Title of Invention

"A PROCESS FOR PRODUCING ALPHA ALUMINA POWDER"

Abstract

The invention relates to a process for producing alpha alumina powder. In the present invention a-AI2O3 particles have been prepared by decomposition of AI (NO3)3 in presence of carbon particles and then decarbonization of the mixture to get rid of carbon particles. To achieve this a sol of carbon particles in nanometer dimension has been prepared by addition of carbon particles (11nm particle size) in a solution of lyophilic non-ionic agent polyoxyethylene (20) sorbitanmonooleate in conductivity grade water with constant stirring at room temperature. The mixure was then evaporated slowly at 100-150°C for a period of 120-600 minutes and it was then pyrolysed under ambient atmosphere at a temperature in the range of 350-400°C for a period of 3060 minutes. The pyrolysed product was then heated at a temperature ranging from 1200-1500°C for 1-8 hours under nitrogen or inert atmosphere having a pressure ranging from 14-50 psig and water vapour less than 10 ppm. The product was characterized by XRD and it has been found that a different composition range of carbon and AI203 (both transition and variety) particles could be obtained at the end. The novelty of the present invention is the easier method of preparation of alpha alumina with less impurities by the inventive step of calcining a solid mass produced by crystallization of AI(NO3), 9H2O on carbon particles of a carbon sol.

Full Text

The present invention relates to a process for producing alpha alumina (a-A!2O3) powder. Dense aluminum oxide is used as ceramic components for use in electronics, structural and high temperature materials due to its interesting properties such as good mechanical, thermal and electrical properties. Porous ceramic materials containing alpha alumina powders are widely used as filters, sensors and catalyst supports because of their permeability and high surface area. The porous structure having high surface area is formed by essentially starting with transient alumina or a mixture of alpha as well as transient alumina such as gamma , delta and theta Alumina. These transitional phases have higher surface areas than alpha alumina and are usually produced by decomposition of A1(OH>3. Good sinterability of transient alumina due to their high surface areas lead to strong bonding among the grains and impart superior mechanical strength. a-AlaOs powder is also used for making ceramic composites and alloys with other ceramics and polymers which find varied uses as structural, electronic and refractory materials. The present day method of making fine alpha alumina powder consists of calcination of transient alumina, produced mainly by heating aluminum hydrate or hydroxides. Careful control of composition of starting raw materials and of reaction atmosphere are necessary for obtaining high purity aluminum oxide powder, otherwise, the reaction often results in coarser alumina particles which are not suitable for engineering applications. Smooth, hollow aggregates to sponge like particles are prepared by thermal reaction of atomized solution (TRAS) of aluminum nitrate. This process consists of atomization and subsequent evaporative decomposition of aluminum nitrate. For the present day method of producing alpha alumina powder reference may bo made to Y.U. Mohri, Y. Sawabe and H.Watanabe in "Alpha- alumina powder and the process for producing the same", in US Pat. No. 6159441, wherein alpha alumina particles having a substantially octahedral or eicosahedral shape, a specific structure, and a narrow primary particle size distribution (0.7 - 30 micrometer) have been produced by calcination (at 600-1400°C for 1-600 minutes) of transition alumina and either aluminum hydroxide or alum or aluminum Sulphate in a gas atmosphere containing a hydrogen halide selected from the group consisting of hydrogen chloride (0.1-30 % v/v concentration with the said gas atmosphere), hydrogen bromide (0.1-33 % v/v concentration with the said gas atmosphere) and hydrogen iodide (0.1-33 % v/v concentration with the said gas atmosphere). The drawbacks of this process are (i) the use of raw material such as Al- hydroxide, Alum or Aluminum Sulphate whose particle size may be coarser and ultimately lead to coarser alpha alumina (ii) seed crystals other than alumina such as oxides and nitrides of Ti, V, Cr, Fe and Ni should remain as impurities unless these were wanted and (iii) use of gaseous atmospheres such as hydrogen halides should involve a costly pollution removal operation. Reference may also be made to S.W.Sucech and J.E. Marhanka in " Small sized alpha alumina particles and platelets", US Pat. No. 5149520 wherein aqueous sodium aiuminate solution (at 20-100°C temperature) containing about 50-400gm/l gm caustic soda and about 0.8 gm of AljCh per gram of caustic soda have been treated with seed material (comprising alumina trihydrate particles) and alpha alumina promoter (comprising about 0.15-2 wt % of the A12O3 content of the precipitated alumina hydrate particles) particle having an average size of less than about 1 micron and then cooled to precipitate alumina hydrate. The precipitated alumina hydrate particles were then separated from aqueous aiuminate solution. The separated alumina hydrate particles were then dried followed by their calcinations in presence of a chlorine- or fluoride-, chloride- or boron-containing mineralizer (comprising about 0.15-2 wt % of the AbOs content of the precipitated alumina hydrate particles) selected from the group consisting of aluminum fluoride, ammonium fluoride, aluminum chloride, ammonium chloride, chlorine, hydrochloric acid, boric oxide, boric acid, and mixtures thereof to produce plate like shaped particle of alpha alumina having average size 'ess than 1.5 micron. Reference may also be made to R.R.Ciminelli and G.L.Messing in "The chemical and structural transformations in the system aluminum nitrate / alumina", Ceramica 30[ 174] 131-38(1984) wherein the process for synthesis of smooth, hollow aggregates to sponge like particles of A^Ch by thermal reaction of atomized solution of Aluminum Nitrate have been described. The drawbacks of this process lie in using excessive steps. The main drawbacks of the above processes are, 1. The substrate particles on which AliO?, are deposited or crystallized or nucleated are not in nanodimension. The possibility of growing large aluminum oxide particles would be there, 2. The substrate particles are not fugitive and hence possibility of forming a core shell structure is not there and consequently the possibility of forming a spherical aluminum oxide particle having a pore or void inside is not obtained 3. Special equipment has to be provided for atomization and subsequent pyrolysis of aqueous solutions. 4. Starting substrate particles are costly in nature. 5. Many steps are involved in the processes. 6. Increase in level of impurity due to additives. The main object of the present invention is to provide a process for producing alpha alumina powder which obviates the above noted drawbacks. Another object of the present invention is to prepare Alpha alumina powder of spherical shape having pore or void inside it. Yet another object of the present invention is to eliminate the use of atomizer for the processing of the same. A further object of the present invention is to use cheaper raw materials for the production of alpha alumina powder. Still another object of the present invention is to reduce the number of the unit operations for the production of alpha alumina to reduce the cost further. In the present invention a-AlaOs particles have been prepared by decomposition of A1(NO3)3 in presence of carbon particles and then decarbonization of the mixture to get rid of carbon particles. To achieve this a sol of carbon particles in nanometer dimension has been prepared by addition of carbon particles (llnm particle size) in a solution of lyophilic non-ionic agent polyoxyethylene(20)sorbitanmonooleate in conductivity grade water with constant stirring at room temperature. The mixture was then evaporated slowly at 100-150°C for a period of 120-600 minutes and it was then pyrolysed under ambient atmosphere at a temperature in the range of 350-400°C for a period of 30-60 minutes. The pyrolysed product was then heated at a temperature ranging from 1200-1500°C for 1-8 hours under nitrogen or inert atmosphere having a pressure ranging from 14-50 psig and water vapour less than 10 ppm. The product was characterized by XRD and it has been found that a different composition range of carbon and AhOs (both transition and variety) particles could be obtained at the end. The novelty of the present invention is the easier method of preparation of alpha alumina with less impurities by the inventive step of calcining a solid mass produced by crystallization of Al(NO3)s,9H2O on carbon particles of a carbon sol. Accordingly the present invention provides a process for producing alpha alumina powder which comprises; characterized in that i) preparing a carbon sol in water by adding carbon particles in a proportion in the range of 6.9 to 23.3 wt% to an aqueous solution of non-ionic lyophillic agent of concentration in the range of 0.7 to 2.4 wt% under constant mechanical stirring, ii) adding water soluble aluminum salt such as herein described to the said carbon sol under constant mechanical stirring so as to obtain an ultimate ratio of C: Al2O3 in the range of 2:1 to 3:2 in the mixed sol, iii) sonicating the resultant mixed sol for a period in the range of 8-20 minutes and evaporating the sonicated sol under constant stirring at a temperature in the range of 100-150° C for a period in the range of 120-600 minutes to obtain a gel, iv) pyrolysing the said gel at a temperature in the range of 350-400°C for a period in the range of 30-60 minutes to obtain a pyrolysed product, v) heating the pyrolysed product at a temperature in the range of 1200-1500°C for a period in the range of 1-8 hours at the hold temperature under a gas pressure in the range of 14-50 psig using gases such as nitrogen, argon and maintaining water vapour in the gas below 10 ppm to obtain a heat treated product, vi) decarbonizing the heat treated product by holding at a temperature in the range of 600-800°C for a period in the range of 2-4 hours under ambient atmosphere to obtain the product. In an embodiment of the present invention the sol of carbon particles in water may be prepared by addition of carbon particles to an aqueous solution of non-ionic lyophillic agent such as polyoxyethylene (20) sorbitanmonooleated which can modify the surface properties of carbon particles. In another embodiment of the present invention the water soluble salt of aluminum used may be such as AI(NO3)39H2O, AI2(SO4)312H2O, AICI3 6H2O. In yet another embodiment of the present invention the water used for dispersion medium may be distilled, deionized and of conductivity grade. In still another embodiment of the present invention, the gel may be pyrolised at a temperature in the range of 350-400°C under ambient, nitrogen or an inert atmosphere and the evolved gas may be dissolved in water for safety of the environment. In still yet another embodiment of the present invention, the pyrolysed product may be heated in ambient atmosphere at a temperature in the range of 700-1000°C for a period of the order of 2 hours to produce materials such as s, mixtures of gamma, delta and theta - Accordingly the present invention relates to a process for producing alpha alumina powder which comprises the following sequential steps: 1. Preparation of a carbon sol in water by addition of carbon particles in the proportion range of 6.9-23.3wt% to an aqueous solution of polyoxyethylene(20)sorbitanmonooleate (0.7-2.4wt% with respect to water) under constant mechanical stirring. 2. Addition of an aqueous solution of aluminum nitrate to the said carbon sol under constant mechanical stirring so as to obtain an ultimate ratio by weight of C: A12O3 in the range of 2: 1 to 3:2. 3. Sonication of the suspension for a period in the range of 8-20 minutes. 4. Drying of the product of step3 under constant stirring at a temperature in the range of 100-150°C for a period in the range of 120-600 minutes. 5. Pyrolysis of the product of step 4 at a, temperature in the range of 350-400°C for a period in the range of 30-60 minutes. 6. Heating the pyrolysed product at a temperature in the range of 1200-1500°C under nitrogen or inert atmosphere having a pressure in the range of 14-50 psig and water vapour less than 10 ppm for a period in the range of 1-8 hours. 7. Decarbonization of the product obtained in step 6 by heating in air at a temperature ranging from 600-800°C for a period in the range of 2-4 hours. The following examples are given as illustration and should not be construed to limit the scope of the present invention. EXAMPLE 1 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(N03)3.9H2O, (equivalent to 5.6 gm of AlaC^ and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for Ih. The product was then cooled and decarbonised in a muffle furnace at 600°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly^ AhO3 with traces of carbon. EXAMPLE 2 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NO3)3.9H2O (equivalent to 5.6 gm of A^Os and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1000C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 600oC under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly^AhOj with traces of carbon. «» EXAMPLE 3 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NO3)3.9H2O (equivalent to 5.6 gm of A1203 and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025% was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 400°C for 30 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 600°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the ac powder is mainly^AhC^ with traces of carbon. EXAMPLE 4 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NO3)3.9H20 equivalent to 5.6 gm of A12O3 and having impurities like chloride (CI) 0.01%, sulphates(SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare* an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2 hours. The product was then cooled and decarbonized in a muffle furnace at 700oC under ambient atmosphere for 2 hours A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainlyAliOa with traces of carbon. EXAMPLE 5 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NO3)3.9H2O (equivalent to 5.6 gm of AhOs and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 14 psig at 1200°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainlyAlaOa with traces of carbon. EXAMPLE 6 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of AlfNOsJs.QHaO (equivalent to 5.6 gm of AliOs and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 600°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly^ AhCh with traces of carbon. EXAMPLE 7 2.6 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 109 gm of distilled water. 26 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 95.59 gm of A1(NO3)3.9H2O (equivalent to 13 gm of A^Ch and having impurities like chloride (Cl) 0.01%, sulphates (SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in '49.8 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 20 minutes. A stable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 150°C within a fume cupboard chamber for 120 minutes. The product was then pyrolysed at 400°C for 30 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 800°C under ambient atmosphere for 4 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly^AbOa with traces of carbon. EXAMPLE 8 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 148 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 54.90 gm of A1(NO3)3.9H2O (equivalent to 7.47 gm of A^Os and having impurities like chloride (Cl) 0.01%, sulphates(SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 28.3 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 150°C within a fume cupboard chamber for 120 minutes. The product was then pyrolysed at 400°C for 30 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under nitrogen pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly/AlaOs with traces of carbon. EXAMPLE 9 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NO3)3.9H2O (equivalent to 5.6 gm of AhOa and having impurities like chloride (Cl) 0.01%, sulphates(SO4) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then directly heated to decarbonize in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is y- A^Os. The morphology of the powder has been observed by SEM and found to be spherical and contain pores. EXAMPLE 10 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(NC>3)3.9H2O (equivalent to 5.6 gm of AlaOs and having impurities like chloride (Cl) 0.01%, sulphates(S04) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 100°C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried mass was then heated in a graphite crucible under argon pressure of 50 psig at 1500°C for 2h. The product was then cooled and decarbonized in a muffle furnace at 600°C under ambient atmosphere for 2 hours. A white powder was obtained. This product was then characterized by XRD, which shows that the composition of the powder is mainly ot-AljOs with traces of carbon. The main advantage of the process lies in producing fine a-A^Os powder by using starting substrate particles in nanometer dimension. Spherical shell ofJ-AlaCh particles having pore or void inside has been obtained by using the nano dimension fugitive substrate carbon particle. Low cost of substrate carbon particles is another advantage. A mixture of gamma, delta and thetaaluminas of sub micron dimension or a mixture of alfa - AhOs and gamma, delta and theta - A^Oj, having different particle sizes could be produced by this process by altering the process variables. Yet another advantage of the process is thatit could also be used to produce a physical mixture of carbon particles and or carbon particles coated with amorphous or crystalline WE CLAIM : 1. A process for producing alpha alumina powder which comprises; characterized in that i) preparing a carbon sol in water by adding carbon particles in a proportion in the range of 6.9 to 23.3 wt% to an aqueous solution of non-ionic lyophillic agent of concentration in the range of 0.7 to 2.4 wt% under constant mechanical stirring, ii) adding water soluble aluminum salt such as herein described to the said carbon sol under constant mechanical stirring so as to obtain an ultimate ratio of C: Al2O3 in the range of 2:1 to 3:2 in the mixed sol, iii) sonicating the resultant mixed sol for a period in the range of 8-20 minutes and evaporating the sonicated sol under constant stirring at a temperature in the range of 100-150° C for a period in the range of 120-600 minutes to obtain a gel, iv) pyrolysing the said gel at a temperature in the range of 350-400°C for a period in the range of 30-60 minutes to obtain a pyrolysed product, v) heating the pyrolysed product at a temperature in the range of 1200-1500°C for a period in the range of 1-8 hours at the hold temperature under a gas pressure in the range of 14-50 psig using gases such as nitrogen, argon and maintaining water vapour in the gas below 10 ppm to obtain a heat treated product, vi) decarbonizing the heat treated product by holding at a temperature in the range of 600-800°C for a period in the range of 2-4 hours under ambient atmosphere to obtain the product. 2. A process as claimed in claim 1, wherein the sol of carbon particles in water is prepared by addition of carbon particles to an aqueous solution of non-ionic lyophillic agent such as polyoxyethylene (20) sorbitanmonooleate. 3. A process as claimed in claim 1, wherein the water soluble salt of aluminum used is selected from Al (NO3)3 9H20, Al2 (SO4)3, 12H20, AICL3, 6H20. 4. A process for producing alpha alumina powder substantially as herein described with reference to the examples.

Documents:

403-DEL-2002-Abstract-(27-02-2009).pdf

403-del-2002-abstract.pdf

403-DEL-2002-Claims-(09-03-2009).pdf

403-DEL-2002-Claims-(27-02-2009).pdf

403-del-2002-claims.pdf

403-DEL-2002-Correspondence-Others-(09-03-2009).pdf

403-DEL-2002-Correspondence-Others-(27-02-2009).pdf

403-del-2002-correspondence-others.pdf

403-del-2002-correspondence-po.pdf

403-DEL-2002-Description (Complete)-(09-03-2009).pdf

403-DEL-2002-Description (Complete)-(27-02-2009).pdf

403-del-2002-description (complete).pdf

403-DEL-2002-Form-1-(27-02-2009).pdf

403-del-2002-form-1.pdf

403-del-2002-form-13-(25-05-2009).pdf

403-del-2002-form-18.pdf

403-del-2002-form-2.pdf

403-DEL-2002-Form-3-(27-02-2009).pdf

403-del-2002-form-3.pdf