FI61726B - SINTRAD SMALL BOXES-VALVES-METALBORIDES-KOLANOD FOER ELECTROCHEMICAL PROCESSER - Google Patents

Description

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Wa W (11) UTLAGGNINGSSKRIFT 61/2 6 <(45) Patent ny one tty 10 09 1932 ι'μρ '; Patent title vv '(51) Kv.ik.Wcu3 c 25 B 11/04 // C 25 c 3/12 FINLAND —FINLAND (21) P * Unttlh »ktmu · - Patmtanaekitlng 773255 (22) H * k« mlspllvl - AiMeknlnpdag 31.10.77 ^ ^ (23) Alkupttvt — GlMghaudag 31.10.77 (41) Tullut iulklMlui - Bllvlt offmtllg 2 ^ .06.78

Patent · ja reki.frlh.Ultu, (44) Νϋ, «ν» ωρ «οη |. MONTHLY Date - 31.05.82

Patent and Registration Office of the United States of America (32) (33) (31) Requested atuoika * —Begird priority 23.12.7 6 01.08.77 USA (US) 75 ^ 025, 82083 * + (71) Diamond Shamrock Technologies SA, 3 Place Isaac Mercier, Geneva,

Switzerland-Switzerland (CH) (72) Vittorio de Nora, Nassau, Bahamas-Bahamas (BS), Antonio Nidola, Lugano, Placido Maria Spaziante, Lugano, Switzerland-Switzerland (CH) (7 * +) Leitzinger Oy (5 * + ) Sintered silicon-carbide-metal-chloride anode for electrochemical processes - Sintrad kiselkarhid-ventil-metallhorid-kolanod för elektrokemiska processer

The invention relates to novel sintered anodes having a composition of at least 40 to 90% by weight of at least one valve metal boride, 5 to 40% by weight of silicon carbide and 5 to 40% by weight of carbon, and which are useful in electrolysis reactions, in particular electrolysis to halide ions. and a new electrolytic cell, a new bipolar electrode and a new method for performing electrochemical methods, in particular for electrolysing molten metal halides.

In the electrochemical industry, the use of dimensionally resistant electrodes in electrolytic cells in anodic and cathodic reactions has recently begun to be widely used, replacing wearing carbon, graphite and lead alloy electrodes. They are particularly useful in cathode cells with mercury flow and in diaphragm cells for chlorine and lye production, in electronic metal recovery cells where pure metal is recovered from aqueous chloride or sulphate solution, and in cathodic protection of ship hulls and other metal structures 2 61 726

Dimensionally resistant electrodes generally comprise a valve-metallic substrate, such as Ti, Ta, Zr, Hf, Nb and W, which under anodic polarization develops a corrosion-resistant but non-conductive oxide layer or "barrier layer" and whose surface is at least partially coated with an electrically conductive and an electrocatalytic layer containing platinum group metal oxides or platinum group metals (see U.S. Patents 3,711,385; 3,763,498 and 3,846,273) and sometimes also valve metal oxides. Molybdenum, vanadium, aluminum and yttrium are also metals which, in certain environments, have clear properties of valve metals, i. they form a covering layer of oxides that substantially protects the metal from further oxidation or corrosion (e.g., anodic treatment of aluminum).

However, electrically conductive and electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are expensive and eventually wear or deactivate in certain electrolytic processes, and therefore spent electrodes must be repaired by reactivation or recoating.

In addition, electrodes of this type cannot be used in many electrolytic processes. For example, in molten salt electrolytes, the valve metal support dissolves rapidly because a thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte, causing the valve metal substrate to dissolve and the catalytic noble metal coating to disappear. In addition, in many aqueous electrolytes, such as bromide solutions or seawater, the decomposition voltage of the protective oxide layer on the bare valve metal substrate is too low, and the valve metal substrate often corrodes under anodic polarization.

It has recently been proposed that other types of electrodes replace fast-wearing carbon anodes and carbon cathodes used in highly corrosive applications such as electrolysis of molten salts, typically electrolysis of molten fluoride baths in the production of, for example, aluminum from molten cryolite. In this particular electrolytic process, which is very important economically, about 500 kg of carbon anodes are consumed per tonne of aluminum produced and an expensive control device is required to maintain a small and uniform gap between the corroding anode surface and the 3,61726 liquid aluminum cathode. Aluminum producers are estimated to consume more than> 6 million tonnes of carbon anodes in one year. The carbon anodes burn according to the following reaction:

Al2 ° 3 + 2A1 + 3/2 CC> 2 but the actual consumption rate is much higher due to rupture and detachment of carbon particles and intermittent sparking across the anode gas films, which often form on parts of the anode surface due to poor wetting of the molten salt electrolytes, or caused by a "bridge" of conductive particles from corroding carbon anodes and dispersed particles of the deposited metal.

British Patent 1,295,117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material containing substantially SnO 2 as well as minor amounts of other metal oxides, namely Fe, Sb, Cr, Nb, Zn, W, Zr, Ta, oxides up to 20%.

Although electrically conductive SnO 2 with minor amounts of other metal oxides such as oxides of Sb, Bi, Cu, U, Zn, Ta, As, etc., has long been used as a durable electrode material with alternating current in working glass melting furnaces (see U.S. Patents 2,490,825; 2,490,826, 3,287,284 and 3,502,597), it wears and corrodes considerably when used as an anode material in the electrolysis of molten salts.

Based on samples taken from the mixtures described in the aforementioned patents, we have found that consumption rates have been up to 0.5 g per hour per cm when used in molten cryolite electrolyte at a current density of 3000 A / m. The high wear rate of the sintered SnO 2 electrodes is believed to be due to several factors: a) the halogens act chemically, in fact SnIV forms complexes with halogen ions with a high coordination number; b) dispersed aluminum of electrolytes reduces SnO 2; and c) anodic gas evolution causes mechanical erosion and salt precipitates in the pores of the material.

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Japanese Patent Application 112589 (Publication 62,114, 1975) discloses electrodes having a conductive support made of titanium, nickel or copper or their alloys, carbon, graphite or other conductive material coated with a layer consisting essentially of spinel and / or perovskite. t-type metal oxides, and alternatively electrodes obtained by sintering mixtures of said oxides. Spinel oxides and perovskite oxides belong to a group of metals characterized by good electrical conductivity and have previously been proposed for use as electrically conductive and electrocatalytic anodic coating materials for dimensionally durable valve metal anodes (see U.S. Patents 3,7,11.

However, coatings made of particulate spinels and / or perovskites have been found to be mechanically weak due to the inherently weak bond between the particulate ceramic coating and the metal or carbon substrate. This is because the crystal structure of spinels and perovskites is not isomorphic to the oxides of the metal support. Attempts have been made to use various binders such as oxides, carbides, nitrides and borides, but little or no improvement has occurred. In molten salt electrolytes, the effect on the substrate material is rapid due to the inevitable pores in the spinel oxide coatings, and the coating rapidly cleaves away from the corrosive substrate. In addition, spinels and perovskites are not chemically or electrochemically stable in molten halide-salt electrolytes and wear out remarkably rapidly due to the reducing effect of the halide ion and the dispersed metal.

Said prior art anodes have been found to have another disadvantage in the preparation of metals from electrolytically molten halide salts. The considerable dissolution of the ceramic oxide material introduces into the solution metal cations which deposit on the cathode together with the metal to be manufactured, so that the amount of impurities in the recovered metal is so great that the metal can no longer be used in electrolyte purity applications. In such cases, the economic advantages of the electrolytic process are partially or completely lost, largely due to the high purity that can be achieved compared to smelting processes.

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The electrode material that can be successfully used under highly corrosive conditions, such as the electrolysis of molten halide salts, and in particular the electrolysis of molten fluoride salts, should primarily be chemically and electrochemically stable under the conditions of use. It should also be catalytic for the anodic generation of oxygen and / or halides, with the lowest potential of the anode and the overall efficiency of the electrolysis process. The electrode should also be thermally stable at operating temperatures, i.e. 200-1100 ° C, it should have good electrical conductivity and should withstand sufficient accidental contact with the molten metal cathode. With the exception of the coated electrode, since hardly any metal substrate could withstand the extremely corrosive conditions that prevail in the electrolysis of a molten fluoride salt, we have systematically tested the performance of a large number of sintered, substantially ceramic, electrodes of varying composition.

U.S. Patent 3,636,856 describes electrodes made of graphite impregnated titanium carbide for preparing manganese dioxide and electrolysing manganese sulfate solutions. U.S. Patents 3,028,324; 3,215,615; Nos. 3,314,876 and 3,330,756 relate to aluminum electrolytic cells using valve metal borides and valve metal carbides as current collectors. The patent 3,459,515 relates to an aluminum electrolytic cell equipped with a current collector consisting of titanium carbide-titanium boride and / or zirconium boride and up to 30% aluminum. U.S. Pat. No. 3,977,959 discloses an electrode made of tantalum, tantalum boride, tantalum carbide and an iron group metal.

It is an object of the present invention to provide novel improved electrodes consisting essentially of silicon carbide-valve metal boride carbon and novel bipolar electrodes.

It is a further object of the invention to provide a novel electrolytic cell equipped with silicon carbide valve metal boride carbon anodes.

It is a further object of the invention to provide a new electrochemical process using the electrodes according to the invention.

These and other objects and advantages of the invention will become apparent from the following detailed description.

The composition of the new sintered electrodes according to the invention is essentially 40 to 90% by weight of at least one valve metal, i-boride, 5 to 40% by weight of silicon carbide and 5 to 40% by weight of carbon.

Said electrodes are useful in electrochemical processes such as electrolysis of aqueous halide solutions, electrical recovery of metals from aqueous sulphate or halide solutions and other processes in which an electric current is passed through an electrolyte to decompose the electrolyte to decompose or oxidize organic and inorganic compounds. , would provide cathode potential in both primary and secondary batteries. The electrodes of the invention can be polarized as anodes or cathodes, or can be used as bipolar electrodes, wherein one side or end of the electrode serves as an anode and the opposite side or end of the electrode serves as a cathode electro cover-catalyst ratio which are respectively in contact with both sides of the electrode, elektrolyysialalla in a known manner.

The term "sintered" refers to a silicon carbide-valve-metal boride-graphite alloy of a defined type as a non-reinforced, substantially rigid body made by any known method used in the ceramic industry, such as applying a certain pressure and temperature to a powder mixture, molding "bonding electrodes", "casting electrodes" or "sintered electrodes", also when used alone, are substantially synonymous, and the components of the substance may be in a crystalline and / or amorphous state. The valve metal covers titanium, tantalum, hafnium, zirconium, aluminum, niobium and tungsten and their alloys which are particularly suitable for anodic polarization, and molybdenum, vanadium and yttrium, which are particularly suitable for cathodic polarization. Electrodes made of valve metal borides such as zirconium boride or titanium boride tend to dissolve when used as an anode in a molten salt bath such as aluminum chloride and have a rather high potential for chlorine.

When valve metal carbides are used in such molten salt baths, they tend to decompose, while pure carbon 61,726 7 or graphite have a poor service life.

In contrast, the electrodes of the invention have good electronic and electrical conductivity, have a lower chlorine excess potential than alloys of graphite and valve metal boride-silicon carbide electrodes, have good corrosion resistance, and are well wetted by the molten salt electrolyte with which the electrode comes into contact. In addition, the electrodes can be used at high current densities, such as 5,000 to 10,000 amps or more per mm.

When the electrodes are made by sintering, the particle size of the powder components can range from 50 to 500 microns, and the powder mixture usually contains a certain range of grain sizes to improve condensation. The electrodes can be manufactured by conventional methods in the ceramic industry. In one preferred method, the mixture of powders is mixed with water or an organic binder to form a plastic mass with flow characteristics suitable for the molding process used. The material can be cast in a known manner either by compressing or pressing the mixture into a mold or by casting into a gypsum mold, or the material can be extruded through a nozzle into various shapes.

The cast electrodes are then dried and heated for 1 to 30 hours at a temperature at which the desired binding can occur, after which they are usually slowly cooled to room temperature. The heat treatment is best carried out in an inert atmosphere or in a slightly reducing atmosphere, for example H2 + N2 (80%). The molding process may be followed by a sintering process at the above-mentioned high temperature, or the molding process and the sintering process may be simultaneous, i. a certain temperature and pressure can be applied to the powder mixture at the same time, for example by means of electrically heated molds. During casting and sintering, lead connectors can be fused to the electrodes or attached to the electrodes after sintering or casting. A metal mesh or core or flexible core material may be placed inside the sintered electrode body to improve current distribution and to make the electrode more easily electrically connected to the power supply system, and to reinforce the sintered body.

The method of the invention can be effectively used to electrolyse many 8,61726 electrolytes. The electrodes can be used as anodes and / or cathodes in electrochemical processes, such as in the electrolysis of aqueous chloride solutions in the preparation of chlorine, alkali, hydrogen, hypochlorite, chlorate and perchlorate; in the electrical recovery of metals from aqueous sulphate or chloride solutions to produce copper, zinc, nickel, cobalt and other metals; and in the electrolysis of bromides, sulfides, sulfuric acid, hydrochloric acid and hydrofluoric acid.

In general, the method of the invention is useful when an electric current is passed through an electrolyte to decompose the electrolyte, in the oxidation and reduction of organic and inorganic compounds, or when a cathode potential in both primary and secondary batteries is applied to a metal structure to be protected from corrosion. When bipolar electrodes are used in the method according to the invention, the composition of the cathode part of the electrodes must be such that it can withstand the respective cathode conditions.

The cathode part of the bipolar electrode may therefore contain other substances which improve the properties of the electrodes according to the invention, such as carbides, borides, silicides, nitrides, sulphides and / or carbonitrides of metals, in particular valve metals, molybdenum, vanadium and yttrium. Bipolar electrodes, a cathode recommended materials are yttrium, titanium or zirconium.

By using a suitable powder mixing technique, the composition of the bipolar electrodes according to the invention can be changed in different parts of the electrode cross section. Ts. the surface layers of the cathode surface of the bipolar electrode can be enriched with yttrium, titanium or zirconium boride during the casting process and before the sintering is completed.

The electrolytic cell according to the invention is a cell provided with at least one separate group of anode and cathode and devices for supplying an electrolytic current to the cell, the anode being a dimensionally durable, three-component electrode as mentioned above. The cell is best used for the electrolysis of molten metal salts such as aluminum chloride.

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The following examples illustrate several preferred embodiments to illustrate the invention. However, the invention is not intended to be limited to certain embodiments.

Example 1

About 250 g of the substances shown in Table I were ground in a mixer for 20 minutes and the powder mixtures were poured into cylindrical plastic molds and pre-pressed by hand with a steel cylinder press.

Each mold was placed in an isostatic pressure chamber and the pressure was raised to about 1500 kg / cm in 5 minutes and then lowered to zero in a few seconds. The samples were then removed from the plastic molds and polished. The compressed samples were placed in an electrically heated oven and heated for 24 hours from room temperature to 1500 ° C under a nitrogen atmosphere. The maximum temperature was maintained for 2-5 hours, after which it was cooled to 20 ° C over the next 24 hours. The sintered samples were then removed from the furnace and weighed after cooling to room temperature. The operating conditions of the electrolytic cell used to make aluminum metal from a molten cryolite bath were monitored in a laboratory-scale test cell. A layer of liquid aluminum was placed on the bottom of the heated graphite crucible and a melt containing 56% by weight of AlCl 3, 19.5% NaCl and 24.5% by weight KCl was poured on top of it. Sample electrodes prepared as described above and to which a Pt wire was attached as a simple electrical connection were immersed in a salt melt and held at a distance of about 1 cm from the liquid aluminum layer. The crucible was maintained at a certain temperature starting at 700 ° C. The current density was 5KA / rtr, and the cell was in operation for 8 hours. The test results obtained are shown in the following diagram.

617 2 6 10

Table I

Electrode No. Composition Dimensions 1 Graphite 20 x 20 x 30 mm 2 ZrB2 (80%) + SiC (20%) 20 x 20 x 30 mm 3 ZrB „(72%) + SiC (18%) + C (10% ) o60 x 10 mm 4 ZrB, (56%) + SiC (14%) + C (30%) o60 x 10 mm

Chlorine potential compared to hogea electrode ...... "(i ...........

2.0- / NSl / 1.5- / __- o · -.n * 2 0.5 - f 50 100 500 1000 5000 10000 A / m2

The above results indicate that graphite has a chlorotent potential. is 1.5 to 1.7 volts higher than the electrodes of the invention. In addition, the chlorine potential of electrodes 3 and 4 according to the invention is lower than that of electrode No. 2, which does not contain any free carbon. No corrosion was observed during 8 hours of operation. In addition, it can be seen from the curves of electrodes 3 and 4 that the chlorine potential is slightly lower, the discipline carbon content increases.

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Example 2

The chlorine potential of electrodes No. 1 to 4 of Example 1 was determined with respect to 2 silver electrodes at a current density of 2.5 KA / m. The results shown in the following graph show that there was no change in chlorine potential at 8 hours.

1-O-O-graphite

^ 2 ‘(V) 1.5 - 1.0-

—O o ---- o-o— · —O— -z r Bz- Si C

0.5 * _ * _ * _> _, _ * _ ZrBfSiC-fC 10%

ZrBfSiCfC 30 ° / o 0- “I l i i k i i I - 12 3 4 5 6 7 g Operating time. (T)