Mechanisms of pressure screening - Part 1: Mechanistic changes occurring with feed concentration and rotor type

摘要

Mechanisms are proposed to account for the shape of curves in semi-log plots of passage ratio versus feed concentration. Mechanisms and sub-mechanisms rely on flocculation effects and assumptions on flow variations through screen apertures induced by screen rotors. A semi-empirical assessment is presented where passage ratio was negatively affected by fibre concentrations greater than a critical concentration. Two passage ratios are introduced including a passage ratio for forward flow from feed to accepts and a passage ratio for reverse flow or back-flushing flow from accepts to feed. Two passage ratios were considered when back-flushing was estimated to occur in the case of a step rotor whereas only pone passage ratio was considered in the case of a bump rotor where back-flushing flow could be ignored. Screening trials were conducted investigating the effect of fibre concentration on fibre passage through a fractionating screen with smooth holes. Reject thickening factors and overall passage ratios were determined spanning fibre concentrations between 0.001% and 2.5% screening pine and eucalypt kraft. Characteristic curves became evident when the overall passage ratio was plotted against feed concentration. The shape of these curves was dependent on the rotor used and differences in shape are accounted for with the existence or absence of back-flushing flow and consequences of this. At fibre concentrations more than 0.1% greater screen capacity was achieved using a step rotor. The greater screen capacity was thought to come from an extended back-flushing period concentrating pulp in the accept chamber.