HK1134115B - Anode for electrolysis - Google Patents

Description

Anode for electrolysis

Background

The generation of chlorine (chloride) is essentially carried out by electrolysis of an alkali chloride solution, in particular a sodium chloride solution, by means of three alternative technologies, based on diaphragms, mercury cathodes or, in the most advanced case, on ion-exchange membrane electrolyzers, equipped with an anode consisting of a porous or differently perforated titanium sheet having an electrocatalytic coating comprising platinum group metals and/or their oxides (optionally in a mixture); such anodes are known, for example, by Industrie De Nora under the trade nameAnd commercialized. A common problem with these three techniques is the need to limit the molar oxygen (oxyden) content in the chlorine to a level below 2% and preferably not above 1%: oxygen is generated by unavoidable water oxidation secondary reactions and hinders most processes utilizing chlorine, particularly in the dichloroethane synthesis, which is the first step in PVC production. According to the prior art teaching, the anodes (the coating of which is obtained by coating the titanium substrate with a noble metal precursor solution and subsequent decomposition by thermal treatment) are subjected to a final thermal treatment in order to obtain a low oxygen content, which thermal treatment is however accompanied by some negative consequences of energy consumption, estimated on average to be about 50-100 kWh/ton of product depending on the temperature and duration applied.

Furthermore, these same anodes are applied to hydrochloric acid electrolysis, which is gaining increasing attention, since hydrochloric acid is a typical by-product of all the major industrial processes using chlorine: the increase in capacity of today's plants involves the production of large quantities of acid, which is rather difficult to market. Hydrochloric acid electrolysis leads to the formation of chlorine, which can be recycled upstream to create a substantially closed cycle without significant environmental impact, which is currently a decisive factor in obtaining construction permission from the competent authorities. The problem of presenting applications of noble metal-coated titanium anodes in this range is directly linked to the strong aggressiveness of hydrochloric acid: hydrochloric acid penetrates through the defects of the electrocatalytic coating, corroding the titanium-coating interface and causing it to detach in a relatively short time, with the result that the plant is shut down. The first strategy suggested by the prior art, which consists in using a substrate made of a titanium-palladium alloy, which is known for its particular corrosion resistance and is used for the construction of critical equipment in chemical plants, does not produce appreciable results. The second remedy, which cannot be applied beyond certain limits, consists in improving the protection of the titanium substrate by increasing the thickness of the catalytic coating, since it is observed that excessively thick coatings become extremely brittle and therefore suffer from significant detachment phenomena of purely mechanical nature. The solution preferred so far provides for electrocatalytic coatings that need to be obtained in a plurality of covering monolayers: the anodes thus obtained exhibit a reduced number of defects and are therefore characterized by a better working life. It was observed, however, that the advantages in terms of extended lifetime are offset by the disadvantages in terms of higher operating voltages, resulting in an increased electrical energy consumption of about 50-150 kWh/ton of chlorine.

Similar problems also arise in all those electrochemical processes, in particular electrometallurgical processes, in which noble metal-coated titanium electrodes are used as oxygen-evolving anodes: these processes typically involve the use of highly concentrated acid solutions, particularly sulfuric acid, which proves aggressive to the titanium substrates currently in use. In order to obtain an acceptable lifetime, measures such as those reviewed above in relation to the hydrochloric acid situation are generally applied.

It is an object of the present invention to provide an anode for industrial electrolytic processes, in particular in terms of energy consumption and chemical resistance to acidic solutions, overcoming the limitations of the prior art.

In another aspect, it is an object of the present invention to provide an anode for industrial chlorine evolution electrolytic processes which overcomes the limitations of the prior art in terms of oxygen content in the product chlorine.

In another aspect, it is an object of the present invention to provide an anode for industrial oxygen evolution electrolysis processes, such as electrometallurgical processes, which overcomes the limitations of the prior art in terms of duration and operating cell voltage.

These and other objects will be apparent from the following description which is not intended to limit the invention, the scope of which is defined solely by the appended claims.

Description of the invention

The anode according to the invention comprises a titanium alloy substrate provided with an electrocatalytic coating based on noble metals and/or oxides thereof, said titanium alloy comprising elements suitable for being oxidized during the formation of said electrocatalytic coating, these elements preferably being in a concentration of 0.01-5% by weight.

