JP2017522457A5 - - Google Patents

Description

Hydrochloric acid electrolysis is an electrochemical process of increasing interest in hydrochloric acid, a common by-product of all major industrial processes that use chlorine, and the production capacity of new concept plants has increased significantly. With the formation of the acid, it is of great difficulty to market it. The electrolysis of the acid, which is usually carried out in a two-compartment cell separated by an ion-exchange membrane, leads to the formation of chlorine in the anode compartment, which is recirculated upstream, substantially with negligible environmental effects. Can provide a closed cycle. The constituent material of the anode compartment must be able to withstand an aggressive atmosphere combining acid, wet chlorine, and anodic polarization while maintaining good electrical conductivity. For such purposes, valve metals such as titanium, niobium, and zirconium are preferably used, and optionally alloyed titanium is the most common example because of cost and ease of machining. For example, nickel alloys including nickel, chromium, and small amounts of noble metals, such as ruthenium and palladium, such as the AKOT® alloy sold by Kobe Steel, are widely used. The anode on which the anodic release of chlorine takes place is, for example, a titanium alloy substrate coated with a suitable catalyst composed of a mixture of titanium and ruthenium oxides, which can usually reduce the overpotential of the anodic release of chlorine. And a valve metal article. The same type of coating has also been used to protect some components of the anode compartment that are not directly involved in the release of chlorine from corrosion, especially in interstitial areas that are prone to electrolyte stagnation. If not enough electrolyte regeneration is performed, a local discontinuity in the passivation layer, which is actually responsible for the protection of the valve metal, can be caused, which can lead to corrosion phenomena, It is more dangerous if more localized to smaller areas. An example of an area where a gap is likely to be defined is provided by the peripheral flanges of both the anode and cathode compartments, on which a sealing gasket is usually mounted. In the most favorable cases encountered in industrial practice, the titanium alloy coated with a catalyst formulation based on ruthenium and titanium oxides results in the passivation of the anode structure and / or the cell components in the flange area. Continuous operation in the hydrochloric acid electrolysis plant in the range of 24 to 48 months can be ensured before corrosion problems causing leakage of water occur. To increase the competitiveness of the industrial hydrochloric acid electrolysis process, the useful life of these components needs to be further increased.

In one form the present invention comprises a catalyst layer containing no titanium, two phases, namely tetragonal including RuO ₂ with any solid solution of SnO ₂ - is mixed with the crystalline phases of the double square dipyramidal Coating of a valve metal surface composed of a mixture of the amorphous phases of Ta ₂ O ₅ as described above. We have in fact observed that the coating without titanium is more resistant to chloride attack in acidic solutions. This, perhaps, titanium (the function of which is to function as a film forming component combined with ruthenium dioxide) oxide, a mixture of crystalline phase containing substantially more fragile anatase TiO ₂ phase than others Because it exists as. We have also observed that mixtures of tantalum and ruthenium oxides in the amorphous phase, even when completely free of titanium, do not contribute to a decisive solution to this problem. . However, coating is typically crystal form similar to rutile (i.e., tetragonal - RuO 2 double square _{dipyramidal),} and if basically formed from a mixture of of Ta ₂ O ₅ which has a amorphous phase The stability of the coating against acid attack is greatly increased. As a further advantage, the overvoltage of the coating, which contributes to anodic chlorine release, is surprisingly reduced. In one embodiment, the weight ratio between the amorphous and crystalline phases of Ta ₂ O ₅ is between 0.25 and 4, which defines the best range of the function of the present invention. In one aspect, the tetragonal - RuO _2-component double tetragonal dipyramidal crystal phase is partially replaced by SnO 2 _{(cassiterite).} The two dioxides, tin and ruthenium, whose tetragonal-digonal bipyramidal crystal forms have been found to be the most stable, can form solid solutions in any weight ratio. In one embodiment, the weight ratio of Ru / Sn in the tetragonal-double tetragonal bipyramid crystalline phase of the coating is in the range of 0.5 to 2, which is important in terms of protecting the substrate and catalytic activity of the coating. Gives the best results. In one embodiment, the coating comprises two separate catalyst layers, one of which is described above, which is in direct contact with the valve metal substrate bonded to the outermost layer to be coated having a higher content of ruthenium oxide. This is, for example, the case where the coating is used for catalytic activation of the anode structure whose outer surface is in direct contact with the electrolyte, on the one hand the protection function on the substrate surface, on the other hand the outermost layer of the catalyst. And the advantage of increasing the conduction properties. In one embodiment, the inner catalyst layer has a weight ratio of amorphous Ta ₂ O ₅ phase / RuO ₂ containing crystalline phase (optionally including SnO ₂ ) in the range between 0.25 and 2.5, Consists of an amorphous phase of Ta ₂ O ₅ mixed with a crystalline phase of a tetragonal-digonal bipyramid of RuO ₂ at a weight ratio of Ru / Ta between 3 and 5. In one embodiment, between the coating described above (one or two coatings) and the substrate, a further protective reserve layer composed of a mixture of oxides of titanium and tantalum is interposed. This can provide the advantage of improving the anchoring of the catalyst layer to the substrate, at the expense of a resistance penalty resulting from the moderate electrical conductivity of the mixture of titanium and tantalum oxide. However, the magnitude of such resistance penalties can be greatly limited if the reserve layer has a suitably limited thickness. The total loading of 0.6 to 4 g / m ² of titanium and tantalum oxide is a suitable value for the reserve layer combined with a catalyst layer containing 20 g / m ² of total oxide.

