Eco-Friendly and Efficient Alumina Recovery from Coal Fly Ash by Employing the CaO as an Additive During the Vacuum Carbothermic Reduction and Alkali Dissolution

With the growing concern of environmental protection and waste recycling, recovery of high-value metal from coal fly ash (CFA) has attracted a lot of attention around the world. In our previous study, CaO was employed as an additive to react with mullite (3Al₂O₃·2SiO₂) in CFA to produce CaO·xAl₂O₃, by which the extraction of alumina was strengthened tremendously. However, the dissolving mechanism of CaO·xAl₂O₃ in alkali liquor has not yet been conducted in in-depth research, which casts a shadow for the further progress of the process. In this paper, with the aim of clarifying the alkali dissolving mechanism as well as developing an eco-friendly alumina recovery process, the effect of CaO addition on the mineralogical transformation of CFA and the dissolving behaviors of CaO·xAl₂O₃ in alkali liquor were investigated thoroughly. The results show that the mullite in CFA can be converted to Fe-Si alloys and CaO·xAl₂O₃ with the addition of CaO. Additionally, it is proved that the liquid/solid ratio, the alkali concentration, and the dissolving temperature would be the most critical impactors to decide the extraction rate of alumina. Under the optimal conditions, the extraction rate of alumina attained 93.8% and the CaO can be recycled for cyclic utilization in this process.