The Role of Contaminant Removal in Kraft Liner Quality

Kraft linerboard demands high burst strength (typically 3.5–5.5 kPa·m²/g), ring crush (1.5–2.5 kN/m), and resistance to tearing. These mechanical properties depend on fiber length preservation and uniform sheet formation—both of which are undermined when contaminants pass through stock preparation into the paper machine.

Shives, uncooked fiber bundles, sand, and stickies from recycled content all act as stress concentrators in the finished sheet. A shive content above 0.5% (by Somerville screen test) correlates with a measurable drop in burst index. The screening and cleaning system is the gatekeeper that determines whether the stock reaching the headbox supports the linerboard’s strength specifications.

System Architecture: Pressure Screens First, Cleaners Second

A well-designed screening system for kraft liner typically follows this sequence:

1. Coarse screening (holes): 2.0–3.0 mm hole basket—removes large debris, protects downstream equipment
2. Fine screening (slots): 0.20–0.35 mm slot basket—removes shives and fiber bundles
3. Forward cleaning (hydrocyclones): Removes high-density contaminants (sand, metal, scale)
4. Reverse or through-flow cleaning: Removes low-density contaminants (stickies, hot melts, wax)

Leizhan’s pressure screen series covers the full range. The coarse screen (Model ZNS series) handles incoming consistency up to 4.5% with a hole basket. The fine screen (Model ZNW series) operates at 1.5–3.5% consistency with wedge-wire slot baskets. Both feature automatic purging and a rotor designed to minimize pulsation—excessive pressure pulsation creates flow variations that reduce slot velocity uniformity and screening efficiency.

Slot Velocity: The Critical Parameter

Screening is not just about opening size—slot velocity determines whether fiber flocs pass or are rejected. For kraft liner stock at 2.5% consistency, the target slot velocity is 1.8–2.5 m/s. Below 1.5 m/s, fiber matting on the basket surface increases reject rate unnecessarily. Above 3.0 m/s, shive passage increases because fiber bundles deform and squeeze through the slot.

Leizhan’s rotor foil design produces a gentle positive-negative pressure cycle: a forward pressure pulse pushes accepted fiber through the slot, followed by a negative suction pulse that pulls matted fiber back into the flow stream. This self-cleaning action allows continuous operation with minimal operator intervention.

Hydrocyclone Bank Configuration

For linerboard stock, forward cleaners (centri-cleaners) operate at a pressure drop of 180–250 kPa with a 75 mm cone diameter, handling approximately 350–450 L/min per cone. Key operating parameters:

• Feed consistency: 0.5–1.0%
• Reject rate: 8–12% of feed flow (adjustable by reject orifice)
• Reject consistency: 0.3–0.5%, routed to secondary and tertiary stages
• Accept consistency: 0.4–0.85%

A four-stage cascade arrangement is standard for kraft liner: primary bank accepts go forward; primary rejects feed secondary; secondary accepts return to primary feed; secondary rejects feed tertiary; tertiary accepts return to secondary; tertiary rejects go to quaternary. This cascading design keeps total fiber loss below 0.3% while achieving >90% removal efficiency for particles above 75 μm.

Maintenance Scheduling

• Basket inspection: Remove and inspect pressure screen baskets every 2,000 operating hours. Measure slot widths with a feeler gauge at 8 positions around the circumference. Replace when any slot exceeds nominal width by more than 0.05 mm.
• Hydrocyclone cone wear: The lower cone section experiences the most abrasive wear. Micrometer-measure wall thickness at the bottom 50 mm section every 6 months; replace cones when wall thickness drops below 3 mm.
• Seal replacement: Pressure screen rotor shaft seals should be replaced annually. A leaking seal pressurizes the bearing housing, forcing grease out and contaminant in.
• Reject orifice checks: Cleaner reject tips erode over time. A tip worn just 0.5 mm oversize changes reject rate by 3–5%. Check diameter quarterly with a go/no-go gauge.

For technical consultation on your kraft liner screening and cleaning system, or to discuss Leizhan’s pressure screen and hydrocyclone specifications, contact our engineering team.

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