Title of Invention	"AN IMPROVED PROCESS FOR THE PRODUCTION OF HIGH-PURITY NANO-CRYSTALLINE ALUMINA POWDERS"
Abstract	An improved process for the production of high purity nano-crystalline alumina powders which comprises reacting aluminium with alcohol in the presence of mercury halides;distilling the so prepared solution in vacuum at a temperature in the range of 100 to 250°C to obtain aluminum alkoxide;hydrolyzing the aluminum alkoxide with excess of water;isolating the alcohol; seeding the sol; drying the transparent sol;calcining the dried gel at a temperature in the range of 1000 and 1400°C for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder.

Title of Invention

"AN IMPROVED PROCESS FOR THE PRODUCTION OF HIGH-PURITY NANO-CRYSTALLINE ALUMINA POWDERS"

Abstract

An improved process for the production of high purity nano-crystalline alumina powders which comprises reacting aluminium with alcohol in the presence of mercury halides;distilling the so prepared solution in vacuum at a temperature in the range of 100 to 250°C to obtain aluminum alkoxide;hydrolyzing the aluminum alkoxide with excess of water;isolating the alcohol; seeding the sol; drying the transparent sol;calcining the dried gel at a temperature in the range of 1000 and 1400°C for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder.

Full Text	This invention relates to an improved process for the production of high purity nano-crystalline alumina powder. The invention particularly relates to the synthesis of nano-crystallinc alumina using a modified sol-gel process. The present invention is a major breakthrough lor the low cost sol-gel synthesis of nano-crystalline high alumina powder having wide range of applications in ceramics, electrical, electronics, chemical, metallurgical, textile and thermal spray coating industries. The product powder produced in the present invention may be used either directly or after processing by conventional ceramic methods like forming into green bodies cither by double action press or isostatic pressing or tape casting or slip casting, followed by presinter machining, sintering in either a conventional resistive heating or microwave or plasma furnace. The powders synthesised by this present invention may also be used as catalyst in chemical industries and as coating materials for thermal and plasma spray. The plasma spray coating of this powder will improve the corrosion and wear resistances of the industrial components. The sintered products prepared from the present invention are likely to be used as, envelope for sodium vapour lamp, substrate for IC chips, high pure alumina crucible, hip joints etc. The hitherto practiced processes for the production of alumina powder are as follows: Bayer alumina leaching process has been the most popular and the cheapest amongst all the known processes which produce alumina powder in bulk quantity. In this process the dry ground bauxite is digested in an autoclave with a solution of sodium hydroxide and sodium carbonate. Red mud is separated from the liquor and then aluminium hydroxide is precipitated by introducing fine seed of aluminum trihydrate. This precipitated aluminium trihydroxide is then calcined at elevated temperatures and ground to get alpha alumina powder. In another known Alum process, ammonium alum is dissolved in water at moderate temperature and then purified by crystallisation. The process of crystallisation is repeated for 2 to 3 times and then the alum is calcined to yield alumina powders. In yet other known processes for the preparation of alumina using Al metal are as follows: a) Hydrothermal oxidation process consists of reacting aluminium metal with steam at high temperature and pressure in an autoclave (350-450oC and 6.9-39.2 Mpa) to generate alumina powder. b) In direct oxidation process, finely divided Al powder is oxidised at high temperature to produce alpha alumina powder. c) In the process of reaction of Al metal with acid, pure Al metal is reacted with an acidic solution (pH alumina powder. d) In the process of reaction with base, aluminium metal is reacted with strong basic (pll> 12) solution. Aluminate forms with the progress of the reaction is subsequently hydrolyscd to form aluminium hydroxide which on calcination gives alumina powder. e) In yet another process aqueous quaternary ammonium hydroxide (choline) is allowed to react with pure Al metal to get aluminium hydroxide which on subsequent calcination gives alpha alumina powder. In alkoxide sol-gel process, aluminium alkoxide is hydrolysed with excess amount of distilled water at 80-90°C for sufficient time and stirred vigorously. The sol is peptiscd with small amount of acetic acid or HNO3 or HC1 and it is subsequently aged to form a transparent gel. This gel on further calcination leads to fine particles of alpha alumina powder. All those processes mentioned above suffer from serious