One-step high efficiency crystallization of zeolite A from ultra-fine circulating fluidized bed fly ash by hydrothermal synthesis method

The present work focused on the crystallization of zeolite A directly from ultra-fine circulating fluidized bed fly ash (UF-CFA) by alkali-activated hydrothermal synthesis method without high temperature calcining and long-term crystallizing. Response surface methodology, namely Box-Behnken design was employed to optimize the synthesis conditions by varying the crystallization temperature, crystallization time, and NaOHaq concentration. The obtained zeolite A was subjected to mineralogical (X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and chemical (Fourier transform infrared spectrometer) characterization. The results showed that zeolite A begins to form at 90 degrees C - 2.5 h with NaOHaq concentration of 2.6 mol/L. The crystallinity at 3 h was approximately 90.1%, and pure zeolite A was formed at 6 h with maximum crystallinity of 97.9%. The solid-liquid separation of the synthesized mixture was carried out under the conditions of the appearance of zeolite A at 2.5 h. Crystal structure and solid-liquid composition of the product during the hydrothermal reaction were analyzed. It is explored the growth mechanism of zeolite A by clarifying the evolution of substances in solid-liquid phases and composition of products in the process of zeolite formation.