If you have a bunch of the powder from firecrackers why not just add sugar to it to slow it down instead of trying to seperate aluminum and oxidizer. The extra aluminum will just make it even hotter for lighting a thermite. I would probably start with 1/3 the weight of powdered sugar to the mix and see how it burns. It should be slow so it transfers a lot of heat to thermite.
That is probably what I would try though you might be able to disolve the perchlorate and filter out the aluminum. You would have to look at a solubility chart and probably use hot water because if I remember right kclo4 has poor solubility in water at room temperature. But I think the aluminum will actually help you quite a bit.
Edit: Not that dismantling fireworks is advised, but I'm sure most of us pulled the powder out of firecrackers as a kid. I only mention it because you did and I assume you have a pile you are wanting to use. Also, try not to include the clay plug from the cracker. It is usually a tan powder instead of a silver one and will slow things down. In this case that's not bad but it might hurt how well heat transfers to another material. Assuming crackers still have a small clay plug as it has been probably 20 years since I've done that. Even then it didn't seem worth the hassle.
Edited by FlaMtnBkr, 16 February 2012 - 11:23 AM.