Title of Invention	"A PROCESS FOR THE MANUFACTURE OF DENSE HIGH ALUMINA REFRACTORY AGGREGATE FROM SILLIMANITE SAND BASED MINERALS"
Abstract	The present invention relates to a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals. This has been made possible by providing a process wherein the starting material is a novel composition consisting of sillimanite sand minerals, ZrO2 and TiO2. The process involves proper milling of the novel mixture and single firing at a temperature between 1500° to 1600°C. The novel composition helps intensify the densification through liquid phase sintering and controls the glassy phase and improves the high temperature properties. The aggregates produced have high alumina content, in the range of 55 to 62% Al203, and have above 98% densification, average grain size of 5 to 10 µm, hot modulus of rupture at 1250°C is 1700 to 2100 kg/cm2. Dense high alumina refractory aggregates are in great demand in refractory industries.

Title of Invention

"A PROCESS FOR THE MANUFACTURE OF DENSE HIGH ALUMINA REFRACTORY AGGREGATE FROM SILLIMANITE SAND BASED MINERALS"

Abstract

The present invention relates to a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals. This has been made possible by providing a process wherein the starting material is a novel composition consisting of sillimanite sand minerals, ZrO2 and TiO2. The process involves proper milling of the novel mixture and single firing at a temperature between 1500° to 1600°C. The novel composition helps intensify the densification through liquid phase sintering and controls the glassy phase and improves the high temperature properties. The aggregates produced have high alumina content, in the range of 55 to 62% Al203, and have above 98% densification, average grain size of 5 to 10 µm, hot modulus of rupture at 1250°C is 1700 to 2100 kg/cm2. Dense high alumina refractory aggregates are in great demand in refractory industries.

