Preparation of hydro-sodalite from fly ash using a hydrothermal method with a submolten salt system and study of the phase transition process

Abstract

Hydro-sodalites are zeolitic materials with a wide variety of applications. Fly ash is an abundant industrial solid waste, rich in silicon and aluminum, from which hydro-sodalite can be synthesized. However, traditional hydrothermal synthesis methods are complex and cannot produce high-purity products. Therefore, there is a demand for processing routes to obtain high-purity hydro-sodalites. In the present study, high-purity hydro-sodalite (90.2wt%) was prepared from fly ash by applying a hydrothermal method to a submolten salt system. Samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry and differential thermal analysis (TG-DTA), and Fourier transform infrared spectroscopy (FUR) to confirm and quantify conversion of the raw material into the product phase. Purity of the samples prepared with an H2O/NaOH mass ratio of 1.5 and an H2O/fly ash mass ratio of 10 was calculated and the conversion process of the product phase was studied. Crystallinity of the product was influenced more by the NaOH concentration, less by the H2O/fly ash mass ratio. The main reaction process of the system is that the $${\rm{SiO}}_3^{2-}$$ ions produced by dissolution of the vitreous body in the fly ash and Na+ ions in the solution reacted on the destroyed mullite skeleton to produce hydro-sodalite. This processing route could help mitigate processing difficulties, while producing high-purity hydro-sodalite from fly ash.