EP1366214A1

EP1366214A1 - Aluminium-wettable porous ceramic material

Info

Publication number: EP1366214A1
Application number: EP02702625A
Authority: EP
Inventors: Vittorio De Nora
Original assignee: Moltech Invent SA
Current assignee: Moltech Invent SA
Priority date: 2001-03-07
Filing date: 2002-03-04
Publication date: 2003-12-03
Anticipated expiration: 2022-03-04
Also published as: US20040149569A1; NZ527306A; RU2003129656A; RU2281987C2; ES2230467T3; CA2438526A1; DE60202265T2; ATE284983T1; EP1366214B1; WO2002070783A1; DE60202265D1

Abstract

A material, for instance used as an aluminium-wettable component (21,21', 41,41', 51), in particular of a cell for the electrowinning of aluminium (60), comprises an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium. The structure is made of ceramic material inert and resistant to molten aluminium, such as alumina, and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal, in particular of manganese, iron, cobalt, nickel, copper or zinc, which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal. The ceramic structure comprises a coating of the aluminium-wettable material on the inert and resistant ceramic material, or is made of a mixture of the inert and resistant material and of the aluminium-wettable ceramic material. The aluminium-wetted component is suitable for use as a cathode (21,21'), as a sidewall (41,41') or as another component (51) which during use is exposed to molten aluminium (60) and/or electrolyte (5), or another oxidising and/or corrosive media at high temperature.

Description

ALUMINIUM-WETTABLE POROUS CERAMIC MATERIAL

Field of the Invention

The invention relates to a ceramic material which can be utilised for the manufacture of aluminium-wettable and aluminium-wetted ceramic components, in particular for use in aluminium production, for example as cathodes, sidewalls and other cell components which during use are exposed to molten aluminium, electrolyte and/or corrosive gases .

Background of the Invention Aluminium is produced conventionally by the Hall- Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950°C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents and corrosive gases. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode forming the cell bottom floor. The cathode is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke and coal tar, or with glue.

It has long been recognised that it would be desirable to make (or coat or cover) the cathode of an aluminium electrowinning cell with a refractory boride such as titanium diboride that would render the cathode surface wettable to molten aluminium which in turn would lead to a series of advantages. Many difficulties were encountered in producing refractory boride coatings which meet up to the rigorous conditions in an aluminium electrowinning cell. Nevertheless, such coatings applied from slurries to carbon bodies have been developed. The most recent slurry-applied coatings are disclosed in WO01/42168 (de Nora/Duruz) and WO01/42531 (Nguyen/Duruz/ de Nora) . US Patent 5,981,081 (Sue) discloses wear and corrosion resistant coatings made of transition metal boride particles dispersed in a matrix of nickel, cobalt or iron. The coatings are applied by explosion or plasma spraying a mixture of powders of a transition metal boride and a boron containing alloy on a metal substrate and heat treating.

Previously, it had been proposed to replace the carbon material of the cathodes of aluminium production cells with ceramic material. For example, US Patent 4,560,448 (Sane/Wheeler/Kuivila) discloses a porous component made of aluminium repellent material covered with an aluminium-wettable metal boride coating which during use is maintained by saturating the molten aluminium infiltrating the porous component with coating constituents. US Patent 4,650,552 (de Nora/Gauger/ Fresnel/Adorian/Duruz) discloses an aluminium production cell component produced from a powder mixture of alumina and aluminium. US Patent 4,600,481 (Sane/Wheeler/Gagescu/ Debely/Adorian/Derivaz) discloses a component of an aluminium production cell which is made of an openly porous matrix, e.g. an alumina matrix, filled with molten aluminium. The openly porous matrix may comprise an aluminium-wettable coating made of a boride or nickel. The infiltration of the matrix with aluminium is carried out at a temperature of 1000° to 1500°C.

Materials made of a ceramic matrix infiltrated with metal have also been described in the following references. US Patents 4,935,055 (Aghaj anian/Claar) , 5,194,202 (Yun/Marra/Gurganus/Kelsey) and 5,676,907

(Ritland/Readey/Stephan/Rulis/Sibold) disclose different methods of infiltrating a ceramic structures, e.g. A1₂0₃,

SiN or SiC, with molten aluminium. US Patent 5,043,182 (Schultze/Schindler/Deisenroth) discloses a porous A1₂0₃- Al₂Ti0₅ structure infiltrated under pressure with a molten aluminium alloy.

US Patent 5,007,475 (Kennedy/Aghajani n) discloses a ceramic structure, e.g. alumina, infiltrated by molten aluminium with the aid of an infiltration enhancer consisting of a metal/gas combination selected from Mg/N,

Sr/N, Zn/O and Ca/N to which the alumina structure is exposed before and during infiltration. It is also contemplated in this patent to use ceramic structures described in US Patent 4,713,360 (Newkirk/Dizio) that discloses porous ceramic structures obtained by oxidising aluminium metal with additives selected from Mg, Zn, Si, Na, Li, Ca, B, P, Y, rare earth metals, and possibly nonfunctional diluents or impurities, such as Mn, Fe, Cu and W, in an amount of much less than 1% of the structure.

Objects of the Invention An object of the invention is to provide an aluminium-wettable component for a cell for the production of aluminium from alumina dissolved in a fluoride-based molten electrolyte.

Another object of the invention is to provide an aluminium-wetted component which is highly conductive and resistant to molten electrolyte for use as a cathode in a drained cell or in a cell operating with a shallow or deep aluminium pool or as a cell sidewall or another component which is exposed to molten aluminium, electrolyte and/or corrosive gases, or as a lining for protecting other cell components against molten electrolyte, or for making other cell components aluminium-wettable .

A further object of the invention is to provide an aluminium-wettable or aluminium-wetted component which can be made from readily available materials.

Yet another object of the invention is to provide an aluminium-wettable component which can be wetted with aluminium outside an aluminium production cell or in-situ by exposure to cathodic molten aluminium.

Another object of the invention is to provide an aluminium-wetted component that retains its protective and wettability properties even when exposed to highly oxidising and/or corrosive environments. Yet a further object of the invention is to provide a ceramic-based or a ceramic-metal material which can be used in an oxidising and/or corrosive media at elevated temperature . Summary of the Invention

A first aspect of the invention relates to an aluminium-wettable component of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte. The component comprises an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium. The structure is made of a ceramic material inert and resistant to molten aluminium and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal . The inert and resistant ceramic material may comprise at least one oxide selected from oxides of aluminium, zirconium, tantalum, titanium, silicon, niobium, magnesium and calcium and mixtures thereof, as a simple oxide and/or in a mixed oxide, for example an aluminate of zinc (ZnAl0₄) or titanium (TiAl0₅) . Other suitable inert and resistant ceramic materials can be selected amongst nitrides, carbides and borides and oxyco pounds , such as aluminium nitride, A10N, SiAlON, boron nitride, silicon nitride, silicon carbide, aluminium borides, alkali earth metal zirconates and aluminates, and their mixtures.

Usually, the reaction of the metal oxide and/or partly oxidised metal with molten aluminium involves the reduction of the metal oxide and/or partly oxidised metal and the oxidation of aluminium. For the metal oxide and/or partly oxidised metal to be reducible by molten aluminium, it is necessary that such a metal be more electronegative than aluminium. For example, the metal of the metal oxide and/or partly oxidised metal reducible by molten aluminium is selected from manganese, iron, cobalt, nickel, copper and zinc and combinations thereof.

The concentration of reactable metal oxide and/or partly oxidised metal at the surface of the ceramic structure affects the speed at which the structure is wetted by molten aluminium. The surface of the ceramic structure should contain the reactable metal oxide and/or partly oxidised metal in an amount of at least 2 to 3 weight%, preferably at least 5 to 25 weight% of the material making the surface of the ceramic structure . When the ceramic structure comprises a coating of the aluminium wettable material as described hereafter, the coating may comprise much more metal oxide and/or partly oxidised metal, e.g. up to 50 or even 80 weight% or possibly even more. The electronegativity of the metal of the reactable metal oxide and/or partly oxidised metal also affects the speed of aluminium wetting. The fastest wetting of the ceramic structure is achieved when the metal of the reactable metal oxide and/or partly oxidised metal is selected from copper, nickel, cobalt, manganese and iron.

In one embodiment of the invention, the openly porous or reticulated ceramic structure comprises a coating of the aluminium-wettable material on the inert and resistant ceramic material. In other words, the openly porous or reticulated ceramic structure consists of a skeleton of the inert and resistant ceramic material coated with the aluminium-wettable material .

Such aluminium-wettable coating is usually a slurry- applied coating comprising particles of the metal oxide and/or partly oxidised metal reactable with molten aluminium in a dried colloidal carrier selected from alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, titanium oxide and zinc oxide, and precursors and mixtures thereof. Further details of such slurry- applied coatings are disclosed in WOOl/42168 (de Nora/ Duruz) , which describes such coatings on solid substrates .

The slurry-applied aluminium-wettable coating may further comprise particles of at least one compound selected from metal borides, carbides and nitrides. For example, the aluminium-wettable coating comprises the particles of the metal oxide and/or partly oxidised metal reactable with molten aluminium and particles of titanium diboride in dried colloidal alumina. Particles of the metal boride, carbide or nitride may be covered with mixed oxides of metal derived from the dried colloidal carrier and metal derived from the metal boride, carbide or nitride. To improve the structure of the coating, the slurry-applied aluminium- wettable coating can be obtained from a slurry comprising metal oxide particles that combine upon heat treatment with metal derived from the dried colloidal carrier to form mixed oxides which are miscible with the mixed oxides covering the particles of the metal boride, carbide or nitride. Suitable slurries producing such a coating are disclosed in WOOl/42531 ( guyen/Duruz/de Nora), which describes such coatings on solid substrates.

In another embodiment of the invention, the openly porous ceramic structure is made of a composition which comprises a mixture of the inert and resistant ceramic material and the aluminium-wettable ceramic material .

Such a ceramic structure should comprise a sufficient amount of inert and resistant ceramic material that upon contact/reaction of the aluminium-wettable ceramic material with molten aluminium, the overall ceramic structure retains sufficient mechanical properties.

Usually, the aluminium-wettable material makes up less than 15 weight%, usually less than 10 weight%, of the ceramic structure.

Furthermore, the openly porous ceramic structure may be formed on a reinforcing metal skeleton, in particular a metal mat. Suitable metals for such a skeleton include iron and iron alloys and other metals which are mechanically resistant at elevated temperature.

For some applications, it may be advantageous to use internal inserts acting as ballast inside a component made of the ceramic structure, for instance to secure the ceramic structure on the bottom of an aluminium production cell as disclosed in Figs. 2 and 3 of US Patent 5,651,874 (de Nora/Sekhar) . The internal inserts may be made of iron or iron alloys or other heavy materials. A reinforcing metal can also act as ballast.

The component of the invention has numerous applications some of which are set out hereafter. For instance, the component may be a cathode or a cathode lining, for example plate- or wedge-shaped, on a cathode body, in particular made of carbon material . The component can also be an aluminium pool stabiliser in the form of a plate having a density which is either lower than that of molten aluminium so that it can float at the surface of the aluminium pool , or higher than that of molten aluminium so that it can rest at the bottom of the aluminium pool. All of the aforementioned components, which are exposed during use to the product aluminium, can be placed as such in the cell and wetted during use. Such components may be top coated with a highly aluminium-wettable start-up layer, for example as disclosed in WOOl/42168 (de Nora/Duruz) . On the other hand, for certain applications the components may need to be wetted with molten aluminium before use. Therefore, the aluminium-wettable component can constitute a skeleton which can be infiltrated with molten aluminium to form for example a cell sidewall or a sidewall lining, or a wedge-shaped connecting body for joining the surface of a cell bottom to an adjacent sidewall at the periphery of the cell bottom.

The invention also relates to an aluminium-wetted component of a cell for the electrowinning of aluminium. The aluminium-wetted component comprises an openly porous or reticulated ceramic structure which has a surface layer containing alumina, aluminium and another metal, e.g. iron, copper or nickel. Such component is obtainable by exposing to molten aluminium an openly porous or reticulated aluminium-wettable component made of ceramic material inert and resistant to molten aluminium, e.g. alumina, and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal, e.g. iron, copper or nickel as oxides and/or partly oxidised metals, which is/are reactable with molten aluminium as described above .

The component comprises an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium. The structure is made of a ceramic material inert and resistant to molten aluminium and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal. Usually, aluminium-wetted components are completely filled and covered with aluminium that shields their openly porous or reticulated ceramic structure from exposure to molten electrolyte and/or corrosive gases during use. The aluminium-wetted component may be a cathode or a cathode lining or an aluminium pool stabiliser wetted by aluminium before or during use. The component may be a cell sidewall or a sidewall lining or a wedge-shaped body for joining the surface of a cell bottom to an adjacent sidewall, all wetted by aluminium before use.

Another aspect of the invention is a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based electrolyte, comprising one or more aluminium-wettable and/or aluminium-wetted components described above.

The cell may in particular comprise a cathode or a cathode body whose surface is lined with a cathode lining as disclosed above. The cathode body and the cathode lining may be joined through a bonding layer, in particular a slurry-applied refractory boride layer as disclosed in WOOl/42168 (de Nora/Duruz) and WOOl/42531 (Nguyen/Duruz/de Nora) . For example, the lined cathode surface is part of a horizontal or inclined cathode bottom, in particular a horizontal cathode bottom lined with a wedge-like cathode lining forming an aluminium- wettable drained sloping cathode surface thereon. Alternatively, the cathode body may be located above a cell bottom that is arranged to collect molten aluminium produced on and drained from the cathode lining. Further aspects of the invention relate to uses of the above described material in fields other than the field of aluminium electrowinning.

One further aspect of the invention relates to a composite ceramic-based material which comprises an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium. This structure is made of a ceramic material inert and resistant to molten aluminium and an aluminium- wettable material that comprises metal oxide and/or partly oxidised metal selected from partly oxidised or oxide of copper, nickel, cobalt, manganese and iron and mixtures thereof, which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal.

Such a material may be used, for instance, for the manufacture of components or linings of apparatus for treating molten aluminium, in particular for purifying molten aluminium or separating alloying metals from an aluminium alloy. Further details of such apparatus can be found in WO00/63630 (Holz/Duruz) .

A yet further aspect of the invention relates to a composite ceramic-metal material which comprises, as before, an openly porous or reticulated ceramic structure which has a surface layer containing alumina, aluminium and another metal. The composite ceramic-metal material is obtainable by exposing to molten aluminium a composite material made of a ceramic material inert and resistant to molten aluminium and an aluminium-wettable material that comprises a metal oxide and/or a partly oxidised metal selected from copper, nickel, cobalt, manganese and iron and mixtures thereof, which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal .

Such a material may be used for the manufacture of aluminium-wetted components for applications in high temperature oxidising or corrosive gases, in particular oxygen and/or fluorine-containing gases, or liquids, such as fluorine-containing liquids or molten metal, in particular molten aluminium.

In particular, the aluminium-wetted components may be used in apparatus for treating molten aluminium. The components may also be used at temperatures below the melting point of aluminium as electrodes, heating elements, structural materials, metallurgical crucibles for containing molten metals other than aluminium, anodes, furnace fixtures, molds etc. Due to the capacity of the ceramic structure to retain molten aluminium within its pores and on its surface by capillary effect, the aluminium-wetted components may be Used in chemically aggressive environments at temperatures above the melting point of aluminium, for instance as linings in furnaces, providing the components are not exposed to substantial mechanical wear.

Brief Description of the Drawings

The invention will be further described with reference to the accompanying schematic drawings, in which Figures 1, 2 and 3 illustrate cells of different configurations fitted with aluminium-wetted components of the invention.

Detailed Description

Figure 1 shows an aluminium production cell of drained configuration. The cell comprises non-carbon metal-based anodes 10, for example as disclosed in WO00/40781 and WO00/40782 (both in the name of de Nora) , which are spaced apart from correspondingly sloped facing cathode surfaces 20, for example as disclosed in WO00/63463 (de Nora) , in a fluoride-based molten electrolyte 5.

The cell bottom 25,25', for example made of carbon material, is covered with aluminium-wetted cathode linings 21,21' which form drained aluminium-wetted sloping cathode surfaces 20 according to the invention, different embodiments being shown in the right and the left hand part of Figure 1. As shown, the cathode surfaces 20 slope down towards the middle of the cell bottom 25,25'. On the left-hand side of Figure 1, the cell bottom 25 is horizontal whereas the cathode lining 21' covering it is a wedge with a small angle forming a sloping cathode surface 20 above the horizontal cell bottom 25. On the right-hand side of Figure 1, the cell bottom 25' is at a slope and covered with cathode lining plates (tiles) 21 of uniform thickness and which form a sloping cathode surface 20 parallel to the sloping cell bottom 25 ' .

The cell bottom 25,25' is only partly covered with the cathode lining 21,21', leaving a central channel 30 formed by the cell bottom 25,25' and the adjacent cathode linings 21,21' which are spaced in the middle of the cell by channel 30. This channel 30 serves to collect product molten aluminium 60 from the sloping cathode surfaces 20.

The cell bottom 25,25', in particular where it forms part of the aluminium-collection channel is preferably protected with an aluminium wettable layer 35, for example a slurry-applied refractory boride layer as disclosed in WOOl/42168 (de Nora/Duruz) or WO01/42531

(Nguyen/Duruz/de Nora) . Such a slurry-applied layer 35 is also wetted by molten aluminium 22 that wets also the bottom of the cathode linings 21,21' providing a continuous and optimal electrical contact.

As shown in Figure 1, the cell comprises sidewalls 40, for example made of silicon carbide, which are protected with an aluminium-wetted sidewall lining 41 according to the invention. The sidewall lining 41 is completely filled with molten aluminium retained in its pores by capillary effect. The sidewall lining 41 extends vertically from the cell bottom 25,25' to above the surface of the molten electrolyte 5, and completely shields the sidewalls 40 from molten electrolyte 5.

The aluminium-wetted sidewall lining 41 and cathode linings 21,21' are joined through generally wedge-shaped aluminium-filled bodies 51 according to the invention located on the periphery of cell bottom 25,25'.

Thus, all the structural elements except anodes 10 are completely shielded from the molten electrolyte 5 by molten aluminium retained in and on the aluminium-wetted components according to the invention, or by the layer of molten aluminium 60 collected in channel 30. Such a cell configuration utilising these cell materials permits use of the electrolyte 5 which is entirely in a molten state, i.e. without frozen electrolyte ledges along the sidewalls 40 and without a frozen electrolyte crust at the surface of the electrolyte 5. Figure 2, where the same reference numerals are used to designate the same elements, illustrates inventive cell components in another cell according to the invention. The cell shown in Figure 2 has a horizontal cell bottom 25 which is covered with an aluminium-wetted cathode lining 21 according to the invention of uniform width forming a horizontal drained cathode surface 20. The sidewalls 40 of the cell are covered with an aluminium-wetted wedge-shaped sidewall lining 41' that extends from the periphery of the cell bottom 25 to above the surface of the molten electrolyte 5.

The cell bottom 25 comprises in the middle of the cell, a channel 30 for collecting product aluminium 60 drained from the adjacent aluminium-wettable cathode surfaces 20.

The aluminium collection channel 30 is preferably coated with a slurry-applied refractory boride layer 35 as described above. The slurry-applied layer 35 is wetted by molten aluminium 22 that wets also the bottom of the aluminium-wetted cathode lining 21.

Similarly to the cell shown in Figure 1, all the internal structural elements except anodes 10 are completely shielded from the molten electrolyte 5 by molten aluminium retained in and on the aluminium-wetted components according to the invention or by the layer of molten aluminium 60 collected in channel 30.

To prevent the electrolyte 5 from freezing along the sidewall lining 41' and on the surface of the electrolyte 5, the cell is thermally well insulated. As shown in Figure 2, the cell is fitted with an insulating cover 45 above the molten electrolyte 5. Further details of suitable covers are disclosed in WO01/31086 (de Nora/ Duruz) . The anodes 10 are preferably made of electrolyte resistant inert metal-based material. Suitable metal- based anode materials include iron and nickel based alloys which may be heat-treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz) , WO00/06804 (Crottaz/Duruz) , WOOl/42535 (Duruz/de Nora) , WOOl/42534 (de Nora/Duruz) and WOOl/42536 (Duruz/Nguyen/de Nora) .

Further oxygen-evolving anode materials are disclosed in W099/36593, W099/36594, WO00/06801, WO00/06805,

WO00/40783 (all in the name of de Nora/Duruz) , WO00/06800 (Duruz/de Nora) , W099/36591 and W099/36592 (both in the name of de Nora) .

To reduce the dissolution of the anodes 10 in the electrolyte, the cell may be operated with an electrolyte 5 at reduced temperature, typically from about 830° to 930°C, preferably from 850° to 910 °C. Operating with an electrolyte at reduced temperature reduces the solubility of oxides, in particular of alumina. Therefore, it is advantageous to enhance alumina dissolution in the electrolyte 5.

Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed devices are disclosed in greater detail in WOOO/63464 (de Nora/Berclaz) . Furthermore, the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in WO00/40781 (de Nora) .

During operation of the cells shown in Figures 1 and 2, alumina dissolved in the electrolyte is electrolysed to produce oxygen on the anodes 10 and aluminium 60 on the drained cathode surfaces 20. The product aluminium 60 drains from the cathode surfaces 20 into the collection channel 30 from where it can be tapped or evacuated into an aluminium reservoir (not shown) , for example as disclosed in WO00/63463 (de Nora) .

Figure 3 where the same reference numerals are used to designate the same elements, illustrates a retrofitted cell utilising aluminium-wetted components according to the invention and conventional consumable carbon anodes 10' .

The cell bottom 25 is horizontal and protected from wear with an aluminium-wetted cathode lining 21 according to the invention forming a drained cathode surface 20. The cell sidewalls 40 are covered with a sidewall lining 41 according to the invention, extending from the cell bottom to above the surface of the molten electrolyte 5. The aluminium-wetted sidewall lining 41 and the aluminium-wetted cathode linings 21 are joined through generally wedge-shaped bodies 51 according to the invention.

The cell bottom 25 is covered with a slurry-applied refractory boride layer 35 wetted by molten aluminium 22 that wets also the bottom of aluminium-wetted cathode lining 21.

Unlike the cell shown in Figures 1 and 2 , the cell shown in Figure 3 operates with a frozen electrolyte crust 70 and ledge 71.

During operation of the cell shown in Figure 3, alumina is dissolved into the electrolyte 5 and electrolysed between the carbon anodes 10' and the drained cathode surface 20 to produce C0₂ at the carbon anodes 10' and aluminium which is drained into channel 30.

In a variation, a retrofitted cell without an aluminium collection groove may operate with a shallow aluminium cathodic pool with little motion of molten aluminium in the shallow cathodic pool. Consequently, the inter-electrode distance may also be reduced which leads to a reduction of the cell voltage and energy savings. Furthermore, compared to conventional deep pool cells, a smaller amount of molten aluminium is needed to operate the cell which substantially reduces the costs involved with immobilising large aluminium stocks in aluminium production plants.

Nevertheless, these aluminium-wetted cathode linings can also be used in deep pool cells operating with a frozen electrolyte ledge and/or an electrolyte crust above the molten electrolyte. Furthermore, one or more large aluminium-wetted conductive plates according to the invention made from a low density openly porous or reticulated ceramic structure may be put into the aluminium pool so that the plates float at the surface of the aluminium pool to restrain aluminium motion and stabilise the aluminium pool. Thus, use of stabiliser plates in a deep aluminium pool permits a reduction of the inter-electrode distance. In further variations of the above cells only one or some of the above described cell components according to the invention, i.e. cathode lining 21,21', sidewall lining 41,41', wedge-shaped bodies 51 and stabiliser plates, may be used in an aluminium production cell, in different combinations.

The invention will be further described in the following examples .

Example 1

An openly porous alumina structure (10 pores per inch which is equivalent to about 4 pores per centimetre) was rendered aluminium-wettable by coating it with two slurry-applied layers of different composition.

The first slurry of the first layer was made of 60 weight% particulate needle-shaped surface-oxidised TiB₂ (-325 mesh) having a Ti0₂ surface oxide film, 3.3 weight% aluminium-wetting agent in the form of particulate Fe₂0₃

(-325 mesh) and 3.3 weight% Ti0₂ powder (-325 mesh) in 33 weight% colloidal Al₂0₃ (NYACOL^® Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer) . When this slurry is heat treated, the colloidal alumina reacts with a Ti0₂ surface oxide and the Ti0₂ powder to form a mixed oxide matrix of Al₂0₃ and Ti0₂ throughout the coating, this matrix containing and bonding the TiB₂ particles and the Fe₂0₃ particles. The second slurry was made of 33 weight% of partly oxidised copper particles, 37 weight% of a first grade of colloidal alumina (NYACOL^® Al-20) and 30 weight% of a second grade of colloidal alumina (CONDEA^® 10/2 Sol, a clear, opalescent liquid with a colloidal particle size of about 10 to 30 nanometer) .

An aluminium-wettable coating was applied onto the porous alumina structure by dipping this structure into the first slurry followed by drying for 4 hours at 40°C and dipping it into the second slurry followed by drying for 15 hours are 40°C. The coated alumina structure was then heat treated for 3 hours in air at 700°C to consolidate the coating.

The resulting structure is aluminium-wettable and is suitable to be wetted by aluminium before use or it can be wetted in-situ when used as a cathode.

The aluminium-wettable porous structure was wetted with aluminium by dipping it in molten aluminium at

850°C. After 20 hours the wetted porous structure was extracted from the molten aluminium and allowed to cool down to room temperature.

Examination of the aluminium-wetted porous structure showed that it was completely filled with aluminium retained in the pores by the wettability of the structure and the capillary effect, and covered over the outer surface with aluminium.

The electrical resistivity of the aluminium-wetted structure was of the order of the resistivity of metal aluminium (2.65 μΩ.cm), whereas before wetting the structure had a resistivity of 35 to 45 kΩ.cm.

Such a wetted alumina structure can be used for various applications in an aluminium electrowinning cell, in particular as a cathode or cathode lining, a cell sidewall or a sidewall lining, or as a non current carrying component of the cell bottom which is exposed to molten aluminium and/or electrolyte. Example 2

An aluminium-wettable ceramic structure was made of a mixture of material inert and resistant to molten aluminium, i.e. alumina and titania, and aluminium- wettable material , i.e. copper oxide . The ceramic structure was prepared by coating a polyurethane foam with a slurry of ceramic particles followed by a heat treatment .

The slurry of ceramic material consisted of a suspension of 40 g particulate Al₂0₃ with an average particle size of 10 to 20 micron, 2.5 g of particulate CuO with a particle size of less than about 45 micron, 2.5 g of particulate Ti0₂ with a particle size of less than about 45 micron in a colloidal alumina carrier consisting of 93 g deionised water and 6.6 g colloidal alumina particles with a colloidal particle size of about 10 to 30 nanometer.

A polyurethane foam having 10 to 20 pores per inch

(equivalent to about 4 to 8 pores per centimetre) was dipped into the slurry and dried in air at 40° to 50°C for 20 to 30 minutes. The dipping was repeated three times .

After dipping, the foam was dried in air at 50°C for 4 to 5 hours. The foam contained about 0.3 to 0.5 g/cm³ of the dried slurry. The drying was followed by a heat treatment at about 850° to 1000°C in air for 4 to 5 hours to eliminate the polyurethane foam and consolidate the ceramic material formed from the slurry into a self- sustaining foam. This heat treatment was followed by an aluminisation treatment by immersion in molten aluminium for 2 hours in molten aluminium at 850°C.

The aluminised foam was extracted from the molten aluminium, allowed to cool to room temperature and cut perpendicular to a surface. Examination of the aluminised foam showed that the polyurethane foam had disappeared. The Ti0₂ had reacted with Al₂0₃ in the ceramic foam to form a titanium- aluminium mixed oxide matrix. CuO present at the surface of the ceramic foam had reacted with molten aluminium to produce an aluminium-wetted surface layer of Al₂0₃ and an alloy of copper and aluminium. The pores of the ceramic foam were completely filled with molten aluminium.

In a variation, the heat treatment step and the aluminisation step are carried out simultaneously as a single step. In a further variation, the copper oxide of the ceramic structure is replaced partly or completely with iron oxide and/or nickel oxide.

Example 3 An aluminium-wettable openly porous ceramic structure as in Example 1 was tested as cathodic material for aluminium production.

The aluminium-wettable ceramic structure was placed on the bottom of a graphite receptacle having an inner diameter of 85 mm. The structure was covered with 120 g aluminium. The receptacle and its content was heated at a rate of 120°C/hour. At a temperature of 700°C, the aluminium had formed an aluminium pool on which the ceramic structure was floating. The temperature was further increased to about 850°C and then maintained for 4 hours so that the molten aluminium completely aluminised and wet the ceramic structure.

After aluminisation, an amount of 1.5 kg electrolytic molten bath consisting of 68 weight% cryolite, 28 weight% aluminium fluoride and 4 weight% dissolved alumina was poured into the receptacle on top of the aluminium pool and aluminium-wetted ceramic structure . A carbon anode was dipped into the electrolyte to face the floating ceramic structure which formed both an aluminium pool stabiliser and a cathode surface. An electrolysis current was passed between the anode and the graphite receptacle at a current density of about 0.8 A/cm² at the anode. A constant cell voltage of about 4 to 4.2 volt was measured throughout electrolysis.^' After 10 hours, electrolysis was interrupted and the floating aluminium-wetted ceramic structure extracted from the graphite receptacle. alloy of copper and aluminium. The pores of the ceramic foam were completely filled with molten aluminium.

Example 3

An aluminium-wettable openly porous ceramic structure as in Example 1 was tested as cathodic material for aluminium production.

After aluminisation, an amount of 1.5 kg electrolytic molten bath consisting of 68 weight% cryolite, 28 weight% aluminium fluoride and 4 weight% dissolved alumina was poured into the receptacle on top of the aluminium pool and aluminium-wetted ceramic structure. A carbon anode was dipped into the electrolyte to face the floating ceramic structure which formed both an aluminium pool stabiliser and a cathode surface. An electrolysis current was passed between the anode and the graphite receptacle at a current density of about 0.8 A/cm² at the anode. A constant cell voltage of about 4 to 4.2 volt was measured throughout electrolysis.

After 10 hours, electrolysis was interrupted and the floating aluminium-wetted ceramic structure extracted from the graphite receptacle.

The ceramic structure was allowed to cool down to room temperature and cut perpendicular to one of its surfaces. Examination of the ceramic structure showed that it was still completely wetted by and filled with molten aluminium. The ceramic structure itself had remained unchanged demonstrating its stability and suitability as cathode material.

Example 4

An openly porous silicon carbide structure (30 pores per inch which is equivalent to about 12 pores per centimetre) was rendered aluminium-wettable by coating it with a slurry-applied layer.

The slurry consisted of 75 g surface oxidised iron particles (-325 mesh) , 75 g Silica sol Nyacol 830 (a milky aqueous liquid containing 32 weight% colloidal silicon hydroxide that is converted into silica upon heat treatment) and 0.35 g of an aqueous solution containing 15% PVA (polyvinyl alcohol) that was used to adjust the viscosity of the slurry.

The openly porous structure was dipped onto the slurry and then dried for 30 min. at 60°C. The impregnated porous structure contained 0.278 g/cm³ of dried slurry including 0.214 g/cm³ surface oxidised iron particles .

The resulting structure was aluminium-wettable and suitable to be wetted by aluminium before use or in- itu when used for example as a cathode.

850°C. After 15 hours the wetted porous structure was extracted from the molten aluminium and allowed to cool down to room temperature.

Examination of the aluminium-wetted porous structure showed that it was filled with aluminium retained in the pores by the wettability of the structure and the capillary effect, and covered over the outer surface with aluminium. The pores had an aluminium filling ratio that was greater than 90 vol%. The aluminium-wetted porous structure can be used as cathodic material like in Example 3.

Claims

1. An aluminium-wettable component of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte, said component comprising an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium, the structure being made of a ceramic material inert and resistant to molten aluminium and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from said oxide and/or partly oxidised metal.

2. The component of claim 1, wherein the inert and resistant ceramic material comprises at least one oxide selected from oxides of aluminium, zirconium, tantalum, titanium, silicon, niobium, magnesium and calcium and mixtures thereof, as a simple oxide and/or in a mixed oxide .

3. The component of claim 1 or 2 , wherein the inert and resistant ceramic material comprises at least one carbide nitride or boride .

4. The component of claim 3 , wherein the inert and resistant ceramic material comprises at least one of aluminium nitride, AlON, SiAlON, boron nitride, silicon nitride, silicon carbide, aluminium borides, alkali earth metal zirconates and aluminates.

5. The component of any preceding claim, wherein the metal of said reactable metal oxide and/or partly oxidised metal is selected from manganese, iron, cobalt, nickel, copper and zinc and combinations thereof.

6. The component of any preceding claim, wherein the openly porous or reticulated ceramic structure comprises a coating of the aluminium-wettable material on the inert and resistant ceramic material.

7. The component of claim 6, wherein the aluminium- wettable coating is a slurry-applied coating comprising particles of said reactable metal oxide and/or partly oxidised metal in a dried colloidal carrier selected from alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, titanium oxide and zinc oxide, and mixtures and precursors thereof .

8. The component of claim 7, wherein the slurry-applied aluminium-wettable coating further comprises particles of at least one compound selected from metal borides, carbides and nitrides.

9. The component of claim 8, wherein the slurry-applied aluminium-wettable coating comprises the particles of said reactable metal oxide and/or partly oxidised metal and particles of titanium diboride in dried colloidal alumina .

10. The component of claim 8 or 9, wherein particles of a metal boride, carbide or nitride are covered with mixed oxides of metal derived from the dried colloidal carrier and metal derived from the metal boride, carbide or nitride .

11. The component of claim 10, wherein the slurry- applied aluminium-wettable coating is obtainable from a slurry that comprises metal oxide particles that combine upon heat treatment with a metal oxide derived from the dried colloidal carrier to form mixed oxides which are miscible with said mixed oxides covering the particles of metal boride, carbide or nitride.

12. The component of any one of claims 1 to 5, wherein the openly porous ceramic structure is made of a composition which consists of a mixture of the inert and resistant ceramic material and the aluminium-wettable ceramic material .

13. The component of any preceding claim, wherein the openly porous ceramic structure is formed on a reinforcing metal skeleton.

14. The component of any preceding claim, which comprises an internal insert acting as ballast.

15. The component of any preceding claim, which is a cathode or a cathode lining.

16. The component of any one of claims 1 to 14, which is an aluminium pool stabiliser in the form of a plate.

17. The component of any one of claims 1 to 14, which is a skeleton of a cell sidewall or a sidewall lining, which skeleton can be filled with molten aluminium to form an aluminium-infiltrated cell sidewall or sidewall lining.

18. The component of any one of claims 1 to 14, which is a skeleton of a wedge-shaped connecting body for joining the surface of a cell bottom to an adjacent sidewall, which skeleton can be filled with molten aluminium to form an aluminium-infiltrated connecting body.

19. An aluminium-wetted component of a cell for the electrowinning of aluminium, said aluminium-wetted component comprising an openly porous or reticulated ceramic structure which has a surface layer containing alumina, aluminium and another metal obtainable by exposing an aluminium-wettable component according to any preceding claim to molten aluminium.

20. The aluminium-wetted component of claim 19, which is filled and covered with aluminium that shields the openly porous or reticulated ceramic structure from exposure to molten electrolyte and/or corrosive gases during use.

21. The aluminium-wetted component of claim 19 or 20, which is a cathode or a cathode lining.

22. The aluminium-wetted component of claims 19 to 20, which is an aluminium pool stabiliser in the form of a plate .

23. The aluminium-wetted component of claim 20, which is a cell sidewall or a sidewall lining.

24. The aluminium-wetted component of claim 20, which is a wedge-shaped body for joining the surface of a cell bottom to an adjacent sidewall.

25. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based electrolyte, comprising at least one aluminium-wettable component as defined in any one of claims 1 to 16 and/or at least one aluminium-wetted component as defined in any one of claims 19 to 24.

26. The cell of claim 25, which comprises a cathode or a cathode lining as defined in claim 15 or 22.

27. The cell of claim 26, which comprises a cathode body having a surface lined with a plate-like or wedge-like cathode lining.

28. The cell of any one of claims 27, wherein the cathode body is joined to the cathode lining through a bonding layer.

29. The cell of claim 28, wherein the lined cathode surface is part of a horizontal or inclined cathode bottom.

30. The cell of claim 29, wherein the cathode bottom is horizontal and lined with a wedge-like cathode lining forming an aluminium-wettable drained sloping cathode surface thereon.

31. The cell of claim 26, 27 or 28, wherein the cathode or cathode lining is located above a cell bottom that is arranged to collect molten aluminium produced on and drained from the cathode or cathode lining.

32. The cell of any one of claims 26 to 31, comprising a cathode or cathode lining as defined in claim 13 which is top coated with an aluminium-wettable start-up layer.

33. The cell of claim 25 or 26, comprising one or more pool stabilisers as defined in claims 16 or 23 floating on an aluminium pool contained in the cell.

34. The cell of any one of claims 25 to 33, which comprises a cell sidewall or a sidewall lining as defined in claim 21.

35. The cell of claim 34, comprising a sidewall lining as defined in claim 23 that covers a sidewall made of carbon-containing material.

36. The cell of any one of claims 25 to 35, comprising at least one wedge-shaped connecting body as defined in claim 24 joining the cell bottom to an adjacent sidewall.

37. A composite openly porous or reticulated ceramic structure whose surface is wettable by molten aluminium, the structure being made of a ceramic material inert and resistant to molten aluminium, and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal selected from partly oxidised or oxide of copper, nickel, cobalt, manganese and iron and mixtures thereof, which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from said metal oxide and/or partly oxidised metal.

38. A composite ceramic-metal material comprising an openly porous or reticulated ceramic structure which has a surface layer containing alumina, aluminium and another metal, said composite ceramic-metal material being obtainable by exposing to molten aluminium a composite openly porous or reticulated ceramic structure as defined in claim 35.