Abstract:
An apparatus for treating, in particular purifying or degassing, molten metal ( 40 ), comprising a component ( 10 ) exposable to molten metal to be treated and means ( 13 ) for imparting a rotary motion to the molten metal ( 40 ) about a substantially vertical axis. The apparatus is so arranged that during use at least part of a wear-exposed surface of the component ( 10 ) is temporarily or permanently in contact with molten metal ( 40 ), the contacting molten metal being in motion relative to the wear-exposed surface. The wear-exposed surface is coated with a slurry-applied protective coating ( 18 A, 18 B) of refractory material in a heat stable binder, in particular an inorganic colloidal and/or polymeric binder, protecting the wear-exposed surface against erosion, oxidation and corrosion.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to an apparatus for treating molten metal, in particular for the purification of molten metal, such as molten aluminium, magnesium, steel, cast iron or copper, having a wear-exposed component, such as a stirrer for dispersing a purifying fluid; and a method for treating molten metal with such an apparatus.  
         BACKGROUND OF THE INVENTION  
         [0002]    For many commercial applications aluminium as well as other metals such as magnesium, steel, cast iron or copper need to be of such high purity that the produced metal needs to undergo a purification process. Such purification process usually takes place immediately after the production of the molten metal or during a recycling process.  
           [0003]    The purity of the aluminium produced in commercial Hall-Héroult cells is typically between 99.7 to 99.9%. The impurities present in the produced aluminium are mainly silicon, zinc, magnesium, manganese and titanium, traces of copper chromium, gallium, sodium, lithium, calcium, vanadium and boron, as well as oxides, carbides and hydrogen (D. G. Altenpohl, “Aluminium: Technology, Applications, and Environment”, 1998, 6th Edition, The Aluminum Association Inc., Washington D.C., p. 15).  
           [0004]    One process for purifying molten metal, in particular molten aluminium includes imparting a rotation to the molten metal around a substantially vertical axis to separate the impurities by centrifugal or gravitational force. U.S. Pat. No. 4,760,066 (Eidem) and U.S. Pat No. 5,106,411 (Miki/Kitaoka/Fujii/ Takeuchi/Sorimachi/Sakuraya/Sudo) both disclose such a process in which the rotation is produced by a rotational magnetic field applied to the molten metal. in WO99/60177 (Trojer), on the other hand, the vessel containing the molten metal is rotated, this process being used to purify the molten metal or to separate it from a second molten metal.  
           [0005]    Another process for purifying molten metal, in particular aluminium, consists of passing a purifying additive which may be a halide gas such as chlorine or fluorine, or nitrogen or argon. These gases remove hydrogen, sodium, lithium, calcium and magnesium from the molten metal.  
           [0006]    U.S. Pat. No. 3,887,172 (Funck/Schummer) discloses an apparatus for the purification of molten steel or cast iron by injecting therein argon and/or nitrogen to reduce the level of silicon and for desulfurisation, dephosphorisation and deoxidation.  
           [0007]    During the purification process, which is often referred to as “metal degassing”, purifying gas bubbles are finely and homogeneously dispersed in the molten metal for 200 to 300 seconds or longer. Gas is finely divided into small bubbles in the molten metal with suitable stirrers, such as rotating dispersers as disclosed in U.S. Pat. Nos. 3,743,263 and 3,870,511 (both in the name of Szekely).  
           [0008]    The design of such apparatus, in particular of devices incorporating means to inject and disperse a purifying gas has been the object of numerous developments and patents. Turbines or rotors, such as “high shear rotors”, provided with blades or baffles have been disclosed for instance in U.S. Pat. No. 3,839,019 (Bruno/Jarrett/Slaugenhaupt/Graziano), U.S. Pat. No. 4,426,068 (Gimond/Gonda/Hicter/Laty) and U.S. Pat. No. 5,198,180 (Pelton). Alternatively, rotors, such as “pump action rotors” may be provided with internal cavities communicating with the molten metal bath and with a purifying gas under pressure, the purifying gas being mixed with the molten metal inside the rotor and evacuated therefrom into the molten metal bath, as described for example in U.S. Pat. No. 4,634,105 (Withers/Pattle), U.S. Pat. No. 5,527,381 (Waite/Dumont) and U.S. Pat. No. 5,660,614 (Waite/Lavoie/Dube/ Dumont). The  
           [0009]    The stirrers or rotating dispersers immersed in the molten metal and rotating therein are usually made of carbon, in particular graphite. During operation, the stirrers or dispersers are eroded by friction with the molten metal in which they are rotating and rapidly oxidise above the meltline of the molten metal at temperatures exceeding 450° C. During continuous operation conventional stirrers or dispersers typically neck down at the region of the meltline. In batch processes the stirrers are in addition exposed to corrosion and/or oxidation when extracted from the molten metal after purification thereof and before immersion into a new molten metal to be purified. Conventional stirrers or dispersers used in batch processes quickly cone down along their length. Therefore, such rotating stirrers or dispersers need to be frequently replaced.  
