Sciencemadness Discussion Board

Drying out aqueous ammonium nitrate

IndependentBoffin - 23-4-2011 at 08:56

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...

ammonium_nitrate_solution.jpg - 97kB

[Edited on 23-4-2011 by IndependentBoffin]

Picric-A - 23-4-2011 at 10:03

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.

hissingnoise - 23-4-2011 at 10:49

Just for your personal reference, i completly disagree that NH4NO3 is hard to get in the UK!

Talking to cops about making explosives might just, er, complicate things for IB . . .
But contaminated NH<sub>4</sub>NO<sub>3 </sub> can be purified somewhat by cautiously heating to its melting point in the hood.
Most of the phenolic contaminants will vapourise; some will be oxidised by nitrate.
If all else fails IB, look for a cleaner sulphate!

IndependentBoffin - 23-4-2011 at 11:07

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.

Can you please let me know the name of the shop (PM or in the thread)?

Why can't I find any ammonium nitrate to buy online?

Picric-A - 23-4-2011 at 11:14

Unfortunatly its an un-branded/loca shop here so there wont be another one like it but my point was im sure there are others that will sell chemicals like that. Do you live in a city? Maybe then it might be a tad more challenging!

IndependentBoffin - 23-4-2011 at 11:23

What does fertiliser grade ammonium nitrate look like? Can you only tell from its NPK rating or does it actually say "Pure Ammonium Nitrate" on the sack?

hissingnoise - 23-4-2011 at 11:42

Ammonium nitrate will normally be rated 35-0-0.
But calcium ammonium nitrate (C.A.N.) may be easier to get than AN.
The clay coating the granules will need to be filtered out.
The IRA used C.A.N. for bombmaking as is, allegedly!

ldanielrosa - 24-4-2011 at 21:15

It sounds like your ammonium sulfate is dirty. Try recrystallizing that before you start. When it's pure, it'll also be white.

IndependentBoffin - 24-4-2011 at 23:46

OK thanks I'll try to recrystallise my ammonium sulphate!

redox - 26-4-2011 at 03:16

For drying, you could desiccate with an appropriate agent in a desiccator bag, or you could just pour the solution into a wide-mouth bowl with a stream of air blowing into it. It would evaporate in a two or three days.

May I ask, How many milliliters are you experimenting with here? 500 mL? 1000 mL?

IndependentBoffin - 26-4-2011 at 08:44

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.

100_3781.JPG - 119kB

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!

100_3775.JPG - 110kB

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]

smuv - 26-4-2011 at 08:48

why not use aqueous ammonia instead of ammonium sulfate? Fert grade ammonium sulfate is extremely dirty. It even smells funny. The precip will now be calcium hydroxide.

IndependentBoffin - 26-4-2011 at 08:56

Ca(NO3)2 (aq) + 2NH4OH (aq) -> Ca(OH)2 (ppt) + 2NH4NO3 (aq)?

Where did the calcium hydroxide come from when methanol is added? If I reacted the stuff in stoichiometric amounts and filtered off the CaSO4 there should barely be any dissolved Ca++ ions left.

Also, if there were dissolved Ca++ ions, where did the OH- ions come from to precipitate them out?

Is it a known fact that fertiliser grade ammonium sulphate is extremely dirty? What about fertiliser grade calcium nitrate?

Sorry for all the questions, not that I don't believe you, I am trying to learn from you.

ScienceSquirrel - 26-4-2011 at 09:04

i would try adding a few grams of finely powdered activated charcoal to the crude solution formed at 3), give it a good boil if you can, and then filter it.
Charcoal takes out lots of glop and boiling will often make precipitates easier to filter.

IndependentBoffin - 26-4-2011 at 09:11

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 :D

100_3783.jpg - 121kB

ScienceSquirrel - 26-4-2011 at 09:20

I have added activated charcoal to truely filthy mixtures. What it likes is big polar organic shit like tars. It will ignore the mainly totally insoluble calcium sulphate and the ammonium nitrate. The fact that the glop dissolves in methanol is good from this point of view.
It is illegal but activated charcoal has been used to decolourise red diesel for agriculural use turning it in to white road, the people concerned would then pocket the duty.
This was a major funding source for the IRA, along with other scams, the used charcoal was dumped often creating local pollution problems.

IndependentBoffin - 26-4-2011 at 09:27

Ah, that is very interesting. I always thought activated charcoal works by having a high surface area and trapping contaminants in its pores. It seems my understanding is inconsistent with what you're saying that it ignores insoluble calcium sulphate and soluble ammonium nitrate.

So how does activated charcoal grab organic stuff like tars? By what mechanism does it work on?

Thanks also for the story about using activated charcoal to decolourise red diesel. Very interesting as well. Does it still work or is a different dye used to counter this? N.B. I have a petrol car but find this very interesting academically.

ScienceSquirrel - 26-4-2011 at 09:46

I really do not know how it works but it has a greater affinity for things with more binding sites.
That is why it is used in aquaria, water filters, etc.
It takes out organic goo produced by the fish or trace contaminants in the water and leaves the water nice and clean with most of the minerals left.
If it just gobbled up everything indiscriminately it would be useless

pjig - 26-4-2011 at 09:59

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]

IndependentBoffin - 26-4-2011 at 10:11

How do CCBs prevent detonation of fertiliser? I would have thought any oxidisable material would just increase the -(delta H) and thus increase sensitivity. Aluminium particles in ammonium nitrate certainly does the job of sensitisation.

By the way even if I filter out the brown gunk how am I going to dry the aqueous ammonium nitrate?

pjig - 27-4-2011 at 07:04

This patent show the reasons and the processes for this Anti-explosive coatings.

IndependentBoffin - 27-4-2011 at 07:17

Yeah, I now totally agree that activated carbon must be used to get rid of any gunk in the fertiliser to get higher purity AN.

I've placed an order for some activated carbon for aquarium filters and will let you all know how I get on.

IndependentBoffin - 30-4-2011 at 00:40

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.

ammonium_sulphate.jpg - 269kB

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]

ldanielrosa - 1-5-2011 at 02:29

What scale do you need to work on for your use? You can clean your ammonium sulfate, but from the pictures it appears that it'll take some effort. How far is the next fertilizer dealer?

I'm sorry I can't find my camera to show my results of the same reaction. Unfortunately (fortunately for me), my (NH4)2SO4 is probably cleaner than yours so my results will not match yours with the same effort- but my ammonium sulfate was spilled on the floor from two different bags and I did recrystallize some for use in other experiments so the before and after pictures would have some meaning here.

Anyway, the price of fertilizer should allow some waste in the spirit of obtaining a clean product. I may have missed something, but it appears that you are cooking to dryness. By dissolving on hot cycles and harvesting on cold cycles you get a cleaner product while the contaminants concentrate. True, you will need to dispose of the filthy liquor at some point, but it's still good fertilizer.

As for drying, I'm working on small scale so I don't feel bad about using sodium hydroxide as a desiccant.

IndependentBoffin - 1-5-2011 at 04:16

I'd like to be able to do at least one static rocket test a day for an invention I am prototyping. The propellant grain is kept under 100g for legal reasons. I'm investigating different propellants - KNO3/sugar based, ammonium nitrate-carbon-aluminium-sulfur based and then moving on to chlorate, perchlorate based once my electrolysis anodes are ready.

Rocket fuels using KNO3 as an oxidiser are easy to mix up and cast, so they can be stored separately.

Ammonium nitrate from fertiliser (especially if an aqueous metathesis reaction is involved) is quite a pain to purify and dry. I'd like to have just under 100g dry + pure ammonium nitrate at any one time (bearing in mind ammonpulver propellant is 85% ammonium nitrate and 15% carbon) and say a kilo in the pipeline of being dried and purified. I don't think impure, wet ammonium nitrate is classified as an explosive although I may stand to be corrected.

I may have missed something, but it appears that you are cooking to dryness.

I've tried different things - placing in a baking tray in the oven, baking tray over the embers of a BBQ, baking tray in the sun, small baking tray with wet crystals in a larger baking tray with silica gel covered with clingfilm.

I guess I have a couple of questions here. I have looked around on Google but no luck.

1) Is indicator silica gel adequate as a dessicant? I have several kilos of it leftover from a separate project.

2) Is it possible to efficienctly recrystallise ammonium sulphate? It is incredibly soluble (70.6 g/100 mL (0 °C), 74.4 g/100 mL (20 °C), 103.8 g/100 mL (100 °C)) and its solubility only varies by about 30% over 100 Celcius.

3) Is it better to recrystallise the ammonium sulphate or the filtered ammonium nitrate? If the former is done, then two drying stages must be performed and two stages for wastage incurred. Of course at the moment the ammonium sulphate is so impure I am probably not even reacting it with the calcium nitrate stoichiometrically.

4) How is activated carbon best used? What volume ratio to the contaminated solution, should the contaminated solution be dilute, saturated or super saturated? How many times can it be reused? Is it better to flush through the activated carbon many times with the same contaminated solution or to just warm/boil it together for some time?

5) It seems from the patent quoted earlier that the contaminant is organic in nature. Does this mean it should support combustion (fuel for the fire, so to speak) while it may inhibit detonation? In which case I don't need to bother purifying my ammonium nitrate at all.

Alright I am going to try to recrystallise some ammonium sulphate now. I'll also take a lot more pictures to document the process better.

ampakine - 1-5-2011 at 06:47

Boil off the water then try recrystallising from water or another suitable solvent.

IndependentBoffin - 1-5-2011 at 15:32

Alright guys here's what I did today:

I got 1 mole of impure (NH<sub>4</sub>;)<sub>2</sub>SO<sub>4</sub> 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.

ammonium_sulphate.jpg - 132kB

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

200ml_water_added.jpg - 106kB

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

dissolved_ammonium_sulphate.jpg - 164kB

The filtered stuff is shown next:

filtered_ammonium_sulphate.jpg - 76kB

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.

filter.jpg - 155kB

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 (NH<sub>4</sub>;)<sub>2</sub>SO<sub>4</sub> fertiliser in the UK?

BTW the boiling point of the (NH<sub>4</sub>;)<sub>2</sub>SO<sub>4</sub> 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]

ldanielrosa - 1-5-2011 at 23:36

You can dissolve more ammonium sulfate in boiling water, then let it cool to room temperature and some crystals will precipitate out. Remove crystals, reheat and repeat until the crystals come out dirty.

As for overnight, a crock pot filled with sand is a wonderful thing.

IndependentBoffin - 2-5-2011 at 04:56

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.

filtered_recrystallised_solution.jpg - 140kB

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

filtered_recrystallised_solution2.jpg - 92kB

But the recrystallised ammonium sulphate crystals are still filthy:

recrystallised_ammonium_sulphate.jpg - 87kB

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.

a_bab - 2-5-2011 at 05:39

I'd dare to say your ammonium sulphate is adulterated with some sulphate, that would somehow react and turn to the insoluble, disgusting-looking iron hydroxide when boiled.

IndependentBoffin - 2-5-2011 at 06:36

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 Fe<sup>3+</sup> catalyses the decomposition of H<sub>2</sub>O<sub>2</sub>.

I added some 35% H<sub>2</sub>O<sub>2</sub> to the brown recrystallised (NH<sub>4</sub>;)<sub>2</sub>SO<sub>4</sub> and got no effervescence. I'm waiting to see if any bleaching of the brown colour occurs.

Good suggestion though, Fe<sup>3+</sup> 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]

pjig - 2-5-2011 at 06:56

Maybe it would be worth ditching that batch of A-sulfate and purchasing from a diff source. It is a cheap material, and not worth the time your spending in trying to clean it up.
Id try to get the hydro-grade crystal A-sulfate. It has way less impurities and dissolves very well.

IndependentBoffin - 2-5-2011 at 08:03

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? :P

watson.fawkes - 2-5-2011 at 19:09

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.
106 &deg;C is the boiling point at atmospheric pressure. Seems like vacuum stripping of the water would be something to try.

pjig - 7-5-2011 at 14:12

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.

I know exactly what you mean.. I too was loosing sleep over trying to extract CCB's from my CAN for quite some time... Very frustrating for sure.