Kraft brownstock leaves the washer with troublemakers—shives, splinters, sand, and even metal—that can scar downstream equipment and quality. Mills lean on two workhorses, pressure screens and centrifugal cleaners, and tune slot sizes and flows to hit a narrow window: high removal, low fiber loss.
In brownstock washing, pulp exiting the washers still carries coarse contaminants—shives (bundles of incompletely cooked fibers), wood fragments, sand, metal—that must be removed to protect downstream equipment and secure pulp quality (www.chinapulppaper.com). Two principal methods are used: pressure screens (mechanical separation by size) and centrifugal cleaners (hydrocyclones that exploit density differences). Pressure screens force pulp through slot-or-holed baskets under pressure, with a rotor to fluidize and pulse the stock (meston.wordpress.com; meston.wordpress.com). Centrifugal cleaners inject the stock tangentially into a conical chamber to generate a swirling vortex: heavy contaminants are thrown to the wall and discharged while lighter fiber follows the inner vortex to the accept outlet (www.researchgate.net; fiberprocessing.kadant.com).
These units are often staged: coarse screens or cleaners first, followed by finer cleaning. Design parameters—basket geometry, rotor profile, feed orientation—and operating conditions—consistency (percent solids), flow, rotor speed, reject rate—must be balanced to maximize debris removal while minimizing fiber lost to rejects (meston.wordpress.com; meston.wordpress.com).
Pressure screen design parameters
Pressure screens typically run at medium consistency (often 0.5–2% solids). The screen basket carries slots or holes (often contoured) and a foil rotor inside, which produces alternating “push‑pull” pulses across the basket to loosen fibers and bulk the mat (meston.wordpress.com; meston.wordpress.com).
Screen design is a trade‑off: larger open area (larger holes or contoured wires) increases throughput but reduces screening efficiency (meston.wordpress.com). Wedge‑wire (contoured) slotted plates can double the open area of machined slots, vastly increasing capacity but letting finer debris through (meston.wordpress.com). Slotted baskets give better shive (large debris) rejection than drilled‑hole screens, though hole screens run faster due to higher open area (meston.wordpress.com). Wider foils and higher rotor speed increase turbulence and capacity but tend to entrain more fibers into rejects (meston.wordpress.com; meston.wordpress.com).
Slot size, velocity, and consistency
The slot (or hole) size and slot‑velocity must match the furnish (the fiber mix). Smaller slots capture finer debris but risk fiber trapping and plugging; modern fine screens run 0.1–0.2 mm slots. Coarse screens after brownstock washers use relatively large apertures (up to several millimeters), with secondary fine screens using ~0.15–0.20 mm to polish the pulp (www.researchgate.net).
There is an optimal slot/flow range: Olson et al. found long‑fiber fractionation peaked at slot fluid velocities of ~0.5–1.0 m/s for slotted plates (www.researchgate.net). Empirical guidelines aim for slot velocities in that 0.5–1 m/s range for fine screening (www.researchgate.net). Screen feed consistency is usually drawn down into 0.3–0.8%: a typical rule‑of‑thumb is to dilute brownstock to 0.4–0.5% for screening; above about 0.85% solids the reject rate rises sharply (below ~0.35% capacity suffers) (www.chinapulppaper.com).
Even at optimal flow, some fiber is lost to rejects. Multi‑stage screening and controlled reject thickening mitigate this. One study reported long‑fiber yield was more sensitive to screen contouring than slot width—slot width had “no effect” on long‑fiber yield in that test (meston.wordpress.com). If apertures are too small or too close together, long fibers can “staple” and reduce capacity (meston.wordpress.com). Operators tune each screen by adjusting feed flow (and dilution water), rotor speed, and the reject valve so slot velocities stay in the sweet spot for that furnish (meston.wordpress.com; www.researchgate.net).
Centrifugal cleaner fundamentals
Centrifugal cleaners (“hydrocyclones”) typically operate at low consistency (~0.5–1% solids) and high velocity, often with dilution water to achieve the required consistency (www.researchgate.net; www.chinapulppaper.com). In a forward cleaner, denser particles—grit, glass, metal—migrate outward and discharge via the underflow; fiber follows the inner vortex to the accept outlet (fiberprocessing.kadant.com; www.researchgate.net).
The cleaner’s cut size is set by geometry and flow, not an aperture. Because even fibers are denser than water, fines and hairy fiber can appear in rejects, so systems use reject cones, smooth walls, and pressurized discharge to thicken rejects and wash fibers back out (www.pulpandpapercanada.com). Multi‑stage cleaner systems can reject ~92–93% of 100 µm sand at 0.6–0.9% consistencies (www.researchgate.net). Heavy contaminants in brownstock are routinely removed at >90% efficiency by well‑graded cycloning. Because fibers also appear in the rejects, mills include fiber recovery steps or additional downstream screens to return lost fiber (www.researchgate.net; www.pulpandpapercanada.com). Newer cleaner designs with vortex‑forming baffles or stepped cones reduce fiber carry‑out; modern pressurized cleaners with special internal profiles cut the fiber volume in rejects (thickening factor) compared to older smooth cones, improving fiber retention, especially on bleached kraft furnishes (www.pulpandpapercanada.com; www.pulpandpapercanada.com).
