Extracting iron

The blast furnace

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron(III) oxide, Fe2O3. The oxygen must be removed from the iron(III) oxide in order to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

Raw materials for the reaction

Raw materialContainsFunction
Iron ore (haematite)Iron(III) oxide (Fe2O3)A compound that the iron is extracted from
CokeCarbon (C)Used as a fuel and reacts to form carbon monoxide (needed to reduce the iron(III) oxide)
LimestoneCalcium carbonate (CaCO3)Helps to remove acidic impurities from the iron by reacting with them to form molten slag
AirOxygen (O2)Provides oxygen to allow the coke to burn, and so produces heat
Blast furnace. Iron ore, carbon, limestone enter at top. Air enters at side near bottom. Three zones. Air into zone 1, waste gases out above zone 3. Slag out below zone 1, iron out at very bottom.

Carbon is more reactive than iron, so it can displace iron from iron(III) oxide. Here are the equations for the reaction.

Step 1 – Hot air (oxygen) reacts with the coke (carbon) to produce carbon dioxide and heat energy to heat up the furnace.

C(s) + O2(g) → CO2(g)

Step 2 – More coke is added to the furnace and reduces the carbon dioxide into carbon monoxide, a good reducing agent.

CO2(g) + C(s) → 2CO(g)

Step 3 – iron(III) oxide is reduced.

iron(III) oxide + carbon → iron + carbon dioxide

2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)

In this reaction, the iron(III) oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide can be used, in place of carbon, to reduce the iron(III) oxide:

iron(III) oxide + carbon monoxide → iron + carbon dioxide

Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

curriculum-key-fact
The blast furnace is a continuous process (ie reactants are added constantly so that the reaction does not stop). This is due to the cost and energy required to start the reaction in the first place meaning it would be too expensive and use too much energy if it was started and stopped regularly.

Removing impurities

The calcium carbonate in the limestone thermally decomposes to form calcium oxide.

calcium carbonate → calcium oxide + carbon dioxide

CaCO3(s) → CaO(s) + CO2(g)

The calcium oxide then reacts with silica (sand) impurities in the haematite, to produce slag – which is calcium silicate. This is separated from the iron and used to make road surfaces.

calcium oxide + silica → calcium silicate

CaO(s) + SiO2(s) → CaSiO3(l)

This reaction is a neutralisation reaction. Calcium oxide is basic (as it is a metal oxide) and silica is acidic (as it is a non-metal oxide).

Choice of blast furnace site

There are a number of important factors to consider when choosing the site of a blast furnace. A blast furnace should be:

  • near the coast to allow for the import of raw materials
  • near roads and railway lines to allow for products to be taken to where they are needed
  • near a town or city, so that workers have somewhere to live close-by
  • away from built-up areas, so that the noise and pollution of the site do not affect the local population

Port Talbot, in south Wales, is a good example of a suitable site for a blast furnace.