CA1093786A - Production of sodium sulfite - Google Patents
Production of sodium sulfiteInfo
- Publication number
- CA1093786A CA1093786A CA270,648A CA270648A CA1093786A CA 1093786 A CA1093786 A CA 1093786A CA 270648 A CA270648 A CA 270648A CA 1093786 A CA1093786 A CA 1093786A
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- Prior art keywords
- solution
- sodium sulfite
- sodium
- crystals
- ppm
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D5/00—Sulfates or sulfites of sodium, potassium or alkali metals in general
- C01D5/14—Preparation of sulfites
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Inorganic Chemistry (AREA)
- Treating Waste Gases (AREA)
- Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
- Paper (AREA)
- Compounds Of Iron (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Anhydrous sodium sulfite is made by process involving introducing substantially anhydrous sodium carbonate concurrently with sulfur dioxide-containing gas into a saturated solution of sodium sulfite maintained at pH of between about 6.5 and about 7.6 at temperature above about 35°C. to form a slurry of anhydrous sodium sulfite crystals, and withdrawing the crystals from the slurry. The process is initiated using a concentrated sodium sulfite solution containing less than about 3 ppm of dissolved iron. iron.
Anhydrous sodium sulfite is made by process involving introducing substantially anhydrous sodium carbonate concurrently with sulfur dioxide-containing gas into a saturated solution of sodium sulfite maintained at pH of between about 6.5 and about 7.6 at temperature above about 35°C. to form a slurry of anhydrous sodium sulfite crystals, and withdrawing the crystals from the slurry. The process is initiated using a concentrated sodium sulfite solution containing less than about 3 ppm of dissolved iron. iron.
Description
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, 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 ~ ' .
, ., ' .. ,:, . .. , ., ' ' : .
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 .
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 ' `.
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 .
.
~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_ ~' 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 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
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, 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 ~ ' .
, ., ' .. ,:, . .. , ., ' ' : .
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 .
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 ' `.
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 .
.
~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_ ~' 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 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.
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.
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~
:
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:
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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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.
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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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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.
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.
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~
:
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:
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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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.
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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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Claims (25)
1. The method of producing anhydrous sodium sulfite comprising;
(a) forming a saturated aqueous solution of sodium sulfite containing less than about 3 ppm of dissolved iron, basis the solutions and adjusting the pH 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 about 706, while maintaining the temperature of said solution above about 35°C. to form a slurry of anhydrous sodium sulfite crystals; and (c) withdrawing anhydrous sodium sulfite crystals from said slurry.
(a) forming a saturated aqueous solution of sodium sulfite containing less than about 3 ppm of dissolved iron, basis the solutions and adjusting the pH 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 about 706, while maintaining the temperature of said solution above about 35°C. to form a slurry of anhydrous sodium sulfite crystals; and (c) withdrawing anhydrous sodium sulfite crystals from said slurry.
2. The method of claim 1 wherein the saturated aqueous solution of sodium sulfite contains less than about 1 ppm of dis-solved iron.
3. The method of claim 1 wherein in step (b) the pH
of the solution is maintained within the range of from about 7.0 to 7.5.
of the solution is maintained within the range of from about 7.0 to 7.5.
4. The method of claim 1 wherein in step (b) the temperature of the solution is maintained between about 50° and 80°C.
5. The method of claim 1 wherein the sodium carbonate is dense soda ash.
6. The method of claim 1 wherein in step (b) the pH
of the solution is maintained within the range of from about 7.25 to about 7.45.
of the solution is maintained within the range of from about 7.25 to about 7.45.
7. The method of claim 1 wherein the saturated aqueous solution of sodium sulfite contains less than about 2 ppm of dissolved iron.
8. The method of claim 7 wherein the sodium carbonate is dense soda ash, as wherein in step (b) the pH of the solution is maintained within the range of from about 7.25 to about 7.45.
9. The method of claim 8 wherein in step (b) the tem-perature of the solution is maintained between about 50° and 80°C.
10. The method of claim 1 wherein the saturated aqueous solution of sodium sulfite contains less than about 1 ppm of dis-solved iron, wherein the sodium carbonate is dense soda ash, and wherein in step (b) the pH of the solution is maintained within the range of from about 7.25 to about 7.45, and the temperature of the solution is maintained between about 50° and about 80°C.
11. The method of claim 1 wherein the saturated solution of sodium sulfite containing less than about 3 ppm of dissolved iron has been obtained by evaporating water from an aqueous solu-tion of sodium sulfite containing in excess of about 3 ppm of dissolved iron, basis the solution, at temperature above about 35°C. to cause precipitation of anhydrous sodium sulfite crystals therefrom and separating the anhydrous sodium sulfite crystals from the solution.
12. The method for making anhydrous sodium sulfite comprising:
(a) forming a saturated aqueous solution of sodium sulfite containing less than about 3 ppm of dissolved iron, basis the solution, and adjusting the pH of said solution to within the range of from about 6.5 to about 7.6;
(b) introducing into said solution substantially anhy-drous sodium carbonate concurrently with mother liquor from which sodium metabisulfite crystals have been obtained by crystallization and a sulfur dioxide-containing gas stream, so proportioned with respect to each other as to maintain the pH of said solution with-in the range of from about 6.5 to about 7.6, while maintaining the temperature of said solution above about 35° C. to form a slurry of anhydrous sodium sulfite crystals; and (c) separating anhydrous sodium sulfite crystals from said slurry.
(a) forming a saturated aqueous solution of sodium sulfite containing less than about 3 ppm of dissolved iron, basis the solution, and adjusting the pH of said solution to within the range of from about 6.5 to about 7.6;
(b) introducing into said solution substantially anhy-drous sodium carbonate concurrently with mother liquor from which sodium metabisulfite crystals have been obtained by crystallization and a sulfur dioxide-containing gas stream, so proportioned with respect to each other as to maintain the pH of said solution with-in the range of from about 6.5 to about 7.6, while maintaining the temperature of said solution above about 35° C. to form a slurry of anhydrous sodium sulfite crystals; and (c) separating anhydrous sodium sulfite crystals from said slurry.
13. The method of claim 12 wherein the saturated aqueous solution of sodium sulfite contains less than about 1 ppm of dissolved iron.
14. The method of claim 12 wherein in step (b) the pH
of the solution is maintained within the range of from about 7.0 to 7.5.
of the solution is maintained within the range of from about 7.0 to 7.5.
15. The method of claim 12 wherein in step (b) the temperature of the solution is maintained between about 50°
and 80°C.
and 80°C.
16. The method o-f claim 12 wherein the sodium carbonate is dense soda ash.
17. The method of claim 12 wherein in step (b) the pH
of the solution is maintained within the range of from about 7.25 to about 7.45.
of the solution is maintained within the range of from about 7.25 to about 7.45.
18. The method of claim 12 wherein the saturated aqueous solution of sodium sulfite contains less than about 2 ppm of dissolved iron.
19. The method of claim 12 wherein the mother liquor from which sodium metabisulfite crystals have been obtained by crystallization provides up to about 25 percent of the total amount of sodium ion introduced into the process as raw material.
20. The method of claim 19 wherein the mother liquor from which sodium metabisulfite crystals have been obtained by crystallization contains less than about 50 ppm of dissolved iron.
21. The method of claim 20 wherein the sodium carbonate is dense soda ash, and wherein in step (b) the pH of the solution is maintained within the range of from about 7.25 to about 7.45.
22. The method of claim 21 wherein in step (b) the temperature of the solution is maintained between about 50° and 80°C.
23. The method of claim 12 wherein the saturated aque-ous solution of sodium sulfite contains less than about 1 ppm of dissolved iron, wherein the sodium carbonate is dense soda ash, wherein in step (b) the pH of the solution is maintained within the range of from about 7.25 to about 7.45, and the temperature of the solution is maintained between about 50° and about 80°C., and wherein the mother liquor from which sodium metabisulfite crystals have been obtained by crystallization provides up to about 70 percent of the total amount of sodium ion introduced into the process as raw material.
24. The method of claim 12 wherein part of the sodium sulfite liquor from which sodium sulfite crystals have been sepa-rated is recycled to the sodium metabisulfite process.
25. The method of claim 23 wherein part of the sodium sulfite liquor from which sodium sulfite crystals have been sepa-rated is recycled to the sodium metabisulfite process.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US653,876 | 1976-01-30 | ||
| US05/653,876 US4003985A (en) | 1976-01-30 | 1976-01-30 | Production of sodium sulfite |
| US711,326 | 1976-08-03 | ||
| US05/711,326 US4112061A (en) | 1976-01-30 | 1976-08-03 | Production of sodium sulfite utilizing mother liquor from the sodium metabisulfite process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1093786A true CA1093786A (en) | 1981-01-20 |
Family
ID=27096610
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA270,648A Expired CA1093786A (en) | 1976-01-30 | 1977-01-28 | Production of sodium sulfite |
Country Status (9)
| Country | Link |
|---|---|
| JP (1) | JPS6054887B2 (en) |
| BR (1) | BR7700565A (en) |
| CA (1) | CA1093786A (en) |
| DE (1) | DE2703480A1 (en) |
| ES (1) | ES455395A1 (en) |
| FR (1) | FR2339573A1 (en) |
| GB (1) | GB1520698A (en) |
| IT (1) | IT1202407B (en) |
| MX (1) | MX143611A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2940697C2 (en) * | 1979-10-08 | 1982-02-25 | Hoechst Ag, 6000 Frankfurt | Process for the production of sodium sulfite |
| US8366793B2 (en) | 2007-11-12 | 2013-02-05 | Solvay Chemicals, Inc. | Method of decarbonation and its use in producing crystalline sodium sulfite or other alkali products |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1096883B (en) * | 1959-09-25 | 1961-01-12 | Basf Ag | Process for the production of solid alkali sulfite |
| DE1186450B (en) * | 1963-04-05 | 1965-02-04 | Basf Ag | Process for the production of pure sodium bisulfite and sulfite |
| DE1252188C2 (en) * | 1963-12-30 | 1973-07-26 | PROCESS FOR MANUFACTURING ANATERIC SODIUM SULPHITE | |
| PL89623B1 (en) * | 1973-12-27 | 1976-11-30 | ||
| JPS5322192A (en) * | 1976-08-13 | 1978-03-01 | Nisso Kinzoku Kk | Manufacturing process for higher purity sodium sulfite anhydride |
-
1977
- 1977-01-20 FR FR7701591A patent/FR2339573A1/en active Granted
- 1977-01-25 IT IT6715177A patent/IT1202407B/en active
- 1977-01-27 ES ES455395A patent/ES455395A1/en not_active Expired
- 1977-01-27 MX MX16784177A patent/MX143611A/en unknown
- 1977-01-28 CA CA270,648A patent/CA1093786A/en not_active Expired
- 1977-01-28 GB GB356677A patent/GB1520698A/en not_active Expired
- 1977-01-28 DE DE19772703480 patent/DE2703480A1/en active Granted
- 1977-01-28 BR BR7700565A patent/BR7700565A/en unknown
- 1977-01-28 JP JP860377A patent/JPS6054887B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE2703480C2 (en) | 1988-03-10 |
| IT1202407B (en) | 1989-02-09 |
| GB1520698A (en) | 1978-08-09 |
| JPS6054887B2 (en) | 1985-12-02 |
| MX143611A (en) | 1981-06-10 |
| ES455395A1 (en) | 1978-03-01 |
| DE2703480A1 (en) | 1977-08-04 |
| JPS5294895A (en) | 1977-08-09 |
| FR2339573B1 (en) | 1982-10-01 |
| FR2339573A1 (en) | 1977-08-26 |
| BR7700565A (en) | 1977-10-04 |
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