Thermal activation of coal fly ash by sodium hydrogen sulfate for alumina extraction

The extraction of alumina from coal fly ash (CFA) is a good way to enhance its value-added utilization. The presence of inert matter with a high degree of polymerization, such as mullite and other aluminosilicates, greatly restricts the reactivity of CFA. Thus, the CFA needs to be activated before use. Thermal activation by calcination with added sodium hydrogen sulfate (NaHSO4) was carried out in this study. The calcination products were characterized by means of X-ray diffraction, thermal gravimetric and differential scanning calorimetric analysis, and scanning electron microscopy to assess the effects of the additive, calcining temperature, and calcining time on the products. The results indicated that NaHSO4 facilitated the decomposition of the polymeric phases and significantly improved the alumina extraction. Alumina in CFA was transformed into soluble Na3Al(SO4)3. When CFA was mixed with sodium hydrogen sulfate at a NaHSO4/Al2O3 molar ratio of 6, the alumina extraction reached 90 % at 500 °C after 2 h.