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

Iron ore, mostly haematite (Fe₂O₃) is mixed with coke and limestone and dropped into the top of the blast furnace on a regular basis. This mixture called charge is fed in by conveyor belts or dropped in by carts. The blast 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 extracted from its ore; haematite in 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. Coke 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, these are large pipes or tubes which surround the bottom of the furnace and through which hot air is blasted into the furnace. The coke reacts or combusts with the oxygen present in the air to form carbon dioxide gas, this reaction also releases lots of heat and the temperature is around 1400⁰C.

   C_(s) + O_2(g) → CO_2(g)

3. The hot carbon dioxide gas (CO₂) gas formed in the above reaction rises up through the blast 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, an equation for this redox reaction is shown below:

   CO_2(g) + C_(s) → CO_(g)

4. The CO gas then reacts with the iron ore (haematite-Fe₂O₃) which was added from the top of the furnace, this reaction produces iron; now it is hot enough inside the furnace to melt the iron which is formed and this heavy dense metal starts to slowly sink down through the furnace, an equation for this reaction is shown below; here the carbon monoxide gas reduces the iron oxide in this reaction to form the liquid molten iron.

   Fe₂O_3(s) + 3CO_(g) → 2Fe_(l) + 3CO_2(g)

   While the carbon monoxide (CO ) gas reduces the iron oxide to iron the (CO) gas itself is oxidised to form CO₂.
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 they were not removed would quickly block up the furnace. The limestone (calcium carbonate) which was one of the ingredients in the charge added to the top of the blast furnace takes care of these 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)

   CaCO_3(s) → CaO_(s) + CO_2(g)

   The calcium oxide (CaO) being a basic substance will react with the acidic silica impurities present in the iron ore:

   calcium oxide_(s) + silica_(s) → calcium silicate_(s)

   CaO_(s) + SiO_2(g) → CaSiO_3(s)

   
   The waste product calcium silicate or slag which is produced is used in the road building and construction industries. The waste slag is a fairly dense material and sinks down through the furnace, now the slag is less dense than the molten iron which collects at the base of the furnace so the molten slag floats on top the layer of molten iron. The waste slag 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. The molten iron 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, unfortunately pig iron has a high carbon content; typically between 3-5% and this makes the iron brittle. However this pig iron is transported to the steelworks where the carbon content of the pig iron is lowered, this turns the pig iron into mild steel which is much stronger and less brittle.
7. The waste gas carbon dioxide which is produced during the reactions taking place inside the blast furnace rise up to top of the furnace and enter the large extraction ducts, labelled 8 in the diagram opposite. Another waste gas which enters the extractor ducts is nitrogen. Now recall that almost 80% of the air is nitrogen, so 80% of the air blasted into the base of the blast furnace is nitrogen and being a very unreactive gas it simply passes through the blast furnace unchanged and leaves with the carbon dioxide gas through the extractor ducts at the top of the furnace.
8. This mixture of gases and other wastes that enter the extractor ducts consists of impurities, fine particles and the gases carbon monoxide, carbon dioxide and nitrogen, these gases next enter the scrubbers where they are cleaned. The carbon monoxide gas is flammable and can be burned to heat the stoves which preheat the air blasted into the furnace, the carbon monoxide gas can also be burned to generate electricity.
9. The molten iron is run off from the furnace base and transported in large torpedo shaped railway wagons 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 carbon and 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 aluminium from its ore, bauxite, by heating with carbon or carbon monoxide or hydrogen as shown below the reactions fail and no aluminium metal is formed:

aluminium oxide_(s) + carbon_(s) → no reaction

aluminium oxide_(s) + carbon monoxide(g) → no reaction

aluminium oxide_(s) + hydrogen _(g) → no reaction

Key points

• The main ore of iron is haematite, it consists mainly of iron(III) oxide. The ore is mixed with calcium carbonate (limestone) and coke and fed into the top of the furnace.
• The fuel for the blast furnace is coke. This burns to release large amounts of heat energy and carbon dioxide gas.
• Carbon monoxide and carbon will reduce the iron oxide to iron.
• The waste products of iron extraction; called slag is removed from near the base of the furnace.
• Molten iron is tapped or removed from the base of the furnace and transported in large railway wagons called torpedoes.
• The iron from the blast furnace traditionally was run into moulds called pigs, so it is often called pig iron. It contains a high percentage of carbon; this unfortunately makes the iron brittle.

Check your understanding - Questions on the blast furnace

Check your understanding - Quick Quiz on the blast furnace