SEPARATION OF SULFIDE FROM HYDROXIDE IN WHITE LIQUOR BY ELECTRODIALYSIS

A process using an electrodialysis (ED) system employing cation-select ive membranes and anion-selective membranes and, in particular monoval ent anion-selective membranes, was used to separate kraft white liquor into two streams: one that is rich in sulphide (to be used in the ini tial stage of pulping), and another that is poor in sulphide (to be us ed in the final stage of pulping). By separating white liquor in this way, the sulphur balance in the kraft process can be maintained while obtaining the benefits of sulphidity profiling in modified pulping. Th e ED system was operated in both the batch and feed-and-bleed modes of operation, producing sulphide-rich white liquors of 57-74% sulphidity (AA = 46-48 g Na2O/L) and sulphide-poor white liquors of 15-17% sulph idity, corresponding to 59-65% sulphide removal (AA = 102-111 g Na2O/L ). Current efficiencies ranged from 43 to 69% for sodium sulphide sepa ration from white liquor Depending on the type of membrane used, the p ower requirement ranged from 297 to 464 kWh/t of Na2S removed from the white liquor Monovalent anion-selective membranes had a higher select ivity for hydrosulphide than general-purpose membranes.