COAL OIL, a volatile oil distilled from petroleum (q.v.).
or the opaque, viscid, black or brownish liquid arising as a by-product from the dry distillation of bitumi nous coal in the manufacture of coal gas, and also in the manufacture of coke. It consists principally of the substances grouped under the name of °aromatic hydrocarbons,>) and varies in composition according to the grade of coal distilled, the degree of heat employed in the distillation, and the shape of the retorts used. In the lower temperatures at which coke is made (about 850° F.) the tar consists of photo gen, paraffin oil, lubricating oils, paraffin, cresol and about 57 per cent of pitch; and the yield of tar may be as high as 20 gallons to the ton of coal. In the high temperatures em ployed in making coal gas (from 1650° to 1830° F.) the tar consists of benzene (benzol), toluene, naphthalene, anthracene, heavy oils, much creosote, and about 82 per cent of pitch; and the amount secured is about nine gallons per ton of coal. In the latter case many of the constituents of coke tar are broken up into permanent gases by the higher temperature. With horizontal retorts the yield of tar is about two gallons less per ton of coal than with vertical retorts.
The specific gravity of coal-tar ranges from 1.0 for low temperature tar to 1.2 for that pro duced at high temperatures.
Up to about the middle of the 19th century coal-tar was regarded as a waste product and thrown away, or burned under the retorts. About 1846 it tiegan to be used in Germany for making roofing felt, and distillations of tar were used to impregnate railway sleepers. A year or two earlier, in England, coal-tar naphtha had been employed successfully as a solvent for India rubber. The discovery in 1856 of coal-tar colors caused a great demand for the crude substance, and it became a com mercial product of increasing value.
Production and The production of coal-tar in the United States in 1915 was 276,745,885 gallons of which 39,021,748 gallons (14 per cent) was used for fuel, and 237,724,137 gallons was sold into commerce. Of the total product 186,278,000 gallons came from old style bench retorts and by-product coke ovens, and 51,446,344 gallons from gas plants. The aggregate value of the product was $6,656,276; an average value of 2.8 cents per gallon. The world's production in 1901 was reported at 2,660,440 tons: the United Kingdom producing 908,000 tons, Germany, 590,200 tons, and the United States, 272,400 tons. These three coun tries thus produced two-thirds of the whole. The principal uses now made of coal-tar are : as fuel — its heat value being nearly double that of coal; in preserving timber, stone and brick work from the effects of moisture; in making roofing felt and roofing papers; as paint for metals; as a cement, in place of pitch; in the manufacture of lampblack and soot ; as a top dressing for roads; in the manufacture of dyes and medicines; in the production of solvents and explosives; and, to a limited degree, in gas manufacture — unprofitable at the prevailing value of coal-tar.
A considerable amount of the coal-tar mar keted is artificially made by mixing coal-tar pitch with enough heavy oil (from petroleum) to give it the required fluidity at normal tem peratures. (See also articles, COAL-TAR COLORS; COAL-TAR PRODUCTS). Consult Lewes, (Car bonization of Coal) (1912) ; Lunge, Tar and Ammonia' (1909) ; Wagner, (Coal Gas Residuals' (1914) ; Warnes, (Coal Tar Distil lation' (1913).