Thermochimica Acta 413 (2004) 23–29 Application of model-free kinetics to the study of dehydration of fly ash-based zeolite I. Majchrzak-Kuc˛ eba ∗ , W. Nowak Department of Heating, Ventilation and Air Protection, Czestochowa University of Technology, ul. Dabrowskiego 73, 42-200 Czestochowa, Poland Received 14 July 2003; received in revised form 29 October 2003; accepted 29 October 2003 Abstract In the present paper, dehydration kinetics of zeolite Na-A synthesized from fly ash was investigated by means of thermogravimetric analysis. Na-A zeolite was formed from coal fly fash by fusion with sodium hydroxide and succeeding hydrothermal treatment at 100 ◦ C after induction period. The model-free kinetic method was applied to calculate the activation energy of the dehydration process of fly ash-based zeolite as a function of conversion and temperature. The Vyazovkin model-free kinetic method also enabled the definition of time, necessary to remove water molecules from the zeolite structure for a given temperature. © 2003 Elsevier B.V. All rights reserved. Keywords: Model-free kinetics; Zeolites; Thermal dehydration; Fly ash 1. Introduction Natural and synthetic zeolites are hydrated aluminosili- cates of porous crystalline structure. Wide variety of charac- teristic features of these compounds has influenced the fact that they are applied in numerous areas of economy. They are especially useful in the processes of drying and cleaning gases, deactivation of radioactive waste as well as in refin- ery, chemical and paper industries [1]. One of the recent zeolite synthesis possibilities is modi- fication of fly ash conducted in hydrothermal conditions, in alkaline environment, with NaOH or KOH solutions. Modi- fication processes may be conducted with the application of different methods: classic alkaline conversion [2–6], fusion with sodium hydroxide before the hydrothermal reaction [7], two-stage process of obtaining pure zeolites [8]. In result of the processes conducted one may obtain the following types of zeolites: Na-A, Na-X, sodalite, analcime, Na-P1. The zeolites synthesized from fly ash have been used for immobilization of environmental pollutants like radioactive ∗ Corresponding author. Tel.: +48-103334-3250432; fax: +48-103334-3250933. E-mail address: izak@is.pcz.czest.pl (I. Majchrzak-Kuc˛ eba). waste, removal of heavy metals and ammonium ions from industrial sludge [2–7]. The research on dehydration kinetics (value of activation energy, conversion rates) is crucial as it is in the dehydration state that zeolites prove their sorption and catalytic quali- ties or molecular-sieve activity. Numerous factors may in- fluence the kinetics of zeolite dehydration. They include the interaction of H 2 O molecules with a framework, of H 2 O molecules with other cations and with each other, geometry of zeolite channel/void systems as well as their transforma- tions during the process of dehydration. Stating the influ- ence of particular factors on the dehydration process is rather difficult. Thermal analysis methods (thermogravimetry, differential thermogravimetry) are widely used in characterization of zeolites and in the process of studying the mechanism and kinetics of zeolite dehydration [1]. The results of thermal analysis can supply very useful information on the processes of dehydration of zeolites and their thermal stability. The aim of the present study is to investigate the pro- cess of dehydration of Na-A zeolite, a product obtained dur- ing hydrothermal treatment of fly ash with the application of thermogravimetry and to determine the kinetic parame- ters of thermal dehydration zeolite by using the Vyazovkin model-free kinetic method. 0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2003.10.021