Editing Hall–Héroult process

{{Short description|Aluminium smelting process}}
The '''Hall–Héroult process''' is the major [[industrial process]] for [[aluminium smelting|smelting aluminium]]. It involves dissolving [[aluminium oxide|aluminium oxide (alumina)]] (obtained most often from [[bauxite]], [[aluminium]]'s chief ore, through the [[Bayer process]]) in molten [[cryolite]] and [[Electrolysis|electrolyzing]] the molten salt bath, typically in a purpose-built cell. The process conducted at an industrial scale, happens at 940–980&nbsp;°C (1700 to 1800°F) and produces [[aluminium]] with a purity of 99.5-99.8%. [[Aluminium recycling|Recycling aluminum]], which does not require electrolysis, is thus not treated using this method.<ref name="georg">{{cite book |last1=Totten|first1=George E.|last2=MacKenzie|first2=D. Scott|date=2003|title= Handbook of Aluminum: Volume 2: Alloy production and materials manufacturing|location=New York, NY|publisher= Marcel Dekker, Inc.|isbn=0-8247-0896-2}}</ref>

The Hall–Héroult process consumes substantial electrical energy, and its electrolysis stage can produce significant amounts of [[carbon dioxide]] if the electricity is generated from high-emission sources. Furthermore, the process generates [[fluorocarbon|fluorocarbon compounds]] as [[byproduct]]s, contributing to both [[air pollution]] and [[climate change]].<ref name="The Role of Anode Manufacturing Pro">{{cite journal|title=The Role of Anode Manufacturing Processes in Net Carbon Consumption|year=2016|doi=10.3390/met6060128|doi-access=free|last1=Khaji|first1=Khalil|last2=Al Qassemi|first2=Mohammed|journal=Metals|volume=6|issue=6|page=128}}</ref><ref>{{cite web|last1=Marks |first1=Jerry |last2=Roberts |first2=Ruth |last3=Bakshi |first3=Vikram |last4=Dolin |first4=Eric |title=Perfluorocarbon (PFC) Generation During Primary Aluminum Production |date=January 2000 |url=https://www.epa.gov/sites/default/files/2016-02/documents/pfc_generation.pdf}}</ref>

==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" />

==History==

===Existing need===
Aluminium is the most abundant [[metal|metallic element]] in the Earth's crust, but it is rarely found in its [[Native aluminium|elemental state]]. It occurs in many minerals, but its primary commercial source is [[bauxite]], a mixture of hydrated aluminium oxides and compounds of other elements such as iron.

Prior to the Hall–Héroult process, elemental aluminium was made by heating ore along with elemental [[sodium]] or [[potassium]] in a [[vacuum]].{{citation needed|date=September 2022}} The method was complicated and consumed materials that were in themselves expensive at that time. This meant that the cost to produce the small amount of aluminium made in the early 19th century was very high, higher than for [[gold]] or [[platinum]].<ref>{{Cite web |last=Kean |first=Sam |date=2010-07-30 |title=Aluminum: It Used To Be More Precious Than Gold |url=http://www.slate.com/articles/health_and_science/elements/features/2010/blogging_the_periodic_table/aluminum_it_used_to_be_more_precious_than_gold.html |access-date=2024-02-23 |website=Slate Magazine}}</ref> Bars of aluminium were exhibited alongside the French [[crown jewels]] at the [[Exposition Universelle (1855)|Exposition Universelle of 1855]], and [[Napoleon III of France|Emperor Napoleon III]] of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.<ref>{{cite web | url=https://www.latimes.com/archives/la-xpm-2001-sep-05-fo-42149-story.html | title=When Aluminum Plates Were Cool | website=[[Los Angeles Times]] | date=5 September 2001 }}</ref><ref>{{cite web | url=https://www.hollymelody.com/history/9/did-napoleon-iii-reserve-a-special-set-of-aluminum-cutlery-for-special-guests | title=Did Napoleon III reserve a special set of aluminum cutlery for special guests? }}</ref><ref>https://www.alcirclebiz.com/blogs/from-emperors-table-to-the-moon-the-fascinating-history-of-aluminium</ref>

Production costs using older methods did come down, but when aluminium was selected as the material for the cap/lightning rod to sit atop the [[Washington Monument]] in [[Washington, D.C.]] upon its completion in 1884, it was still more expensive than [[silver]].<ref>{{cite journal|author = George J. Binczewski|title = The Point of a Monument: A History of the Aluminum Cap of the Washington Monument|journal = JOM|volume = 47|issue = 11|pages = 20–25|year = 1995|url = http://www.tms.org/pubs/journals/JOM/9511/Binczewski-9511.html|bibcode = 1995JOM....47k..20B|doi = 10.1007/BF03221302|s2cid = 111724924}}</ref>

