Defibration of high yield kraft pulps on a laboratory scale

摘要

Valmistettaessa sellua säkkipaperia tai laineria varten, keiton jälkeen tarvitaan erillinen kuidutusvaihe, koska massaan jäänyt ligniini pitää vielä hakkeen koossa. Tämän tutkimuksen tavoitteena oli selvittää laboratoriomittakaavassa kuinka erilaiset terävälit ja erilaiset kemialliset olot kuidutuksessa vaikuttavat valkaisemattoman sulfaattimassan kui-tuominaisuuksiin. Tulokset osoittavat, että sekä kemialliset olot että teräväli vaikuttivat tuotetun sellun laatuun. Tutkimus suoritettiin kahdella sellulla, joiden kappaluvut olivat 90 ja 40. Kor-keampikappaisella massalla terävälin pienentäminen johti suurempaan ligniinin liukenemiseen ja siten alempaan kappalukuun. Odotetusti myös rejektin määrä väheni. Alempikappaisella massalla terävälin muutos ei vaikuttanut rejektin määrään eikä kappalukuun. Kemialliset olot vaikuttivat kappalukuun: alkaalisemmat olot johtivat alempaan massan ligniinipitoisuuteen.%In the manufacture of pulps for sackpaper and linerboard, a separate defibration stage is needed after the cook, since the lignin remaining in the cooked chips is sufficient to hold the fibres together. The object of this investigation was to evaluate on a laboratory scale how different bar gaps and different chemical environments in the defibration process affect the fibre properties of unbleached kraft pulps. The amount of reject, the changes in kappa number, the degree of swelling and the fibre properties resulting from different conditions during the laboratory defibration were evaluated. The results show that both the chemical environment during the defibration and the bar gap affected the pulps produced. The investigation was performed on two pulps with kappa numbers of 90 and 40. For the high kappa number, a decrease in the bar gap led to a higher dissolution of lignin during the defibering and thus a lower kappa number of the pulp. As expected, the amount of reject decreased as the bar gap decreases. For the medium kappa number pulp, the bar gap did not affect the amount of reject or the kappa number. The chemical environment affected the kappa number, a more alkaline environment leading to a lower lignin content of the pulp. During the defibration, increases in WRV and in SR number as well as an increase in zero-span tensile index were observed when the bar gap was decreased. For the medium kappa number pulp, a large difference in the fibre form factor was observed as a result of the decreased bar gap. A similar effect was noted for the high kappa number pulp, but not to the same extent. On a laboratory scale, the defibration is for practical reasons preferable done on a pulp that has been washed in deionized water. If the defibration is done prior to the washing, the residual alkalinity of the cooking liquor can influence the fibre properties of the pulp. In future comparative trials, especially on high yield kraft pulps, we suggest that the defibration should be specified in much more detail. Our study showed that the washed pulp should be defibrated in a single stage where a desirable amount of reject is achieved using deionized water. With the Sprout-Waldron laboratory refiner, it was suitable to use a bar gap of 0,3 mm, although changes may be necessary due to different equipment avaible.