Separation of Sulphide from Hydroxide in White Liquor by Electrodialysis, 1996 Pulping Conference Proceedings

R. Thompson, M. Paleologou, P-Y. Wong,

R. M. Berry

Pulp and Paper Research Institute of Canada

Extended delignification can be improved by applying high sulphidity white liquor at the initial stage of pulping and low sulphidity white liquor at the final stage. Processes that can generate sulphide-rich and caustic-rich white liquors while maintaining the sulphur balance in the kraft process would, therefore, improve the pulping process. An electrodialysis (ED) system was used to separate kraft white liquor into sulphide-rich and caustic-rich components. The ED system was operated in both the batch, and feed-and-bleed modes of operation, producing sulphide-rich white liquors of 57% to 74% sulphidity and caustic-rich white liquors of 15% to 17% sulphidity; these correspond to 59 to 65% sulphide removal. Current efficiencies ranged from 43% to 69% for sodium sulphide separation from white liquor. Monovalent anion-selective membranes were found to have a higher selectivity for sulphide than general-purpose membranes.