Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
  • C25B11/03—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form perforated or foraminous
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/14—Alkali metal compounds
  • C25B1/16—Hydroxides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/49002—Electrical device making

Definitions

• the invention relates to an electrode for electrochemical applications, in particular to an electrode for membrane electrolysis cells made on a metal support. Electrolytic processes carried out in cells separated by ion-exchange membranes are among the most relevant industrial electrochemical applications. Some examples of such applications are the electrolysis of alkali chloride brines (chlor-alkali electrolysis), with particular reference to the electrolysis of sodium chloride brine for the production of chlorine and caustic soda, and the electrolysis of hydrochloric acid solutions.
• the anodic compartment of the electrolysis cell is separated from the cathodic compartment by means of an ion-exchange membrane.
• the anodic compartment of the cell is fed with a sodium chloride brine, for instance at a concentration of about 300 g/l; chlorine evolution takes place on the anode surface, at a current density usually not above 4 kA/m 2 , while brine is consequently depleted down to an outlet concentration usually comprised between 200 and 220 g/l.
• Sodium ions are transported by the electric field across the membrane to the cathodic compartment, where the caustic product is generated at a concentration usually not higher than 33% by weight. The caustic product is then extracted and concentrated by evaporation outside the cell.
• the product gas has the tendency to build-up between the membrane and the electrode surfaces facing the same, increasing the ohmic drop in the contact zone and locally depleting the chloride-ion concentration due to poor electrolyte renewal. Brine dilution favours the local evolution of oxygen with consequent acidification.
• chlorine build-up, oxygen build-up, depletion of trapped brine, acidification accounts for the early deterioration of the membranes, particularly in form of blister generation especially in correspondence of interstitial zones between anode and membrane, leading to voltage increase and electrolysis efficiency decrease.
• a similar deterioration may also take place in the interstitial zones between membrane and cathode: in this case, liquid stagnation leads to an increase in the caustic product concentration, which may reach a value up to 40- 45%.
• Such a high alkalinity can damage the membrane chemical structure, with consequent voltage increase going along with the onset of localised blistering, as described for the anode side.
• US 4,608,144 disclosed an anode surface equipped with vertical parallel channels alternatively directed to brine feed and withdrawal, and further equipped with horizontal channels of lower section reciprocally connecting the feed and withdrawal channels. In this way a forced brine circulation is achieved, somehow preventing the adhesion of chlorine bubbles.
• US 5,1 14,547 discloses an anode aimed at promoting brine circulation at the membrane-anode interface in order to obviate the increase in the electrical resistance associated with the depletion of stagnating brine at the interface by means of a structure consisting of vertical channels connected with slanted secondary channels disposed in a herringbone pattern.
• US 2006/0042935 addresses the same problem by providing an irregular anode surface obtained by sandblasting or acid etching in order to improve the brine supply to the anode. While all of the proposed measures might contribute to some extent to prevent deterioration of ion-exchange membranes in the usual process conditions, they fail to guarantee an optimal functioning in the exasperated process conditions needed to meet the current market requirements aimed at a higher cell productivity.
• One embodiment provides an electrode obtained on a metal substrate having a multiplicity of locally parallel grooves with a depth of 0.005 to 0.02 mm and a pitch - defined as the distance between adjacent grooves - of 0.01 to 0.5 mm.
• locally parallel grooves it is hereby intended a multiplicity of grooves, of open or closed shape, running in parallel at least in part of their length; the path of the locally parallel grooves may assume a generally parallel trend across the whole electrode structure, in straight lines or with curvatures of any type.
• the electrode surface presents locally parallel grooves having a closed shape and intersecting one another reciprocally.
• the electrode as hereinbefore defined can be advantageous in any electrolytic application, especially for working in direct contact with an ion-exchange membrane; in the case of chlor-alkali electrolysis, the above electrode can be assembled with its grooved surface in direct contact with the membrane, with surprisingly advantageous results both used as the anode and/or as the cathode.
• the metal substrate may be made of different materials, including but not limited to titanium and titanium alloys for anode application and nickel, nickel alloys and stainless steels for cathode application.
