KNO3 from Ammonium Nitrate and KCl

The following shows how to obtain KNO3 from Ammonium Nitrate and Potassium Chloride. Ammonium Nitrate is commonly available as a fertilizer as well as KCl which is called Muriate of Potash. The Ammonium Nitrate can be in pure form where it will have 35% nitrogen written on the bag (100% actual Ammonium Nitrate, 0% filler). It is usually mixed with an inert filler and will not be pure and may have 27% nitrogen written on the bag (77.2% actual Ammonium Nitrate, 22.8% filler). The filler is not a problem.

The muriate of potash usually comes pure or fearly pure and will have 52.4% K written on the bag (100% actual KCl, 0% gunk) if using molar percent to describe the content of the bag OR it will have 0-0-63 (63% K2O) written on the bag (using percent K2O to describe what is in the bag). It may be a little less pure and will have ~50% K (or ~62% K2O) written on the bag (95.4% actual KCl, 4.6% gunk).
If the KCl is not pure it should be recrystallized and then used.
Another possible source of KCl is with the people who sell equipment for softening water. Sodium Chloride is usually used in theses devices but some people object to haveing any Sodium ion in their drinking water so they need to use Potassium Chloride instead, so you may be able to get a bag of Potassium Chloride in the same place as where they supply the NaCl for the water solteners

Getting a 50% solution of Ammonium Nitrate

If the Ammonium Nitrate is not pure it will have to be purified. It is difficult to recrystalize the Ammonium Nitrate as the precipitated Nitrate will be mixed with the inert filler. The best (and handiest) way to deal with the Ammonium Nitrate is to make a 50%wt solution of it and use that in the process. To make a 50% you should take about 3KG of the Ammonium Nitrate (pure or otherwise) and dissolve it in one liter of water. An Aluminium or stainless steel container is ok. It will look like an awful lot of Ammonium Nitrate but it will all dissolve. You will need to heat the water as the solution will get very cold (its uses in freezing packs after all) when the Ammonium Nitrate is dissolving and you will want to hurry it up. When all the Nitrate has dissolved (this will have happened at 50C or less) you should stop heating. The solution may be very cloudy because of the inert filler in the Ammonium Nitrate but it's ok. Let the solution settle and all inert filler will fall to the bottom. There may be some of the inert filler floating around on the top which you can lift off with a spoon. Let the solution cool slowly to one of the chosen temperatures in the table below. You should make sure that some Ammonium Nitrate has crystallize out as long pretty needles at your chosen temperature. This will happen if you have proceeded as described above.

When the solution has reached your chosen temperature pour or siphon off as much of the clear(ish) liquid as possible and put into a seperate container. Sometimes the inert filler is very fine and some will still be suspended in the solution but you will be able to get rid of it later. Don't be tempted to be greedy and contaminate your clear(ish) solution with much of the solid material that is at the bottom of the container that you are siphoning from. The solution that you have siphoned (or poured) off will be a certain percentage of Ammonium Nitrate as shown in the table. ie. if you let your solution cool to 20C it will have 65.5g Ammonium Nitrate per 100g of solution. The particular temperature you choose is up to you. The more you cool, before siphoning, the more Ammonium Nitrate will come out of solution and you may feel you are wasting Ammonium Nitrate and may wish to sipnon off at one of the higher temperatures. As stated above, make sure that Ammonium Nitrate is actually precipitating in long needles at your chosen temperature before you start to siphon.

Temperatures, %wt & amount of water per gram solution to add to get a 50%wt solution
TemperatureWeight %Grams water to add per gram solution
3070.40.408
2568.20.364
2065.50.31

Weigh the clear(ish) solution that you have obtained by siphoning (or pouring) and for every gram of solution that you have add the amount of water that it says to add in the table. For example if you have a total weight of 1670g of solution, and you let your solution cool to 20C before siphoning off the clear(ish) liquid then you should add 1670 * 0.31 = 518 grams of water to the solution to obtain 1670 + 518 = 2188g of solution. Stir and dissolve any Ammonium Nitrate that may have came out of solution. You will now have a 50% solution of Ammonium Nitrate for the next stage of the process. Put the solution in a tallish container with a lid and if there is any suspended filler still in it, it will fall to the bottom after about 24 hours. You can then siphon (or pour carefully) the now perfectly clear solution of 50% Ammonium Nitrate for the next stage of the process. Save it in a container with a lid, you don't have to use it all at once.
If you have pure Ammonium Nitrate to start with you don't have to bother with the 50% solution, you can just use as is.
The remainder of the Ammonium Nitrate that was left in the heating container can be discarded if your Ammonium Nitrate has inert filler in it, or if your Ammonium Nitrate was pure (If you are using freezer packs) you can save it for the next time you want to make up a 50% solution of Ammonium Nitrate. Don't throw in drains etc. Dilute it with plenty of water and spread it on grass.

Making the KNO3

You can scale the following up or down.
Take 760cc (760g) of water and heat to about 50C. To this water add 3 mole (223.5 grams) of your recrystalized and dried KCl and dissolve. Add 480grams of your Ammonium Nitrate solution.
You will now have one liter of water with 3 moles of Ammonium Nitrate and 3 moles of KCl dissolved in it. Note: You won't actually have a liter of SOLUTION as dissolving the salts in the water will increase the volume.
If you are using pure Ammonium Nitrate (you did not bother with the 50% solution above) you should take one liter of water, 223.5grams KCl and 240grams solid (pure) Ammonium Nitrate.

All of the salts will be dissolved and if not the solution should be heated more untill they are.
The solution is now cooled slowly, the slower the better. Slow cooling gets you larger crystals of KNO3 which is desirable as they wash better. When the solution has cooled to room temperature it should be placed in the freezer and cooled to about -12C. Don't let any ice form. If you don't have a freezer it is ok but will get much less KNO3 out, in fact you can improvise your own "freezer" by making a freezing mixture of Ammonium Nitrate and water and putting the container which has your solution sitting in it.
The KNO3 will come out of solution. The KNO3 can now be seperated out by filtering through a cloth. The crystals should be squeezed when then are in the cloth to get rid of as much of the liquid as possible. The crystals can now be washed in cold (0C) water by simple putting the cloth containing the crystals into the cold water and moving it up and down a number of times so that the water flows in through the cloth to the crystals. The cloth, still with the crystals in it, it taken out and squeezed so that as much of the washing water is got rid of. The yield of KNO3 is about 175 grams KNO3. The KNO3 should be recrystallized for maximum purity.
The liquid that the KNO3 has come from will have KNO3 and Ammonium Chloride dissolved in it and this can be saved if you want to attempt to take out some Ammonium Chloride from it later. The Ammonium Chloride would be useful if you were making Ammonium Perchlorate from Sodium Perchlorate....but thats another story.

Below is an explaination of what is going on.

SubstanceKNO3 NH4NO3KClNH4Cl
Grams/mole 101.1 80 74.5 53.5
[SOLUBILITYS IN MOLES PER 100 GRAMS WATER]

When you dissolve Ammonium Nitrate and KCl in water you get the substances to dissociate and ions form. The Cl-, N03-, NH4+, and the K+ ions to be precise. The K+ and the NO3- ions decide to pair up and form a solid salt, KNO3, when the temperature goes below about 20C at the concentrations that we are working at.
The graph shows the solubilitys of the relevent salts. The 'stable pair' , that is , the pair of salts that may come out of solution are KNO3 and Ammonium Chloride. We have worked at a concentration of 0.3 moles of KNO3 and 0.3 moles Ammonium Chloride per 100 ml water and the component precipitating is KNO3.

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