SALT FROM SEA WATER 19 



sulfide and calcium carbonate have precipitated out and most of the calcium 

 sulfate been thrown down. Then the nearly saturated brine passes into a second 

 series of shallow reservoirs. The rate of flow of the brine is regulated so that by 

 the time it reaches the last row of these compartments, salt has begun to crystallize; 

 the brine is then at a concentration of 25.2° Be. (1.2104 sp. gr.). The evaporation 

 in reservoirs is allowed to proceed until the brine reaches a concentration of 

 about 27° Be. (1.229 sp. gr.). The saturated brine is then run through the canal 

 into the crystaUizing ponds where the salt crystaUizes more or less rapidly, de- 

 pending upon the rate of evaporation. The evaporation is usually continued until 

 the mother liquor reaches a density of 32° Be. (1.283 sp. gr.). If the crystallization 

 is extended, salt containing large amounts of calcium and magnesium chlorides 

 is obtained. The bittern is then run off into bittern ponds. In the Giraud district 

 in France the mother liquors are worked up into magnesium and potassium salts, 

 according to a process introduced by Balard which is described in the following 

 chapter. After the bittern is run off, the salt remaining is broken up, raked together, 

 and hauled to the salt yards where it is piled in large heaps in the open air to dry. 

 Occasional showers purify it somewhat for the rain dissolves the more soluble 

 calcium and magnesium chlorides. 



Salt produced in this manner contains from 95 to 97 per cent sodium chloride. 

 The impvirities are chiefly due to calcium and magnesium chlorides and sulfates, 

 as is shown by the analyses given in Table 6, on page 15. This salt is hygro- 

 scopic because of its high calcium and magnesium chloride content. It is highly 

 prized for salting fish and hides. It is particularly desii-able for salting hides when 

 they are sold by weight because salted hides do not have a tendency to dry out 

 and lose weight. 



The California Solar Salt Industry. The solar sea salt industry has been par- 

 ticularly successful along the California coast because the rainfall during the salt- 

 making season is very sHght. The largest plants are located in Alameda and San 

 Mateo counties near San Francisco. Three other plants are located in the southern 

 part of the state. The general method of production is nearly the same in all 

 these places although the various operations differ considerably in detail. 



The salt-making process, as it is carried out in the plant of the Western Salt 

 Company at the southern end of San Diego Bay, has been described by Palmer 

 (1917) and Mason (1919) and is typical of California's industrial practice in 

 the manufacture of high grade salt by solar evaporation. San Diego Bay, which 

 is the source of supply of sea water, is a long and narrow "arm" of the Pacific 

 Ocean. The inflow of fresh water is very slight, and consequently the salt content 

 of the water at the southern end where the plant is situated is somewhat higher 

 than that of sea water. According to Palmer samples of the water of San Diego 

 Bay have occasionally run as high as 5 per cent sodium chloride. The net annual 

 evaporation of San Diego Bay is 50 inches, a total evaporation of 60 inches against 

 an average precipitation of 10 inches. 



At high tide bay water is admitted through tide gates into four tide ponds 

 which have a total area of 400 acres. The ponds are surrounded by a levee about 

 10.5 feet high and 4 miles long. The water is retained in the tide ponds for two 

 weeks and then pumped into secondary ponds by means of two centrifugal pumps 

 which have a combined capacity of 15,000 gallons per minute. Each pump has a 

 36-inch suction and a 24-inch discharge, the two discharge pipes uniting in one 



