What are the steps involved in the extraction of aluminium frm bauxite???
The extraction of aluminium from bauxite basically involves two main steps :
1) Concentration of bauxite by leaching.
2) Electrolytic reduction of purified Al2O3.
Leaching of Alumina from bauxite
Leaching of alumina is done by the Bayer's process. The principal ore of aluminium, bauxite, usually contains SiO2, iron oxides and titanium oxide (TiO2) as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This way, Al2O3 is leached out as sodium aluminate (and SiO2 too as sodium silicate) leaving the impurities behind. The solution of sodium aluminate thus obtained is filttered and cooled and its pH is adjusted downward by dilution and neutralization with carbon dioxide. This precipitates hydrated Al2O3. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces the precipitation. The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3. Following equations summarize the above steps
Electrolysis of alumina
In the electrolytic reduction of alumina, purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mix and brings conductivity. The fused matrix is electrolysed. Steel cathode and graphite anode are used. The graphite anode is useful here for reduction to the metal. The overall reaction may be taken as :
The oxygen liberated at the anode reacts with the carbon of the anode to produce CO and CO2. The cell reactions are as follows:
At Cathode:
At Anode: