REFRACTORY FILTER

Abstract

A refractory filter suitable for filtering molten metal, such as steel, and a method and powder composition for producing said filter. The filter comprises refractory material, said refractory material comprising: 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia. The powder composition comprises: 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia, wherein the powder composition comprises less than 12.5% ​​by weight of reactive alumina, calcined alumina, or a mixture thereof, and wherein the remainder of the alumina is tabular alumina. The method comprises: providing a powder composition according to the invention; forming a filter precursor from the powder composition and a liquid component; and baking the filter precursor to form a refractory filter.

Description

REFRACTORY FILTER The present invention relates to a filter suitable for filtering molten metal, such as steel, and to a method for producing such a filter. Molten metals often contain solids such as metal oxides and other impurities that can give the final molten product undesirable characteristics. Filters have been devised to remove these impurities from the molten metal during the smelting process. These filters are typically made of refractory materials, such as ceramics, to withstand the high temperatures associated with molten metals. The different types of refractory filters include cellular and pressed filters. Cellular filters are formed using extrusion techniques, for example, by extruding a plastic or ceramic body through a die, before drying, cutting, and firing the resulting structure in a kiln. Cellular filters typically comprise square, parallel cells that extend through the depth of the filter. Pressed filters are manufactured by forcing molded pins through a plastic or ceramic body in a molded die and usually have parallel, round holes that extend through the filter body. Neither of these filter types provides a tortuous path for molten metal to flow through the structure. The preferred refractory filters have a foam-like appearance and are referred to as foam filters in the metal filtration industry, typically ceramic foam filters. The manufacture of ceramic foam filters is described in EP 0 412 673 A2 and EP 0 649 334 A1. Typically, an open-cell foam (e.g., cross-linked polyurethane foam) is impregnated with an aqueous suspension of refractory particles and a binder. The impregnated foam is compressed to expel the excess suspension and then dried and burned to burn off the organic foam and sinter the refractory particles and binder into the suspension coating. This forms a solid ceramic foam with a plurality of interconnected voids that have substantially the same structural configuration as the initial foam. More recently, it has become possible to create complex ceramic structures, such as filters, using 3D printing (also known as additive manufacturing). Typically, successive layers of material are formed under computer control, for example, based on a 3D virtual or CAD model. To form a ceramic object by 3D printing, an initial structure formed by a 3D printer must be baked at a high temperature (e.g., around 1500-1700°C) to sinter or fuse the ceramic material. Zirconium-based foam filters are widely used in steelmaking because they can withstand the high temperatures required. Zirconium-based filters typically have a very high zirconium content, for example, up to 95% zirconium by weight. However, zirconium is very expensive, and the friability of zirconium-based foam filters can cause small pieces of the filter to break off, potentially contaminating the foundry. Zirconium filters are also dense and difficult to prime, so the molten metal must be heated to a higher temperature before filtration. The present invention has been devised taking these issues into account and aims to reduce the friability, density and cost of the filter by reducing the amount of zirconium in the filter. According to a first aspect of the present invention, a refractory filter is provided for filtering molten steel, the refractory filter comprising refractory material and said refractory material comprising: 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia. In some forms, the refractory material comprises 65 to 80% by weight, or 70 to 75% by weight of alumina. In some forms, the refractory material comprises 10 to 25% by weight or 15 to 20% by weight of zirconium. In some forms, the refractory material comprises 5 to 15% by weight or 7.5 to 10% by weight of magnesia. In some forms, the refractory material comprises 70 to 75% by weight of alumina, 15 to 20% by weight of zirconium, and 5 to 12.5% ​​by weight of magnesia. In some forms, the refractory material comprises 75% by weight of alumina, 20% by weight of zirconium and 5% by weight of magnesia. In some forms, magnesia is partially or completely replaced by ceria. In some versions, the refractory material also includes titania. In some versions, the refractory material comprises up to 0.5% by weight, up to 1% by