Process for the electrolytic deposition of aluminum using a composite anode

• US 4,670,110 A
• Filed: 09/26/1980
• Issued: 06/02/1987
• Est. Priority Date: 07/30/1979
• Status: Expired due to Term

• Current Assignee: Metallurgical Incorporated
• Original Assignee: Metallurgical Incorporated
• Inventors: Withers, James C., Upperman, Gary V.
• Primary Examiner(s): Williams, Howard S.
• Application Number:
  US06/191,344
• Time in Patent Office: 2,440 Days
• Field of Search: 204/67, 204/243 R, 204/294
• US Class Current: 205/362
• CPC Class Codes: C25C 3/06 of aluminium
• CPC Class Codes: C25C 3/12 Anodes

Abstract

A process for the electrolytic deposition of aluminum at low temperatures and low electrical potential in which the anode is the sole source of aluminum and comprises a composite mixture of an aluminous material such as aluminum oxide and a reducing agent. The composite anode is positioned in the electrolyte with at least one active surface of the anode in opposed relationship to but spaced from the surface of the cathode. The greatly increased electrical resistance of the mixture of aluminum oxide and the reducing agent is minimized by passing the anodic current through one or more conductors of low electrical resistivity which extend through the mixture to or approximately to the active reaction face of the mixture in the electrolyte. The position of the ends of said conductors is maintained relative to the reaction face as the mixture is consumed in the electrolysis. These arrangements provide a minimal length of current path through the high resistant mixture and thus result in a low voltage drop of anodic current in its passage to the reaction face. A bipolar electrode arrangement may be employed with the mixture of aluminum oxide and reducing agent covering one face of the electrode with the opposite face of the electrode providing a cathode surface.

Claims

1. The process of producing aluminum from aluminous ore by molten salt electrolysis in a cell having anodic and cathodic bodies and an electrolyte including an aluminum halide salt at a temperature above the melting temperature of aluminum, comprising the steps of:

(a) providing a mixture of aluminous ore and a carbon source with a conductor of higher conductivity than the mixture,

(b) providing a bipolar electrode with two surfaces respectively acting as cathode and anode, and covering the anode surface only with said mixture,

(c) subjecting the mixture to electrolytic reaction at an anodic body reaction site in the electrolyte with said conductor extending below the electrolyte surface to convert ore to an aluminum ionic condition migratory through the electrolyte to the cathodic body and to convert carbon to a carbon oxide gas,

(d) producing an electrolytic reaction at a temperature of the order of 670°

to 900°

C. for consuming at least part of the anodic body to produce aluminum and substantially CO₂ output products.

2. The process of claim 1 including the step of providing an anode to cathode voltage in the range of 1.2 to 3.5 volts.

36. The process of claims 1, 3, 4, 5 or 14 wherein said electrolysis carried out with an electrolyte temperature in the range of 670°

-810°

C., a cell voltage in the range of 2-4 volts and a spacing between the cathode and anode on the order of 0.5 inch or more.

37. The process of claim 36 wherein said electrolysis is carried out with an anode current density in the range of 2-15 amps per square inch.

3. In a process for the electrodeposition of aluminum using an electrolyte cell having a source of electric current, comprising the steps of:

(a) heating a molten electrolyte above the melting temperature of aluminum and below a temperature at which a substantial amount of Al₂ O₃ would dissolve therein and passing a substantially constant voltage of 4 volts or less through the electrolyte from an anode to a cathode,

(b) forming as the anode a combination of an intermixed substance comprising an oxygen containing compound of aluminum in an amount sufficient to provide aluminum during electrolysis and a reducing agent in contact with said compound and presented in a sufficient amount to react at the anode so as to produce aluminum ions in said electrolyte for reduction at the cathode into aluminum metal, the ratio by weight of said aluminum compound expressed as Al₂ O₃ to said reducing agent being at least approximately 1.5 to 1, and an electrically conductive member of higher conductivity than the mixture extending through the substance to the interface between the anode and the electrolyte,

(c) selecting an electrolyte capable of sustaining a reaction at the aluminous ore anode to produce aluminum ions migratory in the molten electrolyte for reduction into aluminum at the cathode,

(d) producing aluminum ions in said electrolyte from said anode compound for reduction to aluminum at the cathode,

(e) depositing electrolytically molten aluminum from the aluminum ions migrating in the electrolyte at the cathode,

(f) positioning a further electrically conductive electrode between said anode and cathode,

(g) contacting one side only of said further electrode with said compound of aluminum and said reduction agent to form an anode, and

(h) operating said electrode in a bipolar mode during the electrodeposition.

4. A process for the electrolytic production of molten aluminum in a cell having a cathode surface and a halide salt electrolyte at a temperature in the range of 670°

to 900°

C., without the evolution of free halogen gases among the gases emanating from the cell as a by product of the electrolytic process, comprising the steps of;

(a) positioning an anode comprising an intimate mixture of an oxygen containing compound of aluminum and an electrically conductive reducing agent in the electrolyte such that at least one surface of said mixture is immersed in the electrolyte and in opposed relationship to but closely spaced from the surface of the cathode, said anodic mixture comprising the sole source of aluminum ore for the electrolytic process,

(b) connecting a source of electrical power to said cathode and through a low resistance conductor to that portion of said anodic mixture approximately adjacent said one surface of said mixture,

(c) applying a voltage on the order of 4 volts or less across said anodic mixture and said cathode to establish a flow of current through a path comprising the reducing agent at said one surface and said electrolyte to said cathode for producing an electrolytic reaction at said one surface of said mixture whereby the aluminum oxide at said one surface is decomposed and converted to aluminum ions which are electrolytically reduced to molten aluminum at the opposing surface of the cathode while maintaining the temperature of the electrolyte above the melting temperature of aluminum, and maintaining the anodic mixture in anodic contact with said power source throughout the electrolytic process.

5. In a process for the electrolytic production of molten aluminum in a cell having a cathode surface, a halide salt electrolyte and an anode body comprising a mixture of an oxygen containing compound of aluminum and an electrically conductive reducing agent, the improvement comprising the steps of:

(a) positioning the anodic mixture in the electrolyte with at least one surface thereof in opposed relationship to but closely spaced from the surface of the cathode,

(b) connecting a source of electrical power to said cathode,

(c) connecting the source of electrical power to said anodic mixture by connecting the source directly by means of a low resistance conductor to that portion of the mixture at least approximately adjacent said one surface of said mixture,

(d) energizing said source of electrical power to apply a voltage across the anode and cathode with the principal anodic current substantially bypassing the bulk of said anodic mixture and following directly to that portion of the mixture at least approximately adjacent said one surface for producing an electrolytic reaction at said one surface in which the aluminum oxide in said mixture is converted to aluminum ions recoverable as molten aluminum at the opposing surface of the cathode, and

(e) maintaining the temperature of the electrolyte above the melting temperature of aluminum in the range of 670°

to 900°

during the electrolytic process.

6. The process of claim 5 and further including the step of feeding said mixture toward the surface of the cathode as said mixture is consumed in the electrolytic process thereby to maintain a relatively constant spacing of said one surface of said anode from the surface of said cathode while maintaining the electrical connection to said anode approximately adjacent said one surface of said anode.

7. The process defined in claim 6 including the step of feeding particles of said aluminous ore mixture into a container surrounding said anode conductor to establish a shortened electrical mixture by a conductivity path flowing to said portion of the mixture adjacent said surface to thereby electrically contact the said anode conductor beneath the surface of the electrolyte.

8. The process defined in claim 7 including the step of providing said container of a construction porous to the molten electrolyte and impervious to passage of aluminous ore particles.

9. The process defined in claim 6 including the step of electrolytically conducting current by way of the anodic body to the cathodic surface through the electrolyte at a substantially constant voltage without producing substantial changes of cell resistance due to changes in aluminum ion concentration in the electrolyte.

10. The process defined in claim 6 including the steps of introducing aluminous ore only as a constituent of the anodic body and maintaining the electrolyte at a substantially constant aluminum ion concentration by means of said reaction at the anodic body.

11. The process of claim 6 including the steps of feeding the anodic body into the electrolyte continuously as the body is consumed to maintain a substantially constant spacing between the electrode bodies.

12. The process of claim 6 including the step of maintaining an anodic to cathodic body spacing of less than 1 inch (2.54 cm).

13. The process defined in claim 6 including the step of providing an aluminum electrical anodic conductor surrounded with a mixture of aluminous ore and carbon to form said anodic body.

14. In a process for the electrolytic production of molten aluminum in a cell having a cathode, an electrolyte including ions selected from the group consisting of chlorides, fluorides or mixtures thereof and an anode comprising a mixture of an oxygen containing compound of aluminum and an electrically conductive reducing agent held together in anodic contact which serves as the sole source of aluminum ore in the electrolytic production of aluminum, the improvement comprising the steps of:

(a) immersing said anode in the electrolyte with at least one surface positioned in opposed relationship to but spaced from the surface of said cathode for providing an active anode surface at which the aluminum oxide may be converted to aluminum ions recoverable as molten aluminum at the opposing surface of said cathode,

(b) providing conductor means of higher electrical conductivity than said anodic mixture and positioning said conductor means with one end thereof below the level of said electrolyte and approximately adjacent to said one active surface with the other end of said conductor extending out of said electrolyte,

(c) connecting a source of electrical power to said other end of said conductor means and said cathode,

(d) energizing said power source whereby substantially the entire anodic current flows directly from said power source through said conductor means to at least the mixture adjacent the end of said conductor means and to said active surface for producing an electrolytic reaction at said active surface, and

(e) replenishing the anodic mixture at said active surface as it is consumed in the electrolytic process while maintaining the position of the end of said conductor means relative to said active anode surface substantially unchanged.

15. The process defined in claim 14 including the step of maintaining the electrolyte in a substantially constant constituency by providing the aluminum ore as a carbon-aluminum oxide mixture of a ratio producing substantially solely CO₂ as an output gas product from the consumption of the anode in the electrolytic process.

16. The process defined in claim 14 including the step of providing an electrolyte including an aluminum halide but excluding aluminum ore.

17. The process defined in claim 14 including the step of providing said aluminum ore as a highly purified aluminum oxide.

18. The process defined in claim 17 including the step of depositing a molten aluminum of a purity of the order of 991/2%.

19. The process defined in claim 14 including the step of providing an electrolyte with a significant percentage of fluoride constituting the halide content.

20. The process defined in claim 19 including the step of operating said electrolyte composition at a temperature too low to dissolve appreciable aluminous ore into the electrolyte but above the melting point of aluminum.

21. The process defined in claim 14 including the step of providing an electrolyte heavier than aluminum so that aluminum floats on top of the electrolyte but is isolated from the anode.

22. The process defined in claim 20 including the step of operating over an uncritical range of temperature exceeding the melting temperature of aluminum by at least 10°

C.

23. The process defined in claim 14 including the step of operating the cell over an uncritical range of voltage and current density in a voltage range between 1.2 and 6 volts.

24. The process of claim 14 including the steps of

positioning a further electrically conductive electrode between said anode and cathode,

contacting said further electrode with said compound of aluminum and said reducing agent, and

operating said electrode in a bipolar mode during the electrodeposition.

25. The process of claim 14 including the step of providing a porous membrane so positioned in said electrolyte to contain said compound and said reducing agent for reaction in conductive contact with the anode electrode.

26. The process of claim 14 including the step of maintaining a spacing between the anodic and cathodic bodies of between 0.25 and 1.0 inches (0.6 and 2.5 cm).

27. The process defined in claims 4 or 14 including the step of providing at least one aluminum electrical anodic conductor positioned to extend through the anodic body substantially to said one surface and proportioned in cross-sectional area, relative to the cross-sectional area of the anodic body exclusive of said conductors, to absorb heat from the electrolyte at a rate sufficient to melt the lower extremities of the aluminum to a position slightly recessed into the mixture of the anodic body approximately at the rate at which the anodic body is consumed by the electrolytic reaction.

28. The process defined in claims 4 or 14 including the step of maintaining approximately constant anode to cathode spacing as the anodic mixture is consumed by the electrolytic reaction.

29. The process of claim 14 including the step of providing said anode electrode structure with one or more aluminum conductors extending through the vertical dimension of said structure so that current passes through said conductors substantially to the anode-electrolyte interface.

30. The process of claim 14 wherein said anodic mixture is replenished by feeding the anodic mixture toward said cathode surface while maintaining said conductor means in a fixed position.

31. The process of claim 14 wherein said conductor means is positioned to extend through said anodic mixture and is of a material and cross-section proportioned to permit said conductor means to melt into the electrolyte during the electrolytic process at a rate substantially corresponding to the rate at which said anodic mixture is consumed,

said anodic mixture being replenished by feeding said anodic mixture and said conductor means toward said cathode.

32. The process of claim 14 wherein said anodic mixture comprises particles of aluminum oxide and said reducing agent and further including the step of containing said particles within a membrane having a pore structure of a size to prevent passage of said particles through the membrane but to permit free passage of ionic aluminum and electrolyte for reaction of the particles within said electrolyte at said active surface.

33. The process of claim 32 wherein said conductor means is positioned to extend substantially through said anodic mixture and is of a graphite material;

the step of replenishing said anodic mixture comprising feeding said mixture into said membrane container to surround said conductor means.

34. The process defined in claims 4, 5 or 14 including the step of retaining undissolved impurities in the region of the anode by confining the anode region by a membrane porous to the electrolyte but not the undissolved impurities.

35. The process of claim 14 wherein a plurality of conductor means are positioned in said anodic mixture and spaced from each other a distance in the range of 1 to 6 inches.

38. A process for producing aluminum by molten salt electrolysis which comprises

electrolyzing an electrolyte melt predominantly containing at least one member selected from the group consisting of alkali halides and alkaline earth halides in an electrolytic cell, said cell containing a cathode resistant to said electrolyte melt and a permanent composite anode comprising a consumable section consisting essentially of a plurality of particles comprising a mixture of aluminum oxide and a carbonaceous material, and a non-consumable graphite section, said non-consumable section having a lower electrical resistance than said consumable section,

the ratio by weight of said aluminum oxide to said carbonaceous material in said consumable anode section being between about 5:

1 and about 3;

1,

replenishing the particulate anode material in said cell as said particulate material is consumed and without halting the operation of said cell, and

recovering aluminum metal in liquid form at the bottom of said cell.

39. An electrolyte cell for producing aluminum by the electrolysis of a molten salt composition which comprises

a cathode consisting essentially of at least one member selected from the group consisting of graphite and titanium diboride,

at least one permanent composite anode comprising a consumable particle portion said particles comprising a mixture of aluminum oxide and carbonaceous materials wherein the ratio by weight of said aluminum oxide to said carbon is between about 5:

1 and 3;

1, and a non-consumable anode portion in electrical contact with said particles, said non-consumable portion comprising graphite and said consumable portion being replaceable without halting operation of said cell,

an electrolyte in said cell consisting of a melt predominantly containing at least one member selected from the group consisting of alkali halides and alkaline earth halides, said electrolyte being in contact with at least a portion of said anode, and

means for replenishing said consumable particles in said anode during continuous operation of said cell.