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Coal gangue is the solid waste from the coal mining process. About 1 billion tons of coal gangue is discharged every year in the world. Coal gangue is usually piled up in the large areas and difficult to be used. Phase transformation and separation of alumina and silica minerals in coal gangue are studied in this paper in order to prompt the resource utilization of coal gangue. The results show that the phase transformation of kaolinite in coal gangue during high-temperature calcination is in the following way: kaolinite → metakaolin → spinel → mullite. Active silica mineral is released during the phase transition process, which can be separated from other minerals by caustic leaching. The optimal calcination temperature of coal gangue is 1050 ± 50 °C while the caustic leaching temperature is 85 °C and the leaching time should be kept between 60 and 80 min. The modulus of the sodium silicate solution can reach 1.0–1.1 and the alumina to silica ratio (A/S) of the desilicated residue can be raised to 1.7.
Xie et al. (Tue,) studied this question.