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AN from AS and CN

ErikSedell - 5-11-2024 at 00:48

Hi,

I know this topic has been up before but I´d like a focussed look on the synthesis of ammonium nitrate (AN) from calcium nitrate (CN) and ammonium sulphate (AS). I have tried this once, but only now realized how unpure my CS was (gardening supply) and I´m not getting the results I expect with the resulting AN.

I now have min 98% AS and analytical grade tetrahydrate CN to start with. What is the best way of obtaining as much and pure AN as possible? Or in other words, precipitate as much Calcium as calcium sulphate as possible.

I have read, that the acidity of the NH4 ions make the calcium sulphate become more soluble, and bisulphate is formed which will contaminate the AN in the solution. It was suggested that one control the pH by adding ammonia as ammonium hydroxide to the solution aiming at pH about 7. What is the best route:

1. adding the AS dry to the CN solution in small steps, and adding NH4OH along the way to control pH?

2. Adding NH4OH to the almost saturated ammonium sulfate solution until pH is about 7 and then just mix the AS and CN solutions and let the precipitate form.

3. Slowly adding the CN dry to the AS solution?

4. Multiple filterings, taking out the calcium sulphate as it forms?

I am thinking to use very concentrated solutions of AS and CN to start with, since all the water has to get boiled off in the end. Is this a mistake, and introduce more problems with the acidity of the NH4 ions?

Hope to hear your experienced opinions and suggestions. Thank you in advance.

//Erik

Syn the Sizer - 5-11-2024 at 22:24

Keep in mind that Calcium Sulfate has the Reverse Solubility property. You need to heat the solution to crash out your Calcium sulfate. As the solution cools the calcium sulfate becomes more soluble.

ErikSedell - 6-11-2024 at 00:33

Compared to AN the solubility is very small. Last time I tried loads of CaSO4 precipitated without needing to adjust temperature.

Nevertheless, it is a good suggestion to elevate the temperature, as AN solubility increases radically with temperature, while the CS solubility drops.

Rainwater - 6-11-2024 at 09:23

If you need to clean the calcium sulfate out of a glass fritz, wash it in Hot diluted sulfuric acid, 5 to 10 percent.

Syn the Sizer - 6-11-2024 at 10:13

Quote: Originally posted by ErikSedell

Compared to AN the solubility is very small. Last time I tried loads of CaSO4 precipitated without needing to adjust temperature.

Nevertheless, it is a good suggestion to elevate the temperature, as AN solubility increases radically with temperature, while the CS solubility drops.

That is a vary good point though, in hind sigh at room temp the solubility is quite low.

yobbo II - 6-11-2024 at 10:42

Decant. And centrifuge

ErikSedell - 6-11-2024 at 23:01

Quote: Originally posted by yobbo II

Decant. And centrifuge

Thank you for adding your thoughts. But what do you think about the 4 points listed in the first post?

bnull - 7-11-2024 at 10:10

Put filter paper on a funnel, add solid ammonium sulfate, then filter a concentrated solution of calcium nitrate through it. Calcium sulfate remains on the filter and ammonium nitrate on the filtrate, as long as the stoichiometry is right. That's how I make sodium hypochlorite from a solution of calcium hypochlorite and solid sodium carbonate.

[Edited on 7-11-2024 by bnull]

ErikSedell - 8-11-2024 at 04:05

Quote: Originally posted by bnull

Put filter paper on a funnel, add solid ammonium sulfate, then filter a concentrated solution of calcium nitrate through it. Calcium sulfate remains on the filter and ammonium nitrate on the filtrate, as long as the stoichiometry is right. That's how I make sodium hypochlorite from a solution of calcium hypochlorite and solid sodium carbonate.

[Edited on 7-11-2024 by bnull]

Thats a nice suggestion. I guess you filter the solution multiple times then, since one must assume that some unreacted CN passes through the first time.

How do you monitor if the process is done? Halfway through I would guess the stuff in the filter is a mixture of unreacted AS and precipitated CS.

bnull - 8-11-2024 at 05:46

Quote:

How do you monitor if the process is done? Halfway through I would guess the stuff in the filter is a mixture of unreacted AS and precipitated CS.

Add a drop of the filtrate to a solution of ammonium sulfate. If it doesn't get cloudy where the drop fell, it's done. The filtrate is supposedly saturated with calcium sulfate, what with the common ion and so on, and the rate of diffusion of the drop into the bulk is not so great as to make the cloud of calcium sulfate dissolve.

There will be some unreacted ammonium sulfate but it can be removed by recrystallisation (which also removes the dissolved calcium) or by adding some calcium nitrate. I'd choose the latter since the process becomes a brute force on the funnel plus a fine tuning on the beaker, without all the hassle of stirring and filtrating a goop.

ErikSedell - 15-11-2024 at 13:09

Quote: Originally posted by bnull

Ok, i just tried out making a new batch. And got a big goop.. of course. I’m making a small batch for trying, just 2 M of AN. Still a lot of goop. You must have a really large funnel?

bnull - 15-11-2024 at 15:43

Damn.

