20 



MARINE PRODUCTS OF COMMERCE 



36-mch line. By this means the water is lifted 8 feet to a sump. It then flows to 

 the high point of the secondary ponds where it is distributed by means of gates 

 between the ponds. This plant has nine secondary ponds, totaling 450 acres. By 

 the time another flow of brine from the tide ponds is due (about two weeks) 

 the water in the upper ponds has reached a density of about 6.77° Be. (1.041 sp. 

 gr.) and is run into the lower secondary ponds to make room for another run. 



(.Courtesy Leslie Salt Co.) 



Fig. 2-1. Evaporation ponds on San Francisco Bay. Their size varies from 40 to 



800 acres. 



The lower secondary ponds are sometimes referred to as "lime ponds" as the cal- 

 cium carbonate settles to the bottom at a brine density of about 7.82° Be. (1.057 

 sp. gr.). The evaporation is continued in the lower secondary ponds until a 

 density of 9.26° Be. (1.0682 sp. gr.) is reached. The brine is then nm to the 

 pickling pond which has an area of 200 acres. In this pond the evaporation is 

 continued until the brine has a density of 24.1° Be. (1.193 sp. gr.). At 13.78° B6. 

 (1.095 sp. gr.) calciimi sulfate begins to settle out and is almost entirely precipi- 

 tated by the time the density reaches 1.193 sp. gr. At this point the brine is 

 run into the crystallizing ponds of 100 acres where salt begins to crystallize at 

 25.2 Be. (1.2104 sp.gr.). 



The density of the brine in the crystallizing ponds is such that 1 inch of salt 

 is thrown down for each 8 to 10 inches of annual evaporation of water; therefore, 

 the annual deposit is from 5 to 7 inches. Since the weight of 1 acre inch of salt is 

 approximately 110 tons, the "crop" is 550 to 770 tons per acre. 



When the density of the brine in the ponds reaches 29.67° Be. (1.2574 sp. gr.), 

 the mother hquor is transferred to the bittern ponds. Sodium chloride will crystal- 

 hze out in a nearly pure form except for a small percentage of magnesium chloride 

 up to a density of 30° Be. (1.2609 sp. gr.); but, above this density other salts are 

 deposited, magnesium sulfate being the first to be precipitated. 



"Harvesting" usually begins the latter part of August and continues until the 



