DD231738A5 - CERAMIC FILTER WITH OPEN CELL FOAM STRUCTURE - Google Patents

Abstract

The invention relates to filters and a manufacturing method for ceramic filter with open-cell foam structure, produced by impraegnieren a polymer foam with a ceramic slurry containing a phosphate binder, squeezing off the excess slip, drying and firing of the foam material at temperatures of 1 660C and more, or the same Verfahrensfuehrung by using a slurry containing a gelled aluminum hydroxide and firing at at least 1 093C. There are provided ceramic filter with open-cell foam structure, wherein the ceramic is substantially free of phosphate and the individual crystals of the ceramic are firmly sintered together. The filters are suitable for the filtration of metal melts and in particular for the filtration of iron and steel melts.

Description

- Λ "Berlin, 6. 5, 85 65 268 17

Ceramic filter with open-cell foam structure. Field of the invention

The invention relates to a ceramic filter with open-cell foam structure for filtering molten metal, wherein the ceramic is substantially phosphate-free and the individual crystals of the ceramic are sintered together and no binding material between the crystals is present and the filters are made such that an open-celled organic Polymer foam is impregnated with an aqueous, thixotropic properties containing slip of ceramic materials * that the excess slurry removed, the coated polymer foam is dried and heated, wherein the organic polymer foam is decomposed and volatilized, that the resulting ceramic foam fired at elevated temperature and thereby sintered becomes, whereby a ceramic develops with open-celled foam structure. Furthermore, the invention comprises the method for producing such filters and their use *

Characteristic of the known technical solutions

From the prior art it is known porous ceramic foam materials as filters for molten metals, in particular to use aluminum, for example broom rubbed in U.S. Patent No. 3,893,917, ο 947 363, 3962081, 4024056, 4024212, 4075 303, 4,265,659, 4,342,644 and 4,343,704 # These filters are primarily made from phosphate

bonded refractory materials, with the addition of various other additives, and firing at temperatures of about 1093 ^° C. to strengthen the bond. A corresponding method is described in US Pat. No. 3,962,081. Such refractory materials are suitable for use in the aluminum industry and easily withstand most of the aluminum alloys commonly cast at 704C, but are unsuitable for many other potential applications due to their low strength and low chemical resistance.

Object of the invention

The aim of the invention is to overcome the limited scope of the known filters, their low strength and low resistance.

Explanation of the essence of the invention

It is the object of the present invention to develop a material which has the desired properties of the known ceramic materials, in particular the high porosity, the low pressure loss, the large geometric surface and tortuous flow paths, but which include the said parts such as mechanical and thermal strength defects and Lack of Chemical Resistance "In addition, it has been desired to develop a special material that is relatively easy to manufacture and suitable for a variety of applications, with particular attention to

aimed at applications in the high temperature range and for applications with ferrous metals and in particular for filtration applications.

Accordingly, the products according to the invention should be distinguished above all by high strength and particular chemical properties, ie. H. offer better mechanical, thermal and chemical properties over previously known ceramic foams «

According to the invention, the filters are characterized in that the ceramic from a slip containing at least 8 weight "- is manufactured% of phosphate binder and the ceramic open-cell foam structure consists essentially phosphate free, ceramic material, whose individual crystals are sintered together among themselves made of ceramic material, the To minimize porosity between the crystals, or that the ceramic is made of a slip containing 3 to 15 wt .-% of a gelled aluminum hydroxide binder and the ceramic foam is sintered at elevated temperatures, so that the ceramic with open-cell display structure of ceramic material with firmly sintered bond exists.

In a preferred embodiment, the filters may consist of a ceramic consisting of a slurry containing at least 8% by weight of phosphate binder. and having open cell foamy ceramic structure of substantially phosphate-free ceramic material whose individual crystals of ceramic material are sintered together to increase porosity between the crystals

minimize.

The filter is characterized in that it is phosphate-free substantially by weight and preferably less than 2 -.% Of phosphate - expressed as ^P ₂ ° 5 ^{"ent 'n} 3 ^ ^{t unc} * Preferably, the individual ceramic grains are densely sintered together to the porosity between the granules as low as possible and to form a microporosity, said microporosity should be less than about 5 % .