weight, up to 1.5% by weight, up to 2% by weight, up to 3% by weight, up to 4% by weight, or up to 5% by weight of titania. In some forms, the refractory filter is a foam filter with a network or lattice of interconnected strands that define interconnected pores or voids, resulting in multiple tortuous paths through the filter. In other forms, the refractory filter is a cellular or pressed filter. A refractory filter is a filter capable of withstanding high temperatures. The refractory filter of the invention must be able to withstand the thermal shock of being heated to the high temperatures required by molten metal, and physically resist the mechanical shock from the impact of molten metal, particularly molten steel. This document describes tests designed to measure these properties, including the filter's compressive strength and its ability to withstand the impact of molten metal. In particular, the refractory filter of the invention should be suitable for filtering molten steel, which may have a temperature of, for example, 1500°C or higher. The filter may also be suitable for filtering other molten metals, such as titanium and its alloys. The refractory filter may have a compressive strength of at least 4, at least 4.5, or at least 5 MPa. In some embodiments, the compressive strength is no greater than 8, no greater than 7, or no greater than 6 MPa. Compressive strength may also be referred to in this document as the crush resistance of the filter. The refractory filter may be suitable for filtering at least 30 kg, at least 40 kg, at least 50 kg, at least 70 kg, at least 100 kg, at least 200 kg, at least 400 kg or at least 600 kg of molten steel, according to the method described herein. In some embodiments, the refractory material comprises less than 1% by weight, less than 0.8% by weight, less than 0.5% by weight, less than 0.3% by weight, less than 0.2% by weight, or less than 0.1% by weight of silica. In some embodiments, the refractory material is substantially silica-free, except for unavoidable impurities that may normally be present in refractory materials. Reducing or eliminating silica is beneficial, as its presence can result in the formation of low-melting-point species that can reduce the filter's hot strength, which in turn can lead to filter deformation and rupture. This is important for filtering metals with high melting points, such as steel and titanium. The refractory filter has at least one first surface forming a lateral face of the filter and two opposing second surfaces forming the continuous flow faces of the filter. The filter may have a circular, square, or rectangular cross-section. A filter with a circular cross-section will have only one first surface, while a filter with a square or rectangular cross-section will have four first surfaces. In some forms, each of the second surfaces has an area of ​​no more than 200 cm2, no more than 300 cm2, no more than 400 cm2 or no more than 500 cm2. Preferably, each of the second surfaces has an area of ​​at least 10 cm2, at least 25 cm2 or at least 50 cm2. In some models, each of the secondary surfaces has an area of ​​no more than 100 cm². In such models, the refractive filter may have a weight of no more than 170 g, no more than 160 g, or no more than 150 g. In some models, the filter has a weight of 140 to 170 g or 140 to 150 g. In some models, each of the secondary surfaces has an area of ​​no more than 70 cm². In such models, the refractive filter may have a weight of no more than 100 g, no more than 90 g, or no more than 80 g. In some models, the filter has a weight of 70 to 100 g or 70 to 80 g. The density of the ceramic in the currently claimed filter is therefore lower than that of zirconium-based filters, and thus the present invention provides a strong but low-weight filter for the filtration of cast steel. The refractory filter of the invention can be a refractory foam filter. The manufacture of refractory foam filters is described in documents EP 0 412 673 A2 and EP 0 649 334 A1. Typically, an open-cell foam (e.g., cross-linked polyurethane foam) is impregnated with an aqueous suspension of refractory particles and a binder. The impregnated foam is compressed to expel the excess suspension and then dried and burned to burn off the organic foam and sinter the refractory particles and binder into the suspension coating. This forms a solid ceramic foam having a plurality of interconnected voids that have substantially the same structural configuration as the initial foam. Alternatively, the precursor filter of refractory formed filter can be derived from one by 3D printing (also known as additive manufacturing). In some embodiments, the filter has at least one closed edge. A closed edge means that most of the pores on at least one of the first surfaces are closed or blocked, for example, by a coating. In embodiments where the filter comprises more than one first surface, the pores on some or all of the first surfaces may be closed. In embodiments where the first surface (in the case of a round