TOWARDS IMPROVEMENTS OF KRAFT PULP BLEACHING BY ADDITIONAL TREATMENTS

During the last decade, many efforts have been made in pulp and paper industry to construct more effective, sustainable and environmentally friendly pulp bleaching sequences. In this work, enzymatic (laccase-mediator system, L), non-oxidative chemical (urea, U) and mechanical (PFI refining, R) treatments have been conducted, alone or in a combination, on unbleached eucalyptus kraft pulps to evaluate their bleaching effects and thus the further potential as additional treatment(s) on a conventional bleaching process. The pulps obtained were characterized in terms of kappa number, hexenuronic acid content, brightness and viscosity. Moreover, the lignin-carbohydrates complexes (LCCs) were fractionated and characterized concerning lignin content and carbohydrate composition and for (HNMR)-H-1 and thioacidolysis-GC and-SEC analysis. It has been found that the effect of the L treatment was limited at delignification by a kappa number reduction of 0.8 units. The effects of the U treatment included delignification (1.4 kappa units), HexA removal (3 mol/g) and pulp brightening (3.4 ISO units). The refining darkened the pulp (3 ISO units) while the subsequent L or U treatment could bleached more than without the R stage. The delignification extent by all the treatments could be noticed also from the isolation yields of the LCC1 (glucan-lignin) fraction. Thioacidolysis-SEC analysis revealed the darkening mechanism of the R treatment and the delignification mechanisms of L and U treatment, showing that the L treatment removed more effectively the condensed parts of the lignin located at LCC2 (xylan-lignin) fraction.