Formation of Na-A and -X zeolites from waste solutions in conversion of coal fly ash to zeolites

Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO2 and Al2O3 of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si4+ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si4+ concentrations. When molar ratio SiO2/Al2O3 of the waste solutions was modified at 1.0≤SiO2/Al2O3≤2.0 by addition of NaOH–NaAlO2 solutions and the solutions were agied at 85°C, a single phase of Na-A zeolite was formed. The Na-X zeolite was formed at SiO2/Al2O3≥2.5 and its crystallinity was increased with increasing the SiO2/Al2O3 ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO2/Al2O3=7.3, a single phase of Na-X zeolite was produced.