A three step process for purification of fly ash zeolites by hydrothermal treatment

Over the decades, synthetic zeolites have been synthesized from the fly ash by resorting to its hydrothermal activation with NaOH, in one step. However, the activated ash (the residues) has been found to exhibit lower cation exchange capacity. Based on X-ray diffraction spectrometry, main culprits have been identified as Quartz and Mullite (i.e., the inactivated or residual fly ash), which are in the form of impurities in the zeolites (i.e., Na-P1, in majority). Also, such impurities have been verified from the field emission gun-scanning electron microscopy of the residues. The purification of these zeolites still remains a big challenge and requires a special attention. In such a situation, this manuscript demonstrates a “three-step hydrothermal process”; the residues obtained after Step-2 and Step-3 of this process significantly gain in their characteristics viz. cation exchange capacity by 185 and 14% and specific surface area by 478 and 33.42%, respectively. In addition, lowering of the specific gravity and improvement in the SiO2/Al2O3 ratio of the residues are indicative of the formation of more porous and silica rich zeolites, in a purified form. Accordingly, an attempt has been made to assimilate the outcome of the adopted process and develop a conceptual model to exhibit various complexities involved in overall conversion of the fly ash to pure zeolites.