Note: Descriptions are shown in the official language in which they were submitted.
il:
~37(;1Z3 ~:
;,:
PRO~UCTIO~ OF ~l~ALT PETAL C~RBO~TATE IN ~ ~R~ E CELL
HAV ~ R~TIO~I
BACKGROUND_OF THE I~VE~TIO~
This invention relates ~enerally to a process for electro~
lytically producing an alkali metal carbonace. ~ore particularly~
it relates to an lmproved~process for elsctrolytically producing an ;
alkali me~al carbonate employing a membrane cell using a pa-~icular
membrane-cathode configuration.
It is~kno~Tn that alkali metal carbonate can be electrolyt-
lcally produced from al~ali metal chlcrides in diaphrag~ and membrane
cclls by introducing carbon dioxide into the ca~holyte compartment or
into recirculatino catholyte outsid~ tl~e cell, as sho~n, for e~a~ple,
lQ in U.S. Patents~557~,~895, 2,967,807, 3,17~,579, 3,374,164 and
~;; 4,080,270. In all but U.S. 552~895, th~e diaphragm or membrane,
separating the electrolytic cell into anode~and cathode compar~m2nts,
is spaced apar~ fro the~ cathode to~permit the introduced C02 gas or
HC03 ions (formed~by the reactlon ot C02 with 0H ions) to more
read~ly reach and react with the OH ions generated at ~he me~brane
side of the cathode, and thus minimize back migration of the OH ions
through the separator into the anode compartment -the primarJ cause
for poor current efficiency in the eiectrolysis process. ~ypic2]1y,
carbonate salt-pr~duction~processes having the cathode separat d from
~he membrane achieve c~rrent efflciencies (measured on cathol~fte
products~ of at leas~ 95,' and often approach theoretical. Further,
U.S. 3,374,164 shows that the diaphragOm cell process is significantly
improved when the diaphraom i5 ~separated from the ca~hode: the
quantity of ~a ions flowing through the diaphrzgm being converted eo
Na2C03 increasing to 80% as compared to only 6~ ~hen the two are
contiguous, as shown in U.S. 552,S95.
"
-2- ~3~
. ~
It is kno~m that the electrolyzing voltage is reduced by
la~inating or juxtaposing the membrane to the cathode as shown, for
example, in U.S. 2t9~7,801, 3,057,7~4~an~ 4,101,395, and Ger~an ~ ~ '
Publications 2,704, 2~3~and 2, 741,956~. However, these expedients have
the dra~back that the OH ions generated at or near the membrane-
cathode interface can more readily escape înto tlle membrane and mi-
grate into the anolyte compart~ent, causing a loss of process current ~i,
efficiency as previously described. Because of this, and the neazly ,
theoretical efficiencies achieved ~hen the cathode is separated from
the membrane, ~he separated cathode-membrane configuration ha's been
employed by the prior art for producing alkali metal carbonates in
membrane cells. '
SU~L~RY OF THE INVE~_ION
Considering this state of the art, it is an object of this
invent'ion to provide a membrane-cell process for t~le production of
alkali metal carbonates operating at low electrolyzing voltage and
high current eEficiency, thereby providing a process more efficien~ '
than thcse now kno~n.
; These and still other objects and advantages, which willbecome apparent from the foliowing descript'ion and claims, are
achieved by electrolyzing an alkali ~.etal chloride in an electrolytic
cell having anolyte and catholyte compartments separated by a perm-
.~ selective, cation-exchange, hydraulically impermeable membrane either ~
laminated or juxtaposed contiguous to the cathode, and introducing '
carbon dioxide into the catholytej either in the cell or being re-
- 25 circulated outside't'ne cell, in a quantity sufficient to convert sub-
stantially all of the alkali metal ~.ydro~ide forming in the cathode
compartment to alkali setal carbonate.' Suprisingly, current effi-
ciencies of ~5-100~ are attained, even at high catholyte total solids,
a~d ~it~ low electrolyzing voltage.
DESCRIPTION OF THE PREFERRED E~ODI~IENTS
.
