REACTIONS OF LIGNIN IN CHLORINE DIOXIDE BLEACHING OF KRAFT PULPS

The reactions between chlorine dioxide and the residual lignin in oxyg en-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxida tion at different pH values and subsequently analysed by oxidative deg radation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins i solated from kraft pulps after each of the bleaching stages in the OD( EOP)DD sequence. The results demonstrate that, in order to minimize ch lorination of the lignin, the first chlorine dioxide stage should be c arried out at a pH around or above three. At this pH level, a high deg ree of lignin oxidation is also achieved. A certain (mono)-chlorinatio n of the lignin in the first D stage cannot be avoided, but this chlor ine is to a large extent removed in the later bleaching stages. The ef ficient and non-selective oxidation of the various phenolic lignin end groups by chlorine dioxide is clearly illustrated by the analytical d ata. Moreover, C-13 NMR reveals that reduced lignin structures formed during the kraft cook survive the oxidative bleaching stages to a larg e extent.