ELECTROLYSIS CELL PROVIDED WITH GAS DIFFUSION ELECTRODES Several industrial processes are carried out in electrochemical cells, such as for example chlor-alkali electrolysis for producing chlorine gas and caustic soda or potash, water electrolysis mainly for producing hydrogen, salt splitting for obtaining the corresponding bases and acids, e. g. caustic soda and sulphuric acid from sodium sulphate, deposition of metals, such as zinc and copper primarily. The intrinsic problem affecting all these processes is the electrical energy consumption which usually constitutes a substantial portion of the total production cost. As the cost of electric energy tends to constantly increase in all geographical areas, the importance of decreasing the electric energy consumption for the above mentioned processes is clearly apparent. The electric energy consumption in an electrochemical process depends mainly on the cell voltage ; it is therefore immediately evident why so many efforts are directed to improving the cell design by resorting to more catalytic electrodes'and by decreasing the ohmic losses in the cell structure and in the electrolytes, for example by reducing the interelectrodic gap.

In the following description reference is made to the chlor-alkali electrolysis which attains undoubtedly the largest industrial relevance but it is to be understood that the discussion of the state of the art and of the improvements disclosed by the present invention certainly applies also to the other electrochemical processes.

In the case of conventional chlor-alkali electrolysis, a solution of sodium chloride, less frequently of potassium chloride, is fed to a cell containing an anode, where chlorine gas is evolved, while hydrogen is evolved at the cathode with the concurrent formation of sodium hydroxide (potassium hydroxide in the case of a potassium chloride feeding). In the most advanced type of cell, the caustic soda which is present close to the cathode is kept apart from the sodium chloride solution present in the anodic side by means of a cationic membrane made of a perfluorinated polymer containing anionic groups, for example sulphonic and/or carboxylic groups. These membranes are commercialised by various companies, such as DuPont/USA, Asahi Glass and Asahi Chemical/Japan.

The design of this type of cell has been thoroughly studied and it can be said that the technology has reached today the optimum state as regards the energy consumption. An example of such a design is given by international patent application No. WO 98/55670.

An analysis of the production cost of chlorine and caustic soda obtained with these advanced types of cells shows however that the impact of the energy consumption is still remarkable. This recognition generated a series of suggestions for a further improvement, whose common element is the use of a gas diffusion electrode, specifically a cathode fed with oxygen (as such or as enriched air, or simply as air deprived of its carbon dioxide content) to replace the hydrogen evolving cathode used in the previously discussed prior art.

A chlor-alkali electrolysis cell comprising a cathode fed with an oxygen- containing gas presents an intrinsically much lower energy consumption than that typical of the conventional prior art. The reason for this is primarily thermodynamic, as the two cells, the conventional one and that containing the oxygen cathode, are characterised by different overall reactions : -Conventional cell 2NaCl + 2H20 2NaOH + C12 + H2 -Oxygen cathode cell 2NaCl + H20 + 1/202 2NaOH + C12 In the practice, the cell voltage of a conventional cationic membrane cell operated at a current density of 4 k} Vm2, is about 3 Volts, while that of a cell equipped with a cationic membrane and oxygen cathode, operating at the same conditions, is about 2-2.2 Volts. As it can be seen, the resulting energy saving is about 30% (the loss of hydrogen production, which is usually employed as a fuel, is of secondary importance). So far, however, cells equipped with oxygen cathodes have found no industrial application.

The reason for this situation resides in the structure of the oxygen cathode and in the requirements that are imposed to the operating conditions to ensure a good efficiency of the cathode. The oxygen cathode is substantially made of a porous support, preferably conductive, provided with a microporous layer consisting of electrocatalytic particles mechanically stabilised by a binder able to withstand the operating conditions. The layer may comprise a further film, also containing particles preferably conductive but not electrocatalytic, and a binder. By suitably selecting the particle size and the chemical nature of the binder it is possible to tailor the characteristics of hydrophobicity and hydrophilicity of the cathode. The porous support may be made by a mesh, a variously perforated sheet, carbon/graphite cloths, carbon/graphite paper or sintered materials. An electrode of this type, with the relevant production method, is described in US Patent No. 4,614,575. When an electrode as the one described above is used as the cathode fed with oxygen in chlor-alkali electrolysis, parallel to the cationic membrane, in direct contact or with a narrow gap, e. g. 2-3 mm, the caustic soda produced by reaction of oxygen on the electrocatalytic particles must be somehow discharged to avoid the progressive filling of the layer microporosity. Should a total filling occur, in fact, the oxygen could no more diffuse through the pores to reach the catalytic particles, where the reaction takes place.

