CA2082473A1 - Sulphate removal process - Google Patents
Sulphate removal processInfo
- Publication number
- CA2082473A1 CA2082473A1 CA 2082473 CA2082473A CA2082473A1 CA 2082473 A1 CA2082473 A1 CA 2082473A1 CA 2082473 CA2082473 CA 2082473 CA 2082473 A CA2082473 A CA 2082473A CA 2082473 A1 CA2082473 A1 CA 2082473A1
- Authority
- CA
- Canada
- Prior art keywords
- liquor
- diluted
- sodium
- conduit
- sodium sulphate
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B11/00—Oxides or oxyacids of halogens; Salts thereof
- C01B11/12—Chloric acid
- C01B11/14—Chlorates
- C01B11/145—Separation; Crystallisation; Purification, After-treatment; Stabilisation by additives
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Paper (AREA)
Abstract
ABSTRACT OF THE INVENTION
This invention relates to an apparatus and process for selectively precipitating crystalline sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride, and containing dissolved sodium sulphate. The process includes diluting the mother liquor with demineralized water thus forming a diluted liquor. The diluted liquor is then chilled to a temperature Tx thereby forming a sodium sulphate crystalline precipitant, where Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts. The sodium sulphate crystalline precipitant is separated, through settling for example, from a resultant clarified liquor.
This invention relates to an apparatus and process for selectively precipitating crystalline sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride, and containing dissolved sodium sulphate. The process includes diluting the mother liquor with demineralized water thus forming a diluted liquor. The diluted liquor is then chilled to a temperature Tx thereby forming a sodium sulphate crystalline precipitant, where Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts. The sodium sulphate crystalline precipitant is separated, through settling for example, from a resultant clarified liquor.
Description
2082~73 FIELD OF THE INVENTION
The invention relates to a process and apparatus to remove sodium sulphate from an aqueous solution of sodium chlorate, sodium chloride, and sodium sulfate.
BACKGROTJND OF THE INVENTION
Sodium chlorate is widely used to produce chlorine dioxide gas for use in pulp bleaching. Sodium chlorate is produced by electrochemical apparatus which electrolize a brine of sodium chloride in an aqueous solution.
Sodium chloride salt is commonly available as rock salt with several impurities including sulfate ions. When ~ i -salt containing sulfate impurities is used in a brine solution to produce sodium chlorate, sulfates may enter the electrolytic system and introduce serious detrimental consequences. As a result, conventional production methods include means to reduce the sulfate concentration to an acceptable level. The present invention relates to an improved method and apparatus for removing the sulfate ion - -in the form of sodium sulfate precipitate.
United States patent No. 4,702,805 to Burkell et al. issued October 27, 1987, describes a method of production of sodium chlorate which includes a secondary crystallization procedure to crystallize a sodium sulfate .. .
sodium chlorate admixture in order to reduce the sulfate concentration to an acceptable level. Other conventional 2082~73 methods include a liquor purge which is less advantageous due to loss of valuable components in the mother liquor.
Conventional processes to produce sodium chlorate include the following steps in general. Water and sodium chloride in the form of rock salt are mixed to form an electrolyte. The electrolyte may be contaminated with various impurities, such as sulfate ion, calcium and ~ ~-magnesium. The sodium chloride brine may be mixed with sodium hydroxide and sodium carbonate to precipitate calcium carbonate and magnesium hydroxide, which are then removed by filtration. The remaining calcium and magnesium ions are reduced in concentration through an ion exchange column.
The purified brine containing sulfate ion contaminants is pH ;
adjusted (mixed with acid) and a portion of the chloride is ' converted to chlorate in an electrolytic cell. The electrolized liquor flows into a primary crystallizer operating under vacuum wherein the chlorate is partially crystallized. The major portion of the chlorate depleted mother liquor of reduced chlorate concentration is recycled in the process, however, a minor portion is removed and forwarded to a secondary crystallizer. In such conventional processes as described in Burkell, the minor portion of mother liquid is further cooled to a temperature wherein dual crystallization of sodium sulfate and sodium chlorate admixture is effected. The spent liquor from the secondary crystallization is also recycled. The crystalline admixture 2~82473 is removed and fresh brine is continually fed to make up the electrolyte.
