Sublimation shouldn't be a worry with 20t. If you start from a powder and heat a bit more than the absolute minimum, you should get an interesting pace. Or boil it if you prefer.
Precisely because iodine sublimes and few substances do, it is a natural way to purify it.
Melting and solidifying does purify substances...
- Iodine has 12kPa vapour pressure at the melting point, so your pot doesn't really need to resist pressure, but it MUST contain the dangerous iodine vapour.
- Molten iodine is almost evaporated. You apparatus must avoid the condensation of the vapour at the wrong places, with walls hot enough and so on.
- Purification occurs only if the impurities are rejected elsewhere during the solidification. This may demand a slow solidification, well oriented, and so on.