At what pH range bayer process operate? and why?

Bayer process is industrial method of the refining of the bauxite to produce alumina which is aluminum oxide.

As alumina is amphoteric in nature, it exhibits a higher solubility at both the extremes of pH range, it is possible to dissolve alumina in low as well as in high pH solutions.

Dissolution of the alumina at high pH is well recognized in Bayer process. Bauxite is digested in very high pH solution (> 13) of alkali like sodium hydroxide at temperature of about 150-250°C and pressure at 20 atm. This is done so that the dissolved alumina is separated from rest of insoluble bauxite minerals.