Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2002-04-27
2004-09-28
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S381000, C205S392000, C204S244000, C204S245000, C204S246000, C204S247000, C204S284000
Reexamination Certificate
active
06797148
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to drained-cathode cells for the electrowinning of aluminium from alumina, of the type comprising a series of anodes spaced by a sloped inter-electrode gap from one or more facing cathodes and arranged so the electrolyte circulates upwardly in the sloped inter-electrode gap assisted by anodically produced gases. The invention also relates to a method of producing aluminium in such cells as well as to cathodes and anodes designed for such cells.
BACKGROUND OF THE INVENTION
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 Héroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
U.S. Pat. No. 3,400,061 (Lewis/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Traugott) disclose aluminium electrowinning cells with sloped drained cathodes and facing anodes sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the center of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
In U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and reimmersion of the assembly during operation, such assembly also operating with a drained cathode.
U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved using a tubular anode with suitable openings.
U.S. Pat. No. 5,651,874 (de Nora/Sekhar) proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. This publication discloses slurry-applied applications and novel drained cathode configurations, including designs where a cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom.
U.S. Pat. No. 5,683,559 (de Nora) proposed a new cathode design for a drained cathode, where grooves or recesses were incorporated in the surface of blocks forming the cathode surface in order to channel the drained product aluminium.
Recently it has become possible to coat carbon cathodes with a slurry which adheres to the carbon and becomes aluminium-wettable and very hard when the temperature reaches 700-800° C. or even 950-1000° C., as mentioned above. Though application of these coatings to drained cathode cells has been proposed, so far the commercial-scale application of this technology has been confined to coating carbon bottoms of cells operating with the conventional deep pool of aluminium. Further design modifications in the cell construction could lead to obtaining more of the potential advantages of these coatings.
While the foregoing references indicate continued efforts to improve cell operations, none suggests the invention and there have been no acceptable proposals for improving the cell efficiency, and at the same time facilitating the implementation of a drained cathode configuration with improved electrolyte circulation.
OBJECTS OF THE INVENTION
An object of the invention is to overcome problems inherent in the conventional design of cells used in the electrowinning of aluminium from alumina dissolved in molten fluoride-containing melts in particular cryolite, notably by proposing a drained cathode configuration incorporating an improved electrode arrangement.
Another object of the invention is to permit more efficient cell operation by modifying the design of the drained cathode(s) and/or of the anodes to improve the electrolyte circulation.
Yet another object of the invention is to provide an arrangement wherein gas release at a sloping anode surface is used to induce electrolyte circulation which in turn is facilitated by a novel cathode and/or anode design.
A further object of the invention is to provide a cell with a cathode of novel design enabling drained cathode operation where efficient electrolyte circulation is combined with ease of removal of the anodically produced gases and with ease of collection of the product aluminium.
A yet further object of the invention is to enhance the efficiency of electrolysis by supplying alumina to a circulating electrolyte to compensate for depletion during electrolysis, this electrolyte circulation being produced by means of a novel electrode configuration.
SUMMARY OF THE INVENTION
One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises one or more anodes and one or more cathodes. The or each anode and cathode respectively have a sloped V-shaped active anode surface and parallel sloped inverted V-shaped drained cathode surface facing one another and spaced apart by two sloped inter-electrode gaps, arranged so that the electrolyte circulates upwardly in the sloped inter-electrode gaps assisted by anodically produced gas and then returns from a top part to a bottom part of each inter-electrode gap along an electrolyte path. Each electrolyte path extends through vertical and horizontal passages as follows: for the cathode, a vertical passage from a top to a lower part of a cathode and then a horizontal passage in or under the lower part of the cathode; and/or for the anode, a horizontal passage in or on an upper part of an anode and then a vertical passage extending from the upper to a bottom part of the anode. Each horizontal passage extends substantially over the entire horizontal length of a corresponding inter-electrode gap.
In this context, a “V-shaped surface” means a surface having a perpendicular cross-section which strictly or generally forms a V, in particular a flattened and/or truncated V. Such a surface may be generally conical, frusto-conical or bi-planar.
The drained-cathode cell according to the invention and the corresponding method of electrowinning aluminium have numerous advantages, including the following
a) The anodically produced gases are rapidly removed due to the slope of the anodic active surfaces.
b) The cell can be operated at high current density, providing for a sufficient upward removal of anodically produced gas to produce a corresponding upward circulation of the electrolyte in the anode-cathode gap.
c) The slope of the cathodic surfaces is sufficient to allow for efficient draining of the product aluminium, despite the counter-current of electrolyte entrained upwardly by the gas release.
d) The generally horizontal passage provides part of a return path for the electrolyte, enabling a steady-state circulation of the electrolyte around the electrodes.
e) An improved electrolyte circulation may be achieved by providing a plurality of return paths associated with both anodic and cathodic electrodes.
e) The induced electrolyte circulation can advantageously be combined with a supply of alumina to compensate for depletion during electrolysis. This supply of alumina may be adjacent to the upper end of the sloping inter-electrode gap or possibly over the anodes.
f) The cathode(s) can easily be made from the usual grades of carbon used for cathode applications, the sloping cathodic surfaces at least being coated with a suitable coating of aluminium-wettable refractory material, for example a slurry-applied coating containing titanium diboride, for example as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar) o
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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