Abstract:
A high alumina ceramic composition has controlled small amounts of MgO and CaO. When refractories made from the composition are fired, the alumina and MgO react to form spinel. The CaO promotes such reaction while minimizing undesirable expansion, and results in a refractory having significantly improved hot strength and other desirable physical properties.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to the art of ceramic compositions and, more particularly, to such compositions that are spinel bondable when fired to high temperatures. The invention is particularly applicable to castable and pressable refractory compositions, and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects, and that it can be used in ceramics other than refractories, and in environments other than those in which refractories are commonly used. 
     It is known that certain properties of refractory castables can be enhanced by utilizing preformed magnesium aluminate spinel in the composition used to make the refractory. However, use of preformed spinel in refractories is generally limited because it is very expensive. 
     Attempts have been made to form spinel bonded refractories by mixing alumina materials with magnesium oxide powder to form spinel in situ during firing to very high temperatures. However, spinel formation during firing results in a high degree of permanent expansion in the refractory, and further permanent expansion occurs when the refractory is subsequently reheated. This expansion is generally greater than 2%, and it can cause a disruption of the refractory structure upon firing. Also, greater expansion causes higher porosity and lower density, and a lowering of performance in severe refractory applications where there is contact with molten metals, slags and glasses. 
     SUMMARY OF THE INVENTION 
     Ordinarily, the presence of CaO is considered to be an impurity in an alumina refractory because it tends to lower the melting point of alumina. Typically, castable refractories having a high CaO content also have a relatively low high temperature strength. 
     In accordance with the present application, it has surprisingly been found that the presence of controlled amounts of CaO and MgO actually enhances spinel bonding that occurs from the reaction of MgO and alumina, and minimizes permanent expansion during initial firing an during reheating. Significant and unexpected improved hot strength has resulted, along with improved resistance to corrosion and penetration by slags and molten materials, all without any deterioration in other desirable physical properties, such as density and porosity. 
     In accordance with a preferred arrangement, the ceramic composition of the present application contains alumina as the principal ingredient, and about 0.10-5.0% MgO by weight of the entire composition, and about 0.10-6.0% CaO by weight of the entire composition. 
     Most preferably, when using raw dolomite as the source of MgO and at least part of the source of CaO, the most preferred amounts of MgO and CaO, in percent weight of the entire composition, are about 0.4-1.7% MgO and about 1.4-3.6% CaO. When raw dolomite is used, and the total MgO and CaO content is in these ranges, the ratio of total CaO content of the composition to total MgO content of the composition is preferably between about 2-5, and most preferably between about 2.2-3.5. 
     The reasons for the improved properties of a fired refractory when using controlled amounts of MgO and CaO in the composition are not fully understood, and it is uncertain whether chemical reactions or mechanical forces, or both, are involved. It is believed that the CaO catalyzes the reaction of alumina and MgO to form spinel when the composition is fired to the MgO-alumina reaction temperature. It is also possible that CaO helps to provide more uniform dispersal of spinel bonding throughout the refractory. 
     The CaO is believed to act as a sintering agent which keeps the refractory dense. It is possible that liquid state sintering is involved, wherein the liquid phase from the CaO controls expansion that tends to take place from spinel formation. At the same time that expansion is tending to take place from spinel formation, contraction is tending to take place due to CaO sintering. 
     In accordance with the present application, both the MgO and CaO can be provided in the composition as a pure oxide, as a hydrate, or as a carbonate. It may also be possible to use other compounds of Mg and Ca to provide the source of MgO and CaO, such as nitrates. In a preferred arrangement, the MgO is provided to the composition in the form of dolomite. Raw dolomite is simply a naturally occurring CaO-MgO carbonate, and it is quite stable and will not hydrate like pure MgO in the presence of water and steam. The total CaO content of the composition includes that which is added in the form of pure oxide, hydrate or carbonate, plus that which is in the composition from any calcium aluminate cement that may be used. 
     Sample mixes have suggested the possibility that raw dolomite may be more effective and more reactive at high temperatures than sintered MgO. The presence of CaO in the dolomite with the MgO may also provide optimum spinel bonding compared to arrangements where the CaO and MgO are added separately. 
     Raw dolomite usually contains about 47% by weight CO 2 , about 22% by weight MgO and about 31% by weight CaO. At around 800° C., all of the CO 2  is driven off, leaving very reactive sources of MgO and CaO. Obviously, limestone can also be used as the source of CaO. 
