Improved fibre fractionation, and selective processing of the resulting pulp fractions is a way to produce higher quality, more uniform pulp A pilot-plant screening study determined the conditions of optimal fractionation, and verified screening performance equations, It showed that fibre length fractionation and pulp consistency changes during screening are both related to volumetric reject ratio, R-v, by a fibre passage ratio, P(I). This ratio quantifies the dependence of the ability of the fibre to go through a single aperture in the screen an the fibre length. The effects of aperture size,fluid velocity through the aperture, and R-v on P(1) were determined. A new definition of fibre length fractionation efficiency was proposed, similar to long fibre removal efficiency, brit which penalizes short fibre removal and is therefore more indicative of the fibre length separation. For a large aperture screen plate, the maximum fractionation took place at relatively large R-v. For smaller diameter apertures, the best fractionation occurred at a significantly lower R-v,. The maximum fibre fractionation occured for an intermediate hole size of 1.0 mm.
