Metal Extraction

Summary: Metals are extracted from ores. Methods depend on reactivity: electrolysis for K—Al; reduction with carbon for Zn—Fe; found native for Cu—Au. Iron from blast furnace using haematite, coke, and limestone; aluminium from Hall—Heroult process using molten cryolite. Tags: igcse chemistry metals Created: 2026-07-14 Last Updated: 2026-07-16

Extraction Method Depends on Reactivity

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

Why more reactive metals need electrolysis: Reactive metals form very stable compounds (strong bonds). Carbon is not a strong enough reducing agent to break these bonds — only electrolysis (electrical energy) provides enough energy.

Extraction of Aluminium by Electrolysis (Hall—Heroult Process)

Aluminium is extracted from its ore bauxite (Al2O3, aluminium oxide) by electrolysis.

Why electrolysis? Aluminium is above carbon in the reactivity series, so carbon cannot reduce Al2O3. Electrolysis is the only viable method.

The problem with Al2O3: Aluminium oxide has a very high melting point of about 2000 degrees C. Melting it directly would be extremely expensive in energy.

The solution — cryolite: Al2O3 is dissolved in molten cryolite (Na3AlF6, sodium aluminium fluoride). This lowers the melting point to about 950 degrees C, making the process far more economical.

Cell setup:

• Electrolyte: Al2O3 dissolved in molten cryolite
• Cathode (negative electrode): carbon lining of the cell
• Anode (positive electrode): large carbon blocks suspended in the electrolyte
• Temperature: ~950 degrees C

Reactions:

• At the cathode (reduction): Al3+ + 3e- → Al(l)
  • Molten aluminium collects at the bottom of the cell and is tapped off.
• At the anode (oxidation): 2O2- → O2(g) + 4e-
  • Oxygen gas is produced at the anode.

Why carbon anodes must be replaced regularly: The oxygen produced at the anode reacts with the carbon anodes at the high temperature:

• C(s) + O2(g) → CO2(g) This burns away the carbon anodes, so they need to be replaced regularly. This is a significant operating cost.

Overall reaction: 2Al2O3(l) → 4Al(l) + 3O2(g)

Extraction of Iron in the Blast Furnace

Iron is extracted from haematite (Fe2O3, iron(III) oxide) in a blast furnace.

Raw materials:

Reactions in the blast furnace:

The furnace is charged from the top (iron ore + coke + limestone) while hot air is blasted in from the bottom. The temperature varies from about 1800 degrees C at the bottom to about 400 degrees C at the top.

Stage 1 — Combustion of coke (bottom of furnace, hottest zone):

• C(s) + O2(g) → CO2(g)
• This reaction is highly exothermic and provides heat for the furnace.

Stage 2 — Production of carbon monoxide (middle of furnace):

• CO2(g) + C(s) → 2CO(g)
• Carbon dioxide reacts with more coke to produce carbon monoxide. Carbon monoxide is the actual reducing agent in the furnace.

Stage 3 — Reduction of iron(III) oxide (upper-middle of furnace):

• Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
• Carbon monoxide reduces iron(III) oxide to molten iron. Iron is produced as a liquid (mp of Fe is 1538 degrees C; temperature in this zone is adequate because impurities lower the melting point).

Stage 4 — Removal of impurities (formation of slag):

Haematite contains sandy impurities, mainly silicon dioxide (SiO2, silica). Limestone removes these:

1. Thermal decomposition of limestone:
   • CaCO3(s) → CaO(s) + CO2(g)
2. Calcium oxide (a basic oxide) reacts with silicon dioxide (an acidic oxide):
   • CaO(s) + SiO2(s) → CaSiO3(l)

Slag (calcium silicate, CaSiO3) is molten and floats on top of the molten iron (slag is less dense than iron). It is tapped off separately and used for road building and cement manufacture.

Products of the blast furnace:

• Molten iron: tapped from the bottom, contains about 4% carbon (cast iron / pig iron)
• Molten slag: tapped from above the iron layer
• Waste gases: CO2, N2, CO — the hot gases are used to pre-heat incoming air (heat exchange)

Cast iron vs pure iron:

Cast iron is converted to steel by removing most of the carbon in a Basic Oxygen Furnace (BOF).

Extraction of Copper by Reduction with Hydrogen

Copper can be extracted from copper(II) oxide by heating with hydrogen gas:

• CuO(s) + H2(g) → Cu(s) + H2O(g)
• Hydrogen is passed over heated black copper(II) oxide.
• Observations: black solid → pink/brown solid (copper metal). Water vapour condenses at the cooler end of the apparatus.

Copper can also be extracted by:

• Heating the ore in air followed by reduction (roasting copper sulfide ores).
• Displacement with iron: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) — used for low-grade ores / scrap.
• Bioleaching and phytomining (newer, more sustainable methods).

