In pulp mills, chip screening determines whether contaminants go to the bin or into the digester—and whether valuable wood becomes pulp or gets burned. Data from mills and vendors show that tuning screen size and flow can cut oversize to under 1%, hold wood loss near 1–3%, and even add roughly two percentage points of pulp yield.
In integrated pulp mills, woodyard chip screening is the small, relentless gatekeeper. The job sounds simple—remove rocks, metal, bark, and fines—but the stakes are big: every extra millimeter in aperture size and every tweak to feed rate shifts removal efficiency and chip loss. Case studies and plant data report oversize fractions reduced to below 1% when screening is designed and operated correctly (ResearchGate), while typical wood loss in chip screens sits around 1–3% of input volume—about 0.051–0.170 m³_sub/ADt (dry basis) or roughly 20–68 kg of dry wood per bone-dry ton rejected as screenings (ResearchGate). ADt (air‑dry tonnes) and o.d. wood (oven‑dry) are standard mill bases; m³_sub/ADt is an industry volume unit per mass on a dry basis.
Screening trains and equipment choices
Most woodyards run multiple stages: coarse scalping, thickness/fines screening, and sometimes air-based classification. Up front, disc (scalper) and grizzly screens remove gross overs—lumps, rocks, frozen chunks—by letting acceptable chips ride forward while oversized pieces discharge off the end; grizzly bars (inclined steel bars) also let fines fall through the gaps (AdvancedBiomass). The basic mechanics mirror what an automatic screen does in continuous debris removal, even though woodyard duty and apertures are chip-specific.
Gyratory and vibratory screens, using stacked decks, perform multi-stage size classification. A common three-deck setup runs ~45 mm holes on top, 13–16 mm mid-deck, and 3–5 mm on the bottom; oversize (“gross overs”) on the top deck goes to re-chipping, mid- and bottom decks separate accept-grade chips from fines (AdvancedBiomass). These screens “scalp” coarse chips, remove dirt‑laden fines, and split short “pin” chips (pins are short slivers).
For sticky, icy, or frozen chip flows, Flip‑Flow/Foil screens use flexible polyurethane panels on counter‑rotating frames to generate high g‑forces that gently but vigorously shake apart clumps and clean fines without blinding; they are noted for high performance in cold‑climate chip streams (AdvancedBiomass). Trommel (drum) screens—heavy‑duty rotating cylinders—are virtually clog‑proof and handle very dirty, high‑volume flows; chips ride the drum, small particles fall through, and larger chips discharge, though long slivers can jam and the units are bulky (often reserved for massive debris duty like logyard clean‑up) (AdvancedBiomass).
At the high end, air‑impulse (jet) screens use strong air blasts to fractionate chips by size, thickness, and density; the Andritz JetScreen claims “very high screening efficiency at high capacity levels” and “very clean chips without dust, sand, stones or scrap” (Andritz). In practice, mills stage these assets: disc or grizzly scalpers first, gyratory/vibratory units to remove fines and thin chips next, and a thickness screen or air screen last to catch over‑thick chips. Standard practices add magnets for ferrous tramp metal, coarse screens to knock out lumps and bark, and fines screens—slots ~3–5 mm—to keep pin chips low and to “screen‑out fine particles that do not make good pulp” (AdvancedBiomass). When the system is sized correctly, case studies report oversize fractions reduced to below 1% of the screened flow (ResearchGate).
Aperture size versus wood loss
Optimizing screen openings is a straight trade‑off: smaller slots (e.g., 3–5 mm) capture more fines and thin chips, but increase wood loss; larger openings reduce loss but let more undersized material (and contaminants) slip through. Industry norms target very low contaminant carryover—e.g., stony debris reduced to <1% of chip flow (ResearchGate)—and multi‑deck arrangements with 45 mm, 13–16 mm, and 3–5 mm apertures route “gross overs” to re‑chipping, accept‑grade chips to the digester, and under‑5 mm fines to rejects (AdvancedBiomass).
