Metallurgy: Metallurgy of aluminium

Metallurgy of aluminium

It is solid in state

Symbol : Al
Color : Silvery white
Atomic number : 13
Electronic
configuration:2, 8, 3
Valency : 3
Atomic mass : 27
Position in the periodic table: period=3, group=13 (III A)

The important ores of aluminium are as follows:

Name of the ore is

Bauxite Al2O3.2H2O
Cryolite Na3AlF6
Corundum Al2O3
The chief ore of aluminium is bauxite (Al2O3.2H2O).

Extraction of aluminium from bauxite involves two stages:

1 st STAGE

Conversion of Bauxite into Alumina by Baeyer’s Process

The conversion of Bauxite into Alumina involves the following steps:

i.Bauxite ore is finely grounded and heated under pressure with concentrated caustic soda

solution at 150°C to obtain sodium meta aluminate.

Al2O3.2H2O + 2NaOH → 2NaAlO2 + 3H2O

Bauxite Sodium Meta aluminate

ii.On diluting sodium meta aluminate with water, aluminium hydroxide precipitate is

obtained.

NaAlO2 + 2H2O → NaOH + Al(OH)3

iii.The precipitate is filtered, washed, dried and ignited at 1000°C to get alumina.

2Al(OH)3 → Al2O3 + 3H2O

Reduction of Alumina by Hall’s process

Aluminium is produced by the electrolytic reduction of fused alumina (Al2O3) in

the electrolytic cell.

Cathode : Iron tank lined with graphite
Anode : A bunch of graphite rods suspended in molten electrolyte
Electrolyte : Pure alumina + moltencryolite + fluorspar (fluorspar lowers thefusion temperature of electrolyte)
Temperature : 900-950°C
Voltage used : 5-6V

The overall equation for aluminium extraction is

2Al2O3 → 4Al + 3O2

Aluminium deposits at cathode and oxygen gas is liberated at anode

Properties of Aluminium

Physical properties:

It is a silvery white metal.
It has low density and it is light
It is malleable and ductile.
It is a good conductor of heat andelectricity.
Melting point: 675°C
It can be well polished to produce attractive shiny appearance.

Chemical properties:

Reaction with air:

It is not affected by dry air.On heating at 800°C,aluminium burns

very brightly forming its oxide and nitride.

4Al + 3O2 → 2Al2O3

2Al + N2 → 2AlN

Reaction with water:

Water has no reaction on aluminium due to the layer ofoxide on it.When steam is passed over red hot aluminium, hydrogen is produced.

2Al + 3H2O → Al2O3 + 3H2↑

Reaction with alkalis: It reacts with strong caustic alkalis forming aluminates.

2Al + 2NaOH + 2H2O → 2NaAlO2 + 3H2↑

Reaction with acids: With dilute and con. HCl it liberates H2 gas.

2Al + 6HCl → 2AlCl3 + 3H2↑

DO YOU KNOW
Dilute or concentrated nitric acid does not attack aluminium. But it renders aluminium passive due to the formation of an oxide film on its surface.

Reducing action : 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 + 2Al → 2Fe + Al2O3

Uses of aluminium