REFRACTORY CASTABLES WITH HYDROPHOBIC AGGREGATES

Abstract

Hydrophobic aggregates for use in refractory castables and gunning mixtures and methods of their preparation. The aggregates here are formed by crushing insulating fire brick and coating the resulting particles with a hydrophobic component. The hydrophobic component may be a polydimethylsiloxane having a terminal silanol group. As a result of the coating process, the coated aggregate has very low levels of alkalis. The aggregates may be used to form refractory castables that do not undergo substantial alkaline hydrolysis due to the reduced levels of alkalis. The castables made from these aggregates display superior physical properties, including lower water content, lower permanent linear change, high strength, and superior thermal conductivity/insulation properties, while at the same time possessing lower density and requiring less water to be used during castable formation. These improved properties also are observed in gunning mixtures formed from these aggregates.

Claims

1. A method of preparing hydrophobic aggregates for use in refractory castables, comprising: (a) creating insulating fire brick (IFB); (b) crushing the IFB to a desired particle size to form an aggregate; (c) coating the aggregate with a hydrophobic component; and (d) drying the coated aggregate to less than 1% moisture.

2. The method of claim 1, wherein the IFB comprises granite/basalt, emery, olivine, chamotte, expanded chamotte, molochite, sillimanite, brown-fused alumina, white-fused alumina, tabular alumina, bubble alumina, calcined alumina, pumice, diatomite, vermiculite, perlite, clay, calcined clay, silica fume, spinel, magnesia, dolomite, silicon carbide, and combinations thereof.

3. The method of claim 1, wherein said hydrophobic component is silicone-based, a nanoscale ceramic, or a siloxane.

4. The method of claim 1, wherein said hydrophobic component is polydimethylsiloxane.

5. The method of claim 4, wherein the polydimethylsiloxane is present as emulsified silanol-terminated polydimethylsiloxane.

6. The method of claim 5, further comprising the step of diluting the emulsified polydimethylsiloxane in water prior to the coating step to a dilution of about 0.5 parts water to about 99.5 parts emulsified polydimethylsiloxane to about 99.5 parts water to about 0.5 parts emulsified polydimethylsiloxane.

7. The method of claim 5, wherein the coating step uses from about 1 to about 3 pounds of emulsified polydimethylsiloxane per pound of aggregate.

8. The method of claim 1, wherein substantially all of the aggregate is substantially covered with said hydrophobic component.

9. The method of claim 1, wherein the coated aggregate has less than about 0.45% by weight alkalis measured as sodium oxide.

10. A refractory castable, comprising: aggregate coated with a hydrophobic component; a cement; and optionally a filler

11. The refractory castable of claim 10, wherein the filler is a clay.

12. The refractory castable of claim 10, wherein said hydrophobic component is polydimethylsiloxane.

13. The refractory castable of claim 10, wherein the refractory castable does not undergo substantial alkaline hydrolysis.

14. The refractory castable of claim 10, comprising less than about 0.45% by weight alkalis, measured as Na.sub.2O.

15. The refractory castable of claim 10, wherein the refractory castable is rated for use at a temperature between about 2300 F. and about 3200 F.

16. The refractory castable of claim 10, having a water content of about 20 to about 50% lower than a prior art castable using non-coated aggregate.

17. The refractory castable of claim 10, having a permanent linear change about 50% less than a prior art castable using non-coated aggregate.

18. The refractory castable of claim 10, having a density between about 75 lb/ft.sup.3 and about 95 lb/ft.sup.3 and a heat storage value between about 30,000 BTU/ft.sup.2 and about 45,000 BTU/ft.sup.2 for a 12 thick block.

19. The refractory castable of claim 10, having a density between about 75 lb/ft.sup.3 and about 95 lb/ft.sup.3 and k-values from about 3 to about 5 BTU-in/ft.sup.2-hr- F.

20. The refractory of claim 19, wherein said refractory castable does not include perlite,

21. The refractory castable of claim 10 adapted for use in furnaces, fired heaters, flues, kilns, catalytic cracking reactors, and flue gas treatment reactors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein like reference characters designate the same or similar elements, which figures are incorporated into and constitute a part of the specification, wherein:

[0024] FIG. 1 is a schematic of an exemplary process by which the coated aggregates of the present invention may be formed.

DETAILED DESCRIPTION OF THE INVENTION

[0025] It is to be understood that the figure and description of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that are well known. The detailed description will be provided herein below with reference to the attached drawing.

