ES2712403T3 - Catalytic coating and manufacturing method - Google Patents

Abstract

Coated valve metal substrate having a coating that comprises a first catalytic layer that does not contain titanium and that consists of an amorphous phase of Ta2O5 mixed with a tetragonal dipyramidal crystalline phase consisting of RuO2 or a solution of solids of RuO2 and SnO2 said first catalytic layer having a weight ratio of said amorphous phase with respect to said crystalline phase varying from 0.25 to 2.5 and the weight ratio of Ru with respect to Sn of said crystalline phase varying from 0 , 5 to 2, and a second catalytic layer applied externally to said first catalytic layer, said second catalytic layer consisting of an amorphous phase of Ta2O5 mixed with a tetragonal dipyramidal dipstragonal crystalline phase of RuO2 with a weight ratio of Ru to Ta 3 to 5, where the content of ruthenium oxide is higher in said second layer than in said first layer.

Description

DESCRIPTION

Cladding catalltico and method of manufacture thereof

Field of the invention

The invention relates to a catallic coating of valve metal articles suitable for use in highly aggressive electrolytic environments, for example in chlorhydride acid electrolysis cells.

Background of the invention

Hydrochloric acid electrolysis is an electrochemical process that is of increasing interest nowadays, since chlorhydrate acid is the typical by-product of all the main industrial processes that use chlorine: the increase in production capacity of the plants of new conception includes the formation of significant quantities of acid, whose placement in the market presents significant difficulties. The electrolysis of the acid, normally carried out in electrolytic cells of two compartments separated by an ion exchange membrane, leads to the formation of chlorine in the anode compartment, which can be recycled upstream resulting in a substantially closed cycle of impact negligible environmental The building materials of the anodic compartment must be able to withstand an aggressive environment that combines acidity, wet chlorine and anodic polarization while retaining an appropriate electrical conductivity; for this purpose, metal valves such as titanium, niobium and zirconium are preferably used, titanium being optionally alloyed as the most common example for reasons of cost and ease of machining. Titanium alloys containing nickel, chromium and small amounts of noble metals such as ruthenium and palladium, such as the AKOT® alloy marketed by Kobe Steel, are for example of wide utility. The anodes on which the chlorine evolution is carried out consist, for example, of a valve metal object such as a titanium alloy substrate coated with an appropriate catalyst, which normally consists of a mixture of titanium oxides and ruthenium, able to reduce the over-tension of the chlorine anodic discharge. The same type of coating is also used to protect against corrosion some components of the anodic compartment not directly involved in the evolution of chlorine, with particular reference to the interstitial areas subject to stagnation of the electrolyte. The absence of sufficient electrolyte renewal can in fact lead to a local discontinuity of the passivation layer intended to protect the valve metal, triggering the corrosion phenomenon, which is more dangerous the more localized it is in small areas. An example of the target areas for delimiting the interstices is given by the peripheral rims of both the anodic and cathodic compartments, on which the sealing gaskets are normally mounted. In the most favorable cases experienced in industrial practice, titanium alloys coated with catalytic formulations based on ruthenium and titanium oxides can guarantee a continuous operation in a hydrochloric acid electrolysis plant within the range of 24 to 48 months, before that the corrosion problems lead to the deactivation of the anode structure and / or the leakage of cell elements in the flange area.

EP 2 757179 A1 discloses chlorine evolution anodes which, in addition to the amorphous ruthenium oxide, can comprise crystalline ruthenium oxide in an intermediate layer or in a catalltic layer. US 3875043 A describes a catallic coating comprising tantalum oxide and ruthenium oxide. US 3,853,739 A describes a coating formed by a solids solution of metal oxides of the platinum group in an amorphous tantalum binder. Carl-Erik Boman et al. describe in Acta Chemica Scandinavica, Vol. 24, January 1, 1970, pp. 116-122 the preparation and characterization of ruthenium dioxide crystals having a rutile structure. US 2011/0209992 A1 discloses an electrode for an electrolysis cell comprising a catalltic layer containing oxides of tin, ruthenium, iridium, palladium and niobium using precursor solutions of hydroxyacetochloride complexes of tin, iridium or ruthenium.

