Dilute HF/Nitric Neutralization MYSTERY

[Aaron William]

My mate and I are neutralizing (what we think is) a 2.5 gallon carboy of 10% HNO3 and 1% HF solution. Reason: We want to pour this stuff down the drain and be done with it.

We use calcium hydroxide because we didn't want to create any sodium fluoride (nasty stuff that it is) from using soda or Na-hydroxide and also because calcium carbonate is very insoluble (and too damn slow).

Initially we slaked the lime in water and added it ever so slowly. Nothing happened. I wanted to at least get the satisfying fizz I get when I neutralize diluted nitric acid with soda; so I started dropping the lime powder in by the scoopful. I had my lab coat on - carboy in the fume hood, what could possibly go wrong?

Still, no heat, no fizz (no carbon dioxide evolved I guess), nothing. We had to stir it up a bit to allow the solid powder to react but once pH was around 7 we called it good and started pouring it down the sink (with lots of excess water). It took 250 g of 95% calcium hydroxide to do the job.

I think the reaction is this:
2HNO₃ + Ca(OH)₂ -> Ca(NO₃)₂ + 2H₂O
2HF + Ca(OH)₂ -> CaF₂ + 2H₂O

Something doesn't feel right. I feel like we had a very uneventful reaction and, at least in theory, that we were "blessed" with fairly benign products. Is there a byproduct we are overlooking? Could the CaF and (dilute) calcium nitrate solution damage our plumbing? Probably our solution was more dilute than we thought (who can keep track of these things - details!). So many questions...

EDIT *I did a stoichiometry and I guess the solution was more like 3-4% acid. Still - am I overlooking something that is going to cause my lab to explode?

 

[Borek]

I must say I feel uneasy about the idea of working with such a mix, about neutralizing it the way you did, and about the idea of pouring it down the drain. Depending on where you are it could be a criminal act.

That being said - if you are still alive and the pH of the solution was around 7, I can't think of anything that could put your lab in the orbit.

 

[Aaron William]

I checked with my local treatment center to see if I can send some fluorides (as CaF) there way and they didn't think it'd be a problem. I'm sure CaF passes right through the treatment process but, in this amount, it certainly won't cause them to bust their emission limit.

If the solution truly is dilute (@5%) what is the worst that could happen? Maybe I don't have a proper gauge of the risk... but it seems like having this big mass of water if making the neutralization a lot safer.

 

[Borek]

If you checked with them, that's much better. My main point is about HF - its a nasty thing to work with. I understand you took necessary precautions, but I still feel creeps. Perhaps that's just me.

 

[Dr_Morbius]

Calcium hydroxide will not "fizz" when added to HNO3 and HF because no gas is produced.

Calcium nitrate is harmless as long as its dissolved in water. If allowed to dry and crystallize and come into contact with an organic material in the presence of a flame or spark or sufficient heat it could cause a fire and possibly an explosion. Nitrate is one of the main ingredients in gunpowder after all.

Calcium fluoride on the other hand is insoluble and just forms an inert precipitate. Although if exposed to concentrated acid, specifically sulfuric acid, it could release fluoride ion.

I would dump excess Calcium Hydroxide into the HNO3 + HF mix until the pH was 8 or more just to be safe. When adding the calcium hydroxide to the acid mix keep an eye on the temperature of the mix as it will heat up.