Synthesis of a Single-Phase Na-A Zeolite from Coal Fly Ash by Dialysis

Abstract

To convert coal fly ash (Fa) into Na-A zeolites, Fa and NaOH−NaAlO2 solutions were added into the tube made by a semipermeable membrane and aged in the same NaOH−NaAlO2 solutions at 85 °C for a given period. The amorphous aluminosilicates in Fa were used as Si and Al sources for synthesis of zeolites. The SiO2/Al2O3 molar ratio of the starting materials was controlled from 2.0 to 5.0. The amorphous aluminosilicate of Fa completely dissolved during the aging and Pc type zeolite was formed in the tube over the whole range of SiO2/Al2O3. On the other hand, the crystalline phase of Fa, such as α-quartz and mullite, was poorly dissolved. After the mixture was aged for 48 h, white precipitates resulted outside the tube. At SiO2/Al2O3 = 2.0, the precipitates were identified as Na-A zeolite and hydroxysodalite. The Na-X zeolite was produced at SiO2/Al2O3 ≥ 2.5 and its crystallinity increased with increasing the molar ratio SiO2/Al2O3, whereas the crystallinity of Na-A zeolite and hydroxysodalite decreased. A single-phase Na-A zeolite was obtained by aging at SiO2/Al2O3 = 2.0 for 24−30 h.