Cermet Anode With Continuously Dispersed Alloy Phase and Process for Making

This invention relates to cermet electrodes for use in the electroytic reduction of a metal from a metal compound dissolved in a molten salt. The invention has specific application in the production of anodes and the electrolytic process for manufacture of aluminum in Hall-Heroult cells.

This invention was made with government support under Contract No. DE-AC06-76RLO 1830 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

Electrolytic cells, such as a Hall-Heroult cell for aluminum production by the electrolysis of alumina in molten cryolite, conventionally employ conductive carbon electrodes. The Hall-Heroult process reduces aluminum metal from alumina in a molten salt electrolyte and consumes carbon from a carbon anode in the process. The anode liberates oxygen from the alumina, which results in aluminum metal being collected on the cathode. The oxygen combines with carbon to produce CO and CO.sub.2. The overall reaction in its simplest form is represented as follows: ##STR1## Approximately 0.33 pounds of carbon are consumed for every pound of aluminum produced. The life of carbon anodes is typically two to three weeks.

Carbon obtained from petroleum coke is typically used for fabrication of such anodes. Such material is becoming increasingly expensive. The petroleum coke also typically contains significant quantities of impurities such as sulfur, silicon, vanadium, titanium, iron and nicke. Such impurities can contaminate the metal being produced as well as cause environmental problems and poor working conditions. Removal of excess quantities of such impurities requires extra and costly steps when high purity aluminum is to be produced.

If no carbon were consumed in the reduction of alumina, the overall reaction would be: 2Al.sub.2 O.sub.3 .fwdarw.4Al +30.sub.2. Accordingly, non-consumable anodes could be used in a process where carbon does not enter into the electrolytic reaction. Such anodes would have a life limited only by corrosion due to the caustic cryolite electrolyte and electrochemical degradation mechanisms. It is anticipated that the life of such anodes could be extended to several months or even a year or more as compared to the two to three week life of a carbon anode which is consumed in the electrolytic reduction reaction. Furthermore, non-consumable anodes would presumably not add the same significant quantities of impurities as do carbon anodes.

Numerous attempts have been made to develop an inert electrode, but apparently without the required degree of success to make it commercially feasible. The entire aluminum industry still uses consumable carbon anodes in the production of aluminum. Many of the newly developed inert electrodes apparently are reactive or corroded by the electrolyte to an extent which results in contamination of the metal being produced as well as consumption of the anode due to corrosion.

There have been numerous suggestions for non-consumable anode compositions based on various ceramic oxides and oxy compounds. Such materials typically behave as a semi-conductor having inherently low electrical conductivities on the order of 1 ohm .sup.-1 cm.sup.-1. (All electrical conductivities referred to in this document related to conductivity at normal cell operating temperatures of 900.degree. C. to 1,000.degree. C.) Attempts have been made to fabricate non-consumable electrodes with special compositions known as cermets. A cermet composition includes both metallic and ceramic phases. Cermets typically have higher electrical conductivity than pure ceramic compositions, and improved corrosion resistance as compared to metals. The conventional method of preparing cermet compositions is to mix metal and ceramic powders. cold press a preform, and sinter the preform at an elevated temperature in a controlled atmosphere. Alternatively, the cermet can be prepared by hot pressing or hot isostatic pressing wherein the sintering operation is carried out under pressure. Other densification methods for forming oxides and metals into cermets may also be usable.

One promising oxide system identified for use with cermets is the NiO-NiFe.sub.2 O.sub.4 system. However, most cermets using this oxide system, or other oxide systems, still have low electrical conductivities, on the order of 1 to 10 ohm.sup.-1 cm.sup.-1. Prior art oxide systems containing nickel in the metal phase (a NiO-NiFe.sub.2 O.sub.4 -Ni cermet) have been fabricated by reaction sintering processes and exhibit excellent electrical conductivity on the order of 300 ohm.sup.-1 cm.sup.-1. This is even an improvement over the conductivity of carbon anodes which typically average approximtely 200 ohm.sup.-1 cm.sup.-1. However, typical cermets have a discontinuous metal phase distributed throughout the oxide phase and are susceptible to anodic dissolution by corrosion. This causes anodes of such material to fail prematurely and also contaminate the produced aluminum with nickel.

Copper has also been incorporated into the NiO-NiFe.sub.2 O.sub.4 matrix creating an NiO-NiFe.sub.2 O.sub.4 -Cu cermet. The Cu metal phase is discontinuously distributed within the oxide matrix, but still provides improved electrical conductivity on the order of 60 to 70 ohm.sup.-1 cm.sup.-1. Such a material had a copper content of 17 weight percent.

For example, U.S. Pat. No. 4,620,905 to Tarcy et al. discloses an NiO-NiFe.sub.4 O.sub.4 -Cu-Ni cermet wherein 17% of the composition is comprised of a metal alloy of copper and nickel. The nickel metal is understood to arise primarily from the reduction of excess NiO in the oxide phase induced by the presence of carbon-based binders used to produce the oxide powders (col. 5, lines 3-14).

U.S. Pat. Nos. 4,374,761; 4,478,693; 4,399,008; and 4,374,050 to Ray and 4,455,211 to Ray et al. also disclose non-consumable cermet electrodes for use in molten salt electrolysis. The electrodes disclosed are stated to be comprised of ceramic oxide compositions having at least one metal powder disbursed therethrough for purposes of increasing electrical conductivity. The metal powder is stated to be selected from the group consisting of Co, Fe, Ni, Cu, Pt, Rh, In, and Ir or alloys thereof. The metal is also stated to be provided in the electrode composition in amounts not constituting more than 30 volume percent metal. Additionally, elemental copper is stated to be includable in an amount up to 30 weight percent of the finished composition using the Ray processes. However, no example in any of these patents supports the broad statements concerning achieving high metal content in a cermet. Further, the metal is indicated as being coated with a wax binder to prevent the metal particles from oxidizing during the sintering step. Additionally, the electrical conductivities of the example electrodes range from 0.4 ohm.sup.-1 cm.sup.-1 to 32 ohm.sup.-1 cm.sup.-1.

Prior to the present invention, 17% metal alloy of copper and nickel was understood to be at or close to the practical upper limit of including an alloy of copper and nickel or pure copper within a NiO-NiFe.sub.2 O.sub.4 oxide system, despite statements in the above prior art patents. Apparently in these processes, additions of copper above this limit merely bled out of the composite during sintering, forming small copper metal beads on the surface of the cermet. Further, the alloy phase is discontinuously distributed throughout the oxide phase.

The following disclosure of the invention is submitted in compliance with the constitutional purpose of the Patent Laws "to promote the progress of science and useful arts" (Article 1, Section 8).

It has been discovered that addition of an effective amount of a metallic catalyst/reactant to a base composition of a nickel/iron/oxide, NiO, copper, and nickel provides a stable electrode having significantly increased electrical conductivity. The metallic catalyst/reactant functions to disperse the copper and nickel as an alloy continuously throughout the oxide phase of the cermet to render the electrode composition more highly electrically conductive than were the third metal not present in the base composition. The third metal is preferably added to the base mixture as elemental metal in micronized form which is converted to a metal oxide during the sintering process. Metallic catalyst/reactants anticipated to be usable in accordance with the invention include aluminum, magnesium, sodium and gallium.

In a preferred process for producing such an electrode, NiO and NiFe.sub.2 O.sub.4 powders are first combined to provide a mixture having a weight concentration of NiO to NiFe.sub.2 O.sub.4 preferably from 2:3 to 3:2. Other oxide powder combinations would also be usable, but a combination that produces a NiO-NiFe.sub.2 O.sub.4 oxide phase in the finished product is preferred. The NiO-NiFe.sub.2 O.sub.4 oxide in this process was a fully reacted, calcined, and spray-dried powder with agglomerates of approximately 50 microns. This was used due to availability, and is of little importance since these agglomerates were broken down to micron size particles during milling. However, small and high surface area particles with a binder provided during spray-drying are considered important in achieving optimum results. The oxide powder combination could also be prepared during milling along with addition of the metals or metal alloy powders, then spray dried to provide the agglomerates to improve powder flowability or packing.

The NiO and NiFe.sub.2 O.sub.4 mixture is combined with quantities of elemental copper powder and elemental nickel powder with an average particle size of approximately 2 microns. Alternatively, a copper-nickel alloy powder can be combined with the combined oxide powders. The third micronized catalyzing metal is also combined with the other materials to produce a base mixture. Broad concentrations of the base mixture should be from 10 to 30 weight percent copper, 0.1 to 10 weight percent nickel, and from 0.5 to 5.0 weight percent of the third catalyzing metal. The remainder of the mixture consists essentially of NiO-NiFe.sub.2 O.sub.4 powder. Preferred concentration ranges are from 20.0 to 30.0 weight percent copper, from 2.0 to 4.0 weight percent nickel, and from 0.5 to 1.5 weight percent of the catalyzing metal.

The base powder mixture is blended by simple shaker-mixing techniques or more preferably, vibrationally milled to more intimately mix the various constituents. Simple blending procedures are typically performed dry. Vibration milling can be used to produce cermets which have a more uniform, homogeneous distribution of the metal phase than possible by using simpler shaker-mixing blending procedures. For vibration milling, stainless steel mixing balls are added to a mixing bottle containing the base powder mixture. A Freon (tm) based liquid solution is preferably used as a milling solution, or the mixture can be milled dry. The Freon (tm) solution functions as a lubricant which volatilizes from the mixture after completion of the milling. The base mixture-solution is vibratory milled from between 0.5 hours to 24 hours and allowed to dry.

The milled base powder mixture is then formed into desired green-body shapes using conventional pressing techniques. The final pressure of the formed mixture will preferably be approximately 25 Kpsi which provides sufficient strength for handling and machining of such green-bodies. The green-bodies are next placed into a sintering furnace having controlled atmosphere capabilities. The furnace preferably has alumina walls as opposed to metal walls which have been shown to cause excessive reduction of nickel and iron from the oxides. The furnace atmosphere is preferably relatively inert containing either argon or nitrogen. Some oxygen is necessary in the range of 100 ppm to 500 ppm, but preferably not higher than 250 ppm to obtain optimum results. The furnace is also preferably ramped to a hold temperature just below the melting point of copper, and held for a period of time up to 50% of the sintering time. The heating rate and hold period allow the alloy to stabilize which contributes to the reduction of metal phase bleedout. This heating cycle is increased to sintering temperatures up to 1300.degree. C. for a holding period of up to 8 hours. This hold time at temperature, as well as heating and cooling down rates, is dependent upon the physical size and mass of the anode being produced.

At the sintering temperatures, the catalyzing metal combines with copper, nickel, NiO, and NiFe.sub.2 O.sub.4 powders to produce a composition of NiO-NiFe.sub.2 O.sub.4 -Cu-Ni having Cu and Ni continuously dispersed as an alloy. Where the third catalysting metal is aluminum, the reaction is represented as follows: ##STR2##

The amount of Fe in Ni.sub.y Fe.sub.1-y O is typically negligible, resulting in the industry accepted abbreviation NiO. The Cu.sub.x Ni.sub.1-x alloy phase generated typically also contains a negligible amount of iron. The quantity of Al.sub.2 O.sub.3 formed in the finished system is also negligible. However, the sintering produces a continuous Cu-Ni alloy phase dispersed throughout the oxide matrix which gives the sintered material metallic properties in terms of electrical conducitivity. Electrical conductivities well in excess of 100 ohm.sup.-1 cm.sup.-1 are achievable.

Although the disclosure refers primarily to the combination of NiO with NiFe.sub.2 O.sub.4, alternate NiO based oxide systems would be usable by skilled artisans without departing from the principles and scope of the invention.

An electrode was produced using the above-described process wherein the third catalyzing metal was aluminum. The concentrations in the base powder mixture were as follows:

71 weight percent NiO-NiFe.sub.2 O.sub.4 powder (spray dried, approximately 50 microns agglomerates)

25 weight percent copper (average 2 micron particles)

3 weight percent nickel (average 3 micron particles)

1 weight percent elemental micronized aluminum.

The base powder mixture was vibratory milled using a Freon (tm) based solution for 2.5 hours. The milled powder was pressed into desired shapes to a final pressure of 25 Kpsi using standard pressing techniques. The sample was slowly heated over a 16 hr period to a diffusion soak temperature below the melting point of copper, (preferably about 1075.degree. C.) and allowed to hold for 2 hours. The sample was then further heated at a rate of approximately 100.degree. C. per hr to a sintering temperature of 1200.degree. C. over a period of 8 hours. Oxygen content in the furnace ranged from 150 to 200 ppm. The remainder of the furnace atmosphere consisted essentially of argon. The sample was maintained under these conditions at 1200.degree. C. for a holding period of an additional 8 hours. The material was then furnace cooled at a rate of approximately 100.degree. C. per hour.

Electrical conductivity of the sample was determined to be in excess of 410 ohm.sup.-1 cm.sup.-1 at 950.degree. C. The sample was tested as an anode in a small Hall-Heroult aluminum electrolysis cell. No wear or corrosion was evident after 6 hours of continuous operation at a current density of 1 A/cm.sup.2.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features described, since the methods herein disclosed comprise a preferred form of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims, appropriately interpreted in accordance with the doctrine of equivalents.