Iron smelting process
Steel is actually a combination of two terms-“steel” and “iron”. Ironmaking must be done before steelmaking. Iron is divided into pig iron and wrought iron. The carbon content of steel is between pig iron and wrought iron.
- Pig iron: Generally refers to an alloy of iron with a carbon content of 2~6.69%. Also known as cast iron. In addition to carbon, pig iron also contains silicon, manganese and a small amount of sulfur and phosphorus. It can be cast and cannot be forged.
- Wrought iron: relatively pure iron refined from pig iron. The carbon content is below 0.02%, also called wrought iron or pure iron. Pure iron requires a very low content of impurity elements such as carbon, phosphorus, and sulfur. It is difficult to smelt, and its manufacturing cost is much higher than that of pig iron and steel.
Iron smelting process
Table of Contents
Iron smelting process
- 1.1 Raw materials: iron ore, solvent, fuel
- 1.2 Physical and chemical process: reduction reaction at high temperature + slagging reaction
- 1.3 Blast furnace products: pig iron + ferroalloy
Ironmaking methods mainly include blast furnace method, direct reduction method, smelting reduction method, etc. The principle is that the ore obtains reduced pig iron through physical and chemical reactions in a specific atmosphere (reducing substances CO, H2, C; suitable temperature, etc.). Except for a small part of pig iron used for casting, most of it is used as raw material for steelmaking.
Blast furnace ironmaking is the main method of modern ironmaking and an important link in steel production. Due to good technical and economic indicators, simple technology, large production, high labor productivity, and low energy consumption, the iron produced by the blast furnace method accounts for more than 95% of the world’s total iron output.
The blast furnace is similar to a cylindrical furnace. Its outer bread is made of steel plates and the inner wall is built with refractory bricks. The entire furnace is built on a deep concrete foundation.
During blast furnace production, iron ore, coke, and flux (limestone) for slag-making are charged from the top of the furnace, and preheated air is blown in from the tuyere at the bottom of the furnace along the periphery of the furnace. At high temperatures, the carbon in the coke is combusted with the oxygen in the air to generate carbon monoxide and hydrogen, and the oxygen in the iron ore is removed during the rise in the furnace, thereby reducing iron. The molten iron produced is discharged from the iron hole.
The unreduced impurities in the iron ore are combined with fluxes such as limestone to form slag, which is discharged from the slag port. The produced gas is led out from the top of the furnace and used as fuel for hot blast stoves, heating furnaces, coke ovens, boilers, etc.
Raw materials: iron ore, solvent, fuel
Naturally mined ore, no matter in terms of chemical composition, physical state, etc., is difficult to meet the requirements of blast furnace smelting. It must be crushed, sieved, beneficiated, agglomerated, mixed and other preparations are processed to achieve high grade, composition, and particle size. Supply the blast furnace in a uniform and stable state.
The gangue in the ore and the ash in the fuel contain some compounds with high melting points (such as the melting point of SiO2 is 1625°C, and the melting point of Al2O3 is 2050°C). They cannot be melted into liquid at the temperature of blast furnace smelting, so they cannot be made. It is well separated from the molten iron and also makes the operation of the furnace difficult.
The purpose of adding flux is to form low-melting slag with these high-melting compounds so as to be completely liquefied at the smelting temperature of the blast furnace and maintain considerable fluidity to achieve the purpose of good separation from the metal and to ensure the quality of pig iron.
According to the properties of flux, it can be divided into alkaline flux and acid flux. Which flux to use depends on the nature of the gangue in the ore and the ash content in the fuel. Since most of the gangue in natural ore is acidic and the coke ash is also acidic, alkaline fluxes such as limestone are usually used. Acidic fluxes are rarely used.
The heat required for blast furnace smelting is mainly obtained by the combustion of fuel. At the same time, the fuel also acts as a reducing agent during the combustion process. Therefore, fuel is one of the main raw materials for blast furnace smelting. The commonly used fuel is mainly coke. There are anthracite and semi-coke.
Physical and chemical process: reduction reaction at high temperature + slagging reaction
The purpose of blast furnace smelting is to reduce iron from iron ore while removing impurities contained in it. In the whole smelting process, the most important thing is to carry out iron reduction reaction and slagging reaction.
In addition, it is accompanied by a series of other complex physical and chemical reactions, such as the evaporation of water and volatiles, the decomposition of carbonates, the carbonization and melting of iron, and the reduction of various other elements. The reaction can only be achieved at a certain temperature. Therefore, the smelting process also needs fuel combustion as a necessary condition.
Combustion of fuel
Decomposition of the charge
Evaporation of water and decomposition of crystalline water; removal of volatiles; decomposition of carbonate.
Reduction reaction in the blast furnace
In the blast furnace, iron is not directly reduced from high-valent oxides, but goes through a process of reducing high-valent oxides to low-valent oxides, and then reducing iron from low-valent oxides:
The reduction of iron mainly relies on carbon monoxide gas and solid carbon as reducing agents. Usually the reduction of carbon monoxide is called indirect reduction, and the reduction of solid carbon is called direct reduction.
The overall reaction of indirect reduction is:
The total reaction of direct reduction is:
Carbonization of iron
The iron reduced from the ore is a solid sponge, and its carbon content is extremely low, usually not more than 1%. Since CO decomposes at a lower temperature, the decomposed C has strong activity. When it comes into contact with iron, it is easy to form iron-carbon alloys.
Therefore, the solid sponge iron begins to carburize at a relatively low temperature (400℃～600℃). The chemical reaction is as follows:
- 2CO+3Fe→Fe3C+CO2 or 3Fe(liquid)+C(solid)→Fe3C
Slag making process
The slagging process is the process in which the gangue in the ore and the ash in the fuel are combined with the flux and removed from the blast furnace. There are two common slagging situations in blast furnaces:
When ordinary acid ore is used for smelting, the flux is loaded into the blast furnace in the form of limestone, and the CaO in the flux cannot be in close contact with the acidic oxides in the ore. Therefore, the initially formed slag is mainly generated by SiO2, Al2O3 and a part of reduced FeO Fe2SiO4. Due to the presence of FeO in the slag, the melting point of the slag is lowered, and the slag has better fluidity. During its descending process (also a process of temperature increase), the contained FeO is gradually reduced and lost, and The content of CaO increases accordingly, and finally the slag flows into the hearth.
When smelting with self-fluxing ore, since the ore contains more CaO and it can be in good contact with acidic SiO2, CaO immediately participates in the slagging reaction at the beginning of smelting, especially when self-fluxing sintering is used. During ore smelting, CaO and SiO2 and Al2O3 formed slag early in the sintering process. Therefore, the content of CaO in the primary slag of this ore is higher, and the composition of the slag changes less during the decline process.
Blast furnace products: pig iron + ferroalloy
The main products of blast furnace smelting are pig iron and ferroalloys, and the by-products include slag, gas and furnace dust.
Pig iron is an iron-carbon alloy with a carbon content of more than 2%, which also contains impurities such as Si, Mn, S, and P.
According to its use and composition, pig iron can be divided into two categories. One type is steelmaking pig iron: the carbon in this type of pig iron exists in the form of a compound, and its cross section is silvery white, also called white iron; the other type is cast iron: directly used to make machine parts.
The alloy of iron and any kind of metal or non-metal is called ferroalloy (some also called alloy pig iron). There are many types of ferroalloys, including ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, and ferro tungsten.
Slag, furnace gas and furnace dust
Slag, furnace gas and furnace dust are by-products of blast furnaces, which were previously discarded as waste, and are now widely used as building materials.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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