Full Text	The present invention relates to a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals. The present invention particularly relates to a process for the production of dense high alumina refractory aggregate from a novel composition consisting of sillimanite sand based minerals, ZrO2, and TiO2 for direct application as refractory or as a component /constituent of castable. Sillimanite beach sand is a byproduct obtained during the extraction of rare earth compounds from beach sand minerals. Mineralogically it is sillimanite (AI2O3.SiO2) associated with small amounts of other mineral phase. Sillimanite decomposes to mullite and silica on heating in the temperature range 1300 to 1700°C. Mullite is the only stable compound with high covalency in the SiO2-AI2O3 system. Mullite has many advantageous properties of refractory materials (a) high melting point (b) low thermal expansion, which yields good thermal shock resistance (c) good corrosion and creep resistance (d) little deformation under load. Due to these unique properties the mullite-based materials is used as refractories in many industrial fields. In steel industry mullite-based bricks are the main constituent of the lining of the upper part of melting furnace, hot blast stoves, hot iron runner and continuous casting furnaces. Mullite refractory bricks are also important components for the lining of high temperature tool in cement industry e.g. calcination and cooler zones of rotary kiln. Sillimanite can be used directly or in combination with alumina depending upon the requirement of the alumina content and temperature of application for production of refractory aggregates. Decomposition temperature of sillimanite sand depends on the particle size and impurity level of the raw material. 3 (AI2O3. SiO2) = 3 AI2O3. 2 SiO2 + SiO2. Silica released partially reacts with the impurities form a glassy phase which impairs the product quality. Product quality can be improved by reducing the amount of glassy phase in the aggregate. Reference may be made to Ceramics International Vol. 20, 299-302 (1994); wherein sillimanite was sintered in presence of Ti02. or ZrO2 in the temperature range of 1520° 1570°C. The TiO2 used is between 6 to 8 wt. % and ZrO2 between 9 to 12.5 wt. %. Liquid phase formation was responsible for improved densification and flexural strength at room temperature varied between 45-94 MPa. Reference may be drawn from Journal of European Ceramic Society, Vol 18, 2081-2087 (1998); wherein densification of sillimanite by wet milling and addition of TiO2 where carried out in the temperature range of 1500° to 1550°C. The pressing is done in two step, i.e., uniaxial as well as isostatic. The high temperature flexural strength measured at 1200°C falls when the sintering temperature increases above 1500°C to 375 MPa. Bloating surface of samples occurred when the aggregate were fired beyong 1550°C due to excessive glassy phase. Reference may be made to Industrial Ceramics Vol. 19, 13-16 (1999); wherein sillimanite were sintered by milling and adding ZrO2. Densification was achieved at 1600°C where the open porosity is almost zero. The hot MOR increases with 2 wt. % addition compared to no additives as the glassy phase is converted to zircon. HMOR at 1200°C is 360 MPa. Reference may be made to US patent 4,960,738 (1990) where mullite -alumina composites sintered body with AI2O3 in the range of 75-85 wt% were developed from powdery aluminosilicate material and sintered in the range of 1500- 1650°C. The sintered body has an apparent porosity of 1%, grain size of 1 Dm and bending strength at room temperature SOOMPa. The main drawbacks of the hitherto known process are: 1. High alumina aggregates produced from sillimanite sand needs higher temperature of 1600°C when ZrO2 is used. On the other hand for TiO2 additive firing above 1500°C deteriorates the product characteristics due to excessive generation of glassy phase and formation of bloated surface. The higher level of the additives would form excessive liquid and reduce the high temperature strength of the aggregates. 2. In all the above process two step pressing i.e uniaxial and isostatic had been done. 3. The process which consists sintered body above 75% alumina the grain size is only 1 urn which may deteriorate some high temperature properties. At present there is a huge demand of high alumina aggregates with content in the range of 55-60% in refractory industries. Hence, there is a definite need to provide a process for the manufacture of dense high alumina refractory aggregate. The main object of the present invention is to provide a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals, which obviates the drawbacks of the hitherto known prior art as mentioned above. Another object of the present invention is to provide a process wherein the starting material is a novel composition consisting of sillimanite sand based mineral, ZrO2 and TiOa, which will provide combined additives to achieve a dense product at a relatively low temperature. Yet another object of the present invention is to develop sintered high alumina refractory aggregate by adopting single step uniaxial pressing. Still another object of the present invention is to produce high alumina aggregate with homogeneous microstructure and improved thermomechanical properties. Still yet another object of the present invention is to provide dense high alumina refractory aggregates in the AI2O3 range of 55 to 62 wt% for application above 1500°C. A further object of the present invention is to provide dense high alumina refractory aggregates having grain size in the range of 5 -10 urn compared to 1um as obtained in the prior art, there by improving the high temperature strength. A still further object of the present invention is to provide a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals, by utilizing low cost sillimanite sand, a byproduct obtained during extraction of rare earth compounds from beach sand minerals. Another object of the present invention is to provide a process which is economical compared to the hithero known prior art processes, introduction of mixed additives, pressing in only one step and sintering. The above process is considered to be cheaper than the earlier processes. The present invention provides a process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals. This has been made possible by providing a process wherein the starting material is a novel composition consisting of sillimanite sand minerals, ZrO2 and TiO2. The process involves proper milling of the novel mixture and single firing at a temperature in the range of 1500° to 1600°C. The novel composition helps intensify the densification through liquid phase sintering and controls the glassy phase and improves the high temperature properties. High alumina aggregates in the range of 55 to 62% AI2O3 thus produced has above 98% densification, average grain size of 5 to 10 urn, hot modulus of rupture at 1250°C is 1700 to 2100 kg/cm2. Accordingly the present invention provides a process for the manufacture of dense high alumina refractory aggregates from sillimanite sand based minerals, which comprises: milling 93 to 96 wt% Sillimanite sand; 6 to1 wt% Titanium dioxide and 1 to 3 wt% Zirconium dioxide in presence of a solvent such as water, isopropyl alcohol, acetone, hexane for a period in the range of 8 to 24 hours to obtain milled slurry, the resultant milled slurry being dried at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, mixing the milled and dried powder so obtained with an organic binder in the proportion of 92 to 98 wt% powder and 2 to 8wt% binder to obtain a prepressed powder, subjecting the pre-pressed powder to uniaxial pressing to obtain pellets, drying the said pellets at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, sintering the dried pellets at a temperature in the range of 1450 to 1650°C for a period in the range of 10 to 24 hours, allowing the sintered pellets to cool naturally. In an embodiment of the present invention, the sillimanite sand contains 55 to 62 wt% AI2O3. In another embodiment of the present invention, the ZrO2 is of 99% purity. In still another embodiment of the present invention, the TiO2 is of 99% purity. In yet another embodiment of the present invention, the milling is done in conventional mill such as attrition mill, ball mill, vibro-mill. In still yet another embodiment of the present invention, the organic binder is poly vinyl alcohol, dextrine, carboxymethyl cellulose. In a further embodiment of the present invention, the solvent used is in the weight ratio of solid : solvent :: 1 : 2. In a still further embodiment of the present invention, the uniaxial pressing is done in the range of 100 to 130MPa. The novelty of the present invention is the production of high density high alumina refractory aggregate from sillimanite beach sand for application at 1500°C or above. Another novel feature of the process is the single pressing rather than conventional and isostatic pressing as in the prior art. Aggregates developed by this process has the advantage for application in kilns and furnaces at high temperature, particularly where the temperature range is between 1500 to 1600°C which is not possible by existing prir art processes due to formation of excessive liquid or poor densification. The non-obvious inventive steps to realize the novel features of the present invention resides in: (i) providing the novel composition consisting sillimanite sand based mineral, Zr02 and TiO2; (ii) utilizing the effect of two additives, TiO2 and ZrO2 in the sintering of high alumina aggregate containing 55 to 62% alumina. TiO2 is the liquid forming additive and facilitates the densification of mullite through liquid phase sintering. While ZrO2 converts the excess glass phase (SiO2) formed during dissociation reaction to ZrSiO4. This is beneficial to control the bloating of aggregates and improve hot properties. The reaction can be expressed as follows: 3(A12 O, .SiO2 ) 3A12 O3 .2SiO2 + SiO22 ZrO2 + SiO2 - ZrSiO4 Therefore, when both the above additives are used in appropriate proportion densification is enhanced without deteriorating the high temperature properties. Thus the aggregate is suitable for application in vessels where the temperature is above 1500°C. The details of the process steps of the present invention are: 1. Mixing of sillimanite sand 93 to 96 wt% with 6 to 1 wt% TiO2 and 1 to 3 wt % ZrO2, 2. Milling the mixture in conventional mill such as attrition mill, ball mill, vibromill in a liquid medium such as water, isopropyl alcohol, acetone, hexane in the weight ratio of solid:solvent:: 1:2 for a period in the range of 8 to 24 hrs to obtained milled slurry. 3. Drying the resultant slurry at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours to obtain a synergistic composition. 4. mixing the synergistic composition with an organic binder to obtain a pre-pressed powder, 5. uniaxial pressing of the pre-pressed powder to form pellets, 6. drying the pellets so obtain at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, 7. firing the pellets so obtain in the range of 1450 to 1650°C for a period in the range of 10 to 24 hrs, 8. allowing the sintered pellets to cool naturally. The sintered high alumina aggregates were characterized by determining properties like 1) bulk density and apparent porosity 2) phase identification by X-ray and IR 3) high temperature flexural strength 4) microstructure. Bulk density apparent porosity were measured by liquid displacement method using Archimedes principle in water medium. High temperature flexural strength was determined by three point bending test at 1250°C. Phase identification was carried out by X-ray and IR and microstructure of the polished and thermally etched sample was observed by scanning electron microscope. The prior art details describes that the additive which is beneficial for low temperature densification (1500°C), resulted bloating of aggregates when used at higher temperature. Alternatively for other additive proper densification of aggregate achieved above 1600°C thereby restricting its use in the range of 1500 to 1600°C. More over the grain size of high alumina aggregate in some of the prior art processes is too low (1um) which will affect the corrosion resistance of the ultimate product. The process of the present invention is described with the help of the 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. Exam pie-1 Beach sand sillimanite was mixed with 4.5 wt% TiO2 and 2 wt% ZrO2. The resultant mixture was attrition milled for 9 hrs. in water as dispersing medium. The slurry thus obtained was dried at 110±5°C for 24 hrs and subsequently crushed to break the agglomerate. The powder was then mixed with 6% PVA solution (5% concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed compacts were dried and fired at 1500°C with a soaking period of 2 hrs. The total firing schedule was 10 hrs. The bulk density and apparent porosity of the sintered aggregate were 2.97 g/cc and 0.6 % respectively. Hot MOR at 1250°C is 2100 kg/cm2. SEM photomicrograph showed compact and uniform microstructure with average grain size of 5 urn. Example-2 Beach sand sillimanite was mixed with 1.5 wt% TiO2 and 2 wt% ZrO2. The resultant mixture was attrition milled for 9 hrs. in isopropyl alcohol as dispersing medium. The slurry thus obtained was air dried followed by drying at 110±5°C for 24 hrs. The powder was then mixed with 6% PVA solution (5% concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed compacts were dried and fired at 1600°C with a soaking period of 2 hrs. The total firing schedule was 10 hrs. The bulk density and apparent porosity of the sintered aggregate were 2.96 g/cc and 0.6 % respectively. Hot MOR at 1250°C is 1760 kg/cm2. SEM photomicrograph showed compact and uniform microstructure with average grain size of 5 urn. Example-3 Beach sand sillimanite was mixed with 3 wt% TiO2 and 2 wt% ZrO2. The resultant mixture was attrition milled for 12 hrs. in water as dispersing medium. The slurry thus obtained was air dried followed by drying at 110±5°C for 24 hrs. The powder was then mixed with 6% PVA solution (5% concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed compacts were dried and fired at 1550°C with a soaking period of 2 hrs. The total firing schedule was 12 hrs. The bulk density and apparent porosity of the sintered aggregate were 3.03 g/cc and 0.6 % respectively. Hot MOR at 1250°C is 1785 kg/cm2. SEM photomicrograph showed compact and uniform microstructure with average grain size of 5 urn. Example-4 Sillimanite rock was crushed and ground to pass through 60 mesh BS sieve. Powdered sillimanite was mixed with 1.5 wt% TiO2 and 2 wt% ZrO2. The resultant mixture was attrition milled for 12 hrs. in alcohol as dispersing medium. The slurry thus obtained was air dried followed by drying at 90±5°C for 24 hrs. The powder was then mixed with 6% PVA solution (5% concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed compacts were dried and fired at 1500°C with a soaking period of 2 hrs. The total firing schedule was 12 hrs. The bulk density and apparent porosity of the sintered aggregate were 2.98 g/cc and 1 % respectively. Hot MOR at 1250°C is 1785 kg/cm2. SEM photomicrograph showed compact and uniform microstructure with average grain size of 5 urn. From the above examples it is observed that the properties of the aggregates of the present invention are much superior than that reported in the prior art. The hot MOR at 1250°C is between 1700 to 2100 kg/cm2. The low glassy phase due to incorporation of ZrO2 has enhanced this property. Furthermore, the grain size of the product is between 5 to 10 urn compared to 1 urn of the prior art. Higher grain size of the aggregate will improve the corrosion resistance of the refractories. The main advantages of the present invention are: 1. Production of dense high alumina aggregates in the AI2O3 range of 55 to 62% for application above 1500°C. 2. The additives intensify the sintering as well as controls the glassy phase in the matrix. Therefore, dense grains with improved hot properties of aggregates are produced. 3. A single uniaxial pressing of powders is sufficient to obtain the properties compared to two step uniaxial and isostatic pressing conducted in the prior art processes. 4. Grain size of aggregates are in the range of 5 to 10 urn compared to 1 urn. We claim: 1. A process for the manufacture of dense high alumina refractory aggregates from sillimanite sand based minerals, which comprises: milling 93 to 96 wt% Sillimanite sand; 6 to1 wt% Titanium dioxide and 1 to 3 wt% Zirconium dioxide in the presence of a solvent such as water, isopropyl alcohol, acetone, hexane, for a period in the range of 8 to 24 hours to obtain milled slurry, the resultant milled slurry being dried at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, mixing the milled and dried powder so obtained with an organic binder such as poly vinyl alcohol, dextrine, carboxymethyl cellulose, in the proportion of 92 to 98 wt% powder and 2 to 8wt% binder to obtain a pre-pressed powder, subjecting the pre-pressed powder to uniaxial pressing to obtain pellets, drying the said pellets at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, sintering the dried pellets at a temperature in the range of 1450 to 1650°C for a period in the range of 10 to 24 hours, allowing the sintered pellets to cool naturally. 2. A process as claimed in claim 1, wherein the sillimanite sand contains 55 to 62 wt% Al2O3. 3. A process as claimed in claim 1-2, wherein the ZrO2 is of 99% purity. 4. A process as claimed in claim 1-3, wherein the TiO2 is of 99% purity. 5. A process as claimed in claim 1-4, wherein the milling is done in conventional mill such as attrition mill, ball mill, vibro-mill. 6. A process as claimed in claim 1-5, wherein the organic binder is selected from a group consisting of poly vinyl alcohol, dextrine, carboxymethyl cellulose. 7. A process as claimed in claim 1-6, wherein the solvent used is in the weight ratio of solid : solvent :: 1 : 2. 8. A process as claimed in claim 1 - 7, wherein the uniaxial pressing is done in the range of 100 to 130MPa. 9. A process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals, substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process for the manufacture of dense high
alumina refractory aggregate from sillimanite sand based minerals.
The present invention particularly relates to a process for the production of
dense high alumina refractory aggregate from a novel composition consisting of
sillimanite sand based minerals, ZrO2, and TiO2 for direct application as
refractory or as a component /constituent of castable.
Sillimanite beach sand is a byproduct obtained during the extraction of rare
earth compounds from beach sand minerals. Mineralogically it is sillimanite
(AI2O3.SiO2) associated with small amounts of other mineral phase. Sillimanite
decomposes to mullite and silica on heating in the temperature range 1300 to
1700°C.
Mullite is the only stable compound with high covalency in the SiO2-AI2O3
system. Mullite has many advantageous properties of refractory materials (a)
high melting point (b) low thermal expansion, which yields good thermal shock
resistance (c) good corrosion and creep resistance (d) little deformation under
load.
Due to these unique properties the mullite-based materials is used as
refractories in many industrial fields. In steel industry mullite-based bricks are
the main constituent of the lining of the upper part of melting furnace, hot blast
stoves, hot iron runner and continuous casting furnaces. Mullite refractory
bricks are also important components for the lining of high temperature tool in
cement industry e.g. calcination and cooler zones of rotary kiln.
Sillimanite can be used directly or in combination with alumina depending upon
the requirement of the alumina content and temperature of application for
production of refractory aggregates. Decomposition temperature of sillimanite
sand depends on the particle size and impurity level of the raw material.
3 (AI2O3. SiO2) = 3 AI2O3. 2 SiO2 + SiO2.
Silica released partially reacts with the impurities form a glassy phase which
impairs the product quality. Product quality can be improved by reducing the
amount of glassy phase in the aggregate.
Reference may be made to Ceramics International Vol. 20, 299-302 (1994);
wherein sillimanite was sintered in presence of Ti02. or ZrO2 in the temperature
range of 1520° 1570°C. The TiO2 used is between 6 to 8 wt. % and ZrO2
between 9 to 12.5 wt. %. Liquid phase formation was responsible for improved
densification and flexural strength at room temperature varied between 45-94
MPa.
Reference may be drawn from Journal of European Ceramic Society, Vol 18,
2081-2087 (1998); wherein densification of sillimanite by wet milling and
addition of TiO2 where carried out in the temperature range of 1500° to 1550°C.
The pressing is done in two step, i.e., uniaxial as well as isostatic. The high
