Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
  • C25C3/12—Anodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/02—Electrodes; Connections thereof
  • C25C7/025—Electrodes; Connections thereof used in cells for the electrolysis of melts

Definitions

• Aluminum is conventionally produced in Hall - Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes. During the reaction, the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction
• Non-consumable electrodes Attempts have been made in the past to use non-consumable electrodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen and/or the cryolite bath. Ceramic compounds, such as oxides with perovskite and spinel crystal structures, usually have too high electrical resistance or are attacked by the cryolite bath.
• Klein discloses an anode of at least 80% SnO 2 , with additions of Fe203, ZnO, Cr 2 0 3 , Sb203, Bi 2 0 3 , V 2 O 5 , Ta 2 O 5 , Nb 2 0 5 or W0 3 .
• Yamada discloses spinel structure oxides of the general formula XYY'O 4 and perovskite structure oxides of the general formula RMO 3 , including the compounds CoCr 2 0 4 , TiFe 2 O 4 , NiCr204, NiCo 2 O 4 , LaCrO 3 , and LaNiO 3 .
• Mochel discloses SnO 2 plus oxides of Ni, Co, Fe, Mn, Cu, Ag, Au, Zn, As, Sb, Ta, Bi and U.
• Belyaev discloses anodes of Fe203, SnO 2 , Co 3 0 4 , NiO, ZnO, CuO, Cr 2 O 3 and mixtures thereof as ferrites. De Nora dis - closes Y 2 0 3 with Y, Zr, Sn, Cr, Mo, Ta, W, Co, Ni, Pd, Ag, and oxides of Mn, Rh, Ir, and Ru.
• the Mochel patents relate to electrodes for melting glass, while the remainder are intended for high temperature electrolysis, such as Hall-Heroult aluminum reduction. Problems with the materials above are related to the cost of the raw materials, the fragility of the electrodes, the difficulty of making a sufficiently large electrode for commercial usage, and the low electrical conductivity of many of the materials above when compared to carbon anodes.
• U.S. Patent No. 3,960,678-Alder, June 1, 1976, Cl. 204/67 discloses a Hall-Heroult process using an anode having a working surface of ceramic oxide, wherein a current density above a minimum value is maintained over the whole anode surface to prevent corrosion.
• the anode is principally Sn0 2 , preferably 80.0 to 99.7 wt. %.
• Additive oxides of Fe, Cu, Sb and other metals are disclosed.
• U.S. Patent No. 4,057,480-Alder, November 8, 1977, Cl. 204/290 R a divisional application from U.S. Patent No. 3,960,678, relates to a ceramic oxide anode for a Hall-Heroult cell using a current density maintained above a minimum value over the contact surface of the anode.
• a protective ring is fitted over the three phase zone at the air-electrolyte- anode junction.
• Anode base material is Sn0 2 , 80.0-99.7 wt. % is shown with additions of 0.05-2.0 wt. % of oxides of Fe, Cu, Sb and other metals as dopants.
• U.S. Patent No. 4,233,148-Ramsey et al., November 11, 1980, Cl. 204/291 discloses electrodes suitable for use in Hall-Heroult cells composed of SnO 2 with various amounts of conductive agents and sintering promoters, principally Ge0 2 , Ca 3 O 4 , Bi 2 O 3 , Sb 2 O 3 , MnO 2 , CuO, Pr 2 0 3' In 2 O 3 and MoO 3 .
• stannic oxide which has a rutile crystal structure, as the basic matrix.
• Various conductive and catalytic compounds are added to raise the level of electrical conductivity and to promote the desired reactions at the working surface of the anode.
• the primary objective of the invention is to provide an improved electrode having a substantially flat working surface and wherein a uniform current density exists at all available regions of the working surface of the electrode during operation thereof in a molten salt electrolysis cell.
• the uniform current density inhibits selective attack of the electrode and provides improved process control.
• Another objective of the invention is to provide an improved method for manufacturing aluminum by the electrolysis of alumina in molten cryolite in a Hall-Heroult cell employing a non-consumable anode comprising the electrode of the invention.
• the invention in one aspect provides a non-consumable electrode particularly, but not exclusively, suitable as an anode for a Hall-Heroult cell having a molten electrolyte bath at cell operating temperature which essentially achieves a uniform current density across its flat working surface, and may be produced from materials having a relatively small difference in electrical resistivity.
• the electrode, and especially an anode is generally produced by the process of: (a) forming, preferably by isostatic pressing, a first conductive ceramic material to produce a core having a substantially flat working surface and a non-working surface; (b) forming a physically adherent coating over the non-working surface of the core on at least the portion thereof which is to be exposed to the electrolyte bath in the cell, the coating consisting of a second conductive ceramic material having a closely matching coefficient of thermal expansion, a close matching of shrinkage during sintering, and a higher electrical resistivity compared to the first conductive ceramic material and capable of being chemical diffusion bonded thereto; and (c) sintering the coated core thus formed to produce a monolithic ceramic electrode having a substantially flat working surface and a non-working surface, the non-working surface having an impervious coating thereon, at least in the portion thereof exposed to the electrolyte bath, of higher resistivity than the core and chemical diffusion bonded thereto, whereby substantially all of the current applied to the
• the invention also provides a method for manufacturing aluminum by the electrolysis of alumina in molten cryolite in a Hall-Heroult cell employing a non-consumable anode which essentially achieves a uniform current density across its flat working surface, and may be produced from materials having a relatively small difference in electrical resistivity.
• the anode is generally produced by the process of: (a) forming, preferably by isostatic pressing, a first conductive ceramic material to produce a core having a substantially flat working surface and a non-working surface; (b) forming a physically adherent coating over the non-working surface of the core on at least the portion thereof which is to be exposed to the electrolyte bath in the cell, the coating consisting of a second conductive ceramic material having a closely matching coefficient of thermal expansion, a close matching of shrinkage during sintering, and a higher electrical resistivity compared to the first conductive ceramic material and capable of being chemical diffusion bonded thereto; and (c) sintering the coated core thus formed to produce a monolithic ceramic anode having a substantially flat working surface and a non-working surface, the non-working surface having an impervious coating thereon, at least in the portion thereof exposed to the electrolyte bath, of higher resistivity than the core and chemical diffusion bonded thereto, whereby substantially all of the current applied to the anode
• the phrase "closely matching coefficient of thermal expansion" refers to the requirement that the CTE of the coating and core materials of the electrode should differ by no more than about 1.0 x 10 /°C. to prevent destruction of the electrode during use. In a preferred system, the . CTE difference is limited to no more than about 0.5%.
• the phrase "a close matching" of shrinkage refers to the requirement that the coating and core materials must undergo an essentially equivalent dimensional or volume change during sintering.
• the preferred conductive ceramic core composition for the electrode consists of 98.0-98.5 wt. % SnO 2 , 0.1-0.5 wt. % CuO and 1.0-1.5 wt. % Sb 2 0 3 .
• a particularly advantageous core composition consists of 98.5 wt. % Sn0 2 , 0.5 wt. % CuO and 1.0 wt. % Sb203.
• the melt was replenished periodically to maintain approximately the starting composition.
• One third of the anode was immersed vertically in the melt.
• the anode retained its structural integrity, exhibiting no visual sign of thermally-induced shock or other indication of separation of the coating from the core.
• the uniform appearance of the working surface of the anode coupled with the absence of corrosion at the lower, sharp edges of the coating presented conclusive evidence that the electrolysis current was constrained substantially to the central core region bounded by the coating.
• the electrochemical corrosion of the working surface of the anode was so slight as to not be readily capable of being quantified by physical measurements.
• the recorded weight and dimensional changes of the anode were of the same order of magnitude as the accuracy of the measurements.
• the coating layer exhibited high corrosion resistance both above and below the melt level and in the region of the melt/ambient interface.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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• 1983
  • 1983-03-30 EP EP83103187A patent/EP0120982A3/de not_active Withdrawn

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