On a single-line bleached kraft fiberline producing 800–1,200 ADt/day, the distribution of alkali between white liquor makeup and oxidative extraction is governed by three linked constraints: the sodium-to-sulfur ratio in the recovery cycle, dead load accumulated as sodium carbonate and reduced-sulfur intermediates, and the terminal pH required for extraction-stage lignin removal. White liquor at the digester feed typically carries an active alkali of 90–120 g/L as Na2O with sulfidity of 25–35%, while the oxidized white liquor diverted to the bleach plant may contain less than 0.5 g/L residual sulfide but elevated thiosulfate. These values are determined by TAPPI T 624 and TAPPI T 625 analyses. Mills that overdraw unoxidized white liquor for bleach extraction without corresponding sulfur purge or saltcake adjustment induce sulfidity drift because the oxidized stream returns thiosulfate and sulfate to the recovery cycle rather than sulfide. The control problem is compounded by sodium carbonate accumulation in the recausticizing loop, which is reported by TAPPI T 624 as residual Na2CO3 in white liquor and is commonly observed at 15–30 g/L as Na2O. Published data for this specific mill configuration is limited, but the compositional bands are consistent with routine mill liquor audits.
Sulfidity, measured by TAPPI T 625, is the ratio of Na2S to NaOH plus Na2S on a Na2O basis, and it is maintained between 25% and 35% for most softwood kraft pulp grades because the hydrosulfide ion accelerates delignification without degrading cellulose at the same rate as hydroxide alone. The oxidation of a white liquor side stream for bleach extraction changes this ratio because the sulfide is converted according to the reaction 2 Na2S + 2 O2 + H2O → Na2S2O3 + 2 NaOH. The sodium hydroxide generated by this oxidation is beneficial for extraction, but the thiosulfate formed is a dead load species in the recovery cycle and does not regenerate hydrosulfide unless the recovery boiler smelt reduction is sufficiently deep. If the residual sulfide in oxidized white liquor remains above 0.5 g/L as Na2O, open extraction washers and filter hoods can release reduced sulfur gases with odour and occupational exposure implications. The sodium-to-sulfur ratio constraint becomes acute when oxidized white liquor is used as the sole alkali source in a two-stage oxygen delignification system: the effective alkali is tied to the hydroxide content of the oxidized liquor, while the thiosulfate and sulfate returns create a sulfur sink that must be balanced with saltcake or elemental sulfur addition at the recovery boiler.
Across a recausticizing line operating with a slaker discharge temperature of 95–104°C and a three-stage lime mud washing filter, causticizing efficiency is controlled between 78% and 85% by adjusting the lime-to-carbonate molar feed ratio. The governing reactions are CaO + H2O → Ca(OH)2 and Ca(OH)2 + Na2CO3 ↔ 2 NaOH + CaCO3. A green liquor total titratable alkali of 125–150 g/L as Na2O with a sodium carbonate dead load of 15–30 g/L is the central constraint on effective alkali because free hydroxide in the product liquor suppresses the forward causticizing equilibrium. Operation below the slaker temperature window leaves unreacted calcium oxide that can armour the causticizer mixing elements, while operation above 104°C can produce steam flashing and reduce slaker residence time. White liquor suspended solids are typically held below 50 mg/L by pressure filters or clarification, and excursions above this band accelerate scaling in digester heaters and extraction stage heat exchangers. Field audits show that a 5 g/L increase in white liquor effective alkali without a corresponding lime mud purge can lower causticizing efficiency by 1.5–2.5 percentage points, although published data for this specific configuration is limited.
Dead load is the difference between total titratable alkali and active alkali and consists primarily of sodium carbonate, sodium sulfate, sodium thiosulfate, chloride, and potassium. Sodium carbonate forms when the causticizing reaction does not go to completion or when black liquor combustion generates carbonate species; sodium sulfate persists when recovery boiler reduction efficiency falls below 90%; sodium thiosulfate originates from white liquor oxidation and from incomplete smelt reduction. Chloride and potassium enter with wood furnish, purchased caustic, and makeup chemicals, and they are not destroyed in the recovery cycle. A white liquor chloride concentration above 5–8 g/L is commonly associated with electrostatic precipitator deposits and smelt bed instability, while potassium above 3–5 g/L modifies the melting behaviour of recovery boiler smelt and can reduce bed stability. Because these species accumulate in the liquor loop, the only practical control mechanisms are purge points such as lime mud, dregs, grits, and electrostatic precipitator ash. The composition bands used for routine alkali charge decisions are summarised in the following matrix.
| Parameter | Typical control range | Standard method | Process consequence |
|---|---|---|---|
| Active alkali as Na2O | 90–120 g/L | TAPPI T 624 | Sets digester cooking alkali charge and delignification rate. |
| Effective alkali as Na2O | 75–105 g/L | TAPPI T 624 | Controls residual hydroxide after sulfide compensation. |
| Sulfidity | 25–35% | TAPPI T 625 | Determines Na2S/NaOH ratio, pulp yield, and selectivity. |
| Causticizing efficiency | 78–85% | TAPPI T 624 | Residual Na2CO3 dead load and lime consumption. |
| Residual Na2CO3 as Na2O | 15–30 g/L | TAPPI T 624 | Dead load in recovery and recausticizing loops. |
During periods of high hardwood kraft production with elevated hexenuronic acid, extraction-stage alkali demand is not linear with kappa number. The acidic intermediates released during oxygen delignification consume sodium hydroxide before the extraction pH can rise above the target threshold, and the oxidation of hexenuronic acid generates additional carboxyl groups that buffer the liquor at lower pH. A medium-consistency Eop tower at 10–12% pulp consistency and 70–85°C is typically fed with oxidized white liquor at an alkali charge of 1.5–3.5% NaOH on OD pulp. If the terminal extraction pH falls below 10.5, chromophoric lignin fragments remain bound to the fiber, and the subsequent D0 stage chlorine dioxide demand increases. If the terminal pH rises above 11.8, alkaline oxidative chain scission accelerates and ISO 5351 viscosity falls, especially in softwood grades. Thus the extraction alkali charge is a threshold-controlled variable rather than a linear adjustment.
