WO2008006909A2

WO2008006909A2 - Chlor-alkali electrolyser equipped with oxygen-diffusion cathode

Info

Publication number: WO2008006909A2
Application number: PCT/EP2007/057279
Authority: WO
Inventors: Giuseppe Faita; Angelo Ottaviani; Fulvio Federico
Original assignee: Uhdenora S.P.A.
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2008-01-17
Also published as: ITMI20061374A1; WO2008006909A3; JP2009543945A; JP5160542B2

Abstract

The invention relates to an electrolytic cell (1) subdivided into two compartments by a cation-exchange membrane (2), the cathodic compartment (3) containing an oxygen- diffusion cathode (5) contacting the membrane through a catalysed hydrophilic porous layer and the anodic compartment (4) comprising one anode (6) provided with a catalytic coating for chlorine evolution spaced apart from the membrane (8). The cell of the invention produces chlorine having a reduced oxygen content and a caustic product of suitable concentration with no need of diluting the brine feed or humidifying the oxygen flow.

Description

CHLOR-ALKALI ELECTROLYSER EQUIPPED WITH OXYGEN-DIFFUSION CATHODE

BACKGROUND OF THE INVENTION

Chlorine and caustic soda, two of the most important products of basic petrochemical industry, are obtained by electrolysis of sodium chloride solutions according to the three technologies making use of mercury cathode, porous diaphragm and cation-exchange membrane-type electrolysers. The success of the latter technology was favoured by the development of perfluorinated-type cation- exchange membranes containing sulphonic ion groups on the anode side and carboxylic ion groups on the cathode side, commercialised by several manufacturers such as DuPont/USA under the trade-mark Nafion^®, Asahi Glass/Japan under the trade-mark Flemion^® and Asahi Kasei/Japan under the trade-mark Aciplex^®.

The same process is applied, albeit in a reduced number of plants, to potassium chloride electrolysis and even less frequently to different alkali chloride solutions (brine in the following). For the sake of simplicity, reference will be made hereafter to chlorine-caustic soda electrolysis, even if it is understood that the various considerations will be substantially applicable to chlor-alkali electrolysis in general. The development of progressively more efficient cation-exchange membranes in terms of ohmic drop, current efficiency and maximum allowable current density went along with the simultaneous evolution of the electrolyser mechanical design: at present, the typical operative conditions comprise cell voltages of 2.9 - 3.1 V, current efficiency around 95 - 98%, current density of 4000 - 6000 A/m² and membrane lifetime of at least 3 years. This framework corresponds to an average consumption of about 2300 kWh/tonne of chlorine, which constitutes a heavy penalty at the present cost of electrical energy. Since further significant improvements of the current technology are not foreseeable, several engineering companies in the field of chlor-alkali electrolysis have been involved in deeply innovative alternative processes, potentially capable of substantially reducing the electrical energy consumption per tonne of product. A first possibility provides the integration of chlor-alkali plants with fuel cell stacks which, taking advantage of hydrogen evolved in the electrolysis, usually considered as a by-product, generate electrical energy to be sent back to the electrolysers with an overall energy saving of about 35%. This kind of integration, with a broad discussion of the interconnection modes of the different fuel cell stacks and of the connection thereof with the electrolysers, is disclosed in US 6,423,203. A second possibility is given by the installation of oxygen-diffusion cathodes in the prior art electrolysers as a replacement of the conventional hydrogen-evolving cathodes. In the context of the present specification the term electrolyser designates a number of equivalent elementary cells assembled in a stack in order to attain the required productive capacity. In the following, for the sake of simplicity, reference will be made to a single elementary cell. The principle whereon the technological evolution introduced by oxygen-diffusion cathodes is based derives from the process depolarisation induced by the different overall reaction which characterises the two electrolytic processes, as indicated hereafter: [traditional process] 2 NaCI + 2 H₂O → Cl₂ + 2 NaOH + H₂ [process with oxygen-diffusion cathode] 2 NaCI + Vi O₂ + H₂O → Cl₂ + 2 NaOH

The two reactions are sensibly different under an energy standpoint, the reaction typical of the depolarised process in particular requiring a substantially reduced amount of energy, with a theoretical saving of 1.23 Volts. Practically, due to unavoidable energy dissipation mechanisms, such as ohmic drops and overvoltage, the achievable cell voltage is 1.9 - 2.1 Volts at current densities of 4000 - 5000 A/m².

The installation of oxygen-diffusion cathodes may be carried out according to two basic mechanical designs, respectively with the cathode directly contacting the membrane (a design known to those skilled in the art as "zero-gap") or with the cathode spaced apart from the membrane by a 1 - 3 mm gap (design known to those skilled in the art as "finite-gap"). In the latter case, the gap may be crossed upwardly by a flow of caustic soda allowing an efficient control of the working temperature and concentration deriving from the mixing of the caustic product with the caustic feed. In another embodiment, a porous planar layer wherethrough the externally fed caustic soda percolates downwards is inserted in the gap. The most important difference between the two embodiments is given by the hydraulic head generated by the caustic soda which turns out to be maximum in the case of upward feed and minimum in the case of caustic soda percolating downwards: considering the poor capability of oxygen-diffusion cathodes to withstand pressure differentials, in the former case it is necessary to subdivide the cell into a certain amount of overlaid internal compartments in order to break the hydraulic head. Such a constructive solution is disclosed in US 5,693,202, while the design with percolating layer is disclosed in WO 03/042430. While the mechanical design required for the finite-gap cell is inevitably complex, the analogous design results very simplified in the case of the zero-gap cell: such type of technology is effectively illustrated for instance in US 4,578,159. In the latter case however, although the cell design is substantially simplified, the working conditions are conversely rather complex. This comes from the fact that, in the absence of an external feed, the concentration of product caustic soda is determined by the amount of water transported across the membrane by aid of the hydrated Na⁺ ion flow and of the natural diffusion between the two solutions of sodium chloride and caustic soda: at the usual water transport rates, the concentration of generated caustic soda is around 35-40%. Such concentrations are not compatible with the commercial membranes, which would suffer a performance decay due to the progressive loss of carboxylic groups. It is also observed that the oxygen concentration in the product chlorine is significantly higher than in the finite-gap cell, in order to overcome these problems, US 6,117,286 suggests humidifying the oxygen feed and/or diluting the brine in the anodic compartment. None of the two measures is entirely satisfactory because it is not possible to uniformly redistribute the oxygen moisture in the product caustic soda, while the dilution of the brine leads to a current efficiency decrease with little or no effect on the level of oxygen concentration in chlorine. Under one aspect the present invention is directed to an electrolytic cell with oxygen-diffusion cathode capable of overcoming the inconveniences of the prior art, in particular to an electrolytic cell requiring no humidification of the oxygen feed or any other form of water injection in the cathodic compartment, and no dilution of the brine in the anodic compartment.

Under another aspect the present invention is directed to an electrolyser comprising a multiplicity of electrolytic cells overcoming the above inconveniences.

DESCRIPTION OF THE INVENTION

The Invention consists of an elementary cell subdivided by an ion-exchange membrane, provided with an oxygen-diffusion cathode in direct contact with the membrane and with an anode comprising a catalytic coating for chlorine evolution kept at a finite distance, preferably not lower than 1 mm, from the membrane. In one embodiment the anode in contact with the membrane is provided with a catalytic coating for chlorine evolution only on the surface opposite the surface contacting the membrane.

The non-activated surface of the anode contacting the membrane may be advantageously provided with notches, which in one embodiment are oriented in the vertical direction.

Alternatively, the non-activated surface of the anode contacting the membrane may consist of a porous hydrophilic and catalytically inert film. In another embodiment, the surface of the anode facing the membrane is provided with a catalytic coating and is kept at a finite distance from the membrane, optionally by interposition of an inert hydrophilic porous layer.

