FIELD OF THE INVENTION
The present invention relates to refractory castable-composition and in particular to a class of high alumina castable compositions with preformed spinel addition in combination with magnesia addition for in situ spinel formation also during exposure to high temperature and, a process for developing such composition, comprising both pre-formed spinel and in situ formed spinel during firing/service at high temperature Advantageously, the high alumina refractory castable product according to the present invention is directed to novel castable compositions for refractory lining having superior properties comprising high hot strength in combination with favourably good corrosion resistance to steel making slag and also thermal shock resistance The invented class of castable is directed to substitute conventional spinel added high alumina castables (AS-castables) and also in situ spinel forming alumina magnesia castables (AM-castables) for use in steel ladle/ladle furnace working lining, functional refractory components, viz , well blocks, nozzles, porous plug and porous plug seating block of steel ladle/ladle furnaces and other critical areas of iron and steel making processes in a cost effective manner and thus having prospects for wide industrial application
BACKGROUND ART
It is well known in the art of developing compositions for refractory castables, spinel structure is such that within its unit cell considerable octahedral and tetrahedral sites remain vacant Therefore, it can form a solid solution with varying amount of Al203 MgO and other oxides Due to this characteristic, spinel can be made to have either excess MgO or Al203 in the form of solid solution Moreover, due to these vacant sites, spinel can trap the rapidly diffusing compounds such as oxides of iron and manganese from the slag within its crystal structure forming complex spinel i e , (Mg, Mn, Fe)0 (Fe, Al)203 Further, the lime of slag reacts with alumina of alumina-rich spinel forming highly refractory hibonite (CaO 6 Al203) Because of the depletion of MnO, FeO and CaO, slag becomes richer in silica and hence more viscous This increase in viscosity of slag, reduces its ability to penetrate inside the castable lining As a consequence, the slag corrosion resistance of high alumina castable containing spinel rich in alumina is improved, which is one of the main qualifying quality criteria of steel ladle refractories Another contribution of spinel addition in high alumina castable is in the improvement of thermal shock resistance of the later owing to lower thermal expansion co-efficient and modulus of elasticity of spinel In conventional spinel added high alumina castables (AS-castables) the above advantages of spinel are utilised by incorporating pre-formed spinel

in the composition to improve performance of the high alumina refractory lining It has also been observed that the above characteristics of spinel tend to improve further if the spinel is formed in situ in the refractory at high temperature during service The in situ formed spinel being very fine is more effective in trapping FeO and MnO of slag and, thereby improves further the corrosion resistance of high alumina castables
The above advantages of in situ formed spinel in the existing art led to the development and use of alumina-magnesia (AM) castable without any pre-formed spinel addition The magnesia present in the AM-castable reacts with alumina and, forms the desired in situ spinel during service/firing at high temperature
It is well established that presence of silica in the matrix of AM-castable accelerates in situ spinel formation at high temperature Silica forms liquid phase in combination with Al203 CaO (from added cement) and MgO present in the castable matrix, at high temperature and, this liquid phase facilitates in situ spinel formation Therefore silica is incorporated in AM-castable matrix to accelerate spinel formation during service at high temperature This liquid phase also helps in controlling the associated large amount of expansion during spinel formation
Though in situ spinel forming alumina-magnesia castables are excellent corrosion resistant against steel making slag but their hot strength is low (hot modulus of rupture (HMOR) at 1400°C is around 30 kg/cm2) Moreover, it has also been observed in the related art that though liquid phase generation in AM-castable helps in situ spinel formation resulting in above mentioned advantages in respect of slag corrosion resistance, but at the same time is the cause for comparatively poor hot strength of in situ AM-castable
There has been therefore a persistent need in the filed of application of AM-castables as refractory lining, to achieve higher hot strength without sacrificing much of the slag corrosion resistance properties and, therefore to develop compositions by effective control of silica addition - the key contributory constituent towards liquid phase formation at high temperature, in combination with control of cement, spinel and magnesia addition for improving performance and longer service life of such castable refractory lining/pre-fabncated shapes in various applications, reducing downtime, ensuring economy in operations and enhancing productivity.

OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing a high alumina castable compositions for refractory lining comprising preformed spinel and magnesia directed to achieving higher hot strength in combination with. reasonably higher slag corrosion resistance and thus ensuring longer and reliable service life of said cast refractory lining/pre-fabricated shapes
A further object of the present invention is directed to achieving improved combination of hot strength and corrosion resistance properties in the high alumina castable compositions by controlling liquid phase formation in the castable at high temperature
A still further object of the present invention is directed to developing a class of high alumina castable compositions with preformed spinel addition in combination with provision of in situ spinel formation by magnesia addition, wherein controlling liquid phase formation at high_ temperature is achieved by selectively lowering the silica content in the castable composition, controlling amount of cement and magnesia addition, in order to have both pre-formed spinel and in situ formed spinel during firing/service at high temperature, providing desired combination of high hot strength and corrosion resistance properties
A still further object of the present-invention is directed to developing a class of high alumina castable compositions with spinel addition in combination with magnesia wherein partial replacement of magnesia for reducing the amount of in situ spinel by equivalent amount of fines of pre-formed spinel with simultaneous reduction in amount of silica and increase in cement content would help to improve hot strength of such castable
A still further object of the present invention is directed to developing a class of high alumina castable compositions with spinel addition in combination with magnesia wherein extensive laboratory work on different formulations with respect to properties and batch compositions were carried out comprising studies on effect of silica, cement and magnesia/pre-formed spinel on hot strength, corrosion resistance and other physical properties and subsequent optimisation thereof

A still further object of the present invention is directed to developing a class of high alumina castable compositions with spinel addition in combination with magnesia wherein the invented castable compositions are tested for reliable performance with respect to parameters comprising green & fired strength, permanent linear change (PLC) after heating to high temperature, apparent porosity after firing, hot modulus of rupture, spinel formation and alumina content of the spinel by XRD analysis, electron probe micro analysis, thermal shock resistance by water quenching from 1200°C and slag resistance test
A still further object of the present invention is directed to developing a class of high
alumina castable compositions with spinel addition in combination with magnesia
adapted for advantageous application in areas comprising Working lining of steel
ladle/ladle furnaces in bottom and metal zone, teeming well blocks & nozzles, Porous
plug and its seating block of steel ladle/ladle furnaces, and any other working lining that
comes in contact with iron, steel and high FeO & MnO containing slag at high
temperature
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to high alumina castable compositions comprising preformed spinel and magnesia for in situ spinel formation also having
Fused Magnesia fines in the range of 1-3 part by weight,
Calcined Micro fine Alumina in the range of 7-12 part by weight,
Calcined Super fine Alumina in the range of 2-3 part by weight,
Micro fine silica in the range of 0-0 2 part by weight,
Sodium hexametaphosphate in the range of 0 04 - 0 lpart by weight,
Citric acid in the range of 0 02- 0 05 part by weight,
Poly propylene fibre in the range of 0 01- 0 03 part by weight, White Fused/Sintered Alumina fines in the range of 11-12 part by weight,
White Fused/Sintered Alumina grains in the range of 55-56 part by weight

Spinel fines in the range of 10-16 part by weight, and High Alumina Cement in the range of 3-7 part by weight,
Another aspect of the present invention is directed to high alumina castable compositions wherein the preferred grain size comprises
Alumina grains (AG) - White fused/ Sintered 0 25 to 5 mm,
White fused / Sintered Alumina fines (AF) < 0 15 mm,
Fused Magnesia fines (FMF) < 0 25 mm,
Calcined Super fine Alumina (CSA) < 10 μm,
Calcined Micro fine Alumina (CMA) < 5 μm,
Spinel fines < 53 μm,
Micro fine Silica (MFS) < 0 5 μm,
A still-further aspect of the present invention is directed to high alumina castable compositions
wherein Al2O3 content of different alumina sources are

