Blast-Furnace Production

the production of pig iron by the reduction smelting of iron ores or pelletized iron-ore concentrates in blast furnaces. It is a branch of ferrous metallurgy.

History. Pig iron was known four to six centuries before the Common Era. The blast-furnace industry took shape as a result of the development of the blooming process—the “direct” production of solid iron from iron ore by reducing it in low hearths or shaft furnaces with charcoal. The first blast furnaces appeared in the mid-14th century in Europe and about 1630 in Russia (near Tula and Kashira). Pig iron was first produced in the Urals in 1701, and in the mid-18th century, as a result of the development of metallurgy in the Urals, Russia became the world’s largest producer, holding this position until the beginning of the 19th century. The only fuel used in blast furnaces until the mid-18th century was charcoal. A. Darby used coal coke in blast-furnace smelting in 1735.

The main stages in the development of blast-furnace production are the use of a steam-powered air blower (I. I. Polzunov, 1766), heating of the air blast (J. Neilson, 1829), and the invention of a regenerative brick air heater (E. Cowper, 1857). Russia held fifth place in the world in 1913, smelting 4.2 million tons of pig iron. In 1940, the USSR smelted 15 million tons (third place in the world), and in 1947 it was surpassed only by the USA. The USSR took first place in world production in 1970; in 1971 it smelted 89.3 million tons of pig iron. An important role in the development of blast-furnace production in the USSR has been played by M. A. Pavlov, M. K. Kurako, and I. P. Bardin. Blast-furnace production in the USSR is characterized by the use of highly mechanized and automated equipment and advanced technology.

Raw materials. The raw materials (the charge or burden) in blast furnaces are iron ore, manganese ore, sinter, and pellets, as well as fuel and fluxes. A fluxed sinter has come into wide use in the USSR in blast furnace burdens (in excess of 90 percent); it contains 50–60 percent Fe, for a basicity of 1.1–1.3. The use of fluxed pellets is becoming more widespread. The most important properties of iron-containing burden materials, which determine the technical and economic indexes for blast-furnace smelting, are the iron content, the composition of the waste matter, the amount of harmful impurities, the granulometric composition, the durability, and the reducibility.

The principal fuel in blast-furnace production is coal coke. Smelting with replacement of part of the coke with gaseous, liquid, or solid fuel blown into the hearth of the blast furnace is becoming widespread. Limestone, and sometimes dolomite, is used as a flux.

Main types of pig iron. The main types of pig iron smelted in blast furnaces are conversion cast iron, which is used to produce steel in steel-smelting units; foundry cast iron, which is used for castings; and special cast irons. The by-products of blast-furnace operations include blast-furnace gas (whose heat of combustion is 3.6–4.6 MJ/m³, or 850–1,100 kcal/m³), which is used (after being cleansed of dust) to heat the blast in air heaters, as well as in factory boiler installations, coke plants, and sintering shops; blast-furnace slag, which is used chiefly in the building-materials industry; and blast-furnace dust, which is taken from the furnace and trapped by a gasscrubbing system. This dust contains 30–50 percent Fe and is returned to the charge of blast furnaces after agglomeration (chiefly through sintering).

Blast-furnace plant. The blast-furnace plant of a mill with a complete metallurgical cycle usually has at least three blast furnaces with air heaters and gas-scrubbing systems. A supply

Figure 1. Blast furnace: (1) protective segments of blast-furnace top, (2) large bell, (3) receiving hopper, (4) small bell, (5) charge spreader, (6) large bell hopper, (7) inclined skip hoist, (8) skip, (9) air tuyere, (10) pig-iron notch, (11) slag notch

of charging materials (coke for 6–12 hours and sinter or ore, as well as fluxes, for 1–2 days of furnace operations) is stored in bins of a trestle that serves all the blast furnaces. Many metallurgical plants also have a so-called ore yard as part of the blast-furnace plant; it is used to store basic stocks of iron ores, which are placed in piles by ore handlers. The formation of piles and the withdrawal of materials from them are performed with a view toward homogenizing the ores. A blast-furnace plant also has machines for pouring out the pig iron.

A blast furnace (Figure 1) is a shaft furnace with a circular cross section and a fire-resistant lining (the upper part is lined with fireclay brick, the lower part largely with carbon brick). Coolers in which water circulates are used to prevent erosion of the lining and to protect the furnace jacket from high temperatures. The furnace jacket and top are supported by columns set on a foundation.

The charge is delivered to the top of the furnace by skips or, less frequently, belt conveyers. The skips are unloaded into the furnace through a receiving hopper and a filling device mounted in the top of the furnace. Air from blowers (the blast) enters the furnace through air heaters (in which it is heated to 1000°-1200°C) and tuyeres installed around the circumference of the hearth. Additional fuel (natural gas, mazut, or coal dust) is also introduced through the tuyeres.

Products of the smelting (pig iron and slag) are released into ladles through notches located in the lower part of the hearth. The blast-furnace gas that forms in the furnace is exhausted through ducts located in the top of the furnace (Figure 2).

The distance between the axis of the pigiron notch and the lower edge of the large charge bell when it is empty is called the working height of the blast furnace, and the corresponding volume is called the working volume. Large blast furnaces in the USSR have a working volume of 2,000–3,000 cu m and are among the largest in the world. Directives in the ninth five-year plan provide for the construction of blast furnaces with volumes of 5,000 cu m.

Main chemical processes. The main chemical processes in a blast furnace are the combustion of the fuel and the reduction of iron, silicon, manganese, and other elements. Part of the coke is consumed in reduction processes, but the main part drops to the hearth and, along with the fuel blown in at the tuyeres, is burned. Gases with temperatures of 1600°-2300°C and containing 35–45 percent CO, 1–12 percent H₂, and 45–65 percent N₂ rise in the furnace and heat the descending charge, during which process the CO and H₂ are partially oxidized to CO₂ and H₂O. The gases leaving the furnace have temperatures of 150°-300°C.

COMBUSTION AT THE TUYERES. Centers of combustion called oxidation zones develop at the tuyeres; the eddying motion of the gases in the zones leads to circulation of pieces of coke. Combustion of the coke develops on the contact surface of the solid and gaseous phases, and in the process oxygen combines with hydrogen into complex C_x O_y groups,

which then decay. In simplified form, the overall process of combustion of the carbon from the solid fuel at the tuyeres reduces to the exothermic reaction 2C + O₂ = 2CO. When natural gas or mazut, in which the main components are hydrocarbons (such as methane), is injected, a reaction occurs in which CO and H₂ are given off. This absorbs a considerable amount of the heat given off when carbon is burned, and consequently the combustion temperature at the tuyere is lowered. To avoid this it is necessary to raise the blast temperature and to enrich it with oxygen. The advantage of the injection of hydrocarbon fuels lies in increasing the concentration of hydrogen in the gas and in thus improving its reducing capacity.

REDUCTION OF IRON AND OTHER ELEMENTS. Copper, arsenic, and phosphorus are reduced in a blast furnace in the same manner as iron and pass almost entirely into the pig iron. Zinc is also fully reduced, after which it sublimates, becomes gaseous, and is deposited in the pores of the furnace lining, which causes the lining to deteriorate. Elements that form more stable compounds with oxygen than does iron are reduced partially or are not reduced at all: vanadium is reduced by 75–90 percent; manganese, 40–75 percent; and silicon and titanium by small amounts. Aluminum, magnesium, and calcium are not reduced.

The oxides Fe₂O₃ and Fe₃O₄, which enter the blast furnace, are reduced by the consecutive splitting-off of oxygen according to the reactions

3Fe₂O₃ + CO(H₂) = 2Fe₃O₄ + CO₂(H₂)

Fe₃O₄ + CO(H₂) = 3FeO + CO₂(H₂)

Iron oxide, FeO, is reduced to Fe by gases (indirect reduction) and by carbon (direct reduction):

FeO + CO(H₂) = Fe + CO₂(H₂O)

FeO + C = Fe + CO

The higher oxides of manganese, MnO₂, Mn₂O₃, and Mn₃O₄, are reduced by gases with the emission of heat. Subsequently, MnO is reduced to Mn only by carbon, with about twice as much heat absorbed as during the reduction of iron. Silicon is also only reduced by carbon at high temperatures according to the endothermic reaction

siO₂ + 2C + Fc = FeSi + 2CO

The degree of reduction of silicon and manganese depends mainly on the coke expended; for every extra percent of silicon in the cast iron, the expenditure of coke is increased 5–7 percent, which increases the amount of hot gas in the furnace and causes the blast-furnace shaft to overheat. Enrichment of the blast with oxygen ensures a high degree of heating of the hearth, reduces the amount of gases formed, and consequently also reduces the temperature in the shaft.

SULFUR IN THE BLAST-FURNACE PROCESS. Sulfur is brought into the blast furnace mainly by the coke and becomes gaseous in the form of SO₂ and H₂S vapors, but a large portion remains in the charge in the form of FeS and CaS; the

Figure 2. Operation of a blast furnace

FeS is dissolved in the pig iron in the process. To remove the sulfur from the pig iron, it must be converted into a compound insoluble in the iron, such as CaS:

FeS + CaO = CaS + FeO

This is brought about by the formation of molten slags with increased CaO content in the blast furnace. The reducing medium has a favorable effect on this process, since it reduces the FeO content in the slag. The degree of desulfurization is rather high, and only in certain cases is the pig iron further desulfurized by various reagents outside the blast furnace.

