Metallurgy — Part 2: Aluminium and alloys: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Aluminium: from bauxite to metal

Part 2 — Aluminium is the most abundant metal in the earth’s crust, yet it was once dearer than gold. Two processes made it cheap: Baeyer’s process to purify bauxite and the Hall–Héroult process to electrolyse alumina.

In this lesson you will

Write the equations of Baeyer’s process for purifying bauxite
Describe the Hall–Héroult cell and state the purpose of cryolite, fluorspar and coke
Write the electrode reactions and explain why the anodes are replaced
State the composition and a use of six alloys and explain why alloys are preferred

Concept

Why aluminium needs electrolysis

Aluminium is high in the activity series: it holds oxygen more strongly than carbon, so coke cannot reduce alumina. Alumina must therefore be reduced by electrolysis. Because impurities in the ore would also be reduced and spoil the metal, the bauxite is purified first.

Table of properties of alumina and their consequences for the extraction of aluminium
Property of alumina	Consequence
Very stable oxide	Not reduced by carbon; electrolysis needed
Melting point about 2050 °C	Solvent (cryolite) needed to work at about 950 °C
Bauxite contains Fe₂O₃ and other impurities	Purified by Baeyer’s process before electrolysis

Concept

Baeyer’s process: step 1, digestion

Finely ground bauxite is heated with concentrated sodium hydroxide solution under pressure (about 150–200 °C). Alumina dissolves as soluble sodium aluminate; the main impurity, iron(III) oxide, does not dissolve and is filtered off.

\ce{Al2O3.2H2O + 2NaOH -> 2NaAlO2 + 3H2O}

Digestion of bauxite with sodium hydroxide

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