COAL TAR, the viscous, oily fraction of the liquid distillates obtained in the manufacture of coal-gas or of coke by the destruc tive distillation (see CARBONIZATION) of bituminous coal at 90o I200° C. Many factors affect both the yield and composition of the tar, particularly the class of coal carbonized, the type of re tort setting used, the temperature at which the coal is heated and the manner in which the heat is applied. The tars obtained when coal is distilled in horizontal retorts for the production of either gas or coke are sufficiently similar to be classed under the head ing of coal tar.
In the manufacture of gas, the coal is often mixed with bog head shale, bituminous lignite, etc., the gas being thereby en riched. The tar is, however, of less value, being mixed with paraffins, which make the purification of tar products much more difficult. Also, by the substitution of vertical for horizontal re tort settings, a tar is obtained which differs in some respects from ordinary coal tar. This new tar is thinner owing to the presence of paraffinoid substances, contains a higher percentage of tar acids, and on distillation yields much less pitch (40-55%) than tars from horizontal retorts. The yield of tar from vertical retorts is slightly higher, and the amount of "free carbon" (i.e., residue insoluble in benzene) is much smaller. These variations from "normal" tar are due to the fact that many of the products of distillation are removed from the reaction chamber before the coal has reached its final high temperature. Vertical retort tar, therefore, has some of thv characteristics of low-temperature tar (see TARS, LOW-TEMPERATURE). Owing to the increasing use of vertical retorts, two types of tar have to be considered as com mercial products. The effect of temperature of carbonization on the yield and composition of tars is very marked. The product commercially accepted as coal tar is therefore that derived from the carbonization of bituminous coal in horizontal retorts at 900 I 200° C, where the primary object is the production of coal-gas or coke. In either case the tar is a by-product of the operation.
The most general characteristic of tar is the occurrence in it of naphthalene, which may represent as much as io% of the crude tar and is the predominant constituent. Other constituents are benzene, anthracene, phenanthrene, carbazole, acenaphthene, fluorene, carbolic acid, cresols, pyridine, quinoline, isoquinoline and cumarone ; the amount of any one of these substances rarely exceeds I %. Valuable as these pure compounds are, from an industrial and commercial point of view, as distinct from that of the organic chemical industry itself, the various crude products obtained from tar are of equal if not greater importance. These coal tar "crudes," including the benzols, toluols, solvent naphtha, heavy naphtha, cresylic acid, creosote, heavy bases, anthracene oil and pitch, form the various "fractions," or can be easily pre pared from these fractions, when the tar is distilled.
(2) Light oil, b.p. IIo° to 18o-2oo° C.
(3) Crude carbolic oil or light creosote, b.p. 180-200° to 240-250° C.
(4) Heavy creosote, b.p. 240-250° to 270-280° C.
(6) Pitch.
These fractions are more or less standard, but rearrangements in the cutting points are to be expected as research develops new plications for substances such as acenaphthene, fluorene, acridine, etc. The value of cumarone and indene as resin-forming constit uents is a case in point. To obtain a fraction rich in these two substances, a distillate b.p. 16o-180° C is isolated.
Dehydration of Tar.—The presence of aqueous liquor leads to many difficulties in the distillation of tar, especially "frothing" and "bumping." Although many efforts have been made to de hydrate the tar before distilling it, prolonged, gentle heating is the only method which has been found satisfactory. For this purpose steam is supplied through a series of closed coils immersed just below the surface of the liquid. By regulating the supply and con trolling the level of the tar, the process is made continuous, and dry, heated tar overflows for use in any distillation apparatus.
In cases where no special preliminary dehydration of the tar is carried out, special mechanical and heating devices are used during the distillation of the light oils and water. A typical tar distillation plant is the Woodall–Duckham still (see DISTILLATION).
