CA1172992A - Halogenation of water using porous anode for on-site halogen production - Google Patents
Halogenation of water using porous anode for on-site halogen productionInfo
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- CA1172992A CA1172992A CA000327615A CA327615A CA1172992A CA 1172992 A CA1172992 A CA 1172992A CA 000327615 A CA000327615 A CA 000327615A CA 327615 A CA327615 A CA 327615A CA 1172992 A CA1172992 A CA 1172992A
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- anode
- porous
- cell
- water
- gap
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Abstract
ABSTRACT OF THE DISCLOSURE
A novel process for the hologenation of water comprising passing an electrolysis current through a porous, permeable anode and a cathode forming an electrodic gap with water passing therethrough, pass an aqueous solution of an alkali metal halide containing at least 25 g/l of said halide through the porous, permeable anode in the electrodic gap and controlling the hydrodynamic flow through the said anode to maintain the weight ratio of active halogen to halide in the water leaving the elect-rodic gap of at least 0.2, preferably near 1, and a novel electrolytic apparatus for effecting the said electrolysis and more generally suited for releasing anodically oxidated chemical species into a supporting electrolyte flowing through the cell while restraining the release of the non-oxidated species into the supporting electrolyte, that is into the effluent solution.
A novel process for the hologenation of water comprising passing an electrolysis current through a porous, permeable anode and a cathode forming an electrodic gap with water passing therethrough, pass an aqueous solution of an alkali metal halide containing at least 25 g/l of said halide through the porous, permeable anode in the electrodic gap and controlling the hydrodynamic flow through the said anode to maintain the weight ratio of active halogen to halide in the water leaving the elect-rodic gap of at least 0.2, preferably near 1, and a novel electrolytic apparatus for effecting the said electrolysis and more generally suited for releasing anodically oxidated chemical species into a supporting electrolyte flowing through the cell while restraining the release of the non-oxidated species into the supporting electrolyte, that is into the effluent solution.
Description
~ 17 ~9g 2 jl STATE OF THE Al~T
I' , . ~
, Active chlorine or hypochlorite is extensively used for water I sterilization, not only to obtain potable water but also for preventing I"the proliferation of bacteria, 'oiological concretions and algae, and for l oxidizing organic matter in swimming pools and in industrial cooling l'water systems. The amount of active chlorine required or this type of , i'trcatment is in the range of 1 to 2 mg/l. The on site electrolytic product-ion of active c'nlorine by electrolysis of alkali metal c`nloride pres-nts I
I',several advantages, particularly as re,,ards safety and control systems, I . .
I,over the utilization of gaseous chlorine from liquid chlorine bottles or ;Ithe use o hypo_ i' i j ), I .
.` ,' ' i .
,.~ ~
~ ~729~`32 , chlorite which involves transportation, storage, and dosing !
apparatu5 of absolute reliability and provision Or costly safety features.
I The advantages of on site electrolytic production Or I,active chlorine can be best appreciated by considering that the active chlorine produced remains completely dissolYed in the electrolyte and, moreover, it may be produced on demand in whatever quantity is required. In fact, the quantity Or !active chlorine produced is proportional to the electrolysis Icurrent and therefore it is sufficient to control the current ~ith respect to the amount Or water to be treated, to rnaint-!ain the chlorine content withinthedesired limits.
j According to knolin electrolytic methods, dilute brine containing from 25 to 45 g/l of sodium chloride is circulatëd at least once through one or more electrolysis Icells in series to obtain an effluent solution containing ¦~irom 0.1 to 8 g/l Or active chlorine and from 24 to l10 g/l ¦of sodium chloride. Gaseous hydrogen produced at the cathode I i5 vented from the effluent solution ~hich is then sultably ¦added to the main stream of water to be treated to obtain a ! concentration of active chlorine of about 1 to 5 mg/l in the water.
~ctive chlorine or active halide is intcnded to be !¦the concentration Or the oxidation equlvalents, expressed as ¦,wei~ht unit per volurne of the elcctrolyte, multiplied by the a~omic wei~ht of the relative halogen (chlorirle = 35.457), liconsidering that: 1 mole of dissolved C12 corresponds to C , ; ~ equiv~lents; 1 mo~e of dL ~o1ved ~IC10 co~respon~ls to ~i 1~ .
l ` ~
r . i 1 7 2 ~ 9 2 ¦ l
I' , . ~
, Active chlorine or hypochlorite is extensively used for water I sterilization, not only to obtain potable water but also for preventing I"the proliferation of bacteria, 'oiological concretions and algae, and for l oxidizing organic matter in swimming pools and in industrial cooling l'water systems. The amount of active chlorine required or this type of , i'trcatment is in the range of 1 to 2 mg/l. The on site electrolytic product-ion of active c'nlorine by electrolysis of alkali metal c`nloride pres-nts I
I',several advantages, particularly as re,,ards safety and control systems, I . .
I,over the utilization of gaseous chlorine from liquid chlorine bottles or ;Ithe use o hypo_ i' i j ), I .
.` ,' ' i .
,.~ ~
~ ~729~`32 , chlorite which involves transportation, storage, and dosing !
apparatu5 of absolute reliability and provision Or costly safety features.
I The advantages of on site electrolytic production Or I,active chlorine can be best appreciated by considering that the active chlorine produced remains completely dissolYed in the electrolyte and, moreover, it may be produced on demand in whatever quantity is required. In fact, the quantity Or !active chlorine produced is proportional to the electrolysis Icurrent and therefore it is sufficient to control the current ~ith respect to the amount Or water to be treated, to rnaint-!ain the chlorine content withinthedesired limits.
j According to knolin electrolytic methods, dilute brine containing from 25 to 45 g/l of sodium chloride is circulatëd at least once through one or more electrolysis Icells in series to obtain an effluent solution containing ¦~irom 0.1 to 8 g/l Or active chlorine and from 24 to l10 g/l ¦of sodium chloride. Gaseous hydrogen produced at the cathode I i5 vented from the effluent solution ~hich is then sultably ¦added to the main stream of water to be treated to obtain a ! concentration of active chlorine of about 1 to 5 mg/l in the water.
~ctive chlorine or active halide is intcnded to be !¦the concentration Or the oxidation equlvalents, expressed as ¦,wei~ht unit per volurne of the elcctrolyte, multiplied by the a~omic wei~ht of the relative halogen (chlorirle = 35.457), liconsidering that: 1 mole of dissolved C12 corresponds to C , ; ~ equiv~lents; 1 mo~e of dL ~o1ved ~IC10 co~respon~ls to ~i 1~ .
l ` ~
r . i 1 7 2 ~ 9 2 ¦ l
2 equivalents; 1 gram ion Or dissolved C10 corresponds to 2 equivalents; and 1 gram ion Or dissolved C103- corresponds to 6 equivalents according to the relative reactions:
ll C12 t H20 , 0 + 2Cl- + 2H+
¦I HC10 - ~ 0 + Cl- ~ H
1~ C10- ~ 0 + Cl-!I C103 ~ 30-+ Cl-In practice, the active chlorine concentration expressed in ligrams/liter is substantially identical to the concentration lC ! in grams/liter of hypochlorite. Therefore, the expressions "active chlorine" and "hypochlorite" in the present descrip-tion are to be considered equivalent in first approximation.
l The known electrolytic process, as commonly intended ¦
,and realized, is subject to well known technical limitations ~5 ¦which have a considerable impact on the economics Or the ¦process. One of the essential conditions necessary to achieve a good overall current efficiency of the on site electrolyti ¦process for the chlorination of water directly ls to obtain la current efficiency for the desired anodic reaction, that i Ichlorine discharge~ as high as possible with respect to the ~undesircd side rcactions, particularly oxygen discharKe.
Cata]ytic anodes having a low overvoltage to chlorine evolu-tion are commonly used to favor this reaction over the unde-¦sircd oxygen discharge reaction which takes place with anodic Ipotentials excceding by 400-450 mV tne chlorine evolution C lanodic potential.
I .
1.~
~ - _3_ l . . , ~ ,.
, 'i. L729g2 ~i evertheless, in order to operate at economically acceptable currentdensities, the sodium chloride content in ~the electrolyte must always exceed 25-35 g/l since lower .
I.concentrations of chloride drastically reduces the faraday I,efriciency ror chlorine evolutiGn whlle the quantity of oxyge:
evolved at the anode increases. This is due to~t~.e well know kinetic problems connected with diffusion Or the chloride 11ions to the anode through the anode double layer. When !l operating at lower chloride concentration in the electrolyte, Isevere depletion of chloride ions in the anodic film occurs .
land this increases oxygen evolution. Oxygen cvolution, ,besides lowering the overall current erficiency of the proces$, jaffects the anode lifetime which, whether made of graphite I
or Or a valve metal coated with an electrocatalytic coating .
C~-5 ¦¦consisting Or noble metals or oxides Or the same, are subject 1 to a rapid wear rate or to a rapid loss of catal.ytic !l activity~ .
However, the necessity to keep a suf'riciently high ~ sodium chloride concentration in the electrolyte entails several d:isadvantages. First of all, a grcat arnount Or salt is lost in the cf'fluent and the salt content in the rnain stream Or water treated by the addition Or thc efrluent liquo r ¦ rrom the cell is remarkably increased. The ratio between ,j active ch].orine and chloride adde~ to thc ~rater, expressed 1 in g/l, is always belo~,r 0.2 and practically seldom excecds ¦ o.o5-o.I g/l with the known processes. ~nother dlsadvantage i s that t ls practic^l1y vlposs1b1c to troat tho body of ¦
I
' .. ' , , .
~ l ~ ~
7299~ ~
water ciirectly by simply passing the water stream through the cell, but a concentrated solution containing hypochlorite and ch loride leaving the electrolysis cell has to be collected in a separate tank and tnen the !I said concentrated solution has to be added to the electrolyte by Ineans I
S 1~ 0 f : d ~ qu .It e do s iD g s y:. tom s . I ¦
OBJECTS OF THE_ENTION
li' ' J .
