Overview

Ferrous chloride (FeCl₂, iron(II) chloride) is an alternate iron-bearing coagulant to ferric chloride (FeCl₃) and ferric sulfate. Wastewater operators use ferrous chloride for phosphate removal (iron precipitation of soluble phosphate as ferrous phosphate), sulfide control in anaerobic digesters (iron precipitates H₂S as iron sulfide), and as a general iron supplement for activated-sludge systems. Industrial users include steel-pickling effluent treatment (where ferrous chloride is both a byproduct and a process chemical) and some specialty metal-finishing operations.

Saturated Solution — Snyder Spec

Commercial ferrous chloride is often sold as concentrated near-saturated solution (roughly 30–35% FeCl₂). Snyder approves HDLPE and XLPE at 1.9 ASTM with a notable bolt-spec upgrade:

• Resin: HDLPE or XLPE — both approved
• Specific Gravity: 1.9 ASTM
• Fittings: PVC
• Gaskets: EPDM
• Bolts: Hastelloy or Titanium — NOT 316SS. This is critical. Ferrous chloride's chloride plus reducing-iron chemistry attacks 316 stainless steel at rates that are operationally unacceptable. The Snyder spec omits 316SS entirely.

Why Ferrous Chloride vs Ferric Chloride?

The distinction between Fe(II) and Fe(III) chemistries matters operationally:

• Ferric chloride (FeCl₃): Oxidized iron. Forms positively-charged floc on hydrolysis. Excellent primary coagulant. See our separate ferric chloride pillar.
• Ferrous chloride (FeCl₂): Reduced iron. Weaker as a primary coagulant but more effective for sulfide precipitation (Fe²⁺ + H₂S → FeS + 2H⁺) and for supplementing the iron cycle in biological-treatment processes. Often generated as a byproduct of steel-pickling and used downstream in the same facility.

Choose ferrous over ferric when sulfide control is a primary driver. Choose ferric when primary coagulation is the need. Both can serve for phosphate removal.

Steel-Pickling Service — The Industrial Supply

The largest industrial source of ferrous chloride is steel-pickling plants. Hydrochloric-acid pickling of steel dissolves surface iron oxide, producing spent pickle liquor containing 10–30% ferrous chloride plus residual HCl. Historically this was a waste stream. Modern plants recycle it — either into their own wastewater treatment or by selling as coagulant-grade ferrous chloride to downstream users. Storage tanks for pickle-liquor-derived ferrous chloride sometimes see residual HCl contamination, which makes the Hastelloy-or-Titanium bolt spec even more important (HCl is aggressive on 316SS too).

Sulfide Precipitation in Anaerobic Digestion

Anaerobic digesters in municipal wastewater plants produce methane biogas for beneficial use — but the biogas contains hydrogen sulfide (H₂S) that must be scrubbed before use in engines or boilers. Adding ferrous chloride to the digester feed precipitates sulfide as iron sulfide in the sludge, reducing H₂S in the biogas to manageable levels (under 100 ppm) without external scrubbing equipment. Typical dose rates are 50–200 mg/L of ferrous as Fe, depending on incoming sulfate/sulfide loading.

Phosphate Removal in Wastewater

Permit-driven phosphate removal is a growing requirement for municipal wastewater treatment facilities — especially those discharging to water bodies with eutrophication concerns. Iron precipitation (either Fe²⁺ or Fe³⁺) captures soluble phosphate as iron phosphate solid that settles with the biosolids. Typical effluent phosphate targets are 1 mg/L or below (increasingly, 0.5 mg/L or 0.1 mg/L for sensitive receiving waters). Iron dose at 1.8–2.5 mg Fe per mg P is typical stoichiometric design.

System-of-Construction Table (Snyder Industries)

This is the exact specification Snyder Industries publishes for this chemistry. Every column is required — changing any of them voids the service rating.

Concentration-Band Compatibility (Enduraplas / Equistar Data)

Polyethylene chemical resistance by concentration and service temperature. Satisfactory (S) = long-term service. Limited (O) = occasional only. Unsatisfactory (U) = do not use.

Frequently Asked Questions

Why isn't 316SS OK for ferrous chloride?

