Everything you need to know for CAPE Unit 2 Module 3 on the chemistry of aluminium extraction. It will allow you to answer any past paper question on this topic. The physical properties of aluminium were left out since you can easily find those yourself.

1. CHEMISTRY OF ALUMINIUM PRODUCTION
THE ORE
Aluminium is extracted from bauxite which contains Al2O3.xH2O
where x = 1 or 3 so that the formula is sometimes given as
Al2O3.2H2O.
PRIMARY OR SECONDARY PRODUCTION
Aluminium production is either primary or through
recycling (secondary). Primary production is energy
intensive and recycling is preferred, since it is
intensive and recycling is preferred since it is
cheaper.
Impurities in bauxite include oxides of Ti, Si and Fe which must be
removed before extraction of aluminium. Varying amounts of iron
oxide give bauxite its colours which range from white (no iron oxide) Primary aluminum production involves obtaining
alumina (Al2O3) from bauxite for electrolysis which
to red (up to 27% Fe2O3).
yields the metal.
SEPARATING ALUMINIUM FROM IMPURITIES
Aluminium separation starts with digestion of bauxite by concentrated NaOH. This takes advantage of the fact that Fe2O3 is
not amphoteric as is Al2O3. Al2O3 is therefore dissolved and Fe2O3 is removed as a solid by filtration. The filtrate contains
two main species: [Al(OH)4]‐(aq) and SiO32‐(aq) which are formed as follows:
Al2O3 + 2OH‐ + 3H2O → 2[Al(OH)4]‐ and SiO2(s) + 2OH‐(aq) → SiO32‐ + H2O
The mixture is diluted with water or treated with CO2 to precipitate aluminium hydroxide:
2[Al(OH)4]‐ + CO2 → 2Al(OH)3 + CO32‐ + H2O ‐ Al(OH)3 is a white gelatinous solid
+ CO
+ CO 2 + H
is a white gelatinous solid.
Carbon dioxide forms a weak acid solution of carbonic acid that neutralizes the NaOH. This neutralization selectively
precipitates Al(OH)3 but leaves the remaining traces of silica in solution.
An alternative route of aluminium separation, not involving use of CO2 or precipitation of hydroxide involves two steps
where conditions are adjusted to promote the same reaction in different directions as required.
1. Dissolving to allow separation of aluminum from impurities: Al2O3.xH2O + 2NaOH → 2NaAlO2 + (x+1)H2O
2. After filtration the reaction is reversed: 2NaAlO2 + 2H2O → 2NaOH + Al2O3.3H2O (reversal is achieved by cooling and
seeding with crystals of Al2O3.3H2O). This method produces fine particles rather than a gelatinous mass.
CALCINATION
This is a term which means heating to a high temperature without
melting so that hydrates, carbonates or other compounds are
decomposed and the volatile material is expelled. Whether alumina or
aluminium hydroxide have been obtained in the separation, the next
step is calcination at 400 to 1200 C (average 1000 C) to give Al
step is calcination at 400 to 1200 °C (average 1000 °C) to give Al2O3:
2Al(OH)3 → Al2O3 + 3H2O or Al2O3.3H2O → Al2O3 + 3H2O.
ELECTROLYSIS
CRYOLITE
Cryolite (Na3AlF6) is a solvent for alumina. It also
serves to lower the melting point of the mixture to
1000 from 2000 °C for pure alumina so that
electrolysis is less costly (consumes less electrical
electrolysis is less costly (consumes less electrical
energy).
Cathode reaction: Al3+ +3e‐ → Al
Primary anode reaction: 2O2‐ ‐ 4e‐ → O2
Secondary (non electrolytic) anode reaction: C + O2 → CO2
Overall cell reaction is 2Al2O3 + 3C → 4Al + 3CO2
From the overall reaction it can be seen that carbon plays
p y
a very indirect role as a reducing agent.
During electrolysis energy is required to both discharge
ions and maintain the electrolyte in a molten state.
Smelting is a process of metal extraction at high
temperature (pyrometallurgy). It involves melting the
concentrates from metal ores to remove impurities and
to reduce metal compounds to metals. Examples of
to red ce metal compo nds to metals E amples of
smelting include the extraction of iron in a blast furnace
and aluminium extraction by electrolysis of a melt.
Put Together By Denison At Global In Cunupia. 739‐2656.