For reasons of improving the competitiveness of the industrial hydrochloric acid electrolysis process, it may be necessary to increase the useful life of these components.

Summary of the invention

Various aspects of the present invention are explained in the appended claims.

In one aspect, the invention relates to a coated metal valve substrate, having a coating as defined in claim 1. The coating includes a catalittic layer that does not contain titanium and consists of the mixing of two phases, specifically, an amorphous phase of Ta2O5 mixed with a dipyramid tetratetragonal tetragonal crystal phase containing RuO2, optionally in solution of solids with SnO2. The inventors, in fact, have observed that coatings that do not contain titanium are more resistant to the attack of chlorine in acid solution, presumably because the titanium oxides - whose function in a combination with ruthenium dioxide is to act as a component of formation of pellula - are present as a mixture of crystalline phases that include an anatase TiO2 phase, substantially weaker than the others. The inventors have also observed that mixtures of tantalum oxide and ruthenium in an amorphous phase do not contribute to solve the problem decisively, even if it is completely free of titanium. When, however, the coating is formed from a mixture of RuO2 in the typical rutile-like crystalline form (ie, tetragonal tetragonal tetragonal) and Ta2O5 in a basically amorphous phase, the stability of the coating increases greatly compared to the acid attack. As an additional advantage, surprisingly the over-tension against the release of anodic chlorine is reduced. The weight ratio between the amorphous phase of Ta2O5 and the crystalline phase is between 0.25 and 2.5, which defines the best operating range of the invention. In one embodiment, the RuO2 component in the tetragonal diradgonal tetrahedral crystal phase is partially replaced by SnO2 (cassiterite). The two dioxides of tin and ruthenium, whose tetrahedral diratragonal dipiramidal crystalline form is the most stable, are capable of forming solids solutions with any ratio in weight; in one embodiment, the weight ratio of Ru to Sn in the tetragonal tetrahedral tetrahedral crystal phase of the coating varies between 0.5 and 2, which provides the best results in terms of substrate protection as well as catalytic activity of the coating. In one embodiment, the coating comprises two different catalytic layers, one as described above in direct contact with the valve metal substrate coupled to an outer one superimposed thereon with a high content of ruthenium oxide. This may have the advantage of improving on the one hand the protective function on the surface of the substrate and on the other, the catalytic and conductive properties of the outermost layer, as required for example in case the coating is used for catalytic activation. of an anodic structure whose external surface is in direct contact with the electrolyte. In one embodiment, the internal catalytic layer has a molar ratio of the amorphous phase of Ta2O5 with respect to the crystalline phase containing RuO2 (optionally including SnO2) varying between 0.25 and 2.5 and the external catalytic layer consists of an amorphous phase of Ta2O5 mixed with a tetrahedral tetrahedral tetrahedral tetrahedral phase of RuO2 with a weight ratio of Ru with respect to Ta between 3 and 5. In one embodiment, between the coating as described above - in one or two coatings - and the substrate is interposed by an additional protective layer consisting of a mixture of titanium oxide and tantalum. This may have the advantage of improving the anchoring of the catalytic layer to the substrate, to the detriment of a resistance penalty from the modest electrical conductivity of titanium oxide and tantalum mixtures. The magnitude of said resistance penalty may, however, be very limited, as long as the pre-layer has an appropriately limited thickness. A total charge of titanium oxides and equal weight of 0.6 to 4 g / m2 is an appropriate value for the combination of the precoat with a catalytic layer containing 20 g / m2 of total oxides.

