Salt and the salt industry

by Albert Frederick Calvert

In order to understand the advantages secured by the operation of the Vacuum System, which comes to us from the United States, it must be remembered that, under atmospheric pressure, brine boils at a temperature of 226° F., whereas in a vacuum of 28 in. mercury, the boiling temperature is reduced to about 100° F. It will thus be seen that evaporation in vacuo renders it possible to use multiple effect apparatus without causing unduly high pressure in the first vessel, and it has this further advantage, that the low-pressure steam, in passing through the evaporation gives up its latent heat, whereas if the steam went to the condenser direct from the engine, the heat employed in the steam engine would be only the difference between the heat contained in steam at 170 lb. and the steam at 5 lb. pressure. By multiple effect evaporation, a great economy in the amount of steam required is effected. Between the evaporation of brine and that of other liquors, the chief difference to be noted is that in the multiple effect system, each pan or unit is supplied with its brine independently of the others, and graining goes on in the pans, whereas in concentrating other liquors the pans are fed from the first to the second and from the second to the third. The removal of the salt from each pan has, therefore, to be arranged for. The method of working a triple-effect plant may be briefly described as follows—

Each of the three pans having been charged with brine to the proper level, exhaust steam from the engines is admitted to the calandria of the first pan in which the highest temperature is maintained. The brine in this pan becomes quickly heated, and the steam given off enters the calandria of the second pan, where it serves to raise the temperature of the brine. After doing its work in the second stage, the steam is condensed, and thus creates a partial vacuum in the first pan. The atmospheric pressure being thus reduced, violent ebullition of the brine in the first pan results. The same process takes place in the second pan, owing to the calandria of the third pan acting as a condenser of the vapour and producing a vacuum. The vapour given off by the brine in the third pan is condensed by means of a jet condenser. It will, therefore, be seen that the highest vacuum and the lowest temperature exist in the third pan, while the highest temperature and lowest vacuum are found in the first pan. As the salt is precipitated it falls to the bottom of the pans. The bottom of each vacuum pan is connected with the boot of a continuous bucket elevator, which is carried in a cast-iron, water-tight casing to a level sufficiently above that of the brine in the pans to ensure that they shall be brine-sealed. The salt is delivered into waggons and the brine drainage returns to the pans. The further treatment of the salt crystals varies with the purpose for which they are required. For table salt they are subjected to grinding, but for export they are simply allowed to drain.

The general aim of the Vacuum apparatus is to divide the boiling process into two stages, in order to prepare the brine beforehand by purification, and out of the purified brine to produce the purest salt possible—chiefly by boiling under atmospheric pressure—and to acquire another liquor of the highest content in medium salt. Balzberg, in his Die Erdesalz Erzeugung, has to admit that the process results in the most complete purification of the common salt, but in the conclusion of his critical summary of the vacuum plant, he says: “At the same time it must be admitted that a complicated machine, which only gains, at a high cost, advantages that can be achieved by more economical and simpler means is of no use in practical business. The question then arises as to whether it is necessary, for the production of domestic or table salt, to have pure chloride of sodium, and whether it pays to use complicated machinery to attain this end.”

THE HODGKINSON PATENT SALT-MAKING PLANT

While the largest size triple-effect vacuum plants are capable of turning out 1,000 tons of salt per diem, with brine at or near saturation, and produce about 6 tons of salt for the combustion of 1 ton of coal, it is a very expensive process to operate as well as to install. The cost of the plant ranges from £26,000 to £100,000, and a large percentage of skilled labour is required in its manipulation. But, despite the high initial cost, and the fact that it only makes one grade of salt, it is extremely complicated, and has to be stopped for 4 hours in each 24 for the purpose of boiling out and cleaning up the pans, the vacuum plant is a highly efficient piece of mechanism, and for a while it remained the best and most economic system on the market.

But the Vacuum process was not destined to remain long without a rival. In point of fact, the merits of the American invention had scarcely obtained recognition when a new furnace was designed which, when applied to the open-pan system and subjected to practical tests, proved an entire success. The late James Hodgkinson, the patentee, was not a salt-man, but the head of a Manchester firm of engineers and machinery manufacturers, and it was a professional visit to a salt-works which revealed to him the crudity of the brine-boiling operation and gave him the idea of adapting to the salt furnaces a mechanical stoker of his own invention, which was already being operated for other manufacturing purposes. In the development of his idea, and with his mechanical stoker as its foundation, he perfected the Hodgkinson Patent Salt-Making Process, the advantages of which over all other processes for the manufacture of salt from brine have been summarized by Sir Thomas H. Holland, D.Sc., F.R.S., under the following six heads—

1. Complete utilization of the heat derived from the fuel employed.

2. The absolute maintenance uniformly of this heat.

3. The fact that finely-divided first-quality table salt can be produced in the dry form fit for dispatch to the market without grinding or other preparation.