Coal Tar and its Products

Coal Tar and its Products

Coal tar is a by-product generated during the high temperature carbonizing of coking coal for the production of the metallurgical coke in the by-product coke ovens. It is a black, viscous, sometimes semi-solid, fluid of peculiar smell, which is condensed together with aqueous ‘gas-liquor’ (ammoniacal liquor), when the volatile products of the carbonization of coking coal are cooled down. Its CAS number is 8007-45-2. It is acidic in nature and is water insoluble.

Coal tar represents a mixture of condensable volatile products formed during the destructive distillation of bituminous coal. Composition is variable, but generally consists of 0 % to 2 % of light oils (chiefly benzene, toluene, and xylene), 16 % to 18 % of middle oils (chiefly phenols, cresols, and naphthalene), 8 % to 10 % heavy oils (naphthalene and derivatives), 16 % to 20 % anthracene oils, and around 50 % pitch. It is composed primarily of a complex mixture of condensed-ring aromatic hydrocarbons. It can contain phenolic compounds, aromatic nitrogen (N2) bases and their alkyl derivatives, and paraffinic and olefinic hydrocarbons.

Coal tar is a complex mixture of chemical compounds, mainly consisting of the aromatic series. Both the method by which the coal tar is produced and the nature of the raw material (coal) influence to a wide extent the chemical composition and physical properties of the coal tar. The specific gravity of coal tar at 15 deg C varies between 1.12 and 1.20, depending upon the temperature of carbonization or kind of coke ovens used. In exceptional cases it can go upto 1.25. The coal tar with lower specific gravity is generally produced when low carbonization temperatures are used. Viscosity of the coal tar is affected in a similar manner. The heavier tars contain lesser benzol than the lighter tars, and more fixed carbon.

The vapour pressure of coal tar at 20 deg C is 0.2 mm to 1.0 mm mercury column. Its flashpoint ranges from 105 deg C to 125 deg C. Its solubility in water is slightly less than 0.2 %.

The absolute chemical composition of coal tar is not known. It contains more than 348 types of chemical compounds, which are very valuable. They are aromatic compounds (benzene, toluene, xylene, naphthalene, and anthracene etc.), phenolic compounds (phenol, cresol, xylenol, cathecol, and resorcinol etc.), heterocyclic nitrogen compounds (pyridine, quinoline, isoquinoline, and indole etc.), and oxygen heterocyclic compound (dibenzofuran etc.), which all have been used as raw materials or intermediates materials in various chemical industries (as anti-oxidant, anti-septic, resin, softener ingredient in plastic industry, paint, perfume, and medicine etc.).

It is seen that the coal tar produced at low temperatures yield on distillation, in addition to phenols of the carbolic acid series, phenols of a different series rather less acid in behaviour and probably of the creosol and guaiacol type. Also, there is a smaller yield of naphthalene and of the benzene hydrocarbons, and a large percentage of hydrocarbons of the paraffin and olefin series. Instead of most of the nitrogen occurring in the form of pyridine bases it appears in the form of aniline and its homologues. The amount of ‘free carbon’ is also small. On the other hand, high temperature tars, i.e., those produced at high heats of carbonization of coal, yield on distillation only traces of paraffinic hydrocarbons, the predominating hydrocarbons being those of the benzene, naphthalene and anthracene series. The nitrogen occurs principally in the form of pyridine bases, and the phenols consist chiefly of carbolic acid and its homologues. The percentage of ‘free carbon’ is generally high. The high percentage of ‘free carbon’ is not due so much to the high heats employed as to the amount of free space. This is borne out by the comparatively low ‘free carbon’ content of tars from heavily charged ovens. In the lightly charged ovens, in which there occurs a good deal of free space, part of the gas and probably some of the tar is ‘cracked’ into lighter materials and ‘free carbon’, and this latter substance to a very large extent is carried up the ascension pipes and arrested by the tar in the hydraulic main.

The aromaticity of the coal tar increases while the content of paraffins and phenols decreases when the carbonization temperature increases. Thus coal tar from coke ovens contains relatively small amounts of aliphatic hydrocarbons. Coke oven tar contains around 3 % of phenolic compounds in the fractions distilling at upto 300 deg C. The coal tar from coke ovens consists predominantly of compounds containing unsubstituted rings.

Some of the main components of coal tars are benzene, toluene, ortho-xylene, meta-xylene, para-xylene, ethyl benzene, styrene, phenol, ortho-cresol, meta-cresol, para-cresol, xylenols, high boiling tar acids, naphtha, naphthalene, alpha-methyl-naphthalene, beta-methyl-naphthalene, acenaphthene, fluorene, diphenylene oxide, anthracene, phenanthrene, carbazole, tar bases, and medium soft pitch etc.