The tar, separated from excess of aqueous liquor by long settling, is charged into the still and heat applied either by means of coal or (as in coke-oven installations) of surplus gas, which permits of very delicate heat control. In all distillations, unless the tar has been previously dehydrated, the first runnings consist of a very light oil and water which separate on standing, the oils forming the upper layer. When no more water distils—a point usually coinciding with the rise of the specific gravity of the oils to 1.o—the crude carbolic oil is collected, and at this stage the condensing apparatus must be maintained at 40° C to avoid deposition of naphthalene in the coils. The end point of the car bolic acid fraction is denoted by the appearance of the distillate, which runs more freely as the naphthalene content decreases. The anthracene oil fraction is collected when the distillate assumes a green colour or when "green grease" makes its appearance, the condensers at this point being maintained at about 100° C. The pitch is run out of the still whilst hot. Towards the end of tar distillations much trouble is often caused by "coking." To pre vent this, and also to help in the distillation of the higher-boiling fractions, use is often made of direct superheated steam and of reduced pressures. These modified methods also improve the yield and quality of the distillates, particularly of anthracene. The temperatures of fractionation are seldom controlled by thermom eters, but use is made of the specific gravity, appearance, or volume of the distillates. The end point of the operation is deter mined. not by temperatures, but by the class of pitch required or the total volume of distillate obtained.
Working Up of the Fractions.—Fraction I. (first runnings), representing 1.5-2% of the crude tar, after separation from the aqueous liquor which has accompanied it in the distillation, con sists of a mobile oil of sp.gr. 0.89 and is composed mainly of ben - zene and toluene with small amounts of phenols and bases. The products of commercial importance isolated from this fraction are pure benzene and toluene and the crude distillates, "9o% benzol" and "50% benzol." The first step in the purification of the oils consists in washing them thoroughly with caustic soda solution (10%) whereby the acidic (phenolic) constituents are extracted as sodium salts soluble in the aqueous solution. A similar extrac tion with dilute sulphuric acid removes the bases as soluble sul phates. If the ultimate aim is the production of pure constit uents, the oils at this stage are agitated with concentrated sul phuric acid, impurities such as olefines and nitriles being destroyed. Successive treatments with alkali and water render the oils neutral. The alkaline and acid washings from this fraction are added to the similar extracts from the light oil and the carbolic oil. The neutral oils, before treatment with concentrated sulphuric acid, consist almost entirely of benzene and toluene, although small quantities of olefines, paraffins, nitriles, carbon disulphide and thiophen are usually present. A further stage in the purifica tion of the oils consists in fractional distillation with or without the aid of direct steam. By careful fractionation and the use of efficient rectifying columns, almost pure benzene (q.v.) and toluene (q.v.) can be obtained. The yields of these hydrocarbons from this and the next higher fraction are approximately 0.4 to o.6% and 0•2 to 0.4% respectively calculated on the crude tar. In many cases the pure hydrocarbons are not isolated, fractions designated as "9o% benzol" and "5o% benzol" being collected. (The expression "per cent." in this case signifies that 9o%, or 5o%, distils below 100°.) Commercial 90% benzol contains about 70% benzene, 24% toluene, traces of xylenes and 4-6% carbon disulphide, light paraffins, etc. ; 5o% benzol is chiefly toluene and xylene with a small proportion of benzene.
Fraction II. (light oil), consisting of hydrocarbons, mostly homologues of benzene, together with phenol and pyridine, amounts to 7-8% of the tar. By application of the treatment described in the case of fraction I, this fraction may be made to yield the pure hydrocarbons, toluene, o-, m- and p-xylene (0.2 to o.5%), pseudocumene, and mesitylene, or alternatively the crude fractions 50% benzol, 3o% benzol, solvent naphtha and heavy or burning naphtha. Solvent naphtha, consisting of xylene and higher homologues, with naphthalene and paraffin in small quantity, is that fraction of the neutral oils distilling between 120° and 160° C, whilst heavy naphtha, containing mostly pseudocumene, mesitylene, naphthalene, with smaller amounts of cumarone and indene, distils between 160° and 190° C.
The acid extracts from fractions I. and II. are worked up for basic compounds. The bases obtained from these low tar distil lates are principally of the pyridine series, contaminated with a small quantity of aniline and resinous products. The two latter are removed from the solution of the sulphates of the bases by frac tional precipitation by ammonia and the bases subsequently lib erated by the addition of aqueous caustic soda or ammonia. The crude bases separating as a black oil are collected and distilled over solid caustic soda in iron stills. Pyridine to the extent of 0.1% of the tar forms the major part of these bases, but methylated derivatives of this constituent are also present.
