EXTRACTIVE METALLURGY OF ALUMINIUM
Aluminium is the metal found most abundantly in the Earth’s crust. Since it is a reactive metal, it occurs in the combined state. The important ores of aluminium are as follows
Bauxite is the chief ore of aluminium. The extraction of aluminium from bauxite involves two steps:
The conversion of Bauxite into Alumina involves the following steps:
Bauxite ore is finely ground and heated under pressure with a solution of concentrated caustic soda solution at 150° C to obtain sodium meta aluminate.
On diluting sodium meta aluminate with water, a precipitate of aluminium hydroxide is formed.
The precipitate is filtered, washed, dried and ignited at 1000°C to get alumina.
Aluminium is produced by the electrolytic reduction of fused alumina (Al2O3) in the electrolytic cell.
Cathode: Iron tank linked with graphite
Anode: A bunch of graphite rods suspended in molten electrolyte.
Electrolyte: Pure alumina+ molten cryolite + fluorspar (fluorspar lowers the fusion temperature of electrolyte)
Temperature: 900 - 950 °C
Voltage used: 5-6 V
Overall reaction: 2 Al2O3 → 4 Al +3 O2↑
Aluminium is deposited at the cathode and oxygen gas is liberated at the anode. Oxygen combines with graphite to form CO2.
· It is a silvery white metal
· It has low density (2.7) and it is light
· It is malleable and ductile
· It is a good conductor of heat and electricity.
· Its melting point is 660 °C.
· It can be polished to produce a shiny attractive appearance.
i. Reaction with air: It is not affected by dry air. On heating at 800 °C, aluminium burns very brightly forming it’s oxide and nitride.
4 Al + 3 O2 → 2 Al2O3(Aluminium oxide)
2 Al + N2 → 2 AlN (Aluminium nitride)
ii. Reaction with water: Water does not react with aluminium due to the layer of oxide on it. When steam is passed over red hot aluminium, hydrogen is produced.
2 Al + 3 H2O → Al2O3 + 3 H2↑
iii. Reaction with alkalis: It reacts with strong caustic alkalis forming aluminates.
2 Al + 2 NaOH +2 H2O → 2 NaAlO2 + 3 H2↑ (Sodium meta aluminate)
iv. Reaction with acids:
With dilute and con. HCl it liberates H2 gas.
2 Al + 6 HCl → 2 AlCl3 + 3 H2↑
Aluminium liberates hydrogen on reaction with dilute sulphuric acid. Sulphur dioxide is liberated with hot concentrated sulphuric acid
2 Al+ 3 H2SO4 → Al2(SO4)3 + 3 H2
2 Al + 6 H2SO4 → Al2(SO4)3 + 6 H2O + 3 SO2 ↑
v. As reducing agent: Aluminium is a powerful reducing agent. When a mixture of aluminium powder and iron oxide is ignited, the latter is reduced to metal. This process is known as aluminothermic process.
Fe2O3 + 2 Al → 2 Fe + Al2O3 + Heat.
Aluminium is used in
· household utensils
· electrical cable industry
· making aeroplanes and other industrial mechine parts