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 . A filter suitable for filtering molten metal comprising an open-pored porous material comprising particles of refractory material embedded in and bonded together by a bonding material comprising a carbon matrix.  
   
   
       2 . A filter as claimed in  claim 1  wherein the refractory material is selected from zirconia, zircon, silica, alumina, titania, silicon carbide, zirconium carbide, titanium carbide, calcium carbide, aluminium carbide, silicon nitride, aluminium nitride, nickel oxide, chromic oxide, magnesia, mullite, graphite, anthracite, coke, active carbon, graphite-magnesia, graphite-alumina, graphite-zirconia, and mixtures comprising two or more of these.  
   
   
       3 . A filter as claimed in  claim 1  wherein the relative proportion of refractory to bonding material by weight is at least 50% refractory: no more than 50% bonding material.  
   
   
       4 . A filter as claimed in  claim 1  wherein the relative proportion by weight of refractory to bonding material lies in the range 65 to 75% refractory: 35 to 25% bonding material.  
   
   
       5 . A filter as claimed in  claim 1  wherein the pores in the open-pored porous material comprise a series of parallel ducts passing through the material.  
   
   
       6 . A filter as claimed in  claim 1  wherein the pores in the open-pored porous material comprise a random distribution of irregular interconnecting passages.  
   
   
       7 . A filter as claimed in  claim 1  wherein the bonding material is a coke or semicoke matrix.  
   
   
       8 . A process for making a material suitable for filtering molten metal comprising forming into an open-pored porous material a mixture comprising a binder and refractory particles and firing the material, wherein the binder is a carbon-rich source selected from one or more of the following: pitch, tar, and aromatic organic polymer that degrades to form carbon on pyrolysis.  
   
   
       9 . A process as claimed in  claim 8  wherein the open-pored porous material is formed by compressing a mixture of the binder and the refractory particles into a disk or slab in a die and piercing the compressed material with a plurality of needles or rods to create perforations traversing the thickness of the disc or slab.  
   
   
       10 . A process as claimed in  claim 8  wherein the open-pored porous material is formed by extruding a mixture comprising the binder and the refractory particles through a suitable die equipped with a plurality of mandrels to form perforations in the extruded article.  
   
   
       11 . A process of producing a refractory article, especially a filter for molten metal, comprising 
 (a) forming a slurry of a particulate refractory material, and a particulate binder comprising mesophase, in a liquid carrier;    (b) forming the slurry into a desired shape; and    (c) firing the shaped slurry.    
   
   
       12 . A process for making an open-pored porous material suitable for filtering molten metal comprising 
 (1.) forming a slurry comprising (a) particles of a refractory material, (b) a binder and (c) a liquid carrier,    (2.) coating a disposable former with the slurry    (3.) drying the coated former,    (4.) optionally applying one or more additional coats of a refractory material and/or a binder, optionally with liquid carrier, and drying the one or more additional coats,    (5.) firing the coated former to produce a porous material,    wherein the binder is a carbon-rich source selected from one or more of the following classes of materials: pitches, tars, and organic polymers that degrade to form carbon on pyrolysis.    
   
   
       13 . A process for making an open-pored porous material suitable for filtering molten metal comprising 
 (1.) forming a slurry comprising (a) particles of a refractory material, (b) a binder and (c) a liquid carrier,    (2.) coating a disposable former with the slurry    (3.) drying the coated former,    (4.) optionally applying one or more additional coats of a refractory material and or a binder, optionally with liquid carrier, and drying the one or more additional coats,    (5.) firing the coated former to produce a porous material,    wherein the binder is a carbon-rich source selected from one or more of the following classes of materials: pitches, tars, and organic polymers that degrade to form carbon on pyrolysis, and wherein (i) the binder has been subjected to stabilisation by pre treatment with an acid and/or with an oxidising agent and/or (ii) a polyfunctional organic compound is included in the slurry or the coating to promote stabilisation of the binder.    
   
   
       14 . A process as claimed in  claim 8  wherein the refractory is selected from zirconia, zircon, silica, alumina, titania, silicon carbide, zirconium carbide, titanium carbide, calcium carbide, aluminium carbide, silicon nitride, aluminium nitride, nickel oxide, chromic oxide, magnesia, mullite, graphite, anthracite, coke, active carbon, graphite-magnesia, graphite-alumina, graphite-zirconia, and mixtures comprising two or more of these.  
   
   
       15 . A process as claimed in  claim 8  wherein the refractory is provided by a mixture of graphite and one or more other particulate refractory materials, and wherein the graphite content is 10 to 40 weight % based on the total weight of the refractory particles.  
   
   
       16 . A process as claimed in  claim 8  wherein the binder is a carbon-rich source selected from coal tar, petroleum pitch, asphalt, bitumen, synthetic pitch, synthetic tar, synthetic bitumen; or residues derived from the pyrolysis of coal, crude oil, coal tar, petroleum pitch, asphalt, bitumen, synthetic pitch, synthetic tar or synthetic bitumen.  
   
   
       17 . A process as claimed in  claim 8  wherein the binder is a solid synthetic particulate pitch.  
   
   
       18 . A process as claimed in  claim 8  wherein the carbon-rich source is an aromatic organic polymer.  
   
   
       19 . A process as claimed in  claim 8  wherein the binder is subjected to stabilisation by heat treatment with nitric acid.  
   
   
       20 . A process as claimed in  claim 13  wherein the polyfunctional compound is polyvinylalcohol.  
   
   
       21 . A process as claimed in  claim 12  wherein the disposable former is reticulated polyurethane foam.  
   
   
       22 . A process as claimed in  claim 11 , wherein the liquid carrier is water.  
   
   
       23 . A ceramic filter for molten metal filtration comprising a ceramic powder and fibers bonded by a network of graphitized carbon.  
   
   
       24 . A filter for molten metal filtration comprising fibers bonded by a network of graphitized carbon.  
   
   
       25 . The filter of  claim 23 , wherein the ceramic powder is selected from a group consisting of zirconia, silica, alumina, brown fused alumina, magnesia, clay, talcum, mica, silicon, carbide, silicon-nitride, graphite and mixtures thereof.  
   
   
       26 . The filter of  claim 23 , wherein the filter comprises 5-15 wt % graphitized carbon.  
   
   
       27 . The filter of  claim 23 , wherein the fibers are selected from a group consisting of ceramic fibers, glass fibers, organic fibers, carbon fibers, meal fibers and mixtures thereof.  
   
   
       28 . The filter of  claim 23 , wherein the filter comprises 1-10 wt % fibers.  
   
   
       29 . The filter of  claim 24 , wherein the fibers are selected from a group consisting of ceramic fibers, glass fibers, organic fibers, carbon fibers, meal fibers and mixtures thereof.  
   
   
       30 . The filter of  claim 29 , wherein the ceramic fibers are selected from a group consisting of alumina fibers, silica fibers, alumosilicate fibers and mixtures thereof.  
   
   
       31 . The filter of  claim 29 , wherein the organic fibers are selected from a group consisting of polyester fibers, polyacrylonitrile fibers, polyethylene fibers, polyamide fibers, viscose fibers, aramid fibers and mixtures thereof.  
   
   
       32 . The filter of  claim 24 , wherein the filter comprises 1-10 wt % fibers.  
   
   
       33 . The filter of  claim 24 , wherein the fibers have a length from 0.1-5 mm.  
   
   
       34 . A method to produce filters for molten metal filtration comprising fibers and a bonded network of graphitized carbon, comprising: a) impregnating a foam comprising a thermoplastic material with a slurry comprising fibers and a graphitizable carbon-bonding precursor; b) drying the impregnated foam; c) firing the impregnated foam in a non-oxidizing atmosphere at a temperature from 500-1000.degree. C., whereby the carbon-bonding precursor is converted at least partially to a bonded network of graphitized carbon.  
   
   
       35 . The method of  claim 34 , wherein the foam is impregnated by a plurality of coatings of the slurry.  
   
   
       36 . The method of  claim 34 , wherein the fibers include organic fiber and the organic fiber is pyrolyzed during firing.  
   
   
       37 . The method of  claim 34 , wherein firing is performed at a temperature from 600-700.degree. C.  
   
   
       38 . The method of  claim 34 , wherein the non-oxidizing atmosphere comprises a reducing atmosphere.  
   
   
       39 . The method of  claim 34 , wherein the slurry includes a ceramic powder.  
   
   
       40 . The method of  claim 34 , wherein the foam comprises polyurethane.  
   
   
       41 . The method of  claim 34 , wherein the slurry includes fibers, carbon-bonding precursor, water, organic binder, and rheology additives.  
   
   
       42 . A method to produce filters for molten metal filtration comprising fibers and a bonded network of graphitized carbon, comprising: a) pressing a semi-damp mixture comprising fibers and a graphitizable carbon-bonding precursor to obtain a perforated article; b) firing the perforated article in a non-oxidizing atmosphere at a temperature from 500-1000.degree. C., whereby the carbon-bonding precursor is converted at least partially to a bonded network of graphitized carbon.  
   
   
       43 . The method of  claim 42 , wherein the slurry includes ceramic powder.  
   
   
       44 . The method of  claim 42 , wherein the graphitizable carbon-bonded precursor comprises high melting pitch.  
   
   
       45 . The method of  claim 42 , wherein the semi-damp mixture comprises: a) 0.1-20 parts fibers; b) 2-15 parts graphitizable carbon-bonding precursor; c) up to 95 parts ceramic powder; d) up to 80 parts anti-oxidation material; e) up to 90 parts graphite; f) up to 10 parts organic binder; and g) up to 4 parts dispersion agent.  
   
   
       46 . The method of  claim 45 , wherein the anti-oxidation material is selected from a group consisting of powders of steel, iron, bronze, silicon, magnesium, aluminum, boron, zirconium boride, calcium boride, titanium boride and mixtures thereof.  
   
   
       47 . The method of  claim 45 , wherein the anti-oxidation material comprises glass frit with 20-30 wt % boric oxide.  
   
   
       48 . The method of  claim 45 , wherein the organic binder is selected from a group consisting of PVA, starch, gums, sugar and mixtures thereof.  
   
   
       49 . The method of  claim 45 , wherein the dispersion agent comprises ligninsulphonate.  
   
   
       50 . The method of  claim 45 , wherein the semi-damp mixture includes up to 2 parts plasticizer.  
   
   
       51 . The method of  claim 45 , wherein the semi-damp mixture includes up to 1 part anti-foaming agent.  
   
   
       52 . The method of  claim 42 , wherein the non-oxidizing atmosphere comprises a reducing atmosphere.  
   
   
       53 . The method of  claim 42 , wherein firing is performed at a temperature from 600-700.degree. C.