To provide a technique of improving the whiteness of causticized light calcium carbonate produced as a by-product in a causticization process for regenerating white liquor for digestion in a pulp production process by a sulfate process or a soda process.

In a causticization process in a pulp production process by a sulfate process or a soda process, the green liquor can be highly clarified by adding thereto a specified cationic polymeric coagulant or by adding thereto the cationic polymeric coagulant and an anionic polymeric coagulant. Therefore, the whiteness of the causticized light calcium carbonate formed from the causticization reaction of the clarified green liquor with calcium oxide or with calcium hydroxide can be stably improved.

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