CA1059535A - Ceramic foam filter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a highly efficient ceramic roam material for use in filtering molten metal, especially molten aluminum. The ceramic roam material of the present invention is characterized by having an open cell structure with a plurality of interconnected voids surrounded by a web of said ceramic material. The filter of the present invention is further characterized by a combination of critical properties, as air permeability, porosity or void fraction and thickness.

Description

l(~S9535 Porous ceramic foam materials are kncwn in the art, for example, having been described in U.S. Patents 3,090,094 and 3,097,930 and are known to be particularly useful in filtering molten metal, as described more specifically in U,S Patent 3,893,917, issued July 8, 1975.
Molten metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have ~oor bright finishing and anodizing characteristics.
The inclusions may originate from several soùrces. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the resultant molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
It is naturally highly desirable to devise an im-proved filter for use in removing or minimizing entrained solids from the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product, such as decorative trim or sheet made from the 5000 series of aluminum alloys, as aluminum Alloys 5252 and 5657. Other aluminum alloys which benefit from improved filtration include: aluminum capacitor foil made from the 1000 series of aluminum alloys, as aluminum Alloys 1145 and 1188 in order to reduce pinhole defects in light gage products and maximize productivity in rolling;
high strength extrusion alloys, such as aluminum Alloys 2024 and 7075 in order to obtain high ultrasonic quality; and .

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~V59535 extrusion alloys of the 6000 series of aluminum alloys, as aluminum Alloy 6061, in order to obtain higher productivity in extrusion operations: etc, Inclusions as a~oresaid cause loss of properties in the finally solidified alloy and lead to degradation of process-ing efficiency and loss of properties in the final product. For example, one type of finishing flaw which is particularly significant in decorative trim or sheet made from aluminum Alloy ~252 is a stringer defect known as a linear defect.
Rigorous melt treatment processes such as gas fluxing minimize the occurrence of such defects; however, these are not successful in reducing them to a satisfactory level for critical applications. Conventionally, melt filtration is , utilized in order to decrease the extent of such defects, and others caused by the presence of inclusions in the melt, The most common form of me]t filtration involves the use of open weave glass cloth screens placed in transfer and pouring :,., troughs or in the molten pool of metal in the top of a solidifying ingot, Such filters have been found to be only ~ii, 20 partially effective since they remove only the larger inclusions, Another type of filter in common use is a bed filter made up, for example9 of tabular alumina. Such filters have many disadvantages, perhaps the most serious of which is the great dif~iculty experienced in controlling and maintain-ing the pore size necessary for efficient filtration. Another difficulty with such filters is their tendency to produce an initial quantity of metal having poor quality at the start up of each successive casting run, This behavior results in a so-called ingot "butt-effect", that~is, ingots having butt portions of relatively poor quality which must be scrapped ,:

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~059S35 and recycled. Still further, the metal in a bed filter mus-t be maintained molten even when the filter is not in use.
In comparison thereto porous ceramic filters appear to be highly desirable. However, the successful use of such articles in exacting functions such as the filtration of -molten metal requires that the article possess particular physical and chemical properties. Specifically, the ceramic foam article requires a certain permeability and structural uniformity to efficiently filter molten metal at commercially acceptable rates and purity levels As a corollary property, the foam material must withstand chemical attack from the molten metal to facilitate its extended use as a filter The prior art suggests various methods for the pre-paration of ceramic foam materials. Particularly, U.S, Patent No. 3,111,396 to Ball suggests that an organic polymer foam impregnated with a refractory material may then be compressed by passage through preset rollers to effect the removal of excess refractory. This technique, which is comparable to a wide variety of conventional expulsion techniques used in the art suffers from an inherent disadvantage in that the slurry is not completely uniformly distributed through the body of the article. ~hus, the outer area of the article tends to be more thinly coated with slurry than that near the center line, Such defects are particularly extended at the extremes of the permeability range found suitable for use in the preparation of molten metal filters; thus, bodies possessing a high permeability tend to have undesirably weak surfaces and edges whereas, bodies possessing relatively low permeability tend to exhibit undesirable center line blockage. Both of the aforenoted defects render the resulting foams unsuitable for 1~5~53S
use in the filtration of molten metal.
It is a principal object of the present invention to devise a ceramic foam filter which has considerable high temperature resistance so that it can be used on a variety of molten metals and so that the structure thereof is resistant to degradation under the severe conditions of use associated with filtration of molten metal.
It is a further object of the present invention to devise a high temperature resistant ceramic foam filter as aforesaid which is convenient to prepare and is characterized by reasonable cost.
It is a still further object of the present invention to provide a high temperature resistant foam filter as afore-said which overcomes the art disadvantages referred to above, -does not contaminate the melt and does not result in degradation of desirable characteristics in the final metal product.
It is a further principal object of the present inven-tion to provide a method for the preparation of ceramic foam articles which is both accurate and expedient.
It is a further object of the present invention to provide a method as aforesaid which yields products possessing ~ `
permeabilities within closely set tolerances, It is yet a further object of the present invention to provide ceramic foam articles as aforesaid which exhibit structural uniformity and freedom from defects such as center line blockage and outer surface weakness~
It is a still further object of the present invention to provide a method as aforesaid which lends itself to rapid commercial-scale production techniques.

' :- ~ , l~S~535 Other objects and advantages will be apparent from a careful review of the ensuing description.
In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily obtained. -The present invention provides a highly efficient ceramic foam material for use in filtering molten metal, espec-ially molten aluminum. The ceramic foam material of the present invention is characterized by having an open cell structure with a plurality of interconnected voids surrounded by a web of said ceramic material. The filter of the present invention is further characterized by a combination of critical charac--teristics. The filter has an air permeability in the range of from 400 to 8000 x 10 7 cm2. In addition, the filter of the present invention has a porosity or void fraction of 0 80 to 0.95 Still further, the filter of the present invention is characterized by from 2 to 20 pores per linear cm and an effective range of filter thickness of from 10 to 100 mm. It has been found that in accordance with the present invention the foregoing filter is particularly useful in the filtration of molten metal, especially molten aluminum.
Numerous advantages are achieved utilizing the filter of ~the present invention, some of which are referred to above and as will be discussed hereinbelow.
In the normal situation one uses a relatively fine filter of the present invention having an air permeability of from 400 to 2500 x 10-7 cm2, a porosity or void fraction of 0 80 to 0 95 and from 8 to 18 pores per linear cm, especially if one is filtering an aluminum alloy of the 5000 series However, if the input metal is particularly dirty, .~

~ S 35 one should preliminarily filter the metal throu~h a relati~ely coarse ceramic foam filter having a pore size of between 2 and 8 pores per linear cm, air permeabilities of 2500 to 8000 x 10 7 cm2 and porosities or void fractions of between 0,90 and 0,95, This may be accomplished by providing a single ceramic filter with a gradation of properties or by using a series of filters of differing porosity, In addition to the foregoing, therefore, the present invention provides a method of filtering molten metal through a ceramic foam filter characterized as aforesaid utilizing a molten metal flow rate through the filter of from 1.25 to 12,~ dm3 per dm of filter area per minute, In accordance with an alte~native embodiment of the method of the presen-t invention, the molten metal may be preliminarily filtered through a relatively coarse ceramic foam filter of the present invention, followed by filtration through a relatively fine, preferred filter of the present invention, This preliminary filtration step may utilize a series of ceramic foam filters of decreasing porosity and is particular-ly useful with especially dirty melts, In accordance with the present invention, the ceramic ;` foam filter described above has been found to be particularly useful in filtering molten metal, especially molten aluminum, ~he ceramic foam filter of the present invention is a low cost material which may be conveniently used on a disposable basis, As indicated hereinabove, the ceramic foam material ofthe present invention is characterized by having an open cell ~, structure with a plurality of interconnec-ted voids surrounded

3, by a web of said ceramic material, It has been found that the characteristics of the filter of the present invention ' ~
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~059535 define a filter which is surprisingly effective in the filtration of ~olten metal, especially aluminum, at a low cost and achieving surprising filtration efficiency with a flexibility not available heretoore.
The combination of properties of the filter of the present invention are critical in obtaining the desirable results of the present invention. As indicated heretofore, the ceramic foam filters of the present invention have an air permeability in the range of 400 to 8000 x 10 7 cm2, and in the normal and preferred case in the range of 400 to 2500 x 10 7 cm2. The air permeability is determined by blowing air through the ceramic foam at a measured rate In accordance with this procedure -the pressure drop is measured by determining the pressure aifferential between the air entering the foam and the air leaving the foam for a defined area and thickness of foam The air permeability is then determined in accordance with the following formula:
K A~P
wherein K = the air permeability, ~ = the dynamic viscosity of air, Q = the flow rate of air through the body, ~ = the length, ; that is, the thickness of the ceramic filter material, A = the area, that is, the defined area of the foam and ~P = the pressure drop~ In accordance with the present invention one utilizes an air flow rate of 0.857 m3 per minute and an area of 73 cm2. The foregoing determination of permeability may be found in the text Micromeretics by J.M. Dallavalle published by Pitman 1948 at Page 263 It may be seen, therefore, that the air permeability is a function of many variables. For example, the bulk density, the pore size, the surface area and ......
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~059535 the tortuosity of the flow paths. In accordance with the present invention i-t has been found that permeabilities greater than 2~00 x 10 7 cm2 give i~adequate filtration unless the melt is particularly dirty in which case permeabilities up to 8000 x 10 7 cm2 may be used, while permeabilities of less than 400 x 10-7 cm2 give impractically high rates of head build up. A particularly preferred range of permeability has been found to be from 1000 to 1500 x 10 7 cm2 wherein optimum filtration and low rates of head build up are obtained.
In addition to the foregoing, the ceramic filters of the present invention should have a porosity or void fraction of from 0.80 to 0 g5 This variable defines the amount of pores or voids in the ceramic body and may be determined in accord-ance with the following formula:
fp = dt - db = 1 _ db dt dt wherein fp = the total porosity or void fraction, dt = -the true density of the solid ceramic body and db = the bulk density of the ceramic foam body. The foregoing formula may be found in the text Introduction to Ceramics by W. D. Kingery, published 1960, by John Wiley at Page 416 It has been found that optimum results are obtained in accordance with porosity values of from 0.8~ to 0 90 Naturally, the specific value for dt will depend upon the par-ticular ceramic body. For example, for alumina-chromia based ceramics, the foregoing porosity values correspond to bulk densities of o.65 to 0.25 gm/cm3 and the optimum values correspond to from 0 35 to 0.45 gm/cm3.
As indicated above the relatively coarse, preliminary filter should have a porosity between 0.90 and 0.95.

1059S:~5 In addition to the foregoing, the ceram~c filter of the present invention should have an effective range of pore size or pore density in terms of number o~ pores per linear cm, namely, ~rom 2 to 20 pores per linear cm, from B to 18 pores per linear cm in the normal and preferred case, and optimally from 10 to 14 pores per linear cm.
The foregoing three variables, namely, the permeability, the porosity and the pore size, are critical in obtaining the greatly improved characteristics of the present inven-tion.
Namely, these variables influence each other in achieving the surprising efficiency of the filter of the present invention They define in effect how many pores or holes are present in the filter, how they are interconnected and how large they are, the surface area of the ceramic web and they define a surprisingly effective ceramic foam filter.
Still further the ceramic foam filter of the present invention should have an e fec-tive range of filter thickness of from 10 to 100 mm, that is, a thickness in the direction of metal flow. The optimum filter thickness is from 35 to 65 mm. It has been found that filters of less than 10 mm in thickness are not effective in removing the bulk of nonmetal-iics from the molten metal, whereas, increments of thicknesses above 100 mm suffer from diminishing filtration rates since i the most effective region of the filter is the first 25 to 35 mm in thickness.
An additional and significant characteristic of the effective filters of the present invention is that they should have substantial structural uniformity. In order to provide an effective filter for molten metal the ceramic foam body must have a high degree of etructural uniformity. Therefore, ,.. ~ . .

~059S3~
although some percentage of blocked pores are helpful and desirable in that they increase flow path tortuosity, these blockages should be homogeneously distributed throughout the ceramic body rather than grouped together. Grouping of blockages will onl~ lead to channeling and inefficient filtration.
A wide variety of materials may be used in the pre-paration of the ceramic foam filter of the present invention. It is an advantage of the present invention that the low cost and ease of preparation of the filter of the present invention makes the filter convenient to use on a throwaway basis The principal component of the ceramic foam material of the present invention is A1203 in an amount from 40 to 95~0 and preferably from 45 to 55% A1203 is particularly de-sirable for use as a ceramic filter since it is not attacked by molten aluminum or molten copper, for example; whereas, silica is attacked by these materials. ~urthermore, the alumina has reasonable strength to stand up to chemical attack and structural and/or mechanical strength to stand up 20 to the particular elevated temperature conditions. In addition ; ;~
to the foregoing, the ceramic material of the present in-vention may contain from 1 to 25% Cr203 and preferably 10 to 17% Cr203 This componen-t is particularly significant ;
since it has been found that it imparts significantly better high temperature resistance 9 that lS, resistance to attack by the molten metal at elevated temperature Furthermore the ceramic foam material contains the products of the thermal decomposition of 0 1 to 12% kaolin and/or bentonite and of 2 5 to 25% of an air setting agent which is substantially 30 non reactive to the molten metal, preferably of aluminum -~ '' ' ~ ................. . .

iO59535 phosphate.
In accordance with the present invention, -the fore-going objects and advantages are readily obtained by -the follow-ing method of preparation: Ceramic foams possessing controlled permeability and structural uniformity are prepared by a process comprising providing an open-celled organic polymer foam material possessing a predetermined permeability and resilience, impregnating said polymer material with an aqueous slurry of a thixotropic ceramic composition while shearing said slurry an amount sufficient to maximize impregnation, and expelling excess slurry from said material by conducting at least two passes of said material through preset rollers to effect a temporary compression ranging from about 50 to 90%
of the thickness of said material for the first pass, and 70 to 90% of said thickness for the second pass. After im-pregnation and expulsion of excess slurry àre complete, the resulting foam material is then heated to remove the organic foam component The resulting article is then ready for use, or may, if desired, be further heated to sinter the ceramic 20 material In accordance with the present invention, the air permeability of the resulting ceramic articles has been found to depend upon the permeability of the organic polymer foam employed in its preparation. For example, ceramic foams having permeabilities in the range of from about 800 to about 2,200 x 10 7 cm2 have been prepared from polyurethane foam ~ -materials having air permeabilities ranging from 4,~00 to ~,400 x 10 7 cm2. Further, the selection of raw foam permeability in the range of +2~o facilita-tes the preparation of a ceramic foam having a permeability predetermined to -- 11 -- .

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1(~5~35 within a range of ~%.
In addition to the control of permeability, the foams of the present invention must possess structural uniformity and a particular range of cell size. Structural uniformity has been found to relate to the resilience of the organic polymer foam precursor. Particularly, resilience may be determined with reference to certain standards set forth in the ASTM-D- ~`
1564-71 which refers to the properties of compression set and resilience as measured by ball rebound. Compression set, determined by the compression load deflection test, measures the extent to which the foam returns to its original size or thickness after compression to a stated reduction such as, for -example, 50%. Foams found suitable in accordance with the invention exhibit a compression set of less than 30% at 50%
compression, and thus, return to at least 70% of their original thickness after compression is released. Resilience, determined by the ball rebound test, measures the resistance that the material exhibits to compression by the height of rebound of a steel ball dropped from a stated distance onto a foam sample. The percentage of return of the ball to the original height is noted, and foams suitable in the present invention have been found to be those possessing ball rebounds of greater than 25%.
The above properties have been measured in terms of --tests run under dry conditions, however, such properties must be ~ -substantially retained in an aqueous environment as, for example, during impregnation with the aqueous ceramic slurry of the present invention. Accordingly, it has been found that hydrophobic foams perform better and are preferred to hydrophilic foams, as the latter suffer considerable loss of ~ -- : . . . .

lOS9S35 resilience in aqueous environments, This loss of resilience is evident in the occurrence of the aforenoted defect of center line blockage, With the above noted criteria in mind, organic polymer foam materials which may be employed in the present invention include a wide variety of highly resilient, reticulated hydrophobic materials such as the polyester and polyether polyurethanes, such as "high resilience" or "cold cure"
; urethane materials which utilize polymeric isocyanates in their formulation; polyvinyl foams such as polyvinyl chloride, polyvinyl acetate, and polyvinyl foams of different copolymers;
polyurethanes coated with polyethylene or polysiloxane polymers and copolymers; and foams prepared from suitable natural resins such as cellulosic derivatives, The foams must burn out or volatilize at below the firing temperature , of the ceramic material with which they are impregnated, As noted earlier, the dimensions of the foam should correspond roughly to the dimensions of the desired ceramic article, Thus, for example, a foam material having a thickness ranging from about 10 to 100 mm is employed when the resulting ceramic foam is to function as a molten metal filter, In addition to the properties of permeability and uniformity, the above noted polymeric materials must possess a pore size within defined limits in order to render them effective in the preparation of molten metal filters, Pore or cell size has been found to be important to the structural uniformity of the ceramic foam and should vary within the ranges of 2 and 20 pores per linear cm, ~ The control of the above noted variables contributes to ; 30 the structural uniformity and permeability of the resulting '~

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filter and directly effects metal flow rate and effectiveness through the tortuosi-ty of the flow path. Though these factors are significant, additional factors will be discussed herein-below which combine to provide further con-trol of the final ceramic foam article.
The organic foam selected wi-th reference to the above discussion is then impregnated with a slurry of a thixotropic ceramic material The property of thixotropy is important to the present invention as it affects the uniformity of structure 10 and strength of the final ceramic foam article. Thixotropic ~
materials are those which display a high resistance to flow ;-under low rates of shear and correspondingly, a low resistance to flow under relatively high rates of shear. As this relates to the method of the present invention, the ceramic slurry must possess sufficient fluidity to rapidly enter and fill the voids of the organic foam material and thereby coat the surrounding polymer web, while possessing a sufficient viscosity to resist running out or draining from the foam once impregnation is complete. It has been found in accordance 20 with the present invention that certain ceramic materials -prepared in combination with particular air setting agents and temporary binders display the desired thixotropic character to successfully conduct impregnation ;
As the ceramic slurry which is employed herein may vary ~
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according to the end use of the foam, a wide variety of i ceramic materials of varying refractoriness may be employed, ;~
'; Particularly, such materials as alumina, chromia, zirconia, magnesia, titania, silica and mixtures thereof may be presen~.
Such materials are noted for their relatively high refractoriness or ability to serve in high temperature .

