Green Liquor Storage & Tank Compatibility

Storing Green Liquor? Start Here

Green liquor is an intermediate process stream in the kraft (sulfate) chemical-recovery cycle of a pulp mill. It is formed when the molten smelt from the recovery boiler — mostly sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S) — is dissolved in water (weak wash) in a dissolving tank. The resulting liquid takes its characteristic green color from colloidal ferrous sulfide (FeS) and carries suspended ‘dregs’ (unburned carbon plus calcium and iron compounds) that are removed in a green-liquor clarifier. Sodium carbonate is the dominant dissolved salt, with sodium sulfide and a little residual caustic making the liquor strongly alkaline (pH about 12–13). Green liquor is then causticized with lime to convert the carbonate to sodium hydroxide, regenerating white liquor for cooking. Material of construction matters because this hot, high-pH, sulfide-bearing, dregs-laden stream actively corrodes carbon steel and rules out common tank plastics — steering storage toward stainless steel.

Is Green Liquor Compatible with Polyethylene (HDPE / XLPE) Tanks?

No — polyethylene is not the right choice for green liquor. The deciding factors are service temperature combined with strong alkalinity. Polyethylene resists carbonate and caustic at ambient conditions, but published tank guidance limits poly alkaline storage to roughly 60°C (140°F), with many vendors recommending solution temperatures well below that for long-term service. Green liquor, however, is generated hot in the smelt-dissolving tank and handled warm through the clarifier and storage circuit, which pushes it past polyethylene's safe operating window, where the resin softens and loses strength. Add a strongly alkaline carbonate/sulfide chemistry (pH ~12–13), sulfide that can react with any acid contact, and abrasive dregs, and HDPE/XLPE is the wrong tank. Industry practice for green-liquor dissolving, clarifying, and storage is austenitic stainless (304L/316L) or duplex stainless, which show very low corrosion rates in kraft liquors. Carbon steel is also unsuitable here — it corrodes actively in green liquor. Always confirm the exact temperature and composition against the supplying mill's SDS.

Material compatibility at a glance

Green liquor is handled in austenitic (304L/316L) or duplex stainless steel because it is a hot, strongly alkaline carbonate/sulfide stream that actively corrodes carbon steel and carries abrasive dregs. Polyethylene is unsuitable: while ambient alkali is poly-compatible, green liquor's elevated service temperature combined with carbonate/sulfide/residual caustic exceeds HDPE/XLPE's thermal and chemical envelope.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Strongly alkaline (pH ~12–13): causes severe skin burns and serious eye damage — wear chemical goggles, face shield, and caustic-rated gloves and apron.
• Handled HOT (generated from molten smelt in the dissolving tank): thermal-burn and scald hazard on top of the chemical-burn risk; insulate and guard hot lines and vessels.
• Contains sodium sulfide: contact with acids can liberate toxic hydrogen sulfide (H₂S) gas — never mix with acidic streams; ensure ventilation and H₂S monitoring.
• Smelt-water contact during dissolving is a documented explosion hazard in kraft recovery — follow safe dissolving-tank operating procedures.
• Corrosive to carbon steel, aluminum, and copper alloys; specify stainless steel and avoid mixed-metal fittings; dregs are abrasive to pumps and linings.
• Always work from the supplying mill's Safety Data Sheet — alkali strength, sulfidity, and temperature vary by mill and by clarified vs. unclarified stream.

Common questions

Can I store green liquor in a poly (HDPE or XLPE) tank?

No. Although polyethylene tolerates carbonate and caustic at room temperature, green liquor is generated and handled hot from the smelt-dissolving tank, exceeding poly's ~60°C / 140°F alkaline service limit. Its sulfide chemistry and abrasive dregs add further risk. Use stainless steel (304L/316L) or duplex stainless, and confirm conditions against the SDS.

What tank material is standard for green liquor?

Austenitic stainless (304L/316L) or duplex (2304/2205) stainless steel. These show very low corrosion rates across kraft liquors regardless of sulfidity. Carbon steel is not used because it corrodes actively in green liquor.

Why is green liquor green, and is it corrosive?

The green color comes from colloidal ferrous sulfide (FeS) and suspended dregs (unburned carbon, calcium and iron compounds). It is strongly alkaline (sodium carbonate plus sodium sulfide and residual caustic, pH ~12–13), which makes carbon steel spontaneously active, with corrosion rates rising 20–75% as sulfidity increases.

Is green liquor flammable or otherwise dangerous to handle?

As a liquid it is non-flammable, but it is corrosive (severe burns), hot (scald risk), and its sulfide content can release toxic H₂S if it contacts acid. Generating it by dissolving molten smelt in water is also a recognized explosion hazard in recovery operations.