NaOH Charge and Kappa Number Reduction in Oxygen Delignification

How Does NaOH Charge Influence Carbohydrate Protection During Delignification in Two-Stage HC Systems?

In two-stage high-consistency (HC) oxygen delignification, the alkali profile across the stages determines both the extent of kappa number reduction and the degree of cellulose chain scission. Industrial systems employing a first-stage upflow reactor followed by a second-stage downflow tower (typical configuration of an Andritz two-vessel HC plant with a reactor diameter of 4.2 m and a total residence time of 60–90 min) rely on split NaOH addition to moderate the concentration of reactive oxygen species. When the total NaOH charge is asymmetrically distributed—for instance, 70% of the total 28–35 kg NaOH/odt applied in the first stage and the remainder in the second—the initial pH reaches 11.8–12.3 at 90–100 °C and 0.6–0.8 MPa O₂ partial pressure. Under these conditions, the superoxide radical (O₂⁻·) concentration is sufficiently low that the rate constant for glycosidic bond cleavage remains below 1.2 × 10⁻⁴ min⁻¹, whereas the rate of phenolic lignin unit oxidation proceeds with a pseudo-first-order constant of approximately 0.15 min⁻¹. Published data from Scandinavian softwood kraft pulp lines (Picea abies, kappa number entering O₂ stage 28–32) indicate that shifting the NaOH split to 50:50 without adjusting total charge elevates the final pulp viscosity loss to 12–14% beyond the baseline 8–10%, measured according to ISO 5351:2010. The critical processing window emerges because the second-stage pH must remain above 10.5 to dissolve re-precipitated lignin fragments; a second-stage NaOH charge below 8 kg/odt in a 12% consistency environment results in kappa stagnation at 14–16, as demonstrated by TAPPI T 236 om-13 measurements. The operational boundary is further constrained by the oxygen mass transfer coefficient (kₗa) in the downflow tower, which drops sharply if gas void fraction exceeds 0.25, limiting the feasible NaOH distribution envelope.

Omission of a dedicated header for the next scenario: The role of magnesium sulfate protection in single-stage medium-consistency oxygen delignification is often mischaracterized as a simple threshold effect. In a Valmet MC system operating at 10–12% consistency, 95–105 °C, and a reactor pressure of 0.5–0.7 MPa, the addition of 0.05–0.10% MgSO₄·7H₂O on oven-dry pulp (equivalent to 500–1000 g/odt) suppresses the Fenton-type reactions catalyzed by transition metals (predominantly Fe²⁺ and Mn²⁺ originating from chip feed and process water). When the total NaOH charge is 22–26 kg/odt—required to achieve a target kappa reduction of 40–45% for Eucalyptus globulus—the absence of magnesium protection causes cellulose viscosity to fall from an initial 1150 dm³/kg to 780 dm³/kg, as per ISO 5351, compared with 950 dm³/kg in the presence of 750 g/odt MgSO₄. The protective mechanism is critically dependent on the NaOH addition sequence: full alkali charge introduced before the oxygen pressurization zone precipitates Mg(OH)₂ prematurely, neutralizing its catalytic passivation effect. Trials on a Sunds Defibrator MC pilot plant (reactor L/D 18:1) revealed that a split alkali injection—60% into the feed line upstream of the mixer and 40% directly into the reactor’s first zone—optimizes the Mg²⁺ availability window, maintaining the dissolved Mg concentration above 30 mg/L for the first 15 minutes of retention time, the interval during which radical flux is highest. This knowledge is codified in mill-specific control logic on Valmet DNA DCS platforms, where the NaOH/MgSO₄ flow ratio is cascaded with inline kappa measurements from a BTG KNA-5000 analyzer.

Published data for this specific configuration is limited with respect to the interaction between magnesium dosage and black liquor carryover into the oxygen stage. Measurement of the residual black liquor COD entering the O₂ reactor—routinely 800–1500 mg/L in a non-washed pulp stream—indicates that organics can chelate divalent cations and reduce the effective Mg²⁺ concentration, necessitating a compensatory increase in MgSO₄ addition to 0.15%. Without such adjustment, the delignification selectivity (defined as Δviscosity/Δkappa) deteriorates from a baseline of 0.9 to above 1.3. The viscosity measurement protocol follows ISO 5351, and the kappa test is performed in accordance with ISO 302:2015, which specifies a reaction time of 10 minutes and a consumption of 50% of the potassium permanganate. These numerical boundaries constitute the operational limits for softwood lines where post-oxygen kappa targets below 12 are required for ECF bleaching sequences.

