To provide an improved alternative method for using heat from black liquor to heat white liquor or a mixture of white liquor with other liquids used for treating fiber materials in a digestion kettle.

The method for heating white liquor or a mixture of white liquor and other liquids used in the digestion vessel system in a chemical pulp plant is provided. The method is (a) to generate black liquor in the digestion kettle system, (b) to extract steam of the black liquor from the digestion kettle, (c) to flash the extracted black liquor to generate flashed black liquor and flashed steam, (d) to pass the flashed steam through at least one heat exchanger so as to indirectly contact the flashed stream by heat exchange with the white liquor and/or the mixture of the white liquor and other liquids to heat the white liquor and/or the mixture used in the digestion kettle system, and (e) to further lead the flashed black liquor from the step (c) to heat recovery or evaporation.

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