Note: Descriptions are shown in the official language in which they were submitted.
7~3~
lL~a:lL~ the Invention
Sodium sulfi-te is commonly made by reacting ~oda ash
with sulfur dioxide in an aqueous medium~ Sulfur dioxide-contain-
ing gas is passed through an aqueous solution of sodium carbonate
to form a solution of sodium bisulfite, which is then neutralized,
as by addition of further sodium carbonate or of sodium hydroxide
to form the desired sodium sulfite. When sodium carbonate is used
for neutralization, the solution is boiled to expel evolved carbon
dioxide. From the neutralized solution sodium sulfite is obtained
by crystallization. If crystallization is carried out below about
35C., the crystals formed are sodium sulfite heptahydrate
~Na2SO3 7H2O~, which can be transformed into the anhydrous form by
heating above about 35C. At about that temperature the hepta-
hydrate melts incongruentiy, forming anhydrous sodium sulfite and
solution. Alternatively, crystallization of sodium sulfite from
the neutralized solution can be conducted at temperatures above
35C. by evaporating water from the solution~ as by boiling it, in
which case the crystals formed are anhydrous sodium sulfite. The
process involved here, however, is a two step process: formation
of sodium bisulfite in the first step, followed by neutralization
thereof to form sodium sulfite in the second step. Processes
for making sodium sulfite involving the above-described reactions
have~ for example, been described in U.S.P. 1/937,075 to Butler;
U.S.P. 2,080,528 to Bowman et al., U~S.P. 2,719,075 to Allen;
U.S.P. 2,899,273 to Murphy; and U.S.Ps. 3~361,524 and 3,216,793 to
Sporman et al. These patents generally are concerned with methods~
for obtaining anhydrous alkali metal sulfite of relatively high
degree of purity, hence include certain further purification
steps not of consequence;here.
Single-step processes for making anhydrous sodium
sulfite are also known and have been described, for example, in
:
' .
.
. .
l~ B6
U.S.P. 3,305,307 to Sporman et al. and U.S.P. 3,213,412 to Carey
et al. According to the Sporman et al. patent~ solid alkali metal
sulfite salt is obtained by contacting an aqueous solution of a
suitable alkali metal compound -- such as sodium hydroxide, sodium
carbonate, sodium bicarbonate, and the like ~- with substantially
dry sulfur dioxide-containing gas at temperature sufficiently high
that the water introduced with the solution and formed by the
reaction of the alkali metal compound with ~he sulfur dioxide is
immediately vaporized. The patent to Carey describes a process
wherein an alkali metal salt, such as carbonate of soda, is
moistened by contact with a small quantity of water or steam, and
the moistened salt is subjected to the action of sulfur dioxide-
con~aining gas. Processes of that kind, however, result in forma-
tion of sodium sulfite of relatively low degree of purity, as
discussed by Carey et al. in U.S.P. 3,213,412.
It is an object of the present invention to provide a
method for producing anhydrous sodium sulfite by reaction of
!` sodium carbonate with sulfur dioxide in an aqueous medium to
obtain crystalline anhydrous sodium sulfite in one step procedure.
It is a further object of the present invention to ~
provide a method for obtaining substantially concentrated solu-
tions of sod um sulfite of high degree of purity from which sodium
sulfite crystals, both anhydrous as well as heptahydrate, may be
crystallized in substantially pure form~ or which solution may be
used in the process for making sodium metabisulfite from sodium
carbonate and sulfur dioxide-containing gas.
9~ , .
In accordance with the present invention there is pro
vided a method for producing anhydrous sodium sulfite comprising
a) forming a saturated aqueous solution of sodium sulfite contain-
ing less than about 3 ppm of dissolved iron~ basis the solution,
--2--
3t7~
and adjusting the p~ of said solution to within the range of from
about 6.5 to about 7.6;
b~ introducing into said solution substantially anhydrous sodium
carbonate concurrently with a sulfur dioxide~containing gas stream
so proportioned with respect to each other as to maintain the pH
of said solution within the range of from about 6.5 to 7.6 while
maintaining the temperature of said solution above about 35C. to
form a slurry of anhydrous sodium sulfite crystals; and
c) withdrawing anhydrous sodium sufi~e crystals from said slurry.
We have made the surprising discovery that sodium
sulfite can be made in a one-step process by contacting sodium
carbonate and sulfur dioxide in an aqueous medium, provided the
process is initiated in a saturated aqueous solution of sodium
sulfite which contains less than about 3 ppm of dissolved iron,
and provided further it is conducted within a certain critical
pH range. We have found that if the initial sodium sulfite
solution contains more than about 3 ppm of iron, addition
thereto of sodium aarbonate and sulfur dioxide will result in
formation of a supersatruated solution of sodium sulfite. Super-
saturation seems to be carried to relatively high degree, and
seems to persist for relatively extended periods of time, until
it is released by sudden precipitation of a dense shower of sodium
sulfite ceystals of extremely fine particle size, resulting in
formation of an intractible mass from which usable sodium sulfite
crystals cannot be recovered ky practical methods such as fil-
tration. We believe that this is the reason why workers in the
art heretofore had resorted to either the above-described two
step processes fo~r making sodium sulfite~ involving first forma-
: tion of sodium bisul~ite, followed by neutralization thereof to
form ssdium sulfite, or to those processes involving formation ofsodium sulfite in substant.ially dry state n
We have further discovered that once reaction of
sodium carbonate with sulfur dioxide has been initiated in a
saturated aqueous solution of sodium sulEite containing less than
about 3 ppm of dissolved iron, basis the solution, and crystals
of anhydrous sodium sulfite are being formed, then iron may be
introduced into the reaction medium~ as e.g. an impurity in the
sodium carbonate, without adverse effect on further formation of
sodium sulfite crystals. Indeed, we have surprisingly found that
when sodium sulfite i5 crystallized at elevated temperature above
about 35C. and up to the boiling point of the solution from a
saturated solution of sodium sulfite containing dissolved iron as
impurity, then the iron reports almost quantitatively to the sodium
sulfite crystals being precipitated 7 leaving a sodium sulfite
mother liquor practically free of iron, that is containing non-
detectible amounts of iron as determined by the ammonium thiocyanate
test. Thus, we have found that in the method of our invention for
producing anhydrous sodium sulfite it is only critical that the
reaction between the sodium carbonate and the sulfur dioxide be
initiated in an aqueous medium containing less than about 3 ppm
of dissolved iron, basis the solution, but that once crystal
formation is under way, the process is capable of tolerating
input o substantial amounts of iron, which will be included in
the sodium sulfite product as an impurity.
We have further found that in the method of producing
anhydrous sodium sulfite in accordance with our invention the p~
of the aqueous reaction medium must be critically maintained within
the range of from about 6.5 to about 7.6. If the pH is permitted
to go above about 7.6 for substantial periods of time while the
process is in progress, conversion of the sodium carbonate to
~ 30 sodium sulfite lS mhibited or does not occur at all~ If, on the
::
other hand, the pH is permitted to Eall below about 6.5 for sub-
stantial periods of time, sodium bisulfite is formed at rapidly
increasing rate, which appears to inhibit growth of sodium sulfite
crystals, resulting in formation of excessive amounts of small
crystals which cannot readily be separated from the reaction
medium, coupled with excessive foaming of the reaction medium.
Further, the method of producing anhydrous sodium sulfite
in accordance with our invention must be conducted at temperatures
above about 35C. and up to the boiling point of the reaction
medium. If conducted below about 35C., anhydrous sodium sulfite
does not crystallize from the reaction medium but the sodium
sulfite heptahydrate is obtained instead~
Brief Description of the Drawing
For purposes of explaining this invention and presenting
one specific embodiment thereof, reference is made to the accom-
panying drawing which represents a simplified schematic flow
diagram of an embodiment of the present invention showing a con-
tinuous process for making sodium sulfite~
Detailed Description of the Invention, of the Preferred ~mhodiments
and of the Best Mode Presentlv Contem~lated for its Practice
. _ .
With reference to the drawing r equipment employed in the
embodiment of the process of the present invention thereby illu-
strated includes gassing tank 1, agitator 2, sparger 3 connected to
sulfur dioxide-Gontaining gas feed line 4, soda ash feed line 5,
water feed line 6, and vent 7, all associated with gassing tank 1.
Equipment further includes centrifuge 9 for separating liquid and
; solid phases of the slurry from gassing tank 1, circulating line 10
for returning mother liquor to gassing tank 1, and dryer 12.
