Increasing packaging grade kraft pulp yield by using sodium methyl mercaptide

The objective of this work was to determine the effect of sodium methyl mercaptide (SMM) on the yield of the packaging-grade kraft pulp. Based on our experience with bleachable grade kraft pulping using SMM, only a co-addition of 4.38% SMM with white liquor was studied. The cooking temperature and H-factor were adjusted to reach the target kappa number. The experimental results showed about a 2.7% (on O.D. (oven dry) wood basis) increase in the pulp yield after the co-addition of 4.38% SMM (on O.D. wood basis). The use of 4.38% SMM allowed lowering of the severity of the following process variables: the white liquor effective alkali charge (EA, on O.D. wood basis) by 2.4%, the cooking temperature from 165 °C to 157 0C, and the H-factor from 535 to 295. For 98 kappa pulp, the increase in pulp yield was mostly due to the increased retention of xylan (1.63%, on wood basis) and lignin (1.71%). The retention of galactoglucomannan was negligible (0.26%). There was a minor loss of cellulose (0.70%). The increase in the pulp yield in the presence of SMM can be explained by two non-exclusive mechanisms: stabilization of carbohydrate-reducing end groups during impregnation, and faster delignification during the bulk cooking phase. Based on the experimental data, we presently do not know which of the two mechanisms for increasing the pulp yield is dominating.