EP2373823A1

EP2373823A1 - Composite materials for wettable cathodes and use thereof for aluminium production

Info

Publication number: EP2373823A1
Application number: EP09817140A
Authority: EP
Inventors: Robert Schulz; Sylvio Savoie
Original assignee: Hydro Quebec
Current assignee: Hydro Quebec
Priority date: 2008-10-02
Filing date: 2009-09-29
Publication date: 2011-10-12
Also published as: CA2739281C; RU2011115571A; AU2009299086B2; RU2487956C2; CN102239272A; EP2373823A4; US20110195228A1; US8741185B2; CA2739281A1; AU2009299086A1; CN102239272B; CA2640206A1; WO2010037220A1

Abstract

The invention relates to a composite material of formula: (C-N-B-MR)x(Al-MR)y(R)Z in which C-N-B-MR is one or more carbides, nitrides or borides of one or more refractory metals of group IV, V or VI of the Periodic Table and/or one or more aluminium carbides, nitrides or borides chosen from Al4C3, AlN, AlB2 and Al1-67B22; Al-MR is one or more aluminides of one or more of the above refractory metals, it being understood that: if MR = Nb, Ta, Hf, Zr, Ti or V, then Al-MR = Al3MR; if MR = W or Cr, then Al-MR = Al4MR; if MR = Mo, then Al-MR = Al8Mo3 or Al17Mo4 (? Al4Mo), and R is a residual component other than carbon, comprising one or more phases chosen from Al4C3, AlN, AlB2, Al1-67B22 and MRtAlu(C-N-B)v in which t, u and v are numbers greater than or equal to zero, and x, y and z are the volume fractions of the respective components with x > y; x + y > 0.5; x + y + z = 1 and 0.01 < y < 0.5. The invention is also aimed at the use of this composite material as a coating in a component that can be wetted by liquid aluminium and can be used in electrolysis cells.

Description

COMPOSITE MATERIALS FOR WELDABLE CATHODES AND USE THEREOF FOR THE PRODUCTION OF ALUMINUM

FIELD OF THE INVENTION

The present invention relates to new composite materials based on refractory metals. It also relates to liquid aluminum wettable components made using these new materials and a method of manufacturing coatings of these materials. Finally, it relates to the use of these components in electrolysis cells for the production of aluminum.

BACKGROUND TECHNOLOGY

Aluminum is conventionally produced by the Hall-Héroult process in electrolysis cells by reducing alumina dissolved in molten cryolite electrolyte at temperatures of about 960X according to the following reaction:

AI ₂ O ₃ + 3/2 C → 2 Al + 3/2 CO ₂

The carbon of the anode is consumed during the reaction and there is release of CO ₂ during the production of aluminum. The counter electrode or cathode <3 is also made of carbon. Today, more and more graphite blocks are used as cathodes to have better electrical conductivity and lower energy losses in the process. The aluminum is deposited at the bottom of the cell and forms a conductive liquid aluminum layer on the surface of the cathodes. This aluminum can react with graphite to form an aluminum carbide (AI ₄ C ₃ ). This is one of the causes that limit the life of cathodes (typically 3 to 8 years). Since liquid aluminum does not wet the graphite, a relatively thick layer of liquid aluminum is usually maintained at the bottom of the cells (15 to 25 cm). The electromagnetic forces generated by the presence of However, strong electric currents and magnetic fields create vacuums on the surface of this layer of liquid aluminum that can short-circuit the anode if the anode-cathode distance (ACD) is not sufficient. To avoid these short circuits, a fairly large value of the ACD of about 4.5 cm is maintained. The relative displacement of the liquid aluminum layer relative to the surface of the cathode is also a source of erosion and deterioration of the cathodes

The resistance of the electrolyte in the gap between the anode and the liquid aluminum conductive layer produces a voltage drop of about 1.5 volts for an ACD of 4.5 cm and for a typical current density of 0. , 7 A / cm ₂ This ohmic drop is the main source of energy loss in the process. For this reason, in recent decades, intense research has been conducted to develop wettable liquid aluminum cathode coatings to reduce the thickness of the aluminum layer at the bottom of cells. By doing so, also reduce the anode-cathode distance. In addition to the wettability, the coating must be a good electrical conductor and improve the resistance to erosion cathodes to increase their life.

The open porosity on the surface of the cathodes, which typically represents 15 to 20% of the volume of the electrodes, is a source of deterioration when the aluminum and / or sodium from the electrolyte is able to penetrate and chemically react with it. A field of research therefore consists in finding products and methods for clogging these pores so as to increase the lifetime of the electrodes. The example of the invention described in the application mentioned below is illustrative in this respect. .

1- WO2000 / 046427 assigned to Carbone Savoie and entitled "Impregnated

Graphite Cathode for Electrolysis of Aluminum "describes a graphite cathode having in its surface pores a cooked carbon product at a temperature below 1600 ° C which increases the erosion resistance of said cathode.

Moreover, since electrolysis cells operate at very high temperatures, researches of protective and wettable cathode coatings by liquid aluminum have mainly focused in the past on refractory materials (MR) or composites based on refractory and carbon materials. In the context of this invention, refractory material is understood to mean a material with a very high melting point, typically greater than 1800 ^° C. With regard to carbon, several types have been used such as coal, anthracite, coke and graphite. The first and most widely used refractory material is T iB _2, which has been known for many years for its wettability, good electrical conductivity and inertness in liquid aluminum.

The following list gives examples of inventions made in order to develop methods and coatings to solve this problem:

2- WO1991 / 018845 in the name of Alcan International Ltd and entitled "Method of Producing Platelets of Borides of Refractory Metals" describes a method of manufacturing refractory metal boride platelets and the platelets thus produced, which consists in reacting an oxide of refractory metals with boric oxide (B ₂ O ₃ ) and carbon in the presence of a small amount of an alkali metal oxide.

3- WO1993 / 020027 in the name of MOLTECH INVENT SA, and entitled "Refractory Protective Coatings, Particularly for Electrolytic CeII Components" describes protective coatings formed by a self-sustaining combustion of a colloidal slurry solution containing refractory materials. . 4- WO1994 / 020651 in the name of MOLTECH INVENT SA and entitled "The Bonding of Bodies of Refractory Hard Materials to Carbonous Suppoits" describes a method for bonding tiles, plates or bricks of refractory materials on a carbon-based cathode to the using a non-reactive colloidal paste solution comprising particles of preformed refractory materials such as TiB ₂ in a colloidal solution containing fine particles of alumina

WO1994 / 021572 in the name of MOLTECH INVENT SA and entitled "Production of Carbon-Based Composite Materials as Component of Aluminum Production CeIIs" describes a method of manufacturing a composite material comprising a mixture of borides, carbides, oxides and / or nitrides of refractory metals and aluminum, silicon, titanium or zircomum which react to form a refractory compound. This mixture is combined with carbon and a colloidal binder containing fine particles such as Al ₂ O ₃

6- WO1997 / 006289 in the name of MOLTECH INVENT SA and entitled "Maintaining Protective Surfaces on Carbon Cathodes in Aluminum Electrowinning CeIIs" describes a cathode composed of carbon blocks having on the surface a wettable layer by liquid aluminum which contains boride particles of titanium or boride of other refractory metals and a porous non-organic binder material containing liquid aluminum whose content of refractory metals and boron is adjusted so as to prevent the dissolution of the wettable layer of refractory meiaux boride

7-WO1997 / 008114 in the name of MOLTECH INVENT SA and entitled "The Production of Bodies of Refractory Bondes for Aluminum Use Electrowinning CeIIs" describes a method of manufacturing a piece of refractory borides selected from Ti ₁ Cr ₁ V borides ₁ Zr, Hf ₁ Nb ₁ Ta Mo and Ce and containing a colloid such as alumina or silica all in the form of a pasty solution which is dried and then heat treated.

8-WO1998 / 017842 in the name of MOLTECH INVENT SA and titled "Slurry and Method for Producing Refractory Borides and Coatings for use in Aluminum Electrowinning CeIIs" describes a piece or coating of refractory boride similar to the previous one but in which the pasty solution may also contain an organic additive such as polyvinyl alcohol.

9- WO2000 / 029644 in the name of Alcan International Ltd. and entitled "Wettable and Erosion / Oxidation-Resistant Carbon-Composite Materials" describes a carbon-based composite material that can be used to fabricate blocks or coatings for cathodes as well as a method of manufacturing a composite material for carbon base that produces in situ TiB ₂ when exposed to liquid aluminum. The method involves mixing quantities of TiO ₂ and B ₂ O 3 to produce a precursor mixture and mixing the product with a carbon-containing component.

10- WO2000 / 036187 in the name of Alcan International Ltd. and entitled "Multilayer Cathode Structure" describes a method of manufacturing one or more layers of a composite refractory material containing a metal boride on a carbon-based (cathode) substrate which roughens the surface of the front substrate. the application of the layers to improve adhesion. When several layers are applied successively, the boride content increases gradually so as to minimize the differences in coefficients of thermal expansion of the various materials.

11-WO2001 / 042531 in the name of MOLTECH INVENT SA and entitled "Dense refractory material for use at high temperatures" describes a component or a coating of a refractory material comprising particles of refractory materials containing boron, nitrogen, water, silicon, carbon or phosphorus all in an oxide matrix. The refractory material is obtained by heat treating a slurry solution.