Sintered alumina grains and fines (SAG and SAF) Al2O3 99 0% min

white fused alumina grains and fines (SAG and SAF) Al2O3 99 3% min
Calcined super fine alumina (CSA) Al2O3 99 3% min
Calcined micro fine alumina (CMA) Al2O3 99 4% min
High alumina cement (HAC) Al2O3 72% min
A still further aspect of the present invention is directed to high alumina castable compositions wherein Al2O3 content of spinel fines is at least 74%
According to yet another aspect of the present invention is directed to high alumina castable compositions wherein MgO content of fused magnesia fines is at least 97%
A still further aspect of the present invention is directed to high alumina castable compositions wherein SiO2 content of micro fine silica is at least 97% According to a further aspect of said high alumina castable compositions of the present invention, wherein the Sodium hexa meta phosphate (SHMP) has at least 60% P2O5
A still further aspect of present invention is directed to high alumina castable compositions wherein the Citric acid (CA) has at least 99 5% C6H8O7 H2O

According to another aspect of high alumina castable compositions of the present invention, wherein the polypropylene fibre has a melting point of 165-170°C
According to another aspect of said high alumina castable compositions wherein the properties of said castable compositions comprise as defined under Table V
According to an important aspect, the present invention is directed to a process for the production of said high alumina castable compositions comprising the steps of
a) providing a mix of (Fused Magnesia Fines (FMF)) + (Calcined Micro fine Alumina (CMA)) + (Calcined Super fine Alumina (CSA)) + (Micro fine Silica (MFS) to thereby obtain a mix M1
b) adding to said mix Ml of step (a) above Sodium hexmeta Phosphate (SHMP) to thereby obtain a mix M2,

c) adding to said mix M2 of step (b) above Citric Acid (CA) to thereby obtain a mix M3,
d) adding to said mix M3 of step (c) above Poly propylene fibre (PPF) to thereby obtain a mix M4,

e) adding to said mix M4 of step (d) above White Fused Alumina Fines (WFAF) / Sintered Alumina Fines (SAF) to thereby obtain a mix M5,
f) adding to said mix M5 of step (e) above White Fused Alumina Grains (WFAG) / Sintered Alumina Grains (SAG) to thereby obtain a mix M6,
g) adding to said mix M6 of step (f) above Spinel Fines (SF) to thereby obtain a
mix M7,
h) adding to said mix M7 of step (g) above High Alumina Cement (HAC) to thereby obtain said Spinel & Magnesia based High Alumina Castables
The present invention and its objects and advantages are described in greater details with reference to the following non limiting illustrative figure and example

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the illustration of the flow chart representing the mixing sequence for spinel and magnesia based high alumina castable compositions according to the present invention
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING NON-LIMITING EXAMPLE
The present invention is directed to developing compositions for high alumina castable containing preformed spinel and aiso magnesia addition for in situ spinel formation adapted to achieving higher hot strength combined with adequate corrosion resistance to slag formed during steel making
The already known properties of conventional high alumina spinel added (AS) castable and in situ spinel forming alumina-magnesia (AM) castable as reported in literatures are presented in Table 1, for clear understanding of the improvements effected by way of the present invention



The formulations/composition for high alumina castables containing preformed spinel and also magnesia addition for in situ spinel formation according to the present invention involve providing (a) Both preformed spinel & magnesia, (b) No silica or very small amount of silica, (c) Low to moderately large quantity of cement and maintaining selective mixing sequence of different components to obtain the final product having desired characteristic property in the castable
The actual chemical composition of the total mix can be altered in order to achieving specific desired end properties depending upon area/condition of applications of the castable composition It is further ensured that during manufacturing stage or during subsequent storage no chemical reaction takes place between various ingredients of the compositions Instead, whatever reactions/changes take place are during application

