CA1044637A - Electrolysis of alkali metal chloride - Google Patents
Electrolysis of alkali metal chlorideInfo
- Publication number
- CA1044637A CA1044637A CA200,273A CA200273A CA1044637A CA 1044637 A CA1044637 A CA 1044637A CA 200273 A CA200273 A CA 200273A CA 1044637 A CA1044637 A CA 1044637A
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- alkali metal
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Abstract
ABSTRACT OF THE DISCLOSURE
Alkali metal hydroxide is produced by electrolysis of alkali metal chloride according to an improved diaphragm process wherein plural cationic ion-exchange membranes are used to divide an electrolytic cell into an anode compartment, at least one middle compartment and a cathode compartment. The ion-exchange membrane confronting the anode is preferably sub-stantially resistant to anodic reaction products such as chlorine or chlorates. Alkali formed by electrolysis is recovered from the cathode compartment or alternatively may be taken from one or more middle compartments at various con-centrations. Alkali metal hydroxide can be produced in high purity and high concentration with excellent efficiency.
Alkali metal hydroxide is produced by electrolysis of alkali metal chloride according to an improved diaphragm process wherein plural cationic ion-exchange membranes are used to divide an electrolytic cell into an anode compartment, at least one middle compartment and a cathode compartment. The ion-exchange membrane confronting the anode is preferably sub-stantially resistant to anodic reaction products such as chlorine or chlorates. Alkali formed by electrolysis is recovered from the cathode compartment or alternatively may be taken from one or more middle compartments at various con-centrations. Alkali metal hydroxide can be produced in high purity and high concentration with excellent efficiency.
Description
`
1S~44637 , "
'- This invention relates to a process for producing ';
. . .
' alkali metal hydroxides, chlorine and hydrogen by electrolysis of aqueous alkali metal chlori~e solutio~s, More particularly, '~ it relates to a process for producing aqueous alkali metal hydroxide solutions (hereinafter referred to as "aqueous alkali `~
.~ .
` solution") efficiently in a high concentration with low sodium . j -.: :
chloride content by the use of cation permselective ion-exchange membranes. `'-~
The we~l known mercury and diaphragm processes have been practiced for production of industrially important alkalis '' ';' and chlorine. These processes have recognized advantages and -' ' disadvantages a~ discussed below. The diaphragm process is ~i~
advantageous for use in small floor areas because an upright ~'~ type system is adopted, and also because of its low electric power ' consumption. On the other hand, the process requires a step for removal of alkali halide, normally sodium chloride, which `;
is ~ in a mixed solution of alkali. Ordinarily, the ;" ' procedure produces a product which is contaminated with sodium ' chloride to the extent of 1 to 2%, so that further purification ~¦~ 20 is required. ' '' The mercury process can produce high purity prod-ucts, but requires large floor areas because mercury flowing ,~,j : .
;;! on a horizontal plane is used as an electrode. Moreover, elec-'~ tric power consumption is high ! In addition, because mercur~ '' is a harmful substance, equipment for protection against leak-age is required. '`-' The use of ion-exchange membranes for obtainin~
high purity alkalis with high efficiency in upright instal-Y, ~S .:
~'' lation systems requiring no mercury ~ an attractive alternative~ "
:~, ~ 30 However, this process has not been used successfully up to this c ~ time. One of the reasons is the inadequacy of presently a~ail-; able ion-exchange membranes for producing NaOH at high concen-. 1 .,, rW/"~?'/
i~44~3~
tration with good efficiency. ~ .
. An object of the present invention is to provide a . process for producing alkali metal hydroxides in high concen~
. tration and high purity with good efficiency by the use of ~.
ion-exchange membranes. `~
In one particular aspect the presen.t.invention . provides a continuous process for the electrolysis of an alkali i ::
~- metal chloride which comprises: 1) dividing an electrolytic . ~
- cell into an anode and a cathode compartment with at least one .::
middle compartment, the compartments being separated one from ~ ~
. ~, .
the other by cK~ cationic io~-exchange membranes; 2) pre~
: ~
:.: charging aqueous alkali metal chloride solution of a first `
concentration into the anode compartment; 3) precharging :-; aqueous alkali metal hydroxide so~ution having the same alkali ~ -.
;;l metal as the alkali metal chloride solution into the first .
~; middle compartment at a second concentration, the ratio of the second concentration ~o the first concentration being such as i to maintain a preselected current efficiency during electrolysis; :.~
^~ 4) precharging the same aqueous metal hydroxide solution into ;~ .
,:,, ' :
.~ 20 the cathode compartment at a third concentration which is - .
~! greater than the second concentration and selected so that the .
!3' ratio of the second concentration to the third concentration is such as to ma~intain the preselected current efficiency during ;:: electrolysis; and 5) applying a voltage across the electrolytic .~.' cell while continuously supplying aqueous alkali metal chloride ~. :
`. to the. anode compartment at the first concentration, and ?
~ continuously adding water to, while withdrawing alkali metal ` :
.;,~ . .
:.; hydroxide solution from, the cathode compartment at rates so as `.
to maintain the third concentration at a constant value.
As one modification of the above process, water is also supplied to the middle compartments, thereby making it pos- .
. sible to recover alkali metal hydroxides of various concentrations .
~ 2 L~' ` - , r~
i~L~ 4/~37 ~ -from said middle compartments. -.. . . . .
~; In order that the invention may be clearly understood and readily carried into effect, embodiments thereof will now be described with reference to the accompanying drawings, in which:
IG. 1 shows the relation between caustic soda con~
centration at the cathode side and at the anode side, of a cationic ion-exchange membrane, required to keep the current effeciency at certa~n constant values;
FIG. 2 shows the relation between the caustic soda '-.. .. . .
concentration at the cathode side of a cationic ion-exchange membrane and the current efficiency; and FIG. 3 shows a schematic diagram of one embodiment =~
of an electrolytic cell which is suitable for practicing the ~ process of the present invention, wherein three cationic ion-,~ ~ exchange membranes are used~
When aqueous alkali solutions such as aqueous caustic ; ~ soda solutions having diferent concentrations are separated by a cationic ion-exchange membrane and electrodes provided at ~.,. . ,.,.~ ~
~20 - both sides, the current efficiency of the current flow there-between depends on the transport numbers of sodium ion and - ~ hydroxide ion in the cationic ion-exchange membrane as~well as on Z~ the diffusion of caustic soda th~ough them. The former is in-fluehced greatly by the concentration of caustic soda at the :
cathode side, the latter by the difference in concentrations of caustic soda at both sides of the cationic ion-exchange membrane.
Accordingly, in order to increase the concentration of aqueous caustic soda at the cathode side, while maintaining a high current efficiency, a very high concentration aqUQOUs caustic soda solu-~.Z . ~.
~Z 30 tion (although, of course, lower than the concentration at the ; .~ ' ; cathode side) may be charged in the anode s~de, which is in `~
'''! contact with the cathode side via the cationic ion-exchange .. . . : -:
.''. ~ , .
... .
,.,: _ , 3 ~ ~
membrane, to reduce the loss by diffusion. :~:
Referring now to FIG.l which shows the relation . ~
.. " . ~ .
determined by experiments between caustic soda concentration . ~ . .
:~ at the cathode side and that at the anode side which is required :~
. to ma~ntain current efficiency at certain constant values, the urves (a),(b),(c) and (d) denote relations between caustic .
. soda concentration at the cathode side and that at the anode ;.
side, when the current efficiencies of the cationic ion-exchange .~
. are 95~, 90~, 80% and 70%, respectively! The line (e) is a :: .
; 10 straight line passing the origin with a gradient of 45~. FIG. ~ .
1S~44637 , "
'- This invention relates to a process for producing ';
. . .
' alkali metal hydroxides, chlorine and hydrogen by electrolysis of aqueous alkali metal chlori~e solutio~s, More particularly, '~ it relates to a process for producing aqueous alkali metal hydroxide solutions (hereinafter referred to as "aqueous alkali `~
.~ .
` solution") efficiently in a high concentration with low sodium . j -.: :
chloride content by the use of cation permselective ion-exchange membranes. `'-~
The we~l known mercury and diaphragm processes have been practiced for production of industrially important alkalis '' ';' and chlorine. These processes have recognized advantages and -' ' disadvantages a~ discussed below. The diaphragm process is ~i~
advantageous for use in small floor areas because an upright ~'~ type system is adopted, and also because of its low electric power ' consumption. On the other hand, the process requires a step for removal of alkali halide, normally sodium chloride, which `;
is ~ in a mixed solution of alkali. Ordinarily, the ;" ' procedure produces a product which is contaminated with sodium ' chloride to the extent of 1 to 2%, so that further purification ~¦~ 20 is required. ' '' The mercury process can produce high purity prod-ucts, but requires large floor areas because mercury flowing ,~,j : .
;;! on a horizontal plane is used as an electrode. Moreover, elec-'~ tric power consumption is high ! In addition, because mercur~ '' is a harmful substance, equipment for protection against leak-age is required. '`-' The use of ion-exchange membranes for obtainin~
high purity alkalis with high efficiency in upright instal-Y, ~S .:
~'' lation systems requiring no mercury ~ an attractive alternative~ "
:~, ~ 30 However, this process has not been used successfully up to this c ~ time. One of the reasons is the inadequacy of presently a~ail-; able ion-exchange membranes for producing NaOH at high concen-. 1 .,, rW/"~?'/
i~44~3~
tration with good efficiency. ~ .
. An object of the present invention is to provide a . process for producing alkali metal hydroxides in high concen~
. tration and high purity with good efficiency by the use of ~.
ion-exchange membranes. `~
In one particular aspect the presen.t.invention . provides a continuous process for the electrolysis of an alkali i ::
~- metal chloride which comprises: 1) dividing an electrolytic . ~
- cell into an anode and a cathode compartment with at least one .::
middle compartment, the compartments being separated one from ~ ~
. ~, .
the other by cK~ cationic io~-exchange membranes; 2) pre~
: ~
:.: charging aqueous alkali metal chloride solution of a first `
concentration into the anode compartment; 3) precharging :-; aqueous alkali metal hydroxide so~ution having the same alkali ~ -.
;;l metal as the alkali metal chloride solution into the first .
~; middle compartment at a second concentration, the ratio of the second concentration ~o the first concentration being such as i to maintain a preselected current efficiency during electrolysis; :.~
^~ 4) precharging the same aqueous metal hydroxide solution into ;~ .
,:,, ' :
.~ 20 the cathode compartment at a third concentration which is - .
~! greater than the second concentration and selected so that the .
!3' ratio of the second concentration to the third concentration is such as to ma~intain the preselected current efficiency during ;:: electrolysis; and 5) applying a voltage across the electrolytic .~.' cell while continuously supplying aqueous alkali metal chloride ~. :
`. to the. anode compartment at the first concentration, and ?
~ continuously adding water to, while withdrawing alkali metal ` :
.;,~ . .
:.; hydroxide solution from, the cathode compartment at rates so as `.
to maintain the third concentration at a constant value.
As one modification of the above process, water is also supplied to the middle compartments, thereby making it pos- .
. sible to recover alkali metal hydroxides of various concentrations .
~ 2 L~' ` - , r~
i~L~ 4/~37 ~ -from said middle compartments. -.. . . . .
~; In order that the invention may be clearly understood and readily carried into effect, embodiments thereof will now be described with reference to the accompanying drawings, in which:
IG. 1 shows the relation between caustic soda con~
centration at the cathode side and at the anode side, of a cationic ion-exchange membrane, required to keep the current effeciency at certa~n constant values;
FIG. 2 shows the relation between the caustic soda '-.. .. . .
concentration at the cathode side of a cationic ion-exchange membrane and the current efficiency; and FIG. 3 shows a schematic diagram of one embodiment =~
of an electrolytic cell which is suitable for practicing the ~ process of the present invention, wherein three cationic ion-,~ ~ exchange membranes are used~
When aqueous alkali solutions such as aqueous caustic ; ~ soda solutions having diferent concentrations are separated by a cationic ion-exchange membrane and electrodes provided at ~.,. . ,.,.~ ~
~20 - both sides, the current efficiency of the current flow there-between depends on the transport numbers of sodium ion and - ~ hydroxide ion in the cationic ion-exchange membrane as~well as on Z~ the diffusion of caustic soda th~ough them. The former is in-fluehced greatly by the concentration of caustic soda at the :
cathode side, the latter by the difference in concentrations of caustic soda at both sides of the cationic ion-exchange membrane.
