EP0046853B1 - 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 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. Although 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.

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 2157 511) have not proven successful in practice, as shown by various post-published patents such as US Pat. No. 3,636,577 and US Pat. No. Re 28,849.

There is therefore a need, generally recognized by experts, to clean the metal construction 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.

Mechanical removal of the coatings by dry or wet blasting has already been described in DE-OSen2815955, 2638218 and 2645414. Although it is probably the most widespread process, it is disadvantageous in that sandblasting by hand incurs very high personnel costs, whereas automatic blasting cannot avoid high losses in construction material. It is also very difficult to recover the noble metals or compounds thereof from the used abrasive, which according to studies 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 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.

DE-PS 1909 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 acids and bases 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 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.

These methods are less suitable for technical use, but more to address as laboratory methods. In particular, processes which work with acidic 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 due to 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 salts according to DE-PS 1 909 757, although only at higher temperatures.

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.

Various physical phenomena come into question as the cause of this non-sticking of the newly formed connection 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 due to different coefficients of thermal expansion, or through the formation of gaseous compounds such as oxides, hydrides etc., or through weakening of the bond in the boundary layer through 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 consist of 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 .

With sheet metal, the thermal treatment 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 help of thermogravimetric and differential thermal analysis, since the available literature primarily relates to the formation of bonds between unprotected metals. Due to the impeded diffusion through the protective layer, e.g. slightly sub-stoichiometric compounds in the intermediate layer, e.g. Oxides that can form on the bare metal surface under significantly different conditions, for example under a very greatly reduced gas partial pressure.

In the process according to the invention, as already mentioned above, 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. For example, air or mixtures with a lower oxygen content can be used as the oxygen-supplying component. 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 in the gas generally has no particular advantage. As the carbon supply component, e.g. serve an atmosphere containing hydrocarbons. As nitrogen or what. The nitrogen-supplying component can be primary nitrogen, its hydrogen compounds or hydrogen. Depending on how the reaction conditions are carried out, it may sometimes be expedient to additionally add a proportion of gas which is inert under the treatment conditions to the gas atmosphere. As such an 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 predrying stage before the thermal treatment. 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 producing the non-adhesive metal compound, it is often preferred to run through low temperature ranges very quickly both in the heating phase and in the cooling phase, and only for a short period at the reaction temperature at which the non-adhesive metal compound is formed, often less than 1 hour, sometimes preferably in the range of 20 to 40 minutes for metal 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 optimal 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, such as are often used in aqueous chlor-alkali electrolysis, ie electrodes which are based on If 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.

However, it is also possible to use the thermal 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. In the case of such electrode constructions, which are often used in aqueous chlor-alkali electrolysis, it is often particularly expedient 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 explanations also apply in part to the production of other non-adhesive valve metal connections or, based on this specific information, an analogous transfer can easily be carried out for the person skilled in the art, if necessary 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 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. Even longer reaction times, thermal shock treatment and an increase in the reaction temperature close to 750 ° C do not result in a complete detachment of 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 linearly proportional to the layer thickness of the growing oxide, r + Ar, the area available for growing up, 2π (r + Δr) · 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 edelmetallbeschichtetem 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 approximately 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, which is screwed into the butterfly and is protected against the attack of the chlorine 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 basket 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 (see the “Chlor-alkali electrolysis” booklet by “Chemie-Ingenieur-Technik”, 47th year 1975, volume 4, in particular page 126, Figs. 1 and 4).

Surprisingly, it has now been found that 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.

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 designs, all coating residues can easily be removed by brushing or by compressed air (without blasting media).

In general, it can be used for the formation of he non-adhesive, oxidic intermediate layers according to the invention, it is essential 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 substoichiometric compound into a compound of 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 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. Furthermore, 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 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 layer of 15 ₁ 1m that contains precious metals and is especially suitable for chlorate electrolysis. 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 clear crystalline habit.

The sheet was no longer pickled before recoating, but only degreased. The new coating adhered extremely well 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 using the amalgam process and 24 months in one technical electrolysis used. It was pre-dried at 200 ° C for 45 minutes, then immediately placed in an oven preheated to 860 ° C and held at 830 ° C for 10 minutes. The anode was cooled in air at room temperature. After this treatment, the coating could be lifted off in large pieces. The remains of the coating 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 (10)

1. Method for the ablation of electrolytically effective noble metal-containing protective coatings from electrodes having valve metal (alloy) core characterised in that for the purpose of exposing clean outer surfaces for the re-coating by means of controlled thermal treatment in a gas atmosphere having at least a proportion of oxygen, carbon, nitrogen or hydrogen releasing component or a mixture thereof at a temperature in the range from 400 to 900°C, a non-adhesive intermediate layer, located between the protective coating and the carrier construction and consisting of oxide, carbide, nitride or hydride of the carrier valve metal is produced.

2. Method according to claim 1 characterised in that the gas atmosphere contains a proportion of gas which is inert under the treatment conditions.

3. Method according to claim 1 characterised in that the O2 releasing component is air or a gas with a lower proportion of O2 and the carbon releasing component is a hydrocarbon gas.

4. Method according to one or more of the proceding claims characterised in that a pre-drying step, in particular in the range of from 130 to 250°C, is introduced before the thermal treatment.

5. Method according to one or more of claims 1 to 3 characterised in that the holding at high temperature is carried out at about 700 to 870°C in gas atmosphere, in particular air.

6. Method according to claim 1 characterised in that the thermal treatment for production of a non-adhesive oxide is carried out at a temperature lying above 650°C in non-oxidizing salt melt by means of anodic oxidation.

7. Use of the method according to one or more of the preceding claims with titanium electrodes the part of which carries the active coating consisting of expanded metal, wire or rods having a maximum diameter below 1 cm.

8. Use of the method according to claim 7, wherein the thermal treatment is carried out between 750 and 870°C in air over a time span of less than fifteen minutes.

9. Use of the method according to one or more of claims 1 to 6 on titanium electrodes of which the part carrying the active coating consists of sheet metal.

10. Use of the method according to claim 9 characterised in that the sheets are treated at a temperature between about 600 and 700°C over a time period of more than twenty minutes, preferably in air.