Extractive Metallurgy of Aluminium: Ores, Properties, and Uses | Class 10 Science Chapter 8

EXTRACTIVE METALLURGY OF ALUMINIUM

Study Material and Lecturing Notes for 10th Science, Chapter 8: Periodic Classification of Elements. This guide provides a detailed explanation of the extractive metallurgy of Aluminium, including its ores, properties, and uses.

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

• 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:

1. 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.
2. On diluting sodium meta aluminate with water, a precipitate of aluminium hydroxide is formed.
3. 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 its 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

• household utensils
• electrical cable industry
• making aeroplanes and other industrial mechine parts