Chromic oxide refractory
BACKGROUND OF THE INVENTION 
     McCreight et al, U.S. Pat. No. 3,378,385 discloses a ceramically bonded. high alumina refractory made from a batch consisting essentially of fused alumina grain containing titania, finely divided chrome sesquioxide, and finely divided nonfused high alumina refractory material. 
     Davis et al, U.S. Pat. No. 3,192,058 discloses a refractory made from a mixture which may contain chrome ore, alumina and magnesia with from about 1 to 15% added finely divided, high purity Cr 2  O 3 . 
     Manigault, U.S. Pat. No. 3,888,687 discloses an alumina-chrome refractory which contains tabular alumina, calcined alumina, chronic oxide, and added phosphate compound. 
     Manigaust, U.S. Pat. No. 3,862,845 discloses an alumina-chromic oxide refractory composition from alumina, chromic oxide, P 2  O 3  and magnesium oxide. 
     The foregoing refractory compositions, while useful in ferrous and non-ferrous industries, specifically in automotive foundry melting and holding furnaces at the slag line, have been restricted in certain applications because they exhibit poor thermal shock resistance and little or no expansion at high temperatures. 
     It is an object of this invention to produce an alumina-chromic oxide refractory composition with improved thermal shock resistance and increased reheat expansion at high temperatures. A specific application of the brick of the present invention is in rotary drum induction furnaces. 
     As the brick of this invention generally are used in contact with various molten slags and metals, it is very important that tight joints between brick be maintained. As the brick of the prior art exhibit only very slight expansion, or even slight shrinkage, when reheated to high temperatures, they must be laid up very carefully to prevent the slag or molten metal penetrating into the joints. Accordingly, it is desirable that brick used in these applications have increased reheat expansion in order to minimize the joint penetration. 
     It has been found that the addition of small, but effective amounts of fume silica to alumina-chromic oxide refractories produce a brick which has improved thermal shock resistance and increased reheat expansion without sacrificing the desirable properties found in present alumina-chromic oxide brick. 
     SUMMARY OF THE INVENTION 
     This invention involves an alumina-chromic oxide refractory composition which contains fume silica and exhibits improved porosity, reheat expansion, and good thermal shock resistance. 
     DETAILED DESCRIPTION 
     The alumina-chromic oxide refractory brick compositions of the prior art comprise the following compositions: 
     
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                   % by weight                                            
                   Brick   Brick                                          
                   A       B                                              
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Tabular Alumina      65-99     59-98                                      
Calcined Alumina      0-10     0-10                                       
Chromic Oxide         1-25     1-25                                       
Phosphate Compounds  --        1-18                                       
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     As mentioned, such products have been useful as brick compositions in both ferrous and non-ferrous industries. However, such products have poor thermal shock resistance, and exhibit very slight shrinkage, neutral behavior, or very slight expansion when reheated to high temperatures. 
     It is the purpose of the present composition to produce refractory brick of the alumina-chromic oxide type with improved thermal shock resistance and increased reheat expansion without sacrificing other properties. 
     It has been found that the foregoing improvements can be obtained by making a small addition of fume silica (volatilized silica) to brick of alumina-chromic oxide type. Thus, the composition covered by the present invention is the following: 
     
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                   % by weight                                            
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Tabular Alumina      55-98                                                
Calcined Alumina     0-10                                                 
Chromic Oxide        1-25                                                 
Fume Silica          1-4                                                  
Phosphate Compounds  0-18                                                 
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     Normal procedures are used in making brick according to this invention. The tabular alumina is used in various sizes ranging from a top size of about 3 to 8 mesh down to and including minus 325 mesh. The calcined alumina, chromic oxide, and fume silica are used in the minus 325 mesh fraction. 
     Various phosphate compounds can be used, including phosphoric acid, mono-aluminum phosphate, mono ammonium phosphate and aluminum dihydrogen phosphate. 
     Fume silica is very fine amorphous silica by-product of the ferro-silicon industry and typically has an average particle size of about 0.2 to about 0.7 microns, as measured on a Fisher sub-sieve-size analyzer. In the total composition, at least 40%, but no more than about 65% of the refractory materials should be minus 3 plus 65 mesh. 
     In preparing the brick of this invention, the different materials, i.e., alumina, chromic acid, phosphoric acid, and fume silica are mixed together with water, and an organic binder, if needed for handling purposes, and formed into shapes. Forming can be done by pressing, ramming, or any conventional means for producing refractory shapes. The shapes are then dried and fired at temperatures ranging from 600° to 3000° F. 
    
    
     EXAMPLE NO. 1 
     In order to more fully describe this invention, the following example is given. The following materials were used to make compositions as follows: 
     
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             % by weight                                                  
             Prior Art                                                    
                     Brick of this invention                              
             A    B      C         D                                      
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Tabular alumina,                                                          
               80     80     80      80                                   
6 mesh/F                                                                  
Tabular alumina,                                                          
               5      5      5       5                                    
325 mesh/F                                                                
Calcined alumina,                                                         
               5      5      3       3                                    
325/F                                                                     
Chromic oxide, 10     10     10      10                                   
325/F                                                                     
Fume silica    --     --     2       2                                    
85% Phosphoric --     1      1       4                                    
acid (extra)                                                              
Dextrin (extra)                                                           
               1      1      1       1                                    
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     These compositions were dry mixed, tempered with water, and then pressed into 9×4-1/2×2-178 &#34; bricks at 10,000 psi pressure. The brick were then dried and burned to about 2900° F. 
     After firing, the bricks were tested to determine their properties which are listed as follows: 
     
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                       Brick of the                                       
             Prior Art Present Invention                                  
             A     B       C        D                                     
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Bulk density, pcf                                                         
               203     204     205    208                                 
Apparent porosity, %                                                      
               16.4    16.1    14.7   11.8                                
Modulus of rupture                                                        
psi            4345    4700    4290   4860                                
Reheat, 3200° F.                                                   
% Linear change                                                           
               +0.11   +0.19   +0.59  +0.22                               
% volume change                                                           
               +1.44   +1.81   +3.10  --                                  
Modified U.S. Steel                                                       
Thermal Shock Test                                                        
Average # of                                                              
cycles before                                                             
first corner                                                              
spalled        19      28      46     --                                  
Average # of                                                              
cycles before                                                             
failure        25      43      50+    50+                                 
2850° F. Cup Slag                                                  
Test using                                                                
Automotive                                                                
Foundry high                                                              
Fe.sub.2 O.sub.3 slag-                                                    
degree of      Very    negli-  negli- negli-                              
reaction       slight  gible   gible  gible                               
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     Based on the foregoing results, it is apparent that a small addition of fume silica to brick of the alumina-chromic oxide type results in increased reheat expansion and improved thermal shock resistance without detrimentally affecting other properties. In fact, porosity was also slightly improved. The use of increased phosphoric acid resulted in improved density, porosity, and strength without detrimentally affecting other properties. 
     The thermal shock test used (Modified U. S. Steel Thermal Shock Test) consists of cutting 2 inch cubes from the brick and subjecting these cubes to cycles of 30 minutes at 2500° F. and 30 minutes at ambient. It should be noted that the brick of the present invention went considerably more cycles before spalling of corners first occurred and had not failed after 50 cycles at which time the test was terminated. This is standard procedure.