Extraction of Metals

Rusting

When a metal is attacked by water, air or acids in their environment, they corrode. Corrosion results in the metal become weaker and brittle.

The corrosion of iron and steel is specifically called rusting due to the red-brown substance called rust that forms in the presence of water and oxygen.

The experiment below shows that an iron nail only rusts if both water and oxygen are present:

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1. Paint or grease

This prevents water or oxygen reaching the iron. However, this is only a temporary step since paint can flake off and grease can be rubbed off quite easily. Bikes are often painted of greased to prevent rust, since this is the cheapest method of prevention.

2. Plastic

Plastic is cheap and acts as a cover for the iron, for instance, it stops water or oxygen reaching the metal surface. Garden chairs are often made from iron coated in plastic.

3. Galvanising

This involves the iron been covered, usually in the form of a paint, by zinc. Since zinc is more reactive than iron, air and water react with the zinc rather than the iron. Outside structures, such as bridges are often galvanised.

4. Chromium plating

Works for the same reason as galvanising. Chromium is a more reactive metal than iron. Car bumpers are often chrome-plated.

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Both chromium plating and galvanising are examples of sacrificial protection. Zinc and chromium are sacrificed for the iron.

The Blast Furnace

The method

Three substances are needed to enable to extraction of iron from its ore. The combined mixture is called the charge:

Iron ore, haematite - often contains sand with iron oxide, Fe2O3.

Limestone (calcium carbonate).

Coke - mainly carbon.

The charge is placed a giant chimney called a blast furnace. The blast furnace is around 30 metres high and lined with fireproof bricks. Hot air is blasted through the bottom.

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Several reactions take place before the iron is finally produced.

Oxygen in the air reacts with coke to give carbon dioxide:

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The limestone breaks down to form carbon dioxide:

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Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide:

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The carbon monoxide reduces the iron in the ore to give molten iron:

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The limestone from 2, reacts with the sand to form slag (calcium silicate):

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Both the slag and iron are drained from the bottom of the furnace.

The slag is mainly used to build roads.

The iron whilst molten is poured into moulds and left to solidify - this is called cast iron and is used to make railings and storage tanks.

The rest of the iron is used to make steel.

The Electrolysis of Bauxite

The method

The bauxite (red-brown solid) - aluminium oxide mixed with impurities - is extracted from the earth.

The extracted aluminium oxide is then treated with alkali, to remove the impurities. This results in a white solid called aluminium oxide or alumina.

The alumina is then transported to huge tanks. The tanks are lined with graphite, this acts as the cathode. Also blocks of graphite hang in the middle of the tank, and acts as anodes.

The alumina is then dissolved in molten cryolite - this lowers the melting point - saves money!

Electricity is passed and electrolysis begins. Electrolysis is the decomposition of a compound using electricity.

When dissolved, the aluminium ions and oxide ions in the alumina can move.

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At the cathode:

Here the aluminium ions receive electrons to become atoms again:

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At the anode:

The oxide ions lose electrons to become oxygen molecules, O2:

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Uses of aluminium:

1. Shiny metal - used as jewellery.

2. Low density - used to make aeroplanes and trains.

3. Non-toxic - used in drink cans.

Exam-style Questions

 

 

 

  1. The diagram below shows the blast furnace:

    Install Flash

    a) Complete the labels A, B and C.

    (3 marks)

    b) What are the main waste gases from the blast furnace?

    (2 marks)

    (Marks available: 5)

    Answer

     

    Answer outline and marking scheme for question: 1

    a) A = iron ore

    (1 mark)

    B = slag

    (1 mark)

    C = molten iron

    (1 mark)

    b) Carbon dioxide

    (1 mark)

    Nitrogen

    (1 mark)

    (Total = 5 marks)

  2. The following questions are about the blast furnace and the production of iron.

    a) Why are hot blasts of air blown in to the blast furnace?

    (1 mark)

    b) Why is limestone added to the blast furnace?

    (1 mark)

    c) Railway lines are welded together using molten iron.

    The molten iron is made using the 'thermit' reaction where iron oxide is reacted with aluminium powder.

    Why is this reaction not used to make iron on a large scale?

    (1 mark)

    (Marks available: 3)

    Answer

     

    Answer outline and marking scheme for question: 2

    a) The air is used to react with the coke/carbon to form carbon dioxide.

    (1 mark)

    b) To remove the impurities that are in the iron ore/to form slag.

    (1 mark)

    c) Any one from:

    • Too expensive/aluminium is more expensive than iron.
    • Too dangerous/aluminium reacts violently with iron oxide.

     

    (1 mark)

    (Total = 3 marks)

  3. Aluminium metal is produced using electrolysis.

    The cell used in this process is shown below:

    Install Flash

    a) Complete the labels A, B and C.

    (3 marks)

    b) Why is cryolite added to the aluminium ore in this process?

    (1 mark)

    (Marks available: 4)

    Answer

     

    Answer outline and marking scheme for question: 3

    a) A = graphite electrodes.

    (1 mark)

    B = molten bauxite/aluminium ore.

    (1 mark)

    C = molten aluminium metal.

    (1 mark)

    b) To lower the melting point of the aluminium oxide (so less energy required).

    (1 mark)

    (Total = 4 marks)

  4. The following questions are about the electrolysis of aluminium oxide.

    a) Why does the anode need to be replaced regularly?

    (1 mark)

    b) Write an equation to show how aluminium ions are changed in to aluminium metal.

    (2 marks)

    c) Why can't the blast furnace be used to make aluminium?

    (1 mark)

    (Marks available: 4)

    Answer

     

    Answer outline and marking scheme for question: 4

    a) The carbon/graphite anode reacts with the oxygen made to form carbon dioxide gas.

    (1 mark)

    b) Al3+(l) + 3e- → Al(l)

     

    (1 mark = formulae, 1 mark = balanced)

    c) Aluminium is more reactive than carbon (not too reactive).

    (1 mark)

    (Total = 4 marks)

Types of Extraction

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Many metals are found in the Earth's crust as ores. An ore is usually a compound of the metal mixed with impurities. When the metal is dug up, a method must be used to separate the metal from the rest of the ore. This is called extracting the metal.

The method of extraction depends on how reactive the metal is. The more reactive the metal, the more difficult it is to extract from its compound.

Electrolysis

Electrolysis is the most powerful extraction method. But it takes a lot of electricity and that makes it expensive. Hence, electrolysis is only used for the most reactive metals.

Metal: Method of extraction:
Potassium Electrolysis
Sodium Electrolysis
Calcium Electrolysis
Magnesium Electrolysis
Aluminium Electrolysis
Zinc Heat with carbon or carbon monoxide
Iron Heat with carbon or carbon monoxide
Lead Heat with carbon or carbon monoxide
Copper Roasting in air
Silver Occur naturally
Gold Occur naturally

Electrolysis: Used in extracting aluminium and extracting sodium from rock salt.

In the case of the rock salt, it is first melted in giant steel tanks:

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The extraction of aluminium is dealt with in a separate learn its within this topic.

Heating with Carbon monoxide: Used for extracting iron from iron ore using the blast furnace.

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Roasting in Air: Used in extractingcopper from copper (I) sulphide (copper pyrites).

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The copper is extracted by roasting the ore in air.

Metals are non-renewable resources. This means once dug up it cannot be replaced. Hence, the supply will eventually run out.

For example: it is expected that tin will run out within the next 15 years and copper in the next 40 years! Therefore,the recycling of these two useful metals and others such as iron and aluminium is most important.

In recycling, metals are melted down before reshaping into their new use. However, this can be costly. Recycling companies will only recycle if it is economical!

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S-Cool Revision Summary

Metals often appear in the Earth's crust as an ore. An ore is the metal usually in the form of a compound, most commonly an oxide. When the ore is dug up, and decomposed to the metal alone, we call this extracting.

The more reactive a metal the more difficult to extract.

Unreactive metals, such as gold and silver can be found as pure elements in the Earth's crust.

There are different methods of extraction:

  1. Eelectrolysis.

  2. Heating with carbon monoxide.

  3. Roasting in air.

Metals are a non-renewable resource. Hence, it is important to recycle used metals.

Aluminium is extracted from the ore bauxite, using electrolysis. At the cathode, aluminium is formed and at the anode oxygen gas forms.

Aluminium has many useful properties:

  1. Shiny.

  2. Good conductor of heat and electricity.

  3. Malleable and ductile.

  4. Low density.

Iron is extracted from the ore haematite (iron oxide) using a blast furnace.

Cast iron and steel are two useful by-products from the extraction of iron.

The corrosion of iron and steel is called rusting.

Rusting of iron occurs if both water and oxygen (usually from the air) is present.

To help prevent rust, several methods may be used:

  1. Paint or grease.

  2. Galvanising.

  3. Sacrificial protection.

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