Fibre Fraction
Hydrocyclone cleaners remove oversized contaminants and dense debris from aqueous pulp suspensions by centrifugal force before the stock reaches the paper machine headbox. Screening rejects represent the concentrated fraction of coarse shives, sand particles, and unfreed wood bundles that fail to pass through the fine aperture slots of the pressure cylinder. Papermakers monitor this heavy discharge stream to prevent excessive fiber loss while maintaining the cleanliness required for high-grade graphic papers.
Secondary sorting stages recover usable long fibers from the primary reject flow, returning them to the main stock preparation loop. Undiluted residue containing dense grit and bark fragments exits the system permanently to protect downstream refining equipment from abrasive wear.
Aperture Sizing
Pressure drop across the cylindrical basket dictates the separation efficiency for varying grammages and furnishes. Slot widths typically range from zero point one five to zero point four zero millimeters depending on whether the furnish consists of bleached hardwood kraft or recycled corrugated containers. Narrower apertures capture smaller shives but increase the volumetric flow of rejected pulp, which demands larger secondary cleaner capacity.
Viscosity fluctuations alter the boundary layer inside the slotted cylinder, shifting the cut point where marginal particles cross into the reject stream.
Refining Yield
Unfiberized wood fragments within the reject fraction possess high lignin content and poor bonding potential if returned directly to the sheet formation zone. Mills direct these recovered bundles through dedicated low consistency refiners to mechanically fracture the stiff cell walls before blending the treated material back into packaging grades. Energy consumption per ton rises sharply when processing heavy reject streams because the raw material requires intense mechanical action to achieve acceptable freeness levels.
Final tensile strength of the paper web depends on proper management of this reject loop, ensuring that abrasive contaminants leave the system entirely while beneficial cellulose fibers rejoin the production cycle.
