Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
January, 2007

Treatment of Hydrofluoric Acid Waste

Q. My question concerns treatment of hydrofluoric acid waste. Is it required to add calcium hydroxide and calcium chloride both to maintain excess Ca ions to precipitate fluorides? What will be the content of the sludge produced if a waste? of 10cum/hr contains 1500 ppm flurides.Pls advise on some references for HF treatment

A. The addition of both would probably speed up the process of neutralizing the HF by making the Ca ion concentration available to the HF higher than without the Ca(Cl)2. The Ca(OH)2 should work by itself but would be slower to react as the Ca(OH)2 has a limited solubility in water, but the Ca(Cl)2 is highly soluble. Ca(Cl)2 by itself would not work.

I am not aware of any references specific to HF neutralization that I could recommend right now.

The sludge would include both Ca(F)2, CaF(OH), Ca(OH)2 and possibly some CaFCl. There will be a substantial amount of unreacted Ca(OH)2 in the sludge so it is difficult to give you a precise number for the amount of sludge produced. The more excess of Ca(OH)2 you add the faster the neutralization step goes but you also get more sludge as a result. The theoretical amount of Ca(F)2 sludge would be meaningless as it would not go to completion and would be too slow. It would leave some unreacted HF and Ca(OH)2 behind if you tried to use stoichiometric amounts.