Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2002-06-04
2003-12-02
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S372000, C205S379000, C205S380000, C205S385000, C205S387000, C205S391000, C205S392000
Reexamination Certificate
active
06656340
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a cell for the production of aluminium by the electrolysis of an aluminium compound dissolved in a molten electrolyte, for example alumina dissolved in a molten fluoride-based electrolyte. It concerns in particular a cell of advanced design having a cathode of drained configuration, and a non-carbon anode facing the cathode both covered by the molten electrolyte.
The invention also relates to methods of operating the cells to produce aluminium.
BACKGROUND OF THE INVENTION
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite-based electrolyte and operating 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.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite-graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode and to which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks. The side walls are also covered with prebaked anthracite-graphite carbon plates or silicon carbide plates.
Conventional aluminium production cells are constructed so that in operation a crust of solidified molten electrolyte forms around the inside of the cell sidewalls. At the top of the cell sidewalls, this crust is extended by a ledge of solidified electrolyte which projects inwards over the top of the molten electrolyte. The solid crust in fact extends over the top of the molten electrolyte between the carbon anodes. To replenish the molten electrolyte with alumina in order to compensate for depletion during electrolysis, this crust is broken periodically at selected locations by means of a crust breaker, fresh alumina being fed through the hole in the crust.
This crust/ledge of solidified electrolyte forms part of the cell's heat dissipation system in view of the need to keep the cell in continuous operation despite changes in operating conditions, as when anodes are replaced, or due to damage/wear to the sidewalls, or due to over-heating or cooling as a result of fluctuations in the operating conditions. In conventional cells, the crust is used as a means for automatically maintaining a satisfactory thermal balance, because the crust/ledge thickness self-adjusts to compensate for thermic unbalances. If the cell overheats, the crust dissolves partly thereby reducing the thermic insulation, so that more heat is dissipated leading to cooling of the cell contents. On the other hand, if the cell cools the crust thickens which increases the thermic insulation, so that less heat is dissipated, leading to heating of the cell contents.
The presence of a crust of solidified electrolyte is considered to be important to achieve satisfactory operation of commercial cells for the production of aluminium on a large scale. In fact, the heat balance is one of the major concerns of cell design and energy consumption, since only about 25% of such energy is used for the production of aluminium. Optimization of the heat balance is needed to keep the proper bath temperature and heat flow to maintain a frozen electrolyte layer (side ledge) with a proper thickness.
Considerations concerning the refractory and insulating materials used in conventional cells to control the the heat flow are discussed in the monograph “Materials Used in the Hall-Heroult Cell for Aluminum Production” by H. Zhang. V. de Nora and J. A. Sekhar, published by The Minerals, Metals and Materials Society, Pennsylvania, USA, 1994, see especially Chapter 6.
In conventional cells, the major heat losses occur at the sidewalls, the current collector bars and the cathode bottom, which account for 35%, 8% and 7% of the total heat losses respectively, and considerable attention is paid to providing a correct balance of these losses.
Further losses of 33% occur via the carbon anodes, 10% via the crust and 7% via the deck on the cell sides. This high loss via the anodes is considered inherent in providing the required thermal gradient through the anodes.
In the literature, there have been suggestions for cells operating with non-carbon anodes with or without a crust of solidified electrolyte, but so far none of these designs has proven to be feasible. Previously this was due principally to the difficulties encountered in developing anode materials that remained sufficiently stable in the aggressive environment.
However, even with available promising non-carbon anode materials such as those based on nickel-iron-aluminium or nickel-iron-aluminium-copper with an oxide surface as described in U.S. Pat. No. 5,510,008 (de Nora et al), there is still a need to provide a redesigned cell of advanced design in order to achieve the potential advantages of the oxygen-evolving anode materials on the one hand and of the drained cathode configuration on the other hand, and to improve the overall cell efficiency.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for a cell operating with non-carbon anodes that can operate without crust formation and which also facilitate the implementation of a drained cathode configuration.
OBJECTS OF THE INVENTION
One object of the invention is to provide an aluminium production cell of advanced design incorporating non-carbon oxygen-evolving anodes which is efficient in operation and can operate without formation of a crust of frozen electrolyte.
Another object of the invention is to provide an aluminium production cell of advanced design wherein the cell efficiency is improved by better control of the thermic losses associated with the anodically-evolved gases.
Another object of the invention is to permit more efficient cell operation by improving the distribution of electric current to the cathode cooperating with non-carbon oxygen evolving anodes.
A further object of the invention is to provide a cell of advanced design with a non-carbon anode in combination with novel cathode which has improved distribution of electric current and can be easily produced and fitted in the cell, and which simplifies dismantling of the cell to replace or refurbish the cathodes.
A yet further object of the invention is to provide a cell of advanced design which facilitates the implementation of a drained cell configuration.
Yet another object of the invention is to provide a cell of advanced design which combines the advantages of a drained cathode configuration and of non-carbon oxygen evolving anodes, is thermally efficient, easy to construct and service, and efficient in operation.
A yet further object of the invention is to provide a cell of advanced design enabling drained cathode operation where ease of removal of the anodically produced gases is combined with ease of collection of the product aluminium.
An even further object of the invention is to provide an aluminium production cell in which fluctuating electric currents that produce a variable electromagnetic field are reduced or eliminated thereby reducing or eliminating the adverse effects that lead to a reduction of the cell efficiency.
SUMMARY OF THE INVENTION
One main aspect of the invention concerns a cell of advanced design for the production of aluminium by the electrolysis of an aluminium compound dissolved in a molten electrolyte, having a cathode of drained configuration and at least one non-carbon anode facing the cathode. Both the cathode and the anode are covered by the electrolyte. In accordance with the invention, the u
de Nora Vittorio
Sekhar Jainagesh A.
Deshmukh Jayodeep R.
King Roy
Moltech Invent S.A.
Nicolas Wesley A.
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