Effect of Furnish Quality on Pressure Screen Performance

The influence of pulp and fibre characteristics on pressure screening and fractionation were studied. Ten different pulp furnishes, including mechanical, chemical and recycled pulps, were pressure screened in equal operational conditions and their thickening behaviour and the quality of fractions analysed. Length weighted average fibre length was observed to well predict the thickening in a pressure screen. The effect of fibre length to thickening was so dominant, regression analysis of results found fibre coarseness, pulp freeness, crowding factor and measured fibre network strength to be insignificant factors in characterization of pulp thickening behaviour. The fibre length difference of the accept and reject fractions could be predicted by the width of the arithmetic fibre length distribution, measured by standard deviation. The results can be utilized in designing screening systems i.e. dimensioning the size of screens for given capacity and quality requirements.