Previous diaphragms for cells for the electrolysis of aqueous solutions of alkali metal halides

Pervious diaphragms for cells for the electrolysis of aqueous solutions of alkali metal halides and the method of manufacture thereof are disclosed; the pervious diaphragms comprise inorganic fibers and a polymer which is selected from polyelectrolytes insoluble in aqueous solutions of alkali metal halides.

BACKGROUND OF THE INVENTION 
The present invention relates to pervious diaphragms based on inorganic 
fibers such as asbestos intended for cells for the electrolysis of aqueous 
solutions of alkali metal halides such as sodium chloride or potassium 
chloride. More particularly it relates to diaphragms of stabilized 
thickness, that is to say diaphragms whose thickness remains substantially 
constant during their whole working life, directly deposited on foraminate 
cathodes. The invention also relates to a method for the manufacture of 
such diaphragms and to electrolytic cells equipped with such diaphragms. 
To manufacture an asbestos diaphragm directly on the foraminate cathode of 
an electrolytic cell, it is known, from U.S. Pat. No. 1,865,152 in the 
name of K E STUART, of June, 28, 1932, to disperse asbestos fibers in an 
aqueous solution, to immerse the cathode in the suspension of asbestos 
thereby obtained, then to suck the suspension through the foraminate 
cathode. During suction of the suspension through the foraminate cathode, 
the asbestos fibers are retained on the cathode where they progressively 
build-up the diaphragm. 
In the known method, the aqueous solution may be a solution of sodium 
chloride or potassium chloride or an alkaline solution obtained from a 
diaphragm cell in which a sodium chloride or potassium chloride brine is 
being electrolyzed. 
The advantage of this known method resides in its simplicity and in the 
capability of applying asbestos diaphragms with precision onto cathodes of 
complex cross-section. It is generally used in the case of cells with 
interleaved vertical electrodes, of the type described in Belgian Pat. 
Nos. 780,912 and 806,280 in the name of the present applicant, applied for 
resepctively on 20.3.72 and 19.10.73. 
The diaphragms obtained by this known method have however the disadvantage 
of suffering changes in thickness, often large changes, during the course 
of electrolysis. Thus, during the first weeks of use, these diaphragms 
generally begin to swell, with the detrimental result a considerable 
increase of the ohmic resistance in the diaphragm. Furthermore, this 
swelling of the diaphragm interferes with the release of the chlorine 
produced at the anodes. In order to avoid accelerated deterioration of the 
diaphragm by erosion owing to turbulent release of chlorine, it is 
necessary to construct the cells so that the distance between the anodes 
and the cathodes is large and generally greater than 10 mm, even as much 
as 15 mm. All other things being equal, this entails the two-fold 
disadvantage of increasing the space occupied by the cells and reducing 
the energy yield of the electrolysis. 
To avoid these disadvantages of diaphragms obtained by this known method, 
there has been proposed, in Belgian Pat. No. 809,822 of Jan. 16, 1974, of 
DIAMOND SHAMROCK CORPORATION, a method wherein an aqueous suspension of 
asbestos fibers and fibers or particles of a thermoplastic polymer is 
formed, the suspension is sucked through the foraminate cathode to deposit 
on it a diaphragm formed of a substantially homogeneous mixture of the 
asbestos fibres and the polymer, and the diaphragm is heated at high 
temperature, for example above 300.degree. C., to melt the polymer and 
allow it to bind together the asbestos fibers. 
Although this known method allows the dimensional stability of asbestos 
diaphragms to be improved, it still has the disadvantage that it is 
expensive because it involves the use of polymers that are difficult to 
manufacture. Moreover, the method is critical and risky to carry out. It 
is specially difficult to ensure a homogeneous dispersion of the polymer 
among the asbestos fibers. Also, the fusion of the polymer necessitates 
heating to very high temperatures, which not only considerably burdens the 
cost of manufacturing the diaphragms but often causes distortion of the 
cathode. 
To improve the dimensional stability of asbestos diaphragms it has also 
been proposed, in German Pat. No. 1,696,259 of Mar. 18, 1967, of SIEMENS 
AG, to treat the asbestos with a solution of alkali metal hydroxide and 
afterwards to heat the diaphragm formed on the cathode, between 
300.degree. and 700.degree. C. This known method allows a reduction in the 
tendency of asbestos diaphragms to swell during use in electrolytic cells. 
