FIELD OF THE INVENTION
The present invention relates to low moisture castables for preshaped and prefired
burner block for reheating furnaces. More particularly, the present invention is
directed to providing 60-65% alumina containing andalusite based low moisture
castable composition and a process for manufacture of such alumina containing
castable. Importantly, the invention provides andalusite based castable composition
which with the help of a vibrator is prefabricated to the shape of burner blocks for
application in heat treatment furnaces with long life and good castable properties.
BACKGROUND ART
Various types of refractory castable composition are available in the market. Main
problem for using these castable compositions are that the thermal shock property of
these compositions are not quite good to achieve a very good life of burner blocks.
Poor thermal shock resistance of these conventional castables leads to development
of crack and ultimate dislodging of the burner block from the furnace and hampers
production of the furnace.
Thermal shock is the property associated with the nature of raw material used for
development of composition and fabrication of burner block. In order to solve the
problem of thermal shock failure of burner blocks, castable composition was made
using andalusite as aggregate material. High alumina refractory castable have been
extensively investigated for production of desired castable because of their
significant advantage over other refractory material. The characteristics that have
made such castable so attractive are higher mechanical strength and better thermal
shock resistant. Unlike other alumino silicate material, andalusite grain is a single
crystal. It contains very low quantities of fluxes and remains unaltered with no
change in density and shape after firing upto 1200°C. At high temperature ,
andalusite becomes a reactive mineral because of mullitization process. During
recrystallization each andalusite crystal is transformed into a composite made of
mullite single crystal with a capillary network filled by silica rich glass. A small
amount of the glass is expelled outside the initial andalusite grain and can provide
secondary mullite by reaction with fine alumina . The liquid glassy phase trapped in
the mullite crystal acts as a healing substance absorber which provides crack

recovery during successive thermal shock. That is why andalusite containing castable
are applied in areas where high hot mechanical properties, high thermal stability,
good thermal shock resistance are required.
There has been thus a need in the art to developing a thermal shock resistive
castable composition and more specifically andalusite based low moisture castable
composition for fabrication of burner blocks for application in reheat/heat treatment
furnaces which would have longer life free of crack development due to thermal
shock in high temperature service conditions, ensuring low down time of furnace and
improved productivity.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing a andalusite
containing low moisture high alumina castable refractory composition with
andalusite as the aggregate material and a process for its production including
forming a preshaped prefired burner blocks for application in heat treatment
furnaces.
A further object of the present invention is directed to providing andalusite based low
moisture castable refractory composition and burner blocks made thereof to reduce
the furnace downtime and lower human involvement.
A still further object of the present invention is directed to providing andalusite based
low moisture castable refractory composition and burner blocks made thereof which
would be thermal shock resistant and avoid cracking of burner blocks in high
temperature applications such as in reheat/heat treatment furnaces.
A still further object of the present invention is directed to providing andalusite based
low moisture castable refractory composition and burner blocks made thereof
wherein andalusite present in the composition forms mullite during heating which in
turn would form needle shape crystal during recrystallisation along with a glassy
phase capillary network which retards the crack propagation in the formed body.

A still further object of the present invention is directed to providing andalusite based
low moisture castable refractory composition and burner blocks made thereof
wherein fine and superfine particles improve particle packing, reducing water
consumption and leading to low porosity castable.
A still further object of the present invention is directed to providing andalusite based
low moisture castable refractory composition and burner blocks made thereof, which
would have very good thermo-mechanical characteristics including high thermal
shock resistance, good abrasion resistance, and favour easy fabrication with
compaction by means of vibrator.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to andalusite based low
moisture and high alumina castable refractory composition comprising:
Andalusite -45-50%,by wt.
Sillimanite: 15-20% by wt.
Reactive alumina- 10-15% by wt,
Calcined alumina - 10—15% by wt,
Microfine silica- 3-5% by wt.
High alumina cement- 3-5% by wt.,
wherein andalusite grains are in the range of 0.25 - 5 mm and adapted to act as
aggregate in the body.
A further aspect of the present invention is directed to andalusite based castable
refractory composition wherein the distribution coefficient of grains is less than 0.35
and grain size is selected such that finer fraction below 300 BSS are less than 20% .
A still further aspect of the present invention directed to andalusite based castable
refractory composition comprises:
Said andalusite of particulate size 5- 0.25mm in an amount of 45-50% by wt.
Said sillimanite of particulate size 0-1mm in an amount of 15-20% by wt.