Metals Found Native (Uncombined)

Silver (Ag) and Gold (Au) are so unreactive that they occur naturally as the free element (native). No chemical extraction is needed — they are separated from surrounding rock by physical methods (crushing, panning, washing).

Copper can occasionally be found native, but most copper is extracted from ores such as chalcopyrite (CuFeS2).

Environmental Considerations

Negative environmental impacts of metal extraction:

• Quarrying/mining: destroys habitats, produces noise and dust pollution, leaves scars on the landscape.
• Energy consumption: electrolysis of aluminium requires huge amounts of electricity — aluminium smelters are often sited near hydroelectric power stations.
• CO2 emissions: the blast furnace produces significant CO2 (C + O2 → CO2; CO2 from CaCO3 decomposition; and the use of fossil-fuel-derived coke).
• Toxic waste: extraction of some metals (e.g., from sulfide ores) can produce SO2 (acid rain).
• Finite resources: metal ores are non-renewable. Eventually, they will run out.

Benefits of recycling metals:

• Saves energy: recycling aluminium requires only about 5% of the energy needed to extract new aluminium from bauxite.
• Conserves finite ore reserves: reduces the need for mining.
• Reduces waste: less material sent to landfill.
• Reduces environmental damage: less quarrying/mining needed.
• Economic benefits: recycled metals are often cheaper than newly extracted ones.

Key Points

• Extraction method depends on reactivity: electrolysis (K—Al), carbon reduction (Zn—Fe), found native (Ag, Au)
• Hall—Heroult process: electrolysis of Al2O3 in molten cryolite (~950 degrees C)
• Cathode: Al3+ + 3e- → Al(l); Anode: 2O2- → O2 + 4e-
• Carbon anodes burn away: C + O2 → CO2 — must be replaced regularly
• Blast furnace raw materials: haematite (Fe2O3), coke (C), limestone (CaCO3)
• Blast furnace reactions: C + O2 → CO2; CO2 + C → 2CO; Fe2O3 + 3CO → 2Fe + 3CO2
• Limestone: CaCO3 → CaO + CO2; CaO + SiO2 → CaSiO3 (slag)
• Slag = calcium silicate, floats on molten iron, tapped off separately
• Copper: CuO + H2 → Cu + H2O (black → pink/brown)
• Recycling aluminium saves ~95% of energy compared to primary extraction

Key Concepts from Past Papers

• Ore: a naturally occurring rock/mineral from which a metal can be profitably extracted
• Electrolysis: the decomposition of a compound using an electric current
• Slag: calcium silicate (CaSiO3) formed from the reaction of calcium oxide with silicon dioxide impurities
• Alloy: a mixture of a metal with one or more other elements (usually other metals or carbon)
• Cryolite lowers the melting point of Al2O3 from about 2000 degrees C to about 950 degrees C, reducing energy costs
• Carbon anodes react with oxygen to form CO2, so they burn away and need replacement
• Limestone decomposes to form CaO, which reacts with SiO2 impurities to form CaSiO3 (slag)

Keywords from Past Papers

iron, copper, reactive, oxide, carbon, oxygen, chlorine, bromine, magnesium, zinc, metal, earth, line, time, high

Related Notes

• Reactivity of Metals
• Uses of Metals
• Rusting of Iron

Sources

• OpenStax Chemistry 2e — Chapter 12: Kinetics, Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — Metal Extraction, BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus topic 9 (Metals), Cambridge Assessment International Education
• CK-12 Chemistry for High School — Metal Extraction, CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q11(c)(ii) (0m), Q33(a)(i) (0m), Q33(a)(ii) (0m) (+1 more)
• 0620/32 Feb/March 2015: Q44(b)(iv) (2m), Q55(a)(i) (2m), Q55(a)(iii) (2m) (+5 more)
• 0620/32 Feb/March 2017: Q11(a)(iii) (1m)
• 0620/32 Feb/March 2018: Q22(a)(iii) (1m), Q55(a)(iii) (1m), Q55(b)(i) (1m) (+3 more)
• 0620/32 Feb/March 2019: Q33(c)(iii) (4m), Q66(a)(i) (1m), Q66(b)(i) (1m) (+1 more)
• 0620/32 Feb/March 2020: Q22(b)(i) (1m), Q22(d)(i) (1m), Q66(a)(iv) (0m) (+1 more)
• 0620/32 Feb/March 2021: Q55(f)(i) (1m), Q77(e)(iii) (1m)
• 0620/32 Feb/March 2022: Q33(b)(ii) (1m), Q88(d)(iii) (1m)
• 0620/32 Feb/March 2023: Q77(a)(ii) (4m)
• 0620/32 May/June 2018: Q88(e)(iii) (0m)
• 0620/32 May/June 2020: Q11(a)(iv) (1m), Q88(e)(ii) (0m)
• 0620/33 May/June 2016: Q33(b)(ii) (1m), Q44(c)(i) (1m), Q77(d)(i) (1m) (+1 more)

Common Misconceptions