FireLion3
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Boiling to dryness - risk of explosion?
Boiling some leftover aqua regia + water mixture off to recover a metal salt, and I need to boil it to dryness to recover the salt. I'm a bit wary of
this as I've never done it. A number of guides online I see say to never boil to dryness, in the case of distillations, or there is the risk of an
explosion.
Why is there this risk? The way my setup is, I have the vapors being boiled off through a hose into a sink with running water. None escape. Am I okay
to continue boiling to dryness to remove the excess Nitrogen Oxide fumes?
Boiling to dryness seems to be the suggested method for recovering palladium salts, I am just wary of doing so after reading about there being a
possible explosion risk, even though it probably refers to organics.
[Edited on 15-12-2015 by FireLion3]
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Detonationology
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An "explosion" caused by distillation could really only occur if there was a highly volatile solvent that was ignited. Even for flammable solvents, I
would assume that the condenser or stillhead would arrest any flame from entering the boiling flask. Anyways, the only issue I can see with boiling
to dryness would be the risk of thermal stress (i.e. cracking, shock, etc.) due to uneven heating, especially for an air bath or when in direct
contact with heat source or flame. Rotovaps are commonly used to dry products completely in a boiling flask while the solvent is evaporated,
condensed and isolated. There is no problem there because the boiling flask is very evenly heated in a water bath. For any non-decomposable salt, I
would just suggest drying in a baking dish with an old toaster oven.
“There are no differences but differences of degree between different degrees of difference and no difference.” ― William James
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Ozone
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Aqua regia residues have been known to blow up for no particular reason. But...
It's probably OK, so long as there is *no* chance you have nitrated organics in there. However, the rule is good because heating to dryness also has a
tendency to, via a contracting sticky mess that sticks to glass, explosive peroxides/nitrates/percholorates, etc., or thermal shock, to wreck your
glassware (and maybe, you). This is why I cannot recommend it.
While you could probably get it to a syrupy paste and finish it off with vacuum (or evaporating dish), it would be better, yet, to ppt the desired
metal and convert to the desired salt.
Further guidance is (as I see it) impossible given the paucity of information you have given us, so far. Mystery metal and mystery salt...I'm guessing
Pd, but that still leaves a pretty wide field-of-play.
O3
[Edited on 15-12-2015 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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FireLion3
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I should be okay then. There are no organics in the mixture, and yes the metal is Pd (as indicated by my past posts), I just didn't think it was
relevant to the question.
There are no flames and all vapors are going into the drain (with water running). The only worry of mine was that there could be a spontaneous
combustion of the nitrates/perchlorates, maybe from the heat. The hot-plate I am using doesn't even heat beyond 250 degrees, so I may be worrying over
nothing.
I may end up drying the final mixture in the oven, but I can't even think about that until I get all the dissolved NOx out of the solution. That is my
main concern at the moment.
Thoughts?
Edit:
Maybe I could just heat it on very low heat, such as 110 degrees, since the hotplate is digital. The contact surface with the glass wouldn't exceed
that.
[Edited on 15-12-2015 by FireLion3]
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ProChem
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i add some mineral oil to the still pot (amount depends on size of batch) which keeps the still from going dry. Explosions occur from peroxides
concentrating during the distillation. Another additive is aliquat 336.
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Sulaiman
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I have been through the gold, aqua regia, urea, smb cycle a few times recently, on a small scale, < 1g Au
I heated residues from almost every stage to dryness in test tubes, not even a hint of explosiveness
things reliably behave differently in bulk so comments only apply at this scale.
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