NO150566B - PROCEDURE FOR MANUFACTURING A CERAMIC FILTER FOR MILT METAL FILTERING - Google Patents

Description

The present invention relates to a method for producing a ceramic filter for filtering melt-

a metal, wherein the filter has predetermined permeability and structural uniformity, and wherein

a polymeric organic foam material with a hydrophobic network with defined permeability and rebound elasticity is used,

an aqueous slurry of a thixotropic ceramic composition with a viscosity in the range (1-80) is produced. 10^ eps,

at least three rollers are arranged in such a way that at least two roller gaps occur between the pairs of rollers adjacent to each other, the first roller gap being set in advance to a space that makes up 50-90% of the thickness of the polymeric organic foam and the second roller gap to 70-90% of the thickness of the polymeric organic foam,

the polymeric organic foam material is passed at least once through the rolling plant and the impregnated foam material is dried and heated to drive off the organic components, and the peculiarity of the method according to the invention is that the aqueous slurry is poured over at least one surface of the polymeric foam material and in the first roll gap, so that a supply of slurry is formed at the first roll gap, the foam material is uniformly impregnated with the slurry.

These features of the invention appear in the patent claim.

When treating impregnated organic sponge or foam materials with open cells, the obtained porous ceramic foam materials belong to the state of the art and are described in e.g. US Patents 3,090,094, 3,097,930 and 3,111,396. In addition to this, the use of such ceramic foams as a filter for liquid metal, especially for filtering liquid aluminum and liquid copper, is described in US patents 3,893,917 and 3,947,363.

As already mentioned, proposals for the production of ceramic foams belong to the state of the art. Thus, it is proposed in US patent 3,111,396 that a polymeric organic foam material after immersion in a slurry of a refractory material is pressed together, as it is passed through press rollers which effect the removal of excess refractory material. This process, which can be compared to a large number of common expulsion methods used in this field, suffers from the inextricably linked disadvantage that the slurry is not uniformly distributed over the entire mass of the object. Thus, it may happen that the outer area of the object is coated less strongly with the slurry than the area in the middle. Such errors are particularly significant at the extreme limits of the permeability range which has yet been found suitable for use in the manufacture of filters for molten metals. Thus, filters with high permeability tend to have undesired weak surfaces and edges, while filters with comparatively lower permeability tend to display an undesired blocking area in the middle. Such defects make the resulting foams unsuitable for use as a filter for liquid metal.

The above-mentioned difficulties are overcome by using the method described in US patent 4,024,212, where it is required that a first method step comprises an impregnation of the polymeric organic foam material with the slurry of a ceramic composition, where completely in the vigorously stirred or vibrated slurry Submerged foams are repeatedly subjected to compression and renewed expansion. The roll gaps are set in advance so that in the first pass a compression of 50-90% is effected, and in the second pass a compression of 70-90%. However, this method presents the disadvantage that a greater number of treatment steps are required for the polymeric foam sheets, which can adversely affect the firmness properties of the finished ceramic foam body. Nor can the processing steps be easily automated or adapted to the application in a continuous or semi-continuous mass production line.

The task underlying the invention is to provide an efficient and economical method for the production of ceramic foam objects with predetermined permeability properties, where the impregnation of polymeric organic foam with a slurry of ceramic composition and the removal of excess slurry from the foam is combined. It is also part of the task to produce products that exhibit improved firmness properties, a structural uniformity, and are without defects such as e.g. tendency to blockage in the middle and weakness in the outer areas. The production method for the ceramic foam objects must be easily automated and quickly lead to production on an industrial scale, where the impregnation with the slurry and the removal of excess slurry are carried out in rapid succession.

Ceramic foams with specific permeability and structural uniformity are produced starting from a polymeric organic foam material with open pores, which has a predetermined permeability and rebound elasticity. This polymer is impregnated with a fluidized aqueous slurry of a thixotropic ceramic composition. The slurry is poured out immediately before passing through the foam material through a pair of roller slots over at least one surface of the foam material and into the first roller slot. In order to enable a sometimes successive compression, which when passing through the first roller gap constitutes 50-90% of the thickness of the used foam material and when passing through the second roller gap constitutes 70-90% of the thickness of the original foam material, set the spaces between the corresponding pairs of rollers fixed in the aforementioned area. After the combined process steps of impregnation and compression, the formed uniformly impregnated foam material is heated to first drive out the moisture, and then the organic foam components. The resulting object is then ready for use, or can, if desired, be heated further to sinter the ceramic material.