           [0010]    When graphite components exposed to air continuously dip in molten metal they oxidise above the melt line. Oxidation occurs more rapidly at higher temperature, in particular above 450° C. The rate of oxidation is controlled by the temperature, by the availability of oxygen and by the structure of the graphite. The more porous the graphite or the larger the grain size, then the easier it is for air to penetrate into the graphite and the faster the breakdown of the component. The interconnectivity of the porosity is known to be an important parameter controlling the oxidation resistance of a graphite component.  
           [0011]    Unfortunately, graphite components having low porosity and/or a low interconnectivity are relatively expensive because of the raw material and manufacturing costs associated with this type of product.  
           [0012]    One method of protecting stirrer or disperser parts consists of covering them with a refractory sleeve, e.g. made of alumina, resistant to oxidation and wear. However, such sleeves are rigid and can only protect a limited number of shapes corresponding to the internal shapes of the sleeves.  
           [0013]    It has also been suggested to protect the stirrers or dispersers with boron nitride coatings bonded to the stirrer with an acrylic binder (PYROTEKT slurries). In practice, such coatings offer only limited protection and permit diffusion of oxygen between the coating and the substrate which leads to oxidation of the stirrers or dispersers from behind the coating. Furthermore, the lifetime of such coatings is limited as they rapidly wear away during use. Moreover, such boron nitride coatings have a rough surface on which particles or amalgams of impurities or other unwanted elements can get caught and carried over between batches of treated metal The cleaning of the coatings can damage or even destroy them.  
           [0014]    A conventional protection method consists of impregnating the stirrers or dispersers with aluminium orthophosphate under pressure in autoclaves and limiting their exposure to oxygen during operation by flushing an inert gas, e.g. argon, in the process chamber.  
           [0015]    Although aluminium orthophosphate impregnation may double the lifetime of the stirrers or dispersers, as demonstrated in the comparative examples below, such lifetime is still very short and the stirrers or dispersers are still quickly consumed.  
           [0016]    Furthermore, such an impregnation with aluminium orthophosphate does not efficiently protect stirrers or dispersers against erosion caused by contact with the molten metal which is in relative motion with the stirrers or dispersers.  
           [0017]    Therefore, there is still a need to enhance protection of the stirrers, dispersers and other components which come into contact with the molten metal against erosion, oxidation and corrosion, and reduce the costs caused by the consumption of stirrers or dispersers during operation by increasing their lifetime.  
         OBJECTS OF THE INVENTION  
         [0018]    A general object of the invention is to provide coatings on components, in particular carbon or carbon-based components, used in metallurgical processes, which components during operation are in moving contact with molten metal, the coatings being such as to protect the components against erosion, oxidation and corrosion.  
           [0019]    An object of the invention is to provide an apparatus for purifying molten metal, in particular molten aluminium, magnesium, steel, cast iron or copper, provided with a component, such as a carbon-based stirrer, having an improved resistance to erosion, oxidation and corrosion.  
           [0020]    Another object of the invention is to provide a carbon-based stirrer having an improved resistance to erosion which can be fitted in existing molten metal purifying apparatus.  
           [0021]    A further object of the invention is to provide a carbon-based stirrer of a molten metal purifying apparatus protected against oxidation and corrosion when it is extracted from the molten metal in a batch process.  
           [0022]    Another object of the invention is to provide a carbon-based stirrer for a molten metal purifying apparatus provided with a low cost and easily applied protection.  
           [0023]    Yet another object of the invention is to provide a method for purifying molten metal in a molten metal purifying apparatus operating with a protected component, such as a carbon-based stirrer.  
