FI59428C - ELECTROCHEMICAL PROCESSER - Google Patents

Description

RSF7! M {11) ANNOUNCEMENT1 £ 9490

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Patent- och registerstyrelsen 'Amökin utlagd oeh utl. * Krlfun publlcerad 30.0U.8l (32) (33) (31) Pyydetty «« lolkaui — Bajlrd prlorltat 20.03.71

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) P 2113676.5 Proven-Styrkt (71) Conradty GmbH & Co. KG Metallelektroden KG, Griinthal, 8505-Röthenbach a.d. Pegnitz, Federal Republic of Germany Förbundsrepubliken Tyskland (DE) (72) Konrad Koziol, Röthenbach a.d. Pegnitz, Karl-Heinz Sieberer, Zim-Dorf iiber Niirnberg, Hans-Carl Rathjen, Röthenbach a.d. Pegnitz,

The present invention relates to an electrode for electrolytic processes, which electrode consists of a valve metal core and a top layer, which in turn contains elemental elements. metal oxides. In particular, the use of an electrode for chlor-alkali electrolysis is explained below, although the electrode can also be used in many other electrolysis processes.

The high level of development of new high-power cells, which is mainly reflected in the low cell voltages, high current and energy values, comfort and reliability of electrolysis equipment, is the result of many measures and improvements, not least for the anodes.

There are several requirements for technical anode materials. These include the corrosion resistance and mechanical strength of the anode material, the sufficient speed of the anode process with the lowest possible overvoltage. To date, the anode materials used in high-power technology only partially meet these requirements. Thus, when graphite anodes are used, a certain amount of waste is always generated. This requires expensive equipment in modern high power cells to maintain a constant distance between the anode and cathode. In addition, this results in relatively high cleaning costs.

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In addition to graphite anodes, anodes made of platinum, platinum group metals or their alloys have also been used. The disadvantages of these anodes are always the very high investment costs and the relatively high consumption of the precious metal. In addition, the limited presence of platinum is quite insufficient to meet the strongly increased need for anodes for electrochemical processes. Recently, for these reasons, anodes made of platinum-titanium have been introduced, which, however, due to their high amalgam sensitivity, have not been used for Hg electrolysis.

It is known that valve metals such as titanium, tantalum, niobium, zirconium, etc., when used in aqueous solutions, passivate very rapidly, forming a thick oxide-like coating on their surface, after which they are able to resist many electrolytes. However, the passive layers of these metals do not conduct electrons in the potential regions in this context, so that there are very high field strengths in the layers. This results in a certain voltage, the so-called. breakdown voltage to destroy passive layers. However, although these metals have a high corrosion resistance, they cannot be subjected to electrolysis processes.

Precious metals are also covered in electrolytes at high voltages by passive layers. For platinum, the already monomolecular oxygen-chemisorption layer on the metal surface causes passivity. It does not matter for this mechanism of passive layers whether this oxide-like coating layer on the noble metal is provided in the electrolyte or whether the noble metal oxide layers are provided before the electrolysis application. In contrast to passive layers of valve metals, these passive layers are characterized by good electron conductivity and thus the anode process can be performed by them.

German information publications 1 814 567 and 1 814 576, which recommend the use of an electrode made of tubular metal with a platinum metal oxide-containing non-base metal oxide layer on the surface of the electrode, are also based on this information. The noble metal oxide moiety then acts as a chlorine release catalyst and donor. In addition to these noble metal oxide-containing coatings, an attempt has also been made here to achieve the protective effect of semiconductor oxide-free semiconductor coatings, although it is obvious from the patent specifications that using such coatings the anode process takes place at a rather unfavorable voltage. My own experience confirms the existence of this drawback associated with the noble metal oxide-free layers deposited on the tubular metal body and shows that the increased anode voltage very quickly leads to passivation and destruction of the coated electrode. As a result, valve metal electrodes coated with oxide-free oxide layers have not been put into technical use to date. Em. platinum metal oxide-containing coatings according to German publications 1 814 567 and 1 814 576, e.g. ruthenium oxide-containing, ceramic semiconductor coatings have known economic disadvantages .

Due to the above-mentioned disadvantages of precious metal oxides, the losses during the electrolysis process are very high. In addition, anodes have recently been invented in which the precious metal and / or the precious metal oxide is coated with a non-conductive enamel or porous, refractory non-conductive oxides for protection against mechanical, chemical and electrochemical wear. However, due to this insulating coating, the local current density of the anode increases and the electrode operates at the same load at a higher anode voltage than the uncoated anode.

