Title of Invention	A PROCESS FOR THE PRODUCTION OF DENSE ALUMINA-RICH MAGNESIUM ALUMINATE SPINEL USEFUL AS REFRACTORY AGGREGATES
Abstract	Spinel has become a popular refractory material owing to its excellent high temperature properties. Alumina rich spinel is extensively used now-a-days as a shaped refractory or a constituent for castables which are used in various secondary refining vessels of steel making. However, densification of spinel by a single firing is a big problem due to volume expansion of 5 to 7 % during spinel formation. The present invention relates to the process by which Al2O3-rich spinel (85 to 95% Al2O3) can be densified to a high degree by a single firing process at a relatively low temperature (less than 1600 deg. C). This can be done by proper milling, incorporation of two additives and controlled firing schedule. One of the additives creates cation vacancy while the other remain in the grain boundary and suppresses grain growth. The firing schedule is particularly controlled around the spinel formation temperature to lower the rate of expansion due to spinelisation reaction and enhance densification. Sintered alumina-rich spinel thus produced has a density of 96 to 98% of theoretical density, average grain size 10 to 15mu m, hot modulus of rupture at 1300deg.C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000deg. C is in the range of 8.7 to 8.9 x 10-6 C-1.

Title of Invention

A PROCESS FOR THE PRODUCTION OF DENSE ALUMINA-RICH MAGNESIUM ALUMINATE SPINEL USEFUL AS REFRACTORY AGGREGATES

Abstract

Spinel has become a popular refractory material owing to its excellent high temperature properties. Alumina rich spinel is extensively used now-a-days as a shaped refractory or a constituent for castables which are used in various secondary refining vessels of steel making. However, densification of spinel by a single firing is a big problem due to volume expansion of 5 to 7 % during spinel formation. The present invention relates to the process by which Al2O3-rich spinel (85 to 95% Al2O3) can be densified to a high degree by a single firing process at a relatively low temperature (less than 1600 deg. C). This can be done by proper milling, incorporation of two additives and controlled firing schedule. One of the additives creates cation vacancy while the other remain in the grain boundary and suppresses grain growth. The firing schedule is particularly controlled around the spinel formation temperature to lower the rate of expansion due to spinelisation reaction and enhance densification. Sintered alumina-rich spinel thus produced has a density of 96 to 98% of theoretical density, average grain size 10 to 15mu m, hot modulus of rupture at 1300deg.C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000deg. C is in the range of 8.7 to 8.9 x 10-6 C-1.

Full Text	The present invention relates to a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates. The present invention particularly relates to a process for production of dense alumina-rich magnesium aluminate spinel for direct application as refractory or as a component/constituent of castables for other engineering applications. Magnesium aluminate spinel, a refractory material, offers a unique combination of high temperature properties including high melting point, excellent resistance against chemical attack, potentially high strength at elevated temperatures and other thermal properties. In the recent times due to the stringency in operational conditions of different kilns and furnaces, magnesium aluminate proved to be an ideal refractory material suitable for many critical applications. The major application areas of magnesium aluminate spinel refractories are transition and burning zones of cement rotary kiln, bottom and side wall of primary and secondary steel ladles and checker bricks of glass tank furnace regenerators where it is either used as spinel body or as a component in a magnesia-rich or alumina-rich matrix. Presently alumina-rich spinel is a popular refractory material for application in steel teeming ladles. The improved service life of these refractories are due to excellent corrosion resistance against slag and metal and good spalling resistance. The improvement in the service life of furnace depends upon densification of spinel. In highly dense spinel refractories fluids cannot penetrate deeply and it becomes immune against corrosion. The densification of spinel is a reaction sintering process. The formation of spinel from reactant oxides is accompanied by 5 to 7 % volume expansion. This expansion does not allow the material to become dense in a single firing. It requires two step sintering which comprises reaction and formation of spinel in the first step followed by densification in the subsequent step. Hence, cost of production rises significantly. Reference may be made to American Ceramic Society Bulletin vol 47, No. 11, 1025-29 (1968); Special Ceramics, 167-87 (1964); wherein a mixture of magnesium oxide and aluminium oxide were fired between 1100 to 1550°C to complete the reaction followed by crushing, grinding, shaping and final sintering between 1600° to 1800°C to achieve high densification. Reference may be drawn for different spinel formation techniques such as coprecipitation (Am. Cer. Soc. Bull vol 48, No.8, 759-62, 1962), sol-gel processing (Ceramic Transactions 1A, 211-17, 1988), spark discharge (Yogyo Kyokaishi Vol 90, No. 10, 603-9, 1982). However, densification of Mag-Al spinel by utilizing the above methods needs two stage firing. Reference may be made to US patent 62,39,051 (2001), wherein it has been described the production of magnesium aluminate refractory with porosity between 3 to 10%. The porosity seems to be high. US Patent 51,71,724 (1992) describes production of 75% Al2O3 magnesium aluminate coclinker by adding up to 3 wt% TiO2 and firing above 1700°C. US patent 39,50,504 (1976) describes preparation of magnesium aluminate spinel of up to 80% Al2O3 content by utilizing fine alumina from ESP and Magnesia. The firing temperature is as high as 2100°C for reaching 90% theoretical density. Reference may be made to US Patent 42, 73,587 (1981), where polyaptelline sintered transparent spinel is prepared by two step sintering. The additive is LiF, the atmosphere is hydrogen, vacuum or inert and temperature up to 1900°C. Reference may be made to Japan Patent 200026354 wherein it has been described that up to 84% Al2O3 spinel can be produced by single firing method within a temperature up to 1750°C. The pressing of compacts is performed in two stages, i.e., uniaxial followed by isostatic pressing. The main drawbacks of the hitherto known processes are: 1. Most of the processes are based on two stage sintering which is time consuming and costly. 2. There are few processes on single stage sintering. But there are limitations in these processes, such as the alumina content in spinel is only up to 84%, firing temperature is relatively higher, and pressing is performed in two steps. At present the refractory industry is mainly concerned about high alumina magnesium aluminate spinel aggregates (above 85%Al203). Al2O3 in this range is difficult to sinter owing to the absence of defective structure in hyperstoichiometric spinel. The present invention is aimed at providing dense alumina rich spinel aggregates at a lower sintering temperature in single firing process by introduction of mixed additives and using only single uniaxial pressing. The main object of the present invention is to provide a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates which obviates the drawbacks as mentioned above. Another object of the present invention is to provide dense alumina-rich magnesium aluminate spinel by adopting single step sintering process. Yet another object of the present invention is to lower the temperature of processing in comparison to the processes developed till now. Still another object of the present invention is to produce spinel with homogeneous microstructure and excellent thermal properties. Spinel has become a popular refractory material owing to its excellent high temperature properties. Alumina rich spinel is extensively used now-a-days as a shaped refractory or a constituent for castables which are used in various secondary refining vessels of steel making. However, densification of spinel by a single firing is a big problem due to volume expansion of 5 to 7 % during spinel formation. The present invention relates to the process by which Al203-rich spinel (85 to 95% AI2O3) can be densified to a high degree by a single firing process at a relatively low temperature ( Accordingly the present invention provides a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates which comprises mixing magnesia and alumina in a ratio to obtain a composition having Al2O3 in the range of 85 to 95% , adding 0.5 to 3 wt.% TiO2 and 0.5 to 3 wt.% ZnO, milling the mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powder, drying the resultant powder at a temperature in the range of 95° to 110°C for a period in the range of 24 to 48hours to obtain a mixed dry powder, adding and mixing an organic binder in the range of 4 to 10 % to the said dried mixed powder to obtain a prepressed powder, subjecting the prepressed powder to uniaxial pressing at a pressure in the range of 800 to 1200 Kg/cm2 to obtain briquettes, firing the briquettes so obtained over a total firing schedule of 8 to 24 hours from room temperature to 1750 °C, allowing holding / soaking period of 1 to 2 hours at the starting temperature of spinelisation and holding / soaking period of 2 to 5 hours at a temperature in the range of 1500 °C to 1750 °C, allowing the sintered briquettes to cool naturally. In an embodiment of the present invention the magnesia used may be such as fused magnesia, sintered magnesia, caustic magnesia of purity above 97% (MgO). In another embodiment of the present invention the alumina used may be lightly calcined at a temperature in the range of 800° to 1400°C and of above 97% purity. In still another embodiment of the present invention the milling may be done by conventional processes such as attrition milling, ball milling, vibro milling. In yet another embodiment of the present invention the raw materials before milling may pass through 60 mesh BS sieve. In a further embodiment of the present invention the solvent used in attrition milling may be such as isopropyl alcohol, acetone, hexane. In still another embodiment of the present invention the binder used before pressing may be such as polyvinyl alcohol, dextrine, glycol in the range of 4 to 10 wt%. In yet another embodiment of the present invention the starting temperature of spinalisation may be in the range of 900 ° to 1200 °C The process of the present invention comprises: 1. mixing magnesia and alumina in a ratio to obtain a composition in the range of 85to95%Al203, 2. adding of 0.5 to 3 wt.% TiO2 and 0.5 to 3 wt.% ZnO to the mixture, 3. milling the mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powders, 4. drying the resultant powders at a temperature in the range of 95° to 110°C for a period in the range of 24 to 48 hours to obtain a mixed dry powder, 5. adding and mixing an organic binder in the range of 4 to 7% to the dried mixed powder to obtain a prepressed powder, 6. uniaxial pressing of the prepressed powder into briquettes at a pressure in the range of 800 to 1200 Kg/cm2, 7. firing the briquettes so obtained at a temperature in the range of room temperature to 1750°C for a total period of 8 to 24 hours, with intermediate soaking / holding for 1 to 2 hours at 900 ° to 1200 °C, the starting temperature of spinelisation, and soaking / holding of 2 to 5 hours at a temperature of 1500° to 1750 °C. 8. allowing the sintered briquettes to cool naturally. The sintered magnesium aluminate aggregates were characterized by determining properties like (1) Bulk density and apparent porosity (2) High temperature flexural strength (3) X-ray and (4) Microstructure. Bulk density was measured by liquid displacement method in xylene medium under vacuum using Archimedes principle. Hot Modulus of Rupture (Hot MOR) was determined by three point bending test at 1300°C. Phase identification was carried out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope. The prior art details reveal that all the earlier processes are based on two stage sintering. There is one reference on single firing process, but the amount of Al2O3 in the sintered spinel was up to 84% only and the pressing was carried out in two steps. Moreover, the firing temperature to achieve 98% densification was above 1600°C. The novelty of the present invention is the achievement of 98% densification of Alumina rich (85 to 95% Al2O3) magnesium aluminate spinel between 1500° to 1750°C. A change in the milling procedure reduces the sintering temperature by 100°C. This is against the present available figure of above 1600°C in spinel containing Al2O3 below 85% by solid oxide reaction sintering. This is achieved by attrition grinding, incorporation of additives and controlling firing schedule. The main inventive step in the present invention is utilizing the effect of two additives in the sintering of Al2O3 rich Mag-Al spinel. TiO2 forms solid solution in MgAl2O4 by substitution of Al2O3 and creation of cation vacancy. While ZnO remains in the grain boundary of spinel and inhibits grain growth. The defect reaction can be expressed as: 2A1203 3Ti02 ► 3TIA] + 60Q + VAI Therefore, when both the above additives are used in optimum quantity grain growth is minimized and densification is enhanced. Reaction-sintering of Mag-Al spinel starts simultaneously above 900°C. Spinellisation is accompanied by a volume expansion between 5 to 7%, which hinders the sintering of spinel. Therefore another inventive step in the above process is controlling the firing by introducing a holding period of 1 to 2 hours at the spinelisation temperature. This reduces the initial rate of reaction and increases the densification at a relatively lower temperature. The invention will now be described with the help of following examples for carrying out the process in actual practice. However, these examples should not be construed to limit the scope of the present invention. Example - 1 Sintered seawater magnesia were crushed and ground to pass through 60 BS mesh. Fine magnesia and alumina were mixed in a proportion to obtain 85% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the mix. The resultant mixture was attrition milled for 5 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm2. Pressed compacts was dried and fired at 1500°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.52 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1350 Kg/cm2. Thermal expansion coefficient is 8.75 x 10"6 °C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of l0mum. Example - 2 Caustic magnesia powders and calcined alumina were mixed in a proportion to obtain 85% A1203. Both Ti02 and ZnO was added 3 wt% each to the batch. The resultant mixture was attrition milled for 4 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 4 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm2. Pressed compacts was dried and fired at 1600°C with a soaking period of 4 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.50 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1320 Kg/cm2. Thermal expansion coefficient is 8.73 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 15um. Example - 3 Sintered magnesia was crushed and ground to pass through 60 BS mesh. Then the magnesia powder and calcined alumina were mixed in a proportion to obtain 90% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the batch. The resultant mixture was attrition milled for 4 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compact was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 11 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.58 gm/cc and apparent porosity 1.2%. Hot MOR at 1300°C is 1410 Kg/cm2. Thermal expansion coefficient is 8.81 x 10-6 °C-l. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12um. Example - 4 Fused magnesia was crushed and ground to pass through 60 BS mesh. Magnesia powder and alumina were mixed in a proportion to obtain 90% Al2O3. Both TiO2 and ZnO was added 1 wt% each to the batch. The resultant mixture was attrition milled for 5 hours in presence of acetone as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm . Pressed compact was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinealisation. The bulk density of sintered spinel was 3.58 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1450 Kg/cm2. Thermal expansion coefficient is 8.80 x 10-6 0C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 1 lpm Example - 5 Sintered seawater Magnesia were crushed and ground to pass through 60 BS mesh. Fine Magnesia and alumina were mixed in a proportion to obtain 95% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the mix. The resultant mixture was attrition milled for 5 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compacts was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.59 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1425 Kg/cm2. Thermal expansion coefficient is 8.88 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 10mum. Example - 6 Sintered seawater Magnesia were crushed and ground to pass through 60 BS mesh. This MgO powder was fed to tubular vibro mill and milled in dry condition. The residence period of powders in the mill was 2 hours. The milled magnesia powders was mixed with calcined alumina (surface area 2 m2/gm) in a proportion to obtain 90% Al2O3. 2 wt.% Ti02, 2 wt.% ZnO and 5wt. % PVA (5% solution) were added to the batch and meixed in a fluidised bed mixed for 10 minutes. The mixed powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compacts was dried and fired at 1700°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1150°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.56 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1425 Kg/cm2. Thermal expansion coefficient is 8.80 x 10-6 °C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12mum. Example - 7 Sintered seawater Magnesia were crushed and ground to pass through 100 BS mesh. Magnesia powder was mixed with calcined alumina powder in a proportion to obtain 90% A1203 in the batch. 3 wt.% TiO2 and 3 wt.% ZnO were added to the mix. The batch was then ball milled for 24 hours. 7wt. % PVA (5% solution) was added to the batch and mixed in a fluidised bed mixed for 10 minutes. The mixed powder was then pressed uniaxially at a pressure of 1200 Kg/cm . Pressed compacts was dried and fired at 1750°C with a soaking period of 4 hours. The total firing schedule was 12 hours and an intermediate soaking of 1 hour was also provided at 1200°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.54 gm/cc and apparent porosity 1.5%. Hot MOR at 1300°C is 1320 Kg/cm2. Thermal expansion coefficient is 8.79 x 10-6 °C_1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 14mum. The main advantages of the present invention are: 1. Production of dense alumina rich spinel aggregates in the Al2O3 range of 85 to 95% by single firing process. 2. The firing temperature is between 1500° to 1750°C, which is relatively low compared to the earlier processes of reaction-sintering. 3. The powders are pressed uniaxially compared to both uniaxial and isostatic pressing conducted in the earlier process. 4. The additives controlled the grain growth and intensified sintering. Therefore, dense grains with homogeneous microstructure are produced.

Full Text

The present invention relates to a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates.
The present invention particularly relates to a process for production of dense alumina-rich magnesium aluminate spinel for direct application as refractory or as a component/constituent of castables for other engineering applications.
Magnesium aluminate spinel, a refractory material, offers a unique combination of high temperature properties including high melting point, excellent resistance against chemical attack, potentially high strength at elevated temperatures and other thermal properties. In the recent times due to the stringency in operational conditions of different kilns and furnaces, magnesium aluminate proved to be an ideal refractory material suitable for many critical applications.
The major application areas of magnesium aluminate spinel refractories are transition and burning zones of cement rotary kiln, bottom and side wall of primary and secondary steel ladles and checker bricks of glass tank furnace regenerators where it is either used as spinel body or as a component in a magnesia-rich or alumina-rich matrix.
Presently alumina-rich spinel is a popular refractory material for application in steel teeming ladles. The improved service life of these refractories are due to excellent corrosion resistance against slag and metal and good spalling resistance.
The improvement in the service life of furnace depends upon densification of spinel. In highly dense spinel refractories fluids cannot penetrate deeply and it becomes immune against corrosion. The densification of spinel is a reaction sintering process. The formation of spinel from reactant oxides is accompanied by 5 to 7 % volume expansion. This expansion does not allow the material to become dense in a single firing. It requires two step sintering which comprises reaction and formation of spinel in the first step followed by densification in the subsequent step. Hence, cost of production rises significantly.

Reference may be made to American Ceramic Society Bulletin vol 47, No. 11, 1025-29 (1968); Special Ceramics, 167-87 (1964); wherein a mixture of magnesium oxide and aluminium oxide were fired between 1100 to 1550°C to complete the reaction followed by crushing, grinding, shaping and final sintering between 1600° to 1800°C to achieve high densification.
Reference may be drawn for different spinel formation techniques such as coprecipitation (Am. Cer. Soc. Bull vol 48, No.8, 759-62, 1962), sol-gel processing (Ceramic Transactions 1A, 211-17, 1988), spark discharge (Yogyo Kyokaishi Vol 90, No. 10, 603-9, 1982). However, densification of Mag-Al spinel by utilizing the above methods needs two stage firing.
Reference may be made to US patent 62,39,051 (2001), wherein it has been described the production of magnesium aluminate refractory with porosity between 3 to 10%. The porosity seems to be high. US Patent 51,71,724 (1992) describes production of 75% Al2O3 magnesium aluminate coclinker by adding up to 3 wt% TiO2 and firing above 1700°C. US patent 39,50,504 (1976) describes preparation of magnesium aluminate spinel of up to 80% Al2O3 content by utilizing fine alumina from ESP and Magnesia. The firing temperature is as high as 2100°C for reaching 90% theoretical density.
Reference may be made to US Patent 42, 73,587 (1981), where polyaptelline sintered transparent spinel is prepared by two step sintering. The additive is LiF, the atmosphere is hydrogen, vacuum or inert and temperature up to 1900°C.
Reference may be made to Japan Patent 200026354 wherein it has been described that up to 84% Al2O3 spinel can be produced by single firing method within a temperature up to 1750°C. The pressing of compacts is performed in two stages, i.e., uniaxial followed by isostatic pressing.

The main drawbacks of the hitherto known processes are:
1. Most of the processes are based on two stage sintering which is time consuming and costly.
2. There are few processes on single stage sintering. But there are limitations in these processes, such as the alumina content in spinel is only up to 84%, firing temperature is relatively higher, and pressing is performed in two steps.
At present the refractory industry is mainly concerned about high alumina magnesium aluminate spinel aggregates (above 85%Al203). Al2O3 in this range is difficult to sinter owing to the absence of defective structure in hyperstoichiometric spinel. The present invention is aimed at providing dense alumina rich spinel aggregates at a lower sintering temperature in single firing process by introduction of mixed additives and using only single uniaxial pressing.
The main object of the present invention is to provide a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates which obviates the drawbacks as mentioned above.
Another object of the present invention is to provide dense alumina-rich magnesium aluminate spinel by adopting single step sintering process.
Yet another object of the present invention is to lower the temperature of processing in comparison to the processes developed till now.
Still another object of the present invention is to produce spinel with homogeneous microstructure and excellent thermal properties.
Spinel has become a popular refractory material owing to its excellent high temperature properties. Alumina rich spinel is extensively used now-a-days as a shaped refractory or a constituent for castables which are used in various secondary refining vessels of steel making. However, densification of spinel by a single firing is a big

problem due to volume expansion of 5 to 7 % during spinel formation. The present invention relates to the process by which Al203-rich spinel (85 to 95% AI2O3) can be densified to a high degree by a single firing process at a relatively low temperature ( Accordingly the present invention provides a process for the production of dense alumina-rich magnesium aluminate spinel useful as refractory aggregates which comprises mixing magnesia and alumina in a ratio to obtain a composition having Al2O3 in the range of 85 to 95% , adding 0.5 to 3 wt.% TiO2 and 0.5 to 3 wt.% ZnO, milling the mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powder, drying the resultant powder at a temperature in the range of 95° to 110°C for a period in the range of 24 to 48hours to obtain a mixed dry powder, adding and mixing an organic binder in the range of 4 to 10 % to the said dried mixed powder to obtain a prepressed powder, subjecting the prepressed powder to uniaxial pressing at a pressure in the range of 800 to 1200 Kg/cm2 to obtain briquettes, firing the briquettes so obtained over a total firing schedule of 8 to 24 hours from room temperature to 1750 °C, allowing holding / soaking period of 1 to 2 hours at the starting temperature of spinelisation and holding / soaking period of 2 to 5 hours at a temperature in the range of 1500 °C to 1750 °C, allowing the sintered briquettes to cool naturally.
In an embodiment of the present invention the magnesia used may be such as fused magnesia, sintered magnesia, caustic magnesia of purity above 97% (MgO).

In another embodiment of the present invention the alumina used may be lightly calcined at a temperature in the range of 800° to 1400°C and of above 97% purity.
In still another embodiment of the present invention the milling may be done by conventional processes such as attrition milling, ball milling, vibro milling.
In yet another embodiment of the present invention the raw materials before milling may pass through 60 mesh BS sieve.
In a further embodiment of the present invention the solvent used in attrition milling may be such as isopropyl alcohol, acetone, hexane.
In still another embodiment of the present invention the binder used before pressing may be such as polyvinyl alcohol, dextrine, glycol in the range of 4 to 10 wt%.
In yet another embodiment of the present invention the starting temperature of spinalisation may be in the range of 900 ° to 1200 °C
The process of the present invention comprises:
1. mixing magnesia and alumina in a ratio to obtain a composition in the range of 85to95%Al203,
2. adding of 0.5 to 3 wt.% TiO2 and 0.5 to 3 wt.% ZnO to the mixture,
3. milling the mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powders,
4. drying the resultant powders at a temperature in the range of 95° to 110°C for a period in the range of 24 to 48 hours to obtain a mixed dry powder,
5. adding and mixing an organic binder in the range of 4 to 7% to the dried mixed powder to obtain a prepressed powder,
6. uniaxial pressing of the prepressed powder into briquettes at a pressure in the range of 800 to 1200 Kg/cm2,

7. firing the briquettes so obtained at a temperature in the range of room temperature to 1750°C for a total period of 8 to 24 hours, with intermediate soaking / holding for 1 to 2 hours at 900 ° to 1200 °C, the starting temperature of spinelisation, and soaking / holding of 2 to 5 hours at a temperature of 1500° to 1750 °C.
8. allowing the sintered briquettes to cool naturally.
The sintered magnesium aluminate aggregates were characterized by determining properties like (1) Bulk density and apparent porosity (2) High temperature flexural strength (3) X-ray and (4) Microstructure. Bulk density was measured by liquid displacement method in xylene medium under vacuum using Archimedes principle. Hot Modulus of Rupture (Hot MOR) was determined by three point bending test at 1300°C. Phase identification was carried out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope.
The prior art details reveal that all the earlier processes are based on two stage sintering. There is one reference on single firing process, but the amount of Al2O3 in the sintered spinel was up to 84% only and the pressing was carried out in two steps. Moreover, the firing temperature to achieve 98% densification was above 1600°C.
The novelty of the present invention is the achievement of 98% densification of Alumina rich (85 to 95% Al2O3) magnesium aluminate spinel between 1500° to 1750°C. A change in the milling procedure reduces the sintering temperature by 100°C. This is against the present available figure of above 1600°C in spinel containing Al2O3 below 85% by solid oxide reaction sintering. This is achieved by attrition grinding, incorporation of additives and controlling firing schedule.
The main inventive step in the present invention is utilizing the effect of two additives in the sintering of Al2O3 rich Mag-Al spinel. TiO2 forms solid solution in MgAl2O4 by substitution of Al2O3 and creation of cation vacancy. While ZnO remains in

the grain boundary of spinel and inhibits grain growth. The defect reaction can be
expressed as:
2A1203
3Ti02 ► 3TIA] + 60Q + VAI
Therefore, when both the above additives are used in optimum quantity grain growth is minimized and densification is enhanced.
Reaction-sintering of Mag-Al spinel starts simultaneously above 900°C. Spinellisation is accompanied by a volume expansion between 5 to 7%, which hinders the sintering of spinel. Therefore another inventive step in the above process is controlling the firing by introducing a holding period of 1 to 2 hours at the spinelisation temperature. This reduces the initial rate of reaction and increases the densification at a relatively lower temperature.
The invention will now be described with the help of following examples for carrying out the process in actual practice. However, these examples should not be construed to limit the scope of the present invention.
Example - 1
Sintered seawater magnesia were crushed and ground to pass through 60 BS mesh. Fine magnesia and alumina were mixed in a proportion to obtain 85% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the mix. The resultant mixture was attrition milled for 5 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm2. Pressed compacts was dried and fired at 1500°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.52 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1350 Kg/cm2. Thermal expansion coefficient is 8.75 x 10"6

°C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of l0mum.
Example - 2
Caustic magnesia powders and calcined alumina were mixed in a proportion to obtain 85% A1203. Both Ti02 and ZnO was added 3 wt% each to the batch. The resultant mixture was attrition milled for 4 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 4 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm2. Pressed compacts was dried and fired at 1600°C with a soaking period of 4 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.50 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1320 Kg/cm2. Thermal expansion coefficient is 8.73 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 15um.
Example - 3
Sintered magnesia was crushed and ground to pass through 60 BS mesh. Then the magnesia powder and calcined alumina were mixed in a proportion to obtain 90% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the batch. The resultant mixture was attrition milled for 4 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compact was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 11 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.58 gm/cc and apparent porosity 1.2%. Hot MOR at 1300°C is 1410 Kg/cm2. Thermal expansion coefficient is 8.81 x 10-6

°C-l. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12um.
Example - 4
Fused magnesia was crushed and ground to pass through 60 BS mesh. Magnesia powder and alumina were mixed in a proportion to obtain 90% Al2O3. Both TiO2 and ZnO was added 1 wt% each to the batch. The resultant mixture was attrition milled for 5 hours in presence of acetone as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1000 Kg/cm . Pressed compact was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinealisation. The bulk density of sintered spinel was 3.58 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1450 Kg/cm2. Thermal expansion coefficient is 8.80 x 10-6 0C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 1 lpm
Example - 5
Sintered seawater Magnesia were crushed and ground to pass through 60 BS mesh. Fine Magnesia and alumina were mixed in a proportion to obtain 95% Al2O3. Both TiO2 and ZnO was added 2 wt% each to the mix. The resultant mixture was attrition milled for 5 hours in presence of isopropyl alcohol as dispersing medium. The material was then dried at 100°C for 24 hrs and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compacts was dried and fired at 1550°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1000°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.59 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1425 Kg/cm2. Thermal expansion coefficient is 8.88 x 10"6

°C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 10mum.
Example - 6
Sintered seawater Magnesia were crushed and ground to pass through 60 BS mesh. This MgO powder was fed to tubular vibro mill and milled in dry condition. The residence period of powders in the mill was 2 hours. The milled magnesia powders was mixed with calcined alumina (surface area 2 m2/gm) in a proportion to obtain 90% Al2O3. 2 wt.% Ti02, 2 wt.% ZnO and 5wt. % PVA (5% solution) were added to the batch and meixed in a fluidised bed mixed for 10 minutes. The mixed powder was then pressed uniaxially at a pressure of 1200 Kg/cm2. Pressed compacts was dried and fired at 1700°C with a soaking period of 3 hours. The total firing schedule was 10 hours and an intermediate soaking of 1 hour was also provided at 1150°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.56 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1425 Kg/cm2. Thermal expansion coefficient is 8.80 x 10-6 °C-1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12mum.
Example - 7
Sintered seawater Magnesia were crushed and ground to pass through 100 BS mesh. Magnesia powder was mixed with calcined alumina powder in a proportion to obtain 90% A1203 in the batch. 3 wt.% TiO2 and 3 wt.% ZnO were added to the mix. The batch was then ball milled for 24 hours. 7wt. % PVA (5% solution) was added to the batch and mixed in a fluidised bed mixed for 10 minutes. The mixed powder was then pressed uniaxially at a pressure of 1200 Kg/cm . Pressed compacts was dried and fired at 1750°C with a soaking period of 4 hours. The total firing schedule was 12 hours and an intermediate soaking of 1 hour was also provided at 1200°C, which is the starting temperature of spinelisation. The bulk density of sintered spinel was 3.54 gm/cc and apparent porosity 1.5%. Hot MOR at 1300°C is 1320 Kg/cm2. Thermal expansion

coefficient is 8.79 x 10-6 °C_1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 14mum.
The main advantages of the present invention are:
1. Production of dense alumina rich spinel aggregates in the Al2O3 range of 85 to 95% by single firing process.
2. The firing temperature is between 1500° to 1750°C, which is relatively low compared to the earlier processes of reaction-sintering.
3. The powders are pressed uniaxially compared to both uniaxial and isostatic pressing conducted in the earlier process.
4. The additives controlled the grain growth and intensified sintering. Therefore, dense grains with homogeneous microstructure are produced.

Documents:

86-DEL-2002-Claims-(19-02-2008).pdf

86-DEL-2002-Correspondence-Others-(19-02-2008).pdf

86-DEL-2002-Description (Complete)-(19-02-2008).pdf

abstract.pdf

claims.pdf

correspondence-others.pdf

correspondence-po.pdf

description complete.pdf

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Patent Number

215810

Indian Patent Application Number

86/DEL/2002

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

03-Mar-2008

Date of Filing

31-Jan-2002

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	ARUP GHOSH	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
2	BARUNDEB MUKHERJEE	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
3	HIMANSU SEKHAR TRIPATHI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
4	MANAS KAMAL HALDAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
5	SAMIR KUMAR DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032
6	HIMADRI SEKHAR MAITI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032

PCT International Classification Number

C04B 35/443

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA