(I) From Natural Brine.—When an aqueous solution of sev eral salts is evaporated, the salts separate in the order of their solubility, the least soluble salt being deposited first and the most soluble last. Hence in the case of sea water and brines the order of deposition is calcium carbonate, calcium sulphate, sodium.
chloride, magnesium sulphate, carnallite (potassium magnesium chloride) and finally magnesium chloride. These salts are not, however, deposited completely within sharply defined limits of concentration ; there is some overlapping and each salt is con taminated to a greater or less extent with others. Further, when solar evaporation is employed the difference of temperature be tween day and night will affect the character of the salt deposited. The art of the salt-maker is to produce grades of salt suitable for the particular use to which it is to be put ; e.g., table salt, industrial salt, fish salt.
Salt is produced by solar evaporation from sea water in France, Portugal, Spain, Italy, India, California and China, and from natural brine or salt lakes in India, Russia and the United States.
The process generally adopted is similar in principle, although details of evaporating pans and of manufacturing plant vary with local conditions. A preliminary concentration is usually carried out by allowing the brine to flow through a series of channels to concentrating ponds constructed of wood, puddled clay or con crete. The areas of the ponds vary from 28o to 20,000 sq. feet in different countries. The solution is concentrated first to specific gravity of about 1.21. At this stage suspended impurities (sand and clay) and the less soluble salts (calcium carbonate or chalk and calcium sulphate, gypsum and anhydrite) are removed. The clear brine is now run successively into crystallizing pans, usually three, of similar construction to the evaporating pans, where the salt is deposited. The total area of crystallizing pans is approxi mately one tenth of that of the evaporating pans. In the first crystallizing pan the brine is concentrated to a specific gravity of 1.25 and here the finest grade of salt is produced. The mother liquor reaches a specific gravity 1.26 in the second pan, where the second grade of salt separates. In the third pan a specific gravity 1.275 is attained when the coarsest salt is deposited. The final
mother liquor, termed bitterns, is used in some countries, e.g., France, India and America, for the manufacture of potash, bromine, Epsom salts and magnesium chloride.
The salt from each crystallizing pan is raked into rows and allowed to drain for some days, then collected into heaps, drained again, lifted from the pans, washed and finally dried. As in most European countries a tax is levied on salt, it is obviously of advantage to trade in the dried material. The salt from the first pan is frequently utilized locally as table salt, that from the second pan goes as a rule into chemical industry, and that from the third pan is used for pickling fish, refrigerating, and as bath salt. Typical compositions of the salts thus produced are given in the following table: In England, Germany, most of the eastern states of the United States and elsewhere where it is impracticable to manufacture salt by means of solar heat, the brines are concentrated by artificial heat. Formerly the brine was concentrated in open pans over a fire until crystallization of the salt occurred. More recently steam jacketed vessels were used, but now much of the salt pro duced in colder countries is manufactured in triple-effect vacuum pans. In the older method the brine, natural or artificial, is first pumped into settling tanks, where lime and magnesium salts may be precipitated by the addition of milk of lime. The excess of lime is then removed by soda ash and, after settling, the brine is delivered into the steam-heated evaporating pan (grainer) at approximately the same rate as that at which evaporation is taking place, and at a temperature only slightly below that of the brine in the grainer. To obtain a high-grade salt the bitterns are removed every day, but at less frequent intervals for coarser salt. The bitterns may be further concentrated by waste steam in a separate or "dividend" grainer, when salt suitable for refrigerat ing is deposited. Where multiple-effect vacuum evaporators are employed, the vacuum in each vessel is so adjusted that the steam from the first vessel is sufficiently hot to boil the brine in the second vessel, and so on. In a triple-effect system, vacua of 15, 24 and 27 inches of mercury have been found efficient, the vacuum of the third vessel representing a high degree of evacu ation. With ordinary open pans, one ton of coal will produce about 2 to 21 tons of salt, whereas it will yield 5 to 6 tons of salt with an efficient triple effect plant.