Empirical data put typical screening loss at 1–3% of input volume: 0.051–0.170 m³_sub/ADt (dry basis) or about 20–68 kg of dry wood per bone‑dry ton—mostly fine chips that never reach the digester (ResearchGate). Mills manage the balance by adjusting weir heights, feeder speeds, or trommel tilt to “open” or “close” the chip bed without swapping screens. In related pre‑treatment contexts, manual screen configurations similarly live or die by slot choice and bed depth control.
Chip quality expectations are tight. In Kraft digesters (alkaline chemical pulping vessels), undersized chips (pins and fines) tend to overcook and weaken fiber, while oversized ones disrupt digesting uniformity and reduce yield. Plants therefore aim to keep over‑thick chips and fines each to a few percent or less in digester feed. Industry reviews document that advanced thickness screening has delivered pulp yield boosts of about two percentage points by cutting rejects; one North American summary reported ~2% yield gain and a 50% drop in chip rejects after installing thickness screens (AdvancedBiomass).
Throughput, bed depth, and flow-rate effects
Throughput matters as much as aperture size. Higher chip loading per square meter can overwhelm the screen face; if the bed is too deep or fast, not all particles contact the surface and fines ride over, cutting removal efficiency. Running below capacity cleans up the cut but underutilizes the asset. Mills modulate feed and, where relevant, moisture (if a wet screen) to keep bed depth steady. On disk screens, operators throttle drives so a predictable fraction goes “through” versus “over”; faster rotation (or lower feed) tends to increase the fraction passing over (accepts) relative to through (rejects), while slowing the screen lets more material fall through, raising fines rejection (US5503712A).
One industry flow‑management approach puts a disk flow screen ahead of a fine thickness screen to shoulder bulk flow and grit; changing disk speed or upstream conveyor rate tunes chip distribution without altering apertures, effectively balancing mass flow through vs. over each screen to meet targets (for example, 90% of >8 mm‑thick chips removed) (US5503712A). While proprietary curves dominate vendor literature, one simulation indicated that vertical loads beyond about 70% of screen capacity began to drop oversize‑capture by several percent. In practice, mills aim to run each screen near but below rated capacity so the specified size cut and contaminant removal are achieved.
Operational checks align with this: fines removal efficiency rises as effective open area goes down—e.g., shrinking a slot from 10 mm to 8 mm can raise capture of >8 mm chips from roughly 90% to about 98%, with a corresponding drop in the accept fraction; similarly, increasing disk screen speed to throttle accepts can lift oversize removal but requires more re‑blending (US5503712A).
Removal outcomes and the cost balance
A well‑designed line removes nearly all stones, metal, and bark and cuts fatty, resin‑laden fines that absorb pulping chemicals. One case study reduced oversize particles to under 1% of feed and “substantially” cut fines (ResearchGate). Screening losses of good wood generally stay in the 1–3% band (ResearchGate). For a mill processing 1000 ADt/day, a 2% loss is ~20 tons of o.d. wood per day—often burned in the boiler. By contrast, oversized apertures or excessive flow can double those losses or let fines enter the digester, forcing chemical overcharge or burdening downstream cleaners.
Advanced air‑based screens maintain high efficiency at scale (Andritz). Finnish pulp research quantified screening wood loss at about 0.05–0.17 m^3_sub/ADt, matching the 1–3% benchmark (ResearchGate). These stats help engineers set targets: for example, if a mill consumes 2000 tons of chips per day, limiting screens losses to <25 tons/day could save >5% in wood consumption (worth significant fuel and chemical savings). The throughline: multi‑stage screening with correctly sized apertures and well‑managed flow reduces contaminant carryover to under 1% and holds chip loss around 1–2%, with reported yield boosts of ~2% and chip‑reject reductions of ~50% after improving thickness screening (AdvancedBiomass).
Sources and data
Peer‑reviewed studies and industry reports underpin the figures above. Screening trials show wood‑loss ~1–3% (ResearchGate); a mobile screen study cut oversize to <1% with fines also “substantially” reduced (ResearchGate). Industry reviews cite typical yield gains of 2% and reject reductions of 50% after chip thickness screening (AdvancedBiomass), and manufacturer data emphasize very high efficiency at scale (Andritz). Practical guidelines enumerate screening tasks—remove metals, lumps, fines; limit pin chips (AdvancedBiomass)—and patents detail flow‑management tactics and performance trade‑offs (US5503712A).
