INTERACTION OF HYDROGEN-PEROXIDE AND CHLORINE DIOXIDE STAGES IN ECF BLEACHING

Changing the order of the hydrogen peroxide (P) and chlorine dioxide ( D) stages in an ECF (elemental chlorine free) bleach sequence was show n to affect the final brightness of kraft pulps. Application of a D(EO P)DPP bleach sequence to a southern softwood pulp produced consistentl y higher interstage and final brightnesses than a D(EOP)PDP sequence. These results were confirmed by bleaching a northern softwood and a eu calypt pulp using DEDP and DEPD sequences. In all cases, higher bright ness were achieved when the second D stage (D-2) preceded the P stage. The effect was especially evident with the hardwood pulp, where brigh tness of the DEDP pulp was three points higher than the DEPD pulp. NMR analysis of extracted lignin showed that the D-2 stages reduced the p henolic lignin content of softwood pulp by 60%, while the P stage had no effect. For the hardwood pulp, the D-2 stage reduced the phenolic l ignin content by 90-95%. The greater bleaching efficiency of a DEDP se quence (compared with DEPD) is consistent with known reactivities of h ydrogen peroxide and chlorine dioxide toward specific lignin structure s. The D-2 stage lowers the phenolic lignin content of pulp while gene rating hydrogen peroxide-sensitive, nonphenolic lignin species. A foll owup P stage reacts effectively with this lignin.