Modes of Occurrence, Migration, and Evolution Pathways of Lithium and Gallium during Combustion of an Al-Rich Coal, Inner Mongolia, China

The modes of occurrence, migration, and evolution pathways of lithium (Li) and gallium (Ga) during combustion of an Al-rich coal from Inner Mongolia, China, were investigated using methods of simulated combustion experiments, the sequential chemical extraction procedure (SCEP), and the thermodynamic equilibrium calculation. Mineralogical and chemical compositions of the feed coal and combustion ash were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy (SEM). The study reveals that Li and Ga are significantly enriched in the ash after combustion, with the contents reaching up to 1086 mu g/g and 133 mu g/g, respectively. The primary modes of occurrence of Li and Ga in the ash are quartz and aluminosilicates, and sulfides, respectively. Li, in the form of LiAlSi4O10 (s), primarily occurs in hematite, glass, and quartz below 800 degrees C. However, it migrates into the glass phase, mullite, and quartz above 1000 degrees C. On the other hand, Ga exists as Ga4S5 (s) and transforms into Ga2S (g) as the temperature rises from 800 degrees C to 1000 degrees C, maintaining this gaseous form until 1200 degrees C. Ga4S5 (s) predominantly occurs in the glass phase at 600 degrees C, whereas mullite and quartz become its dominant modes of occurrence in industrial combustion ashes and ashes obtained from simulated combustion above 600 degrees C.