DE2213528A1 - Process for removing used coatings from metallic electrodes - Google Patents

Description

St71 / 22

Solvay & Gie., Rue du Prince Albert 33, B-1050 Brussels / Belgium

Process for removing used coatings from metallic electrodes

Priority: April 21, 1971, Luxembourg, No. 63 028

The invention relates to a chemical process for the removal of used coatings from electrodes for electrochemical processes, which electrodes made of a metallic carrier, which conducts the electric current., and is resistant to corrosion under the conditions prevailing in the electrochemical cell, and consist of a coating which is firmly attached to the carrier, also against the electrochemical attack is resistant and favors the exchange of electrons between this carrier and the ions of an electrolyte.

The use of metallic electrodes of this type has developed considerably in recent years, in particular in the electrolysis of aqueous solutions of alkali halides Here one tends to use them as a substitute for the graphite anodes to use because these wear out relatively quickly, frequent readjustment of the distance between Anode and cathode require and periodic renewal of the used anodes, which stops the electrolysis and the opening of the cell with it.

209844/1157

As a result of their increased resistance to the electrochemical attack in the cell venueiden the metallic electrodes with a support made of titanium or Tantttl these libelstands.

Although the wear and tear of these metallic electrodes is exceptionally slow in operation however, their effective life is not unlimited due to their progressive passivation and slow degradation their coatings during operation, especially through partial dissolution or sometimes even through local crumbling, which progressively increases the charge voltage of the Halogen at the anode surface is so increased that it is inevitable There comes a time when one is beneficial

has replaced the / fashions whose coating is degraded ^ or this renewed if you want to maintain the optimal yield in the cell.

If the metallic electrodes are put out of service they still retain a noticeable portion of the initial coating on their surface, which is often Precious metals or their oxides or relatively inert substances and which are difficult to use by common ones Chemical or electrochemical means to be eliminated is «If one wishes, new ones on the surface of this carrier Forming coatings and ensuring their adherence has been found to be inevitable in every bpur of the degraded coating. Even if pickling a surface made of titanium or tantalum is no problem makes, but this is not the case with decommissioned metallic electrodes, where the presence of Noble metal, however small its amount on the surface of the support, is a considerable chemical one Transferring inertia through anodic protection.

The invention involves the complete and rapid removal of the spent coating from the surface of a metallic one Electrode with minimal damage for the wearer to the target. Its aim is also the easy re-

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ORIGINAL INSPECTED

,: / 13 5 28

Obtaining the "remove coating," either for its reuse or its other utilization. In addition, it also aims at the entirety of the metallic carrier to reproduce its original surface condition without requiring additional treatment, e.g. usual Pickling, which more or less damages the carrier metal. After all, she also has the later goal as her goal

sden
To enable a well-adhering coating to be deposited, which in the cell has electrochemical properties comparable to those of the first coating.

The invention relates to a method for rapid and complete elimination of used coatings without substantial damage to the supports of electrodes for electrochemicals Processes which electrodes are made of a metallic support, which conducts the electric current and against electrochemical corrosion is resistant under the operating conditions prevailing in the cell, and from one conductive, fixed on the carrier and also resistant to electrochemical attack and the Favoring electron exchange between the carrier and the ions of an electrolyte in contact with the electrode Coating, this process consists in placing the spent electrodes in a bath of molten salt, which consists essentially of at least one hydrogen sulfate or pyrosulfate of an alkali metal or ammonium is formed, namely at a temperature between 300 and 550 ° C, to immerse and then the electrodes treated in this way to rinse with water after cooling.

Under a metallic support, which conducts the electric current and against electrochemical corrosion under the resistant to the conditions prevailing in the cell is, one generally understands a carrier which at least superficially consists of a film-forming metal, such as titanium, tantalum, niobium, zirconium and tungsten, or an alloy composed mainly of at least one The carrier can optionally have a core made of a more conductive metal or alloy

209 8 A4 / 1 1 57

ORIGINAL JNSPECTH)

possess such _o as copper, aluminum or alloys thereof.

