Coal tar, a black, sticky liquid thicker than water, is produced when coal is heated in the absence of air, a process called destructive distillation. Much coal tar is produced by the steel industry as it produces millions of tons of coke each year to fuel the furnaces used in separating iron from its ores. A modern coke oven makes about 22 metric tons of coke from 30 tons of coal in less than a day. About one fourth of the coal is converted into gases that are piped out of the oven. Cooling them produces about 82 pounds (37 kilograms) of coal tar for each ton of coal. The remaining gases then rise through a tower called a scrubber. Oil with a high boiling point, called wash oil, is sprayed into the top of this tower, and the falling drops of oil absorb vapors of light oil from the gas. Light oil is a mixture of chemical compounds that does not completely liquefy upon cooling. The gas leaving the top of the scrubber is used as fuel. The oil leaving the bottom is distilled to remove the light oil from the wash oil.
Crude coal tar is a mixture of hundreds of organic chemical compounds (see Organic Chemistry). Most of these are aromatic compounds, the molecules of which have one or more rings of carbon atoms. Most of the compounds in coal tar contain only the elements carbon and hydrogen, but a few of them also contain oxygen, nitrogen, or sulfur.
From about 1860 until the 1940s, coal-tar products became the main raw materials of a large branch of the chemical industry. They were converted into dyes, drugs, explosives, plastics, fibers, films, pesticides, paints, rubber, and other useful substances. When the demand for these products grew too great for the coke producers to meet, petroleum became the main source of aromatic compounds.
To separate crude coal tar into all of its different components would cost more than the compounds are worth. Therefore, the refining processes are designed to produce only the most valuable compounds. The first step in refining coal tar is distillation to get three different liquid parts, or “fractions,” of the tar and an undistillable residue called pitch.
About 5 percent of the tar distills at temperatures up to 410° F (210° C). This fraction consists of light oil like that obtained from the scrubber. About 17 percent—called middle oil—distills at temperatures between 410° and 460° F (240 ° C). About 16 percent—called heavy oil—distills at temperatures above 460° F. The remainder of the coal tar is pitch.
Light oil can be separated into benzene, toluene, xylene, naphtha, and other substances for use in the chemical industry. It can also be used without separation to make gasolines that keep automobile engines running smoothly.
The important compounds obtained by separating middle oil are naphthalene, phenol, and cresylic acids. The heavy oil contains several substances used in making dyes. Removing these from the oil leaves a sticky black liquid called creosote. It is used to saturate railroad ties and telephone poles to protect them from organisms that destroy wood.