Title of Invention	A process for Manufacturing Mullite Aggregates from pyrophyllite, High Alumina Minerals and AL-Hydroxy Hydrogel
Abstract	A process for manufacturing mullite aggregates from pyrophillite an apparently unusable mineral to produce quality mullite aggregate (of the order of 45 - 65 % mullite). Pyrophyllite, an alumino silicate mineral, abundantly available in nature finds very limited use in refractory industry. Theoretically, pyrophyllite on heating yields mullite, but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. In the present invention the process produces mullite aggregates from a powder precursor containing pyrophyllite, kyanite minerals and aluminium hydrate and intimately mixing the same with AI-O-OH hydroxy hydrogel network produced by flash condensation techniques. The active precursor on firing at a temperature in the range of 1550°C to 1600°C produces mullite aggregates (of the order of 45 - 65 % mullite) with dense microstructure. Percentage of mullite is estimated by x-ray analysis. Dense mullite aggregate has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C.

Title of Invention

A process for Manufacturing Mullite Aggregates from pyrophyllite, High Alumina Minerals and AL-Hydroxy Hydrogel

Abstract

A process for manufacturing mullite aggregates from pyrophillite an apparently unusable mineral to produce quality mullite aggregate (of the order of 45 - 65 % mullite). Pyrophyllite, an alumino silicate mineral, abundantly available in nature finds very limited use in refractory industry. Theoretically, pyrophyllite on heating yields mullite, but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. In the present invention the process produces mullite aggregates from a powder precursor containing pyrophyllite, kyanite minerals and aluminium hydrate and intimately mixing the same with AI-O-OH hydroxy hydrogel network produced by flash condensation techniques. The active precursor on firing at a temperature in the range of 1550°C to 1600°C produces mullite aggregates (of the order of 45 - 65 % mullite) with dense microstructure. Percentage of mullite is estimated by x-ray analysis. Dense mullite aggregate has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C.

Full Text	The present invention relates to a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel. Dense mullite aggregate has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C. The first identification of mullite (3Al2O3: 2SiO2 ; 71.80 wt% Al2O3) in the Al2O3 - S1O2 binary system is credited to the historic work of Bowen and Greig. Toropov and Galakov prepared mullite from alumina gel and quartz. Mullite as solid solution range was first proposed by Shears and Archibold. Aramaki and Roy identified the solid solution range of 71.8-74.3 wt.% Al2O3 They also determined that the solid solution could be extended to 77.7% Al2O3 under metastable conditions. However mullite is generally represented as 3Al2O3 2SiO2 with * 72% Al2O3 that melts incongruently at >1800°C. Generally, mullite is made by using conventional powder mixing techniques in which alumina and silica powder (having a particle size of approximately 1 micron) are mixed and sintered above 1600°C. However, mullite prepared using these techniques tends to contain traces of other components, such as, a-Al2O3 and a-cristobalite, which indicates that the reaction to mullite is not complete even at such high temperatures. As a result there is decreased density for compositions formed from such mullite. Reference may be made to M. D. Sacks & H-W Lee, "A Review of Powder Preparation Methods and Densification Procedures for Fabricating High Density Mullite", appearing in Mullite and Mullite Matrix Composites, Ceramic Transaction 6, 167-207 (S. Somiya, R. F. Davis, & J. A. Pask eds. 1990), the disclosures of which are herein incorporated by reference in their entirety. When used in industrial applications, it is desirable to have mullite compositions or compacts that have a high density because it enhances the structural integrity of the ceramic component i.e., prevents cracks and brittleness) and enhances the useful life of the components. It is also desirable to lower sintering temperatures for mullite i.e., lower the mullitization temperature, and such attempts have been made by trying to reduce the effective particle size of the starting materials used for making the mullite powder. For example, colloid techniques, sol-gel techniques, and solution techniques, which generally operate based on precipitation principles, using typical starting materials that have smaller particle sizes (i.e., less than 50 nm) and higher surface area (i.e., greater than 200 m2/g), which can lead to more complete mullitization even at lower sintering temperatures. Using these methods, sintering temperatures less than about 1200°C. have been obtained. However, despite these various methodologies employed to make mullite powders using lower temperatures, there remains the problem of producing mullite composites also having high density. In addition, prior art techniques for making mullite powder typically use organic solvents or organic components as part of the reaction conditions, which is ecologically undesirable. There has been a move toward trying to prepare water-based mullite sols. For example, it is known that a water-based silica sol is stable in high pH (≈ 8) and a water-based alumina sol is stable in low pH (≈2) for which reference may be made to R. K. Her, in "The Chemistry of Silica"(John Wiley & Sons 1979) and B. E. Yoldas, Ceramic Bulletin, 54(3), 1975, 286-88 & 289-290, the disclosures of which are herein incorporated by reference in their entirety. When the alumina sol and the silica sol are mixed together, a gel or precipitation is immediately formed. The sol has to stay in a solution state in order to effectively impregnate the preform so that it may be processed into a mullite composite, thus, these methods are not suitable for making mullite composites. Water-based mullite sols have also been reported in the literature for monolith and powder preparations. Reference may be made to, C. J. Brinker & G. W. Scherer, in Sol-Gel Science: " The Physics and Chemistry of Sol-Gel Processing", p. 216-223 (Academic Press, Inc. 1990), the disclosures of which are herein incorporated by reference in their entirety. In these applications, the sol is converted to a gel in a short time (i.e., minutes to days) so a sol having months stability is not required. However, this rapid conversion to a gel makes mullite sols prepared according to this methodology poor candidates for use in impregnating preforms, which is an essential step in making mullite composites. Water-based mullite sols having sintering temperatures of about 1000°C have been synthesised, but they contain hazardous fluoride ion and react with reinforcing preform material. Further, these mullite sols have a high exothermic phase transition from amorphous to crystalline mullite at crystallisation temperatures (about 980° C), which is an indication that mullite composites made from the sol will have poor density characteristics. Therefore, the mullite sol is an important factor in the success of composite processing. The sol has to be stable for easy handling and processing, it should have a high solids contents for high yield, its components should have a small particle size, it should be homogeneous, it should exhibit little or no exothermic reaction during heat treatment or densification procedures before mullitization, and should be easily converted into substantially crack-free and dense mullite composites at relatively low sintering temperatures. Also, to comply with environmental issues, the sol should preferably be water based. Reference may be made to United States patent no.: 5,932,168 wherein the inventors: Su; Wei-Fang Anne, have attempted to address the above noted requirements as well as other needs by presenting aqueous mullite precursor sols, unlike the prior art mullite powders, that can be made into mullite composites. The said invention produces stable, water-based mullite precursor sols by hydrolysing an aqueous silane solution with a base and then adding an aqueous solution of aluminium nitrate to the solution. These stable sols can then be used to prepare, substantially crack-free and dense mullite composites of the invention. The above said process uses silane and the process primarily depends on the stability of mullite sol. Tetra ethoxysilane is used as source of silicon. Therefore the above process suffers from two limitations, that is the requirement of stability of the sol and specificity of source of silica which is generally an alkoxysilane. The alkoxysilane during processing is likely to cause environmental pollution and yield of the product will also be low as this process is limited by solid content of the reaction system. The micro structure development of mullite crystals, were not specifically defined. Reference may be made to Inui; Tomoyuki, Inoue; Masashi in US Patent No. 5338707 seeks to provide a process for the production of high purity mullites which are highly resistant to heat in their application as various structural materials. Briefly stated, the process involves reacting a mixture of aluminum alkoxide and silicon alkoxide with an atomic ratio (weight) of Al/Si in the range of 2:1 to 7:1 in an aromatic hydrocarbon solvent at a temperature of 200°C-350°C and calcining the resulting reaction product at temperature above 900°C. Reference may also be made to Yoldas; Bulent E., Partlow; Deborah P., in US Patent No. 4687652 where in provided a novel method for preparing high purity mullite which comprises the steps of (a) partially hydrolyzing a dilute silicon alkoxide solution; (b) adding an aluminum alkoxide to the partially hydrolyzed, dilute silicon alkoxide solution; and (c) eliminating terminal alkoxide groups. In one embodiment of the invention, the liquid is evaporated out and the remaining material is calcined at about 500°C to yield an amorphous structure having the mullite composition. Further heating to about 650°C eliminates alkoxide terminal groups still present in the amorphous material. Firing the material to about 985°C converts the amorphous material to crystalline mullite. In another embodiment of the invention, the solution resulting from step (b), above, is further hydrolyzed using a relatively small amount of water A solution prepared in this manner will yield a clear gel and may be used to deposit a coating on a substrate. Firing the deposited coating to about 985°C converts the amorphous material to crystalline mullite. In yet another embodiment of the invention, the solution resulting from step (b), above, is further hydrolyzed using a relatively large amount of water to produce a precipitate. This precipitate converts to mullite at about 985°C. Reference may further be made to Ismail; M. G. M. U., Nakai; Zenjiro, Tsunatori; Hideo in US Patent No. 5045514 with a primary object of the present invention to provide a method for making composite mullite/cordierite ceramics having an increased sintering strength and an enriched mechanical strength, in which sintering can be carried out even at a relatively low temperature, say, 1450°C. It is another object of this invention to provide a method for making composite mullite/cordierite ceramics having an increased sintering density and so not much pores, in which sintering can be carried out even at a relatively low temperature, say, 1450°C. When this ceramic product is used as an IC substrate, a signal pattern formed on its surface is unlikely to break down, since it is substantially rid of pores. Nor does a temperature rise produce an adverse influence on an integrated circuit such as an LSI mounted on the substrate, because water is unlikely to stay therein due to the absence of pores Yet another object of this invention is to provide a method for making composite mullite/cordierite ceramics having a dielectric constant as low as 5.2. By using this ceramic product as an IC substrate, it is possible to improve the transmission properties of an integrated circuit mounted on it. Still another object of this invention is to provide a method for making composite mullite/cordierite ceramics having a coefficient of thermal expansion, say, 3-4.times.10.sup.