Optimizing Pressure Screening for Higher Fiber Yield in Kraft Liner Production
Every kraft liner mill manager knows the tension: you need clean stock for good machine runnability, but aggressive screening throws away valuable long fiber. In a market where OCC prices fluctuate and margins are squeezed, fiber yield is everything.
The Screening Tradeoff Nobody Talks About
Pressure screens are essential — they remove stickies, plastics, and shives that cause web breaks and sheet defects. But here’s the uncomfortable truth: a poorly configured pressure screen can reject 5–8% of good fiber along with the contaminants. That fiber goes straight to your sludge press or effluent treatment. You paid for it in raw material cost, and now you’re paying again to dispose of it.
For a mill running 500 TPD of OCC-based kraft liner, a 5% fiber loss means 25 tons of usable fiber per day going to waste. At typical OCC prices, that’s a significant annual loss.
Basket and Rotor Design: The Two Levers That Matter
The key to maximizing yield lies in basket selection and rotor foil geometry. Leizhan’s pressure screens use advanced profile bar baskets (also called wedge-wire or C-bar baskets) with precisely engineered slot widths:
- Coarse screening (primary): 0.20–0.35 mm slots — removes large contaminants while passing virtually all fiber
- Fine screening (secondary): 0.12–0.20 mm slots — targets stickies and micro-contaminants
- Fractionation screening: 0.15–0.25 mm slots — separates long fiber from short fiber for multi-layer forming
The Multi-Foil rotor generates positive and negative pressure pulses that keep the basket surface clean without forcing fiber through the slots. The result: fiber rejection rates below 2% while maintaining 90%+ stickies removal efficiency.
Two-Stage Cascaded Screening for Maximum Recovery
The industry best practice is a two-stage pressure screen system:
- Primary screen: Accepts go to the paper machine approach flow. Rejects (typically 15–20% of feed) go to the secondary screen.
- Secondary screen: Accepts return to the primary screen feed. Rejects go to the sludge system — but by this point, they represent only 1.5–2.5% of total feed fiber.
This cascaded configuration recovers over 97% of long fiber while achieving stickies removal rates that keep the paper machine running at full speed.
Key Operating Parameters
| Parameter | Primary Screen | Secondary Screen |
|---|---|---|
| Feed consistency | 1.5–3.0% | 0.8–1.5% |
| Pressure drop | 20–50 kPa | 15–35 kPa |
| Reject rate | 15–20% | 20–30% |
| Basket slot width | 0.20–0.35 mm | 0.15–0.20 mm |
| Rotor speed | 12–18 m/s | 10–15 m/s |
Maintenance Schedule for Consistent Performance
- Daily: Check differential pressure across the screen. A rising delta-P trend indicates basket plugging.
- Weekly: Inspect reject chamber for abnormal buildup.
- Monthly: Remove and inspect basket for damage, wear patterns, and slot deformation.
- Every 6–8 months: Replace rotor foils if wear exceeds 5 mm from original profile.
- Annually: Full basket replacement under normal operating conditions; sooner if processing high-ash OCC.
The bottom line: you can have both clean stock and high yield. It comes down to the right equipment configuration and disciplined operating practices.
📧 Contact us at leizhanzhang@gmail.com to discuss pressure screen upgrades, basket selection for your furnish, or a complete stock preparation audit.