how i have been thinking about another concept of making potassium nitrate (used in black powder) from a garden fertiliser, potassium carbonate (k2co3). potassium carbonate can be bought from here in Australia at Bunnings warehouse so it should be available anywhere else. the other substance you need is any nitrate (eg. calcium nitrate). the metal in the nitrate should have 2 electrons in its outer shell so there are 2 potassiums and 2 nitrates.
You dissolve both the chemicals in water but do not over saturate so there is solid on the bottom. next you mix the two solutions together. a solid should form and move to the bottom. then filter out the carbonate by pouring it through a paper towel and collecting the leftover liquid and throw away the filtered out solid ((metal) carbonate). boil off the water in the solution and then leave to dry.
this should be potassium nitrate shouldn't it?
any suggestions or criticism? reply back