1~ 30 The electrolytic cell, having the contiguous cathode-~em- 'brane configuration used in the invention process~ may be a single
cell or a plurality of cells combined together in a single electro-
lyzing unit either ln serles using bipolar ~lect-=des or parallel
', ~
~3~7~2~ ~
_3-
using monopolar electrodes. The cells are generally conventional
having a housing reslstant to the electrolytes, and being separated 3
by the membrane into anolyte and catholyte compartments--the anolyte
compartment having an inlet and outlet for the al~ali m~tal chloride
brine and outlet for chlorine gas; and the catholyte compartment
having an inlet(s~ for wa~er and/or recirculated catholyte and out-
lets for product catholyte and hydrogen, and a C02 inlet, preferably
at or near the bottom of the cell, if CO2 is to be introduced into
the catholyte in the cell.
The membrane dividing the cell housing into anolyte and 1
catholyte compartments may be, in gPneral, any hydraulically imper- I
meable cation-exchange membrane electrolytically conductive in the
hydrated state obtaining under cell operating conditions and useful
for electrolyzing alkali metal chloride brines. These membranes com-
prise a film of a polymer, chemically re~istant to the anolyte and
catholyte, containing hydrophylic, ion-exchange groups ~such as sul-
fonic groups, carboxylic groups and/or sulfonamide groups. Membralles
made from polymers containing sulfonic and/or carboxylic groups have
been found to have good selectivity (tnat is~ they tran.sport vir-
20 tually only alkali metal ions) and low-voltage characteristics for :
the prcduction of both sodium and potassium carbona,es, while mem-
branes containing sulfonamide groups appear to be useful ~or sodium
~I carbonate production but r;equire a somewhat higller electroly~ing
~¦ voltage. Typically, these ~embrane polymers have an ion-exchangegroup eql~ivalent weight of about 800-15~0 and the capacity to absorb,
on a dry basis, in excess of 5 weight percent gel water.
The cation~of the ion-exchange group (-C02H, -S03H, -SO~N~H
and the like) in the membrane will mostly be the same alkali metal as
present in the chloride salt being electrolyzed to the carbonate salt.
While the acid or other alkali metal salt form can be employed at
start-up, it will be appreciated that the membrane will exchange
virtually all of these cations for the cation of the salt being
electrolyæed within a relatively short period of cell operation.
Polymers having all of its carbon hydrogens replaced with fluorine
;~ 35 atoms or the majority with -t~e~}s=~ atoms and the balance with
chlorine atom~, and ha~ing the ion-exchange gro~ps attached to a
carbon atom having at least one fluorine atom connected thereto, are
partic~larly pref.rred for maximum hemical resistance to -he anolyte.
. :
.
. , .
1 1 37023
.;
To mini~i~e electrolyzing voltage, the membrane preferably has a
thicknes~ in the range of about 3 to 10 mils, with thic~er membranes
in this range being used for better durability and selectivity. Be-
- cause of the large cross-sectional areas of co~lercial cells, whe~
the me~brane is not supported on both sides by contiguous electrodes,
lt typically will be laminated to and i~pregnated into a hydrauli~
cally permeable, electrolytically nonconductive, inert reinforcing
member such as a w ~ en or nonwoven fabric made from fibers of asbes-
tos9 glass, TEFLON and the like. In film-fabric composite membranes,
it is preferred that the laminate have an unbroken surface of ~he
film resin on both sides of the fabric to prevent leakage through the
membrane caused by seepage along the fabric yarns. Such composites
and methods for their manufacture are disclosed in U.S. 3,770,567.
Alternativcly, films of the membrane polymer may be ~aminatcd to
each side of the fabric.
Suitable membranes are available from the E. I. duPont de
Nemours & Co. under the trademar'~ NAFION. The preparation and de-
scription of suitable NAFION and other types of membranes is provided,
among others, in British Patent 1,184,321, German Patent Publication
20 1,341,847, U.S. Patent Nos. 3,041,317, 3,282,875, 3,624,053,
3,784,399, 3,S4g,243, 3,~09,37S, 4,025,405, ~,080,270, 4,1~1,395,
and 4,147,599.
