Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2003-05-16
2004-11-02
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C204S241000, C204S243100, C204S244000, C204S247500, C429S006000
Reexamination Certificate
active
06811677
ABSTRACT:
FIELD OF INVENTION
The present invention relates to an electrolytic cell for the production of aluminium, a method for maintaining a crust on the sidewall of an electrolytic cell for producing aluminum and a method for recovering electricity from an electrolytic cell for producing aluminum.
BACKGROUND ART
Aluminium is produced in electrolytic cells comprising an electrolytic tank having a cathode and an anode which is either a selfbaking carbon anode or a plurality of prebaked carbon anodes. Aluminum oxide is supplied to a cryolite-based bath in which the aluminum oxide is dissolved. During the electrolytic process aluminum is produced at the cathode and forms a molten aluminum layer on the bottom of the electrolytic tank with the cryolite bath floating on the top of the aluminum layer. CO-gas is produced at the anode causing consumption of the anode. The operating temperature of the cryolite bath is normally in the range of about 920 to about 950° C.
The electrolytic tank consists of an outer steel shell having carbon blocks in the bottom. The blocks are connected to electrical busbars whereby the carbon blocks function as a cathode. The sidewalls of the electrolytic tank are generally lined with refractory material against the steel shell, and a layer of carbon blocks or carbon paste is formed on the inside of the refractory material. There are several types of lining materials and ways of arranging the sidewall lining.
During the operation of the electrolytic cell, a crust or ledge of frozen bath forms on the sidewalls of the electrolytic tank. This layer may, during operation of the electrolytic cell, vary in thickness. The formation of this crust and its thickness are critical to the operation of the cell. If the crust becomes too thick, it will disturb the operation of the cell as the temperature of the bath near the walls becomes cooler than the temperature in the bulk of the bath, thereby disturbing the dissolution of aluminum oxide in the bath. On the other hand, if the frozen layer of crust becomes to thin or is absent, the electrolytic bath may attack the sidewall lining of the electrolytic tank, which ultimately can result in failure of the tank. If the bath attacks the sidewalls, the electrolytic cell has to be shut down, the electrolytic tank has to be removed and a new one has to be installed. This is one of the main reasons for reduced average lifetime of electrolytic tanks.
In order to maintain a proper thickness of the frozen layer of electrolytic bath on the sidewall lining, it is necessary to design the sidewall lining in such a way that the flow of heat from the bath through the sidewall lining is sufficiently high to maintain a frozen crust on the inside of the sidewall lining. The heat losses through the sidewalls of the electrolytic tank may thus account for up to 40% of the total heat losses from the electrolytic cell. However, even with a proper design of the sidewall lining it is impossible to obtain and maintain a thin stable layer of frozen bath on this sidewall lining due to variations in bath composition and other process variables not under operator control.
SUMMARY OF INVENTION
It is an object of the present invention to provide an electrolytic cell for the production of aluminum where the heat losses through the sidewalls of the electrolytic tank are partially recovered as electricity and wherein a thin, stable layer of frozen electrolytic bath is obtained and maintained on the inside of the sidewall lining. It is a further object of this invention that the frozen layer is not influenced by differences in temperature of the molten electrolytic bath or of the bath composition.
Accordingly, the present invention relates to an electrolytic cell for the production of aluminum comprising an anode and an electrolytic tank where the electrolytic tank comprises an outer shell made from steel and carbon blocks in the bottom of the tank forming the cathode of the electrolytic cell, said electrolytic cell being characterized in that at least a part of the side wall of the electrolytic tank has one or more evaporation cooled panels, and wherein high temperature, heat resistant and heat insulating material is arranged between the evaporation cooled panels and the steel shell.
According to a preferred embodiment, all the sidewalls of the electrolytic cell are equipped with evaporation cooled panels.
According to another embodiment, the evaporation cooled panels are intended to contain a first cooling medium which has a boiling point in the range between 850 to 950° C., preferably between 900 and 950° C. at atmospheric pressure.
Suitably, the evaporation cooled panels contain molten sodium, a sodium-lithium alloy or zinc as a cooling medium.
According to yet another embodiment of the present invention, each evaporation cooled panel has means, in its upper part, for circulation of a second cooling medium for convective heat removal to condense the cooling medium in the evaporation cooled panel.
According to yet another embodiment of the present invention, the means for circulation of the second cooling medium is a first closed loop, and a part of said first closed loop runs through the upper part of each evaporation cooled panel in the electrolytic cell.
The parts of the first closed loop for the second cooling medium that are not situated inside the upper part of the evaporation cooled panels are preferably arranged in the heat resistant and heat insulating material arranged between the evaporation cooled panels and the steel shell.
The first closed loop for circulating the second cooling medium is preferably connected to a heat exchanger for transferring heat from the second cooling medium to a third cooling medium contained in a second closed loop. After being heated in the heat exchanger, the third cooling medium is pumped through a generator for producing electrical energy. The heat exchanger is preferably arranged in the heat resistant and heat insulating material arranged between the evaporation cooled panels and the steel shell.
The second closed loop for circulating the third cooling medium is preferably connected to heat exchangers for a plurality of electrolytic cell, and more preferably is connected to heat exchangers for all electrolytic cells in a potline.
When operating a potline with a plurality of electrolytic cells according to the present invention, each evaporation cooled panel in an individual cell is set to operate such that the temperature on the side of the panels facing the interior of the electrolytic cells is slightly below the temperature of the molten electrolytic bath, preferably between 2 and 50° C. lower than the temperature of the electrolytic bath. Thus, due to the small temperature drop between the evaporation cooled panels and the molten electrolytic bath, a thin, solid and stable crust of electrolytic bath will form on the side of the evaporation cooled panels facing the molten electrolytic bath. This crust will protect the sides of the evaporation cooled panels facing the molten electrolytic bath. As an example, if the temperature of the electrolytic bath is 940° C., the evaporation cooled panels are set to operate at 920° C. Further, due to the heat resistant and heat insulating material arranged between the evaporation cooled panels and the steel shell, the heat flow through the sidewall is negligible.
Heat will be transferred from the electrolytic bath to each evaporation cooled panel, and the first liquid cooling medium in the lower part of the evaporating cooled panels will transfer this heat to the upper part of the evaporation cooled panels through evaporation of a part of the first liquid cooling medium. In the upper part of the evaporation cooled panels, the vapour will condense as it comes into contact with the first closed loop for circulating the second cooling medium and the heat of condensation will be transferred to the second cooling medium. The condensed first cooling medium will flow down into the lower part of the evaporation cooled panels.
The heat transferred to the second cooling medium will cause a
Aune Jan Arthur
Johansen Kai
Nos Per Olav
Elkem ASA
Muserlian Lucas and Mercanti
Valentine Donald R.
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