Title of Invention

A PROCESS FOR MAKING DENSE MULLITE AGGREGATE

Abstract A process for making dense mullite aggregate with high density and preferred lathe-shaped microstructure wherein the process utilises flash polycondensation of reactive silica and alumina in aqueous medium. The process is economic and ecofriendly and is capable of producing very high purity product.
Full Text The present invention relates to a process for making dense mullite aggregate. Dense mullite aggregate is used as a raw material for making different types of refractories, both shaped and unshaped. It is also used as one of the constituent for making aircraft brake pad materials.
The first identification of mullite (3A12: 2SiO2; 71.80 wt% Al2 O3) in the A12O3 - SiO2 binary system is credited to the historic work of Bowen and Greig. Toropov and Galakov prepared mullite from alumina gel and quartz. Mulite 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.% A12O3 .They also determined that the solid solution could be extended to 77.7% A12O3 under metastable conditions. However mullite is generally represented as 3A12O3 .2SiO2 with « 72% A12O3 that melts incongruently at >1800°C.
Generally, mullite is made 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-Al2 O3 and a-cristobalite, which indicates that the reaction to mullite is not complete

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, have been investigated. As referred by M.D. Sacks and H-W Lee in " A review of powder preparation methods and densification procedure for fabricating high density mullite" appearing in mullite and mullite. These methods typically use starting materials that have smaller particle sizes (i.e., less than 50 nm) and higher surface

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 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

(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 synthesized, 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 crystallization 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

Wei-Fang Anne, United States Patent: 5,932,168 addresses these 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.
The general disadvantages of above processes are : (1) Incomplete reaction of the reactants,

(3) Ecologically undesirable reaction in cases, where organic solvents or organic components are used as part of the reaction conditions,
(4) Poor densification characteristics of the prepared mullite,
(5) Low stability of mullite sol in some cases.
The main object of the present invention is to provide a process for making dense mullite aggregates which obviates the drawbacks as mentioned above.
Another object of the present invention is to provide a process which is eco-friendly.
Yet another object of present invention is to use aqueous medium for reaction for the preparation of mullite precursor powder.
Still another object of the present invention is to prepare sintered mullite aggregates with lathe shaped crystals of mullite in the dense microstructure.
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

the silica rich matrix. The ease of formation of mullite is strongly dependent on the starting raw materials, alpha- A12O3 has strong bonds requiring a relatively large amount of energy for release of A12O3 molecules in comparison with the energy requirement for dissociation from a sol-gel type of starting material. Similarly silica has a high degree of covalence which makes restructuring more difficult. The more reactive precursors are those chemically prepared wherein homogenous atomic mixtures are formed. In such system reaction between SiO2 and Al2O3 proceed in the following sequence: (i) A12O3 dissolves in amorphous SiO2 matrix, (ii)mullite nuclei formed within this matrix as the matrix composition exceeds the
saturation limit with respect to mullite and, (iii) mullite crystals grow as more A12O3 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.
In the present invention source of silica was reactive silica which reacts with water, forming silicic acid which polymerises into three dimensional network.

they will mutually arrange in their respective preferred crystallographic sites. A flash polycondensation in alkaline medium will freeze the atomic site and will produce a solid substance in a metastable reactive phase with preferred ionic distribution. The novelty of the process of the present invention is ease of manufacture of sintered mullites aggregates in large amounts with lathshaped microstructure and inventive steps lies in flash poly condensation of ionic spices in metastable state by mixing Al+3- Si+4 containing suspension by excess hydroxyl ions.
Accordingly the present invention provides a process for making dense mullite aggregate which comprises preparing a slurry of reactive silica in 20 to 80 % alumina salt solution so as to maintain a proportion of 28 wt % SiO2: 340 to 528 wt % aluminium salt, subjecting the slurry to flash poly condensation by pouring the said slurry into a saturated ammonia solution and maintaining the pH of the reaction system in the range of 3.5 to 5.5 to obtain a gel like mass, ageing the gel like mass for a period in the range of 6 to 36 hours, removing the liquid by conventional method to obtain a solid gel like mass, washing the solid gel like mass with water to remove soluble salts, drying the gel like mass at a temperature in the range of 50°C to 150°C , calcining the dried gel like mass at a temperature

calcined powder under a pressure of 50 to 250 MPa to obtain pelletised material, sintering the said pelletised material at a temperature in the range of 1400°C to 1700°C.
In an embodiment of the present invention, reactive silica used may be such as silica fumes, silica flowers, silica gel, precipitated silica.
In another embodiment of the present invention, aluminium salt used may be such as aluminium nitrate, aluminium chloride, aluminium sulphate.
The process steps of the present invention are as follows:
(1) A concentrated (20 to 80 %) aluminium salt solution is prepared.
(2) Reactive silica is mixed with 20 to 80 % aluminium salt solution, so as to maintain a proportion of 28 wt% SiO2: 340 to 528 wt % aluminium salt.
(3) Mixture is stirred vigorously to obtain a slurry.
(4) The slurry is subjected to flash polycondensation by pouring into saturated ammonia solution and maintaining the pH of the reaction system in the range of 3.5 to 5.5. A gel like mass is obtained.
(5) Ageing the material obtained in (4), for a period in the range of 6 to 36
hours

(6) Filtering the aged mixture and washing with water to remove soluble salt.
(7) Drying the washed material at a temperature in the range of 50 to 150 ° C.
(8) Calcining the dried material at a temperature in the range of 550°C to 900°C.
(9) Pelletising the calcined material in a press under a pressure of 50 to 250 Mpa.
(10) Sintering the pelletised material at a temperature in the range of 1400°C to 1700°C.
The following examples are given as illustrations and should not be construed to limit the scope of the present invention.
Example 1 :
528 grms A1(NO3)3. 9 H2O is dissolved in 2640 ml deionised water to obtain a 20 % solution of the salt. 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 3.5 .It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were
calcinated at1400 deg C to 100 deg C • 1 r\r\ 11 • .