The c~thode used in the electrolysis cell of the invention
process, may be any conventionaI electrically conduc~ive material
resistant to the catholyte, such as iron, mild steel, stainless
steel, nickel, and the like. The cathode is foraminous and gas per-
meable, preferably having at least 25% of its surface area open to
- facilitate the generation, flow and removal of hydrogen gas in the
catholyte compart~ent and the circulation of carbon dioxide and/or
bicarbonate ions to the cathod~-me~brane i~terface. To reduce the
electrolyzing vol~age, all or part of the surface of the cathode may
bear a coating orlayer of a material lowering the hydrogen over-
~oltage of the cathode, such as are disclosed in U.S. 4,024,044
~melt-sprayed and leached coating of particulate nickel and aluminum),
35 U.S. 4~104,133 (electrodeposited coating of a nic~el-zinc alloy), and
U.S. 3,350,294 (coating of molybdenum and tungsten and cobalt, nickel
or iron). When some of the csthode surface is devoid of such coating
or layer, it typically will be the area of the cathode juxtaposed or
.
, '~
~ ~ 37~:3
--5--
laminated to the membrane. Suitable cathodes can be made fro~, for
example, expanded mesh sheet, woven wire screen or perforated plates.
Especially preferred are cathodes having an opening (void) area of
at least about 50X and good gas-rel~ase characteristics, such as the
S parallel-plate electrodes described in S. African Patent No. 73/B433
This
type of cathode is particularly effective for concentrated catholy~e
solutions (e.g. 80% to lOOX saturated), especially K~C03 solu~ions,
in which higher viscosities and other solution hydrodyna~ic effec~s
impede the formation~ flow and escape of hydrogen gas. Retained
hydrogen causes "gas blinding or blanketing" oE the cathode, thus
increasing the electrolyzing vol~age. Because the ~ertical parallel
configuration of the electrode elemen~s minimizes gas holdup and
hence gas blinding of the electrode, electrolyzing voltage is mini-
mized. While parallel plates are described and illustrated in the
South African Patent, it is e~ident that other elongated electrode
element~ having other cross-sectional shapes, such as round, elipsoid,
triangu~ar, diamond, and square, can be utiliæed for these preferred
2Q catholes, so long as they are disposed in substantially vertical
alignmel~t and with sufficient spacing between adj~cellt eiements to `~ ~;
~ provide good electrolyte circulation and uni~peded flow and release
;~ of gas in the catholyte co~partment.
In the invention process, the cathode and membrane are
~; 25 juxtaposed such that at least a major portion of the c2thode-membrane
~; interface is contiguous. While as little as 50% touching is advan-
tageous, lowest IR drops will be achieved when 90 to 100% of their
interfnce is contiguous. Means ~or achieving this are varied and
well-known: as for example, employing a greater anolyte hydrostatic ~ `~
pressure to force the membrane against ~he cathode as shown in U.S.
3,057,794; sandwiching the membrane between the anode and cathode
with zero clearance at their interfaces; eompressing the membrane
between the cathode and anode with suitable resilient compressing
means such as shown in U.S. 39873,437; forming the membrane in situ
upon the su~face of ~he cathode ~by means such as coating, dipping,
- spraying, polymeriæing or fusing together suitablc polymcr precur-
sors, solutions, dispersions~ powders or fibers) as shown in U.S.
4,036,728 and 4,101,395; or la~inating a membrane film to the cathode
.~ .
':~ ' . ~
ii
.
~137~3 ~
-6- ! -
using heat and pressure as shown in U.S 4,101,3g5. In any o these
methods, an intel~ediate layer of suitable inert, nonconducting poly-
meric maeerial or polymeric precursor (nonhydrophylic and substan-
tially free of ion e~change groups) may first be applied to the
cathode (by spraying, brushing, dipping and the liXe) to inactivate
cathode surfaces to be abutted against the membrane and/or to improve
interfacial adhesion when the membrane is formed in situ upon or is
laminated to the cathode. Considering the foregoing it is apparent l
that the expressions "ju~taposing, abutting and contiguous," as used `
ln the specifi^ation and in the following claims, mean and are meant
to encompass, unless otherwise indicated, not only a touching of the
membrane and cathode at their interface, but also configurations in
- which the membrane is formed upon or is laminated to the cathode.
The anode used in the electrolysis cell of the invention
process, similar~ly, may be any conventional, electrically-conductive,
electrocatalytically active material resistant to the anolyte such as
graphite or, more preferably, a valve metal such as titanium, tantalum
or alloys thereof bearing on its surface a noble metal, a noble metal
oxide (either alone or in combination with a valve metal oxide), or
other electrocatalytically active, corrosion-resis~tant material.