The combination of chloride ion and reducing-iron conditions is more aggressive on 316SS than chloride-only chemistries (like sodium chloride) or oxidized iron (like ferric sulfate). Chloride pitting is accelerated by the Fe(II)-to-Fe(III) cycling that occurs at the steel surface. Hastelloy C-276 family and titanium resist this combined attack mechanism. It's not a theoretical issue — documented field failures show 316SS bolts pitting through in 12-36 months of continuous ferrous chloride service.

Can I use 316SS elsewhere on the tank?

Limited, with caveats. Internal ladders or inspection platforms (not wetted continuously) may tolerate 316SS. Flange bolts (directly wetted or in vapor space) should be Hastelloy/Titanium. Inspection hardware that would require refastening after inspection should be Hastelloy/Titanium. When in doubt, use Hastelloy — the cost increment vs 316SS is less than the cost of failed bolts and emergency repair.

Does ferrous chloride stain like ferric?

Yes — any iron chemistry leaves rusty iron-oxide stains on concrete and light-colored surfaces. Ferrous solutions are initially clear green but oxidize to rust-brown quickly in air contact. Design containment with dark-colored coatings, HDPE liners, or epoxy floor systems to protect against staining.

Is ferrous chloride stable in storage?

Partially. Ferrous chloride slowly oxidizes to ferric chloride in air contact — the two chemistries are interconvertible and the storage product can drift toward ferric over time. This doesn't invalidate the product (ferric chloride is an equivalent coagulant) but may shift dosing calculations. Tanks with tight seal (low headspace oxygen exchange) stay ferrous longer.

Can I make ferrous chloride in-house from iron and HCl?

Yes in principle — iron + HCl → FeCl2 + H2 — but the reaction releases hydrogen gas (explosion hazard) and is slower than industrial supply. Commercial ferrous chloride is cheap enough that in-house generation is rarely cost-effective. Purchase is the industry norm.

Source Citations

• Snyder Industries — Chemical Resistance Recommendations (current edition)
• Enduraplas / Equistar Technical Tip — Chemical Resistance of Polyethylene (12-page reference)

Field Operations Addendum — Ferrous Chloride

Expanded Compatibility Matrix. Ferrous chloride (FeCl₂, CAS 7758-94-3) is a divalent-iron chloride salt typically handled as 30–40% aqueous solution, most commonly produced as a byproduct of steel pickling liquors and used in municipal wastewater phosphate precipitation and hydrogen-sulfide odor control. The solution is strongly acidic (pH 1–2) and contains both chloride and ferrous iron species, driving an aggressive corrosion profile. HDPE and XLPE are A-rated at all concentrations up to saturated solution and at temperatures up to 100°F; polymer tanks are the industry standard for bulk storage. Polypropylene is A-rated. FRP vinyl ester with iso-NPG C-veil is A-rated; FRP isophthalic polyester is C-rated; FRP epoxy is B-rated. PVC is A-rated at ambient; CPVC is A-rated to 140°F. PVDF is A-rated. 316L stainless steel is NR — chloride pitting and iron-chloride galvanic-cell attack both destroy 316L within weeks of continuous exposure. 304 SS is NR. Super-duplex and high-nickel alloys (Hastelloy C-276, AL6XN) are A-rated but rarely cost-justified versus polymer. Carbon steel is NR. Aluminum is NR. Copper, brass, and bronze are NR. Titanium Grade 2 is C-rated under chloride attack at elevated temperature. EPDM and Viton gaskets are A-rated; PTFE is A-rated; nitrile is C-rated.

Hazard Communication Refresh. Ferrous chloride solution (CAS 7758-94-3) is classified under GHS as Category 1B Skin Corrosive, Category 1 Eye Corrosive, and Category 4 Acute Oral Toxicity. NFPA 704 placard is Health 3, Flammability 0, Instability 0. DOT hazard class is UN1760 Corrosive Liquid NOS, Packing Group III. OSHA has no specific PEL for ferrous chloride; ACGIH sets a 1 mg/m³ TWA for iron-soluble-salt dust plus 0.5 mg/m³ for chloride-salt aerosol. NSF/ANSI 60 certified product is required for any drinking-water-adjacent service; typical wastewater applications do not require NSF certification. The greenish-to-yellowish solution oxidizes in air to the reddish-brown ferric form over days of aerobic exposure — the color change indicates oxidation-state conversion but does not render the product unusable for wastewater service.

