Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-07-15
2002-07-02
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S389000, C205S392000, C204S243100, C204S247300, C204S290010
Reexamination Certificate
active
06413406
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to non-carbon metal-based anodes having an electrocatalytically active surface for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, 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 Heroult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still not less than 950° C. in order to have a sufficiently high solubility and rate of dissolution of alumina and high electrical conductivity of the bath.
The carbon anodes have a very short life because 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.
The frequent substitution of the anodes in the cells is still a clumsy and unpleasant operation. This cannot be avoided or greatly improved due to the size and weight of the anode and the high temperature of operation.
Several improvements were made in order to increase the lifetime of the anodes of aluminium electrowinning cells, usually by improving their resistance to chemical attacks by the cell environment and air to those parts of the anodes which remain outside the bath. However, most attempts to increase the chemical resistance of anodes were coupled with a degradation of their electrical conductivity.
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 compounds 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 barrier layer. However, full protection of the alloy substrate was difficult to achieve.
U.S. Pat. No. 4,039,401 (Yamada/Hashimoto/Horinouchi) discloses anodes for the production of aluminium which are made of a metallic substrate coated with various electronic conductive oxide ceramics. However, protection of the substrate against corrosion is not addressed.
A significant improvement described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar/Liu/Duruz) involved a micropyretically produced body of nickel, aluminium, iron and copper whose surface is oxidised before use or in-situ. By said micropyretic methods materials have been obtained whose surfaces, when oxidised, are active for the anodic reaction and whose metallic interior has low electrical resistivity to carry a current from high electrical resistant surface to the busbars. However it would be useful, if it were possible, to simplify the manufacturing process of these materials and increase their life to make their use economic.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry because of their poor performance.
OBJECTS OF THE INVENTION
An object of the invention is to reduce substantially the consumption of the electrochemically active anode surface of a non-carbon metal-based anode for aluminium electrowinning cells which is attacked by the nascent oxygen by enhancing the reaction of nascent oxygen to gaseous biatomic molecular gaseous oxygen.
Another object of the invention is to provide a coating for a non-carbon metal-based anode for aluminium electrowinning cells which has a high electrochemical activity and also a long life and which can easily be applied onto an anode substrate.
A further object of the invention is to provide a coating for a non-carbon metal-based anode for aluminium electrowinning cells which lowers the cell voltage compared to the voltage of cells having metal-based anodes which are not provided with this coating.
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 high cell operating costs.
SUMMARY OF THE INVENTION
The invention relates to a non-carbon, metal-based high temperature resistant anode of a cell for the production of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte. The anode has a highly conductive metal-based substrate coated with one or more electrically conductive adherent intermediate protective layers and an outer layer which is electrically conductive and electrochemically active. The outer electrochemically active layer contains one or more electrocatalysts fostering the oxidation of oxygen ions as well as fostering the formation of biatomic molecular gaseous oxygen from the monoatomic nascent oxygen obtained by the oxidation of the oxygen ions present at the surface of the anode in order to inhibit ionic and/or monoatomic oxygen penetration. The intermediate layer(s) constitute(s) a substantially impermeable barrier to ionic, monoatomic and/or biatomic gaseous oxygen to prevent attack of the metal-based substrate.
In this context, metal-based substrate means that the anode substrate contains at least one metal as such or as alloys, intermetallics and/or cermets.
The electrocatalyst(s) may be selected from iridium, palladium, platinum, rhodium, ruthenium, silicon, tin or zinc metals, Mischmetal and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof.
In addition to the electrocatalyst(s), the electrochemically active layer usually comprises mainly electrochemically active constituents selected from the group consisting of oxides, such as iron oxides, oxyfluorides, for instance cerium oxyfluoride, phosphides, carbides and combinations thereof.
An oxide may be present in the electrochemically active layer 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 electrochemically active layer may in particular comprise spinels and/or perovskites, such as ferrite which may be selected from cobalt, manganese, molybdenum, nickel, magnesium and zinc ferrite, and mixtures thereof. Nickel ferrite may be partially substituted with Fe
2+
. Additionally, ferrites may doped with at least one oxide selected from the group consisting of chromium, titanium, tantalum, tin and zirconium oxide.
Optionally the electrochemically active layer may comprise a chromite, such as iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, magnesium, nickel and zinc chromite.
The electrochemically active layer may be applied in the form of powder or slurry onto metal-based substrate, dried as necessary and heat-treated.
Typically, the electrochemically active layer may
Bell Bruce F.
Deshmnkh Jayadeep E.
Moltech Invent S.A.
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