Softwood fiber damage – a newly discovered enigma in modern kraft fiber lines.


Studies of pulp strength delivery have shown that cooking is the most significant step causing strength losses in softwood pulp. In the early 1990s it was demonstrated that different cooking processes resulted in different strength delivery and that displacement kraft batch cooking systems resulted in a pulp strength which comes very close to the strength of the laboratory reference pulp, giving a strength delivery of almost 100 %. Since many such systems have been started up, the authors have carried out multiple pulp strength audits. Surprisingly, the strength delivery values showed a variability which was difficult to understand at first. After intensive mill studies it was found that all displacement batch cooking systems produced close to 100 % strength delivery, if the pulp sample was taken from the digester discharge line. However, if the sample was taken later in the fiber line, after various brown stock process components, clearly lower values were noted in several tests. The variability was caused by the location of the sampling point rather than the cooking system itself. This strongly suggests that the cooked alkaline pulp in situ seems to be exceptionally sensitive to fiber damage. This situation seems worrying: the good pulp strength achieved in a modern cooking plant can be lost in the succeeding brown stock fiber line. The present paper demonstrates various pulp strength delivery profiles along fiber lines consisting of different cooking systems and fiber line equipment. The results of various fiber measurements and possible mechanisms and consequences for pulp quality are discussed.