INSULATING RAW MATERIAL FOR HIGH TEMPERATURE APPLICATIONS

Abstract: A new refractory insulating aggregate for high temperature applications has been developed. The product contains approximately 92 % Al2O3 and 7.5 % CaO; its mineralogical composition is calcium hexaluminate. The product is a homogeneous material with a high porosity of typically 75 % and a small pore radius of 0.5-2.5 mu m. The product is produced by a process which includes hydrothermal hardening in an autoclave.

Description: INSULATING RAW MATERIAL FOR HIGH TEMPERATURE APPLICATIONS Technical Field The present invention relates to insulating materials and methods of making refractory bodies. Background Art Insulating materials are important products for many industrial applications. They are used to minimize heat and energy losses and to improve safety of equipment and working environment. Depending on the application, insulating products have to meet various requirements: • highly efficient thermal insulation properties in order to minimize wall thickness. • excellent thermal shock resistance, • easy to handle and safe to install, and • high durability to minimize downtimes and related costs. Recent changes in the European legislation have started a new discussion around the health aspect of various insulating products. In December 1997, the European Union classified ceramic mineral fibers as a category 2 carcinogen. This has stimulated an intensive search for alternative products with similar refractoriness, low thermal conductivity, and less concerns around potential health hazards. The thermal conductivity properties of refractory materials are determined by their heat transport characteristics, which are conduction in solids/gases, convection, and radiation. The kinetics are well known in the art and are explained in detail by Schulle (Feuerfeste Werkstoffe. Dt. Verlag fur Grundstoffindustrie, Leipzig, 1990). An article by Seifert, H.: Mikroporδse Feuerbeton-Leichtzuschlagstoffe. Silikattechnik 38 (1987) 4, p. 129-131, specifically discusses the thermal characteristics of microporous materials. The development of material of the present invention was targeted to obtain an insulating aggregate with high refractoriness for use at temperatures above 1200°C. At high temperatures the major heat transfer mechanism is the radiation. The radiation emission rises with the 4 th power of the temperature. Pores behave like a vacuum for radiation (so-called short circuit of pores). Thermal conductivity by radiation is proportional to the pore size. Smaller pore sizes result in a lower radiant heat conductivity. Figure 1 shows an example for two products with similar total porosity, but different pore sizes. The product with the smaller pore size has the lower thermal conductivity at high temperatures. Therefore to provide optimum insulating efficiency, the target microstructure for the new insulating aggregate was that the pore size should be as small as possible to minimize radiation, and that the total porosity should be high. To allow the use at high temperatures, the mineral phase should have a high refractoriness and a low impurity level. Accordingly, it would be advantageous to provide a replacement material in many of the applications where ceramic fibers are currently used. An object of the invention is to provide a new insulating aggregate having a small pore size to minimize heat transfer by radiation. Another object of the present invention is to provide a new insulating aggregate having a high total porosity. Still another object of the present invention is to provide a new insulating aggregate having a high total porosity. Brief Description of the Drawings Other features of the present invention will be further described in the following related description of the preferred embodiment which is to be considered together with the accompanying drawings wherein like figures refer to like parts and further wherein: Figure 1 is a graph showing the effect of temperature on the thermal conductivity of two refractory materials with similar porosity. Figure 2 is a graph showing the micro pore distribution of the product of the present invention. Figure 3 is a scanning electron micrograph of the present invention. Figure 4 is a graph comparing the thermal conductivity of the present invention to other materials. Figure 5 is a graph showing the micro pore size distribution of several variations of the present invention. Figure 6 is a graph showing the refractoriness of the present invention under load curves. Figure 7 is a graph showing the creep in compression of the present invention. Figure 8 is a graph showing the thermal conductivity of the present invention. Figure 9 is a process flow diagram