Mechanism Of Microwave-Assisted Activation Of Coal Fly Ash

Cascade utilization of aluminum, silicon and other resources is an important method for the reduction and resource utilization of coal fly ash (CFA). However, the energy consumption of conventional thermal activation process with the addiction of assistant activators is huge, which hinders the industrialized scale-up of related technologies. In previous work, a new activation method assisted by microwave has been developed with an optimized scheme. In this research, the microwave effects in solid reaction process under microwave irradiation are investigated systematically using electromagnetic simulation along with experiments.Under microwave irradiation, electric field (E-field) energy would be collected in the gap between solid particles, which contribute to higher E-field intensity in these certain areas. Ultra-high temperature caused by E-field intensification leads to the generation of fusing of solid materials at contact zone and enhances the mass transportation and the so called "hot spot". The intensification degree varied with dielectric constant of compound, size of the particles, distance between two particles, angle between the direction of E-field and the normal to the surface at the adjacent point of two particles.High temperature caused by strong E-field intensification would lead to fast formation of liquid phase and neck growth between particles in dielectric-air systems. With the formation of liquid phase and neck growth between particles, the air in the gap would be extruded and E-field intensification disappears. The fused dielectric materials continuously absorb microwave to enhance sintering even after the disappearance of E-field intensification which causes fast and anisothermal sintering of dielectric powders. The temperature of melted metal with low viscosity at neck zone, however, would cool down instantly after the disappearance of E-field intensification since metal materials are good conductor of heat, after which more grain cores and small size crystals correspondingly form, i.e. restriction of grain growth.The introduction of CaCO3 could enhance the intensification of E-field, and reduced the activation temperature of CFA. The formation of a large amount of fused phase under microwave irradiation accelerates the activation process.A conclusive explanation on microwave effects was obtained, with which a more scientific utilization of microwave irradiation as a green method on sintering process would be expected.