Editing Hall–Héroult process (section)

===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>