In Bayer's process leaching is carried out using:

The seperation of the alumina from the impurities in the baurite ore is usually accomplished by bayer process. The bauxite ore is leachedwith caustic soda (NaOH) to produce soluble sodium Aluminate $(NaAl{O}_2)$ leaving the impurities in the insoluble residue. The aluminate solution is then decomposed producing $Al(OH{)}_3$ which is calcined to produced $A{l}_2{O}_3$ so, leaching is carried ot by caustic soda