You need something to change the shape and rate of growth of calcium sulfate crystals. According to a patent (EP0057519B1, "A method for increasing the filtration rate of the calcium sulphate crystals in the production of wet process phosphoric acid"), gelatin can increase the rate of filtration.

Or, and I don't know why I hadn't thought of this before suggesting the "salted funnel" process (which appears to work well only with carbonates), you can mix small amounts of calcium nitrate solution to a solution of ammonium sulfate and filter it after each addition. It will be a long and quite boring process that manages to be less annoying than to mix everything at once and then (hope) to filter the goop.

Nothing can be done for the gooped up stuff except for washing it into a beaker and letting it settle. It will eventually settle enough so you can drain the liquid. Sorry for that.

I hate calcium sulfate.

Attachment: EP0057519B1.pdf (382kB)
This file has been downloaded 22 times

Precipitates - 16-11-2024 at 00:55

Vacuum filter.

I gravity filter when I can, but with calcium sulfate you'll be waiting a month of Sundays.

Rainwater - 16-11-2024 at 10:08

All these seperation methods except for the decant and centrifuge are going to be a waste of time.
The by product to be filtered out is the same stuff used to make sheetrock for houses.

https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

ErikSedell - 16-11-2024 at 11:31

Quote: Originally posted by bnull

Damn.

You need something to change the shape and rate of growth of calcium sulfate crystals. According to a patent (EP0057519B1, "A method for increasing the filtration rate of the calcium sulphate crystals in the production of wet process phosphoric acid"), gelatin can increase the rate of filtration.

Or, and I don't know why I hadn't thought of this before suggesting the "salted funnel" process (which appears to work well only with carbonates), you can mix small amounts of calcium nitrate solution to a solution of ammonium sulfate and filter it after each addition. It will be a long and quite boring process that manages to be less annoying than to mix everything at once and then (hope) to filter the goop.

Nothing can be done for the gooped up stuff except for washing it into a beaker and letting it settle. It will eventually settle enough so you can drain the liquid. Sorry for that.

I hate calcium sulfate.

Thank you for helping out! I put most of the goop in a filter over night and at least some filtrate came out. But I see now, as I start to boil it down that quite some part of it is very small CS particles. Do you have a suggestion for filter paper quality that is good for removing CS? I’m using a coffee filter now… I just added more water to the goop in the filter to wash out as much AN as I can.

bnull - 16-11-2024 at 14:32

You have two options: a lab grade filter, which I have never used and can't recommend a grade of (20 micrometer, like a Whatman 41 perhaps?); or cotton.

When I use cotton, I pick up a small wad and push it into the bottom of the funnel, taking care to shape it like a golf "tee" with a large head. Stick a glass rod into the stem and compress the cotton, holding the flat of the "tee" in place with your fingers. When the solution passes through, the particles will find enough surface to stop them without slowing down the filtration. Pass it twice to remove the loose cotton fibers.

[Edited on 16-11-2024 by bnull]

You can increase the size of the crystals by letting the solution age for a couple of days. It should help with the filtration.

[Edited on 17-11-2024 by bnull]

Precipitates - 16-11-2024 at 19:57

Quote: Originally posted by Rainwater

All these seperation methods except for the decant and centrifuge are going to be a waste of time.
The by product to be filtered out is the same stuff used to make sheetrock for houses.

https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

Maybe i've been very lucky, but i've never had a problem vacuum filtering calcium sulfate.

Sure gravity filtration is difficult, but running a vacuum for 30 mins or so pulls most of the water through. The calcium sulfate precipitate becomes cracked and dries out to a reasonable extent.

Sure, calcium sulfate is not the easiest to filter out, but it's not too hard.

The thing that's perhaps more annoying is that it doesn't have a negligible solubility in water and so contaminates your product.

Quote: Originally posted by bnull

You have two options: a lab grade filter, which I have never used and can't recommend a grade of (20 micrometer, like a Whatman 41 perhaps?)
[Edited on 16-11-2024 by bnull]

I use cheap 15-20 μm Chinese filter paper.

It's worth noting that it really doesn't matter if some calcium sulfate makes it through on the first filtration, and is to be somewhat expected, just repeat to gain a solution of clarity (second filtration will be much faster).

ErikSedell - 18-11-2024 at 02:01

thank you for your replies! bnull: I will try the stepwise filtering method next time.

However now I have some mysterious result. I started boiling down my filtrate, approx 400 ml of solution to see how it goes. Some is still in the filter with the goop so the yield will not be fantastic but I´ll see if I can improve on that later.

I started with 1 M of CN tetrahydrate dissolved in abou 200 ml of water (almost conc solution) and 1 M of AS dissolved in about the same amount of water, quite conc solution. I mixed them, got the goop, filtered over night through a paper coffee filter lined with some synthetic fabric with seemingly fine pores.

