At this period, the evaporation is usually brought to that degree, that a crust of salt is formed on the surface of the water, which the workmen break, and it immediately falls to the bottom. They continue to do this until the quantity is sufficient to be raked out and dried in heaps: this is called bay-salt.
In several parts of Fiance, and on the coast of China, the sands of the seashore are washed, and the brine thus obtained is subsequently evaporated in boilers. In various places of Germany and France, the salt waters are pumped up to the top of very extensive sheds, filled with brushwood, over which it is duly distributed by means of gutters, whence, falling in drops from sprig to sprig, a rapid evaporation takes place over an immense surface; the same water is pumped up many times before it is sufficiently concentrated to be drawn off into boilers, which complete the operation. See an account of several works of this kind in Dr. Ure's Dictionary of Chemistry.
Under the article Evaporation, we have given several ingenious illustrations of the process of "Salt-making." In this place we shall add a notice of two or three more recent inventions.
Messrs. Jump and Coart's patent method consists in concentrating the salt water, by a simple arrangement, previous to its entering the pans. For this purpose, the reservoir of salt water is elevated above the pans, and the pipe which supplies them with the brine first passes through all the furnaces beneath, which brings the liquid quickly to a boiling temperature, in which state it is discharged, by means of a curved pipe, into the pans above, thereby greatly facilitating and abridging the process of concentration. A stop-cock is placed in the supply-pipe, so that, as often as it is desired to replenish the pan, this cock is opened, and the superincumbent pressure of the water in the reservoir forces out the boiling brine from the pipe into the pan, the pipe receiving, in lieu thereof, the cold liquid from the reservoir.
Mr. Johnson, of Droitwich, according to his patent of 1827, employs steam of different degrees of heat to produce the evaporation in pans closed from the atmosphere, so that the vapour arising from the first pan, where the fine salt is produced, is employed in heating the second, where the broad salt is formed: and the vapour arising from the latter is employed in like manner, to produce in the third pan British bay-salt.
A sketch of the steam boiler is represented in the annexed drawing, divided into three portions, a, b, and c; and steam is generated in one or more of these divisions, according to the supply required. When the steam in a is raised to a pressure of twenty-five pounds on the square inch, that in b will be twelve, and that in c five pounds. When only one of the divisions, a, of a steam-boiler about seventeen feet by ten is employed, it will heat pans to the extent of 2,400 square feet up to 164o Fahr.: and when the three divisions, a, b, and c, are used together, an extent of 4,300 square feet will be heated to the same temperature.
The steam is conveyed in a pipe from the boiler to a steam-vessel e, under the fine salt-pan f. This pan is made steam-tight, and the steam arising therein is conveyed by a pipe to a similar vessel under the broad salt-pan k. Over the broad salt-pan k is placed the bay salt-pan m, and the space between them is enclosed by thin boards, or other light material, to confine the vapour arising from k, in order to produce the required heat in the pan m. This pan is made lowest in the middle, as represented in the drawing. so that water condensed on its lowest surface may be collected in one place, where it is received and carried off in a spout, to prevent its return into the brine in the lower pan k.
The patentee considers it of great importance to keep the bottom of the pans clear of salt; and for that purpose employs rakes, which are kept constantly in motion by a steam engine. These rakes deposit the salt in receptacles at the sides of the pans. The rods by which the rakes are moved pass through stuffing boxes in the pans, to prevent the escape of steam.
Mr. Furnival, a spirited manufacturer of salt on an extensive scale, has taken out several patents for improvements in the mechanical arrangements of the process. His last patent, which embodies the leading characteristics of his previous plans, may be explained with reference to the annexed diagram, which represents a vertical section of the apparatus, with two tiers of pans. a a a a are four furnaces, the flues from which are extended under a considerable range or surface of the pans, which are of the shape represented at b c, the deep part c being made to receive the salt thrown over by the ebullition, and also such portion as may be scraped from the surface of b, by means of the instruments shown at d d. The deep chambers being removed from the direct action of the fire, prevent the salt deposited therein from becoming burned; and these receptacles, being at the sides, the salt is conveniently scooped out. The steam raised from the lower range of pans is then employed to heat an upper range e e of less area, supported upon suitable framing, lined interiorly, to confine the steam, with boards.
In order that the water resulting from the condensation of the steam against the bottoms of the upper pans, may not fall back into the lower pans, two inclined planes are formed, which receive the condensed water, and conduct it into a pipe, whence it is carried off by a gutter underneath. In these inclined planes suitable apertures are made for the passage of the ascending steam from the lower to the upper boilers.
The patentee also proposes to heat a third set of pans above the second; for this purpose there is a central aperture to conduct the steam to them; this aperture is covered with a cap.