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The blast furnace works by the principle of countercurrent. Burdening materials – agglomerate, coke, etc. – load from above by means of the zasypny (loading) device. Towards to the falling materials from below the flow of the hot gases which are formed at fuel combustion (coke), and also natural gas moves up.
In the blast furnace the following basic processes proceed:
Iron recovery. This process comes consistently from the highest oxides to the lowest and further to pure metal: Fe2O3 – Fe3O4 – FeO – Fe
The main reducers of iron in the blast furnace are carbon oxide (I) and solid carbon of coke. Carbon oxide (I) is formed at interaction of carbon dioxide gas with the heated coke:
C + CO2=2CO
Recovery by carbon oxide is called indirect (indirect) recovery and happens on reactions
3Fe2O3 + CO = 2Fe3O4 + CO2 + Q;
Fe3O4 + CO = 3FeO + CO2 - Q;
FeO + CO = Fe + CO2 + Q.
Recovery of Fe2O3 begins at rather low temperatures (400-5000C) in upper part of the mine of the furnace. In process of lowering of ore materials temperature and content WITH in blast-furnace gases increase; at the same time conditions for final recovery of iron are created. These processes come to an end in the bottom of the mine of the furnace at temperatures about 900-9500 Pages.
Value of indirect recovery is very big. Depending on operating conditions of the furnace SO carbon oxide recovers 60-80% of all iron. Other part of iron is recovered by solid carbon.
Recovery by solid carbon is called direct recovery. It occurs at temperatures over 950-10000 With (zone of raspar of the furnace) on reaction
FeO + C = Fe + CO – Q.
It should be noted that this reaction reflects only the end result of process of direct recovery which proceeds in two stages:
FeO + CO = Fe + CO2 + Q
CO2 + C = 2CO– Q
FeO + C = Fe + CO2 – Q
Thus, at direct recovery only coke carbon is spent though the reagent interacting with FeO is SO carbon oxide. Direct recovery of iron oxides at contact with coke carbon practically does not happen.
Already in the mine of the blast furnace at temperatures over 400-5000 With along with recovery of iron there is also its carburizing at the expense of SO carbon oxide on reaction:
3Fe + 2CO = Fe3 C + CO2 + Q.
Fe3C iron carbide is well dissolved in hard iron and iron alloy with carbon is gradually formed. With increase in carbon content alloy melting temperature considerably goes down and reaches the minimum value 11470C at 4,3%. In furnace zones with high temperatures – usually in the bottom of the mine – alloy melting begins. Liquid alloy – cast iron, flowing down down, washes pieces of the heated coke and it is in addition intensively carbonized. In it the recovered manganese, silicon, sulfur and other impurity are also dissolved. The final composition of cast iron is established in horn. At the same time the structure, properties and amount of slag are of great importance.
Recovery of other elements. Manganese, silicon, sulfur and other elements in the form of different chemical compounds get to the blast furnace with burdening materials. These elements are partially or completely recovered and are part of cast iron, improving or worsening its properties.
Unavoidable useful impurities of cast iron are manganese and silicon, harmful – sulfur and phosphorus.
Manganese – unavoidable impurity of iron ores. When smelting chugun with the increased content of manganese manganic ore is loaded into the blast furnace.
The highest manganese oxides are recovered to MnO manganese oxide by carbon monoxide, similar to iron oxides: MnO2 - Mn2O3 - Mn3O4-MnO. Manganous oxide is recovered by solid carbon on reaction:
MnO + C = Mn + CO – Q.
This reaction proceeds at temperatures over 11000 With with absorption of heat. Therefore for recovery of manganese it is required to increase consumption of coke and temperature of blasting. For example, when smelting mirror cast iron from 10-25% of Mn the consumption of coke increases by 2-2,5 times. Considerable part of MnO is in type of silicates from which it can be allocated with lime.
Thus, additional condition for increase in extent of recovery of manganese is enough CaO lime in slag, i.e. its increased basicity.
Silicon is in dead rock of ore and in coke sol in the form of free SiO2 silicon dioxide or in the form of silicates (SiO2 · 2sao, etc.).
Recovery of silicon comes from SiO2 silicon dioxide on reaction:
SiO2 + 2C = Si + 2CO – Q.
Apparently, silicon is recovered from SiO2 and Fe3C iron carbide.
This reaction proceeds with absorption of heat at temperatures not lower than 14500 Pages. Therefore for smelting of cast iron with the raised silicon content it is necessary to increase considerably consumption of coke and to apply the high-temperature blasting enriched with oxygen. For increase in amount of free silicon dioxide in slag it is necessary to reduce in it the content of CAO lime, i.e. to lower its basicity.
Other useful impurity – nickel, vanadium, titanium, etc. – get to the blast furnace in the form of impurity of iron ore. At domain melting nickel is recovered and passes into cast iron completely, is lame – for 85-95%, vanadium – for 70-80%.
Phosphorus – harmful impurity of iron ores is in them mainly in the form of P2O5 · 3sao. Recovery of phosphorus happens SO carbon monoxide, hydrogen, and also solid carbon. All phosphorus brought by furnace charge is recovered and passes into cast iron almost completely.
Sulfur – especially harmful impurity in cast iron (and also in steel). The main amount of sulfur brings coke, part – iron ore. In the blast furnace of 10-20% of sulfur is removed in the form of connections. Other part of sulfur passes into cast iron and into slag in the form of sulfides FeS, CaS, etc. FeS iron sulfide is well dissolved in cast iron.
In the conditions of domain melting the main way of desulphuration, i.e. sulfur removal from metal, is formation of CaS calcium sulfide on FeS reaction + CaO = FeO + CaO + Q.
CaS calcium sulfide is insoluble in cast iron and is in slag. Most intensively this reaction proceeds when passing drops of cast iron through slag layer.
Follows from this reaction that one of the main conditions of sulfur removal of metal is enough CaO lime in slag. Sulfur removal is promoted by high temperature in horn; with heating viscosity of slag decreases that improves diffusion of sulfides and promotes FeO recovery.
Part of sulfur is removed by means of MgO (always contained in slag), and also manganese on FeS reactions + MgO = FeO + MgS and FeS + Mn = Fe + MnS.
MgS magnesium sulfide is insoluble in metal, and MnS manganous sulfide is dissolved slightly. Extra domain sulfur removal from cast iron was widely adopted. At its endurance in ladles-chugunovozakh and in the mixer part of sulfur can pass from metal into slag in the form of MnS manganous sulfide as solubility of this connection in metal at fall of temperature decreases. Such way yields good results at the content in cast iron more than 2% of Mn.
One of the ways of extra domain sulfur removal tested commercially is processing of cast iron in final trench or in chugunovoza NaCO3 soda (1% of the mass of cast iron). Cera leaves on reaction:
FeS + NaCO3 = FeO + Na2S + CO2.
The Na2S sodium sulfide which is formed at the same time passes into slag. Now conduct research of work on research of other not scarce and cheap reagents. Shlakoobrazovaniye begins approximately in furnace raspara.
Primary slag is formed as a result of alloyage of CaO, SiO2, Al2O3 and other oxides which are in structure of gumboil and dead rock of ore. At certain ratios on weight these refractory oxides can form fusible mixes – alloys with T пл = 1150-12000 C. Flowing down down and collecting in horn, slag significantly changes the structure. As a result of interaction with molten iron and the remains of not burned down coke in slag iron oxides and manganese are recovered, in it FeS, MnS, coke ashes, etc. are dissolved. Chemical composition of slag defines composition of cast iron and therefore when smelting conversion, cast and other irons always select slag of the corresponding structure. Standard composition of slag: 40-50% of CaO; 38-40% of SiO2; 7-10% of Al2O3.
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