Corrugated medium and test liner production relies heavily on recycled fiber — primarily old corrugated containers (OCC). The economics of corrugated paper mills are tightly linked to fiber yield: every percentage point of fiber lost through the cleaning and screening system represents raw material that must be replaced, typically at a cost. Optimizing fiber recovery while maintaining effective contaminant removal is the central challenge of OCC stock preparation.

Understanding OCC Contaminant Profile

OCC arrives at the mill carrying a complex mixture of contaminants that must be removed before the fiber can be used for papermaking:

• Heavy contaminants: Staples, paper clips, sand, glass fragments, stones — removed by high-density cleaners and detrashing systems
• Lightweight contaminants: Plastic films, tapes, polystyrene, wax coatings — removed by reverse cleaners or forward cleaners with lightweight rejects
• Adhesives and stickies: Hot-melt adhesives, pressure-sensitive adhesives, wax — removed by fine screening and, increasingly, by specially designed stickies-removal screens
• Fiber contaminants: Fines, ash, and very short fiber fractions that reduce strength and drainage — managed through fractionation and selective rejection

Each contaminant type requires different removal technology, and the system must be designed to maximize fiber recovery at each stage. A typical OCC line processes 300–800 tpd, so even a 0.5% improvement in fiber recovery can be very significant for mill economics.

High-Density Cleaning: First Stage Fiber Recovery

High-density cleaners are the first major separation point in OCC treatment. Operating at 3.5–5.0% consistency, these hydrocyclones use centrifugal force to separate heavy particles from the fiber suspension. The key to maximizing fiber recovery is the cascade arrangement and proper reject rate control:

• Primary cleaners: Accept rate 90–95% (reject rate 5–10%), with rejects containing concentrated heavy contaminants
• Secondary cleaners: Process primary rejects, recovering 85–92% of the fiber from this stream
• Tertiary cleaners: Final stage, recovering 75–85% of fiber from secondary rejects

A properly designed three-stage HD cleaner system achieves overall fiber recovery above 99% while maintaining heavy contaminant removal efficiency above 95%. The tertiary stage, while adding capital cost, typically pays for itself within the first year of operation through fiber savings in larger mills.

The reject rate at each stage must be carefully controlled. Too low a reject rate allows contaminants to pass forward. Too high a reject rate increases fiber loss. For the primary stage, a reject rate of 8–12% provides the best balance for typical OCC furnishes.

Fine Screening for Stickies Control

Stickies are the most troublesome contaminants in OCC-based corrugated production. These adhesive particles cause deposits on forming fabrics, press felts, and dryer cans — leading to sheet defects, web breaks, and increased maintenance downtime.

Fine screening with slotted baskets is the primary defense against stickies. For corrugated medium and test liner grades:

• Primary fine screen: 0.20–0.25 mm slots for effective stickies removal
• Secondary screen: 0.25–0.30 mm slots for fiber recovery from primary rejects
• Reject screen: 0.30–0.35 mm slots for final-stage fiber recovery

Modern profiled slot baskets with contoured inlet surfaces (rather than sharp-edged slots) improve stickies capture efficiency by 5–10% while reducing the tendency for long fibers to align with and pass through the slots. This is particularly important for OCC fiber, which contains longer fiber fractions than many other recycled grades.

Fractionation: Selective Fiber Recovery

For mills producing both corrugated medium and test liner, fiber fractionation offers additional recovery optimization. A pressure screen configured with 0.15–0.20 mm slots can separate the feed into:

• Long-fiber fraction (accepts): Used in test liner for strength development
• Short-fiber fraction (through slots): Used in corrugated medium where bulk and drainage are more important than strength

This approach maximizes the value extracted from OCC fiber. The long-fiber fraction receives intensive refining (60–100 kWh/t) to develop strength for the liner ply, while the short-fiber fraction requires minimal refining (15–25 kWh/t) for the medium ply. Total energy consumption is lower than refining the entire stream to liner quality, and fiber utilization is optimized.

Process Water Management and Fiber Recovery

Significant fiber loss often occurs not through the main process streams but through the water system. Clarified white water from the disc filter or dissolved air flotation (DAF) unit should be monitored for fiber content. A well-operated save-all system recovers fiber with the following performance expectations:

• Disc filter: Fiber recovery efficiency >98%, producing clear filtrate with <50 mg/L suspended solids
• DAF unit: Fiber recovery efficiency >97%, with clarified water TSS <100 mg/L

Regular inspection of disc filter sectors for fabric damage and DAF units for proper air saturation and flocculation chemistry ensures fiber recovery targets are maintained.

Maintenance for Sustained Fiber Recovery

Fiber recovery systems lose effectiveness gradually as equipment wears. Key maintenance attention points:

• HD cleaner cones: Measure lower cone diameter quarterly; a 3–5% increase from wear changes the separation cut point and increases fiber loss
• Screen baskets: Slot width increases with wear based on throughput volume; baskets handling OCC typically require replacement at 18–24 month intervals
• Disc filter sectors: Inspect fabric condition monthly; torn or holed sectors allow fiber-laden water to bypass to the clear leg
• Pump performance: Reduced pump output (from impeller wear) changes flow distribution in cascade systems, affecting reject rates and fiber recovery

A systematic fiber balance — tracking fiber content at every major process point from pulper feed to effluent discharge — is the most effective tool for identifying fiber loss sources and prioritizing corrective actions.

📧 For technical support on fiber recovery optimization: leizhanzhang@gmail.com