Sciencemadness Discussion Board - Large Scale NH4NO3 Processing

Large Scale NH4NO3 Processing

MrHomeScientist - 1-5-2018 at 11:45

My current project involves processing a rather large amount of dilute(ish) ammonium nitrate solution to make potassium nitrate crystals. This is "waste" from an educational outreach activity I did a while back where we dissolved nitrate cold packs in tap water and measured the temperature change. I collected the solutions afterward and was left with 16 liters (!) of solution of unknown concentration.

The first step is to determine concentration. This step would be unnecessary had I taken better notes, but here we are. To do this, I took a 500mL sample and boiled it down until crystals began to appear upon cooling to room temperature. This occurred at 144mL, and the crystals that had formed weighed in at 3.64g. Thus:

NH₄NO₃ solubility at 20C: 192g/100g
NH₄NO₃ molecular weight: 80g/mol

(192g/100mL)*(144mL/1) = 276.48g + 3.64g = 280.12g
(280.12g/500mL)*(1mol/80g)*(1000mL/1L) = 7M

So my bucket of solution is about 7 molar.

According to the equation:
NH₄NO₃ + KCl == NH₄Cl + KNO₃

We should have the following amounts of reactants/products in the remaining solution:
(7mol/1L)*(15.5L/1)*(80g/mol) = 8680g NH₄NO₃
(7mol/1L)*(15.5L/1)*(1/1)*(74.5g/mol) = 8083.25g KCl
(7mol/1L)*(15.5L/1)*(1/1)*(53.5g/mol) = 5804.75g NH₄Cl
(7mol/1L)*(15.5L/1)*(1/1)*(101g/mol) = 10958.5g KNO₃

Now, based on the solubilities of each of these at 20C, we have:

(8680g/1)*(192g/100mL) = 4.5L for NH₄NO₃
(8083.25/1)*(34.2g/100mL) = 23.6L for KCl
(5804.75g/1)*(37.2g/100mL) = 15.6L for NH₄Cl
(10958.5g/1)*(31.6g/100mL) = 34.7L for KNO₃

Thus, if I just dump in the requisite amount of KCl, a lot of it won't dissolve and some ammonium chloride will also fall out. The last thing I want to do is add more water to all this; does anyone have any suggestions on how to proceed?

=========================

One other important observation I made was during my initial test with a separate sample (not part of the 16L, but from the same event). I noticed not as much KCl would dissolve as it theoretically should have. For this experiment, I had a saturated solution of ammonium ntirate at 202mL. To this was added 75.13g KCl, the amount theoretically soluble in that volume. This was heated with stirring. Some of this didn't fully dissolve, which it turns out is because I made a mistake and actually only 69g is soluble. I filtered this off. Upon cooling, 59.6g of very clean crystals of KNO₃ was recovered.
Now the strange part. To this solution, now totaling 233mL after some water rinses), I added 70.12g more KCl. Theoretically about 80g is soluble, but I lowballed it to be safe. However a surprisingly large amount of KCl refused to dissolve this time, and I filtered off 52.5g undissolved salt! I'm not sure, but I suspect this is due to maybe a common ion effect, or the fact that solubilities may be quite different in this soup of ions as opposed to pure water. This result doesn't bode well for my main solution, and I don't want to start adding things until I have all the kinks worked out. Any suggestions on how to proceed would be appreciated!

Since you made it all the way to the end, here's a Pretty Picture of nice tabular KNO₃ crystals growing on a bed of cubic KCl crystals, from this second experiment.

SWIM - 1-5-2018 at 12:11

So do you actually want ammonium chloride too?

If not, how about just dumping KOh in there and boil off the ammonia?

If you want the ammonium chloride too, catch the vapors by bubbling through HCl.

MrHomeScientist - 1-5-2018 at 12:47

No, I only want the potassium nitrate.

Have you considered the scale of this problem? Using the KOH reaction would produce 108.5 moles of ammonia gas, or roughly 2,500 LITERS OF GAS. If you want to be anywhere near a gas release of that magnitude, be my guest. I'll gladly send you the bucket for processing.

phlogiston - 1-5-2018 at 13:23

Yes, that's a lot. You might be able to handle the gas by passing it through a few buckets of water.

That amount is a little less than 2 kg of ammonia gas, and a liter of room temperature water can dissolve as much as 500 grams. A train of buckets with a several liters of cold water each should trap much of it.

VSEPR_VOID - 1-5-2018 at 13:31

Very good. Did you keep the NH4Cl? You can use it to generate ammonia

SWIM - 1-5-2018 at 13:36

Ammonia burns, doesn't it?

MrHomeScientist - 1-5-2018 at 13:47

It sure burns the lungs! I used to be OK around it, but now even dilute ammonia gives me an instant, intense headache. I avoid it whenever possible. So while I realize the KOH method is way better for product purity, I'd rather not do it at this scale. The bucket train is a good idea, but then I'd have many liters of ammonia solution to deal with! I suppose if I can isolate the ammonium chloride reasonably well I'll hang onto it for future reactions.

VSEPR_VOID - 1-5-2018 at 13:59

Quote: Originally posted by MrHomeScientist

I suppose if I can isolate the ammonium chloride reasonably well I'll hang onto it for future reactions.

You could use it to make nitric acid if you have a U-tube and some platinum sitting around.

Generate your ammonia by reacting sodium, calcium, or potassium hydroxide with with your ammonium chloride, dry it, and then pass it through the hot noble metal with some dry O2. You will need to condense the HNO3 and rid it of excess NOx dissolved in it.

Deathunter88 - 1-5-2018 at 22:02

Think what you want to do with that much KNO3 before you react the solution. KNO3 is one of the least soluble nitrates, so once you get to it you're pretty much stuck. NH4NO3 on the other hand is much more soluble, so you can use it to make many more interesting nitrate salts. I would suggest you just evaporate the solution as is and collect the NH4NO3.

MrHomeScientist - 2-5-2018 at 04:31

I use the potassium salt far more than the ammonium salt, hence my desire to convert it. If I ever want more ammonium nitrate, I'll just buy more cold packs. I'm sort of surprised everyone seems to be trying to convince me to abandon my plan.

Sigmatropic - 2-5-2018 at 06:23

Use potassium carbonate. Only the first equivalent of ammonia will be released immediately, but is taken up by the solution. the 2nd equivalent is released as the ammonium bicarbonate decomposes upon boiling. Which also frees up the dissolved ammonia. It is quite a gentle boil IIRC. I have done something similar a long time ago but only with about 1-2 liter of saturated AN solution, so goodluck scaling 10x.
And yeah I agree you may want to capture the gasses or simply remove yourself from the area after you've set the heating.

unionised - 2-5-2018 at 12:35

If it works, it works.
The only thing I'd have done differently would have been to measure the density to get the concentration.

Texium - 2-5-2018 at 20:45

Quote: Originally posted by VSEPR_VOID

Quote: Originally posted by MrHomeScientist

I suppose if I can isolate the ammonium chloride reasonably well I'll hang onto it for future reactions.

Ah yes, armchair chemist casually suggests doing the Ostwald process as if it's an easy and fun time

PirateDocBrown - 2-5-2018 at 20:51

Around here, KNO3 is lots easier to come by OTC (Stump remover) than NH4NO3. Cold packs I have found are made of urea.

j_sum1 - 2-5-2018 at 21:05

Quote: Originally posted by MrHomeScientist

I use the potassium salt far more than the ammonium salt, hence my desire to convert it. If I ever want more ammonium nitrate, I'll just buy more cold packs. I'm sort of surprised everyone seems to be trying to convince me to abandon my plan.

I would be reticent to evaporate 16L unless I had to.
My inclination would be to fertilise my lovely Florida lawn. And then, if I needed/wanted more ammonium nitrate, buy some fresh cold packs. Or if I was after some potassium nitrate, get it from the most convenient source (whatever that was.)

(Around here of course, ammonium nitrate is not an OTC product. In my location it would actually be worth evaporating 16L to get some AN. KNO3 is far more readily available.)

MrHomeScientist - 3-5-2018 at 09:15

Yeah, the humidity here is not conducive to evaporation at all. I've had this bucket on my back porch for about a year now (!) and it hasn't appreciably reduced in volume. I transferred it to another bucket wrapped in black electrical tape and have it somewhere where it gets afternoon sun now, so maybe that will help.

Fertilizer isn't a bad idea, really.

Melgar - 3-5-2018 at 22:00

Make up a bunch of KOH or K2CO3 solution and add a few hundred mL to your bucket every day, if you're not in a rush. You'll eventually get KNO3 crystallizing on the bottom and the solution will lower in volume as the water is able to evaporate. Sure, you'll get a whiff of ammonia when you walk by now and then, but just do it like, right before you go to bed or something.