Comprehensive Extraction of Silica and Alumina from High-Alumina Coal Gangue (HACG): Hematite Involved Roasting − Alkaline Leaching − Bayer Digestion Process

This paper describes a novel process, hematite involved roasting − alkaline leaching − Bayer digestion, to extract silica and alumina from high-alumina coal gangue (HACG). The phase transformations during roasting were investigated through X-ray diffraction, X-ray photoelectron spectroscopy, scanning electronic microscope, and energy-dispersive spectrometer. The silica and alumina extraction were carried out through alkaline leaching. Results shown that, by reductively roasting with hematite, kaolinite converted into hercynite (FeAl2O4), and cristobalite solid solution (SiO2), which dispersed as the form of fine grain dissemination. During cooling in oxidizing atmosphere, the cristobalite solid solution showed insignificant change. In contrast, the hercynite oxidized into α-Fe2O3 and α-Al2O3 through the conversion of intermediate products of Fe3O4, γ-Fe2O3, and γ-Al2O3. The migration of iron oxides to hercynite surface during cooling process prevented the formation of Al–Si compounds. > 96% of silica and > 97% of alumina were extracted by alkaline leaching at 110 °C and Bayer digestion at 260 °C, respectively. This work demonstrates a new process to treat and recover the high-value components in HACG and decrease the red mud output through recycling the hematite.