In the Bayer's process of purification of red bauxite leaching agent is aqueous solution of

Bauxite, with symbol $\left({\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}\right)$ is an important ore of aluminum with large impurities and is hence first roasted and then treated with a concentrated solution of caustic soda. This dissolves aluminum oxide present in bauxite due to the formation of sodium aluminate, $\left({\mathrm{NaAlO}}_2\right)$.

${\mathrm{Al}}_2{\mathrm{O}}_2·{\mathrm{H}}_2\mathrm{O}+2\mathrm{NaOH}\to 2{\mathrm{NaAlO}}_2+3{\mathrm{H}}_2\mathrm{O}$

Since $\mathrm{NaOH}$ is a strong reducing agent, it is used in Bayer's process.