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Author: Subject: Purifying calcium/ammonium nitrate fertilizer, how to determine product?
Junk_Enginerd
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Purifying calcium/ammonium nitrate fertilizer, how to determine product?

Hi, I finally decided to make an account. Been reading for quite some time, but I'm starting to need a bit more interactive help... I'm an electrical engineer but I've always had a thing for chemistry. Lately I've picked up interest in it again, and I've started dabbling for fun with no particular goal in mind.

I recently discovered that a local gardening store had a big sack of fertilizer named "Lime Saltpetre" which piqued my interest. Until now I've been satisfying my nitrate needs with KNO3 stump remover, but it's a little expensive at ~$25 per 1 kg, compared to this saltpetre for ~$25 per 25 kg...

I checked the safety sheet for the product to get a better understanding of what's actually in there but it didn't provide much clarity. It just says "Calcium ammonium nitrate" which as far as I know is not a compound but some mixture.

I've read up on topics regarding this product and the general consensus seems to be that it's ammonium nitrate and limestone, but I'm not so sure that's what I have here. I'm in Sweden, so quite often american sources of information don't really apply, so I have to take everything with a grain of salt...

I've started the process of trying to purify it and a get a somewhat pure and dry nitrate out of it.

The product is white spherical prills, and almost all of it dissolves in water. This is what leads me to believe there isn't a significant amount of limestone in there, contrary to what people's experience here seems to be. The insoluble parts of it are present as a cloudyness which ends up being a surprisingly small amount of material once filtered, and a horribly sticky and slimey substance that mostly floats, and is a nuisance to clean off anything it sticks to(what is this??).

Absolutely crazy amounts of it can be dissolved, which I understand is quite typical of ammonium nitrate.

The method of purification I've gone with so far is the same as I've done with potassium nitrate; dissolving in water, heating, filtering and recrystallization by cooling. It's been a lot more finnicky than KNO3, but once I got the amount of water dialed in right(which didn't leave me with one giant solid chunk nor nothing at all) I've started to get good crystallization.

But... Those aren't ammonium nitrate crystals as far as I can tell. Looking up what ammonium nitrate crystals look like, they seem to be similar to KNO3 crystals: slender hexagonal rods. What's recrystallized in my jar is chunky, blocky and angular. I'll try to upload a photo later, and look more closely at the crystal structure, but from what I can tell it looks a lot more like calcium nitrate crystals, or maybe a double salt crystal? That would align with what the safety sheet calls calcium ammonium nitrate I guess?

I think for the experiments I want to attempt it won't matter very much what nitrate it is, I'm mostly after the nitrate ion anyway, but I'd still like to know exactly what it is I've gotten here.

If it's a double calcium ammonium nitrate salt, how would one separate these? Is it practical/possible at all?

And a final question on safety... One of the things I want the nitrate for is to produce nitric acid. I've done it successfully before by bubbling NO2 from hydrochloric acid, nitrate and copper before, but I'd rather not have to spend lots of hydrochloric acid in the process. I understand most nitrates will release NO2 upon sufficient heating, and I'd like to try that route instead and eliminate the need for HCl. The explosive nature of ammonium nitrate has me a tiny bit worried though.

I don't *really* think there's any danger, considering how difficult it is to make it detonate, but I'd feel better if someone could confirm that heating ammonium nitrate in a mostly sealed container to temps in the 700-1000°C range isn't going to cause anything dramatic to happen with the nitrate..

hissingnoise
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 Quote: I don't *really* think there's any danger, considering how difficult it is to make it detonate, but I'd feel better if someone could confirm that heating ammonium nitrate in a mostly sealed container to temps in the 700-1000°C range isn't going to cause anything dramatic to happen with the nitrate..

My experience with C.A.N was that the filtered crystals were cream-coloured, so I heated the filtrate above its MP (170°C) in a casserole dish until decomposition had just begun, about 210°C.

When the bubbling liquid solidified, it was snow-white as the gas-liquor contaminents it had contained had been oxidised.

Recrystalising then gave long, clean crystals...

For producing NA, though, it's more convenient (and efficient) to react boiling sols of AN and KCl and distill the acid from the KNO3 formed, using H2SO4.

Pumukli
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Rest assured: heating ammonium nitrate to the 700-1000 C range in a closed container WILL LEAD to dramatic things happen!

Ammonium nitrate decomposes if heated above 120 C - 150 C or so! In a closed container it will not necessarily mean detonation but deflagration can be almost as bad!

In a semi-closed container it is not that bad - depending on quantity and placement of course. (A pound on your kitchen stove of this stuff slowly simmering can be still considered bad.)
Ammonium nitrate releases N2O and H2O vapour at those temps, along with some NOx, although not much of this later in my experience.

Btw. if "almost all of it" dissolves in water then you have the ammonium nitrate - calcium nitrate double salt.

Junk_Enginerd
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Quote: Originally posted by hissingnoise
 Quote: I don't *really* think there's any danger, considering how difficult it is to make it detonate, but I'd feel better if someone could confirm that heating ammonium nitrate in a mostly sealed container to temps in the 700-1000°C range isn't going to cause anything dramatic to happen with the nitrate..

My experience with C.A.N was that the filtered crystals were cream-coloured, so I heated the filtrate above its MP (170°C) in a casserole dish until decomposition had just begun, about 210°C.

When the bubbling liquid solidified, it was snow-white as the gas-liquor contaminents it had contained had been oxidised.

Recrystalising then gave long, clean crystals...

For producing NA, though, it's more convenient (and efficient) to react boiling sols of AN and KCl and distill the acid from the KNO3 formed, using H2SO4.

Nope, no cream color. The crystals look very pure and clear.

For that I can probably just use kno3 from stump remover instead. But H2SO4 is hopeless to find unfortunately. New regulations here state that it may only be sold to those with a permit, and selling it also requires a permit. I do have a car battery's worth of it, about 2 liters of 30%, but seeing as I have no clear way of getting more for a reasonable cost I'm avoiding using it as much as possible. Would sodium bisulfate do the trick?

I've also felt a bit held back on materials when it comes to distilling nitric acid. Boiling concentrated nitric acid as about as bad as it gets for materials, and almost nothing can withstand it. I don't have any suitable glassware.

I did see that 304L stainless steel could handle it, which is good news because I have plenty of stainless steel pneumatic tubing. A second choice which surprised me was that PVC tube should be able to handle it as well, but I have doubts that it'll perform well enough temperature wise.

 Quote: Originally posted by Pumukli Rest assured: heating ammonium nitrate to the 700-1000 C range in a closed container WILL LEAD to dramatic things happen! Ammonium nitrate decomposes if heated above 120 C - 150 C or so! In a closed container it will not necessarily mean detonation but deflagration can be almost as bad! In a semi-closed container it is not that bad - depending on quantity and placement of course. (A pound on your kitchen stove of this stuff slowly simmering can be still considered bad.) Ammonium nitrate releases N2O and H2O vapour at those temps, along with some NOx, although not much of this later in my experience. Btw. if "almost all of it" dissolves in water then you have the ammonium nitrate - calcium nitrate double salt.

Yeah no not actually closed. A sealed container with a tube to collect the NO2 and bubble it into water, so I'm not talking about anything that could build pressure. And the NO2 being generated is the purpose. But deflagration would definitely be bad. Is that an actual risk?
Pumukli
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Ammonium nitrat thermal decomposition gives N2O.
Not NO2.

As I said, NO2 containing gas was just a minor byproduct in my case.

Deflagration: I meant quick thermal decomposition into high temperature gaseous products.

I didn't try the reaction in a sealed vessel, only in a small, (few grams scale) semi-closed tinbox. The mixture was heated from the bottom with a small candle flame. With time (temperature), the reaction became very active, like a smoke-bomb. :-)

This way you surely won't produce nitric acid in appreciable quantity.
Ubya
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as other said, ammonium nitrate on decomposition gives N2O, not NO2. i have at home 5kg of ammonium-calcium nitrate (but without tge slimy sticky stuff you have), i'm converting it to sodium nitrate by adding sodium hydroxide as i can buy 1kg of anhydrous lye for just 2 euros, calcium hydroxide precipitates and ammonia gas is freed from solution (some heating helps removing any NH3 and to coalesce calcium hydroxide particles to make filtering easier later). sodium nitrate could be thermally decomposed first at nitrite (380°C) and at higher temperatures to the oxide and NO2), or used to distill nitric acid using sulphuric acid or sodium hydrogensulphate

[Edited on 27-5-2019 by Ubya]

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Junk_Enginerd
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 Quote: Originally posted by Pumukli Ammonium nitrat thermal decomposition gives N2O. Not NO2. As I said, NO2 containing gas was just a minor byproduct in my case. Deflagration: I meant quick thermal decomposition into high temperature gaseous products. I didn't try the reaction in a sealed vessel, only in a small, (few grams scale) semi-closed tinbox. The mixture was heated from the bottom with a small candle flame. With time (temperature), the reaction became very active, like a smoke-bomb. :-) This way you surely won't produce nitric acid in appreciable quantity.

