DEFLUORIDATION
Fluoride removal for lithium, photovoltaic and plating wastewater
A two-stage route: pH adjustment, defluoridation reagent, then PAC coagulation and PAM flocculation so the precipitate actually settles. Built for streams where fluoride has to reach single-digit mg/L, not just drop a little.
Why fluoride removal fails
Fluoride removal fails more often on sequence than on chemistry. Above pH 6.5 the reagent reacts incompletely, and the fluoride that does precipitate stays as fine crystals that pass straight through the clarifier.
The reliable order is: adjust pH to 5.5 – 6.5, dose the defluoridation reagent with rapid mixing, add PAC to co-precipitate the fine crystals, then flocculate with a low dose of anionic PAM and settle. Each stage needs its own mixing intensity.
The four stages
- 01
Equalise and characterise
Balance the batch, then test fluoride, calcium, aluminium, sulphate and pH. High sulphate or phosphate competes with the fluoride reaction.
- 02
Adjust pH to 5.5 – 6.5
Acid dosing under mixing. Too high and the reaction stalls; too low and you dissolve the precipitate again.
- 03
Defluoridation and co-precipitation
Reagent first with rapid mixing, then PAC to enmesh the fine crystals into a settlable floc.
- 04
Flocculate, settle, polish
Anionic PAM at 0.2 – 1 mg/L, settlement, then a polishing step if the limit is below 2 mg/L.
Working parameters
| Parameter | Typical value | Method / note |
|---|---|---|
| Typical feed fluoride | 20 – 500 mg/L | Case by case |
| Working pH | 5.5 – 6.5 | Process |
| Reagent form | Powder, dissolved on site | Application |
| Coagulant | PAC, 30 – 100 mg/L | Application |
| Flocculant | Anionic PAM, 0.2 – 1 mg/L | Application |
| Sludge | Fluoride-bearing, dewater with cationic PAM | Application |
Doses depend on the fluoride load and on the ions competing for the reagent. The figures above are starting points for a jar test, not a design basis.
Defluoridation FAQ
Why does fluoride rebound after treatment?
Usually because pH drifted up before the reagent had finished reacting, or because the precipitated crystals were never captured in a floc and carried over the weir. Check the pH at the reaction tank, not at the outlet, and confirm that PAC is dosed after the reagent rather than together with it.
Can PAC alone remove fluoride?
Only partly. PAC can adsorb and co-precipitate some fluoride, but the capacity is low compared with a dedicated reagent, and the dose needed makes sludge volumes uneconomic. In practice PAC is the capture and settling aid, not the primary removal step.
How do we handle the sludge?
Fluoride-bearing sludge is usually dewatered with a cationic PAM and sent for compliant disposal. Check the local classification for your country before choosing a disposal route, because fluoride sludge is often regulated separately from ordinary biological sludge.