Extraction of Metals

Most metals are found as compounds in ores. Metals below carbon are extracted by reduction with carbon (heating with coke). Metals above carbon must be extracted by electrolysis of the molten compound, which costs more energy. Iron is made in the blast furnace from haematite ( $\mathrm{Fe_2O_3}$ ), coke and limestone. Aluminium is made by electrolysing aluminium oxide dissolved in molten cryolite.

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Section 1 of 14

Learning objectives

Relate extraction method to reactivity.
Describe the blast furnace with equations.
Describe the electrolysis of aluminium oxide.

Pause and think

Iron has been used for 3000 years, aluminium only for about 150. Yet aluminium is the more common metal in the Earth's crust. Why?

Section 2 of 14

Core idea

Start hereRocks into metal

Every steel beam and aluminium can began as dull rock. Getting the metal out means pulling oxygen away from it, and the method depends on how tightly the metal holds on.

The big idea

Metals below carbon can be reduced by heating with carbon. Metals above carbon need electrolysis, which uses much more energy.

1. The blast furnace

Reactions in the blast furnace

Drawing diagram…

Coke provides heat and the reducing agent.

Worked exampleIdentifying redox

$\mathrm{Fe_2O_3+3CO\rightarrow2Fe+3CO_2}$

Goal: What is reduced and what is oxidised?
Strategy: Track the oxygen.

$\mathrm{Fe_2O_3\rightarrow Fe}$
Loses oxygen: reduced.
$\mathrm{CO\rightarrow CO_2}$
Gains oxygen: oxidised (reducing agent).

AnswerIron(III) oxide is reduced; carbon monoxide is oxidised.

✓ CheckFe goes from

+3

0

✓.

2. Aluminium

Electrolyte: $\mathrm{Al_2O_3}$ dissolved in molten cryolite (about $950\,^\circ\mathrm{C}$ ).
Cathode: $\mathrm{Al^{3+}+3e^-\rightarrow Al}$ .
Anode: $\mathrm{2O^{2-}\rightarrow O_2+4e^-}$ ; carbon anodes burn away.

Exam tipWhy electrolysis is costly High temperatures and large currents need lots of electricity, and the anodes must be replaced.

Pause and think

Why is recycling aluminium so much more worthwhile than recycling iron?

Summary

Below carbon: reduce with carbon.
Above carbon: electrolysis.
Limestone removes silica as slag.

Ore: A rock containing enough metal compound to extract profitably.
Slag: Calcium silicate waste from the blast furnace.
Cryolite: Compound that lowers the melting point in aluminium extraction.

HistoryHall and Héroult. In 1886, Charles Hall (USA) and Paul Héroult (France), both aged 22, independently invented the cryolite electrolysis method. They were born and died in the same years too.

Fun factRecycling wins. Recycling an aluminium can uses about 5% of the energy needed to make it from bauxite. A recycled can can be back on the shelf in 60 days.

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