Spinel bondable ceramic composition

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: 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. 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 highter porosity and lower density, and a lowering of performance in severe refractory applications where there is contact with molten metals, slags and glasses. H. UCHIKAWA et. al; Proceedings 83; Interceram, Special Issue 1984, Vol.33, p. 40-43 describes a sintered spinel clinker containing low amounts of SiO2, CaO and Fe2O3. 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 spine] bonding that occurs from the reaction of MgO and alumina, and minimizes permanent expansion during initial firing and 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 the invention, the ceramic composition of the present application contains alumina as the principal ingredient, and 0.10 - 5.0 % MgO by weight of the entire composition, and 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 0.4 - 1.7 % MgO and 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 prefereably between 2 - 5, and most preferably between 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 applicaton, both the MgO band CaO can be provided in the compositon 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 arrangemant, 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 compostion 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 CO₂, about 22 % by weight MgO and about 31 % by weight CaO. At around 800°C, all of the CO₂ 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 0.10-5.0 % MgO by weight, 0.10-6.0 % CaO by weight. The refractory oxide includes a substantial amount of alumina. The castable is heated to an initial drying out temperature of around 149 - 204°C (300-400°F) before firing to around 1482°C (2700°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 "consisting essentially of" 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 "consisting essentially of," 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 compositons 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 Figure 1 is a graph showing the effect of varying amounts of MgO on the hot modulus of rupture of a refractory at 1482°C (2700°F) when using two different calcium aluminate cements containing different amounts of CaO; Figure 2 is a bar graph showing the effect of varying amounts of MgO on the hot modulus of rupture of a refractory at 1482°C (2700°F) when using different amounts and types of calcium aluminate cements; Figure 3 is a graph showing the effect of varying amounts of MgO from dolomite on the hot modulus of rupture of a refractory at 1482°C (2700°F) when using a low amount of calcium aluminate cement; and Figure 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 1482°C (2700°F) when using a low amount of calcium aluminate cement. Description of Preferred Embodients Unless otherwise specifically indicated, when the term "% by weight" 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 0.10 - 5.0 % by weight MgO, 0.10 - 6.0 % by weight of CaO and the balance alumina. Conventional additives such as dispersants can be used. This is so even though the additive is not specifically listed, and may be important to the processing, casting, shaping and curing of the compostion, 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 abouve, are known to those skilled in the art. Compostions in accordance with the present application may contain between 0 - 40 % by weight calcium aluminate cement. Compostions 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 compositon 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. The alumina may include 60 - 90 % by weight tabular alumina and 1 - 40 % by weight fine alumina powder. Tabular alumina is a refined high purity Al₂O₃ having a very dense grain structure. 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 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 1482°C (2700°F) of at least 16,5 N/mm² (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. HIGH CEMENT REFRACTOPRY ALUMINA CASTABLES WITH AND WITHOUT MgO Formulation, wt. % without MgO with MgO - Tabular Alumina 0,074-6,35 mm (3/200 Mesh) 70.0 67.5 - Fine Alumina -0,074 mm (-200 Mesh) 9.0 9.0 - 80/20 Calcium Aluminate Cement 21.0 21.0 - Sintered Magnesium Oxide 0.0 2.5 Chemical Composition, wt.% - Alumina (Al₂O₃) 93.7 91.0 - Magnesia (MgO) 0.0 2.5 - Lime (CaO) 5.8 5.8 Casting Properties - Water Added, % 6.0 - 7.5 6.0 - 7.5 - Flow Value, % 80 - 100 80 - 100 - Set Time, Hrs. 12 - 24 12 - 24 After Drying 110°C (230°F) - Bulk Density g/cm³ (lbs/ft³) 2.87 (179) 2.82 (176) - Apparent Porosity, % 13 13 - Modulus of Rupture, N/mm² (psi) 18.5 (2700) 17.2 (2500) After Firing, 1599°C (2910°F) - Bulk Density, g/cm³ (lbs/ft³) 2.69 (168) 2.74 (171) - Apparent Porosity, % 27 24 - Modulus of Rupture, N/mm² (psi) 14.4 (2100) 19.2 (2800) - Permanent Linear Change, % + 0.10 + 0.10 Hot Modulus of Rupture, 1482°C (2700°F), N/mm² (psi) 8.2 (1200) 12.3 (1800) LOW CEMENT REFRACTOPRY ALUMINA CASTABLES WITH AND WITHOUT MgO AND WITH DOLOMITE Formulation, wt. % without MGO with MgO with Dolomite - Tabular Alumina 0,074-6,35 mm (3/200 Mesh) 70.0 70.0 66.0 - Fine Alumina -0,074 mm (-200 Mesh) 25.0 21.5 25.0 - 80/20 Calcium Aluminate Cement 5.0 5.0 5.0 - Sintered Magnesium Oxide 0.0 3.5 0.0 - Raw Dolomite 0.0 0.0 4.0 -1.19 mm (-14 Mesh) Chemical Composition, wt.% - Alumina (Al₂O₃) 98.4 95.0 96.2 - Magnesia (MgO) 0.0 3.4 0.9 - Lime (CaO) 1.4 1.4 2.8 Casting Properties - Water Added, % 4.5-5.5 5.5-6.0 4.5-5.0 - Flow Value, % 35-50 35-50 35-50 - Set Time, Hrs. 6-18 6-18 6-18 After Drying 110°C (230°F) - Bulk Density g/cm³ (lbs/ft³) 3.12 (195) 3.03 (189) 3.04 (191) - Apparent Porosity, % 15 17 16 - Modulus of Rupture, N/mm² (psi) 13.0 (1900) 10.3 (1500) 11.7 (1700) After Firing, 1599°C (2910°F) - Bulk Density, g/cm³ (lbs/ft³) 3.0 (187) 2.88 (180) 2.95 (184) - Apparent Porosity, % 21 23 22 - Modulus of Rupture, N/mm² (psi) 25.4 (3700) 28.8 (4200) 26.7 (3900) - Permanent Linear Change, % +0.20 +0.40 +0.70 Hot Modulus of Rupture, - 1482°C (2700°F), N/mm² (psi) 6.86 (1100) 16.5 (2400) 19.9 (2900) - 1371°C (2500°F), N/mm² (psi) 22.6 (3300) - 1093°C (2000°F), N/mm² (psi) 27.4 (4000) - 816°C (1500°F), N/mm² (psi) 18.5 (2700) PRESSED ALUMINA REFRACTORY MATERIAL WITH AND WITHOUT CaO Formulation, wt. % without CaO with CaO - Tabular Alumina 0.074-1.19 mm (14/200 Mesh) 80.5 80.1 - Fine Alumina -0.074 mm (-200 Mesh) 17.5 15.9 - Sintered Magnesium Oxide -0.074 mm (-200 Mesh) 2.0 2.0 - Limestone, -0.074 mm (-200 Mesh) 0.0 2.0 Chemical Composition, wt.% - Alumina (Al₂O₃) 98.0 97.0 - Magnesia (MgO) 2.0 2.0 - Lime (CaO) 0.0 1.0 Pressing Information - Water Added for Pressing, % 3.2 2.9 - Tons of Pressure 100 100 - Bulk Density, g/cm³ (lbs/ft³) 2.80 (175) 2.79 (174) After Firing 1585°C (2885°F) - Bulk Density g/cm³ (lbs/ft³) 2.66 (166) 2.63 (164) - Apparent Porosity, % 30 30 - Permeability, cgs 0.54 0.29 Hot Modulus of Rupture, 1482°C (2700°F), N/mm² (psi) 2.7 (400) 7.5 (1100) Figure 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. Figure 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. Figure 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. Figure 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 occured 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 0.4 - 1.7 % MgO and 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 2 - 5, and most preferably between 2.2 - 3.5. A fired refractory made with the composition of the present application has a hot modulus of rupture at 1482°C (2700°F) of at least 13.7 N/mm² (2000 psi), and most preferably of at least 16.5 N/mm² (2400 psi). The spinel bonding renders the refractory more resistant to corrosion, and to penetration by slags and molten materials. The use of -0.074 mm (-200 mesh) dolomite suggests that slightly less dolomite is required to obtain a hot strength comparable to refractories in which -1.19 mm (-14 mesh) dolomite was used. However, the reheat properties of a refractory made using -0.074 mm (-200 mesh) dolomite were not as good as a refractory made using -1.19 mm (-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 22.9 x 11.4 x 6.4 cm (9 by 4.5 by 2.5 inch) slab was molded with two cup-shaped depressions, each 2.54 cm (one inch) deep and 5.08 cm (two inches) in diameter. All slabs were prefired to 1482°C (2700°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 1538°C (2800°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.

Claims: Spinel bondable refractory composition for burned bricks and castables consisting of alumina as the principal ingredient and compounds providing as a source of MgO and CaO 0.10 - 5.0 % MgO and 0.10 - 6.0 % CaO by weight of the entire composition, excluding SiO₂, said composition when fired forming spinel by reaction between MgO and alumina. Composition of claim 1 wherein the compounds providing MgO and CaO are in the form of a pure oxide, hydrate, carbonate, nitrate, raw dolomite and/or calcium aluminate cement. Composition of claim 2 wherein the raw dolomite content is 0.1 - 12.0 % by weight, especially 2 - 6 % by weight. Composition of claim 2 wherein the content of calcium aluminate cement is 0 - 40 % by weight and especially 5 - 40 % by weight. Castable comprising the refractory composition of claim 1 containing as a source of CaO 0 - 40 % by weight calcium aluminate cement and especially 5 - 40 % by weight calcium aluminate cement. Method of making burned bricks from a mixture comprising the refractory composition of claim 1, forming the bricks and firing the bricks to a temperature for reacting MgO and alumina to form spinel.

Inventors: VEZZA THOMAS FRANCIS, VEZZA, THOMAS FRANCIS
Applicants: NORTH AMERICAN REFRACTORIES, NORTH AMERICAN REFRACTORIES COMPANY
Application year: 1990
Main classification: C04B35
IPC codes: C04B35