disadvantages as mentioned below: In the Bayer process, alumina contain excessive amount of sodium and silicon leading to degradation of mechanical, electrical and optical properties. Though the aluminium hydroxide from the Bayers process can be reacted with strong acids or bases to give high purity producls, there are difficulties with both the approaches. The reaction with a strong base produces a hydroxide, which still contains significant amount of alkali metal impurities from the caustic solution. On the other hand, reaction with acids such as hydrofluoric or sulphuric acids produces hydrated aluminium salts which must be calcined to A1203. This calcination produces corrosive acid vapours which are environmentally unacceptable. Alum process is well known lor the preparation of high purity product but the process is environmentally unacceptable as it is beset with corrosion problems due to generation of corrosive acid gases such as ammonia and sulphuric acid gases into the environment during thermal decomposition of ammonium alum. Hydrothermal process requires expensive high pressure reactors and long processing time for the completion of the reaction. In direct oxidation, the exothermic reaction of aluminium powder with oxygen or oxidising agent is extremely rapid and therefore the product alumina is melted by the heat of reaction and the generated alumina powders become lumpy which require superfine grinding to yield ultrafmc powders. The oxidation of Al powder is very difficult to control and potentially hazardous. In the process of reaction of Al with acid is beset with the corrosion and contamination problems because of the reactive nature of the acid solution and generation hazardous gases. Alumina obtained by the reaction of Al metal with bases is contaminated with alkali metal ion impurities and these impurities cannot be removed by any cost effective methods. Though reaction of aluminium metal with choline produces high purity alumina powder, these powders need considerable amount of superfine grinding to get the fine particle size. Alkoxide process is well known for preparation of fine particles of high purity alumina but it suffers due to high cost of starting material (alkoxide) and coarsening of the grains during calcination. The main object of the present invention is to provide an improved process for the production of high purity ultrafine alumina powder. Another object of the present invention is to provide an improved process for the production of high purity ultrafine alumina powder using less costly raw materials thereby reducing the cost of production. Yet another object of the present invention is to provide an improved process lor the production of high purity ultrafine alumina powder wherein raw material used such as alcohol is recycled thereby further reducing the cost of production. Still another object is to provide an improved process using a modified sol-gel process. The process of the present invention provides an improved sol-gel process for the preparation of ultrafine alumina powders using Al-alkoxide. In the process of the present invention, alcohol generated during the hydrolysis of alkoxide is isolated and the isolated alcohol after proper treatment and distillation has been reused for the preparation of alkoxide. The recovery of alcohol has shown that only aluminium is mainly consumed for the preparation of alumina powder and the consumption of costlier alkoxide is no longer necessary. In the process of the present invention, nanometric alpha alumina particles have been seeded to the nanometric sol at molecular level. These seeds act as nuclei and the phase transformation temperature for thcta to alpha alumina reduces considerably and alpha alumina forms only at 1050oC, thereby the resultant powder consist of submicron sized alpha alumina particles. The present invention uses aluminium, alcohol and distilled water as raw materials for the production of high pure nano-crystalline alumina powder. Aluminum reacts with alcohol to produce Al-alkoxide which is hydrolysed to yield Al-sol and finally produce alumina powder after calcination. Accordingly, the present invention provides an improved process for the production of high purity nano-crystalline alumina powder which comprises: i) reacting aluminium with alcohol in a proportion range of 1:8 to 1:12 at a temperature in the range of 80 to 90oC for a period in the range of 20 to 28 hours in the presence of 50 to 150 mg of mercury halides, ii) distilling the so prepared solution of step (i) in vacuum at a pressure in the range of 1 to 30 cm of mercury and at a temperature in the range of 100 to 250°C to obtain aluminium alkoxide, iii) hydrolysing the aluminium alkoxide obtained in step (ii) with excess of water in a propution usingat a temperature in the range of 80 to 90°C under stirring for a period in the range of 3 from to 4 