The discharge of the product caustic soda may be effected essentially in two ways, either towards the membrane in the case of a cathode parallel and spaced apart from the membrane, or towards the oxygen atmosphere, on the opposite side with respect to the one facing the membrane, in the case of a cathode in contact with the membrane.

In the first case, a liquid film, about 2-3 mm thick as previously said, is formed and is normally circulated upwards (the electrodes are vertically arranged in the cells) both for extracting the caustic soda produced by the cells and for withdrawing the heat naturally produced by the reaction, as well as for maintaining the caustic soda concentration within defined limits, which allow to prolong the ion-exchange membrane active life. This situation establishes a pressure gradient between the caustic soda and the oxygen at the two sides of the cathode, which in fact acts as a separation wall. The gradient may be positive (the caustic soda pressure is higher than the oxygen one) and in this case it is increasing from top to bottom due to the hydraulic head. Conversely, the gradient may be negative (pressure of oxygen higher that the one of the caustic soda) and in this case it is decreasing from top to bottom once again due to the hydraulic head of caustic soda.

The materials nowadays available and the known production methods allow to obtain cathodes able to withstand pressure differentials of the order of 30 cm (intended as water column). As a consequence, for the optimum operation of oxygen cathodes the cells intended for housing the same cannot be higher than 30 cm. With higher cells, cathode flooding occurs with the caustic soda completely filling the porosity in the case of positive differentials while a severe leak of oxygen into the caustic soda is experienced in the case of a negative differentials. This fact is extremely detrimental in the case of large size electrolysis plants, as the total number of cells, each one of small dimensions, should be very high with heavy additional costs for the auxiliary equipment (electrical connections, piping, pumps). It must be considered that the industrial cells of the conventional type, that is equipped with hydrogen evolving cathodes, usually have heights comprised in the range of 1-1.5 metres. To overcome the above inconvenience it is known that a structure could be used wherein the cathode is maintained about 2-3 mm spaced apart from the membrane, the total height being again 1-1.5 meters but the cell being subdivided in a number of sub-units, each one being about 30 cm high. This design involves a remarkable complexity for the piping connecting the various sub-units and in fact a complex operation and a cost non compatible with industrial applications. A different structure is described in US Patent No.

5,693,202. The design foresees a unitary structure of the cell which is equipped with oxygen cathodes divided in strips. The pressure of the oxygen which is fed to each strip is automatically adjusted exploiting the hydraulic head of caustic soda by means of a bubble system. This type of cell overcomes the complexity of the design illustrated above with the splitting in sub-units, even if it appears intrinsically complicated in view of the need of ensuring a hydraulic/pneumatic seal for each strip.

Furthermore, it imposes particular procedures for the shut-down and start- up in order to avoid, in these transient phases, a loss of the pressure compensation due to the lack of oxygen feeding. A further proposal is disclosed in the European Publication EP 0082514 A1, wherein a cathode made of graphite particles coated with a catalyst and a hydrophobic material, packed together to form a layer which fills the gap between the cationic membrane and the cell wall facing the same, is described. The cathodic liquor (in this specific case herein discussed, caustic soda) flows downwards not as a continuous film but rather as drops falling down by gravity, through the open spaces existing among the packed particles, as a consequence of the hydrophobic nature of such particles. The oxygen, fed from the top (preferably) or from the bottom, crosses the layer of particles flowing through the spaces statistically left free by the droplets of liquid.

This type of cathode is negatively affected by the complexity of its installation, which is quite difficult in the vertical position, that is the usual one for operations in industrial cells. Moreover, the capability in terms of current output is relatively small in consideration of the limited catalyst surface area in contact at the same time with the oxygen and the cathodic liquor. As the economical surveys clearly demonstrate that the investment required for constructing an industrial plant becomes acceptable only when the current density is at least 4 kA/m2, the cathode claimed in EP 0082514 A1 is practically not suitable. A modification of the structure claimed in EP 0082514 A1 is disclosed in the US patent no. 4,061,557, which describes an arrangement of anodes and cathodes in the form of wires or rods, preferably having a circular cross-section, superimposed to form a vertical plane having rods or bars of insulating material sandwiched in-between.

