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.
0 comments :
Post a Comment