Said reactive alumina of particulate size 300 BSS and above in an amount of
12-15% by wt.
Said calcined alumina of particulate size 5-8mm in an amount of 10-15% by wt.
Said microfine silica of particulate size 250 BSS in an amount of 3-5% by wt.
Said high alumina cement of particulate size 150 to 200 BSS in an amount of 3-
5% by wt. and wherein the distribution coefficient of grain is less than 0.35 and
the grain size is selected such that finer fraction below 300BSS are less than 20%
by wt.
A still further aspect of the present invention is directed to said andalusite based
castable refractory composition which comprises:
Andalusite 45-50% of wt. size <90BSS
Sillimanite 15-20% by wt.
Reactive alumina 12-15% of wt.size 300-350BSS
Calcined alumina 10-15% of wt. size 5-8mm.
Microfine silica 3-5% of wt. size 250BSS
High alumina cement 3-5% of wt. size 150-200BSS
Importantly also, in said andalusite based castable refractory composition according
to the invention, said dispersants are selected from usual dispersants and wherein
the dispersant is used in the range of 0.05 to 0.1%.
According to yet another aspect of the present invention is directed to a process for
production of andalusite based castable refractory composition as described above
which comprises (i) providing andalusite -45-50%,by wt., sillimanite-15-20% by
wt., reactive alumina- 12-15% by wt, calcined alumina - 10-15% by wt, microfine
silica- 3-5% by wt. and high alumina cement-3-5% by wt. with dispersant in an
amount upto 0.1% by wt. and (ii) subjecting the mix to dry mixing for a period of
about five minutes: adding water in desired proportion to the mix slowly and
continuing mixing to thereby provide castable composition having distribution
coefficient of grains less than 0.35 and the grain size such that finer fraction below
300BSS constitute less than 20% by wt.
A still further aspect of the present invention is directed to said process of
manufacture of the andalusite based castable refractory composition wherein

andalusite of particulate size 5- 0.25mm , sillimanite 1-0 mm, reactive alumina of
particulate size 300 BSS , calcined alumina of particulate size 5-8mm, microfine silica
of particulate size 250BSS preferably 300BSS and high alumina cement of
particulate size 150-200BSS preferably 170BSS are used.
A still further aspect of the present invention is directed to said process wherein the
coarser fractions are taken in a mixer , the finer fractions are then added to it
alongwith a dispersant and the whole ingredients dry mixed, then measured amount
of water by volume is added to it and mixed with constant stirring for at least 5
minutes to produce the composition, thereafter the mix is unloaded in a format and
vibrating casting done for making burner block shape followed by curing for about 24
Hrs. ,demolding and thereafter whole block is kept in natural weather for about 24
hrs. by covering it with wet gunny bag for natural curing,subjeceting to drying in a
dryer at about 120oC and kept there for 24-30hrs. to remove inside moisture,
followed by firing in a kiln at 1000oC by raising the temperature slowly and
maintaining the same at 1000oC for 12hrs, cooling and taking out from furnace and
finally surface finished by grinding the surface and packaging.
Also in said process the dispersant used in the process is selected from usual
dispersants and the dispersant used in the range of 0.05 to 0.1% of the total
substances.
The objects and advantages of the present invention are described in greater details
with reference to the following accompanying non limiting illustrative example.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING
EXAMPLE
The present invention relates to improved quality andalusite based low moisture
castable for making preshaped and prefired burner block for reheating furnaces having
high strength and high thermal shock resistance properties.

According to the present invention andalusite containing castable have been
extensively investigated in laboratory for production of the desired castable because
of their significant advantage over other refractory materials. The characteristics that
have made such castables so attractive are high strength and high thermal shock
resistant, versatility and good performance at high temperature. Andalusite present
in the composition forms mullite during heating which forms needle shape crystal
during recrystallisation along with a glassy phase capillary network which retards the
crack propagation in the formed body.
Unlike other alumino silicate material , andalusite grain is a single crystal. It contains
very low quantities of fluxes and remains unaltered with no change in density and
shape after firing upto 1200°C. At high temperature , andalusite becomes a reactive
mineral because of mullitization process. During recrystallization each andalusite
crystal is transformed into a composite made of mullite single crystal with a capillary
network filled by silica rich glass. A small amount of the glass is expelled outside the
initial andalusite grain and can provide secondary mullite by reaction with fine
alumina . The liquid glassy phase trapped in the mullite crystal acts as a healing
substance absorber which provides crack recovery during successive thermal shock.
That is why andalusite containing castable are applied in areas where high hot
mechanical properties, high thermal stability, good thermal shock resistance are
required. The object is to provide a low moisture castable which can be preshaped
and prefired at 1000°C with 12 hrs. soaking time for making of burner blocks.
The objects and advantages of the invention is described in details with reference to
the following example I.
EXAMPLE I:
According to an embodiment of the present invention following raw materials and
ingredients were provided to produce burner blocks using the desired low moisture
andalusite based thermal shock resistant castable composition.
Andalusite: 45-50%
Sillimanite: 15-20%
Reactive Alumina: 12-15%

