TWI480086B - Method for the production of a refractory filter - Google Patents

Description

Method for making a refractory filter

The present invention relates to a method of making a filter suitable for filtering molten metal and a filter produced by the method.

Molten metal typically contains solids such as metal oxides and other impurities which can cause the final cast product to have undesirable characteristics. Filters have been designed to remove such impurities from the molten metal during the casting process. Conventionally, such filters are made of a refractory process to withstand the high temperatures associated with the molten metal.

One type of filter is a honeycomb type filter that includes a series of parallel conduits or channels through which metal passes. The filter is formed by extrusion or by swaging. Although they are robust and easy to handle, their filtration efficiency is relatively poor because the molten metal travels only short and the diameter passes through the filter.

A preferred refractory filter has a foam-like appearance and is referred to as a foaming filter in the metal filtration industry. These are generally ceramic foam filters, but more recently carbon-bonded filters in which the refractory material is bonded by a material comprising a carbon matrix, as described in WO 2002/018075, have begun to be established in several applications. The foaming filter has a strand network defining a plurality of interconnected open cells. Because the flow path through the filter is roundabout, the filtration efficiency is much higher than the honeycomb filter.

The manufacture of ceramic foaming filters is described in EP 0 412 673 A2 and EP 0 649 334 A1. Typically, open cell foams (e.g., reticulated polyurethane foams) are impregnated with an aqueous slurry of refractory particles and a binder. The impregnated foam is compressed to discharge excess slurry, which is then dried and baked in a slurry coating to burn off the organic foam and to sinter the refractory particles and the binder. Thereby, a solid ceramic foam having a plurality of interconnected voids having substantially the same structural configuration as the starting foam is formed. Although the filtration efficiency is much improved much better than the honeycomb filter described previously, the ceramic foam filter is mechanically weak (the strand tends to break particularly at the edge of the filter).

A filter can be used to place a hole in the wall between the molten metal inlet and the molten metal outlet to filter the metal. An example of placing a filter on a refractory wall is described in U.S. Patent 4,940,489. Because the foaming filter is porous in all directions and the filter edges are irregular, it is possible for several molten metals to flow around the filter or only through a portion of the filter, thereby reducing filtration efficiency. This problem is exacerbated if a strand break has occurred during filter transport or during placement of the filter in the refractory wall position (it will be noted that the broken strand itself may contribute to impurities in the final casting).

Increasing the amount of slurry used for impregnation (i.e., coating the foam) in the manufacture of the filter increases its strength, but also reduces filtration efficiency due to higher weight and reduced porosity.

The filtration process requires a priming filter in which the filter holes are filled with metal and reach a continuous flow of metal. The priming involves replacing the air in the hole (on the surface of the filter) and the required pressure is inversely proportional to the pore size. In addition, temperature loss in the metal will increase the viscosity of the metal, so a filter with a high heat capacity will result in increased heat loss and reduced priming. It is therefore not desirable to coat a heavier filter with a thicker strand because it will have a greater heat capacity. This means that the molten metal will need to be heated to a higher temperature, ensuring that it will not freeze as it passes through the filter. This is disadvantageous from an economic and environmental point of view because it increases the amount of energy required to heat the metal to the desired temperature.

In addition to the extra weight, filters made with increased slurry usage will have a reduced metal flow rate due to increased strand thickness and smaller pores and will have a greater tendency to block. Reducing flow rate and premature fracture can have an adverse effect on metal casting, for example by increasing casting time or causing incomplete molding, and may have to increase filter size or increase foam pore size. Increasing the level of the slurry is therefore an unrealistic solution to increase the strength of the foaming filter, especially the edge of the foaming filter.

No. 5,039,340 describes a method of making a foaming filter in which an adhesion promoting material is preferably applied to a foam together with a flocking agent. The adhesion promoting material and the flocking agent increase the amount of slurry which is subsequently adhered to the foam. The final result is a stronger but heavier filter.

It has previously been proposed to supply a protective layer to the edge of the foaming filter that is in contact with the mold/mold wall. The purpose of this protective layer may include increased mechanical strength, prevention of metal passage between the mold or mold wall and the filter (metal by-pass), and reduction of the end of the ceramic foam filter strand will be operated (especially mechanical / Robot operating the filter) and the possibility of breaking during transportation. The protective layer also facilitates the use of robotic operations to allow automatic configuration of filters into the model.

EP 0 510 582 A1 discloses a ceramic foaming filter which is surrounded in a rigid metal or ceramic frame. The ceramic frame filter can be produced by winding a dough-like extruded strip around a filter (which may or may not be pre-baked) to form a pellet of the pellet and then drying and baking.

CN 200991617Y discloses a ceramic foaming filter having a protective layer of organic material around its edges which decomposes at high temperatures during use of the filter. The protective layer is said to reduce damage to the filter during transportation and installation and also allows it to be used in automated production lines.

