XII Chemistry BIEK Topic - Aluminium (Extraction)

Aluminium (Extraction)
Extraction of Aluminium
Aluminium does not occur free in nature, but it widely distributed in combine state. Aluminium is mainly extracted from Bauxite ore (Al2O3.nH2O). The extraction of Aluminium from Bauxite proceeds through following two steps.

1. Purification of Bauxite to Alumina.

2. Electrolysis of Pure Bauxite (Alumina)


1. Purification of Bauxite

Bauxite contains iron oxide (Fe2O3) and Silica (SiO2) as chief impurites. These impurities must be removed from Bauxite, because they make the aluminium brittle and liable to corrosion. Bauxite ore may be purified by the following methods.


a. Hall's Method

This method is used for the purification of Bauxite containing Fe2O3 and SiO2 as impurities. The finely divided Bauxite is fused with sodium carbonate (Na2CO3), as a result sodium aluminate is formed while the impurities are left unaffected.

Al2O3.nH2O + Na2CO3 ----> 2NaAlO2 + CO2 + nH2O


The fused mass is rapidly extracted with water leaving behind both the impurites Sodium aluminate is heated up to 50ºC - 60ºC in the presence of CO2. As a result while glatinous precipitates of aluminium hydroxide are formed.


The precipitates of Al(OH)3 are separated, washed to remove Na2CO3, dried and ignited about 1500ºC to get pure bauxite (alumina)


b. Bayer's Method

This method is used for the purification of Bauxite containing excess of Fe2O3 as impurity. The finely divided Bauxite is treated with concentrated solution of Sodium Hydroxide. As a result soluble sodium Aluminate is formed, while the impurities are removed by filtering the solution.

Al2O3.nH2O + NaOH ----> 2NaAlO2 + 2(n)H2O

Soluble sodium Aluminate is treated with excess water to form white gatinous precipitates of Aluminium Hydroxide.

2NaAlO2 + 4H2O ----> 2Al(OH)3 + 2NaOH

The precipitates of Al(OH)3 are separated, washed to remove NaOH, dried and ignited about 1500ºC to get pure bauxite (alumina)


c. Serpek's Method

This method is used for the impurities of Bauxite ore containing excess of SiO2 as impurity. The finely divided Bauxite is mixed with carbon and heated up to 1800ºC in the current of Nitrogen. As a result Aluminium Nitride is formed.


2. Electrolysis of Pure Bauxite (Alumina)

The electrolysis of pure bauxite is carried out in a steel tank lined with carbon (graphite). The carbon lining serve as cathode. The anode consist of carbon rods hanging in the molten mass.

In fused state pure alumina is bad conductor of electricity and its melting point is about 2050ºC. So flourspar and cryolite is added in alumina to increase the fluidity of the melt and lower the melting point respectively. When electric current is passed through this mixture, the aluminium is obtained at cathode in liquid state. It sink to bottom from where it drawn, periodically through the tapping hole.

Aluminium produced by this method is 98% and contains traces of Fe, Si and Al2O3 etc.




Refining of Aluminium
The molten aluminium so obtained contains 2% impurities which is further purified by electrolysis in hoop's cell.




Construction
The Hoop's cell consist of an iron box lined with carbon (graphite), the carbon lining served as anode. This iron box consist of three layers, the upper most layer is of pure aluminium the middle layer molten flourides (AlF3, BaF2 and NaF) act as electrolyte, where as the lower layer consist of impure aluminium. The cathode consist of carbon rods hanging in pure aluminium.




Working
When electric current is passed through the impure aluminium goes into the middle layer as all leaving impurities and the pure aluminium is deposited at the top of molten flourides at cathode. The aluminium layer grows and drawn off time to time from tapping hole. The refined Aluminium so obtained is 99.99% pure.