In the cell according to the invention the membrane may be kept in contact with a non catalysed surface of the anodic structure by aid of a pressure differential obtained by setting the pressure in the cathodic compartment at a higher value than the pressure in the anodic compartment. Alternatively, the whole of the anodic structure and the membrane may be spaced apart by a gap occupied by the process brine, the membrane being kept in contact with the oxygen-diffusion cathode by aid of a pressure differential obtained by setting the pressure in the anodic compartment at a higher value than the pressure in the cathodic compartment.

In one embodiment, the oxygen-diffusion cathode of the electrolytic cell of the invention has a porous hydrophobic structure provided with a catalyst for oxygen reduction, further equipped with an external porous conductive hydrophilic layer, also provided with a catalyst for oxygen reduction. The hydrophilic external layer may be physically separated or it may be integral to the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereafter making reference to the following drawings:

- Figure 1 : side-view of a chlor-alkali electrolysis cell of the prior art.

- Figure 2: side-view of an anode of an electrolysis cell of the prior art. - Figure 3: top-view of a section of an anode for electrolysis cell in accordance with a first embodiment of the invention.

- Figure 4: side-view of an anode for electrolysis cell in accordance with a second embodiment of the invention.

- Figure 5: side-view of an anode for electrolysis cell in accordance with a third embodiment of the invention.

- Figure 6: side-view of a chlor-alkali electrolysis cell in accordance with a fourth embodiment of the invention.

- Figure 7: oxygen-diffusion cathode for electrolytic cell in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 is a side-view of a chlor-alkali elementary electrolysis cell of the prior art wherein 1 indicates the cell as a whole, 2 the membrane, preferably a cation- exchange perfluorinated membrane, subdividing the cell into a cathodic compartment 3 and an anodic compartment 4, 5 the oxygen-diffusion cathode, 6 the anode provided with catalytic coating for chlorine evolution, 7 the chlorine bubbles dispersed in the brine, 8 the elastic supports for keeping the cathode in contact with membrane 2. Cell 1 is further provided with nozzles 9 for feeding oxygen or an oxygen-containing gas, 10 for discharging the exhaust oxygen, 11 for extracting the product caustic soda, 13 for releasing the mixture consisting of chlorine and exhaust brine. Membrane 2 is further supported by the anode 6 under the thrust of pressure differential obtained by setting pressure P₂ of the cathodic compartment 3 at a value higher than pressure Pi in the anodic compartment 4. Figure 2 shows magnified detail B of figure 1 wherein the side-view of an anode of the prior art is shown, for instance consisting of a titanium expanded sheet whose surface is completely coated with a catalytic film 14 for chlorine evolution.

When the cell of figure 1 equipped with the anode of figure 2 is fed with oxygen or with an oxygen-containing gas in the cathodic compartment and with a sodium chloride brine in the anodic compartment, chlorine is produced in the anodic compartment and caustic soda in the cathodic compartment: in particular, the caustic soda generated at the membrane-cathode interface percolates across the cathode porous structure and is extracted at the bottom of the cathodic compartment which is essentially occupied by the gaseous phase alone. If the concentration of the brine in the anodic compartment is maintained in the conventional range of 180 - 220 g/l, it can be noticed that the oxygen concentration in chlorine is significantly higher than the value of 1.5 - 2% commonly observed in industrial plants, the current efficiency is lower than 94 - 95% and the cell voltage quickly rises to unacceptable values. This negative behaviour can be correlated to the high concentration of product caustic soda largely exceeding 35%: such high concentrations cause a sensible back-migration toward the anode and the release of carboxylic groups from the membrane, which in this way progressively loses the cationic conductivity required for its correct functioning. The caustic soda emerging on the membrane anodic surface comes in direct in contact with the anode catalytic coating, causing its immediate conversion to oxygen according to the following reaction: 4 OH^" → O₂ + 2 H₂O According to the prior art, for instance as disclosed in US 6,117,286, the above inconveniences may be overcome by humidifying the oxygen fed to the cathodic compartment and by diluting the brine in the anodic compartment down to a concentration of 150 - 170 g/l: with these measures, the concentration of product caustic soda is reduced to 33 - 35% extending the membrane lifetime and decreasing the oxygen content in chlorine. Nevertheless the current efficiency remains unsatisfactory and the overall process is difficult to control due to local variations in the concentration of caustic soda, also contributing to membrane deterioration. By an appropriate test campaign, the inventors have determined that the above seen inconveniences can be overcome making use of suitable anode structures coupled to an appropriate oxygen-diffusion cathode design. A first embodiment of the invention as shown in figure 3 is given by an anode whose catalytic coating 14 for chlorine evolution is applied only to the surface opposite the one contacting the membrane: the catalytic coating-free anode surface may be advantageously provided with notches 15, for instance grooves, preferably oriented in the vertical direction. This embodiment leads to a content of oxygen in chlorine below 1.5% and in the most favourable cases below 1 %. At the same time, even after a prolonged operation, the membrane results free from damages such as the release of carboxylic groups or the delamination of the carboxylic and sulphonic layers, keeping the main operative parameters practically constant. The excellent preservation of the membrane is likely due to the concentration of caustic soda which surprisingly turned out to be comprised between 30 and 34%, even with a 180 - 220 g/l brine concentration in the anodic compartment as commonly employed in the industrial electrolysers of the prior art. Without wishing to be bound to any particular theory, it may be supposed that this result of great practical interest could be associated to the higher fraction of membrane surface accessible by or in contact with the brine, going along with a higher diffusion of water across the membrane. In the prior art, a sensible fraction of membrane surface corresponding to the area directly contacting the anode is likely to be blinded by chlorine and oxygen gas initially evolved from the film of solution seeping in-between the contacting anode and membrane surfaces. Similar positive results were obtained making use of the cell of figure 1 equipped with the anode of figure 4, provided with a catalytic coating only on the surface opposite the one facing the membrane and characterised by being further provided with an inert, hydrophilic and non conductive coating 16, whose porosity can be optionally adjusted by adding compounds made soluble by suitable pre- treatments, optionally carried out directly in the cell. As an alternative, coatings in form of hydrophilic films characterised by high surface roughness, expressed as maximum peak height (R_m) of at least 50 micrometres, prove particularly advantageous. Suitable films comprise titanium dioxide, zirconium dioxide, niobium oxide and mixtures thereof, obtainable by the known methods of thermal decomposition of paints containing appropriate precursors or by thermal spraying, for instance flame-spray or plasma-spray. A similar solution is illustrated in figure 5, wherein the anode in accordance with the invention has a composite structure consisting of the anode itself, optionally catalysed on the whole surface, and of a layer 17 formed for instance by a mesh of inert hydrophilic material, for example a titanium mesh free of catalytic coating and having a reduced abutment surface with the membrane (a high expansion factor is for instance preferred when expanded sheets are employed for this purpose). A high degree of hydrophilicity of the porous film is preferred in order to prevent the anode gas bubbles from adhering to the anode-membrane interface. It was finally found that satisfactory results are obtained with the embodiment illustrated in figure 6: in this case the anode 6 is spaced apart from the membrane by a gap 18 filled with the brine, while the cathode 5 is fixed on stiff supports 19. In this case membrane 2 is pushed by the pressure differential obtained by setting pressure Pi of the anodic compartment at a higher value than pressure P2 of the cathodic compartment. Such a constructive solution, although as adequate as the previous ones in terms of oxygen concentration in chlorine and long-term stability of membrane performances, presents the drawback of allowing brine and chlorine to penetrate into the cathodic compartment in case the membrane is punched, with consequent problems of corrosion of the construction materials, moreover lessening the commercial value of product caustic soda due to increased chloride and hypochlorite level.

The oxygen-diffusion cathode of elementary cells of figures 1 and 6 preferably consists of a porous layer provided with a catalyst and with additives directed to impart a predetermined ratio of hydrophilicity to hydrophobicity as necessary to allow both the passage of product caustic soda at the membrane interface (hydrophilic pores) and the flow of oxygen (hydrophobic pores). With this structure, the product caustic soda is discharged to the cathode back-wall and percolates to the bottom of the cathodic compartment: to decrease the possibility that caustic soda may flood also the hydrophobic pores after some time, US 6,117,286 disclosed the insertion of a hydrophilic layer of appropriate porosity between membrane and cathode, whose purpose is to allow the percolation of product caustic soda, diminishing if not eliminating the passage thereof across the cathode. However, the inventors observed that cells equipped with this type of cathodic structure exhibit a pretty fast membrane decay, which is proportional to the frequency of cell shut-downs. These problems are likely associated to a massive lack of uniformity in the current density which takes place during the startup phase after a prolonged shut-down. In order to improve this situation a common practice of the prior art, as disclosed for instance in US 4,578,159, is to fill the cathodic compartment with caustic soda and to drain it just before the start-up, with the apparent purpose of flooding the hydrophilic layer. It is apparent to those skilled in the art that these filling and drainage operations are definitely not recommendable as routine procedures on industrial plants. The inventors observed that this problem is completely overcome when the hydrophilic layer is electrically conductive and is also catalytic versus oxygen reduction, which can be obtained by adding the same catalyst of the cathode in similar amounts. In an another embodiment, the catalytic hydrophilic layer is integral to the cathode. The cathode structure according to the invention is depicted in figure 7 which represents a magnification of detail A in figure 1 , wherein 20 indicates the cathode, pressed against membrane 2 by the current distributor 21 fixed on elastic supports 22, and 23 the conductive and catalytic hydrophilic layer. A suitable porosity of the hydrophilic layer allows discharging the product caustic soda by percolation to the bottom of the cathodic compartment: the cathode may therefore be substantially hydrophobic so as to ensure an optimal oxygen transfer to the catalyst particles. Without wishing to be bound to a particular theory, the inventors presume that the presence of catalyst in the hydrophilic layer allows maintaining a homogeneous current distribution also in the critical phase of cell start-up, in which the hydrophilic layer is not yet filled with caustic soda, drained at the time of the previous shut-down. In the course of the start-up, the product caustic soda fills the porosity of the hydrophilic layer and percolates to the bottom part of the cathodic compartment. The catalyst of the hydrophilic layer turns out to be completed flooded by the product caustic soda and stops functioning (start-up catalyst) since the oxygen diffusion is practically blocked: at this stage, the prosecution of the electrolysis is made possible by the intervention of the catalyst contained in the hydrophobic cathode (operation catalyst). The presence of catalyst in the hydrophilic layer is therefore essential, since during the start-up it allows preventing harmful current density inhomogeneities (with the relevant lack of uniformity in the caustic soda concentration and possible hydrogen generation which may lead to formation of flammable mixtures) without having to resort to procedures not compatible with the normal operation of industrial plants.

EXAMPLE

For the testing activity reported in the following, 100 cm high and 10 cm wide chlor-alkali cells were employed, each subdivided into an anodic and a cathodic compartment respectively made of titanium and nickel by means of a Flemion^® 893 perfluohnated cation-exchange membrane commercialised by Asahi Glass (Japan). The pressure of the two compartments was adjusted so as to maintain a differential of 200 mm of water capable of keeping the membrane pressed against the anodic structure. The anodes were welded to stiff supports and configured as follows: - Test 1 : 1 mm thick titanium expanded sheet with rhomboidal meshes (4 x 8 mm diagonals) provided with catalytic coating for chlorine evolution comprising titanium, iridium and ruthenium oxides according to the prior art, only applied to the surface opposite the one facing the membrane, obtained by deposition of the coating on one side of a solid sheet, followed by mechanical expansion and final flattening.

- Test 2: expanded sheet as in test 1 , added with 5.5 mm high and wide notches, vertically oriented according to the embodiment shown in figure 3.

- Test 3: expanded sheet as in test 1 , added with a hydrophilic inert film of high superficial roughness consisting of about 500 micrometres of zirconium dioxide

(Alfa Aesar GmbH, Germany) applied to the surface facing the membrane by plasma-spray, according to the embodiment shown in figure 4.

