Impact of Residual Extractives on Lignin Determination in Kraft Pulps

The amount of residual extractives in kraft pulps and oxygen-delignified pulps from loblolly pine, white birch, and aspen, as well as their impact on the kappa number and Klason lignin determinations were investigated. In the hardwoods, significant amounts of extractives remained after kraft pulping and oxygen delignification. These residual extractives had an impact on the lignin determination, more so on the acid lignin method than kappa number. For an accurate lignin determination, solvent extraction of hardwood kraft and oxygen-delignified pulp is needed, but not for softwood pulps.