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
• • 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

Definitions

• the invention relates to a process for the removal of electrocatalytically active, noble metal-containing protective coatings from electrodes with a valve metal (alloy) core and the application of the process.
• Electrodes of this type have been used increasingly for a number of years, especially for the aqueous electrolysis of the alkali halides, since they work more economically than the graphite anodes which were customary in the majority of cell types.
• the service life of the coatings is constantly increasing due to improved coating techniques and the trend towards low currents, the activity of the anode surface has been reduced as far as possible after continuous use over several years due to anodic passivation, formation of foreign deposits, partial destruction of the construction due to short circuits or due to mechanical wear of the cover layer that re-coating is necessary.
• DE-OS 2213528 discloses a method in which the used electrodes are converted into a molten salt, which is essentially formed from at least one hydrogen sulfate or pyrosulfate of an alkali metal or ammonium. immersed at a temperature between 300 and 500 ° C and then this treated electrode after cooling is subjected to a rinsing with water.
• US Pat. No. 3,684,577 describes a method for removing the electrically conductive coating from a titanium structure by bringing the carrier structure into contact with a molten salt bath which consists of a mixture of 1 to 15 parts by weight of an alkali metal hydroxide and one part by weight. Part of an alkali salt of an oxidizing agent.
• US Pat. No. Re 28,849 describes an electrolytic cleaning process, the electrode to be cleaned being connected as an anode in an electrolyte which contains 5 to 70% of a sulfate, nitrate, perchlorate, chlorate, a persulfate or a mixture thereof. Electrolysis is carried out at a current density of 1 to 100A / dM2.
• the invention has for its object to provide a simple and inexpensive method for removing used, precious metal-containing coatings from valve metal (alloy) electrodes exposing a clean surface for re-coating, wherein the removal of metal is minimal and above all uniform and the valuable components of the protective covers can be recovered completely and easily.
• This method is said to be particularly applicable to valve metal electrodes with protective coatings containing precious metals.
• This object is achieved by the creation of a method of the type mentioned at the outset, which is characterized in that, in order to expose clean surfaces for the re-coating by targeted thermal treatment in a gas atmosphere with at least a proportion of oxygen, carbon, nitrogen or hydrogen-supplying component or a mixture thereof at a temperature in the range from 400 to 900 ° C., a non-adhesive intermediate layer of oxide, carbide, nitride or hydride of the carrier valve metal is located between the protective coating and the carrier structure.
• the metallic carrier core can consist of any valve metal or valve metal alloy on which a non-adhesive connection can be produced.
• the type of coating itself on the metallic support is not critical.
• the electrocatalytically active protective layers used for the chlor-alkali electrolysis and related electrochemical processes generally consist of oxidic components of the platinum metals and have a layer thickness of a few micrometers.
• the chemical composition of the coating and its thickness can be varied within wide limits without the solid-state diffusion of cations and / or anions necessary for the formation of the non-adhesive connecting layer being impeded by the coating which is still present, in particular in the case of used coatings, in particular at higher temperatures .
• the thermal treatment can also consist of several cycles.
• every new combination of metallic substrate and protective coating expediently requires a few specific tests, if necessary with the help of thermogravimetric and differential thermal analysis, since the available literature primarily relates to the formation of bonds between unprotected metals.
• the thermal treatment is carried out in a gas atmosphere with at least a proportion of oxygen, carbon, nitrogen or hydrogen-supplying component, or a mixture thereof, depending on the desired non-adhesive compound.
• oxygen-supplying component e.g. serve an atmosphere containing hydrocarbons.
• nitrogen-supplying component can be primary nitrogen, its hydrogen compounds or hydrogen.
• an inert gas e.g. Noble gases, preferably argon etc., are considered.
• Predrying can be carried out in the range from 130 to 250 ° C. in particular.
• a valve metal core contains a coating of platinum metal or compounds or mixtures thereof, it is often favorable to keep the desired compounds in the range from 700 to 870 ° C. in a gas atmosphere, these conditions being particularly suitable for the production of oxides, for example by Treatment in air, have proven.
• thermo treatment to produce a non-stick oxide, e.g. at a temperature above 650 ° C in non-oxidizing molten salt, by anodic oxidation.
• the method according to the invention is used to remove deactivated protective coatings from electrodes with a core made of valve metal or a valve metal alloy and in particular titanium or alloys thereof.
• the method can also be used particularly advantageously on electrodes in which the parts carrying the active coating consist of expanded metal, wire or rods of a maximum diameter of less than 1 cm.
• the thermal treatment between 800 and 870 ° C. is carried out in air for less than 15 minutes, this temperature range being achieved by very rapid heating the electrode is reached very quickly.
• the method according to the invention is also particularly applicable to electrode sheets which carry the active coating.
• the sheets are treated at a temperature between about 600 to 700 ° C. for a period of more than 20 minutes, preferably in air.
• the shape of the metallic base body can also play an important role in determining the reaction conditions.
• a non-adherent oxide can be produced on a coated titanium molded body with a planar planar structure by exposing it to a temperature treatment at 650 to 700 ° C. in air. A white titanium oxide is formed, which peels off and jumps off when the molded body cools. If, however, coated round material is treated under the same reaction conditions, for example wire of 3 to 5 mm in diameter, the titanium oxide formed as an intermediate layer adheres firmly to the substrate and can hardly be removed by brushing with wire brushes or similar processes.
• valve metal anodes in the above-described embodiments whose activated surface essentially consists of wires, rods or expanded metal, can be freed from the used coating by the process according to the invention.
• the titanium anodes with the used coating being held at 800 to 860 ° C. for about 5 to 10 minutes, preferably 7 to 8 minutes.
• the coating peels off slightly when it cools down. In the case of complicated designs, all coating residues can easily be removed by brushing or by compressed air (without blasting media).
• the samples should be pre-dried because traces of water favor the formation of adherent compound and especially oxide skins.
• the favorable temperature ranges determined by orientation tests should be adhered to very precisely so that a certain compound, such as oxide, forms.
• the low temperature ranges should be passed very quickly both when heating up and when cooling down, if an adhesive bond can form in them.
• a certain duration of treatment must not be exceeded so that e.g. does not convert a non-adherent substoichiometric compound into a compound of high oxidation levels adhering to the metal surface. This applies particularly to the manufacture of oxides.
• short reaction times should be aimed at so that the intermediate layer does not become unnecessarily thick.
• the method according to the invention has the considerable advantage that the removal of the deactivated coating is very uniform, complete and easy to control, even in the case of complicated constructions.
• the new surface of the support structure obtained can be directly recoated without further process steps, such as pickling, greasing, rinsing, etc.
• the new coatings adhere just as firmly as the first coating and they have the same favorable electrochemical properties.
• the procedure is very little labor, time and cost intensive.
• the deactivated old coating is obtained in pure form, so that it is easy to recover the valuable precious metals that are still preserved without cumbersome enrichment from abrasive abrasives or aggressive salt melts and pickling.
• the sheet was used in a technical chlorate electrolysis for 3 years.
• the remaining coating still had an average layer thickness of 10 1 1 m based on gamma head examinations.
• the sheet was pre-dried at 175 ° C for 20 minutes, then held in a preheated oven at 650 ° C for 40 minutes, then immediately removed and cooled by the ambient air.
• the coating could be lifted off in large pieces. It had a white oxide skin on its underside, which could be separated from the original black coating by soaking it in an HF / HN0 3 mixture for 20 hours.
• the metal surface was shiny metallic. Scanning electron micrographs of the metal surface show hexagonal stepped depressions with clear step formation parallel to the 001 surfaces. The back of the oxide skin showed growths that fit into the depressions of the metal surfaces. However, they do not have a
• the sheet was no longer pickled before recoating, but only degreased.
• the new coating adhered extremely well and had better electrochemical values than before.