Anodes of this preferred class are called di~ensionaliy stable anodes
and are well-known and widely used in industry. See, for example,
U.S. Patents 3,117,0~3, 3,632,4g8, 3,840,44~ and 3,846,273. ~ile
solid anodes may~be used when the anode is spaced apart from the
membrane, foraminous anodes having about ~5~ or more of their surface
area open, such as an expanded mesh sheet, woven mesh screen, or per-
forated plate, are preferred since they have greater electrocatalytic
surface area and facilitate the fo~-mation, flow and removal of the
chlorine gas in the anolyte compartment. As previously described,
good gas-release electrodes having SOZ or more open area, such as
disclosed in South ~frican Patent No. 73/8433, and discussed herein-
before, may especially be preferred when the anode also is juxtaposed
contiguous to the anode and/or when nearly saturated brines are used. ;
With respect to the spacing of the anode from the membrane,
this distance ideally is the minimum that maintains high current ef-
ficiency with respect to chlorine generation, and minimizes ~oltage.
Usually, minimum voltage is achieved when thc anode is contiguous to
the membrane or the membrane is laminated to the anode. ¦
~37~3
_7_
The invention process can be used to produce any alkali
metal carbonate starting with the corresponding al~ali metal chloride.
l~us, sodium, potassium and lithium carbonaees are made from sodium, I
potassium and lithium chlorides respectively.
As is the conventional electrolysis of alkali mecal halides
to form chlorine and alkali metal hydroxide and hydrogens, the
"
alkali metal chloride is charged to the anode compartment to become
the cell anolyte as an aqueous solution commonly referred to as ;
"brine." The brine typically is acidified with an acid, such as
hydrochloric acid, to a p~l of about 4 or less to minimize o.Yygen
evolution at the anode and to minimize the formation of insoluble
precipitates on or in the membrane from polyvalent cation salts, such ;
as calcium or magnesium chlorides, present in the brine.
Alternatively or in addition to the aforedescribed control
of pH, the deleterious effect of polyvalent cation salts may be minl-
.
~ized by addin& to the brine a compound capable of forming ~.Tith the
polyvalent cation saIts at a p~l of greater than 5.5 an insoluble gel
at the anolyte-membrane interface, the gel being reversible at a pH
of less ~han 3.0, as disclosed in U.S. Patent 3,793,l63. Illustra-
20 tive of su&h gel forming compounds, which can be used in the present ~
invention, are alkali metal phosphates, orthophosphates, and meta- 1 -
phosphates (preferably having the sa~e all;ali metal as the charged
brine), or the free acid form~of thes2`phosphates.
Typically,~ the brine;is charged at or close to saturation
25 in order to maximize the anolyte c~oncentraticn, and hence minimize ~ ;
the vol~age requirements of the cell. Also affecting anolyte con-
.
centration are the rate of charging the brine and the current density
-of the cell. MGre rapid brine-charging rates increase anolyte
solids! while higher current densities, conversely, deplete anolyte
solids more rapidly. Ideally, these three interrelated parameters
are chosen and controlled so that the anolyte will have a solids
concentration of about 75% or greater of saturation to mini~ize volt-
.
age requirements. Anolyte concentrations of less than 75~ ofsaturation, of course, are equally suitable when higher cell voltages
are acceptable.
