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Chlorine : Common Salt |
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After air and water, there is probably no material so familiar as common salt. It is an essential constituent of food, about 29 lb. per head of population being annually consumed in this way. In 1907 nearly 2,000,000 tons of salt were recovered from brine and rock-salt in Great Britain alone. Common salt is contained in small quantities in primary rocks. From these it has passed by the action of water to rivers, and thence to the sea, where the water re-evaporates whilst the salt remains. Average seawater contains about 3 per cent, of salt. The extensive deposits of rock-salt, found in the earth in many localities, appear to have been produced by the evaporation of former seas and lakes. Rock-salt, or halite, is the crystalline variety, occurring in cubic crystals, colourless when pure, but often tinged yellow, brown, or sometimes blue, by impurities, or else in large more or less coloured masses, which have a cubic cleavage. The richest English deposits are in the Cheshire district, at Northwich and Winsford, in the Upper Trias formation. Besides rock-salt, there are brine springs, yielding a nearly saturated solution of salt. A saturated solution contains 35.78 parts of salt per 100 of water at 15°, or about 26 per cent. The solubility increases only very slowly with rise of temperature. The densities of salt solutions at 15° are:
From brine, salt was prepared by the Romans during their occupation of Britain, by evaporation in square lead pans holding a few gallons. With the difference that flat iron pans holding several thousand gallons of brine are now used, the modern process of salt manufacture in Cheshire is the same as that of the Romans. The brine is tapped by bore-holes sunk through the marl; if no brine is found, water is sent down, becomes nearly saturated with salt, and is pumped directly to the evaporating pans. Large cavities are formed by the dissolving out of the salt deposits, and serious subsidences of land often occur. An analysis of Northwich brine is as follows:
The more slowly the evaporation proceeds, the larger are the crystals deposited. The different grades, according to fineness, are: fine, or table, salt; manufacturer's salt; fishery salt, and bay salt (usually in the form of floating "hoppers," or cubes with hollow faces). In some works the brine is evaporated in vacuum pans under reduced pressure. These are iron boilers heated by steam coils,
the steam produced by evaporation in one pan passing to the coils of the next. The steam from the last pan, which is under low pressure, is condensed by injecting cold water into it at P, and removing the extricated air along with the water by a pump top reserve the vacuum. Each pan has a long leg dipping into an open trough, into which the salt falls. The length of this liquid column balances the vacuum in the pan, and thus acts as a brine barometer. In warm climates (e.g., in the South of France) sea-water is evaporated in large flat ponds, called salt meadows, by the heat of the sun; the salt so made is called solar salt. The mother-liquor, called bittern, contains the magnesium salts and bromides of the sea-water. This process was formerly carried on, previous to boiling, at Hayling Island, near Portsmouth, and at Lymington. |
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