MX2008001958A - Improved ceramic foam filter for better filtration of molten iron - Google Patents

Abstract

A ceramic precursor, and ceramic filter prepared therewith, with 35-70 wt%refractory aluminosilicate;10-30 wt%colloidal silica;0-2 wt%modified bentonite;0-35 wt%fumed silica;0-10 wt%pore formers and liquid..

Description

IMPROVED CERAMIC FOAM FILTER FOR BETTER CAST IRON FILTRATION Background The present invention relates to an improved ceramic foam filter. More particularly, the present invention relates to a ceramic foam filter for cast iron with sufficient mechanical properties for use in capturing and retaining liquid slag of high FeO content, other metal oxide slags and other entrained particulates. A significant portion of iron smelters use pressed or extruded mulita sieves. A ceramic foam filter is expected to perform better than a sieve due to its tortuous fluid flow path, but mulita sieves perform better than would be expected in ductile iron applications considering the straight flow configuration of the sieves. It is hypothesized that the liquid slag in molten ductile iron moistens the surface of the mulite better than what SiC moistens. Mulite filters can better retain liquid slag during filtration but mulite foam filters for casting applications have not been available. Slag retention is difficult in the sieve type filters. The liquid inclusions, although they easily moisten the mulite sieve material, are very easily deformable by the flow of molten iron and simply flow down from the filter wall to the outlet. Frequently the sieves act to coalesce numerous inclusions of small scoria (non-critical size) and release them back into the iron flux as a larger inclusion. The state of the art for silica-bonded SiC foam filters used for the filtration of molten iron is described in US Pat. No. 6,663,776. The filter described therein produces the highest temperature resistance commercially known for this particular type of filter. There are many companies that manufacture SiC filters linked to silica for the iron industry because it is quite easy to make a relatively robust filter of this type. During heating, the grain of SiC in the body is believed to be partially oxidized to silica glass. The silica glass allows the SiC grain to bond well with the silica binder matrix to thereby create a relatively robust foam. In the torrential of iron castings, particularly ductile iron, a liquid slag of high FeO is formed. The high FeO slag does not wet the SiC foam filters due to a carbothermic reaction between the carbon constituent of the SiC grain and the graphite impurities in the SiC. The FeO slag reacts and the CO gas is formed at the slag-filter interface, which prevents the slag from wetting and adhering to the filter. The substitution of SiC with mulite has proved to be elusive and proper design of the binder for the formulation of a filter with the appropriate mechanical properties at both ambient and high temperature has not been previously discovered. Thus, an acceptable SiC filter could easily be made even if a substandard matrix design is used, but not with mulita. A mulita filter with enough resistance for the filtration of liquid iron has so far encouraged the researchers. Brief Description of the Invention It is an object of the present invention to provide a mulite foam wherein the wetting characteristics in the iron are used within the tortuous path of the foam to produce a filter which is superior to either the foam of SiC bonded to silica or mulita sieves, creating more machinable, cleaner iron castings. It is another object of the present invention to provide an improved method for filtering iron. It is another object of the present invention to provide a filter with improved wettability to FeO, improved heat shock resistance and a tortuous flow path. Provided herein is a ceramic precursor. The ceramic precursor has 35-70% by weight of refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of modified bentonite; 0-35% by weight of fumed silica; 0-10% by weight of pore former and solvent. Also provided herein is a ceramic filter prepared by the method of preparing a ceramic suspension precursor. The ceramic precursor has 35-70% by weight of refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of modified bentonite; 0-35% by weight of fumed silica; 0-10% by weight of pore former and solvent. An organic foam is impregnated with the ceramic precursor. The impregnated organic foam is heated to a temperature sufficient to volatilize the organic foam and the pore former and to sinter the ceramic precursor. Also provided herein is a process for filtering molten iron. The process includes preparing a ceramic foam filter. The filter is prepared by preparing a ceramic precursor with 35-70% by weight refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of modified bentonite; 0-35% by weight of fumed silica; 0-10% by weight of pore former and solvent. An organic foam is impregnated with the ceramic precursor by heating the impregnated ceramic foam to a temperature sufficient to volatilize the organic foam and the pore former and to synthesize the ceramic precursor to form a filter. The molten iron is passed through the filter where the FeO slag is retained by the filter. Brief Description of the Figures Fig. 1 is a macroscopic view of a filter of the present invention. Fig. 2 is an electron microscopic view of a filter of the present invention taken in 50x magnification. Fig. 3 is a cross-sectional, electron microscopic view of a filter of the present invention after the use of filtering molten iron. Fig. 4 is a scanning electron microscope (SEM) image. Fig. 5 is a dispersive electron spectroscopic image (EDS). Fig. 6 is an EDS image. Fig. 7 is an image of SEM. Fig. 8 is an image of SEM.

