Disclaimer: This information has purely educational and scientific intend.
Often Potassium nitrate is not available or is too expensive so I decided to describe an economically viable way of synthesizing KNO3 from cheap and available resources. The process is not difficult, requires no specialized chemicals and the yield is excellent. Oversimplified description would be:
Calcium Nitrate + Potassium Chloride = Potassium nitrate + Calcium Chloride
In reality things are a bit more complicated, but nothing to worry.
Potassium Chloride (KCl)
The best source of cheap and very pure Potassium Chloride is salt for water softener installations. It is sold on many places in different sizes – just make sure you are getting Potassium and not Sodium chloride softener. Don’t bother buying table salt replacement as it is too expensive and very impure.
Calcium Nitrate (Ca(NO3)2)
Calcium Nitrate is sold as fertilizer in most hydroponic shops and agriculture suppliers in different bag sizes. In reality it is not pure Calcium Nitrate but rather a Calcium-Ammonium nitrate decahydrate [NH4(NO3)*5Ca(NO3)2*10H2O], which means that it contains Calcium nitrate, ammonium nitrate and water. What we are interested is the nitrate ion (ammonium or calcium both work).
Because of the presence of ammonium nitrate we will get additionally some ammonium chloride, but this doesn’t really change anything for us. Hence a proper description of the ion-exchange reaction would be:
NH4(NO3)*5Ca(NO3)2*10H2O + 11KCl = 11KNO3 + NH4Cl + 5CaCl2 + 10H2O
To help you calculating the proper weights that you need instead of playing with molar masses you can use the following on-line chemistry calculator (just click on the picture) where you can enter the weight of one of the input reagents or desired weight of the reaction products and the rest will be automatically calculated. The corresponding solubility can be found in the Solubility table.
Basically we dissolve the reagents in hot water, mix them to allow the ion exchange and cool down the solution to obtain the desired product (KNO3) which will crystalize in the reaction vessel since KNO3 has much lower solubility than the other products.
In order to finish with high purity KNO3 we need to do preferably 2 re-crystallizations of the Potassium nitrate. However every time we do that inevitably some KNO3 remains dissolved in the water, which lowers our final yield. To keep the final yield high we need to work in batches to minimize the water used and recycle it while “playing” with the solutions temperatures depending on the values from the Solubility table.
STEP3 – cool down to (negative) -10*C – no point of further cooling it down because you will get water crystals. About 90% of the KNO3 (90gr) will crystalize, however contaminated with Calcium chloride and Ammonium chloride. Discard the remaining water.
STEP5 – cool down to (negative) -5*C. About 90% of the KNO3 will crystalize giving appr 80gr relatively pure KNO3. Keep the remaining water, which contains about 10gr dissolved KNO3 to be used in Batch 2.
STEP6 – dissolve the 80gr KNO3 from STEP5 in 80ml boiling distilled water for second re-crystallization.
STEP7 – cool down to 0*C. About 90% of the KNO3 will crystalize giving appr 70gr very pure KNO3. Keep the remaining water, which contains about 10gr dissolved KNO3 to be used in Batch 2.
STEP1 – BATCH2 – repeat as in Batch1 but now instead of tap water, for this step use the water from the two re-crystallizations in the previous Batch.
STEP2 …. STEP7 – repeat as in Batch 1.
Normally if we cool water solution of KNO3 below 0 degrees it will start forming water crystals together with the KNO3 crystals but because the solution in STEP3 of each batch contains high amount of Calcium Chloride we can cool it down to -10 degrees without forming water crystals which provides few additional percent to the final yield of KNO3. Additionally the remaining dissolved KNO3 from the both re-crystallizations in the preceding batch is carried on to the next batch and recuperated. This gives a final yield of about 85-90 percent of very pure KNO3.
The following diagram describes the “near ideal” process. If you want to scale it up (or down) you have to prorate all amounts. The temperatures and quantities of water are not critical as long as all reagents gets dissolved. More water and higher temperatures during the KNO3 crystallization will leave more KNO3 in the solutions.
Finally if you want really pure KNO3 after drying the crystals you can wash them with pure denatured Ethanol sold in most hardware stores. Potassium Nitrate is insoluble in ethanol while Calcium Chloride and Ammonium Chloride are somewhat soluble. After washing the KNO3 you can recycle the Ethanol by distilling it – the dissolved chlorides will remain in the distilling vessel. However in most cases this will be an “overkill”.
Every time when you separate the KNO3 crystals from the liquor it is strongly advisable to remove (not evaporate) as much liquor from the crystals as possible. Remember that all contaminants (calcium and ammonium chlorides) remain in the liquor and if you leave wet crystals then you leave also some of the chlorides at the crystal’s surface –this makes the Potassium nitrate more hygroscopic.
Removing the liquor is best done with centrifuges or vacuum filters. The easiest way is DIY vacuum filter (courtesy of good friend) made from an old vacuum cleaner, a small bucket and a coffee filter – like the one that I am using shown on the picture. It is a good practice during vacuum filtering to rinse the crystals by spraying some water over the crystals.
As always the best time to do the conversion is Winter! All heat from the appliances remains inside and outside the chill doesn’t cost anything – almost no electricity is wasted.