Coal Tar
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Coal Tar
Description
A black viscous residue from coal or oil, varies in consistency from semi-liquid to hard. The product has a naphthalene-like odour; sharp, burning taste.
Coal tar is a product of the destructive distillation of bituminous coal. Coal tar can be distilled into many fractions to yield a number of useful organic products, including benzene, toluene, xylene, naphthalene, anthracene, and phenanthrene. These substances, called the coal-tar crudes, form the starting point for the synthesis of numerous products—notably dyes, drugs, explosives, flavorings, perfumes, preservatives, synthetic resins, and paints and stains. The residual pitch left from the fractional distillation is used for paving, roofing, waterproofing, and insulation.
Coal Tar Products
Coal, like oil, is chemically a complex substance but unlike oil, it is somewhat deficient in hydrogen and rich in carbon. This not only makes for a lower heating value but also makes it more difficult to convert coal into clean liquid products than it is to refine into such products. Moreover, processes developed for one type of coal may not work with another and conversion of coal requires the handling of vast tonnages of the raw material. However, the original industrial source of hydrocarbons is the residual coal tar derived when coal is carbonized to produce coal gas, and although its importance has been dissipated by the large scale use of petrochemicals, the potential for coal tar products appears brighter now than it has for some time. Coal tar is a black, viscous , thick liquid from which a great number of chemical compounds are derived. The first fraction is obtained in the distillation of coal tar contains many aromatics, among them benzene, toluene and xylem.
Benzene is the lightest fraction obtained from the distillation of coal tar hydrocarbons. It is a thin, colourless, highly flammable liquid with a characteristic smell. It is a good motor fuel solvent as well as being a feedstock for other benzene derivatives used in dye, insecticides and plastics production. At a later stage in the distillation process, a heavy coal tar solvent naphtha and a naphthalene fraction are produced. The fraction is further treated to yield naphthalene, a white crystalline solid with a penetrating tarry smell. Naphthalene is the most abundant of the aromatic hydrocarbons, being important as the starting point for a number of dyes and also as a source of phthalic acid which in turn leads to indigo and to certain resins used in very large quantities in paint. Naphthalene can be reduced to a series of compounds such as tetralin and decalin which have great value as solvents.
Benzene is by far the most important of the aromatics and its principal derivatives are cyclohexane, cumene, ethyl benzene and alkyl benzene. Cyclohexane accounts for approximately 23% of the benzene usage. Ethyl benzene represents approximately 45% of the benzene production and is used in the manufacture of styrene which in turn is used to produce polystyrene and ABS (acrylonitrile/butadiene/styrene) resins, both of which should enjoy strong growth rates. Cumene is used to make phenol and alkyl benzene goes toward the manufacture of detergents.
After coal tar has been distilled and the necessary hydrocarbon cut obtained, the next step in the processing procedure is the extraction of the tar acids. Caustic Soda is used to extract the phenolic compounds – phenol and creosol. Before 1960 virtually all phenols used commercially were derived from coal tar but since that time synthesis of phenol and creosol has become so inexpensive that coal tar supplied only 10% of these products now. Phenol, formerly called carbolic acid still is used as an intermediary in disinfectant manufacture. It is also sued in the production of detergents, dyestuffs, selective weed killers, certain types of nylon and pharmaceuticals. Phenol played a prominent part in the birth of the plastics industry when ‘Bakelite’, the hard synthetic resin, was produced by reaction of formaldehyde and phenol in 1907. Cresosoles are rather stronger disinfectants than phenols and from the basis of creosote. The below figure shows many of the derivates of the coal carbonization process.
Application
Raw material for plastics, solvents, dyes, drugs and other organic chemicals. The crude or refined product or fractions thereof are also used for waterproofing, paints, pipe coating, roads, roofing, insulation, as pesticides and sealants.
Shipment / Storage
Packed in second-hand metal drums. Is highly odorous and detrimental to other cargoes. According to wear of drums loss through leakage may be more or less considerable.
The most suitable packing for the tropics are sealed metal containers. Is highly odorous and detrimental to other cargoes. May be combustible at high temperature. Keep containers/drums tightly closed and stow in a well ventilated area.
Risk factors
Coal tar is a human carcinogen. Toxic by inhalation. TLV: (volatiles) 0,2 mg/m3 in air. Reference is made to the relevant IMO regulations on hazardous cargo and the applicable MSDS sheet.