hours, iv) isolating the alcohol so generated in step (iii) by known methods such as distillation, v) purifying the isolated alcohol so obtained by known methods such as azeolropic fractional distillation, vi) seeding of the sol so prepared in step (iv) with nano sized alpha alumina and peptizing the sol by adding acids in the proportion range of 1:0.08 to 1:0.1 respectively in molecular weight, vii) aging the sol so prepared in step (vi) at a temperature in the range of 80 to 90°C for a period in the range of 4 to 6 hours to yield transparent sol, viii) drying the transparent sol at a temperature in the range of 120 to 500°C to obtain dried gel, ix) calcining the dried gel at a temperature in the range of 1000 and 1400oC for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder. According to a feature of the invention, aluminium metal used may have a purity of 99%. Alcohol used may be such as butyl, isopropyl, ethyl alcohol and may have a purity of 99.5% in anhydrous condition. Mercury halides used may be such as mercuric chloride, mercuric iodide and may have a purity of 99%. The hydrolysation of aluminium alkoxide may be effected using water in a proportion in the range of 1:4 to 1:10. Nano sized alpha alumina used may have particle size less than 10 nm. Drying of the transparent sol may be effected using means such as spray dryer, resistive heating furnace. The calcined powder may be ground using high alumina ball mill. The recovered alcohol from the process may be recycled. Acids used for peptising may be such as nitric, acetic, hydrochloric acid. The following examples are presented by way of illustration of the process of the present invention and should not be construed to limit the scope of the present invention. Example - 1 1 Kg of aluminium metal is reacted with 11 litres of isopropyl alcohol in a 20 litre glass container in anhydrous condition in presence of 50 mg of mercuric chloride at 82oC for 24 hours. Vigorous reaction takes place and aluminium isopropoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 20 cm of llg at 180oC. The pure aluminium alkoxide is hydrolysed with 1:5 ratio of water at 82°C under vigorous stirring. During hydrolysis alcohols boils out which is collected for isolation. The isopropyl alcohol collected is essentially an azeotropic mixture of isopropyl alcohol and water (12%). This alcohol is fractionally distilled for azeotropic separation reused for the alkoxide synthesis. Acetic acid is added in small amount i.e. 0.08 mol of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 85oC and stirring is continued. Fine particles of alpha alumina is added in 0.08 wcight% of the alkoxide to the solution to provide nuclealion site at later stage. Transparent sol formed after 3 hours of stirring is aged at 85"C for 4 hours. At this stage the sol looses the water and viscosity rises and therefore gelation takes place. The gel is rapidly heated to 500°C and soaked for 1 hour where it converts to gamma alumina. The dried gel is subjected to grinding in high alumina ball mill which gives essentially very fine particles of transition alumina. The ground powder is heated to 1050oC and soaked for 30 minutes to yield alpha alumina powders. The results obtained are presented in Table-1. Example - 2 1 Kg aluminium metal is reacted with 11 litres of butyl alcohol in a 20 litre glass container in anhydrous condition in presence of 100 mg of mercuric iodide at 82°C for 24 hours. Vigorous reaction takes place and aluminium butoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 25 cm of Hg at 200oC. The pure aluminium alkoxide is tiydrolysed with 1:10 ratio of water at 85oC under vigorous stirring. During hydrolysis alcohols boils out which is collected or isolated. The alcohol collected is essentially an a/colropic mixture of alcohol and water. This aicohol is fractionally distilled lor the a/cotropic separation and reused for the alkoxide synthesis. Nitric acid is added in small amount i.e. 0.08 mo] of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 85°C and stirring is continued. Fine particles of alpha alumina is added in 0.1 weight% of the alkoxide to the solution to provide nucleation site at later stage. Transparent sol formed after 3.5 hours of stirring is aged at 87°C for 5 hours. The transparent sol is spray dried at 165°C. The spray dried powder is heated to 1050oC and soaked for 30 minutes to yield alpha alumina powders. The results obtained are presented in Table - 1. Example - 3 1.5 Kg of aluminium metal is reacted with 15 litres of isopropyl alcohol in a 20 litre glass container in anhydrous condition in presence of 100 mg of mercuric chloride at 82oC for 24 hours. Vigorous reaction takes place and aluminium isopropoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 20 cm of Ilg at 18()oC. The pure aluminium alkoxide is hydrolysed with 1:7.5 ratio of water at 84oC under vigorous