Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
D76a~
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method and an apparatus for
installation of membranes in an electrolytic cell7 more particularly, to a finger
type electrolytic cell for electrolysis of an alkali metal chloride solution.
Sodium hydroxide has been commercially produced using asbestos or
modified asbestos diaphragm cells. However, sodium hydro~ide prepared using these
cells is poor in quality as about 0.9 to about 1.2% by weight of sodium chloride
is usually contained in a 50% sodium hydroxide liquor. Sodium chloride contained
in sodium hydroxide liquor produced in these cells may be removed by ammonia
extraction, the use of hydrated sodium hydroxide or the like, but on an
industrial scale, the sodium hydroxide liquor can only be purified, at best, to
an extent ranging from about 500 to l,000 ppm, and still more significant, a
relatively large expenditure is required`for puriEication. Sodium hydroxide
used for the rayon industry can contain only 200 ppm or less of sodium chloride in
a 50% sodium hydroxide liquor. Accordingly, it is rather difficult to produce
sodium hydroxide for the rayon industry at a reasonable and moderate cost by thepurification of sodium hydroxide from the asbestos or modified asbestos diaphragm
cells.
When asbestos or modified asbestos diaphragm cells are converted to the
~on exchange membrane cells according to the present invention, not only is the
quality of the product improved, but also the operation of an electrolytic plant
becomes feasible. That is, due to the fact that there is no precipitation of
- salts in the evaporation system, washing of the slurry lines and the vessels and
the like is not required, and the operation may be carried out automatically.
; Another advantage obtained by the conversion to the ion exchange membrane cell
- method is ~hat a cell liquor containing hardly any NaCl is obtained. A further
:
advantage is that sodium hydroxide which is to be used directly in the same plant
or the like may be supplied for use without being concentrated by evaporation to
:
' " ,~
~7~
45 to 50~, a low concentration being satisfactory. Asbestos or modified
asbestos diaphragm cell liquor containing a large amount of sodium chloride,
however, must be concentrated to from 45 to 50~ for use, even though used
directly in the plant. By converting to the ion exchange membrane method,
sodium hydroxide containing substantially no sodium chloride is obtained, and thus
may be supplied for a variety of uses immediately by being cooled to the desired
temperature, or may be mixed with a 50% sodium hydroxide to the desired concen-
tration and then supplied for use.
However, it is very difEicult to install an ion exchange membrane to
a finger type electrolytic cell comprising winding portions. The installation
in a plain way of an ion exchange membrane onto the curved surfaces of a cathode
always results in increased concentration of alkali metal chlorlde in the
product alkali metal hydroxide liquor.
On the other hand, hydrodynamically permeable perforated membranes are
known as microporous membranes. It is also difficult to locate these membranes
in a finger type electrolytic cell which is typical and common as an asbestos -;
diaphragm cell, but these are not yet in use on an industrial scale because of
this.
In order to solve the aforementioned problèms, the present inventors
have studied a method and an apparatus which enable simple and easy installation
of a substantially flat membrane in a finger type electrolytic cell.
Thus the present invention provides a feasible method and an apparatus
for installing a membrane in a finger type electrolytic cell.
Secondly, the present invention provi.des a method and an apparatus for
converting an asbestos or modified asbestos diaphragm cell to a cation exchange
membrane cell, and preparing and alkali metal hydroxide liquor containing little
alkali metal chloride or the like.
Thirdly, the present invention provides a method and an apparatus for
- 2 -
,
.
,
7Ç~
producing an alkali metal hydroxide of high quality containing hardly any alkali
metal chloride, by the use of a catlon exchange membrane installation frame.
Fourthly, the present invention provides a method and an apparatus
which are free from danger to health and from environmental contamination due to
asbestos.
Lastly, the present invention provides a method and an apparatus ~hich
enable the production of an alkali metal hydroxide in an economically advantageous
way.
According to the invention, the horizontal surfaces of a cathode not
opposing an anode of the finger type electrolytic cell are covered with a membrane
installation frame having a collar, the membrane is positioned substantially
parallel to the vertical surfaces of the cathode, and the membrane is installed
; and secured to the collar by mechanlcal securing means.
