Synthesis of faujasite (FAU) and tschernichite (LTA) type zeolites as a potential direction of the development of lime Class C fly ash

The study presents an evaluation of the possibility of using Class C fly ash for the synthesis of faujasite (Type X) and tschernichite (Type A) type zeolite materials. In order to obtain the well-formed zeolites, syntheses were carried out. The variables were: the ratio of NaOH to fly ash, water, the filtrate (post-reaction solutions obtained during the hydrothermal synthesis of zeolites rich in Si), and the amount of added aluminium foil. The analysis showed that three of the most effective reactions (from which Samples 21–23 were derived) occurred under the following conditions: the ratio of NaOH/fly ash was 1.6, 2.0 and 1.25, respectively, with a fusion temperature of 550 °C (for each of the three reactions), fusion time of 1 h and reaction time of 4 h (for each of the three reactions); the amount of H₂O was 100, 100 and 50 ml; the amount of filtrate was 0, 0 and 50 ml; the amount of added aluminium foil was 0.5 g (for each of the three reactions); and the reaction temperature was 80 °C (for each of the three reactions). The three best zeolite materials (Samples 21–23) that were derived were subjected to mineralogical (X-ray powder diffraction [XRD], scanning electron microscopy-energy-dispersive X-ray spectroscopy [SEM-EDS]) and chemical (X-ray fluorescence [XRF]) characterization, with the addition of textural analysis (Brunauer-Emmett-Teller [BET] specific surface area and pore volume and size). Studies have shown that the obtained zeolites have a Type A (Samples 21–22) and Type X (Sample 23) structure, including well-formed grains with isometric and cubic characteristics. The calculated unit cell parameters of the obtained zeolites indicate a cubic crystal system and are very close to the reference values for structures of X and A type zeolites. The ratio of SiO₂/Al₂O₃ in each of the three tested zeolite materials was as follows: 2.16, 1.98 and 2.41. The specific surface area amounted to 106, 104 and 256 m²/g for Samples 21–23, respectively. Obtained results were similar to the type of zeolite structures obtained from Class F fly ash. Therefore, we can conclude that the analysed Class C fly ash may also be a productive substrate for the synthesis of zeolite materials of Type X and A.