-6/°C, the figures being close to that of silicon. According to the present invention, the above-mentioned objects are achievable by the provision of a method for making composite mullite/cordierite ceramics characterized by including the steps of: preparing a mullite-intensive sol by mixing alumina and silica sols together, preparing a cordierite-intensive so! by mixing alumina, silica and magnesia sols together, gelating a mixture of the mullite- and cordierite-intensive sols, calcinating the thus obtained gel, and compacting and sintering the thus calcinated material. The general disadvantages of above described prior art processes are: 1. Incomplete reaction of the reactants, 2. High mullitization temperature, 3. Ecologically undesirable reaction in cases, where organic solvents or organic components are used as part of the reaction conditions, 4. Low stability of mullite sol in some cases. The main object of the present invention is to provide a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel which obviates the drawbacks as mentioned above. Another object of the present invention is to provide a process of making dense mullite aggregates from a cheap and abundantly available mineral pyrophyllite. Yet another object of the present invention is to provide a process of making dense mullite aggregates from pyrophyllite, wherein mullite formation is over 40%. Still another object of the present invention is to prepare sintered mullite aggregates with dense microstructure. Yet another object of the present invention is to prepare sintered dense mullite aggregate which has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C. In the present invention there is provided a process for manufacturing mullite aggregates from pyrophillite an apparently unusable mineral to produce quality mullite aggregate (of the order of 45 - 65 % mullite). Pyrophyllite, an alumino silicate mineral, abundantly available in nature finds very limited use in refractory industry. Theoretically, pyrophyllite on heating yields mullite, but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. In the present invention the process produces mullite aggregates from a powder precursor containing pyrophyllite, kyanite minerals and aluminium hydrate and intimately mixing the same with AI-O-OH hydroxy hydrogel network produced by flash condensation techniques. The active precursor on firing at a temperature in the range of 1550°C to 1600°C produces mullite aggregates (of the order of 45 - 65 % mullite) with dense microstructure. Percentage of mullitre is estimated by x-ray analysis. Mullite is formed by two broad and distinctly different mechanisms. It forms at alumina interface if reaction is carried out at high temperature. If the reaction is conducted at 1. AI2O3 dissolves in amorphous SiO2 matrix, 2. Mullite nuclei formed within this matrix as the matrix composition exceeds the saturation limit with respect to mullite and, 3. Mullite crystals grow as more Al2O3 dissolved in to the matrix and then is incorporated into the growing mullite grains. Therefore to obtain a grown and elongated crystals of mullite in the matrix, temperature of formation of mullite should be low. This is achievable by using highly reactive powder precursor in the hydroxy hydrogel form. Pyrophyllite, reference for which may be made to Singer F & Singer S.S., "Industrial Ceramics" pp 93-94, Edited by Chapman and Hall, London & New York,(Reprint 1984), is a hydrous aluminium silicate minerals, Al2O3. 4 SiO2. H2O (28.3% Al2O3, 66.7% SiO2l 5.0% H20) and chemically more akin to the clay minerals having a crystal structure similar to the ideal structure for montmorillonite. It is perfectly crystalline and hence does not absorb water or ions. However, pyrophyllite is not preferred as a source of AI2O3 and SiO2 to produce mullite due to the following reason: Theoretically, pyrophyllite decomposes during heating producing mullite and silica as per the following reaction: (Formula Removed) but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. The mineral pyrophyllite used contained alkali, alkaline earth and iron bearing impurities and these largely combine with the siliceous material to form low viscous glassy phase. The amount of silica which readily dissolves in glassy phase at elevated temperature, enhances the viscosity of the silica rich melt and negatively influences the relative amount of mullite. The ultimate structure normally consist of small amount of secondary mullite embedded in a glassy matrix which make the ultimate product unsuitable for refractory application. The troublesome glassy phase may be significantly reduced by addition of aluminous substances like kyanite so as to produce secondary mullite in addition to primary mullite derived from kyanite. Kyanite starts changing to mullite at 1300°C [3(Al2O3.SiO2) = 3 Al2O3.2SiO2 + SiO2] and the formation of small nuclei assist in the further crystallization of well developed mullite crystals from the mixture. The number of favorable sites for crystallization is enhanced and the rate of nucleation passes through a maximum. As the temperature is increased the rate of nucleation increases exponentially. Since both the nucleation rate and the growth rate are strongly temperature dependent the over all rate of recrystalization increases rapidly with the temperature. To make the system further efficient, an alumina precursor material may be incorporated in the reaction system in such a way that the nascent silica formed from pyrophyllite will have reactive encounter with it and will combine to produce further mullite. This is possible by preparing a powder precursor containing pyrophyllite along with kyanite particles entrapped in the hydroxy-hydrogel cages of Al - O - OH network.. Gel derived mullite grains are sufficiently fined to develop better bonding with the fused mullite grains during sintering thus enhancing the over all refractory properties of the aggregates. Accordingly the present invention relates to a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, which comprises; (a) mixing calcined at a temperature in the range of 650°C to 850°C and pulverized pyrophyllite alongwith high alumina minerals such as pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 with aluminium hydrate in the range of 3 to 6 mole% of aluminium hydrates per mole of pyrophyllite to obtain a mixed material, (b) adding aluminium hydroxy hydro gel in the range of 0 to 0.2 mole% into the mixed material to obtain an active precursor, (c) pressing the active precursor into brickets by conventional methods such as herein described and firing the brickets at a temperature in the range of 1550°C to 1600°C, for a period in the range of 1 to 4 hours at the peak temperature to obtain the desired product. In an embodiment of the present invention the pyrophyllite is calcined at a temperature in the range of 650°C to 850°C. In another embodiment of the present invention china clay is used instead of kyanite proportionately. In yet another embodiment of the present invention the aluminium hydrate is of commercial grade. In still another embodiment of the present invention aluminium hydrate used is such as Bayer's alumina, diaspore. In yet another embodiment of the present invention the aluminium hydroxy hydrogel is produced by flash polycondensation technique in alkaline medium. In a further embodiment of the present invention the quantum of mullite obtained is increased by firing at a temperature in the range of 1600°C to 1750°C, for a period in the range of 1 to 4 hours at the peak temperature. The novelty of the process of the present invention is the formation of mullite at lower temperature in higher amount, of the order of 45 - 65 %, with considerable densification characteristics and well developed propeties from an apparently non-usable pyrophyllite. The inventive step lies in forming a powder precursor containing pyrophyllite along with kyanite particles entrapped in the hydroxy-hydrogel cages of Al - O - OH network produced by flash polycondensation technique in alkaline medium. The stepwise details of process of the present invention are given below: a) Calcined and pulverized pyrophyllite along with pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 is mixed with commercially available aluminium hydrate in the range of 3 to 6 mole % of aluminium hydrate per mole of pyrophyllite to obtain a mixture. b) Aluminium hydroxy