Aluminium extraction

ALUMINIUM EXTRACTION

It forms about 7% of earth’s crust; it is the most abundant metal

Aluminium extraction may be done from the following ores.

Bauxite Al2O3.2H2O

Corundum Al2O3

Cryolite Na3AlF6 (sodium hexafluoro aluminate)

 Aluminum extraction from bauxite

It contains impurities such as iron (III) oxide, titanium oxide and silica (silicon (IV) oxide) (SiO2)

The ore is heated at low temp to convert any iron (II) oxide to iron (III) oxide

Water is also removed at this point

Bauxite is then ground into fine powder and treated with hot concentrated sodium hydroxide at high pressure

Aluminium oxide is amphoteric and thus dissolves in hot concentrated sodium hydroxide

Al2O3 (s)   +   OH(aq)    + 3H2O (l)   →   2[Al (OH) 4] (aq)

Fe2O3 and TiO are not amphoteric hence do not dissolve and are filtered off as residue




Silica is acidic hence dissolves in sodium hydroxide forming sodium silicate

SiO2 (s)  +  2NaOH (aq)   →   Na2SiO3 (aq)     +    H2O (l)

The filtrate containing sodium silicate and [Al (OH) 4]  is treated with carbon (IV) oxide precipitating Al(OH)3 which is filtered  and obtained as residue leaving Na2SiO3 as the filtrate

2[Al (OH) 3] (aq) + CO2 (g)   →  2Al (OH) 3 (s) + CO32- (aq)+   H2O (aq)

An alternative method is adding a little pure Al (OH) 3 to the filtrate after dilution with water (hydrolysis) to cause precipitation in process called seeding

[Al (OH) 3] (aq)   → Al (OH) 3 (s) + OH(aq)

The solid aluminium hydroxide is filtered washed and dried.

It is then heated to give pure aluminium oxide

2Al (OH) 3(s)   →    Al2O 3 (s) + H2O (l)

 

Electrolysis of molten aluminium oxide

Aluminium

Aluminium oxide has high melting point of 2015oC

Na2AlF6 (Cryolite) is added to lower the Melting point to about 900oC

Cryolite also improves conductivity of molten Al2O3

Electrolytic cell is a steel tank lined with graphite (carbon) which acts as the cathode

The anode consists of graphite dipped in molten Al2O3

Al is formed at the cathode

Al3+ (l)   + 3e  →   Al (s)

Oxygen is liberated at the anode.

2O 2- (l)    →   O 2 (g) + 4e(aq)

The oxygen gas produced may react with carbon anode at high temperature hence the anode wears and requires replacement from time to time

C (s)   + O 2 (g)   → CO 2 (g)

Molten aluminium being denser than the electrolyte collects at the bottom of the cell from where it is tapped

Uses

  1. Alloy of aluminium and magnesium is used to make airplane parts, railway trucks, trains, buses, tankers, furniture and cars because of its low density.
  2. Duralumin an alloy of aluminum that is 4 percent copper and contains small amounts of magnesium, manganese, iron, and silicon: used for applications requiring lightness and strength, as in airplane construction.
  3. Cooking vessels such as sufurias, since it is a good conductor of heat and not easily corroded since it forms a protective layer of aluminium oxide
  4. Making overhead cables, because it is light and a good conductor of electricity
  5. Reducing agent in thermite process

Cr2O3 (s)   +   2Al (s)                 2Cr (s)     +     Al2O3 (s)

 



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