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 is Alumina + 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.
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.