In another aspect, the invention relates to a method of manufacturing a valve metal substrate coated as described hereinabove comprising the optional application of a solution of titanium and tantalum compounds, for example TiOCl 2, TiCl3 and TaCls, to a valve metal substrate in one or more coatings, with subsequent thermal decomposition after each coating; the application of a solution of tantalum, ruthenium and optionally tin compounds in one or more coatings, with the subsequent thermal decomposition after each coating, until a first catalytic layer is obtained; the optional application of a solution of tantalum and ruthenium compounds on the first catalytic layer with subsequent thermal decomposition after each coating, until a second catalytic layer is obtained. In one embodiment, the ruthenium and tin compounds applied in view of the subsequent thermal decomposition are hydroxyacetochloride complexes; this may have the advantage of obtaining more regulatory and compact layers, which have a more homogeneous composition, in comparison with hydrochloric precursors and other precursors. The thermal decomposition step after each coating can be affected between 350 and 600 ° C, depending on the selected precursor compounds. In the case of the decomposition of mixtures of precursors consisting of tantalum chloride and complexes of ruthenium hydroxyacetochloride and optionally of tin, the thermal decomposition, for example, can be carried out between 450 and 550 ° C.

The following examples are included to demonstrate the particular embodiments of the invention, whose viability has been verified in the claimed range of values. Those skilled in the art should appreciate that the compositions and techniques disclosed in the following examples represent techniques and compositions discovered by the inventors to function well in the practice of the invention; however, those skilled in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a similar or similar result without departing from the scope of the invention.

Example 1

An alloy of 1 mm thick AKOT® titanium was degreased with acetone in an ultrasonic bath and subjected to chemical etching with 20% HCl at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of size 10 cm x 10 cm for the subsequent preparation of the electrode samples.

A solution of precursors was obtained for the preparation of the protective pre-layer by means of a mixture of 150 g / l of TiOCl2 and 50 g / l of TaCls in 10% by weight hydrochloric acid.

A first series of catalytic solutions was obtained by mixing 20% by weight of RuCh and 50 g / l TaCls in 10% by weight hydrochloric acid according to various proportions.

Solutions of hydroxyacetochloride complexes of Ru (0.9 M) and Sn (1.65 M) were obtained by dissolving the corresponding chlorides in aqueous acetic acid at 10% by volume, evaporating the solvent, mixing with 10% aqueous acetic acid with subsequent evaporation of the solvent for two more times, finally dissolving the product again in 10% aqueous acetic acid to obtain the specified concentration.

A second series of catallic solutions was obtained by mixing the hydroxyacetochloride complexes of Ru and Sn according to various proportions.

Electrode samples were obtained with different formulations with the following procedure:

- a protective pre-coating was applied to the samples cut from titanium mesh by means of brushing the solution containing precursors of TiOCh and TaCl5 in two coatings, with subsequent drying at 50 ° C for 5 minutes and treatment of thermal decomposition to 515 ° C for 5 minutes after each coating, until obtaining a deposit of tantalum and titanium oxides with a charge of approximately 1 g / m2;

- Catallic layers of various formulations were applied on the protective pre-coat of the previous samples by means of the alternative application of catallic solutions of the first and second series. The catallic solutions of the first series were applied by means of brushing in 8-10 coatings and subjected to subsequent drying at 50 ° C for 10 minutes and treatment of thermal decomposition at 500 ° C for 5 minutes after each coating, until a tantalum and ruthenium oxide deposit with a total ruthenium load of approximately 20 g / m2. At the end of the thermal decomposition process, the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 ° C, until obtaining a crystalline phase of tetragonal diradgonal tetrahedral ruthenium dioxide mixed with an amorphous phase of tantalum oxide, as verified by means of later XRD research. Some electrode samples obtained in this way are indicated in Table 1 as RuTa type. The catalltic solutions of the second series were applied by means of brushing in 8-10 coatings and subjected to subsequent drying at 60 ° C for 10 minutes and treatment of thermal decomposition at 500 ° C for 5 minutes after each coating, until a tantalum, tin and ruthenium oxide deposit with a total ruthenium load of approximately 20 g / m2. Also in this case, at the end of the thermal decomposition process, the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 ° C, until a solids solution of ruthenium dioxide and tin dioxide was obtained in a ditetragonal dipiramidal crystal phase tetragonal mixed with the amorphous phase of tantalum oxide, as verified by means of subsequent XRD investigation. Some samples of electrodes obtained in this way are indicated in Table 1 as RuTaSn type;

