Editing Hall–Héroult process (section)

==Process==

===Difficulties faced===
Elemental aluminium cannot be produced by the electrolysis of an [[aqueous solution|aqueous]] [[aluminium salt]], because [[hydronium]] ions readily [[redox|oxidize]] elemental aluminium. Although a [[molten salt|molten]] aluminium salt could be used instead, [[aluminium oxide]] has a melting point of 2072&nbsp;°C (3762°F)<ref>{{cite book |last1=Haynes|first1=W.M.|date=2015|title=CRC Handbook of Chemistry and Physics|edition=96th|location=Boca Raton, FL|publisher=Taylor & Francis|isbn=978-1-4822-6096-0}}</ref> so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al<sub>2</sub>O<sub>3</sub>, is dissolved in molten synthetic [[cryolite]], Na<sub>3</sub>AlF<sub>6</sub>, to lower its melting point for easier electrolysis.<ref name="georg" /> The carbon source is generally a [[Coke (fuel)|coke (fossil fuel)]].<ref name="The Role of Anode Manufacturing Pro"/>

===Theory===
[[File:Hall-heroult-kk-2008-12-31.png|alt=|thumb|A Hall–Héroult industrial cell]]
In the Hall–Héroult process the following simplified reactions take place at the carbon electrodes:

[[Cathode]]:
{{block indent|Al<sup>3+</sup> + 3 [[electron|e<sup>−</sup>]] → Al}}

[[Anode]]:
{{block indent|O<sup>2-</sup> + C → [[Carbon monoxide|CO]] + 2 e<sup>−</sup>}}

Overall:
{{block indent|Al<sub>2</sub>O<sub>3</sub> + 3 C → 2 Al + 3 CO}}

In reality, much more [[carbon dioxide|CO<sub>2</sub>]] is formed at the anode than CO:
{{block indent|2 O<sup>2-</sup> + C → [[Carbon dioxide|CO<sub>2</sub>]] + 4 e<sup>−</sup>}}
{{block indent|2 Al<sub>2</sub>O<sub>3</sub> + 3 C → 4 Al + 3 CO<sub>2</sub>}}

Pure cryolite has a melting point of {{val|1009|1|u=degC}} (1848°F). With a small percentage of alumina dissolved in it, its [[Freezing-point depression|melting point drops]] to about 1000&nbsp;°C (1832°F). Besides having a relatively low melting point, cryolite is used as an electrolyte because, among other things, it also dissolves alumina well, conducts electricity, dissociates electrolytically at higher voltage than alumina, and is less dense than aluminum at the temperatures required by the electrolysis.<ref name="georg" />

[[Aluminium fluoride]] (AlF<sub>3</sub>) is usually added to the electrolyte. The ratio NaF/AlF<sub>3</sub> is called the cryolite ratio and it is 3 in pure cryolite. In industrial production, AlF<sub>3</sub> is added so that the cryolite ratio is 2–3 to further reduce the melting point, so that the electrolysis can happen at temperatures between 940 and 980&nbsp;°C (1700 to 1800°F). The density of liquid aluminum is 2.3&nbsp;g/ml at temperatures between 950 and 1000&nbsp;°C (1750° to 1830°F). The density of the electrolyte should be less than 2.1&nbsp;g/ml, so that the molten aluminum separates from the electrolyte and settles properly to the bottom of the electrolysis cell. In addition to AlF<sub>3</sub>, other additives like [[lithium fluoride]] may be added to alter different properties (melting point, density, conductivity etc.) of the electrolyte.<ref name="georg" />

The mixture is electrolysed by passing a low voltage (under 5&nbsp;V) [[electric current|direct current]] at {{val|100|-|300|u=kA}} through it. This causes liquid aluminium to be deposited at the [[cathode]], while the oxygen from the alumina combines with carbon from the [[anode]] to produce mostly carbon dioxide.<ref name="georg" />

