All the processes of metal production – its smelting and purification from impurities – proceed at high temperatures, for the creation of which large quantities of solid, liquid and gaseous fuels are expended. In addition, the processing of already finished metal (rolling, forging, heat treatment and other operations) is usually associated with preheating the metal. To metallurgical, as well as in general to any fuel used in a specific technological process, special requirements are imposed – in fact it is not only a source of heat, but also a direct participant in chemical reactions.
The value of metallurgical fuel, like any other, is determined by the amount of thermal energy enclosed in its unit of volume – the heat of combustion. Therefore, choosing the type of fuel, take into account its economy, and the size of fuel stocks, and the convenience of transportation, and the impact on the environment, and much more. Thus, the fuel loaded into the blast furnace must be strong (withstand high loads during transportation and not abrade in the furnace itself), porous (do not obstruct the passage of gases moving in the blast furnace), clean enough (contain as little ash and sulfur as possible).
For many centuries in a row coal was used as fuel for smelting metal. They poured wood, piled it up, covered it with turf and burned it, and then they were brought to the blast furnaces in baskets. They destroyed huge forest tracts, but received little, however, clean (containing little ash and harmful impurities), but very fragile, not suitable for high blast furnaces, expensive fuel. Therefore, as soon as pig iron was started to be smelted in large-volume furnaces, metallurgists had to search urgently for what to replace charcoal. This fuel became coke: to get it, special coking grades of coal are heated to a high temperature without access to air (subjected to dry distillation).
Unlike coal, from which coke is produced, it is durable, porous, with less ash and volatile compounds. Coking is carried out at coke plants. It, as a rule, is included in the full metallurgical cycle. As well as ore, coal before coking enriches – get rid of most of the mineral impurities (ash). The coal is sintered in special vertical furnaces, connected to whole batteries, which are a series of identical chambers adjoining one another. They look from the outside like books on the bookshelf-giants up to 7 meters high, up to 450 millimeters wide, in whose “bindings” they burn gas. Each battery has between 60 and 77 cameras. Pre-crushed coal in the loading car is fed to a coking chamber where it is heated without access of air to a temperature of 1000-1100 ° C.
Coking takes place in several stages. First, at a temperature of 200-350 ° C, the substances entering into the coal composition begin to decompose, then the coal softens, becomes plastic, and at a temperature of 450-500 ° C a deeper decomposition occurs – a semi-coke (thick and viscous mass) is formed, and at 1000-1100 ° C this mass is caked in a solid and strong coke. The sintering process lasts 12-14 hours. Everything is calculated and envisaged: both the sintering time, and the width and height of the chambers. As a good housewife, all the concerns of the technologists are aimed at making the cake pie. The door of the oven opens, a special ejector helps to unload the cake into the car. Then the coke is cooled, sorted and sent to the blast furnace shop.
But what only do not get, except for coke, at coke plants – about 80 different products! And they, in turn, are the starting material for manufacturing the most diverse substances that different industries, agriculture, transport need! For example, coke oven gas. It is characterized by high heat of combustion (at the combustion of one cubic meter of this gas, up to 4,500 kilocalories of heat are released), which makes it possible to use it as fuel right there at a metallurgical plant. But the coke oven gas burns with a strong smoky flame. Therefore, it is blended with blast furnace gas (one that is produced in the production of pig iron – a gas with a rather low heat of combustion) and is used for heating open hearth furnaces and heating furnaces of rolling mills, so-called heating wells, used for public utility and domestic needs – And cooking – in those areas where there is no natural gas and a nearby coking plant.
The main consumer of coking coal is the same ferrous metallurgy, with which we get acquainted in the pages of our book – about 90 percent of produced coke and 60 percent of coke oven gas it takes. Coke products are also required for the chemical industry; For example, from benzene, phenol, naphthalene, anthracene and other substances, up to two thousand different dyes are obtained. The chemical and pharmaceutical industry produces a lot of drugs and disinfectants from coking products. Familiar to us all, aspirin, ammonia, carbolic acid, naphthalene – because they are also from coal! Various coking products are used in the plastics industry – in the production of synthetic rubber, leather, artificial fiber.
In some chemical plants sulfur is used to produce sulfuric acid, which is contained in the hydrogen sulfide of coke oven gas. Sulfur is sent to the pulp and paper industry, and to the production of explosives (used for peaceful purposes) substances. Pure benzene and toluene also arrive here. To protect from decay, wooden structures are covered with coal lacquer, and wooden railway sleepers, telegraph and telephone poles are impregnated with tie-off impregnating oil. In the textile industry, ammonium thiocyanate is used, in the food for canning – benzoic acid, in perfume – pure toluene, phenol-cresols and other compounds – for the production of fragrant substances.
In livestock breeding, crein, lysol is used as a disinfectant. An equally important consumer of coking products is agriculture. By processing a small amount of ammonia contained in the coke oven gas into ammonium sulfate, artificial nitrogen fertilizers are obtained. The same ammonium thiocyanide is used for the preparation of preparations for the control of pests of agricultural crops. And ammonium nitrate, and plant protection products, and pesticides for combating weeds and pests of agriculture – and everything from coal! All that gives us coke production is impossible to enumerate. The role of this branch of industry in our national economy, the industry entering into a full metallurgical cycle, can not be overestimated.