Describe extraction of aluminium from named ore

Extraction of Aluminium

Ore: Bauxite (Al₂O_3.xH₂O)

 

Major impurities are

• Silica or SiO₂
• Iron salts

 

Principles in extraction

Extraction of aluminium involves removal of impurities (purification) and then reduction to metal by electrolysis.

 

Steps in extraction of Aluminium

1. The ore is heated
   • to remove water and,
   • To convert iron salts to iron III oxide

2. The powdered ore is heated with concentrated sodium hydroxide to dissolve aluminium oxide and silica such that the insoluble iron oxide is filtered off.

Aluminium oxide form aluminate

Al₂O₃ (s) + 2NaOH (aq) + 7H₂O (l) → 2Na[Al(OH)₄(H₂O)₂] (aq)

 

Or the ionic form

Al₂O₃ (s) + OH^–(aq) → 2AlO₂^–(aq) + H₂O(l)

 

Silica also dissolves forming sodium silicate.

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

 

3. To the filtrate a little aluminium hydroxide is added to precipitate Aluminium hydroxide,(seeding).

NaAlO₂(aq) + 2H₂O(l)  → NaOH(aq) + Al(OH)₃(s)

 

Alternatively carbon dioxide bubbled through the filtrate to precipitate aluminium hydroxide as follows

2NaAl(OH)₄(aq) + CO₂(g) → 2Al(OH)₃(s) + Na₂CO₃(aq) + H₂O(l)

 

4. The precipitated aluminium hydroxide is filtered off, washed and ignited to give pure aluminium oxide (alumina).

2Al(OH)₃(s)  → Al₂O₃(s) + 3H₂O (g)

Alumina

5. Aluminum is obtained form aluminium oxide by electrolysis.

Cryolite, Na₃AlF₆,  is added to

• lower the melting point of alumina from2050⁰C to 900⁰C
• and improve conductivity of aluminium oxide

 

At the cathode (carbon) aluminium is liberated

Al³⁺ (aq) + 3e^–  → Al (s)

At the anode (carbon) oxygen is liberated

2O^2- – 4e → O₂ (g)

The anode is eaten up by oxygen

C + O₂ (g) → CO₂ (g)

 

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