Calcined Alumina: 10-15%
Microfine Silica: 3-5%
High Alumina Cement: 3-5%
Dispersant: 0.05-0.1%
wherein andalusite used as grain in the range of 0.25-5mm and act as aggregate in
the body and wherein the distribution coefficient of grains is less than 0.35 and grain
size is selected such that finer fraction below 300 BSS are less than 20% by wt.
In accordance with a preferred aspect in the process of manufacture of castable
refractory composition according to the invention, andalusite of particulate size 0.25-
5mm, sillimanite of particulate size 0-lmm, reactive alumina of particulate size
300BSS or above, calcined alumina of particulate size 5-8mm, microfine silica of
particulate size 250BSS, high alumina cement of particulate size 150BSS are used .
The process of production of andalusite containing low moisture castable for
fabrication of preshape prefired burner blocks according to the present invention
comprise the steps of:
(i) the coarser fraction of particulate size comprising 8-0.25mm is first taken
in a mixer;
(ii) the finer fractions comprising 150-300BSS are then added to it alongwith
the dispersant up to 0.1% by wt and the rest of the ingredients are dry
mixed for a period of at least five minutes;
(iii) then 4-5% water by volume is added and mixed with stirring for two to
three minutes to provide for castable composition of good consistency;
(iv) It is then cast in a steel format to give the shape of burner block of
suitable size,
(v) Demolding of the shape is done after 24hrs. of curing,
(vi) The cast blocks are then kept in wet conditions by covering gunny bags
over it and then they are kept in open condition for normal curing,
(vii) In the next step they are put into dryer at a temperature of 120°C for
drying the sample by driving out the moisture for another 24hrs.

(viii) Then they are put into a furnace and fired at 1000°C with 12hrs. soaking.
So as to obtain the desired thermal shock resistant burner blocks for
application in reheat/heat treatment furnace.
In the above process, the dispersant used in the process may be selected from usual
dispersants and is used in the range of 0.05-0.1% of the total substances and is
added alongwith the finer fractions before addition of water in the composition.
Granulometry and grain size are the most important factors and are required to be
closely controlled.
The developed castable is andalusite based and CaO in small amount is used in the
form of high alumina cement. The castable contains silica to favour the formation of
mullite and cement is added to impart green strength. An intergranular liquid is
formed in this system that significantly reduced the thermo mechanical performance
of the castable. In order to diminish the amount of liquid phase at high temperature
and simultaneously improve the refractoriness of the castable, a reduction of cement
content was necessary.
Fine and superfine particles improve particle packing, reducing water consumption
and leading to low porosity castable. The incorporation of fine and superfine particles
requires the use of specific defloculants for dispersion. So optimum particle packing
at high solid content could be attained.
Following are the physico-chemical properties of the castable obtained as above.
Chemical, wt(%)
AI203- 60-65
CaO - 1.2,max
Physical properties:
BD,gm/cc at 110oC/24hrs.- 2.5,min
AP (%) after firing the sample at 1450oC/2hrs.- 12-14.

CCS, kg/cm2, at 110oC/24hrs. 300, 800oC/3hrs.- 700
HMOR,kg/cm2, at 1400oC/30min.- 25
PLC(%) at 1500Oc/2hrs =0.5
Spalling resistance (cycles) at 1200oC( water Quenching)- > 10
It is evident from the above physical properties that the castable of the said
invention have significantly improved thermo-mechanical characteristics.
Advantageously, the andalusite containing castable of the invention have the
following advantages:
(i) Very good thermal shock resistant and abrasion resistant material;
(ii) Easy fabrication with compaction by means of vibrator;
(iii) It can greatly reduce the downtime of the furnace and hence lower involvement
of manpower.
(iii)The castables can be installed in walking beam reheat furnaces as burner block
for improved life.
It is thus possible by way of the present invention to providing andalusite based high
alumina low moisture castable for production of burner blocks with high strength and
high thermal shock resistance favouring high temperature application of such burner
blocks in reheat/heat treatment furnace with longer life reducing down time and
improving productivity.

