Title of Invention	"A PROCESS FOR THE PRODUCTION OF DENSE MAGNESIA-RICH MAGNESIUM ALUMINATE SPINEL USEFUL AS REFRACTORY AGGREGATES"
Abstract	In the present invention there is to provided a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein Al2O3-rich spinel (56 to 66% AI2O3) is densified to a high degree by a single firing process at a relatively low temperature (<1600°C). This is done by proper milling, introduction of mixed additives such as incorporation of two additives, ZnO and ZrSiO4, using only single pressing and controlled firing schedule. One of the additives creates lattice strain, while the other additive develops viscous liquid and facilitates liquid phase sintering. The firing schedule is particularly controlled around the spinel formation temperature to lower the rate of expansion due to spinellisation reaction and enhance densification. Sintered magneisa-rich spinel thus produced has a density of 96 to 98% of theoretical density, average grain size 10 to 15 micro-m, hot modulus of rupture at 1300°C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000°C is in the range of 9.2 to 9.6 x 10-6 °C"1. Spinel is a popular refractory material owing to its excellent high temperature properties. Magnesia-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

Title of Invention

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

Abstract

In the present invention there is to provided a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein Al2O3-rich spinel (56 to 66% AI2O3) is densified to a high degree by a single firing process at a relatively low temperature (<1600°C). This is done by proper milling, introduction of mixed additives such as incorporation of two additives, ZnO and ZrSiO4, using only single pressing and controlled firing schedule. One of the additives creates lattice strain, while the other additive develops viscous liquid and facilitates liquid phase sintering. The firing schedule is particularly controlled around the spinel formation temperature to lower the rate of expansion due to spinellisation reaction and enhance densification. Sintered magneisa-rich spinel thus produced has a density of 96 to 98% of theoretical density, average grain size 10 to 15 micro-m, hot modulus of rupture at 1300°C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000°C is in the range of 9.2 to 9.6 x 10-6 °C"1. Spinel is a popular refractory material owing to its excellent high temperature properties. Magnesia-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

Full Text	The present invention relates to a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates. The present invention particularly relates to a process for the production of dense magnesia-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 cntical 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, magnesia-nch spinel is becoming an increasingly popular refractory material for application in transition and burning zones of cement rotary kilns. The improved service life of these refractories are due to excellent corrosion resistance against slag and metal and good spalling resistance. It is mostly replacing magnesite-chrome refractories because of better performance and environment friendliness. 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); 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), etc. However, densification of Mag-AI spinel by utilizing the above methods needs two stage firing. Reference may be made to US patent no. 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% Al203 magnesium aluminate coclinker by adding up to 3 wt% Ti02 and firing above 1700°C. US patent 39,50,504 (1976) describes preparation of magnesium aluminate spinel of up to 80% Al203 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 no. 42, 73,587 (1981), wherein 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. 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 processes on single stage sintering. But there are limitations in these processes, such as the alumina content in spinel is between 85 to 95%. Spinel has become a popular refractory material owing to its excellent high temperature properties. Magnesia-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. The demand for magnesia-rich spinel is rising for use as a high performance environment friendly refractory matenal for lining in cement rotary kilns. The prior art details reveal that all the earlier processes are based on two stage sintering. The firing temperature to achieve 98% densification was above 1600°C However, densification of spinel by a single firing is a big problem due to volume expansion of 5 to 7 % during spinel formation. Hence, it is clear that there is a definite need to provide a single firing process which aims at lowering the sintering temperature. The main object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, which obviates the drawbacks of the hitherto known prior art as mentioned above. Another object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel by single step sintering process. Yet another object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel at a lower temperature of processing in comparison to the hitherto known prior art processes. Still another object of the present invention is to provide a process for the 'production of dense magnesia-rich magnesium aluminate spinel with homogeneous microstructure and excellent thermal properties. In the present invention there is to provided a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein Al203-nch spinel (56 to 66% AI2O3) is 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 magnesia-rich magnesium aluminate spinel useful as refractory aggregates, which comprises mixing magnesia and alumina in a ratio so as to obtain a composition in the range of 56 to 66% Al203 and 43 to 28 % MgO , characterized in the steps of adding to the said magnesia and alumina mixture 0 5 to 3 wt.% ZnO and 0.5 to 3 wt.% ZrSi04, milling the resultant mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powders, 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, adding and mixing an organic binder in the range of 4 to 7% to the to the said mixed dry powder to obtain a pre-pressed compact, briqueting the pre-pressed powder by subjecting to uniaxial pressing at a pressure in the range of 800 to 1400 Kg/cm2, firing the briquettes so formed obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes, allowing the said sintered briquettes to cool naturally. In an embodiment of the present invention the magnesia is such as fused magnesia, sintered magnesia, caustic magnesia, sintered seawater magnesia. In another embodiment of the present invention the alumina is pre-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 is be done by conventional processes such as attrition milling, ball milling, vibro milling In yet another embodiment of the present invention the solvent in attrition milling is such as isopropyl alcohol, acetone, hexane. In still yet another embodiment of the present invention the organic binder used before pressing is such as polyvinyl alcohol, dextrine, glycol in the range of4tol0wt% In a further embodiment of the present invention an intermediate soaking of the order of 1 hour is provided at 1000°C, the starting temperature of spinallisation, during the firing of the briquettes at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes. The present invention provides a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein the additive ZnO forms solid solution in MgO. In the high MgO spinel (MgAI204) there is an ionic substitution of Mg2+ by Zn2+. During this substitution lattice strain is created, which enhances the surface energy and ultimately the sintering of spinel. On the other hand incorporation of the additive ZrSi04 develops high viscous Zr02 and Si02 containing glassy phase and promotes liquid phase sintering. Thus, the introduction of mixed additives such as incorporation of the two additives, ZnO and ZrSi04, results in one of the additives creating lattice strain, while the other additive develops viscous liquid and facilitates liquid phase sintering. This coupled with milling, single "pressing and controlled firing schedule produces dense magnesia-rich magnesium aluminate spinel. The novelty of the present invention is obtaining dense Magnesia rich (56 to 66% AI2O3) magnesium aluminate spinel by single firing process at a lower firing temperature between 1500° to 1700°C, which is relatively low compared to the earlier processes of reaction-sintering temperature. The powders are pressed uniaxially compared to both uniaxial and isostatic pressing conducted in the earlier process. The process of the present invention produces dense grains with homogeneous microstructure. The main non-obvious inventive step in the present invention is utilizing the effect of two additives ZnO and ZrSi04 in the sintering of MgO rich Mag-AI spinel. The additives creates lattice strain as well as liquid phase and intensifies sintering. In the process of the present invention the additive ZnO forms solid solution in MgO. In the high MgO spinel (MgAI204) there is an ionic substitution of Mg2+ by Zn2+. During this substitution lattice strain is created, which enhances the surface energy and ultimately the sintering of spinel. On the other hand incorporation of the second additive ZrSi04 develops high viscous Zr02 and S1O2 containing glassy phase and promotes liquid phase sintering. Therefore, when both the above additives are used in optimum quantity densification is substantially enhanced. The other inventive steps are attrition grinding, and controlling the firing schedule. A change in the milling procedure reduces the sintering temperature by 100°C. Therefore, another inventive step is reducing the particle size through attntion milling, which ultimately sinters at a lower temperature. Reaction-sintering of Mag-AI spinel starts simultaneously above 900°C. Spinellisation is accompanied by a volume expansion between 5 to 7%, which hinders the sintering of spinel. Yet another inventive step in the above process is controlling the firing by introducing a holding period of 1 to 2 hours eit the spinelisation temperature. This reduces the initial rate of reaction and increases the densification at a relatively lower temperature. The process of the present invention comprises- 1. Mixing magnesia and alumina in a ratio to obtain a composition in the range of 56 to 66% Al203. 2. Adding 0.5 to 3 wt.% ZnO and 0 5 to 3 wt.% ZrSi04 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 1400 Kg/cm2. 7. Firing the briquettes so obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours. An intermediate soaking of the order of 1 hour was provided at 1000°C, which is the starting temperature of spinallisation. 8. Allowing the sintered briquettes to cool naturally. The following examples are given by way of illustration of the process of the present invention 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 56% AI2O3. Both ZnO and ZrSi04 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 spinellisation. 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 earned out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope. The bulk density of sintered spinel was 3.51 gm/cc and apparent porosity 0.5%. Hot MOR at 1300°C is 1400 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 10 micro-m. Example - 2 Caustic magnesia powders and calcined alumina were mixed in a proportion to obtain 56% Al203. 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 dned 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 spinallisation. 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 bulk density of sintered spinel was 3.53 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1400 Kg/cm2. Thermal expansion coefficient is 9 58 x 10"6 °C'1 SEM photomicrograph showed compact and uniform texture of grains with average grain size of 15 micro-m. 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 60% Al203. Both Ti02 and ZnO was added 2 wt% each to the batch. The resultant mixture was attntion 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 spinallisation. 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 earned out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope. The bulk density of sintered spinel was 3.55 gm/cc and apparent porosity 1.2%. Hot MOR at 1300°C is 1410 Kg/cm2. Thermal expansion coefficient is 9.32 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12 micro-m. 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 60% AI2O3. Both T1O2 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/cm2. 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 spinallisation. 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 pnnciple. 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 bulk density of sintered spinel was 3.56 gm/cc and apparent porosity 1 %. Hot MOR at 1300°C is 1420 Kg/cm2. Thermal expansion coefficient is 9.35 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 11 micro-m. 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 60% Al203. Both Ti02 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 spinallisation. 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 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 10 micro-m. 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 60% Al203. 2 wt.% Ti02, 2 wt.% ZnO and 5wt. % PVA (5% solution) were added to the batch and mixed in a fluidised bed mixer 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 penod 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 spinallisation. 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 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 12 micro-m. Sintered magneisa-rich spinel thus produced by the process of the present Invention has a density of 96 to 98% of theoretical density, average grain size 10 to 15 micro-m, hot modulus of rupture at 1300°C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000°C is in the range of 9.2 to 9.6 x 10"6 OQ-1 The main advantages of the present invention are: 1. Production of dense magnesia-rich spinel aggregates in the AI2O3 range of 56 to 66% by single firing process. 2. The firing temperature is between 1500° to 1700°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. Dense grains with homogeneous microstructure are produced. We claim: 1. A process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, which comprises mixing magnesia and alumina in a ratio so as to obtain a composition in the range of 56 to 66% Al2O3 and 43 to 28 % MgO , characterized in the steps of adding to the said magnesia and alumina mixture 0.5 to 3 wt.% ZnO and 0.5 to 3 wt.% ZrSi04, milling the resultant mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powders, 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, adding and mixing an organic binder in the range of 4 to 7% to the to the said mixed dry powder to obtain a pre-pressed compact, briqueting the pre-pressed powder by subjecting to uniaxial pressing at a pressure in the range of 800 to 1400 Kg/cm , an intermediate soaking of 1 hour being provided at 1000°C for starting temperature of spinallisation , firing the briquettes so formed obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes , allowing the said sintered briquettes to cool naturally. 2. A process as claimed in claim 1, wherein the magnesia used is selected from fused magnesia, sintered magnesia, caustic magnesia, sintered seawater magnesja. 3. A process as claimed in claim 1-2, wherein the alumina is pre-calcined at a temperature in the range of 800° to 1400°C and is of above 97% purity. 4. A process as claimed in claim 1-3, wherein the milling is be done by conventional processes such as attrition milling, ball milling, vibro milling. 5. A process as claimed in claim 1-4, wherein the solvent in attrition milling is such as isopropyl alcohol, acetone, hexane. 6. A process as claimed in claim 1-5, wherein the organic binder used before pressing is such as polyvinyl alcohol, dextrine, glycol in the range of 4 to 10 wt%.

Full Text

The present invention relates to a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates.
The present invention particularly relates to a process for the production of dense magnesia-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 cntical 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, magnesia-nch spinel is becoming an increasingly popular refractory material for application in transition and burning zones of cement rotary kilns. The improved service life of these refractories are due to excellent corrosion resistance against slag and metal and good spalling resistance. It is mostly replacing magnesite-chrome refractories because of better performance and environment friendliness.
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); 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), etc. However, densification of Mag-AI spinel by utilizing the above methods needs two stage firing.
Reference may be made to US patent no. 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% Al203 magnesium aluminate coclinker by adding up to 3 wt% Ti02 and firing above 1700°C. US patent 39,50,504 (1976) describes preparation of magnesium aluminate spinel of up to 80% Al203 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 no. 42, 73,587 (1981), wherein 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.
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 processes on single stage sintering. But there are limitations in these processes, such as the alumina content in spinel is between 85 to 95%.
Spinel has become a popular refractory material owing to its excellent high temperature properties. Magnesia-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. The demand for magnesia-rich spinel is rising for use as a high performance environment friendly refractory matenal for lining in cement rotary kilns. The prior art details reveal that all the earlier processes are based on two stage sintering. The firing temperature to achieve 98% densification was above 1600°C However, densification of spinel by a single firing is a big problem due to volume expansion of 5 to 7 % during spinel formation. Hence, it is clear that there is a definite need to provide a single firing process which aims at lowering the sintering temperature.
The main object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, which obviates the drawbacks of the hitherto known prior art as mentioned above.
Another object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel by single step sintering process.
Yet another object of the present invention is to provide a process for the production of dense magnesia-rich magnesium aluminate spinel at a lower
temperature of processing in comparison to the hitherto known prior art processes.
Still another object of the present invention is to provide a process for the 'production of dense magnesia-rich magnesium aluminate spinel with homogeneous microstructure and excellent thermal properties.
In the present invention there is to provided a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein Al203-nch spinel (56 to 66% AI2O3) is 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 magnesia-rich magnesium aluminate spinel useful as refractory aggregates, which comprises mixing magnesia and alumina in a ratio so as to obtain a composition in the range of 56 to 66% Al203 and 43 to 28 % MgO , characterized in the steps of adding to the said magnesia and alumina mixture 0 5 to 3 wt.% ZnO and 0.5 to 3 wt.% ZrSi04, milling the resultant mixture in the presence of an organic solvent for a period in the range of 3 to 24 hours to obtain milled powders, 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, adding and mixing an organic binder in the range of 4 to 7% to the to the said mixed dry powder to obtain a pre-pressed compact, briqueting the pre-pressed powder by subjecting to uniaxial
pressing at a pressure in the range of 800 to 1400 Kg/cm2, firing the briquettes so formed obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes, allowing the said sintered briquettes to cool naturally.
In an embodiment of the present invention the magnesia is such as fused magnesia, sintered magnesia, caustic magnesia, sintered seawater magnesia.
In another embodiment of the present invention the alumina is pre-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 is be done by conventional processes such as attrition milling, ball milling, vibro milling
In yet another embodiment of the present invention the solvent in attrition milling is such as isopropyl alcohol, acetone, hexane.
In still yet another embodiment of the present invention the organic binder used before pressing is such as polyvinyl alcohol, dextrine, glycol in the range of4tol0wt%
In a further embodiment of the present invention an intermediate soaking of the order of 1 hour is provided at 1000°C, the starting temperature of spinallisation, during the firing of the briquettes at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes.
The present invention provides a process for the production of dense magnesia-rich magnesium aluminate spinel useful as refractory aggregates, wherein the additive ZnO forms solid solution in MgO. In the high MgO spinel (MgAI204) there is an ionic substitution of Mg2+ by Zn2+. During this substitution lattice strain is created, which enhances the surface energy and ultimately the sintering of spinel. On the other hand incorporation of the additive ZrSi04 develops high viscous Zr02 and Si02 containing glassy phase
and promotes liquid phase sintering. Thus, the introduction of mixed additives such as incorporation of the two additives, ZnO and ZrSi04, results in one of the additives creating lattice strain, while the other additive develops viscous liquid and facilitates liquid phase sintering. This coupled with milling, single "pressing and controlled firing schedule produces dense magnesia-rich magnesium aluminate spinel.
The novelty of the present invention is obtaining dense Magnesia rich (56 to 66% AI2O3) magnesium aluminate spinel by single firing process at a lower firing temperature between 1500° to 1700°C, which is relatively low compared to the earlier processes of reaction-sintering temperature. The powders are pressed uniaxially compared to both uniaxial and isostatic pressing conducted in the earlier process. The process of the present invention produces dense grains with homogeneous microstructure.
The main non-obvious inventive step in the present invention is utilizing the effect of two additives ZnO and ZrSi04 in the sintering of MgO rich Mag-AI spinel. The additives creates lattice strain as well as liquid phase and intensifies sintering. In the process of the present invention the additive ZnO forms solid solution in MgO. In the high MgO spinel (MgAI204) there is an ionic substitution of Mg2+ by Zn2+. During this substitution lattice strain is created, which enhances the surface energy and ultimately the sintering of spinel. On the other hand incorporation of the second additive ZrSi04 develops high viscous Zr02 and S1O2 containing glassy phase and promotes liquid phase sintering. Therefore, when both the above additives are used in optimum quantity densification is substantially enhanced.
The other inventive steps are attrition grinding, and controlling the firing schedule.
A change in the milling procedure reduces the sintering temperature by 100°C. Therefore, another inventive step is reducing the particle size through attntion milling, which ultimately sinters at a lower temperature.
Reaction-sintering of Mag-AI spinel starts simultaneously above 900°C. Spinellisation is accompanied by a volume expansion between 5 to 7%, which hinders the sintering of spinel. Yet another inventive step in the above process is controlling the firing by introducing a holding period of 1 to 2 hours eit the spinelisation temperature. This reduces the initial rate of reaction and increases the densification at a relatively lower temperature.
The process of the present invention comprises-
1. Mixing magnesia and alumina in a ratio to obtain a composition in the range of 56 to 66% Al203.
2. Adding 0.5 to 3 wt.% ZnO and 0 5 to 3 wt.% ZrSi04 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 1400 Kg/cm2.
7. Firing the briquettes so obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours. An intermediate soaking of the order of 1 hour was provided at 1000°C, which is the starting temperature of spinallisation.
8. Allowing the sintered briquettes to cool naturally.
The following examples are given by way of illustration of the process of the present invention 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 56% AI2O3. Both ZnO and ZrSi04 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 spinellisation.
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 earned out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope.
The bulk density of sintered spinel was 3.51 gm/cc and apparent porosity 0.5%. Hot MOR at 1300°C is 1400 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 10 micro-m.
Example - 2
Caustic magnesia powders and calcined alumina were mixed in a proportion to obtain 56% Al203. 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 dned 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 spinallisation.
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 bulk density of sintered spinel was 3.53 gm/cc and apparent porosity 1%. Hot MOR at 1300°C is 1400 Kg/cm2. Thermal expansion coefficient is 9 58 x 10"6 °C'1 SEM photomicrograph showed compact and uniform texture of grains with average grain size of 15 micro-m.
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 60% Al203. Both Ti02 and ZnO was added 2 wt% each to the batch. The resultant mixture was attntion 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 spinallisation.
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 earned out by x-ray and microstructure of the polished and thermally etched sample was observed by scanning electron microscope.
The bulk density of sintered spinel was 3.55 gm/cc and apparent porosity 1.2%. Hot MOR at 1300°C is 1410 Kg/cm2. Thermal expansion coefficient is 9.32 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 12 micro-m.
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 60% AI2O3. Both T1O2 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/cm2. 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 spinallisation.
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 pnnciple. 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 bulk density of sintered spinel was 3.56 gm/cc and apparent porosity 1 %. Hot MOR at 1300°C is 1420 Kg/cm2. Thermal expansion coefficient is 9.35 x 10"6 °C"1. SEM photomicrograph showed compact and uniform texture of grains with average grain size of 11 micro-m.
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 60% Al203. Both Ti02 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 spinallisation.
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 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 10 micro-m.
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 60% Al203. 2 wt.% Ti02, 2 wt.% ZnO and 5wt. % PVA (5% solution) were added to the batch and mixed in a fluidised bed mixer 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 penod 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 spinallisation.
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 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 12 micro-m.
Sintered magneisa-rich spinel thus produced by the process of the present Invention has a density of 96 to 98% of theoretical density, average grain size 10 to 15 micro-m, hot modulus of rupture at 1300°C is 1300 to 1450 Kg/cm2 and thermal expansion coefficient at 1000°C is in the range of 9.2 to 9.6 x 10"6
OQ-1
The main advantages of the present invention are:
1. Production of dense magnesia-rich spinel aggregates in the AI2O3 range of 56 to 66% by single firing process.
2. The firing temperature is between 1500° to 1700°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. Dense grains with homogeneous microstructure are produced.

We claim:
1. A process for the production of dense magnesia-rich magnesium aluminate spinel useful as
refractory aggregates, which comprises mixing magnesia and alumina in a ratio so as to
obtain a composition in the range of 56 to 66% Al2O3 and 43 to 28 % MgO , characterized in
the steps of adding to the said magnesia and alumina mixture 0.5 to 3 wt.% ZnO and 0.5 to 3
wt.% ZrSi04, milling the resultant mixture in the presence of an organic solvent for a period
in the range of 3 to 24 hours to obtain milled powders, 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, adding and mixing an organic binder in the range of 4 to 7% to the to
the said mixed dry powder to obtain a pre-pressed compact, briqueting the pre-pressed
powder by subjecting to uniaxial pressing at a pressure in the range of 800 to 1400 Kg/cm
, an intermediate soaking of 1 hour being provided at 1000°C for starting temperature of spinallisation , firing the briquettes so formed obtained at a temperature in the range of 1500° to 1700°C for a period of 8 to 24 hours to obtain sintered briquettes , allowing the said sintered briquettes to cool naturally.
2. A process as claimed in claim 1, wherein the magnesia used is selected from fused magnesia, sintered magnesia, caustic magnesia, sintered seawater magnesja.
3. A process as claimed in claim 1-2, wherein the alumina is pre-calcined at a temperature in the range of 800° to 1400°C and is of above 97% purity.
4. A process as claimed in claim 1-3, wherein the milling is be done by conventional processes such as attrition milling, ball milling, vibro milling.
5. A process as claimed in claim 1-4, wherein the solvent in attrition milling is such as
isopropyl alcohol, acetone, hexane.
6. A process as claimed in claim 1-5, wherein the organic binder used before pressing is such as polyvinyl alcohol, dextrine, glycol in the range of 4 to 10 wt%.

Documents:

1211-del-2004-abstract.pdf

1211-DEL-2004-Claims-(15-05-2012).pdf

1211-del-2004-claims.pdf

1211-DEL-2004-Correspondence Others-(15-05-2012).pdf

1211-del-2004-correspondence-others.pdf

1211-del-2004-description (complete).pdf

1211-del-2004-form-1.pdf

1211-del-2004-form-18.pdf

1211-del-2004-form-2.pdf

1211-DEL-2004-Form-3-(15-05-2012).pdf

1211-del-2004-form-3.pdf

1211-del-2004-form-5.pdf

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Patent Number

265435

Indian Patent Application Number

1211/DEL/2004

PG Journal Number

09/2015

Publication Date

27-Feb-2015

Grant Date

24-Feb-2015

Date of Filing

30-Jun-2004

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	MANAS KAMAL HALDAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032, INDIA.
2	SAMIR KUMAR DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032, INDIA.
3	ARUP GHOSH	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032, INDIA.
4	BARUNDEB MUKHERJEE	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032, INDIA.

PCT International Classification Number

C09K2 11/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA