Extraction of Metals

This lesson covers the extraction of metals from their ores, as specified in the AQA GCSE Chemistry syllabus (4.4.1). You need to understand why different extraction methods are used for different metals, how the reactivity series determines the method of extraction, and the economic and environmental considerations involved. This is a core topic that connects reactivity, reduction, and real-world industrial chemistry.

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Metals and Their Ores

Most metals are found in the Earth's crust combined with other elements in compounds called ores. An ore is a naturally occurring rock that contains enough metal or metal compound to make extraction economically worthwhile.

Metal	Ore name	Chemical compound in the ore
Iron	Haematite	Iron(III) oxide (Fe2O3)
Aluminium	Bauxite	Aluminium oxide (Al2O3)
Copper	Chalcopyrite	Copper iron sulfide (CuFeS2)
Zinc	Sphalerite	Zinc sulfide (ZnS)
Tin	Cassiterite	Tin dioxide (SnO2)
Lead	Galena	Lead sulfide (PbS)

Only very unreactive metals such as gold and platinum are found native (uncombined) in the Earth's crust. This is because they do not react with oxygen, water, or other substances in the environment.

Exam Tip: The term "ore" specifically means a rock from which a metal can be extracted economically. If the cost of extraction is greater than the value of the metal obtained, the rock is not considered an ore — even if it contains the metal.

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Choosing an Extraction Method

The method used to extract a metal from its ore depends on the metal's position in the reactivity series:

graph TD
    A["Metal in Ore"] --> B{"Where is the metal<br/>in the reactivity series?"}
    B -->|"Above Carbon<br/>(e.g. Aluminium, Sodium)"| C["Electrolysis"]
    B -->|"Below Carbon<br/>(e.g. Iron, Zinc, Tin)"| D["Reduction with Carbon"]
    B -->|"Very Unreactive<br/>(e.g. Gold, Platinum)"| E["Found Native<br/>(no extraction needed)"]

    C --> F["Very expensive<br/>due to electricity costs"]
    D --> G["Cheaper — uses<br/>carbon (coke) in a furnace"]
    E --> H["Separated from rock<br/>by physical methods"]

    style A fill:#4a90d9,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#f39c12,color:#fff

Key Principle

• Metals more reactive than carbon must be extracted using electrolysis (passing an electric current through the molten ore). This includes potassium, sodium, lithium, calcium, magnesium, and aluminium.
• Metals less reactive than carbon can be extracted by reduction with carbon in a blast furnace. This includes zinc, iron, tin, lead, and copper.

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Reduction with Carbon

For metals less reactive than carbon, the ore is heated with carbon (in the form of coke or charcoal) in a furnace. The carbon acts as a reducing agent — it removes the oxygen from the metal oxide, leaving the pure metal behind.

General word equation:

metal oxide + carbon → metal + carbon dioxide

Example: Extraction of Iron

Iron is extracted from its ore (haematite, Fe2O3) in a blast furnace:

iron(III) oxide + carbon → iron + carbon dioxide

The process involves:

1. Iron ore, coke (carbon), and limestone are loaded into the top of the blast furnace.
2. Hot air is blasted in at the bottom, causing the coke to burn and reach very high temperatures (about 1500 degrees C).
3. Carbon reacts with oxygen to form carbon dioxide, which then reacts with more carbon to form carbon monoxide.
4. Carbon monoxide acts as the reducing agent, removing oxygen from the iron ore.
5. Molten iron sinks to the bottom of the furnace and is tapped off.
6. Limestone reacts with impurities (sand/silica) to form slag, which floats on the iron and is removed.

Example: Extraction of Copper (Traditional)

Copper can be extracted by heating copper ore with carbon:

copper oxide + carbon → copper + carbon dioxide

However, copper ores are becoming scarce, so alternative methods are increasingly important (see below).

Exam Tip: When writing word equations for extraction by reduction, always state that carbon removes the oxygen — this is the definition of reduction. The carbon is the reducing agent because it donates electrons (or removes oxygen) from the metal oxide.

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Extraction by Electrolysis

Metals more reactive than carbon (aluminium, sodium, magnesium, etc.) cannot be extracted by reduction with carbon because carbon is not reactive enough to displace them. Instead, electrolysis of the molten ore is used.

Electrolysis involves:

1. Melting the ore (or dissolving it) to allow the ions to move freely.
2. Passing an electric current through the molten compound.
3. The metal ions are attracted to the cathode (negative electrode), where they gain electrons and are reduced to form the metal.
4. Non-metal ions are attracted to the anode (positive electrode), where they lose electrons and are oxidised.

Example: Extraction of Aluminium

Aluminium is extracted from purified bauxite (aluminium oxide) by electrolysis. The aluminium oxide is dissolved in molten cryolite to lower the melting point and reduce energy costs.

• At the cathode: aluminium ions gain electrons → aluminium metal is formed
• At the anode: oxide ions lose electrons → oxygen gas is produced

The process requires enormous amounts of electricity, making it very expensive. This is why aluminium recycling is economically and environmentally important.

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Alternative Methods of Extracting Copper

As high-grade copper ores become scarce, scientists have developed alternative methods to extract copper from low-grade ores and recycled materials:

Phytomining

Phytomining involves growing plants on soil that contains copper compounds. The plants absorb and accumulate the copper ions. The plants are then harvested and burned. The copper is extracted from the ash by displacement or electrolysis.

Bioleaching

Bioleaching uses bacteria to extract copper from low-grade ores. The bacteria feed on the minerals in the ore and produce a solution called a leachate that contains copper ions. The copper is then extracted from the leachate by displacement (using scrap iron) or electrolysis.