Title of Invention

"A MAGNESIA BASED BASIC REFRACTORY CASTABLE HAVING IMPROVED CHARACTERSTICS AND THE PROCESS OF PREPARATION THEREOF"

Abstract

The present invention provides magnesia based basic refractory castable having improved characteristics and the process of preparation thereof. The magnesia based basic refractory castable using sintered magnesites of various size fractions, reactive alumna fines and high temperature barium aluminate cement bond. Incorporation of reactive alumina fines and barium aluminate bond resulted in a castable material, which does not deteriorate in mechanical strength significantly with heating, compared to presently available magnesia based basic castable material. The basic castable material produced by the present invention possesses high refractoriness with improved strength retaining capacity and thus can be used for application as monolithic lining in the basic processes of steel making technology where excellent resistance against basic slag and high temperature withstanding properties are the prime requirement.

Full Text

Field of the invention The present invention relates to a magnesia based basic refractory castable having improved characteristics and the process of preparation thereof. The present invention particularly relates to a magnesia basic castable mix, for the production of magnesia based basic refractory castable having improved characteristics which can be used in the iron and steel industries in the severe slag corrosion areas requiring enhanced strength, durability and stability. Background of the invnetion The present invention is useful for producing magnesia based basic refractory castable for application as monolithic lining in the basic processes of steel making technology where excellent resistance against basic slag as well as low vulnerability to attack by iron oxide and alkalis are the prime requirement, particularly for the electric arc furnace, basic oxygen furnace, steel ladle lining and flow control devices for continuous casting. Such types of basic castable also find applications in the glass tank furnace regenerators. Magnesia based refractory castable is produced by mixing suitably graded sintered / fused magnesia refractory aggregates, fines, bond materials and some property modifying additives. Magnesia is the mostly accepted basic refractory raw material with high melting point, very good chemical stability, excellent corrosion resistance against high basicity slag and iron oxide & alkalis. In spite of these many advantageous properties of magnesia, magnesia based castables are not commercially highly successful even today due to the susceptibility of magnesia towards hydration leading to volume expansion and cracking. This hydration of magnesia finally results a castable body with very poor mechanical characteristics. In the present day method suitably graded sintered or fused magnesia aggregates, fines, various types of bonding materials such as calcium alumina cement, magnesium oxysulphate cement, magnesium silicate hydrate, mono aluminium phosphate, and other admixtures are thoroughly mixed with required amount of water. To improve the hydration resistance of magnesia, different workers in this field have used various additives which improves the strength retainment properties of the basic castable up to certain limit. Reference may be drawn to the work of A. Watanabe, H. Takahashi, S. Takanaga and M. Uchida, paper presented at UNITECR 95, in Kyoto, Japan, Nov 19-22, 1995, wherein the hydration resistance of magnesia was improved by the addition of fine alumina. It is also cited in the literature that incorporation of medium sized fused silica grains improved the slag penetration resistance and volume stability of the basic castable. In another reference, Transactions of the Indian Ceramic Society 56 [6] Nov -Dec 1997, by R. Basu, P. K. Das Poddar and S. K. Das, it is reported that incorporation of MgO-AI203 gel in the magnesium oxysulphate based basic castable has a beneficial effect on the improvement and retainment of room temperature and high temperature strengths. However it is also reported that the dehydration and decomposition of the bond material are accompanied with a decrease in strength of the castable at the intermediate temperatures. Reference may also be made to S. Narayanan, D. G. Banawalikar