hydrogel in the range of 0 to 0.2mole% is added to the mixture obtained in step 'a' to obtain an active precursor. c) The active precursor obtained in step 'b' is then pressed into brickets by conventional methods. d) The brickets obtained in step 'c' are then fired at a temperature in the range of 1550°C to 1600°C for a period in the range of 1 to 4 hours at the peak temperature. e) Estimating the formation of mullite percentage by x-ray analysis. The following examples are given by way of illustration of the process of the present invention in actual practice and therefore, should not be construed to limit the scope of the present invention. Example -1 500 gms of calcined and pulverized pyrophyllite alongwith 500 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 3 mole % of aluminium hydrate per mole of pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.125 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1600°C with 2 hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 65%. Example - 2 300 gms of calcined and pulverized pyrophyllite alongwith 200 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 6 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.15 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1600°C with 1 hour soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 58%. Example - 3 750 gms of calcined and pulverized pyrophyllite alongwith 1200 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 5 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.075 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1550°C with 4 hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 48%. Example -4 750 gms of calcined and pulverized pyrophyllite alongwith 1000 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 4 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.10 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1550°C with 2hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-rsy analysis. Percentage of mullite present in the fired material was estimated to be 55%. The main advantages of the process of the present invention are: a) Utilization of apparently unsuable mineral pyriphyllite to produce quality mullite aggregate. b) Low mullitization temperature, thus saving of energy c) Lower cost of production of mullite aggregates. d) Produces dense mullite aggregate which has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C. We Claim: 1. A process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, which comprises; (a) mixing calcined at a temperature in the range of 650°C to 850°C and pulverized pyrophyllite alongwith high alumina minerals such as pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 with aluminium hydrate in the range of 3 to 6 mole% of aluminium hydrates per mole of pyrophyllite to obtain a mixed material, (b) adding aluminium hydroxy hydro gel in the range of 0 to 0.2 mole% into the mixed material to obtain an active precursor, (c) pressing the active precursor into brickets by conventional methods such as herein described and firing the brickets at a temperature in the range of 1550°C to 1600°C, for a period in the range of 1 to 4 hours at the peak temperature to obtain the desired product. 2. A process as claimed in claim 1 wherein, china clay is used instead of kyanite proportionately 3. A process as claimed in claim 1-2 wherein, the aluminium hydrate is of commercial grade. 4. A process as claimed in claim 1-3 wherein, the aluminium hydrate used is such as Bayer's alumina, diaspore. 5. A process as claimed in claim 1-4 wherein, the aluminium hydroxy hydrogel is produced by flash polycondensation technique in alkaline medium. 6. A process as claimed in claim 1-5 wherein, the quantum of mullite obtained is increased by firing at a temperature in the range of 1600°C to 1750°C, for a period in the range of 1 to 4 hours at the peak temperature. 7. A process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel.
Dense mullite aggregate has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C.
The first identification of mullite (3Al2O3: 2SiO2 ; 71.80 wt% Al2O3) in the Al2O3 - S1O2 binary system is credited to the historic work of Bowen and Greig. Toropov and Galakov prepared mullite from alumina gel and quartz. Mullite as solid solution range was first proposed by Shears and Archibold. Aramaki and Roy identified the solid solution range of 71.8-74.3 wt.% Al2O3 They also determined that the solid solution could be extended to 77.7% Al2O3 under metastable conditions. However mullite is generally represented as 3Al2O3 2SiO2 with * 72% Al2O3 that melts incongruently at >1800°C.
Generally, mullite is made by using conventional powder mixing techniques in which alumina and silica powder (having a particle size of approximately 1 micron) are mixed and sintered above 1600°C. However, mullite prepared using these techniques tends to contain traces of other components, such as, a-Al2O3 and a-cristobalite, which indicates that the reaction to mullite is not complete even at such high temperatures. As a result there is decreased density for compositions formed from such mullite. Reference may be made to M. D. Sacks & H-W Lee, "A Review of Powder Preparation Methods and Densification Procedures for Fabricating High Density Mullite", appearing in Mullite and Mullite Matrix Composites, Ceramic Transaction 6, 167-207 (S. Somiya, R. F. Davis, & J. A. Pask eds. 1990), the disclosures of which are herein incorporated by reference in their entirety. When used in industrial applications, it is desirable to have mullite compositions or compacts that have a high density because it enhances the structural integrity of the ceramic
component i.e., prevents cracks and brittleness) and enhances the useful life of the components.
It is also desirable to lower sintering temperatures for mullite i.e., lower the mullitization temperature, and such attempts have been made by trying to reduce the effective particle size of the starting materials used for making the mullite powder. For example, colloid techniques, sol-gel techniques, and solution techniques, which generally operate based on precipitation principles, using typical starting materials that have smaller particle sizes (i.e., less than 50 nm) and higher surface area (i.e., greater than 200 m2/g), which can lead to more complete mullitization even at lower sintering temperatures. Using these methods, sintering temperatures less than about 1200°C. have been obtained. However, despite these various methodologies employed to make mullite powders using lower temperatures, there remains the problem of producing mullite composites also having high density.
In addition, prior art techniques for making mullite powder typically use organic solvents or organic components as part of the reaction conditions, which is ecologically undesirable. There has been a move toward trying to prepare water-based mullite sols. For example, it is known that a water-based silica sol is stable in high pH (≈ 8) and a water-based alumina sol is stable in low pH (≈2) for which reference may be made to R. K. Her, in "The Chemistry of Silica"(John Wiley & Sons 1979) and B. E. Yoldas, Ceramic Bulletin, 54(3), 1975, 286-88 & 289-290, the disclosures of which are herein incorporated by reference in their entirety. When the alumina sol and the silica sol are mixed together, a gel or precipitation is immediately formed. The sol has to stay in a solution state in order to effectively impregnate the preform so that it may be processed into a mullite composite, thus, these methods are not suitable for making mullite composites. Water-based mullite sols have also been reported in the literature for monolith and powder preparations. Reference may be made to, C. J. Brinker & G. W. Scherer, in Sol-Gel Science: " The Physics and Chemistry of Sol-Gel Processing", p. 216-223 (Academic Press, Inc.
1990), the disclosures of which are herein incorporated by reference in their entirety. In these applications, the sol is converted to a gel in a short time (i.e., minutes to days) so a sol having months stability is not required. However, this rapid conversion to a gel makes mullite sols prepared according to this methodology poor candidates for use in impregnating preforms, which is an essential step in making mullite composites. Water-based mullite sols having sintering temperatures of about 1000°C have been synthesised, but they contain hazardous fluoride ion and react with reinforcing preform material. Further, these mullite sols have a high exothermic phase transition from amorphous to crystalline mullite at crystallisation temperatures (about 980° C), which is an indication that mullite composites made from the sol will have poor density characteristics.
Therefore, the mullite sol is an important factor in the success of composite processing. The sol has to be stable for easy handling and processing, it should have a high solids contents for high yield, its components should have a small particle size, it should be homogeneous, it should exhibit little or no exothermic reaction during heat treatment or densification procedures before mullitization, and should be easily converted into substantially crack-free and dense mullite composites at relatively low sintering temperatures. Also, to comply with environmental issues, the sol should preferably be water based.
Reference may be made to United States patent no.: 5,932,168 wherein the inventors: Su; Wei-Fang Anne, have attempted to address the above noted requirements as well as other needs by presenting aqueous mullite precursor sols, unlike the prior art mullite powders, that can be made into mullite composites. The said invention produces stable, water-based mullite precursor sols by hydrolysing an aqueous silane solution with a base and then adding an aqueous solution of aluminium nitrate to the solution. These stable sols can then be used to prepare, substantially crack-free and dense mullite composites of the invention.
The above said process uses silane and the process primarily depends on the stability of mullite sol. Tetra ethoxysilane is used as source of silicon. Therefore the above process suffers from two limitations, that is the requirement of stability of the sol and specificity of source of silica which is generally an alkoxysilane. The alkoxysilane during processing is likely to cause environmental pollution and yield of the product will also be low as this process is limited by solid content of the reaction system. The micro structure development of mullite crystals, were not specifically defined.
Reference may be made to Inui; Tomoyuki, Inoue; Masashi in US Patent No. 5338707 seeks to provide a process for the production of high purity mullites which are highly resistant to heat in their application as various structural materials. Briefly stated, the process involves reacting a mixture of aluminum alkoxide and silicon alkoxide with an atomic ratio (weight) of Al/Si in the range of 2:1 to 7:1 in an aromatic hydrocarbon solvent at a temperature of 200°C-350°C and calcining the resulting reaction product at temperature above 900°C.
Reference may also be made to Yoldas; Bulent E., Partlow; Deborah P., in US Patent No. 4687652 where in provided a novel method for preparing high purity mullite which comprises the steps of (a) partially hydrolyzing a dilute silicon alkoxide solution; (b) adding an aluminum alkoxide to the partially hydrolyzed, dilute silicon alkoxide solution; and (c) eliminating terminal alkoxide groups. In one embodiment of the invention, the liquid is evaporated out and the remaining material is calcined at about 500°C to yield an amorphous structure having the mullite composition. Further heating to about 650°C eliminates alkoxide terminal groups still present in the amorphous material. Firing the material to about 985°C converts the amorphous material to crystalline mullite. In another embodiment of the invention, the solution resulting from step (b), above, is further hydrolyzed using a relatively small amount of water A solution prepared in this manner will yield a clear gel and may be used to deposit a coating on a substrate.
Firing the deposited coating to about 985°C converts the amorphous material to crystalline mullite. In yet another embodiment of the invention, the solution resulting from step (b), above, is further hydrolyzed using a relatively large amount of water to produce a precipitate. This precipitate converts to mullite at about 985°C.
Reference may further be made to Ismail; M. G. M. U., Nakai; Zenjiro, Tsunatori; Hideo in US Patent No. 5045514 with a primary object of the present invention to provide a method for making composite mullite/cordierite ceramics having an increased sintering strength and an enriched mechanical strength, in which sintering can be carried out even at a relatively low temperature, say, 1450°C. It is another object of this invention to provide a method for making composite mullite/cordierite ceramics having an increased sintering density and so not much pores, in which sintering can be carried out even at a relatively low temperature, say, 1450°C. When this ceramic product is used as an IC substrate, a signal pattern formed on its surface is unlikely to break down, since it is substantially rid of pores. Nor does a temperature rise produce an adverse influence on an integrated circuit such as an LSI mounted on the substrate, because water is unlikely to stay therein due to the absence of pores Yet another object of this invention is to provide a method for making composite mullite/cordierite ceramics having a dielectric constant as low as 5.2. By using this ceramic product as an IC substrate, it is possible to improve the transmission properties of an integrated circuit mounted on it. Still another object of this invention is to provide a method for making composite mullite/cordierite ceramics having a coefficient of thermal expansion, say, 3-4.times.10.sup.-6/°C, the figures being close to that of silicon. According to the present invention, the above-mentioned objects are achievable by the provision of a method for making composite mullite/cordierite ceramics characterized by including the steps of: preparing a mullite-intensive sol by mixing alumina and silica sols together, preparing a cordierite-intensive so! by mixing alumina, silica and magnesia sols together, gelating a mixture of
the mullite- and cordierite-intensive sols, calcinating the thus obtained gel, and compacting and sintering the thus calcinated material.
The general disadvantages of above described prior art processes are:
1. Incomplete reaction of the reactants,
2. High mullitization temperature,
3. Ecologically undesirable reaction in cases, where organic solvents or organic components are used as part of the reaction conditions,
4. Low stability of mullite sol in some cases.
The main object of the present invention is to provide a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel which obviates the drawbacks as mentioned above.
Another object of the present invention is to provide a process of making dense mullite aggregates from a cheap and abundantly available mineral pyrophyllite.
Yet another object of the present invention is to provide a process of making dense mullite aggregates from pyrophyllite, wherein mullite formation is over 40%.
Still another object of the present invention is to prepare sintered mullite aggregates with dense microstructure.
Yet another object of the present invention is to prepare sintered dense mullite aggregate which has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory
and kiln furniture for use at comparatively higher temperatures, greater than 1400°C.
In the present invention there is provided a process for manufacturing mullite aggregates from pyrophillite an apparently unusable mineral to produce quality mullite aggregate (of the order of 45 - 65 % mullite). Pyrophyllite, an alumino silicate mineral, abundantly available in nature finds very limited use in refractory industry. Theoretically, pyrophyllite on heating yields mullite, but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. In the present invention the process produces mullite aggregates from a powder precursor containing pyrophyllite, kyanite minerals and aluminium hydrate and intimately mixing the same with AI-O-OH hydroxy hydrogel network produced by flash condensation techniques. The active precursor on firing at a temperature in the range of 1550°C to 1600°C produces mullite aggregates (of the order of 45 - 65 % mullite) with dense microstructure. Percentage of mullitre is estimated by x-ray analysis.
Mullite is formed by two broad and distinctly different mechanisms. It forms at alumina interface if reaction is carried out at high temperature. If the reaction is conducted at 1. AI2O3 dissolves in amorphous SiO2 matrix,
2. Mullite nuclei formed within this matrix as the matrix composition exceeds the saturation limit with respect to mullite and,
3. Mullite crystals grow as more Al2O3 dissolved in to the matrix and then is incorporated into the growing mullite grains.
Therefore to obtain a grown and elongated crystals of mullite in the matrix, temperature of formation of mullite should be low. This is achievable by using highly reactive powder precursor in the hydroxy hydrogel form.
Pyrophyllite, reference for which may be made to Singer F & Singer S.S., "Industrial Ceramics" pp 93-94, Edited by Chapman and Hall, London & New York,(Reprint 1984), is a hydrous aluminium silicate minerals, Al2O3. 4 SiO2. H2O (28.3% Al2O3, 66.7% SiO2l 5.0% H20) and chemically more akin to the clay minerals having a crystal structure similar to the ideal structure for montmorillonite. It is perfectly crystalline and hence does not absorb water or ions. However, pyrophyllite is not preferred as a source of AI2O3 and SiO2 to produce mullite due to the following reason:
Theoretically, pyrophyllite decomposes during heating producing mullite and silica as per the following reaction:
(Formula Removed)
but this reaction is retarded due to the formation of large amount of free silica and alumino-silicate glassy phase. The mineral pyrophyllite used contained alkali, alkaline earth and iron bearing impurities and these largely combine with the siliceous material to form low viscous glassy phase. The amount of silica which readily dissolves in glassy phase at elevated temperature, enhances the viscosity of the silica rich melt and negatively influences the relative amount of mullite. The ultimate structure normally consist of small

