Note: Descriptions are shown in the official language in which they were submitted.
~6~
This .is a divisional application of copending ¦ -
application serial no. 357,494, filed August l, 1980. t
B/~.G~GROUI~ID_0~1 T~[E IN~IE~TI~I~
¦ e
This invention relates ~o a novel rnethod ~f g~nera-;
ting chlorin~ or other halogens by electrolysis of ~n acueousl .
; halide ioncontaining solution such as hydrochloric ~ci~ an~o~ r
al~ali metal chloride or other corres~ondin~ elec~rolys~ble ¦ r
halide. Chlorine has been produced for a long ~ime by suc~
~,electrolysis in a cell ~rherein the anode and the cathode are ¦
,separated by an ion permeable membrane or diaphra~m ~nd ln
;,cells .having a liquid permeable diaphra~m, th~ al~ali me,~a~
,chloride or other halide is circula~ed throu~h the anol~e
chamber and a portion thereo~ ~lolls throu~h the di~nra~ in~c
,Ithe catholyte.
~Ihen an alkali met~l chloride solution is electro-
lyzed, chlor.ine is evolved at the anode and alkali, whi~n may
be all~ali metal carbo.~te or bicarbonate DUt more co~only is~
alkali me~al hydroxide solution, is ~or~ed at the c~ho~
This alkali solution also contains alkal~ met21 chloridQ ~h.~h
must be separated from the.allcali in a s-~bse~uent oo~a~lon
.;
and the said solu~ion is relatively dilu,e, r~rel~ be~ng in
excess Or 12-15% allcali by weih'~. Since com~.ercial co~cen~ ¦
.ration Or sodium hydro~ide normally is abou~ 50~ or hig~er by !
~.eight, t~e water in the dilute solution ~us~. be ev~po~~ed tJ
I ~achieve this conc.entration. ¦ .
; More r~cently~ considerable st~lcl~ h~ en u~ ~n!
~; '
:~ 25 re;pectillg the llse nf ion e~ch~n~g~? resin~ or pol~-;ne~s ~ the I .
.~ .
, ~
'lon perineable diaE~hragrn which polyrners are in the form of
,~thin sheets or mer,lbranes. Generally, they are i~perforate
and do not permit a flotJ of anolyte into the cathode chamber ¦
but it has also been su~gested that such membranes ~ay be
Iprovided with some small perforations to permit a small flow
jlof anolyte therethrourrh, although the bulk of ~he work appears
to have been accomplished with imperforate memDranes. ¦
Typical polymers ~hich ma~ be used for this pur- ¦
~pose include fluorocarbon polymers such as polymers o~ an
,,unsaturated florocarbon. For example, polymers o~ trifluoro-
,~ethylene or tetrafluoroethylene or copolymers ~her~of whichcontain ion exchange ~roups are used for this purpose. The
iion exchange groups normally are cationic ~rou~s including
llsulfon:ic acid, sulfonamide, carboxylic acid~ phosphoric acid
land the like, wh.ich are attached to the fluorocarbon polymer ~
Ichain through carbon and which exchange cat:ions. Ho~e~er, I .
'.they may also contain anion exchange groups. Thus they have ~ ¦
`.the general formula: I
.. j I I 1, .
- C - C - C - C - or
C
~' S0
!!
20 !j - C - C - C -
,~ , C - 0~ 1 '
o
, !
1l 1
!i 1
.
Such mernbranes are t~pically those manllractured b~
the Du Pont Company under the tra(le name of ~afion and by
~Asahi Glass ~o. o~ Japan under the trade narne o~ "F~emion"
"and patents which describe such membranes include British
IPatent No. 1,184,321 and U.S. Patents No. 3,2~2,875 and
,INo. 4,075,405.
~I Since these diaphragms are ion permeable but do no~
!~ permit anolyte flow therethrough, little or no halide ion
llmigrates through the diaphragm of such a material in an a~kaii
',chloride cell and therefore the alkali thus produced contains
,llittle or no chloride ion. Furthermore, it is possible to
i! produce a more concentrated alkali,metal hydroxide in which
the catholyte produced may contaln ~rom 15 to 45% NaO~I by
llweight or ev~n hi~her. Patents ~h:lch descrlbe such a process
! include U.S. Patents No. 4~111,779 and No. 4,100,050 and many
,others. The application of an ion exchan~e membrane as an I
. lion permeable diaphragm has been proposed for other uses such, .
; ~as in water electrolysis.
; It has also been proposed to conduct such electro- ¦
lysis between an anode and cathode separated by a diaphragm,
Ino~ably an ion exchange membrane ~herein the anode or cathode
¦or both are in the form of a thin porous layer o~ electro-
¦,conductive material resistant to electro-chemical attack and
,bonded or otherwise incorpor2ted over the sur~ace o~ the .
,'diaphra~m. Similar electrode-~embrane assemblies h~ve been
.
,l . I :
l l i
~pr~oposed ~or ?. long time for use in fuel cells ~Ihich cells
have been called "solid polymer electrol~te" cells. Such
cells have been used for a long ti7.'e as ~aseous-fuel cells~
and only recently have been successfully adapted for the
~ electrolytic production of chlorine from hydrochloric acid
'lor alkali metal chloride brines.
For ~he production of chlorine in a solid polymer
lelectrolyte cell, the electrodes usually consist of a ~hin,
! porous layer of electroconductive,electrocataly~ic material
~Ipermanently bonded onto the surface of an ion-e~c~ange mem-
,~brane with a binder, usually composed of a fluorinated polyme
such as polytetrafluoroethylene (PT~E~ for example.
According to one of the preferred proce~ures of
,forming the gas permeable electrodes as described in the U.S.
'Paten~ No. 3,297,484~a po~ider of elec~roconduct~ve and
lelectrocatalytic m~terial is blended with an aa~eous disper-
,~sion of poly~etrafluo~ocarbon particles to ~btain a doughymi~ture containing 2 to 20 grams ofpo~rder per gram of poly-
tetrafluoroethylene. The mi~ture, which may be dilu~ed if
Idesired, is then spre2d onto a supporting metal sheet and
,Idried after ~Yhich the po~rder layer is then covered with
alu7~inum foil and pressed at a temperature suEf-icient to
effect sintering of the polytetrafluoroethylene particles to
lobtain a thin, coherent film. After remo~al Or the alu~inum-
'foil by caustic leachlng, the preformed electrode is applie~
I'i
1,i
4-
--: !l
''' 'I
i . ,
to thc~ surface o~ th~ rrl~mbrane ar~d pressed at a temperature I r
su~f:iclent to cause the polytetr~fluoroethylene matrlx t~ ¦
sinter onto the membrane. ~rter rapid quenching, the support
~ing metal sheet is removed and the electrode rernains bonded
Illonto the membrane. ¦
,l As the electrodes of the cell are Intimately bonded
¦onto the opposite surlaces of the membrane separating the
¦anode and the cathode chambers, and are not therefore se~a-
llrately supported by metal struc~ures, it has been d,scovered r
~that the most efficient way to carry and distribute the cur- l c
rent to the electrodes consists in resor~ing to multiple
~contacts uniformly distributed all over the electrode surfacei
by means of current-carrying structures provided with a series
! of project:ions or ribs wh~ch, during the assembly o~ the cell
contact the electrode sur~ace at a mult:Lplic:Lty o~ evenly
,Idistributed points. The membrane, carryin~ on its opposite
surfaces the bonded electrodes, must then be pressed be~reen ¦
the two current-carrying structures or collect~rs, respec-
tively anodic and cathodic.
~ Contrary to ~hat happens in ~uel cells wherein the
reactants are gaseous, the current densïties are s~all and l ¦
,,wherein practically no electrodic side-reactions can occur,
i'the solid electrolyte cells ~sed for electrolysis o~ solutior.s~
llas i~ the particular instance of sodium chloride brine~, give
rise t, proolams Or a dirficult resolutirn In a cell for
- 5 -
~` ! !
~:~J6~Z~ I ~
the electrol~sis of so~ium chloridc brirle~ the follo-;Jing
reactions take place at the various parts of the cell:
- main anode reaction: 2 Cl ~ C12 ~ 2e
' - transport across the membrane: 2 Nai- ~ H20 i~'
il- cathode reaction: 2 H20 ~ 2e~ ~ 20H ~ ~I2
j- anode side-reaction: 2 OH ~ 2 ~ 2H20 ~ ~e~
_ main overall reaction: 2 NaCl 1~- 2H20 ~2NaOH~C12+H2
Therefore,at the anode, besides the desired main
reaction of chlorine discharge~ a certain water oxidation ¦
~lalso occurs wikh consequent oxygen evolution although to an
'lextent held as low as possible. This trend ko oxygen evolu- ¦
tion is particularly enchanced by an alkaline environment at
the active sites of the anode consisting Or the catalyst ~i
,Ipartlcles contacti~lg the membrane. In fact, the catio~l-ex-
¦change membra~ssuitable for khe eleckrolysls of all~all metal
halides have a transfer number different from the unit and, ¦
under khe conditions of high allcalinity existin~ in the catho-
lyte, some of these membranes allo~ some migration of h~-dro;~y
anions from the catholyte to the anolyte across the membrane.l ~
iMoreover, the conditions necessary for an efficient transfer l 'r
¦¦of liquid electrolytes to the active surfaces of the elect- ¦
; l¦rodes and for gas evolution there-~require anode and c2thode
llchambers characterized by f]ow sections for the electrolytes ¦
Z1
; ,iand ~ases much larger relatively than those adopted in fuel
Icells
l .~
I - 6 -
I ~ 1
The electrodes mu t conversely nave a minirnum
,thickness, usually in the range of 40 - 150 ~mg to allolr an
~efficient mass exchange with t~le bulk o~ the liquid electro-
,'lyte. Because of this requirement as ~ell as the fact that
¦I the electrocatalytic and electroconductive materials con-
Istituting the electrodes, particularly the anode, are fre v
l! quently a mixed oxide comprising a platinum group metal oxide
¦¦or a pulverulent metal bonded by a binder having little or
- I no electroconductivity, the electrodes are barely conduc~ive
!¦in the direction of their major dimension. Therefore, a hign
lldensity of contacts with the collector is required ~s well as!
,la uniform contact pressure to limit the ohmicdrop through the,
~cell and to afford a uniform current density all over the
~la~tive surface of the cell.
~l ~hese requirements have been so far extremely hard
'Ito ~ulfill, especially ln cells characterized by large sur- ,
faces such as the ones industrially employed in plants for
the production of chlorine having capacities generally
~greater than one hundred tons of chlorine a day. Industrial '
! electrolysis cells require, for economic reasons, electrodic ,
surfaces in the range of at least 0.5~ pre~erably 1 to 3~ squarie
`meters or greater and are often electrically connected în j
¦¦series to form electrolyzers comprising up to several tens of¦ ~-
,¦bipolar cells asseTnbled by means of tie rods or hydraulic or ~`
,! pneumatic jac~s in a filter~resstype arrangement. ~
'' 7 1~
, , 1i I .
~36~
Cell.s of l;his slze pose gre~t technologic~l pro~
blems Wit~l respect to ~roducing curren~ carrying structures,
that is current collec~ors, with extremely low toleYances for;
the planarity of the contacts and to pro~ide a uniform con-
tact pressure over the electrode surface after the assemblin~ ;
if the cell. Moreover, the membrane used in such cells must ¦ F
,be very thin to limit the ohmic drop acrossthe solid electro~
jlyte in the cell which thickness is often less than 0.2 mm j ~~
and rarely more than 2 millimeters and the membrane may be ¦
~easily ruptured or u~d~lythinned out at the points where an
,excessive press~re is applied thereto during the closing of j
~the-cell. Therefore, both the anodic and the cathodic collecL
tor, besides being alMost perfectly planar, must also be ¦
lalmost exactly parallel.
'I In cells of small size, a high degree of planarity ¦
and parallelism can be maintained while providing a certain
~lexlbility of th~ collectors to make up for the slight
deviations from an exact planarity and parallelism. In
commonly assigned, copending Canadian ~PPlication Serial ~o.
332,470 f~led July 24, 1979, there is disclosed a solid elec~ro
lyte monopolar type cell for the electrolysis of sodum chlo-
rid~ ~rherein both the ~nodic and the cathodic current collec-¦
,, ~.
tor consist of screens or expanded sheets welded onto
respec~ive series of vertical metal ribs which are offse~
~ 25 from one another whereby a certain bending of the screens s-
; Iduring the assembly o~ the cell is permi~ted to exert a more .
~nif`oFm pressure on the membrane surfaces
1 ~1
~, ~
: .
-.`' ' ' : ~'
~3~2~ ~ f',
In commonl~ assioned, co ~ clinq ~an~an P~tent P~ tion
Serial Mo. 3~9,714 ~iled ~. June 14, 1979, now ~nadi~n ~atent no.
1,127,706, a solid electrolvte ~iDolar- ~ e cell is de~cribe~ for t~e
electrolysis of s ~ u~ chloride~.7herein the ~iFolar se~a~abors are
~provided on both sides thereof and in the area correspondin~
ito the electrodes with a series of ribs or pro~ections. To
lmake up for the slight deviations from planarity and paral- i
lelism, the insertion o~ a resilient means consisting of two
'or more valve metal screens or expanded sheets coated lti ~h a ¦ ~.
Inon-passivatable material is con~emplated, said resiLient
f means being compressed between the anode-sid~ ribs and the
anode bonded to the anodic side of the nlembrane.
It has been observed, however, tha~ both of these
solutions as proposed in the sa:Ld t~ro Patent ~pplications
, _ I
jentail serlous llm-ltations a~d disadvantages in cells
characterized by large electrod:ic surfaces. In the first ¦
instance, the desired uniformity of contact pressure tends to
; -be lacking, thus Giving rise to current concentrations at
`points Or greater contact pressure with consequen-t polariza-
tion phenomena and the related deactivation Or the membrane t
,and of the catalytic electrodes and localized ruptures of ¦
Ithe membrane and localized mechanical losses o~ catalytic
~material often occur during the assembly o~ the cell. In the I ~
! second instance, a very high planarity and parallelism o.~ th ! .
Ibipolar separator surfaces must be provided for but this
,,1,
''I
g _
, 1l . l
,
~3~
requires precise and costly machining of the ribs and of the
seal surface of the bipolar separator. Moreover, the high
rigidity of the elements entails pressure concentrations which
tend to accumulate along the series thereby limiting the number
of assemblable elements in a single filter-press arrangement.
As a result of these dificulties, a current
distributor screen when pressed against the electrode may even
leave some electrode areas untouched or contacted so lightly
that they are essentially ineffective. Comparative tests which
have been made by pressing the distributor screen against
pressure sensitive paper capable o-f showing a visible
impression corresponding to the screen have shown that
substantial areas ranging about 10 percent to as high as 30 to ~-
40 percent of the screen area produce no marking o~ the paper
and this indicates that these unduly large areas remain
untouched. Applying this ohservation to the electrodes, it
appears that substantial electrode surface areas are
inoperative or substantially so.
THE I~VENTION
In a first claimed aspect of the invention there is
provided an ion-permeable diaphragm-type or membrane-type
electrolysis cell provided with at least one set of anode and
cathode separated by an ion-permeable diaphragm or membrane,
the improvement comprising at least-one of the anode and
cathode being an electrode structure comprising a planar coarse
mesh metal screen and a thin fine mesh metal screen having an
-- 10 --
;
rm/~
.~
.
i ,~
.' . " .
~ .
:~36~
electrocatalytic surface and having at least ten strands or
meshes per inch and being disposed over the coarse mesh metal
screen and in electrical contact therewith, the fine mesh
screen directly facing the ion-permeable diaphragm or membrane.
In a second claimed aspect of the invention there is
provided a foraminous anode structure ~or electrolytic cells
consisting essentially of a thin fine mesh valve metal screen
coated with an electrocatalytic material and having at least
ten strands or meshes per inch disposed over a planar coarse
mesh valve metal screen substantially thicker and more rigid
than the coated fine mesh screen and in electrical contact
therewith, the coarse mesh, more rigid metal screen being
mechanically and electrically connected to support means to
supply electrical current to the anode.
The novel electrolysis cell of the invention
compri ses a cell housing containing at least one set of
electrodes of an anode and a cathode separated by an
ion-permeable diaphragm or membrane, means for introducing an
electrolyte
~ -- 11 --
.~'
.
'! 1236
to be electroiyzed, m~ans for removal of electrolysis p.od
~and means fo~ impressing an electrolysi~ current thereon, at !
lleast one of the electrodes being pressed a~ainst the dia-
,¦phragm or membrane by a resiliently compressible layer co-ex-
¦tensive ~ith the electrode surface, said layer being compres-
¦Isible against the diaphragm while exertin~ an elastic reactio n
'¦force onto theelectrode in contact with the diaphragm or mem l
¦brane at a plurality of evenly distributed contact points and¦ j
l¦being capable of transfering excess pressure actin~ on indivi-
,'dual contact points to less charged adjacent points laterallyt
¦along any axis lying in the plane of the resilient layer
¦whereby the said resilient layer distributes the press~re
~over the entire electrode surface, the said resilient lz~er ¦
,having an open structure to permit gas an~ electrolyte flow ¦
lkherethrough. ¦
The novel me~hod of the invention generating halo- f
! gen comprises electrolyzing an aqueous halide containing
lelectrolyte at an anode separated from a cathode in contact
;~ith an aqueous electrolyte by an ion-permeable diapnrag~ o~ !
,Imembrane and an aqueous electrolyte at the ca~hode, at least
¦one of the said anode and cathode having a gas ~nd elec~ro-
lyte permeable surface held in direct contact at a plurality
¦of poinks with the diaphragm or membrane by an electroconduc-
lltive, resiliently compressible layer open to electrolyte and
gas flow and capable of ~ppl; ng pressur~ to the said sur~ace
-
.` :' .
æ~ I ~
and l~terally distri.buting pressure ~rhereby the
pressure on the suriace of the diaphrag~. or membrane is
uniform
l According to the invention, eflective electrical
5 l¦ contact between the porous electrode surf~ce and the membrane
' or diaphragm is achieved and polarity imparted thereto readil-J
¦l and without induclng an excessive pressure in local areas by
~I pressing the current distributing or electric~lly charging ¦ 3
surface against the electrode layer by means of a readily co~-
pressible resilient sheet or layer or mak whi~h extends alon~
¦l a major part and usually substantially all of the surf`ace
1 of the porous electrode layer in direct contact with the
'I membrane.
- Thi~ compressible layer is springlike in character
5 ! and, t~hile capable of bein~ compressed to a reduction of up
to 60% or more of i.ts uncompressed thickness against the meml
.~ brane carrying the electrode layer by applica~.ion of pressur~ .
from a bac~;all or pressure member, it is also capaole f t
; , springing back substantially to its initial thickriess upon j
,' release of the clarnping pressure. Thus, by i~s elastic
,i memory, it applies substantially uniform pressure against khl
membrane carrying the electrode layer since i~ .is capable ofl .
distributing pressure stress and of compensating ~or irregu-l
¦ larities in the surfaces with which it is ln contact. The .
I compressible sheet should also provide ready access o~ the
~ .
1 -13-
~electrolyte to the electrode and ready escape of the electro-
s~sproducts ;~hether gas or liquid from ~he elec~rode.
Thus, lt is open in s~u~ture and encloses a large i
~free volume and is electr
,cally conductive, being generally made of a rne~al resistant
¦to the electrochemical attack of the electrolyte in contact
¦~there~with and thus distributes polarity and current over the .
¦¦enkire electrode layer. It may engage ~he electrode la~ers
¦directly, but alternatively and preferably it ma~
o ~! have a pl~able electroconducti~e screen of nickel,
titanium, ni~obium or other resistant metal inter~osed
' ~etween itself and the membrane. .
The screen is a thin, foraminous sheet which readil
I¦ f~exes and accommodates any surface irregularities in the
,l electrode surface. It may be a screen of fine net ~ork or
a perf`orated film. Usually, it is finer in mesh or pore
size than the compressible laye~ and less compressible or
substantially non-compressible. In either case, an open
mesh la~er bears against and is compressed against the mem-
brane with the opposite or counter electrode or at least a
,I gas and electrolyte permeable surface thereof, being pressed~
'¦ against the opposite side o~ the diaphragm. Since the com-
pressib]e layer and the finer screen, if present, are no~
! bonded to the membrane, the~ are ~ove~ble (sli~eahle) along the
~ membrane surface and therefore can readily adapt to the
i
; j -14-
~36~2~ 1
contours of th~ mcmbr~ne and of the counter el~ctrode.
It is therefore an object of this invention to co.~-
~duct the electrolysis of an alkali metal chloride ~ith an
electrolysis cell having an electrode in direct con~act with
il a membrane or diaphragm which electrode, or a section thereof
¦is easily compressed and has high resiliency and i5 capable
of effectively distribu~ing a clamping pressure on the cell ¦
.lin a substantially unlform manner over the en~ire electrode
surface.
o l! A preferred embodiment of the resilien~ current
~collector or electrode of the inven;tion is charac~erized in
,¦that it consists of a substantially open mesh, planar electro
conductive metal-wire art.icle or screen having an o~en net-
l,w~rk and composed o~ wire fabric resistant to the electrol~tel
l,and the electrolysis products and in tha~ some or all of th~ I i
wires form a series of coils, waves or cri~ps or o~her undulà.l ~
ting contour whose diameter or amplitude are substantially ~ ¦
in excess Or the wire thickness and prefera~ly correspon~ to
'the article thickness, along at least one directrlx paralle~ i
Ito the plane of the article. Of course, such crimps or
¦wrinkles are disposed in the direction across the thickness
~1f the screen.
These wrinkles in the form of crimps, coils, wa~es
llor the like ha~e side portions ~hich are sloped or curved .
With respect to the axis n;rmal to the thickness Or the
~23~
rinkled ~abrlc so that, when the collector is compressed,
some displacement and pressure is transmi,Jted laterally so ¦
~as to make distribution of pressure more uniform o~.~er the ¦
electrode area. Some coils or wire loops ~Ihich~ becaus~- of
lirregularities in the planarity or parallelism of the sur- ~'
Ifaces compressin~ the fabric, may be subjected to 2 compres-
l¦sive force greater than ,hat actin~ on ajdacent are~s are cap-
,lable of yielding more and to discharge-the excess force by
!~ transmitting it to neighboring coils or ~lire lo~s.
'' Therefore, the fabric is effective in actlng as a
llpressure equalizer to a substantial extent and ln pre~enting !
Itheelasticreactionforceactingonasinglecontacpoint ! ~1
,ifrom exceeding the limit ~hereby the membrane is ~xcessively ¦
,Ipinched or pierced. Of course, such self-adjust~ns capabili-¦
~ es o~ the resilient collector ls instrumen,al ~n obtaining ,
a good and uniform contact distribution o~er the entire sur- ! ~
-face of the electrode. ~ i
One very ef~ective embodiment desi~ably consists o
a series of helicoidal, cylindrical spirals ol wire ~Jhose
l~coils are mutually wound t~ith the ones of the ad~acent s~ira?
iin an intermeshed or interlooped relationship The spirals ¦~
~are of a length substantially corresponding to the height or ¦
,width of the electrode chamber or at least 10 or ~ore centi-
~neters in length and the number of intermeshed spirals is su
!ficient to span the entire width thereof and the` diameter fl
the spirals is 5 ~o 10 or more times the diameter of the wire ~
-16- !
- l . .
1,, 1 ~.
! .
o~` the s~,irals. According to this preferr--d arran~ement, the
wire heli~ itself represents a ver~ small por~ion of the
section o~ the electrode chamber enclosed by the helix and
,therefore the helix is open on all sides thereby providing
' 5 ~an interior channel to allo:l circulation of the electrolyte
and the rise of the gas bubbles along the chamber. .
Ho~rever, it is not necessary for the helicoidal
'icylindrical spirals to be wound in an intermeshed relation-
lship with the adjacent spirals as described above and they ¦
Irnay also consist of single adjacent metal wire spirals In
,~this case~ the spirals are juxtaposed one beside ano~her with~
,,the respective coils beinG merely engaged in an alternate ¦
~sequence. ln this manner, a higher contact point density may,
lbe achieved with the cooperating planes represented by the
I;counter e]ec~rode or counter current collector and ~he cell
end-plate.
~ccording to a further embodiment~ the current I ¦
collector or distributor consists of a crimped knit~ed mesh
~or fabric of metal wire ~lhereby e~rery single lire for~s a
series o~ waves of an amplitude corresponding to the ma~imu~ ¦
,height o~ the crimping o~ the knitted mesh or fabric. Every !
''metal wire thus contacts, in an alternaking sequenc~, the j
''cell end-plate which serves as the plate to apply the pressu~
',and the porous electrode layer bonded on the membrane sur~ace ¦ .
2S or the intermediate ~lex~le sc7een interposed betueeh the
~ '' . .
lelectrode l~yer or th~ membrane and the compressible layer.
;,~t least a portion Or the mssh e~.tends across the thic~ness
,¦ of the fabric and is open to electrolyte flo~l in an edge~ise
,Idirection
1 As an alternative, two or more l~nitted meshes or
¦fabrics~ after being individually crimped by formin~ may be
superimposed one upon another to obtain a collector of ~he
desired thickness.
ll The crimping of the metal mesh or fabric imparts
llto the collector a great compressibility and an ou~standing
resiliency to compression under a load which may be at least
,labout 50 to 2000 grams per sqUare centimeter (g~cm2) of sur-
face applying the load, i.e. the baclc or end-pl~te.
I The electrode o~ the invention, after asse~bling of
'the cell, has a thickness pre~erably corresponding to th2
depth of the electrode chamber but the dep~h of the chamber ¦
,may conveniently be made larger. In this instance, a forami-
nous and substantially rigid screen or a plate spac5d from
the surface of the bac~-;rall of the chamb2r may ac t 2s tne
~compressing surface against the compressible res~lient collec-
Itor mat. In that case, the space behind the, at least rela- ¦
iitively, rigid screen is open and provides an electrolyte I
¦ch~nnel through which evolved gas and electrolyte may f~ow. ¦
I¦The mat is capable of being compressed to a much lo~er ~hick-~ .
¦ness and volume For example~ it may be compressed to cbout ¦
;150 to gO percent or even lesser percent o~ its initial volume
.~ :
lZ36~124 ~ ~
and/or thiclness and i~ is, thereforc-, pressed o~ compressed
bet~leen the m~-mbrane and the conducting back-plate of the
¦cell by clamping these members together. The co~pressi~le
llsheet is moveable, i.e. it is nok welded or bonded to the
¦¦ cell end-plate or to the interposed screen and ~ransmits the
l! current essentially by mechanical contact with t~e same,
¦¦suitably connected to the electrical source and ~ith th2
l~electrode.
!I The mat is moveable or slideable with respect to
o ¦! the adjacent surfaces of these elements ~ith ~hich it is in
~Icontact. When clamping pressure is applied, t~e ~rire loops
,¦or coils consituting the resilient mat may defle~t and slide
¦ laterally and distribute pressure uniformly over the entire
I surface w.ith which it contacts. In this ~ay, it ~unctions
l ln a manner superior to ind:ividual springs distributed over
an electrode sur~ace since the springs are fixed and there is
no interaction bet~een pressure points to compensate ~or sur-t
face irre~ularities of the bearing surfaces.
, A large por.ion of the clampin~ pressuPe of the
Ij cell is elastically memorized by every single coll or wave of~
¦~ the metal wires forming the current collector. As subs~anti-
¦¦ ally no severe mechanical strains are created by the differ-
¦ ential elastic deformation of one or more singl~ coils or
I ¦ crimps of the article with respect to the adjacent ones~ the
I resilient collector of the invention can effect~vely prevent
j or avoid the pieFcin6 or undue thlnnin6 of the membrahe at
.
~6~æ~
~'the more strained points or are~s during the as~embl-~ of the
cells. Rath~r high deviations from the planariky of the cur-
l~rent-carrying structure of the opposed electrode can be thus
,,tolerated, as well as deviations from the paral1elism between
~, said structure and the cell back-plate or rear pressure plate .
- il The resilient electrode of the invention is adv~nt-
, ageously the cathode and is associated ~ith or opposed by an
Il anode which may be of the more rigid type, which means tha~ th~ .
i electrode on the anode side may be supported more or less
1I rigidly. In the cells for the electrolysis of sodiu~ chlorid e
¦¦ brines, the cathode mat or compressible sheet prleferably con-
¦I sists of a nickel or nickel-alloy wire or stainless steel
because of the high resistance of these materials to caustic
Il and hydrogen embrittlement. The mat may be coa~d w~th a ¦
ll platinum group metal or metal ox:ides, cobalt or nxide thereo~'
or other electrocatalysts ~o reduce hydrogen over~ol~age. !
Any other metal capable of retaining i~s resilienc~
.
during use including titanium optionally coated with a non- ,
'~ passlvating coating such as ~or example, a platinum group
', metal or oxide thereof may be used. The latter is particu-
l', larly useful when used in contact with acidic anoly~es.
,l As has been mentioned~ an electrode layer of elect
¦! rode particles o~ a platinum group metal or oxide thereo~ or
l¦ other resistant electrodic material may be bonded to the .
¦I membrane. This layer is usually at leas~ about 40 to 150
¦ microns in thicl{ness and may be produced substantially as
I!
-I 11.
,
descr1bed in U.S. Pat~nt No. 3,297,484 and, if desired, the
layer may be applied to both sides of the diaphragm or meMbranf
Since the layer is substantially continuousg althouGh gas and
¦electrolyte permeable, it shielùs the compressible mat and
!laccordingly most, if not all, of the electrolysis occurs on
j¦the layer with little, if any, electrolysis e g. gas e~olu-
¦ltion, taking place on the compressed mat which engages the
¦ back side of the layer. Thisi~sparticularly true where p~rticles
l of the layer have a lower hydrogen (or chlorine~ ~er~oltage
than the mat surface. In that case, the mat serves largely
¦as a current distributor or collector distributing current
¦ove-r the less electrically conducting layer.
In contrast thereto, ~hen the compressible ma~
I,directly enga~es the diaphragm or membrane or even ~hen there
l~ is an intervening foraminous electroconducti~re screen or other
i perforate conductor bet-~een the mat and ~he diaphra~m, ~he
open mesh structure ensures the existence Or unob~tructed
paths for electroly~e to rear a~eas ~hich are spaced fro~
the m~mbrane includinJ areas which may be on the ~ront, the
~!
20 ~! interior and on the rear portion of the compressible f`abric.
~ I Thus, the compressed mat being open and not completely shield~ ¦
; can itself provide an active electrode surface which ma~ be
~ ¦ 2 or 4 or more times the total projected surface in direct
I ! contact with the diaphragm.
Some recognition of the ir.crease in surface area
i¦ of a multilayered electrode has been su~ested in British
11 .'
ll -21-
æ~ I ~
Patent No. 1,26~ 2 ~Ihich describes a multl~ayered cathode
;comprising ou~er layers of exp~nded metal and inne~ layers CL !
thinner and smaller mesh (~rhich may be' kni.tted mesh) ~Ji~h the
,icat;hode touching a cation exchan~e membra~e ~lith electrolvte
;flowin~ in an edgel1ise direction through the catho~e.
i According to the present invention, it has been
found that lower voltage is achieved by recourse ~o a com-
pressible mat ~hich by virtue of crimping, ~rrinkli~ curling
l'or other design has a substantial portion of the wi~es or
~'conductors tJhich extend across the thickness of the mat a
distance at least a portion of such thickness. Usually these~
wires are curved so that as the mat is compressed~ ~hey ben~ ¦
resiliently to distribute the pressure and these cross ~Jires
I,impart substant:i~lly the same potential to the wires in the
' rear- as exists on the ~ires contact;lng the membrane
hen such a mat is compressed against the diaphra~m I
including or excludin~ any interposed screen, a ~ol~age which~
is lower by 5 to 150 millivoltsJcan be achieved at the sar,e
current flol~ as can be achie~ed when the mat or i~s inter- 1
" posed screen simpl~- touches the diaphragm. This can represent
'substantial reduction in lcilowatt-hour consumption per ton
'of chlorine evol~ecl. As the mat is compressed, its portions
~rhich are spaced from the membrane approach, h~t remain spaced
~Ifrom the' membrane! and the likehood and indeed extent o~ . .
I ., electrolysis thereon increases, T~is increase in surface
; llarea permits a ~reater amount of electrolysistiitho~ excessive
~oltage increase.
-22-
'i
, ,~.
, There is also a further advant;age even wnPre little
,actual elPctrolysis takes place on the rear portion~ of the
mat because the mat is better polarized aGainst corrosio~.
or example, when a nickel compressible mat is butted against
¦a continuous layer of highly conductive electrode p~rticles
bonded to the diaphragm, electrical shielding ma~ ~e so ~reat
that little or no electrolysis takes place on the ma~. In
such a case, it has been observed that the nickel ~2t ~ende~
llto corrode, particularly when alkali metal hydroxide exceeded¦
ji15 percent by weight and some chlorides were prese~. Wlth
~! an open foraminous structure directly in contact ~r~ h the
diaphragm~ enough open path to the spaced por-tions and even
the rear of the mat is provided so that the expose~ surfaces
Ith'ereof at least become negatively polarized or ca~odically !
Iprotected against corrosion. This applies even to surfaces
ilwhere no gas evolution or other electrolysis takes ~lace~ j
~,These advan~ages are especially no~able at current ~ensities
"above 1000 amperes per square meter of electrode sT~face
i~measured by the total a~ea enclosed by the electro~ extre- I
~! meties. I
Preferably, the resilient mat ls compressed to about
~j80 to 30 percent of its original uncompressed thickness under
a compression pressure betwen 50 and 2000 grams per square
llcentimeter of projected area. Even in its compressed sta~,e~
,,lthe resilient mat must be highly porous as the ratio bet~een ¦
jthe voids volume and the apparen~ volume of ~he co~ressed
!~
ll -23-
,
,
:~L23~2~
mat expre~se(l ~n percen~age is advantageousl~ at least 75%
(rarely below 50%) ancl preferably is bet~reen 85~ and ~6~.
This may be computed by rneasuring the volume occupied by the
mat compressed to the desired de~ree and ~eighing the mat.
IKno~ling the density of the metal of the mat, its solid volume
can be calculated by dividing the volume by the den3ity ~.lhich
Igives the volume of the solid mat structure and the Yolume of
Ivoids is then obtained by substracting this figure from the
JItotal volume. -
l It has been found that when this ratio becomes
i~exceedingly low, for example, by exceedingly co~pressing the
~Iresilient mat belolr 30% of its uncompressed thic~ness, the
,Icell voltage begins to increase probably due in par~ to a
l decrease in the rate of mass transport to the ac~ surfaces
of the electrode and/or the ability of the electrode system
to allow adequate escape of evolved ~as. A typical charac-
teristic of cell voltages as function of the degrees of com-
pression and of the void's ratio of the compressible ~at is
~ reported later in the examples.
The diameter of the wire utili~ed may ~ar~ within
l a wide range depending on the type of forming or te~turing
,I but is lo~r enough in any event to obtain the deslred ch~rac-
teristics of resiliency and deformation at the cell-assembly
', pressure. An assembly pressure corresponding to a 102d o~
li 5 to 500 g/cm2 of electrodic surface is normally required t~
obtain a good electriccl contact between the membrane-bonded
Il
; jl
1ll ' . I
~2~6~æ~
e:Le~ro(tes and ~he rec,pective current-carryin~ structures or
collect;ors althou~h higher pressures may be used~usuall~ up
~to 2000 g/cm ~
I It has been found that by providin~ a de~ormation
~,of the resilient electrode of the invention of about 1.5 to
3 millimeters ~mm) which corresponds to a compression not
¦ ~reater than 60% of the thickness of the non-compr2ssed artic
'~ at a pressure of about 400 g/m2 of projected surf~ce, a
l~ contact pressure ~ith the electrodes may also be obtained
ll within the above cited limits in cells ~rith a high surface
development and with deviations from planarity up ~o 2 milli-
l meters per meter (mm/m).
I The meta,l wire diameter ls pr~ferably be~ween O.l
I or even les~ and 0.7 mlll:imeters whlle the ~h:ickness o;~ the
ll non-compressed article~ that -ls, either the coils t diamQter
j or the amplitude of the crimping, is 5 or more times the wire .
~! diameter, preferably in the range of 4 and 20 milllmeters,
'I Thus, i~ is a~parent that the compressible section encloses
~, a large free volume i.e. the proportion of occupi~ volume
WhiCIl iS free and open to electrolyte flo~r and gas ~low, In ,
¦ the ~Irlnlcled fabrics described above which includ~ these
compr~ssing wire helixesJ this percent of free volu-,ne is
¦ above 75% of the total volume occupied by the fa~ric This I
!I percent of free volume rarely should be less ~han 25~ and prè- .