     The composition of the present application consists essentially of about 0.10-5.0% MgO by weight, about 0.10-6.0% CaO by weight, 0-5% fibers by weight, and the remainder refractory oxide other than SiO 2 . The refractory oxide preferably includes a substantial amount of alumina. The refractory oxide can include one or more refractory oxides other than SiO 2  and alumina. For example, in addition to the amounts of MgO, CaO and fibers set forth above, the composition may have 0-50% by weight refractory oxide other than SiO 2  and alumina, such as one or more of zirconia and chromic oxide, and the balance alumina. A refractory oxide other than SiO 2  and alumina may enhance shock resistance of refractories made with the composition. 
     The use of small fibers in castable refractories to minimize hydration and cracking is well known in the art. The castable is heated to an initial drying out temperature of around 300°-400° F. before firing to around 2,700° F. for spinel bonding. 
     Obviously, a dispersant can also be used with the castable refractory to reduce the amount of water needed for casting. While a dispersant is desired when casting the composition as a refractory in order to obtain optimum properties, the use of same is well known in the castable refractory art and is not part of the present invention. However, the phrase &#34;consisting essentially of&#34; is intended to cover compositions that contain conventional additives, such as dispersants, even though such additives are not specifically listed as an ingredient following the phrase &#34;consisting essentially of,&#34; and even though such conventional additives can impart desired properties to the finished refractory. 
     It is a principal object of the present invention to provide ceramic compositions that are spinel bondable upon firing to high temperatures, and that will not undesirably hydrate during curing, drying or after firing. 
     It is another object of the invention to provide such compositions that form end products having superior physical properties. 
     It is also an object of the invention to provide such compositions that can be used to form refractories having improved strength at high temperatures, along with excellent resistance to corrosion and penetration by slag and molten materials. 
     It is a further object of the invention to provide such compositions that have controlled expansion behavior during and after firing to produce refractories having a very high density and a very low porosity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a graph showing the effect of varying amounts of MgO on the hot modulus of rupture of a refractory at 2,700° F. when using two different calcium aluminate cements containing different amounts of CaO; 
     FIG. 2 is a bar graph showing the effect of varying amounts of MgO on the hot modulus of rupture of a refractory at 2,700° F. when using different amounts and types of calcium aluminate cements; 
     FIG. 3 is a graph showing the effect of varying amounts of MgO from dolomite on the hot modulus of rupture of a refractory at 2,700° F. when using a low amount of calcium aluminate cement; and 
     FIG. 4 is a graph showing the effect of varying amounts of MgO and CaO from dolomite on the hot modulus of rupture of a refractory at 2,700° F. when using a low amount of calcium aluminate cement. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Unless otherwise specifically indicated, when the term &#34;% by weight&#34; is used in this specification, it is meant the percent by weight of the particular ingredient based upon the weight of the entire composition. 
     A ceramic composition in accordance with the present application consists essentially of about 0.10-5.0% by weight MgO, about 0.10-6.0% by weight of CaO, about 0-0.5% by weight fibers, about 0-50% by weight refractory oxide other than alumina or silica, and the balance alumina. Conventional additives such as dispersants can be used, and it is intended to cover compositions containing such additives when using the phrase &#34;consisting essentially of.&#34; This is so even though the additive is not specifically listed, and may be important to the processing, casting, shaping and curing of the composition, and may help impart desired properties to the final product. Thus, it is intended to cover conventional additives whose benefits to the composition and the final product, such as those identified above, are known to those skilled in the art. 
     Compositions in accordance with the present application may contain between 0-40% by weight calcium aluminate cement. Compositions having no calcium aluminate cement have been made using limestone as the source of CaO. 
     When calcium aluminate cement is used, it may contain varying amounts of alumina and CaO. Sample mixes have been made using 80/20 calcium aluminate cement having about 80% alumina by weight of the entire cement, and about 20% CaO by weight of the entire cement. Other sample mixes have been made using 71/29 calcium aluminate cement having about 71% alumina by weight of the entire cement, and about 29% CaO by weight of the entire cement. Obviously, cements having other alumina to CaO ratios can be used. 
     When CaO content of the composition is identified in this application, it includes the CaO in any calcium aluminate cement that may be added to the composition, plus any CaO that may be added to the composition independently of the cement. It is possible to rely solely upon the CaO contained in a calcium aluminate cement that may be used, without making any independent addition of CaO in another form. 
     It is believed that it may be possible to impart other desired properties to the composition for certain applications, such as improved shock resistance, by the use of a refractory oxide in addition to alumina but other than SiO 2 , such as zirconia or chromic oxide. The refractory oxide other than alumina or SiO 2  may be used in the range of 0-50% by weight. Such material could be used in a combined form, such as fused alumina zirconia. 
