DISSOLUTION OF LIGNIN IN KRAFT PULP BY OXYGEN BLEACHING

Gel permeation chromatography of spent liquors from oxygen bleaching s howed that cleavage of covalent linkages in the fiber was a prerequisi te for the dissolution of the lignin. The reactions continued in the l iquor phase. The cleavage was related to the oxidation of the lignin. With pulps pretreated with nitrogen dioxide an appreciable number of l ignin linkages were cleaved also by the direct influence of the hydrox ide ions on the nitrated lignin. In the alkaline medium the acidic sit es in the fiber and in dissolved lignin fragments are ionized. Donnan- exclusion and an increased fiber swelling promote the transfer of the fragments into the liquor. Hydrophilic groups introduced into the lign in during oxygen bleaching promote the delignification.