Login

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:

  • 43
View Full Answer

The extraction of aluminium involves two steps:-

1. Purification of bauxite by Baeyer's process.

 2. Electrolysis of alumina.

1. Purification of bauxite by Baeyer's process:-

In the Baeyer's process, the bauxite ore is heated with concentrated NaOH solution under pressure (Aluminum is purified by leaching method). The alumina dissolves as sodium meta aluminate. The other materials present in the ore are left as insoluble part. This solution is filtered in ore are left as insoluble part. This solution is filtered off. From the solution Al(OH)3 is precipitated by adding freshly prepared Al(OH)3 to the cold dilute solution and agitating.


Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2H2O -------> NaOH + Al(OH)3


The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O

2. Electrolysis of alumina:-


The alumina is dissolved in a mixture of molten cryolite and fluorspar which lowers the melting point. It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused Al2O3. The temperature is maintained between 1200 and 1310K . Oxygen is evolved at the anode.


Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
The metal obtained by this method is about 99% pure. Further purification is carried out by Hoop's electrolytic method. The electrolytic refining of aluminum is carried out in Hoop's cell. The cell consist of iron tank having a lining of carbon. It has three layers of molten liquids with different densities. Molten impure aluminum in the bottom layer which along with the carbon lining acts as the cathode. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum in the molten fluorides of sodium, barium and aluminum in the molten state. This act as the electrolyte. Top layer consist of molten pure aluminum in which a number of carbon electrodes are suspended. These carbon electrodes act as the cathode.When electric current is passed, the aluminium ions from the middle layer move to the top layer and are discharged at the cathode as pure aluminium. At the same time, an equivalent amount of aluminum from the bottom layer migrates to the middle layer leaving behind the impurities. The pure aluminum is removed from the tapping hole from time to time.

 


 

  • 16
What are you looking for?