9.4 - Extracting Metals from Their Ores
Ores, native metals, and extraction as reduction
- Most metals are found combined with other elements as metal compounds in rocks, usually as oxides.
- An ore is a rock containing enough metal compound to make extracting the metal worthwhile:
- Haematite (or hematite) - An iron oxide ore
- Bauxite - An aluminium oxide ore
Native metals
- Unreactive metals, such as gold, do not readily react with other elements and are found as pure metal.
- Metals found like this are described as uncombined, or in their native form.
Extraction is reduction
- Extracting a metal from its oxide requires removing the oxygen.
- Losing oxygen is reduction, so extracting a metal from its oxide is a reduction reaction.
Reactivity and extraction
- A reactive metal forms stable compounds and holds its oxygen tightly, making it harder and more expensive to extract.
- A less reactive metal forms less stable compounds and releases its oxygen more easily.
Choosing an extraction method
The method used to extract a metal from its ore depends on where the metal sits relative to carbon in the reactivity series.
Below carbon - Reduction with carbon
A metal below carbon is extracted by reduction with carbon. The ore is heated with carbon, which removes the oxygen from the metal oxide:
iron(III) oxide + carbon → iron + carbon dioxide
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)
- The iron oxide loses oxygen, so it's reduced; the carbon gains oxygen, so it's oxidised.
- Zinc sits below carbon too and is extracted in the same way:
- zinc oxide + carbon → zinc + carbon dioxide
- 2ZnO(s) + C(s) → 2Zn(g) + CO2(g)
This only works for metals below carbon because carbon can only displace less reactive metals from their oxides.
Above carbon - Electrolysis
- A metal above carbon must be extracted by electrolysis. The compound is melted and an electric current is passed through it, splitting it into its elements.
- Aluminium sits above carbon, so it must be extracted from aluminium oxide using electrolysis.
- Electrolysis is also used if a metal reacts with carbon to form a carbide instead of the pure metal.
Cost
- Electrolysis is much more expensive than reduction with carbon because:
- Melting the compound requires large amounts of thermal energy
- Passing the electric current uses large amounts of electrical energy
- Reduction with carbon is cheaper because carbon is cheap and acts as both fuel and reducing agent.
Worked example - Choosing an extraction method
A company has a sample of magnesium oxide and a sample of zinc oxide, and wants to obtain the metal from each. Using this extract from the reactivity series, decide which extraction method to use for each, and why.
| More reactive |
|---|
| magnesium |
| carbon |
| zinc |
| Less reactive |
Step 1 - Find each metal in the series
Magnesium and zinc are both in the list, with carbon sitting between them.
Step 2 - See whether each is above or below carbon
- Magnesium is above carbon.
- Zinc is below carbon.
Step 3 - Read the method off each position
- Magnesium is above carbon, so carbon cannot displace it. Magnesium oxide must be melted and extracted by electrolysis.
- Zinc is below carbon, so carbon can displace it. Zinc oxide is extracted by reduction with carbon:
- zinc oxide + carbon → zinc + carbon dioxide
Step 4 - Interpretation
Electrolysis requires large amounts of energy to melt the compound and supply the electrical current, making magnesium much more expensive to extract than zinc.
Extracting iron in the blast furnace
- Iron is extracted from iron ore in a blast furnace.
- It runs continuously: fresh raw materials are added at the top while molten iron and slag are tapped off at the bottom.
The charge
Three solid raw materials are added at the top:
- Iron ore - Haematite (iron(III) oxide, Fe2O3); source of iron
- Coke - Mostly carbon, C; fuel and source of reducing agent
- Limestone - Calcium carbonate, CaCO3; removes impurities
Reactions in the blast furnace
Hot air, the fourth raw material, is blasted in near the bottom, raising the temperature to around 1,500 °C:
- Combustion of coke - Carbon reacts with oxygen to form carbon dioxide, releasing heat:
- C + O2 → CO2
- Reduction of carbon dioxide - Carbon dioxide reacts with more coke to form carbon monoxide:
- C + CO2 → 2CO
- Reduction of iron(III) oxide - Carbon monoxide reduces the iron(III) oxide to molten iron:
- Fe2O3 + 3CO → 2Fe + 3CO2
- Thermal decomposition of limestone - Calcium carbonate decomposes into calcium oxide:
- CaCO3 → CaO + CO2
- Formation of slag - Calcium oxide (a base) reacts with silicon dioxide (an acidic impurity) to form molten calcium silicate (slag):
- CaO + SiO2 → CaSiO3
Tapping off the products
- Molten iron is denser than molten slag, so it sinks to the bottom.
- Slag forms a separate layer on top of the iron and is tapped off above it.
- Waste gases escape from the top of the furnace.