~I ferably should not be less than 50P dS the pressure drop in
, the flow of gas an~ electrolyte through such a ~abri¢ is
,¦ negligib1e. -2
,. . i
When the use o~ particulate elcctrodeO or~ othsr
~porous electrode layers directly bonded to the membrane sur-
face is not contemplated~ the resilient mat or fabric directl, ,
engages the mernbrane and acts.as the electrode As it has .
i~ 'i r)e,5~ /e. ~,
5 1l now been surprisingly found, only a substantially ~}gea-~-le
¦¦cell voltage penalty with respect to the use of bonded porous¦
¦electrode layers is achieved by providing a sufficien~ density
¦of resiliently established contact points between the elect-
l¦rode surface and the membrane. The density of contact points
,¦should be at least about 30 points per square..centimeter of
membrane surface and more preferably, about 50 points or more
jlper ;square centimeter. Conversely, the contact area of
~single contact points should be as small as possible and the 1-
ratio of total contact area versus the corresponding engaged
`,membrane area should be smaller than o.6 and prererably, ¦
!i smaller than 0.4, I .
ij I
In practice ? it has been found convenient to use a ¦
' pliable metal screen havinga mesh num~er of at ~east 10,
.,prefera~ly above 20 and usually bstwesn 20 and 200 or a fine
.'mesh of expanded metal of similar characteristics interposed .
between the resiliently compressed mat and the membrane. The i
mesh number is lntended to indicatethe number of threads or
! ~ires per inch.
I! It has been proven that under these condition3 o~
'Iminute and dense contacts, resiliently esta~lished between th,e
; I! electrode screen and the surface of the membrane, a major
!l
~ 2~-
11 .
: ....... !! ,
1~ . .
. ' ' ` ~ ~ I
.
.. .
~3~
'I
portion of the electrode reactlon takes place at the contact ¦ }
interface bet~reen the electrode and the ion exchang~ groups
contained in the membrane material ~lith most of the ionic conl ~
.ductiontaking place in or across the membrane and llttle or r,
none taking place in the liquid electrol~te in cont2ct ~,lith r
the electrode. For example, electrolysis of pure, twice
distill;ed water having a resisti~rity of over 2,000,000Q cm.
llhas been successfully conducted in a cell of thiS type equip~
¦Iped ~rith a cation exchange membrane at a surprisingly lo~r cel t'
¦¦ voltage. '
!¦ Moreover~ ~hen electrolys,is of a- kali me~l brin~
l¦iS performed in the same cell, no appreciable change Or cell
¦Ivolt2.ge is experienced by varying the orientation of ~he cell¦ r
llfrom the horlzontal to khe vertical, indicating ~hat; the con-l i
~tribution to the cell voltage drop attributable to ~he So
called ~'bubble effect" is negligible. This behavior is in
good 2greement with that of a solid elec~rolyte cell having
particulate electrodes bonded to the memo~ane whicn ~ontras~s
,~rith that of traditional me~brane cells equipped ~J}~h coarse
foraminous electrodes, either in contact or slightl~ spaced
i~ from the membrane, ~rherein the bubble effect ha5 a ~reat con-
,¦ tribution to the cell voltage which is normally lower when the
gas evolving foraminous electrode is kept horizontal below a
certain head of electrolyte and is maximum when the electrode
jl is vertical because of a reduction o~ the rate Or gas disen-
I gagement and because of ln reasing gas buùble population along
!
!j !
,the heig'nt of the electrode due to accumulation.
, An explanation of this unexpec~ed behavior is
Ijcertainly due in part to the f'act tha~ the cell behaves sub-
l stantially as a solid electrolyte cell since the major portio
1 of the ionic conduction takes place in the membrane~ and also
because the resiliently established cont-icts o~ eY~remely-
!, small individual contact areas bet~een the fine mesh screen
¦electrode layer and the membrane are capable of easily releas
in~ the infinitesimal amount of ~as which forms at the contact
interface and ,o immediatel~ re-es~ablish the conbact once
¦,the gas pressure is relieved. The resiliently compressed
electrode mat insures a substantially unlform contact pressur,e
and a uniform ~dsubstantlally complete covera~e Or high densi~y
, m~nute contact points between the elec~rode sur~ace and the
~, membrane and it e~fectively acts as a gas release spring I
to maintain a substantially constant contact bet~le~n the -¦
electrode surface and the func~ional ior exch~noe ~roups on ~,
the surface of the membrane ~rhich acts as th~ elec~rolyte
~, of the cell. ' ¦
1 Both electrodes of the cell may ccmprise a resili-
ently compressible mat and a fir~e mesh screen pro~iding for $
i number o~ contacts over at least 30 contact points per square .
Il centimeter respectively made Or materials resistant to ~he
¦ 11 anolyte and to the catholyte. More prefera~y5 only one
¦1 electrode of thecell co~)rises the resiliently compressible ma~
¦¦ of the invention associated with the fin~ mesh electrode
-28- ~ ~
~,
. :
~ll
~ 236~
!
¦Iscreen ~I'nile the other electrode of the cell is a substantl-
¦lall~ rigid~ foraminous structure preferably also havin2 a
l,fine mesh screen interposed bet~:een the coarse rigid structur
iland the membrane.
I To better illustrate the various charac~eristics
of the invention, the following drawings are included to
¦illustrate practical embodiments of the inven~ion:
Fig~ 1 is a photographic reproduction o~ an embodi-
~ment of a typical resiliently compressible mat used in the
o !I practice of this invention.
Fig. 2 is a photographic reproduction of another
I embodiment of the resiliently compressible mat which may be
,j used accordlng to ~his invention.
Il ~ig. 3 :i~. a photographic reproduction of a ~urther
I'l embodiment of the r~siliently co~pressible mat used accordinq
'' to this invention. ¦
" Fig. 4 is an exploded sec~ional horizontal ~i2W of '
,~ a solid electrolyte cell of the invention havin~ a typical
compressible electrode system OL~ the claimed t~pe ~herein
~I the compressible port on comprises helical spiral wires.
ll Fig, 5 is a horizontal sectlon~l view of the
I assembled cell of Fig. 4
li Fig, 6 is an exploded perspective vie~ of ano~her .
preferred embodiment of the current collector ~f the cell of
!¦ Fig. 4,
I
I ,
I -~9-
:~ !
l .. . l
1~:364~ ~
I Fig. 7 is an exploded perspecti~/e view of another
preferred embodiment of the current collec~or ol the cell of
Fig. 4.
¦I Fig. 8 is an exploded sectional view of another
5 llpreferred embodiment of the electrolyte cell of the invention
Fig. 9 is a horizontal sectional vie~ of the :
~ass~ d cell of Fig. 8.
Fig. 10 is a horizontal sectional view of ano~her
I,lpreferred embodiment of the cell of the invention~
il Fig. 11 is a diagrammatic fragmetary ~ertical cross- .
section o~ t~le cell of Flg. 10.
~i~. 12 :Ls a schematic diagram illustrating the
electrolyte circulatlon system used in connection with the
I! cell herein contemplated.
15 ll Fig. 13 is a graph illustrating the voltage re~uc-
¦tion achieved as the pressure on the elec~rode and diaphrag~
is increased.
The compressible-e~ e or section thereo~ ¦
.,illustrated in Fi~. 1 is comprised of a series o~ interlaced~ !
I'helicoidal cyclindrical spirals consis~ing o~ a o.6 mm ~or
less) diameter nickel wire with their coils mutually ~round
¦lone inside the adjacent one respectively and having a coil
lidiameter of 15 mm.
I ¦ A typical embodiment of the structure of Fig. 2 com-
l prises substantially helicoidal spirals 2 having a fla~tened
or eliptical section made o~ 0.5 mm di2meter nickel wire
30-
I
.
,.
.
lZ36~Z~
i
'Iwith their coils mutually ~ound one inside th~ adjacent one
! respectively and the minor axis of the helix being 8 mm
j A typical embodiment of the struct~re of Fig. 3
Ijconsists of a 0.15 mm diameter nickel wire-knltted mesh
1¦ crimped by forming and the amplitude or heigh~ or depth of th ,
llcrimping is 5 mm ~lith a p:itch bet~reen the wa~es of 5 mm; .
j¦The crimping may be in the form of interesec~lnb~p~rallel
crimp banks in the form of a herring bone pat~ern as shown
l in Fig. 3. -
¦ Referring to ~ig. 4, the,solid elec~rolyte cell
which is particularly useful in sodium chlorine brine elect-
rolys:is and embodies one of' the current collectors of the .
invention is essentlally comprised of a verti~al anod~c
'I ... .
end-plate 3 provided with a seal surface 4 al~ng the entire
~¦perimeter thereof to sealably contact the p~ripheral edges
of the membrane 5 with the lnsertion, i~ des~red~ of a li~uid
impermeable insulatin~ gasl~et (not illus~rate~). The anodic !
end-plate 3 is also provided wi~h a central ~2cessed area 6
'I with respect to said seal surface with a su~.lace correspond-
1 ing to the a~ a of anode 7 bondedtothe membr-~ne surface.
, The end-plate may be made of steel with its side contacting
~¦ the anolyte clad with titanium or other passiva~able ~alve
metal or it may be made of graphite or moul~a~le mi~t.~l~es of
graphite. and a chemically resistan~ resin binder.
!l
11 .
-31-
:
lZ36~4
The anodic collector pre~erabl~ consists of a
titanium, niobium or other valve metal screen or expanded
.lshee~ 8 coated ~rith a non-passivatable and electrolys~-r~
.Isistant material such as noble metals and/or oxides and mixed¦
.i I ',
Iloxides of platinum group metals. The screen or expanded shee
¦8 is welded or more s~mply rests on the series of ribs or
iprojections 9 of titanium or other valve metal ~7elded on the
¦central recessed zone 6 of the cell end-plate so tha~ the
;screen plane is parallel and preferably coplanar With the
i¦plane of the seal surface 4 of the end-plate.
¦l The vertical cathodic end-plate 10 has on its inner
! side a central recessed zone 11 with respect to the periphera:
¦seal sur~aces 12 and said recessed zone 11 is subst2n-~ially
l!planar, that is rlbless and parallel to the seal sur~aces
'Iplane~. Inside said recessed ~one of the cathodic end-plate,¦
there is positioned a resilient co~.pressible current collecto~
'~13 of the invention, preferably made of nickel--a
The thickness of ~he non-co~pressed resil~-ent col-
,ilector is pre~erabl~ from 10 to 60~ greater than the depth ofl
jlthe recessed central zone 11 ~ith respect to t~e plane of the¦
seal surfaces 2nd during the assembly of the cell, th~ col-
llector is compressed frorn 10 to 60% of its original thickness
¦Ithereby e~erting an elastic reaction ~orce, preferably in the
: ilrange of 80 to 600 g/cm of proJected surface The cathodic
llend plate 10 may be m2de of steel or any other electricall~
; l~material reSistant to Caustic and hydrogen.
; The membrane 5 is pr fera51ya ~luid-imper~ious and ¦
~1
Il
,¦cation-permselective ion-exchange membrane such as, for
,¦example, a me~brane consisting of a 0.3 rum thick polymeric
film of a copolymer of tetrafluoroethylene and perfluoro-
. ¦ sulfonylethoxyvinyl ether having ion exch~nge groups such as
, sulfonic, carboxylic or sulfonamide groups. Because of its
¦thinness, the mem~rane is relatively flexible and tends to
¦sag, creep or otherwise deflect unless supported. Such mem-
branes are produced by E.I. DuPont de Nemours under the trade
mark o~ Nafion.
