Title of Invention	"A SYNERGISTIC COMPOSITION FOR THE MANUFACTURE OF IMPROVED BASIC COMPOSITE REFRACTORY AND A PROCESS FOR THE MANUFACTURE OF IMPROVED BASIC COMPOSITE REFRACTORY THEREFROM"
Abstract	A novel synergistic composition for the manufacture of improved basic composite refractory and a process for the manufacture of improved basic composite refractory therefrom is provided. The synergistic composition consists of sintered magnesia and milled synthetic reactive preformed magnesium aluminate spinel, which improves the thermal shock resistance as well as the hot strength characteristics of the refractory. This synergistic composition for the manufacture of basic composite refractory bricks provides a retained cold strength, after 5 cycles of thermal shock at 1000°C, of the order of 550 kg/cm2, as against 256 kg/cm2 of only magnesite composition and a hot strength at 1400°C of more than 400 kg/cm2, as against less than 150 kg/cm2 for magnesite bodies.

Title of Invention

"A SYNERGISTIC COMPOSITION FOR THE MANUFACTURE OF IMPROVED BASIC COMPOSITE REFRACTORY AND A PROCESS FOR THE MANUFACTURE OF IMPROVED BASIC COMPOSITE REFRACTORY THEREFROM"

Abstract

A novel synergistic composition for the manufacture of improved basic composite refractory and a process for the manufacture of improved basic composite refractory therefrom is provided. The synergistic composition consists of sintered magnesia and milled synthetic reactive preformed magnesium aluminate spinel, which improves the thermal shock resistance as well as the hot strength characteristics of the refractory. This synergistic composition for the manufacture of basic composite refractory bricks provides a retained cold strength, after 5 cycles of thermal shock at 1000°C, of the order of 550 kg/cm2, as against 256 kg/cm2 of only magnesite composition and a hot strength at 1400°C of more than 400 kg/cm2, as against less than 150 kg/cm2 for magnesite bodies.

