Abstract:
The invention is directed to a low density material suitable for use as an expendable liner in a molten metal handling vessel such as tundish. The low density materials include a first component having at least one decomposable compound capable of producing gas, a second component having a reactable compound for reacting with the decomposable compound in the presence of water to cause the decomposable compound to generate a gas, and a refractory aggregate. The invention also relates to a method for making the low density material and to the resultant material as well.

Description:
FIELD OF THE INVENTION 
     This invention relates to low density linings for molten metal handling vessels such as tundishes. 
     BACKGROUND OF THE INVENTION 
     In the continuous casting of molten metals such as steel, molten metal is poured from a ladle into a continuous casting mould via an intermediate vessel called a tundish which acts as a constant head reservoir. The tundish has a metal floor, sidewalls, and one or more outlets set in the floor or sidewall. To protect the metal floor and sidewalls from the molten metal, it is usual to line the floor and sidewalls of the tundish with a relatively permanent lining, often made of bricks or a castable. The tundish additionally may be provided with an inner, expendable lining of refractory. For example, see British patent No. 1,364,665. 
     While it is desirable for the expendable lining to be highly heat-insulating, it is also desirable for the expendable lining to have substantial resistance to erosion and corrosion by molten metal and slag. However, any change in the lining that improves its heat-insulating properties is usually associated with reduced density which tends to lessen erosion/corrosion resistance. 
     Various proposals have been made for expendable linings. For example, U.S. Pat. No. 5,139,239 shows forming an expendable lining in a molten metal handling vessel that has an outer metal casing lined with a permanent lining. Portions of the surface of the permanent lining are covered with a removable material. A slurry of refractory material is applied over both the removable material and the remaining exposed portions of the permanent lining. The slurry of refractory material can be applied by spraying, although gunning or trowelling may be used. The removable material may be in the form of consumable material, for example cardboard, strawboard, fiberboard or polystyrene. Alternatively, the removable material may be in the form of a consumable cellular material, e.g. polyurethane foam. The slurry is dried to form the expendable lining. 
     Traditional expendable liner materials as described above have employed combustible fillers to reduce the density of the formed expendable lining. These filler materials may be organic, inorganic, or synthetic fibers or spheres. The drawbacks to using fillers to reduce the density of the lining material are that the amount of density reduction is limited to the amount of filler added. Also, the temperature required to remove any combustible filler materials can generate undesirable stress in the lining as the combustible materials are oxidized. The filler also can reduce the strength and chemical durability of the resulting lining, and produce undesirable interconnected porosity. The filler can also increase water requirements, thereby necessitating extra drying time and expense, and as well increasing the risk of shrinkage cracking. In addition, the filler may cause handling and flow difficulties. 
     U.S. Pat. No. 5,188,794 shows a lining obtained by application of burnt dolomite in the form of grains mixed with an organic binder such as a thermosetting resin. After application of the lining material, it is heated to decompose the carbonate and/or the binder of the composition to provide a porous layer. 
     Although the methods of the art have provided porous materials which are useful as expendable linings such as on vertical surfaces, these methods have required high temperatures to produce a porous lining. Also, these methods often yield undesirable interconnected porosity. A need therefore exists for expendable linings which can be formed without the high temperature requirements of the art, and which avoid interconnected porosity. 
     SUMMARY OF THE INVENTION 
     According to the invention, a composition suitable for providing a low density expendable liner is provided. This composition comprises first component which generates a gas such as SO 2 , H 2 , or CO 2 , preferably CO 2 , when the first component decomposes, and a second component for causing the first component to decompose in the presence of water to generate the gas, and a refractory aggregate. The first component can include compounds such as sodium bicarbonate, calcium carbonate, dolomitic carbonate, barium carbonate, preferably calcium carbonate, to provide a source of CO 2 . More preferably, the first component includes calcium carbonate in an amount of about 1-5 percent by weight of the composition. 
     Examples of materials useful as the first component to generate SO 2  include but are not limited to aluminum sulfate, and calcium lignosulfonate; examples of materials useful as the first component to generate H 2  include but are not limited to metal powders such as Fe-Si alloys of -150 mesh, Al-Si alloys of -20 mesh; Si alloys of -325 mesh; Mg-Al alloys of -200 mesh; and Al alloys of -325 mesh. Combinations of these materials, such as blends of aluminum sulfate and calcium carbonate, to generate a mixture of CO 2  and SO 2  gases also can be used. 
     The second component employed in the composition of the invention may be any compound capable of reacting with the first component to cause the first component to generate a gaseous product. Preferably, acidic materials such as aluminum sulfate are useful as the second component, and are present in an amount of about 1/3-5/3 the weight of the first component. A preferred expendable liner composition includes three percent of calcium carbonate as the first component and 1-5 percent of aluminum sulfate as the second component, based on the total weight of the composition. 
     The invention further relates to a method for producing a low density lining material on a substrate such as the permanent lining of a tundish. The method entails applying the expendable liner composition of the invention to the substrate by methods such as spraying or gunning to produce a green, porous layer on the substrate. The green porous layer readily can be dried and heat treated to yield an expendable liner. 
     Having briefly summarized the invention, the invention will now be described in detail by reference to the following specification and non-limiting examples. Unless otherwise specified, all percentages are by weight and all temperatures are in degrees Celsius. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a gunning system for applying the compositions of the invention to provide an expendable liner. 
     FIG. 2 shows a spraying system for applying the compositions of the invention to provide an expendable liner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, a low density lining according to the invention is formed from a mixture which contains a refractory aggregate and a composition that includes a first component having at least one decomposable compound capable of producing gas when the first component decomposes. The composition further includes a second component having a reactable compound for reacting with the decomposable compound in the presence of liquid, preferably water, to cause the decomposable compound to generate a gaseous product. A sufficient amount of liquid is provided to the composition to enable the first and second components to react to internally generate gas in the composition to reduce the density of the composition. 
     The decomposable compound of the first component preferably is a material which, when added to water, yields a pH above 7. Materials useful as decomposable compounds include carbonates in which CO 2  can be evolved therefrom by a reactable component such as an acid. Useful carbonates include but are not limited to calcium carbonate, dolomitic carbonate, and the like. Reactable materials useful as the second component include cation type acids such as aluminum sulfate, chromium sulfate, citric acid, tartaric acid, and organic acids. 
     The particle size distribution of the decomposable compound of the first component depends on the method for applying the composition to a substrate. If the composition is applied by spraying, the particle size distribution of the decomposable compound, for example, calcium carbonate, is ≦8 mesh. If the composition is intended to be applied by gunning, the particle size of the decomposable compound such as calcium carbonate is ≦50 mesh, preferably ≦200 mesh. 
     The refractory aggregate employed may be an acidic, neutral, or basic water insoluble refractory such as olivine, silica, siliceous sand, chamotte, graphite, alumina, corundum, mullite, spinel, dolomite, magnesia, calcia, chromia, or zirconia as originated from brick or ore. The aggregate typically is present in the range of 45 to about 97% of the composition. The composition may be combined with at least one setting agent to increase the strength of the applied material. Useful setting agents include inorganic phosphates, lime, sodium acid phosphates, sodium silicate and the like, and mixtures thereof. 
     The low density expendable linings can be provided in a range of thicknesses depending on the application method used. Since the expendable lining formed by the invention has high strength as well as high porosity, the thickness of the lining can be reduced as compared with linings formed by conventional heat treating processes. 
     Useful non-limiting compositions illustrative of the types of inorganic materials and acids useful as the first and second components in the invention are given in Table 1. 
     