i e , on mixing the castable composition with water followed by placement, curing, drying and subsequent exposure to high operating/firing temperature
It is experienced that liquid phase formation in AM-castable need to be selectively
controlled to achieve desired properties of improved hot strength in combination with
high slag corrosion resistance So for improvement of hot load properties of in situ spinel
based high alumina castable, it is necessary to reduce the amount of liquid phase
formation in the body at high temperature This reduction in liquid phase formation is
possible if in situ spinel formation in the castable is reduced by reducing the amount of
magnesia addition But to keep the total spinel content at the same level at -20% so
that corrosion resistance is not affected too much, this decrease of in situ formed spinel
should be supplemented by adding equivalent amount of pre-formed spinel In the
matrix containing CaO, MgO, Al2O3 and SiO2, the key liquid forming component is silica
Therefore, reduction in the amount of silica will reduce the amount of liquid phase
formation in the castable Further in alumina-magnesia castable, cement content is kept
at a low level of ~3 wt % in order to avoid too much liquid formation in presence of
moderately high silica content in the range of ~0 5 to 1 wt % Once the amount of silica
is reduced, cement content can be increased for formation of more hibonite (calcium
hexa aluminate) in the matrix of the castable that would contribute in improving hot
strength further Therefore partial replacement of magnesia for reducing the amount of
in situ spinel by equivalent amount of fines of pre-formed spinel with simultaneous
reduction in amount of silica and increase in cement content would help to improve hot
strength of such castable
As already described the castable compositions of the present invention is directed to achieving desired end properties of higher hot strength without sacrificing the corrosion resistance to slag by selectively controlling the liquid phase formation at high service / firing temperature by controlling the components comprising primarily silica, magnesia and cement The details of the castable compositions of the invention and the steps of developing the preformed and in situ spinel forming high alumina and AM-castable with magnesia addition is illustrated in greater details with the help of the following example
Example I
The basic ingredients used for the preparation of desired batch composition of castables according to the present invention used in selective wt proportion, and the detailed

constitution of the basic ingredients to form a batch being as given in the accompanying Table II, comprising
Table II Details of different Raw Materials & Additives:
1 White Fused Alumina Grains (WFAG)
A1203 99 3 % min
T1O2 0 001% max
Fe2O3 0 10% max
Na2O 0 40 % max
S1O2 0 05 % max
Particle Size 0 25 to 5 mm
2 White Fused Alumina Fines (WFAF)
Al2O3 99 3 % min
T1O2 0 001% max
Fe2O3 0 10% max
Na2O 0 40 % max
S1O2 0 05 % max
Particle Size < 0 15 mm
3 Sintered Alumina Grains (SAG)
AI2O3 99 0 % mm
Fe2O3 0 5 % max
Na2O 0 5 % max
Paiticle Size 0 25 to 5 mm
4 Sintered Alumina Fines (SAF)
Al2O3 99 0 % min
Fe2O3 0 5 % max
Na2O 0 5 % max
Particle Size <0 15 mm
5 Calcined Super fine Alumina (CSA)
A12O3 99 3 % Mm
Na2O 0 5 %Max
LOI 0 5 % Max
Particle Size <10 μm

Selective mixing sequence of different components.
An important aspect of the invention is that the different components are to be added during mixing in the order as indicated in accompanying Table-Ill Sequence of additions and mixing of different raw materials and additives as stated above are carried out as follows

Where 'M' symbolises a particular mix comprising specified blend of components contained therein represented by the number suffixed to it, which would be amply clear when read in combination with accompanying Figure 1.
The mixing sequence of the different ingredients and the additives having desired grain size are also represented in the form of a flow chart in accompanying Figure 1, wherein the each component has been numbered according to the ordered sequence of mixing in a mixer device to arrive at the desired castable composition dry mix according to the invention with selective weight percent of the constituents

Method of application of the castable composition
The Spinel and Magnesia based High Alumina Castable mix according to the invention as obtained in the previous step following selective mixing sequence of various components, can be applied following the procedure that is normally followed for any standard low moisture castable, which comprises following steps
(i) providing selective mix of spinel and Magnesia based High Alumina Castable mix
castable composition in desired quantity depending on application, (ii) Mixing said castable mix with 5 5-6% by weight of water in a suitable mixer, (iii) Placing the moistened mix by vibro-casting in desired lining location or in mould
cavity for pre-fabricated shapes (iv) curing the cast mix under covered condition, (v) air drying, drying and preheating up to at least 1000°C, as per schedule
normally followed for low moisture castables, (vi) the lining or the prefabricated shape made out of this castable is then put in
actual service
Batch composition of castable formulations:
The batch compositions for the preformed spinel and magnesia based high alumina castable refractory product derived from the basic ingredients following specified process steps starting with the above said basic ingredients is as given in accompanying Table IV
Table IV Batch composition of Spinel & Magnesia based High Alumina Castable Compositions
Ingredients Wt %
• White fused / Sinteied Alumina grains (0 25 -5 mm) 55 -56
• White Fused/Sinteied Alumina fines (-0 15 mm) 11-12 -• Calcined Super fine Alumina (-10 μm) 2-3

• Calcined Micro fine Alumina (-5 μn) 7-12
• Fused Magnesia Fines (-0 25 mm) 1 -3
• Spinel fines (-0 053 mm) 10-16
• Micro fine Silica (-0 5 μm) 0-02
• High Alumina Cement 3 -7
• Sodium hexametaphosphate 0 04 - 0 1
• Citric Acid 0 02-0 05
• Polypropylene fibie 0 01-0 03
Testing and Results achieved:
The synergistic effect/ of the presence of different ingredients as per above stated
batch compositions results in the invented high alumina castable compositions the
following characteristics
a Excellent hot strength,
b Controlled expansion on exposure to high temperature, c Very good resistance to steel making slag,
d Excellent thermal shock resistance,

Extensive Laboratory works have been conducted during development stage of the invented castables and the final outcome of Laboratory scale trials are summarised in Table V below These, are the properties achieved for the invented castables as obtained on samples made after mixing the castable with specified amount of water following standard procedure of sample making by vibro-casting and after curing, drying
and firing at different temperatures For this different tests / evaluation were conducted for green & fired strength, permanent linear change (PLC) after heating to high temperature, apparent porosity after firing, hot modulus of rupture, spinel formation and

alumina content of the spinel by XRD analysis, electron probe micro analysis, thermal shock resistance by water quenching from 1200°C and slag resistance test


It is thus possible by way of the present invention to achieve a class of high alumina spinel forming low moisture refractory castable wherein spinel is added in preformed condition with the provision of in situ spinel formation also by incorporating controlled amount of magnesia such that the liquid phase formation during application at high temperature is reduced Such reduction of the liquid phase volume favor desired expansion of castable at high temperature and improvement in the hot strength while retaining the corrosion resistance to slag at the desired level by way of selective addition of desired weight proportion of components like Silica, magnesia and cement in castable mix compositions The castable compositions according to the present invention is thus capable of applications comprising steel ladle/ladle furnace working lining, functional refractory components viz , well blocks, nozzles, porous plug and porous plug seating block of steel ladle/ladle furnaces and other critical areas of iron and steel making processes in a cost effective manner and ensuring longer life of the castable lining without failure The laboratory test result established improvement properties for the spinel and magnesia based high alumina castable compositions of the present invention as compared to the conventional AM-castable, viz , CCS of 800-1000kg/cm2 after 1500°C/3hr, HMOR of 90-185 at 1400°C/l/2 hr on sample pre-fired at 1600°C/3hr, RUL (Ta °C) under 2 kg/cm2 load, for samples pre-fired at 1500°C/3h in the range of over 1650°C, apparent Porosity (%) after l500°C/3h in the range of 18 -22, thermal spalling resistance number of quenching cycles from 1200°C in water in the range of + 30 cycles, having reasonably good Corrosion Resistance to slag (on a scale of 10) of ~ 9 and the like, making such castable compositions suitable for reliable and durable performance under severe service conditions and thus having prospects for wide industrial application in steel plants and the like

We Claim:
1 High Alumina castable compositions comprising preformed spinel and magnesia for in situ spinel formation also having
Fused Magnesia fines in the range of 1-3 part by weight,
Calcined Micro fine Alumina in the range of 7-12 part by weight,
Calcined Super fine Alumina in the range of 2-3 part by weight,
Micro fine silica in the range of 0-0 2 part by weight,
Sodium hexametaphosphate in the range of 0 04 - 0 1 part by weight,
Citric acid in the range of 0 02- 0 05 part by weight,

Poly propylene fibre in the range of 0 01- 0 03 part by weight,
White Fused/Sintered Alumina fines in the range of 11-12 part by weight,
White Fused/Sintered Alumina grains in the range of 55-56 part by weight,
Spinel fines in the range of 10-16 part by weight, and
High Alumina Cement in the range of 3-7 part by weight, providing for a
combination of both pre-formed spinel and in situ formed spinel during firing
/service at high temperature with desired combination of hot strength and corrosion
resistant properties


3 High Alumina castable compositions as claimed in anyone of claims 1 or 2 wherein Al203
content of different alumina sources are

4 High Alumina castable compositions as claimed in anyone of claims 1 to 3 wherein Al203 content of spinel fines is at least 74%
5 High Alumina castable compositions as claimed in anyone of claims 1 to 4 wherein MgO content of fused magnesia fines is at least 97%
6 High Alumina castable compositions as claimed in anyone of claims 1 to 5 wherein Si02 content of micro fine silica is at least 97%
7 High Alumina castable compositions as claimed in anyone of claims 1 to 8, wherein the Sodium hexametaphosphate (SHMP) has at least 60%P2O5
8 High Alumina castable compositions as claimed in anyone of claims 1 to 8 wherein the Citric acid (CA)has at least 99 5% C6H807 H20
9 High Alumina castable compositions as claimed in anyone of claims 1 to 8 wherein the polypropylene-fibre has a melting point of 165-170°C

10. A process for the production of high Alumina castable compositions as claimed in
anyone of claims 1 to 9 comprising the steps of

a) providing a mix of (Fused Magnesia Fines (FMF)) + (Calcined Micro fine Alumina
(CMA)) + (Calcined Super fine Alumina (CSA)) + (Micro fine Silica (MFS) to thereby obtain a mix M1

b) adding to said mix Ml of step (a) above Sodium hexmetaphosphate (SHMP) to thereby obtain a mix M2,

c) adding to said mix M2 of step (b) above Citric Acid (CA) to thereby obtain a mix M3,
d) adding to said mix M3 of step (c) above Poly propylene fibre (PPF) to thereby obtain a mix M4,
e) adding to said mix M4 of step (d) above White Fused Alumina Fines (WFAF) / Sintered Alumina Fines (SAF) to thereby obtain a mix M5,
f) adding to said mix M5 of step (e) above White Fused Alumina Grains (WFAG) /
Sintered Alumina Grains (SAG) to thereby obtain a mix M6,

g) adding to said mix M6 of step (f) above Spinel Fines (SF) to thereby obtain a
mix M7,
h) adding to said mix M7 of step (g) above High Alumina Cement (HAC) to thereby obtain said Spinel & Magnesia based High Alumina Castable

ABSTRACT
High alumina spinel forming low moisture refractory castable compositions comprising preformed spinel and magnesia for in situ spinel formation also along with controlled silica and cement addition such that high hot strength with desirable corrosion resistance to slag is achieved The castable compositions are suitable for applications comprising steel ladle/ladle furnace working lining, functional refractory components of steel ladle/ladle furnace viz , well blocks, nozzles, porous plug and seating block and the like, ensuring longer lining life without failure The castable compositions of the invention having improved properties as compared to the conventional AM-castable comprising
CCS of 800-1000kg/cm2 at 1500°C/3hr, HMOR of 90-185 at 1400°C/1/2 hr on sample pre-fired at 1600°C/3hr, RUL (Ta °C) under 2 kg/cm2 load for samples pre-fired at 1500°C/3h in the range of over 1650°C, making such castable compositions reliable and durable under severe service conditions and thus having prospects for wide industrial application in steel plants and the like