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.

Tuyere

• The Iron Ore fed into the blast furnace may be a number of different compounds:

I. Haematite Fe2O3 (this is the one you learnt at IGCSE)

II. Magnetite Fe3O4

III. Iron Pyrites FeS2 (Fool’s gold!)

FeS2 must be converted to the oxide:

4FeS2 + 11O2 2Fe2O3 + 8SO2

• Coke is also fed in at the top.• This provides a source of carbon to reduce the

iron oxide.

• Fe2O3 + 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 + O2 CO2

• CO2 + C 2CO

• This carbon monoxide then reduces the iron ore.

• Either

• Fe2O3 + 3CO 2Fe + 3CO2

• Or

• Fe3O4 + 4CO 3Fe + 4CO2

• 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.

• 2CH4 + O2 2CO + 4H2

• Then either the CO can act as the reducing agent as before, or the H2 can act as the reducing agent.

• Write balanced equations for hydrogen reducing both the forms of iron oxide present.

• Fe3O4 + 4H2 3Fe + 4H2O

• Fe2O3 + 3 H2 2Fe + 3H2O

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.

• CaCO3 CaO + CO2

• Calcium oxide is “quicklime”• Quicklime is very basic and reacts with the

impurities we have mentioned.

• CaO + SiO2 CaSiO3

• CaO + Al2O3 CaAl2O4

• 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 SiO2

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