High temperature coal tar distillate fractions and their mixtures

During the distillation of high temperature coal tar the common products obtained are carbolic acid, naphthalene oil, creosote oil, anthracene oil (light and heavy), and coal tar pitch (Fig 1). These are the higher boiling (greater than 250 deg C) distillate fractions and their mixtures are given below.

Fig 1 Coal tar and its products

Carbolic oil – It is also known as light oil and is a complex combination of hydrocarbons obtained by distillation of coal tar. It consists of aromatic and other hydrocarbons, phenolic compounds and aromatic nitrogen compounds and distills at the approximate temperature range of 150 deg C to 210 deg C. Its CAS number is 84650-03-3. Its density at 20 deg C is 900 kg/cum to 1030 kg/cum. It is a flammable liquid which has moderate solubility in water. Its colour is yellow to dark brown. It has aromatic odour. It is not a self-igniting liquid and has flash point which is greater than 21 deg C. Its auto ignition temperature is greater than 500 deg C. Its explosion limits is 1.3 % to 9.5 % by volume in air. The vapour pressure of carbolic oil at 20 deg C is 0.1 kg/sq cm.

Naphthalene oil – It is a liquid with a brown to dark brown colour. It is complex combination of hydrocarbons and consists primarily of aromatic and other hydrocarbons, naphthalene, phenolic compounds, benzothiophene, and aromatic nitrogen compounds. It has boiling range of 210 deg C to 220 deg C and solidifying point of 65 deg C to 75 deg C. It is a flammable liquid. Its CAS number is 84650-04-4. It has a moth ball odour. Its flash point is 56 deg C to 81 deg C. The specific gravity is in the range of 0.987 to 0.993.

Creosote oil – It is also known as wash oil and has boiling range of 230 deg C to 290 deg C. It is reported to have some 162 compounds. However, only a limited number (lesser than 20) are at levels exceeding 1 % but these constitute the major portion of creosote oil. It contains besides small amounts of naphthalene, naphthalene derivatives, indole, diphenyl, acenaphthene, diphenylene oxide, fluorene, phenol derivatives (2 %) and quinoline base (4 % to 6 %). A small amount of benzo(a)pyrene concentration of 0.01 % is also found in wash oil.

Creosote oil has a varying composition but can be described generally as a yellow-dark-green brown oily liquid, consisting of aromatic hydrocarbons, e.g. anthracene, naphthalene and phenanthrene derivatives, some tar acids (e.g. phenol, cresols and xylenols) and tar bases (e.g. pyridine and lutidines derivatives). Its density at 20 deg C is 1.025 kg/cum to 1.04 kg/cum. Its CAS number is 90640-84-9. It has a characteristic odour. Its flash point is less than 90 deg C. The ignition temperature of creosote oil is greater than 500 deg C. The oil is not self-igniting and is not an explosion hazard. The vapour pressure at 20 deg C is less than 0.001 kg/sq cm. Its miscibility with water at 20 deg C is around 30 milligrams per litre.

Anthracene oil – Anthracene oil is a complex combination of polycyclic aromatic hydrocarbons having an approximate distillation range of 300 deg C to 450 deg C. It is obtained from the primary distillation of coal tars in two fractions. The light boiling fraction (light anthracene oil) has a high content of phenathrene, anthracene, and carbazole, while the higher boiling fraction (heavy anthracene oil) has a high content of fluoranthene and pyrene. Benzo(a)pyrene concentrations ranges between 0.01 % and 0.06 % in anthracene oil.

The CAS number of anthracene oil is 90640-80-5. It i semisolid, greenish brown pasty crystalline material below melting temperature and liquid above melting point. Its colour id dark brown to black and odour is aromatic. Its density is 1.004 kg/cum to 1 15 kg/cum. Its melting range is 40 deg C to 60 deg C. It is not self-igniting and has the flash point which is greater than 100 deg C and ignition temperature is more than 450 deg C. The vapours of anthracene oil may form explosive mixture with air. Its vapour pressure at 20 deg C is less than 0.002 kg/sq cm. It is slightly soluble in water at 20 deg C. The dynamic viscosity of anthracene oil at 80 deg C is in the range of mPas to 14 mPas.

Coal tar pitch – It is a dark-brown-black, shiny, amorphous residue produced during the distillation of coal tars. It is composed of many different compounds which interact to form eutectic mixtures and thus it does not show a distinct melting or crystallization point. Rather, it is characterized by a softening point (the temperature at which a given viscosity is reached). Depending upon the extent of distillation, pitches of different softening point can be obtained. Examples are medium soft pitch or very hard pitch. The details of coal tar pitch are given in the article

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