Separation of aluminum and silica from coal gangue by elevated temperature acid leaching for the preparation of alumina and SiC

• High-temperature direct acid leaching is highly effective for separating metal ions and silica in coal gangue.
• An increase in temperature significantly improves the extraction efficiency of aluminum and iron.
• Alumina with a purity of 98.70% is obtained from acid leach liquor.
• The synthesis yield and purity of SiC could reach up to 72.72% and 76.01%, respectively.

Coal gangue is regarded as a potential source of aluminum and silica. In this work, acid leaching behaviors of aluminum, iron, and silica in coal gangue were investigated using hydrochloric acid as leaching agent. Chemical and mineralogical compositions of coal gangue and the leached residue were determined by X-ray diffraction (XRD), X-ray fluorescence, and Fourier transform infrared spectroscopy. XRD results indicated that kaolinite and boehmite were the predominant minerals in coal gangue. During elevated temperature acid leaching, mineral phases gradually decomposed, and aluminum and iron ions were dissolved in the acid leaching solution. However, the carbonaceous material and the product of silica were not dissolved in acid and were abundant in leached residue. The extraction efficiency of Al and Fe from coal gangue depended on coal gangue size, liquid–solid ratio, temperature and leaching time. Extraction efficiency of Al and Fe increased with increased liquid–solid ratio, temperature, and leaching time. Under the same conditions, SiO2 content in the ash of leached residue also increased. The optimum technological conditions for aluminum and iron extraction were obtained with a coal gangue size of 74 μm at a liquid–solid ratio of 3:1 to react at 180 °C for 4 h. After evaporation and crystallization, selective precipitation by adding sodium hydroxide solution, carbon decomposition, and calcination, alumina with a purity of 98.70% was prepared from acid leaching solution. Finally, a suitable amount of low ash anthracite was introduced to the leached residue, and the product of SiC was produced by carbon thermal reduction method. The product had a yield and purity of 72.72% and 76.01%, respectively.

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