Conversion of Coal Fly Ash Into Zeolite 4A and Its Applications in Waste Water Treatment and Greenhouse Gas Reduction

This study applied a novel conversion method to convert coal fly ash (CFA) into pure form (without mixture of CFA residue), single phase and high crystalline zeolite 4A. This novel conversion method allows a reduction by half of the total conversion time while maintaining a high degree of crystallinity of zeolite 4A which exists in a narrower particle size distribution. Applications of the CFA converted zeolite 4A (C4A) in waste water treatment of multi-heavy-metal-ions and in catalytic methane combustion were evaluated. In waste water treatment, for C4A and commercial zeolite 4A, the equilibrium sorption data were well fitted by the Langmuir model and showed the affinity order: Cu²⁺ > Cr³⁺ > Zn²⁺ > Co²⁺ > Ni²⁺. Compared to commercial zeolite 4A, the C4A and the treated CFA residue (TCFAR) were effective in removing multi-heavy-ions in water and could be an alternative material for treatment of wastewater. In catalytic methane combustion, post-treatment of C4A was performed to enhance catalytic activity of the catalyst. Catalytic methane combustion was conducted at atmospheric pressure and gas hourly space velocity (GHSV) between 3230 and 16150 h⁻¹ under different lean fuel concentrations (equivalence ratio of 0.1–0.4) at 500 °C. Thermogravimetry analysis (TGA) results showed the catalyst (M(250)-C4A) could be operated at a temperature of 700 °C without damage to the zeolite structure. At 500 °C, higher combustion efficiency was achieved by either reducing the GHSV under the same fuel concentration or reducing the fuel concentration under the same GHSV. Comparing to a commercial catalyst 2%Pd/Al₂O₃, the catalyst (M(250)-C4A) achieved a higher methane conversion % in the GHSV range of 3230–9690 h⁻¹. Finally, economic and environmental aspects of converting CFA to zeolite 4A were discussed.