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lOS9535 situations. However, other materials of lesser refractoriness such as mullite, calcined clay and various glasses of high softening temperature may be employed herein either alone or in combination with each other and with more refractory materials to prepare the resulting foam article, for example, ; in an amount up to 1~%. Insofar as the ~tility of the resulting article as a molten metal filter is concerned, the only requirement placed on a selection of the particular ceramic materials is that they provide the article with sufficient resistance to the chemical attack of the molten alloys over the exposure times involved in filtration. A
particular cornposition which has been successfully employed herein comprises a mixture of alumina and chromia.
The above composition also includes a room temperature binder or air setting agent which provides green strength to the slurry, particularly during the bake out and the optional sintering operations where the foam is subjected to thermal stress.
According to the inventlon one provides from 2 5 to 25%
of an air setting agent which is substantially nonreactive to the molten metal. The air setting or bonding agent sets up or hardens the ceramic slurry without the need for heating, and preferably by drying, normally by a chemical reaction, while heating to moderate temperatures The preferred air setting agent is aluminum orthophosphate, preferably in the form of a 50% aqueous solution. Other air setting agents which may be employed include, for example, magnesium orthoborate, aluminum hydroxy chloride, etc. Alkaline metal silicates such as sodium silicates may be employed at least in part; however, these are less desirable since melting and .

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consequent loss of set occurs at temperatures around 1500F
(815,56C). Furthermore, the silicon contents thereof, and perhaps the sodium content, may become dissolved in the melt, Similarly, ethyl silicate and other phosphates may be employed but are less desirable. Aluminum orthophosphate is particu-larly preferre~ due to its very desirable combination of ~ s properties, that is, nonreactivity, stability over a wide range of temperatures and good setting properties.
As indicated hereinabove, the air setting agent is preferably added as an aqueous suspension including equal parts of binder and water particularly in the case of aluminum ~
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orthophosphate, The binder provides green strength before the :
formation of the ceramic bond, that is, after the burning off or volatilization of the web of flexible foam material, The binder material provides sufficien-t strength to hold the mixture together for formation of the final product, In fact, the stability and strength of the chemical bond provided by the preferred air setting agent is sufficient for many i :
applications to enable use of the product at this stage without high temperature sintering, This strength is substantial and exists over a wide temperature range, The preferred embodiment utilizes from 12 to 17% of aluminum orthophosphate, In addition to the binder noted above, certain agents herein referred to as rheological agents are employed which serve to promote the desired thixotropic property of the slurry, ~-Several materials are known which may serve as rheological agents, among them certain organic materials such as :
carboxymethyl cellulose and hydroxyethyl cellulose, and certain inorganic materials such as bentonite and kaolin, of : 30 the materials available in this regard, bentonite has been :. :. .: .. ~.. : ....................... , ~ .
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found to be particularly preferred. Bentonite is a naturally occurring clay composed primarily of aluminum and various silicates, usually including quantities of magnesium and iron.
In addition to its promotion of -the thixotropic properties of the slurry, bentonite performs a small setting or binding function, as certain glassy phases are produced upon firing of the article which yield increased strength in the ~inal ~-foam structure, In addition to bentonite, a small amount of kaolin may also be employed which provides both binding and rheological improvement to the final slurry in the same manner as bentonite. Kaolin is a clay composed of primarily of alumina and silica. Naturally, one could employ the chemical equivalents of the aforenoted materials to approximate their compositions. The general range of addition of the rheological agents of the present invention is within about 0.1 to about 12% by weight of the slurry. In a preferred embodiment, the rheological agents are added in an amount ranging from about 0.5-5% by weight.
Though, as indicated above, the thixotropic ceramic 20 material may be prepared in a wide variety of formulations, a -~
particular composition has been determined to be eminently suitable which comprises alumina in an amount ranging from about 40-80%, and preferably from about 45-50%, chromia in an amount ranging up to about 20%9 and preferably from about 10-15%, kaolin in an amount ranging up to about 10%, and preferably from about 2-5%, bentonite in an amount ranging from about 0 l-lO~o, and preferably from about 0 5-2%, colloidal aluminum orthophosphate (50% solution) in an amount of from about 5-50%, and preferably from about 25-35%.
Additional water may be added to the above formulation in ,~ , 1~5gS35 amounts ranging up to about 20%, and preferably from about 5-lO~o for the purpose of adjusting viscosity, discussed in detail hereinbelow. Generally from 10 to 40~0 water is present ;
in the slurry Though the foregoing formula-tion is suggested in its preferred ranges, it is to be understood that the invention is not limited thereto, as other formulations may be prepared from the ingredients recited earlier.
In addition to its thixotropic properties, the ceramic -~slurry of the present invention must possess a carefully controlled viscosity at and during the time of impregnation The character of viscosity has been found to exert a material effect on the achievement of a reproducibly uniform ceramic article. The desired range of viscosity has been found to be from 1 x 103 to 80 x 103 cps (centipoise), and preferably within the range of 10 x 103 to 40 x 103 cps. Viscosity is regulated during the formulation of the slurry and must be within the forenoted ranges at the time the slurry is to impregnate the organic polymer foam. As noted above, a convenient way of regulating and thereby controlling viscosity is through the variation in excess water content within the ranges specified above. For the purposes of the present invention, viscosity is measured at 25C with a #6 spindle, Brookfield RVT Viscometer at 20 rpm after 20 minutes rotation, .
the slurry having previously been mixed in an 80-quart Hobart*
Mixer at 60 rpm for 30 minutes.
Once the ceramic slurry is prepared to within the -`
aforenoted limitation of viscosity, the impregnation of the organic foam material can be conducted. Thus, slabs of reticulated polyurethane foam having pore sizes lying between 30 2 and 20 pores per linear cm are immersed in the slurry until ~ ;

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' ' ~: ., ' : . ' the interstices of the foam are fully saturated therewith, Impregnation may be conducted by one of many techniques, For example, the foam slab may be totally immersed in t~e slurry and passed through a pair of rolls likewise immersed therein to expel air from the pores of the foam on compression whereby the reexpanding foam emerging from the rolls draws in the slurry and is thereby filled therewith, Another technique which may be employed would involve placing the foam over a bath of the slurry in an enclosed vessel, evacuating the vessel to a reduced pressure, immersing the foam in -the bath of slurry and then releasing the vacuum in the vessel, This method which would comprise a modification of vacuum impregnation would likewise result in the total saturation of the foam with slurry, Naturally, other forms of impregnation including the standard vacuum impregnation technique of merely exerting the vacuum force on one side of the foam while drawing slurry through the opposite side, would be applicable herein and the invention should not be limited to a par-ticular technique per se, A preferred impregna-tion technique which has been employed herein comprises complete immersion of the foam in a bath of slurry and repeated compression and expansion of the foam by a mechanical plunger device made from perforated steel sheet, This process is conducted for from 30 seconds to 1 minute, or, of course, until the interstices of the foam are ~ ;
complètely saturated. In view of the thixotropic nature of the slurry discussed earlier, it is particularly advantageous and important that the slurry be continually sheared during -the impregnation to maintain the desired rate of flow into the foam material, This shearing may be accomplished in a wide .. . . .
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~059535 variety of ways such as the continual high speed agitation of the slurry. A technique which has been employed in the present invention comprises the continual vibration of the slurry during impregnation. It should be noted at this point that ~;
all impregnation techniques previously discussed would require that the slurry be maintained in its highly flowable state by some form of shearing action such as vibration or the like~
Upon completion of the impregnation of the foam material, ~-shearing is stopped; the slurry residing within the foam becomes resistant to flow and is substantially completely retained therein, with little loss due to drainage during the subsequent transfer of the foam from the impregnation area.
Upon completion of the impregnation of the foam with slurry, the impregnated foam material is then treated to remove excess slurry therefrom. ~his removal or expulsion of excess slurry must be closely controlled and uniformly conducted throughout the body of the foam in order to obtain a uniform ceramic article. As noted earlier, a wide variety of conventional methods are known for the removal of slurry from impregnated organic foam, however, such methods which include squeezing, blowing out by compressed air, centrifuging, and even passage through rolls, do not provide satisfactory results in this respect. Generally, in the case where passage through rolls is employed, the resulting article possesses either the defect of center line blockage, wherein excess slurry remains and agglomerates within the center of the article, or outer surface weakness, wherein insufficient ceramic material remains after expulsion and the article is mechanically weak.

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,' ~0S9535 In accordance with the present invention, it has been found that expulsion is advantageously conducted which results in consistently uniform ceramic foam articles by a process comprising conducting at least two passes of the impregnated foam material through preset rollers to effect the compression of said material in the range of about 50 to 90% of its thickness for one pass and 70 -to 90% of thickness for a further pass. Thus, multiple rolling passes conducted at the same or increased percent reductions yield a ceramic article possessing increased strength and freedom from center line blockage. Further, the employment of multiple rolling passes affords the careful control of the permeability of the final ceramic article which is particularly critical when such articles are prepared for use as molten metal filters.
An additional aspect of significance relating to the ; expulsion technique of the present invention is the use which is made of the thixotropic nature of the slurry in the rolling operation, Thus, because the slurry flows freely Imder high rates of shear but remains virtually static once shearing ceases, close control over slurry removal is available through control of roll gap (percent reduction), roll speed and/or roll diameter. Specificallyl control over the roll ; gap and roll speed determine the rate of shear experienced by the slurry andr hence, the extent of its removal and the geometry of its redistribution on the web of the rolled foam, The preferred rolling technique of the present invention utilizes a double pass schedule, although a multiple pass ;
schedule may be desirable in certain instances such as where the foams possess a thickness greater than 5 cm. As noted earlier, the respective percent reductions which have been GON-31-r~

lOS9S35 determined wlth a double pass schedule are 50 to 90% for the flrst pass and 70 to 90% ~or the second pass. Preferred percent reductions within the aforenoted ranges are 70 to 80%
for the first pass and 70 ~o 90% ~or the second pass.
Expulsion may be conducted wlth a conventional roll stand apparatus comprising two cooperating rolls. Thus, the foam material would be passed through the roll stand a first time, and thereafter recirculated for the second pass. In the instance where the second pass is to be conducted at a dif-ferent percent reduction, a palr of roll stands could be suitably provided in spaced relation to each other to permit the foam material to pass throu~h the respective reductions in a sequential manner. A further variation contemplated within the present invention to provide successive roll gap settings contemplates a single pass through a roll forming mill having three rolls defining the respective successive roll gap settings. This technique offers the advantages of a double pass schedule in a single operation using only a single roll stand.
In addition to the roll stands provided above, the rolls employed therein-may be advantageously coated with a material such as grit or the like to increase friction between the ~oam material and the roll and to thereby prevent or minimize slippage in rolling. A ~urther feature of the apparatus employed in the expulsion of slurry comprises a moving run-out table placed at the exit of the roll stands to support and transfer the newly rolled foam as it emerges therefrom. In sum, the employment of coated rolls and a movin~ run-out table would serve to add to the integrity, unlform.ity of structure ar.d ~hape ^f the ~olled ~roduct, s~rce ~;~ey . CON-31-M
~ ~ ~()5S3535 alleviate un;~anted dlstortlon efrects and mlnlmi~e unnecessary handllng O F' ~ne fcam article which might disturb slurry distribution.
As noted earller, the provlsion o~ multlple rolling passes has as its advantage the unexpected increase in the permeabillty of the final foam artlcle. Thus, in the lnstance where two passes through the rolls are conducted at the same percent reduction, the permeabillty of the flnal article has increased by 30 to 50~. This increase is significant -~hen it is considered that the pass schedule leads to a more uniform slurry distribution and ultimately stronger and more uniform articles than would be obtainable by comparable single pass reductions. Further, pass schedules conducted wherein the second or subsequent rolling pass is of a higher percent reduction than the initial pass, have resulted in permeaDility increases of greater than 100% over the single pass reduction schedule.
After expulsion of excess slurry is complete, the resulting foam structures may be dried and, if desired, fired to provide a ~used ceramic foam article. The drying sequence is employed for the prirnary purpose of removing the organic -polymer foam from the article. Generally, conventional drying techniques may be emplo~ed in this regard, however, it should be born in mind that a suitable heating rate for the removal of the foam should take into account the heat provided by the oxidation of the foam itself. The effect of - this phenomenon is particularly noticeable in the heating of large masses of the foam whe~e a significant volume of the heating chamber may be occupied by the article. In ~uch cases, ~ may be r.ecessa~Y ~o ma~ntain 'he mate-lal ~ a : , .~ , ' ~ .
: , 1~)S5~35 temperature ranglng from 200-370C to a~old excessive heat up resulting from chemical reactlon which may cause the ceramic filaments to rupture under thermal stress. The exact temper-ature will be dictated by the particular organic foam base used and need not be further developed herein.
As indicated above, the ceramic roam may, if desired, . ~
be further heat treated or fired to fuse the ceramic particles lnto a highly refractory structure. As noted earller, this practlce is optlonal, as, for example, in the employment of 10- the foam artlcles of the present invention as filters for molten aluminum, it has been found that the foam mater~al need only be heat treated at a temperature of from 540 to 600C to remove the organic component. ~he resulting article would be suitable as such for use with aluminum alloy mel~s at temperatures as high as 760C. In such an instance, the alr setting or binding agent would provide the necessary strength to the article, and the full sintering treatment would not be required.
Utilizing the method disclosed above, ceramic foams may be-prepared which range in thickness from 6 to 100 mm and may be of an area ranging up to about 1 m2 or more. The foams would possess, based on the raw foam employed, pore counts of from about 2 to 20 pores per linear cm with permeabilities ranging from about 100 x 10-7 cm2 to 10,000 x 10-7 cm2 and bulk densities of from 0.2 gm/cm3 to 1 gm/cm3 In the lnstance where the foam articles of the present lnventlon are utllized as fllters for molten metal, alr permeabllities may range from about ~00 to 8,~00 x 10-7 cm2 ; and pore counts may range from about 2 to 18 ~ores per linear cm. Natur 11~, as ~.oted -arlier, ~oth p-rmeabi'~i^s ~r.d .1 -24-.' .
". , - .~ ; , CO~-31-M

1(~5~i35 pore sizes may be varied to suit the particular mode of end use of the artlcle. Thus, ~or example, a relatively fine fllter may be prepared whlch would possess an alr permeability of from 400 to 2,500 x 10-7 cm2 and a pore count of from 8 to 18 pores per linear cm. Such an article would be useful in 'the filtration of aluminum alloys of the 5000 series. However, lr, ln the instance of molten metal filtration, the input ~`
metal is particularly dirty, one should prellminarily filter the metal through a relatively coarse ceramic foam filter having a pore size of between 2 and 8 pores per linear cm, and an air permeability ranging from 2,500 to 8,000 x 10-7 cm2.
This may be accomplished by providing a single filter with a gradation of properties or by using a series of filters of differing porosity.
The method of the present invention enables the accurate ~' control of the permeability of the resulting ceramic foam article. Further, foams prepared in accordance with the lnvention exhibit structural uniformity, as neither center llne blockage nor weak outer surfaces are encountered. When the articles'of the present invention are employed in the filtration o~ molten metal, it has been found that they can ' successfully withstand the rigors of the extended exposure to metal flow without blockage or fracture, and the resulting metal filtrate possesses unexpectedly improved purity.
- ' The resultant product is a bonded ceramic foam material having an open cell structure characterized by a pl'urality of lnterconnected voids surrounded by a web of ceramic, with the foam material having the characteristics defined herei-.above.
If one desires a single foam filter having a gradation of properties from. -oarse ~o f ne throu :~out the thi~'~ness ';^er~

.

' ' ' .
,.. , . . ~

~ 105~53S

one may combine two or more slabs of polyurethane foam having approprlately different pore sizes. Naturally, the ceramic foam may have any deslred configuration based on the configuration needed for the particular molten metal filtration process. Although naturally these configurations can be many and varied~ particular conflgurations may be preferred for filtration in a transfer trough between the furnace and the casting mold in filtering molten aluminum.
A wide variety of suitable configurations may be readily and convenlently prepared in view of the flexibility afforded by the preparation process utilized herein. It ls a particular advantage of the ceramic foam material of the present invention that said ceramic foam has sufficient strength property to withstand attack by molten metal and also advantageously does not require excessive heads of molten metal in order to start the filtration process.
As lndicated hereinabove, the present invention also provides a method of filtering molten metal throug~ a dlsposable ceramic filter characterized as aforesaid by pouring sa~d molten metal through the ceramic material at a rate of ~rom 1.25 to 12.5 dm3 per dm2 Of filter area per minute, and preferably from 2.5 to 7.5 dm3 per dm2 of filter area per minute for aluminum. Metal flow rates in normal aluminum casting operations vary from a mInimum of about 90 kg. per minute to a maximum in excess of 900 kg. of metal per minute, with a typical bulk metal flow rate being about 225 kg. per minute. In accordance with the present invention the ceramic materlals o~ the present invention are well suited to opera'e success~ully utili~ing bulk metal flow rates as described above. ~Torma lJ~ ~or ~lum'r.um, the ~artlclllar s~ecific f;ow .

i OS ~ 5 ~ S

rate of metal within the filter should not exceed 35 kg. of metal per dm2 (square decimeters) of filter cross section per minute and preferably should be less than 21 kg. per dm2 per minute. Hlgher flow rates through the filter than indlcated above results in the filters passing too many undesirable nonmetalllcs for the production of a high grade sheet product. -The lower limit is governed by practical size considerations and would require an impractically large filter to handle bulk metal flow rates in excess of 450 kg. per minute, that is, a ceramlc filter greater than 114 cm square or 130 dm2 would be required. A typical filter of the present inventlon may, therefore, be defined as being 40 cm square or about 16 dm2 ;
designed to pass 225 kg. of metal per minute at a specific ~low rate of 14 kg. per dm2 per minute.
In addition to the foregoing, the quality of the input metal is an important variable. If the input metal is -~
particularly dirty, and if the preferred, relatively finer foam is used, it will quickly block the ceramic fiIter of the present invention. The output quality, that is, the resultant filtered metal is a function of input quality. Hence, a minimum input quality should be provided in order to assure a good ouput quality. In order to lnsure a good output ;
quality, in accordance with the present invention one may preliminarily filter the molten metàl through a relatively coarse ceramic foam filter and optimally utilize a series of foam filters of decreasing porosity. Thus, as indicated hereinabove, in accordance with the present invention a typical preliminary fi'tration step would utilize a relatively coarse ceramic roam filter having a oore si~e of between 2 and 8 pores per linear cm, ~ir permeabil~ties of .. . .. ..

, CON-31-M
S'~5;~5 2500 to 800o x 10-7 cm2, porosities or void fractions of between 0.90 and 0.95, bulk densities of between 0.20 and 0.35 and thicknesses of 10 to 100 mm. A series of such filters having decreasing permeability is particularly suitable.
Alternatively, a single preliminary filter or a single fiiter of the present invention may be utilized having a gradation o~ properties from coarse (high permeability) to fine (low permeability) through its thickness may be used.
In accordance with the present invention the specific features t,hereof will be more readily understandable from a consideration of the following illustrative examples.
EXAMPLE I
A polyurethane foam was provided having a thickness of 5 cm. An aqueous ceramic slurry was provided having the following composition: 47% A12O3, 13% Cr2O3, 3-5% kaolin, 1%
bentonite and 14-1/2~ colloidal aluminum orthophosphate added as an aqueous solution with an equal amount of water. The slurry contained 82% solids and 18% water. The foam material was immersed in the slurry and kneaded to remo~e air and substantially fill voids with the slurry and also to coat the fibrous webs of the foam with the slurry. The resultant impregnated foam was removed from the slurry and sub~ected to compression through preset rolls to squeeze approximately 80~o of the slurry out of the foam by passing the impregnated foam through the preset rollers. The f'oam material sprung back to its original dlmension,after passing through the rollers and had the fibrous polyurethane filaments coated with a substanti 211~ uniform re~idue of the ceramic 31urry. The materia' W2S o~en d~ied at 125C for 1 hour~ heated slowly at a heat up ra'.e of 0.5C per .~inute to 500C to drIve off the _28-- , ' ' ~! lQ5~ S 3S
water and allow the polyurethane flbers to ~olatilize and/or burn out without collapsing the ceramic and wlthout destroying the filamentary ceramic configuration. The foam was held at 500C for 1 hour and was subsequently heated to 1350C at a rate of 1C per minute, held at 1350C for 5 hours to permit ' the ceramic to sinter together and thereby provide an open cell ceramic foam material having a configuration o~ the origlnal polyurethane foam material. The characteristics of the resultant foam were as follows:
Permeabil'ity 1425 x 10-7 cm2 ' Porosity 0.87 Pore Size 12 pores per linear cm Thickness 5 cm Structural Uniformity Excellent - EXAMPLE II
A 22,500 kg. charge of aluminum Alloy 5252 contalning 2.3% magnesium, 0.04% silicon, 0.05% iron and 0.06% copper was melted in a gas fired open hearth furnace and ~luxed with chlorine gas according to conventional practice. This unfiltered metal was then cast into three 50 x 135 cm cross section rolling ingots weighing 4,500 kg. each.
A second charge of the same composition was melted and prepared for casting in accordance with the same practice except, however, the metal was passe'd ~ith a rate of 14 kg/
dm2~min. through the ceramic foam filter prepared in Example I lnstalled in the pouring trough be~ore the metal was cast into rolling ingots. A metal head of only 15 cm was required to prime the fllter and during the casting the running head loss built up from 0.3 - 2.5 cm after 13,500 kg. of metal had been ~ ered l~.eret'nrough.

~ .
-29- ' ~ ~
.

- , '.
!s i~5~535 Pressure filter tests were run on metal taken from upstream and downstream of the filter during the castlng of the filtered metal and from the pourlng trough durlng the casting of the unfiltered metal. The flltratlon affect of the ceramic foam filter of the foam of the present invention proved to be excellent. Cross sectlons of the pressure filter disc from the unfiltered metal were compared with cross sections of the pressure filter disc from the filtered metal. These cross sections clearly showed that little or no residue was present in the metal filtered with ceramic foam filter of the present invention; whereas, considerable amounts of residue e~isted in the cross section of the pressure filter disc from the unfiltered metal. Similarly, a pressure filter disc was obtained upstream of the ceramic foam filter and resulted in the presence of considerable residue therefrom. This clearly shows the effectiveness of the ceramic foam filter of the present invention.
The pressure filter test described above is a method of concentrating and examining the nonmetallic particulate in a 9 to 11.3 kg. sample of molten aluminum. The molten aluminum in this test is carefully ladled into a preheated 11.3 kg.
clay graphite crucible into the base of which is set a 30 mm diameter, 3 mm thick porous silica disc plug. 90% o~ the metal is forced through the disc by application of air pressure and the remaining metal solidified in situ. The disc and -ad~acent metal are then sectioned, polished and examined by normal metallographic techniques to reveal the quantity of nonmetallics filtered o~t.
EXAMPLE III
The lngots prepared in E~ample II were all rolled down - --3~-.

:~ . .. . . .
, 105~3S

to 0.75 mm gage and samples o~ the sheet from coil locations corresponding to the head and butt sections of the lngots were tested to reveal the incldence of llnear defects per unit area of sheet. The sheet manufactured from flltered metal was found to contain about 10 times fewer linear defects than did the sheet manufactured from unfiltered metal as indicated in Table I below. Thls is a further strong indication of the effectiveness of the ceramic foam filter of the present inventlon.
In addition to the foregoing, the spent ceramic foam filter utllized in Example II was examined metallographically.
Considerable oxide stringers and fine nonmetallic particulates were found to be captured in the web of the filter, showing ~urther evidence of the filtration power of the filter of the present invention.
In addition, mechanlcal properties and composition were tested in metal filtered in accordance with the present invention. Good mechanical properties were obtained and no metal contamination was found to exist in the resultant product due to the use of the ceramic filter of the present -invention.
EXAMPLE IV
A further 22,500 kg. melt of aluminum Alloy 5252 was prepared as ln Example II. In this case, the metal was filtered through a tabular alumina bed in accordance with conventional practice prior to casting into rolling ingots for comparative purposes. The ingot thus cast was rolled down to 0.75 ~m gage sheet and samples were removed at locations in the resultant coil corresponding to the ingot butt~ 2 location 50 cm ~rom the ingot butt and the ingot head.

"~

lVS9~35 The samples were then tested to reveal the lncldence of llnear defects. The resu]ts of thls inspection are shown in Table I
below together wlth slmllar data carrled out on unfiltered metal and ceramic foam filter metal from Example II. The results in this table are given as percentages using the unfiltered metal as basis for comparlson.
TABLE I
Comparative Llnear Defect Counts Type of Metal Butt 20" From Butt Head Unfiltered Metal 100% 100% 100%
Bed Flltered Metal 150% 25% 10%
Ceramic Foam Filter Metal 10% 10% 10%
These results clearly show the reduced quality of the butt reglon of the ingot produced by filtration with the commercial bed filter as compared with that produced by the filter of the present invention.
EXAMPLE V
An addltional 22,500 kg. charge of aluminum Alloy 5252 was prepared as in Example II. The metal was passed through a ceramic foam filter prepared in Example I, with the filter being installed in the pouring trough in a manner similar to that described in Example II. The filter used was identical to that of Example I except that the available area of the filter was cut by two thirds with a result that the specific flow rate of metal withln the filter was 42 kg. per dm2 per minute ~hich is higher than the maximum specified flow rate of the present invention.
The resultant ingots were all rolled down to 0~75 mm gage and samples were removed and tested to reveal the incidence of linear defects in the manner of Example I~. The results of ' -32-1055~535 this lnspection are set out ln Table II below and compared to equivalent inspections carried out on unflltered metal and metal filtered through the ceramic ~oam as in Example II at a flow rate of 14 kg. per dm2 per minute. The results are glven as percentages using the unflltered metal as a basis for comparison.
TABLE II
Comparative Linear Defect Counts Type of Metal Butt20" From Butt Head Un~lltered Metal 100% 100% 100%

Ceramic Foam Filtered Metal (14 kg./dm2~min.) 10% 10% 10%

Ceramic Foam Filtered Metal (42 kg./dm2/min.) 150~ 50% 150%
The foregoing clearly demonstrates the reduced product quality obtained when one exceeds the specified flow rate range of the present invention.
EXAMPLE VI
:, A further 22j500 kg. charge of aluminum Alloy 5252 was prepared as in Example II. The metal was passed through a ceramic foam filter prepared in accordance with Example I
installed in the pouring trough in a like manner to that described in Example II. The filter was similar to that of Example I in composition, thickness and structural uniformity but di~fered in the following respects:
Permeability 2140 x 10-7 cm2 Porosity 0.92 Pore Slze 8 pores per linear cm In addition, the specific metal flow rate in the filter was about 28 kg. per dm2 per minute. Thus, the physical properties of the filter and metal flow rate are within the present ,r.~. . . . . .

lOS~535 lnventlon, but outside the preferred range in terms of permeability, porosity, pore size and speciflc metal flow rate in the ~ilter.
The ingots thus prepared were all rolled down to 0.75 mm gage and samples from the coils at locations corresponding to the heads and butts of the ingots were tested to reveal the incidence of linear defects in a like manner to that described in Example IV. The results of this investigation are set forth in Table III below and are compared to the results of equivalent inspections carried out on unfiltered metal and ; metal ~iltered through the pre~erred ceramic foam of Example I. The results are given as percentages using the unfiltered metal as a basis for comparison.
TABLE III
Comparative Linear Defect Counts Type of Metal Butt Head Unfiltered Metal 100% 100%

Ceramic Foam Filtered Metal (12 p/cm filter - 14 kg./dm2/min.) 10% 10%

Ceramic Foam Filtered Metal (8 p/cm filter - 28 kg./dm2/min.) 40% 60%
Thus, the foregoing clearly shows that the use of the filter within the broad specifics of the present invention resulted in an approximately 50% decrease in the incidence of linear - defects as opposed to a 90% increase when operating in accordance with the preferred speci~ication of the present invention.
In accordance with the present invention, further advantages and features of the above method o~ preparing ceramic ~oams will be more readily understandable from a consideration of he following illustratire examples.
' ~OS9S35 EXAMPLE_VII
A polyester polyurethane foam material was provided havlng a thickness of 5 cm, containing 12 pores per llnear cm and having an air permeability of 4,600 x 10-7 cm2. An aqueous ceramic slurry comprising 47% alumina, 13% chromia, 3.5%
kaolin, 1% bentonite, 29% of a 50% aqueous solution of aluminum orthophosphate (= 14.5% aluminum orthophosphate) was mixed in an 80-quart Hobart mixer at 60 rpm for 1 hour. After 1/2 hour of mixing a sample was removed for viscosity testing.
This sample showed that the slurry had a viscosity of 25.5 x 103 cps at 25C measured with a #6 spindle, Brookfield RVT
Vlscometer at 20 rpm after 20 minutes testing. A sample of the foam material was immersed in the slurry and repeatedly compressed and expanded with a plunger device for about 30 seconds while the bath of slurry was vibrated at 50 cycles - per second in order to fill the voids with slurry. The foam sample thus impregnated was taken from the slurry and passed through grit-coated rolls preset to provlde a 70% reduction in thlckness to expel the excess slurry. The rolls were of 76 mm diameter and were driven at a speed of 12.5 rpm. The sample exhibited substantlally complete spring-back after rolling was completed.
The sample was then dried in an oven at 65C for 1 hour and at 95C for 2 hours. The dried sample was then heated from 95C to 260C at 56C/hr., then ~o 315C at 11C/hr., and to 345C at 56C/hr. and held at this temperature for 4 hours in order to remove the polyurethane fibers without collapsing the ceramic web. The slow heating rate from 260C
to 315C was needed to prevent a sudden temperature excursion res~11ting from oxidation of the polyureth2ne.
;

J~

1C)59S35 The resulting sample was then fi.red in a klln using heatlng rates of 56C/hr. to 540C and 224C/hr. to 1315C
followed by a ~urnace cool.
The flred sample proved to be sound and the surface was resistant to spalling. Its permeability was measured as 400 x 10-7 cm2 and its bulk density 0.74/gm/cm3. The body possessed good physical strength and its modulus of rupture was found to be 17.6 g/mm2. However, sectioning the body revealed that it possessed a non-uniform structure, as it suffered from a center line blockage which would preclude its use in many applications such as filtration of molten metal.
EXAMPLE VIII
Another sample was prepared in accordance with the practice employed in Example ~II, with the exception that a variation in the percent reduction in rolling was made. In the present example, the roll gap was set to give an 86%
reduction, comprising a significant increase over that of Example VII.
The fired sample appeared to be sound but possessed rather weak surfaces and edges which made it susceptible to spalling in rough handling. Its permeability was measured as 1,600 x 10-7 cm2 and its bulk density as 0.39 gm/cm3. The body possessed adequate physical strength and its modulus of rupture was found to be 12.7 g/mm2. Sectioning the body revealed that it was quite uniform although the outer fibers were somewhat finer than those of the bulk which explained its characteristic weak surfaces. This reasonably uni~orm body would be suitable for applications where surface strength is of lesser importance compared to overall unlformity, and does not comprise a drawbac~. The ~ragility of the body, however, 1~5g535 would render lt unsuitable for use as a fllter, partlcularly of molten aluminum.
EXAMPLE IX
A further sample prepared in acccrdance with the practice described in Example VII was sub~ected to a two pass rolling schedule employing, respectively, roll reductions of 75%
(first pass) and 86% (second pass). The above double pass schedule was within the purview of this invention.
The fired sample proved to be sound and possessed strong surfaces and edges. Its permeabllity was measured as 1,700 x 10-7 cm2 and its bulk density as 0.41 gm/cm3. The body possessed excellent strength and its modulus of rupture was found to be 19 g/mm2. Sectioning the body revealed that it was exceptionally uniform having a moderate number of blocked pores evenly distributed through its bulk. This uniform, strong sample would be suitable for critical applications such as filtration of molten metal.
EXAMPLE X
An additional sample was prepared with a polyester polyurethane foam possessing a thickness of 5 cm, containing 12 pores per linear cm and having an air permeability of 6,ooo x 10-7 cm2. The ceramic slurry employed was of the same composition as in Example VII, but possessed a viscosity of 31 x 103 cps due to some water addition.
The foam sample was impregnated in the manner disclosed in Example VII. Expulsion of slurry was carried out with a double-pass rolling schedule, wherein the first pass was set at a reduction of 62%, and the second pass at a reduction of 86%.
The resulting sample was dried ard fired, an~, on ~7 '''' .

:: , . .

S3~

inspection, was observed to have an air permeability of 1,700 x 10-7 cm2. The sample possessed a uniform structure which was free from surface fragility and center llne blockage, and was therefore suitable for use as a molten metal filter.
EXAMPLE XI

.
An additional sample was processed in accordance with the procedure of Example VII to further lllustrate the present $nvention. A polyester polyurethane foam was employed which dlffered from the previous samples in possessing an air per~.eability of 4,700 x 10-7 cm2. Likewise, the ceramic slurry was the same with the exception that its viscosity was 25 x 103 cps.
As noted, processlng was identical to Example ~I~, with the difference that expulsion was conducted with a double-pass rolllng schedule wherein the first pass was performed to an 84% reduction, while the second pass was set at 86%.
Upon drying and firing, the resulting sample was likewise sound and uniform, and free from both surface weakness and center line blockage. The sample possessed a permeability of 2,650 x 10-7 cm2, and is considered suitable for use as a filter of molten metal.
Unless otherwlse specified, all percentages expressed herein are in terms of percent by weight.
; As indicated hereinabove, the present invention provides considerable advantages in the filtration of molten metal, especially aluminum. Thus, for example, the present invention enables one to filter molten metal with a conveniently removable filter plate which may be easily and quickly inserted in the filtration apparatus and easily and conveniently removed therefrom. In accordance with the l~)SS~S35 preferred embodiment of the present lnventlon when a ceramic foam filter plate is utllized, extremely high flltratlon - efflciencles are obtained and these efficlencies are obtained utillzlng a dlsposable ~ilter plate which can be easlly and convenlently inserted and removed ~rom the filtration apparatus, as will appear from the accompanying drawings showing as examples varlous appllcations of the ceramic foam filter and shapes of said filter.
Figure 1 is a top view of a filter chamber including the -filter plate ln place therein substantially horizontally dlsposed;
Figure 2 is a sectional view along the lines II-II of Figur~ l;
; Figure 3A is a perspective view of the filter plate as shown in Figures 1 and 2;
Figure 3B is a perspectlve view of a modifled filter plate similar to the filter plate shown in Figure 3A;
Figure 4 is a top view of another embodiment of the i fllter chamber wherein the filter plate is substantially vertically disposed;
Figure 5 is a sectional view along the lines V-V of Figure 4;
Figure 6 is a perspective view of the filter plate as shown ln Figures 4 and 5; and Figure 7 is a sectional view of another embodiment wherein the filter plate is disposed above an individual . . .
pouring spout.
' A filter chamber is illustrated in Flgures 1 and 2, as .. }
in a molten metal transfer system, pouring pans, pouring troughs, transfer troughs, metal treatment bays, or the l~ke.

` ~39~

55~5;~5 The filter apparatus 2 may lf deslred be constructed ln two sections 2a and 2b which may be bolted together by any sultable means, such as by flan~es at the peripheries thereof, not shown. The particular filter apparatus ill~strated in Figures 1 and 2 is a transfer trough containing a central filter chamber 3 fed by inlet 4 with the metal passing out of the filter chamber vla outlet 5. The molten metal may enter the inlet 4 by any suitable means, such as pouring spout 6.
` The filter chamber 3 is a bowl shaped chamber, the bottom of which is recessed below the level of the inlet 4 so that the molten metal passing into the filter chamber 3 may travel downwardly through the filter plate of the present ~nvention in place in'the ~ilter chamber. Thus, the filter chamber 3 is characterized by a peripheral rim 7 which may completely surround the upper portion of the filter chamber. As shown in Figure 1, the filter chamber rim, 7 surrounds the filter chamber on all si~des except ad~acent the area of inlet 4. The filter chamber rim 7 is connected to side wall 8 which~extends downwardly to filter chamber floor 9 which has a circumfer-ential bevelled portion or aperture 10 (Figure 2) extending around the periphery thereof to mate with the bevelled wall surface of the filter plate. The filter plate 11 has a corresponding bevelled peripheral surface 12 adapted to mate with the bevelled wall surface 10 of the filter chamber. The bevelled peripheral surface of filter 12 is provlded with a resilient sealing means 13 thereon resistant to the molten metal, and the filter plate 11 and sealing means 13 are ' inserted in the filter chamber 3 so that the filter plate-sealing means assembly engages the bevelled wall surface of the filter chamber.

, _40_ - :

Thus, as shown in Figures 1 and 2, the filter 11 is substantially horizontally disposed in a trough. The filter as shown has a square configurationj however, any convenlent shape may be readlly employed for the filter, such as round, hexagonal or the like. The filter plate ll is positloned ln a recessed section of a filter chamber or filter bowl 3, such as in the floor portion 9 thereof. Molten metal is fed to the filter ll via inlet 4 into the filter chamber 3. The molten metal passes downwardly through filter ll lnto the recess 14 beneath the fllter plate 11. The filter ll is sealed in place by means of resilient seal 13 so that the filter plate may be readily inserted by pressure vertically downwards and easily removed by pressure vertically upwards.
Alternatively, as indicated above, the filter chamber may be :t split and moved laterally for positioning the gasketed or ;
sealed filter plate in the filter chamber, with the filter v plate held therein with a vise type action. Preferably, the bevelled peripheral surface of the filter chamber 10 is - -bevelled at an angle of from 2 to 20 and the filter plate is preferably provided with a bevelled surface 12 corresponding thereto at an angle of from 2 to 20. The filter plate ll is preferably substantially horizontally disposed at an angle of ` from 1 to 5 upwardly sloped towards the metal outlet 5 in order to prevent entrapment of air against the underside of the filter. In addition, the floor 15 of recess 14 beneath filter plate 11 is preferably sloped downwardly at an angle of from l to 5 sloped towards outlet 5 in order to facilitate drain-age of metal during operation and at the completion of the pouring or transfer operation.

-4:-CON-3l-r~
105~S~

Alternatively, ~f desired, the filter chamber may be ~plit horlzontally, 2S along a horizontal plane beneath floor 9, or on an angle beneath floor 9, particularly to enable easy cleaning of recess 14. It may be deslrable to reverse the direction of the taper of bevelled portion 10, effecting a positive seal by means of the wall portion in recess 14.
As shown in Figure 3A, the filter plate of the present lnvention 11 has a bevelled peripheral surface 12 adapted to mate with a bevelled surface 10 of the filter chamber.
Naturally, variations in design are contemplated within the scope of the present invention, such as shown in Figure 3B
wherein a corresponding flat surface 16 is provlded around the entire perlphery of filter plate 11 ad~acent bevelled surface 12. Figures 3A and 3B show filter plates wherein the bevelled sur~ace extends around t'ne entire periphery of the plate; however, it may be more convenient to provide that the bevelled surface extends around less than the entire periphery, as in Figure 6 wherein the bevelled surface extends around two faces of the plate.
Thus, it can be seen that the filter plate of the present invention may be conveniently utilized in a variety of locations, including pouring pans, pouring troughs, transfer troughs, pouring spouts and metal treatment bays. The filter should not be placed in the immediate vicinity of turbulent molten metal flow, especially where such turbulence results in oxide formation and entrainment. This is true for the case - of turbulence both upstream and downstream of the filter.
Turbulence upstream o~ the filter with attendant oxide entrainment tends to lead to channeling of the ~llter, inefflcient filtration and in severe cases premature blockage -4~-,; - , ~ :- . .

``~ 11)5~S35 of the filter. Turbulence downstream of the fllter will tend to undo the ~ood rendered by the filter and once agaln load the molten metal with oxide or other nonmetallics which are present or are formed on the surface o~ the metal. Frequently - encountered sources o~ turbulence are Yurnace tap holes, pouring spouts and other devices whlch cause rapld changes in ;
flow cross section and consequent high velocity gradients. `!
Naturally, the particular filter lnstallation must be chosen with care to ensure that lt too does not become another source of turbulence. The foregoing considerations of turbulence are, of course, particularly relevant to chemically reactive metals, such as aluminum and magneslum and their alloys which readily oxidize on contact with air, however, these considerations are .j, .
also significant for less reactive metals, such as copper and ; its alloys. Naturally, devices are available in the art to mitigate turbulence~ as, for example, appropriately placed vanes.
As can be seen from the foregoing, the filter plate of the present invention may be conveniently disposed substantially horizontally. In addition, i~ desired, the filter may be .
disposed substantially vertically or at an intermediate angle with respect to the flow of molten metal. The vertlcal disposition of the filter plate of the present invention wlll : be described in a specific embodiment hereinbelow. The ;, horizontally disposed filter, however, has two principal advantages over a vertically disposed filter. First, by utilizing the significant length of a pouring trough or length and breadth of a pouring pan a large filter area can be easily and conveniently accommodated. On the other hand, in order to accommodate a large filter area in a vertical installation, , ~.~ , . . . . .

~os95 one must generally resort to a deep trough, pan or tundish.
Such geometrlc constraints are often of great practical signiflcance, especially where adaptation of existing casting facilities is contemplated. A second advantage of a horizontal filter is the fact that it ensures that all parts of the filter will prime under substantially the same metallostatic head; whereas, a vertlcal filter will naturally prime under a head varying from top to bottom. For this reason naturally a vertical filter primes in a non-uniform manner. The upper portlons o~ a vertical filter will not pass as much metal as the lower portions thereof and in fact may ; not prime at all. Furthermore, changes of head upstream of the filter will have more effect on a vertical filter than on a horizonta~ filter and may result ln momentary or significant loss of prime to the upper portions of the filter and even freezing off thereof. To minimize loss of primed area in operation, a filter should remain buried under a minimum head of molten metal. This is easier to achieve with a horizontal filter than with a vertical one. Also, exposure of unprimed areas of a vertical filter above the metal line can result in ` cracking of the filter due to thermal stress from the high temperature gradients obtaining under such conditions. For the foregoing reasons, a horizontally or substantially horizontally disposed filter is preferred in the present invention.
A disadvantage, however, of horizontally placed filters is that air can be trapped beneath these filters. This in turn can lead to oxide formation downstream of the filter and to channeling of ~low through the ~ilter and, hence, less than optimum filtration. This type OL entra2ment is obviated by a . . .

~ON-31-M

lQ5~535 vertical disposltion of the fllter.
In accordance wlth the present inventlon, it has been found that the foregoing aisadvantages of horlzontally dls! ;~
posed fllters can be greatly minlmized whlle retainlng essentially all advantages of a horizontally disposed filter by canting the filter at a small angle of from 1 to 5 to the horiæontal. Such a dlsposltlon allows escape of the air during lnitlal primlng of the filter without relinquishlng the uniform or substantially uniform depth of lmmerslon of the filter body resultlng from horlzontal placement. Preferably, as shown in Figure 2, the high point on the horlzontally dlsposed fllter surface should be at the extreme downstream end of the filter so that the escape of air is augmented by the sweeping action of the metal str~am. It can be seen that the feature of a substantlally horizontally disposed, upwardly ' sloped fllter is highly advantageous so that excellent results may be obtained u~ing said feature without the use of a bevelled peripheral surface, as, for example, using a spllt ; filter chamber and holding the filter therein wlth a vise type ; 20 action. Naturally, an appropriate sealing means should also - be used and also the floor 15 of recess 14 beneath filter plate 11 ls preferably sloped as described above. -Figures 4 and 5 show a vertically dlsposed filter . . .
installation in a transfer trough according to the present ~; inventlon. In the embodiment of Figures 4 and 5, a filter plate 20 ls held in place by a refractory dam 21 and positloned ln a slot 22 in a filter chamber 23. Molten metal is fed to the fllter chamber 23 via inlet trough 24 and passes horizontally into fllter chamber well 25 and thence through ; 30 filter plate 20 into outlet trough 26. Filter plate 20 is :

, ~(~5~S~S
sealed into slot 22 by means of a ceramic fiber gasket 27 which completely circumscribes the filter plate 20, The pregasketed filter plate 20 and dam 21 are placed into the slot 22 and sealed in place by means of wedges 2~, A drain hole 29 is provided to drain well 25 of metal at the completion of pouring or transfer. In operation, the drain hole 29 may be closed by a stopper rod or other convenient closure means, not shown, The filter plate of the present invention, as filter plate 20, is a frustum or segment of a solid figure with sloping sides so that the peripheral surface thereof has a bevelled configuration, Filter chamber well 25 has a corre-sponding bevelled wall surface 30 (Figure 4) to mate with the bevelled peripheral surface 31 of the filter plate (Figure 6), Filters up to several mm thick and several square meters in area can be conveniently located in troughs in the foregoing manner, The dam 21 and the filter chamber 23 may be made of conventional materials of construction, The filter well 25 and corresponding trough linings may be conveniently prepared of castable refractory or ceramic tile, The dam 21 and wedges 28 may be made of refractory boards such as MARTINITE if the metal to be filtered is aluminum or some lower melting alloy, MARTINITE is a trademark which stands for a refractory material having the chemical composition ~a5H2(P04)4, Naturally, the sealing means 27 is preferably adjacent the bevelled filter plate surface 31; however, às shown in Figures 4-6 where the filter plate is bevelled on only two (2) peripheral faces thereof, the sealing means is preferably adjacent all peripheral ;
surfaces of the filter plate including non-bievelled peripheral surfaces, Figure 7 shows an example of a horizontally disposed frustoconical filter plate installation designed to feed a - ; :
.

CO~-31-M

iOS~S35 single feeding pouring spout. In this unit, filter plate 40 ls located in a recess 41 in the refractory base 42 of a pouring pan or tundish 43. Durlng casting metal ~rom pan 43 flows vertically through filter plate 40 lnto channel 44 beneath filter plate 40 and thence into pourlng spout 45 feeding an ingot or casting below. The fllter plate is provided wlth a bevelled peripheral surface 46 for matlng wlth a corresponding bevelled surface 47 in recess 41. A resilient sealing means 48 is provided between the corresponding bevelled surfaces so that the presealed filter 40 is positioned and sealed in place by pressure from above in a manner similar to the preceding embodiments. Preferably, some means should be provided for ventlng alr bubbles from the bottom of the fllter.
This invention may be embodied in other forms or,carrled out in other ways without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered as in all respects illustratlve and not restrictive, the scope of the lnventlon being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

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Claims (35)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A ceramic foam filter for use in filtering molten metal having an open cell structure characterized by a plurality of interconnected voids surrounded by a web of said ceramic, said filter having an air permeability in the range of from 400 to 8000 x 10-7 cm2, a porosity of 0.80 to 0.95, a pore size of 2 to 20 pores per linear cm and a thickness of from 10 to 100 mm.

2. A ceramic foam filter according to Claim 1, said filter having an air permeability in the range of from 400 to 2500 x 10-7 cm2, preferably 1000-1500 x 10-7 cm2, a porosity of 0.80 to 0.95, preferably 0.85 - 0.90, a pore size of 8 to 18, preferably 10 to 14, pores per linear cm and a thickness of from 10 to 100 mm, preferably 35 to 65 mm.

3. A filter according to Claim 1 wherein said ceramic foam filter has a gradation of properties from coarse to fine throughout the thickness thereof.

4. A filter according to Claim 1 having a gradation of properties throughout the thickness thereof, with one side of said filter having an air permeability in the range of 2500 to 8000 x 10-7 cm2, a porosity of 0.90 to 0.95 and a pore size of 2 to 8 pores per linear cm, and the opposite side of said filter having an air permeability of from 400 to 2500 x 10-7 cm2, a porosity of 0.80 to 0.95 and a pore size of 8 to 18 pores per linear cm.

5. A high temperature resistant ceramic foam filter according to Claim 1 for use in filtering molten metal having an open cell structure characterized by a plurality of interconnected voids surrounded by a web of said ceramic, said filter having a density of less than 30% of the theoretical density for ceramic material of the same size, said foam having the following composition: 40 to 95% A12O3; up to 25% Cr2O3;
0.1 to 12% calcination products of bentonite and/or kaolin;
and 2.5 to 25% calcination products of an air setting agent which is substantially nonreactive to the molten metal, preferably of aluminum orthophosphate.

6. A filter according to Claim 5 containing from 45 to 55% A12O3.

7. A filter according to Claim 5 containing from 10 to 17% Cr2O3.

8. A filter according to Claim 5 containing from 0.5 to 5% calcination products of bentonite and/or kaolin.

9. A filter according to Claim 5 containing from 12 to 17% aluminum orthophosphate.

10. A ceramic foam filter plate according to Claim 1 having the shape of a plate with bevelled peripheral surface adapted to removably mate with a corresponding peripheral wall surface in a filter chamber.

11. A filter plate according to Claim 10 including a resilient sealing means adhered to said bevelled peripheral surface.

12. A filter plate according to Claim 10 wherein said peripheral surface is bevelled at an angle of from 2 to 20°.

13. A filter plate according to Claim 10 wherein said bevelled surface extends around the entire periphery of said plate.

14. A filter plate according to Claim 10 wherein said bevelled surface extends around two faces of said plate.

15. A filter plate according to Claim 10 wherein said filter plate has a flat surface around the entire periphery thereof adjacent the bevelled filter plate surface.

16. A method of preparing a ceramic foam particularly a ceramic foam filter according to claim 1 possessing controlled permeability and structural uniformity which comprises:
A. providing a hydrophobic, reticulated organic polymer foam possessing a predetermined permeability and resilience;
B. preparing an aqueous slurry of a thixotropic ceramic composition having a viscosity within the range 1 x 103 -80 x 103 cps, preferably 10 x 103 to 40 x 103 cps;
C. impregnating said polymer foam material with said slurry while shearing said slurry an amount sufficient to maximize impregnation and completely saturate said foam material;
D. expelling excess slurry from said foam material by conducting at least two passes of said material through preset rollers to effect a temporary compression ranging from about 50-90%, preferably 70-80%, for one pass and a further pass at a greater reduction, preferably 70-90%;
and E. drying and heating said foam material to remove the organic component therefrom.

17. The method of Claim 16 wherein said organic polymer foam possesses a resilience measured by the ball rebound test, of greater than 25%, and a compression set of less than 30% at 50% compression, and a pore size ranging from 2 to 20 pores per linear cm.

18. The method of Claim 17 wherein said polymer material is selected from the group consisting of polyester and polyether polyurethanes, polyvinyl foam materials, and foams prepared from cellulosic derivatives.

19. The method of Claim 16 wherein said aqueous slurry contains a ceramic composition comprising materials selected from the group consisting of alumina, chromia, zirconia, magnesia, titania, silica, mullite, calcined clay, high softening temperature glasses and mixtures thereof, an air setting agent selected from the group consisting of colloidal aluminum orthophosphate, alkali metal silicates, ethyl silicate, aluminum hydroxychloride, magnesium orthoborate and mixtures thereof, in an amount of from 2.5 - 25% of the total slurry, and a rheological agent is selected from the group consisting of organic cellulosic derivatives, bentonite, kaolin and mixtures thereof, and is present in an amount ranging from 0.1-12% by weight of said slurry.

20. The method of Claim 19 wherein said slurry comprises from about 40-95% alumina, up to 20% chromia, aluminum orthophosphate in an amount of from 2-25%, kaolin in an amount up to about 10% and bentonite in an amount up to about 10%.

21. The method of Claim 20 wherein said slurry comprises 45-50% alumina, 10-15% chromia, 12-18% aluminum orthophosphate, 2-5% kaolin, and 0.5-2% bentonite.

22. The method of Claim 16 wherein impregnation is conducted by the immersion compression of said foam within said slurry, followed by the release of said compression.

23. The method of Claim 22 wherein said compression is provided by a pair of rolls situated within a bath of said slurry.

24. The method of Claim 23 wherein said compression is conducted by repeated compression and expansion of said foam for a period of time ranging from 30 seconds-1 minute.

25. The method of Claim 16 wherein impregnation is conducted by the placement of a vacuum force on said foam while said foam is in contact on an opposite side thereof with said slurry.

26. The method of Claim 16 wherein said slurry is sheared by mechanical agitation.

27. The method of Claim 26 wherein said agitation comprises continual high speed stirring.

28. The method of Claim 26 wherein said agitation comprises continual high speed vibration of said slurry.

29. The method of Claim 16 wherein said drying step is conducted at a temperature ranging up to 600°C, preferably from about 200 to 370°C.

30. The method of Claim 29 wherein said foam material is further heated to sinter said ceramic composition entrained therein, at temperatures ranging from about 1100 to 1400°C, preferably 1300 to 1400°C.

31. A method of filtering molten metal by means of a ceramic foam filter according to claim 1, in which said molten metal is poured through said filter material at a rate of from 1.25 to 12.5 dm3 per dm2 of filter area per minute, thereby removing entrained solids from the molten metal.

32. A method according to Claim 31 including the step of preliminarily filtering the molten metal through a relatively coarse ceramic foam filter having an air permeability of 2500 to 8000 x 10-7 cm2, a porosity of from 0.90 to 0.95, and a pore size of from 2 to 8 pores per linear cm.

33. A method according to Claim 32 wherein said preliminary filtration utilizes a series of ceramic foam filters of decreasing porosity.

34. A method according to Claim 32 wherein said preliminary filtration utilizes a single ceramic foam filter having a gradation of properties from coarse to fine throughout the thickness thereof.

35. A method according to Claim 31 wherein said molten metal is aluminum.