DESICCATION, the operation of drying or removing water from a substance. It is of particular importance in practical chemistry. If a substance admits of being heated to say 100°, the drying may be effected by means of a steam bath, which is simply an oven heated by steam ; an air-bath may be used for higher temperatures. Otherwise a desiccator must be employed ; this is essentially a closed vessel in which a hygroscopic substance is placed together with the substance to be dried. The process may be accelerated by exhausting the desiccator; this so-called vacuum desiccation is especially suitable for the concentration of aqueous solutions of readily decomposable substances. Hygro scopic substances in common use are phosphoric anhydride, concentrated sulphuric acid, potassium and sodium hydroxides, soda lime, anhydrous sodium sulphate and calcium chloride.
Two common types of desiccator are in use. In one the ab sorbent is placed at the bottom, and the substance to be dried above; in the other, this arrangement is inverted. Liquids are dried either by means of the desiccator, or, as is more usual, by shaking with a substance which removes the water. Fused calcium chloride is the commonest absorbent; but it must not be used with alcohols and certain other liquids with which it forms corn pounds. Quicklime, barium oxide and dehydrated copper sul phate are especially applicable to alcohol and ether; the last traces of water may be removed by adding metallic sodium and distilling. Gases are dried by leading them through towers or tubes containing an appropriate drying material. The experiments of H. B. Baker on the influence of moisture on chemical combination have shown the difficulty of removing the last traces of water—see DRYNESS, CHEMICAL.
In chemical technology, apparatus on the principle of the laboratory air-bath is mainly used. Crystals and precipitates, deprived of as much water as possible by centrifugal machines or filter-presses, are transported by means of a belt, screw or other form of conveyor, on to trays staged in brick chambers heated directly by flue gases or steam pipes; the latter are easily controlled, and if the steam be superheated a temperature of 3oo° and over may be maintained. In some cases the material tra verses the chamber from the coolest to the hottest part on a con veyor or in wagons. Rotating cylinders are also used; the material to be dried is placed inside, and the cylinder heated by a steam jacket or otherwise.