Abstract
The present study emphasized the conversion of fly ash from lignite coals into zeolites by a double-stage fusion-hydrothermal synthesis. The raw and the converted fly ashes were characterized with respect to their morphology, composition, and crystallinity. The effects of process parameters such as fusion temperature, concentration of the alkaline activator, and the duration of the hydrothermal incubation on the zeolitization process were studied. The experimental results indicate that the fusion temperature does not influence the process mechanism in the range 550-850°C, but it has an acceleration effect. The elevation of the sodium hydroxide/fly ash ratio from 1.6 to 2.0 enhances the degree of zeolitization and leads to conversion of the zeolite phase from linde to faujasite type. The observed regularities are discussed taking into consideration thermodynamic and kinetic factors. The best attempt in the selective preparation of faujasite in the framework of this study was observed at a sodium hydroxide/fly ash ratio of 2.0, fusion temperature of 550°C, and a hydrothermal synthesis for 2 hours at 90°C.
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