In the cathode compartment, electrolyte is charged at the
start-up of the process to~provide initial catholyte. Typically,
this electrolyte will have the same alkali metal as the brine and
. . - , ' ~i
'' i:
. ~,
,
~3~ 3
will be a carbonate salt to facilitat~ rapid equilibriu~. After
star~-upj the catholyte is continuously rep].enished during electrol-
ysis by the alkali metal ion of the chargcd brine migrating through
the membrane; and the catholyte solids are adjusted to the deslred
S concentration by adding water to the catholyte. If it is desired
to ~inimize the energy required for drying the carbonate salt product,
the catholyte concentration will be maintaine.d at or near the sa~ura-
tion point of the carbonate salt, e.g. 75-100% of the saturation con-
centration. Conversely, if lower electrolyzing voltages are the para-
mount consideration, then lower catholyte concentrations wili be
used--with the optimum concentration being determined by the cell and
cathode design, the type of carbonate sal~, and the catholyte tempera-
ture. For e~ample, when a small mesh 316 stai.nless steel cathode
~having dia~ond-shaped openings 0.25 X 0.5 inches) ~as used to pro-
lS duce K2C03 at 2 asi, voltage dropped from 4.77 volts @ 640 g/l to 4.2
volts @ 490 g/l or a decrease of 0.375 volts per 100 g/l decrease in
catholyte solids. Using a nickel parallel-pl~te cathode, such as de-
scribcd in the examples and having better gas release characteristics,
it was observed that voltage dropped from 3.95 volts @ 640 g/l to 3~7
20 volts ~ ~90 g/l or a decrease of 0.167 volts per 100 g/l decrease in
catholyte solids. -~
In the invention process, carborl dioxide gas is introduced
into the catholyte so that it and/or bicarbona~te ions, resulting from
~he reaction of ~he carbon dioxide ~ith OH ionsJ react with the
alkali metal hydroxide produced in the catholyte co~partment from the
alkali ~etal ions migrating through the ~embrane and the hydroxyl
ions generated at the cathode. This may be accomplished by directly
.. injecting carbon dioxide into the catholyte compartment usually at or
near the bottom ~o pro~ide maximum ~ixing and contact time oE the
carbon dio~ide with the catholyte, and preferably with sufficient
exit velocity to minimize plugging of the ca~bon dioxide inlet port~s)
and provide good mixing. Alternatively, carbon dioxide can be passed
in a carbonator into recirculating catholyte outside the cell. This
mode of operation and typical carbonators are sho~m in U.S. 557,895,
3,17~,579 or 3,819,813. The recirculation rate preferabl.e is at a
rate sufficient to ensure that the catholyte solids in the cell con-
tain at least 90% by weight of the carbonate salt~
:
~ ' ' ' , "' ' '' . `
~'' , . . .
~1~37~3
.~ _ g _ .
The quantity of carbon dio~i~e introduced into the ~ I
catholyte should be su~ficient to give catholyte solids containing
at least about 90% by weight of the desired carbonate salt if high
current efficiencies, i.e. on the order of about 90% or greater, are ~¦
to be attained~ ~Sore preferred, ho~ever, is the use of carbo~ ,
dioxide in a quantity producing about 95% by weight or more of alkali
metal carbonate in the catholyte solids, since current efficiencies
(with respect to catholyte product solids) are maximized in this
range, generally exceeding 95%. For this reason, a quantity of car~
bon dioxide producing substantially only carbonate salt is ideally
and most preferably used. When less is used, the carbonate product
will contain minor amounts of the alkali metal hydroxide, ~hile more
may give carbonate product con~aining a minor quantlty of the bicar-
bonate salt.
. .
The carbon dioxide employed in the invention process may be
essentially ].00% pure or may be adMixed with other gases such as
nitrogen and oxygen, as for example when flue gases resulting from
the comhustion of coal, gas, oil and the like are used as the source
of the carbon dioxide. However~ flue-gas carbon dioxide will not ~
20 normally be used when high-purity hydrogen gas is desired. ~s
The temperatures of the anolyte and catholyte in the inven-
tion process are not especlally critical with respect to achieving
-high curre~t efficiency. ~owever, because voltage diminishes as the
teMperature increases, temperatures o~f about 90C or more are pre- ¦
ferably utilized when it is desired to minimize power consumption.
In the invention process, ~ypically a magnitude of current
density in excess of one ampere per square inch ~asi) of membrane
area is utilized ~o reduce the alkali metal chloride level in the
catholyte solids to less than 400 parts per million ~ppm) as described
in U.S. 4,080,270. The magnitude of current density required to
achieve this low level of salt i~purity will vary depending upon
~he thickness, ion-exchange groups and equivalent weight of the mem-
brane utilized and can be readily ascertained. If higher chloride
salt impurity levels are acceptable ~hen lower current density levels-
~ay be used. Typical current densities that may be used are 1 to 4
asi (15.5 to 62 asdm).
Typically, the catholyte is discharged from the cathode
com~artmene or dra n off from che recircula~mg c= holyC- tt a -aee
!
'.