The type of conductive coating fixed on the carrier is not critical for the method according to the invention, which makes it possible to remove a large variety of conductive coatings and in particular of coatings which are at least an active substance that promotes the exchange of electrons between the carrier and the electrolyte include a metal of the platinum group in a metallic state, as an alloy and / or MKaSc »iba5n ^ Mx» K especially ^a xabc Oxydioaxoc. A metal of the platinum group is understood to mean platinum, iridium, rhodium, osmium, ruthenium and palladium. Such coatings include, for example, the dioxides of ruthenium and titanium, iridium-platinum alloys or stoichiometric compounds such as 2 IrC ^ -WO ^. They are quickly eliminated by the method according to the invention, which also enables them to be recovered very easily. To do this, the bath of molten salt, which includes a sufficient amount of the coating, is allowed to cool, and an aqueous solution is produced. The generally insoluble coating substance2 is easily separated by a suitable process, for example by filtration or centrifugation, and can then be used again for coating electrodes or for any other use.

The time required for immersion in the bath of molten salt to completely remove the conductive coating from its support is a function of the temperature and composition of the bath, the type of coating, its thickness and its crystallinity and 55O ⁰ C work. Even if a temperature below 300 ^° C. is sufficient to keep the bath in the molten state, there is no advantage in working below this temperature if one wants to keep the immersion time required for the complete removal of the coating within acceptable limits. It is also advisable ^{not to noticeably exceed the temperature of 55O 0} C if there is any risk of ignition of the metallic

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cs carrier as well as a strong reduction in the bath volume avoid evaporation after a few hours of use. Immersion for 15 to 60 minutes in the bath enables all coatings to be treated at temperatures compatible with the treatment.

At temperatures between 400 and 500 ^° C., the removal of the coating generally proceeds / by keeping the pieces immersed in the bath for 30 minutes.

A bath of molten salt can be used according to the invention any hydrogen sulfate or pyrosulfate of an alkali metal or ammonium, optionally a mixture of two or several of them and optionally in a mixture with a smaller proportion of one or more salts of the alkali metals or ammonium, especially sulfate, sulfite, hydrogen sulfite, pyrosulfite, dithionite, thiosulfate, des

use
Sulphide and the SuIf hydra ts /. Alkali metal is understood to mean lithium, sodium and potassium. In particular, baths with a low melting point are used, for example lithium hydrogen sulfate LiHSO. (Melting point: 120 ⁰ C), ammonium hydrogen sulfate NH ^ HSO, (melting point: 147 ° C) / the equimolecular mixture of Na ₂ S ₂ O ₇ + K ₂ S ₂ O ,, (melting point: 145 ⁰ C), the mixture Na ₂ S ₃ O ₇ + Na ₂ SO ₄ (10 moles / 1 mole - melting point: 190 ⁰ C) and the like. Nevertheless, there is no direct relationship between the melting point of the bath and the treatment temperature, and it is useful in any case to use the bath to bring from molten salt to a minimum temperature of about 400 ⁰ C, if you want to achieve a quick and complete removal of the coating.

The method according to the invention has the considerable advantage on, to a certain extent to leave the metallic support intact, which after its removal from the molten Salt bath, cooling and rinsing with water to loosen the adhesion of the salt is ready, if necessary by means of a light pickling with dilute oxalic acid, a new coating was received, which was just as firmly adhering as the previous one and favorable electrochemical properties:

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possesses, as the following examples show, which serve to further illustrate the invention (Examples 1 to 4) _T

example 1

The bath of molten salt consists of ammonium hydrogen sulfide NH ₄ HSO ₄ (melting point: 147 ^° C.).

