CA1058555A - Electrolytic method for the manufacture of dithionites - Google Patents
Electrolytic method for the manufacture of dithionitesInfo
- Publication number
- CA1058555A CA1058555A CA212,484A CA212484A CA1058555A CA 1058555 A CA1058555 A CA 1058555A CA 212484 A CA212484 A CA 212484A CA 1058555 A CA1058555 A CA 1058555A
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- CA
- Canada
- Prior art keywords
- compartment
- cell
- cathode
- anode
- sulfite
- 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.)
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Classifications
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
- C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/14—Alkali metal compounds
Abstract
ABSTRACT OF THE DISCLOSURE
Dithionites are made by a process which begins with the production of high concentration chloride-free sodium hydroxide solution and chlorine at a high current efficiency from a three-compartment electrolytic cell having membranes of a cation-active permselective membrane material separating anode and cathode compartments from a buffer compartment. Hydroxide ions migrating into the buffer compartment from the cathode compartment are converted to sulfite by reaction with sulfur dioxide, improving the current efficiency of the three-compartment cell, and the sulfite is removed. Subsequently, the sulfite resulting and additional sulfur dioxide are fed to the cathode compartment of a two-compartment electrolytic cell wherein the anode and cathode compartments are separated by a cation-active perm-selective membrane and in which chloride solution is being electrolyzed to chlorine and caustic. The caustic reacts with the sulfite and sulfur dioxide in the cathode compartment to produce sulfite and dithionite.
Dithionites are made by a process which begins with the production of high concentration chloride-free sodium hydroxide solution and chlorine at a high current efficiency from a three-compartment electrolytic cell having membranes of a cation-active permselective membrane material separating anode and cathode compartments from a buffer compartment. Hydroxide ions migrating into the buffer compartment from the cathode compartment are converted to sulfite by reaction with sulfur dioxide, improving the current efficiency of the three-compartment cell, and the sulfite is removed. Subsequently, the sulfite resulting and additional sulfur dioxide are fed to the cathode compartment of a two-compartment electrolytic cell wherein the anode and cathode compartments are separated by a cation-active perm-selective membrane and in which chloride solution is being electrolyzed to chlorine and caustic. The caustic reacts with the sulfite and sulfur dioxide in the cathode compartment to produce sulfite and dithionite.
Description
~CI S85S~i This invention relates to the electrolytic manufacture of dithionites, More specifically, it is of a process for making alkali metal dithionite from alkali metal chloride and sulfur dioxide, utilizing a combination of electrolytic cells, one having three compartments and the other having two compart-ments, the compartments of each being separated by a cation-active permselective membrane which, in the best embodiments of the invention, is of a hydrolyzea polymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether or is a sulfostyrenated perfluorinated ethylene propylene polymer.
The cation-active membranes mentioned allow ~ .
proportio~ of hydroxyl ion generated at the cathodes of the cells to migrate to the buffer compartmen~ of the three-compartment cellsand to the anode compartmen~ of the two-compartment cells~
In the former case this portion of the hydroxyl generated i5 reacted with sulfur dioxide to produce sulfite and in the latter case may be converted to oxygen, thereby interfering with the efficiency of the two-compartment cell portion of the process.
However, the proportion of hydroxide entering the anode compart-ment of the two-compartment cell is very low because it is consumed in the catholyte of that cell by reaction with sulfur dioxide therein to form larger anions, such as sulfite and dithionite, which do not readily.penetrate the cation-active permselective membrane. Thus, dithionite and sulfite ions are ~, .. ' -. , ' , ~ . .....
1~5~3555 preven~ed from migrating from the catholyte or buffer solution to the buffer solution or anolyte, chloride is prevented from migrating from the anolyte to the buff~r or catholyte compart-ments and hydroxyl ion is effectively prevented from passing ~5 into the anolytes.
^ Dithionites and in particular, alkali metal dithionites, especially sodium dithionite, are useful bleaching agents and have been found to brighten or bleach wood pulps appreciably.
Such a brightening or bleaching operation is an essential portion of many papermaking processes. Usually, the dithionite employed in the past has been zinc dithionite but to prevent water pollution the discharging of zinc ions into streams has been limited. Therefore, it has been found desirable to utilize other dithionites which are less objectionable. It has been suggested that dithionites could be made by the electrolysis of acidic solutlons of sulfur dioxide, utili~ing separating permselective membranes between anode and cathode compartments~ Such a process ha~ been described in Pulp and Paper Magazine of Canada~ in the issue of Dec. 19, 1969, at pages 73-78. Such methods are feasible to some extent but the process of the present invention is far superior. It eIectrolytically produces hydroxide employ-ed to make sulfite reactant, manufactures useful chlorine simultaneously, rather than useless oxygen,and makes a hydroxide and the bleaching product, both of which are low in chloride 2S content. Such low chloride contents are advantageous since the .
' . .' ' -- - . . . .: . ,~
~q~585S5 proportion of chloride which may be discharged into streams and ground water is also limited. Although sulfite accompanies the dithionite, it may be usefully employed with it and is useful in making white liquor, utilized in papermaking processes. A
~5 special advantage of the present invention is in the utilization of the various products of the process in industrial plants, such as papermaking plants. The chloride-free hydroxide, dithionite, sulfite and chlorine are all useful products for papermaking and are produced in usable forms, without objection-able contaminants. They are made from a limited number of starting materlals, primarily sources of chloride, e.g., salt, and sulfur dioxide, which may be obtained from the burning of sulfur or sulfur-containing ores.
In accordance with the present invention a method for electrolytically manufacturing a dithionite, chlorine, hydroxide and a sulfite from sulfur dioxide and a chloride comprises feeding chloride solution to the anode compartment of an electro-lytic cell having anode, buffer and cathode compartments separated b~ cation-active permselective membranes, an anode in the anode compartment and a cathode in the cathode compartment and feeding sulfur dioxide to the buffer compartment, withdrawing chlorine from the anode compartment, hydroxide from the cathode compart-; ment and ~ulfite from the buffer compartment, feeding such sulfite and sulfur dioxide to the cathode compartment of a two-compartment electrolytic cell having ~n anode in an anode ~ 3 ~L~5~555 compartment, a cathode in a cathode compartment and a cation-~-` active permselective membrane dividing the compartm~nts, feeding chloride to the anode compartment thereof and withdrawing chlorine from the anode compartment and dithionite and sulfi~e from the cathode compartment.
Important advantages of this process include the maml-facture of chloride-free, high concentration caustic in the three compartment cell at a high current efficiency, together with use-ful chlorine from both cells, and the production of sodium di-thionite in the cathode compartment of the two compartment cellat a pH which is about neutral, preferably about 6 to 8, in which range the dithionite is comparatively stable, so that it may be used commercially as the aqueous solution produced, with sulfite, for ~he bleaching of wood pulp and other analogous pro-cesses.
According to another aspect of the invention there is provided an electrolytic cell system for manufacturing a dithionite, chlorine, a hydroxide and a sulfite from sulfur dioxide and a chloride which comprises a first cell comprising a housing having an anolyte compartment, containing an anode adapted to be connected to a positive terminal of an electrical input source a catholyte compartment containing a cathode, and a buffer com-partment be~ween said anolyte compartment and said catholyte compartment defined by a pair of spaced apart cation-active perm-selective membranes, and at least a second electrolysis cel.l comprising a housing having an anode compartment containing an anode adapted for connection to a positive terminal of an electrical input source; and a cathode compartment containing a cathode, said anode compartment being separated from said cathode compartment by a cation permselective membrane; said buffer compartment having a first inlet for sulfur dioxide and a first outlet for sulfite communicating with said cathode com-~5~555 partment of said second cell, said anolyte and anode compartmentseach including an inlet for chloride solution and an outle~ for gaseous chlorine, said catholyte compartment including an outlet for hydroxide, said cathode compartment including an inlet for sulfur dioxide and an outlet for sulfite and dithionite.
The invention will be readily understood by reference to the following description of an embodiment thereof, taken in conjunction with the drawing of apparatuses utilized in carrying out the inventive process.
In the drawing:
The FIGURE is a schematic representation of a pair of electrolytic cells and auxiliary equipment for producing dithionite by the method of this inve~ion.
In electrolytic cell 11, outer wall 13 and bottom 15 enclose anode 17, -cathode 19 and conductive means 21 and 23, respectively, for connecting the anode and cathode to sources of positive and negative electrical potentials, respectively.
~.. ,~
~0513S~5 Cation-active permselective membranes 25 and 27 divide the cell volume into anode or anolyte compartment 29, buffer compartment 31 and cathode or catholyte compartment 33. An acidic aqueous solution of a halide or brine is indicated as passîng into the anode compartment through line 35. Such brine is used for initial charging of the anolyte and for make-up eed, although make-up may also be added before recirculated anolyte is admit~
ted to the resaturator, to be descrihed. Also, it may be desirable to dispense with brine line 35 and charge the cell initially and ~eed make-up through the resaturator piping. The chloride solution ~or the anolyte compartment, which may be maintained at a desired acidity by add~ions of acid, e.g., HCl, by conventional means, not shown, is circulated from the anode compartment through resaturator 37 via line 39 and exits from ; 15 the resaturator through line 41, from whence it returns to the anode compartment. In a normal opera~ion, utilizing sodium chloride solution or other alkali metal chloride, ~he anolyte compartment is charged with a suitable chloride r e.g., a 25%
salt solution, and that withdrawn for r~satur~ion is at ~
lower concentration, e.g., about 22~ NaCl. Chlorine, generated in the anode compartment by electrolysis of the halide solu-tion, is taken off through line 43.
Water may be added to the cathode compar~ment 33 through piping 45 to maintain the desired level thereof ~nd of the buffer compartment. Hydrogen is removed from this . .
~5~555 s compartment through ventin~ means 47. The bufer compar~ment has sulfur dioxide and water added to it through lines 4g and -51, respectively, and alkaline sodium sulfite is taken off through piping 53, through which it is transmitted to cathode ; 5 compartment 55 of two-compartment electrolytic cell 57.
~ To increase circulation in the buffer compartment, effectively increase the volume of the compartment and to allow greater reaction times between the caustic and sulfur dioxide there may be provided a recirculation loop, for the bufer compaxtment including lines 50, 52 and 54g pump 56 and "holding tank" 58. The volume of such system may be 10 to 100,000 times that of t~.e bu-Efer compartment, preferably from 100 to 10,000 times such volume. High strength sodium hydroxide is removed from the cell through take-off piping 40, at a con-lS centration of about 20 to 30% hydroxide, as sodium hydroxide, in water, and with a low chloride content, usually less than one gram per liter of NaCl. Some of the hydroxide produced ~
in the cathode compartment 33 penetrates the cation-active perm-selective membrane 27 and passes into buffer compartment 31, wherein it reacts with the sulfur dioxide to produce sodium sulfite. The passage of the hydroxide into the buffer compart-ment is represented by arrow 42. Because of the reaction of the hydroxide in th buffer compartment and because the sulfite ion and S02 do not penetrate the membrane 25, very little h,droxide passes into the anode compa~tment 29 and thereLore, ' - . . .
~58SS5 the chlorine efficienc~ is maintained high. Also, of course, chloride ion does not pass from the anolyte into the buffer compartment, due to the repulsive effect of the permselective membrane. Additionally, the membranes and buffer zone prevent ; 5 hydrogen or other cathode-produced gases from being mixed with chlorine, preventing the production of combustible gas mixtures.
