Types of Extraction
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Types of Extraction
Methods of extraction
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
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Silver Occur naturally
Gold Occur naturally
Examples of the different methods of extraction
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:
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.
Roasting in Air: Used in extractingcopper from copper (I) sulphide (copper pyrites).
The copper is extracted by roasting the ore in air.
Recycling metals
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!
The Blast FurnaceoreThe 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).
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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.
Several reactions take place before the iron is finally produced.
Oxygen in the air reacts with coke to give carbon dioxide:
The limestone breaks down to form carbon dioxide:
Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide:
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):
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
How to extract aluminium from its ore
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.
At the cathode:
Here the aluminium ions receive electrons to become atoms again:
At the anode:
The oxide ions lose electrons to become oxygen molecules, O2:
Uses of aluminium:
1. Shiny metal - used as jewellery.
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2. Low density - used to make aeroplanes and trains.
3. Non-toxic - used in drink cans.
Rusting
The corrosion of iron
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:
How to prevent rusting
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 thecheapest 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 oftenchrome-plated.
Both chromium plating and galvanising are examples ofsacrificial protection. Zinc and chromium are
sacrificed for the iron.