The anodic side of the membrane has bonded thereto
¦the anode 7 consisting of a 20-150 ~m thick porous layer of
¦particles of electroconductive and electrocatalytic material,
¦preferably consisting of oxides and mlxed oxides of a~ least
¦one of the platinum group metals. The cathodic side of the
~membrane has bonded thereto the cathode 14 consisting of a
~20-150Jum thick porous layer of particles of a conduc~i~e
material with a low hydro~en-overvoltage, preferably consist-
jing of graphite and platinum-black in a ~;eight ratio of 1:1
llto 5~
ll The binder utilized to bond the particles to the
I i¦membrane surface is preferably polytetrafluoroethylene (PTFE)
¦¦and the electrodes are formed by sintering a mixture of PTFE
and the conductive catalytic-material particles to form the
mixture into a porous film and pressing the film onto the mem
brane at a high enough temperature to effect bonding. This
¦ bonding is effected by assembling a sand~ich of the electrodi~
I
: : j
lz36~æ4 ~,
l,sheets with the membrane bet~,~een them and pressing the
I~assembly together to embed the electrode particles into the
~membrane. ~-I
I¦ Usual]y, the membrane has been hydrated by boiling ~,
l~in an aqueous electrolyte such as a salt solution~an acid or ;
alkali metal hydroxide solution and therefore are highly
¦hydrated and contain a considerable amount, 10 to 20% or more
ilby weight, of water either combined as hydrate or simply
!l absorbed In this case, care,must be exerted to prevent
¦¦excessive loss of water during the lamination process_
Since this lamlnation is achieved by applyinO heat ¦
,as well as pressure to the laminate, water may tend to evapor-
ate and this may be held to a minimllm by one or more o~ the
l~lfoilowing: (1) Enclosing the laminate in an impermeable
~¦ envelope l.e. bet~een metal foils pressed
,¦ or sealed at their edges to maintain ~ -.~ater
ii saturated atmosphere about the lamin~te;
(2) Proper design of the mold to quiclcly retur
,I water to the laminate; and
i~ (3) Molding in a steam atmosphere. The elect-
rodes bonded on the membrane surfaces have a projected area
i¦practically corresponding to the central recessed areas 6 and
11 o~ the two end-plates. r
: 1l
:` 11 1 1
~. I
.
~23~i~'~
I!
Fig. 5 represents th~ cell of Fig~ 4 in the
assembled state ~herein the parts corresponding to both
¦drawings are labelled with the same numbers~ ~s sho-m in this
¦view, the end plates 3 and 10 havebeen clam~ed together ~-
Itthereby compressing the helical coil sheet or ma~ 13 against
the cathode 14. During the cell operation, the anolyte con
¦sisting for example, of a saturated sodium chloride brine is
circulated through the anode chamber, more desirably bY feeding
¦fresh anolyte through an inlet pipe (not illus~rated~ in ~he
,Ivicinity ofthe chamber bottom and discharging the spent
!l anolyte through an outlet pipe (not illustrated) in ~he
l¦proximity of the top of the said chamber together with the
j¦evolved chlorine.
I: The cathode chamber is fed w~th water or dilute
Icaustic through an inlet pipe (not illustrated) at tlle botto
of the chamber, ~hile the caustic produced is recovered as a
lconcentrated solution through an outle~ pipe ~ot illustrated
;iin the upper end o~ said cathode chamber. The h~dro~en
'evolved at the cathode may be recovered ~rom the cathode ti
~chamber either together with the concentrated caustic solu-
¦tion or through another outlet pipe at the top o~ the
'¦chamber.
Because the mesh of the resilient collector is .
¦ open, there is little or no resistance to gas or electrolyte
j flow throu~h the com~ressed collector. The anodic and cathodi
end-plate are both properly connected to an externa1 current¦
-35- I
ll
1
.
~L236~24
Isource and the current passes through the series of ribs 9
~to the anodic current collector 8 wheref`rom it is then dis- ¦ .
tributed to anode 7 th.ough the multiplicity Qf contact points
l~between the expanded sheèt 8 and the anode 7. The ionic con- t
5 l~l duction essentially occurs across the ion-exchan~e m~m,brane
¦5 with the cvrrent being substantially carried by sodlum ions .
, migrating across the cationic membrane 5 from the anode 7 to
I the cathode 14 of the cell. The curren.t collect~r 13 collects
I the current from cathode 14 throu~h the multiplicity of con- ¦
tact points between the nickel wire and the cath~de and then ¦
. transmits it to the cathode end-plate 10 throu~h .~ p~-uralit~
; ,~of contact points.
~fter the assembling of the cell, the c~rrent col-
lllector 13 in its compressed state ~/hich enta:ils a deformat:ion
ll preferably bet~leen 10 and 60% of the original thic~ness of .
~the article, that is o. the single coils or crimps thereof, :
exerts an elastic reaction force a~ainst the catho~e 1~ sur- ¦
face and therefore against the restrainin~ surface represented
by the substantially non-defor~able anodic current collector j
,~8. Such reaction force maintains the desired pressu~e on the!
contact points between the cathodic collector and the anodic
~! collector wit~ the cathode 14 and the anode 7, respectively.
il The absence o~ mechanical restraintsto the differ- . ~.
'¦entlal elastic deformation between adjacent spira~s or adjà-
! cent crimps o~ the resilient current collector allows the
¦!same to adj~st to unavoidable slight deviations ~rom pla~rity
'11 - .
I -36- . .
,,, 1, 1`
!l .
~236~
lor parallelism between the cooperating planes repre~enked by
'Ithe anoclic collector 8,and the surface 11 of the ca~hode com-
partment, respecti~ely. Such slight devia~ions which normall; r
occur in standarcl fabrication processes may theref`ore be co~-
,pensated for to a substantial degree.
~l In Figs, 6 and 7, there are schematicall~ sho~Jn, by
,¦exploded perspective partial views, two preferred e~bodiments
¦of the resilient compressible current collec~or mat 13 of the
'lcell illustrated in Figs. ~ and 5. For simplicity's sake~
I~only the relevant parts are depicted and they are in~icated
'¦by the same numerals as in Figs. 4 ~nd 5. The resillently
! compressible mat of Fig. 6 is a series of helicoida~ cylind-
, rical spirals o~ o.6 ~n diameter nickel wire 13 whose coils
¦! are pre~era~ly mu~ually ~lound one inside the other 2s more
Iclearly seen :;n the photographic reproduction of ~1~. 1 and
the diameter of the coils is 10 mm. Bet~een the resilient ¦-
fabric or sheet 13a and the membrane 5 carryin~ on e~s sur~
faces the cathode layer 14, there is disposed a thi~ ~oramin-; ~
ous sheet 13b which may advantag~ously,be an e~pand_-d 0.3 mm-, ~,
~,thick nickel sheet. The foraminous shee~ 13 is readily fle~
ble or pliable and off`ers negligible resistance to bending
¦~and f'lexing under the elastic reaction forces exerte~ by the ¦ ~
! wire loops of sheet 13a upon compression against the membranel .
',l5. Fig. 7 depicts a similar emhodiment as that described in
1¦ Fig. 6 but wherein the resilien~ly compressible rab~ic or
¦layer 13a is a crimped knitted fabric of 0.15 mm-diamet:er
~niclcel wire such as that illustrated in the photogr~phic
reprocluction of Fig. 3.
i~
I!
, Fig. 8 represents another embodiment of t,he inYen-
:'ition wherein the cell ~Jhich is particularly use~ul in tne
.Isodium chloride brine elec~rolysis embodies a compressible
llelectrode or current collector of the invention~ associated
llwith a vertical anodic end-plate 3 provided with a seal sur-
'Iface 4 along the entire perimeter thereo~ to sealably contact
! the peripheral edges of the diaphragm or membrane 5 with the
i optional insertion of a li~uid impermeable, insulating Deri-
. ¦ pheral gasket (not illustrated). The anodic end-plate -3 is
.10 i a.lso provided with a central recessed area 6 with respect to
l said seal surface with a surface extending from a.lower area
I I where br:lne is lntr.oduced to a top area where spent or parti-
ally spent brine and evolved chlorine are discharged ~Jhich
I I said areas are usually in ready communication at top and
Ij bottom. The end-plate may be made of steel witb its s.ide .
contacting the anolyte clad with titanium or another passlva~
able valve metal or it may be o~ graphite or moul~able mix- i
tures of graphite and a chemically resistant resin Dinder or;
'' of other anodically resistant material.
l~ The anode pre~erably consists of a gas and elect-
rolyte permeable titanium, niobium or other va~ve metal .
'I screen or expanded sheet 8 coated with a non-passivatable .
; il and electrolysis-resistant material such as noble metals
il and~or oxides and mixed oxides of platinum group metals or
jl other électrocatalytic coating ~hich serves as a~ anodic sur-
¦¦ face when d~sposed on an electroconductive substrate. The
, 11
I -38- ~ ~
12~ig24 1 1
anode is substantiall~ rigid and the screen is sufficien~ly ¦
thick to carry the electrolysis current from the ribs 9 ~rith-¦
out excessive ohmic losses. More preferably, a ~ine mesh ~
Ipliable screen which may be of the same ~aterial as the coarse
iscreen 8 is disposed on the surface of the coarse screen 8
~to provide fine contacts ~lith the membrane ~rith a densit~ of
¦30 or ~ore, preferably 60 to 100, contact points per square
jcentimeter of membrane surface. The fine mesh screen may be
~¦ispot welded to the coarse screen or may just be sandT~iched
,'between screen 8 and the membrane. The fine mesh screen is
i! coated with noble metals or conductive oxides resis~ant to
~lthe anolyte.
¦I The vert.lcal cathodlc end-plate lO has ~Dn its inner
Ijside a central recessed zone 11 with respect to the periphera~
,~seal surrace 12 and the said recessed zone 11 is substantially
planar, t:hat is ribless and is parallel to the se~l sur~ace
plane. The resilient compressible electrode ele~.ent 13 con-
templated by the invention, advanta~eously madeo~ nicke--alloy
is positioned inside said recessed ~one of the ca~hodic end-
,plate. In the embodiment illustrated in this drawing, the
,~electrode is an helix of ~rire or a plurality of interlaced
jlhelixes and these heli~es may engage the membrane directly. r,
However, a screen 14 is preferably interposed as illustra~ed
~¦between the wire helix and the membrane so that the helîx
; 25 'land the screen slideably enga~e each other and the membrane.
i
'I '
-39-
,
: ::
..
~3~
The spaces bet~leen adjacent spirals of the heliY~
~should be large enough to ensure ready flow or movement of
gas and electrolyte bet~een the spirals, for example, into
,1 and out of the central areas enclosed by the helix. These
5 1l spaces generally are substantially large, of~en ~-5 times or ¦
larger, than the diameter of the wire~ The thicXness of the I
non-compressed helical wire coil is preferably ~rom 10% to 60 t
greater than the depth of the recessed central zone 11 ~.Jith
¦¦respect to the plane of the seal surfaces. During the
lassembly of the cell, the coil is compressed fro~ 10 to 60%
I of its original -thickness thereby eXerting an ela5tic react-
,1 ion force, preferably in the ran~e of 80 to 10~ ~ cm2 of pro-
jlJected surface.
;l - The cathodlc end-plate 10 may be made o~ steel or
lany other electrically conductive materi~l resistant to
caustic and-hydrogen. The membrane 5 is preferably a fluid- ¦
impervious and cation-permselective ion exchange me~brane ~s i
~mentioned above. The screen 14 is conveniently m~de of nicke
:, i
Iwire or other material capable of resisting corrosion under
',! cathodic conditions. While the said screen may have rigidi~y;
it preferably should be flexible and essentially non-ri~id so
that it can readily bend to accom~odate the irregularities of
the membrane cathodic sur~ace. These irregularities ma~ be
¦in the membrane surface itself? but more commonly are due b~
1l lrregularities in the more ri~id anode against w~lch the
membrane bears. Generally, t~le screen is more ~le~ible than
the helix. _ ¦ .
~ ,
i~236~
Il For most purposes~the mesh si~e of the scre~n shoul~l
,Ibe smaller than the size of` the openings betl"een the sp~rals
lof the helix and screens with openings of 0.5 to 3 milli-
,¦meters in width and length are suitable although the finer
~Imesh screens are particularly preferred embodiments o~ the
¦linvention. The intervening screen can serve a plurality o~
¦ functions~ First,since it is electroconductive and ~hus has
¦ an acti~e electrode surface. ~econd, it serves to prevent
¦the helix or other compressible electrode element from locall Y
abrading, penetrating or thinning out the membrane and as the
i compressed electrode presses against the screen in a local
¦ area, the screen helps to distribute the pressure along the
¦ membrane surface bet~een ad~acent pressure points and also
I prevents a distorted spiral section from penetra~ing or
i, abrading the membrane.
In the course of electrolysis, hydrogen and alkal
I metal hydroxide are evolved on the screen and generall~ on
I some poriion or even all of the helix. As the helical spir21s
¦ are compressed, their rear sur~aces i.e. those remot2 or
,! spaced from the membrane surface, approach the screen and the
membrane and of course the greater the degree of compression
l the smaller the average space of the spirals ~rom the mem- l
I brane and the greater the electrolysis on or at least c~tho~c
l polarization of the spiral surface. Thus, the effect of coml
pression is to increase theoverall effective surface area of
the cathode-
-41-
.
