EP2041335B1 - Verfahren zum betrieb einer elektrolytischen membranzelle - Google Patents

Verfahren zum betrieb einer elektrolytischen membranzelle Download PDF

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Publication number
EP2041335B1
EP2041335B1 EP06847451.9A EP06847451A EP2041335B1 EP 2041335 B1 EP2041335 B1 EP 2041335B1 EP 06847451 A EP06847451 A EP 06847451A EP 2041335 B1 EP2041335 B1 EP 2041335B1
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EP
European Patent Office
Prior art keywords
diaphragm
catholyte
alkali metal
fibers
anolyte
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EP06847451.9A
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English (en)
French (fr)
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EP2041335A1 (de
Inventor
Henry W. Schussler
Joseph R. Cimini
Robert A. Fast
Berry G. Garrard
Steven T. Richardson
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Westlake US 2 LLC
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Axiall Ohio Inc
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    • 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
    • C25B13/00—Diaphragms; Spacing elements
    • C25B13/04—Diaphragms; Spacing elements characterised by the material
    • 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
    • C25B15/00—Operating or servicing cells
    • C25B15/02—Process control or regulation

Definitions

  • the present invention relates to electrolytic diaphragm cells for the electrolysis of inorganic materials, and to methods for operating such electrolytic cells.
  • the method relates to minimizing the effect of perforations that occur in the diaphragm of the electrolytic cell, e.g., a chlor-alkali electrolytic cell.
  • the electrolytic cell generally comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and a microporous diaphragm that separates the anolyte compartment from the catholyte compartment.
  • Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
  • a non-limiting example of a diaphragm electrolytic cell is the electrolytic cell that is used for the electrolysis of aqueous alkali metal halide solutions (brine).
  • the diaphragm is generally formed on the cathode and separates an acidic liquid anolyte from an alkaline catholyte liquor.
  • the electrolysis of alkali metal brine generally involves introducing liquid brine into the anolyte compartment of the cell and allowing the brine to percolate through the brine-permeable microporous diaphragm into the catholyte compartment.
  • the microporous diaphragm is sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte
  • halogen gas is evolved at the anode
  • hydrogen gas is evolved at the cathode
  • an aqueous alkali metal hydroxide solution is formed in the catholyte compartment.
  • the halogen produced is chlorine and the alkali metal hydroxide formed is sodium hydroxide.
  • Catholyte liquor comprising alkali metal hydroxide and unconverted brine is removed from the catholyte compartment of the cell.
  • the present invention provides a method of operating a chlor-alkali electrolytic cell as set out in claim 1.
  • dopant materials and/or halogen-containing polymer fibers e.g., fluorocarbon fibers, are introduced into the anolyte compartment in conjunction with the ceramic fibers.
  • ceramic fiber means inorganic, non-metallic fibers comprising one or more of the oxides, nitrides, carbides, borides and silicates of metals or semi-metals that are at least partially resistant to the corrosive conditions within the anolyte compartment of the electrolytic cell into which the ceramic fibers are introduced.
  • the metals and semi-metals include, but are not limited to, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum, titanium, tungsten, silicon, aluminum, boron, iron, cobalt, nickel, copper, zinc, cadmium, cerium, lanthanum, yttrium, calcium, barium, magnesium, beryllium, tin, lead, gallium and germanium.
  • the metals and semi-metals will be chosen from zirconium, titanium, silicon, aluminum, boron, and magnesium.
  • the ceramic fiber can be a synthetic material or a naturally occurring mineral, and in a non-limiting embodiment is nonconductive.
  • chlor-alkali cell or terms of like import means an electrolytic cell for the production of halogen, e.g., chlorine, and alkali metal hydroxide, e.g., sodium hydroxide and potassium hydroxide, by the electrolysis of aqueous alkali metal halide solutions, e.g., sodium chloride brine.
  • halogen e.g., chlorine
  • alkali metal hydroxide e.g., sodium hydroxide and potassium hydroxide
  • aqueous alkali metal halide solutions e.g., sodium chloride brine.
  • the chlor-alkali cell described in this description is a diaphragm electrolytic cell.
  • diaphragm means a microporous, liquid electrolyte permeable material that separates the anolyte compartment from the catholyte compartment of a diaphragm electrolytic cell.
  • the diaphragm may be, but is not limited to, an asbestos-type diaphragm, including the so-called polymer- or resin-modified asbestos diaphragm, e.g., asbestos in combination with polymeric resins such as fluorocarbon resins, or it may be a synthetic diaphragm.
  • electrolytic diaphragm cell or "electrolytic cell” means an electrolytic cell for conducting an electrochemical process wherein an electrolyte is passed through a diaphragm that separates the anolyte and catholyte compartments of the cell.
  • electrolyte is dissociated to synthesize chemical materials, e.g., inorganic materials.
  • the electrolytic cell is a chlor-alkali cell wherein, for example, aqueous sodium chloride brine undergoes electrolysis to produce sodium hydroxide in the catholyte compartment and chlorine gas in the anolyte compartment.
  • perforation means openings, e.g., holes, tears, etc, in the diaphragm through which the anolyte passes, which openings are of a size that cause the concentration of the principal product formed in the catholyte compartment to be reduced to a level below that which is desired, e.g., to a level below that which generally occurs during normal operation of the electrolytic cell.
  • synthetic diaphragm means a diaphragm that is primarily comprised of fibrous organic polymeric materials that are substantially resistant to the internal corrosive conditions present in the electrolytic cell, e.g., a chlor-alkali electrolytic cell, particularly the corrosive environments found in the anolyte and catholyte compartments of the cell.
  • the synthetic diaphragm is substantially free of asbestos, i.e., the synthetic diaphragm contains not more than 5 weight percent of asbestos.
