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Full text of "How to make salt from sea-water"

HOW TO MAKE SALT 




^ATER. 



BY 



Professor JOHN LeCONTE 



PUBLISHED BY THE 



GOVERNOR AND COUNCIL OF SOUTH CAROLINA. 




COLUMBIA, S. C: 
CHARLES P. PELHAM, STATE PRINTER. 

1862 



EJmbersttp of Jgorti) Carolina 




€nbotoeb bp Gtfje dialectic 

ant 

$Jnlantf)ropic Societies; 

O90-L^6h 

Conned. 




RARE BOOK 
COLLECTION 




THE UNIVERSITY OF 

NORTH CAROLINA 

LIBRARY 



2923 

Conf . 



: 



HOW TO MAKE SALT 



FROM SEA- WATER. 



BY 



Pkofessor JOHN" LeCONTE 



PUBLISHED BY THE 



GOVERNOR AND COUNCIL OF SOUTH CAROLINA. 



COLUMBIA, S. C: 
CHAKLES P. PELHAM, STATE PEINTEK. 

1862. 



Digitized by the Internet Archive 

in 2012 with funding from 

University of North Carolina at Chapel Hil 



http://archive.org/details/howtomakesaltfroOOIeco 



MANUFACTURE OF SALT FROM SEA-WATER. 



FIRST METHOD. 



BY THE SPONTANEOUS EVAPORATION OP SEA-WATER IN LARGE SHALLOW 
BASINS, CALLED " SALT GARDENS," OR " SALINES." 

The process is simple, and requires but little apparatus of any kind. The 
first condition for the establishment of a "saline" is a low, broad, level 
ground on the border of the sea, bay, or inlet, which can be protected by 
dykes or dams from the action of the tides, % A shallow artificial pond, or 
reservoir, is made near the sea, and, if possible, below the level of high 
tides : so that it may receive water from the sea at high tides, by means of 
flood-gates. When, however, t it is above the sea-level, the water may 
be elevated by means of hydraulic machines. The bottom of the pond or 
reservoir must be laid out perfectly even, the soil must be clayey, to 
retain the water, and should be beaten hard and smooth. Its outline may 
be irregular, and its depth should be from three to six feet. Of course its 
size must depend on the extent of the ''saline" which is to be established. 
In this reservoir the sea-water deposits its sediments, becomes warmed by 
the sun's rays, and begins to evaporate. 

From this reservoir the partially concentrated brine is led by a canal to a 
series of "rectangular basins," No. 1, from ten to sixteen inches in depth. 
These " basins" are divided into a series of compartments, by means of little 
cross banks, through which the brine flows successively, in a slow current, 
which can be regulated at pleasure. These "basins" should be carefully 
prepared like the "reservoir," so as to retain water. Here, by the action 
of the sun and wind, the water is rapidly evaporated, and deposits a portion 
of its lime, in the form of sulphate. When the concentrated brine marks 
15° to 18° of Baume's hydrometer, it deposits a considerable quantity of 
sulphate of lime; and when it marks 25°, the whole of the lime is deposited. 
From "basins" No. 1 the brine passes, in like manner, to another series 
of similar "rectangular basins," No. 2, larger and more shallow, but more 
carefully constructed. In these "basins," No. 2, the remainder of the sul- 
phate of lime is deposited, and the evaporation is carried to such a point 



that the water becomes a saturated brine by the time it reaches the lower 
side of them. At this period the volume of the concentrated brine is greatly 
diminished, and, marking 25° of Baume's hydrometer, is ready to be trans- 
ferred to the "salting tables." 

From "basins" No. 2 the concentrated brine passes, in like manner, to a 
third and last series of similar "rectangular basins," No. 8, called " salting 
tables." These are smaller and shallower "basins," carefully constructed, 
and divided into compartments communicating with each other, and the 
layer of water should not be more than three inches in depth. The point 
at which the brine has reached a sufficient degree of concentration to be 
introduced into the "salting tables," is sometimes judged of by the water 
becoming red, but the hydrometer is. perhaps, a more reliable indicator — 
it should mark 25° of Baunie. In the "salting tables" the brine soon be- 
gins to deposit salt, in the form of crystalline crusts, which are either col- 
lected with rakes as soon as they form, or allowed to accumulate at the 
bottom, until they form masses of several inches in thickness. The brine 
on the "tables" is renewed daily, or every two days, according to the 
evaporation; whilst- the "resewMir," as well as "basins" No. 1 and No. 2, 
are constantly supplied with fresh brine. The concentration of the brines 
in the "salting tables" must be carefully watched, and their density never 
allowed to exceed 28 i° of Baume's hydrometer, otherwise a deposit of sul- 
phate of magnesia (Epsom salts) would be formed, rendering the salt impure. 
The mother-liquors, as they are called, are run off as soon as they have 
reached the above density : it is usually necessary to draw them off three 
or four times during the season. When the salt has. attained a sufficient 
thickness, it is broken up and piled upon the sides of the "tables" in large 
pyramids, which are covered with clay. In these heaps the salt undergoes 
a process of purification — the moisture from the clay, or from occasional 
rains, penetrates slowly through the mass, removing the more soluble foreign 
matters, and leaving the salt much purer than before. If the salt in these 
heaps is too much soiled to be sent into market, it requires a process of 
refining. For this purpose two methods are employed ; the one consists in 
simply washing the crude salt with a concentrated brine, which removes 
the foreign salts, and a large portion of the earthy impurities. The other, 
more perfect, but more costly process, consists in dissolving the impure salt 
in water, and adding a little lime to precipitate the salts of magnesia, always 
present, after which the filtered brine is slowly evaporated, to obtain the 
large-grained salt used for salting provisions. When the "salines" yield 
the coarsely crystalline " bay-salt," there is no need of these refining 
processes. 

At some of the French " salines" the concentration of the brines is carried 
as far as 32° of Baume's hydrometer, and the salt separated into three. 

m ' 



qualities. Between 25° and 26° of Baunie, the brine deposits 25 per cent, 
of the salt extracted, which is kept apart, on account of its great purity, 
and sold at a higher price than the rest. In passing from a density of 26° 
to 28J° of Baunie, 60 per cent, more of salt of second quality is deposited, 
and from this point to 32° the remaining 15 per cent, is obtained, some- 
what impure and deliquescent, from the magnesian salts which it contains, 
but preferred for the salting of fish, on account of its tendency to keep 
them moist. 

The "salines" are usually cultivated from March to September, so that 
the process is suspended during a large part of the year. But the French 
have recently introduced improvements, by which the works are carried on 
throughout, the whole year, with an increase of the produce by about 50 per 
cent. During the months of autumn the evaporation is still carried on, 
though more slowly, and brine is furnished marking from 10° to 20° of 
Baume. This is stored away in pits or large wells, where the diluting effect 
of the winter and spring rains is but little felt, and at the commencement 
of the warm season this brine is elevated into the evaporating " basins/' so 
that the summer's labors are commenced with concentrated brine, and the 
salt is all harvested in the months of August and September. 

The foregoing method, when Conducted on a large scale, constitutes the 
most economical means of making salt from sea-water. But the nature of 
our climate offers serious obstacles to the manufacture of salt by this pro- 
cess : jirst, on account of the large quantity of rain which falls; and secondly, 
from the great uncertainty of haviug dry weather after mid-summer, which 
is necessary during, the deposition of salt on the '-salting tables." In the 
moist and rainy climate of Venice these difficulties are in a measure over- 
come, by so arrauging the "salting tables" that "in case of heavy rains the 
concentrated brines are rapidly run off into deep reservoirs or large wells, 
from which they are again elevated when the weather becomes fine. 

The marsh lands, provided they have an underlying stratum of clay, 
might be prepared for extensive "salines." In using sea-water of full 
strength, the proportionate area of the several evaporating "basins" may be 
as follows : 

Area of reservoir, J of total evaporating surfaces. 

Basins No. 1, I of total evaporating surfaces. 

Basins No. 2, J of total evaporating surfaces. 

Basins No. 3, i of total evaporating surfaces. 
When the water used is one-half the strength of that in the open-ocean, the 
area of basins No. 3, the "salting tables," need not be more than one-tenth 
of the aggregate evaporating surfaces. Whenever it is practicable, the dif- 
ferent basins should be nearly on the same plane, so that the brine may 
flow from one series to the other, as its level is reduced by evaporation. 



6 

But when it is necessary to construct the basins at different levels, the brines 
may be raised from one series to another by means of wooden drums, from 
eight to sixteen feet in diameter, moved by steam or horse power. 

The " saline" at Baynas, in the South of France, covers an aggregate 
area of nearly 371 acres, and yields per annum 2,125 bushels (56 pounds 
to the bushel) of salt to each acre of evaporating surface. At other 
French "salines" the average yearly produce is about 1,594 bushels to 
each acre of evaporating surface. At the salt works of the Lake of Berre, 
near Marseilles, the brine is scarcely half the strength of sea-water, and yet 
the annual yield is 966 bushels of salt to each acre of evaporating surface. 
The average price of salt at the last-named "salines" is only jive cents per 
bushel, but an outrageous impost upon it, amounting to ten times that sum, 
enhances the price to about fifty-five cents per bushel. At these works the 
"mother-liquors" are used for the production of large quantities of sulphate 
of soda (from which soda is manufactured), as also the salts of potash and 
magnesia. 

SECOND METHOD. 

BY "salines" and artificial evaporation combined. 

In this method, the evaporating basins are constructed as before indicated, 
excepting that basins No. 3 (salting tables) are omitted. In fact, by enlarg- 
ing the area of basins No. 1, we might likewise dispense with basins No. 2, 
thus retaining only the reservoir and the series of basins No. 1 as the 
aggregate evaporating surfaces. The sea-water is first concentrated in 
these basins by spontaneous evaporation, and the brine thus obtained is 
boiled down, and the salt extracted. For this purpose furnaces, evaporating- 
pans, and a store-house must be provided. The pan is a shallow four-sided 
vessel of sheet-iron, from nine to twelve inches deep, with flat bottom, some- 
what deepened towards the middle,, and from ten to twenty feet or more in 
length and breadth, according to the extent of the salt-works. The bottom 
of the pan is supported by small pillars of brick-work, built from the foun- 
dation of the furnace, so as to form, at the same time, flues for the distribu- 
tion of the heat. These flues are calculated to disseminate the flame as 
uniformly as possible over the bottom of the pan, after which it is made to 
pass out behind, to one of the rooms of the store-house, called the drying- 
chamber, which it heats, and then escapes by the chimney. 

The process of boiling consists of two distinct operations : First, the puri- 
fication and evaporation of the brine up to the point of saturation, which is 
called sludging ; and second, the crystallization of the salt, which is called 
soccage. 



Sludging. — The pans are rather more than half filled with clear con- 
centrated brine, which is brought rapidly to a state of violent boiling, the 
evaporated portion being replaced from time to time by fresh brine. The 
surface soon becomes covered with a dirty brown scum, which, with the 
salts precipitated at the same time, collects as a thick mud. As far as pos- 
sible, this must be removed by means of rakes, but some attaches itself to 
the bottom of the pans, forming the pan scale. After twelve or fifteen 
boilings, it often increases to the thickness of an inch, and must then be 
broken up by the chisel and removed. In the meantime the solution of 
salt becomes more concentrated by the constant evaporation and renewal of 
the brine, until at last it begins to crystallize. This process lasts from 20 
to 24 hours. When the scum of crystals begins to form on the surface, the 
fire is lessened, until the temperature of the brine falls to 194° or 167° 
Fahrenheit, when, with slow evaporation, the soccage begins? and lasts sev. 
eral days. 

Soccage. — During this iime the small floating crystals gradually increase 
in size and sink to the bottom. When the pan is kept at 194° the crystals 
have no time for growing, and fine-grained salt falls to the bottom ; at 167° 
they remain floating a longer time, and produce salt of a coarser grain. 
The temperature must not be too much reduced, otherwise the chloride of 
magnesium is a source of obstruction to the process, unless removed by the 
addition of sulphate of soda or slaked lime ; and, better still, of both. To 
remedy the crusting of the surface of the brine, which sometimes occurs, a 
half an ounce of butter, or a small quantity of powdered rosin, may, if 
necessary, be added to the pan. Sometimes the sludging and soccage are 
conducted in separate pans, the brine being transferred at the proper time. 

The purity of the salt diminishes towards the end of the process of soccage; 
for this reason it must be stopped before all the salt is deposited. It need 
not, however, be rejected at the end of each boiling; a second, and some- 
times even a third, charge may be boiled down before the residue — the 
mother-liquor — is removed. 

During the whole process of soccage the salt is raked up from the bottom 
with long cullender-shovels, to the edge of the pan, and placed either in 
wicker-baskets of peeled willow, or heaped upon boards which are thrown 
back for the purpose, when, in both cases, the brine runs back to the pan. 
The moist salt, either in the same baskets or spread out upon hurdles, is 
then placed in the drying-chamber, where it is exposed to a heat of 120° 
or 130° Fahrenheit as long as it loses moisture, when it is packed up for 
sale. 

At Lymington, in Hampshire, England, the process of obtaining salt 
from sea-water is similar to the foregoing, but somewhat simpler. The sea- 
water is concentrated, by spontaneous evaporation, in shallow basins, to about 



s 

one-sixth its bulk, before admitting it into the boilers. The salt is not 
fished out of the pan and drained in baskets, but the water is almost entirely 
evaporated, and the whole mass of salt taken out at once, and removed to 
troughs with holes in the bottom : through these it drains into pits under 
ground, which receive the mother-liquor. Under the troughs, and in a line 
with the holes, are fixed upright stakes (old broom-handles), and on these 
the salt concretes and forms, in the course of ten or twelve days, on each 
stake a mass of sixty or eighty pounds. These lumps are called salt-cats. 
They bear the proportion to the salt made from the same brine of one to 
one hundred. The mother-liquor is reserved for the manufacture of' Epsom 
salt during the winter season. 

Cost of Boiling. — This will, of course, depend on the extent to which 
the brine has been concentrated by the process of natural evaporation in the 
basins. If practicable, the brine should mark about 20° of Baume before 
it is introduced into the boilers. To do this, ordinary sea-water must be 
evaporated to about one-eighth of its bulk. Calculation shows that 10,000 
gallons (of 231 cubic inches each) of this concentrated brine contains 305 i 
bushels (of 56 pounds) of salt. Assuming that only eighty-five per cent, of 
it can be conveniently extracted, this quantity of brine should yield nearly 
260 bushels of salt. With tolerably good arrangements for boiling, each 
pound of wood should evaporate 3} pounds of water. Taking the weight of 
a cord of dry pine to be equal to 3,200 pounds (a low estimate), it will re- 
quire about 7J cords to boil down the 10,000 gallons so as to extract the 260 
bushels of salt ; that is, it will require the burning of one cord of wood to 
every 35} bushels of salt manufactured. 

Supposing the concentration of the sea-water to be carried only as far as 
at Lymington, viz : to one-sixth -of its bulk, or marking 15° of Baum6, 
10,000 gallons will contain about 194} bushels of salt capable of being ex- 
tracted. In this ca*se about 26} bushels of salt ought to be made for each 
cord of wood burnt. 

THIRD METHOD. 

BY SPONTANEOUS EVAPORATION IN GRADUATION-HOUSES AND BOILING 

COMBINED. 

The advantages of this method are : 1. That all the processes are con- 
ducted under shelter, so that the operations are independent of rainy weather. 
2. That the space required for the works is. comparatively small. 3. That 
the arrangements are not costly, and may be put up on any desired scale. 

A large reservoir is constructed at some point conveniently situated for 
receiving the brine from the sea. Erom this the sea-water is pumped up 



into a large wooden cistern on the top of a tower 25 or 30 feet in height, 
from whence it is conducted to the graduation-house, to be concentrated. 

Graduation-House. — This is a long, open frame building, the longest 
side of which is exposed to the prevailing wind of the locality. The floor 
of the building is made of a large clay basin, or of strong wooden planks, 
intended to collect the brine which has been concentrated by evaporation. 
The spaces between the frame of the shed are filled with bundles of twigs 
or fagots, so that the building looks like a vast wall of brush, 20 or 30 feet 
high, and from 100 to 1,000 feet in length. The pyramid of fagots should 
be ten feet thick at its base and six feet at its upper part. Under the ridge 
of the graduation-house, and consequently over the middle of the long wall 
of fagots, is a long wooden trough or spout, perforated with holes at small 
intervals, through which the sea-water flows from the large cistern on the 
tower. In this manner the weak brine is made to fall like a perpetual 
shower over the vastly extended surface of the fagots into the basin below, 
during which it is rapidly concentrated by evaporation. This partially con- 
centrated brine is again pumped up, and undergoes the same operation suc- 
cessively, until it is sufficiently concentrated for boiling. The same brine is 
usually passed four or five times through the fagots before it is fit for boil- 
ing; it is then passed into, a reservoir, and kept for the boiling operations. 
During these successive concentrations the fagots gradually become coated 
with a crust of the salts of lime, but one set will last about eight or ten 
years. 

Of course it is best that this operation should be conducted during the 
warm season ; in fact, when the temperature is below 27° Fahrenheit, a 
portion of salt is lost by the reaction of the sulphate of magnesia. But the 
boiling may be carried on at any season. In general, the brine jit for boiling 
should contain about 20 per cent, of salt, that is, should mark 20° of Baume\ 
The boiling process is the same as that indicated in the second method, and 
its cost will be as there estimated. By adding slaked lime to the reservoir 
of brine fit for boiling, the magnesia might be precipitated, and a purer salt 
obtained. 

FOURTH METHOD. 

BY LIXIVIATING SALINE SAND AND THEN BOILING THE BRINE. 

This method is employed in Lower Normandy, and is used for making 
salt on a small scale. A level shore with a clean sand is selected, and the 
necessary buildings are erected (covered sheds, evaporating-pans, store- 
houses, &c), and an area of three or four acres is chosen, a little below the 
level of spring tides. The surface is carefully levelled, and rolled smooth 
and hard. It is then filled with sand taken from the shore at low water, to 
2 



10 

the depth of several inches, and the sand is also drenched with sea-water at 
high tide. It then lies exposed to the sun and wind, which soon evaporate 
the water, and the surface is covered with a white efflorescence of salt. It 
is then turned over frequently with the shovel, changing the surface several 
times a day, till the whole is perfectly dry. This saline sand is carried to 
the sheds, and the process repeated with fresh sand, till a large quantity is 
collected. i 

To make the brine, the dry saline sand is taken out of the sheds and 
thrown into small round pits, 2£ feet in diameter and 12 inches deep, the 
bottoms of which are lined with hard rammed clay, mixed with straw. The 
sand is then covered with sea-water, or with the weaker ley of former 
operations, and, after standing some hours, is drawn off into reservoirs or 
barrels,' whence the evaporating-pans are supplied. The sand is lixiviated 
a second time, and this ley is reserved for a fresh portion of saline sand. 
The boilers used on the western coast of France are about 3 \ feet square, 
and 4 or 6 inches in depth — a charge is worked off in from 2 to 3 hours. 
Three pans of this size yield together about 50 pounds of salt, but it is of 
rather inferior quality for preserving meat. 

FIFTH METHOD. 

BY BOILING SEA-WATER. 

This method cannot be made economical except where fuel is very cheap. 
In Scotland a very good quality of salt is made in this manner. The boil- 
ing processes are identical with those detailed under the second method, 
excepting that, on account of the weakness of the brine, the first operation 
(sludging) is very much prolonged. 

Cost op Boiling. — Ordinary sea-water contains about 2£ per cent, of its 
weight of pure salt. Hence, it follows that 10,000 gallons contain a little 
more than 38 bushels of salt, of 56 pounds each. Assuming that 85 per 
cent, of this can be extracted by boiling, 10,000 gallons of sea-water will 
yield nearly 32 \ bushels of salt. To boil down this quantity of sea- water will 
require the combustion of 7i cords of wood; that is, not quite 4$ bushels 
of salt to each cord of wood burnt. When the brine is weaker than ordi- 
nary sea-water, of course the yield will be proportionally smaller, and the 
operations more expensive. To obtain a good quality of salt, all the precau- 
tions in boiling indicated under the second method must be observed. 

The foregoing abstract of the several processes of manufacturing salt 
from sea-water has been prepared at the request of a member of the Exec- 
utive Council of the State of South Carolina.