Metals: Extraction, Alloys, and Applications

How a metal is extracted from its ore, what alloys it forms, and whether it is essential or toxic to living things all follow from its chemistry. The reactivity series determines the extraction method; the structure of an alloy determines its improved properties; and the chemistry of heavy metals explains their toxicity.

Extraction of Metals¶

The method of extraction depends on position in the reactivity series. Reactive metals hold their electrons weakly but lose them easily to form stable compounds — this means a lot of energy is required to reverse this and reclaim the metal.

Extraction of Iron (Blast Furnace)¶

Two extraction processes come up repeatedly on the syllabus: iron, which sits below carbon and can be reduced by carbon monoxide, and aluminium, which sits above carbon and must be extracted by electrolysis.

Iron is extracted from iron ore (haematite, Fe₂O₃) by reduction with carbon monoxide in a blast furnace. Coke (carbon), iron ore, and limestone are fed in at the top; hot air is blasted in at the bottom.

Key reactions:

1. Carbon burns to form CO₂: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$
2. CO₂ reacts with more coke to form CO: $\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$
3. CO reduces iron oxide: $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$
4. Limestone removes silica impurities as slag:

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

$\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \quad \text{(slag)}$

Molten iron sinks to the bottom and is tapped off. The product is pig iron, which is brittle due to its high carbon content (~4%). It is converted to steel by controlled oxidation to reduce carbon content to 0.1–1.5%.

Extraction of Aluminium¶

Because aluminium is above carbon in the reactivity series, carbon cannot reduce it. Electrolysis is the only option.

Aluminium is above carbon in the reactivity series, so carbon cannot reduce it. It is extracted by electrolysis of molten aluminium oxide. Key points:

• Ore is bauxite (Al₂O₃, aluminium oxide)
• Bauxite is dissolved in molten cryolite to lower the melting point from ~2000 °C to ~950 °C
• The molten mixture is electrolysed: Al³⁺ is reduced at the cathode, O²⁻ is oxidised at the carbon anodes
• Carbon anodes burn away as they react with the oxygen produced: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$

The high energy cost of electrolysis makes aluminium more expensive to produce than iron despite aluminium being more abundant in the Earth's crust.

Alloys¶

Pure metals are rarely used as extracted. Most applications need something stronger, harder, or more corrosion-resistant than any single pure metal provides, which is why alloys exist.

Pure metals are often too soft or too brittle for practical uses. Mixing metals (or adding non-metals) produces alloys with improved properties.

Alloys have different properties from their component metals because the different-sized atoms disrupt the regular lattice arrangement, preventing layers from sliding over each other. This is why alloys are stronger and harder than pure metals.

Metals in Living Systems¶

Not all of this is industrial. Several metals are biologically essential, filling roles in living organisms that no other element can take over.

Metals play essential biological roles:

Harmful Effects of Toxic Metals¶

Some metals are toxic rather than essential. What makes heavy metals resistant and durable also means they accumulate in living tissue rather than breaking down.

Heavy metals are particularly dangerous because they are not biodegradable — they accumulate in organisms (bioaccumulation) and become more concentrated at higher trophic levels in a food chain (biomagnification).