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

Abstract Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO 2 and Al 2 O 3 of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si 4+ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si 4+ concentrations. When molar ratio SiO 2 /Al 2 O 3 of the waste solutions was modified at 1.0≤SiO 2 /Al 2 O 3 ≤2.0 by addition of NaOH–NaAlO 2 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 SiO 2 /Al 2 O 3 ≥2.5 and its crystallinity was increased with increasing the SiO 2 /Al 2 O 3 ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO 2 /Al 2 O 3 =7.3, a single phase of Na-X zeolite was produced.