Study on the Occurrence of Gallium in Gallium-Enriched Coal Gangue and Migration During Thermal Treatment

The occurrence of gallium in gallium-enriched coal gangue and the migration of gallium during thermal treatment were systematically studied. The phase and chemical composition of gallium-enriched coal gangue were determined, and the gallium content was determined by inductively coupled plasma mass spectrometer (ICP-MS). The mineralogical characteristics of coal gangue and liberation characteristics of main mineral phases, where gallium occurs, were analyzed using mineral liberation analyzer (MLA). The migration and occurrence of gallium in coal gangue during phase evolution after heat treatment were investigated through thermogravimetric and differential scanning calorimetry (TG-DSC) and step sequential chemical extraction procedure. The results show that the main minerals in the coal gangue are kaolinite, pyrite, and illite, and the gallium content is 38.82 ppm, of which about 84% is present in kaolinite. It is indicated that gallium mainly occurs in aluminosilicate minerals in the form of isomorphism. MLA results reveal that kaolinite in coal gangue shows a good liberation degree, while pyrite and muscovite are encapsulated with another or more minerals, reflecting lower liberation characteristics. Organic matter combustion, pyrite oxidation, calcite decomposition, aluminosilicate dehydroxylation (transformation from kaolinite to metakaolin), and conversion of metakaolin into mullite sequentially occur during thermal treatment for coal gangue. As the mineral phases evolve, gallium gradually migrates from the aluminosilicate phase, predominantly kaolinite, to the oxides of iron, potassium, magnesium, and calcium, and then to aluminosilicate at 1000 ℃. Present work sheds light on the efficient utilization for coal gangue in terms of beneficiation and gallium extraction.