By-Product Coke Chemistry

an area of chemistry and chemical industry concerned with the processing of natural fuels (mainly coal) to produce coke and other valuable products by the method of coking (carbonization). The main by-products of coke chemistry (in addition to coke) are coke-oven gas, products of the processing of raw benzene, coal tar, and ammonia (ammonia fertilizers). The complete list of products includes about 80 items (about 160 grades). A number of products similar to coke by-products are also produced in petroleum refineries.

Coking is performed in high-output coke ovens heated by poor (blast-furnace) gas or high-BTU (coke-oven) gas. Raw materials for coking include special types of bituminous coals, which are capable of sintering. However, because of the shortage of these types of coal, a mixture of various kinds of coal (the charge) is made up before coking. The charge may contain considerable quantities of noncoking coals even though the ultimate aim is to achieve a charge that permits a normal coking process. The main property that determines the suitability of the charge for coking is its ability upon heating to form a plastic layer of sufficient thickness and viscosity and with the necessary rate of thermal decomposition. Coal is tested in the laboratory for coking ability. The significant characteristics are the thickness of the plastic layer and shrinkage of the coal during coking; the latter is essential for the removal of the coke cake from the oven. In addition, the charge composition is controlled for ash content, sulfur, moisture, and other impurities. To remove undesirable impurities from the charge, the coals undergo preliminary enrichment and are then carefully blended to obtain a uniform charge and crushed (the content of 3–mm fraction in the charge must be about 95 percent).

The raw gas formed during coking contains, in g/m³ (referred to the standard state or normal conditions), water vapor, 250–450; tar vapor, 100–120; benzene hydrocarbons, 30–40; ammonia, 8–13; and hydrogen sulfide, 5–30. The hot gas (800°C) is cooled in the gas collector by spraying water and then in condensers (to 30°–35°C). The condensate is separated into ammonia water and coal tar. The cooled gas is sucked up by powerful gas blowers and pumped into a collection and purification system, where the raw benzene vapor, most of the remaining ammonia, the hydrogen sulfide, and the cyanide compounds are removed. The raw benzene, tar, and ammonia water are converted into industrial products.

An example illustrating the material balance of coking of one variety of coal is given in Table 1. The quantity of dry coal charge (input) equals 100.0. Purified coke-oven gas, called the return gas, is used as fuel, and as a raw material in ammonia synthesis.

Hydrogen sulfide, an undesirable impurity that accompanies the coke-oven gas, is converted into elemental sulfur or sulfuric

acid. Cyanide compounds may be used in the production of salts of rhodanic acid and other products.

Ammonia (0.3 percent yield, based on the weight of the charge), which partially dissolves in the aqueous condensate to form ammonia water and partially remains in the coke-oven gas, is separated from the water by distillation and returned to the gas, after which it is separated from the gas and used mainly for the production of ammonium sulfate, a nitrogen fertilizer, containing 25.8 percent NH₃. Ammonium phosphate fertilizers are produced by absorption of ammonia in phosphoric acid. Aqueous solutions of ammonia are also used as liquid fertilizers after addition of potassium salts and other components.

Raw benzene is a mixture of aromatic hydrocarbons boiling up to 180°C. In addition, it contains a number of nonaromatic unsaturated compounds (for example, cyclopentadiene). Hydrocarbons are extracted from coke-oven gas by passing it through scrubbers containing light oil. After separation from the oil by distillation, fractionation, purification, and repeated rectification, pure industrial-grade products are obtained, the most important of which are benzene, toluene, and xylenes (the content of these materials in raw benzene is 65–70, 13–15, and 2.5–4.0 percent, respectively). Hydrocarbons boiling above 145°C are produced under the general name “solvent naphthas.” Individual components may also be obtained from these materials for the synthesis of dyes and other products. Unsaturated compounds from raw benzene yield coumarone-indene resins, which are used in varnishes, paints, and linoleum, as well as in the rubber industry. Cyclopentadiene is also a promising raw material for the synthesis of organic compounds.

Coal tar is the raw material for the production of naphthalene and other individual aromatic compounds, special-purpose oils (for absorption, impregnation of railroad ties, and so on), and pitch, which is the initial material for such materials as electrode pitch coke.

Other types of important by-products are the pyridine bases and phenols. The lowest-boiling fractions of pyridine bases are separated with ammonia, and the phenols are separated with raw benzene. The high-boiling fractions of the bases and phenols pass into the tar. Pyridine bases consist mainly of pyridine and its homologues. Their total yield is 70–80 g per ton of the charge. After separation and fractionation, they are widely used in the synthesis of organic compounds, particularly in the pharmaceutical industry. The phenol yield is about 700 g per ton of charge. After extraction with an alkali and separation into the individual compounds or groups of isomers, they are used in the production of phenol-formaldehyde resins and other polymers, as well as in the synthesis of dyes, pharmaceutical and perfume preparations, and pesticides.