Chemistry in Industry and Technology Option C

The Blast Furnace

This follows closely from IGCSE work. If you can’t remember the work you did at IGCSE then you need to revise! You will learn about some different sources of iron, and you will learn more detail about the chemical reactions that happen in a blast furnace.

Syllabus Statements C1.1 State the main sources of iron. C1.2 Describe the main reactions that occur in the blast furnace.

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The Iron Ore fed into the blast furnace may be a number of different compounds: I.Haematite Fe 2 O 3 (this is the one you learnt at IGCSE) II.Magnetite Fe 3 O 4 III.Iron Pyrites FeS 2 (Fool’s gold!) FeS 2 must be converted to the oxide: 4FeS O 2  2Fe 2 O 3 + 8SO 2

Coke is also fed in at the top. This provides a source of carbon to reduce the iron oxide. Fe 2 O 3 + 3C  2Fe + 3CO This DOES happen to a small extent – but from IGCSE you know its not really that simple!

Most of the reduction is done by Carbon Monoxide CO. C + O 2  CO 2 CO 2 + C  2CO

This carbon monoxide then reduces the iron ore. Either Fe 2 O 3 + 3CO  2Fe + 3CO 2 Or Fe 3 O 4 + 4CO  3Fe + 4CO 2

Both these reactions are exothermic. The iron produced is molten because of the high temperature. ( Iron melts at 1536 °C or 2797 °F (1811 K). It boils at 2862 °C or 5182 °F (3134 K).) It sinks to the bottom of the furnace where it can be “tapped”

There are 2 more possible reaction mechanisms that you need to learn: If methane (“natural gas”) is blown into the furnace, then incomplete combustion can take place. 2CH 4 + O 2  2CO + 4H 2 Then either the CO can act as the reducing agent as before, or the H 2 can act as the reducing agent. Write balanced equations for hydrogen reducing both the forms of iron oxide present.

Fe 3 O 4 + 4H 2  3Fe + 4H 2 O Fe 2 O H 2  2Fe + 3H 2 O

Getting rid of impurities The iron ore fed into the blast furnace is NOT a pure chemical. It contains impurities such as silicon (IV) oxide and aluminium oxide. These come from the rocks which contain the iron compounds.

Relative abundance of elements in the Earth’s crust Oxygen 46.6% Silicon 27.7% Aluminum 8.1% Iron 5.0% Calcium 3.6% Potassium 2.6% Magnesium 2.6%

The third raw material added at the top of the blast furnace is powdered limestone. This decomposes at the high temperatures involved. CaCO 3  CaO + CO 2 Calcium oxide is “quicklime” Quicklime is very basic and reacts with the impurities we have mentioned.

CaO + SiO 2  CaSiO 3 CaO + Al 2 O 3  CaAl 2 O 4 Silicon oxide is a non-metal oxide and is therefore acidic. Aluminium oxide is amphoteric!!

Both the compounds formed are liquids. We call them “slag” They float on top of the molten iron and can be removed through a second outlet. They are used for roads and making cement. Because they can be easily removed the blast furnace can be run continuously for a number of years (good economics!). Eventually the lining of the furnace breaks down and we have to let the whole thing cool down so we can replace it.

Reminder... C1.1 State the main sources of iron. C1.2 Describe the main reactions that occur in the blast furnace.

Summary... Iron Ore Carbon Limestone Reduction reactions Reactions to remove impurities Making CO CO reducing oxides methane giving H2 H2 reducing oxides Forming quicklime reaction with Al2O3 reaction with SiO 2

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