The invention relates to plant operations for increasing the cycle times of heat exchangers by the digestion of the aluminium oxide content of a slurry according to the Bayer technology, said slurry consisting of bauxite and sodium aluminate liquor, using an equipment in which the material is heated stepwise and in more parallel material streams, then the material streams are combined and further heated, or combined and kept at the same temperature, or after combination further heated and kept at the same temperature, characterized in that a part of the parallel material streams is slurry, another part of same is aqueous sodium hydroxide solution, preferably sodium aluminate liquor containing sodium hydroxide, or water, and the parallel material streams are exchanged in the heat exchangers in a cyclical manner.