Landscapes

• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Metals (AREA)
• Conductive Materials (AREA)
• Catalysts (AREA)
• Exhaust Gas Treatment By Means Of Catalyst (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Insulated Metal Substrates For Printed Circuits (AREA)
• Battery Electrode And Active Subsutance (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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Citations (1)

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• 1980
  • 1980-08-28 DE DE3032480A patent/DE3032480C2/de not_active Expired
• 1981
  • 1981-07-09 DE DE8181105342T patent/DE3167929D1/de not_active Expired
  • 1981-07-09 EP EP81105342A patent/EP0046853B1/de not_active Expired
  • 1981-07-09 AT AT81105342T patent/ATE10955T1/de not_active IP Right Cessation
  • 1981-07-14 NO NO812421A patent/NO155974C/no unknown
  • 1981-07-14 IE IE1591/81A patent/IE52090B1/en unknown
  • 1981-07-16 ZA ZA814889A patent/ZA814889B/xx unknown
  • 1981-07-17 CA CA000381959A patent/CA1176600A/en not_active Expired
  • 1981-08-10 US US06/291,407 patent/US4379723A/en not_active Expired - Fee Related
  • 1981-08-14 JP JP56126862A patent/JPS5754289A/ja active Pending
  • 1981-08-24 SU SU813324655A patent/SU1306485A3/ru active
  • 1981-08-26 BR BR8105449A patent/BR8105449A/pt unknown

Patent Citations (1)

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