Accordingly, in order to increase the concentration of aqueous caustic soda at the cathode side, while maintaining a high current efficiency, a very high concentration aqUQOUs caustic soda solu-~.Z . ~.
~Z 30 tion (although, of course, lower than the concentration at the ; .~ ' ; cathode side) may be charged in the anode s~de, which is in `~
'''! contact with the cathode side via the cationic ion-exchange .. . . : -:
.''. ~ , .
... .
,.,: _ , 3 ~ ~
membrane, to reduce the loss by diffusion. :~:
Referring now to FIG.l which shows the relation . ~
.. " . ~ .
determined by experiments between caustic soda concentration . ~ . .
:~ at the cathode side and that at the anode side which is required :~
. to ma~ntain current efficiency at certain constant values, the urves (a),(b),(c) and (d) denote relations between caustic .
. soda concentration at the cathode side and that at the anode ;.
side, when the current efficiencies of the cationic ion-exchange .~
. are 95~, 90~, 80% and 70%, respectively! The line (e) is a :: .
; 10 straight line passing the origin with a gradient of 45~. FIG. ~ .
2 shows the relation between the caustic soda concentration at .~.
the cathode side of a cationic ion-exchange membrane and the current efficiency~lexperimentally , ..................................................................... . .
`.sil determined, when an aqueous sodium chloride solution with a con-.~ . .... .
I centration of 300 g/l is charged at the anode side of said cat- ...
; ionic ion~exchange membrane... FIG. 3 shows a schematic diagram `.
~l of one embodiment of an electrolytic cell which is suitable for ~.. -, practicing the process of the present invention, wherein three ..
cationic ion-exchange membranes Kl, K2 and K3 are used. r.~
~ . ' ~ , .
~: 20 The electrolytic cell shown in FIG. 3 consists of :. .
the anode chamber 3, two middle compartments 4 and 5 and the l:
.l cathode chamber 6 which are partitioned between anode 1 and : . ,~ . . .
cathode 2 with three cationic ion-exchange membranes K1, K2, -.
and K3. Take a case wherein electrolysis is performed by j charging the anode compartment 3 with, for example, an aque-~ ous sodium chloride solution at a sodium chloride concentrati.on : .
. -, ~ , . . .
of 300 g/l and charging the middle compartments 4, 5 and the `
cathode compartment 6 with a~ueous caustic soda solutions. In ~... .
order to attain a current efficiency of 95%, the necessary alkali ..
concentration in cathode compartment 6 and the alkali concen~
.- trations in the middle compar.tments 4 and 5 are determined as ~ .
follows: From FIG. 2, the alkali concentration in the middle .
, compartment 4 neighboring the anode compartment 3 must be 15~
,;~ :: -rw/l~
~ .
: The alkali concentration in the next middle compartment 5 must be such that the current efficiency of th~ cationic ion-. .
` exchange membrane K2 is 95%. Hence, in FIG. 1, the line of the cathode side alkali concentration Cl=15~ is drawn and the ~ point at which this line intersects the line (e) i5 determined -: as El~ From El is drawn a line parallel to the ordinate and ~ -.
the point at which this line intersects the line (a) is deter~
mined as A2. The reading on the cathode side alkali concentra-tion corresponding to A2~namely C2=23% denotes the alkali con~
.~ 10 centration at the cathode side (middle compartment 5) to attain -~
~ 1 , current efficiency of 95% or the cationic ion-exchange membrane :~r, : K2, when the alkali concentration at the anode side (middle `~
' compartment 4) is Cl=15%. By a similar procedure, namely by vl drawing a line parallel to the abscissa from A2, determining ~:-I the intersected point E2 on the.line (e), then drawing a line r'' parallel to the ordinate f~om E2~ determining the intersected point A3 on the line (a), followed by reading the cathode .. ;., . !, side alkali concentration corresponding to A3, it is apparent that the alkali concentration in cathode compartment 6 must be adjusted to about 30%.
l As. described above, when using an electrolytic cell having four compartments partitioned with three cationic ion- -¢ -~
: exchange membranes, caustic soda is obtained from the cathode chamber in a concentration of 30% at a current efficiency of ~ 95~, whereas the alkali concentrations in the-.middle compar~ments .. j ..
are 15~ and 23~, respectively. It can be seen from FIG. 1 that . alkali concentration is increased when the numbex of partitioned ;~
j compartments is increased by increasing the numher of cationic ::
ion-exchange membranes. Thus, by suitably selecting the number ,.; 30 of cationic ion-exchange membranes, an aqueous alkali solution ~.
- can be obtained in any desired concentration with high current . . .
:; efficiency.
, ' ~; ' ..
, ... . .
, ~- i , , .. , ,. . . .. :
1~ ti37 ~ `" . .
For practice of the present invention, there may be used a unit cell consisting of an anode compartment, at least one middle compartment and a cathode compartment which are partitioned with two or more cationic ion-exchange membranes between electrodes.
Alternatively, a multiple cell constructed from a combination of unit cells may also be used.
The anodes and cathodes utilized in this invention ; may be any of those normally employed in electrolytic cells.
