To provide a method for stabilizing the operation of causticizing process and obtaining calcium carbonate as byproduct.

The method comprises the following procedure: A green liquor obtained in the kraft pulp production process is causticized with quicklime or slaked lime obtained by reacting quicklime with water, 10 wt.% or more of calcium carbonate from the resultant lime sludge is extracted from the causticizing process and used as an industrial feedstock, the rest of 90 wt.% or less of calcium carbonate is fed to a calcining step to make it into quicklime, which is put to recycling use in the causticization process, thus stabilizing the operation of the causticization process. Furthermore, the calcium carbonate obtained by solid/liquid separation of the resulting lime sludge followed by washing with water is utilized as an industrial feedstock.

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