Anthraquinone (AQ) Pulping Liquor Storage & Tank Compatibility

Storing Anthraquinone (AQ) Pulping Liquor? Start Here

Anthraquinone (AQ) pulping liquor is not a single pure chemical but a process formulation: a strongly alkaline cooking liquor in which a small charge of anthraquinone catalyst is dispersed. In the soda-AQ and kraft-AQ processes, the bulk of the liquor is caustic soda (sodium hydroxide) at roughly 13-16 wt%, with the kraft variant also carrying sodium sulfide and carbonate. AQ is dosed at only about 0.05-0.1% on wood, where it acts as a redox catalyst that accelerates delignification, raises pulp yield, and reduces carbohydrate degradation.

Because AQ is present in trace amounts and is largely consumed in the cook, the material-of-construction decision for tanks, piping, and seals is governed almost entirely by the caustic-soda fraction and by service temperature. Caustic is highly corrosive to skin, eyes, aluminum, and (when hot) carbon steel, which is why correct MOC selection protects both equipment life and worker safety.

Is HDPE / XLPE Safe for Anthraquinone Pulping Liquor?

Yes — for ambient make-up and storage service. The dominant component is sodium hydroxide, and caustic soda is one of the classic, well-proven good fits for polyethylene. Published HDPE/XLPE resistance charts rate sodium hydroxide solutions as resistant at ambient temperatures, and polyethylene does not suffer the caustic stress-corrosion cracking that threatens carbon steel. The trace anthraquinone catalyst is a poorly soluble solid that does not change that verdict.

The honest limit is temperature, not chemistry: the actual digester cook runs at 140-185°C, far above the service window of any polyethylene tank. HDPE/XLPE is therefore rated S (suitable) for ambient-temperature make-up tanks, day tanks, and secondary containment of AQ-charged alkaline liquor — but in-process hot liquor must run in stainless or coated steel. Always confirm against the specific liquor SDS and your operating temperature.

Material compatibility at a glance

Storage tank selection is driven by the caustic-soda carrier, not by the trace anthraquinone catalyst. For ambient make-up and day-tank storage of AQ-charged alkaline liquor, HDPE/XLPE and PP are well suited (high-alkali resistance, low cost, no caustic embrittlement at ambient). Hot digester and recovery service (140-185°C) requires 316 stainless or properly specified/coated carbon steel; aluminum is unsuitable. Match elastomers to caustic (EPDM yes, FKM no).

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 caustic (pH 13-14): the liquor causes severe skin burns and serious eye damage — treat as a corrosive base.
• Carcinogen flag: anthraquinone is classified UN GHS Carc. 1B (may cause cancer); minimize dust and skin contact when charging the catalyst.
• Skin sensitizer: anthraquinone may cause an allergic skin reaction; use impervious gloves and aprons.
• Hot-liquor scald + pressure hazard: in-process liquor is handled at 140-185°C under pressure; relieve and cool before opening systems.
• Metal incompatibility: the alkaline liquor attacks aluminum and can cause caustic stress-corrosion cracking of hot carbon steel.
• PPE / response: chemical splash goggles, face shield, and caustic-rated gloves; provide emergency eyewash and safety shower; have dilute-acid neutralization and spill control ready.

Common questions

Is anthraquinone pulping liquor a single chemical?

No. It is a process formulation — a strongly alkaline caustic-soda cooking liquor (kraft-AQ also carries sodium sulfide and carbonate) dosed with a trace charge of anthraquinone catalyst (about 0.05-0.1% on wood). Tank selection is driven by the caustic fraction, not the AQ.

Can I store AQ pulping liquor in an HDPE or XLPE tank?

Yes for ambient make-up, day-tank, and containment service. Sodium hydroxide is well-resisted by HDPE/XLPE and does not cause the stress-corrosion cracking that threatens carbon steel. The limit is temperature: hot in-digester liquor (140-185°C) must use stainless or coated steel, not polyethylene.

What controls the corrosion behavior, the anthraquinone or the caustic?

The caustic soda. Anthraquinone is present in trace amounts, is poorly soluble, and is largely consumed in the cook, so the strongly alkaline carrier governs material compatibility, pH, and most of the corrosivity.

What metals should I avoid?

Avoid aluminum entirely — strong alkali attacks it rapidly. Carbon steel is usable for caustic liquor but is vulnerable to caustic stress-corrosion cracking when hot, so monitor or coat it; 316 stainless is the standard for hot in-process service.