The liquid fed to the top of the cell by means of a perforated distributing pipe falls by gravity forming a vertical film wetting the assembly. The lower part of the cell may be supplied with a gas reactant, for example oxygen. In this case the oxygen reacts onto the cathodic surface only after diffusion through the liquid film which soaks the cathodes completely. As thin as the film may be, undoubtedly the diffusion rate is small and consequently the current output is also small, as demonstrated by the data of the specification itself. A design similar to the one illustrated in US patent 4,061,557 is disclosed in the European patent application EP 0150018 A1, which however is directed to a cell optionally containing a diaphragm or ion-exchange membrane interposed between the anode and the cathode, with a falling film of liquid which wets the electrodes, whereon gas evolving reactions take place. Therefore, EP 0150018 A1 is directed to solve not the previously mentioned problems typical of oxygen consuming cathodes, but rather the one associated with the release of gas bubbles produced by the reaction liquid wherein they are formed. In fact, at the beginning of the description of EP 0150018 A1 it is clearly stated that the devices of the invention are intended to solve the drawbacks connected with the withdrawal of gas bubbles from the liquids, that is anodic and cathodic pressure variations, dangerous vibrations for ion-exchange membranes, partial blinding of the electrodes due to gas bubbles adhering thereto and increased ohmic drops, as the electrical conductivity of the electrolytes is obviously decreased by the presence of gas. It is therefore clear that for EP 0150018 A1 the fact that the electrodes are completely covered by a liquid film, even having a variable thickness, is not a main problem as the process considered is the formation of gas bubbles and their withdrawal from the liquid phase and not that of gas diffusion and its consumption on the electrode surface (a problem typical, as aforementioned, of oxygen consuming cathodes).

In the second case of operation with the oxygen cathode in direct contact with the membrane, the only possibility of releasing the caustic soda is towards the atmosphere of oxygen, on the cathode side opposed to the one in contact with the membrane. In this case a series of problems is experienced, as listed here below : -The caustic soda which is forced to flow through the cathode tends to fill the porosity hindering the oxygen diffusion. To avoid this inconvenience the structure of the cathode must be provided with two series of pores, respectively a hydrophobic one, allowing oxygen diffusion, and a hydrophilic one, directed to facilitate the flow of caustic soda. Additionally, in order to further facilitate the release of caustic soda and to minimise the risks of total occlusion of the porosity, it has been proposed to subdivide the cathodes into strips and to interpose a porous element between the membrane and the cathode strips, along whose surface part of the product caustic soda may be released.

-The caustic soda released on the oxygen atmosphere side exhibits a strong tendency to wet the rear surface of the cathode forming a continuous film which again hinders the oxygen diffusion. To prevent this harmful effect, it is necessary that the rear surface of the cathode be strongly hydrophobic, which may decrease the electrical conductivity of the surface with the consequent complications for the electrical contact necessary to feed electric current.

-The concentration of the product caustic soda is necessarily the one generated by the reaction and no control within predetermined limits is possible, as happens instead in the former case of oxygen cathode where forced circulation is applied. The concentration value of the caustic soda generated by the reaction is about 37-45% depending on the quantity of water carried across the membrane, which in its turn depends on the type of membrane and the operating conditions in terms of current density, temperature and concentration of the alkali chloride solution.

The ion-exchange membranes available on the market are irreversibly deteriorated when they come in contact even for relatively short times with caustic soda above 35% concentration. Therefore, it has been suggested to operate the cell having the oxygen cathode in direct contact with the membrane with diluted solutions of alkali chloride, as it is known that the amount of water carried across the membrane increases as the alkali chloride concentration decreases. However, the operation flexibility afforded by this factor is limited, as too low concentrations of alkali chloride lessen the efficiency of the membrane, increase the content of oxygen in the chlorine and may decrease the active lifetime of the anodes. For this reason it has been proposed, as additional measure, to saturate the oxygen with water at temperatures close to that of operation of the cell ; the diffusion of water vapour through the pores of the cathode permits to further decrease the caustic soda concentration towards acceptable values for the membrane. This measure, however, is only partially effective as part of the water vapour is absorbed by the caustic soda released at the rear surface of the cathode. The technology of the cathode in direct contact with the membrane and the above mentioned solutions (dilution of the alkali chloride solution, oxygen saturation with water vapour, cathode strips with interposed porous elements) are described in the patent literature.

It is an object of the invention to provide a chlor-alkali electrolysis cell structure equipped with an oxygen consuming cathode, overcoming the inconveniences of the prior art. In particular, the cell structure of the invention eliminates the problems of the pressure differential, at the same time ensuring an operation stable with time even with cell heights in the range of 1-1.5 m. Additionally, the cell structure of the invention permits to easily maintain the concentration of caustic soda below 35%, with the consequent high efficiency of the membrane in absence of degradation phenomena.

The method for obtaining these results of high industrial interest will be described in the following detailed description of the invention.

The cell of the invention is made of two compartments separated by an ion- exchange membrane and containing an anode and an oxygen cathode respectively. It is known to the experts in the field that industrial plants are actually made of cell stacks, as the one above described, pressed one against the other to form an electrolyser, which may be of the monopolar or bipolar type depending on the way of supplying the electric current thereto.

In the case of chlor-alkali electrolysis, and more specifically chlorine- caustic soda, taken as a particularly representative example but not intended as a limitation, the ion-exchange membrane is of the cationic type, that is capable of selectively transferring the Na+ ions from the anodic compartment containing the sodium chloride solution to the cathodic compartment wherein caustic soda is produced. The membrane is also crossed by a flow of water transported by the sodium ions as a hydration shell and by diffusion as a consequence of the different activities of water in the sodium chloride solution and in caustic soda. The anode is made, as known in the art, by a titanium sheet provided with apertures, for example an expanded or perforated sheet, coated with an electrocatalytic film of mixed oxides of at least one platinum group metal and at least one metal of the so-called valve metals (titanium, zirconium, niobium, tantalum). The oxygen cathode consists of a porous conductive support having electrocatalytic particles stabilised by a binder applied thereto. On the surface opposed to the one facing the membrane a layer may optionally be applied, having distinctly hydrophobic characteristics, consisting of conductive particles and a binder, both having hydrophobic characteristics.

The porous support may be a carbon cloth or paper, optionally graphitised at least partially, or a metallic layer, such as for example a mesh of woven wires, a perforated sheet or an expanded and flattened sheet, a foam, a sintered fabric or a mattress made of wire coils. The metal is preferably silver, but also nickel, stainless steels, nickel alloys, are suitable, in particular if coated by a thin layer of silver deposited by a chemical or electrochemical procedure. Silver is preferred due to its high chemical resistance in caustic soda, even in the presence of oxygen and small traces of oxygen peroxide which is formed as a by-product. Under these conditions silver forms a thin film of conductive oxide which ensures an efficient electrical contact with the conductive particles of the supported layer or layers. Further, as silver oxide is highly protective, there is no release of metal in the form of ions which could poison the membrane.

Conversely, nickel gets covered by an oxide which is less conductive and releases nickel ions which are adsorbed by the membrane, as well known to the experts of the art. As a consequence the performances of the cell result at least partially spoilt when the porous support of the oxygen cathode is not made of silver or coated with silver. The electrocatalytic particles comprise a catalyst suitable for favouring the oxygen reaction to form alkalinity. Known catalysts are the platinum group metals and above all platinum, oxides thereof, sulphides and more generally chalcogenides, pyrochlores, in particular ruthenium, silver or gold pyrochlores. An interesting analysis of the scientific knowledge in this topic is given in Electrochemical Hydrogen Technologies, edited by H. Wendt, Elsevier, 1990, Chapter 3"Electrocatalysis of the Cathodic Oxygen Reduction", K.

Wiesener, D. Ohms. These catalysts can be used as bulk powders, optionally in admixture, with the addition of graphite powders with the twofold aim of increasing the transversal electrical conductivity of the layer and decreasing the quantity of catalyst so as to reach an optimum compromise between performance and cost. This last goal may be attained also by resorting to catalysts supported on conductive but catalytically inert material, such as carbon optionally partially graphitised, for example those commercialised by Cabot Corp. under the trademark Vulcan XC-72, Shawinigan Acetylene Black (SAB in the following description) well known to the expert of the art. The conductive hydrophobic particles of the hydrophobic layer optionally applied to the cathode are made of low surface area carbon at least partially graphitised, such as for example the aforementioned SAB or equivalent material or graphite. The binder, which must be resistant to caustic soda in the presence of oxygen and peroxide traces, is preferably a fluorinated material, such as polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, <BR> <BR> perfluoroalcoxypolymers, polyvinylidenfluoride, polychlorofluoroethylene and copolymers thereof. It is also possible to use sulphonated fluorinated polymers, such as those commercialised by Du Pont under the trademark Nations : these materials exhibit a lower hydrophobicity due to the presence of ionic groups. Cathodes suitable for consuming oxygen and incorporating the above mentioned materials are described, together with their fabrication method, in the patent literature, for example in US patents 4,614,575 and 5,584,976.

The electric current is supplied to the oxygen cathode by a current distributor made of a conductive sheet, preferably metallic, provided with apertures, for example a mesh of woven wires, perforated sheet, flattened expanded sheet. Preferably, the current distributor consists of a first rigid conductive sheet, with apertures, having a second conductive sheet with smaller apertures superimposed thereto. The metal is silver or stainless steel or nickel, preferably coated with silver. The contact between the oxygen cathode and the current distributor may be provided by mechanical connection or simply by compression. In the latter case, the anode or preferably the cathode itself are fixed to elastic supports allowing the required compression of the anode/membrane/oxygen cathode/current distributor assembly, when the two half-shells of the cell forming the anodic and cathodic compartments are clamped together, for example by means of metal tie-rods or hydraulic jacks. There are two convenient alternatives for the realization of the elastic supports of the cathode : either the current distributor is connected to the wall of the compartment by means of conductive elastic supports, such as springs made of corrosion resistant metal or a resilient conductive layer is placed between the cathode itself and the current distributor which in this case is connected to the wall of the compartment by means of rigid supports. An example of resilient layer, not to be taken as limiting the field of the invention, is a mattress comprising a number of interlaced coils made with a wire of corrosion resistant metal.

The compression exerted as above described is comprised between 1 and 10 metres of water column, preferably between 3 and 5 metres of water column. The peripheral sealing between the various elements is provided by peripheral gaskets, well known in the prior art. Alternatively, instead of relying on the elasticity of the supports, the required compression between the oxygen cathode and the current distributor may be obtained by applying a higher pressure to the anodic compartment than to the cathodic one. The higher anodic pressure pushes the membrane against the oxygen cathode, which in turn is compressed against the current distributor which constitutes in fact the mechanical support of the membrane/oxygen cathode assembly.

The invention foresees that a planar porous element be inserted between the membrane and the oxygen cathode and that a flow of electrolyte, in this specific case caustic soda with a suitable concentration, be fed to the upper portion of the planar element. The form of the planar element may vary to a large extent. For example purpose not intended as a limitation, suitable materials are foams, preferably with open pores, mattresses made of coils of wires, planar meshes made of crossed and overlapped layers of wires, planar meshes of woven wires, wire meshes profiled so as to create surfaces with protruding portions and recessed portions, expanded meshes. The planar element may consist of one of these elements alone or of an overlapping of some of these. The correct choice of the type of planar element (percentage of empty volume, average dimensions of the pore diameter, thickness) and of the caustic soda flow-rate permit to achieve the following results, which allow for completely overcoming the drawbacks of the prior art, and precisely: Equalisation of the pressure of the caustic soda flowing inside the planar element from top to bottom under the gravity force.

Therefore, the pressure differential between the two faces of the cathode, the front one in contact with the planar element and the back one facing the oxygen atmosphere, is small along the whole length of the cathode, with the pressure of the liquid marginally higher than that of the gas (positive differential). It is however possible, by suitably selecting the type of planar element and the caustic soda flow-rate, to obtain negative differentials, that is with the pressure of caustic soda marginally lower than that of the gas, always under the condition that the absolute value of the differential is small, in particular below 30 cm (of water column), preferably below 10 cm. This situation substantially facilitates the operation of the oxygen cathodes, as the low value of the pressure differential prevents an excessive liquid flooding of the cathode structure, thus avoiding the complete blocking of the pores and of the oxygen diffusion, or conversely a substantial release of oxygen in the liquid with a total emptying of the cathode interior with consequent decay of the performances (an extended concurrent contact of catalytic particles, caustic soda and oxygen is important to obtain high current densities). It may be stated that with the device of the invention the oxygen cathode structure is no more critical and practically any type of prior art oxygen cathode is suitable for a regular operation, at least on a hydraulic standpoint.

Control of the concentration of caustic soda in contact with the membrane.

As previously said, the caustic soda formed by the reaction at the cathode has a concentration naturally comprised between a minimum of 37% and a maximum of 45% and results absolutely incompatible with the requirements of modern low ohmic resistance membranes, suitable for high current densities. These membranes have long active lifetimes with a high efficiency of operation only when they are kept in contact with caustic soda at 30-35% max.

This goal is easily obtained according to the present invention with an appropriate flow of suitably concentrated caustic soda flowing from top to bottom through the planar element of the invention (under the gravity force effect). Just as an example, operation may be carried out at a current density of 4 kA/m2 with a caustic soda flow rate of about 60 lihim2 at an input concentration of 32%, obtaining a concentration in correspondence of the lower edge of the planar element of 33.7%, with a longitudinal concentration gradient of only 1.7%. The concentration gradient is moreover scarcely affected by the operating conditions of current density, temperature and concentration of the sodium chloride solution, which obviously permits a simple and reliable operation of the plant. As the control of the caustic soda concentration is carried out according to the invention by feeding the planar element interposed between membrane and cathode, it is no more necessary to saturate the fed oxygen with water, with a remarkable reduction of investment cost and a better energy balance.

A further positive effect, of smaller importance but still useful, is the more reliable regulation of the membrane temperature, due to the possibility of withdrawing heat from the caustic soda flow.

The invention will be now described making reference to the figures, wherein: Fig. 1 shows the structure of the cell of the invention.

Fig. 2 shows a mock-up of the cell of fig. 1 Fig. 3 shows a detail of the structure of the porous planar element consisting of a metallic foam Fig. 4 shows a porous planar element consisting of a mesh obtained by superimposing two layers of crossed wires Fig. 5 shows a porous planar element consisting of a mesh of wires having a pyramidal profile. The wires are illustrated only in detail, for simplicity sake.

