Extraction of Aluminium: Hall-Heroult Process Class 10

Quick answer: Aluminium is very reactive, so carbon cannot reduce aluminium oxide; aluminium is extracted by electrolysis of molten alumina (the Hall–Héroult process). Alumina ( $\mathrm{Al_2O_3}$ ) is purified from bauxite, then dissolved in molten cryolite, which lowers the working temperature from over 2000 °C to about 950 °C and improves conductivity. Aluminium forms at the cathode; oxygen at the anode burns the carbon anodes to $\mathrm{CO_2}$ , so they must be replaced.

Syllabus note: NCERT Class 10 says that metals high in the reactivity series, such as aluminium, are obtained by electrolytic reduction of their molten compounds, and that carbon cannot reduce their oxides. The details below (the Bayer process, cryolite and the Hall–Héroult cell) go beyond the Class 10 book and are given as background.

Why carbon cannot be used

Less reactive metals such as zinc can be obtained by heating their oxides with carbon:

$\mathrm{ZnO + C \longrightarrow Zn + CO}$

Aluminium is high in the reactivity series and has a stronger attraction for oxygen than carbon has. Heating aluminium oxide with carbon does not give aluminium. So aluminium is extracted by electrolysis of its molten oxide.

Step 1: purifying bauxite (Bayer process)

The main ore of aluminium is bauxite, a hydrated aluminium oxide ( $\mathrm{Al_2O_3 \cdot xH_2O}$ ) containing impurities such as iron(III) oxide and silica.

Dissolving in sodium hydroxide. Aluminium oxide is amphoteric, so it dissolves in hot concentrated sodium hydroxide, forming sodium aluminate. Iron(III) oxide does not dissolve and is filtered off as "red mud".

$\mathrm{Al_2O_3 + 2NaOH \longrightarrow 2NaAlO_2 + H_2O}$

Getting aluminium hydroxide back. The solution is cooled and seeded with aluminium hydroxide crystals, so aluminium hydroxide precipitates.

Heating. The aluminium hydroxide is strongly heated to give pure alumina:

$\mathrm{2Al(OH)_3 \longrightarrow Al_2O_3 + 3H_2O}$

Step 2: electrolysis (Hall–Héroult process)

Pure alumina melts at about 2070 °C, which would need a huge amount of energy. So it is dissolved in molten cryolite ( $\mathrm{Na_3AlF_6}$ ), often with a little fluorspar ( $\mathrm{CaF_2}$ ).

Why cryolite is used	Effect
Lowers the melting point of the electrolyte	The cell works at about 950 °C instead of over 2000 °C, saving energy
Improves electrical conductivity	Current passes more easily through the melt

 (+) carbon anodes  ↓ ↓ ↓
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  molten alumina + cryolite
       (about 950 °C)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  molten aluminium (bottom)
 [ carbon-lined steel tank ] (−)

Reactions at the electrodes

At the cathode (−): aluminium ions gain electrons.

$\mathrm{Al^{3+} + 3e^- \longrightarrow Al}$

Molten aluminium is slightly denser than the electrolyte, so it collects at the bottom and is tapped off.

At the anode (+): oxide ions lose electrons.

$\mathrm{2O^{2-} \longrightarrow O_2 + 4e^-}$

Why the anodes must be replaced: At about 950 °C, the oxygen reacts with the carbon anodes:

$\mathrm{C + O_2 \longrightarrow CO_2}$

So the anodes burn away and have to be replaced regularly. Roughly 0.4–0.5 kg of carbon is used for every kilogram of aluminium produced.

Summary table

Stage	What happens	Key equation or fact
Ore	Bauxite, hydrated $\mathrm{Al_2O_3}$	Contains $\mathrm{Fe_2O_3}$ and silica as impurities
Purification	Dissolve in NaOH, precipitate, heat	$\mathrm{2Al(OH)_3 \to Al_2O_3 + 3H_2O}$
Electrolyte	Alumina dissolved in molten cryolite	Works at about 950 °C
Cathode	Aluminium forms	$\mathrm{Al^{3+} + 3e^- \to Al}$
Anode	Oxygen forms and burns the carbon	$\mathrm{C + O_2 \to CO_2}$

Exam tips

For "why is aluminium not obtained by reduction with carbon?", write: aluminium has a greater affinity for oxygen than carbon, so carbon cannot remove oxygen from $\mathrm{Al_2O_3}$ .
For "how is aluminium extracted?", the Class 10 answer is electrolytic reduction of molten aluminium oxide.
Give both roles of cryolite: lowers the melting point and improves conductivity.
Write the cathode reaction with 3 electrons, $\mathrm{Al^{3+} + 3e^- \to Al}$ .

Common mistakes

Saying aluminium is extracted by heating with coke, like iron or zinc.
Electrolysing an aqueous solution. In water, hydrogen forms at the cathode instead of aluminium, so the electrolyte must be molten.
Writing the anode reaction as gaining electrons. Oxide ions lose electrons at the anode.
Forgetting why the anodes are replaced: they react with oxygen and burn away.

Frequently Asked Questions

Why can't aluminium be extracted by reduction with carbon?

Aluminium is highly reactive and has a greater affinity for oxygen than carbon, so carbon cannot remove oxygen from aluminium oxide.

How is aluminium extracted?

Aluminium is extracted by electrolysis of molten aluminium oxide (alumina) dissolved in molten cryolite. This is called the Hall–Héroult process.

Why is cryolite added to alumina?

Cryolite lowers the melting point of the electrolyte from over 2000 °C to about 950 °C and improves its electrical conductivity, which saves a lot of energy.

Why do the carbon anodes have to be replaced?

Oxygen released at the anode reacts with the hot carbon anodes to form carbon dioxide, so the anodes slowly burn away.

What is the ore of aluminium?

The main ore of aluminium is bauxite, which is hydrated aluminium oxide.

Extraction of Aluminium: Hall–Héroult Process | Class 10