Primary Tar

a product of the semicoking of solid fuels, such as coal, lignite, peat, and shales. It is called primary to distinguish it from resins produced by coking. Primary tar is a complex mixture of organic compounds; it is a liquid whose consistency ranges from readily mobile to viscous and whose color ranges from pale to dark brown. Primary tar is made up of various classes of hydrocarbons, phenols, organic acids, pyridine and quinoline bases, and inert oxygen compounds. The yield, physical properties, and chemical composition of primary tar are determined by the type of fuel (see Table 1), the processing temperature, the rate of heating, and the type of furnace.

The most valuable components of peat primary tars are waxes and phenols. However, these tars have not yet been used on an industrial scale. Lignite primary tars (from bituminous coal) are characterized by a high paraffin content (up to 19 percent). The phenols in the tar include phenol, cresols, xylenols, and higher alkyl phenols. One of the methods of processing such primary tars is distillation at atmospheric pressure and under vacuum, separation of phenols, and aromatization of the hydrocarbon raffinate, which yields aromatic hydrocarbons, solvents, and paraffin. The second method is distillation with subsequent processing of the fractions to yield commercial products (phenols, pyridine bases, and antiseptics). The distillation residue is used for making paving bitumen and briquetting and founding binders.

Primary coal tar is characterized by a high phenol content (up to 40 percent). It may be efficiently processed by hydrogenation. However, a simpler method is distillation with removal of the light and medium fractions, which are then purified and redistilled. Steam distillation of the phenolates from alkali treatment yields neutral oils, and subsequent treatment of the phenolates with carbon dioxide gives so-called crude phenols (a mixture of phenol, cresols, xylenols, and higher phenols). Fractional distillation may be used to separate the crude phenols into narrow phenol fractions, with the evolution of individual phenols, or the crude phenols may be used to make alkyl phenols (carbolite resin), antiseptics, resol glues, and other products. The distillation residue is oxidized to produce bituminous materials for various purposes.

Shale primary tars are composed of phenol, cresols, xylenols, higher methyl and ethyl phenylic acids, dihydric phenylic acids, naphthols, and dihydroxynaphthalenes. The neutral oxygen compounds contain mainly aldehydes and ketones. Among the hydrocarbons are alkenes, alkanes, cycloparaffins, dienes, and aromatic hydrocarbons with at least one ring. The processing of primary shale tar yields motor fuels, solvents, paving and construction bitumen, electrode carbon, anthracene oil, shale lacquer, softeners for the rubber industry, detergents, resol resins, and toxic chemicals.