Aluminium — Solidified Electricity
The third most abundant element in the crust, once more valuable than gold. What changed was not geology but electrochemistry — and the chain still reflects that.
Ore to Vehicle · Part 2 — The Metals · Chapter 5 · 20 min read
Aluminium is the third most abundant element in the crust and was once more valuable than gold. What changed was not geology but electrochemistry — and the chain still reflects that: aluminium is, in a real sense, solidified electricity.
13–15 MWh
Per tonne of primary metal
2,072 → 960 °C
What cryolite does
4–5 : 2 : 1
Bauxite to alumina to aluminium
~5 %
Energy to recycle instead
5.1 — Bauxite is a rock, not a mineral
Bauxite is formed by intense tropical weathering that leaches away silica and leaves aluminium hydroxides behind. Its useful minerals are gibbsite Al(OH)₃, boehmite AlO(OH) and diaspore — which matters, because gibbsite dissolves at 140 °C and diaspore needs 250 °C and more caustic.
Guinea’s gibbsitic bauxite is cheap to process; China’s diasporic bauxite is not, which is a large part of why China imports so much of it. Guinea, Australia, Brazil, China, India and Indonesia supply the roughly 400 Mt mined each year.
5.2 — The Bayer process, 1888
In plain English
The trick is that aluminium hydroxide is amphoteric — it dissolves in strong base, while iron and titanium oxides do not. That single difference in solubility does the whole separation.
- •1 · Digestion at 150–250 °C, 5–30 bar, hot caustic soda. Al(OH)₃ + NaOH → NaAl(OH)₄, dissolved. Fe₂O₃, TiO₂ and silicates do not dissolve.
- •2 · Clarification. The undissolved residue is red mud — iron oxide, titania, silica and residual caustic at pH 12–13, at 1–1.5 tonnes per tonne of alumina. Billions of tonnes are impounded worldwide, and it is the industry’s largest unresolved environmental liability.
- •3 · Precipitation. NaAl(OH)₄ → Al(OH)₃↓ + NaOH, the reverse of digestion, driven by cooling and seed crystals. The caustic is regenerated and recycled — the elegance of the process.
- •4 · Calcination at 1,000–1,100 °C. 2 Al(OH)₃ → Al₂O₃ + 3 H₂O → smelter-grade alumina, 99.5 % Al₂O₃.
5.3 — Hall–Héroult, and why cryolite is the invention
Alumina melts at 2,072 °C. Electrolysing it directly is impossible at industrial scale. Hall and Héroult independently found that alumina dissolves in molten cryolite, Na₃AlF₆, giving a bath that works at about 960 °C.
Important
That solvent is the invention. Everything else is engineering.
- •Cathode — the carbon lining of the pot: Al³⁺ + 3e⁻ → Al(l), and molten aluminium pools on the floor to be siphoned off.
- •Anode — consumable carbon blocks: 2 O²⁻ + C → CO₂ + 4e⁻. The anode is consumed at roughly 0.4–0.45 t of carbon per tonne of aluminium, an unavoidable direct CO₂ emission of the current process.
- •Overall: 2 Al₂O₃ + 3 C → 4 Al + 3 CO₂, at 4–5 V per cell and 300,000–600,000 A, with cells wired in series into potlines a kilometre long.
5.4 — The number that defines the industry
Technical framing
13 to 15 MWh per tonne. A smelter is an electricity buyer that happens to produce metal, and it cannot be switched off — if a potline freezes it is destroyed.
That is why smelters sit next to hydro dams in Quebec, Iceland, Norway and Odisha, and next to captive coal plants elsewhere. It is also why “green aluminium” is a real distinction rather than marketing: the same metal can carry 4 or 16 tonnes of CO₂ per tonne depending only on which wire is plugged in.
5.4.1 — The anode effect
One more emission worth knowing: when alumina in the bath runs low, an anode effect occurs and the cell starts electrolysing the cryolite itself, producing CF₄ and C₂F₆ — perfluorocarbons with global warming potentials in the thousands. Modern process control exists largely to prevent this.
