Desorption of water during the drying of clay minerals. Enthalpy and entropy variation

The purpose of this paper is to characterize the energy associated to the d rying of three based clay raw materials containing mostly the minerals kaol inite, illite, and smectite, respectively. These natural clays, are commonl y employed in many ceramic processes. Shaped samples were dried in a convec tion dryer at 55 degrees C and 70 degrees C, with an air humidity ratio ran ging between 0.99 and 0.2. From the desorption isotherms, we have determine d the enthalpy and entropy variations corresponding to the water desorption phenomena. They were calculated using the interpretation of the Gibbs free energy of the desorption mechanism, H2Oadsorbed-->H2Ofree.liq.water. The b ehavior observed for shaped raw material containing kaolinite is similar to what is described in literature for kaolinite mineral. A maximum in bindin g heat of 17 kJ mol(-1) is observed for a complete monolayer coverage locat ed between the layers of kaolinite clay. For shaped raw material containing smectite the behavior is fairly similar and the maximum in binding heat is about 18 kJ mol(-1). It corresponds to a situation for which 2/3 of the in terlayer water is present. For shaped raw material containing illite, it wa s only possible to observe the beginning of the strong variation of Delta H and Delta S degrees when the number of water layers on clay surface is low . (C) 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved.