Pitch—lipophilic wood extractives—can gum up washers and screens, forcing shutdowns and off‑quality paper. The fix is a disciplined mix of wood selection, washing finesse, and low‑dose chemistry.
In pulp production, pitch is the catch‑all for hydrophobic wood extractives—resin acids, fatty acids, sterols, waxes, and triglycerides—that survive cooking and then stick to equipment. These lipophilic compounds often make up 2–5% of wood dry weight (researchgate.net) (bioresources.cnr.ncsu.edu).
Unbound pitch “tends to concentrate in downstream processes and form sticky deposits on equipment, which…result[s] in off‑quality paper” (researchgate.net). The consequences—paper breaks, higher maintenance, and lost production—are well‑documented; one study links untreated wood resin in pulp to higher maintenance costs and defects in finished paper (researchgate.net). Efficient brownstock washing plus targeted additives are therefore essential to prevent costly fouling.
Wood extractives and pulping variables
Extractives—and the pitch they create—vary strongly by wood species and pulping method. Softwoods (pine, spruce) contain abundant resin‑rich extractives, while hardwoods (eucalyptus, acacia) tilt toward more polar phenolics. Typical extractives span 2–5% of wood (bioresources.cnr.ncsu.edu), and resins (rosin acids) occur only in softwoods (bioresources.cnr.ncsu.edu).
Brazilian eucalyptus clones, for example, averaged ~2.7% total extractives—mainly polar steroids plus aromatic and fatty acids (mdpi.com). Hardwoods carry a higher proportion of polar polyphenolic compounds, whereas conifers have more non‑polar extractives due to resins (mdpi.com). Tropical (warm‑climate) species yield more saturated fatty acids in extractives (bioresources.cnr.ncsu.edu); Indonesian eucalypts likely fit this trend.
Mechanical pulping retains most extractives as colloids, while kraft and sulfite cooking remove the bulk. “Mechanical pulping removes accessory compounds…just slightly, but chemical pulping…removes them to a large extent” (researchgate.net). In kraft mills, softwood cooking produces crude tall oil (resin acid soap)—Southern pine mills often recover ~10–20 kg tall oil per tonne pulp—leaving only trace resin in brownstock (bioresources.cnr.ncsu.edu). Hardwood kraft yields negligible rosin oil, but small amounts of fatty and sterol residues may remain.
Pulp furnishes with higher softwood content generate more pitch; mills often quantify aggregate extractives (fatty + resin acids) in the black liquor (researchgate.net). Bleaching and washer stages further leach extractives, making brownstock washing decisive (researchgate.net) (researchgate.net). At high pH (≥12), fatty/resin acids form sodium salts that help keep them in solution; localized pH drops (e.g., acidified washer filtrate) can precipitate them (bioresources.cnr.ncsu.edu).
Netting it out: mills using mostly tropical hardwood chips should expect pitch mainly as saturated fatty acids and sterols—with low resin acid content (bioresources.cnr.ncsu.edu). Cold‑season or softwood chips (if used) will spike the pitch load disproportionately. Pulping variability (e.g., higher active alkali) tweaks lipophilic extraction. Monitoring wood inventory and black liquor soaps helps predict pitch tendency (researchgate.net).
Brownstock washers and screen stations
Brownstock washers—vacuum drum, belt/press, and diffusion washers—sit immediately after the digester. Their job is to displace spent cooking liquor with clean water, removing dissolved chemicals and residual lignin; a collateral benefit is leaching out residual extractives. Good washing “assists in the removal of undesirable material such as… pitch and wood extractives” (researchgate.net).
Modern mills run multi‑stage counter‑current washer trains; poor washing or screening allows pitch to carry over into green liquor or bleaching, where it fouls surfaces. Pitch can deposit on washer wires, screens, and the headbox lip, or accumulate in knotters/screens. Undispersed pitch bodies agglomerate and float; finer dispersions exit with the filtrate. Anti‑foam aids (defoamers) help by reducing entrained air that can trap pitch (researchgate.net).
Screening (coarse screen stations) then removes knots and large pitch lumps. As Santos and Hart note, substances not removed in washing “tend to concentrate in downstream processes and form sticky deposits” causing quality issues (researchgate.net). Mills monitor wash effluent for extractive load and adjust wash parameters or additives if pitch levels rise (researchgate.net).
On the chemical side of foaming control, mills commonly deploy antifoam to collapse bubbles and release any emulsified pitch—so it can be removed in the pulp mat or filtrate—supporting washer performance (researchgate.net).
Chemical dispersants and detackifiers
Chemical dispersants and detackifiers (polymers that reduce stickiness) are dosed into the pulp line to prevent deposition. They adsorb to pitch particles, lowering interfacial tension and adhesiveness, or convert pitch to a more stable colloid (researchgate.net) (buckman.com).
Vendor examples include Buckman’s polymer “Buckman 280,” which “keeps pitch particles from melting and agglomerating,” and its “Busperse” dispersants that “keep pitch dispersed, reduce tackiness, and prevent deposition” (buckman.com). Non‑ionic modified celluloses or polyvinyl alcohol derivatives (hydrophobic cellulose ethers or PVA‑co‑VAc) are known detackifiers that adsorb onto pitch droplets and resist coalescence (freepatentsonline.com). Cationic coagulants (e.g., poly‑DADMAC, cationic PVOH) are sometimes used to pull residual pitch out with the fiber, improving retention (buckman.com).
In practice, these chemicals are added to the brownstock filtrate or pulp stage, often injected just above or below vacuum dryers/screens. Typical lab and pilot dosages range from 0.10 ppm to 100 ppm (parts per million) on dry fiber, with ~15–50 ppm often optimal in kraft pulp (freepatentsonline.com). Mills source dispersant chemicals for this service chemistry and meter them precisely with a dedicated dosing pump to keep addition rates in the single‑ to few‑tens‑of‑ppm window.
Key performance metrics include deposit reduction and sheet cleanliness. One indirect payoff: where a modern WRV polymer system nudged pulp discharge consistency up by ~2% (from ~16% to 18%), the next‑stage caustic demand dropped significantly—roughly a 50% saving—by improving extraction of organics (including pitch) from the pulp mat (pulpandpapercanada.com).
Defoamers serve indirectly in pitch control: residual pitch emulsified in foam can be removed when bubbles collapse, releasing the pitch into the mat or filtrate (researchgate.net). Where retention‑style strategies are favored, mills can also turn to coagulants to bind and carry pitch with fiber.
Observed outcomes and case signals
Data from industry underline the benefits. As noted, a 2% increase in pulp dryness enabled roughly 50% savings in caustic in one brownstock washer upgrade (improved wash retention reduces residual Y₂O requirement) (pulpandpapercanada.com).
In mills where talc was added (e.g., packaging paper), colloidal pitch in white water dropped dramatically even at low dosages (docta.ucm.es). Similarly, polymeric detackifiers have cut felt cleaning frequency by 30–50% on some paper machines.
Environmental constraints and regional practice
In Indonesia, environmental and safety considerations shape chemical choices. Major producers commit to “safe chemicals” and tighter effluent targets (okipulppaper.co.id) (okipulppaper.co.id). One mill’s roadmap mandates COD emissions 30% below regulatory limits (okipulppaper.co.id), and OKI reports cutting overall chemical use by ~14% via process optimization (okipulppaper.co.id).
That puts a premium on highly efficient, low‑dosage pitch dispersants (including multipurpose polymers and enzymes) with minimal effluent impact. Mills routinely pilot‑test candidates—nonionic vs. anionic polymers, enzyme aids—to satisfy both technical and environmental criteria (buckman.com).
Operating practices for pitch risk
Key practices in the brownstock line include: screening aggressively to remove knots (and macroscopic pitch), maintaining high‑wash consistency by optimizing vacuum and water flow, and dosing dispersants/detackifiers upstream of washers so they complex with pitch before it deposits (researchgate.net).
Monitoring wash filtrate for pitch content—by GC or colorimetry—and housekeeping (regular cleaning of washers/screens) further reduce buildup and unplanned downtime (researchgate.net).
Bottom line for mill runnability
Pitch control in brownstock washing is a combination of source control (low‑resin wood, good chip prep), process optimization (effective multi‑stage washing and screening), and chemical treatment (dispersants and detackifiers). Each mill tunes these to its furnish and washer configuration, using data—pitch load measurements and runnability indicators—to refine dosages. The payoff is cleaner brownstock lines, fewer interruptions, higher yields, and better product quality at manageable environmental impact (researchgate.net) (buckman.com).
Sources: Wood extractive contents and behavior summarized in Sjöström and Alén et al. (mdpi.com) (bioresources.cnr.ncsu.edu); process impacts on pitch in Allen et al. and Gutiérrez et al. (researchgate.net) (mdpi.com). Brownstock washing fundamentals in “Brown Stock Washing – Review of the Literature” (researchgate.net). Chemical control methods in industry literature and patents, including Buckman pitch brochures (buckman.com) and Hercules patents (freepatentsonline.com) (freepatentsonline.com). Indonesian chemical and effluent commitments from corporate sustainability reports (okipulppaper.co.id) (okipulppaper.co.id) (okipulppaper.co.id).
