Extractive Metallurgy of Aluminium

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

Ores of Aluminium Formula

Bauxite Al₂O₃^.2H₂O

Cryolite Na₃AlF₆

Corundum Al₂O₃

Bauxite is the chief ore of aluminium. The extraction of aluminium from bauxite involves two steps:

(i) Conversion of bauxite into alumina – Baeyer’s Process

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.

(ii) Electrolytic reduction of alumina – Hall’s Process

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.

Physical Properties of Aluminium

·                 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.

Chemical Properties of Aluminium

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 chloride)

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.

Uses

Aluminium is used in

·                 household utensils

·                 electrical cable industry

·                 making aeroplanes and other industrial mechine parts