===Independent discovery===
The Hall–Héroult process was invented independently and almost simultaneously in 1886 by the [[United States|American]] chemist [[Charles Martin Hall]]<ref name="Hall-patent">{{US patent reference|number = 400664|y = 1889|m=04|d=02|inventor=[[Charles Martin Hall]] |title=Process of Reducing Aluminium from its Fluoride Salts by Electrolysis}}</ref> and by the [[France|Frenchman]] [[Paul Héroult]]<ref>Héroult, Paul; French patent no. 175,711 (filed:  23 April 1886; issued:  1 September 1886).</ref>—both 22 years old. Some authors claim Hall was assisted by his sister [[Julia Brainerd Hall]];<ref name = "Kass">{{cite book|editor1-last=Kass-Simon |editor1-first= Gabrielle|editor2-last=Farnes|editor2-first=Patricia|editor3-last=Nash|editor3-first=Deborah |title=Women of Science: Righting the Record|url=https://books.google.com/books?id=Ez7DCJM57esC&q=%22Julia+Brainerd+Hall%22&pg=PA173|year=1990|publisher= Indiana University Press|isbn=0-253-20813-0|pages=173–176}}</ref> however, the extent to which she was involved has been disputed.<ref name= "Sheller">{{cite book|last1=Sheller|first1=Mimi|title=Aluminum dreams : the making of light modernity|date=2014|publisher=MIT Press |location=Cambridge, MA|isbn=978-0262026826|page=270|url=https://books.google.com/books?id=VOjaAgAAQBAJ&pg=PA270|access-date=19 April 2016}}</ref><ref name="Giddens">{{cite journal|last1=Giddens |first1=Paul H.|title=Alcoa, An. American Enterprise. By Charles C. Carr. (Book review)|journal=Pennsylvania History|date=1953|volume=20|issue=2|pages=209–210 |url=https://journals.psu.edu/phj/article/view/22279/22048}}</ref> In 1888, Hall opened the first large-scale aluminium production plant in [[Pittsburgh]]. It later became the [[Alcoa]] corporation.

In 1997, the Hall–Héroult process was designated a [[National Historic Chemical Landmarks|National Historic Chemical Landmark]] by the [[American Chemical Society]] in recognition of the importance of the process in the commercialization of aluminum.<ref>{{cite web | title = Production of Aluminum: The Hall–Héroult Process | work = National Historic Chemical Landmarks | publisher = American Chemical Society | url = http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/aluminumprocess.html | access-date = 2014-02-21 }}</ref>

===Economic impact===
Aluminium produced via the Hall–Héroult process, in combination with cheaper [[electric power]], helped make aluminium (and incidentally [[magnesium]]) an inexpensive commodity rather than a precious metal.

This, in turn, helped make it possible for pioneers like [[Hugo Junkers]] to utilize aluminium and [[Magnesium alloy#Aluminium alloys with magnesium|aluminium-magnesium alloys]] to make items like metal airplanes by the thousands, or Howard Lund to make aluminium fishing boats.<ref>{{cite web|url= https://www.in-depthoutdoors.com/community/forums/topic/ftlgeneral_69525/|title= Lund Boat Company Founder Dies at 91|date =October 24, 2003|website = In-Depth Outdoors}}</ref> In 2012 it was estimated that 12.7 tons of CO<sub>2</sub> emissions are generated per ton of aluminium produced.<ref name= "Das2012">{{cite journal|last1=Das|first1=Subodh|title=Achieving Carbon Neutrality in the Global Aluminum Industry|journal=JOM|volume=64|issue=2|year=2012|pages=285–290|issn=1047-4838|doi= 10.1007/s11837-012-0237-0 |bibcode=2012JOM....64b.285D|s2cid=59383624}}</ref>

In the 20th and 21st century the aluminum industry due to its large-scale requirements for cheap electricity has often been sited in locations where such electricity is available. For example Iceland, a country with no notable bauxite reserves and [[demographics of Iceland|a population]] of less than half a million, is the world's [[List of countries by aluminium production|twelfth largest aluminum producer]] due to the availability of [[energy in Iceland|cheap and plentiful]] electricity, particularly [[hydropower in Iceland|hydropower]]. Similarly [[Aluminerie Alouette]] in [[Sept-Îles, Quebec]] is dependent for its electricity needs on the 5,428 MW [[Churchill Falls Generating Station]] operated by [[Churchill Falls (Labrador) Corporation Limited]]. The [[company town]] of [[Kitimat]] in [[British Columbia]] was built by [[Alcan]] to meet the growing demand for aluminum in the postwar era. It makes use of the [[Kenney Dam]] built to power the smelters. The [[Tiwai Point Aluminium Smelter]] on the South Island of [[New Zealand]] consumes some 570 MW of electricity, most of which is supplied by nearby [[Manapōuri Power Station]]. This amounts to around a third of the electricity demand of South Island and 13% of that of the entirety of New Zealand. [[Borssele Nuclear Power Station]] was built primarily to supply electricity to an aluminum smelter operated by French [[Pechiney]] at the time.

== See also ==
* [[Bayer process]]
* [[History of aluminium]]
* [[Tantalum#Electrolysis|Solid oxide Hall–Héroult process]]
* [[Hoopes process]]
* [[Downs cell]]

==References==
{{reflist}}

==Further reading==
* Grjotheim, U and Kvande, H., [http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=199501420050MD&recid=199501P10015AI Introduction to Aluminium Electrolysis. Understanding the Hall–Heroult Process], Aluminium Verlag GmbH, (Germany), 1993, pp.&nbsp;260.
* {{cite journal |first1=Shiva |last1=Prasad |title=Studies on the Hall-Heroult Aluminum Electrowinning Process |journal=Journal of the Brazilian Chemical Society |volume=11 |issue=3 |pages=245–251 |date=May–June 2000 |doi=10.1590/S0103-50532000000300008|doi-access=free }}

{{electrolysis}}

{{DEFAULTSORT:Hall-Heroult Process}}
<!-- Categories -->
[[Category:Industrial processes]]
[[Category:Chemical processes]]
[[Category:Aluminium industry]]
[[Category:Electrolysis]]