Alumina refractories
BACKGROUND OF THE INVENTION 
     This invention relates to alumina containing refractories having a volatilized silica binder. It has been known that a volatilized silica binder produces superior characteristics when used in alumina refractory mixtures. The properties which may be improved by the inclusion of the proper proportions of volatilized silica include modulus of rupture, apparent porosity, linear change, volume change and spalling. 
     Volatilized silica, which is also referred to as fumed silica, is an amorphous silica which has been deposited from a vapor phase. A typical silica of this type results from the reduction of silica to form silicon alloys such as ferrosilicon. A similar silica fume can also be produced by reducing quartz with carbon or other suitable reducing agent, treating the vaporous products of the reduction with an oxygen yielding gas, and condensing the silica in finely divided form. An analysis of such a silica would run at least 90 percent SiO 2  and normally runs about 95 percent, with between 2 and 3 percent of total FeO, MgO and Al 2  O 3 . 
     Refractories of this type are prepared by grinding, screening and blending the desired alumina material to give the desired alumina content with the remainder consisting essentially of the other natural constituents of the alumina ore used. The exact screen sizing to be used is dependent upon such factors as the raw materials used and the purpose to which the refractory is to be put but this is a matter well within the knowledge and skill of those familiar with the refractory field. It is then conventional practice to blend into the batch the volatilized silica in an amount from at least about 1 percent up to about 10 percent by weight of the batch. This volatilized silica is added in the dry form. A temporary binder, of which a wide variety are known and used in the manufacture of refractory materials, is then added to the batch and the batch is tempered with water to give the desired consistency. For a further discussion of the use of volatilized silica in alumina refractories, reference may be had to U.S. Pat. Nos. 3,067,050 and 3,652,307. Refractories of this type may be employed to form brick or they may be used as ramming or gunning materials. 
     SUMMARY OF THE INVENTION 
     The present invention involves an alumina refractory material with a volatilized silica binder and its method of preparation and has the objective of improving the properties by forming a dispersed, acidified, aqueous slurry of the volatilized silica which is then mixed with the refractory batch. The batch may also include graphite, silicon carbide and green strength binders. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to alumina refractories and more particularly to those alumina refractories which are classified as having a high alumina content. This is generally considered to include refractories containing from about 50 to 99 percent Al 2  O 3 . Although the invention will be described with reference to such high alumina refractories, it is to be understood that the invention is not so limited and it includes all refractories which can employ volatilized silica as a binder. 
     Volatilized silica, as previously indicated, is an amorphous silica which has been deposited from a vapor phase. The particles are substantially all finer than 0.3 microns and have a typical particle size range of from 0.3 to 0.03 microns. 
     The volatilized silica is prepared in an acidified slurry form prior to being mixed with the refractory particles and other ingredients. The slurry is prepared by mixing the volatilized silica with water, an acid and a dispersant. The following table gives the preferred composition and the range of permissible compositions for the slurry: 
     
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               Preferred Range                                            
               Weight %  Weight%                                          
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Water            76.3        60.0 - 90.0                                  
Concentrated HC1 0.5         0.1 - 2.0                                    
Dispersant       0.2         0.05 - 1.5                                   
Volatilized Silica                                                        
                 23.0        10.0 - 40.0                                  
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     This slurry differs from a colloidal silica suspension in that the silica particles are perhaps 10 times as large. A colloidal suspension is much more expensive to produce because it starts with a sodium silicate solution and uses ion exchange procedures. The volatilized silica is an inexpensive source of silica. The use of the acidified, aqueous slurry instead of the dry blending of the silica with the refractory promotes solution of at least a part of the silica in the water and thereby enhances the bonding properties. Also, an aqueous slurry assures a more uniformed dispersion of the silica in the refractory particles than would be obtained with dry blending. The slurry form also reduces dust losses of the fume during blending. The fact that the slurry is acidified produces qualities superior to those obtained with an unacidified slurry. The acid has the effect of reacting with the volatilized silica to form an adhesive silicic acid so that it performs better as a binder. Although hydrochloric acid is the preferred acid, phosphoric acid may also be used since it forms a bond with the silica which is the same as or equivalent to the silicic acid bond formed by the hydrochloric acid. The concentrated hydrochloric acid referred to in the above table is 20°Be (32% HCl) but equivalent amounts of other concentrations may be used. 
     The volatilized silica slurry also contains a dispersant that permits a high solids loading of the slurry. Any substance capable of imparting a like charge on the particulate matter in the acidified slurry will cause the particles to repel each other and thereby remain mobile. This will prevent the solids from aggregating into a viscous mass. One such dispersant which can be employed in a water soluble powder sold under the trademark &#34;Toranil B&#34; which is a desugared extract of coniferous wood consisting of 96 percent calcium salt of lignosulfonic acid and 1.2 percent glucose. Some other dispersants which can be used are the highly polymerized napthalene sulfonates and the alkali salts of polyacrylates. Other dispersants can be used so long as they are compatible with the system. Highly alkaline dispersants such as sodium silicates would cause precipitation in the acid medium and would not be compatible. 
     The volatilized silica slurry of the present invention is mixed with an aggregate refractory material of relatively large particle size, calcined alumina or relatively fine particle size, graphite and a green strength binder. Other option ingredients may be included as discussed hereinafter. 
     The aggregate material may be selected from alumina or alumina containing materials such as fused alumina, tabular alumina and calcined kaolin. The aggregate materials will have particle size distributions primarily in the range of 3 mesh to 100 mesh or what are conventionally referred to as minus 3 mesh, minus 4 mesh, minus 6 mesh or minus 8 mesh aggregate. Some examples are as follows: 
     
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Crushed Fused Alumina, -4 Mesh                                            
U.S. Standard Mesh                                                        
                Minimum %   Maximum %                                     
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+4              0.0          5.0                                          
+6              7.5         12.5                                          
+8              15.0        25.0                                          
+10             5.0         10.0                                          
+14             15.0        25.0                                          
+20             8.5         14.5                                          
-20             23.0        33.0                                          
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Tabular Alumina, -6 Mesh                                                  
U.S. Standard Mesh                                                        
                Minimum %   Maximum %                                     
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+6              0.0          1.0                                          
+10             10.0        30.0                                          
+20             30.0        40.0                                          
+30             6.0         11.0                                          
+70             8.0         19.0                                          
-70             5.0         18.0                                          
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Calcined Kaolin, -8 Mesh                                                  
U.S. Standard Mesh                                                        
                Minimum %   Maximum %                                     
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+8              0.0          1.0                                          
+20             30.0        50.0                                          
+40             17.0        33.0                                          
+60             8.0         18.0                                          
 +100           6.0         16.0                                          
 -100           6.0         15.0                                          
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     The fine calcined alumina particles preferably have a particle size of about 325 mesh or smaller and no more than about 5 percent should be retained on a 325 mesh screen. The calcined alumina particles probably form a strong mullite (an aluminum silicate) bond with the silica fume. The graphite in the mixture retards erosion of the resulting refractory material. Any naturally occuring graphite is suitable which contains at least 75 to 80 percent carbon. Lower grade graphites will provide too much harmful ash. The graphite preferably has a particle size of what is commonly referred to as minus 40 mesh. A typical 40 mesh Mexican graphite has the following particle size distribution: 
     
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U.S. Standard Mesh    Maximum %                                           
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+10                   0.0                                                 
+20                   1.5                                                 
+30                   4.0                                                 
 -200                 36.9                                                
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     The green strength binder is selected from the conventional materials used for this purpose such as western bentonite and the various bond and ball clays. These materials are all of relatively fine particle size and typically have no more than about 7 percent retained when washed through a 270 mesh screen. Other bonding materials may be used such as the &#34;Toranil B&#34; binder previously discussed and asphaltite binders. One asphaltite binder which can be used is sold commercially as &#34;Corez Resin&#34; and it is a composition of a naturally occuring asphaltite of complex molecular structure and additives of organic binder and inert filler. 
     Optional ingredients in the mixture of the present invention are kyanite and silicon carbide. The kyanite expands on heating by a process referred to as mullitization to compensate for the normal shrinkage of clay refractories and seals cracks due to the shrinkage. The silicon carbide has high thermal conductivity and a good refractoriness. It aids the development of a ceramic bond further away from the hot face of the refractory by improving the heat flow to the cooler regions. The preferred particle size distribution is -30 +50 mesh as follows: 
     
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U.S. Standard Mesh                                                        
                Minimum %   Maximum %                                     
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+20             0.0         0.0                                           
+30             0.0         5.0                                           
+40             70.0        90.0                                          
+50             2.0         10.0                                          
-50             0.0         5.0                                           
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     Some examples of alumina refractory materials according to the present invention are as follows: 
     Example I 
     
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               Preferred Range                                            
               Weight %  Weight %                                         
Crushed fused Alumina, -4M                                                
                 24.0        15 -50                                       
Silicon Carbide, -30 +50M                                                 
                 7.25        5 - 30                                       
Graphite, -40M   7.25        5 - 20                                       
Asphaltite Binder                                                         
                 6.08        3 - 15                                       
Calcined Alumina, -325M                                                   
                 41.37       15 - 45                                      
Clay Binder      2.55        1 - 15                                       
Volatilized Silica Slurry                                                 
                 9.61        2 - 20                                       
Water            1.87        0 - 5                                        
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     Example II 
     
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               Preferred Range                                            
               Weight %  Weight %                                         
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Tabular Alumina, -6M                                                      
                 52.5        40 - 60                                      
Calcined Alumina, -325M                                                   
                 25.0        15 - 35                                      
Western Bentonite                                                         
                 4.0          1 - 6                                       
Graphite, -40M   7.5          5 -20                                       
Dispersant - Binder                                                       
                 1.0         0.5 -2                                       
Volatilized Silica Slurry                                                 
                 10.0         2 - 20                                      
Water            0            0 - 5                                       
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     Example III 
     
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               Preferred Range                                            
               Weight %  Weight %                                         
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Calcined Kaolin, 8-40M                                                    
                 50.0        40 - 60                                      
Calcined Alumina, -325M                                                   
                 8.0         6 - 20                                       
Raw Kyanite, -35M                                                         
                 8.7         4 - 15                                       
Bond Clay        16.4        6 - 35                                       
Graphite         5.9         5 - 20                                       
Dispersant - Binder                                                       
                 1.0         0.5 - 2                                      
Volatilized Silica Slurry                                                 
                 10.0        2 - 20                                       
Water            0           0 - 5                                        
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     Example IV 
     
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               Preferred Range                                            
               Weight %  Weight %                                         
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Alumina Aggregate, -4M                                                    
                 50.0        15 - 60                                      
Calcined Alumina, -325M                                                   
                 22.5        6 - 45                                       
Graphite         7.5         5 - 20                                       
Binder           10.0        1 - 35                                       
Volatilized Silica Slurry                                                 
                 10.0        2 - 20                                       
Water            0           0 - 5                                        
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     While specific formulations for the present invention have been described, it is to be understood that these are by way of illustrations and that the invention is to be limited only by way of the claims as appended.