When the extraction-stage alkali demand exceeds the buffering capacity of oxidized white liquor, the mill must either split the alkali addition with purchased sodium hydroxide or accept a terminal pH depression that increases bleaching chemical consumption downstream. Oxidized white liquor contains sodium hydroxide, sodium thiosulfate, and residual sodium carbonate. The latter two species do not provide the same neutralization capacity as hydroxide per mole of sodium, so the effective alkali available to neutralize acid groups generated in oxygen delignification is lower than the total sodium concentration suggests. In a medium-consistency two-stage oxygen delignification system operating at 0.4–0.8 MPa oxygen partial pressure and 90–105°C, the total alkali charge is divided between the first and second reactors to prevent local pH depletion. A production-scale failure mode occurs when the white liquor oxidation air valve does not track the extraction-stage flow demand: oxidized liquor with residual sulfide enters the Eop tower, and the resulting hazard is not only odour but also product quality variability because thiosulfate generation is time-dependent. The control matrix below summarises the extractive oxidation constraints.
| Control parameter | Test method | Control band | Consequence of deviation |
|---|---|---|---|
| Terminal extraction pH | TAPPI T 252 | 10.8–11.5 | Below 10.5 leaves chromophoric lignin fragments unextracted; above 11.8 accelerates viscosity loss. |
| Extraction stage alkali charge | TAPPI T 624 for liquor strength and mill flow integration | 1.5–3.5% NaOH on OD pulp | Low charge depresses terminal pH; high charge raises yield loss and recovery sodium load. |
| O2 partial pressure | Plant distributed control system pressure transmitter, calibration per mill ISO 9001 procedure | 0.4–0.8 MPa | Below band limits oxidative extraction; above band increases oxygen delignification and viscosity loss. |
| Post-Eop kappa number | ISO 302:2015 | 6–10 | Higher kappa increases D0 chlorine dioxide demand; lower kappa may indicate excessive carbohydrate degradation. |
| Pulp viscosity | ISO 5351:2010 | 650–850 mL/g | Below 650 mL/g on market softwood indicates strength loss; above band is acceptable but not always required. |
In bleach plants operating with alkaline filtrate recycle, the sodium/sulfur closure constraint is directly influenced by extraction-stage washer carryover and the split between D0 and Eop filtrates. Alkaline filtrate from the Eop washer contains sodium hydroxide, carbonate, and dissolved organic matter. If this stream is returned to brownstock washing, it displaces fresh caustic and improves sodium recovery, but it also carries chloride and organic acids back into the fiberline. Sodium recovery from bleach plant alkaline filtrate may range from 60–85% depending on the degree of segregation, but published data for this specific configuration is limited because fiberline integration differs from mill to mill. The limiting boundary is usually chloride: bleach plant filtrate picks up chloride from wood furnish and from chlorine dioxide decomposition, and excessive recycle can raise white liquor chloride above the mill-specific threshold of 5–8 g/L. Mills operating with closed filtrate loops must therefore monitor chloride in white liquor by ion chromatography and purge a side stream from lime mud or electrostatic precipitator ash when the threshold is approached.
The choice among saltcake, soda ash, caustic, and elemental sulfur is a mass-balance constraint, not an operating preference. Saltcake adds two moles of sodium per mole of sulfate and contributes sulfur; caustic adds sodium only; elemental sulfur adds sulfur only. For a bleached kraft mill producing 1,000 ADt/day, sodium losses from dregs, grits, lime mud purge, and stack particulates are commonly reported in the range 5–15 kg Na2SO4 equivalent per ADt, while sulfur losses range from 3–8 kg S per ADt, depending on wood species and equipment condition. These loss rates are not universal; each mill requires a site-specific sodium-sulfur balance audit. The constraint is that over-addition of saltcake to cover sodium losses can push sulfidity above 35%, increasing digester corrosion risk and changing delignification selectivity; under-addition can allow sulfidity to fall below 20%, slowing softwood delignification and lowering effective alkali. Recovery boiler dead load increases as sulfate, thiosulfate, chloride, and potassium accumulate, and the black liquor heating value declines accordingly. Published data for this specific configuration is limited, but the direction of the mass flow is well established.
Oxidized white liquor is segregated from acidic D0 filtrate before the extraction stage because acidification of thiosulfate-laden liquor releases sulfur dioxide and precipitates elemental sulfur, causing scaling in filtrate transfer piping and odour release. The storage temperature of oxidized white liquor is maintained below 70°C to reduce evaporative steam loss and to slow the further oxidation of thiosulfate to sulfate, which would increase dead load. Oxidized white liquor with residual sulfide above 0.5 g/L is not routed to open Eop washers without local ventilation upgrades. These operational boundaries constrain the maximum diversion of white liquor to bleach extraction and are applied independently of the digester alkali charge.
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