~!`
-10- ~ ~L37~Z3
.. ~ .
proportional to the ra~e of transport of the hydrated al~ali metal
ions t~llough the membrane (proportional to current denslty) and the
rate of any external ~a~er added to the catholyte so as to maintain
an esscn~ially constant catholyte vol~me. After being discharged,
the catholyce typically is transported to a holding ~ank prior to '~
further i1rocessing such as concentrating, dr~Jing or packaging for
shipmen~ At this poine any residual by-product hydroxide or bicar-
bonate can be chemically removed if deemed undesirable i~ the final
produc~, i
Alkali metal carbonates, and particularly the sodium and
potassiul" carbonates, are well-known large volume industrial chemi-
cals. Like the products of the prior art, the alkali metal carbonate
producc~l by the invention process can be marketed either as liquors
or as anhydrous or hydrated solid materials and are produced from the
lS d~schar~,ed catholyte by means conventional to the industry such as
concentr.lting, drying and ~he like. Similarly, t~ey can be used for
like encl uses such as: in the manufacture of glass, alumina, paper
and de~rgents; as the precursor of other alkali metal compounds; and
as regel~crable absorbents for carbon dioxide and hydrogen s~lfide.
While the preceding description and following examples are
; directctl, for clarity, to single cells, it Will be obvious that in
com~ercial operation a plurality of such cells will usually be com-
bined in a single electrolyzing unit either in a series arrangement
usin~ bipolar electrodes or in a`parallel configuration using mono-
polar electrodes.
:~ .
` EXAMPLES 1-7
In the examples, a cylirLdrical laboratory electrolysis cell
separated by a cation-exchange memb~ane into anolyte and catholyte
compartments and having an inside diameter of 2 inche~ (50.3 mm) was
used. Tllc anode and cathode, both slightly smaller in diame~er,
were po~itioned contiguous to the membrane, except in Examples5 to 7
where th~ anode ~as~spaced 0.125 inch (3.2 ~) from the membrane.
Th~ anod~ in all the examples was an expanded mesh of titanium metal
bearing n 2TiO2:Ru02 coating. The cathode in Examples 1-4 was a
0.063 inch (1.6 mm) thick low-carbon steel plate ha~ing 0.25 inch
(6.4 mm) diameter holes therethrough spaced apart from each other
.
0.62 incl~ ~15.8 mm) center to center (58% open area), while Examples
5 to 7 olllrloy.d a~ array of verticel nic~el plates ~.062 inch ~1 6 t )
',
! ``
.
.` ~: . .
131L3~23 ',
" -11-- . ,
thick a~ld 0.5 inch (12.7 mm) wide disposed ver~ically and perpendi- ~ ;
cular to the ~embrane and spaced about 0.19 inch (4.?3 m~) apart from
each other. In all the examples, the hydrostatic pressure of the
anolyte upon the membrane exceeded that of the catholyte. Except for
Example 6 run as a compariso~ experiment, carbon dioxide was intro-
duced into the rear of the cathode compartment at the bottom of the
cell and in a quantity theoretically sufficiene to convert all the
alkali metal hydro~ide electroly~ically formed to carbonate salt.
In E~amples 1 ta 4, the membrane utiLized was a supported
film (T-12 square-wo~en TEFLON fabric), having an average thickness
of abou~ 3.5 mils (0.09 mm) in Examplesl-3 and 5 mil ~0,13 mm) in
Example 4, of a copolymer having recurring lmi~s of:
,
2 2
and
- CF2-CF-
-C~2 CF--CF2-CF2-S3H .~
CF3 i
~,
and an -S03H equivalent weight of 1100. The membrane of Examples
~ 5 and 6 also utilized the same copolymer and had a thickness oE 5
- 20 mils (0.13 m~ but was reinforced with the more~open T-900 G
square-woven TEFLO~ fabric. j
' In Example 7, the membrane used (N~FIO~-415~ was a 7.0
mil (0.18 mm) film comprised o~ two integral layers of differen~ co-
- polymers laminated to the T-900 G fabric. The laye~ laminated to25 the fabric had a thickness of about 6.1 mils (O.lS5 mm) and com- ,
; prised a copolymer having recurring uni~s of:
~ ' ' .
- - CF2-C~2-
and
CF -CF
3~ O-CF2-CF-O-CF2-CF2-SO3H
~- ~ . .
and an -SO3~ e~uivalent weight of 1100. The second layer had a thic~-
ness of about 0.9 mils (0.023 m~) and comprised a copolymer having
'--` ' '. ',,, ' ~
2-~37~
recurri~g units of~
- CF -CF -
and
~CF2-CF- ~1
O~CF2-~F-O-CF2-COOH
CF3
and an -COOH equivalent weight of 1014. In the cell, the layer con~
taining carboxyl groups faced the cathode.