Two-compartment cell 57 has sides 59 and bottom 61 enclosing anode 63 and cathode 65, which are connected to sources of positive and negative electrical potentials, respectively, through conductive means 67 and 69. Cation-acti~7e permselective membrane 71 divides the two-compartment cell volume into anode or anolyte compartment 73 and cathode or catholyte compartment 55. Acidic aqueous halide, e.g., chloride solution or brine passes into the anode compartment through line 77 for initial charging of the anolyte andt i~
desired, for make-up feed. ~he ha-ide or chloride solution for the anolyte compartment, also maintained at desired acidity in the same manner described for the three-compart-ment cell, is taken off from the anode compartment through line 79 and passes through resaturator 81, exiting through line 83 and returning to the anode compartment. Concentrations of chloride solution taken off and returned to that compaxt-ment are about the same as with respect to the three-compart-ment cell, already described. As with the three-compartment ~ ~ .
.
~5~5~5 cell operation the use of the separate brine line may be discontinued in favor of utilization of the resaturator elements instead, to feed brine and make-up for any losses thereof. Also, i~stead of separate resaturatoxs and attendant . 5 lines a single resaturator and appropriate piping may be used to maint~ain halide concentrations in both cell anolytes.
Chlorine generated in the anode compartment of the two-compart-ment cell is removed therefrom through piping 85.
Cathode compartment 55 is~charged with gaseous sulfur dioxide through line 87 and water is added through line 89. A
mixture of dithionite and sulfite is removed via piping 91 and any hydrogen or other gases which may be produced in the cathode compartment are vented off via venting means 93.
Analogously to the buffer solution recirculation in the three-compartment cell, catholyte of the two-compartment cell may also be recirculated, utilizing lines 60, 62.and 64, tank 66 and pump 68. The ratio of the total circulating system volume to that of the cathode compartment may be from 2:1 to 100,000:1 and is preferably 100:1 to 10,000:1.
During operations of the cells high concentration, low chloride caustic is taken off from the three-compartment-cell and is ready for use in wood pulping, bleaching or other opera-tions and chlorine removed from the anode compartment of the three-cvmpartment cell is useful in the bleaching of wood pulp or for other pulp and paper mills' industrial purposes. The sulfite, produced in alkaline form due to the content ol hydroxide thorein, _ g _ ' , . - .
.
~3585~5 is converted in the two-compartment cell to dithionite and addi-tional sulfite is made by reaction of sulfur dioxide with hydroxide generated in the cathode compartment. As is clear from the diagram, the two-compartment cell also makes chlorine, useful in pulp bleaching. The sulfibe made by reaction of the sulfur dioxide with hydroxide in the cathode compartmen~ is useful in pulping operations and may be converted to white liquor after completion of bleaching of pulp by the accompanying dithionite.
The sulfur dioxide performs the important function of regulati~g the pH in the cathode compartment of the two-compartment cell so - as to maintain it in the range of 6 to 8, thereby stabilizingthe dithionite produced. Although the mechanism of the reaction . has been described, applicants should not be considered as being bound by this description, since it may also be theorized that the sulfur dioxide charged is reduced to dithionic acid, which is -then neutralized by hydroxyl present to form dithionite. In such case, the presencs of the sulfite can help to exert a buffering effect to maintain the desired pH. . .
As is illustrated schematically ~y arrow 95 the ~20 dithionite (and sulfite) ions do not penetrate the permselectivemembrane 71 and therefore, are held in the.cathode compartment 55. Similarly, halide ions, the path of which is indicated by an arrow identified by numeral 97, do not pass from the anolyte to the catholyte of the two-compartment cell. However, cations :25 such as alkali metal ions, e~g., Na+, indicated by M~ in the illustration, the direction of which is represented by the ar-row99 headed toward the right on the right side of the .
iC~S15~S55 drawing, may pass from anolyte to catholyte. A small propoxtion of hydroxyl ion may penetrate the membrane 71 but usually the concentration of free hydroxyl is low in the catholyte, due to reaction with sulfur dioxide and reduction of the pH to the 6 to 8 range, so that the hydroxyl entering the anolyte, if any, has li~tle effect on chloride current efficiency.
By the described process, utilizing a co~bination of three~compartment and two-compartment cells, the sulfur dioxide feed to the buffer compartment of the three-compartment cell ties up the sodium hydroxide penetrating the membrane between the catholyte and buffer solution and prevents it from reaching the anode; where it could be converted to useless oxygen, thereby decreasing current efficiency. At the same time, high strength, chloride-free caustic is made, which is important in various chemical operations, e.g., pulp bleaching, where chloride dls-charges from industrial plants are undesirable and may be strictly limited.
The chlorine and chloride-free caustic made are both useful chemicals for many industrial processesr including wood pulpiny and pulp bleaching. Thus, the invention has a distinct advantage over an electrolytic method for producing dithionite by charging sulfite or sulfur dioxide to a two-compartment cell and producing dithionite in the cathode compartment by reduction of sulfite or reduction of sulfur dioxide, followed by neutrali-zation to the dithionite. That is~ the sulfur dioxide which would _, ... , ,., . . . .. .... . . . . . . , ... .... .. .......... .... .. . , . . , ~
be required to make sulfite for the two-compartment cell electrolytic reaction, makes the sulfite in the buffer compart-ment of the three-compartment cell while chloride-free caustic is made in the cathode compartment, and increases chlorine current efficiency of the cell. These additional advantages improve the efficiency of the present process and make it commercially advantageous over similar or related processes.
Instead of adding suIfur dioxide to the cathode compart-ment, wherein it acts as a source of sulfite for reduction to dithionite and at the same time serves to help regulate the pH
in the desired 6 to 8 range, sulfite may be fed to the catholyte, with other means employed for pH regulation. By such a process, although the results may not be as satisfactory as with that previously described, utilizing sulfur dioxide, dithionite can be made. However, unless the means of reducing the àlkaline pH
caused by the presence of the hydroxide generated at the ca~hode is a chemical which produces a useful proauct ~and which is non-; interfering with the dithionite process), there will ~e a waste of hydroxide and po~sibly, even sreation of a disposal problem~
The halide solution fed to the anode compartment of both cells is an aqueous solution of a water soluble metal chloride in the usual case, preferably of sodium chloride. The concentra-tion thereof is generally in the range of 200 to 320 grams/liter for sodium chloride and 200 to 360 g./1. ~or potassium chloride.
Preferably such solutions contain 20 to 25% of the alkali metal .
~S85SS
halide salt, as the solutions are charged to the cell or deliver-ed to it from the resaturator. Generally the chloride content will be reduced to 5 to 30% less than the original content, preferably to 10 to 20% less and normally, as with sodium `5 chl~ride, ~he concentration of the halide xemoved from the anode compartment for resaturation and return to such compartment is about 22~, as NaCl, or equivalent. Although the anolyte may be nsutral, it is often acidified so as to be of a pH in the range of about 1 to 6, preferably 2 to 4, with acidification normally being effected with a suitable acid, such as hydrochloric acid.
Water utilized to make the initial brine charge or added as make-up feed to the ano~e compartments and watex added to the other compartments of the cells will preferably be deionized, ; containing less than 10 p.p.m. hardness, as CaCO3, although tap water of comparatively low hardnesst è.g., under 150 p.p.m., preferably under 50 p.p.m., can be used.
The sulfur dioxide charged to the buffer compartment of the three-compartment cell is usually s~bstantlally pure, e.g., over 90~ SO2, but lower concentrations thereof, e.g., as low as 20%, are usable because of the desirable attributes of the membrane material in preventing gas in~erchanges between cell portions. Thus, the unreacted gas, e.g., 2~ N2, may be removed from line 53 at a suitable point, before the sulfite produced is charged to the cathode compartment of the two-compartment cell.
In the three-compartment cell hig~ concentration .
.
.
'. ' ~0585S5 hydroxide solution, such as alkali metal hydroxide, preferably sodium hydroxide, is produced, normally of 20 to 30% hydroxide, although lesser concentrations may also be made, e.g., down to as low as 5%. The chloride content thereof is low, usually being less than 5 g./l. and often less than 1 g.fl. The concentration of the hydroxide may be regulated by controls of the rate of feed of water to the catholyte, flow of elec~ric current and, in some cases, nature o the feed to the cathode compartment (dilute caustic may somètimes be fed in at least partial replacement of water).
The sulfite produced by reaction of the sodium hydroxide and sulfur dioxide in the bu~fer compartment may be of any of various concentrations. These are controllable by regulating the feed of sulfur dioxide to the buffer compartment.
m e more sulfur dioxide charged, the greater the quantity of sulfite in the buffer effluent, in comparison to that of the hydroxide. Generally, the sulfite will be an aqueous solution of 1 to 15% strength and the hydroxide removed from the buffer compartment will also be a corresponding 15 to 1% soluticn r with more sulfite than hydroxide in the buffer compartment.
Preferably the sulfite and hydroxide concentrations total about 10 to 20%, e.g., about 15%, and in more preferred embodiments of the invention the concentration of hydroxide is maintained at less than 5~ while that of the sulfite is up to about 10%.
In the two-compartment cell the feeds to the catholyte _ 14 ~L058SSS
of sulfite-hydroxide solution from the buffer compartment and SO2 are so regulated as to maintain the desired pH for the formation of a stable dithionite. Such a pH should be in the range of about 6 to 8, preferably 6 to 8 and most preferably about 7.
~5 It may be regulated by controlling the feed of sulfur dioxide, which has the additional beneficial effect of diminishin~ the hydroxide concentration to a very small proportion, preventing all but a very minor proportion of the hydroxide generated at the cathode ~rom migrating through the membrane to the anolyte, where it could have been converted to oxygen, with a loss of electrical efficiency. The effluent from the cathode compartment is a mixture of dithionite and sulfite and the concentrations of these components are usually in the ranges of 0.5 to 30% sulfite and 0.5 to 10% dithionite. Within such ranges the noxmal ranges are from 10 to 20% of sulfite and 1 to 5~ of dithionite. The conversion of sulfite or sulfur dioxide to dithionite will usually .
be at a current efficiency of *rom about 40 to 80%, normally within the 60 to 75% range. The dithionite removed from the cathode compartment of the two-compartment cell will generally have a concentration of 10 to 70 g./l., within which range 30 to 50 g./l. is usual. From 100 to 250 g./l. will be the concen-tration of the sulfite drawn off wikh it.
To obtain the desired operation of these cells, as described, the voltage drop across the three-compartment cell is maintained at about 3 to 6 volts, preferably 4 to 5 volts .. _ ... . .. . .. . . . . . . . .. . . . .. . . . . . . .... .
~51~SS5 and that across the two-compartment cell is about 3 to 5 vol~s, pre~erably 3.S to 4.5 volts. The current density for the three-compartment cell is about 1 to 3 amperes/sq. in., preferably 1.5 to 2.5 a.s.i., and that of the two-compartment cell is 0.1 to
The cation-active membranes mentioned allow ~ .
proportio~ of hydroxyl ion generated at the cathodes of the cells to migrate to the buffer compartmen~ of the three-compartment cellsand to the anode compartmen~ of the two-compartment cells~
In the former case this portion of the hydroxyl generated i5 reacted with sulfur dioxide to produce sulfite and in the latter case may be converted to oxygen, thereby interfering with the efficiency of the two-compartment cell portion of the process.
However, the proportion of hydroxide entering the anode compart-ment of the two-compartment cell is very low because it is consumed in the catholyte of that cell by reaction with sulfur dioxide therein to form larger anions, such as sulfite and dithionite, which do not readily.penetrate the cation-active permselective membrane. Thus, dithionite and sulfite ions are ~, .. ' -. , ' , ~ . .....
1~5~3555 preven~ed from migrating from the catholyte or buffer solution to the buffer solution or anolyte, chloride is prevented from migrating from the anolyte to the buff~r or catholyte compart-ments and hydroxyl ion is effectively prevented from passing ~5 into the anolytes.
^ Dithionites and in particular, alkali metal dithionites, especially sodium dithionite, are useful bleaching agents and have been found to brighten or bleach wood pulps appreciably.
Such a brightening or bleaching operation is an essential portion of many papermaking processes. Usually, the dithionite employed in the past has been zinc dithionite but to prevent water pollution the discharging of zinc ions into streams has been limited. Therefore, it has been found desirable to utilize other dithionites which are less objectionable. It has been suggested that dithionites could be made by the electrolysis of acidic solutlons of sulfur dioxide, utili~ing separating permselective membranes between anode and cathode compartments~ Such a process ha~ been described in Pulp and Paper Magazine of Canada~ in the issue of Dec. 19, 1969, at pages 73-78. Such methods are feasible to some extent but the process of the present invention is far superior. It eIectrolytically produces hydroxide employ-ed to make sulfite reactant, manufactures useful chlorine simultaneously, rather than useless oxygen,and makes a hydroxide and the bleaching product, both of which are low in chloride 2S content. Such low chloride contents are advantageous since the .
' . .' ' -- - . . . .: . ,~
~q~585S5 proportion of chloride which may be discharged into streams and ground water is also limited. Although sulfite accompanies the dithionite, it may be usefully employed with it and is useful in making white liquor, utilized in papermaking processes. A
~5 special advantage of the present invention is in the utilization of the various products of the process in industrial plants, such as papermaking plants. The chloride-free hydroxide, dithionite, sulfite and chlorine are all useful products for papermaking and are produced in usable forms, without objection-able contaminants. They are made from a limited number of starting materlals, primarily sources of chloride, e.g., salt, and sulfur dioxide, which may be obtained from the burning of sulfur or sulfur-containing ores.
In accordance with the present invention a method for electrolytically manufacturing a dithionite, chlorine, hydroxide and a sulfite from sulfur dioxide and a chloride comprises feeding chloride solution to the anode compartment of an electro-lytic cell having anode, buffer and cathode compartments separated b~ cation-active permselective membranes, an anode in the anode compartment and a cathode in the cathode compartment and feeding sulfur dioxide to the buffer compartment, withdrawing chlorine from the anode compartment, hydroxide from the cathode compart-; ment and ~ulfite from the buffer compartment, feeding such sulfite and sulfur dioxide to the cathode compartment of a two-compartment electrolytic cell having ~n anode in an anode ~ 3 ~L~5~555 compartment, a cathode in a cathode compartment and a cation-~-` active permselective membrane dividing the compartm~nts, feeding chloride to the anode compartment thereof and withdrawing chlorine from the anode compartment and dithionite and sulfi~e from the cathode compartment.
Important advantages of this process include the maml-facture of chloride-free, high concentration caustic in the three compartment cell at a high current efficiency, together with use-ful chlorine from both cells, and the production of sodium di-thionite in the cathode compartment of the two compartment cellat a pH which is about neutral, preferably about 6 to 8, in which range the dithionite is comparatively stable, so that it may be used commercially as the aqueous solution produced, with sulfite, for ~he bleaching of wood pulp and other analogous pro-cesses.
According to another aspect of the invention there is provided an electrolytic cell system for manufacturing a dithionite, chlorine, a hydroxide and a sulfite from sulfur dioxide and a chloride which comprises a first cell comprising a housing having an anolyte compartment, containing an anode adapted to be connected to a positive terminal of an electrical input source a catholyte compartment containing a cathode, and a buffer com-partment be~ween said anolyte compartment and said catholyte compartment defined by a pair of spaced apart cation-active perm-selective membranes, and at least a second electrolysis cel.l comprising a housing having an anode compartment containing an anode adapted for connection to a positive terminal of an electrical input source; and a cathode compartment containing a cathode, said anode compartment being separated from said cathode compartment by a cation permselective membrane; said buffer compartment having a first inlet for sulfur dioxide and a first outlet for sulfite communicating with said cathode com-~5~555 partment of said second cell, said anolyte and anode compartmentseach including an inlet for chloride solution and an outle~ for gaseous chlorine, said catholyte compartment including an outlet for hydroxide, said cathode compartment including an inlet for sulfur dioxide and an outlet for sulfite and dithionite.
The invention will be readily understood by reference to the following description of an embodiment thereof, taken in conjunction with the drawing of apparatuses utilized in carrying out the inventive process.
In the drawing:
The FIGURE is a schematic representation of a pair of electrolytic cells and auxiliary equipment for producing dithionite by the method of this inve~ion.
In electrolytic cell 11, outer wall 13 and bottom 15 enclose anode 17, -cathode 19 and conductive means 21 and 23, respectively, for connecting the anode and cathode to sources of positive and negative electrical potentials, respectively.
~.. ,~
~0513S~5 Cation-active permselective membranes 25 and 27 divide the cell volume into anode or anolyte compartment 29, buffer compartment 31 and cathode or catholyte compartment 33. An acidic aqueous solution of a halide or brine is indicated as passîng into the anode compartment through line 35. Such brine is used for initial charging of the anolyte and for make-up eed, although make-up may also be added before recirculated anolyte is admit~
ted to the resaturator, to be descrihed. Also, it may be desirable to dispense with brine line 35 and charge the cell initially and ~eed make-up through the resaturator piping. The chloride solution ~or the anolyte compartment, which may be maintained at a desired acidity by add~ions of acid, e.g., HCl, by conventional means, not shown, is circulated from the anode compartment through resaturator 37 via line 39 and exits from ; 15 the resaturator through line 41, from whence it returns to the anode compartment. In a normal opera~ion, utilizing sodium chloride solution or other alkali metal chloride, ~he anolyte compartment is charged with a suitable chloride r e.g., a 25%
salt solution, and that withdrawn for r~satur~ion is at ~
lower concentration, e.g., about 22~ NaCl. Chlorine, generated in the anode compartment by electrolysis of the halide solu-tion, is taken off through line 43.
Water may be added to the cathode compar~ment 33 through piping 45 to maintain the desired level thereof ~nd of the buffer compartment. Hydrogen is removed from this . .
~5~555 s compartment through ventin~ means 47. The bufer compar~ment has sulfur dioxide and water added to it through lines 4g and -51, respectively, and alkaline sodium sulfite is taken off through piping 53, through which it is transmitted to cathode ; 5 compartment 55 of two-compartment electrolytic cell 57.
~ To increase circulation in the buffer compartment, effectively increase the volume of the compartment and to allow greater reaction times between the caustic and sulfur dioxide there may be provided a recirculation loop, for the bufer compaxtment including lines 50, 52 and 54g pump 56 and "holding tank" 58. The volume of such system may be 10 to 100,000 times that of t~.e bu-Efer compartment, preferably from 100 to 10,000 times such volume. High strength sodium hydroxide is removed from the cell through take-off piping 40, at a con-lS centration of about 20 to 30% hydroxide, as sodium hydroxide, in water, and with a low chloride content, usually less than one gram per liter of NaCl. Some of the hydroxide produced ~
in the cathode compartment 33 penetrates the cation-active perm-selective membrane 27 and passes into buffer compartment 31, wherein it reacts with the sulfur dioxide to produce sodium sulfite. The passage of the hydroxide into the buffer compart-ment is represented by arrow 42. Because of the reaction of the hydroxide in th buffer compartment and because the sulfite ion and S02 do not penetrate the membrane 25, very little h,droxide passes into the anode compa~tment 29 and thereLore, ' - . . .
~58SS5 the chlorine efficienc~ is maintained high. Also, of course, chloride ion does not pass from the anolyte into the buffer compartment, due to the repulsive effect of the permselective membrane. Additionally, the membranes and buffer zone prevent ; 5 hydrogen or other cathode-produced gases from being mixed with chlorine, preventing the production of combustible gas mixtures.
Two-compartment cell 57 has sides 59 and bottom 61 enclosing anode 63 and cathode 65, which are connected to sources of positive and negative electrical potentials, respectively, through conductive means 67 and 69. Cation-acti~7e permselective membrane 71 divides the two-compartment cell volume into anode or anolyte compartment 73 and cathode or catholyte compartment 55. Acidic aqueous halide, e.g., chloride solution or brine passes into the anode compartment through line 77 for initial charging of the anolyte andt i~
desired, for make-up feed. ~he ha-ide or chloride solution for the anolyte compartment, also maintained at desired acidity in the same manner described for the three-compart-ment cell, is taken off from the anode compartment through line 79 and passes through resaturator 81, exiting through line 83 and returning to the anode compartment. Concentrations of chloride solution taken off and returned to that compaxt-ment are about the same as with respect to the three-compart-ment cell, already described. As with the three-compartment ~ ~ .
.
~5~5~5 cell operation the use of the separate brine line may be discontinued in favor of utilization of the resaturator elements instead, to feed brine and make-up for any losses thereof. Also, i~stead of separate resaturatoxs and attendant . 5 lines a single resaturator and appropriate piping may be used to maint~ain halide concentrations in both cell anolytes.
Chlorine generated in the anode compartment of the two-compart-ment cell is removed therefrom through piping 85.
Cathode compartment 55 is~charged with gaseous sulfur dioxide through line 87 and water is added through line 89. A
mixture of dithionite and sulfite is removed via piping 91 and any hydrogen or other gases which may be produced in the cathode compartment are vented off via venting means 93.
Analogously to the buffer solution recirculation in the three-compartment cell, catholyte of the two-compartment cell may also be recirculated, utilizing lines 60, 62.and 64, tank 66 and pump 68. The ratio of the total circulating system volume to that of the cathode compartment may be from 2:1 to 100,000:1 and is preferably 100:1 to 10,000:1.
During operations of the cells high concentration, low chloride caustic is taken off from the three-compartment-cell and is ready for use in wood pulping, bleaching or other opera-tions and chlorine removed from the anode compartment of the three-cvmpartment cell is useful in the bleaching of wood pulp or for other pulp and paper mills' industrial purposes. The sulfite, produced in alkaline form due to the content ol hydroxide thorein, _ g _ ' , . - .
.
~3585~5 is converted in the two-compartment cell to dithionite and addi-tional sulfite is made by reaction of sulfur dioxide with hydroxide generated in the cathode compartment. As is clear from the diagram, the two-compartment cell also makes chlorine, useful in pulp bleaching. The sulfibe made by reaction of the sulfur dioxide with hydroxide in the cathode compartmen~ is useful in pulping operations and may be converted to white liquor after completion of bleaching of pulp by the accompanying dithionite.
The sulfur dioxide performs the important function of regulati~g the pH in the cathode compartment of the two-compartment cell so - as to maintain it in the range of 6 to 8, thereby stabilizingthe dithionite produced. Although the mechanism of the reaction . has been described, applicants should not be considered as being bound by this description, since it may also be theorized that the sulfur dioxide charged is reduced to dithionic acid, which is -then neutralized by hydroxyl present to form dithionite. In such case, the presencs of the sulfite can help to exert a buffering effect to maintain the desired pH. . .
As is illustrated schematically ~y arrow 95 the ~20 dithionite (and sulfite) ions do not penetrate the permselectivemembrane 71 and therefore, are held in the.cathode compartment 55. Similarly, halide ions, the path of which is indicated by an arrow identified by numeral 97, do not pass from the anolyte to the catholyte of the two-compartment cell. However, cations :25 such as alkali metal ions, e~g., Na+, indicated by M~ in the illustration, the direction of which is represented by the ar-row99 headed toward the right on the right side of the .
iC~S15~S55 drawing, may pass from anolyte to catholyte. A small propoxtion of hydroxyl ion may penetrate the membrane 71 but usually the concentration of free hydroxyl is low in the catholyte, due to reaction with sulfur dioxide and reduction of the pH to the 6 to 8 range, so that the hydroxyl entering the anolyte, if any, has li~tle effect on chloride current efficiency.
