Fundamentals of Screening: Effect of Rotor Design and Fibre Properties, 1999 Pulping Conference Proceedings

François Julien Saint Amand & Bernard Perrin

Centre Technique du Papier

An experimental pressure screen has been designed and methods were developed to investigate the fundamental parameters of the screening process on industrial scale. The work described in this paper deals with the effect of rotor design on the screening performance, which is reported in terms of contaminant removal efficiency and fibre passage ratio with respect to fibre length. These data allow the pulp reject thickening factor and the fractionation efficiency of full size screens to be assessed according to existing models. Two rotors were compared under various screening conditions (0.10 to 0.25 mm slots, 1 to 3 % consistency, 0.5 to 3.5 m/s passing velocity) with two chemical pulps bleached softwood and bleached hardwood) and two mechanical pulps (softwood TMP and hardwood SGW).

All the results confirmed the general trends observed previously with the experimental screen. The fibre passage ratio was increased as the slot width and the passing velocity were increased and as fibre length was reduced. The consistency showed no large effect in the tested range, except on the screening capacity limits. The screening efficiency, which was determined for shives with the mechanical pulps and for flat shaped model contaminants with the chemical pulps, generally decreased as the fibre passage increased. The screening selectivity showed to be difficult to improve significantly and was in general slightly lower at high consistency.

A rotor with long blades has been designed in such a way to produce extended reverse pulses. Compared to a foil rotor, which produced short reverse pulses, the long blade rotor showed similar trends regarding the effect of the screening parameters, with however some significant differences. These included a lower influence of the passing velocity, higher fibre passage ratios and generally lower efficiencies under given screening conditions, and a slightly higher selectivity, especially with the finest slots. These differences should mainly be due to the high effective passing velocity of the extended reverse pulse rotor.

The two rotors gave the same results with respect to the screening ability of the tested pulps. The pulp passage ratio, which depends mainly on the fibre length distribution, was shown to depend also on the type of fibres. The coarse SGW fibres and to a lesser extend the TMP fibres had lower passage ratio than flexible chemical fibres of the same length. Long and stiff fibres showed to be very difficult to screen with both rotors, especially with the 0.10 mm slots which achieved the best shives and contaminant removal efficiencies.