In a preferred embodiment of the invention, the anode of the invention comprises a substrate composed of a titanium alloy comprising one or more elements selected from the group consisting of aluminum, niobium, chromium, manganese, molybdenum, ruthenium, tin, tantalum, vanadium and zirconium; in another embodiment, the alloy further comprises one or more elements selected from the group consisting of nickel, cobalt, iron, and copper.

In a particularly preferred embodiment of the invention, the titanium alloy used as anode substrate comprises 0.02 to 0.04 wt.% ruthenium, 0.01 to 0.02 wt.% palladium, 0.1 to 0.2 wt.% chromium and 0.35 to 0.55 wt.% nickel.

Independent of the final application of the titanium anode, a titanium anode having a noble metal based active coating is produced by the following procedure comprising: the titanium substrate is pretreated by sandblasting and/or etching in an acidic solution, and the electrocatalytic coating based on platinum group metals or their oxides (optionally in a mixture) is applied by thermal decomposition of a coating containing suitable precursors of the final metals and/or oxides at 450-550 ℃.

The coating may present defects in the form of pores or cracks, which are considered to be a significant cause of reduced working life in the particular case of working in the presence of aggressive acidic solutions, such as in the case of hydrochloric acid solutions for the reconversion of hydrochloric acid to chlorine and sulfuric acid solutions used in many electrometallurgical treatments: these solutions can penetrate into the defects until the interface with the titanium substrate is reached and an etching process is started, which in a short time can lead to coating detachment and consequent shut-down of the electrolysis apparatus.

The number of defects is shown to be a strain function (function) of the coating application process: in particular, past experience has shown that the higher the thickness (or specific loading), the fewer defects are present in the electrocatalytic coating; on the other hand, for a given thickness or specific loading, the more portions are applied (in other words, the higher the number of individual layers applied), the fewer defects are present. In the latter case, it is evident that the total heat treatment as a function of the strain of the number of individual layers can last for a considerable time.

In a preferred embodiment, said single layer constituting said catalytic coating is obtained by successive thermal decomposition steps of total duration higher than 1 hour.

For anodes used in the electrolysis of acidic solutions, a similar long-term heat treatment must also impart sufficient resistance to dissolution of the coating: this positive effect can be associated with a crystallization process of the coating material which leads to the elimination of the more vulnerable amorphous parts.

Similar situations are also experienced when using such anodes in chlor-alkali electrolysis, where industrial users typically require that the oxygen content in chlorine be kept below certain limits, such as less than 2% and preferably less than 1%: such a result is in fact obtained by subjecting the anode to a further final heat treatment.

Industrial experience has shown that extending the duration of the treatment at a temperature of 450-.

As an example of these drawbacks, the relative electrochemical potential E is reported in table 1 below_C12，SCEData of (SCE ═ saturated calomel reference electrode) and oxygen content in chlorine as a function of total heat treatment time (d, given in hours) obtained for the anode for chlorine evolution in chlor-alkali electrolysis, the other production parameters were kept constant (pure titanium substrate grade 1 according to ASTM B265, rutritio, an oxide of ruthenium, iridium and titanium mixed by non-stoichiometric ratio, namely rutritio_xA structured electrocatalytic coating).

Entirely similar results were obtained using titanium palladium alloys as the substrate (ASTM B265, grade 7, palladium 0.12-0.25 wt%), with higher costs even acceptable, at least in some applications, in exchange for possible voltage and lifetime increases.

The inventors have unexpectedly observed that when the substrate is composed of a suitable titanium alloy, it is possible to manufacture anodes with a long total heat treatment time without experiencing a significant deterioration of the electrochemical operating potential, unlike the teaching of the prior art: the invention thus provides an anode of higher quality which is capable of operating both in hydrochloric acid solution electrolysis or in the sulfuric acid-containing electrolytes currently used in electrometallurgy with an extended working life and of generating chlorine with a low oxygen percentage in chlorine-caustic soda electrolysis.