Ohhh. Right. Okay. I was under the impression that all nitrates eventually decomposed to NO2 if heated enough, but ammonium nitrate contains more nitrogen so that makes sense. No, N2O and water doesn't make any acid of course. That would make laughing gas a little uncomfortable...

Huh. I wouldn't have thought ammonium nitrate would be very energetic on its own like that, but it's irrelevant since the reaction I had in mind won't get me NO2 anyway.

Although as far as I can tell now, this isn't ammonium nitrate, but mostly calcium nitrate with 20% ammonium nitrate if I've understood this double salt thing correctly...

 Quote: Originally posted by Ubya as other said, ammonium nitrate on decomposition gives N2O, not NO2. i have at home 5kg of ammonium-calcium nitrate (but without tge slimy sticky stuff you have), i'm converting it to sodium nitrate by adding sodium hydroxide as i can buy 1kg of anhydrous lye for just 2 euros, calcium hydroxide precipitates and ammonia gas is freed from solution (some heating helps removing any NH3 and to coalesce calcium hydroxide particles to make filtering easier later). sodium nitrate could be thermally decomposed first at nitrite (380°C) and at higher temperatures to the oxide and NO2), ore used to distill nitric acid using sulphuric acid or sodium hydrogensulphate

Oh wow, where do you find lye that cheap? Cheapest I've found is grocery store drain cleaner, which is at least 6€ for 0.75-1 kg. I'd love to have a cheaper source, since I use quite a lot of it for sodium silicate refractory making...

Can sodium hydrogen sulfate be used instead of H2SO4 in distilling HNO3? That's great! I have no reliable source of sulfuric acid, but sodium hydrogen sulfate is dead simple to buy.

[Edited on 27-5-2019 by Junk_Enginerd]

I uploaded a photo of a small purified crysyal as well. I'm guessing the structure can reveal quite a bit. I'm thinking calcium ammonium nitrate double salt?

[Edited on 27-5-2019 by Junk_Enginerd]

MrHomeScientist
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Unfortunately using sodium bisulfate will only result in dilute nitric acid. This is because when distilling off the acid, all of that water also comes over. The standard distillation method uses concentrated (98%) sulfuric acid because it has no water (and also aggressively holds onto it), so you get concentrated acid to distill. Concentrated in, concentrated out. Plus it's difficult to get the right apparatus because, as you know, nitric is not friendly to a lot of materials.

If you can't get concentrated sulfuric, you can boil down your battery acid. If you're saving the acid, then you're better off doing the NO2 into cold water (or better, hygrogen peroxide) method. Although I don't know how concentrated this route will get you.

It seems like the consensus for any of these methods is potassium nitrate is better than ammonium nitrate. People are worried about ammonium nitrate's instability, but personally I think those concerns are overblown (pun intended). It takes quite a lot to detonate; those warnings are usually meant for factories that stockpile tons of the stuff. That being said, it can't hurt to be careful and convert to the potassium salt if you're able.
AJKOER
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 Quote: Originally posted by Junk_Enginerd ............ But H2SO4 is hopeless to find unfortunately. New regulations here state that it may only be sold to those with a permit, and selling it also requires a permit. I do have a car battery's worth of it, about 2 liters of 30%, but seeing as I have no clear way of getting more for a reasonable cost I'm avoiding using it as much as possible. Would sodium bisulfate do the trick?

Would sodium bisulfate do the trick?

In particular, the section dealing with the creation of CuSO4 via a microwave assisted reaction:

Cu + H2O2 + H+ + HSO4- --> Cu(ll) + SO4(2-) + 2 H2O = CuSO4 (aq)

in the presence of a bit of sea salt and an activated carbon electrode created from burnt bread.

Your story is becoming more of the norm, but perhaps I have found a different path that is also safer (referring to the use of NaHSO4 in place of H2SO4), efficient and low cost!
Ubya
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