The sources of salt are: -
2. Rock salt.
3. Salt brines derived from springs, lakes, or wells.
Atlantic sea-water, except near the mouths of large rivers, averages about 3.4 per cent of solid matter, of which about 75 per cent is sodium chloride, the remainder consisting of chlorides, bromides, and sulphates of potassium, magnesium, calcium, lithium, etc., with minute amounts of other salts. The concentration of sea-water for salt is carried on to some extent in warm, dry countries by solar evaporation, the water usually being exposed in shallow tanks or ponds to the full force of the sun's rays. Sea-water is seldom evaporated over fire because of the cost of fuel. In Russia it is allowed to freeze over the surface, and the ice, which contains but little salt, is removed. This is repeated until the brine is sufficiently concentrated to make the evaporation over fire profitable. Salt made from sea-water ("sea-salt") is very coarse and is usually damp, owing to the presence of some magnesium chloride, which, being a deliquescent substance, attracts moisture from the air. It is of less importance in this country than that made from other brines; the greater part of the sea-salt produced here in 1894 was made in California.
Rock salt is found in many countries, and often very pure. In
Austria, Germany, Spain, and Louisiana it occurs in large deposits, so pure that it is only necessary to grind it for use, but in most cases it is contaminated with iron oxides, clay, sand, and other impurities, which often necessitate its purification. In this country it is mined in New York, Kansas, California, Utah, and Louisiana.
As it does not dissolve so readily as finely crystallized salt, it is preferred for many purposes, such as curing meat, preserving green hides, and feeding to live stock.
The salt of principal interest in this country is derived from
natural brines, found chiefly in :New York, :Michigan, Kansas, and Ohio, while "'Vest Virginia, Utah, Texas, and Pennsylvania produce lesser quantities.
The New York deposits are near Syracuse and in the neighborhood of Warsaw and Batavia. The Onondaga (Syracuse) deposit has been known since the middle of the seventeenth century, and since 1797 has been the property of the state. That at Warsaw, opened in 1883, is now the most important. The Michigan deposits are near Saganaw Bay and Manistee, a strong brine being obtained by boring. Large amounts of brine are evaporated near Salina, Kansas.
The Ohio and West Virginia deposits are in the valley of the
Ohio River, near Pomeroy and Wheeling.
In the Onondaga district, the brine is obtained by boring wells, 8 inches in diameter and from 300 to 350 feet deep, and lined with iron casings to exclude surface water. The state owns and operates these wells, furnishing the brine to the salt makers, and collecting a tax of one cent per bushel of the salt made. This plan is not profitable to the state at present, and it is not improbable that the reservation may be sold.
The brine is raised by pumps, worked by an endless wire rope, the power being furnished by an engine. As it comes from the pumps the brine has a slightly turbid appearance, due to clay or fine sand raised from the well, together with minute bubbles of carbon dioxide, with which the brine is charged. It also contains some ferrous carbonate, which is held in solution by the carbon dioxide. As the latter escapes, the ferrous salt absorbs oxygen from the air, and hydrated ferric oxide separates as a yellowish red turbidity, which settles after a time, leaving the brine perfectly clear.
For the manufacture of "solar salt," of which a considerable
quantity is still made at Syracuse, three sets of tanks are used. The first are called" deep rooms"; here the brine is received from the pump-house, and the ferric hydroxide and sediment deposit. The clear brine is then drawn into the "deep rooms," - tanks about 8 inches deep, 18 feet wide, and from 100 to 400 feet long. In these the evaporation by the sun's rays goes Oil until small crystals of salt appeal'. The brine contains calcium sulphate and chlorides of calcium and magnesium. The calcium sulphate deposits as gypsum (CaS04• 2 H20), in long slender crystals, usually in clusters; but a small portion remains in solution with the salt. The concentrated
brine is then drawn into the" salt rooms," which are very similar
to the lime rooms, but only about 6 inches deep. In these the salt is deposited, more brine being admitted as the water evaporates, until the layer of salt on the vat floor is about 3 inches deep. About three times in a season the salt is "harvested"; i.e. it is raked together and put into tubs having perforated bottoms, through which the mother-liquor drains off. It is then taken to the storehouse, where, according to the :New York law, it must drain 14 clays before being marketed.
All the vats are built of wood and provided with movable covers to keep out rain. During fair weather these are rolled back. The vats are built on piles, so that the" deep rooms" stand higher than the" lime rooms," and these in turn are higher
than the salt vats; thus the liquor runs by gravity from one set to the next.
The mother-liquors contain considerable quantities of calcium
and magnesium chlorides.
The large cubical salt crystals are usually not perfect; they
are skeleton crystals, with the edges nearly complete, but with cavities in the crystal faces. This makes the salt more bulky than is the case with fine solid crystals. Moreover, the cavities hold small drops of the mother-liquor, even after draining for some days; consequently, calcium and magnesium chlorides remain in the salt, and these being deliquescent, cause it to become moist in damp weather.
In some foreign countries, dilute brine is concentrated by flowing over brush wood "ricks," prior to final solar evaporation.
