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Effect of high boron on lime consumption in Fluoride precipitation process
We have a waste water treatment system that uses lime to reduce
180 ppm of fluoride to 10 ppm. The system was designed to handle a maximum boron content of 130 mg/l in the waste water. In real life, the waste stream contains 1300 mg/l of boron and now we can not reduce the fluoride in the effluent stream below 18 to 25 ppm. We tripled the lime flow and still we are not able to go below the 15 ppm (the pH of the precipitation basin will not increase above 8.7). The system can not be changed, however, we can get a variance on the
10 ppm if we can show by a technical document the effect of boron on the precipitation of fluoride. Modification of the system is not feasible at this time. Please provide any references or technical articles that may address this or a similar problem.
Thank you for your attention
- Barberton, Ohio
"Boron Fluoride and Its Compounds as Catalysts in Organic Chemistry"
by A. V. Topchiev
from Abe Books
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Here is an excerpt from a UN report on Boron. You may find some insight. I can run some thermodynamic equilibrium curves on the concentrations, if I knew more about the water chemistry. In the absence of such data, I can suggest you add a little phosphoric acid to the water in a beaker/jar test and see if the precipitation performance gets better. I suspect the borate ions are adsorbing onto the calcium, which already has limited surface availability when you use lime, and not CaCl2. The concentration and pH would drive the reaction in that direction. If I find any more references, I will post them.
"Boric acid is a very weak acid, with a p Ka of 9.15, and therefore boric acid and the sodium borates exist predominantly as undissociated boric acid [B(OH)3] in dilute aqueous solution below pH 7; above pH 10, the metaborate anion B(OH)4 becomes the main species in solution. Between pH 6 and pH 11 and at high concentration(>0.025 mol/litre), highly water soluble polyborate ions such as B3O3(OH)4, B4O5(OH)4, and B5O6(OH)4 are formed.
Biggar & Fireman (1960) determined that the fixation of boron in soils occurs by one of three mechanisms: physical (molecular) adsorption, in which the boron is held to the surface of the soil by van der Waals bonds; anion exchange; or chemical precipitation. Chemical adsorption involves ionic and covalent bonding. The investigators speculated that the initial adsorption is probably molecular in nature, followed by the formation of surface compounds that result in an increase in adsorption sites, particularly at higher boron concentrations in the soil solution. At higher concentrations, chemical bonding of borate ions with hydroxyl ions on the soil surface results in boron fixation to soluble aluminium, silicon, and iron.
This same mechanism (chemisorption) was observed by Couch & Grim (1968) for the uptake of borate ions to clay mineral surfaces. The presence of calcium ions, drying, and high pH values will tend to increase the amount of fixed boron. Wetting and drying of the soil will increase the maximum adsorption capacity and bonding energy of the soil for boron."
- Brea, California, USA
Boron and Fluoride do indeed form complexes. My experience on this subject comes from the treatment of fluoborate
(BF4-) plating bath rinses. It is just about impossible to meet a total fluoride limit when treating these rinses. The BF 4- ion hydrolyzes rapidly to BF3(OH)- and then to BF2(OH)2-, and then very,very slowly after this. Since most of these fluoborate solutions have been replaced by methane sulfonic acid solutions, this particular fluoride treatment problem has largely disappeared.
It is likely that you are forming some borofluoride complexes such as BF(OH)3-. These complexes are not precipitated by calcium chloride or by lime. However, in a total fluoride analysis, preceded by a distillation step, it will test as fluoride ion.
Is it possible to locate the source of the boron and isolate it from the fluoride? That would probably make the lime precipitation work.
Other possibilities include adding some phosphate to your precipitation process, or a final polish using activated alumina. Phosphates such as hexametaphosphate and sources of phosphate like bone char have been used to enhance fluoride removal in precipitation systems.
Activated alumina is used as an ion exchange media that can remove and concentrate fluorides. The fluoride ion can also be regenerated off of the media and sent back to the lime treatment system. Activated alumina is capable of removing fluoride to < 1 mg/L.
consultant - Cleveland Heights, Ohio
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