Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-07-15
2002-03-26
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S384000, C204S290010, C204S243100, C204S247300, C427S126300, C427S372200, C427S376200, C427S383700, C427S419200, C427S419300, C106S286100
Reexamination Certificate
active
06361681
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a slurry for coating anodes for use in cells for the electrowinning of metals from their oxides dissolved in molten salts, and to methods for their fabrication and reconditioning, as well as aluminium electrowinning cells containing coated anodes and their use to produce aluminium.
BACKGROUND ART
The production of metals by the electrolysis of their oxides is usually carried out in very chemically aggressive environments. Therefore, the materials used for the manufacture of components of production cells must be resistant to attack by the environment of such cell. Anodes of cells for the production of metals by the electrolysis of their oxides dissolved in molten salts need to be resistant to attack by the electrolyte and by the oxygen which is anodically produced during electrolysis.
Unfortunately, for the dissolution of the raw material a highly aggressive electrolyte, such as a fluoride-based electrolyte is required.
The surface of the anode must be electrochemically active, substantially insoluble in the electrolyte and resistant to attacks by the nascent monoatomic oxygen and by the subsequently formed molecular oxygen gas which are anodically produced. Since monoatomic oxygen is far more aggressive than biatomic molecular gaseous oxygen, the constituents of the active surface of the anode should contain electro-catalytic materials for the reaction which forms molecular oxygen from the monoatomic oxygen to reduce monatomic oxygen attack.
The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.
In the field of aluminium production, it has been described in U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan), and U.S. Pat. No. 5,510,008 (Sekhar/Liu/Duruz) that a metal core could be protected by barrier layers and/or by oxidised metals but these results have not as yet been commercially and industrially applied.
OBJECT OF THE INVENTION
An object of the invention is to provide a method for coating an anode for metal electrowinning cells, in particular aluminium electrowinning cells, which substantially reduces the consumption of the active anode surface that is attacked by nascent monoatomic oxygen by enhancing the reaction of nascent oxygen to gaseous molecular gaseous oxygen.
Another object of the invention is to provide a slurry for coating anodes for metal electrowinning cells, in particular aluminium electrowinning cells, which provides a coating with high electrolytic activity, a long life and which can be re-coated onto the anode as soon as such activity decreases or when the coating is worn out.
A major object of the invention is to provide an anode for metal electrowinning cells, in particular aluminium electrowinning cells, which has no carbon so as to eliminate carbon-generated pollution and reduce the cell voltage and the high cost of cell operation.
SUMMARY OF THE INVENTION
The present invention concerns a method of applying a slurry onto a conductive, heat resistant anode substrate to form an oxide coating on those parts of the substrate which are exposed to oxidising or corrosive cell environments.
The invention in particular relates to a method of coating an electronically conductive and heat resistant substrate of a non-carbon metal-based anode of a cell for the electrowinning of metals from their oxides dissolved in molten salt, to protect and make the surface of the anode substrate active for the oxidation of the oxygen ions present in the electrolyte. The method comprises applying onto the substrate a slurry comprising at least one oxide or a precursor thereof as a non-dispersed but suspended particulate in a colloidal and/or inorganic polymeric carrier, the slurry is then solidified and made adherent to the substrate upon heat treatment to form an adherent, protective, predominantly oxide-containing coating.
An oxide may be present in the oxide-containing coating 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.
A typical application for this method is the coating of anodes for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, such as a cryolite-based electrolyte or cryolite.
The colloidal and/or inorganic polymeric carrier may be selected from alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide, zinc oxide and mixtures thereof.
Advantageously, the colloidal and/or inorganic polymeric carrier forms upon heat treatment the same chemical compound as the non-dispersed particulate.
The oxides which may be used as a non-dispersed particulate and/or as a carrier may be in the form of spinels and/or perovskites or precursors thereof. Spinels may be doped, non-stoichiometric and/or partially substituted spinels, the doped spinels comprising dopants selected from the group consisting of Ti
4+
, Zr
4+
, Sn
4+
, Fe
4+
, Hf
4+
, Mn
4+
, Fe
3+
, Ni
3+
, Co
3+
, Al
3+
, Cr
3+
, Fe
2+
, Ni
2+
, Co
2+
, Mg
2+
, Mn
2+
, Cu
2+
, Zn
2+
and Li
+
.
The spinels may comprise a ferrite which can be selected from cobalt, copper, chromium, manganese, nickel and zinc ferrite, and mixtures and precursors thereof. The ferrites may also be doped with at least one oxide selected from chromium, titanium, tin, zinc and zirconium. Nickel-ferrite is a preferred compound for an electrochemically active coating for its high chemical resistance and may be present as such or partially substituted with Fe
2+
.
Alternatively, the spinels may also comprise a chromite which can be selected from iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, yttrium, magnesium, nickel and zinc chromite, and mixtures and precursors thereof.
The slurry advantageously comprises one or more electrocatalysts or a precursor thereof, however such a constituent is not always necessary. When an electrocatalyst is used, it may be advantageously selected from iridium, palladium, platinum, rhodium, ruthenium, silicon, tin, zinc, Mischmetal oxides and metals of the Lanthanide series, and mixtures and compounds thereof.
For the formation of the coating onto the substrate, the oxide constituents of the slurry may react among themselves. Alternatively the constituents of the slurry may react with constituents of the electronically conductive and heat resistant substrate. However, a reaction is not always necessary for the formation of the coating from the slurry.
The slurry may be applied onto the substrate by conventional techniques such as brushing, spraying dipping, electrodeposition or by using rollers.
The substrate can be chosen among metals, alloys, intermetallics, cermets, and conductive ceramics. It may for instance comprise at least one of chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and their combinations and compounds.
The substrates may advantageously have a self-healing effect, i.e. when exposed to electrolyte the substrate passivates under the effect of the electrical current and becomes substantially inert to the electrolyte.
The adherence of the coating on the substrate may be enhanced by applying onto the substrate a pre-coat before applying the slurry. Several methods are known to obtain an oxide pre-coat on a metal substrate, e.g. heating in air for prolonged periods at high temperatures (>1000° C.).
However, a pr
de Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent S.A. Luxembourg
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