Abstract:
A refractory composition characterized by relatively high abrasion resistance, good flowability and long working time, comprised of 0.5 to 5 weight percent volatilized silica, 3.0 to 15 weight percent -65 mesh alumina, 20-40 weight percent cement, and the balance a refractory aggregate. Better results are obtained with a composition comprised of from 30-40 weight percent cement.

Description:
This is a continuation-in-part application of U.S. Ser. No. 748,260 filed 06/24/85. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a refractory composition characterized by high abrasion resistance, and in particular, to such a composition which may be used as a refractory castable. 
     Refractory castables are hydraulic setting compositions. They comprise granular refractory aggregates and chemical binders. The refractory castables are shipped in dry form, and when mixed with water to the desired consistency, may be poured like concrete, tamped or rammed into place, troweled or applied with an air gun. Refractory castables take a strong hydraulic set at room temperatures and maintain good strength until the desired ceramic bond is developed as the temperature is increased. Castables are specially suited for furnace linings of irregular contours, for patching brick work and for casting special shapes which may be urgently required. Numerous castable compositions are known, with each of the known compositions, having different properties, making each one useful for different applications. 
     One such application involves the use of refractory castables in lining transfer lines employed in fluid catalytic cracking units used in petrochemical processes. In such units, highly abrasive catalysts travel at high speeds, thereby creating extreme erosion potential throughout the catalytic cracking unit. In such units, early abrasion resistant linings were formed from phosphate bonded refractories, which required extensive anchoring and hand ramming to install. To reduce the expense of installing phosphate bonded refractories, the refining industry began using castables with field additions of stainless steel fibers which required less anchoring on the metal shell, and which could be poured relatively quickly. Although the foregoing improved on the time and cost of installation, increased abrasion resistance was desired. 
     Abrasion resistant linings in petrochemical vessels are typically chemically bonded or cement bonded refractory compositions. Abrasion resistance is generally obtained by utilizing a strong, dense refractory grain such as calcined fireclay, and a strong bond consisting of aluminum orthophosphate, or calcium aluminate cement. In the case of cement, the abrasion resistant bond is achieved by using large amounts of cement, or a combination of fumed silica, cement in amounts less than ten percent, and a surface active agent which allows flow at low water contents. Improved density, which is achieved by casting at low water contents, results in a highly abrasion resistant bond at low cement levels. 
     Trying to effect further economies in installation, operators of the fluid catalytic cracking units started casting larger sections of transfer lines, eliminating the assembly of many smaller sections. The refractory castables used on the transfer lines were made with relatively fast setting cements, and did not stay flowable a sufficient time for use in such applications. Refractory manufacturers reformulated their abrasion resistant castables to incorporate casting grade cements to lengthen working time. These products provided the flowability and working time needed, but strengths and abrasion resistance were often lower than similar mixes containing regular calcium aluminate cement. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a refractory composition characterized by good flowability and relatively long working times, with improved densities, strength and abrasion resistance. The foregoing objective is achieved in a refractory composition comprised of 0.5-5%, by weight, volatilized silica; 3.0-15.0%, by weight, -65 mesh alumina; 20-40%, by weight, calcium aluminate cement; and the balance a refractory aggregate selected from the class consisting of silica, alumina, or fireclay. Better results are obtained with a refractory composition wherein the range of calcium aluminate cement is from 30-40%. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The utilization of refractory castable compositions in highly abrasive environments, requires the castables to have excellent abrasion resistant properties. In addition, when used to line relatively large members, such as lining of transfer lines used in fluid catalytic cracking units, the castable should have good flowability and relatively long working times, so that the castable can be installed. 
     A first series of mixes was prepared (see Table I below). In describing the various mixes, all percentages will be on a weight percent basis unless otherwise indicated. This mix series shows the effects of increasing the fine alumina content from 0-15% in a fireclay castable. As may be observed, density, strength and abrasion resistance improve as the alumina content increases. Abrasion resistance improvement from 8.4 cc loss in the mix having 0% alumina to 7.0 cc loss in the mix having 15% alumina, is significant for a type of refractory which has relatively good abrasion resistance before the alumina addition. Fine, synthetic aluminas are commonly used in the refractory industry to improve the refractoriness of the bonding portion of the refractory, and as a source of fine material to insure that the refractory has a proper grain size distribution. 
     
