Within the kraft recovery loop, the causticizing reaction is the controlled conversion of sodium carbonate present in green liquor to sodium hydroxide by calcium hydroxide, expressed as Ca(OH)2 + Na2CO3 ⇌ 2NaOH + CaCO3. Causticizing efficiency is calculated as the percentage ratio of sodium hydroxide to the sum of sodium hydroxide and residual sodium carbonate on an equivalent Na2O basis, and operating values in continuous single-vessel or three-compartment causticizing trains are commonly maintained between 75 % and 85 % when the green liquor total titratable alkali is held within 110–140 g/L as Na2O and sulfidity is kept between 25 % and 35 %. Laboratory titration under TAPPI T 624 cm-02 partitions the ABC alkalinity into active alkali, effective alkali, and residual carbonate, and that measured carbonate fraction directly controls lime demand, mud mass flow, calcium carbonate seed recirculation, and sodium losses across the lime mud washing circuit. In a mill environment, causticizing efficiency drifts downward when the carbonate load exceeds available lime, when dregs carryover coats lime particles with iron sulfide and carbon fines, when slaker discharge temperature falls below 90 °C, or when the lime kiln delivers underburned or overburned calcium oxide with poor slaking reactivity. Excess lime addition may raise measured efficiency for a short period, but it also increases the unreacted Ca(OH)2 mass reaching the white liquor clarifier, producing sticky lime mud, higher filter cloth blinding rates, and increased soda carryover into the kiln. The maintenance objective is therefore not isolated lime ratio adjustment, but the alignment of reactive carbonate load, lime physical availability, slaking temperature, retention time, and solid-liquid separation capacity so that residual carbonate remains stable at the mill’s equilibrium limit and the lime mud returning to the kiln contains a minimum of soluble sodium compounds.
At a fixed temperature and total titratable alkali, the residual concentration of sodium carbonate after causticizing is governed by the equilibrium between dissolved calcium hydroxide and the increasing hydroxide ion activity produced by the reaction. Because the equilibrium quotient is approximately constant for a given temperature and ionic strength, the ratio [OH−]2/[CO32−] imposes a maximum conversion for a given final sodium hydroxide concentration. This thermodynamic constraint means that the practical ceiling for causticizing efficiency is typically 85–88 % at a green liquor TTA near 130 g/L as Na2O; attempts to exceed this ceiling by adding excessive lime result in only marginal conversion of the remaining carbonate and generate a larger calcium carbonate cake that must be washed, dewatered, and recalcined. The ABC titration method measures residual carbonate using phenolphthalein and methyl orange endpoints at pH 8.3 and pH 4.5, or by automated potentiometric titration with calibration records maintained under ISO 9001:2015 clause 7.1.5 for monitoring and measuring equipment. Causticizing efficiency is computed from the titration values as CE = 100 × NaOH/(NaOH + Na2CO3) on an equivalent Na2O basis, and the calculation assumes that the analyzed sample has been filtered to remove calcium carbonate before titration. In practice, the observed efficiency in a mill is often lower than the equilibrium ceiling because the slaker and causticizer train operate with finite retention time and with mass-transfer limitations between the solid calcium hydroxide surface and the bulk liquid. The first 10–15 minutes of contact in the slaker and first causticizer produce the largest increase in sodium hydroxide concentration; after that period, the reaction rate declines as calcium carbonate product layers form on the lime particles and as the remaining calcium hydroxide surface area becomes restricted. Mills that shorten total retention below 90 minutes frequently observe a decline of 2–5 percentage points in causticizing efficiency even when the lime dosage remains constant, because the reaction does not reach the equilibrium residual carbonate concentration before the liquor enters the white liquor clarifier.
Before green liquor reaches the slaker, dregs removal has a direct influence on the subsequent solid-liquid separation and lime utilization. Green liquor dregs are composed of non-reactive smelt solids, carbon char, iron sulfide, silicates, and fine suspended impurities that remain after the smelt dissolution tank and green liquor clarifier. Mills using pressure disc filters or candle filters after the green liquor clarifier typically maintain suspended solids below 50 mg/L, while conventional sedimentation alone may leave between 100 mg/L and 300 mg/L of suspended dregs entering the slaker. When suspended solids exceed 300 mg/L, the calcium carbonate particles formed during causticizing become finer and more gelatinous because the dregs provide heterogeneous nucleation sites and interfere with the growth of dense calcium carbonate crystals. The resulting lime mud settles more slowly in the white liquor clarifier, and the increase in fine fraction forces the lime mud filter to operate at lower drum speed or with higher vacuum to maintain acceptable cake moisture. The dregs removal system requires routine attention to filter leaf pressure drop, because a pressure drop increase of more than 15 % per week usually indicates precoat fouling or channeling that permits unfiltered green liquor bypass. A green liquor clarifying system that is not maintained also increases erosion in the slaker impeller and classifier screw because the abrasive dregs accumulate in the slaker bottom. Suspended solids in green liquor are measured gravimetrically using membrane filtration, and the result is used to trigger filter precoat renewal or clarifier desludging before dregs reach the slaker in concentrated slugs.
Of the lime-side variables, available calcium oxide content in kiln product is the single largest factor controlling lime reactivity and causticizing efficiency. Kiln lime with available CaO below 85 % contains uncalcined limestone, silica, iron oxide, or alumina that does not participate in slaking and causticizing; the inert fraction contributes to grit load, increases mud solids without increasing conversion, and forces operators to raise the lime-to-carbonate molar ratio. The optimum available CaO range is generally 88–93 %, with a loss on ignition below 5 % by weight and grit retained on a 45 µm sieve below 3 %. Lime with high loss on ignition indicates underburning and persisting calcium carbonate that recirculates through the liquor loop without providing available calcium oxide. Overburned lime, by contrast, may have low loss on ignition but develops a dense crystalline structure that hydrates slowly in the slaker; its slaking temperature rise measured by ASTM C110-20 may remain below 20 °C in the first 2 minutes, and unreacted CaO can pass into the causticizers where delayed hydration creates localized hot spots and liquor density fluctuations. The lime feed system must deliver a consistent particle size distribution to the slaker, because segregation in the lime storage silo changes the bulk density and reactivity of the feed, causing the slaker temperature controller to cycle and the causticizing efficiency to vary even when the lime conveyor speed is constant.
| Lime parameter | Standard method | Typical control range | Effect on causticizing loop |
|---|---|---|---|
| Available CaO | ASTM C25-19 | 88–93 % | Below 85 % requires increased lime ratio and raises inert mud flow |
| Loss on ignition | ASTM C25-19 | 2–5 % | Above 8 % indicates underburning and recirculation of uncalcined CaCO3 |
| Grit retained on 45 µm | ASTM C110-20 | <3 % | High grit accelerates classifier wear and increases dregs load in slaker |
| Slaking reactivity | ASTM C110-20 | rise to 70 °C within 2 min | Slow slaking lowers causticizing efficiency at fixed retention time |
In high-sulfidity kraft operations, the green liquor contains a larger proportion of sodium sulfide relative to sodium hydroxide and sodium carbonate, and the total titratable alkali can no longer be used directly as the carbonate load for lime feed control. When sulfidity increases from 28 % to 38 % on an Na2O basis, the sodium sulfide hydrolysis contributes hydroxide activity to the liquor before causticizing begins, and the actual sodium carbonate concentration must be determined by subtracting the measured sodium hydroxide and sodium sulfide from the total titratable alkali. If the lime dosage control loop does not apply this correction, chronic overliming occurs because the total alkali appears to demand more lime than the carbonate portion actually requires. Overliming in high-sulfidity liquors is particularly damaging because the excess calcium hydroxide raises the hydroxyl concentration beyond the equilibrium value for the remaining carbonate, accelerates calcium carbonate nucleation on existing mud particles, and produces a lime mud with poor settling characteristics. The target molar ratio of available CaO to reactive sodium carbonate is typically 0.95–1.05 mol/mol, but the operating ratio must be recalculated when green liquor sulfidity changes by more than 3 percentage points. High-sulfidity operation also intensifies the common-ion effect on calcium hydroxide solubility, because the total sodium ion concentration is higher at the same TTA, reducing the dissolution rate of slaked lime and increasing the sensitivity of the system to slaker temperature and agitation intensity. In such configurations, causticizing efficiency can remain acceptable only if the slaker is operated with sufficient turbulence to suspend the dense lime mud and if the retention time in the causticizers is extended to compensate for the slower dissolution of calcium hydroxide in the high-ionic-strength liquid.
| Green liquor parameter | Analytical method | Typical range | Causticizing impact |
|---|---|---|---|
| Total titratable alkali | TAPPI T 624 cm-02 | 110–140 g/L as Na2O | Higher TTA lowers equilibrium causticizing degree; dilution may be required |
| Sulfidity | TAPPI T 624 cm-02 | 25–35 % | Above 35 % requires carbonate load correction; overliming risk increases |
| Suspended dregs | gravimetric membrane filtration | <50 mg/L | Above 300 mg/L reduces lime availability and lowers mud settling rate |
| Temperature to slaker | calibrated thermocouple | 85–95 °C | Low temperature slows slaking and reduces causticizing efficiency |
When the slaker discharge temperature remains below 90 °C, the hydration of calcium oxide is incomplete and the lime slurry entering the causticizers contains a residual CaO core surrounded by a calcium hydroxide shell. The unreacted CaO continues to hydrate in the first causticizer, consuming water and releasing heat that can exceed the agitator’s ability to maintain uniform mixing, producing localized boiling and density-driven short-circuiting. At temperatures above 105 °C, the slurry approaches atmospheric boiling, water evaporation increases the apparent solids concentration, and the grit classifier becomes overloaded with fine particles that no longer settle through the viscous liquid. The slaker temperature window is therefore maintained between 90 °C and 100 °C, with a tolerance of ±5 °C, by controlling the green liquor inlet temperature, lime feed rate, and wash water addition. Total retention time in the slaker is typically 12–16 minutes, followed by 90–120 minutes in a three-compartment causticizer train equipped with baffled agitators and overflow transfer boxes. Slaker agitator power draw is monitored continuously because a sudden increase of more than 10 % indicates grit accumulation or overliming, while a decrease suggests diluted slurry flow and reduced mixing intensity. Kiln lime feed to the slaker should be interrupted automatically when the green liquor TTA falls below 80 g/L as Na2O, because the resulting high initial hydroxide concentration suppresses further carbonate conversion and increases the risk of scale formation on the slaker internals and downstream piping. The slaker temperature control loop is often combined with a variable-frequency drive on the lime conveyor and a ratio controller tied to the green liquor carbonate flow, but the response of the system is limited by the residence time of the slaker and the heat capacity of the incoming green liquor.
Between the last causticizer and the white liquor storage tank, solid-liquid separation determines whether the sodium hydroxide produced in the causticizing reaction actually reaches the digester without excessive lime mud carryover. White liquor clarifiers or pressure filters separate the crystallized calcium carbonate from the hot liquor, and the rake mechanism in a conventional clarifier must be maintained so that the mud bed depth remains below the manufacturer’s setpoint. When the rake torque increases by more than 15 % above baseline, the mud bed deepens, clarified white liquor turbidity rises above 100 NTU, and fine calcium carbonate particles carry over to the digester feed, causing scaling on digester screens and extraction lines. The lime mud washing circuit, typically a vacuum rotary drum filter or a horizontal belt filter, removes residual sodium compounds from the mud before the kiln. If the wash water flow provides a displacement ratio below 0.8, the soda content of the lime mud sent to the kiln exceeds 1.5 % Na2O by dry mud weight, increasing kiln fuel consumption and promoting sticky ring formation in the mid-kiln zone. Wash water temperature is maintained between 60 °C and 70 °C to lower liquor viscosity and improve filtration rate, but water above 80 °C can reduce vacuum pump capacity through increased vapor pressure and may soften older filter cloth adhesives. The lime mud filter vacuum level is typically held between 40 kPa and 60 kPa; lower vacuum produces wetter mud, while higher vacuum can compact the cake and reduce washing efficiency. Operators should avoid feeding sodium aluminate-containing streams into the lime mud system because aluminate reacts with calcium hydroxide to form poorly settling precipitates that destabilize the clarifier and increase soda loss.
Inside a rotary lime kiln operating with satellite coolers and chain sections, the returned lime mud is dried and calcined at a burning zone temperature of 1000–1100 °C, with back-end oxygen maintained between 1.0 % and 2.0 % dry to avoid reductive burning and excessive sulfur salt buildup. The lime mud entering the kiln should be dewatered to 70–85 % dry solids; residual moisture above 35 % by weight consumes additional fuel and depresses the burning zone temperature if the combustion air supply is not increased. Underburned kiln lime with residual carbonate above 8 % as CO2 reduces available CaO and forces a higher slaker lime ratio, while overburned lime with a dense microstructure hydrates slowly and may not fully slake before the third causticizer. The kiln fuel-to-lime ratio is adjusted through the mud feed rate, and a sudden increase in mud feed moisture of 5 percentage points without a corresponding fuel increase can reduce the calcination temperature by 50–80 °C, producing lime that is unreactive in the slaker and directly lowering causticizing efficiency. Continuous measurement of kiln feed-end oxygen, burning zone temperature, and lime loss on ignition forms the main feedback loop for maintaining causticizing efficiency across the lime cycle.
Committed to quality, innovation, and reliable chemical solutions for a sustainable future.
