Extracting Metals
How metals are obtained from their ores using reduction
Metals and Ores
Most metals are found in the Earth's crust as compounds, usually as metal oxides or metal sulfides. The method used to extract a metal depends on its position in the reactivity series.
| Position in Reactivity Series | Extraction Method | Examples |
|---|---|---|
| Above carbon (very reactive) | Electrolysis | Potassium, sodium, lithium, calcium, aluminium |
| Below carbon (less reactive) | Reduction with carbon (or hydrogen) | Zinc, iron, copper |
| Very bottom (unreactive) | Found uncombined (native metals) | Gold, platinum |
Oxidation and Reduction in Extraction
Reduction = loss of oxygen (or gain of electrons)
When we extract a metal from its ore, we need to remove the oxygen from the metal oxide. This means the metal oxide is being reduced.
The substance that removes the oxygen (e.g. carbon) gains oxygen, so it is oxidised.
Extraction by Reduction with Carbon
Metals below carbon in the reactivity series can be extracted by heating their oxide with carbon. Carbon is more reactive than these metals, so it can “steal” the oxygen away.
Extracting Iron (Blast Furnace)
- Iron oxide is reduced – it loses oxygen
- Carbon is oxidised – it gains oxygen
Extracting Zinc
Extracting Copper
Extraction by Reduction with Hydrogen
Hydrogen can also be used as a reducing agent to extract metals from their oxides. When hydrogen reduces a metal oxide, water is produced instead of carbon dioxide.
- Copper oxide is reduced – it loses oxygen
- Hydrogen is oxidised – it gains oxygen
Using hydrogen instead of carbon avoids producing CO2 (a greenhouse gas), but hydrogen is more difficult and dangerous to handle.
Metals Found Uncombined
Some metals are so unreactive that they do not combine with other elements. They are found in the Earth's crust as pure elements (native metals).
- Gold – found as nuggets or flakes in rocks and rivers
- Platinum – found uncombined
- Silver – sometimes found uncombined (though also found as compounds)
These metals are at the bottom of the reactivity series. They do not react with oxygen, water, or acids under normal conditions, which is why they have been used for coins, jewellery, and decorations throughout history.
Biological Methods of Extraction Triple only
Copper-rich ores are being used up. New methods allow copper to be extracted from low-grade ores (ores that contain only small amounts of metal) and from waste copper solutions.
Phytomining
Plants are grown on soil that contains copper compounds. The plants absorb and accumulate copper ions through their roots. The plants are then harvested and burned. The ash produced contains a high concentration of copper, which can be extracted by smelting or electrolysis.
Bioleaching
Bacteria are used to extract copper from low-grade ores. The bacteria feed on the metal compounds in the rock and produce a solution called a leachate that contains copper ions. The copper can then be extracted from the leachate by displacement (using scrap iron) or by electrolysis.
Metals Above Carbon: Electrolysis
Very reactive metals like aluminium, sodium, and potassium cannot be extracted by reduction with carbon because they are more reactive than carbon.
These metals must be extracted using electrolysis (passing an electric current through the molten ore or a solution). This is more expensive because it requires a lot of electrical energy.
For example, aluminium is extracted from its ore bauxite (aluminium oxide) by electrolysis of the molten aluminium oxide.
Practice Questions
Which method is used to extract aluminium from its ore?
Aluminium is above carbon in the reactivity series, so it cannot be reduced by carbon. Electrolysis of molten aluminium oxide is needed.
Iron is extracted from iron oxide using carbon in a blast furnace. Write the word equation and the balanced symbol equation for this reaction. Identify which substance is oxidised and which is reduced.
Word equation: iron oxide + carbon → iron + carbon dioxide [1 mark]
[1 mark for balanced equation]
Iron oxide is reduced (it loses oxygen). Carbon is oxidised (it gains oxygen). [1 mark]
Why is gold found uncombined in the Earth's crust?
Gold is at the very bottom of the reactivity series. It is so unreactive that it does not form compounds with other elements, so it exists as the pure element in the Earth's crust.
Copper oxide is heated with hydrogen gas.
- Write the balanced symbol equation for this reaction. [1 mark]
- Identify which substance is oxidised and which is reduced. [2 marks]
(a)
(b) Copper oxide is reduced because it loses oxygen [1 mark]. Hydrogen is oxidised because it gains oxygen (to form water) [1 mark].
Explain why the extraction of aluminium is more expensive than the extraction of iron.
Aluminium is more reactive than carbon [1 mark], so it cannot be extracted by reduction with carbon like iron can [1 mark]. Instead, aluminium must be extracted by electrolysis, which requires a large amount of electrical energy and is therefore more expensive [1 mark].
In the reaction: ZnO + C → Zn + CO2, what is the reducing agent?
Carbon is the reducing agent because it causes the zinc oxide to be reduced (lose its oxygen). The carbon itself is oxidised in the process.
A student suggests that sodium could be extracted from sodium oxide by heating it with carbon. Explain why this would not work.
Sodium is more reactive than carbon [1 mark]. Carbon can only reduce the oxides of metals that are less reactive than carbon, so it cannot remove the oxygen from sodium oxide [1 mark].