Does drying potassium nitrate in an oven cause any health hazards

[erdem]

I have recently gotten a bag of potassium nitrate and i want to learn how to crystallize and purify it, in this process i have learnt that using an oven to dry the potassium nitrate after crystallizing it is very efficient but im concerned that this will leave residue or anything else in the oven and cause some kind of poisoning or anything else that will be dangerous for my health if someone with some chemistry knowledge answered it would be greatly appreciated!

[Arthur]

Unless your bag of nitrate says it's low purity don't start down the purifying route, it's work for no purpose.

Potassium nitrate is a food ingredient used as a preservative and oxidiser -it's principally used in cured meats -Bacon, ham and salt beef. However never let any pyro chemical contaminate food equipment.

[mx5kevin]

For Black Powder, Flash Powder, and what is milled and necessary i dry every oxidizer. What cannot be oxidizers that decompose at too low a temperature close to 100°C. For ball milling if the oxidizer heated before milling, and after milling we heat it up again a second time, then the powder loosens up further if sifted through a sieve, after this, if milled together with another ingredients, like in Black powder, it grinds much better and does not stick together. In compositions like FP, it also significantly affects its storage stability and performance. Oxidizers with a high decomposition temperature are 300-400°C, Like KNO3, KClO3, KClO4, and much more can heated in a gas stove using a stainless steel pan. Oxidizers with a lower decomposition point, what are higher than 150°C can heated in a oven like NH4ClO4, Ba(ClO3)2 under 100°C. Oxidizers with lower or close to 100°C decomposition point cannot be dried this way. In terms of recrystallization, the potassium nitrate we get as a fertilizers in my country does not need to be further purified because it is 99% pure. If done in a well-ventilated place, you won't have any problems.

Edited September 17 by mx5kevin

[davidh]

If it didn't start out horribly impure, and you've recrystalized it and rinsed it (with ice-cold distilled water), it should be fine in an inside oven. That said, you can put it on some sort of tray and just let it air dry too. It won't be even slightly hydroscopic.

[PyroGnome]

KNO3 fertilizer is probably relatively pure in most places, unless they're intentionally mixing something neutral in to keep people from using it as an oxidizer.  There are a limited number of things you can add to fertilizers which won't affect plant growth through either soil pH change or nutrient imbalance since plants need roughly the same trace nutrient profile as humans.  Doing some quick looking around, the chilean stuff dudadiesel sells is listed as 99.8%+. 

Haifa Multi-K GG (Greenhouse Grade) is listed with a min / max insoluble content of 350-700ppm and an upper moisture content of 0.2%.  10% solutions have a relatively wide pH range of 6-11 though (9 typical)...  This is in contrast to the EU food additive range of 4.5-8.5.  Random batches sitting at the upper end there might be part of why some people will have no issues with glitters and some people need to add boric acid.  Likewise for possible less pure / lower range pH with magnalium.      this is actually about the same for most nitrate fertilizers (they used to have a higher purity grade called TOP with a range from 6-8.5 but I can't find it for sale anymore)..  pH and everything else are the same.  FWIW pyrochemsource lists their potassium nitrate as this product, hammer-milled, and I haven't had problems using it for anything. 

Haifa Multi-K Reci can be found fairly commonly, it's meant for recirculating hydroponic systems where sodium buildup can't really be allowed and only contains 150-300ppm sodium, which is important for those applications as well as fireworks in cases when yellow color contamination in the KNO3 flame isn't acceptable.   Everything else is the same as the GG listed above which makes me suspect it might just be a sodium tested version of the same product and the production process itself keeps sodium from being a major issue. 

They have another form which is likely also low sodium since it's designed for foliar application / liquid fertilizer called Multi-K pHast which is low-pH version (typical 4, minimum 3, maximum 6) with a maximum water content of 0.12%.  I'm guessing they need to add some small amount of an acid to achieve this, but plants treat nearly every organic acid as a hormone, chlorides are massively detrimental to leaves, and it can't be anything toxic so who knows...  If you're making a ton of aluminum glitter / other aluminum containing KNO3 comets at once it might actually be worth buying this since it's guaranteed to be at least slightly acidic. There used to be a Multi-K TOP which was probably pure enough to be reagent grade but they don't seem to make it anymore. 

Unless you need an even higher temp to remove waters of crystallization, you can get standalone food dehydrators that'll go up to 190F for less than $100 and just run the mess outside and not have to worry about destroying your expensive range with horrifying fumes (especially since the temperatures widely vary inside regular ovens unless they're expensive enough to adjust heat via current control to the heating elements rather than just turning them on and off at intervals.  Another option might be the vacuum de-gassing systems for epoxy casting which can usually be had in the ~$100 range. 

I wouldn't dry Ba(ClO3)2 in a household oven if you paid me (I don't think it's particularly hygroscopic anyway), and I have no qualms making parlon-bound electric stars with it.  I don't think most things need to be dried in the first place.  The multitudes of formulas where they directly substitute Ba(NO3)2 for Sr(NO3)2 without adjusting for M.W. probably indicate the original strontium formulas just used it without heating high enough to produce the anhydrous form.  Strontium nitrate tetrahydrate is ~6.3% heavier per mol than standard (anhydrous) barium nitrate which is fairly negligible.  Anhydrous Sr(NO3)2 is ~19% lighter and would require formula modifications to avoid undershooting the oxidizer by quite a bit.