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potassium chloride
sodium carbonate
sodium sulphate
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600 mm
depth of 1 metre
Number GB402324A
EN Title Process of and apparatus for the production of coarse
crystals by evaporation in vacuo and stagewise cooling
<PICT:0402324/III/1> In crystallizing liquids by evaporative cooling
in stages under increasing vacuum in presence of culture crystals, the
solution being admitted in each stage at such a depth below the
surface of the liquid that evaporation first begins in the cooling
vessel itself, the difference between the admission and outlet
temperature of the liquid in each stage is increased, to a maximum of
about double, above the temperature range of metastable saturation,
e[[g]] from between 3 to 6 DEG C. The liquid is admitted through a
pipe 3 into the top compartment 4 of a three stage cooling vessel and
passes thence through a pipe 5 into the middle compartment 6 finally
passing through a pipe 7 into the bottom compartment 8. Stirrers 10,
11, 12 are provided in each compartment and the vapors evolved are led
off through outlets 13, 14, 15, to condensers in which the liquid to
be treated is preheated.
Jn';\' t lt A'N;S
PATENT SPECI FICATION Application Date: April 18, 1933 No 11,316/33
402,324 Complete Accepted: Nov 30, 1933.
Process of and Apparatus for the Production of Coarse Ciystals by
Lvaporation in Vacuo and Stagewise Cooelng.
under the Laws of Germany, of 45, Boci enjieimier Anlage, bran
Kiurt-on-theMain, Germany, and Professor Dr.
W 1 LHELM GENSECKE, a German Citizen, of by, Homburgerstrasse,
Gonzenlieim, near Erankfurt-on-the-\lain, Germany, do hereby declare
the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the
following statement:This invention relates to a process of and apparatus for the
production of coarse crystals by evaporation in vacuo and stagewise
It is known that the gradual cooling of saturated saline solutions in
vessels fitted with cooled stirring mechanism furnishes crystals the
size of which depends on the length of the cooling period According to
published reports, the liquid from which the crystals are being
deposited must be maintained in the range of the metastable zone while
cooling If the solution be cooled more rapidly so that this zone is
surpassed, saline crystals are deposited at once, and a more or less
finely divided salt is obtained If however, the cooling be conducted
so gradually that the supersaturation of the cooled solution remains
willii the metastable range, a coarsegrained salt will be obtained,
provided a sufficient number of crystals be maintained in suspension,
a condition that can be fulfilled by stirring or keeping the entire
cooling apparatus in motion.
Apparatus have also been proposed in which the contents of the cooling
apparatus are passed over a cooling surface with such velocity that
the cooling effect that is, the difference in the temperature of the
solution admitted on to the coolinf surface and on passing away
therefrom-is less than 10 C This slightly supercooled solution is
brought into contact with so-called " culture " crystals, by which
to the degree of
supersaturation is deposited.
Consequentl, the known methods of t Price A cooling consist in
effecting the cooling very slowly or in the case of momentary 55
cooling, in permitting merely a small drop in temperature, less than 1
Apparatus of this kind are not easy to operate, since undesired saline
deposits are very liable to occur should the maxi: 6 mum limit of
decrease in temperature be exceeded In dealing with large volumes of
liquor, for example, in the potash industry, attempts have long been
made to render the heat of the vapours formed 65 in cooling available
for utilisation in the process For this reason, the system of indirect
cooling or cooling by gradual evaporation has been abandoned, and the
production of coarsely crystalline salts 71 sacrificed, and cooling by
spontaneous evaporation in vacuo has been adopted.
In such case, the potassium chloride solution has generally been
cooled from a temperature of about 90 C to 300 C as in four or five
cooling stages so that the average temperature drop in each appara tus
is about 15 C, under which con ditions it is impossible to cultivate
coarse-grained crystals so Moreover, vacuum-cooling plant, consisting
of an elongated vessel divided int(D a plurality of compartments
transversely in relation to its longitudinal axis, are Ott known The
partitions in said 85 vessel are provided with openings at their
deepest parts only, so that the soluffoin from which the dissolved
salt is to be) deposited in crystalline form, can flowthrough the
several compartments in succession The vapour spaces of the 90
compartments are not in relative communication, and the vapours from
each of such spaces are led away separately.
The vacuum increases uniformly in each successive compartment, in the
direction of flow of the solution, so that uniform differences in
vapour pressure exist between the several successive compartments If,
with such an adjustment of the negative pressures in the several
eonmpartments, the cooling in each compartment is greater tlinii the
temnerature range of metastable saturation, no ordered cultivation of
crystals is possible, O even when the pressure difference main402,324
tained between the several compartments is small and the compartments
a-re fitted with stirring mechanism, an arrangement also known in
connection with other vacuuin-cociin, plants TH 1 it were desired to
perform the vacuum cooling with 1 in the zone of metastability, the
number of serially connected apparatus would be so large as to cause
to be regarded, for _practical and economic,
considerations, as unsultable-.
According to the present invention, the cultivation of crystals by
subjecting to vacuum cooling salts -such 1 as sodium sulpha-te ,
sodium carbonate , m esnuxw sulphate or potassium chloride -from
solutions depositing saline constituents, is carried out in such a
manner that in the several cooling stages of the vacuum plant, tbe
dieiarence between the inlet and outlet temperature of tlhe slowly
evaporated solution is ine'eased to ab Du T dotible (at the maximum)
the temperature ravge of metastable saturation, and by maintaining
culture crystals in suspension in the solution by means of stirrers
The solution is adniitl ed, at each evaporation stage in a manner
known per se, at such a depth below the surface of the liquid that the
pressure of the column of liquid above the inlet is approximately
equal to the -pressure drop sustained by the liquid in that stage, so
that the evaporation only com mences in the evaporation vessel itself
and not in the inlet pipe This arrangement ensures sufficiently
gradual evaporation in all cases.
The pphenomenon which, though apparently contrary to theory, is
confirmed by numerous experiments, can be explained in the following
manner:When a saturated solution is admitted into a cooling apparatus
at such a depth below the level of the liauor that no evaporation can
occur in lihe feed pipe, then the freshly admitted liquor will begin
to evaporate at a certain level below the surface If for example, the
absolute pressure above the solution is mm mercury gauge-corresponding
to a vapour saturation temperature of 400 C -and the temperature of
the enterin solution is 450 C, the first buble of vapour will form at
about 230 mn below the surface of the liquor, provided solution with a
specific gravity of 1 be r Present above said bubbhle However, since
the solution cools progresO sively in ascending to the evarioratin T
surface, the column of lianid above the bubbles already forming at the
deepest point is not uniform but is a mixtire -65 of va-pour and
lionid of considerablv lower specific gravity than the solution itself
This layer of vapour and liquid must therefore be made considerably
higher than is hereinbefore specified for the case of liquid alone It
must amount to 600 mm and more, so that, for cooling -o the solution
from 450 to 400 O, a vertical path exceeding 600 mm must be available
to the solution in passing from the inlet to the surface of the liquid
in the cooling apparatus If, under these con 75 -ditions an aideqi-fte
quantity of culture crystals be introduced into the liquor, for
example, by gradual stirring, no finegrained but only coarsely
crystalline salt will be formed, because the path of the 8 w freshly
admitted solution to the surface of the liquid in the cooling
apparatus and therefore, the time taken by the solution in transit, is
so great that the supersaturations occurring as the cooling 85
progresses can become equalised while the solution admitted below is
rising to the surface.
the range of cooling in one apparatus cannot be
indefinitely in 90 creased, because, in sllc event, the liberation of
vapour would becomie so intensive as to carry the admifted fresh
liquid verv rapidly up to the surface, and the deposition of the salt
would no longer 95 occur within the zone of metastabtililv.
By maintaining a temperaturc difference of 3-60 C, uniform eb Uliitio
21 occurs, thereby producin the desired effect In such case, the
eva-porative sir i-0 (G face plays merely a secondary part On the
other hand, it is necessary to nmaintain saline crystals in suspension
all tlh time by gradual movement There is also no need to have an
excessive volunle lt Jo of liquid in the apparatus, a depth of 1 metre
being sufficient, inpractice, with the temperatures
subdividing existing
conical-bottoni evapora H O tors (for example, those 6 to S metres
high, employed for cooling solutions of potassium chloride ) b)v means
of horizontal partitions, so as to create in each apparatus for
example, three cooling 15 siages operating under different pressures.
If a four-stage vacuum plant be modifil-d in this manner, twelve
cooling stages will be obtained, in place of four, so that a
temperature drop of 50 C is present in 120 eachl of the newly created
stag es, this enabling crystals of any convenient size to be
In order more clearly to understand the invention, reference is made
to the 125 a-comne-nvi-rqig drawing which illustrates diagrammatically
and by way of example, one embodiment of apparatus for cooling and the
cultivation of crystals 3 according to the hereindescribed process.
402,324 In said drawing, 1 denotes a known cooling apparatus divided
according to the invention, into three stages by the horizontal
partitions 2 Each of these h three compartments contains a certain
depth of liquid The solution to be cooled is admitted, through the
pipe 3 into the bottom of the compartment 4.
When the temperature range for the metastable saturation of the
solution is for example, 20 C, the solution is cooled by 40 C in this
compartment, and is then passed through an overflow pipe 5 into
compartment 6, the mouth of said pipe being also situated as low as
possible in the compartment The solution is finally passed from
compartment 6 into compartment 8 by way of a similar pipe 7 In the
compartments 6 and 8, the solution is also cooled by 40 C in each case
The blades 10, 11 and 12 are actuated by a stirrer shaft 9 and thus
maintain a certain quantity of crystals in suspension The stirring
mechanism may also be of different design, so that, for example, the
solution is kept in motion in each compartment by means Qf a
horizontal instead of a vertical shaft.
The pipes 3 5 and 7 are preferably located outside the apparatus, for
facility of access The vapours formed in each stage are led away
through the pipes 13, 14 and 15, either jointly to a condenser in
which the liquid required for preparing the solution to be treated is
preheated, or separately into different condensers In the former case,
the vapour from compartments 4 and 6 is throttled to the pressure of
compartment 8 In the second case a more extensive preheating of the
solvent liquid can be obtained inasmuch as the vapours issuing from
the compartments 4 and 6 have a higher temperature than those from
compartment 8.
The solution and the deposited crystals are discharged jointly from
the compartment through an overflow pipe 16, leading for example, into
the next cooling apparatus It is not essential that the apparatus 1
should be divided bv means of horizontal partitions These latter may
also be of conical form, especially when local incrustations are
formed as the result of the deposition of larger quantities of salts
during the cooling process In such case, the removal of the liquor can
be effected from the conical bottom by means of a syphon, instead of
from the surface.
Having now particularly described and ascertained the nature of our
said invention and in what manner the same is to be
1) performed, we declare that what we claim is:1) A process for the
cultivation of 65, crystals in vacuum-cooling plants, in which the
mother liquor is subjected to gradual evaporation in vacuo and cooling
in several stages by admitting the mother liquor in each evaporation
vessel, at such 70 a depth below the surface of the liquid it said
vessel that the evaporation only commences in the evaporation vessel
itself and in which the difference between the admission and outlet
temperature of 75the slowly evaporated mother liquor is increased (to
a maximum of about double) above the temperature range of metastable
saturation, whilst culture crystals are maintained in suspension in 80
the mother liquor for example, by means of stirring mechanism.
2) Process as set forth in claims 1 and 2, in which, during the
cooling in several stages, the temperature difference 85 in each stage
is not less than 30 C and not more than 60 C.
3) A vacuum-cooling apparatus for carrying out the process set forth
in claims 1 and 2, in which the cooling 90 vessels of the several
stages of the vacuaum plant-which are preferably assembled to form a
unit-are provided with vertically or horizontally disposed stirring
mechanism 95
4) Apparatus as set forth in claim 3, comprising vessels of the shaft
type, divided into two or more superimposed evaporating compartments
by means of partitions 100
5) Apparatus as set forth in claim 4. comprising pipes located outside
the evaporators, for transferring the liquid that is to be cooled from
one evaporator compartment to another 105
6) The process for the production of coarse crystals by evaporation in
vacuo and stagewise cooling, substantially as described.
7) The apparatus for the production t 1 oof coarse crystals,
arranged and adapted to operate, substantially as
described with reference to the accompanying drawing. Dated this l Ath
day of Auril, 1933. AL Bl ERT L MOND, 19, Southampton Buildings,
Chancery Lane, London, W C 2, Agent for the Applicants. Redhill:
Printed for His Majesty's Stationery Office, by Love and Malcom-son,
Ltd -1933
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