In another aspect, the present invention provides a compound of any of titanium and tantalum, for example TiOCl _2, TiCl _3, and a solution of TaCl ₅ is subjected to the valve metal substrate in one or more covering, then each coated Is followed by one or more coatings of a solution of a compound of tantalum, ruthenium and optionally tin, until a first catalyst layer is obtained, followed by pyrolysis after each coating. Optionally, applying a solution of the tantalum and ruthenium compound on the first catalyst layer until a second catalyst layer is obtained, followed by pyrolysis after each coating. 3. The method for producing a coating according to (1). In one aspect, the ruthenium and tin compound that is applied in view of subsequent pyrolysis is a hydroxyacetochloride complex, which has a more ordered composition with a more uniform composition compared to hydrochloric acid or other precursors. For obtaining a dense layer. The pyrolysis step after each coating can be performed between 350 and 600 ° C., depending on the precursor compound selected. In the case of the decomposition of a mixture of tantalum chloride and a precursor composed of a ruthenium and optionally a hydroxyacetochloride complex of ruthenium, the thermal decomposition can be carried out, for example, between 450 and 550 ° C.

-The catalyst layers of various formulations were applied over the protective reserve layer of the above sample by selectively applying the first or second set of catalyst solutions. The first set of catalyst solutions was brushed with 8-10 coatings until a coating of tantalum and ruthenium oxide was obtained at a total ruthenium loading of about 20 g / m ² , and then each coating was coated. This was followed by 10 minutes of drying at 50 ° C. and 5 minutes of pyrolysis at 500 ° C. At the end of the pyrolysis process, a crystalline tetragonal-digonal bipyramidal ruthenium dioxide phase is obtained which is subsequently mixed with an amorphous tantalum oxide phase, as demonstrated by subsequent XRD analysis The electrode was subjected to a thermal cycle at 500 ° C. for 2 hours. Some samples of the electrodes thus obtained are shown in Table 1 as RuTa type. The second set of catalyst solutions was brushed with 8-10 coatings until a coating of tantalum, tin and ruthenium oxide was obtained at a total ruthenium loading of about 20 g / m ² , then each After drying for 10 minutes at 60 ° C. and 5 minutes for pyrolysis at 500 ° C. In this case, too, at the end of the pyrolysis process, subsequently, as demonstrated by subsequent XRD analysis, the crystalline tetragonal-bigonal pyramid mixed with the amorphous phase of tantalum oxide The electrode was thermally cycled at 500 ° C. for 2 hours until a solid solution of the phases ruthenium dioxide and tin dioxide was obtained. Some samples of the electrodes thus obtained are shown in Table 1 as RuTaSn type.
-Another electrode sample with a catalyst coating composed of two layers was obtained by selectively applying the first or second set of catalyst solutions. The first set of catalyst solutions was brushed with 6-7 coats until a first coat of ruthenium and tantalum oxide was obtained, then dried at 50 ° C for 5 minutes after each coat. By subjecting it to a pyrolysis treatment at 500 ° C. for 5 minutes; then a first type of solution having a weight ratio of Ru / Ta equal to 4 is obtained with a total ruthenium loading of about 20 g / m ^2. Until then, two coats were applied by brushing and after each coat were subjected to the same drying and pyrolysis cycle. At the end of the pyrolysis process, the crystalline tetragonal-digonal bipyramidal phase of ruthenium dioxide mixed with the amorphous phase of tantalum oxide was subsequently detected, as demonstrated by subsequent XRD analysis. Until obtained, the electrodes