Mineral phase transition of desilicated high alumina fly ash with alumina extraction in mixed alkali solution

• Alumina was effectively leached from desilicated high alumina fly ash.
• Mechanism of mineral phase transition for alumina extraction was explored.
• The influencing factors on the alumina extraction were investigated.
• The mineral phase of alumina extracted ash can be selectively synthesized.

High alumina fly ash (HAFA), a main industrial solid waste produced in increasing amounts in western China, is a type of alumina-rich resource that can replace bauxite in the preparation of alumina. A new process of alumina extraction from HAFA is proposed in this paper. This method involved the raw HAFA being treated first by a low concentration alkali solution to form desilicated HAFA and then alumina was effectively extracted from desilicated HAFA in mixed alkali solution (NaOH + Ca(OH)2). The effects of the reaction conditions on alumina leaching from desilicated HAFA were investigated. The extraction efficiency of alumina could reach 92.0% under the following optimal conditions: alkali to ash ratio of 6, calcium to silicate ratio of 1.0, and reaction temperature of 280 °C. After alumina extracted from desilicated HAFA, the solid residues were collected and characterized to better understand the mechanism of mineral phase transition of desilicated HAFA with alumina extraction in mixed alkali solution. By changing the calcium to silica ratio or alkali to ash ratio, the mineral phases could be transformed from 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O to NaCaHSiO4. A relatively high alkali to ash ratio or calcium to silicate ratio was preferred for the extraction of alumina from desilicated HAFA.