Higher and foundation tier

The blast furnace

The blast furnace is used industrially to extract iron from its ore. Most of the iron produced from the blast furnace is usually turned into steel. The diagram below shows the main parts of the furnace with a brief description of what they do.

Labelled diagram of a blast furnace

Iron ore, mostly haematite (Iron oxide) is mixed with coke and limestone and dropped into the top of the furnace on a regular basis. This mixture called charge is fed in by conveyor belts or dropped in by carts. The furnace operates 24 hours a day, 365 days an year and is never allowed to cool down, it would simply take too long and be very expensive to shut down the furnace and restart it again. Below is an outline of how iron is produced in a blast furnace.

cross section of a blast furnace

  1. Crushed iron ore (haematite), coke and limestone are added to the top of the furnace.
  2. The fuel for the blast furnace is coke, this is made by heating coal in the absence of air. The solid which is left has a very high carbon content and burns at a high temperature. The coke (carbon) reacts with the hot air blasted in from the tuyeres to form carbon dioxide, this reaction also releases lots of heat and the temperature is around 14000C.
    C(s) + O2(g) → CO2(g)
  3. The hot CO2 gas formed in above rises up through the furnace and reacts with more coke (carbon) to form carbon monoxide gas (CO). The carbon dioxide is reduced and the carbon is oxidised in this reaction.
    CO2(g) + C(s) → CO(g)
  4. The CO gas then reacts with the iron ore (haematite) which was added from the top of the furnace, but sinks slowly down the furnace.
    Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
    The CO gas reduces the iron oxide to iron and the CO gas is oxidised to CO2. It is hot enough in the furnace to melt the iron which is formed.
  5. The iron ore (haematite) is mainly iron oxide but it contains a number of impurities, mainly acidic impurities like silicon dioxide (silica or sand). These impurities if not removed would block up the furnace. The limestone (calcium carbonate) which was one of the ingredients in the charge takes care of the impurities. The calcium carbonate will decompose due to the heat in the furnace to form the basic substance calcium oxide:
    calcium carbonate(s) → + calcium oxide(s) + carbon dioxide(g)
    CaCO3(s) → CaO (s) + CO2(g)
    the calcium oxide (CaO) being a basic substance will react with the acidic impurities present in the iron ore
    calcium oxide(s) + silica(s) → calcium silicate(s)
    CaO(s) + SiO2(g) → CaSiO3(s)
    The waste product calcium silicate or slag is used in the road building and construction industries. It is a fairly dense material and sinks down through the furnace. It is removed daily from the furnace and collected in large pits, where it cools and solidifies. It is then sold to the construction and building industries. One of its main uses is to make beese blocks for house building.
  6. The liquid iron produced in step 4 is dense (heavy) and sinks to the bottom of the furnace. It is tapped off and run into a torpedo shaped railway wagon before being transported to the steel works. The iron produced from the blast furnace is called pig iron, it has a high carbon content, typically between 3-5% and this makes it brittle. The carbon content of this iron is lowered at the steel works.
  7. The waste gases from the reactions rise up through the furnace and enter the large extraction ducts, labelled 8.
  8. This mixture of gases and other impurities and fine particles is mainly carbon monoxide, carbon dioxide and nitrogen (the main gas present in the air blasted in at the bottom of the furnace, nitrogen is very unreactive and passes through the furnace unchanged), these gases next enter the scrubbers where they are cleaned. The carbon monoxide gas is flammable and can be burned to heat the stoves and also to generate electricity.
  9. The molten iron is transported in large torpedoes holding many hundreds of tonnes of molten iron on its way to the steel works.

The next steps........

So far we have seen that it is possible to extract metals from their ores using hydrogen and carbon or carbon monoxide as reducing agents. However how do you extract a metal above carbon in the reactivity series? For example if you try to extract aluminum from its ore, bauxite, by heating with carbon or carbon monoxide or hydrogen as shown below:

aluminium oxide(s) + carbon(s) → no reaction
aluminium oxide(s) + carbon monoxide(g) → no reaction
aluminium oxide(s) + hydrogen (g) → no reaction
Unfortunately all these reactions FAIL. Aluminium is too reactive a metal to be extracted by any of the methods above. None of them provide enough energy to remove aluminium from its ore. What is needed is a method which can supply large amounts of energy to break the very strong bonds between highly reactive metal and non-metal present in metal ores. The method used is electrolysis.

Key points

Practice questions

Check your understanding - Questions on the blast furnace