Some factors affecting the amount of residual phase lignin during kraft pulping

Abstract: The amount o ALIGN="JUSTIFY">It is shown, that the amountsof residual phase lignin in constant composition cooks (highliquor-to-wood ratio) of softwood (spruce), hardwood (birch) andgrass (wheat straw) are affected by the same factors in theinvestigated range. The amount of residual phase lignin isstrongly reduced by higher hydroxide ion concentration during thekraft cook and is to some extent decreased by higherhydrogensulfide ion concentration and lower sodium ionconcentration. These effects were not, however, found to bepurely additive for spruce wood, instead interactions occurred.In order to decrease the amount of residual phase lignin, it isessential to have a high concentration of hydrogensulfide ionswhen cooking with a low hydroxide ion concentration. Thetemperature has no effect on the amount of residual phase ligninin the range investigated.It is also shown that the lignin reacting according to theslow delignification phase is not a homogeneous lignin sincealteration of the conditions in the cook can make part of itreact as bulk phase lignin. This indicates that most of theresidual phase lignin is determined by the prevailing conditions,which determine how much of the native lignin reacts according tothe bulk and how much according to the residual delignificationmechanism.Pulping experiments carried out under the same conditions onwheat straw, birch wood and spruce wood also showed that theamount of residual phase lignin in wheat straw is only 30% of theamount of residual phase lignin in spruce wood, and that theamount of residual phase lignin in birch wood, is50%of the amount of residual phase lignin in sprucewood.The amount of residual phase lignin was affected by apolysulfide treatment. The amount of residual phase lignin waslowered by 30% for spruce wood and 10% for birch wood with apolysulfide treatment, compared with cooks without polysulfide.An explanation for this may be the finding that polysulfide canintroduce a carboxylic acid function into the lignin, which willincrease its water solubility, and reduce the need forfragmentation. An additional explanation may be the ability ofpolysulfides to degrade enol ether structures, which are normallymore or less stable during kraft pulping.Keywords:Deligniflcation, Residual phase lignin,Hydrogensulfide ion, Hydroxide ion, Polysulfides, Ionic strength,Temperature, Lignin,Kraftpulping, Selectivity, viscosity, Kappa number,chemical degradation, Disproportionation.