The principle of the blast furnace operation lies in the uninterrupted nature of the metallurgical process during the entire life of the furnace until another complete overhaul, which is performed every 3 to 12 years; the overall service life of a blast furnace can exceed 100 years. The blast furnace is a shaft-type one: it is periodically charged from the top and in batches with the so-called burden consisting of the iron ore, the limestone flux and the coke, while from the bottom and also periodically, the molten iron is discharged and the molten slag is drained, i. e. the column of the raw materials in the blast furnace shaft gradually settles down turning into cast iron and slag, and that column keeps being replenished from the top. However, the road of the ferrous metallurgy to the implementation of that seemingly unsophisticated principle was long and tortuous.
Blast furnace operating principle
The indispensable condition for the successful operation of a blast furnace is the excessive amount of carbon in it throughout the entire blast-furnace process. For the thermochemical (highlighted in red) and the technical and economic chart of the blast-furnace process, see the figure. The production of the cast iron in a blast furnace is as follows. A new blast furnace or a reconditioned one after Category 3 complete overhaul (see below) is filled with the relevant materials that are ignited with the gas; also fired is one of the Cowper stoves (see below). Then the air is started to be blown in. Coke burning immediately intensifies raising the temperature in the blast furnace; decomposition of the flux begins with the release of the carbon dioxide. Its excess in the atmosphere of the furnace with the abundance of the air being blown in does not allow the coke to completely burn out, and carbon monoxide, also known as ‘whitedamp’, is formed in large quantities. But in this case, it is not a poison, but a powerful reducing agent greedily taking oxygen away from the iron oxides that make up the ore. The reduction of the iron by a gaseous monoxide, instead of by the less active solid free carbon, is the fundamental difference of the blast furnace from the bloomery one.
As the coke burns out and the flux breaks up, the column of materials in the blast furnace settles down. Basically, a blast furnace is two truncated cones put together by their bases (see below). The upper, high one is the stack of the blast furnace. In it, the iron is reduced from various oxides and the hydroxide to the iron monoxide FeO. The widest part of the blast furnace (the juncture of the bases of the cones) is called the belly (known as ‘rozpar’ in Ukrainian but ‘rozpara’ is incorrect). In the belly, settling down of the charge slows down, and the iron is reduced from FeO to pure Fe that precipitates in drops and flows down into the hearth of the furnace. The ore seems to be steaming and sweating with the molten iron, hence the name in Ukrainian where ‘para’ stands for ‘steam’.