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| United States Patent |
5,765,763
|
|
Kroner
, et al.
|
June 16, 1998
|
Recycling of diaphragms
Abstract
The invention relates to a process for recycling diaphragms, in particular
a process for recycling used asbestos-free diaphragms from alkali metal
chloride electrolysis, the diaphragm material being comminuted, the
diaphragm material being washed with a wash substrate and a reusable
material being obtained.
| Inventors:
|
Kroner; Rudi (Mannheim, DE);
Leutner; Bernd (Frankenthal, DE);
Schlafer; Dieter (Ludwigshafen, DE);
Steiner; Wolfgang (Friedelsheim, DE);
Friedrich; Holger (Bad Durkheim, DE)
|
| Assignee:
|
BASF Aktiengesellschaft (Ludwigshafen, DE)
|
| Appl. No.:
|
585350 |
| Filed:
|
January 11, 1996 |
Foreign Application Priority Data
| Jan 13, 1995[DE] | 195 00 871.5 |
| Intern'l Class: |
B02C 019/12 |
| Field of Search: |
241/20,21,23,24.11,24.18,29,24,28,DIG. 38,22
|
References Cited
U.S. Patent Documents
| 2879005 | Mar., 1959 | Jarvis | 241/24.
|
| 3154255 | Oct., 1964 | Schulman et al. | 241/21.
|
| 3162380 | Dec., 1964 | Cohn et al. | 241/21.
|
| 3690569 | Sep., 1972 | Leverett | 241/21.
|
| 3717307 | Feb., 1973 | Beck | 241/24.
|
| 3726483 | Apr., 1973 | Kometani et al. | 241/5.
|
| 3815833 | Jun., 1974 | Vliet et al. | 241/5.
|
| 3992350 | Nov., 1976 | Bensa et al. | 241/18.
|
| 4073661 | Feb., 1978 | Buzga et al. | 241/20.
|
| 4199109 | Apr., 1980 | Watanabe | 241/24.
|
| 4226672 | Oct., 1980 | Absolon et al. | 241/21.
|
| 4367147 | Jan., 1983 | Asami et al. | 210/636.
|
| 4406411 | Sep., 1983 | Gall et al. | 241/24.
|
| 4526904 | Jul., 1985 | Kishida et al. | 521/26.
|
| 4650126 | Mar., 1987 | Feder et al. | 241/23.
|
| 4830188 | May., 1989 | Hannigan et al. | 241/DIG.
|
| 5133843 | Jul., 1992 | Eisman.
| |
| 5156343 | Oct., 1992 | Sueyoshi et al. | 241/21.
|
| 5236603 | Aug., 1993 | Sampson | 241/23.
|
| 5255859 | Oct., 1993 | Peacock et al. | 241/21.
|
| 5375778 | Dec., 1994 | Lundquist | 241/20.
|
Other References
Chem. Abst., vol. 81, No. 20, Nov. 18, 1974; Abst. No. 122250 (English
abstract of Levitin I.A., "Use of Radiation Reclaimed Butyl Rubber in
Diaphragm Rubbers" and Tr. Mosk. Inst. Tonkoi Khim. Teknol., vol. 2, No.
1, 1972).
|
Primary Examiner: Rosenbaum; Mark
Attorney, Agent or Firm: Keil & Weinkauf
Claims
We claim:
1. A process for recycling asbestos-free diaphragms, which comprises
comminuting the diaphragm material,
washing the diaphragm material with a wash solution whereby a reusable
diaphragm material is obtained.
2. The process of claim 1, wherein the asbestos-free diaphragm material is
diaphragm material from alkali metal chloride electrolysis.
3. The process of claim 1, wherein the reusable diaphragm material is
further comminuted.
4. The process of claim 1, wherein an inorganic material is added to the
comminuted diaphragm material.
5. The process of claim 4, wherein the inorganic material is added to a
slurry of the comminuted diaphragm material.
