Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-10-16
2003-06-03
Nguyen, Nam (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S380000, C204S245000
Reexamination Certificate
active
06572757
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, fitted with an alumina feed device for feeding alumina over substantially the entire surface of the molten electrolyte; an alumina feed device for such a cell; and a method for producing aluminium in such a cell.
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.
An important aspect of the production of aluminium in such cells resides in the way in which alumina is fed to the molten electrolyte for its subsequent dissolution and electrolysis, as described hereafter.
Conventional cells are usually operated with a crust of frozen electrolyte above the molten electrolyte. This crust needs to be periodically broken to form an opening for feeding alumina into the molten electrolyte situated underneath. Various systems have been provided to locally break the frozen electrolyte crust and feed alumina into the molten electrolyte, for instance as described in U.S. Pat. Nos. 3,664,946 (Schaper/Springer/Kyburz), 4,049,529 (Golla), 4,437,964 (Gerphagnon/Wolter), 5,045,168 (Dalen/Kvalavag/Nagell), 5,108,557 (Nordquist), 5,294,318 (Grant/Kristoff), 5,324,408 and 5,423,968 (both in the name of Kissane).
One drawback of feeding alumina to the molten electrolyte by initially breaking the electrolyte crust resides in the introduction of a mass of frozen electrolyte into the molten electrolyte which generates a thermal shock therein. Moreover, after the crust is broken cold alumina is added to the molten electrolyte which inevitably freezes the bath, thereby forming dense alumina and/or electrolyte aggregates increasing the chance of sludging.
Therefore, with the trend towards more automated systems, the frequency of feeding alumina has been increased. Feeding may take place every 20 to 90 min., sometimes even shorter, for instance every 1 to 5 min. as described in U.S. Pat. No. 3,673,075 (Kibby), while smaller amounts of alumina are introduced with each feed. The advantages are in particular maintaining a more constant concentration of dissolved alumina in the electrolyte and reducing the temperature variation in the electrolyte. A typical automated break and feed system comprises a pneumatically-operated crust breaker beam and an ore bin capable of discharging a fixed amount of alumina (K. Grjotheim & B. J. Welsh, “Aluminium: Smelter Technology ”, 1988, 2
nd
Edition, Aluminium Verlag GmbH, D-4000 Düsseldorf 1, pp. 231-232).
U.S. Pat. No. 5,476,574 (Welsh/Stretch/Purdie) discloses a feeder arranged to continuously feed alumina to an aluminium electrowinning cell. The feeder is associated with a point breaker which is operated to maintain a hole in a frozen electrolyte crust on the surface of the molten electrolyte.
In order to enhance dispersion, dissolution and control of the amount of fine particulate alumina fed to the electrolytic bath, various alumina feed devices have been developed involving fluidisation of alumina powder by using compressed gas such as compressed air, for instance as disclosed in U.S. Pat. Nos. 3,901,787 (Niizeki/Watanabe/Yamamoto/Takeuchi/Kubota), 4,498,818 (Gudmundur/Eggertsson) and 4,525,105 (Jaggi).
Although substantial efforts have been made to enhance the feeding of alumina as described above, such feeding is still locally limited to one or more feeding points over the electrolytic bath between dipping carbon anode blocks. Furthermore, the above described processes still necessitate to periodically form or continuously maintain as many holes into the frozen electrolyte crust above the molten bath as there are feeding points.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed alumina to substantially the entire anode's surface.
A further object of the invention to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to operate with a substantially crustless molten electrolyte.
Another object of the invention is to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed and disperse pre-heated alumina powder to the molten electrolyte to minimise the risk of sludging and enhance dissolution of the fed alumina.
Yet another object of the invention is to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed continuously or intermittently alumina to the molten electrolyte.
A still further object of the invention is to provide an alumina feed device for such aluminium electrowinning cells as well as a method to produce aluminium in such cells.
SUMMARY OF THE INVENTION
The invention relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises a plurality of anodes immersed in the molten electrolyte, each anode having an oxygen-evolving active surface of open structure facing and spaced by an inter-electrode gap from a cathode; a thermal insulating cover above the molten electrolyte surface; and an alumina feed device arranged above the molten electrolyte surface for supplying alumina to the molten electrolyte surface from where the alumina is dissolved as it enters the electrolyte to enrich the electrolyte in dissolved alumina. Alumina-containing electrolyte is electrolysed in the inter-electrode gaps to produce oxygen gas on the anodes an aluminium on the cathode.
The alumina feed device comprises means for spraying and/or blowing alumina between the molten electrolyte surface and the thermal insulating cover and over an entire expanse of the surface of the electrolyte, hereinafter called the “alumina feeding area”, so that upon dissolution of alumina sprayed and/or blown to the electrolyte, electrolyte enriched in dissolved alumina flows to the inter-electrode gaps where is electrolysed.
In other words, the anode feeding area is at least a portion of the surface of the electrolyte whose size is substantially greater than that with conventional point feeders. Thus, alumina powder fed with this feeder is spread over a substantially greater surface of molten electrolyte and can much easier dissolve. The size of the expanse may be at least a tenth or a fifth of the surface area of the anode structure, in particular from a quarter to a half of the full surface area. Typically, the expanse may have a size of at least 0.1 m
2
, such as 0.5 or 1 or 2 m
2
to 6 or 10 m
2
or more.
Conveniently, the spraying and/or blowing means are arranged to spray and/or blow alumina into an area which corresponds approximately to the perpendicular projection on the surface of the molten electrolyte of the active anode surface. For example, a spraying and/or blowing means may be arranged to spray and/or blow alumina over an expanse which covers entirely or at least partly the perpendicular projection onto the molten electrolyte surface of an active anode surface. The alumina feeding area may correspond to the feeding area of one anode or several anodes.
In one embodiment, the anode feeding area corresponds to a projection onto the surface of the electrolyte of the active anode surfaces, this projection possibly being smaller or greater than the corresponding area(s) of the active anode surfaces. This anode feeding area is usually, but not necessarily, situated above the active anode surfaces.
The alumina feeding area
Berclaz Georges
de Nora Vittorio
Deshmukh Jaydeep R.
Moltech Invent S.A.
Mutschler Brian L.
Nguyen Nam
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