The following is a more specific description of the practice of the invention as applied to the electrolysis of sodium chloride. The anode compartment 3 is charged with an aqueous . , .
sodium chloride solution. Said compartment 3 may be arranged in a -circulation system, equipped with inlet and outlet tubes. In this circulation system, an aqueous sodium chloride solution is supplied to the anode compartment. After said solution is lowered in con-centration by electrolysis, it is taken out of the compartment, and ;
after addition of sodium chloride or removal of impurities, if necessary, recycled again to the anode compartment. An aqueous , caustic soda solution of selected high concentration and of high purity (low sodium chloride content) is charged into the cathode compartment 5. The cathode compartment may also be arranged in a circulation system for circulating an aqueous caustic soda solution which is kept at a constant concentration by removing the caustic soda formed and supplying water from outside the cathode compartment. ~ -The middle compartments 4 and 5 which are separated from each other and from the adjacent anode or cathode compartments, respectively,~
by cationic ion-exchange membranes are charged with aqueous caustic soda solutions having different concentrations. The concentration of the aqueous caustic solution charged in the middle compartment 5 is higher than that in the middle compartment 4. Each concentration is determined as described above.
. .
.
` ~' .
jl/ ,, ,, "
1~ 3~7 In the procedure as described above, the operations ~
are performed while maintaininy the middle compartments at -equilibrium concentrations so that current efficiencies of the cationic membranes Kl, K2 and K3 are equal. In other words, the concentrations of the solutions in the middle compartments are equilibrated so that the alkali coming into each compartment ~
through the cationic ion-exchange membrane at the cathode side ~-and that going out through the cationic ion-exchange membrane at the anode side are quantitatively equal.
Alternatively, the present process may also be oper- -~
ated by varying the concentration of the middle compartment .. . .
I from the equilibrium concentration thereof so that the cationic . ., ion-exchange membranes differ in current efficiency. For such an operation, a line parallel to the ordinate is drawn f~om E
in FIG,l and the point at which this line intersects the line (b) is determined as B2. From B2 is drawn a line parallel to the abscissa and the point intersecting the line (e) is deter-mined as E2'. From E2~ is drawn a line parallel to ordinate and ~-the point intersecting the line (d) is determined as D .
Take a case where an aqueous sodium chloride solution with a concentration of 300 g/l is charged in anode compartment
the cathode side of a cationic ion-exchange membrane and the current efficiency~lexperimentally , ..................................................................... . .
`.sil determined, when an aqueous sodium chloride solution with a con-.~ . .... .
I centration of 300 g/l is charged at the anode side of said cat- ...
; ionic ion~exchange membrane... FIG. 3 shows a schematic diagram `.
~l of one embodiment of an electrolytic cell which is suitable for ~.. -, practicing the process of the present invention, wherein three ..
cationic ion-exchange membranes Kl, K2 and K3 are used. r.~
~ . ' ~ , .
~: 20 The electrolytic cell shown in FIG. 3 consists of :. .
the anode chamber 3, two middle compartments 4 and 5 and the l:
.l cathode chamber 6 which are partitioned between anode 1 and : . ,~ . . .
cathode 2 with three cationic ion-exchange membranes K1, K2, -.
and K3. Take a case wherein electrolysis is performed by j charging the anode compartment 3 with, for example, an aque-~ ous sodium chloride solution at a sodium chloride concentrati.on : .
. -, ~ , . . .
of 300 g/l and charging the middle compartments 4, 5 and the `
cathode compartment 6 with a~ueous caustic soda solutions. In ~... .
order to attain a current efficiency of 95%, the necessary alkali ..
concentration in cathode compartment 6 and the alkali concen~
.- trations in the middle compar.tments 4 and 5 are determined as ~ .
follows: From FIG. 2, the alkali concentration in the middle .
, compartment 4 neighboring the anode compartment 3 must be 15~
,;~ :: -rw/l~
~ .
: The alkali concentration in the next middle compartment 5 must be such that the current efficiency of th~ cationic ion-. .
` exchange membrane K2 is 95%. Hence, in FIG. 1, the line of the cathode side alkali concentration Cl=15~ is drawn and the ~ point at which this line intersects the line (e) i5 determined -: as El~ From El is drawn a line parallel to the ordinate and ~ -.
the point at which this line intersects the line (a) is deter~
mined as A2. The reading on the cathode side alkali concentra-tion corresponding to A2~namely C2=23% denotes the alkali con~
.~ 10 centration at the cathode side (middle compartment 5) to attain -~
~ 1 , current efficiency of 95% or the cationic ion-exchange membrane :~r, : K2, when the alkali concentration at the anode side (middle `~
' compartment 4) is Cl=15%. By a similar procedure, namely by vl drawing a line parallel to the abscissa from A2, determining ~:-I the intersected point E2 on the.line (e), then drawing a line r'' parallel to the ordinate f~om E2~ determining the intersected point A3 on the line (a), followed by reading the cathode .. ;., . !, side alkali concentration corresponding to A3, it is apparent that the alkali concentration in cathode compartment 6 must be adjusted to about 30%.
l As. described above, when using an electrolytic cell having four compartments partitioned with three cationic ion- -¢ -~
: exchange membranes, caustic soda is obtained from the cathode chamber in a concentration of 30% at a current efficiency of ~ 95~, whereas the alkali concentrations in the-.middle compar~ments .. j ..
are 15~ and 23~, respectively. It can be seen from FIG. 1 that . alkali concentration is increased when the numbex of partitioned ;~
j compartments is increased by increasing the numher of cationic ::
ion-exchange membranes. Thus, by suitably selecting the number ,.; 30 of cationic ion-exchange membranes, an aqueous alkali solution ~.
- can be obtained in any desired concentration with high current . . .
:; efficiency.
, ' ~; ' ..
, ... . .
, ~- i , , .. , ,. . . .. :
1~ ti37 ~ `" . .
For practice of the present invention, there may be used a unit cell consisting of an anode compartment, at least one middle compartment and a cathode compartment which are partitioned with two or more cationic ion-exchange membranes between electrodes.
Alternatively, a multiple cell constructed from a combination of unit cells may also be used.
The anodes and cathodes utilized in this invention ; may be any of those normally employed in electrolytic cells.
The following is a more specific description of the practice of the invention as applied to the electrolysis of sodium chloride. The anode compartment 3 is charged with an aqueous . , .
sodium chloride solution. Said compartment 3 may be arranged in a -circulation system, equipped with inlet and outlet tubes. In this circulation system, an aqueous sodium chloride solution is supplied to the anode compartment. After said solution is lowered in con-centration by electrolysis, it is taken out of the compartment, and ;
after addition of sodium chloride or removal of impurities, if necessary, recycled again to the anode compartment. An aqueous , caustic soda solution of selected high concentration and of high purity (low sodium chloride content) is charged into the cathode compartment 5. The cathode compartment may also be arranged in a circulation system for circulating an aqueous caustic soda solution which is kept at a constant concentration by removing the caustic soda formed and supplying water from outside the cathode compartment. ~ -The middle compartments 4 and 5 which are separated from each other and from the adjacent anode or cathode compartments, respectively,~
by cationic ion-exchange membranes are charged with aqueous caustic soda solutions having different concentrations. The concentration of the aqueous caustic solution charged in the middle compartment 5 is higher than that in the middle compartment 4. Each concentration is determined as described above.
. .
.
` ~' .
jl/ ,, ,, "
1~ 3~7 In the procedure as described above, the operations ~
are performed while maintaininy the middle compartments at -equilibrium concentrations so that current efficiencies of the cationic membranes Kl, K2 and K3 are equal. In other words, the concentrations of the solutions in the middle compartments are equilibrated so that the alkali coming into each compartment ~
through the cationic ion-exchange membrane at the cathode side ~-and that going out through the cationic ion-exchange membrane at the anode side are quantitatively equal.
Alternatively, the present process may also be oper- -~
ated by varying the concentration of the middle compartment .. . .
I from the equilibrium concentration thereof so that the cationic . ., ion-exchange membranes differ in current efficiency. For such an operation, a line parallel to the ordinate is drawn f~om E
in FIG,l and the point at which this line intersects the line (b) is determined as B2. From B2 is drawn a line parallel to the abscissa and the point intersecting the line (e) is deter-mined as E2'. From E2~ is drawn a line parallel to ordinate and ~-the point intersecting the line (d) is determined as D .
Take a case where an aqueous sodium chloride solution with a concentration of 300 g/l is charged in anode compartment
3, an aqueous caustic soda solution with a concentration of 15~
in the middle compartment 4, an aqueous caustic soda solution with a concentration of 30.5%, which is the alkali concentration at cathode side corresponding to the point B2~ in the middle compart-ment 5, and an aqueous caustic soda solution with a concentration of 49~, which is the alkali concentration at cathode side cor-. . .
responding to the point D3, in the cathode compartment 6, respec--` tively. When electrolysis is carried out under these conditions, the current efficiencies of the cationic ion-exchange membranes `
Kl, K2 and K3 become 95~, 90% and 70~, respectively, as seen from FIG. 1. If electrolysis is further continued, the alkali con-.
' ~ :- `
: ... , . ~
,.................................... .
``-- 16~4~t;3~7 ~
-centrations in the middle compartm~nts change so as to equal-ize current efficiencie~ of the three cationic ion-exchange mem-t~ branes Kl, K2 and K3. However, airculation systems may also be adopted for the middle compartments by providing outlets .~, . :: .
and inlets as in the anode compartment 3 and cathode compart-ment 6. The caustic soda concentration in the middle com- , ~
~; partments 4 and 5 are maintained at the initial concentrations ~ -,7~ 15~ and 30%, respectively, by removal of caustic soda and addit- r-~ ~-ion of water similarly as in the cathode chamber 6. When -~ ;
electrolysis is continued under these conditions, caustic soda ~-is removed from the respective middle compartments, in an amount ' corresponding to the difference between the cationic ion-exchange ~ ~ -~1 membranes at the cathode and anode sides of each compartments, f"! as one portion of the caustic soda formed, namely aqueous -~
caustic soda soltuions having concentrations of 49%, 30%
~ and 15% are obtained with current efficiency of 70%, 20% and i~ 5%~ respectively. As a whole, aqueous caustic soda solutions ;~ are obtained with overall current effi¢iency of 95%. , Any of the usual cationic ion-exchange membranes , -i~ 20 known to the art may be used in the present invention. Prefer- ~r~
ably membranes wherein carboxylic groups are fixed as exchange groups on a polymeric substrate are employed. Such membranes -~,,; are produced, for example, by copolymerlzation of divinyl benzene ~;
with acrylic acid, anhydride, ester, acid chloride or other `~
~- derivatives in a suitable solvent. It is also desirable in the present process to use a plurality of ion-exchange mem-' branes having different properties. For example, the membrane r;~ confronting the anode may be selected to be resistan~ to ~`;! ' ~
anodic reaction products. Sulfonated ion-exchange mem~ranes '!,.'.` '., derived from fluorine containing compounds such as trifluoro styrene are preferably employed.
;~, :.~
Thus, when the process of the present invention is ~..... . ..
: ~ ` j 8 ``-~ rw/
practiced, caustic soda can be produced in various desired concentrations.
Although the process of the present invention is described mainly with reference to production of caustic soda by electrolysis of sodium chloride, it is also applicable in a similar manner to other alkali metals.
The following non-limiting examples are given by way of illustration only.
An electrolytic cell, as shown in FIG. 3, is used.
; Membranes derived from a copolymer of acrylic acid and styrene crosslinked with divinyl benzene are used for three cationic ion-exchange membranes. A platinum coated titanium plate is used for the anode and a nickel plate for the cathoae. The effictive area for electric conduction is 10 cm in length and 5 cm in breadth.
; The anode compartment is equipped at top and bottom with openings for circulation of aqueous sodium chloride solution. ;-.~
An aqueous sodium chloride solution with a concentration of about 300 g/l is supplied f~om the opening at the bottom and overflow out of the opening at the top. The cathode compartment is simi- ~
larly equipped with openings at top and bottom. A vessel contain- -ing aqueous caustic soda solution supplies aqueous caustic soda ` solution to the compartment through the opening at the bottom~
; The caustic soda overflowing from the opening at the top is recycled in selected amounts to the alkali vessel. During flow of current, the caustic soda concentration is maintained at 30~ 1% `-by supplying water to the alkali vessel. The two middle compart-ments are filled with aqueous caustic solutions, the middle com-partment at the side of anode with 15% and that at the side of cathode with 23% aqueous caustic solutions, respectively, Under these conditions, current is passed at a current ~ h:
' ~;''' :
rw/ ~
:
l~D4~37 density of 15 A/dm for a total of 30 hours. The current ~`~
efficiency, which is determined from the increased amount of caustic soda in the vessel, is 94.7~. The concentrations of `-caustic soda in the middle compartments are also analyzed after electrolysis and are~found to bé substanitally the same as the initial concentrations, namely 15.3% and 22.6%, respectively.
EXAMPLE 2 ;--. . - - -- . .
The electrolytic cell used in Example 1 is modified so that each of compartments 4, S and 6 is equipped with a cir- i - 10 culation system for circulating, by pump, an aqueous caustic soda solution between vessels containing said solution and each compartment.
The circulation system for the compartment 4 at the anode side and the circulation system for the compartment 5 at the cathode side are charged with 15 and 30% aqueous caustic soda solutions, respectiveIy. The circulation system for the ! cathode compartment is charged with 48~ aqueous caustic soda solution. The anode compartment is supplied, as in Example 1, ~-with aqueous sodium chloride solution in a concentration of 300 g/l. ~ `
$
Under these conditions, current is passea at a cur-rent density of 15 A/dm2. Each vessel containing aqueous `~
caustic soda solution is maintained at the initial concentration ;
~, + 1%. ' ,';,,~
The current efficiencies are determined from the ~r'"' increased amount of caustic soda in each vessel to be 71.0%, -~: :, .
19.1% and 4.3%, resepctively. The concentrations of caustic -~
soda in the respective solutions are 48~, 30% and 15%. The over-all current efficiency is 94.4~.
:; ,.~ . .
An electolytic cell consisting of a one unit cell, com-prising an anode, a middle and a cathode compartment partitioned ~ ., ~ with t~o ion-exchange membranes, is used. A membrane derived ;~.,' ' :
~ from a three dimensionally crosslinked polymer obtained by cross~
; linking acrylic acid-styrene copolymer with divinyl benzene ... . .. .
~ is used as the ion-exchange membrane between the cathode and ., .
~ the middle compartments. A sulfonated ion-exchange membrane ... ..
~` having partially sulfonated bridges, obtained by treating a ~
. .
mem~rane polymer of ~ -trifluoro styrene at about 70C with ~ .~............................................................... . .
~ an excess of chlorosul~onic acid and thereafter boiling the .,, .. :?', treated membrane in boiling water overnight.is used as the ion-;
exchange membrane ~etween the anode and the middle compartments.
The same cathode and anode as used in Example 1 are used. The ,.. .
structure and the circulation system in the cathode and the anode compartments are also the same as in Example 1. The anode com-.'':, . ':
~ partment is supplied with 290 g/l of aqueous sodium chloride . :
solution. The alkali concentration in the cathode compartment is ~-~ maintained at 18 to 21% by supplying water to the vessel contain-~:, ;~ ing aqueous caustic soda solution. Under these conditions, cur-`' rent is passed at a current density of 15 A/dm2 for 32 hours.
The current efficiency determined from the increased amount of -;.~. ,, ~':.
l alkali is 92.7% in terms of the average current efflclency over ~
.... .
the total period. During the curEent flow, there is a gradual .,.,; , .
~ decrease in amount of the alkali solution in the mi~dle com~
'- partment. Accordingly, electrolysis is discontinued at the end ~, of each hour, and all of the alkali solution in the middle com~
partment is drawn out for analysis of alkali concentration~ Thenr ,'. 1! . ... .
` the middle compartment is filled again with an aqueous alkali .: ..,~
;d`,, solution having the same concentration as analyzed, electrolysis ; continued. During these operations, the alkali concentration in the middle compartment is found to be invariably within the .;! range of 8 to 9% after 10 hours. The alkali contained no trace ~ ................................................................... . .
of anodic reaction products such as hypochlorite or chlorate ions.
For comparison, electrolysis is conducted, under the same conditions as in Example 3, in a two-compartment system ?' ' 1 1 :' ~, ``:
in the middle compartment 4, an aqueous caustic soda solution with a concentration of 30.5%, which is the alkali concentration at cathode side corresponding to the point B2~ in the middle compart-ment 5, and an aqueous caustic soda solution with a concentration of 49~, which is the alkali concentration at cathode side cor-. . .
responding to the point D3, in the cathode compartment 6, respec--` tively. When electrolysis is carried out under these conditions, the current efficiencies of the cationic ion-exchange membranes `
Kl, K2 and K3 become 95~, 90% and 70~, respectively, as seen from FIG. 1. If electrolysis is further continued, the alkali con-.
' ~ :- `
: ... , . ~
,.................................... .
``-- 16~4~t;3~7 ~
-centrations in the middle compartm~nts change so as to equal-ize current efficiencie~ of the three cationic ion-exchange mem-t~ branes Kl, K2 and K3. However, airculation systems may also be adopted for the middle compartments by providing outlets .~, . :: .
and inlets as in the anode compartment 3 and cathode compart-ment 6. The caustic soda concentration in the middle com- , ~
~; partments 4 and 5 are maintained at the initial concentrations ~ -,7~ 15~ and 30%, respectively, by removal of caustic soda and addit- r-~ ~-ion of water similarly as in the cathode chamber 6. When -~ ;
electrolysis is continued under these conditions, caustic soda ~-is removed from the respective middle compartments, in an amount ' corresponding to the difference between the cationic ion-exchange ~ ~ -~1 membranes at the cathode and anode sides of each compartments, f"! as one portion of the caustic soda formed, namely aqueous -~
caustic soda soltuions having concentrations of 49%, 30%
~ and 15% are obtained with current efficiency of 70%, 20% and i~ 5%~ respectively. As a whole, aqueous caustic soda solutions ;~ are obtained with overall current effi¢iency of 95%. , Any of the usual cationic ion-exchange membranes , -i~ 20 known to the art may be used in the present invention. Prefer- ~r~
ably membranes wherein carboxylic groups are fixed as exchange groups on a polymeric substrate are employed. Such membranes -~,,; are produced, for example, by copolymerlzation of divinyl benzene ~;
with acrylic acid, anhydride, ester, acid chloride or other `~
~- derivatives in a suitable solvent. It is also desirable in the present process to use a plurality of ion-exchange mem-' branes having different properties. For example, the membrane r;~ confronting the anode may be selected to be resistan~ to ~`;! ' ~
anodic reaction products. Sulfonated ion-exchange mem~ranes '!,.'.` '., derived from fluorine containing compounds such as trifluoro styrene are preferably employed.
;~, :.~
Thus, when the process of the present invention is ~..... . ..
: ~ ` j 8 ``-~ rw/
practiced, caustic soda can be produced in various desired concentrations.
Although the process of the present invention is described mainly with reference to production of caustic soda by electrolysis of sodium chloride, it is also applicable in a similar manner to other alkali metals.
The following non-limiting examples are given by way of illustration only.
An electrolytic cell, as shown in FIG. 3, is used.
; Membranes derived from a copolymer of acrylic acid and styrene crosslinked with divinyl benzene are used for three cationic ion-exchange membranes. A platinum coated titanium plate is used for the anode and a nickel plate for the cathoae. The effictive area for electric conduction is 10 cm in length and 5 cm in breadth.
; The anode compartment is equipped at top and bottom with openings for circulation of aqueous sodium chloride solution. ;-.~
An aqueous sodium chloride solution with a concentration of about 300 g/l is supplied f~om the opening at the bottom and overflow out of the opening at the top. The cathode compartment is simi- ~
larly equipped with openings at top and bottom. A vessel contain- -ing aqueous caustic soda solution supplies aqueous caustic soda ` solution to the compartment through the opening at the bottom~
; The caustic soda overflowing from the opening at the top is recycled in selected amounts to the alkali vessel. During flow of current, the caustic soda concentration is maintained at 30~ 1% `-by supplying water to the alkali vessel. The two middle compart-ments are filled with aqueous caustic solutions, the middle com-partment at the side of anode with 15% and that at the side of cathode with 23% aqueous caustic solutions, respectively, Under these conditions, current is passed at a current ~ h:
' ~;''' :
rw/ ~
:
l~D4~37 density of 15 A/dm for a total of 30 hours. The current ~`~
efficiency, which is determined from the increased amount of caustic soda in the vessel, is 94.7~. The concentrations of `-caustic soda in the middle compartments are also analyzed after electrolysis and are~found to bé substanitally the same as the initial concentrations, namely 15.3% and 22.6%, respectively.
EXAMPLE 2 ;--. . - - -- . .
The electrolytic cell used in Example 1 is modified so that each of compartments 4, S and 6 is equipped with a cir- i - 10 culation system for circulating, by pump, an aqueous caustic soda solution between vessels containing said solution and each compartment.
The circulation system for the compartment 4 at the anode side and the circulation system for the compartment 5 at the cathode side are charged with 15 and 30% aqueous caustic soda solutions, respectiveIy. The circulation system for the ! cathode compartment is charged with 48~ aqueous caustic soda solution. The anode compartment is supplied, as in Example 1, ~-with aqueous sodium chloride solution in a concentration of 300 g/l. ~ `
$
Under these conditions, current is passea at a cur-rent density of 15 A/dm2. Each vessel containing aqueous `~
caustic soda solution is maintained at the initial concentration ;
~, + 1%. ' ,';,,~
The current efficiencies are determined from the ~r'"' increased amount of caustic soda in each vessel to be 71.0%, -~: :, .
19.1% and 4.3%, resepctively. The concentrations of caustic -~
soda in the respective solutions are 48~, 30% and 15%. The over-all current efficiency is 94.4~.
:; ,.~ . .
An electolytic cell consisting of a one unit cell, com-prising an anode, a middle and a cathode compartment partitioned ~ ., ~ with t~o ion-exchange membranes, is used. A membrane derived ;~.,' ' :
~ from a three dimensionally crosslinked polymer obtained by cross~
; linking acrylic acid-styrene copolymer with divinyl benzene ... . .. .
~ is used as the ion-exchange membrane between the cathode and ., .
~ the middle compartments. A sulfonated ion-exchange membrane ... ..
~` having partially sulfonated bridges, obtained by treating a ~
. .
mem~rane polymer of ~ -trifluoro styrene at about 70C with ~ .~............................................................... . .
~ an excess of chlorosul~onic acid and thereafter boiling the .,, .. :?', treated membrane in boiling water overnight.is used as the ion-;
exchange membrane ~etween the anode and the middle compartments.
The same cathode and anode as used in Example 1 are used. The ,.. .
structure and the circulation system in the cathode and the anode compartments are also the same as in Example 1. The anode com-.'':, . ':
~ partment is supplied with 290 g/l of aqueous sodium chloride . :
solution. The alkali concentration in the cathode compartment is ~-~ maintained at 18 to 21% by supplying water to the vessel contain-~:, ;~ ing aqueous caustic soda solution. Under these conditions, cur-`' rent is passed at a current density of 15 A/dm2 for 32 hours.
The current efficiency determined from the increased amount of -;.~. ,, ~':.
l alkali is 92.7% in terms of the average current efflclency over ~
.... .
the total period. During the curEent flow, there is a gradual .,.,; , .
~ decrease in amount of the alkali solution in the mi~dle com~
'- partment. Accordingly, electrolysis is discontinued at the end ~, of each hour, and all of the alkali solution in the middle com~
partment is drawn out for analysis of alkali concentration~ Thenr ,'. 1! . ... .
` the middle compartment is filled again with an aqueous alkali .: ..,~
;d`,, solution having the same concentration as analyzed, electrolysis ; continued. During these operations, the alkali concentration in the middle compartment is found to be invariably within the .;! range of 8 to 9% after 10 hours. The alkali contained no trace ~ ................................................................... . .
of anodic reaction products such as hypochlorite or chlorate ions.
For comparison, electrolysis is conducted, under the same conditions as in Example 3, in a two-compartment system ?' ' 1 1 :' ~, ``:
4~ '7 ,, . ` ` ~ .
usin~ only one membrane, oE the type used in Example 3, betwecn `~ the anode and the middle compartments. The current efficiency i~ obtained is 76% (+3%).
