Editing Aluminium (section)

=== Hall–Héroult process ===

[[File:Tovarna glinice in aluminija Kidričevo - kupi aluminija 1968.jpg|thumb|upright=0.75|right|[[Extrusion]] billets of aluminium]]

{{Main|Hall–Héroult process|Aluminium smelting}}
{{See also|List of countries by aluminium oxide production}}

The conversion of [[alumina]] to aluminium is achieved by the [[Hall–Héroult process]]. In this energy-intensive process, a solution of alumina in a molten ({{convert|940|and|970|C|F}}) mixture of [[cryolite]] (Na<sub>3</sub>AlF<sub>6</sub>) with [[calcium fluoride]] is [[electrolysis|electrolyzed]] to produce metallic aluminium. The liquid aluminium sinks to the bottom of the solution and is tapped off, and usually cast into large blocks called [[Bar stock|aluminium billets]] for further processing.<ref name="Ullmann" />

Anodes of the electrolysis cell are made of carbon—the most resistant material against fluoride corrosion—and either bake at the process or are prebaked. The former, also called Söderberg anodes, are less power-efficient and fumes released during baking are costly to collect, which is why they are being replaced by prebaked anodes even though they save the power, energy, and labor to prebake the cathodes. Carbon for anodes should be preferably pure so that neither aluminium nor the electrolyte is contaminated with ash. Despite carbon's resistivity against corrosion, it is still consumed at a rate of 0.4–0.5&nbsp;kg per each kilogram of produced aluminium. Cathodes are made of [[anthracite]]; high purity for them is not required because impurities [[Leaching (chemistry)|leach]] only very slowly. The cathode is consumed at a rate of 0.02–0.04&nbsp;kg per each kilogram of produced aluminium. A cell is usually terminated after 2–6 years following a failure of the cathode.<ref name="Ullmann" />

The Hall–Heroult process produces aluminium with a purity of above 99%. Further purification can be done by the [[Hoopes process]]. This process involves the electrolysis of molten aluminium with a sodium, barium, and aluminium fluoride electrolyte. The resulting aluminium has a purity of 99.99%.<ref name="Ullmann" /><ref>{{cite book
|url=https://books.google.com/books?id=KpgTrFloOq0C&pg=PA40|title=Handbook of Aluminum|last1=Totten|first1=G.E.|last2=Mackenzie|first2=D.S.|date=2003
|publisher=[[Marcel Dekker]]|isbn=978-0-8247-4843-2|page=40
|archive-url=https://web.archive.org/web/20160615132126/https://books.google.com/books?id=KpgTrFloOq0C&pg=PA40|archive-date=15 June 2016|url-status=live}}</ref>

Electric power represents about 20 to 40% of the cost of producing aluminium, depending on the location of the smelter. Aluminium production consumes roughly 5% of electricity generated in the United States.<ref name="Emsley2011" /> Because of this, alternatives to the Hall–Héroult process have been researched, but none has turned out to be economically feasible.<ref name="Ullmann" />