The theoretical minimum energy requirement for this process is 6.23 kWh/(kg of Al), but it commonly requires 15.37 kWh.<ref>{{cite journal |date=17 April 2018 |doi=10.3390/su10041216 |doi-access=free |title=Energy and Exergy Analyses of Different Aluminum Reduction Technologies |last1=Obaidat |first1=Mazin |last2=Al-Ghandoor |first2=Ahmed |last3=Phelan |first3=Patrick |last4=Villalobos |first4=Rene |last5=Alkhalidi |first5=Ammar |journal=Sustainability |volume=10 |issue=4 |page=1216 |bibcode=2018Sust...10.1216O }}</ref>

===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" />

===Electrodes===
Electrodes in cells are mostly [[Coke (fuel)|coke]] which has been purified at high temperatures. [[Pitch (resin)|Pitch resin]] or [[tar]] is used as a binder. The materials most often used in anodes, coke and pitch resin, are mainly residues from the petroleum industry and need to be of high enough purity so no impurities end up into the molten aluminum or the electrolyte.<ref name="georg" />

There are two primary anode technologies using the Hall–Héroult process: '''Söderberg''' technology and '''prebaked''' technology.

In cells using '''Söderberg''' or self-baking anodes, there is a single anode per electrolysis cell. The anode is contained within a frame and, as the bottom of the anode turns mainly into CO<sub>2</sub> during the electrolysis, the anode loses mass and, being [[Amorphous solid|amorphous]], it slowly sinks within its frame. More material to the top of the anode is continuously added in the form of briquettes made from coke and pitch. The lost heat from the smelting operation is used to bake the briquettes into the carbon form required for the reaction with alumina. The baking process in Söderberg anodes during electrolysis releases more [[carcinogen]]ic [[Polycyclic aromatic hydrocarbon|PAHs]] and other pollutants than electrolysis with prebaked anodes and, partially for this reason, prebaked anode-using cells have become more common in the aluminium industry. More alumina is added to the electrolyte from the sides of the Söderberg anode after the crust on top of the electrolyte mixture is broken.<ref name="georg" />

'''Prebaked''' anodes are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution. There are usually 24 prebaked anodes in two rows per cell. Each anode is lowered vertically and individually by a computer, as the bottom surfaces of the anodes are eaten away during the electrolysis. Compared to Söderberg anodes, computer-controlled prebaked anodes can be brought closer to the molten aluminium layer at the bottom of the cell without any of them touching the layer and interfering with the electrolysis. This smaller distance decreases the resistance caused by the electrolyte mixture and increases the efficiency of prebaked anodes over Söderberg anodes. Prebake technology also has much lower risk of the anode effect (see below), but cells using it are more expensive to build and labor-intensive to use, as each prebaked anode in a cell needs to be removed and replaced once it has been used. Alumina is added to the electrolyte from between the anodes in prebake cells.<ref name="georg" />

Prebaked anodes contain a smaller percentage of pitch, as they need to be more solid than Söderberg anodes. The remains of prebaked anodes are used to make more new prebaked anodes. Prebaked anodes are either made in the same factory where electrolysis happens, or are brought there from elsewhere.<ref name="georg" />

The inside of the cell's bath is lined with cathode made from coke and pitch. Cathodes also degrade during electrolysis, but much more slowly than anodes do, so their purity and maintenance requirements are lower than the anodes. Cathodes are typically replaced every 2–6 years. This requires the whole cell to be shut down.<ref name="georg" />

===Anode effect===
The anode effect is a situation where too many gas bubbles form at the bottom of the anode and join, forming a layer. This increases the resistance of the cell, because smaller areas of the electrolyte touch the anode. These areas of the electrolyte and anode heat up when the density of the electric current of the cell focuses to go through only them. This heats up the gas layer and causes it to expand, thus further reducing the surface area where electrolyte and anode are in contact with each other. The anode effect decreases the energy-efficiency and the aluminium production of the cell. It also induces the formation of [[tetrafluoromethane]] (CF<sub>4</sub>) in significant quantities, increases formation of CO and, to a lesser extent, also causes the formation of [[hexafluoroethane]] (C<sub>2</sub>F<sub>6</sub>). CF<sub>4</sub> and C<sub>2</sub>F<sub>6</sub> are not [[Chlorofluorocarbon|CFCs]], and, although not detrimental to the [[ozone layer]], are still potent [[greenhouse gas]]es. The anode effect is mainly a problem in Söderberg technology cells, not in prebaked.<ref name="georg" />