The application of an oxygen delignification simulator employing a Parr 4848 reactor(2-L vessel, 0–14 MPa rating)enables the mapping of NaOH charge–kappa response surfaces while controlling heat-up time to 5 minutes and maintaining a consistency of 10% under continuous stirring at 800 rpm. A series of isothermal runs at 100 °C with a mixed northern hardwood furnish (Acer, Betula, Fagus) at a NaOH charge range of 15–35 kg/odt yielded a non-linear kappa reduction profile: the differential kappa drop per kg NaOH declined from 0.8 units/kg at 15–20 kg/odt to 0.3 units/kg beyond 28 kg/odt. At charges exceeding 32 kg/odt, end pH rose above 11.7, initiating alkaline peeling reactions that reduced the degree of polymerization by an additional 200–250 units compared with the optimum at 26 kg/odt. This behavior was confirmed by size-exclusion chromatography (SEC) analysis of the pulp dissolved in CED according to ASTM D1795-13. The optimum NaOH charge for this species mix, defined as the charge producing maximum kappa reduction per unit viscosity loss, coincides with a terminal pH of 10.8–11.0 and a residual NaOH concentration of 4–6 g/L. Operating below this residual threshold leads to lignin re-condensation, manifesting as a kappa floor at 10.5, while exceeding it accelerates cellulose degradation without commensurate delignification gain. These lab-scale findings have been scaled to a continuous pilot plant (GL&V MC O₂ system, throughput 50 kg/h) and are consistent with published isothermal kinetic models.

Terminal pH Depression and Its Consequences on Post-Oxygen Wash Press Performance

In a two-stage oxygen delignification plant where the post-oxygen wash press is a twin-roll press (e.g., Metso TwinRoll TRPB93, design pressure 0.25 MPa, nip force 120 kN/m), the NaOH charge directly modulates the filtrate pH entering the press section and consequently the dissociation of lignin-carbohydrate complexes. When the total NaOH charge is pushed to the upper boundary of 35 kg/odt to force kappa below 10 for a softwood pulp entering the O₂ stage at kappa 34, the residual alkali in the post-reactor pulp suspension reaches 8–10 g NaOH/L. The high pH in the press vat (pH 11.5) causes dissolution of xylan-based hemicelluloses at a rate of 1.2–1.5 kg/odt per hour of contact time, as quantified by carbohydrate analysis using Dionex ICS-6000 HPIC with a CarboPac PA20 column. This hemicellulose loss reduces the fibre charge density from approximately 120 µeq/g to 95 µeq/g, impairing subsequent bleaching chemical uptake in the D₀ stage. The viscosity drop across the press, attributable to alkali-induced fibre swelling and mechanical shear in the roll nip, can account for an additional 5–8% loss beyond what is measured in pre-press pulp sampled according to SCAN-CM 15:99. This phenomenon sets a soft upper NaOH charge limit of 32 kg/odt for softwood grades where tensile index (measured per ISO 1924-2:2008) must remain above 90 N·m/g after full ECF bleaching. Mill data from a Stora Enso mill in Scandinavia confirmed that reducing the O₂ NaOH charge from 34 to 30 kg/odt with a compensating increase in ClO₂ in the D₁ stage improved overall pulp strength by 3.5% while meeting the same final brightness of 89% ISO.

When the topic concerns single-stage MC oxygen delignification with a short retention time (20–30 minutes) in a single reactor (e.g., Kvaerner compact reactor, L/D 12:1), the NaOH charge becomes the dominant lever for kappa control because temperature and O₂ pressure cannot be raised appreciably without risk of excessive oxygen delignification non-uniformity. The narrow residence time distribution (RTD) of the plug-flow reactor, characterized by a Péclet number exceeding 50, ensures that all pulp fibres experience nearly identical chemical conditions. A NaOH charge of 20 kg/odt for an incoming softwood kraft pulp at kappa 28 in such a configuration yields an exit kappa of 17–18 and a viscosity of 1050 dm³/kg. Incrementing the charge to 26 kg/odt depresses the kappa to 13–14 but drives viscosity to 920 dm³/kg. The threshold at which the NaOH charge ceases to yield incremental delignification and instead triggers “alkaline darkening”—the formation of quinoid chromophores—is 24 kg/odt for this reactor geometry, because beyond that the pH remains above 11.5 for the entire plug-flow transit, favouring alkaline degradation over oxygen radical-mediated lignin fragmentation. The distinct inflection in the NaOH–kappa curve correlates with a change in the Arrhenius activation energy from 45 kJ/mol (lignin oxidation regime) to 80 kJ/mol (carbohydrate peeling regime), as derived from pilot trials on a pressurized screen-equipped reactor.

What Happens to Extraction-Stage Carryover When NaOH Charge Exceeds Process Design Limits?

In integrated pulp mills where the brownstock washing efficiency varies diurnally due to chip quality shifts, the sodium hydroxide charge to the oxygen delignification stage must compensate for the variable weak black liquor carryover entering the O₂ reactor. A North American mill processing a mix of Pinus taeda and Pinus elliottii reported that when the washing loss, measured as COD entering the O₂ reactor (Hach method 8000 with dichromate digestion), increased from a baseline of 120 kg COD/odt to 165 kg COD/odt, the effective alkali demand rose by 4–5 kg NaOH/odt just to neutralize the organic acids in the carryover. If the mill’s DCS NaOH flow controller (Emerson DeltaV with FF fieldbus) is capped at a total of 30 kg/odt due to a pump sizing constraint (Goulds 3196 MTX with 7.5 kW motor for caustic service), the actual alkali available for delignification drops below the threshold required to maintain pH above 10.5 in the reactor’s latter half. The resulting operational failure mode is a kappa number drift upward from a target of 14 to 17–18 within 4 hours, as measured by the inline KNA-5000. The solution implemented at this site was a pre-O₂ stage hot water displacement press upgrade to a twin-roll press (TRPB93) with a nip force of 140 kN/m, enabling a reduction in COD carryover to 90 kg/odt, effectively reclaiming 3–4 kg NaOH/odt of alkali equivalent. This case illustrates that the NaOH charge cannot be considered independently of the upstream washing system; the combined hydraulic and chemical balance must be managed as a single unit. The safe zone for NaOH charge is therefore defined not solely by delignification chemistry but by the coupling of the recausticizing loop capacity and the brownstock washing efficiency, a principle embedded in the mass balance modules of WinGEMS simulation software used widely in the industry.

The interaction of NaOH charge with oxygen mass transfer in an agitated MC reactor is critical: the alkali influences pulp consistency fluidization and gas dispersion. In a high-intensity mixer (e.g., Kamyr MC mixer with rotor-stator clearance 0.5 mm, tip speed 25 m/s), the immediate rise in pH upon alkali injection reduces the viscosity of the fiber suspension by de-swelling the cell wall, thereby improving turbulence and kₗa. When NaOH charge is below 18 kg/odt at 10% consistency for a softwood furnish, the suspension exhibits a yield stress above 1500 Pa (measured via a Brookfield RST-SST rheometer with vane spindle), leading to channeling of oxygen gas and reduced interfacial area. At 26 kg/odt, the yield stress falls to 800 Pa, permitting a gas holdup of 0.20 and a volumetric mass transfer coefficient of 0.12 s⁻¹, which supports a delignification rate of 2.2 × 10⁻³ min⁻¹. This fluidization effect creates a secondary incentive to maintain NaOH charge above 20 kg/odt even if the stoichiometric requirement is lower, as suboptimal mixing leads to local lignin re-condensation from oxygen-depleted zones, evidenced by an increase in the proportion of carbonyl groups detected by FTIR at 1720 cm⁻¹. The process window thus has a lower bound set by rheology and not just chemistry. Pulp samples from a commercial line operating at 18 kg/odt showed 15% higher residual lignin in the final bleached pulp after D₀-Eop-D₁ compared with identical furnish processed at 23 kg/odt, attributable to unreactive lignin structures generated during oxygen starvation in the first 5 minutes of the reaction.