The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Fluoride removal from waste water below 5 ppm

June 10, 2011

Q. Hy,
my name is Andrija Proso and I am chemical engineer in Croatia.
We are producing laminated aluminum foil (lacquer, aluminum with chromate conversion coating in it, adhesive, CPP).
Our production speed is around 70 m/h of laminate.
Chemical pretreatment before lamination goes like this: first foil is entering in acid cleaning (H2SO4+ non-ionic tenside; Ridoline 124) bath, after that it is rinsed with water, after that it enters in chromating bath (Alodine 45+Alodine 401; mainly chromic, phosphoric an HF acid) then rinsed in several stages with DI. Water.
The rinse water goes to waste water treatment plant, the chromium rinse water (contains Cr6+) are reduced with sodium bisulphite at pH=2.0-2.5, then is mixed with rinse water from acid cleaner bath and alkalized with lime milk to pH=10 which is best for removal for phosphate (limit is <2.00 ppm), and also suitable for heavy metals precipitations. We also add some poly-electrolyte to improve sedimentation of the sludge in the settling pool.
At the end of the pool we are pumping clear water (pH = 10) to another reactor where we polish the pH to 6.5-7.0 (discharge limit is 6.5-8.0) with HCl and discharge the water in sewer.
The problem is next, we still have high fluoride in the finish around 14-18 ppm (limit is <5.00 ppm).
Everything else is below limits (total Cr <0.1 ppm; Cr6+ <0.05 ppm; P <2.0 ppm; Al <2.0 ppm.

I try in jar test to remove fluoride by adding Al2SO4 to water (pH=10; one that is pumped from the end of the settling pool to polishing reactor) and bringing the pH to around 4-4.2, and than neutralize it to pH 6.5-7.0 with NaOH to start Al(OH)3 precipitation, I also add polyelectrolyte to speed up the settling. I run test for fluoride and they were below limit of 5 ppm.

The problem is that we have no space to install another settling unit (it demands a big one), is it possible to resolve this in one step?
I am also afraid to use Al2SO4 because if sedimentation is not perfect, we will have a new problem, that is high Al (> 2.00 ppm) at finish.

Can you please give me some advice regarding this problem?

Kind regards,

Andrija Proso
Employee - Croatia

---

---

Q. We plate "leaded Brass" at our facility and using a Fluoride based preparation chemical is a problem because the fluoride ends up in the rinse water and without any special treatment for fluorides it goes down the sewer line. Our POTW limits our fluoride discharge to 3 ppm. Is there any other chemical that can be used?

B Ishtiaq
- New York, USA
July 8, 2013

A. Is the plating bath based on fluoboric acid? If so, then you have a very difficult problem. The fluoborate ion hydrolyses rather quickly to release the first two of the four fluoride ions it contains, but after this, only very slowly. This BF2(OH)2- ion will not precipitate with calcium, but will test positive for fluoride if the analysis includes a pre-distillation step. Other solutions to your problem are to use a plating bath formulated with MSA instead of fluoboric acid, or to close-loop the rinses by using vacuum distillation. Lyle Kirman consultant - Cleveland Heights, Ohio July 12, 2013 A. That is a tough limit. What I used to do is: add lime to a pH of about 9. Then, add calcium chloride such that there's about 400 ppm excess. More does not help. (I did a study of this) Then let settle, decant, dewater solids, etc.This will get you down to 10 - 15 mg/l. Then, I guess you will have to apply a polish treatment to the combined decant and filter press return water. Alumina adsorbs fluoride. Dave Wichern Consultant - The Bronx, New York July 12, 2013