Title of Invention

"A PROCESSOF MANUFACTURING LITHIUM ALUMINOSILICATE POWDERS"

Abstract

The present invention relates to a process for the manufacture of lithium aluminosilicate powders. They find applications in industry as furnace materials and as gas turbine heat exchangers. They are also used as a matrix of ceramic matrix composites (CMC). The process is economical, environment friendly, high yielding and item saving which makes the process suitable for industrial manufacture. It is difficult to obtain a solid precipitate from a system containing A13+, Li+ and Si + in solution or suspension at a fixed pH following the co precipitation technique as the method involves inhomogeneous and non-stoichiometric precipitation of the hydrated metal oxides due to their different solubilities. This problem was obviated by using active silica in aqueous medium which undergoes hydrolysis forming silicic acid network. In case both Al3+ and Li+ are entrapped in the silicic acid network, on further hydrolysis, A13+ also forms a A1-O_OH network with H2O molecules contributing towards fulfillment of co-ordination requirement.

Full Text

The present invention relates to a process of manufacturing lithium aluminosilicate powders. This invention particularly relates to a process for manufacturing lithium aluminosilicate powders from water-based sols and gels. The lithium aluminosilicate powders like p-spodumene, fl-eucryptite have ultra low and even negative thermal expansion coefficients along with good thermal and chemical durability, and are particularly important from a tecnological standpoint. They find applications in industry as furnace materials and as gas turbine heat exchangers where the dimensional stability and ability to resist thermal shock are necessary. They are also used as a matrix of ceramic matrix composites (CMC). The present day methods of preparing lithium aluminosilicate powders mainly consists of solution phase reactions. Reference may be made to S.-F. Ho, L. C. Klein and R. Caracciolo in "J. Non-Cryst. Solids 120 (1990) pp. 267-274" wherein lithium aluminosilicate powder was prepared using lithium nitrate, aluminium nitrate and tetraethylorthosilicate (TEOS) as the precursors for Li, Al and Si respectively. The solutions were prepared by mixing methanol, water, hydrochloric acid and salts at 90°C for Ih followed by cooling TEOS was added to the solution. The sols were cast into polystyrene molds or spun onto polished silica disks. The main drawback of the above process is that the synthesis of material is carried out entirely in organic solution, which during drying creates pollution in the atmospheres. Reference may also be made to G. Orcel, J. Phalippou and L.L. Hench in "J. Non-Cryst. Solids, 82 (1986) pp 301-306" wherein the gel composition chosen for study corresponded to the mineral eucryptite. The gels were prepared by hydrolyzing a methanolic solution of the metal organic precursors, tetramethoxysilane (TMOS), aluminium secondary butoxide (ABS), lithium methoxide (LM) and sodium methoxide (SM). The gelation occurs within minutes at room temperature. The gels were dried at 60°C for a few hours and oxidised at 400°C for Ih. For the crystallization study, the samples were directly introduced at the specified temperature and treated for Ih in air atmosphere. The main drawbacks of the above process are: (i) The alkoxides used as the precursor materials are costly and moisture-sensitive, (ii) An intermediate oxidation step at 400°C is necessary. Reference may further be made to M. H. Lin and M. C. Wang in "J. Mat. Sci. 30 (1995) pp 2716-2721" wherein precursor powder with a spodumene composition was prepared from tetraethoxy silane (TEOS), A1(OC2H5)3, LiOCH3 and Zr(OC2H5)4 as the starting materials using formamide as a drying control chemical additive. TEOS was partially hydrolyzed at 60°C for 60h in presence of hydrochloric acid as the catalyst. The TEOS ethanol solution reacted with aluminium, lithium and zirconium alkoxides and resulted in a transparent solution with a cation ratio similar to the composition of spodumene. This solution was completely hydrolyzed by adding a large amount of excess water (FbOiTEOS = 30). The gel was then obtained via the sol. The gel was dried at 120°C for 24h and calcined at 400°-1200°C for Ih. The main drawbacks of the above process are: (i) The method involves long processing steps and time, (ii) The alkoxides used as the precursor materials are costly and moisture sensitive. Reference may also be made to L. Zhien, S. Yihui, D. Xijiang and C. Jijian in "J. Mat. Sci. 30 (1995) 390-394" wherein lithium aluminosilicate powders containing Mg, Zn and Ti (or Zr) were prepared using tetraethylorthosilicate (TEOS), titanium butoxide (or ZrCU) and inorganic salts of Li, Al, Mg (and Zn). In this process, inorganic salts of Li, Al, Mg (and Zn) were dissolved in a solvent and mixed under stirring with a prehydrolysed solution of TEOS, titanium butoxide (or ZrCU) when a homogeneous sol was obtained. The sol was converted to the gel in presence of ammonia as the catalyst. The gel was dried and finally calcined for obtaining lithium aluminosilicate powders. The method suffers the following drawbacks: (i) The method requires an additional prehydrolysis step to prevent rapid hydrolysis of titanium butoxide and ZrCU. (ii) Synthesis of material is carried out in organic solvent which during drying might pollute the atmosphere. Reference may further be made to N. N. Ghosh and P. Pramanik in " Brit. Ceram. Trans. 96 (1997) pp 155-159 wherein the lithium aluminosilicate powders with the spodumene and eucryptite compositions were prepared using Si(OC2Hs)4 (TEOS), Li2CO3> A1(NO3)3. 9H2O and formic acid. The method involves the preparation of freshly precipitated aluminium hydroxides from aluminium nitrate solution using ammonium hydroxide followed by its dissolution in aqueous formic acid to give aluminium formate solution. Lithium formate was prepared by reacting lithium carbonate with aqueous formic acid. The metal formate solutions were then added to TEOS according to the desired compositions. Gel formation was accomplished by adopting slow stirring and heating. The gels after drying at 100°C/2h were subjected to calcination in air from 500° to 800°C. The main drawbacks of the above process are: (i) The method involves long processing time. (ii) Synthesis reaction is carried out in organic medium which pollutes atmosphere during drying of the precursor gel materials. The main objective of the present invention is to provide a process for the manufacture of lithium aluminosilicate powders which obviates the drawbacks as detailed above. Another object of the present invention is to provide a process for manufacturing lithium aluminosilicate powders using water-based sols and gels which are not health hazardous and does not create any atmospheric pollution during heat treatment. Yet another object of the present invention is to provide a process for the manufacture of lithium aluminosilicate powders which uses precursor chemicals that are cost-effective, and can be handled without any specific precautions. Still another object of the present invention is to provide a process for the manufacture of lithium aluminosilicate powder which crystallizes at a very low temperature, thus making the process energy efficient. Another object of the present invention is to provide a process of manufacturing lithium aluminosilicate powder which is very simple and cost-effective. Accordingly, the present invention provides a process for the manufacture of lithium aluminosilicate powders, which comprises preparing an aqueous solution of aluminium salt such as aluminum nitrate nonahydrate, Al(NOs)39H2O, aluminum chloride hexahydrate, A1C13.6H2O with A13+ concentration in the range of 1-3 M, filtering, adding a water soluble Lewis base such as ammonia solution with a concentration in the range of 15-25 wt% under stirring at ambient temperature to obtain a solution of pH in the range of 3.0 - 4.5, heating the resulting solution at a temperature in the range of 70 - 90 C to a sol with pH in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPas, adding further, ammonia solution drop by drop to the sol kept at a temperature in the range of 70 - 90°C thereby increasing the pH in the range of 3.5-4.5 and viscosity in the range of 20-35 mPa s, adding a water soluble salt of lithium such as lithium nitrate, LiNOs, lithium chloride, LiCl with Al: Li mole ratio in the range of 1:1 to 1:3, into the resulting alumina sol kept at a temperature in the range of 70 - 90°C to obtain a alumina-lithia bi-component sol, adding an active silica-containing material such as fumed silica, rice husk ash silica, precipitated silica sol which will form silicic acid in aqueous medium, continuous heating of the resulting mixture at a temperature in the range of 85 - 95°C to obtain a dry powder, calcining the dry powder at a temperature in the range of 800 - 1000 C for a period in the range of l-6h. The process comprises the following operations : 1. An aluminium metal salt solution was prepared by dissolving aluminium nitrate nonahydrate, A1(NO3)3.9H2O, aluminium chloride hexahydrate, A1C13.6H2O in water with A13+ concentration in the range of 1-3 M, and the solution was filtered. 2. A water soluble Lewis base such as ammonia solution with a concentration in the range of 15-25 wt% was added to the aluminium metal salt solution under stirring at ambient temperature to obtain a solution of pH in the range of 3.0 - 4.5. 3. The resulting solution was heated at a temperature in the range of 70° - 90°C for polymerizing it to a sol with pH and viscosity in the ranges 3.0 - 4.0 and and 5-15 mPa s respectively. 4. Ammonia solution was further added drop by drop to the alumina sol kept at a temperature in the range of 70° - 90°C to accelerate further polymerization, thereby increasing the pH and viscosity in the range of 3.5-4.5 and 20-35 mPa s respectively. 5. An water soluble salt of lithium such as lithium nitrate, LiNOs, lithium chloride, LiCl with Al: Li mole ratio in the range of 1:1 to 1:3, was added to the alumina sol kept at a temperature in the range of 70° - 90°C, thereby resulting in the formation of a clear bi- component alumina-lithia sol. 6. An active silica-containing material such as fumed silica, rice husk ash silica, precipitated silica sol which will form silicic acid in aqueous medium, was added to the resulting alumina-lithia sol. 7. The mixture was then continuously heated at a temperature in the range of 85°-95°C untill a dry powder is obtained. 8. The powder when calcined at a temperature in the range of 800°-1000°C for a period in the range of l-6h, produced crystalline lithium aluminosilicate powder. The novelty of the present invention primarily resides in providing a process which is economical, environment friendly, high yielding and time saving which makes the process suitable for industrial manufacture and the non-obvious inventive steps lies as follows: It is difficult to obtain a solid precipitate from a system containing A13+, Li+ and Si4+ in solution or suspension at a fixed pH following the coprecipitation technique as the method involves inhomogeneous and non-stoichiometric precipitation of the hydrated metal oxides due to their different solubilities. This problem was obviated by using active silica in aqueous medium which undergoes hydrolysis forming silicic acid network. In case both A13+ and Li+ are entraped in the silicic acid network, on further hydrolysis, A13+ also forms a Al-O-OH network with H2O molecules contributing towards fulfilment of coordination requirement. Alkaline earth elements such as Mg2+ may remain in gel structure by hydroxy bridging which is not possible with alkali metals as those ions have a natural tendency of seeping out into solution. This problem may be counteracted by creating an environment where alkali metal cations will be with solid substance by forming bonds with non-bridging oxygen of SiO44" tetrahedra or associated with A1O4" units in the pores of the gel structure (Ref: B. Wang, S. Szu, M. Greenblatt and L. C. Klein in "Chem. Mat. 4 (1992) pp 191-197"). Thus in a complex networking between silicic acid network and Al-O-OH network, Li+ is involved for charge balancing, maintaining homogeneity and quantitative stoichiometry in the aqueous system. Thus the inventive steps lies in using active silica in aqueous medium for the above mentioned reaction. The invention is described herein in details in the following examples, which are cited by way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1 20.1751 g of Al(NOs)3.9H2O was dissolved in 55 mL of deionized water to make Al(NO3)s concentration of about 1 M. The solution was filtered to remove the undissolved impurities. To this solution, concentrated ammonia solution (25wt%, GR) was added under vigorous stirring until the pH of the solution becomes 3. The resulting solution was then heated at 75° ± 1°C for Ih for obtaining a sol by polymerization. The pH and viscosity of the sol thus obtained was 3.0 and 15 + 1 mPa s respectively. The sol was further heated at 75° ± 1°C in which concentrated ammonia solution (25 wt%) was again added drop by drop for further polymerization, maintaining the pH of the sol at 3.5. The viscosity of the resulting clear sol was 20+1 mPa s. To this sol, 3.7080 g of lithium nitrate, LiNOs was dissolved under stirring to obtain a mixed sol. To this mixed sol, 6.7305 g of rice husk ash silica was added under stirring. The mixture containing alumina, lithia and silica was continuously heated at 80° + 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed p-spodumene phase after calcination of the gel powder at 1000°C for Ih. Example 2 29.7869 g of Al(NO3)s.9H2O was dissolved in 55 mL of deionized water to make Al(NOs)3 concentration of about 1.5 M. The solution was filtered to remove the undissolved impurities. To this solution, ammonia solution (15wt%, GR) was added under vigorous stirring until the pH of the solution becomes 3.3. The resulting solution was then heated at 80° ± 1°C for 1.5h for obtaining a sol by polymerization. The pH and viscosity of the sol thus obtained was 3.5 and 20 ± 1 mPa s respectively. The sol was further heated at 75° ± 1°C in which concentrated ammonia solution (25 wt%) was again added drop by drop for further polymerization, maintaining the pH of the sol at 3.5. The viscosity of the resulting clear sol was 30 ± 1 mPa s. To this sol, 5.4746 g of lithium nitrate, LiNOs was dissolved under stirring to obtain a mixed sol. To this mixed sol, 4.9684 g of rice husk ash silica was added under stirring. The mixture containing alumina, lithia and silica was continuously heated at 85° ± 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed p-eucryptite phase after calcination of the gel powder at 900°C for 3h. Example 3 20.1751 g of A1(NO3)3.9H2O was dissolved in 55 mL of deionized water to make A1(NO3)3 concentration of about 1 M. The solution was filtered to remove the undissolved impurities. To this solution, ammonia solution (20wt%, GR) was added under vigorous stirring until the pH of the solution becomes 4. The resulting solution was then heated at 80° ± 1°C for Ih for obtaining a sol by polymerization. The pH and viscosity of the sol thus obtained was 3.5 and 20 ± 1 mPa s respectively. The sol was further heated at 75° + 1°C in which concentrated ammonia solution (25 wt%) was again added drop by drop for further polymerization, maintaining the pH of the sol at 3.5. The viscosity of the resulting clear sol was 25 ± 1 mPa s. To this sol, 3.7080 g of lithium nitrate, LiNO3 was dissolved under stirring to obtain a mixed sol. To this mixed sol, 6.7336 g of fumed silica was added under stirring. The mixture containing alumina, lithia and silica was continuously heated at 90° ± 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed P-spodumene phase after calcination of the gel powder at 1000°C for Ih. Example 4 29.7869 g of A1(NO3)3.9H2O was dissolved in 40 mL of deionized water to make A1(NO3)3 concentration of about 2 M. The solution was filtered to remove the undissolved impurities. To this solution, ammonia solution (20wt%, GR) was added under vigorous stirring until the pH of the solution becomes 3.3. The resulting solution was then heated at 80° + 1°C for 1.5h for obtaining a sol by polymerization. The pH and viscosity of the sol thus obtained was 3.5 and 20 ± 1 mPa s respectively. The sol was further heated at 75° ± 1°C in which concentrated ammonia solution (25 wt%) was again added drop by drop for further polymerization, maintaining the pH of the sol at 3.5. The viscosity of the resulting clear sol was 35 + 1 mPa s. To this sol, 5.4746 g of lithium nitrate, LiNO3 was dissolved under stirring to obtain a mixed sol. To this mixed sol, 4.9708 g of fumed silica was added under stirring. The mixture containing alumina, lithia and silica was continuously heated at added under stirring. The mixture containing alumina, lithia and silica was continuously heated at 80° ± 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed ß-eucryptite phase after calcination of the gel powder at 800°C for 5h. The main advantages of the present invention are: (i) The process for the manufacture of lithium aluminosilicate powders does not require any sophisticated instrument. (iii) It is a tailor-made process, as the particle size and size distribution can be varied according to the necessity by changing the process parameters, (iv) The process is simple and cost-effective. (v) It uses water-based sols as the precursor materials which are not health-hazard and does not create any atmospheric pollution during heat-treatment of gel materials, thus the process is environment friendly. (vi) The process uses raw materials which can be handled without any specific precautions. We Claim: 1. A process for the manufacture of lithium aluminosilicate powders, which comprises preparing an aqueous solution of aluminium salt such as aluminum nitrate nonahydrate, A1(NO3)39H2O, aluminum chloride hexahydrate, A1C13.6H2O with A13+ concentration in the range of 1-3 M, filtering, adding a water soluble Lewis base such as ammonia solution with a concentration in the range of 15-25 wt% under stirring at ambient temperature to obtain a solution of pH in the range of 3.0 - 4.5, heating the resulting solution at a temperature in the range of 70 - 90°C to a sol with pH in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPas, adding further, ammonia solution drop by drop to the sol kept at a temperature in the range of 70 - 90 C thereby increasing the pH in the range of 3.5-4.5 and viscosity in the range of 20-35 mPa s, adding a water soluble salt of lithium such as lithium nitrate, LiNOs, lithium chloride, LiCl with Al : Li mole ratio in the range of 1:1 to 1:3, into the resulting alumina sol kept at a temperature in the range of 70 - 90°C to obtain a alumina-lithia bi-component sol, adding an active silica-containing material such as fumed silica, rice husk ash silica, precipitated silica sol which will form silicic acid in aqueous medium, continuous heating of the resulting mixture at a temperature in the range of 85 - 95°C to obtain a dry powder, calcining the dry powder at a temperature in the range of 800 - 1000°C for a period in the range of l-6h. 2. A process for the manufacture of lithium aluminosilicate powders substantially as herein described with references to the examples.

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