C9: Industrial inorganic chemistry
Steel-making
The iron ore is reduced by carbon monoxide (temperature about 600 ºC).
The iron produced by the blast furnace is known as ‘pig iron’ or ‘cast iron’ and is not pure. It contains about 4% carbon, and other impurities. This amount of carbon makes the iron brittle.
carbon iron + dioxide 2Fe + 3CO2
Carbon dioxide is reduced as it rises through the furnace – carbon monoxide is produced (temperature about 1000 ºC). carbon carbon + carbon monoxide dioxide 2CO CO2 + C
Molten iron flows down the furnace.
Hot gases rise up the furnace.
iron(III) carbon + oxide monoxide Fe2O3 + 3CO
Carbon burns strongly at the base of the furnace (temperatures reach 1900 ºC). carbon + oxygen carbon dioxide C + O2 CO2
Figure C9.02 Iron is produced in the blast furnace by a series of reactions. Carbon monoxide is thought to be the main reducing agent.
Most of the pig iron produced is taken to make steel. The carbon content is reduced by burning it off as carbon dioxide. This basic oxygen process is carried out in a tilting furnace (Figure C9.03). Scrap steel is added to the molten pig iron for recycling. A high-speed jet of oxygen is blown into the vessel through a water‑cooled lance. Some impurities, for example silicon and phosphorus, do not produce gaseous oxides, so lime (CaO) is added to the furnace. The impurities form a ‘slag’, which floats on top of the molten iron. The molten iron is poured off by tilting the furnace. Controlled amounts of other elements such as chromium, manganese, tungsten or other transition metals are added to make different types of steel (see Tables C9.01 and C9.02).
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The hot waste gases escape from the top of the furnace. They are used in heat exchangers to heat the incoming air. This helps to reduce the energy costs of the process. The extraction of iron is a continuous process.
a
fumecollecting hood
water-cooled oxygen lance
The blast furnace extraction of iron: ■
uses iron ore, coke, limestone and hot air ■ involves the reduction of iron(iii) oxide by carbon monoxide ■ uses limestone to remove the main impurity (sand) as slag (calcium silicate).
oxygen
b
scrap steel
molten iron and scrap steel, lime
molten iron c
slag slag
molten steel
Tip
For the blast furnace it is important that you are aware of the different aspects of how it works. You should be able to label a diagram of it and know what is fed into it. Importantly, you should also know the key reactions of the furnace, including the formation of slag.
Figure C9.03 The different stages of the steel-making process (the basic oxygen process). a The furnace is charged with scrap steel and molten iron. b Oxygen is blown in through an ‘oxygen lance’. c The molten steel, and then the slag, are poured from the furnace by tilting it in different directions.
Original material © Cambridge University Press 2017