IN INDUSTRIAL ELECTROLYTIC PROCESSES

FIELD OF THE INVENTION

The present invention relates to an electrode suitable for functioning as anode in electrochemical cells, for instance as oxygen-evolving anode in electrolytic cells for electrometallurgical processes.

BACKGROUND OF THE INVENTION

The invention relates to an electrode suitable for use in industrial electrochemical processes, for instance in electrolytic applications involving an oxygen evolution anodic reaction. Oxygen-evolving anodes are widely used in several electrochemical applications, many of which fall in the domain of electrometallurgy and cover a wide range in terms of applied current density, which can be very low (for instance few hundreds A/m ² , as in electrowinning processes) but also very high (for instance fast plating, in which 10 kA/m ² referred to the anodic surface can be exceeded); another field of application for oxygen-evolving anodes is given by impressed current cathodic protection. Electrodes suitable for anodic oxygen evolution can be obtained starting from valve metal substrates, for example titanium and alloys thereof, coated with catalytic compositions based on transition metals or alloys thereof, characterised by their capability to lower the oxygen discharge anodic overvoltage, too high to allow carrying out industrial processes in the absence of catalytic systems. A composition suitable for catalysing anodic oxygen evolution consists for instance of a mixture of oxides of iridium and tantalum, wherein iridium constitutes the catalytically active species and tantalum favours the formation of a compact coating, capable of protecting the valve metal substrate from corrosion phenomena especially when operating with aggressive electrolytes. An anode formulation suitable for anodic oxygen evolution in many industrial electrochemical processes comprises a titanium substrate and a catalytic coating consisting of iridium and tantalum oxides of molar composition referred to the metals of 65% Ir and 35% Ta. In some cases, for examples to be able to operate with very acidic or otherwise aggressive electrolytes, it can be advantageous to interpose a protective interlayer between the titanium substrate and the catalytic coating, for instance consisting of titanium and tantalum oxides of molar composition referred to the metals of 80% Ti and 20% Ta. This kind of electrode can be prepared in several ways, for instance by thermal decomposition at high temperature, for instance 400 to 600 ⁰ C , of a precu rsor solution. An electrode with the above-specified composition can meet the needs of many industrial applications, both at low or high current density, with reasonable operative lifetimes. The economics of some productive processes especially in the metallurgical domain (for instance copper deposition in galvanic processes for the manufacturing of printed circuits and copper foil) nevertheless requires that electrodes have a higher and higher duration, versus a suitably reduced oxygen evolution potential also at high current density: oxygen evolution potential is in fact one of the main factors in determining the process operative voltage and therefore the overall energy consumption. Moreover, the operative lifetime of anodes based on noble metals or oxides thereof on valve metal substrates is remarkably reduced in the presence of particularly aggressive pollutants, capable of establishing accelerated phenomena of corrosion or of fouling of the anode surface. An example of the former type is given by fluoride ions, which determine a specific attack on valve metals such as titanium deactivating the electrodes very quickly; in some industrial environments, remarkable costs have to be faced to reduce fluoride concentration to extremely low levels, since a fluoride ion content higher than 0.2 parts per million (ppm) can already be capable of showing sensible effects on the duration of the anodes. An example of the latter type is conversely given by manganese ions, present in several industrial electrolytes in a typical amount of 2-30 g/l, which starting from concentrations as low as 1 g/l have the tendency to film the anode surface with an MnO2 layer liable to blind their catalytic activity and difficult to remove without causing damages.

It has been therefore made evident the need to provide anodes for oxygen evolution characterised by higher operative lifetimes even in particularly critical process conditions, such as a high current density or the presence of particularly aggressive electrolytes, for instance due to the presence of contaminant species.

SUMMARY OF THE INVENTION Various aspects of the invention are set out in the accompanying claims.

In one embodiment, the present invention relates to an electrode suitable for functioning as anode in electrolytic processes, comprising a substrate of titanium or other valve metal and a catalytic coating consisting of one or more layers, whose outermost layer, suitable for working in contact with the electrolyte, consists of oxides of iridium, tantalum and titanium with a molar composition referred to the metals of Ir 76-84%, Ta 15-23%, Ti 0.2-1.3%. The inventors surprisingly observed that the addition of small amounts of titanium to a catalytic composition very rich in iridium has the capability of increasing the duration of an electrode used for anodic oxygen evolution to a remarkable extent, also improving its tolerance to the presence of contaminant species. In one embodiment, a catalytic coating consisting of multiple layers comprises an internal layer consisting of oxides of iridium and tantalum with a molar composition referred to the metals of Ir 60- 70% and Ta 30-40%, and an external layer consisting of oxides of iridium, tantalum and titanium with a molar composition referred to the metals of Ir 76-84%, Ta 15-23%, Ti 0.2-1.3%. This can have the advantage of better distributing iridium, by far the most expensive component of the electrode, so as to maximise its efficacy for a given overall loading. The optimum overall loading of iridium in the catalytic coating depends on the specific type of application whereto the electrode is destined and on the minimum operative lifetime required to the anode in such application: in one embodiment, the overall specific loading of iridium in the catalytic coating is 20 to 50 g/m ² , optionally subdivided into an internal layer containing 15 to 25% of total loading and an external layer containing the rest.

In one embodiment, a protective interlayer, for instance based on titanium and/or tantalum oxides, is interposed between the substrate and the catalytic layer. In one embodiment, the interlayer consists of a mixture of oxides of titanium and tantalum with a composition of Ti 75-85%, Ta 15-25% referred to the metals. In another embodiment, the protective interlayer consists of titanium oxide formed by a process of thermal oxidation of the titanium substrate. These embodiments can have the advantage of providing the substrate with an adequate form of protection against the electrolyte corrosive attack without excessively burdening the cost of the electrode. A person skilled in the art will nevertheless be able to identify other types of protective interlayer, such as titanium and/or tantalum oxides applied by flame or plasma spraying, galvanically or by means of different techniques of chemical or physical vapour deposition, optionally under vacuum (CVD, PVD, IBAD, sputtering) according to the specific requirements.

Some of the most significant results obtained by the inventors are presented in the following examples, which are not intended as a limitation of the extent of the invention.

EXAMPLE 1

A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100 ⁰ C for 20 minutes.

After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m ² referred to the metals (equivalent to 0.87 g/m ² referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ - and subsequent thermal decomposition at 500 ⁰ C. A catalytic coating based on oxides of iridium, tantalum and titanium at a 78:20:2 weight ratio (corresponding to a molar ratio of about 80.1 :19.4:0.5) was then applied on the protective layer, with an overall loading of iridium of 45 g/m ² . The application of the catalytic coating was carried out by painting in 22 coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ , then of H ₂ IrCIe until reaching an Ir concentration of 195 g/l - and subsequent thermal decomposition at 480°C.

Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 5245 hours. Similar series of three samples were subjected to the same test in the presence of 1 mg/l and 5 mg/l of fluoride ion: following such test, an average deactivation time of 3715 and 980 hours was detected, respectively.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 3900 hours was detected. EXAMPLE 2

A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100 ⁰ C for 20 minutes.

After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m ² referred to the metals (equivalent to 0.87 g/m ² referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ - and subsequent thermal decomposition at 500 ⁰ C.

A catalytic coating consisting of two distinct layers was then applied on the protective layer: a first (internal) layer based on oxides of iridium and tantalum in a 65:35 weight ratio (equivalent to a molar ratio of about 66.3:36.7), with an overall loading of iridium of 10 g/m ² , and a second (external) layer based on oxides of iridium, tantalum and titanium, at a 78:20:2 weight ratio (corresponding to a molar ratio of about 80.1 :19.4:0.5), with an overall loading of iridium of 35 g/m ² . The application of the internal catalytic layer was carried out by painting in 8 coats of a precursor solution - obtained by addition of H ₂ IrCIe to an aqueous TaCI ₅ solution, until reaching an Ir concentration of 76 g/l - and subsequent thermal decomposition at 520°C. The application of the external catalytic layer was carried out by painting in 14 coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution Of TiCI ₄ , then of H ₂ IrCIe until reaching an Ir concentration of 195 g/l - and subsequent thermal decomposition at 480 ⁰ C.

Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 6270 hours.

Similar series of three samples were subjected to the same test in the presence of 1 mg/l and 5 mg/l of fluoride ion: following such test, an average deactivation time of 4080 and 1360 hours was detected, respectively.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 4420 hours was detected.

EXAMPLE 3 A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100°C for 20 minutes.

After drying, the sheet was subjected to a thermal treatment at 650 ⁰ C for 3 hours in the presence of air, obtaining the growth of a protective titanium oxide layer.

A catalytic coating based on oxides of iridium, tantalum and titanium at a 80:15:5 weight ratio (corresponding to a molar ratio of about 83.9:14.8:1.3) was then applied on the protective layer, with an overall loading of iridium of 45 g/m ² . The application of the catalytic coating was carried out by painting in 20 coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ , then of H ₂ IrCIe until reaching an Ir concentration of 195 g/l - and subsequent thermal decomposition at 480 ⁰ C.

Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 4980 hours.

