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Iron is too reactive a metal to be found as the pure element in the Earth's crust. Instead, it occurs in compounds within rocks and minerals known as metal ores. The two most common ores of iron are haematite (Fe₂O₃) and magnetite (Fe₃O₄). Typical haematite ores contain around 50–60% iron, while magnetite ores usually contain 45–65% iron. The rest is made up of impurities such as silica and other rock, which must be removed during processing these ores.

• Haematite (Fe₂O₃): This is the most abundant iron ore. Its name comes from the Greek word for "blood" due to its reddish-brown colour which is particularly evident in its powdered form; this metal ore typically contains around 50-60% iron with the rest of the ore consisting of impurities such as silica and other minerals.


• Magnetite (Fe₃O₄): Magnetite is perhaps best known for its strong magnetic properties. It's often black or dark gray in colour; magnetite ores usually contain 45–65% iron

The blast furnace-extracting iron

An overview of the blast furnace

Before we look in detail at how a blast furnace works the image below will give you an idea of the main parts of a blast furnace and what each part does; it also shows the truly huge scale of these massive furnaces.

How a blast furnace works

Iron ore, mainly haematite (Fe₂O₃) and magnetite (Fe₃O₄) along with coke and limestone are the three main ingredients that are added to the top of the blast furnace on a regular basis to ensure that the blast furnace produces iron continually. These three substances are added to the blast furnace at point 1 in the diagram opposite.

The mixture of iron ores which are added to the top of the blast furnace are firstly crushed and heated to a high temperature with coke (which is mainly carbon) and limestone; at this high temperature; which is not sufficient to melt these substances but it is high enough to cause this mixture of substances to stick together to form small highly porous lumps of material called sinter.

The Sinter along with more coke and limestone are added to the top of the blast furnace on a regular basis, this mixture of sinter, coke and limestone is often called charge and is fed into the furnace by conveyor belts or dropped in by carts as outlined in the image opposite. 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 one of its ore haematite in a blast furnace.

How a blast furnace operates

1. Sinter, coke and limestone are added to the top of the blast furnace at point 1 in the diagram opposite.


2. The blast furnace operates at a temperature of around 1600⁰C; the fuel which is burned to maintain this high temperature in the blast furnace is coke. Coke is made by heating coal in large ovens in the absence of air. Heating coal in the absence of air drives off water, gases, tar and any other volatile components present in the coal to leave behind a solid which has a very high carbon content which burns at a high temperature. The coke (carbon) burns with the hot air blasted in from the tuyeres. The tuyeres are large pipes or tubes which surround the bottom of the furnace and through which hot air is blasted into the furnace. The blast furnace gets it name from the blasts of hot air from these tuyeres.
   
   The coke in the furnace now reacts or combusts with the oxygen present in the air blasted in to form carbon dioxide gas, this reaction also releases lots of heat and the temperature is around 1600⁰C. This happens at point 2 in the diagram opposite and an equation for this combustion reaction is:

   C_(s) + O_2(g) → CO_2(g)

3. The hot carbon dioxide gas (CO₂) gas formed in the above reaction now rises up through the blast furnace and reacts with more coke (carbon) to form carbon monoxide gas (CO). The carbon dioxide gas is reduced and the carbon/coke is oxidised in this reaction, an equation for this redox reaction is shown below:

   CO_2(g) + C_(s) → CO_(g)

4. Carbon monoxide (CO) gas is a good reducing agent; it will remove oxygen from substances. This is ideal since it can react with the haematite (iron oxide) which was tipped into the top of the furnace, being a dense material the iron ore will slowly sink through the furnace where it can react with the CO gas, this reaction will produce iron. Now it is hot enough inside the furnace to melt the iron which is formed and this heavy dense liquid 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. The carbon monoxide is the main reducing agent inside the blast furnace; however in the hotter lower parts of the blast furnace carbon present in the coke can also reduce the iron oxide to form iron:



2Fe₂O_3(s) + 3C_(s) → 4Fe_(l) + 3CO_2(g)



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 in the blast furnace 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 breeze 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. The 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 blast 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.

Self-check: True or false

Sort the statements below into either the true or false bins, when you are done press the check answers button.

Self-check: How a blast furnace works

Place the cards in the left had pool in the correct order to show the reactions which take place in a blast furnace.

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 three main substances which are tipped into the top of a blast furnace are sinter, limestone and coke.


• 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 - Quick Quiz on the blast furnace

Check your understanding - Questions on the blast furnace