Extraction of Metals

This lesson covers the extraction of metals from their ores, as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 4: Extracting Metals and Equilibria. You need to understand why different metals require different extraction methods, describe how iron is extracted in a blast furnace, explain why aluminium is extracted by electrolysis, and link extraction to the reactivity series.

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Metals and Ores

Most metals are found in the Earth's crust as ores — rocks that contain enough of a metal compound to make extraction economically worthwhile. Only the most unreactive metals (gold, platinum) are found native (uncombined) in the Earth's crust.

Metal	Ore Name	Compound in the Ore
Iron	Haematite	Iron(III) oxide, Fe₂O₃
Aluminium	Bauxite	Aluminium oxide, Al₂O₃
Copper	Malachite / Chalcopyrite	Copper carbonate, CuCO₃ / Copper iron sulfide, CuFeS₂
Zinc	Sphalerite	Zinc sulfide, ZnS
Tin	Cassiterite	Tin dioxide, SnO₂

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The Reactivity Series and Extraction Method

The method of extraction depends on the position of the metal in the reactivity series relative to carbon.

flowchart TD
    A["Which method of extraction?"] --> B{"Is the metal MORE reactive than carbon?"}
    B -->|"Yes<br/>(K, Na, Ca, Mg, Al)"| C["Extract by<br/><b>ELECTROLYSIS</b><br/>(using electricity)"]
    B -->|"No<br/>(Zn, Fe, Sn, Pb, Cu)"| D{"Is the metal found native?"}
    D -->|"No"| E["Extract by<br/><b>REDUCTION WITH CARBON</b><br/>(heating with carbon/coke)"]
    D -->|"Yes (Au, Pt)"| F["Found <b>NATIVE</b><br/>(uncombined in the Earth)"]

    style C fill:#e74c3c,color:#fff
    style E fill:#f39c12,color:#fff
    style F fill:#27ae60,color:#fff

Summary of Extraction Methods

Reactivity	Metals	Extraction Method	Reason
Above carbon	K, Na, Ca, Mg, Al	Electrolysis	Carbon is not reactive enough to reduce their oxides
Below carbon	Zn, Fe, Sn, Pb, Cu	Reduction with carbon	Carbon can reduce their oxides because carbon is more reactive
Very unreactive	Au, Pt	Found native (uncombined)	Too unreactive to form compounds in the Earth's crust

Exam Tip: The key question is: "Is the metal more or less reactive than carbon?" If MORE reactive → electrolysis. If LESS reactive → reduction with carbon. This is a very common exam question.

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Extraction of Iron: The Blast Furnace

Iron is extracted from its ore, haematite (iron(III) oxide, Fe₂O₃), in a blast furnace by reduction with carbon (in the form of coke).

Raw Materials

Raw Material	Purpose
Iron ore (haematite, Fe₂O₃)	Source of iron
Coke (carbon, C)	Fuel and reducing agent
Limestone (calcium carbonate, CaCO₃)	Removes impurities (forms slag)
Hot air (blown in at the bottom)	Provides oxygen for combustion of coke

Reactions in the Blast Furnace

Step 1: Coke burns in hot air

Word equation: carbon + oxygen → carbon dioxide

Symbol equation: C(s) + O₂(g) → CO₂(g)

This is exothermic and heats the furnace to about 1 500 °C.

Step 2: Carbon dioxide reacts with more coke to form carbon monoxide

Word equation: carbon dioxide + carbon → carbon monoxide

Symbol equation: CO₂(g) + C(s) → 2CO(g)

Step 3: Carbon monoxide reduces the iron ore

Word equation: iron(III) oxide + carbon monoxide → iron + carbon dioxide

Symbol equation: Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)

This is the key reduction reaction. The iron(III) oxide is reduced (it loses oxygen). The carbon monoxide is oxidised (it gains oxygen). This is a redox reaction.

The iron produced is molten (liquid) because of the high temperatures. It sinks to the bottom of the furnace and is tapped off.

Step 4: Limestone removes impurities

The limestone decomposes in the heat:

CaCO₃(s) → CaO(s) + CO₂(g)

The calcium oxide reacts with sandy impurities (silicon dioxide) to form slag (calcium silicate):

CaO(s) + SiO₂(s) → CaSiO₃(l)

The slag floats on top of the molten iron and is tapped off separately.

Exam Tip: In the blast furnace, the reducing agent is carbon monoxide (not carbon directly). Make sure you write the correct equation: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Identify that iron oxide is reduced and carbon monoxide is oxidised.

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Extraction of Aluminium: Electrolysis

Aluminium is more reactive than carbon, so it cannot be extracted by reduction with carbon. Instead, it is extracted by electrolysis of molten aluminium oxide (Al₂O₃).

The Process

1. Aluminium oxide (from bauxite ore) is dissolved in molten cryolite (Na₃AlF₆).
2. Cryolite lowers the melting point of aluminium oxide from about 2 050 °C to about 950 °C.
3. This saves a huge amount of energy and reduces costs.
4. The electrolyte (molten mixture) is electrolysed using large carbon electrodes.

Electrode Reactions

At the cathode (negative electrode) — reduction:

Al³⁺ + 3e⁻ → Al

Aluminium ions gain electrons and are reduced to form molten aluminium metal. The aluminium sinks to the bottom of the cell and is tapped off.

At the anode (positive electrode) — oxidation:

2O²⁻ → O₂ + 4e⁻

Oxide ions lose electrons and are oxidised to form oxygen gas.

Why Is It Expensive?

The electrolysis of aluminium is very expensive because:

1. Huge amounts of electricity are needed to maintain the high temperatures and to provide the electrical current for electrolysis.
2. The carbon anodes burn away (they react with the oxygen produced: C + O₂ → CO₂) and must be replaced regularly.
3. The process must run continuously — if it stops, the electrolyte solidifies and the equipment is damaged.

Exam Tip: When explaining why aluminium extraction is expensive, the main reason is the large amount of electricity required. Also mention that the carbon anodes need regular replacement because they react with the oxygen produced.

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Unreactive Metals Found Native

Metals that are very unreactive, such as gold and platinum, are found native — uncombined in the Earth's crust. They do not need to be chemically extracted because they do not react with other elements in the environment.

Gold can simply be found as nuggets or particles in river beds and rocks. It may need physical separation (e.g. panning, sifting) but not chemical extraction.

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Reduction and Oxidation in Extraction

Metal extraction is fundamentally about reduction — the metal ions in the ore are reduced to form metal atoms.

Extraction Method	How Reduction Occurs
Reduction with carbon	Carbon (or CO) removes oxygen from the metal oxide; metal oxide is reduced
Electrolysis	Electrons from the cathode reduce the metal ions to metal atoms

In both cases, the metal compound is reduced to the metal. This links directly to the definitions of reduction: loss of oxygen or gain of electrons.

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Copper Extraction

Copper is below carbon in the reactivity series, so it can be extracted by heating copper ore with carbon:

Word equation: copper oxide + carbon → copper + carbon dioxide

Symbol equation: 2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

However, copper obtained this way is impure. For electrical wiring, copper must be very pure (99.99%). Pure copper is obtained by electrolysis (purification of copper), using:

• Impure copper as the anode
• Pure copper as the cathode
• Copper sulfate solution as the electrolyte

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Summary