BOOSTING PULP YIELD OF WESTERN SOFTWOOD WITH ANTHRAQUINONE/POLYSULFIDE KRAFT PULPING, 2002 Kraft Pulp Yield Symposium

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Current trends toward higher yields and lower kappa number have sparked renewed interest in anthraquinone/polysulfide pulping and prompted this study. The effect of anthraquinone (AQ) and polysulfide (PS), both separately and in combination, on kraft cooks of western softwood was investigated in a laboratory batch digester. Brownstock kappa number, brownstock yield, ease of bleaching and strength measurements for AQ, PS and AQ/PS pulps were compared to conventional kraft pulps. The addition of 0.07% anthraquinone (on oven dry wood) resulted in a 13% reduction in the brownstock kappa number with no significant change in pulp yield. Polysulfide alone, at a charge of 1.3% (on oven dry wood), increased pulp yield 1.3% at constant brownstock kappa number. An increase of 1.8% in active alkali, over the conventional kraft cook, was required to maintain a constant kappa number with polysulfide use. When using both anthraquinone and polysulfide, the brownstock kappa number decreased 19%. This decrease was greater than that obtained with anthraquinone alone. The resultant pulp yield increased by 1.5%. The yield increases achieved with PS and AQ/PS were carried through the bleaching stage. Strength results showed that there was no decrease in strength for PS and AQ/PS pulp; if anything, there was an increase in pulp strength. These results showed that, in combination, AQ and PS can be used to decrease kappa number and increase pulp production with no detrimental effect on pulp strength. Furthermore, the data showed that PS enhances the decrease in kappa number achievable with AQ. An explanation for this effect is proposed.