  • the synthetic diaphragm contains not more than 3, e.g., not more than 2 or 1, weight percent of asbestos.
  • the synthetic diaphragm is totally free of asbestos (a non-asbestos-containing diaphragm).
  • the term "at least partially resistant to the corrosive conditions within the anolyte compartment" or a term of like import, as used in reference to the ceramic fiber, means that the ceramic fiber is resistant to chemical and/or physical degradation, e.g., chemical dissolution and/or mechanical erosion, by the conditions within the anolyte compartment for a reasonable period of time. Generally, a reasonable period of time will depend upon and be a function of the cell's operating conditions. In a non-limiting embodiment, a cell treated with ceramic fiber will return to acceptable levels of operation for at least 2 weeks before the addition of further amounts of the ceramic fiber may be required.
  • the cell that has been treated with ceramic fiber will return to acceptable levels of operation for from 2 to 12 weeks or more before the addition of further amounts of ceramic fiber may be required.
  • Acceptable levels of operation are generally the operating conditions that existed for the particular treated cell prior to the event(s) that necessitated addition of the ceramic fiber.
  • dopant material means inorganic particulate material that is applied to the diaphragm, e.g., to the surface of the diaphragm, to regulate the microporosity of the diaphragm.
  • Dopant materials are applied to the diaphragm when it is first prepared, and during operation of the electrolytic cell to adjust the microporosity of the diaphragm.
  • the dopant material includes inorganic particulate material that comprises the topcoat applied to the diaphragm.
  • dopant materials include clay minerals, the oxides of valve metals, e.g., titanium and zirconium, and the oxides and hydroxides of alkaline earth metals, e.g., magnesium.
  • fluorocarbon fiber means fluorine-containing polymeric hydrocarbon fibers, e.g., polytetrafluoroethylene.
  • the fluorocarbon fiber may also contain other halogens, e.g., chlorine, such as polychlorotrifluoroethylene, and can be comprised of a mixture of halogen-containing polymer fibers.
  • the following disclosure is directed specifically to chlor-alkali electrolytic cells; but as one skilled in the art can appreciate, the method of the present invention is also applicable to other diaphragm-containing electrolytic cells that are used for the conducting an electrochemical process.
  • the electrochemical process is used for the electrolysis of inorganic materials, e.g., aqueous inorganic metal salt solution such as sodium chloride brine.
  • electrolytic cells A variety of electrolytic cells (electrolyzers) known to those skilled in the art can be used for the electrolysis of aqueous alkali metal halide solutions.
  • the electrolyzers are monopolar or bipolar cells that contain planar and non-planar electrodes, e.g., cathodes.
  • electrolysis is performed in a plurality of housings comprising a plurality of individual electrolytic cell units wherein a succession of anode units alternate with cathode assemblies.
  • the electrolyzer is a bipolar electrolyzer wherein substantially vertical cathodes are interleaved or positioned within and spaced from substantially vertical anodes.
  • This type of electrode assembly has been referred to as a fingered configuration, e.g., wherein a series of cathode fingers and anode fingers are interleaved with one another.
  • the cathode of a diaphragm electrolytic cell generally comprises a liquid-permeable substrate, e.g., a foraminous metal cathode.
  • the cathode is electroconductive and may be a perforated sheet, a perforated plate, metal mesh, expanded metal mesh, woven screen, an arrangement of metal rods or the like having equivalent openings (nominal diameter) generally in the range of from 0.05 inch (0.13 cm) to 0.125 inch (0.32 cm).
  • the openings in the foraminous metal cathode range from 0.07 inch (0.17 cm) to 0.1 inch (0.25 cm).
  • the cathode is typically fabricated of iron, an iron alloy or some other metal, such as nickel, that is resistant to the corrosive conditions within the operating electrolytic cell environment to which the cathode is exposed, e.g., the corrosive conditions with the anolyte and catholyte compartments of an operating chlor-alkali electrolytic cell.
  • Electrolysis of alkali metal halide brines typically involves charging an aqueous solution of the alkali metal halide salt, e.g., sodium chloride brine, to the anolyte compartment of the cell.
  • the alkali metal halide brine typically contains alkali metal halide in an amount of from 24 to 26 percent by weight.
  • the aqueous brine percolates through the liquid-permeable microporous diaphragm into the catholyte compartment and then is withdrawn from the cell.
  • halogen gas e.g., chlorine
  • hydrogen gas is produced at the cathode.
  • An aqueous solution of alkali metal hydroxide, e.g., sodium hydroxide, is produced in the catholyte compartment from the combination of alkali metal ions with hydroxyl ions.
  • the resultant catholyte liquor which comprises principally alkali metal hydroxide and depleted alkali metal halide brine, is withdrawn from the catholyte compartment.
  • the alkali metal hydroxide product is subsequently separated from the catholyte liquor.
  • asbestos has been the most common diaphragm material used in chlor-alkali electrolytic diaphragm cells for the electrolysis of alkali metal halide brines because of its chemical resistance to the corrosive conditions that exist in such electrolytic cells.
  • Asbestos in combination with various polymeric resins, particularly fluorocarbon resins have been used also as diaphragm materials in such electrolytic cells.
  • Synthetic diaphragms are generally fabricated from fibrous polymeric materials that are resistant to the corrosive conditions present within the electrolytic cell, such as a chlor-alkali cell, e.g., the corrosive environments found in the anolyte and catholyte compartments.
  • the synthetic diaphragm is formed on the foraminous cathode by vacuum depositing (in one or more steps) the materials comprising the diaphragm onto the foraminous cathode substrate from an aqueous slurry of those materials.
  • synthetic diaphragms used in chloralkali electrolytic cells can be prepared using fibrous organic polymers.
  • fibrous organic polymers include, but are not limited to, a polymer, copolymer, graft polymer or combinations of polymers that are substantially chemically resistant to the corrosive conditions in which the diaphragm is employed, e.g., chemically resistant to degradation by exposure to the reactants, products and co-products present in the anolyte and catholyte compartments.
  • Such products include, but are not limited to sodium hydroxide, chlorine and hydrochloric acid.
  • the fibrous organic polymers are halogen-containing polymer fibers.
  • the halogen-containing polymer fiber is a fluorocarbon fiber.
  • Non-limiting examples of halogen-containing polymer fibers include fluorine- and fluorine and chlorine-containing polymers, such as perfluorinated polymers, and chlorine-containing polymers that include fluorine.
  • halogen-containing polymers and copolymers include, but are not limited to, polymers, such as polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene (PTFE), polyperfluoro(ethylenepropylene), polytrifluoroethylene, polyfluoroalkoxyethylene (PFA polymer), polychlorotrifluoroethylene (PCTFE polymer) and the copolymer of chlorotrifluoroethylene and ethylene (CTFE polymer).
  • polymers such as polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene (PTFE), polyperfluoro(ethylenepropylene), polytrifluoroethylene, polyfluoroalkoxyethylene (PFA polymer), polychlorotrifluoroethylene (PCTFE polymer) and the copolymer of chlorotrifluoroethylene and ethylene (CTFE polymer).
  • the synthetic diaphragm is formed from a composition comprising polytetra
  • the synthetic diaphragm generally further comprises ion-exchange materials having cation selective groups thereon, e.g., acid groups.
  • the acid groups include, but are not limited to, sulfonic acid groups, carboxylic acid groups and their derivatives, e.g., esters, phosphonic acid groups, and phosphoric acid groups.
  • the acid group is either a sulfonic acid groups or a carboxylic acid group.
  • the ion-exchange material is a perfluorinated copolymer material prepared from the polymerization of a fluorovinyl ether monomer containing a functional group, e.g., an ion-exchange group or a functional group easily converted into an ion-exchange group, and a monomer chosen from fluorovinyl compounds, such as vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene and perfluoro(alkylvinyl ether), with the alkyl being an alkyl group containing from 1 to 10 carbon atoms.
  • a functional group e.g., an ion-exchange group or a functional group easily converted into an ion-exchange group
  • a monomer chosen from fluorovinyl compounds such as vinyl fluoride, vinylidene fluoride, tri
  • ion-exchange materials can be found in column 5, line 36 through column 6, line 2 of U.S. Patent No. 4,680,101 , which disclosure is incorporated herein by reference.
  • an ion-exchange material with sulfonic acid functionality is used.
  • a perfluorosulfonic acid ion-exchange material (5 weight percent solution) is available from E. I. du Pont de Nemours and Company under the trade name NAFION.
  • Other appropriate halogenated ion-exchange materials that can be used to allow the diaphragm to be wetted by the aqueous brine fed to the anolyte compartment of the electrolytic cell include, for example, the ion-exchange material available from Asahi Glass Company, Ltd. under the trade name FLEMION.
  • Organic polymeric materials in the form of microfibrils are also generally used to prepare synthetic diaphragms.
  • Such microfibrils can be prepared in accordance with the method described in U.S. Patent No. 5,030,403 , the disclosure of such method being incorporated herein by reference.
  • the fibers and microfibrils of the organic polymeric material e.g., PTFE fibers and PTFE microfibrils, generally comprise the predominant portion of the diaphragm solids.
  • the diaphragm generally comprises from 65 to 93 percent by weight combined of such fibers and microfibrils and from 0.5 to 2 percent by weight of the ion-exchange material.
  • the organic fibrous polymers of the synthetic diaphragm are generally used in particulate form, e.g., in the form of particulates or fibers, as is well known in the art.
  • the organic polymer material generally has a fiber length of up to 0.75 inch (1.91 cm) and a diameter of from 1 to 250 microns.
  • Polymer fibers comprising the diaphragm can be of any suitable denier, e.g., commercially available fibers.
  • the PTFE fiber used to prepare synthetic diaphragms is a 0.25 inch (0.64 cm) chopped 6.6 denier fiber; however, other lengths and fibers of smaller or larger deniers can be used.
  • the liquid slurry used to deposit the synthetic diaphragm on the foraminous cathode can also include other materials.
  • Such other materials include, but are not limited to, materials such as thickeners, surfactants, antifoaming agents, antimicrobial agents and other polymers, e.g., polyethylene.
  • materials such as fiberglass can also be incorporated into the diaphragm.
  • a non-limiting example of the components of a synthetic diaphragm material useful in a chlor-alkali electrolytic cell can be found in Example 1 of U.S. Pat. No. 5,188,712 , the disclosure of which synthetic diaphragm composition is incorporated by reference.
  • Synthetic diaphragms can also comprise various modifiers and additives, including but not limited to, inorganic fillers, such as clays, metal oxides, pore formers, wetting agents, etc, as is well known in the art.
  • Synthetic diaphragms can comprise one or more layers of modifiers and additives that are deposited on and within the interstices of the diaphragm comprising the fibrous halogen-containing polymer, e.g., one or more top coats of vacuum deposited modifiers and additives, as is known to those skilled in the art.
  • the diaphragm of an electrolytic cell is generally deposited onto the foraminous cathode from a slurry of components comprising the diaphragm.
  • the slurry comprises an aqueous liquid medium such as water.
  • Such an aqueous slurry generally comprises from 1 to 6 weight percent solids, e.g., from 1.5 to 3.5 weight percent solids, of the diaphragm components, and has a pH of between 8 and 11.
  • the appropriate pH can be obtained by the addition of an alkaline reagent, such as alkali metal hydroxide, e.g., sodium hydroxide, to the slurry.
