GB766887A - Improvements relating to the purification of salicylic acid and other substances - Google Patents

Abstract

In a process for the purification of a sublimable organic chemical substance, e.g. salicyclic acid, benzoic acid or anthracene, the substance in the form of a vapour that contains entrained non-volatile impurities is passed through an electrostatic precipitator and the impurities are thereby removed. Salicylic acid, e.g. containing non-volatile impurities is introduced in a finely divided state into a carrier gas stream, e.g. carbon dioxide, or nitrogen, and the mixture which is incapable of causing an explosion, is heated to volatilize the salicylic acid. Alternatively salicylic acid may be volatilized into a stream of hot carrier gas. The mixture of gas, vapour and non-volatile impurities is then passed through an electrostatic precipitator, heated to prevent condensation of salicylic acid, where the impurities are removed. The apparatus may be made of aluminium or stainless steel. The carrier gas may be re-cycled but it may be necessary to prevent the build-up of volatile impurities, e.g. of phenol therein. In an example, salicylic acid is introduced into a stream of carbon dioxide by means of a screw conveyer and volatilized by passing the mixture through a heated stainless steel tube. The stream of gas and vapour is then passed through an electrostatic precipitator consisting of steel cylinder with a central electrode in the form of a bar of spare cross-section. The precipitator and the piping between the heater and precipitator are steam-heated. Salicylic acid vapour emerging from the precipitator is condensed with a condenser fitted with a vibrator.