Apparatus for preparing high-concentration alkali

An apparatus for producing high-concentration caustic alkali by concentrating a caustic alkali solution by evaporation at an elevated temperature, comprising:

• a high pressure evaporation vessel operated at elevated temperature and under a predeterminate pressure above atmospheric pressure;
• a low pressure expansion vessel operating under a predeterminate pressure lower than said operating pressure of said high pressure evaporation vessel;
• pressure difference absorbing means having a predeterminate length and connecting said high pressure evaporation vessel and said low pressure expansion vessel to each other for gradually absorbing over said predeterminate length a pressure difference between the predeterminate pressure above atmospheric pressure in said high pressure evaporation vessel and the predeterminate pressure prevailing in said low pressure expansion vessel;
• said pressure difference absorbing means is a liquid column having a predeterminate vertical extension defining said predeterminate length of said pressure difference absorbing means and generating a hydrostatic pressure substantially corresponding to said pressure difference between said predeterminate pressure above atmospheric pressure in said high pressure evaporation vessel and said predeterminate pressure prevailing in said low pressure expansion vessel;
• said liquid column being part of a substantially U-shaped siphon tube connected between said high pressure evaporation vessel and said low pressure expansion vessel;
• an intermediate vessel disposed in a lower region of said liquid column;
• a rise-pipe connecting an upper end of said intermediate vessel to a lower end of said low pressure expansion vessel;
• and
• a recirculation pipe connecting a lower end of said intermediate vessel to said lower end of said low pressure expansion vessel.