Similar series of three samples were subjected to the same test in the presence of 1 mg/l and 5 mg/l of fluoride ion: following such test, an average deactivation time of 3630 and 920 hours was detected, respectively.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 2100 hours was detected.

EXAMPLE 4

A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected to the deposition of a protective layer of oxides of titanium and tantalum at a 70:30 weight ratio, with a thickness of about 25 μm, by plasma spraying.

A catalytic coating based on oxides of iridium, tantalum and titanium at a 75:24:1 weight ratio (corresponding to a molar ratio of about 76.6:23.1 :0.3) was then applied on the protective layer, with an overall loading of iridium of 20 g/m ² . The application of the catalytic coating was carried out by painting in 15 coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ , then of H ₂ IrCIe until reaching an Ir concentration of 195 g/l - and subsequent thermal decomposition at 480°C. Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 3600 hours.

Similar series of three samples were subjected to the same test in the presence of 1 mg/l and 5 mg/l of fluoride ion: following such test, an average deactivation time of 870 and 120 hours was detected, respectively.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 2460 hours was detected.

COUNTEREXAMPLE 1

A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100 ⁰ C for 20 minutes.

After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m ² referred to the metals (equivalent to 0.87 g/m ² referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ - and subsequent thermal decomposition at 500°C. A catalytic coating based on oxides of iridium and tantalum at a 65:35 weight ratio was then applied on the protective layer, with an overall loading of iridium of 45 g/m ² . The application of the catalytic coating was carried out by painting in 29 coats of a precursor solution - obtained by addition of H ₂ IrCIe to an aqueous TaCI ₅ solution until reaching an Ir concentration of 76 g/l - and subsequent thermal decomposition at 520 ⁰ C. Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 2800 hours. Similar series of three samples were subjected to the same test in the presence of 1 mg/l of fluoride ion: for all the three samples, a deactivation time lower than 100 hours was detected.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 1550 hours was detected.

COUNTEREXAMPLE 2 A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100 ⁰ C for 20 minutes.

After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m ² referred to the metals (equivalent to 0.87 g/m ² referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution of TiCI ₄ - and subsequent thermal decomposition at 500°C.

A catalytic coating based on oxides of iridium and tantalum at a 80:20 weight ratio was then applied on the protective layer, with an overall loading of iridium of 45 g/m ² . The application of the catalytic coating was carried out by painting in 30 coats of a precursor solution - obtained by addition of H ₂ IrCIe to an aqueous TaCI ₅ solution until reaching an Ir concentration of 76 g/l - and subsequent thermal decomposition at 520 ⁰ C.

Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² . The average deactivation time for the three samples was 2940 hours.

Similar series of three samples were subjected to the same test in the presence of 1 mg/l of fluoride ion: for all the three samples, a deactivation time lower than 100 hours was detected. A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 1020 hours was detected.

COUNTEREXAMPLE 3

A titanium sheet grade 1 of 200 x 200 x 3 mm size was degreased and subjected first to sandblasting with iron grit until obtaining a value of superficial roughness R _z of 70 to 100 μm, then to etching in 20% by weight HCI at a temperature of 90-100°C for 20 minutes. After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m ² referred to the metals (equivalent to 0.87 g/m ² referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution Of TiCI ₄ - and subsequent thermal decomposition at 500 ⁰ C.

A catalytic coating based on oxides of iridium, tantalum and titanium at a 63:35:2 weight ratio was then applied on the protective layer, with an overall loading of iridium of 45 g/m ² . The application of the catalytic coating was carried out by painting in 29 coats of a precursor solution - obtained by addition of an aqueous TaCI ₅ solution, acidified with HCI, to an aqueous solution Of TiCI ₄ , then of H ₂ IrCIe until reaching an Ir concentration of 76 g/l - and subsequent thermal decomposition at 520 ⁰ C. Three samples of 10 cm ² surface were cut out from the thus obtained electrode and submitted to an accelerated lifetime test under anodic oxygen evolution, measuring the deactivation time (defined as the operating time required to observe a potential increase of 1 V) in H ₂ SO ₄ at 150 g/l, at a temperature of 60 ⁰ C and at a current density of 30 kA/m ² .

The average deactivation time for the three samples was 2170 hours.

Similar series of three samples were subjected to the same test in the presence of 1 mg/l of fluoride ion: for all the three samples, a deactivation time lower than 100 hours was detected.

A similar series of three samples was subjected to the test in the presence of 20 g/l of manganese ions: following such test, an average deactivation time of 940 hours was detected.

The previous description is not intended to limit the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is univocally defined by the appended claims.

Throughout the description and claims of the present application, the term "comprise" and variations thereof such as "comprising" and "comprises" are not intended to exclude the presence of other elements or additives.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.