Ferric chloride
The coagulant you reach for when the consent has a phosphorus number on it, when the stream smells of sulphide, or when pH cannot be brought down into the aluminium window. Also the most corrosive product on this site — the materials list is not advisory.
Ferric chloride, FeCl₃, is supplied as a 38–42% solution containing 13.0–14.5% iron. It coagulates across an unusually wide pH range of 4.0–11.0, precipitates orthophosphate as ferric phosphate, and precipitates sulphide as iron sulphide — three duties aluminium coagulants perform poorly or not at all.
Typical dissolved air flotation doses run 50–250 mg/L as supplied. For phosphorus removal, the practical requirement is 1.5–3.0 moles of iron per mole of phosphorus removed, well above the 1:1 stoichiometry, because iron is also consumed by hydroxide formation.
Specification
| Parameter | Industrial grade | Drinking-water grade |
|---|---|---|
| FeCl₃ content | 38.0–42.0% | 40.0–42.0% |
| Total iron as Fe | 13.0–14.5% | 13.8–14.5% |
| Ferrous iron as Fe²⁺ | ≤ 0.5% | ≤ 0.1% |
| Free hydrochloric acid | ≤ 1.0% | ≤ 0.5% |
| Insolubles | ≤ 0.5% | ≤ 0.1% |
| Density at 20 °C | 1.42–1.47 kg/L | 1.42–1.47 kg/L |
| Crystallisation point | −5 to −10 °C at 40% | −5 to −10 °C at 40% |
| Appearance | Dark brown to black liquid | Dark brown liquid |
| Heavy metals | Not specified | NSF/ANSI 60 or EN 888 limits |
The three duties it wins
Phosphorus removal
Ferric iron precipitates orthophosphate as FePO₄ and adsorbs further phosphate onto the ferric hydroxide floc. On paper the reaction is 1 mole Fe to 1 mole P; in practice, because hydroxide formation competes, plan on 1.5 moles Fe per mole P for a consent around 2 mg/L, 2.0–2.5 for 1 mg/L, and 3.0 or more to approach 0.5 mg/L.
| Target total P | Molar Fe:P | kg Fe per kg P removed | Litres of 40% FeCl₃ per kg P removed |
|---|---|---|---|
| 2.0 mg/L | 1.5 : 1 | 2.7 | 13 |
| 1.0 mg/L | 2.0–2.5 : 1 | 3.6–4.5 | 17–22 |
| 0.5 mg/L | 3.0 : 1 | 5.4 | 26 |
| < 0.3 mg/L | 4.0 : 1 plus filtration | 7.2 | 35 |
Sulphide precipitation
In tannery beamhouse liquor, in septic sewage and in anaerobic industrial effluent, dissolved sulphide is both an odour problem and a hydrogen sulphide hazard. Ferric iron oxidises and precipitates it as iron sulphide, which the flotation cell then removes with the rest of the floc. The stoichiometric requirement is about 3.5 kg of Fe per kg of S²⁻, and dosing is normally continuous rather than as a shock treatment.
High-pH coagulation
Ferric hydroxide has a much lower solubility than aluminium hydroxide at pH 8–10. Where the stream carries high alkalinity or lime, and bringing pH down into the aluminium window would cost more in acid than the coagulant is worth, iron simply keeps working. Tannery and some mining streams fall here.
Dose ranges by application
| Application | Dose, 40% FeCl₃ | As Fe | Coagulation pH | Polymer |
|---|---|---|---|---|
| Municipal primary, DAF | 30–120 mg/L | 4–17 mg/L | 6.5–7.5 | Anionic, 0.5–2 mg/L |
| Municipal phosphorus removal | 40–150 mg/L | 6–21 mg/L | 6.0–7.0 | Anionic, 0.5–1.5 mg/L |
| Tannery, beamhouse | 150–400 mg/L | 21–56 mg/L | 7.5–9.0 | Anionic, 2–5 mg/L |
| Abattoir and meat | 80–250 mg/L | 11–35 mg/L | 6.0–7.0 | Anionic, 2–5 mg/L |
| Sludge conditioning before dewatering | 3–8% on dry solids | — | — | Cationic, 4–10 kg/t DS |
| Potable water, colour and organics | 20–80 mg/L | 3–11 mg/L | 4.5–6.0 | Usually none |
| Mining and metals-bearing water | 50–300 mg/L | 7–42 mg/L | 8.0–10.0 | Anionic, high MW |
Corrosion and materials of construction
Ferric chloride is an acidic solution of a strong oxidising chloride. It attacks almost every metal used in plant construction, including stainless steels that resist most other water treatment chemicals. Chloride pitting of 316L in ferric chloride service is fast enough to be measured in months, not years.
- Suitable: HDPE, polypropylene, PVC, uPVC, CPVC, PVDF, PTFE, FRP with vinyl ester resin and a synthetic veil, rubber-lined steel, ceramic, titanium, Hastelloy C-276.
- Unsuitable: mild steel, galvanised steel, aluminium, copper and brass, 304 and 316 stainless in any wetted duty, unprotected concrete, natural rubber in prolonged contact.
