Drying out aqueous ammonium nitrate

As some of you might know NH4NO3 can be quite hard to get in the UK unless you are a farmer or run a quarry.

I've been able to produce impure NH4NO3 through the metathesis of Ca(NO3)2.4H2O and (NH4)2SO4, both of which are commonly available as fertiliser.

My goal is to have the whole thing dry enough for use as ammonpulver solid rocket fuel, or 85% NH4NO3 and 15% C.

The problem is drying it and removing a brownish soluble contaminant in the system. Any advice from the experienced chemists to an amateur chemist with an engineering background will be much appreciated.

Here are the steps I've taken:
1) Ca(NO3)2.4H2O has a molar weight of 236.15 g/mol for the calcium nitrate tetrahydrate. It comes as white prills about 2-3mm in diameter as Yaraliva Tropicote Calcium Nitrate fertiliser. It is dissolved fully in water.
2) (NH4)2SO4 has a molar weight of 132.14 g/mol. It comes as a moist brown crystalline salt of size ~0.5mm as Sinclair Sulphate of Ammonia. It looks like brown sugar and tastes spicy if accidentally confused with brown sugar. It is also dissolved fully in water.
3) The two mixtures are added together at a mole fraction of 1:1. Immediately a white precipitate of CaSO4.2H2O forms.
4) The mixture is gravity filtered. A clear dark brown solution is produced (see picture)
5) I do not know why the solution is brown but presumably it is because there is an excess of (NH4)2SO4. I used stoichiometric amounts.
6) I tried drying the solution in a vacuum chamber (built professionally for about £400) that goes down to almost -1 atmosphere. Initially the solution at room temperature boils, but absorbs heat of vapourisation and cools, then stops boiling until heat conducts into the chamber and raises the solution's temperature.
7) I've tried boiling and sun-drying it. The former is energy intensive and the latter is slow and ends up full of other impurities (dirt, grass, etc.)

My questions are:
1) How do I determine the impurities and their percentages in these fertilisers?
2) What is the principal impurity in the filtered solution?
3) How do I economically dry the solution?
4) For both the boiling and sun drying methods, eventually I have a wet mass of crystals. How much further can I heat it, bearing in mind my goal is bone dry NH4NO3 suitable for rocket fuel (even dry NH4NO3 obtained from Instant Kool Paks used in Ammonpulver can sputter out unless ignited by a very hot source). Ammonpulver quenched an igniting match head stuffed straight into it. It needed KNO3 + steel wool to start it going.
5) Is there any other way to purify the NH4NO3 apart from dissolving it in methanol and drying it? I do have methanol but it isn't the safest chemical to have drying around the house in large quantities...

[Edited on 23-4-2011 by IndependentBoffin]

Quote: Originally posted by Picric-A

Just for your personal reference, i completly disagree that NH4NO3 is hard to get in the UK!! I buy 5kg sacks of the stuff from my local gardening/hardware shop or £5.99 a sack. I can also buy NaNO3, (NH4)2SO4, H3PO4, H2O2 and all sorts of other chemicals from the same place. The NH4NO3 is in the form of white prills, which completly disolve in water giving a colourless solution. (which shows it had no CaCO3 contaminents like some forms of prills do) One can also obtain the stuff from ice packs, in around 50g quantities which cost around 99p each. More expencive i know but this is certainly avaiable to everyone and is very pure NH4NO3.

Would silica gel be an appropriate dessicator? Also, I have a professionally built vacuum chamber connected to an electric vacuum pump.

The following picture is the result of leaving the saturated solution of (NH4)2SO4 + Ca(NO3)2, with the CaSO4 precipitate filtered out, to dry in the sun for many days. It is a moist brown crystalline salt that seems to boil at around 140C, absorbing latent heat of vapourisation. If you keep heating it, it eventually melts into a brown liquid.



I still have no idea what the brown impurity from the (NH4)2SO4 is.

On dissolving the moist brown salt in methanol, an off white powder is produced while the brown contaminant remains in solution in methanol. So the contaminant must be something that can dissolve in methanol. Not very helpful as so does NH4NO3!



If you guys think trying to dissolve it in acetone will help you identify the impurity, let me know. I do have some of that lying around.

[Edited on 26-4-2011 by IndependentBoffin]

Would it be better to have a first pass filtration to remove the CaSO4 first? Otherwise won't the activated charcoal be just swamped by the CaSO4?

I tried dissolving it in acetone. Solution remains clear but an off white powder is seen at the bottom (see picture below)

I was planning on pulverising activated charcoal for my ammonpulver so that compound is definitely on my shopping list

From some research I have found that Yara brand fert . uses a "anti-explosive" C.C.B. coal combustion byproducts to contaminate the fertilizer for its missuses as a explosive making material.
Much time and money has gone into the making of a non-explosive fertilizer. This is so they can sell it on the market as a direct replacement for AN . The nitrate is under 25% (15.5%)making it hard if not impossible to det when mixed with a fuel, as well as these dang CCB's that can not be filtered out by conventional means.

The charcoal filtratration is a promising idea as well as using solvents to extract and separate out the goods.

It stinks that in some areas , such a cheap source for rocket composite fuel is now a really expensive regulated chem.

[Edited on 26-4-2011 by pjig]

I've bought some activated charcoal and added an equal volume to a saturated solution of ammonium sulphate (bottom flask). Edit, clarification: The solution in the bottom flask is very dark but it is a saturated solution of ammonium sulphate only with no carbon added.

I kept the thing warm (about 50 - 60C) over and electric heater for a few hours and let it sit at room temperature for several more hours.

I then filtered it. The lighter brown solution is what I obtained (top flask).

So using activated charcoal to get rid of the gunk inside the fertiliser grade ammonium sulphate from Sinclair was only a partial success.



What about if I buy cheap white copier paper, put some sheets vertically in a solution of the ammonium nitrate + unidentified brown gunk....after a few days I just scrape the white crystallised salt (ammonium nitrate) which should be at around the same height above the now dry salt bath, or cut the portion with ammonium nitrate, redissolve, filter and dry it?


[Edited on 30-4-2011 by IndependentBoffin]

Alright guys here's what I did today:

I got 1 mole of impure (NH₄₂SO₄ as Sinclair Sulphate of Ammonia fertiliser of mas 132.1g. As you can see from the picture it is extremely dirty when it should be white crystals.



I then added roughly 200ml of water so that the crystals should dissolve completely. 200ml should dissolve 148.8g at 20C.



The dissolved stuff is shown below. Note the insoluble sediment stuck to the walls of the flask and near opaque brown colour.



The filtered stuff is shown next:



And what gunk we find in the filter paper! Clarification: no activated carbon was used yet and the black stuff in the filter paper was what was in the fertiliser, and not added by me.



I will boil off as much water as possible and then try to recrystallise. 25kg of dirty sulphate of ammonia fertiliser cost about £50 incl. shipping. Not worth the trouble, in terms of time, hassle and electricity.

Does anyone know a supplier who will sell a 25kg sack of purer (NH₄₂SO₄ fertiliser in the UK?

BTW the boiling point of the (NH₄₂SO₄ solution is about 106 Celcius. I'm not going to risk leaving the electric hotplate on overnight even on low to dry the sample so I'll call it a day and continue tomorrow.

[Edited on 1-5-2011 by IndependentBoffin]

Well folks after boiling down the solution to about half volume late at night and letting it cool down + recrystallise overnight, I filtered out the remaining aqueous solution and scooped out the crystals.

With heating, more brown insoluble sediment has appeared in the solution, which started off clear brown. The insoluble sediment is caught in the filter paper as seen below.



The filtered solution is a lighter brown than in previous pictures.



But the recrystallised ammonium sulphate crystals are still filthy:



So it seems that the impurity in the (NH₄₂SO₄ is something which decomposes or undergoes a chemical change at about 106 Celcius (the BP of the saturated (NH₄₂SO₄ solution) to produce impurities with a dark brown colour and insoluble brown-black precipitates.

I don't have the chemicals to do a thiocyanate test (which might be too sensitive anyway) and carbonate test (would sodium bicarbonate do?) but AIUI Fe³⁺ catalyses the decomposition of H₂O₂.

I added some 35% H₂O₂ to the brown recrystallised (NH₄₂SO₄ and got no effervescence. I'm waiting to see if any bleaching of the brown colour occurs.

Good suggestion though, Fe³⁺ is brown and soluble with the right anion.

There can't be many brown soluble salts that go even more brown and produce a brown/black precipitate when heated at 106 Celcius. Has to be organic to have such a low decomposition temperature, but if so why can't activated carbon get rid of it? The organic molecule has to be simple or with lots of polar groups to be so soluble in water.

[Edited on 2-5-2011 by IndependentBoffin]

Yeah I had already decided to do that. I just find it really annoying that I don't know what the impurity is and my methods to separate it out are failing.

You know that feeling when you have a puzzle you can't solve in front of you? I'm just trying to purify it now for the science.

P.S. I have about 25 kg of impure ammonium sulphate fertiliser for sale. Any takers?

Quote: Originally posted by IndependentBoffin

So it seems that the impurity in the (NH<sub>4</sub><sub>2</sub>SO<sub>4</sub> is something which decomposes or undergoes a chemical change at about 106 Celcius (the BP of the saturated (NH<sub>4</sub><sub>2</sub>SO<sub>4</sub> solution) to produce impurities with a dark brown colour and insoluble brown-black precipitates.

Quote: Originally posted by IndependentBoffin

You know that feeling when you have a puzzle you can't solve in front of you? I'm just trying to purify it now for the science.