GIS-type zeolite synthesis from Greek lignite sulphocalcic fly ashes promoted by NaOH solutions

Abstract GIS-type zeolite (gobbinsite–NaP1) was synthesized from sulphocalcic lignite fly ashes, from the Agios Dimitrios and Amynteon-Filotas power plants of the Lignite Center of Ptolemais-Amynteon, Macedonia, Greece. The fly ash samples were treated with 0.1 and 0.5 mol dm −3 NaOH solutions at 90 °C and at room temperature. Both fly ash samples contained calcite, quartz, anhydrite and lime as major mineral phases, and mainly consisted of SiO 2 , CaO, Al 2 O 3 . Their Si/Al ratio was 2.08 and 2.28, respectively. The experiments (duration 95–260 h) were conducted under continuous rotational stirring. The solid experimental products have been analyzed by means of X-ray diffraction, FT-IR, and scanning electron microscopy–energy dispersive spectrometer. The treatment of both fly ashes at 90 °C with 0.5 mol dm −3 NaOH resulted in GIS-type zeolite (gobbinsite–NaP1) with a 10–15% yield. FT-IR absorption bands assigned to CaCO 3 , CaSO 4 , CaO, and silicates were observed in the starting materials, whereas bands attributed to CaCO 3 , amorphous silicate polymers (intermediate products of zeolitization), hydrated aluminum silicates, and increased amount of silicates, attributed to the zeolite synthesis, were identified in the solid experimental products.