Acid recovery after RDX pption - WARNING!!!

[Mr Cool]

Maybe you wouldn't be stupid enough to try this, but I was :o .
I had read that residual organics (formaldehyde etc) hindered acid recovery from the spent acid after making RDX on an industrial scale, and therefore the spent acid was commonly used to make fertilisers instead.
I thought that maybe the problem was how to deal with the formaldehyde fumes on such a large scale, and I was willing to give it a go on a small scale.
So anyway, 180g of distilled acid, 60g of HDN, gave me slightly over 41g of crude RDX which is awaiting recrystalisation. I haven't had a chance to do this yet, my lab smells too bad still...
Obviously, if the residual acid in the solution after dumping it into water could be recovered, as it can for PETN etc, then the process would become much more economical. So I tried it...
There was approx 400mL of solution in total. My plan was to reflux it for a while to drive out/destroy most of the remaining organic compouns, before distilling off azeotropic nitric acid, then concentrating this with sulphuric acid and making another batch of RDX.
The strange thing was, it refluxed happily for almost 15 minutes, then in the space of around 5 seconds it turned into an erupting volcano of nitrogen oxides, nitric acid and formaldehyde.
This experiment cost me about £30 for a quickfit leibeg condenser, £7 for a quickfit reduction adapter, and a few hours clearing up acidic formaldehyde solution from all over my lab and the ceiling :( :(.
So, I thought I would warn you about it, just in case anyone was thinking of doing the same!

[Einsteinium]

Poor glassware, lab, chemicals, human! <img border="0" title="" alt="[Wink]" src="wink.gif" /> Quite freak story, at least fire didn't start up. I think what happened here is the reaction between ammonia (from ammonium nitrate/water equilibrium) and formaldehyde in acidic media to give methylamine, ethylamine, formic acid, HEAT, etc... (this reaction is quite known). Ammonium nitrate and formaldehyde are byproducts of the hydrolysis of HMTA (I assumed HDN = HMTA, hexamethylenetetratemine, Hexamine, Urotropine?) Could well be something else like a radicalar chain reaction though, that would explain the 15 minutes peacefull reflux.

If one is really in the mood of recycling nitric from RDX mixture, he could probably neutralize the residual liquor (with say, sodium bicarbonate), evaporate the solution, then add sulfuric acid until all sodium nitrate has reacted and finally distill out the acid (ammonia wouldn't complicate things as it would have been removed during the evaporation step). I'm not certain about this, but it does seems possible to do. In any case, I would rather purchase KNO3 from the drugstore rather than losing a couple of hours recycling the acid, that's my 2 cents. Hoping your lungs will recover properly from this experience :D

[rikkitikkitavi]

ammonia cant exist in a acidic environment, it will all react to ammoniumions...

but the remaining nitric acid plus the ammoniumnitrate formed probably started a runaway oxidation of the organic residue in solution, when it was heated enough beoynd a certain threshold.

Fortunately you had a dilute solution which prevented a even more feriocous reaction, possible explosion and fire!

maybe by distilling under reduced pressure, the majority of the organic can be stripped of , and the remaining carefully oxidized ...
but it is probably very difficult to control this reaction and get a good separation since it is difficult to judge at what temperature and pressure maximum organic/minimum nitric acid is done.

on an industrial scale this is probably one of the secrets of each manufacturer , since recovery of strong nitric is determinal for the cost of product.

/rickard

[Einsteinium]

The reaction I was referring to can be found here : <a href="http://www.rhodium.ws/chemistry/methylamine.html" target="_blank">http://www.rhodium.ws/chemistry/methylamine.html</a>. If one does the study of this reactions, he find out that it proceeds through this equilibrium :
NH₄⁺ + H₂O -&gt; NH₃ + H₃O⁺
I agree there's not much free ammonia in a such acidic solution (a couple micrograms, maybe), but there is enough to start this reaction. HCl is a stronger acid than HNO₃, thus there should be even more free NH₃ in Mr. Cool's acidic solution than in the actual reaction I quoted. Quantitativly :

NH₄⁺ + H₂O -&gt; NH₃ + H₃O⁺

K_A = [NH₃][H₃O⁺]/[NH₄⁺] = 6,31x10^-10

If we set the activity in hydronium cations to be of 0,1M (just for the purpose of the calculation, this means pH = 1,0) we find :

[NH₃]/[NH₄⁺]= 6,31x10^-9

This means that in this solution, if there are, for example, 1,0M in ammonium cations, there will always be 6,31x10^-9M in ammonia, this seems small but this is enough molecules to start a reaction. To convince yourself, use Avogadro's number and find out that in 400mL of this hypothetical solution, there is 1,5x10¹⁵ free ammonia molecules fooling around. That's 1,5 million of billion of ammonia molecules!.

<small>[ April 13, 2003, 05:32 AM: Message edited by: Einsteinium ]</small>

[Mr Cool]

Einsteinium, HDN = hexamethylenetetramine dinitrate. In my experience (and others), it gives better yields with less acid than free base hexamine.

Next time I'll probably recover the acid as HDN, by adding some hexamine, cooling, filtering etc, then using this in subsequent batches (although if I recover it all this way I'll soon end up with a lot of HDN!). If I recover it as NaNO₃, NH₄NO₃ etc, then I will actually be losing money that way since the prices for the hydroxides from the sources I have would make the resulting nitrates significantly more expensive than fertiliser nitrates, plus it'd be a lot more effort to recover them.

What a pitty it can't be purified like I was trying to do :(. Maybe under reduced pressure it could be done safely, but now I am reluctant to try it!!

I know it's really cheap just to make some more acid, but it seemed senseless to throw away 80% of the acid that you made! If that remaining 80% could be recovered, giving 100% efficiency based on acid (I know that wouldn't be possible, it's just to make it simpler to work out!), then the 200mL of 70% HNO₃ that I started with could have made well over 200g of RDX!

Well, from now on I'll only recover the acid after making PETN or MHN. I've been doing this safely for a while now, and there aren't really any organic by-products so I'm sure it's safe. But first I'll have to find another £40 to replace my glaswear :(.

[Microtek]

I remember seeing a patent for HNO3 recovery from waste flows after RDX production. I'm not completely sure, but I think H2SO4 was added before heating the waste. At least that way was used for recovering acids from HMX production via DADN. Another way was to use Mg(NO3)2 to break the azeotropic coupling between water and HNO3 which could then be distilled off.
Anyway, the mechanism in the method with H2SO4 was IIRC, that the byproducts from the RDX synthesis were destroyed by the hot sulfuric acid ( as in the nitrometer ) and volatilized.