Recycling of Waste Solution after Hydrothermal Conversion of Fly Ash on a Semi-Technical Scale for Zeolite Synthesis

Nowadays, using fly ash for zeolites production has become a well-known strategy aimed on sustainable development. During zeolite synthesis in a hydrothermal conversion large amount of post-reaction solution is generated. In this work, the solution was used as a substrate for Na-A and Na-X zeolites synthesis at laboratory and technical scale. Obtained materials were characterized using particle size analysis, X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FTIR), and nitrogen adsorption/desorption isotherm. Produced zeolites revealed high purity (>98%) and monomineral zeolitic phase composition. The SiO₂ content was in the range 39-42% and 40-38%, whereas Al₂O₃ content was 23-22% and 25-26% for Na-X and Na-A, respectively. TEM and BET analyses revealed Na-X zeolite pores were almost identical to commercial 13X with S_BET in the range 671-734 m²/g. FTIR indicated slight differences between materials obtained at laboratory and technical scale in Si-O-(Si/Al) bridges of the zeolitic skeleton. The results showed good replicability of the laboratory process in the larger scale. The proposed method allows for waste solution reusability with a view to highly pure zeolites production in line with circular economy assumptions.