Patent Publication Number: US-2003221955-A1

Title: Aluminium-wettable protective coatings for carbon components used in metallurgical processes

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates to aluminium-wettable protective coatings for components used in processes for the production, purification or recycling of metals in a molten state during which the components are exposed to high temperature oxidising and/or corrosive environments, methods of manufacturing such components, apparatus for the production, purification or recycling of metals comprising such components and their use to produce, purify or recycle metals.  
       BACKGROUND OF THE INVENTION  
       [0002] The production, purification or recycling of metals, such as aluminium or steel, is usually carried out at high temperature in very aggressive environments, in particular in molten metal, molten electrolyte and/or corrosive gas. Therefore, the materials used for the manufacture of components exposed to such environments must be thermally and chemically stable.  
       [0003] Graphite and other carbonaceous materials are commonly used for components, especially conductive components. Unfortunately, carbon components do not resist oxidation and/or corrosion and must be periodically replaced.  
       [0004] Several proposals have been made to reduce wear of carbon components in such technologies to achieve a higher operation efficiency, reduce pollution and the costs of operation.  
       [0005] In the field of steel recycling using arc electrode furnaces, it has been sought to reduce oxidation wear of inactive lateral faces of carbon arc electrodes, which is caused by exposure to oxygen at the high operating temperature. For instance, in U.S. Pat. No. 5,882,374 (Hendrix) it has been proposed to coat the inactive lateral face of the arc electrode with silica material to avoid consumption of the lateral face.  
       [0006] For the purification of molten metals, in particular molten aluminium, by the injection of a flux removing impurities towards the surface of the molten metal, it has been proposed to coat carbon components which are exposed to the molten metal with a coating of refractory material as disclosed in WO00/63630 (Holz/Duruz).  
       [0007] In aluminium production, some components are exposed to molten fluoride-containing electrolyte, molten aluminium and/or anodically produced oxygen. In conventional Hall-Héroult cells these components are still made of consumable carbonaceous materials.  
       [0008] However, it has been proposed to replace consumable non-carbon anodes with inert anodes. U.S. Pat. No. 5,720,860 (Sekhar/zhang) for example proposed the use of colloidal cerium acetate to produce anode bodies by a micropyretic reaction of a powder mixture of nickel, aluminium and copper. WO99/36593 (de Nora/Duruz) discloses slurry-applied metal oxide coatings used for aluminium electrowinning non-carbon anodes. The slurry is applied from a metal oxide or its precursor in a colloidal and/or polymeric carrier.  
       [0009] 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. However, as described in the following patents, such coatings have recently been successfully introduced.  
       [0010] U.S. Pat. Nos. 5,310,476, 5,364,513 (both in the name of Sekhar/de Nora) and 5,651,874 (de Nora/Sekhar) all disclose applying a protective coating of a refractory material such as titanium diboride to a carbon component of an aluminium electrowinning cell, by applying thereto a slurry of particulate refractory material and/or precursors thereof in a colloid in several layers with drying between each layer.  
       [0011] These coatings with preformed refractory material have shown outstanding performance compared to previous attempts to apply refractory coatings to components of aluminium electrowinning cells. These aluminium-wettable refractory boride coated bodies can be used in conventional cells with a deep aluminium pool and also permit the elimination of the thick aluminium pool required to partially protect the carbon cathode, enabling the cell to operate with a drained cathode.  
       [0012] These refractory boride coated bodies have the following attributes: excellent wettability by molten aluminium, inertness to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks, durability in the environment of an aluminium production cell, and ease of production and processing. The boride coating also acts as a barrier to sodium penetration.  
       [0013] During the start-up of aluminium electrowinning cells provided with cathodes having such aluminium-wettable refractory boride coatings, the coatings should be protected against oxidation and/or corrosion until the they are covered with molten aluminium.  
       [0014] WO98/17843 (de Nora/Sekhar/Duruz/Liu) discloses a method of starting up a cell which comprises a carbon cathode which can be coated with an aluminium-wettable refractory boride coating. During start-up the cathode is covered with temporary layers of protective materials. The layers applied to the cathode include inter-alia layers obtained from a slurry of particulate metal, such as aluminium, nickel, iron, titanium, cobalt, chromium, zirconium or copper, in a colloidal carrier or in a polymer.  
       [0015] U.S. Pat. No. 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.  
       OBJECTS OF THE INVENTION  
       [0016] An object of the invention is to provide an aluminium-wettable protective coating on a component, in particular made of carbon or another oxidisable or corrodable and/or wear-exposed material, for use at elevated temperature in oxidising and/or corrosive metallurgical environments, in particular in the production, purification or recycling of metals, to achieve improved protection.  
       [0017] Another object of the invention is to provide an apparatus for the production, purification or recycling of metals in a molten state, provided with components coated with coatings that maintain their protective and wettability properties even when exposed to highly oxidising and/or corrosive environments.  
       [0018] Yet a further object of the invention is to provide a method of operating an apparatus for the production, purification or recycling of metals in a molten state provided with coated components which maintain their protective and wettability properties even when exposed to highly oxidising and/or corrosive environments.  
       SUMMARY OF THE INVENTION  
       [0019] A first aspect of the invention relates to an aluminium-wettable protective coating on a substrate, in particular made of carbon material, for use at elevated temperature in an oxidising and/or corrosive gaseous and/or molten environment. The coating is applied to protect the substrate from liquid and/or gaseous attack. The coating comprises particles of at least one metal oxide and/or at least one partly oxidised metal in a dried and/or cured colloidal carrier and/or organic binder. The metal oxide and/or partly oxidised metal is reactable with molten aluminium so that the particles react with aluminium when exposed thereto to form an alumina matrix containing metal of said particles and aluminium.  
       [0020] Usually, the particles of said at least one partly oxidised metal contain at least 10 atomic % of the metal(s) as oxide(s), typically 20 to 30 atomic % or more.  
       [0021] 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 substantially more electronegative than aluminium.  
       [0022] The particles may be non-dispersed but suspended or suspendable in the colloidal carrier and/or organic binder. The metal(s) of the particles can selected from manganese, iron, cobalt, nickel, copper, zinc, niobium, molybdenum and tantalum.  
       [0023] The coating may comprise a dried colloidal carrier selected from colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate and cerium acetate and combinations thereof. The coating can alternatively or additionally contain a cured organic binder selected from polyurethane, ethylene glycol, polyethylene glycol, resins, esters or waxes.  
       [0024] Upon drying and/or curing the aluminium-wettable coating can be heat-treated before exposure to molten aluminium. However, the heat treatment can be carried out solely by exposure to molten aluminium.  
       [0025] Compared to known refractory boride coatings, it has been observed that the coatings according to the invention are better wettable by molten metal and are less likely to be impaired when exposed to highly oxidising and/or corrosive environments, for example during start-up procedures of aluminium electrowinning cells. The improved wettability comes initially from the oxide particles which can react with the molten aluminium and consequently draw molten aluminium into the coating. The oxide protection on the metal particles improves their resistance against oxidation during start-up. Hence, cell components protected with such a coating remain aluminium-wettable even if the start-up procedure is not carried out under optimal conditions.  
       [0026] Refractory boride coatings of cathodes used in aluminium electrowinning cells require strict cell start-up conditions to avoid detrimental alteration of the coatings. During start-up such refractory boride coatings need to be properly protected against oxidation, for example as disclosed in WO98/17843 (de Nora/Sekhar/Duruz/Liu), otherwise they lose their capacity to be wetted by aluminium.  
       [0027] However, for certain applications, it may be advantageous to combine an aluminium-wettable coating according to the present invention with known refractory boride coatings, for instance as disclosed in U.S. Pat. Nos. 5,364,513 (Sekhar/de Nora) and 5,651,874 (de Nora/Sekhar) or in PCT/IB00/01551 (Nguyen/Duruz/de Nora). Indeed, the aluminium wettability of the coating according to the present invention can be such that molten aluminium infiltrates the coating down to the substrate. To avoid contact between molten aluminium and the substrate, one or more sub-layers of conventional boride coatings, which are inert and impervious to molten aluminium, can be used to separate a coating according to the invention from the substrate. For example a thin layer, e.g. 0.5 to 2 mm thick, of refractory boride material, as disclosed in 5,651,874 (de Nora/Sekhar), can be used as a sub-layer under a thicker aluminium-wettable coating according to the invention, e.g. 3 to 8 mm thick.  
       [0028] Furthermore, the long term mechanical resistance and wettability of the protective coatings according to the invention can be improved by adding a refractory hard metal boride, such as titanium diboride and/or zirconium diboride. For instance, the particles may be made of a core of a refractory hard metal boride covered with a metal oxide or partly oxidised metal. The coating may also contain a mixture of metal oxide particles and/or partly oxidised metal particles with particles of refractory hard metal boride. The metal oxide and/or partly oxidised metal thus combined with the refractory hard metal boride may include at least one of copper, nickel and iron as an oxide or partly oxidised metal.  
       [0029] A coating according to the invention with refractory hard metal boride particles in a dried colloidal carrier that consists of dried colloidal metal oxide, preferably comprises particles of the refractory hard metal boride covered with mixed oxides of metal derived from the refractory hard metal boride and metal derived from the dried colloidal metal oxide. Moreover, the slurry-applied aluminium-wettable coating can be obtained from a slurry comprising metal oxide particles that combine upon heat treatment with dried colloidal metal oxide to form mixed oxides which are miscible with said mixed oxides covering the particles of said refractory hard metal boride, as disclosed in PCT/IB00/01551 (Nguyen/Duruz/de Nora).  
       [0030] Optionally, the coating may contain carbon powder which is mixed to the colloid and/or inorganic binder before application of the coating.  
       [0031] Thick or thin coatings may be applied to the substrate, e.g. having a thickness of 0.1 to 5 mm, typically 0.3 to 3 mm.  
       [0032] In one embodiment, the coating is made of a plurality of layers of different compositions. For example, one or more long-lasting aluminium-wettable layers comprising a mixture of boride particles and particles of the metal oxide and/or partly oxidised metal reactable with molten aluminium underlie one or more highly aluminium-wettable layers made of the reactable particles of metal oxide and/or partly oxidised metal in the dried and/or cured colloidal carrier and/or organic binder. As mentioned above, the coating may comprise one or more boride-based sub-layers serving as anchorage layers on the substrate, the sub-layers being inert and impervious to molten aluminium.  
       [0033] The invention relates also to an aluminium-wetted protective coating on a substrate obtainable by exposing an aluminium-wettable coating as described above to molten aluminium. The wetted coating an alumina matrix contains metal of the reactable metal oxide and/or partly oxidised metal particles and aluminium.  
       [0034] The aluminium-wettable coating may be used as such in an aluminium electrowinning cell or in any apparatus in which during use the coating is exposed to molten aluminium, e.g. an aluminium purification apparatus. During initial use, the aluminium-wettable coating is wetted by molten aluminium so that the metal oxide particles and/or partly oxidised metal particles of the coating react with the molten aluminium to give the final aluminium-wetted coating described above.  
       [0035] For applications in which the coating does not (sufficiently) contact molten aluminium, the precursor coating should be exposed to molten aluminium before use so that the reaction between the metal oxide particles and/or partly oxidised metal particles and aluminium takes place before use.  
       [0036] Another aspect of the invention is a method of forming an aluminium-wettable protective coating on a substrate as described above. The method comprises applying particles of at least one metal oxide and/or at least one partly oxidised metal in a colloidal carrier and/or organic binder onto the substrate as one or more layers, drying and/or curing the applied layer(s).  
       [0037] Yet another aspect of the invention is a method of forming an aluminium-wetted protective coating on a substrate comprising exposing the coating as described above to molten aluminium so that the particles of at least one metal oxide and/or at least one partly oxidised metal react with aluminium to form an alumina matrix containing metal of the particles and aluminium.  
       [0038] A further aspect of the invention is an apparatus for the production, purification or recycling of a metal in a molten state. This apparatus comprises at least one component having a substrate, in particular a carbon substrate, coated with an aluminium-wettable protective coating as described above. In particular when the component is not for use in molten aluminium, the component is preferably wetted by molten aluminium before use. During use, such a component is exposed to an oxidising and/or corrosive gaseous and/or molten environment at elevated temperature and is protected from liquid and/or gaseous attack by the aluminium-wettable protective coating.  
       [0039] The apparatus may be an aluminium electrowinning cell, in which case the coated component is a cell component. The cell component can be part of a cell bottom coated with said aluminium-wettable protective coating protecting the cell bottom component from molten aluminium and/or molten electrolyte. The cell component may also be a cathode having an aluminium-wettable surface, in particular a drained surface, formed by an upper surface of the aluminium-wettable protective coating protecting the cathode from molten aluminium and/or molten electrolyte. Alternatively, the cell component is an anode having one or more electrochemically inactive surfaces coated with the aluminium-wettable protective coating protecting the inactive surface from molten electrolyte and/or anodically produced gas. Another cell component is a cell sidewall coated with the aluminium-wettable protective coating protecting the sidewall from molten electrolyte, molten aluminium and/or anodically produced gas.  
       [0040] The apparatus can also be an arc furnace for the recycling of steel, the coated component being an arc-electrode or an arc-electrode holder with the aluminium-wettable protective coating protecting it from oxidising gas and/or molten steel.  
       [0041] Another embodiment is an apparatus for the purification of a molten metal, such as aluminium, magnesium, cast iron or steel, by the injection of a purifying fluid into the molten metal to remove impurities towards the surface thereof. The coated component may be a rotatable stirrer or a vessel for containing molten metal to be purified with the aluminium-wettable protective coating protecting it from the molten metal to be purified, the purifying fluid and/or impurities of the molten metal by the aluminium-wettable protective coating.  
       [0042] Yet another aspect of the invention relates to a method of producing, purifying or recycling a metal in an apparatus as described above and comprising at least one component coated with the aluminium-wettable protective coating. The method comprises exposing said coated component to an oxidising and/or corrosive gaseous and/or molten environment at elevated temperature. The product of the method may be primary aluminium, recycled steel, or a purified metal such as purified aluminium, magnesium, iron or steel. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0043] Embodiments of the invention will now be described by way of example with reference to the accompanying schematic drawings, wherein:  
     [0044]FIG. 1 shows a schematic cross-sectional view of an aluminium production cell with carbonaceous drained cathodes having an aluminium-wettable protective coating in accordance with the invention;  
     [0045]FIG. 2 schematically shows an arc electrode furnace incorporating protective coatings according to the invention;  
     [0046]FIG. 3 shows an apparatus for the purification of a molten metal having a carbonaceous stirrer protected with an aluminium-wettable protective coating according to the invention;  
     [0047]FIG. 3 a  is an enlarged schematic sectional view of part of the stirrer shown in FIG. 3; and  
     [0048]FIG. 4 schematically shows a variation of the stirrer shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION  
     [0049] Aluminium Electrowinning Cell  
     [0050]FIG. 1 shows an aluminium electrowinning cell comprising a series of carbonaceous anode blocks  5  having operative surfaces  6  suspended over drained sloping flattened generally V-shaped cathode surface  21  in a fluoride-containing molten electrolyte  42  containing dissolved alumina.  
     [0051] The drained cathode surface  21  is formed by the surface of an aluminium-wettable protective coating  20 A applied to the upper surfaces of a series of juxtaposed carbon cathode blocks  15  extending in pairs arranged end-to-end across the cell. The aluminium-wettable protective coating  20 A is deposited from metal oxide particles and/or partly oxidised metal particles in a colloidal carrier and/or organic binder according the invention, for instance as embodied in Examples 1 to 3.  