Full Text	The present invention relates to a synergistic composition for the manufacture of improved basic composite refractory and a process for the manufacture of improved basic composite refractory therefrom. The present invention particularly relates to a synergistic composition and a process for the manufacture of basic composite refractory brick / block with improved thermal shock resistance and hot strength from the novel synergistic composition consisting of reactive preformed magnesium aluminate spinel in basic sintered magnesite refractory composition. Basic composite refractory in the shape of bricks / blocks are used as refractory lining in converters and ladles of iron and steel industries, rotary kilns of cement manufacturing units, checker work of regenerators of glass tank furnaces. Production of iron and steel is governed by the quality of refractory and so iron and steel manufacturers are in continuous search for improved quality refractory lining . Basic process of steel making has enhanced the use of basic refractories and increased the scope of further improvement in quality for basic refractories. This has emphasized extensive work on magnesite refractories, the only basic refractory available. Excellent resistance against basic slag as well as low vulnerability to attack by iron oxide and alkalis has made magnesite bodies an essential refractory lining material for electric arc furnaces, basic oxygen furnaces, glass tank checkers and flow control devices for continuous casting of steel. But these refractories are not suitable for applications requiring severe thermal fluctuations due to its poor resistance against thermal shock. High thermal expansion and brittle bonding phase (mainly silicate) in conventional magnesite refractories are main drawbacks. Hence the development of basic refractory with better bonding and thermal shock resistance is a time demand requirement. Using of fine reactive alumina to form magnesium aluminate spinel is a development on these refractories. Spinel has very high thermal shock resistance and corrosion resistance and improves the drawbacks of magnesite refractories. But the addition of fine alumina in magnesite body causes spinel formation, which is associated with 5% volume expansion as referred by E. Rayshkeistch, "Oxide Ceramics", page 257-74, Academic Press, New York, 1960, which can cause cracking of the matrix phase and deteriorate the final properties. Reference may be made to G. R. Eusner and D. H. Hubble, "Technology of spinel bonded periclase brick", Journal of the American Ceramic Society, Vol 43, No. 6 page 292-6 (1960), wherein the use of 8-10 wt% fine alumina in magnesite brick is described. Another reference can be made to S. C. Cooper and T. A. Hudson, "Magnesia-magnesium aluminate spinel as a refractory", Transactions and Journal of the British Ceramic Society, 81, p 121-8 (1982), wherein is described a process of making magnesia-magnesium aluminate spinel refractory co-clinker and further use of the co-clinker for brick development. Still another reference may be made to Aksel and others, "Investigation of thermal shock resistance in model magnesia spinel refractory material", IV Ceramic Congress Proceedings book - 1999, page 193-9, Elsevier Science, Tokyo; wherein it is described that extent of interlinking of the thermal expansion mismatch (between spinel and magnesia) microcracking is due to addition of spinel and this finally imparts resistance against crack initiation and propagation. Yet another reference may be made to Videtto of Kaiser aluminium corporation, US Patent No. 4126479, wherein the development of spinel bonded magnesite brick was claimed without any undue expansion using 4 - 15% alumina fines. The product consists of 60 to 80 percent refractory aggregate coarser than 44 micron size and 40 to 20 percent of material finer than 44 micron made of 15 to 25 percent fine magnesia and 4 to 15 percent fine alumina (size less than 5 micron). The said composition was heated to at least at 1400°C. A further reference may be made to Nazirizadeh and Naefae of Didier Werke AG, in US Patent No. 4729934, wherein it has been claimed that refractory shapes, consisting essentially of 82 to 90 weight percent MgO and 18 to 10 weight percent A\2Oit were fired in the temperature range of 1450°C to 1600°C. The refractory shapes were reported to show refractoriness under load of more than 1740°C and shrinkage of 3 to 5% at 1400°C after 24 hrs under a compressive load of 0.2 N/mm2. The said product was suggested for cement rotary kiln applications. The main drawbacks of the hitherto known prior art are: 1. Presence of brittle bond between magnesia grains and high thermal expansion of the conventional magnesite refractory resulted in very poor resistance against thermal fluctuations. 2. Use of fine alumina in matrix phase to form spinel in situ during firing causes cracking due to spinel formation and deteriorates mechanical characteristics. 3. Making of magnesia-spinel co-clinker requires high temperature firing. Hence use of such co-clinker for brick making is costlier. The main object of the present invention is to provide a synergistic composition for the manufacture of improved basic composite refractory, which obviates the drawbacks as mentioned above. Another object of the present invention is to provide a synergistic composition consisting synthetic reactive preformed magnesium aluminate spinel powder. Still another object of the present invention is to provide a process for the manufacture of improved basic composite refractory from the novel synergistic composition of the present invention, which obviates the drawbacks as mentioned above. Yet another object of the present invention is to significantly improve the thermal shock resistance and hot strength of the basic composite refractory without affecting the other properties. From hitherto known prior art details it is ascertained that conventional basic composite refractory bricks / blocks have poor thermal shock resistance due to the brittle bond present as silicates. Improvement of bond character by using purer grade raw material and using alumina fines to produce spinel bond can improve spalling characteristics as well as the high temperature properties, but formation of spinel phase by reaction between added alumina fines and fine magnesia of basic refractory causes expansion during reaction and subsequent cracking of the refractory produced, thus deteriorating the refractory qualities. In the present invention there is provided a novel synergistic composition consisting of synthetic reactive preformed magnesium aluminate spinel in basic magnesite refractory composition and a process for the manufacture of basic composite refractory with improved thermal shock resistance and hot strength. This is achieved by the inventive step of incorporating a reactive preformed spinel in basic magnesite refractory composition (purity of magnesia >98%) and selecting a composition of spinel and magnesia that leads to an improved basic composite refractory. The reactive preformed spinel does not react further during firing and rules out any chance of expansion and associated crack formation. Further, during firing the spinel makes diffusion bonding with magnesite (not brittle like glassy bonding as in the case of conventional magnesite refractories), which results in improved thermal shock resistance and hot strength in the basic brick so produced. Accordingly the present invention provides a synergistic composition for the manufacture of improved basic composite refractory, which comprises sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range of 5-40 wt%. In an embodiment of the present invention the particle size distribution of the composition is within the range of 5 No. BS (British standard) to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 15 wt% and particle size less than 150 No. BS: 30 wt%. In another embodiment of the present invention the particle size distribution of the 60 95 wt% sintered magnesia is within the range of 5 No. BS to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No BS: 5 wt%, within the range of 60 No. to 150 No. BS: 5 to 15 wt% and particle size less than 150 No. BS: 0 to 30 wt%. In another embodiment of the present invention the particle size distribution of the 60 - 95 wt% sintered magnesia is such as 5 # BS to 10 # BS: 30 wt%., 10 # BS to 30 # BS: 20 wt%, 30 # BS to 60 # BS: 5 wt%, 60 # to 150 # BS: 5 to 15 wt% and less than 150 # BS: 0 to 30 wt%. In yet another embodiment of the present invention the sintered magnesia used is such as sea water magnesia, dead burnt magnesia of purity above 98%. In still another embodiment of the present invention the synthetic reactive preformed magnesium aluminate spinel consists of MgO in the range of 20 to 40 wt% and A12O3 in the range of 60 to 80 wt%. The novel composition of the present invention, for the manufacture of improved basic composite refractory, is not a mere admixture but a synergistic mixture having properties which are distinct and different from the mere aggregation of the properties of the individual ingredients. Further, there is no chemical reaction in the said novel synergistic composition. Accordingly the present invention provides a process for the manufacture of improved basic composite refractory from the synergistic composition of the present invention, which comprises mixing sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range of 5 - 40 wt% to obtain a homogenous mixture of the synergistic composition, adding to the said mixture 4 to 8 wt% green binder and mixing thoroughly, pressing the resultant mixture under an uniaxial pressure in the range of 600 to 1500 kg/cm2 to obtain pressed shapes, drying the pressed shapes at a temperature in the range of 110 ± 10°C for a period of 16 to 24 hours and firing the dried pressed shapes at a temperature in range of 1450°C to 1650°C for a period in the range of 2 to 8 hours, allowing the fired shapes to cool naturally. In an embodiment of the present invention the synthetic reactive preformed magnesium aluminate spinel consisting of MgO in the range of 20 to 40 wt% and A12O3 in the range of 60 to 80 wt% is preformed by calcination at a temperature in the range of 1200°C to 1500°C for a soaking period in the range of 2 to 6 hours. In another embodiment of the present invention the synthetic reactive magnesium aluminate spinel is made using magnesia sources such as sea water magnesium hydroxide, commercial magnesium hydroxide. In still another embodiment of the present invention the synthetic reactive magnesium aluminate spinel is made using alumina sources such as commercial grade hydrated alumina, aluminium hydroxide. In yet another embodiment of the present invention milled synthetic preformed reactive magnesium aluminate spinel is obtained by milling in a conventional mill such as attrition mill, ball mill, vibro mill, for a period in the range of 2 to 6 hours, in the presence of liquid such as isopropyl alcohol, acetone, hexane. In a further embodiment of the present invention the green binder used for pressing is such as polyvinyl alcohol, dextrin, glycol. The steps of the process of the present invention comprises: 1. Mixing magnesium hydroxide and hydrated alumina in a pot mill for 30 to 60 minutes to obtain a composition with MgO in the range of 20 to 40 wt % and A12O3 in the range of 60 to 80 wt%. 2. Calcining the mixture obtained in step-1 , at a temperature in the range of 1200°C to 1500°C with a soaking period in the range of 2 to 6 hours to form synthetic reactive preformed magnesium aluminate spinel. 3. Milling the preformed spinel in a liquid for a time period in the range of 2 to 6 hours to obtain milled synthetic reactive preformed magnesium aluminate spinel 4. Preparing a homogenous mixture of the synergistic composition comprising: sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range 5. Adding 4 to 8 wt% green binder to the mixture obtained in step-4, and mixing thoroughly, pressing the resultant mixture under an uniaxial pressure in the range of 600 to 1500 kg/cm2 to obtain pressed shapes. 6. Drying the pressed shapes at a temperature in the range of 1 10 ± 10°C for a period of 16 to 24 hours 4. Firing the dried products at a temperature in the range of 1450°C to 1650°C with a soaking period in the range of 2 to 8 hours. 5. Allowing the fired products to cool naturally to obtain improved basic composite refractory. The specific inventive step in the present invention is the incorporation of preformed magnesium aluminate spinel to obtain a synergistic composition consisting of sintered magnesia and milled synthetic reactive preformed magnesium aluminate spinel. This synergistic composition is used for the manufacture of improved basic composite refractory. This step of preformation of spinel rather than producing spinel by reaction of individual ingredients like magnesia and alumina at high temperature during the refractory manufacturing process eliminates the formation of cracks and deterioration of other properties of the refractory. In general practice of magnesia - spinel refractory manufacturing a holding time at the spinellisation temperature is required to smoothen the spinel formation reaction and to reduce the chances of cracking. In the present invention such holding period is eliminated and this makes the firing schedule easier and economical. Thus there is provided a synergistic composition consisting of preformed spinel and sintered magnesia which is processed to manufacture improved magnesia spinel refractory in the shape of bricks / blocks. The invention is described with the help of the following examples for illustration of the novel synergistic composition and the process for the manufacture of improved basic composite refractory. However, the examples should not be construed to limit the scope of the present invention. The sintered magnesia spinel composite basic brick / blocks prepared as described in the following examples were characterized by determining: 1. Bulk density (BD), apparent porosity (AP) and volumetric shrinkage (VS). 2. Reversible thermal expansion (RTE).3. Hot strength (HMOR) at a temperatures of 1000°C, 1200°C and 1400°C. 4. Retainment of cold strength (R-CMOR) after different number of thermal cycles, each thermal cycle comprises of 10 mins of heat at 1000°C and 10 mins of air quenching. Bulk density and apparent porosity were measured by liquid displacement method in xylene medium under vacuum using Archimedes principle. Reversible thermal expansion was measured in a horizontal dilatometer up to 1450°C. Hot strength was measured as 3 point bending test. Abbreviations used in the examples: BD - bulk density, AP - apparent porosity, VS - volumetric shrinkage, RTE - reversible thermal expansion, HMOR - hot modulus of rupture and R-CMOR - retained cold modulus of rupture. Example 1 Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : Al2O3 at ratio 35:65 in a pot mill for 30 minutes. The mixture was then calcined at 1450°C for 3 hours and then milled in attrition mill for 4 hours. 5 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No.BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 15 % and less than 150 No. BS: 25 wt%. The batch was mixed, pressed at 1400 Kg/cm2, dried at 100±10°C for 24 hours, sintered at 1650°C for 2 hours. Sintered products showed a BD of 2.79 gm/cc, AP of 21.8%, VS of 3.3%, RTE of 1.97%, HMOR of 245 Kg/cm2 at 1400°C and R-CMOR after 5 cycles was 550 Kg/ciri2. Example 2 Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : A1203 weight ratio 28 : 72 in a pot mill for 30 minutes, then calcined at 1400°C for 2 hours. The formed spinel was attrition milled for 3 hours. 10 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 15 wt% and less than 150 No. BS: 20 wt%. The mixture was mixed and pressed at 1000 Kg/cm pressure, then dried at 110±10°C for 24 hours and sintered at 1600°C for 2 hours. Sintered products showed a BD of 2.88 gm/cc, AP of 19.3 % VS 3.8% RTE of 1.94%, HMOR at 1400°C of 296 Kg/cm2 and R-CMOR after 5 cycles 392 Kg/cm2. Example 3 Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : A1203 weight ratio 30:70 in a pot mill for 30 minutes, then calcined at 1450°C for 2 hours. The formed spinel was attrition milled for 3 XA hours. 30 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 5 wt% and less than 150 No. BS: 10 wt%. The mixture was mixed and then pressed at 1100 Kg/cm2, dried at 110±10°C for 24 hours and sintered at 1550°C for 4 hours. Sintered products exhibited a BD of 3.02 gm/cc, AP of 15.4%, VS 4.8%, RTE 1.72 % HMOR at 1400°C of 314 Kg/cm2 and R-CMOR after 5 cycles 409 Kg/cm2. Example 4 Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed in a pot mill for 1 hour for the composition of MgO : A1203 weight ratio 25 : 75. Mixed material was calcined at 1450°C for 2 hours and the formed spinel was attrition milled for 3 hours. 40 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 5 wt%. The mixture was then pressed at 1200 Kg/cm2, dried at 110±10°C, sintered at 1550°C for 4 hours. Sintered products resulted a BD of 3.23 gm/cc, AP of 9.5 %, VS of 7.7 %, RTE of 1.51%, HMOR at 1400°C of 472 Kg/cm2 and R-CMOR after 5 cycles 412 Kg/cm . Sintered products exhibited a BD of 3.02 gm/cc, AP of 15.4%, VS 4.8%, RTE 1.72 % HMOR at 1400°C of 314 Kg/cm2 and R-CMOR after 5 cycles 409 Kg/cm2. Example 4 Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed in a pot mill for 1 hour for the composition of MgO : A12O3 weight ratio 25 : 75. Mixed material was calcined at 1450°C for 2 hours and the formed spinel was attrition milled for 3 hours. 40 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 # BS to 10 # BS: 30 wt%, 10 # BS to 30 # BS: 20 wt%, 30 # BS to 60 # BS: 5 wt%, 60 # BS to 150 # BS: 5 wt%. The mixture was then pressed at 1200 Kg/cm2, dried at 110±10°C, sintered at 1550°C for 4 hours. Sintered products resulted a BD of 3.23 gm/cc, AP of 9.5 %, VS of 7.7 %, RTE of 1.51%, HMOR at 1400°C of 472 Kg/cm2 and R-CMOR after 5 cycles 412 Kg/cm2. The characteristics of the sintered magnesia spinel composite basic brick / blocks prepared as described in the above examples are: 1. Bulk density (BD): 2.79 to 3.23 gm/cc. 2. Apparent porosity (AP): 9.5 to 21.8%. 3. Volumetric shrinkage (VS): 3.3 to 7.7%. 4. Reversible thermal expansion (RTE): 1.51 to 1.97 %. 5. Hot strength (HMOR): 245 to 472 Kg/cm2. 6. Retainment of cold strength (R-CMOR): 392 to 550 Kg/cm2. From the above it is clearly seen that the synergistic composition of the present invention for the manufacture of improved • basic composite refractory bricks provides a retained cold strength, after 5 cycles of thermal shock at 1000°C, of the order of 550 kg/cm2. This is a distinct improvement in the retained cold strength of 256 kg/cm2 of only magnesite composition. Further, the hot strength at 1400°C of more than 400 kg/cm2 is obtained as against less than 150 kg/cm2 for magnesite bodies. The main advantages of the present invention are: (i) Improved thermal shock resistance of the order of 550 Kg/cm2, as against 256 Kg/cm2 in the case of conventional magnesite composition, (ii) Improved hot strength of more than 450 Kg/cm2, as against 150 Kg/cm2 in the case of conventional magnesite composition. (iii) Low reversible thermal expansion, (iv) Increased densification. (v) Firing schedule easier and economical. We claim: 1. A synergistic composition for the manufacture of improved basic composite refractory, which comprises sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel (MgO - A12O3) in the range of 5-40 wt%. 2. A process for the manufacture of improved basic composite refractory from the synergistic composition as claimed in claim 1, which comprises mixing sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range of 5 - 40 wt% to obtain a homogenous mixture of the synergistic composition, adding to the said mixture 4 to 8 wt% green binder such as herein described and mixing thoroughly, pressing the resultant mixture under an uniaxial pressure in the range of 600 to 1500 kg/cm to obtain pressed shapes, drying the pressed shapes at a temperature in the range of 110 ± 10°C for a period of 16 to 24 hours and firing the dried pressed shapes at a temperature in range of 1450°C to 1650°C for a period in the range of 2 to 8 hours, allowing the fired shapes to cool naturally to obtain improved basic composite refractory. 3. A synergistic composition as claimed in claim 1, wherein the particle size distribution of the composition is within the range of 5 No. BS (British standard) to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 15 wt% and particle size less than 150 No. BS: 30 wt%. 4. A synergistic composition as claimed in claim 1, wherein the particle size distribution of the 60 - 95 wt% sintered magnesia is within the range of 5 No. BS to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 5 to 15 wt% and particle size less than 150 No. BS:0to30wt%. 5. A synergistic composition as claimed in claim 1, wherein the sintered magnesia used is sea water magnesia, dead burnt magnesia of purity above 98%. 6. A synergistic composition as claimed in claim 1, wherein the synthetic reactive preformed magnesium aluminate spinel consists of MgO in the range of 20 to 40 wt% and A12O3 in the range of 60 to 80 wt%. 7. A process as claimed in claim 2, wherein the milled synthetic preformed reactive magnesium aluminate spinel is obtained by milling in a conventional mill such as attrition mill, ball mill, vibro mill, for a period in the range of 2 to 6 hours, in the presence of liquid such as isopropyl alcohol, acetone, hexane. 8. A process as claimed in claim 2, wherein the green binder used for pressing is polyvinyl alcohol, dextrin, glycol. 9. A synergistic composition for the manufacture of improved basic composite refractory, substantially as herein described with reference to the examples. 10. A process for the manufacture of improved basic composite refractory, substantially as herein described with reference to the examples.

Full Text

The present invention relates to a synergistic composition for the
manufacture of improved basic composite refractory and a process for the
manufacture of improved basic composite refractory therefrom.
The present invention particularly relates to a synergistic composition and a
process for the manufacture of basic composite refractory brick / block with
improved thermal shock resistance and hot strength from the novel
synergistic composition consisting of reactive preformed magnesium
aluminate spinel in basic sintered magnesite refractory composition.
Basic composite refractory in the shape of bricks / blocks are used as
refractory lining in converters and ladles of iron and steel industries, rotary
kilns of cement manufacturing units, checker work of regenerators of glass
tank furnaces.
Production of iron and steel is governed by the quality of refractory and so
iron and steel manufacturers are in continuous search for improved quality
refractory lining . Basic process of steel making has enhanced the use of
basic refractories and increased the scope of further improvement in quality
for basic refractories. This has emphasized extensive work on magnesite
refractories, the only basic refractory available.
Excellent resistance against basic slag as well as low vulnerability to attack
by iron oxide and alkalis has made magnesite bodies an essential refractory
lining material for electric arc furnaces, basic oxygen furnaces, glass tank
checkers and flow control devices for continuous casting of steel. But these
refractories are not suitable for applications requiring severe thermal
fluctuations due to its poor resistance against thermal shock. High thermal
expansion and brittle bonding phase (mainly silicate) in conventional
magnesite refractories are main drawbacks. Hence the development of basic
refractory with better bonding and thermal shock resistance is a time demand
requirement.
Using of fine reactive alumina to form magnesium aluminate spinel is a
development on these refractories. Spinel has very high thermal shock
resistance and corrosion resistance and improves the drawbacks of
magnesite refractories. But the addition of fine alumina in magnesite body
causes spinel formation, which is associated with 5% volume expansion as
referred by E. Rayshkeistch, "Oxide Ceramics", page 257-74, Academic
Press, New York, 1960, which can cause cracking of the matrix phase and
deteriorate the final properties.
Reference may be made to G. R. Eusner and D. H. Hubble, "Technology of
spinel bonded periclase brick", Journal of the American Ceramic Society,
Vol 43, No. 6 page 292-6 (1960), wherein the use of 8-10 wt% fine alumina
in magnesite brick is described.
Another reference can be made to S. C. Cooper and T. A. Hudson,
"Magnesia-magnesium aluminate spinel as a refractory", Transactions and
Journal of the British Ceramic Society, 81, p 121-8 (1982), wherein is
described a process of making magnesia-magnesium aluminate spinel
refractory co-clinker and further use of the co-clinker for brick development.
Still another reference may be made to Aksel and others, "Investigation of
thermal shock resistance in model magnesia spinel refractory material", IV
Ceramic Congress Proceedings book - 1999, page 193-9, Elsevier Science,
Tokyo; wherein it is described that extent of interlinking of the thermal
expansion mismatch (between spinel and magnesia) microcracking is due to
addition of spinel and this finally imparts resistance against crack initiation
and propagation.
Yet another reference may be made to Videtto of Kaiser aluminium
corporation, US Patent No. 4126479, wherein the development of spinel
bonded magnesite brick was claimed without any undue expansion using 4 -
15% alumina fines. The product consists of 60 to 80 percent refractory
aggregate coarser than 44 micron size and 40 to 20 percent of material finer
than 44 micron made of 15 to 25 percent fine magnesia and 4 to 15 percent
fine alumina (size less than 5 micron). The said composition was heated to at
least at 1400°C.
A further reference may be made to Nazirizadeh and Naefae of Didier
Werke AG, in US Patent No. 4729934, wherein it has been claimed that
refractory shapes, consisting essentially of 82 to 90 weight percent MgO and
18 to 10 weight percent A\2Oit were fired in the temperature range of 1450°C
to 1600°C. The refractory shapes were reported to show refractoriness under
load of more than 1740°C and shrinkage of 3 to 5% at 1400°C after 24 hrs
under a compressive load of 0.2 N/mm2. The said product was suggested for
cement rotary kiln applications.
The main drawbacks of the hitherto known prior art are:
1. Presence of brittle bond between magnesia grains and high thermal
expansion of the conventional magnesite refractory resulted in very
poor resistance against thermal fluctuations.
2. Use of fine alumina in matrix phase to form spinel in situ during firing
causes cracking due to spinel formation and deteriorates mechanical
characteristics.
3. Making of magnesia-spinel co-clinker requires high temperature
firing. Hence use of such co-clinker for brick making is costlier.
The main object of the present invention is to provide a synergistic
composition for the manufacture of improved basic composite refractory,
which obviates the drawbacks as mentioned above.
Another object of the present invention is to provide a synergistic
composition consisting synthetic reactive preformed magnesium aluminate
spinel powder.
Still another object of the present invention is to provide a process for the
manufacture of improved basic composite refractory from the novel
synergistic composition of the present invention, which obviates the
drawbacks as mentioned above.
Yet another object of the present invention is to significantly improve the
thermal shock resistance and hot strength of the basic composite refractory
without affecting the other properties.
From hitherto known prior art details it is ascertained that conventional basic
composite refractory bricks / blocks have poor thermal shock resistance due
to the brittle bond present as silicates. Improvement of bond character by
using purer grade raw material and using alumina fines to produce spinel
bond can improve spalling characteristics as well as the high temperature
properties, but formation of spinel phase by reaction between added alumina
fines and fine magnesia of basic refractory causes expansion during reaction
and subsequent cracking of the refractory produced, thus deteriorating the
refractory qualities.
In the present invention there is provided a novel synergistic composition
consisting of synthetic reactive preformed magnesium aluminate spinel in
basic magnesite refractory composition and a process for the manufacture of
basic composite refractory with improved thermal shock resistance and hot strength. This is achieved by the inventive step of incorporating a reactive preformed spinel in basic magnesite refractory composition (purity of magnesia >98%) and selecting a composition of spinel and magnesia that leads to an improved basic composite refractory.
The reactive preformed spinel does not react further during firing and rules out any chance of expansion and associated crack formation. Further, during firing the spinel makes diffusion bonding with magnesite (not brittle like glassy bonding as in the case of conventional magnesite refractories), which results in improved thermal shock resistance and hot strength in the basic brick so produced.
Accordingly the present invention provides a synergistic composition for the manufacture of improved basic composite refractory, which comprises sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range of 5-40 wt%.
In an embodiment of the present invention the particle size distribution of the composition is within the range of 5 No. BS (British standard) to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 15 wt% and particle size less than 150 No. BS: 30 wt%. In another embodiment of the present invention the particle size distribution of the 60 95 wt% sintered magnesia is within the range of 5 No. BS to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No BS: 5 wt%, within the range of 60 No. to 150 No. BS: 5 to 15 wt% and particle size less than 150 No. BS: 0 to 30 wt%.

In another embodiment of the present invention the particle size distribution
of the 60 - 95 wt% sintered magnesia is such as 5 # BS to 10 # BS: 30 wt%.,
10 # BS to 30 # BS: 20 wt%, 30 # BS to 60 # BS: 5 wt%, 60 # to 150 # BS:
5 to 15 wt% and less than 150 # BS: 0 to 30 wt%.
In yet another embodiment of the present invention the sintered magnesia
used is such as sea water magnesia, dead burnt magnesia of purity above
98%.
In still another embodiment of the present invention the synthetic reactive
preformed magnesium aluminate spinel consists of MgO in the range of
20 to 40 wt% and A12O3 in the range of 60 to 80 wt%.
The novel composition of the present invention, for the manufacture of
improved basic composite refractory, is not a mere admixture but a
synergistic mixture having properties which are distinct and different from
the mere aggregation of the properties of the individual ingredients. Further,
there is no chemical reaction in the said novel synergistic composition.
Accordingly the present invention provides a process for the manufacture of
improved basic composite refractory from the synergistic composition of the
present invention, which comprises mixing sintered magnesia in the range of
60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate
spinel in the range of 5 - 40 wt% to obtain a homogenous mixture of the
synergistic composition, adding to the said mixture 4 to 8 wt% green binder
and mixing thoroughly, pressing the resultant mixture under an uniaxial
pressure in the range of 600 to 1500 kg/cm2 to obtain pressed shapes, drying
the pressed shapes at a temperature in the range of 110 ± 10°C for a period
of 16 to 24 hours and firing the dried pressed shapes at a temperature in
range of 1450°C to 1650°C for a period in the range of 2 to 8 hours, allowing
the fired shapes to cool naturally.
In an embodiment of the present invention the synthetic reactive preformed
magnesium aluminate spinel consisting of MgO in the range of 20 to 40 wt%
and A12O3 in the range of 60 to 80 wt% is preformed by calcination at a
temperature in the range of 1200°C to 1500°C for a soaking period in the
range of 2 to 6 hours.
In another embodiment of the present invention the synthetic reactive
magnesium aluminate spinel is made using magnesia sources such as sea
water magnesium hydroxide, commercial magnesium hydroxide.
In still another embodiment of the present invention the synthetic reactive
magnesium aluminate spinel is made using alumina sources such as
commercial grade hydrated alumina, aluminium hydroxide.
In yet another embodiment of the present invention milled synthetic
preformed reactive magnesium aluminate spinel is obtained by milling in a
conventional mill such as attrition mill, ball mill, vibro mill, for a period in
the range of 2 to 6 hours, in the presence of liquid such as isopropyl alcohol,
acetone, hexane.
In a further embodiment of the present invention the green binder used for
pressing is such as polyvinyl alcohol, dextrin, glycol.
The steps of the process of the present invention comprises:
1. Mixing magnesium hydroxide and hydrated alumina in a pot mill for
30 to 60 minutes to obtain a composition with MgO in the range of 20
to 40 wt % and A12O3 in the range of 60 to 80 wt%.
2. Calcining the mixture obtained in step-1 , at a temperature in the range
of 1200°C to 1500°C with a soaking period in the range of 2 to 6
hours to form synthetic reactive preformed magnesium aluminate
spinel.
3. Milling the preformed spinel in a liquid for a time period in the range
of 2 to 6 hours to obtain milled synthetic reactive preformed
magnesium aluminate spinel
4. Preparing a homogenous mixture of the synergistic composition
comprising: sintered magnesia in the range of 60 - 95 wt% and milled
synthetic reactive preformed magnesium aluminate spinel in the range
5. Adding 4 to 8 wt% green binder to the mixture obtained in step-4, and
mixing thoroughly, pressing the resultant mixture under an uniaxial
pressure in the range of 600 to 1500 kg/cm2 to obtain pressed shapes.
6. Drying the pressed shapes at a temperature in the range of 1 10 ± 10°C
for a period of 16 to 24 hours
4. Firing the dried products at a temperature in the range of 1450°C to
1650°C with a soaking period in the range of 2 to 8 hours.
5. Allowing the fired products to cool naturally to obtain improved basic
composite refractory.
The specific inventive step in the present invention is the incorporation of
preformed magnesium aluminate spinel to obtain a synergistic composition
consisting of sintered magnesia and milled synthetic reactive preformed
magnesium aluminate spinel. This synergistic composition is used for the
manufacture of improved basic composite refractory. This step of
preformation of spinel rather than producing spinel by reaction of individual
ingredients like magnesia and alumina at high temperature during the
refractory manufacturing process eliminates the formation of cracks and
deterioration of other properties of the refractory. In general practice of
magnesia - spinel refractory manufacturing a holding time at the
spinellisation temperature is required to smoothen the spinel formation
reaction and to reduce the chances of cracking. In the present invention such
holding period is eliminated and this makes the firing schedule easier and
economical. Thus there is provided a synergistic composition consisting of
preformed spinel and sintered magnesia which is processed to manufacture
improved magnesia spinel refractory in the shape of bricks / blocks.
The invention is described with the help of the following examples for
illustration of the novel synergistic composition and the process for the
manufacture of improved basic composite refractory. However, the
examples should not be construed to limit the scope of the present invention.
The sintered magnesia spinel composite basic brick / blocks prepared as
described in the following examples were characterized by determining:
1. Bulk density (BD), apparent porosity (AP) and volumetric shrinkage
(VS).
2. Reversible thermal expansion (RTE).3. Hot strength (HMOR) at a temperatures of 1000°C, 1200°C and 1400°C.
4. Retainment of cold strength (R-CMOR) after different number of thermal cycles, each thermal cycle comprises of 10 mins of heat at 1000°C and 10 mins of air quenching.
Bulk density and apparent porosity were measured by liquid displacement method in xylene medium under vacuum using Archimedes principle. Reversible thermal expansion was measured in a horizontal dilatometer up to 1450°C. Hot strength was measured as 3 point bending test.
Abbreviations used in the examples: BD - bulk density, AP - apparent porosity, VS - volumetric shrinkage, RTE - reversible thermal expansion, HMOR - hot modulus of rupture and R-CMOR - retained cold modulus of rupture.
Example 1
Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : Al2O3 at ratio 35:65 in a pot mill for 30 minutes. The mixture was then calcined at 1450°C for 3 hours and then milled in attrition mill for 4 hours. 5 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No.BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 15 % and less than 150 No. BS: 25 wt%. The batch was mixed, pressed at 1400 Kg/cm2, dried at 100±10°C for 24 hours, sintered at 1650°C for 2 hours.