filter having only one first surface) or all of the first surfaces (in the case of square or rectangular filters) are closed, the filter may be described as framed. The closed edge or frame can help increase the strength of the filter. US4568595, US4331621, and WO2011 / 114080 describe examples of the preparation of closed-edge filters. The use of framed filters can improve performance by significantly increasing the mass of metal that the filter can support.In some cases, framed filters have been found to increase the filter's capacity from 30 kg to 100 kg before failure. In some embodiments where the filter already has an inherent capacity of more than 100 kg of metal, framing the filter is not necessarily required for strength, but it can help to further enhance the improved friability performance achieved through the composition of the present invention. According to a second aspect of the invention, a powder composition is provided comprising 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia, wherein the powder composition comprises less than 12.5% ​​of reactive alumina, calcined alumina, or a mixture thereof. In some forms, the powder composition comprises 65 to 80% by weight or 70 to 75% by weight of alumina. In some forms, the powder composition comprises 10 to 25% by weight or 15 to 20% by weight of zirconium. In some forms, the powder composition comprises 5 to 12.5% ​​by weight or 7.5 to 10% by weight of magnesia. In some forms, the powder composition comprises 70 to 75% by weight of alumina, 15 to 20% by weight of zirconium, and 5 to 12.5% ​​by weight of magnesia. In some forms, the powder composition comprises 75% by weight of alumina, 20% by weight of zirconium and 5% by weight of magnesia. In some forms, magnesia is partially or completely replaced by ceria. The powder composition comprises less than 12.5% ​​by weight of reactive alumina, calcined alumina, or a mixture thereof, the remainder being tabular alumina. In some embodiments, the powder composition comprises no more than 10% by weight of reactive and / or calcined alumina, or no more than 5% of reactive and / or calcined alumina. In some embodiments, the powder composition comprises only tabular alumina and no reactive or calcined alumina. The powder composition may comprise from 0 to 10% by weight, from 1 to 9% by weight, or from 2 to 8% by weight (e.g., 5% by weight) of reactive alumina, calcined alumina, or a mixture thereof. The powder composition may comprise at least 60% by weight, at least 65% by weight, at least 70% by weight, or at least 75% by weight of tabular alumina. In general, reactive alumina has a fluffier or featherier texture due to the precipitation techniques used to produce it. Therefore, reactive alumina absorbs more water than tabular alumina (known as higher water demand), resulting in greater shrinkage after baking. This, in turn, can reduce the strength of the resulting filter. When a slurry is formed, higher amounts of reactive alumina in the powder composition can also make the slurry more difficult to pump and process due to reduced flow. In some embodiments, the powder composition comprises less than 1% by weight, less than 0.8% by weight, or less than 0.5% by weight of silica. In some embodiments, the powder composition is substantially free of silica. In some forms, zirconium is reactive zirconium. In some forms, the powder composition comprises 70% by weight of tabular alumina, 5% by weight of reactive or calcined alumina, 20% by weight of zirconium and 5% by weight of magnesia. The tabular alumina present in the powder composition may have a D50 particle size of less than 500 pm, less than 400 pm, less than 300 pm, less than 200 pm, less than 100 pm, or less than 50 pm. In some embodiments, the tabular alumina has a D50 particle size of at least 20 pm, at least 30 pm, at least 40 pm, at least 50 pm, at least 100 pm, or at least 200 pm. In some embodiments, the tabular alumina has a D50 particle size of 20 to 500 pm, 40 to 400 pm, or 40 to 300 pm. In some embodiments, tabular alumina comprises a mixture of different grades of alumina. In some embodiments, tabular alumina comprises a mixture of finer-grade tabular alumina (for example, having a D50 particle size of less than 50 pm, or from 20 to 50 pm) and coarser-grade tabular alumina (for example, having a D50 particle size of 100 to 500 pm). In some embodiments, the finer-grade tabular alumina has a D50 particle size of around 40 pm and the coarser-grade tabular alumina has a D50 particle size of around 200 pm. In some forms, the ratio of finer grade tabular alumina to coarser grade is 20:80 to 80:20, 30:70 to 70:30, 40:60 to 60:40 or 50:50. Compositions comprising coarser-grade tabular alumina (e.g., D50 size particles from 100 to 500 pm, or around 200 pm) can exhibit extremely low water requirements and produce stronger filters with molten metal capacities significantly exceeding 100 kg of metal, e.g., 600 kg of metal. Therefore, compositions comprising coarser-grade tabular alumina can be used to produce larger filters (e.g., filters where the first surfaces are around 150 mm in diameter or up to 500 cm² in area). Tabular alumina may have a specific surface area (SSA) of no more than 1.0, no more than 0.8, no more than 0.5, or no more than 0.3 m² / g. The specific surface area may be characterized by standard methods, for example, the Brunauer-Emmett-Teller nitrogen adsorption method (ISO 9277:2010). Reactive and / or calcined alumina, when present in the powder composition, may have a D50 particle size of less than 20 pm, less than 10 pm, less than 5 pm or less than 3 pm, less than 2 pm or less than 1 pm. Reactive and / or calcined alumina may have a specific surface area (SSA) of no more than 5, no more than 3, no more than 2 or no more than 1 m2 / g. The magnesia present in the powder composition may have a D50 particle size of less than 50 pm, or less than 30 pm, for example, 20 pm. Magnesia can have a specific surface area (SSA) of no more than 10, no more than 5, no more than 3, or no more than 2 m2 / g. ML / t / ZUZZ / UOO Ί 1 l The zirconium present in the powder composition may have a D50 particle size of less than 10 pm, less than 5 pm, less than 3 pm, less than 1 pm or less than 0.5 pm. Zirconium can have a specific surface area (SSA) of no more than 10, no more than 8, no more than 6, or no more than 3 m2 / g. It can be beneficial for the powder composition to have a wide range of particle sizes. For example, the powder composition may comprise relatively coarse tabular alumina particles (e.g., D50 from 40 pm to 200 pm) and relatively fine zirconium particles (e.g., D50 of 0.4 pm). The fine zirconium particles act as a bonding agent and form complexes with the alumina. In some embodiments, a coarser grade of zirconium (e.g., having a D50 particle size of 5 to 20 pm) may be used, either alone or in combination with a finer grade of zirconium (e.g., D50 of less than 1 pm). However, in such embodiments, the amount of coarser grade zirconium in the powder composition should preferably be less than 15% by weight. In some embodiments, the powder composition includes titania. In some embodiments, the powder composition comprises up to 0.5% by weight, up to 1% by weight, up to 1.5% by weight, up to 2% by weight, up to 3% by weight, up to 4% by weight, or up to 5% by weight of titania. The addition of titania to the powder composition can further increase the cold strength, metal capacity, and friability performance of the filter. Titania can also cause a slight increase in filter shrinkage during firing, but without a corresponding loss of strength. This effect can be particularly useful in compositions comprising coarser grades of tabular alumina (e.g., 200 pm), which exhibit very little shrinkage after firing and may therefore not produce a finished filter with the exact dimensions or pore size required when manufactured using a standard-sized foam precursor. Therefore, adding titania to compositions comprising coarser grades of tabular alumina can allow the final filter dimensions to be tailored to the requirements, eliminate the need to stockpile special-sized foam precursors, and allow standard-sized foam precursors to be used to produce the filter. According to a third aspect of the present invention, a powder composition is provided for use according to the second aspect to form a refractory filter. According to a fourth aspect of the present invention, a method for producing a refractory filter is provided, comprising: to provide a powder composition in accordance with the second aspect of the invention; form a filter precursor from the powder composition and a liquid component; and bake the filter precursor to form a refractory filter. In some embodiments, the step of forming the filter precursor comprises impregnating a cross-linked foam substrate with a suspension comprising the powder composition and the liquid component to form the filter precursor. Impregnation of foam substrates with a refractory suspension is well known in the art. The cross-linked foam substrate can be impregnated with the suspension by spraying, roller impregnation, immersion, centrifugation, or any combination thereof. Excess suspension can be removed by pressing, rolling, and / or centrifugation. In some forms, the suspension is applied by a combination of roller application (e.g., 60% by weight of the suspension can be applied by roller) and spraying (e.g., 40% by weight of the suspension can be applied by spraying). It will be noted that it may be necessary to adjust the viscosity of the suspension according to the impregnation method, and a person skilled in the art will be able to adjust the viscosity accordingly. For example, for roller impregnation, the suspension may have a viscosity of 25 to 100, 35 to 60, 40 to 55, or 45 to 49 Pa·s. For spray or dip application, the suspension may have a viscosity of 1 to 5, 1.5 to 4, or 2 to 3.1 Pa·s. For centrifugation, the suspension may have a viscosity of 2 to 50 Pa·s. The suspension can be formed by mixing the powder composition with at least one liquid component. Therefore, the method may further comprise combining the powder composition and at least one liquid component to form a suspension. The liquid component of the suspension can comprise any suitable liquid diluent, for example, water, methanol, ethanol, or light oil. However, water is normally used because it provides suspensions with good coating properties and is environmentally safe. One or more additives can also be added to the suspension to modify its rheological properties. The use of such additives in filter preparation is well known in the art and includes: suspension aids, such as clays; antifoaming agents, such as silicone-based liquids; binders, such as polyvinyl acetate (PVA); dispersants, such as lignosulfonates and / or carboxylic acids; viscosity modifiers, such as xanthan gum; and wetting agents, such as propylene glycol. The cross-linked foam substrate can be a polymer foam, such as a polyether, a polyurethane (including polyether-polyurethane and polyester-polyurethane), or a cellulose foam. The cross-linked foam substrate serves as a template for the resulting filter, so its porosity provides an indication of the filter's porosity. Porosity can be defined in terms of the number of pores and the percentage of void volume (pores) in the substrate. The porosity of a foam filter is generally specified in terms of pores per linear inch (ppi), and for metallurgical applications, porosity typically ranges from 5 ppi to 60 ppi, and from 10 ppi to 30 ppi for most foundry applications. In the foundry industry, the ppi of a filter is, strictly speaking, a reference to the ppi of the foam substrate from which it is made. The cross-linked foam substrate used in the embodiments of the invention can have a porosity of 5 ppi to 40 ppi, 8 to 30 ppi or 10 to 20 ppi, for example, 15 ppi. Like the refractory filter used to form it, the cross-linked foam substrate has at least one first surface, which ultimately forms a side face of the filter, and two opposing second surfaces, which form the continuous flow faces of the filter. In some embodiments, the method further comprises forming a closed edge on the cross-linked foam substrate. The closed edge can be formed by applying an organic coating to at least one first surface of the cross-linked foam substrate before impregnating the foam substrate with the suspension. Upon baking, the organic material burns away, leaving a closed edge. The organic coating can be applied, for example, by spraying organic fibers (e.g., polyurethane) onto at least one first surface of the cross-linked foam substrate. Alternatively, the coating can be applied by impregnation, wrapping at least one first surface in a strip of organic coating material, or by melting the edge of the cross-linked foam substrate. This results in the formation of a unitary closed edge that is indistinguishable from the filter body. In some modalities, the stage of forming the filter precursor includes 3D printing. 3D printing is a well-known technology that encompasses a variety of different techniques and processes for manufacturing 3D objects using various materials. The term 3D printing is often used synonymously with additive manufacturing. Typically, in a 3D printing process, successive layers of a material are formed under computer control, for example, based on a virtual or CAD design, which can allow the creation of an object of almost any shape or geometry. The use of 3D printing to form complex structures, such as refractory filters, is desirable because the technique allows precise control over the size and shape of the filter's pores and flow paths. 3D printing can also be used to create consistent, regular shapes. The filter precursor can be formed using any suitable additive manufacturing / 3D printing technique. Examples of suitable methods include extrusion deposition, powder bed fusion, fused deposition modeling, and ceramic inkjet printing. In some modalities, for example, in fused deposition modeling and ceramic inkjet printing, 3D printing is performed by premixing the powder composition and the liquid component before deposition. In such modalities, the method may involve mixing the powder composition and the liquid component to provide a paste or suspension, and then shaping the paste or suspension using a 3D printer to form the filter precursor. In some alternative methods, such as powder bed fusion, 3D printing is performed by depositing the powder composition and then applying the liquid component using a 3D printer to selected regions of the deposited powder composition. The liquid component (which can be a liquid solvent or a binder) selectively bonds a layer of the powder composition in the regions where it is applied. Loose powder can then be removed by blowing or vacuuming. The process can then be repeated to build a 3D filter precursor. In some embodiments, the method also includes the deagglomeration of the filter precursor. Deagglomeration can be carried out in embodiments where the filter precursor has been formed using an organic binder, as may be required by some 3D printing processes. Deagglomeration can be carried out by heating the filter precursor to a temperature of up to 400°C. A constant temperature ramp can be applied over a period of 2 to 10 or 3 to 8 hours, for example, 5 hours. The deagglomeration process can be incorporated into the firing stage or can be a separate step in the refractory filter forming method. A deagglomeration step can be useful for large filters. In some embodiments, the method also includes drying the filter precursor before cooking. A drying step is beneficial when the filter precursor is formed from an aqueous mixture. Drying can be carried out (for example, in an oven) at a temperature of 110°C to 200°C. Above 180°C, any organic components present, such as cross-linked foam substrates and organic binders, will burn. Therefore, drying at higher temperatures is carried out for a shorter period of time than at lower temperatures. For example, at 110°C drying can be carried out for 60 minutes, whereas at 180°C it can take only 5 minutes. The filter precursor can be cooked at a temperature between 1500 and 1700°C. In some embodiments, the filter precursor is cooked at a temperature above 1500°C, above 1550°C, or between 1550 and 1650°C, for example, 1600°C. The cooking time can be at least 30 minutes, for example, from 0.5 to 5 hours, or from 1 to 3 hours, for example, for about 2 hours. In some embodiments, the filter precursor is burned in an oxidizing atmosphere, for example, an atmosphere comprising more than 0.5% oxygen. The following will now describe embodiments of the invention by way of example and with reference to the accompanying figures in which: Figure 1 is a graph showing the friability of refractory filters, measured by the level of broken particles of the filter material after vibration. Example 1 Preparation of a refractory filter A piece of cross-linked polyurethane foam was impregnated with a slurry using a combination of rollers and spraying until the desired weight was achieved. The slurry comprised approximately 90% powder composition and 10% rheology modifiers (defoamers, dispersants, wetting agents, binders, and viscosity modifiers). Water was added to provide the required slurry viscosity. The impregnated foam piece was then dried in an oven set to 150°C before firing. Firing took place in a tunnel (continuous) oven set to a temperature of 1620°C. Cold crush resistance The cold crush resistance test is used to evaluate the compressive strength of a filter at ambient temperature. The cold crush resistance was determined using a test method as specified by the German Foundry Association (BDG (Bundesverband der Deutschen Giesserei-Industrie) Directive P100, September 2012 edition), in accordance with DIN EN. 993-5: Test methods for densely shaped refractory products - Part 5: Determination of cold crush resistance. Briefly, a refractory filter (100 x 100 x 25 mm, 10 ppi, unframed), prepared as described in 5 above, was placed on a 25 mm diameter support. Using a ram of the same diameter, the filter was subjected to load at a rate of 20 mm / min until rupture occurred. The resulting maximum force was used to determine the cold crush resistance. Metal spill test Molten stainless steel at a temperature of 1610-1620°C was poured through a refractory filter (100 x 100 x 25 mm, 15 10 ppi, frameless), prepared as described above. The filter was held in a two-sided support and positioned 700 mm below a lower pouring spoon with a 30 mm nozzle. The filter was considered to have passed this test if it remained intact and did not break when a minimum of 30 kg of molten stainless steel was poured through the filter. Results Filters (without frame) that had the dimensions 100 x 100 x 25 mm were prepared from pieces of 8 ppi crosslinked polyurethane foam using the method described above. The filters were made using different compositions in 10 powder according to the recipes in Table 1 below. The compressive strength of the filters and their ability to withstand a molten steel pouring test were tested as described above. The results are shown in Table 1. Table 1 Compoundings: Alumina (% by weight) Zirconium (% by weight) Magnesia (% by weight) Other (% by weight) Cast Steel Test Compressive Strength (MPa) Tabular (D50: 40 pm) Reactive (D50: 2.5 pm) D50: 0.4 pm D50: 15 pm A 70 0 30 0 0 0 Failure 4.5 B 70 29 0 0 0 lb Failure 6.3 c 70 20 10 0 0 0 Fall 2.3 D 72 5 20 0 3 0 Pass (30 kg) 3.6 E 70 5 20 0 5 0 Pass (150 kg) 4 . 6 F 75 0 20 0 5 0 Pass (50 kg) 4.5 G 67.5 5 20 0 7.5 0 Pass (30 kg) 4.4 H 62.5 5 20 0 12.5 0 Pass (50 kg) 4.0 0 65 5 16 0 14 0 Pass (50 kg) . 4 . 6 J 65 5 10 0 20 0 Pass (30 kg) 3.1 K 82 5 8 0 5 0 Pass (50 kg) 6.3 L 74 5 16 0 5 0 Pass (50 kg) 5.4 M 55 5 20 0 20 0 Pass (50 kg) 4.4 N 24 0 5 0 Pass (50 kg) 9 0 65 5 25 0 5 0 Pass (30 kg) 4.1 P 60 0 30 0 10 0 Pass (50 kg) 4 . 9 Q 70 5a 20 0 5 0 Pass (100 kg) 4.7 ΜΛ / t / ZUZZ / UOO 11 t R 72.5 0 20 0 7.5 0 Pass (50 kg) 5.4 S 62 5 20 8 5 0 Pass (30 kg) 4.5 T 60 9 10 15 6 0 Fall 3.6 U 65 10 10 10 5 0 Pass (30 kg) 4.8 V 62.5 12.5 0 20 5 0 Fault 2.1 W 67 15 15 0 3 0 Fault 2.3 X 70 5 20 0 1.7 3.3C Pass (50 kg) 2.6 Y 70 5 20 0 1.7 3.3d Fault 5.9 aD50: 0.4 pm;bSilica;cCeria;dYttria Filters made using the AC compositions, which did not contain magnesia, failed the molten steel test and broke on impact. The DJ compositions, comprising 3–20 wt% magnesia and 67.5–77 wt% alumina, passed the molten steel test. The KP compositions, comprising 8–30 wt% zirconium (D50 0.4 pm), also passed the molten steel test. Composition E, comprising 5 wt% magnesia, 20 wt% zirconium (D50 0.4 pm), 70 wt% tabular alumina, and 5 wt% reactive alumina, was found to provide a strong filter capable of holding up to 150 kg of molten steel. Composition Q, comprising reactive alumina with a smaller D50 particle size (0.4 pm vs. 2.5 pm for composition E), also showed good strength in the metal pouring test. The RW compositions showed that filters containing higher levels of reactive alumina (e.g., 12.5 wt% or more) and / or higher levels of zirconium with a D50 particle size of 15 pm (e.g., 15 wt% or more) were weaker and did not pass the molten steel test, although filters comprising a zirconium mixture having both smaller and larger particle sizes (e.g., compositions S and U) passed the molten steel test. Composition X, in which magnesia was partially replaced by ceria, passed the molten steel pouring test, while Composition Y, in which magnesia was partially replaced by yttria, failed. Example 2 Powder composition E was selected for further testing. Friability test The friability of a filter prepared from powder composition E (referred to as Filter E) was compared with three commercially available framed and frameless zirconium-based filters of the same dimensions (75 x 75 x 25 mm, prepared from a 10 ppi cross-linked polyurethane foam filter), with zirconium levels >90%. One hundred and seventeen filters of each type were packed in a box, placed on their sides in three layers. The box was vibrated on a table for 20 minutes. After vibration, the crumbs resulting from filter breakage were weighed. It was observed that Filter E had a significantly lower friability than commercially available filters (comparative examples X, Y, Z) (Figure 1). Comparison of the Filter E structure with a standard zirconium filter by SEM analysis indicated that the sintering of the refractory material is more complete in Filter E. This is believed to be the reason why the filter of the invention has lower friability than standard zirconium filters. Deformation test A refractory filter with a circular cross-section (150 mm in diameter, 30 mm deep) was prepared from a cross-linked polyurethane foam impregnated with a suspension formed from powder composition E (Filter E'). The deformation of Filter E' was compared to that of a commercially available filter with the same dimensions but a zirconium content greater than 90%. The filters were supported along a 110 mm span. A 170 g weight was placed on top of each filter, in the middle of its upper surface. The filters were exposed to a temperature of 1620°C for 2.5 hours. Following the test procedure, the deformation (i.e., sagging) of filter E' was measured at 3 mm, while for the commercially available filter the deformation was 5 mm. Example 3 An additional composition (Composition Z) was formulated based on Composition E, replacing half of the 40 pm grade tabular alumina with a coarser grade of tabular alumina having a D50 particle size of 200 pm. The water demand of Composition Z was found to be 15% lower than that of Composition E, and Composition Z showed even less shrinkage after firing (around 4.5% shrinkage compared to 6% shrinkage for Composition E). Filters made from composition Z (dimensions: 75 x 75 x 25 mm) were tested using the cold crush and metal pouring strength tests described in Example 1. The filters were found to have higher crush strength than filters made from composition E, and were easily able to withstand 100 kg of molten steel poured at ~1640°C without any sign of breakage. Example 4 Composition Z was tested with the addition of small amounts of titanium dioxide. The addition of 0.5 wt% titania to composition Z was found to increase shrinkage by an additional 1.5%, bringing the total shrinkage to 6% (in line with conventional zirconium filters). The addition of 2 wt% titania was found to increase shrinkage by an additional 4%. The metal capacity of filters with composition Z and containing 0.5% by weight of titania improved dramatically compared to filters made with composition E. Circular filters with a diameter of 150 mm were able to withstand 600 kg of molten steel without breaking. It was also found to improve the filter's cold crush resistance and friability performance. A filter made using a composition comprising 10% by weight of zirconium, 5% by weight of magnesia and 1% by weight of titania, with the remainder consisting of a 50:50 mixture of tabular alumina of 40 pm and 200 pm, also worked well and the slurry was found to be easier to pump.

