Aluminium (Extraction)
2nd Year Chemistry Notes
http://karachiboardnotes.blogspot.com/
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
Tags:
2nd Year Chemistry Notes