Kraft linerboard lives and dies by its strength properties — ring crush, STFI, burst, and tensile. The paradox that every linerboard production manager faces is that refining improves strength up to a point, then tears it back down. Over-refining shortens fiber length, reduces tear resistance, and drives up energy costs with diminishing returns. The art is knowing exactly where to stop.

Why High-Consistency Refining Matters

Low-consistency refining (3–5%) primarily cuts fibers. High-consistency refining (25–35%) fibrillates them — opening up the fiber wall structure to expose more bonding sites without drastically shortening fiber length. For kraft liner grades where tear and ring crush are both critical, this distinction is everything. A disc refiner running at 30% consistency can deliver a 15–20% increase in burst strength with less than a 5% drop in tear, a trade-off that low-consistency refining cannot match.

Leizhan’s ZDP Series Double Disc Refiner

The ZDP double disc refiner is engineered specifically for this task. It operates in the 25–35% consistency range with plate diameters from 450 mm to 600 mm, depending on throughput requirements. The counter-rotating disc design doubles the effective refining zone compared to a single-disc machine of the same diameter, meaning more fiber treatment per pass and fewer recirculation loops.

Power consumption for the ZDP series typically runs 18–22 kWh per ton at typical refining intensities for OCC-based kraft liner. That’s a meaningful number in an operation where refining can account for 30–40% of total mill electricity consumption. Precision plate gap control — maintained within ±0.02 mm — ensures energy isn’t wasted on no-load horsepower or uneven treatment.

Plate Selection and Wear Patterns

Refiner plate geometry — bar width, bar angle, groove depth — should be matched to the furnish. For long-fiber virgin kraft, wider bars with shallow grooves provide more fibrillation action. For OCC-based furnishes with higher fines content, narrower bars with deeper grooves help prevent plate plugging. Plate life varies with contamination levels, but operators should inspect every 2,000–2,500 operating hours and replace when bar edge rounding exceeds 1.5 mm.

Process Control: What to Monitor

Track freeness (CSF) before and after each refining stage. For standard kraft liner, a freeness drop of 80–120 mL CSF per pass is a good working target. Anything steeper suggests over-refining, while a drop below 50 mL CSF indicates plates need gapping tighter or replacing. Fiber length analysis (using a Kajaani or similar analyzer) should be run weekly — the weighted average fiber length should stay above 1.2 mm for acceptable tear strength in linerboard.

Energy Optimization Strategy

Install a kWh/ton meter on the refiner motor circuit and trend it against production rate. When specific energy climbs above baseline for the same freeness target, it’s almost always plate wear or changes in incoming fiber quality. Address the root cause rather than just tightening the plates — you’ll save both fiber and electricity.

Our engineers can help you specify the right refiner configuration, plate pattern, and operating window for your specific kraft liner furnish mix.