Refractory articles

Abstract

A filter for molten metal is an open-pored porous material comprising particles of refractory material embedded in and bonded together by a carbon matrix bonding material. The filter can be made by forming a porous article from refractory particles, e.g. refractory oxide, carbide or graphite, and a carbon-rich binder, e.g. tar, pitch or an organic (preferably aromatic) polymer that degrades to form carbon on pyrolysis, and then firing the porous article to generate the carbon matrix in which the refractory particles are embedded. The porous article is preferably made by coating a reticulated polyurethane foam with binder and refractory particles, and firing at preferably no higher than 800° C.

1 - 22 . (canceled)  
     
     
         23 . A ceramic filter suitable for molten metal filtration comprising a bonded network of graphitized carbon.  
     
     
         24 . The filter of  claim 23  wherein the graphitized carbon is present in a positive amount up to 15% by weight.  
     
     
         25 . Use of a filter comprising a bonded network of graphitized carbon for the filtration of molten steel.  
     
     
         26 . A method to produce ceramic filters according to  claim 23 , comprising the steps a) impregnating a foam made of thermoplastic material with a slurry containing a graphitizable carbon bonding precursor, ceramic powder, and optionally other additives, b) drying, optionally followed by one or two coatings of the same slurry in order to increase the mass, followed by final drying, c) firing the impregnated foam in non-oxidizing and/or reducing atmosphere at a temperature in the range of from 500 to 1000° C., whereby the carbon bonding precursor is least partially converted to a bonded network of graphitized carbon.  
     
     
         27 . A method to produce filters suitable for molten metal filtration comprising a bonded network of graphitized carbon, comprising the steps a) impregnating a foam made of thermoplastic material with a slurry containing a graphitizable carbon bonding precursor, and optionally other additives, b) drying, optionally followed by one or two coatings of the same slurry in order to increase the mass, followed by final drying, c) firing the impregnated foam in non-oxidizing and/or reducing atmosphere at a temperature in the range of from 500 to 1000° C., whereby the carbon bonding precursor is at least partially converted to a bonded network of graphitized carbon.  
     
     
         28 . The method of  claim 26  utilizing a thermoplastic foam that contains polyurethane.  
     
     
         29 . The method of  claim 26  wherein the carbon bonded precursor is mixed with ceramic powder, water, organic binder, and additives to control the rheology, prior to impregnating the foam.  
     
     
         30 . The method of  claim 27 , wherein the carbon bonded precursor is mixed with water, organic binder, and additives to control the rheology, prior to impregnating the foam.  
     
     
         31 . A method to produce the ceramic filters according to  claim 23 , comprising the steps a) pressing a semi-damp mixture comprising ceramic powder and a graphitizable bonding precursor, and optionally other additives in a hydraulic press, b) pressing to obtain a perforated article, c) firing the perforated article in non-oxidizing and/or reducing atmosphere at a temperature in the range of from 500° C. to 1000° C., whereby the carbon bonding precursor is at least partially converted to a bonded network of graphitized carbon.  
     
     
         32 . A method to produce the filters suitable for molten metal filtration comprising a bonded network of graphitized carbon, comprising the steps a) pressing a semi-damp mixture comprising a graphitizable bonding precursor, and optionally other additives in a hydraulic press, b) pressing to obtain a perforated article, c) firing the perforated article in non-oxidizing and/or reducing atmosphere at a temperature in the range of from 500° C. to 1000° C., whereby the carbon bonding precursor is converted partially or fully to a bonded network of graphitized carbon.  
     
     
         33 . The method of  claim 26  wherein high melting pitch (HMP) is used as the graphitizable carbon bonding precursor.  
     
     
         34 . The method of  claim 26  wherein a slurry or semi-damp mixture is used comprising: graphitizable carbon bonding precursor in the range of from 2 to 15 parts by weight, ceramic powder in the range of from 0 to 95 parts by weight, anti-oxidation material in the range of from 0 to 80 parts by weight, graphite in the range of from 0 to 90 parts by weight, organic binder in the range of from 0 to 10 parts by weight and, dispersion agent in the range of from 0 to 4 parts by weight.  
     
     
         35 . The method of  claim 34  wherein zirconia, silica, alumina, brown fused alumina, magnesia, any type of clay, talcum, mica, silicon carbide and silicon nitride or a mixture thereof, or graphite, is used as the ceramic powder.  
     
     
         36 . The method of  claim 34  wherein metallic powders such as steel, iron, bronze, silicon, magnesium, aluminium, boron, zirconium boride, calcium boride, titanium boride and the like, and/or glass frits containing 20 to 30 weight percent of boric oxide are used as the anti-oxidation material.  
     
     
         37 . The method of  claim 33  wherein a green binder such as PVA, starch, gums, sugar or the like or a combination thereof is used as the organic binder.  
     
     
         38 . The method of  claim 33  wherein lignisulphonate is used as the dispersion agent.  
     
     
         39 . The method of  claim 33  wherein a slurry or semi-damp mixture is used that further comprises: a plasticizer in the range of from 0 to 2 parts by weight, an anti-foam agent in the range of from 0 to 1 part by weight.  
     
     
         40 . The method of  claim 27  utilizing a thermoplastic foam that contains polyurethane.  
     
     
         41 . The method of  claim 39  wherein said plasticizer is polyethylene glycol.  
     
     
         42 . The method of  claim 35  wherein said anti-foam agent is silicon.  
     
     
         43 . The filter of  claim 24  wherein the graphitized carbon is present in a positive amount up to about  10  by weight.  
     
     
         44 . The method of  claim 26  wherein step c) is carried out at a temperature of between about 600° C. to 700° C.  
     
     
         45 . The method of  claim 27  wherein step c) is carried out at a temperature of between about 600° C. to 700° C.  
     
     
         46 . The method of  claim 31  wherein step c) is carried out at a temperature of between about 600° C. to 700° C.  
     
     
         47 . The method according to  claim 34  wherein said organic binder is present in an amount of between about 0.2 to 2 parts by weight.  
     
     
         48 . The method according to  claim 34  wherein said dispersing agent is present in an amount of between about 0.1 to 2 parts by weight.