For kraft liner board producers, ring crush test (RCT) and short-span compression test (SCT) values determine everything — from pricing premiums to customer loyalty. Yet many mills still operate screening and refining systems that inadvertently sacrifice fibre strength for throughput. The right combination of screening technology and refining strategy can lift SCT index values by 8-15% on the same furnish.

Screen Basket Selection and Fibre Fractionation

In OCC-based kraft liner production, the screening system must handle a notoriously contaminated furnish. Screen baskets with slot widths of 0.20-0.30 mm offer the best balance between contaminant removal and fibre recovery. Slotted screens consistently outperform hole screens for stickies removal — a critical factor when producing high-quality kraft liner from recycled fibre.

Multi-stage screening with cascaded accepts delivers the best fibre yield. A typical three-stage system should achieve overall fibre recovery rates exceeding 95% while maintaining reject thickening ratios below 2.5:1. When slot velocity exceeds 2.5 m/s, passage ratios for desirable long fibres drop significantly, hurting both yield and strength properties.

Refining Strategy for Maximum Strength Development

Kraft liner strength depends on fibre-to-fibre bonding, which refining develops — but over-refining fibre cutting destroys the very long-fibre fraction that gives liner its compression strength. The target is achieving 28-35°SR for the top ply (which contributes to printability) while keeping the bottom ply at 22-28°SR to preserve the long-fibre fraction for bulk and stiffness.

Disc refiners with optimized plate patterns — typically using wide-bar, shallow-groove designs for the primary stage and finer patterns for secondary refining — produce fewer fines and more fibrillation per unit of specific energy applied. Target specific edge load (SEL) values of 1.5-2.5 Ws/m for primary refining of kraft liner stock, with net specific energy consumption in the range of 60-100 kWh/t depending on the incoming freeness.

Screening-Refining Integration

One of the most common mistakes is placing the refiner before the screen in the process flow. For recycled fibre-based kraft liner, the correct sequence is: screening first, then refining the accepts. This prevents contaminants from being broken down into smaller particles that are harder to remove, and avoids wasting refining energy on reject material that will ultimately be removed.

Practical Equipment Maintenance Guidelines

• Screen Basket Life: Monitor Delta-P across screen baskets daily. A rise of 20% above baseline indicates either blinding or wear requiring inspection. Well-maintained chrome-plated baskets can process over 80,000 tonnes before replacement.
• Refiner Plate Replacement: Track specific energy consumption relative to freeness development. When energy input rises 10-15% for the same freeness drop, plate wear has reached the replacement threshold.
• Consistency Control: Screening efficiency drops sharply when inlet consistency exceeds 1.5%. Verify consistency meters are calibrated weekly.

Optimizing the screening and refining loop is one of the highest-ROI investments a kraft liner mill can make. The combination of proper screen basket selection, correct refining strategy, and the right process sequence delivers measurable improvements in both product quality and operating cost.

For technical guidance on upgrading your kraft liner screening or refining equipment, reach out to our team.

📧 leizhanzhang@gmail.com