Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-12-01
2002-08-20
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S387000, C205S383000, C204S243100, C204S247000, C204S247300, C204S244000, C204S245000, C204S293000, C204S290010, C204S290120, C204S290130
Reexamination Certificate
active
06436274
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to non-carbon, metal-based, slow consumable anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, and to methods for their fabrication and reconditioning, 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
An object of the present invention is to provide a non-carbon, metal-based anode for the electrowinning of aluminium so as to eliminate carbon-generated pollution and reduce the frequency of anode replacement, such an anode having an outside layer well resistant to chemical electrolyte attack whose surface is electrochemically active for the oxidation of oxygen ions contained in the electrolyte and for the formation of gaseous oxygen.
A further object of the invention is to provide a metal-based anode capable of generating during normal electrolysis at its surface an electrochemically active oxide layer which slowly and progressively dissolves into the electrolyte.
A major object of the invention is to provide an anode for the electrowinning of aluminium which has no carbon so as to eliminate carbon-generated pollution and reduce the high cell voltage.
SUMMARY OF THE INVENTION
The invention relates to a non-carbon, metal-based slow-consumable anode of a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-based electrolyte. The anode self-forms during normal electrolysis an electrochemically-active oxide-based surface layer, the rate of formation of said layer being substantially equal to its rate of dissolution at the surface layer/electrolyte interface thereby maintaining its thickness substantially constant forming a limited barrier controlling the oxidation rate.
In this context, metal-based anode means that the anode contains at least one metal as such or as an alloy, intermetallic and/or cermet.
During normal operation, the anode thus comprises a metallic (un-oxidised) anode body (or layer) on which and from which the oxide-based surface layer is formed.
The electrochemically active oxide-based surface layer may contain an oxide as such, or in a multi-compound mixed oxide and/or in a solid solution of oxides. The oxide may be in the form of a simple, double and/or multiple oxide, and/or in the form of a stoichiometric or non-stoichiometric oxide.
The oxide-based surface layer has several functions. Besides protecting in some measure the metallic anode body against chemical attack in the cell environment and its electrochemical function for the conversion of oxygen ions to molecular oxygen, the oxide-based surface layer controls the diffusion of oxygen which oxidises the anode body to further form the surface layer.
When the oxide-based surface layer is too thin, in particular at the start-up of electrolysis, the diffusion of oxygen towards the metallic body is such as to oxidise the metallic anode body at the surface layer/anode body interface with formation of the oxide-based surface layer at a faster rate than the dissolution rate of the surface layer into the electrolyte, allowing the thickness of the oxide-based surface layer to increase. The thicker the oxide-based surface layer becomes, the more difficult it becomes for oxygen to reach the metallic anode body for its oxidation and therefore the rate of formation of the oxide-based surface layer decreases with the increasing thickness of the surface layer. Once the rate of formation of the oxide-based surface layer has met its rate of dissolution into the electrolyte an equilibrium is reached at which the thickness of the surface layer remains substantially constant and during which the metallic anode body is oxidised at a rate which substantially corresponds to the rate of dissolution of the oxide-based surface layer into the electrolyte.
In contrast to carbon anodes, in particular pre-baked carbon anodes, the consumption of the non-carbon, metal-based anodes according to the invention is at a very slow rate. Therefore, these slow consumable anodes in drained cell configurations do not need to be regularly repositioned in respect of their facing cathodes since the anode-cathode gap does not substantially change.
To practically realise the invention, the anode body can comprise an iron alloy which when oxidised will form an oxide-based surface layer containing iron oxide, such as hematite or a mixed ferrite-hematite, some of which adheres to the iron alloy, providing a good electrical conductivity and electrochemical activity, and a low dissolution rate in the electrolyte.
Optionally, the anode body may also comprise one or more additives selected from beryllium, magnesium, yttrium, titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhodium, silver, aluminium, silicon, tin, hafnium, lithium, cerium and other Lanthanides.
Suitable kinds of anode materials which may be used for forming the oxide-based surface layer comprise high-strength low-alloy (HSLA) steels.
It has been observed that low-carbon HSLA steels such as Cor-Ten™, even at high temperature, form under oxidising conditions an iron oxide-based surface layer which is dense, electrically conductive, electrochemically active for oxygen evolution and, as opposed to oxide layers formed on standard steels or other iron alloys, is highly adherent and less exposed to delamination and limits diffusion of ionic, monoatomic and molecular oxygen.
HSLA steels are known for their strength and resistance to atmospheric corrosion especially at lower temperatures (below 0° C.) in different areas of technology such as civil engineering (bridges, dock walls, sea walls, piping), architecture (buildings, frames) and mechanical engineering (weld
De Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
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