Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1999-10-22
2002-03-26
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C204S243100, C204S247300, C204S290100, C204S290010, C204S290120, C204S290130, C029S592100
Reexamination Certificate
active
06361680
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to cell components, particularly anodes, for use in the electrowinning of aluminum by the electrolysis of alumina in a molten fluoride electrolyte, in particular cryolite.
The invention is more particularly concerned with the production of cell components, particularly anodes, of aluminum production cells made of composite materials by the micropyretic reaction of a mixture of reactive powders, which reaction mixture when ignited undergoes a micropyretic reaction to produce a reaction product.
BACKGROUND ART
U.S. Pat. No. 4,614,569 (Duruz et al) describes anodes for aluminum electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the. cell or pre-applied, this coating being maintained by the addition of certain to the cryolite electrolyte.
U.S. Pat. 4,948,676 (Darracq et al) describes a ceramic/metal composite material for use as an anode for aluminum electrowinning particularly when coated with a protective cerium oxyfluoride based coating, comprising mixed oxides of cerium and one or more of aluminum, nickel, iron and copper in the form of a skeleton of interconnected ceramic oxide grains interwoven with a metallic network of an alloy or an intermetallic compound of cerium and one or more of aluminum, nickel, iron and copper.
U.S. Pat. No. 4,909,842 (Dunmead et al) discloses the production of dense, finely grained composite materials with ceramic and metallic phases by self-propagating high temperature synthesis (SHS) with the application of mechanical pressure during or immediately after the SHS reaction.
U.S. Pat. No. 5,217,583 (Sekhar et al) describes the production of ceramic or ceramic-metal electrodes for electrochemical processes, in particular for aluminum electrowinning, by micropyretic reaction of particulate or fibrous reactants with particulate or fibrous fillers and binders. The reactants included aluminum usually with titanium and boron; the binders included copper and aluminum; the fillers included various oxides, nitrides, borides, carbides and silicides. The described composites included copper/aluminum oxide-titanium diboride etc.
U.S. Pat. No. 5,316,718 (Sekhar et al) describes an improvement of U.S. Pat. No. 5,217,583 with specific fillers. The described reactants included an aluminum nickel mixture, and the binder could be a metal mixture including aluminum, nickel and up to 5 weight % copper.
U.S. Pat. Nos. 4,374,050 (Ray) and 4,374,761 (Ray) disclose anodes for aluminum electrowinning composed of a family of metal compounds including oxides. It is stated that the anodes could be formed by oxidizing a metal alloy substrate of suitable composition. However, it has been found that oxidized alloys do not produce a stable, protective oxide film but corrode during electrolysis with spalling off of the oxide. U.S. Pat. No. 4,620,905 (Tarcy et al) also discloses oxidized alloy anodes.
U.S. Pat. Nos. 4,454,015 (Ray/Rapp) and 4,678,760 (Ray) disclose aluminum production anodes made of a composite material which is an interwoven network of a ceramic and a metal formed by displacement reaction. These ceramic metal composites have not been successful.
U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen et al) disclose aluminum production anodes with an oxidized copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
U.S. Pat. No. 5,284,562 (Beck et al) discloses alloy anodes made by sintering powders of copper nickel and iron. However, these sintered alloy anodes cannot resist electrochemical attack.
Published international application WO94/24321 (Sekhar et al), discloses aluminum production anodes comprising ordered aluminide compounds of nickel, iron and titanium produced by micropyretic reaction with a cerium based colloidal carrier.
A significant improvement was described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar et al). Prior to this, all attempts to produce an electrode suitable as anode for aluminum production and based on metals such as nickel, aluminum, iron and copper or other metals had proven to be unsuccessful in particular due to the problem of poor adherence due partly to thermal mismatch between the metals and the oxide formed prior to or during electrolysis.
This teaching provided an anode for aluminum production where the problem of poor adherence due partly to thermal mismatch between a metal substrate and an oxide coating formed from the metal components of the substrate was resolved, the metal electrode being covered with an oxide layer which remained stable during electrolysis and protected the substrate from corrosion by the electrolyte.
Such an anode for the production aluminum by the electrolysis of alumina in a molten fluoride electrolyte comprises a porous micropyretic reaction product derived from particulate nickel, aluminum and iron, or particulate nickel, aluminum, iron and copper, optionally with small quantities of doping elements such as chromium, manganese, titanium, molybdenum, cobalt, zirconium, niobium, cerium, oxygen, boron and nitrogen included in a quantity of up to 5 wt % in total.
The porous micropyretic reaction product contained metallic and/or intermetallic phases, and a composite oxide surface formed in-situ from the metallic and intermetallic phases contained in the porous micropyretic reaction product, by anodically polarizing the micropyretic reaction product in a molten fluoride electrolyte containing dissolved alumina. The in-situ formed composite oxide surface comprised an iron-rich relatively dense outer portion, and an aluninate-rich relatively porous inner portion.
Comparative anodes of similar compositions (i.e. similar to those of the anodes of U.S. Pat. No. 5,510,008 and WO 96/12833, Sekhar et al), but prepared from alloys not having a porous structure obtained by micropyretic reaction, show poor performance. This is believed to be a result of the mismatch in thermal expansion between the oxide layer and the metallic substrate with the alloy anodes. The differences in thermal expansion coefficients allow cracks to form in the oxide layer, or the complete removal of the oxide layer from the alloy, which induced corrosion of the anode by penetration of the bath materials, leading to short useful lifetimes.
In contrast, the porous anodes according to U.S. Pat. No. 5,510,008 and WO 96/12833 (Sekhar et al) accommodate the thermal expansion, leaving the dense protective oxide layer intact. Bath materials such as cryolite which may penetrate the porous metal during formation of the oxide layer become sealed off from the electrolyte, and from the active outer surface of the anode where electrolysis takes place, and did not lead to corrosion but remain inert inside the electrochemically inactive inner part of the anode.
These in-situ oxidized anodes represent a considerable improvement over earlier proposals. However, the composite oxide layer of these in-situ oxidized anodes of U.S. Pat. No. 5,510,008 and WO96/12833 (Sekhar et al) may grow to a thickness that reduces process efficiency, which limits the useful lifetime of the anodes. Attempts to remove this limitation of the anodes by including the additives disclosed in U.S. Pat. No. 5,510,008 and WO96/12833 (Sekhar et al) were not successful, in that such additives were found either not to have an effect of limiting the growth rate of the thickness of the oxide layer, or a thickness limiting effect was achieved but to an inadequate amount and/or this effect would be offset by problems of contamination of the product aluminum.
SUMMARY OF THE INVENTION
The invention is based on the discovery that the performance of the anodes of U.S. Pat. No. 5,510,008 and WO96/12833 is unexpectably improved by certain additive elements.
The invention relates to a cell component, preferably an anode, for the electrowinning of aluminum by the electrolysis of alumina dissolved in a molten fluoride electrolyte, comprising a porous micropyretic reaction product of pa
de Nora Vittorio
Sekhar Jainagesh Akkavaju
Deshmukh Jayadeep R.
Moltech Invent S-A.
Valentine Donald R.
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