The present invention is related to the field of fabrication of micro-porous
ceramic membranes, which can be adapted for variety of purposes/applications
in particular to the filtration/separation technologies. More particularly, the
present invention relates to a methodology for fabricating disk/pellet type
alumina ceramic membranes of consistently uniform pore size and its distribution
with superior mechanical strength and surface finish through maintenance in the
uniformity of thickness of the membrane, all of the said can further be tailored to
give rise alumina membranes with predetermined levels of pore size in the range
of 0.3 to 0.6 micrometer. The present method also provides a guideline to
fabricate alumina ceramic membranes in the entire spectrum of micro-filtration
besides the defined range of 0.3 to 0.6 micron.
BACKGROUND OF THE INVENTION
Membrane based processes are known for numerous applications, each being
different in their mode and with separation characteristics. Pressure driven
filtration processes, e.g. micro-, ultra- and nano-filtration, reverse osmosis;
Concentration driven processes, e.g. gas separation, pervaporation, dialysis;
Temperature driven processes, e.g. membrane distillation; Electrically driven
processes, e.g. electrodialysis. It also can be used as a support for
heterogeneous catalysis reactions and photo-catalysis reactions and processing
involving adsorption or absorption reactions. The produced ceramic membranes
can further be used as a support material for the fabrication of ultra-filtration,
nano-filtration, reverse osmosis membranes. The main criterion considered in
membrane filtration/separation process is the ability of the membrane to

permeate selectively. Indeed, approximately 60 - 75% of synthetic polymer
membranes are today employed as semi-permeable barrier layers. Organic
Polymeric membranes have numerous advantages like flexibility to fabricate in
the form of tubes and sheets. However they have great limitations like, they are
susceptible to biodegradation (organic ones), have relatively short shelf and
operating life, less resistant to organic solvents, chlorines, and extreme pH
conditions, relatively less mechanical strength.
In order to overcome the above-mentioned disadvantages, there is a need to
develop new membrane materials, which can overcome the disadvantages of the
polymeric/organic membranes, and at the same time, it should provide similar
advantages comparable to the polymer/organic membranes.
During the past few decades, ceramic membranes have been receiving greater
attention because of their advantages over polymeric /organic and metallic
membranes. Compared to polymeric based membranes, ceramic membranes
exhibit unquestionable advantages, essentially due to their inherent properties.
They can withstand high temperatures, high pressure (> 100 bar), abrasion, and
chemical attack, which makes them a very good materials for applications in
harsh and extreme environments. Recent studies have showed that ceramic
membranes exhibit high resistance against microbiological attack. Generally, they
are stable up to 1000°C or more under pressure which enable them for high-
temperature applications and thereby permitting sterilization operation in
biochemical applications. Ceramic membranes can well tolerate chemical /
mechanical cleaning, can readily accommodate the abrasion encountered in
slurries, and resist the build up of high pressure (up to 30 atm) often used in
back flushing techniques. Ceramic membranes have excellent reliability and long
operating life. There is no structural deformation even under a wide range of

operating conditions. Moreoever, the well-developed production technology
provides membranes with high permeability and desired pore size. Due to the
stringent operating conditions (wide pH range, repeated vapor sterilization,
organic solvents, and temperatures up to 500°C) several industries have thought
to immensely benefit from ceramic membranes in the field of agro-foodstuffs
industries, biotechnology, biomedicine, paper industries, textile, and petroleum
industries. Some membranes, which have been commercialized recently
haveunequivocally longer life under harsh industrial conditions and their
industrial use have shown a good degree of reliability in several applications. In
some cases, the lifespan of membranes is greater than five years and have also
significantly decreased the cost of maintenance in industrial facilities.
However, even though the basic principles and methodology of ceramic
membrane processing have already been known, tailoring/regulating pore size
and the distribution of pore size, consistency in the uniformity in thickness of the
membranes are still a major task to accomplish. Besides, improvement in
mechanical strength and good surface finish despite allowing the membrane to
possess a high porosity level is also a major task.
The present invention aims to provide a fabrication method to obtain
microporous alumina membranes with above-mentioned characteristics.

PRIOR ART REFERENCE
There are many prior arts reported in the preparation of ceramic membranes.
Hay et al. in U.S. Pat. No. 4,968,426 has described the procedure of preparation
of strong and durable alpha phase alumina ultrafitration membranes by seeding
boehmite sols, and further coating on the support with a thin layer of gel to
obtain a final pore size of less than about 500µm. The preparation of membranes
by sol gel technology as disclosed by Mori et al. in U.S. Pat. No. 4,770,908
describes a procedure of membrane preparation by disposing a layer of alumina
sol on a porous membrane formed by hydroly2ing an alcoholate or a chelate of
alumina followed by drying and burning thereof. Anderson, Marc A et al.
conducted significant work on preparation of membrane by various procedures
reported in EP No.0537944B1, U.S. Pat. No. 5.006,248, U.S. Pat. No. 5.096745,
U.S. Pat. No. 5,104,539, U.S. Pat. No. 5,169,576, U.S. Pat No.5,194,200, U.S.
Pat. No. 5,208,190, U.S. Pat. No. 5,215,943, U.S. Pat No. 5,610,109, U.S. Pat.
No. 5,712,037, and was able to create a stable, transparent metal oxide ceramic
membrane, particularly alumina, titanium, and silica based with a narrow pore
size distribution and with pore diameter manipulable in the range of 5 - 100 Å.
Ail those inventions were based on preparation of membranes using sol-gel or
improved sol-gel techniques using different precursors. The same group also
invented the procedure (U.S. Pat. No.5,268,101) of preparing combined alumina
and silica membranes having high surface area, small pore size with high
temperature stability. Various other patent literatures like Van T Veen et al. U.S.
Pat.No. 5,089,299, Maier US Pat. No. 5,250,184, Nishio et al. U.S. Pat. No.
5,250,242, de Jong et al. U.S. Pat. No. 5,407,703, Chen et al. U.S. Pat.

No. 6,165,553 have described the method of creating membranes suing sol-gel
techniques. Liu et ai. in U.S. Pat. No. 5,645,891 has described the method of
preparation of mesoporous ceramic membranes by inserting a substrate into the
reaction chamber at pressure and thereby allowing the deposition of nucleates
on the substrate leading to the formation of a membrane layer therein. Yet in the
same patent, it is reported one more method of preparation of ceramic
membranes by placing a substrate between two solutions permitting the
formation of membrane on the surface by or within the pores of the porous
substrate. Webster et al. in U.S. Pat. No. 5,269,926 has disclosed the creation
of supported microporous membranes by placing a porous support on one side in
a colloidal suspension and drying the other side by exposing it to the drying
stream of air or a reactive gas stream so that the particle are deposited on the
drying side as membrane. Shimai et al. in U.S. Pat No. 5,405,529 has described
yet another method of preparing ceramic filter consisting of bone like structure
using ceramic-slurring foaming techniques. Bartton et al. in U.S. Pat. No.
5,935,440 has disclosed a process for treating memebrane comprising a film of
crystalline ceramic zeo-type material which process comprises treating the
membrane with a silicic acid and or a polysilicic acid or a mixture of the both.
Takahashi et al. in U.S. Pat. No. 6,007,800 has described a method of
preparation of ceramic porous membrane and ceramic filter by the deposition of
titania from titania slurry (1-70% by weight) on the surface on the porous
substrate followed by thermally treating the membrane at 100-300°C in an
aqueous vapor phase environment. Herrmann et al. in U.S. Pat. No. 6,309,546
discloses various examples of preparing the micro and ultra pore membrane with
controlled pore size like, single membrane layer by immersing the substrate in a

