Extraction and Uses of Metals

The big idea. The method used to extract a metal from its ore depends on how strongly the metal holds onto its oxide / sulfide / halide. More reactive metals hold their compounds tightly; less reactive metals release them readily. Carbon is the natural reference: if C can take the oxygen FROM the metal oxide, carbon reduction is sufficient. If not, more energetic electrolysis is needed.

The three categories.

Position in reactivity series	Examples	Method
ABOVE carbon	K, Na, Ca, Mg, Al	ELECTROLYSIS of molten ore
BELOW carbon (and above H)	Zn, Fe, Sn, Pb	Reduction with C / CO
BELOW hydrogen (very unreactive)	Au, Ag, Pt, (Cu)	Found NATIVE; physical separation

1. Electrolysis — for K, Na, Ca, Mg, Al.

Carbon CANNOT reduce these oxides because the metal is more reactive than carbon — i.e. the metal holds the oxygen more strongly than carbon does. We must use an even more powerful reducing process: ELECTROLYSIS forces electrons onto the metal cation using electrical energy.

Examples:

• Sodium: electrolysis of MOLTEN NaCl in the Downs cell.
• Magnesium: electrolysis of molten MgCl₂.
• Aluminium: electrolysis of Al₂O₃ dissolved in cryolite.

Electrolysis is EXPENSIVE because it consumes huge amounts of electrical energy. Used only when necessary.

2. Carbon reduction — for Zn, Fe, Sn, Pb, (Cu).

When the metal is LESS reactive than carbon, carbon CAN take the oxygen and form CO₂ (or CO), reducing the metal oxide. This is done at high temperature in a furnace.

Examples:

• Iron: Fe₂O₃ + 3CO → 2Fe + 3CO₂ in a blast furnace.
• Zinc: ZnO + C → Zn + CO at high temperature.
• Copper (low-grade ore): Cu₂O + C → 2Cu + CO.

Carbon reduction is MUCH CHEAPER than electrolysis — it only needs heat, not electricity. So we use it whenever the metal allows.

3. Native (found uncombined) — for Au, Ag, Pt, sometimes Cu.

Some metals are so unreactive that they exist in nature as the FREE ELEMENT. Gold is the classic example — found as nuggets or fine flakes in alluvial deposits and quartz veins. No chemical extraction is needed; just physical separation:

• Panning: dense gold sinks in water-washed gravel.
• Crushing + cyanide leaching: dissolves gold from finely-crushed ore as a complex; precipitated and refined.
• Electrorefining: final purification by electrolysis.

Silver and platinum also occur native and are extracted by similar physical methods.

Why not electrolysis for everything? Electrolysis WOULD work for iron, copper etc., but it's MUCH more expensive than the alternative. With ~1.5 billion tonnes of steel produced worldwide each year, the cost difference is enormous. Use the cheapest method that works.

Choosing the method — algorithm.

1. Look up the metal in the reactivity series.
2. Above C? Use electrolysis.
3. Below C but above H? Use C/CO reduction.
4. Below H and very unreactive? Look for native deposits.

The challenge. Aluminium is the third-most-abundant element in Earth's crust (~8% by mass), but it took until 1886 for an economic extraction process (the Hall–Héroult process) to be developed. Why so hard? Because aluminium is ABOVE carbon in the reactivity series, so carbon reduction doesn't work — and pure Al₂O₃ melts at the eye-watering temperature of ~2050 °C.

The breakthrough — using cryolite as a solvent. Cryolite (Na₃AlF₆) is a mineral that DISSOLVES Al₂O₃ at a much lower temperature. The mixture's mp is ~950 °C — energy-affordable.

The Hall–Héroult cell — apparatus.

• Large steel tank lined with carbon (graphite) — this lining is the CATHODE (negative electrode).
• Several carbon blocks dipped from above into the molten mix — these are the ANODES (positive electrodes).
• The molten mixture: Al₂O₃ dissolved in cryolite at ~950 °C.
• A very large DC current (~100,000-300,000 A) is passed through the cell.

Process. Al₂O₃ in the melt ionises (essentially):

$\text{Al}_2\text{O}_3 \rightarrow 2\text{Al}^{3+} + 3\text{O}^{2-}$

The ions migrate to the electrodes and are discharged.

Cathode (reduction):

$\text{Al}^{3+}\text{(l)} + 3e^- \rightarrow \text{Al(l)}$

Molten aluminium forms at the cathode. Because Al (mp 660 °C) is liquid at 950 °C, and is DENSER than the cryolite mixture, it pools at the bottom of the cell. It is tapped off periodically and cast into ingots.

Anode (oxidation):

$2\text{O}^{2-}\text{(l)} \rightarrow \text{O}_2\text{(g)} + 4e^-$

Oxygen gas is evolved at the carbon anode. (Note the stoichiometry — each O²⁻ has charge 2−, so two O²⁻ ions release 4 electrons to form one O₂ molecule.)

Why the carbon anode wears away.

The anode operates at ~950 °C and is in direct contact with hot O₂. Hot carbon reacts with oxygen:

$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}$

Some CO is also formed. The carbon anode is gradually 'eaten away' as CO₂. The anodes get thinner over weeks of operation and must be REPLACED REGULARLY — a significant running cost.

Energy and cost. Aluminium production is extremely energy-intensive: ~14 MWh per tonne of Al produced. Worldwide aluminium electrolysis consumes ~3.5% of global electricity. For this reason, smelters are often located near cheap hydroelectric or geothermal power (Iceland, Norway, Canada).

Why we still use aluminium despite the cost. Aluminium has a unique combination of properties:

• Low density (~2.70 g/cm³) — about 1/3 that of steel. Critical for aircraft, cars, drink cans.
• Strong (especially in alloys like duralumin).
• Excellent corrosion resistance — forms a thin tough Al₂O₃ layer that protects from further oxidation.
• Good electrical conductor — used for long-distance power transmission lines (lighter than copper).
• Good thermal conductor and reflector — used for cooking foil and insulation.
• Non-toxic — used for food packaging.
• Easily recycled — 95% energy saving versus primary production.

The setup. A blast furnace is a tall (40+ m) tower lined with refractory bricks. It is continuously charged from the top with three solid raw materials and has hot air blown in near the bottom.

Raw materials.

1. Iron ore — usually haematite (Fe₂O₃) or magnetite (Fe₃O₄).
2. Coke — refined coal, mostly C. Provides both heat (combustion) and the reducing agent (via CO).
3. Limestone — CaCO₃. Removes the acidic silica (SiO₂) impurity from the ore.

Hot air (~1200 °C) is blasted in near the bottom of the furnace.

The four key reactions.

Reaction 1 — Combustion of coke (heat source).

$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}$

Highly exothermic. The temperature near the bottom reaches ~2000 °C.

Reaction 2 — CO₂ reduced to CO (creates the reducing agent).

$\text{CO}_2\text{(g)} + \text{C(s)} \rightarrow 2\text{CO(g)}$

The CO₂ rises through more hot coke and is partially reduced to carbon monoxide. CO is the ACTUAL REDUCING AGENT for the iron ore.

Reaction 3 — Main extraction: CO reduces Fe₂O₃.

$\text{Fe}_2\text{O}_3\text{(s)} + 3\text{CO(g)} \rightarrow 2\text{Fe(l)} + 3\text{CO}_2\text{(g)}$

The KEY reaction. Carbon monoxide reduces the iron oxide to molten iron, which drains to the bottom of the furnace. This happens in the middle of the furnace at ~1000-1500 °C.

Reaction 4 — Thermal decomposition of limestone.

$\text{CaCO}_3\text{(s)} \rightarrow \text{CaO(s)} + \text{CO}_2\text{(g)}$

At the cooler upper levels of the furnace, limestone breaks down. The CaO produced is needed for the next step.

Slag formation — removing silica impurities.

Iron ore contains silica (sand, SiO₂) as an ACIDIC impurity. CaO is a BASE, so it reacts:

$\text{CaO(s)} + \text{SiO}_2\text{(s)} \rightarrow \text{CaSiO}_3\text{(l)}$

The product, calcium silicate (CaSiO₃) — known as SLAG — is a molten liquid LESS DENSE than molten iron. It floats on top of the iron at the bottom of the furnace and is tapped off SEPARATELY.

Slag is NOT waste. It is sold for road-building (aggregate), fertiliser ingredients (calcium and silica) and breeze blocks for construction.

Products of the furnace.

• Molten iron ('pig iron'), tapped from the bottom. Contains ~4% C and other impurities. Sent for further refining (basic oxygen process) to make steel.
• Slag (CaSiO₃), tapped separately.
• Waste gases (CO₂, CO, N₂) exit the top. Cleaned and partially recycled to preheat the incoming hot air ('regenerative heating') — improves efficiency.

Why continuous operation?

1. The furnace takes DAYS to heat up to operating temperature — restarting after a shutdown costs enormous energy.
2. Cooling and reheating stresses the refractory brick lining → cracks → expensive repairs.
3. Production economics — a typical blast furnace produces ~10,000 tonnes of iron PER DAY.

So blast furnaces run 24/7/365 for years between major rebuilds (typically 15-20 year 'campaigns').

From pig iron to steel. Pig iron is brittle because of its high carbon content (~4%). To make steel, the carbon content is reduced to 0.05-2% by blowing O₂ through molten pig iron in a Basic Oxygen Furnace. Some carbon and other impurities oxidise and are removed. Steel is harder, stronger, and tougher than pure iron or pig iron.

Choosing the right metal for a job is about matching the properties to the application. Below are the most common cases tested in 4CH1.

Aluminium — used for aircraft, cans, foil, power cables.

Properties: LOW density (1/3 of steel), STRONG (in alloys), CORROSION-RESISTANT (Al₂O₃ layer), GOOD conductor (heat + electricity), NON-TOXIC, RECYCLABLE.

Applications:

• Aircraft bodies + frames — low density essential for fuel efficiency; alloys (duralumin = Al + Cu + Mg + Mn) make it as strong as steel.
• Drink cans + food packaging — light, non-toxic, easily recycled, doesn't react with most foods/drinks (oxide layer protects).
• Power cables (especially long-distance) — lighter than copper, so cables can sag less between pylons. Slightly lower conductivity than copper, but compensated by larger cross-section.
• Cooking foil and pans — good thermal conductor; light.
• Window frames + cladding — corrosion-resistant outdoors.

Copper — used for electrical wiring, plumbing, electronics.

Properties: EXCELLENT electrical conductor (2nd to silver, but cheaper), DUCTILE (drawn into wires), MALLEABLE (bent and shaped), CORROSION-RESISTANT (in water), ANTIMICROBIAL, easily JOINED by soldering.

Applications:

• Electrical wiring — power cables, household wiring, electronics. ≥99.99% pure copper (electrolytically refined) for lowest resistance.
• Plumbing pipes — corrosion-resistant in water, soldered easily, malleable for bending.
• Heat exchangers — good thermal conductor.
• Roofs — develops green patina (basic copper carbonate) which is itself corrosion-resistant; lasts centuries (cathedrals, statues).

Iron and steel — used for construction, cars, ships, tools.

Properties: STRONG, CHEAP, easily SHAPED (rolled, pressed, forged), MAGNETIC, RECYCLABLE.

Mild steel (Fe + 0.1-0.3% C):

• Construction: bridges, beams, reinforced concrete.
• Cars: body panels, chassis.
• Ships: hulls.
• Tools: hammers, wrenches.

Limitation: rusts. Must be painted or galvanised.

Stainless steel (Fe + ~18% Cr + ~8% Ni):

• Cutlery — corrosion-resistant.
• Surgical instruments — non-corroding, sterilisable, biocompatible.
• Kitchen sinks, food-processing equipment.
• Architectural cladding (e.g. the Gateway Arch in St Louis).

The Cr forms a thin Cr₂O₃ layer that prevents corrosion of the iron beneath — even when scratched.

Common alloys.

Alloy	Composition	Key properties	Uses
Steel	Fe + 0.1-2% C	Strong, cheap	Construction, cars
Stainless steel	Fe + Cr + Ni	Corrosion-resistant	Cutlery, surgery
Brass	Cu (~70%) + Zn (~30%)	Hard, decorative, antimicrobial	Door handles, instruments
Bronze	Cu + Sn	Hard, corrosion-resistant	Statues, bells, propellers
Duralumin	Al + Cu + Mg + Mn	Strong + low density	Aircraft
Solder	Sn + Pb (older) or Sn + Cu + Ag (lead-free)	Low mp	Joining metal parts

Why alloys are stronger than pure metals. In a pure metal, atoms are all the same size and arranged in regular layers that slide past each other under stress (making the metal soft). In an alloy, atoms of DIFFERENT SIZES disrupt the regular lattice → layers cannot slide as easily → harder and stronger.

This is why pure copper is too soft for door handles (brass is preferred) and pure iron is too soft for construction (steel — Fe + C — is much stronger).

Why recycle metals?

Most metal ores are FINITE resources — there is only so much in Earth's crust, and economic-grade ores are limited. Mining causes environmental damage (deforestation, habitat loss, water pollution, dust). Smelting and electrolysis consume enormous energy. Metal objects don't biodegrade and accumulate in landfill.

Recycling addresses all of these problems by recovering metals from used products and reprocessing them into new ones.

Key advantages of recycling.

1. Massive energy savings.

   Metal	Energy saving (vs primary)
   Aluminium	~95%
   Copper	~80%
   Steel	~60-70%
   Lead	~65%

   For aluminium, recycling melts existing metal (~660 °C) instead of running the energy-intensive electrolysis. The 95% saving is one of the highest of any material.

2. Reduces fossil-fuel use + CO₂ emissions. Lower energy use → less coal/gas burned in power stations → lower greenhouse-gas emissions.

3. Conserves finite ore reserves. Bauxite, copper ores, iron ores are not infinite. Recycling stretches their lifespan.

4. Less mining. Less land disturbance, less deforestation, less water pollution from acidic mine drainage.

5. Less waste / landfill. Metals can be recycled indefinitely without losing quality. Aluminium and steel can be melted down and reused many times.

6. Economic benefits. Often cheaper than primary extraction (especially for aluminium and copper); creates recycling industry jobs.

Disadvantages and limitations.

1. Cost of collection + sorting. Kerbside collection, transport, automated sorting facilities all cost money and energy.

2. Contamination. Mixed scrap, paint, food residues, plastic labels need to be separated.

3. Some items too small/light to be economic. Aluminium foil is light, contaminated, expensive to sort → less commonly recycled than cans.

4. Some alloys hard to separate. Stainless steel can be recovered, but separating Cr and Ni from Fe is difficult.

5. Quality control. Some applications (electronics) demand very pure metal; recycled may not always meet specifications.

Practical strategies to increase recycling.

1. Kerbside recycling programmes — separate bins for cans, plastics, paper.
2. Deposit-return schemes — pay-back on cans/bottles (Germany, Sweden, some US states).
3. Producer responsibility — manufacturers must take back their products at end-of-life (EU WEEE Directive for electronics).
4. Design for recycling — products designed to be easily disassembled and materials separated.
5. Public education + labelling — clear instructions on what can/can't be recycled.

Life-cycle assessment (LCA).

An LCA is a systematic comparison of the ENVIRONMENTAL IMPACT of a product over its FULL LIFE CYCLE — from raw materials to disposal. Key stages:

1. Raw material extraction. Mining ore, processing.
2. Manufacture. Smelting, refining, alloying, forming the product.
3. Distribution. Transport, packaging.
4. Use phase. Energy used during operation; emissions; wear.
5. End of life. Disposal — recycling, incineration, or landfill.

For each stage, the LCA quantifies: energy used, CO₂ emitted, water used, waste produced, other environmental harms.

Example — aluminium can vs glass bottle for soft drinks.

Stage	Aluminium can	Glass bottle
Raw materials	Bauxite + cryolite	Silica sand + soda + lime
Manufacture	Smelting + rolling + forming	Melting + moulding
Distribution	Lightweight → less fuel	Heavy → more fuel
Use	Single use, recyclable	Often refilled
End of life	95% energy saved on recycling	Refill avoids manufacture

LCAs often reveal that the BEST environmental choice depends on context. A glass bottle that's refilled 25 times beats a single-use can; but a single-use can is better than a glass bottle used only once.

The circular economy. Modern environmental thinking goes beyond recycling to a 'circular economy' — designing products and materials to circulate continuously, not flow once from extraction to landfill. Principles: design for durability, repair, reuse, recycling; share rather than own; turn waste into resource. Many companies (especially in EU + Asia) now commit to circular-economy goals.

Mining vs urban mining. Conventional mining digs ore from the ground. 'Urban mining' recovers metals from already-built infrastructure — old wiring, demolished buildings, scrap cars, electronic waste. As the easy ores are depleted, urban mining will become increasingly important.