Cathode for aluminum producing electrolytic cell

#This invention relates to electrolytic production of aluminum and more particularly, it relates to an improved cathode suited for use in a low temperature electrolytic cell for the production of aluminum.

#In the electrolytic production of aluminum, there is great interest in utilizing a cathode that is highly conductive and does not react with molten aluminum deposited thereon. Carbon cathodes which are traditionally used in the Hall-Heroult cells have the problem that they are not readily wettable with molten aluminum. The carbon cathode surface reacts with the molten aluminum and forms aluminum carbide. Thus, the cathode is depleted at about 2 to 5 cm/yr. during operation of the cell. Or, the carbon cathode has the problem that it permits formation of cyanides, presenting a disposal problem. Thus, the carbon cathode has been replaced or modified with materials to improve its performance.

#For example, U.S. Pat. No. 5,961,811 discloses an improved carbonaceous material suitable for use as a cathode in an aluminum producing electrolytic cell, the cell using an electrolyte comprised of sodium containing compounds. The carbonaceous material is comprised of carbon and a reactive compound capable of suppressing the formation or accumulation of sodium cyanide during operation of the cell, of reacting with sodium to reduce problems associated with sodium intercalation, and of reacting with one of titanium or zirconium to form titanium or zirconium diboride during operation of the cell to produce aluminum.

#FIG. 1 is a cross-sectional view of a test cell employed in testing the cathode of the invention.

#Referring now to FIG. 1, there is shown a laboratory, electrolytic cell 10 for use in electrolytically reducing alumina to aluminum, in accordance with the invention. Cell 10 is comprised of an alumina crucible 12 containing an anode 14 and a cathode 16. An electrolyte 18 also is provided in cell 10. Alumina crucible 12 is positioned within a stainless steel container 20. The inner surface of container 20 and outer surface of the sidewall of crucible 12 are provided in abutting relationship (see FIG. 1).

#Anode 14 is provided in the form of a disc covering bottom 22 of crucible 12. A vertical copper conductor 24 has a lower end thereof connected to disc 14 and upper end thereof connected to a source of electrical current. In FIG. 1, conductor 24 is covered with alumina tube 26 to confine the electrolysis current to anode disc 14. Cathode 16 is also connected to the source of electric current. For purposes of performing tests, cell 10 is placed in a furnace and held at a temperature at which electrolyte 18 is molten, for example, 680° to 800° C. The temperature of electrolyte 18 may be measured continuously using a chromel-alumel thermocouple contained in a closed-end fused alumina tube.

#The electrolytic cell can have an operating temperature less than 900° C. Further, electrolytes that can be employed in the cell can comprise NaF and AlF3 eutectics, KF and AlF3 eutectic and LiF. The electrolyte can contain 6 to 26 wt. % NaF, 7 to 33 wt. % KF, 1 to 6 wt. % LiF and 60 to 65 wt. % AlF3. More broadly, the cell can employ electrolytes that contain one or more alkali metal fluorides and at least one metal fluoride, e.g., aluminum fluoride, and use a combination of fluorides as long as such baths or electrolytes operate at less than about 900° C. For example, the electrolyte can comprise NaF and AlF3. That is, the bath can contain 62 to 53 mol. % NaF and 38 to 47 mol. % AIF3. Examples of electrolytes are described in detail in U.S. Pat. Nos. 5,006,209 and 5,284,562, incorporated herein by reference.