Fraction III. (crude carbolic oil or light creosote) is one of the most important fractions of the tar of which it represents 8 to 12%. Its major constituents are naphthalene, which may amount to as much as 4o% of the fraction, and the lower phenols, carbolic acid and the cresols. At ordinary temperatures it is solid or semi-solid with a specific gravity of 1•2, but at 45° C is wholly liquid. The oils are chilled and the naphthalene, of which about four-fifths crystallizes out, is isolated by filtration or by exuding the oil under hydraulic pressure. Further purification of this hydrocarbon consists in treating the melted product with con centrated sulphuric acid and caustic soda, followed by distillation or sublimation. The content of naphthalene, by far the largest single constituent of the tar, varies from 6 to 10%.
The oily filtrate from the crude naphthalene, containing phenols, cresols, xylenols, pyridine bases and dissolved naphthalene, is called crude carbolic acid. It contains 25-35% of phenols and 5% of bases. The former are extracted in the manner described in the purification of fraction I., and the alkaline solution with the corresponding extracts from fractions I. and II. are steamed to remove neutral substances and then decomposed by means of mineral acids or carbon dioxide. The phenols, which separate as an oil, are collected, washed with water and distilled, when crude carbolic acid, b.p. about 180° C, is separated from the cresols (q.v.), xylenols and higher phenols, the mixture of which is termed liquid carbolic acid or cresylic acid. Repeated fraction ation of crude carbolic acid finally yields a pure white product which crystallizes on cooling after distillation.
The oils left after extraction of the phenols can be worked up for pyridine bases, naphthalene and heavy solvent naphtha in the following manner. Agitation with sulphuric acid extracts the bases, which can be recovered as described above, whilst the re maining neutral oils are chilled and filtered, a second yield of naphthalene being thus obtained.
Fraction IV. (heavy creosote), a greenish-yellow, fluorescent oil amounting to 10-12% of the tar, contains carbolic acid, cresol, naphthalene, anthracene and other hydrocarbons, and is employed in the crude state as creosote oil. The naphthalene content in this fraction may be as high as 20% and the hydrocarbon often crystal lizes out on standing. It is removed by distillation, being found in the first runnings of the distillate, from which it crystallizes in a pure condition. The crude oily residue of the oils containing nearly 3o% of high phenols is employed in commerce without further purification. The "creosote oil" contains several constit uents which are of increasing importance.
Isoquinoline (see QUINOLINE) is obtained from the mixture of crude quinoline bases which are extracted from the creosote oil by sulphuric acid. Its purification, a matter of some difficulty, consists in fractional precipitation of the crude bases with ammonia, and the conversion of the most basic components into sulphates, followed by crystallization of the sulphate most in-, soluble in alcohol until the m.p. rises to 206° C. Decomposition of this sulphate liberates the pure base, b.p. 242° C.
Indole (q.v.), extracted as its potassium salt when the neutral, oils of the creosote fraction are heated with solid caustic potash, is liberated when the potassium salt is treated with water, and purified by distillation under reduced pressure.
Acenaphthene (q.v.) appears in the creosote fraction distilling at 26o-27o° C. On cooling, this fraction yields a solid product moderately rich in hydrocarbon. Further purification is carried out by crystallization from alcohol.
Fraction V. (anthracene oil or green oil), which represents on an average 12% of the tar, is wholly liquid at 6o° C, but on cool ing to ordinary temperatures it becomes semisolid, yielding a crystalline deposit of crude anthracene amounting to 6—io% of the fraction. This crude product is freed from oils by centrifugal plant or by hydraulic or filter presses. After pressing, first cold then hot, a crude anthracene cake containing 3o to 4o% of an thracene is obtained. Other hydrocarbons present in this crude product include methylanthracene, phenanthrene, acenaphthene, diphenyl and pyrene, whilst nitrogen compounds such as car bazole and acridine are also present. As a general rule phenan threne and carbazole (q.v.) are the chief constituents together with anthracene in the crude filtered product. These three substances can be separated by treatment first with naphtha, which dis solves the phenanthrene, and then with pyridine, in which car bazole is much more soluble than anthracene (q.v.). The latter is then purified by sublimation.
From the anthracene oil fraction, by treatment at 28o° C with solid caustic potash, the hydrocarbon fluorene (q.v.) can be isolated through its potassium salt, which is freed from oils and other hydrocarbons, and decomposed by means of water. The accompanying chart (Bunbury and Davidson) illustrates the pre liminary distillation of tar and the nature and average yield of the commercial products.
Although the number of pure products isolated commercially is small, their value can hardly be assessed. Many industries are largely dependent on these few substances either directly or in directly. In fact, the majority of the industrially important or ganic chemicals are prepared from the five first-mentioned con stituents of tar. Naphthalene is one of the main starting points in the manufacture of intermediates for the important dyestuff industry, whilst from anthracene springs the whole of the anthra quinone series of vat colours and also the mordant dyes of the alizarin group.
The pharmaceutical use of coal-tar products takes its place in the front rank, not so much because of the amount of material used, which is comparatively small, but on account of its impor tance in the prevention and suppression of disease. As antiseptics, antipyretics, hypnotics, anaesthetics, etc., the derivatives of ben zene, toluene and phenol particularly have received wide and successful application. Further, the growth of synthetic organic chemistry from coal-tar products has enabled the medical pro fession to fight a winning battle against bacteria and trypanosomes. The value of these specialized applications of tar products lies not in finance or in economics but in their significance in human affairs.
Toluene and phenol are respectively the starting points for two well-known explosives, T.N.T. and picric acid, whilst the World War led to the manufacture of lachrymators ("tear gas"), such as diphenylchloroarsine, designed to produce disablement. In the manufacture of these, large quantities of benzene, toluene and phenol are used.
Another important industrial development is the manufacture of synthetic resins (q.v.), wherein large quantities of phenols and cresols are used in one type of resin and cumarone and indene in another.
Derivatives of tar products have had successful application as rubber accelerators, particularly nitrosophenol, aniline and nitrosodimethylaniline.
The manufacture of perfumes, the photographic industry, and the synthetic organic chemical industry employ large quantities of the pure constituents of coal tar. An interesting development is the manufacture of hydrogenated naphthalenes and phenols which are powerful solvents, especially for fatty oils, waxes, gums, resins, etc. One of the most important properties of hydro genated phenols is their power of dissolving or dispersing in aqueous soap solutions, yielding a colloidal solution with powerful detergent and emulsifying powers. They are used in soaps, pastes and polishes, and in certain varnishes and lacquers.
Crude Products.—In addition to the value of the pure con stituents isolated from the tar, there are products obtained by distillation which have far-reaching technical and industrial uses, and which are consumed in quantities many times greater than all the pure products combined. The coal-tar industry is indeed dependent on the commercial usefulness of the "crudes," i.e., the distilled fractions and the residue, which together amount to 9o% of the tar. The following are some of the most important appli cations of tar and its crude distillates.
(I) Crude Tar.—Commercial application of crude tar is some what restricted, as in most processes the presence of water is detrimental. In the crude however, tar is used as an anti corrosive paint in the protection of iron, wood, brick and stone work against atmospheric conditions. This application is par ticularly useful in large manufacturing works where iron-work is exposed to an acid atmosphere. As a preservative, the crude product is used in the tarring of ropes and other accessories in ships. By the application of the Bergius process of hydro genation (q.v.) tar, when heated with hydrogen at 43o° C under zoo atmospheres pressure, is converted into a product resembling petroleum, 6o% of which distils below 25o° C and from which fuel oils and spirits can be obtained by distillation.