The ceramics * from which the filters can be made are preferably made from a slurry containing 8-30% by weight, preferably 10-25% by weight, of phosphate binder.

A preferred binder is aluminum orthophosphate, but other binders can be used "such. B, phosphoric acid, aluminum phosphate, alkali metal phosphates such as sodium hexametaphosphate, etc. The slip can have at least 8% by weight, preferably more than 10% by weight of phosphate binder, based on the amount of ceramic materials in solid form. The quantities are based on aluminum orthophosphate; If other phosphate-binding agents are used, their amount, based on aluraiura orthophosphate, must be calculated.

In a further preferred embodiment, the filters can consist of a ceramic which is produced from a slip containing 3 to 15 % by weight of a gelled aluminum hydroxide binder and the ceramic foam is sintered at elevated temperatures, so that the ceramic is sealed with open

fenzelliger foam structure consists of ceramic material with firmly sintered bond. Such a filter is substantially binder-free, d, h. free of glassy phases, which reduce the strength of the product. The structure therefore has a form of mechanical solidification, also referred to as dry sintering.

The ceramic is preferably made of a slurry containing 8x30% by weight, preferably 10-25% by weight, of phosphate binder.

The individual crystals of ceramic material are sintered together and the porosity between the grains is kept so low that a microporosity arises. This is preferably less than 5 %,

The phosphate binder used is advantageously aluminum orthophosphate. In this case, the ceramic with open-cell foam structure expediently has less than 2% by weight of phosphate, expressed as ^{ρ o} ° 5 ' ^{au =}

The ceramic of the filters can also be made from a slurry containing 3 to 15% by weight of gelled boehmite.

For the ceramic of the filter is preferably used as the ceramic material alumina or alumina.

The amount stated with regard to the binder always refers to the entire amount of dry material of the ceramic composition, ie "without taking into account the mixing water of the slip.

The process for producing the filters is carried out so that a reticulated, open-celled organic polymer foam with aqueous, thixotropic properties containing slip containing the ceramic materials and binder * is impregnated, then that the excess slurry is removed and the coated polymer foam is dried and heated wherein the organic polymer is decomposed and volatilized to fire the resulting ceramic foam at an elevated temperature to form a ceramic foam whose network is ceramic, and in that the filter is an open cell structure of a plurality of interconnected, networked ones ceramic cavities surrounded.

In accordance with the method of the present invention, a hydrophobic open-celled organic polymer foam, preferably a polyurethane foam, is used.

As a binder, the slurry contains at least 8 weight percent phosphate binder and is sintered to form a ceramic having an open-celled foam structure of substantially phosphate-free material whose individual crystals of ceramic material are sintered together to minimize porosity between the crystals. or the slip contains from 3 to 15% by weight of a gelled aluminum hydroxide binder and the ceramic foam is sintered at elevated temperatures to form a ceramic having an open-cell foam structure of a sintered bond ceramic material.

According to a process according to the invention, a hydrophobic reticulated organic polymer foam, preferably polyurethane urea, impregnated with an aqueous and thixotropic properties slurry of ceramic materials and at least 8 wt .-%, preferably more than 10 wt - . % Phosphatbindemittel, dried and are heated to remove the organic polymer foam and fired at elevated temperature to volatilize the phosphate to form a substantially phosphate-free material, after which the refractory material is sintered to produce a material whose individual ceramic granules are so tightly fused that the Porosity between the grains is kept as low as possible.

According to the invention, the open-cell foam ceramic is made from an open-cell, preferably hydrophobic, flexible organic foam. Suitable materials are polymer foams, such as. B. Polymerurethane foams and cellulose derivative foams. Normally, any combustible and deformable organic foam which is elastic and capable of recovering to its original shape after deformation is suitable. The foam may have between 2 to 40, preferably between 20 and 30, pores per linear cm length. The foam must burn out or volatilize at a temperature lower than the firing temperature of the ceramic material used.

The aqueous, ceramic slurry used is thixotropic and should have a good flow behavior as well as the

ceramic foam materials intended for use suspended in the water, the proportion of which is not critical and should be IO - 50 wt .-%, contain "Typical: 'ceramic materials that can be used include alumina, alumina, zirconia, zirconia, mullite and silicon carbide or mixtures thereof. The grain size should be 75 mesh and finer and preferably 270 to 400 mesh and finer. Although clay is preferred as the refractory material, any refractory suitable for use with a phosphate binder and having a sintering temperature higher than the temperature required to drive off the phosphorus can be used.

A piece of flexible organic foam is so impregnated with aqueous ceramic slurry that its network is coated therewith and the voids filled therewith, usually the foam is simply immersed in the slurry for a short period of time sufficient to insure complete impregnation of the foam and optionally compressed and expanded.

The impregnated foam is then squeezed to remove so much of the slurry that the network of organic foam remains coated with slurry and evenly and without agglomeration through the entire foam piece, a number of pores remain closed by the slurry achieved strongly tortuous flow channels in the final product. For example, in a continuous process, the impregnated foam

passed through one or more preset pairs of rollers to squeeze the desired amount of the slurry from the foam piece and to obtain the desired degree of impregnation. Of course, the pressing can also be done manually by expressing the flexible foam to the desired extent. In this phase, the foam is still flexible and may, if desired, be formed into structures suitable for specific filtration tasks, d, h, curved plates, hollow cylinders, etc. It is then necessary to mold the shaped foam in the usual manner in the given form hold until the organic substrate is disintegrated, or preferably until the ceramic is sintered. The impregnated foam is then dried by means known per se, for example by air drying, rapid drying at a temperature of 100-700 ° C. for a period of 15 minutes to 6 hours or by microwave drying. Air drying takes about 8 to 24 hours. After drying, the treated foam piece is fired and heated until sintering of the ceramic topcoat.

The firing should be carried out at a temperature of at least 1660 C, preferably at least 1676 ⁰ C, and this temperature should be maintained for 15 min to 10 h to decompose and expel the network of flexible organic foam, then expel the phosphate content and finally the to sinter ceramic material.

The resulting product is a porous sintered ceramic in foam structure which is essentially phosphate-free,

ie less than 2% by weight of phosphate - expressed as ^P 2 ° 5 ", and characterized by better mechanical, thermal and chemical properties than those of the hitherto known ceramic foams." The ceramic in an open-cell foam structure is characterized by a large number of different ones In the network of ceramics, the individual ceramic granules are densely sintered to minimize porosity between the granules and to achieve a microporosity that is less than about 5 % For example, the ceramic in open-cell foam structure has a number of closed pores regularly distributed throughout the structure, increasing the tortuosity of the flow paths.

The said method according to the invention results in the entire structure shrinking by up to 15 % per elongation, resulting in ceramic bodies which, starting from Polyraer foams having 2 to 40 pores per cm length, have 1.7 to 40 pores per cm length and there is a reduction in the macroporosity or coarse porosity of the entire filter body. In a filter produced by the process according to the invention, the macroporosity is about 85 %, but without the process steps according to the invention 90 %,

Furthermore, the process of the present invention can be carried out such that substantially a gelled aluminum hydroxide binder is used instead of the phosphate binder.

The inventive method is per se as mentioned above

The resulting changes can be seen in the following explanations «

Upper accordance with the present invention, the ceramic with open-cell foam structure of an open cell _f is preferably hydrophobic, flexible foam material is produced which comprises a plurality of interconnected voids surrounded by a web of said flexible foam material. Typical material that can be used includes polymeric foams such as polyurethane foam and cellulosic foams. In general, any combustible organic plastic foam having the resiliency and ability to recover to its original form may be used. The foam must burn out or evaporate at temperatures below the firing temperature of the ceramic material used.

The aqueous ceramic slurry which is used is said to have pseudoplastic behavior and be thixotropic, have a relatively high degree of fluidity and consist of an aqueous suspension of the ceramic in the intended material. Typical ceramic materials that may be used include preferably alumina and alumina and others such as zirconia, zircon sand, chromia, cordierite, mullite, etc., each in a 75 mesh and finer, and preferably 270 to 400 mesh and finer grain size .

The use of a phosphate or other inorganic binder is not necessary according to this procedure.

The aqueous slurry contains a gelled aluminum hydroxide (alumina hydrate) ♦

In accordance with this procedure, aluminum hydroxides, preferably boehmite, but also the other alumina monohydrates (alumina hydroxides) and alumina trihydrate (aluminum hydroxide), are employed as a temporary binder and rheological agent. First, a collidual dispersion with an acid which is substantially less Firing temperature is volatile, preferably nitric acid, but also hydrochloric acid, sulfuric acid or others produced. This process step is followed by the preparation of a pseudoplastic and thixotropic slip of the desired ceramic material, preferably aluminum oxide.

Generally, 3-15 weight percent aluminum hydroxide based on the dry materials and a hydrate: acid ratio of from 2: 1 to 5: 1, preferably 3: 1, based on 70 % acid is used. In the subsequent preparation of the thixotropic slurry, if desired, small amounts of organic additives, for example, theological additives, additional binders, dispersants and the like can be added. The water content is not particularly critical, for example 10 to 50 % by weight of water, based on the total weight of the slurry. "The water component easily creates the desired fluidity to impregnate the polymer foam and coat the organic network.

An advantage of this process procedure and the resulting products is that no residues such as phosphates potentially reactive with molten metal remain in the final product.

Thus, in accordance with the practice of the present invention, one will first gel the aluminum hydroxide with the acid and then add the ceramic components and the water to form the slurry. If desired, all components can be mixed together. "The slurry is then used to make the open-cell foam ceramic.

The flexible foam material is impregnated with the aqueous ceramic slurry such that the net-like structure is coated therewith and the cavities are filled with it. "Normally, the process is given the preference to immerse the polymer foam in the slurry for a short time to complete To achieve impregnation of the foam.

The impregnated foam is then compressed to squeeze out a portion of the slurry such that the remaining portion of the slurry covers the reticulated framework and also forms a plurality of uniformly dispersed, bare pores in the body that create tortuosity of the flow paths the blocked pores should be evenly distributed and not accumulated in the ceramic body. In the following process, the impregnated foam can be passed through pre-set rollers to achieve the desired

Of course, this can also be done by hand, by simply manual squeezing of the flexible foam material to the desired degree. "At this stage, the foam is still flexible and flexible may, if desired, be formed into various shapes suitable for specific filtering purposes, e.g., B, bent plates, hollow cylinders, etc. It is then necessary to hold the formed foam in a mold by known means until the organic skeleton decomposes is or preferably until the ceramic is sintered. The impregnated foam is then dried in a suitable manner, such as air drying, accelerated drying at temperatures of 100,700 ^° C. for times of 15 minutes to 6 hours or by microwave drying. "Air drying takes 8-24 hours. Upon drying, the material is elevated to elevated temperatures to sinter the ceramic coating on the net-like framework and leave a plurality of uniformly distributed blocked pores, as described above.

In accordance with the present invention, the use of an aluminum hydroxide in the drying process first results in crystallites of the alumina, initially gamma alumina, and finally converted to alpha alumina. This provides sufficient green strength for processing and firing,

The firing conditions depend on the ceramic material. Usually, temperatures of about 10-93 ⁰ C and preferably of about 1371 ⁰ C for at least 15 min and

usually for at least 1 hour, and usually less than 10 hours, to drive out the framework of the flexible foam and sinter the ceramic to form a solid sintered ceramic bond.

The resulting product is a porous, solid sintered ceramic foam material which is substantially free of phosphate and organic components, and is characterized by mechanical, thermal and chemical properties superior to known ceramic foam materials. The ceramic foam is characterized by an open face Cellular structure with a multitude of interconnected cavities surrounded by a ceramic framework dry sintered and absolutely free from potentially hazardous binder materials or glass or clay phases. The open-cell foam structure ceramic is a solid sintered product, particularly suitable for High temperature applications, such as iron, iron alloy and steel filtration.

The aforementioned structure has an increase in the desired physical properties, such as increased mechanical, thermal and chemical properties, Any small amounts of organic additives are expelled in the firing process. The small amount of additives that could be used, if desired, would not degrade the properties, eg, less than 1-3 parts by weight of sintering aids, such as zinc oxide, grain growth inhibitors , such as magnesium oxide, or inorganic rheological aids, such as clays, Montmorillonite, bentonite, kaolin or organic

- 16 rhsological aids »

Various methods of making ceramic foams for molten metal filters are also described in U.S. Patent 3,962,081, U.S. Patent 4,075,303 and U.S. Patent 4,024,212, the teachings of which may be applied to the present invention.

The filters according to the invention are suitable on account of special mechanical, thermal and chemical properties, for the filtration of molten metals, "such as iron, iron alloys and molten steel"

Working Example example 1

Procedure with phosphate binder:

Various samples were prepared by impregnation of a commercial, hydrophobic, open-cell polyurethane foam with ceramic slurry containing as a solid 98 % by weight of alumina and 2 % by weight of montmorillonite as a theological aid. Added were about 30 % by weight of aluminum orthophosphate in a 50% by weight aqueous solution based on the total weight. Slide samples were dried and fired at temperatures of 1093 ⁰ G ₁ 1577 ⁰ C, 1635 ⁰ C, 1660 ⁰ C, 1676 C, and 1699 C over a period of 5 hours. "" The samples were then subjected to chemical analysis and compression strength testing and chemical

Resistance "The results are explained below.

The results clearly show that fired at about 1093 G samples substantially over 2 Gevv »- ^ phosphate - expressed as P ₂ O 5 - contained, but at 1660 ⁰ C and higher fired samples were substantially phosphate-free, that is less than 2 Gevu % Phosphate - expressed as ^ρ ₂ ^ 5 ~. Further, in all cases, the samples burned at 1,660 ° C and higher were characterized in that the individual ceramic granules were densely fused and the porosity between the granules was found to be minimum, the microporosity being less than 5 % .

Tables I and II below show the various properties at the various firing temperatures, with the thickness of the starting material uniformly 5.01 cm (2.01 inches) each.

- 18 Table I

Compressive strength, bulk density and thickness of ceramic foams fired at different temperatures

Burning ⁰ C 1093 1577 1635 1660 1676 1699

Number of tested

Samples 8 10 9 11 10 10

Compressive strength

kg / cm ² 10.28 11.01 9.70 10.12 14.30 18.52

ρsi ^X 146 157 138 144 203

Apparent density

gm / cm ³ 0.31 0.36 0.33 0.37 .0.37 0.39

lb / ft ^{3 XX} 19.2 22.3 20.4 22.9 22.9 24.1

cm 5.10 4.87 4.82 4.64 4.41 4.48

in. ^XXX 2.01 1.92 1.90 1.33 1.74 1.77

χ psi = pound per square inch (pounds per 6.45 cm) xx Ib = pounds (pounds); ft = foot = 30.5 cm xxx in. = inch = 2.54 cm

The strength data listed above in Table I show a sharp increase at temperatures above 1576C, whereas acceptable strength at 1660C is achieved. The data also shows that the apparent density increases and the thickness decreases, indicating a shrinkage caused by the loss of porosity in the walls, i.e., through

, caused the forming microporosity

Table II

Chemical resistance of ceramic foams fired at different temperatures

Burning temperature weight loss (%) in:

⁰ F °> 1OfSNaOH 10 = ^ Ν0 ₃ 10SH ₂ SO ₄ 10 & HCL 10 ^ CH ₃ CCO 2000 1093 prob crumble in d Solutions 2870 1577 13.4 12.5 12.6 10.5 1.5 2980 1635 10.4 10.5 10.6 10.5 1.4 3020 1560 8.8 10.1 8.1   9.2 1.6 3050 1676 3.6 4.9 4.4 5.5 1.1 3090 1699 2.7 2.0 2.2 1.4 0.9

The chemical resistance of the samples was determined by immersing the previously weighed filter samples in the specific solutions for 5 1/2 days. The _peaks above were then removed from the solutions, washed, dried and weighed again. The samples fired at 1093 G disintegrated or softened to such an extent that they could not be removed from the solutions intact; therefore, no weight loss data was registered. For the other samples, the weight losses are listed above. At 3660 ⁰ C, and even more at 1676 C, a strong change seems to occur.

In this way, as is clear from the above,

.- 20 -

becomes a ceramic foam with improved strength properties and chemical durability,

Example 2

Procedure with aluminum hydroxide binder:

A thixotropic ceramic slip was prepared such that first a gel consisting of:

1S35 g boehmite

475 ml of concentrated nitric acid 9060 ml of water

* This gel was made into a dry powder mixture:

74 kg of aluminum oxide 79 g of magnesium oxide

Another 300 ml of water was added to the mixture and the whole batch was completely mixed in a high-speed mixer.

The above-mentioned thixotropic ceramic slurry was used to impregnate an open-cell, flexible polyurethane foam block of nominally 8 pores per cm to a green density of about 10 percent by volume of the theoretical density so that the fibrous framework of the foam is covered and the voids filled therewith , The impregnation was

carried out by immersing the foam in the slurry and the use of preset rollers to compress the foam and expel a portion of the slurry, wherein the net-like skeleton part remains covered with slurry, a plurality of blocked pores evenly distributed through the body, which forms increase the tortuosity of the flow paths.

The resulting impregnated foams are dried and heated to remove the organic components and fired to achieve a solid sintered ceramic bond by heating to about 1648 G for 1 h. The resulting product is a porous, sintered, ceramic foam material substantially free of organic components and characterized by excellent mechanical, thermal and chemical properties. The linear shrinkage by firing was about 15 %, which means that the resulting ceramic foam is a fired one nominal Porsn number of 9 pores per cm and a fired density of 15 % of the theoretical density «

Example 3

Ceramic foams prepared according to Example 1 with a size of 5 × 5 × 2.5 cm after firing are used in a precision casting process for filtering high quality stainless steel. The ceramic filters are placed directly in the casting opening and cast from below , The resulting castings are free of defects,

- 22 -

caused by inclusions

Example 4

Ceramic foams prepared according to Example I ₁ having dimensions after 3 × 10 5 × χ of 2.5 cm are used for filtering fan casings of stainless steel, for which a top pouring device is used. A high quality casting was achieved * A welding temperature, normally necessary for such casting and caused by inclusions, was thus prevented

Example 5

Ceramic foams prepared according to Example 1, with dimensions after firing of 10.times.10.times.2.5 cm, are used as filters for a high-temperature nickel-aluminum-bronze alloy test casting. A high quality casting was achieved. This casting process would normally require several hours of welding repair, in the present case a casting resulted with significantly reduced need for welding work,

Example 6

A thixotropic slip was prepared according to Example I and for the Impregnation of a nominal 4 pores per running ciii, _t offenzeliigen flexible polyurethane foam block used as dried according to Example 1, the resulting foam was impregnated with microwaves and then at

1643 C to produce a solid sintered ceramic bond and a porous fused ceramic foam material substantially free of organic components characterized by excellent mechanical, thermal and chemical properties. The fired ceramic foam, with the dimensions of IO χ IO χ 2.5 cm and with a pore number of 4.7 pores per cm, was used for filtering low-alloy steel. Defects originating from inclusions were eliminated.

Thus, as apparent from the foregoing, a ceramic foam having the best strength properties and chemical resistance is obtained.

Claims (23)

Erfindunqsanspruch The ceramic is substantially phosphate-free and the individual crystals of the ceramic are densely sintered to each other and no bonding material is present between the crystals and the filters are made such that an open-celled organic polymer foam impregnated with an aqueous slurry having thixotropic properties of ceramic materials, the excess slurry is removed, the coated polymer foam is dried and heated, whereby the organic polymer foam is decomposed and volatilized by firing the resulting ceramic foam at an elevated temperature and sintering it in which a ceramic having an open-cell foam structure is produced, characterized in that the ceramic is produced from a slurry containing at least 8% by weight of phosphate binder and the ceramic having an open-cell foam structure consists of substantially phosphate-free ceramic material whose individual crystals of ceramic material are firmly sintered together to minimize the porosity between the crystals, or that the ceramic is made of a slurry containing 3 to 15 wt .-% of a gelled Alurainiumhydroxid-3inders and sintering the ceramic foam at elevated temperatures so that the open cell foam ceramic material is a sintered bond ceramic material. 2. Filter according to item 1, characterized in that the ceramic is made of a slurry containing at least 8 wt .- ^ phosphate binder and the ceramic with an open-cell foam structure consists of substantially phosphate-free ceramic material, the individual crystals of ceramic material are sintered together, to minimize the porosity between the crystals, 3. Filter according to item 1, characterized in that the ceramic is made of a slip containing 3 to 15 gavu% of a gelled aluminum hydroxide Sinders and the ceramic foam is sintered at elevated temperatures, so that the ceramic with open-cell foam structure of ceramic Material with firmly sintered bond, 4. Filter according to item 1 and 2, characterized in that the ceramic of a slurry containing 8-30 Gew.-J - preferably 10 - 25 wt - . % - Phosphate binder, is produced. 5. Filter according to item 1 and Z, characterized in that the individual crystals of ceramic material are sintered together and the porosity between the grains is kept so low that a microporosity is ent, 6. Filter according to item 1, 2 »4 and 5, characterized in that the microporosity is less than 5; o. 7. Filter according to item 1, 2 and 4 to 6, characterized in that is used as the phosphate binder aluminum orthophosphate. 8. The filter according to items 1, 2 and 4 to 7, characterized in that the ceramic with an open-cell foam structure has less than 2% by weight of phosphate - expressed as _{2 2} ⁰ S ~ « Q. Filter according to item 1 and 3 / characterized in that the ceramic is produced from a slurry containing 3 to 15 % by weight gelled boehmite. 10. Filter according to item 1 to 8, characterized in that is used as ceramic sesource alumina or alumina * 11. A method of making a filter according to item 1 by impregnating an open-celled organic polymer foam with an aqueous thixotropic-grade slurry of ceramics, removing the excess slurry, drying and heating the coated polymer shea, whereby the organic polymer foam decomposes and is volatilized, that the resulting ceramic foam is fired at elevated temperature and sintered to form a ceramic having an open-cell foam structure »characterized in that the slurry contains at least 8 % by weight of phosphate binder and sintering a ceramic with an open-cell Schaurastruktur from essentially phosphate-free material is formed, the individual crystals of ceramic material are firmly sintered together to minimize the porosity between the crystals, or that the slurry 3 to 15 Gevi, -% of a gelled aluminum hydroxide binder and the ceramic foam at elevated temperatures sintered to form a ceramic open-cell foam structure of ceramic material having a firmly sintered bond, 12. The method according to item 11, characterized in that the slurry contains at least 8 % by weight of phosphate binder and on sintering a ceramic with open-cell foam structure of substantially phosphate-free material is formed, the individual crystals of ceramic material are sintered together to the porosity between the crystals to minimize. 13. The method according to item 11, characterized in that the slurry contains 3 to 15 Gevw-% of a gelled aluminum hydroxide binder and the ceramic foam is sintered at elevated temperatures, that a ceramic with , open-cell foam structure made of ceramic material with firmly sintered bond, 14. The method according to item 11 and 12, characterized in that the firing temperature during a period of 15 minutes to 10 hours is at least 1660 ⁰ C. 15. Method according to items 11, 12 and 14, characterized in that the firing temperature during a period of 15 minutes to 10 hours is at least 1676 ° C. 16 * Procedure according to item 11, 12, 14 and 15, characterized
by using aluminum orthophosphate as the phosphate binder « 17. The method of item H ₁ 12 and 14 to 16, characterized in that 8 to 30 wt .- ^, suitably at least 10 wt -% and preferably 10 to 30 wt -%, Phosphatbindemittsl be used » 18. The method according to item 11 and 12, characterized in that the kollidale dispersion of the aluminum hydroxide with an acid, the volatile under firing temperature
is produced and a thixotropic slip with pseudoplastic behavior together with ceramic material. 19. The method according to item 11, 12, 18, characterized in that boehmite is used as aluminum hydroxide. 20. The method according to item 11, 12, 18 and 19, characterized
in that nitric acid is used as the acid. 21. The method according to item 11, 12 and 13 to 20, characterized in that the firing temperature is at least 1093 ⁰ C for at least 15 minutes, 22. The method according to item 11, 12 and 18 to 21, characterized in that the firing temperature of at least 1371 ⁰ C. while at least 15 minutes, 23. Method according to item 11, 12 and 18 to 22, characterized in that the firing temperature is maintained for 15 minutes to 10 hours, 24 «Process according to P _u nkt 11 to 23, characterized in that as a ceramic material alumina or alumina is used * 25. The method according to item 11 to 24, characterized in that as the polymer foam, a polyurethane foam: with 2 to
20 pores per cm length is applied. 26. Use of the filters according to items 1 to 10 for
Filtration of ferrous metal, ferrous metal alloys and steel melts.