- Other electrode samples were obtained, provided with a catallic coating consisting of two layers, by means of the alternative application of catallic solutions of the first and second series. The catalltic solutions of the first series were applied by means of brushing in 6-7 coatings and subjected to subsequent drying at 50 ° C for 5 minutes and treatment of thermal decomposition at 500 ° C for 5 minutes after each coating, until a first deposit of ruthenium oxide and tantalum; A later solution of the first type was applied with a ratio in weight of Ru with respect to Ta equal to 4, by means of brushing in 2 coatings and subjected to the same cycle of drying and thermal decomposition after each coating, until obtaining a total ruthenium load of approximately 20 g / m2. At the end of the thermal decomposition process, the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 ° C, until obtaining a tetragonal tetrahedral tetrahedral ruthenium dioxide crystal phase mixed with the amorphous phase of tantalum oxide, as verified by means of later XRD research. Some electrode samples obtained in this way are indicated in Table 1 as RuTa_TOP type. The catalltic solutions of the second series were applied by means of brushing in 6-7 coatings and subjected to subsequent drying at 60 ° C for 5 minutes and treatment of thermal decomposition at 500 ° C for 10 minutes after each coating, until a deposit of oxides of tantalum, tin and ruthenium; one placed a deposit of ruthenium oxide and tantalum, obtained by means of brushing in 2 coatings of a solution of the first type with a weight ratio of Ru with respect to Ta equal to 4, subjected to drying at 50 ° C for 5 minutes and thermal decomposition at 500 ° C for 10 minutes after each coating, on it, until obtaining a catallic coating in two layers with a total ruthenium load of approximately 20 g / m2. At the end of the thermal decomposition process, the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 ° C, until a solution of solids of ruthenium dioxide and tin dioxide was obtained in a tetrahedral diratragonal tetrahedral crystal phase mixed with the phase amorphous oxide of tantalum in the inner layer and of a crystalline phase of tetragonal tetrahedral tetrahedral ruthenium dioxide with the amorphous phase of tantalum oxide in the outer layer, as verified by means of further XRD investigation. Some electrode samples obtained in this way are indicated in Table 1 as RuTaSn TOP type.

Table 1

CONTRAEJMPLO 1

An alloy of 1 mm thick AKOT® titanium was degreased with acetone in an ultrasonic bath and subjected to chemical etching with 20% HCl at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of size 10 cm x 10 cm for the subsequent preparation of the electrode samples.

A solution of precursors for the preparation of the protective pre-layer was obtained by means of a mixture of 150 g / l of TiOCl2 and 50 g / l of TaCls in 10% hydrochloric acid.

A series of catalytic solutions was obtained by mixing 20% by weight of RuCh and 150 g / l of TaCl2 in 10% hydrochloric acid according to various proportions.

- a protective pre-coat was applied to the samples cut from the titanium mesh as in the case of Example 1 - catalytic layers of various formulations were applied on the protective pre-coat of the previous samples by means of brushing the catalytic solutions previous in 8-10 coatings and was subjected to subsequent drying at 50 ° C for 5 minutes and treatment of thermal decomposition at 500 ° C for 5 minutes after each coating, until obtaining a deposit of ruthenium and titanium oxides with a total charge of ruthenium of approximately 20 g / m2 At the end of the thermal decomposition process, the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 ° C. Some electrode samples obtained in this way are indicated in Table 2 as RuTi type.

Table 2

Example 2

The electrode samples of the table were subjected to a conventional potential test under an anodic chlorine release at the current density of 3 kA / m2, in 15% HCl at a temperature of 60 ° C. The potential data obtained are presented in Table 3 (SEP). The table also shows the related data of accelerated life test, expressed in terms of hours of operation before deactivation under anodic chlorine release at the current density of 6 kA / m2, in HCl at 20% by weight at a time. temperature of 60 ° C, using a zirconium cathode as a counter electrode. The deactivation of the electrode is defined by means of an increase of 1 V in the cell with respect to the initial value.

