Chemistry: electrical and wave energy – Processes and products
Patent
1981-07-24
1985-11-12
Metz, Andrew H.
Chemistry: electrical and wave energy
Processes and products
204 64R, 204243R, 204292, 264 65, C25C 306, C25B 1104
Patent
active
045526303
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to the electrolysis of molten salts particularly in an oxygen-evolving melt, such as the production of aluminium from a cryolite-based fused bath containing alumina, and to anodes for this purpose comprising a body of ceramic oxide material which dips into the molten salt bath, as well as to aluminium production cells incorporating such anodes.
BACKGROUND ART
The conventional Hall-Heroult process for aluminium production uses carbon anodes which are consumed by oxidation. The replacement of these consumable carbon anodes by substantially non-consumable anodes of ceramic oxide materials was suggested many years ago by Belyaev who investigated various sintered oxide materials including ferrites and demonstrated the feasibility of using these materials (Chem. Abstract 31 (1937) 8384 and 32 (1938) 6553). However, Belyaev's results with sintered ferrites, such as SnO.sub.2.Fe.sub.2 O.sub.3, NiO.Fe.sub.2 O.sub.3 and ZnO.Fe.sub.2 O.sub.3, show that the cathodic aluminium is contaminated with 4000-5000 ppm of tin, nickel or zinc and 12000-16000 ppm of iron, which rules out these materials for commercial use.
Considerable efforts have since been made to design expedients which offset the defects of the anode materials (see for example U.S. Pat. Nos. 3,974,046 and 4,057,480) and to develop new anode materials which stand up better to the operating conditions. Some of the main requirements of the ideal non-consumable anode material for aluminium production are: thermal stability and good electrical conductivity at the operating temperature (about 940.degree. C. to 1000.degree. C.); resistance to oxidation; little solubility in the melt; and non-contamination of the aluminium product with undesired impurities.
U.S. Pat. No. 4,039,401 discloses various stoichiometric sintered spinel oxides (excluding ferrites of the formula Me.sup.2+ Fe.sub.2.sup.3+ O.sub.4) but recognized that the spinels disclosed had poor conductivity, necessitating mixture thereof with various conductive perovskites or with other conductive agents in an amount of up to 50% of the material.
West German published patent application (Offenlegungsschrift) DE-OS No. 23 20 883 describes improvements over the known magnetite electrodes for aqueous electrolysis by providing a sintered material of the formula ##STR1## where M represents Mn, Ni, Co, Mg, Cu, Zn and/or Cd and x is from 0.05 to 0.4. The data given show that when x is above 0.4 the conductivity of these materials drops dramatically and their use was therefore disconsidered.
DISCLOSURE OF THE INVENTION
The invention, as set out in the claims, provides an anode material resistant to the conditions encountered in molten salt electrolysis and in particular in aluminium production, having a body consisting essentially of a ceramic oxide spinel material of the formula ##EQU1## where: M.sub.I is one or more divalent metals from the group Ni, Co, Mg, Mn, Cu and Zn; and Fe, but excluding the case where M.sub.I and M.sub.II are both the same single metal (preferably, M.sub.II is Fe or is predominantly Fe with up to 0.2 atoms of Ni, Co or Mn); Sn.sup.4+, Fe.sup.4+, Hf.sup.4+ Mn.sup.4+, Fe.sup.3+, Ni.sup.3+, Co.sup.3+, Mn.sup.3+, Al.sup.3+ and Cr.sup.3+, Fe.sup.2+, Ni.sup.2+, Co.sup.2+, Mg.sup.2+, Mn.sup.2+, Cu.sup.2+ and Zn.sup.2+, and Li.sup.1+, where n is 1, 2, 3 or 4 depending upon the valence state of M.sub.III ; and ##EQU2## in the spinel lattice, providing that y.noteq.0 when (a) x=1, (b) there is only one metal M.sub.I, and (c) there is only one metal M.sub.II or there are two metals M.sub.II in an equal whole atom ratio.
Ceramic oxide spinels of this formula, in particular the ferrite spinels, have been found to provide an excellent compromise of properties making them useful as substantially non-consumable anodes in aluminium production from a cryolite-alumina melt. There is no substantial dissolution in the melt so that the metals detected in the aluminium produced remain at sufficiently low levels to be tolerated in commercial production.
In the prefe
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Derivaz Jean-Pierre
Duruz Jean-Jacques R.
Sane Ajit Y.
Wheeler Douglas J.
Chapman Terryence
Collins Arthur S.
Eltech Systems Corporation
Metz Andrew H.
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