FILED OF THE INVENTION
The present invention relates to a method of producing a ceramic filter for
filtration of water on developing an alumina composition which is fired at low
temperature in the range of 1200 - 1250° C resulting in fine porous structures,
which are used as filters.
BACKGROUND OF THE INVENTION
The necessity to fulfill the societal requirements for high quality drinking water is
the driving force for the development of novel and efficient methods for
the purification of water. Activated carbon, the traditional way of purifying
water has been useful for treatment of water for undesirable tastes and
odors. However, despite its relatively broad range of effectiveness in

absorbing organic substances from aqueous solutions, it can no longer be viewed
as a water treatment panacea. Thus, there is an emerging need of industry for
the development of improved, alternative purification systems viz. ceramic
membrane system.
Ceramic membrane tubes are most preferred than other membrane materials
because of many advantages. Most important of them are:
a) long and reliable lifetime
b) high stability in organic media c) rigid with no deformation

d) stable over a wide PH range
e) corrosion and abrasion resistant
f) insensitive to bacteria action
g) can be repeatedly sterilized by steam or chemicals
h) greater void area per unit area of filtration surface
i) capable of regeneration after fouling
j) membrane surface-bonded to substrate by strong ceramic bonds
k) can process highly viscous fluids

Ceramic membranes normally have an asymmetrical structure composed of at
least two, mostly three, different porosity levels. Indeed, before applying the
active, microporous top layer, a mesoporous intermediate layer is often applied
in order to reduce the surface roughness. The macroporous support ensures the
mechanical resistance of the nanofilter. The ceramic membranes are often
formed into an asymmetric, multi-channel element. These elements are grouped
together in housings, and these membrane modules can withstand high
temperatures extreme acidity or alkalinity and high operating pressures, making
them suitable for many applications where polymeric and other inorganic
membranes cannot be used. Several membrane pore sizes are available to suit
specific filtration needs covering the microftltration, the ultrafiltration, and
nanofiltration ranges (from 5 µm down to 1000 Daltons).
The supports for the ceramic membrane elements are made from alpha
aluminium oxide or silicon carbide with open pores. This material can provide
not only maximum permeability but can also fulfill high requirements relating to
mechanical stability. These supports are either for a single channel or a multi-

channel design. A membrane layer of a defined texture only a few um thick is
applied to the inner side of the channels in a sandwich-type process and
connected monolithically.
The alumina ceramic support tubes are generally extruded to multi channel form
with a pore former and sintered at high temperature of > 1500° C. The high
temperature processing is not cost effective for the production of support tubes.
Therefore, a long standing need of industry was felt to develop a suitable
alumina composition, which can be sintered at a lower temperature in the range
of 1200° - 1250 C without significantly altering the properties of the support
tubes for microfiltration applications.
The present invention is aimed to solve the above prior art difficulties.

DESCRIPTION OF THE INVENTION
One of the object of the present invention is to provide a composition having
alumina as the major element with a pore former as Graphite which can be
sintered at a temperature range of 1200 - 1250°C resulting in strong and fine
porous material.
Another object of the invention is to develop a process for fabricating multi
channel alumina porous tubes in the range of 1.2 m long suitable for
microfiltration.
A further object of the present invention is to test the porous alumina support
tubes for microfiltration and decide on the suitability of its use for the intended
application.
Yet another objective of the invention is to monitor bending strength and pore
size distribution of the formed material.

A still further objective of the invention is to study extrudability of the product
with the pore former and straightness of the product during drying to ensure
optimization of the properties of the product.
PRIOR STATE OF THE ART
Tatsuki Ohji and Zhen-Yan Deng Aichi in United States Patent No. 6565825 and
in Porous alumina fabrication procedures (2003) have reported a method of
producing an alumina porous material using a mixed powder of alumina powder
and aluminium hydroxide represented by the chemical formula Al (OH)3 at
different percentages as the starting material, comprising the steps of heating
this mixed powder to decompose the aluminium hydroxide and further heat
treating it within a temperature range of 1000 to 1600 deg C. The authors
reported that the alumina porous material produced by the above mentioned
method processed a porosity exceeding 40 vol. % and specific surface area of 8
to 40 m2 / g. This method however used a very fine and pure powder which is
costly and difficult to scale up and the said prior art is not related to the
technology field of approach of the present invention.

To approach the objects, and purpose of present invention, a method has been
developed which allows sintering of an alumina composition at a low
temperature range of 1200 -1250 deg C with ~ 40 % apparent porosity with
Graphite as the pore former. The modified process enables the use of low
temperature glass composition to bind the alumina particles together when fired
at a substantially lower temperature. Further, the improved method uses single
firing step in contrast to the double firing steps used in the earlier invention. The
new improved process will thus be beneficial for commercialization because of
drastic reduction in the cost of the process by reducing the sintering temperature
by ~ 300 deg. C and reducing one firing step. Further, an extrusion process is
developed to extrude multi channel alumina tubes and tested for the water
permeability after characterizing the basic properties. The new method is
suitable for reducing the pore size drastically by glass penetration in the pores
created by burning the pore formers. The developed cost effective method has
the potential of a commercial success in this field of the ceramic membrane
product development.

According to the invention there is provided a method of producing a ceramic
filter for filtration of fluids from a mixture of powder in wt % of alumina - 70 -
80, graphite - 15 - 20, optimized mixture of clay materials as flux (felcite based"
composition) - 0.5 - 1.5 and additives (e.g. clays, magnesia, alkali stearate etc.)
of different percentages - 5 - 7, comprising the steps of dry ball miIIing the said
constituents using alumina (grinding media for more than 30 hours) wet mixing
the grinded well mixed homogenous dry powder in a zigma mixture to form a
dough, extruding the dough in a preformed die to form the shaped filter
preferably as a multi-channeled tubular structure, subjecting the formed filter to
controlled drying in combination of natural drying and oven drying, firing in one
step in a gas fired Kiln at a temperature in the range of 1200 - 1250° C obtaining
a porous filter on sintering of the formed filter material having its porosity
exceeding 40 volume %, bulk density in the range of 2.0 - 2.10 g/cc and three
point bending strength in the range of 65 - 75 MPa.
The alumina porous material produced from the method has a porous structure
having major pore distribution in the range of 10 to 10, 000 nm and the median
pore diameter is in the range of 250 to 300 nanometer. This has been confirmed
both by the results of Mercury porosimetry and by scanning electorn microscopy.

The alumina porous material as produced has phase composition confirmed as
alpha alumina.
A filter as produced is tested for water quality test using a mobile filtration
module and confirmed the water flow through the multichanneled tubes in the
range of 0.2 -0.25 jiters per min at an inlet water pressure of 3 bars.
The present invention will be better understood by the narrated description for
an embodiment below with reference to the accompanying drawings in which
Figure 1 represents the microstructure of the sintered product
as revealed by scanning electron microscope (SEM)
Figure 2 represent the pore size distribution in the sintered
porous tubes as revealed by Mercury Porosimetry Test.
Figure 3 represents the x-ray diffraction pattern
confirming alumina as the single phase.

Figure 4 represents the multi-channeled porous alumina
tubes developed from the new composition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be explained in further detail:
The method of producing an alumina porous material related in the present
invention is characterized in that a mixed powder of alumina and graphite is used
as the starting material along with certain propriety additives and this is heated
to a temperature sufficient to form a glassy phase to bind the alumina particles
together without complete densification resulting in a fine porous structure
having porosity exceeding 40 volume % and sufficient strength in the range of
65 - 75 MPa to use in practical application.
The additives comprising of suitable mixture of clay, traditional flux and other
minor additives like magnesia were mixed with the alumina graphite to form a
batch.

The graphite particles act as pore formers and on initial heating stage the
graphites burn leaving the pores inside the alumina body. The additives used in
the present invention form a glassy phase and binds the alumina particles
together. This avoids the complete densification of alumina which otherwise
requires a very high temperature in the range of 1500 - 1600 deg C. The glassy
phase also flow inside the pores and thus reduces the pore size resulting in very
fine pores in the range of 250 - 300 nanometer as observed in the present
invention.
Moreover, during studying various factors pertaining to the development of
composition for firing at low temperature, it was suddenly discovered that
porosity, pore diameter and bending strength can be controlled by adding the
required ratio of the raw materials for forming the glassy phase for binding the
alumina particles.
Thus, it is possible to obtain an alumina porous material that has high porosity
and bending strength and a narrow micropore diameter distribution by the
method described in the present invention.

When the method of the present invention is explained in further detail, the
ceramic porous material by the present invention is basically made by using
commercial alumina powders and mixing graphite with low melting temperature
glass composition (flux) as an additive, molding the product to form multi-
channeled alumina tubes, drying and sintering at a temperature range of 1200 -
1250 deg C. In this case, it is preferred that the temperature be low so as to
achieve better surface area and incomplete densification of alumina. It is
preferred that the sintering temperature be ~ 1210 - 1230 deg C in order to
obtain sufficient strength. However, when the sintering temperature exceeded
1250 deg C, there is a reduction in porosity and when the sintering temperature
is lowered than 1210 deg C, there is not sufficient strength for practical
application. Two grades of alumina powders were used in this study. The finer
alumina had an average particle size of <0.5 micron and that of the second
grade was of the order of 1.0 - 1.5 micron. The ratio of the coarse of fine
alumina was kept at 2:1. Both the alumina powder were of purity exceeding
99.5 % and the ratio of alumina to graphite is kept in the range of 4 to 6: 1 to
1.5.
Moreover, the graphite powder with a mean particle size of 10 microns or smaller
is used in order to make pore distribution uniform as observed in Figure 2. The
mixing and grinding of the starting materials has been done by dry method.
Dough is prepared by wet mixing in zlgma mixture.

By means of the method of producing an alumina ceramic material of the
present invention, the additive contents is varied in the range of 2 - 12 wt % in
the alumina - graphite mix, then ball milled in dry condition and homogenously
mixed for effectiveness. The paste is prepared by wet mixing of grinded
materials using zigma mixture. The paste extrusion method was used to form
the multi-channeled alumina porous tubes. The alkali stearate in the range of
0.2-0.8 % was used as a plasticizer for extrusion. The number of channels can
vary from 1 to 20.
The characteristic features of the alumina porous material produced by the
method of the present invention are listed below:
Porosity: In the range of 30-50%
Pore distribution: At least one pore distribution peak within the range of 10 to
1,000 nanometer and distribution of pores with 1000 nm or more as observed in
Figure 3.
Phase composition: Alpha alumina
Strength (Three point bend): 25 - 140 MPa in the porosity range 30 - 50 %,

The alumina porous sintered compact of the present invention is useful as filter
using the above mentioned features.
In the porous material in the present invention, it is preferred that fine alumina
particles be distributed uniformly throughout the molded article of the mixed
powder, which has been confirmed from the scanning electron micrographs
presented in Figure 1.
The main advantages of the new and improved processes are:
a) The improved method reduce the firing temperature in the range of 200 -
300 deg C than the conventionally used for this product.
b) The present method can be processed by single step firing.
c) The present method satisfies all the characteristics as a product for
ceramic membrane applications using a cost-effective processing route.

d) The present method is useful in avoiding coating process for use of these
products directly for ultrafiltration applications.
The invention as narrated herein with an embodiment and illustrated with test
results and microstructure should not be read and construed in a restrictive
manner as various adaptations, changes and modifications are possible within
the limit and scope of the invention as defined and encompassed in the
appended claims.

WE CLAIM:
1. A method of producing an alumina based multichannel ceramic
filter upto 1.2 m length processed at low temperature for filtration
of water having its porosity in the range of 38-43 volume % and
median pore size in the range of 250-300 nm, bulk density in the
range of 2.0-2.10 g/cc and three point bending strength in the
range of 65-75 MPa, the method comprising the steps of subjecting
a mixture of 70-80 wt% of alumina, 15 to 20 wt% graphite as pore
former, 0.5 to 1.5 wt% optimized mixture of clay materials as flux
and 5-7 wt% of additives (e.g. clays, magnesia etc.), to dry ball
milling using alumina grinding media to obtain a ground
homogeneous powder of required characteristics:
- wet mixing the homogeneous powder to form a dough;
- extruding the dough in a reformed die to form the shaped filter
preferably as a multi-channeled tubular structure of 1.2 m long;
-subjecting the formed filter to controlled drying in combination of
natural drying and oven drying to obtain the dried filter;
-firing the dried filter at a temperature in the range of 1200-
1250°C to obtain a porous filter.

2. The method as claimed in claim 1, wherein the alumina powder
comprises a mixture of two grades of average particle size i.e. fine
alumina and coarse alumina of <0.5 micron and of 1.0-1.5 microns
respectively and the ratio of coarse to fine alumina powders are
maintained as 2:1, both of the grades having purity greater than
99.5%.
3. The method as claimed in claim 1, wherein the additives are
powdered and then added to alumina - graphite mix in the ratio of
4 to 6:1 to 1.5.
4. The method as claimed in claim 1, wherein the mean particle size
of the graphite powder is ≤ 10 microns.
5. The method as claimed in the preceding claims, wherein additive
content in the mixture is varied in the range of 2-12 wt% in the
alumina-graphite mix.
6.The method as claimed in the preceding claims, wherein during
extrusion of the dough from the mixer, alkali stearate in the range
of 0.2-0.8 wt% is used as a plasticizer.
7.The ceramic alumina porus material prepared by the method as
claimed in claim 1, wherein the pore distribution of the filter
material is in the range of 10 to 10,000nm and the median pore
diameter is in the range of 250 to 300 nanometer.

8. The ceramic alumina porus material as claimed in claim 7, wherein
the porous material has an alpha phase with micro pores resulting
from the graphite former.
9. A tube filter produced by the method as claimed in claim 1,
wherein water flows through the multichanneled tubes in the range
of 0.20-0.25 liters per minute at an inlet water pressure of 3 bars.

This invention relates to a method of producing an alumina based
multichannel ceramic filter upto 1.2 m length processed at low
temperature for filtration of water having its porosity in the range of
38-43 volume % and median pore size in the range of 250-300 nm,
bulk density in the range of 2.0-2.10 g/cc and three point bending
strength in the range of 65-75 MPa, the method comprising the steps
of subjecting a mixture of 70-80 wt% of alumina, 15 to 20 wt%
graphite as pore former, 0.5 to 1.5 wt% optimized mixture of clay
materials as flux and 5-7 wt% of additives (e.g. clays, magnesia etc.),
to dry ball milling using alumina grinding media to obtain a ground
homogeneous powder of required characteristics: wet mixing the
homogeneous powder to form a dough; extruding the dough in a
reformed die to form the shaped filter preferably as a multi-channeled
tubular structure of 1.2 m long; subjecting the formed filter to
controlled drying in combination of natural drying and oven drying to
obtain the dried filter; firing the dried filter at a temperature in the
range of 1200-1250°C to obtain a porous filter.