Increasing softwood kraft pulp yield using sodium methyl mercaptide

The objective of this work was to determine the effect of sodium methyl mercaptide (SMM) on the minimization of peeling reactions of southern pine chips in the kraft pulping process. Two methods were evaluated for SMM addition to the pulping process: (1) pre-treatment before pulping or (2) co-addition with white liquor. The effect of SMM charge, pre-treatment temperature and time, and pH of pre-treatment liquor was studied.The experimental results showed about 1.5 to 2.5% (on O.D. (oven dry) wood basis) increase in the pulp yield after pre-treatment with or co-addition of 4.38% SMM (on O.D. wood basis). The use of 4.38% SMM allowed a decrease of the white liquor effective alkali charge (EA, on O.D. wood basis) by 3%. 4.38% SMM charge seemed to be optimum for the pre-treatment. Pre-treatment at lower pH resulted in a significant decrease in yield and an increase in rejects. The increase in pulp yield was mostly due to the increased retention of cellulose and xylan. The retention of galactoglucomannan was negligible. The degree of polymerization of cellulose increased from 5180 to about 6000 due to SMM pre-treatment. About 80% of the cellulose yield increase is due to the suppression of primary peeling. The remainder (0.3-0.4% of the yield increase (on O.D. wood basis) is due to reduced alkaline hydrolysis and subsequent secondary peeling. [GRAPHICS] .