Extended Oxygen Delignification for Increased Yield, 1999 Pulping Conference Proceedings

Donna A. Johnson, Seongkyung Park, H. Kwon, Joseph M. Genco, Lewis D. Shackford, D. Patrick Sullivan

The objective of this study was to examine the effect of initial kappa number on subsequent oxygen delignification and bleaching. Southern softwood chips were conventionally pulped to high (37) and low (29) kappa numbers using industrially significant conditions in a laboratory digester. The pulps were prepared by varying the time-at-temperature during cooking, while keeping the sulfidity and effective alkali constant. These pulps were then delignified using oxygen in a two-stage process by varying the caustic charge. The target kappa number prior to bleaching was 16. Bleaching was accomplished by using the D100-Eo-D-E-D sequence. Data are presented for pulp yield as a function of caustic charge in the oxygen stage, overall yield following bleaching, and selectivity, COD and wet zero-span breaking length.

The yield after pulping and screening was 44.8 and 43.2%, respectively, for the high (37) and low (29) kappa number pulps, while the rejects were 2.7% and 2.5%, respectively, resulting in a total yield of 47.5 versus 45.6%. In the oxygen stage, there was less yield loss at the same caustic charge for the high (37) kappa pulp when compared to the low (29) kappa number pulp. However, the yield at the target 16-kappa number into the bleach plant was 97% for both the 29- and 37-kappa pulps. Additional caustic was required to reach this target for the 37-kappa pulp; 3.0% caustic was needed for the 37-kappa pulp, and 1.8% caustic for the 29-kappa pulp. The selectivity for lignin removal was very similar for both pulps, and was better at lower caustic charges.

Both the high and low kappa number pulps were easily bleached to the target brightness of 88%+ ISO using the D100-Eo-D-E-D sequence. The chemical charge for each stage of bleaching was kept constant. The final brightness was higher for the 37-kappa pulp, which may indicate easier bleaching if the pulp is removed from the digester at higher kappa numbers. The change in kappa/permanganate numbers, viscosity, and wet zero-span breaking length was similar for both pulps at various stages during the bleaching sequence. The 37-kappa pulp had a 2% decrease in yield in the D100 stage, which decreased the total yield. However, the 37-kappa pulp still had a 1% total yield advantage over the 29-kappa pulp after bleaching. These results suggest that it may be advantageous to raise the kappa target in the digester, provided the rejects from the screens can be adequately handled and there is sufficient washing capacity available to handle the extra dissolved solids. However, each mill is different and whether high kappa cooking is practical would depend upon the equipment available and would require appropriate study.