Hard coal contains aromatic hydrocarbons in the form of high molecular weight compounds (average molecular weight about 3000 and according to new measurements up to 500000) which are cracked, or depolymerized, and rearranged by the coking process at 1000- 1400 °C (high-temperature coking). The low molecular aromatics which are formed in the three coking products - crude gas, coke-oven water, and coke tar - can be isolated using special techniques. The residue, coke, is essentially pure carbon. Coking can therefore also be seen as a dismutation of H-rich and H-poor products.

The aromatics are removed from the crude gas or coke-oven gas either by washing with higher boiling hydrocarbons (anthracene oil) or by adsorption on active carbon. The aromatic mixture is then distilled from the wash liquid or desorbed from the carbon with steam. Unsaturated hydrocarbons and N- and S-containing compounds are then removed either by refining with sulfuric acid or, as is preferred today, in a hydrogenative catalytic reaction. Distillation gives a product known as 'crude benzene' or 'cokeoven benzene' with a typical composition shown in the following table:



Most of the naphthalene is present in the residue.

The coke-oven water contains about 0.3 wt% of a mixture of phenol and its homologues which is isolated from the aqueous phase by extraction, e. g., with benzene or butyl acetate (Phenosolvan process). It consists mainly of phenol (52 wt%), cresols, xylenols, and higher phenols.

contains other important aromatic parent substances which can be separated by fractional distillation into the following product mixtures:

The further processing of individual fractions to enrich or recover important components, e. g., naphthalene and anthracene, is done by crystallization, extraction, or other separation processes.