stirring. During hydrolysis alcohols boils out which is collected for isolation. The isopropyl alcohol collected is essentially an azeotropic mixture of isopropyl alcohol and water (with 12% water). This alcohol is fractionally distilled for azeotropic separation and reused for further alkoxide synthesis. Acetic acid is added in small amount i.e. 0.08 mol of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 87°C and stirring is continued. Fine particles of alpha alumina is added in 0.08 weight% of the alkoxide to the solution to provide nucleation site at later stage. Transparent sol formed after 3 hours of stirring is spray dried at 175"C. The spray dried powder is heated to 1050"C and soaked for 30 minutes to yield alpha alumina powders. The results obtained arc presented in Table - 1. Table -1 (Table Removed) By the process of the present invention, high purity uano-crystalline alpha alumina powder have been produced having properties in the range of fropcilics Values Phase present Crystallite sizes Agglomerate sizes Purity Sp. Surface area Bulk density Th. Density Particle shape 100% alpha 10 to 100 nanometer lOOnmto 100 mm 99 to 99.99% 0.1 to 20 m2/g 0.1 to I gm/cc. 3.9 to 3.95 g/cc Spherical -liregular Further, the powders produced by the present invention show excellent sprayability during thermal aud plasma spray deposition on several industrial components. The coating of mis product powder will improve the corrosion and wear resistances of the components. The properties of the powders synthcsised by the present invention show the possibility of wide ranges of high technology ceramic applications such as spray coating, 1C substrates, catalysts and envelope for sodium vapour lamp etc. The main advantages of the present invention are: 1) High purity nano-crystalline alumina powder is synthestsed by a modified alkoxide sol-gel process. 2) More than 85% alcohol is recovered and recycled thus making the process economically viable. 3) Low temperature processing reduces the energy consumption. 4) An environment friendly process due to non-generation of any toxic product and rccoveiy of alcohol. 5) Provides die control of the product composition and characteristics by varying different process p arameter s. 6) Provides the possibility for the preparation of spherical free flowing alumina powder. We Claim: 1. An improved process for the production of high purity nano-crystalline alumina powders which comprises: i) reacting aluminium with alcohol in a proportion in the range of 1:8 to 1:12 at a temperature in the range of 80 to 90°C for a period in the range of 20 to 28 hours in the presence of 50 to 150 mg of mercury halides; ii) distilling the so prepared solution of step (i) in vacuum at a pressure in the range of 1 to 30 cm of mercury and at a temperature in the range of 100 to 250°C to obtain aluminum alkoxide; iii) hydrolyzing the aluminum alkoxide obtained in step (ii) with excess of water in a proportion ranging from 1:4 to 1:10 at a temperature in the range of 80 to 90°C under stirring for a period in the range of 3 to 4 hours; iv) isolating the alcohol so generated in step (iii) by known methods such as distillation; v) purifying the isolated alcohol so obtained by known methods such as azeotropic fractional distillation; vi) seeding the sol so prepared in step (iv) with nano sized alpha alumina and peptizing the sol by adding acids in the proportion range of 1:0.08 to 1:0.1 respectively in molecular weight; vii) aging the sol so prepared in step (vi) at a temperature in the range of 80 to 90°C for a period in the range of 4 to 6 hours to yield transparent sol; viii) drying the transparent sol at a temperature in the range of 120 to 500°C to obtain dried gel, ix) calcining the dried gel at a temperature in the range of 1000 and 1400°C for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder. 2. An improved process as claimed in claim an wherein the aluminum metal used is about 99% pure. 3. An improved process as claimed in claims1andwherein the alcohol used is selected from isopropyl, butyl and ethyl alcohol. 4. An improved process as claimed in claims 1 to 3, wherein the mercuric halides used is selected from mercuric chloride and mercuric iodide. 5. An improved process as claimed in 1 to 44Wierein the particle size of alumina seeds used are less than 10 nm. 6. An improved process claimed in claims 1 to5, wherein the drying of the transparent sol is effected using spray drier or resistive heating furnace. 7. An improved process as claimed in claims 1 to 6, wherein calcined powder is ground using high alumina ball mill. 8. An improved process as claimed in claims 1 to 8; wherein the acids used for peptizing is selected from nitric, acetic and hydrochloric acid. 9. An improved process for the production of high purity nano-crystalline alumina powders substantially as herein described with reference to the example.

