Aluminum is mostly extracted from the bauxite ore by an electrolytic process. The process involves the following steps.
(i) Purification of Bauxite Ore
The Chief impurities present in bauxite are: Ferric Oxide (Fe2O3) and Silica (SiO2). These impurities can be removed from the bauxite ore by means of the following processes.
a) Baeyer’s process:
The finely divided bauxite ore is heated with concentrated solution of sodium hydroxide in an autoclave at about 150°C. The aluminium oxide present in the ore gets dissolved in NaOH solution forming sodium aluminates. The impuritieS being insoluble separate out and removed by filtrations.
Al2O3. 2H0 + 2 NaOH → 2 Na Al02 + 3H2O
The filtrate containing sodium aluminate solution is diluted with water, agitated and cooled. As a result a precipitate of Al(OH)3 is obtained.
Na AlO+2 2H2O → NaOH + Al(OH)3↓
The filtratetontaining sodium hydroxide is concentrated and used again.
b) Hall’s process
The finely divided bauxite ore is treated with concentrated solutionof Na2CO3 in an autoclave. Aluminium Oxide present in the ore dissolves in the solution as it is amphoteric. The impurities Fe2O3 and SiO2 are insoluble hence settles down and removed by filtration.
Al2O3. 2H2O+ Na2 CO3 → Na AlO2+2H2O + CO2.
The filtrate is warmed at a temperature of 50-60°C and a current of CO2 gas is passed. As a result, a precipitate of Aluminium hydroxide, Al(OH)3 is obtained.
2Na AlO2 + 3H2O + CO2 → Na2CO3 + 2AI (OH)3
This precipitate of Al(OH)3 is filleted, washed and dried.
c) Serpeck’s process
This process is used for the purification of bauxite ore containing silica (SiO2) as the main impurity. The powdered ore is mixed with coke and the mixture is heated at about 1800°C in the presence of Nitrogen gas, when aluminium nitride is formed.
Al2O3. 2H2O + 3C + N2 → 2AI N + 3CO+ 2H2O
Aluminium nitride thus obtained is hydrolysed with water to get a precipitate of Al(OH)3.
2Al N + 6H2O → 2NH3 + Al(OH)3
The precipitate of Al (OH)3 is filtered, washed and dried. The silica present as impurity in bauxite is reduced to silicon which being volatile at high temperature. is removed easily.
SiO2 + 2C → Si + 2CO
Calcinations or Conversion of Al(OH)3 to Al2O3
The dried precipitate of Al(OH)3 is ignited in a rotary kiln at about 1500°C. As a result, pure anhydrous aluminium oxide or alumina is obtained.
2Al(OH)3 → Al2O3 + 3H2O
ELECTROLYSIS OF ALUMINA
Aluminum is obtained from pure alumina by means of Hall’s process. In this process, alumina (Al2O3) is dissolved in fused cryolite (Na3AlF6) to get the molten electrolyte.
The electrolysis is carried out in a steel tank, internally lined with a layer of “graphite to act as cathode. The anode made up of carbon rods is suspended from above in the molten electrolyte. A low voltage must be used to avoid thedecomposition of the cryolite. The temperature is maintained at 950-1900°C by the passage of electric current.
The aluminum metal obtained is 99% pure. The trace impurities present are iron, silicon and Al2O3 which can be removed during the refining of Aluminium.
4. REFINING OF ALUMINIUM (HOOP’S PROCESS)
Aluminium is refined by an electrolytic method. The electrolysis is carried out in a carbon lined iron tank, which is filled with three molten layers of different specific gravity one over the other. The top layer consists of pure molten Aluminium and acts as cathode. The middle layer consists of fused mixture of fluorides of Sodium, Aluminium and Barium (Na3 AlF6 + Ba F2) and acts as the electrolyte. The bottom layer consists of molten impure aluminium containing the impurities. This layer acts as anode.
When electric current is passed, Al+3 ions from the middle layer go to the top layer and are deposited as pure Aluminium. At the same time an equivalent amount of Al from the bottom layer passes into the middle layer. The impurities remain in the bottom layer as they do not dissolve in the electrolyte. The pure aluminium containing 99.99% Al is removed from time to time from the top layer whereas impure aluminium is added into bottom layer.