Conversion of coal fly ash by-products into high-grade zeolite by a quasi natural crystallization process

Recently, the synthesis of zeolites from coal fly ash (FA) has been extensively studied regarding the process optimization, industrial upscaling and technological commercialization. The obtaining of fly ash zeolites (FAZ) is based on the transformation of aluminosilicates of FA by alkaline activation which undergoes several stages: alkaline dissolution of Si-Al-components; hydrogel formation; crystallization of zeolite phases over the insoluble matter. Different synthesis methods of FAZ have been studied in the quest to achieve the highest transformation rate in a particular zeolitic phase. Generally, zeolites occur in the nature due to the interaction between rock aluminosilicates and sea salts in a course of thousands of years. This natural example provides an idea of simplifying the conversion of FA into zeolite under atmospheric conditions avoiding the external energy and special equipment. The equipment convenience and the zero energy demands are of great importance for scaling up the coal ash utilization.This study focuses on technologically viable approach to obtain zeolites by aging of FA and alkaline solution mixtures in ambient conditions. Reaction products were characterized for their phase composition, morphology and surface properties. The effect of the alkaline activator concentration and the crystallization time was investigated as important variables for zeolitization rate. Acknowledgements This work was financially supported by the National Science Fund, Ministry of Education and Science of Bulgaria under contract DN 17/18 and Grant DNTS Slovak Republic 01/6.