Extraction & Refining
Of Aluminium
ALUMINIUM
Symbol: Al
Relative atomic mass : 27
Colour: Silvery white metal Specific gravity: 2.2
Atomic number: 13
Electronic configuration: 2,8,3
Position in Periodic Table: Period 3, Group IL4(13)
Valency: 3
- Aluminium is the most abundant metal in the earth crust.
- Aluminium is a reactive metal and so does not occur free in nature.
- Aluminium was discovered by Oersted in 1825.
Extraction of Aluminium
Hall and Héroulr, in 1885, developed the process for the extraction of aluminium. Aluminium is extracted from its main ore bauxite Al2O3.2H2O. Bauxite contains 60% Al2O3, the rest being sand, ferric oxide and titanium oxide.
It is concentrated or purified by chemical method
Purification of bauxite ore (conversion of bauxite into alumina)
(Bayer's process)
Bauxite is ground finely and heated to remove volatile impurities. It is then heated under pressure with concentrate. caustic soda solution for 2 to 8 hours at 140 degrees*C to 50 degrees*C (leaching).
Al2O3.2H2O+2NaOH ⟶ 2Na AlO2+3H2O
Bauxite dissolves and forms sodium meta aluminate because of the amphoteric nature of aluminium. leaving behind insoluble impurities called red mud. Red mud consists of ferric oxide. sand, etc., which are removed by filtration.
On diluting sodium meta aluminate with water and cooling to 50 degrees * C it gets hydrolysed to give aluminium hydroxide as a precipitate. The other soluble impurities remain dissolved in sodium hydroxide.
NaAlO₂ + 2H₂O ⟶ NaOH + Al(OH)3
The precipitate is filtered, washed, dried and ignited at 1000 degrees * C to get alumina (Al₂O3).
The filtrate containing sodium hydroxide is again used to extract aluminium from bauxite,
Aluminium oxide due to its great affinity for oxygen is a very stable compound. It is not reduced easily by common reducing agents like carbon, carbo monoxide or hydrogen. Hence, the electrolytic reduction was chosen as the method for reducing alumina.
Difficulty faced in obtaining aluminium from alumina
The electrolytic reduction can be done in a molten state only. Alumina melts at approximately 2050°C. At this temperature, aluminium vaporises, so it was not easy to collect aluminium. This difficulty was removed by Hall who found a suitable solvent in another ore of aluminium, cryolite (Na3AlF6). When he applied the current to the molten mixture of alumina (20%), cryolite (60%) and calcium fluoride (20%), he saw some shining globules of aluminium appearing at the cathode. He got elated over the success of his three years' concerted efforts and immediately ran excitedly to his professor and presented to him the few small pieces of the metal he had collected. It was February 23, 1886. Hall's discovery revolutionised the aluminium industry. Aluminium which was once a costlier and rarer metal than gold became a cheap metal of common use in every home.
Electrolytic reduction of fused aluming (Hall-Héroult's process)
Electrolytic cell: Rectangular iron tank with a sloping bottom. The sloping bottom of the tank facilitates the removal of the molten aluminium. The tank is lined with gas carbon. Electrolyte: It is a mixture of molten alumina 20%, cryolite 60% and fluorspar 20%. Powdered coke is sprinkled over the surface of the electrolyte. Temperature: The molten electrolyte is kept at 950°C by electrical heating. The voltage used: is 5 to 6 volts. A low voltage is used to avoid the decomposition of the molten cryolite.
Reactions Cryolite : Na3AlF 6 ⇌ Na+ + +Al^3＋ +6F^ -
Fluorspar : CaF₂ ⇌Ca²+ + 2F
Alumina :Al2O3⇌2Al^3+ +30^2-
Positive ions formed in the solution are Na+, Ca^2+ and Al^ 3+ while negative ions formed in the solution are F, O^2-. Aluminium being lower in the electrochemical series is preferentially discharged at the cathode while oxygen is at the anode.
Cathode: (Inner carbon lining of the (electrolytic cell)
4Al^ 3+ +12e- ⟶ ^4Al
Anode: Thick rods of graphite are suspended into the fused electrolyte.
6O^ 2- -12e- ⟶ 6[O]
3O+ 3O⟶ 3O2
An anode is oxidised to carbon monoxide, which further forms carbon dioxide.
2C+O2⟶2CO
2CO+O2⟶2CO2
The anode has got to be replaced from time to time, as it gets oxidised by the oxygen evolved at the anode.
When the supply of alumina decreases, the voltage suddenly jumps from 40 to 60 volts. If a bulb is attached to the apparatus, it will glow brightly. Then fresh alumina is added. The aluminium obtained is 99.8% pure.
Further purification can be done by electrolysis.
Note:
- Cryolite: Lowers the fusion temperature from 2050 degrees * C to 950 degrees * C and enhances conductivity.
- Fluorspar and cryolite (i) act as a solvent for the electrolytic mixture. (ii)increases its conductivity since pure alumina is almost a non-conductor of electricity.
- Powdered coke is sprinkled over the surface of the electrolytic mixture. It : (i) reduces heat loss by radiation. (ii) prevents the burning of the anode.
Anode : Al - 3e- ⟶ Al3+Cathode : Al3+ + 3e- ⟶ Al
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