Dissolution of Lignin from Kraft Pulp after Treatment with Nitrogen Oxides

Laboratory experiments show that pretreatment with NO₂ including heating and ripening at 5% consistency and 90°C for 3 hours in the presence of NO₃- and H⁺ led to a dissolution of approximately 50–60% of the lignin in unbleached kraft pulp. Only 5–15% were dissolved when the treatment was terminated after heating to 90°C. An extensive fragmentation of the lignin during the ripening explains the extensive delignification in acid medium.

Fractionated dissolution of the remaining lignin by alkaline extraction gave fractions with small variation in N-contents (2.9–3.4%) and hydrophilicity. The last fraction exhibited the highest sorption of water vapor. The delignification was related to the decrease in molecular size during the pretreatment and during the subsequent alkaline treatments.