However, it has the same disadvantage of requiring a thermal treatment 
which is expensive and is likely to damage the cathode. 
In order to improve the firmness and the mechanical properties of 
diaphragms not directly formed on the cathode but made from sheets of 
asbestos fibers, it has been proposed, in U.S. Pat. No. 3,694,281 of Apr. 
9, 1969, in the name of J A LEDUC, to impregnate the asbestos sheets with 
a liquid medium containing a polymer, then to heat the impregnated sheets 
at high temperature, so as to melt the polymer. 
This known method has the disadvantage of requiring a long and expensive 
thermal treatment. It has the further and important disadvantage of 
affecting the permeability and the hydrophilic nature of the diaphragms, 
the molten polymer having a tendency to block the pores formed between the 
fibers of asbestos. 
SUMMARY OF THE INVENTION 
The applicant has now found that the stability of the thickness of 
diaphragms based on inorganic fibres can be largely ensured while avoiding 
the disadvantages of the aforesaid known methods. 
The invention therefore provides pervious diaphragms for cells for the 
electrolysis of aqueous solutions of alkali metal halides comprising 
inorganic fibers and a polymer which is selected from polyelectrolytes 
insoluble in aqueous solutions of alkali metal halides. 
DESCRIPTION OF THE INVENTION 
By "polyelectrolytes" the applicant means all polymeric substances which 
comprise monomer units containing ionizable groups, following the 
generally accepted definition (Encyclopedia of Polymer Science and 
Technology, vol, 10, p. 781, 1969, John Wiley and Sons). 
In the context of the present invention, the applicant prefers to use as 
polyelectrolytes, the polyacids of weakly acid character, which are well 
known in the art (Op. cit., p. 781-784). When they are dissociated, these 
polyacids give rise to polymeric anions (polyanions) and to elementary 
cations, for example protons or monovalent cations derived from alkali 
metals. The polyacids that are very weakly dissociated in pure water, such 
as the polyvinyl alcohols and the polyvinylpyrrolidones, also belong to 
this class, although they are sometimes considered as being non-ionic 
polymers. In fact these polyacids are dissociated in strongly polar liquid 
environments. 
By polyacids of weakly acid character the applicant means polyacid 
polyelectrolytes that have a pH, measured on a 0.01N solution in pure 
water, greater than 4 and preferably greater than 6 (Op. cit., p.787 and 
788). 
The polyelectrolytes that can be used in the context of the present 
invention may be insoluble in aqueous solutions of alkali metal halides so 
as not to be removed from the diaphragms when these are in use. It is 
therefore advisable that the polyelectrolytes employed be insoluble under 
the conditions of operation of the cells where the diaphragms are used 
(temperature, concentration of the electrolyte in respect of alkali metal 
halide and products of electrolysis among others). It is easy to comply 
with this condition, because it is well known that the addition of 
non-polymeric electrolytes such as the alkali metal halides in relatively 
small amounts to aqueous solutions, even diluted, of polyelectrolytes 
causes precipitation of the latter (Op. cit., p.827-830). Thus the 
addition of 210g/liter of sodium chloride to a 5% aqueous solution of 
polyvinyl alcohol having a degree of hydrolysis equal to 99 moles % and a 
degree of polymerization between 1700 and 1800 is sufficient to cause 
precipitation of polyvinyl alcohol. Since aqueous solutions of alkali 
metal halides submitted to electrolysis are in general as concentrated as 
possible, it is not difficult to find a polyelectrolyte that is insoluble 
in the electrolysis medium. 
In general, polyelectrolytes that have a solubility in aqueous solutions 
containing 250g/liter of sodium chloride, measured at 20.degree. C., of 
less than 1% are suitable. 
The polyacids of weakly acid character well suited for use in the context 
of the present invention are in general polymeric substances (of molecular 
weight greater than 1000) derived from polymers containing at least one 
hydroxyl group to 10 carbon atoms and preferably at least one hydroxyl 
group to 5 carbon atoms. They may be used in the form of acids or in the 
form of alkali metal salts. 