FORMATION OF PIG IRON AND SLAG. The reduced iron in the blast furnace is partially carbonized in the solid state and then in the molten state. The carbon content in pig iron depends on the temperature of the iron and on its composition. The slag consists of the unreduced oxides SiO₂, Al₂O₃, and CaO (90–95 percent), MgO (2–10 percent), FeO (0.1–0.4 percent), MnO (0.3–3 percent), and sulfur (1.5–2.5 percent; mainly in the form of CaS). Slags are usually described by the CaO/SiO₂ or (CaO + MgO)/SiO₂ basicity index. The CaO/SiO₂ basicity for various smelting conditions varies from 0.95 to 1.35 percent. When cast iron is smelted on coke with a high sulfur content (Donbas coke), slags with maximum basicity are used, and an effort is made to keep the MgO content in the slag at 6–8 percent or more to improve its flowability.

Operation of a blastfurnace. The operation of a blast furnace begins with the blowing in. At that point the hearth and the blast furnace boshes are charged with coke, and the shaft is charged with the so-called blow-in burden. A small amount of hot blast is delivered to the fully charged furnace, the coke ignites, and the materials begin to descend. The first tapping of pig iron and slag takes place 12–24 hours later, after which the amount of the hot blast and the ore charge (the ratio of the weight of ore to the weight of coke in the charge) are gradually increased; several days after blowing in, the blast furnace reaches normal productivity.

The continuous operation (campaign) of a blast furnace from blowing in to blowing out (stopping for major repairs) lasts five to six years and in some instances eight to ten years or more, during which the furnace is shut down once or twice for so-called medium overhaul to replace the worn-out shaft lining. The amount of pig iron smelted in high-capacity furnaces in a single campaign may be as high as 5–8 million tons, or even higher.

The control of the operation of a blast furnace involves the regulation, according to the quality of raw materials and the type of pig iron smelted, of the composition of the charge; the amount, temperature, and humidity of the air blast; and the amount of the charge (burden) or the sequence of introducing individual components of the charge and the level of charging. The operations of a blast furnace are controlled by measuring instruments that record the main parameters of load, blast, and blast-furnace gas and the temperature of the furnace lining at various levels.

Smelting with the injection of additional types of fuel, enrichment of the blast with oxygen, and work with blast-furnace gases at higher pressures are widespread. When pressure in the blast-furnace top is decreased, the pressure gradient between the lower and upper parts of the blast furnace is reduced, which causes a more even descent of the charge, improves the reducing work of the gases, and decreases the discharge of dust.

Blast-furnace production is characterized by a high degree of automation. All operations involved in delivering the charge are automated in modern blast furnaces: the assembling of the components of the charge and the sifting out of burden fines, weighing, hauling to the furnace top, and loading into the furnace according to a preset schedule. The optimum level of the charge and the distribution of the charge material in the furnace top, the pressure of the blast-furnace gas, the consumption of water for cooling, the temperature and moisture of the blast, its oxygen content, and the consumption of natural gas are all automatically controlled. The switching of air heaters and the control of the heating rate are automated, and automatic analyzers ensure constant recording of the composition of the furnace gas and the blast. Systems are being introduced for automatic control of the supply of the blast and the natural gas, both for total consumption and for individual tuyeres.

New blast furnaces are equipped with centralized monitoring and control systems that ensure averaging of instrument readings and calculation of complex indexes of the operation of the furnace. Work is under way for the integrated automation of blast furnaces, including automation of control over the thermal conditions of a blast furnace with the aid of an electronic computer.

INDEXES. Performance indexes for blast furnaces depend mostly on the quality of the raw materials and the degree to which they are prepared for smelting. The main indexes are the daily productivity of the blast furnace in tons and the consumption of coke per ton of pig iron. In the USSR the productivity of blast furnaces is sometimes characterized by the coefficient of use of the working volume, that is, by the ratio of the working volume in cu m to the daily output of conversion pig iron in tons. The productivity of a blast furnace with a volume of 3,000 cu m is 7,000 tons of pig iron per day. In 1970 the average coefficient of use of the working volume was 0.597 (in some instances, 0.43–0.45). The consumption of coke per unit of smelted pig iron is of great economic importance because of the high cost of coke. The use of additional fuel permits a decrease in the consumption of coke by 8–20 percent and, consequently, a reduction in the prime cost of the pig iron. In the USSR, when conversion pig iron is smelted from a well-prepared iron-rich charge, the consumption of coke is 550–600 kg per ton, and at some mills not more than 450–500 kg per ton.

Advances in the blast-furnace industry are directed toward improving the preparation of raw materials for smelting, increasing the capacity (volume) of blast furnaces, introducing modern technology, and automating control of blast-furnace operations.