In particular, very interesting results are obtained with titanium alloys comprising one or more elements of a first group consisting of aluminum, niobium, chromium, manganese, molybdenum, ruthenium, tin, tantalum, vanadium and zirconium, optionally added with elements of a second group comprising nickel, cobalt, iron, copper. It has also been found that titanium alloys containing only one or more elements of the second group prove to be less effective in preventing electrochemical potential deterioration under long-term heating. Furthermore, the presence of iridium, rhodium, palladium and platinum in the alloy has proved to be insignificant, even though the addition of these elements may in any case produce advantageous results for preventing certain kinds of corrosive attacks, which occur when the anodes remain immersed in aggressive solutions during the electrolyzer shutdown procedure, as is known to the person skilled in the art.

Without being bound by any particular theory, first considering the cause of the increase in electrochemical potential of a titanium anode subjected to a long heat treatment, a possible explanation of the positive effect of the elements of the first group defined above can be given: it is a widely accepted view that the growth of a titanium oxide film at the interface between the coating and the substrate during the coating layer forming step causes a potential decay: because the heat treatment is carried out in the presence of air at 450 ℃ - & 550 ℃, titanium metal actually tends to be oxidized by oxygen diffusing through the coating. Titanium oxide generated in this way is hardly conductive and thus becomes a location of a resistance voltage drop that increases the actual galvanic potential during operation: such a resistive voltage drop is of a moderate degree, so that its effect on the galvanic potential remains negligible until the titanium oxide film is sufficiently thin. This situation is only true when the total heat treatment duration does not exceed a certain value, which is different from the requirement to produce an anode as follows: the anode is characterized by a satisfactory working life in aggressive environments (reduced number of individual layers still having significant residual defects) or a low oxygen percentage in chlor-alkali applications.

The first group of elements defined above is primarily characterized by being susceptible to oxidation in the typical processing conditions under which electrocatalytic coatings are applied, in particular with respect to temperature and the presence of air: it can therefore be considered that these elements act as dopants for titanium oxide, which thus obtains a significantly higher conductivity than the corresponding oxide grown on unalloyed titanium. The second aspect can be given by the ability to form solid solutions, at least when used at low levels (typically 0.01-5 wt%): a solid solution in which the alloying elements are uniformly dispersed will allow the same elements to be dispersed in a similarly uniform manner in a shallow (super) titanium oxide phase, even at moderate alloying element contents, giving it the same conductivity characteristics described above. However, it is known that the second group of elements, which can also be oxidized during the formation of the coating, generally causes segregation phases in the form of particles dispersed in the metal matrix and whose position corresponds in particular to the crystal grain boundaries: as a possible consequence of such discontinuous distribution on a microscopic scale, their presence inside the titanium oxide may also be non-uniform, having a less pronounced effect on the electrical conductivity.

Some of the more significant results obtained by the inventors are given in the following examples, which are not intended to limit the scope of the invention.

Example 1

Some anodes for chlorine evolution from hydrochloric acid electrolysis were prepared by using the following procedure:

a. the following titanium alloys were obtained in sheets 1mm thick (content of additional elements in percent by weight):

■ alloy 1: titanium-ruthenium (0.08/0.14%)

■ alloy 2: titanium-aluminium (1.0/2.0%)

■ alloy 3: titanium-tantalum (5%)

■ alloy 4: titanium-aluminium (2.5/3.5%) -vanadium (2.0/3.0%)

■ alloy 5: titanium-molybdenum (0.2/0.4%) -nickel (0.6/0.9%)

■ alloy 6: titanium-chromium (0.1/0.2%) -nickel (0.35/0.55%) -ruthenium (0.02/0.04%) -palladium (0.01/0.02%)

■ alloy 7: titanium-palladium (0.12/0.25%) (cf. prior art)

■ alloy 8: titanium-iron (0.5%)

■ alloy 9: grade 1 pure titanium according to ASTM B265 (see prior art)

b. Cold cutting the sheet into square plates with 5cm sides

c. One side of each panel was pretreated by sand blasting followed by degreasing and hydrochloric acid etching

d. On the pretreated side, a coating consisting of a mixed oxide of ruthenium and titanium was applied, this coating consisting of a plurality of monolayers, each layer being obtained by thermal decomposition of an aqueous coating comprising chlorides of the two metals at 480-490 ℃ for 10 minutes, to a total of 25 layers corresponding to a total ruthenium loading of 50 mg.

