u/InternationalPlay920

So, I recovered some KNO3 crystals that I had dissolved in water using the most frugal method possible: I let the sun evaporate the water. I had spread the solution in an aluminum baking tray and now there are deposits that I can't remove in any way. I've tried several times to dissolve these flakes in water, but they remain stuck and won't dissolve.

Could anyone suggest a simple method to remove them?