amount of secondary mullite embedded in a glassy matrix which make the ultimate product unsuitable for refractory application.
The troublesome glassy phase may be significantly reduced by addition of aluminous substances like kyanite so as to produce secondary mullite in addition to primary mullite derived from kyanite. Kyanite starts changing to mullite at 1300°C [3(Al2O3.SiO2) = 3 Al2O3.2SiO2 + SiO2] and the formation of small nuclei assist in the further crystallization of well developed mullite crystals from the mixture. The number of favorable sites for crystallization is enhanced and the rate of nucleation passes through a maximum. As the temperature is increased the rate of nucleation increases exponentially. Since both the nucleation rate and the growth rate are strongly temperature dependent the over all rate of recrystalization increases rapidly with the temperature. To make the system further efficient, an alumina precursor material may be incorporated in the reaction system in such a way that the nascent silica formed from pyrophyllite will have reactive encounter with it and will combine to produce further mullite. This is possible by preparing a powder precursor containing pyrophyllite along with kyanite particles entrapped in the hydroxy-hydrogel cages of Al - O - OH network.. Gel derived mullite grains are sufficiently fined to develop better bonding with the fused mullite grains during sintering thus enhancing the over all refractory properties of the aggregates.
Accordingly the present invention relates to a process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, which comprises; (a) mixing calcined at a temperature in the range of 650°C to 850°C and pulverized pyrophyllite alongwith high alumina minerals such as pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 with aluminium hydrate in the range of 3 to 6 mole% of aluminium hydrates per mole of pyrophyllite to obtain a mixed material, (b) adding aluminium hydroxy hydro gel in the range of 0 to 0.2 mole% into the mixed material to obtain an active precursor, (c) pressing the active precursor into
brickets by conventional methods such as herein described and firing the brickets at a temperature in the range of 1550°C to 1600°C, for a period in the range of 1 to 4 hours at the peak temperature to obtain the desired product.
In an embodiment of the present invention the pyrophyllite is calcined at a temperature in the range of 650°C to 850°C.
In another embodiment of the present invention china clay is used instead of kyanite proportionately.
In yet another embodiment of the present invention the aluminium hydrate is of commercial grade.
In still another embodiment of the present invention aluminium hydrate used is such as Bayer's alumina, diaspore.
In yet another embodiment of the present invention the aluminium hydroxy hydrogel is produced by flash polycondensation technique in alkaline medium.
In a further embodiment of the present invention the quantum of mullite obtained is increased by firing at a temperature in the range of 1600°C to 1750°C, for a period in the range of 1 to 4 hours at the peak temperature.
The novelty of the process of the present invention is the formation of mullite at lower temperature in higher amount, of the order of 45 - 65 %, with considerable densification characteristics and well developed propeties from an apparently non-usable pyrophyllite. The inventive step lies in forming a powder precursor containing pyrophyllite along with kyanite particles entrapped in the hydroxy-hydrogel cages of Al - O - OH network produced by flash polycondensation technique in alkaline medium.
The stepwise details of process of the present invention are given below:
a) Calcined and pulverized pyrophyllite along with pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 is mixed with commercially available aluminium hydrate in the range of 3 to 6 mole % of aluminium hydrate per mole of pyrophyllite to obtain a mixture.
b) Aluminium hydroxy hydrogel in the range of 0 to 0.2mole% is added to the mixture obtained in step 'a' to obtain an active precursor.
c) The active precursor obtained in step 'b' is then pressed into brickets by conventional methods.
d) The brickets obtained in step 'c' are then fired at a temperature in the range of 1550°C to 1600°C for a period in the range of 1 to 4 hours at the peak temperature.
e) Estimating the formation of mullite percentage by x-ray analysis.
The following examples are given by way of illustration of the process of the present invention in actual practice and therefore, should not be construed to limit the scope of the present invention.
Example -1
500 gms of calcined and pulverized pyrophyllite alongwith 500 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 3 mole % of aluminium hydrate per mole of pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.125 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1600°C with 2 hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was
done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 65%.
Example - 2
300 gms of calcined and pulverized pyrophyllite alongwith 200 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 6 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.15 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1600°C with 1 hour soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 58%.
Example - 3
750 gms of calcined and pulverized pyrophyllite alongwith 1200 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 5 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed material, mixing aluminium hydroxy hydrogel 0.075 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1550°C with 4 hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-ray analysis. Percentage of mullite present in the fired material was estimated to be 48%.
Example -4
750 gms of calcined and pulverized pyrophyllite alongwith 1000 gms of pulverized kyanite were mixed with commercially available aluminium hydrate of 4 mole % of aluminium hydrate per mole pyrophyllite to obtain a mixed
material, mixing aluminium hydroxy hydrogel 0.10 mole% produced by flash polycondensation technique in alkaline medium to obtain an active precurser. Brickets were prepared and fired at 1550°C with 2hours soaking at the peak temperature. The fired material was crushed and estimation of mullite % was done by x-rsy analysis. Percentage of mullite present in the fired material was estimated to be 55%.
The main advantages of the process of the present invention are:
a) Utilization of apparently unsuable mineral pyriphyllite to produce quality mullite aggregate.
b) Low mullitization temperature, thus saving of energy
c) Lower cost of production of mullite aggregates.
d) Produces dense mullite aggregate which has industrial usage as a raw material for making different types of refractories, both shaped and unshaped, such as refractory and kiln furniture for use at comparatively higher temperatures, greater than 1400°C.

We Claim:
1. A process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, which comprises; (a) mixing calcined at a temperature in the range of 650°C to 850°C and pulverized pyrophyllite alongwith high alumina minerals such as pulverized kyanite in the mole ratio ranging from 1:0.5 to 1:7 with aluminium hydrate in the range of 3 to 6 mole% of aluminium hydrates per mole of pyrophyllite to obtain a mixed material, (b) adding aluminium hydroxy hydro gel in the range of 0 to 0.2 mole% into the mixed material to obtain an active precursor, (c) pressing the active precursor into brickets by conventional methods such as herein described and firing the brickets at a temperature in the range of 1550°C to 1600°C, for a period in the range of 1 to 4 hours at the peak temperature to obtain the desired product.
2. A process as claimed in claim 1 wherein, china clay is used instead of kyanite proportionately
3. A process as claimed in claim 1-2 wherein, the aluminium hydrate is of commercial grade.
4. A process as claimed in claim 1-3 wherein, the aluminium hydrate used is such as Bayer's alumina, diaspore.
5. A process as claimed in claim 1-4 wherein, the aluminium hydroxy hydrogel is produced by flash polycondensation technique in alkaline medium.
6. A process as claimed in claim 1-5 wherein, the quantum of mullite obtained is increased by firing at a temperature in the range of 1600°C to 1750°C, for a period in the range of 1 to 4 hours at the peak temperature.
7. A process for manufacturing mullite aggregates from pyrophyllite, high alumina minerals and Al-hydroxyhydrogel, substantially as herein described with reference to the examples.