6. The process of claim 4, wherein the inorganic material has a
distribution of particle size with a maximum smaller than 100 .mu.m.
7. The process of claim 4, wherein 5 to 50 parts by weight of zirconium
oxide per 100 parts by weight of the dry mass of the comminuated diaphragm
material are used as the inorganic material.
8. The process of claim 1, wherein the comminuted diaphragm material is
washed with an acid-containing aqueous solution.
9. The process of claim 1, wherein the comminuted diaphragm material is
washed with an HCl-containing wash solution having an HCl concentration in
the range of 0.1-13 mol/l.
10. The process of claim 1, wherein the comminuted diaphragm material is
washed with a solution containing sodium chloride.
11. The process of claim 1, further comprising adding a nonionic surfactant
to the reusable diaphragm material.
12. The process of claim 1, wherein the reusable diaphragm material is
mixed with original diaphragm material.
13. The process of claim 1, wherein the reusable diaphragm material is
impregnated with a zirconium compound.
14. The process of claim 1, wherein the reusable diaphragm material is
separated from the washing solution.
15. The process of claim 14, wherein the separated diaphragm material is
dried.
Description
The invention relates to a process for recycling diaphragms. In particular,
it is aimed at a process for recycling used asbestos-free diaphragms from
alkali metal chloride electrolysis.
The electrolysis of aqueous solutions of NaCl or KCl (alkali to metal
chloride electrolysis) is of great industrial importance for obtaining
many different products. In industry, alkali metal chloride electrolysis
is primarily used to obtain chlorine and sodium hydroxide solution. In
order to obtain a sodium hydroxide solution which is as free of chloride
as possible, several processes are available. In the is diaphragm process,
cathode and anode spaces are separated by a diaphragm which consists, for
example, of fibers entangled in one another. Conventional diaphragm
materials consist essentially of asbestos. Recently, diaphragms have also
been employed which consist of chemically inert synthetic materials.
These diaphragms become unusable after several months or years of
operation, as impurities deposit in the diaphragm. These impurities lead,
for example, to hydrogen occurring on the anode side. The cell has to be
switched off, and the diaphragm is removed and added to the landfill.
Apart from the fact that at regular intervals new diaphragms have to be
prepared, which are produced from new, expensive diaphragm material,
storage and disposal costs additionally accumulate for the diaphragms
removed. No process is known from the prior art to reuse used diaphragm
material.
It is an object of the invention to avoid said disadvantages in the
disposal of used diaphragms, in particular to provide a process which can
be specifically employed with asbestos-free diaphragm materials, and
avoids the high storage and disposal costs, in particular in connection
with special landfills.
We have found that this object is achieved according to the invention by
providing a recycling process for the diaphragm material, as is defined in
claim 1. The process is distinguished in that the diaphragm material is
comminuted, and the diaphragm material is washed with a wash substrate, in
particular a wash solution, a reusable material being obtained.
As preferred diaphragm materials, polyfluorohydrocarbons such as
polyvinylidene fluoride, polytetrafluoroethylene (PTFE) or
polychlorotrifluoroethylene may be mentioned here. For the purpose
mentioned, fibers or fibrils of polychlorotrifluoroethylene or PTFE are
suitable which contain up to 80% by weight of an inorganic Is hydrophilic
material, e.g. ZrO.sub.2 or titanium dioxide. The typical mean fiber
length (measured as a maximum of the fiber length distribution) of the
starting material to be processed is in the range 0.25-0.5 mm. The mean
fiber length in the recycling process according to the invention is
typically reduced by from 5 to 80%, in particular by from 25 to 50%. The
typical fiber diameter of the starting material to be processed, which can
be produced by any desired process, is in the range 0.05-100 .mu.m.
Special fibers can also have a length of 2-30,000 .mu.m, in particular of
1,000-7,000 .mu.m, and a diameter of 1-1000 .mu.m, in particular of 10-100
.mu.m. The fibers used can also be irregularly shaped, branched fibrils.