As a result of the secondary crystallization and conventional processes, the sulfate ion concentration in the electrolyte is maintained at an acceptable level. The admixture of sodium sulfate and sodium chlorate as described in Burkell are blended with the pure chlorate crystals obtained from the primary crystallization process to produce a blended product having an acceptably low concentration of sodium sulfate.
The advantage of removing sodium sulfate in conventional systems is that it prevents sulfate ions from building up in the electrolytic system and increasing to a concentration that adversely effects the electrolytic power consumption and avoids problems due to localized precipitation in the electrolytic cells. A disadvantage of the process described in Burkell, of significance, is the inability to separate sodium sulfate as a contaminant from the sodium chlorate produced. Although, for many applications, a low concentration of sodium sulfate contaminant in the sodium chlorate crystals is acceptable, a more pure product would be desirable to increase environmental acceptability and efficiency of the pulp bleaching process.
The term "mother liquor", as used herein, refers to an aqueous solution of sodium chlorate and sodium ~ ~ 2~82473 chloride. Sodium sulfate is considered to be a contaminant, not an integral part of the mother liquor by definition.
Therefore, it is desirable to produce sodium chlorate free of sodium sulfate contaminants, especially where only minor modification of existing procedures and equipment need be implemented.
SUM~ARY OF THE INVENTION ~;
The invention overcomes the disadvantages of the prior art in a novel manner in the provision a of a process for selectively precipitating crystalline sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride and aontaining dissolved sodium sulphate. The process includes diluting the mother liquor with demineralized water thus ~ -forming a diluted liquor. The diluted liquor is then chilled to a temperature Tx thereby forming a sodium sulphate crystalline precipitant, where Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts. The sodium sulphate crystalline precipitant is separated, through settling for example, from a resultant clarified liquor.
Further according to the invention is provided an apparatus for carrying out the above described process for selectively precipitating sodium sulphate from a mother 2082~73 liquor. The apparatus includes dilution means having an inlet communicating with a mother liquid input conduit, and with a water input conduit, and having an outlet communicating with a diluted liquor conduit. A chilling ;
unit has an inlet communicating with the diluted liquor conduit and an outlet communicating with a chilled liquor conduit. The chilling unit is capable of chilling the diluted liquor to temperature Tx. A settling clarifier tank has an inlet communicating with the chilled diluted liquor conduit, a lower outlet communicating with a crystalline sodium sulphate slurry conduit, and an upper outlet ~
communicating with a clarified liquor conduit. ~ ;
Further aspects of the invention will become ~ ~
apparent upon review of the following detailed description. ;
: '~ .
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be readily understood, a preferred embodiment of the invention will be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a graph showing the relationship between the solubility of sodium chlorate, temperature, and relative concentrations of sodium chlorate to sodium chloride in mother liquor;
Figure 2 is a graph showing the sodium sulphate saturation curve relating temperature and dissolved sodium sulphate in diluted liquor; and 2082~73 Figure 3 is a schematic representation of a process apparatus according to the invention.
DETAILED_DESCRIP~ION OF PREFE~RED EMBODIMENTS
OF THE INVENTION
The invention proposes a method and apparatus to remove sodium sulfate from a mother liquor. The mother ~-liquor is an aqueous solution saturated with sodium chlorate and containing varying concentrations of sodium chloride.
In general the process in a preferred embodiment follows conventional sodium chlorate production methods in taking a slip stream of mother liquor from the output of a primary crystallization of chlorate crystals system. The novel process involves diluting the mother liquor with just enough demineralized water to reduce the saturation temperature of sodium chlorate in the diluted liquor to near the "low temperature boundary of the ice field". The term "ice field" pertains to the area of Figure 1 indicated. This area graphically represents any composition of sodium chlorate, sodium chloride, and water where reducing the temperature of the solution will cause the solution to freeze before any of the chlorate or chloride will precipitate out.
