Method of separating sodium hydroxide from white liquor or polysulfide liquor using diffusion dialysis

A method of processing a circulating aqueous liquor, containing sodium hydroxide and sodium sulfide, which liquor has been obtained from a caustic treating step of a pulping process, comprising:

• passing said aqueous liquor containing sodium hydroxide through a diffusion dialysis process using a cationic ion exchange membrane under conditions sufficient to separate sodium hydroxide from said aqueous liquor and to produce a leaner aqueous liquor diminished in sodium hydroxide;
• recovering separated sodium hydroxide;
• and then
• oxidizing said leaner aqueous liquor under conditions sufficient to convert sulfide to polysulfide.

The method of processing an aqueous liquor, containing sodium hydroxide and sodium sulfide, obtained from a caustic treating step of a pulping process, comprising:

• oxidizing said aqueous liquor under conditions sufficient to obtain an oxidized aqueous product comprising polysulfide and sodium hydroxide;
• passing said oxidized aqueous product through a diffusion dialysis process using a cationic ion exchange membrane whereby separating at least part of said sodium hydroxide from said oxidized aqueous product;
• and
• recovering said separated sodium hydroxide.

A method of treating an aqueous liquor, containing sodium hydroxide and sodium sulfide, to obtain sodium hydroxide and polysulfide therefrom, which aqueous liquor has been obtained from a caustic treating step of a pulping process, comprising:

• passing a first aqueous liquor containing sodium hydroxide through a diffusion dialysis process using a cationic ion exchange membrane to separate sodium hydroxide therefrom and forming a second aqueous liquor containing sodium sulfide;
• oxidizing said second aqueous liquor containing sodium sulfide to convert sodium sulfide to sodium polysulfide;
• and
• recovering, respectively, said sodium hydroxide and said sodium polysulfide.