Effects of lignin chemistry on oxygen delignification performance

The present work focused on characterizing the chemical and structural properties of isolated lignin from six hardwoods and their kraft pulps in an attempt to better understand the relationship between lignin’s chemical properties and resultant oxygen delignification performance. Several hardwood samples were cooked under the same conditions with varying alkali charges to obtain unbleached pulps with kappa numbers between 19 and 20. These pulps were then subjected to an oxygen delignification stage. Both processes were evaluated for pulp quality, residual lignin, and O-stage delignification efficiency. The oxygen delignification stage was carried out under fixed conditions and evaluated with regards to kappa number, which was corrected for hexenuronic acid (HexA) contributions.Results revealed that different hardwood species exhibited differing oxygen delignification efficiencies. A high correlation was found between the O-stage delignification efficiency and the content of phenolic groups in the unbleached lignin, which confirmed that free phenolic groups are the reactive site for molecular oxygen attack. When different hardwood species were compared, the HexA contents were not found to affect O-stage delignification efficiencies.