The processing of tars remains an important industry in countries including the US and Japan. Tar distillate is considerably less of a ‘rag bag’ of organic compounds than petroleum distillates are.
At temperatures up to about 100°C, the distillation product of coal tar is benzol. This consists of benzene, toluene and xylenes (BTX) in whatever proportions. The kinematic viscosities of these compounds singly are all around 1 cSt, the smallest value being that for m-Xylene which is 0.68 cSt at 25°C. Separation of BTX from coal tar into its constituent compounds in high purity by further distillation is straightforward. The heavier distillate, when cooled to below its cloud point, yields crystals of useful compounds including phenol, o-cresol.
Coal tar pitch
The residue of distillation of coal tar is pitch. It can be the primary product in tar processing in which case the conditions of distillation including the maximum temperature can be controlled to give a pitch of desired properties. Pitches vary in nature from semi-solid at room temperature to hard and brittle. Most start to melt at temperatures in the region of 100°C and their theology is of some importance.