5.5 — Why recycling wins so decisively
Remelting takes about 5 per cent of the energy of making it from bauxite, because the electrochemical reduction has already been paid for once and metal does not forget. Roughly three-quarters of all aluminium ever smelted is still in use.
Commercially: China smelts around 60 per cent of world primary aluminium under a self-imposed capacity cap near 45 Mt. India is the second-largest producer, with domestic bauxite and integrated players. Guinea supplies a very large share of seaborne bauxite, which makes its politics a genuine input to the world aluminium price. Both alumina and aluminium trade on the LME with deep, liquid markets — unlike most of what follows in this series.
An EV carries 250 to 300 kg of aluminium: battery enclosure, crash structure, motor housing, wheels, heat exchangers, and the cathode current collector foil inside every cell.
Quick check: test yourself
1.Why does the Bayer process separate aluminium from iron so cleanly?
Show answer
2.Why can’t an aluminium smelter be used for demand response?
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3.Two tonnes of aluminium have identical specifications but carbon footprints of 4 and 16 tonnes CO₂. How?
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Chapter summary
- ✓Bauxite mineralogy decides processing cost: gibbsite digests at 140 °C, diaspore needs 250 °C and more caustic.
- ✓Bayer works on one fact — aluminium hydroxide is amphoteric and iron and titanium oxides are not — and regenerates its own caustic.
- ✓Red mud at 1 to 1.5 tonnes per tonne of alumina is the industry’s largest unresolved environmental liability.
- ✓Cryolite is the whole Hall–Héroult invention, dropping the working temperature from 2,072 °C to about 960.
- ✓At 13 to 15 MWh per tonne aluminium is essentially solidified electricity, which is why recycling at 5 per cent of the energy is the strongest recycling case of any metal.
Frequently asked questions
Why does the Bayer process work at all?+
Because aluminium hydroxide is amphoteric — it dissolves in strong base, while the iron and titanium oxides alongside it do not. That single difference in solubility does the entire separation. Hot caustic soda at 150 to 250 °C digests gibbsite and boehmite into dissolved sodium aluminate, the undissolved residue is filtered off as red mud, cooling and seeding reverses the reaction to precipitate aluminium hydroxide, and calcination at 1,000 to 1,100 °C gives smelter-grade alumina. The caustic is regenerated and recycled, which is the elegance of the process.
Why is cryolite the whole invention in Hall–Héroult?+
Because alumina melts at 2,072 °C, which makes direct electrolysis impossible at industrial scale. Hall and Héroult independently found that alumina dissolves in molten cryolite, Na₃AlF₆, giving a bath that works at about 960 °C. That solvent is the invention; everything else is engineering. The cell then runs at 4 to 5 V and 300,000 to 600,000 A, with a consumable carbon anode giving 2 Al₂O₃ + 3 C → 4 Al + 3 CO₂.
Why is green aluminium a real distinction rather than marketing?+
Because 13 to 15 MWh per tonne is the number that defines the industry. A smelter is an electricity buyer that happens to produce metal, and it cannot be switched off — if a potline freezes it is destroyed. The same metal can therefore carry 4 or 16 tonnes of CO₂ per tonne depending only on which wire is plugged in, which is why smelters sit next to hydro dams in Quebec, Iceland, Norway and Odisha and next to captive coal plants elsewhere.
Why is aluminium the strongest recycling case of any metal?+
Because remelting takes about 5 per cent of the energy of making it from bauxite — the electrochemical reduction has already been paid for once, and metal does not forget. Roughly three-quarters of all aluminium ever smelted is still in use. An EV carries 250 to 300 kg of it: battery enclosure, crash structure, motor housing, wheels, heat exchangers and the cathode current collector foil inside every cell.
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Ore to Vehicle is an original educational series on the battery and EV materials supply chain. Country shares, grades, prices and policy status are approximate, drawn from public reporting as of mid-2026, and move year to year — treat them as orders of magnitude rather than as a ledger, and verify before relying on them commercially.