Where brine is concentrated by the use of fuel, the product is generally called "boiled salt." This is prepared in several ways. By the Kettle Process, Fig. 31,* the brine is evaporated in cast-iron kettles (A, A) about 4 feet in diameter by 2 feet deep. They are set in rows of from 16 to 25, over a flue leading from the fire box (G) to the chimney. The contents of the kettles near the fire boil down rapidly and produce small
| || dense crystals, but in those near the chimney, evaporation is slow, and large crystals similar to solar salt are formed. The two products are generally mixed and sold as "common fine salt." The brine coming from the wells must be purified by adding "milk of lime," and stirring well, otherwise the product will be colored by ferric hydroxide. The lime combines with the carbon dioxide and precipitates the iron. After settling, the brine is supplied to each kettle by a wooden pipe (P). The first effect of heating and concentrating the brine is the separation of "bittern," consisting of calcium sulphate and a little magnesium sulphate. This is removed by the bittern pan (B), a shallow wrought-iron dish, somewhat like a frying pan with very sloping sides and a vertical handle near the centre. Its sides fit closely against the walls of the kettle; as the bittern collects in the pan, the latter is emptied and replaced several times; but as soon as the salt crystals begin to form, it is removed. |
Part of the calcium sulphate, together with some salt, deposits as a scale on the sides of the kettle. This incrustation soon becomes so thick that it causes loss of heat, and it is removed by filling the kettle with fresh water, which dissolves the salt, leaving the calcium sulphate so porous that it is easily scraped away. This deposit collects faster in the front kettles than in those nearer the chimney; when the latter
become coated, it is customary to shut down the entire system and clean them all. The salt is removed as it separates, and drained for a short time in baskets (D) placed over the kettles; it is then dumped into the storage bins (E), where it must remain 14 days.
The salt block, as the plant is called, is in continuous operation, night and day, for about 14 days, two" runs" being made each month. A good average "run" at Syracuse produces about 7800 bushels of salt.
Sometimes the kettles are heated by steam jackets; as all have the same steam pressure the temperature is uniform, and only one quality of salt is produced.
Salt is also made by the "pan process" (Fig. 32), of direct evaporation over fire. Large wrought-iron pans (H, H), 24 feet wide, 100 feet long, and 12 inches deep are used. These pans are divided into two sections by a loose partition, which allows the brine to flow slowly from the rear to the front section. A second. smaller pan is set behind and slightly above the first, so that its contents may be siphoned into the front pan. Both are heated by flues from grates (G), but the rear one gets only the waste heat, before the gases pass into the chimney. The ends of each pan are made perpendicular to the bottom, but the sides are inclined, and sloping wooden platforms (F, F), called "drips," are joined to them: on these the salt is drained when removed from the pans. The brine is purified with" milk of lime," as in the kettle process.
The pan process permits an easy control of the size of the grain. For the preparation of a very fine grained product, called "factory filled salt," it is customary to add a small amount of sodium carbonate to the brine; this decomposes the chlorides of calcium and magnesium and any excess of caustic lime from the" liming." Then a small quantity of butter, glue, or soft soap is added, and forms an insoluble calcium soap with the remaining traces of lime, and this is removed by skimming.
For both the kettle and the pan process, coal dust is used as fuel. In :Michigan and in western New York, brine is evaporated in "grainers" (Fig. 33) ; these are long, shallow vats of wood or iron, containing steam pipes (P, P), through which live or exhaust steam is passed. '1'he pipes are about 4 inches in diameter and are hung about G inches above the floor of the" grainer," which is some 20 inches deep. Once a day the salt is raked up and deposited on draining platforms over the grainers. The brine is purified before evaporation, as in the pan process, and is supplied to the grainer in just sufficient quantities to replace the water evaporated. 'When the mother-liquors become too highly charged with calcium and magnesium chlorides, they are drawn into special grainers, and a low grade of salt is made from them.
Brine is sometimes evaporated in vacuum pans, and a very finely crystalline product, the best grade of table and dairy salt results. It is separated from adhering mother-liquor by the centrifugal machine.
Strong brine boils at 105°-l09° C., and so cannot be boiled by free steam, although evaporation of very dilute brine can be slowly accomplished in this way. The heat in the kettle and pan process is sufficient to dehydrate any calcium sulphate in the salt; when dissolved in water, such products cause a slight milkiness which disappears after a time, owing to the hydration of the calcium sulphate and its solution in the water.
Sometimes pure water is introduced into rock salt deposits through tube wells; when saturated with salt, it is pumped to the surface and evaporated. A much stronger' brine than is found in nature is secured in this way.
In some places, chiefly in 'Vest Virginia and in Germany, large quantities of bromine are recovered from the mother-liquors (also called "bittern") from the salt industry.
In Italy, Austria, and China the manufacture and sale of salt is a government monopoly. In France, Germany, and India salt used for seasoning food is subject to tax. When used for technical purposes, or in agriculture, the tax is very small. To prevent fraud, all German salt, not intended for table use, must be mixed with certain substances to render it unfit for eating. Some of these adulterants are iron oxide, crude petroleum, coal dust, pyrolusite, carbolic acid, mineral acids, sodium sulphate or carbonate, alum, soot, etc.
Organic Chemistry for the industry
Inorganic Chemistry for the industry
Lixiviation Levigation Evaporation Distillation Sublimation Filtration Crystallization Calcination Refrigeration Density Fuels Liquid fuels Gaseous fuels Water Sulphur Sulphur Derivatives Sulphuric Acid Sulphuric acid burners Fuming Sulphuric acid Salt Hydrochloric Acid Soda Industry Caustic Soda Treatment of tank Ammonia Soda Cryolite Soda process Chlorine Industry Electrolytic Chlorine Hypochlorites Chlorates Nitric Acid Nitrates Ammonia Potash Industry Fertilizers Lime, Cement Cement Glass Ceramic Industries Pigments Bromine Iodine Phosphorus Boric Acid Arsenic Compounds Peroxides Oxygen Sulphates Alum