Aluminium is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic.
Instead, it is extracted by electrolysis. The ore is first converted into pure aluminium oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminium compound. The aluminium oxide has too high a melting point to electrolyse on its own.
The usual aluminium ore is bauxite. Bauxite (Al2O3)i s essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.
The usual aluminium ore is bauxite. Bauxite (Al2O3)i s essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.
Firstly, we purify the bauxite by Baeyer's process
1.The bauxite ore is first crushed and heated in a concentrated solution of sodium hydroxide(NaOH) under high pressure.The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.
2.Sodium aluminate is produced as it is dissolved from the ore. The solution is then filtered to remove any insoluble products, namely other elements aside from aluminum.
3.From this filtrate, pure aluminum hydroxide is precipitated by adding freshly prepared aluminum hydroxide to it.
4.The precipitated aluminium hydroxide is then dried and ignited to obtain pure alumina(Al2O3)(the oxide of aluminum in its original state).
Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2h2o -------> NaOH + Al(OH)3
The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O
Then, we carry out the Electrolysis of Alumina
1.Firstly, we dissolve the alumina in a mixture of molten cryolite(Na3AlF6) and fluorspar(CaF2) in order to lower the melting point of the alumina.
2.It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused alumina. The temperature is maintained between 1200 and 1310kelvin(926.85 to 1036.85degrees celsius). Oxygen is produced at the anode.
Al2O3 -------> 2Al3+ + 3O2-
At cathode: 2Al3+ + 6 electron -------> 2Al
At cathode: 3O2- --------> 1.5 O2 + 6 electrons
3.The aluminum formed at the cathode gets collected at the bottom of the electric cell where it is collected from time to time.(The metal is 99% pure)
More about the electrolysis cell : The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.
Further purification by Hoop's electrolytic method
1.The electrolytic refining of aluminum is carried out in Hoop's cell, consisting of an iron tank with a lining of carbon and three layers of molten liquids with different densities.
2.The bottom layer would be the impure aluminum. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum, acting as the electrolyte. The top layer consists of pure aluminum in which carbon electrodes are suspended. These carbon electrodes act as the cathode.
3.When an electric current passes through, the aluminum ions are discharged at the cathode as pure aluminum. The impurities are then left behind.
RESOURCES:
http://entrancechemistry.blogspot.sg/2011/05/extraction-of-aluminium-from-bauxite.html
VIDEO LINKS:
http://www.youtube.com/watch?v=IG7NSDG1mGQ
http://www.youtube.com/watch?v=EK4dEEoKNiE
http://www.youtube.com/watch?v=fa6KEwWY9HU&feature=related
Environmental Impact
1. The process of converting bauxite into pure aluminum is incredibly energy consuming, requiring a lot of resources like electricity and water to produce.
2.The release of per fluorocarbons during the heating and melting process are 9200 times more harmful than carbon dioxide, trapping more heat in our atmosphere, leading to more global warming.
3.There will be a loss of wildlife habitats as all vegetation within mining areas will be removed.(This causes soil erosion as well)
4.The red sludge and toxic mine chemicals are deposited into excavated mine pits where they end up seeping into the ground, contaminating water sources.
5.Air quality is also affected as particulates such as combustion byproducts, caustic aerosols, dust from bauxite, limestone, charred lime, alumina and sodium salt are released during the processing of bauxite.
Economic Impact
1. The high cost of the process because of the huge amounts of electricity it uses. This is so high because to produce 1 mole of aluminium which only weighs 27 g you need 3 moles of electrons. You are having to add a lot of electrons (because of the high charge on the ion) to produce a small mass of aluminium (because of its low relative atomic mass).
2. Energy and material costs in constantly replacing the anodes.
Uses of Aluminium
Aluminium is usually alloyed with other elements such as silicon, copper or magnesium. Pure aluminium isn't very strong, and alloying it adds to it strength.
Aluminium is especially useful because it
- has a low density;
- is strong when alloyed;
- is a good conductor of electricity;
- has a good appearance;
- resists corrosion because of the strong thin layer of aluminium oxide on its surface. This layer can be strengthened further by anodising the aluminium.
Aluminium can be used to build aircrafts because it is light, strong and resists corrosion (due to tiny meteorites and dust particles hitting the surface). It can also be used to make power cables since aluminium is a good conductor of electricity. Saucepans are also made with aluminium since it is a good conductor of heat.
RESOURCES: http://1800recycling.com/2010/11/aluminum-extraction-recycling-environment/http://www.chemguide.co.uk/inorganic/extraction/aluminium.html
VIDEO LINKS:
http://www.youtube.com/watch?v=OvPWvg8XJvE
PICTURES
http://www.ntnews.com.au/images/uploadedfiles/editorial/pictures/2007/11/13/23mines.jpg