         SUMMARY OF THE INVENTION  
         [0024]    One aspect of the invention relates to an apparatus for treating molten metal. The apparatus comprises a component exposable to molten metal to be treated and means for imparting a rotary motion to the molten metal about a substantially vertical axis. The apparatus is so arranged that during use at least part of a wear-exposed surface of the component is temporarily or permanently in contact with molten metal, the contacting molten metal being in motion relative to the wear-exposed surface. The wear-exposed surface is coated with a slurry-applied protective coating of refractory material in a heat stable binder, in particular an inorganic binder, protecting the wear-exposed surface against erosion, oxidation and corrosion.  
           [0025]    In this context, heat stable means that the binder does not lose its integrity at the operating temperature at which the molten metal is treated, i.e. at or above the melting point of the metal.  
           [0026]    Such a coating protecting these components provides the following advantages.  
           [0027]    1) The coating can be applied to a component of any shape, even complex shapes, without corrupting the original design.  
           [0028]    2) The coating can be applied to selected parts of the component, the thickness of the coating being adaptable to the wear exposure of the selected part. A typical coating thickness is in the range of 100 to 1500 micron, in particular 250 to 800 micron.  
           [0029]    3) The coating can be applied by simple techniques, e.g. painting, brushing, spraying and dipping, and does not require complex application devices such as autoclaves.  
           [0030]    4) By having a heat stable binder, the overall wear resistance and oxygen impermeability of the coating is not affected at elevated temperatures, even above 450° C. or higher, e.g. above the melting point of magnesium, aluminium, copper or iron.  
           [0031]    5) The coating is sufficiently resistant to and protective against oxidation that no particular inert atmosphere is needed during use, even at high temperature. This feature is particularly advantageous since older systems which are not designed to operate under inert atmosphere may be economically retrofitted.  
           [0032]    6) The coating is not merely oxidation or corrosion resistant but also resistant to erosion.  
           [0033]    7) The coating substantially increases the possible duration of use of the component as shown in the comparative examples below.  
           [0034]    8) The coating fully protects the component which can be re-coated and re-used after the coating is worn or needs otherwise to be replaced.  
           [0035]    9) The coating is suitable to protect numerous substrate materials, e.g. carbon-based materials, such as graphite, carbide materials, such as silicon carbide, and metals, such as iron, steel and titanium, the substrates being thereby protected particularly against chemical attack and dissolution.  
           [0036]    10) The protective effect of the coating is such that different materials may be used, in particular grades of carbon cheaper than dense graphite.  
           [0037]    11) The coating has such a smooth surface that no unwanted elements carried in and/or on the molten metal get caught on the surface and carried between batches of treated metal. Hence, the coating needs less or cleaning during use.  
           [0038]    The apparatus according to the invention can be used for separating molten metal from impurities and/or separating constituents of an alloy metal by centrifugal and/or gravitational force.  
           [0039]    As mentioned above, the coated part of the coated component may be made of carbon-based or carbide-based material, in particular petroleum coke, metallurgical coke, anthracite, graphite, amorphous carbon or mixtures thereof. Alternatively, the coated part of the coated component can be made of metal-based material.  
           [0040]    The apparatus may comprise one or more of such coated components. Suitable components include vessels for containing the rotating molten metal, stators that in use dip in the molten metal and are arranged to deliver treating fluid into the molten metal, rotatable stirrers arranged to dip in and rotate the molten metal during operation, and other dispersers or components thereof and.  
           [0041]    In one embodiment, the apparatus comprises a rotatable stirrer that is surrounded in the molten metal by a stator, e.g. a tube-like stator, the stirrer extending through the stator and protruding therefrom into the molten metal. The rotor and the stator define a passage between them for the delivery of the treating fluid.  
           [0042]    For some applications, the apparatus comprises means for generating a rotational magnetic field in the molten metal, usually arranged around a vessel containing the molten metal.  
           [0043]    Some coated components like stirrers have an upper and a lower part, the lower part being during operation partly exposed to molten metal up to a meltline. In this configuration, the component has an interface portion extending from below to above said meltline which may be coated with the slurry-applied protective coating.  
           [0044]    In one application the apparatus is arranged for the purification of molten metal, in particular the removal of impurities from molten metal and/or degassing molten metal. The apparatus comprise a vessel for containing molten metal to be purified or is associated during use with such a vessel. The apparatus has means for injecting a fluid, in particular a gas, a liquid, and/or a flowable solid, such as particulate e.g. powder and granules, for instance a flux, into the molten metal to remove impurities towards the surface thereof, and a rotatable stirrer forming a coated component which during operation dips in the molten metal, whereby upon rotation of the stirrer the molten metal is stirred and fluid injected therein is dispersed. The stirrer has an upper part engaged with a rotary drive means and a carbon-based lower part which during operation is partly immersed in molten metal up to a meltline.  
           [0045]    In this apparatus, the carbon-based lower part of the stirrer has an interface portion which extends from below to above the meltline, the interface portion being coated with the slurry-applied protective coating.  
           [0046]    The coating offers the advantage of being suitable to protect any stirrer, including prior art stirrers of complex shapes and stirrers disclosed in the background art discussion.  
           [0047]    Another advantage is that the protective coating can be applied selectively onto the stirrer and tailored to control the lifetime of the stirrer.  
           [0048]    A further advantage is that the coating can protect any kind of stirrer design in any operating conditions, and be adapted to the type and grade of the carbon-based material to which it is applied.  
           [0049]    The entire carbon-based lower part of the stirrer or other coated component, or even substantially- the entire stirrer or component, may be fully coated with the refractory material to improve its resistance. However, it has been found sufficient to coat the stirrer or component only on the meltline region where protection is most needed. The coating may also be selectively applied to other parts of the stirrer or component, depending on its design and operating conditions.  
           [0050]    The upper part of the stirrer or other coated component may be coated with a thin coating of refractory material against oxidation and corrosion, while the lower part of the stirrer or component is preferably coated with a thick coating of refractory material against erosion, oxidation and corrosion.  
           [0051]    Although, it is possible to protect efficiently a carbon-based upper part of the stirrer or other coated component with a refractory material as stated above or with different coatings and/or impregnations as will be further described below, it is not essential to the invention that also the emerging upper part be made of carbon-based material. The upper part of the stirrer or component which is engaged with the drive means, and which is not exposed to molten metal may be made of long-lasting materials other than carbon, in particular oxidation and temperature resistant metals, such as suitable kinds of steel, and/or ceramics.  
           [0052]    Therefore, the stirrer or other coated component may be a two part assembly having a coated carbon-based lower part assembled to a long-lasting upper part. Hence, when the carbon-based lower part of the stirrer needs to be replaced, in particular when it is eroded, replacement may be limited to the carbon-based lower part by substituting the used lower part with a new one, which enables cost savings.  
           [0053]    The protective coating of the stirrer or other coated component may comprise a refractory hard metal selected from borides of titanium, zirconium, vanadium, tantalum, nickel, molybdenum, chromium and iron, in particular titanium diboride. The protective coating can also comprise at least one carbide or oxide of aluminium, silicon, titanium, zirconium, vanadium, tantalum, nickel, molybdenum, chromium, copper and iron, or a combination thereof, in particular corundum or fused alumina.  
           [0054]    The protective coating may be applied from a slurry, in particular a colloidal slurry, containing particulate reactant or non-reactant substances, or a mixture of particulate reactant and non-reactant substances, which when the stirrer is heated at a sufficient temperature, usually above 500° C., form the protective coating by reaction sintering and/or sintering without reaction. Alternatively, the coating may be consolidated by fusion and solidification of one or more of its constituents.  
           [0055]    The slurry for such a coating may comprise at least one colloid or an inorganic polymer, for example selected from colloidal or polymeric silica, alumina, yttria and ceria. For instance, the slurry consists of preformed particulate titanium diboride in colloidal alumina and/or colloidal or polymeric silica. Methods for applying sintered slurries have been described in U.S. Pat. No. 5,364,513 (Sekhar/de Nora) and U.S. Pat. No. 5,651,874 (de Nora/Sekhar).  
           [0056]    However, the colloidal slurries described in U.S. Pat. Nos. 5,364,513 and 5,651,872 and other non-sintered slurries may be improved by the addition of 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 protective coating. For instance, the liquid content of the colloidal slurry may contain between 0.5 and 6 weight % typically 2 to 5 weight %, polyethylene glycol (PEG 300) or a corresponding amount of a polyethylene glycol of different concentration.  
           [0057]    The slurry may be applied to the component by painting, spraying or dipping. Use of standard mixing tanks for keeping the slurry in continuous motion without introduction of air bubbles is also possible but neither necessary nor preferred. The rheology of the slurry is such that substantially no settling takes place and the slurry remains in a state of readiness without using expensive and elaborate mechanical mixing device. It is usually sufficient to simply dip and agitate or rotate the component in the static slurry.  