  • each of the components comprising the synthetic diaphragm can vary in accordance with variations known to those skilled in the art.
  • the following approximate amounts (as a percentage by weight of the total slurry having a percent solids of between 1 and 6 weight percent) of the diaphragm components in a slurry used to deposit a synthetic diaphragm can be used: polyfluorocarbon fibers, e.g., PTFE fibers, - from 0.25 to 1.5 percent; polyfluorocarbon microfibrils, e.g., PTFE microfibrils, - from 0.6 to 3.8 percent; ion-exchange material, e.g., NAFION resin, - from 0.01 to 0.05 percent; fiberglass - from 0.0 to 0.4 percent; and polyolefin, e.g., polyethylene, such as SHORT STUFF, - from 0.06 to 0.3 percent.
  • the aqueous slurry comprising the synthetic diaphragm components can also contain a viscosity modifier or thickening agent to assist in the dispersion of the solids, e.g., the perfluorinated polymeric materials, in the slurry.
  • a viscosity modifier or thickening agent such as CELLOSIZE ® materials can be used.
  • a thickening agent such as CELLOSIZE ® materials can be used.
  • from 0.1 to 5 percent by weight of thickening agent can be added to the slurry mixture, basis the total weight of the slurry.
  • from 0.1 to 2 percent by weight thickening agent can be used.
  • a surfactant may, if desired, be added to an aqueous slurry of synthetic diaphragm components to assist in obtaining an appropriate dispersion.
  • the surfactant is a nonionic surfactant and is used in amounts of from 0.1 to 3 percent, e.g., from 0.1 to 1 percent, by weight, based on the total weight of the slurry.
  • the nonionic surfactant is a chloride capped ethoxylated aliphatic alcohols, wherein the hydrophobic portion of the surfactant is a hydrocarbon group containing from 8 to 15, e.g., 12 to 15, carbon atoms, and the average number of ethoxylate groups ranges from 5 to 15, e.g., 9 to 10.
  • a non-limiting example of such an nonionic surfactant is AVANEL ® N-925 surfactant.
  • additives that can be incorporated into the aqueous slurry of synthetic diaphragm forming components include, but are not limited to, antifoaming amounts of an antifoaming agent, such as UCON ® 500 antifoaming compound, to prevent the generation of excessive foam during mixing of the slurry, and an antimicrobial agent to prevent the digestion of cellulose-based components by microbes during storage of the slurry.
  • an antimicrobial is UCARCIDE ® 250, which is available from the Dow Chemical Company.
  • Other antimicrobial agents known to those skilled in the art also can be used.
  • antimicrobials are incorporated into the aqueous slurry of synthetic diaphragm components in amounts of from 0.05 to 0.5 percent by weight, e.g., between 0.08 and 0.2 weight percent.
  • the diaphragm of an electrolytic cell e.g., a chlor-alkali electrolytic cell
  • an electrolyte such as sodium chloride brine
  • the pressure gradient in a diaphragm electrolytic cell is the result of a hydrostatic head on the anolyte side of the cell, e.g., the liquid level in the anolyte compartment will be on the order of from 1 to 25 inches (2.54-63.5 cm) higher than the liquid level of the catholyte compartment.
  • the specific flow rate of electrolyte through the diaphragm can vary with the type of the cell, and how it is used.
  • the diaphragm In a chlor-alkali cell, the diaphragm is microporous and is prepared in such a manner that it is able to pass from 0.001 to 0.5 cubic centimeters of anolyte per minute per square centimeter of diaphragm surface area.
  • the flow rate is generally set at a rate that allows production of a predetermined, targeted concentration of the principal product formed in the catholyte compartment.
  • the principal product formed in the catholyte compartment is alkali metal hydroxide, e.g., sodium hydroxide.
  • synthetic diaphragms used in chlor-alkali cells will have a porosity (permeability) similar to that of asbestos-type and polymer resin modified asbestos diaphragms.
  • diaphragms used in electrolytic cells can vary and will depend on the type of electrolytic cell used and the nature of the electrochemical process being performed.
  • diaphragms e.g., synthetic diaphragms
  • diaphragms generally have a thickness of from 0.075 to 0.25 inches (0.19 to 0.64 cm), and a weight per unit area ranging from 0.3 to 0.6 pounds per square foot (1.5 to 2.9 kilograms per square meter) of the cathode.
  • the inorganic particulate material present in the top coat slurry may be selected from (i) oxides, borides, carbides, silicates and nitrides of valve metals, (ii) clay mineral, and (iii) mixtures of (i) and (ii).
  • the inorganic particulate material is substantially water-insoluble.
  • valve metal includes the metals vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, titanium, tungsten and mixtures of such metals.
  • valve metals titanium and zirconium are generally the metals chosen.
  • valve metal oxides borides, carbides and silicates
  • valve metal oxides and silicates are generally the materials used.
  • Non-limiting examples of valve metal oxides include titanium oxide and zirconium oxide.
  • Non-limiting examples of clay minerals that may be present in the topcoat slurry include the naturally occurring hydrated silicates of metals, such as aluminum and magnesium, e.g., kaolin, meerschaums, augite, talc, vermiculite, wollastonite, montmorillonite, illite, glauconite, attapulgite, sepiolite and hectorite.
  • metals such as aluminum and magnesium
  • attapulgite and hectorite and mixtures of such clays are generally chosen.
  • Such clays are hydrated magnesium silicates and magnesium aluminum silicates, which materials may also be prepared synthetically.
  • Attapulgite clay is available commercially under the trade name
  • the mean particle size of the inorganic particulate material used in the topcoat slurry or as a dopant material can vary.
  • the mean particle size may range from 0.1 to 20 microns, e.g., from 0.1 to 0.5 microns.
  • one commercially available attapulgite clay has a mean particle size of 0.1 microns.
  • the amount of inorganic particulate material in the topcoat slurry can vary and will depend on the amount that is required for the particular diaphragm.
  • the topcoat slurry can contain from 1 to 15 grams per liter (gpl) of inorganic particulate material.
  • the amount of inorganic particulate in the topcoat slurry may vary from 5 to 15 gpl, e.g., 8 to 12 gpl.
  • the topcoat slurry may also comprise alkali metal polyphosphate, e. g., sodium polyphosphate, potassium polyphosphate and mixtures of such polyphosphates.
  • the polyphosphate may be a hydrated polyphosphate, a dehydrated polyphosphate or a mixture of hydrated and dehydrated polyphosphates.
  • the alkali metal polyphosphate may be present in the topcoat slurry in an amount of at least 0.01 weight percent. In an alternate non-limiting embodiment, the alkali metal polyphosphate may be present in amounts of at least 0.1 weight percent. Generally, the alkali metal polyphosphate is present in the topcoat slurry in amounts of less than 2 weight percent.
  • the alkali metal polyphosphate is present in the topcoat slurry in amounts of less than 1 weight percent, e.g., less than 0.5 weight percent.
  • the amount of alkali metal polyphosphate present in the topcoat slurry can range between any of the aforedescribed upper and lower values, inclusive of the recited values.
  • alkali metal polyphosphates include tetraalkali metal pyrophosphate, e.g., tetra sodium pyrophosphate and tetra potassium pyrophosphate, alkali metal triphosphate, e.g., sodium triphosphate and potassium triphosphate, alkali metal tetraphosphate, e.g., sodium tetraphosphate, alkali metal hexametaphosphate, e.g., sodium hexametaphosphate, and mixtures of such polyphosphates.
  • alkali metal polyphosphates include tetraalkali metal pyrophosphate, e.g., tetra sodium pyrophosphate and tetra potassium pyrophosphate, alkali metal triphosphate, e.g., sodium triphosphate and potassium triphosphate, alkali metal tetraphosphate, e.g., sodium tetraphosphate, alkali metal hexametaphosphate,
  • one or more perforations in the diaphragm e.g., tears, holes, etc can develop.
  • Such perforations are larger than the pores that are present in the microporous diaphragm during normal operation of the electrolytic cell, e.g., the pores that define the microporosity of the diaphragm.
  • the root cause of such perforation(s) is not known for certain.
  • the catholyte in the catholyte compartment is diluted with electrolyte due to the increase in the flow of electrolyte through the diaphragm.
  • the dilution effect is evidenced, for example, by a decrease in the concentration of the principal product formed in the catholyte compartment.
  • the concentration of the aqueous alkali metal hydroxide in the catholyte liquor, e.g., sodium hydroxide, in the catholyte liquor decreases.
  • the decrease in the concentration of the aqueous alkali metal hydroxide that is observed as a result of perforations occurring in the diaphragm can be 2 percent or more.
  • the observed decrease in alkali metal hydroxide in the catholyte liquor as a result of perforations in the diaphragm can be as much as from 3 to 70 percent, e.g., from 3 to 40 percent.
  • Periodic chemical analysis of the catholyte liquor withdrawn from the catholyte compartment will evidence this decrease in the alkali metal hydroxide concentration and indicate that there are perforations in the diaphragm.
  • An aqueous alkali metal hydroxide solution product of diminished concentration results in increased process costs in order to evaporate the excess water present in the alkali metal hydroxide recovered from the catholyte liquor in order to bring the alkali metal hydroxide solution to a concentration that is sold commercially.
  • hypohalite ion e.g., hypochlorite ion
  • the concentration of hypohalite ion in the catholyte liquor of a good operating chlor-alkali electrolytic cell will range from 0 to 10 parts per million (ppm), e.g., 3 to 10 ppm.
  • ppm parts per million
  • the hypohalite ion concentration in the catholyte liquor can increase to levels of 150 ppm or more.
  • the concentration of hypohalite ion in the catholyte liquor as a result of perforation(s) in the diaphragm can range from 20 to 150 ppm.
  • the increase in hypohalite concentration in the catholyte liquor as a result of perforation(s) in the diaphragm can range from 25 to 100 ppm, e.g., 25 to 50 ppm.
  • the increase in hypohalite concentration in the catholyte liquor can range between any combination of the described concentrations, inclusive of the recited concentrations.
  • ceramic fiber is introduced into the anolyte compartment in an amount sufficient to reduce the flow of liquid anolyte (electrolyte) through the diaphragm into the catholyte compartment, e.g., an effective amount.
  • ceramic fiber is introduced into the anolyte compartment in amounts sufficient to reduce the flow of anolyte (electrolyte) through the diaphragm to a value within the desired operating range chosen for the treated cell.
  • the flow rate of anolyte through the diaphragm is typically within the range of from 0.001 to 0.5 cubic centimeters per minute per square centimeter of effective diaphragm surface area.
  • ceramic fiber is introduced into the anolyte compartment while the cell is operating.
  • the amount of ceramic fiber introduced into the anolyte compartment is an amount sufficient to increase the concentration of alkali metal hydroxide, e.g., sodium hydroxide, in the catholyte liquor.
  • the amount of ceramic fiber introduced into the anolyte compartment is an amount sufficient to reduce the concentration of hypohalite ion, e.g., hypochlorite ion such as sodium hypochlorite.
  • the increase in alkali metal hydroxide concentration and the decrease in hypohalite ion concentration in the catholyte liquor are to at least substantially the same respective concentrations that existed in the catholyte liquor prior to the conditions that gave rise to the need for adding ceramic fiber to the anolyte compartment.
