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In: Chemistry

Describe how aluminum metal is isolated from its natural ore, as well as the processes incolved...


Describe how aluminum metal is isolated from its natural ore, as well as the processes incolved in its purification


Solutions

Expert Solution

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.

  • Purification of Bauxite
  • Electrolytic reduction of Alumina, (AI2O3)
  • Purification of AI.

•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.


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