12- WO2001 / 061076 in the name of Alcan International Ltd. The present invention describes a method for protecting a carbon-based electrolysis cell component against deterioration by preparing a solution of refractory materials in a binder. lignosulfonate "and applied as a protective coating ^that is allowed to dry thereafter.

13- WO2001 / 061077 in the name of Alcan International Ltd. and entitled "Refractory coating for components of an aluminum electrolysis cell" describes a refractory coating for the components of an electrolysis cell manufactured by coating a aqueous slurry comprising particles of a refractory material such as as TiB ₂ dispersed in an oxalic aluminum complex. When exposed to high temperatures in the electrolysis bath, the compound produces aluminum oxide which binds the refractory particles to each other and to the cathode.

14-WO2002 / 070783 in the name of MOLTECH INVENT SA and entitled "Aluminum-Wettable Porous Ceramic Material" describes a material comprising a liquid aluminum resistant ceramic such as alumina and an aluminum-wetted material comprising a metal oxide or a partially oxidized metal including Mn, Fe, Co, Ni, Cu or Zn that reacts or reacts with Al (I) to form a surface containing alumina, aluminum and the metal derived from the metal oxide.

15- WO2003 / 018876 in the name of Alcoa Inc. and titled "Method for Protecting

Electrodes During Electrolysis CeII Start-up describes a method for applying a protective layer to the cathode of an electrolysis cell comprising a plurality of layers with an inner layer of TiB ₂ of preferably and a protective layer which protects the cathode from the hot gases used for preheating the cell during startup

16-WO2004 / 092449 in the name of MOLTECH INVENT SA and entitled "Alumimum-Wettable Carbon Based Body" describes a carbon component having an aluminum wettable outer component comprising a carbon-rich mixture containing metal-based particles which can react with aluminum The metal-based particles are metal oxides or partially oxidized metal particles selected from Fe, Cu, Co, Ni, Zn and Mn

17, WO2004 / 011697 to Alcoa Inc and entitled "Interlocking Wettable Ceramic Tiles" describes an electrolysis cell for aluminum comprising cathode tiles interlocked with one another and positioned on the graphite blocks. Each tile includes a main body with vertical latches (notches) to prevent movement of tiles out of the surface of the graphite blocks during operations

18-WO2005 / 052218 in the name of Alcan International Ltd and entitled "Stabilizers for Titanium Diboid-Containing Cathode Structure" describes a method of stabilizing the surface of the cathodes which consists in preparing a mixture of materials based on carbon, TiB ₂ and up to 25% by weight of a finely divided additive comprising two closely related compounds one of which has a melting point higher than the annealing temperature of the mixture The mixture is applied to the surface of the cathodes and when it is placed in contact with liquid aluminum, the aluminum reacts with the additive to form a dense surface phase which has low solubility in aluminum Examples of additives are the compounds T1O ₂ / B ₂ O ₃ or the compounds T1C / B ₂ O ₃

Among the methods of manufacturing liquid aluminum wettable cathode coatings are pressing and bonding techniques in the case of tiles of dense refractory materials that are adhered to or applied to the surface of the graph or coatings. application techniques, aerosol style paint or spray followed by thermal annealing when the refractory product is in the form of a paste or an aqueous or colloidal solution. Furthermore, thermal deposition techniques have never been proposed except for the plasma technique (Air Plasma Spray - APS) because when it comes to melting ceramics or refractory materials, very high temperatures of a few thousand degrees Celsius are required (typically between 2000 and 3500 ⁰ C). Only a plasma technique involving ionized gases makes it possible to reach these temperatures and these energy levels. The invention below is an example.

19- US 3,856,650 in the name of Swiss Aluminum Ltd. in 1974 and titled: "Cathode for an Aluminum Fusion Electrolysis CeII and Method of Making the Same" describes a coating of electrically conductive and insoluble ceramic material in cryolite and liquid aluminum as well as a method of manufacturing plasma coating which consists in applying the ceramic material in a finely dispersed form with a snow such that a consolidated and adherent coating is produced.

In spite of all the above efforts, it appears that none of the inventions has been widely implemented in the industry to date for the following reasons: a) The majority of these inventions describe composite materials that still include carbon added in the form of anthracite, graphite coke or the like and therefore, these additives may further react with the liquid aluminum or sodium of the electrolyte to form aluminum carbonate and lead to deterioration of the coating. b) Many of the coatings that have been inventions decolleni from the substrate to use because of the difference in thermal expansion between the coating and the graphite cathode. (c) Inventions of using dense tiles of refractory materials such as TiB ₂ can hardly be applied to large scale because of the high costs of materials and manufacturing processes. d) Most patented coating materials are not thermodynamically stable in liquid aluminum and have a high physical and chemical erosion rate leading to insufficient life.

In parallel with these developments of protective coatings based on refractory metals wettable by liquid aluminum, there are also a number of inventions concerning the "design" of electrolysis cell that take advantage of the use of protective coatings wettable by liquid aluminum. The following patents are examples:

US 3,400,061 issued in 1968 to Kaiser Aluminum & Chemical Corporation and entitled "Electrolytic CeII for Producing Aluminum and Method of Making the Same" describes an electrolytic cell in which the conventional carbon flat cathode bottom is replaced by a cathode structure. drained, sloped, and wettable by aluminum. The drained cathode surface is covered with a refractory substance comprising a mixture of refractory metals and at least 5% of carbon.

US Pat. No. 5,203,971 issued in 1993 to Moltech Invent SA and entitled "CeII Bottom Composite for Aluminum Electrowinning" describes an electrolysis cell whose base is made up in part of conductive carbon sections and partly of non-refractory material sections. - drivers juxtaposed next to each other. Carbon sections are at the same level or lower than sections of refractory materials.

WO 1999/002764 issued to MOLTECH INVENT SA and entitled "A Drained Cathode CeII for the Production of Aluminum" describes a drained cell in which the surface of the wettable cathode is dimensionally stable and inclined thus allowing the liquid aluminum produced from the surface. The distance between the anode and the inclined cathode (ACD) is not more than 3 cm.

23- WO 2002/097168 and WO 2002/097169 in the name of MOLTECH INVENT SA, titled "Aluminum Electrowinning CeIIs Having a Drained Cathode Bottom and an Aluminum Reservoir Collection", describe electrolysis cells for the production of aluminum comprising a cathode formed of a series of carbon blocks having a wettable surface by liquid aluminum.

This section can not be completed on the prior art without mentioning the inventions that are intended to channel the product aluminum to a collecting tank. To do this, channels are usually made

(grooves) on the surface of the graphite cathodes which serve not only to drain the liquid aluminum but also in some cases to break the waves on the surface of the liquid aluminum as described above. The following two inventions are examples.

24-WO1996 / 007773 in the name of MOLTECH INVENT SA and titled "Aluminum Electrowinning CeII with Improved Carbon Cathode Block"> describes a cathode made of graphite blocks whose surface has been scratched by a series of parallel channels (grooves) in the direction current bars to reduce the waves and movements of the liquid aluminum layer.

25- WO 2000/063463 in the name of MOLTECH INVENT SA and titled "Aluminum Electrowinning CeIIs Having a V-Shaped Cathode Bottom" as well as WO2001 / 031088 entitled "Drained-Cathode Aluminum Electrowinning CeII with Improved Electrolyte Circulation" describe a wettable cathode surface by drainable liquid aluminum containing channels (grooves) for collecting aluminum and having V-shaped surface sections to facilitate the flow of aluminum. The problem with these recent inventions is that they add additional costs to the technologies in addition to complicating the processes since the wettable coatings are usually applied to the whole of a drainable cathode surface now hilly and complex geometry.

It is interesting to note that none of the many documents describing cathode coatings based on liquid aluminum wettable refractory materials has specifically described the degree of wettability of the coatings in question. Indeed, to characterize the wettability, these inventions are usually limited to applying the coating on a graphite sample often cylindrical and dip this coated sample in a liquid aluminum bath. The microscopic analysis of the interface between the coating and the aluminum after bath solidification makes it possible to identify whether or not the liquid aluminum is wetting the coating well. Or the wettability of a surface by liquid aluminum is defined by the contact angle that develops when a drop of aluminum is deposited on this surface. A contact angle of less than 90 ° indicates good wettability. None of the inventions mentioned above specifies the degree of wettability of the patented coatings. It should be mentioned, however, that the mechanisms responsible for the wettability of the surfaces of refractory materials such as TiB ₂ by liquid aluminum have never been precisely elucidated.

In the course of the work that led to the present invention, the inventors realized that there was a connection to be made between the cathode coating materials of the electrolysis cells and the grain refiners in the electrolysis industry. aluminum; two topics that at first sight seem unrelated. Grain refiners are additives that are added in small amounts to molten aluminum alloys to refine their microstructure during solidification or in other words, to reduce the size of the aluminum crystals. A fine and uniform microstructure improves the properties of the solidified metal and facilitates its subsequent shaping. During cooling, the surface of the grain refiner acts as a heterogeneous nucleation site to crystallize the alloy. To act effectively, the liquid aluminum must well wet the surface of the grain refiner. At the commercial level, there are two types of grain refiners. The system Al-Ti-B (for example, Al-5Ti-1B in wt%) and the Al-Ti-C system (eg Al 3Ti-O 15C _1% by weight). The best known, the Al-Ti-B system consists of small particles of TiB ₂ in an aluminum matrix.

But recent works published in the literature (26-31):

26- P. Schumacher and A. L. Gréer, Mater. Sci. Eng. A, A178 (1994) p. 309 27. P. Schumacher and A. L. Gréer, Mater. Sci. Eng. A, A181 / 182 (1994) p.1335

P. Schumacher and A. L. Gréer, Proc. Conf. "Light metals 1995" (J.W. Evans ed.) P.869 (1995), Warrendale, PA, TMS

29- P. Schumacher and AL Gréer, Proc. Conf. "Light metals 1996" (W. HaIe ed.) P.745 (1996), Warrendale, PA ₁ TMS

30- P. Schumacher, A.l. Greer, J. Worth, P.V. Evans, MA Keams, P. Fisher, and A. H. Green, Mater. Sci. Technol., 14 (1998) p.394

31- AM Bunn, P. Schumacher, A. Keams, CB Boothroyd, and AL Gréer, Mat. Sci. Technol., (1999) p.1115 suggest that it may not be the surface of TiB ₂ as such that acts as a germination site but a thin layer of AbTi that would be present between the surface of TiB ₂ and the 'aluminum. This thin layer of Ti aluminide a few nanometers thick, responsible for the wettability, would be stabilized by the presence of TiB ₂ (see reference 29).

In the work that led to the present invention, the inventors, influenced by these recent studies on grain refiners, have assumed that to ensure good wettability of cathode coatings, the presence of refractory metal aluminides is desirable. . Cassie's law, which describes the contact angle of a liquid on a composite material made of a refractory ceramic (carbide, nitride or boride) and a refractory metal aluminide, can be expressed as follows:

Cos θc = S ₁ Cos Θ1 + S ₂ Cos Θ2

where θc is the contact angle of the liquid with the composite material, Θ1 and 02 are the respective contact angles on the ceramic and the refractory aluminide while Si and S ₂ are the respective surfaces of the two phases in contact with the liquid . If + S ₂ = 1.

When the wettability on the refractory ceramic is not very large and Θ1 close to 90 °, the first term of the preceding equation becomes negligible (cos Θ1 ~ 0) and if the wettability on the aluminide is very high and that Θ2 neighbor the 0 (cos Θ2 ~ 1) so under these conditions, the cosine of the contact angle on the composite becomes directly proportional to the surface of the refractory metal aluminide. The higher this surface, the smaller the contact angle on the composite. For example, for a surface fraction of aluminide of 30%, the contact angle on the composite would be 72.5 °, ie cos ^"1 (0.3) .Following the same idea, if we imagine a composite consisting of refractory ceramic grains (eg NbB ₂ or NbC grains) with refractory metal aluminide grain boundaries (eg Al ₃ Nb), plus the volume fraction and therefore the surface fraction of grain boundaries will be large, the wettability of the composite will be high.Therefore it will be advantageous to reduce the size of grains or crystallites and increase the density of grain boundaries as is the case in nanocrystalline materials where the density of grain boundaries can reach values as high as 30% when the grain size is a few nanometers By definition a nanocrystalline material is a material for which the size of the grains or crystals is less than 100 nm. SUMMARY OF THE INVENTION

The present invention firstly relates to a composite predominantly composed of carbide and / or nitride and / or boride of refractory metals or aluminum (C-N-B-MR). It contains in volume at least 1% but less than 50% of refractory metal aluminide (AI-MR) and less than 50% of a residual component (R) which stabilizes the composite. This composite is in the following form:

(CNB-MR) _x (AI-MR) _y (R) _z wherein: Al, C, N and B are respectively aluminum, carbon, nitrogen and boron;

MR is one or more refractory metals of the IV, V or VI series from table to periodic;

CNB-MR is one or more carbides, nitrides or borides of the refractory metal or metals mentioned above and / or one or more carbides, nitrides or borides of aluminum chosen from Al ₄ C ₃ , AlN, AIB ₂ and Al 1.67B22;

AI-MR is one or more aluminides of the refractory metal or metals mentioned above, it being understood that: if MR = Nb ₁ Ta, Hf, Zr, Ti, V then AI-MR = AI ₃ MR; if MR = W, Cr then AI-MR = AI ₄ MR; if MR = MB, then AI-MR = AI ₈ MB ₃ or AI ₁₇ MB ₄ (≥ AI ₄ MB)

R is a non-carbon residual component comprising one or more of Al ₄ C ₃ , AlN, AlB ₂ , Al ₁ ₇ B _22, and MRtAlu (CNB) V where l, u, and v are larger or equal numbers to zero, and x, y, z are the volume fractions of the respective components with x>y; x + y>0.5; x + y + z = 1 and 0.01 <y <0.5, provided that when CNB-MR = TiB ₂ , then AI-MR is not AI ₃ Ti.

The residual component is usually multiphase and may contain aluminum carbide (AI ₄ C ₃ ) if the composite contains carbides and a excess of aluminum or carbon and / or aluminum nitride (AIN) if the composite contains nitrides and an excess of aluminum or nitrogen and / or aluminum borides (AIB ₂ , AI _V67 B ₂₂ ) if the composite contains borides and an excess of aluminum or boron. This residual component may also contain one or more mixed compounds of the MR type _t Al _u (CN-Fi) _v if the composite contains carbides, nitrides or borides and an excess (I refractory metals MR. An example of such mixed compound is Ta ₂ AIC (t = 2, u = 1, v = 1) (see Fig.ib).

The composite of the invention has great advantages over the materials of the prior art for applications in electrolysis cells for the production of aluminum. First, the carbides, nitrides and borides of the refractory metals (CNB-MR) in question are good electrical conductors when compared to graphite. Moreover, even in the case where the CNB-MR is a carbide, nitride or aluminum boride (AI ₄ C ₃ , AlN, AIB ₂ , AI1.67B22) which is not an electrical conductor, the composite itself is thanks to its component AI-MR which it is conductive. Indeed, all the aluminides of the refractory metals of the series IV ₁ V and Vl are good electrical conductors compared to graphiie. Second, it has been discovered in the context of this invention that the wettability of liquid aluminum on surfaces of refractory metal aluminides is much greater than the wettability on carbides, nitrides or borides of refractory metals or aluminum. Therefore, even if the carbide, nitride or boride (CNB-MR) is not wetted by the liquid aluminum, the composite will be wet thanks to the refractory metal aluminide component (AI-MR). According to Cassie's law and following the same reasoning as previously described, the contact angle on the composite would vary between 89 ° (cos ^"1 (0.01)) and 60 ° (cos ^{" 1} (0.5)) when the volume or surface fraction "y" of the aluminide component in the composite varies between 0.01 (1%) and 0.5 (50%). This calculation is based on the assumption of a zero contact angle on the aluminide and 90 ° on the refractory ceramic. Finally, adding to the composite an excess of aluminum and / or refractory metals via the component "R" allows exposure of the electrolysis cells (960 ^° C.) at temperatures close to the temperatures of the electrolysis cells, in situ forming very stable compounds (Al ₄ C ₃ , AIN, AIB ₂ , AIL ₆₇ B ₂₂ , MR _t Al _u (CNB) ") which improves the stability of the composite and its resistance to wear during prolonged exposures of the material in high temperature liquid aluminum.

Certain inventions such as that described in application WO2005 / 052218 entitled "Stabilizers for Titanium Diboride-Containing Cathode Structure" have already described the addition of additives for stabilizing TiB ₂ and carbon cathode structures. These additives react with the liquid aluminum produced during the electrolysis reaction to form a dense phase on the surface which seals the open porosity and stabilizes the surface of the coating. However, this stabilization of the material takes place only on the surface whereas in our case, the entire volume of the composite is stabilized since the component R is within the composite itself.

The invention as claimed also has for its second object a method of manufacturing a coating made of a composite as defined above, which consists in consolidating the material by partial melting or by sintering at a temperature below 1800. ⁰ C or to project at high speed on a substrate of fine particles of the composite using the HVOF thermal spraying technique.

The invention as claimed also has for its third object a component wettable by liquid aluminum which consists of a solid body on which is applied a coating made of a composite as defined above, but without the reserve expressed above at the end of the definition of the composite formulation.

The invention finally relates to the use of a component as defined above, in electrolysis cells for the production of aluminum. The invention will be better understood on reading the detailed but non-limiting description of the invention which follows, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1d show the ternary phase diagrams of the Nb-Al-C, Ta-Al-C, W-Al-C and Ti-Al-C systems, respectively. Two-phase triangles surrounded by dashed lines include compounds of the type (CNB-MR) χ (AI-MR) _y . The three-phase regions within the tiiet lines include (CNB-MR) _x (AI-MR) y) R) _z compounds.

FIGS. 2a and 2b show, at a temperature in the vicinity of 1000 ^° C., the wettability as a function of time of a 90 mg liquid aluminum bead on a dense substrate of TiC and TiB _2, respectively. The table in the figures shows the contact angles on the left, on the right and the average value.

FIGS. 3a to 3c show, at a temperature in the vicinity of 1000 ^° C., the wettability as a function of time of a 90 mg liquid aluminum ball on Al ₃ Ta, Al ₄ W and Al ₈ Mo ₃ respectively.

FIG. 4a shows, as a function of time, the average contact angle between a 90 mg liquid aluminum bead and various refractory ceramics. The curves corresponding to titanium aluminide (Al ₃ Ti) and to graphite are also presented for comparison purposes.

Figure 4b shows, as a function of time, the average contact angle between a 90 mg liquid aluminum bead and various refractory metal aluminides. The curves corresponding to TiB ₂ and TiC are also presented for comparison purposes. FIG. 5 shows x-diffraction spectra of composites according to the invention comprising titanium aluminide (Al ₃ Ti) and various refractory ceramics.

Figure 6 shows a table showing the melting points of various refractory metal aluminides as well as various refractory ceramics.

Figures 7a and 7b show the phase diagrams of the Λl-Nb and Al-Ta systems respectively.

Figure 8 shows a schematic view of a composite material coating manufacturing method according to the invention which involves the use of thermal deposition by HVOF.

DETAILED DESCRIPTION OF THE INVENTION

As previously indicated, FIGS. 1a, 1b, 1c and 1d show high temperature phase diagrams (close to that used in the electrolysis cells) of Nb-Al-C, Ta-Al-C ternary systems. , W-Al-C and Ti-Al-C respectively. The composites (CNB-MR) _x (AI-MR) _y without residual component (z = 0) are inside the triangles surrounded by dashed lines. Moreover, the space delimited by lines in tii and indicates compositions according to the invention with residual components having an excess of refractory metal on the one hand or aluminum and carbon on the other hand (z ≠ 0) .

Figures 2a and 2b show wettability experiments of liquid aluminum on component surfaces (CNB-MR) that is to say, refractory ceramics where MR is Ti. Note that the melting point of pure Ti is slightly lower than 1800 ⁰ C but nevertheless, it is considered a refractory metal (MR) in the present context. Figure 2a is the case of TiC while Figure 2b is the case of TiB ₂ . These photographs show a liquid aluminum ball that wets the surface as a function of time. Temperature, time and contact angle mean with the surface of TiC or TiB ₂ is indicated on each image, the table under each figure summarizes the results of the experiment. It is noted that durations longer than 300 minutes are required to reach an average contact angle of the order of twenty degrees. Cos refractory materials that are wet with liquid aluminum (contact angle less than 90 °) are known to be good grain refiners (see previous discussion).

Figures 3a, 3b and 3c show wettability experiments of liquid aluminum on surfaces of refractory metal aluminides (Al-MR). Figure 3a is the case of AI ₃ Ta, Figure 3b is the case of AI ₄ W and Figure 3c is that of AI ₈ Mo ₃ . Unlike previously, it only takes a few tens of minutes or at most a hundred minutes to get a contact angle of a few tens of degrees. The velocity (wettability) on a refractory metal aluminide is therefore an order greater than that of the corresponding refractory ceramic.

FIG. 4a shows the variation over time of the contact angle of the liquid aluminum on various refractory ceramics (CNB-MR) at 1000 ^° C. As previously mentioned, the graphite is not wetted by the liquid aluminum and this is also the case for boron nitride (BM). The contact angle for these materials is very high and hardly changes over time. Aluminum nitrides (AIN) or titanium (TiN) are very little wet by aluminum. The corresponding contact angles are for the most part greater than 90 °. Moreover, the refractory materials known to be good grain refiners (TiC and TiB ₂ ) are well wetted by liquid aluminum. The contact angles become less than 90 ° after about 100 minutes. Figure 4a also shows the case of titanium aluminide (AI ₃ Ti) to compare it with refractory ceramics. The wettability rate on AI ₃ Ti is extremely fast when compared to others. This observation supports the hypothesis previously discussed of the existence of a thin layer of Al ₃ Ti on the surface of TiB ₂ to ensure the wettability of this grain refiner. Figure 4b shows the contact angle as a function of time for aluminides of refractory metals (AI-MR). The time scale is a few tens of minutes which is much shorter than that of the previous figure. The wettability is the fastest on the aluminides of Mo, Ti, Hf and W followed by Nb, V finally Ta. The same figure for comparison shows the result of the wettability experiments on TiC and TiB ₂ .

The previous results clearly show the advantage of combining these two types of materials within a composite to on the one hand to ensure wettability via the component (AI-MR) and on the other hand to ensure the stability, the resistance wear and durability via the component (CNB-MR). Moreover, these two components can coexist with the thermodynamic equilibrium at the operating temperatures of the electrolysis cells (~ 960 ⁰ C) as shown in FIGS. 1a) to d) (two-phase space in the triangle surrounded by lines dotted).

FIG. 5 shows nanocrystalline x-ray diffraction spectra of composites (CNB-MR) χ (AI-MR) _y obtained by intensive mechanical grinding. The aluminide component (Al-Mr) is Al ₃ Ti and the refractory ceramic is 5a) AlN, 5b) TiN and 5c) TiC.

There is an additional advantage to the addition of a component (AI-MR) to that of the refractory ceramic: that of the shaping and densification of composite materials. Indeed, as shown in the table of Figure 6, the refractory ceramics (CNB-MR) have very high melting points typically between 2000 and 4000 ⁰ C. It therefore requires temperatures (and often very high pressures). ) to consolidate them at high density by sintering or to form them in coating. In thermal projection techniques, only plasma techniques (AF ¹ S, VPS "air and vacuum plasma spray") are possible as mentioned above. Unfortunately, these techniques often result in very porous materials. In contrast, aluminum aluminides refractory metals, that is to say the components (AI-MR) ₁ have melting points much lower, below 1800 ⁰ C and typically between 1300 and 1700 ⁰ C (see table of Figure 6). This temperature range is ideal for high velocity oxyfuel (HVOF) thermal spraying which results in very dense coatings. Furthermore, sintering (I composite (CNB-TB) _x (AI-RM) _y containing an aluminide component of refractory metal with a low melting point might also be carried out at much lower temperatures and more feasible a In addition, refractory metal aluminides of the type contemplated in the present invention often have congruent melting points as shown in Figures 7a and 7b for Al ₃ Nb and Al ₃ Ta respectively. the liquid state directly the material under the correct composition (AI-MR) without chemical segregation.

FIG. 8 shows a schematic view of a nonlimiting example of application in the form of a coating of the material according to the invention by the HVOF technique. A groove is first etched with a grinding wheel on the surface of a solid body which may be a series of graphite cathodes juxtaposed next to each other. The depth of the furrow can typically vary between a few tens of microns and a few centimeters. The composite of the invention is deposited using the HVOF technique within the groove so as to produce a wettable channel by liquid aluminum. The groove thus makes it possible to channel the liquid aluminum towards the recovery basin in addition to creating an obstacle to the movements of aluminum generated by the Lorentz forces as discussed previously. By limiting the coating to the grooves rather than coating the entire surface of the cathodes, the costs are reduced accordingly.

Claims

1. A composite material of formula:

(CNB-MR) _x (Al-MR) _y (R) _z in which: Al ₁ C, N and B represent respectively aluminum, carbon, nitrogen and boron;

MR is one or more refractory metals of the IV, V or Vl series of the periodic table;

CNB-MR is one or more carbides, nitrides or borides of or refractory metals previously stated and / or one or more carbides, nitrides or borides of aluminum selected from (AI ₄ C ₃ , AIN, AIB ₂ , AI1.67B22) ;

AI-MR is one or more aluminides of the refractory metal or metals stated above, it being understood that: if MR = Nb, Ta, Hf, Zr, Ti, V then AI-MR = AI ₃ MR; if MR = W, Cr then AI-MR = AI ₄ MR; if MR = MB, then AI-MR = AI ₈ MB ₃ or AI ₁₇ MB ₄ {= AI ₄ MB)

R is a residual component other than carbon comprising one or more of Al ₄ C ₃ , AlN, AlB ₂ , AlI. ₆₇ B ₂₂ and MR _t Al _u (CNB) _v where t, u and v are numbers greater than or equal to zero, and x, y, z are the volume fractions of the respective components with x>y; x + y>0.5; x + y + z = 1 and 0.01 <y <0.5, provided that when CNB-MR is TiB ₂ , then AI-MR is not AI ₃ Ti.

2. A composite material according to claim 1, wherein MR is selected from Nb, Ta, W, Mo, Hf, Zr, Cr, V and Ti.

3. A composite material according to claim 1, wherein MR is selected from Nb, Ta and W.

4. A composite material according to claim 1, wherein MR is Pi.

5. A composite material according to any one of claims 1 to 4, wherein the material is nanocrystalline.

6. A method of manufacturing composite material coating according to any one of claims 1 to 5, which consists in consolidating the material by partial melting or by sintering at a temperature below 1800 ⁰ C or to project at high speed on a substrate of fine particles ^, composite material using the HVOF thermal spraying technique.

7. A liquid aluminum wettable component which consists of a solid body to which is applied a coating of a composite according to any one of claims 1 to 5, but without the proviso mentioned in claim 1.

A component according to claim 7 wherein the coating is made using the method of claim 6.

9. A component according to claim 7 or 8 wherein the coating is applied solely in grooves etched on the surface of the solid body.

10. Use of a component according to any one of claims 7 to 9, in electrolysis cells for the production of aluminum.

11. Use of a component according to any one of claims 7 to

9. as a wettable cathode for the production of aluminum.