COAL TAR

DescriptionCoal tar is a complex hydrocarbon mixture produced by thermal destruction (pyrolysis) of coal, typically a dark viscous liquid or semisolid with a smoky or naphthenic odor. The composition of coal tar will be influenced by the process used for pyrolytic distillation as well as by the original composition of the coal; however all coal tars will be comprised of a variable mixture of organic compounds including benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene, acenaphthene, methylnaphthalenes, fluorine, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene, and pyrene. The number of specific chemical constituents is in the thousands. Coal tar creosotes and coal tar distillates, oily liquids generally lighter in color and of lower viscosity than coal tar, are fractions produced by additional distillation of crude coal tar. Coal tar pitch is a highly viscous dark semisolid byproduct of coal pyrolysis. Coal tar volatiles are the vapors produced from heated coal tar or coal tar pitch, containing lower molecular weight (smaller ring number) polycyclic aromatic hydrocarbons (PAHs). Coal tar is noteworthy as one of the first – if not the first – chemical substances documented to cause cancer through occupational exposures. In the eighteenth century, Sir Percival Pott, a British surgeon, noticed a higher incidence of cancers in chimney sweeps chronically exposed to soot and coal tar. He then demonstrated excess cancers occurring in laboratory animals when coal tar is applied to the ears and skin. In the early twentieth century, polycyclic aromatic compounds isolated from coal tar were identified as chemical carcinogens.

UsesCoal tars and coal-tar pitches have many uses in industry and in consumer products. Coal tars are used primarily for the production of refined chemicals and coal-tar products, such as creosote, coal-tar pitch, and crude naphthalene and anthracene oils from the distillation of crude coal tar. Coal tar has been used as a fuel in open-hearth furnaces and blast furnaces in the steel industry, as a binder and filler in surface-coating formulations, and as a modifier for epoxy-resin surface coatings. U.S. Pharmacopeia–grade coal tar is approved for use in denatured alcohol (IARC 1985). Coal-tar preparations have been used for many years to treat various skin conditions, such as eczema, psoriasis, seborrheic dermatitis, and dandruff. Both prescription and nonprescription preparations are available and include cleansing bars, creams, gels, lotions, ointments, shampoos, and other topical solutions and suspensions (DermNet NZ 2010). Coal tar is also registered as an active ingredient in pesticides with the U.S. Environmental Protection Agency (EPA 2003). Coal-tar pitches are used primarily as the binder for aluminumsmelting electrodes (IARC 1984). They are also used in roofing materials, to impregnate and strengthen refractory brick (for lining industrial furnaces), and in surface coatings, such as pipe-coating enamels and black varnishes used as protective coatings for industrial steelwork and as antifouling paints for boats. Hard pitch is used as a binder for foundry cores. Coke-oven pitch is used to produce pitch coke, which is used as the carbon component of electrodes, carbon brushes, and carbon and graphite articles. Distillation fractions and residues from high-temperature coal tars are used for road paving and construction and in the production of naphthalene, recovery of benzene, production of anthracene paste, briquetting of smokeless solid fuel, impregnation of electrodes and fibers, and manufacture of electrodes and graphite (IARC 1985).

UsesCoal tar is primarily used as a raw material in the manufacture of plastics, solvents, dyes, and in the manufacturing of other chemicals. Most coal tar undergoes further distillation. Industries that use coal tar include road paving, roofing, smelting, and coking. Coal tar creosote is used as a wood preservative. Coal tar products are also ingredients in medicine (Coal Tar United States Pharmacopeia) used to treat skin diseases such as psoriasis or eczema.

UsesCoal tar is used for constructing roads, electricity generation and coking. It has been used as topical medical treatment for psoriasis and dandruff and is formulated as a soap or ointment. It is also used for other types of rashes (such as eczema, atopic dermatitis, chronic exudative dermatitis).