     [0052] The cathode blocks  15  comprise, embedded in recesses located in their bottom surfaces, current supply bars  22  of steel or other conductive material for connection to an external electric current supply.  
     [0053] The drained cathode surface  21  is divided by a central aluminium collection groove  26  located between pairs of cathode blocks  15  arranged end-to-end across the cell. The aluminium collection groove  26  is situated at the bottom of the drained cathode surface  21  and is arranged to collect the product aluminium draining from the cathode surface  21 . The aluminium collection groove  26  is coated with an aluminium-wettable protective coating  20 B according to the invention.  
     [0054] The anode blocks  5  too are coated with an aluminium-wettable refractory protective coating  20 C on their inactive surfaces, but not on the operative anode surfaces  6  which are immersed as such in the molten electrolyte  42 .  
     [0055] The cell comprises carbonaceous sidewalls  16  exposed to molten electrolyte and to the environment above the molten electrolyte, but protected against the molten electrolyte  42  and the environment above the molten electrolyte with an aluminium-wettable protective coating  20 D according to the invention.  
     [0056] The method of application of the aluminium wettable coatings  20 A, 20 B, 20 C, 20 D comprises applying to the surface of the component a slurry of metal oxide particles and/or partly oxidised metal particles in a colloidal carrier and/or organic binder as specified above, followed by drying. The cathode coating  20 A and the collection groove coating  20 B can be heat treated before or after installation in the aluminium production cell, and the reaction of the coating with molten aluminium can be done before or during use. However, the anode coating  20 C and the sidewall coating  20 D should be heat treated and reacted with molten aluminium before use in the cell since these components do not contact molten aluminium during use.  
     [0057] The method of coating the components  5 , 15 , 16  of the present invention by application of the slurry involves painting (by brush or roller), dipping, spraying, or pouring the slurry onto the components  5 , 15 , 16  and allowing to dry before another layer is added. The coating  20 A, 20 B, 20 C, 20 D does not need to be entirely dry before the application of the next layer. It is preferred to heat at least the final coating  20 A, 20 B, 20 C, 20 D with a suitable heat source so as to completely dry it and improve densification. Heating and drying take place preferably at about 80-200° C., usually for half an hour to several hours, and further heat treatments are possible.  
     [0058] The surfaces of the carbon components  5 , 15 , 16  to be coated with this slurry may be treated by sand blasting or pickled with acids or fluxes such as cryolite or other combinations of fluorides and chlorides prior to the application of the coating  20 A, 20 B, 20 C, 20 D. Similarly the surfaces may be cleaned with an organic solvent such as acetone to remove oily products and other debris prior to coating. These treatments will enhance the bonding of the coatings to the component.  
     [0059] Before or after application of the coating  20 A, 20 B, 20 C, 20 D and before use, the surfaces of the components  5 , 15 , 16  can be painted, sprayed, dipped or infiltrated with reagents and precursors, gels and/or colloids.  
     [0060] In operation of the cell illustrated in FIG. 1, alumina dissolved in the molten electrolyte  42  at a temperature of 750° to 960° C. is electrolysed between the anodes  5  and the cathode blocks  15  to produce gas on the operative anodes surfaces  6  and molten aluminium on the aluminium-wettable drained cathode coating  20 A.  
     [0061] The cathodically-produced molten aluminium flows down the inclined drained cathode surface  21  into the aluminium collection grooves  26  onto the aluminium-wettable protective coating  20 B from where it flows into an aluminium collection reservoir for subsequent tapping.  
     [0062]FIG. 1 shows a specific aluminium electrowinning cell by way of example. It is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. For instance, the cell may have one or more aluminium collection reservoirs across the cell, each intersecting the aluminium collection groove to divide the drained cathode surface into four quadrants as described in PCT/IB99/00698 (de Nora). The anodes may be made of inert materials and have an electrochemically active structure of grid-like design to permit electrolyte circulation, as for example disclosed in PCT/IB99/01739 (de Nora/Duruz).  
     [0063] Arc Furnace:  
     [0064] The arc furnace shown in FIG. 2 comprises three consumable electrodes  15 A arranged in a triangular relationship. For clarity, the distance between the electrodes  15 A as shown in FIG. 2 has been proportionally increased with respect to the furnace. Typically, the electrodes  15 A have a diameter between 200 and 500 mm and can be spaced by a distance corresponding to about their diameter.  
     [0065] The electrodes  15 A are connected to an electrical power supply (not shown) and suspended from an electrode positioning system above the cell which is arranged to adjust their height.  
     [0066] The consumable electrodes  15 A are made of a carbon substrate laterally coated with an aluminium-wettable protective coating  20  protecting the carbon substrate from oxidising gas. According to the invention, the coating is deposited from particles of at least one metal oxide and/or at least one partly oxidised metal in a colloidal carrier and/or organic binder applied onto the carbon substrate as one or more layers which are dried and/or cured and then exposed to molten aluminium so that the particles react with aluminium to form an alumina matrix containing metal of the particles and aluminium.  
     [0067] The method of application of the coating  20  is similar to the method described in relation to the cathode blocks  15  described above.  
     [0068] The bottom of electrodes  15 A which is consumed during operation and constitutes the electrodes&#39; operative surface is uncoated. The coating  20  protects only the electrodes&#39; lateral faces against premature oxidation.  
     [0069] The electrodes  15 A dip in an iron source  41 , usually containing iron oxide or oxidised iron, such as scrap iron, scrap steel and pig iron. Preferably, the iron source  41  further comprises reductants selected from gaseous hydrogen, gaseous carbon monoxide or solid carbon bearing reductants. The reductants may also comprise non-iron minerals known as gangue which include silica, alumina, magnesia and lime.  
     [0070] The iron source  41  floats on a pool of liquid iron or steel  40  resulting from the recycling of the iron source  41 .  
     [0071] During use, a three phase AC current is passed through electrodes  15 A, which directly reduces iron from the iron source  41 . The reduced iron is then collected in the iron or steel pool  40 . The gangue contained in the reduced iron is separated from the iron by melting and flotation forming a slag (not shown) which is removed, for example through one or more apertures (not shown) located on sidewalls of the arc furnace at the level of the slag.  
     [0072] The pool of iron or steel  40  is periodically or continuously tapped for instance through an aperture (not shown) located in the bottom of the arc furnace.  
     [0073] Molten Metal Purification Apparatus:  
     [0074] The molten metal purification apparatus partly shown in FIG. 3 comprises a vessel  45  containing molten metal  40 ′, such as molten aluminium, to be purified. A rotatable stirrer  10  made of carbon-based material, such as graphite, is partly immersed in the molten metal  40 ′ and is arranged to rotate therein.  
     [0075] The stirrer  10  comprises a shaft  11  whose upper part is engaged with a rotary drive and support structure  30  which holds and rotates the stirrer  10 . The lower part of shaft  11  is carbon-based and dips in the molten metal  40 ′ contained in vessel  45 . At the lower end of the shaft  11  is a rotor  13  provided with flanges or other protuberances for stirring the molten metal  40 ′.  
     [0076] Inside the shaft  11 , along its length, is an axial duct  12 , as shown in FIG. 3 a , which is connected at the stirrer&#39;s upper end through a flexible tube  35  to a gas supply (not shown), for instance a gas reservoir provided with a gas gate leading to the flexible tube  35 .  
     [0077] The axial duct  12  is arranged to supply a fluid to the rotor  13 . The rotor  13  comprises a plurality of apertures connected to the internal duct  12  for injecting the gas into the molten metal  40 ′, as shown by arrows  51 .  
     [0078] The lower part of the shaft  11 , i.e. the immersed part and the interface region at or about the meltline  14  of the shaft, as well as the rotor  13  are coated with a protective coating  20 E deposited from metal oxide particles and/or partly oxidised metal particles in a colloidal carrier and/or organic binder according to the invention which improves the resistance to erosion, oxidation and/or corrosion of the stirrer during operation.  
     [0079] As shown in FIG. 3, the upper part of shaft  11  is also protected with a protective coating  20 F, against oxidation and/or corrosion. The upper part of the carbon-based shaft  11  is coated with a thin coating of refractory material  20 F providing protection against oxidation and corrosion, whereas the coating  20 E protecting the immersed part of the shaft  11  and the rotor  13  is a thicker coating of refractory material providing protection against erosion, oxidation and corrosion.  
     [0080] Likewise, surfaces of the vessel  45  which come into contact with the molten metal may be protected with an aluminium-wettable protective coating according to the invention.  
     [0081] The method of application of the coating  20 E, 20 F is similar to the method described in relation to the cathode blocks  15  described above.  
     [0082] During operation of the apparatus shown in FIG. 3, a reactive or non-reactive fluid, in particular a gas  50  alone or a flux, such as a halide, nitrogen and/or argon, is injected into the molten metal  40 ′ contained in the vessel  45  through the flexible tube  35  and stirrer  10  which dips in the molten metal  40 ′.  
     [0083] The stirrer  10  is rotated at a speed of about 100 to 500 RPM so that the injected gas  50  is dispersed throughout the molten metal in finely divided gas bubbles. The dispersed gas bubbles  50 , with or without reaction, remove impurities present in the molten metal  40 ′ towards its surface, from where the impurities may be separated thus purifying the molten metal.  
     [0084] The stirrer  10  schematically shown in FIG. 4 dips in a molten metal bath  40 ′ and comprises a shaft  11  and a rotor  13 . The stirrer  10  may be of any type, for example similar to the stirrer shown in FIG. 3 or of conventional design as known from the prior art. The rotor  13  of stirrer  10  may be a high-shear rotor or a pump action rotor.  
     [0085] In FIG. 4, instead of coating the entire shaft  11  and rotor  13 , parts of the stirrer  10  liable to erosion are selectively coated with an aluminium-wettable protective coating according to the invention.  
     [0086] The interface portion at and about the meltline  14  of the carbon-based lower part of the shaft  11  is coated with a refractory interface coating  20 E 1 , for instance over a length of up to half that of the shaft  11 . Excellent results have been obtained with a coating over a third of shaft  11 . However, the length of coating  20 E 1  could be a quarter of the length of shaft  11  or even less, depending on the design of stirrer  10  and the operating conditions.  
     [0087] In addition to the interface portion of such stirrers, other areas may be liable to erode, again depending on the design and operating conditions of the stirrers. The schematically shown stirrer  10  in FIG. 4 illustrates further coated surfaces which are particularly exposed to erosion. The lower end of the shaft  11  adjacent to the rotor  13  is protected with a coating  20 E 2 , the lateral surface of rotor  13  is protected with a coating  20 E 3  and the bottom surface of the rotor  13  is coated with a coating  20 E 4 .  
