Apparent and actual delignification response in industrial oxygen-alkali delignification of birch kraft pulp.

Application: Improved procedures for measuring and reporting the results of delignification could lead to better evaluation and control of a modern oxygen delignification process.

Two different ways to look at lignin content are the "apparent" and "actual" responses to oxygen delignification. In the present work, the transient-i.e., time-dependent-behavior of residual material affecting the kappa number of birch kraft pulp was experimentally tested step-wise in an industrial two-stage oxygen-alkali delignification process. The behavior was characterized by leaching the pulps and dividing the leached material into four different fractions representing wash loss, easily leachable, slowly leachable and stagnant fractions of the lignin in pulps. The lignin apparently removed during delignification includes the actual residual lignin, extractives, HexA, and other chemical structures that contribute to the kappa number of the pulp. In this study, the apparent delignification response of birch kraft pulp was calculated from the ordinary kappa number measurements, and the actual response was calculated from the Ox-Dem kappa number measurements.

Although components other than lignin contribute to the kappa number, the ordinary kappa number still has practical value because it indicates the efficiency of different parts of fiber processing. All of the oxidizable pulp components consume some amount of bleaching chemicals. On the other hand, the ordinary kappa number does not give us detailed information about the constituents of the total lignin. These constituents include residual lignin, extractives, HexA, and other components. Each structure behaves differently in the bleaching process and influences the properties of the bleached pulp differently.

The experimental results reflected these differences in the responses of the different chemical fractions. The degree of delignification measured as a change in kappa number based on actual residual lignin was 50%, with 15% from HexA and 33% from extractives. However, the degree of delignification as a total percentage of the change in kappa number based on the apparent lignin was only 32%. Consequently, how the kappa number is measured and reported could be misleading when mill managers are trying to access the performance characteristics of the oxygen delignification process. It is important to understand that the transient behavior of lignin affects the response measured both for cooking and for oxygen delignification.

Kari Ala-Kaila is with the Technology Development Dept. at the Metsa-Botnia Corp., FIN-26101 Rauma, Finland. Li, Sevastyanova, and Gellerstedt are with the Dept. of Fiber and Polymer Technology, Royal Institute of Technology, S-10044, Stockholm, Sweden. Email Ala-Kaila at [e-mail].