Calcium Hydrosulfide O-Ring Compatibility: Which Compounds Hold Up

Calcium hydrosulfide (Ca(HS)2) is an aqueous sulfide source used in leather dehairing, heavy-metal precipitation in wastewater treatment, pulp and chemical processing, and related sulfide chemistry. Seals on dosing pumps, transfer lines, tank fittings, and process valves see alkaline hydrosulfide solutions that can release hydrogen sulfide if mishandled. Pick the wrong O-Ring family and you get swell, softening, or early leak-off.

This guide maps the handbook Chemical Compatibility Chart row for calcium hydrosulfide to practical compound choices.

Where it shows up

• Leather tanning: dehairing / depilatory baths that break down keratin on hides before further wet processing
• Wastewater and environmental treatment: sulfide precipitant for removing heavy metals such as copper, lead, cadmium, and zinc as insoluble metal sulfides
• Pulp, paper, and chemical manufacture: sulfide-related process streams and intermediate chemistry tied to calcium sulfide / hydrosulfide loops
• Ore flotation and specialty reagents: sulfide-ion sources in mineral and chemical service where controlled HS- delivery matters

Service is usually aqueous and alkaline, near ambient to moderately warm. Concentration, pH, and any free H2S risk still matter. Chart ratings assume a generic commercial compound near room temperature.

How calcium hydrosulfide stresses an O-Ring

Hydrosulfide solutions are polar, alkaline, reducing media. Elastomers built for petroleum oils often take more swell or property loss here than they do in hydraulic oil. Water-friendly polymers (EPDM, butyl, neoprene in many grades) are the usual starting point. Alkalinity and any dissolved solids also raise the stakes on continuous immersion seals. See Chemical Compatibility and Failure Analysis for swell and softening modes.

Handbook chart ratings for calcium hydrosulfide

Same rating badges and layout as the interactive chart. Search calcium hydrosulfide on the Chemical Compatibility Chart to pin the live row.

Best family on the chart row: EPDM (E). Neoprene, fluorosilicone, butyl, FFKM, and PTFE also rate excellent (1). Silicone rates good (2). Nitrile, HNBR, and FKM rate poor (3). Polyurethane is not recommended (4). Aflas lacks data (X) on the handbook matrix.

Recommended O-Ring materials

First choice: EPDM

EPDM is rated excellent (1) and is the chart’s preferred letter for calcium hydrosulfide. It prefers polar and aqueous service over petroleum oils, which matches leather, wastewater dosing, and pulp-related hydrosulfide duty. Use 70 Shore A for most static covers and transfer seals; step hardness or add a back-up ring if pressure and gap climb.

Also usually serviceable

• Neoprene (CR) 1Excellent Strong aqueous / alkaline rating; confirm grade for continuous immersion.
• Fluorosilicone (FVMQ) 1Excellent Useful when cold flexibility or mixed hydrocarbon traces matter on the same plant.
• Butyl (IIR) 1Excellent Good polar-fluid resistance; watch heat and dynamic wear on pumps.
• FFKM 1Excellent Overkill for many dosing skids; use for mixed aggressive sulfide streams.
• PTFE 1Excellent Chemically inert but not an elastomer. Consider PTFE-encapsulated when you need rebound.
• Silicone (VMQ) 2Good Usually OK for static aqueous service; watch tear and abrasion in dynamic glands.

Materials to use with caution or avoid

• Nitrile (Buna-N) 3Poor Petroleum workhorse, weak here. Prefer EPDM for hydrosulfide solutions.
• HNBR 3Poor Same polarity mismatch as standard nitrile for this fluid.
• Viton® / FKM 3Poor Not the first pick for aqueous hydrosulfide; save FKM for fuels and hot oils.
• Polyurethane 4Not recommended Avoid in continuous calcium hydrosulfide service.
• Aflas® (FEPM) XNo data Insufficient chart data. Do not assume steam or amine grades are automatic fits here.

Physical properties to match (after chemistry)

A chart 1 means the polymer family usually survives the fluid. It does not finish the spec. After you shortlist EPDM (or another excellent family above), lock the physical properties the gland and process actually need.

Hardness (Shore A / durometer)

• 70 Shore A is the everyday default for static covers, flanges, and low-to-moderate pressure dosing and transfer lines.
• 90 Shore A (or 70 plus a back-up ring) when pressure and clearance gap push extrusion risk on pumps and filter housings.
• Soft grades (50–60) seal rough faces better but give up extrusion margin.

See Shore A Hardness for the extrusion-gap chart and feel references.

Temperature

Handbook chemical ratings are near room temperature. Leather baths, wastewater reactors, and pulp loops can run warmer than ambient. As a starting frame:

Confirm solution temperature and any heat tracing as well as the elastomer’s window. Hot alkaline sulfide service ages seals faster than a room-temperature chart letter implies.

Static vs dynamic duty

• Static face and cover seals: chemical rating + hardness + squeeze usually dominate.
• Dynamic shafts, rods, and reciprocating glands: add abrasion, extrusion, and friction. A harder EPDM often outlasts a soft general-purpose grade in slurry or frequent cycling. Check groove design and seal types.

Size and squeeze

Lock ID × CS (or the AS568 dash) on the Master O-Ring Size Chart before you argue compound. Wrong size fails faster than a second-choice polymer that fits the groove.

Alkalinity and H2S notes

Calcium hydrosulfide solutions are alkaline and can evolve hydrogen sulfide if acidified or poorly vented. Seal choice does not replace safe handling, materials of construction, or gas monitoring. If the loop also sees strong acids, oxidizers, or mixed sulfide grades, say so when you specify the ring. Related chart rows to compare: other hydrosulfides and sulfide salts on the Chemical Compatibility Chart.

Practical selection checklist

1. Confirm the solution. Concentration, pH, solids, and whether free H2S is expected.
2. Temperature. Room-temperature dosing is milder than warm leather or process baths.
3. Hardness and pressure. Default 70 Shore A; step to 90 or add a back-up ring when gap and psi demand it.
4. Static or dynamic. Metering pumps and valves need abrasion and extrusion resistance beyond a chemical rating alone.
5. Confirm on the chart, then test. Pin Calcium Hydrosulfide on the Chemical Compatibility Chart. For critical service, contact The O-Ring Store with fluid grade, temperature, hardness, and duty cycle.

Quick answers

Which O-Ring for calcium hydrosulfide?
Start with EPDM at 70 Shore A for general static duty, or 90 Shore A / back-up rings when pressure and gap climb. Neoprene, fluorosilicone, butyl, FFKM, and PTFE also rate excellent on the handbook row. Silicone is usually good for static service. Avoid polyurethane; treat nitrile, HNBR, and standard FKM as poor fits; treat Aflas as unproven here until you test.

Is this the same as sodium hydrosulfide or calcium polysulfide?
Related sulfide chemistry appears in leather, mining, and wastewater plants. Always match the exact fluid name on the chart and on the SDS.

Chart ratings are guidance for generic commercial compounds near room temperature. Concentration, temperature, and co-fluids change outcomes. Evaluate specific parts before use in critical applications.