Why Recycled Fiber Processing Demands Special Attention for Coating Board

Coating board mills increasingly rely on recycled fiber—old corrugated containers (OCC), mixed office waste, and sorted white grades—to control raw material costs. However, recycled fiber brings challenges that virgin fiber does not: residual inks, toner particles, stickies, hot melts, and the degraded fiber quality from previous drying cycles.

For coating board, the bar is especially high. Toner specks visible on the uncoated sheet will telegraph through the coating layer. Stickies that escape the screening system deposit on forming fabrics and press felts, creating sheet defects. And hornified fibers—fibers that have lost swelling capacity from repeated drying—produce a weaker sheet with lower bulk, directly hurting the stiffness that coating board converters depend on.

Pulping: The First Process Decision

The pulper sets the tone for everything downstream. For OCC-based coating board production, a drum pulper or continuous high-consistency pulper (operating at 15–18% consistency) offers advantages over batch low-consistency pulping:

• Lower specific energy: 30–40 kWh/ton for drum pulping vs. 55–70 kWh/ton for batch pulping at the same defibering level
• Better contaminant integrity: Gentle tumbling action keeps plastic films and wet-strength materials intact so downstream screens can remove them efficiently; high-shear batch pulpers break them into harder-to-remove fragments
• Uniform stock consistency: Continuous discharge at controlled consistency simplifies downstream dilution control

Leizhan supplies drum pulpers with capacities from 100 to 800 TPD, featuring wear-resistant screen plates with 8–14 mm perforations and a two-zone design: a high-consistency defibering zone followed by a dilution/screening zone.

Dispersion: The Key to Coating Board Quality

Hot dispersion is arguably the most important single unit operation for recycled-fiber coating board. A properly operated disperser:

1. Reduces visible stickies below the threshold where they cause coating defects
2. Breaks down toner and ink particles so they’re no longer visible
3. Reactivates hornified fibers, partially restoring swelling and bonding potential

Leizhan’s disc disperser (Model ZDR series) operates on these parameters:

• Feed consistency: 28–35% (achieved via a screw press or twin-wire press upstream)
• Operating temperature: 85–105°C (steam-heated housing)
• Disc gap: 0.2–0.8 mm, depending on stock type and treatment goals
• Specific energy: 50–90 kWh/ton, with higher energy applied for stickies-intensive grades
• Throughput: 50–400 TPD per unit, with multiple units in parallel for larger mills

The critical metric: after dispersion, visible stickies (TAPPI T 277 method) should measure below 500 mm²/kg. Below this level, stickies no longer cause roll picking or coating defects. Achieving this requires both the right equipment and consistent operational discipline.

Deinking for White-Top and Coated Grades

When producing white-top liner or coated board with a bright surface layer, deinking of the top-ply stock is essential. A flotation deinking system for coating board typically includes:

• Primary flotation cells: 4–6 cells in series, 10–15 minutes retention, removing 85–92% of ink particles
• Secondary flotation: 2–3 cells treating primary accepts for brightness polishing
• Post-flotation washing: A disc thickener or twin-wire press to remove dispersed ink and ash, boosting brightness by an additional 2–4 ISO points

Leizhan’s flotation cells use a venturi aeration design that generates fine air bubbles (0.3–0.8 mm diameter) without the maintenance burden of compressed-air injection or porous diffusers that plug over time. Brightness gain of 8–12 ISO points is achievable on mixed office waste, depending on incoming furnish quality and chemical dosing (typically 0.3–0.6% sodium hydroxide, 1.0–2.0% sodium silicate, 0.3–0.8% hydrogen peroxide, and 0.1–0.3% collector soap—all on oven-dry fiber basis).

Practical Operation Tips

• Disperser plate life: Fine-tooth disperser plates (1.5–2.5 mm tooth width) last 1,200–1,800 operating hours before edge rounding reduces treatment effectiveness. Monitor motor load at constant gap—a gradual drop in power draw at constant gap and throughput indicates plate wear.
• Pulper detrashing: Empty the pulper junk trap every 2 hours during continuous operation. Accumulated heavy contaminants reduce effective pulping volume and increase wear on the rotor.
• Flotation consistency: Maintain 0.8–1.2% consistency in flotation cells. Above 1.3%, bubble-ink contact probability drops; below 0.7%, throughput suffers without proportional improvement in ink removal.
• Dispersion temperature check: Use an infrared thermometer on the disperser housing at the discharge flange weekly. An unexpected temperature drop signals steam system issues or reduced shear from worn plates.

To discuss recycled fiber processing for your coating board line, contact Leizhan’s process engineering team for an equipment recommendation tailored to your furnish and quality targets.

Contact Leizhan for Your Paper Machinery Needs

Zhengzhou Leizhan Technology Paper Machinery Co., Ltd. supplies complete stock preparation systems and paper machine equipment for tissue, kraft liner, coating board, and corrugated medium production worldwide. For technical consultation or equipment inquiries, reach our team at:

📧 leizhanzhang@gmail.com