Effect of LC refining intensity on fractionated and unfractionated mechanical pulp

Abstract

This pilot scale study evaluates the benefits of fibre fractionation by pressure screening prior to low consistency (LC) refining. The hypothesis was that the long, coarse fibre fraction could be LC refined at higher intensity without excessive fibre length reduction. The study was conducted with primary HC refined pulp, a mix of spruce, pine and fir. Four screen basket designs were evaluated (with 0.8, 1.0 and 1.5 mm holes, and 0.15 mm slots), and fractionation was carried out. The coarse screen reject pulp underwent subsequent LC refining at low and high intensity (0.25 and 0.5 J/m). LC refined reject fractions at different freeness levels were combined with screen accept pulp, and final pulp qualities were compared with unfractionated pulps LC refined at the same intensities. In this study, fractionated LC refined pulps (as well as low-intensity LC refined pulp) showed less fibre length and bulk reduction than that obtained at high intensity LC refining when compared at the same specific energy. Overall, fractionation plus high-intensity LC refining provided similar pulp qualities as low-intensity LC refining. Fractionation thus enabled the use of higher intensity LC refining, allowing LC refiners to operate at higher power and a reduced number of stages.