Coating processes – Electrical product produced – Electron emissive or suppressive
Reexamination Certificate
2002-03-14
2003-12-02
Bell, Bruce F. (Department: 1746)
Coating processes
Electrical product produced
Electron emissive or suppressive
C427S078000, C427S126300, C427S126600, C427S249100, C427S249170, C427S249180, C427S249190, C427S255190, C427S327000, C427S328000, C427S331000, C427S372200, C427S379000, C427S383100, C427S383300, C427S383700, C427S419200, C427S419300, C427S419700, C427S421100, C427S427000, C427S428010, C427S429000, C427S430100, C427S431000, C427S436000
Reexamination Certificate
active
06656520
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to non-carbon, metal-based anodes provided with an electrochemical active surface coating 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 Hèroult, 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.
In the second largest electrochemical industry following aluminium, namely the chlorine and caustic industry, the invention of dimensionally stable anodes (DSA®) which were developed around 1970 permitted a revolutionary progress in chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine caustic purity.
The substitution of graphite anodes with DSA® increased drastically the life of the anodes and reduced substantially the cost of chlorine production and operating the cells.
In the case of aluminium production, an additional problem is the pollution due to the materials used in the process and to the primitive cell design and operation which have remained the same over the years.
Progress has been made in the operation of modern plants which utilise cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes and cathodes is carefully controlled.
However, 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.
Thus, the dimensionally-stable anode technology used in chlorine production has not yet been successfully adapted to the aluminium electrowinning cells.
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 et al.) 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 barrier layer. However, full protection of the alloy substrate was difficult to achieve.
A significant improvement described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar/Liu/Duruz) involved micropyretically producing a body from nickel, aluminium, iron and copper and oxidising the surface 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. As described hereabove, 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 an applied electrochemically active anode surface coating of a metal-based non-carbon anode for aluminium electrowinning cells which coating is in contact with the electrolyte.
Another object of the invention is to provide a surface coating for a metal-based anode for aluminium electrowinning cells which in addition to a long life has a high electrochemical activity and can easily be applied onto an anode substrate.
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 operating costs.
SUMMARY OF THE INVENTION
The invention relates to a non-carbon, metal-based, high temperature resistant, electrically conductive and electrochemically active anode of a cell for the production of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte. The anode has a metal-based substrate to which an adherent surface coating is applied prior to its immersion into the electrolyte and start up of the electrolysis by connection to the positive current supply. The coating is obtainable from one or more layers applied from: a liquid solution, a dispersion in a liquid or a paste, a suspension in a liquid or a paste, and a pasty or non-pasty slurry, and combinations thereof with or without one or more further applied layers, with or without heat treatment between two consecutively applied layers when at least two layers are applied. The coating is after final heat treatment electrically conductive and during operation in the cell electrochemically active for the oxidation of oxygen ions present at the surface of the anode to form monoatomic nascent oxygen which as such or as biatomic molecular gaseous oxygen oxidises or further oxidises the surface of the coating, or part or most of the coating or the surface of the substrate, to form a barrier to the ionic and gaseous oxygen and even a barrier to the nascent monoatomic oxygen, the coating having a slow dissolution rate in the fluoride-containing electrolyte.
In the context of this invention:
a metal-based anode means that the anode contains at least one metal in the anode substrate as such or as alloys, intermetallics and/or cermets.
a liquid solution means a liquid containing ionic species which are smaller than 5 nanometers and/or polymeric species of 5 to 10 nanometers and no larger particles;
a dispersion means a liquid containing particles in colloidal form, wherein the size of the largest particles is comprised between 10 and 100 nanometers;
a suspension means a liquid containing particles in which the largest particles are comprised between 100 and 1000 nanometers; and
a slurry means a liquid co
Bell Bruce F.
Deshmukh Jaydeep R.
Moltech Invent-SA
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