Green Liquor (Kraft Recovery Smelt-Dissolved Intermediate) Storage — Na2CO3 + Na2S Pre-Causticizing Solution Tank Selection
Green Liquor (Kraft Recovery Smelt-Dissolved Intermediate) Storage — Na2CO3 + Na2S Pre-Causticizing Solution Tank Selection at Integrated Kraft Pulp and Paper Mill Recovery Loops
Green liquor is the intermediate cooking-liquor stage of the kraft chemical recovery cycle: an aqueous solution of sodium carbonate (Na2CO3) and sodium sulfide (Na2S) produced by dissolving the molten reduced smelt from the recovery-boiler floor in weak wash water inside the dissolving tank. The solution gets its name from the characteristic green tint imparted by trace iron + manganese sulfide colloids carried over from the smelt + dissolving stage. Typical green liquor at the clarifier feed runs 100-130 g/L total titratable alkali (TTA, expressed as Na2O equivalent) with sulfidity 25-35% (Na2S as a fraction of active alkali), residual smelt-carryover dregs (iron + manganese + silicate insolubles) at 100-2,000 mg/L before clarification, and trace sodium thiosulfate + sodium chloride + sodium sulfate dead-load. Operating temperature 80-100°C through the dissolving tank + green-liquor storage + clarifier + slaker feed; bulk pH 12.5-13.5; density at delivery temperature 1.10-1.18 g/cm3.
Green liquor is the upstream feed to the recausticizing chain. The dissolving tank operates directly under the recovery-boiler smelt spout with steam-jet shatter dispersion that fractures the molten smelt into droplets as it contacts dilute weak wash; the resulting agitated suspension is gravity-fed to the green liquor storage tank for buffering + dregs settling. Green liquor is then clarified (modern Andritz LXTE pressure-filter or conventional gravity-clarifier) to remove residual smelt dregs + colloidal iron-manganese-sulfide solids before the clarified liquor is reacted with quicklime (CaO) in the slaker + causticizer chain. The slaker hydrolyzes the lime to calcium hydroxide which reacts with sodium carbonate to convert it to sodium hydroxide while precipitating calcium carbonate (lime mud); the result is white liquor for digester feed plus lime mud for re-calcination in the lime kiln. Green liquor is therefore the equilibrium-buffer + dregs-removal stage between the recovery boiler and the white-liquor production chain.
The eight sections below cite Andritz (continuous-digester + recausticizing-system + green-liquor pressure-filter OEM), Valmet (recovery-boiler + evaporator + dissolving-tank OEM, formerly Metso Paper), TAPPI Press technical references for kraft chemistry + recausticizing-loop design, BLRBAC (Black Liquor Recovery Boiler Advisory Committee) Recommended Good Practice, and operating mill specifications from International Paper, WestRock, Georgia-Pacific, Domtar, Resolute Forest Products, and Suzano (Brazil) for green-liquor storage, dregs-handling, and tank-selection practice at modern kraft installations.
1. Material Compatibility Matrix
Green liquor at delivery condition (80-100°C, 100-130 g/L TTA, sulfidity 25-35%, residual dregs 100-2,000 mg/L pre-clarifier or under 50 mg/L post-clarifier) is a hot moderately-strong alkaline + sulfide solution with abrasive carryover at the dissolving + pre-clarifier stages. Material selection mirrors white-liquor service with additional erosion + abrasion considerations at the dissolving-tank + dregs-handling sections.
| Material | Hot Green Liquor 80-100°C | Cooled / Buffered Storage | Notes |
|---|---|---|---|
| HDPE | C | B | Hot at over 80°C exceeds standard HDPE rating; acceptable for cooled buffer + dregs-collection day-tank service only |
| XLPE | B | A | Cross-linked structure improves hot performance somewhat; not industry standard at active dissolving + clarifier service |
| Polypropylene (PP) homopolymer | B | A | Acceptable for piping + lined-tank trim at lower-temperature service; limited at hot dissolving-tank + clarifier overflow |
| Polypropylene (PP) copolymer | B | A | Slight improvement at hot service over homopolymer |
| Carbon steel (A516-70 / A36) | A | A | Industry-standard at all green-liquor + dissolving-tank + clarifier construction; corrosion rate under 5 mpy at typical operating chemistry |
| 304 stainless | A | A | Acceptable; chloride pitting risk at high-chloride mills (recovery-cycle dead-load buildup) |
| 316L stainless | A | A | Standard for high-velocity + erosion-prone trim at dissolving-tank shatter-jet + dregs-handling |
| 2205 / duplex stainless | A | A | Premium for high-stress + high-chloride + erosion-prone dregs-handling sections |
| FRP vinyl ester (Derakane 411 / 470) | B | A | Acceptable at lower-temperature service under 80°C; thermal degradation accelerates at hot service |
| FRP isophthalic polyester | NR | NR | Caustic resin attack; never specified for kraft liquor service |
| EPDM | A | A | Standard gasket + O-ring at hot caustic + sulfide service |
| Chlorobutyl-lined steel | A | A | Historical tank + pump lining at heavy-industrial mills; abrasion-resistant at dissolving-tank + dregs-handling |
| Buna-N (Nitrile) | NR | C | Caustic + sulfide attack; never specified |
| Viton (FKM) | C | B | Hot caustic attack; not industry-standard at hot liquor service |
| Aluminum | NR | NR | Severe caustic attack; never specified at any kraft liquor service |
| Concrete (epoxy-novolac coated) | A | A | Standard at secondary-containment pits + spill-control basins; not for primary tank construction |
The dominant industrial pattern at modern kraft mills is bare carbon-steel API 650 atmospheric storage at green-liquor buffer + clarified-storage tanks with 316L stainless cladding or chlorobutyl-rubber lining at high-erosion sections (dissolving-tank shatter-jet inlet + dregs-handling underflow + clarifier rake mechanisms), 304L or 316L stainless piping at the green-liquor delivery loop, EPDM or chlorobutyl gasket at all flange + pump-seal interfaces, and 316L vertical-turbine submersible pumps at clarifier-underflow + dregs-collection day-tank service. HDPE rotomolded tanks find use at the dilute side of the cycle: dregs-washing dilution-water tanks, clarifier-overflow polishing receiver, dilution-water makeup buffer, and recovered-condensate stripping receivers at room or near-room temperature with dilute alkalinity (under 5 g/L NaOH eq).
2. Real-World Industrial Use Cases
Recovery-Boiler Smelt Dissolution. The integrated kraft mill recovery boiler reduces the inorganic fraction of concentrated black liquor (65-80% solids) under reducing-zone combustion to a sodium-carbonate + sodium-sulfide molten smelt that flows from the boiler floor through smelt spouts into the dissolving tank. Smelt temperature 800-900°C; smelt flow 30-150 t/hr at modern recovery boilers feeding 1,500-3,500 ADt/day pulp lines; dissolving-tank capacity 30-90 m3 with shatter-jet steam dispersion + agitator mixing. Modern installations include shatter-jet redundancy + smelt-flow continuous monitoring + boiler-side smelt-temperature interlocks per BLRBAC Recommended Good Practice + ANSI / NFPA 85 Boiler and Combustion Systems Hazards Code.
Green Liquor Storage and Buffering. Green-liquor storage at modern bleached-kraft mills runs 1,000-3,000 m3 (300,000-800,000 gallons) at carbon-steel API 650 atmospheric construction with insulation + heat-tracing maintaining 80-95°C operating temperature, gentle agitation to maintain dregs suspension during clarifier feed-rate balancing, dual primary + redundant overflow weirs, and tank-mounted radar or differential-pressure level instrumentation. Tank-roof vapor-space connection to mill non-condensable gas (DNCG) collection header captures TRS + carryover for thermal-oxidation incineration in the lime kiln or dedicated NCG burner.