.~ '
~2364;~
Compression of the electrode is found to effecti.vel~
reduce the overall voltage required to s~s~ain. a current flo~
of 1000 ~mperes per square rneter of active membran~urface
.~or more. At the same time, compression should be limited so
5.~ ~! that the compressible electrode remains open ~o electrolyte
and gas flow. Thus, as illustrated in Fig. 9, ~he spirals
remain open to provide central vertical channels ~hrough whic~ .
¦electrolyte and gas may rise. Furthermore, the spaces betwee
¦spirals re~ain spa~.ed to permit access of catholyt~ to the
I!membrane and the sides of the spirals. The wire ~ the Spir?lS
Igenerally is small rangirIg from 0.05 to 0.5 millimeters in
diameter. While larger wires are permlssible~ they ~end to b~
more rigid and less compressible and so it is rare for the
I ¦wire to exceed 1.5 mm.
ll Fig. 9 represents the cell of Fig. 4 in the l .
¦assembled state wherein the parts corresponding to both draw-
Iings are labeled with the same numbers. As sho~m in this
'Iview, the end plates 3 and 10 have been clamped together
! thereby compressing the helical coil sheet or mat 13 agaînst
~ the electrode ~5. During the cell operation~ the anolyte
consisting, for example, of a saturated sodium chloride b~ine
is circu].ated through the anode chamberJ more desirably feedinc
¦fresh anolyte through an inlet pipe tnot illustrated) in the
vicinity of the .chamber bottom and discharging the spent
~ anolyte through an outlet pipe (not illustrated~.in the proxi
mity of the top of said chamber together ~ith ~he evol~ed
¦clorine.
I -42-
I . ;
~. ~ . ' .
:
~23G424
jj The cathode cham~er is fed ~lith Jater or dilute
aqueous alkali through an inlet pipe (not illustrated) a~ the
~Ibottom of the chamber, while the alkali produced is reco~ered
llas a concentratedsolution through an outl~t pipe. ~no~ illust-
,¦rated) in the upper end of said cathode chamber, The h~dro-
en evolved at the cathode may be recovered from the cathode
chamber either together ~ith the concentrated caustic solu-
tion or through another outlet pipe at the top of the cham~er .
. ¦ The anodic and cathodic end-plates are both properl
connected to an external current source and the current J
¦passes through the series of ribs 9 to the anode 8. The
!ionic conduction essentially occurs across the ion-exchange .
¦membrane 5 with the current being substantially carried by ~he
I¦ sodium ions migrating across the cakionlc membrane 5 from
I;the anode 8 ~o the cathode 14 of the cell. The electrodes .
,provide a plurality of contact points on the membrane with
. i~ current ultimately flowing to the cathode end-plate 10 thr~ug
j, a plurality of contact points.
i After assembling of the cell, the current collector'
il 13 in its compressed state which entails a deformation pre- '.
Il ferably bet~een 10 and 60% of the original thickness of the
¦¦ article, that is of the single coils or crimps thereof,
¦¦ exerts an elastic reaction force against the cathode surface
~¦ 14 and therefore against the restraining surface represen~ed
1¦ by the rela~,ively more rigid, substantially non~e~o~ma~le-
!! anode or anodic current collector 8. Such reaction ~orce
l . ll
~ :~ ~ -43-
. : .,
.1 .
', '''''
". . ,
. . .
;,
123642~ ~
Imaln~ans the desired pressur~ on the contact poin~s bet~.leen
.the cathode and the membrane as ~lell as the screen portion
an~l the helical portion of the catho~e 14.
'. Because the helix spirals and the screen are slide- .
~lable with respect to ea~h.other and ~Jith respect to the mem-
i.brane as ~1ell as the rear bearing ~rall, absence of mechan1cal
irestraints to the differential elastic deformation between
: I,adjacent spirals or adjacent crimps of the resilient elect-
ilrode allows the same to laterally adjust to unavoidable
l~slight deviations from planarity or parallelism between the
llcooperating planes represented by the anode 8 and t~e bearing
surf~ce 11 of the cathode compartment respecti-~ely. 5uch
sllght deviations normally occurring in standard fab.ricat~on
p~ocesses a.re there~ore compensate~ to a substankial degr~
: 15 . The advan~ages of the resilient electrode of tne ~
invention are fully realized and appreciated in industrial .
` filtQr press-type electrolyzers :ihich co~prise a great numbert
~of elementary cells clamped together in a series-arrangemen~
to form modules of high production capacity In this instance
Ithe end plates of the intermediate cells are represente~ by
llthe surfaces of thebipolar separators bearing the anode and b'
cathode current collector on each respective sur~ace. The
bipolar separators, therefore, besides acting as the defining
walls of the respective electrode chambers, electrica1ly ¦-
,,connect the anode of one cell to the cathode o~ the adjacent ¦ ~
cell in the series. ¦ :
-4/l-
,1
~ i
,I Due to their elevated deformability, the resilient ¦
',compressible electrodes of the invention afford a mor~ unifor~
~distribution of the clamp1ng pressure of the filker-press
'~module on every single cell and this is particularly ~ru~ when
1l the opposite side of each membrane is rigidly supported by a ¦
~relatively rigid anode,8. In such series cells, ~he use of ¦
resilient gaskets on the seal-surfaces of the single cells is¦
! recommended to avoid limiting the resiliency o~ the compressea
ilfilter~re,ss module to the membranes resiliency.- ~ greater
,~advantage may be thus taken of the elas~ic deforma~ion pro-
'Iperties of the resilient collectors ~ithin each cell of the
l~series-
Il ~ig. 10 cliagrammat:icaly illustrates a ~urther em-
,',bodlment wherein a cri~ped fabr:ic of interlaced wires is used
' as the compressible element Or the electrode in lieu ofhelicoi!,
, dal spirals and an additional electrolyte channel is provided,
for electrolyte circulation. As sho~in, ~he cell comprises
an anode end plate 103 and a cathode end plate 110, both
,mounted in a vertical plane ~lith each end-pla~e in the lorm
I,of a channel having side IJalls enclosing an anode space 106 ¦
and a catho~e space 111. Each en~ plate also has a perip- ¦
~jheral seal surface on a side-~all projec~ing from the plane I
¦of the respective end plake 104 being the anode seal sur- ¦
~Iface and 112 being the cathode seal surface. These surfaces
libear against a membrane or diaphragm 105 which skretches
,lacross the enclosed space bet~;leen the side walls.
-45-
li
`"' ' 11 ' .
12364Z4
l The anode 10~ comp-rises a relat:i~tely rigid uncom-
¦pressible sheet of expanded titanium metal or other perforate,
anodicall~ resistant substrate, preferably having ~ non-passi' ~-
,vable coatin~ thereon such as a metal or oxide or mixed oxide
il of a platinurn group metal. This sheet is sized to fit within
the side ~alls of the anode plate and is supported rather ¦
rigidly by spaced electroconductive metal or graphite ribs
~,109 which are fastened to and project from the web ~r base of
¦¦the anode end plate 103. The spaces bet~reen the ribs pro~ide ¦-
llfor ready flow of anolyte ~hich is fed into ~he bo~om and
j¦withdrawn from the top of such spaces. The entire end plate ¦
l~and ribs may be of graphite and alternatively, it may be of ¦ .
I¦titanlum clad steel or other suitable material. l'he rib erldsj
llbc~lrin~ against the anode sheet 108 may or not be coated, e.g.
ll wlth platinu~l, to im~rove electrical~ contact and the
: l¦anode sheek 108 may be also welded to the ribs 109. The anod~
rigid foraminous sheet 108 is held firmly in an upright posi-
.tion. This sheet may be of expanded metal having upwardly , ¦
inclining openings di.rected a~ay frorn the membrane (see Fi~ ~ ~
20 ~ 11) to derlect rising gas bubbles towards the spac~ g. ¦ ~;
I~ More preferably~ a fine mesh pliable screen 108
! f ~itanium or other valve metal coated with a non-passivat-
'able l~yer which is advantageousl~ a noble metal or conductive
','oxides having a lo-l overvoltage for the anodic reac~ion (e.g.
ll chlorine evolution), is disposed bet~een the rigid foraminous¦
! sheet 108 and the membrane 105. The fine mesh scr~en 108a
.
,1 -46-
.
12al6~Z4
provides a density of contacts of extremely 10T~ area ~ith the
~membrane in excess of at least 30 contac~s pe-r ~quare centi- I
,meter. It may b~ spot ~elded to the coarse screen 108 or not. F
I~ On the cathode side, ribs 120 extend outT;~ard fro~ ~-
'Ithe base of the cathode end plate 110 a distance ~Jhich is a
~¦fraction of the entire depth of the cathode spac~ 111. These
Ijribs are spaced across the cell to pro~ide parallel spaces
¦Ifor electrolyte flow. ~s in the embodiments discussed above,
~¦the cathode end plate and ribs may be made of steel or a
1 nickel iron alloy or other cathodically resistan~ material.
On the conductive ribs 120 is welde~ a relati~ely ~igid pres-~
sure plate 122 which is perforate and read:ily allo~rs circula-~
¦tion o~ electrolyte from one side thereof to the other~
ilG~nerally~ these openings or louvers are inclined upward and ¦
,~a~ay from the membrane or compressible electrode ~o~ard the
space 111 (see also :fig. lll). The pressure pla~ is electro-
~,conductive and serves to impart polarity to the electrode andl
'to apply pressure thereto and it may be made of expanded metal
! I
,or heavy screen of steel, nic~el, copper or alloys ~hereof.
~1 A relati~ely fine flexible Screen 114 bears againstl ~
the cathode side of the active area of diaphragm 105 ~Ihich ~'
because of its flexibility and relative thinness~ assumes
jthe contours of the diaphragm and therefore that of anode 1081.
IIThis screen serves substantially as the cathode and thus ~s b
¦electroconductive e.g. a screen of nickel ~ire or other
cathodically resistant ~rire and may have a sur~ace of lo~
_1~7_
36~æ4L
~hydrogen overvoltag~. The screen prefera~ly provide3 a
density of contacts of` extremely lo~ are~ ~rith the-mem~rane ¦
in excess of at least 30 contacts per square centirneter. A
Icompressible rnat 113 is disposed bet~ieen the cathode screen t
l114 and the cathode pressure plate 1~2.
As illustrated in Fi~. lO the mat is a crimpe~ or
l¦wrinkled wire-mesh fabric which fabric is advantageousl~ an
,lopen mesh knitted-wire mesh of the type illus~rated in Fig. 3
~Iwherein the wire strands are knitted into a relatively fla~
Ifabric ~lith interlocking loops. This ~abric is then crimped
or wrinkled into a ~;7ave or undulating form with the waves ¦
¦being clo~e together, for example, 0.3 to 2 centime~ers ap~rt
and the overall thickness of` the compressible fabric is 5 ~o
~llO~millimeters. The crlmps may be in a æig-zag or herring 5
bone patter~ as illustrated in ~'ig. 3 and the ~esh of the
fabric is coarser, i.e. has a larger pore size ~han that o~ l ¦
screens 114. -
As illustrated in Fig. 10, this undulatin~ fabric
113 is dis2osed in the space between the finer mesh screen
,114 and the more rigid expanded metal pressure plate 1~2.
The undulations extend across the space and the void ratio of
~the compressed fabric is still preferably higher than 75
preferably between 85 and 96%, of the apparent volume occupie~
,~by the fabric. As illustrated, the waves extend in a vertical
'lor inclined direction so that channels for upward ~ree ~low of
gas and electrolyte are prov ded which channels are not sub- !
:
2~6424 1;
llstantially obstructed by the rrire of the ~abric, This is ¦
.! true even when the waves ex~end across the cell from one side
to the other because the mesh openings in the sides of the
~Iwaves permit free flow Or fluids. ,~
.l As described in connection with o~her embodiments,
the end-plates llO.and 103 are clamped together and bear
against membrane 105 or a gasket shielding the membrane from
the outside atmosphere disposed beteen the end w~lls The
, clamping pressure compresses the undulating fab~ic 113 a~ns~,
1 the finer screen 114 which in turn presses the me~brane
I against the opposed anode 108a.and this compressio.n appears
!~ to permit a lower overal]. voltage. One test was performed
¦where the uncompressed fabric 113 had an overal~ t~ickness
,~ o~ 6 millimeters an,d lt was found that at a c~lrren,~ dens1ty
'i of 3000 Amperes per square meter of projected elec~rode area,l ~.
,~ a voltage reduction of about 150 millivolts ~as achieved when;
the compressible sheet was compressed to a thickness of 4
millimeters and also to 2.0 millimeters over that obser~ed
~ ~or the same current density at zero compression.
