On the application of grits to thermomechanical pulp refining

Abstract

The objective of this thesis was to develop a mechanical pulping process capable of producing good-quality pulp, while consuming less electrical energy. The study focused on applying of grits to TMP refining to enhance the breaking of the fiber cell wall promoting faster development of pulp fibers to the desired quality for papermaking. The study comprises tests at laboratory scale and development of an application for industrial, including trials at pilot scale. A preliminary trial with the grit application in TMP refining was conducted at laboratory scale. The results showed that the grits should be introduced between the first and second stages in TMP refining. The grit treatment on the TMP fibers caused disruption of the wall structure, opening of the outer layers and peeling-off of the cell wall. The efficient disruption with minimal shortening and weakening of fibers was found to be operated at a low-intensity and high-frequency of treatment. According to an experiment with first-stage TMP pulp, the disrupted pulp developed faster during subsequent refining, while the energy consumption was reduced by up to 30% without a significant loss of pulp quality. With the aim of developing an industrial application, the refiner segments were modified by applying grits on the refiner segment surfaces. The grits were made from self-fluxing tungsten-carbide powder and a Ni-base alloy powder, which were laser-clad onto the surface of breaker bars, the inner part of a segment. Trials with grit segments were carried out on a pilot refiner. The grit segments were applied in first-stage TMP refining, followed by treatment with base segments operated under normal mill conditions. The grit segments were found to have no negative effects on the refining system. A refiner equipped with grit segments, operated at a speed of 2400 rpm, produced pulp with a higher level of disruption of fiber cell walls than a refiner equipped with the reference segments. According to the results, the energy consumption can be reduced by at least 10% with minimal negative impacts on pulp and paper properties.