BE902297R - Electrode for chlor-alkali cell - comprising coarse and fine metal cloths spot-welded together - Google Patents

Abstract

Improved electrolysis cell of the diaphragm type comprising one set of anode and cathode sepd. by a diaphragm is claimed in which at least one of the electrodes is a structure comprising a plane cloth of coarse metal fibre and a cloth of fine metal fibre with an electrocatalytic surface and comprising at least 4 wires or openings per cm. disposed above the coarse mesh and in contact with it. The fibre metal mesh cloth faces the diaphragm directly.

Description

       

  New electrolysis cell and method for electrolysis

  
halides.

  
The present invention relates to a new process for the production of chlorine where other halogens by electrolysis of an aqueous solution containing a halide ion

  
such as hydrochloric acid and / or a corresponding alkali metal chloride or other electrolyzable halide.

  
Chlorine has been produced for a long time by such electrolysis in a cell where the anode and the cathode are separated by a membrane or diaphragm permeable to ions and in cells comprising a diaphragm permeable to liquids, chloride or any other halide of alkali metal flowing through an anolyte chamber and part of it flowing through the diaphragm towards the catholyte.

  
When an alkali metal chloride solution is electrolyzed, chlorine is released at the anode and a

  
alkali which can be a carbonate or a bicarbonate of

  
alkali metal, but more commonly an alkali metal hydroxide solution, forms at the cathode. This alkaline solution also contains alkali metal chloride which must be separated from the alkali in a subsequent operation, and said solution is relatively dilute rarely containing above 12 to 15% alkali by weight. As the commercial concentration of hydroxide

  
sodium is normally about 50% by weight or more, the water in the diluted solution must be evaporated to obtain this concentration.

  
More recently, considerable studies have been undertaken regarding the use of ion exchange resins or polymers as ion permeable diaphragms, said polymers being in the form of thin sheets or membranes. In general, they are only

  
not perforated and do not allow anolyte to flow into the cathode compartment; it has also been suggested that such membranes may be provided with a few small perforations to allow a small flow of anolyte through them, but the essential

  
work appears to have been carried out with

  
non-perforated membranes.

  
\

  
Typical polymers that can be used for this purpose include: fluorocarbon polymers such as polymers of an unsaturated fluorocarbon. For example, polymers of trifluoroethylene, tetrafluoroethylene or their copolymers containing ion exchange groups are used for this purpose. Ion exchange groups are normally cationic groups comprising sulfonic acid, sulfonamide, carboxylic acid, phosphoric acid and the like which are attached to the fluorocarbon polymer chain via carbon and are able to exchange cations. They can however also contain anion exchange groups. This is how they correspond to the general formula:

  

  <EMI ID = 1.1>


  
Such membranes are typically those manufactured by the Du Pont Company under the brand "Nafion" and by the Japanese company Asahi Glass Co under

  
the trademark "Flemion", the patents describing such membranes are the G.B. 1,184,321 patent and the patents

  
U.S. 3,282,875 and 4,075,405.

  
. As these diaphragms are permeable to ions but do not allow the flow of the anolyte flow through them, there are little or no halide ions which migrate through the diaphragm of such a material. in an alkali chloride cell, which is why the alkali so produced contains little or no chloride ion. On the other hand, it is possible to produce a more concentrated alkali metal hydroxide in which the catholyte obtained can contain 15 to 45% or more of NaOH by weight.

  
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The patents describing such a process are the U.S. patents
4,111,779 and 4,100,050 and many others. The application of an ion exchange membrane as an ion permeable diaphragm has been proposed for other uses, for example in the electrolysis of water.

  
It has also been proposed to conduct such electrolysis between an anode and a cathode separated by a diaphragm, in particular an ion exchange membrane in which the anode and / or the cathode are in the form of a thin porous layer d 'an electroconductive substance resistant to electro-chemical attack and bonded or otherwise incorporated into the surface of the diaphragm. Similar electrode-membrane assemblies have long been proposed for fuel cells, which have been referred to as "polymer-based solid electrolyte" cells. Such cells have been used for a long time as gaseous fuel cells and it is only recently that they have been successfully adapted to the electrolytic production of chlorine from hydrochloric acid or alkali metal chloride brines.

  
For the production of chlorine in a solid electrolyte cell based on polymer, the electrodes usually consist of a thin porous layer of electroconductive and electrocatalytic material permanently bonded to the surface of an ion exchange membrane by means of a binder usually composed of a fluoropolymer such as for example polytetrafluoroethylene (PTFE).

  
According to one of the preferred techniques for making gas permeable electrodes as described in US Pat. No. 3,297,484, a powder of electroconductive and electrocatalytic material is mixed with an aqueous dispersion of polytetrafluorocarbon particles with a view to obtaining a pasty mixture containing 2 to 20 g of powder per gram of polytetrafluoroethylene. The mixture / which can be diluted if necessary, is then spread on a metal sheet support and dried, after which the layer of powder is covered with aluminum foil and pressed.

  
at a temperature sufficient to effect sintering of the polytetrafluoroethylene particles in order to obtain a thin coherent film. After removal of the aluminum foil by caustic leaching, the preformed electrode is applied to the surface of the membrane and pressed at a temperature sufficient to cause sintering of the polytetrafluoroethylene matrix on the membrane. After rapid quenching, the metal support sheet is

  
removed and the electrode remains bonded to the membrane.

  
As the electrodes of the cell are intimately linked to the opposite surfaces of the membrane separating the anode and cathode compartments and are therefore not supported by metallic structures, it has been discovered that the most effective means for bringing and distributing the current to the electrodes consists of using multiple contacts uniformly distributed over the entire surface of the electrode by means of current transport structures provided with a series of projections or ribs, which, during the assembly of the cell , come into contact with the electrode surface at multiple, uniformly distributed points. The membrane carrying on its opposite surfaces bonded electrodes must then be pressed between two current transport structures or collectors, respectively anodic and cathodic.

  
Contrary to what happens in fuel cells where the reagents are gaseous and the current densities are low, and where there can hardly be any side reactions to the electrodes, the solid electrolyte cells used for electrolysis

  
of solutions, as in the particular example of sodium chloride brines, raise difficult questions

  
to solve. In a cell for the electrolysis of sodium chloride brine, the following reactions take place

  
in various places of the cell:
- main anode reaction:
- transport through the membrane:
  <EMI ID = 2.1>
 - reaction at the cathode:
- secondary reaction at the anode
  <EMI ID = 3.1>
  <EMI ID = 4.1> main chlorine discharge reaction desired, some oxidation of the water with consequent release of oxygen, albeit at a level kept as low as possible. This tendency to give off oxygen is particularly accentuated by an alkaline environment at the active sites of the anode consisting of particles of catalyst in contact with the membrane.

   In fact, the cation exchange membranes suitable for the electrolysis of alkali metal halides have a different transfer number from the unit and, under the conditions of high alkalinity existing in the catholyte, some of these membranes allow a certain migration of hydroxyl anions from the catholyte to the anolyte through the membrane. In addition, the conditions necessary for efficient transfer of liquid electrolytes to the active surfaces of the electrodes and

  
to an evolution of gases require anode and cathode compartments characterized by flow sections for electrolytes and gases much larger relatively than those adopted in cells

  
fuel.

  
Conversely, the electrodes must have a

  
  <EMI ID = 5.1>

  
that the fact that the electrocatalytic and electroconductive materials constituting the electrodes, in particular the anode, are frequently a mixed oxide composed of a metallic oxide of the platinum group or of a pulverulent metal fixed by a binder having little or no conductivity electric, the electrodes are barely conductive in the direction of their large dimension. This is why a high density of contacts with the collector is necessary as well as a uniform contact pressure to limit the ohmic drop.

  
through the cell and provide a current density

  
  <EMI ID = 6.1> uniform over the active surface of the cell.

  
These conditions have hitherto been extremely difficult to fulfill, in particular in cells characterized by large areas such as those used industrially in installations for the production of chlorine with a capacity generally greater than 100 tonnes of chlorine per day. For economic reasons, industrial electrolysis cells require electrode surfaces of the order of at least 0.5,

  
  <EMI ID = 7.1>

  
electrically in series to form electrolysers comprising up to several tens of bipolar cells assembled by means of tie rods, hydraulic or pneumatic cylinders in an assembly of the filter press type.

  
Cells of this size pose major technological problems with regard to current transport structures, namely current collectors, with extremely low tolerances for the flatness of the contacts and the uniform contact pressure on the electrode surface after assembly. of the cell. In addition, the membrane used in such cells must be very thin to limit the ohmic drop through the solid electrolyte in the cell, said thickness often being less.

  
0.2 mm and rarely more than 2 mm, and the membrane can be easily ruptured or thinned to the extreme

  
in places where too high pressure is applied

  
during cell closure. That is why

  
the anode and cathode collectors, apart from being almost perfectly flat, must also be exactly parallel.

  
In small cells, one can maintain a high degree of flatness and parallelism while ensuring a certain flexibility of the collectors to compensate for small deviations from a flatness and a precise parallelism. The patent application FR 79.

  
.19.346 filed on July 26, 1979, describes a solid-state electrolyte cell of the monopolar type for the electrolysis of sodium chloride in which the anodic and cathodic current collectors consist of grate screens or deployed sheets welded on respective series of metal ribs offset verticals

  
between them, which allows a certain flexion of the grid screens during the assembly of the cell so

  
to exert a more uniform pressure on the membrane surfaces.

  
In published patent application EP 79.102310.4

  
on February 6, 1980 under the n [deg.] 0007078 a cell was described

  
with a solid electrolyte of bipolar type for the electrolysis of sodium chloride in which the bipolar separators are provided on their two faces and, in the zone corresponding to the electrodes, with a series of ribs or projections. To compensate for small differences in flatness and parallelism, provision is made for the insertion of elastic means consisting of two

  
or more screens or sheets of expanded metal made of stopper coated with a material that cannot be made passive, said elastic means being compressed between the ribs on the anode side, and the anode fixed on the anode face of the membrane .

  
However, it has been observed that the two solutions proposed in the two aforementioned patent applications lead to serious limitations and disadvantages in cells characterized by large electrode surfaces. In the first case, the uniformity of the desired contact pressure tends to be lacking, which gives rise to

  
of concentrations of current at the places of greatest contact pressure, accompanied by phenomena of polarization, and it often occurs during the assembly of the cell a deactivation of the membrane and the catalytic electrodes as well as localized ruptures of the membrane and localized mechanical losses of the catalytic material. In the second case, a very high flatness and parallelism of the surfaces of the bipolar separator must be ensured, but this requires precise and costly machining of the ribs and of the joint surface of the separator bipolar. In addition, the

  
high rigidity of the elements causes pressure concentrations which tend to accumulate along the series, thus limiting the number of elements which can be assembled in a single device of the filter-press type.

  
As a result of these difficulties, the current distributor screen or grid can even, when pressed against the electrode, leave certain areas of the electrode without contact or in such a light contact that they are practically ineffective. Comparative tests carried out by pressing the screen of the dispenser grid on pressure-sensitive paper capable of revealing a visible impression corresponding to the sieve, have shown that a large area extending from approximately 10% to 30 to

  
40% of the surface of the grid screen does not produce a mark on the paper, which indicates that these too large areas remain intact. By applying this observation to the electrodes, it appears that large areas

  
of the electrode are almost inoperative.

  
The invention relates to a new electrolysis cell characterized in that it comprises a cell casing or housing containing at least one set of electrodes formed by an anode and a cathode separated by a diaphragm or a membrane permeable to ions, means for introducing an electrolyte to be electrolyzed, means for discharging the products of electrolysis and means for applying an electrolysis current thereto, at least one of the electrodes being pressed against

  
the diaphragm or the membrane by an elastically compressible layer which can take an extension equal to the electrode surface, said layer being able to be pressed against the diaphragm while exerting an elastic reaction on the electrode in contact with the diaphragm or the membrane a number of uniformly distributed contact points capable of transmitting excess

  
of pressure applied on individual contact points to laterally adjacent less loaded points

  
on an axis located in the plane of the elastic layer, said elastic layer thus distributing the pressure over the total surface of the electrode and having an open structure to allow the flow of gas and

  
of the electrolyte through it.

  
The new process according to the invention, of electrolysis with production of halogen, is characterized in that an electrolyte containing an aqueous halide is electrolyzed at an anode separated from a cathode in contact with

  
an aqueous electrolyte through an ion-permeable diaphragm or membrane and an aqueous electrolyte at the cathode, at least one of said anode and cathode having a gas-permeable surface and the electrolyte maintained in direct contact, in a number of points, with the diaphragm or the membrane by a compressible electroconductive layer elastically open towards the electrolyte and

  
the gas flow and capable of applying pressure over the whole of said surface and of distributing laterally

  
pressure, the pressure at the surface of the diaphragm or

  
the membrane being uniform.

  
According to the invention, an effective electrical contact between the porous electrode surface and the membrane or The diaphragm is produced and a polarity is communicated therein easily and without creating excessive pressure in local areas by pressing the distribution surface of the

  
current or electric charge against the electrode layer by means of a sheet, a mattress (hereinafter referred to as "mat") or an easily compressible elastic layer which

  
extends along most of and nearly

  
almost the entire surface of the porous electrode layer in direct contact with the membrane.

  
This compressible layer has elasticity

  
of a spring, and, while being able to be compressed

  
up to 60% or more of its thickness in the uncompressed state against the membrane supporting the electrode layer by application of pressure from a back wall or a pressure element, it is also capable to return substantially to its initial thickness after the clamping pressure has been released. Thus, by its "elastic memory", it applies a substantially uniform pressure against the membrane supporting the electric layer.

  
  <EMI ID = 8.1>

  
pressure and compensate for irregularities in the surfaces with which it is in contact. The compressible sheet must also allow easy access of the electrolyte to the electrode and easy release of gaseous or liquid electrolysis products from the electrode.

  
The leaf is of open structure and encloses

  
a large free volume and the compressible elastic sheet is electrically conductive, being generally made of a metal resistant to the electrochemical attack of the elect
-trolyte in contact with it which thus distributes the polarity and the current on the whole of the electrode layer. It can be engaged directly with the layers

  
  <EMI ID = 9.1>

  
trice, elastic, compressible, can include an electroconductive screen or grid, flexible in nickel, titanium, niobim or another resistant metal interposed between the sheet or a mat and the membrane.

  
The grid or screen is a thin perforated sheet which easily folds and adapts to all surface irregularities of the electrode surface. This can be a fine grating screen grid or a perforated film. It usually has a finer mesh or pore size than the compressible layer and is less compressible or substantially non-compressible. In either case, an open mesh layer presses against it and is pressed against the membrane with the opposite electrode or counter electrode, or else at least one of its surfaces permeable to gas and electrolyte, pressed against the opposite side of the diaphragm.

   Since the compressible layer and the finer grid, when present, are not linked to the membrane, it is mobile (sliding) along the surface, of the ^ membrane ^ and can therefore marry easily contours

  
membrane and counter electrode.

  
The present invention therefore has for one of its objects to conduct the electrolysis of an alkali metal chloride with an electrolysis cell comprising an electrode in direct contact with a membrane or a diaphragm, said electrode or a part of it being easily compressed, having a high elasticity and being capable of effectively distributing a clamping pressure exerted on the cell in a practically uniform manner over the entire surface of the electrode.

  
A preferred embodiment of the elastic current collector or of the electrode according to the invention is characterized in that it consists of an article of electroconductive metallic wire, flat, with substantially open meshes or in an open network grid composed of a wire fabric resistant to electrolyte and electrolysis products, and that some, or all

  
wires form turns, corrugations, folds or any other corrugated outline whose diameter or amplitude is greater than the thickness of the wire and preferably corresponds to the thickness of the article along at least one direction parallel to the plane of the article. Of course, these folds or corrugations are arranged in the direction through the thickness of the grid.

  
These ripples in the form of folds, turns,

  
waves or the like have lateral parts which are bevelled or curved with respect to the axis normal to the thickness of the pleated fabric in such a way that, when the collector is compressed, the displacement and the pressure are transmitted laterally so as to distribute more uniformly the pressure on the surface of the electrode. Certain coils or wire loops which, due to irregularities in the flatness or parallelism of the surfaces compressing the fabric, may be subjected to a compressive force greater than that acting on neighboring areas, are capable of bending more and discharging the excess by force by transmitting it to

  
  <EMI ID = 10.1>

  
Consequently, the fabric is effective by its action as a pressure equalizer to a significant extent and by preventing the elastic reaction force acting on a single point of contact from exceeding the limit, the membrane being pinched to the extreme or pierced. It is quite obvious that such an aptitude of the elastic collector for self-regulation contributes to ensuring a good uniform distribution of contact over the entire surface of the electrode.

  
A very efficient embodiment advantageously consists of a series of helical cylindrical metal wire spirals, the turns of which are wound mutually with those of the adjacent spiral in an interlaced connection. The spirals have a length substantially corresponding to the height or width of the electrode chamber or at least a length of 10 cm or more and the number of intertwined spirals is sufficient to cover its entire width, and the diameter of the spirals is five to ten times or more that of the spiral thread.

  
According to this preferred arrangement, the actual wire helix represents a very small part of the section of the electrode chamber surrounded by the spiral and this is why the spiral is open on all sides, thus providing a channel: interior for allow the electrolyte to circulate and the gas bubbles to rise along the chamber.

  
However, it is not necessary for the helical cylindrical spirals that they are wound in an intertwined manner with the adjacent spirals as indicated above; they can also consist of simple adjacent metal wire spirals. In this case, the spirals are juxtaposed one beside the other, the respective spirals being simply engaged in an alternating sequence. In this way, a higher density of contact points with the planes can be obtained.

  
  <EMI ID = 11.1>

  
from the end the current counter-collector and the end plate or / of the cell.

  
According to another embodiment, the current collector or distributor consists of an embossed woven mesh or of metallic wire fabric, each single wire forming a series of undulations of an amplitude corresponding to the maximum height of the fold of woven mesh or fabric. Each metal wire thus comes into contact, in an alternative sequence, with the: end plate of the cell which serves as a plate for applying the pressure and the

  
  <EMI ID = 12.1>

  
or the flexible intermediate grid inserted between the electrode layer, or the membrane and the compressible layer. At least part of the mesh extends through the thickness of the fabric and is obliquely open to the electrolyte current.

  
In a variant, two or more woven meshes or fabrics, after having been individually embossed by shaping, can be superimposed on each other to obtain a collector having the desired thickness.

  
The embossing of the metallic mesh or of the fabric gives the collector great compressibility and remarkable elasticity under compression under a load which can be at least 50 to 2,000 g / cm 2 approximately of surface of application of the load, namely the back plate or end plate.

