Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-01-29
2003-03-18
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S247400, C205S372000, C205S377000, C205S380000, C205S384000, C205S386000, C205S387000
Reexamination Certificate
active
06533909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to bipolar cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte provided with bipolar electrodes having carbon cathodes and oxygen-evolving anodes, methods for the fabrication and reconditioning of such electrodes, and the operation of such cells to maintain the anodes dimensionally stable.
BACKGROUND ART
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
A major drawback of conventional cells is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD), also called inter-electrode gap (IEG), must be kept at a safe minimum value of approximately 5 cm to avoid short circuiting between the aluminium cathode and the anode or re-oxidation of the metal by contact with the CO
2
gas formed at the anode surface.
The high electrical resistivity of the electrolyte causes a voltage drop in the inter-electrode gap which alone represents as much as 40% of the total voltage drop with a resulting low energy efficiency.
All aluminium production cells commercially used today have carbon anodes and carbon cathodes. Only recently has it become possible to make the carbon cathode surface aluminium-wettable by means of an applied coating obtained from an applied slurry or colloidal dispersion containing titanium diboride as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar). Making the cathode surface aluminium-wettable allowed the design of drained cells with reduced anode-cathode distance (ACD) and therefore to save energy as described in U.S. Pat. No. 5,683,559 (de Nora).
Twenty years of intense and costly research made it possible to design non-carbon anodes which eliminate the severe pollution during their fabrication and utilisation. Improvements have been achieved, as described in co-pending applications WO99/36591 and WO99/36592 (both in the name of de Nora), WO99/36593 and WO99/36594 (both in the name of de. Nora/Duruz) which disclose anodes having a metal core resistant to cryolite and oxygen, and an electrochemically active coating.
Several past attempts were made to design bipolar cells in order to overcome the problems encountered with conventional aluminium electrowinning cells. In order to make their use economic, bipolar cells need electrodes which are resistant to the products of electrolysed aluminium salts. Using consumable electrodes in bipolar cells is not acceptable as their replacement is much more difficult and their consumption enlarges the anode-cathode gap (ACG) and cannot be compensated by repositioning the electrodes as in Hall-Héroult cells.
U.S. Pat. Nos. 3,822,195 and 3,893,899 (both in the name of Dell/Haupin/Russel) and U.S. Pat. No. 4,110,178 (LaCamera/Trzeciak/Kinosz) all describe bipolar cells operating with carbon electrodes and with an electrolytic bath containing aluminium chloride instead of alumina. However, these cell designs have not been commercially adopted.
U.S. Pat. No. 3,578,580 (Schmidt-Hatting/Huwyler) discloses bipolar cells, in particular having inclined electrodes, wherein the anodes are made of oxygen-resistant material such as platinum or a conductive oxide or wustite (ferrous oxide FeO). The cathode is made of carbon, or other electrically conductive material resistant to fused melt, such as a carbide of titanium, zirconium, tantalum or niobium.
U.S. Pat. No. 3,930,967 (Alder) describes a bipolar cell electrode comprising an anode, an intermediate plate and a cathode plate held together in an alumina or magnesium oxide frame. The anode plate is made of ceramic oxide material, the preferred material being tin oxide with copper oxide and antimony oxide. The cathode is graphite or made of borides, carbides, nitrides, silicides, in particular of metals such as titanium, zirconium or silicon. The intermediate plate, for instance made of silver, nickel or cobalt, prevents direct contact between the anode and the cathode plates to avoid any reaction between them when exposed to high temperature.
U.S. Pat. No. 5,019,225 (Darracq/Duruz/Durmelat) discloses a bipolar electrode for an aluminium electrowinning cell having a cerium oxyfluoride anode surface and a cerium hexaboride cathode surface, which was specially designed for use in the process of U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) wherein cerium species dissolved in the electrolyte maintain the anode surface stable.
Despite all previous attempts, the bipolar technology has never been successfully implemented in industrial aluminium production cells due to many problems of cell operation.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a bipolar electrode for aluminium electrowinning bipolar cells, which has an oxygen resistant anode surface.
Another object of the invention is to provide a bipolar electrode for aluminium electrowinning bipolar cells, which contains carbon but which is not exposed to carbon oxidation so as to eliminate carbon-generated pollution and high costs of carbon consumption.
Yet another object of the invention is to provide a bipolar electrode for aluminium electrowinning bipolar cells whose anodic surface has a sufficient operative lifetime to make its use commercially acceptable.
An important object of the invention is to provide a bipolar electrode for aluminium electrowinning bipolar cells, which may be maintained dimensionally stable, without excessively contaminating the product aluminium.
Yet another object of the invention is to provide an aluminium electrowinning bipolar cell operating under such conditions that the contamination of the product aluminium is limited.
The invention relates to a bipolar cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, having a terminal cathode, a terminal anode and thereinbetween at least one bipolar electrode. The bipolar electrode comprises a carbon cathode body having on one side an electrochemically active surface on which aluminium is produced and connected on the other side through an oxygen impermeable barrier layer to an anode layer having a metal oxide outer surface which is electrochemically active for the oxidation reaction of oxygen ions into nascent monoatomic oxygen.
More generally, the metal oxide may be present in the electrochemically outer surface in a multi-compound mixed oxide, in mixed crystals 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.
Oxygen Barrier Layers & Protective Layers
The oxygen barrier layer may be made of a metal or an oxidised metal, an intermetallic compound, a mixed perovskite ceramic, a phosphide, a carbide, a nitride such as titanium nitride, or a combination thereof.
Suitable metals or oxides of metals acting as a barrier to oxygen may be selected from chromium, chromium oxide, niobium, niobium oxide, nickel and nickel oxide. The oxygen barrier layer may in particular consist of a 5 to 20 micron thick layer of noble metal, such as platinum, palladium, iridium or rhodium. Intermetallic compounds such as silver-palladium, chromium-manganese and chromium-molybdenum also act as a barrier to oxygen.
The oxygen barrier may contain a mixed perovskite ceramic which may be chosen among zirconate, cobaltite, chromite, chromate, manganate, ruthenate, niobiate, tantalate and tungstate. The perovskite preferably contains strontium zirconate to enhance the conductivity of the oxygen barrier layer. A conductive phosphide resistant to oxygen may be chosen among a phosphide of titanium, chromium and tungsten. A suitable carb
de Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent S.A.
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