The Effect of Aluminum on the Two-Stage Pe rox i d e B I each i n g of Eucalyptus regnans Stone Groundwood Pulp
The addition of aluminum nitrate (up to 100 ppm A1 on pulp) to a two-stage, acidalkali, hydrogen peroxide bleach of Eucalyptus regnans stone groundwood pulp enhanced the brightness gain of the pulp, over a range of consistencies. It is proposed that the presence of aluminum ions modifies the catalytic behaviour of the transition metal ions present in the pulp, especially manganese, reducing peroxide consumption while increasing the brightness of the pulp. The two-stage process, under optimized conditions at low consistency, consumes considerably less peroxide while achieving a greater brightness increase than a singlestage alkaline peroxide bleach.
Peroxide Bleaching Reactions Under Alkaline and Acidic Conditions
Abstract An examination of previously reported kinetic expressions describing peroxide bleaching of wood pulp under alkaline conditions reveals that the overall process can be considered as a combination of two parallel reaction routes. The first route corresponds to a reaction involving direct participation of the perhydroxyl anion in chromophore elimination. This mechanism can be identified with the classical explanation for peroxide bleaching. The second route can be associated with reactions in which chromophores are eliminated through the action of free radical intermediate species. New experimental evidence is presented to show that processes catalysed by transition metal ions can lead to enhancement of bleaching. A two stage peroxide bleaching sequence, initially under acidic conditions in the presence of chromium, followed by alkaline conditions produces an acceleration in bleaching rate, without significant additional consumption of peroxide.
Simulation and Interpretation of Kinetic Phenomena for Soda-Additive Delignification
Abstract Computer simulations of kinetic phenomena have been carried out for a general soda-additive delignification mechanism consisting of an equilibrium process, followed by an irreversible step in which the additive participates. Simulations have been performed under conditions where the concentration of additive is either at steady state, or declines with time. The results clearly show that caution should be exercised in interpretation of experimental data when attempting to use kinetic results to elucidate details of reaction mechanisms. In particular, it is shown that the origin of exprimentally observed dependencies on additive concentrations such as a “square-root” relationship can be explained by assuming different ratios of rate constants for formation and subsequent decomposition of a quinone methide intermediate.