In mechanical pulp brightening, the alkali charge to a hydrogen peroxide stage—expressed as mass percentage of sodium hydroxide on oven-dry pulp—simultaneously activates perhydroxyl anion formation, buffers the reaction pH, exchanges metal ions from fibre carboxylates, and saponifies a portion of the accessible wood resin. The active bleaching anion is generated by the equilibrium H₂O₂ + OH⁻ ⇌ HOO⁻ + H₂O, with a published pKa of 11.6 for hydrogen peroxide; therefore the initial pH must be high enough to maintain a sufficient HOO⁻ concentration, but not so high that alkali-induced chromophore formation overwhelms the oxidative brightening. Mill experience with spruce thermomechanical pulp and chemithermomechanical pulp indicates that the practical initial pH window for a single stage is 10.5–11.2, and final pH typically falls to 8.8–9.5 after peroxide consumption. At a typical charge of 2.0–2.5% NaOH on oven-dry pulp, and 1.5–3.0% hydrogen peroxide, brightness determined according to ISO 2470-1:2016 can increase by 8–12 ISO points in a single high-consistency stage; however, the gain is strongly dependent on the buffering behaviour of the specific furnish. Sodium silicate at 1.5–3.0% on oven-dry pulp and magnesium sulphate at 0.05–0.1% are commonly introduced to suppress transition-metal-catalysed peroxide decomposition, while diethylenetriaminepentaacetic acid applied at 0.2–0.5% before the stage chelates iron, manganese, and copper. The alkali charge also mobilises anionic polysaccharides and lipophilic extractives into the liquid phase, raising cationic demand in the surrounding paper machine white-water system and increasing the load on the bleaching filtrate treatment plant. This dual function—activation of peroxide and generation of anionic dissolved organic matter—makes the alkali charge the most consequential operational variable in mechanical pulp brightening, yet it cannot be optimised independently of furnish metal content, wood species, refiner energy, or white-water closure level.

What Limits Alkali Charge in Peroxide Brightening of TMP and CTMP?

The practical ceiling is set by four interacting mechanisms: alkali darkening of lignin-derived phenols, peroxide decomposition by transition metals released at elevated pH, silicate-related deposit formation, and anionic trash generation. Alkali darkening arises when ortho-quinoid and conjugated carbonyl structures are formed by base-catalysed condensation of phenolic fragments; the resulting chromophores absorb at 420–500 nm and reduce brightness gain before peroxide can oxidise them. In process terms, this becomes measurable when the initial pH exceeds approximately 11.5, and the effect is most severe with high-lignin mechanical pulps because the lignin concentration is not reduced by prior delignification. Transition-metal release is equally important: at pH values above 10.0, precipitated iron and manganese species can redissolve as hydroxide complexes, and the Fenton cycle accelerates peroxide decomposition; this can consume 15–40% of the charged hydrogen peroxide within the first 10–20 min of retention. Sodium silicate stabilises the peroxide by passivating metal surfaces and buffering alkalinity, but its efficiency declines when the NaOH charge exceeds 3.0% because calcium and magnesium silicate precipitation can deposit on tower walls, medium-consistency pump rotors, and discharge screw conveyors. Mechanical pulp mills running closed white-water systems report that high alkali charges increase the concentration of dissolved pectic acids, acetylated galactoglucomannans, and resin soaps, which raise the cationic demand of the filtrate by 0.2–0.8 meq/L and reduce first-pass retention on the paper machine. This is why the optimum alkali charge for a given furnish is normally verified through a matrix trial using ISO 2470-1:2016 brightness, post-color number, residual peroxide titration, and filtrate COD rather than through a single pH setpoint. Published data for the specific configuration of a closed-loop TMP mill with a disc-filter white-water system and a two-stage peroxide plant is limited, but the boundary conditions identified above are consistent across northern European and North American mechanical pulp operations.