                       TABLE I______________________________________Mixes Containing A-17 Reactive AluminaWithout Volatilized SilicaMix Designation: B       C       D     E______________________________________Mix:Calcined Super Duty Flint,            50%     50%     50%   50%-3 meshCalcined Super Duty Flint,            20      17      11    5BMFA-17 Reactive Alumina, -325            0       3       9     15meshCA-25 C Casting Grade            30      30      30    30CementCasting Water Required, %:            8.7     8.6     8.4   8.2Bulk Density, pcfAfter Drying at 250° F.:            144     145     150   152After Heating 5 Hrs.            137     139     141   145at 1500° F.:Cold Crushing Strength After            14,260  15,940  16,880                                  16,900Heating 5 Hrs. at 1500° F., psi:Abrasion Test (ASTM C-704)            8.4     7.2     7.1   7.0Volume Loss of SamplesHeated 5 Hrs. at 1500° F.,cu cm:______________________________________ 
    
     A second series of mixes were made to determine the effect of adding 0-5% volatilized silica to the same abrasion resistant fireclay castable. Volatilized silica additions of 0.5% and 2% resulted in improved densities, strength and abrasion resistance. At volatilized silica levels of 3% and 5%, the mix became sticky and did not flow as well as previously. Densities and strength suffered, but the improved abrasion resistance is maintained. Volatilized silica is a sub-micron, amorphous bi-product of ferrosilicon production and is a well known refractory raw material. It is used primarily as a source of ultrafine particles, as a source of reactive silica and as an additive to improve flow properties. As Table II illustrates, only small amounts can be used in cement containing mixes or flow properties would be adversely affected. 
     
                                           TABLE II__________________________________________________________________________Mixes Containing Volatilized Silica Without Fine AluminaMix Designation:   B    F   G    H   J__________________________________________________________________________Mix:Calcined Super Duty Flint, -3 mesh              50%  50% 50%  50% 50%Calcined Super Duty Flint, BMF              20   19.5                       18   17  15Volatilized Silica --   0.5 2    3   5CA-25 C Casting Grade Cement              30   30  30   30  30Casting Water Required, %:              8.7  8.0 7.9  7.8 7.6Casting Characteristics:              At volatilized silica levels up to 2%, the mix              flowed well during vibration casting. At a 3%              volatilized silica level, the mix flowed well,              but was sticky. At a 5% volatilized silica              level, the mix had poor flow during casting, was              very sticky to the point that handling was              difficult, and dried out quickly, making large              installations difficult.Bulk Density, pcfAfter Drying at 250° F.:              144  147 147  147 145After Heating 5 Hrs. at 1500° F.:              137  140 140  138 138Cold Crushing Strength After Heating              14,260                   16,800                       16,290                            12,730                                11,4205 Hrs. at 1500° F., psi:Abrasion Test (ASTM C-704)              8.4  6.8 6.4  6.1 6.3Volume Loss of Samples Heated5 Hrs. at 1500° F., cu cm:__________________________________________________________________________ 
    
     A third series of mixes were made to determine the effects of adding volatilized silica to a fireclay castable containing fine alumina. As the alumina is replaced by up to 2% silica, density, strength and abrasion resistance improve. At a silica level of 3%, the mix becomes sticky and flow is impaired. The abrasion resistance, however, continues to improve. Mix P has outstanding abrasion resistance, but mix N is preferred because of its superior flowability, a necessary property when the composition is employed as a refractory castable for use in relatively large applications. The synergistic effect of using volatilized silica and fine alumina together should be noted. The abrasion resistances of mixes N and P are superior to any of the mixes set forth in Tables I and II, where each material was used separately. 
     
                                           TABLE III__________________________________________________________________________Evaluation of Alumina/Silica Ratio when Fine Alumina + Volatilized SilicaEquals 10%Mix Designation:   K     L     N     P__________________________________________________________________________Mix:Calcined Super Duty Flint, -3 mesh              50%   50%   50%   50%Calcined Super Duty Flint, BMF              10    10    10    10A-17 Reactive Alumina, -325 mesh              10    9     8     7Volatilized Silica --    1     2     3CA-25 C Casting Grade Cement              30    30    30    30Casting Water Required, %:              9.4   8.15  8.0   8.65Casting Characteristics:              All mixes flowed well during vibration casting.              At a volatilized silica level of 3%, the mix              was sticky and difficult to handle.Bulk Density, pcfAfter Drying at 250° F.:              141   148   151   141After Heating 5 Hrs. at 1500° F.:              135   140   141   136Cold Crushing Strength After Heating              6,990 12,460                          12,440                                9,6605 Hrs. at 1500° F., psi:Abrasion Test (ASTM C-704)              10.1  6.7   5.7   4.6Volume Loss of Samples Heated5 Hrs. at 1500° F., cu cm:__________________________________________________________________________ 
    
     A fourth series of mixes were made, with each of the mixes being compounded according to the teachings of the present invention. Each of the mixes contained 2% volatilized silica and 8% fine alumina of three different types. All four mixes had high cold crushing strengths and outstanding abrasion resistance. The A-17 and A-15 reactive aluminas are almost entirely composed of fine, sintered corundum (alpha-alumina) crystals. Their high surface area and small crystal size makes them thermally reactive, that is, they will further sinter or react with other compounds at relatively low temperatures. T-61 tabular alumina is also essentially 100% corundum crystals, but this material has been fired to a high temperature, resulting in coarse, tablet-shaped, non-reactive crystals. A-2 calcined alumina is about 90% corundum crystals and 10% beta-alumina (Na 2  O.11Al 2  O 3 ) crystals. The thermal reactivity of the A-2 calcined alumina is between tabular alumina and reactive alumina. Table VIII lists the various properties of these aluminas. 
     
                       TABLE IV______________________________________Alumina Type EvaluationMix Designation: N       Q       R     S______________________________________Mix:Calcined Super Duty Flint,            50%     50%     50%   50%-3 meshCalcined Super Duty Flint,            10      10      10    10BMFA-17 Reactive Alumina,            8       --      --    ---325 meshA-15 Reactive Alumina,            --      8       --    ---325 meshT-61 Tabular Alumina,            --      --      8     ---325 meshA-2 Calcined Alumina,            --      --      --    8-325 meshVolatilized Silica            2       2       2     2CA-25 C Casting Grade            30      30      30    30CementCasting Water Required, %:            8.0     7.9     8.0   8.3Bulk Density, pcfAfter Drying at 250° F.:            151     149     150   149After Heating 5 Hrs.            141     142     142   142at 1500° F.:Cold Crushing Strength After            12,440  11,320  14,410                                  15,960Heating 5 Hrs. at 1500° F., psi:Abrasion Test (ASTM C-704)            5.7     5.3     5.6   5.4Volume Loss of SamplesHeated 5 Hrs. at 1500° F.,cu cm:______________________________________ 
    
     Three further mixes were made according to the invention with 1% volatilized silica and 9% fine alumina. Mix T was based on a calcined fireclay grain. This type of mix would be used where good abrasion resistance is necessary. Mix U is based on a vitreous silica grain. This mix would be used where a combination of good abrasion resistance and low thermal conductivity are desired. Mix V is based on coarse, tabular alumina, and represents the ultimate strength and abrasion resistance. Since tabular alumina is over ten times more expensive than calcined fireclay, the increased cost may not be justified by the modest property improvements. The three mixes are intended to illustrate the types of base grains which may be used from 100% silica to a fire clay of roughly 50% silica and 45% alumina to 100% alumina. There are a variety of high alumina grains having alumina contents between fireclay and tabular alumina, such as calcined bauxitic kaolin, calcined bauxite, kyanite and andalusite, which would also work satisfactorily in this invention. In addition, non-aluminosilicates such as silicon carbide, silicon nitrides or any acid aggregate would be satisfactory. 
     
                       TABLE V______________________________________Base Grain EvaluationMix Designation:     T       U       V______________________________________Mix:Calcined Super Duty Flint,                60%     --      ---3 mesh through finesVitreous Silica, -3 mesh through fines                --      60%     --Tabular Alumina, -6 mesh through                --      --      60%finesCalcined Super Duty Flint, BMFA-17 Reactive Alumina                9       9       9Volatilized Silica   1       1       1CA-25 C Casting Grade Cement                30      30      30Casting Water Required, %:                8.15    7.9     8.2Bulk Density, pcfAfter Drying at 250° F.:                148     130     176After Heating 5 Hrs. at 1500° F.:                140     123     166Cold Crushing Strength After Heating                12,460  6,890   16,7705 Hrs. at 1500° F., psi:Abrasion Test (ASTM C-704)                6.7     12.6    6.5Volume Loss of Samples Heated5 Hrs. at 1500° F., cu cm:______________________________________ 
    
     The next series of mixes was intended to show the effects of variations of the cement content. As may be observed, as cement content increases from 10 to 40%, cold crushing strength and abrasion resistance generally improved. 
     
