Extraction of metals
Extraction methods linked to reactivity, including the blast furnace and electrolysis of aluminium oxide.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 9.6.1Relate the method of extraction of a metal to its position in the reactivity series.
- 9.6.2Describe the extraction of iron in the blast furnace, including the main equations.
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Extraction of metals
The method used to extract a metal from its ore depends on the metal's position in the reactivity series — the general rule is that the extraction method needed becomes more powerful (and more expensive) the more reactive the metal is.
The general rule
- Metals more reactive than carbon (potassium down to aluminium) cannot be extracted using carbon, because carbon cannot remove oxygen from their compounds. They are extracted by electrolysis of a molten compound instead.
- Metals less reactive than carbon (zinc down to gold, and iron) can be extracted by heating with carbon, which reduces the metal oxide by removing oxygen from it.
- The very least reactive metals (like gold) are found naturally as the uncombined element, needing no chemical extraction at all.
Extracting iron: the blast furnace
Iron ore (mainly haematite, iron(III) oxide), coke (carbon) and limestone are added to the top of the blast furnace, and hot air is blasted in near the bottom.
- Coke burns in the hot air:
C + O₂ → CO₂ - Carbon dioxide reacts with more coke to form carbon monoxide:
CO₂ + C → 2CO - Carbon monoxide reduces the iron(III) oxide, removing its oxygen:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Molten iron collects at the bottom and is drawn off. The limestone decomposes and reacts with impurities (mainly sand) to form slag, which floats on top of the molten iron and is removed separately.
Extracting aluminium: electrolysis
Aluminium is more reactive than carbon, so it must be extracted by electrolysis of molten aluminium oxide. Because aluminium oxide has a very high melting point, it is dissolved in molten cryolite to reduce the operating temperature (and cost) of the process. Molten aluminium forms at the negative electrode (cathode) and oxygen at the positive electrode (anode).
Think of it like this
Extracting a metal is like trying to free someone tightly holding onto something valuable: a weak request (heating with carbon) works on someone with a loose grip (a less reactive metal), but for someone gripping very tightly (a very reactive metal), you need a far more forceful method (electrolysis) to pry it away.
Common misconceptions
- Thinking any metal oxide can be reduced by carbon. This only works for metals below carbon in the reactivity series — carbon cannot reduce the oxides of metals more reactive than itself, such as aluminium.
- Believing the blast furnace uses carbon directly to reduce iron oxide. It is specifically carbon MONOXIDE, formed from the carbon, that does the reducing in the main reaction.
- Assuming limestone is added to react with the iron ore. Limestone reacts with impurities (mainly silica/sand) in the ore to form slag — it is not involved in reducing the iron itself.
In the exam
- For the blast furnace, know all three key equations in order — many questions ask for "the equation that shows iron oxide is reduced" specifically, which is the third one, not the first.
- When asked why a particular extraction method is used, always link it back to the metal's position relative to carbon in the reactivity series, rather than just describing the method itself.