Chemistry in Industry and Technology

Option C

Aluminium

Syllabus Statements• C.1.8 Describe and explain the production of

aluminium by electrolysis of alumina in molten cryolite

• C1.9 Describe the main properties and uses of aluminium and its alloys.

• C.1.10 Discuss the environmental impact of iron and aluminium production.

• Aluminium is mined as Bauxite• This is Al2O3. x H2O• Plus impurities of Iron (III) Oxide and Silicon

Dioxide • Fe2O3 and SiO2

• These impurities need to be removed.

• This is done in a number of steps.• (1) Add concentrated NaOHThis reacts with SiO2 (SiO2 is a non-metal oxide

and is acidic)SiO2 + 2NaOH H2O + Na2SiO3

Na2SiO3 , Sodium Silicate is soluble and dissolves.

How about the Iron (III) oxide?

• The Iron (III) Oxide is basic (metal oxide)• It doesn’t react and so remains as a solid.

• How about the Aluminium Oxide?

• Aluminium Oxide is amphoteric• Al2O3 + 2NaOH H2O+ 2NaAlO2

• The Sodium Aluminate is soluble

• Now the only solid present is Iron Oxide. This can be filtered out

• If the solution is diluted, the Aluminate precipitates out as a Hydroxide.

• NaAlO2 + 2H2O Al(OH)3 + NaOH

• This can be filtered and heated to give pure Aluminium Oxide.

• 2Al(OH)3 Al2O3 + H2O

Add NaO

H

• Bauxite with SiO2 and Fe2O3 impurities

Filter

• Fe2O3 is insoluble• Everything else reacts and dissolves

Dilute

• Mixture of sodium silicate and sodium aluminate

Filter and heat

• Aluminium Hydroxide precipitates out

• Aluminium is higher in the reactivity series than iron and Aluminium Oxide can’t be reduced using CO or C.

• It is produced by electrolysis of the molten ore

• Why can’t we just dissolve it in water and electrolyse the solution?

• We would get Hydrogen produced instead of Aluminium!

• There are two problems with this:The melting point of aluminium oxide is >2000°CAluminium Oxide has a high degree of covalent

character and so it doesn’t conduct very well even when it is molten.

• To get round these problems, the Aluminium Oxide is dissolved in molten cryolite – the mineral Na3AlF6

• This lowers the melting point to ≈900°C and increases the conductivity.

Production of aluminium

• Note:• Anode and cathode are both carbon – it can

withstand high temperatures• The aluminium can be tapped as it is formed.

This is a continuous process• The electrolyte is maintained at a high

temperature by the current passing through it

• Now write half equations for the anode and cathode

• At the cathode:• Al3+ + 3e- Al• This is gain of electrons • Hence reduction

• At the anode• 2O2- O2 + 4e-

• This is loss of electrons• Hence oxidation

• To complete the overall equation, we have to balance the number of electrons in the half equations.

• In this case 12 electrons in each half equation

• 4Al3+ + 6O2- 4Al + 3O2

• The oxygen produced at the anode reacts with the carbon and gradually wears it away. The anode has to be regularly replaced.

• Each tonne of aluminium produced uses ½ tonne of carbon!

• C + O2 CO2

Uses of Aluminium

• Aluminium is lightweight, corrosion resistant and malleable. It has high electrical and thermal conductivity.

• It is used in aircraft bodies; overhead electrical cables; cooking pans; food packaging

• It is also used in alloys:Alloyed with Properties Uses

Copper Increased strength Cars

Iron and silicon Strong and heat resistant

Food packaging (foil!)

Up to 12% silicon Lower melting point

Welding

Magnesium Hard; corrosion resistant

Marine work

Zinc High strength Airplanes

• Aluminium is higher in the reactivity series than iron is.

• So why is it corrosion resistant?• It is coated with an oxide layer• This layer is impermeable to oxygen and water• (or at least nearly impermeable)• So it protects the aluminium from further

attack.• Compare this with iron oxide (rust!)

• Anodised aluminium

• We can protect aluminium even more by artificially thickening the oxide layer.

• This is done by anodising the aluminium.• The aluminium is made the anode during the

electrolysis of dilute sulphuric acid. • Oxygen is produced and this reacts with the

aluminium to make a thicker oxide layer.• What’s the half equation for the reaction at

the anode?

• 4OH- O2 + 2H2O + 4e-

• Mp3 players, flashlights, cookware, cameras, sporting goods, window frames, roofs

• If coloured dyes are present during anodising, these are incorporated into the oxide layer.

• This gives a permanent coloured finish

The Environmental Impact of Iron and aluminium Production

• Some points to consider:

• All the processes mentioned use a great deal of energy. This is usually from fossil fuels.

• One ton of coal is needed to produce one ton of iron.

• Ten times more energy is needed to produce a ton of Al than a ton of Fe.

• The ores used have to be mined.• This may have a severe environmental impact

depending on the location and type of mining.

• Waste products of mining can be visually unattractive and can produce environmental damage.

• Similarly waste products from purification can be damaging.

• Recycling Al is time consuming and labour intensive.

• Recycling uses only 5% as much energy as producing Al from its ore.

• Only about half the Al produced is recycled.• Much less iron and steel is recycled.