Interaction of Hydrogen Peroxide and Chlorine Dioxide Stages in ECF Bleaching, 1996 International Pulp Bleaching Conference Proceedings

The order of hydrogen peroxide (P) and chlorine dioxide (D1 ) stages in an ECF sequence has been shown to affect the final brightness in Southern and Northern softwood and eucalypt kraft pulps. The Southern softwood pulp bleached using the sequence DEopDPP had consistently higher interstage and final brightnesses than the DEopPDP sequence. Northern softwood and eucalypt pulps were each bleached using DEDP and DEPD sequences. In each case higher brightnesses were achieved when the D1 stage preceded the P stage. The effect was especially evident with the hardwood pulp where over 3% ISO points higher brightness was seen for the DEDP sequence. Lignin was extracted from the brownstock, DE-stage, DEP stage and DED stage pulps and analyzed by 31 P NMR. The aromatic lignin content of softwood pulp was reduced by 60% following the D, stage (DED) but was essentially unchanged when treated by peroxide (DEP). For the hardwood pulp, lignin content following the D, stage (DED) was reduced by 90-95%. The improved bleaching response of the DEDP pulp in comparison to the DEPD pulp is consistent with documented reactivities of hydrogen peroxide and chlorine dioxide toward specific lignin structures. In the DEDP sequence, chlorine dioxide of the D, stage reduces the aromatic content of pulp so that the following hydrogen peroxide stage can more effectively react with non-aromatic species. Conversely, during bleaching with a DEPD sequence, the P-stage is less effective in reducing aromatic lignin content prior to the D1 stage and therefore is beneficial to a lesser degree prior to the D1 stage. This effect is clearly manifested as a lower brightness. It is also reflected as a reduced sensitivity to bleaching with increasing hydrogen peroxide charges.