· General overview
Aluminium is remarkable for the metal's low density and for its ability to resist corrosion. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials.
The usual aluminium ore is bauxite. Bauxite is essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.
Aluminium is extracted by electrolysis. The aluminium oxide has too high a melting point to electrolyse on its own. The ore is first converted into pure aluminium oxide by the Bayer Process, and then electrolysed in solution to get aluminium. To purify it further, Hoope’s method is used.
This is the general process of how aluminium is extracted from bauxite:
 |
Bauxite
--------------------------> |
 |
| Alumina (Aluminium oxide) -----------------------> |
Let’s now go deeper into each stage of the extraction process.
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| Aluminium |
· Method of extraction
1) Purification of bauxite (aluminium oxide) by Baeyer's process
2) Electrolysis of alumina
3) Hoope’s electrolytic method
· How the metal is extracted from its ore
Purification of bauxite (Baeyer's process)
1) Reaction with sodium hydroxide solution
Al2O3 + 2NaOH + 3H2O -------> 2NaAl(OH)4
2) Precipitation of aluminium hydroxide
NaAl(OH) 4 -------> Al(OH)3 + NaOH
3) Formation of pure aluminium oxide(also known as alumina)
2Al(OH)3 --------> Al2O3 + 3H2O
- Bauxite ore is heated with concentrated NaOH solution under pressure -- purified by leaching method. (Temp.: ~ 140°C to 240°C; Pressure: up to about 35 atmospheres)
- With hot concentrated NaOH solution, Al2O3 reacts to give a solution of sodium tetrahydroxoaluminate.
- The solution is filtered from sodium tetrahydroxoaluminate solution as solids remaining are impurities
- The solution is cooled, and sowed with some previously produced Al(OH)3 to precipitate the new Al(OH)3
- The precipitated Al(OH)3 is dried and ignited to get pure alumina, Al2O3. (Temp.: ~ 1100 - 1200°C)
Electrolysis of alumina (Converting Al2O3 into Al)
1) Al2O3 -------> 2Al3+ + 3O2-
2) Formation of Aluminium at cathode
Al3+ + 3e- -------> Al
3) Initial production of oxygen at anode
2O2- -------> O2 + 4e-
- The alumina’s melting point is lowered by dissolving it in a solution of molten cryolite, Na3AlF6 (Cryolite is another aluminium ore).
- It is then electrolysed in a rectangular steel tank with carbon lining and a set of thick carbon rods suspended from the top into the fused Al2O3.
- Aluminium forms and collects at the cathode, where it is removed periodically.
- Cathode: carbon lining, molten aluminium that forms on the bottom of the cell
Anode: carbon rods
(Voltage: low, ~ 5-6 volts; Current: high, at least 100,000 ampereàheat keeps temp.: ~ 1000°C.)
Anode: carbon rods
(Voltage: low, ~ 5-6 volts; Current: high, at least 100,000 ampereàheat keeps temp.: ~ 1000°C.)
- Oxygen is evolved at the anode. However, given the temperature of the cell, the carbon anodes will burn in oxygen to give carbon dioxide and carbon monoxide.
- The metal obtained by this method is about 99% pure and contains impurities of Fe, Si and Al2O3. Further purification is carried out by Hoope's electrolytic method.
(Additional): Hoope’s electrolytic refining of aluminium
1) Na3AlF6 -------> 3NaF + AlF3
AlF3 -------> Al+3 + 3F-
2) Formation of pure Aluminium at cathode
Al3+ + 3e- -------> Al
3) At anode
Al -------> Al+3 + 3e-
4) Overall reaction
Al+3 + Al -------> Al + Al+3
- This carried out in Hoope's cell. The cell consists of iron tank having a lining of carbon. It has three layers of molten liquids with different densities.
- Bottom layer: alloy of molten impure aluminium and copper (anode)
Middle layer: solution of cryolite and barium fluoride (electrolyte)
Top layer: molten pure aluminium (suspends carbon electrodes which act as the cathode, along with carbon lining)
Middle layer: solution of cryolite and barium fluoride (electrolyte)
Top layer: molten pure aluminium (suspends carbon electrodes which act as the cathode, along with carbon lining)
- When electric current is passed, the Al3+ ions from the middle layer move to the top layer, gaining 3e- at the cathode to become pure aluminium. At the same time, an equal number of Al3+ ions from the bottom layer moves up to the middle layer, leaving behind the impurities.
- Pure aluminum is removed from the tapping hole from time to time. Hoope’s method gives 99.99% pure aluminium.
Acknowlegements
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