By way of examples of these polyacids, there may be mentioned polymers of 
acrylic acid and of methacrylic acid, copolymers of maleic acid, 
carboxylic derivatives of cellulosic ethers, sulphonated and phosphonated 
polymers, polymers of vinyl esters partially or completely hydrolyzed, 
polyalphahydroxyacrylic acids and their alkali metal salts. 
Polyacids very specially preferred by the applicant are the polyvinyl 
alcohols which are products of hydrolysis of polymers containing vinyl 
esters as monomer units such as the polyvinyl acetates. Among these, the 
applicant prefers to use polyvinyl alcohols derived from homopolymers of 
vinyl esters, and more particularly from vinyl acetate as well as those 
having a degree of hydrolysis greater than 80 moles % and a degree of 
polymerization greater than 500. The best results are obtained with 
polyvinyl alcohols that have a degree of hydrolysis between 85 and 95 
moles % and a degree of polymerization between 1500 and 2500. 
Another class of polyacids very specially preferred by the applicant is the 
class of polymers derived from alpha-hydroxyacrylic acids. These polymers 
contain in their molecule monomeric units of formula: 
##STR1## 
where R.sub.1 and R.sub.2 represent hydrogen or an alkyl group containing 
1-3 carbon atoms which may be substituted by a hydroxyl group or a halogen 
atom, R.sub.1 and R.sub.2 being identical or different, and where M 
represents hydrogen, an alkali metal atom or an ammonium group. 
Preferably, M represents an atom of sodium or potassium and R.sub.1 and 
R.sub.2 represent hydrogen or an unsubstituted methyl group. The best 
results are obtained when M represents a sodium atom and R.sub.1 and 
R.sub.2 represent hydrogen. 
Also, the applicant prefers to use polymers containing 50 molar % of 
monomer units such as those defined above. The best results are obtained 
with polymers containing only such units. 
The applicant also prefers to use polymers such as defined above in which 
the degree of polymerization is greater than 100. 
Diaphragms according to the invention also contain inorganic fibers 
interlaced so as to form a structure analogous to that of paper. For 
making the diaphragms there may be used any inorganic fiber suitable for 
this purpose, and in particular the fibers of asbestos that are in current 
use for the manufacture of pervious diaphragms. The applicant prefers more 
particularly to use fibers of chrysotile asbestos. 
The amount of polyelectrolyte employed is in general more than 10g per kg 
of inorganic fibers. It is preferably greater than 40g per kg. To achieve 
good results, it is generally unnecessary to employ more than 500g of 
polyelectrolyte per kg of inorganic fibers. 
It will be understood that, besides inorganic fibers and polyelectrolytes, 
the diaphragms according to the invention may contain other conventional 
ingredients of pervious diaphragms, such as particles of fluorinated 
polymers, inorganic particles, organic fibers, etc. 
The present invention also includes a method for the manufacture of 
pervious diaphragms such as those described above. 
Although the polyelectrolyte can be incorporated into the diaphragm in any 
form whatever, the applicant nevertheless prefers to apply it for the 
manufacture of the diaphragm in the form of a solution. For this purpose 
any type of solvent may be used, for example alcohols such as methanol and 
ethanol, acetone and dimethylformamide. However, for reasons of 
availability, the applicant prefers to use water alone, which dissolves 
almost all polyelectrolytes. The concentration of the polyelectrolyte in 
the solution may vary widely and is chosen in relation to the amount of 
polyelectrolyte that it is desired to incorporate into the diaphragm. The 
temperature of the solution may also vary widely and is chosen with regard 
to the solubility of the polyelectrolyte in the solvent; in general the 
temperature is between 20.degree. and 100.degree. C. 
The method according to the invention lends itself equally well to the 
manufacture of pervious diaphragms starting from prefabricated coherent 
sheets made of inorganic fibers, and to diaphragms made directly on a 
rigid foraminate support (for example the foraminate cathode of a 
diaphragm cell), starting from a suspension of asbestos fibers, using the 
technique described in the aforesaid U.S. Pat. No. 1,865,152 of KE STUART 
or in German patent application No. 2,134,126 of NIPPON SODA CO LTD, of 
July 8, 1971. 
Thus according to a first embodiment of the method according to the 
invention, a flat coherent sheet of inorganic fibers is made, for example 
by the methods used in papermaking. Then this sheet is impregnated with a 
solution of polyelectrolyte, for example by immersion or by spraying. 
Finally, the impregnated sheet may be dewatered, for example by 
calendering, and/or dried. 
According to another embodiment, a coherent sheet of inorganic fibers is 
made on a foraminate support by sucking through the support a suspension 
of inorganic fibers in a liquid medium such as a relatively viscous 
aqueous solution. Thereby there is obtained a sheet that follows the 
contours of the foraminate support. The sheet is afterwards impregnated 
with a solution of polyelectrolyte as in the preceding embodiment and may 
be dried. In this embodiment, the foraminate support may remain in place 
at the end and is preferably the cathode itself. 
The applicant prefers, however, to use another embodiment, wherein the 
inorganic fibers are formed into a suspension in the solution of 
polyelectrolyte. This suspension is sucked through the foraminate support 
on which the diaphragm is thus formed directly. In this embodiment, the 
foraminate support may be a temporary one. This may be for example an 
endless guaze from which the diaphragm is removed; the diaphragm is then 
flat and may be dewatered and/or dried. The applicant prefers, however, to 
use a foraminate support which remains in place at the end and which is 
preferably constituted by the cathode itself. 
In this preferred embodiment, there may be dissolved a thickening agent 
that does not affect the solubility of the polyelectrolyte so as to 
increase the viscosity of the suspension and consequently its stability. 
In general it is advantageous, in order to obtain a diaphragm having a 
good permeability and good electrical properties, to keep the absolute 
viscosity of the suspension between substantially 1 and 30 centipoises, 
preferably 2 and 10 centipoises, at 20.degree. C. 
According to an advantageous feature of the invention, the thickening 
action may be provided by the polyelectrolyte itself. This is the case, 
for example, when there is used polyacrylic acid, a polymer derived from 
alpha-hydroxyacrylic acid or polyvinyl alcohol, which are available in 
various qualities differentiated from each other by the degree of 
polymerization. 
In this same preferred embodiment of the method according to the invention, 
in order to improve the permeability of the diaphragm, a phosphate of 
ammonium or of an alkali metal may be dissolved in the suspension, so as 
to help towards obtaining as homogeneous dispersion provided that the 
solubility of the polyelectrolyte is not effected. However, diaphragms 
with poorer electrical properties are then obtained. 
In the method according to the invention, the diaphragm may be put into the 
cell immediately after being impregnated with the solution of polymer. 
It is, however, preferable, in order to improve the mechanical and 
electrical properties of the diaphragm, to dry it at least partially 
before putting it into the cell. The drying of the diaphragm is carried 
out at a temperature below the melting point of the polyelectrolyte, for 
convenience and so as to avoid damaging the diaphragm. It may for example 
be carried out in a current of air at ambient temperature or by heating 
the diaphragm, preferably to a temperature lower than the boiling point of 
the solvent. In general, the drying is carried out between 20.degree. and 
150.degree. C. and preferably between 40.degree. and 100.degree. C. 
According to another particular embodiment of the method according to the 
invention, the diaphragm impregnated with the solution of polyelectrolyte 
is treated with a liquor in which the polyelectrolyte is insoluble so as 
to precipitate the polyelectrolyte, for example by immersion, spraying or 
washing. As a modification, the diaphragm may then be dried under the 
conditions described above to remove the liquor from the diaphragm. This 
particular embodiment of the invention may for example be applied to the 
manufacture of diaphragms that are to be put into storage before use in 
electrolytic cells. As the liquor in which the polyelectrolyte is 
insoluble there may be used the electrolyte which is to be treated in the 
cell for which the diaphragm is intended, for example an aqueous solution 
of sodium chloride or a caustic liquor. 
The diaphragms according to the invention may be used in any type of 
diaphragm cells where there is percolation of the solution of electrolyte 
through the diaphragm, such as vertical cells with an alternating sequence 
of anodes and cathodes separated by diaphragms and horizontal cells. They 
are particularly well suited to the electrolysis of aqueous solutions of 
sodium chloride and of potassium chloride. 