                                           TABLE VI__________________________________________________________________________Mix Designation:           W  A  B   C   X   D   Y__________________________________________________________________________Mix:Calcined Super Duty           70%              65%                 60% 55% 50% 50% 50%Flint Clay, -3 meshCalcined Super Duty           10%              10%                 10% 10% 10% 5%  --Flint Clay, BMFA-17 Reactive Alumina           8  8  8   8   8   8   8Volatilized Silica           2  2  2   2   2   2   2CA-25C Casting Grade Cement           10 15 20  25  30  35  40Casting Water Required, %:           8.0              8.0                 7.9 8.2 8.0 9.2 9.0Bulk Density, pcfAfter Drying at 250° F.:           138              141                 145 147 151 150 151After Heating 5 Hours           136              138                 140 140 141 139 141at 1500° F.:Cold Crushing Strength           5,600              9,610                 11,950                     12,410                         12,440                             13,050                                 12,420After Heating 5 Hoursat 1500° F., psi:Abrasion Resistance:           14.9              9.5                 7.3 6.8 5.7 5.3 5.5(ASTM C-704) Volume Lossof Samples Heated 5 Hoursat 1500° F., cu cm:__________________________________________________________________________ 
    
     The last series of mixes establishes the upper and lower lower limits of volatilized silica and fine alumina contents as well as illustrates the preferred mix which was chosen for its good combination of flow properties and physical properties. 
     
                       TABLE VII______________________________________Fine Alumina and Volatilized Silica RangesWith Preferred Mix          Mix Designation:          Z     X       AA______________________________________Mix:Calcined Super Duty            50%     50%     50%Flint Clay, -3 meshCalcined Super Duty            4.5     10      12Flint Clay, BMFA-17 Reactive Alumina,            15      8       3-325 meshVolatilized Silica            0.5     2       5CA-25C Casting   30      30      30Grade CementCasting Water Required, %:            8.4     8.0     7.6CastingCharacteristics: The mix containing 0.5% volatilizedsilica had acceptable flow properties duringcasting, but not as good as the mix containing 2%volatilized silica. The mix containing 5%volatilized silica had poor flow, was difficult tohandle because of its stickiness and dried outquickly during casting.Bulk Density, pcfAfter Drying at 250° F.:            154     151     149After Heating 5 Hrs      145     141     141at 1500° F.:Cold Crushing Strength   15,050  12,440                                  11,540After Heating 5 Hrsat 1500° F., psi:Abrasion Resistance (ASTM                    6.5     5.7   4.9C-704) Volume Loss ofSamples Heated 5 Hrsat 1500° F., cu cm:______________________________________ 
    
     
                       TABLE VIII______________________________________Alumina Brand:        A-17     A-15     T-61   A-2Manufacturer:        ALCOAAlumina Type:        Reactive Reactive Tabular                                 CalcinedChemical Analysis(Typical)Alumina (Al.sub.2 O.sub.3)        99.5+%   99.5+%   99.5+% 99.2%Silica (SiO.sub.2)        0.02     0.02     0.06   0.02Iron Oxide (Fe.sub.2 O.sub.3)        0.02     0.02     0.06   0.04Soda (Na.sub.2 O)        0.07     0.07     0.10   0.45Ultimate Crystal Size,        3-3.5    2-2.5    10+    3-5microns:MineralogicalCompositionCorundum, %  100      100      100    90Beta Alumina, %        --       --       --     10Particle SizeDistribution% Finer than 60        100%     100%     100%   100%microns:% Finer than 40        100      100      99     100microns:% Finer than 20        100      100      80     94microns:% Finer than 10        98       98       56     86microns:% Finer than 5        87       90       37     74microns:% Finer than 1        21       24       9      9micron:% Finer than 0.5        12       15       2      4microns:______________________________________ 
    
     The composition of the present invention provides a refractory castable which may be used in applications requiring high abrasion resistance, good flowability and long working times. Such properties are required in lining transfer lines of fluid catalytic cracking units. 
     In the present specification, all percents have been provided on a weight percent basis, and all mesh sizes have been determined in acordance with Taylor Standard Series. 
     While only calcined clay has been used in forming the refractory aggregate for each of the mixes, other refractory aggregates such as silica, and alumina as well as other acid aggregates, can also be used. 
     While a preferred embodiment of the present invention has been described and illustrated, the invention should not be limited thereto but may be otherwise embodied within the scope of the following claims.