CA1186489A - Purification of alkali metal hydroxides - Google Patents
Purification of alkali metal hydroxidesInfo
- Publication number
- CA1186489A CA1186489A CA000420374A CA420374A CA1186489A CA 1186489 A CA1186489 A CA 1186489A CA 000420374 A CA000420374 A CA 000420374A CA 420374 A CA420374 A CA 420374A CA 1186489 A CA1186489 A CA 1186489A
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- CA
- Canada
- Prior art keywords
- hydroxide
- ammonia
- alkali metal
- container
- solution
- 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.)
- Expired
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Abstract
Abstract of the Disclosure A method to purify an aqueous solution of an alkali metal hydroxide by countercurrently contacting the ammonia and hydroxide, in a volume ratio of from 0.6:1 to 1:1, in a packed bed. The temperature in the lower portion of an extractor in which the process is carried out is controlled and maintained within a range of from 68° to 77°C.
Description
4~3~
PURIFICATION OF ALKALI METAL HYDROXIDES
This invention xelates to the purification of alkali metal hydroxides and more in particular to the removal of soluble impurities from alkali metal hydrox-ides by contacting such hydroxides with ammonia.
Alkali metal hydroxides, such as sodium hydroxide, are generally produced commercially in the so-called mercury cells or diaphragm cells. In sodium hydroxide produced by mercury cells, a sodium chloride impurity may be present in an amount of roughly from 0.01 to 0.001 weight percent. A typical diaphragm cell aqueous effluent contains from 8 to 10 weight percent sodium hydroxide and 15 weigh-t percent sodium chloride.
The diaphragm cell effluent is concentra-ted by evapora-tion to raise the sodium hydroxid~ concentration to about 50 weight percent and to reduce the sodium chloride concentration. However, even after such reduction of the sodium chlcride, the 50 percent aqueous sodium hydroxide solution from the diaphragm cell still contains from 1 to 2 weight percent sodium chloride.
A number of processes have been developed throughout the years to reduce the sodium chloride con-centration in aqueous sodium hydroxide. Many patents 28,888-F -1-
PURIFICATION OF ALKALI METAL HYDROXIDES
This invention xelates to the purification of alkali metal hydroxides and more in particular to the removal of soluble impurities from alkali metal hydrox-ides by contacting such hydroxides with ammonia.
Alkali metal hydroxides, such as sodium hydroxide, are generally produced commercially in the so-called mercury cells or diaphragm cells. In sodium hydroxide produced by mercury cells, a sodium chloride impurity may be present in an amount of roughly from 0.01 to 0.001 weight percent. A typical diaphragm cell aqueous effluent contains from 8 to 10 weight percent sodium hydroxide and 15 weigh-t percent sodium chloride.
The diaphragm cell effluent is concentra-ted by evapora-tion to raise the sodium hydroxid~ concentration to about 50 weight percent and to reduce the sodium chloride concentration. However, even after such reduction of the sodium chlcride, the 50 percent aqueous sodium hydroxide solution from the diaphragm cell still contains from 1 to 2 weight percent sodium chloride.
A number of processes have been developed throughout the years to reduce the sodium chloride con-centration in aqueous sodium hydroxide. Many patents 28,888-F -1-
-2-have issued describing the removal of sodium chloride and sodium chlorate from aqueous solutions containing a 50 percent concentration of caustic soda. U.S. Patent -2,196,594 discloses an apparatus and process to purify a 50 percent or greater concentration of caustic liquor by countercurrently contacting the aqueous caustic liquor with liquid ammonia at a superatmospheric pres sure and temperatures of from 50 to 100C (122 to 212F~. U.S. Patents 2,196,595; 2,285,299; 2,285,300;
2,349,596; 2,325,339; 2,354,823 and 2,373,257 disclose still other processes for treating an aqueous sodium hydroxide containing solution with ammonia. The process of U.S. Pa-tent 2,622,009 removes salt impurities from an aqueous caustic soda solution, containing from 45 to 50 percent caustic, using liquid ammonia and contact temperatures of from 54 to 71C (130 to 160F) at pressures above 21 kg/cm2 (300 psi). These processes have been successful in eliminating a major portion of the sodium chloride impurity, but the existing ammonia -aqueous sodium hydroxide impurity extracting processes have generally not been able to consistently reduce the sodium chloride impurity in -the caustic to a concentra-tion level closely approachin~ that of mercury cell caustic.
