Title of Invention

"A PROCESS FOR THE MANUFACTURE OF SPHERICAL MAGNESIUM ALUMINATE (MgAl2O4) POWDER"

Abstract

The present invention provides a process for the manufacture of spherical magnesium aluminate (MgAI2O4) powder process which process directly produces end product of minimum agglomeration and does not require any post-processing steps like high energy ball-milling, screening, thus avoiding possibility of contamination and further cost saving. The spherical MgAl2O4 powder of the present process is manufactured using water-based sols and gels such as aluminium salt selected from aluminium nitrate nonahydrate, AI(NO3)3.9H2O, aluminium chloride hexahydrate, AICl3.6H2O with AI3+ concentration in the range of 1 - 3 M, and water salt of magnesium selected from magnesium nitrate hexahydrate, Mg(NO3)26H2O, magnesium chloride hexahydrate MgCI2. 6H2O .

Full Text

The present invention relates to a process for the manufacture of spherical magnesium aluminate (MgAI204) powder This invention particularly relates to a process for the manufacture of spherical magnesium aluminate (MgAI204) powder from water-based sols. MgAl2O4 spinel is known to be used as an important ceramic material:, because of its high melting point (2135°C), high resistance against chemical attack, superior mechanical strength both at room temperature and elevated temperature, low dielectric constant and excellent optical characteristics. It also finds applications as a sensing element for humidity measurement devices in the form of porous compacts. Dense spinel ceramics find potential application in diverse engineering fields. The spherical morphology of the powder exhibits good flowability characteristics and can be useded for making monolothic refractories. It also helps in rapid die filling during the preparation of spinel ceramics. The present day methods of preparing MgAl2O4 powders mainly consists of solution phase reactions. Reference may be made to R. J. Bratton in "Am. Ceram. Soc. Bull. 48 (1969) pp 759-62" wherein a mixed solution of magnesium and aluminium chloride was prepared (Mg2+/A13+ = 1:2) by dissolving 0.5 moles of MgCI2.6H2O and 1 mole of A1C13.6 H2O per litre of distilled water at 80°C. The chloride solution was then added to a stirred excess solution of ammonium hydroxide. The pH of the solution was maintained between 9.5-10 during precipitation. The gelatinous precipitate was kept at about 80°C for 1/2 h followed by cooling to room temperature, washed and dried slowly at 100°C. The main drawbacks of the above process are: (i) It suffers lack of homogeneity, (ii) Crystallization temperature is high. Reference may also be made to J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori and Y. Yajima in "J. Eur. Ceram. Soc. 21 (2001) pp 139-148" wherein spinel precursor was made by adding 400 mL of the mixed solution of aluminium nitrate nonahydrate and magnesium nitrate hexahydrate at a speed of 5 mL/min into 600 mL of the ammonium bicarbonate solution under stirring followed by ageing the precipitate at the reaction temperatures of 50°C in a suspension of pH 1 1.07. The precipitate after washing with ethanol was dried at room temperature and subsequently calcined at different temperature under flowing oxygen. The main drawbacks of the above process are: (i) Precursor precipitate consists of several phases which at high temperature produces spinel phase. (ii) Calcination at various temperatures is carried out under flowing oxygen (100 ml/min). Reference may also be made to R. K. Pati and P. Pramanik in "J. Am. Ceram. Soc. 83 (2000) pp 1822-1824" wherein nanocrystalline (10-20 nm) MgAl2O4 spinel powder was synthesized by the pyrolysis of complex compounds of aluminium and magnesium with triethanolamine (TEA). Aluminium nitrate nonahydrate, magnesium nitrate hexahydrate and TEA were used as the starting materials. The soluble metal ion-TEA complexes formed the precursor material on complete dehydration of the complexes of aluminium-TEA and magnesium-TEA. When complete dehydration occurs, the nitrates themselves are decomposed with the evolution of brown fumes of nitrogen dioxide, leaving behind a voluminous, organic-based, black, fluffy powder, i.e. precursor powder. The precursor powders after grinding are calcined at 650°C and above to obtain MgAl2O4 powders. The main drawbacks of the above process are: (i) The experimental procedure involves too many steps. (ii) Crystallization temperature is high, about 675°C. Reference may further be made to Z. Nakagawa, K. Hamano, M. Sakaguchi and S. Kanzaki in "Yogyo-Kyokai-Shi, 90 (1982) pp. 313-319" wherein a freeze-dried mixed sulphate precursor was prepared from an aqueous solution of Mg- and Al-sulphates. The mixed sulphate precursor was then calcined at 900° - 1400°C/lh. During this heat-treatment, the mixed sulphate completely decomposed and converted to spinel above 1000°C. The method is associated with the following darwbacks: (i) The preparative technique is dependent on a freeze-dryer and hence is costly, (ii) Decomposition temperature of the mixed sulphate precursor is quite high. Reference may also be made to P. W. D. Mitchel in "J. Am. Ceram. Soc., 55 (1972) p. 484" wherein stoichiometric quantities of aluminium hydroxychloride, Al2(OH)5C1.2-3 H2O in an aqueous solution and a suspension of Mg(OH)2 in water was mixed. Rapid reaction between the two formed a gel. The gel was dried and ball-milled to a fine powder. At 900°C, the gel partly converted to spinel; complete conversion to spinel takes place at around 1300°C. The main difficulty of the above method is that the spinel formation occurs at a considerably high temperature. Reference may also be made to M. Sugiura and O. Kamigaito in "Yogyo-Kyokai-Shi, 92 (1984), pp. 605-611" wherein a mixed alkoxide, MgAl2(i-C3H7)8, was used for the synthesis of spinel. In this method, the alkoxide was dissolved in a 1 : 1 mixture of benzene and ethanol under Ar atmosphere. Water was added dropwise under stirring, when a white powdery material was precipitated. The precipitate was dried under vacuum at 90°C and then calcined at 1000°C for crystallization to spinel phase. The main drawbacks of the above process are : (i) It is difficult to obtain the mixed alkoxide of Mg-Al. (ii) Dissolution of the alkoxide is carried out in Ar atmosphere. (iii) Drying of the precipitated material id performed under vacuum and (iv) The crystallization temperature is quite high. The main objective of the present invention is to provide a process for the manufacture of spherical magnesium aluminate (MgAI2O4) powder which obviates the drawbacks of the hitherto known processes. Another object of the present invention is to provide a process for manufacturing spherical MgAI2O4 powder using water-based sols and gels which are not health hazard 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 spherical MgAI2O4 powders which uses precursor chemicals that are cost effective, and safe. Still another object of the present invention is to provide a process for the manufacture of spherical MgAI2O4 powder which crystallizes at a lower temperature, thus making the process energy efficient. Yet another object of the present invention is to provide a tailor-made process of spherical powder synthesis as the particle size and size distribution can be varied according to the necessity. Still another object of the present invention is to provide a process of manufacturing spherical MgAI2O4 powder which is very simple and cost-effective. Another object of the present invention is to provide a process which directly produces end product of minimum agglomeration and does not require any post-processing steps like high energy ball-milling, screening, thus avoiding possibility of contamination and further cost saving. Accordingly, the process for the manufacture of magnesium aluminate (MgAI2O4) powder which comprises preparing an aqueous solution of aluminium salt such as aluminium nitrate nonahydrate, AI(NO3)3.9H2O, aluminium chloride hexahydrate, AICI3.6H2O with AI3+ concentration in the range of 1 - 3 M, dissolving a water salt of magnesium such as magnesium nitrate hexahydrate, Mg(NO3)26H2O, magnesium chloride hexahydrate MgCI2. 6H2O in aluminium metal salt solution maintaining a Al : Mg mole ratio in the range of 1:1 to 1:3 to obtain to 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 ( 25 °C ) to obtain a solution of pH in the range 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 mPa s, 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, dispersing the said aqueous mixed sol as droplets into a solution of a water - immiscible organic solvent and non-ionic surfactant of hydrophilic-lipophilic balance (HLB) value in the range 4 - 15, in a proportion in the range of 1 : 2 - 1:5 (v/v) under constant stirring to obtain a water - in oil ((w/o) type emulsion , gelling the stabilized sol droplets by using an organic amine such as triethylamine, under stirring until the stirred mixture attains a pH value in the range of 8 - 10, filtering the gel microspheres under vacuum and subsequent washings with ketones such as acetone, drying the washed gel microspheres at a temperature in the range of 100° - 200°C for a period in the range of 1 - 5h, calcining the dried gel microspheres at a temperature in the range of 500° - 800°C for a period in the range of 1 - 3 h. In an embodiment of the present invention the surfactant solution may be prepared by mixing water immiscible organic solvent such as cyclohexane, n-hexane, n-heptane, xylene, benzene with a non-ionic surfactant such as sorbitan monooleate, sorbitab monopalmitate, sorbitan monolaurate, polyethoxyetylene sorbitan monooleate, polyethoxyetylene sorbitan monoostearate with a concentration in the range of 1 - 5 vol% with respect to the volume of the water immiscible organic solvent under stirring. In another embodiment, of the present invention the gelling agent used may be such as triethylamine, diethylamine, monoethylamine. 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 Al3* concentration in the range of 1-3 M5 and the solution was filtered. 2. A water soluble salt of magnesium such as magnesium nitrate hexahydrate, Mg(NO3)2.6H2O, magnesium chloride hexahydrate MgCl2. 6H2O with Al: Mg mole ratio in the range of 1:1 to 1:3 was dissolved to the above aluminium metal salt solution, and the mixed solution was 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 in the range of 3.0 - 4.0 and viscosity in the range of 5-15 mPa s. 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 in the range of 3.5-4.5 and viscosity in the range of 20-35 mPa s respectively. 6. The aqueous mixed sol was dispersed as droplets by pouring the sol under stirring into a solution prepared by mixing water immiscible organic solvent such as cyclohexane, n- hexane, n-heptane, xylene, benzene with a non -ionic surfactant such as sorbitan monooleate, sorbitan monopalmitate, sorbitan monolaurate, polyethoxyetylene sorbitan monooleate, polyethoxyetylene sorbitan monoostearate with a concentration in the range of 1-5 vol% with respect to the volume of the water immiscible organic solvent under stirring. 7. The volume ratio of the mixed sol to the organic solution of surfactant was kept in a proportion in the range of 1:2 -1:5 (v/v). 8. The stabilized sol droplets was subjected to gelation by using an organic amine, such as triethylamine, under stirring until the mixture attains a pH value in the range of 8 -10. 9. The gel microspheres were filtered under vacuum and subsequenty washed with ketones such as acetone to remove the adhered impurities. 10. The washed gel microspheres were dried at a temperature in the range of 100°-2000C for a period in the range of l-5h. 11. The dried gel microspheres when subjected to calcination at a temperature in the range of 500°-800°C for a period in the range of l-3h produced single phase, crystalline spherical MgAl2O4 powders. The novelty of the present invention primarily resides in providing a process of obtaining a spherical magnesium aluminate (MgA12O4) powder which is economical, environment friendly, energy efficient, high yielding and time saving which makes the process suitable for industrial manufacture and the non-obvious inventive steps lies in the dispersion of Mg-Al aqueous mixed sol as droplets into a solution of non-aqueous, water-immiscible organic solvent and non-ionic surfactant of hydrophilic-lipophilic balance (HLB) value in the range 4 - 15 to obtain an water-in oil (w/o) type emulsion, gelling the stabilized sol droplets by using an organic amine to produce the corresponding gel powders in which the sphericity of sol droplets is properly retained. The surfactant added to the system reduces the interfacial tension between the sol droplets and the water immiscible organic solvents and prevents the coalescence of the sol droplets by steric hindrance after absorbing on their surfaces. This stabilized sol droplets produces the corresponding gel microspheres with minimum of agglomeration and avoids grinding and milling operations, prevents dust hazards and possibility of contamination. Judicious selection of surfactant of different HLB values and process parameters, spherical MgAl2O4 powders with desired size and size distribution are prepared. 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 52.8351g of A1(NO3)3.9H2O was dissolved in 140 mL of deionized water to make A1(NO3)3 concentration of about 1 M. To this solution, 18.0570 g of magnesium nitrate hexahydrate, Mg(NO3)2 6H2O was dissolved under stirring to obtain a mixed solution. The mixed solution 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 1h 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. The bi-component sol was then dispersed as droplets under stirring to a mixture of cyclohexane and sorbitan monooleate (HLB value 4.3) under stirring to obtain an w/o type emulsion. The volume ratio of the sol to the organic solution of surfactant was kept in the proportion of 1:4. The stabilized sol droplets were then subjected to gelation by using triethylamine under stirring until the pH of the mixture becomes 8. The gel microspheres were filtered under vacuum and subsequently washed with acetone to remove the adhered impurities. The washed gel microspheres were then dried at 100°C for 5h. The dried gel microspheres after calcination at 500°C for 3h, produced phase-pure, crystalline, spherical ^ microspheres with the size range of 3 - 30 micron. Example 2 41.2230 g of Al(NO3)3.9H2O was dissolved in 55 mL of deionized water to make Al(NOs)3 concentration of about 2 M. To this solution, 28.1768 g of magnesium nitrate hexahydrate, Mg(NO3)2 6H20 was dissolved under stirring to obtain a mixed solution. The mixed solution 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 80° ± 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. The bi-component sol was then dispersed as droplets under stirring to a mixture of n-hexane and sorbitan monopalmitate (HLB value 6.7) under stirring to obtain an w/o type emulsion. The volume ratio of the sol to the organic solution of surfactant was kept in the proportion of 1:5. The stabilized sol droplets were then subjected to gelation by using diethylamine under stirring until the pH of the mixture becomes 9. The gel microspheres were filtered under vacuum and subsequently washed with acetone to remove the adhered impurities. The washed gel microspheres were then dried at 150°C for 3h. The dried gel microspheres after calcination at 600°C for 2h, produced phase-pure, crystalline, spherical MgA^C^ microspheres with the size range of 5 - 40 micron. Example 3 33.7955 g of A1(NO3)3.9H2O was dissolved in 36 mL of deionized water to make Al(NO3)3 concentration of about 2.5 M. To this solution, 34.6499 g of magnesium nitrate hexahydrate, Mg(NO3)2.6H20 was dissolved under stirring to obtain a mixed solution. The mixed solution was filtered to remove the undissolved impurities. To this mixed solution, ammonia solution (20wt%, GR) was added under vigorous stirring until the pH of the solution becomes 4. The solution was then heated at 80° ± 1°C for 2h 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 85° ± 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 4. The viscosity of the resulting clear sol was 30+1 mPa s. The bi-component sol was then dispersed as droplets under stirring to a mixture of n-heptane and sorbitan monolaurate (HLB value 8.6) under stirring to obtain an w/o type emulsion. The volume ratio of the sol to the organic solution of surfactant was kept in the proportion of 1:3. The stabilized sol droplets were then subjected to gelation by using monoethylamine under stirring until the pH of the mixture becomes 8,5, The gel microspheres were filtered under vacuum and subsequently washed with acetone to remove the adhered impurities. The washed gel microspheres were then dried at 150°C for 2h. The dried gel microspheres after calcination at 700°C for 2h, produced phase-pure, crystalline, spherical MgAl2O4 microspheres with the size range of 4 - 60 micron. Example 4 33.7955 g of A1(NO3)3.9H2O was dissolved in 30 mL of deionized water to make A1(NO3)3 concentration of about 3 M. To this solution, 34.6499 g of magnesium nitrate hexahydrate, Mg(NO3)2.6H2O was dissolved under stirring to obtain a mixed solution. The mixed solution was filtered to remove the undissolved impurities. To this mixed solution, ammonia solution (15wt%, GR) was added under vigorous stirring until the pH of the solution becomes 4. The 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 85° ± 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 4. The viscosity of the resulting clear sol was 30 ± 1 mPa s. The bi-component sol was then dispersed as droplets under stirring to a mixture of xylene and polyoxyethylene sorbitan monooleate (HLB value 15) under stirring to obtain an w/o type emulsion. The volume ratio of the sol to the organic solution of surfactant was kept in the proportion of 1:2. The stabilized sol droplets were then subjected to gelation by using triethylamine under stirring until the pH of the mixture becomes 10. The gel microspheres were filtered under vacuum and subsequently washed with acetone to remove the adhered impurities. The washed gel microspheres were then dried at 200°C for 1h. The dried gel microspheres after We Claim: 1. A process for the manufacture of magnesium aluminate (MgAI2O4) powder which comprises preparing an aqueous solution of aluminium salt such as aluminium nitrate nonahydrate, AI(N03)3.9H2O, aluminium chloride hexahydrate, AICI3.6H20 with AI3+ concentration in the range of 1 - 3 M, dissolving a water salt of magnesium such as magnesium nitrate hexahydrate, Mg(NO3)26H20, magnesium chloride hexahydrate MgCI2. 6H20 in aluminium metal salt solution maintaining a Al : Mg mole ratio in the range of 1:1 to 1:3 to obtain to 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 ( 25 °C ) to obtain a solution of pH in the range 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 mPa s, 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, dispersing the said aqueous mixed sol as droplets into a solution of a water - immiscible organic solvent and non-ionic surfactant of hydrophilic-lipophilic balance (HLB) value in the range 4 - 15, in a proportion in the range of 1 : 2 - 1:5 (v/v) under constant stirring to obtain a water - in oil ((w/o) type emulsion , gelling the stabilized sol droplets by using an organic amine such as triethylamine, under stirring until the stirred mixture attains a pH value in the range of 8 - 10, filtering the gel microspheres under vacuum and subsequent washings with ketones such as acetone, drying the washed gel microspheres at a temperature in the range of 100° - 200°C for a period in the range of 1 - 5h, calcining the dried gel microspheres at a temperature in the range of 500° - 800°C for a period in the range of 1 - 3 h. 2. A process as claimed in claim 1 wherein the surfactant solution is prepared by mixing water immiscible organic solvent such as cyclohexane, n-hexane, n- heptane, xylene, benzene with a non-ionic surfactant such as sorbitan monooleate, sorbitan monopalmitate, sorbitan monolaurate, polyethoxyetylene sorbitan monooleate, sorbitan monopalmitate, sorbitan monolaurate, polyethoxyetylene sorbitan monooleate, polyethoxyetylene sorbitan monostearate with a concentration in the range of 1 - 5 vol% with respect to the volume of the water immiscible organic solvent under stirring. 3. A process as claimed in claims 1 and 2 wherein, the gelling agent used is selected from triethylamine, diethylamine, monoethylamine. 4. A process for the manufacture of spherical magnesium aluminate ( MgAI204) powder substantially as herein described with reference to the examples.

Documents:

1267-del-2001-abstract.pdf

1267-del-2001-claims.pdf

1267-del-2001-correspondence-others.pdf

1267-del-2001-correspondence-po.pdf

1267-del-2001-description (complete).pdf

1267-del-2001-form-1.pdf

1267-del-2001-form-18.pdf

1267-del-2001-form-2.pdf

1267-del-2001-form-3.pdf