Kraft White Liquor Causticizing Degree Drop Thresholds for Direct NaOH Makeup

Kraft white liquor strength and composition are not adequately described by active alkali alone. The causticizing degree, defined as the NaOH fraction of the sum of NaOH and Na₂CO₃ expressed on a Na₂O basis, determines how much of the white liquor total titratable alkali is pulping-active hydroxide. When the causticizing reaction in green liquor is incomplete, residual Na₂CO₃ remains as dead load that travels through the digester, washer, evaporator, and recovery cycle without contributing to delignification. Routine liquor analysis in accordance with TAPPI T 624 cm-00 and SCAN-N 2:88 provides NaOH, Na₂S, and Na₂CO₃ concentrations; the causticizing degree is calculated from those titrimetric values. In continuous fiberlines, a stable white liquor typically exhibits a causticizing degree between 76% and 82% when the green liquor total titratable alkali is 100–110 g/L Na₂O and the sulfidity target is 25–32% as Na₂O. The remaining 18–24% of the NaOH + Na₂CO₃ sum is sodium carbonate, which for a 100 g/L Na₂O total corresponds to 18–24 g/L Na₂O as Na₂CO₃. This carbonate load reduces the concentration of effective alkali per unit volume and increases the risk of evaporator and recovery boiler fouling.

The industrial causticizing system consists of a slaker, a series of agitated causticizer vessels, and a white liquor clarifier or filter. Burnt lime is slaked to Ca(OH)₂, and the resulting slurry is contacted with clarified green liquor. The stoichiometry is CaO + H₂O → Ca(OH)₂, followed by Ca(OH)₂ + Na₂CO₃ ⇌ 2NaOH + CaCO₃. The equilibrium conversion is limited, and practical industrial installations are seldom operated above 85% causticizing degree because the excess lime required to force higher conversion increases lime mud load, raises white liquor turbidity, and degrades mud dewatering. The reaction is temperature- and residence-time-dependent; green liquor is maintained near 90–95°C, and causticizer trains of three or more vessels in series provide nominal residence times of 90–150 minutes depending on white liquor demand. Direct sodium hydroxide addition does not pass through this recausticizing circuit. It is injected after filtration and therefore raises the NaOH concentration without increasing lime consumption, but it also leaves the carbonate dead load unchanged. The causticizing degree thus increases arithmetically, not because the recausticizing reaction improved.

The direct NaOH makeup decision is a mass-balance decision constrained by the liquor cycle sodium-to-sulfur ratio, carbonate purge capacity, and digester alkali profile. If purchased 50 wt% sodium hydroxide is added to a white liquor stream at a rate equivalent to 10 g/L Na₂O, it reduces the demand on the lime kiln but can depress sulfidity by dilution and can increase white liquor effective alkali above the digester setpoint. The result is that direct NaOH makeup is best applied as a short-term response to a causticizing degree drop, not as permanent replacement for recausticizing capacity. The thresholds described below are operational rules built from the relationship between residual Na₂CO₃, white liquor demand, and the mass of NaOH needed to restore a target causticizing degree.

When Does a Falling Causticizing Degree Justify Direct Sodium Hydroxide Addition?

Direct NaOH makeup is considered when the causticizing degree falls below 75% and the lime kiln or slaker-causticizer circuit cannot restore target white liquor strength within the same production shift. At a total titratable alkali of 100 g/L Na₂O, a causticizing degree of 75% yields 25 g/L Na₂O as Na₂CO₃ and 75 g/L Na₂O as NaOH. At 70%, the carbonate residual increases to 30 g/L Na₂O and the NaOH fraction declines to 70 g/L Na₂O. The corresponding loss in effective alkali can be offset by adding 50 wt% NaOH directly to white liquor storage, but the carbonate dead load remains fixed unless a purge is taken. The operational threshold therefore has two simultaneous criteria: a causticizing degree below 75% and a white liquor flow demand above the recausticizing plant maximum. In mills where a single continuous digester consumes 600–800 m³/h of white liquor, a causticizing degree reduction from 78% to 72% at 100 g/L Na₂O represents a soda demand gap of roughly 6 g/L Na₂O as NaOH, which must be supplied either by increased lime conversion or purchased alkali.

A causticizing degree below 70% is generally considered a severe operational deviation because the sodium carbonate dead load exceeds 30 g/L Na₂O under the same TTA assumption. In that condition, the black liquor delivered to the recovery area carries additional non-alkali sodium carbonate, which tends to increase evaporator scaling and can reduce recovery boiler throughput due to higher inorganic dead load. Direct NaOH addition at this severity level is an immediate bridge, but it does not correct the recausticizing failure. The recausticizing plant must be audited for lime quality, slaker grit removal, green liquor clarity, causticizer agitation, and temperature control. In parallel, direct NaOH addition is typically limited to 20–30% of total effective alkali demand as Na₂O to prevent excessive sulfidity depression and heat-of-mixing excursions. These limits are not universal; published data for specific mill configurations is limited, but they are consistent with white liquor mass-balance logic used in causticizer sizing and digester alkali control.

