Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-07-02
2003-04-01
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S247300, C204S247400, C204S247500, C205S372000, C205S375000, C205S378000, C205S380000, C205S381000
Reexamination Certificate
active
06540887
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte such as cryolite, provided with non-carbon, metal-based, anodes designed for such aluminium electrowinning cells.
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 Héroult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. 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.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
U.S. Pat. No. 4,999,097 (Sadoway) describes anodes for conventional aluminium electrowinning cells provided with an oxide coating containing at least one oxide of zirconium, hafnium, thorium and uranium. To prevent consumption of the anode, the bath is saturated with the materials that form the coating. However, these coatings are poorly conductive and have not been used.
U.S. Pat. No. 4,504,369 (Keller) discloses a method of producing aluminium in a conventional cell using massive metal oxide anodes having a central vertical through-opening for feeding anode constituents and alumina into the electrolyte, to slow dissolution of the anode.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal 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 during electrolysis by the addition of small amounts of a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack and to a certain extent from gaseous oxygen but not from nascent monoatomic oxygen.
Several designs for oxygen-evolving anodes for aluminium electrowinning cells were proposed in the following documents. U.S. Pat. No. 4,681,671 (Duruz) discloses vertical anode plates or vertical blades operated in low temperature aluminium electrowinning cells. U.S. Pat. No. 5,310,476 (Sekhar/de Nora) discloses oxygen-evolving anodes consisting of roof-like assembled pairs of anode plates. U.S. Pat. No. 5,362,366 (de Nora/Sekhar) describes non-consumable anode shapes, such as roof-like assembled pairs of anode plates, as well as a downwardly curved flexible sheet or wire or bundle of wires. U.S. Pat. No. 5,368,702 (de Nora) discloses vertical tubular or conical oxygen-evolving anodes for multimonopolar aluminium cells. U.S. Pat. No. 5,683,559 (de Nora) describes an aluminium electrowinning cell with oxygen-evolving bent anode plates which are aligned in a roof-like configuration facing correspondingly shaped cathodes. U.S. Pat. No. 5,725,744 (de Nora/Duruz) discloses vertical oxygen-evolving anode plates, preferably porous or reticulated, in a multimonopolar cell arrangement for aluminium electrowinning cells operating at reduced temperature.
While the foregoing references indicate continued efforts to improve the operation of aluminium electrowinning cell operations by using oxygen-evolving anodes none of them has found any commercial acceptance yet.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes.
It is also an object of the invention to provide an aluminium electrowinning cell with one or more anodes which have a large surface area and a high electrochemical activity for the evolution of oxygen and which permit fast oxygen gas release and circulation of alumina rich electrolyte between the anodes and a facing cathode.
An object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are easy and economic to manufacture.
Another object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are made of a long lasting anode material leading to commercially acceptable produced aluminium and which can be shaped at will.
A further object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are made of an anode material having a low solubility in the electrolyte.
An important object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which can be maintained dimensionally stable and do not excessively contaminate the product aluminium.
SUMMARY OF THE INVENTION
The invention provides a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises at least one non-carbon metal-based anode having an electrically conductive metallic structure with an electrochemically active anode surface on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode. Such metallic structure comprises a series of parallel horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved, the electrochemically active surfaces being in a generally coplanar arrangement to form said active anode surface. The anode members are spaced apart to form longitudinal flow-through openings for the circulation of electrolyte driven by the fast escape of anodically evolved oxygen.
Depending on the cell configuration some or all of the flow-through openings may serve for the flow of alumina-rich electrolyte to an electrolysis zone between the anode(s) and the cathode and/or for the flow of alumina-depleted electrolyte away from the electrolysis zone. When the anode surface is horizontal or inclined these flows are ascending and descending. Part of the electrolyte circulation may also take place around the metallic anode structure.
A substantially uniform current distribution can be provided from a current feeder through conductive transverse metallic connectors to the anode members and their active surfaces.
As opposed to known oxygen-evolving anode designs for aluminium electrowinning cells, in an anode according to this invention the coplanar arrangement of the anode members provides an electrochemically active surface extending over an expanse which is much greater than the thickness of the anode members, thereby limiting the material cost of the anode.
The electrochemically active anode surface is usually substantially horizontal or inclined to the horizontal.
In special cases, the electrochemically active anode surface may be vertical or substantially vertical, the horizontal anode members being spaced apart one above the other, and arranged so the circulation of electrolyte takes place through the flow-through openings. For example, the anode members may be arranged like venetian blinds next to a vertical or substantially vertical cathode.
In one embodiment, two substantially vertical (or downwardly converging at a slight angle to the vertical) spaced apart adjacent anodes are arranged between a pair of substantially vertical cathodes, each anode and facing parallel cathode being spaced apart by an inter-electrode gap. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyt
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent SA
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