Metal Extraction
Higher Tier
All content in this note is Higher Tier chemistry-only content (spec points 2.22C to 2.27C).
Sources of Metals
Most metals are found combined with other elements in rocks and minerals called ores in the Earth’s crust. They must be extracted by chemical reactions.
Unreactive metals (such as gold and silver) are found uncombined as the native metal element in the Earth’s crust. Gold is so unreactive it does not need extracting chemically; it is simply mined and refined.
Method of Extraction and the Reactivity Series
The method of extraction is linked to the metal’s position in the reactivity series:
| Position in Reactivity Series | Method of Extraction |
|---|---|
| Very reactive (above carbon) | Electrolysis of molten compounds |
| Less reactive than carbon | Reduction by carbon (or carbon monoxide) |
| Very unreactive | Found as native metal; physical separation |
Key principle: Carbon can only displace metals that are less reactive than carbon from their oxides. For metals more reactive than carbon, electrolysis is required.
Extraction by Carbon Reduction (Iron)
Iron is extracted from iron ore (mainly iron(III) oxide, Fe2O3) in a blast furnace by reduction with carbon (as coke):
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)
- Iron oxide loses oxygen and is therefore reduced; carbon is the reducing agent.
- Carbon gains oxygen and is therefore oxidised; iron oxide is the oxidising agent.
The iron produced from the blast furnace is impure and quite brittle; it is mostly converted into steel.
Extraction by Electrolysis (Aluminium)
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, extracted from bauxite ore).
Process:
- Aluminium oxide is mixed with cryolite (sodium aluminium fluoride) to lower its melting point (from about 2050 °C to about 940 °C), saving energy.
- Carbon electrodes (anodes) are used.
- Electrolysis separates Al3+ ions (which go to the cathode and are reduced to aluminium metal) from O2- ions (which go to the anodes and are oxidised to oxygen gas).
- At the anode, oxygen reacts with the carbon electrodes to form CO2, so the anodes need to be replaced regularly.
Electrolysis is expensive because:
- Large amounts of electrical energy are needed
- The compound must be melted (very high temperatures required)
- This is why aluminium is more expensive to produce than iron
Alternative Extraction Methods
For low-grade ores or ores in which the metal concentration is too small for conventional smelting, alternative methods may be used:
Phytoextraction (bioleaching using plants):
- Some plants absorb metal compounds through their roots and concentrate them.
- The plants are burned; the ash contains the metal compounds in higher concentration.
- This ash can then be processed to extract the metal.
Bacterial extraction (bioleaching using bacteria):
- Some bacteria absorb metal compounds from ores.
- They produce solutions called leachates that contain the metals.
- The metals can then be extracted from the leachate.
These methods are more environmentally friendly and useful for low-grade ores.
Uses of Metals
Aluminium
Properties: low density, corrosion-resistant (forms a protective oxide layer), good conductor of heat and electricity, malleable.
Uses: aircraft construction, trains, overhead power cables (combines low density with conductivity), saucepans, cooking foil, drink cans.
Copper
Properties: soft and easily bent, excellent conductor of electricity, does not react with water, easily shaped.
Uses: electrical wiring, water pipes, plumbing fittings.
Iron and Steel
Properties of pure iron from blast furnace: hard but brittle; too brittle for most structural uses.
Properties of pure iron (annealed): soft, relatively low tensile strength; too soft for many uses.
Iron is most useful when converted to steel (an alloy of iron and carbon, sometimes with other metals):
| Type of Steel | Composition | Properties | Uses |
|---|---|---|---|
| Low-carbon (mild) steel | ~0.25% carbon | Easily shaped and welded, not very hard | Car body panels, girders |
| High-carbon steel | ~2.5% carbon | Very hard, brittle | Cutting tools, drill bits |
| Stainless steel | Iron + ~18% chromium + ~8% nickel | Resistant to corrosion, hard | Cutlery, surgical instruments, kitchen sinks |
Alloys
An alloy is a mixture of a metal with one or more other elements (usually other metals or carbon).
Pure metals are often too soft for everyday use. Alloying makes them harder and more useful.
Examples of alloys:
- Steel: iron + carbon
- Brass: copper + zinc
- Bronze: copper + tin
- Gold alloys: gold + silver + copper + zinc (pure gold is too soft for jewellery)
Why Alloys are Harder than Pure Metals
In a pure metal, all ions are the same size and arranged in regular layers that can slide over each other easily, making the metal soft.
In an alloy, atoms of different sizes are present. These disrupt the regular arrangement of the ions and prevent the layers from sliding over each other as easily. The alloy is therefore harder and stronger than the pure metal.
Exam Questions
Q: Explain why aluminium is extracted by electrolysis rather than by reduction with carbon.
Aluminium is more reactive than carbon and therefore cannot be displaced from its ore by carbon. Electrolysis of molten aluminium oxide must be used instead.
Q: Give two reasons why the extraction of aluminium is more expensive than the extraction of iron.
Electrolysis requires large amounts of electrical energy; the aluminium oxide must be heated to a very high temperature (or mixed with cryolite) to make it molten so the ions can move.
Q: Explain why alloys are harder than pure metals.
In a pure metal, all the ions are the same size and arranged in regular layers that can slide over each other easily. In an alloy, atoms of different sizes disrupt the regular arrangement, preventing the layers from sliding easily, so the alloy is harder.
Q: State two properties of aluminium that make it suitable for making aircraft.
Aluminium has a low density (so aircraft are not too heavy) and is corrosion-resistant (so it does not rust or degrade).
Exam Tip
The key rule for extraction: if the metal is LESS reactive than carbon, it can be extracted by reduction with carbon (cheaper). If the metal is MORE reactive than carbon, electrolysis is needed (more expensive). This is why aluminium is more expensive than iron despite being the most abundant metal in the Earth’s crust.