Contaminant Removal
Heavy and coarse contaminants separated from recycled pulp slurries by perforated or slotted screening plates are continuously purged from the stock preparation system. In recycled papermaking, screen rejects represent the reject stream containing plastic film fragments, sticky adhesives, wet-strength paper pieces, and shives. Purging non-papermaking solids protects paper machine equipment and maintains finished paperboard cleanliness.
Scope applies to coarse and fine screening stages in secondary fiber pulping operations.
Slurry Separation
Dilute pulp slurry flows under pressure across stationary or rotating screen plates featuring precise slot widths between point-one and point-three millimeters. Acceptable flexible cellulose fibers pass through screen openings, while rigid plastic fragments, unpulped flake bundles, and adhesive particles are retained on the inlet side. Continuous reject flow prevents hole blinding and maintains screening efficiency across the basket surface.
Multi-stage screening systems re-pass reject streams through secondary and tertiary screens to recover entrapped good fiber before final discharge. De-watering equipment concentrates rejected debris into semi-solid cake for energy recovery or municipal landfill disposal.
Fiber Loss
Screening operations must minimize good fiber losses while maximizing contaminant removal efficiency. Overly aggressive screening discards long softwood fibers, reducing final paperboard strength properties. Deflocculation agents and optimized pressure drops maintain high yield without clogging screen plates.
Proper reject management stabilizes pulp quality and reduces raw fiber waste in recycled board mills.