The plate thus activated, with the addition of a further plate identified as alloy 9B and provided with the same composition and loading (but obtained by applying only 13 monolayers and then subjecting the alloy of type 9 to a final heat treatment for a total duration of 4 hours) is subjected to 60 ℃, in an electrolytic cell charged with 14% by weight of hydrochloric acid at 0.5A/m²The current density of (a) is operated. The perfluorinated Nafion 324 ion exchange membrane commercialized by DuPont/USA divides the cell into two compartments, an anode compartment and a cathode compartment, containing the plate under test and a zirconium cathode of the same size, respectively. During electrolysis, the electrochemical potential E of the plate operating as chlorine-evolving anode is measured_C12，SCE(V, reference: saturated calomel), and periodic tests of coating adhesion were performed: the data are summarized in tables 2a and 2 b.

TABLE 2

The data of tables 2a and 2b show that the use of a titanium alloy comprising elements of the first group according to the invention allows firstly to meet the aim of working at low galvanic potentials with an electrical energy saving of about 50-100 kWh/ton chlorine, despite the fact that the manufacturing process comprising the deposition of a high number of monolayers follows in order to obtain a coating substantially free of through-defects. Such high industrial relevance results are also accompanied by a significant stability of the coating, which is not affected by a severe detachment from the substrate.

The data in tables 2a and 2b demonstrate that the elements of the second group defined above, if present in significant amounts, are themselves capable of ensuring improved electrochemical potentials relative to the prior art, albeit to a lesser extent than those obtainable with the alloying elements of the first group (see alloy 8).

Finally, the data in tables 2a and 2B show that the performance of the anodes according to the invention proves to be significantly better with respect to the performance of anodes comprising a coating made of a few but highly defective monolayers (see alloy 9B, prior art) and with a coating consisting of a number of monolayers applied to pure titanium or to a titanium alloy containing an element that cannot be oxidized (such as palladium) (see alloy 9 and alloy 7, prior art).

Example 2

Some anodes for electrolysis of sodium chloride solutions were prepared by using the following procedure:

a. the following titanium alloys were obtained in sheets 1mm thick (content of additional elements in percent by weight):

■ alloy 2: titanium-aluminium (1.0/2.0%)

■ alloy 5: titanium-molybdenum (0.2/0.4%) -nickel (0.6/0.9%)

■ alloy 6: titanium-chromium (0.1/0.2%) -nickel (0.35/0.55%) -ruthenium (0.02/0.04%) -palladium (0.01/0.02%)

■ alloy 9: grade 1 pure titanium according to ASTM B265 (see prior art)

b. Cold cutting the sheet into square plates with 5cm sides

c. One side of each panel was pretreated by sand blasting followed by degreasing and hydrochloric acid etching

d. On the pretreated side, a coating consisting of a mixed oxide of ruthenium, iridium and titanium, consisting of a plurality of monolayers, each obtained by thermal decomposition of an aqueous coating comprising the chlorides of the three metals at 490-500 ℃ for 10 minutes, to a total of 11 layers corresponding to a total loading of 55mg of ruthenium + iridium, was applied. The plate is further subjected to a final heat treatment for a period (d) of 1-4 hours.

The thus activated sheet is placed in an electrolytic cell at 90 ℃ at a rate of 0.4A/m²The current density of (a) is operated. The perfluorinated Nafion 982 ion exchange membrane commercialized by DuPont/USA divides the cell into two compartments, an anode compartment and a cathode compartment, in which the plate under test and a nickel cathode of the same size are installed. The two compartments contained respectively a sodium chloride solution with a concentration of 220g/l and a pH of 3 and a sodium hydroxide solution of 32% by weight.

During electrolysis, the electrochemical potential E of the plate operating as chlorine-evolving anode is measured_C12，SCE(V, reference: saturated calomel) and oxygen content in product chlorine: the relevant data are summarized in table 3.

TABLE 3

The data in table 3 show that in the case of the anodes of the invention comprising a suitable titanium alloy as substrate, a final heat treatment can be carried out in order to reduce the oxygen content in the chlorine to a completely industrially satisfactory level, without any significant loss of potential. Such a result is not achieved with anodes according to the prior art, in which the titanium substrate, which does not contain the alloying element according to the invention, forms a non-conductive oxide which grows thicker with the extension of the heat treatment carried out on the anode (see alloy 9): the growth of the non-conducting oxide brings about a significant deterioration of the operating potential of the anode, which can be quantified to about 100 kWh/ton of chlorine.