WE CLAIM:
1. Andalusite based low moisture and high alumina castable refractory
composition comprising:
Andalusite -45-50%,by wt.
Sillimanite: 15-20% by wt.
Reactive alumina- 10-15% by wt,
Calcined alumina - 10--15% by wt,
Microfine silica- 3-5% by wt.
High alumina cement- 3-5% by wt.,
wherein andalusite grains are in the range of 0.25 - 5 mm and adapted to act as
aggregate in the body.
2. Andalusite based castable refractory composition as claimed in claim 1 wherein
the distribution coefficient of grains is less than 0.35 and grain size is selected such
that finer fraction below 300 BSS are less than 20% .
3. Andalusite based castable refractory composition as claimed in anyone of claims 1
or 2 comprising:
Said andalusite of particulate size 5- 0.25mm in an amount of 45-50% by wt.
Said sillimanite of particulate size 0-lmm in an amount of 15-20% by wt.
Said reactive alumina of particulate size 300 BSS and above in an amount of
12-15% by wt.
Said calcined alumina of particulate size 5-8mm in an amount of 10-15% by wt.
Said microfine silica of particulate size 250 BSS in an amount of 3-5% by wt.
Said high alumina cement of particulate size 150 to 200 BSS in an amount of 3-
5% by wt. and wherein the distribution coefficient of grain is less than 0.35 and
the grain size is selected such that finer fraction below 300BSS are less than 20%
by wt.
4. Andalusite based castable refractory composition as claimed in anyone of claims 1
to 3 comprising:
Andalusite 45-50% of wt. size <90BSS

Sillimanite 15-20% by wt. size 0-1mm
Reactive alumina 12-15% of wt.size 300-350BSS
Calcined alumina 10-15% of wt. size 5-8mm.
Microfine silica 3-5% of wt. size 250BSS
High alumina cement 3-5% of wt. size 150-200BSS
5. Andalusite based castable refractory composition as claimed in anyone of claims 1
to 4 comprising dispersants selected from usual dispersants.
6. Andalusite based castable refractory composition as claimed in claim 5wherein the
dispersants is used in the range of 0.05 to 0.1%.
7. A process for production of andalusite based castable refractory composition as
claimed in anyone of claims 1 to 6 comprising (i) providing andalusite -45-
50%,by wt., sillimanite-15-20% by wt., reactive alumina- 12-15% by wt,
calcined alumina - 10-15% by wt, microfine silica- 3-5% by wt. and high
alumina cement-3-5% by wt. with dispersant in an amount upto 0.1% by wt. and
(ii) subjecting the mix to dry mixing for a period of about five minutes: adding
water in desired proportion to the mix slowly and continuing mixing to thereby
provide castable composition having distribution coefficient of grains less than
0.35 and the grain size such that finer fraction below 300BSS constitute less than
20% by wt.
8. A process of manufacture of the andalusite based castable refractory composition
as claimed in claim 7 as claimed in claim 7 wherein andalusite of particulate size
5- 0.25mm , sillimanite 1-0 mm, reactive alumina of particulate size 300 BSS ,
calcined alumina of particulate size 5-8mm, microfine silica of particulate size
250BSS preferably 300BSS and high alumina cement of particulate size 150-
200BSS preferably 170BSS are used.
9. A process as claimed in anyone of claims 7 or 8 wherein the coarser fractions are
taken in a mixer , the finer fractions are then added to it alongwith a dispersant
and the whole ingredients dry mixed, then measured amount of water by volume
is added to it and mixed with constant stirring for at least 5 minutes to produce
the composition, thereafter the mix is unloaded in a format and vibrating casting

done for making burner block shape followed by curing for about 24 Hrs. ,
demolding and thereafter whole block is kept in natural weather for preferably
about 24 hrs. by covering it with wet gunny bag for natural curing, subjecting to
drying in a dryer at 120oC and kept there for 24-30hrs. to remove inside
moisture, followed by firing in a kiln at 1000oC by raising the temperature
slowly and maintaining the same at 1000oC for about 12hrs, cooling and taking
out from furnace and finally surface finished by grinding the surface and
packaging.
10. A process as claimed in anyone of claims 7 to 9 wherein the dispersant used in
the process is selected from usual dispersants.
11. A process as claimed in claim 10 wherein the dispersant used in the range of
0.05 to 0.1% of the total substances.
12. High alumina andalusite containing castable refractory composition and a process
for production of high alumina andalusite castable refractory composition
substantially as herein described.

The present invention relates to low moisture castables for preshaped and prefired
burner block for reheating furnaces. More particularly, the present invention is
directed to providing high alumina containing andalusite based low moisture castable
refractory composition and a process for manufacture of such castables and burner
blocks therefrom having high strength and improved thermal shock resistance
properties. In the above composition andalusite grains are in the range of 0.25 - 5
mm and adapted to act as aggregate in the body and wherein the distribution
coefficient of grains is less than 0.35 and grain size is selected such that finer
fraction below 300 BSS are less than 20% The andalusite based castable refractory
composition favour prefabrication to the shape of burner blocks using vibrator for
application in heat treatment furnaces with long life and good castable properties.