Oxygen Delignification Kinetics and Selectivity Improvement

Pulp strength loss during oxygen delignification is caused by cellulose chain cleavage resulting fiom attack by oxygen-based radicals generated through reactions with lignin. The objective of this research was to improve the lignin-to-cellulose selectivity during oxygen delignification by using radical-scavenging polymeric additives that adsorb on the cellulose surfaces where protection fiom radicals is needed. Among the polymers evaluated were starch, carboxymethyl-cellulose, galactomannan, xylan, and glucomannan. Oxygen delignification experiments were conducted on Northeast softwood Kraft pulps at different temperatures, sodium hydroxide charges, reaction times, and pretreatments. The effect of polymer additives on the selectivity and kinetics was studied. Polymer adsorption on pulp was measured by sugar analysis using high performance anionic exchange chromatography. It was determined that increasing alkali charge between 1.5% and 5.0% sodium hydroxide decreases the selectivity of oxygen delignification, and that an increase in temperature from 90°C to 120°C does not significantly affect the selectivity. The observed Kappa nunlber changes were corrected for alkali extraction in the absence of oxygen. The corrected Kappa number change was shown to be proportional to the number of cellulose chain scissions, confirming that the cellulose degradation is directly related to oxygen-mediated reactions. Kinetic equations were developed for the rate of change of Kappa number and cellulose chain scission. The alkali consumption was found to be linearly related to the change in Kappa number. Significant increases in lignin-cellulose selectivity were observed for galactomannan and glucomannan as the adsorbed additives, with significant improvements at charges as low as 0.3% added on pulp. At 2.0% galactomannan or glucomannan on pulp, the selectivity improvement was about 30% to 40%. None of the other polymers tested gave results as favorable. The selectivity improvements were found to increase linearly with the amount of adsorbed polymer additive. This supports the hypothesis that the adsorbed additives on cellulose provide a protective barrier. Lastly, extended delignification up to 65% Kappa number reduction was successfully achieved by using high temperature, extended reaction time, and split NaOH addition to keep the alkali concentration low throughout two- and multi-stage delignification experiments