How aluminum is made

Aluminum is made by running a huge electric current through molten salt, and the metal sinks to the bottom.

stations: 5
things upstream: 10
raw sources: 6
steps from the ground: 3

The short version

Aluminum is made by electrolysis. Alumina powder is dissolved in a bath of molten cryolite at 940 to 980 °C, and a direct current of 100 to 300 kiloamperes runs between carbon anodes and the cell floor. Liquid metal collects at the bottom, is siphoned out every day or two, and is cast into ingots.

Bake the carbon anodes

Petroleum coke, the solid left over from refining crude oil, is bound with pitch from coal tar and baked in big gas-fired ovens into carbon blocks. These are the anodes, and they are used up in the reaction, so smelters must make them continuously.

Goes in
Crude oil Coal Petroleum coke
Dissolve alumina in cryolite

Alumina melts at 2072 °C, far too hot to be practical. So it is dissolved in molten cryolite, a sodium aluminum fluoride that melts near 1000 °C, with some aluminum fluoride added to lower the temperature further.

940 to 980 °C

Goes in
Alumina
Run the current through the pot

A low direct-current voltage, under 5 volts, but at 100 to 300 kiloamperes, runs through the bath. Oxygen from the alumina burns the carbon anode away to carbon dioxide, and aluminum is deposited on the cell floor. Pots run around the clock so the bath never freezes.

about 15.37 kWh per kilogram
Siphon off the metal

Liquid aluminum is denser than the molten bath, so it sinks and pools at the bottom. It is siphoned out every 1 to 3 days, which avoids hot-metal valves and pumps. Fresh alumina is fed in to replace what was used.

Goes in
Alumina
Alloy and cast

The metal goes to a holding furnace, where other metals such as magnesium, manganese, copper, zinc or silicon are added for strength, and then is cast into ingots, billets or slabs.

End of the lineAluminum Next it goes into deodorant, lipstick, microchip and 61 more.

Worth repeating

Recycling aluminum takes only 5 percent of the energy of making it from ore.
Cells in a smelter run 24 hours a day, because if the molten bath were left to cool it would solidify.
Iceland has no notable bauxite, yet it ranks among the larger aluminum producers because its hydropower is cheap and plentiful.