JPS6022069B2 - sintered anode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary of the Disclosure] Essentially at least one borated valve metal 40-9
0 (weight)%, and 5 to 40 (weight)% silicon carbide,
A novel non-condensing anode and a novel bipolar electrode useful for electrolysis, particularly for electrolyzing halide ions to the corresponding halogen, comprising 5 to 40% (by weight) carbon are disclosed.

[Description of prior art]

Dimensionally stable electrodes for anodic and cathodic reactions in electrolytic cells have recently become commonly used in the electrochemical industry to replace consumable electrodes of carbon, graphite and lead alloys.

Dimensionally stable electrodes are used in particular in flow-through mercury cathode electrolyzers and diaphragm electrolyzers for the production of chlorine and caustic soda, metal electrowinning for the recovery of pure metals from aqueous chloride and sulfate solutions, and for ship hulls and other metal structures. Useful for cathodic protection. Dimensionally stable electrodes generally consist of valve metal substrates such as Ti, Ta, Zr, Hf, Nb and W.

These substrates are corrosion resistant upon anodic polarization, but contain conductive and electrocatalytic layers containing platinum group metal oxides or platinum group metals (U.S. Pat.
No. 3,846,273) and sometimes also a non-conductive oxide layer or "weir layer" whose surface is coated at least in part with the oxide of the valve metal. Molybdenum, vanadium, aluminum and yttrium are also metals that in some environments exhibit distinct valve metal properties, such as the formation of a thin layer of oxide that substantially protects the metal from oxidation or corrosion (e.g. anodizing). This is because conductive electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are expensive and can be depleted or deactivated in certain electrolytic treatments. It is necessary to reactivate the exhausted electrode by reactivating or recoating it.

．． Furthermore, this type of electrode does not work in many electrolytic processes.

For example, in molten salt electrolysis, the valve metal support rapidly dissolves. Either the thin protective oxide layer does not form at all, or it is rapidly destroyed by the electrolyte, dissolving the valve metal substrate and depleting the catalytic precious metal coating. Furthermore, in many electrolytes such as bromide solutions and in seawater, the breakdown voltage of the protective oxide layer on the exposed valve metal substrate is very low, and the valve metal substrate often corrodes during anodic polarization. Instead of using rapidly depleting anodes and carbon cathodes for electrolysis that is subject to numerous corrosive effects, such as the electrolysis of molten salts, primarily the electrolysis of molten rock oxide baths such as those used to produce aluminum from molten cryolite, It has been proposed to use other types of electrodes.

In this economically important electrolytic process, the carbon anode consumes about 500k9 carbon per ton of aluminum produced, and in order to maintain a uniform gap of 4 mm between the corroding anode surface and the liquid aluminum cathode. Uses expensive constant adjustment equipment. 60 per year by aluminum manufacturers
It is estimated that more than 1,000,000 tons of carbon anodes have been consumed. The carbon anode disappears through the following reaction. That is, mountain 20313 / next 1 2N13 / book Q However, the actual consumption rate is much higher.

This is due to the fragility and destructive nature of the carbon particles, as well as the intermittent sparks that form in the anode gas thin film that can cover the entire anode surface because the carbon is not sufficiently wetted by the molten salt electrolyte. or short circuits due to conductive particle "bridging" caused by corrosive carbon anodes and dispersed particles of deposited metal. GB 1295117 discloses a precipitated ceramic oxide consisting essentially of Sn02 and small amounts of other metal oxides, namely Fe, Sb in concentrations up to 20%.
, Cr, Nb, Zn, W, Zr, and Ta. Conductive SN
In addition to 02, metal oxides such as Sb, Bi, Cu,
Additions of small amounts of oxides such as U, Zn, Ta, etc. have long been used as durable electrode materials in AC glass melting furnaces (U.S. Pat. No. 2,490,825; U.S. Pat.
No. 0826, No. 32872, No. 3502597
When used as an anode body for molten salt electrolysis, it wears out and corrodes considerably.

When a sample of the composition described in said patent was operated in molten cryolite electrolysis at 300 m/m, the wear rate was 0.0 m/s. I learned that it could reach the ship's side as well.

The high wear rate of the sintered twill SBI02 electrode is thought to be due to the following numerous factors. That is, due to the chemical action of halogens, in fact, SnW exhibits a rust body with a high conodination rate (conodination nm) due to halogen ions.

;'b}Sn0 due to aluminum dispersed in electrolyte
Ring element 2: [c] Mechanical erosion due to generation of anode gas and deposition of salt in the pores of the material. 1975 Patent Application Publication No. 62
No. 114 discloses electrodes in which a conductive support of titanium, nickel or steel or alloys thereof, carbon, graphite or other conductive material is coated with a substantially spinel-type and/or berovskite-type metal oxide, and in its place. discloses an electrode made of a competitive mixture of the above oxides.

Spinel-type oxides and velovskite-type oxides belong to metal oxides with excellent electronic conductivity and have already been proposed as suitable conductive and electrocatalytic anode coating materials for dimensionally stable valve metal anodes. It is. (See US Pat. No. 3,711,382, US Pat. No. 3,711,297, Belgian Patent No. 780,303). However, certain spinel and/or velopskite coatings have been found to be mechanically weak due to the inherently weak bond between certain ceramic coatings and metal or carbon substrates. This is because the crystal structure of spinel and that of velopskite are not isomorphic to the oxide of the metal support, and numerous binders such as oxides, carbides, nitrides and borides have been tested. Little or no improvement. In molten salt electrolysis, the substrate material is rapidly eroded due to the presence of porosity, which is inevitable for spinel-type oxide coatings, and the coating rapidly destroys the corroded substrate.
Furthermore, spinels and berovskites are not chemically or electrochemically stable in molten halide salt electrolytes and have high wear rates due to the action of halide ions and ring element action of dispersed metals. In the electrolytic production of metals from Z-fused halide salts, we have found that the anodes described in the prior art have additional disadvantages.
Dissolution of the ceramic oxide brings the metal cations into solution.
This solution is deposited on the cathode along with the metal produced. 2 The content of impurities in the recovered metal is so high that it can no longer be used for applications requiring electrolytic purity.

In such cases, electrolytic treatment has a wider range and can yield products with higher purity than melting treatment.
Some or all of the economic advantages mentioned above will be lost. Electrode materials that can be used satisfactorily in highly corrosive conditions, such as the electrolysis of molten halide salts, especially molten fluoride salts, are primarily chemically and electrochemically Must be stable.

It should also be catalytic for the anodic generation of oxygen and/or halides such that the anodic overpotential is minimal to increase the overall efficiency of the electrolytic process. The dew electrode must also be thermally stable at operating temperatures, e.g., from about 200°C to 1100°C, and must be sufficiently conductive to withstand accidental contact with the molten metal cathode. . Few metal substrates can withstand the highly corrosive conditions of molten fluoride salt electrolytes;
Excluding coated metal electrodes, the performance of a large number of scale silk ceramic electrodes with different compositions was systematically tested. U.S. Pat. No. 363-56 describes an electrode made of titanium carbide-impregnated graphite for the electrolysis of manganese sulfate solutions to produce manganese dioxide; U.S. Pat.
No. 5615, No. 3314876, No. 330756
This issue concerns an aluminum electrolytic cell using valve metal borides and valve metal carbides as a flow collector.

U.S. Pat. No. 3,459,515 relates to an aluminum electrolytic cell with a current collector consisting of titanium carbide-titanium boride and/or zirconium boride and aluminum up to 3 P0. US Pat. No. 3,977,955 describes electrodes made of tantalum, tantalum boride, tantalum carbide, and iron group metals. OBJECTS OF THE INVENTION It is an object of the present invention to provide a new and improved electrode and a new bipolar electrode consisting primarily of silicon carbide-valve metal boride-carbon.

These and other objects and advantages of the invention will become apparent from the following detailed description. [Description of the Invention] The novel phosphorescent electrode of the present invention mainly contains 40 to 90% (by weight) of at least one boride valve metal, 5 to 40% (by weight) of silicon carbide, and 5 to 40% (by weight) of at least one boride valve metal. weight)%
It consists of carbon.

Said electrodes may be used for electrochemical processes such as electrolysis of aqueous halides, electrowinning of metals from aqueous sulfate or halides, and other methods, i.e. for decomposition of electrolytes, oxidation and oxidation of organic and inorganic compositions. It is useful in processes such as reducing, applying an anodic voltage to metal structures to prevent corrosion, and passing electrical current through electrolytes for primary and secondary batteries.