                       TABLE 1______________________________________Example  First Component                Second Component                               WaterNo.    (g)           (g)            (g)______________________________________1      Dolomitic     Powdered Citric                               16.0  Limestone (2.0)                Acid (4.0)2      Dolomitic     Powdered Citric                               24.0  Limestone (2.0)                Acid (8.0)  High CaO  Limestone (2.0)3      Dolomitic     Granular Citric                               16.0  Limestone (2.0)                Acid (4.0)  High CaO  Limestone (2.0)4      Dolomitic     Granular Citric                               24.0  Limestone (2.0)                Acid (8.0)  High CaO  Limestone (2.0)5      Dolomitic     Powdered Citric                               24.0  Limestone (2.0)                Acid (4.0)  High CaO      Granular Citric  Limestone (2.0)                Acid (4.0)6      High CaO      Sulfamic       16.0  Limestone (4.0)                Acid (4.0)7      Dolomitic     Sulfamic       16.0  Limestone (2.0)                Acid (4.0)8      Dolomitic     Sulfamic       24.0  Limestone (2.0)                Acid (4.0)  High CaO  Limestone (2.0)9      Dolomitic     Malic Acid (4.0)                               16.0  Limestone (2.0)  High CaO  Limestone (2.0)10     Albaglos.sup.1 (4.0)                Oxalic Acid (4.0)                               16.011     Dolomitic     Oxalic Acid (4.0)                               12.0  Limestone (2.0)12     Dolomitic     Oxalic Acid (2.0)                               16.0  Limestone (2.0)______________________________________ .sup.1 Precipitated calcium carbonate from Minerals Technologies Co., Inc., New York, New York. 
    
     EXAMPLES 13-24 
     Examples 13-24 illustrate various combinations of carbonate materials (first component) and acid (second component), and refractory aggregate useful in the invention. 
     
                       TABLE 2______________________________________Example  Refractory  CarbonateNo.    Aggregate(g)              (g)       Acid(g) Water(%)______________________________________13     Base mix.sup.1              DF5025.sup.2                        PCA.sup.5                                12.0  (94.0)      (5.0)     (1.0)14     Base mix.sup.1              DF2055.sup.3                        PCA     12.0  (94.0)      (5.0)     (1.0)15     Base mix.sup.1              DF5025    PCA     12.0  (93.0)      (5.0)     (1.0)16     Base mix.sup.1              DF2055    PCA     12.0  (93.0)      (5.0)     (2.0)17     Base mix.sup.1              MS LIME.sup.4                        PCA     12.0  (93.0)      (5.0)     (2.0)18     Base mix.sup.1              DF5025    PCA     12.0  (92.0)      (5.0)     (3.0)19     Base mix.sup.1              DF2055    PCA     12.0  (92.0)      (5.0)     (3.0)20     Base mix.sup.1              DF5025    PCA     12.0  (91.0)      (5.0)     (4.0)21     Base mix.sup.1              DF2055    PCA     12.0  (91.0)      (5.0)     (4.0)22     Base mix.sup.1              DF2055    GCA.sup.6                                12.0  (94.0)      (5.0)     (1.0)23     Base mix.sup.1              DF5025    GCA     12.0  (93.0)      (5.0)     (2.0)24     Base mix.sup.1              MS LIME   GCA     12.0  (93.0)      (5.0)     (2.0)______________________________________ .sup.1 Mixture of 21.5% Chinese 9010 MgO, 8 × 18 m; 41.3% Chinese 9010 MgO, -18 m; 26.7% Chinese 9010 MgO, pulverized; 7.9% Ube MgO, pulverized; 2.1% Bentonite, 0.5% Calcium Ligno Sulfonate. .sup.2 DF5025 = Dolofill 5025 limestone, from Minerals Technologies, New York, New York. .sup.3 DF2055 = Dolofill 2055 limestone, from minerals Technologies, New York, New York. .sup.4 MS Lime = Ca(OH).sub.2 .sup.5 PCA = Powdered citric acid .sup.6 GCA = Granular citric acid 
    
     EXAMPLES 25-38 
     Examples 25-38 illustrate the effects of varying the amount of granular citric acid (second component) with varying size of MW Limestone (first component). 
     
                       TABLE 3______________________________________                         Granular  Refractory MW          CitricExample  Aggregate  Limestone.sup.2                         AcidNo.    (g)        (g)         (g)     Water %______________________________________25     94.0       +50 mesh    1.0     20.0             5.026     94.0       50 × 100 mesh                         1.0     22.0             5.027     94.0       50 × 200 mesh                         1.0     22.0             5.028     94.0       50 × 325 mesh                         1.0     22.0             5.029     93.0       +50 mesh    2.0     22.0             5.030     93.0       50 × 100 mesh                         2.0     22.0             5.031     93.0       50 × 200 mesh                         2.0     22.0             5.032     93.0       50 × 325 mesh                         2.0     22.0             5.033     92.0       +50 mesh    3.0     22.0             5.034     92.0       50 × 100 mesh                         3.0     22.0             5.035     92.0       50 × 200 mesh                         3.0     22.0             5.036     92.0       50 × 325 mesh                         3.0     22.0             5.037     91.0       +50 mesh    4.0     22.0             5.038     91.0       50 × 100 mesh                         4.0     22.0             5.0______________________________________ .sup.1 Mixture of 21.5% Chinese 9010 MgO, 8 × 18 m; 41.3% Chinese 9010 MgO, -18 m; 25.7% Chinese 9010 MgO, pulverized; 7.9% Ube MgO, pulverized; 2.1% Bentonite, 0.5% Calcium Ligno Sulfonate; 1.0% coarse paper fibers. .sup.2 MW Limestone = marble white limestone from Minerals Technologies, New York, New York. 
    
     EXAMPLES 39-50 
     Examples 39-50 illustrate various compositions useful for providing low density expendable linings in accordance with the invention. Each of these compositions are prepared by hand mixing. 
     
                                           TABLE 4__________________________________________________________________________Example/Material (%)    39 40 41 42 43 44 45 46 47 48 49 50__________________________________________________________________________MB3-1.sup.3    -- 95.0.sup.1          93.0.sup.1             -- 92.5.sup.1                   -- 91.5.sup.1                         -- 92.0                               91.5                                  92.0                                     91.5MB1-3.sup.4    95.0.sup.1       -- -- 93.0.sup.1                -- 92.5.sup.1                      -- 91.5.sup.1Bentonite    --    2.0.sup.1             2.0.sup.1                2.0.sup.1                   2.0.sup.1                      2.0.sup.1                         2.0.sup.1                            2.0                               2.0                                  -- --Pwd. Al. 2.5.sup.1       2.5.sup.1          2.5.sup.1             2.5.sup.1                2.5.sup.1                   2.5.sup.1                      2.5.sup.1                         2.5.sup.1                            2.5                               3.0                                  2.5                                     3.0SulfateMW Limestone.sup.5    2.5.sup.1       2.5.sup.1          2.5.sup.1             2.5.sup.1                2.5.sup.1                   2.5.sup.1                      2.5.sup.1                         2.5.sup.1                            3.0                               3.0                                  3.0                                     3.0(-325m)Monocalcium    -- -- -- -- 0.5.sup.1                   0.5.sup. 1                      0.5.sup.1                         0.5.sup.1                            0.5                               0.5                                  0.5                                     0.5PhosphateCoarse Paper    -- -- -- -- -- -- 1.0.sup.1                         1.0.sup.1                            -- -- -- --FibersWater    26.sup.2       15.sup.2          15.sup.2             26.sup.2                15.sup.2                   26.sup.2                      20.sup.2                         30.sup.2                            15 15 15 15Set Time 1.5       .25          .25             2.0                .25                   2.5                      2  3.5                            .167                               .167                                  .167                                     .167MinutesAluminum -- -- -- -- -- -- -- -- -- -- 2.0                                     2.0HydrateDensity  96.4       82.1          -- -- -- -- -- -- -- 77.9                                  89.2                                     78.0(lb/ft.sup.3)__________________________________________________________________________ .sup.1 % by weight of Total Solids .sup.2 % by weight of overall composition .sup.3 Refractory Aggregate of Composition of 25% Chinese 9010 MgO, 8 × 18 m; 35% Chinese 9010 MgO, -18 m; and 40% Chinese 9010 MgO, pulverized. .sup.4 Refractory Aggregate of Composition of 100% Ube 95 MgO, -40 mesh. .sup.5 MW Limestone = marble white limestone from Minerals Technologies, New York, New York. 
    
     The method of providing and applying the compositions of the invention to provide an expendable liner will now be explained in detail by reference to the drawings where like numerals indicate like components. 
     In FIG. 1, a gunning system 1 for applying the compositions of invention to a vertical substrate such as the permanent lining of tundish to form an expendable liner thereon is shown. As shown therein, composition 5 of the invention that includes the aforementioned first and second components, and which is substantially free of water is provided in feed hopper 10. Composition 5 is transported by pressurized air provided to end portion 15A of hose 15 by a compressed air generator (not shown). The pressurized air forces composition 5 to move through hose 15 to contact a substrate 30 such as the permanent lining of a tundish to form expendable lining 25 thereon. Prior to exiting hose 15, however, composition 5 is contacted by water distributed by water ring nozzle 20 on hose 15 to achieve a moisture content of 10-15% in composition 5 prior to contacting substrate 30. The aforesaid first and second components of composition 5, after having been formed into a layer on substrate 30, react in the presence of the moisture provided by water ring nozzle 20 to generate a gas within the layer causing the material to expand to thereby yield a porous, green liner. The porous, green liner is dried prior to exposure to high temperature corrosive environments such as those produced by steel. 
     In an alternative embodiment as shown in FIG. 2, spraying system 2 may be used to produce an expendable lining formed from the compositions of the invention. As shown in FIG. 2, composition 5 of the invention that includes the aforementioned first and second components, and which is substantially free of water is provided in feed hopper 35 for supply to wet mixing chamber 40 wherein water provided by a water source (not shown) is added to composition 5 to achieve a moisture content of 20-30%. The resulting moist composition 5&#39; is transported to mixing/feed chamber 45. Mixing/feed chamber 45 is provided with feed screw 50 for mixing and feeding of composition 5&#39; through supply hose 55 to air injector nozzle 60. Composition 5&#39; exits nozzle 60 for spraying against wall 70 to form lining 65. The aforesaid first and second components of composition 5&#39;, after having been formed into a layer on wall 70, react to generate a gas within the layer to thereby yield a porous, green liner. The porous, green liner is dried prior to exposure to high temperature corrosive environments such as those produced by molten steel. 
     In an alternative embodiment of spraying system 2, the second component containing a reactable compound may be added to the first component while the first component is transported through hose 15. The second component can be supplied to the first component in the hose through, for example, a valve (not shown) positioned on the side of hose 15. Such valves are known in the art. The amount of the reactable compound in the second component can be determined from stoichiometry to yield the desired extent of reaction of the first and second components. The specific pressures required to spray or gun the compositions of the invention onto the substrate also can readily be determined by the art skilled. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent.