32 MARINE PRODUCTS OF COMMERCE 



used. Impurities of light weight are removed from the salt by air-suction over the 

 screens in the screening machine. 



Shaker salt is obtained by adding about 1 per cent fine magnesium carbonate. 

 Some of the salt manufacturers in the eastern part of the United States use fine 

 sodium carbonate in place of magnesium carbonate; but sodium carbonate de- 

 velops an unpleasant odor when it is used with sea salt unless the latter has been 

 recrystallized. To make the salt "free-running" IM per cent white air-floated talcum 

 powder is sometimes added instead of magnesium carbonate. 



Refining by Recrystallization in Grainers. In some American sea salt factories 

 the refining is carried out by recrystallizing the salt in grainers. The crude solar 

 salt is dissolved in a tank containing fresh water until a nearly saturated solution 

 is prepared. This brine is limed, settled, preheated, and then evaporated in the 

 grainers while live or exhaust steam is passed through pipes immersed in it. 

 A grainer consists essentially of a long, narrow, shallow vat built of wood or metal, 

 supported on a framework, or of reinforced concrete, supported on a foundation 

 of sand. 



Crystallization begins as the little cubes which float on the surface are gradually 

 submerged in the hot brine. During evaporation new crystals form a cup-shaped 

 concrement on the sides of the original cube until it becomes heavy enough to 

 sink to the bottom. A scraping conveyor carries the salt out of the grainer and 

 deposits it on a platform where it is allowed to drain. Then it is usually dried in a 

 revolving horizontal drier. After the salt is in solution a small amount of lime 

 may be added to it before it leaves the dissolving tank. It is allowed to stand 

 long enough for the precipitated impurities to settle and for a clear white salt 

 to be produced. Prior to the evaporation in the grainer the brine is preheated and 

 the gypsum precipitated. 



The Alberger Salt Refining System. The Alberger process is a relatively new 

 system of refining salt. At present it is not used in refining solar salt, but only 

 in producing salt from brine obtained from salt wells. To produce salt by the 

 Alberger process a preheater, three pressure heaters, two gravellers, three flash 

 evaporators, and an open evaporating pan, shaped like a figure 8 (Fig. 3-3), are 

 needed. Although the brine, which has been purified and clarified by treatment 

 with lime and soda ash, followed by settling, is raised in temperature, it does not 

 boil as it passes through the preheater and pressure heaters. In the third pressure 

 heater the brine reaches 290° F ( 144.3° C) , the temperature at which the optimimi 

 removal of calcium sulfate is effected. The brine, passing through the graveller 

 where the calcium sulfate precipitates, is deposited on cobble stones. 



The purified brine is now cooled by passing through a series of three flash 

 evaporators. The critical crystallizing temperature 226° F (107.8° C) is reached 

 in the third flasher, at which point about half of the salt crystals have crystallized 

 into microscopic cubes. The crystal-seeded brine is ejected into the open evaporat- 

 ing pan where the surface evaporation of the rapidly cooling brine results in the 

 formation of delicate, hopper-shaped, flake-like crystals. After they have settled, 

 the crystals are drawn from the pan and whirled in a basket-type centrifuge to 

 keep the brine from adhering. The drying is completed in a steam-heated, rotary, 

 counterflow air-drier. 



Recrystallization in Vacuum Pans. A number of factories on the Pacific Coast 

 have adopted vacuum pans for recrystallization of the crude solar salt. Although 



