Vacuum Pan Salt Refining
This page is available here: http://www.saltinstitute.org/13.html
Excerpt:
Table salt is typical of the fine, granulated-evaporated salt produced in vacuum pan evaporators. Virtually all food grade salt sold or used in the United States is produced by vacuum evaporation of brine.
Table salt is often fortified with iodine and sometimes fluoride and other additives.
Kali und Salz (Germany) and Salt Union (UK) have further information about vacuum pan production on their company websites.