  • the increase in alkali metal hydroxide concentration and decrease in hypohalite ion concentration are those respective concentrations that are within the range established for a good operating electrolytic cell, e.g., standard operating conditions for a cell of the type treated.
  • the ceramic fiber may be introduced batch wise into the anolyte compartment. In an alternate non-limiting embodiment, ceramic fiber may be introduced continuously into the anolyte compartment. Regardless of the manner by which ceramic fiber is introduced into the anolyte compartment, e.g., periodically or continuously, the ceramic fiber can in alternate non-limiting embodiments be introduced dry, as a wetted fiber or in the form of a slurry, e.g., an aqueous slurry.
  • the aqueous portion of the slurry can be, but is not limited to, water, anolyte feed, e.g., brine, recycled anolyte liquor, or mixtures of such aqueous liquids.
  • water or brine feed is used to prepare the slurry.
  • the ceramic fiber is introduced periodically, e.g., batchwise, into the anolyte compartment.
  • ceramic fiber is introduced into the anolyte compartment until the concentrations of alkali metal hydroxide and/or hypohalite ion have returned to their desired levels.
  • Chemical analysis of the catholyte liquor subsequent to the initial introduction of ceramic fiber to the anolyte compartment and after equilibrium within the cell is substantially attained will determine if sufficient ceramic fiber has been introduced to bring the cell back to its desired operating conditions, or whether additional amounts of ceramic fiber are required to rectify the increased flow of anolyte through the diaphragm.
  • Such chemical analyses are good indicators of whether the flow of anolyte liquor through the diaphragm is excessive or whether it is within the range of standard cell operating conditions.
  • Periodic chemical analysis of the catholyte liquor after ceramic fiber addition avoids producing an operating condition where the flow of anolyte liquor through the diaphragm becomes too low.
  • the steps of catholyte liquor analysis and ceramic fiber addition to the anolyte compartment can be repeated until the cell returns to a desired operating condition.
  • the amount of ceramic fiber introduced into the anolyte compartment can vary.
  • the amount of ceramic fiber charged to the anolyte compartment during each occurrence of ceramic fiber addition can range from 0.1 to 30 grams of ceramic fiber per square foot of effective diaphragm surface area (the surface area through which electrolyte passes into the catholyte compartment).
  • the amount of ceramic fiber introduced into the anolyte compartment can range from 0.1 to 10 grams of ceramic fiber per square foot of effective diaphragm surface area, e.g., from 0.1 to 8.5 grams per square foot of effective diaphragm surface area.
  • the amount of ceramic fiber introduced into the anolyte compartment can range from 0.1 to 5, e.g., 0.3 to 3, grams of ceramic fiber per square foot of effective diaphragm surface area.
  • the amount of ceramic fiber introduced into the anolyte compartment can vary between any combination of the stated values, including the recited amounts. Care should be observed that the amount of ceramic fiber added to the anolyte is not excessive, thereby resulting in either plugging of the diaphragm or reducing the flow of electrolyte through the diaphragm to rates significantly below that of normal operating conditions.
  • the ceramic fiber is at least partially resistant to the corrosive conditions within the anolyte of the electrolytic cell, e.g., oxidizing conditions, pH and temperature.
  • the pH of the anolyte is generally acidic.
  • corrosive conditions within the anolyte compartment of the chlor-alkali cell can be caused by the presence of chlorine, hydrochloric acid, hypochlorous acid, chlorate ions and oxygen within the anolyte compartment.
  • the ceramic fibers may be exposed to alkaline materials, e.g., hydroxides, that are present on or in the diaphragm or that back migrate from the catholyte department, which may also cause chemical degradation of the ceramic fibers.
  • the ceramic fibers may be eroded by mechanical forces operating within the anolyte compartment or be dissolved chemically by the chemicals present within the anolyte compartment.
  • perforations in the diaphragm are likely to reoccur. Generally, the perforations will reoccur gradually, as evidenced for example by the dilution of the concentration of the principal product within the catholyte compartment. In such an event, the addition of further quantities of ceramic fiber to the anolyte compartment may be required.
  • the ceramic fiber material introduced into the anolyte compartment is at least partially resistant to degradation/dissolution by the chemical and mechanical forces within the anolyte compartment for a reasonable period of time.
  • the period of time that the ceramic fibers perform their function of reducing the flow of anolyte into the catholyte compartment can vary, and will be a function of the ceramic fiber used, the conditions within the electrolytic cell, e.g., turbulence, power (load) variations, outages and the previously described chemically corrosive conditions.
  • the electrolytic cell will operate at substantially the operating conditions for that particular cell after the addition of ceramic fiber to mend perforations in the diaphragm for from approximately 2 to 12 weeks or longer, although shorter periods of time can be expected in some cases.
  • Ceramic fiber materials include, but are not limited to, silicon dioxide, silicon nitride, silicon carbide, zirconium dioxide, zirconium diboride, zirconium silicate, boron nitride, boron oxide (B 2 O 3 ), germanium dioxide, aluminum oxide, aluminum silicates, aluminum nitride, silicon carbide, tin oxide, iron silicide, molybdenum disilicide, hafnium oxide, titanium suboxides, titanium dioxide, titanium carbide, titanium diboride, titanate fibers, such as the alkali titanates represented by the formulae M 2 O.4TiO 2 and M 2 O.6TiO 2 , wherein M is the alkali metal sodium, potassium, rubidium or cesium, e.g., potassium tetratitanate (K 2 Ti 4 O 9 ), mixtures of alumina and silica, e.g., blends of from 46 to 96 weight percent alumina and 4 to 54 weight percent silic
  • the ceramic fibers can vary in length. In one non-limiting embodiment, the fiber length can range from 0.03 to 10 inches (0.07 to 25.4 centimeters). In alternate non-limiting embodiments, the fiber length can vary from 0.05 to 4 inches (0.13 to 10 centimeters), e.g., from 0.5 to 2 inches (1.3 to 5.1 centimeters).
  • the fibers can be fibril-like, and of irregular morphology, e.g., beads, tear-drop shapes, bent-branch shapes and blobular rods. They can be amorphous, crystalline, isotropic, anisotropic and branched and/or unbranched. In one non-limiting embodiment, the width of the fibers can range from 0.1 to 10,000 microns.
  • the width of the fibers can range from 0.5 to 10 microns, e.g., 3 to 5 microns.
  • the cross-sectional morphology of the ceramic fibers is circular, e.g., as a result of circular dies used to prepare the fibers.
  • Ceramic fibers can be prepared by methods known to those skilled in the art. Such methods include drawing the fibers from a molten state of the chemical composition comprising the fiber and rapidly cooling the fiber. Another method that can be used is that described in column 3, lines 11 - 25 of U.S. Patent 3,385,915 .
  • That described method comprises (1) impregnating a preformed organic polymeric fiber material with one or more compounds, e.g., salts or hydrolysis products of salts, of the chosen metal elements, e.g., metal elements that form oxides, and (2) heating the impregnated organic material under controlled conditions in the presence of an oxidizing gas to (a) convert the organic material to predominantly carbon and removing the carbon as a carbon-containing gas and (b) oxidize the metal compound(s) to their respective metal oxide(s).
  • one or more compounds e.g., salts or hydrolysis products of salts
  • the chosen metal elements e.g., metal elements that form oxides
  • That method comprises (1) forming a dispersion of particles of the refractory material, e.g., particles of less than 100 microns, (2) mixing the dispersion with a carrier solution of a salt of cellulose xanthate to form a spin mix, (3) forming filaments of regenerated cellulose from the spin mix using wet spinning techniques, and (4) heat treating the filaments to remove substantially all of the regenerated cellulose and sinter the particles of refractory material to form the desired fibers.
  • dopant material can be added to the anolyte compartment at substantially the same time as the ceramic fiber.
  • dopant material can be added before or subsequent to, e.g., sequentially, to the addition of the ceramic fiber.
  • fibers comprising halogen-containing polymers e.g., fluorocarbon polymers
  • fibers comprising halogen-containing polymers can be added to the anolyte compartment of the electrolytic cell in conjunction with the ceramic fiber, e.g., at substantially the same time as the ceramic fiber.
  • the fibers comprising halogen-containing polymers can be added before or subsequent to, e.g., sequentially, the addition of ceramic fibers to the anolyte compartment.
  • dopant material and fibers of halogen-containing polymer can be added to the anolyte compartment of the electrolytic cell to work in conjunction with the ceramic fibers.
  • the order in which the ceramic fibers, halogen-containing polymer fibers and dopant material are added to the anolyte compartment can vary. Generally, for reasons of convenience, a mixture of one or more of the aforementioned materials, e.g., a slurry of all three of the materials, is prepared and the slurry added to the anolyte compartment.
  • Each electrolyzer element contained 44 substantially vertical cathode fingers interleaved within and spaced from substantially vertical anodes.
  • the cathode area for each element was 416 square feet (33.6 square meters).
  • the cathodes were provided with a non-asbestos synthetic diaphragm comprising fibrous polytetrafluoroethylene (PTFE), PTFE microfibers (fibrils), NAFION® ion exchange material having sulfonic acid functional groups, fiberglass and SHORT STUFF polyethylene fibers.
  • the synthetic diaphragms were deposited onto the cathodes by vacuum deposition of an aqueous slurry of the materials comprising the diaphragm.
  • the synthetic diaphragms were coated (by vacuum deposition) with inorganic particulate material.
  • the coating comprised either ATTAGEL® attapulgite clay and zirconium dioxide, or ATTAGEL® attapulgite clay zirconium dioxide and magnesium hydroxide
  • the electrolyzers were used for the electrolysis of sodium chloride brine.
  • concentration of the brine fed to the anolyte compartment of each electrolyzer element was in the range of 318 to 322 grams per liter (gpl).
  • the voltage and amperage applied to each electrolyzer element was in the range of 3.32 to 3.36 volts at approximately 72 kilo amperes.
  • analysis of the catholyte liquor was performed at approximately seven-day intervals. The reported Oxy '6' efficiencies are for the electrolyzer.
  • a doping solution comprising 3 pounds (1.36 kg) of CERACHEM® HM-12 ceramic fibers and 5 pounds (2.27 kg) of ATTAGEL® 36 attapulgite clay (Engelhard Corporation) in 25 gallons (94.6 liters) of water was prepared and added to the element brine box from where it was introduced into the anolyte compartment of the electrolyzer element.
  • ATTAGEL® 36 attapulgite clay Engelhard Corporation
  • a doping solution having the composition described in Example 1 was prepared and added to the brine box of element 11 from where it was introduced into the anolyte compartment of the electrolyzer element.
  • analysis of the catholyte liquor showed that the NaOH concentration to be 134.1 gpl and the NaOCl concentration to be zero (0) ppm.
  • the Oxy'6' efficiency of the electrolyzer at this time was calculated to be 95.8 %.
  • a doping solution comprising 3 pounds (1.36 kg) of CERAFIBER® 112 ceramic fibers, 5 pounds (2.27 kg) of ATTAGEL® 36 attapulgite clay (Engelhard Corporation), 2 gallons (7.6 liters) of a 10% PTFE microfibril suspension (17.7 pounds microfibrils, 8 kg), approximately 1 pound (0.5 kg) of shredded synthetic PTFE diaphragm and 25 gallons (94.6 liters) of water was prepared and added to the element brine box from where it was introduced into the anolyte compartment of the electrolyzer element. Analysis of the catholyte liquor 12 days later reported that the NaOH concentration was 140.3 gpl and the concentration of NaOCl had dropped to zero (0).
  • CERAFIBER® 112 is a ceramic fiber of approximately 46 weight % alumina and 54 weight % silica having a fiber length of up to 10 inches (25.4 cm), which is available from Thermal Ceramics Inc.
  • Shredded PTFE synthetic diaphragm is synthetic diaphragm material that has been passed through a paper shredder. The dimensions of the shredded diaphragm were approximately 1/8 inch wide x 1.5 inches long x 1/8 inch thick (0.3 cm x 3.8 cm x 0.3 cm).

Claims (10)

  1. Verfahren zum Betrieb einer Chloralkali-Elektrolytzelle, die ein Anolyt-Kompartiment, ein Katholyt-Kompartiment und ein Diaphragma, das die Anolyt- und Katholyt-Kompartimente trennt, umfasst, wobei wässriges Alkalimetallchlorid kontinuierlich in das Anolyt-Kompartiment eingeleitet wird und durch das Diaphragma in das Katholyt-Kompartiment gelangt, das Katholytflüssigkeit enthält, die Alkalimetallhydroxid und Hypochloritionen umfasst, wobei das Verfahren Folgendes umfasst:
    - Überwachen der Konzentration von Alkalimetallhydroxid und/oder Hypochloritionen in der Katholytflüssigkeit durch chemische Analyse der Katholytflüssigkeit; und
    - Zugeben von Keramikfasern in das Anolyt-Kompartiment, wenn die Konzentration an Alkalimetallhydroxid in der Katholytflüssigkeit unter der gewünschten Konzentration liegt und/oder die Konzentration an Hypochloritionen in der Katholyt-Flüssigkeit über der gewünschten Konzentration liegt, bis die Alkalimetallhydroxid- und/oder Hypochloritionenkonzentration auf den gewünschten Pegel zurückgekehrt ist.
  2. Verfahren nach Anspruch 1, wobei die chemische Analyse der Katholytflüssigkeit periodisch durchgeführt wird.
  3. Verfahren nach einem der Ansprüche 1 oder 2, wobei die Stufen der Analyse der Katholytflüssigkeit und der Zugabe der Keramikfasern in das Anolyt-Kompartiment wiederholt werden, bis die Zelle zu einem gewünschten Betriebszustand zurückgekehrt ist.
  4. Verfahren nach einem der vorstehenden Ansprüche, wobei es sich bei dem Diaphragma der Chloralkali-Elektrolytzelle um ein synthetisches Diaphragma handelt und es sich beim Alkalimetallchlorid um Natriumchlorid handelt.
  5. Verfahren nach Anspruch 4, wobei die Keramikfasern aus Fasern ausgewählt werden, die Oxide, Nitride, Carbide, Boride und Silicate von Metallen oder Halbmetallen, die aus Zirkonium, Titan, Silicium. Aluminium, Bor und Magnesium ausgewählt sind, und Gemische derartiger Metall- oder Halbmetalloxide, -nitride, -carbide, -boride und -silicate umfassen.
  6. Verfahren nach Anspruch 5, wobei die Keramikfasern aus Fasern ausgewählt sind, die mindestens eines der Oxide von Silicium. Aluminium und Zirkonium umfassen.
  7. Verfahren nach Anspruch 5, wobei die Keramikfasern zumindest teilweise gegen die korrosiven Bedingungen im Anolyt-Kompartiment beständig sind.
  8. Verfahren nach Anspruch 7, wobei die Keramikfasern während des Betriebs der Zelle in das Anolyt-Kompartiment gegeben werden.
  9. Verfahren nach Anspruch 8, wobei anorganische teilchenförmige Materialien dem Anolyt zusammen mit den Keramikfasern zugesetzt werden.
  10. Verfahren nach einem der Ansprüche 8 oder 9, wobei Fasern mit einem Gehalt an einem halogenhaltigen Polymer zusammen mit den Keramikfasern zugesetzt werden.
EP06847451.9A 2006-09-07 2006-09-07 Verfahren zum betrieb einer elektrolytischen membranzelle Active EP2041335B1 (de)

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US8784620B2 (en) * 2010-05-13 2014-07-22 Axiall Ohio, Inc. Method of operating a diaphragm electrolytic cell
US9222182B2 (en) 2013-06-14 2015-12-29 Simple Science Limited Electrochemical activation device
WO2019055815A1 (en) * 2017-09-15 2019-03-21 Dow Global Technologies Llc ELECTROLYTE PERMEABLE DIAPHRAGM
WO2019055801A1 (en) * 2017-09-15 2019-03-21 Dow Global Technologies Llc TEMPORARY MODIFICATION OF THE PERMEABILITY OF A PERMEABLE DIAPHRAGM TO ELECTROLYTES

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US4174266A (en) * 1975-05-14 1979-11-13 Ppg Industries, Inc. Method of operating an electrolytic cell having an asbestos diaphragm
SU916603A1 (ru) * 1976-07-19 1982-03-30 Kudinov Vladimir Способ получения хлора и щелочи 1
MX169225B (es) * 1984-09-17 1993-06-24 Eltech Systems Corp Compuesto de fibras no organicas/polimero metodo para elaborarlo y uso del mismo, incluyendo un separador dimensionalmente estable
US5266350A (en) * 1992-07-14 1993-11-30 The Dow Chemical Company Processes and materials for treatment and repair of electrolytic cell separators
DE19650316A1 (de) * 1996-12-04 1998-06-10 Basf Ag Verfahren zur Modifikation des Durchflußwiderstandes von Diaphragmen

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