• the substrate geometry can be of any type: as a non limiting example, the grooved surface can be provided on punched or expanded sheets, meshes and structures comprised of parallel strips optionally rotated along the horizontal axis, also called louvered electrodes.
• the electrode substrate can be provided with a known catalytic coating on its grooved surface: for instance, when use as anode for chlorine evolution in chlor-alkali cells is intended, the electrode substrate may be provided with a coating based on noble metals or oxides thereof. Electrodes obtained on the substrate as hereinbefore defined can be particularly useful in chlor-alkali electrolysis cells, both as anodes for chlorine evolution and as cathodes for hydrogen evolution, especially when assembled with the grooved surface in direct contact with the membrane. In case of straight grooves running parallel across the whole structure, orienting the grooves in the vertical direction can provide an improved circulation of electrolyte and gas- bubble release from the surface.
• Life-tests were also carried out with excellent results at anolyte concentrations below 200 g/l (in particular down to 150 g/l), with caustic product concentrations above 33% (in particular up to 37%) and maintaining pressure differentials across the two compartments higher than 3000 Pa (in particular up to 10000 Pa), conditions which normally led to a quick deterioration of the membranes when prior art electrodes were employed.
• the electrode obtained on a grooved substrate as defined allows a particularly efficient release of the gas bubbles, also in comparison with grooved electrodes of the prior art, possibly because the densely packed and shallow grooves favour capillary transport phenomena as opposed to an electrolyte circulation.
• the electrode as defined can be obtained by simple and cheap methods such as a superficial erosion carried out by means of abrasive paper or fabric - optionally in a continuous rolling process - lamellar grinding wheels or grindstones; other techniques include the use of draw-benches or rolling mills, besides more sophisticated technologies such as laser etching or lithographic techniques, according to the selected geometry.
• the erosion by grindstone for instance can be suitable for obtaining locally parallel grooves of closed shape and intersecting one another, while a lamellar grinding wheel, a draw-bench or a rolling mill can be more suitable for obtaining generally parallel grooves along the whole surface.
• An electrode obtained with the above mentioned techniques can allow a sensible cost reduction compared to other grooved electrodes known in the art and characterised by a much higher groove depth, which cannot be obtained by simple abrasion.
• All samples prepared in the previous example were cut into 150 mm x 200 mm wide pieces and characterised, coupled in various combinations, in a multiple bench for chlor-alkali electrolysis accelerated lifetime tests.
• Each station of the multiple bench was equipped with one membrane electrolysis cell suitable for accommodating one anode and one cathode of 1 mm thickness in direct contact with a reference sulphonic/carboxylic double layer membrane (Nafion ® 982 produced by DuPont, U.S.A.).
• the electrode samples of tables 1 and 2 were assembled with vertically oriented grooves.
• the lifetime test was carried out simultaneously starting-up all cells with the various combinations of anodes and cathodes at process conditions much more severe than the common industrial practice, determining the time of ion- exchange membrane decay, defined as the time required for the cell voltage to increase by 0.5 V with respect to the initial value at the process current density.
• An electrolysis cell as in example 2, equipped with an anode sample A4 and a cathode sample C2, and a second analogous electrolysis cell equipped with a non- grooved anode sample AO and a non-grooved cathode sample CO were subjected to a lifetime test at process conditions sensibly more severe than the common industrial practice.
• the cell equipped with electrode samples AO and CO had to be shut down because the progressive deterioration of the membrane had caused a strong increase in the cell voltage, which attained high values strongly fluctuating in time.
• the cell disassembly evidenced a general formation of blisters on the surface, with a higher population in correspondence of the brine exhaust outlet nozzle, where an incipient local delamination of the two layers of the membrane could also be observed.
• the cell equipped with anode A4 and cathode C2 was dismantled after 2400 hours of continuous testing at practically constant voltage. Upon disassembling the cell, no particular phenomenon of membrane deterioration was observed.

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• Chemical & Material Sciences (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Metals (AREA)
• Secondary Cells (AREA)
• Primary Cells (AREA)
• Manufacture Of Macromolecular Shaped Articles (AREA)

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