Fibre length fractionation caused by pulp screening, slotted screen plates

Fibre fractionation and selective processing of each fraction produces high er quality, more uniform pulp. Engineering efficient pressure screening sys tems requires appropriate performance equations. This study extends previou s screening performance equations by comparing pilot screening trials with a mechanistic model of fibre passage through the screen to predict the effe cts of reject rate, slot width, contour type, and slot velocity on fraction ation and consistency changes. Smooth-hole screen plates were found to frac tionate more efficiently than contour-slot screen plates. Fibre passage thr ough slotted screen plates was strongly dependent on the fluid velocity pas sing through the slot, while smooth-hole apertures were approximately indep endent of aperture fluid velocity. The maximum fractionation efficiency for slotted plates occurred at slot velocities between 0.5 and 1.0 m/s. The na ximum fractionation efficiency for both slot and hole plates occurs at a re ject ratio that results in a reject thickening factor greater than two.