and S. D. Majumdar, Proceedings of the 3rd India International Refractory Congress, vol. I, 1998, wherein the dissolution precipitation reaction in presence of moisture was identified as the main cause of magnesia hydration. The workers formed a phosphate based chelate compound by capturing the dissolved magnesium ions from the surface of the magnesia grains. It was reported that this phosphatic material formed lime alkali phosphate compound, which was stable up to 1700°C and imparts strength in the body. Hydraulically bonded magnesia based refractory castable may also be referred from the work of Guy Denier and Romain Henrion, Institute de Recherches de la Siderugie Francaise, Arbed, S. Africa, US patent no. 4696456 and 4779846. This hydraulically bonded castable although provides sufficient fluidity, the hydraulic bond breaks at the intermediate temperature and finally leads to 12 - 15% deterioration in strength values. In another reference, Proceeding of the 5th India International Refractory Congress, vol. I, 2002, B. Myhre, C. Odegard and H. Feldborg, used MgO-Si02-H20 bond system for the production of magnesia based basic castable and found poor refractoriness under load properties when combinations of different magnesites were used in presence of micro silica. In all the above noted prior art of present day methods the main drawbacks are: a) slaking of magnesia due to dissolution which brings about precipitation reaction of magnesia in presence of water leading to volume expansion and cracking, b) poor strength retaining capacity due to disrupture of bonds at the intermediate temperatures, c) structural spalling caused by absorbing slag into the structure and expansion and contraction on thermal cycling of heating and cooling, d) calcium aluminate cement binder used as hydraulic bond forms low melting compounds and eutectics at higher temperature thus restricts the high temperature applications, e) dehydration and decomposition of magnesium oxysulphate binder system accompanied by a decrease in mechanical strength of the castable up to a certain temperature which is critical during application, f) tailored made properties are not achievable. From the above noted drawbacks of the hitherto known prior art, it is clear that there is a definite need for magnesia based basic refractory castable having improved characteristics. The main object of the present invention is to provide a magnesia based basic refractory castable having improved characteristics and the process of preparation thereof, which obviates the drawbacks of the hitherto known prior art as described above. Another object of the present invention is to provide magnesia based basic refractory castable which does not develop crack and deteriorate its strength during heating. Yet another object of the present invention is to provide magnesia based basic refractory castable which does not form low eutectic liquid phases at higher temperatures. In the present invention magnesia based basic refractory castable developed using sintered magnesite aggregates of different size fractions, high temperature bonding material such as barium aluminate, reactive alumina fines as major raw materials. The binary phase diagram of BaO-AI203 system provides the existence of various phases (BA, B3A and BA6; B = BaO and A = AI2O3) and all these phases have higher melting points (BA - 1815°C, B3A - 1620°C and BA6 - 1915°C) than the calcium aluminate phases (CA - 1600°C, CA2 - 1720°C and C12A7 - 1455°C; C = CaO and AI2O3). The higher melting point of barium aluminate cement phases resulted in a castable material in the present invention with higher refractoriness. It is also reported in the literature by M. Drozdz and W. Wolek, Bull. Soc. Fr. Ceram., No 107, 1975 that barium aluminate cement posses low coefficient of thermal expansion as compared to calcium aluminate cement and this particular advantageous property of barium aluminate cement offered higher resistance against thermal shock of the presently developed castable material. Use of barium aluminate cement in magnesia based refractory castable resulted only 3 - 4% deterioration in strength with heating in the range of 110°C to 1400°C, as compared to 12 - 15% deterioration in case of presently available calcium aluminate cement bonded basic castable and as high as 60 - 68% deterioration