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Патент USA US2403789

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Patented July 9, 1946
2,403,789
‘UNITED STATES PATENT OFFICE
2,403,7 89 .
METHOD OF PURIFYIN'G CAUSTI
ALKALI' SOLUTION
‘
Claude A. Cummins, Midland, Mich., assignor to
The Dow Chemical Company, Midland, Mich,
a corporation of Michigan
No Drawing. Application June 2, 1943,
Serial‘ No. 489,387
6 Claims.
1
(Cl. 23-484)
2
This invention concerns an improved method
' have reacted to reduce the chlorate to the chlo
of purifying caustic solutions, particularly of re
ride (and the article indicates that at least part
moving chlorates therefrom.
of the corrosion may be due to other causes)
>
t
It is known that sodium and potassium hy
droxide solutions, as obtained by the electrolysis
of corresponding alkali metal chloride-‘contain
the chlorate thus destroyed would amount to
only 9 per cent of that originally present in
the solution, leaving at least 91 per cent of the
ing brines, usually contain a small amount of
chlorate in the solution.
.
the alkali. metal chlorate and that the presence
This showing by Rudensky that an extended
of the chlorate frequently is objectionable. Cer
surface, e. g. a strip, of iron will not cause appre
tain methods for removing the chlorate, e. g.’by 10 ciable reduction of a chlorate in an alkali solu
dehydrating the alkali and heating it to a tem
perature above that at which the chlorate is de
composed or by subjecting the alkali solution to
tion and, in fact, will often be rendered passive
and corrosion-resistant during contact with such
alkaline chlorate-containing solution accords
electrolysis under certain special conditions, are
with the facts observed over a period of many
known, but these previously known methods are 15 years in the manufacture of caustic by the elec
trolytic process. In such usual manufacturing
inconvenient or di?icult to carry out and add ,
appreciably to the cost of the caustic product.
process, the crude chlorate-containing caustic
It has long been known that a chlorate may be
solution, as obtained from the electrolytic cells, is
evaporated at‘ elevated temperatures, stored,
reduced, while in a neutral‘or acidic solution,
by agitating the same with ?nely divided iron, 20 shipped, and otherwise handled in iron or steel
but the literature on the subject indicates that
equipment, but such prolonged contact of the
such reduction would occur sluggishly and in?
solution with iron surfaces results in very little,
completely, or not at all, in an alkaline solution. .
or no,’ reduction in the chlorate content of the
For instance,rin vol. II, pages 316-18 ‘of Mellor’s
“Inorganic andv Theoretical Chemistry,” pub
lished in 1922 by Longmans, Green and Com
pany of London, a number of examples are ‘cited
showing that halates may be reduced more read
solution. Furthermore, it has been-observed that
25 when iron apparatus is ?rst placed in service in
such manufacturing process, corrosion sometimes
occurs to a noticeable extent without, however,
appreciable reduction in the chlorate content of
ily in an acidic solution than in the presence
the solution, but that after the apparatus has
of alkali, and it is reported‘ that an aqueous solu 30 been in service for a short time the rate of cor
tion of an alkali metal chlorate may be reduced
rosion decreases, indicating that the iron has been
with‘ iron ?lings and that the reduction may be
carried out quantitatively by adding sulphuric
rendered resistant to corrosion by the solution.
acid. Again, Rudensky et al., Khimstroi (USSR)
extended surfaces is not effective in reducing
I have now found that although iron having
5 2053-4 (1933), in reporting the results of a 35 the chlorate in an aqueous. caustic alkali solu
study on the corrosive action of aqueous alkali.
tion and, in fact, is often rendered passive and
solutions toward iron and certain of its alloys,
corrosion-resistant during contact with such solu
show that the corrosion of iron occurs in two
tion, ?nely divided iron, e. g. iron powder or ?l
ings, is highly effective in reducing chlorates
ways, i. e. with and without evolution of hydro
gen; that chlorates, if present, may accelerate 40 to chlorides in the presence of alkali. I have
further found that the reduction of the chlorate
the rate of corrosion; but that iron in the presence
of alkali and a chlorate often behaves asif it
with ?nely divided iron occurs more rapidly and
were passive and (in these instances the corro
is more readily carried to completion when the
sion was‘. least. The experimental data in this
chlorate-containing vcaustic solution is of quite
article by Rudensky shows that the corrosion of 45 high concentration, e. g. when it contains from
iron which occurred was in all instances such
30 to 60 per cent by weight of sodium or potas
as to‘ reduce very little, if any, of the chlorate
sium hydroxide, than when it is of lower con
in the alkali solution. For instance, the weight
centration. This is contrary to the result ex
of iron lost by corrosion per part of chlorate
pected in view of the repeated showing in the
in the alkali solution was greatest in- the experi
prior art that the reduction of halates usually
ment in Table 6 ‘of the article on the actionof
occurs more readily in acidic solutions than under
chlorate-containing caustic soda on an iron strip
alkaline conditions.
_
which was immersedin the. solution for 6 hours
However, the treatment of an aqueous caustic
while heating at 90°». C. _.In this.v experiment, if‘
alkali‘solution. with ?nely divided iron usually
all‘ of theiron lost by corrosion. is‘assumed to. 55 results in the introduction of iron-containing
2,403,789
3
4
impurities into the solution, and in discoloration
of the latter. For instance, a colorless chlorate
containing caustic alkali solution which initially
usually contains from 0.03 to 0.12 per cent of iron
(presumably as a ferrate or ferrite) based on the
dry weight of the alkali metal hydroxide.
The dissolved or dispersed iron compounds are
contains less than 0.01 per cent of iron, based on
precipitated by treating such iron-containing
the dry weight of the alkali metal hydroxide,
after being treated with ?nely divided iron to
destroy the chlorate is of greenish color and
causticalkali solutionwith a minor amount of
chlorine or a hypochlorite, e. g. ‘a solution of
chlorine in cold water or in a cold aqueous alkali
usually contains from 0.03 to 0.12 per cent of dis
metal hydroxide solution, preferably while agi
solved iron, based on the dry weight of the alkali
metal hydroxide. The iron is presumed to be 10 tating the mixture. The treatment is carried out
at a temperature below 30° C., and preferably
present as an alkali metal ferrate or ferrite, but
between 15° and 25° (2., since the precipitation of
the identity of the iron-containing impurity has
the iron compounds‘ occurs most favorably at
not de?nitely been established.
‘
‘
these moderate or low temperatures. During the
I have found that the dissolved or disperse
iron-containing impurity may readily be removed 15 treatment, a white precipitate is formed and the
greenish color of the caustic alkali solution dis
by treating the caustic alkali solution at a low or
appears, 1. e. the mixture becomes white or color
moderate temperature with a minor amount of
less. After completing the treatment, the pre
chlorine or a soluble hypochlorite. By this treat-I
cipitate is removed, e. g..>by ?ltration, preferably
ment the iron-containing impurity is. converted
into an insoluble white, substance which may be 20 while at a temperature below 30° C. The caustic
alkali solution obtained as the ?ltrate is substan
removed by ?ltration.‘ The precipitation of the
tially colorless. It retains only a minute amount
iron-containing impurity vis accompanied by a
of iron-containing compounds, e. g. an amount
disappearance of the greenish color and this color
corresponding toless' than 0.01 per cent, and
change serves as a convenient indicator as to the
. amount of chlorinev or hypochlorite required to 25 usually less than 0.0005 per cent, of the dry weight
of the alkali metal'hydroxide.
complete the precipitation.
.
' When the caustic alkali solution subjected to
The invention then comprises two new steps’;
viz., that of'destroying a chlorate, occurring in' an
the treatment is subsequently to be concentrated
aqueous caustic alkali solution by treating-the
I by evaporation in nickel apparatus, it is important
30 that the employment of excess chlorine or hypo
solution with ?nely divided iron and that of sub
sequently removing dissolved or dispersed iron
chlorite over the amount‘ required to precipitate
the iron compounds be avoided, since the pres
compounds from the solution by treating the
latter with a minor amount of chlorine or a hypo
ence of a hypochlorite, or other oxidizing agent,
chlorite.
during such evaporation may result in corrosion
In order to be satisfactory for use in the ?rst
of these steps, i. e. that of destroying a chloram,
the iron must be of a physical form such that the
ratio of the square millimeters total surface of
the metal to its cubic millimeters absolute volume
is 10 or higher, and preferably above 30. Iron 40
drawing a‘ portion of the solution and analyzing
it for dissolved iron. This is a somewhat incon
venient method for controlling the addition to
avoid'the employment of an excess of chlorine or
powder or ?lings of 100 mesh particle size or_?ner
is'preferably used. The iron may be in the form
of a powder of wrought iron, or cast iron, or steel.
The iron or steel should be of a form and com
of the apparatus. The addition of chlorine or a
hypochlorite to the iron-containing caustic alkali
solution may bev controlled by periodically with
a hypochlorite, but when the treatment is carried
out ina closed container so that the mixture can;
- notbeobserve'd ,duringl'the addition, it may be
position susceptible to corrosion in the presence 45 applied. A moreconvenient procedureis t'o'con
trol the addition of ‘the chlorine or hypochlorite
of moisture and oxidizing agents. ' '
'
in accordance with the color change which occurs
In practicing the invention, a caustic soda or a
during the treatment. It so happens that the
caustic potash solution of between 30 and 60, and
preferably between 415 and 55 per cent by weight . initial green color of the caustic alkali solution
concentration and containing a soluble chlorate 60 disappears when precipitation of the iron-con
taining compounds is substantially complete, i. e.
is treated with the iron powder, preferably in
amount exceeding that theoretically required to
reduce the chlorate to the chloride.
when the ironcompounds have been precipitated
as'completely as possible by the present method.
The iron
The disappearance of thegreenish color occurs
powder'ise?ective in causing reduction of the
chlorate regardless of the amount of iron em
55
quite sharply at temperatures below 30° C. and
ployed and, when substantially complete. removal
serves as a convenient indicator in determining
of the chlorate is not desired it may, of course, be
the amount of chlorine or hypochlorite to be used.
By discontinuing ‘the addition when the green
color disappears, the employment of an excess of
chlorine or a hypochlorite may be avoided? In
used in a smaller proportion,
;
v I
'
Reaction between the iron powder and vthe
alkali metal chlorate in the caustic solution to
destroy the chlorate takes place smoothly, but
some-‘Q instances 1 the" treatment is ‘discontinued
quite slowly, on permitting the mixture to stand
at room temperature. It occurs fairly rapidlyat
when theeolor ‘of the caustic alkali solution has
become'faint, but is still discernible. In case an
temperatures from ‘70° C. to the boiling tempera
excess of chlorinev orahypochlorite. is added dur
ture of the caustic solution and is advantageously 65 ing the step of.‘ precipitating the iron-containing
carried out at temperatures within this range,
impurity, .it may be removed by, adding to the
preferably at temperatures above 80° C. By oper
mixture a further amount of the iron-containing
ating ‘at the preferred temperatures just given,
90 per cent or more of the chlorate impurities
ordinarily present in caustic solutions may usually
caustic alkali solution.
~ :
By ‘operating as just described, fairly concen
trated' aqueous caustic solutions which‘ are liquid
at room temperature and which contain anal
kali metal ‘chlorate may readily be freed. of the
?ltered to remove unreacted iron powder. The
latter and be recovered in a substantially colorless
?ltrate is a greenish colored caustic alkali solution
which, although substantially free. of chlorate,‘ 75 formiwhichis ‘as free, and'u'sually is freenof dis-1
be destroyed in less than 3 hours.
‘
7
After completing the reaction, the mixture is
5
soivéa- dr dispersed; non cbnipoundsvthari i_ before
tnetreatment~<1~
I:
'
Q'
pounds of iroii per‘: tonfof c‘austic'“solution.v Each
mixture'was stirred“ continuously,” one at tem
=
Thelfollowing ‘examples illustrate certain ways
in which the principle'of the invention has been
peratures-between ‘90° 3 and ‘100° C. and the. other
' at approximately 1145° 'C., and aliquot portions
of" each mixture were withdrawnirom' time to
applied, but? are not to be construed as limiting
the'vinvention.
4
4-’
-
;
-
time and analyzed to determine‘ the‘ chlorate-con
'~ “
v
tent thereof. 'After completing the treatment
eachsolutio'n was'of greenish color.‘ _-'It waspfound
The purpose of 'thisexample is‘to show thatth‘e
reduction with iron'powder of a chlorate in‘ van
aqueous caustic, 'soda/solutionj'occurs more rap‘
idly and completelyv and ‘requires asmall'er excess
of, the'iron for, rapid. nd complete reaction when
the; caustic solution . ‘1s faijr1y§eoncentrated than
when it-is" dilute.“ ' Threéfvsamplesjof. caustic‘ soda
solution whichfwere-collected abdi?erent, stages
10
90°—l00‘_’ C.’ in the presence of the iron. A It was
also found’that 94 per cent of‘ the chlorate origi
nally present in theother sample‘was‘destroy'ed
"15
proximately 0.156. per cent of the weight?of?the
sodium hydroxidej'present thereinQ-Each solu.
tion wastreated with 100 mesh iron powder in
the . amount ‘indicated; in» the following table and
the mixture was heated at the temperature and
for the time also given. The treated solutions
were of greenish color. After completion of the
heating operation each solution was analyzed to
determine its chlorate content, whereby the pro
by 5- minutes of he'atingof’th'e sample with’jthe
iron‘ at ‘145° C., and that all of the'chlorate'had
been "removed after only 15- minutesof ‘heating
in the ,electrolytic'process'. for the manufacture of
caustic soda and were of di?‘ere'nt concentrations
were employed. Althoughthe solutions were .of
different ‘concentrations, they each contained so;
diuinl chlorate in amount corresponding to ‘ap
that 93" per ‘cent ofthe chlorate inI‘one'ofY-the
samples'was destroyed by {3 hours ‘of heating at
at said ‘temperature. '
,
v20
~.;
.
;.
~
EXAMPLE
4
..
.
.
A 60-ton batch of a- 50 per centconcentrated
aqueous-v caustic soda- solution which contained
0.25s per cent by weight of sodium ‘chlorate ‘was
treated with 1480 pounds of iron powder ofsmaller
than vv100v mesh particle size- and the mixture was
heat'e'di'at temperatures between"90° and"l00° C.
for 2.5 hours. An additional 25 pounds of iron
powder was then added and the heating was con
tinued for 0.5 hour. The mixture was cooled and
iron and insoluble iron compounds suspended
were removed. The treated liquor, which
portion of the chlorate initially present which 30 therein
was of greenish color, contained only 0.0047 per
had been destroyed by treatment with the iron‘
cent by weight of sodium chlorate.
powder was determined. The table gives the con
EXAMPLE 5
centration of each caustic soda solution as per
cent by weight of sodium hydroxide therein and
A greenish colored aqueous caustic soda solu
the proportion of iron powder added in terms of
tion of 50 per cent concentration, and containing
the pounds of iron powder‘per ton dry weight 7 approximately 500 parts per million of dissolved
of the sodium hydroxide. .It also gives the tem
iron (i. e. iron in amount corresponding to 0.10
' perature and time of heating in each experiment
per cent of the dry weight of the alkali), was
and the per cent by weight of the chlorate initial
treated with chlorine until colorless. This treat
ly present in each caustic solution which was re 40 ment, which was carried out with agitation, re—
moved by the treatment with‘ iron powder.
sulted in the formation of a white precipitate‘ of_
an iron-containing compound. After adding the
Table
chlorine, the mixture was agitated for three hours
45 and then ?ltered. The ?ltrate was a colorless
Reaction condi
Run No.
Percent
NaOH
cone.
9
25
37
Lbs. Fe
per ton
of NaOH
twns
Time’
hours
Temp’,
°O.
10
10
4
110
90-110
90-100
104
37
22
Percent
of NaClOa
removed
76
88
99
caustic soda solution containing only 15 .parts per
million of iron.
This application is a continuation-in-part of
my copending application, Serial No. 462,417, ?led
50 October 17, 1942, which is, in turn, a continua
tion-in-part of an earlier application, Serial No.
375,257, ?led January 21, 1941,
Other modes of applying the principle of the
invention may be employed instead of those ex
EXAMPLE 2
A 50 per cent by weight concentrated aqueous
caustic soda solution which contained approxi
mately 0.08 per cent by weight of sodium chlorate
was treated with an approximately 100 mesh iron _
plained, changebeing made as regards the meth..
od herein disclosed, provided the vstep or steps
stated by any oi‘ the following claims or the
equivalent of such stated step or steps be em
powder in amount corresponding to 8 pounds of
ployed.
iron per ton of the solution. The mixture was
stirred at room temperature to maintain the iron
distributed throughout the same and aliquot por
tions of the solution were withdrawn from time‘
to time and analyzed to determine the chlorate
claim as my invention:
1. In a method of treating a chlorate-contain
I therefore particularly point out and distinctly
ing aqueous caustic alkali solution to remove the
chlorate, the steps of contacting the solution with
content thereof. It was found that 6.6 per cent 65 ?nely divided iron and destroying the chlorate
of the chlorate originally present was destroyed
in 2 hours and that 72 per cent of the original
chlorate was destroyed in 24 hours. The treated
solutions were of greenish color.
EXAMPLE 3
>
Each of two samples of a 50 per cent con
centrated caustic soda solution containing ap
proximately 0.08 per cent by weight of sodium
‘by reaction'with the ?nely divided iron, introduc
ing into the resultant caustic alkali solution,
which is enriched in iron compounds and is of
greenish color, while the solution is at a tempera’
ture below 30° C., an agent selected from the class
consisting of chlorine and soluble hypochlorites,
said agent being employed in amount suiiicient to
render the solution substantially colorless, and
removing the iron-containing substance which is
chlorate was treated with powdered iron of 100
meshparticle size in amount corresponding to 8 75 thereby precipitated.
2,408,789 ,
60. Per -_ cent concentration to remove. the ‘ chlorate,
from the ‘clues consisting, 01.. chlorine ‘and soluble
hypochlorites, said agent being adwdetllin amount
su?loient to. rende;
migture substantially col
ins‘ tempexjature‘ thereafter ?ltering, the 1mlxture,
while ,a_.,t a, temperature Vbe1ow_25°v C., chlpijlne in
poundsLwhi-le the .solution is at agtemperature -
tneteby Precipitated from theisolution
' 2. Inamethodoitzjeatlnz achloratewontain:
ing aqueous caustic alkali solution or vn'ornoxtimtvok
the steps ofeontacting, the ‘solution wlth‘?nely'
orlessnand thereafter remevinz?the preqlnltate
5-‘. The method‘ wmcn comprises removinz dis:
divided iron and destroying. the chlorate by heat
ing, the ..solution together with the ?nely volivided f solved iron compounds from an aquewswcaustic
alkali solution by introducing into the solution,
iron at a temperature between 70° C.’ and the boil
introducingintoxthe resultant solution, Whlohls
amount sumcient to‘ render the, solution substan,
of greenish” color and-11s enrlohedin iron com; 19 tially colorless. and removinzlme iron comnounds
censis?fng of chlozine endsoluble- mochloriteo.
Said asént'tlbeina emplovedin approximately the
amountirequireq to render the solution lsubstanf
15
tielly._‘<'=<>iorless.~and removine-theiron-wntaininc , -
4; In‘a method of treating an aqueous caustic
alkali- solution to remove impurities therefrom,
the ste?‘ of precipitating an iron ‘compound- from
the solution by ‘introducing. into the latter; while
at atempei-ature ‘below 3010., an agent selected 7
iron wmnounqimmjan aqueous caustlp soda 50+
lution 0! between. 30mm ,69. percent wncentra:
tion, which :solution has ‘been freed :ol' chloxateg
‘by treatment with ?nely divided‘irbais. 021mm:
ish color . and, contains moretheul?omartsper
substané'e which is thereby’ précipitated,~
3. The method as, described in'claim 2 wherein
the caustic alkali is sodium hydroxide and the
chlorateis sodium chlorate. -
'
6-..The method - whim ‘comprises .removlnz an
below 530°- C,“ an agent selected ‘from, the-c1885.
million. of main. the ‘91311.9! .dlspersedltbn com:
mums, by lntmduqma into. the solution,“ while
20
at a tengpemtuge between 15}? and 25° cq-‘chlonne
‘in amount sumolent-to lender the lnlictute vall!»
stantlallylcoloxflesg anel jthexeaftel- ‘removingvthe
iron ‘compound which a is. precipitated ;by this
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