Extraction of alumina from high-alumina coal ash using an alkaline hydrothermal method

As a type of industrial solid waste with huge emission and heap stock, high-aluminum pulverized coal ash has a significant negative impact on resources and the environment. At the same time, as a secondary resource, its aluminum content can reach 50%, which is close to that of medium-grade bauxite; thus, it can be used as a substitute for bauxite resources. We propose a new method for extracting aluminum from fly ash via leaching. By designing the thermal reaction system of the molten brine, high-purity hydroxyl sodalite (90.2%) was prepared from fly ash. In this study, aluminum and other valuable elements were extracted using a hydrochloric acid leaching method from prepared high-purity hydroxyl sodalite in the form of ions by converting fly ash into a fixed hydroxyl sodalite solid product system. The major factors affecting the leaching rate are the hydrochloric acid concentration, liquid-solid product quality ratio, reaction temperature, and reaction time. Through analysis and calculation, within the scope of study, the leaching rate of aluminum reached more than 90%. The optimal experimental conditions are a leaching concentration of 1.5 mol/L, a liquid-solid volume-mass ratio of 20, a leaching time of 120 min, and a leaching temperature of 85 degrees C. Aluminum exists in the leaching solution in the form of Al3+, while the silica is partly dissolved in the hydrochloric acid solution; the other part remains in the solid phase as the main part of the hydrochloric acid leaching residue in the form of silica gel. In addition, this method not only can treat high-alumina fly ash, but also can be applied to other types of fly ash.