- Test 4: 1 mm thick titanium expanded sheet with rhomboidal meshes (4 x 8 mm diagonals) provided with catalytic coating for chlorine evolution comprising titanium, iridium and ruthenium oxides applied to the whole surface, with an additional second titanium expanded sheet free of catalytic coating interposed between anode and membrane. The second expanded sheet with rhomboidal meshes of 6 and 10 mm diagonals was characterised by a reduced abutment surface with the membrane with respect to the one of the catalysed expanded sheet.

- Test 5 (comparative): 1 mm thick titanium expanded sheet with rhomboidal meshes (4 x 8 mm diagonals) provided with catalytic coating for chlorine evolution comprising titanium, iridium and ruthenium oxides applied to the whole surface in accordance with the prior art.

In every cell, the cathode consisted of an 80 mesh net made out of a silver thread (0.2 mm diameter) with a layer of catalyst particles applied thereto suitable for oxygen reduction (20% by weight silver-platinum alloy on Shawiningan Acetylene Black carbon produced by Chevron Chemical Co./USA with a total silver loading of 50 g/m²) mixed with polytetrafluoroethylene particles in a 1 :1 weight ratio; the whole assembly was sintered at 350⁰C leading to a final thickness of about 0.5 mm. The structure of the cathode so obtained resulted porous and clearly hydrophobic as indicated by contact angle determinations with water droplet. A porous conductive and hydrophilic layer, suitable for allowing the catalytic reduction of oxygen in the early stages of start-up and the percolation of product caustic soda, was interposed between cathode and membrane. This layer was obtained starting from an open cell foam of polyurethane, nickel-plated and further coated with a 5 micrometre silver layer, with an average pore diameter of about 0.2 mm and an initial thickness of 2 mm, in whose meshes a mixture of catalyst particles and zirconium oxide particles (Alfa Aesar GmbH, Germany) was pressed at a 1 :1 weight ratio for a total silver loading of 40 g/m², followed by a final compression to reduce the thickness to a final value of about 1 mm. It was observed that such a layer, characterised by a high hydrophilicity, allowed percolation of caustic soda produced at current densities comprised between 2000 and 5000 kA/m² without generating any harmful overpressure. It was demonstrated that similar results are obtainable with cathodes with the above disclosed catalytic hydrophilic layer integral thereto.

The current distributor consisted of a 1 mm thick nickel expanded sheet with rhomboidal openings (4 x 8 mm diagonals), fixed to flexible supports, with an additional fine mesh nickel expanded sheet (2 x 4 mm diagonals) welded to the surface facing the cathode. Both of the expanded sheets were provided with a silver coating about 10 micrometre thick, and were subdivided into four portions in order to favour a better adaptation of the cathode-hydrophilic layer assembly to the anode supported-membrane surface. The anodic and cathodic compartments of all cells were respectively fed with sodium chloride brine whose concentration was kept within the range of 190 - 210 g/l and with dry pure oxygen with an about 10% excess. Temperature and current density were respectively set at 86 - 88°C and at 4000 A/m². The product caustic soda was extracted from the bottom of the cathodic compartments. The results obtained are collected in the following table. TABLE

After tests 1 , 2, 3 and 4, all membranes appeared well preserved and no damage could be evidenced by a visual inspection. Conversely, the membrane extracted from the cell of test 5 presented several blisters of a few millimetre size showing a delamination of the carboxylic and sulphonic layers of the membrane. The above tests shows that by operating in accordance with the invention, with the anodic surface provided with a catalytic film for chlorine evolution spaced apart from the facing membrane surface and with the cathodic structure contacting the membrane comprised of the hydrophobic catalysed cathode and the interposed catalysed hydrophilic layer (optionally integrated in a single assembly) it is possible to obtain a time-constant operation in terms of cell voltage, current efficiency and oxygen content in chlorine with an increased membrane lifetime, even operating with the conventional brine concentrations in the anodic compartment and in the absence of any oxygen humidification or pre-wetting of the cathodic structure with caustic soda.

Test 4 was repeated, the only difference being the use of a carbon cloth made hydrophilic (Zoltek PWB - 3 boiled in nitric acid) as the interposed catalyst-free conductive layer for oxygen reduction. When the cell was started with the dry cloth an abnormally prolonged period of instability was observed, with wide voltage oscillations associated with a performance decay that could not be completely recovered at a later stage. Conversely, when the cell cathodic compartment was previously filled with caustic soda and then drained in order to dampen the cloth, the start-up proved very simple and completely equivalent to that of tests 1 - 5. Test 5 was repeated, the only variation being the dilution of the sodium chloride solution in the anodic compartment to 160 - 170 g/l and the pre-humidification at 85°C of the oxygen feed. With these operative conditions, a sensibly more stable functioning was observed, presumably associated to the lower concentration of product caustic soda (33 - 34%).

Nevertheless it is apparent to those skilled in the art that the operation of an industrial plant which needs wetting the cathodic hydrophilic layer prior to each start-up and/or diluting the sodium chloride solution and/or humidifying the oxygen feed results scarcely practical and certainly not welcome by the operative staff.

The previous description shall not be intended as limiting the invention, which may be practised according to different embodiments without departing from the scopes thereof, and whose extent is exclusively defined by the appended claims. Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other additives, components, integers or steps. The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

Claims

1. Chlor-alkali electrolysis cell subdivided by a perfluorinated cation-exchange membrane into a cathodic compartment and an anodic compartment, the cathodic compartment containing an oxygen-diffusion cathode in contact with the membrane and means for feeding and extracting oxygen, the anodic compartment fed with an alkali chloride solution containing an anode provided with catalytic coating for chlorine evolution, characterised in that the catalytic coating for oxygen evolution is kept at a distance not lower than 1 mm from the facing membrane surface.

2. Cell according to claim 1 characterised in that said distance is not higher than 3 mm.

3. Cell according to claim 1 or 2 characterised in that a first surface of the anode is in contact with the membrane and the catalytic coating for chlorine evolution is applied only to a second surface of the anode opposite the one facing the membrane.

4. Cell according to claim 3 characterised in that said first surface of the anode in contact with the membrane is provided with notches having a vertical orientation.

5. Cell according to claim 1 or 2 characterised in that the anode comprises a porous catalytically inert hydrophilic film in contact with the membrane.

6. Cell according to claim 5 characterised in that said porous catalytically inert hydrophilic film is physically distinct from the anode.

7. Cell according to claim 5 or 6 characterised in that said hydrophilic film has a surface roughness expressed in terms of maximum peak height R_m not lower than 50 micrometres.

8. Cell according to any one of claims 1 to 7 characterised in that the pressure in the cathodic compartment is higher than that in the anodic compartment.

9. Cell according to claim 1 or 2 characterised in that the anode and the membrane are separated by a gap occupied by the alkali chloride solution.

10. Cell according to claim 9 characterised in that in that the pressure in the anodic compartment is higher than that in the cathodic compartment.

11. Cell according to any one of the preceding claims characterised in that the oxygen-diffusion cathode comprises a first conductive hydrophobic porous structure and a second conductive hydrophilic porous structure, said first hydrophobic structure and said second hydrophilic structure being provided with a catalyst for oxygen reduction, said second hydrophilic structure being in contact with the membrane.

12. Cell according to claim 11 characterised in that said first hydrophobic structure and said second hydrophilic structure are physically distinct.

13. Electrolyser for a chlor-alkali process characterised in that it comprises a multiplicity of cells according to any one of the preceding claims.

14. Process of chlor-alkali electrolysis characterised by feeding the electrolyser according to claim 13 with oxygen, pure or in admixture, in the cathodic compartment and with an alkali chloride solution in the anodic compartment, applying an electrical current so as to obtain a caustic product at a concentration comprised between 31 and 34% by weight.