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
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
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.