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

• Aluminum is 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
• the problems caused by the consumption of the anode carbon are related to the cost of the anode consumed in the reaction above and to the impurities introduced to the melt from the carbon source.
• the petroleum cokes used in the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing particularly troublesome workplace and environmental pollution.
• the metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
• Klein discloses an anode of at least 80% SnO 2 , with additions of Fe 2 O 3 , ZnO, Cr 2 O 3 ' sb 2 O 3' Bi 2 O 3' V 2 O 5' Ta 2 O 5 , Nb 2 O 5 or WO 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 O 4 , TiFe2O 4 , NiCr 2 O 4 , NiCo 2 O 4 , LaCrO 3 , and LaNiO 3 ;
• Alder discloses SnO 2 , Fe 2 O 3 , Cr 2 O 3 , Co 2 O 4 , NiO, and ZnO;
• Mochel discloses
• the Mochel patents are of electrodes for melting glass, while the remainder are intended for high temperature electrolysis such as Hall 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.
• Electrodes of oxycompounds of metals including Sn, Ti, Ta, Zr, V, Nb, Hf, Al, Si, Cr, Mo, W, Pb, Mn, Be, Fe, Co, Ni, Pt, Pa, Os, Ir, Rh, Te, Ru, Au, Ag, Cd, Cu, Sc, Ge, As, Sb, Bi and B, with an electroconductive agent and a surface electrocatalyst.
• Electroconductive agents include oxides of Zr, Sn, Ca, Mg, Sr, Ba, Zn, Cd, In, Tl, As, Sb, Bi, Sn, Cr, Mn, Ti; metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd & Ag; plus borides, suicides, carbides and sulfides of valve metals. Electrocatalysts include Ru, Rh, Pd,
• 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 surface of the electrode.
• An electrode useful as the anode in Hall aluminum cells is manufactured by sintering a mixture of SnO 2 with various dopants. Ratios used are commonly less than 80% SnO 2 with approximately 20% GeO 2 or Co 3 O 4 and 1-3% Sb 2 O 3 CuO, Pr 2 O 3, In 2 O 3 , or Bi 2 O 3 . DETAILED DESCRIPTION OF THE INVENTION
• Tin oxide is sintered with additives to increase the electrical conductivity and to promote sintering.
• the resulting solid is a ceramic body with a rutxle crystal structure.
• Tin oxide falls into the class of materials denoted as having 'rutile' structures. Other compounds found in this class are TiO 2 , GeO 2 , PbO 2 and MnO 2 .
• the structure is formed by a distorted cubic-close-packed array of oxygen anions with cations (Sn, Ge, etc.) filling half of the octahedral voids in the oxygen array.
• the cations occupy the octahedral positions because of the radius ratio (cation radius/anion radius) being > 0.414 but ⁇ 0.732.
• the large radius of the cations prevents them from occupying tetrahedral voids.
• SnO 2 is primarily a covalent compound and not ionic. This is accounted for by the high electronegativity of elemental tin. The greater the differences in electronegativities of two elements, the greater the likelihood of an ionic compound. However Sn and O 2 are of relatively comparable electronegativities. This results in a sharing of electrons (covalent bonding) instead of a loss or gain (ionic).
• X A electronegativity of element A
• X B electronegativity of element B. By inserting electronegativity values for tin and oxygen (1.8 and 3.5 respectively) it is found that the structure is approximately 40% ionic with the remainder covalent. Evidence has been found that structures of this nature will have fluctuations in bonding which could attribute for the electrical conductivity being high.
• SnO 2 is difficult to sinter.
• SnO 2 is classed as an n-type semi-conductor. Higher conductivity can be induced by doping with a cation having more electrons in its external shell than does Sn.
• the outer elec tronic configuration of Sn is 5s 2 5p 3 . Therefore each added atom of Sb donates an extra electron to the conduction band of SnO 2 . This reasoning also holds true for other doping agents.
• EXAMPLE 1 An anode was prepared for comparison of properties and compared to a standard carbon anode as the control in a Hall aluminum reduction cell as follows: The sample anodes were made by milling the powders, pressing them into pellets 0.8 in. (2 cm) diam. by 1 in. (2.54 cm) length at 2000 psi (140.6 kg/cm 2 ), then sintering them with the temperature rxsxng to a maxxmum of 1250oC xn 16 hrs. The power leads were attached by a threaded rod with melted copper powder. Cell Resistance at lA/cm 2 a) Carbon- -100% 0.03 ⁇ b) Sn0 2 - - 77%
• Sample (a) above is a standard carbon anode run as a control. After 4 hrs. the normal loss of carbon as a fraction of the aluminum produced was found.
• Sample (b) above SnO 2 , GeO 2 & Sb 2 O 3 , was run at lA/cm. 2 with 11.2A total current at 0.2V, giving a resistance of 0.017 ⁇ a very favorable value. During the test the resistance fluctuated between 0.0085-0.018 ⁇ . After four hours the sample showed no attack, but had several thermal shock cracks.
• EXAMPLE 2 An anode was prepared in the same manner as in Example 1 from:
• An anode of the composition SnO 2 75%
• Sb 2 O 3 2%_ 100% was made as in Example 1, and run in the Hall cell at lA/cm 2 , showing a resistance of 0.048 ⁇ . After 8 hrs, the current was increased to 2A/cm 2 , the resistance dropping to 0.041 ⁇ , for another 8 hrs. At the end of this period, the anode showed a crack due to the expansion of the metal lead, and the run was discontinued. No attack on the body of the anode was seen.
• EXAMPLE 4 The anode composed of the following compounds was prepared as in Example 1: SnO 2 60%
• a conductive phase (SnO 2 & Sb 2 O 3 ) was dispersed in a nonconductive phase (ZrO 2 ) at two levels in order to determine their utility as electrodes in Hall cells, and prepared as in Example 1. These were of the following compositions:
• Samples of the SnO 2 -Sb 2 O 3 system in an Al 2 O 3 matrix were made at the following levels, as in Example 1 with firxng carried up to 1500°C.:
• An anode of the following composition prepared as in Example 1 was sintered in a 16 hr. cycle of rising temperature with the temperature reaching 1250oC.:
• compositions incorporating PbO 2 were prepared by mixing and pressing at 10,000 psi (703 kg/cm 2 ), as in Example 1, then fired in a cycle rising to 1050oC. They were tested for weight loss with the following results:
• sample (a) indicates a solubility limit of the system PbO 2 -SnO 2 of below 50% PbO 2 at the 1050oC. firing temperature. PbO 2 melted and noticeably stained the support brick.
• EXAMPLE 11 An anode was prepared and tested as in Example 1 with the following composition:

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Citations (5)

• 1979
  • 1979-10-01 US US06/080,430 patent/US4233148A/en not_active Expired - Lifetime
• 1980
  • 1980-04-28 DE DE8080901089T patent/DE3069095D1/de not_active Expired
  • 1980-04-28 WO PCT/US1980/000475 patent/WO1981000865A1/en not_active Ceased
  • 1980-04-28 JP JP50128680A patent/JPS56501246A/ja active Pending
  • 1980-05-22 CA CA000352479A patent/CA1147292A/en not_active Expired
  • 1980-05-30 AR AR281260A patent/AR223528A1/es active
• 1981
  • 1981-04-08 EP EP80901089A patent/EP0037398B1/en not_active Expired
  • 1981-05-29 NO NO811819A patent/NO811819L/no unknown

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