The production of ceramic objects from foam according to the invention depends on the observance of certain critical properties and parameters, as described in US patent 3,962,081. In order to obtain an adjustable permeability and regression elasticity for the ceramic end product, one must start from polymeric organic foam material with open pores. These organic foams have, when the end product is to be used as a filter for molten aluminium, preferred an air permeability in the range (400-8000)

-7 2 •10 cm . Determination of the air permeability can be made according to the book "Micromeretics" by J. M. Dallavalle, Pitman forlag, 1948, page 263. In accordance with the present invention, it has been found that the air permeability in the ceramic objects produced as a final product primarily depends on the permeability of the used polymeric organic foams. E.g. have been produced ceramic foams, which have perme-- 7 2

abilities in the range from (800-2200)»10 cm , from a polyurethane foam with an air permeability in the range from (4500—

- 7 2

5400)«10 cm . A starting foam material, which with regard to permeability exhibits a permeability variation of - 2%, allows the permeability of the ceramic foam to be determined in advance within an accuracy in the range of - 5%.

In addition to the specific permeability, the ceramic foams produced according to the invention must exhibit a structural uniformity, and a particular range for the number of pores or pore density. It has been found that the structural uniformity depends on the rebound elasticity of the polymeric organic foam used as starting material. In particular, the regression elasticity can be measured using specific standard regulations, which are set out in

standard ASTM-D-1564-71. These standard regulations concern the properties measured according to the ball rebound method with regard to compressibility and rebound elasticity. to 50%, returns to its original size or thickness. Foam materials that have been found suitable within the scope of the present invention show, after compression to 50%, a permanent deformation of less than 30%, i.e. that they return to at least 70% of their original thickness after the loss of pressure. The rebound elasticity determined after the ball rebound test is a measure of the resistance of the foam to pressure, which is measured by means of the rebound height of a steel ball, which

falls from a pre-set height onto the foam material sample. Thereby, the setback height for the ball is expressed in percentages, calculated from the original height. Foams suitable for the present invention have a bullet rebound elasticity of greater than 25%.

The above-mentioned properties, which are otherwise determined as usual for test methods under dry conditions, must essentially be maintained also in an aqueous environment, such as e.g. during the impregnation with a uniform distribution of an aqueous ceramic slurry. Consequently, it has been found that hydrophobic foams give better results and are therefore preferred over hydrophilic foams, the latter being exposed to a considerable loss of rebound elasticity in an aqueous environment. This loss in rebound elasticity is noticeable by a blockage in the middle of the ceramic foam material used as a filter.

Taking into account the above-mentioned criteria, the starting substances that can be used in the invention include many polymeric organic foams with highly elastic hydrophobic properties, such as e.g. polyurethanes based on polyester or polyethers, highly elastic or cold-cured urethanes based on polymeric isocyanates, polyvinyl foams, such as polyvinyl chloride, polyvinyl acetate and polyvinyl copolymers, polyurethanes coated with polyethylenes or polysiloxanes or their mixed polymers, and of suitable resins such as cellulose derivatives manufactured foams. All foam substances used must burn up or volatilize below the sintering temperature of the ceramic material with which they are impregnated. As already mentioned, the dimensions of the foam must roughly correspond to the dimensions of the desired ceramic object. Thus, e.g. an organic foam with a thickness of approximately 6-100 mm when the ceramic foam is to be used as a filter for molten metal. In addition to the necessary properties with regard to permeability and uniformity, the above-mentioned polymers must exhibit a pore number within defined limits, so that they can be used for the production of filters for molten metals. The number of pores or cells is also important for the structural uniformity of the ceramic foam material, and should preferably lie within a range of 2-20 pores per cm length, especially 10-14 pores per cm length.

The control of the above variables contributes to the structural uniformity and permeability of ceramic filters. The complicated shape of the flow path determines the metal flow rate and performance. Although the above factors are important, further factors are described which contribute to an advanced control of the end products of ceramic foams.

The organic foams are selected with reference to the

criteria listed above are impregnated with a slurry of a thixotropic ceramic material. The thixotropic properties are of essential importance in the present invention, as they help to determine the uniformity of the structure and the firmness of the final product of ceramic foam material. Thixotropic materials show a high resistance to flow under a small shear stress, and a low resistance to flow under a relatively strong shear stress. The ceramic slurry, which is used in the present invention, must exhibit sufficient flowability to be able to quickly penetrate into the cavities, i.e. the pores in the organic foam material and fill them, whereby the surrounding polymer network is coated. At the same time, upon loss of pressure, the slurry must quickly assume a sufficient viscosity again to prevent runoff or dripping from the impregnated foam material. Within the scope of the present invention, it has been found that certain ceramic materials, together with special, air-binding agents and together with temporary binders, which exhibit the desired thixotropic properties, are successfully used for an impregnation and the subsequent uniform distribution in the polymeric foam material.

While the ceramic slurry used here can vary according to the purpose of use for the foam, it is possible to use a larger number of ceramic materials with different refractoriness. In particular, materials such as aluminum oxide, chromium oxide, zirconium oxide, magnesium oxide, titanium oxide, silicon oxide and mixtures thereof may be present in the slurry. These materials are known for their relatively high refractoriness and usability at high temperatures. For the production of objects made of ceramic foam, other materials with less refractoriness can also be used, such as mullite, calcined clay and various types of glass with a high softening temperature, and then either alone or together with the others, as well as together with more refractory materials. In the case of filters for molten metal, the only requirement for the selection of the special ceramic materials is that during the filtration period they give the foam material sufficient resistance to the chemical attack from the liquid alloys. A ceramic composition which has proved particularly suitable in the practice of the invention consists of a mixture of aluminum oxide and chromium oxide.

The ceramic compositions also include a binder that acts at room temperature or an agent that binds under the influence of air, so that the slurry in its raw state achieves a certain firmness, especially during the expulsion of the organic components and the eventual sintering, where the foam material is subjected to thermal stresses. A large number of air-binding agents or other binding agents are known which can be used for this. Thus, e.g. the composition used in the invention includes materials such as colloidal aluminum orthophosphate, alkali metal silicates, such as sodium and potassium silicate, ethyl silicates, aluminum hydroxychloride, magnesium orthoborate etc. The binder or the air binding agent is usually used in a 50% aqueous solution, which in turn can be added to the slurry in a proportion of 5 to 50%. The slurry thus contains 2.5-25% of the air binding agent or other binding agents. The proportion of binder solution is preferably in the range of 25-35%, calculated on the entire slurry.

r

In addition to the above-mentioned binders, rheological substances can be used which serve to promote the desired thixotropic properties of the slurry. Several substances are known which can be used successfully as rheological agents, including certain organic compounds, especially cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, as well as certain inorganic substances such as bentonite and kaolin. Of the substances available for this purpose, bentonite is particularly preferred. Bentonite is a naturally occurring clay that consists mainly of alumina and various silicates and usually also contains some magnesium and iron. In addition to promoting the thixotropic properties of the slurry, bentonite provides a weak binding agent effect, as certain glass phases are produced during calcination of the object which gives the final structure of the foam an increased strength. In addition to bentonite, a small amount of kaolin can also be used, which results in both an improved debonding and also a rheologically more high-quality finished slurry. Kaolin is also a clay that mainly consists of aluminum oxide and silicon oxide. Of course, one can use the chemical equivalents of the above-mentioned substances to approximate their composition. The usual range for the added rheological agents lies in the invention between 0.1 and 12 percent by weight calculated on the slurry. In the preferred embodiment, the rheological agents are added in a proportion of between 0.5 and 2% by weight.

Although a greater number of recipes can be used for the production of the thixotropic ceramic material, a particularly suitable composition has been found which contains aluminum oxide in an amount of 40-80%, preferably 45-50%, chromium oxide in an amount of up to 20%, preferably 10-15%, kaolin in an amount of up to 10%, preferably 2-5%, bentonite in an amount of 0.1-10%, preferably 0.5-2%, colloidal aluminum orthophosphate (50% solution) in a amount of 5-50%, preferably 25-35%. Water can be further added to the above recipe in an amount of up to approximately 20%, preferably 5-10%, so that, as described below, setting the viscosity to a specific value can be achieved. Although the recipe is proposed with the above-mentioned areas, you can of course also use other recipes, which contain the previously mentioned other constituent parts.

Apart from the thixotropic properties, the ceramic slurry used in the invention must exhibit a carefully adjusted viscosity during the impregnation period, so that uniform ceramic objects can be produced reproducibly. It has been found that the desired viscosity range is within 1-80.10^ eps (centipoise), preferably within a range of 10-40*10 3 eps, especially 25-35*10<3 >eps. The viscosity is set during the preparation of the slurry, but must also lie in the above-mentioned range during the impregnation of the organic foam. As already mentioned, a suitable way of setting and controlling the viscosity consists in changing the water content within the specified range. For the purpose of the invention, the viscosity is measured at 25°C with a Brookfield RVT viscometer at 20 rpm, after 15 min. rotation. In advance, the slurry has been mixed in a Hobart mixing device that holds 76.5 liters for 30 minutes. with 60 rpm.

In the attached drawings, two preferred embodiments of the invention are schematically shown in longitudinal section. They show the production of a uniformly impregnated polymer foam sheet by means of a rolling process, in which an aqueous slurry of a ceramic composition is applied immediately before passing through the first roll gap, and thus: Fig. 1 shows a rolling plant with three pressure rolls, in which the impregnated sheet in rapid succession is compressed, expanded again, compressed again and expanded again, and Fig. 2 shows compression and expansion of the plate using two pairs of rollers.

In fig. 1, sheets 1 of spongy polyurethane foam with a pore count of between 2 and 20 pores per cm length, preferably between 10 and 14 pores per cm length, successively through a 12 mm wide roller gap between rollers 2 and 3, and then through the immediately adjacent 3-4 mm wide roller gap between rollers 2 and 4. The foam sheets used for this can be of any suitable length , a width of 100-900 mm and a thickness of 6-100 mm, e.g. a width of 430 mm and a thickness of 50 mm. The used press rollers 2, 3 and 4 have a sandblasted surface, exhibit a suitable diam. on e.g. 75 mm and is somewhat longer than the width of the foam boards used. The rollers are inserted in a stand, not shown, which is supported by the support 5. An undrawn drive unit ensures that all rollers turn at the same speed, e.g. with 12.5 rpm. The front of the plate can be immersed in the slurry before it is introduced into the first roll gap. During the passage of the plate through the rolling mill, the slurry 6 flows from the container 7 over the surface of the roll 2 and the plate 1 lying in the area of the inlet gap. The outflow velocity of the slurry is measured so that a supply 8 of slurry is formed on the inlet side of the first roll gap. After the plate has left the first roll gap, the plate 1 rapidly expands again due to rebound elasticity to the conceivable thickness. However, the plate is immediately compressed again by being passed through the second roller gap between rollers 2 and 4. Even after the plate has stepped out of this roller gap, it immediately expands to its original size, and is then carried away by the conveyor belt 9. The series of alternate compression and expansion of the plate serves to keep the slurry in a flowable state and thus facilitate its uniform distribution over the entire plate. Immediately after passing through the last gap of the rolling mill, the slurry retained in the pores of the foam sheet is in a state of rest, and its viscosity rises rapidly due to its thixotropic properties, and for this reason the slurry does not flow during the following treatments and during the drying process out of the disc.

The above-described passing of the plate through the rolling plant can be repeated once or twice, so that the desired uniformity of the distribution of the retained in the plate

slurry is ensured. For further passes through the rolling mill, the plate is preferably rotated 180° with consideration

to the plate ends. The impregnated sheet emerging from a first rolling mill can also be processed in one or more similarly arranged rolling mills. Underneath the roller plant, a drainage container 10 is arranged to catch excess slurry and the drops that fall from the edge of the plate and the rollers. The slurry collected in the drainage container 10 can be continuously or at specific time intervals returned to the not shown main container for the slurry, where it is kept in a flowable and usable

condition by sufficient stirring or vibration.

Among the various design options for the device described above, one is highlighted where the rollers 2 and 3

are arranged next to each other in such a way that their axes lie essentially in the same horizontal plane.

On this, the supply of slurry upon entry into the roll gap is kept in contact with both plate surfaces. The roller 4 is arranged below the rollers 2 and 3, approximately midway, so that the plate is not bent to an unduly strong degree during its passage through the roller gap.

The one in fig. The device shown in 2 consists of two tandem roller pairs

which is used instead of that in fig. 1 showed a wood rolling mill.

The first and second roller gaps are arranged between it

first pair of rollers 11 and 12 respectively between the second pair of rollers 13 and 14 and works in the same way as is explained for the embodiment shown in fig. 1.

The uniformly impregnated plates are carried away by means of the conveyor belt 9, and can then, if desired, be burned for the production of molten ceramic foam objects. The drying and heating are carried out primarily to first expel the water and then the organic polymeric foam substances from the object. Normally, normal drying processes can be used, but it should be noted that when choosing a suitable heating rate for the expulsion of the foam material,

the heat released by the oxidation process for the foam must be taken into account. The impact of this pheno-

but particular account must be taken of when large quantities of the foam material are heated, and a significant proportion of the volume of the heating chamber is taken up by the objects. In such cases, it may be necessary to keep the material at a temperature of 205-370°C to avoid excessive heating resulting from the chemical reaction. Excessive heating can cause the ceramic network to crack due to thermal stresses. The exact temperature is determined by the organic foam used in each individual case.

The ceramic foam can, if desired, be heated and fired further to sinter the ceramic particles to form a highly refractory structure. As previously mentioned, however, this sintering is not optional, and it has been found that the foam material, if it is to be used as a filter for molten aluminium, only needs to be treated at a temperature in the range from 540-595°C to drive out the organic components . The resulting filter can be used for cleaning molten aluminum alloys at temperatures up to 760°C. The debonding in the air of the binder imparts sufficient strength to the object, and a complete sintering process is not necessary. When the method described above is used, ceramic foams can be produced with a thickness in the range of 6-100 mm, which can have a surface area of one or more square metres. The foams can, depending on the organic starting foam material used, have a pore number of 2-20 pores per cm length, a permeability of (100-10000)» 10 - 7 cm <2> and a bulk weight of 0.2-1 g/cm <3>

If the foam articles produced by the present invention are to be used as a filter for molten metal, the air permeability is preferably in the range of approximately

- 7 2

(400-8000)»10 cm and the number of pores between 2 and 18 pores per cm length. As previously mentioned, both parameters, namely permeability and pore number, can of course be varied according to the particular area of application for the object. Thus, e.g. a relatively fine ceramic filter

set with an air permeability of (400-2500)•10 - 7 cm<2>

and a pore number of 8-18 pores per cm length. Such a filter is advantageous when filtering aluminum alloys of the 5000 series. However, when the metal to be filtered is particularly contaminated, the liquid metal should first be cleaned with a relatively coarser ceramic filter that has a pore number between 2 and 8 pores per cm length, and an air permeability in the range (2500-8000)• 10~7 cm<2>. This differentiated filtration can be achieved by using a single filter with different properties or a series of filters with different porosity.

Further advantages and characteristics of the method described above are more readily apparent from the following examples. Example 1 listed for comparison purposes shows the method according to US patent 4,024,212, while example 2 relates to the present invention.

Example 1 (Comparison example)

The starting material is a polyester-polyurethane foam with a thickness of 50 mm, pore number 12 pores per cm length and an air permeability of 4697*10 - 7 cm 2. An aqueous ceramic slurry of 47% alumina, 13% chromium oxide, 3.5% kaolin, 1% bentonite and 29% aluminum orthophosphate (50% aqueous solution) was mixed thoroughly in 1 hour at 60 rpm. in a Hobart mixer which held 76.5 litres. After 1/2 hour mixing time, a sample was taken for viscosity determination. The test showed that the slurry had a viscosity of 32.0*10 eps, at 25°C determined using a Brook

field RVT viscometer at 20 rpm. after 15 minutes of turning. A plate of the organic foam was completely immersed in the slurry and for about 30 seconds was repeatedly compressed with a ram and again unloaded, while the slurry bath was vibrated at a frequency of 60 Hz to fill the voids in the foam with slurry. The foam board impregnated in this way was taken out of the slurry and passed through sandblasting

rollers arranged in a three-roll mill, which were previously fixed at a 75 and 87.5% reduction to squeeze out excess slurry. The rollers had a diameter of 75 mm and were driven at a speed of 12.5 rpm. After the passage through the roller slots, the foam sheet practically completely bounced back to its original thickness in the individual case.

Then the plate was dried in an oven at 65°C for one hour and then for two hours at 95°C, then heated with a temperature gradient of 55°C/hour from 95°C to 260°C, and held for 8 hours at a temperature of 260°C to drive out the polyurethane substance without the ceramic skeletal structure collapsing. The material obtained was then sintered in an incinerator, heating it at a rate of 95°C/hour to achieve a temperature of 980°C.

The sintered ceramic plate was cooled in the oven.

This plate was free of defects and the surface was resistant to chipping. The permeability was measured at 1554»10~<7 >cm 2 , and the bulk weight at 0.39 g/cm 3. The plate showed an average modulus of rupture of 12.8 g/mm 2.

Example 2

For this example, foam boards were used which originated from the same series of a polyester-polyurethane material and a slurry with the same composition as in example 1, with a measured viscosity of 30.5*10^ eps. These foam panels were treated according to the combined in fig. 1 illustrated impregnation roller method according to the present invention. Thereby, a rolling mill with 3 rolls of 75 mm diam was used. which was rotated at a speed of 12.5 rpm. The polymeric foam sheet was introduced into the first roll gap which was fixed at a 75% reduction in thickness. At the first roll gap, a slurry was poured onto the foam sheet and the roll surface at such a rate that a supply of the slurry was constantly maintained. After passing through the first roll nip, the plate immediately expanded to its original thickness, and immediately then entered the second roll nip which was fixed at an 87.5% reduction in thickness. After passing through the second roll gap, the plate immediately expanded again to its original thickness. After exiting the plate from the rolling mill, it was rotated 180°/ with respect to the plate ends and the same impregnation-rolling procedure was repeated, this first to a uniform impregnated plate which showed a complete expansion to the original thickness. Bi-retention of a, in comparison with example 1, greater regression elasticity during the process was obvious.

The plates were then dried and heated as described in example 1, the product being without defects and exhibiting a chip-resistant surface. The permeability was measured at 1456*10 - 7 cm 2 , bulk weight 0.38 g/cm 3 and average modulus of rupture was 14.8 g/mm 2 .

With this method as indicated in this example, in comparison with the previous example, time-consuming processing steps can be eliminated, and the product formed exhibits improved firmness properties. This result is possibly achieved due to the reduced deformation effects, the smaller internal crushing of the polymeric fibers and networks and/or structural disturbances, which could otherwise occur during the processing steps previously employed as necessary but redundant according to the present invention.

In addition to this, it can easily be seen that significant processing steps from example 1 are not as well suited for automation as is desirable for mass production. In contrast, the processing steps from Example 2 can be easily automated, including the forward movement of the individual polymeric foam sheets, the photoelectric control of the inflow of slurry, which must hit the roll and the surface of the foam sheet just before the first roll gap, and the transfer of the impregnated boards for the drying and heating zone. It must also be taken into account that the method according to the invention is also suitable for the combi-

nert impregnation and rolling of an endless polymer foam

fabric web, which can be divided into finite length pieces at arbitrary times, e.g. after the drying and heating treatment. Especially for such a continuous application, it can be advantageous to arrange it so that the lowest roller, or the lowest rollers, are partially immersed in the slurry contained in the draining container, so that it is ensured that the underside of the polymeric foam

web during the impregnation rolling process comes into contact

ing with a surplus of slurry.

In summary, it can therefore be said that a preferred embodiment"-

form of the method according to the invention includes the use of polymeric organic foam material, which is charac-

served at a pore number of 10-14 pores per cm length, a permeability of (4000-8000)•10 7 cm ? and a thickness of about

trained 50-100 mm. The use of aqueous slurries of fine-particle refractory composition, preferably with a certain

cosity of 25-35*10 eps allows the production of a ceramic foam product with a permeability of (1000-3000)»

-7 2 10 cm. Immediately after the first contact of the organic foam board with the resulting slurry,

of several treatment steps consisting of short-term compression

ning and renewed expansion due to the passage through immediately adjacent roller slots,

where the first compression is 50-80% and the subsequent

end renewed compression is equal to or greater than the first, and at most 90%, leads to a uniform distribution of the slurry in all pores of the organic foam material.

Unless otherwise specifically stated, all figures are percentages

calculated on a weight basis.

Claims (1)

Process for the production of a ceramic filter for filtering molten metal, where the filter has predetermined permeability and structural uniformity, and where a polymeric organic foam with a hydrophobic network with defined permeability and rebound elasticity is used, an aqueous slurry of a thixotropic ceramic composition is prepared with a viscosity in the range (1-80)* 10 eps, at least three rollers are arranged in such a way that between the pairs of rollers at least two roller gaps occur adjacent to each other, the first roller gap being set in advance to a space of 50-90 % of the thickness of the polymeric organic foam material and the second roll gap to 70-90% of the thickness of the polymeric organic foam material, - the polymeric organic foam material is passed at least once through the rolling plant and the impregnated foam material is dried and heated to drive off the organic components, characterized in that the aqueous slurry is poured over at least one surface of the polymeric foam material and in the first roll gap, so that a supply of slurry is formed at the first roll gap and the foam material is uniformly impregnated with the slurry.