Process Technology

Conventional oxygen delignification cannot remove more than 35–50% of the residual

lignin before sustaining detrimental oxidative carbohydrate degradation,

which is expressed as a loss in viscosity and fiber strength. The low selectivity of

oxygen delignification can be explained in part by the unfavorable conditions for

lignin leaching that seem necessary to achieve access to the more resistant lignin

structures (e.g., condensed phenolics). If lignin leaching does not take place, the

more resistant structures might not be attacked as easily as the carbohydrates

under typical conditions of a one-stage oxygen delignification process [93].

A clear overview of the physical and chemical behavior of leachable residual lignin

from a Scandinavian softwood kraft pulp during oxygen delignification has

been provided by Ala-Kaila and Reilama [114]. The residual lignin across an industrial

two-stage oxygen delignification process was divided into four fractions by

different leaching operations, representing wash loss lignin, easily leachable,

slowly leachable, and resistant fraction of lignin [114]. The investigated pulp samples

originate from the positions prior to the first reactor (brownstock), the transition

line from the first reactor blow tank to the second reactor (O1 blow), and

finally from the second reactor blow tank (O2 blow). The experiments revealed

that most of the leachable lignin is only weakly attached on the fiber–liquid interface,

and can be removed by a 5-min washing operation at 90 °C after being centrifuged

to a consistency of 38% (Tab. 7.24). The amount of wash loss lignin fraction

increases during the course of delignification. It can be assumed that part of the

resistant lignin fraction is converted into the wash loss lignin as a result of

delignification reactions in the first reactor. The unbleached softwood kraft pulp

contains approximately one kappa unit of each easily (30 min at 90 °C) and slowly

(24 h at 90 °C) leachable fractions, respectively. In the first reactor, the easily leachable

lignin fraction diminishes almost quantitatively, whereas the slowly leachable

lignin decreases significantly in the second reactor only. There, predominantly

resistant and slowly leachable lignin fractions are involved in the delignification

reactions. The results confirm the importance of mass-transfer processes between

fiber and liquor phases in the transition from cooking over washing to oxygen

delignification.

Tab. 7.24 Residual lignin contents measured as kappa number

of a softwood kraft pulp after different leaching operations in the

course of a two-stage oxygen delignification process (according

to [114]).

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