EP0046853A1 - Process for removing electrocatalytically acive protective layers of electrodes comprising a metal core, and application of this process - Google Patents

Abstract

A method of removing electrocatalytically active protective coatings from electrodes with metal cores, in which a non-adhesive intermediate layer of a compound of the substrate metal is produced in a position between the protective coating and the substrate structure by means of controlled thermal treatment. By using the method, deactivated protective coatings can be removed in a particularly easy manner from electrodes with valve metal cores.

Description

The invention relates to a method for removing electrocatalytically effective protective coatings from electrodes with a metal core, and to the application of the method.

Electrodes of this type have been used increasingly for some years, especially for the aqueous electrolysis of the alkali halides, since they work more economically than the usual ones in the majority of cell types.

Graphite anodes. Although the life of the coatings is constantly increasing due to improved coating techniques and the trend towards low amperages, the activity of

Anode surface after continuous use over several years by anodic passivation, formation of foreign deposits, partial destruction of the construction by short circuits or due to mechanical wear of the covering layer, degraded to such an extent that re-coating is necessary.

Before the metal construction can be coated again, the remaining coating residues containing the precious metal must be removed appropriately. Attempts to apply the new coating on titanium electrodes directly to the remains of the old coating (DE-OS 21 57 511) have not proven themselves in practice, as have been various post-published patents such as US Pat. No. 3,684,577 and US Pat. No. Re 28,849 demonstrate.

There is therefore a need, generally recognized by experts, to clean the metal structure as completely as possible from the used coatings with the least possible loss of carrier material. The newly formed surface of the construction should show good adhesive properties when applying the new coating. It is also very important for an economical recoating process that the valuable coating metals can be recovered from the used coating.

A mechanical removal of the coatings by dry or wet blasting has already been described in DE-OSes 28 15 955, 26 38 218 and 26 45 414. Although it is probably the most common process, it is disadvantageous in that sandblasting by hand incurs very high personnel costs, whereas automatic blasting cannot avoid high losses in construction material. That too is It is quite difficult to recover the noble metals or their compounds from the used abrasive, which according to investigations contains a maximum of 3% coating material, due to the abrasive properties of the abrasive.

However, other methods for removing used coatings from metal anodes are also known: For example, DE-OS 22 13 528 discloses a method in which the used electrodes are melted in a salt, which essentially consists of at least one hydrogen sulfate or pyrosulfate of an alkali metal or ammonium is 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 a part by weight Part of an alkali salt of an oxidizing agent.

DE-PS 19 09 757, according to which the anodes are treated at a temperature of 250 ° C. with a melt of potassium or sodium nitrate, which also contains a strong inorganic base, is almost identical to this.

A somewhat different method is disclosed in U.S. Patent 3,761,312. Here the electrodes are subjected to a two-stage pickling process, the first pickling bath containing 0.3 to 3% H ₂ 0 _{2 in} addition to any acid and base and the second pickling liquid consisting of 20-30% hydrochloric acid. Finally, US Pat. No. Re 28,849 describes an electrolytic cleaning process, the electrode to be cleaned being switched 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 100 Afdm ² .

These methods are less suitable for technical use, but more to address as laboratory methods. In particular, processes which work with acid salts or acids are not suitable for the treatment of titanium anodes of technical construction and after a technical use, since these have construction parts which either have not received a protective layer from the outset or have completely lost them through short circuits. Treating them with acidic chemicals will therefore immediately attack them massively, while the top layer to be removed will only dissolve weakly or not at all.

In processes of the type described in US Pat. No. 3,684,577, there are considerable dangers, since the oxidizing salt melts used there sometimes react explosively with titanium even when they are only slightly warmed (GMELIN, Handbuch der inorganic Chemie, system number 41, 198 (1951)). This also applies to the molten salt according to DE-PS 19 09 757, although only at higher temperatures.

The invention has for its object to provide a simple and inexpensive method for removing used coatings from metal electrodes while exposing a clean 'surface for recoating, in which the removal of

Metal is minimal and, above all, uniform and the valuable components of the protective coatings can be completely and easily recovered. 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 a non-adhesive intermediate layer of a connection of the carrier metal located between the protective coating and the carrier structure is produced by targeted thermal treatment.