As finger type electrolytic cells useful according to the present
invention there are included not only a finger type construction cell such as that
described at page 93, Chlorine Its Manufacture, Properties and Uses, edited by
J. S. Scone, issued Reinhold Publishing Corporation, New York, 1962, but also a
flattened tube type construction cell. Nowadays, the flattened tube type .
construction is also generally referred to as a finger type electrolytic cell.
As alkali metals herein, there are included sodium, potassium and the like.
An anode in a finger type electrolytic cell is commonly located and
-~ embedded in an electroconductive bottom plate or side plate. According to the
invention a membrane installation frame is positioned to cover the horizontal
surfaces of a cathode not opposite an anode. That is, when the anode is embedded
in the bottom plate, the installation frame is positioned to cover the horizontal
surfaces of the cathode. When the membrane installation frame is electrically
conductive, it is usually insulated from the cathode by insertion of packing,
lining or by any other suitable means.
- 3 -
.~ I
37~
It is a feature of the present invention that installation of the
substantia]ly flat membrane to the curved surfaces of the cathode can be
facilitated by providing a collar with the installation frame. The collar is
preferably located at an angle of about 90 degrees or less, more preferably
from about 90 degrees to about 30 degrees, to the flat portion of the membrane
installation frame. The most preferred angle is about 90 degrees since neither
expansion nor contraction of the membrane is required. However, it is desirable
to locate the collar close to the bottom plate or side plate in which the anode
is embedded, at an angle of about 60 degrees to about 30 degrees, thereby
facilitating assembly of the anode and the cathode to which the membrane has been
positioned. In the case of an angle of less than 90 degrees, the membrane
has to be flared to conform with the collar angle. The smaller the angle, the
larger the flare of the membrane. While the flared membrane can be made from
a polymer by heat-moulding without much difficulty, it is rather difficult to
form a flared membrane from a flat membrane. Accordingly, the collar angle is
decided upon in accordance with the degree of ease of processing the membrane
and the operation efficiency. Therefore, the lower limit of an angle of 30
degrees i~ a matter of practice, not theory. Inversely, in the case of an angle
of more than 90 degrees, the membrane must be contracted or folded to secure
it to the collar. Care must be taken that the efficiency of assembly of the
anode and the cathode is not disrupted, when the collar angle close to the
bottom or side plate in which the anode is embedded, is more than 90 degrees.
The membrane is secured to the collar and the membrane installation
frame by a mechanical securing ~eans. One such securing means is the use of
~ bolts. The use of a press plate and bolts provides better attachment. Nuts,
; washers, spring washers or ~he like may also be used, if desired. Another
suitable securing means is clips. An effective seal is obtained by the combined
use of a press plate and clips. Securing by clips is preferred because there
- 4 -
~J~
:
is no need to make holes in the membrane. The use of clips formed as part of the
press plate increases efficiency of assembly. Combined use of bolts and clips
is effective. For example, it is rather difficult ~o secure the membrane to
the curved portion by clips alone or with the press plate, but this can be
done with bolts alone or together with the press plate, while the membrane can
be secured to the straight portion by clips singly or in combination with the
press plate.
When press plates and bolts are used, it is preferred to use a press
plate having a longitudinal edge flange or longitudinal edge flanges for
strength considerations so that waving does not occur which can result in an
imperfect seal. Without flanges on the press plate, more bolts are required
to prevent the waving phenomenon from occurring. Obviously, a decrease in the
number of bolts required increases efficiency of installation.
The membrane is secured to the collar even without using the press
plate, but the press plate enables easier and more complete assembly. In cases
where no press plate is employed, it is necessary that the secured portion of
the membrane be hardened by thickening that portion during production of the
;- membrane or by folding the membrane twofold or threefold and heat-fusing. More
effective securing is also obtained by applying bolts or clips at smaller
;~ 20 intervals than in the case where the press plate is used. When a microporous
membrane is employed, insufficient securing is not necessarily problematical as
compared with ion exchange membrane, since it is inherently hydrodynamically
permeable.
The membrane installation frame, the press plate and the like have to
be made from materials which are resistant for example to anolyte. Titanium,
zirconium, tantalum or alloys thereof such as titanium-palladium, titanium-
tantalum, Hastelloy* or the like are suitable. Organic materials other than
the metallic materials as aforesaid, are suitable, as for example FRP ~glass
*Trade Mar~ for nickel-base, corrosion resistant alloys
,. .. ..