The internals of the cell of the invention, previously described, are graphically represented in fig. 1. Cell 1 comprises two shells, anodic 2 and cathodic 7, made of titanium and nickel respectively. The two shells are clamped together by means of tie-rods or hydraulic jacks, not shown in the figure, and include the cationic membrane 16 which separates the anodic and cathodic compartments.

The cationic membrane used in chlor-alkali, and specifically chlorine- caustic soda electrolysis, is made of a perfluorinated polymer on whose backbone sulphonic groups (side facing the anode 3) and carboxylic groups (side facing the cathode 10) are inserted. Membranes of this type, characterised by internal low ohmic drops and capable of operating at high current densities, typically 3-5 kA/m2, are supplied by DuPont/USA, Asahi Glass and Asahi Chemical/Japan under the trademarks Nation@, Flemiono and Aciplexd9 respectively. The peripheral sealing, necessary to avoid that chlorine, oxygen, sodium chloride and caustic soda solutions be released in the surrounding environment, is provided by peripheral gaskets 8. The anodic shell 2 contains the anode 3, made of a titanium sheet with apertures, for example an expanded or perforated sheet, provided with an electrocatalytic coating for chlorine evolution from chlorides, based on platinum group metals or oxides thereof and containing also valve metal oxides, in particular titanium oxide. The anode 3 is fixed to the shell 2 by means of supports 4 which permit also to transmit electric current from the shell wall to the anode. The cathodic current distributor 11, consisting of a metallic sheet with apertures, for example metallic mesh, perforated or expanded sheet, made preferably of silver or alternatively nickel, stainless steel, nickel alloys, coated with silver for an optimal electrical contact, is fixed to the cathodic shell 7 by means of supports 12, which permit an easy current transmission between the shell itself and the distributor surface.

Between the distributor 11 and the membrane 16, the oxygen cathode 10 and the planar element of the invention 9 are inserted on the current distributor side and the membrane side respectively. The pressure necessary to ensure an intimate contact between membrane and planar element, between planar element and oxygen cathode, between oxygen cathode and current distributor, may be mechanically provided if the supports 12 of the current distributor are elastic and undergo a deflection when the shells with the various constitutive elements are clamped together. Alternatively, the required compression may be obtained by maintaining the pressure inside the anodic shell 2 higher than that of the cathodic shell 7. This higher pressure pushes the membrane against the planar element/oxygen cathode/current distributor assembly, whose supports 12 are in this case rigid, thus causing an efficient contact over the whole extension of the various interfaces. During operation, the negative and positive poles of the current generator are respectively connected to the anodic shell 2 and to the cathodic shell 7, the fresh sodium chloride solution is fed to the nozzle 5, the depleted sodium chloride solution and the product chlorine are withdrawn from the nozzle 6, the oxygen- containing gas is fed to the nozzle 14, the fresh caustic soda solution if fed through the distributor 13, for example a perforated pipe, and flows longitudinally from top to bottom through the planar element 9, the product caustic soda solution, consisting of a mixture of fed caustic soda and caustic soda produced at the cathode 10 by the oxygen reaction, is discharged through the nozzle 15 together with the exhaust oxygen- containing gas (indicatively a quantity of oxygen 10-20% higher with respect to that necessary for the reaction, which is a direct function of the total current supplied to the cathode, is fed through nozzle 14).

In the following examples further details on the method of carrying out the invention will be given. Said examples are not to be intended as a limitation of the same, as what described therein may be modified by the experts in the art without departing from the field of protection of the claims. For example, although the cathode fed with a gas containing oxygen has been assumed as the preferred type of gas diffusion electrode in the description of the invention and in the following examples, also anodes-fed with hydrogen may use the device of the invention comprising the porous planar element and the flow of electrolyte.

EXAMPLE 1 The efficiency in discharging liquids of different types of planar elements has been tested using a cell mock-up as illustrated in fig. 2. The mock-up 17 is made of two shells 18 and 19 made of transparent plastic clamped together by means of tie-rods not shown in the figures in order to house a membrane 16 analogous to those used for chlor-alkali electrolysis. The peripheral sealing is provided by peripheral gaskets 20. The shell 19 is hollow in order to form a chamber 25 connected through a nozzle 26 to a compressed air generator. The pressure in chamber 25 is controlled by the meter 27 and is regulated so as to ensure a compression of the membrane 16 equivalent to the one of the electrolysis cell, as previously discussed.