This ice field boundary is at approximately -20~C
in the relevant range as shown in Fig. 1. The dilution reduces the saturation temperature of sodium sulfate in the diluted liquor only to nominally below 0~C. As a result 2~8~73 chilling the diluted liquor to between 0~C and -20~c results in precipitation of sodium sulfate as a decahydrate. The total operable range of temperature is 0~C to -20~C, however -15~C represents the preferred temperature in operation determined by experiment.
Importantly, the process does not result in precipitation of sodium chlorate crystals. Therefore, the invention represents an improvement on conventional methods which rely upon the co-crystallization of sodium sulfate and sodium chlorate crystals. As described above, this co-crystallization is less than optimal in that the preferred method provided by the invention separates sodium sulfate and sodium chlorate formation.
Referring to Figure 1, the composition of aqueous solutions saturated with sodium chlorate and containing various concentrations of sodium chloride, and the influence of temperature of such solutions is illustrated. The upper boundary limitations represent the sodium chloride saturation temperatures in degrees celsius, whereas the lower boundary represents the "ice field" at approximately -20~C.
Where the invention is used in the production of sodium chlorate, the normal operating temperature of the mother liquor is 35 to 40~C and the initial concentrations of sodium chlorate, sodium cloride, and sodium sulfate in the mother liquor are 40 to 42 wt%, 8.25 wt~ and no greater ';, than 2.5 wt% respectively. This mother liquor condition is represented in Figure 1 by a point designated as A.
Dilution of the mother liquor with water results in concentrations graphically illustrated as a water dilution locus in the form of a sloping line from point A to the intersection of the co-ordinate axes of the graph in Figure 1. Intersecting with this water dilution locus are shown three solubility curves, namely at temperatures 0~C, -15~C
and the boundary of the ice field, approximately -20~C.
These intersecting points are respectively designated as B, C and D for reference.
Referring to Figure 2, the sodium sulfate saturation curve is shown. In a process where sodium chlorate is produced from sodium chloride, sodium sulfate is present as a contaminant in relatively low concentrations in the mother liquor, in general no greater than 2.5%. As graphically illustrated in Figure 2 when the mother liquor is chilled below 10~C sodium sulfate begins to precipitate out of the solution until equilibrium in the solution at its saturation point is achieved, as illustrated in Figure 2.
For example, if the mother liquor is chilled to 0~C, as shown in Figure 2, the concentration of sodium sulfate would approximate 0.95 wt%. Chilling the mother liquor to -10~C
would result in a concentration of sodium sulfate at approximately 0.45 wt%. -2082~73 With reference to Figure 3, the process and apparatus proposed by the invention for selectively precipitating sodium sulfate from a mother liquor will be described below. The mother liquor is an aqueous solution of sodium chlorate and sodium chloride and containing dissolved sodium sulfate which is conducted along the mother liquor input conduit 1. Water, preferably demineralized water, flows through a water input conduit 2 and is mixed with the mother liquor with conventional dilution means 3.
The resulting diluted liquor is conveyed via diluted liquor conduit 4. It will be understood that any suitable conventional dilution means may be used, such as an in-line mixer, a mixing tank and impeller, or a simple T-joint as illustrated.
As shown in Figure 3 after the step of diluting the mother liquor with demineralized water to form a diluted liquor, the diluted liquor is pre-chilled through a heat exchanger 5. It will be apparent to those skilled in the art that the heat exchanger is not an essential component, however, in a preferred embodiment the use of a heat exchanger results in cost savings in respect of chilling the diluted liquor. The operation of the heat exchanger 5 will be explained in detail below.
The next step in the process is to chill the diluted liquor to a temperature Tx. A chilling unit 6 has an inlet communicating with the diluted liquor conduit 4 and ' 2~2~73 .~.,.
an outlet communicating with a chilled liquor conduit 7.
