Reactivity of hexenuronic acid in bleaching of eucalyptus kraft pulps

Abstract

Bleached Eucalyptus kraft pulps form the fastest growing global source of virgin papermaking fibers. Modern Eucalyptus pulp bleaching lines are designed to produce pulps of high brightness and brightness stability with minimal consumption of bleaching chemicals. In most cases a mildly acidic stage is included to hydrolyze most of the uncolored hexenuronic acid that would otherwise consume bleaching chemicals and/or give rise to thermal discoloration. In this paper alternative ways of bleaching Eucalyptus kraft pulps are reviewed with special emphasis on the fate of hexenuronic acid. The review is complemented with recent data (obtained by UV resonance Raman spectroscopy) on relative reaction rates of residual lignin and hexenuronic acid through complete bleaching sequences. The complex reaction mechanisms in ECF bleaching sequences are analyzed through kinetic data from competing and/or consecutive model reactions of hexenuronic acid and lignin. The various active chlorine compounds, chlorine dioxide, chlorite and hypochlorous acid, differ largely on their reactivity towards the two substrates. Therefore the chemistries of hexenuronic acid and lignin in pulp bleaching are interdependent. The paper ends up with a conclusion how hexenuronic acid reacts under various bleaching conditions and how its reactivity is related with the amount and type of residual lignin and/or various dissolved compounds that might be present in the bleaching filtrates. The information provided can be used in planning of bleaching sequences. The chemistry alone is not enough to rank between alternative bleaching sequences as they may differ from each other in respect of several incommensurable features.