The metallic carrier core can consist of any metal or metal alloy on which a non-adhesive connection can be produced.

Various physical phenomena come into question as the cause of this non-sticking of the newly formed connecting layers, for example due to the Pilling-Bedworth principle, according to which e.g. Oxides occupy a larger volume than the metals from which they are formed, or by different. Liche thermal expansion coefficient, or by formation of gaseous compounds, such as oxides, hydrides, etc., or by weakening the bond in the boundary layer by diffusion of the cations from the metal (Kirkendall effect) and the like.

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 exist from oxidic components of the platinum metals and have a layer thickness of a few micrometers. However, 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 .

When carrying out the process according to the invention, the formation of oxides, carbides, nitrides, hydrides or combinations thereof is particularly advantageous.

In general, the formation of the non-adhesive intermediate layer between the coating and the metallic substrate is achieved in that the thermal treatment is carried out at a temperature of 400 to 900 ° _C. In particular, the thermal treatment is carried out in a gas atmosphere with at least a proportion of the component which supplies oxygen, carbon, nitrogen or hydrogen or a mixture thereof, depending on the desired compound. For sheet metal, this can also consist of several cycles. To optimize the conditions, every new combination of metallic substrate and protective coating expediently requires a few specific tests, if necessary with the aid of thermogravimetric and differtial thermal analysis, since the existing literature primarily relates to the formation of bonds between unprotected metals. As a result of the impeded diffusion through the protective layer, slightly sub-stoichiometric compounds, for example oxides, are formed in the intermediate layer, which can form on the bare metal surface under substantially different conditions, for example under very greatly reduced gas partial pressure.

In the context of the method according to the invention, as already mentioned above, it is preferred that 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. As an oxygen-supplying compo. For example, air or mixtures with a lower. Oxygen content can be used. Since the diffusion of the gas through the protective layer to be removed is often the rate-determining step, an increase in the oxygen content generally brings about

Gas no special advantage. As the carbon supply component, e.g. serve as an atmosphere containing hydrocarbons. Primarily nitrogen, its hydrogen compounds or hydrogen can be used as the nitrogen or hydrogen supplying component. Depending on the implementation of the reaction conditions, it may sometimes be expedient to additionally add a proportion of the gas which is inert under the treatment conditions to the gas atmosphere. As such inert gas e.g. Noble gases, preferably argon, etc., are considered.

To carry out the method according to the invention, it is particularly preferred to precede a pre-drying stage before the thermal treatment. The predrying can be carried out in the range from 130 to 250 ° C. in particular.

When carrying out the process according to the invention for the preparation of the non-adhesive metal compound, it is often preferred to go through low temperature ranges very quickly both in the heating phase and in the cooling phase and at the reaction temperature, at The formation of the non-adhesive metal compound takes place only for a short period of time, often less than 1 hour, sometimes preferably in the range from 20 to 40 minutes for sheets, and even with a shorter reaction time if a very reactive gas is used. Treatment times of less than 15 minutes are preferred for the treatment of electrodes as wire gratings. Although these times refer primarily to the treatment of titanium cores, it is nevertheless easily possible for a person skilled in the art on the basis of preliminary tests to easily determine the optimum temperature and time conditions for other valve metals. It is obvious that the temperature and time conditions can vary to a certain extent, depending on the type and in particular the detailed geometry of the electrode construction, the thickness of the coating to be removed, the type of reaction gas used and its pressure.

In the treatment of electrodes which are often used in aqueous chlor-alkali electrolysis, i.e. Of electrodes, which contain a coating of platinum metal or compounds or mixtures thereof 'on a valve metal core, 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 Manufacture of oxides, for example by treatment in air, have proven.

However, it is also possible to use the thermal treatment to produce a non-stick oxide, e.g. to be carried out at a temperature above 650 ° C. in non-oxidizing molten salt by anodic oxidation.

The method according to the invention finds its preferred application for the removal of deactivated protective coatings from Electrodes with a core made of valve metal or a valve metal alloy and in particular made of titanium or alloys. of this. The method can also be applied particularly advantageously to 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.

With such electrode constructions, which are often used in aqueous chlor-alkali electrolysis, it is often particularly advantageous if 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. In this case, it is particularly advantageous if 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 process according to the invention and its use is further explained below with regard to the preferred formation of oxides. These statements also apply in part analogously to the production of other non-adhesive metal compounds or, based on this specific information, an analogous transfer can easily be carried out for the person skilled in the art, possibly by carrying out a few simple orientation tests.

