The Influence of Ultrasound on Fly Ash Zeolitisation Process Efficiency

Abstract

ABSTRACT The zeolitisation of fly ash originating from hard coal combustion in a Polish power plant was conducted using the alkaline hydrothermal method with and without the application of ultrasound. The output ash primarily contained glass, mullite, quartz and hematite, and its dominant chemical components included SiO2 (over 50 wt.%) and Al2O3 (over 28 wt.%). The synthesis of two zeolitic materials was conducted under the following conditions: 3 M NaOH solution, synthesis time – 8 hours, temperature – 80°C. For one of the syntheses, ultrasound with a frequency of 40 kHz and power of 480 W was applied for the first two hours. The identification of mineral components of initial ash and post-synthesis products was conducted using following methods: X-ray diffraction, X-ray energy dispersive spectroscopy, scanning electron microscopy, differential thermal analysis and thermogravimetric analysis. The obtained zeolitic materials varied greatly in phase composition. The product phase composition for the non-ultrasound-assisted synthesis was practically identical to the initial fly ash (mullite, quartz and hematite). The glass underwent minor chemical metamorphosis, recognizable by the presence of new phases in small quantities, typically mutually interlayered, mostly in the form of thin incrustation on the ash grains. The phases included: hydrosodalite, NaP-type zeolite, LTA-type zeolite and A-type zeolite. These phases, where Na+ cations should be dominant, contained K+, Ca2+ and Mg2+ cation substitutions. The zeolitic material obtained with the application of ultrasound, apart from initial fly ash phase residues, contained new phases in significant quantities, such as hydrosodalite and NaP-type zeolite, and A-type zeolite in smaller quantities.