Cast cermet anode for metal oxide electrolytic reduction

Abstract

The invention is a method for producing a cast cermet anode for metal oxide electrolytic reduction by feeding metallic iron and metallic nickel in solid form to an oxidizing reactor; melting and oxidizing the iron and nickel and forming molten nickel ferrite; mixing the molten nickel ferrite with a base metal of high electrical conductivity such as nickel, copper, silver, or alloys thereof in a holding vessel such as ladle or tundish, and casting the mixture into a mold to form a near net shape of the desired anode. Apparatus for carrying out the method, and the resulting product are also disclosed.

Description

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for producing a cast cermet anode for metal oxide electrolytic reduction, and a cast cermet anode product.

BACKGROUND OF THE INVENTION

Since the advent of the electrolytic reduction process for producing aluminum, the anodes used have been made of carbon which is consumed during the electrolytic reduction process. In the more recent past (20 years), there has been an effort to produce an inert anode or electrode that is not consumed during reduction. Metal anodes, ceramic anodes, and cermet anodes have been proposed for development. Of these, the cermet anode has been at the forefront of the development race. According to published patents, the best available practice to produce a cermet anode heretofore has been to mix ceramic and metal powders with a binder, press at very high pressures, then sinter at high temperature. Specifically, nickel ferrite (NiFe₂O₄) powder has been mixed with metallic copper powder and copper/silver alloy powder, a binder added, and the mixture pressed and sintered to make the cermet anode. The manufacture of a nickel ferrite powder is a complex and expensive process. The subsequent processing of the nickel ferrite by blending and mixing with copper powder and organic binder, followed by pressing at high pressure, then followed by sintering at high temperatures (greater than 1300 C) for long times, is also quite complex and expensive.

DESCRIPTION OF THE PRIOR ART

Applicant is aware of the following U.S. patents concerning cermet electrodes for electrolytic reduction of aluminum:

U.S. Pat. No.	Inventor	Title
5,865,980	Ray et al.	ELECTROLYSIS WITH A INERT
		ELECTRODE CONTAINING A
		FERRITE, COPPER AND SILVER
6,126,799	Ray et al.	INERT ELECTRODE CONTAINING
		METAL OXIDES, COPPER AND
		NOBLE METAL
6,030,518	Dawless et al.	REDUCED TEMPERATURE
		ALUMINUM PRODUCTION IN AN
		ELECTROLYTIC CELL HAVING AN
		INERT ANODE
6,217,739	Ray et al.	ELECTROLYTIC PRODUCTION OF
		HIGH PURITY ALUMINUM USING
		INERT ANODES
6,332,969	Ray et al.	INERT ELECTRODE CONTAINING
		METAL OXIDES, COPPER AND
		NOBLE METAL
6,372,119	Ray et al.	INERT ANODE CONTAINING
		OXIDES OF NICKEL IRON AND
		COBALT USEFUL FOR THE
		ELECTROLYTIC PRODUCTION OF
		METALS

SUMMARY OF THE INVENTION

The invention provides a method for producing a cast cermet anode for metal oxide electrolytic reduction by feeding metallic iron and metallic nickel in solid form to an oxidizing reactor; melting and oxidizing the iron and nickel and forming molten nickel ferrite; mixing the molten nickel ferrite with a base metal of high electrical conductivity, such as nickel, copper, silver, or copper/silver alloy, in a holding vessel such as a ladle or tundish, and casting the mixture into a mold to form a near net shape of the desired anode.

The invention also comprises apparatus for producing a cast cermet anode for metal oxide electrolytic reduction, comprising an oxidizing reactor; means for feeding metallic iron and metallic nickel to the oxidizing reactor; a ladle or tundish positioned for receiving molten material from the reactor; means for adding high electrical conductivity metal to the ladle or tundish; a mold positioned to receive molten material from the ladle or tundish; and means for discharging molten material from the ladle or tundish into the mold to form the anode.

The invention also comprises the product of the method, a cast cermet anode for metal oxide electrolytic reduction comprising from about 75 to about 95% ceramic material, consisting of one or more of nickel ferrite, iron ferrite and nickel oxide, and from about 5 to about 25% of a base metal or base metal alloy, preferably copper, copper-silver alloy, nickel, nickel-copper alloy, silver, or nickel-copper-silver alloy.

OBJECTS OF THE INVENTION

The principal object of the present invention is to provide a process for the manufacture of cast cermet type inert anodes for the electrolytic reduction of metal oxides.

Another object of the invention is to provide a process for the manufacture of cermet type inert anodes that is simpler and more cost efficient than the current state of the art of cermet anode manufacture.

Another object of the invention is to produce a cermet anode that has as good or better properties of conductivity, strength, and resistance to attack by the electrolyte than sintered cermet anodes.

Another object of the invention is to provide a process that allows near net shape casting of an inert cermet anode.

A further object of this invention is to provide apparatus for the manufacture of cermet type cast inert anodes.

Another object of the invention is to provide an anode useful in the chlor-alkali industry for the electrolysis of brine to produce sodium hydroxide and chlorine.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will become more readily apparent by referring to the following detailed description and the appended drawings in which:

FIG. 1 is a schematic diagram of the method and apparatus of the preferred embodiment of the invention.

FIG. 2 is a schematic diagram of the method and apparatus of an alternative embodiment of the invention.

FIG. 3 is a schematic diagram of the method and apparatus of another alternative embodiment of the invention.

FIG. 4 is a schematic diagram of the method and apparatus of another alternative embodiment of the invention.

FIG. 5 is a schematic diagram of the method and apparatus of another alternative embodiment of the invention.

FIG. 6 is a schematic diagram of the method and apparatus of a further alternative embodiment of the invention which utilizes a melting furnace rather than an oxidizing furnace.

FIG. 7 is a schematic diagram of alternative embodiment of the invented method utilizing a variation of feed materials to the oxidizing furnace.

DETAILED DESCRIPTION

Referring now to the drawings, and particularly to FIG. 1, molten metal oxide is formed by oxidizing iron and other metal(s) to form a molten ferrite of the general formula A_(x)B_(1-x)Fe₂O₄ where A&B are divalent metal ions such as Mg, Ni, Mn, Co, Fe and Zn; and x can vary from 0 to 1.0. The molten ferrite is then mixed with a base metal of high electrical conductivity in percentages of base metal from about 5% up to about 25%. The molten mixture of the ferrite and the base metal such as nickel, copper, silver, copper-silver alloy, nickel-copper alloy, or nickel-silver-copper alloy, is then cast, solidified and cooled. The mixture may be cast into a near net shape of the desired cermet anode. An electrical connector may be attached to the cermet anode by cementing after cooling, or by insertion during the time the anode is molten in the mold.

In a preferred embodiment, metallic iron and metallic nickel in briquet form from source 10 are fed to an oxidizing reactor 12 wherein the iron and nickel are melted and oxidized by oxygen from a source 14. The iron and nickel are fed into the reactor in a molar ratio of: Fe/Ni=2/1. A molten nickel ferrite of formula Ni Fe₂O₄ is formed.

It is possible to utilize a molar ratio of Fe/Ni of greater than 2/1 to produce a mixture of nickel ferrite (NiFe₂O₄) and iron ferrite (Fe₃O₄). It is also possible to operate with a molar ratio or Fe/Ni less than 2 in order to produce a nickel ferrite plus excess nickel oxide (NiO).

The molten nickel ferrite is discharged from the oxidizing reactor at a temperature sufficient to maintain it in the molten state plus sufficient superheat to melt the base metal being added thereto. The molten nickel ferrite is discharged through outlet 16 into a receiving and holding vessel 18 such as a tundish or ladle. Copper or copper/silver alloy 20 is added and mixed into the molten nickel ferrite in the holding vessel in which the base metal melts. The base metal is kept in suspension by gas stirring and not allowed to separate from the nickel ferrite and settle to the bottom of the ladle. The gas 22 used for stirring can be an inert gas or it can be an oxygen-containing gas, including oxygen and air. The gas can be injected through a gas injection port or inlet 24 or through a lance 26. The copper that is added can be in the form of powder or larger particles that are readily melted. The ladle can be heated to prevent the molten mixture from solidifying.

The injection of oxygen containing gas, in addition to stirring the molten material, can be used to oxidize all or part of any nickel metal carried over from the oxidizing reactor 12 or melter 13 (FIG. 6). Nickel metal from the oxidizing reactor or melter that is not oxidized in the ladle will become part of the base metal system of nickel, copper, silver and combinations or alloys thereof. It is preferable that oxidation be incomplete, leaving some free metal in the product to be cast. Injection of an inert gas for stirring may be used to insure that nickel metal is not oxidized when it is desired to have metallic nickel in the base metal system. Inert gas will generally be used when adding copper and/or silver.

Vacuum degassing of the ladle may be employed to remove entrapped gases and minimize porosity of the resulting final cast anode product.

The mixture of nickel ferrite and copper is then removed from the holding vessel and cast into a mold to form a near net shape inert anode 32, after which it is allowed to cool. Controlled cooling rates, post-heat treatment, and bubbling of argon gas for coalescing and removal of entrapped gases may be employed as methods for reducing stresses and porosity in the cast anode. During the solidification process an electrical connector rod 34 made of nickel may be inserted into the still-molten nickel ferrite/copper casting. The finished product is a cast inert anode 32 of correct shape with the electrical connector 34 attached.

A suitable post-heat treatment can be annealing in the presence of an oxygen-containing gas. This controls the cooling rate, and assures that the metal on the outer surface of the anode is oxidized, which makes it resistant to attack by electrolyte solutions.

Molten and cast material inherently has better resistance to attack by molten salt bath solution than a sintered material because the true density of the cast material is greater than that of sintered material because of the lack of voids.

Alternative Embodiments

Iron and nickel feed material may be provided in metallic form other than briquets, such as from punchings, turnings, or other high purity solid form.

In an alternative embodiment of the method, as shown in FIG. 2, copper or base metal alloy may be added between the melting vessel and the tundish or ladle by introducing it into the discharge runner 16 between the melting vessel 12 and the tundish 18. The base metal or base metal alloy is advantageously added in the form of wire or wire rod for accurate control of the molten cermet composition, or the base metal may be added in powder form.

In another alternative embodiment of the method, as shown by dotted lines in FIG. 2, copper or base metal alloy 20 may be added directly to the casting mold 30 before or during filling of the mold with the molten nickel ferrite. The base metal or alloy may be added in the form of wire, wire rod, or powder.

In another alternative embodiment, shown in FIG. 3, copper oxide is fed into the reactor in which it is reduced to copper by reaction with metallic iron and metallic nickel, and it oxidizes the metallic iron and metallic nickel. Alternatively, copper oxide, along with metallic iron and metallic nickel (in the same ratio as it is or would be fed into the reactor) may be fed from source 40 to the tundish by feed 42, or to the mold 30 by feed 44, as shown in FIG. 4.

As shown in FIG. 5, copper oxide, iron oxide and nickel oxide from source 48 can be fed into the reactor 12, with metallic nickel and iron, to form molten nickel ferrite. Additional base metal or base metal alloy can be introduced to the tundish 18 or the mold 30, as desired. An electrical connector rod 34 (as in FIG. 2), preferably made of nickel, may be inserted into the still molten anode casting 32. The resulting product is as described earlier, a cast inert anode of correct shape with the electrical connector attached.

In the alternative embodiment shown in FIG. 6, solid nickel ferrite 50, or a mixture 52 of nickel ferrite, nickel oxide and iron oxides, such as hematite or iron ferrite, is melted in a melting furnace or vessel 13, which in this case is not an oxidizing vessel, to form molten nickel ferrite. The melting vessel 13 is a gas fired furnace, induction furnace, or electric arc furnace. Other iron-containing or nickel-containing compounds, such as metal sulfides or carbonates, can be used as feed material in place of the metal oxides 50, 52.

In the alternative embodiment shown in FIG. 7, mostly metallic iron and nickel 10 are fed to the reactor 12 along with some iron oxide and/or nickel oxide, which are melted and oxidized to form molten nickel ferrite. There is sufficient exothermic heat available from the oxidation of nickel and iron to allow the use of nickel oxide and iron oxide as feed materials to the reactor. The molten nickel ferrite is then discharged into a ladle or tundish 18 and further treated according to the remaining steps of the method to form a cast cermet anode 32. In this embodiment, solid nickel oxide 54, or iron oxide 58, or a mixture of nickel oxide and iron oxide 60 is introduced to and melted in an oxidizing reactor or vessel 12, to form molten nickel ferrite.

It is also to be understood that a cermet type inert anode made from a ferrite may be used in electrolytic reduction processes besides aluminum reduction, such as electrolytic reduction of magnesium, lithium, or calcium. A cast cermet anode useful in the chlor-alkali industry for the electrolysis of brine to produce sodium hydroxide and chlorine, comprises about 75 to about 95% ceramic, selected from the group consisting of nickel ferrite, iron ferrite, nickel oxide, and mixtures thereof, and from about 5 to about 25% base metal or base metal alloy.

Summary of the Achievement of the Objects of the Invention

From the foregoing, it is readily apparent that we have invented an improved process for the manufacture of cast cermet type inert anodes that is simpler and more cost efficient than the current state of the art of cermet anode manufacture, and that allows near net shape casting of an inert cermet anode; a cast cermet anode product that has as good or better properties of conductivity, strength, resistance to attack by the electrolyte than sintered cermet anodes, and apparatus for the manufacture of cermet type cast inert anodes.

It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the apparatus by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.

Claims

1. A method for producing a cast cermet anode for metal oxide electrolytic reduction, comprising the steps of: feeding metallic iron and metallic nickel in solid form to an oxidizing reactor; melting and oxidizing the iron and nickel and forming molten nickel ferrite; discharging molten nickel ferrite from the oxidizing reactor at a temperature sufficient to maintain the molten nickel ferrite in the molten state; adding a base metal of high electrical conductivity to the nickel ferrite to form a mixture; and casting the mixture into a mold to form a near net shape of the anode.

2. A method according to claim 1 further comprising attaching an electrical connector to said anode.

3. A method according to claim 1 wherein the metallic iron and metallic nickel are fed in briquet form to the oxidizing reactor.

4. A method according to claim 1 wherein the base metal is selected from the group consisting of nickel, copper, silver, copper-silver alloy, nickel-copper alloy, and nickel-copper-silver alloy.

5. A method according to claim 1, further comprising feeding iron oxide and nickel oxide to the oxidizing reactor.

6. A method according to claim 1 wherein the molten nickel ferrite is discharged into a holding vessel and base metal is kept in suspension in the holding vessel by gas stirring.

7. A method according to claim 6 wherein gas stirring is carried out with an inert gas or an oxygen-containing gas.

8. A method according to claim 7, wherein said oxygen-containing gas is air or oxygen.

9. A method according to claim 1 wherein the base metal is maintained in a molten state.

10. A method according to claim 1 wherein the metal oxide for electrolytic reduction is selected from the group consisting of aluminum, magnesium, lithium, and calcium oxides.

11. A method according to claim 1 wherein the base metal forms 5 to 25% of said mixture.

12. A cast cermet anode product for metal oxide electrolytic reduction made by the method of claim 1.

13. A method for producing a cast cermet anode for metal oxide electrolytic reduction, comprising the steps of: feeding at least one compound selected from the group consisting of nickel oxides, iron oxides, nickel ferrite, iron sulfides, nickel sulfides, iron carbonates, nickel carbonates, and mixtures thereof to the melting vessel; melting the compounds and forming molten nickel ferrite; discharging molten nickel ferrite from the melting vessel at a temperature sufficient to maintain the molten nickel ferrite in the molten state; adding a base metal of high electrical conductivity to the nickel ferrite to form a mixture; and casting the mixture into a mold to form a near net shape of the anode.

14. Apparatus for producing a cast cermet anode for metal oxide electrolytic reduction, comprising: an oxidizing reactor; means for feeding metallic iron and metallic nickel to said oxidizing reactor; means for discharging molten material from said oxidizing reactor; a ladle or tundish positioned for receiving molten material from said oxidizing reactor; means for adding high electrical conductivity metal to said ladle or tundish; a mold positioned to receive molten material from said ladle or tundish; and means for discharging molten material from said ladle or tundish into said mold to form said anode.

15. Apparatus according to claim 14, further comprising means for providing stirring action in said ladle or tundish.

16. Apparatus according to claim 15, wherein said means for providing stirring action is a gas injection system.

17. Apparatus according to claim 15, wherein said means for providing stirring action is a lance communicating with a source of gas, said gas being an inert gas or an oxygen-containing gas.

18. Apparatus according to claim 14, further comprising means for adding heat to molten material within said ladle or tundish.

19. Apparatus according to claim 14, further comprising means for heating said ladle or tundish to prevent molten material therein from solidifying.

20. Apparatus for producing a cast cermet anode for metal oxide electrolytic reduction, comprising: a melting vessel; means for feeding iron and nickel compounds to said melting vessel; means for discharging molten material from said vessel; a ladle or tundish positioned for receiving molten material from said vessel; means for adding high electrical conductivity metal to said ladle or tundish; a mold positioned to receive molten material from said ladle or tundish; and means for discharging molten material from said ladle or tundish into said mold to form said anode.

21. Apparatus according to claim 20, further comprising means for providing stirring action in said ladle or tundish.

22. Apparatus according to claim 21, wherein said means for providing stirring action is a gas injection system.

23. Apparatus according to claim 21, wherein said means for providing stirring action is a lance communicating with a source of gas, said gas being an inert gas or an oxygen-containing gas.

24. Apparatus according to claim 20, further comprising means for heating said ladle or tundish to prevent molten material therein from solidifying.

25. Apparatus according to claim 20, wherein said melting vessel is selected from the group consisting of a gas fired furnace, an induction furnace, or an electric arc furnace.

26. A cast cermet anode for metal oxide electrolytic reduction comprising: from about 75 to about 95% ceramic, selected from the group consisting of nickel ferrite, iron ferrite, nickel oxide, and mixtures thereof; and from about 5 to about 25% base metal or base metal alloy.

27. A cast cermet anode according to claim 26, wherein said base metal or base metal alloy is selected from the group consisting of nickel, silver, copper, copper-silver alloy, copper-nickel alloy, and copper-nickel-silver alloy.

28. A cast cermet anode according to claim 26, comprising: from about 75 to about 95% nickel ferrite; and from about 5 to about 25% copper or copper-silver alloy.

29. A cast cermet anode according to claim 26, comprising: about 85% nickel ferrite; and about 15% copper or copper-silver alloy.

30. A cast cermet anode useful in the chlor-alkali industry for the electrolysis of brine to produce sodium hydroxide and chlorine, said anode comprising: from about 75 to about 95% ceramic, selected from the group consisting of nickel ferrite, iron ferrite, nickel oxide, and mixtures thereof; and from about 5 to about 25% base metal or base metal alloy.