United States Patent (19) (11) 4,111,765 De Nora Et Al
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United States Patent (19) (11) 4,111,765 De Nora et al. 45) Sep. 5, 1978 (54) SLCON CARBDE-VALVE METAL (56) References Cited BORIDES-CARBON ELECTRODES U.S. PATENT DOCUMENTS 75 Inventors: Vittorio De Nora, Nassau, The 3,330,756 7/1967 Ransley ................................ 204/279 Bahamas; Antonio Nidola; Placido 3,632,498 1/1972 Beer ............... 204/290 F Maria Spaziante, both of Lugano, 3,645,862 2/1972 Cotton et al... ... 204/56 R Switzerland 3,661,736 5/1972 Holliday ................................ 204/67 73 Assignee: Diamond Shamrock Technologies 3,687,724 8/1972 Keith et al..... 204/290 FX S.A., Geneva, Switzerland 3,788,968 1/1974 Miller et al. ...... - O - O - 204/290 R Primary Examiner-F.C. Edmundson (21) Appl. No.: 820,834 Attorney, Agent, or Firm-Hammond & Littell 22 Filed: Aug. 1, 1977 57 ABSTRACT Related U.S. Application Data Novel sintered electrodes consisting essentially of 40 to 90% by weight of at least one valve metal boride, 5 to 63 Continuation-in-part of Ser. No. 754,025, Dec. 23, 40% by weight of silicon carbide and 5 to 40% by 1976, abandoned. weight of carbon useful for electrolysis reactions, par (51) int. Cl. ........................ C25B 1/34; C25B 11/00; ticularly electrolysis of halide ions to the corresponding C25B3/06; C25B 7/02 halogen and to a novel electrolytic cell, a novel bipolar 52 U.S. C. ........................................ 204/67; 204/98; electrode and to a novel process for effecting the elec 204/243 R; 204/252; 204/254; 204/290 R; trochemical processes, particularly electrolysis of mol 204/290 F; 204/291 ten metal halides. 58) Field of Search ........... 204/290 R, 290 F, 290 G, 204/291, 67,98, 243, 252,254 5 Claims, 2 Drawing Figures U.S. Patent Sept. 5, 1978 4,111,765 Chlorine potential referred to silver electrode 2 t (V) .5 O.5 O 5O OO 5OO IOOO 5OOOOOOO A/m? FG. 2 o-o-GRAPHITE (V) .5 emes mess saw aaaaaass seems mod uses 8 were A ZrB-SiC O.5 is ceases as a soon b areases e. Zr B-SiC+CIO% Zr B-SiC+C 30% 2 3 4. 5 6 7 8 Working time (hrs) 4,111,765 2 anodes are consumed in one year by aluminum produc SLICON CARBIDE-VALVE METAL ers. The carbon anodes are burned away according to BORDES CARBON ELECTRODES the reation: PRIORAPPLICATION AlO3/223 C-2Al + 3/2 CO, This application is a continuation-in-part of our co pending, commonly assigned U.S. patent application but the actual consumption rate is much higher due to Ser No. 754,025 filed Dec. 23, 1976, now abandoned. fragilization and breaking away of carbon particles and to intermittent sparking which takes place across anodic STATE OF THE ART 10 gas films which often form over areas of the anode Dimensionally stable electrodes for anodic and ca surface since carbon is poorly wetted by the molten salt thodic reactions in electrolysis cells have recently be electrolytes, or to short circuiting caused by “bridges' come of general use in the eletrochemical industry re of conductive particles coming from the corroding placing the consumable electrodes of carbon, graphite carbon anodes and from dispersed particles of the de and lead alloys. They are particularly useful in flowing 15 positing metal. mercury cathode cells and in diaphragm cells for the British Pat. No. 1,295,117 discloses anodes for molten production of chlorine and caustic, in metal electrowin cryolite baths consisting of a sintered ceramic oxide ning cells wherein pure metal is recovered from chlo material consisting substantially of SnO, with minor ride or sulfate aqueous solution as well as in the ca amounts of other metal oxides, namely, oxides of Fe, Sb, thodic protection of ships' hulls and other metal struc 20 Cr, Nb, Zn, W, Zr, Ta in concentrations of up to 20%. tures. While electrically conducting sintered Sno with Dimensionally stable electrodes generally comprise a minor additions of other metal oxides, such as oxides of valve metal base, such as Ti, Ta, Zr, Hf, Nb and W, Sb, Bi, Cu, U, Zn, Ta, As, etc., has been used for a long which under anodic polarization develope a corrosion time as a durable electrode material in alternating cur resistant but non-electrically conductive oxide layer or 25 rent glass smelting furnaces (see U.S. Pat. Nos. “barrier layer,' coated over at least a portion of the 2,490,825; 2,490,826; 3,282,284 and 3,502,597), it shows surface with an electrically conductive and electrocata considerable wear and corrosion when used as anode lytic layer containing platinum group metal oxides or material in the electrolysis of molten salts. platinum group metals (see U.S. Pat. Nos. 3,711,385; We have found wear rates of up to 0.5 grams per hour 3,763,498 and 3,846,273) and sometimes oxides of also 30 per cm from samples of the compositions described in valve metals. Molybdenum, vanadium, aluminum and the patents mentioned above when operated in fused yttrium are also metals which, within certain environ cryolite electrolytes at 3000 A/m. The high wear rate ments, show distinct valve metal characteristics, i.e. the of sintered SnO, electrodes is thought to be due to sev formation of a filming layer of oxides substantially pro eral factors: a) chemical attack by the hologens, in fact tecting the metal from further oxidation or corrosion. 35 (e.g. anodic treatment of Al). Sn gives complexes of high corrodination numbers Electroconductive and electrocatalytic coatings with halogen ions; b) reduction of SnO, by aluminum made of or containing platinum group metals or plati dispersed in the electrolyte; and c) mechanical erosion num group metal oxides are, however, expensive and by anodic gas evolution and salt precipitation within the are eventually subjected to consumption or deactiva pores of the material. tion in certain electrolytic processes and, therefore, Japanese Patent Application No. 112589 (Publication reactivation or recoating is necessary to reactivate ex No. 62,114,Of 1975) discloses electrodes having a con hausted electrodes. ductive support of titanium, nickel or copper or an alloy Furthermore, electrodes of this type are not operable thereof, carbon, graphite or other conductive material in a number of electrolytic processes. For example, in 45 coated with a layer consisting substantially of spinel molten salt electrolytes, the valve metal support is rap and/or perovskite type metal oxides and alternatively idly dissolved, since the thin protective oxide layer is electrodes obtained by sintering mixtures of said oxides. either not formed at all or is rapidly destroyed by the Spinel oxides and perovskite oxides belong to a family electrolyte with the consequent dissolution of the valve of metal oxides which typically show good electronic metal base and loss of the catalytic noble metal coating. 50 conductivity and have been proposed previously as Moreover, in several aqueous electrolytes,such as bro suitable electroconductive and electrocatalytic anodic mide solutions or in sea-water, the breakdown voltage coating materials for dimensionally stable valve metal of the protective oxide layer on the exposed valve metal anodes (see U.S. Pat. Nos. 3,711,382 and 3,711,297; base is too low and the valve metal base is often cor Belgian Pat. No. 780,303). roded under anodic polarization. 55 Coatings of particulate spinels and/or perovskites Recently, other types of electrodes have been sug have been found, however, to be mechanically weak as gested to replace the rapidly consumed anodes and the bonding between the particulate ceramic coating carbon cathodes in severely corrosive applications, and the metal or carbon substrate is inherently weak, such as the electrolysis of molten salts, typically for the because the crystal structure of the spinels and of the electrolysis of molten fluoride baths such as those used perovskites are not isomorphous with the oxides of the to produce aluminum from molten cryolite. In this par metal support and various binding agents such as ox ticular electrolytic process, which is of great economic ides, carbides, nitrides and borides have been tried with importance, carbon anodes are consumed at a rate of little or no improvement. In molten salt electrolytes, the approximately 500 kg of carbon per ton of aluminum substrate material is rapidly attacked due to the inevita produced and expensive constant adjustment apparatus 65 ble pores through the spinel oxide coating and the coat is used to maintain a small and uniform gap between the ing is quickly spalled off the corroding substrate. Fur corroding anode surface and the liquid aluminum cath thermore, spinels and perovskites are not chemically or ode. It is estimated that over 6 million tons of carbon electrochemically stable in molten halide salt electro 4,111,765 3 4. lytes and show an appreciable wear rate due to halide The said electrodes are useful in electrochemical ion attack and to the reducing action of dispersed metal. processes such as the electrolysis of aqueous halide In the electrolytic production of metals from molten solutions, for electrowinning of metals from aqueous halide salts, the mentioned anodes of the prior art have sulfate or halide solutions and for other processes in been found to have another disadvantage. The apprecia which an electric current is passed through an electro bie dissolution of the ceramic oxide material brings lyte for the purpose of decomposing the electrolyte, for metal cations into the solution which deposit on the carrying out oxidations and reduction of organic and cathode together with the metal which is being pro inorganic compounds or to impress a cathodic potential duced and the impurity content in the recovered metal to a metallic structure which has to be protected for is so high that the metal can no longer be used for ap O corrosion, as well as for primary and secondary batter plicatins requiring electrolytic grade purity.