Chemistry: electrical and wave energy – Processes and products
Patent
1984-08-20
1986-09-30
Williams, Howard S.
Chemistry: electrical and wave energy
Processes and products
204 64R, 204 39, 204 64T, 204 65, 204 68, 204 69, 204 70, 204290R, 204291, C25C 300, C25C 306, C25C 312, C25C 318
Patent
active
046145699
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to the electrowinning of metals from molten salt electrolytes as well as to molten salt electrolysis anodes and methods of manufacturing these anodes.
BACKGROUND ART
Electrowinning of metals from molten salt electrolytes involves numerous difficulties. A typical process is the production of aluminum by the Hall-Heroult process which involves the electrolysis of alumina in a molten cryolite-based bath usng carbon anodes. These carbon anodes are consumed by the anodic oxidation process with the formation of CO2/CO and their life-time is very short, typically about two to three weeks for the pre-baked type or anode. They may also add impurities to the bath. There have been numerous suggestions for non-consumable anode compositions based on various ceramic oxides and oxycompounds usually with added electro-conductive agents and electrocatalysts. Many difficulties have been encountered in practice with such anodes, the major difficulty being that the anodes are invariably consumed more or less slowly and undesirably contaminate the molten bath and the aluminum or other metal produced.
For example, U.S. Pat. Nos. 4,146,438 and 4,187,155 describe molten-salt electrolysis anodes consisting of a ceramic oxycompound matrix with an oxide or metallic conductive agent and a surface coating of an electrocatalyst e.g. oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver. One of the problems with these electrodes is that the catalytic coating wears away.
Another approach, described in U.S. Pat. Nos. 3,562,135, 3,578,580 and 3,692,645, was to separate the anode and cathode by an oxygen-ion conducting diaphragm, typically made of stabilized zirconium oxide or other refractory oxides with a cubic (fluorite) lattice, including thorium oxide/uranium oxide and cerium oxide suitably stabilized with calcium oxide or magnesium oxide. In one arrangement, the ion-conductive diaphragm was applied to the operative anode surface which was either liquid or was porous, perforated or reticulated and provided with means for releasing the oxygen generated at the anode under the diaphragm. This involved considerable problems in anode design and in manufacture of the composite anode/diaphragm. Another arrangement was to separate the diaphragm from the anode surface; here, it would appear that tests failed to identify any feasible diaphragm material.
DISCLOSURE OF INVENTION
According to one of the main aspects of the invention, as set out in the claims, a method of electrowinning metals and typically the electrowinning of aluminum from a cryolite-based melt containing alumina, is characterized in that the anode dipping in the molten electrolyte has as its operative surface a protective coating which is maintained by the presence of constituents of the coating dissolved in the melt, usually with substantially no cathodic deposition of said constituents.
Generally, cerium is dissolved in the a fluoride melt and the protective coating is predominantly a fluorine-containing oxycompound of cerium. When dissolved in a suitable molten electrolyte, cerium remains dissolved in a lower oxidation state but, in the vicinity of an oxygen-evolving anode, oxidizes in a potential range below or at the potential of oxygen evolution and precipitates as a fluorine-containing oxycompound which remains stable on the anode surface. It has been found that the thickness of the electrodeposited fluorine-containing cerium oxycompound coating can be controlled as a function of the amount of the cerium introduced in the electrolyte, so as to provide an impervious and protective coating which is electronically conductive and functions as the operative anode surface, i.e. usually an oxygen evolving surface. Furthermore, the coating can be self-healing or self-regenerating and can be maintained permanently by having a suitable concentration of cerium in the electrolyte.
The term fluorine-containing oxycompound is intended to include oxyfluoride compounds and mixtures and solid solutions of oxides and fluori
REFERENCES:
patent: 3562135 (1971-02-01), Marincek
patent: 3578580 (1971-05-01), Hatting et al.
patent: 3692645 (1972-09-01), Marincek
patent: 4146438 (1979-03-01), de Nora et al.
patent: 4173518 (1979-11-01), Yamada et al.
patent: 4187155 (1980-02-01), de Nora et al.
patent: 4192724 (1980-03-01), Minami et al.
Adorian Iudita L.
Debely Pierre-Etienne
Derivaz Jean-Pierre
Duruz Jean J.
Eltech Systems Corporation
Freer John J.
Williams Howard S.
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