These fibers do not lose their shape as a result of the recycling process,
but are shortened by from about 5 to 80%.
By washing the diaphragm material, iron-containing residues especially can
also be reduced or removed.
This process is preferably employed in order to recycle asbestos-free
diaphragm material, preferably diaphragm material from alkali metal
chloride electrolysis. Suitable diaphragms for carrying out the recycling
process according to the invention are in particular those which have been
prepared by deposition of a suitable fibrous material (fibrils) on the
cathode of the electrolysis cell, e.g. diaphragms of titanium
dioxide-containing polychlorotrifluoroethylene fibers or Polyramix.RTM.
fibers from Oxytech. The latter are fibers which consist essentially of
PTFE (about 20%) and zirconium oxide (about 80%).
An advantageous further development of the process proposes that the
comminution is carried out in at least one stage, preferably in several
stages, in particular that the diaphragm material is comminuted to fiber
size. The diaphragm mats detached from the cathode are dried and
comminuted such that preferably finely divided, fibrous is material is
available for the subsequent washing. This fiber size is typically in the
range 2-10,000 .mu.m, in particular in the range 0.5-5 mm. In order to
achieve this, the number of individual comminution stages and the type of
comminution in the individual stages can be suited to the economic and
technical requirements.
Preferably, the process comprises several comminution steps, in particular
a coarse comminution and a finer comminution. This is particularly
advantageous if the detached diaphragm mats have to be stored in between a
coarse precomminution or transported before a finer comminution can take
place, since the coarsely precomminuted material can be handled better in
transfer processes, in particular the precomminution serves to facilitate
the operating procedures on charging the mill in which the breaking-up
into fibers takes place. As a result of a first coarse precomminution in
which the surface area of the diaphragm material has been increased, the
drying process, for example, can also be shortened. The finer comminution
can particularly comprise a cutting in which the diaphragm material is
comminuted to fiber size.
The process according to the invention advantageously proposes that the
diaphragm material is washed after at least one comminution stage and then
again comminuted in at least one stage. By this means, diaphragm material
obtained, for example, after a pre-comminution is subjected to a first
washing, large amounts of deposits, which, for example, can contain iron
compounds, already being removed here. The material thus obtained now
takes up less space, so it can be more economically transported, in
particular more easily pneumatically transported, and stored.
A preferred embodiment of the invention proposes a process in which the
diaphragm material is dried after washing and before the subsequent
comminution. This intermediate drying is particularly advantageous if the
type of mill employed for the breaking-up into fibers yields more usable
fibers from a technical point of view using dried diaphragm pieces. The
process according to the invention can advantageously be refined by first
subjecting dry diaphragm material to the various comminution stages and
then washing and, if desired, additionally rewashing the finely divided
(fibrous) material obtained in this process. After washing, the diaphragm
material is separated from the washing solution by filtration and
additionally rewashed several times with water. The filter cake can then
be employed without further treatment for the production of a new
diaphragm, or else dried after washing. In the former case, a working step
is saved by immediate processing without drying. For example, in the case
in which it is wished additionally to store the material before
processing, the storage weight can be reduced and thus storage costs can
be saved by drying. In particular, if required the amount necessary in
each case can now be comminuted to the fiber size according to the
technical requirements and even employed without further purification for
diaphragm production.
The process according to the invention can advantageously be refined by the
comminution only comprising a coarse comminution, the coarse comminution
preferably producing diaphragm elements having a mean particle size of
5-25 mm, in particular one of not under 3 mm. If desired, after additional
washing the diaphragm material thus obtained can be economically stored
and transported.
It is furthermore provided according to a particular embodiment of the
process according to the invention that at least one stage of the
comminution is carried out in a mill. As a result of breaking up into
fibers in the mill, the diaphragm material can be comminuted to fiber
size.
In a further preferred embodiment of the process according to is the
invention, inorganic material is added to the comminuted diaphragm
material. By adding of an inorganic material, preferably an inorganic
solid, the functionality of the recycled diaphragms can be significantly
improved. Especially preferred is the adding of an inorganic solid with a
certain distribution of the size of particles. By adding the inorganic
solid to the comminuted diaphragm material or the fibres, respectively, in
first place the behaviour with the difusion through the diaphragm is
controlable, that means that the difusion of the brine through the
recycled diaphragm can be reduced. With this, the behaviour of the
recycled diaphragms with respect to the difusion can be set to a
technically required value.
It is especially preferred that materials or solids, respectively, are
employed as inorganic material which are hardly soluble in an aqueous
solution, which do not react with fluorine plastics and which are
preferably stable over a large scale of pH values (pH approximately from 2
to 14). Thus, materials which are suitable as inorganic materials are
oxides, silicates, carbides, sulfates, borides, silicones, nitrides. It is
preferred to use oxides of titanium, vanadium, cromium, zirconium,
molybdenum, hafnium, tungsten, tantalum, niobium, and these substances can
either be used pure or as a mixture. The inorganic solids are either
extremely pure or technical substances. It is specially preferred to use
zirconium oxide since it is very stable under the conditions of the alcali
metal chloride electrolysis.
In a further preferred embodiment of the present invention, the inorganic
material is mixed with the comminuted diaphragm material, especially the
inorganic material is added to a slurry made of the comminuted diaphragm
material. The addition of for instance zirconium oxide can either be
accomplished by a simple mixing with the fibres. It is, however, preferred
to make a slurry from the comminuted diaphragm material, that means the
fibres, in which the inorganic material is then mixed. This slurry can
afterwards be used for producing the diaphragms, for instance by means of
vacuum deposition.
It is preferred to add the inorganic material to the washed, comminuted
diaphragm material. The used amount of inorganic material is determined
preferably in dependence of the distribution of particle sizes of the
inorganic material or the mixture of inorganic solids, respectively, and
in dependence of the technical requirements which the recycled diaphragm
has to meet. It was for instance found that amounts of 5 to 50 parts in
weight of zirconium oxide, preferably 10 to 45 parts in weight of
zirconium oxide, per 100 parts in weight of fibres--wherein these values
refer to the dried mass of the fibres--lead to a desired reduction of the
diffusion of the brine through the diaphragm. Diaphragms, which have been
made from a material without any addition of zirconium oxide, for instance
have a diffusion of the brine through the diaphragm of approximately 0.1
to 0.3 m.sup.3 /hm.sup.2. By adding of approximately 30% of zirconium
oxide of the given particle size distribution, this value can be reduced
to approximately 0.02 to 0.05 m.sup.3 /hm.sup.2. Thus, the diffusion can
be reduced by a factor of 4 to 10.
In an embodiment of the present invention it is especially preferred to use
zirconium oxide with the following distribution in particle sizes: The
particles are mainly--that means more than 90%, preferably more than 99%
of the particles--smaller than 100 .mu.m, preferably smaller than 40
.mu.m, in their largest extension. Preferably, material is employed in
which the upper limit of the particle size of 10% of the particles ranges
between 0.3 and 0.9 .mu.m, for 50% of the particles ranges between 0.9 and
3 .mu.m and for 90% of the particles ranges between 3 and 20 .mu.m,
wherein the values of the percentages refer to the volume of the inorganic
material. It is especially preferred to have an upper limit of the
particle size for 10 % of the particles in the range of 0.5 and 0.7 .mu.m,
for 50% of the particles in the range of 1 to 2 .mu.m and for 90% of the
particles in the range of 3 to 10 .mu.m. The values of the percentages are
volume percentages (Q3, i.e. the distribution of the volume, that means
the sum of the volumes of all particles of a given size (diameter) or of a
given range of particle size, respectively) which result from a measuring
of the particle size distribution.
Preferably, an HCl-containing wash solution having an HCl concentration in
the range 0.1-13 mol/l is used for washing. Owing to the hydrochloric
acid, deposits which contain iron compounds can be removed.
In a preferred embodiment of the process according to the invention, the
wash substrate, in particular the acidic wash solution, contains sodium
chloride. This has the advantage that the brine anyway employed in the
alkali metal chloride hydrolysis can be used after addition of, for
example, hydrochloric acid. By means of this, costs both with respect to
the disposal of the brine, and in the production of the wash solution can
be reduced.
An advantageous refinement of the process proposes washing at from room
temperature to the boiling point of the wash solution. The temperature to
be selected is predetermined by overall economic and technical conditions,
expediently, however, the wash substrate is heated to a temperature of or
below the boiling point. In particular, to the temperature in a wash
solution can be 50.degree.-100.degree. C.
In a further preferred embodiment of the process according to the
invention, the washed diaphragm material is used for diaphragm production.
Thus, the material obtained, which is present, for example, in a filter
cake, can be employed without further treatment for the production of a
new diaphragm. To do this, it is, for example, suspended and the mixture
thus obtained is applied to the cathode as a covering layer on one side.
This can be achieved by vacuum filtration, brushing or spraying. If
desired, the diaphragm is then impregnated with a zirconium compound, e.g.
Zirconium oxychloride (ZrOCl.sub.2) or a zirconium alkoxide or a solution
of these compounds and then brought into contact with an aqueous sodium
hydroxide solution, preferably by means of immersion, in order, for
example, to precipitate water-containing zirconium oxide in the
interstices of the diaphragm matrix. Finally, the preformed diaphragm is
dried, preferably by heat treatment below the sintering temperature, if
appropriate also with application of pressure, whereby the total strength
and dimensional stability of the diaphragm is increased. After drying, the
diaphragm is heated to a temperature above the sintering temperature of
the synthetic material of which the fibers consist, the fibers sticking
together without the pore structure of the diaphragm being destroyed.
In this manner, final storage of the diaphragm material is avoided, as the
diaphragm material is fed back into the production cycle. A further
advantage can be seen in the fact that no additional new diaphragm
material has to be produced if diaphragms are produced from the diaphragm
material recovered by the process according to the invention. Furthermore,
the material can also be used in other areas, e.g. in the production of
filter presses. The recovered material can be suspended, applied via the
vacuum process and baked solid by appropriate action of pressure and
temperature.
Preferably, the washed diaphragm material can be subjected to a further
treatment, namely at least partial drying of the diaphragm material and/or
at least a further washing of the diaphragm material. Thus, the washed
diaphragm material can be directly dried, or the washed diaphragm material
can be additionally rewashed several times with, for example, water. As a
result of the rerinsing, the washing with other wash substrates and/or the
drying of the diaphragm material, diaphragm material can be produced for
further use in any desired starting condition.
Another advantageous further development according to the invention
proposes adding a nonionic surfactant to the washed diaphragm material,
the nonionic surfactant preferably being added to the diaphragm material
after at least one wash and/or after drying. By this means, the
suitability of the washed material for further processing is improved.
This process step is particularly recommended if dry recycling material is
to be employed for the production of a new diaphragm material.
Preferably, the washing of the diaphragm takes place at the end of the
various comminution stages, after which the washed material is employed
again in the damp condition. In this manner, the material can be
economically reused, as additional drying stages can be omitted. In this
process, the material is precomminuted in as dry a form as possible, then
broken up into fibers, then subjected to washing and if possible employed
again without drying. The already damp material can then also be more
simply suspended and applied to the cathode.
Preferably, original material is admixed to the reusable material in the
process according to the invention. As a result of the admixture of the
original material, the quality of the reusable material can be modified in
any desired manner. Thus 1-99% of the original material, in particular
10-70% by weight of the original material, can be admixed, and the mixture
thus obtained can be suspended and applied to a recipient surface,
preferably a cathode surface, and solidified by heating, a diaphragm being
obtained.
A zirconium compound can be added to the diaphragm in the initial operating
period such that, for example, water-containing zirconium oxide
precipitates between the diaphragm material. In this manner, the flow of
the brine can be regulated and thus a greater lifetime of the diaphragm
can be achieved.
The process according to the invention is intended to be illustrated in
greater detail by the following examples, which contain further preferred
details of the invention.
EXAMPLE 1
A diaphragm used for alkali metal chloride electrolysis, which according to
DE 27 56 720 consisted of polychlorotrifluoroethylene fibers which contain
about 70% by weight of titanium dioxide (mean particle diameter <1 .mu.m)
was detached from the cathode, spread out and stored at room temperature
for one day.
It was then precomminuted in a suitable mill and thereafter broken up into
fibers in a second mill. To remove deposits which contain, inter alia,
iron compounds, 1,000 g of the fiber material were contain, inter alia,
iron compounds, 1,000 g of the fiber material were boiled under reflux for
one hour in 3,000 g (2.87 l) of 10% strength hydrochloric acid. The fiber
material was then filtered off with suction through a frit and the filter
cake was rinsed four times with 1 l of water until free of acid. A white,
fibrous material remained, which still contained moisture (<10% by
weight).
The recycling material thus obtained was employed for the production of a
diaphragm, the same process being used which was also used in the
production of the diaphragms from unused material.
EXAMPLE 2
A Polyramix.RTM. diaphragm from Oxytech used in alkali metal chloride
electrolysis and consisting essentially of PTFE fibers containing 82% by
weight of ZrO.sub.2 was detached from the cathode of the alkali metal
chloride electrolysis cell in large surface-area mats, dried at
25.degree.-50.degree. C. for 24 hours in air, comminuted according to
Example 1 and freed from impurities and deposits which also contained iron
compounds by heating 1 kg of the material in 3 l of 10% strength by weight
hydrochloric acid for one hour.
TABLE 1
______________________________________
Analytical monitoring of fiber cleaning with 10% strength by
weight hydrochloric acid via the iron content
Fe content,
calculated as
ZrO.sub.2 content
Fe.sub.3 O.sub.4
______________________________________
Original fibers:
82% by weight (after
--
dissolving out
adhering NaCl)
Fibers, before
70% by weight 2.5% by weight
washing with HCl:
Fibers, after washing
74% by weight 0.05% by weight
with HCl:
Wash solution
0.77 g 24.6 g
(1997 g): (0.15% by weight of
(98% by weight of
the total ZrO.sub.2)
the total Fe.sub.3 O.sub.4)
______________________________________
The fiber material thus obtained (solid content >90%) was used in the solid
state for the production of a diaphragm by means of vacuum deposition, the
same process being used which was also employed for the production of the
diaphragms from unused Polyramix.RTM. fibers containing 82% by weight of
ZrO.sub.2.
The electrolysis of sodium chloride using a pilot plant cell which was
equipped with a diaphragm of recycling material produced sodium hydroxide
solution and chlorine. The cell voltage was 2.9 V. The sodium hydroxide
solution at the cell exit had a sodium hydroxide concentration of 67 g/l.
It was possible to obtain chlorine in a purity of 98.5% by weight.
In order to regulate the flow of brine through the diaphragm, the cell was
fed in the first five hours of electrolysis with a sodium chloride
solution which contained 500 ppm of Zirconium oxychloride (ZrOCl.sub.2).
EXAMPLE 3
Used diaphragm from alkali metal chloride electrolysis was processed
according to Example 1. Differing from Example 1, the comminuted diaphragm
material was covered with 3 l of boiling half-concentrated hydrochloric
acid for washing and the mixture was stirred with a stirrer at a low speed
of rotation (about 100 rpm) without further addition of heat for 30
minutes.
EXAMPLE 4
Polyramix.RTM. diaphragms were removed from the electrolysis cells and
immediately precomminuted in a saw mill without further drying. The
precomminuted diaphragm pieces (mean weight: about 1 kg) were covered with
3 l of half-concentrated hydrochloric acid heated to 90.degree. C. and
allowed to stand in a Dewar vessel for 24 hours. The acid was then
decanted off and the diaphragm pieces were washed several times with water
until acid-free. The rinsed pieces were dried at 120.degree. C. for 5
hours and broken up into fibers in a mill. The fiber material was then
passed through a 1 mm sieve in order to retain residual lumps. The
recycling fibers obtained were then used for the production of a
diaphragm, the same process again being employed which was also used for
the production of the original diaphragms.
EXAMPLE 5
Diaphragms of Polyramix.RTM. were removed from the electrolysis cell, dried
in air and precomminuted in a saw mill. Pieces having a mean diameter of
about 7 mm resulted. The precomminuted material (1 kg) was boiled under
reflux with a solution of 500 g of sodium chloride in 1,500 g of I molar
hydrochloric acid for 1 hour and then filtered off with suction through a
frit. In this manner, about 94% of the iron-containing impurities were
washed out. The moist substrate was then washed with 6 l of water and
subsequently stirred with 500 ml of water and 5 g of a nonionic
fluorosurfactant (Fluorad.RTM. FC-171 from 3M). The aqueous solution was
then largely removed by repeated filtering off through a frit. The
material was then dried at 70.degree. C. for 10 hours and broken up into
fibers in a suitable mill. Further processing to give diaphragms was
carried out according to Example 1.
TABLE 2
______________________________________
Analytical monitoring of fiber purification using 1 molar HCl
solution via the iron content
Fe content, calculated as Fe.sub.2 O.sub.3
______________________________________
Fibers, before washing with HCl,
3.01% by weight
dried
Fibers, after washing with HCl,
0.17% by weight
dried
______________________________________
EXAMPLE 6
A diaphragm was comminuted according to Example 1, washed and then dried at
about 70.degree. C. 320 g of the dry recycling material were intimately
stirred with 10 g of Fluorad.RTM. FC-171 (chemically and thermally stable
wetting agent based on polyfluorocarboxylic acid (derivatives) and
perfluorosulfonic acid derivatives from 3M for use in electroplating) and
200 ml of water before use for forming a diaphragm on the cathode. The
suspension thus formed was used for the production of a diaphragm for the
electrolysis cell according to Examples 1 and 2.
EXAMPLE 7
A 50% solution of sodium hydroxide was added to 12.5 kg water until a pH
value of approximately 11.5 was reached. In additon, 26.25 g of thickening
agent on the basis of polysaccharid (Welan Gum, trademark of Kelco,
Division of Merck), 26.25 g of the bacteriozide Proxel.RTM. of the company
ICI, which is based on 1.2 benzisothiazin-3-on, and 3.1 g of a silicone
antifoaming agent (for instance silicone antifoaming agent DB 10010A.RTM.
of the company Dow Corning) were added and homogenized with a ultraturrax.
For the deposition of a diaphragm with a surface of 75 cm.sup.2, 434 g of
the above described solution was mixed with 34,4 NaCl, 1 g of a tenside
for effecting hydrophilicity and 42.8 g of the fibres prepared according
to example 2 (dry mass 65.7%=28.1 g) and the suspension was mixed with a
mixer. 11 g of unstabilized ZrO.sub.2 with 10% of particles smaller than
0.54 .mu.m, 50% of the particles smaller than 1.4 .mu.m and 90% of the
particles smaller than 3.45 .mu.m were dispersed in 20 g water and
subsequently mixed into the suspension of fibres.
The slurry suspension was now applied to a cathode mash, which was covered
with a nylon web having a small pore size. The solution was poured through
for 30 minutes, thus building up a filter layer. Afterwards a solution was
sucked through by applying a vaccum on the other side (200 mbar), and then
the diaphragm was left for another 90 minutes under the sucktion fan.
The diaphragm resulting from this process was dried for 6 hours at
95.degree. C. and afterwards sintered in an oven at 320.degree. to
360.degree. C.
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