pH 12 Alkaline Extraction Stage Chloride Management in Kraft Pulp

In a modern elemental chlorine-free kraft bleach plant, the alkaline extraction stage following the initial chlorine dioxide delignification tower operates at pH 11.8–12.2 and 70–85°C to solubilize oxidized lignin fragments, hydrolyse residual chlorinated organic material, and promote additional delignification through oxygen-reinforced alkali. Chloride ion enters this stage through multiple convective and internal chemical pathways: seawater-saturated coastal chips can carry 0.5–1.5 kg Cl⁻/adt; sodium hydroxide makeup may contain 0.02–0.2 wt% NaCl; chlorine dioxide generator carryover, recycled bleach-plant filtrates, and mill fresh water contribute additional loads that typically sum to 3–8 kg Cl⁻/adt across the fibre line. Because chloride is a conservative anion under the oxidizing alkaline conditions of the E-stage, it does not form insoluble precipitates with sodium, calcium, or magnesium at the concentrations normally encountered, and it is not volatilized in the E-stage vent gases. The chloride therefore distributes between the pulp mat and the filtrate according to the liquor-to-fibre ratio and the displacement washing efficiency. In closed-cycle bleach plants, E-stage shower water drawn from filtrate tanks may exhibit chloride concentrations between 150 mg/L and 900 mg/L, while the corresponding pulp mat chloride can reach 0.1–0.4 kg Cl⁻/adt depending on final wash loss. This concentration range defines both the corrosion risk for wetted metal surfaces and the purity requirement for final bleached pulp sold to dissolving-grade or food-contact converters.

The extraction stage chemistry at pH 12 is dominated by base-catalyzed hydrolysis of chlorinated lignin fragments and by oxygen-driven free-radical degradation of residual chromophores. Hydrogen peroxide, when added as an EOP reinforcement, generates perhydroxyl anion that participates in nucleophilic substitution and ring-opening reactions; these reactions release chloride from aliphatic and aromatic chlorinated intermediates. The conversion of organically bound chlorine to inorganic chloride in a conventional O₂-reinforced E-stage typically ranges from 40% to 60% of the AOX present in the incoming pulp, but the exact split depends on incoming kappa number, temperature, oxygen partial pressure, and retention time. Because the alkali charge is usually 15–25 kg NaOH/adt and the consistency is 10–12%, the liquid phase is a concentrated electrolyte, and the ionic strength suppresses the activity of carbonate and sulfate ions. Chloride contributes to this ionic strength without being consumed, so chloride entering with the pulp or alkali exits either with the washed pulp or with the filtrate. The partition coefficient for chloride between retained liquor and displacement liquor approaches unity after sufficient contact time, which means the washing system rather than the chemical reaction system governs the steady-state chloride inventory.

Why Does Chloride Accumulate in the pH 12 Extraction Stage?

The accumulation of chloride in the E-stage filtrate loop is governed by the balance between chloride input and chloride purge. In a closed-cycle fibre line, the E-stage washer uses filtrate from its own or downstream stages as shower water; if the chloride concentration in the shower water rises, the mass of chloride carried back with the washed pulp increases proportionally. The steady-state concentration C in the E-stage filtrate tank can be described by a simple mass balance: C = (m_Cl,in + C_ps × Q_ps) / (Q_ps + Q_purge + Q_carry), where m_Cl,in is the incoming chloride mass flow from pulp and chemicals, C_ps is the chloride concentration in the pressate or shower recycle, and Q terms are volumetric flows. This equation illustrates the dominant process conflict: any reduction in purge flow to the effluent treatment system or any increase in shower recycle ratio raises the steady-state chloride concentration. Mill water conservation programs that reduce E-stage effluent from 1.5 m³/adt to 0.5 m³/adt can therefore increase filtrate chloride by a factor of two to three unless chloride is removed elsewhere. In a 1000 adt/d line operating with 4 kg Cl⁻/adt total input and an E-stage shower flow of 8 m³/adt, a purge of 1.0 m³/adt gives a theoretical steady-state chloride concentration of approximately 500 mg/L if all other losses are ignored; reducing purge to 0.5 m³/adt raises the calculated value to approximately 1000 mg/L. These are mass-balance reference points rather than measured plant data, but they define the operational window within which wash press metallurgy must be selected.

Chloride also accumulates because the extraction stage receives recycled filtrates from multiple sources. Acidic D-stage filtrate, alkaline E-stage filtrate, and neutral washer filtrate are often mixed in a common seal tank or buffer tank. If the combined filtrate is used for brown-stock washing showers, chloride that was purged from the bleach plant can be carried backward into the unbleached pulp storage chest and then re-enters the bleach plant. This cross-stage recycle loop is a common field-identified bottleneck; its severity is highest when the brown-stock washer discharge consistency exceeds 12%, because carryover liquor is enriched in dissolved solids. Operators typically monitor chloride in the brown-stock pulp ash and in the bleach-plant filtrate to identify such hidden recycle paths. TAPPI T 256 cm-07 and ASTM D512-23 provide laboratory methods for water-soluble chloride in pulp and in water, but these methods require sampling points that are representative of the actual shower header rather than the bulk tank after settling. On a modern fibre line, automated ion chromatography with sample preconditioning can provide chloride values at 15-minute intervals; the sampling loop must be purged with 5–10 volumes of filtrate to avoid static-line artefacts.

Because chloride is a ubiquitous contaminant in bleach-plant filtrates, analytical selection depends on sample matrix, required detection limit, and interference tolerance. The following methods represent the commonly accepted reference procedures for E-stage chloride control. Values are method detection limits or practical working ranges reported in the cited standards.

Chloride-Induced Corrosion and Scale Deposition Mechanisms

At pH 12, carbon steel is generally protected by a passive oxide film, but the presence of chloride produces localized breakdown of passivity, especially in crevices, under gasketed joints, at flanged connections, and in stagnant zones behind wash press screen plates. The pitting potential of austenitic stainless steel in oxygenated alkaline chloride solution decreases with increasing chloride concentration and temperature; cyclic potentiodynamic polarization tests conducted according to ASTM G61-86 are used to measure repassivation potential, while ASTM G48-11 Method A provides a ferric chloride exposure test that ranks alloys but does not reproduce the actual pH 12 environment. Type 316L stainless steel has a PREN calculated as %Cr + 3.3(%Mo) + 16(%N) of approximately 25 and is usually restricted to chloride concentrations below 200–500 mg/L at 75°C in oxygen-containing E-stage filtrate. Duplex alloy 2205 with a PREN of approximately 35 extends the tolerable chloride range to 1000–2000 mg/L, but weld heat tint, surface roughness, and crevice geometry can reduce this tolerance by half. Super duplex 2507 with a PREN of approximately 42 is specified for high-chloride E-stage filtrate tanks and shower headers, yet it remains susceptible to crevice corrosion at chloride concentrations above 2000 mg/L when the temperature exceeds 80°C and oxygen is present. Titanium Grade 2 has excellent general corrosion resistance in alkaline chloride but is not immune to crevice corrosion in oxygenated aqueous service; manufacturer corrosion tables frequently derate its maximum service temperature to 60–70°C when chloride exceeds 200 mg/L at pH 12 in gasketed joints.

For material selection, the chloride threshold is not a single value but a function of temperature, oxygen concentration, geometry, and surface condition. The following table consolidates typical engineering-recommended limits for E-stage service based on published alloy supplier data and referenced corrosion test methods.

Scale deposition in the E-stage at pH 12 is dominated by calcium carbonate, magnesium hydroxide, and fibre fines, but chloride influences deposition indirectly through ionic strength and conductivity. The calcium carbonate saturation index in E-stage filtrate often exceeds 1.5–2.5 when calcium input from wood and fresh water is high, leading to hard deposits on wash press filtrate baskets and doctor blades. Chloride does not form a separate crystalline scale, but increasing chloride concentration raises the conductivity of the liquid film inside the deposit, which accelerates underdeposit pitting of the metal substrate. The pressure drop across a wash press screen is a practical indicator of scale accumulation; a rise from a clean-condition baseline of 20–40 kPa to 80–120 kPa usually indicates the need for acid washing with inhibited sulfamic or formic acid. The removal of calcium carbonate scale is more difficult when the scale contains polymerized wood resin, which is typical in mills using softwood furnish with high extractives. In such cases, high-pressure hydroblasting at 70–100 MPa may be required before chemical cleaning. Because chloride accumulates in the scale pores, the cleaned surface must be passivated with alkaline solution before returned to service to avoid flash rusting of carbon steel components.

When Chloride Concentration Exceeds Process Tolerance in E-stage Washing

When chloride concentration in the E-stage shower water exceeds the corrosion threshold of the wetted metallurgy, the operating envelope narrows in three measurable ways. First, the maximum permissible shower temperature must be reduced to maintain the material below its pitting and crevice corrosion range; a shower header designed for 85°C operation with 2205 may need to be derated to 60–70°C if chloride rises from 500 mg/L to 1500 mg/L. Second, the washer dilution factor must be increased to maintain the same displacement efficiency, which increases filtrate volume and energy demand for subsequent evaporation or effluent treatment. Third, the carryover of chloride into the recovery cycle rises because the pulp mat exits the washer with a higher chloride concentration at constant wash loss. A displacement wash press operating at 10% discharge consistency and 2.5 m³/adt dilution factor can remove approximately 85–95% of the incoming soluble chloride when the shower water chloride is below 50 mg/L; when the shower water chloride is 500 mg/L, the calculated removal efficiency falls to 60–75% because the concentration gradient across the mat is reduced. Published data for specific washer types is limited, but the general behaviour is consistent with standard displacement washing theory used in sizing twin-roll presses and EPDM-lined medium-consistency pumps.

The operational response to high chloride in the E-stage is therefore forced by the wash press hydraulics. A twin-roll press operating at a vat consistency of 2–4% and a nip pressure of 2–4 MPa requires a shower flow sufficient to displace entrained liquor from the web; if the shower chloride is high, the displaced liquor recontaminates the downstream seal tank. The chloride concentration in the seal tank then determines the concentration in the next stage's wash liquor, creating a cascading effect through the final D-stage and peroxide stages. To break this cascade, mills segregate the high-chloride E-stage filtrate from low-chloride final wash filtrates and direct the contaminated stream to the chemical recovery system or to a dedicated chloride removal step. A chlorine dioxide stage final washer using clean water with <20 mg/L chloride can produce pulp with water-soluble chloride below 50 mg/L even when the E-stage filtrate chloride exceeds 800 mg/L, provided the washer discharge consistency is maintained at 12% or higher and the wash factor is at least 1.5 times the theoretical displacement requirement.

Managing Chloride Partitioning Across the Brownstock Washing Line

Managing chloride at the extraction stage requires a systems approach that begins before the bleach plant. Brownstock washing determines the quantity of dissolved solids, including chloride, that enters the first chlorine dioxide stage. If the brownstock washer line discharges pulp with a carryover of 20–40 kg dissolved solids/adt, the chloride load entering the bleach plant may be 0.5–2.0 kg Cl⁻/adt even when the wood supply is low-chloride. Improving brownstock washing from a soda loss of 15 kg Na₂SO₄/adt to 5 kg Na₂SO₄/adt reduces the bleaching chemical demand and the chloride load simultaneously, but the capital cost of an additional washing stage must be justified by recovery boiler tube life and bleach-plant corrosion maintenance. In mills with seawater-logged chips, chip pile drainage and chip washing can remove a substantial fraction of the chloride before the fibre reaches the digester; high-pressure chip washing at 0.5–1.0 m³/t reduces chip water-soluble chloride by 50–70%, but the effluent is saline and must be managed separately.

Within the bleach plant, chloride partitioning is controlled by the configuration of filtrate recycle loops. The E-stage filtrate is the most contaminated stream and should not be used as shower water for the final bleach washer or for the brownstock washing line unless chloride removal is installed. The standard segregation strategy is to use E-stage filtrate only for the D0-stage washer or for the first brownstock washer after the digester, while the final D1 or P-stage washer uses fresh water or condensate. This creates a countercurrent flow of chloride toward the recovery cycle, but it also increases the chloride concentration in the black liquor. The recovery boiler then becomes the ultimate chloride purge point, because sodium chloride is volatile at recovery boiler temperatures and is captured in the electrostatic precipitator ash. Electrostatic precipitator ash from a closed-cycle mill may contain 1–5 wt% chloride as NaCl, and this ash is often purged from the cycle by dissolution in weak wash and discharge to the aerated lagoon. The purge must be balanced against sodium and sulfur losses; a mill with a high chloride input may require a purge of 0.5–1.5 kg NaCl/adt from the precipitator ash to maintain black liquor chloride below 1.0 wt% on dry solids.

When purge capacity is constrained, chloride removal from E-stage filtrate can be achieved by membrane processes, but the alkaline pH and high temperature require careful softening and pH adjustment. Nanofiltration membranes with a molecular weight cut-off of 150–300 Da can reject 30–60% of chloride while retaining larger organics, but the permeate is not sufficiently chloride-free for reuse in the final washer. Reverse osmosis can reject 90–99% of chloride when the feed is softened and cooled to 25–35°C; however, the concentrated reject stream still requires a purge destination. Electrodialysis with monovalent-selective membranes has been piloted for bleach-plant filtrates, achieving chloride removal of 60–90% in published process development reports, but stack fouling from precipitated calcium and fibre fines remains a limitation. Ion exchange with strong-base anion resin in chloride form is generally not economical for such high chloride loads because the regeneration brine would be very saline. The most common industrial practice remains purge through the recovery cycle and precipitator ash, with chemical recovery performance monitoring by black liquor chloride titration using ASTM D512-23 or ion chromatography.

The final bleached pulp chloride specification is determined by the end use. Dissolving pulp for cellulose acetate and viscose production often requires water-soluble chloride below 50–100 mg/kg on pulp, while bleached softwood kraft for food-contact board may require chloride below 200 mg/kg to avoid corrosion of downstream converting equipment. The pH 12 extraction stage is the primary chloride removal point because the high pH keeps chloride in the aqueous phase and the subsequent wash efficiently displaces it; therefore, final pulp chloride is more sensitive to washer performance than to the extraction stage chemistry itself. A final washer with a dilution factor of 1.5–2.5 m³/adt and shower water chloride below 20 mg/L typically achieves a final pulp chloride of 50–150 mg/kg, while shower water chloride of 200 mg/L can produce a final pulp chloride above 400 mg/kg even with the same wash factor. These values are calculated from displacement washing mass balances and should be verified with TAPPI T 256 cm-07 extraction.

Field observations on E-stage washers identify the filtrate distribution bar, the doctor blade holder, and the seal tank agitator as the most frequent chloride-related failure points. On a twin-roll press, the centre rolls are made from cast duplex stainless steel or have a welded overlay; the overlay transition zone at the roll edges is susceptible to chloride pitting because of dilution and residual ferrite. A typical failure signature is a series of pits 0.5–2.0 mm in diameter aligned along the weld toe, discovered during annual shutdown inspection. The repair procedure requires grinding to sound metal, dye penetrant inspection according to ISO 3452-1, and re-passivation with 20–30% nitric acid at 40–50°C for 20–30 minutes. If the pits are not removed before restart, the chloride-rich scale remaining in the pit accelerates crevice corrosion, and the roll can fail within 6–12 months. This operational failure mode has driven many mills to replace the original 316L centre roll overlay with 2205 or 2507 during the first major maintenance window.