C1aL12 Aluminium and Recycling

Key Words

Alloys - A metal in which other metals are mixed as impurities.

Aluminium Oxide - Al2O3

Bauxite - the most common ore for aluminium.

Corrode - rotting away of metals due to oxidisation

Cryolite - another ore of aluminium

Density - mass per unit volume.

Electrolysis - extracting a metal from its ore by electricity.

Energy - ability to do work.

Expensive - costs lots of money (really?).

Extracting - getting the metal from its ore, or getting the ore out of the ground.

Immunity - resistance to disease

Ions - Charged atoms.

Pollutants - materials that harm the environment

Rainforest - tropical forests that contain a wide range of species of animals and plants.

Recycle - using scrap materials to make new materials.

Waste - material that is thrown away.

Grade E

Aluminium is very common in the Earth's crust, even more common than iron.  A couple of hundred years ago, it was more precious than gold, because it was hard to extract using chemical means.

Aluminium is a reactive metal, so it needs electricity to extract it from its ore, bauxite.  The process is called electrolysis and requires a huge amount of electricity.  It is therefore expensive.  The process is summed up in the word equation:

aluminium oxide ®  aluminium + oxygen

Aluminium has properties that make it very useful:

• Easy to cut and shape;

• Forms strong alloys;

• Very low density;

• Good electrical and thermal conductor

• Resistant to corrosion

• Non magnetic.

There are lots of uses including:

• Drink cans;

• Car bodies;

• Saucepans

• Overhead electrical wires.

Aluminium can be left unpainted.  Many of the trains on the Underground railway in London had unpainted aluminium bodies (until they got covered in graffiti).

Aluminium is easily recycled.  Scrap aluminium melts easily and can be re-used.  This saves a lot of energy and reduces the waste sent to landfill.

Grade C

Aluminium oxide cannot be reduced using carbon, as aluminium is more reactive than carbon.  It is extracted using electrolysis in an electrolysis cell.

Bauxite has a high melting point but another ore called cryolite melts at a much lower temperature and can dissolve the bauxite, enabling the aluminium and oxygen ions to move about.

The aluminium ions go to the cathode:

aluminium ions + electrons ® aluminium atoms

At the anode we get:

oxide ions  ® oxygen + electrons

The oxygen almost immediately reacts with the graphite to form carbon dioxide.  The anode is eaten away, so needs to be replaced every so often.

Aluminium smelters use huge amounts of electricity.  A current of 100 000 amps will make about 8 grams of aluminium per second, about the mass of a drinks can.  They are often near a power station.

Aluminium atoms are less tightly packed than iron atoms, so the density is much less.  Density of aluminium is 2.7 g/cm³ while iron is 7.6 g/cm³.  It is quite soft when pure, but much stronger when alloyed with other metals like titanium.

Since aluminium is a very reactive metal, it may seem a surprise that aeroplanes and trains have unpainted bodies.  However the bare metal very rapidly acquires a thin oxide coat that binds to the surface very strongly.  This prevents further oxygen from getting in and there is no corrosion.

Steel forms a rust layer, which is flaky, allowing air to get at the bare metal and oxidising it.  Corrosion in steel can be rapid.

Extraction of bauxite can lead to serious damage to the environment:

• the open cast mines are found where there are rainforests;

• there can be a lot of pollution from badly run mines, for example, litter and waste oil;

• the miners bring in diseases that can kill local people who have no immunity to the diseases.

Grade A

The electrolysis of aluminium involves both oxidation and reduction.  Oxidation involves the loss of electrons; reduction involves the gain of electrons.

The aluminium oxide is reduced.  At the cathode, aluminium ions gain electrons to become aluminium metal.  This is summed up by the equation:

Al³⁺ + 3e^- ® Al

At the same time, at the anode, the oxide ions lose electrons:

2O^2- ® O₂ + 4e^-

The simultaneous reduction of the aluminium ions and the oxidation of the oxygen is an example of a redox reaction.

In the rainforests there are many undiscovered plants and animals.  Many plants give us useful products like medicines.  Therefore it's essential to protect them.  Recycling reduces the demand for bauxite.