Consequently, it is desired to provide a pro-cess ~hich can reduce the impurity (such as sodium chloride) concentration in an aqueous alkali metal hydroxide solution to a level approaching that of mercury cell caustic.
The hereinafter disclosed invention resides in a method to purify in liquid phases, an impure alkali metal hydroxide by countercurrently contacting ~8,888-F -2 an aqueous solution of the hydroxide with ammoni~ in a container having a first inlet for the impure hydroxide solution upwardly disposed from a second inlet for the ammonia, a first outlet for the purified hydroxide solution and a second outlet ~or the ammonia containing impurities extracted from the impure hydroxide solution, the steps of (a) feeding an aqueous solution containing from 40 to 60 weight percent alkali metal hydroxide and at least a sodium chloride impurity into the first inlet;
(b) feeding sufficient ammonia into the second inlet to provide an ammonia to alkali metal hydroxide volume ratio of from 0.6:1 to 1:1;
~c~ countercurrently flowing the hydroxide and ammonia through a bed of a substantially inert, insoluble par-ticulate material in the container; and ~d) controlling the temperature of the purified hydroxide solution in the lower portion of the container within a range of from 68~ to 77C.
The two figures of the drawing schematically depict different embodiments of an apparatus in which the improved method of the present invention can be carried out. Identical numerals, distinguished by a letter suffix, within the figures represent parts having a similar function within the differen-t embodi-ments.
In practice of one embodiment of the present invention, an aqueous solution containing an alkali metal hydroxide such as potassium hydroxide or more preferably sodium hydroxide with impurities dissolved 28,888-F -3-therein, is Eed into a generally cylindrical container 10 through a first inlet 12 in the container 10.
Liquid ammonia (N~I3) with a preferred purity of at least about 90 (and more preferably at least about 95) weight percent and containing less than about 30 (and more preferably less than about 10) parts per million (ppm) sodium chloride, is fed into a second inlet 14 of the container 10 and flows upwardly through the container countercurren-tly to the flow of the alkali metal hydro~ide solution passing downwardly through the con~ainer.
In operation, -the pressure within the container 10 is maintained at a sufficient level to insure that the ammonia and hydroxide solution are liquid during the extraction or purification process.
When this is done, and the ammonia to alkali metal hydroxide volume ratio is from 0.6:1 to l:l and more preferably from 0.7:1 to 0.8:1, an ammonia-alkali metal hydroxide interface 16 appears in the con-tainer 10 a-t a posi-tion located between the first and second inlets 12 and ].4, respectively.
A purified alkali me-tal hydroxide solution containing significantly less sodium chloride and sodium chlorate impurities than were present in the impure hydroxide solution is removed from the container 10 through a first outlet 18 in a lower portion of the container 10. The temperature of the purified hydroxide solution in the lower portion 11 of the container 10 is preferably controlled to a -temperature within a range of from 68 to 77C (155 to 170F). ~ore preferably, this temperature is controlled and maintained ~ithin a range of from 71 to 74C (160 to 165F) and most preferably at 72.2C (162F3. The temperature of the 28,888-F -4-purified hydroxide solution as it is removed from the container 10 through the first outlet 18 is at a tem-perature which is substantially the same as that in the lower portion of the container 10.
It has been surprisingly found that processing the hydroxide solution within the above-described temperature range combined with use of a bed of a substantially inert, insoluble par-ticulate material in -the container through which the hydroxide solution and ammonia flows results in the production of a purified alkali metal hydroxide solution, e.g. sodium hydroxide, with a reduced sodium chlorate concentration and a sodium chloride impurity level approaching that achievable in the so-called mercury cell caustic.
The particulate material is essentially insoluble and non-reactive with either the ammonia or the hydroxide solution at the temperatures and pres-sures utilized during the process. For example, polypropylene, polyte-trafluoroethylene, nickel, Inconel, and the like, are satisfactory for use in the presen-t invention. Other sui-table materials will be readily apparent to -those skilled in the art. The size and configuration of the particulate material useful in the present invention will vary depending upon the size of the container in which the impuri-ty extraction process is operated. The particulate material should provide a packed bed of sufficient porosity to permit adequate flow of the hydroxide solution and the ammonia therethrough without an undesired reduction in flow through the cylinder. Simultaneously, the particulate material should be sufficiently small to form a packed 28,888-F -5-L~ ~ ~
bed sufficiently dense to cause the hydroxide solution to flow in an agitated or somewhat turbulent manner through the cylinder toward the first outlet 18. It has been determined that polypropylene "Fle~irings"
with average diameters of 1.6 and 2.54 cm (5/8 inch and 1 inch) are satisfactory materials for the bed when sodlum hydroxide is treated in a column with a diameter of about 1.07 meters (3.5 feet).
An upper support plate 20 is suitably disposed across substantially the entire transverse cross section of the container 10 below the fixst inlet 12. The support plate 20 has a plurality of holes therein of a size suf~icient to permit the desired flow of the hydroxide solution and ammonia therethrough and insufficient to permit passage of the particulate material 22. In like manner, a lower support plate 2~
is disposed between the upper support plate 20 and the second inlet 14. The particulate material 22 in the bed supported by the support plate 24 can be the same or a different material and/or size and confiyura-tion than the material retained by -the upper support plate 20.
Alternatively, the entirety or any portion of the cylinder between the first inlet 12 and the second inl~t 14 can be packed with the particulate material 22 to form a bed of any desired depth.
Treatment of the impure ammonia withdrawn from a second outlet 26 to repurify the ammonia so that it may again be used in the disclosed process is performed by using technology readily available and kno~n in the art. Like~ise, further treatment of the 28,888-F -6~
-purified hydroxide solution to, for example, form a more concentrated form of sodium hydroxide is carried out using techniques and equipment common in the industry.
The following examples are further illus-trative of the invention.
Examples 1-~
A 12.2 meter (40 feet) high, generally centrally positioned section of a 20.3 cm (8 inch) diameter by 18.3 meter (60 feet) hi.~h column extractor 10a shown in Figure 2 was packed between an upper retainer plate 27 and a lower support plate 24a (a distance of 12.2 meters) with 2.54 cm polypropylene "pall rings" (Example 4 was packed with "Flexipack" type I rings). Operation of the extractor 10a was begun by setting a pressure valve 28 to release when the pressure within the extractor 10a exceeded 26.4 kg/cm2 (375 psi). Suf-ficient amounts of water and purified liquid ammonia were fed into the extractor 10a through first and second inlets 12a and 14a, respectively, to provide a liquid concentration within the reactor of about 75 weight percent ammonia and about 25 weight percen-t water. When this composition had been obtained and the entire extractor was filled with liquid, the feed was stopped and the contents of the extractor heated to 70C (158F).
When the temperature had reached 70C l158F) within the extrac-tor, the flow of a solution of about 50 weight percent sodium hydroxide and water was started through the first inlet 12a and liquid ammonia slmul-taneously passed into the lower portion of the extractorthrough the second inlet 14a. The volume ratio of 28,888 F -7-liquid ammonia to caustic solution flowing into the extractor was initiall~ controlled at 0.75. Valve 30 remained closed until -the ammonia-caustic solution -interface level reached about the middle of the extractor, i.e. about one-half the height of the extractor; until this interface level was reached, effluent was removed from the e~tractor through a second outlet 26a. The valve 30 was opened after reaching this interface level and a purified 50 percent sodium hydroxide-water solution was continuously xemoved from the e~tractor through the first outlet 18a.
The Table lists operating parameters ~or Examples 1-8. It will be observed from the Table that the described process can reduce the sodium chloride concentration in the sodium hydroxide solu-tion removed from the extractor to extremely low levels.
Examples 9 and 10 Using an extractor with a diameter o~ 1.07 meters (42 inches) and a height of 18.3 meters (60 feet) with a 12.2 meter (40 foot) portion thereof packed with 2.54 cm ~1 inch) (Example 9) of 1.6 cm (5/8 inch) (E~ample 10) polypropylene "Flexirings", a 50 percent sodium h~droxide solution was pumpe~ as described above for Examples 1-8.
