FIELD OF INVENTION
The present invention relates to improved process for the production of dense high alumina
refractory aggregates from sillimanite sand.
The present invention particularly relates to a cost efficient process for production of dense high
alumina refractory aggregates from a composition consisting of sillimanite sand and bauxite for
production of shaped refractory or as a component/constituent of castable.
BACKGROUND OF INVENTION
Beach sand sillimanite is a by-product generated in huge quantities during the extraction of rare
earth compounds from beach sand. Sillimanite is actually aluminosilicate (Al2SiOs) and it is
associated with small amounts of other mineral phase. Mullite is an aluminosilicate compound
with the specific formula 3Al203.2Si02 and it is a solid solution phase of alumina and silica.
Very rarely mullites occur as a natural mineral. It is the only stable intermediate phase in the
Al203-Si02 system at atmospheric pressure. Mullite has many advantageous properties of
refractory material such as:
i) High temperature stability i.e. high melting point.
ii) Low thermal conductivity and low thermal expansion coefficient, which leads to good
thermal shock resistance.
Hi) Good corrosion and creep resistance.
iv) Low under load deformation
Due to the above fundamental refractory properties the mullite-based material is widely used as
refractories in many industrial fields. In iron and steel industry mullite based bricks are used in
blast furnace, stove, torpedo ladle, iron ladle. It is also important in kiln lining, burner blocks and
kiln furniture in ceramic industry. For the production of refractory aggregates sillimanite can be
used directly or in combination with alumina depending upon the requirement of alumina content
and temperature of application. Particle size and impurity level of the raw material control the
decomposition temperature of sillimanite sand. In the high temperature range 1300 to 1700°C
sillimanite decomposes to mullite and silica.
u> 3(Al203.Si02) = 3AI203.2Si02 + Si02 CO
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SilicaJs_pro.duc.ed_in this_pro.cess_.from glass_y_phase_by partiaLreaction.with imp.urities.Jlhis
glassy phase reduces the product quality. Removal of glassy phase improves the quality of the
g product.
o Again Ti02 is the liquid forming additive which facilitates the densification of mullite through
liquid phase sintering.
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DESCRIPTION OF PRIOR ART & DRAWBACKS
o
C! Reference may be made to Ceramics International Vol. 20. 299-302 ^r (1994V
Wherein sintering of sillimanite was observed in the temperature range 1520-1570 °C in
presence of TiC>2 (6-8 wt.%) and ZrC>2 (9-12 wt.%) additives. Large amount of liquid formation
during sintering is responsible for the densification. With additive flexural strength of sintered
sillimanite varies from 45.2 to 93.5 MPa. at room temperature.
Reference may be made to Journal of European Ceramic Society, Vol. 18, 2081-2087 (1998);
Wherein high alumina aggregate were prepared from beach sand sillimanite and calcined
alumina using titania (TiCh) additive (0- 6 wt%). It was observed that alumina/silica ratio
determines densification, flexural strength and microstructural development of the aggregates. At
a sintering temperature of 1500 °C aggregate were prepared with zero porosity with the help of
titania additive. Pressing is completed in two stages i.e. uniaxial as well as isostatic.
Reference may be made to Indian patent Ref. no. NF495/02; wherein dense high alumina
aggregate refractory were prepared from sillimanite sand based material using Ti02 and Zr02
additive. Here the aggregate contains 55 to 62 % alumina with 98 % densification. Average grain
size of the aggregate was between 5 to 10 (im and HMOR at 1250°C ranges from 1700 to 2100
kg/cm2
Reference may be made to Industrial Ceramics Vol. 19, 13-16 (1999); wherein sillimanite were
sintered by milling and adding Zr02. At 1600 °C open porosity becomes almost zero i.e.
densification achieved. Sillimanite sand with 2 wt % ZrC>2 additive shows high HMOR compared
to without additive as the glassy phase is converted to ZrSi04. HMOR of sillimanite sand with 2
wt% Zr02 at 1200 °C is 360 MPa.
The main drawback of the previously known process is:
1. In the previously known process synthetic chemicals are used as additive to reduce the
sintering temperature which is not cost effective.
New generation high alumina refractory aggregate is very much needed in refractory industries
due to its excellent properties. Again the aggregate which is prepared in cost efficient method is
very important industrially. So there is an obvious need to provide a cost efficient method for the
preparation of high alumina refractory aggregate.
OBJECTS OF THE INVENTION
The-main-object-of-the-present-inventioa is-to provide improved-_process-for-the-production-of- ....
dense high alumina refractory aggregates from sillimanite sand, which removes the drawbacks of
the previously known prior art as detailed above.
Another object of the present invention is to provide a process wherein the starting material
consists of sillimanite sand, alumina and bauxite. Here without using any synthetic additive raw
bauxite is used directly. The presence of Ti02 and Fe203 in bauxite will act as additive and
finally develop dense high alumina refractory aggregate.
Still another object of the present invention is to provide sintered high alumina refractory
aggregate by uniaxial pressing.
Yet another object of the present invention is to provide dense high alumina (60-70 %) refractory
aggregate which is applicable above 1600 °C.
Still yet another object of the present invention is to provide a process for the production of high
alumina refractory aggregate by using low cost sillimanite sand and bauxite which is
economically important compared to the previously known process.
A further object of the present invention is to provide high alumina refractory aggregate from
sillimanite sand, alumina and impure bauxite without effecting hot properties of the aggregate.
SUMMARY OF THE INVENTION
The present invention provides improved process for the production of dense high alumina
refractory aggregates from sillimanite sand. The process involves proper wet milling of
sillimanite, alumina and bauxite mixture and firing at the temperature range 1500 °C to 1650 °C.
Bauxite raw material (composition ranging between 10-12 wt% Ti02 and 3-5 wt% Fe203)
induces densification of the aggregate through the liquid phase sintering without affecting the hot
properties of the aggregate. The high alumina aggregate produced has almost zero porosity and
RULis 1640 °C.
Accordingly the present invention provides improved process for the production of dense high
alumina refractory aggregates from sillimanite sand, which comprises: attrition of 68 to 90 wt%
sillimanite sand; 10 to 20 wt% bauxite; 0 to 14 wt% alumina; in presence of water, for a period
of 2-12 hours to get milled slurry, the resultant milled slurry being dried at a temperature in the
range of 100 to 110 °C for a period of 24 hours, milled and dried powder mixed with an organic
binder (5 wt%).to get a pre-pressed powder, the pre-pressed powder subjected to uniaxial
pressing to obtain pellets, the said pellets dried at a temperature in the range of 100 to 110 °C for
a period of 24 hours, the dried pellets sintered at 1500 to 1650 °C for the period in the range of
10 to 24 hours, the sintered pellets allowed to cool naturally.
In an embodiment of the present invention, the sillimanite sand contains 59.32 wt% AI2O3.
In another_embodiment_of_the_present.invention,, commerciaLcalcined Al20-3_is_o£99.6 %_purity.__
In yet another embodiment of the present invention, the bauxite contains 69 wt% AI2O3.
In still another embodiment of the present invention, the milling is done in usual mill like
attrition mill, ball mill, vibro mill etc.
In still yet another embodiment of the present invention, the organic binder like poly vinyl
alcohol, carboxymethyl cellulose, dextrin is used.
In a further embodiment of the present invention, the solid: solvent used is 1: 1
In a still further embodiment of the present invention, the uniaxial pressing of pre- pressed
powder is done at 1200 Kg/cm2
NOVELTY & NON-OBVIOUS INVENTIVE STEPS
The novelty of the present invention is the manufacture of dense high alumina refractory
aggregate by cost efficient method from sillimanite beach sand for the application at 1600 °C and
above. Another novel features is use of natural impure raw material as an additive, so the process
is cost efficient. Aggregate produced in this process is industrially very important where the
temperature is 1600 °C or above.