Complementary performance roles
Screens (especially fine slotted baskets) excel at removing fibrous impurities and bundles (shives, knots, splinters) and at fractionating fiber lengths. Cyclone cleaners excel at inorganic/grit contaminants that may pass through screen slots. In practice, brownstock lines use both: coarse knotting or coarse screens first, fine pressure screens next, and cleaner banks to polish out fine grit.
Comparative metrics underscore the trade‑offs: pressure screens will reject on the order of 10–30% of fiber mass at each stage, depending on settings (www.pulpandpapercanada.com). A typical first‑stage cleaner might be set to reject ~20–30 vol% at a feed consistency of ~0.5%, capturing most heavy grit. Screens might run at 0.8% with slot velocities ~0.6 m/s to maximize shive capture while holding fiber loss to a few percent. The overall goal is >90% removal of critical contaminants (shives, sand, bark) with minimal pulp yield loss.
Flow tuning and slot selection
Smaller slots and lower flow favor removal but raise fiber carry‑to‑reject and plugging risk; larger slots and higher flow favor capacity and yield but let more debris slip. For slotted plates, fiber separation efficiency peaks at intermediate slot velocity (~0.5–1.0 m/s), underperforming below ~0.5 m/s (insufficient fluidization) and washing fibers through above ~1.0 m/s (www.researchgate.net). Mills evaluate screens empirically: for example, testing 0.15 mm versus 0.10 mm slots at varied rotor speeds to measure reject freeness and fiber yield.
Increasing slot width or adding contour can roughly double open area (and capacity) (meston.wordpress.com), but typically halves screening efficiency (debris in accept) (meston.wordpress.com). Raising slot velocity increases throughput and accept consistency but tends to decrease removal efficiency (meston.wordpress.com; meston.wordpress.com). Operators often set pump speeds or restrictors to maintain a modest slot flow—in tens of liters per minute per square meter—tuned to each screen’s slot width.
Accepted practice shows that, all else equal, a very fine slot (e.g., 0.10 mm) can remove perhaps 95% of 200–300 µm shives, whereas a coarser slot (0.20 mm) might only remove ~75%. For a fiber length of 30–40 mm, the percentage of fiber caught by slotting is small—performance curves often show fiber retention losses of only 2–5% for well‑tuned fine screening, while poor tuning can double the loss. In parallel with pressurized baskets, continuous debris removal in screening applications is handled by automatic screen systems (automatic screen), while coarse interception above 1 mm can use manual configurations (manual screen).
Instrumentation and operational window
Modern mills instrument screens with pressure and flow sensors; deviations in differential pressure or accept consistency trigger adjustments. Screen plugging correlates strongly with the ratio of fiber length to aperture; if this ratio exceeds ~1.5, normal operation persists, but if fibers are very long relative to slot, plugging can occur unless flow/rotor are increased (bioresources.cnr.ncsu.edu). Adjusting slot velocity and rotor speed is crucial as furnish changes (e.g., switching hardwood vs. softwood pulps).
Practical setpoints and staged gains
A practical configuration: a first‑stage screen at 0.8% consistency, 0.6 m/s slot velocity, and 20% reject, yielding ~15% debris removal with ~3% fiber in reject; then a second‑stage cyclone bank at 0.5% consistency and 20% reject, catching >90% of remaining grit with <1% incremental fiber loss. In one documented case, tuning increased overall debris removal by ~10% while reducing fiber loss by ~2% by widening foils on a secondary screen and slightly raising feed dilution (www.pulpandpapercanada.com; meston.wordpress.com).
The objective holds constant across designs: maximize debris removed per ton of pulp while keeping the reject‑stream fiber fraction as low as practical. Pressure screens and centrifugal cleaners are essential partners in modern brownstock washing; choosing the right slot size and carefully adjusting flows/rotor speeds enables mills to remove well over 90% of unwanted debris while holding fiber losses to a few percent. Fine slots and moderate feed velocities help screens capture unwanted material; high‑g vortex action in cyclones sweeps out dense impurities. Continuous monitoring of accepts/rejects keeps each stage at its sweet spot (meston.wordpress.com; www.researchgate.net).
Sources and technical references
Authoritative studies and industry reports inform these conclusions. Design principles and parameter effects are summarized in Pulp and Paper technical literature (meston.wordpress.com; meston.wordpress.com). Performance data, including ~92–93% removal of 100 µm sand at 0.6–0.9% consistencies, come from peer‑reviewed experiments (www.researchgate.net; www.researchgate.net) and detailed industry guidelines (meston.wordpress.com; www.chinapulppaper.com).