for making the product of the present invention. Summary of the Invention The present invention is a refractory insulating product having an alumina content greater than 60%, a density less than 1.2 g/cm 3 , a porosity greater than 60% and a median pore diameter greater than 15 μm. The material of the present invention is useful for insulation up to high temperatures (>1400°C). Preferably the total impurities of the material of the present invention is less than or equal to 5%. This results in: • thermal stability against sintering or creep at high temperature usage; • chemical stability at reducing or alkaline atmospheres (i.e., for petrochemical, glass, ceramics, or ammonia production). The material of the present invention can be used as a raw material for refractory products or other production processes that run under high energy input, have a traditionally used ceramic mineral fibres and need replacement, or have critical working conditions regarding durability of refractory insulating products under high temperature, chemical attack and reducing atmosphere. Examples of such applications are steelworks, foundries, non-iron metal production, cement production, glass production, firing kilns, petrochem industry, chemical industry, or other. Another aspect of the invention is the use of hydrothermal hardening of low loose bulk density (< 1 t/m J ) dry powder compound, followed by crushing, sintering, sizing, and bagging. The hydrothermal hardening is accomplished in an autoclave with saturated steam at 100-300°C, 1-100 bar, 3-50 h. By Autoclaving of dry powder instead of the previously patented slurry route: increased throughput and drastically reduced production costs. The resulting product is a light weight, microporous, high porosity insulating material. Mode for Carrying Out the Invention Physical product properties of the present invention The base mineral composition for material of the present invention is calcium hexaluminate (CA 6 - where C=CaO, A=Al 2 O 3 ). Theoretical composition: CA 6 = CaO 6 Al 2 O 3 = 91.6% Al 2 O 3 , 8.4% CaO Target is an alumina-rich CA 6 to prevent the formation of calcia-rich, hydratable calciumaluminate-phases (CA, CA 2 , C, 2 A 7 ). CaO content range 7-8%, target 7.5%. CA 6 exhibits the most excellent thermal properties of the calcium aluminate system with a melting point above 1850°C. CA 6 has high thermal shock resistance, a thermal expansion and fracture toughness K, c similar to alumina, moderate flexural strength due to anisotropic grain growth, and stability in reducing atmosphere. In addition, European Patent Application EP 0 615 959 Al reports that calcium aluminate-based material have good stability in contact with alkalioxides. Table 1 shows that the product chemistry of material of the present invention. In comparison to the present invention, pure CA 6 is 91.6% Al 2 O 3 , 8.4% CaO. The present invention is on the alumina-rich side in order to suppress the formation of hydratable calcium aluminate phases (CA, CA 2 ). The impurity level is low, SiO 2 and Fe 2 O 3 contents are max. 0.1%. The bulk density is around 0.65-0.7 g/cm 3 . Figure 2 is a graph showing the micro pore distribution of the product of the present invention. The micro pore size distribution in Figure 2, as measured by Hg-intrusion method, shows a narrow range of 1 -5 μm with an average pore size of 3-4 μm. Figure 3 is a scanning electron micrograph of the present invention. The scanning electron microscope image in Figure 3 shows the microporous, homogeneous internal structure of the material of the present invention. The interlocking nature of the platelet-shaped CA 6 -crystals results in a high level of internal porosity (typically 75%), as well as enhancing the strength of the aggregates. The free distance between the CA 6 -platelets determines the small pore diameter. The material of the present invention is produced via a wet chemical process which includes sintering of the final product at high temperatures. Its performance has been evaluated at temperatures up to 1500°C. The material of the present invention is produced in four sizes, three closed sizes (1-3, 3-6, 6-10 mm) and one open (0-1 mm). The loose bulk density is around 400 kg/m 3 for the closed sizes and around 700 kg/m 3 for the fine fraction. Thermal conductivity of material of the present invention grains The thermal conductivity of 1-3 and 3-6 mm material of the present invention grains was tested in the form of beds with packed density. Testing was done up to 1400°C according to the hot wire method (parallel wire) (DIN EN 993- 15). The packing density of the grain bed was achieved by densification of the loose fill trough tapping. Figure 4 is a graph comparing the thermal conductivity of the present invention (listed as SLA-92) to other materials. These other materials are commercially available bubble alumina, insulating fire clay grains, and ceramic fiber blanket. As shown in Figure 4, at 25°C, material of the present invention has a thermal conductivity of 0.15 W/m K which gradually rises up to 0.5 W/m K at 1400°C. The curve of the ceramic fiber blanket runs parallel to that of SLA-92, the distance between both lines is approx. 0.15 W/m K. Both the bubble alumina as well as fire clay grain bed have a rapid rise of thermal conductivity from 600°C up to 1400°C. The thermal conductivity of the bubble alumina bed is much higher than that of material of the present invention over the entire temperature range. The data illustrates the large effect that pore size has on thermal conductivity. The pore size of bubble alumina is nearly equivalent to the grain diameter which is in the mm-range, whereas material of the present invention shows microporosity in the μm-range. This leads to much lower thermal conductivity of material of the present invention especially in the high temperature range where the conductivity is dominated by radiation. The different packed densities of material of the present inventionl-3 and 3-6 mm (0.61 vs. 0.47 g/cm 3 ) have no effect on the thermal conductivity. The material of the present invention in insulating products The material of the present invention is currently under evaluation in several applications, where low weight of the castables, refractoriness and thermal insulation properties are key factors. High purity is essential in applications such as petrochemical and ammonia industry to provide a product with CO- and alkali resistance. Brick pressing trials of the material of the present invention showed that the grain crushing strength is sufficient for insulating brick production. The relatively high grain strength also provides grain stability when the mix casting is helped by ramming or poking. Ease of installation as well as of destruction leads to shorter downtimes and helps to reduce costs. The porosity of material of the present invention provides good stickiness for gunning. Examples The tests described above give some examples for the application of material of the present invention aggregate in castables or bricks and show the physical and insulating properties of the refractory products produced from this aggregate. The composition of the Test Mix 16/1 castable is shown in Table 2. Preparation and treatment of the test pieces and testing followed the European standard DIN ENV 1402 "Unshaped refractory products". Part 5 and Part 6. The dry ingredients were mixed for 1 minute. After addition of 60% of a solution with 0.5% cellulose (Blanose ex Aqualon Henkel), the mixing was continued for another 4 minutes. The castable was cast under vibration into the mould. Vibration time was 30-60 sec with an amplitude of 0.2-0.5 mm depending on the specimen size. The following test pieces were prepared: • 230 mm x 64 mm x 54 mm for testing the conventional properties (DIN ENV 1402-6) • 230 mm x 1 14 mm x 64 mm for testing thermal conductivity (hotwire (parallel) method DIN EN 993-15) - 150 mm x 25 mm x 25 mm for testing the hot modulus of rupture (DIN EN 993-7) The vibrated test pieces were cured at room temperature in the mold for 24h and after de-molding for another 24h in air with a relative humidity of ^90%. The pieces were dried for 24h at 1 10°C and fired from 850°C up to 1500°C with a soaking time of 5h. For the brick preparation the same composition as for the vibration castable (Table 2) was used, but only 30% of the cellulose solution was added. Bricks (250 mm x 100 mm x 65 mm) and plates (250 mm x 100 mm x 30 mm) were pressed without problems on an industrial hydraulic press with an applied pressure of 0.2 N/mm 2 . Edges and corners of the pressed pieces are strong enough for an automatically transport. The modulus of rupture of the green pressed plates is about 0.8 N/mm 2 . The physical properties of the prepared test pieces after drying and firing up to 1500°C are compiled in Table 3 below. T AB LE 3 00 CO CO m CO X m m TO Z m CO CO O TO CZ r- m ro en The drying shrinkage of the Mix 16/1 is about 0.05 % and the firing shrinkage at 1400°C/5h reaches about 0.4 % for both castable and pressed brick. The firing temperature of 1500°C results in a shrinkage of 0.95 % for the castable and 0.77 % for the pressed brick. There is practically no change in bulk density and open porosity at firing temperatures from 850 up to 1400°C. In this temperature range the castables has a bulk density of about 1.1 g/cm 3 , the pressed brick of about 1.28 g/cm 3 . The apparent porosity is about 70 % for the castable and 66% for the pressed brick. The slightly higher shrinkage at 1500°C results in a small increase of bulk density and decrease in porosity. Figure 5 is a graph showing the micro pore size distribution of several variations of the present invention. The total microporosity is roughly 60%. Even at firing at 1500°C the median pore diameter of the Mix 16/1 material remains at about 3-5 μm similar to that of the material of the present invention grain, but the castable and the brick contain a small amount of larger pores up to 60 μm. The gas permeability of the fired 16/1 castable and pressed brick at 1400-1500°C is in the range of about 5.5 nPm, a relatively low value for these high porous materials. The low value is the effect of the very small pore size. Up to a firing temperature of 1000°C, the flexural strength remains at about 0.3 N/mm 2 for the castable and about 1 N/mm 2 for the pressed brick because of its higher bulk density. Firing temperature of 1500°C increases strength about a factor of three. The development of cold crushing strength is similar to the changes in the cold modulus of rupture. After firing at 1500°C, the cold crushing strength reaches 6.5 N/mm 2 for the castable and 11.5 N/mm 2 for the pressed brick. The test results indicate that the modulus of rupture from room temperature up to 1500°C testing temperature remains constant at about 1 N/mm 2 , emphasizing that no melting phase in the material develops. Figure 6 is a graph showing the refractoriness of the present invention under load curves (DIN EN 993-8) of pre-fired specimen of Mix 16/1 castable and brick. The pressed brick pre-fired at 1400°C reaches practically the same high T 0 5 value as the 1500°C pre-fired castable of about 1490°C. Figure 7 is a graph showing the creep in compression of the present invention. From the creep curves (DIN EN 993-9) in Figure 7 the test results as given in Table 4 below were calculated. They demonstrate the good creep resistance of material of the present invention materials up to 1400°C. The air quenching method, according to DIN ENV 993-1 1 was used to determine thermal shock resistance. The thermal shock test was stopped after 10 cycles and the strength measured and compared to the untreated material. The results are reported in Table 5 below and show that the Mix 16/1 castable fired at 1000°C up to 1500°C shows no sign of cracks and no strength decrease after 10 cycles. The pressed brick fired at 1400°C failed at 10 cycles by one crack passing through the material, probably due to the higher density of the material or not yet optimized pressing parameters when forming the brick (laminations). Figure 8 is a graph showing the thermal conductivity of the present invention (Mix 16/1) castable and pressed brick. For the materials pre-fired to 1000°C, the thermal conductivity decreases from 25°C down to 500°C and then levels out in the temperature range from 800°C to 1300°C at about 0.4 W/m K. Pre-firing at 1300°C results in a steady decrease of thermal conductivity from 0.5 W/m K to 0.4 W/m K. The differences of thermal conductivity between castable and brick are small, even though the brick has a higher bulk density. The Mix 16/1 material competes with the thermal conductivity of high temperature insulating bricks. The increasing thermal conductivity of ceramic fiber modules at 1300°C leads to values similar or higher as the Mix 16/1 material. In Table 6 below selected properties of the Mix 16/1 castable pressed brick are compared to two just available refractory materials for high temperature insulating applications: a bubble alumina castable and an insulating mullite brick (Group ASTM 30). TABLE 6 CO e CO CO e —I m O x m m TO en It is to be noted that the two other products can only give a rough comparison because of the wide variety of the properties of these two refractory product types. The material of the present invention which includes the concept of an optimized insulating aggregate for high temperatures with a high porosity and a small pore size, has shown to give the targeted low thermal conductivity up to 1300°C. Tests with material of the present invention as a grain bed showed a slow rise in thermal conductivity up to 1300°C, an effect of its microporosity. A similar slow rise in thermal conductivity was seen for the ceramic fiber blanket, its conductivity was at approx. 0.1-0.15 W/m K lower. This correlation could indicate some future potential. The tested bubble alumina and fire clay grain beds had a distinctly higher and steeper rising thermal conductivity in the critical high temperature range above 600°C where radiation becomes the main contributor to thermal conductivity. The material of the present invention Mix 16/1 based test castable and brick showed good hot properties up to the tested 1400-1500°C, which reflects the stability of the product and its low impurity level. Thermal shock cycling revealed excellent stability of the material of the present invention castable after 10 cycles, a similar specimen remained undamaged after 20 cycles in a separate external test. The brick had a higher strength and lower creep rate than the castable due to higher densification (fired 1000°C: 1.08 vs. 1.28 g/cm 3 ), but still the thermal conductivity properties of the brick were similar to the lighter castable. This again shows that the microporosity is the major contributor to the insulating properties. An important characteristic of the material of the present invention Mix 16/1 materials is the constant low thermal conductivity level with little or no rise up to 1300°C, where an average value of 0.4 W/m K is found. This indicates competitiveness of its properties to high temperature insulating bricks. The increasing thermal conductivity of ceramic fibers modules at 1300°C lead to values similar or higher as the Mix 16/1 material, a finding which could open up some future potentials for material of the present invention. The material of the present invention presents a new concept for insulating aggregates with a high potential for high performance, innovative insulating products. It could be the key to new solutions for technical requirements such as decreased lining thickness, longer lifetime, increased volume/capacity, higher operating temperatures, or higher energy savings. The new insulating aggregate of the present invention can be an alternative material for existing insulating products. Its benefits include high creep resistance, high thermal shock resistance, high purity, and low thermal conductivity at temperatures above 1200°C. It seems to have a potential to be a replacement material in many applications that currently use ceramic fibers. Material of the present invention could assist in alternative products solutions with similar refractoriness, low thermal conductivity, and fewer concerns around potential health hazards. Figure 9 is a process flow diagram for the present invention. Raw Materials Oxide content finer than d50 Calcium Hydroxide Ca(OH) 2 73.55 % CaO 63 μm 20 μm Aluminium Hydroxide Al(OH) 3 65.20 % Al 2 O 3 32 μm 10 μm Raw Mix Composition Mixing ratio: 100 kg Al(OH) 3 + 6.66 kg Ca(OH) 2 ^7% CaO + 7.70 kg Ca(OH) 2 ^>8% CaO Grinding The grinding in the process shown in Figure 9 is to produce a high fineness so as to achieve high loose bulk density of the raw mix of < 1 t/m ~ \ This results in low density, small pore size and high porosity of final product. Filling of autoclaving forms Filling under high agitation of dry powder to full desagglomeration and to generate free flowability. Optional spraying of water mist in layers during filling on powder and on top (stabilizes bulk density) and/ or granulation to balls with water and filling of forms therewith (stability/pre-sizing of aggregates/optimium usage of autoclaving capacity). Autoclaving Autoclaving conditions: saturated steam at 100-300°C, 1-100 bar, 3-50 h holding time. Hardens/solidifies dry powder to solid body with consistency of soft to hard chalk. Crushing Crushing autoclaved material close to the requested final product size (0.2 mm to <10 mm). Crushers must be suitable to downsize chalky material without grinding, smearing, or other destruction of the product structure. Conversion/Sintering Heat treatment is performed in kiln with high end temperature. The target temperature for sintering is greater than 1500°C. Screening, Bagging, Shipping Final screening to desired final aggregate sizes, bagging and shipment to consumers. The key step in the method of the present invention is the autoclaving step. The ground material is hydrothermally hardened in an autoclave with saturated steam at 100-300°C, 1-100 bar, 3-50 h. The performance of material of the present invention is suited to higher application temperatures. The option for the development of the family of insulating raw materials with different chemistries is under evaluation for the future. Members of the family include: (a) calcium hexaluminate CA 6 CaO-6Al 2 O 3 (b) corundumAl 2 O 3 (c) MA-Spinel MgO-Al 2 O 3 (d) beta-alumina Na 2 O- l lAl 2 O 3 (also contains LiO 2 , K 2 O, CaO, BaO) (e) mullite 3Al 2 O 3 2SiO 2 (f) aluminum titanate Al 2 O 3 TiO 2 The invention is obviously not limited to the embodiments just described, and numerous changes and modifications can be made to the latter without departing from the scope of the invention. What is believed to be the best mode of the invention has been described above. However, it will be apparent to those skilled in the art that numerous variations of the type described could be made to the present invention without departing from the spirit of the invention. What is claimed is:

