Naphtha, not namphtha.
Normally I would just link you to the FAQ because it answer your question on how many pulls, and how to recrystallize, but for some reason the wiki is down now.
Anyways, recrystallize your dirty crystals, and keep pulling and freezing each pull separately (pre-evaporating it till its cloudy before freezing), and as long as a significant amount of alkaloids are still crashing out, keep pulling.