By the described process, utilizing a co~bination of three~compartment and two-compartment cells, the sulfur dioxide feed to the buffer compartment of the three-compartment cell ties up the sodium hydroxide penetrating the membrane between the catholyte and buffer solution and prevents it from reaching the anode; where it could be converted to useless oxygen, thereby decreasing current efficiency. At the same time, high strength, chloride-free caustic is made, which is important in various chemical operations, e.g., pulp bleaching, where chloride dls-charges from industrial plants are undesirable and may be strictly limited.
The chlorine and chloride-free caustic made are both useful chemicals for many industrial processesr including wood pulpiny and pulp bleaching. Thus, the invention has a distinct advantage over an electrolytic method for producing dithionite by charging sulfite or sulfur dioxide to a two-compartment cell and producing dithionite in the cathode compartment by reduction of sulfite or reduction of sulfur dioxide, followed by neutrali-zation to the dithionite. That is~ the sulfur dioxide which would _, ... , ,., . . . .. .... . . . . . . , ... .... .. .......... .... .. . , . . , ~
be required to make sulfite for the two-compartment cell electrolytic reaction, makes the sulfite in the buffer compart-ment of the three-compartment cell while chloride-free caustic is made in the cathode compartment, and increases chlorine current efficiency of the cell. These additional advantages improve the efficiency of the present process and make it commercially advantageous over similar or related processes.
Instead of adding suIfur dioxide to the cathode compart-ment, wherein it acts as a source of sulfite for reduction to dithionite and at the same time serves to help regulate the pH
in the desired 6 to 8 range, sulfite may be fed to the catholyte, with other means employed for pH regulation. By such a process, although the results may not be as satisfactory as with that previously described, utilizing sulfur dioxide, dithionite can be made. However, unless the means of reducing the àlkaline pH
caused by the presence of the hydroxide generated at the ca~hode is a chemical which produces a useful proauct ~and which is non-; interfering with the dithionite process), there will ~e a waste of hydroxide and po~sibly, even sreation of a disposal problem~
The halide solution fed to the anode compartment of both cells is an aqueous solution of a water soluble metal chloride in the usual case, preferably of sodium chloride. The concentra-tion thereof is generally in the range of 200 to 320 grams/liter for sodium chloride and 200 to 360 g./1. ~or potassium chloride.
Preferably such solutions contain 20 to 25% of the alkali metal .
~S85SS
halide salt, as the solutions are charged to the cell or deliver-ed to it from the resaturator. Generally the chloride content will be reduced to 5 to 30% less than the original content, preferably to 10 to 20% less and normally, as with sodium `5 chl~ride, ~he concentration of the halide xemoved from the anode compartment for resaturation and return to such compartment is about 22~, as NaCl, or equivalent. Although the anolyte may be nsutral, it is often acidified so as to be of a pH in the range of about 1 to 6, preferably 2 to 4, with acidification normally being effected with a suitable acid, such as hydrochloric acid.
Water utilized to make the initial brine charge or added as make-up feed to the ano~e compartments and watex added to the other compartments of the cells will preferably be deionized, ; containing less than 10 p.p.m. hardness, as CaCO3, although tap water of comparatively low hardnesst è.g., under 150 p.p.m., preferably under 50 p.p.m., can be used.
The sulfur dioxide charged to the buffer compartment of the three-compartment cell is usually s~bstantlally pure, e.g., over 90~ SO2, but lower concentrations thereof, e.g., as low as 20%, are usable because of the desirable attributes of the membrane material in preventing gas in~erchanges between cell portions. Thus, the unreacted gas, e.g., 2~ N2, may be removed from line 53 at a suitable point, before the sulfite produced is charged to the cathode compartment of the two-compartment cell.
In the three-compartment cell hig~ concentration .
.
.
'. ' ~0585S5 hydroxide solution, such as alkali metal hydroxide, preferably sodium hydroxide, is produced, normally of 20 to 30% hydroxide, although lesser concentrations may also be made, e.g., down to as low as 5%. The chloride content thereof is low, usually being less than 5 g./l. and often less than 1 g.fl. The concentration of the hydroxide may be regulated by controls of the rate of feed of water to the catholyte, flow of elec~ric current and, in some cases, nature o the feed to the cathode compartment (dilute caustic may somètimes be fed in at least partial replacement of water).
The sulfite produced by reaction of the sodium hydroxide and sulfur dioxide in the bu~fer compartment may be of any of various concentrations. These are controllable by regulating the feed of sulfur dioxide to the buffer compartment.
m e more sulfur dioxide charged, the greater the quantity of sulfite in the buffer effluent, in comparison to that of the hydroxide. Generally, the sulfite will be an aqueous solution of 1 to 15% strength and the hydroxide removed from the buffer compartment will also be a corresponding 15 to 1% soluticn r with more sulfite than hydroxide in the buffer compartment.
Preferably the sulfite and hydroxide concentrations total about 10 to 20%, e.g., about 15%, and in more preferred embodiments of the invention the concentration of hydroxide is maintained at less than 5~ while that of the sulfite is up to about 10%.
In the two-compartment cell the feeds to the catholyte _ 14 ~L058SSS
of sulfite-hydroxide solution from the buffer compartment and SO2 are so regulated as to maintain the desired pH for the formation of a stable dithionite. Such a pH should be in the range of about 6 to 8, preferably 6 to 8 and most preferably about 7.
~5 It may be regulated by controlling the feed of sulfur dioxide, which has the additional beneficial effect of diminishin~ the hydroxide concentration to a very small proportion, preventing all but a very minor proportion of the hydroxide generated at the cathode ~rom migrating through the membrane to the anolyte, where it could have been converted to oxygen, with a loss of electrical efficiency. The effluent from the cathode compartment is a mixture of dithionite and sulfite and the concentrations of these components are usually in the ranges of 0.5 to 30% sulfite and 0.5 to 10% dithionite. Within such ranges the noxmal ranges are from 10 to 20% of sulfite and 1 to 5~ of dithionite. The conversion of sulfite or sulfur dioxide to dithionite will usually .
be at a current efficiency of *rom about 40 to 80%, normally within the 60 to 75% range. The dithionite removed from the cathode compartment of the two-compartment cell will generally have a concentration of 10 to 70 g./l., within which range 30 to 50 g./l. is usual. From 100 to 250 g./l. will be the concen-tration of the sulfite drawn off wikh it.
To obtain the desired operation of these cells, as described, the voltage drop across the three-compartment cell is maintained at about 3 to 6 volts, preferably 4 to 5 volts .. _ ... . .. . .. . . . . . . . .. . . . .. . . . . . . .... .
~51~SS5 and that across the two-compartment cell is about 3 to 5 vol~s, pre~erably 3.S to 4.5 volts. The current density for the three-compartment cell is about 1 to 3 amperes/sq. in., preferably 1.5 to 2.5 a.s.i., and that of the two-compartment cell is 0.1 to
2 a.s.i., preferably 0.2 to 1 a.s.i. The operating temperature of the three-compartment cell is about 50 to 100C.~ pre~erably 80 to 100C., whereas that of the two-compartment cell is 3 to 40C., preferably 3 to 25C. A low temperature is desirable for operation of the two-compartment cell because of the ~reater stabilit~ of the dithionite at such low temperatures~
The anodes employed are preferably dimensionally stable anodes o~ a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides and mixtures thereof, on a valve metal, whereas the cathodes are preferably o stainless steel. Instead of the dimensionally stable anodes, anodes of noble metals or oxides thereof may also be employed, e.g~, platinum, iridium, ruthenium or rhodium. Alternatively, other anodes resistant to the anolytes can be used, although t~ey are not usually preferred. The anodes and cathodes.may be connected to sources of electrical potential by conductive metals, such as copper, silver, aluminum~ steei and iron but these materials are normally shielded fro~. contact with the electrolytes.
Pre~erable dimensionally stable anode surfaces, all on titanium or tantalum substrates, are ruthenium oxide-titanium oxide , . . . . . , . .. ....... . . . . . . . , , _ _ _ 5~555 mixtures, platinum, ruthenium, platinum oxide and mixtures of ruthenium and platinum and mixtures of their oxides. A preferred dimensionally stable anode is a ruthenium oxide-titanium dioxide mixture on a titanium substrate, connected to a source of positive electrical potential by a titanium-clad copper conductor.
The cathodes employed should be resistant to the corrosive catholyte and therefore it has been found that no~le metal, noble metal oxide and stainless steel cathodes are prefer-red. Ordinary iron or steel cathodes soon become deteriorated in use, although they may be employed for short term operations.
Graphite cathodes are not preferred because of their poorer ; conductivity and other physical properties. Of the noble metals,those previously described are satisfactory and of the stainless steels those containing small proportions of molybdenum, in addition to chromium, nickel and iron, are preferred. These include Stainless Steel Types Nos. 316 and 317. However, other stainless steels of high resistances to corrosion by the catholyte environments may also be employed, many of which may contain about 18% of chromium and 8~ of nickel. The various stainless steels from which corrosion-resistant anodes may be made are described in Section 24 of the Steel Products Manual, issed by the American,Iron and Steel Institute in February, 1949, under the heading 'IStainless and ~eat-Resisting Steels". A summary of such steel formulations and correspond-ing type numbers is found in the Handbook of Engineering Fundamentals by Eshbach, Second Edition r published in 1952 by John Wiley & Sons, Inc., New York, page 12-40 and discussions .
' . , ~ .. .. ~ .
of such s-teels and their corrosion resistances is at page 12-39. In addition to the s-tainless steels, other corrosion resistant steels such as silicon steels, nickel steels, and other conduc-tive materials resistant to corrosion may also be employed as cathode materials or surfaces.
The presently preferred cation-permselective membrane is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated ; hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and most especially th~se o;f-2 toi3lca~rbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether ~hich is most useful is that of -the formula FS02CF2-CF20~F(CF3)CF20CF=CF2- Such a j material, named as perfluoro e2-(2-fluorosulfonylethoxy)-propyl vinyl ether~, referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying the inter-nal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substi-tution of the sul-fonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer ~s~s~
is described in Example XVII o U.S. patent 3,~82,875 and an alternative method is mentioned in Canadian patent 849,670, which also discloses the use of the finished men~rane in fuel cells, characterized therein as electrochemical cells. In short, the copolymer may be made by reacting PSEPVEor equiva-lent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular wei~ht is indeter-minate but the equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400 and the percentage of PSEPVEor corresponding compound is about 10 to 3~/O preferably 15 to 2~/o and most preferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0~02 to 0.5 mm. These are then further treated to hydrolyze pendant -SO2F groups to -S03H groups, as by treating with l~/o sulfuric acid or by the methods of the patents previously mentioned.
The presence of the -S03H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian patent 752,427 and U~S. patent 3,0~1,317.
Because it has been found that some expansion ~58~i5 accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably applied to the backing polytetrafluoroethylene filaments or other suitable filaments prior to hydrolysis, when it is still thermoplastic, and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, the films becoming slightly thinner in the process, where they cover the filaments.
The membrane described is far superior in the present processes to all other previously suggested membrane materials. It is more stable at elevated temperatures, e.g., above 75CC. It lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected to chlorine at high cell temperatures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become inordinately high, as it does with many other membrane materials, when the caustic concentration in the cathode compartment increases to above about 200 g./l. of caustic. The~selectivity of the membrane and its compatibility with the electrolyte do not decrease ~s~ss detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane materi-als. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly as it does with o~her membranes when the hydroxyl ion concentration in the catholyte increases. Thus, these differences in the present process make it practicable~ whereas previously described processes have not attained commercial acceptance. While the more preferred copolymers are those having equivalent weights of 900 to 1,600, with ltlOo to 1,400 being most preferred, some useful resinous membranes produced by the present me~hod may be o~ equivalent weights from 500 to 4,003. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell operations.
Improved versions of the above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -S03H group thereon. For example, ~he sulfonic group may be altered or may be replaced in part with other moieties. Such changes may be made in the manufacturing process or after production of the membrane; ;~
When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to Q.Ol mm.
Caustic efficiencies of the inven~ed processes r usiny su~h modified versions of the present improved membranes can - increase about 3 to 20%, often about 5 to 15%. Exemplary of - 21 - ~
.
..... ~ ' :, " '' ;5S
such treatments is th~t described in French patent publication 2,i52,194, in which one side of the membrane is treated with NH3 to form SO2NH2 groups.
In addition to the copolymers previously discussed, including modifications thereof, it has been found that another type of membrane material is also superior to`prior art films for applications in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not usefu~ for making satisfactory cation-active permselective memhranes for use in the present electrolytic processes it has been established that perfluori-nated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred lS copolymers) may not be obtained,the sulfostyrenated FEP~s are surprisingly resis~ant to hardening and otherwise failing in - use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes a stanaard FEP, such as manufactured by E. I. DuPont de Nemours & Co. Inc., is styrenated and the styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 to 20~ is prepared and a sheet of FEP polymer having a thick-ness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., is dipped into the solution. Aiter removal it is sub~ected , '.
.'.
' ' - :. - ~ ... .
.. .
__ __ __ .. ~ _ ._ _ _ _ _ .. __ ... _ __ _ _ .. _ .. _ ... _ _ .. . .. _ . ... _ . . .. .. . _ .. _ ._ .. .... _ _ _ _ _ _ . _ ..
_ . ; . ...... _ .... ..
ss~
to radiation treatment, using a cohalt60 radiation source.
The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarad. After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated~ preferably in the para position, by treatment with chlorosulfonic acid, uming sulfuric acid or S03. Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about 1/2 hour.
~xamples of useful membranes made by the described process are products of RAI Research Corporation, ~auppauge, New York, identified as 18ST12S and 16ST13S, the form~r being 18~ styrenated and having 2/3 of the phenyl groups mono~
sulfonated and the latter being 16% s~yrenated and having 13/16 of the phenyl groups monosulfonated. To obtain 18% styrenation a solution of 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16% styrenation a solution of 16%
of styrene in methylene chloride is employed.
The products resulting compare favorably with ~he preferred copolymers previously described, givi~g voltage drops of about 0.2 volt each in the present cells at a current density of 2 amperes/sq~ in., the same as is obtained rom the copolymer.
- ~ ~r~ ~R~k ~ . . ' '' .' , _ 23 _,, , , . . .. . . . .. . .. . . ... . , .. .. . .: ... .... . ., .. . _, .
~L~ 8 ~S ~j The membrane walls will normally be from 0.02 to 0.5 mm.
thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm. When the membrane is mounted for support on a network oF fila-ments or fibers of polytetrafluoroethylene perfluorinated ethylene propylene polymer, polypropylene, asbestos, titanium, niobium and noble metals or other suitable network filaments, the filaments or fibers of the network will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thick-ness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thicknesses greater than that of the f;lm may also be successfully employed, e.g., 1.1 to five times the film thickness. The networks, screens or cloths have an area percentage of openings therein from about ~3 to 80%, preferably 10 to 70% and most preferably 30 to 70%.
Generally the cross-sections of the filaments will be circular but other shapes, such as elipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to the membrane it is preferred that it be fused to it by high temperature, high pressure comp-ression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fastened in place in a holder or support. It is preferred to employ the described backed membranes as walls of the cell between the anolyte and catholyte
The anodes employed are preferably dimensionally stable anodes o~ a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides and mixtures thereof, on a valve metal, whereas the cathodes are preferably o stainless steel. Instead of the dimensionally stable anodes, anodes of noble metals or oxides thereof may also be employed, e.g~, platinum, iridium, ruthenium or rhodium. Alternatively, other anodes resistant to the anolytes can be used, although t~ey are not usually preferred. The anodes and cathodes.may be connected to sources of electrical potential by conductive metals, such as copper, silver, aluminum~ steei and iron but these materials are normally shielded fro~. contact with the electrolytes.
Pre~erable dimensionally stable anode surfaces, all on titanium or tantalum substrates, are ruthenium oxide-titanium oxide , . . . . . , . .. ....... . . . . . . . , , _ _ _ 5~555 mixtures, platinum, ruthenium, platinum oxide and mixtures of ruthenium and platinum and mixtures of their oxides. A preferred dimensionally stable anode is a ruthenium oxide-titanium dioxide mixture on a titanium substrate, connected to a source of positive electrical potential by a titanium-clad copper conductor.
The cathodes employed should be resistant to the corrosive catholyte and therefore it has been found that no~le metal, noble metal oxide and stainless steel cathodes are prefer-red. Ordinary iron or steel cathodes soon become deteriorated in use, although they may be employed for short term operations.
Graphite cathodes are not preferred because of their poorer ; conductivity and other physical properties. Of the noble metals,those previously described are satisfactory and of the stainless steels those containing small proportions of molybdenum, in addition to chromium, nickel and iron, are preferred. These include Stainless Steel Types Nos. 316 and 317. However, other stainless steels of high resistances to corrosion by the catholyte environments may also be employed, many of which may contain about 18% of chromium and 8~ of nickel. The various stainless steels from which corrosion-resistant anodes may be made are described in Section 24 of the Steel Products Manual, issed by the American,Iron and Steel Institute in February, 1949, under the heading 'IStainless and ~eat-Resisting Steels". A summary of such steel formulations and correspond-ing type numbers is found in the Handbook of Engineering Fundamentals by Eshbach, Second Edition r published in 1952 by John Wiley & Sons, Inc., New York, page 12-40 and discussions .
' . , ~ .. .. ~ .
of such s-teels and their corrosion resistances is at page 12-39. In addition to the s-tainless steels, other corrosion resistant steels such as silicon steels, nickel steels, and other conduc-tive materials resistant to corrosion may also be employed as cathode materials or surfaces.
The presently preferred cation-permselective membrane is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated ; hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and most especially th~se o;f-2 toi3lca~rbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether ~hich is most useful is that of -the formula FS02CF2-CF20~F(CF3)CF20CF=CF2- Such a j material, named as perfluoro e2-(2-fluorosulfonylethoxy)-propyl vinyl ether~, referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying the inter-nal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substi-tution of the sul-fonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer ~s~s~
is described in Example XVII o U.S. patent 3,~82,875 and an alternative method is mentioned in Canadian patent 849,670, which also discloses the use of the finished men~rane in fuel cells, characterized therein as electrochemical cells. In short, the copolymer may be made by reacting PSEPVEor equiva-lent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular wei~ht is indeter-minate but the equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400 and the percentage of PSEPVEor corresponding compound is about 10 to 3~/O preferably 15 to 2~/o and most preferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0~02 to 0.5 mm. These are then further treated to hydrolyze pendant -SO2F groups to -S03H groups, as by treating with l~/o sulfuric acid or by the methods of the patents previously mentioned.
The presence of the -S03H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian patent 752,427 and U~S. patent 3,0~1,317.
Because it has been found that some expansion ~58~i5 accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably applied to the backing polytetrafluoroethylene filaments or other suitable filaments prior to hydrolysis, when it is still thermoplastic, and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, the films becoming slightly thinner in the process, where they cover the filaments.
The membrane described is far superior in the present processes to all other previously suggested membrane materials. It is more stable at elevated temperatures, e.g., above 75CC. It lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected to chlorine at high cell temperatures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become inordinately high, as it does with many other membrane materials, when the caustic concentration in the cathode compartment increases to above about 200 g./l. of caustic. The~selectivity of the membrane and its compatibility with the electrolyte do not decrease ~s~ss detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane materi-als. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly as it does with o~her membranes when the hydroxyl ion concentration in the catholyte increases. Thus, these differences in the present process make it practicable~ whereas previously described processes have not attained commercial acceptance. While the more preferred copolymers are those having equivalent weights of 900 to 1,600, with ltlOo to 1,400 being most preferred, some useful resinous membranes produced by the present me~hod may be o~ equivalent weights from 500 to 4,003. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell operations.
Improved versions of the above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -S03H group thereon. For example, ~he sulfonic group may be altered or may be replaced in part with other moieties. Such changes may be made in the manufacturing process or after production of the membrane; ;~
When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to Q.Ol mm.
Caustic efficiencies of the inven~ed processes r usiny su~h modified versions of the present improved membranes can - increase about 3 to 20%, often about 5 to 15%. Exemplary of - 21 - ~
.
..... ~ ' :, " '' ;5S
such treatments is th~t described in French patent publication 2,i52,194, in which one side of the membrane is treated with NH3 to form SO2NH2 groups.
In addition to the copolymers previously discussed, including modifications thereof, it has been found that another type of membrane material is also superior to`prior art films for applications in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not usefu~ for making satisfactory cation-active permselective memhranes for use in the present electrolytic processes it has been established that perfluori-nated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred lS copolymers) may not be obtained,the sulfostyrenated FEP~s are surprisingly resis~ant to hardening and otherwise failing in - use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes a stanaard FEP, such as manufactured by E. I. DuPont de Nemours & Co. Inc., is styrenated and the styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 to 20~ is prepared and a sheet of FEP polymer having a thick-ness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., is dipped into the solution. Aiter removal it is sub~ected , '.
.'.
' ' - :. - ~ ... .
.. .
__ __ __ .. ~ _ ._ _ _ _ _ .. __ ... _ __ _ _ .. _ .. _ ... _ _ .. . .. _ . ... _ . . .. .. . _ .. _ ._ .. .... _ _ _ _ _ _ . _ ..
_ . ; . ...... _ .... ..
ss~
to radiation treatment, using a cohalt60 radiation source.
The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarad. After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated~ preferably in the para position, by treatment with chlorosulfonic acid, uming sulfuric acid or S03. Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about 1/2 hour.
~xamples of useful membranes made by the described process are products of RAI Research Corporation, ~auppauge, New York, identified as 18ST12S and 16ST13S, the form~r being 18~ styrenated and having 2/3 of the phenyl groups mono~
sulfonated and the latter being 16% s~yrenated and having 13/16 of the phenyl groups monosulfonated. To obtain 18% styrenation a solution of 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16% styrenation a solution of 16%
of styrene in methylene chloride is employed.
The products resulting compare favorably with ~he preferred copolymers previously described, givi~g voltage drops of about 0.2 volt each in the present cells at a current density of 2 amperes/sq~ in., the same as is obtained rom the copolymer.
- ~ ~r~ ~R~k ~ . . ' '' .' , _ 23 _,, , , . . .. . . . .. . .. . . ... . , .. .. . .: ... .... . ., .. . _, .
~L~ 8 ~S ~j The membrane walls will normally be from 0.02 to 0.5 mm.
thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm. When the membrane is mounted for support on a network oF fila-ments or fibers of polytetrafluoroethylene perfluorinated ethylene propylene polymer, polypropylene, asbestos, titanium, niobium and noble metals or other suitable network filaments, the filaments or fibers of the network will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thick-ness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thicknesses greater than that of the f;lm may also be successfully employed, e.g., 1.1 to five times the film thickness. The networks, screens or cloths have an area percentage of openings therein from about ~3 to 80%, preferably 10 to 70% and most preferably 30 to 70%.
Generally the cross-sections of the filaments will be circular but other shapes, such as elipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to the membrane it is preferred that it be fused to it by high temperature, high pressure comp-ression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fastened in place in a holder or support. It is preferred to employ the described backed membranes as walls of the cell between the anolyte and catholyte
3 C~S8~SS
compartments and the buffer compartment(s) but if desired, that separating the anolyte and buffer compartments may be of conventional diaphragm material, e.g., deposited asbestos fibers or synthetic polymeric fibrous material (polytetrafluoroethylene, polypropylene).
Also, treated asbestos fibers may be utilized and such Pibers mixed with synthetic organic polymeric fibers may be employed. ~owever, when such diaphragms are used efforts should be made to remove hard-ness ions and other impurities from the feed to the cell so as to prevent these from prematurely depositing on and blocking the diaphragm5 The material of construction of the cell body may be con-ventional, including concrete or stressed concrete lined with mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, TFE polymers or other suitable plastic or may be similarly lined boxes of other structural materials. Substantially self-supporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with molded-in fibers, cloths or webs.
The processes of this invention obtain good current efficiencies for the manufacture of chlorine and acceptable ~-1 ~ , ~5~35S~
current efficiencies for producing hydroxide, sulfite and dithionite. In preferre~ embodiments of the invention, when sodium chloride is utilized and sodium sul~ite and sodium dithionite are made, the current efficienc~for the productions . 5 of chlorine in both cells ~e from 90 to 99%, usually be1ng ~4 to 97%, e.g., 96%-.The production of caustic in the ~hree-compart-ment cell, including caustic produced in the cathode compartment, whether removed therefrom or from the buffer compartment and whether removed from the buffer compartmPnt as caustic or sulfite, is at a current efficiency or sodium ion efficiency of about 70 or 75 to 90%. Approximately 5 to 50~ of the hydroxide produced in the cathode compartment migrates to the buffer compartment and usua~y this will be from 5 to 25%. In the two-compartment cell the current efficiency for the production of the dithionite will normally be from 40 to 80%, u sually 60 to 75~, with the conver-sion of sulfur dioxide or sulfite to dithionit~ being about 20 to 50%. Such efficiencies are acceptable and althou~h ~he efficiency for the manufacture of dithionite might appear low, considering that useful sulfite is also made, it is satisfactory~
The present cells may be incorporated in large ox small electrochemical plants, those producing bleaching dithionite and accompanying sulfite while also making from 20 to 1,000 tons per day of chlorine or equivalent derivative. In all cases the efficiencies obtainable are such as to make the processes economically desirable. It is highly preferred, however, that .
.
~58555 the installation should be located near to and should be used in conjunction with a groundwood or woodpulp bleaching plant so that the dithionite produced can be employed promptly as a bleach and the other chemicals may also be us~d for pulping or bleaching purposes without the need to ship ~hem long distances ~o ultimate consumers. Of course, if desired, the chlorine and caustic may be so shipped or may be chemically converted to other matexials.
In some instances the chlorine may be liquefied and the caustic may be evaporated to a higher concentration so as to facilitate shipment or transfer.
The following examples illustrate but do not limit the invention~ Unless otherwise indicated, all parts are by weight and all temperatures are in C.
lS Utilizing the apparatus illustrated in the FIGURE, useful sodium dithionite in a~ueous solution, accompanied by sodium sulfite, is produced and is successfully employed in the bleaching of groundwood pulp.
The materials of construction of the ~hree-compartment and two-compartment cells include as a preferred material, asbestos filled polypropylene. The anodes are dimensionally stable anodes of titanium having ruthenium-titanium oxide coatings. The titanium mesh-based anodes are connec~ed to sources o~ electricity by titanium-clad copper rods. The cathodes are of Typ- 316 ' ' , . " ~
' -- . .
.
~63 5~35S5 .
stainless steel. In other experiments, yielding essentiall~ the same results, the internal cell walls are of such materials as chlorinated polyethylene or chlorinated polypropylene, the anodes are o~ platinum or platinum-ir~dium alloy and the cathodes are ; 5 of Type 317 stainless steel.
The cation-active permselective membranes employed have a wall thic~ness of 7 mils (about 0.2 mm.) and the membrane portion thereof is joined to a backing or supporting network of polytetra-1uoroethylene tTe~lon~ filaments having a diameter of about 0~1 mm. and woven into cloth form such that the ar~ percentage of openings therein is of about 25%. The cross-sectional shape of the filaments is substantially circular and the membranes mounted on them are originally flat and are fused onto ~he screen or cloth by high temperature, high compression pressing, with portions of the membranes actually flowing around the filaments during the fusion processes to lock onto the cloth. The described perm-selective membranes are obtainable from E. I. Du Pon~ de Nemours and Company, Inc., Plastics Department, Wilmington, Delaware 19898, as XR Perfluorosulfonic Acid Membranes. The material thereof is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The hydrolyzed copolymer is of tetrafluoroethylene and FSO2CF2CF2OCFtCF3)CF2-OCF-CF2 and has an equivalent weight in the 1,100 to 1,40~ range, about 1,250.
Although in the FIGURE, for clarity of presentation, - ,, -;;'- ;' .; , . .
1C~585S5 the electrodes are apart from the membranes, in the practice of ~he present process the electrodes are in contact with the membranes in the three-compartment cell, with the "flatter" sides of the membranes facing the contacting electrodes. In ~he three-` 5 compartment cell the buffer compartment volume is about 10~ ofthe total of the anode and cathode compartment volumes, which are of about the same volume. In the two-compartment cell, cell volumes are about equal and the electrodes are about 1/4 inch or 6.3 mm. apart.
The feeds to the anode compartments of both cells are 25% sodium chloride solutions in water and the depleted anolytes in both cases are at 22% sodium chloride contents, with circula-tions of the depleted anolytes through the resaturators (ox a single resaturator) being controlled by sensors, valves and pumps to maintain this desired difference in concentration between feed and take-off solutions to/from the anode compartment.
In the case of the three-compartment cell the feed of sulfur dioxide to the buffer compartment is regulated so as to produce an effluent from that compartment comprising about 10% of sodium sulfite and 10% of sodium hydroxide i~ water. Water feed to the buffer compartment and wzter feed and caustic producing conditions in the cathode compartment may also be regulated to adjust the proportion of sulfite to hydroxide leaving the buffer compartment. The pH of such solution is that of the caustic, 14.
Under best operating conditions of the three-compartment cell ~he ~ 29 - . -- . . . ^. :- .
. _, .. . . .. . .. . . . . . . . ... . . . . ., . _ ... _ . _ ~ .. .. .. . . . . . _ ~L~51!~5S5 proportion of hydroxide passing from the ca~hode compartment to the buffer compartment is or averages about 25~ of that produce~
at the cathode and this ratio is in the range of 5 to 50%. The high concentration, low chloride content hydroxide taken off from ;s 5 the cathode compartment is a 25% hydroxide and has a chloride content of about 0.05~. The temperature of the electrolyte is maintained at about 90C. during the process, with 4.5 volts impressed across the electrodes and a current density of 2 a.s.i., the current flow being 90 kiloamperes.
In the two-compartment cell khe feed to the catholyte is the effluent from the buffer compartment of a three-compartment cell and preferably it is cooled en route by cooling means, not illustrated in the drawing, so as ~o enter the cathode compartment of the two-compartment cell at the desired cell temperature r about 20C. (within a range of 15 to 35C.), Sulfur dioxide is added to the cathode compartment at such a rate as to maintain the pH
of the catholyte at 7, although it may vary between 6 and 8.
Under flow rates described,about 60% of the cathodic current is utilized in the production of dithionite and about 40% to make sulfite from hydroxide and sulfur dioxide. The effluent from the cathode compartment is an aqueous solution containing 16% of sodium sulfite and 3,7~ of sodium dithionite.
The installation described produ~es 0.36 ton per day of sodium dithionite, in a 3.7~ concentration aqueous solution, with 33% conversion of sulfur dioxidP to dithionite and with the 1~35~S~5 dithionite obtained at a 75c current efficiency, calculated on the basis of useful products obtained. The chlorine produced from the two-compartment cell is at the rate of 0.3 ton per day and the current efficiency is 95~. With respect with the three-compartment S cell, the chl~rine production is at ~he rate of 3 tons per day, also with a 95% current efficiency. The sodium hydroxide taken off the cathode ~ompartment of the three-compartment cell is produced at the rate of 2.28 tons per day and is in 25% aqueous solution. The sulfur dioxide feed to the buffer compartment cell is 0.49 ton per day with production of sodium sulfite from ~hat compartment being at 0.97 ton per day and with 0.39 ton per day of sodium hydroxide accompanying it. Current efficiency for the production of sulfite and hydroxide in the three-compartment cell, or sodium ion efficiency, is about 90%.
The solution of dithionite and sodium sulfite from the cathode compartment of the t~o-compartment cell is continuously employed to bleach groundwood pulp, aft r ailution to a 1%
dithionite solution. The groundwood charge is an 85.15 mixture of West Coast hemlock and balsam, the rate of applicatior. is 1.1% of sodium dithionite, on a dry pulp basis and the pulp is in - a 3~ aqueous slurry buffered to a pH of about 6.5 with potassium hydrogen phosphate before addition of the dithionite. A b~ight-ness increase of about 10 units is obtained at a brightening temperature of 60-70C. after about 30 minutes treatment.
Reversion in such cases is about 2 units.
' .
1~5~55~
The bleach liquor is recovered and mixed with black liquor which is subsequently converted to white liquor used in pulping.
E_AMPLE 2 The procedure of Example 1 is followed except for the addition of sulfur dioxide to the catholyte of the two-compart-ment cell. Instead of the sulfur divxide, additional sulfi.te is added and the desired pH of 7 is maintained in the cathode compart-ment by continuous addition of sulfuric acid, sodium bisulfatP, sodium bisulfite or any other suitable acidic or alkaline neutralizing agent or buffer. A~though the current efficiency is not as good as in the processes utilizing a sulfur dioxide feed to the catholyte of the two-compartment cell, thè pxocess is operative and production of dithionite and other product is at essentially the same rate as previously described. The dithionite solution obtained is effective for groundwood bleaching, as described in Example 1, and is useful for other bleaching purposes, too.
In variations of this process and that of Example 1 the sulfur dioxide is fed to the buffer compartment of the three-compartment cell and to the cathode compartment of the two-compartment cell as aqueous solutions containing about 8% of sulfur dioxide. Utilizing the solutions fewer problems of gas bubbling and interference with electrode reactions are experienced .-~L~58S~S
but weaker product is obtained. In other modifications of the experiments, batch and continuous processes are employed. The continuous processes, sometimes with recycles o~ each of the compartment contents, are generally superior, yielding a more consistent product a n d readily lending themselves to automatic control.
The invention has been described with respect ko working examples and illustrative embodiments but is not to be limited to these because it is evident that one of ordinary skill in the art will be able to utilize substitutes and equivalents without departing from the spirit of the invention or the scope of the claims.
- ' ' ' '~ '~
, - 33 - ~
'; ' ' .
compartments and the buffer compartment(s) but if desired, that separating the anolyte and buffer compartments may be of conventional diaphragm material, e.g., deposited asbestos fibers or synthetic polymeric fibrous material (polytetrafluoroethylene, polypropylene).
Also, treated asbestos fibers may be utilized and such Pibers mixed with synthetic organic polymeric fibers may be employed. ~owever, when such diaphragms are used efforts should be made to remove hard-ness ions and other impurities from the feed to the cell so as to prevent these from prematurely depositing on and blocking the diaphragm5 The material of construction of the cell body may be con-ventional, including concrete or stressed concrete lined with mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, TFE polymers or other suitable plastic or may be similarly lined boxes of other structural materials. Substantially self-supporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with molded-in fibers, cloths or webs.
The processes of this invention obtain good current efficiencies for the manufacture of chlorine and acceptable ~-1 ~ , ~5~35S~
current efficiencies for producing hydroxide, sulfite and dithionite. In preferre~ embodiments of the invention, when sodium chloride is utilized and sodium sul~ite and sodium dithionite are made, the current efficienc~for the productions . 5 of chlorine in both cells ~e from 90 to 99%, usually be1ng ~4 to 97%, e.g., 96%-.The production of caustic in the ~hree-compart-ment cell, including caustic produced in the cathode compartment, whether removed therefrom or from the buffer compartment and whether removed from the buffer compartmPnt as caustic or sulfite, is at a current efficiency or sodium ion efficiency of about 70 or 75 to 90%. Approximately 5 to 50~ of the hydroxide produced in the cathode compartment migrates to the buffer compartment and usua~y this will be from 5 to 25%. In the two-compartment cell the current efficiency for the production of the dithionite will normally be from 40 to 80%, u sually 60 to 75~, with the conver-sion of sulfur dioxide or sulfite to dithionit~ being about 20 to 50%. Such efficiencies are acceptable and althou~h ~he efficiency for the manufacture of dithionite might appear low, considering that useful sulfite is also made, it is satisfactory~
The present cells may be incorporated in large ox small electrochemical plants, those producing bleaching dithionite and accompanying sulfite while also making from 20 to 1,000 tons per day of chlorine or equivalent derivative. In all cases the efficiencies obtainable are such as to make the processes economically desirable. It is highly preferred, however, that .
.
~58555 the installation should be located near to and should be used in conjunction with a groundwood or woodpulp bleaching plant so that the dithionite produced can be employed promptly as a bleach and the other chemicals may also be us~d for pulping or bleaching purposes without the need to ship ~hem long distances ~o ultimate consumers. Of course, if desired, the chlorine and caustic may be so shipped or may be chemically converted to other matexials.
In some instances the chlorine may be liquefied and the caustic may be evaporated to a higher concentration so as to facilitate shipment or transfer.
The following examples illustrate but do not limit the invention~ Unless otherwise indicated, all parts are by weight and all temperatures are in C.
lS Utilizing the apparatus illustrated in the FIGURE, useful sodium dithionite in a~ueous solution, accompanied by sodium sulfite, is produced and is successfully employed in the bleaching of groundwood pulp.
The materials of construction of the ~hree-compartment and two-compartment cells include as a preferred material, asbestos filled polypropylene. The anodes are dimensionally stable anodes of titanium having ruthenium-titanium oxide coatings. The titanium mesh-based anodes are connec~ed to sources o~ electricity by titanium-clad copper rods. The cathodes are of Typ- 316 ' ' , . " ~
' -- . .
.
~63 5~35S5 .
stainless steel. In other experiments, yielding essentiall~ the same results, the internal cell walls are of such materials as chlorinated polyethylene or chlorinated polypropylene, the anodes are o~ platinum or platinum-ir~dium alloy and the cathodes are ; 5 of Type 317 stainless steel.
The cation-active permselective membranes employed have a wall thic~ness of 7 mils (about 0.2 mm.) and the membrane portion thereof is joined to a backing or supporting network of polytetra-1uoroethylene tTe~lon~ filaments having a diameter of about 0~1 mm. and woven into cloth form such that the ar~ percentage of openings therein is of about 25%. The cross-sectional shape of the filaments is substantially circular and the membranes mounted on them are originally flat and are fused onto ~he screen or cloth by high temperature, high compression pressing, with portions of the membranes actually flowing around the filaments during the fusion processes to lock onto the cloth. The described perm-selective membranes are obtainable from E. I. Du Pon~ de Nemours and Company, Inc., Plastics Department, Wilmington, Delaware 19898, as XR Perfluorosulfonic Acid Membranes. The material thereof is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The hydrolyzed copolymer is of tetrafluoroethylene and FSO2CF2CF2OCFtCF3)CF2-OCF-CF2 and has an equivalent weight in the 1,100 to 1,40~ range, about 1,250.
Although in the FIGURE, for clarity of presentation, - ,, -;;'- ;' .; , . .
1C~585S5 the electrodes are apart from the membranes, in the practice of ~he present process the electrodes are in contact with the membranes in the three-compartment cell, with the "flatter" sides of the membranes facing the contacting electrodes. In ~he three-` 5 compartment cell the buffer compartment volume is about 10~ ofthe total of the anode and cathode compartment volumes, which are of about the same volume. In the two-compartment cell, cell volumes are about equal and the electrodes are about 1/4 inch or 6.3 mm. apart.
The feeds to the anode compartments of both cells are 25% sodium chloride solutions in water and the depleted anolytes in both cases are at 22% sodium chloride contents, with circula-tions of the depleted anolytes through the resaturators (ox a single resaturator) being controlled by sensors, valves and pumps to maintain this desired difference in concentration between feed and take-off solutions to/from the anode compartment.
In the case of the three-compartment cell the feed of sulfur dioxide to the buffer compartment is regulated so as to produce an effluent from that compartment comprising about 10% of sodium sulfite and 10% of sodium hydroxide i~ water. Water feed to the buffer compartment and wzter feed and caustic producing conditions in the cathode compartment may also be regulated to adjust the proportion of sulfite to hydroxide leaving the buffer compartment. The pH of such solution is that of the caustic, 14.
Under best operating conditions of the three-compartment cell ~he ~ 29 - . -- . . . ^. :- .
. _, .. . . .. . .. . . . . . . . ... . . . . ., . _ ... _ . _ ~ .. .. .. . . . . . _ ~L~51!~5S5 proportion of hydroxide passing from the ca~hode compartment to the buffer compartment is or averages about 25~ of that produce~
at the cathode and this ratio is in the range of 5 to 50%. The high concentration, low chloride content hydroxide taken off from ;s 5 the cathode compartment is a 25% hydroxide and has a chloride content of about 0.05~. The temperature of the electrolyte is maintained at about 90C. during the process, with 4.5 volts impressed across the electrodes and a current density of 2 a.s.i., the current flow being 90 kiloamperes.
In the two-compartment cell khe feed to the catholyte is the effluent from the buffer compartment of a three-compartment cell and preferably it is cooled en route by cooling means, not illustrated in the drawing, so as ~o enter the cathode compartment of the two-compartment cell at the desired cell temperature r about 20C. (within a range of 15 to 35C.), Sulfur dioxide is added to the cathode compartment at such a rate as to maintain the pH
of the catholyte at 7, although it may vary between 6 and 8.
Under flow rates described,about 60% of the cathodic current is utilized in the production of dithionite and about 40% to make sulfite from hydroxide and sulfur dioxide. The effluent from the cathode compartment is an aqueous solution containing 16% of sodium sulfite and 3,7~ of sodium dithionite.
The installation described produ~es 0.36 ton per day of sodium dithionite, in a 3.7~ concentration aqueous solution, with 33% conversion of sulfur dioxidP to dithionite and with the 1~35~S~5 dithionite obtained at a 75c current efficiency, calculated on the basis of useful products obtained. The chlorine produced from the two-compartment cell is at the rate of 0.3 ton per day and the current efficiency is 95~. With respect with the three-compartment S cell, the chl~rine production is at ~he rate of 3 tons per day, also with a 95% current efficiency. The sodium hydroxide taken off the cathode ~ompartment of the three-compartment cell is produced at the rate of 2.28 tons per day and is in 25% aqueous solution. The sulfur dioxide feed to the buffer compartment cell is 0.49 ton per day with production of sodium sulfite from ~hat compartment being at 0.97 ton per day and with 0.39 ton per day of sodium hydroxide accompanying it. Current efficiency for the production of sulfite and hydroxide in the three-compartment cell, or sodium ion efficiency, is about 90%.
The solution of dithionite and sodium sulfite from the cathode compartment of the t~o-compartment cell is continuously employed to bleach groundwood pulp, aft r ailution to a 1%
dithionite solution. The groundwood charge is an 85.15 mixture of West Coast hemlock and balsam, the rate of applicatior. is 1.1% of sodium dithionite, on a dry pulp basis and the pulp is in - a 3~ aqueous slurry buffered to a pH of about 6.5 with potassium hydrogen phosphate before addition of the dithionite. A b~ight-ness increase of about 10 units is obtained at a brightening temperature of 60-70C. after about 30 minutes treatment.
Reversion in such cases is about 2 units.
' .
1~5~55~
The bleach liquor is recovered and mixed with black liquor which is subsequently converted to white liquor used in pulping.
E_AMPLE 2 The procedure of Example 1 is followed except for the addition of sulfur dioxide to the catholyte of the two-compart-ment cell. Instead of the sulfur divxide, additional sulfi.te is added and the desired pH of 7 is maintained in the cathode compart-ment by continuous addition of sulfuric acid, sodium bisulfatP, sodium bisulfite or any other suitable acidic or alkaline neutralizing agent or buffer. A~though the current efficiency is not as good as in the processes utilizing a sulfur dioxide feed to the catholyte of the two-compartment cell, thè pxocess is operative and production of dithionite and other product is at essentially the same rate as previously described. The dithionite solution obtained is effective for groundwood bleaching, as described in Example 1, and is useful for other bleaching purposes, too.
In variations of this process and that of Example 1 the sulfur dioxide is fed to the buffer compartment of the three-compartment cell and to the cathode compartment of the two-compartment cell as aqueous solutions containing about 8% of sulfur dioxide. Utilizing the solutions fewer problems of gas bubbling and interference with electrode reactions are experienced .-~L~58S~S
but weaker product is obtained. In other modifications of the experiments, batch and continuous processes are employed. The continuous processes, sometimes with recycles o~ each of the compartment contents, are generally superior, yielding a more consistent product a n d readily lending themselves to automatic control.
The invention has been described with respect ko working examples and illustrative embodiments but is not to be limited to these because it is evident that one of ordinary skill in the art will be able to utilize substitutes and equivalents without departing from the spirit of the invention or the scope of the claims.
- ' ' ' '~ '~
, - 33 - ~
'; ' ' .
Claims (16)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for electrolytically manufacturing a dithionite, chlorine, a hydroxide and a sulfite from sulfur dioxide and a chloride which comprises feeding chloride solution to the anode compartment of an electrolytic cell having anode, buffer and cathode compartments separated by cation-active permselective membranes, an anode in the anode compartment and a cathode in the cathode compartment, and feeding sulfur dioxide to the buffer compartment, withdrawing chlorine from the anode compartment, hydroxide from the cathode compartment and sulfite from the buffer compartment, feeding such sulfite and sulfur dioxide to the cathode compartment of a two-compartment electrolytic cell having an anode in an anode compartment, a cathode in a cathode compartment and a cation-active permselective membrane dividing the compartments maintaining the catholyte at pH 6-8, feeding chloride to the anode compartment thereof and withdrawing chlorine from the anode compartment and dithionite and sulfite from the cathode compartment.
2. A method according to claim 1 wherein the material of the cation-active permselective membranes is selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydro-carbon and a fluorosulfonated perfluorovinyl ether, and a sulfo-styrenated perfluorinated ethylene propylene polymer, the cells employed are three- and two-compartment cells, and the pH of the catholyte of the two-compartment cell is in the range of about 6 to 8.
3. A method according to claim 2 wherein the perm-selective membrane is of a hydrolyzed copolymer of tetrafluoro-ethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2, which copolymer has an equivalent weight of about 900 to 1,600.
4. A method according to claim 3 wherein the voltage drop across the three compartment cell is about 3 to 6 volts, that across the two-compartment cell is about 3 to 5 volts, the current density for the three-compartment cell is about 1 to 3 amperes/sq. in., that for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and the operating temperature of the three-compartment cell is about 50 to 100°C. and that of the two-compartment cell is about 3 to 40°C.
5. A method according to claim 4 wherein the membrane walls are from about 0.02 to about 0.5 mm. thick, the membranes are mounted on a network screen or cloth of filaments of a material selected from the group consisting of polytetrafluoro-ethylene, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to about 80%, with the filaments having a thickness of about 0.01 to about 0.5 mm.
6. A method according to claim 5 wherein the voltage drop across the three-compartment cell is from 4 to 5 volts, that across the two-compartment cell is from 3.5 to 4.5 volts, the current density in the three-compartment cell is from about 1.5 to 2.5 amperes/sq. in., the current density in the two-compart-ment cell is 0.2 to 1 ampere/sq. in., the operating temperature of the three-compartment cell is 80 to 100°C., the operating temperature of the two-compartment cell is 3 to 25°C., the feed to the anode compartment of the three compartment cell is a chloride solution containing 20 to 25% of chloride, the hydroxide removed from the cathode compartment of that cell is an aqueous solution at a concentration of 20 to 30% hydroxide, the sulfite withdrawn from the buffer compartment of the same cell is an aqueous solution at a concentration of 1 to 15% sulfite and accompanying it is sodium hydroxide, at a concentration of 15 to 1% hydroxide, the chloride feed to the anolyte compartment of the two-compartment cell is essentially the same as that of the feed to such compartment of the three-compartment cell, and the dithionite and sulfite removed from the catholyte compartment of the two-compartment cell are in aqueous solution at a concen-tration of 10 to 70 g./l. of the dithionite and 100 to 250 g./l. of the sulfite.
7. A method according to claim 6 wherein the anodes are dimensionally stable anodes of material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, and the cathode is stainless steel.
8. A method according to claim 7 wherein the chloride is sodium chloride, the hydroxide is sodium hydroxide, the sulfite is sodium sulfite and the dithionite produced is sodium dithionite, the anolytes are recirculated and the depleted anolytes are increased in concentration to about 25% NaC1, at which concentration they are fed to the anode compartments, by dissolving solid sodium chloride therein.
9. A method according to claim 8 wherein the membrane copolymer equivalent weight is from 1,100 to 1,400, the membrane wall thickness is 0.1 to 0.3 mm., the anode is ruthenium oxide on titanium, the pH's of the anolytes are about 2 to 4 and the dithionite withdrawn is in an aqueous solution with sodium sulfite, wherein the dithionite concentration is from 30 to 50 g./l.
_ 3?
_ 3?
10. A method for electrolytically manufacturing a dithionite, chlorine and a hydroxide from sulfur dioxide and a chloride which comprises feeding chloride solution to the anode compartment of an electrolytic cell having anode, buffer and cathode compartments separated by cation-active permselective membranes, an anode in the anode compartment and a cathode in the cathode compartment, and feeding sulfur dioxide to the buffer compartment, withdrawing chlorine from the anode compart-ment, hydroxide from the cathode compartment and sulfite from the buffer compartment, feeding such sulfite to the cathode compart-ment of a two-compartment electrolytic cell having an anode in an anode compartment, a cathode in a cathode compartment and a cation-active permselective membrane dividing the compartments, maintaining the pH in the cathode compartment of the two-compartment electrolytic cell at about 6 to 8, feeding chloride to the anode compartment of such cell and withdrawing chlorine from the anode compartment and dithionite from the cathode com-partment.
11. A method according to claim 10 wherein the cation-active permselective membranes are selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydro-carbon and a f1uorosulfonated perfluorovinyl ether, and a sulfo-styrenated perfluorinated ethylene propylene polymer, the wall thickness of the membranes is from about 0.02 to 0.5 mm. the hydroxide produced in the cathode compartment of the three-compartment cell is of a high concentration and chloride-free, the sulfite is of a concentration of 1 to 15%, the voltage drop across the three-compartment cell is about 3 to 6 volts, that across the two-compartment cell is about 3 to 5 volts, the current density for the three-compartment cell is about 1 to 3 amperes/sq. in., that for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and the operating temperature of the three-compartment cell is about 50 to 100°C. and that of the two-compartment cell is about 3 to 40°C.
12. A method according to claim 11 wherein the perm-selective membrane is of a hydrolyzed copolymer of tetrafluoro-ethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2, which copolymer has an equivalent weight of about 1,100 to 1,400, the membrane thickness is from 0.1 to 0.3 mm., the anodes are dimensionally stable anodes of material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, the cathodes are stainless steel, the chloride, hydroxide, sulfite and dithionite are sodium salts, the sodium chloride is charged to the anode compartments of the cells in an aqueous solution at a concentration of about 20 to 25% NaC1 and the dithionite produced is in aqueous solution at a concentration of 5 to 50 g./l.
13. An electrolytic cell system for manufacturing a di-thionite, chlorine, a hydroxide and a sulfite from sulfur dioxide and a chloride which comprises a first cell comprising a housing having an anolyte compartment, containing an anode adapted to be connected to a positive terminal of an electrical input source;
a catholyte compartment containing a cathode; and a buffer com-partment between said anolyte compartment and said catholyte compartment defined by a pair of spaced apart cation-active permselective membranes and at least a second electrolysis cell comprising a housing having an anode compartment containing an anode adapted for connection to a positive terminal of an electrical input source, and a cathode compartment containing a cathode, said anode compartment being separated from said cathode compartment by a cation permselective membrane; said buffer compartment having a first inlet for sulfur dioxide and a first outlet for sulfite communicating with said cathode compartment of said second cell, said anolyte and anode compart-ments each including an inlet for chloride solution and an out-let for gaseous chlorine, said catholyte compartment including an outlet for hydroxide; said cathode compartment including an inlet for sulfur dioxide and an outlet for sulfite and dithionite.
a catholyte compartment containing a cathode; and a buffer com-partment between said anolyte compartment and said catholyte compartment defined by a pair of spaced apart cation-active permselective membranes and at least a second electrolysis cell comprising a housing having an anode compartment containing an anode adapted for connection to a positive terminal of an electrical input source, and a cathode compartment containing a cathode, said anode compartment being separated from said cathode compartment by a cation permselective membrane; said buffer compartment having a first inlet for sulfur dioxide and a first outlet for sulfite communicating with said cathode compartment of said second cell, said anolyte and anode compart-ments each including an inlet for chloride solution and an out-let for gaseous chlorine, said catholyte compartment including an outlet for hydroxide; said cathode compartment including an inlet for sulfur dioxide and an outlet for sulfite and dithionite.
14. A cell system according to claim 13, wherein said anolyte and anode compartments include recirculation means for re-circulating anolyte liquor in said compartments, said recirculation means including resaturating means for increasing the chloride concentration to maintain the chloride concentration in said anolyte and anode compartments, said buffer compartment includes an inlet and outlet communicating with a first recirculation loop effective to allow a greater reaction time for sulfite production; said first recirculation loop having a volume 10 to 10,000 times that of the buffer compartment, said cathode com-partment including an inlet and outlet communicating with a second recirculation loop effective to allow a greater reaction time for dithionite and sulfite production, and said second recirculation loop is designed such that a ratio of the total circulating system volume to that of the cathode compartment volume is from 2:1 to 100,000:1.
15. A cell system according to claim 13 or 14 wherein the material of the cation-active permselective membranes is selected from the group consisting of a hydrolyzed copolymer of a perfluori-nated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, said first cell is a three compartment cell and said second cell is a two compartment cell.
16. A cell system according to claim 13 or 15, wherein said permselective membranes of said first and second cells are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2;
said copolymer having an equivalent weight of about 900 to 1600.
said copolymer having an equivalent weight of about 900 to 1600.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US411613A US3905879A (en) | 1973-11-01 | 1973-11-01 | Electrolytic manufacture of dithionites |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1058555A true CA1058555A (en) | 1979-07-17 |
Family
ID=23629632
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA212,484A Expired CA1058555A (en) | 1973-11-01 | 1974-10-29 | Electrolytic method for the manufacture of dithionites |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US3905879A (en) |
| CA (1) | CA1058555A (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4166014A (en) * | 1973-12-27 | 1979-08-28 | Tokuyama Soda Kabushiki Kaisha | Electrolytic diaphragms, and method of electrolysis using the same |
| US4124477A (en) * | 1975-05-05 | 1978-11-07 | Hooker Chemicals & Plastics Corp. | Electrolytic cell utilizing pretreated semi-permeable membranes |
| DE2646825A1 (en) * | 1976-10-16 | 1978-04-20 | Basf Ag | PROCESS FOR THE CONTINUOUS PRODUCTION OF SODIUM DITHIONITE SOLUTIONS BY CATHODIC REDUCTION |
| US4213833A (en) * | 1978-09-05 | 1980-07-22 | The Dow Chemical Company | Electrolytic oxidation in a cell having a separator support |
| FR2485395B1 (en) * | 1980-06-24 | 1986-04-11 | Commissariat Energie Atomique | CATION EXCHANGE MEMBRANE, MANUFACTURING METHOD THEREOF AND APPLICATION AS A SOLID ELECTROLYTE |
| US4976835A (en) * | 1988-03-08 | 1990-12-11 | Hoechst Celanese Corporation | Electrosynthesis of sodium dithionite |
| US5126018A (en) * | 1988-07-21 | 1992-06-30 | The Dow Chemical Company | Method of producing sodium dithionite by electrochemical means |
| DE19954299A1 (en) * | 1999-11-11 | 2001-05-17 | Eilenburger Elektrolyse & Umwelttechnik Gmbh | Simultaneous production of sodium peroxodisulfate and sodium dithionite, useful as bleach, e.g. for pulp, involves anodic oxidation of sodium sulfate in cell with cation exchange membrane and sodium dithionite formation in cathode chamber |
| JP6368374B2 (en) * | 2013-10-23 | 2018-08-01 | ネマスカ リチウム インコーポレーテッド | Method and system for preparing lithium hydroxide |
| US20160375402A1 (en) * | 2015-06-23 | 2016-12-29 | Anni Environmental, Llc | Method And Apparatus For Chemical Removal Of Acid Gas From Combustion Emissions And Natural Gas Streams |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3220941A (en) * | 1960-08-03 | 1965-11-30 | Hooker Chemical Corp | Method for electrolysis |
| US3344053A (en) * | 1964-05-04 | 1967-09-26 | Dow Chemical Co | Chlorine cell |
| US3523755A (en) * | 1968-04-01 | 1970-08-11 | Ionics | Processes for controlling the ph of sulfur dioxide scrubbing system |
| US3784399A (en) * | 1971-09-08 | 1974-01-08 | Du Pont | Films of fluorinated polymer containing sulfonyl groups with one surface in the sulfonamide or sulfonamide salt form and a process for preparing such |
-
1973
- 1973-11-01 US US411613A patent/US3905879A/en not_active Expired - Lifetime
-
1974
- 1974-10-29 CA CA212,484A patent/CA1058555A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US3905879A (en) | 1975-09-16 |
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