Electrolysis is carried out in large steel vessels lined with a graphite cathode, and into which an array of carbon-based anodes is suspended. Other features include:
The cell voltage is in the region of 4v, but the current can be as much as 300,000 amps in each cell. The heating effect of the electric current is responsible for maintaining the temperature of the cell.
At the cathode that lines the cell, aluminium is produced and collects below the less dense electrolyte.
At the anode oxygen is produced, which reacts with carbon in the anode to produce carbon dioxide. This eats away at the anode, which has to be replaced about every 20 days.
Electrolyte composition is important, and it is necessary to replace
fluorine lost as fluorocarbons and inorganic fluorides. This is achieved
by adding aluminium fluoride, made from aluminium hydroxide available
from the Bayer process.
Pre-baked anodes are constructed from graphite or coke, mixed with pitch (made from coal tar) and baked. Used, crushed anode stubs are included in the mix. Each anode lasts about 20 days, and so 20 anodes will typically be used in a cell, with one being changed every day. This allows production to continue more or less without interruption.
Anode production forms a significant part of an aluminium plant. A 200,000 tonnes per year aluminium facility will consume in the region of 100,000 tonnes of anodes, generating a lot of carbon dioxide in the process. Much research is therefore directed towards designing inert, non-consumable anodes. This would significantly reduce carbon dioxide emissions from the electrolysis, and remove the need for the continual anode preparation and baking.