Aluminium oxide adsorbs water, organic liquids and gases, without undergoing changes of form or characteristics in the process. Activated aluminium oxide is a particularly strong adsorber of molecules with high polarity. Water possesses such high polarity so that aluminium oxide is suitable as a drying medium. Aluminium oxide, whether found in nature or artificially produced, is a powdery material and has to be granulated by means of a binding substance.
Starting from aluminium hydroxide Al(OH)3 , a transition clay is first obtained through partial dehydration. This is formed, provided certain conditions are observed, through calcination in the temperature range 200 - 800°C. These transition clays find particularly frequent use as adsorbents. Viewed under an electron microscope, highdrargillite crystal Al(OH)3 shows the first surface cracks and fissures at temperatures of 200 - 300°C. Through longer exposure or correspondingly higher temperatures, these become larger, leading to the formation of porous structures and more or less disturbed crystal grids as well as high surface activities. These are all the more considerable, the higher the percentage of X-ray amorphous phases and imperfectly developed crystal phases. Aluminium oxide as a drying medium displays very good chemical resistance, resists liquid water and displays a high load capacity per grain. However, it has minor catalytical effects which are often undesirable. The internal surface of activated aluminium oxide lies in the range of 100 - 400m2/g. At low degrees of humidity, the drying effect is limited.
The regeneration temperature lies between about 170 - 320°C. When reactivating aluminium oxide, one should ensure that the gas used for regeneration is relatively dry, as high temperatures and an increased water content are particularly inclined to inflict hydrothermal damage upon activated aluminium oxide.
