Extraction of Metals

Position in the reactivity series determines how the metal can practically be obtained:

Reactivity	Metals	Method
Most reactive	K, Na, Ca, Mg, Al	Electrolysis of molten compound
Mid (below C)	Zn, Fe, Pb, Sn, Cu	Reduction with carbon
Least reactive	Ag, Au, Pt	Often found native; or heat alone

A metal can only be reduced by something BELOW it in the series. So carbon (placed effectively between Al and Zn) can reduce iron oxide but not aluminium oxide. For metals above carbon, you need a source of electrons stronger than carbon → electrolysis (electrons supplied directly by an electric current).

Inputs: iron ore (haematite, Fe₂O₃), coke (carbon, C), limestone (CaCO₃). Hot air is blown in at the bottom.

The key reactions:

1. Coke burns: C + O₂ → CO₂.
2. CO₂ reacts with more coke: CO₂ + C → 2 CO. (Carbon monoxide is the actual reducing agent.)
3. CO reduces the iron oxide: Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂.
4. Molten iron sinks to the bottom and is tapped off.
5. Limestone decomposes: CaCO₃ → CaO + CO₂.
6. CaO reacts with SiO₂ impurities (sand): CaO + SiO₂ → CaSiO₃ (slag), which floats on the iron and is tapped off separately.

The blast furnace runs continuously, producing pure-ish iron at ~1 500 °C. The iron contains 4-5% carbon ('pig iron') — too brittle to use directly. It's later converted into steel by adjusting the carbon content and adding other elements (Cr, Ni, Mn).

Aluminium ore (bauxite) is purified to alumina (Al₂O₃). Al is too high in the reactivity series for carbon reduction, so we use electrolysis.

Setup: alumina dissolved in molten cryolite (Na₃AlF₆) at ~950 °C. Cryolite lowers the melting point from 2 050 °C (pure Al₂O₃) to a manageable 950 °C, saving huge amounts of energy.

Electrode reactions:

• At cathode (−): Al³⁺ + 3 e⁻ → Al. (Molten aluminium collects at the bottom, drained off periodically.)
• At anode (+): 2 O²⁻ → O₂ + 4 e⁻. (Oxygen gas reacts with the carbon anodes: C + O₂ → CO₂.)

Because the anodes burn away, they have to be REPLACED REGULARLY. This is one of the biggest costs and a major source of CO₂.

Energy: electrolysis of aluminium uses ~15 kWh per kg of aluminium produced — about 1% of all global electricity goes into aluminium production!

Metal extraction has major environmental costs:

• CO₂ emissions: iron/steel produces ~7% of global CO₂; aluminium electrolysis adds more from electricity generation and burning of carbon anodes.
• Mining destruction: open-pit mines disturb large areas, destroying habitats and sometimes displacing people.
• Toxic waste: bauxite mining produces 'red mud' rich in toxic heavy metals.
• Energy use: aluminium consumes ~1% of global electricity; iron/steel consumes 6-8%.

Ways to reduce impact:

• Recycling: making aluminium from scrap uses only ~5% of the energy of primary extraction. Recycling steel uses ~25%.
• Renewable electricity: most aluminium smelters are built near hydropower for cheap and low-CO₂ electricity.
• Hydrogen direct reduction: a new method using H₂ instead of CO to reduce iron oxide — produces water rather than CO₂.

MYP criterion D: discuss whether recycling or improving extraction is the more important step. (Often the recycling option dominates: aluminium can be recycled an unlimited number of times without losing quality.)