A process using an amphoteric ion-exchange resin, also known as a "snake-cage polyelectrolyte" resin, contained in an ion retardation unit (9) separates kraft white liquor (8) into sulphide-rich (12) and caustic-rich (10) components. The sulphide-rich component (12) can be used in the initial stage of pulping (1), pretreatment of wood chips prior to pulping, or it can be used to make polysulphide-rich liquor. The caustic-rich component (10) can be used in the final delignification phase, in place of sodium hydroxide or white liquor in oxygen delignification, pH adjustment and flue gas scrubbing. The same system can be used to separate green (6) and polysulphide liquors (8a) into sulphide-rich and sulphide-poor components, and to remove sulphide from other mill caustic streams contaminated with sulphide.