Storage Protocol Specifics. Freeze management: 30–40% ferrous chloride freezes at approximately 25°F. Outdoor unheated tanks in USDA Zone 6 and colder require heat trace and insulation, or indoor heated shelter. Vented storage is standard with 20-mesh screen atmospheric vent; no significant vapor evolution at ambient but some HCl vapor at elevated temperature. Containment berms must be acid-chloride-resistant coated concrete or polymer-lined. Pump wetted parts: PVDF, PP, or polymer-lined cast iron; never stainless steel. Transfer hose: EPDM-lined or PVDF-lined with polymer fittings. Segregate from strong bases (produces ferrous hydroxide precipitate and can evolve HCl vapor if acidic aerosol is present). Segregate from strong oxidizers (hypochlorite, peroxides) because oxidation of Fe²⁺ to Fe³⁺ can drive exothermic reaction and chlorine evolution. Wastewater plant tanks are typically 5,000–20,000 gallons XLPE double-wall. Inventory rotation within 6 months prevents excessive air-oxidation to ferric.

Three Additional FAQs.

Why does my wastewater plant use ferrous chloride instead of the more aggressive ferric chloride for phosphate removal? Ferrous chloride is cheaper (byproduct pricing from steel pickling) and is adequate for phosphate precipitation under reducing/anoxic conditions. Ferric chloride is preferred where oxic conditions or higher-demand coagulation warrants the premium pricing.

My ferrous chloride solution turned from green to reddish-brown during storage. Is it still usable? Yes for wastewater phosphate and sulfide control applications — the ferric form is equally or more effective. For applications specifically requiring ferrous-state chemistry (some reduction-chemistry processes) the oxidized solution is off-spec. Minimize headspace and consider nitrogen blanket for ferrous-specific service.

Can I use 316L stainless steel pump internals for short-duration ferrous chloride transfer? Batch-service transfer over hours is marginally acceptable but continuous-duty service fails in weeks. Specify PVDF or polymer-lined cast-iron pumps from the start; the polymer hardware cost premium is trivial compared to unplanned pump-replacement labor.

Operational Supplement — Ferrous Chloride Dosing and Byproduct Economics

Typical Dosing and Process Performance. Ferrous chloride dosing at municipal wastewater plants runs 5 to 50 mg/L as Fe for phosphate precipitation to meet typical NPDES effluent-phosphorus permits of 1.0 mg/L or tighter. Hydrogen-sulfide odor-control dosing in anaerobic digester and collection-system applications runs 20 to 200 mg/L as Fe depending on sulfide load; the iron precipitates as insoluble ferrous sulfide and prevents H2S off-gassing at lift stations and headworks. Dosing equipment at plant scale is peristaltic or polymer-diaphragm metering pumps; day-tank sizing at 200 to 500 gal allows dose-rate adjustment without bulk-tank manipulation. Operators monitor residual iron in effluent against EPA iron Secondary MCL 0.3 mg/L and against receiving-water quality criteria that vary by state.

Byproduct Economics and Supply Chain. Ferrous chloride is a byproduct of steel pickling operations where HCl acid is used to descale hot-rolled steel coil; the spent pickle liquor contains 10-15% iron as ferrous chloride plus residual free HCl that is neutralized at specialty processors. This byproduct pricing typically undercuts virgin-synthesis iron chemistry (ferric chloride, ferric sulfate, ferrous sulfate) at 30 to 60 percent cost per pound of active iron, driving widespread adoption in wastewater and odor-control markets where the less aggressive ferrous chemistry is adequate. Supply chain resilience depends on regional steel-industry activity; Rust Belt wastewater utilities have multiple regional suppliers while southeastern and mountain-west utilities face longer supply lines and higher delivered cost.

Related Chemistries: Iron Chemistry + Water Treatment

Related to:

• Ferric Chloride (FeCl3) — Oxidized-iron counterpart
• Ferric Sulfate (Fe2(SO4)3) — Iron sulfate form
• Ferrous Sulfate (FeSO4) — Same oxidation state, sulfate