What Makes Kraft Liner Production Different

Kraft linerboard demands high strength properties — ring crush, burst, and short-span compression — that directly depend on fiber quality and the production process. Unlike tissue or fine paper, linerboard production at 100–400 gsm requires equipment capable of handling long fibers, high consistency, and heavy sheet weights while maintaining formation quality and strength development.

1. Pulping System Design

The pulping stage sets the foundation for everything downstream. For kraft liner mills using virgin fiber (softwood pulp), key considerations include:

• Pulper capacity and consistency: High-consistency pulpers operating at 12–18% consistency provide better fiber separation with lower specific energy (15–25 kWh/t vs. 30–45 kWh/t for low-consistency pulping). The rotor design — helical or Vokes-type — must handle the high viscosity at these consistencies.
• Deflaking after pulping: Even at high consistency, 2–5% of fiber bundles remain. A deflaker with 0.2–0.5 mm gap setting downstream of the pulper ensures complete fiberization without cutting fibers.
• Dump chest sizing: Minimum 20–30 minutes retention time at design throughput to buffer consistency variations. Agitation power of 0.10–0.15 kW/m³ is sufficient for pulp storage.

2. Screening & Cleaning for Strength Retention

Long softwood fibers are valuable — over-screening loses strength potential. The right screening configuration balances cleanliness with fiber recovery.

• Hole screening first: Primary screens with 2.0–3.0 mm holes running at 1.5–2.5% feed consistency remove shives and oversized contaminants. Reject rates of 15–20% go to secondary screening.
• Secondary & tertiary screening: Cascade screening with progressively smaller hole sizes (1.6 mm → 1.2 mm) recovers good fiber. The tertiary stage reject rate — typically 1–3% of total feed — goes to the sludge system.
• Slot screening for final cleaning: After hole screening, slot screens (0.25–0.40 mm slots) remove small debris that holes miss. Note: slots reject more long fiber, so positioning matters — place them after the hole screen cascade, not before.
• Forward cleaners: Hydrocyclone cleaners at 0.8–1.2% consistency remove high-density contaminants (sand, metal). A 3–4 stage cascade recovers fiber while concentrating rejects to less than 0.5% of feed flow.

3. Refining Strategy: Develop Strength, Don’t Kill Drainage

Refining is the most critical energy investment in kraft liner production. The goal: maximize fibrillation while minimizing fiber cutting.

• Disc refiners with wide plates: For softwood kraft, use plates with 2.5–3.5 mm bar width and 4.0–5.5 mm groove width. Narrow-bar plates designed for hardwood simply cut softwood fibers rather than fibrillate them.
• Specific edge load (SEL): Target SEL of 1.5–2.5 W·s/m for softwood. Lower SEL gives better fibrillation; higher SEL increases cutting. The net specific energy input typically ranges 80–150 kWh/t for liner grades.
• No-load power management: A refiner running at 60–80% of motor capacity wastes energy in no-load losses. Size refiners so the operating point uses 75–90% of installed power for best efficiency.
• Refining consistency: 3.5–4.5% consistency produces the best fibrillation-to-cutting ratio. Above 5%, fiber mat thickness between plates reduces refining uniformity.

4. Approach Flow & Headbox

Kraft liner requires higher headbox consistency (0.6–1.2%) than finer grades. This demands careful approach flow design:

• Machine screen: The last screen before the headbox — typically with 1.4–2.0 mm holes — must run at low pressure drop (20–30 kPa) and generate minimum pulsation. A damper-style machine screen with controlled rotor speed provides the cleanest accept flow.
• Headbox design: For multi-ply liner, multi-layer headboxes with adjustable slice openings (8–15 mm) per layer allow precise basis weight profiling. Turbulence generator tubes with 14–18 mm diameter provide sufficient micro-turbulence for good formation at heavy sheet weights.

5. Press & Drying Section

• Press configuration: A double-felted first press followed by a single-felted second press with 90–120 kN/m line load achieves 42–47% dryness entering the dryer section. Shoe presses with extended nips (200–250 mm) can push post-press dryness to 48–52% on liner grades.
• Dryer section sizing: Linerboard typically requires 1.3–1.5 m² of drying surface per kg/h of evaporation at 3–5 bar steam pressure. Multi-cylinder configurations with 1,500–1,800 mm diameter dryers are standard.

Energy Benchmark: Kraft Liner

Get Expert Guidance on Kraft Liner Equipment

Every mill’s fiber furnish and production targets are unique. Equipment selection — from pulper rotor geometry to refiner plate specifications — should be matched to your specific conditions. For technical consultation on kraft liner production equipment, contact our team.

📧 leizhanzhang@gmail.com