Preparation and Characterization of Zeolites from Coal Ash

Abstract

Coal ash, which was formed as a by-product waste at coal-fired power station, was converted into zeolites by hydrothermal reaction in alkaline solution.
The reaction of coal ash with sodium hydroxide at 373 K under atmospheric pressure resulted in formation of Na-P 1, Na-A type zeolite, and hydroxysodalite. X-Ray microanalysis (EPMA) revealed that the zeolite zone was formed like egg white, covering the central core of fly ash particles (see Fig.2). Comparing SEM images at various times of hydrothermal treatment shown in Fig.5, formation of coal ash zeolites was suggested to proceed by the following sequence: (1) dissolution of zeolite components from the starting fly ash particles, (2) deposit and growth of amorphous microspheres on the surface of fly ash core, and (3)crystalization of the amorphous aggregates toward Na-P 1 or Na-A type zeolite. Na-A type zeolite, which was obtained through the above sequence, gradually changed with time to afford hydroxysodalite.
The zeolite was soluble in acid, and thus the weight percent of zeolite zone in fly ash zeolite could be evaluated quantitatively. The cation exchange capacity per unit weight of the acid soluble (zeolite) zone measured by NH4+ adsorption was quite close to that of synthetic zeolite (see Table 5).