Fig. 9 is an image of SEM. Fig. 10 is an image of SEM. Detailed Description The invention provides a mulite filter bonded to silica with sufficient strength, at both room temperature and high and sufficient heat shock resistance and a method for making the filter. The invention also provides an improved method for filtering molten metal. The filter is made via the technique of foam replication, which is a common method used to manufacture cross-linked ceramic foam for use as molten metal filtration devices. In the process, the polyurethane foam is coated with the ceramic suspension, then dried and heated. During heating, the polyurethane foam inside the ceramic coating vaporizes, but the ceramic structure remains, resulting in an exoskeleton-like ceramic foam that has empty voids where the polyurethane once resided. Figure 1 provides a macroscopic image of ceramic foam. The structure is essentially a connection of structures with porosity that resides around and within these structures. In the preparation of a ceramic filter, the foam is impregnated with the ceramic suspension. The ceramic suspension is then dried, the foam is vaporized and the ceramic is sintered. The process for forming a ceramic filter is provided in U.S. Patent Nos. 4,056,586; 5,456,833 and 5,673,902 each of which is incorporated herein by reference. The suspension used depends on the ceramic material desired for the selected application. Sufficient properties must be had in the final product to withstand the particular application with respect to the chemical attack and must have sufficient structural and / or mechanical strength to withstand the particular elevated temperature conditions. In addition, the suspension must have a relatively high degree of fluidity and be comprised of an aqueous suspension of the proposed ceramic for use in the filter. Normally, the suspension contains water. Additives, such as linkers and surfactants, can be employed in the suspension. The flexible foam material is impregnated with the aqueous ceramic suspension so that the fiber-like tapes are coated therewith and the voids are filled therewith. Normally, it is preferred to repeatedly immerse the foam in the suspension and compress the foam between dips to ensure complete impregnation of the foam. The impregnated foam is preferably compressed to expel from 25 to 75% of the suspension while leaving the portion of fiber-like tape coated therewith. In a continuous operation, the impregnated foam can be passed through a predetermined roller to affect the desired ejection of the foam suspension and leave the desired amount impregnated therein. This can be done manually by simply pressing the flexible foam material to the desired degree. In this step, the foam is still flexible and can be formed in suitable configurations for the specific filtering task, ie in curved plates, hollow cylinders, etc. It is necessary to keep the formed foam in place by conventional means until the polymeric substrate decomposes, or preferably until the ceramic is sintered. The impregnated foam is then dried either by air drying or accelerated drying at a temperature of 35 ° to 700 ° C for 2 minutes to 6 hours. After drying, the material is heated to an elevated temperature to bond the ceramic particles constituting the fiber-like tapes. It is preferred to heat the dry impregnated material in two stages, with the first stage being to heat to a temperature of 350 ° to 700 ° C and to keep it within this temperature range of 2 minutes to 6 hours in order to burn or volatilize the flexible foam tape. Clearly this stage can be part of the drying cycle, if desired. The second stage is to heat at a temperature of 900 ° to 1700 ° C and keep it within that temperature range of 2 minutes to 10 hours to bond the ceramic. The resulting product is a fused ceramic foam having an open cell structure characterized by a plurality of interconnected recesses surrounded by a ceramic tape. The ceramic foam can have any desired configuration based upon the configuration necessary for the particular molten metal filtration process. The process for forming the inventive filter comprises the formation of a suspension of ceramic precursors. For the purposes of the present invention, the