Green Liquor Clarification or Pressure Filtration. Conventional gravity clarification at older mills uses a mechanical-rake clarifier 15-25 m diameter at hydraulic loading 0.5-1.5 m3/m2/hr with dregs underflow at 5-15% solids for further dewatering at the dregs filter. Modern installations replace the clarifier with the Andritz LXTE pressure filter (Liquid Polishing Pressure Filter) which combines clarification + filtration + dregs-cake-discharge in a single skid producing crystal-clear green liquor with under 50 mg/L residual TSS. The LXTE pattern reduces capital footprint + operator burden + sees dominant share at greenfield + capacity-expansion projects from 2010 forward.
Dregs Filter and Mud Disposal. Green-liquor dregs (concentrated iron + manganese sulfide + silicate + carbonate insolubles, 5-15% solids underflow) are washed + dewatered at a dedicated dregs filter (vacuum drum or pressure plate-and-frame) producing dregs cake at 30-50% solids. Dregs cake disposal varies by mill + state environmental permit: landfill at non-hazardous solid-waste classification (most US mills); land-application as soil amendment at limited-permit installations; cement-plant-coprocessing or recovery-boiler-recirculation at integrated-vendor partnership programs. Filtrate weak-wash from the dregs filter recycles to the dissolving tank.
Slaker + Causticizer Feed. Clarified green liquor flows to the slaker + causticizer chain where reaction with quicklime converts sodium carbonate to sodium hydroxide. Standard slaker capacity 50-150 m3/hr at typical bleached-kraft mill scale; causticizer chain 3-5 stirred-tank reactors in series providing 60-90 minutes residence time at 95-105°C for causticizing-equilibrium approach. Modern installations achieve causticizing efficiency 78-85% at the white liquor pressure filter feed.
Smelt-Water Explosion (SWE) and Recovery-Boiler Hazards. The dissolving tank + smelt spout chemistry carries the dominant safety hazard of the kraft recovery cycle: introduction of water (rain, condensate, weak-wash backflow, tube-leak feedwater) onto molten smelt at 800-900°C causes flash steam generation + violent mechanical disruption of the boiler floor + dissolving-tank vapor space. BLRBAC Recommended Good Practice requires Emergency Shutdown Procedure (ESP) installation, automatic ESP actuation at boiler tube-leak detection + dissolving-tank water-ingress detection, and operator training + drill cadence at all member mills.
3. Regulatory Hazard Communication
OSHA HazCom GHS Classification. Green liquor mill product carries hazard statements similar to dilute-caustic + sulfide chemistry: H290 May Be Corrosive to Metals Category 1, H314 Causes Severe Skin Burns and Eye Damage Category 1B, H318 Causes Serious Eye Damage Category 1, H335 May Cause Respiratory Irritation Category 3, plus EUH031 Contact with acids liberates toxic gas (hydrogen sulfide release at acidification). Mill operators handling green-liquor sample draw + tank-cleaning + emergency-spill cleanup require chemical-resistant PPE and at confined-space entry require hydrogen-sulfide gas monitoring + ventilation per OSHA 29 CFR 1910.146 + 29 CFR 1910.1000 H2S 8-hour TWA 10 ppm + 15-minute STEL 15 ppm.
NFPA 704 Diamond. Health 3 (severe corrosive + eye + skin burn hazard plus H2S release on acidification), Flammability 0, Instability 0, Special hazard COR (corrosive).
DOT and Shipping. Green liquor is generated + consumed inside the integrated mill recausticizing loop and is not transported off-site as a commercial commodity.
EPA Regulations. Kraft mill recovery + recausticizing wastestreams are subject to EPA Effluent Guidelines for Pulp + Paper + Paperboard Manufacturing 40 CFR Part 430, Subpart B Bleached Papergrade Kraft + Soda + related subparts. The Pulp + Paper Cluster Rule (1998) integrated air + water effluent + best-management-practice requirements at bleached kraft + papergrade sulfite mills under 40 CFR Part 63 Subpart S NESHAP for Pulp + Paper + Paperboard Industry. Specific limits cover BOD5, TSS, AOX, chlorinated phenolics, dioxins + furans (TCDD/TCDF), and pH at the mill final discharge. Recovery + recausticizing-cycle emissions (recovery-boiler + dissolving-tank + lime-kiln stack + non-condensable gases) are managed under MACT subpart-S best-controls + concentrated-non-condensable-gas (CNCG) + dilute-non-condensable-gas (DNCG) collection + incineration in the lime kiln or dedicated thermal oxidizer.
RCRA + Spill Reporting. Concentrated kraft liquor spills are managed under EPA RCRA general corrosivity D002 (pH above 12.5) characteristic. Significant spills to surface water trigger Clean Water Act Section 311 + CERCLA Section 103 reporting at sodium hydroxide reportable quantity 1,000 lb (or equivalent caustic-potash-conversion). Mills maintain Spill Prevention Control + Countermeasure (SPCC) plans under 40 CFR Part 112 covering chemical + recovery + bleaching tank farms. Confined-space entry into liquor + smelt + dissolving + clarifier tanks requires 29 CFR 1910.146 permit-required entry program with H2S + low-oxygen monitoring + ventilation + standby attendant.
Solid-Waste Classification. Green-liquor dregs cake disposal varies by state environmental permit; most US installations classify at non-hazardous solid waste D-listed by absence of RCRA characteristic toxicity (TCLP under regulatory limits for As, Ba, Cd, Cr, Hg, Pb, Se, Ag) and non-corrosivity (cake pH 8-10 at typical washed condition). Dredged-cake disposal under state-specific landfill or land-application permit is standard.
BLRBAC Recommended Good Practice. The Black Liquor Recovery Boiler Advisory Committee (BLRBAC) publishes industry-consensus Recommended Good Practice for recovery-boiler design + operation + maintenance + emergency-shutdown including the ESP protocol for smelt-water-explosion (SWE) prevention. Member mills (representing the substantial majority of US + Canada bleached-kraft + unbleached-kraft pulp mill operators) commit to ESP-actuation-system installation + maintenance + operator-training-drill cadence as condition of mutual-information-sharing membership.
4. Storage System Specification
Dissolving Tank. The dissolving tank is a heavy-walled carbon-steel + stainless-clad open-top vessel directly under the recovery-boiler smelt spouts with internal capacity 30-90 m3. Construction includes shatter-jet steam-injection redundancy at multiple smelt spouts, internal agitator (top-mounted at large mills; side-mounted at smaller installations), 316L stainless or chlorobutyl-rubber-lined wear surfaces at smelt-droplet impact zones, integral overflow weir to green-liquor storage transfer line, vapor-space connection to non-condensable-gas collection header, and emergency-vent stack with vapor-arrester at boiler-side ESP actuation.
Green Liquor Storage Tank. Standard industrial construction is bare carbon-steel API 650 atmospheric storage in the 1,000-3,000 m3 range (300,000-800,000 gallons) with insulation + heat-tracing maintaining 80-95°C delivery temperature, gentle agitation to maintain residual dregs in suspension during feed-rate balancing, dual primary + redundant overflow weirs, and tank-mounted radar or differential-pressure level instrumentation.
Green Liquor Clarifier or LXTE Pressure Filter. Conventional gravity clarifier construction is welded carbon-steel cone-bottom or flat-bottom with 316L stainless cladding at rake mechanism + underflow scraper, tank diameter 15-25 m, sidewall depth 4-6 m. Modern Andritz LXTE pressure-filter installation is a packaged skid with carbon-steel + 316L pressure-vessel main shell, internal candle-element filtration media, automated cake-discharge cycle, and integration to clarified-liquor storage downstream.