were subjected to a thermal cycle at 500 ° C. for 2 hours. Some samples of the electrodes thus obtained are shown in Table 1 as RuTa-TOP type. The second set of catalyst solutions was brushed with 6-7 coats until a tantalum, tin, and ruthenium oxide coat was obtained, then dried at 60 ° C for 5 minutes after each coat. By subjecting it to a pyrolysis treatment at 500 ° C. for 10 minutes; a first having a Ru / Ta weight ratio equal to 4 until a two-layer catalyst coating is obtained at a total ruthenium loading of about 20 g / m ² . Ruthenium and tantalum oxide coatings obtained by brushing the solution of the type with two coats and subjecting each coat to 5 minutes of drying at 50 ° C. and 10 minutes of pyrolysis at 500 ° C. Coated thereon. At the end of the pyrolysis process, the ruthenium dioxide and tin dioxide of the tetragonal-bigonal bipyramidal crystalline phase mixed with the amorphous phase of tantalum oxide in the inner layer, as demonstrated by subsequent XRD The electrode was subjected to a thermal cycle at 500 ° C. for 2 hours until a solid solution of a tetragonal-bipyramidal ruthenium dioxide crystalline phase mixed with an amorphous phase of tantalum oxide in the outer layer was obtained. Some samples of the electrodes thus obtained are shown in Table 1 as RuTaSn-TOP type.

Claims (7)

A first titanium- free catalyst comprising an amorphous phase of Ta ₂ O ₅ mixed with a tetragonal-diagonal bipyramidal crystalline phase of either RuO ₂ or a solid solution of RuO ₂ and SnO ₂ A layer, wherein the weight ratio of the amorphous phase to the crystal phase is in the range of 0.25 to 2.5, and the weight ratio of Ru to Sn in the crystal phase is in the range of 0.5 to 2. , And
A second catalyst layer externally applied to the first catalyst layer, wherein the second catalyst layer comprises a tetragonal-polytetragonal RuO ₂ with a weight ratio of Ru to Ta in the range of 3-5; Consisting of an amorphous phase of Ta ₂ O ₅ mixed with a bipyramidal crystal phase , wherein the content of RuO ₂ in the second catalyst layer is higher than the content of RuO ₂ in the first catalyst layer ,
The having including coating,
Coated valve metal substrate. A protective pre-layer comprising a mixture of the oxide of titanium is interposed between said valve metal surface first catalyst layer and the oxide of tantalum, coated valve metal substrate according to claim 1 . Wherein the substrate is made of titanium or a titanium alloy, coated valve metal substrate according to claim 1 or 2. A chlorine generator electrolytic cell comprises an anode and / or the flange are manufactured from the coated valve metal substrate according to claim 1, wherein the chlorine generating electrolytic cell. The chlorine producing electrolytic cell according to claim 4 , wherein the electrolytic cell is a hydrochloric acid electrolytic cell. A method for producing a coated valve metal substrate according to any one of claims 1 to 3 ,
The following sequential steps:
Optionally, applying a solution of a compound of titanium and a compound of tantalum to the valve metal substrate in one or more coatings, followed by pyrolysis after each coating;
- until the first catalyst layer is obtained, carried out with a compound of the compound and ruthenium tantalum, and optionally subjected solution one or more coating of a compound of tin, pyrolysis after then each coated Process;
- until the second catalyst layer can be obtained, the step of performing a solution of a compound with a compound of ruthenium tantalum subjected to the first catalyst layer in one or more coating, pyrolysis after then each coated ;
Including, the coating process for the preparation of a valve metal substrate. 7. The method of claim 6 , wherein the ruthenium and tin compound is a hydroxyacetochloride complex.