Removal or reduction of fluoride ion concentration in saturated sodium sulfate
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There is a client who needs to remove fluoride ions at a concentration of nearly 2% from a nearly saturated sodium sulfate solution; reducing that concentration would also be acceptable. I’m wondering if there are any ways to do this
Does anyone who knows know? Help please
If anyone knows, please let me know. Thanks!
Generally, Ca(OH)2 is added to form CaF2, and then trace amounts of F ions can be filtered out using a ceramic membrane. You can look for Suzhou Weitao; they have this ceramic membrane technology. Hope it’s useful to you
When lime water is added, doesn’t the reaction with sodium sulfate produce a precipitate of calcium sulfate? This estimate can’t be right, can it? Can trace amounts of fluoride ions be filtered using ceramic membranes?
It still depends on the fluoride ion concentration in your raw water. If it’s not high, around 10 milligrams or so, adsorption can be used for removal; if the concentration is high, it’s recommended to use calcium ions for removal first, and then deal with the remaining calcium ions. Fluoride ions are in ionic form; ceramic membrane filtration is essentially similar to ordinary filtration and belongs to the micron-level filtration category, so it has little effect on removing fluoride ions
This post was last edited by qugd on 2016-9-18 at 12:20. The original poster mentioned around 2% fluoride ions, which is not in the same order of magnitude as several dozen milligrams. This situation is actually not that difficult to handle. By adding an appropriate amount of calcium sulfate dihydrate (gypsum) slurry, fluoride ions can form calcium fluoride with lower solubility in combination with trace amounts of calcium. This reaction is slow, and proper stirring as well as an appropriate pH range are required. Finally, the precipitate is filtered out; the trace amount of calcium sulfate remaining should not affect the quality of sodium sulfate. Please refer to some basic data: the solubility of gypsum is approximately 0.2 grams, while that of calcium fluoride is about 0.002 grams; thus, most fluoride ions can be removed using calcium sulfate. This may require some process condition tests.
The method you mentioned is theoretically feasible, but in practice, overall operation control and the amount of chemicals added must be managed with great precision. I have conducted numerous experiments on fluoride-containing wastewater; whether lime, calcium chloride, or other calcium-containing substances are used, to enable fluoride ions to form precipitates within an appropriate pH range, an excess of calcium ions is required. Since the water contains fluoride ions at a concentration of 2%, there ends up being more than just trace amounts of calcium ions in the sodium sulfate
So, if calcium sulfate is used for precipitation, what range would the concentration of calcium ions in the final solution be? Calcium ions are much easier to handle than fluoride ions; if the amount is a few hundred milligrams, resins can be used for treatment, but at gram levels it’s likely to be quite difficult.