Documents:

915-del-2003-abstract.pdf

915-del-2003-claims.pdf

915-del-2003-complete specification (granded).pdf

915-del-2003-correspondence-others.pdf

915-del-2003-correspondence-po.pdf

915-del-2003-description (complete).pdf

915-del-2003-form-1.pdf

915-del-2003-form-19.pdf

915-del-2003-form-2.pdf

915-del-2003-form-3.pdf

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Patent Number

199640

Indian Patent Application Number

915/DEL/2003

PG Journal Number

38/2008

Publication Date

19-Sep-2008

Grant Date

08-Dec-2006

Date of Filing

22-Jul-2003

Name of Patentee

Council of Scientific and Industrial Research

Applicant Address

Rafi Marg New Delhi, 110001,India

Inventors:

#	Inventor's Name	Inventor's Address
1	Tapas Kumar Mukhopadhyay	Central Glass & Ceramic Research Institute, Kolkata 700032,India
2	Syamal Ghosh	Central Glass & Ceramic Research Institute, Kolkata-700032 India.
3	Sankar Ghatak	Central Glass & Ceramic Research Institute, Kolkata-700032 India.
4	Sukhen Das	Central Glass & Ceramic Research Institute, Kolkata-700032 India
5	Sachchidananda Chakrabarti	Central Glass & Ceramic Research Institute, Kolkata-700032 India

PCT International Classification Number

B015 021/12

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA