Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-10-04
2002-08-20
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S376000, C205S378000, C205S380000, C205S386000, C205S387000, C205S394000, C205S395000, C205S396000, C204S243100, C204S244000, C204S245000, C204S247300, C204S247400, C204S280000
Reexamination Certificate
active
06436272
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a low temperature aluminum reduction cell and more particularly, it relates to a low temperature aluminum reduction cell using an improved method for removing molten aluminum from the cell.
The present invention relates generally to methods and apparatuses for the electrolytic reduction of alumina to aluminum. More particularly, the subject matter herein relates to the subject matter disclosed in Beck et al. U.S. Pat. No. 4,592,812; 4,865,701; 5,006,209; 5,284,562; and U.S. patent application Ser. No. 09/247,196, the disclosures thereof which are incorporated herein by reference.
The aforementioned patents of Beck et al. are directed to a series of developments relating to the electrolytic reduction of alumina to aluminum. The developments culminated in an electrolytic reduction cell containing a relatively low melting point, molten electrolyte composed of fluorides, a non-consumable anode composed of a particular alloy of copper, nickel and iron, and a cathode, composed of titanium diboride (TiB
2
), that is wettable by molten aluminum. A plurality of the non-consumable anodes are vertically disposed within a vessel containing a bath composed of molten electrolyte. A plurality of the cathodes are also vertically disposed within the vessel, with the cathodes being arranged in close, alternating, spaced relation with the vertically disposed anodes. In a preferred embodiment, the vessel has an interior metal lining electrically connected to the anodes and having essentially the same composition as the anodes. The lining can function as an auxiliary anode.
The bath of molten electrolyte contains dissolved alumina and additional alumina in the form of finely divided particles. The molten electrolyte has a density less than the density of molten aluminum and less than the density of alumina. As noted above, some alumina is dissolved in the molten electrolyte. When an electric current is passed through the bath, aluminum ions are attracted to the cathodes, and oxygen ions are attracted to the anodes. Bubbles of gaseous oxygen form at each of the anodes, and aluminum forms at each of the cathodes. The bubbles of gaseous oxygen pass upwardly from the anodes and maintain the undissolved, finely divided alumina particles suspended in the bath of molten electrolyte, forming a slurry. The metallic aluminum formed at the cathodes wets the surface of each cathode and flows downwardly along the cathode.
The electrolytic reduction cell is operated at a relatively low temperature, substantially below 950° C. The composition of the electrolyte employed in the cell enables operation of the cell at a relatively low temperature, because the electrolyte is molten at that low temperature. The low cell temperature allows the use of non-consumable anodes composed of the Ni—Cu—Fe alloys described below without subjecting the anodes to deterioration in the molten electrolyte.
In conventional electrolytic cells for production of aluminum from alumina, molten aluminum is removed by tapping the cell periodically by removal of a plug in the bottom of the cell where the molten aluminum has collected. In such cells, the density of the molten aluminum is greater than the density of the electrolyte. Consequently, molten aluminum collects on the floor of the cell which may be comprised of the cathode. In another embodiment, the aluminum is removed by siphoning molten aluminum from the pool of metal collected on the cell floor. However, when the aluminum is not permitted to collect on the floor of the cell, then its removal becomes much more difficult and thus various processes have been proposed.
U.S. patent application Ser. No. 09/247,196 discloses capillary action for collecting metal product. One problem with capillary action is clogging of the capillary conduit which can result from freezing of the electrolyte when a shift in current density occurs. Another problem resides in removing metal from the capillaries because the molten metal is drawn towards the smallest cross section. In addition, there is the problem of excluding alumina particles from the product metal when an alumina slurry electrolyte is used with capillaries.
U.S. Pat. No. 5,284,562 discloses an oxidation resistant, non-consumable anode, for use in the electrolytic reduction of alumina to aluminum, that has a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF
3
and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF. In this patent, one embodiment of a removal device is a pierced, titanium diboride member 31 which is wet internally and externally by aluminum and is mounted in the lower, inlet end of a suction tube 32 disposed above tap location 34. Member 31 has a lower-most extremity at tap location 34. A sump (not shown) may be provided at tap location 34 to assist in accumulating molten aluminum there. Titanium diboride member 31 will remove molten aluminum from the cell.
U.S. Pat. No. 4,740,279 relates to a process of producing lithium metal by the electrolysis of fused mixed salts comprising electrolyzing fused mixed salts consisting of lithium chloride and potassium chloride in a diaphragmless electrolytic cell, withdrawing molten lithium metal from the cell to a receiver and cooling the lithium metal which has been withdrawn. To decrease the content of impurities in a continuous process, molten mixture which rises in the interelectrode space in the cell and contains lithium metal is collected in an annular zone, which surrounds the top end of the cathode adjacent to the surface level of the molten mixture, the molten mixture is withdrawn from the annular zone through a siphon pipe and is supplied from the latter to a separating chamber, which communicates with the electrolytic cell and is sealed from the chlorine gas atmosphere in the electrolytic cell. Electrolyte and lithium are separated in the separating chamber under a protective gas atmosphere. Lithium metal is discharged from the separating chamber into a receiver under a protective gas atmosphere and the electrolyte is recycled from the separating chamber to the electrolytic cell.
U.S. Pat. No. 4,165,272 discloses an electrolytic cell cathode having a hollow cathode finger with fins extending outwardly therefrom for electrolysis of alkali metal chlorides. A synthetic separator surrounds the cathode and rests upon the fin-like extensions.
U.S. Pat. No. 4,681,671 discloses a method of producing aluminum by electrolysis of alumina dissolved in molten cryolite at temperatures between 680°-690° C.
The method comprises the employment of permanent anodes the total surface of which is increased up to 5 times compared to the total surface of anodes in a classical Hall-Heroult cell of comparable production rate. By this means the anodic current density is lowered to a degree which permits the discharge of oxide ions preferentially to fluoride ions at an acceptable rate. Additionally, the electrolyte is circulated by suitable means whereby it passes from an enrichment zone where it is saturated with alumina to an electrolysis zone and back.
U.S. Pat. No. 5,498,320 discloses a method and apparatus for electrolytic reduction of alumina using a porous cathode. The patent discloses in aluminum smelting by electrolysis, a double salt of KAlSO
4
, as a feedstock, heated with a eutectic electrolyte, such as K
2
SO
4
, at 800° C. for twenty minutes to produce an out-gas of SO
3
and a liquid electrolyte of K
2
SO
4
with fine-particles of Al
2
O
3
in suspension having a mean size of six to eight microns. This is pumped into a cell with an electrolyte comprised of K
2
SO
4
with fine-particles of Al
2
O
3
in suspension, an anode and a porous cathode of open-cell ceramic foam material. The cell is maintained at 750° C. and four volts of electricity applied between the ano
Brown Craig W.
Frizzle Patrick B.
Alexander Andrew
Bell Bruce F.
Northwest Aluminum Technologies
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