Part of the Bayer process involves the digestion of crushed ore in concentrated aqueous sodium hydroxide. This process is carried out at high pressure

In Bayer’s process bauxite is heated with NaOH solution in an autoclave at about ${150}^{0}C$ for few hours. If this process is carried at normal pressure the water evaporate. To prevent the boiling and evaporation of water, the process is carried at high pressure.