Extracting and reusing metals

Reactivity decides the method

Whether a metal is extracted by reduction with carbon or by electrolysis depends on its position in the reactivity series.

• More reactive than carbon (K, Na, Ca, Mg, Al) → extracted by electrolysis of a molten ore (energy-intensive but the only way).
• Less reactive than carbon (Zn, Fe, Sn, Cu) → extracted by reduction with carbon in a blast furnace (cheaper, well-established).
• Below hydrogen (Ag, Au, Pt) → often found native (uncombined).

Iron in the blast furnace

Iron(III) oxide is reduced by carbon monoxide:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Coke burns to make heat; limestone removes silicate impurities as molten slag (CaSiO₃).

Aluminium by electrolysis

Al₂O₃ is dissolved in molten cryolite (lowers melting point from ~2050°C to ~950°C → energy saving).

• Cathode (−): Al³⁺ + 3e⁻ → Al
• Anode (+): 2O²⁻ → O₂ + 4e⁻ (oxygen reacts with carbon anode → CO₂; anodes burn away and must be replaced).

Biological extraction (Higher)

Used for low-grade copper ores where mining and smelting are uneconomic:

• Phytomining: plants absorb metal ions; ash is smelted.
• Bioleaching: bacteria oxidise sulphide ores → leachate solution; copper recovered by displacement (e.g. with scrap iron) or electrolysis.

Biological methods are slower but use less energy, less land disturbance and lower CO₂ emissions.

Life-cycle assessment (LCA)

A cradle-to-grave audit of environmental impact, with four stages: raw materials → manufacture → use → disposal/recycling. Compares two products by total energy/water use, CO₂ produced, and waste at each stage.

Recycling lowers raw-material extraction, energy use and landfill — but transport and reprocessing themselves use energy. LCAs are partly subjective: some impacts are easy to quantify (kWh) and others are not (visual amenity).

OCR exam tip

When choosing between extraction methods, justify "why this metal" — link reactivity, cost, energy, and supply.