KoiosPhoebus
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Preparing Ammonium Nitrate from Common Fertilizers without Nitric Acid
Hi everyone! This is my second post on here, so apologies in advance if this
in the wrong forum. I have some background working with chemicals used in agriculture/horticulture so that's what I'll be posting about.
Outside of the US, ammonium nitrate (NH4NO3) is typically restricted and pretty hard to get hold of. Depending on where you are,
this might mean "you can recrystallise it from a complex fertilizer like ammonium sulphate nitrate" or "it's mixed with three different fertilizers,
some of which are somewhat soluble in ethanol lol good luck". However, it can be prepared from other fertilizers; the most common combination I've
heard of is ammonium sulphate and calcium nitrate. The resulting calcium sulphate is minimally soluble in water and precipitates out, leaving mostly
ammonium nitrate in solution.
(NH4)2SO4 + Ca(NO3)2 → CaSO4 ↓ + 2 NH4NO3
This is a very viable way to make ammonium nitrate. Unfortunately, many commercial ammonium sulphate fertilizers are fairly impure, with the most
common impurity typically being another metal sulphate such as iron sulphate. This makes purification difficult as the products will include various
metal nitrates which are commonly soluble in alcohols the same way ammonium nitrate is.
Instead, I've had success using:
Diammonium phosphate, (NH4)2HPO4
Calcium nitrate, Ca(NO3)2
Diammonium phosphate (DAP) is both a very common fertilizer, as well as a commonly-used yeast nutrient source in brewing. The latter use means that
it's often widely available in food-grade form, which is usually much purer than the fertilizer grade. Get the food-grade from your local brewing
supplies store if you can; otherwise you might need to recrystallise the DAP to remove impurities.
Calcium nitrate can be found either by itself, or as the double salt calcium-ammonium nitrate (5
Ca(NO3)2.NH4NO3.10 H2O). This is commonly found as a soluble calcium fertilizer or in
hydroponic fertilizers; because of this, it's typically reasonably pure as long as stored properly.
The reaction proceeds as:
(NH4)2HPO4 + Ca(NO3)2 → CaHPO4 ↓ + 2 NH4NO3
Or in the case of the double salt:
5 (NH4)2HPO4 + 5 Ca(NO3)2.NH4NO3.10 H2O → 5
CaHPO4 ↓ + 11 NH4NO3 + 10 H2O
The side-product, dicalcium phosphate (DCP), is very insoluble in water, with a solubility of 0.018 g/100 mL as opposed to calcium sulfate's 0.26
g/100 mL. My procedure is as follows:
Dissolve the calcium nitrate into water. Filter off insolubles if need be.
Add a stir bar, or be prepared to stir the mixture regularly.
Add diammonium phosphate dry to the solution of calcium nitrate and stir. DAP produces an alkaline solution when dissolved; if you dissolve it
into solution first and don't add it rapidly enough, the solution will lose ammonia and become a mix of DAP and monoammonium phosphate. This messes up
the stoichiometry and results in monocalcium phosphate when reacted, which is a lot more soluble than dicalcium phosphate.
After stirring becomes difficult, leave the mixture alone for a few hours to age and for everything to react.
Give it one last stir, then start filtering out the dicalcium phosphate. Typically, you'll want to add more water to get as much ammonium
nitrate out as possible as some will be trapped in the DCP. I usually add about as much water to the dicalcium phosphate mix as I used in the first
step.
One way to test if you've washed out as much ammonium nitrate as possible is to test the pH of the water coming out of the filter; if it's lower than
the water you're pouring in, that probably means that some ammonium nitrate is still coming through.
Evaporate off as much water as possible. Don't worry if you don't remove all of it, the next few steps will produce much drier crystals.
Break up the chunks of evaporated ammonium nitrate into a reasonably fine powder. Then, wash them a few times with acetone to remove excess
calcium nitrate. Calcium nitrate has a high solubility in acetone (33 g/100 g) but ammonium nitrate is minimally soluble in acetone (0.15g/100g).
Leave the washed crystals to dry. Once the crystals seem mostly dry, weigh them.
Measure out enough ethanol to dissolve all the crystals if they were pure ammonium nitrate and the ethanol is near-boiling. I typically use a
rate of about 100g ethanol per 7g crystals based on this data (https://en.crystalls.info/Solubility_curve), but I've definitely heard of people getting more ammonium nitrate into boiling ethanol.
Once you dissolve the crystals into the ethanol, there may be some precipitate formed. This precipitate is usually various ammonium phosphates
and trace calcium phosphates, which can be filtered off.
