Mechanism study on green high-efficiency hydrothermal activation of fly ash and its application prospect

Efficient activation of fly ash is necessary for its further applications in zeolite synthesis and useful element extraction. However, traditional fusion activation is energy-intensive and complicated. Herein, a simple, highly efficient and green activation method of fly ash was presented. The activation degree of fly ash could be 80% by adding 1.0 mol/L NaOH solution at 200 °C. The fly ash was fully activated at 350 °C. The mechanism and kinetics of hydrothermal activation of fly ash were further studied. The outmost amorphous glass body of the fly ash was firstly activated. The fly ash was activated from the surface of the glass microspheres to the center gradually. The mullite in the center of the glass microspheres was finally activated. Therefore, the morphology and crystal phase of the microspheres in the fly ash can be controlled by adjusting the activation conditions. The activation process can be divided into two stages under mild conditions. The hydrothermal activation process was controlled by internal diffusion at initial stage and chemical reaction at later stage. Under more severe conditions, the glass microspheres in fly ash were rapidly activated and converted into flower-liked microcrystal clusters. This work provides a progressive strategy of conversion and utilization of fly ash for its practical application in valuable element extraction and zeolite synthesis.