     Although formation of spinel in situ in the presence of CaO is the preferred arrangement, it may also be possible to impart improved properties to compositions using preformed spinel by the use of controlled amounts of CaO. 
     The use of 0-0.5% by weight of fibers may be provided along with a drying procedure to minimize hydration and cracking. 
     The alumina may include about 60-90% by weight tabular alumina and about 1-40% by weight fine alumina powder. Tabular alumina is a refined high purity Al 2  O 3  having a very dense grain structure. It is possible to mix tabular alumina and dolomite, and feed the mixture through a calcining furnace to provide a polycrystalline spinel bonded alumina coating on the tabular alumina. Around 0.1-4.0% dolomite by weight of the entire tabular alumina/dolomite mixture could be used. The calcining furnace heats the mixture to a temperature at which spinel will form. The tabular alumina may be in the form of pellets or briquets when coated with dolomite and processed in a calcining furnace. 
     Although the MgO may be provided to the composition in the form of a pure oxide or a hydrate, significantly improved results have been obtained by the use of about 0.1-12.0% by weight raw dolomite. 
     Fired refractories can be formed in accordance with the present application to have a hot modulus of rupture at 2,700° F. of at least 2,400 psi. The hot modulus of rupture is also more than two times such modulus for a refractory made using the same composition but without any MgO. 
     The following Tables 1, 2 and 3 give examples of refractories that have been formed with and without the use of MgO. Table 2 also gives an example of a refractory formed using dolomite. 
     
                       TABLE 1______________________________________HIGH CEMENT REFRACTORY ALUMINA CASTABLESWITH AND WITHOUT MgO             Without MgO                      With MgO______________________________________Formulation, wt %Tabular Alumina 3/200 Mesh               70.0       67.5Fine Alumina -200 Mesh               9.0        9.080/20 Calcium Aluminate Cement               21.0       21.0Sintered Magnesium Oxide               0.0        2.5Chemical Composition, wt. %Alumina (Al.sub.2 O.sub.3)               93.7       91.0Magnesia (MgO)      0.0        2.5Lime (CaO)          5.8        5.8Casting PropertiesWate Added, %       6.0-7.5    6.0-7.5Flow Value, %        80-100     80-100Set Time, Hrs.      12-24      12-24After Drying @230° F.Bulk Density, lbs/ft.sup.3               179        176Apparent Porosity, %                13         13Modulus of Rupture, psi               2,700      2,500After Firing @2,910° F.Bulk Density, lbs/ft.sup.3               168        171Apparent Porosity, %                27         24Modulus of Rupture, psi               2,100      2,800Permanent Linear Change, %               +0.10      +0.10Hot Modulus of Rupture, psi@2,700° F.   1,200      1,800______________________________________ 
    
     
                       TABLE 2______________________________________LOW CEMENT REFRACTORY ALUMINA CASTABLESWITH AND WITHOUT MgO AND WITH DOLOMITE            Without                   With    With            MgO    MgO     Dolomite______________________________________Formulation, wt. %Tabular Alumina 3/200 Mesh              70.0     70.0    66.0Fine Alumina -200 Mesh              25.0     21.5    25.080/20 Calcium Aluminate Cement              5.0      5.0     5.0Sintered Magnesium Oxide              0.0      3.5     0.0Raw Dolomite -14 Mesh              0.0      0.0     4.0Chemical Composition, wt %Alumina (Al.sub.2 O.sub.3)              98.4     95.0    96.2Magnesia (MgO)     0.0      3.4     0.9Lime (CaO)         1.4      1.4     2.8Casting PropertiesWater Added, %     4.5-5.5  5.5-6.0 4.5-5.0Flow Value, %      35-50    35-50   35-50Set Time, Hrs.      6-18     6-18    6-18After Drying @230° F.Bulk Density, lbs/ft.sup.3                195      189     191Apparent Porosity, %                15       17      16Modulus of Rupture, psi              1,900    1,500   1,700After Firing @2,910° F.Bulk Density, lbs/ft.sup.3                187      180     184Apparent Porosity, %                21       23      22Modulus of Rupture, psi              3,700    4,200   3,900Permanent Linear Change, %              +0.20    +0.40   +0.70Hot Modulus of Rupture, psi@2,700° F.  1,000    2,400   2,900@2,500° F.                   3,300@2,000° F.                   4,000@1,500° F.                   2,700______________________________________ 
    
     
                       TABLE 3______________________________________PRESSED ALUMINA REFRACTORY MATERIALWITH AND WITHOUT MgO             Without MgO                      With MgO______________________________________Formulation, wt. %Tabular Alumina 14/200 Mesh               80.5       80.1Fine Alumina -200 Mesh               17.5       15.9Sintered Magnesium Oxide               2.0        2.0-200 MeshLimestone -200 Mesh 0.0        2.0Chemical Composition, wt. %Alumina (Al.sub.2 O.sub.3)               98.0       97.0Magnesia (MgO)      2.0        2.0Lime (CaO)          0.0        1.0Pressing InformationWater Added for Pressing, %               3.2        2.9Tons of Pressure    100        100Bulk Density, lbs/ft.sup.3               175        174After Firing @2,885° F.Bulk Density, lbs/ft.sup.3               166        164Apparent Porosity, %                30         30Permeability, cgs    0.54       0.29Hot Modulus of Rupture, psi@2,700° F.   400        1,100______________________________________ 
    
     FIG. 1 shows the effect of MgO on the hot strength of two different refractory compositions containing different types and amounts of calcium aluminate cements. The curves show that hot strength increases when the amount of cement (and thereby CaO content) is decreased. Curve A shows that maximum strength was reached at an MgO content of about 3.5% by weight when using 5% by weight of the 71/29 calcium aluminate cement. 
     FIG. 2 illustrates preferred levels of added MgO which increase hot strength in alumina castable refractories containing different types and amounts of calcium aluminate cement. The graph shows a trend for increased hot strength regardless of type and content of cement. However, too much MgO results in decreased hot strength. 
     FIG. 3 shows the hot strength of alumina castable refractories using 5% by weight of 71/29 calcium aluminate cement and 0-12% by weight raw dolomite. The relationship between cement content and MgO content is not linear, and specific preferred levels of MgO and CaO result in optimum hot strength. 
     FIG. 4 shows the hot strength for alumina castable refractories using 5% by weight of 71/29 calcium aluminate cement and raw dolomite. The graph shows that maximum hot strength occurred when raw dolomite content was 2-6% by weight, giving a total CaO content ranging from 1.97-3.22% by weight and a total MgO content ranging from 0.44-1.32% by weight. This graph shows that when raw dolomite is used as the source of MgO and at least part of the CaO, the most preferred amounts of MgO and CaO, in percent weight of the entire composition, are about 0.4-1.7% MgO and about 1.4-3.6% CaO. When raw dolomite is used, and the total MgO and CaO content is in these ranges, the ratio of total CaO content of the composition to total MgO content of the composition is preferably between about 2-5, and most preferably between about 2.2-3.5. 
     A fired refractory made with the composition of the present application has a hot modulus of rupture at 2,700° F. of at least about 2,000 psi, and most preferably of at least about 2,400 psi. The spinel bonding renders the refractory more resistant to corrosion, and to penetration by slags and molten materials. 
     The use of -200 mesh dolomite suggests that slightly less dolomite is required to obtain a hot strength comparable to refractories in which -14 mesh dolomite was used. However, the reheat properties of a refractory made using -200 mesh dolomite were not as good as a refractory made using -14 mesh dolomite. As previously mentioned, the preferred arrangement uses raw dolomite as the source of at least part of the MgO and at least part of the CaO. Most preferably, the raw dolomite is the source of substantially all of the MgO and at least part of the CaO. The intimate relationship between the MgO and CaO in the dolomite is believed to enhance the beneficial effect of the CaO on the reaction between MgO and alumina, and on reducing expansion due to the reaction. 
     Refractories made with the compositions of the present application take on a characteristic pink color after firing. Obviously, other firing temperatures could be used, the importance of the temperature is simply that it be one at which spinel will form by the reaction of MgO and alumina. 
     Fired refractories made according to the present application were tested using a special slag cup test to evaluate slag and steel penetration. For each refractory sample, a 9 by 4.5 by 2.5 inch slab was molded with two cup-shaped depressions, each one inch deep and two inches in diameter. All slabs were prefired to 2,700° F. to cure same and form spinel. After cooling, 40 grams of #1 BOP slag were placed in one depression and 90 grams of steel shot were placed in the other depression. The slabs were then reheated to 2,800° F. for five hours. After cooling to room temperature, the slab is cut in half through the depressions and perpendicular to the face to expose the internal structure. Measurements of the slag and steel penetration are taken to determine resistance properties. Fired refractories made with the compositions of the present application exhibited far less penetration by the slag or steel compared to samples using standard compositions. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.