FI58657B - METALLANOD FOER ELECTROCHEMICAL PROCESSER - Google Patents

Description

ΓβΊ ^ juhutusjulka.su ς ο * e 7 jjggAa LBJ (11) UTLÄGGNI NGSSKRIFT ~ 00 ° 'C Patent granted 10 03 1981 (45) Patent raeddelat (51) Kv.lk?/lnt.CI.3 C 25 B 11 / 10 FINLAND —FINLAND (21) PKunttlhukumu * —P * hour «» i * 6kn »nf 195 ^ / 71 (22) Hakwnitptwt - AnaOknlngadtg 09.07.71 (23) Alkupllvft — GlMghutidaf 09.07.71 (41) Tullut luikikuksi - Bllvlt offuncI 17/1/72

National Board of Patents and Registration / 44) NlhttviksJpunon js kuL. -

Patent · och reglsterstyrslssn 'Antttkan utlugd och utUkrtfun publicurud 28.11.80 (32) (33) (31) Pyy4 * «y Utuolkus — Buglrd priority 16.07.70

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) P 2035212.9 Proven-Styrkt (71) Conradty GmbH & Co. KG Metallelektroden KG, Grunthal, D-8505 Röthen- Bach a.d. Pegnitz, Federal Republic of Germany Förbundsrepubliken Tyskland (DE) (72) Konrad Koziol, Röthenbach a.d. Pegnitz, Hans-Carl Rathjen, Röthen-Bach a.d. Pegnitz, Karl-Heinz Sieberer, Zirndorf uber Niirnberg,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (TM Oy Kolster Ab (5 * 0 Metal anode for electrochemical processes - Metallanod för elektro-kemiska processer

The invention relates to a metal anode for electrochemical processes. In particular, its use in chlor-alkali electrolysis will be explained below, although this anode can also be used in many other electrolysis processes.

The high level of development of many large electrolysis plants, manifested primarily in low cell voltages, high current and energy efficiencies, ease of use, and reliability of electrolysis equipment, is due to a number of measures and improvements that are not least targeted at the anode.

A number of requirements are set for technical anode materials. These requirements include, among other things, the corrosion resistance of the anode material, the operation of the anode process at a sufficiently high speed and with the lowest possible overvoltage. Until now, industrially used anode materials only partially meet these ever-increasing requirements. Thus, when graphite anodes were used, a certain, inevitable burnout occurred. This requires expensive equipment in modern large plants to maintain a constant distance between the anode and cathode 2 58657. In addition, sol purification requires relatively high costs.

Thus, in addition to graphite anodes, anodes made of platinum, platinum group metals, or their alloys began to be used. However, these anodes always had the disadvantage of high investment costs and relatively high precious metal consumption. Anodes made of platinum-titanium have now become known, mainly for cost reasons, but cannot be used in Hg electrolysis due to their amalgamation sensitivity.

It is known that valve metals such as titanium, tantalum, niobium, zirconium, etc., placed in aqueous solutions, passivate very rapidly due to the formation of a dense oxide-like surface layer and are therefore highly resistant to corrosion in many electrolytes. But the passive layers of these metals then have no electron conductivity at all in the potential regions in question, so that very high field strengths occur in the layers. This eliminates a certain potential - the so - called. to break up the breakthrough-drop-potential-passivating layers. Thus, although these metals are very resistant to corrosion, these passive metals cannot be used in the anode process.

In most cases, it is not taken into account that also in the case of precious metals, the surface potential, the potential at which the state of the metal changes from active to passive, is considerably more negative than the normal potential. Therefore, precious metals, at higher potentials, are also covered with passive layers in electrolytes. In the case of platinum, the already monomolecular oxygen chemisorption layer on the metal surface leads to passivity. This passive layer mechanism is not affected by whether this oxide-like surface layer is formed on the noble metal in the electrolyte or whether the noble metal oxide surface layer is formed even before being placed in the electrolysis, as proposed for anodes with constant dimensions according to DT-OS 18 14 5 ^ 7. These passive layers on a noble metal base are more advantageous compared to passive layers of valve metals due to their good electron conductivity and can thus be used in the anode process.

However, it is apparent that the incorporation of a foreign substance into the support, such as the incorporation of cubic platinum at the temperatures used, is usually hexagonally more tightly packed titanium, is problematic. Likewise, the mechanical stability of the okeid layers attached to the metal is unsatisfactory, because in rapid temperature changes, due to the different shrinkage behavior, stresses are generated at the oxide-metal interface, causing oxide release, which is clearly shown by specimens oxidized at higher temperatures in air for some time. As is known, this method of rapid temperature variation is widely used in the art to remove slag layers. Thus, the weakness of 5 58657 anodes with ceramic semiconductor coatings according to DT-OS 18 14 567, in which the active cover layer provided with a chlorine release catalyst is located against a bare or oxide film-covered valve metal surface, would also be sufficiently explained.