The electrodes of this invention can be polarized as anodes or cathodes, or can be utilized as bipolar electrodes, so that one side of the electrode and the opposite side of the electrode are in contact, respectively, as is well known in the art of electrolysis. For the electrolyte, one side or end of the electrode acts as an anode and the opposite side or end of the electrode acts as a cathode. o
The term "competitive" refers to the process by which a particular mixture of carbide diosoborate valve metal and lead is prepared by applying pressure and temperature to a powder mixture, by casting the material in a mold, by extruding it, or by applying a binder to the mixture. Used to describe the creation of a free-standing, rigid body by well-known methods used in the ceramic industry, such as bonding.

The terms "coupled electrode,""castelectrode," or "competitive anode," even when used separately, are synonyms;
The composition material can be in a crystalline or amorphous state. Valve metals include titanium, tantalum, hafnium, zirconium, aluminum, niobium, tungsten and their alloys, which are especially suitable for anodic polarization, and molybdenum, vanadium, yttrium, which are particularly suitable for cathodic polarization. Electrodes made of boride valve metals, such as Zirconium boride or titanium boride, tend to dissolve when used as anodes in molten salt baths such as aluminum chloride, and have somewhat high overpotentials for chlorine. Carbonized valve metal disintegrates when used in this type of molten salt bath, and carbon or graphite alone has a short life. In contrast, the electrode of the present invention has excellent electronic and electrical conductivity, has a chlorine overvoltage that is 2 lower than that of a graphite and borated valve metal-diastochemistry carbide mixture electrode, and has a chlorine overvoltage that is lower than that of a graphite and borated valve metal-diastochemistry carbide mixture electrode, relative to the molten salt electrolyte in contact with it. It has excellent corrosion resistance and excellent wettability.

Furthermore, the electrode of this invention has a density of 5,000 per square meter.
It can operate as an anode at high current densities ranging from 1000 amperes to more. When an electrode is made by condensing the powder for 2 hours, the particle size of the powder composition is 50
to 500 microns, which is usually the particle size range in which powder mixtures consolidate well. The electrodes of this invention can be manufactured by methods commonly used in the ceramic industry. In one preferred method, the powder mixture is mixed with water or an organic binder to form a plastic form with flow properties suitable for the particular molding process used.

It can be molded by three well-known methods, such as by compacting or pressing the mixture in a mold, or by casting, or it can be extruded from a mold to form various shapes. The molded electrode is then dried and heated for 1 to 30 hours at a temperature that produces the desired bond, and then allowed to gradually cool, usually to room temperature.

The heat treatment is preferably carried out in an inert atmosphere, or alternatively with some reduction, for example in 100% N2 (80%).
The molding process can be followed by a high-temperature fringing process as described above, or the molding process and the striation process can be carried out simultaneously, i.e. by applying pressure and temperature to the powder mixture, for example using an electrically heated mold. be able to.

Drop-in connections may be twilled to the electrode during molding and fringing, or attached to the electrode after sintering or molding. A wire mesh or core or flexible core material can be provided inside the competitive electrode body to improve current distribution, facilitate electrical connection of the electrode to the power supply system, and reinforce the aggregate.

The method of this invention can be effectively used for the electrolysis of many electrolytes.

The electrode of this invention is suitable for electrochemical processes such as electrolysis of persalt solutions for the production of chlorine, caustic soda, hydrogen, hypochlorites, chlorates and perchlorates: steel, zinc, nickel, Electrowinning of metals from sulfate or chloride solutions for the production of cobalt and other metals; and can be used as anode and/or cathode in the electrolysis of bromide, sulfide, sulfuric acid, hydrochloric acid, hydrofluoric acid . Generally, the method of the invention involves passing an electrical current through an electrolyte to decompose the electrolyte to effect oxidation and reduction of organic and inorganic compositions, or applying a cathodic potential to a metal structure to prevent corrosion of the metal structure. It is useful when adding a battery or making it into a primary battery or a secondary battery. When using the method of the invention with bipolar electrodes, the composition of the cathode portion of the electrode must be adapted to withstand the particular anodic conditions.

Therefore, the cathode part of the bipolar electrode may contain other substances which improve the properties of the electrode of the invention, such as metals, in particular bulb metals, carbides, borides, silicides, chambers, sulfides of molybdenum, vanadium and yttrium. and/or carbonitride.

Yttrium boride, titanium boride or zirconium boride are preferred materials for the cathode side of the bipolar electrode. By appropriate powder mixing techniques, the composition of the bipolar electrode of this invention can be varied across the cross-section of the electrode.

That is, zirconium boride, titanium boride,
Alternatively, additional zirconium boride may be present. The electrolytic cell of this invention comprises a cell having at least one set of spaced apart anodes and cathodes and a device for applying an electrolytic current to the electrolytic cell, the anode being dimensionally stable and comprising the three components described above. Use as an electrode. The electrolytic cell of this invention is preferred for use in the electrolysis of molten metal salts such as aluminum chloride. Next, some examples of preferred embodiments of this invention will be listed.

However, the invention is intended to be limited to these specific embodiments. Example 1 Approximately 25 samples of the material listed in Table 1 were ground in a mixer for 20 minutes and the powder mixture was placed in a cylindrical plastic mold and pre-compacted by hand in a steel cylindrical press.

Place each mold in an equal pressure chamber and increase the pressure to approximately 1500k9/cm in 5 minutes.
I raised it to zero, then lowered it to zero within a few seconds. The sample was removed from the plastic mold and polished. The pressed sample was placed in an electric heating furnace and heated from room temperature to 1500oO over 2 hours in a nitrogen atmosphere, kept at the maximum temperature for 2 to 5 hours, and then cooled to 20OO over 2 hours. The phosphorized sample was removed from the oven and cooled to room temperature before being calibrated. The operating conditions of an electrolytic cell for producing aluminum metal from molten cryolite were matched to experimental electrolytic cells at a laboratory.

In a heated graphite crucible with a layer of liquid aluminum on the bottom, 56% (by weight) of AIC13 and 19.5% (by weight)
A melt consisting of % NaCI and 24.5% (by weight) KCI was injected on top. As a simple means of electrical connection, a Pt wire was soldered to a sample electrode prepared by the method described above, and the electrode was immersed in a salt melt at a distance of approximately 1 c from the liquid aluminum layer. The crucible was kept at a temperature of 700 oo, the current density was arc A/〆, and the electrolyzer was operated for 8 hours. The experimental data obtained thereby are shown in FIG. Table 1 The above results show that the chlorine voltage of graphite is 1.5 to 1.7 volts higher than that of the electrode of the present invention.

Furthermore, the chlorine voltage of electrodes No. 3 and No. 4 of the invention is less than electrode No. 2, which does not contain any free carbon. No corrosion was observed during 8 hours of operation. Further, from the curves of electrodes No. 3 and No. 4, it can be seen that the chlorine voltage decreases somewhat as the carbon content increases. [Example 2] The chlorine voltage of the first to fourth electrodes of Example 1 was set to 2. Song A/
When the silver electrode was measured at the final stage, the results were as follows.
As shown in the figure, no change was observed in the chlorine voltage after 8 hours.

Numerous modifications of the electrodes, electrolyzers, and electrochemical processes of this invention may be made without departing from the spirit or scope of this invention, and this invention is within the scope of the following claims. limited as set forth in .

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams showing chlorine voltage data for a silver electrode to explain an example of the present invention. Heron 2

Claims (1)

[Claims] 1 Titanium, tantalum, zirconium, aluminum,
At least one boride of a valve metal selected from the group consisting of hafnium, niobium, tungsten, yttrium, molybdenum and vanadium 40 to 90 (by weight)
%, 5 to 40% (by weight) silicon carbide, and 5 to 40% (by weight) carbon. 2. The anode according to claim 1, wherein the boride of the valve metal is zirconium boride. 2 Titanium, tantalum, zirconium, aluminum,
At least one boride of a valve metal selected from the group consisting of hafnium, niobium, tungsten, yttrium, molybdenum and vanadium 40 to 90 (by weight)
%, an anode portion consisting of 5 to 40% (by weight) silicon carbide, and 5 to 40% (by weight) carbon, titanium, tantalum, aluminum, niobium, tungsten, molybdenum,
A cathode portion made of at least one member selected from the group consisting of borides, carbides, nitrides, silicides, sulfides, and carbonitrides of valve metals selected from the group consisting of vanadium, yttrium, zirconium, hafnium, and mixtures thereof. A bipolar electrode comprising: