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. Prior to mechanical evaporation, the brine may be treated
to remove minerals that can cause scaling in the evaporators and adversely affect salt
purity. Chemical treatment of the brine, followed by settling, reduces levels of dissolved
calcium, magnesium and sulfate. Sulfuric acid treatment or chlorination may be used to
remove hydrogen sulfide, and hydrochloric acid will neutralize brine used in diaphragm
cell production of chlorine and caustic soda. Brine
purification has become increasingly important to produce high purity salt for use in
chlor-alkali production, particularly in Europe where dry salt is used extensively for
this purpose.
Water is evaporated from purified brine using multiple-effect or vapor
recompression evaporators. Multiple-effect (calandria, pictured right) systems typically
contain three or four forced circulation evaporating vessels connected together in series.
Steam from boilers supplies the heat for evaporators and is fed from one evaporator to the
next to increase energy efficiency in the multiple effect system. Vapor recompression
forced-circulation evaporators (pictured below)
consist of a
crystallizer, compressor and vapor scrubber. Feed brine enters the crystallizer vessel
where salt is precipitated. Vapor is withdrawn, scrubbed and compressed for reuse in the
heater. Recompression evaporators are more energy efficient than multiple effect
evaporators, but require higher cost electrical power for energy input. The development of
single stage compressors has significantly reduced costs.
Ultimately, weak brine from either process is recycled to the solution mined cavern.
Crystallized salt is produced as slurry which is dewatered first by centrifuging or vacuum drying and then in kiln or fluidized-bed dryers where moisture content of the final product is reduced to 0.05% or less. During this century, salt producers have made significant advances in lowering energy consumption and in reducing salting and scaling in evaporators.
Evaporated salt sales in the U.S. reached 4.36 million tons in 2005.
Most evaporated salt is processed for packaging, using various quality control methods to assure, for example, that food grade salt is safe for human consumption. Table salt is often fortified with iodine and sometimes fluoride and other additives. Evaporated salt has always been important in the U.S. In early U.S. history, Syracuse, NY produced most of the nation's salt. The newest U.S. evaporated salt plant is in Baytown, TX.
Kali und Salz (Germany) has further information about vacuum pan production on their company websites. Also see KPatents for information on salt evaporation.
For further information, contact the Salt Institute or manufacturers of salt evaporator/crystalizer units such as Krebs Swiss or HPD. Note our disclaimer.
If you'd like salt-related information delivered automatically via a
newsreader, you can subscribe to our Salt
Institute NewsCentral service or add any of our
feeds to your
newsreader. The RSS feed for evaporated
salt production news is also available via your Web browser.
 |
| [About Salt Institute] [About salt] [About the salt industry] [News] [SI Member Business (password required] [E-Mail Salt Institute] |