11 Between zero and compression to ~ millimeters, a
! comparable voltage drop of 5 to 150 millivolts was obser~ed.
! The cell voltage remained practically constant down to a :~
compression of about 2.0 millimeters and then started to ris~
slightly as compression went belorr 2.0 millimeters, that is
to about 30~ Or the ori~inal thickness o~ the fabrlc. This
Il -49- .
3~4æ~ 1 ~
represented a substantial energy saving -~hich may be 5 or more
Ip~rcent for brine electrolysis process.
! In ~he operation of this embodi~ent, substant~ally
Isaturated sodium chloride aqueous solution is fed into the
,Ibottom of the cell and flo~7s up~Jard throu~h channe~s or
/o~
spaces ~4~ bet~een ribs 109 and depleted brine and e~ol~ed
~chlorine escapes f'rom the top of the cell. Water or dilute
¦sodium hydroxide is fed into the bottom of the cathode
¦chambers and rises through channels 111 as wel~ as ~hrough
,Ithe voids of the cornpressed mesh sheet 113 and evo~ved hydro-~
'Igen and alkali is withdrawn from the top of the ce~l.
I¦Electrolysis is caused by imparting a direct current electric
potential bet~leen ~he anode and cathode end plates~
ll Fig. 11 is a diagrammatic vertical secti~nal frag- ¦
1 15 ment ~ihich illustrates the flow patterns of this cell rherein! .
'at least the upper openings in pressure plate 122 a~e louvere
to provide an inclined outlet directed u?~rardly aw~7 from t~le
compressed fabric 113 whereby some portion of evol~d h~dro- I
gen and/or electrolyte escapes to the rear elcctro~yte chamber
'llll (Fig. 10). Therefore, the vertical spaces at t;he back of
'l,the press~lre plate 122 and the space occupied by co~np~essed
,Imesh 113 are provided ~or upl,lard catholybe and gas flo~;r.
, By recourse to t~o such chambers~it is possible to .
I,reduce the gap between pressure plate 122 and the membran~
,land to increase tne comPression of sheet 113 while still leav ,~
ing the sheet open to fluid flo~ and this serves to increase
-50-
! - !
,~i 1
i ~.23~2~
the overall effective surface area of active portions o~ the
cathode. ¦
Fig. 12 diagrammatically illuskra~es the ~,an.~er of
~loperation of the cell herein contemplated. As shown therein,
,la vertical cell 20 of the type illustrated in ~he cross-
section-il view in Figs. 5,9 or 10 is provided with anolyte ¦
inlet line 22 which enters the bottom of the anolyte chamber ¦
,I'(anode area) of the cell and anolyte exlt line 24 which exits¦.
Ilfrom the top of the anode area. Similarly, catholyte inlet
I¦line 26 discharges into the bottom of the catholyte chamber
iof cell 20 and the cathode area has an exit line 28 locz,ed
~a~ the top of the cathode area. The anode area is separated
~rom the cathode area by membrane 5 which has anode 8 pressed
llon the anode side and cathode 1l' pressed on the cathode side.
.',The membrane-electro~ extends in an upward direction and .
generally, its height ranges ~rom about 0.4 to 1 ~eter or
.higher.
The anode chamber or area is bounded by the ~e,~brzne~
'and anode on one side and the anode end ~all 6 (see Figs. 5~9 !
,or 10) on the other~ while the cathod.e area is bounded ~y the¦
¦Imembrane and the cathode on one side and the upright cathode I
.llend wall on the other. In the operation of the syste~, the .
aqueous brine is fed from a feed tank 3Q into line 22 tkrougn
',a valved line 32 which runs. from tank 30 to line 22 and a
jlrecirculation tank 34 is pro~ided to dischar~e brine f.r~ a
¦llower part thereof through line 5. The brine concentra~ion
1' .
-51-
a
3~
of t~e solution enterin~ the bottom of the anode area is ¦
;,controlled so as to be at least close to saturation by pro-
,portioning the relative flo-ls through line 32 and the brine
!l entering the bottom of the anode area flo:.s upl.~ard ~nd in
i¦ contact ~lith the anode. Consequently, chlorine is evol~red
l~and rises with the anolyte and both are discharged through
¦ line 24 to tank 34. Then chlorine is separated and escapes
llas indicated through exit port 36 and the brine is collec~d
¦jin tank 34 and is recycled. Some portion o~ th-s brine is
¦Iwithdrawn as d~pleted brine through,overflow line 4~ and is
sent to a source of solid alkall metal halide ~or resatura-
I t:ion and purif'ication~ Alkaline earth metal in the ~orm O:e
¦halide or other compounds is held low, ~lell below one part
,Iper million parts of allcali metal halide and frequen~ly as
I~low as 50 to 100 parts of alkaline earth metal per billion
',parts by weight of alkali halide
On the cathode side, water is fed to line 20 from
a tank or other source 42 through line 44 ~rhich discharOes
'linto recirculating line 26 where it is mixed .~ith recircula-
20 1I ting alkali metal hydroxide (NaOH) comin~ through line 26
,lifrom recirculation tank. The water-alkali me~al hydroxi~le
¦,mixture enters the bottom of the cathode area and rises
¦i~oward the top thereof through the compressed gas permeable
¦Imat 13(Fi~s. 5,9 or 10) or current collector. During the
¦,flow, it contacts the cathode and hydrogen gas as well as
lalkali metal hydroxide is formed, The catholyte liquor
-52-
.
::~Z364Z~
lldischarges through line 28 into tank 46 ~Ihere hydrogen is
separated through port 48. Alkali metal hydro~ de solution
~,is ~rithdrawn through line 50 and ~ater fed through line 41~ is
ll'controlled to keep the concentration of rTaOH ~r other alkali
~iat the desired level. -This concentration may be as low as 5
¦or 10~ alkali metal hydroxide by weight but norm~lly this
concentration is above about 15%, preferably in the range of
l15 to 40 percent by weight.
¦ Since gas is evolved at both elec~rodes, it is pos-
Isible and indeed advantageous to take advanta~e `o~ the gas
lift properties of evolved gases which is accomplished by
Irunning the cell in a flooded condition and ke~pin~ th~ anode
¦jand cathode electrolyte chambers relati~ely narrow~ ~or
;lexample~ 0.5 to 8 cent:imeters in width. Under such circumst-
!¦ances, evolved gas rapidly rises carrying elect~olyte ~here-
with and slugs of electrolyte and gas are disc~ar~ed throu~h
,,the discharge pipes into the recirculatin~ tan~s and this
,fcirculation may be supplementecl by DUmps, if desired.
il Knitted metal fabric which is suitab~e for use as
1l the current collector of the invention is manù~actured by
¦Knitmesh Limited, a British Company having an office at south
Croyd~n. Surrey and the knitted fabric may vary in size and
¦degree of fineness. Conveniently used wire ranges ~rom 0.1 t~
10.7 millimeters, althou~h larger or smaller wires may ~e
resorted to and these wires are knitted to pro~ide aboui 2 5
. to 20 sti ches per inch (` to ~I stitches per centimeter), pre
~. '
~ ':
:
3~Z3~
I ferably in the range of about 8 to 20 stitches o~ o~enings
! per inch, (2 to 4 openings per centimeter). Of course, it ¦
llwill be understood that wide variations are possible and thus,
'¦undulating wire screen having a fineness ranging from 5 to
1 100 mesh may be used.
The interwoven, interlaced or knitted metal sheets
are crimped to provide a repeatino wavelike contour or are
loosely woven or other:rise arranged to provide a thickness ~o
the fabric which is 5 to 100 or more times the diameter o~
the wire so that the sheet is compressible. Ho~Yever~ because
the structure is interlaced and movement is restricted by ~he
structure, elasticity of the fabric is preserved. This is
particularly true when it is crimped or corrugated in an
Illorderly arrangement of spaced ~-raves such as in a herring bone
I!pattern. Several layers of thii, knitted fabric may be super- .
Il imposed if desired.
~here helix construction illustrated in Fi~. 3 is ¦
; res~rted to~ the ~iire helices should be elastically com- I
Ipressible. The diameter of the wire and the diameter o~ the
¦ helices are such as to provide the necessary compressibility and
¦~ resiliency. The diameter of the helix is genera~ly 10 or I
!I more times the diameter of the wire in its uncompressed con- ~ .
il dition. For example, 0.6 mm dlar,leter nickel wire wound in
i helices of about 10 mm diameter has been used satisfac~orily.
l Nickel wire is suitable when the t~ire is cathodic
as has been descrlbed above and illustrated in the dra~ings.
.
64~
llo::cvcr, ~-Lrly o~her rr~el~a:l cap~lbLc 01 resi~,tin,; C?A'~ iC ~ ~ac~
or corrosion b~ th~ electrolyt~ or h~cirGc~en e"~br-i~tlL-.~.ent In~
.~ used an~ ~hese ma~ include stainless steel, co.rJper, silve-r i'
coa~ed copper or the lil~e . -~i
While in the embodim2nts described a~o-~e~~~he com-
pressi~le collector is S~o~Jn as cathodic, i~ i5 ~0 be un~er-
'.stood that the polarity of the cells may be reversed so ~hat ~,
~he compressible collector is anodlc 0~ course, in tha~ ~:
~event, the electrode wire mus~ be resistan'c to chlorine and
. anodic attaclc a-nd the ~iires may be o~f a valve ~2tal such as L:
, ti~anium or niobium,,prelerably coated ~rith an elec~rocon~v.c-
i tive, non-passivatinc~, la~Jer resistan~ to anod-~c a~ac~ such ~ t
~i as plati.nurn c~roup metal or oxide, bime~allic ~pinel, pero~r-
k-it t ¦ t
In some cas~esJ appl:ication of the col~lpressible mern~
er to the L~node sldernay create a pr~lernbecause h2lide elect~ly~e I r
n supp].y to the elec~rode-msmbran~ in~erface may be rcs~ric'ed
" ~Jhen the anod-~c areas do no~ have sulfici.er~ access ~o ~he
,t anolyte flo~lirl~ thro-l~h the cell, ~he halide concen.. ra~ion r~
. may beco~e reduced in local areas due ~o ~he elec~rolys.is t
~! and, Jhen it is reduced to too great-an~xten~, oxy5~n ra~her :
i! than halQs~n tends ko be evol~ed as a resul~ of wa~r ele~c~-
. rolysi.s. This is avoided by main~alnln~ the are~s of p~ints .
Il Or ~lecCrode-membrane con~act small l,e: rar~ly ~ore than -
; ~5 i' 1;0 millim~te~is and o~en less ~han o~e~hal~ im2ter in .,
i ~rid~h and it can also be .~ef`f'ec~.ively avoi~e~ b~ main~ain-~n~ t
, .
~ ~55-
,, , ' ''' ' . .
,
,a screen of rela~ively fine mesh, 10 mesh or greaker, be~een
lthe compressible mat and the membrane surface.
~1 Although these problems are also importa~t on the
'Icathode, less difficulty is encountered since the ~athodic
¦reaction is to evolve hydrogen and there is no occurence o~
a side reaction as the products are generated even though the
~!Points of contact are relatively large because water and ~he
,lalkali metal ion migrate through the membrane so ~hat e~en if
Ijthe cathode presents some restriction, by-product
l¦formation is less likely to occur. Therefore, it is advant-
~¦ageous to apply the compressible mat to the cathode side.
I In the following examples there are described
¦several preferred embodlments to ~llustrate the in~rention.
jHo~^rever, it is to be understood that the invention is nok
1l intended to be limited to the specific embodiments
il' .
!, EXAMPLE 1
,,. ! ~
A first test cell ~A) was constructed according to i
lthe schematic illustration shown in Figs. 10 and 11. Dimens-
!1 ions of the electrodes were 500 mm in width and 5~O mm in
~I hei~ht and the cathodic end plate 110~ cathodic ri~s 120 and
¦ the cathodic foraminous pressure plate 122 were masde of steel
¦~ galvanically coated with a layer of nickel. The ~oraminous ¦
pressure plate was obtained by slitting a 1 5 ~m thick ~late ¦
! Or steel forming diamond shaped apertures having their major
d1mension of 12 and 6 mm. The anodic end plate 103 was made,
~ lZ;~i6424 ~
,¦of titanium cladded steel and the' anodic ribs 10~ ~Jere made o: ~ ¦
~itanium.
