Chemical character of the response of softwood kraft pulp towards industrial two-stage oxygen-alkali delignification

• The transient behavior of residual material affecting the kappa number of softwood kraft pulp (termed residual lignin), was tested experimentally step-wise in an industrial two-stage oxygen-alkali delignification process. The behavior was characterized by leaching the pulps and dividing the residual lignin into four different fractions to represent the wash loss fraction, the easily leachable fraction, the slowly leachable fraction and the stagnant fraction of the lignin in pulp. Further characterization was made regarding the chemical character of these fractions, i.e. residual lignin, extractives, hexenuronic acid, and other chemical structures that contribute to the kappa number of the pulps. The componential delignification response in the two oxygen-alkali reactors and the effects of the leaching operation were comprehensively assessed regarding the behavior of these different pulp components. The results were compared with results obtained earlier for birch kraft pulp originating from the same industrial process and treated using identical experimental procedures. Application: This study gives a new perspective with which to evaluate apparent and actual responses in industrial oxygen-alkali delignification of kraft pulps.

Keyword

kappa-number

lignin removal

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