Example 2 :
528 grms A1(NO3)3. 9 H2O is dissolved in 1760 ml deionised water to obtain a 30 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.0 . It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 3 :
528 grms A1(NO3)3. 9 H2O is dissolved in 1321 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.5 . It was kept for 06 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were

Example 4:
528 grms A1(NO3)3. 9 H2O is dissolved in 1056 ml deionised water to obtain a 50 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.0. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 5:
528 grms Al (NO3)3. 9 H2O is dissolved in 880 ml deionised water to obtain a 60 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.5. It was kept for 14 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were

Example 6 :
528 grms A1(NO3)3. 9 H2O is dissolved in 755 ml deionised water to obtain a 70 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 3.5. It was kept for 10 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1450°C to obtain 100 gms mullite.
Example 7 :
528 grms A1(NO3)3.9 H2O is dissolved in 660 ml deionised water to obtain a 80 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.0. It was kept for 30 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 8 :
340 grms A1C13. 6 H2O is dissolved in 1700 ml deionised water to obtain a 20 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.5. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 9:
340 grms AlCI3. 6 H2O is dissolved in 1134 ml deionised water to obtain a 30 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.0 . It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1550°C to obtain 100 gms mullite.
Example 10 :
340 grms A1C13. 6 H2O is dissolved in 850 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.5. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 11 :
450 grms A12(SO4)3. 18 H2O is dissolved in 2250 ml deionised water to obtain a 20 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 3.5. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 12 :
450 grms A12(SO4)3. 18 H2O is dissolved in 1500 ml deionised water to obtain a 30 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.0. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1500°C to obtain 100 gms mullite.
Example 13 :
450 grms Al2(SO4)3. 18 H2O is dissolved in 1125 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.5. It was kept for 20 hours for ageing. The aged mixture was filtered and solid residue was washed with

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 14 :
450 grms A12(SO4)3. 18 H2O is dissolved in 1125 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 4.5. It was kept for 08 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1700°C to obtain 100 gms mullite.
Example 15 :
340 grms A1C13. 6 H2O is dissolved in 850 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.5. It was kept for 14 hours for ageing. The aged mixture was filtered and solid residue was washed with

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 16 :
340 grms A1C13. 6 H2O is dissolved in 850 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.5. It was kept for 12 hours for ageing. The aged mixture was filtered and solid residue was washed with distilled water to remove soluble salt. The solid mass was then dried at 60°C for 01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1400°C to obtain 100 gms mullite.
Example 17:
340 grms AlCl3. 6 H2O is dissolved in 850 ml deionised water to obtain a 40 % solution of the salt 28 grms reactive silica is added into it. The mixture is stirred vigorously. A slurry was obtained. The slurry was poured into concentrated NH4OH solution till the pH of the mixture becomes 5.5. It was kept for 24 hours for ageing. The aged mixture was filtered and solid residue was washed with

01 hour. The dried material was calcined at 550°C for 02 hours. Pellets were sintered at 1500°C to obtain 100 gms mullite.
In the present invention, as described and illustrated herein above, is a process for making dense mullite aggregrate with high density and preferred lathe-shaped microstructure wherein the process utilises flash polycondensation of reactive silica and alumina in aqueous medium. The process is economic and ecofriendly and is capable of producing very high purity product.
The main advantages of the process of the present invention are :
(1) Complete reaction of the reactants.
(2) Low mullitization temperature.
(3) Ecological friendly as no organic solvents or organic component are used as part of the reaction conditions.
(4) Better densification characteristics of the prepared mullite.
(5) Presence of lathe shaped crystals in the microstructure.

Documents:

119-del-2002 abstract.pdf

119-del-2002 claims.pdf

119-del-2002 coreespondence po.pdf

119-del-2002 corspondence others.pdf

119-del-2002 descrijpdtion complete.pdf

119-del-2002 form-1.pdf

119-del-2002 form-18.pdf

119-del-2002 form-2.pdf

119-del-2002 form-3.pdf


Patent Number 221966
Indian Patent Application Number 119/DEL/2002
PG Journal Number 32/2008
Publication Date 08-Aug-2008
Grant Date 12-Jul-2008
Date of Filing 15-Feb-2002
Name of Patentee COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA
Inventors:
# Inventor's Name Inventor's Address
1 SANKAR GHATAK CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
2 ASIS KUMAR BASU CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
3 KRISHNENDU ADHIKARY CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
4 SUKHEN DAS CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
5 SURAJIT GUPTA CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
6 HIMADRI SHEKHAR MAITI CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
PCT International Classification Number C01B 33/26
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA