In: Chemistry
Aluminium is the most abundant metal in the earth’s crust. Aluminium does not occur free in nature, but its compounds are numerous and widely distributed.
The chief and important ore from which aluminium is exclusively and profitably obtained is Bauxite, AI2O3.2H2O. The extraction of the metal from bauxite involves the three main steps.
•Ores of Aluminium
Name of Ore |
Formula of Ore |
Bauxite |
Al2O3.2H2O |
Cryolite |
Na3AlF6 |
Feldspar |
K2Oal2O3.6SiO2 or KalSi3O8 |
Mica |
K2O.3Al2O3.6SiO2.2H2O |
Corundum |
Al2O3 |
Aluminium is mainly extracted from bauxite ore.
•Extraction of Aluminium :
•Purification of Bauxite
By Bayer’s process comercially it is being carried out (for red bauxite not for the white bauxite).
Flow sheet of Bayer’s process for the preparation of pure Al2O3
•Hall’s process
Crude bauxite at 1100°C reacts with Na2CO3, little CaCO3 when CaSiO3, NaSiO2, NaFeO2 etc. form
Al2O3 + Na2CO3 → 2NaAlO2 + CO2
Fe2O3 + Na2CO3 → 2NaFeO2 + CO2
SiO2 + Na2CO3 → Na2SiO3 + CO2
CaO + SiO2 → CaSiO3
Then at 50° – 60°C CO2is passed through NaAlO2 solution and produces thereby Al(OH)3
2NaAlO2 + CO2 + 3H2O ¾® 2Al(OH)3¯ + Na2CO3
2Al(OH)3Al2O3 + 3H2O
•Serpeck’s Process
Bauxite containing high percentage of silica can be purified by Serpeck’s process. In this process finely powdered bauxite is mixedf with coke and the mixture is heated to 1800°C in a current of nitrogen. The AlN thus obtained is reacted with hot and dilute NaOH, produced NaAlO2 and excess AlN is hydrolysed and Al(OH)3 is formed.
Al2O3 + 3C + N2 → 3AlN + 3CO
SiO2 + 2C → Si + 2CO
AlN +NaOH → NaAlO2 + NH2+
NaAlO2 + 2H2O → Al(OH)3¯ + NaOH
AlN + 3H2O → Al(OH)3¯ + NH3
2Al(OH)3Al2O3 + 3H2O
•Electrolytic Reduction of Al2O3
Pure alumina melts at about 2000°C and is a bad conductor of electricity. If fused cryolite AlF3.3NaF and CaF2 (Fluorspar) is added the mixture melts at 900°C and Al2O3 becomes a good conductor of electricity. Metallic Al is liberated at the cathode
Alumina is mixed with cryolite (Na3AIF3), fluorspar (CaF2) in
the ratio 20 : 60 whereby, it not only becomes good conductor but
also fuses at about 900oC which is much below the b.p.
of aluminium.
The electrolysis of the fused mass is carried out in an iron box,
which lined with gas carbon. The lining serves as the cathode, the
anode consists of carbon rods dipped in the fused mass. The fused
electrolyte is kept covered with a layer of powdered coke to
prevent any action of air. The voltage employed in the electrolysis
is 5.3 volts. The current passed (about 50,000 amperes) serves to
purposes: (i) heating and (ii) electrolysis. Thus the fused mass is
automatically kept at 900oC during electrolysis.
Aluminium is obtained at the cathode and being heavier than the electrolyte sinks to the bottom and is tapped off periodically from the tap hole. Oxygen liberated at the anode attacks carbon rods and forms CO and CO2. During electrolysis the concentration of the electrolyte goes on falling thereby increasing the resistance of the cell which is indicated by the glowing of a lamp placed parallel. Much of the alumina is then added and the process is made continuous.
•Electrolysis of molten mixture
Cathode: Carbon
Anode: Graphite rods
Electrolyte: 60 parts cryolite + 20 parts fluorspar + 20 parts pure Al2O3
Temperature: 900°C
Reactions
According to the 1st theory the following reaction occurs
Al2O3 2Al+3 + 3O–2
At cathode : 2Al+3 + 6e → 2Al
At anode : 3O–2 – 6e → 3O2
As cryolite has greater electrochemical stability it does not dissociate. It only increases the dissociation of Al2O3
But the second theory states that, cryolite undergoes electrolytic dissociation first then Al+3 goes to the cathode, produced F2 at anode then reacts with Al2O3 produces AlF3.
AlF3.3NaFAl+3 + 3Na+ + 6F–
At cathode : Al+3 + 3e → Al
At anode : 6F– – 6e → 3F2
Overall Reaction : Al2O3 + 6F2 → 4AlF3 + 3O2
•Refining of Aluminium
The aluminium metal obtained by the electrolysis of fused almina is about 99.5% pure. It can be further refined by Hoope’s electrolytic process
Aluminium as produced by the electrolysis of AI2O3 is 90% pure. It can be refined further up to 99.9% purity by Hoope’s process.
The electrolytic cell consists of an iron tank lined with carbon. It is filled with three liquids differing in specific gravity. The upper layer is of pure fused aluminium and serves as cathode.
The bottom layer is that of impure metal in the fused state and serves as anode. The central layer is that of molten mixture of the fluorides of AI, Ba and Na and serves as an electrolyte
On passing electric current, pure aluminium goes to the top layer from the central layer and an equivalent amount of the metal from the bottom layer passes into the central layer. There is thus gradual transference of aluminium from bottom layer to the top and the impurities are left behind. Crude aluminium is added from time to time.