Sea water may be considered as a dilute saline solution, and its treatment for the recovery of common salt (sodium chloride) affords an example of the utilisation of natural heat (the sun's rays) for evaporation on a commercial scale.
The works in which the sea-salt industry is carried on consist of several series of basins communicating with one another, and possessing extensive evaporating surfaces. Through these, the sea-water is led until arriving in the last, which are very shallow; the already concentrated salt water is allowed to stand till most of the salt has crystallised out. The mother-liquor or "bittern" is drawn off, and the salt is collected and drained to dryness.
The first of the series of basins is usually a large shallow pond, into which the sea-water is admitted, and where it is allowed to settle, and is stored for use. Sometimes two such large basins are employed, one for settling, the other for storage. Hence the water is carried through a series of other basins, each set in its turn being smaller and shallower. In the last, the salt principally deposits; it is then collected, drained, and stacked for sale.
These works are called by various names, according to the countries in which they are situate. In England, they were known as "salt-marshes," "salterns," "salt-gardens," and by other local names. In France, they are called marais salants or salins; in Portugal, marinhas; in Germany, Meersalinen or Salzgdrten.
Fig. 119 shows a marais salant as now in use on the Atlantic coast of France. The spot chosen is generally some little bay or creek protected from the direct action of the waves; from this is led a small canal, through which at springtides the sea-water can be conducted into the large reservoir A, the jas or casiere ("settler") where the water is allowed to clarify. This reservoir is usually placed higher than the rest of the marais salant, so that the water can be run off at pleasure into the first set of basins or couches c, without pumping. The jas may be of any moderate dimensions, and often covers 2 1/4 acres, the depth varying from a yard to a fathom. The water, having become thoroughly clarified in the jast is allowed to run by the underground channel B, titled with a suitable sluice, to the coaches, which are frequently about 23-2* ft. long, 12 ft. wide, and 1-1 1/2 ft. deep, arranged in sets of 8 or 10 in a doable row, as shown, separated by low walls or dams, bat communicating with each other in such a manner that the water entering from A by the sluice B can circulate slowly through them, as shown by the lines and arrows, and be drawn off by the sluice G. In fine weather, the water has already undergone some degree of in the jasA, nnd as it passes in an almost insensible current through the couches, it continues to evaporate.
It is led by the sluice G into a cannal D, which nearly encircles the marais salant, and serves to conduct the water on to the tables E, arranged similarly to the ouchies; over these, it flows as before in an almost insensible current into other basins R, called aderaes or muants, whence it is fed as required by small channels cut in the soil into the ceuillets f g, small basins where the salt crystallises.
On the shores of the Mediterranean, about Cette, Marseilles, and the Etang de Berre, immense quantities of salt are produced by a somewhat similar arrangement. As, however, there are no tides in that sea, the arrangement with the separate reservoir A is not essential. A series of basins whose bottoms are levelled and plugged with clay, are made by sets in gradients (usually 3), so arranged with channels and sluices that the water can flow from basin to basin and from one set to another. The general principles involved are much the same as on the Atlantic coast. They differ, however, in the degree of circulation of the water. In the western works, the water is allowed to almost stagnate, as it were, no differences of level being maintained so as to promote its. flow, except in respect of the jasy which is usually placed on a rather higher level. In the salins du midi, on the contrary, when the flowing water has reached its lowest gradient, it is collected in large wells, whence it is drawn up and thrown back by a pump or water-wheel to its former level, and again traverses a like set of gradients, to return once more to another set of wells.
The first set are called "wells of green water," the second are called "salt water" wells.
Sometimes brine, whether derived from springs or otherwise, is not brought to the surface at a sufficient degree of concentration to be evaporated by artificial heat, without too great a consumption of fuel. It then becomes necessary to concentrate the brine. The most economical mode of doing this is obviously spontaneous evaporation by exposure to the air; and in places by the seaside where high winds prevail, and where land may be of little value, large quantities of salt are economically produced, as already detailed, by this means. But in other places, this arrangement would be inconvenient, and other means of exposing the liquid to evaporation on an extended surface are resorted to. Such is the so-called "graduation" system invented by Abith in the 16th century, and still practised in a few places on the Continent. A graduation-house (Gradishaus) is generally a huge shed, 300-400 yd. long, presenting one end to the prevailing wind, and open at both ends. The interior is filled with rows of fagots; the floor is a large flat reservoir or basin, and on the top, by means of pumps and other arrangements, the water is sprinkled profusely over the fagots, and in course of descending into the trough below, trickles over the sticks, and exposes a large evaporating surface.
By several repetitions of this process, the liquor loses water, and a concentrated brine is the result. Fig. 120 represents the general construction of a graduation-house. A description of that at Schonebeck, one of the largest and most important establishments of this kind, will suffice, as the system is not required in England, and is becoming less used elsewhere. The building is 916 yd. long, and 11-14 yd. high. It is filled with a double tier of fagots, presenting a thickness of 5 1/2-7 1/2yd. at its base, and 3 1/4-5 1/2 yd. at the top, consequently offering an immense superficies for evaporation. The illustration shows the whole arrangement in profile, end on. a is the large reservoir for the salt water. It is excavated in the ground, and widens out at the top to c to catch any drip the wind may carry away; d e are merely stays to support the walls of the reservoir, and to sustain the building against the lateral pressure of the wind; / is the wooden framework in which may be arranged 4 vertical walls or tiers of fagots. These fagots are made of white- or black-thorn, the branches of which are especially crooked and angular. The water is elevated by pumping to the reservoir h at the top, which is so arranged that the outflow can be altered according to the way of the wind.