Leave the hot solution of ammonium nitrate to cool and form crystals; place it in the freezer if possible to maximise crystal recovery.
Filter out the crystals. Congrats, you now have your ammonium nitrate!
I've also heard from friends who had success adding the DAP in batches; i.e. adding half the DAP, filtering, then adding the other half.
Questions I've been asked about this before
Can I use other ammonium phosphates?
You can hypothetically use triammonium phosphate ((NH4)3PO4), which would precipitate hydroxyapatite or tricalcium
phosphate instead of dicalcium phosphate. The reaction should be similar; however my impression is that apatite and tricalcium phosphate have greater
solubility in acidic conditions than dicalcium phosphate. As ammonium nitrate is a slightly acidic salt, this likely means greater contamination and
hence reduced yield.
(I'd be surprised if you could get hold of triammonium phosphate and not diammonium phosphate though)
Monoammonium phosphate (NH4H2PO4) should be avoided as it produces monocalcium phosphate
(CaH2PO4) instead of dicalcium phosphate. MCP is 100x more soluble than dicalcium phosphate according to standard accepted
solubility figures; some literature even argues that MCP is highly water-soluble and the standard solubility estimate of 18g/L is inaccurate due to
the precipitation of DCP from MCP decomposition in solution.
Can I use other nitrates?
Any metal nitrate whose hydrogen phosphate (HPO42-) salt is minimally soluble in water is fine as long as it doesn't
co-crystallise with ammonium. This rules out magnesium nitrate as magnesium co-crystallises with ammonium and phosphates to give struvite
(MgNH4PO4), messing up the stoichiometry. I'm not sure if there's many other commonly available nitrates which fit the above
criteria.
You could use sodium and potassium nitrate and then try to remove the resulting sodium/potassium hydrogen phosphate through the ethanol
recrystallisation step. I haven't done it this way myself; I typically try to minimize the amount of precipitates to be removed through ethanol
recrystallisation due to the fact that ammonium nitrate isn't very soluble in ethanol. Hence, it's not always easy to tell if a bunch of precipitate
is from impurities, or if it's leftover ammonium nitrate which hasn't dissolved into the ethanol.
Can I use denatured acetone/ethanol?
It depends on what they've been denatured with. Denatonium benzoate is fine, but sometimes manufacturers add weird solvents which could mess things
up. In particular, check if the acetone has been denatured with anything that would dissolve ammonium nitrate (methanol comes to mind) and check if
the ethanol has been denatured with anything that would dissolve ammonium phosphates.
I'd also recommend trying to get solvents which are as close to anhydrous as possible. Ammonium nitrate and ammonium phosphate dissolve readily in
water, so e.g. if you use a 90% acetone solution to wash your product, you might lose a bunch of ammonium nitrate when washing out calcium nitrate. Or
if you use 90% ethanol, you might introduce a greater amount of ammonium phosphate into the final product.
Can I use methanol instead of ethanol?
I'd say...probably? Never done it before myself. Methanol dissolves ammonium phosphates at slightly higher rates than ethanol but it's not a huge
difference. On the other hand, ammonium nitrate is a lot more soluble in methanol than it is in ethanol, so you can work with smaller amounts of
solvent. Be sure to work in an environment where you won't come into contact with fumes; methanol is quite toxic if it accumulates in your body.
I hope someone finds this helpful on their home scientist journey!
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Texium
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Thread Moved
6-2-2023 at 06:37 |
MadHatter
International Hazard
Posts: 1332
Registered: 9-7-2004
Location: Maine
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Mood: Enjoying retirement
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CAN
This is a link to my post on using CAN(Calcium Ammonium Nitrate). It really
depends on the purity you need. For me it's energetics for propellants so purity
isn't that important.
http://www.sciencemadness.org/talk/viewthread.php?tid=154741...
From opening of NCIS New Orleans - It goes a BOOM ! BOOM ! BOOM ! MUHAHAHAHAHAHAHA !
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KoiosPhoebus
Hazard to Self
Posts: 51
Registered: 23-1-2023
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Great addition, thanks. I've used both before and I typically get better
yields + purer crystals with the DAP/CAN method as compared to the AS/CAN method.
FYI, the paraffin coating is partly for slower release in soil but more importantly for anti-caking purposes. If you ever find soluble CAN (usually
declared as hydroponics grade, or "soluble calcium"), you'll notice it's quite slimy/sticky to the touch. The coating can probably be removed with a
non-polar organic solvent like toluene or xylene, but honestly dissolution in water + filtration is probably better anyway - gets out other particles
which may have ended up in the mix.
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