Hall-Héroult electrolysis makes aluminum by passing direct current through alumina dissolved in a molten fluoride bath near 960 °C, with a carbon anode that burns away. It takes about 14.4 kWh and 2,980 ampere-hours per kilogram, so a continuous 15 kW supply at 4.5 V, which is 3,333 A, yields only about 25 kg of metal a day. A village without surplus power, refined alumina, a fluoride source and a carbon-anode shop should remelt scrap instead.
Part of Aluminum Smelting
The electrolytic step of aluminum production, what it needs as a chain of inputs, and when a village should not attempt it.
Carbon Cannot Reduce Alumina, So Electricity Must
Aluminum holds its oxygen too tightly for ordinary smelting; the Phase 5 article Aluminum Production says so. The carrier’s answer is electrolysis: dissolve alumina (melting point 2,072 °C, carrier) in molten cryolite so the bath melts near 960 °C, put carbon anodes above and a carbon-lined floor below, pass DC, and molten aluminum, denser than the bath, collects on the floor. The anode burns: 2Al₂O₃ + 3C → 4Al + 3CO₂.
Worked Example: What 15 kW Buys
Charge per kilogram. Aluminum needs 3 electrons per atom. With Faraday’s constant 96,485 C/mol and molar mass 26.98 g (recalled values), 1 A·h gives 26.98 / (3 × 26.8) = 0.336 g at 100% efficiency, so 1 kg takes about 2,980 A·h. At an assumed 93% current efficiency (the Phase 5 article quotes 92-95%) it is 3,200 A·h.
Energy per kilogram. At 4.5 V: 3.2 kAh × 4.5 V = 14.4 kWh/kg. At 4 V it is 12.8 and at 5 V 16.0 kWh/kg. The carrier’s cell range is 4-5 V. Operating Parameters derives this fully.
Output of a 15 kW supply. 15,000 W / 4.5 V = 3,333 A. Metal per hour: 3,333 × 0.336 × 0.93 = 1.04 kg. Per day: about 25 kg. The carrier’s own estimate is 15-20 kW per kg/h, which matches; it allows for the heat loss of a small cell, which I recall is proportionally larger than a big plant’s.
The conductor. 3,333 A at the Bus Bars rule of 1.5-2 A per mm² of copper needs 3,333 / 2 = 1,670 to 3,333 / 1.5 = 2,220 mm², for example a 50 × 40 mm bar, for one cell.
A hydro source. With 10 m head and 50% efficiency (both example values) in the watts = head × flow(L/s) × 9.81 × efficiency formula of Hydro Generator, 15,000 W needs 15,000 / (10 × 9.81 × 0.5) = 306 L/s, running day and night. The output is 4.5 V at 3,300 A, far below any ordinary alternator’s voltage, so it needs rectified low-voltage supply the corpus does not describe.
The Chain You Need Before The First Kilogram
| Input | Why it is hard | Article |
|---|---|---|
| Pure alumina, 1.89 kg per kg of metal | Pressure digestion in caustic | Simplified Bayer Process |
| Fluoride bath | Cryolite is nearly extinct in nature; synthetic routes use hydrofluoric acid | Cryolite Flux |
| Carbon anodes, about 0.4 kg per kg of metal | Pitch, press and baking kiln | Carbon Electrode Production |
| A cell that survives molten fluoride | Carbon-lined, with no leaks | Cathode Lining |
| Continuous DC | 3,300 A at 4.5 V | above |
The Verdict
Do not attempt this unless a surplus of continuous power already exists, caustic soda is produced reliably, a fluoride source is secured, and a carbon-electrode shop is running. Each is a project. Until then, aluminum comes from scrap: Why Aluminum Matters, Ingot Casting.
Common Mistakes
- Sizing the cell by area and ignoring the supply. Current, not size, is the limit.
- Starting without alumina in stock.
- Treating fluoride as an ordinary flux. See Cryolite Flux.
Safety
Molten fluoride bath at about 960 °C and thousands of amps
The bath and metal are far hotter than molten aluminum scrap, water or damp alumina flashes to steam and throws the bath, and a cold tool is as dangerous. Fluoride fume attacks the lungs and bone; the details are in Cryolite Flux. Carbon monoxide and carbon dioxide come off the anode: work outdoors or under a flue. A disconnect for the DC supply must be where an operator outside the splash zone can reach it.
What’s Next
- Learn next: Cryolite Flux - the bath and its hazards
- Combine with: Operating Parameters - controlling the cell