Kraft linerboard demands high burst strength, ring crush resistance, and consistent CD (cross-direction) profiles. For mills using recycled fiber — particularly old corrugated containers (OCC) — the challenge is maintaining these strength properties while managing the contaminants that come with post-consumer fiber streams.

The OCC-to-Kraft Liner Challenge

OCC fiber inherently carries more variables than virgin kraft pulp: starch residues, wax coatings, pressure-sensitive adhesives, wet-strength resins, and varied fiber length distribution from mixed sources. Each re-pulping cycle shortens average fiber length by approximately 6–10%. For kraft liner grades requiring burst index of 3.0–4.5 kPa·m²/g and ring crush above 1.8 kN/m, fiber treatment must be precise.

Stage 1: High-Consistency Pulping for Fiber Preservation

The pulping stage sets the foundation for the entire line. For OCC processing, a D-type hydrapulper operating at 14–18% consistency is recommended:

• Extraction plate hole diameter: 8–12 mm for first-stage extraction; this determines the maximum contaminant size entering downstream equipment
• Pulping time: 15–25 minutes at 14–16% consistency achieves >95% defibering while minimizing fiber cutting
• Specific energy: 20–35 kWh per tonne of OCC pulp — excessive pulping time drives up both energy consumption and fines generation
• Temperature: Maintaining 45–55°C improves ink detachment and reduces pulping energy by up to 15%

Stage 2: Screening — The Critical Quality Gate

Multi-stage screening with progressively finer slot dimensions is essential for kraft liner quality:

• Primary screen: 0.25–0.35 mm slots, 3.5–4.5% feed consistency, reject rate 20–25%
• Secondary screen: 0.20–0.30 mm slots, processing primary rejects to recover good fiber
• Tertiary screen: 0.15–0.20 mm slots, minimizing fiber loss in final rejects
• Total screening efficiency: >95% stickies removal with <3% total fiber loss across all stages

Basket design matters significantly. Profile bars with rounded leading edges and sharp trailing edges create micro-turbulence that resists hole-plugging while maintaining high removal efficiency for lightweight contaminants.

Stage 3: Refining — Building Strength Without Killing Drainage

Refining for kraft liner from OCC requires balancing strength development against freeness reduction:

• Refining consistency: 3.5–5.0% — higher consistency favors fiber fibrillation over cutting
• Specific edge load (SEL): 1.0–2.0 J/m for OCC — lower than virgin kraft (1.5–3.0 J/m) because recycled fiber is more brittle
• Target freeness drop: 60–100 mL CSF reduction from inlet to outlet
• Specific energy input: 60–120 kWh/t, depending on target ring crush and source fiber quality

Stage 4: Lightweight Cleaning

Reverse cleaners and through-flow cleaners operating at 0.8–1.2% consistency are standard after screening. They remove lightweight contaminants — polystyrene, wax particles, hot-melt adhesives — that pass through screens. Cleaner diameter of 75–150 mm with pressure drop of 0.08–0.12 MPa provides optimal separation efficiency for particles with density less than 1.0 g/cm³.

Stage 5: Fractionation (Optional but Recommended)

For mills producing both top and bottom liner plies, fiber fractionation using multi-stage pressure screens (0.10–0.15 mm slots) separates long fiber (>1.5 mm) for the top ply from short fiber (<1.0 mm) for the filler/bottom ply. This approach can improve top ply burst strength by 15–25% while reducing overall refining energy by 10–15% since only the long fiber fraction requires intensive refining.

Complete Line Performance Benchmarks

A well-designed OCC-to-kraft-liner pulping line should achieve:

• Stock preparation total energy: 220–320 kWh/t (including pulping, screening, cleaning, refining, thickening)
• Fiber loss: <3% of incoming OCC weight
• Stickies removal efficiency: >95% (macro stickies >100 μm)
• Final stock freeness: 380–480 mL CSF (dependent on target grade)

Whether upgrading an existing OCC line or planning new capacity, equipment selection at each stage directly determines your final linerboard quality. Our team can help evaluate your fiber furnish characteristics and recommend the right screen basket configurations, refiner plate patterns, and process flow layout.

📧 Contact us at leizhanzhang@gmail.com for OCC pulping line equipment specifications.