First off, watch yourself for self-incrimination. Even though SWIY may legally posses a prescription that doesn't mean that he should be telling people he is not taking it as directed. Just a friendly reminder since you're new.
As for purifying it, I'm really not sure about a method that works for sure. A friend of mine claims that he created "jump juice," a mixture of purified amphetamine
salts and water in an afrin-like inhaler, with a relatively easy method. His method was to take 100mg worth of Adderall
tablets, grind them up to a powder, and mix them with water. His creation hinged on the fact that amphetamine salts are water soluable (which is why they can be efficiently insuffulated). He said using a simple coffee
filter he filtered out all the filler and was left with a mixture that was water and amphetamine salts.
He claimed it worked, and I'm not going to doubt him because he knows his stuff, but I do question how efficient a process this could be. Besides, there is certainly the chance that other fillers are water-soluable as well, and then there is the issue of figuring out what those fillers are for both XR and IR pills. There really wasn't much scientific method behind his experiment, so he never actually quantified what the resultant was. However, as I said, he did say it worked.
The reason I mention this is because it could be possible, if this method is efficient enough, to run a powder/water mixture through a coffee filter and then evaporate the water. If my friend's theory is true, you'll be left with just amphetamine salts. In my opinion you'll find some other things there too, for example generics are noted to have some kind of sugar as a filler, and I believe that would make its way through the filter as well.
I would suggest reading up on the chemical properties of amphetamines more if SWIY is actually serious about pursing this. There might be an extraction that is more efficient or easy, or one that could take advantage of the properties of the fillers (for example doing a cold water extraction if the fillers crystalized and amphetamines didn't).