(2) Prepared Tars.—By far the widest application of tar is in the surfacing of roads and binding of macadam, granite chips or wood blocks. For this purpose the crude tar is "prepared" by distillation up to the point when the creosote oil begins to distil. The residue in the still, consisting of pitch dissolved in the heavier oils of the tar, constitutes road tars. Prepared tars, or tars from which the water and more volatile oils have been removed, can be regarded as soft pitch. These may be diluted further with distilled creosote and are often mixed with asphalt, mineral pitch or bitumen to improve the binding power. In tarring stones about 10 gal. of tar are used per ton of stone, and in surfacing i gal. covers about 6 square yards. Prepared tars are also used in the manufacture of steel linings for converters of the Bessemer type, and in the manufacture of roofing felt, when the woollen fabric is impregnated with the hot material and covered with a layer of sand on both sides.
(3) Pitch.—In the distillation of tar the residue in the still is pitch, constituting from 5o to 6o% of the original tar. Until the modern demand for road tar developed, this pitch formed the most valuable fraction of the tar commercially. It is used chiefly as a binder for small coal in the preparation of briquettes, the proportions used being approximately 90% of coal and io% pitch. Some of the more important specifications for pitch for briquettes are : softening point not to be below 6o° C, amount of "free car bon" not to exceed 3o% and absence of excessive amounts of volatile matter. In the preparation of briquettes, the coal slack is "wetted" with molten pitch and compressed into uniformly shaped blocks which are resistant to ordinary handling and transport. Mixtures of coal and pitch or the coke formed by the carbonization of the pitch have been used as domestic fuel. These products are attractive by reason of their low ash content and the high radiant heat obtained on combustion.
Pitch is often used as one of the components in protective paints and varnishes. For this purpose it is dissolved in heavy naphtha, other ingredients such as creosote being added as desired. A special bituminous paint is made by heating pitch to 16o° to 200° C and adding cumarone resin as a solvent ; a waterproof paint consists of a mixture of i part of pitch, i part of hot tar, and a suspension of i part clay in z part of water.
Other minor applications of pitch are the production, by further carbonization, of a coke suitable for the manufacture of elec trodes, and as a filling material for resins in the manufacture of cold moulded articles.
(4) Crude Benzols.—The neutral oils of tar distillate up to IIo° C constitute the commercial "crude benzol." For employ ment as a motor fuel it is further purified by redistillation and careful fractionation, eliminating the carbon disulphide which dis tils in the first runnings. If a high-grade fuel is required, the thiophen, which cannot be separated from the benzene by distil lation, is removed by treatment with concentrated sulphuric acid. Motor benzol gives a ton-mileage per gallon 25% higher than petrol and has other advantages, such as less tendency to knock, whilst allowing a higher compression ratio to be used. It is also used in admixture with petrol, the fuel being known as "benzole mixture." Motor benzol has the following properties. Its sp.gr. is o.87–o.885, and on distillation not more than 75% should pass over below ioo° C and 9o% below I20° C, whilst at 125° C there should be no residue. The sulphur content does not exceed and the product is water-white in colour and free from water. It is free from acids, bases and sulphuretted hydrogen, does not freeze above —14° C, and when shaken with 9o% sulphuric acid for 5 minutes does not give more than a light brown colour in the acid layer.
As a solvent, the crude benzol is used extensively in the paint and varnish trade, and for dry cleaning. Mixed with acetone it dissolves nitrocellulose, and with anthracene oil it is used as a solvent for resins. Purified benzol is one of the most important solvents used in fine organic chemical work. It is also used as a fuel in heating lamps and blow-lamps.
(5) Solvent Naphtha, as its name implies, is almost wholly used as a solvent. It also dissolves paints and varnishes and is used extensively in the composition of grease and oil removers.
(6) Heavy Naphtha.—High boiling, heavy or burning naphtha contains two substances, cumarone and indene, which when treated with suitable reagents polymerise to form valuable resinous prod ucts. In the manufacture of the resins (q.v.) the pure compounds are not isolated, the crude naphtha, b.p. 16o-18o° C after purifica tion, being used. This crude distillate is also employed as a fuel for burning in illuminating lamps of the Lecigen type for use out of doors.
(7) Anthracene Oil.—After removal of the crystalline deposit of anthracene, the green oil is used, like creosote, as a fuel in Diesel engines and as an absorbent in gas washing. It is also employed as a lubricant for clay moulds in brick making, and as a component in grease for lubricating wire ropes such as are used in mines, ships, etc.
(8) Light Creosote.—Large quantities of light creosote, gen erally in the form of cresylic acid, are used in the preparation of general disinfectants, sheep dips and timber preservatives. It is usually profitable to extract the carbolic acid from the light creosote by using insufficient alkali to neutralize all the phenols in the fraction. The "sharp oil" or cresylic acid which remains is the basis of many common disinfectants. Saponified by emul sifying agents it is found in commerce under the trade names of "Lysol," "Creolin," etc. When dispersed in water, these ducts are powerful disinfectants and are used for general purposes.
(9) Creosote Oil or heavy creosotes have provided excellent fuels for oil-burning engines of the Diesel and semi-Diesel type. These oils have a high ignition temperature, approximately 450 500° C, and difficulties are met in starting the engine from the cold. Two methods have been used to overcome this difficulty— either starting the engine on a low fuel such as petrol and chang ing over to the higher fuel when the engine is sufficiently hot, or using a mixture of io% petrol and 9o% creosote as fuel. Creosote for this purpose must be thoroughly freed from solids such as naphthalene, inorganic salts and tarry impurities. It also forms a good fuel for oil-burning furnaces such as are employed in glass works and in heating rivets and small metal work. Like naphtha it is burned in lamps for out-door use.
Heavy creosote is also used as an absorbent in the recovery of light hydrocarbons from coal gas. In this process, called "gas washing" or "gas stripping," the gas is made to ascend towers down which the heavy oil is sprayed. The absorbed light oils, consisting chiefly of benzene, toluene and xylene, are recovered by subsequent distillation and amount to 1 to 3 gal. per ton of coal fivefold by creosoting.
Creosote finds its largest application in the preservation of wood, especially railway sleepers, piles, telegraph posts and paving blocks, being frequently applied under pressure, sometimes after subjecting the timber to a vacuum. The oil may also be used at a temperature sufficient to boil the sap out of the wood. The advantages of treating wood with creosote are the closing of pores, thus rendering the material a non-absorbent of water, the inhibi tion of germ life, and the coagulation of albumen by the tar acids. It is estimated that the life of railway sleepers is increased fivefold by creosoting.
Heavy creosote, after elimination of deposited naphthalene, is frequently converted into general disinfectants by admixture with about f our times its volume of slaked lime, when "disin fectant powder" is obtained. This is used for railway cars, farm buildings, etc. When saponified by soaps, these oils form disin fecting emulsions with water and are used as sheep dips. Being less caustic than carbolic acid, the tar acids of the heavy creo sote are less harmful to animals (see TARs, LOW-TEMPERATURE, for wood-tar creosote) .
Creosote has been suggested as a soil sterilizer. A direct con version of creosote oils into permanent gas has been evolved, but high temperatures are necessary. A ton of oil is capable of yielding 13,00o cu.ft. of gas of 14 candle power. Sulphonation products of light and heavy creosotes, either alone or on further condensation with formaldehyde, have been used as tanning agents.
(Io) Crude Tar Acids, i.e., the phenolic substances found in the higher fractions of the tar distillates have been applied as froth-producing agents in the "froth flotation" process, which mechanically and economically separates associated solids such as coal and stone, ore and earth, etc.
(I 1) Pyridine Bases, and more particularly crude pyridine, are used as denaturants to make alcohol non-potable. They are also employed as solvents, notably in the purification of anthracene.
(12) Crude Solid Aromatic Hydrocarbons.—The sulphonated products of crude phenanthrene, fluorene and carbazole have been applied as tanning agents. More extensive use has been made of crude naphthalene. It is claimed that a solution of naphthalene in petrol functions as an anti-detonating fuel from which greater efficiency can be obtained than from petrol. Motor cars, omni buses and motor boats have been successfully run on naphthalene itself as a fuel, although the use of this product necessitated special adaptations in the engine for feeding and carburetting. It is claimed that 1 lb. of naphthalene is equivalent to 0.3 gal. of petrol. As a fuel, however, naphthalene finds its widest application in the manufacture of fire lighters which consist of wood waste impreg nated and consolidated by immersion in the molten hydrocarbon.