Synthesis of Na-zeolites from fly ash

Abstract

The present study focuses on the synthesis of zeolites after fly ash activation, the major objectives being: (a) to study the activation processes; (b) to elucidate the activation conditions for maximum synthesis efficiency; (c) to optimize the conditions for the monomineral synthesis; and (d) to test synthesis efficiencies for different fly ash types. Fly ash was activated by NaOH and KOH solutions in a closed system. The zeolite conversion was studied as a function of temperature (150–200°C), reaction time (8–100 h) and solution concentration (0.1–1 m). The estimated pressure during activation ranged from 0.48 MPa at 150°C to 1.55 MPa at 200°C. The activation was performed using a sample concentration of 0.055 g mL −1. The zeolites synthesized were NaPl, NaP derivatives (mainly sodalite hydrate), analcime, gmelinite and nepheline hydrate after NaOH activation, and phillipsite after KOH activation. Subsequent experiments focused on short reaction times for high conversion efficiencies and monomineral synthesis of zeolites. High synthesis efficiencies for NaPl zeolite and analcime were obtained with the fly ash types studied. The results highlight the importance of the mineralogical composition of the fly ash. Fly ashes with very similar SiO 2 Al 2O 3 ratios show different zeolites synthesis behaviour under the same activation conditions. These differences could be attributed to different SiO 2 Al 2O 3 ratios of the glass matrix inferred from the differences in mineralogy at the same bulk chemical composition.