LATEST METHODS OF SALT-MAKING
In tracing the development of the salt-making industry in this country,
it will be observed that, until the last quarter of a century, the old
open-pan system defied improvement, and the salt-makers from generation
to generation successfully resisted the endeavours of all who suggested
innovations or hinted that better methods could be introduced in the
manufacture. It is true that experiments were made with the sizes and
arrangement of the pans, that coal replaced wood and straw as fuel,
that the locomotive superseded the wain as a means of transporting salt
from the works to the markets, and that pumps were employed instead
of buckets to raise the brine and deposit it in the cisterns which
supplied the pans; but these several developments produced no change
in the system of manufacture, which consisted of lighting a fire
beneath a pan of brine, driving off the water in the form of vapour,
and collecting the salt crystals that form and sink to the bottom of
the pan. The salt-men were devoted to their primitive, rule-of-thumb
methods, and the most enterprising among them regarded the process as
unimprovable. In the construction of salt-works there was no attempt at
engineering exactness; the size of the pans was regulated roughly by
the dimensions of the plates of which they were made; and the heights
of the brickwork of the furnaces, etc., was usually reckoned by courses
of bricks.
The fireman, the real salt-maker, whose business it was to attend to
the fires and see that the proper degree of heat was maintained to
produce the variety of salt required, did his work almost entirely
by rule-of-thumb. It was only rarely that a thermometer was used. The
technical knowledge acquired by experience enabled a man to see at
a glance whether the pan was working properly, and the quantity and
quality of the salt showed whether his work had been well or ill done.
The late Thomas Ward was a greatly respected authority, one of the
most reliable experts of the Salt Union, and a voluminous writer and
indefatigable lecturer on every aspect of the subject of salt, but he
failed to persuade himself that it was even thinkable that the open-pan
system should ever be abandoned in favour of a more scientific, more
rapid, or more economical process.
Mr. Ward admitted that the process was archaic, but he was at pains to
demonstrate that the trade was justified in desiring it to remain so.
He argued that the price of salt was so low, and the product was so
bulky, that costly and elaborate apparatus was both inappropriate and
ineffective. He compared the life of an ordinary open salt-pan with
that of any of the innumerable patented pans that had been tried, and
found that the ancient article produced salt at less cost than the
patent contraptions, and was far easier to repair. “The chief business
of the salt manufacturer,” Mr. Ward wrote in 1894, “is to utilize to
the best purpose, for the production of salt, the heat obtained from
the fuel. To this end, innumerable patents have been taken out, but
few have been so successful as the simple application of direct heat
to open pans. The method seems a very primitive one, and most visitors
to salt-works think they can improve upon what they consider a rude,
antiquated system. I have had brought before me, and have seen working,
scores of patented plans. In all, or nearly all, the idea was to
economize heat; and if the whole of salt manufacturing consisted in
evaporating the greatest quantity of water with the least quantity of
fuel, doubtless many of the schemes would succeed instead of fail, as
they do now.”
Since the open-pan system of manufacturing salt from brine was in
general and uninterrupted use in this country from the time of
Julius Caesar to within a few years ago, we must study the interim
developments from direct-fire to vacuum pan evaporation in the industry
of the State of New York. The salt springs in New York State were
discovered by Jesuit missionaries about the middle of the seventeenth
century, but the manufacture of salt on a commercial scale was not
begun until 1788, when the industry was established in the vicinity
of Syracuse. Solar salt is still manufactured in large quantities
at Syracuse, where the evaporating surface covers an area of over
12,000,000 square feet, and the season’s output amounts to about
3,500,000 bushels of salt, but between the solar and the vacuum
processes the American salt-men have exploited the Pan and the Kettle
processes of direct-fire evaporation, and the Steam Kettle and the
Grainger processes of steam evaporation; all of which methods are
employed to-day in the State of New York.
In the Pan process, several pans, having a width of 20 to 24 ft., a
length of 100 ft. in two sections, and a depth of 12 in., are placed
under one roof. Adjoining this front row of pans at the back are
arranged a second row of pans, 20 to 24 ft. wide, 30 ft. long, and
12 in. deep, set from 12 to 16 in. higher than the front pans, to
enable the easy transfer of brine by syphon from the back to the front
pan. The grates are 3 to 4 ft. wide, by 5 to 6 ft. long, and the pan
bottoms, which are directly over the fires are protected from a too
intense heat by fire-brick arches, which decrease in width from the
front to the back of the pan, while the air spaces between the arches
increase in width in the same direction. Beyond 20 ft. from the front
of the first section of the pan they cease altogether. To convey the
heat as close to the pan bottom as possible, beyond the last arch,
the flues are usually filled in with earth or plaster, and thus the
distance between the pan and flue bottom is between 3 and 4 ft., or
even less, at the end of the first pan, where a perpendicular wall,
called a bridge wall, reduces the space to about 1½ to 2 ft., through
which the products of combustion pass under the back pan and finally
into a common chimney.
After the pans are properly cleansed they are white-washed with a thin
milk of lime to prevent their rusting before they become thoroughly
heated; the fires are started, and the pans are filled by syphons to a
depth of about 6 in. with brine from the back pans. The former are so
inserted that a constant flow of brine passes from the back pans into
the last section of the front pans, and from these under the partition
into the first section. Into the back pan flows a constant stream from
the outside cistern, until the front pans are sufficiently full, when
the flow is stopped. After a sufficient amount of salt has collected
in the first section of the front pan it is removed to the “drip” for
drainage. This is called drawing or raking the pans. The front pans are
refilled from the back pan in which the brine has become considerably
heated, and thus is prevented a too rapid cooling of the brine in the
front pan, which would seriously interfere with the formation of a
properly grained salt. For the same reason, the partition is placed in
the front pan, since it prevents any cold brine from coming suddenly
into the first section, but is compelled to enter at the bottom of the
pan, where the temperature is at the highest.
For the purpose of aiding the formation of fine grained salt, butter,
specially prepared soft soap, gelatine, or white glue are added, and
when this variety of salt is made the pans are drawn every 45 to 60
minutes. In the manufacture of coarser grained salt, the drawing of the
pans take place at intervals of from two to twelve hours, while the
temperature is reduced from 229° F. to as low as 148° F., according to
the size of the grain.
The Kettle process, which is exclusively employed on the Onondaga Salt
Reservation, consists of from 60 to 100 hemispherical cast-iron kettles
suspended or hung on “lugs” or pins in two parallel flues, called
arches, ending in one chimney, which has a height of 50 to 100 ft.,
according to the length of the flues. In front the arches are provided
with cast-iron, flat-topped grates, 3 ft. in width and 5 ft. long,
perforated with holes ⅜ in. in diameter and 1 in. apart. These are well
adapted for the burning of anthracite dust, which is now exclusively
used for the purpose. The necessary artificial draught is furnished by
a pressure blower. The kettles are from 23 to 26 in. in depth, and from
3 ft. 10 in. to 4 ft. 2 in. in diameter, with a capacity of 100 to 150
gallons. The distance from the bottom of the kettle to the top of the
grate is 3 ft. 6 in., with a solid fire-brick arch in each, extending
somewhat beyond the length of the grate. The distance from the bottom
of the kettle to the crown of this arch is 10 to 12 in. Beyond the
grate the fire-brick arches are constructed in sections, the air spaces
between the arches increasing in size with the advancing distance from
the grates. This construction allows the heated gases to pass through
these spaces without striking the kettle bottoms directly. While the
distance between the bottom of the front kettle and the top of the
grate is 3 ft. 6 in., these flues decrease in depth as they advance
towards the chimney, so that under the last kettle the distance is but
6 or 8 in. The kettles are hung as close as possible with their rims
against each other, and the space between the walls and kettles above
the lugs is properly covered by masonry, etc., for the purpose of
confining all the heat as much as possible within the two arches.
The system of kettles is fed by means of a conduit connected with large
wooden cisterns situated outside the building and sufficiently elevated
to supply the brine contained therein by gravity to the kettles in the
block.
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: 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.
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.”
[Illustration: 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.
4. The fact that coarsely crystallized salt can be produced at the same
time as the finest table salt.
5. That the proportion of the different grades of salt can be varied
at will, as well as maintained constantly, to suit the varying
requirements of the market.
6. The automatic and continuous removal of the salt as fast as it is
precipitated from the brine.
The essential features of the Hodgkinson plant consist of (_a_) a
mechanically-stoked furnace for the production of heat; (_b_) a primary
closed evaporating pan, 30 ft. in diameter; (_c_) two secondary
circular pans, 25 ft. in diameter; (_d_) four open rectangular pans,
60 ft. by 25 ft.; (_e_) a series of folded steam-jacketed pipes for
heating the inflowing brine by the waste steam; and (_f_) a condensing
arrangement to produce a partial vacuum in the closed pans.
The Hodgkinson furnace is not placed under the pan, as in the old
system, but in front of the plant, and the heated gases pass under
the primary pan, where the temperature ranges between 1,800 and
2,000°F. In this primary pan is made a finer and better salt than
can be manufactured by any other system in the world. Moreover, by
means of the mechanically-stoked furnace, and the consequent uniform
high temperature, it is possible, for the first time, to control the
character of the salt produced. Where the temperature varies, as in the
open-pan system, crystals of varying shapes and sizes are produced,
and this mixed salt must be ground to make it suitable for table
purposes. Where steam heat is employed, as in the vacuum process, the
temperature is not high enough to make crystals of the smallest size.
By the Hodgkinson system the primary pan produces a precipitation which
requires no grinding, which flows in a cascade of salt from the pan,
and can be delivered to the consumer without having come into contact
with the hand of man in the whole course of the operation.
The heated gases, having passed under the primary pan, are then divided
and sent under the two secondary pans, and from thence they pass
under the open rectangular pans, the gases being distributed by the
broken columns of brickwork on which the pans stand. The temperature
of the gases passing under the open pans commences at about 600° F.,
and gradually decreases to about 200° F. under the farthest pans.
By the automatic regulation of the temperature, the waste gases
are utilized to produce salts of the various degrees of coarseness
required for the dairy, the stock-yard, and fishery purposes. In the
two secondary closed pans, finely divided table salt is also produced,
but it is possible, by opening the manhole traps in the covers, to
increase the size of the crystal and make dairy salt in these pans.
The coarser crystals and flake salts are made in the open pans in
which the crystallization is at the lowest rate. The grain of the salt
can be altered at will. In order to meet any change in the market
requirements, coarser salt can be produced at a moment’s notice in the
secondary pans. One very marked superiority of the whole system over
all other processes is seen in the fact that a change in the type of
salt produced can be immediately effected, and a constant and uniform
output of any combination of products can be absolutely guaranteed.
The improvements which the Hodgkinson plant has effected in the
open-pan system are: the increased production of from 2 to 7 tons
of salt from the combustion of 1 ton of coal, the production of the
finest table salt without grinding, and of every grade of salt from
the flour-fine table to the coarsest fishery salt, in one and the
same operation, and the saving of time that is required in all other
processes for scraping and cleaning the pans. Its superiority over the
Vacuum system lies in the facts that its initial cost is about £4,000,
as against anything from £26,000 to £100,000; that the majority of the
work being automatic, the expense of specially trained, skilled labour
is dispensed with; that it is operated for 24 hours a day as against
20; requires no grinding process in the manufacture of table salt;
and produces every grade of salt simultaneously. Sir Thomas Holland,
while studying the Hodgkinson process in operation, is said to have
exclaimed: “This is not an improvement, it is a revolution”; and in his
subsequent report upon the process, he has declared that it “has an
enormous advantage over any known process for the production of salt.”