Effects of carbonate on delignification of birch wood in Kraft cooking

A laboratory study of the kraft cooking of birch wood (Betula pendula) chips and the effect of carbonate in the cooking liquor was undertaken. When sodium carbonate was added to a carbonate free laboratory white liquor, a significantly higher rate of delignification of birch wood was attained. The cooking time to reach kappa 16 was halfed when 0.1 M sodium carbonate was added to a carbonate-free laboratory white liquor. In addition, the carbohydrate yield in the unbleached pulps produced with carbonate-free white liquor decreased significantly at kappa 18, whereas the carbohydrate yield of the pulps produced with added carbonate was practically unaffected at kappa numbers above 14-15. The higher rate of delignification in the kraft cook in the presence of carbonate is most probably due to precipitation of calcium carbonate. The concentration of calcium in black liquor from cooks performed without added carbonate was twice that when carbonate was added from the beginning.