Surprisingly, the shape of the metallic base body can also play an important role in determining the reaction conditions. For example, a non-adherent oxide can be produced on a coated, planar, titanium-shaped body by subjecting it to a temperature treatment at 650 to 700 ° C Exposes air. A white titanium oxide is formed, which peels off and jumps off when the molded body cools. However, if coated round material is treated under the same reaction conditions, for example wire with a diameter of 3 to 5 mm, 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. Longer reaction times, thermal shock treatment and an increase in the reaction temperature even into the

Close to 750 ° C does not completely detach the coating from the substrate. This phenomenon can be explained if it is assumed that the oxide formed in the temperature range mentioned, due to the Pilling-Bedworth principle, i.e. because of its larger volume, grows on round material without tension during radial growth, since it is linearly proportional to the layer thickness of the growing oxide, r + △ r, the area available for growing up, 2 π (r + △ r) · h, also increases.

• (a) Bei Anoden für Horizontalzellen wird die eigentliche Anodenfläche aus parallelen Titandrähten von etwa 3 bis 5 mm Durchmesser gebildet, die im Abstand von wenigen mm auf ein Stromverteilungssystem aus mehreren massiven Titanstegen aufgeschweisst sind (Schmetterling). Die Stromzuführung erfolgt durch einen Kupferstab, der in den Schmetterling eingeschraubt wird und gegen den Angriff des Chlors durch eine aufgeschweisste Titanhülse geschützt ist.
• (b) Für die Alkalichloridelektrolyse nach dem Diaphragma- bzw. Membran-Verfahren verwendet man Kastenanoden mit äusseren Abmessungen von ca. 0,5 bis 2 m Kantenlänge und einer Tiefe von einigen Zentimetern. Die Korbwände bestehen aus gewalztem oder ungewalztem edelmetallbeschichteten Streckmetall mit Steghöhe und Stegbreite von meist 0,5 bis 3 mm. Zur Stromzuführung wird ein titanplattierter Kupferstab an die Korbwände angeschweisst (vgl. Themaheft "Chloralkali-Elektrolyse" von "Chemie-Ingenieur-Technik", 47. Jahrgang 1975, Heft 4, insbesondere Seite 126, Abb. 1 und 4).

However, the removal of used coatings from wires with a diameter of less than 1 cm or from expanded metal with a bar width and bar height of less than 0.5 cm is of particular importance, since activated metal anodes used in technical electrolysis can mainly be assigned to the following two types of construction:

• (a) In the case of anodes for horizontal cells, the actual anode surface is formed from parallel titanium wires of about 3 to 5 mm in diameter, which are welded at intervals of a few mm onto a current distribution system made of several massive titanium bars (butterfly). The power is supplied by a copper rod that is screwed into the butterfly and against the attack of the chlorine is protected by a welded-on titanium sleeve.
• (b) Box anodes with external dimensions of approximately 0.5 to 2 m edge length and a depth of a few centimeters are used for alkali metal chloride electrolysis using the diaphragm or membrane method. The cage walls consist of rolled or non-rolled expanded metal coated with noble metal with web height and web width of usually 0.5 to 3 mm. To supply power, a titanium-plated copper rod is welded to the basket walls (cf. thematic booklet "Chloralkali Electrolysis" by "Chemie-Ingenieur-Technik", 47th year 1975, booklet 4, especially page 126, fig. 1 and 4).

Surprisingly, it has now been found, that even Me- _t allanoden in the above-described embodiments, the activated surface is substantially made of wires, rods, or expanded metal, can entcoaten according to the inventive method.

This requires a very targeted thermal treatment, 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. This creates a very fine black, X-ray amorphous, substoichiometric titanium oxide in the intermediate layer. The coating peels off slightly when it cools down. In the case of complicated constructions, all coating residues can easily be removed by brushing or by compressed air (without blasting media).

• Die Proben sollten vorgetrocknet werden, da Spuren von Wasser die Bildung festhaftender Verbindungs- und insbesondere Oxidhäute begünstigen.

In general, it may be essential for the formation of the non-adhesive, oxidic intermediate layers according to the invention that the following parameters are observed:

• 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. In particular, 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. Furthermore, a certain duration of treatment must not be exceeded so that e.g. does not convert a non-adherent .stoichiometric compound into a compound with high oxidation levels adhering to the metal surface. This applies particularly to the manufacture of oxides. In general, 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 re-coated 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 process is very little labor, time and cost intensive. Furthermore, the deactivated old coating is obtained in pure form, so that the valuable precious metals that are still preserved can be recovered without cumbersome enrichment from highly abrasive blasting media or aggressive salt melts and pickling is easily possible.

The invention is illustrated below with the aid of a few exemplary embodiments:

example 1

A titanium sheet, 860 x 420 x 3 mm, was coated with a noble metal coating that is especially suitable for chlorate electrolysis and has a layer thickness of 15 µm.

The sheet was used in a technical chlorate electrolysis for 3 years. The residual coating still had an average layer thickness of 10 µm due to 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 was soaked in an HF / HN0 ₃ mixture by soaking for 20 hours. the original black coating. 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 clear crystalline habit.

The sheet was no longer pickled before recoating, but only degreased. The new coating stuck excellent and had better electrochemical values than before.

Example 2

A titanium anode with an effective anode area of 420 x 495 mm, consisting of titanium wires with a diameter of 4 mm, which were welded in parallel at a distance of 3 mm to the power distribution structure, was provided with a coating suitable for chloralkali electrolysis according to the amalgam process and 24 months in one technical electrolysis used. She was pre-dried at 200 ° C for 45 minutes then immediately placed in a preheated 860 ° C oven and held for 10 minutes at 830 ⁰ C. The anode was cooled in air at room temperature. After this treatment, the coating could be lifted off in large pieces. The coating residues hanging in the corners of the construction were brushed off. In places, the otherwise bare metal surface was covered with fine black oxide powder, which was rinsed off in the normal degreasing process. The titanium structure was then coated again, which could then be used again in technical electrolysis.

Claims (15)

1. A method for the removal of electrocatalytically active protective coatings from electrodes with a metal core, characterized in that a non-adhesive intermediate layer of a connection of the carrier metal between the protective coating and the support structure is produced by targeted thermal treatment. 2. The method according to claim 1, characterized in that an oxide, carbide, nitride or hydride is produced as the non-adhesive compound of the carrier metal. 3. The method according to claim 1 or 2, characterized in that the thermal treatment is carried out at a temperature in the range of 400 to 900 ° C. 4. The method according to one or more of the preceding claims, characterized in that 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 compound. 5. The method according to claim 4, characterized in that the gas atmosphere contains a proportion of gas inert under the treatment conditions. 6. The method according to claim 4, characterized in that the O ₂ supplying component is air or a gas with a lower proportion of 0 ₂ and the carbon supplying component is a hydrocarbon gas: 7. The method according to one or more of the preceding claims, characterized in that the thermal treatment is preceded by a predrying step, in particular in the range from 130 to 250 ° C. 8. The method according to one or more of the preceding claims, characterized in that the thermal treatment consists in a rapid passage through low temperature ranges and a short hold at a higher temperature. 9. The method according to one or more of claims 1 to 6, characterized in that the holding is carried out at a higher temperature at about 700 to 870 ° C in a gas atmosphere and in particular air. 10. The method according to one or more of claims 1 to 3, characterized in that the thermal treatment for producing a non-stick oxide is carried out at a temperature above 650 ⁰ C in non-oxidizing molten salt by anodic oxidation. Use of the method according to one or more of the preceding claims for removing deactivated protective coatings from electrodes with a core made of valve metal or a valve metal alloy and in particular titanium. 12. Application of the method according to claim 11 to titanium electrodes, the parts carrying the active coating of expanded metal, wire or rods of a maximum diameter of less than 1 cm. 13. Application of the method according to claim 12, wherein the thermal treatment between 750 and 870 ° C in air is carried out for a period of less than 15 minutes. 14. Application of the method according to claim 11 to titanium electrodes, the parts carrying the active coating consisting of sheets. 15. Application of the method according to claim 14, characterized in that the sheets are treated at a temperature between about 600 to 700 ° C for a period of more than 20 minutes, preferably in air.