solution of desired metal (dip coating) followed by sintering to remove organic
binder (double dip coating), two layer graded membrane by dip coating, tape
casting, screen printing, and finally two layer hybrid membrane on a metallic
substrate
via spin coating. Fain, Sr, et al. in U.S. Pat. No. 6,649,255 reports an unexpected
invention, a process for controlling the ultimate pore size of a fine-pored
inorganic membrane, particularly chosen from the group of alumina, zirconia,
titania, silica alumina/silica mixtures, can be achieved by depositing one
monolayer at a time of an inorganic compound where the layer thickness
approximately equals to the size of the molecule, such as metal oxide, metal
nitride or the pore walls of the inorganic membrane followed subsequently by
drying the membrane. Piner et al. in U.S. Pat. No. 6,528,214 presents yet
another method of preparing inorganic membranes by suspending the mixture of
fine and coarse particles in a liquid to form a slurry followed by poring the thus
prepared slurry into a mould such as one made up of 'Piaster of Paris' thereby
obtaining green intermediate product and finally baking, firing or sintering the
green intermediate so as to obtain a finished membranes having a density
distribution of fine particles that increases in one direction (surface in use) across
the finished membrane and a density distribution of the coarse particles that
decreases in the same direction across the finished membrane. The method of
preparation of ceramic filters or membranes by sol-gel, tape casting methods is
believed to suffer from the disadvantage like higher chances for cracks formation
on the membrane surface during the binder burn-off and sintering due to large
shrinkage ratio between the film and the substrate. Presence of even small
cracks, crevices can have a remarkable deleterious effect on the performance of
membranes and can render them substantially little value in many operations.
This is because in many separation operations the effect of defects is essentially
to provide a channel where the un-separated products can pass through.

Though some existing methods of membrane preparation like slurry suspension,
sol-gel, tape casting claim that a defect free membrane is obtained on a
laboratory scale, but attempts to provide a substantially defect free membrane
on a larger scale production have proved to be unsuccessful.
In the background of aforesaid disadvantages of prior art, the present invention
has devised a novel fabrication procedure for the preparation of ceramic
membranes, in particular alpha phase of alumina ceramic having a consistent
and uniform pore size, uniform pore distribution, uniform thickness appended
with superior mechanical strength and surface finish characteristics.
OBJECTIVES OF THE INVENTION:
Hence, it is the objective of the present invention to provide a fabrication
procedure to produce microporous aluminum oxide (alumina) ceramic
membranes in the form of disks/pellets with consistently uniform pore size,
uniform pore distribution and uniform thickness with superior mechanical
strength with surface finish characteristics by varying the same base
composition.
One of the objective of the present invention is to provide a method for
fabrication of ceramic membranes by routes other than sol-gel route, tape
casting method, solution or colloidal route, isostatic pressing.
Another objective of the present invention is to fabricate ceramic membranes, in
disk/pellet forms using alpha phase of aluminum oxide (alumina) in an efficient
and predictable manner.

Still another objective of the present invention is to fabricate alumina disk/pellet
membranes with controlled and uniform pore size and its distribution along with
uniform thickness of the membrane.
Still further objective of the proposed invention is to provide a fabrication
method for generating reproducible and reliable alumina ceramic disk/pellet
membranes for its applications in various filtration/separation processes.
Yet another objective of the present invention is to define various process
parameters for producing the above alumina membranes, which are very
significant for the continuous and beneficial production of the process.
Yet further objective of the present invention is to provide alumina membrane
substrates (supports) for generating ultra-filtration, nano-filtration, hyper-
filtration, reverse osmosis membranes by applying further ceramic/polymer
layer/s on the support structure thereby generating the said membranes, which
has great applications in the other separation/purification technologies.
Other objects, novel features, advantages, and applications of the present
invention will be better understood in the description that follows.
According to the invention there is provided a method of producing 'micro-porous
ceramic membranes' of disk/pellet type for filtration/separation
processes/systems comprising the sequential steps of

a) mixing/grinding of alumina powders of different sizes and other additives
like, pore former, binder, dispersant, stabilizers in a water solvent in a ball mixer
using different sizes of alumina balls followed by transferring batch mix in a
teflon/nylon or similar jars placed in a pot mill for thorough mixing of the batch
for a period of 24 to 48 hours to form a slurry,
b) drying the said slurry in ambient conditions on pouring the suspension
into 'plaster of paris'/clay moulds or similar and to allow the water to drip out
from the slurry yielding 'dry batch composition',
c) sieving the said dry batch composition using sieves of 100-200 mesh or
similar, to result a binder-mixed batch composition in granular form,
d) pressing the resultant granules into the form of green disks/pellets with
varied diameter and thickness in stainless steel die/s by processing in an uni-
axial pressing machine at varied pressure levels to obtain green disks/pellets,
e) drying the said green disks/pellets in an oven maintaining a temperature
in the range of 50-120 degC so as to get dry green disks/pellets,
f) heat treating the dry green disks/pellets in presence of air or similar
oxidizing atmosphere in a suitable kiln/furnace in the range of 1558°C. - 1580°C,
to get an intermediate membrane disks/pellets and
g) heat treating (sintering) the intermediate disks/pellets in presence of air
or similar oxidizing atmosphere in a suitable kiln at set temperature/s in the
range of 1550°C - 1580°C to result the micro-porous alumina disk/pellet
membrances followed by characterization and testing of the resulted micro-
porous alumina membranes