In comparison with the known diaphragms described in USP 1865152, the 
diaphragms according to the invention have a considerably improved 
stability of thickness in service. They also allow a considerable 
reduction to be made in the anode-cathode distance of diaphragm cells. 
They have a stability of thickness comparable to that of diaphragms 
obtained by the aforesaid improved methods described in Belgian Pat. No. 
809,822, German Pat. No. 1,696,259 and U.S. Pat. No. 3,694,281. They 
possess the advantage over these of having a lower electrical resistivity 
and of allowing, all other things being equal, the use of lower 
electrolyzing voltages. 
Moreover, the diaphragms according to the invention generally have a higher 
permeability than the asbestos diaphragms obtained by the known methods. 
From this stems for the invention the supplementary advantage of 
permitting higher current densities in the electrolytic cells and, 
consequently, an increase in the productivity of cells, without increasing 
too greatly the concentration of alkali metal hydroxide in the catholyte. 
The various examples of use which follow will illustrate the invention, 
without however limiting its scope. 
In each of these examples, an asbestos diaphragm was made directly on a 
cathode consisting of a disc of 120 cm.sup.2 surface area made of a steel 
lattice. The cathode, covered with the diaphragm, was then set up 
vertically in a laboratory-type electrolytic cell, facing an anode made up 
of a succession of vertical titanium vanes carrying an electrocatalytic 
coating consisting of a mixture of ruthenium oxide and titanium dioxide. 
The distance between the cathode and the vanes of the anode was adjusted 
to 5 mm (except in Example 2, 3 and 4, where the distance was made 
respectively 10, 6 and 4 mm). In the cell made up in this manner a brine 
saturated with sodium chloride was electrolyzed at 85.degree. C., at an 
anodic current density of 2kA/m.sup.2 and a hydrostatic pressure on the 
diaphragm equal to a 30 cm head of electrolyte. For each diaphragm there 
were recorded the voltage across the cell terminals and the permeability 
of the diaphragm after several days of electrolysis, the said permeability 
being defined by the relationship: 
EQU K = Q/S.H, 
where 
Q is the rate of flow of electrolyte through the diaphragm (in cm.sup.3 
/h), 
S is the useful cross-section of the diaphragm (in cm.sup.2), and 
H is the hydrostatic pressure of the electrolyte on the diaphragm, 
expressed as head of electrolyte in cm (30 cm in the examples). 
FIRST SERIES OF TESTS 
These tests relate to diaphragms by the prior art methods described above.

EXAMPLE 1 
17.5g of chrysotile asbestos fibers were dispersed in 0.9 liter of an 
aqueous solution of sodium chloride and sodium hydroxide containing about 
170g/liter of NaCl and 120g/liter of NaOH coming from a diaphragm cell in 
which a sodium chloride brine was being electrolyzed. The suspension thus 
obtained was then filtered through the cathode lattice of the laboratory 
cell, by applying suction corresponding to 200 mm of mercury. The 
recovered filtrate was filtered a second time through the cathode lattice 
covered by the diaphragm, under a suction of 200 mm of mercury. The 
diaphragm was then dried at ambient temperature, applying beneath the 
cathode lattice successively a suction of 200 mm of mercury for 15 minutes 
than a suction of 400 mm of mercury for 30 minutes. The cathode furnished 
with the diaphragm was then set up in the laboratory cell, where an 
electrolysis test was carried out under the conditions stated above. After 
20 days' electrolysis a voltage of 3.56V was recorded at the cell 
terminals and the permeability of the diaphragm was measured as K = 
0.118h.sup.-1. 
EXAMPLE 2 
The test of Example 1 was repeated, but with the distance separating the 
anode from the cathode adjusted this time to 10 mm. After 20 days' 
electrolysis a voltage of 3.59V was recorded at the cell terminals and the 
diaphragm showed a permeability K = 0.105h.sup.-1. 