The object of the present invention is to provide an electrode which does not have the above-mentioned drawbacks. This object is solved in connection with a coated valve metal electrode according to the invention, so that the top layer is formed in two layers, the bottom layer adjacent to the core being non-valve metal and / or carbon and the bottom layer having a surrounding gas- and liquid-tight cover layer consisting of: a) T.iO 2) and / or tantalum oxide (Ta 2 O 5); (b) electrically conductive oxides of niobium, tantalum, tungsten, antimony, iron, cobalt, manganese and / or tin to increase the electrical conductivity of TiO2 and / or Ta2 ^ 5 as a sufficient proportion; and (c) electrochemically stable Spinell-type metal oxides. . Due to this structure, the base layer arranged on the valve metal body does not come into contact with the electrolyte. The existence of a cover layer allows the use of a much smaller precious metal in the base layer than if the anode process were allowed to take place immediately on the precious metal layer. This saving of precious metal results in a sharp reduction in the cost of manufacturing the electrode according to the invention. Since the base layer is not in direct contact with the e-electrolyte, it is now possible for the first time to place on the valve metal body also substances normally consumed in electrolysis, such as base metals and graphite, but meet the important requirement that the valve-metal body does not come into contact during coating and operation. oxidize. Furthermore, the base layer must also prevent the valve metal body from being passivated by the penetrating electrolyte in the event that the cover layer is no longer completely sealed in order to ensure that the electric current is conducted from the valve metal body to the cover layer.

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The cover layer of the anode according to the invention does not contain any expensive precious metals, therefore this layer can be so thick that very long service times are achieved. In addition to the low manufacturing costs of the electrode according to the invention, its cover layer provides considerably better amalgam resistance compared to a conventional noble metal anode.

Suitable for this coating layer are chemically and electrochemically particularly resistant, electrically conductive oxides, such as Spinnell-type oxides R 2 O 2, in the base electrolysis medium, in which case R is in most cases one or more divalent or trivalent metals. In addition, the oxides of titanium and tantalum are known to be very durable in the electrolysis medium, but titanium dioxide and tantalum pentoxide conduct electricity very poorly. In classical methods for improving the electrical conductivity of these poorly conductive oxides, the oxides are mixed with an oxide of another metal of chemical value or mixed with a large amount of highly electrically conductive oxides. For titanium dioxide, oxides of tantalum, niobium, tungsten, antimony, iron, cobalt, manganese and tin are known to be suitable for this purpose. Although experience has shown that titanium or tantalum oxides mixed in this way should not be used in the manufacture of an electrically conductive surface layer to be placed on top of a valve metal body for the anode process, these materials have surprisingly been shown to be suitable as electrode materials between the valve metal and these conductive oxides. a conductive layer, as is the case in the electrode according to the invention. It has proven advantageous to combine valve metal oxides and Spinell-type oxides, above all because the Spinell-type oxide part further improves the conductivity of the valve metal oxide. For cover layers containing conductive valve metal oxides, it has proven expedient to produce a base layer of materials which generally conduct electricity well or also form volatile oxides. Thus, precious metals, base metals such as cadmium and cadmium alloys, as well as carbon in various forms, are well suited as materials for this base layer.

The structure and operation of the anodes according to the invention will now be explained in more detail by means of the accompanying drawing and the following examples, without, however, limiting them in any way.

Figures 1 and 2 show the structure of an anode according to the invention in section, where a is a valve metal body, b a base layer of metal without valve action and / or carbon and c a cover layer of electrically conductive oxides of base metals.

In Fig. 2, the base layer b is divided into segments by means of grooves.

Example 1

A 100 x 100 x 1 mm titanium plate is etched for 60 minutes in boiling 20% hydrochloric acid steam, rinsed with water and dried. This plate thus pretreated is galvanically coated in a conventional bath with a thin layer of platinum metal to a base layer. A solution is then prepared containing 1 * 00 g of Fe (NO ^. 9 ^ 0, 120 g of MnUiO ^. HE ^, 150 g of CoiNO. ^ G .6H? 0, 225 g of εηΟΐ, -, ^ Η 150 g of TiCl2 in 5 liters of 3% hydrochloric acid, an excess of 30% H2O2 is added to the solution, which results in continuous gasification, and then a further 125 g of completely dilute TaCl2 are added to this solution. on top of the plate and incinerated for 10 minutes at 300 ° C. In this way, 20 layers are placed, and after the last layer is applied, the electrode is incinerated for another 30 minutes at 1 + 00 ° C.

The anodes prepared according to this example operate in the NaCl-Laboratory cell after about 3000 operating hours without any noticeable presence of coatings and without an increase in cell voltage.

The most obvious advantage of the anode according to the invention was demonstrated by using the anode prepared according to Example 1 in 20% HCl electrolyte at an operating temperature of T0 ° C. After two months of electrolysis, no deterioration in the performance of this electrode was observed, whereas a platinum electrode made in the same bath as the base layer of the electrode according to the invention was already largely passivated after this time. The ruthenium oxide anode also shows a clear increase in cell voltage after an equally long operating time.

It must be assumed that the high durability of the electrode according to the invention is due to the presence of tantalum oxide. Thus, substantially better corrosion resistance of titanium is also achieved by coating it with a layer of tantalum oxide. If these tan tallow oxide layers are made thick enough, the corrosion resistance of pure tan tallow can be almost achieved, even though the support structure consists mainly of substantially cheaper titanium. The advantages are particularly evident in the use of electrodes in hot hydrochloric acid electrolytes, e.g. technical HCl electrolysis, and in sulfuric acid electrolytes, e.g. electrolytic sulfuric sulphate solution electrolysis,

The following example shows the preparation of such a special corrosion-resistant oxide layer.

Example 2

A titanium plate measuring 100 x 100 x 2 mm and pretreated as in the previous example is coated with a layer of a hydrochloric acid solution containing 15 g of TiCl 3 and 11 + 0 S TaCl 2 containing an excess of H 2 O 2. Each layer is incinerated at Loo C and after the last layer incinerated for an additional 60 minutes at 700 ° C. It turns out that the breakdown voltage of this plate is substantially higher than that of an uncoated titanium plate.

This oxide layer is mainly suitable for conducting current and parts of the electrode structure which are located on the side away from the cathode and do not actually participate in the electrolysis process. To make this corrosion-resistant coating, the proportion of more durable valve metal is selected to be 50-90% of the titanium fraction to be coated.

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Example 3

A 100 x 100 x 2 mm titanium plate is etched for 10 minutes in 50% hydrofluoric acid, then rinsed with water and dried. This plate is galvanically coated with a thin layer of ruthenium metal. A solution of 32 g of FeCl 2, 14 g of CoCl 2, H 2 O, 2 g of MnCl 2 is then prepared.

4Η ^ 0 and 30 g of TiCl2 in one liter of 3 - $ * hydrochloric acid. 30 g of HgO 2 are added to the solution until the excess causes continuous light gas formation. To this solution is then added an additional 20 g of HgOgie completely dissolved

UbCl, .. The UbClc solution must no longer contain any part in colloidal form.

5 5

Any NboOc present must be carefully removed from the solution and made up to έ 5 with an appropriate amount of niobium. This solution is then divided into equal portions into 20 containers. The embedded titanium plate is lifted by a hoist at a speed of about 5 cm / min from the solution and the coating is burned in for 15 minutes at a temperature of 400 ° C. To coat the next layer, the following tank is used and the incineration of the cover layer is repeated. When the plate is coated in this way with 20 layers, the annealing treatment is performed for one hour at a temperature of 500 ° C. This coating method ensures that there is no base layer material in the cover layer.

The anode prepared according to this method operated in NaCl electrolyte 2 with a current density of 6.7 kA / m after 5000 operating hours at a cell voltage of 4.1 V. Example 4

As in Example 1, a pre-etched titanium sheet is coated with four layers of a solution of 51 x 5 g of ruthenium chloride and 40 g of TiCl in one liter of 20- $ hydrochloric acid to provide a base layer. Each floor

^ O

It is burned in argon gas at a temperature of 500 ° C. The cover layer is coated with a further 18 layers of a solution of 80 g FeCl2.4H2O, 60 g Co (NO5) 2.6H2O, 50 g Μη (Ν05) 2 · 4Η20, 90 g SnCl2.2H2O and 60 g TiCl2 per liter of 3 ~ / * hydrochloric acid to which an excess of H 2 O 2 is added and which further contains 50 g of TaCl 2. Each layer is fired for 15 minutes at 400 ° C.

The electrode made in this way worked completely satisfactorily for 2,500 hours, while the electrode coated only with ruthenium-containing layers already showed a clear voltage rise in such a short operating time.

Example 5 A tantalum plate having a size of 100 x 100 x 2 mm etched in 50 hydrofluoric acid is galvanically coated with a layer of cadmium. This cadmium base layer is divided by 2 mm wide grooves extending into the tantalum plate into 5 * 5 mm squares which are not connected to each other. A liter of sulfuric acid solution is then prepared with 50 g of FeSO 4 .7H 2 O, 50 g of SnSO 4, 10 g of MnSO 4 .H 2 O, 25 g of CoSO 4 .7H 2 O and 80 g of TiO 2. JO solution HgOgt is added to the solution until continuous light gas evolution is observed. 15 g of barium phosphorus tungstate prepared by conventional methods are then dissolved in hot water while adding hydrochloric acid, barium is precipitated with sulfuric acid and the filtrate is added to the above solution. The plate is coated with 30 layers of this solution and each layer is fired for 30 minutes at 350 ° C. After the last layer, the final treatment is carried out for a further 60 minutes at 500 ° C.

The anode prepared according to this method operated in the ITaCl-Laboratory cell 2 at a current density of 10 kA / m at a voltage of 4.3 V.

Example 6

In the usual way, the carbon layer of a carbon-coated, pre-etched titanium rod with a diameter of 10 mm and a length of 20 cm is cut at 2 cm intervals with milled grooves about 1 mm wide. The rod is then coated with a 20 layer of a solution of 20 g PeCl 2 .111 ^ 0, 15 g

Co ^ O ^), -, .6 ^ 0, 12 g MniNO ^ Jg. 4H 2 O, 22 g SnCl 2. 211 ^ 3 and 3 g TaCl 2. With an excess of H 2 O 2. Each layer is fired for 15 minutes at 400 ° C.

The anode made according to this method operated in a laboratory cell at 4.2 V with a current density of 10 kA / m. After 500 hours of operation, the cover was broken at a few points with a chisel. After an additional 1000 hours of operation, it turned out that the operation in the damaged areas had deteriorated, but no deterioration could be observed in the operation of the other zones.

If the base layer is a material that can be destroyed during electrolysis by damage to the cover layer (e.g. due to a short circuit or mechanical reasons), then it is appropriate to cut the base layer into suitable pieces and separate the individual segments with an insoluble cover layer material. Thanks to this operation, only individual segments are destroyed in the event of damage while the rest of the electrode is still operating.

Example 7 A titanium plate measuring 100 x 100 x 2 mm is coated with a base layer of polycrystalline graphite. A solution is then prepared with 20 g of SnCl2.2II2O, 40 g of ΡβΐΝΟ2.22.22, 12 g of Mn (NO3) 2.4H2O, 15 g of Co (NO2) 2.

6H 2 O, 20 g Ti 2 (SO 2) 3 and 10 g SbCl 3 in 500 ml of 3% hydrochloric acid. To this solution is added excess H 2 O 3 until continuous light gas evolution can be observed. The plate is coated with 20 layers of this solution and each layer is fired for 15 minutes at 300 ° C.

The anode prepared according to this method operated for 8 weeks without any noticeable weight loss.

Example A 50 x 10 x 2 mm titanium plate etched in oxalic acid at 95 ° C is vaporized in vacuo with a platinum-carbon mixture. The precious metal accounts for $ 10 of the amount of steam. The plate is then coated with 20 layers of θ 59428 solution containing 32 g of FeCl 2, 15 g of CoCl 2 .6H 2 O, 25 g of MnCl 2 .4H 2 O, 60 g of TaCl 5 and 15 g of barium phosphorus tungstate from which barium has been removed according to Example 5, per liter 3- #: hydrochloric acid containing an excess of H 2. Each layer was fired for 20 minutes at 350 ° C.

The anode prepared according to this method operated at a current density of 10 kA / m at a supply voltage of 10 kA / m in a NaCl electrolyte.

Claims (3)

9 59428 An electrode for electrolytic processes, which electrode consists of a valve metal core and a main layer, which in turn contains elemental metal and base metal oxides, characterized in that the top layer is formed in two layers, the adjacent bottom layer being metal and / or carbon without a valve effect. a surrounding gas- and liquid-tight cover layer, which in turn consists of: a) titanium dioxide (TiO 2) and / or tantalum oxide (Ta 2 O 5); b) highly conductive oxides of niobium, tantalum, tungsten, antimony, iron, cobalt, manganese and / or tin to increase the electrical conductivity of TiO 2 and / or Ta 2 O 5 in a sufficient proportion; and c) electrochemically stable Spinell-type metal oxides. Electrode according to Claim 1, characterized in that the base layer consists of platinum, ruthenium, cadmium, polycrystalline graphite or a platinum-carbon alloy. Electrode according to Claim 1 or 2, characterized in that the base layer has breaks within which the cover layer is in direct contact with the valve metal.