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2,349,596; 2,325,339; 2,354,823 and 2,373,257 disclose still other processes for treating an aqueous sodium hydroxide containing solution with ammonia. The process of U.S. Pa-tent 2,622,009 removes salt impurities from an aqueous caustic soda solution, containing from 45 to 50 percent caustic, using liquid ammonia and contact temperatures of from 54 to 71C (130 to 160F) at pressures above 21 kg/cm2 (300 psi). These processes have been successful in eliminating a major portion of the sodium chloride impurity, but the existing ammonia -aqueous sodium hydroxide impurity extracting processes have generally not been able to consistently reduce the sodium chloride impurity in -the caustic to a concentra-tion level closely approachin~ that of mercury cell caustic.
Consequently, it is desired to provide a pro-cess ~hich can reduce the impurity (such as sodium chloride) concentration in an aqueous alkali metal hydroxide solution to a level approaching that of mercury cell caustic.
The hereinafter disclosed invention resides in a method to purify in liquid phases, an impure alkali metal hydroxide by countercurrently contacting ~8,888-F -2 an aqueous solution of the hydroxide with ammoni~ in a container having a first inlet for the impure hydroxide solution upwardly disposed from a second inlet for the ammonia, a first outlet for the purified hydroxide solution and a second outlet ~or the ammonia containing impurities extracted from the impure hydroxide solution, the steps of (a) feeding an aqueous solution containing from 40 to 60 weight percent alkali metal hydroxide and at least a sodium chloride impurity into the first inlet;
(b) feeding sufficient ammonia into the second inlet to provide an ammonia to alkali metal hydroxide volume ratio of from 0.6:1 to 1:1;
~c~ countercurrently flowing the hydroxide and ammonia through a bed of a substantially inert, insoluble par-ticulate material in the container; and ~d) controlling the temperature of the purified hydroxide solution in the lower portion of the container within a range of from 68~ to 77C.
The two figures of the drawing schematically depict different embodiments of an apparatus in which the improved method of the present invention can be carried out. Identical numerals, distinguished by a letter suffix, within the figures represent parts having a similar function within the differen-t embodi-ments.
In practice of one embodiment of the present invention, an aqueous solution containing an alkali metal hydroxide such as potassium hydroxide or more preferably sodium hydroxide with impurities dissolved 28,888-F -3-therein, is Eed into a generally cylindrical container 10 through a first inlet 12 in the container 10.
Liquid ammonia (N~I3) with a preferred purity of at least about 90 (and more preferably at least about 95) weight percent and containing less than about 30 (and more preferably less than about 10) parts per million (ppm) sodium chloride, is fed into a second inlet 14 of the container 10 and flows upwardly through the container countercurren-tly to the flow of the alkali metal hydro~ide solution passing downwardly through the con~ainer.
In operation, -the pressure within the container 10 is maintained at a sufficient level to insure that the ammonia and hydroxide solution are liquid during the extraction or purification process.
When this is done, and the ammonia to alkali metal hydroxide volume ratio is from 0.6:1 to l:l and more preferably from 0.7:1 to 0.8:1, an ammonia-alkali metal hydroxide interface 16 appears in the con-tainer 10 a-t a posi-tion located between the first and second inlets 12 and ].4, respectively.
A purified alkali me-tal hydroxide solution containing significantly less sodium chloride and sodium chlorate impurities than were present in the impure hydroxide solution is removed from the container 10 through a first outlet 18 in a lower portion of the container 10. The temperature of the purified hydroxide solution in the lower portion 11 of the container 10 is preferably controlled to a -temperature within a range of from 68 to 77C (155 to 170F). ~ore preferably, this temperature is controlled and maintained ~ithin a range of from 71 to 74C (160 to 165F) and most preferably at 72.2C (162F3. The temperature of the 28,888-F -4-purified hydroxide solution as it is removed from the container 10 through the first outlet 18 is at a tem-perature which is substantially the same as that in the lower portion of the container 10.