During 30 minutes in this held at 400 ⁰ C bath, a non-coated titanium plate and two plates of titanium coated with RuO ₂ + TiO ₂ (10-12 g / m ²⁾ or ₀ Pt + Ir (7.3 g / m ² These two coatings were obtained by applying several layers of a liquid composition to the titanium, each time followed by a heat treatment. For the first coating, a solution of RuCl, .3H ₂ O and TiCl ₄ in isopropyl alcohol was used and the thermal treatments were 380 to 525 ⁰ C. Pur carried out the second coating, a solution of H ₂ PtCl ₆ was .6HpO and H ₂ IrCl ₆ .xH ₂ 0 used in isopropyl alcohol and thermal treatments were carried out between 350 and 400 ⁰ C.

After removal from the molten salt bath, cooling, rinsing with water and drying of the treated plate, the aggressiveness of the bath towards the titanium on the uncoated plate and towards the coating on the other two plates was determined. This aggressiveness is expressed as the depth of penetration (in microns) for the titanium and as the degree of removal (in $>) of the initial coating for the two coated plates, the determination of the former of these two plates was carried out by spectrography and by X fluorescence, while this latter method was used only for the second plate, where the removal of Pt and Ir was determined separately. The results follow in Table 1 after the examples. It can be seen there that under the test conditions the molten bath of NH ₄ HSO ₄ has negligible aggressiveness towards titanium, while under the same conditions it contains the entirety of the coating of RuO ₂ + TiO ₂ and on average 90% of the coating of pi Pt + Ir eliminated.

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After the treatment in the bath of molten salt and rinsing with water, the metallic support has a homogeneous and rough surface, which favors the anchoring of the paintwork. There are four layers of the solution of RuCl, .3HpO and TiGl on titanium plates treated in this way. applied in isopropyl alcohol and subjected it to heat treatments between 380 and 525 G so as to achieve coatings of about 10 g / m of RuO ₂ + TiOo. When subjected to the peeling test using tape applied under pressure, the coatings thus obtained showed excellent adhesiveness.

When tested as anodes in a cell with a mercury cathode for the electrolysis of a ^{brine saturated with sodium chloride and chlorine at 80 °} C. and with the anode-cathode potential difference kept constant, the coated plates treated in this way had recorded electrochemical properties as shown in Table 2 at the end of the Examples is evident.

Example 2

The bath of molten salt consisted of potassium pyrosulfate K ₂ S ₂ O ₇ (melting point above 300 ^° C.).

For 30 minutes, the three plates of the previous example and, in addition, a plate made of uncoated W and a plate made of ^{were in this bath kept at 425 ° C.}

2
Ti with a coating of 4.6 g / m 2 of 2 IrO ₂ -WO. *, Obtained by applying several layers of a solution of

g and H ₂ IrClg in dimethylformamide with subsequent thermal final treatment for 5 hours at 475 ° C.

After removing q, us from the molten bath, cooling, rinsing with water and drying the five plates treated in this way, the aggressiveness of the bath towards Ti and W on the two uncoated plates and towards the coating on the other three plates was determined. This aggressiveness is expressed (Table 1) as the depth of penetration (in microns for Ti and W and as the degree of removal (in %) of the initial coating for the three coated panels. "

209844/1157

For the latter plate were separated (by X fluorescence the degrees of elimination of W and Ir are determined.

The molten bath of K ₂ S ₂ CU has practically no effect on Ti and its aggressiveness towards W is negligible. In contrast, it removes the entirety of the coating of RuO ₂ + TiO ₂ , more than $ 70 (on average) of the coating of Pt + If and about 92 % of the noble metal contained in the coating of 2.

After the treatment in a molten salt bath and V / ater, the metallic carrier to a homogeneous and rough surface, which had the anchoring of the spreads favored As in Example 1, the thus treated sheets of Ti four layers of the solution RuCl ₅ + tiol ₄ applied in such a way as to obtain coatings of about 10 g / m ² of RuO ₂ + TiO ₂ after the thermal treatment. When subjected to the tear-off test by means of adhesive tape applied under pressure, the coatings thus obtained also demonstrated excellent adhesion.

When tested as anodes under the same conditions as in Example 1, those so treated and coated Ti plates also demonstrated excellent electrochemical properties, as shown by the figures in Table 2 testify.

Example 5

The molten salt bath consisted of sodium pyrosulfate S ₂ O ^ (melting point: 401 ⁰ C).

For 30 minutes were immersed in this Example 2 held at 45O ° C bath four plates of Ti. After removal from the molten bath, cooling, rinsing with water and drying of these plates, the aggressiveness of the bath towards Ti on the uncoated plate and towards the coating on the other three plates was determined. This aggressiveness is expressed (see Table 1) as the depth of penetration (in foikron) for Ti and the degree of removal (in i ») of the initial coating on the three coated ones

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Panels, as in the previous example. If the aggressiveness towards Ti is negligible, the molten bath of Na ₂ SpO ₇ removes the entire coating of RuOp + TiOp, Q9f ° (on average) of the coating of Pt + Ir and almost 83% of that in the coating of 2 IrO ₂ -WO * contained precious metal.

After treatment in the molten salt bath and with water, the metallic carrier has a homogeneous and rough surface, which promotes the anchoring of the spreads. As in Example 1, four layers of the solution RuCl ^ + ^ iCl. applied in such a way as to obtain coatings of about 10 g / m ₂ of RuOp + TiO 2 after the thermal treatment. When subjected to the tear-off test using tape applied under pressure, the coatings obtained in this way also had excellent adhesion.

When tested as anodes under the same conditions as in Example 1, those so treated and coated Plates made of Ti also have excellent electrochemical properties, as indicated by the figures in Table 2 prove.

Example 4

The bath of molten salt consisting of potassium hydrogen sulfate KHSO ₄ (melting point: 210 ⁰ C). The three titanium plates of Example 1 were immersed in this ^{bath, which was kept at 50O 0 G, for 30 minutes.} After removal from the bath of molten salt, cooling, rinsing with water and drying of these plates, the aggressiveness of the bath relative to Ti at the ni _c htüberzogenen plate and opposite the coating on the two other plates was determined. This aggressiveness is expressed (see Table 1) as the penetration depth (in microns) for Ti and as the degree of removal (in %) of the initial coating in the two coated plates, as in Example 1. Although greater than that of the baths in FIG previous examples, the aggressiveness of the weld pool made of KHSO, compared to Ti, remains within permissible limits, while it is completely eliminated

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of the coating of RuOp + TiO ₂ and the 90 ^ ige removal (on average) of the coating of Pt + It.

After treatment in the molten bath and with water, the metallic carrier has a homogeneous and rough surface, which promotes the anchoring of the spreads. As in Example 1, four layers of the solution RuCl ^ + TiCl were applied to the titanium plates thus treated. applied in such a way as to obtain coatings of about 10 g / m 2 of RuOp + TiO ₀ after the thermal treatment.

The plates treated and coated in this way were tested as anodes under the same conditions as in Example 1 Excellent electrochemical properties have also been demonstrated from Ti, as can be seen from the figures in Table 2.

"Comparative example

For comparison, a sheet of Ti having a coating of 10-12 g / m RuOp + TIOP prepared after the same method as in Example 1, a pickling under similar conditions by immersion during 5o minutes in a bath maintained at 45O ⁰ C molten, consisting of a mixture of 1/3 KOH + 2/3 KNO ₅ (parts by weight). As can be seen from Table 1, the degree of removal varies

between $ 59 and $ 71 of the initial coating depending on the applied
/ Determination technique, which is clearly inferior to the 99-100% elimination, which in this type of coating under the ^; the same working conditions is obtained with the baths of molten salts according to the invention. ^:

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ORIGINAL INSPECTED

Table 1

Aggressiveness of the baths of molten salt to the carrier and coating

Example no.

Type of bath Melting point Immersion time Bath temperature Attack on Ti (in yu) Attack on W (in u) Removed RuO «+ TiO _? (Spectro) Id. (By X fluorescence) Eliminated Pt + Ir Pt (by X fluorescence) j

Eliminated 2 IrO ₂ -Wo, γ (by X fluorescence) _w

NH ₄ HSO ₄

147 ⁰ C

30 minutes

40O ⁰ C
7.2

99.86%

100%

94%
84%

K ₂ S ₂ O _7> 300 ° C for 30 minutes 425 ⁰ C 2.3 6.2

99

100%

75% 67%

91.6%

401 ⁰ C i'iin.

450 ⁰ C 7.2

99.83%

100%

94% 84%

88.9% 100%

KHSO ₄

210 ⁰ C

30 min,

500 ^° C 69.1

98.60%

"100%

94% 85%

Comparative example

+ I KnO

235 ⁰ C

30th

450 ⁰ C

71% 59%

Tabel

Anodic verbal behavior in a cell with a mercury cathode, the Plates made of Ti, the coating of which was removed by immersion in a bath of molten salt (see Table 1) and on which then a new coating of RuOp + TiOp was applied

Example no.

rt of the pickle used

Thickness of the new coating (g / m)

Initial anodic current density (kA / m2)

Id. After 24 days (n / a / m) Id. After 157 days (n / a / m ² )

Amount of chlorine produced after 157 days (in tons per m ^ of active anodic Surface)

NH ₄ HSO ₄

10.87

37 34 22

127

KHSO

9.73

41 37

22nd

137

10.87

39
34
22nd

127

10.27

37 35 26

138

After 157 days of uninterrupted operation, the limit of usability was not yet reached and the * test was successful be continued"

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Claims (12)

QjV experience for the quick and complete removal and without substantial damage to the carrier of the coatings of electrodes for electrochemical processes, which electrodes consist of a metallic carrier that conducts electrical current and is resistant to electrochemical corrosion under the conditions prevailing in the cell, and of a conductive carrier , firmly attached to the support, also resistant to electrochemical attack and favoring the exchange of electrons between this support and the ions of an electrolyte in contact with the electrode, characterized in that the spent electrodes are placed in a bath of molten salt, which essentially of an alkali metal or of ammonium is formed of at least one hydrogen sulfate or Pyro.ßulfat, immersing at a temperature between 300 and 55O ⁰ C, and then these electrodes so treated, after cooling a rinsing with wat r submits, 2. The method according to claim 1, characterized in that that the bath of molten Mäxx essentially is formed by ammonium hydrogen sulfate. 3. The method according to claim 1, characterized in marked-n e t that the molten salt bath is formed essentially of potassium hydrogen sulfate. 4. The method according to claim 1, characterized in that dciaü the bath of molten salt essentially is formed by potassium pyrosulfate. 5 · The method according to claim 1, characterized in that there is no t that the molten salt bath consists essentially of sodium pyrosulfate. 6. The method according to claim 1, characterized in that the bath of molten salt of an equimolecular Mixture of potassium and sodium pyrosulphate is formed. 7 β method according to claim 1, characterized in that that the immersion time in the bath of molten τ bii-i hü I-ijii Uten is. 2098AA / 1157 8. The method according to any one of the preceding claims, characterized in that the bath of molten Salt also includes at least one alkali sulfate or ammonium sulfate. 9. A method according to claim 1, characterized gekennzeichne tj that the immersion period 400 is 30 minutes and the temperature of the bath of molten salt to 500 ⁰ G. 10. The method according to claim 1, characterized in that that the metallic conductive carrier at least superficially made of a film-forming metal such as titanium, tantalum, Niobium, Zirconium and IwοIfram, or from one mainly is formed from at least one of these metals consisting alloy. 11. The method according to claim 1, characterized in that that the conductive coating is at least one metal from the platinum group in a metallic state, as an alloy and / or as a compound contains, 12. Electrode for electrochemical processes, characterized in that that they are made by coating a carrier made of film-forming metal or alloy, which was previously after a process of the preceding claims has been obtained. ?. U a 8 4 4/1157