Salt and Chlorine

Salt (the mineral halite – NaCl) is one of society's basic needs. Its use goes back to earliest recorded history (3,500 B. C.). Pure halite consists entirely of sodium and chlorine in an atomic ratio of 1:1. Rock salt is the solid form. When dissolved in water it becomes saltwater – or brine if the salt content is high. Salt has many uses in modern society and is an important raw material of industry. It is the starting material for the manufacture of both the bulk of elemental sodium and chlorine used in industry, and most compounds of either element. Among other things, sodium is used to make caustic soda, large quantities of which are used in the pulpwood and metallurgical industries.

Chlorine is used to manufacture polyvinylchloride (PVC, an important plastic) and muriatic (hydrochloric) acid, as a water purifier, and as a bleaching agent. Huge tonnages of rock salt are used to de-ice highways, streets, and sidewalks. Much salt is used as a flavor-enhancer and a preservative in the food-processing industry. A volumetrically minor, but very important product, is the table salt used in most homes as a seasoning agent.

The most important commercial source of salt is rock salt. It occurs in evaporite layers deposited by the evaporation of water from oceans or saline lakes. Commonly, rock salt is interbedded with shale, gypsum, and anhydrite. Because rock salt may flow under pressure, it also occurs in domes that were forced or extruded into younger overlying sedimentary rocks. Salt is often mined underground or recovered by the evaporation of water obtained from the seas, saline lakes, and brine springs or wells. In Arkansas's past, Native Americans and early settlers obtained salt from evaporation of small bodies of saline water and brine springs in several counties, mostly in the central part of the state.

Although there are major deposits of both rock salt and brine in the subsurface of south Arkansas, there has been no commercial recovery of salt or chlorine from the subsurface. The salt-bearing Louann Formation (Jurassic) in southern Arkansas extends in the subsurface from Miller and Hempstead Counties in the west to Chicot County in the east. The Louann in south Arkansas represents the northernmost edge of salt-bearing units in the Gulf Coast Basin, the largest of 4 major basins containing salt in the United States. Some deep wells have encountered more than 1,000 feet of salt in the Louann. Eventually, it may be economical to utilize these bedded deposits by deep-well injection of water into the salt-bearing units and extracting the resultant brines for processing. Consequently, the remaining cavity might have value for storage.

Oil-field brines, most notably those in the Smackover Formation (Jurassic), are present throughout southern Arkansas. A collection of 284 Smackover brine samples was analyzed for chlorine and determined to average more than 171,000 parts per million, or nine times the chlorine content of typical seawater. Presently, the oil-field brines in southern Arkansas have unknown potential for halite and other salts.

References

Bell, H. W., 1933, Discovery of rock salt in deep well in Union County: Arkansas Geological Survey Information Circular 5, 24 p.

Collins, A. G., 1974, Geochemistry of liquids, gases, and rocks from the Smackover Formation: U. S. Bureau of Mines Report of Investigations 7897, 84 p.

Imlay, R. W., 1949, Lower Cretaceous and Jurassic Formations of southern Arkansas and their oil and gas possibilities: Arkansas Resource and Development Commission, Division of Geology Information Circular 12, 64 p.