The anode was comprised of a co~rse, substantially
¦rigid expanded metal screen of titanium 108 obtained by slitt- .
,'ing a 1.5 n~ thick titanium plate forming diamond shaped aper
¦tures having their major dirnensions of 10 and 5 mm, and a fi~l ,
mesh screen 108a of titanium obtained by slitting a 0.20 mm
~thiclc titanium sheet forming diamond shaped apertures ha~ g .
~ltheir major dimensions of 1.75 and 3.00 mm spot we~ded on the
llinner surface of the coarse s~reen. Both screens were coated
¦¦with a layer of mi~ed oxides of ruthenlum and titanium corres
¦Iponding to a load of 12 granls of ruthenium (as metal) per E
llsq~lare meter o~ projected surface. , ~'
; ~I The cathode was comprised of three layers of crimpe
,llcnitted nickel fablic forming the resilient mat 113 and ~he ¦ ~'
~fabric ~las knitted wit~ nickel ~ire llith a diamete~ of 0.15m~
'.The crimping had a herring bone pattern, the ~ave a~.pli~lde
of ~hich ~.~as 4.5 n~ and the pitch bet~een adjacen~ crests of
iwaves was 5 mm. After a pre-packing of the t,hree layers of
1l the crimped fabric carried out by superimposing the layers
and applying a moderate pressure, on the order of 100 to 20U ; j~
l g/cm2 the ma~ assumed an uncompressed thickness of about ~-
,! 5.6 mm. That is, af`ter relieving the pressure, ~he mat ~
I~retu~ned elastically to a thickness o~ about 5.6 mm. The
~! cathod~ also contained a 20 mesh'nickel screen 114 ~rm2d
'I -57-
~L2~6~
with a nickel wire having a diameter of 0.15 mm whereby the
screen provided about 64 points of contact per square
centimeter with -the surface of the membrane 105 verified by
obtaining impressions over a sheet of pressure sensitive
paper. The membrane was a hydrated film, 0.6 mm thick, of a
~afion 315 cation exchange membrane produced by Du Pont de
~emours i.e. a perfluorocarbon sulfonic acid type of membrane.
A reference test cell (B) of the same dimensions was
constructed and the electrodes were formed according to normal
commercial practice, with the two coarse rigid screens 108 and
122 described above directly abutting against the opposite
surfaces of the membrane 105 without the use of either the fine
mesh screeens 108a and 114 and without being uniformly
resiliently compressed against the membrane 105 (i.e. the
compres~ible mat 113 is missing). The test circuits were
similar to the one illustrated in Fig. 8.
The operating conditions were as follows:
- inlet brine concentration 300 g/l of ~aCl
- outlet brine concentration 180 g/l of NaCl
- temperature of anolyte 80C
- pH of anolyte 4
- Caustic concentration in catholyte 18% by weight of NaOH
- current densi.ty 300 A/m2
Test cell A was put in operation and the resilient
mat was increasingly compressed to relate the operating
characteristics of the cell, namely cell voltage and current
- 58 -
:~ .
~ rm/,~
~ ~'
,,
,,
- ,.,
¦efficiency, to the de~ree of com~ression. In Fi~. 13, curve 1
'Ishows the relation of cell voltaOe to the degreee ol compres-
ision or to the corresponding pressure applied. It is observe
; r ~ c n æ~s~ / é
that the cell voltage de'~rea-sed t~ith increasing compression
of the resilient mat down to a thickness corresponding to
about 30% of the original uncompressed thickness of the ma~.
Beyond this degree o~ compression, the cell voltage tende~-to
rise slightly.
By reducing the degree of compression to a ma~
thickness of 3 mm, the operation of the cell A compared ~tith
¦that of parallely operated reference cell B-~n the follo~
ing resul,s:
. ____ ~ .. _ _ _ .. _ _ _ _ .. _ _._.. _ .. _ . . .. _, .. _ . . _ _ _ .. _ . . . _ _ __ _. .. ...
Cell Volt~e I Cathocl7c Current 2 in C12
Eff:~c:iencJ~ ~ ~ by volwme
I Test cell ~ 3.3 85
;ITest cell B 3.7 85 4 5
In order to have an assess~ent of the contribu~io~
~of the bubble effect on the cell ~Jolta,Q,e, the ~ells were ¦
rotated first 45and finally 90 from the vertical T.lith the
llanode remaininO horizontally on top of the mlembrane. The ~;
,~,opera~ characteristics of the cells are reported herein- ~'
belo~
......... .
. . .
~,
~36
,
,,, . ~
_, . . _ ~ . _ ., . _ . . . . _ .. . .. .... . _ _ _ , _
nclination Cell Voltage !Cathodic Current O2 in C12
, Effieienc,y .
() I V i ~ % by vol.
_ .. .. . . ~ . _. . .. _ . __ .~ _ _ . . .... ... ___ .. _ __ _ _.. _ _ __ _ . _ _ _ _
ITest cell A , 45 , 3.3 ' 85 ; 4.4
;jReference I I , i .
'jcell B , ~ __ __ L 5 1 4.4 i
-I,rTest cell ~ horizontal', 3.3 (x) I 86 i~ 4O3
i! Reference i ¦ ¦ i
Il ceil B ' ~I j 3.6 (xx) : 85 4.5
_ __._ . ._.__._.__ _._______ _ __.__ _. !_ _ _ ___ _ li
Il (X) ~ne cell voltage started slowly to rise and stabilized at about
li 3.6 V. .
,1 (xx) The cell voltage rose abruptly to w~l over 12 V and electrolysis
,I was therefore.interrupted.
I~ These results are interprcted as rOl10- s a) by
,'rotating the cells from the vertical and to-~ards ~he horizon-
.tal orientation, the bu.bble effect contribution to th~ cell ¦ i
voltaGe decreases in cell B, ~Jhile the relative in-sens~ rit~J d
of cell A is apparently due to a substantially negliaihle
~bubble effect which would in part explain the much lower cell
,volta~e of cell A with respect to cell B. b~ Upon reaching
~Ithe horiæontal position, the hydrogen gas begins ~o pocket I ~
"~under the membrane and tends to insulate more and more the ~,
! acti~Te surface of the cathode screen from ionic current con-
,¦ductlon throu~h the catholyte in the reference cell B, ~lhile
,the same e,ffect is outstandingly lower in ~he test cell ~.
-60-
! ~,
.
.
~3~4~ ~
' This can only be explained by the fact .t~at a major portion
., of the ionic conduction is limited to within the thickness of!
'i the membrane and the cathode pro-~ides SUL ficient contac~
~¦points with the ion exchange groups on the membrane surface
,~ to effectively support the electrolysis current.
~! It has been found that by increasingly reducing th
density and fineness of the contact points be~ween the elect-
rodes and the membrane by replacing the fine mesh screens 10 a 6
I¦ and 114 with coarser and coarser ~creens, the behavior of ¦'
ll the test cell ~ approaches more and more that of the referenc.
Il cell B. Moreover, the resiliently compressible cathode la~e~
!¦ 113 insures a coverage of themembrane sur~ace with the dense
: distributed fine conta'ct points consisten~ly abo~e 90~ and
I, more often above 98% of the ent:lre surface even in presence ¦
' Or substantial deviations from planarity and parallelism o~
the compresslon plates 108 and 122
; EX~MPI.E 2
For comparison purposes, test cell A was ope~ed and
~' membrane 105 was replaced by a similar membrane carrying a
,' bon~d anode and a bonded cathode.' The anode was a po~ous~ ¦
l~ 80 ~Im - thic~ layer of particles of mixed oxides of ru~hen~u~
ll and titanium with a Ru~T.i ratio of 45~55 being bonded to the
!, surface of the membrane with polytetrafluoroethylene. The
Il cathode was a porous~ 50 ~m thick layer of particles o~
i! platinum black and graphite in a weight ratio of 1/1 b~ing
~1l .
,-61-
, bonded lith ~olyt~r~-luoroethyl~ne to the opposite surface ¦
of the membrane.
The cell was operated under e~actly the sam~ condi-
' 'tions of ~xarnple 1 and the relation between the cell voltage
, and the degree of compression of the resilient cathode cur-
! rent collector layer 113 is shown by curve 2 on the dia~ram
;lof Fig. 13. It is significant that the cell voltage o~ this ¦
,itruly solid electrolyte cell is only approximately 100 to ¦
,, 200 mV lower than that of test cell A under the same operat-
''ing conditions.
, I ' ,
ji EXAMPLE 3
~i
,,1
~I To verify unexpected results, test cell A~ras rnodi-
! fied by replacing all the anodic structures made of titanium
I with comparable structures made of nickel coated steel ~anodi c
I end plate 103 and anodic ribs 109) and pure nickel (coarse ¦
screen 108 and fine mesh screen 108a). The membrane used was~
a 0.3 ~n thick cation exchange membrane Nafion 120 manufactu
red by Du Pont de Nemours. I
Pure twice-distilled water having a resisti~ity o~ ~t
, more than 200,000~Qcrn was circulated in both the anodic and
l~ cathodic chambers. An increasing difference of potential ~ac ~,~
j' applied to the two end plates of the cell and an electrolysi
; li current started to pass with oxygen being evolved o~ the
~, nickel screen anode 108a and hydrogen belng evolved on the
'I nickel screen cathode 114. After a fcw hours of operatlon, ¦
.
~ 11
-
~L23~ 4 I j
. ~I the follo~iin~ voltage-current characteristics were observed: ¦
!j -.. _ .. . _.. . .. _ .. ` . .. ._.. _ . _ .. . .. .. __ . . . .. _ .. . . -- - _
I' Current Densi~y Cell Voltage Te~;~era~ure of Operati~ n
ii A~m2 ! V "C
_.
l 3000 , 2.7 65 .
ll 50 3.5 . 65
j, 10,000 5.1 ' 65
__ _, , , , , ,,, ., . .... , .. . , .. ..
The concluctivity of the electrolytes being insigni-
ficant, the cell proved to operate as a true soli~ electrolyt ~
llsystem. .
li~y replacing the fine mesh electrode screens 108a
¦and 114 with coarser screens, thereby reducing t~e density of~
contacts bet~een the electrodes and the membrane surface from
lO0 points/cm to 16 points/cm ; a dramati~ rise of the cell
l¦volta~e ~las obser-~ed as reported hereinbelo~ !
! Current Density i Cell Voltage :Temperature o~ ~era~icn
: 3ooo~ 8.8 , ~5
1 5 12.2 ~5
: ~! 10,000, - -
~j As will be obvious to the skilled ~nthe 2rt, it is j
possible to increaSe the density o~ contact points between
the electrodes and the membrane by mean~ of various expedients.
For example, the fine electrodic mesh screen may be sprayed ¦
-ith metal particles throuDh plasma ~et depos~tion, or the
:~ ~ __
,,~.
l ~,,
..
'Imetal wire rorming thc surface in contact with the Membr~ane
rnay be m~de co~rser through a controlled chemical attac~ to
increase the denslty of contact points~ Nevertheles3~ the
, 'structure must be sufficiently pliable to provide an e~en
Idistribution of contacts over the entire surface of the mem-
~brane so that the elastic reaction pressure exerted ~y the
'resilient mat to the electrodes is evenly distribute~ to all
l¦thecontact points.
! The electric contact at the interface between the ~
I,electrodes and the membrane mày be improved by increasing the :
density of functional ion exchange groups, or by reducing the
equival.ent weight of the copolymer onthe surfa^e of t~e mem-- ~
~¦brane. in contact with the resi.lient mat or the intervening .
I~s~reen or particulate.electrode In this way, the exc~nange
,Ipropertles of the diaphra~m matrix rema:Ln unal~ere~1 and i.t is
possible to increase the contact points density of th~ elect- Ç
rodes with the sites of ion transport to the membrane. For
example, the membrane may be formed by laminatin~ one or t~ro ¦
`thin films having a thickness in the range of ~.05 to 0.15 ~ml
, of copolymer exhibiting a lo~ equivalent ~Jei~ht, over ~he
,Isurface or surfaces of a thicker ~ilm, in the range of 0.1~ .
to o.6 mm~ ol a copolymer having a higher equivalent weight ,~
lor, a weight apt to optimize the ohmic drop and selectivity of
! the membrane.
,1
-611-
:
~3~æ~ ~ ~
Various o~her modifications o~ the method and
apparatus of the invention may be made wi~ho~ departing
~rom the spirit or scope thereof and it is to, be understood
,lthat the invention is to be limited only as d~ined in the
appended claims. ,
I'
1 ~
~ -65-