The planar element 9 of the invention is interposed between shell 18 and membrane 16 and is thus pressed by the membrane against the wall of shell 18. In this way a duct for the passage of liquid is formed, whose thickness coincides with that of the planar element. The available cross section for the passage of liquid is that of the duct less the encumbrance of the planar element. The shell 18 is provided with nozzles 21 and 23 for feeding and withdrawing the liquid, having a diameter sufficient for a passage of the channel type, and 22 to ensure that the chamber 28a remain at atmospheric pressure.

The two upper and lower chambers 28a and 29a each contain a support element, 28b e 29b respectively, to support the membrane 16 and avoid deformation of the same, preventing the free passage of liquid. The shell 18 is further provided with pressure meters 24 positioned at different levels along the height of the planar element.

An evaluation of various geometries of planar elements has been carried out at ambient temperature, 20-25°C, using water. In the following table the flow-rates of liquid which may be used for each planar element in order to establish 1-2 cm of hydraulic head in chamber 28a maintained at atmospheric pressure through nozzle 22 are indicated. In the case of a planar element with a finer porosity, wherein the capillary forces begin to show their effects, the maximum flow-rates to obtain a hydraulic head not higher than 1-2 cm and the minimum flow-rates to keep the planar element completely flooded, without any air entrapment inside, are reported.

The table shows also the range of pressure values (coinciding with the pressure differential existing in the cell between the two faces of the oxygen cathode) detected by the meters 24 : the positive values indicate the cm of water column overpressure with respect to the atmospheric pressure.

TABLE Water flow-rate, T= 20-25°C, and overpressures detected with various types of planar elements having dimensions of 20 x 120 cm. Type Flow-rate (I/hour) Pressure (cm water) 1 0. 1 not detected 2 3 +3. +5 3 5 not detected 4 5 (min)-20 (maux)-2-+2 5 10 +2. +15 6 35 +1. +2 7 120 +1. +2 200 +1. +2 The types of planar elements in the table are: 1. Carbon cloth PWB-3, produced by Zoltek/USA, made of fibre bundles with a total thickness of 0.3 mm 2. Nickel foam with an average porosity of 100 ppi (pores per inch) and original thickness of 2.2 mm, compressed to a thickness of 1.2 mm.

Material supplied by Nitech/France and Sumitomo/Japan 3. Nickel foam with an average porosity of 100 ppi (pores per inch) and original thickness of 2.2 mm, with a thickness after compression of 1.8 mm 4. Nickel foam with an average porosity of 100 ppi (pores per inch) and original thickness of 2.2 mm, not compressed (fig. 3) 5. Polypropylene mesh made of two layers of wires with a diameter of 1 mm superimposed to form a square mesh with continuous channels on each side. Thickness 1.9 mm, mesh 5 x 5 mm (fig. 4) 6. Polypropylene mesh with wires having a diameter of 0.5 mm, mesh dimensions 1 x 1 mm, machined to form an array of pyramids (fig. 5), thickness under compression 1.4 mm 7. Polypropylene mesh similar to type no. 5, made of two layers of wires with a diameter of 1 mm superimposed to form a rhomboidal mesh (diagonals 5 x 10 mm), with continuous channels on each face.

Thickness 2 mm 8. Mesh as for no. 7, made of high density polyethylene, diagonals 7 x 15 mm, thickness 2 mm The types of tested planar elements are to be intended as an example and innumerable modifications of the structure and dimensions are obviously acceptable. In any case the results reported in the table show that the planar elements allow for a wide range of flow-rates with overpressure (pressure differentials between the two faces of the oxygen cathode) extremely small, thus making the optimisation of the operation in electrolysis cells extremely easy. The construction materials reported herein are absolutely not binding. In fact, several alternatives are represented by various metals and plastic materials, which permit to obtain long operating times without appreciable deterioration by a suitable selection. Although the thickness may be varied within a wide range, 2-3 mm are preferred as an upper limit in order to reduce the ohmic drops associated with the passage of electric current through the liquid layer.

EXAMPLE 2 A test of type no. 6 of the table has been repeated using, always at ambient temperature, a magnesium sulphate solution at 15% instead of water to simulate the effect of the higher viscosity of 32% caustic soda at 80°C (2.5 centipoise).

The resulting flow-rate was 15 I/h, thus showing the laminar nature of the hydraulic regimen which characterises the liquid flow in the planar element.

