Editing Hall–Héroult process (section)

===Cell operation===
Cells in factories are operated 24 hours per day so that the molten material in them will not solidify. Temperature within the cell is maintained via electrical resistance. Oxidation of the carbon [[anode]] increases the electrical efficiency at a cost of consuming the carbon electrodes and producing carbon dioxide.<ref name="georg" />{{clarify|date=January 2025}}

While solid cryolite is [[density|denser]] than solid aluminium at room temperature, liquid aluminium is denser than molten cryolite at temperatures around {{convert|1000|°C|°F}}. The aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The liquid aluminium is [[Siphon|siphoned]] every 1 to 3 days to avoid having to use extremely high temperature valves and pumps. Alumina is added to the cells as the aluminum is removed. Collected aluminium from different cells in a factory is finally melted together to ensure uniform product and made into metal sheets. The electrolytic mixture is sprinkled with coke to prevent the anode's oxidation by the oxygen involved.<ref name="georg" />

The cell produces gases at the anode, primarily CO<sub>2</sub> produced from anode consumption and [[hydrogen fluoride]] (HF) from the cryolite and [[Flux (metallurgy)|flux]] (AlF<sub>3</sub>). In modern facilities, fluorides are almost completely recycled to the cells and therefore used again in the electrolysis. Escaped HF can be neutralized to its sodium salt, [[sodium fluoride]]. [[Particulate]]s are captured using [[Electrostatic filter|electrostatic]] or bag filters. The CO<sub>2</sub> is usually vented into the atmosphere.<ref name="georg" />

Agitation of the molten material in the cell increases its production rate at the expense of an increase in cryolite impurities in the product. Properly designed cells can leverage [[Magnetohydrodynamics|magnetohydrodynamic]] forces induced by the electrolysing current to agitate the electrolyte. In non-agitating static pool cells, the impurities either rise to the top of the metallic aluminium, or sink to the bottom, leaving high-purity aluminium in the middle area.<ref name="georg" />