My dutch oven for alloying/smelting gets no special care, unless it's so rusty that I feel the need to hit it with steel wool and some 60 grit sandpaper, before getting the melt going.
Anything that's loose, or that I scrape off while stirring, will float to the top with the rest of the trash.
As a bonus, none of the lead will stick in a rusty dutch oven. However, some
will stick on perfectly clean cast iron surfaces. ...In my mind, minor surface rust is actually helpful.
I use a turkey fryer burner rated for 55k BTUs. It's more than enough for my 8" dutch oven, and I can't imagine anything I could put on that burner, that it couldn't handle.