     [0088] For each specific stirrer design, the coating or different protective coatings on different parts of the stirrer, such as coatings  20 E 1 ,  20 E 2 ,  20 E 3  and  20 E 4  shown in FIG. 4, may be adapted as a function of the expected lifetime of the stirrer. For optimal use, the amount and location of such coatings can be so balanced that they each have approximately the same lifetime.  
     [0089] In an alternative embodiment (not shown), the coating on such stirrers may be continuous as illustrated in FIG. 3 but with a graded thickness or composition so as to adapt the resistance against erosion to the intensity of wear of each part of the stirrer, thereby combining the advantages of the different coatings shown in FIG. 4.  
     [0090] Various modifications can be made to the apparatus shown in FIGS. 3, 3 a  and  4 . For instance, the shaft shown in FIG. 3 may be modified so as to consist of an assembly whose non-immersed part is made of a material other than carbon-based, such as a metal and/or a ceramic, which is resistant to oxidation and corrosion and which, therefore, does not need any protective coating, whereas the immersed part of the shaft is made of carbon-based material protected with a protective coating according to the invention. Such a composite shaft would preferably be designed to permit disassembly of the immersed and non-immersed parts so the immersed part can be replaced when worn.  
     [0091] Likewise, a carbon-based non-immersed part of the shaft may be protected from oxidation and corrosion with a coating and/or impregnation of a phosphate of aluminium, in particular applied in the form of a compound selected from monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, aluminium metaphosphate, and mixtures thereof as disclosed in U.S. Pat. No. 5,534,119 (Sekhar). It is also possible to protect the non-immersed part of the shaft with a coating and/or impregnation of a boron compound, such as a compound selected from boron oxide, boric acid and tetraboric acid as disclosed in U.S. Pat. No. 5,486,278 (Manganiello/Duruz/Bellò) and in co-pending application WO97/26626 (de Nora/Duruz/Berclaz).  
     [0092] In a modification, the protective coating of the invention may simply be applied to any part of the stirrer in contact with the molten metal, to be protected against erosion, oxidation and/or corrosion during operation.  
     [0093] The invention will be further described in the following examples.  
     EXAMPLE 1  
     [0094] Particle Encapsulation:  
     [0095] A particulate consisting of titanium diboride particles encapsulated in partly oxidised copper was prepared as follows:  
     [0096] An amount of 100 g of TiB 2  powder having a particle size of less than 15 micron and a surface area of about 0.2 m 2 /g was added to 300 ml of an etching solution containing 30 vol. % concentrated HCl and 70 vol. % deionised water and mixed in a beaker at room temperature with a magnetic stirrer. After 15 minutes of vigorous stirring, the TiB 2  powder was fully dispersed in the etching solution which had become a violet dispersion.  
     [0097] An amount of 40 g CuSO 4 .5H 2 O solid was added to the dispersion (the amount corresponding to about 8.5 g Cu per 100 g of TiB 2 ) followed by vigorous stirring for 15 minutes. The temperature of the mixture increased to about 50° C. and the violet colour of the dispersion became more intense.  
     [0098] The dispersion was filtered to separate the powder from the solution. The powder was then washed several times with deionised water until a pH of 4.5 was reached in the filtrate. Finally the powder was dried in air at 50 to 60° C.  
     [0099] Analysis of the powder showed that it consisted of TiB 2  particles encapsulated in partly oxidised copper with a copper content of about 9.8 weight % which corresponds to an average copper coating thickness of 0.06 micron on the TiB 2  particles. About 20 atomic % of the copper was in oxide form.  
     [0100] Coating:  
     [0101] A coating on a carbon substrate was prepared from a slurry of the partly oxidised copper-encapsulated titanium diboride particles as follows:  
     [0102] The slurry was obtained by mixing 125 g of the partly oxidised copper-encapsulated titanium diboride particles with 52 ml of colloidal alumina which consisted of 28 ml Nyacol® (Al-20, a milky liquid with a colloidal particle size grade of about 40 to 60 nanometer) and 24 ml Condea® (CONDEA® 10/2 Sol, a clear, opalescent liquid with a colloidal particle size grade of about 10 to 30 nanometer).  
     [0103] Using a multigrade colloidal carrier such as a mixture of Nyacol® and Condea®, provides a combination of properties, namely better homogeneity/bonding without cracking and enhanced protection against sodium penetration when the coating or body is used as a component in an aluminium production cell. Moreover, the slurry with this mixed colloidal carrier has improved thixotropic properties, in particular it remains in suspension much longer than standard colloids, which leads to significant cost savings.  
     [0104] A coating having a thickness of 2.5 mm was formed on a carbon substrate by successively painting and drying at room temperature 10 layers of this slurry on the substrate.  
     [0105] To aluminise the coating, the substrate was placed in an alumina crucible and covered with a sheet of aluminium having a thickness of 3 to 5 cm. The crucible was then heat treated in a furnace at 970° C. in air for 10 hours.  
     [0106] After cooling, a cross-section of the coated substrate was subjected to microscopic examination. The coating showed a good penetration of aluminium metal. Aluminium had reacted with the oxygen from the partly oxidised copper of the encapsulation to form an alumina matrix and the remaining metallic copper was alloyed to the aluminium which had infiltrated the coating providing an excellent aluminium-wettability at high temperature.  
     [0107] Such a coating is particularly suitable for cathodes or the cell bottom of an aluminium electrowinning cells, as well as for use in an apparatus for the purification of a molten metal, in particular molten aluminium, as described above.  
     [0108] Compared to known refractory boride coatings, for instance as disclosed in U.S. Pat. No. 5,651,874 (de Nora/Sekhar), the partly oxidised copper encapsulating the titanium diboride particles improved the wettability of the present coating. Furthermore, when subjected to an oxidising environment, the present coating still remains aluminium-wettable in contrast to known refractory boride coatings.  
     EXAMPLE 2  
     [0109] A coating from a slurry of partly oxidised particulate copper in a colloid was prepared as follows:  
     [0110] The slurry was obtained by mixing 300 g of partly oxidised particulate copper having a particle size of less than 10 micron with 52 ml colloidal alumina which consisted of 28 ml Nyacol® (a milky liquid with a colloidal particle size of about 40 to 60 nanometer) and 24 ml Condea® (a clear, opalescent liquid with a colloidal particle size of about 10 to 30 nanometer).  
     [0111] A coating having a thickness of 1.5 mm was formed on a carbon substrate by successively painting and drying at room temperature three layers of this slurry on the substrate.  
     [0112] To aluminise the coating, the substrate was placed in an alumina crucible and covered with a sheet of aluminium having a thickness of 3 to 5 cm. The crucible was then heat treated in a furnace and at 970° C. in air for 10 hours.  
     [0113] The dimensions of the coating on the substrate had remained substantially unchanged after cooling. A microscopic examination of a cross-section of the coating showed a continuous matrix of alumina filled with aluminium and 2-3 atomic % copper in the form of copper-rich copper-aluminium alloy particles uniformly dispersed in the aluminium. This makes use of the aluminium-wettable protective coating according to the invention particularly advantageous for cell start-up.  
     EXAMPLE 3  
     [0114] A coating from a slurry of particulate iron in a colloid was prepared as follows:  
     [0115] The slurry was obtained by mixing 260 g of partly oxidised iron particles having a particle size of less than 10 micron with 28 ml colloidal alumina such as Condea®.  
     [0116] A coating having a thickness of 1.5 mm was formed on a carbon substrate by successively painting and drying at room temperature three layers of this slurry on the substrate.  
     [0117] The coating was aluminised and heat-treated as described in Examples 1 and 2.  
     [0118] The final coating had a metallic aspect and its surface was clearly defined under the above aluminium mass originating from the aluminisation treatment.  
     [0119] A microscopic examination of a cross-section of the coating showed a continuous matrix of alumina filled with aluminium and less than 1 atomic % iron in the form of iron-aluminium alloy particles uniformly dispersed in the aluminium, the iron-aluminium alloy particles containing 35 to 50 atomic % aluminium.  
     [0120] As a variation, part or all of the iron particles may be replaced with a corresponding amount of nickel particulate or with a nickel-iron alloy particulate of similar particle size.  
     EXAMPLE 4  
     [0121] A slurry free of any metal oxide and/or partly oxidised metal reactable with molten aluminium for use as an anchorage layer for a coating according to the invention was prepared by suspending a refractory hard metal boride consisting of 47.5 g surface-oxidised particulate spherical TiB 2  (−325 mesh) having a TiO 2  surface film and a metal oxide in the form of 2.5 g TiO 2  (−325 mesh) in a colloidal carrier consisting of 20 ml colloidal Al 2 O 3  (NYACOLD Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer) and 1 ml PEG (polyethylene glycol) which increases the viscosity of the slurry and enhances its capacity to be applied by painting as well as the adherence and coherence of the final coating.  
     [0122] This slurry produces upon heat treatment an oxide matrix of titanium-aluminium mixed oxide from the reaction of the colloidal oxide Al 2 O 3  and TiO 2  present as suspended oxide particles and oxide film covering the suspended TiB 2  particles. The oxide matrix contains and bonds TiB 2  particles.  
     Example 4a  
     [0123] The constituents of the slurry of Example 4 may be changed as shown in the following Table in which each line represents possible combinations of constituents:  
                                               Suspended Surface-               Oxidised       Colloidal or   Suspended Metal   Refractory Metal       Polymeric Oxides   Oxides   Compounds                  Al 2 O 3     TiO 2 , MgO or SiO 2     TiB 2 , SiC, TiC or TiN       TiO 2     Al 2 O 3  or MgO   SiC or SiN       SiO 2     Al 2 O 3  or MgO   TiB 2 , TiC or TiN                  
 
     EXAMPLE 5  
     [0124] A slurry for producing a coating according to the invention was prepared by suspending 2.5 g particulate Fe 2 O 3  (−325 mesh), a refractory hard metal boride consisting of 92.5 g particulate needle-shaped surface-oxidised TiB 2  (−325 mesh) having a TiO 2  surface oxide film, and 2.5 g particulate TiO 2  (−325 mesh) in a colloid consisting of a combination of two grades of colloidal Al 2 O 3 , namely 28 ml of a first grade of colloidal Al 2 O 3  (NYACOL® Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer) and 24 ml of a second grade of colloidal Al 2 O 3  (CONDEA® 10/2 Sol, a clear, opalescent liquid with a colloidal particle size of about 10 to 30 nanometer).  
     [0125] This slurry produces upon heat treatment a matrix of mixed oxides consisting of titanium-aluminium mixed oxide and a small amount of iron-titanium-aluminium mixed oxide from the reaction of TiO 2 , Fe 2 O 3  and Al 2 O 3 . This matrix contains and bonds the TiB 2  and Fe 2 O 3  particles.  
     Example 5a  
     [0126] Example 5&#39;s slurry composition consists of Fe 2 O 3  and a reaction mixture made of the colloid (Al 2 O 3 ), the suspended refractory metal boride (TiB 2 ) the suspended metal oxide (TiO 2 ). This Example can be modified by completely or partly substituting Fe 2 O 3  with partly oxidised or oxides of copper and/or nickel, and/or by varying the composition of the reaction mixture as in Example 4a.  
     EXAMPLE 6  
     [0127] A further slurry for producing a highly aluminium-wettable start-up layer according to the invention, was prepared as follows. An amount of 60 g of surface oxidised copper particles (−325 mesh) was suspended in a carrier consisting of 13 ml of colloidal Al 2 O 3  (7 ml NYACOL® Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer and 6 ml CONDEA® 10/2 Sol, a clear, opalescent liquid with a colloidal particle size of about 10 to 30 nanometer) and 1 ml of PEG (polyethylene glycol) which increases the viscosity of the slurry and enhances its capacity to be applied by painting as well as the adherence and coherence of the final coating.  
     [0128] Upon heat treatment the slurry produces an alumina matrix containing and bonding the oxidised copper particles.  
     [0129] As a modification, oxidised or partly oxidised particles of nickel and/or iron may be used to substitute in part or completely the oxidised copper particles in colloidal alumina (CONDEA 25/5 with a pH&gt;7).  
     EXAMPLE 7  
     [0130] Three carbon cathodes for use in a drained cell for the production of aluminium were each coated with the slurries of Examples 4, 5 and 6 as follows:  
     [0131] First, an anchorage layer having a thickness of about 100 micron was painted onto the exposed surface of the carbon cathode from the slurry of Example 4. The anchorage layer was allowed to dry for 30 minutes.  
     [0132] The anchorage layer was covered with an aluminium-wettable layer obtained by painting 8 layers of the slurry of Example 5. Each applied layer was allowed to dry for 30 minutes before application of the next layer. The final aluminium-wettable layer had a thickness of about 1.8 mm.  
     [0133] The aluminium-wettable layer was then covered with a temporary start-up layer obtained by painting 1 layer of the slurry of Example 6. The start-up layer had a thickness of about 100 to 150 micron.  
     [0134] The coating formed by the anchorage layer, the aluminium-wettable layer and the start-up layer on the carbon cathode was allowed to dry for 24 hours.  
     [0135] Two of the three cathodes were then covered with an aluminium sheet having a thickness of about 1.5 cm and heated in an oven at a temperature of about 850-900° C. in air.  
     [0136] The first cathode was extracted from the oven after 30 minutes and allowed to cool down to ambient temperature. Examination of a cross-section of the coating showed that aluminium had infiltrated the start-up layer so that the coating was superficially wetted by molten aluminium. No reaction between aluminium and iron oxide had yet taken place.  
     [0137] The second cathode was extracted from the oven after 24 hours and allowed to cool down to ambient temperature. Examination of a cross-section of the coating showed that aluminium had infiltrated the start-up layer and the aluminium-wettable layer. Part of the aluminium had reacted with Fe 2 O 3  to form Al 2 O 3  and Fe metal. Aluminium infiltration had been stopped on the anchorage layer for lack of oxide reactable with aluminium, which demonstrated that the anchorage layer is an effective barrier layer against penetration of aluminium into the carbon cathode.  
     [0138] The aluminium metal infiltration into the start-up layer and the aluminium-wettable layer enhanced the conductivity of the coating. At ambient temperature, the perpendicular electrical resistance through the coating was less than 1 ohm after infiltration versus more than 500 ohm before infiltration.  
     [0139] The coatings on both cathodes showed a continuous matrix of titanium-aluminium mixed oxides between the anchorage layer and the aluminium-wettable layer which guarantees an excellent adherence between the two layers. In both cases the particles of TiB 2  had not been oxidised by the heat treatment and wettability of the coating by aluminium was very good. The angle of wettability was less than 10 deg.  
     [0140] The third coated carbon cathode was used in an aluminium production drained cell as follows:  
     [0141] The cathode covered with the dried coating according to the invention was covered in the cell with a 1.5 cm thick sheet of aluminium. The cell was heated to a temperature of about 850-900° C. by passing an electrical current between the cathode and facing anodes through carbon powder. Other start-up heating procedures could also have been used, e.g. using gas burners to generate heat.  
     [0142] After 30 minutes at 850-900° C., the start-up coating was superficially wetted by molten aluminium which constitutes a barrier against damaging fluoride-based molten electrolyte constituents, such as sodium compounds, and a cryolite based electrolyte was filled into the cell.  
     [0143] The cell was further heated to 960° C. at which temperature the cell was operated under an electrolysis current density of 0.8 A/cm 2  to produce aluminium under conventional steady state conditions.