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

• Published in: Microporous and mesoporous materials Vol. 127; no. 1; pp. 41 - 49
• Main Authors: Gross-Lorgouilloux, Marion, Soulard, Michel, Caullet, Philippe, Patarin, Joël, Moleiro, Edgar, Saude, Isabelle
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 2010; Elsevier
• Subjects: Additional silica; Chemistry; Coal fly ashes; Colloidal state and disperse state; Conversion rate; Exact sciences and technology; Faujasite; General and physical chemistry; Ion-exchange; Porous materials; Soft conditions; Surface physical chemistry; Zeolites: preparations and properties; Additional silica; Soft conditions; Conversion rate; Faujasite; Coal fly ashes; Binary compound; Atmospheric pressure; Purity; Porous material; Silica; Molecular sieve; Fly ash; Synthesis; Coal; Aluminium oxide; Composition; Crystals; Zeolite; Conversion; X ray diffraction; Trace; Adsorption; Silicon oxides; Kinetics; Alumina; Hydrothermal condition
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2009.06.026

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., 1SiO 2:0.36Al 2O 3:3.8NaOH:72.7H 2O, the Al 2O 3/SiO 2 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: 1SiO 2:0.17Al 2O 3:4.0NaOH:75.5H 2O. The zeolitisation kinetics – followed mainly by XRD and N 2 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.