Synthesis of zeolites Na-A and Na-X from tablet compressed and calcinated coal fly ash.

Zeolites Na-A and Na-X are important synthetic zeolites widely used for separation and adsorption in industry. It is of great significance to develop energy-efficient routines that can synthesize zeolites Na-A and Na-X from low-cost raw materials. Coal fly ash (CFA) is the major residue from the combustion of coal and biomass containing more than 85% SiO and AlO, which can readily replace the conventionally used sodium silicate and aluminate for zeolite synthesis. We used NaCO to replace the expensive NaOH used for the calcination of CFA and showed that tablet compression can enhance the contact with NaCO for the activation of CFA through calcination for the synthesis of zeolites Na-A and Na-X under mild conditions. We optimized the control variables for zeolite synthesis and showed that phase-pure zeolite Na-A can be synthesized with CFA at reactant molar ratio, hydrothermal reaction temperature and reaction time of 1.3NaO: 0.6AlO: 1SiO: 38HO at 80°C for 6 h, respectively, while phase-pure zeolite Na-X can be synthesized at 2.2NaO: 0.2AlO: 1SiO: 88HO at 100°C for 8 h, respectively. The composition, morphology, specific surface area, vibration spectrum and thermogravimetry of synthesized Na-A and Na-X were further characterized.