..~
':
~76~4L
fibre-reinforced plastic), hea~-resistant polyvinyl chloride, polypropylene,
fluorocarbon polymer, fluorocarbon polymer or rubber lined metals or the like.
Examples of fluorocarbon polymers are polymers of tetrafluoroethylene,
hexafluoropropylene, perfluoroalkylvinylether, copolymers thereof, polytri-
fluorochloroethylene, polyfluorovinylidene and the like.
I~hen the membrane installation frame is made of an electro-conductive
.,
~'
.,~
~ - 5a
.~
material, insulating packing is interposed between the frame and the cathode,
otherwise, a lining or coating is applied to at least the surface of the cathode.
When the membrane installation frame is of plastics, washers should be used.
The press plate is preferably made from metallic materials but a plastic press
plate is desirably used with washers. As suitable materials for bolts, nuts,
washers, clips and the clips formed as part of a press plate, metals such as
titanium, tantalum, titanium-tantalum alloy and the like are preferred. Organic
materials such as fluorocarbon polymer bolts may also be used.
In order to accomplish more complete securing and sealing, it is
preferred to interpose a polyfluorocarbon packing, as for example, manufactured
by E. I. Du Pont de Nemours & Company under the trade mark "Teflon", between thecollar and the membrane, and further between the membrane and the press plate,
~`! if necessary. As packing, "Teflon" thin film or the like may be suitably used
which are known as a porous film or tape seal. A thin film of fluorocarbon
polymer other than "Teflon", such as tetrafluoroethylene-hexafluoropropylene co-
polymer, may also be used. Sealants such as "Teflon" paste, silicone grease
and the like may be used with the packing to provide more perfect securing
and sealing.
A membrane formed in a cylindrical shape is preferably used. The
cylindrical membrane is obtained by extrusion moulding, heat-sealing, use of
adhesives and the like.
` When a cation exchange membrane is employed, the cell liquor concentra-
tion (sodium hydroxide concentration) can be increased over 30 to 40%. As
cation exchange membranes, polyfluorocarbon membrane with sulfonic acid group,
sulfonamide group, carboxylic acid group, as for example, "Nafion* #110",
"#215", "#315", "#425" and the like which are produced and sold by E. I. Du Pont
de Nemours & Company may be used.
In cases where the present invention is applied to a microporous
* Trade Mark
~1
'
:.
3 ~ 6~
membrane, there are advantages that back flow of OH is reduced due to the
amount of flux per unit area of membrane being increased, and the sodium
hydroxide concentration in the cell liquor can be raised at the same oxygen
concentration in chlorine. The operation costs can therefore be reduced on
account of the decrease in cell voltage and the increase in the cell liquor
concentration. Although it is difficult to apply an ion exchange membrane method -
to an electrolysis plant where subterranean brine is used, if a microporous
membrane method is carried out according to the method and apparatus of the
present invention, operation costs can be reduced. As microporous membranes,
"Nafion* #701", "#710" or the like which are manufactured and sold by E. I. ~u
Pont de Nemours & Company are Stli table.
Figure 1 is a perspective vertical sectional view of a finger type
electrolytic cell wherein the membrane installation frame for attachment of the
membrane is located DO as to cover the surfaces of the cathode at right angles
to an anode positioning direction between the cathodes.
` Figure 2 illustrates an enlarged partial sectional view of Figure 1.
Figure 3 is a sectional view illustrating attachment of the membrane
to the installation frame.
Figures 4 and 8 illustrate perspective vertical sectional views of
a finger type electrolytic cell to which the membrane is installed using clips
according to the present invention.
Figures 5 and 9 are enlarged partial sectional views of Figures 4 and 8,
respectively.
Figures 6 and 10 illustrate sectional views of the attachment of the
membrane to the installati.on frame.
Figure 7 is a perspective view of a clip and Figure 11 is a perspective
view of clips formed as part of a press plate.
Figure 12 is a perspective vertical sectional view of a finger type
* Trade Mark
-- 7 --
.~
. . .
76~9l
electrolytic cell wherein one collar close to the bottom plate provided with the
anode is located at about ~5 degrees to, and the other collar is located at
about 90 degrees to, the flat portion of the membrane installation frame, to
which collars the membrane is attached.
Figure 13 is an enlarged partial sectional view of Figure 12 and
Figure 1~ is a sectional view illustrating the membrane attachment to the lower
collar (the collar close to the bottom plate embedding the anode).
The present invention will be illustrated in more detail by way of
particular embodiments and examples, which are not, however, to be construed
` 10 in any manner as limiting to the invention. `
EXAMPI.E 1
. . .
As is shown in Figure 1, membranes 4, formed in a cylindrical shape
are positioned adjacent to collars 7 located at about 90 degrees to the flat
portion 7a of a membrane installation frame. The membranes are positioned
substantially parallel to the vertical surfaces 11 of the cathodes and secured by
means of bolts and nuts 3 to the collars using press plates 2, a packing 6 being
interposed between the collar and the membrane (Figure 3). Thus, a cathode com-
, . t:
partment 8 and an anode compartment 9 are completely isolated from each other
tFigure 2). The membrane installation frames 1 are located in a manner that the
surfaces 10 of the cathode at right angles to the anode 12 embedded in bottom
plate 13 are covered with the flat portion 7a of the installation frame. The
bottom plate 13 is insulated from the cathode box 5 by inserting an insulator 14
between the two. As the membrane, cation exchange membrane Nafion* #315 produced L
by E. I. Du Pont de Nemours & Company is employed.
To the anode compartment of such a cell was supplied`a hydrochloric ~-
acid-containing sodium chloride solution, deionized water`was fed to the cathode
compartment, and then 2,000A electric current supplied to the cell. The anode
current density was 25A/dm . The brine supplied was 3N with respect to the Na~l L
*Trade Mark
- 8 ~
~7~4
and the HCl was 0.2N in the brlne. After continuous operation for 7 daysJ the
NaCl concentration was as low as 20 ppm in the catholyte and the NaOH concentration
was 16~ in the catholyte.
EXAMPLE 2
:
As illustrated in Figure 4, cylindrical membrane 4 is positioned
substantially parallel to the vertical surfaces of the cathode and secured by the
use of the press plate 2 and clips 15 to the collars 7 located at about 90
degrees to the flat portion 7a of the membrane installation frame 1. At the curved
portion securing is effected using bolts and nuts. Packing 6 is inserted between
10 the collar and the membrane (Figure 6). The cathode compartment and the anode
compartment are thus iso]ated from each other (Figure 5). A suitable number of clips
shown in Figure 7 are employed at desired intervals to accomplish securing and
sealing. Operation efficiency is improved as compared with Example l.
EXAMPLE 3
A cylindrically formed membrane 4 is positioned, as is depicted in
; Figure 8 substantially parallel to the vertical surfaces of the cathodes and
secured to the collars 7 which are located at about 90 degrees to the flat portion
7a of the membrane installation frame, using clips 16 which are part of a press
plate. At the curved portion using a U-form press plate 2a is secured to the
collars 7 with packing inserted therebetween, by bolts and nuts. The labor and
time required for the installation of membranes to the electrolytic cell are
markedly reduced. I
EXAMPLE 4 - i
A cylindrical membrane 4 having one edge flared is positioned parallel
to the surfaces of the cathode, then applied to the collars 7. While the upper
part is located at about 90 degrees, the lower (close to the bottom plate in which
the anode is embedded) is located at about 45 degrees to the flat portion 7a of
the membrane installation frame l. The membrane is secured and sealed by means
~1 .
., ... : , ,:
: ,' '
: ' " ' '
7~
of clips and the press plate to the collars. At .the curved portion, securing i5
achieved by screw bolts. Between the collar and the membrane a packing is inter-
posed (Figure 14). The cathode compartment 8 is thus isolated from the anode
compartment 9 (Figure 13). Assemblage of the membrane installed cathode box and
the anode embedded bottom plate is effected by rotating the cathode box at an
angle of 180 degrees. By locati.ng the collar 7 at 45 degrees to the flat portion
7a of the insta].lation frame, operation efficiency of assembly i~ greatly
improved.
'
- 10 -
:, ~
