Occupational Exposures during Coal-tar Distillation

Coal tar is obtained by cooling the gas that is formed during the destructive distillation of coal to approximately ambient temperature. It is a black, viscous liquid composed primarily of a complex mixture of condensed-ring aromatic hydrocarbons. It may contain phenolic compounds, aromatic nitrogen bases and their alkyl derivatives, and paraffinic and olefinic hydrocarbons. Coal-tar pitch is the residue from the distillation of coal tar (Betts, 1997). The largest source of tar and pitch is the pyrolysis or carbonization of coal. Until the end of the Second World War, coal tar was the main source of benzene, toluene, xylenes, phenol, cresols and cresylic acids, pyridine and methylpyridines, naphthalene, anthracene, creosote, tar paints, road tars and pitch binders (Betts, 1997). However, by the 1990s, over 90% of the world production of these aromatic chemicals was derived from the petrochemical industry, and coal tar became chiefly a source of anti-corrosion coatings, wood preservatives, feed-stocks for the manufacture of carbon black and binders for electrodes (Betts, 1997). Coal-tar distillation products comprise essentially the distillate (primarily a complex mixture of monoand polycyclic aromatic hydrocarbons) and the residue from the distillation (pitch) (Betts, 1997). The part of coke-oven tar that is normally distillable at atmospheric pressure boils at up to ~400 °C and contains principally aromatic hydrocarbons. These include (in order of the distillation fraction): benzene, toluene and the xylene isomers, triand tetra-methylbenzenes, indene, hydrindene (indane), and coumarone; polar compounds, including tar acids (phenol and cresols) and tar bases (pyridine, picolines (methylpyridines) and lutidines (dimethylpyridines)); naphthalene, contaminated with small but significant amounts of thionaphthene, indene and other compounds; methylnaphthalene isomers; biphenyl, acenaphthene and fluorene; anthracene and phenanthrene; pyrene and fluoranthene (Betts, 1997). Much less is known about the composition of pitch, the residue from coal-tar distillation. Studies of coke-oven pitch indicate that it contains: four-membered aromatic hydrocarbon OCCUPATIONAL EXPOSURES DURING COAL-TAR DISTILLATION