If you bought the KNO3 from
herethen it definately contains fire suppressant. I am not sure of the percentage but its bad enough to make your BP burn really slow, and in some pyro formulas wont even ignite. They are also a rip off too. I never got much of a residue, but I suspect that if the fire supressant in yours is anything like whats in sodium chlorate weedkiller then it will be borax.
If this is the case you can purify it by looking up the solubility of both the borax and the potassium nitrate and then dissolving in hot water and collecting the precipitate when it cools, or evaporating the solution and getting rid of the precipitate, whichever one is most soluble.
You can never have a long enough fuse...