EXAMPLE 3 A cell with an available cross section of 20 cm (width) x 120 cm (height), likewise that of the simulator of Examples 1 and 2, was assembled according to the scheme of fig. 1 using the following components: Anode made of a 1 mm thick titanium sheet expanded to form rhomboidal apertures (diagonals = 4 mm x 8 mm), partially flattened to eliminate the asperities capable of damaging the membrane and coated with an electrocatalytic film for chlorine evolution made of a mixed oxide of titanium, ruthenium and iridium membrane : Flemion 893 (Asahi Glass/Japan) cathodic current distributor fixed to the wall of the cathodic shell by flexible supports and made of a nickel sheet, expanded to form rhomboidal apertures (diagonals 4 mm x 8 mm), acting as a support for a second fine mesh or expanded sheet, galvanically coated with 10 microns of silver oxygen cathode made of a 80 mesh supporting mesh of nickel wires having a diameter of 0.2 mm, coated with 10 microns of silver having a layer of electrocatalytic particles applied thereto (silver 30% on Vulcan XC-72) mixed with polytetrafluoroethylene particles (ratio 1.1 by weight) on a face and a second layer of hydrophobic carbon particles (SAB) mixed with polytetrafluoroethylene particles (ratio 1.1 by weight) on the other face, both layers being sintered at 330 °C. The resulting thickness was about 0.5 mm planar element, type no. 6 of the table The pressure exerted by the flexible supports of the cathodic current distributor on the anode/membrane/planar element/oxygen cathode assembly was about 300 g/cm2.

The cell was fed with a sodium chloride solution at a concentration of about 200 grams/litre in the anodic compartment and a 32% caustic soda solution.

In order to maintain a hydraulic head of about 1-2 cm on the planar element a flow-rate of 23 Uh of caustic soda was required. To avoid the complication of a regulation, the flow-rate was effectively maintained at about 30 I/h, with the excess of about 7 1/h purged through an overflow device. The cathodic shell was fed also with 99% oxygen (nitrogen to balance) with an excess of 10% with respect to the quantity required by the reaction. With a total current of 1000 Amps, corresponding to a current density of 4 kA/m2, the flow of oxygen was regulated at 250 I/h. After an initial period necessary to reach the operating temperature of 80°C, the cell voltage was stabilised at 2.10 volts, a value which was maintained without appreciable variations over a period of 35 days. At the end of the electrolysis test, the membrane displayed no damages under visual and microscope inspection.

EXAMPLE 4 The test of Example 3 was repeated with a similar cell, with the only difference that the supports of the cathodic current distributor were rigid and not flexible. Upon assembling the cell, the average distance between the anode surfaces and the cathodic current distributor was 3.5 mm. The operating conditions were the same of Example 3, with the additional detail of an anodic pressure of 1.3 bar (the cathodic compartment fed with oxygen was operated at atmospheric pressure). The higher anodic pressure pressed the membrane against the planar element/oxygen cathode/current distributor/rigid supports assembly. The same performances detected for Example 3 were obtained, with a cell voltage of 2.2 Volt stable with time.

EXAMPLE 5 Three cells of the type described in Example 3 were used in an assembly of the filter-press type to form a small electrolyser. The operating conditions of each cell were the same as in Example 3, with quite similar voltages.

The electrical connections were of the bipolar type initially and subsequently of the monopolar type. During the first phase (bipolar connection), the operating voltage of the three cells was 2.15,2.05,2.10 Volts respectively with an overall voltage of the electrolyser of 6.3 Volts at a current density of 1000 Amps. The flow-rate of the oxygen fed to the central cell was then decreased below the value required by the reaction to simulate a condition of anomaly of an industrial electrolyser. The cell voltage of the central cell rapidly increased to 3.5 Volt and in the gas contained in the cathodic compartment increasing quantities of hydrogen were detected. A similar result was obtained by progressively decreasing the flow rate of caustic-soda. Taking into account the rapidity of this phenomenon, it is soon evident that to avoid dangerous situations on a bipolar industrial electrolyser the voltages of the cells must be monitored predetermining threshold values, beyond which a quick shut-down must be carried out.

During the second phase (monopolar connection) an overall voltage of 2.12 Volt was detected in steady state conditions with a total current of 2950 Amps, subdivided in 900,1100,950 Amps for the individual cells.

When decreasing the oxygen or caustic soda flow-rate, a certain increase in the voltage was detected, but much slighter than the one detected with the bipolar connection. For example with an oxygen reduction of 50% in the central cell (and thus with an increase of the oxygen fed to the other two cells), the voltage rose to 2.4 Volts with a current distribution of 1100, 500 e 1350 Amps in the three cells.

This result indicates that an electrolyser made of cells incorporating gas diffusion electrodes of the monopolar type is capable of withstanding anomalies on single cells better than the bipolar type as the current may be re-distributed to the normally operating remaining cells, without creating immediate conditions of risk. This behaviour permits to proceed with schduled shut-downs without the need for emergency shut-downs, resulting in a better conservation of those components more affected by temperature variations, in particular the membranes.