It has been surprisingly found that processing the hydroxide solution within the above-described temperature range combined with use of a bed of a substantially inert, insoluble par-ticulate material in -the container through which the hydroxide solution and ammonia flows results in the production of a purified alkali metal hydroxide solution, e.g. sodium hydroxide, with a reduced sodium chlorate concentration and a sodium chloride impurity level approaching that achievable in the so-called mercury cell caustic.
The particulate material is essentially insoluble and non-reactive with either the ammonia or the hydroxide solution at the temperatures and pres-sures utilized during the process. For example, polypropylene, polyte-trafluoroethylene, nickel, Inconel, and the like, are satisfactory for use in the presen-t invention. Other sui-table materials will be readily apparent to -those skilled in the art. The size and configuration of the particulate material useful in the present invention will vary depending upon the size of the container in which the impuri-ty extraction process is operated. The particulate material should provide a packed bed of sufficient porosity to permit adequate flow of the hydroxide solution and the ammonia therethrough without an undesired reduction in flow through the cylinder. Simultaneously, the particulate material should be sufficiently small to form a packed 28,888-F -5-L~ ~ ~
bed sufficiently dense to cause the hydroxide solution to flow in an agitated or somewhat turbulent manner through the cylinder toward the first outlet 18. It has been determined that polypropylene "Fle~irings"
with average diameters of 1.6 and 2.54 cm (5/8 inch and 1 inch) are satisfactory materials for the bed when sodlum hydroxide is treated in a column with a diameter of about 1.07 meters (3.5 feet).
An upper support plate 20 is suitably disposed across substantially the entire transverse cross section of the container 10 below the fixst inlet 12. The support plate 20 has a plurality of holes therein of a size suf~icient to permit the desired flow of the hydroxide solution and ammonia therethrough and insufficient to permit passage of the particulate material 22. In like manner, a lower support plate 2~
is disposed between the upper support plate 20 and the second inlet 14. The particulate material 22 in the bed supported by the support plate 24 can be the same or a different material and/or size and confiyura-tion than the material retained by -the upper support plate 20.
Alternatively, the entirety or any portion of the cylinder between the first inlet 12 and the second inl~t 14 can be packed with the particulate material 22 to form a bed of any desired depth.
Treatment of the impure ammonia withdrawn from a second outlet 26 to repurify the ammonia so that it may again be used in the disclosed process is performed by using technology readily available and kno~n in the art. Like~ise, further treatment of the 28,888-F -6~
-purified hydroxide solution to, for example, form a more concentrated form of sodium hydroxide is carried out using techniques and equipment common in the industry.
The following examples are further illus-trative of the invention.
Examples 1-~
A 12.2 meter (40 feet) high, generally centrally positioned section of a 20.3 cm (8 inch) diameter by 18.3 meter (60 feet) hi.~h column extractor 10a shown in Figure 2 was packed between an upper retainer plate 27 and a lower support plate 24a (a distance of 12.2 meters) with 2.54 cm polypropylene "pall rings" (Example 4 was packed with "Flexipack" type I rings). Operation of the extractor 10a was begun by setting a pressure valve 28 to release when the pressure within the extractor 10a exceeded 26.4 kg/cm2 (375 psi). Suf-ficient amounts of water and purified liquid ammonia were fed into the extractor 10a through first and second inlets 12a and 14a, respectively, to provide a liquid concentration within the reactor of about 75 weight percent ammonia and about 25 weight percen-t water. When this composition had been obtained and the entire extractor was filled with liquid, the feed was stopped and the contents of the extractor heated to 70C (158F).
When the temperature had reached 70C l158F) within the extrac-tor, the flow of a solution of about 50 weight percent sodium hydroxide and water was started through the first inlet 12a and liquid ammonia slmul-taneously passed into the lower portion of the extractorthrough the second inlet 14a. The volume ratio of 28,888 F -7-liquid ammonia to caustic solution flowing into the extractor was initiall~ controlled at 0.75. Valve 30 remained closed until -the ammonia-caustic solution -interface level reached about the middle of the extractor, i.e. about one-half the height of the extractor; until this interface level was reached, effluent was removed from the e~tractor through a second outlet 26a. The valve 30 was opened after reaching this interface level and a purified 50 percent sodium hydroxide-water solution was continuously xemoved from the e~tractor through the first outlet 18a.
The Table lists operating parameters ~or Examples 1-8. It will be observed from the Table that the described process can reduce the sodium chloride concentration in the sodium hydroxide solu-tion removed from the extractor to extremely low levels.
Examples 9 and 10 Using an extractor with a diameter o~ 1.07 meters (42 inches) and a height of 18.3 meters (60 feet) with a 12.2 meter (40 foot) portion thereof packed with 2.54 cm ~1 inch) (Example 9) of 1.6 cm (5/8 inch) (E~ample 10) polypropylene "Flexirings", a 50 percent sodium h~droxide solution was pumpe~ as described above for Examples 1-8.
28,888-F -8 ~ ~ o o X ~ r~ ~ o ~ u~ ~ ~ t~ ~o o ~ o ~ oo o ~ ~ o o r~
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,, æ ~, O O ~ O O O O O O O
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~ ~ ~ ~ ~ 5~
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Claims (9)
1. A method to purify in liquid phases, an impure alkali metal hydroxide by countercurrently contacting an aqueous solution of the hydroxide with ammonia in a container having a first inlet for the impure hydroxide solution upwardly disposed from a second inlet for the ammonia, a first outlet for the purified hydroxide solution and a second outlet for the ammonia containing impurities extracted from the impure hydroxide solution, the steps of (a) feeding an aqueous solution containing from 40 to 60 weight percent alkali metal hydroxide and at least a sodium chloride impurity into the first inlet;
(b) feeding sufficient ammonia into the second inlet to provide an ammonia to alkali metal hydroxide volume ratio of from 0.6:1 to 1:1;
(c) countercurrently flowing the hydroxide and ammonia through a bed of a substantially inert, insoluble particulate material in the container; and (d) controlling the temperature of the purified hydroxide solution in the lower portion of the container within a range of from 68° to 77°C.
(b) feeding sufficient ammonia into the second inlet to provide an ammonia to alkali metal hydroxide volume ratio of from 0.6:1 to 1:1;
(c) countercurrently flowing the hydroxide and ammonia through a bed of a substantially inert, insoluble particulate material in the container; and (d) controlling the temperature of the purified hydroxide solution in the lower portion of the container within a range of from 68° to 77°C.
2. The method of Claim 1 wherein the solution contains from 48 to 52 weight percent of the alkali metal hydroxide and wherein the ammonia fed into the container is at least 90 weight percent pure and contains less than 30 parts per million sodium chloride.
3. The method of Claim 1 or 2 wherein the alkali metal hydroxide is sodium hydroxide containing less than 1.2 weight percent sodium chloride and the purified hydroxide solution contains less than 0.006 weight percent sodium chloride.
4. The method of Claim 1 wherein the temperature is controlled within the range of from 71° to 74°C and the ammonia to alkali metal hydroxide ratio is from 0.7:1 to 0.8:1.
5. The method of Claim 1 including before step (a), filling the container with ammonia and water to provide a liquid concentration within the container of about 75 weight percent ammonia and about 25 weight percent water and thereafter heating the contents of the container to operating temperature.
6. The method of Claim 5 wherein the temperature is 70°C to 73.9°C.
7. The method of Claim 5 wherein the temperature is 72.2°C.
8. The method of Claim 5 wherein the alkali metal hydroxide is sodium hydroxide.
9. The method of Claim 5 wherein the alkali metal hydroxide is sodium hydroxide and the sodium hydroxide -11a-solution is agitated as said solution flows through the particulate material.
-11a-
-11a-
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000420374A CA1186489A (en) | 1983-01-27 | 1983-01-27 | Purification of alkali metal hydroxides |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000420374A CA1186489A (en) | 1983-01-27 | 1983-01-27 | Purification of alkali metal hydroxides |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1186489A true CA1186489A (en) | 1985-05-07 |
Family
ID=4124431
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000420374A Expired CA1186489A (en) | 1983-01-27 | 1983-01-27 | Purification of alkali metal hydroxides |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1186489A (en) |
-
1983
- 1983-01-27 CA CA000420374A patent/CA1186489A/en not_active Expired
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