  
After assembly of the cell, the electrode of the invention has a thickness preferably corresponding to the depth of the electrode chamber but the

  
  <EMI ID = 13.1>

  
bigger. In this case, a screen or a pierced grid

  
and substantially rigid or a plate spaced from the surface of the rear wall of the chamber can act as a compression surface against the elastic and compressible mat of the collector. In this case, the space behind the at least relatively rigid grid is open and provides for the electrolyte a channel through which the released gas and the electrolyte can flow. The mat can be compressed to a much smaller thickness and volume. For example, it can be compressed up to around 50 to 90% or even up to a smaller percentage compared to its volume. and to its initial thickness and it is therefore pressed or compressed between the membrane and the conductive bottom plate of the cell by clamping these elements.

   The compressible sheet is movable, that is to say that it is not welded or fixed to the end plate of the cell or to the interposed grid and essentially transmits the current by mechanical contact with it. , appropriately connected to the power source and to the electrode.

  
The mat is movable or slidable relative to the adjacent surfaces of these elements with which it is in contact. When applying the clamping pressure, the loops or coils of wire constituting the elastic mat can deform and slide laterally while uniformly distributing the pressure over the entire surface with which they are in contact. In this way, the mat acts better than individual springs distributed over the surface of the electrode since the springs are fixed and there is no interaction between the pressure points to compensate for surface irregularities of the surfaces. bearing.

  
A large part of the cell clamping pressure is stored elastically by each of the turns or undulations of the metal wires forming the collector

  
current. Since no serious mechanical stress is practically created by the differential elastic deformation of one or more individual turns or undulations of the article with respect to the adjacent turns, the elastic collector of the invention can effectively prevent or avoid drilling or excessive thinning

  
of the membrane in places or in areas subject to more stress during cell assembly. Rather large deviations in the flatness of the current carrying structure of the opposite electrode may thus be

  
7 tolerated as well as differences in parallelism between said structure and the bottom or end plate of the cell or the rear pressure plate.

  
The elastic electrode of the invention is advantageously the cathode and is associated or opposed to an anode which may be of a more rigid type, which means that the electrode can, on the anode side, be supported in a more or less rigid. In cells for the electrolysis of sodium chloride brines, the mat, or the compressible cathode sheet. Is preferably made of a wire of nickel or of nickel alloy or of stainless steel because of the high resistance of these materials to caustic substances and embrittlement by hydrogen. The mat can be coated with a metal or metal oxide

  
platinum group, cobalt or their oxides or any other electrocatalyst to reduce the hydrogen overvoltage.

  
Any other metal capable of retaining its elasticity during use may be used, in particular titanium, optionally coated with a non-passivating layer such as for example a platinum group metal or

  
of its oxide. This is particularly useful when there

  
for use in contact with acid anolytes.

  
As mentioned, an electrode layer made of particles of a platinum group metal or

  
of its oxide from another resistant electrode material may be bonded to the membrane. This layer generally has a thickness of at least about 40 to 150 microns and can be essentially produced as described in U.S. Patent 3,297,484; the layer can be applied on both sides of the diaphragm or the membrane if necessary. Since the layer is substantially continuous, although permeable to gas and electrolyte, it protects the compressible mat and, therefore, most if not all of the electrolysis occurs on the layer, weak electrolysis, for example a release of gas, taking place on the compressed mat in contact with the rear face of the layer.

  
\

  
This is true, especially when the particles in the layer have a hydrogen (or chlorine) surge lower than that of the surface of the mat. In this case, the mat largely serves as a distributor or current collector, distributing the current over the less electrically conductive layer.

  
Conversely, when the compressible mat is in direct contact with the diaphragm or the membrane and even if there is a perforated electrically conductive screen grid or any other perforated conductor between the mat and the diaphragm, the open mesh structure ensures the existence of unobstructed passages for the electrolyte to the rear areas which are spaced from the membrane, including areas which may be on the front, inside and

  
at the back of the compressible fabric. Thus, the open, not fully protected, compressed mat can itself provide an active electrode surface which can represent two or four or more times the total protruding surface in direct contact with the diaphragm.

  
The finding of an increase in the area of a multilayer electrode was suggested in the G.B.
1.268.182 which describes a multilayer cathode comprising

  
outer layers of expanded metal and inner layers of a smaller, thinner mesh which may be woven mesh, the cathode touching a cation exchange membrane with the electrode extending at an angle through the cathode .

  
In accordance with the present invention, it has been found that a lower tension is obtained by using a compressible mat which, due to embossing, wrinkling, waving or any other design, has a large part of the wires or conductors which pass through the thickness of the mat over a distance representing at least part of this thickness. In general, these wires are curved so that when the mat is compressed, they flex elastically to distribute the pressure and these crossed wires communicate substantially the same potential to the wires in the rear part as that existing on the wires in contact with the membrane.

  
When such a mate is compressed against the diaphragm including or excluding any interposed grid screen, a voltage which is less than 5 to 150 millivolts can be obtained with the same current flow rate as that obtained when the mate and its interposed grid screen just touch the diaphragm. This represents a substantial reduction in. consumption in kilowatt-hours per tonne of chlorine released. When the mat is compressed, the parts which are spaced from the membrane approach but remain spaced from the membrane, and the possibility or probability as well as the degree of electrolysis increases, this increase in the surface allowing a greater amount of electrolysis without excessive increase in tension.

  
In addition, an additional advantage is achieved even if there is a weak electrolysis on the rear parts of the mat because it is better polarized against corrosion. For example, when a compressible nickel mat is pressed against a continuous layer of highly conductive electrode particles bonded to the diaphragm, the electrical protection can be so strong that only a weak or even no at all occurs. electrolysis on the mat. In this case, it has been found that the nickel mat tends to corrode, especially when the hydroxide

  
of alkali metal exceeded 15% by weight and that chlorides were present. With an open perforated structure, directly in contact with the diaphragm, a sufficiently open path is provided towards the spaced parts and even towards the rear of the mat so that the exposed surfaces polarize at least negatively or are cathodically protected against corrosion. This even applies to surfaces where there is no evolution of gas or other electrolysis. These advantages are particularly notable for current densities greater than 1,000 amperes per square meter of electrode area, measured by the total area surrounded by the electrode ends.

  
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The elastic mat is preferably compressed to about 80 to 30% of its initial thickness in the uncompressed state under a compression pressure between
50 and 200 g per square centimeter of protruding surface. Even in this compressed state, the elastic mat must be highly porous, since the ratio between the void volume and the apparent volume of the compressed mat expressed as a percentage is advantageously at least 75% (rarely below 50)) and preferably between 85% and

  
96%. This can be calculated by measuring the volume occupied by the mate. In the compressed state to the desired degree and by weighing the mate. Knowing the density of the metal of the mate, its solid volume can be calculated by dividing the volume by the density which gives the volume of the structure of solid mate, and the volume of the voids is then obtained by subtracting this value from the total volume.

  
It has been found that when this ratio becomes excessively low, for example by much too high compression of the elastic mat. below 30% of its thickness in the uncompressed state, the cell voltage begins to increase, probably due to a decrease in the rate of mass transport to surfaces

  
  <EMI ID = 14.1>

  
electrode to allow adequate release of the released gas. A characteristic of typical cell voltages, as a function of degrees of compression and vacuum ratio

  
compressible mat, is shown later in the examples.

  
The diameter of the wire used can vary over a wide range depending on the type of shaping or texturing, but it is small enough to obtain the desired elasticity and deformation characteristics under the assembly pressure of the cell. An assembly pressure corresponding to a load of 50 to 500 g / cm <2> electrode surface is normally necessary to obtain good electrical contact between the electrodes linked to the membrane and the transport structures of \

  
current or respective collectors, although higher pressures up to 2,000 g / cm2 can usually be used.

  
It has been found that, by causing a deformation of the elastic electrode of the invention of approximately 1.5 to 3 mm, which corresponds to a compression of not more than 60% of the thickness of the article in the non-state. compressed under a pressure of about 40 g / mp of projecting surface, one can also obtain a contact pressure with the electrodes included within the limits mentioned above in cells having a high developed surface and deviations from flatness up to 2 mm / m.

  
The diameter of the metal wire is preferably between 0.1 mm or less than 0.7 mm while the thickness of the article in the uncompressed state, i.e. the diameter of the coil or the amplitude of the ripple représente represents 5 or more times the diameter of the wire, preferably in the range of

  
4 to 20 mm. It is therefore clear that the compressible section surrounds a large free volume, namely the proportion of occupied volume which is free and open to the flow of electrolyte and gas. In the embossed fabrics described above which include these compression thread helices, this percentage of free volume is greater than 75% of the total volume occupied by the fabric. This percentage of free volume is rarely less than 25% and should preferably not be less than
50%, because the pressure drop when the gas and the electrolyte flow through such a tissue is negligible.

  
When the use of particulate electrodes or other directly bonded porous electrode layers

  
on the surface of the membrane is not envisaged, the mat or the elastic fabric is in direct contact with the membrane and acts as an electrode. The surprising fact has been found that only a substantially negligible loss of cell voltage is recorded compared to what happens when using porous electrode layers bonded by providing a sufficient density of points of elastic contact between the electrode surface and the membrane.

  
The density of the contact points should be at least 30 points per square centimeter of membrane surface, and preferably about 50 points or more per square centimeter. Conversely, the contact area of the individual contact points should be as small as possible and the ratio of the total contact area to the corresponding membrane area should be less than 0.6 and preferably 0.4.

  
In practice, it has been considered convenient to use a foldable metallic screen-grid with a number of meshes at least equal to 10, preferably greater than 20 and usually between 20 and 200, or a fine mesh of expanded metal. with similar characteristics, sandwiched between the elastically compressed mat and the membrane. The number of stitches is given as an indication of the number of threads for 2.54 cm.

  
It has been proven that under these conditions -, of minimal and dense contact established elastically between the electrode screen grid and the membrane surface, most of the reaction at the electrode takes place at the interface of contact between the electrode and the ion exchange groups contained in the membrane material, most of the ionic conduction taking place in or through the membrane and little or no conduction taking place in the liquid electrolyte

  
in contact with the electrode. For example, the electrolysis of pure bi-distilled water with a resistivity greater than
2,000,000 ohms.cm has been successfully conducted in a cell of this type equipped with a cation exchange membrane. surprisingly low cell voltage.

  
Moreover, when the electrolysis of an alkali metal brine is carried out in the same cell, there is no appreciable change in cell voltage by varying the orientation of the cell from horizontal to vertical, which indicates that the contribution to the cell voltage drop, attributable to the so-called "bubble" effect, is negligible.

   This behavior is in good agreement with a cell with a solid electrolyte comprising particulate electrodes linked to the membrane which contrasts with that of traditional membrane cells equipped with electrodes with coarse perforations either in contact with or slightly spaced from the membrane, in which the effect of bubbles strongly contributes to the voltage of the cell which is normally lower when the perforated electrode giving off gas is kept horizontal below a certain level of electrolyte and is maximum when the electrode is vertical in due to a decrease in the rate of gas evolution and the increasing population of gas bubbles along the electrode, due to accumulation.

  
An explanation for this unexpected behavior is

  
to be partly seen in the fact that the cell behaves almost like a solid electrolyte cell since the major part of the ionic conduction has place in the membrane and also in the fact that the contacts of the extremely small individual contact zones elastically established between the electrode layer in the form of a fine mesh screen grid and the membrane are capable of easily releasing the infinitesimal amount of gas that forms

  
at the contact interface and restore contact immediately once the gas pressure has dissipated. The elastically compressed electrode mat ensures a substantially uniform contact pressure and a uniform and almost total covering of small high density contact points between the electrode surface and the membrane and it acts effectively as a spring for releasing the gas to maintain almost constant contact between the surface of the electrode and the functional ion exchange groups on the surface of the membrane which act as the electrolyte of the cell.

  
The two electrodes of the cell may include an elastic compressible mat and a fine mesh screen grid ensuring a number of contacts greater than at least 30 contact points per square centimeter, made of materials resistant to anolyte and to catholyte. More favorably, an electrode only of the \

  
cell comprises the elastically compressible mat according to the invention associated with the screen grid of fine mesh electrode, while the other electrode of the cell is a substantially rigid perforated structure preferably comprising a fine mesh grid inserted between the rigid coarse structure and the membrane.

  
Various embodiments of the invention have been described below, with reference to the accompanying drawings in which:
Fig. 1 is a photographic reproduction of an embodiment of an elastically typical compressible mat used for the implementation of this invention. Fig. 2 and 3 represent photographic reproductions of two embodiments of the compressible mat usable according to the invention. Fig. 4 is an exploded horizontal section view of a solid electrolyte cell according to the invention, comprising a compressible electrode system in which the compressible part consists of helical spiral wires. Fig. 5 is a view in horizontal section of the assembled cell of FIG. 4. Fig. 6 is an exploded perspective view of another preferred embodiment of the current collector of the cell of FIG. 4.

   Fig. 7 is an exploded perspective view of another preferred embodiment of the current collector of the cell of FIG. 4. Fig. 8 is an exploded sectional view of another preferred embodiment of the electrolyte cell of the invention. Fig. 9 is a horizontal sectional view of the assembled cell of FIG. 8. Fig. 10 is a view in horizontal section of another preferred embodiment of the cell of the invention:
tion. Fig. 11 is a schematic fragmentary vertical section of the cell of FIG. 10. Fig. 12 is a schematic diagram illustrating the circulation system of the electrolyte used in connection with the envisaged cell. Fig. 13 is a graph illustrating the reduction in voltage obtained as a function of the increase in pressure on the electrode and the diaphragm.

  
The compressible electrode or its section shown in fig. 1 is formed by a series of interwoven helical cylindrical spirals made of nickel wire with a diameter of 0.6 mm (or less); their turns being mutually wound one inside the adjacent turns and having a diameter of 15 mm.

  
The example of the structure of fig. 2 comprises substantially helical spirals 2, of an elliptical or flattened section made of a nickel wire with a diameter of 0.5 mm, their turns being wound one inside the adjacent turns and the small

  
axis of the propeller being 8 mm.

  
In the example of structure of FIG. 3, one sees a mesh woven in nickel wire with a diameter of

  
  <EMI ID = 15.1>

  
the depth of the crimping ripple being 5 mm

  
with a peak between the corrugations of 5 mm. The crepe can. be in the form of groups of parallel folds intersecting in a herringbone pattern as shown in fig. 3.

  
In the example of FIG. 4, the cell with a solid electrolyte) which is particularly suitable for the electrolysis of a sodium chloride brine / and comprises one of the current collectors of the invention essentially consists of a terminal plate or d vertical anode end 3 comprising a joint surface 4 all along its perimeter so as to ensure tight contact

  
  <EMI ID = 16.1>

  
Lement interposition of an insulating seal impermeable to liquids (not shown). The anode end plate 3 is also provided with a central zone 6 set back with respect to said seal surface 4 with a surface corresponding to the area of the anode 7 linked to the surface of the membrane. The end plate can be made of steel, its face in contact with the anolyte being plated with titanium

  
or another passivable metal for valve, it can also be made of graphite or of moldable mixtures of graphite and a resinous binder, chemically resistant.

  
The anode collector preferably consists of a screen grid or an expanded sheet 8 of titanium, niobium or another metal for valve, coated with a non-passivable material and resistant to electrolysis such as

  
than noble metals and / or oxides and mixed oxides of platinum group metals. The screen grid or the expanded sheet 8 is welded or more simply rests on the series of ribs or projections 9 made of titanium or of another metal for valve welded to the recessed central zone 6 of

  
the cell end plate so that the plane of

  
the grid is parallel and preferably co-planar with the plane of the seal surface 4 of the end plate.

  
The vertical cathode terminal plate 10 has on its internal face a central recessed area

  
11 relative to the peripheral seal surface

  
12 and said recessed area 11 is substantially planar, that is to say without ribs, and parallel to the planes of the surfaces, sealing. Inside the zone set back from the cathode end plate is placed an elastic compressible current collector 13 according to the invention, preferably made of a nickel alloy.

  
The thickness of the uncompressed elastic manifold

  
of

  
is preferably / 10 to 60% greater than the depth of

  
the central zone set back 11 relative to the plane of the sealing surfaces and, during assembly of the cell, the collector is compressed from 10 to 60% of its initial thickness, thus exerting an elastic reaction,

  
preferably of the order of 80 to 600 g / cm 2 of protruding surface. The cathode end plate 10 can be made of steel or any other material resistant to caustic soda and to hydrogen.

  
The membrane 5 is preferably an ion exchange membrane permselective to cations and impermeable to the fluid, such as, for example, a membrane composed of a polymer film with a thickness of 0.3 mm of a copolymer of, tetrafluoroethylene and perfluorosulfonylethoxyvinyl ether having ion exchange groups such as sulfonic, carboxylic or sulfonamide groups. Because of its thinness, the membrane is relatively flexible and tends to sag, creep or flex unless supported. Such membranes are produced by

  
the firm E.I. du Pont de Nemours under the name Nafion.

  
On the anode face of the membrane is fixed or bonded the anode 7 consisting of a porous layer with a thickness of 20 to 150 microns of particles of electroconductive and electrocatalytic material preferably composed of oxides and mixed oxides of minus one of the platinum group metals. The cathode 14 consisting of a porous layer of particles of a conductive material with a thickness of

  
  <EMI ID = 17.1>

  
preferably killed by graphite and platinum black / in a weight ratio of 1: 1 to 5: 1.

  
The binder used to bind the particles to the surface of the membrane is preferably polytetrafluoroethylene (PTFE) and the electrodes are obtained by sintering a mixture of PTFE and particles of conductive catalytic material to transform the mixture into a porous film, and by pressing the film on the membrane at a temperature high enough to make the bond. This connection is made by assembling a sandwich of electrode sheets with the membrane between them and pressing the assembly to incorporate the electrode particles in the membrane.

  
In general, the membrane has been hydrated by heating to boiling in an aqueous electrolyte such as saline, acid or alkali metal hydroxide solution and therefore it is highly hydrated and contains a considerable amount, 20% by weight or more of water, either combined as a hydrate or simply absorbed. In this case, care should be taken to avoid excessive water loss during the lamination process.

  
Since this stratification is achieved by application of heat as well as pressure on the laminate, the water may tend to evaporate and this can be kept to a minimum by one or more of the following procedures:

  
1) inclusion of the laminate in an impermeable envelope, that is to say between metal sheets pressed or sealed on their edges to maintain a water-saturated atmosphere around the laminate;

  
2) appropriate mold design to quickly return water to the laminate and

  
3) molding in a vapor atmosphere. The electrodes linked to the surfaces of the membrane have a projecting zone corresponding practically to the recessed central zones 6 and 11 of the two end plates.

  
In Figure 5, we see the cell of fig. 4 in the assembled state, the corresponding parts of these two figures bearing the same references. As seen in the drawing, the end plates 3 and 10 have been tightened together, thus compressing the sheet with helical coils or mat 13 against the electrode 14. During the operation of the cell, the anolyte is circulated, consisting of example in a saturated sodium chloride brine, through the anode chamber, more advantageously by sending fresh anolyte through an intake manifold (not shown) in the vicinity of the bottom of the chamber and discharging the anolyte worn through outlet tubing
(not shown) near the upper part of said chamber, at the same time as the chlorine released. The cathode chamber is supplied by

  
water or caustic soda diluted through an intake manifold (not shown) located at the bottom of the chamber while the caustic soda produced is recovered in the form of a concentrated solution through an outlet manifold (not shown) at the upper end of said cathode chamber. The hydrogen released at the cathode can be recovered from the cathode chamber by

  
at the same time as the concentrated caustic solution, or through another outlet tube at the top of the chamber.

  
Since the mesh of the elastic manifold is open, there is little or no resistance to the flow of gas or electrolyte through the compressed manifold. The anodic and cathodic end plates are both suitably connected to an external current source and the current passes through the series of ribs 9 in the direction of the anodic current collector 8, from which it is then distributed in the direction of the anode 7 through the multiplicity of points of contact between the expanded sheet 8 and the anode 7. The ionic conduction takes place essentially through the ion exchange membrane 5, the current being largely transported by the

  
sodium ions migrating through the cationic membrane 5 from the anode 7 to the cathode 14 of the cell. The current collector 13 collects the current from

  
the cathode 14 through the multiplicity of contact points between the nickel wire and the cathode, and then transmits it to the cathode end plate 10, through a plurality of contact points.

  
After assembly of the cell, the current collector 13 in the compressed state which causes a deformation, preferably between 10 and 60% of the initial thickness of the article, that is to say of its turns. or individual undulations, exerts an elastic reaction force against the surface of the cathode 14 and consequently against the retaining surface represented by the anode current collector \

  
8 substantially non-deformable. Such a reaction force maintains the desired pressure at the points of contact between the cathode collector and the anode collector with the cathode 14 and the anode 7 respectively.

  
The absence of mechanical barriers to the differential elastic deformation between the adjacent spirals or the adjacent undulations of the elastic current collector allows it to adapt to slight inevitable differences in flatness or parallelism between the cooperating planes represented. by the anode collector 8 and the

  
surface 11 of the cathode compartment. These slight deviations

  
which normally occur in standardized manufacturing processes can therefore offset each other to a significant extent.

  
In Figs 6 and 7, there is shown schematically partial exploded perspective views of two modes

  
preferred embodiments of the compressible and elastic current collector mate 13 of the cell of FIGS. 4 and 5. For the sake of simplification, only the useful parts are described and designated by the same figures as in FIGS. 4 and 5. The elastic mate compressible of fig. 6 is constituted by a series of helical cylindrical spirals of nickel wire 13 with a diameter of 0.6 mm, the coils of which are preferably wound one inside the other (as can be seen more clearly on the photographic reproduction of Fig. 1) and the diameter of the turns is 10 mm. Between the fabric or the elastic sheet 13 a and the membrane 5 comprising on its surface the cathode layer 14, is disposed a thin perforated sheet 13 b which may advantageously be an expanded sheet of nickel with a thickness of 0.3 mm.

   The perforated sheet 13 is flexible and offers negligible resistance to folding and bending under the effect of the elastic reaction forces exerted by the loops of the sheet 13 a during compression against the membrane.

  
5. In FIG. 7, we see an identical embodiment

  
to that of fig. 6, but in which the elastically compressible fabric or layer 13 a is a corrugated woven material of nickel wire with a diameter of 0.15 mm such as that of the photographic reproduction of the rod 3.

  
In another embodiment of the invention shown in FIG. 8, the cell which is particularly useful for the electrolysis of a sodium chloride brine comprises a compressible electrode or a current collector according to the invention associated with a anodic end plate 3 provided with a seal surface 4 all along its perimeter, so as to establish a sealed contact with the peripheral edges of the diaphragm or of the membrane 5, with the possibility of possibly interposing an insulating peripheral seal impermeable to the liquid (not shown).

   The anode end plate 3 is also provided with a recessed central zone 6 relative to the sealing surface, with a surface extending from a lower zone where the brine is introduced to an upper zone where the spent or partially used brine and the chlorine released are evacuated, said zones being generally in communication by the top and the

  
low. The end plate can be made of steel, its face in contact with the anolyte being coated with titanium

  
or another passivable metal for valve; it can also be made of graphite or of a moldable mixture of graphite and of a chemically resistant resinous binder, or of another anodically resistant material.

  
The anode is preferably constituted by a screen grid or an expanded sheet 8 of titanium, niobium

  
or other metal for valve permeable to gas and electrolyte, coated with a non-passivable and resistant material

  
electrolysis such as noble metals and / or

  
oxides and mixed oxides of metals from the platinum group or another electrocatalytic layer which serves as

  
anodic surface when it is placed on an electroconductive support. The anode is practically rigid and the screen grid is thick enough to transport the electrolysis current from the ribs 9 without excessive ohmic losses. It is even more favorable to use :. a flexible, foldable, fine-mesh screen that can be of the same material as the coarse screen

  
8 and disposed on the surface: of this grid 8 to ensure fine contacts with the membrane with a density of 30 or more, preferably from 60 to 100 points of contact per square centimeter of membrane surface. The fine mesh grid can be spot welded to the coarse grid or can simply be sandwiched between grid 8 and the membrane. The fine mesh grid is coated with noble metals or conductive oxides resistant to the anolyte.

  
The vertical cathode terminal plate 10 has on its internal face a central recessed area

  
  <EMI ID = 18.1>

  
said recessed area 11 is substantially flat, that is to say free from ribs and is parallel to the plane of the sealing surface. The elastically compressible electrode element 13 envisaged by the invention, advantageously made of a nickel alloy, is placed inside the zone set back from the cathode end plate. In

  
the embodiment shown in this figure, the electrode is a wire propeller or a plurality of interlaced propellers and these propellers can come into direct contact with the membrane. However, a screen grid 15 is preferably interposed, as can be seen in the drawing, between the wire propeller and the membrane, so that the propeller and the grid are in sliding engagement, between them as well as with the membrane.

  
The spaces between the adjacent turns of the propeller must be large enough to allow an easy flow or movement of the gas and the electrolyte between the spirals, for example inwards or out of the central areas surrounded by the propeller. . In general, these gaps are sensitive, often three to five times larger than the diameter of the wire. The thickness of the uncompressed helical coil is preferably from 10% to 60% greater than the depth of the recessed central zone 11 relative to the plane of the sealing surfaces. During the assembly of the cell, the coil is compressed by

  
10 to 60% of its initial thickness, thus exerting a

  
elastic reaction force, preferably between

  
80 and 100 g / cn? ' protruding surface.

  
The cathode terminal plate 10 can be made of steel or any other electroconductive material resistant to caustic soda and to hydrogen. The membrane

  
5 is preferably an ion exchange membrane permselective to cations and impermeable to the fluid, as already indicated. The screen grid 15 is advantageously made of wire

  
nickel or any other material capable of withstanding corrosion under cathodic conditions. Although said grid screen has rigidity, it should preferably be flexible and practically non-rigid so that it can easily fold to adapt to irregularities in the cathode surface of the membrane. These irregularities can be found in the surface of the membrane itself,

  
  <EMI ID = 19.1>

  
the more rigid anode on which / carries the membrane. In general, the screen grid is more flexible than the propeller.

  
For many uses, the mesh size of the screen grid must be smaller than the size of the openings between the helix spirals and the grids:

  
  <EMI ID = 20.1>

  
are particularly preferred embodiments of the invention. The interleaved screen grid ^ can fulfill several functions. First, it has an active electrode surface due to its electroconductivity. Secondly, it serves to prevent the propeller having any other compressible electrode element from wearing out locally, penetrating into, or thinning the membrane, and as the compressed electrode presses on the screen grid in a local area, this grid helps distribute pressure throughout

  
the surface of the membrane between the adjacent pressure points and also prevents a deformed spiral section from entering, or wearing out the membrane.

  
During electrolysis, hydrogen and an alkali metal hydroxide are released on the screen grid and generally on part or even all of the propeller. When the helical spirals are compressed, their rear surfaces, i.e. those distant or spaced from the surface of the membrane, approach the screen grid and the membrane and the higher the degree of compression, the higher the average space between the spirals and

  
the membrane is weak and stronger will be the electrolysis or at least the cathodic polarization of the spiral surface. Thus, compression has the effect of increasing the overall effective surface of the cathode.

  
Compression of the electrode has been shown to be effective in reducing the overall voltage required to maintain a current of 1,000 amps per square meter of active membrane area or more. At the same time, compression must be limited so that the compressible electrode remains accessible to the flow of electrolyte and gas. As seen in Figure 9, the spirals remain open to provide vertical central channels through which can rise electrolyte and gas. Furthermore, the spacings between the spirals remain spaced to allow catholyte access to the membrane, and to the sides of the spirals. The spiral wire is generally thin, with a diameter of 0.05 to 0.5 mm. We can admit larger wires but they tend to be more rigid and less compressible; that's why he

  
it is rare that the wire exceeds 1.5 mm.

  
In fig. 9, the cell of FIG. 4

  
in the assembled state, the parts corresponding to the two figures being given the same references. As seen in the drawing, the end plates 3 and 10 were assembled by clamping, thus pressing against the electrode 15 the sheet or mate with helical turns / During the operation of the cell, the anolyte consisting for example of brine of saturated sodium chloride, circulates through the anode chamber, very advantageously bringing fresh anolyte to \

  
through an inlet pipe (not shown) in the vicinity of the bottom of the chamber and discharging the spent anolyte through an outlet pipe (not = shown) in the vicinity of the top of said chamber, at the same time as the chlorine released.

  
The cathode chamber is supplied with water or diluted aqueous alkali through an intake manifold

  
(not shown) at the bottom of the chamber, the alkali produced being recovered in the form of a concentrated solution through an outlet tube (not shown) at the upper end of said cathode chamber. The hydrogen released at the cathode can be recovered from the cathode chamber, either with the concentrated caustic solution, or

  
through another outlet tube at the top of the chamber :. bre.

  
The anodic and cathodic end plates are both suitably connected to an external current source and the current passes through the series of ribs 9 towards the anode 8. The ionic conduction takes place essentially through the ion exchange membrane 5 ,

  
the current being largely transported by the sodium ions migrating through the cationic membrane 5 from the anode 8 towards the cathode 14 of the cell. The electrodes provide a plurality of contact points on the membrane, the current ultimately flowing to the end plate of the cathode 10 through a plurality of contact points.

  
After assembly of the cell, the collector

  
of current 13. in its compressed state which causes a deformation, preferably between 10 and 60% of the initial thickness of the article, i.e. individual turns or plies, exerts an elastic reaction force against

  
the surface of the cathode 14 and therefore against the retaining surface represented by the anode or the anode current collector 8 relatively more rigid and substantially non-deformable. Such a reaction force maintains the desired pressure on the contact points between the cathode and the membrane as well as on the screen grid part and the helical part of the cathode 14.

  
Since the helical spirals and the screen grid can be slid relative to each other and to the membrane as well as to the rear wall, the absence of mechanical barriers to differential elastic deformation between the spirals adjacent or the adjacent folds of the elastic electrode allows these to compensate laterally for the slight inevitable deviations in flatness or parallelism between the cooperating planes represented by the anode 8 and the bearing surface

  
11 of the cathode compartment. Such slight deviations which normally occur in common manufacturing processes are therefore largely corrected.

  
The advantages of the elastic electrode of the invention are fully realized and appreciated in industrial electrolysers of the filter press type which include a large number of elementary cells assembled by clamping in a series arrangement so as to form modules of a high production capacity. In this case, the end plates of the intermediate cells are represented by the surfaces of the bipolar separators carrying the anode and cathode current collector on each respective face. Consequently, the bipolar separators, in addition to acting as walls, of delimitation for the respective electrode chambers, electrically connect the anode of a cell to the cathode

  
of the adjacent cell in the series.

  
Due to their high deformability, the elastic compressible electrodes of the invention ensure a more uniform distribution of the clamping pressure of the filter press module on each individual cell, this is particularly true when the opposite face of each membrane is rigidly supported

  
by a relatively rigid anode 8. In such cells in series, it is recommended to use elastic seals on the sealing surfaces of the individual cells - to avoid limiting the elasticity of the compressed press filter module to the elasticity of the membranes. One can thus derive a greater advantage from the elastic deformation properties of the elastic collectors inside each cell of the series.

  
In the variant shown schematically in FIG. 10, a fabric crimped with interlaced wires is used as the compressible element of the electrode in place of the helical spirals, and a channel for additional electrolyte is provided for the circulation of the electrolyte. As can be seen, the cell comprises an anode end plate 103 and a cathode end plate 110, both mounted in a vertical plane, each end plate in the form of a channel comprising side walls which delimit an anode space 106 and a cathode space
111. Each end plate also has a sealing surface or peripheral seal on a side wall projecting from the plane of the respective end plate, 104 being the sealing surface of the anode.

  
and 112 the sealing surface of the cathode. These surfaces are supported by a membrane or diaphragm 105 which extends

  
through the space enclosed between the side walls.

  
The anode 108 comprises a relatively rigid incompressible sheet of expanded metallic titanium or any other perforated anodically resistant substrate with preferably-a non-passivable coating-such as a metal,

  
an oxide or mixed oxide of a platinum group metal. This sheet is sized to fit the side walls of the anode plate and is supported

  
rather rigidly by spaced ribs of electrically conductive metal or graphite 109 which are fixed to the

  
and protrude from the fabric or base of the anode end plate 103. The spaces between the ribs ensure an easy flow of the anolyte which is brought from below and is evacuated at the top of these spaces. The entire end plate and the ribs can be made of graphite and, alternatively, they can be made of steel

  
  <EMI ID = 21.1>

  
plated with titanium or another suitable material. The ends of the ribs bearing on the anode sheet

  
108 may or may not be coated, for example with platinum, to improve electrical contact and the anode sheet 108 may also be welded to the ribs 109. The rigid perforated anode sheet 108 is held securely in the vertical position. This sheet may be made of expanded metal having orifices inclined upwards away from the membrane (see FIG. 11) to deflect the ascending gas bubbles towards space 105.

  
It is even more preferable to have, between the rigid perforated sheet 108 and the membrane 105, a flexible screen mesh with fine mesh 108 a, made of titanium or another metal for valve, coated with a non-passivable layer which is advantageously a noble metal or conductive oxides having a low overvoltage for the reaction

  
example

  
anodic (by '/ release of chlorine). The fine mesh screen grille 108 a provides the membrane with an extremely low surface contact density, greater than at least
30 contacts per square centimeter. It may or may not be spot welded to the coarse screen grid 108 ...

  
On the cathode side, ribs 120 extend outward from the base of the cathode end plate 110 at a distance which is a fraction of the total depth of the cathode space 111. These ribs are spaced along the cell to provide parallel spaces for the flow of electrolyte. As in the abovementioned embodiments, the cathode end plate and the ribs can be made of steel, a nickel-iron alloy or any other cathodically resistant material. On the conductive ribs 120 is welded a relative pressure plate =. rigid ment 122 which is perforated and which allows easy circulation of the electrolyte from one face to the other.

   Generally, these orifices or flaps are inclined upward away from the membrane or from the compressible electrode in the direction of the space 111. (see also fig. 11). The pressure plate is electrically conductive, used to communicate polarity to the electrode and to apply pressure there; it can be made of expanded metal

  
or be made up of a thick screen grid made of steel, nickel, copper or their alloys.

  
A relatively fine flexible screen grille 114 rests on the cathode face of the active surface of the diaphragm 105, because of its flexibility and its relative thickness, it follows the contours of the diaphragm and consequently those of the anode 108. This screen grid largely serves as a cathode; it is therefore electrically conductive, for example a screen grid made of nickel wire or another cathodically resistant wire, and can have a surface with low hydrogen overvoltage. The screen grid preferably provides an extremely low surface contact density with the membrane, and greater than at least 30 contacts per square centimeter. A compressible mat 113 is placed between the cathode screen grid 114 and the cathode pressure plate 122.

  
  <EMI ID = 22.1> creped or pleated mesh fabric, is advantageously made of open mesh woven wire of the type shown in fig. 3 where the strands of yarn are woven into a relatively flat fabric with interwoven loops. This fabric is then creped or pleated in a wavy shape, the corrugations being close to each other, for example spaced 0.3 to 2 cm and the total thickness of the compressible fabric being

  
  <EMI ID = 23.1>

  
herringbone as shown in fig. 3 and the mesh of the fabric is coarser, that is to say that it has a larger pore dimension than that of screen grids
114.

  
We see in fig. 10, that this corrugated fabric 113 is placed in the space between the finer mesh screen grille 114 and the more rigid pressure plate of expanded metal 122. The corrugations extend through

  
this space and the vacuum ratio of the compressed tissue is preferably greater than 75%, preferably between 85 and 96% of the apparent volume occupied by the tissue. As shown, the waves or waves extend in a vertical or inclined direction so as to provide channels for the free upward flow of the gas and the electrolyte, said channels not being obstructed by the fabric thread. This is true even when the ripples extend across the cell from one side to the other, since

  
the mesh openings in the walls of the corrugations allow free flow of fluids.

  
As described with regard to other embodiments, the end plates 110 and 103 are clamped together and are supported on the membrane 105 or on a seal protecting the membrane from the external atmosphere and disposed between the end walls. Clamping pressure compresses the fabric

  
  <EMI ID = 24.1>

  
In turn, press the membrane against the opposite anode 108a and this compression appears to allow a lower total tension. We did a test where the fabric

  
  <EMI ID = 25.1>

  
found that a current density of 3,000 amperes per square meter of protruding electrode surface and a voltage reduction of about 150 millivolts were obtained by compressing the compressible sheet to a

  
  <EMI ID = 26.1>

  
that observed for the same current density at zero compression.

  
Between zero compression and compression up to 4 mm, a comparable voltage drop from 5 to 150 millivolts was observed. The cell tension remained practically constant until a compression of approximately

  
2 mm and then started to increase slightly when the compression reached below 2 mm, i.e. about 30% of the initial thickness of the fabric. This represents a substantial energy saving which can be

  
5% or more for a brine electrolysis process.

  
In the implementation of this embodiment, an aqueous solution of sodium chloride substantially \

  
saturated is sent to the bottom or bottom of the cell and flows upward through channels or spaces
105 between the ribs 109, and the depleted brine and the chloride released escape from the top of the cell.

  
Diluted water or sodium hydroxide is sent to the bottom of the cathode chambers and rises through the channels 111 as well as through the voids of the compressed mesh sheet 113, the hydrogen released and l 'alkali

  
being withdrawn at the head of the cell '. Electrolysis is caused by applying a direct current electric potential between the anode and cathode end plates.

  
In FIG. 11 "a partial view in vertical section is shown showing the flow diagrams of this cell in which at least the upper orifices of the pressure plate 122 are bevelled so as to provide an inclined outlet oriented upwards in moving away from the compressed tissue 113, part of the hydrogen released and / or the electrolyte thus escaping towards the rear electrolyte chamber 111 (fig. 10). This is why, the vertical spaces at the back of pressure plate
122 and the space occupied by the compressed mesh 113 are provided for the upward flow of the catholyte and the gas.

  
By using two such chambers, it is possible to reduce the interval between the pressure plate
122 and the membrane and to increase the compression of the sheet 113, while still leaving the sheet open to the flow of fluid, which serves to increase the total effective area of the active parts of the cathode.

  
In Figure 12, there is shown schematically the mode of operation of the cell considered here.

  
As can be seen, a vertical cell 20 of the type shown in section in FIGS. 5, 9 or 10, is provided with an anolyte inlet pipe 22 which penetrates into the bottom of the anolyte chamber (anode area) of the cell, and a

  
  <EMI ID = 27.1>

  
upper part of the anode zone. Likewise, the catholyte inlet pipe 26 opens out at the bottom of the chamber \

  
catholyte of cell 20 and the cathode zone has an outlet tube 28 located at the top of the cathode zone. The anode zone is separated from the cathode zone by a membrane 5 which. has a pressed anode 8

  
on the anode face and a cathode 14 pressed against the

  
cathodic side. The membrane electrode is in a vertical position and its height is generally between 0.4 and 1 meter or more.

  
The anode chamber or zone is linked to the membrane with the anode on one side and the end wall of the anode
(see fig. 5, 9 or 10) on the other side, while the cathode surface is linked to the membrane, the cathode being on one side and the end wall of the vertical cathode on the other side. During operation of the system, the supply of aqueous brine takes place from a supply tank 30 in the pipe 22, through a pipe, fitted with a valve 32, which goes from the tank

  
30 to line 22; a recycling or recirculation tank 34 is provided for pouring brine from the lower part through the pipe 5. The brine concentration of the solution entering the bottom of the anode zone is adjusted so as to be at least close to saturation, by proportioning the relative flows through line 32. and the brine entering the bottom of the anode zone flows upwards and in contact with the anode. Consequently, chlorine is released and rises with the anolyte and the two are evacuated through line 24 to tank 34. The chlorine is then separated and escapes as shown through an orifice

  
outlet 36. and brine is collected in the tank

  
34 and recycled. Part of this brine is removed as spent or depleted brine through the overflow line 40 and is sent to a source of solid alkali metal halide for resaturation and purification.

  
The alkaline earth metal as the halide or other compounds is kept low, well below one part per million parts of the alkali metal halide and frequently as low as 50 to 100 parts alkaline earth metal per trillion parts by weight of alkali halide.

  
On the face of the cathode, water is brought

  
to line 26 from a reservoir or other source 42 through line 44 which pours into recycling line 26 where it is mixed with recycled alkali metal hydroxide (NaOH) from line 26 from the recycling tank. The water-alkali metal hydroxide mixture enters the

  
bottom of the cathode zone and goes up towards the top through the compressed mat, gas permeable (fig. 5,9 or 10) or current collector. During its journey,

  
it comes into contact with the cathode and it forms hydrogen gas as well as metal hydroxide

  
alkaline. The catholyte liquor pours out through the canali-

  
  <EMI ID = 28.1>

  
to port 48. The alkali metal hydroxide solution

  
is drawn off through line 50 and the water supplied through line 44 is adjusted so as to maintain the concentration of NaOH or any other alkali at the desired level. This concentration can be as low as 5 or 10% by weight of alkali metal hydroxide, but this concentration is usually greater than about 15%, preferably between
15 and 40% by weight.

  
Since gas is released at both electrodes, it is possible and in fact advantageous to draw-

  
take advantage of the ascension properties of the gases released, which is obtained by operating the cell in the submerged state and by keeping the chambers with anodic and cathodic electrolyte relatively narrow, for example at a

  
width from 0.5 to 8 cm. Under these conditions, the gas released rises rapidly, entraining electrolyte and the

  
electrolyte sludge as well as the gas are discharged through

  
  <EMI ID = 29.1>

  
this circulation can be supplemented by pumps if necessary.

  
Woven metal material suitable as a collection

  
current generator according to the invention is manufactured by KNITMESH Limited, an English company with an office in

  
  <EMI ID = 30.1>

  
can use larger or finer yarns, and these yarns are woven to provide about 2.5 to

  
20 points for 2.54 cm (1 to 4 points per cm), preferably about 8 to 20 opening points for 2.54 cm (2 to 4 openings per cm). It is obvious that large variations are possible: this is how we can use a wavy screen grid with a fineness of about 5 to 100 meshes.

  
Intertwined, intertwined, or woven metal sheets are crimped to provide a repetitive wave-like outline, or are loosely woven or otherwise arranged, to give the fabric a thickness that is 5 to 100 times or the larger the diameter of the wire so that the sheet is compressible. However, since the structure is intertwined and movement is limited by the structure, the elasticity of the fabric is retained. This is especially true when it is crimped or wavy in a spaced wave arrangement as in a herringbone pattern. Several layers of this woven material can be superimposed if necessary.

  
When using the helical structure

  
of fig. 3, the wire propellers will be elastically compressible. The diameter of the wire and the diameter of the propellers are

  
  <EMI ID = 31.1>

  
required. The diameter of the propeller is generally 10 times where more the diameter of the wire in its uncompressed state. For example, a nickel wire with a diameter of 0.6 mm wound in helices of about 10 mm. In diameter has been used satisfactorily.

  
Nickel wire is suitable when the wire is cathode as described and as shown in the drawings. We can however use any other metal capa &#65533;

  
ble to resist cathode attack or corrosion by electrolyte or embrittlement by hydrogen and among these are stainless steel, copper, copper coated with silver and the like.

  
Although in the embodiments described above the compressible collector: is represented as being cathodic, it is quite obvious that the polarity of the cells can be reversed so that the compressible collector is anodic. It goes without saying that in this case the electrode wire must resist chlorine and attack
- anodic and, that the wires can be made of a valve metal such as titanium or niobium, preferably coated with a non-passivating electrically conductive layer resistant to anodic attack, such as a metal or a <EMI ID = 32.1>

  
perovskite, etc ...

  
In some cases, the application of the compressible element on the anode side or side may pose a problem because the arrival of a halide-based electrolyte on the electrode-membrane interface may be limited.

  
When the anode areas are not sufficiently accessible to the anolyte flowing through the cell, the concentration of halide may become reduced in local areas due to electrolysis and, when it is too reduced, there is a tendency to release oxygen rather than halogen as a result of the electrolysis of water. This is avoided by maintaining the surfaces

  
electrode-membrane contact points at a low value, that is to say rarely at a width greater than

  
1 mm and often less than 1/2 mm and can also be effectively avoided by maintaining a screen grid

  
a relatively fine mesh, (ten mesh or more)

  
between the compressible mat and the surface of the membrane.

  
Although these problems are also important -: = -. tants on the cathode, one encounters less difficulties since the cathodic reaction consists in a release of hydrogen and there is no secondary reaction because \

  
products are formed even if the contact points are

  
relatively large because water

  
and the alkali metal ion migrate through the membrane

  
so that, even if the cathode has some restrictions, there is less likelihood of secondary product formation. Therefore, it is advantageous to apply the compressible mat on the cathode face.

  
In the following examples, several preferred embodiments have been described which do not limit the implementation of the invention.

  
EXAMPLE 1

  
A first upright cell (A) was constructed according to the schematic representation of FIGS. 10 and 11. The dimensions of the electrodes were 500 mm for the

  
  <EMI ID = 33.1>

  
cathode 110, the cathode ribs 120 and the perforated cathode pressure plate 122 were made of steel galvanically coated with a layer of nickel. The perforated pressure plate was obtained by incising a steel plate 1.5 mm thick so as to form diamond-shaped openings whose main dimensions are 12 and 6 mm. The anode end plate 103 was made of titanium plated steel and the anode ribs 109 were made of titanium.

  
The anode consisted of an expanded screen grid made of coarse and substantially rigid titanium 108 obtained by incision of a titanium plate 1.5 mm thick by forming diamond-shaped orifices with the main dimensions of 10 and 5 mm. and by a screen mesh with fine mesh 108 a of titanium obtained-by incision of a titanium sheet of 0.20 mm thick by forming lozenge orifices whose main dimensions were 1.75 and 3 mm, welded by points on the internal surface of the coarse grid. The two grids were coated with a layer of mixed ruthenium and titanium oxides corresponding to a load of 12 g of ruthenium
(in the form of metal) per square meter of projecting surface.

  
The cathode consisted of three layers of crimped nickel fabric forming the elastic mat 113 and

  
the material was woven with nickel wire with a diameter of 0.15 mm. The crepe had a herringbone pattern, the amplitude of which was 4.5 mm and the distance between neighboring ridges of 5 mm. After prior compaction of the three layers of crepe fabric produced by superposition of the layers and application of a moderate pressure of the order of 100 to 200 g / cm 2 the mat had an uncompressed thickness of approximately 5.6 mm . In other words, the mat covered elastically, after release of the pressure, a thickness of approximately

  
5.6 mm. The cathode also contained a nickel screen screen 114 to 20 mesh formed of a nickel wire with a diameter of 0.15 mm, the grid provided about 74 points of contact per square centimeter with the surface of the membrane.
105, which was verified by printing on a sheet of pressure-sensitive paper. The membrane was a hydrated film 0.6 mm thick, of a Nafion 315 cation exchange membrane manufactured by Du Pont de Nemours, namely a type of membrane based on perfluorocarbon sulfonic acid.

  
A control test cell (B) of the same size was constructed and the electrodes were formed

  
according to normal industrial practice, the two coarse and rigid screen grids 108 and 122 described above resting directly on the opposite surfaces of the membrane 105 without the intervention of the fine mesh grids
108 a and 114 and without being uniformly compressed and

  
elastic against the membrane (see compressible mat 113). The test circuits were identical to those

  
of figure 8.

  
The operating conditions were as follows:

  

  <EMI ID = 34.1>
 

  

  <EMI ID = 35.1>


  
Test cell A has been put into service and the elastic mat has been compressed in an increasing manner to bring the operating characteristics of the cell, namely the cell voltage, the current efficiency, in accordance with the degree compression. In fig. 13, curve 1 shows the relationship between the cell voltage and the degree of compression or the corresponding pressure applied. It is observed that the cell tension decreased with increasing compression of the elastic mat to a thickness corresponding to approximately 30% of the initial uncompressed thickness of the mat. Beyond this degree of compression, the cell tension tended to rise slightly.

  
By reducing the degree of compression to a thickness of the mat of 3 mm, the operation of cell A, compared to that of control cell B operating in parallel has given the following results:

  

  <EMI ID = 36.1>


  
To evaluate the contribution of the bubble effect on the tensior of the cell, the cells were rotated

  
o

  
  <EMI ID = 37.1>

  
wedge, the anode remaining horizontal at the top of the membrane. The operating characteristics of the cells were as follows.

  

  <EMI ID = 38.1>


  
(x) The cell voltage started to rise slightly

  
and stabilized at around 3.6 V.

  
(xx) The cell voltage rose suddenly

  
well above 12 V and the electrolysis was therefore interrupted.

These results are interpreted as follows:

  
a) by rotating the cells from vertical to horizontal, the contribution of the bubble effect to the cell tension decreases in cell B, while the relative insensitivity of cell A is apparently due to a significantly negligible bubble effect which would partly explain the much lower cell voltage of cell A compared to cell B. b) Once the horizontal position is reached, the hydrogen gas begins to form a pocket under the membrane and tends to increasingly isolate the active surface of the grid cathode-ray screen from the ion current conduction at <EMI ID = 39.1>

  
same effect is significantly weaker in the test cell. A..This can only be explained by the fact that the main fraction of the ionic conduction is limited within the thickness of the membrane and that the cathode provides sufficient

  
&#65533;

  
enough points of contact with the ion exchange groups on the surface of the membrane to effectively maintain the electrolysis current.

  
It has been found that by increasing the density and the fineness of the points of contact between the electrodes and the membrane by increasing the replacement of fine mesh grids 108 a and 114 by increasingly coarse grids, the behavior of the test cell A is getting closer and closer to that of the control cell B. Moreover, the elastically compressible cathode layer 113 ensures covering of the surface of the membrane by the fine contact points distributed in such a way more than 90% dense and more often more than 98% of the total surface, even in the presence of large deviations in flatness and parallelism of the compression plates 108 and 122.

Example 2

  
For comparison, the test cell A was opened and the membrane 105 replaced by an identical membrane comprising a bonded anode and a bonded cathode.

  
  <EMI ID = 40.1>

  
particles of mixed ruthenium and titanium oxides with a Ru / Ti ratio of 45/55, bound to the surface of the membrane by means of polytetrafluoroethylene. Cathode

  
  <EMI ID = 41.1>

  
platinum black and graphite in a weight ratio

  
1/1 bonded by polytetrafluoroethylene to the opposite surface of the membrane.

  
The cell was operated under conditions exactly the same as. those of Example 1 and is shown by curve 2 of the diagram in FIG. 13, the relationship between the cell voltage and the degree of compression of the cathode current collector layer
-elastic 113. It is significant that the cell voltage of this really solid electrolyte cell is only about 100 to 200 mV lower than that of test cell A under the same operating conditions.

Example 3

  
To verify the unexpected results, the test cell A was modified by replacing all the anodic structures in titanium by comparable structures in steel coated with nickel (anodic end plate 103 and anodic ribs 109) and in pure nickel (grid coarse 108 and fine mesh grid 108 a). The membrane used was a 0.3 mm thick Nafion 120 cation exchange membrane manufactured by Du Pont de Nemours.

  
In the two anode and cathode compartments, pure bi-distilled water having a resistivity of more than 200,000 ohms.cm was circulated. An increasing potential difference was applied to the two end plates of the cell and an electrolysis current started to pass, oxygen being released at the 108 a nickel grid anode, and hydrogen being released to the nickel grid cathode 114. After a few hours of operation, the following voltage-current characteristics were observed:

  

  <EMI ID = 42.1>


  
Since the conductivity of the electrolytes is insignificant, the cell has been found to function as a <EMI ID = 43.1>

  
By replacing the fine mesh electrode grids 108 a and 114 with coarser grids, thereby reducing the density of the contacts between the electrodes and

  
  <EMI ID = 44.1>

  
a considerable increase in cell voltage was observed as indicated below:

  

  <EMI ID = 45.1>


  
As will be obvious to those skilled in the art, it is possible to increase the density of the contact points between the electrodes and the membrane by various expedients. For example, you can spray on the electrode grid

  
fine mesh of metal particles by deposition at

  
plasma jet or the metallic wire forming the surface in contact with the membrane can be made coarser by a controlled chemical attack to increase the density of the contact points. However, the structure

  
must be sufficiently elastic to ensure a uniform distribution of the contacts over the entire surface of the membrane after the elastic reaction pressure exerted by the flexible mat on the electrodes is uniformly distributed over all the contact points.

  
The electrical contact at the interface between the electrodes and the membrane can be improved by increasing the density of the ion exchange functional groups or by reducing the equivalent weight of the copolymer on the surface of the membrane in contact with the

  
elastic mat, the interposed grid or the particulate electrode. In this way, the exchange properties

  
of the diaphragm matrix remain unchanged and it

  
It is possible to increase the density of the contact points of the electrodes with the ion transport sites towards the membrane. For example, the membrane can be formed by laminating or laminating one or two thin films, having a thickness of 0.05 to 0.15 mm, of copolymer having an equivalent low weight on the surface (s) of a thinner film. , of the order of 0.15 to 0.6 mm, of a copolymer having a higher equivalent weight or a weight capable of optimizing the ohmic drop and the selectivity of the membrane.

  
Various other process modifications and

  
of the apparatus of the invention can be made without departing from its spirit.

CLAIMS

  
1.- Improved electrolysis cell of the diaphragm type comprising at least one set of an anode and a cathode separated by a diaphragm, characterized in that at least one of the anode and the cathode is an electrode structure comprising a flat coarse metal wire fabric and a fine metal wire fabric having an electrocatalytic surface and comprising at least 4 strands or meshes per cm arranged above
- top of the fabric - of coarse metal wire and in electrical contact with it, the fabric of fine metal wire directly facing the diaphragm.


    

Claims (1)

2.- cell according to claim 1, characterized in. this - the = flat coarse wire fabric --- is thicker and stiffer than the thin wire fabric. 3.- Cell according to claim 1, character- <EMI ID = 46.1> wire mesh or expanded metal. 4.- cell according to claim 1, characterized in that the metal wire fabrics are made of titanium, -nobium, tantalum, iron, iron alloy, stainless steel, copper or nickel. 5. A cell according to claim 1, characterized in that the coarse wire fabric is electrically connected to a support device which supplies electricity to the electrode structure. 6. A cell according to claim 1, characterized in that the electrode structure is the anode of the cell and the metal wire fabrics are made of titanium. 7.- Cell according to claim 6, characterized in that the fine titanium wire fabric is coated with noble metals or catalytically conductive oxides resistant to the anolyte and having a low overvoltage for the anodic reaction. 8. A cell according to claim 6, characterized in that the fine titanium wire fabric is coated with mixed oxides at least of titanium and ruthenium. 9. A cell according to claim 7, characterized in that the fabric of fine titanium wire is spot welded to the fabric of coarse titanium wire. 10.- Structure of foraminated anode for electrolysis cells, characterized in that it essentially consists of a thin wire fabric of a metal whose oxide is semiconductor, coated with an electrocatalytic material and comprising at least 4 strands or meshes per cm disposed above a flat fabric of coarse wire of a metal of which the oxide is semiconductor, which is substantially larger and more rigid than the fabric of coated fine metal wire and in electrical contact therewith, the more rigid coarse metal wire fabric being mechanically and electrically connected to a support device which supplies the anode with current. 11.- electrode structure according to claim 10, characterized in that the coated fine metal wire fabric is spot welded on the coarser metal wire fabric. 12.- electrode structure according to claim 10, characterized in that the foraminous tissue of coarser wire is a wire mesh or expanded metal. 13.- electrode structure according to claim 10, characterized in that the coated fine metal wire fabric and the coarser metal wire fabric are both made of titanium. 14.- electrode structure according to claim 10, characterized in that the fine metal wire fabric is coated with noble metal or conductive oxides resistant to the anolyte.