ceramic precursors include specific ratios of refractory aluminosilicate, colloidal silica, fused or fused silica, and modified bentonite. The suspension may comprise a surfactant to decrease the surface tension of the aqueous phase below 80 mN / m to improve the wetting characteristics. The term "refractory aluminosilicate" as used herein refers to refractory crude materials comprising predominantly mulite and having a pyrometric cone equivalent (PCE) of at least 20. This class of raw materials is also known in the literature of refractory materials by the synonyms of calcined refractory clay, calcined aggregate, refractory calcina, mulite calcina, refractory aggregates, calcined cyanite, electrofused mulberry and refractory clays. The ceramic precursor of the present invention comprises about 35-70% by weight of refractory aluminosilicate, about 10-30% by weight of colloidal silica, about 0 to 2% by weight of bentonite or modified bentonite having a rheology modifier added polymer, approximately 0 to 35% by weight of fumed or fused silica and approximately 0-10% by weight of pore former with the remainder being a solvent, preferably water, present in an amount sufficient to allow the composition flow in the foam. Approximately 5-8% by weight of water is particularly preferred as the solvent. More preferably, the ceramic composition comprises 40-60% by weight and much more preferably 50-60% by weight refractory aluminosilicate. Below about 40% by weight of refractory aluminosilicate the FeO can not adequately wet the interior surfaces of the filter to allow absorption in the crevices where it is retained. Filters made with less than 50% by weight of refractory aluminosilicate can also be sensitive to thermal shock in the application. Above about 60% by weight of refractory aluminosilicate the resistance of the filter is compromised. More preferably, the ceramic precursor comprises 10-23% by weight of colloidal silica. More preferably, the ceramic precursor comprises about 0.6 to 1.5% by weight of modified bentonite or bentonite and much more preferably about 0.8% by weight of modified bentonite or bentonite. More preferably, the ceramic precursor comprises approximately 10-20% by weight of wet silica. The fused and fused silica can be used interchangeably in the present invention in any ratio up to the total amount of fumed or fused silica as set forth herein. The resulting filter provides a hot MOR, measured at 1,428 ° C, from 20 to 80 psi at an average relative density of about 14%. The resulting filter density is preferably at least 8% by weight of the theoretical density to not more than 18% by weight of the theoretical density. Up to 18% by weight of the theoretical density the filtration ratio is also low to be effective. Below 8% by weight of the theoretical density the filter resistance is insufficient for use in the filtration of molten iron. Refractory aluminosilicate is a naturally occurring material with a nominal composition of 3Al203.Si02. In practice, the refractory aluminosilicate comprises from about 45% by weight to 70% by weight of A1203 and about 25% by weight to about 50% by weight of SiO2. Impurities that occur naturally are present and one of skill in the art would perform that removal completely that the impurities are prohibitive in cost. In practice, the refractory aluminosilicate has about 1.5-3% by weight of TiO2, up to about 1.5% by weight of Fe203, up to about 0.06% by weight of CaO, up to about 0.8% by weight of MgO, up to about 0.09% by weight of Na20 , up to about 0.9% by weight of K20 and up to about 0.12% by weight of P205. For the purposes of the present invention the preferred refractory aluminosilicates are Mulcoa 47®, Mulcoa 60® and Mulcoa 70® all available from C-E Minerals of Americus, GA but any commercially available refractory aluminosilicate powder is suitable for the application. It is preferable to add volatile organic materials in the ceramic suspension for further increase in porosity. In an alternative embodiment a ceramic precursor comprising spherically shaped holes therein can be formed into the desired shape of the porous and heated ceramic as described in US Patent No. 6,773,825 which is incorporated herein by reference in the same.

A mixture of ceramic or metal particles and flexible organic spheres such as the pore former is prepared in a liquid or suspension and the mixture is formed into a shaped article. The formed article is dried and heated so that the particles are bonded by sintering. The organic spheres and other organic additives are volatilized. The spheres are preferably of low density and more preferably hollow. The size of the holes can be pre-selected by selecting the appropriate polymer spheres. The porosity is also easily controlled by the number of polymer spheres added. It is much more preferred that the polymer spheres are each in contact with at least two other spheres such that a network of voids is created in the eventual diffuser. To a suspension of the ceramic precursor are added flexible organic hollow spheres which are simultaneously suspended in the solvent as a pore former. The ceramic precursor is then incorporated into the foam as also described herein and dried to remove the solvent. When the ceramic precursor is heated to form a ceramic, the spheres are volatilized resulting in uniformly distributed voids throughout the filter network. By using this method a range of porosities can be achieved, however, for use in the filtration of molten iron it is preferable that the porosity is not more than 60% due to insufficient thermal stress resistance at higher levels of porosity. Porosity and pore size are easily controlled by the number and sizes of polymer spheres used. After heating the gap is substantially the same shape and size as the included sphere. It is much more preferable to use spheres with an average diameter of 20 to 150 microns and more preferably 20-80 microns. A sphere of 80 micras is much more preferred. Other organic pore formers including flour, cellulose, starch and the like can be included. The hollow organic spheres are much more preferred because of the low volume of organic to pore volume that can be achieved and the minimum level of organic residue that remains after heating. It is much more preferred that the suspension comprises up to about 10% by weight of pore formers based on a hollow sphere of 80 microns. The material is either formed to size or cut to size. The material can be cut to size as a green ceramic or as a sintered ceramic. EXAMPLES Example 1 A ceramic precursor composition was prepared using the materials listed in Table 1. In Table 1, the refractory aluminosilicate used was Mulcoa 60®, ground to a -325 mesh, as is available from C-E Minerals. Colloidal silica was used as obtained from Nyacol. Modified bentonite was used as obtained from Wyo-Ben, Inc. Fumed silica was used as obtained from CE Minerals. The composition was heated at 1200 ° C for 30 min. The fused silica could be replaced by fumed silica to improve the ability to cut the ceramic foam in the established heater. The rupture modulus was measured at room temperature as reported in Table 2. The rupture modulus was measured at a high temperature as reported in Table 3. Table 1: The average rupture ambient temperature (MOR) filter of the filter is approximately 90-psi over the given density range. This value is acceptable for most applications of molten metal filtration. Table 2: MOR at room temperature To measure the hot MOR of mulite ceramic foam filters bonded to silica the samples at room temperature were inserted directly into an oven maintained at 1428 ° C, then tested in the three point curve configuration approximately 45 seconds after which the filter was inserted. This test is analogous to the conditions that the filter is subjected to during iron filtration. The results are reported in Table 3. Table 3: Hot MOR Example 2 To improve the liquid slag retention and capacity a composition was prepared as in Table 1 with the additional inclusion of 4% by weight of hollow organic spheres with a diameter of 80 μm. The resulting filter was examined under an electron microscope and the resulting microstructure is shown in Figure 2. When the liquid slag moistens the mulita body, it can be extended into the micropores via the capillary action. Example 3 A filter was prepared under the same conditions as Example 1 with the composition provided in Table 4. Mulcoa 70® was used as the refractory aluminosilicate as available from C-E Minerals. Table 4: The resulting average hot MOR was measured to be 34 psi at a mean foam relative density of 14%. Example 4 A filter was prepared under the same conditions as Example 1 with the composition provided in Table 5. Mulcoa 47® was used as the refractory aluminosilicate as available from CE Minerals and the ceramic was heated at 1225 ° C for 5 minutes. . Table 5: The resulting average hot MOR was measured to be 63 psi at a mean foam relative density of 14%. Example 5 Filters of the type described in Example 1 were made and tested in an iron melting shop. These filters were tested in comparison with standard silicon carbide ceramic foam filters. In the test, the gray cast iron was emptied through a standard metal gate shaft and filtering housing, although the filter tested and in a mold to make a standard commercial cast iron component. After solidification of the metal and the cooling system, the test filters were removed, cut and polished through the standard metallurgical specimen preparation techniques, and examined in cross-section for evidence of slag capture liquid, the retention and absorption in the micro-porosity of the filter material. Figure 3 shows the micrograph of the sample of cross section produced. In the figure, there is evidence of the penetration of liquid slag into the filter body. In this case, the capture of metal oxide slag was a mixture of metal oxide impurities (silicon, titanium, calcium, manganese and aluminum) as determined through energy dispersive spectroscopy (EDS). The evaluation of the samples from this test indicates the deeper penetration of the metal oxide slag into the micropore structure of the filter compared to that indicated with a standard silicon carbide filter run under the same operating conditions. In a similar inventive test the filters prepared as described in Example 1 and the standard silicon carbide filters were tested in a standard gray iron casting application forming cylindrical gray iron blocks. The casting pattern contains two filter impressions. Each of the individual filter print filters the required iron fills an individual cylindrical block. For each test an inventive filter was placed in a filter print and a standard silicon carbide filter was placed in the other filter print as control. Four casts were produced with each one being free of inclusion flaws. There was no measurable difference in each emptying. The gate shaft was cleaned and the filter print segments were removed from the gate shaft for metallurgical evaluation. The filter print segment of the gate shaft was sectioned to expose the filter filaments. The inventive filter print was examined for signs of mechanical failure. There was no evidence of deflection or deterioration in the filament structure. There were no signs of cracking or shaping of the inventive filter in order to confirm that the filter is able to withstand sufficient mechanical stress and thermal for iron filtration applications. The filters were examined to determine the amount and type of inclusion material captured by the filter filaments. Figures 4 and 10 illustrate the captured inclusion material. The examination of both the inventive filter and the control revealed a large quantity of the grains of earth captured on the led edge of both filters. In some areas the filter was completely blocked by the grains of earth. Examples of ground grains are clearly shown in Fig. 4 for the control filter and in Figs. 8 and 9 for the inventive filter. Figure 5 shows the results of an X-ray energy dispersive spectroscopy (EDAX) microanalysis confirming that the inclusion is a Si02 earth particle or silica. The major component of the inclusion was a metal oxide slag phase shown in Figs. 4, 7, 8, 9 and 10. This slag was examined and found to contain silicon oxide, calcium oxide, manganese oxide, aluminum oxide and titanium oxide. This scum was found in all four filters. The amount of oxide slag varied by the location within each individual filter but this inclusion material was abundant in each filter and easily found during examination in each filter. In addition, this metal oxide slag contained small beads or droplets of pure iron. These iron droplets are usually formed by the reduction of iron oxide to elemental iron by the carbon precipitate as the iron solidifies. These iron droplets were observed in both the control filters and the inventive filters. These droplets are usually created by turbulence within the gate tree system. There was no significant difference in the composition of the slag captured by the control filter material against the inventive filter material. The difference was only observed in the two different ceramic materials which was the penetration depth of the metal oxide slag in the inventive filter against the control. Figures 9 and 10 clearly show that the inventive filter seems to absorb the slag in the filter much more effectively than the control. The penetration of the slag into the inventive filter was much more prominent than in the comparative filter. Since both the inventive and the comparative filters were placed within the same mold, it is not surprising that they captured similar inclusion materials. Based on the visual inspection it seems that the inventive filter has captured more inclusions than the comparative filter. The inventive filter maintained the thermal and mechanical stress of a gray cast iron application production. There was no evidence of any mechanical or chemical deterioration of the filter. The invention has been described with particular reference to the preferred embodiments without limitation thereto. One of skill in the art would be driven to modalities which do not depart from the scope of the present invention as more specifically set forth in the claims appended hereto.

Claims (43)

1. CLAIMS 1. A ceramic precursor, characterized in that it comprises: 35-70% by weight of refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of bentonite; 0-35% by weight of fumed silica; 1-10% by weight of pore formers; and liquid.
2. 2. The ceramic precursor according to claim 1, characterized in that the pore formers are hollow spheres.
3. 3. The ceramic precursor according to claim 2, characterized in that the hollow spheres have a diameter of 20 to 150 μm.
4. 4. The ceramic precursor according to claim 1, characterized in that the ceramic precursor comprises 40-60% by weight of refractory aluminosilicate.
5. 5. The ceramic precursor according to claim 1, characterized in that the ceramic precursor comprises 10-23% by weight of colloidal silica.
6. The ceramic precursor according to claim 1, characterized in that the ceramic precursor comprises 0-1.5% by weight of bentonite.
7. 7. The ceramic precursor according to claim 5, characterized in that the ceramic precursor comprises 0.
8. 8% by weight of bentonite. The ceramic precursor according to claim 6, characterized in that the ceramic precursor comprises 0-20% by weight of wet silica.
9. 9. The ceramic precursor according to claim 8, characterized in that the ceramic precursor comprises 10-20% by weight of wet silica.
10. 10. The ceramic precursor according to claim 1, characterized in that the liquid is water.
11. 11. A filter, characterized in that it is prepared by impregnating a foam with a ceramic precursor of claim 1 and heating, wherein the filter has a hot MOR of 25 to 120 psi.
12. The filter according to claim 11, characterized in that the filter has a relative density that is approximately 12% of the theoretical density.
13. The filter according to claim 11, characterized in that the filter has a density of 8- 18%.
14. 14. The filter according to claim 11, characterized in that the bentonite is modified bentonite.
