosilicate. Portions of the Li react with Fe, Mn, and S to form lithium manganese/iron oxides and lithium sulfate. This study evaluates the extraction potential of Li from coal by experimental heating under different atmospheres and the study of mineralogical changes of Li-hosts, providing practical guidance for the recovery of Li from coal.