Extracting Metals: GCSE Chemistry Guide to Reduction and Electrolysis

Why does reactivity determine the extraction method?

The reactivity series ranks metals from most reactive (potassium, at the top) to least reactive (gold, at the bottom). A more reactive metal forms stronger bonds with oxygen and other non-metals, so it requires more energy to extract from its compound.

• Less reactive than carbon (e.g. iron, copper, lead, zinc): can be extracted by heating the ore with carbon (reduction). Carbon "steals" the oxygen from the metal oxide because carbon is more reactive than these metals at high temperatures.
• More reactive than carbon (e.g. aluminium, magnesium, calcium, sodium): carbon cannot displace these metals. They must be extracted by electrolysis — passing an electric current through the molten ore to force the ions to become atoms.
• Very unreactive metals (gold, platinum, silver): found as native elements; no extraction needed.

How is iron extracted using the blast furnace?

Iron is extracted from its ore (haematite, Fe₂O₃) in a blast furnace:

1. Iron ore, coke (carbon) and limestone are added at the top.
2. Hot air (oxygen) is blasted in at the bottom; coke burns to form carbon dioxide: C + O₂ → CO₂
3. Carbon dioxide reacts with more coke to form carbon monoxide: CO₂ + C → 2CO
4. Carbon monoxide reduces the iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (the key reduction step)
5. Molten iron sinks to the bottom and is tapped off.
6. Limestone (CaCO₃) decomposes and removes acidic silica rock impurities as slag.

The overall equation for the key step is: Fe₂O₃ + 3CO → 2Fe + 3CO₂

Carbon monoxide is the reducing agent — it gains oxygen (is oxidised) while iron oxide loses oxygen (is reduced).

How is aluminium extracted by electrolysis?

Aluminium is more reactive than carbon, so carbon cannot reduce it. Electrolysis of molten aluminium oxide (Al₂O₃), known as the Hall–Héroult process, is used:

• Aluminium oxide is dissolved in molten cryolite (a naturally occurring fluoride mineral) to lower the melting point from 2072 °C to about 950 °C — a crucial energy-saving step.
• A large electric current passes through the molten mixture between carbon electrodes.
• At the cathode (negative electrode): Al³⁺ ions gain electrons (reduction) → Al atoms: Al³⁺ + 3e⁻ → Al
• At the anode (positive electrode): O²⁻ ions lose electrons (oxidation) → O₂ gas: 2O²⁻ → O₂ + 4e⁻
• Molten aluminium sinks to the bottom and is tapped off.

Why is aluminium more expensive than iron?

Although aluminium is the most abundant metal in the Earth's crust, it is more expensive than iron because:

• Electrolysis consumes large amounts of electrical energy, which is costly.
• The carbon anodes are slowly oxidised and must be replaced regularly.
• The process requires the aluminium oxide to be molten, maintaining temperatures above 950 °C continuously.

For every tonne of aluminium produced, roughly 13–14 megawatt-hours of electricity are consumed. This is why recycling aluminium saves about 95% of the energy compared with extracting it from ore.

What are phytomining and bioleaching?

Traditional mining and smelting of low-grade ores is economically impractical. Two newer methods can extract copper and other metals from ores that are too lean for conventional processing:

• Phytomining: certain plants (hyperaccumulators) absorb metal ions through their roots. The plants are harvested and burned; the metal is extracted from the ash.
• Bioleaching: bacteria metabolise metal sulfide ores, producing a leachate solution containing dissolved metal ions. Copper is recovered from the leachate by displacement with scrap iron: Fe + CuSO₄ → FeSO₄ + Cu

These methods are slower than conventional smelting but have a lower environmental impact and can be used on sites where bulk mining is not economical.

Frequently asked questions

Why can carbon reduce iron oxide but not aluminium oxide?

Reduction by carbon depends on carbon being more reactive than the metal at the temperature used. At blast furnace temperatures (~1200–1500 °C), carbon is more reactive than iron, so it can displace iron from its oxide. Aluminium is more reactive than carbon under all practical conditions, so carbon cannot take oxygen away from aluminium oxide — the reaction simply does not occur. Electrolysis, which supplies energy directly to the ions, overcomes the reactivity barrier.

What is the role of limestone in the blast furnace?

Limestone (calcium carbonate, CaCO₃) serves as a flux: it decomposes at high temperature to form calcium oxide (CaO) and carbon dioxide. The calcium oxide then reacts with acidic silica impurities (SiO₂) in the ore to form calcium silicate slag (CaSiO₃), which is molten and sinks below the iron. Slag is not waste — it is used in road construction and as a fertiliser. Without limestone, silica impurities would contaminate the iron.

Why must the ore be molten (not just dissolved in water) for electrolysis to work?

Electrolysis works by moving ions towards oppositely charged electrodes. In a solid ore, ions are fixed in a lattice and cannot move. In aqueous solution, water molecules can also be discharged — for example, electrolysing dissolved aluminium sulfate would produce hydrogen gas at the cathode, not aluminium. Using a molten ore or a molten ionic compound ensures that only the target metal ions are mobile and present in sufficient concentration to be extracted efficiently.

How does recycling reduce environmental impact compared with primary extraction?

Recycling metals avoids the need for mining (which destroys habitats and produces large amounts of waste rock), reduces energy consumption dramatically (95% less for aluminium, ~70% less for copper), and reduces carbon dioxide emissions from smelting. It also preserves finite ore reserves. For aluminium especially, the energy cost of recycling is so much lower than primary extraction that much of the world's aluminium comes from recycled sources.

---

For particle-model chemistry tutoring that connects industrial processes to examination questions, explore Professor Curie at aitutors.me.