PURPOSE: To improve production efficiency and to save energy by recycling waste liquid portion to a temperature adjusting kettle from a pressure crystallizer, feeding, while lowering the temperature in the kettle according to the lowering of the concentration of the objective components in the starting material mixture in the kettle, the mixture to the pressure crystallizer and performing crystallization and solid-liquid separation.

CONSTITUTION: Waste liquid 18 discharged from a pressure crystallizer 10 is recycled to a temperature adjusting kettle 2 to mix it with the residual starting material in the kettle 2 and also while lowering the temperature in the kettle 2 according to the lowering of the concentration of the objective components in the starting material mixture in the kettle 2, the mixture is fed to the pressure crystallizer 10 to perform crystallization and solid-liquid separation. When the temperature in the kettle 2 attains the prescribed final liquid feeding temperature, it is maintained to perform the feeding to the pressure crystallizer 10 and pressure crystallization. Consequently since pressure crystallization is performed in what is called unstationary state and a cooling process of the waste liquid in an the temperature adjusting kettle 2 is carried forward at the same time with a pressure crystallization process, production efficiency and saving of energy are greatly improved.