[0026] The present invention addresses the limitations currently existing within the art and provides refractory castables having superior combinations of physical and insulation properties, while at the same time avoiding commonly encountered degradation processes. The castables of the present invention are formed with aggregate that is substantially coated with a hydrophobic material. The coated aggregate is able to be mixed in standard castable formulations while maintaining those hydrophobic properties. Additionally, the aggregates of the present invention bond with common components of castables, including calcium aluminate cements, Portland cements, hydratable aluminas, and phosphate binders, thus not requiring any special treatment after aggregate generation. Because the aggregate is coated with a hydrophobic component, the amount of water required to form the castable form is dramatically reduced. That provides the further benefit of reducing the time needed for dry out during initial heat up in industrial applications, as well as lowering the water-to-cement ratio of the castable, thereby improving the strength of the castable.

[0027] Refractory castables formed from those aggregates display the unique combination of low density and excellent thermal conductivity properties. Thus, the castables of the present invention represent a novel refractory castable for industry.

[0028] Furthermore, the hydrophobic coating largely eliminates free alkaline content in the castables. As a result, one of the key components of undesirable alkaline hydrolysis reactions is largely missing from the castables and degradation by alkaline hydrolysis is largely, if not entirely, avoided. As such, the refractory castables of the present invention are physically stable over longer periods of time when compared to the refractory castables of the prior art.

[0029] FIG. 1 displays an exemplary process by which the aggregates of the present invention may be formed. Initially, fire brick is formed by processes well known in the art. Following standard procedures of the art, fire brick are made from high purity refractory clays and other ceramic raw materials, as discussed further below. The fire brick may then be crushed (e.g., by using a horizontal shaft impact crusher 100) to form a blend of various particle sizes. The crushed firebrick may then be transferred to a screening system 104. The screens permit aggregate particles smaller than a particular size to pass through the screening system 104 and into a rotary coating drum 108. Typically, the particles exist as a mixture and may range from about 7/16 to about 200 mesh screen.

[0030] The present invention may employ any commonly used aggregate base. That includes granite/basalt, emery, olivine, chamotte, expanded chamotte, molochite, sillimanite, brown-fused alumina, white-fused alumina, tabular alumina, bubble alumina, calcined alumina, pumice, diatomite, vermiculite, perlite, clay, calcined clay, silica fume, spinel, magnesia, dolomite, silicon carbide, and combinations thereof. Those of skill in the art will also recognize other commonly employed aggregates that may be used in the context of the present invention. Further, since practicing the present invention results in encapsulation of the aggregate particles, the skilled practitioner may consider the use of aggregates that have been previously avoided over concerns of reactivity with the environment of the installed castables.

[0031] In some embodiments, the aggregates of the present invention have a base that ranges from about 35 to 85% aluminum oxide, about 15 to 55% silicon dioxide, about 0.1 to 1% ferric oxide, about 0.1 to 12% calcium oxide, and about 0.1 to 2.5% alkalis (as Na.sub.2O). Additional components may be included as desired, as noted above.

[0032] The crushed fire brick may then be transferred to a system where the aggregate is sprayed with a fluid composition containing the hydrophobic component. In the embodiment displayed in FIG. 1, the hydrophobic-containing composition is delivered using a plurality of spray nozzles in a rotary coating drum 108, though one of skill in the art will recognize that numerous, well-known fluid administration mechanisms may be employed (e.g., a conventional conveyor belt with spray nozzles).

[0033] During the coating process, the hydrophobic component preferably adheres to the aggregate, and allows it to substantially coat the aggregate particles. Examples of compounds that may be used as hydrophobic components in the context of the present invention include various silicone-based formulations, nanoscale ceramic coatings, and other commonly known hydrophobic components used in commercial waterproofing of concrete.

[0034] In some embodiments, the hydrophobic component is an emulsion of polydimethylsiloxane. The polydimethylsiloxane may be terminated with a silanol moiety and formulated as an emulsion with water. When emulsified polydimethylsiloxane is used, it may be further diluted in water prior to being applied to aggregate. Generally, dilutions of the emulsified polydimethylsiloxane may range from about 0.5% emulsion to 99.5% water to an undiluted polydimethylsiloxane emulsion. In certain embodiments, the emulsified polydimethylsiloxane may be mixed with water at a 1% emulsion to 99% water ratio. One of skill in the art may select the appropriate dilution based on the aggregate base and the particular end-use application for the aggregate and refractory castable.

[0035] The amount of hydrophobic component applied to the aggregate is sufficient to substantially coat substantially all of the aggregate particles. The specific amount of hydrophobic may vary depending on the hydrophobic component, aggregate base, and dilution employed. Generally, the amount of solution containing the hydrophobic component applied to the aggregate will be greater than or equal to the weight of aggregate. In embodiments where 1:99 diluted polydimethylsiloxane emulsion is employed, the solution may be applied to the aggregate at 150 weight percent. That is, for every pound of aggregate to be coated, 1.5 pounds of polydimethylsiloxane-containing solution is applied to the aggregate. Again, the amount of hydrophobic component used may vary widely, with the goal being substantially complete coating of substantially all of the aggregate particles.

[0036] The coated aggregate is then dried under heat. The specific temperature chosen for drying may vary widely and depends, in part, on the specific hydrophobic component employed, the aggregate base, and resources available. Generally, sufficient heat is applied to drive off a substantial amount of the water present in the coating composition so as to allow the coating composition to set. The specific temperature to be used will vary with the type of hydrophobic component and composition employed. In the embodiment shown in FIG. 1, the heat is administered in a rotary aggregate dryer 112. The specific manner of heating may be varied widely with one of skill in the art recognizing many common ways of heating and drying (e.g., through a natural gas heater and a blower) the coated aggregate. The dried aggregate may then be packaged for shipment using a packing machine 116.

[0037] The coated aggregate of the present invention may be used as a raw material in forming a wide variety of castable components. The aggregates may be used as a component of cementitious compositions that are cast for use in any industrial and domestic setting where heat insulation is desired. For example, the refractory castables of the present invention may be used as hot-face refractory linings or as back-up insulation behind other refractories in furnaces, flues, kilns, catalytic cracking reactors, fired heater linings, and flue gas treatment reactors. One of skill in the art will recognize the utility of the refractory castables of the present invention in numerous additional applications.

[0038] As noted above, one particularly useful implementation of the present invention is in the formulation of refractory castables. The novel aggregates of the present invention may be formed into castables using the same components as used in the prior art. The novel aggregates of the present invention may be substituted for the prior art aggregates without changing the relative levels of other components of the formulation. The amount of water used in generating castables using aggregates of the present invention, however, is dramatically reduced due to the hydrophobic nature of the coated aggregate. The following table displays two comparable formulationsone using prior art aggregate and one using the aggregate of the present inventionas an example.

TABLE-US-00001 Formulation A Formulation B Component (Prior art) (Present invention) Aggregate (Wt. %) 54 54 Wilson Clay (Wt. %) 7 7 Lumnite Cement (Wt. %) 28 28 Water (gal) 55.7 31.4

[0039] As is shown in this illustrative example, the water usage is dramatically reduced when forming castables using the coated aggregates of the present invention. It is believed that during prior art castable formation the aggregate absorbs a substantial amount of water. The hydrophobic coating of the aggregates of the present invention dramatically reduces or substantially eliminates the water absorption that occurs. The reduction is water usage in this example is typical for castables formed using the aggregates of the present invention, with reductions of about 40% to about 60% commonly observed.

[0040] The reduced water content in the castables formed from the aggregates of the present invention leads to additional benefits. As the water content is reduced in any castable, the water-to-cement ratio is reduced with the resulting benefit in the cement forming strong bonds. Further, because less water is present, heating the castables during drying and firing causes smaller changes to the physical properties of the castables. Additionally, the castables formed from aggregates of the present invention display better strength. The physical properties of Formulations A and B are shown in the table below as illustrative examples.

TABLE-US-00002 Formulation A Formulation B Component (Prior art) (Present invention) Density at 220 F. (lb/ft.sup.3) 71 63 Density at 1500 F. (lb/ft.sup.3) 65 58 Cold crushing strength at 314 357 1500 F. (psi) Water content (%) 55.7 31.4 Permanent linear change (%), 0.13 0.05 cast to dried Permanent linear change (%), 0.40 0.22 cast to fired (1500 F.)

[0041] The values in the table are representative of the physical properties commonly observed in castables when using the aggregates of the present invention. Water content of the novel castables of the present invention are commonly about 20 to about 50% lower than castables using prior art aggregate. The permanent linear change is reduced by approximately 50%, while the cold crushing strength is consistently comparable or greater. The test method for assessment of permanent linear change is that included in ASTM method C113-02, which is hereby incorporated by reference. The strength of the castable, of course, may be manipulated to a desired level by addition of other components such as silica fume for the particular application at hand.

[0042] The reduced density of the castables of the present invention is a particularly striking feature of the present invention that carries additional benefits. Typically, prior art castable formulations include high levels of perlite to achieve desired densities and strengths. The use of perlite results in high levels of soluble alkali (approximately 1-2.9%, as Na.sub.2O), which as discussed above, leads to alkaline hydrolysis and eventual physical failure of the castable. Additionally, the expected result of using a castable having lower density, as in the present example, is a lower strength castable. In contrast, the castables of the present invention have a lower density and, at the same time, a higher strength. Without being bound to theory, it is believed that because of the lower water content of the castables of the present invention, the bonds between aggregate and the calcium aluminate cements used in castable formation are stronger. In addition, the use of the coated aggregate of the present invention allows castables having the recited densities and strengths to be achieved without the use of perlite. As a result, the soluble alkali levels of the present invention may be very low (approximately 0.45% and less, as Na.sub.2O). Accordingly, alkaline hydrolysis is dramatically reduced, if not substantially eliminated, in castables formed from the coated aggregates of the present invention.