US 4,568,595 relates to a ceramic foaming filter with a ceramic coating. The coating is provided by smearing, brushing or spraying a ceramic slurry over a baked ceramic foam filter and then baking the composite structure.

No. 4,331,621 describes a ceramic foaming filter having an integrally bonded ceramic gasket secured to its surrounding surface. It is made possible by placing the flexible foamed material in a slurry, placing it in a model having the desired size of the final filter product, and then feeding the ceramic fiber slurry into the crack between the foamed material and the mold. The molded article is then dried and baked to burn off the foam and the sintered ceramic material.

In a specific example, GB 2 227 185 proposes to suspend the foamed plastic starting block with a ceramic slurry and then squeeze the foam to drive the excess mud into the surrounding layer of the solid prior to baking. In another embodiment, GB 2 227 185 proposes forming a closure layer on a ceramic foam filter by adhering a web of further foamed material or fine plastic filament to the foam. During the impregnation of the mud, the small holes or intermediate spaces in the surrounding side edge regions become (and remain) filled with mud, thereby forming a closed layer upon baking. In both specific examples, the resulting coating is thick, thereby reducing the useful volume of the filter and also increasing its heat capacity.

It is an object of the present invention to provide an improved method of making a foaming filter and an improved filter made in this manner. In particular, it is an object of the present invention to provide a method and filter for imparting one or more of the following advantages:

(i) simplified manufacturing methods;

(ii) lower manufacturing costs;

(iii) reduced filter fragility;

(iv) increased filter porosity (and thus increased flow rate and capacity);

(v) greater filter operating strength;

(vi) easier filter installation;

(vii) filter automatic (robot) operation;

According to a first aspect of the present invention, there is provided a method of making a closed edge refractory foam filter comprising: providing a reticulated foamed substrate having at least one first surface forming a filter side and two opposing forming filters a second surface of the flow surface; applying a liquid comprising an organic coating component to the first surface; curing the organic coating component to form a filter precursor having a continuous volatile coating on the first surface; to include refractory particles, bonding The slurry of the agent and the liquid carrier impregnates the filter precursor; and the impregnated filter precursor is dried and baked to form a filter having a closed edge.

The closed edge filter is a filter in which the holes in the edge (i.e., the surrounding surface or sides) of the filter are closed (i.e., blocked). The filters described in U.S. Patent 4,568, 859 and U.S. 4,331,221 are examples of closed edge filters. The term "edge" is commonly used in the art to which the present invention pertains to refer to the surrounding surface/side of the filter.

The liquid must be applied to the foamed substrate to provide a continuous volatile coating on the first surface of the filter precursor such that after impregnation and baking with the slurry, the resulting filter has a continuous closed edge. It will be appreciated that small discontinuities can be created in the volatile coating due to the coating process and the curing of the organic coating. Small discontinuities can also be created in the closed edge of the filter as a result of any defects in the volatile coating layer and subsequent volatilization of the organic coating components as the filter is dried. The discontinuous surface will constitute no more than 5% of the coated first surface area.

The slurry adheres to both sides of the volatile coating and to the foamed substrate such that as it is baked, the volatile coating and the foamed substrate are volatilized to produce a single closed edge filter. Single means that it is impossible to distinguish the end of the strand from the beginning of the paint. It will be appreciated that a single closed edge is different from the protective coating obtained by applying a slurry to a filter that has been baked, such as US 4,568,595, discussed above. In these cases, there will be a visible dividing line between the filter strand and the closed edge.

The invention also pertains to a refractory foam filter for filtering molten metal that can be manufactured by a method of the first aspect, the filter comprising a three-dimensional network of refractory strands and having at least one side and two opposing flow paths The face has at least one side having a unitary closed edge.

The method of the invention allows the edge of the filter to be protected to establish the interior of the filter without the use of non-essential refractory materials. Thus, this property can be utilized to produce filters having lower weight/density or higher porosity than conventional filters while maintaining or improving the properties of conventional filters (e.g., edge strength or friability).

According to a second aspect of the present invention, there is provided a refractory foam filter which can be manufactured by the method of the first aspect, the filter comprising a three-dimensional grid/network of refractory strands and having at least one side and two opposite sides The flow-through surface has at least one side having a single closed edge, characterized in that the single closed edge has a thickness of less than 1 mm.

According to a third aspect of the present invention, there is provided a refractory foam filter which can be manufactured by the method of the first aspect, the filter comprising a three-dimensional grid/network of refractory strands and having at least one side and two opposite sides The flow-through surface has at least one side having a single closed edge, characterized in that the single closed edge includes a bore.

In several embodiments, the bore is substantially longer in a direction parallel to the sides than in a direction perpendicular to the sides.

In a particular series of specific examples, the closed edge has a thickness of less than 0.7 mm, less than 0.5 mm, less than 0.45 mm, less than 0.4 mm, less than 0.35 mm, or less than 0.3 mm. In another series of specific examples, the closed edge has a thickness of at least 0.15 mm, at least 0.25 mm, at least 0.35 mm, at least 0.45 mm, at least 0.55 mm, at least 0.65 mm, at least 0.75 mm, at least 0.85 mm, or at least 0.95 mm.

To understand the variability of the thickness of the closed edge. As referred to herein, the thickness is typically determined halfway between adjacent surface nodes in the filter. A node is defined as the point of an irregular network of strands of two or more strands. The thickness can be determined by reference to a plurality of such measurements between pairs of different surface nodes.

The thickness of the closed edge can be compared to the thickness of the filter strand. In a specific example, the ratio of the thickness of the closed edge to the thickness of the strand is from 0.5 to 2:1, or from 0.75 to 1.75:1, or from 1 to 1.5:1.

In a particular example, the filter of the present invention has a reduced density of at least 10% compared to a similar conventional filter. In a further embodiment, the reduction is at least 15%, at least 18%, or at least 20%.

Comparing the thermal and physical properties of the filter control assesses the suitability of the filter for use. The filter must be able to withstand thermal shocks that are heated to high temperatures, physically refrain from mechanical shock from the impact of molten metal, allow sufficient molten metal to pass through the filter (ie, filter priming and capacity), and have sufficient strength to withstand operation and transportation. Tests designed to measure such properties include friability, air and/or water flow rates, mechanical strength, and molten metal impact (as described herein).

As used herein, a refractory foaming filter is a filter having irregular network or mesh interconnecting strands that can withstand elevated temperatures (eg, above 500 ° C or even above 1500 ° C in the case of filters for molten steel). The strand defines interconnecting holes or voids therebetween such that multiple bypass paths exist through the filter. The foaming filter is conveniently, but not necessarily, formed using, for example, a reticulated foaming substrate as defined in the method of the present invention.

The reticulated foamed substrate may be a polymeric foam such as a polyether, a polyurethane (including a polyether-polyurethane and a polyester-polyurethane), or a fiber foam. . The reticulated foam substrate serves as a template for the resulting filter, so its porosity provides a porosity indicative of the resulting filter. Porosity can be defined in terms of the number of pores in the substrate and the volume fraction of voids (holes). The porosity of the foaming filter is generally indicated by the number of holes per linear enthalpy (ppi). For metallurgical applications, the porosity generally ranges from 5 ppi to 60 ppi, typically 10 ppi to 30 ppi for most casting applications. In fact, in the foundry industry, the ppi mentioned filter is strictly stated as a ppi made of a foamed substrate. The reticulated foamed substrate of the present invention may have a porosity of from 5 ppi to 60 ppi, typically from 10 to 40 ppi or from 10 to 30 ppi.

The pores in the filter are not of uniform size (due to the structure of the foamed substrate), and the pore size is further affected by the method and level of impregnation of the foam. For example, for a 10 ppi foam, the average pore size typically ranges from 4,800 to 5,200 microns, and the resulting filter made from the foam will have an average pore size order of 1200 to 1500 microns. Similarly for 30 ppi, the foamed substrate has an average pore size order of 2800 to 3200 microns and an average filter pore size of 650 to 900 microns. The overall porosity of the foaming filter is typically from about 75% to about 90% by volume.

The shape of the reticulated foamed substrate is not critical and will generally depend on the future application of the resulting filter. The reticulated foamed substrate will generally have a round, square or rectangular cross region. A reticulated foamed substrate having a circular cross region will have only one first surface, while a foamed substrate having a square or rectangular cross region will have four first surfaces.

The liquid can be applied to one or more of the first surfaces of the reticulated foamed substrate. The organic liquid will generally be applied to all of the first surfaces of the reticulated foamed substrate.

The liquid can be applied by spraying. Alternatively, the liquid can be applied using a roller or brush or by sinking the edge of the filter in a large amount of liquid.

The physical properties of the liquid will be determined in part by the coating method. When using a roller, brush or liquid immersion, the solids content and viscosity should be adjusted to impart sufficient adhesion to adhere the precursor and completely enclose the transverse holes to promote smooth surface coating with minimal penetration of liquid into the interior of the precursor. The liquid (containing the organic coating component) should also have good and rapid skin coating properties in order to minimize collapse and maintain a regular and flat coating.

The coating thickness is controlled to allow for greater control of the coating thickness, thus allowing the use of a minimum amount of organic coating composition. As with other coating methods, spraying also requires the liquid to have good outer coating properties, plus low viscosity to promote spraying.

Volatile coatings can be established by applying an additional amount of liquid to ensure a continuous coating. This may be necessary when applying a liquid by spraying, and several coatings may be required. The total liquid to be applied will depend on the desired properties of the volatile coating and the method of applying the liquid, such as the nature of the spraying process.

After application to the reticulated foamed substrate, the organic coating component can be dried and hardened at room temperature. In several embodiments, the drying is accelerated by drying at elevated temperatures (e.g., 80 to 140 °C). When cured (e.g., dried or cured), the organic coating component forms a volatile coating that is compatible with the subsequent impregnation step, and is finally burned out (i.e., volatilized) during the baking so that it is not present in the resulting filter.

In a particular embodiment, the organic coating component is cured to form a flexible, volatile coating. Flexibility means that the coating is bendable and durable; it can be flexed or bent without breaking, breaking or becoming separated from the reticulated foam, and it recovers and retains its shape when any coating pressure is removed . This property is particularly important if the filter precursor is impregnated with a slurry using a method that requires compression (squeezing) to remove excess slurry.

The liquid may be the organic coating component itself or the liquid may include an organic coating component along with other components such as a solvent, a curing agent, and a pigment. In a specific example, the liquid system is non-aqueous.

Curing the organic coating component to form a volatile coating can be cured by simply drying the liquid comprising the organic coating component (evaporating the solvent) at room temperature, or by applying heat and/or air flow, or by exposure to moisture in the atmosphere. The organic coating component, either by the addition of a chemical accelerator, or by combining one or more of these.

The organic coating component can be a polymeric material such as polyurethane, polyvinyl chloride (PVC), polyester (PET, PVA), polystyrene, a mixture of two or more polymer types, and a copolymer. In a specific example, the organic coating component forms an elastomer upon drying. The above-mentioned organic coating ingredients are considered to be particularly useful because they are known to form durable elastomers upon drying.

The organic coating component can be, for example, a single component system or a two component system that is briefly mixed prior to application to a reticulated foamed substrate.

The liquid may include an organic solvent to dilute the organic coating component. The solvent should not adversely affect (ie dissolve) the foamed substrate and should evaporate rapidly at room temperature. Depending on the chemical nature of both the substrate and the coating, a wide variety of solvents may be suitable, including ethers such as tetrahydrofuran (THF) and diethyl ether, hydrocarbons such as pentane, cyclopentane and xylene, ketones such as acetone and methyl b. Ketones, esters such as ethyl acetate, and fluorinated/chlorinated hydrocarbons. In a particular embodiment, the liquid comprises a solvent selected from the group consisting of acetone, THF, ethyl acetate, xylene, and mixtures thereof.

In several embodiments, the organic coating component is a single component moisture cured polyurethane that is diluted to the desired viscosity with a ketone/ether solvent blend.

The liquid can include a pigment to color it. This provides a useful indicator of the amount of liquid that has been applied to the first surface and assists in ensuring that the surface has been completely coated.

The refractory material may be selected from the group consisting of zirconia, zircon, cerium oxide, aluminum oxide (including brown-brown fused aluminum oxide), talc, mica, titanium dioxide, tantalum carbide, zirconium carbide, titanium carbide, calcium carbide, aluminum carbide, and nitriding. Bismuth, aluminum nitride, nickel oxide, chromium oxide, magnesium oxide, mullite, graphite, anthracite, coal char, activated carbon, graphite-magnesia, graphite-aluminum oxide, graphite-zirconia, zirconium boride, boron Calcium, titanium boride, glass (frosted glass), and a mixture comprising two or more of these.

The refractory particles utilized may be, for example, powders, fines, fine gravels, fibrous materials or microspheres (hollow and/or solid). In a specific example, the fibrous material constitutes a refractory material that is utilized up to 5%. Such a small increase in fiber material is known to improve the mechanical strength and heat resistance of the filter.

A small amount of additional material can be added to the slurry to modify the mechanical and thermal properties of the resulting filter. In a specific example, other materials (e.g., metal powder and metal alloy powder) are present in an amount equal to up to 5% by weight of the refractory material. Suitable materials include steel, iron, bronze, bismuth, magnesium, aluminum, and boron.

The binder can be any conventional binder used in the manufacture of refractory foam filters. The binder may be an inorganic binder such as silicate glass (such as borosilicate, aluminosilicate, magnesium silicate) or phosphate glass, or carbon-rich selected from one or more of the following types of materials. Source: asphalt, hydrazine, and organic polymers that degrade in a non-oxidizing atmosphere to form carbon upon pyrolysis.

The skilled person will be able to select a suitable refractory or refractory mixture depending on the particular mechanical and thermal requirements of the filter. For example, glass bonded by aluminum oxide and aluminum oxide/graphite mixtures is often used to filter aluminum alloys. Aluminum carbide and tantalum carbide bonded glass are often used for iron filtration, and zirconia bonded glass is used for steel filtration, while carbon The bonded aluminum oxide and graphite mixture is used for both iron and steel filtration.

The liquid carrier in the slurry can be any suitable liquid diluent such as water, methanol, ethanol or petroleum ether. However, water is generally utilized because it provides a good coating property of the slurry and is environmentally safe.

Other materials may also be added to the refractory slurry to modify its rheological properties. The use of such materials in the preparation of filters is well known in the art and includes suspending aids such as clays, antifoaming agents such as helium-based liquids, polymeric stabilizers and dispersing agents.

The refractory slurry impregnated filter precursor system is well known in the art and can be squeezed by immersing the precursor into the slurry and/or by rolling the slurry onto the precursor or rolling into the precursor and/or spraying. / or roll and / or centrifugation to remove any excess slurry.

One or more additional coatings of refractory material and/or binder, optionally with a liquid carrier, may be applied to the filter precursor and the additional coatings may be dried.

DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described by way of example only with reference to the accompanying drawings, in which: FIG. 1 is a schematic cross-sectional view of a partial filter according to an embodiment of the present invention; and Figure 2a is a cross section of a conventional filter obtained by CT X-ray imaging. Figure 2b is a negative view of a filter according to an embodiment of the present invention, and Figure 3b is a negative view of the same image; SEM image of the filter.

1 is a highly schematic cross section of a partial filter 10 in accordance with the present invention. The filter 10 has a closed peripheral edge 12 and includes a network of irregular strands 14 that surround and define the holes/voids 15. Both the closed rim 12 and the strands 14 are slurries formed from refractory material. The strands 14 have cavities 16 due to the burning (volatilization) of the reticulated foam during firing of the filter precursor. The cavities are also present in the closed edge 14 of the pre-existing volatile coating. These are located along the dotted line.

A node can be defined in a filter in which two or more strands 14 meet. Several nodes in the filter have been marked A. The thinnest portion of the closed edge 12 is between the midpoints of the two surface nodes. The midpoint instance has been marked B. It can be seen that the thinnest point of the closed edge 12 has a comparable thickness to the strands 14.

Preparation of standard niobium carbide foaming filter

Using a roller machine in combination with spraying, the web-like polyurethane foam block having a square cross section is impregnated with a refractory slurry until the desired weight is achieved. The slurry comprises about 60% lanthanum carbide, 15% aluminum oxide, 5% cerium oxide, 10% rheology modifier (antifoaming agent, dispersant, stabilizer, binder, etc.), and 5 to 10% water. The amount of water added is adjusted to give the desired slurry viscosity.

The impregnated foam block was then dried in an oven at 150 ° C prior to baking. The baking is carried out in a tunnel (continuous) kiln where the temperature in the hottest section does not exceed a maximum of 1200 °C.

system Sealed edge carbonized bismuth foaming filter

A polyurethane foam block having a square cross section is spray coated on its four lateral sides (sides) with a liquid spray comprising a non-recessed single component moisture-curing polyurethane adhesive to a ketone/ether The solvent blend was diluted to a 15% dry solids solution and colored via a 5% addition compatible pigment. The liquid is applied using a standard type pistol spray gun with a pressure tank (5 bar atomization (air) pressure and 2 bar liquid supply pressure). Several layers are applied to the sides until the liquid coating on the foam block is continuous. The edge-coated foam block is then allowed to dry at room temperature.

In addition to using a lower slurry coating rate when preparing the inventive filter, the edge coated foam block was used to prepare a foaming filter using the same method as described above in connection with a standard foaming filter. Decreasing the viscosity and solids content of the slurry by dilution, and/or by adjusting the ratio of roll:spray coating until the desired weight is achieved while maintaining uniform coverage of the foam, reducing the amount of slurry applied (coating rate) . After the slurry is applied, the impregnated edge coated foam block is dried in the same manner as a standard foam filter and then baked.

Evaluation filter

The following means were used in order to evaluate the properties of the filter. It will be important to understand that the test system is for comparison, so it is important to use the same parameters for all filters tested.

Average weight

The weight of a given number of samples is measured and the average is calculated. As previously stated, there is a compromise between using sufficient slurry to provide sufficient strength and good priming and filtration efficiency. In general, a lower weight filter slurry would be preferred as long as the filter is sufficiently strong to apply.

Water flow rate test (capacity)

The water flow rate test is an internally designed device in which the water system circulates and passes through a vertical steel tube in which the filter is sealed to the bottom and perpendicular to the flow so that water flows down the surface and through the filter. For all of the filters tested, the device was arranged such that the surface area of the filter exposed to water was 40 mm in diameter. The height of the water on the filter (head) was 125 mm and the average flow rate through the filter water was measured. The test is used to compare the expected relative flow rates (capacities) of the various filters in order to give an indication of how the filter will be performed with molten metal. The values quoted are averaged by the results of several filters tested.

Pressure drop test (capacity and priming)

The pressure drop test is a standard test for filters where the difference in atmospheric pressure across the filter is determined using a fluid pressure gauge. The filter is sealed in a print in a test fixture connected to a constant flow air pump. An inlet valve is used to vary the air flow and a flow meter is coupled to the outlet end to record the flow rate through the tool. The fluid pressure gauge is attached to the fixture on either side of the sample and measures the difference in air pressure across the filter. The device is arranged such that the surface area of the filter surface exposed to air changes according to the size of the filter, as is the flow of air. For 50mm x 50mm, 75mm x 75mm and 100mm x 100mm filter, the exposed area and the flow rates were 2025mm ^2, 4096mm ^2, 6400mm ² and ^{40m 3 / hr, 57m 3 /} hr, 100m 3 / hr. Similar to the water flow rate test, the pressure drop test is used for comparison purposes to indicate the relative flow characteristics through the filter. It is believed that the lower the pressure drop, the easier it is for the metal to primate and pass through the filter. The values quoted are averaged by the results of several filters tested.

Fragility measurement

Two methods are used to measure the fragility of the filter. In the first type, the weight of the filter fragments that break the filter during the automated packaging phase of the commercial filter manufacturer is measured. Open the bundled carton of the filter and remove each filter. Any broken bulk fragments are separated from the filter and collected with any debris remaining in the box and package. The friability value is then given by the total weight of the bulk debris to the percentage of the total weight of the filter.

An alternative and more rigorous test involves placing six filters in a 200 mm diameter lidded metal pan and then securing this to a standard shaker. Turn on the vibration base (speed setting number 3) and the metal can vibrate for 3 minutes. After three minutes, remove the filter from the pan and separate any bulk debris. Then return the filter to the pan and repeat the shake for a further 3 minutes. The pan is then removed and the friability value is calculated as described above by separating and weighing the total weight of the bulk filter fragments.

Direct impact test (mechanical strength measurement)

A direct impact test was used to test the filter with molten iron, in which 50 kg of gray cast iron at a given temperature was cast from a bottom pouring pail installed below a 450 mm gate to the filter face, which was bonded with resin-bonded sand. The printed print is supported on two opposite sides. The test provides measurement of mechanical strength, thermal shock resistance, mechanical strength at temperature, and erosion resistance of filters from initial metal impact. The test temperature may vary depending on the thickness of the filter being tested and the stringency required, such as a temperature of 1530 ° C and a more stringent test of the filter performance than the use of metal at 1480 ° C. After the test (and cooling), the filter is inspected and if it has a void that passes completely through, it is marked as failed. For each sample, the maximum number of five filters is tested. If at least four filters pass (if the first four passes, the fifth filter is not tested), the result is considered "pass". In addition, scrutinize the filter to see the level of erosion and any cracks in the filter.

Cold crush strength

The cold crush strength of the filter was measured using a Hounsfield compressive strength tester. The test sample was placed centrally on the test pedestal, and a piston of known diameter was moved downward toward the sample at a constant speed of 50 mm per minute until the sample was crushed. The values quoted are averaged by the results of several filters tested.

Comparative Example 1 - Standard Carbide Foaming Filter

A filter having a size of 50 mm x 50 mm x 22 mm was prepared from a 10 ppi mesh polyurethane foam block of appropriate size using the method described above.

Fig. 2a is a cross section of Comparative Example 1 having all filters having a size of 50 mm x 50 mm x 22 mm. The irregular arrangement of the refractory strands can be clearly seen as a light-colored area contrasting with a dark background. The refractory strands include cavities in which the reticulated foam has been burned during the bake. These can be seen in dark areas within the tinted strands. For the sake of clarity, the negative film of this image is shown in Figure 2b, where the refractory strands are shown as dark areas.

Example 1 - Carbide foaming filter with closed edge coating

A closed edge filter having a size of 50 mm x 50 mm x 22 mm (as described above) was prepared from a 10 ppi mesh polyurethane foam block of appropriate size. The coating rate of the slurry was reduced to compare with Comparative Example 1.

Figure 3a is a cross section of Example 1 having all filters of dimensions 50 mm x 50 mm x 22 mm. The irregular network of refractory strands can be clearly seen along with the continuous closed edges. See the closed edge filter in a light-colored area with holes/voids in dark areas. The paint and strands have a similar thickness, i.e., the ratio of the edge coating diameter to the strand diameter is about 1:1. For the sake of clarity, the negative film of this image is shown in Figure 3b, where the refractory material is shown as a dark area.

Figure 4a is a scanning electron microscope (SEM) image of the corner portion of the filter of Example 1, and Figures 4b, 4c and 4d are enlarged images of the same portion. In each case, the line indicates 1 mm. The thickness of the closed edge changes from a maximum of about 1 mm at the node to less than 0.5 mm at the midpoint between the nodes. The closed edge is of comparable thickness to the strand and, at several points, is significantly thinner than the strand. Because the volatile organic coating has been burned off during baking, the closed edge includes a cavity. Several long narrow cavities are visible, which show the location of the volatile coating prior to baking. These have been highlighted by arrows in Figures b, c and d. Because the volatile coating is continuous, it may be expected that a continuous cavity will occur in place of the discontinuous cavity. The inventors believe that many pores occur because the refractory composition of the dry slurry hardens but retains fluid while the coating is volatilizing, so when cracks are creating, the coating can move to fill the crack.

result

The mechanical, physical and thermomechanical properties of the filters of Comparative Example 1 and Example 1 are shown below.

Average filter weight

The average weight (and density) of the filter of the present invention (Example 1) was 18.5% lower than the average weight of the prior art filter having an open edge (Comparative Example 1).

Water flow rate test (volume)

The water flow rate of Example 1 was greater than about 11% greater than Comparative Example 1, indicating that the filter would have a higher metal flow rate and capacity for application. Example 1 had a lower slurry impregnation level and thus a lower filter weight than Comparative Example 1, resulting in a filter with a greater porosity (thinner strands and less barrier pores).

Fragility measurement

The friability (conventional packaging line) was measured using the first test described above. Measurements show that Example 1 is less fragile, i.e., despite the fact that the strands are generally thinner (less refractory load) and thus weaker than Comparative Example 1, the closed edge protects the ends of the filter strands and thus reduces the amount of chip breakage .

Direct impact test (mechanical strength measurement)

Using the standard test, all of the Example 1 filters passed, showing no signs of failure, i.e., breaking. The same results as in Comparative Example 1 show that the Example 1 closed edge filter remains suitable for filtering molten metal despite lower filter weight (dipping level).

The closed edge filter therefore provides an advantage over prior art filters. Protecting the filter edges without the need to establish an internal portion of the filter, and even reducing the level of impregnation inside the filter. As a result of this, the flow rate and capacity of the filter can be increased, and in several instances, it will be possible to manufacture a filter having a smaller pore size while maintaining the flow rate and capacity of a standard foam filter having a large pore size. This means that the filtration efficiency can be improved without adversely affecting the entire casting (molding filling) process.

Comparative Examples 2 and 3 and Examples 2 and 3

A conventional and closed edge filter having a size of 50 mm x 50 mm x 15 mm was prepared from a polyurethane foam block. Comparative Example 2 and Example 2 were prepared from 20 ppi pieces, and Comparative Example 3 and Example 3 were prepared from 30 ppi pieces. The filter properties series are described below.

Note 1) Product specifications (and results) of commercial products

As expected, the 20 ppi filter has a higher water flow rate than the 30 ppi filter due to the larger pore size. Example 3 had a pore size of 30 ppi but a comparable water flow rate to a 20 ppi conventional filter (Comparative Example 2). The filters of Example 2 and Example 3 passed the direct impact test at a higher (and thus more stringent test) temperature than the specifications of the current commercial product Comparative Example 2 and Comparative Example 3. At the same time, if the 30 ppi filter and the larger filtration efficiency together with the capacity are conventionally associated with the lower ppi product, the result means that the Example 3 filter can be used instead of Comparative Example 2.

Comparative Examples 4 and 5 and Examples 4 and 5

A conventional and closed edge filter having a size of 50 mm x 50 mm x 22 mm was prepared from a polyurethane foam block. Comparative Examples 4 and 4 were prepared from 20 ppi pieces, and Comparative Examples 5 and 5 were prepared from 30 ppi pieces. The filter properties series are described below.

Note 1) Product specifications (and results) of commercial products

Both filters of the present invention exhibit improved (reduced) friability, despite having lower filter weight and cold crush strength than conventional filters. They also have a greater water flow rate than conventional filters, indicating that they will have greater application capacity when filtering metals. Example 5 can be used in place of Comparative Example 4 to provide greater filtration efficiency while maintaining flow rate.

Comparative Example 6 and Example 6

A conventional and closed edge filter having a size of 75 mm x 75 mm x 22 mm was prepared from a polyurethane foam block having a porosity of 20 ppi. The filter properties series are described below.

Note 1) Product specifications (and results) of commercial products

Comparative Examples 7 and 8 and Examples 7 and 8

A conventional and closed edge filter having a size of 100 mm x 100 mm x 22 mm was prepared from a polyurethane foam block. Comparative Examples 7 and 7 were prepared from 20 ppi pieces, and Comparative Examples 8 and 8 were prepared from 30 ppi pieces. The filter properties series are described below.

Note 1) Product specifications (and results) of commercial products

Comparative Example 9 and Examples 9A, 9B and 9C

A conventional and closed edge filter having a size of 50 mm x 50 mm x 15 mm was prepared from a polyurethane foam block having a porosity of 20 ppi. Different slurry compositions were used compared to the previous examples, including approximately 55% niobium carbide, 15% aluminum oxide, 10% ceria, 10% rheology modifier with binder, and 5 to 10% water. The amount of slurry applied is adjusted to produce a range of filters having different weights. The impregnated foam block was dried as in the previous example, however the bake filter was carried out in a batch kiln to a maximum of 1150 ° C at the highest point of the bake cycle.

The second method is used to measure the fragility of the filter and is shown below along with other properties of the filter.

The results show that increasing the level of impregnation (and thus the weight of the filter) increases filter strength and reduces filter friability. Further, Example 9B had the same overall weight as Comparative Example 9, however, since it also had a closed edge, less impregnation of the foam (i.e., the thickness of the coated strand) was achieved. The filter will therefore have a higher capacity and higher flow rate than a standard filter, as indicated by a lower pressure drop value without an increased filter friability (edge).

Example 9C has a higher overall filter weight than Comparative Example 9, however, as indicated by the pressure drop data, the strand immersion level is lower. Therefore, it is expected to have a slightly higher capacity and metal flow rate than conventional filters. In addition, Example 9C will be more resistant to fracture and thus particularly robust to mechanical (including robotic) operation due to its significant improvement (reduction) of friability.

real Examples 10, 11 and 12

A zirconia-based filter having a closed edge was prepared from a polyurethane foam block having the same dimensions as those used in the manufacture of Example 6 (Example 10) and 7 (Examples 11 and 12). The slurry composition comprises about 75% zirconia, 10% magnesium oxide, 10% rheology modifier and binder, and 5 to 10% water. The impregnated foam block was dried as in the previous examples. The zirconia filter is baked at a higher temperature than the previous niobium carbide filter, and the highest point of the baking cycle in the batch kiln reaches a maximum of 1600 °C. The results are shown below.

Examples 13 and 14 and Comparative Examples 13 and 14

A 20 ppi closed edge niobium carbide filter and a standard filter were prepared as previously described and evaluated in a horizontally separated model. For each test, twenty sets of steering joint castings were cast, two for each molded case, one for each filter. Two different series of castings were made, and larger filters were used for larger/heavier casting types. The results are shown below.

The closed edge filter is considerably lighter than the corresponding standard filter. Again, these results show that the closed edge filter has a higher flow rate, which reduces casting casting time by about 6% compared to an equivalent standard filter. All castings showed satisfactory results in visual inspection, with no obvious (filter related) defects.

10. . . filter

12. . . Closed edge

14. . . Strand

15. . . Hole/void

16. . . Cavity

A. . . node

B. . . midpoint

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross-sectional view of a partial filter according to an embodiment of the present invention.

Figure 2a is a cross-section of a conventional filter obtained using CT X-ray imaging, and Figure 2b is a negative of the same image.

Fig. 3a is a cross section of a filter according to an embodiment of the present invention obtained by CT X-ray imaging, and Fig. 3b is a negative film of the same image.

Figures 4a through 4d are SEM images of a filter in accordance with an embodiment of the present invention.

10. . . filter

12. . . Closed edge

14. . . Strand

15. . . Hole/void

16. . . Cavity

A. . . node

B. . . midpoint

Claims (11)

A method of making a closed edge refractory foam filter, comprising: providing a reticulated foamed substrate having at least one first surface forming a side of the filter and two opposing second surfaces forming a filter flow surface; Coating a first surface with a liquid comprising an organic coating component; curing the organic coating component to form a filter precursor having a continuous volatile coating on the first surface; impregnating and filtering with a slurry comprising refractory particles, a binder and a liquid carrier Precursor; and drying and drying the impregnated filter precursor to form a filter having a closed edge. The method of claim 1, wherein the reticulated foaming filter has pores (ppi) having a porosity of 5 to 60 per linear enthalpy. The method of claim 1 or 2 wherein the liquid system is applied to all of the first surfaces of the reticulated foamed substrate. The method of claim 1 or 2, wherein the liquid system is applied by spraying. The method of claim 1 or 2, wherein the volatile coating is flexible. The method of claim 1 or 2, wherein the organic coating component is selected from the group consisting of one or more polyurethanes, polyvinyl chloride (PVC), polyester (PET), or polystyrene. A refractory foaming filter obtainable by a method according to any one of the preceding claims, comprising a three-dimensional network of refractory strands and having At least one side and two opposing flow passages, at least one side having a unitary closed edge, wherein the closed edge includes a bore. A filter according to claim 7 wherein the single closed edge has a thickness of less than 1 mm. A filter according to claim 7 or 8, wherein the cavities are significantly longer in a direction parallel to the side than in the direction in which the equal width is perpendicular to the sides. A filter according to any one of claims 7 or 8, wherein the single closed edge has a thickness of less than 0.5 mm. A filter according to any one of claims 7 or 8, wherein the ratio of the thickness of the single closed edge to the thickness of the strand is 0.5 to 2.