Desirably, the equipment is constructed of corrosion resistant
material such as stainless steel.
On start-up of operation, there is provided in gassing
tank 1 a saturated solution of sodium sulfite. It is essential
-5-
~ ' .
, ., ' .. ,:, . .. , ., ' ' : .
that the solution contains less than about 3 ppm of dissolved iron,
basis the solution. Sodium sulfite solution of such low iron
content may, for example, be prepared by dissolving iron-free
sodium sulfite in water. Alternatively, such solution may be
prepared by subjecting a concentrated solution of sodium sulfite
containing more than akout 3 ppm of dissolved iron, basis the
solution, to crystallization at temperature above about 35C. as
by boiling the solution to precipitate anhydrous sodium sulfite
crystals therefrom, and separating the sodium sulfite crystals
from the mother liquor. The mo~her liquor from which sodium
sulfite crystals have been thus separated will be essentially
iron-free, that is to say it will contain less than about 3 ppm of
dissolved iron. Such iron-free sodium sulfite solution may also
be prepared by reacting iron-free sodium carbonate with sulfur
dioxide-containing gas in aqueous solution at pH in the
neighborhood of about 7 in substantially iron-free water. In
any ev~nt, the method by which the saturated sodium sulfite
solution containing less than about 3 ppm iron, basis the solu-
tion, is prepared is not critical.
In operation of the embodiment illustrated by the
drawing, the substantially iron-free (containing less than about
3 ppm of dissolved iron, basis the solution) concentrated sodium
sulfite solution in gassing tank 1 is adjusted to pH within the
range of from about 6.5 to about 7 6, as by addition of soda ash
or sodium hydroxide if its pH is below about 6.5, or as by bubbling
sulfur dioxide-containing gas through it, if its pH is above
about 7.6. It is heated to temperature above about 35C. by means
of heating equipment ~not shown). Soda ash is introduced into
gassing tank 1 via soda ash feed line 5 while concurrently sul-fur
dioxide-containing gas is bubbled through the solution by means of
sparger 30 Inert gases, such as nitrogen, which may be introduced
-6-
.
with the sulfur dioxide-containing gas stream, as well as carbon
dioxide formed in the reaction between the sodium carbonate and
sulfur dioxide in accordance with the equation
Na2C03 ~ S2 ~ Na2S03 + C2
are vented from gassing tank 1 through vent 7. Substantially
anhydrous sodium carbonate in the form of l.ight or dense soda ash,
preferably dense soda ash, and sulfur dioxide-containing gas are
fed to gassing tank 1 so proportioned with respect to each other
as to maintain the pH of the solution within gassing tank 1
within the range of from about 6.5 to ahout 7.6 throughout the
operation. This can be simply accomplished by continually or
intermittently monitoring the pH, as by means of a pH meter, and
adjusting either one or both of the soda ash and sulfur dio~ide
feed responsive to changes in the pH. Thus, should the pH tend
to increase and threaten to become more basic than indicated by
pH of 7.6, one could reduce the soda ash feed rate or increase
the sulfur dioxide feed rate, or make both adjustments concurrently.
Conversely, should the pH tend to drift towards the acidic side,
one could increase the soda ash feed rate or decrease the sulfur
dioxide feed rate, or both.
The temperature within the vessel during the gassing
operation must be maintained above about 35C. Ordinarily, the
heat of reaction between the soda ash and the sulfur dioxide
will be sufficient to maintaln the temperature at that level.
However, under certain circumstances it may be necessary or
desirable to apply heat to gassing tank 1 to maintain temperature
above about 35C.
As the soda ash and sulfur dioxide are being fed to the
saturated aqueous solutlon of sodium sulfite in the gassing tank,
anhydrous sodium sulfite will precipitate in crystalline form,
forming a slurry of sodium sulfite crystals in saturated sodium
7-
' `.
3~
sulfite mother liquor. The crystals are held in suspension bymeans of agitator 2. Crystal slurry is withdrawn from gassing
tank 1 via slurry line 8 and fed to centrifuge 9 wherein liquid
and solid phases are separated. The liquid phase (sodium sulEite
mother liquor) is returned to gassing ~ank 1 by means of circulat-
ing pump 10 via mother liquor return line 11. Cyrstals of anhy-
drous sodium sulfite which are separated in centrifuge 9 may, if
desired, be washed using small amounts of water to remove adhering
mother liquor, and the crystals so washed may then be dried in
dryer 12, as by intimately contacting them wi-th heated air to
obtain dry anhydrous sodium sulEite product. Liquor level within
the system is maintained constant by adding water, as required,
via water feed line 6 to gassing tank 1, although water may also
be introduced to other points within the system (not shown), if
desired.
Any commercial form of sodium carbonate (soda ash) is
suitable for use in our process. We have found, however, that
that form of commercial grade sodium carbonate known as dense
soda ash is particularly desirable Eor use in our process, since
dense soda ash readily disperses and dissolves in the reaction
medium and reacts quickly witb the sulfur dio~ide. Commercial
grade light soda ash is also suitable. However its use seems
to require more efficient agitation of the reaction medium, or
else the soda ash tends to agglomerate and to acquire a surface
coating of sodium sulfite, which seemingly retards the rate of
reaction. For these reasons we prefer to use dense soda ash.
It should be understood, however, that water-containing crystal-
line forms of sodium carbonate are also suitable for use in our
proces~, subject onIy to the limitation that the water introduced
with the sodium carbonate may not be of sucb amount as to upset the
water balance in the sys~em. Thus, sodium carbonate monohydrate
-8-
.
.
~3;~
is suitable for use in our process. It is also possible to par-
tially substitute sodium bicarbonate, sodium hydroxide7 or sodium
bisulfite for the sodium carbonate, in solid form or in solution,
and the appended claims are intended to cover partial use of such
materials in our process.
Sulfur dioxide-containing gas suitable for use in our
process may be obtained from any convenient source, such as combus-
tion of sulfur or roasting of sulfide ores. The volume ratio of
sulfur dioxide in the sulfur dioxide-containing gas is not criti-
cal. Sulfur dioxide-containing gas may contain as little as
about 1 percent by volume of sulfur dioxide, or it may consist
of 100 percent sulfur dioxide. In usual commercial plant opera-
tion, sulfur dioxide-containing gas as obtained by combustion of
sulfur or roasting of sulfide ores usually contains about 8 to
about 20 pexcent by volume of sulfur dioxide. If desired~ the
sulfur dioxide-containing gas stream may, prior ~to introduction
into the process, be puriEied, e.g. by removal of dust therefrom
as by scrubbing~ precipitation or filtration, or by washing it so
as to minimize contamination of the process liquor.
The process of our invention can be effectively con-
ducted at p~ within the range of from about 6.5 to about 7.6,
is preferably conducted at pH within the range of from about 7.0
to 7.5 and, more preferably yet, within the range of from about
7.25 to 7.~5.
; Preferably, the reaction between the sodium carbonate
and the sulfur dioxide in accordance with our invention is ini-
tiated in an aqueous medium containing less than about 2 ppm of
dissolved iron, basis the solution and, more preferably yet, in
an aqueous medium containing less than about 1 ppm of dissolved
~ 30 iron.
; _9_
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3~
The temperature of the reaction medi.um wherein sodium
sulfite is Eormed in accordance w.ith the method of our invention
must be maintained above about 35C., or else anhydrous sodium
sulfite is not obtained but instead crystals formed in the liquor
will be those of the sodium sulfite heptahydrate, Na2SO3 7~2
The upper temperature limit is the boiling point of the reaction
medium at prevailing pressure conditionsO The preferred tempera-
ture range if from about 50 to about 80C. The reaction may be
conducted under subatmospheric or superatmospheric pressure, as
desired, although ordinarily atmospheric pressure conditions
would be preferred for the sake of convenience.
The concentration of solid sodium sulfite crystals
within the reaction medium may vary within wide ranges, depending
on the ability of the agitator to maintain the suspension of
sodium sulfite crystals sufficiently homogeneous. Typical solids
concentration may range from about 1 to about 60 percent by volume,
preferably from a~out ~0 to 40 percent by volume.
EXAMPLE I
A stainlees steel reactor equipped with agitator,
temperature control and sparger for introducing sulfur dioxide-
containing gas, having a volume of 10 gallons and a working capa-
city of about 9 gallonsl is charged with about 9 gallons of a
saturated solution of sodium sulfite at temperature of about
60C., containing less than about 1 ppm of dissolved iron, basis
the solution. Under constant agitation commercial grade dense
soda ash is charged to the reactor at the rate of 16 grams per
minute, while concurrently sulfur dioxide-containing gas contain-
ing about 20 percent by volume of sulfur dioxide is sparged through
the liquor within the reactor at a rate sufficient to provide 9.7
grams per minute of sulfur dioxide. Throughout the operation
the temperature of:the liquid reaction medium within the reactor
--10--
is maintained at tempeature between 50 and 75C., and its p~
is controlled between about 7.2 and 7.5 by making minor adjust-
ments on the soda ash and sulfur dioxide feed rates~ Solids oE
anhydrous sodium sulfite crystallize from the reaction medium at
the rate of about 19 grams per minute as the soda ash and the
sulfur dioxide-containing gas are Eed to the reactor. These
crystals are permitted to accumulate within the reaction medium
to solids level of between about 14 to 40 percent by volume.
Periodically, liquid reaction medium is withdrawn from the reactor;
sodium sulfite crystals are separated from the mother liquor by
filtration and the mother liquor is returned to the reactor,
thereby maintaining the crystal volume within the reactor between
about 14 and 40 percent by volume~
During a run of continuous operation, liquor samples
are taken at approximatly two-hour intervals, crystals and mother
li~uor are separated and the concentration within the mother
liquor of sodium sulfite tNa2SO3) and sodium blsulfite (NaHSO3)
are determined. Results are shown in Table I, below.
TABLE I
Sample _E~_Na2SO3 (~ by~wt.)_ 3 ~% b~ wt-?
1 7.00 24 o 41 2. 15
2 7 15 23.30 ~.73
3 7 35 22 . 73 0 . 93
4 7.15 ~2.91 1. 52
7 ~ 6024 . 94 0. 59
7 . 4525 . 04 0.33
7 7.20 23.53 0.73
8 7 . 3523 . 85 0 . 57
9 7.35 23.39 0. 79
The anhydrous sodium sulfite thus obtained contains 9802
percent by weight of Na2SO3; I.5 percent by weight oE Na~SO4; and
5.5 ppm iron. The p~ of a 5 percent solution thereof is 10.1. The
product consists of white crystals; a 20 percent solution of the
solids in water is clear. The product has the following screen
analys l s:
3`~
Mesh (U.S. Screen)
on 30 0.7
3.1
36.7
100 38.0
200 19.1
325 2.4
through 325 ---
The product is of good commercial quality.
As above described, the process of the present invention
is initiated using a saturated sodium sulfite solution containing
less than about 1 ppm dissolved iron, basis the solution. Such
solution mayt as above described, be obtained by evaporating water
from an aqueous solution of sodium sulfite containing in excess
of 1 ppm of dissolved iron, basis solution, as by boiling, to
cause precipitation of anhydrous sodium sulfite crystals therefrom,
and separating the anhydrous sodium sulfite crystals. During the
formation of the sodium sulfite crystals, iron and calcium
impurities unexpectedly become associated with the growing crystals
and are thereby removed from the solution. It is, of course, then
possible to further evaporate the purified liquor to obtain a
further crop of anhydrous sodium sulfite crystals, which are of
~0 purity suitable for use in photographic applications. Purification
of sodium sulfite solutions by this method is most effectively
carried out by evaporating water from such solutions, as by boiling
at temperature in the order of 102 to 104C~ Temperatures less
than boiling are also suitable, but not ordinarily desirable because -
of the lower evaporation rate of the water.
Puriication of sodium sulfite solution by this method
is illustrated by Experiment 1/ set forth below.
EXPERIMENT 1
Eight gallons of~sodium sulfite solution containing
15 ppm dissolved iron and 45 ppm dissolved calcium, basis the
~olution, are heated to boiling under agitation for a period of
-12~
three hours~ During this period~ sodium sulfite crystallizes
from the solution, forming crystals in amounts of about 6 percent
by volume of the combined volume of crystals and liquor. Analysis
of iron and calcium in the crystals and the liquor are shown in
Table II below:
TABLE II
Sample Fe (ppm) Ca (ppm)
Original Solution 15 45
Purified Solution 3 15
Solids Obtained 84
Experiment 2 set forth below -further illustrates puri-
fication o-f sodium sulfite solution by the above-described method.
EXPERIMENT 2
Saturated sodium sulfite solution containing 27 ppm
dissolved iron is heated to boiling, causing precipitation of
sodium sulfi-te crystals as a result of evaporization of water
therefrom. Samples of the solution are taken on periodic basis,
; crystals and mother liquor are separated, and the mother liquor i5
analyzed for iron~and calcium. Results are summarized in Table
III below:
TABLE III
Time, Minutes Fe (ppm) ~ pm~
0 27 0.8
26 1.0
~3 1.4
5~ 3 5 0.6
1 0.6
As these experiments demonstrate, evaporization of
water at elevated temperature to effect crystalllzation of anhy-
;~ ~ drous~sodium sulfite from a concentrated solution thereof contain-
.
ing in excess of~1 ppm of dlssolved iron, basis the solution, is
an effective means for providiny a concentrated sodium sulfite
solution containing less than l~ppm of dissolved iron, basis the
solution, suitable for use as starting liquor for making anhydrous
sodium sulfite in accordance with ~the method of our lnvention~
-13
:
3~
In the manufacture of sodium metabisulfite by reacting
sodium carbonate with sulfur dioxide in accordance with the follow
ing equations:
( 1 ) Na2Co3 ~ SO2 ~ Na2So3 ~ C2
~ 2) Na2SO3 + S2 ~ Na2S2O5
soluble iron, calcium and sulfate impuri~ies inevitably accumulate
in the mother liquor. These impuritles are introduced by the raw
materials and through plant operation. As they accu~ulate, these
impurities tend to contaminate the produc~ and render it unac-
cep~able for photographic and certain other uses. As a result,some of the mother liquor from the process mus~ be purged from the
system in order to maintain contamination of the sodium metabî-
sulfite product within permissible limits. 5ince the purge liquor
; contains considerable sodium and sulfur values, recovery or puri-
fication of the purged liquor by some economical means is desirable.
The literature is replete with suggestions for removing soluble
impurities from solutions containing the same by methods such as
coagulation, absorptionl precipitation, extraction, ion exchange,
electrolysis, or the like. All of these however, have disadvan-
tages such as expense or interference with normal plant operation,
or they may raise disposal and/or pollution problems.
We now have found that the purge liquor from the sodium
metabisulfite process can be used as partial raw material for
making sodium sulfite in accordance with our invention process,
thereby permitting ready recovery of sodium and sulfur values there'
from. Since sodium sulfite and sodium metabisulfite operations are
in many instances carried on concurrently, ready means for
disposal of sodium metabisulfite process purge liquor i5 provided~
The amount of sodium metabisulfite purge liquor which can be used
as partial source of raw material in our process is principally
limited by two considerations: (1) the need for maintaining the
-14~
3'~6
p~ in our reaction medium within the range of from about 6.5 to
about 7.6; and (2) the level of iron contamination in the purge
liquor. Sodium metabisulfite process purge liquor containing
dissolved iron as impurity may be introduced into the reaction
medium only at such rate that the iron impurity substantially
immediately associates wi~h the new and growing sodium sulfite
crystals. If the sodium metabisulfite process purge liquor is
introduced at a rate ~reater than that at which the iron intro-
duced through it becomes associa~ed with the newly forming and
growing sodium sulfite crystals, then the concentration of dis-
solved iron in the mother liquor will build up, t~nding to
cause massive supersa~uration of the liquor with respect to
sodium sulfite and subsequent rapid precipitation of large
quantities of very small sodium sulfite crystals, resulting in
production of an intractible mass, foaming and ultimate termina-
tion of the reaction~
Typical composition of purge liquor from the sodium
metabisulfite process from which sodium metabisulfite crystals
have:been obtained by crystallization may vary within the ranges
2D stated below:
NaHSO3 about 20 to about 40 ~ by weight
Na2S 3 ~about 0.1 to about 3 % by weight
Na2SO4~ about 0.5 to about 15 ~ by weight
Feabout 5 to about 50 ppm
pHabout 4.3 to about 5.2
Ca~about 3 to about 50 ppm
Generally, the sodium metabisulfite proces5 mother
liquor may be ~ed~to the sodium sulfite process in accordance
with our invention:in amount to prov.ide up to abou~t 70 percent
of the to~al amount of sodium .ion introduced as raw material,
ordinarily up to about 30 percent, preferably up to about 15
: -15-
~L~9 3r7;~6~
percent of the total amount of sodium ion introduced as raw
material to the sod.ium sulfite process. For reasons above ex-
plained, higher proportions of such purge liquor can be utilized,
if the purge liquor is relatively low in iron impurities, and,
conversely, increasing amounts of impurities, especially iron
impurities, will tend to limit the amount of purge li~uor that
can be tolerated by the sodium sulfite process.
Sodium sulfite mother liquor from our process from
which anhydrous sod.ium sulfite crystals have been separated and
from which dissolved iron and calcium impurities have been sub-
stantially removed by coprecipi~ation w.ith the sodium sulfite
crystals can be returned to the sodium metabisulfite process.
In substance, the above-described process provides a means for
removing impurities from the mother liquor of the sodium metabi-
: sulfite process.
An embodiment of the process for making anyhdrous sodiumsulfite in accordance with our invention utilizing sodium metabi-
sulite process purge liquor as partial source of raw material is
illustrated in Example II, set forth below:
EXAMPLE II
In the process o~ Example I the sodium carhona-te feed
rate is reduced to 13.3 grams per minute, the sulfur dioxide feed
rate is reduced to 8.0 grams per minute, and concurrently with
the sulfur dioxide and sodium carbonate there is fed purge liquor
~rom a sodium metabisulfite process at a rate o~ 5.8 millillters
per minute. The purge liquor has the fcllowing composition:
-16-
NaHSO3 34.4 percent (by weight)
2 3 1.8 percent
2 4 3.6 percent
Fe 31 ppm
Ca 26 ppm
pH 5.0
Otherwise, the process is conducted as described in Example I.
Table IV below, wherein percent are by weight, shows
typical reaction liquor and product analysis on samples taken
pe~iodically during this run:
~ .
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'
, ~ :
:~
; ~ -17-
:
~ .
~3tjJ~
_ ~ o
o
~:s
o
~ _~
P~ o~
~_
I` CO
~ ~ r~
.,, o
C~ o o
~:5
U~ Z
.,.,
o
U~
,_
oP
~ o~
o t`
u~
Z;
H ¦ _ .
o ~ o
_
~ o o o o
C) ,_
:~ ~
.,~ ~ ~r ~ r
U~ o~
o
~ U~ ~ o o _I
o
.,,
~ æ
~a
P~
_~
OP
00 U~
LO
o
U~
Z
C~
~;~
U~
~ ~ .
~n
. . ~ . .
--18--
~37~
EXA~PLE III
The procedure of Example II is repeated, adding sodium
me~abisulfite process purge liquor at a rate of 23 milliliters
per minute over an 8-hour period, adjusting feed rates of sodium
carbonate and sulfur dioxide to compensate for the increased rate
of addition of the sodium metabisulfite process purge liquor to
maintain pH within the required limits. The sodium metabisulfite
process purge liquor has the following composition.
NaHSO3 26.2 percent (by wei~ht)
Na2SO3~ 0.15 percent
Na~SO4 10.21 percent
Fe 50 ppm
Ca 13 ppm
; Purified sodium sulfite solution is withdrawn from the
reactor at a rate of 33 millimeters per minute. A total of about
18 liters of purified sodium sulfite solution saturated with
respect to sodium sulfite are thus obtained. Analytical results
on periodically taken samples of reaction liquor and sodium sul-
ite crystal solids are summariæed in Table IV, below, wherein
percent are by weight. The purified sodium sulfite solution is
recycled to the sodium metabisulfite process.
3~
--19--
3 ~6
Q~
,
~,
J-
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oU~ ~ ~ o ~ ~o
o _ u~"~
Q)
E4
J-
~J
\o
U~
o ~ ~ ~ ~ r u~
o ~ ~I~ ~ oo,_
~U~ . . . . . .
.,,
~a
o Z
U~
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Since various changes may be made in carrying out the
process of our invention without departing from its scope and
essential characteristics, all matter contained in the above
description shall be interpreted as illustrative only, the ~cope
of our invention being defined by the appended claims.
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