Claims: C L A I M S I . A refractory insulating aggregate for high temperature, the composition of said aggregate comprising more than about 92% Al 2 O 3 and less than about 8% CaO. 2. The refractory insulating aggregate of claim 1, in which said composition is an alumina-rich calcium hexaluminate. 3. The refractory insulating aggregate of claim 1, in which said composition comprises about 92 to about 94% Al 2 O 3 . 3. The refractory insulating aggregate of claim 1, in which said composition comprises about 7 to about 8% CaO. 4. The refractory insulating aggregate of claim 1, in which said aggregate has porosity greater than about 60%. 5. The refractory insulating aggregate of claim 1, in which said aggregate has a small pore radius of 0.5-2.5 μm. 6. The refractory insulating aggregate of claim 1, in which said aggregate has an average thermal conductivity of less than 0.5 W/m K. 7. A method a making a refractory insulating aggregate comprising: (a) forming a mixture of hydrated alumina and alumium hydroxide; (b) grinding said mixture; (c) thermally autoclaving said mixture to form a solid body; (d) crushing said solid body to form a plurality of particles having a size of less than about 15 mm; and (e) sintering said plurality of particles. 8. The method of claim 7, in which (c) further comprises spraying said mixture with water prior to autoclaving. 9. The method of claim 7, in which (c) further comprises thermally hardening said mixture with saturated steam at a temperature between about 100°C and about 300°C. 10. The method of claim 7, in which (c) further comprises thermally hardening said mixture with saturated steam at a temperature between about 100°C and about 300°C at a pressure of between about 1 to about 100 bar. I I . The method of claim 7, in which (e) further comprises sintering said plurality of particles at a temperature greater than about 1000°C. 12. The method of claim 7, in which (e) further comprises sintering said plurality of particles at a temperature greater than about 1400°C. 13. The method of claim 7, in which (e) further comprises sintering said plurality of particles to form an aggregate with particles group selected from calcium hexaluminate (CaO-6Al 2 O 3 ), corundum (Al 2 O 3 ), MA-Spinel (MgO-Al 2 O 3 ), beta- alumina (Na 2 O- l lAl 2 O 3 ), mullite (3Al 2 O 3 2SiO 2 ), aluminum titanate (A1,0 3 TiO,) and mixtures thereof. 14. The method of claim 7, in which (e) further comprises sintering said plurality of particles to form an aggregate with particles group selected Li0 2 , K 2 O, CaO, BaO and mixtures thereof. 15. A refractory insulating aggregate for high temperature, the composition of said aggregate comprising an alumina-rich calcium hexaluminate containing about 92 to about 94% Al 2 O 3 . 16. The refractory insulating aggregate of claim 15, in which said alumina- rich calcium hexaluminate comprises about 7 to about 8% CaO. 17. The refractory insulating aggregate of claim 15, in which said aggregate has porosity greater than about 60%. 18. The refractory insulating aggregate of claim 15, in which said aggregate has a small pore radius of 0.5-2.5 μm. 19. The refractory insulating aggregate of claim 15, in which said aggregate has an average thermal conductivity of less than 0.5W/m K. 20. A method a making a refractory insulating aggregate comprising: (a) forming a mixture of hydrated alumina and alumium hydroxide; (b) grinding said mixture; (c) hydrothermally hardening said mixture with saturated steam at a temperature between about 100°C and about 300°C to form a solid body; (d) crushing said solid body to form a plurality of particles having a size of less than about 15 mm; and (e) sintering said plurality of particles at a temperature greater than about 1000°C. 21. The method of claim 20, in which (e) further comprises sintering said plurality of particles to form an aggregate with particles group selected from calcium hexaluminate (CaO-6Al 2 O 3 ), corundum (Al 2 O 3 ), MA-Spinel (MgO-Al 2 O 3 ), beta- alumina (Na 2 O- l lAl 2 O 3 ), mullite (3Al 2 O 3 2SiO 2 ), aluminum titanate (Al 2 O 3 TiO 2 ) and mixtures thereof.

Inventors: GNAUCK VOLKER, KRIECHBAUM GANGOLF W, VAN GARSEL DORIS, WOLFRAM LINGENBERG, GNAUCK, VOLKER, KRIECHBAUM, GANGOLF, W., VAN GARSEL, DORIS, WOLFRAM, LINGENBERG
Applicants: ALCOA CHEMIE GMBH, GARSEL DORIS VAN, GNAUCK VOLKER, KRIECHBAUM GANGOLF W, WOLFRAM LINGENBERG, GNAUCK, VOLKER, KRIECHBAUM, GANGOLF, W., VAN GARSEL, DORIS, WOLFRAM, LINGENBERG
Application year: 1999
Main classification: C04B35
IPC codes: C04B35, C04B38