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Патент USA US2138347

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Patented Nov. 29, 1938
Raymond M. Law and Harry ,0. Britton, Syra
cuse, N. Y., assignors tov The Solvay Process
Company, New York, N. Y., a corporation of
New York
N0 Drawing.
Application March '19, 1934, Serial
No. 716,431. Renewed June.12, 1937
9 Claims.
This invention relates to the puri?cation of
caustic solutions and is especially. directed to a
method for reducing the sulfate content of such
In the manufacture of caustics such as sodium
hydroxide, the product frequently is contami
nated with impurities such as sulfates, etc., con
tained in the raw materials or formed in the
process. Thus, although sodium sulfate is rela
tively insoluble in caustic solution, it appears to
exist in the solution in a state of supersatura
tion or in an extremely ?nely divided condition
so that ?ltration does not give a maximum re
moval of the sulfate and settling is exceedingly
715 slow. Hence, for satisfactory puri?cation a long
settling period is required.
In View of the high viscosity and density of
concentrated caustic solution (such as an aque
ous ‘solution containing more than about 40%
NaOH) , which render optimum settling even more
di?icult, the above characteristics are especially
undesirable in connection with such a solution.
It has long been known that the content of
sodium sulfate in aqueous caustic solution may
be reduced by addition of sodium carbonate in
solid form.
Such a process is disclosed, for ex
ample, in an article by Alfons Krause Roczniki
Chemji 5, (1925) pages 395 to 402. Although
such a process assists in the removal of impuri
ties, it does not give in commercial operation as
complete elimination of sodium sulfate as is
sometimes desirable.
In accordance with the present invention, it
has been found that when an alkali-metal car
bonate is ‘formed in situ, that is, its crystalliza
tion or precipitation is caused to take place in
an alkali-metal hydroxide solution, the solid so
formed possesses a physical structure that assists
in the removal of sulfate and other ?nely dis
persed or suspended impurities so that a more
effective elimination of such impurities is ob
tainable than is the case using the solid car
The precipitate not only appears to bring down
readily suspended impurities, but possesses a
highly bene?cial effect upon ?ltration. In carry
ing out the ?ltration of sodium hydroxide solu
tion to remove sodium sulfate and iron com
pounds for example, the impurities are normally
50 present in such a ?nely divided form that they
pass through the ?lter diaphragm. The sodium
carbonate precipitate formed in accordance with
the present invention serves as a ?lter-aid and
prevents the passage of such ?nely dispersed ma
terials through the interstices of the ?lter dia
phragm" and at the same time is of such a char
acter that it does not clog the diaphragm or pre
vent the easyflow of liquid therethrough.
In order'to obtain crystallization of the sodium
carbonate 'in'thesodium hydroxide solution, a T5
water-soluble‘carbonate e. g., an alkali carbonate
may be added to the sodium hydroxide as an
aqueous solution whereupon sodium carbonate
will crystallize out.
A satisfactory solution for treating or mixing'lO
with the sodium hydroxide'solution may be pre-'
pared containing either carbonate or bicarbonate
or both; for example, a solution containing 290
grams of sodium carbonate and 80 grams of
sodium bicarbonate per liter has been found to 15
be very satisfactory.‘ . The solid material formed
in situ upon additioni'o‘f a solution of sodium car
bonate or '‘ sodium bicarbonate to sodium hy
droxide liquors differs from the material result
ing-from the addition of solid sodium carbonate 20
or sodium bicarbonate since the former material
has a physical form that renders it an efficient
?lter-aid Whereas the latter material has a form
that tends to block the pores of the ?lter cloth
rather‘ than to assist the ?ltration.
It is not necessary that a large quantity of the
carbonate‘ be added to obtain satisfactory sepa
ration but the results obtained appear to depend
somewhat upon the ‘ratio of carbonate to sulfate.
Thus, if su?icient sodium carbonate is added to 30
raise the ratio of Na2CO3:Na2SO4 in a concen
trated ‘NaOH solution to about 1.4:1, and espe
cially 1.5:1 or above, NaOH solutions containing
considerably less than .1% NazSO4 may be ob
In‘ its preferred embodiment the process of this
invention ‘involves the addition of su?icient car
bonate. solutioniat ‘an elevated temperature to
form a‘precipitate and subsequent cooling of the
hydroxide solution to reduce the solubility of 40
the carbonate and e?ect further precipitation of
carbonate on the already form-ed carbonate pre
cipitate. In this‘manner a precipitate is formed
that is especially effective as a ?lter-aid and
causes a very e?ective removal of sulfate im- 45
As illustrative of the method that we prefer to
employ for carrying out the puri?cation of caustic
solutions, the following examples are given:—
Example 1.-880 parts of a 48% aqueous solu- 50
tion of sodium hydroxide at about 60° C. and
containing about 0.10% of sodium sulfate was
vigorously agitated with 1 part of sodium car
bonate, added thereto as a 28% aqueous solu
tion, for about twelve hours, during which the 55
solution cooled. to about 25° C. The solution
was then ?ltered through ?ne mesh wire ?lter
cloth to separate the ?occulent precipitate so»
formed. The ?ltrate contained only about 0.04%
of sodium sulfate.
therewith at a temperature of about 60° C. an
aqueous solution of a sodium carbonate in suffi
cient amount to form a carbonate precipitate at
Example 2.—100 parts of an aqueous 48%
sodium hydroxide solution containing about .12%
said temperature, cooling the solution, and ?lter
ing the solution to separate therefrom the result
ant precipitate.
5. The method of separating sodium sulfate
from a concentrated aqueous solution of sodium
of sodium sulfate was agitated at about-60° C. hydroxide, which comprises mixing with the
with about 0.5 part of a sodium carbonate solu
hydroxide solution a sodium carbonate 10
10 tion (containing 290 grams of sodium carbonate solution su?icient to form in situ a carbonate
and 80 grams of sodium bicarbonateper liter of
precipitate the quantity of carbonate solution
solution) and was cooled as in Example 1 to
being sufficient to produce a weight ratio of
between 20° and 30° C. Thecso‘dium hydroxide
sodium carbonate to sodium sulfate in the hy
solution was then ?ltered'to separate solids. The droxide solution greater than about 15:1, and 15
?ltering the solution to separate therefrom the
Example 3.—-Another sample of the same NaOH
solution after being subjected totagitation and
6. The method of separating sodium sulfate
?ltration in a manner similar to;that:.of Example
from a 46% to 52% aqueous solution of sodium
2 but without addition of NaHCOs or NazCOa hydroxide, which comprises mixing with the 20
20 contained 0.09% of Na2SO4.
sodium hydroxide solution ‘at an elevated :tem
Solutions prepared in accordance with this in
.vention contain not only.a:red-uced sulfate con
tent but a reducedicontent of'carbonates, lime,
and iron, since the’flocculentcharacter of the
25 precipitate produced (apparently :due to the
peculiar branching structureof the crystals) ap
pears to be highly-conducive .to the removal of
the ?ne dispersion or. suspension of these mate
rials and makes possible their effective removal by
30 means of ?ltration.v Thus with relatively simple
equipment, an aqueous sodium hydroxide solution
containing 40% or more NaOH may be'prepared
which at temperatures above 25°C. is water- clear
and contains only minimal quantities of impuri
ties. The invention ?nds special application in
the :puri?cation v"of commercial concentrated
caustic solutions that are liquidat. ordinary tem
perature more particularlythosercontaining from
around 46% ,to 52% NaOH.
We claim:
1. The ‘method of v‘separating ‘sodium :sulfate
from an aqueous sodium hydroxidesolution con
taining sodium sulfate, which‘comprises adding
thereto an aqueous solution ofasodium ‘carbon
ate in su?icient amount to form’ in situ acarbon
ate precipitate, and ?ltering the:sodium-hy'drox
ide solution to separate the precipitate, from the
2. The method of purifying, an aqueous sodium
perature aquantity of sodium .carbonate solution
,su?ici-ent to form a carbonate precipitate in the
sodium hydroxide solution and also sufficient to
produce a weight ratio of sodium carbonate to 25
sodium sulfate in the sodium hydroxide solution
greater than about 1.5:1, cooling the solution,
and then ?ltering it to separate therefromthe
resultant precipitate.
7.>The method of separating sodium sulfate '30
from a 46% to 52% aqueous solution of sodium
hydroxide, which comprises mixing with the
sodium hydroxide solution at a temperature of
about 60° C. a quantity of aqueous sodium car
bonate solution su?icient to form a carbonate x35
precipitate in the sodium-hydroxide solution and
also suf?cient to produce a weight ratio of sodium
carbonate‘ to sodium sulfate in the solution greater
than about 1.5:1, cooling the sodium hydroxide
solution to between 20° to 30° C., and then ?lter
ing the solution to separate therefrom the result
ant precipitate.
8. .The method of purifying an aqueous concen
trated sodium hydroxide solution, which com
prises mixing therewith at an elevated tempera
ture'an aqueous alkali carbonate. solution in suffi
cient amount and concentration to form a sodium
carbonate precipitate at said temperature, cooling
the :solution, and?ltering the cooled solution to
‘hydroxide solution, whichcomprises adding there
separate the precipitate therefrom.
to an aqueous solution-20f zsodiumicarbonaterin
suf?cient amount to form in situ-a carbonate pre
centrated sodium hydroxide solution, which com
prises mixing with the solution to be puri?ed at
cipitate, and ?ltering the sodium hydroxide-solu
tion to separate the precipitate.
3. The method of purifying an aqueous .sodium
hydroxide solution, which comprises mixing there
with at an elevated temperaturean aqueous solu
tion of a sodium carbonate‘ in sui?cient-amount to
form a carbonate precipitate at said ‘tempera
GO ture, cooling the solutionpand' ?ltering the solu
tion to separate therefrom the precipitate.
4. The method of purifying an aqueous sodium
hydroxide solution, which :comprises mixing
9. The method of purifying an aqueous con
an elevated temperature, a sodium carbonate
suspension obtained by addition of an aqueous
.alkali carbonate solution to an aqueous concen
trated sodium hydroxide solution at an elevated
temperature, cooling the mixture, and ?ltering
the cooled mixture to separate the precipitate
from the sodium hydroxide solution.
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