The non-obvious inventive steps of the present invention with respect to prior art are given
below:
1. Cost efficient composition which consists of sillimanite beach sand, bauxite and alumina.
In this process chemical grade sintering aid is not added as well as the amount of calcined
AI2O3 is reduced. Therefore, the cost reduction in the overall process comes down in the
range of 15-17%.
2. Utilization of substantial amount of bauxite as sintering aid as well as AI2O3 source for
the production of high alumina refractory aggregate. Ti02 exist as impurities in bauxite
act as sintering additive. Ti02 and Fe203 are liquid forming sintering additives which
facilitates the densification through liquid phase sintering.
DETAIL DESCRIPTION OF THE INVENTION
Therefore, when easily available raw material bauxite used in appropriate proportion it act as
additive and help in densification of the aggregate without affecting the high temperature
properties. Thus this aggregate is suitable for the use above 1600°C
The complete step of the present invention comprises:
1. Mixing of sillimanite sand 68 to 90 wt% with 10 to 20 wt% bauxite and 0 to 14 wt%
AI2O3.
2. Milling of the mixture in a usual mill such as attrition mill, ball mill, vibro mill for a
period of 2 hour in a liquid medium.like water in the w_eight ratio of solut_e_: spjvent:: 1:1
to get milled slurry.
3. Drying of the resultant milled slurry at the temperature in the range of 100 to 110 °C to
obtain dry powder of collective composition.
4. Mixing the collective composition with an organic binder to get pre-pressed powder.
5. Uniaxial pressing of the pre-pressed powder in the pressure 1200 Kg/cm to get pellets.
6. The pellets dried at a temperature in the range 100 to 110 °C for 24 hours.
7. The pellets fired at the temperature range 1500 to 1650 °C for a period 10 to 24 hours.
8. Sintered pellets are allowed to cool naturally.
Characterization of the sintered high alumina aggregate was done by determining the properties
like 1) bulk density and apparent porosity 2) phase identification by X-ray diffraction 3)
refractoriness under load. Bulk density and apparent porosity was measured by liquid
displacement method utilizing Archimedes principle in water medium.
The prior art revels that in presence of Ti02 and ZrC>2 additive production of dense high alumina
refractory aggregate at relatively low temperature. Again the additive used in the above process
is synthetic chemical. Therefore use of synthetic additive which is used for densification is not
cost effective process.
EXAMPLES
The following examples are given by way of illustration of the working of the invention in actual
practice and should not be construed to limit the scope of the present invention in any way.
EXAMPLE-1
Sillimanite beach sand of 80 wt% was mixed with 20 wt% bauxite (-60 BS mesh) and the
resultant mixture was attrition milled in water dispersing medium for 2 hours. The slurry
produced was dried 110±5 °C for 24 hours and cooled at room temperature. Then it was crushed
to break the aggregate and fine powder produced. 5 wt% PVA was mixed with the powder and
pressed uni-axiaJJy at a pressure 1200 Kg/cm2. The pressed compacts were first air dried for 24
hours and subsequently oven dried for 24 hours. The dried pressed compacts were fired at 1600
°C with a two hours soaking period. Total firing time was 10 hours. The apparent porosity of the
sintered aggregates was almost zero and the bulk density was 2.95 gm/cc. The RUL value of the
refractory produced from this aggregate was above 1600 °C.
EXAMPLE-2
Sillimanite beach sand of 68 wt% was mixed with 17 wt% bauxite (-60 BS mesh), and 15 wt%
calcined AI2O3, Then the resultant mixture was attrition milled in water dispersing medium for 2
hours. The slurry produced was dried at 110±5 °C for 24 hours and cooled at room temperature.