.,, :;' An electrolytic cell as used in Example 3 is modified ~ such that the middle compartment, in the same manner as the cathode , compartment, is equipped with a system to circulate solution from an alkali vessel to a pump, the middle chamber and again back to the ; vessel. In order to maintain the concentration of the circulated .,:, :' alkali in the middle compartment at 5% (+1%) and also to maintain the concentration of the circulated alkali in the cathode compartment -.. . . .
at 18 to 21%, water is supplied continuously into each of the vessels which are eqoipped with circulation systems.
, Under these conditions, current is passed at a .'~ current-density of 15 A/dm2 for 27 hours. The average current ,,,,, ,.
efficiency was 87.5% as determined from the increased alkali in the cathode circulation system and 8.2% as determined from the alkali concentration in the middle circulation system. No trace of anodic reaction products such as hydrochlorite or chlorate ions were found in the alkali in the middle circulation system.
~:", :~
.:':`~ .. . .
" :-., :
,,~,i ~ ~ .:
1 2 -~
: ~"j , .
., ,`' j . `:
i , ..
-...................................................................... :
.~',', jl/ .
., ,.i, r, J j
usin~ only one membrane, oE the type used in Example 3, betwecn `~ the anode and the middle compartments. The current efficiency i~ obtained is 76% (+3%).
.,, :;' An electrolytic cell as used in Example 3 is modified ~ such that the middle compartment, in the same manner as the cathode , compartment, is equipped with a system to circulate solution from an alkali vessel to a pump, the middle chamber and again back to the ; vessel. In order to maintain the concentration of the circulated .,:, :' alkali in the middle compartment at 5% (+1%) and also to maintain the concentration of the circulated alkali in the cathode compartment -.. . . .
at 18 to 21%, water is supplied continuously into each of the vessels which are eqoipped with circulation systems.
, Under these conditions, current is passed at a .'~ current-density of 15 A/dm2 for 27 hours. The average current ,,,,, ,.
efficiency was 87.5% as determined from the increased alkali in the cathode circulation system and 8.2% as determined from the alkali concentration in the middle circulation system. No trace of anodic reaction products such as hydrochlorite or chlorate ions were found in the alkali in the middle circulation system.
~:", :~
.:':`~ .. . .
" :-., :
,,~,i ~ ~ .:
1 2 -~
: ~"j , .
., ,`' j . `:
i , ..
-...................................................................... :
.~',', jl/ .
., ,.i, r, J j
Claims (7)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A continuous process for the electrolysis of an alkali metal chloride which comprises:
1. dividing an electrolytic cell into an anode and a cathode compartment with at least one middle compartment, said compartments being separated one from the other by cationic ion-exchange membranes;
2. precharging aqueous alkali metal chloride solution of a first concentration into said anode compartment;
3. precharging aqueous alkali metal hydroxide solution having the same alkali metal as said alkali metal chloride solution into said first middle compartment at a second concentration, the ratio of said second concentration to said first concentration being such as to maintain a preselected cur-rent efficiency during electrolysis;
4. precharging the same aqueous metal hydroxide solution into the cathode compartment at a third concentration which is greater than said second concentration and selected so that the ratio of said second concentration to said third concentration is such as to maintain the said preselected current efficiency during electrolysis; and 5. applying a voltage across the electrolytic cell while continuously supplying aqueous alkali metal chloride to said anode compartment at said first concentration, and continuously adding water to, while withdrawing alkali metal hydroxide solution from, said cathode compartment at rates so as to maintain said third concentration at a constant value.
2. precharging aqueous alkali metal chloride solution of a first concentration into said anode compartment;
3. precharging aqueous alkali metal hydroxide solution having the same alkali metal as said alkali metal chloride solution into said first middle compartment at a second concentration, the ratio of said second concentration to said first concentration being such as to maintain a preselected cur-rent efficiency during electrolysis;
4. precharging the same aqueous metal hydroxide solution into the cathode compartment at a third concentration which is greater than said second concentration and selected so that the ratio of said second concentration to said third concentration is such as to maintain the said preselected current efficiency during electrolysis; and 5. applying a voltage across the electrolytic cell while continuously supplying aqueous alkali metal chloride to said anode compartment at said first concentration, and continuously adding water to, while withdrawing alkali metal hydroxide solution from, said cathode compartment at rates so as to maintain said third concentration at a constant value.
2. A process as in claim 1 wherein there is a plur-ality of middle compartments each precharged with an alkali metal hydroxide solution at different concentrations, the concentrations in each separate compartment being selected to increase with increasing proximity to the cathode compartment all such concen-trations being less than the said third concentration in the cathode compartment and being selected so as to maintain said preselected current efficiency.
3. A process as in Claim 1 wherein the cationic ion exchange membrane closest to the anode is substantially resistant to anode reaction products.
4. A process as in Claim 1 wherein the alkali metal chloride is sodium chloride.
5. A process as in Claim 2 wherein the alkali metal chloride is sodium chloride.
6. A process as in Claim 5 wherein the number of middle compartments is two, said first concentration is 300 g/l, said second concentration is 15%, the concentration in the second middle compartment is 23%, the third concentration is 30%, and the preselected current efficiency is 95%.
7. A process as in Claim 1 wherein water is supplied to at least one middle compartment and alkali metal hydroxide solution is withdrawn therefrom, the concentration of the said alkali metal hydroxide solution being different from the concentra-tion of the alkali metal hydroxide solution withdrawn from the cathode compartment.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA200,273A CA1044637A (en) | 1974-05-17 | 1974-05-17 | Electrolysis of alkali metal chloride |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA200,273A CA1044637A (en) | 1974-05-17 | 1974-05-17 | Electrolysis of alkali metal chloride |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1044637A true CA1044637A (en) | 1978-12-19 |
Family
ID=4100110
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA200,273A Expired CA1044637A (en) | 1974-05-17 | 1974-05-17 | Electrolysis of alkali metal chloride |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1044637A (en) |
-
1974
- 1974-05-17 CA CA200,273A patent/CA1044637A/en not_active Expired
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