EXAMPLE 3 
17.5g of chrysotile asbestos fibers and particulate polytetrafluoroethylene 
(about 20 micron diameter) were dispersed in 0.9 liter of an aqueous 
solution of sodium chloride and sodium hydroxide coming from a diaphragm 
cell in which a sodium chloride brine was being electrolyzed. The 
polytetrafluoroethylene content of the suspension was fixed so that it 
represented about 8% of the total weight of asbestos and 
polytetrafluoroethylene. Starting with this previously homogenized 
suspension, a diaphragm was formed on the cathode, using the procedure of 
Example 1. 
The cathode furnished with the diaphragm was then heated successively t 
90.degree. C. for 1 hour then at 240.degree. C. for 1 hour. After cooling, 
the cathode with the diaphragm was set up in the cell, the distance 
between the anode and the cathode being adjusted to 6 mm. At the end of an 
electrolysis test of 17 day a voltage of 3.20 V was recorded at the cell 
terminals and the permeability of the diaphragm had risen to 0.065 
h.sup.-1. 
EXAMPLE 4 
17.5g of chrysotile asbestos fibers and particulate polytetrafluoroethylene 
(having a particle diameter of about 20 micron) were dispersed in 0.9 
liter of a sodium chloride brine. The polytetrafluoroethylene content of 
the suspension was fixed so as to correspond to 10% of the total weight of 
asbestos and polytetrafluoroethylene. Starting with this previously 
homogenized suspension, a diaphragm was formed on the cathode using the 
procedure of Example 1. The cathode furnished with the diaphragm was then 
heated, successively at 90.degree. C. for 16 hours then at 280.degree. C. 
for 1 hour. After cooling, the cathode with its diaphragm was set up in 
the cell with a distance of 4 mm between the anode and the cathode. At the 
end of an electrolysis test of 20 days a voltage of 3.28V was recorded at 
the cell terminals and the diaphragm showed a permeability of 
0.101h.sup.-1. 
EXAMPLE 5 
A diaphragm of chrysotile asbestos was formed on the foraminate cathode of 
the cell using the procedure described in Example 1. The cathode furnished 
with the diaphragm was then heated successively at 90.degree. C. for 1 
hour then at 240.degree. C. for 1 hour. After cooling, the cathode 
furnished with its diaphragm was set up in the cell with a distance of 5 
mm between the anode and the cathode. After 20 days' electrolysis, there 
were recorded a voltage of 3.18V at the cell terminals and a permeability 
of the diaphragm K = 0.099h.sup.-1. After 60 days' electrolysis the 
voltage had risen to 3.21V and the and the permeability had fallen to 
0.089h.sup.-1. 
EXAMPLE 6 
The trial of Example 5 was repeated with, however, the thermal treatment of 
the diaphragm modified so that it was heated successively at 90.degree. C. 
for 16 hours, then at 240.degree. C. for 1 hour. After an electrolysis 
test of 20 days, there were recorded a voltage of 3.222V at the cell 
terminals and, for the diaphragm, a permeability K = 0.108h.sup.-1. At the 
end of 50 days' electrolysis the voltage had increased to 3.33V and the 
permeability had fallen to 0.098h.sup.-1. 
The result of the six tests that have been described, carried out in 
accordance with the prior art methods, are recorded in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Electrolysis 
Anode-cathode 
Test 
Initial Thermal distance 
Duration 
Voltage 
Permeability 
No. 
Suspension 
treatment 
(mm) days (V) K(h.sup.-1) 
__________________________________________________________________________ 
1 Chrysotile in 
none 5 20 3.56 0.118 
solution NaCl + 
NaOH 
2 idem none 10 20 3.59 0.105 
3 Chrysotile + 8% 
1 hr at 90.degree. C 
PTFE in + 1 hr at 
solution NaCl 
24.degree. C 
+ NaOH 6 17 3.20 0.065 
4 Chrysotile + 10% 
16 hrs at 
PTFE in solution 
90.degree. C 
NaCl + NaOH 
+ 1 hr at 280.degree. C 
4 20 3.28 0.101 
5 Chrysotile in 
1 hr at 90.degree. C 
20 3.18 0.099 
solution NaCl 
+ 1hr at 
+ NaOH 240.degree. C 
5 60 3.21 0.089 
6 idem 16 hrs at 90.degree. C 
20 3.22 0.108 
+ 1 hr at 
240.degree. C 
5 50 3.33 0.098 
__________________________________________________________________________ 
SECOND SERIES OF TESTS 
These tests relate to asbestos diaphragms made by the method according to 
the invention. 
EXAMPLE 7 
A diaphragm of chrysotile asbestos was formed on the foraminate cathode of 
the cell, using the method described in Example 1. The diaphragm was then 
treated on the cathode with 0.5 liter of a solution of polyvinyl alcohol 
in water of concentration 40g/liter, the polyvinyl alcohol being that sold 
under the trade mark POLYVIOL W25/140 (WACKER-CHEMIE GmbH), and the 
diaphragm was then dried at 90.degree. C. for 16 hours. The cathode with 
the diaphragm was then set up in the cell, the anode-cathode distance 
being adjusted to 5 mm. In the cell, the diaphragm was treated with a 
brine saturated with sodium chloride, while proceeding to electrolyze the 
brine under the conditions stated above. At the end of a period of 20 
days' electrolysis the electrolyzing voltage measured at the cell 
terminals was 3.18V and the permeability of the diaphragm has risen 
0.114h.sup.-1. 
EXAMPLE 8 
Polyvinyl alcohol sold under the trade mark ELVANOL 52/22 (E I du Pont de 
NEMOURS & Co) and sodium metaphosphate were dissolved in water so as to 
produce an aqueous solution containing 12g of alcohol and 2g of phosphate 
per liter, to obtain an absolute viscosity of about 2.5 centipoises at 
20.degree. C. 17.5g of chrysotile asbestos were then dispersed in 0.9 
liter of the solution. From the suspension thus prepared an asbestos 
diaphragm was formed on the cathode, using the procedure described in 
Example 1, then the cathode and the diaphragm were set up in the 
electrolytic cell with a distance of 5 mm separating the anode from the 
cathode and the electrolysis of the sodium chloride brine was begun. At 
the end of a 10 day period of electrolysis a voltage of 3.15V was recorded 
at the cell terminals and the permeability of the diaphragm proved to be K 
= 0.127h.sup.-1. After 40 days the voltage was found to be 3.13V and the 
permeability of the diaphragm 0.178h.sup.-1. 
EXAMPLE 9 
The test of Example 8 was repeated, but this time using an aqueous solution 
containing 12g of alcohol per liter and no phosphate for forming the 
diaphragm on the cathode. At the end of the test (20 days), a voltage of 
3.15V was recorded at the cell terminals and the permeability of the 
diaphragm was found to be 0.139h.sup.-1. 
EXAMPLE 10 
A diaphragm of chrysotile asbestos was formed on the cathode using the 
stages of Example 9. After formation of the diaphragm on the cathode, the 
diaphragm was treated, on the cathode, with a solution of 40G of alcohol 
per liter, then the cathode furnished with the diaphragm was set up in the 
cell with the separation between the anode and the cathode set at 5 mm. At 
the end of the test (20 days), a voltage of 3.09V was recorded at the cell 
terminals and the permeability of the diaphragm was found to be 
0.126h.sup.-1. 
EXAMPLE 11 
An asbestos diaphragm was formed on the foraminate cathode of the cell 
using the procedure described in Example 9. The diaphragm was then dried 
on the cathode, by heating it for 16 hours at 90.degree. C., then the 
cathode furnished with the diaphragm was set up in the electrolyte cell, 
the anode-cathode distance being adjusted to 5 mm. 
At the end of an electrolysis test of 20 days, the voltage at the cell 
terminals was found to be 3.12V and the permeability of the diaphragm had 
settled down at 0.113.sup.-1. 
EXAMPLE 12 
A diaphragm was formed on the foraminate cathode of the cell using the 
stages of the procedure described in Example 8, then the diaphragm was 
dried by heating it on the cathode for 16 hours at 90.degree. C. After 20 
days of electrolysis the voltage measured at the cell terminals was 3.10V 
and the diaphragm showed a permeability K = 0.138h.sup.-1. After 75 days 
of electrolysis the measured voltage was still 3.10V and the diaphragm 
showed a permeability K = 0.120h.sup.-1. 
EXAMPLE 13 
The test of Example 12 was repeated, but this time using an aqueous 
solution containing 40g of alcohol per liter and no phosphate for 
preparing the diaphragm. the suspension of asbestors thus obtained had an 
absolute viscosity of about 22 centipoises at 20.degree. C. 
At the end of a period of electrolysis of 20 days, a voltage of 3.01V was 
recorded at the cell terminals and the diaphragm had a permeability of 
0.116h.sup.-1. 
EXAMPLE 14 
The test of Example 12 was repeated, but this time using for preparation of 
the diaphragm an aqueous solution free from phosphate and containing 100g 
of polyvinyl alcohol sold under the trade mark ELVANOL 70/05 per liter. 
The absolute viscosity of the asbestos suspension has risen to 27 
centipoises at 20.degree. C. After an electrolysis trial of 10 days a 
voltage of 3.01V was recorded, while the diaphragm had a permeability of 
0.123h.sup.-1. 
EXAMPLE 15 
17.5g of chrysotile asbestos were dispersed in 0.9 liter of an aqueous 
solution containing 12g of polyvinyl alcohol sold under the trade mark 
POLYVIOL W 25/140 per liter and free from phosphate. After homogenizing 
the suspension (having an absolute viscosity of 2.5 centipoises at 
20.degree. C.) a diaphragm was formed from this suspension on the cathode, 
using the procedure described in Example 1.The diaphragm thus obtained was 
then treated with 0.5 liter of an aqueous solution containing 40g of 
alcohol W25/140 per liter, then dried by heating it at 90.degree. C. for 1 
hour. After an electrolysis test of 20 days, there were recorded an 
electrolyzing voltage of 3.02V and a permeability of the diaphragm equal 
to 0.131h.sup.-1. 
EXAMPLE 16 
17.5g of chrysotile asbestos were dispersed in 0.9 liter of an aqueous 
solution free from phosphate and containing 40g of sodium 
polyhydroxyacrylate per liter. A diaphragm was formed on the foraminate 
cathode from this suspension, using the stages of the procedure described 
in Example 1, then the diaphragm was dried by heating it on the cathode at 
90.degree. C. for 1 hour. After an electrolysis test of 20 days, the 
voltage measured at the cell terminals was 3.04V and the diaphragm had a 
permeability K = 1.160h.sup.-1. 
The results of the second series of tests, according to the invention, are 
recorded in Table 2. 
A comparison of Tables 1 and 2 demonstrates the beneficial effect of the 
method according to the invention on the electrolyzing voltage and on the 
permeability of the diaphragm. 
TABLE 2 
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Electrolysis 
Solution Anode-Cathode 
Test 
of Asbestos 
Solution of Distance 
Duration 
Voltage 
Permeability 
No (s.a.) suspension 
Polymer Drying 
(mm) (days) 
(V) K(h.sup.-1) 
__________________________________________________________________________ 
7 Solution NaCl 
Solution of 
16 hrs 
5 20 3.18 0.114 
+ NaOH 40g/l Polyviol 
90.degree. C 
8 Elvanol 52/22 
Solution s.a. 
None 5 20 3.15 0.127 
(12g/l) 
+NaH.sub.2 PO.sub.4 40 3.13 0.178 
(2 g/l) 
9 Elvanol 52/22 
Solution 
None 5 20 3.15 0.139 
(12g/l) s.a. 
10 idem Solution 
None 5 20 3.09 0.126 
s.a. + 
Elvanol 52/ 
22(40g/l) 
11 idem Solution 
16 hrs 
5 20 3.12 0.113 
s.a. at 90.degree. C 
12 Elvanol 52/22 
Solution 
idem 5 20 3.10 0.138 
(12g/l) s.a. 
+ NaH.sub.2 PO.sub.4 75 3.10 0.120 
(2g/l) 
13 Elvanol 52/22 
Solution 
idem 5 20 3.01 0.116 
(40g/l) s.a. 
14 Elvanol 70/05 
Solution 
idem 5 10 3.01 0.123 
(100g/l) s.a. 
15 Polyviol Solution 
W25/140 s.a. + 
(12g/l) Polyviol 
W25/140 
(40g/l) idem 5 20 3.02 0.131 
16 Sodium poly- 
Solution 
idem 5 20 3.04 0.160 
hydroxy s.a. 
acrylate 
(40g/l) 
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