Abstract: The process according to the invention to raise simultaneously to an optimum level the temperature and concentration of an aqueous medium intended for use in bauxite ore digestion in an alumina production plant according to the Bayer process, which includes successive phases (a) to (f) in which: a) the aqueous medium coming from digestion containing pregnant liquor is cooled and evaporated in a multiple staged flash evaporation zone; b) the sterile residue is separated, the aluminium trihydroxide is precipitated and the aqueous medium, consisting of spent liquor, intended for use in digestion is recovered; c) if need be, the required quantity of water, additional to that already evaporated in phase (a) is eliminated by evaporation in a specific evaporation section; d) the aqueous medium intended for use in digestion is heated in several stages; e) the flash vapour generated in each of the stages of the evaporation phase (a) is used in each of the stages of the heating phase (d); f) the aqueous medium in