Editing Aluminium (section)

=== Bayer process ===

{{Main|Bayer process}}
{{See also|List of countries by bauxite production}}

[[Bauxite]] is converted to alumina by the Bayer process. Bauxite is blended for uniform composition and then is ground fine. The resulting [[slurry]] is mixed with a hot solution of [[sodium hydroxide]]; the mixture is then treated in a digester vessel at a pressure well above atmospheric, dissolving the aluminium hydroxide in bauxite while converting impurities into relatively insoluble compounds:<ref name="UllmannOxide" />

{{block indent|Al(OH)<sub>3</sub> + Na<sup>+</sup> + OH<sup>−</sup> → Na<sup>+</sup> + [Al(OH)<sub>4</sub>]<sup>−</sup>}}

After this reaction, the slurry is at a temperature above its atmospheric boiling point. It is cooled by removing steam as pressure is reduced. The bauxite residue is separated from the solution and discarded. The solution, free of solids, is seeded with small crystals of aluminium hydroxide; this causes decomposition of the [Al(OH)<sub>4</sub>]<sup>−</sup> ions to aluminium hydroxide. After about half of aluminium has precipitated, the mixture is sent to classifiers. Small crystals of aluminium hydroxide are collected to serve as seeding agents; coarse particles are converted to alumina by heating; the excess solution is removed by evaporation, (if needed) purified, and recycled.<ref name="UllmannOxide">{{Ullmann
|last1=Hudson|first1=L. Keith|last2=Misra|first2=Chanakya|last3=Perrotta|first3=Anthony J.|last4=Wefers|first4=Karl|last5=Williams|first5=F.S.|date=2005
|publisher=Wiley-VCH|title=Aluminum Oxide|display-authors=3|doi=10.1002/14356007.a01_557}}</ref>