Aluminum release from microwave-assisted reaction of coal fly ash with calcium carbonate

Coal fly ash (CFA) is a potential aluminum resource alternative to bauxite. Because most aluminum in CFA exists in an inert mullite phase, the way in which CFA can be activated is becoming important. Thermal activation with the addition of calcium carbonate (CaCO3) is a common method for recovering aluminum from CFA; however, the lower energy efficiency of conventional heating has limited its application. In this investigation, microwave irradiation (MWI) was found to be an energy-saving method for CFA activation, and 95 wt% of aluminum was released from the specimen treated by MWI at 800 °C for 60 s. Compared to conventional thermal activation, MWI reduced the sintering temperature by nearly 400 °C and shortened the reaction time by 20 times. Anisothermal reactions, which occurred due to the selective heating of the microwave, controlled the decomposition rate of CaCO3 and the formation rate of anorthite. MWI also accelerated the formation of fused phase and contributed to the formation of well-ordered hexagonal anorthite.