Claims

1. A refractory filter for filtering molten steel, comprising refractory material, said refractory material comprising: 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia.

2. The refractory filter of claim 1, wherein the refractory material is substantially free of silica.

3. The refractory filter of claim 1 or claim 2, wherein the refractory filter has a compressive strength of at least 4 MPa.

4. The refractory filter of any of the preceding claims, wherein the refractory filter has at least a first surface forming a side face of the filter and two opposing second surfaces forming the continuous flow faces of the filter, the second surfaces having an area no greater than 500 cm2.

5. The refractory filter of any of the preceding claims, wherein the filter is framed.

6. The refractory filter of any of the preceding claims, wherein the refractory filter is a foam filter, a cellular filter, or a pressed filter.

7. The refractory filter of any of the preceding claims, wherein the refractory material further comprises up to 5% by weight of titania.

8. The refractory filter of any of the preceding claims, wherein the refractory material is manufactured using less than 12.5% ​​by weight of reactive or calcined alumina.

9. A powder composition for manufacturing a refractory filter comprising 60-90% by weight of alumina; 8-30% by weight of zirconium; and 3-20% by weight of magnesia, wherein the powder composition comprises less than 12.5% ​​by weight of reactive alumina, calcined alumina or a mixture thereof, and wherein the remainder of the alumina is tabular alumina.

10. The powder composition of claim 9, wherein the powder composition comprises 0 to 10% by weight of reactive alumina, calcined alumina, or a mixture thereof.

11. The powder composition of claim 9 or claim 10, wherein the powder composition comprises at least 60% by weight of tabular alumina.

12. The powder composition of any of claims 9 to 11, wherein the tabular alumina has a D50 particle size of less than 500 pm.

13. The powder composition of any of claims 9 to 11, wherein the tabular alumina comprises a mixture of finer grade tabular alumina having a D50 particle size of 20 to 50 pm and coarser grade tabular alumina having a D50 particle size of 100 to 500 pm.

14. The powder composition of claim 13, wherein the ratio of finer grade tabular alumina to coarser grade tabular alumina is 40:60 to 60:

40. 5 15. The powder composition of any of claims 9 to 14, wherein the reactive alumina, when present, has a D50 particle size of less than 10 pm.

16. The powder composition of any of claims 9 to 15, wherein the magnesia has a D50 particle size of less than 30 pm.

17. The powder composition of any of claims 9 to 16, wherein the zirconium has a D50 particle size of less than 1 pm.

18. The powder composition of any of claims 9 to 17, wherein the powder composition comprises less than 1% by weight of silica.

19. The powder composition of claim 18, wherein the powder composition is substantially free of silica.

20. The powder component of any of claims 9 to 19, wherein the magnesia is at least partially replaced by ceria.

21. The powder composition of any of claims 9 to 20, further comprising up to 5% by weight of titania.

22. Use of a powder composition according to any of claims 9 to 21 to form a refractory filter.

23. A method for producing a refractory filter, comprising: providing a powder composition according to any of claims 9 to 21; forming a filter precursor from the powder composition and a liquid component; and baking the filter precursor to form a refractory filter.

24. The method of claim 23, wherein the filter precursor is dried before cooking.

25. The method of claim 23 or claim 24, wherein the formation of the filter precursor comprises 3D printing.

26. The method of claim 23 or claim 24, wherein forming the filter precursor comprises: combining the powder composition and the liquid component to form a suspension, and impregnating a crosslinked foam substrate with the suspension to form the filter precursor.

27. The method of claim 26, wherein the cross-linked foam substrate is impregnated with the suspension by spraying, roller impregnation, immersion, centrifugation, or any combination thereof.

28. The method of any of claims 23 to 27, wherein the filter precursor is cooked at a temperature above 1500°C.

29. The method of any of claims 23 to 28, wherein the filter precursor is cooked for at least 30 minutes.

30. The method of any of claims 23 to 29, wherein the filter precursor is baked in an oxidizing atmosphere.