- Gaskets and elastomers: EPDM, Viton and PTFE-envelope. Nitrile fails.
- Dosing pumps: diaphragm pumps with PVDF or PTFE wetted ends, ceramic ball checks. Peristaltic with Norprene or Chem-Sure tube is the low-maintenance alternative and is the usual choice where the site has limited maintenance capability.
The consequence for the plant is that ferric chloride staining is permanent and highly visible. A weeping flange on ferric service produces a rust-coloured stain across concrete that will still be there in ten years. Bund properly, use full-face gaskets, and plan on a wash-down point at the dosing skid.
Storage
- Temperature: 5–35 °C. Below about −5 °C at 40% strength it crystallises; the 35–38% winter grade exists for cold climates and is not needed anywhere in Africa. Above 40 °C it hydrolyses slowly and drops solids.
- Vessels: HDPE or FRP with a chemically resistant liner, vented, shaded, on a plinth over a chemically resistant bund at 110% of the largest vessel.
- Shelf life: 6–12 months sealed. Practical limit is set by sediment rather than by loss of iron.
- Protection: splash goggles, face shield, PVC or neoprene gloves and apron. Eyewash and safety shower within ten seconds of the transfer point.
- Do not mix with: alkalis, hypochlorite, sulphide-bearing liquor or any aluminium coagulant in the same line or vessel.
When ferric chloride is the wrong choice
- Where treated water is reused in a process sensitive to colour. Overdosed iron leaves a yellow-brown tint. In a laundry, a beverage plant or a paper mill making white grades, that is a product-quality problem.
- Where 316 stainless is already installed and cannot be replaced. Choose ferric sulphate instead — same iron chemistry, no chloride attack.
- Where the site has no competent chemical handling. This is the product most likely to cause an injury or a permanent mess on a plant with untrained operators. PAC is far more forgiving.
- Where alkalinity is low and pH control is manual. Ferric chloride destroys about 0.9 mg of alkalinity as CaCO₃ per mg dosed. On soft water at a 150 mg/L dose that is 135 mg/L of alkalinity gone, and pH will crash.
- Where sludge is destined for a chromium-sensitive route. On tannery duty, iron-conditioned sludge carries the chromium with it — which is usually the intention, but it dictates the disposal route.
Supply
Frequently asked questions
How much ferric chloride is needed to remove phosphorus?
Plan on 1.5 moles of iron per mole of phosphorus for a consent around 2 mg/L, 2.0–2.5 for 1 mg/L and 3.0 or more to approach 0.5 mg/L. In mass terms that is roughly 2.7 to 5.4 kg of iron per kg of phosphorus removed, or 13 to 26 litres of 40% ferric chloride. Below about 0.3 mg/L, chemical precipitation alone will not hold the consent and a polishing filter is required.
Ferric chloride or ferric sulphate — which should I use?
The iron chemistry is the same and the doses are comparable on an iron basis. Ferric chloride is more concentrated, usually cheaper per kilogram of iron, and slightly better on sulphide. Ferric sulphate does not carry chloride, so it does not pit stainless steel and does not add chloride to a stream heading for reuse or reverse osmosis. If you already have 316 stainless in the wetted path, or a chloride limit downstream, use the sulphate.
Will ferric chloride corrode my 316 stainless dosing line?
Yes, and faster than most people expect. Ferric chloride is the standard laboratory reagent for testing pitting resistance precisely because it attacks austenitic stainless so aggressively. Use PVDF, CPVC, PTFE-lined or rubber-lined components in every wetted part. Titanium and Hastelloy C-276 are the metals that survive, and they are rarely worth the cost against plastic.
Does ferric chloride lower pH?
Substantially. It destroys roughly 0.9 mg of alkalinity as CaCO₃ per mg of product dosed, so a 150 mg/L dose consumes about 135 mg/L of alkalinity. On a soft or low-alkalinity water this will drive pH below the coagulation window and performance will collapse. Either dose lime or caustic to replace the alkalinity, or use a lower-alkalinity-demand coagulant such as PAC.
Why is my treated water tinted yellow?
Almost always overdosing, sometimes ferrous iron in the delivered product. Excess iron that is not incorporated into floc stays in solution or as fine colloid and passes the flotation cell. Reduce the dose to the jar-test optimum first. If the tint persists at the correct dose, check the ferrous iron figure on the certificate of analysis — above about 0.5% Fe²⁺ the product is out of specification for water treatment duty.
Can ferric chloride be dosed with sodium hypochlorite?
Not in the same line or vessel. They react. Where both are needed — hypochlorite to oxidise sulphide or ferrous iron, ferric chloride to coagulate — dose them at separate injection points with sufficient separation and reaction time between them, and confirm the sequence on the bench before committing the plant to it.
What HS code and UN number does ferric chloride ship under?
HS 2827.39 for customs classification, and UN 2582, Class 8 corrosive, packing group III for dangerous goods purposes. The dangerous goods declaration is straightforward but must be correct — a mis-declared corrosive is a common reason for a container being held at an African port.