Internal fibre length concentration in a pressure screen

Abstract

Localised axial consistency profiles within a pressure screen of a Pinus Radiata kraft pulp are reported. Axial Samples were also analysed using a Kajaani FS-200 to obtain fibre length distribution data. Localised consistency in the feed annulus was found to vary considerably and the consistency was found to be less than the feed consistency over some portions of the screen (annular dilution). Changes in consistency along the accept side was fairly constant although subtle changes were observed. Pulp passage ratios for both the bulk and individual fibre length fractions were calculated using the consistency profiles and fibre length data. In all cases fibre passage decreased along the screen length. Fibre passage was affected by a position effect which is comprised of two factors: flocculation effects, and flow and rotor effects. Fibre fractionation efficiency was found to increase along the length of the screen. Mechanisms that account for the observed annular dilution, passage ratio and efficiency changes are proposed. These involve flow of both fluid and fibre in the forward and reverse directions across the screen plate, increased flocculation in the feed annulus and the slip velocity between incoming pulp and the rotor tip.