Across mill-scale high-consistency peroxide towers operating at 20–30% discharge consistency, the alkali charge interacts with refiner latency, pressate recycling, and tower retention time in ways that are not visible in laboratory bleaching trials. A twin-wire press or screw press thickens the pulp after the refiner, and the alkali-containing impregnation liquor is often introduced through a high-shear mixer immediately upstream of the retention tower. The effectiveness of alkali distribution depends on mixer rotor tip speed, which is typically 15–30 m/s in industrial medium-consistency mixers, and on the residence time under plug-flow conditions, which is commonly 60–120 min at 70–80°C. When the alkali charge is raised above the mill baseline, the thickened pulp becomes more slippery and can exhibit reduced friction in a plug screw feeder or discharge screw, but the larger immediate concern is the dissolution of anionic colloidal material and its return to the bleached thickener filtrate. The filtrate is often recirculated as shower water or pulp dilution water; a COD increase of 5–15 kg/t in the peroxide-stage filtrate can be observed in mill data when the NaOH charge is increased from 1.5% to 3.0%, though the exact increment depends on wood species and closure level. Magnesium sulphate and sodium silicate are not fully retained in the fibre mat; a fraction exits with the pressate and can form deposits in downstream evaporators or on the paper machine felt. Some Canadian TMP mills have reduced alkali-induced scaling by limiting the sodium silicate charge to 1.5–2.0% and substituting a portion of the NaOH with sodium hydroxide/carbonate mixtures, but this substitution is constrained by the need to maintain the initial pH above 10.5. The operating window is therefore narrow: a deviation of ±0.2% NaOH at constant peroxide charge can shift the final pH by 0.3–0.4 units and alter the residual peroxide by 10–15% of the initial charge.

When Alkali Charge Exceeds 2.5% in Mechanical Pulp Brightening

At this threshold, the economics change from additional brightness gain to brightness reversion and process instability. In spruce TMP, raising the NaOH charge from 2.5% to 3.5% at constant 3.0% hydrogen peroxide moves the initial pH from approximately 11.0 to 11.8. The peroxide residual falls from 20–30% of the initial charge to below 15%, and the post-color number measured after 24 h at 105°C can increase by 1.5–2.0 units, indicating greater thermal reversion. The exact response is furnish-specific, but the general pattern is robust: the brightness gain per kilogram of NaOH decreases, and the incremental alkali goes disproportionately into organic dissolution and undesirable chromophore generation. Laboratory and mill data also show that high residual alkali in the bleached pulp consumes cationic retention aids at the paper machine, and the dissolved lignin fragments can precipitate when the pH is reduced at the paper machine or in the effluent treatment plant. In closed-loop systems, this precipitation contributes to sticky deposits on press rolls and forming fabrics. For these reasons, a charge of 3.0% NaOH or greater is generally reserved for high-brightness mechanical pulps used in woodfree coated paper or multiply board, where the added cost is offset by the higher ISO 2470-1:2016 brightness specification of 78–82%. Even in those applications, the preferred strategy is to split the peroxide and alkali into two stages rather than exceeding the single-stage threshold.

Table 1 — Representative mill-scale peroxide brightening response as a function of alkali charge for spruce TMP at 3.0% H₂O₂, 70°C, 90 min retention, compiled from published ranges.

Sodium hydroxide charge in chemical pulp alkaline extraction is operationalised differently: it is applied to an already delignified fibre and is intended to dissolve alkali-soluble lignin fragments, hexenuronic acid-bearing xylan, and oxidation products from preceding oxygen or chlorine dioxide stages. The alkaline extraction stage following oxygen delignification or D0 receives pulp at 8–15% consistency and at a temperature of 60–90°C for 60–90 min. The effective alkali charge, expressed as NaOH on oven-dry pulp, is typically 1.0–2.5% for a conventional E stage and 1.5–3.0% for an oxygen-reinforced extraction stage. The alkaline conditions convert residual lignin phenols to phenolate ions, hydrolyse acetyl groups to sodium acetate, and ionise hexenuronic acid, which improves solubility in the wash filtrate. Under these conditions, the kappa number determined by ISO 302:2015 can decrease from 10–18 to 6–10, depending on the incoming kappa number, wood species, and oxygen charge. The same alkali charge also attacks the cellulose reducing end groups through alkaline peeling, so the viscosity measured by ISO 5351:2010 falls as the NaOH charge and temperature increase. Process control in the extraction stage therefore involves balancing kappa reduction against viscosity retention and COD generation, with the target endpoint pH typically between 10.5 and 11.5 for Eop stages. Insufficient alkali allows the dissolved lignin to re-precipitate onto fibre surfaces during washing, where it appears as increased kappa number after the stage and can reduce subsequent chlorine dioxide or peroxide efficiency.