The basic object of the invention is now to develop an anode in which the active substances J against the passivation of the valve metal J

1. are better attached to their substrate, 2. are attached to a much larger metal conductor surface capable of conducting electrons, 3. extend deep into the valve metal substrate and thus meet the intensive chemical, mechanical and erosion requirements of the electrolysis bath, 4. do not need to meet epitaxial with this arrangement; the high demands placed on the conductivity of the high electron, whereby the constraints on choice are decisively dropped.

This object is particularly advantageously achieved by an anode in which the anti-passivation active substances are attached to a porous support layer sintered in the valve metal base. The carrier layer sintered to the cleaned valve metal base may be a powder of the same metal or a powder of a crystallographically similar metal. The pretreatment of the metal base can be carried out by any suitable method, such as etching, degreasing by steam, rinsing, grinding, etc. The size, shape and surface area of the metal powder particles vary depending on the material and the manufacturing method. The introduction of powder particles onto the metal substrate prior to sintering can be accomplished by spraying, rolling, electrically depositing, brushing, and other suitable methods. To facilitate the attachment of the material prior to the sintering process, binders and / or adhesives may be mixed into the powder. Preferably, various valve metal powders are used as the powder, such as titanium or tantalum powder or a mixture of valve metal powders or a valve metal alloy in powder form.

Titanium metal has the special property of being obtained as a powder in a purer form than in a molten form. Nevertheless, depending on the method of manufacture and storage, the surface of ordinary commercial powders is most often covered by an absorbed gas layer. When stored in air, oxide layers are most often formed, while when stored in a nitrogen atmosphere, partial nitration occurs. Prior to the sintering process, reducing pretreatment may be necessary. However, the powders can be used on a case-by-case basis for sintering without pretreatment. the dimension is calculated from the diameter of the particles.

In many cases, when a somewhat higher porosity is desired, it is advisable to add fillers which, when sintered, for the most part 4,5857 evaporate or are removed by thermal decomposition. Examples include, but are not limited to, useful ammonium salts such as ammonium perchlorate, ammonium chromate and ammonium sulfate, and alcohol-diluted resin.

In order to avoid oxidation of the valve metal powder during the sintering process -7, work is carried out either in a vacuum between lsn and 5 x 10 torr or in a certain gas atmosphere, e.g. argon. The heating rate depends on the degree of vacuum used or the effect of the decomposing substances on heating without damaging the sintered layer. The sintering temperature varies from 800 ° C to 2800 ° C depending on the metal powder or base metal, with further 1/4 hour interruptions further depending on the temperature.

Anti-passivation agents can be added by impregnation and drying and / or incineration, separation from the vapor phase by galvanic means or from the gas phase. The addition of surface tension reducing agents often improves the result. The active ingredients can already be present in the sintering mix.

Suitable active substances are all substances which are sufficiently corrosion-resistant in electrolysis and have good electron conductivity in the potential areas used, in which the anode process is possible. Such are all metals and oxides of the platinum group meeting the requirements mentioned above, intermediate and mixed oxides of precious and / or base metals, or oxides of base metals alone. Surprisingly, it has been found that even conductive base substances give excellent results with this arrangement. Thus, the widespread notion that the active layer must always contain a noble metal or precious metal compounds in order to remain effective has been shown to be false for the first time.

The desired active ingredients must not yet be in oxide form during the combining step, but this is achieved by post-treatment of the sintered layer during or after the heat treatment and / or sintering process.

As a result of this treatment, a composite material is formed on the valve metal surface, i.e. a metal-metal or metal-ceramic combination; and even a mechanically strong, yet porous, crystallographically similar, highly stable valve metal carrier layer containing the active ingredients in a firmly attached form. This layer, which has proved to be partly cermet-like, is good in that the active substances here are contained in a support which has the same crystal structure as the parent metal and thus forms an integral with the parent metal. The electrical conductivity of this activated carrier layer is thus primarily of a metallic nature. Although the electrical resistance of the sintered layers is greater than the resistance of the massive parts of the same metal, with sufficient mechanical strength, the valve metal base body can be dispensed with and a sintered body containing an anti-passivation activator alone can be used. In addition, the sintered layer containing activators protects them from mechanical and to a certain extent · also from chemical effects 5 58657. Thus, as an additional advantage, a substantially lower amalgamation tenderness is achieved. This provides an unambiguous improvement over conventional platinum-plated titanium anodes, which, when used in the event of a short circuit with the mercury cathode, partially remove the thin platinum layer due to amalgamizing due to price considerations, rendering the anode ineffective in a short time. If, instead, a platinum layer is placed inside the sintered layer according to the invention, contact between the mercury and the precious metal is hardly possible due to the high surface tension of the mercury and thus a reduction through amalgamation need not be feared.

Until now, it has been necessary for the active layers to be attached to the anode to be relatively firmly attached and thus to withstand mechanical stresses. Thus, a number of substances fell out in practice initially, even if they were of interest from an electronic and economic point of view. Thus, for example, it has not hitherto been technically possible to use spinel-derived surface layers on a valve metal base because the adhesion of the spinels to the bare or oxidizing metal base is not sufficient. This also confirms an experiment in which an titanium plate coated with iron chromium spinel was tarnished after 27 days of use in a laboratory cell at a current density of 1 A / cm, with Spinelli lasting about 250 days when applied to the sintered layer. Similar results were obtained using oxides and oxide alloys of lead, manganese, iron, cobalt, nickel and tungsten.

Due to the high porosity of the sintered layers and the consequent larger anode surface, a lower actual anode current density than with conventional metal anodes is effectively achieved with the same load. This also results in voltage savings of several tenths of a volt.

The invention thus offers the electrochemical industry a wide variety of electrode materials which are considerably cheaper in terms of price, durability and economy than electrode materials to date.

The theories presented are intended only to illustrate and explain the uses of the electrodes according to the invention and are in no way binding or in any way restrictive of the use of the electrodes.

Example 1

The titanium plate, measuring 100 x 100 x 1 mm, is etched for 30 minutes in boiling 20% hydrochloric acid, washed with water and rinsed with propanol. The thus-pretreated plate is sprayed with a compressed air syringe using a mixture of titanium powder, polyglycol 6000 and hexanol.

After drying for 20 minutes at 120 ° C, the titanium powder coating is sintered in an induction oven using a heating rate of 300 ° C / h with a final temperature of 1100 ° C.

6 58657 The base body thus prepared is moistened with a 1 molar solution of ruthenium chloride to which a slightly surface tension reducing agent, "Erkantol", has been added. This is followed by a heat treatment at 450 ° C for 30 minutes. This delivery is repeated in the same way three times.

The anode thus prepared has a much higher active surface layer compared to an anode coated with the same solution in a known manner (i.e. without a sintered layer), resulting in a lower actual anode current density and thus a lower cell voltage at the same load. In addition, the anode according to the invention is even more resistant to amalgamation and short circuit.

Example 2 A 400 mm long 3 mm diameter titanium rod is etched for 30 minutes in boiling 20 wt% hydrochloric acid, washed with water and rinsed with propanol. A 1 mm thick titanium layer is sintered on the rod thus pretreated by means of a carbon mold in a tube furnace at 1200 ° C. This sinter rod is impregnated several times with a solution of Mn (NO 2) 2 * and AgNO 2 in a ratio of 1: 1 and air-dried. For activation, the rod is kept for 5 minutes in the steam state of boiling 20% by weight hydrochloric acid. This is followed by a final firing for 40 to 5 minutes at temperatures of 200 to 450 ° C.

Example 3

Fine-mesh, commercially available platinum-plated titanium braided mesh metal is drawn through a paste made of a titanium-tantalum powder alloy and higher alcohol and sintered in a rotary kiln. Due to the higher melting point of the precious metal, sintering occurs mainly on the precious metal-free surfaces of the valve metal base body.

This valve metal sinter layer provided the best protection for the platinum layer.

Claims (5)

58657 An anode for electrolytic processes, in particular cloziral alkali electrolysis, comprising a solid metal core with an electronically active surface layer consisting of valve metals such as titanium, tantalum, niobium, zirconium, hafnium, tungsten, molybdenum and platinum metals, oxides of precious metals - including intermediate and mixed oxides - in particular platinum, platinum-iridium alloys, platinum-ruthenium alloys, platinum-palladium oxide, platinum-ruthenium oxide, ruthenium-palladium oxide, iron, manganese, cobalt, cobalt, oxides of tin, antimony and mixtures thereof - including intermediate and mixed oxides - characterized in that the activating agents are attached to a porous support layer made of valve metal sintered on top of the valve metal core. Anode according to Claim 1, characterized in that the activating agents form a valve-metal-precious metal-type composite with the sintered support layer. Anode according to Claim 1, characterized in that the activating agents form a valve-metal-ceramic type composite with the sintered support layer. Anode according to Claim 3, characterized in that the ceramic part consists of oxide mixtures of noble and / or base metals. Anode according to Claims 1 to 4, characterized in that the porous support layer consists of oxygen-oxidized or nitrogen-nitrogenated valve metal.