Conversion of coal fly ashes into faujasite under soft temperature and pressure conditions: Influence of additional silica

The hydrothermal conversion of a French coal fly ash (FA) into NaX zeolite (faujasite) was studied under soft conditions (30 °C, atmospheric pressure). One used molar composition had previously been found during a preliminary study aiming to determine an optimal composition for the synthesis of NaX from this FA, i.e., 1SiO2:0.36Al2O3:3.8NaOH:72.7H2O, the Al2O3/SiO2 ratio corresponding to the one of the FA, and the zeolitisation procedure consisting only in adding a NaOH solution to the FA. As this composition is much poorer in silica than the one given in the literature for the synthesis from commercial reactants, new experiments were performed with addition of external soluble silica, leading to the following composition: 1SiO2:0.17Al2O3:4.0NaOH:75.5H2O. The zeolitisation kinetics – followed mainly by XRD and N2 adsorption – were compared in presence or absence of additional silica. The addition of silica leads to a final higher content in faujasite (30 wt.% instead of 20–25 wt.%), a higher purity of the zeolitic fraction (co-formation of traces of other more stable zeolites avoided). In presence of external silica, the faujasite crystals are also smaller and display a slightly higher Si/Al framework ratio (1.2 instead of 1.1). One significant point is that the addition of soluble silica leads to a better solubilisation of the FA, about 20% more Al being mobilised. Finally, the zeolisation reaction is delayed when silica is added, the maximum faujasite content being reached after about 50 days instead of about 20 days.