The Economics of Fiber Yield in Corrugated Mills

Corrugated paper production — testliner and fluting medium — relies overwhelmingly on recycled fiber. Old corrugated containers (OCC) typically make up 80–100% of the furnish. At current recovered fiber prices, every 1% of fiber lost to the reject system represents significant raw material cost per ton of production. For a 500 tpd mill, that adds up to substantial daily losses.

But maximizing fiber yield isn’t simply about accepting everything that enters the mill. Low-quality fiber increases energy consumption in refining, reduces machine runnability, and damages final product quality. The challenge is achieving high yield while removing only what must be removed.

1. OCC Pulping: The First Decision Point

The pulper is where fiber recovery begins — and where significant losses can occur if poorly designed or operated.

• Consistency matters: Batch pulpers at 12–16% consistency provide mechanical action (fiber-to-fiber rubbing) that defibers OCC with minimal fiber damage. Continuous pulpers at 4–6% consistency are gentler but require longer retention time (20–30 minutes vs. 12–18 minutes for batch).
• Pulper extraction hole size: Extraction plates with 8–12 mm holes remove defibered stock. The hole size is a trade-off: larger holes (14–16 mm) increase throughput but pass more contaminants downstream; smaller holes (6–8 mm) protect downstream equipment but reduce capacity and can trap good fiber.
• Ragger and junk removal: A well-designed pulper ragger continuously removes wire, plastic strapping, and wet-strength materials. The ragger should remove 2–4% of the incoming OCC weight — if it’s removing more, pulping time may be insufficient; if less, contaminants are passing through.

2. High-Density Cleaning: Protect Downstream Equipment

After the pulper, high-density cleaners (HD cleaners) at 3–5% consistency remove heavy contaminants — stones, metal, glass, sand — that would damage screening equipment. Key parameters:

• Pressure drop: 150–200 kPa across the cleaner for effective separation.
• Reject rate: 0.5–1.5% of feed flow. The reject stream should be visibly enriched in heavy material.
• Fiber loss in HD cleaners: Minimal — typically below 0.1% of total fiber, since the separation mechanism is density-based rather than size-based.

3. Screening: Where Yield vs. Cleanliness Battles Play Out

Screening is the single largest source of fiber loss in an OCC system. The right screen configuration — hole size, slot width, reject rate, and cascade design — determines yield.

• Coarse screening (holes): Primary screens with 2.0–3.0 mm holes remove large contaminants (plastic, wet-strength paper, shives). Reject rate: 12–20%. A secondary screen with 2.0–2.5 mm holes recovers over 60% of the fiber in the primary reject. A tertiary screen (1.8–2.2 mm) recovers fiber from the secondary reject.
• Fine screening (slots): Slot screens with 0.20–0.35 mm slots remove stickies, hot melts, and small debris. This is where the biggest yield trade-offs occur — tighter slots remove more contaminants but also more fiber. A three-stage cascade is standard: primary at 0.25–0.35 mm, secondary at 0.20–0.25 mm, tertiary at 0.15–0.20 mm.
• Reject thickening: Rather than sending tertiary screen rejects directly to sludge, a reject thickener (sidehill screen or screw press) recovers additional fiber from the reject stream. A 15–20% consistency reject cake exits the system with minimal fiber content.

4. Fractionation: The Modern Approach to Yield Optimization

Instead of cascading all rejects through progressively tighter screens, fractionation uses a primary slotted screen (0.15–0.25 mm) to split the stock into two streams:

• Long-fiber fraction (accepts): 60–75% of flow, containing longer fibers that provide strength. Goes to refining and ultimately to the testliner top layer.
• Short-fiber fraction (reject/through): 25–40% of flow, containing fines, short fibers, and fine contaminants. After separate treatment (washing or fine screening), used in the back ply or fluting medium where strength requirements are lower.

This approach typically increases overall fiber yield by 2–4% compared to conventional screening because fines that would otherwise be lost in tertiary rejects are recovered and used.

5. Refining Recovered Fiber

OCC fiber has been dried, used, and rewetted multiple times — it responds differently to refining than virgin fiber:

• Lower energy input: OCC fiber reaches its strength maximum at 60–90 kWh/t refining energy, compared to 100–150 kWh/t for virgin softwood kraft. Over-refining OCC produces excess fines without strength gain — the fibers have already lost much of their swelling capacity.
• Plate selection: Wide-bar plates (2.5–4.0 mm bar width) with open grooves (5.0–6.5 mm) prevent fines accumulation between bars, which causes plugging and uneven refining on recycled stocks.
• Consistency: 3.0–4.0% — slightly lower than for virgin pulp because recycled fiber suspensions have higher viscosity at the same consistency due to fines content.

6. Yield Benchmarks for OCC Mills

A 500 tpd mill reducing fiber loss from 7% to 4% saves approximately 15 tonnes of fiber per day — that’s roughly 5,500 tonnes per year. At typical OCC costs, this represents significant annual savings in the range of hundreds of thousands of dollars.

Talk to a Recycling Fiber Specialist

OCC processing equipment — from pulper design to screen cascade configuration — directly determines your mill’s fiber yield and operating economics. For technical consultation on stock preparation systems for corrugated paper production, contact our team.

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