Note: Descriptions are shown in the official language in which they were submitted.
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Back~round_of t~e Invention
Field of ~he Invention
The in~ntion rélates to ultrafiltration membranes.
More particularly, it relates to the production of dry,
crack-free membranes having enhanced stability.
Description of the Prior Art
The production and use of inorganic, semipermeable
membranes for ultrafiltration purposes are well known in the
art. Most such inorganic membranes are vf advantage in
their resistance to temperature and solvent effects. In
some instances, the membranes also possess molecular perm-
selectivity and ion-exchange properties. Berger, US 3,497,394,
thus disclosed an ion exchange membrane made by forcing a
metal oxide gel into a porous support.
In the practical application of ultrafiltration
membranes, high flux is an essential feature, and it has
been found desirable to have a highly porous support and a
thin, fine membrane. Colloidal particles thus should not
be imbedded in depth into the filter body. In a dry, in-
organlc, semipermeable filter disclosed in French patent
No. 1,440,105, however, fine colloidal particles are said
to create a thin membrane in the surface of a porous com-
bined membrane - support formed from a suspension of coarse
and eolloidal size particles of -A1203 by slip casting
in a plaster mold.
Ultrafiltration membranes should also have good
mechanical and chemical stability for use in practical
commercial applications. Dehydration of ordinary particulate
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membranes always result, however, in "mud cracks" that ruin
the membranes' semipermeable characteristics. In the pre~
paration of a support surface, the ormation of cracks can,
of course, be tolerated. In the Thomas patent, US 3,926,799,
for example, a membrane support is made by coating a zirconia
slurry onto a porous substrate, followed by drying and fir-
ing the resulting composite at high temperature to form a
rugged precoat. Lar~e pores or cracks would be expected to
form in this process and would be unacceptable in the form-
ation of the membrane itself. Ultrafiltration membranes
susceptible to such crack formation must be maintained wet
at all times. Such membranes include those taught by the
Trulson et. al. patent, US 3,977, 967, which discloses
hollow tubular members having a well defined porosity and a
substantially uniform, continuous, adherent, porous coating
of preformed, aggregated inorganic metal sxide particles
deposlted thereon through permeation means. The cohesiveness
of membranes of this type is due to relatlvely weak physical
forces, and the cohesive forces between the particles, and
the stability of the particulate membrane, would be ~nhanced
by the dehydration o the metal oxide particles. Such
dehydration is precluded, however, by the necessity for
maintalning the membranes wet at all times to avoid the
formation of cracks that would destroy the semipermeable
characteristics of the membrane.
Enhanced mechanical and chemical stability are,
of course, desired characteristics for inorganic ultra-
filtration membranes In addition,' enhanced flexibility
would be achieyed by the development of ultrafiltration
membranes that need not be maintained wet at all
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times. Thus, the handling, transport and storage of mem-
branes would be facilitated by the elimination of this
require~ent.
It is an object of the invention, there~ore, to
provide an improved ultrafiltration membrane.
It is another object of the invention to provide
a process for t~e production of a dry, crack-free, inorganic
ultrafiltration membrane.
It is another obJec~ of the invention to provide
a crack-free, mechanically and chemically stable membrane.
It is a further object of the invention to provide
a stable, crack-free, dry ultrafiltration membrane having
good perselectivity and flux.
With these and other objects in mind, the inven^
tion is hereinafter described in detail, the novel features
thereof being particularly pointed out in the appended
claims.
Summa~y of the Invention
The objects of the invention are accomplished by
~he coating of a microporous suppor~ with an inorganic
membrane coating material in the pre.sence of a volatile
liquid capable of drawing the coating material into the
suppor~ and desolvating said coating. The desolvation of
the coating, prior to the complete removal of the volatile
liquid, results in a shrinking of the coating and the
consequent filling of voids regulting from such shrinkage
by the coating material. As a result, the development of
cracks during desolvation of the coated membrane is
avoided. The membrane thus produced is a dry, essentially
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crack-free inorganic membrane of enhanced mechanical and
~hemical stability. The membrane support may be pretreated
with the volatile liquid prior to application of the coat-
ing or the coating material, preferably zirconia, can be
dispersed in a suitable volatile liquid to form a suspen-
sion that is coated onto the membrane support. Following
removal of the volatile liquid from the treated mem~rane,
as by air drying, firing to a desolvating temperature
further enhances the stability of the membrane.
Detailed Descr ption_of the Inven~ion
The ultrafiltration membrane of the invention
comprises a crack-free, dry, inorganic coating anchored
to a microporous support. The membrane has desirable
mechanical and chemical stability, exhibiting good physical
coherence, resistance to acid, alkali, soap and deter~ent
washing and the ability to withstand ultrasonic stress and
abrasion. Unlike prior art membranes that develop cracks
tending to destroy their semipermeable characteristics
when dry, the membrances of the invention remains essentially
crack-free upon drying with their physical stability
actually being improved by drying and firing.
In the process of the invention, a volatile liquid
medium miscible with the membrane coating suspension is
employed to draw the coating material into the membrane
support and to desolvate said coating material. As a resul~
of such desolvation, a shrinking of the coating material
occurs, accompanied by a consequent filling of the voids
produced as a result of said shrinkage by said coating
material. Such action, made possible by the desolvation of
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the coating material prior to complete removal of said
liquid medium, avoids the development of microscopic cracks
that otherwise occur upon desolvation of the coated
membrane.
/ In one embodiment of the process of the invention,
a microporous membrane support is pretreated with a volatile
liquid medium non-solvating to the coating material and
capable of drawing said material into the support and of
desolvating said coating material. The pretreated ~upport
is then contacted with a suspension of the coating material.
After draining excess suspension from the surface of the
membrane, the thus-treated mem~rane is exposed to a
temperature capable of volatilizing the liquid medium to
remove said liquid from the microporous membrane support
and said coating material~ As indicated above, the treated
membrane may thereafter be fired, if desired, to a dehydra-
tion or desolvation temperature to enhance the stability o~
the me~brane by sintering the coating material. The pre-
treatment can be carried out by wetting the entire micro-
poro~s ~upport with the volatile liquid medium until the
support is saturated. This usually takes a very short
time, e.g., less than a minute. While still wet, the
support is contacted, on one surface, with the coating
suspension, usually again for about`one minute. When the
porous support is of convenient tubular shape t it is
generally preferred to wet the inside surface with the
support tube positioned in a vertical manner. The suspension
can be fed conveniently through the bottom opening of the
tubular support, by gravity, injection ~leans or vacuum
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so entrapped air can be avoided. The tube is filled to the
top with the coating suspension, and the supply of suspension
is replenished as liquid is drawn into the porous support.
The coa~ing prooedure can be completed in about one minute.
In treating a number of tubes at one time, sufficient space
should desirably be maintained between the tubes to assure
that the volatile liquid medium vaporizes freely. After the
support has been coated as indicated above, e~cess
suspension can be drained therefrom in a few seconds time.
The treated tube is then air dried or otherwi~e exposed to a
temperature capable of volatilizing the liquid medium while
the tube is conveniently maintained in its vertical position.
Air drying is usually carried out for about an hour.
In another embodiment, the coating material is
dispersed directly in the liquid medium that is non-solvating
to said coating material and is capable of drawing the coat-
ing material into the support and of de~ol~a~ing ~he coating
material The resulting suspension is pplied to the
membrane support withou~ pretreatment of the support. Excess
suspension is drained away from ~he membrane, as in the
previous embodiment, and the membrane is dried and, if de-
sired, fired at a desolvation tempera~ure to enhance the
stability of the membrane
It will be understood that ~he desolvation of thP
- coating material by the volatile liquid medium includes not
only the removal of a separate liquid employe.d to form a
suspension of the coating material, as in the support prP-
treatment embodiment described above, but the possible removal
of ~ater of hydration associated with the coating material.
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Removal of such water by dehydration upon drying
and/or firing of conventional membranes would result in
shrinkage and of the development of cracks as noted above.
In the latter embodiment, the particles of coating material
in the suspension are drawn into the porous surface as the
solvent evaporates. The thickness of the membrane in-
creases with contact time, however, so time control must
be observed to avoid excess membrane thickness.
The membrane of the invention is formed from sus-
pensions of inorganic refractory materials. Most such
inorganic refractory materials are in the form of oxides 7
e.g., metal oxides. In particular, the oxides of metals
of Groups III-A, III-B, IV-A, IV-B, V-A, V-B, VI-B, VII-B
and VIII and lanthanides and ac~inides, as described in
the Berger patent, US 3,497,394, can be employed in the
practice of the present invention. Zirconia is a particu-
larly preferred coating material, as it is known to be
chemically inert to strong and weak acids, alkalis and
solvents, even at high temperature, and advantageous for
practical commercial applications.
The finely dispersed colloidal oxides employed in
the invention usually are solvated or have hydrous or
hydroxyl surfaces. When deposited to form a filtration mem-
brane, only weak van der Waals or hydrogen bonding inter-
actions in close proximity are responsible for the cohesive
force holding the membrane together. Upon heat treatment to
a desolvating or dehydrating temperature, or to a sintering
temperature, strong metal-oxygen-metal bonds can be formed,
thus increasing the cohesion between the membrane coating
particles. Unlike previous wet particulate membranes that
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developed macroscopic cracks due to shrinkage upon dehydra-
tion, the present invention inherently overcomes the problem
of shrinkage because most of the dehydration of the particu-
late surface occurs by the use of said miscible,dehydrating
liquid, i.e., solvent, during deposition o~ the mem~rane
material. The desolvated particles are thus preshrunk and
coating particles continue to fill the voids produced by
solvent removal. The formation of cracks during the sub-
sequent drying of the membrane is thereby avoided, resulting
in the production of a heat-treated membrane composed mainly
of dehydrated oxide, that is microporous in nature, and free
of observable cracks.
The coating particles employed are in the particle
size range that will form a good semipermeable filter.
Typically, such particles are in the range of from about
5 mu to about 10 u, with a range o from about 10 mu to
a~out 1 u being generally preerred for ultrafiltration
purposes, Wh~le it is generally preferred that the
disper8ion be ~n an aqueous medium for convenlence ln
handling and good stab~lity, it will be appreciated that
other liquid ~edia can also be employed. When the support
pretreatment embodiment is employed~ the suspension itself
should be miscible with the pretreatment solvent. If
the medium used for coating material dispersion is a
volatile, nonsolvating liquid, comp~tible with said
dispersion so as to preelude the flocculation thereof, then
~ ~he alternate embodimen~ can be employed with direct contact
of the suspension with~the untreated microporou~ suppvrt. As
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noted above, however, the contact time must be carefully
controlled in this embodiment as such suspensions would
continue to coat the support substrate as long as they main-
tain contact with said substra~e. An undesired thickness
can thereby be formed if contact of the suspension with the
support substrate is unduly prolonged, The thickness of the
membrane is also influenced by the concentration of eoating
particles in the suspension. Concentrations of from about
0.5 to about 20% by weight based on the total weight of the
suspension are generally satisfactory, depending on the type
of coating material employed, with a concentration of about
6% by weight being generally preferred to form an optimum
coating thickness. The membrane coating will generally be
from submicron up to about 20- micron. i.e. 20 u. ~hickness .
The microporous support, or substrate, should
consist of a material as chemically and thermally reslstant
as the membrane itself. Sintered metal inorganic oxides,
such as metal oxides, carbon and graphite are lllu~trative
examples of suitable substrate materials. The substrate
shouId have a high porosity with pore sizes that can support
the colloidal particles used to form the membrane coating.
Thus, it is generally desirab~e that the substrate have a
pore volume of from about 5 to about 60% with pore sizes of
from about 5 mu to about 40 u, More preferably, the sub-
strate should have a pore volume of from about 20% to about
40%, with pore sizes of from about 100 mu to about 2 u.
The microporous membrane support of the invention
is not limited to any particular shape. Thus, the support
can be flat, spiral, hollow-fiber, or any other convenient
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shape although tubular shapes are generally preferred.
Porous carbon ~ubing has been found to be particularly con-
venient, having ex elle~t resistance to both chemicals and
high temperature. When firing such tubing, appropriate
care should be taken to avoid its oxidation or the r~duction
of some metal oxide particles. A carbon tube having a pore
.t ,
volume of about 0.19 ml/g with a pore size distribution
peaked at abou~ 0.3 u has been found to constitute an espe-
cially pre~erred membrane support material. In coating the
support, the direction of flow of the coating suspension
can be inside-out or outside-in depending on various design
parameters such as hydraulic flows, pressure and the like.
The volatile liquid medium used to pre~reat the
microporous support should be nonsolvating to the coating
material and capable of drawing said material into the
support and of de~olvat~ng the coating material. Said
volatile liquid should thus be miscible with the coating
suspension ~edium so as to draw the coating material in~o
the support. It is also preferred tha~ said liquid be
2V volatile a~ a convenient temperature, such as from about
15C to about 100C. ) Most ketones and alcohols are suitable
pretreatment liquids, wi~h acetone and methanol being
preferred liquids, and with acetone being particul~rly
preferred and highly suitable or use in conjunction with
aqueous suspensions of the coatlng material. In the em-
bodiment in which the coating material suspenslon medium is
volatile and is a nonsolva~ing liquid to the particles, the
coating operation can be carried ~ut directly without mem-
brane support pretreatment, the suspension medium serving to
draw the coating matsrial lnto the support and to desolvate
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said coating material particles. Methanol is a sui~able
suspension medium for use in this em~odiment of the inven-
tion, and can readily ~e employed as a suspension medium for
the preferred zirconia coati.ng material without pretreatment
of the supporting substrate.
The exposing of the treated membrane to a temper-
ature capable o~ volatilizing the liquid medium to remove
it from the membrane support and coating material can
readily be accomplished in the atmosphere, i.e., by air
drying as at from about 15C to about 100C. If prolonged
and high temperature is required for an oxidizable material,
such as carbon and metals, the baking can be carried out in
an inert atmosphere. The temperature should be above the
desolvation or dehydration temperature. Wh~n firing to
enhance the stability of the membrane~ temperatures above
that at which the particles will be sintered should
advantageously be employed. Firing will generally be at
a dehydration or desoluation temperature in the range of
from about 25C to about 1500C, raore particularly from
about 60C to about 1200C. Firing temperatures in the
range o~ ~rom about 400C to about 600C, with firing ~imes
on the order o~ thirty minutes, have been preferred. The
furnace can be preheated to a desired temperature, or the
temperature can be raised gradually while the membrane and
support are in place. The temperature is usually brought
gradually up to ~ preset maximum and then held for a period
of from about ten minutes to a couple of hours. ~-
Fired zirconia membranes prepared in accordance
with the invention have been found to maintain the coating
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after exposure to a circulating wash employing acid,
base, detergent and soap was~es, and to abrasion and
ultrasonic tests~ A conventional wet membrane as des-
cribed above, on the other hand, was found to have only
partial retention in acidic, base and detergent circulat-
ing washes. The coating of the conventional membrane was
found also ~o have sloughed off upon exposure to a cir-
culating soap wash and upon exposure to abrasion and
ultrasonic tests. Whereas the membrane of the inventlon
had good rejection properties on drying, the conventional
wet membrane had poor rejection properties when dry. Be-
cause of its superior stability, a stable hydrous zirconia
coating can be permanently deposited onto the membrane of
this invention to provide it with a hyperfiltration charac-
teris~ic that can be used for the retention of low molecular
weight macromolecules. The invention is further illustrated
by the following examples falling within the scope of the
invention disclosed and claimed herein.
Example 1
A microporous carbon tube having a length of
63.5 cm, an inside dia. of 6 mm, an outside dia. of 10 mm,
and a pore volume of 0.185 cm3/g was employed as a mem-
brane support. 75% of the pores were between 0.1 mu and
1.0 mu, with the peak distribution at about 0.3 mu. About
0.025 cm3/g of the tube had pores of from about 2 to about
10 u. The air permeation rate of the tube was about 1500
cm3/min. at 25C and 0.68 atm. differential pressure.
Water flux was measured at about 200 ml/min. at 38C and
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6,8 atm. Th tube was tilted on one end, and sufficien~
acetone was ~ntroduced from the top opening untll the tube
was full, with additional acetone belng supplied as the
level decreased due to absorption, Upon saturatlon in
about 30 secon~sJ the acetone was drained from the tube. A
suspension o zirconia was then quickly injected through
the bottom cork seal until the tube was full~ The
suspension was held in the tu~e in the vertical position
for one minute with the level of the suspension maintained
at the top opening continuously, after which the suspension
was drained from the tube. The tube wa~ then air dried
in a Yertical position for one hour. It was then fired
in a furnace, starting at 25C and with the temperature
increased to 650C in about 15 minutes and maintained
a~ that temperature or an additional 15 minute~, The
eoating suspenslon was a 6~/o weigh~/per volume aqueous
suspension of zirconia (88%) ~tabilized wit~ yttria ~12~/o)~
The particle surface area was about 45 m2/g having an
aggregate si~e of 3.1 ~o 1,0 u. The coated tube wa~
found to have about 1,7 mg/cm2 of zirconia on the carbon
tube, The essentially crack-free membrane coating
remained intact after washing or 10 minutes with circulat-
ing water, then with 0.5% aqueous oxalie acid ~or 20
minutes, and then with 0,1 M Na~H or lO minutes, and
finally again with water ~or 10 minutes. The coating
likewise remained ln~act in an ultrasonic test in which
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a one inch piece of the coated tube was broken open,
submerged in a 200 ml beaker half filled with water
and subjected to an ultrasonic stress of 2 x 104 cps
at about 70 watts for 15 minutes. After measuring
the water flux which was 173 gfd (gallons per
square foot per day) at 60 psi and 40C, the membrane
was tested with a 1% soluble oil in water emulsion
using cutting oil as a feed at a flux of 173 gfd
at 60 psi and 40C, with 2.5 gpm circulation.
Rejection of emulsified oil by turbidity testing was
99.5%. Concentration was carried out until a 5%
oil concentration was reached. The flux at this
point was 167 gfd at 60 psi and 40C with 2.5 gpm
circulation. Rejection of emulsified oil, by
turbidity test, was 99.7%. A separate zirconia
membrane prepared in the same manner on the same
type tube was tested with a 400,000 mol. wt hydro-
lyzed starch. Rejection of better than 99% was
observed.
Example 2
A membrane was prepared by the procedure
of Example 1 except that the acetone pretreatment
was omitted. The suspension was prepared by diluting
one volume of a 30% aqueous zirconia suspension with
methanol to a total of five volumes. Using the
evaluation procedures and conditions of Example l,
the crack-free membrane of the invention achieved the
following results: water flux -180 gfd; flux at the
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beginning feed concentration of 1% oil - 177 gfd, with a
rejection of 99.8%; and flux after concentration to 5% oil -
173 gfd with a rejection of 99~8~/o~
Example 3
A fired zirconia membrane was prepared as in
Example 1 except that an alumina tube was employed as ~he
membrane support. The tube had an initial water-wetted
bubble point pressure of 22 psi in air. Performance under
the same conditions and procedures as in Example 1, for
said 1% cutting oil, were: flux - 340 gfd; rejection -
98.4%.
Example 4
A zirconia membrane was prepared on a carbon tube,
as in Example 1, except that the maximum furnace temperature
for firing was 1100C for one hour under a nitrogen atmo-
sphere. Performance under the conditions and procedures of
Example 1 give a water flux of 306 gfd and a flux ~or 1%
oil of 272 gfd) with a rejection of 99.4%.
, Example 5
The procedures of Example 1 were again employed,
except tha~ the coating suspension was made from 5%
tantalum oxide. The particles had an initial surface area
of 5.14 m2/g, and were ground with ceramic balls at a`p~
of 4 for 72 hours. Performance under the conditions and
procedures of Example 1 were: water flu~ - 258 gfd; flux
for 1% oil - 200 gfd, with a rejection of 96%.
In other applications of the invention, various
other alternatives were employed, e.g., empLoying me~hyl
87
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ethyl ketone as the pretreatment volatile liquid in the
preparation of a zirconia membrane in accordance with the
procedures of Example 1 and, likewise employing said pro-
cedures, except for the substitution of silica in p~ace of
zirconia for the production of a silica membrane. In other
applications,~zirconia membranes can be prepared ~n a
variety of porous support materials, such as a porous
sintered metal tube, a fiber glass tube, a paper tube and
the like.
The ultrafiltration membrane of the invention, in
its various embodiments, represents a ~ignificant advance
in the art. In addition to having good flux and rejection
properties, the membrane resists chemicals, detergents and
extremes of pH and temperature and has a superior stability
to previously available particulate membranes. By providing
these advantages in a dry, crack-free membrane, the inventior
overcomes the appreciable limitations and restrictions here-
tofore encountéred and provides greatly enhanced flexibility
in the handling, storage and application of lnorganic ultra-
filtration membranes.
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