Table 3

Example 3

Duplicates of electrode samples 2, 6 and C2 were subjected to a corrosion test that simulates the corrosion conditions in cracks that may occur on the flanges of the electrolysis devices for the production of chlorine or other occluded areas. A first series of samples was immersed in a known volume of 20% by weight of HCl at 45 ° C under a stream of nitrogen, to simulate electrolyte stagnation conditions; a second series (control) was immersed in the same volume of 20% by weight of HCl at 40 ° C under a stream of oxygen, in order to maintain passivation. In both cases, the concentration of chromium and nickel released from the substrate was detected during the course of 24 hours: for samples 2 and 6, the concentration of both metals in the HCl volume was less than 2 mg / l, time that sample C2 showed slightly elevated concentrations of 2 mg / l of Cr and 4 mg / l of Ni under current of oxygen, which increased significantly under nitrogen current (up to 6.5 mg / l for nickel).

This test was repeated with another set of samples, confirming a substantial increase in corrosion resistance for the formulations of the invention.

The foregoing description should not be interpreted as limiting the invention, which can be used according to different embodiments without departing from the scope thereof, and whose scope is defined only by the appended claims.

Throughout the description and the claims of the present application, it is not intended that the word "comprise" and its variations such as "understand" and "comprise" exclude the presence of other additional elements or components. additional components or process steps.

The discussion of the documents, records, materials, devices, articles or the like that have been included in the present specification is 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 form part of the basis of the prior art or are general knowledge common in the field relevant to the present invention as it existed prior to the priority date of each claim of the present application.

Claims (7)

1. A metal substrate of a coated valve having a coating comprising a catalytic first layer which does not contain titanium and which consists of an amorphous phase of Ta2O5 mixed with a dipyramid tetratetragonal tetragonal crystal consisting of RuO2 or a solution of solids of RuO2 and SnO2 having said first catalytic layer a ratio by weight of said amorphous phase with respect to said crystalline phase varying from 0.25 to 2.5 and the weight ratio of Ru with respect to Sn of said crystalline phase varying from 0.5 to 2, and a second catalytic layer applied externally to said first catalytic layer, said second catalytic layer consisting of an amorphous phase of Ta2O5 mixed with a tetrahedral ditatragonal tetrahedral crystal phase of RuO2 with a weight ratio of Ru with respect to Ta of 3 to 5, where the content of ruthenium oxide is higher in said second layer than in said first layer. 2. The coated metal valve substrate according to claim 1 comprising a protective pre-coat consisting of a mixture of titanium and tantalum oxides interposed between the metal valve surface and said first catalytic layer. 3. The coated valve metal substrate according to one of the preceding claims, wherein said substrate is formed of titanium and titanium alloy. 4. The use of an anode and / or a flange formed by the coated valve metal substrate of any one of claims 1 to 3 in an electrolysis device that produces chlorine. 5. The use of claim 4 wherein said electrolysis device is an electrolysis device of hydrochloric acid. 6. METHOD OF MANUFACTURE OF A PLATED VALVE METAL SUBSTRATE IN ACCORDANCE WITH ANY OF CLAIMS 1 TO 3 COMPRISING THE FOLLOWING SIMULTANEOUS OR SECRETARY STAGES: - optional application of a solution of titanium compounds and tantalum to a valve metal substrate in one or more coatings, with subsequent thermal decomposition after each coating; - application of a solution of tantalum, ruthenium and optionally tin compounds in one or more coatings, with the subsequent thermal decomposition after each coating, until a first catalytic layer is obtained; - optional application of a solution of tantalum and ruthenium compounds to said first catalytic layer in one or more coatings, with the subsequent thermal decomposition after each coating, until a second catalytic layer is obtained. 7. The method according to claim 6, wherein said ruthenium and tin compounds are dihydroxyacetochloride complexes.