Full Text

This invention relates to an improved process for the production of high purity nano-crystalline alumina powder. The invention particularly relates to the synthesis of nano-crystallinc alumina using a modified sol-gel process.
The present invention is a major breakthrough lor the low cost sol-gel synthesis of nano-crystalline high alumina powder having wide range of applications in ceramics, electrical, electronics, chemical, metallurgical, textile and thermal spray coating industries. The product powder produced in the present invention may be used either directly or after processing by conventional ceramic methods like forming into green bodies cither by double action press or isostatic pressing or tape casting or slip casting, followed by presinter machining, sintering in either a conventional resistive heating or microwave or plasma furnace. The powders synthesised by this present invention may also be used as catalyst in chemical industries and as coating materials for thermal and plasma spray. The plasma spray coating of this powder will improve the corrosion and wear resistances of the industrial components. The sintered products prepared from the present invention are likely to be used as, envelope for sodium vapour lamp, substrate for IC chips, high pure alumina crucible, hip joints etc.
The hitherto practiced processes for the production of alumina powder are as follows:
Bayer alumina leaching process has been the most popular and the cheapest amongst all the known processes which produce alumina powder in bulk quantity. In this process the dry ground bauxite is digested in an autoclave with a solution of sodium hydroxide and sodium carbonate. Red mud is separated from the liquor and then aluminium hydroxide is precipitated by introducing fine seed of aluminum trihydrate. This precipitated aluminium trihydroxide is then calcined at elevated temperatures and ground to get alpha alumina powder.
In another known Alum process, ammonium alum is dissolved in water at moderate temperature and then purified by crystallisation. The process of crystallisation is repeated for 2 to 3 times and then the alum is calcined to yield alumina powders.

In yet other known processes for the preparation of alumina using Al metal are as follows:
a) Hydrothermal oxidation process consists of reacting aluminium metal with steam at high
temperature and pressure in an autoclave (350-450oC and 6.9-39.2 Mpa) to generate
alumina powder.
b) In direct oxidation process, finely divided Al powder is oxidised at high temperature to
produce alpha alumina powder.
c) In the process of reaction of Al metal with acid, pure Al metal is reacted with an acidic
solution (pH alumina powder.
d) In the process of reaction with base, aluminium metal is reacted with strong basic (pll> 12)
solution. Aluminate forms with the progress of the reaction is subsequently hydrolyscd to
form aluminium hydroxide which on calcination gives alumina powder.
e) In yet another process aqueous quaternary ammonium hydroxide (choline) is allowed to
react with pure Al metal to get aluminium hydroxide which on subsequent calcination gives
alpha alumina powder.
In alkoxide sol-gel process, aluminium alkoxide is hydrolysed with excess amount of distilled water at 80-90°C for sufficient time and stirred vigorously. The sol is peptiscd with small amount of acetic acid or HNO3 or HC1 and it is subsequently aged to form a transparent gel. This gel on further calcination leads to fine particles of alpha alumina powder.
All those processes mentioned above suffer from serious disadvantages as mentioned below: In the Bayer process, alumina contain excessive amount of sodium and silicon leading to degradation of mechanical, electrical and optical properties. Though the aluminium hydroxide from the Bayers process can be reacted with strong acids or bases to give high purity producls, there are difficulties with both the approaches. The reaction with a strong base produces a hydroxide, which still contains significant amount of alkali metal impurities from the caustic solution. On the other hand, reaction with acids such as hydrofluoric or sulphuric acids produces hydrated aluminium salts

which must be calcined to A1203. This calcination produces corrosive acid vapours which are environmentally unacceptable.
Alum process is well known lor the preparation of high purity product but the process is environmentally unacceptable as it is beset with corrosion problems due to generation of corrosive acid gases such as ammonia and sulphuric acid gases into the environment during thermal decomposition of ammonium alum.
Hydrothermal process requires expensive high pressure reactors and long processing time for the completion of the reaction.
In direct oxidation, the exothermic reaction of aluminium powder with oxygen or oxidising agent is extremely rapid and therefore the product alumina is melted by the heat of reaction and the generated alumina powders become lumpy which require superfine grinding to yield ultrafmc powders. The oxidation of Al powder is very difficult to control and potentially hazardous.
In the process of reaction of Al with acid is beset with the corrosion and contamination problems because of the reactive nature of the acid solution and generation hazardous gases.
Alumina obtained by the reaction of Al metal with bases is contaminated with alkali metal ion impurities and these impurities cannot be removed by any cost effective methods.
Though reaction of aluminium metal with choline produces high purity alumina powder, these powders need considerable amount of superfine grinding to get the fine particle size.
Alkoxide process is well known for preparation of fine particles of high purity alumina but it suffers due to high cost of starting material (alkoxide) and coarsening of the grains during calcination.
The main object of the present invention is to provide an improved process for the production of high purity ultrafine alumina powder.
Another object of the present invention is to provide an improved process for the production of high purity ultrafine alumina powder using less costly raw materials thereby reducing the cost of production.

Yet another object of the present invention is to provide an improved process lor the production of high purity ultrafine alumina powder wherein raw material used such as alcohol is recycled thereby further reducing the cost of production.
Still another object is to provide an improved process using a modified sol-gel process.
The process of the present invention provides an improved sol-gel process for the preparation of ultrafine alumina powders using Al-alkoxide. In the process of the present invention, alcohol generated during the hydrolysis of alkoxide is isolated and the isolated alcohol after proper treatment and distillation has been reused for the preparation of alkoxide. The recovery of alcohol has shown that only aluminium is mainly consumed for the preparation of alumina powder and the consumption of costlier alkoxide is no longer necessary. In the process of the present invention, nanometric alpha alumina particles have been seeded to the nanometric sol at molecular level. These seeds act as nuclei and the phase transformation temperature for thcta to alpha alumina reduces considerably and alpha alumina forms only at 1050oC, thereby the resultant powder consist of submicron sized alpha alumina particles. The present invention uses aluminium, alcohol and distilled water as raw materials for the production of high pure nano-crystalline alumina powder. Aluminum reacts with alcohol to produce Al-alkoxide which is hydrolysed to yield Al-sol and finally produce alumina powder after calcination.
Accordingly, the present invention provides an improved process for the production of high purity nano-crystalline alumina powder which comprises:
i) reacting aluminium with alcohol in a proportion range of 1:8 to 1:12 at a temperature in the range of 80 to 90oC for a period in the range of 20 to 28 hours in the presence of 50 to 150 mg of mercury halides,
ii) distilling the so prepared solution of step (i) in vacuum at a pressure in the range of 1 to 30 cm of mercury and at a temperature in the range of 100 to 250°C to obtain aluminium alkoxide,

iii) hydrolysing the aluminium alkoxide obtained in step (ii) with excess of water in a
propution usingat a
temperature in the range of 80 to 90°C under stirring for a period in the range of 3 from to 4 hours,
iv) isolating the alcohol so generated in step (iii) by known methods such as distillation, v) purifying the isolated alcohol so obtained by known methods such as azeolropic
fractional distillation,
vi) seeding of the sol so prepared in step (iv) with nano sized alpha alumina and peptizing the sol by adding acids in the proportion range of 1:0.08 to 1:0.1 respectively in molecular weight, vii) aging the sol so prepared in step (vi) at a temperature in the range of 80 to 90°C for
a period in the range of 4 to 6 hours to yield transparent sol, viii) drying the transparent sol at a temperature in the range of 120 to 500°C to obtain
dried gel,
ix) calcining the dried gel at a temperature in the range of 1000 and 1400oC for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder.
According to a feature of the invention, aluminium metal used may have a purity of 99%. Alcohol used may be such as butyl, isopropyl, ethyl alcohol and may have a purity of 99.5% in anhydrous condition. Mercury halides used may be such as mercuric chloride, mercuric iodide and may have a purity of 99%. The hydrolysation of aluminium alkoxide may be effected using water in a proportion in the range of 1:4 to 1:10. Nano sized alpha alumina used may have particle size less than 10 nm. Drying of the transparent sol may be effected using means such as spray dryer, resistive heating furnace. The calcined powder may be ground using high alumina ball mill. The recovered alcohol from the process may be recycled. Acids used for peptising may be such as nitric, acetic, hydrochloric acid.

The following examples are presented by way of illustration of the process of the present invention and should not be construed to limit the scope of the present invention.
Example - 1
1 Kg of aluminium metal is reacted with 11 litres of isopropyl alcohol in a 20 litre glass container in anhydrous condition in presence of 50 mg of mercuric chloride at 82oC for 24 hours. Vigorous reaction takes place and aluminium isopropoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 20 cm of llg at 180oC. The pure aluminium alkoxide is hydrolysed with 1:5 ratio of water at 82°C under vigorous stirring. During hydrolysis alcohols boils out which is collected for isolation. The isopropyl alcohol collected is essentially an azeotropic mixture of isopropyl alcohol and water (12%). This alcohol is fractionally distilled for azeotropic separation reused for the alkoxide synthesis. Acetic acid is added in small amount i.e. 0.08 mol of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 85oC and stirring is continued. Fine particles of alpha alumina is added in 0.08 wcight% of the alkoxide to the solution to provide nuclealion site at later stage. Transparent sol formed after 3 hours of stirring is aged at 85"C for 4 hours. At this stage the sol looses the water and viscosity rises and therefore gelation takes place. The gel is rapidly heated to 500°C and soaked for 1 hour where it converts to gamma alumina. The dried gel is subjected to grinding in high alumina ball mill which gives essentially very fine particles of transition alumina. The ground powder is heated to 1050oC and soaked for 30 minutes to yield alpha alumina powders. The results obtained are presented in Table-1.
Example - 2
1 Kg aluminium metal is reacted with 11 litres of butyl alcohol in a 20 litre glass container in anhydrous condition in presence of 100 mg of mercuric iodide at 82°C for 24 hours. Vigorous reaction takes place and aluminium butoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 25 cm of Hg at 200oC. The pure aluminium alkoxide is

tiydrolysed with 1:10 ratio of water at 85oC under vigorous stirring. During hydrolysis alcohols boils out which is collected or isolated. The alcohol collected is essentially an a/colropic mixture of alcohol and water. This aicohol is fractionally distilled lor the a/cotropic separation and reused for the alkoxide synthesis. Nitric acid is added in small amount i.e. 0.08 mo] of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 85°C and stirring is continued. Fine particles of alpha alumina is added in 0.1 weight% of the alkoxide to the solution to provide nucleation site at later stage. Transparent sol formed after 3.5 hours of stirring is aged at 87°C for 5 hours. The transparent sol is spray dried at 165°C. The spray dried powder is heated to 1050oC and soaked for 30 minutes to yield alpha alumina powders. The results obtained are presented in Table - 1.
Example - 3
1.5 Kg of aluminium metal is reacted with 15 litres of isopropyl alcohol in a 20 litre glass container in anhydrous condition in presence of 100 mg of mercuric chloride at 82oC for 24 hours. Vigorous reaction takes place and aluminium isopropoxide is formed. The product aluminium alkoxide is vacuum distilled out under low pressure i.e. at 20 cm of Ilg at 18()oC. The pure aluminium alkoxide is hydrolysed with 1:7.5 ratio of water at 84oC under vigorous stirring. During hydrolysis alcohols boils out which is collected for isolation. The isopropyl alcohol collected is essentially an azeotropic mixture of isopropyl alcohol and water (with 12% water). This alcohol is fractionally distilled for azeotropic separation and reused for further alkoxide synthesis. Acetic acid is added in small amount i.e. 0.08 mol of alkoxide, in the hydrolysed sol. The sol temperature is maintained at 87°C and stirring is continued. Fine particles of alpha alumina is added in 0.08 weight% of the alkoxide to the solution to provide nucleation site at later stage. Transparent sol formed after 3 hours of stirring is spray dried at 175"C. The spray dried powder is heated to 1050"C and soaked for 30 minutes to yield alpha alumina powders. The results obtained arc presented in Table - 1.
Table -1

(Table Removed)
By the process of the present invention, high purity uano-crystalline alpha alumina powder have been produced having properties in the range of

fropcilics

Values

Phase present Crystallite sizes Agglomerate sizes Purity
Sp. Surface area Bulk density Th. Density Particle shape

100% alpha 10 to 100 nanometer lOOnmto 100 mm 99 to 99.99%
0.1 to 20 m2/g 0.1 to I gm/cc. 3.9 to 3.95 g/cc Spherical -liregular

Further, the powders produced by the present invention show excellent sprayability during thermal aud plasma spray deposition on several industrial components. The coating of mis product powder will improve the corrosion and wear resistances of the components.
The properties of the powders synthcsised by the present invention show the possibility of wide ranges of high technology ceramic applications such as spray coating, 1C substrates, catalysts and envelope for sodium vapour lamp etc.
The main advantages of the present invention are:
1) High purity nano-crystalline alumina powder is synthestsed by a modified alkoxide sol-gel
process.
2) More than 85% alcohol is recovered and recycled thus making the process economically viable.
3) Low temperature processing reduces the energy consumption.
4) An environment friendly process due to non-generation of any toxic product and rccoveiy of
alcohol.
5) Provides die control of the product composition and characteristics by varying different
process p arameter s.
6) Provides the possibility for the preparation of spherical free flowing alumina powder.

We Claim:
1. An improved process for the production of high purity nano-crystalline alumina powders which comprises:
i) reacting aluminium with alcohol in a proportion in the range of 1:8 to 1:12 at a temperature in the range of 80 to 90°C for a period in the range of 20 to 28 hours in the presence of 50 to 150 mg of mercury halides;
ii) distilling the so prepared solution of step (i) in vacuum at a pressure in the range of 1 to 30 cm of mercury and at a temperature in the range of 100 to 250°C to obtain aluminum alkoxide;
iii) hydrolyzing the aluminum alkoxide obtained in step (ii) with excess of water in a proportion ranging from 1:4 to 1:10 at a temperature in the range of 80 to 90°C under stirring for a period in the range of 3 to 4 hours;
iv) isolating the alcohol so generated in step (iii) by known methods such as distillation;
v) purifying the isolated alcohol so obtained by known methods such as azeotropic fractional distillation;
vi) seeding the sol so prepared in step (iv) with nano sized alpha alumina and peptizing the sol by adding acids in the proportion range of 1:0.08 to 1:0.1 respectively in molecular weight;
vii) aging the sol so prepared in step (vi) at a temperature in the range of 80 to 90°C for a period in the range of 4 to 6 hours to yield transparent sol;
viii) drying the transparent sol at a temperature in the range of 120 to 500°C to obtain dried gel,
ix) calcining the dried gel at a temperature in the range of 1000 and 1400°C for a soaking period in the range of 0.5 to 1 hour followed by grinding for a period in the range of 15 to 60 minutes to produce high purity nano-crystalline alpha alumina powder.

2. An improved process as claimed in claim an wherein the aluminum metal
used is about 99% pure.
3. An improved process as claimed in claims1andwherein the alcohol used
is selected from isopropyl, butyl and ethyl alcohol.
4. An improved process as claimed in claims 1 to 3, wherein the mercuric
halides used is selected from mercuric chloride and mercuric iodide.
5. An improved process as claimed in 1 to 44Wierein the particle size of
alumina seeds used are less than 10 nm.
6. An improved process claimed in claims 1 to5, wherein the drying of the
transparent sol is effected using spray drier or resistive heating furnace.
7. An improved process as claimed in claims 1 to 6, wherein calcined powder
is ground using high alumina ball mill.

8. An improved process as claimed in claims 1 to 8; wherein the acids used for
peptizing is selected from nitric, acetic and hydrochloric acid.
9. An improved process for the production of high purity nano-crystalline
alumina powders substantially as herein described with reference to the
example.

Documents:

1116-del-1998-abstract.pdf

1116-del-1998-claims.pdf

1116-del-1998-correspondence-others.pdf

1116-del-1998-correspondence-po.pdf

1116-del-1998-description (complete).pdf

1116-del-1998-form-1.pdf

1116-del-1998-form-19.pdf

1116-del-1998-form-2.pdf

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Patent Number

216035

Indian Patent Application Number

1116/DEL/1998

PG Journal Number

13/2008

Publication Date

28-Mar-2008

Grant Date

05-Mar-2008

Date of Filing

27-Apr-1998

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH.

Applicant Address

RAFI MARG, NEW DELHI 110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	PATCHA RAMACHANDRA RAO	NATIONAL METALLURGICAL LABORATORY, P.O. BURMAMINES, JAMESHEDPUR, BIHAR, INDIA
2	TEJ BAHADUR SINGH	NATIONAL METALLURGICAL LABORATORY, P.O. BURMAMINES, JAMESHEDPUR, BIHAR, INDIA
3	LOKESH CHANDRA PATHAK	NATIONAL METALLURGICAL LABORATORY, P.O. BURMAMINES, JAMESHEDPUR, BIHAR, INDIA
4	ANIL KUMAR VERMA	THE TATA IRON, AND STEEL COMPANY LTD., JAMSHENPUR, P.O. BISTUPUR, BIHAR, INDIA

PCT International Classification Number

C01F 7/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA