In aluminum extraction by the Baeyer's process, alumina is extracted from bauxite by sodium hydroxide (NaOH) at high temperature and pressure.

${\mathrm{Al}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+2{{\mathrm{OH}}^{-}}_{\left(\mathrm{aq}\right)}⟶2{\mathrm{AlO}}_2^{-}\text{(aq) }+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$

Solid impurities such as ${\mathrm{Fe}}_2{\mathrm{O}}_3\text{ and }{\mathrm{SiO}}_2$ are removed and then Al(OH)₃ is reprecipitated

$2\mathrm{Al}(\mathrm{OH}{)}_4^{-}⟶{\mathrm{Al}}_2{\mathrm{O}}_3\cdot 3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+2{{\mathrm{OH}}^{-}}_{\left(\mathrm{aq}\right)}$

In the industrial world:

Bayer's method is used to extract alumina from bauxite, the primary source of aluminum. Bauxite (Al₂O₃) contains 30–60% aluminum along with iron oxides, silica and so on. Bauxite is treated with NaOH during the extraction process to turn it into a soluble material called NaAlO₂. This process is known as Bauxite digestion process.

${\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{NaOH}\to 2{\mathrm{NaAlO}}_2+{\mathrm{H}}_2\mathrm{O}$

Other impurities won't dissolve, therefore in this way, bauxite is free of other minerals.

To precipitate aluminum hydroxide, the formed alkaline solution is passed through carbon dioxide.

$2{\mathrm{NaAlO}}_2+3{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\to 2\mathrm{Al}{\left(\mathrm{OH}\right)}_3+{\mathrm{Na}}_2{\mathrm{CO}}_3$

During extraction process, seeding of supersaturated solution is feasible by reducing the temperature and pressure of the system.