Overview
Sodium hydrosulfide (NaHS) is a highly reactive inorganic salt widely used in copper–molybdenum separation, paper pulping, leather unhairing and the production of high-purity hydrogen sulfide. Commercial solid products are sold as ≥70 % flakes or crystals. They are extremely hygroscopic, spontaneously deliquescing and decomposing exothermically to yield NaOH, Na₂S and the highly toxic H₂S gas, posing severe challenges for safe storage, transport and process stability.
I. Deliquescence Mechanism
1. Physical moisture uptake
NaHS is a typical ionic crystal. The strong ionic bond between Na⁺ and HS⁻ allows water molecules to intercalate rapidly into the lattice and form NaHS·nH₂O hydrates, with n increasing as relative humidity (RH) rises. At 25 °C and 80 % RH, the mass gain after 24 h can reach 25 %.
2. Chemical decomposition
Once moist, the system becomes strongly alkaline and a chain reaction ensues:
NaHS + H₂O ⇌ NaOH + H₂S↑
2 NaOH + 2 NaHS → 2 Na₂S + 2 H₂O
Overall: 2 NaHS → Na₂S + H₂S↑ ΔH = –55 kJ mol⁻¹ (exothermic)
The reaction rate increases 2–3-fold for every 10 °C rise in temperature.
3. Phase change and caking
Repeated hydration–dehydration cycles fracture the crystals, dissolve and recrystallise the surface, and ultimately form dense, hard lumps that plug pipelines and cause metering errors exceeding 10 %.
II. Hazard Assessment
1. Safety: H₂S is acutely toxic (LC₅₀ 444 ppm, 30 min) and forms explosive mixtures with air (LEL 4.3 %).
2. Quality: Declining active NaHS content and rising free alkalinity lead to Cu–Mo separation “tail loss” and excessive residual sulfur in pulping.
3. Equipment: Caking blocks screw feeders and metering screws, causing unplanned shutdowns.
III. Systematic Control Technologies
1. Design-level control
• Materials: Use 316 L or 904 L stainless steel for tanks and pipelines, lined with PTFE or PE; avoid copper and copper alloys to prevent CuS deposition.
• Process route: Prefer on-line synthesis from liquid caustic and H₂S to minimise solid NaHS inventory. If solid must be purchased, choose flake form in 25 kg Al-foil vacuum pouches.
2. Storage and transport environment
• Warehouse: ≤25 °C, RH ≤50 %; rotary desiccant dehumidifiers (dew point ≤–20 °C).
• Packaging:
– Inner layer: 0.12 mm seven-layer PE/PA/Al co-extruded vacuum bag.
– Middle layer: 2 kg silica-gel desiccant (≥50 g per pouch, regenerable).
– Outer layer: 200 L HDPE tight-head drum with EPDM gasket and 0.02 MPa explosion-proof breather valve.
• Stacking: On pallets with stretch wrap, ≥30 cm from walls and ≥15 cm above floor; install on-line H₂S detectors (alarm 5 ppm, interlocked exhaust).
3. On-site operating procedures
• Receiving inspection: Sample each batch for moisture (Karl Fischer ≤2 %) and free alkali (acid–base titration ≤1 %).
• Bag opening & feeding: Perform inside a nitrogen-purged glove box (O₂<0.5 %, RH<10 %); reseal and vacuum-backfill with N₂ within 4 h.
• Equipment cleaning: Flush lines with de-oxygenated water, then nitrogen-purge to H₂S<1 ppm; collect washings in a 10 % NaOH scrubber.
4. Emergency response
• Spillage: Wear Level A chemical suit, collect solids with a negative-pressure vacuum system, cover floors with NaHCO₃ slurry; ventilation ≥12 air changes h⁻¹.
• Poisoning: Remove victim to fresh air, administer 100 % oxygen, intravenously inject 3 % sodium nitrite (10 mL) as antidote, and transfer to hospital.
IV. Conclusion
NaHS deliquescence results from the coupling of physical moisture uptake and chemical decomposition. Lifecycle management—from design through packaging, storage, transport, use and emergency response—is essential. Low-humidity storage, vacuum Al-foil pouches, nitrogen-protected feeding and on-line monitoring can markedly reduce safety risks and quality fluctuations, ensuring long-term stable operation of the plant.，
