Industrial Refining of Unbleached Kraft Pulps-The Effect of pH and Refining Intensity, 2002 Technology Summit Proceedings

Ulla-Britt Mohlin

The effect of refining intensity and pH on sheet properties for machine-made papers of unbleached softwood kraft pulp was evaluated using the dynamic conditions of modern papermaking. An industrial never-dried unbleached kraft pulp was refined in a commercial disc refiner (Beloit DD 24. refiner) at two production levels and at three pH-levels. Different specific energy levels were obtained by changing the power input at constant production. Papers were produced both on a laboratory scale and on the EuroFEX pilot paper machine at 600 m/min using the twin-wire roll former unit. The papermachine was run under constant conditions except for two levels of jet-to-wire speed differences.

The refining efficiency at a certain power input improved, as expected, with increasing pH. The improvement could be explained by a plate gap reduction when pH was increased. The results also showed that refining efficiency evaluated as the WRV-increase times production was directly related to plate gap independent of pH and production level in the refiner. Changes in fiber length and fiber curl were also controlled by plate gap. In this case, plate gap was a better way of comparing refining conditions than power input or specific edge load.

For the handsheets, strength was directly related to WRV. No differences in strength could be observed that were related to the production level in the refiner. For the machine-made sheets, lower strength was observed than for the handsheets due to higher forming consistency. In addition, however, pulps refined at the lower production level in the refiner gave machine-made sheets with lower strength and density than the pulps refined at the higher production level. Micrographs of fiber cross sections in the machine-made sheets showed a lower degree of fiber collapse for the pulps refined at lower production.

One interpretation could be that differences in the stiffness of the fiber wall due to the different production levels in the refiner affect sheet strength and sheet density only when the dynamic wet pressing of a fast papermachine is applied. When the static wet pressing of handsheet making is used, however, these differences are concealed by the longer pressing time. Thus, the results here demonstrate the severe limitations in our ability to evaluate refining results by laboratory testing only.