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Fluoride precipitation by lime, magnesium hydroxide, alum
We have a wastewater that consists of mixture of hydrofluoric and and ammonium fluoride. The total fluoride ion concentration in the solution is about 65,000 mg/L. My objective is to treat and remove the fluoride from this wastewater down to 20 to 30 mg/L because of wastewater discharge regulations. I have conducted jar tests using CaCl2, CaCl2 plus MgOH2), and Alum, with pH adjustment to pH 7 to 9. The best result / precipitation was with CaCl2 with pH adjusted to 8 using Mg(OH)2, but the residual fluoride concentration in the decant effluent was still about 1000 mg/L (filtered sample). While the solubility of both CaF2 and MgF2 are very very low in water, I am not getting good results! What's the problem and can you suggest any enhanced fluoride precipitation/removal without going into RO or other membrane technologies?
Zareh Maserejian- Dedham, Massachusetts
2005
2005
I treated lots of waste like this, back in the day.
If I were you, I'd add calcium hydroxide to a pH of about 10, then add calcium chloride until the dissolved calcium in a filtered sample is at least 500 mg/l. Then, add flocculant to drop CaF2 as a sludge. I never had trouble meeting our limit, which was 15 mg/l F.
Dave Wichern
Consultant - The Bronx, New York
I agree with the previous response. A combination of lime and calcium chloride usually works better than using just one of them.
Lyle Kirmanconsultant - Cleveland Heights, Ohio
2005
Hi I have a similar fluid, we are aiming to reduce fluoride from 20 g/L to 10 mg/L
We have trialled CaF precipitant with CaCl, filterability is poor.
What floc have you had success with?
Did you have a filter press on the back end of the process to give a good useable solid?
Thanks,
- Perth, WA, Australia
2007
I work at a Semiconductor industry and we too have been facing the problem of Fluoride precipitation for a while. The fluoride concentration in the effluent stream is around 100-130 ppm which we have managed to reduce to 10 ppm using Calcium Hydroxide. But the problem at present remains the long time taken for the settlement of the sludge formed (Calcium Fluoride). Kindly suggest/advice what flocculant/coagulant should be used for the purpose.
Jacob JoseEffluent Treatment - Fujairah, U.A.E.
April 18, 2011
April 25, 2011
I had similar problems - the cake coming out of the filter press was very gummy. It formed a kind of a "skin" up against the filter cloth, leaving the center of the cake quite wet.
I'd suggest contacting Nalco, or any other large polymer vendor, for technical assistance.
Dave Wichern
Consultant - The Bronx, New York
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