for the magnesium oxysulphate bonded magnesia based basic castable even with gel additives. Significantly less deterioration in strength during heating in case of barium aluminate cement bonded basic refractory castable is obtained in the present invention due to its quick rate of bonding and strong binding power. Owing to the great water sensivity of barium aluminate cement and slaking tendency of magnesia in water, reactive alumina fine is used in the presently developed castable material which instantly reacts with water and forms a strong bond with other ingredients, thereby reducing the slaking tendency of magnesia and negative effect on the consolidation of the barium aluminate cement bond. The magnesia basic castable mix, a synergistic composition, of the present invention is particularly useful for the production of magnesia based basic refractory castable which can be used in the iron and steel industries in the severe slag corrosion areas where enhanced strength, durability and stability characteristics are required. Summary of the invention Accordingly the present invention provides a magnesia based basic refractory castable having improved characteristics, which comprises 85 to 87 wt% sintered magnesite aggregate mix of size fractions in the range of below 6 to above 100 BS sieve, 3 to 7 wt% reactive alumina fines, and 8 to 10 wt% of barium aluminate cement binder characterized in that the use of barium aluminate allows alumina fines to form strong bond with other ingredients. In an embodiment of the present invention the sintered magnesite aggregate is of crystal size in the range of 70 to 90 micron. In another embodiment of the present invention the sintered magnesite aggregate has chemical constituents such as: 99 to 99.5 wt% MgO, 0.5 to 0.6 wt% CaO, 0.01 to 0.05 wt% Si02, 0.02 to 0.04 wt% Fe203 and 0.02 to 0.05 wt% AI203. In yet another embodiment of the present invention the sintered magnesite aggregate mix constitutes of size fractions in the range of 34 to 39 wt% below 6 above 18, 26 to 30 wt% below 18 above 60, 22 to 27 wt% below 60 above 100 and 8 to 14 wt% below 100 BS sieve. In still another embodiment of the present invention, the reactive alumina fines are of average particle size (d50) in the range of 1.8 to 2.5 microns. In yet another embodiment of the present invention, the reactive alumina fines has chemical constituents such as: minimum 99.7 wt% AI2O3 with alpha content over 97%, maximum 0.08 wt% Na20, and maximum 0.06 wt% Si02. In a further embodiment of the present invention the barium aluminate cement binder has specific surface area of the order of 2300 ± 200 cm2/gm. In a further embodiment of the present invention the barium aluminate cement binder has chemical constituents such as: 56 to 57 wt% BaO, 38 to 39 wt% Al203, 1.0 to 1.5 wt% Si02, maximum 0.50 wt% Fe203, 1.5 to 2.0 wt% CaO, maximum 0.25 wt% MgO, maximum 0.2 wt% S03. The novelty of the present invention resides in magnesia based basic refractory castable mix as herein above described, and the process for the preparation thereof. The magnesia based basic castable having improved characteristics such as: (i) only 3 to 4% deterioration in strength with heating in the range of 110°C to 1400°C. This is much lower than the currently available castable mixtures having deterioration in strength of 12 to 15% in the case of calcium aluminate cement bonded castable mixture and 60 to 68% in the case of magnesium oxysulphate cement bonded castable mixture; (ii) no significant change in strength, cracking and disintegration is observed during its exposure in moist atmosphere, which is commonly noticed in the currently available similar castables; (iii) the refractoriness of the presently developed novel magnesia based basic castable material is higher (orton cone 36+, > 1804°C) than the currently available basic castables (orton cone 34+, > 1763°C). The non-obvious inventive step of the present invention lies in providing a mixture of sintered magnesite aggregate of a mix of size fractions, high temperature barium aluminate cement binder along with reactive alumina fines to produce the magnesia based basic castable material which not only increases the strength retainment capacity significantly with heating but also increases the refractoriness property. The details of the process steps for the production of magnesia based basic castable having improved characteristics from the synergistic composition of the present invention are: i) intimate mixing of 85 to 87 wt% of sintered magnesite aggregate of different size fractions as detailed herein above, ii) addition of 3 to 7 wt% reactive alumina fines and 8 to 10 wt% of barium aluminate cement binder and then mixing the batch uniformly, iii) mixing thoroughly with 7 to 10 wt% water of the total batch to obtain a castable dough, pouring in a mould and allowing to set, followed by curing and drying. The following examples are given by the way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1 600 gms of below 6 above 18, 500 gms of below 18 above 60, 400 gms of below 60 above 100 and 200 gms of below 100 BS sieve of sintered magnesite, 100 gms of reactive alumina fines, 200 gms of barium aluminate cement are thoroughly mixed with 160 gms of water for a period of 10 mins in a Hobart type mixture. To evaluate the properties of the castable mix the mass was poured in a cube mould of 50 mm x 50 mm x 50 mm size and allowed for setting. Then the cube shapes were released from the mould and allowed for 72 hrs air curing followed by 24 hrs drying at 110°C. The dried samples were finally heated at 900°C and 1400°C for a period of 2 hrs soaking. The resultant product properties were: (Table Removed) Example 2 600 gms of below 6 above 18, 500 gms of below 18 above 60, 400 gms of below 60 above 100 and 200 gms of below 100 BS sieve of sintered magnesite, 140 gms of reactive alumina fines, 160 gms of barium aluminate cement are thoroughly mixed with 140 gms of water for a period of 10 mins in a Hobart type mixture. To evaluate the properties of the castable mix the mass was poured in a cube mould of 50 mm x 50 mm x 50 mm size and allowed for setting. Then the cube shapes were released from the mould and allowed for 72 hrs air curing followed by 24 hrs drying at 110°C. The dried samples were finally heated at 900°C and 1400°C for a period of 2 hrs soaking. The resultant product properties were: (Table Removed) Example 3 600 gms of below 6 above 18, 500 gms of below 18 above 60, 400 gms of below 60 above 100 and 240 gms of below 100 BS sieve of sintered magnesite, 60 gms of reactive alumina fines, 200 gms of barium aluminate cement are thoroughly mixed with 200 gms of water for a period of 10 mins in a Hobart type mixture. To evaluate the properties of the castable mix the mass was poured in a cube mould of 50 mm x 50 mm x 50 mm size and allowed for setting. Then the cube shapes were released from the mould and allowed for 72 hrs air curing followed by 24 hrs drying at 110°C. The dried samples were finally heated at 900°C and 1400°C for a period of 2 hrs soaking. The resultant product properties were: (Table Removed) Example 4 650 gms of below 6 above 18, 450 gms of below 18 above 60, 450 gms of below 60 above 100 and 150 gms of below 100 BS sieve of sintered magnesite, 100 gms of reactive alumina fines, 200 gms of barium aluminate cement are thoroughly mixed with 160 gms of water for a period of 10 mins in a Hobart type mixture. To evaluate the properties of the castable mix the mass was poured in a cube mould of 50 mm x 50 mm x 50 mm size and allowed for setting. Then the cube shapes were released from the mould and allowed for 72 hrs air curing followed by 24 hrs drying at 110°C. The dried samples were finally heated at 900°C and 1400°C for a period of 2 hrs soaking. The resultant product properties were: (Table Removed) In the experiments as given in the above examples it was observed that the deterioration in strength with heating in the range of 110°C to 1400°C is low. This is much lower than the currently available castable mixtures having deterioration in strength of 12 to 15%. Moreover, significant change in strength, cracking and disintegration was observed during its exposure in moist atmosphere, which is commonly noticed in the currently available similar castables. Further, the refractoriness of the presently developed novel magnesia based basic castable material is higher (orton cone 36+, > 1804°C) than the currently available basic castables (orton cone 34+, > 1763°C). The main advantages of the synergistic composition of the present invention are: 1) Enables faster rate of bonding thereby permits quicker application. 2) Enables achieve tailor made properties. 3) Enhances the durability and volume stability of the product. 4) Enables production of magnesia based basic refractory castable which does not develop cracks. 5) Enables production of magnesia based basic refractory castable which does not deteriorate in strength during heating. 6) Enables production of magnesia based basic refractory castable which does not form low eutectic liquid phases at higher temperatures. 7) Enables production of magnesia based basic refractory castable with improved mechanical characteristics such as high refractoriness with improved strength retaining capacity. We claim: 1. A magnesia based basic refractory castable having improved characteristics, comprising: 85 to 87 wt% sintered magnesite aggregate mix of size fractions in the range of below 6 to above 100 BS sieve, 3 to 7 wt% reactive alumina fines, and 8 to 10 wt% of barium aluminate cement binder, characterized in that the use of barium aluminate allows alumina fines to form strong bond with other ingredients. 2. A magnesia based basic refractory castable as claimed in claim 1, has the following characteristics: i) cold crushing strength (Kg/cm2) in the range of 515-575 at a temperature of 110°C for 24 hrs heating, ii) cold crushing strength (Kg/cm2) in the range of 495-551 at a temperature of 900°C for 2 hrs heating, iii) cold crushing strength (Kg/cm2) in the range of 510-569 at a temperature of 1400°C for 2 hrs heating, iv) refractories (°C) in Orton cone 36+ > 1804°C at a temperature of 110°C for 24 hrs, v) refractories (°C) in Orton cone 36+ > 1804°C at a temperature of 1400°C for 2 hrs, 3. A product as claimed in claims 1&2, wherein the sintered magnesite aggregate is of crystal size in the range of 70 to 90 micron. 4. A product as claimed in claims 1-3, wherein the sintered magnesite aggregate has chemical ingredients of 99 to 99.5 wt% MgO, 0.5 to 0.6 wt% CaO, 0.01 to 0.05 wt% Si02, 0.02 to 0.04 wt%Fe203 and 0.02 to 0.05 wt% Al203. 5. A product as claimed in claims 1—4, wherein the sintered magnesite aggregate mix constitutes of size fractions in the range of 34 to 39 wt% below 6 above 18, 26 to 30 wt% below 18 above 60, 22 to 27 wt% below 60 above 100 and 8 to 14 wt% below 100 BS sieve. 6. A product as claimed in claims 1-5, wherein the reactive alumina fines used has average particle size (d50) in the range of 1.8 to 2.5 microns. 7. A product as claimed in claims 1-6, wherein the reactive alumina fines has chemical ingredients of 99.7 wt% AI2O3 with alpha content over 97%, maximum 0.08 wt% Na20, and maximum 0.06 wt% Si02 8. A product as claimed in claims 1-7, wherein the barium aluminate cement binder used has specific surface area of the order of 2300 ± 200 cm2/gm. 9. A product as claimed in claims 1-8, wherein the barium aluminate cement binder used has chemical ingredients of 56 to 57 wt% BaO, 38 to 39 wt% A1203, 1.0 to 1.5 wt% Si02, maximum 0.50 wt% Fe203, 1.5 to 2.0 wt% CaO, maximum 0.25 wt% MgO, maximum 0.2 wt% SO3. 10. A process for the preparation of magnesia based basic refractory castable having improved characterictics as claimed in claim 1, which comprises 85 to 87 wt% sintered magnesite aggregate mix of size fractions in the range of below 6 to above 100 BS sieve, 3 to 7 wt% reactive alumina fines, and 8 to 10 wt% of barium aluminate cement binder in water and pouring it in a cube mould for settings, releasing the above said cube shapes from the mould, followed by air drying for 70- 80 hrs and further heating at a temperature of 110-120°C for 20-30 hrs, and finally heated at temperature of 900-1450°C for a period of 2-3 hrs to obtain the desired product. 11. A magnesia based basic refractory castable having improved characteristics and the process for the preparation thereof, substantially as herein described with reference to the examples.

Documents:

56-DEL-2003-Abstract-(28-07-2008).pdf

56-del-2003-abstract.pdf

56-DEL-2003-Claims-(28-07-2008).pdf

56-del-2003-claims.pdf

56-DEL-2003-Correspondence-Others-(28-07-2008).pdf

56-del-2003-correspondence-others.pdf

56-del-2003-correspondence-po.pdf

56-DEL-2003-Description (Complete)-(28-07-2008).pdf

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

56-del-2003-form-1.pdf

56-del-2003-form-18.pdf

56-DEL-2003-Form-2-(28-07-2008).pdf

56-del-2003-form-2.pdf

56-DEL-2003-Form-3-(28-07-2008).pdf

56-del-2003-form-3.pdf