15. 15. A ceramic filter, characterized in that it is prepared by the method of: preparing a ceramic precursor, comprising: 35-70% by weight of refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of modified bentonite; 0-35% by weight of fumed silica; 0-10% by weight of pore formers; and solvent; impregnate an organic foam with the ceramic precursor; heating the impregnated organic foam to a temperature sufficient to volatilize the organic foam and bind the ceramic precursor, where the filter has a density of 8-18%.
16. 16. The ceramic filter according to claim 15, characterized in that the pore formers are hollow spheres.
17. 17. The ceramic filter according to claim 16, characterized in that the hollow spheres have a diameter of 20 to 150 μm.
18. 18. The ceramic filter according to claim 15, characterized in that the ceramic precursor comprises 50-60% by weight of refractory aluminosilicate.
19. 19. The ceramic filter according to claim 18, characterized in that the ceramic precursor comprises 40-60% by weight of refractory aluminosilicate.
20. The ceramic filter according to claim 15, characterized in that the ceramic precursor comprises 10-23% by weight of colloidal silica.
21. 21. The ceramic filter according to claim 15, characterized in that the ceramic precursor comprises 0-1.5% by weight of bentonite.
22. 22. The ceramic precursor according to claim 21, characterized in that the ceramic precursor comprises 0.8% by weight of bentonite.
23. 23. The ceramic filter according to claim 15, characterized in that the ceramic precursor comprises 0-20% by weight of wet silica.
24. 24. The ceramic filter according to claim 23, characterized in that the ceramic precursor comprises 10-20% by weight of fumed silica.
25. 25. The ceramic filter according to claim 15, characterized in that the solvent is water.
26. 26. The ceramic filter according to claim 15, characterized in that the filter has a hot MOR of 25 to 120 psi.
27. 27. The ceramic filter according to claim 26, characterized in that the filter has a density that is approximately 12% of the theoretical density.
28. 28. The ceramic filter according to claim 15, characterized in that the bentonite is modified bentonite.
29. 29. A process for filtering molten iron, characterized in that it comprises: preparing a ceramic foam filter by the steps of: preparing a ceramic precursor comprising: 35-70% by weight refractory aluminosilicate; 10-30% by weight of colloidal silica; 0-2% by weight of bentonite; 0-35% by weight of fumed silica; 0-10% by weight of pore formers; and solvent; impregnate an organic foam with the ceramic precursor; heating the impregnated organic foam to a temperature sufficient to volatilize the organic foam and sintering the ceramic precursor to form a filter; and passing the molten iron through the filter where the FeO slag is retained by the filter.
30. 30. The process for filtering molten iron according to claim 29, characterized in that the pore formers are hollow spheres.
31. 31. The process for filtering molten iron according to claim 30, characterized in that the hollow spheres have a diameter of 20 to 150 μm.
32. 32. The process for filtering molten iron according to claim 29, characterized in that the ceramic precursor comprises 50-60% by weight of refractory aluminosilicate.
33. 33. The process for filtering molten iron according to claim 32, characterized in that the ceramic precursor comprises 40-60% by weight of refractory aluminosilicate.
34. 34. The process for filtering molten iron according to claim 29, characterized in that the ceramic precursor comprises 10-23% by weight of colloidal silica.
35. 35. The process for filtering molten iron according to claim 29, characterized in that the ceramic precursor comprises 0-1.5% by weight of bentonite.
36. 36. The process for filtering molten iron according to claim 35, characterized in that the ceramic precursor comprises 0.8% by weight of bentonite.
37. 37. The process for filtering molten iron according to claim 29, characterized in that the ceramic precursor comprises 0-20% by weight of fumed silica.
38. 38. The process for filtering molten iron according to claim 37, characterized in that the ceramic precursor comprises 10-20% by weight of fumed silica.
39. 39. The process for filtering molten iron according to claim 29, characterized in that the solvent is water.
40. 40. The process for filtering molten iron according to claim 29, characterized in that the filter has a hot MOR of 25 to 120 psi.
41. 41. The process for filtering molten iron according to claim 40, characterized in that the filter has a relative density that is approximately 12% of the theoretical density.
42. 42. The process for filtering molten iron according to claim 29, characterized in that the filter has a relative density of 8-18%.
43. 43. The process for filtering molten iron according to claim 29, characterized in that the bentonite is modified bentonite.