Separation of alumina and silica from metakaolinite by reduction roasting−alkaline leaching process: Effect of CaSO4 and CaO

Abstract

Limestone (CaCO3), which could promote sulfur fixation, was added to coal gangue during roasting in a circulating fluidized bed (CFB) boiler. CaO and CaSO4 were the main Ca-bearing minerals while metakaolinite was the major Al-bearing mineral in CFB slag. The effect of CaSO4 and CaO on the separation of alumina and silica from metakaolinite by reduction roasting−alkaline leaching process was studied. Results showed that metakaolinite was completely converted into hercynite and silica solid solutions (i.e., quartz and cristobalite solid solutions) by reduction roasting with hematite. More than 95% of silica in the reduced specimen was removed by alkaline leaching. The addition of CaSO4 and CaO remarkably decreased the separation efficiency of alumina and silica in metakaolinite, which could be attributed to the formation of Si-bearing minerals: (1) Fayalite and anorthite were formed during the reduction roasting process; (2) Fayalite was stable while anorthite was converted into sodalite and wollastonite during the alkaline leaching process. This study demonstrates that sulfur in coal gangue should be fixed by treating the exhaust gas instead of controlling the combustion process of CFB to achieve the comprehensive recovery of silica and alumina from the CFB slag.