Recovery of Alumina from Coal Fly Ash by CaCl2 Calcination Followed byH2SO4 Leaching

In this paper, a calcination-leaching method by using CaCl2 and H2SO4 to recover alumina from coal fly ash (CFA) is developed. The process conditions including the parameters of CaCl2 content, calcination temperature and time, acid concentration and leaching time are studied. Heat treatment under 900°C for 1 h of the ash with 80 wt.% CaCl2 significantly enhances the alumina leachability (>96.6%) in comparison with the directing acid-leaching (2.31-4.78%) method. The recovery rate increases dramatically with the increasing of CaCl2 content from 20% to 80%, and a H2SO4 leaching time from 10 minutes to 1 hour as well, but it remains near constantly as the increasing of CaCl2 dosage >80% or acid-leaching time >1 h. The reaction mechanism is studied by X-ray diffraction (XRD) analysis to determine the crystalline phases change. The results indicate that quartz, mullite and amorphous phases have been destroyed after calcination with CaCl2 and transformed to gehlenite, calcium aluminum silicate, andradite, wollastonite and mayenite, which are susceptible to acid leaching. The mineralogical characteristics of roasted clinker indicate that molten CaCl2 can promote the incorporation of Ca into silica and silicates and result in the progressive susceptibility to acid attack. The potential chemical reactions during calcinating and leaching processes are also studied via chemical equations. The reuse and resource of coal fly ash can reduce the environmental impact of ash heap and the heavy metal pollution to the surrounding environment.