Alkaline Extraction Stage Variants in Kraft Pulp Lines

The conventional E stage removes alkaline-soluble lignin and extractives with a comparatively low chemical demand, while EO and Eop variants introduce oxygen to degrade additional lignin through superoxide and hydroxyl radical intermediates. In a conventional E stage at 60–75°C with 1.0–1.8% NaOH, the kappa reduction is typically 10–20%, and the viscosity loss measured by ISO 5351:2010 is commonly 10–30 mL/g. In an EO stage with 1.5–2.5% NaOH, 70–85°C, and an oxygen partial pressure of 200–400 kPa, the kappa reduction increases to 20–35%, while the viscosity loss rises to 20–50 mL/g. In an Eop stage at 80–90°C, 2.0–3.0% NaOH, and 400–600 kPa oxygen, the kappa reduction can reach 35–50%, but the viscosity loss may exceed 50 mL/g if the alkali charge is not matched to the oxygen consumption profile. The reason for the higher NaOH demand in oxygen-reinforced stages is that oxygen delignification produces carboxylic acid fragments and carbon dioxide, which consume hydroxyl ions and lower the pH during the reaction. If the endpoint pH falls below 10.5, oxygen delignification decelerates, and the residual alkali is insufficient to keep the solubilised lignin in solution. Conversely, if the initial pH exceeds 12.0, the rate of carbohydrate peeling increases and the pulp viscosity loss becomes economically unacceptable for high-strength packaging grades. Mill-scale extraction stages are therefore equipped with online pH and temperature sensors at the mixer and at the tower discharge, and the NaOH charge is frequently adjusted in response to post-stage kappa and viscosity.

Table 2 — Comparative alkaline extraction stage conditions and typical responses from published mill-scale ranges.

Oxygen-Reinforced Extraction Kinetics and Caustic Partitioning

In oxygen-reinforced extraction, the NaOH charge is not a single stoichiometric demand but a partitioned quantity that neutralises acidic groups, maintains the oxygen reduction potential, and compensates for carbon dioxide generated by lignin oxidation. The overall oxygen consumption in an Eop stage is typically 5–15 kg O₂/t pulp, and the associated acid production consumes 0.5–1.2 kg NaOH/t pulp before the desired endpoint pH is re-established. Oxygen solubility at the operating pressure of 400–600 kPa and temperature of 80–90°C is the primary mass-transfer variable, and industrial reactors maintain gas-liquid contact through medium-consistency mixers with high-shear zones and static mixers in the first section of the tower. The alkali charge must be introduced before the oxygen charge, or simultaneously with it, because a local pH below 10.5 in the mixer creates conditions under which oxygen radical attack on cellulose rather than lignin becomes significant. Magnesium sulphate is added at 0.05–0.1% to limit metal-catalysed cellulose degradation, and the ratio of magnesium to transition metals in the pulp feed must be maintained above the Fenton-catalysis threshold; this is typically achieved when iron and manganese residuals are below 10–20 mg/kg. At 2.5% NaOH on oven-dry pulp, the endpoint pH in an Eop stage is usually 10.8–11.2, and the kappa reduction is sufficiently high to reduce the subsequent chlorine dioxide charge by 15–25% in an ECF sequence. However, the same NaOH charge can raise the COD load of the stage effluent to 20–40 kg/t pulp, which is a critical parameter for mills subject to COD-based discharge limits under national permits.

Effluent streams from alkaline extraction contain dissolved xylans, hexenuronic acids, acetyl groups, resin soaps, and oxidised lignin fragments, and the COD measured by ISO 6060:1989 increases with both alkali charge and oxygen charge. In mill-scale balances, the extraction-stage filtrate alone can contribute 20–40 kg COD/t pulp, and the additional alkali-induced dissolution of hemicelluloses increases the load on the aerobic basin and can lower sludge settleability if the food-to-microorganism ratio is not adjusted. The dissolved organic matter also carries anionic charge that affects the performance of retention aids and sizing agents when bleach plant filtrates are used for paper machine showers or pulp dilution. Alkali charge optimisation in chemical pulp extraction is therefore not solely a bleaching question; it is an effluent-management decision, because every incremental 0.5% NaOH may solubilise additional 2–5 kg/t of organic material, depending on the furnish and preceding delignification stage. Published data for this specific configuration is limited when the pressate is recirculated to the brown stock washing system, but the general operating boundary is clear: the NaOH charge must exceed the acid-neutralisation demand by a margin sufficient to maintain the final pH above 10.5, while remaining below the point at which viscosity loss and COD generation outweigh the downstream bleaching cost reduction.