Abstract
The aluminosilicate materials kaolinite, calcium montmorillonite and emathlite have been tested as solid sorbents for alkali vapour in controlled gaseous environments, in order to study their sorption characteristics. The study used pan pelletised and extruded pellets in single pellet and fixed bed reactor systems under gaseous environmental conditions containing water vapour, hydrogen chloride and nitrogen at a temperature of 850°C. The means of producing the pellets and the composition of the gaseous environment were shown to determine the sorption performance of the sorbent pellets. The physical properties of the pellet (particle size, total pore volume, surface area, crush strength) significantly affect the sorption effectiveness, while the formation of reaction products is dependent upon the sorbents' chemical composition and on the sorption conditions. Reaction products identified under the various sorption conditions indicated possible pathways for alkali capture. Hydrogen chloride mixtures were shown to cause a reverse of the sorption process for some sorbents. A leaching method for extracting sodium from treated pellets enabled the type of bonding to be determined but not necessarily the prime sorption mechanism. With the introduction of water vapour, some evidence was presented that the alkali sorption rate can change significantly. The previously reported effect of hydrogen chloride upon alkali capture by the sorbent material, calcium montmorillonite; shown by McLaughlin (1990) was confirmed and the effect was also investigated for emathlite and kaolinite. Reversibility depended upon the presence or absence of hydrogen chloride. Both water vapour and hydrogen chloride determine sorption activity and capacity. Likewise the structural characteristics of the pellet influences the sorption activity and capacity. The sorption process is not due to aluminosilicate reaction alone but also due to replacement mechanisms. Conclusions are drawn regarding mechanism and theoretical model proposed.