In all the examples, KCl or NaCl brine, containing about
350 parts per million of H3P04 and acidified with HCl ~o a pH of
about 2.0, was charged to the anoly~e compartment at a rate of
0.75-l.Oml p&r ampere-minute, while aqueous carbonate sal~ was
charged to the catholyte compartment to provide initial catholyte.
During electrolysis water was added to the catholyte compartment at
a rate sufficient to provide the desired catholyte concentration.
The temperature of the~anolyte was controlled at about 90C, while
catholyte temperatures varied between i0 to 90C depending upon
the current density used. Other process parame~ers and the results
obtained were as shown in Table 1. `
: , ~
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~ o l ~
a ~ ' o ,~ ~ co ~ O ~ ~ ~ ~
h ~rl o~ I O a~ O cr~ ~ O
__ ,~ . X~i
: ~ ~ . ~
~ N V ~ O 1 ~
~0 ~ C`~ Ei ~ :
I h ~ ~ ~ L^~ ~
C~ ,1:~ : ~ ~ ~
,, x . a a :
d ~:1 ~ V
o ~ : ~
L',_l 1.1 ~ ~ ~1 ~ C~ 1 C`l V X
a ~ : ~ ?~
, ~ . ~ _.. ____ ~ ~: : : ' ~.
~. r~ L'~ ~ O L^~ L'l `J 1 : U~ :
`.~ C~ C~ -1 O` ~ O O C`l : ~
O ~ C~ . . . . . . . h ~ :S
. . Z `' ~; 0 ~ 0 0 0 :: O ~ O ~ ~ `- ~ .
. ~ ~_ ~ ~ :
!~ ~ ~/ : ~ ~ : ~t L) ~ .,
.: .4 oh ~ U C~ O~ ~ O Ul :
a ~ ~J o h
~7 0 ~1: 1~ ~t ~CO ~ I ~D a~ ( o :
~ ho ~ U ~ ~ ~ 11 I ~ L_(
~ : ,c~ ~ , a.~ U a
:-
'. _ ~ ....... __ __. ~ .. _.. _.. __.. ___ O O ~ O ,~
~ ~ : ~8.~ ~ ~
~ ~ , ~
h ,1 O LO L~ O O O ~ ,C ~ ri
. JJ`_ ~ 0: 0 CO CO ca o ' c
C~l :~ ` C`l ~ C~l ~ ` ~ ~ '~
~ ~ :' .~ ' ,
~ ~ O J-' ~ h
~ , ~::1 ~) ' .,. .. ____.__ O 'O C~ O ~
~ ~ ~ 6 .
.~ . ~ ~ æ ~: ~ ~ æ o
_ _ ___ ___ a~ aJ '
~; aJ ~ ~ ~ ~
.~ E ~ L'~ ~ Z I ,:
. P1 ~-~
:~
~'' : ' I :'
,
,
. ' .
2.~
" -14- ~
F~om the data in Table l, it can be seen tha~ the invention 1,
process provides nearly cheoretical (100%) currcnt efficiency with
respect to ca~1olyte product and operates at low electrolyzing vol~- !
ag~s. In this connection, the advantage of having the membrane con-
tiguous to the cathode was demonstrated by backing ~he cathode away
from the ~embralle in increments at about one minute intervals in
Example l, and noting the effect upon electrolyzing voltage. The re-
sults of this e~periment are summarized in Table 2, which shows the
considerable voltage savings that are realized when the invention
a process is used.
Table 2
- Effect of Cathode~~lembrane ap in Example l
Cathode-~lembrane Gap tinches) Electrolyzing Voltage
0 (start) 3.18
lS 0.03 3.20
0.12 3.34
0.25 3.57
0.38 3.77
0~50 4.0l
0.62 4.23
O (return) 3.l8
, ~
Exa~ple 5 shows that at higher catholyte concentrations (50-100% of
saturation - particularly ~or potassium carbonate, which can give 50%
total solids catholyte) lowest electrolyzing voltages are obtained
with a more-open, good-gas-release cathode. Lastly, E~ample 6,illus-
- trating the prior art, demonstrates the poor current efficlencies
which characterize membrane-cell electrolysis process for caustic
run at high catho]yte concentrations and with the membrane contiguous
to the c-thode.
_ i3
