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 Al 2 O 3. x H 2 O Plus impurities of Iron (III) Oxide and Silicon Dioxide Fe 2 O 3 and SiO 2 These impurities need to be removed.

This is done in a number of steps. (1) Add concentrated NaOH  This reacts with SiO 2 (SiO 2 is a non-metal oxide and is acidic)  SiO 2 + 2NaOH  H 2 O + Na 2 SiO 3  Na 2 SiO 3, 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 Al 2 O 3 + 2NaOH  H 2 O+ 2NaAlO 2 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. NaAlO 2 + 2H 2 O  Al(OH) 3 + NaOH This can be filtered and heated to give pure Aluminium Oxide. 2Al(OH) 3  Al 2 O 3 + H 2 O

Add NaOH Bauxite with SiO2 and Fe2O 3 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°C  Aluminium 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 Na 3 AlF 6 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: Al e -  Al This is gain of electrons Hence reduction At the anode 2O 2-  O 2 + 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 4Al O 2-  4Al + 3O 2 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 + O 2  CO 2

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 withPropertiesUses CopperIncreased strengthCars Iron and siliconStrong and heat resistant Food packaging (foil!) Up to 12% siliconLower melting point Welding MagnesiumHard; corrosion resistant Marine work ZincHigh strengthAirplanes

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 -  O 2 + 2H 2 O + 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.