Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1989-04-28
1991-12-03
Niebling, John F.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204 67, 204242, 204290R, C25B 1104, C25B 1100, C25C 306
Patent
active
050697714
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The invention relates to methods of electrowinning metals by electrolysis of a melt containing a dissolved species of the metal to be won using an anode immersed in the melt wherein the anode has a metal, alloy or cermet substrate and an operative anode surface which is a protective surface coating containing a compound of a metal less noble than the metal to be electrowon, the protective coating being preserved by maintaining in the melt a suitable concentration of a species of this less noble metal. The invention further relates to non-consumable anodes for the electrowinning of metals such as aluminum by molten salt electrolysis, and to methods of manufacturing such anodes as well as molten salt electrolysis cells incorporating them.
BACKGROUND OF INVENTION
The electrowinning method set out above has been described in U.S. Pat. No. 4,614,569 and potentially has very significant advantages. Usually the protective anode coating comprises a fluorine-containing oxycompound of cerium (referred to as "cerium oxyfluoride") alone or in combination with additives such as compounds of tantalum, niobium, yttrium, lanthanum, praesodymium and other rare earth elements, this coating being maintained by the addition of cerium and possibly other elements to the electrolyte. The electrolyte can be molten cryolite containing dissolved alumina, i.e. for the production of aluminum.
To date, however, there remain problems with the anode substrate. When this is a ceramic, the conductivity may be low. When the substrate is a metal, alloy or cermet, it may be subject to oxidation leading to a reduced life of the anode, despite the excellent protective effect of the cerium oxyfluoride coating which protects the substrate from direct attack by the corrosive electrolyte.
A promising solution to these problems has been the use of a ceramic/metal composite material of at least one ceramic phase and at least one metallic phase, comprising mixed oxides of cerium with aluminum, nickel, iron and/or copper in the form of a skeleton of interconnected ceramic oxide grains which skeleton is interwoven with a continuous metallic network of an alloy or intermetallic compound of cerium with aluminum, nickel, iron and/or copper, as described in EP-A-O 257 708. When used as electrode substrates, these materials have promise, particularly those based on cerium and aluminum because even if they corrode, this does not lead to corrosion products that contaminate the electrowon aluminum. Nevertheless corrosion of the substrate remains a problem.
Generally speaking, materials used as non-consumable anodes in molten electrolytes must have a good stability in an oxidising atmosphere, good mechanical properties, good electrical conductivity and be able to operate for prolonged periods of time under polarising conditions. At the same time, materials used on an industrial scale should be such that their welding and machining do not present unsurmountable problems to the practitioner. It is well known that ceramic materials have good chemical corrosion properties. However, their low electrical conductivity and difficulties of making mechanical and electrical contact as well as difficulties in shaping and machining these materials seriously limit their use.
In an attempt to resolve well known difficulties with conductivity and machining of ceramic materials, the use of cermets was proposed. Cermets may be obtained by pressing and sintering mixtures of ceramic powders with metal powders. Cermets with good stability, good electrical conductivity and good mechanical properties, however, are difficult to make and their production on an industrial scale is problematic. Also the chemical incompatibilities of ceramics with metals at high temperatures still present problems. Composite materials consisting of a metallic core inserted into a premachined ceramic structure, or a metallic structure coated with a ceramic layer have also been proposed. Cermets have been proposed as non-consumable anodes for molten salt electrolysis but to date probl
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Chemical Abstracts, vol. 103, 1985, p. 226, Abstract #9850e.
Doan Kim S.
Lazouni Adbelkrim
Nguyen Thinh
Freer John J.
Koestner Caroline
Moltech Invent S.A.
Niebling John F.
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