Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-01-29
2003-02-18
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S372000, C205S384000, C205S385000, C204S247300, C204S243100, C204S291000, C204S293000
Reexamination Certificate
active
06521115
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to non-carbon, metal-based, anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte such as cryolite, and to methods for their fabrication, as well as to electrowinning cells containing such anodes and their use to produce aluminium.
BACKGROUND ART
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950° C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Héroult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO
2
and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes anodes for aluminium 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 cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As mentioned hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry.
OBJECTS OF THE INVENTION
A major object of the invention is to provide an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and increase the anode life.
A further object of the invention is to provide an aluminium electrowinning anode material with a surface having a high electrochemical activity for the oxidation of oxygen ions for the formation of bimolecular gaseous oxygen and a low solubility in the electrolyte.
Another object of the invention is to provide an anode for the electrowinning of aluminium which is covered with an electrochemically active layer with limited ionic conductivity for oxygen ions.
Yet another object of the invention is to provide an anode for the electrowinning of aluminium which is made of readily available material(s).
An important object of the invention is to substantially reduce the solubility of the surface layer of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable.
Yet another object of the invention is to provide operating conditions for an aluminium electrowinning cell under which the contamination of the product aluminium is limited.
SUMMARY OF THE INVENTION
The invention is based on the fact that iron-nickel alloys when oxidised form a dense and coherent oxide layer consisting essentially of iron oxide, in particular hematite. As this oxide layer is well adherent to the non-oxidised iron-nickel alloy and also electrochemically active for the oxidation of oxygen ions, it can be used as an electrochemically active surface for the oxidation of oxygen ions of an anode for the electrowinning of aluminium. Small scale tests have also shown that such an iron oxide-based layer has a slow dissolution rate in fluoride-containing molten electrolyte which can even be substantially suppressed under favourable cell operating conditions.
Therefore, the invention relates to an anode of a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte. The anode comprises an iron-nickel alloy body or layer whose surface is oxidised to form a coherent and adherent outer iron oxide-based layer, in particular a hematite-based layer, the surface of which is electrochemically active for the oxidation of oxygen ions and which reduces diffusion of oxygen from the electrochemically active surface into the iron-nickel alloy body or layer.
The surface oxidation of the iron-nickel alloy body may be such as to form an iron oxide-based layer comprising a dense iron oxide outer portion, a microporous iron oxide portion which separates the outer portion from a two-phase inner portion, one phase containing iron oxide, the other phase containing a nickel metal.
The surface of the iron-nickel alloy body or layer may be oxidised in a molten electrolyte at 800 to 1000° C. for 5 to 15 hours. Alternatively, the surface of the iron-nickel alloy body or layer may be oxidised at 750 to 1150° C. for 5 to 100 hours, in particular 20 to 75 hours at average temperature or below 25 hours at elevated temperature, in an oxidising atmosphere such as air or oxygen.
Usually, the iron-nickel alloy body or layer comprises 50 to 95 weight % iron and 5 to 50 weight % nickel, preferably 50 to 80 weight % iron and 20 to 50 weight % nickel, and even more preferably 60 to 70 weight % iron and 30 to 40 weight % nickel, i.e. with optionally up to 45 weight % of further constituents providing it is still capable of forming an iron oxide-based electrochemically active layer. Normally, the iron-nickel alloy comprises less than 30 weight %, in particular less than 20 weight % and often less than 10 weight %, of further constituents. Such constituents may be added to improve the mechanical and/or electrical properties of the anode substrate, and/or the adherence, the electrical conductivity and/or the electrochemical activity of the anode layer.
Alternatively, the iron-nickel alloy body or layer may comprise more than 50 weight % nickel, as described below.
The iron-nickel alloy body or layer may in particular comprise in addition to iron and nickel the following constituents in the given proportions: up to 15 weight % of chromium and/or additional alloying metals selected from titanium, copper, molybdenum, aluminium, hafnium, manganese, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium, in a total amount of up to 5 weight %. Furthermore, nickel present in the iron-nickel alloy may be partly substituted with cobalt. The iron-nickel alloy may contain up to 30 weight % of cobalt.
The anode may comprise a layer of iron-nickel alloy on an oxidation resistant and preferably highly electrically conductive metallic core, such as copper or a copper alloy, possibly containing minor amounts of at least one oxide reinforcing the mechanical properties of the metallic core. The reinforcing oxides may be selected from alumina, hafnia, yttria and zirconia.
This metallic core may be coated with at least one metal selected from nickel, chromium, cobalt, iron, aluminium, hafnium, manganese, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and alloys, intermetallic compounds and combinations thereof.
The metallic core may be coated with an intermediate protective layer against oxidation.
A layer of iron-nickel alloy may be applied on an oxidation resistant metallic core before or after formation of said outer iro
Crottaz Olivier
de Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent S. A.
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