temperature flexural strength measured at 1200°C falls when the sintering
temperature increases above 1500°C to 375 MPa. Bloating surface of samples
occurred when the aggregate were fired beyong 1550°C due to excessive
glassy phase.
Reference may be made to Industrial Ceramics Vol. 19, 13-16 (1999); wherein
sillimanite were sintered by milling and adding ZrO2. Densification was
achieved at 1600°C where the open porosity is almost zero. The hot MOR
increases with 2 wt. % addition compared to no additives as the glassy phase
is converted to zircon. HMOR at 1200°C is 360 MPa.
Reference may be made to US patent 4,960,738 (1990) where mullite -alumina
composites sintered body with AI2O3 in the range of 75-85 wt% were developed
from powdery aluminosilicate material and sintered in the range of 1500-
1650°C. The sintered body has an apparent porosity of 1%, grain size of 1 Dm
and bending strength at room temperature SOOMPa.
The main drawbacks of the hitherto known process are:
1. High alumina aggregates produced from sillimanite sand needs higher
temperature of 1600°C when ZrO2 is used. On the other hand for TiO2
additive firing above 1500°C deteriorates the product characteristics due
to excessive generation of glassy phase and formation of bloated
surface. The higher level of the additives would form excessive liquid
and reduce the high temperature strength of the aggregates.
2. In all the above process two step pressing i.e uniaxial and isostatic had
been done.
3. The process which consists sintered body above 75% alumina the grain
size is only 1 urn which may deteriorate some high temperature
properties.
At present there is a huge demand of high alumina aggregates with
content in the range of 55-60% in refractory industries. Hence, there is a
definite need to provide a process for the manufacture of dense high alumina
refractory aggregate.
The main object of the present invention is to provide a process for the
manufacture of dense high alumina refractory aggregate from sillimanite sand
based minerals, which obviates the drawbacks of the hitherto known prior art
as mentioned above.
Another object of the present invention is to provide a process wherein the
starting material is a novel composition consisting of sillimanite sand based
mineral, ZrO2 and TiOa, which will provide combined additives to achieve a
dense product at a relatively low temperature.
Yet another object of the present invention is to develop sintered high alumina
refractory aggregate by adopting single step uniaxial pressing.
Still another object of the present invention is to produce high alumina
aggregate with homogeneous microstructure and improved thermomechanical
properties.
Still yet another object of the present invention is to provide dense high alumina
refractory aggregates in the AI2O3 range of 55 to 62 wt% for application above
1500°C.
A further object of the present invention is to provide dense high alumina
refractory aggregates having grain size in the range of 5 -10 urn compared to
1um as obtained in the prior art, there by improving the high temperature
strength.
A still further object of the present invention is to provide a process for the
manufacture of dense high alumina refractory aggregate from sillimanite sand
based minerals, by utilizing low cost sillimanite sand, a byproduct obtained
during extraction of rare earth compounds from beach sand minerals.
Another object of the present invention is to provide a process which is
economical compared to the hithero known prior art processes,
introduction of mixed additives, pressing in only one step and sintering. The
above process is considered to be cheaper than the earlier processes.
The present invention provides a process for the manufacture of dense high
alumina refractory aggregate from sillimanite sand based minerals. This has
been made possible by providing a process wherein the starting material is a
novel composition consisting of sillimanite sand minerals, ZrO2 and TiO2. The
process involves proper milling of the novel mixture and single firing at a
temperature in the range of 1500° to 1600°C. The novel composition helps
intensify the densification through liquid phase sintering and controls the glassy
phase and improves the high temperature properties. High alumina aggregates
in the range of 55 to 62% AI2O3 thus produced has above 98% densification,
average grain size of 5 to 10 urn, hot modulus of rupture at 1250°C is 1700 to
2100 kg/cm2.
Accordingly the present invention provides a process for the manufacture of
dense high alumina refractory aggregates from sillimanite sand based minerals,
which comprises: milling 93 to 96 wt% Sillimanite sand; 6 to1 wt% Titanium
dioxide and 1 to 3 wt% Zirconium dioxide in presence of a solvent such as
water, isopropyl alcohol, acetone, hexane for a period in the range of 8 to 24
hours to obtain milled slurry, the resultant milled slurry being dried at a
temperature in the range of 90 to 120°C for a period in the range of 24 to 48
hours, mixing the milled and dried powder so obtained with an organic binder in
the proportion of 92 to 98 wt% powder and 2 to 8wt% binder to obtain a prepressed
powder, subjecting the pre-pressed powder to uniaxial pressing to
obtain pellets, drying the said pellets at a temperature in the range of 90 to
120°C for a period in the range of 24 to 48 hours, sintering the dried pellets at a
temperature in the range of 1450 to 1650°C for a period in the range of 10 to 24
hours, allowing the sintered pellets to cool naturally.
In an embodiment of the present invention, the sillimanite sand contains 55 to
62 wt% AI2O3.
In another embodiment of the present invention, the ZrO2 is of 99% purity.
In still another embodiment of the present invention, the TiO2 is of 99% purity.
In yet another embodiment of the present invention, the milling is done in
conventional mill such as attrition mill, ball mill, vibro-mill.
In still yet another embodiment of the present invention, the organic binder is
poly vinyl alcohol, dextrine, carboxymethyl cellulose.
In a further embodiment of the present invention, the solvent used is in the
weight ratio of solid : solvent :: 1 : 2.
In a still further embodiment of the present invention, the uniaxial pressing is
done in the range of 100 to 130MPa.
The novelty of the present invention is the production of high density high
alumina refractory aggregate from sillimanite beach sand for application at
1500°C or above. Another novel feature of the process is the single pressing
rather than conventional and isostatic pressing as in the prior art. Aggregates
developed by this process has the advantage for application in kilns and
furnaces at high temperature, particularly where the temperature range is
between 1500 to 1600°C which is not possible by existing prir art processes
due to formation of excessive liquid or poor densification.
The non-obvious inventive steps to realize the novel features of the present
invention resides in:
(i) providing the novel composition consisting sillimanite sand based mineral,
Zr02 and TiO2;
(ii) utilizing the effect of two additives, TiO2 and ZrO2 in the sintering of high
alumina aggregate containing 55 to 62% alumina. TiO2 is the liquid forming
additive and facilitates the densification of mullite through liquid phase
sintering. While ZrO2 converts the excess glass phase (SiO2) formed during
dissociation reaction to ZrSiO4. This is beneficial to control the bloating of
aggregates and improve hot properties.
The reaction can be expressed as follows:
3(A12 O, .SiO2 ) 3A12 O3 .2SiO2 + SiO22
ZrO2 + SiO2 - ZrSiO4
Therefore, when both the above additives are used in appropriate proportion
densification is enhanced without deteriorating the high temperature properties.
Thus the aggregate is suitable for application in vessels where the temperature
is above 1500°C.
The details of the process steps of the present invention are:
1. Mixing of sillimanite sand 93 to 96 wt% with 6 to 1 wt% TiO2 and 1 to 3
wt % ZrO2,
2. Milling the mixture in conventional mill such as attrition mill, ball mill,
vibromill in a liquid medium such as water, isopropyl alcohol, acetone,
hexane in the weight ratio of solid:solvent:: 1:2 for a period in the range
of 8 to 24 hrs to obtained milled slurry.
3. Drying the resultant slurry at a temperature in the range of 90 to 120°C
for a period in the range of 24 to 48 hours to obtain a synergistic
composition.
4. mixing the synergistic composition with an organic binder to obtain a
pre-pressed powder,
5. uniaxial pressing of the pre-pressed powder to form pellets,
6. drying the pellets so obtain at a temperature in the range of 90 to 120°C
for a period in the range of 24 to 48 hours,
7. firing the pellets so obtain in the range of 1450 to 1650°C for a period in
the range of 10 to 24 hrs,
8. allowing the sintered pellets to cool naturally.
The sintered high alumina aggregates were characterized by determining
properties like 1) bulk density and apparent porosity 2) phase identification by
X-ray and IR 3) high temperature flexural strength 4) microstructure. Bulk
density apparent porosity were measured by liquid displacement method using
Archimedes principle in water medium. High temperature flexural strength was
determined by three point bending test at 1250°C. Phase identification was
carried out by X-ray and IR and microstructure of the polished and thermally
etched sample was observed by scanning electron microscope.
The prior art details describes that the additive which is beneficial for low
temperature densification (1500°C), resulted bloating of aggregates when used
at higher temperature. Alternatively for other additive proper densification of
aggregate achieved above 1600°C thereby restricting its use in the range of
1500 to 1600°C. More over the grain size of high alumina aggregate in some
of the prior art processes is too low (1um) which will affect the corrosion
resistance of the ultimate product.
The process of the present invention is described with the help of the 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.
Exam pie-1
Beach sand sillimanite was mixed with 4.5 wt% TiO2 and 2 wt% ZrO2. The
resultant mixture was attrition milled for 9 hrs. in water as dispersing medium.
The slurry thus obtained was dried at 110±5°C for 24 hrs and subsequently
crushed to break the agglomerate. The powder was then mixed with 6% PVA
solution (5% concentration) and pressed uniaxially at a pressure of 1200
kg/cm2. Pressed compacts were dried and fired at 1500°C with a soaking
period of 2 hrs. The total firing schedule was 10 hrs. The bulk density and
apparent porosity of the sintered aggregate were 2.97 g/cc and 0.6 %
respectively. Hot MOR at 1250°C is 2100 kg/cm2. SEM photomicrograph
showed compact and uniform microstructure with average grain size of 5 urn.
Example-2
Beach sand sillimanite was mixed with 1.5 wt% TiO2 and 2 wt% ZrO2. The
resultant mixture was attrition milled for 9 hrs. in isopropyl alcohol as dispersing
medium. The slurry thus obtained was air dried followed by drying at 110±5°C
for 24 hrs. The powder was then mixed with 6% PVA solution (5%
concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed
compacts were dried and fired at 1600°C with a soaking period of 2 hrs. The
total firing schedule was 10 hrs. The bulk density and apparent porosity of the
sintered aggregate were 2.96 g/cc and 0.6 % respectively. Hot MOR at 1250°C
is 1760 kg/cm2. SEM photomicrograph showed compact and uniform
microstructure with average grain size of 5 urn.
Example-3
Beach sand sillimanite was mixed with 3 wt% TiO2 and 2 wt% ZrO2. The
resultant mixture was attrition milled for 12 hrs. in water as dispersing medium.
The slurry thus obtained was air dried followed by drying at 110±5°C for 24 hrs.
The powder was then mixed with 6% PVA solution (5% concentration) and
pressed uniaxially at a pressure of 1200 kg/cm2. Pressed compacts were dried
and fired at 1550°C with a soaking period of 2 hrs. The total firing schedule
was 12 hrs. The bulk density and apparent porosity of the sintered aggregate
were 3.03 g/cc and 0.6 % respectively. Hot MOR at 1250°C is 1785 kg/cm2.
SEM photomicrograph showed compact and uniform microstructure with
average grain size of 5 urn.
Example-4
Sillimanite rock was crushed and ground to pass through 60 mesh BS sieve.
Powdered sillimanite was mixed with 1.5 wt% TiO2 and 2 wt% ZrO2. The
resultant mixture was attrition milled for 12 hrs. in alcohol as dispersing
medium. The slurry thus obtained was air dried followed by drying at 90±5°C
for 24 hrs. The powder was then mixed with 6% PVA solution (5%
concentration) and pressed uniaxially at a pressure of 1200 kg/cm2. Pressed
compacts were dried and fired at 1500°C with a soaking period of 2 hrs. The
total firing schedule was 12 hrs. The bulk density and apparent porosity of the
sintered aggregate were 2.98 g/cc and 1 % respectively. Hot MOR at 1250°C
is 1785 kg/cm2. SEM photomicrograph showed compact and uniform
microstructure with average grain size of 5 urn.
From the above examples it is observed that the properties of the aggregates
of the present invention are much superior than that reported in the prior art.
The hot MOR at 1250°C is between 1700 to 2100 kg/cm2. The low glassy
phase due to incorporation of ZrO2 has enhanced this property. Furthermore,
the grain size of the product is between 5 to 10 urn compared to 1 urn of the
prior art. Higher grain size of the aggregate will improve the corrosion
resistance of the refractories.
The main advantages of the present invention are:
1. Production of dense high alumina aggregates in the AI2O3 range of 55 to
62% for application above 1500°C.
2. The additives intensify the sintering as well as controls the glassy phase in
the matrix. Therefore, dense grains with improved hot properties of
aggregates are produced.
3. A single uniaxial pressing of powders is sufficient to obtain the properties
compared to two step uniaxial and isostatic pressing conducted in the prior
art processes.
4. Grain size of aggregates are in the range of 5 to 10 urn compared to 1 urn.

We claim:
1. A process for the manufacture of dense high alumina refractory aggregates from sillimanite sand based minerals, which comprises: milling 93 to 96 wt% Sillimanite sand; 6 to1 wt% Titanium dioxide and 1 to 3 wt% Zirconium dioxide in the presence of a solvent such as water, isopropyl alcohol, acetone, hexane, for a period in the range of 8 to 24 hours to obtain milled slurry, the resultant milled slurry being dried at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, mixing the milled and dried powder so obtained with an organic binder such as poly vinyl alcohol, dextrine, carboxymethyl cellulose, in the proportion of 92 to 98 wt% powder and 2 to 8wt% binder to obtain a pre-pressed powder, subjecting the pre-pressed powder to uniaxial pressing to obtain pellets, drying the said pellets at a temperature in the range of 90 to 120°C for a period in the range of 24 to 48 hours, sintering the dried pellets at a temperature in the range of 1450 to 1650°C for a period in the range of 10 to 24 hours, allowing the sintered pellets to cool naturally.
2. A process as claimed in claim 1, wherein the sillimanite sand contains 55 to 62 wt% Al2O3.
3. A process as claimed in claim 1-2, wherein the ZrO2 is of 99% purity.
4. A process as claimed in claim 1-3, wherein the TiO2 is of 99% purity.
5. A process as claimed in claim 1-4, wherein the milling is done in conventional mill such as attrition mill, ball mill, vibro-mill.
6. A process as claimed in claim 1-5, wherein the organic binder is selected from a group consisting of poly vinyl alcohol, dextrine, carboxymethyl cellulose.
7. A process as claimed in claim 1-6, wherein the solvent used is in the weight ratio of solid : solvent :: 1 : 2.

8. A process as claimed in claim 1 - 7, wherein the uniaxial pressing is done in the range of 100 to 130MPa.
9. A process for the manufacture of dense high alumina refractory aggregate from sillimanite sand based minerals, substantially as herein described with reference to the examples.

Documents:

221-DEL-2003-Abstract-(05-03-2009).pdf

221-del-2003-abstract.pdf

221-DEL-2003-Claims-(05-03-2009).pdf

221-del-2003-claims.pdf

221-DEL-2003-Correspondence-Others-(05-03-2009).pdf

221-del-2003-correspondence-others.pdf

221-del-2003-correspondence-po.pdf

221-del-2003-description (complete).pdf

221-DEL-2003-Form-1-(05-03-2009).pdf

221-del-2003-form-1.pdf

221-del-2003-form-18.pdf

221-del-2003-form-2.pdf

221-del-2003-form-3.pdf

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Patent Number

235106

Indian Patent Application Number

221/DEL/2003

PG Journal Number

31/2009

Publication Date

31-Jul-2009

Grant Date

25-Jun-2009

Date of Filing

05-Mar-2003

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	HIMANSU SEKHAR TRIPATHI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE,KOLKATA 700 032, INDIA
2	ARUP GHOSH	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE,KOLKATA 700 032, INDIA
3	BARUNDEB MUKHERJEE	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE,KOLKATA 700 032, INDIA
4	SAMIR KUMAR DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE,KOLKATA 700 032, INDIA

PCT International Classification Number

C04B 035/10

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA