CHAPTER 2 

 Salt from Sea Water 



Common salt is one of the necessary components of man's diet. Consequently, 

 its manufacture has been of great importance since prehistoric days. Large quan- 

 tities of salt are needed for human and animal food, and, in addition enormous 

 quantities are used by the chemical and other industries. 



The oldest method of making salt is by the evaporation of sea water, salt lakes, 

 or inland seas. Enormous quantities of salt are obtained from underground deposits; 

 much is also indirectly derived from such deposits by the evaporation of brines 

 obtained from salt wells and springs. Many salt wells are made to yield great 

 quantities of brine by pumping fresh water down into these deposits and drawing 

 the brine out after the water has dissolved the salt. The original source of salt is 

 the ocean. All the large deposits in the earth's crust resulted from solar evaporation 

 of inland seas or "arms" cut off from the ocean. 



The Importance, Properties and Uses of Salt 



As long as man's diet consisted of uncooked food, the fresh meat that he con- 

 sumed provided sufficient salt; but, when he began to boil his food, a consider- 

 able proportion of the salt was removed. This deficiency had to be compensated 

 for in some way. It is not known how long ago man first began using salt from 

 the ocean, but the Chinese obtained it before 1000 B.C. by evaporating sea water. 

 However, the production of salt from sea water by solar evaporation is a com- 

 paratively modern practice. 



In medieval times the production of solar sea salt was very crude, with little 

 effort made to eliminate impurities. The sea water was run through long trenches 

 into a series of shallow basins. After the sun and wind had effected a preliminary 

 evaporation, the water was run into a second series of basins. After considerable 

 concentration had occurred, the brine was run into a third series of shallow basins 

 where it was allowed to remain until crystallization was complete. The salt was 

 then raked into piles and removed for use. 



The world's annual production of salt now exceeds 35 million metric tons 

 and is increasing with the expansion of the chemical industries. The United 

 States alone produces about 15 million metric tons (Table 5). At one time most 

 of the commercial salt was produced either by the mining of rock salt or by 

 the evaporation of brine produced by dissolving rock salt in situ. Nevertheless, 

 the manufacture of solar salt from sea water is still an important industry in 

 many countries. The amount of salt which could be obtained from the ocean is 

 tremendous. The average salt content of a gallon of sea water is about 0.2547 

 pound. Since the average density of rock salt is 2.24 times that of water, the 

 evaporation of the entire ocean, which has a volume of 331,000,000 cubic miles, 



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