Salt and the salt industry

by Albert Frederick Calvert

The manufacture proper of salt is commenced by lighting the fires under the kettles and filling them partly with brine as soon as they become warm, and from within 3 to 4 in. of the top when evaporation has well commenced. When salt commences to separate, the pan is withdrawn and the evaporation is allowed to go on undisturbed till a sufficient amount of salt has separated, when the contents of the kettle are well stirred with the ladle and dipped into the basket resting on the so-called basket-sticks laid across the rim of the kettle. While the process of taking the salt from the kettle is going on, the workman opens the faucet for a few minutes to add some fresh brine to the concentrated pickle of the kettle, and washes the salt, so to speak, with this mixture, thereby freeing it as much as possible from the adhering calcium sulphate and the calcium and magnesium chlorides.

ILLUSTRATION OF FOUR SCOTT PATENT DOUBLE EFFECT SALT EVAPORATORS, WITH AUTOMATIC SALT DISCHARGERS, SALT CONVEYERS, AND HYDRO-EXTRACTORS

The panning process, though carried out in the best possible manner, will not completely remove from the kettle all the separated calcium sulphate, but some of it, together with separated salt, will bake on the bottom and sides, forming an incrustation constantly increasing in thickness, though at every refilling of the kettle with fresh brine much of this adhering salt re-dissolves. This incrustation increases much more rapidly in the front kettles than in those nearer to the chimney, since, a front kettle is usually drawn every 4 or 5 hours, while a back kettle often requires from 24 to 36 hours before a sufficient amount of salt has separated. Moreover, a front kettle holds 150 gallons of brine, while those nearest the chimney contain but 100 gallons. Usually, in 5 or 6 days the incrustation becomes so thick that it interferes very materially with the evaporation, causing a great loss of fuel, as gypsum is one of the poorest conductors of heat. The workman therefore draws the salt from the kettle, removes the remaining brine to within a few gallons, and refills the kettle with fresh water. After a continuous boiling of about half an hour, the greater part of the adhering salt has dissolved and the rest of the incrustation can easily be removed.

The time a salt block is in operation is between 10 and 15 days, and the manufactured salt, according to the State laws, remains undisturbed for 14 days for drainage. A salt block usually cools sufficiently in 24 hours for the kettles, grates, arches, etc., to be properly cleaned and made ready for the next run, so that about two runs can be accomplished per month. The quantity of salt produced in 24 hours in a good salt block, with average good coal dust and brine, is from 500 to 600 bushels of 56 lbs. each, and the amount obtainable by the burning of 1 ton of 2,000 lb. of this fuel varies from 45 to 50 bushels.

There are two salt blocks at the Wyoming Valley, at Warsaw, in which the Onondaga kettles are heated by steam instead of direct fire. Here, in place of the brick arches in which the kettles are hung at Syracuse, they are supported by a framework, and each kettle is surrounded by a steam jacket covered with a non-conductor. Moreover, the kettle is made much thinner for the better transmission of the heat. The steam enters the jacket at the upper end of the kettle at one side, and the condensed water escapes by a valve below it, to be returned to the steam boiler. The method of manufacture of the salt does not differ in any particular from the Onondaga method.

The grainer or Michigan process is, like the “kettle method,” a purely American invention, and consists in passing live or exhaust steam through a set of iron pipes immersed in long, shallow wooden or iron vats. These vats rest on a strong wooden frame. They are from 100 to 150 ft. long, usually 12 ft. wide, and from 20 to 24 in. deep; provided with four or six steam pipes having a diameter of 4 to 5 in., and hung on pendants 4 to 6 in. above the bottom of the vats. These pipes are within a few feet of the same length as the grainer, and so arranged that the salt can be conveniently removed towards the outer side of the grainer.

To obtain the best effect in a grainer system, the temperature of the heated brine is kept at or near the boiling-point when no lifting or removal of salt is in progress. To do this an abundance of high-pressure steam must first be supplied to the grainers, and, secondly, the constant supply of brine required for the grainers while evaporation is going on, must enter at a temperature but little lower than that of the brine in the grainer. For this purpose two large tanks, called settlers, are employed, which are usually as long and wide as the grainers, but 6 ft. deep, and provided with four rows of steam pipes about 1 ft. above the floor to heat the cold brine drawn into them from the outside cisterns as required. Although the six rows of steam pipes in the grainer have an entire length of from 550 to 750 ft. (suspended in the brine 4 to 6 in. above the bottom of the grainer and with 8 to 10 in. of brine above them) and a heating surface of from 700 to 1,000 square feet, a great deal of the steam supplied to them is not condensed, and, therefore, passes from the grainer pipes into the settler pipes (sometimes passing through a steam trap to separate the condensed water) to heat the brine of the settlers.

The main difficulty with which the manufacturers of New York State have to contend is the calcium sulphate. In fact, it is this impurity which causes the interruption of the process, and the laborious cleaning out, whether the kettle, the pan, the grainer, or the vacuum pan is used. It not only entails a great loss of heat in consequence of its slow conductivity, but it also causes the overheating of the metal exposed to direct fire, wherever this is employed. Suggestions and experiments have been made to overcome this difficulty, involving the expenditure of great sums of money, but without any practical results as far as mechanical means are concerned.

From the time of the introduction of the open-pan system in Cheshire, until the beginning of the present century it was found impossible, owing to the nature of the furnaces employed in the process, to maintain a sufficiently high and uniform temperature to produce salt which, without grinding, is marketed as finest table salt, or to make more than 2 tons of salt from the consumption of 1 ton of coal. Experiments for the purpose of economizing fuel appeared destined to perpetual failure, and the hand-stoking of the furnaces entailed so many variations of temperature that the production of salt crystals of uniform size was impossible. Then, within the same decade, two processes were invented which, between them, solved the problems that had hitherto eluded all the efforts of the scientist, the engineer, and the practical salt-man.