Dregs Filter and Filtrate Receivers. Vacuum drum or pressure plate-and-frame dregs filter with 316L stainless construction + filter-cloth media; filtrate receiver at carbon-steel construction returning weak-wash to the dissolving tank. Dregs cake conveyor + load-out hopper at 316L construction + chlorobutyl-lined chute surfaces.
Secondary Containment. EPA SPCC + state environmental + mill-specific tank-farm regulations require secondary containment sized 110% of largest single tank capacity at all liquor + caustic + saltcake storage installations. Concrete-pit construction with epoxy-novolac coating is the dominant tank-farm pattern; HDPE rotomolded containment pans serve smaller installations.
Pump Selection. Magnetic-drive ductile-iron + 316L stainless centrifugal pumps with 316L wear surfaces and EPDM or chlorobutyl seal sets at green-liquor + weak-wash recirculation + transfer service. Vertical-turbine submersible pumps at clarifier underflow + dregs-collection sump. Slurry-rated centrifugal or progressive-cavity pumps with chlorobutyl-rubber-lined wear surfaces at dregs-handling service. Air-operated diaphragm pumps with EPDM diaphragm + EPDM check-valves at sample + drain + emergency-sump service.
5. Field Handling Reality
Operator PPE. Workers handling green-liquor sample draw + clarifier rake-arm inspection + dregs-handling tasks require chemical-resistant gloves (PVC, neoprene, butyl rubber), chemical splash goggles plus full-face shield, chemical-resistant apron + sleeves + boots, and at confined-space entry full-face APR or supplied-air respirator plus H2S + low-oxygen continuous monitor. Mill safety programs require pre-shift PPE inventory + decontamination shower + emergency-eyewash maintenance + annual confined-space + hot-work + lockout-tagout retraining per OSHA 29 CFR 1910.
Skin and Eye Hazards. Hot caustic + sulfide solution causes immediate severe skin burns + eye injury at any contact. Emergency eyewash + safety shower within 10 seconds travel time per ANSI Z358.1 are mandatory at all liquor sample + valve + transfer locations. First aid: immediate flush with copious water for 15 minutes minimum + medical attention for any deep or extensive contact.
H2S and TRS Atmosphere. Green liquor + dissolving-tank + clarifier vapor space + concentrated-non-condensable-gas (CNCG) header carry hydrogen sulfide + methyl mercaptan + dimethyl sulfide + dimethyl disulfide (collectively Total Reduced Sulfur, TRS) at concentrations 100-10,000 ppm depending on stream + cycle position. H2S is acutely toxic at 100-300 ppm + immediately dangerous to life and health (IDLH) at 100 ppm per NIOSH; the chemistry is the dominant pulp-mill fatality risk at confined-space + emergency-response + tank-cleaning operations.
Smelt-Water Explosion (SWE). The dominant catastrophic hazard at the dissolving tank is the SWE event: water ingress (rain, condensate, weak-wash backflow, recovery-boiler tube-leak feedwater) onto molten smelt at 800-900°C causes flash steam generation + violent mechanical disruption. BLRBAC Recommended Good Practice mandates Emergency Shutdown Procedure (ESP) installation + automatic actuation + operator training + drill cadence at all member mills. Operator situational awareness of weather (rain water-ingress risk), tube-leak indicators (sudden steam-drum-level + makeup-water-flow-rate + furnace-pressure changes), and ESP-readiness verification is part of standard recovery-operator certification + ongoing training.
Spill Response. Green-liquor spill response: (1) evacuate area + activate emergency response + verify wind direction (H2S vapor-cloud risk on acidification + dilution), (2) PPE-equipped responders contain with sand or vermiculite, (3) avoid acid-bearing absorbents + acid-decontamination chemistry until H2S vapor cleared + extensive water dilution achieved, (4) collect liquid + solid + decontamination-rinse waste into mill weak-wash sump for processing through normal recovery loop, (5) document spill volume + decontamination + mill HSE-incident reporting under EPA RCRA + EPCRA + CWA 311 reportable quantities (sodium hydroxide RQ 1,000 lb).
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