I buy KNO3 in prilled form, which is advertised at 99.8% purity - but in the interest of eking out that last little bit of efficiency, I recently ran a batch of it through a boil and precipitation cycle using distilled water in the pot. It was pretty easy to do and I reclaimed all but about a tablespoon of solids from the 3 lbs I started with. However, my cynical nature tends to think it’s a waste of time to attempt wringing out that last .2%.
Does anyone else refine your nitrate if it’s already fairly pure, and at what percentage does it become a worthless exercise?