COAL-TAR PRODUCTS. Coal-tar is separated into its many valuable constituents by distillation at gradually increasing tem peratures. This is done usually in an iron still cylindrical in shape, about as high as it is in diameter, with a domed top and a concave bottom. The charge is usually 20 tons of tar, which has previously been completely freed from its water content. The distillate passes through a con densing coil surrounded with water which is cold at first but is changed at intervals to hot — to prevent blocking of the coil with solid sub stances which crystallize out.
The products of this primary distillation are (1) light oils of a specific gravity of 0.90 to 0.95, coming over up to the temperature of 342° F., and in amount varying from 2 to 8 per cent of the charge of tar; (2) middle, or car bolic oils of a specific gravity of 1.01, coming over above 342° and up to 42-6°, in atnount, 8 to 10 per cent of the tar; (3) heavy, or creosote oils of a specific gravity of 1.04, coming over above 446° and up to 518°, in amount, 8 to 10 per cent of the tar; and (4) anthracene oils of a specific gravity of 1.10, coming over above 518° and up to• 752°, in amount, 16 to 20 per cent of the tar. The residue in the still is pitch, in amount, about 50 per cent of the onginal charge of tar.
These primary distillation products are treated in various ways to secure the large variety of coal-tar products known to com merce and utilized in the economic arts.
The light oils are first redistilled, to separate them from a small admixture of the middle or carbolic oils. Then the purer liquid is agitated at a low temperaturc with concentrated sul phuric acid in cast iron or lead-lined wooden vessels. This operation fixes the bases, and the unsaturated hydrocarbons, phenols, and sulphur compounds arc dissolved or resinified. The acid portion is drawn off and yields anunonia sulphate and pyridine. The oil is then washed with water and a wealc solution of caustic soda, and, after drying, is fractionally distilled.
The fractions are collected in four portions: (1) the ((first runnings," up to a temperature of 158° F.—consisting chiefly of carbon disul phide, pentane, hexane and aceto-nitrite; (2) light benzol, coming over above 158° and up to 252° ; (3) heavy benzol, coming over above 252°, and up to 350°; and (4) carbolic oils, which are turned back into that portion of the primary products.
The °crude° benzol thus obtained is again distilled, and separated into pure benzol, to1uol, xylol and a residue which is called °solvent naphtha.° The pure benzol and toluol thus ob tained yield coal-tar dyes. The 90 per cent benzol is used in making a motor fuel, in laundry work and in the manufacture of illu minating gas. Solvent naphtha is used ex tensively in the rubber industry.
The middle or carbolic oils contain naphtha lene, phenol and the cresols. Naphthalene crystallizes out when the oil is allowed to stand, and the crystals are separated in a centrifugal machine and by pressure. After the naphtha lene is removed, phenol to the amount of 30 to 40 per cent is obtained by fractional extraction with a 10 per cent caustic .soda solution, arid then neutralized by sulphunc or carbonic acid — which precipitates the phenol fluid. This, again, is fractionally distilled and subjected. to cold, when the phenol crystallizes out, leaving the cresols still liquid. The pure phenol is used in making picric and salicylic acids. The cresols remain in three forms mingled together — ortho-cresol, para-cresol and meta-cresol —and are used in the crude form as a disinfectant.
The heavy or creosote oils are a combination of neutral and acid oils, chiefly: naphthalene, dinaphthalene, methylnapthalene, xylenol, naph thol and paraffins. The creosote oils are used principally for preserving timber.
The anthracene oils contain some carbolic acid besides the anthracene. The latter is separated out by chilling the oils to a tempera ture where the anthracene crystallizes into what is lcnown as crude anthracene, from 10 to 12 per cent pure. The impurities consist chiefly of phenanthrene, methylanthracene, diphenyl, naph thalene, pyrene, cretene and carbazol. The crude anthracene is subjected to a pressure of 3,000 to 4,000 pounds to the square inch in a hydraulic press heated by steam. Nearly all the phenan threne and naphthalene flow out in liquid form. The remainder is washed with solvent naphtha, and with the acid pyridine liquid from the first treatment of the oils, and anthracene of 90 per cent purity remains.