' It is an object of the invention to provide a novel method and a Il novel apparatus for halogenatlng water for steril~zation purposes in a ~.
ii manner so as to obtain an effluent solution from wherein the ratio ¦¦ between the active halogen and the halide content ls higher than 0. 2, and !
preferably not lower than 0. 3, and operatinE~ with high efficiency.
It is another object of the invention to provide a method and appar-Il atus for halogenating water for sterili~zation purposes by producing the !i hc~lo~c,- ' l I¦ desired active h~lo~ on concentration with a low salt content directly in 1I the stream of water to be treated.
li It is anot-ner object of tne invention to provide for a rnethod and ¦, apparatus for the electrolytic production of aqueous hypochlorite solut-¦l ionfi exhibiting a low chloride content.
Il It is a furhter object of the invention to provide a nuvel diaphrag ,j less electrolytic cell particularly suited for releasincg anodically ~1 ¦I chemical species into the supporting electrolyte flowing tnrougih the cell Il ' oY/~
Il while restrail-ing the release of the non-cxi1;1ted- :;pecies into the sllppOlt-c l' ing electrolyte. ¦ ~.
These and other o'ojects and advantages will 'oecome obvious !
from the follo~ving detailed description.
1, _5_ I, . i ,, . i ., ' . I
T~E INVENTION ¦
I'The novel process Or the invention for the halogena-j ( lltlon Or ~Jater comprises passlng anelectrolysis current ! throu~h a porous,permcable anode and a cathode forming an l¦elcctrodic gap with ~rater passing therethrou~h, passing an l~aqueous solution.of an alkali metal halide containing at .
i! least 25 g/l of said halide through the porous permeable ,anode into the electrodic gap and cpntrolling the hydrodynami jlflow through the said anode to maintain the weight ratio of llactive halogen to ;nalide in the water leaving the electrodic ¦¦gap of at least 0.2; -The said process makes it possible to produce directl ! by electrolysis an aqueous solution of alkali metal hypo-halites with a very low contcnt of alkali metal halides and ~Ithererore with a reduced halide consumption while operating l¦with high efriciency and at high current density. This is achieved utilizing two separate liquid circuits, one ror the ali metal halide solution and another for the electrolyte, that is for the Iormed hypohalite solution.
~nd will~e described Thc process of the ~.nvention is best described~by ¦rererri.nr~ to the production Or hypochloritc utilizing sodium Ichloride becauseorthe ~eat commercial acceptance of this i! process, but it is obvious that other electrolytic processes llsuch as the production Or potassium hypochlorite from ¦! potassiurn chloridc, sodium chlorate from sodiwn chloride, sodium hypobromit.e from sodium bromide, may be pérformed by the present invention. ~.
C I . ' , I -6- . .
:.~72592 11 ¦~ l'referably, the brine used is as concentrated as possible and,inthe caseorsodium chloride, it is preferably llmaintalned in the range of 100 g/l to 300 g/l. Brine con-( llcentration can easily~kept constant by continuously circulatin J
¦Ithc brine through an external concentration stage and recycl-¦ing the concentrated brine back tG the cell compartment which ls separated from the water flowing t.hrough the electrolytic cell by the porous anode.
l The porous anode is preferably made of a sintered ln ¦ valve metal having a thickness in the range from 0.5 mm to 5 ¦¦millimeters, a porosity ranging from 20 to 70~ and an average ¦Ipore diameter from 5 to 100 microns. More preferably, the anode has a porosity ranging from 30 to 65% and the average diameter of the pores ranges from 5 to 50 microns and is 15 ¦I made of sintered titanium. However, it is obvious that a I
Isuitable valve metal porous anode can be prepared otherwise, !
¦for example by plasma jet deposltion of a porous layer of a valvc mctal onto a foraminous structure of a valve metal, l prefcrably an e~panded sheet, which, besides actlng as a 23 support.Lng matrix and mechanical stiffener, also behaves as a ourrcnt distrlbutor for the whole anodic surface.
'rhe porous valve metal anode is preferably activate ! by impregnation and deposition on the valve metal of a thin layer of non-passivatable material which is reslstant to the ~5 ¦ anodi.c environmenta~ ~ electrocatalytiC to evolut-ion Or ! halogen from aqucous solutio!ls~ Suitable materials are, for l example, the platinum group metals applied by thcrMal deposi C t~on to he porous st-ucCuro Or the valve metal by first i ~ 1729~ I
~mpregnating the porous valve metal with a solutlon containin~
¦,decomposa~le salts of the platlnum group metals. Galvanic id~oosition Or noble metals such as platinum, palladium, ir~dium, ruthenium, rhodium and osmium may also be used.
l Oxides of the metals belonging to the Group VIII of !llth~ Periodic Table, catalytic oxldes of other metals such as i!manganese, lead, tin or mixed oxides or mixtures of the oxide ¦lof the above metals and oxides of valve metals may also be iiut~lized for activating the porous anode. The most preferred llmaterial is a mixed oxide of titanium and rutheni~m applied Iby thermal decomposition in the presence of oxygen of a ¦!solution containing titanium and ruthenium salts and possibly¦
jsmall quantities of other metals salts, such as cobalt, nicke L
¦ and tin, to the sintered structure of titanium or ¦o~her valve metal. The electrocatlytic coatings are more fully described in U.S. patents No. 3,711,385 and l~o.
ll C12 t H20 , 0 + 2Cl- + 2H+
¦I HC10 - ~ 0 + Cl- ~ H
1~ C10- ~ 0 + Cl-!I C103 ~ 30-+ Cl-In practice, the active chlorine concentration expressed in ligrams/liter is substantially identical to the concentration lC ! in grams/liter of hypochlorite. Therefore, the expressions "active chlorine" and "hypochlorite" in the present descrip-tion are to be considered equivalent in first approximation.
l The known electrolytic process, as commonly intended ¦
,and realized, is subject to well known technical limitations ~5 ¦which have a considerable impact on the economics Or the ¦process. One of the essential conditions necessary to achieve a good overall current efficiency of the on site electrolyti ¦process for the chlorination of water directly ls to obtain la current efficiency for the desired anodic reaction, that i Ichlorine discharge~ as high as possible with respect to the ~undesircd side rcactions, particularly oxygen discharKe.
Cata]ytic anodes having a low overvoltage to chlorine evolu-tion are commonly used to favor this reaction over the unde-¦sircd oxygen discharge reaction which takes place with anodic Ipotentials excceding by 400-450 mV tne chlorine evolution C lanodic potential.
I .
1.~
~ - _3_ l . . , ~ ,.
, 'i. L729g2 ~i evertheless, in order to operate at economically acceptable currentdensities, the sodium chloride content in ~the electrolyte must always exceed 25-35 g/l since lower .
I.concentrations of chloride drastically reduces the faraday I,efriciency ror chlorine evolutiGn whlle the quantity of oxyge:
evolved at the anode increases. This is due to~t~.e well know kinetic problems connected with diffusion Or the chloride 11ions to the anode through the anode double layer. When !l operating at lower chloride concentration in the electrolyte, Isevere depletion of chloride ions in the anodic film occurs .
land this increases oxygen evolution. Oxygen cvolution, ,besides lowering the overall current erficiency of the proces$, jaffects the anode lifetime which, whether made of graphite I
or Or a valve metal coated with an electrocatalytic coating .
C~-5 ¦¦consisting Or noble metals or oxides Or the same, are subject 1 to a rapid wear rate or to a rapid loss of catal.ytic !l activity~ .
However, the necessity to keep a suf'riciently high ~ sodium chloride concentration in the electrolyte entails several d:isadvantages. First of all, a grcat arnount Or salt is lost in the cf'fluent and the salt content in the rnain stream Or water treated by the addition Or thc efrluent liquo r ¦ rrom the cell is remarkably increased. The ratio between ,j active ch].orine and chloride adde~ to thc ~rater, expressed 1 in g/l, is always belo~,r 0.2 and practically seldom excecds ¦ o.o5-o.I g/l with the known processes. ~nother dlsadvantage i s that t ls practic^l1y vlposs1b1c to troat tho body of ¦
I
' .. ' , , .
~ l ~ ~
7299~ ~
water ciirectly by simply passing the water stream through the cell, but a concentrated solution containing hypochlorite and ch loride leaving the electrolysis cell has to be collected in a separate tank and tnen the !I said concentrated solution has to be added to the electrolyte by Ineans I
S 1~ 0 f : d ~ qu .It e do s iD g s y:. tom s . I ¦
OBJECTS OF THE_ENTION
li' ' J .
' It is an object of the invention to provide a novel method and a Il novel apparatus for halogenatlng water for steril~zation purposes in a ~.
ii manner so as to obtain an effluent solution from wherein the ratio ¦¦ between the active halogen and the halide content ls higher than 0. 2, and !
preferably not lower than 0. 3, and operatinE~ with high efficiency.
It is another object of the invention to provide a method and appar-Il atus for halogenating water for sterili~zation purposes by producing the !i hc~lo~c,- ' l I¦ desired active h~lo~ on concentration with a low salt content directly in 1I the stream of water to be treated.
li It is anot-ner object of tne invention to provide for a rnethod and ¦, apparatus for the electrolytic production of aqueous hypochlorite solut-¦l ionfi exhibiting a low chloride content.
Il It is a furhter object of the invention to provide a nuvel diaphrag ,j less electrolytic cell particularly suited for releasincg anodically ~1 ¦I chemical species into the supporting electrolyte flowing tnrougih the cell Il ' oY/~
Il while restrail-ing the release of the non-cxi1;1ted- :;pecies into the sllppOlt-c l' ing electrolyte. ¦ ~.
These and other o'ojects and advantages will 'oecome obvious !
from the follo~ving detailed description.
1, _5_ I, . i ,, . i ., ' . I
T~E INVENTION ¦
I'The novel process Or the invention for the halogena-j ( lltlon Or ~Jater comprises passlng anelectrolysis current ! throu~h a porous,permcable anode and a cathode forming an l¦elcctrodic gap with ~rater passing therethrou~h, passing an l~aqueous solution.of an alkali metal halide containing at .
i! least 25 g/l of said halide through the porous permeable ,anode into the electrodic gap and cpntrolling the hydrodynami jlflow through the said anode to maintain the weight ratio of llactive halogen to ;nalide in the water leaving the electrodic ¦¦gap of at least 0.2; -The said process makes it possible to produce directl ! by electrolysis an aqueous solution of alkali metal hypo-halites with a very low contcnt of alkali metal halides and ~Ithererore with a reduced halide consumption while operating l¦with high efriciency and at high current density. This is achieved utilizing two separate liquid circuits, one ror the ali metal halide solution and another for the electrolyte, that is for the Iormed hypohalite solution.
~nd will~e described Thc process of the ~.nvention is best described~by ¦rererri.nr~ to the production Or hypochloritc utilizing sodium Ichloride becauseorthe ~eat commercial acceptance of this i! process, but it is obvious that other electrolytic processes llsuch as the production Or potassium hypochlorite from ¦! potassiurn chloridc, sodium chlorate from sodiwn chloride, sodium hypobromit.e from sodium bromide, may be pérformed by the present invention. ~.
C I . ' , I -6- . .
:.~72592 11 ¦~ l'referably, the brine used is as concentrated as possible and,inthe caseorsodium chloride, it is preferably llmaintalned in the range of 100 g/l to 300 g/l. Brine con-( llcentration can easily~kept constant by continuously circulatin J
¦Ithc brine through an external concentration stage and recycl-¦ing the concentrated brine back tG the cell compartment which ls separated from the water flowing t.hrough the electrolytic cell by the porous anode.
l The porous anode is preferably made of a sintered ln ¦ valve metal having a thickness in the range from 0.5 mm to 5 ¦¦millimeters, a porosity ranging from 20 to 70~ and an average ¦Ipore diameter from 5 to 100 microns. More preferably, the anode has a porosity ranging from 30 to 65% and the average diameter of the pores ranges from 5 to 50 microns and is 15 ¦I made of sintered titanium. However, it is obvious that a I
Isuitable valve metal porous anode can be prepared otherwise, !
¦for example by plasma jet deposltion of a porous layer of a valvc mctal onto a foraminous structure of a valve metal, l prefcrably an e~panded sheet, which, besides actlng as a 23 support.Lng matrix and mechanical stiffener, also behaves as a ourrcnt distrlbutor for the whole anodic surface.
'rhe porous valve metal anode is preferably activate ! by impregnation and deposition on the valve metal of a thin layer of non-passivatable material which is reslstant to the ~5 ¦ anodi.c environmenta~ ~ electrocatalytiC to evolut-ion Or ! halogen from aqucous solutio!ls~ Suitable materials are, for l example, the platinum group metals applied by thcrMal deposi C t~on to he porous st-ucCuro Or the valve metal by first i ~ 1729~ I
~mpregnating the porous valve metal with a solutlon containin~
¦,decomposa~le salts of the platlnum group metals. Galvanic id~oosition Or noble metals such as platinum, palladium, ir~dium, ruthenium, rhodium and osmium may also be used.
l Oxides of the metals belonging to the Group VIII of !llth~ Periodic Table, catalytic oxldes of other metals such as i!manganese, lead, tin or mixed oxides or mixtures of the oxide ¦lof the above metals and oxides of valve metals may also be iiut~lized for activating the porous anode. The most preferred llmaterial is a mixed oxide of titanium and rutheni~m applied Iby thermal decomposition in the presence of oxygen of a ¦!solution containing titanium and ruthenium salts and possibly¦
jsmall quantities of other metals salts, such as cobalt, nicke L
¦ and tin, to the sintered structure of titanium or ¦o~her valve metal. The electrocatlytic coatings are more fully described in U.S. patents No. 3,711,385 and l~o.
3,632,498.
¦ After activation, the porous anode surface facing ¦the cathode surface may optionally be provided with a thin 1! porous layer of a material such as polytetrafluoroethylene ¦lor other halogenated polymert~ m~ X surfacehydrophobic for !!retarding the transport of the aqueous phase across the ¦jporous anode. The porous layer of polytetrafluoroethylene or ~¦other suitable material may be applied onto the porous anode ¦,sur~ace~ for example, by depositing a solution or an emulsion 'tor the polymer, removing the solvent and subjecting the sur-~ace to heat treatment. Spray applicatlon o~ polymer may bc used as well~
11 ' .
1 17299~
I' I
!I The proccss Or the present invention presents the !Igreat advantage over conventional techniques by permitting ( loperation at vcry high current densities.Presently, in the Ikno~ln indust'rial processes, the current density seldom .
lexceeds 400-1000 A/m . In fact, above these limits, a~
electrolyte containing a greater quantity of chloride has to be utilized to achieve acceptable current efficiency for .
chlorine evolution, but a solution with a concentration above l40 g/l of chlor-ide is economically and technically disadvant-10 ¦lageous because of the large waste of chloride and an excessiv , ~¦chloride content in the effluent would occur.
¦ By the process of the present invention, it is posslble to operate at a current density which may be 4000_5000 A/m2 with a chlorine discharge current efficiency ¦¦higher than 85%. When opcrating with a current density, for ¦jexamplc within 2000-3500 A/m2, it is possible to maintain a ¦current efficiency for chlorine evolution higher than 96~
with an extraordinary increase in the anode lifeti~e ~ecause.
Ithe anodes arc no longer subjected to the undesired, exces-¦sive oxygen evolution which causes the ].oss of catalytic activity of the noble metal oxide coating. Considering that chlorine evolution is thermodynami.cally favored ~ith rcl;pcct to oxygcn evolution, t~le fact that the major portion !or the active anodic surfacc is const<lntly wetted by con-~centrated brine favors chlorine discharge and effectively ¦prevents oxygen discharge, even at relatively high current (.~ ~ denslty l . .
! Meanwhile~ the chloride ions whlch tend to ldirfuse ! rrom the concentrated solution through the porous anode to iithe aqueous electrolyte circulating in the interelectrodic 1' where necessary ligap because Or the concentration gradient and~dlso to a !, sl~ght pressure gradient ~urposedly maintained across the l!Porous anode, are oxidized to chlorine while passing across ¦~the porous anode. Therefore, the chloride losses in the :' c ~" ,'" "- s ¦electrolyte,which ic in the hy~ochlorite cclution produced, Itare greatly reduced. The wei~ht ratio between the active ~Ichlorine and the chloride ion contained in the effluent may ~Ibe easily maintained above 0.2 and, preferably it is maint-¦ ained between 0.3 and 0.8.
Ii Reduction of water takes place at the surface Or the ¦¦cathode which is made of titanium, steel or other suitable !?material-and the evolved gaseous hydrogen is vented from the il . I
. - I cell together with the electrolyte, while hydroxyl ions are formed according to the reaction:
H20 + e ' 2 2 IlThe chlorine evolved at the anode readily reacts with the l~hydroxide ions producing hypochlorite through the kno~n ¦ chemical reaction.
Practically~ depending upon the desired minimum ratio between the concentrations of the active chlorine and I the chloride contained in the cell efrluent water, the more ¦ - concentrated the brine is, the higher is the current t density which can be used while maintaining a very high cur-¦ rent efficiency for the halogen discharge and therefore a , low oxygen evolutionO
~ .172D92 1~ To provide a surricient supply Or chloride lons to the anode and to inhibit oxygen evolution, especially when jusin~ a porous anode with low permeability which is an anode ¦with a lower porosity and/or smaller mean diameter pores, a ¦certain pressure differential may be maintained through the Iporous anode to provide ror a sufficient hydrodynamic flow the concentrated brine across the porous anode towards the electrolyte flowing through the interelectrodic gap. It has l been found that the required pressure differential seldom exceeds 1 meter of water column. Practical~, Zto 25 centimete~so water column are sufficient when utilizing commercial sinter~
valve metal. Under the above conditions, an automatic contro~
of the pressure differential between the brine compartment and the water compartment should be provided to maintain the optimal process operation under all conditions, since possible pressure variations of the electrolyte due to transiL
tory causes could adversely affect the electrolytic water chlorination process~
According to a preferred embodiment ~ith an auto-matic control system, two pressure gauges are provided in the brine and in the electrolyte compartments respectively.
The algebraic difference between the pressure in the t~o com-partments is compared with a fixed rererence signal which ~¦can be preset for establishing a certain pressure difference ¦¦and acts on a control valve placed in the brine circuit downstream from the brine compartment so that the pressure difference across the porous anode can b~ kept constant inde-¦
pendent of pressure variations which may occur either in the !
water or in the brine circuit.
~., .~
t -~ ~ 7 ? ~ ~3 ,~ t The brine conccntration in the cell compartmcnt is maintained substantially constant by controllin~ the brine recyclin~ speed, while il t ~the quantity of the water flo~ving between the porous anode and the cathode . '.
may vary witnin large limits depending on the desired hypochlorite (tnat ''is activc chlorine) content in the efflucnt, Therefore, the electrolyte ~, jspeed through the interelcctrodic gap may vary within limits ranging ¦from I cm/second to 1. 0 m/second.
¦i, Alternatively, instead of utilizing re-cycllng o the brine for ,maintaing constant the brine concentration, conce~trated orine may be !fed continously into the anode compartrnent by means of an adjustable i'c~r automatically controlled dosing pump capable of providing for the ¦~necessary hydraulic flow of tne brine through the porous anode.
t~ Other factors, besides tne current efficiency for chlorine discharge at the ancdç, may affect the overall current efficiency of the electrolyte. f In particular, tne hypochlorite formed at the anode tends to decompose ~vitn consequent oxygen evolution or to dismutate to chlorate and further-more cathodic reduction of the hypochlorite occurs at the cathode. While the first two chemical reactions are readily minimized by operating at jlow temperatures, that is to say below 25 C, the cathodic reduction of ;hypochlorite according to the reaction ¦
C10- + H20 + 2e~ Cl- + 20H
iLS competitive with hydrogen evolution since its standard potential is ¦about 1 Volt greater than tne water reduction potcntial. Accordingly, ~the kinetic of tnis reaction is controlled by the convective and diffusive transport of hypochlorite from the anode towards the c~thode. This fecondary reaction is a major cause of the overall current efficiency ~loss. The loss due to this secondary reaction may be reduced by impart-ing to the electrolyte a laminar-flow throu~h the , .~ I - 12 -~729~2 I
interelectrodic gap. Nevertheless, the mass transfer of hypochlorite from the bul'~ Or the electrolyte to the cathode "surface is proportional to the concentration of the hypo-chlorite in the electrolyte.
In the known processes, the necessity to maintain a rclative]y high chloride concentration in the electrolyte flowing throuc~,h the cell does not permit direct chlorination ¦of water, that is with a concentration of active chlorine (hypochlorite) as low as 1 to 2 mg/l, and therefore it is o !i necessary to obtain a more concentrated solution containing ¦¦up to 6-8 g/l of activc chlorine (hypochlorite), which is ,Ithen dilutcd into the main stream of water. At thess~
¦Ihigh concentrations of hypochlorite, the loss of efficiency due to the cathodic reduction of hypochlorLte is very ;
-5 ¦ h:igh and the ovcrall current efficiency may drop to lcss than 60%. In contrast thereto, the process of the inventionp~-rsnitC
chlorination Or water directly to concentration on the order ¦
1-2 mg/l of active chlorine rcducing the loss Or efficienc Y
Idue to the cat;hodic reduction of hypochlorite. These con-¦ditions are easily achieved by increasin~ the speed of the lelcctrolyte (water) in the ccll and/or by allowing the ¦ ~rcatcr part o~ the water stream to by-pass the interelect-I rodic eap of thc cell. Ilowcver, it ls also possible, by the ¦¦ process of the invcntion, to produce an effluent solution I with an hypochlorite contcnt as high as 8-10 ~/1 by increas-ing the residcnce tirnc Or the electrolytc in the ccll or by using more cells in serics in the usual manner.
(. I ~
1 . _ . . ~. ,1 ~ .
I
Another remarkable loss in the overall current lerriciency is caused by the anodic oxidation of hypochlorite !
¦~to chlorate according to the reaction:
6 C10- ~ 3H20 - _ 2C103 + 4Cl- + 6H+ + ~2 + 6e The higher the hypochlorite concentration in the electrolyte and the current density are, the greater is the loss due to this reaction. Considering that hypochlorite is formed at the anode surface, it is practically impossible in cbnvent-ional cells to control or to reduce this loss since the laminar flow imposed on the electrolyte for reducing hypochlo-rite diffusion towards the cathode and to prevent the ¦cathodic re~uction of the hypochlorite can only ma~.e even -¦worse the conditions of a fast removal of hypochlorite which continuo~sly forms at the anodic surface. By the process ~of the present invention, the loss of efficiency from this cause . is also effectively reduced.
As a matter of fact, it has been found that ~y provid-ing a small hydrodynamic flow of the brine through the porous anode, the overall current efficiency is drastically increased. Thls is presum~.bly due to various factors which positively reduce the tendency of hypochlorite to be o~idized to chlorate at the anode. One of these factors is represente 1 by the fact that chloride ions are effectively and constantly supplied directly ~t the anodic double layer over the entire ~ctive surface Or the ano~e. Another factor is the component of the hydrodynamic flow which tends to drag out, that is to remove from the active surf~ce of the -. . .
, . '.
i 29g~
¦~anodc, the hypochlorite produced. Thc brinc flow thro~ h tnc porons .anode offcrs furtherrnorc the aclvantage of reducing the ccll voltage.
.On the other hand, within certain limits, this 1O~v does not substantially ~lower the ratio betwcen the active chlorinc and the chloride con~ainecl in 1 i the effluent as has Deen observed during the experiments.
The novel diaphragmless electlolytic cell of the invcntion is ,comprised of a first compartment containing a porous, permeable anode lland a cathode forming an interelectrodic gap, a second cornpartment ¦'separated from the first compartment by the porous, permeable anode, l means for introducing a solution containing anodically oxidable chemical I
¦¦species into the second compartment, means for circulating an electrolyto l through tne interelectrodic gap of the flrst compartment, means for !l controlling the .hydrodynamic flow of the solution, contained in said llsecond compartment, tnrough the porous anode into the sUpportina ¦¦electrolyte and means for impressing an electrolysis current between jthe said anode and cathode.
. ¦ The novel diaphragmless electrolytic cell of tne invention is parti- ¦
¦¦cularly suited for releasing anodically oxidated chemical species (e. g.
IICl2) into a supporting electrolyte 1owing through the cell (e. g. water) while restraining tne release of the non-oxidated species (e. g. Cl-) into Ithe supporting electrolyte.
The porous anode is capable of restraining tne intermixing Detween 1 the solution fe~ into the second compartment of tne cell and the support- ' ¦ing electrolyte flowing throu~h the interelectrodic space of the cell where !by a large portion of the oxidizable cihemical species ~vhich are passed I -_ 15- 1~
-' Il' , ,, ~ i 7 2 ~ 9 ~ , , . . . .
!l ,.
through thc porous anode by diffusion and, whcre necessary, also by !
hydrodynamic flow, are anodically oxidi~sed before being released in the supporting clectrol~te flowing through the cell. !
l The porous anode of the cell has a thickness comprised in the ¦ range from 0. 5 to 5 mm, a porosity ranging from 20 to 70% and an average pore diameter ranging from 5 to 100 microns. More preferably, the anode has a porosity ranging from 30 to 65 and an average diameter ' of the pores comprised between 5 and 50 microns. .
j; The porous anode is made o an anodically resistant conductive !
Imaterial and is preferably made of sintered valvc metal activated with a non passivating catalytic material such as a noble metal or a noble metal oxide. ¦
' The cat`node is made of a corrosion resistant conductive material l,presenting a low overpotential for the cathodic reaction supporting the , electrolysis current impressed on the cell. In case of hydrogen evolution ¦
~, ¦from aqucous supporting electrolytcs the cathode is preferably made of
¦ After activation, the porous anode surface facing ¦the cathode surface may optionally be provided with a thin 1! porous layer of a material such as polytetrafluoroethylene ¦lor other halogenated polymert~ m~ X surfacehydrophobic for !!retarding the transport of the aqueous phase across the ¦jporous anode. The porous layer of polytetrafluoroethylene or ~¦other suitable material may be applied onto the porous anode ¦,sur~ace~ for example, by depositing a solution or an emulsion 'tor the polymer, removing the solvent and subjecting the sur-~ace to heat treatment. Spray applicatlon o~ polymer may bc used as well~
11 ' .
1 17299~
I' I
!I The proccss Or the present invention presents the !Igreat advantage over conventional techniques by permitting ( loperation at vcry high current densities.Presently, in the Ikno~ln indust'rial processes, the current density seldom .
lexceeds 400-1000 A/m . In fact, above these limits, a~
electrolyte containing a greater quantity of chloride has to be utilized to achieve acceptable current efficiency for .
chlorine evolution, but a solution with a concentration above l40 g/l of chlor-ide is economically and technically disadvant-10 ¦lageous because of the large waste of chloride and an excessiv , ~¦chloride content in the effluent would occur.
¦ By the process of the present invention, it is posslble to operate at a current density which may be 4000_5000 A/m2 with a chlorine discharge current efficiency ¦¦higher than 85%. When opcrating with a current density, for ¦jexamplc within 2000-3500 A/m2, it is possible to maintain a ¦current efficiency for chlorine evolution higher than 96~
with an extraordinary increase in the anode lifeti~e ~ecause.
Ithe anodes arc no longer subjected to the undesired, exces-¦sive oxygen evolution which causes the ].oss of catalytic activity of the noble metal oxide coating. Considering that chlorine evolution is thermodynami.cally favored ~ith rcl;pcct to oxygcn evolution, t~le fact that the major portion !or the active anodic surfacc is const<lntly wetted by con-~centrated brine favors chlorine discharge and effectively ¦prevents oxygen discharge, even at relatively high current (.~ ~ denslty l . .
! Meanwhile~ the chloride ions whlch tend to ldirfuse ! rrom the concentrated solution through the porous anode to iithe aqueous electrolyte circulating in the interelectrodic 1' where necessary ligap because Or the concentration gradient and~dlso to a !, sl~ght pressure gradient ~urposedly maintained across the l!Porous anode, are oxidized to chlorine while passing across ¦~the porous anode. Therefore, the chloride losses in the :' c ~" ,'" "- s ¦electrolyte,which ic in the hy~ochlorite cclution produced, Itare greatly reduced. The wei~ht ratio between the active ~Ichlorine and the chloride ion contained in the effluent may ~Ibe easily maintained above 0.2 and, preferably it is maint-¦ ained between 0.3 and 0.8.
Ii Reduction of water takes place at the surface Or the ¦¦cathode which is made of titanium, steel or other suitable !?material-and the evolved gaseous hydrogen is vented from the il . I
. - I cell together with the electrolyte, while hydroxyl ions are formed according to the reaction:
H20 + e ' 2 2 IlThe chlorine evolved at the anode readily reacts with the l~hydroxide ions producing hypochlorite through the kno~n ¦ chemical reaction.
Practically~ depending upon the desired minimum ratio between the concentrations of the active chlorine and I the chloride contained in the cell efrluent water, the more ¦ - concentrated the brine is, the higher is the current t density which can be used while maintaining a very high cur-¦ rent efficiency for the halogen discharge and therefore a , low oxygen evolutionO
~ .172D92 1~ To provide a surricient supply Or chloride lons to the anode and to inhibit oxygen evolution, especially when jusin~ a porous anode with low permeability which is an anode ¦with a lower porosity and/or smaller mean diameter pores, a ¦certain pressure differential may be maintained through the Iporous anode to provide ror a sufficient hydrodynamic flow the concentrated brine across the porous anode towards the electrolyte flowing through the interelectrodic gap. It has l been found that the required pressure differential seldom exceeds 1 meter of water column. Practical~, Zto 25 centimete~so water column are sufficient when utilizing commercial sinter~
valve metal. Under the above conditions, an automatic contro~
of the pressure differential between the brine compartment and the water compartment should be provided to maintain the optimal process operation under all conditions, since possible pressure variations of the electrolyte due to transiL
tory causes could adversely affect the electrolytic water chlorination process~
According to a preferred embodiment ~ith an auto-matic control system, two pressure gauges are provided in the brine and in the electrolyte compartments respectively.
The algebraic difference between the pressure in the t~o com-partments is compared with a fixed rererence signal which ~¦can be preset for establishing a certain pressure difference ¦¦and acts on a control valve placed in the brine circuit downstream from the brine compartment so that the pressure difference across the porous anode can b~ kept constant inde-¦
pendent of pressure variations which may occur either in the !
water or in the brine circuit.
~., .~
t -~ ~ 7 ? ~ ~3 ,~ t The brine conccntration in the cell compartmcnt is maintained substantially constant by controllin~ the brine recyclin~ speed, while il t ~the quantity of the water flo~ving between the porous anode and the cathode . '.
may vary witnin large limits depending on the desired hypochlorite (tnat ''is activc chlorine) content in the efflucnt, Therefore, the electrolyte ~, jspeed through the interelcctrodic gap may vary within limits ranging ¦from I cm/second to 1. 0 m/second.
¦i, Alternatively, instead of utilizing re-cycllng o the brine for ,maintaing constant the brine concentration, conce~trated orine may be !fed continously into the anode compartrnent by means of an adjustable i'c~r automatically controlled dosing pump capable of providing for the ¦~necessary hydraulic flow of tne brine through the porous anode.
t~ Other factors, besides tne current efficiency for chlorine discharge at the ancdç, may affect the overall current efficiency of the electrolyte. f In particular, tne hypochlorite formed at the anode tends to decompose ~vitn consequent oxygen evolution or to dismutate to chlorate and further-more cathodic reduction of the hypochlorite occurs at the cathode. While the first two chemical reactions are readily minimized by operating at jlow temperatures, that is to say below 25 C, the cathodic reduction of ;hypochlorite according to the reaction ¦
C10- + H20 + 2e~ Cl- + 20H
iLS competitive with hydrogen evolution since its standard potential is ¦about 1 Volt greater than tne water reduction potcntial. Accordingly, ~the kinetic of tnis reaction is controlled by the convective and diffusive transport of hypochlorite from the anode towards the c~thode. This fecondary reaction is a major cause of the overall current efficiency ~loss. The loss due to this secondary reaction may be reduced by impart-ing to the electrolyte a laminar-flow throu~h the , .~ I - 12 -~729~2 I
interelectrodic gap. Nevertheless, the mass transfer of hypochlorite from the bul'~ Or the electrolyte to the cathode "surface is proportional to the concentration of the hypo-chlorite in the electrolyte.
In the known processes, the necessity to maintain a rclative]y high chloride concentration in the electrolyte flowing throuc~,h the cell does not permit direct chlorination ¦of water, that is with a concentration of active chlorine (hypochlorite) as low as 1 to 2 mg/l, and therefore it is o !i necessary to obtain a more concentrated solution containing ¦¦up to 6-8 g/l of activc chlorine (hypochlorite), which is ,Ithen dilutcd into the main stream of water. At thess~
¦Ihigh concentrations of hypochlorite, the loss of efficiency due to the cathodic reduction of hypochlorLte is very ;
-5 ¦ h:igh and the ovcrall current efficiency may drop to lcss than 60%. In contrast thereto, the process of the inventionp~-rsnitC
chlorination Or water directly to concentration on the order ¦
1-2 mg/l of active chlorine rcducing the loss Or efficienc Y
Idue to the cat;hodic reduction of hypochlorite. These con-¦ditions are easily achieved by increasin~ the speed of the lelcctrolyte (water) in the ccll and/or by allowing the ¦ ~rcatcr part o~ the water stream to by-pass the interelect-I rodic eap of thc cell. Ilowcver, it ls also possible, by the ¦¦ process of the invcntion, to produce an effluent solution I with an hypochlorite contcnt as high as 8-10 ~/1 by increas-ing the residcnce tirnc Or the electrolytc in the ccll or by using more cells in serics in the usual manner.
(. I ~
1 . _ . . ~. ,1 ~ .
I
Another remarkable loss in the overall current lerriciency is caused by the anodic oxidation of hypochlorite !
¦~to chlorate according to the reaction:
6 C10- ~ 3H20 - _ 2C103 + 4Cl- + 6H+ + ~2 + 6e The higher the hypochlorite concentration in the electrolyte and the current density are, the greater is the loss due to this reaction. Considering that hypochlorite is formed at the anode surface, it is practically impossible in cbnvent-ional cells to control or to reduce this loss since the laminar flow imposed on the electrolyte for reducing hypochlo-rite diffusion towards the cathode and to prevent the ¦cathodic re~uction of the hypochlorite can only ma~.e even -¦worse the conditions of a fast removal of hypochlorite which continuo~sly forms at the anodic surface. By the process ~of the present invention, the loss of efficiency from this cause . is also effectively reduced.
As a matter of fact, it has been found that ~y provid-ing a small hydrodynamic flow of the brine through the porous anode, the overall current efficiency is drastically increased. Thls is presum~.bly due to various factors which positively reduce the tendency of hypochlorite to be o~idized to chlorate at the anode. One of these factors is represente 1 by the fact that chloride ions are effectively and constantly supplied directly ~t the anodic double layer over the entire ~ctive surface Or the ano~e. Another factor is the component of the hydrodynamic flow which tends to drag out, that is to remove from the active surf~ce of the -. . .
, . '.
i 29g~
¦~anodc, the hypochlorite produced. Thc brinc flow thro~ h tnc porons .anode offcrs furtherrnorc the aclvantage of reducing the ccll voltage.
.On the other hand, within certain limits, this 1O~v does not substantially ~lower the ratio betwcen the active chlorinc and the chloride con~ainecl in 1 i the effluent as has Deen observed during the experiments.
The novel diaphragmless electlolytic cell of the invcntion is ,comprised of a first compartment containing a porous, permeable anode lland a cathode forming an interelectrodic gap, a second cornpartment ¦'separated from the first compartment by the porous, permeable anode, l means for introducing a solution containing anodically oxidable chemical I
¦¦species into the second compartment, means for circulating an electrolyto l through tne interelectrodic gap of the flrst compartment, means for !l controlling the .hydrodynamic flow of the solution, contained in said llsecond compartment, tnrough the porous anode into the sUpportina ¦¦electrolyte and means for impressing an electrolysis current between jthe said anode and cathode.
. ¦ The novel diaphragmless electrolytic cell of tne invention is parti- ¦
¦¦cularly suited for releasing anodically oxidated chemical species (e. g.
IICl2) into a supporting electrolyte 1owing through the cell (e. g. water) while restraining tne release of the non-oxidated species (e. g. Cl-) into Ithe supporting electrolyte.
The porous anode is capable of restraining tne intermixing Detween 1 the solution fe~ into the second compartment of tne cell and the support- ' ¦ing electrolyte flowing throu~h the interelectrodic space of the cell where !by a large portion of the oxidizable cihemical species ~vhich are passed I -_ 15- 1~
-' Il' , ,, ~ i 7 2 ~ 9 ~ , , . . . .
!l ,.
through thc porous anode by diffusion and, whcre necessary, also by !
hydrodynamic flow, are anodically oxidi~sed before being released in the supporting clectrol~te flowing through the cell. !
l The porous anode of the cell has a thickness comprised in the ¦ range from 0. 5 to 5 mm, a porosity ranging from 20 to 70% and an average pore diameter ranging from 5 to 100 microns. More preferably, the anode has a porosity ranging from 30 to 65 and an average diameter ' of the pores comprised between 5 and 50 microns. .
j; The porous anode is made o an anodically resistant conductive !
Imaterial and is preferably made of sintered valvc metal activated with a non passivating catalytic material such as a noble metal or a noble metal oxide. ¦
' The cat`node is made of a corrosion resistant conductive material l,presenting a low overpotential for the cathodic reaction supporting the , electrolysis current impressed on the cell. In case of hydrogen evolution ¦
~, ¦from aqucous supporting electrolytcs the cathode is preferably made of
4' l'valve metals, nickel, iron, silver Or alloys thereof;
I' The cell of the invention is useful for a variety of electrochemical :
: processes wherein it is advantageous to restrai n the release of un-oxid-'lated chcmical species into the supporting electrolyte flowing through the interelectrodic gap of the cell.
Referring now to tne drawings:
~¦ r`ig. 1 is a schematic diagram of the process of the present invent-ion and ~5 1' Fig. 2 is a partial cross-sectional view of an electrolysis cell of jlthe present invention.
Fig. 3 is a cross-sectional view of the cell of Fig. 2.
Fig. ~ is a cross-sectional viev. of another embodirtlcntof an lelectrolysis ccll of the present inventiOn.
I Fig. 5 is a partial cross-sectional vicw of an alternative embodi-~rneDt of an lectrolysis ccll of the prasent iQvc~tion and - 15 bis -'i I! , . . l ~7>9g,~
Fig. 6 is a cross-sectional view of' the cell of Flg. 5 With reference to Flg. 1, the apparatus of the jlinvention comprises an electrolysis cell 1 consisting Or a !i compartment 2 containing a cathode 3 made of steel or nickel ¦¦or Hastelloy*or titanium or other electrically conducting !j material exhibiting a low overvoltage to hydrogen evolution ¦land a compartment 3 separated from compartment 2 by a porous lanode 5 made of a sintered valve metal activated with non , ¦passivatable and electrocatalytic material resistant to the anodic environment. The brinè circuit is comprised Or a saturation tank 6 where salt and the brine solution are con-¦ tacted, a circulating pump 7, preferably upstream to the ¦electrolytic cell 1, a second valve 8 downstream to the electrolytic cell and a suitable direct curr,ent supply.
The electrolyte leaving the compartment 2 after lelectrolysis contains hypochlorite and the hydrogen which is ¦produced at the cathode. A suitable gas separation tank l allows the venting of gaseous hydrogen from the effluent ¦ hypochlorite solution. The automatic control of the optimal ¦ pressure differential through the porous anode 5 may be effected by two pressure gauges 10 and 11 in the brine com-partment 5 and in the water compartment 2, respectively.
Pneumatic or electric signals rrom the two gauge~ are suit-jlably operated by a control device which, through an actua~in sYstem, acts on control valve 8.
. ¦ ~ Trade mark for high-strength, nickel-base, corrosion- .
. ~ resistant alloys.
' -16-i !i j .
i ~729~2 I
. . .
Fig. 2 shows schematlcally a typical embodiment of llthe cell according to thc present invention wherein the cell ( l¦is cornprised of a first body consisting of a container 20, ,Ipreferably of titanium, provided with a flange 21. A porous !~ anode 22 made o~ ~intered titanlum activated preferably with mixed oY.ides of ruthenium and titanium is welded to the llinternal perimeter of the flan~e 21. ~ne inlet 23 and one ¦loutlet 24 allow brine to recycle through the compartment.
IIThe second cell body consists Or a steel container 25 on llwhose flange a cathode 25, preferably made of stainless steel ¦lis welded and an inlet 27 and an outlet, not illustrated, are provided at the two ends of container 25. The cathode 26 may consist of a plate or mesh or ex~anded sheet and it ¦ stops short Or the two ends of containcr 25 to allow flowing l Or the electrolyte between the porous anode 22 and cathode 26 .
I An insulating gasket 2~ electrically insulates the cathode from the anode and suitable electric terminals, ~hich are no ¦illustrated, supply electric current to the anodic flange 21 land to the cathodic flange. Fig. 3 is a transversal cross Isection of the ce]l Or Fig. 2 and in both figures, the same parts are indicated by the same number.
l~i~. 4 shows another embodiment of the cell Or the inventlon particularly suitable for small capacities. It l consists Or a porous titanium tube 41 suitably activated wit ¦ an clectrocatalytic coating which is l~elded at one end to a titanium tube l12 and is provlded with a sealing flange 43 an l at the other end to anothcr titanium tube 411. Thc assembly ( is then introduced into a flanged tube l15 made of steel or I
! ~i 172992 stainless steel or nickel ~Jhich is connected to a line of the electrolyte solution by two elbo~rs 46 and 47, preferably mad~
ior inert, non conductive material and provided with suitable ¦
jextensions 48 and ll9 for effecting a hydraullc seal with thc ,titanium tubes. Sealing is effected on one end by gasket 50 ¦and on the other end by pacl~ing box 40 or equivalent. During operation, concentrated brine is continuously circulated in-~side the titanium tube which is connected to the positive pol , IOr the current supply, ~!hile tube 45 is connected to the ~corresponding negative pole. The porous titanium tube 41 ¦acts as the anode and the internal surface of pipe 45 act as jthe cathode.
j By the embodiment incorporating concentric, tubular ¦électrodes such as that illustrated in Fig. 4, high capacity cells can be realized since as large a number of the elements las desired can be assembled in a fashion similar to the con-¦figuration of conventional tube heat exchangers. For example a cell similar to a tube heat exchanger can be realized utili~
z~ng activated porous titanium tubes which allow brine circu-lation inthe casing and water circulation inside the porous tubes. The cathode may consist of an arra~r of rods welded to one of the cell heads and inserted concentrically inside ever I
lanodic tube. Alternatively, brine may be circulated through ¦the porous titanium tubes and water in the tube sheet casing.
IIn this case, foraminous screens concentrically arranged out-¦side the porous titanium pipes and electrically connected to ;the casing act as cathodes.
. .
Il Fig. 5 shows another embodiment Or the cell Or the ¦
j,invention which consists Or a cylindrical container 51 flan~ed at both ends and this container may be easily inserted into a!
jline of the water to be treated. The cylinder may be provide~
liwith two rectangular, flangcd openings, diametrically opposedL
Anode 52 consists Or a rectangular box closed on all sldes an I .
made Or titani.urn or other valve metal, and it is inserted l~into one Or the two openings. The box is closed on one side .
¦,by titanium flange 53 adapted to the flanged opening Or the llcylinder. The large surfaces Or the anode 52 consist, at ¦least partially, Or porous p]ates 54 made Or titanium or othe~
¦sintered valve metal and the porous plates are preferably activated by mixed oxides Or titantium and ruthenium.
TitaniuM flan~e 53 is provided with at.least two . ¦no~zles ,5 and 56 for brine inlet and outlet, respectively an ¦brine is continuo~sly circulated inside the anodic box 52 to ¦
maintain the brine concentration at a constant level. An ¦insulating gasket 57 provides electrical i.nsulation o~ the l anode from container 51 and terminal 58 supplies the anode wlth 1 thc electric currcnt. The cathode Or the cell, place~ in the op~osltc opening, consists Or two sheets 59a and 59b rnade Or steel, n:i.ckel or sta:i.nlcss stccl welded to flange 60 which is adapted to the containcr flarlged openin~ suitable terminal¦
~61 supp].ies the cathode with elcctrlc current. The two shcetls ¦constitutln~ the ccll cathodc are placcd some rnlllirneters ¦~part frorn anode 52 surrace and suitable spacers such as a serles Or pol~v~nyl ~hl r;ide buttons bonded to th~ s~rfacc of 1~
- ' .
.i72992 - l~catho~es 59a and 59bto pr~ventthe electrodes from contactin~
each othcr during assembly or operation.
Fig. 6 is a transversal cross-section Or the cell Or Fig. 5 and in both figures, the same parts are indicated by the same numbers. Fig. ~ shows the spacers 62 consistin~ of Teflon*rods having a diameter Or 3.5 mm bonded to the surface the anodic box 52.
During operation, the electrolyte or the dilute l hypochlorite solutions flows through the container 51 and ¦ part Or the rlow passes between the opposed surfaces of the anode, which is substantially constituted by the activated titanium porous plates on both the larger sides of box 52 and of the cathode consisting of the two sheets 5ga and 5~b. The ¦chlorine evolved at the anode reacts with the hydroxide of the alkali metal to generate hypochlorite, ~hile hydrogen is evolved at the cathode as a result of ~rater electrolysis.
¦The cell is particularly suited for direct chlorination of water, for example in potable ~ater systems or for swimming pool sterilization, etc. In fact, the hypochlorite or active chlorine quantity produced in the unit of time can be easily controlled by varying the current supplied to the cell, pro-portionately to the overall water flo~ through container 51.
The free cross-section of container 51 can be suit-ably dimensioned with respect to the overall current suppliec !to the cell to obtain directly the required concentration of ¦ hypochlorite in the effluent, that is to say concentrations lin the range of 0.5 - 1 mg/l. In this way, collecting means ¦of the concentrated hypochlorite solution and dosing systems ¦ * Trade mark for tetrafluoroethylene fluoroc&rbon reslns.
I -20- .
~ I ~ ~ ~
i ;172992 il L
¦!are no longer required. Moreover, an automatic control can b ~easily rcali~cd for regulating the overall current supplied to the cell proportionate to the electrolyte flow to maintain ,Ithc active chlorine concentration ~n the effluent under the !~ rnaxirnum tolerable limit.
In the following examples there are described several prererred embodiments to illustrate the invention.
IlHowever, it is to be understood that the invention is not ¦lintended to be limited to the specific embodiments.
l EXAMPLE l ¦
The test was conducted with the apparatus of Fig. l using the electrolysis cell of Fig. 2 having an apparent (projected) anodic surface area of about lO cm2. The anode iconsiste_ Or a porous sintered titanium plate with a thicknes of l.5 mm, a porosity of 65% and an average pore diameter Or 25 microns. The porous titanium substrate was de~reased and thenpickled in dilute hydrochloric acid solution for one minute and was then completely immersed in a solution comprised Or 5 ml of 5% hydrochloric acid, lO.8 m~ of TiCl3 (calcu]ated as metal), 9 mg of ~uCl3 (calculated as metal), 0.2 m~ Or SnCll~ 51l20 (calculated as metal), 5 drops Or iso-propyl alcohol and 2 to Ij drops of hydrogen peroxide. The porous ti1;anium was then dried in air at 30C and was heated in a forced air circula~ion oven at 350C for lO m;nutes.
The proccss was repeated a number of times untll the porous titanium showed a weight increase corresponding to 25 ~/m2 and with the resulting electrocatalytic coating. The brine I' .
~ ~ 172992 !, I
¦!PermeabilitY Or the activated porous titanium anode decreased ,,to about 60% o~ the original permeability. !
j, Brinc cont2ining 300 g/l Or sodium chloride was "continuously circulated in the anode compartment Or the cell ~Ito always maintain a sodium chloride content in the brine ,erflucnt above 298 g/l and water was circulated through the cathode compartment. During the test, the temperature in , llthe cell was Mainta-,ined between 14 to 17~C by the circulating ¦Iwater. The hydrostatic pressure differential through the 1 porous anode was kept at zero while the current density was J increased as well as the water flow to keep the concentration ¦of active chlorine equivalent between 100 and 500 mg/l in the ¦effluent electrolyte. The cell voltages, amount Or chloride ¦ion in thc erf'luent and the overall current efriciency are ¦ reportcd~in Table I.
l TAB~E I
_~ _______ _ _ _. _.
Currcnt Cell Cl? equivalent NaCl Active C12/ Overall C~ ,_ density Voltag,e Concentration Concen. ~aCl rent Effi-¦
IA/m2 _ _ V_ _ mg/l _ mg/l Ratio ciency % _ ~ 500 4 98 400 0.2ll5 54 l 1000 4.5 150 600 0.25 68 ¦!2GOO 6 300 1000 0.30 7o 3000 7 350 1100 0.32 65 11ll000 8 350 1090 0.32 55 16 0 , 12__ 500 1000 __ 0.ll6 _ _ _8_ ' The rcsults of Table I show that the ovcra],l currcnt jeffic:icncy tendcd to dirninish as the current density lncreasc _ beyond the limit of between 1000 and 3000 ~/m2 and this was ( Idue to an increase in anodic polarization with conscquent 3o ¦oxygen evolution due to ~atcr electro]ys:is and also due to ~17299~ 1 ~anodic oxidation of the hypoehlorite to chlorate whieh eon- ¦
'tributed to the decrease Or the overall eurrent erfieiency.
,' At thls point, the eurrent density was held at 2000 ,~A/m2 and an increasing hydrostatie pressure differenee was l,applicd through the porous anode by increasing the hydrostati llpre~sure on the brine with respeet to the water in order to ,,supply rnore chloride ions to the anodic surface through the ~iporosity Or the anode. The results are reported on the llrollo~ing Tab]e II.
10 ~I TABLE II
Cell C12 equiv. NaCl Aetive Overall eurl em Voltage eoncent. Concent. C12/NaCl 'rent Effi-~r 1l 0 _ _ _,__ mg/lmg/l Ratio !Cl~n C I 0 -, 6 300 1050 l 0.30 1 70 2.5 1 5.8 310 1050 0.33 74 4' ' 5.1 3O3, 1020 0.37 88.5 6 . o ll.7 394 '1200 33 ' 91.2 The results of Table II elearly show that when a l min:imum hydrodynarnic brine rlow is ereated through the porous ¦anode by m~intaining a certain pressure dirrercnce aeross the porous anode, the eell voltage and conscquently the proeess enerey erficiency and the overall current efriciency are exceptionally :irnproved, Al the same time, under these con-,dit:ions, the ratio betwecn the active chlorine and the Iehloride ion contalned in the electrol,yte leaving the cell ( /as not t ~11 reduced ùut lJghtly ,ml~roved, llhich is .
~ ..
. . .
: !
`
~ 172992 explainablc by the exceptional increase of the overall curren~
efficiency .
Various ~odifications Or the process and elcctrolysi ! cell of the invention may be made ~ithout departing rrom the ¦,spirit or scope.thercof and lt should be understood that the ~! inventlon is intended to be limited only as defined in the jappended claims.
Il .
C
~ . -24-I
I' The cell of the invention is useful for a variety of electrochemical :
: processes wherein it is advantageous to restrai n the release of un-oxid-'lated chcmical species into the supporting electrolyte flowing through the interelectrodic gap of the cell.
Referring now to tne drawings:
~¦ r`ig. 1 is a schematic diagram of the process of the present invent-ion and ~5 1' Fig. 2 is a partial cross-sectional view of an electrolysis cell of jlthe present invention.
Fig. 3 is a cross-sectional view of the cell of Fig. 2.
Fig. ~ is a cross-sectional viev. of another embodirtlcntof an lelectrolysis ccll of the present inventiOn.
I Fig. 5 is a partial cross-sectional vicw of an alternative embodi-~rneDt of an lectrolysis ccll of the prasent iQvc~tion and - 15 bis -'i I! , . . l ~7>9g,~
Fig. 6 is a cross-sectional view of' the cell of Flg. 5 With reference to Flg. 1, the apparatus of the jlinvention comprises an electrolysis cell 1 consisting Or a !i compartment 2 containing a cathode 3 made of steel or nickel ¦¦or Hastelloy*or titanium or other electrically conducting !j material exhibiting a low overvoltage to hydrogen evolution ¦land a compartment 3 separated from compartment 2 by a porous lanode 5 made of a sintered valve metal activated with non , ¦passivatable and electrocatalytic material resistant to the anodic environment. The brinè circuit is comprised Or a saturation tank 6 where salt and the brine solution are con-¦ tacted, a circulating pump 7, preferably upstream to the ¦electrolytic cell 1, a second valve 8 downstream to the electrolytic cell and a suitable direct curr,ent supply.
The electrolyte leaving the compartment 2 after lelectrolysis contains hypochlorite and the hydrogen which is ¦produced at the cathode. A suitable gas separation tank l allows the venting of gaseous hydrogen from the effluent ¦ hypochlorite solution. The automatic control of the optimal ¦ pressure differential through the porous anode 5 may be effected by two pressure gauges 10 and 11 in the brine com-partment 5 and in the water compartment 2, respectively.
Pneumatic or electric signals rrom the two gauge~ are suit-jlably operated by a control device which, through an actua~in sYstem, acts on control valve 8.
. ¦ ~ Trade mark for high-strength, nickel-base, corrosion- .
. ~ resistant alloys.
' -16-i !i j .
i ~729~2 I
. . .
Fig. 2 shows schematlcally a typical embodiment of llthe cell according to thc present invention wherein the cell ( l¦is cornprised of a first body consisting of a container 20, ,Ipreferably of titanium, provided with a flange 21. A porous !~ anode 22 made o~ ~intered titanlum activated preferably with mixed oY.ides of ruthenium and titanium is welded to the llinternal perimeter of the flan~e 21. ~ne inlet 23 and one ¦loutlet 24 allow brine to recycle through the compartment.
IIThe second cell body consists Or a steel container 25 on llwhose flange a cathode 25, preferably made of stainless steel ¦lis welded and an inlet 27 and an outlet, not illustrated, are provided at the two ends of container 25. The cathode 26 may consist of a plate or mesh or ex~anded sheet and it ¦ stops short Or the two ends of containcr 25 to allow flowing l Or the electrolyte between the porous anode 22 and cathode 26 .
I An insulating gasket 2~ electrically insulates the cathode from the anode and suitable electric terminals, ~hich are no ¦illustrated, supply electric current to the anodic flange 21 land to the cathodic flange. Fig. 3 is a transversal cross Isection of the ce]l Or Fig. 2 and in both figures, the same parts are indicated by the same number.
l~i~. 4 shows another embodiment of the cell Or the inventlon particularly suitable for small capacities. It l consists Or a porous titanium tube 41 suitably activated wit ¦ an clectrocatalytic coating which is l~elded at one end to a titanium tube l12 and is provlded with a sealing flange 43 an l at the other end to anothcr titanium tube 411. Thc assembly ( is then introduced into a flanged tube l15 made of steel or I
! ~i 172992 stainless steel or nickel ~Jhich is connected to a line of the electrolyte solution by two elbo~rs 46 and 47, preferably mad~
ior inert, non conductive material and provided with suitable ¦
jextensions 48 and ll9 for effecting a hydraullc seal with thc ,titanium tubes. Sealing is effected on one end by gasket 50 ¦and on the other end by pacl~ing box 40 or equivalent. During operation, concentrated brine is continuously circulated in-~side the titanium tube which is connected to the positive pol , IOr the current supply, ~!hile tube 45 is connected to the ~corresponding negative pole. The porous titanium tube 41 ¦acts as the anode and the internal surface of pipe 45 act as jthe cathode.
j By the embodiment incorporating concentric, tubular ¦électrodes such as that illustrated in Fig. 4, high capacity cells can be realized since as large a number of the elements las desired can be assembled in a fashion similar to the con-¦figuration of conventional tube heat exchangers. For example a cell similar to a tube heat exchanger can be realized utili~
z~ng activated porous titanium tubes which allow brine circu-lation inthe casing and water circulation inside the porous tubes. The cathode may consist of an arra~r of rods welded to one of the cell heads and inserted concentrically inside ever I
lanodic tube. Alternatively, brine may be circulated through ¦the porous titanium tubes and water in the tube sheet casing.
IIn this case, foraminous screens concentrically arranged out-¦side the porous titanium pipes and electrically connected to ;the casing act as cathodes.
. .
Il Fig. 5 shows another embodiment Or the cell Or the ¦
j,invention which consists Or a cylindrical container 51 flan~ed at both ends and this container may be easily inserted into a!
jline of the water to be treated. The cylinder may be provide~
liwith two rectangular, flangcd openings, diametrically opposedL
Anode 52 consists Or a rectangular box closed on all sldes an I .
made Or titani.urn or other valve metal, and it is inserted l~into one Or the two openings. The box is closed on one side .
¦,by titanium flange 53 adapted to the flanged opening Or the llcylinder. The large surfaces Or the anode 52 consist, at ¦least partially, Or porous p]ates 54 made Or titanium or othe~
¦sintered valve metal and the porous plates are preferably activated by mixed oxides Or titantium and ruthenium.
TitaniuM flan~e 53 is provided with at.least two . ¦no~zles ,5 and 56 for brine inlet and outlet, respectively an ¦brine is continuo~sly circulated inside the anodic box 52 to ¦
maintain the brine concentration at a constant level. An ¦insulating gasket 57 provides electrical i.nsulation o~ the l anode from container 51 and terminal 58 supplies the anode wlth 1 thc electric currcnt. The cathode Or the cell, place~ in the op~osltc opening, consists Or two sheets 59a and 59b rnade Or steel, n:i.ckel or sta:i.nlcss stccl welded to flange 60 which is adapted to the containcr flarlged openin~ suitable terminal¦
~61 supp].ies the cathode with elcctrlc current. The two shcetls ¦constitutln~ the ccll cathodc are placcd some rnlllirneters ¦~part frorn anode 52 surrace and suitable spacers such as a serles Or pol~v~nyl ~hl r;ide buttons bonded to th~ s~rfacc of 1~
- ' .
.i72992 - l~catho~es 59a and 59bto pr~ventthe electrodes from contactin~
each othcr during assembly or operation.
Fig. 6 is a transversal cross-section Or the cell Or Fig. 5 and in both figures, the same parts are indicated by the same numbers. Fig. ~ shows the spacers 62 consistin~ of Teflon*rods having a diameter Or 3.5 mm bonded to the surface the anodic box 52.
During operation, the electrolyte or the dilute l hypochlorite solutions flows through the container 51 and ¦ part Or the rlow passes between the opposed surfaces of the anode, which is substantially constituted by the activated titanium porous plates on both the larger sides of box 52 and of the cathode consisting of the two sheets 5ga and 5~b. The ¦chlorine evolved at the anode reacts with the hydroxide of the alkali metal to generate hypochlorite, ~hile hydrogen is evolved at the cathode as a result of ~rater electrolysis.
¦The cell is particularly suited for direct chlorination of water, for example in potable ~ater systems or for swimming pool sterilization, etc. In fact, the hypochlorite or active chlorine quantity produced in the unit of time can be easily controlled by varying the current supplied to the cell, pro-portionately to the overall water flo~ through container 51.
The free cross-section of container 51 can be suit-ably dimensioned with respect to the overall current suppliec !to the cell to obtain directly the required concentration of ¦ hypochlorite in the effluent, that is to say concentrations lin the range of 0.5 - 1 mg/l. In this way, collecting means ¦of the concentrated hypochlorite solution and dosing systems ¦ * Trade mark for tetrafluoroethylene fluoroc&rbon reslns.
I -20- .
~ I ~ ~ ~
i ;172992 il L
¦!are no longer required. Moreover, an automatic control can b ~easily rcali~cd for regulating the overall current supplied to the cell proportionate to the electrolyte flow to maintain ,Ithc active chlorine concentration ~n the effluent under the !~ rnaxirnum tolerable limit.
In the following examples there are described several prererred embodiments to illustrate the invention.
IlHowever, it is to be understood that the invention is not ¦lintended to be limited to the specific embodiments.
l EXAMPLE l ¦
The test was conducted with the apparatus of Fig. l using the electrolysis cell of Fig. 2 having an apparent (projected) anodic surface area of about lO cm2. The anode iconsiste_ Or a porous sintered titanium plate with a thicknes of l.5 mm, a porosity of 65% and an average pore diameter Or 25 microns. The porous titanium substrate was de~reased and thenpickled in dilute hydrochloric acid solution for one minute and was then completely immersed in a solution comprised Or 5 ml of 5% hydrochloric acid, lO.8 m~ of TiCl3 (calcu]ated as metal), 9 mg of ~uCl3 (calculated as metal), 0.2 m~ Or SnCll~ 51l20 (calculated as metal), 5 drops Or iso-propyl alcohol and 2 to Ij drops of hydrogen peroxide. The porous ti1;anium was then dried in air at 30C and was heated in a forced air circula~ion oven at 350C for lO m;nutes.
The proccss was repeated a number of times untll the porous titanium showed a weight increase corresponding to 25 ~/m2 and with the resulting electrocatalytic coating. The brine I' .
~ ~ 172992 !, I
¦!PermeabilitY Or the activated porous titanium anode decreased ,,to about 60% o~ the original permeability. !
j, Brinc cont2ining 300 g/l Or sodium chloride was "continuously circulated in the anode compartment Or the cell ~Ito always maintain a sodium chloride content in the brine ,erflucnt above 298 g/l and water was circulated through the cathode compartment. During the test, the temperature in , llthe cell was Mainta-,ined between 14 to 17~C by the circulating ¦Iwater. The hydrostatic pressure differential through the 1 porous anode was kept at zero while the current density was J increased as well as the water flow to keep the concentration ¦of active chlorine equivalent between 100 and 500 mg/l in the ¦effluent electrolyte. The cell voltages, amount Or chloride ¦ion in thc erf'luent and the overall current efriciency are ¦ reportcd~in Table I.
l TAB~E I
_~ _______ _ _ _. _.
Currcnt Cell Cl? equivalent NaCl Active C12/ Overall C~ ,_ density Voltag,e Concentration Concen. ~aCl rent Effi-¦
IA/m2 _ _ V_ _ mg/l _ mg/l Ratio ciency % _ ~ 500 4 98 400 0.2ll5 54 l 1000 4.5 150 600 0.25 68 ¦!2GOO 6 300 1000 0.30 7o 3000 7 350 1100 0.32 65 11ll000 8 350 1090 0.32 55 16 0 , 12__ 500 1000 __ 0.ll6 _ _ _8_ ' The rcsults of Table I show that the ovcra],l currcnt jeffic:icncy tendcd to dirninish as the current density lncreasc _ beyond the limit of between 1000 and 3000 ~/m2 and this was ( Idue to an increase in anodic polarization with conscquent 3o ¦oxygen evolution due to ~atcr electro]ys:is and also due to ~17299~ 1 ~anodic oxidation of the hypoehlorite to chlorate whieh eon- ¦
'tributed to the decrease Or the overall eurrent erfieiency.
,' At thls point, the eurrent density was held at 2000 ,~A/m2 and an increasing hydrostatie pressure differenee was l,applicd through the porous anode by increasing the hydrostati llpre~sure on the brine with respeet to the water in order to ,,supply rnore chloride ions to the anodic surface through the ~iporosity Or the anode. The results are reported on the llrollo~ing Tab]e II.
10 ~I TABLE II
Cell C12 equiv. NaCl Aetive Overall eurl em Voltage eoncent. Concent. C12/NaCl 'rent Effi-~r 1l 0 _ _ _,__ mg/lmg/l Ratio !Cl~n C I 0 -, 6 300 1050 l 0.30 1 70 2.5 1 5.8 310 1050 0.33 74 4' ' 5.1 3O3, 1020 0.37 88.5 6 . o ll.7 394 '1200 33 ' 91.2 The results of Table II elearly show that when a l min:imum hydrodynarnic brine rlow is ereated through the porous ¦anode by m~intaining a certain pressure dirrercnce aeross the porous anode, the eell voltage and conscquently the proeess enerey erficiency and the overall current efriciency are exceptionally :irnproved, Al the same time, under these con-,dit:ions, the ratio betwecn the active chlorine and the Iehloride ion contalned in the electrol,yte leaving the cell ( /as not t ~11 reduced ùut lJghtly ,ml~roved, llhich is .
~ ..
. . .
: !
`
~ 172992 explainablc by the exceptional increase of the overall curren~
efficiency .
Various ~odifications Or the process and elcctrolysi ! cell of the invention may be made ~ithout departing rrom the ¦,spirit or scope.thercof and lt should be understood that the ~! inventlon is intended to be limited only as defined in the jappended claims.
Il .
C
~ . -24-I
Claims (15)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for halogenating water comprising passing an electrolysis current through a porous permeable anode and a cathode forming an electrodic gap with water passing therethrough, passing an aqueous solution of an alkali metal halide containing at least 25 g/l of said halide through the porous, permeable anode into the electrodic gap and controlling the hydrodynamic flow through the said anode to maintain a weight ratio of active halogen to halide in the water leaving the electrodic gap of at least 0.2.
2. The process of claim 1 wherein the alkali metal halide is sodium chloride and the active halogen contained in the water effluent from the cell is present substantially in the form of sodium hypochlorite.
3. The process of claim 1 wherein the porous and permeable anode is comprised of a rigid element with a layer of sintered valve metal powder having a porosity between 20 and 70% and an average pore diameter between 5 and 100 microns thereon, said porous valve metal layer having been activated by impregnation with an electro-catalytic material exhibiting a low overvoltage to halogen evolution.
4. The process of claim 3 wherein the valve metal is titanium and the electrocatalytic material is at least one metal selected from the group consisting of platinum, palladium, ruthenium, iridium, osmium and rhodium in metallic form and oxides thereof.
5. The process of claim 1 wherein a hydrostatic pressure difference through the thickness of said porous and permeable anode is kept constant and is up to lm of water column in the positive sense from the side opposite the electrodic gap to the side of the anode facing the electrodic gap.
6. The process of claim 1 wherein the electrodic surfaces of the electrolysis cell are parallel to each other and both lie in parallel planes with respect to the flux lines of the water flowing through the cell.
7. The process of claim 1 wherein the concentration of the alkali metal halide solution in contact with the porous and permeable anode is kept substantially constant by con-tinuously re-concentrating the metal halide solution in contact with the anode.
8. The process of claim 5 wherein the hydrostatic pressure difference through the thickness of said porous and permeable anode is automatically kept constant by an auto-matic control means acting on a control valve placed in the alkali metal chloride solution circuit downstream of the electrolysis cell, said control means being actuated by two pressure gauges, one set in the alkali metal chloride solution in contact with the side of the porous anode opposite to the electrodic gap, the other in the water passing through the electrodic gap.
9. A process for the electrolytic production of an aqueous solution of sodium hypochlorite by electrolysis of sodium chloride carried out at a temperature below 25°C, characterized in that water is flowed through an electrolysis cell consisting of a cathode and a porous, permeable anode having low overvoltage to chlorine evolution, the anode surface not exposed to the contact to the water flow being maintained in contact with brine containing at least 25 g/l of sodium chloride and hydrodynamic flow through said porous, permeable anode being limited by controlling the hydrostatic pressure difference through the thickness of said anode to maintain a weight ratio between the active halogen and the halide contained in the solution effluent from the cell greater than 0.2.
10. A diaphragmless electrolysis cell comprising a first compartment containing a porous, permeable anode and a cathode forming an interelectrodic gap, a second compartment separated from the first compartment by the porous permeable anode, means for circulating an electrolyte through the second compartment, means for circulating a second electrolyte through the interelectrodic gap of the first compartment, means for maintaining a constant hydrostatic pressure differential between the two compartments through the said porous anode and means for impressing an electrolysis current between the said cathode and anode.
11. The cell of claim 10 wherein the porous, permeable anode is comprised of a rigid element with at least a layer of sintered valve metal powder, said layer having a porosity between 20 and 70% and an average pore diameter between 5 and 100 microns and having been activated by impregnation with an electrocatalytic material.
12. The cell of claim 11 wherein the valve metal is titanium and said electrocatalytic material is at least one metal selected from the group consisting of platinum, palladium, ruthenium, iridium, rhodium and osmium in metallic form and oxides thereof.
13. The cell of claim 12 wherein the electrocatalytic material is titanium dioxide and ruthenium dioxide in mixed crystal form.
14. The cell of claim 10 wherein the anodic is a body made of sintered titanium powder.
15. The cell of claim 10 wherein the anode is a valve metal mesh provided with a layer of sintered valve metal particles.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000327615A CA1172992A (en) | 1979-05-15 | 1979-05-15 | Halogenation of water using porous anode for on-site halogen production |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000327615A CA1172992A (en) | 1979-05-15 | 1979-05-15 | Halogenation of water using porous anode for on-site halogen production |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1172992A true CA1172992A (en) | 1984-08-21 |
Family
ID=4114203
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000327615A Expired CA1172992A (en) | 1979-05-15 | 1979-05-15 | Halogenation of water using porous anode for on-site halogen production |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1172992A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112759038A (en) * | 2021-01-29 | 2021-05-07 | 陕西科技大学 | Penetrating type electrocatalytic anode and preparation method thereof |
-
1979
- 1979-05-15 CA CA000327615A patent/CA1172992A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112759038A (en) * | 2021-01-29 | 2021-05-07 | 陕西科技大学 | Penetrating type electrocatalytic anode and preparation method thereof |
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