J. Mater. Environ. Sci. 5 (2) (2014) 615-624 Khachani et al. ISSN : 2028-2508 CODEN: JMESCN 615 Non-isothermal kinetic and thermodynamic studies of the dehydroxylation process of synthetic calcium hydroxide Ca(OH) 2 M. Khachani * , A. El Hamidi, M. Halim, S. Arsalane Laboratoire de Physico-Chimie des Matériaux, Catalyse et Environnement, Université Mohammed V- Agdal, Faculté des Sciences, Avenue Ibn Batouta, BP:1014, 10000 Rabat Principal, Morocco Received 9 Oct 2013, Revised 10 Dec 2013, Accepted 10 Dec 2013 *Corresponding author. E-mail: mariamkhachani@gmail.com ; Tel: (+ 212 537 77 54 40) Abstract The non-isothermal kinetics of dehydroxylation of Ca(OH) 2 have been studied in dynamic helium atmosphere using TG, DTG, DTA and XRD techniques at different heating rates. The apparent activation energy E α is determined using Friedman and advanced isoconversional methods (Model-Free). The results indicate that dehydroxylation process occurs predominantly by an irreversible major step, preceded by a rapid and reversible one. The Malek's kinetic procedure associated with non-linear regression approach was used to determine the pre-exponential factor (A) and the kinetic model function. The autocatalytic SB (Systak-Berggren) reaction model corresponding to differential function f(α) = α m .(1-α) n is the most probable for the dehydroxylation process of Ca(OH) 2 . The best fit has led to following kinetic triplet: Ln A = 16.85, E = 132.20 kJ.mol -1 and f(α) = α 0.203 .(1-α) 0.380 . The thermodynamic functions (ΔS*, ΔH*, ΔG*) of the studied reaction are calculated using activated complex theory and show that dehydroxylation process requires heat. Keywords: Calcium hydroxide, Non-isothermal kinetics, Thermal dehydroxylation, Thermodynamic parameters 1. Introduction Calcium hydroxide Ca(OH) 2 is an important inorganic compound belonging to the group of hydroxide with brucite structure type. The packing is formed by layers of octahedral Ca sites linked together in (001) plane by strong hydrogen bonds. Ca(OH) 2 has been widely used in various technological processes of making new materials or compounds with particular characteristics such as building materials, adsorbents for wastewater including radioactive elements, desulphurizing agents, materials for thermal energy storage and food additives [1-4]. Synthetic Ca(OH) 2 compound can be prepared by various methods using different alkaline media and in presence of organic additives or surfactants [5, 6]. However, carbonation by atmospheric CO 2 causes many problems in its use, mainly in the building industry. Several kinetic studies of the carbonation of Ca(OH) 2 were performed to examine the possible mechanisms governing this reaction. The carbonation process seems to depend on the particle size and moisture which significantly affects the surface properties of the calcium hydroxide [7, 8]. On the other hand, thermal decomposition of synthetic calcium hydroxide has been subject of several studies because it leads to the formation of calcium oxide CaO nanoparticles more reactive than that derived from the respective limestone, CaCO 3 and even from commercial CaO [9, 10]. But some controversial results have been revealed in kinetic studies notably on the estimation of the reaction mechanism and the model which give the best approximation of experimental kinetics. The main divergence is due to experimental conditions, which can be adopted, and to reversible nature of the dehydration produced by the pressure effect of vapor water [11-13]. In the majority of cases, the dehydroxylation process of Ca(OH) 2 has been observed under different atmospheres, for temperatures ranging from 300 to 500°C [14, 15]. It is confirmed that dehydroxylation is extremely sensitive to the texture, the form of particles and to the choice of heating rate. The basic mechanism describing the kinetic scheme is often represented by two steps. Thus, Mikhail and Brunner's examination indicate that the dehydroxylation took place at Ca(OH) 2 /CaO interface through two separate steps [16]. In addition, Brett stated that dehydroxylation started at crystal edges and surface defects followed by the reaction moving inwards into the crystal [17]. According to the nature of decomposition reactions, various mathematical models, listed by Sharp et al [18] were used in order to estimate the kinetic parameters and to simulate the experimental data. Recently, the recommended isoconversional method (Model-Free) proposed by Vyazovkin et al [19, 21] seems the most