Sintered products showed a BD of 2.79 gm/cc, AP of 21.8%, VS of 3.3%, RTE of 1.97%, HMOR of 245 Kg/cm2 at 1400°C and R-CMOR after 5 cycles was 550 Kg/ciri2.
Example 2
Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : A1203 weight ratio 28 : 72 in a pot mill for 30 minutes, then calcined at 1400°C for 2 hours. The formed spinel was attrition milled for 3 hours. 10 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 15 wt% and less than 150 No. BS: 20 wt%. The mixture was mixed and pressed at 1000 Kg/cm pressure, then dried at 110±10°C for 24 hours and sintered at 1600°C for 2 hours. Sintered products showed a BD of 2.88 gm/cc, AP of 19.3 % VS 3.8% RTE of 1.94%, HMOR at 1400°C of 296 Kg/cm2 and R-CMOR after 5 cycles 392 Kg/cm2.
Example 3
Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed for MgO : A1203 weight ratio 30:70 in a pot mill for 30 minutes, then calcined at 1450°C for 2 hours. The formed spinel was attrition milled for 3 XA hours. 30 wt% of this milled preformed spinel was added to sintered

seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 5 wt% and less than 150 No. BS: 10 wt%. The mixture was mixed and then pressed at 1100 Kg/cm2, dried at 110±10°C for 24 hours and sintered at 1550°C for 4 hours.
Sintered products exhibited a BD of 3.02 gm/cc, AP of 15.4%, VS 4.8%, RTE 1.72 % HMOR at 1400°C of 314 Kg/cm2 and R-CMOR after 5 cycles 409 Kg/cm2.
Example 4
Sea water magnesium hydroxide and commercial grade hydrated alumina were mixed in a pot mill for 1 hour for the composition of MgO : A1203 weight ratio 25 : 75. Mixed material was calcined at 1450°C for 2 hours and the formed spinel was attrition milled for 3 hours. 40 wt% of this milled preformed spinel was added to sintered seawater magnesia having particle size distribution: 5 No. BS to 10 No. BS: 30 wt%, 10 No. BS to 30 No. BS: 20 wt%, 30 No. BS to 60 No. BS: 5 wt%, 60 No. BS to 150 No. BS: 5 wt%. The mixture was then pressed at 1200 Kg/cm2, dried at 110±10°C, sintered at 1550°C for 4 hours.
Sintered products resulted a BD of 3.23 gm/cc, AP of 9.5 %, VS of 7.7 %, RTE of 1.51%, HMOR at 1400°C of 472 Kg/cm2 and R-CMOR after 5 cycles 412 Kg/cm .

Sintered products exhibited a BD of 3.02 gm/cc, AP of 15.4%, VS 4.8%,
RTE 1.72 % HMOR at 1400°C of 314 Kg/cm2 and R-CMOR after 5 cycles
409 Kg/cm2.
Example 4
Sea water magnesium hydroxide and commercial grade hydrated alumina
were mixed in a pot mill for 1 hour for the composition of MgO : A12O3
weight ratio 25 : 75. Mixed material was calcined at 1450°C for 2 hours and
the formed spinel was attrition milled for 3 hours. 40 wt% of this milled
preformed spinel was added to sintered seawater magnesia having particle
size distribution: 5 # BS to 10 # BS: 30 wt%, 10 # BS to 30 # BS: 20 wt%,
30 # BS to 60 # BS: 5 wt%, 60 # BS to 150 # BS: 5 wt%. The mixture was
then pressed at 1200 Kg/cm2, dried at 110±10°C, sintered at 1550°C for 4
hours.
Sintered products resulted a BD of 3.23 gm/cc, AP of 9.5 %, VS of 7.7 %,
RTE of 1.51%, HMOR at 1400°C of 472 Kg/cm2 and R-CMOR after 5
cycles 412 Kg/cm2.
The characteristics of the sintered magnesia spinel composite basic brick /
blocks prepared as described in the above examples are:
1. Bulk density (BD): 2.79 to 3.23 gm/cc.
2. Apparent porosity (AP): 9.5 to 21.8%.
3. Volumetric shrinkage (VS): 3.3 to 7.7%.
4. Reversible thermal expansion (RTE): 1.51 to 1.97 %.
5. Hot strength (HMOR): 245 to 472 Kg/cm2.
6. Retainment of cold strength (R-CMOR): 392 to 550 Kg/cm2.
From the above it is clearly seen that the synergistic composition of the
present invention for the manufacture of improved • basic composite
refractory bricks provides a retained cold strength, after 5 cycles of thermal
shock at 1000°C, of the order of 550 kg/cm2. This is a distinct improvement
in the retained cold strength of 256 kg/cm2 of only magnesite composition.
Further, the hot strength at 1400°C of more than 400 kg/cm2 is obtained as
against less than 150 kg/cm2 for magnesite bodies.
The main advantages of the present invention are:
(i) Improved thermal shock resistance of the order of 550
Kg/cm2, as against 256 Kg/cm2 in the case of conventional
magnesite composition,
(ii) Improved hot strength of more than 450 Kg/cm2, as against
150 Kg/cm2 in the case of conventional magnesite
composition.
(iii) Low reversible thermal expansion,
(iv) Increased densification.
(v) Firing schedule easier and economical.

We claim:
1. A synergistic composition for the manufacture of improved basic
composite refractory, which comprises sintered magnesia in the range of
60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate
spinel (MgO - A12O3) in the range of 5-40 wt%.
2. A process for the manufacture of improved basic composite refractory from the synergistic composition as claimed in claim 1, which comprises mixing sintered magnesia in the range of 60 - 95 wt% and milled synthetic reactive preformed magnesium aluminate spinel in the range of 5 - 40 wt% to obtain a homogenous mixture of the synergistic composition, adding to the said mixture 4 to 8 wt% green binder such as herein described and mixing thoroughly, pressing the resultant mixture under an uniaxial pressure in the range of 600 to 1500 kg/cm to obtain pressed shapes, drying the pressed shapes at a temperature in the range of 110 ± 10°C for a period of 16 to 24 hours and firing the dried pressed shapes at a temperature in range of 1450°C to 1650°C for a period in the range of 2 to 8 hours, allowing the fired shapes to cool naturally to obtain improved basic composite refractory.
3. A synergistic composition as claimed in claim 1, wherein the particle size distribution of the composition is within the range of 5 No. BS (British standard) to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 15 wt% and particle size less than 150 No. BS: 30 wt%.

4. A synergistic composition as claimed in claim 1, wherein the particle size distribution of the 60 - 95 wt% sintered magnesia is within the range of 5 No. BS to 10 No. BS: 30 wt%., within the range of 10 No. BS to 30 No. BS: 20 wt%, within the range of 30 No. BS to 60 No. BS: 5 wt%, within the range of 60 No. to 150 No. BS: 5 to 15 wt% and particle size less than 150 No. BS:0to30wt%.
5. A synergistic composition as claimed in claim 1, wherein the sintered magnesia used is sea water magnesia, dead burnt magnesia of purity above 98%.
6. A synergistic composition as claimed in claim 1, wherein the synthetic reactive preformed magnesium aluminate spinel consists of MgO in the range of 20 to 40 wt% and A12O3 in the range of 60 to 80 wt%.
7. A process as claimed in claim 2, wherein the milled synthetic preformed reactive magnesium aluminate spinel is obtained by milling in a conventional mill such as attrition mill, ball mill, vibro mill, for a period in the range of 2 to 6 hours, in the presence of liquid such as isopropyl alcohol, acetone, hexane.

8. A process as claimed in claim 2, wherein the green binder used for pressing is polyvinyl alcohol, dextrin, glycol.
9. A synergistic composition for the manufacture of improved basic composite refractory, substantially as herein described with reference to the examples.

10. A process for the manufacture of improved basic composite refractory, substantially as herein described with reference to the examples.

Documents:

965-DEL-2002-Abstract-(06-01-2009).pdf

965-del-2002-abstract.pdf

965-DEL-2002-Claims-(06-01-2009).pdf

965-DEL-2002-Claims-(27-01-2009).pdf

965-del-2002-claims.pdf

965-del-2002-complete specification (granted).pdf

965-DEL-2002-Correspondence-Others-(06-01-2009).pdf

965-del-2002-correspondence-others.pdf

965-del-2002-correspondence-po.pdf

965-DEL-2002-Description (Complete)-(06-01-2009).pdf

965-del-2002-description (complete)-27-01-2009.pdf

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

965-DEL-2002-Form-1-(06-01-2009).pdf

965-del-2002-form-1.pdf

965-del-2002-form-18.pdf

965-DEL-2002-Form-2-(06-01-2009).pdf

965-del-2002-form-2.pdf

965-DEL-2002-Form-3-(06-01-2009).pdf

965-del-2002-form-3.pdf

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Patent Number

228363

Indian Patent Application Number

965/DEL/2002

PG Journal Number

08/2009

Publication Date

20-Feb-2009

Grant Date

03-Feb-2009

Date of Filing

24-Sep-2002

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	RITWIK SARKAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA
2	ARUP GHOSH	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA
3	BARUNDEB MUKHERJEE	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA
4	SAMIR KUMAR DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA

PCT International Classification Number

C04B 2/06

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA