White Liquor Preparation for Kraft Pulp Digestion with 5 kg Sodium Hydroxide

White liquor for kraft digestion is an aqueous alkaline solution in which sodium hydroxide and sodium sulfide are present in a controlled ratio, expressed either as sulfidity or as active alkali on a Na₂O basis. A 5 kg sodium hydroxide charge, corresponding to 125 mol NaOH, is sufficient to prepare approximately 47 L to 50 L of finished liquor depending on sulfidity and target effective alkali. The analytical reference point is ANSI/TAPPI TAPPI T 624 cm-11 for sulfidity and TAPPI T 625 cm-85 for sodium hydroxide, sodium sulfide, and sodium carbonate in white and green liquors. A 100 L jacketed 316L stainless steel vessel with a variable-speed agitator at 250 rpm to 300 rpm is representative for a 5 kg caustic charge; a production-scale analogue is a 10 m³ caustic dilution tank with a 45° pitched-blade turbine. The order of addition is controlled: flaked or prilled sodium hydroxide is fed into water at a rate that keeps solution temperature below 95°C, and sodium sulfide is withheld until the caustic solution is homogeneous. The integral heat of solution for sodium hydroxide at 25°C to infinite dilution is −44.5 kJ/mol; a 5 kg charge therefore liberates approximately 5.56 MJ. When dispersed into 50 L of water initially at 25°C, this raises the bulk temperature by roughly 25 K to 26 K assuming no heat loss through the jacket, which is within the safe operating window for a closed, vented stainless steel vessel.

Sulfide Addition and Causticizing Balance at 5 kg NaOH Charging

To produce a white liquor with a sulfidity of 30% as Na₂O, the molar basis must account for stoichiometric equivalence. Sulfidity is defined as Na₂S/(NaOH + Na₂S), both expressed as Na₂O; one mole of NaOH contributes 0.5 mol Na₂O, whereas one mole of Na₂S contributes 1 mol Na₂O. For 125 mol NaOH, the NaOH equivalent is 62.5 mol Na₂O. The sulfide addition solves S = x/(0.0625 kmol + x) = 0.30, giving x = 0.0268 kmol Na₂S, equivalent to 2.09 kg of 100% Na₂S. If the available raw material is 60% sodium sulfide flake, the required mass becomes 3.48 kg. This produces an active alkali charge of 0.0893 kmol Na₂O, or 5.53 kg Na₂O equivalents, and an effective alkali charge of 0.0759 kmol Na₂O, or 4.70 kg Na₂O equivalents. To maintain a target effective alkali concentration of 100 g/L Na₂O, the finished liquor volume is 47.0 L. These calculations must be corrected after titration because commercial sodium sulfide flakes contain polysulfides, thiosulfate, and carbonate; TAPPI T 625 cm-85 includes procedures for acid titration and the reporting of carbonate blank corrections. In a jacketed 100 L mixing vessel, the sodium sulfide is predissolved in 15 L of deaerated water at 40°C under a nitrogen sweep; the solution is then transferred by gravity through a 50 µm stainless steel filter to the caustic tank. The transfer line from the sulfide dissolving tank to the caustic tank is fitted with a pH 11–13 tolerant elastomer-lined check valve, and the vent is connected to a 0.1 M NaOH scrubber to prevent hydrogen sulfide release. Addition of the sulfide solution to the caustic over a 10–15 minute period at 300 rpm prevents local sulfide-rich zones that could darken the liquor or precipitate iron sulfide from trace ferrous contamination.

In the analytical verification of this liquor, the titrimetric distinction between hydroxide, sulfide, and carbonate determines whether the batch can be used for a 2 m³ pilot digester or a 20 L laboratory autoclave. The raw titration values from TAPPI T 625 cm-85 are commonly reported as total titratable alkali, active alkali, and effective alkali, all converted to grams per litre Na₂O. A 30% sulfidity liquor prepared from solid caustic and flaked sodium sulfide should be titrated in triplicate after 30 minute mixing, and the relative standard deviation between replicates should be below 1.5% of the active alkali value. If the measured sulfidity differs from the target by more than 2 percentage points, the batch is not corrected by simple addition of flake because the hydroxide present shifts the Na₂S/HS⁻ equilibrium; the measured pH is above 13.5, at which sulfide exists predominantly as S²⁻, but pH measurement alone cannot substitute for titration under high-alkalinity conditions. The liquor is transferred to a nitrogen-blanketed storage vessel with a 5–10 mbar overpressure and a pressure/vacuum relief valve; the dissolved oxygen concentration is checked with an optical oxygen probe before digester charging.

How Does Sulfidity Affect Effective Alkali and Digester Charge for a 5 kg NaOH Basis?

Because effective alkali determines delignification rate and digester endpoint control, changing the sulfidity from 20% to 35% for a fixed 5 kg NaOH charge shifts both the sulfide mass and the total liquor volume. At 20% sulfidity, the sulfide mole requirement is 0.0156 kmol Na₂S; at 35% sulfidity, it is 0.0337 kmol Na₂S. The corresponding Na₂S masses are 1.22 kg and 2.63 kg as 100% Na₂S. The active alkali values are 4.84 kg and 5.96 kg Na₂O, and the effective alkali values are 4.36 kg and 4.92 kg Na₂O. This nonlinear relationship arises because sulfidity is expressed on a Na₂O-equivalent basis, not on simple mass. In practical digester control, a fixed total active alkali charge is often preferred; a laboratory cook with 5 kg NaOH cannot simultaneously fix both sulfidity and total active alkali unless the final volume is adjusted. For a softwood digester with an effective alkali demand of 18% on oven-dry wood, a 5 kg NaOH batch at 30% sulfidity supplies approximately 4.70 kg Na₂O effective alkali, sufficient for 26.1 kg oven-dry softwood at a 4:1 liquor-to-wood ratio when the target effective alkali is 18%. If hardwood cooking requires only 13%–15% effective alkali, the same batch may suffice for 31 kg–36 kg oven-dry chips. The actual charge must be verified against the kappa target, chip moisture, and digester temperature schedule.

Liquor Clarification, Dregs Removal, and Oxidation Limits

White liquor clarity affects digester scaling and chip penetration. After the sulfide and caustic streams are combined, the liquor should be recirculated through a 20–50 µm sintered metal filter for 20–30 minutes to remove dregs, undissolved sodium carbonate, and metal sulfides. A target suspended solids content below 5 mg/L is common for continuous digesters; a 316L filter housing with differential pressure monitoring can detect cake buildup. The combined liquor has pH above 13.5; at this pH, sulfide oxidation by entrained air produces thiosulfate according to 2 Na₂S + 2 O₂ + H₂O → Na₂S₂O₃ + 2 NaOH. TAPPI T 625 cm-85 titration may overestimate hydroxide if thiosulfate and carbonate are not corrected. Therefore storage tanks for white liquor should be blanketed with nitrogen at 5–10 mbar overpressure and fitted with a pressure/vacuum relief valve. Published data for a 5 kg NaOH batch in a 100 L stirred vessel is limited; however, near-atmospheric oxygen ingress into a vented tank reduces sulfidity by measurable amounts over 24 h, and the TAPPI method can detect the change as a drop in sulfide concentration. At dissolved oxygen concentration above 0.5 mg/L, thiosulfate formation accelerates; at pH below 12.5, hydrogen sulfide partial pressure becomes measurable. Therefore the storage vessel is maintained under nitrogen and the liquor is consumed within 24 h to keep thiosulfate below 5% of total sulfide. Wetted parts should be 316L, PTFE, or EPDM; aluminum, zinc, tin, and galvanized steel are incompatible with concentrated caustic. If the liquor is heated above 60°C and the NaOH concentration exceeds 30%, austenitic stainless steel may be subject to caustic stress corrosion cracking, and low-carbon grades or nickel alloy cladding should be selected.

When Sodium Carbonate Carryover Lowers Causticizing Efficiency Below 75%

When sodium carbonate carryover is encountered in a closed kraft recovery cycle, the liquor must be re-causticized with lime mud or blended with fresh NaOH before use. For a 5 kg NaOH batch used as a mill-scale model, blending with 10–20 g/L Na₂CO₃ equivalent can simulate high dead load conditions; however, TAPPI T 624 cm-11 and TAPPI T 625 cm-85 should be run in triplicate because the carbonate endpoint in alkaline sulfide solutions is pH 8.3 and the hydroxide endpoint is pH 4.5. A liquor with 75% causticizing efficiency and 30% sulfidity is on the edge of acceptable mill operation and should not be used for final kappa optimization unless the carbonate is corrected. The limiting parameter for storage is not alkali strength but sulfide stability: the sodium sulfide component is subject to oxidation, and the free hydroxide component can absorb carbon dioxide from air to form sodium carbonate. In a 100 L batch exposed to air for more than 8 h, carbonate can increase by 0.1–0.3 g/L Na₂O depending on air exchange and surface area; this is small in absolute terms but affects causticizing efficiency calculations. The batch should therefore be sealed, purged, and analyzed immediately before digester injection.