The causticizing degree threshold interacts with the target effective alkali residual at the digester blowline. If the white liquor causticizing degree is 72% and the effective alkali is 90 g/L Na₂O, the soda component available for delignification is less than the active alkali calculation would suggest because sodium carbonate does not participate in the kraft delignification reaction at typical digester temperatures of 150–170°C. Direct NaOH addition raises the effective alkali, but the ratio of NaOH to Na₂S changes. A mill operating at a sulfidity of 30% with 100 g/L Na₂O active alkali has 30 g/L Na₂O as Na₂S and 70 g/L Na₂O as NaOH. Adding 15 g/L Na₂O as purchased NaOH drops sulfidity to approximately 26% because the denominator increases while Na₂S remains constant. This interaction means that direct NaOH makeup is not a simple soda replacement; it must be accompanied by sulfur addition or by a deliberate acceptance of lower sulfidity.

Recausticizing plant instability is often the origin of a falling causticizing degree, and direct NaOH addition is most effective when the rectifying response is mass-balanced against the residual carbonate. When the causticizing degree has fallen below target, the required direct NaOH addition is not a fixed volume. It is determined by the initial NaOH and Na₂CO₃ concentrations and the desired restored causticizing degree. For a white liquor stream with 100 g/L Na₂O total titratable alkali and a target causticizing degree of 80%, the NaOH addition required per cubic metre to restore the target is calculated by holding Na₂CO₃ constant and increasing NaOH. The equation is (NaOH_initial + NaOH_added)/(NaOH_initial + NaOH_added + Na₂CO₃) = 0.80. The following table illustrates the arithmetic for a white liquor flow of 1 m³; at mill scale, the per-cubic-metre addition is multiplied by white liquor flow in m³/h.

The 50 wt% NaOH volume assumes solution density of 1.53 kg/L at 20°C and NaOH mass fraction of 0.50. At actual storage temperatures, density changes must be compensated. In a recausticizing plant with three causticizer vessels operated at 90–95°C, the residence time available for the reaction declines as white liquor production increases. A three-vessel train with a total working volume of 300 m³ at a white liquor flow of 600 m³/h provides only 30 minutes of nominal retention, which is below the 90–150 minutes commonly required for high conversion. In such a condition, direct NaOH addition can maintain digester hydroxide demand during the production peak, but the carbonate dead load remains in the white liquor and returns to the recovery cycle. The mass balance in the table therefore represents a sodium hydroxide supplement that elevates causticizing degree without changing the actual causticizing conversion. Repeated use without recausticizing improvement gradually raises the total sodium inventory in the liquor cycle, which can increase white liquor total titratable alkali, alter the liquor-to-wood ratio, and shift digester temperature response.

A field observation from recausticizing circuits is that the first indication of a causticizing degree drop is often a decline in white liquor clarity due to unreacted lime mud carryover when lime feed is increased rapidly to compensate. In such cases, direct NaOH addition after the white liquor filter is preferred because purchased caustic soda does not add suspended solids; however, it can destabilize the white liquor sodium balance if the addition point is placed upstream of the digester feed heater where calcium carbonate precipitation and scale deposition are more likely. The addition point is therefore selected after the white liquor storage tank or after the pressure disc filter, with a side-entry mixer or static mixer sized for a residence time of 10–20 seconds before the digester feed pump suction. This mixing time is not standardised but is applied to avoid local high-pH pockets in carbon steel lines.

If the Lime Kiln Is Bottlenecked, Direct Sodium Hydroxide Addition Depresses Sulfidity and Delays Carbonate Purge

Lime kiln capacity limits the rate at which calcium carbonate from the causticizing reaction can be converted back to calcium oxide. When the kiln is at maximum throughput, the recausticizing plant cannot produce additional hydroxide from carbonate, and the causticizing degree falls as the white liquor production rate increases. In this condition, direct NaOH addition is often the only immediately available source of pulping alkali. The practical consequence is that the sodium inventory rises while the sulfur inventory remains fixed, producing a lower sulfidity. A white liquor with 100 g/L Na₂O active alkali and 30% sulfidity contains 70 g/L Na₂O as NaOH and 30 g/L Na₂O as Na₂S. If 20 g/L Na₂O is added as purchased sodium hydroxide, the sulfidity declines to 25% because the Na₂S concentration remains 30 g/L and the active alkali becomes 120 g/L Na₂O. At sulfidities below 20%, the rate of delignification may decline and the screened kappa at a fixed H-factor can increase, although the exact response depends on wood species, chip thickness, and cooking temperature.

The sodium-to-sulfur ratio is not corrected by direct caustic soda. Sulfur addition must be made in the recovery cycle, typically as sodium sulfate or elemental sulfur added to the black liquor or recovery furnace. The amount of sulfur required to maintain a target sulfidity can be calculated from the sodium addition rate and the white liquor sulfidity setpoint. If the desired sulfidity is 30% and direct NaOH addition adds 20 g/L Na₂O as NaOH, the Na₂S concentration required to restore 30% sulfidity with 90 g/L Na₂O as NaOH is approximately 38.6 g/L Na₂O, meaning an incremental sulfur requirement of about 8.6 g/L Na₂O as Na₂S. This adds to the reduction burden in the recovery furnace and may conflict with reduction efficiency constraints. Direct NaOH makeup therefore interacts with recovery boiler operation, not only with the recausticizing plant. Lime kiln bottlenecks that persist beyond 4–6 weeks often force a mill to choose between reduced digester production, increased purchased sulfur, or curtailed causticizing degree.

The carbonate dead load remains a hidden consequence. Direct NaOH addition raises the causticizing degree measurement but does not purge the existing Na₂CO₃. The carbonate entering the black liquor evaporators raises the inorganic solids load, reduces heat-transfer coefficients, and can increase the probability of sodium carbonate precipitation in the strong black liquor circuit. Control of the carbonate purge therefore requires a separate action, such as a controlled white liquor spill, green liquor dregs purge, or adjustment of the lime mud washing system. Without a purge, the liquor cycle total sodium inventory increases and the apparent causticizing degree can become artificially high because the ratio of NaOH to Na₂CO₃ has been altered by purchased alkali. This is why direct NaOH makeup is monitored using both causticizing degree and sodium carbonate concentration; the two variables are not redundant when purchased alkali is used.

Direct Sodium Hydroxide Injection Point and White Liquor Oxidation Compatibility Limits

The purchasing and unloading of 50 wt% sodium hydroxide introduces a high-freeze-point electrolyte into the mill liquor system. The freezing point of 50 wt% NaOH is approximately 12°C, which means unheated storage and transfer lines in cold climates can solidify. Storage tanks are typically fabricated from carbon steel with stress relief or nickel alloy and are maintained at 15–25°C to avoid crystallisation and to keep viscosity within pumpable limits. The addition to white liquor should be made through a chemical metering pump and a static mixer, not by direct splash into an open tank, because the heat of dilution is exothermic. The temperature rise can exceed 10°C at high addition rates, and localised temperatures above 100°C in white liquor at atmospheric pressure can generate steam and cause cavitation at the digester feed pump suction. Materials in contact with heated 50 wt% NaOH should comply with NACE SP0403 for avoidance of caustic stress corrosion cracking in carbon steel.

The compatibility of direct NaOH addition with white liquor oxidation must be checked when the mill operates an oxygen delignification or white liquor oxidation stage. Purchased sodium hydroxide does not contain reduced sulfur species, so it does not add chemical oxygen demand to the white liquor; however, it increases the pH and may alter the absorption of oxygen in a white liquor oxidation system if added upstream. The preferred injection point is therefore downstream of white liquor oxidation and filtration. In mills where white liquor is heated to 85–95°C before the digester, the direct NaOH stream is introduced into the suction of the digester feed pump or into a side-entry mixer on the white liquor storage tank. The addition point should be placed after the flow meter and sampling connection so that the purchased soda addition can be accounted for in the digester alkali charge. Analytical verification under TAPPI T 624 cm-00 should be performed every 2–4 hours during periods of direct NaOH addition because the causticizing degree can shift rapidly when the purchased alkali addition rate changes.

Operational limitations on direct NaOH addition are not solely chemical. The shipping, storage, and handling of 50 wt% caustic soda introduce a separate alkalinity source that bypasses the lime kiln and the causticizing area. If the mill exceeds a direct addition rate of roughly 20% of total white liquor effective alkali demand as Na₂O, the sodium-to-sulfur ratio drift and the white liquor carbonate dead load become difficult to manage without deliberate purge actions. The causticizing degree may appear to recover, but the carbonate concentration can remain at or above 25–30 g/L Na₂O, indicating that the actual causticizing reaction has not improved. Therefore, direct NaOH addition is governed by the dual threshold of a causticizing degree below 75% and a recausticizing plant at maximum throughput, with an upper operational boundary of approximately 20% of total effective alkali demand unless a structured sodium purge is available.