DETAILED DESCRIPTION OF THE INVENTION:
The present invention describes the fabrication of a disk/pellet type ceramic
membrane using alpha phase of alumina material that generates consistently
uniform pore diameter which is manipulable in the range of 0.3 to 0.6 µm. For
the said purpose, alumina powders with different particle size distribution but
similar physical and chemical properties along with other additive materials such
as binders, dispersants, deflocculant, sintering agent, pore former etc were used
as the starting raw materials. In general, the fabrication process starts with a
wet-mixing operation using predetermined quantities of the said raw materials
using water in a ball mill mixer using nylon pot or a similar kind of container for a
mixing period for about 24 to 48 hours.
The sequence of addition of the constituents within the raw materials has an
important effect on the characteristics of the membrane. As per the present
invention, alumina powders were first mixed with water. Then, the dispersant
sodium stearate was added to alumina powders to aid good dispersion
consistency in the mixture followed by addition of other organic additives, i.e.
polyvinyl alcohol, polyethylene glycol, ammonium polyarcylate, carboxymethyl
cellulose etc. The mixing is carried out using different sizes of alumina balls in a
ball mixer. The alumina balls are used in the present invention to avoid
entrainment in the batch composition. However other kinds of balls can be used
for mixing the above said raw materials by taking enough precaution to avoid
entrainment or contamination of the starting materials from the balls. The
aqueous suspension thus obtained after 24 to 48 hours of mixing is poured into a
gypsum mould to drain out the solvent (water).

The suspension was then dried under controlled conditions ambient or in an oven
or both or other means in order to get the dry batch. The drying temperature is
kept in the range of 60-100°C. Only water is eliminated during drying as the
organic additives that have been incorporated in the batch during processing
have decomposition temperature of higher than 600°C. The slurry is dried until
the moisture content is reduced to < 7 weight %. Drying of the slurry beyond 7
weight % moisture level results over-dried powder, which subsequently looses its
binding properties and hence makes the pressing of the powders difficult in the
subsequent steps, thereby leading formation of cracks and or pinholes during the
green stage. The resultant dry batch is further mixed either of the organic
binders, like polyethylene glycol (PEG), carboxymethyl cellulose (CMC), polyvinyl
alcohol (PVC) etc or similar, with appropriate quantity in the range of 1-5 weight
% using a dry mixer or similar so as to obtain a binder-mixed batch composition
that subsequently provides good strength to the uni-axially pressed green
disk/pellets. The binder-mixed batch composition is then sieved using a 200
mesh or similar so as to get the binder-mixed batch composition in granular
form. The resultant granular composition is then weighed (depending on the
thickness of the membrane to be fabricated) and poured in the stainless steel die
of desired dimensions depending on the diameter/dimension of the membrane to
be fabricated. The granular composition is then uniformly spread over the
surface of the die cavity and pressed in the uni-axial direction at definite levels of
pressure in order to obtain a predetermined geometrical shape of disk or pellet
type.

During uni-axial pressing, compaction occurs by crushing of the granules and
mechanical redistribution of the particles into a closed packed array. The
lubricant and binder usually aid in this redistribution and the binder provides
cohesion of the batch material. A compaction pressure in the range of 5000 -
5500 kPa was chosen to ensure breakdown of the granules and uniform
compaction during uni-axial pressing. After the above uni-axial pressing, the
disks/pellets those obtained are conventionally called as the green disks/pellets.
The green disks/pellets need to be carefully dislodged form the die using a
suitable scrapping mechanism. These green disks/pellets are dried in an oven in
the temperature range of 50-100°C until the moisture content of the body is
reduced to