Ferric sulphate
Everything ferric chloride does, minus the chloride. That single difference matters more often than the price gap between them.
Ferric sulphate, Fe₂(SO₄)₃, is supplied as a solution containing 11–12.5% iron, equivalent to about 40–45% Fe₂(SO₄)₃. It coagulates over the same wide pH range as ferric chloride, precipitates phosphate and sulphide equally well, and destroys about 0.75 mg of alkalinity as CaCO₃ per mg dosed.
It is chosen over ferric chloride wherever chloride is a problem: existing 316 stainless in the wetted path, a chloride limit on the discharge, or treated water heading for reuse or reverse osmosis. Typical dissolved air flotation doses run 100–350 mg/L as supplied.
Specification
| Parameter | Liquid | Granular |
|---|---|---|
| Total iron as Fe | 11.0–12.5% | 19.0–21.0% |
| Fe₂(SO₄)₃ content | 39.0–45.0% | 68.0–75.0% |
| Ferrous iron as Fe²⁺ | ≤ 0.5% | ≤ 1.0% |
| Free acid as H₂SO₄ | ≤ 1.0% | ≤ 1.5% |
| Insolubles | ≤ 0.5% | ≤ 1.0% |
| Density at 20 °C | 1.50–1.60 kg/L | Bulk 1.0–1.2 kg/L |
| pH, 1% solution | 1.5–2.5 | 1.5–2.5 |
| Appearance | Dark red-brown liquid | Reddish-brown granule |
Against ferric chloride
| Ferric sulphate | Ferric chloride | |
|---|---|---|
| Iron content | 11–12.5% | 13–14.5% |
| Attacks 316 stainless | No | Yes, rapidly |
| Adds chloride to the stream | No | Yes, ~90 mg/L per 100 mg/L dosed |
| Adds sulphate to the stream | Yes | No |
| Alkalinity destroyed per mg | 0.75 mg CaCO₃ | 0.9 mg CaCO₃ |
| Phosphorus removal | Equivalent per mole Fe | Equivalent per mole Fe |
| Sulphide precipitation | Good | Slightly better |
| Cost per kg of iron | Higher | Lower |
Dose ranges
| Application | Dose, 12% Fe liquid | As Fe | Coagulation pH |
|---|---|---|---|
| Municipal primary DAF | 40–150 mg/L | 5–18 mg/L | 6.5–7.5 |
| Phosphorus removal | 50–180 mg/L | 6–22 mg/L | 6.0–7.0 |
| Fish and seafood processing | 100–350 mg/L | 12–42 mg/L | 6.0–7.0 |
| Textile and dyehouse | 150–400 mg/L | 18–48 mg/L | 7.0–8.5 |
| Potable colour and organics | 25–100 mg/L | 3–12 mg/L | 4.5–6.0 |
| Sludge conditioning | 4–10% on dry solids | — | — |
| Water reuse feed to RO | 50–200 mg/L | 6–24 mg/L | 6.5–7.5 |
Handling
- Suitable: HDPE, polypropylene, PVC, CPVC, PVDF, FRP with vinyl ester, rubber-lined steel, 316L stainless at ambient temperature.
- Unsuitable: mild steel, galvanised steel, aluminium, copper alloys, unprotected concrete, 304 stainless.
- Storage: 5–35 °C, shaded and vented, in a bund at 110% of the largest vessel. Shelf life 6–12 months; the practical limit is sediment rather than iron loss.
- Staining: as permanent as ferric chloride. Plan a wash-down point at the dosing skid and bund in a material that does not discolour.
- Protection: splash goggles, face shield when transferring, PVC or neoprene gloves, apron, eyewash within reach.
Frequently asked questions
When should I choose ferric sulphate over ferric chloride?
When 316 stainless is already installed in the wetted path and cannot be replaced; when the discharge consent or a reuse application sets a chloride limit; when treated water feeds reverse osmosis, where added chloride raises the conductivity the membrane must overcome. Otherwise ferric chloride is more concentrated and cheaper per kilogram of iron.
Does ferric sulphate remove phosphorus as well as ferric chloride?
Yes. Phosphorus removal depends on the iron, not on the anion. Dose on an iron basis — 1.5 to 3.0 moles of Fe per mole of P depending on the target — and the two products perform equivalently. Because ferric sulphate is less concentrated, the product dose is roughly 15–20% higher for the same iron.
Will the added sulphate cause a problem downstream?
It can. Where an anaerobic digester, anaerobic lagoon or long rising main follows the treatment, sulphate feeds sulphate-reducing bacteria and can generate hydrogen sulphide — the exact problem iron dosing is often intended to prevent. On those plants ferric chloride is normally the better choice.
Is granular ferric sulphate worth the handling?
On a long freight route, often yes. Granular carries roughly 20% Fe against 12% for the liquid and ships without the water, so landed cost per kilogram of iron can be substantially lower. It needs a dissolving station and a competent operator; where those exist, run the numbers.