In pulp bleaching, every degree, pH unit, and percent consistency moves the needle—sometimes dramatically. Mills are now pairing tight operating windows with online brightness sensors to hit spec, cut chemicals, and protect viscosity.
Bleaching to high ISO brightness—often exceeding 88–90% for printing grades—demands precision. Temperature ramps can double reaction rates, pH tweaks can stall or supercharge delignification, and pulp consistency defines how evenly chemicals actually hit fibers. Today, mills translate those limits into DCS (distributed control system) setpoints and close the loop with real-time optical measurements.
The result: reproducible brightness with less over-dosing, fewer rework loops, and better pulp strength. Vendor and mill data even point to 30–50% tighter brightness control and lower reagent costs when online sensors and advanced controls are in play (new.abb.com) (new.abb.com).
Temperature windows in bleach towers
Temperature governs kinetics. Industry literature shows that raising temperature accelerates oxidative reactions; a 10 °C rise roughly doubles reaction rates (researchgate.net). Typical chlorine dioxide (ClO₂) D₀ stages (the first chlorine dioxide treatment) run at 50–75 °C, with hardwoods sometimes up to ~95 °C (researchgate.net).
Higher temperatures generally boost brightness gain and can cut brightness reversion by about 50% according to studies (researchgate.net). But the ceiling is real: in eucalyptus kraft trials, running a high‑temperature chlorine dioxide stage (DHT) above 85 °C delivered no net brightness gain while severely reducing viscosity and yield (researchgate.net). Ozone stages, by contrast, must stay below 50 °C to avoid yellowing (researchgate.net).
In practice, mills target around ~80 °C for D₀ and ~100 °C for peroxide or alkaline extraction steps, then adjust steam flow to hold the line (researchgate.net) (researchgate.net).
Stage pH setpoints and selectivity
Each chemical stage has a sweet spot. Chlorine dioxide stages are acidic—targeting a reaction pH near ~3.0—and, if left unneutralized, typically exit at pH 3.0–3.8 (researchgate.net). Operating at ~pH 3 maximizes lignin removal; dropping below ~2.5 yields diminishing returns and harms viscosity (researchgate.net).
Alkaline stages behave differently. The E stage (alkaline extraction) requires a terminal pH above 10.5 (25 °C basis) to fully solubilize oxidized lignin; if end‑pH drops below about 9, lignin salts can re‑precipitate and stall brightness (researchgate.net). Chemical dosages—NaOH or Mg(OH)₂—are tuned to hold each tower at its setpoint (researchgate.net). For precise feed control, mills commonly rely on accurate chemical metering; an industrial dosing pump helps maintain steady ClO₂, H₂O₂, or caustic addition at the pH targets.
Not every brightening step must be strictly acidic. In one plant test, a final peroxide stage run at pH 5.5 (after quench) delivered better brightness than the conventional pH 3.5–4.0, suggesting near‑neutral can pay off in specific conditions (researchgate.net).
Pulp consistency, mixing, and residence
Pulp consistency (solids concentration) shapes mixing and heat transfer. Medium‑consistency towers typically run ~8–12% solids—literature also cites 10–15%—while high‑consistency towers operate around ~20–30%, often near ~25% (patents.google.com). One high‑yield process example used 10% solids in initial bleaching and 25% in a final high‑consistency stage (patents.google.com).
At medium consistency, stock mixes easily and heats quickly—but mills burn more energy heating water. High consistency concentrates chemicals at the fiber, enabling shorter retention, but requires stronger agitation and pumping, and it raises the risk of channeling if control falters. As a guidepost, a 10%‑consistency step might run 5–8 minutes of residence; ~25% can need less time but more aggressive mixing (patents.google.com).
Case data: three‑stage tuning and outcomes
In a DHT–(PO)–D sequence on eucalyptus kraft, careful tuning hit 89% ISO brightness with just 9.7 kg ClO₂, 4.5 kg H₂O₂, and 8.6 kg NaOH per oven‑dry ton (ODt) (researchgate.net). After ramp‑up, the mill reported ~21% lower ClO₂ use, 58% less H₂O₂, and 18% less NaOH versus its prior process (researchgate.net).
The same work underscored the guardrails: pushing the DHT tower above ~85 °C or driving pH below 3.0 hurt viscosity and darkened the pulp (researchgate.net). Those findings translated directly into control targets now common in mill DCS logic—temperature ~80–85 °C, pH ≈3.0 in the D stage (researchgate.net), and pH ≥10.5 in the E stage (researchgate.net).
Online brightness sensors and APC feedback
Modern bleach plants deploy inline optical probes that measure reflectance at 457 nm—the ISO brightness wavelength—in real time, typically at washer discharge or after each stage. Valmet’s Cormec family is one example, measuring brightness and related optical parameters such as OBA (optical brightening agent) effect or ERIC ink content (new.valmet.com) (valmet.com).
Bringing those measurements into the DCS or advanced controls (APC, including model‑predictive controllers) closes the loop. Brightness drift can trigger on‑the‑fly adjustments to ClO₂, H₂O₂, or caustic dosing (valmet.com). Valmet highlights “accuracy and reliability for bleaching control” and “extremely fast measurements” that “shorten control loops” (valmet.com). Mills running APC tied to online brightness (and pH/Kappa) report a 30–50% reduction in brightness variation and 3–10% lower chemical use, with one vendor’s system (“Expert Optimizer Bleach”) advertising up to 50% tighter brightness spread and significant reagent savings (new.abb.com) (new.abb.com).
Quality, cost, and compliance linkage
Temperature, pH, and consistency deviations immediately impact brightness. Inline sensors enable rapid correction—for example, a sudden drop at tower outlet brightness can prompt a slight pH tweak or increased ClO₂ dose—stabilizing final brightness and cutting unspent chemical residuals (which lowers effluent load) (valmet.com) (new.valmet.com).
These gains complement ECF/TCF approaches (elemental chlorine‑free/totally chlorine‑free), which are designed to reduce AOX (adsorbable organic halides) in effluent (researchgate.net). In sum, the consistent picture from technical literature, mill trials, and vendor data is clear: hold temperature, pH, and consistency at their optimal values—verify with online brightness monitoring—to reach target brightness with high yield, low cost, and regulatory compliance (researchgate.net) (new.abb.com).
References and technical notes
Technical literature, mill studies, and vendor data cited: Hart (2019); Milanez & Colodette IPBC 2005; Suess 2010; Valmet product literature; ABB APC; Indonesian industry experience—each citation corresponds to lines in the sources (researchgate.net) (researchgate.net) (researchgate.net) (researchgate.net) (patents.google.com) (new.abb.com) (valmet.com) (new.valmet.com) (researchgate.net) (researchgate.net).