The chilling unit 6 has an associated refrigeration unit 8 of conventional design.
The chilling unit 6 is capable of chilling the diluted liquor to a temperature Tx. Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor (referred to hereinafter as Tc) and the saturation temperature of sodium sulfate in the diluted liquor (Ts).
Chilling the diluted liquor to a temperature Tx results in the formation of sodium sulfate crystalline precipitant in the chilled liquor.
In order to separate the sodium sulfate crystalline precipitant from the resultant clarified liquor, the chilled diluted liquor is conveyed to a clarifier tank 9 having an inlet communicating with the chilled diluted liquor conduit 7 in its mid portion. Through conventional settling processes the precipitant is conveyed through a lower outlet communicating with a crystalline sodium sulphate slurry conduit 10 and is pumped to further processing with a positive displacement pump 11.
Through an upper outlet communicating with a clarified liquor conduit 12, the resultant clarified liquor is withdrawn and returned to processing of the mother liquor through a return conduit via the heat exchanger 5. The diluted liquor is pre-chilled in the heat exchanger 5 which 2082~73 .~ .
comprises a counter-flow heat exchanger thermodynamically communicating between the clarified liquor conduit 12 and the diluted liquor conduit 4.
Energy consumption may be reduced in the chilling of the diluted liquor as a result of using the heat exchanger to pre-chill the diluted liquor with the clarified liquor to a temperature greater than Tx. Therefore, the chilling unit 6 need only further reduce the temperature to Tx. Referring to Figures 1 and 2 therefore, an example of the process will be described. Diluting of the mother liquor is graphically illustrated in Figure 1 resulting in the change in concentrations from point A to point C. As a result the saturation temper~ture of sodium chlorate in the diluted liquor is reduced to Tc equalling -15~C.
Referring to Figure 2 the same dilution of the mother liquor reduces the concentration of sodium sulfate to approximately 0.95 wt%. From the sodium sulfate saturation curve it can be seen that this concentration of sodium sulfate results in a ~aturation temperature of sodium ~ :
sulfate in the diluted liquor Ts equalling nominally less than 0~C. :~
Therefore, if the diluted liquor is chilled by the heat exchanger 5 and chilling unit 6, to a temperature Tx, ;
in this example between Ts 0~C and Tc -15~C, sodium sulfate ~ :
crystalline precipitant will be formed in the chilled diluted liquor.
r r 2082~ 73 ':, Since the saturation temperature Ts of sodium chlorate in this example is -15~C, the disadvantage o~
conventional methods involving co-crystallization of sodium sulfate and chlorate crystals together is avoided.
It will be understood that, although various features of the invention have been described with respect to one or another of the embodiments of the invention, the various features and embodiments of the invention may be combined or used in conjunction with other features and embodiments of the invention as described and illustrated herein.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to these particular embodiments. Rather, the invention ' .
includes all embodiments which are functional or mechanical ..
equivalents of the specific embodiments and features that have been described and illustrated herein.
j ., ~, ..
: ~ ... ...
The invention relates to a process and apparatus to remove sodium sulphate from an aqueous solution of sodium chlorate, sodium chloride, and sodium sulfate.
BACKGROTJND OF THE INVENTION
Sodium chlorate is widely used to produce chlorine dioxide gas for use in pulp bleaching. Sodium chlorate is produced by electrochemical apparatus which electrolize a brine of sodium chloride in an aqueous solution.
Sodium chloride salt is commonly available as rock salt with several impurities including sulfate ions. When ~ i -salt containing sulfate impurities is used in a brine solution to produce sodium chlorate, sulfates may enter the electrolytic system and introduce serious detrimental consequences. As a result, conventional production methods include means to reduce the sulfate concentration to an acceptable level. The present invention relates to an improved method and apparatus for removing the sulfate ion - -in the form of sodium sulfate precipitate.
United States patent No. 4,702,805 to Burkell et al. issued October 27, 1987, describes a method of production of sodium chlorate which includes a secondary crystallization procedure to crystallize a sodium sulfate .. .
sodium chlorate admixture in order to reduce the sulfate concentration to an acceptable level. Other conventional 2082~73 methods include a liquor purge which is less advantageous due to loss of valuable components in the mother liquor.
Conventional processes to produce sodium chlorate include the following steps in general. Water and sodium chloride in the form of rock salt are mixed to form an electrolyte. The electrolyte may be contaminated with various impurities, such as sulfate ion, calcium and ~ ~-magnesium. The sodium chloride brine may be mixed with sodium hydroxide and sodium carbonate to precipitate calcium carbonate and magnesium hydroxide, which are then removed by filtration. The remaining calcium and magnesium ions are reduced in concentration through an ion exchange column.
The purified brine containing sulfate ion contaminants is pH ;
adjusted (mixed with acid) and a portion of the chloride is ' converted to chlorate in an electrolytic cell. The electrolized liquor flows into a primary crystallizer operating under vacuum wherein the chlorate is partially crystallized. The major portion of the chlorate depleted mother liquor of reduced chlorate concentration is recycled in the process, however, a minor portion is removed and forwarded to a secondary crystallizer. In such conventional processes as described in Burkell, the minor portion of mother liquid is further cooled to a temperature wherein dual crystallization of sodium sulfate and sodium chlorate admixture is effected. The spent liquor from the secondary crystallization is also recycled. The crystalline admixture 2~82473 is removed and fresh brine is continually fed to make up the electrolyte.
As a result of the secondary crystallization and conventional processes, the sulfate ion concentration in the electrolyte is maintained at an acceptable level. The admixture of sodium sulfate and sodium chlorate as described in Burkell are blended with the pure chlorate crystals obtained from the primary crystallization process to produce a blended product having an acceptably low concentration of sodium sulfate.
The advantage of removing sodium sulfate in conventional systems is that it prevents sulfate ions from building up in the electrolytic system and increasing to a concentration that adversely effects the electrolytic power consumption and avoids problems due to localized precipitation in the electrolytic cells. A disadvantage of the process described in Burkell, of significance, is the inability to separate sodium sulfate as a contaminant from the sodium chlorate produced. Although, for many applications, a low concentration of sodium sulfate contaminant in the sodium chlorate crystals is acceptable, a more pure product would be desirable to increase environmental acceptability and efficiency of the pulp bleaching process.
The term "mother liquor", as used herein, refers to an aqueous solution of sodium chlorate and sodium ~ ~ 2~82473 chloride. Sodium sulfate is considered to be a contaminant, not an integral part of the mother liquor by definition.
Therefore, it is desirable to produce sodium chlorate free of sodium sulfate contaminants, especially where only minor modification of existing procedures and equipment need be implemented.
SUM~ARY OF THE INVENTION ~;
The invention overcomes the disadvantages of the prior art in a novel manner in the provision a of a process for selectively precipitating crystalline sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride and aontaining dissolved sodium sulphate. The process includes diluting the mother liquor with demineralized water thus ~ -forming a diluted liquor. The diluted liquor is then chilled to a temperature Tx thereby forming a sodium sulphate crystalline precipitant, where Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts. The sodium sulphate crystalline precipitant is separated, through settling for example, from a resultant clarified liquor.
Further according to the invention is provided an apparatus for carrying out the above described process for selectively precipitating sodium sulphate from a mother 2082~73 liquor. The apparatus includes dilution means having an inlet communicating with a mother liquid input conduit, and with a water input conduit, and having an outlet communicating with a diluted liquor conduit. A chilling ;
unit has an inlet communicating with the diluted liquor conduit and an outlet communicating with a chilled liquor conduit. The chilling unit is capable of chilling the diluted liquor to temperature Tx. A settling clarifier tank has an inlet communicating with the chilled diluted liquor conduit, a lower outlet communicating with a crystalline sodium sulphate slurry conduit, and an upper outlet ~
communicating with a clarified liquor conduit. ~ ;
Further aspects of the invention will become ~ ~
apparent upon review of the following detailed description. ;
: '~ .
