Aluminium
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ALUMINIUM
OCCURENCE
Aluminium is the third most abundant element in the earth’s crust (after oxygen and silicon). It forms nearly 7.28% of the earth’s crust. It occurs as aluminosilicate minerals. It is also an important constituent of clay, slate and many types of silicate rocks. Weathering of clay yields bauxite, the principal ore of aluminium (Al203.H2O)
Deposits of bauxite occur throughout the world but are mainly found in tropical and subtropical regions. Another ore of aluminium is Corundum. Corundum is a hard mineral of aluminium oxide, Al2O3. The pure oxide is colourless, but the presence of impurities can give various colours to it. Sapphire(usually blue) and ruby (deep red) are gems.
EXTRACTION
The ore of aluminium is bauxite, Al2O3. 2H2O. It contains 2 chief impurities Ferric oxide (Fe2O3) and Silica (SiO2). These impurities are removed according to the following steps
PURIFICATION OF BAUXITE
1. HALL’S PROCESS
Bauxite is purified mainly by the Hall’s process. In this Process the ore is fused with sodium carbonate. Aluminium oxide being amphoteric dissolves in it to form sodium meta-aluminate while the insoluble iron oxide and silica are left as residue:
Al2O3. 2H2O + Na2CO3 è 2NaAlO2 + 2H2O + CO2
The fused mass is extracted with water, the solution is heated to 50OC-60OC and a current of carbon dioxide is passed through when aluminium hydroxide precipitates
2NaAlO2 + 3H2O + CO2 è 2Al (OH)3 + Na2CO3
The precipitate is washed, dried and ignited to obtain pure alumina.
2Al (OH)3 è Al2O3 + 3H2O
2. BAEYER’S PROCESS
Baeyer’s method is especially used for the red variety of bauxite, that contains ferric oxide as the chief impurity. In the Baeyer’s method powdered bauxite first roasted to convert any ferrous oxide to ferric oxide. Then it is digested with concentrated solution of an alkali solution in an autoclave at 150OC, when the alumina present in bauxite dissolves, leaving behind the impurities
Al2O3.2H2O + 2NaOH è 2NaAlO2 + 3H2O
The mass is filtered and the filtrate is agitated with some freshly precipitated Al(OH)3. As a result of this the sodium meta-aluminate present in solution is hydrolysed to Al (OH)3
NaAlO2 + 2H2O è NaOH + Al (OH)3
Al(OH)3 is filtered off and washed. The filtrate containing caustic soda is concentrated and used again.
3. SERPECK’S PROCESS
The Serpek’s Process is especially implied for bauxite containing a lot of silica as an impurity. In this process the ore is heated with carbon and nitrogen gas passed over the mixture. The aluminium nitride formed is hydrolysed by water :
Al2O3.2H2O + 3C + N2 è 2AlN + 3CO + 2H2O
AlN + 3H2O è Al (OH)3 + NH3
The residue consisting of Al (OH)3 is ignited to produce pure alumina
Al (OH)3 è Al2O3+ 3H2O
ELECTROLYSIS OF ALUMINA
Because of its high melting point pure alumina cannot be hydrolysed, besides it is a bad conductor of electricity. In order to overcome this problem alumina is dissolved in molten cryolite (Na3AlF6) and calcium Fluoride, in order to lower its melting point. It is then electrolysed in a steel cell lined with graphite (which acts as the cathode) using carbon or copper anode.
4AlF3( from cryolite) è 4Al3+ + 12 F-
at the cathode
4Al3+ + 12e è 4Al
At the anode
12F- è 6F2 + 12e
Uses:
- As strong alloys especially in the aircraft industry.
One of the most important alloys of aluminium is duralumin ( consisting of Al, Cu, Mg). It is used in the framework of airplanes and dirigibles. Light alloys are aluminium bronze (Al, Cu) and magnalium (Mg, Al). All metal transport and military planes are made of aluminium alloys nowadays.
- Electrical cables and insulators.
- As a packaging material.
- House-hold utensils.