The CN is analytical grade due to seller, the AS technical grade >98%. I used tap water for all steps, since I´m happy to get AN of reasonable pure quality. Maybe this was the mistake?

Theoretical yield should be 2M of AN, or about 160 g which at room temperature is dissolved in about 160 ml of water, and very easily dissolved in this amount of water at elevated temperature.

Nevertheless I got som crystallization when turning on the stove with clearly more than 160 ml water in the solution. This amount of water left should contain all AN as dissolved so I dont know whats going on.

I tried preparing some AS and CN solutions and dripped some of the filtrate in them without any cloud precipitate appearing. Hence there should be no Ca ions or SO4 ions in my filtrate, respectively. What possibilities are left, to me there is just NH4 or NO3 remaining.

I did add some ammonium hydroxide when mixing the CN and AS to get the pH to about 8, but that was very little. One could suspect CaOH forming, but not in this quantity.

also, the bottle with wich I collected the filtrate has some crystals in it stuck to the wall. see pic. Could this be CaSO4 which is crystallizing on the wall?

Could something else be happening? Is concentrated AS solution just SO4 ions, or can some bisulfate turn up? Could some double salts appear?

I´m puzzled, since I thought this reaction should be easy.

Now Im keeping on boiling the filtrate down, filtering out the crystals and I´m keeping them since I dont know what they are, maybe its the real AN crystals?

I guess some of you experienced people could figure out what is going on.

one more clue

ErikSedell - 18-11-2024 at 03:34

Kept boiling down slowly. Now the filtrate is kind of brown and has some charachteristic of AN, like turning more solid as heat decreases (solubility goes down) and being a thick liquid at about 100 degC.

I wonder why it is not white anymore. Maybe my coffee filters left something in the solution. Or the tap water.

//Erik

Precipitates - 18-11-2024 at 07:11

Quote: Originally posted by ErikSedell

Hence there should be no Ca ions or SO4 ions in my filtrate, respectively. What possibilities are left, to me there is just NH4 or NO3 remaining.

Calcium sulfate is slightly soluble in water (approx 2.0 g per litre).

You can dissolve the product in an alcohol e.g., methanol, for which calcium sulfate is truly insoluble, to get a purer AN product.

Brown colour likely impurities - but the AN should still be fine for most purposes. It will probably become more white as it dries out.

ErikSedell - 18-11-2024 at 23:35

Indeed, the powder is now off-white after drying and grinding.

I got 134 g, which is about 83% yield from theoretical 2M of AN. But, I do not know how much water there is left. Drying AN is not very easy. So the yield is smaller than that. But I’m happy and hope the quality is sufficient for propellant use. Will test a first comp tonight.

Next time I will filter stepwise, use proper filters and destilled water.

ErikSedell - 18-11-2024 at 23:58

Quote: Originally posted by Precipitates

Yes, it is. But 0.2% solubility is within my margin of error. My AS is not pure enough to make this matter.

I´d like to try recrystallization with methanol, but methanol is not so easy to source where I live (Sweden)

bnull - 19-11-2024 at 04:48

The brown color my come from iron hydroxides/oxides. We have problems at home with yellow-tinted calcium carbonate stones forming inside showers; water here is too heavy on calcium and iron.

Try ethanol then. 30% of ethanol should be good enough to remove most of the calcium sulfate and iron while keeping ammonium nitrate in solution (see attached paper for a similar idea involving calcium sulfate in brine).

Take a look at the links at the bottom of the wiki page for ammonium nitrate.

Attachment: Use of ethanol to precipitate calcium sulfate from brine.pdf (157kB)
This file has been downloaded 18 times

Edit: I forgot this one. Look the entry for Ammonium Nitrate in Bretherick's Handbook of Reactive Chemical Hazards (starts on page 1749 in the 7th edition). To have an idea:

Quote:

The decomposition, fire and explosion hazards of this salt of positive oxygen balance have been adequately reviewed. In the absence of impurities it is difficult, but not impossible, to cause ammonium nitrate to detonate. Use of explosives to break up the caked double salt, ammonium nitrate—ammonium sulfate (2:1), caused a 4.5 Mkg dump to detonate, even though some 45% of ammonium sulfate was present, effectively as a diluent, in the salt. (...) The ammonium sulphate is unlikely to be totally inert, having a negative enthalpy of decomposition to steam, nitrogen and sulfur, which last is capable of oxidation to sulfur dioxide, bringing the double salt into oxygen balance. The effect of various impurities and additives on the thermal stability of ammonium nitrate has been widely studied. Impure ammonium nitrate now constitutes more than 95% of all civil explosives used in the USA. A few incidents involving explosive decomposition of aqueous solutions of the salt during evaporation, or transfer operations (some in presence of oil) have been recorded. Flame cutting a mild steel pipe blocked by the solid (impure) salt caused it to explode.

[Edited on 19-11-2024 by bnull]

Rainwater - 19-11-2024 at 14:47

Please be carefully when mixing an oxidizer with an organic solvent. Even a wet one

[Edited on 19-11-2024 by Rainwater]