Get stump remover, then dissolve it in water. Pour the solution through a filter, through a funnel, then into a steel/pyrex container you don't want anymore. Boil the water off and wait for the crystals to start forming. Drive off the remaining moisture by placing the container with the crystals and a little bit of solution in a warm, dry place until the water is completely gone. What you are left with should be 98%+ purity KNO3.NOTE: The stump remover must be mainly KNO3 for this to work.