           [0058]    After application of a layer of slurry onto the component, the layer can be dried in air at room temperature or at higher temperature, typically 200° to 300 20  C. The drying process involving heating should be controlled so that the coating does not mud crack.  
           [0059]    The coating should be heat treated at a temperature of at least about 450 to 500° C. to permit consolidation by sintering the coating on the component before use or in-situ, or by melting and then solidifying at least one constituent of the coating. The melting point of the coating should be above the temperature of operation of the component. The consolidation temperatures may be in the range of 500° to 1500° C. The temperature of consolidation by sintering tend to be lower than by melting and solidifying.  
           [0060]    Care should be taken when the coating is applied onto an oxidisable substrate, in particular carbon. Above 450° C., carbon is exposed to oxidation in the presence of oxygen. Hence, only surfaces of the substrate which are coated with the protective coating should be heat treated at elevated temperature in an oxidising atmosphere.  
           [0061]    For instance, if only one end of the component is coated, only this end need be introduced in an oven for heat treatment, the protective coating extending at least over the part of the surface of the component which is inside the oven. Similarly, if an intermediate portion of the component is coated, the coating may be heat treated using a hot collar which is not larger than the coating. When the substrate is only partly coated, the coating preferably extends beyond the heating element during the firing step. Furthermore, when a coating of a component which is exposed to an oxidising atmosphere above the molten metal meltline is heat treated insitu, the coating should extend above the meltline to a level at which the temperature is sufficiently low that no substantial oxidation of un-coated parts of the substrate occurs.  
           [0062]    The drying and heat treatment of the coating may be achieved in a single heat exposure before use of the component. However, if the component is to be consolidated in-situ, drying should take place before using the component.  
           [0063]    The upper part of the component, in particular of the stirrer, especially when it is carbon-based, may be protected from an oxidising and/or corrosive atmosphere with a coating and/or impregnation of a phosphate of aluminium, such as monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, aluminium metaphosphate, aluminium orthophosphate and mixtures thereof.  
           [0064]    Alternatively, the upper part of the component, in particular the stirrer, especially when it is carbon-based, may also be protected against an oxidising or corrosive atmosphere by coating and/or impregnating it with a boron compound, such as a compound selected from boron oxide, boric acid and tetraboric acid.  
           [0065]    When the upper part of the component is treated with aluminium phosphate or a boron compound, the extent of the slurry-applied protective coating above the meltline may be reduced.  
           [0066]    The stirrer may be arranged to rotate about an axis passing through the lower part of the stirrer, or passing outside its lower part.  
           [0067]    The stirrer may be a rotary shaft and/or a rotor with at least one stirrer blade. Rotors may be of any type including prior art rotors as disclosed in the background description, such as high shear rotors provided with vanes for shearing gas and/or flux into fine bubbles, or pump action rotors in which gas and/or flux supply ducts are intercepted by mixing ducts for sucking in, mixing with the gas and/or flux, and expelling molten metal.  
           [0068]    The apparatus of the invention may be used in particular for the purification of molten aluminium, molten magnesium, cast iron, molten steel or copper, or for the separation of alloys thereof.  
           [0069]    The invention also relates to a rotatable stirrer of an apparatus for the purification of a molten metal as described above. The rotatable stirrer dips during operation in the molten metal, whereby upon rotation of the stirrer the molten metal is stirred and the fluid injected therein is dispersed. It comprises an upper part arranged to be engaged with a rotary drive means of the apparatus and a carbon-based lower part which during operation is partly immersed in molten metal up to a meltline.  
           [0070]    The carbon-based lower part has an interface portion extending from below to above the meltline. The interface portion is coated with a protective coating of a refractory material to improve the resistance against erosion, oxidation and/or corrosion during operation.  
           [0071]    A further aspect of the invention is a method of treating molten metal in an apparatus as disclosed above. The method comprises temporarily or permanently exposing the wear-exposed surface of the coated component to molten metal and imparting a rotary motion to the molten metal about a substantially vertical axis, the contacting molten metal being in motion relative to the wear-exposed surface.  
           [0072]    One particular embodiment of the method relates to the purification of molten metal in a purifying apparatus comprising or associated with a vessel for containing molten metal and a coated stirrer as described above. The method comprises injecting a purifying fluid, in particular a gas and/or a flux, into a molten metal contained in the vessel, and rotating the coated stirrer therein to stir and disperse the injected fluid in the molten metal to remove impurities towards the surface.  
           [0073]    The method of the invention may be applied to purify the molten metal either in batches or continuously.  
           [0074]    Different purifying gases may be injected into the molten metal to purify it, such as a halide gas, in particular chlorine and/or fluorine, or an inert or substantially inert gas, in particular nitrogen and/or argon. One gas or a mixture of different purifying gases may be injected into the molten metal.  
           [0075]    A flux for the purification process, for instance a solvent such as an organic or inorganic fluoride, e.g. aluminium, magnesium or potassium fluoride powder or fused fluxes, carbonates, sulphates, nitrides and oxides, may also be injected into the molten metal as such or together with a purifying gas, the flux being usually in the form of a liquid, powder or granule.  
           [0076]    Usually, the purifying fluid is injected into the molten metal through a duct in the stirrer or through an auxiliary tube which dips in the molten metal.  
           [0077]    During operation in the molten metal, the stirrer may be rotated at a speed of the order of 100 to 500 RPM, in particular in the range of 250 to 400 RPM.  
           [0078]    Usually, the coating is exposed to contacting molten metal that has a tangential speed of at least 0.1 m/s relative to the coated component. Depending on the application, the relative speed may be in range of 1 to 10 m/s or even higher, in particular from 2.5 to 4 m/s.  
           [0079]    Another aspect of the invention is the use of a refractory material as protective coating against erosion, oxidation and/or corrosion on a rotatable stirrer of an apparatus for the purification of a molten metal.  
           [0080]    The invention also relates to the use of a carbon-based component coated with a protective coating of refractory material against erosion, oxidation and/or corrosion as rotatable stirrer in an apparatus for the purification of a molten metal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0081]    [0081]FIG. 1 schematically and by way of example shows part of one embodiment of an apparatus for the purification of a molten metal according to the invention, part of it being shown in an enlarged view and partly cut away; and  
         [0082]    [0082]FIG. 2 schematically shows a stirrer dipping in molten metal and selectively coated with a refractory material according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0083]    The apparatus partly shown in FIG. 1 comprises a vessel  20  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 and disperse a fluid injected therein.  
         [0084]    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  20 . At the lower end of the shaft  11  is a rotor  13  provided with flanges or other protuberances for stirring the molten metal  40 .  
         [0085]    Inside the shaft  11 , along its length, is an axial duct  12  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 .  
         [0086]    The axial duct  12  is arranged to supply a fluid, e.g. gas and/or a molten or solid flux, to the rotor  13 . The rotor  13  comprises a plurality of apertures connected to the internal duct  12  for injecting the fluid into the molten metal  40 , as shown by arrows  51 .  
         [0087]    According to the invention, 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  18 A of a refractory material which improves the resistance to erosion, oxidation and corrosion of the stirrer during operation.  
         [0088]    As shown in FIG. 1, the upper part of the carbon-based shaft  11  is coated with a thin coating  18 B of refractory material  18 B providing protection against oxidation and corrosion, whereas the coating  18 A 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.  
         [0089]    Such a coating gradation is adapted for coatings of stirrers used in batch processes, the lower part of the stirrers being exposed alternately to molten metal and oxidising atmosphere. For stirrers used in continuous metal treatment systems, the gradation may be inverted, i.e. thicker coating above the meltline and thinner coating below the meltline, or the coating may be equally thick above and below the meltline because the lower part of the stirrer is only exposed to molten metal, not to an oxidising atmosphere.  
         [0090]    The coatings  18 A and  18 B may be made of an aluminium-wettable material, such as a refractory hard metal (RHM) having little or no solubility in molten aluminium. Useful RHM include borides of titanium, zirconium, tantalum, chromium, nickel, cobalt, iron, niobium and/or vanadium, for example as disclosed in U.S. Pat. No. 5,364,513 (Sekhar/de Nora) and U.S. Pat No. 5,651,874 (de Nora/Sekhar) as well as in the examples below.  
         [0091]    A suitable coating consists of particulate refractory hard metal boride in a colloid applied from a slurry of the particulate refractory hard metal boride in a colloid carrier, wherein the colloid comprises at least one of colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate, as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar). The colloidal carrier has been found to considerably improve the properties of the coating produced by non-reactive sintering.  
         [0092]    The method of application comprises applying to the surface of the component a slurry of particulate preformed refractory boride in a colloidal carrier as specified above, followed by drying, and by heat treatment. This method involves painting (by brush or roller), dipping, spraying, or pouring the slurry onto stirrer  10  and allowing to dry before another layer is added. Each applied layer does not need to be entirely dry before the application of the next layer. It is preferred to heat the coating with a suitable heat source so as to completely dry it and improve densification of the coating. Heating and drying take place preferably in non-oxidising atmospheres at about 80-200° C., usually for half an hour to several hours, and further heat treatments are possible.  
         [0093]    The surfaces of stirrer  10  to be coated with this slurry may be treated by sand blasting or pickled with acids or fluxes. Similarly the surfaces may be cleaned with an organic solvent such as acetone to remove oily products and other debris prior to the application of the coating. These treatments- will enhance the bonding of the coatings to the stirrer.  
         [0094]    After coating the surfaces of the stirrer by dipping, painting or spraying the slurry or combinations of such techniques in single or multi-layer coatings and drying, a final coat of the colloid alone may be applied lightly prior to use.  
         [0095]    Before or after application of the RHM coating and before use, the surfaces of the stirrer can be painted, sprayed, dipped or infiltrated with reagents and precursors, gels and/or colloids.  
         [0096]    To assist rapid wetting of the immersed part of stirrer  10  by molten aluminium, before use the refractory coating may be exposed to molten aluminium in the presence of a molten flux assisting penetration of aluminium into the refractory material, the molten flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof. Such treatment favours aluminization of the refractory coating by the penetration therein of aluminium.  
         [0097]    Likewise, if a stirrer or other component is used for the treatment of a metal other than aluminium, the refractory material may be exposed to the corresponding metal before use, if appropriate in the presence of a flux.  
         [0098]    During operation of the apparatus shown in FIG. 1, a reactive or non-reactive fluid, in particular a gas alone or a flux, such as a halide, nitrogen and/or argon, is injected into the molten metal  50  contained in the vessel  20  through the flexible tube  35  and stirrer  10  which dips in the molten metal  40 .  
         [0099]    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.  
         [0100]    The stirrer  10  schematically shown in FIG. 2 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. 1 or other designs known from the prior art. The rotor  13  of stirrer  10  may be a high-shear rotor or a pump action rotor.  
         [0101]    In FIG. 2, instead of coating the entire shaft  11  and rotor  13 , parts of the stirrer  10  liable to erosion are selectively coated with a refractory material, such as a refractory hard metal coating, for example a titanium diboride coating applied in a colloidal slurry as described above.  
         [0102]    According to the invention 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  18 A 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  18 A 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.  
         [0103]    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. 2 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  18 A 2 , the lateral surface of rotor  13  is protected with a coating  18 A 3  and the bottom surfaces of the rotor  13  is coated with a coating  18 A 4 .  
         [0104]    For each specific stirrer design, the coating or different protective coatings on different parts of the stirrer, such as coatings  18 A 1 ,  18 A 2 ,  18 A 3  and  18 A 4  shown in FIG. 2, 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.  
         [0105]    In an alternative embodiment (not shown), the coating on such stirrers may be continuous as illustrated in FIG. 1 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. 2.  
         [0106]    The invention is understood not to be limited to the details of the apparatus shown in the Figures or described in the above specification. Additional embodiments and modifications within the scope of the claimed invention will be apparent to one of ordinary skill in the art.  
         [0107]    For instance, the shaft shown in FIG. 1 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.  
         [0108]    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, aluminium orthophosphate and mixtures thereof. 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.  
         [0109]    In a modification of the invention, the protective coating may simply be applied to any part of the stirrer in contact with the molten metal, to be protected against erosion, oxidation and corrosion during operation.  
         [0110]    In a further modification, the stirrers shown in FIGS. 1 and 2 may each be surrounded by a tube-like stator extending from above into the molten metal  40  along shaft  11  down to above rotor  13 , the stator being arranged to contain between its internal walls and shaft  11  a purifying gas or flux for supplying the gas or flux to the molten metal  40  above rotor  13 . Similarly, the stator is preferably coated with a protective coating at least at and about a level corresponding to the meltline  14 .  
         [0111]    Coatings of refractory material for protecting stirrers or other coated components according to the invention will be further described in the following examples.  
       EXAMPLE 1 (COMPARATIVE)  
       [0112]    Three non-treated conventional graphite shafts having a diameter of 60 mm were tested in a batch treatment process during which the shafts were rotated at a speed of about 200 rpm in molten Al-7Si-0.5 Mg (A356), i.e. containing in weight % 92.5% Al, 7% Si and 0.5% Mg, at about 750-760° C. The tests were carried out in a standard Ramsell Nabers™ electric resistance furnace.  
         [0113]    Every 60 min. the shafts were extracted from the molten metal for examination and exposed to the oxidising atmosphere for approximately 20 min. After 15, 20 and 24 hours operation respectively, the three conventional shafts were coned down to such an extent that they needed to be replaced.  
       EXAMPLE 2 (COMPARATIVE)  
       [0114]    Example 1 was repeated with three similar shafts but impregnated with aluminium orthophosphate by a conventional method in an autoclave and dried in an air oven.  
         [0115]    Under the same conditions as in Example 1 and after 36, 72 and 72 hours operation respectively, the three impregnated shafts were coned down to such an extent that they needed to be replaced  
       EXAMPLE 3  
       [0116]    Three rotary shafts of the same type as the shafts of comparative Example 1 were protected with a refractory coating according to the invention.  
         [0117]    A slurry was prepared by mixing particulate TiB 2 , two grades of colloidal alumina and polyethylene glycol in the following proportions: 100 mg particulate TiB 2  -325 mesh (99% of particles smaller than 45 micron and a mean particle size of 6 micron), 28 ml of a first grade of colloidal alumina (NYACOL® Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer) 24 ml 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) and 0.5 ml of polyethylene glycol (PEG 300).  
         [0118]    Before applying the coating, the surface of each shaft was mechanically roughened by shot blasting to increase anchorage of the coating.  
         [0119]    A coating was formed by brushing several layers of the above slurry onto the roughened surface of each shaft. Each applied layer was allowed to dry for a few minutes, typically 10 min., at 300° C. before application of the next layer. After having reached a thickness of 800 micron, the slurry-applied coating was subjected to a heat treatment in an oxygen-containing atmosphere at 950° C. for sintering and consolidation. Alternatively, the coating may be sintered and consolidated without prior drying heat treatment.  
         [0120]    The coated shafts were tested in a molten metal treatment process under the same conditions as in comparative Examples 1 and 2. After 168 hours operation, the shafts coated according to the invention were removed from the process. The graphite of each shaft and its coating had remained substantially intact and could have been further used.  
       EXAMPLE 4  
       [0121]    As a modification of Example 3, the shafts are coated with a slurry of 100 g particulate TiB 2  in 500 g of a cement consisting of approximately 57 weight % mullite (3Al 2 O 3 .2SiO 2 ), 12 weight % silica (SiO 2 ) and 31 weight % waterglass. The waterglass is an aqueous solution containing 33.4 weight % sodium silicate in the approximate formula Na 2 O.3.8Si 0   2 .  
         [0122]    Several layers of this slurry are successively applied onto the roughened surface of the shafts. Each applied layer is preferably allowed to dry at room temperature or with heating up to about 200° C. before application of the next layer until a desired thickness, e.g. 0.4 mm, is obtained. The coating is then heat treated at about 950° C. for consolidation to form a glassy protective coating.  
         [0123]    The coated shaft can be used in molten aluminium as described in Examples 1 to 3 or for treating of other metals, such as magnesium, iron, steel or copper.  
       EXAMPLE 5  
       [0124]    A further modification of Example 3 involves mixing 10 g particulate aluminium-iron alloy made of 60 weight % aluminium and 40 weight % iron to 90 g particulate TiB 2  and suspending this particulate mixture in a carrier consisting of colloidal alumina and polyethylene glycol as in Example 3. Layers of this slurry containing particulate aluminium-iron alloy are applied onto a graphite shaft and dried thereon as in Example 3.  
         [0125]    The coating is consolidated under an inert atmosphere by heat treatment at a temperature of 1200° C. followed by cooling whereby the aluminium-iron alloy particles are fused into a continuous matrix in which TiB 2  particles are embedded. Such a coated shaft can be used for treating molten metals up to a temperature of about 1100° C., e.g. for treating molten magnesium, aluminium or copper.