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David Dunlop:


BY-PRODUCTS OF COKING

As the name implies, the recovery of coal chemicals is an important aspect of the by-product coking process. During the cokemaking process, approximately 30% by weight of the initial coal charge is given off as mixed gases and vapours. This raw coke oven gas is drawn off from the ovens, collected and put through a refining process in the By-Product area of cokemaking. The recovery system is extremely complex, but essentially it involves the condensation or extraction of the desired products from the gases of the coke-oven exhaust stream. The gases and vapours leave the ovens at a temperature ranging from 315 to 370 C. Water sprays at strategic locations in the collecting system reduce the temperature to a range of 145 to 215 C.

The carbonization or destructive distillation of a ton (0.9 metric ton) of coal produces an average yield of about 0.7 ton (0.63 metric ton) of coke; 11,500 cubic feet (345 cubic meters) of gas; 12 gallons (45.6 Litres) of tar; 27 pounds (12 kilograms) of ammonium sulfate; 50 gallons (190 litres) of benzol; 0.9 gallon (3.4 litres) of toluol and naphtha; and 0.5 pound (0.2 kilograms) of naphthalene.

Coal tar,a thick black, sticky liquid, is the source of a great number of chemical compounds, including creosote, pitch, toluene, and naphthalene; is extracted from the stream of gases evolved during coking. Coal tar is recovered by partially condensing (changing to liquid) the hot vapours from the coke oven. This is done by means of water spray cooling towers, mechanical impingement of tar particles, and electrostatic precipitation.

Ammonia, a gaseous compound, is used in products ranging from smelling salts to agricultural fertilizers. The most common method to extract ammonia from coke-oven gas,involves the concentration of ammonia bearing vapours by use of a still and an ammonia absorber. In the absorber, the rising gases are sprayed with dilute sulphuric acid to form ammonia sulphate, which, when dried, contains about 26 5 ammonia.

Phenol (C6H5OH) is recovered from coal tar and ammonia liquor. Sometimes called carbolic acid, phenol is used in the manufacture of plastics, perfumes, picric acid, salicylic acid, cutting oils, antiseptics, and wood preservatives. The method of recovering phenol uses benzol as a solvent to remove the phenol from the ammonia liquor. Caustic soda is then used to extract the phenol from the benzol, and the final product is sodium phenolate.

Cresols (C7H8O) are used extensively in insecticides, weed killers, resins, pharmaceutical,and photographic compounds. They are in the phenol family and are extracted by a process using sulphuric acid as a converter and benzene as an extracting agent.
Toluene (C6H5OH3) is recovered both from the coke-oven gas and from the coal tar. It is used in the manufacture of chemicals, explosives, detergents, solvents, and dyes, and is also converted to benzene.

Light oil, which is one of the components of coke-oven gas contains more than a hundred compounds.The most important elements recovered from light oil are benzene, toluene,xylene,and solvent naphtha. These chemicals are then used in the preparation of solvents,plasticizers, resins, and synthetic fibres.

The coke oven gas resulting from the refinement of the raw gas is used throughout the plant as a fuel for reheating furnaces, soaking pits, coke oven batteries, boilers, and numerous other uses.