Aluminum

Assessment statement

• Describe and explain the production of aluminum by electrolysis of alumina in molten cyrolite. (Include the use of cyrolite as a solvent because of the very high melting point of Al2O3 and the type of materials used for the construction of the cell and choice of electrodes.)

Production of Aluminum

• Aluminum is primarily made by the electrolytic reduction of aluminum oxide.

• Aluminum oxide or aluminum does not occur naturally.

• The main ore of aluminum is bauxite.• Bauxite is in the form of mainly

aluminum hydroxide Al(OH)3, and other impurities such as iron(III) oxide and titanium hydroxide, and other few oxides.

Bayer process

• Bayer process is the process of extracting aluminum oxide from bauxite.

• Bauxite is powdered prior to the process.

• As aluminum oxide is amphoteric, it dissolves in sodium hydroxide solution.

• Al(OH)3(s) + NaOH(aq) NaAlO2(aq) + 2H2O(l)

• The aluminate solution is filtered leaving behind the impurities.

• Seeding with aluminum hydroxide then reverses the reaction.

• The pure recrystallized aluminum hydroxide is then heated to produce aluminum oxide (alumina).

• 2Al(OH)3 Al2O3(s) + 3H2O(l)

Use of cryolite

• To make the electrolysis happen, alumina has to be in aqueous state.

• Alumina has a very high melting point of 2045°C.

• Cryolite (sodium hexafluoroaluminate(III), Na3AlF6) melts at about 950°C and the solution can dissolve alumina sufficiently well.

• Considerable energy is still required to heat cryolite and dissolve alumina, but it is much more energy-efficient than melting the alumina itself.

• Natural cryolite is depleted long ago, thus synthetic cryolite is needed.

• The aluminate solution (NaAlO2(aq)) from Bayer process is used in separate process to make synthetic cryolite.

• Hydrogen fluoride and sodium carbonate is added to the solution to precipitate cyrolite.

• NaAlO2(aq) + 6HF(g) + Na2CO3(aq) Na3AlF6(s) +3H2O(l) + CO2(g)

Hall-Heroult process

• Hall-Heroult process is an electrolysis process that extracts aluminum out of alumina (aluminum oxide).

• Overall cell reaction isAlumina + carbon = aluminum + carbon dioxide

Assessment statement

• Describe the main properties and uses of aluminum and its alloys.

Properties of aluminum

• Malleable and can be shaped easily• Excellent conductor of heat and electricity• Has much a lower density than iron, yet

can form alloys that are stronger than steel

• Resistant to corrosion due to its protective layer of aluminum oxide

• The thickness of the oxide layer can be further increased by a process known as anodizing.

Uses of aluminumUse Examples

Transport Superstructures of trains, ships and airplanes. Alloy engines for cars.

Construction Window frames, doors roofing

Power transmission

Overhead electricity cables, capacitor foil

Kitchen utensils Kettles, saucepans

Packaging Drink cans, foil wrapping

Chemical industry Al(OH)3 – flame retarder, paper makingAl2(SO4)3 – flocculant in sweage treatment and to precipitate PO43-Al2O3 – catalyst and catalytic support material, abrasive

Assessment statement

• Discuss the environmental impact of iron and aluminum production.

Environmental impact

• Smelting processes of aluminum requires enormous amount of electricity.

• Also, the main process which is the electrolysis emits carbon dioxide which is greenhouse gas.

• Recycling aluminum is an important method of saving energy and minimizing the environmental damage.

• Recycling aluminum requires only 5% of the energy to produce the same amount of aluminum from bauxite.