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be readily understood, a preferred embodiment of the invention will be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a graph showing the relationship between the solubility of sodium chlorate, temperature, and relative concentrations of sodium chlorate to sodium chloride in mother liquor;
Figure 2 is a graph showing the sodium sulphate saturation curve relating temperature and dissolved sodium sulphate in diluted liquor; and 2082~73 Figure 3 is a schematic representation of a process apparatus according to the invention.
DETAILED_DESCRIP~ION OF PREFE~RED EMBODIMENTS
OF THE INVENTION
The invention proposes a method and apparatus to remove sodium sulfate from a mother liquor. The mother ~-liquor is an aqueous solution saturated with sodium chlorate and containing varying concentrations of sodium chloride.
In general the process in a preferred embodiment follows conventional sodium chlorate production methods in taking a slip stream of mother liquor from the output of a primary crystallization of chlorate crystals system. The novel process involves diluting the mother liquor with just enough demineralized water to reduce the saturation temperature of sodium chlorate in the diluted liquor to near the "low temperature boundary of the ice field". The term "ice field" pertains to the area of Figure 1 indicated. This area graphically represents any composition of sodium chlorate, sodium chloride, and water where reducing the temperature of the solution will cause the solution to freeze before any of the chlorate or chloride will precipitate out.
This ice field boundary is at approximately -20~C
in the relevant range as shown in Fig. 1. The dilution reduces the saturation temperature of sodium sulfate in the diluted liquor only to nominally below 0~C. As a result 2~8~73 chilling the diluted liquor to between 0~C and -20~c results in precipitation of sodium sulfate as a decahydrate. The total operable range of temperature is 0~C to -20~C, however -15~C represents the preferred temperature in operation determined by experiment.
Importantly, the process does not result in precipitation of sodium chlorate crystals. Therefore, the invention represents an improvement on conventional methods which rely upon the co-crystallization of sodium sulfate and sodium chlorate crystals. As described above, this co-crystallization is less than optimal in that the preferred method provided by the invention separates sodium sulfate and sodium chlorate formation.
Referring to Figure 1, the composition of aqueous solutions saturated with sodium chlorate and containing various concentrations of sodium chloride, and the influence of temperature of such solutions is illustrated. The upper boundary limitations represent the sodium chloride saturation temperatures in degrees celsius, whereas the lower boundary represents the "ice field" at approximately -20~C.
Where the invention is used in the production of sodium chlorate, the normal operating temperature of the mother liquor is 35 to 40~C and the initial concentrations of sodium chlorate, sodium cloride, and sodium sulfate in the mother liquor are 40 to 42 wt%, 8.25 wt~ and no greater ';, than 2.5 wt% respectively. This mother liquor condition is represented in Figure 1 by a point designated as A.
Dilution of the mother liquor with water results in concentrations graphically illustrated as a water dilution locus in the form of a sloping line from point A to the intersection of the co-ordinate axes of the graph in Figure 1. Intersecting with this water dilution locus are shown three solubility curves, namely at temperatures 0~C, -15~C
and the boundary of the ice field, approximately -20~C.
These intersecting points are respectively designated as B, C and D for reference.
Referring to Figure 2, the sodium sulfate saturation curve is shown. In a process where sodium chlorate is produced from sodium chloride, sodium sulfate is present as a contaminant in relatively low concentrations in the mother liquor, in general no greater than 2.5%. As graphically illustrated in Figure 2 when the mother liquor is chilled below 10~C sodium sulfate begins to precipitate out of the solution until equilibrium in the solution at its saturation point is achieved, as illustrated in Figure 2.
For example, if the mother liquor is chilled to 0~C, as shown in Figure 2, the concentration of sodium sulfate would approximate 0.95 wt%. Chilling the mother liquor to -10~C
would result in a concentration of sodium sulfate at approximately 0.45 wt%. -2082~73 With reference to Figure 3, the process and apparatus proposed by the invention for selectively precipitating sodium sulfate from a mother liquor will be described below. The mother liquor is an aqueous solution of sodium chlorate and sodium chloride and containing dissolved sodium sulfate which is conducted along the mother liquor input conduit 1. Water, preferably demineralized water, flows through a water input conduit 2 and is mixed with the mother liquor with conventional dilution means 3.
The resulting diluted liquor is conveyed via diluted liquor conduit 4. It will be understood that any suitable conventional dilution means may be used, such as an in-line mixer, a mixing tank and impeller, or a simple T-joint as illustrated.
As shown in Figure 3 after the step of diluting the mother liquor with demineralized water to form a diluted liquor, the diluted liquor is pre-chilled through a heat exchanger 5. It will be apparent to those skilled in the art that the heat exchanger is not an essential component, however, in a preferred embodiment the use of a heat exchanger results in cost savings in respect of chilling the diluted liquor. The operation of the heat exchanger 5 will be explained in detail below.
The next step in the process is to chill the diluted liquor to a temperature Tx. A chilling unit 6 has an inlet communicating with the diluted liquor conduit 4 and ' 2~2~73 .~.,.
an outlet communicating with a chilled liquor conduit 7.
The chilling unit 6 has an associated refrigeration unit 8 of conventional design.
The chilling unit 6 is capable of chilling the diluted liquor to a temperature Tx. Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor (referred to hereinafter as Tc) and the saturation temperature of sodium sulfate in the diluted liquor (Ts).
Chilling the diluted liquor to a temperature Tx results in the formation of sodium sulfate crystalline precipitant in the chilled liquor.
In order to separate the sodium sulfate crystalline precipitant from the resultant clarified liquor, the chilled diluted liquor is conveyed to a clarifier tank 9 having an inlet communicating with the chilled diluted liquor conduit 7 in its mid portion. Through conventional settling processes the precipitant is conveyed through a lower outlet communicating with a crystalline sodium sulphate slurry conduit 10 and is pumped to further processing with a positive displacement pump 11.
Through an upper outlet communicating with a clarified liquor conduit 12, the resultant clarified liquor is withdrawn and returned to processing of the mother liquor through a return conduit via the heat exchanger 5. The diluted liquor is pre-chilled in the heat exchanger 5 which 2082~73 .~ .
comprises a counter-flow heat exchanger thermodynamically communicating between the clarified liquor conduit 12 and the diluted liquor conduit 4.
Energy consumption may be reduced in the chilling of the diluted liquor as a result of using the heat exchanger to pre-chill the diluted liquor with the clarified liquor to a temperature greater than Tx. Therefore, the chilling unit 6 need only further reduce the temperature to Tx. Referring to Figures 1 and 2 therefore, an example of the process will be described. Diluting of the mother liquor is graphically illustrated in Figure 1 resulting in the change in concentrations from point A to point C. As a result the saturation temper~ture of sodium chlorate in the diluted liquor is reduced to Tc equalling -15~C.
Referring to Figure 2 the same dilution of the mother liquor reduces the concentration of sodium sulfate to approximately 0.95 wt%. From the sodium sulfate saturation curve it can be seen that this concentration of sodium sulfate results in a ~aturation temperature of sodium ~ :
sulfate in the diluted liquor Ts equalling nominally less than 0~C. :~
Therefore, if the diluted liquor is chilled by the heat exchanger 5 and chilling unit 6, to a temperature Tx, ;
in this example between Ts 0~C and Tc -15~C, sodium sulfate ~ :
crystalline precipitant will be formed in the chilled diluted liquor.
r r 2082~ 73 ':, Since the saturation temperature Ts of sodium chlorate in this example is -15~C, the disadvantage o~
conventional methods involving co-crystallization of sodium sulfate and chlorate crystals together is avoided.
It will be understood that, although various features of the invention have been described with respect to one or another of the embodiments of the invention, the various features and embodiments of the invention may be combined or used in conjunction with other features and embodiments of the invention as described and illustrated herein.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to these particular embodiments. Rather, the invention ' .
includes all embodiments which are functional or mechanical ..
equivalents of the specific embodiments and features that have been described and illustrated herein.
j ., ~, ..
: ~ ... ...
Claims (10)
1. A process for selectively precipitating sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride and containing dissolved sodium sulphate, the mother liquor being at an initial temperature To, said process comprising:
diluting the mother liquor with demineralized water thus forming a diluted liquor;
chilling the diluted liquor to a temperature Tx thus forming a sodium sulphate crystalline precipitant while maintaining sodium chlorate in solution; and separating the sodium sulphate crystalline precipitant from a resultant clarified liquor.
diluting the mother liquor with demineralized water thus forming a diluted liquor;
chilling the diluted liquor to a temperature Tx thus forming a sodium sulphate crystalline precipitant while maintaining sodium chlorate in solution; and separating the sodium sulphate crystalline precipitant from a resultant clarified liquor.
2. The process according to claim 1, wherein Tx is in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts.
3. The process according to claim 2, wherein the clarified liquor is conducted via a counter-flowing heat exchanger wherein the diluted liquor is pre-chilled by the clarified liquor to a temperature greater than Tx.
4. The process according to claim 2, wherein Tc is in the range of 0 to -20°C.
5. The process according to claim 4, wherein Tc is -15°C.
6. The process according to claim 2, wherein To is in the range of 35 - 40°C.
7. The process according to claim 3, wherein the diluted liquor is pre-chilled to a temperature of approximately 0°C.
8. The process according to claim 1, wherein the initial concentrations of sodium chlorate and sodium sulphate in the mother liquor are in the range from 40 - 42 wt% and no greater than 2.5 wt% respectively.
9. An apparatus for selectively precipitating sodium sulphate from a mother liquor, the mother liquor being an aqueous solution of sodium chlorate and sodium chloride, and containing dissolved sodium sulphate, said apparatus comprising:
dilution means having an inlet communicating with a mother liquor input conduit, and with a water input conduit, and having an outlet communicating with a diluted liquor conduit;
a chilling unit having an inlet communicating with the diluted liquor conduit and an outlet communicating with a chilled liquor conduit, the chilling unit having the capacity to chill the diluted liquor to a temperature Tx, Tx being in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts;
a clarifier tank having an inlet communicating with the chilled diluted liquor conduit, a lower outlet communicating with a crystalline sodium sulphate slurry conduit, and an upper outlet communicating with a clarified liquor conduit.
dilution means having an inlet communicating with a mother liquor input conduit, and with a water input conduit, and having an outlet communicating with a diluted liquor conduit;
a chilling unit having an inlet communicating with the diluted liquor conduit and an outlet communicating with a chilled liquor conduit, the chilling unit having the capacity to chill the diluted liquor to a temperature Tx, Tx being in the range between the saturation temperature of sodium chlorate in the diluted liquor Tc and the saturation temperature of sodium sulphate in the diluted liquor Ts;
a clarifier tank having an inlet communicating with the chilled diluted liquor conduit, a lower outlet communicating with a crystalline sodium sulphate slurry conduit, and an upper outlet communicating with a clarified liquor conduit.
10. An apparatus according to claim 9, including a diluted liquor pre-chiller comprising a counter-flow heat exchanger thermodynamically communicating between the clarified liquor conduit and the diluted liquor conduit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2082473 CA2082473A1 (en) | 1992-11-09 | 1992-11-09 | Sulphate removal process |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2082473 CA2082473A1 (en) | 1992-11-09 | 1992-11-09 | Sulphate removal process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2082473A1 true CA2082473A1 (en) | 1994-05-10 |
Family
ID=4150669
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2082473 Abandoned CA2082473A1 (en) | 1992-11-09 | 1992-11-09 | Sulphate removal process |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2082473A1 (en) |
-
1992
- 1992-11-09 CA CA 2082473 patent/CA2082473A1/en not_active Abandoned
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Dead |