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

Definitions

• This invention relates to metal-based anodes for aluminium production cells, aluminium production cells operating with such anodes as well as operation of such cells to produce aluminium.
• the anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting C0 2 and small amounts of CO and fluorine-containing dangerous gases .
• the actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than 1/3 higher than the theoretical amount of 333 Kg/Ton.
• metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
• US Patent 6,077,415 discloses a metal-based anode comprising a metal-based core covered with an oxygen barrier layer and an electrochemically active outer layer, the barrier layer and the outer layer being separated by an intermediate layer to prevent dissolution of the oxygen barrier layer.
• EP Patent application 0 306 100 and US Patents 5,069,771, 4,960,494 and 4,956,068 disclose aluminium production anodes having an alloy substrate protected with an oxygen barrier layer that is covered with a copper- nickel layer for anchoring a cerium oxyfluoride operative surface coating.
• a major object of the invention is to provide an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and increase the anode life.
• An important object of the invention is to reduce the solubility of the surface of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable without excessively contaminating the product aluminium.
• Another object of the invention is to provide a cell for the electrowinning of aluminium utilising metal-based anodes, and a method to produce aluminium in such a cell and preferably maintain the metal-based anodes dimensionally stable.
• a main object of the invention is to provide a metal- based anode for the production of aluminium which is resistant to fluoride attack.
• a subsidiary object of the invention is to prevent diffusion of chromium in a metal-based anode that comprises chromium as an oxygen barrier layer.
• metal oxides present at the surface of metal-based anodes like oxides of iron, nickel, copper, chromium etc., combine during use with fluorides of the electrolyte to produce soluble oxyfluorides .
• the invention is based on the observation that silver can be used as a barrier layer to fluoride attack. At high temperature, i.e. above 450°C, silver does not form an oxide and remains as a metal . It follows from the above theory that during use fluorides cannot form oxyfluorides by exposure to the silver layer which is devoid of oxide, and the fluorides cannot corrode the silver layer. Therefore, the invention relates to a metal-based anode substrate of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte.
• the substrate comprises a nickel-alloy based core, a layer of silver on the core and a layer comprising nickel and iron covering the silver layer and serving as an anchorage layer for anchoring an electrochemically active surface coating on top of the anode substrate.
• the silver layer inhibits diffusion of fluoride species into the core and prevents interdiffusion of constituents of the core and constituents of the anchorage layer.
• the silver layer may have an average thickness in the range of 5 to 100 micron.
• the anode substrate comprises a further layer of silver and a layer of chromium, the chromium layer being located between the core and the anchorage layer and separated therefrom by the silver layers.
• the chromium layer may have an average thickness in the range of 10 to 100 micron.
• the oxygen barrier layer is separated from the core by a first layer of silver and from the anchorage layer by a second layer of silver.
• the silver layers prevent interdiffusion of chromium from the barrier layer with constituents of the core and with constituents of the anchorage layer.
• the silver layers confine the chromium within the barrier layer and do not mix with the chromium, thereby securing a long-lasting integrity of the chromium barrier layer.
• this embodiment of the anode according to the present invention is efficiently protected against oxidation for a longer period of time than prior art anodes .
• the chromium barrier layer contacts miscible metals such as nickel and/or copper.
• miscible metals such as nickel and/or copper.
• the anchorage layer and/or the core may comprise one or more additives selected from yttrium, tantalum and niobium in a total amount of 0.1 to 5 weight% .
• the anchorage layer and/or the core comprise yttrium, for instance in an amount of less than 1 weight% .
• the anchorage layer may have an average thickness in the range of 30 to 300 micron.
• the anchorage layer can be made of a bottom layer of nickel and iron and a top layer of copper.
• the nickel-iron bottom layer may have an average thickness in the range of 30 to 300 micron and the copper top layer an average thickness in the range of 5 to 50 micron.
• the copper top layer is usually partly interdiffused with the nickel-iron layer adjacent to it.
• the invention also relates to a metal-based anode that comprises an anode substrate as described above which is coated with an electrochemically active surface coating made of one or more cerium compounds, in particular cerium oxyfluoride.
• the electrochemically active surface coating may comprise at least one additive selected from yttrium, tantalum and niobium.
• the electrochemically active surface coating can be an electrolytically deposited coating or applied before use, for instance from a cerium-based slurry.
• Another aspect of the invention is a cell , for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte.
• the cell according to the invention comprises at least one of the above described metal-based anodes.
• the electrolyte of the cell preferably comprises cerium species to maintain the electrochemically active surface coating dimensionally stable.
• the cell of the invention may be operated with or without a crust and/or a sideledge of frozen electrolyte.
• the cell has an electrolyte at reduced temperature, i.e. below 960°C, for instance in the range from 860° to 930°C.
• a further aspect of the invention is a method of producing aluminium in the above described cell.
• the method of the invention comprises dissolving alumina in the electrolyte and passing an electrolysis current between the or each anode and a facing cathode whereby oxygen is anodically evolved on the electrochemically active surface coating and aluminium is cathodically reduced.
• An anode substrate made of a nickel-iron core covered with a silver barrier layer and a nickel-iron anchorage layer according to the invention was prepared as follows :
• a he i-spherical nickel-containing anode core having a diameter of 20 mm and a length of 30 mm was machined from a nickel-iron alloy rod made of 80 weight% nickel and 20 weight% iron.
• the surface of the anode core was sandblasted, degreased and rinsed carefully with deionised water .
• the anode core was then immersed in an AgCN/KCN bath at room temperature and polarised in order to electrolytically deposit silver thereon from a silver counter electrode .
• a cathodic current with a current density of about 50 mA/cm 2 was passed at the surface of anode core.
• the AgCN/KCN bath was moderately agitated. After 10 minutes electrodeposition was interrupted.
• the anode core was removed from the AgCN/KCN bath and carefully rinsed with deionised water.
• An electrodeposited silver layer having an average thickness of about 25 to 30 micron had been formed on the anode core .
• the silver plated anode core was then immersed and polarised in a FeS0 4 -NiS0 4 -NiCl 2 /Boric - Salicylic acid bath at a temperature of 55°C.
• a nickel-iron alloy was deposited onto the silver plated anode core from an alloy counter electrode made of a 50 weight% nickel and 50 weight% iron.
• An electrolysis current was passed between the plated anode core and the counter-electrode at a current density of about 60 mA/cm 2 at the surface of the plated anode core. As before, the bath was moderately agitated during the electrolytic deposition.
• the anode substrate was oxidised in air at a temperature of about 1100°C for 1 hour.
• An iron oxide based black adherent layer consisting of 95-97 weight% iron oxide and 3-5% nickel oxide was formed on the anode substrate.
• the oxidised anode substrate was then immersed and anodically polarised in ' a laboratory aluminium electrowinning cell operating with a cryolite-based electrolyte consisting of about 21 weight% AlF 3 , 4 weight%
• the cell used an aluminium pool as a cathode.
• the cell was periodically supplied with a powder feed of Al 2 0 3 containing 1 weight% CeF 3 .
• the feeding rate corresponded to 30% of the cathodic current efficiency.
• the anode was removed from the molten bath and cooled down to room temperature.
• the anode was cut perpendicular to a cerium oxyfluoride coated surface and the section was examined under a SEM microscope .
• the cerium-based coating had a thickness of about 500 to 700 micron. Underneath the cerium-based coating, the nickel-iron anchorage layer had been completely oxidised and transformed into a black and adherent matrix of iron-nickel mixed oxyfluorides . The electroplated silver layer had remained un-oxidised. Underneath the silver layer, the anode core showed no sign of corrosion or exposure to fluorides. However, a surface layer containing a uniform distribution of iron oxide inclusions and having a thickness of about 200 micron had been formed on the core.
• An anode substrate made of a nickel-iron core covered with a silver barrier layer, a chromium barrier layer, a further silver layer and a nickel-iron anchorage layer according to the invention was prepared as follows: An anode core was plated with a layer of silver as in Example 1.
• An oxygen barrier layer of chromium was then formed on the plated anode core by immersing and polarising it in a Cr0 3 /H 2 S0 4 bath at a temperature of 35°C.
• a dimensionally stable counter electrode was used.
• An electrolysis current was passed between the plated anode core and the counter- electrode at a current density of about 300 mA/cm 2 on the plated anode core in order to deposit chromium from the bath onto the anode core.
• the bath was moderately agitated during the electrolytic deposition.
• the plated anode core was removed from the bath and rinsed carefully with deionised - water.
• a mat chromium electrodeposited layer of about 15 micron had been deposited onto the silver layer.
• the chromium layer was activated in a NiCl 2 /HCl bath by anodic polarisation at a current density of about 30 mA/cm 2 for 3 minutes followed by a cathodic polarisation at the same current density for 6 minutes.
• a layer of nickel having a thickness of about 1 micron was deposited onto the chromium coating.
• the plated anode core was removed from the activation bath, rinsed carefully with deionised water and immediately plated with a further layer of silver following the above-described silver plating procedure and then with a nickel-iron anchorage layer air oxidised as described in Example 1.
• the anode substrate was coated with a cerium oxyfluoride electrochemically active layer to form an anode according to the invention and then used for 24 hours in a cell as described in Example 1.
• the anode was cut " perpendicular to a cerium oxyfluoride coated surface and the section was examined under a SEM microscope .
• the cerium-based coating had a thickness of about 500 to J00 micron. Underneath the cerium-based coating, the nickel-iron anchorage layer had been completely oxidised and transformed into a black and adherent matrix of iron-nickel mixed oxyfluorides . The electroplated silver layers had remained un-oxidised. The chromium oxygen barrier layer was oxidised to a depth of about 2 to 5 micron.
• the anode core Underneath the silver layers and the chromium layer, the anode core showed no sign of corrosion or exposure to fluorides . No oxide was found in the anode core demonstrating the efficiency of the chromium oxygen barrier layer.

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• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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• 2002
  • 2002-03-04 AU AU2002236142A patent/AU2002236142B2/en not_active Ceased
  • 2002-03-04 ES ES02702624T patent/ES2239709T3/es not_active Expired - Lifetime
  • 2002-03-04 AT AT02702624T patent/ATE296367T1/de not_active IP Right Cessation
  • 2002-03-04 EP EP02702624A patent/EP1381716B1/de not_active Expired - Lifetime
  • 2002-03-04 NZ NZ527307A patent/NZ527307A/en unknown
  • 2002-03-04 DE DE60204307T patent/DE60204307T2/de not_active Expired - Fee Related
  • 2002-03-04 WO PCT/IB2002/000667 patent/WO2002070786A1/en active IP Right Grant
  • 2002-03-04 CA CA002437671A patent/CA2437671A1/en not_active Abandoned

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