Example 3

Two pairs of 1mm thick square plates with 2cm sides obtained by cold cutting sheets of alloy 6 and alloy 9 (prior art) were treated as follows:

a. one side of each panel is pretreated by heavy sandblasting in order to produce a high surface roughness, followed by degreasing and hydrochloric acid etching

b. On the pretreated side of each plate a coating consisting of mixed oxides of iridium and titanium, consisting of a plurality of monolayers, each obtained by thermal decomposition of an aqueous coating comprising chlorides of the two metals at 490-500 ℃ for 10 minutes, reaching a total of 16 layers corresponding to a total loading of iridium of 32 mg.

The plates were mounted in an undivided cell containing a 10 wt.% sulfuric acid solution and a zirconium cathode of the same size at 60 °. At 2A/cm²In order to simulate operating conditions that are significantly more severe than the typical operating conditions in an electrometallurgical treatment, such as rapid electrogalvanizing of steel sheets or copper foil deposition of controlled thickness.

During operation, the galvanic potential of the plate is detected: the measured values are 1.35V/SCE and 1.55V/SCE for anodes according to the invention comprising a catalytic coating applied to alloy 6 and for anodes according to the prior art in which an electrocatalytic coating is applied to titanium without alloying elements (alloy 9), respectively. Thus, similarly to what is seen in example 1 for the electrolysis of hydrochloric acid solutions, also the anodes suitable for working in electrometallurgical processes in contact with aggressive sulfuric acid (sulfuric) solutions, can advantageously be applied with electrocatalytic coatings consisting of a plurality of monolayers, allowing the presence of defects that may hinder lifetime to be eliminated or at least reduced to tolerance levels without at the same time generating a loss of galvanic potential.

The foregoing description is not intended to limit the invention, which may be applied in different embodiments without departing from the scope thereof, which is expressly defined by the claims appended hereto.

In the description and claims of the present invention, the term "comprising" and its variants, such as "comprises" and "comprising", are not intended to exclude the presence of other elements or additives.

Claims (10)

1. An anode for electrochemical treatments comprising a metal substrate provided with an electrocatalytic coating containing platinum group metals and/or oxides thereof and formed by a plurality of single layers obtained by thermal decomposition of soluble precursors, said metal substrate being made of a titanium alloy containing at least one element that can be oxidized under said thermal decomposition conditions, said at least one element that can be oxidized being selected from a first group consisting of aluminum, niobium, chromium, manganese, molybdenum, ruthenium, tin, tantalum, vanadium and zirconium, said anode further comprising a titanium oxide layer interposed between the metal substrate and the electrocatalytic coating, wherein the oxide of said at least one element that can be oxidized obtained during said thermal decomposition is partially dispersed in said titanium oxide layer; and wherein the titanium alloy of the metal substrate further comprises at least one element selected from a second group consisting of nickel, cobalt, iron, and copper.

2. The anode of claim 1, wherein the at least one oxidizable element is present in a concentration of 0.01 to 5% by weight.

3. The anode of claim 1 or 2 wherein the titanium alloy comprises 0.02 to 0.04 wt% ruthenium, 0.01 to 0.02 wt% palladium, 0.1 to 0.2 wt% chromium and 0.35 to 0.55 wt% nickel.

4. Anode for electrochemical treatment comprising a metal substrate provided with an electrocatalytic coating containing platinum group metals and/or oxides thereof and formed of a plurality of monolayers obtained by thermal decomposition of soluble precursors, said metal substrate being made of a titanium alloy comprising 0.02-0.04 wt.% ruthenium, 0.01-0.02 wt.% palladium, 0.1-0.2 wt.% chromium and 0.35-0.55 wt.% nickel.

5. The anode of any one of the preceding claims wherein said single layer constituting said catalytic coating is obtained by successive thermal decomposition steps of total duration higher than 1 hour.

6. The anode according to claim 5, wherein said electrocatalytic coating is further subjected to a final heat treatment.

7. An electrolysis cell, characterized by being equipped with an anode according to any one of the preceding claims.

8. Use of the cell of claim 7 for the electrolysis of hydrochloric acid solutions.

9. Use of the cell of claim 7 for chlorine-caustic soda electrolysis treatment.

10. Use of the cell of claim 7 for electrometallurgical processing with oxygen anodic evolution in acidic electrolytes.