Effect of Ambient Atmosphere on Solid State Reaction of Kaolin-salt Mixtures

-The reaction of kaolin with NaC1 was followed by dynamic thermal analysis and mass spectrometry under/'42, CO2, and air atmospheres and in a 10-5-torr vacuum. The weight loss was a function of the atmosphere used and, according to mass spectrometry, was due to the evolution of H20, HC1, and very small amounts of H2. HC1 was formed only after the release of 85% of the hydroxyl content of the kaolin. When the clay was pretreated with saturated salt solution, H20 and HC1 evolved in more or less the same temperature range, indicating that only some of the OH groups reacted with the chloride ion. High-temperature X-ray powder diffraction patterns showed that the sodium ion reacted with the noncrystalline metakaolin to give NaA1SiO4. Chemical analysis showed that the reaction of kaolinite and sodium chloride started below 400~ The rate of the reaction increased at higher water vapor concentration. From mass spectrometric data, the NaCl-treated kaolin appeared to adsorb CO2. Desorption at several distinct temperatures suggests that CO2 was adsorbed by different parts of the structure, i.e., holes and channels. X-ray powder diffraction and infrared absorption data indicate that the kaolinite structure persisted even after it had been heated with NaC1 in a CO2 atmosphere to as high as 800"C. Key Words--Infrared spectroscopy, Gaseous atmosphere, Kaolin, Metakaolin, Salt, Thermal analysis. I N T R O D U C T I O N The atmosphere surrounding solids greatly influences the physical and chemical processes occurring at particle boundaries. Catalytic and adsorptive properties, color, and the shape, size, and distribution of pores of solids are all influenced by the nature of the surrounding gaseous atmosphere (Hedwall, 1966), and chemically inert gases may dissolve in solids and minerals (Freund et aL, 1983), causing topochemical effects on the heated substances. The effect of ambient atmospheres on the thermal reactivity of clays has been investigated by only a few authors. By heating kaolin to 1000~ in N2, air, CO, CO2, and H20 vapor, Sanford (1951) found that the reaction of the heated products with water vapor at 25*(2 depended on the type of atmosphere employed. Mackenzie (1968) showed that the presence of 02 and CO2 hindered the formation of crystalline phases from metakaolin at 1100~ whereas water vapor and a 10-4torr vacuum promoted the process. The reaction ofmetakaolin with alkali salts was studied at atmospheric pressure by Jagitsch (1958) who measured the diffusion of soda into metakaolin at 500*-700~ in a water-free system. The diffusion rate increased with temperature. Farmer (1966) demonstrated that the temperature of dehydroxylation of clays changed if they were pressed into discs with alkali halide. Yariv (1975) found that the layered structure of kaolinite was destroyed even at room temperature when the kaolinite was ground with KBr. Heller-Kallai (1975) showed during the solid state reaction of montmori l lonite and KBr and KC1 at 300~176 that excess cations were taken up by the clay with deprotonation Copyright 9 1986, The Clay Minerals Society of structural hydroxyls. She later (1978) showed that during reactions of kaolinite with salts of alkali metals the clay became reactive after dehydroxylation. Alkali ions were apparently incorporated into the aluminosilicate structure without the formation of detectable metakaolinite. The aim of the present investigation was to investigate the role of the non-reactive gas atmospheres on the reaction of kaolin with sodium chloride.