Title of Invention

" A PROCESS OF MANUFACTURING CORDIERITE (2MgO.2AL2O3.SiO2) POWDER"

Abstract

Title : A process for the manufacture of cordierite powders A process for the manufacture of cordierite powders characterized by preparing an aqueous solution of aluminium salt such as aluminium nitrate nonahydrate (A1(N03)3.9H20), aluminium chloride hexahydrate (AlCl36H20) with A13+ concentration in the range of 1 -3 M, dissolving a water soluble salt of magnesium such as magnesium nitrate hexahydrate, Mg(N03)2.6H20, magnesium chloride hexahydrate MgCl2.6H20 in aluminium metal salt solution maintaining a Al: Mg mole ratio in the range of 1: 1 to 1:3 to obtain a mixed solution, 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 in the mixed solution to obtain a mixed 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 obtain a sol with pH in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPa s, adding further, ammonia solution drop by drop to the said 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 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 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 1000°-1300°C for a period in the range of 1-1O h.

Full Text

The present invention relates to a process of manufacturing cordierite (2MgO.2Al2O3.SiO2) powder. This invention particularly relates to a novel technique for the manufacture of cordierite powders from water-based sols and gels. The cordierite powders are used in various fields such as microelectronic packaging, automobile exhaust catalysis support, turbine engine, refractory application. The present day methods of preparing cordierite powders mainly consists of solution phase reaction. Reference may be made to R. Petrovic and D. Janackovic in "J. Mater. Res. 16 (2001) pp. 451-458" wherein cordierite powder was prepared using silica sol, boehmite sol and an aqueous solution of Mg(N03)2 .6H20. The silica sol (pH = 9.8) was obtained from water glass by the ion exchange method. The boehmite sol (pH = 3.8) was prepared by peptization of freshly prepared AI(OH)3 with nitric acid under reflux. The boehmite and silica sol were mixed and the mixture was stirred for 2h. An aqueous solution of Mg(N03)2.6H20 was added and the stirring was done further for 2h. the multi component colloidal dispersion was subjected to gelation by the addition of ammonium carbonate. The resulting gel after drying at 40°C for 2 days was further dried for 24h at 110°C. The dried gel was ground, calcined at 600°C for 4h to remove the volatiles. Further calcination at 1300°C for 2h produced cordierite powders. The drawbacks of the above process are: (i) Since source of silica is water glass i.e. sodium silicate, their is every possibility of contamination by Na+ ions, (ii) Preparation of boehmite sols by peptization involves too many steps. Reference may also be made to D. Pal, A K. Chakraborty, S. Sen and S. K. Sen in "J. Mater. Sci. 33 (1996) pp. 3995-4005" wherein cordierite powder was prepared by the sol-gel route in an organic solvent, i.e. 2-methoxy ethanol using silicon tetraethoxide (TEOS), aluminium sec-butoxide and magnasium acetate tetrahydrate in presence of. a chelating agent, glacial acetic acid. The molar ratio of acetic acid to TEOS used were 0.50 and 0.34. The samples were made with two different molar ratios of water to TEOS, 19.6 and 6.6. The pH of the water added was also varied, i.e. 7 and 2. The gels after drying in room temperature and subsequently drying at 130°C in an air oven were thereafter heat-treated at 700°C in oxygen atmosphere to get the amorphous powder. Cordierite was obtained after calcination at 1200°C for 2h. The main drawbacks of the above process are: (i) The alkoxides used as the precursor materials are costly and moisture-sensitive, (ii) A chelating agent, i.e acetic acid is used to control the hydrolysis of the aluminium sec-butoxide. (iii) Synthesis of material is carried out entirely in organic solution, which during drying creates pollution in the atmospheres. Reference may further be made to M. Nogami, S. Ogawa and K. Nagasaka in "J. Mater. Sci., 16 (1989) pp. 4339-4342" wherein cordierite powder was prepared following a sol-gel route using Al (OC4H9)3, Si(OC2H5)4 and magnesium powder were used as the raw materials. In this process, magnesium powder (1.66g), Al(OC4He)3 (33.63g) and Si(OC2H3)4 (35.61g) were dissolved in 1-propanol (50g), followed by addition of 1.0 mL of CCl4 as a catalyst, and refluxed at 90°C for l0h under nitrogen. The clear sol was checked poured into the polystyrene container and left at room temperature for about 5 days for gel formation. The gel was dried under different conditions. Cordierite powder was obtained at 1600°C for 3h. The method suffers from the following drawbacks: (i) Drying/calcination of the material of product derived through organic medium pollutes atmosphere. (ii) Reflux in N2 atmosphere is necessary, (iii) Crystallization temperature is high. Reference may further be made to Y. F. Chen and S. Vilminot in "J. Sol-Gel Sci. Technol. 5 (1995) pp. 41-47" wherein synthesis of cordierite powder was carried out using Mg(NO3)2 -6H2O, Al (NO3)3 .9H2O, and Si(OC2H5)4 in isopropanol at 60°C. Gel formation was accomplished under basic condition using ammonia solution (20wt%). The dried gel when calcined at 1100°C for 4h is converted to cordierite as the major phase along with spinel as the minor phase. The main difficulties of the above process are: (i) The reaction is carried out in organic medium, (ii) Even at 1100°C/4h, phase-pure cordierite was failed to obtain. Reference may be made to U. Selvaraj, S. Komarneni and R. Roy in "J. Am. Ceram. Soc. 73 (1990) pp. 3663-3669" wherein cordierite powder was prepared from a multi-component metal alkoxide gel from a mixture of single metal alkoxides, like magnesium ethoxide, aluminium isobutoxide and tetraethylorthosilicate in 2-methoxyethanol and 2, 4-pentanedione, 2, 4-pentanedione was used as a complexing agent to overcome different hydrolysis rates of metal alkoxides. The mixture was refluxed in argon atmosphere at 121°C for 12h, The refluxed solution after acidified with HNO3, was hydrolysed with an aqueous 2-methoxyethanol solution. The hydrolysed solution during stirring at 60°C converted to the corresponding gel within 12h. A white powder was obtained by drying the gel in air at 60°C and heating at 800°C for 8h. At 900°C/6h, µ-cordierite and at 1040°C/6h, a-cordierite was obtained. In another method, an acidified hydrolysed solution of silicon tetraethoxide was mixed with the ethanolic solution of A1(NO3)3 -9H2O and Mg(NO3)2 .6H2O at 25°C and the mixed solution after heating at 60°C converted to a cordierite gel. The gel was converted to the corresponding powder at 800°C/8h which further heating at 1300°C/6h converted to a-cordierite powder. The main drawbacks of the above process are: (i) The alkoxides are costly and moisture sensitive, (ii) A complexing agent is required to control the hydrolysis rate of the alkoxides. (iii) A long time refluxing step in Ar atmosphere is essential. Reference may further be made to S. Kumar, K. K. Singh and P. Ramchandrarao in "J. Mat. Sci. Lett., 19 (2000), pp. 1263-1265" wherein cordierite powder was prepared from fly ash, a by-product of thermal power plants resulting from the combustion of pulverized coal in furnaces. In addition to the fly ash, other raw materials used for the synthesis of cordierite was calcined alumina and talc. As per the required proportion the raw materials were wet-milled for l0h to get the desire fineness. The milled product after drying was powdered and granulated using liquid binder. The granules were compacted in the desire shapes under pressure and heated at 1350°C/2h in air. The product contained other phases in addition to cordierite. The main drawbacks of the above method are: (i) The cordierite phase is always contaminated with other unwanted materials, (ii) A long time milling steps is necessary for the desire fineness. The main objective of the present invention is to provide a process for the manufacture of cordierite (2MgO.2Al2O3.SiO2) powders which obviates the drawhacks as detailed above. Another object of the present invention is to provide a process for manufacturing cordierite powders using water-basgd_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 cordierite 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 cordierite 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 cordierite powder which is very simple and cost-effective. Accordingly, the present invention provides a process for the manufacture of cordierite (2MgO.2Al2O3.SiO2) powder characterized by preparing an aqueous solution of aluminium salt such as aluminium nitrate nonahydrate (A1(N03)3.9H20), aluminium chloride hexahydrate (A1C136H20) with A13+ concentration in the range of 1 -3 M, dissolving a water soluble salt of magnesium such as magnesium nitrate hexahydrate, Mg(N03)2.6H20, magnesium chloride hexahydrate MgCl2.6H20 in aluminium metal salt solution maintaining a Al: Mg mole ratio in the range of 1: 1 to 1:3 to obtain a mixed solution, 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 in the mixed solution to obtain a mixed 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 obtain a sol with pH in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPa s, adding further, ammonia solution drop by drop to the said 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 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 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 1000°-1300°C for a period in the range of 1-lOh. The process comprises the following operations : 1. An aluminium metal salt solution was prepared by dissolving aluminium nitrate nonahydrate, A1(N03)3.9H20, aluminium chloride hexahydrate, A1C136H20 in water with A13+ concentration in the range of 1-3 M. 2. A water soluble salt of magnesium such as magnesium nitrate hexahydrate, Mg(N03)2. 6H20, magnesium chloride hexahydrate MgCl2. 6H20 with Al: Mg mole ratio in the range of 1: 1 to 1::3 was dissolved to the above aluminium metal salt solution and filtered. 3. A water soluble Lewis base such as ammonia solution with a concentration in the range of 15-25 wt% was added to the mixed aluminium-magnesium metal salt solution under stirring at ambient temperature to obtain a solution of pH in the range of 3.0 - 4.5. 4. The resulting solution was heated at a temperature in the range of 70° - 90°C for polymerizing it to a clear alumina-magnesia bi-component sol with pH and viscosity in the ranges 3.0 - 4.0 and and 5-15 mPa s respectively. 5. Ammonia solution was further added drop by drop to the above 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. 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-magnesia 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 1000°-1300°C for a period in the range of l-10h produced single phase, crystalline cordierite 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+, Mg2+ 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 Mgz+ are entraped in the silicic acid network , on further hydrolysis, A13+ also forms a Al-O-OH network with H20 molecules contributing towards fulfilment of coordination requirement. Thus in a complex networking between silicic acid network and Al-O-OH network, Mg2+ 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 25.692g of A1(NO3)3.9H2O was dissolved in 70 mL of deionized water to make A1(NO3)3 concentration of about 1M. To this solution, 8.77 g of magnesium nitrate hexahydrate, Mg(NO3)26H20 was dissolved under stirring to obtain a mixed solution. The mixed solution containing A13+ and Mg2+ ions was filtered to remove the undissolved impurities. Concentrated ammonia solution (25wt%, GR) was then added to the above mixed solution 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 resulting clear, bi-component alumina-magnesia 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 mixed sol, 5.356g of rice husk ash silica was added under stirring. The mixture containing alumina, magnesia and silica was continuously heated at 80° ± l°C until dried gel powder was obtained. X-ray diffraction analysis confirmed cordierite phase after calcination of the gel powder at 1200°C for 2h. Example 2 25.692g of Al(N03)3.9H20 was dissolved in 35 mL of deionized water to make M(NO3)3 concentration of about 2M. To this solution, 8.77g of magnesium nitrate hexahydrate, was dissolved under stirring to obtain a mixed solution. The mixed solution containing A13+ and Mg2+ ions was filtered to remove the undissolved impurities. Ammonia solution (15wt%, GR) was then added to the above mixed solution 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 resulting clear, bi-component alumina-magnesia 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 mixed sol, 5.356g of rice husk ash silica was added under stirring. The mixture containing alumina, magnesia and silica was continuously heated at 85° ± l°C until dried gel powder was obtained. X-ray diffraction analysis confirmed cordierite phase after calcination of the gel powder at BOOT for Ih. Example 3 25 692g of A1(NO3)3.9H2O was dissolved in 70 mL of deionized water to make A1(NO3)3 concentration of about 1M. To this solution, 8.77g of magnesium nitrate hexahydrate, Mg(NO3)2.6H2O was dissolved under stirring to obtain a mixed solution. The mixed solution containing A13+ and Mg2+ ions was filtered to remove the undissolved impurities. Ammonia solution (20wt%, GR) was then added to the above mixed solution 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 resulting clear, bi-component alumina-magnesia 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 mixed sol, 5.358g of fumed silica was added under stirring. The mixture containing alumina, magnesia and silica was continuously heated at 90° ± 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed cordierite phase after calcination of the gel powder at 1100°C for 4h. Example 4 25.692g of A1(NO3)3.9H2O was dissolved in 35 mL of deionized water to make A1(NO3)3 concentration of about 2M. To this solution, 8.77g of magnesium nitrate hexahydrate, Mg(NO3)2,6H2O was dissolved under stirring to obtain a mixed solution. The mixed solution containing A13+ and Mg2+ ions was filtered to remove the undissolved impurities. Ammonia solution (20wt%, GR) was then added to the above mixed solution 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 resulting clear, bi-component alumina-magnesia 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 mixed sol, 5.358g of fumed silica was added under stirring. The mixture containing alumina, magnesia and silica was continuously heated at 80° ± 1°C until dried gel powder was obtained. X-ray diffraction analysis confirmed cordierite phase after calcination of the gel powder at 1000°C for lOh. The main advantages of the present invention are: (i) The process for the manufacture of cordierite 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 cordierite (2MgO.2Al2O3.SiO2) powder characterized by preparing an aqueous solution of aluminium salt such as aluminium nitrate nonahydrate (A1(N03)3.9H20), aluminium chloride hexahydrate (A1C136H20) with A13+ concentration in the range of 1 -3 M, dissolving a water soluble salt of magnesium such as magnesium nitrate hexahydrate, Mg(N03)2.6H20, magnesium chloride hexahydrate MgCl2.6H20 in aluminium metal salt solution maintaining a Al : Mg mole ratio in the range of 1: 1 to 1:3 to obtain a mixed solution, 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 in the mixed solution to obtain a mixed 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 obtain a sol with pH in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPa s, adding further, ammonia solution drop by drop to the said 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 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 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 1000°-1300°C for a period in the range of 1-1O h.. 2. A process for the manufacture of cordierite powders as herein described with references to the examples.

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