THE REFINING OF SEA SALT 



31 



table and dairy]. The wash-solution, obtained by dissolving high-grade salt in fresh 

 water, removes a part of the magnesium salts and other impurities from the crude 

 salt. A fine precipitate of magnesium hydroxide forms in the wash liquor. The 

 solution runs out through an overflow pipe and is then returned to storage tanks 

 where it becomes alkaline again. The alkalies which are used in the wash solu- 

 tion include lime and sodium carbonate and bicarbonate. Sometimes crude trona 



(Courtesy Leslie Salt Co.) 



Fig. 3-2. Rotary kiln used for drying refined sea salt. Powerful fans draw heated air 

 through the salt as it is tumbled over and over. 



from the alkali lakes of California is used. In most cases lime is sufficient; how- 

 ever, an excess of lime causes a fine white flocculent precipitate in the solution 

 of the washed salt. In some plants the washed salt is crushed and washed again 

 with a dilute solution of ultramarine in a centrifugal washer. This produces a 

 white salt. 



The washed salt must be dried before grinding as it contains from I/2 to 2/2 per 

 cent water. Ordinarily, this is accomplished by heating it in a revolving horizontal 

 drier at a temperature of 190° to 210° F (87.8° to 98.9° C). At higher tempera- 

 tures the salt becomes discolored ( reddish-gray ) . The drier is usually a rotary 

 cylinder which revolves slowly as the salt passes through it. It is provided with 

 the steam pipes of a steam jacket, or other heating equipment, which heats the 

 salt to the proper temperature. Hot air is pumped through the drier to facilitate 

 evaporation of the moisture contained in the salt. 



Various types of machines are used for the grinding of dried salt. Carr disin- 

 tegrators and other grinders, equipped with beaters, are quite satisfactory 

 although seldom used. Roller mills, having equal speed rollers, are commonly 