[0043] The reduced densities of the castables of the present invention also contribute to superior thermal insulation properties. It is believed that the lower densities of the castables of the present invention are the result of air captured in the aggregate during the encapsulation process. As is well known, air is an excellent insulator, and the heat storage capacity of the refractory castables fabricated from the coated aggregates of the present invention may be approximately 50 to 60% lower (i.e., able to act as a better insulator) than castables made with prior art aggregates. The following example provides illustrative data demonstrating the superior combination of density and heat storage displayed by castables of the present invention.

[0044] Refractory castables are graded for use at particular temperatures set by the final industrial application for the castable. This example evaluates the thermal and physical properties of various prior art materials to inventive castables that might be used in a refractory lining of a cyclic furnace that operates at 2800 F. and which use a base of approximately 60% alumina. For a refractory castable of the present invention, the service temperature (i.e., the maximum temperature at which the castable may be used before it physically fails) may be varied widely by modifying numerous factors, including the aggregate base employed. The results observed here for operation at 2800 F. are representative of results using castables employed for other operating and service temperatures. Typically, the prior art employed medium-to-high density refractory components to line the walls of furnaces operating in this temperature range. In the prior art, high-density materials are used to achieve a lining having the thermal properties needed to maintain adequate insulation of the furnace. Examples of prior art refractory materials include 85 alumina-based, clay-bonded plastic, 3000 general performance, and 3000 high performance (having low cement of <5% by weight) castables. The following data are generated presuming a hot face temperature of 2800 F. and a lining thickness of 12 inches.

TABLE-US-00003 Cold Heat k-value Density face temp. storage (BTU-in/ft.sup.2-hr- Material (lb/ft.sup.3) ( F.) (BTU/ft.sup.2) F.; at 1000 F.) Castable of the 76.1 332 28,974 3.03 present invention rated to 2800 F. 85 plastic 156 463 54,677 7.6 3000 General 130 468 51,925 9.5 Performance 3000 High 154 596 62,350 14.5 Performance

[0045] As is clear from the above example, high temperature castables formed from the coated aggregates of the present invention have a combination of low density and superior thermal conductivity (measured either as heat storage or as a k-value). The refractory castables of the present invention achieve superior insulation (approximately 40% better heat storage and approximately 45-50% better k-value in this example) while at the same time having a density that is more than 40% less than prior art refractory castables. The present invention allows coated aggregates to be used in thermally stable, high-temperature castable formulations that provide much lower densities and better insulating value than the prior art, while maintaining equivalent material strengths.

[0046] The high-temperature refractory castables of the present invention thus possess densities of approximately 75-95 lb/ft.sup.3 and heat storage values ranging from approximately 30,000 BTU/ft.sup.2 to approximately 45,000 BTU/ft.sup.2for a 12 block and k-values from about 3 to about 5 BTU-in/ft.sup.2-hr-T. This combination of low density and high insulation properties is not found in the prior art. As such, the castables of the present invention represent a novel tool for the castables industry. The prior art used much heavier refractory castables to obtain the same insulation. The use of the high-temperature castables of the present invention will dramatically lessen the engineering requirements of insulation systems because of the lighter physical load and will open new opportunities for the use of refractory castables as a result.

[0047] The aggregates of the present invention may also be used in gunning mixtures. Because of the hydrophobic nature of the coated aggregate, the fluid properties of gunning mixtures including the aggregates of the present invention are improved for pumping and gunning applications. Additionally, the use of the aggregates of the present invention lowers the water range required to be used during gunning. Because the aggregates of the present invention may include reduced water during use as gunning mixtures, the density will similarly be lower when the composition is gunned. This, in turn, dramatically reduces the pumping demands as this lower-density gunning mixture will flow better at lower water content. As a result, pumping of the gunning mixture is dramatically improved compared to prior art gunning mixtures.

[0048] Nothing in the above description is meant to limit the present invention to any specific materials, geometry, or orientation of elements. Many part/orientation substitutions are contemplated within the scope of the present invention and will be apparent to those skilled in the art. The embodiments described herein were presented by way of example only and should not be used to limit the scope of the invention.

[0049] Although the invention has been described in terms of particular embodiments in an application, one of ordinary skill in the art, in light of the teachings herein, can generate additional embodiments and modifications without departing from the spirit of, or exceeding the scope of, the claimed invention. Accordingly, it is understood that the drawings and the descriptions herein are proffered only to facilitate comprehension of the invention and should not be construed to limit the scope thereof.