The first cells for fused-salt electrolysis built after 1886 were pots one metre in diameter and they had a single anode (positive electrode). Around 1900, there was a change to using rectangular containers with a surface area of 20 square metres or more and up to a dozen anodes. In principle, today"s pots are still built in the same way but many details have been changed and the pots improved:
- A steel shell up to 20 metres long, four metres wide and 1.5 metres deep is lined with carbon, which serves as the cathode (negative electrode). There are 20 to 40 carbon anodes, which can be moved vertically, arranged above the pot. The anodes are prebaked to carbon blocks from petroleum coke and coal-tar pitch, the cathodes from hard coal and binder or graphite.
- The pot is filled with (up to 20 tonnes of) molten cryolite at about 950 °C, additives and liquid metal. Two to four per cent aluminium oxide is dissolved in this bath. The aluminium oxide breaks down into aluminium and oxygen.
- A direct-current voltage of four to five volts is applied between anode (which protrudes into the bath) and cathode, which results in a current density of 0.6-1.0 amperes per square centimetre and a total current of up to 300,000 amperes in the bath.
- Approximately 40 per cent of the current is converted into heat as a result of the electrical resistance in the bath and keeps it at working temperature (about 950 °C).
- The other part of the current causes the actual electrolysis, in other words the release of the aluminium at the cathode and the oxygen at the anode.
- As the molten aluminium with 2.3 grams per cubic centimetre is heavier at the operating temperature than the cryolite with 2.1 grams per cubic centimetre, it collects at the bottom of the pot as a "bath of metal" (up to 2.5 tonnes a day). It is then siphoned off from there every one or two days.
- The oxygen reacts with the carbon of the anode to form carbon monoxide and carbon dioxide, i.e. the anode burns away (0.42-0.45 kilograms of graphite per kilogram of extracted aluminium, so that the anode is consumed within three to four weeks).
- A solid crust containing cryolite and aluminium oxide forms on the surface of the bath. It is pierced every few minutes in order to add a few kilograms of aluminium oxide.
- The aluminium oxide concentration is kept as constant as possible in order to obtain optimal efficiency or the lowest energy requirement. Control is carried out via the aluminium oxide dosing or, if necessary, by raising and lowering the anodes, to maintain the most favourable separation between anode and metal bath (which takes on the role of the cathode), namely some five centimetres.
- The cell is hooded (enclosed) in order to extract and clean all of the gaseous emissions; for each tonne of aluminium, about 750 cubic metres of "anode gas" (carbon dioxide mixed with fluorides, and containing traces of carbon monoxide) and dust, together with up to 150,000 cubic metres of air from the potroom.
In modern aluminium smelters, the pots are connected electrically and, in the interests of occupational health and environmental protection, linked via waste-gas ducts to a gas cleaning (and fluoride recovery) plant. With the exception of the removal of the aluminium from the pot and the replacement of the anodes, practically all steps are automated.