Then it was crushed to break the aggregate and fine powder produced. 5 wt% PVA was mixed
with the powder and pressed uni-axially at a pressure 1200 Kg/cm2. The pressed compacts are
first air dried for 24 hours and subsequently oven dried for 24 hours. The dried pressed compacts
were fired at 1600 °C with a two hours soaking period. Total firing time was 10 hours. The
apparent porosity of the sintered aggregates was almost zero and the bulk density was 2.95
gm/cc. The RUL value of the aggregate was above 1640 °C.
EXAMPLE-3
Sillimanite beach sand of 90 wt% was mixed with 10 wt% bauxite (-60 BS mesh) and the
resultant mixture was vibro milled in water dispersing medium for 12 hours. The slurry produced
was dried 110±5 °C for 24 hours and cooled at room temperature. Then it was crushed to break
the aggregate and fine powder produced. 5 wt% PVA was mixed with the powder and pressed
0
uni-axially at a pressure 1200 Kg/cm . The pressed compacts were first air dried for 24 hours and
subsequently oven dried for 24 hours. The dried pressed compacts were fired at 1650 °C with a
two hours soaking period. Total firing time was 10 hours. The apparent porosity of the sintered
aggregate was almost zero and the bulk density was 2.92 gm/cc. The RUL value of the refractory
produced from this aggregates was above 1600 °C.
From the above examples it is clear that the properties of high alumina aggregates of the current
invention are excellent without using synthetic additive. Ti02 and Fe2C>3 present in the bauxite
act as an additive and help the densification of the aggregate.
ADVANTAGES OF THE INVENTION
The main advantages of the present invention are:
1. Manufacture of dense high alumina aggregate in the AI2O3 range of 60 to 70 % for the
application above 1600 °C.
2. In this invention instead of adding synthetic additive, bauxite is used which supply
necessary additive (Ti02, Fe203)3 help in sintering and densification without detoriating
the hot property,
3. Instead of isostatic pressing single stage uni-axial pressing is done which is sufficient to
get the desired properties.
4. Due to use bauxite as sintering additive instead of synthetic additive (Ti02) the current
-inv-ention-is-highly^c.QSt_effective.
5. In general 27 wt% of alumina needed to convert sillimanite beach sand to mullite. But in
presence of 20 wt% bauxite only 14 wt% of calcined alumina is needed to convert in
mullite. Which is also cut the price of production of high alumina aggregate.

We claim
1. An improved process for the production of dense high alumina refractory aggregates
from sillimanite sand, which comprises : attrition/vibro milling of 68 to 90 wt%
sillimanite sand; 10 to 20 wt% bauxite; 0 to 14 wt% alumina; in presence of water, for a
period of 2-12 hours to get milled slurry, the resultant milled slurry being dried at a
temperature in the range of 100 to 110 °C for a period of 24 hours, milled and dried
powder mixed with an organic binder (5 wt%) to obtain a pre-pressed powder, the prepressed
powder subjected to uniaxial pressing to obtain pellets, the said pellets dried at a
temperature in the range of 100 to 110 °C for a period of 24 hours, the dried pellets
sintered from 1500 to 1650 °C for the period of 10 to 24 hours, the sintered pellets
allowed to cool naturally.
2. An improved process as claimed in 1? wherein the sillimanite sand contains 59.32 wt%
A1203.
3. An improved process as claimed in 1-2, wherein calcined AI2O3 is of commercial variety
with 99.6 % purity.
4. An improved process as claimed in 1-3, wherein the bauxite contains 69 wt% AI2O3.
5. An improved process as claimed in 1-4, wherein the milling is done in usual mill such as
attrition mill, ball mill, vibro-mill.
6. An improved process as claimed in 1-5, wherein the organic binders like poly vinyl
alcohol, carboxymethyl cellulose, dextrin are used.
7. An improved process as claimed in 1-6, wherein the weight ratio of solid : solvent used
is 1: 1
8. An improved process as claimed in 1-7, wherein the uniaxial pressing of pre-pressed
powder is done at }200Kg/cm2