Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to the design
and manufacture of filtering structures and elements and
devices designed to be used in the practising of ultra-
filtration techni~ues.
Of course, the term ultrafiltration is applied
to methods of separation by filtration which permit the
selective retention of the constituents on the scale of
their molecular sizes. There can be employed, for
example, separate constituents remaining in suspension
in solution, and even to retain selectively chemical
substances of a particular molecular weight in the
presence of other more or less similar substances, of a
different molecular weight. Organic molecules can, in
particular, be separated in this way, whence the pro-
mising prospects of their use in the separation of bio-
logical substances for the food industry or the pharma-
ceutical industry. It may, for example, concern freeing
blood serum from globulins which interfere with its
preservation, by removing the proteins of high molecular
weight or increasing the casein concentration of milk
for the manufacture of cheeses, and here exploi-tation on
the industrial scale is envisaged in the near future.
The difficulties which must still be faced are
connected with the nature of the elements and ultra-
filtration structures available. At the present time
there are u0ed as filtering element~, organicmembranes,
based,in particular,on cellulo~e acetate, deposited on
porous ~upport~ HoweYer the ~tructure~ thu~ co~titut0d
are very badly adapted to suit employment in the treatment
of current industrial produc-t~ as ~ould be neces3ary
for the particular application~ cont~mplated above. Their
manufacture i9 laboriou~ and the membranes are very
sensitive from all points of vie~, mechanical, thermal
or chemical, ~hich re~ults not only in troublesome
fragility for mounting the membranes on their supports
and for the manipulation of the completed structures, but
also theimpossibility of subjecting these structuxes
to chemical or thermal treatments, for -the purposes
for example9 of cleaning or sterilization, or of exposing
them to chemically aggressive med~a during operation. The
chemical i~stability of the membranes is ~uch that in
practice rapid degradation by the product~ treated
themselves cannot be avoided~ with consequent variations
in the filtering properties. In addition, organic mem-
branes must imper t~vely be al~ay~ preserved in the moist
state and their relatively slight permeability necessitates
the provi~ion of very large surfaces to achieve processing
flowrates compatible with the requirements of industrial
exploitatio~.
The same draw~acks are also to be found in their
quasi-totality in ultrafiltration structures thQt the
owner oE French patent pub]ica-tion 2228518 has proposed
to use for the treatment of oily products, in an appa-
ratus with a bundle of filtering tubes. In this case,
the membrane is of inorganic nature, but it is a layer
of inorganic oxide sueh as zirconia simply deposited in
the state of hydroxide in colloidal suspension. It
suffers especially from very great meehanical fragility.
It is further known r from US patents 3359622
or 3984044, to produee filtering elements eomprising two
inorganic layers made of sintered me-tals, the one being
a relatively coarse-grained porous support and the other
a relatively fine-grained filtering layer But such
porous metal struetures eannot generally be used in the
presenee of sterilized or pure substanees, when metal
eontamination is prohibited.
A purpose of the invention is to provide a new
filtration strueture eomprising two sueh layers made of
eeramic materials instead of metals, so that it is com-
patible with biologic liquids such as milk or blood and
it is suitable for filtering sueh biologie liquids with
no eontamination or any similar fluids, especially in
the pharmaeeutieal industry and the food industry. A
further object of the invention is to enable the pro-
duction of such structures from two different ceramic
eompositions, by a method easy to praetiee and leading
to a filter element of high permeability, which com-
prises essentially the forming and baking of a first
1.~
- 4a -
sinterable ceramic composition with relatively coarse
grains constituting a support, and a second sinterable
ceramic composition with ~
__ _ _____~ _
relatively fine grain~ con~tituting a thin filtering
l~yer on said ~upport. This method i8 characterized
in addition in that said thin layer i9 depo~ited
on -the surface of the ~upport, ~hils-t the latter
contains an organic binder sub~tantially fiiling its
pores at least at the surface, and then subjected
to baking at a temperature causing its sintering a~
well as the decomposi-tion of the organic b~nder.
One of the pre~erred embo~iment~ of this method,
the baking of the support and that of the ~iltering
layer is carried out simultaneously, the organic b~nder
being incorporated in the sinterable composition of -the
support. In such case, the method according to the
present invention comprises essentially -the forming of
a support of a first sinterable composition with
relatively coarse grains, containing an orga~ic binder
sub~tantially filling the pores, at least at the surface,
the deposition at the surface o~ the support of a thin
layer of second sinterable composition ~ith relatively
fine grains and the simultaneous baking of said support
provided with said layer at a temperature causing the
sintering of said compositions and the decomposition~
of the organic binder.
The simultaneous baking of the two sinterable
compositions constituting respectively the support and
the~filtering layer enables th~ latter to be consolidated
by the sintering9 whil~t theorganic binder is still
present in tbe composition of the support. In this way
a structure i~ obtained wherein the .filtering layer
is bond~d to -the suppor~ at the surface ~ufficiently
to be made fast thereto~ but Nithout substantial
interpenetration betwee~ the t~o materials, a~d without
the fine grains of the filtering layer becomir.g
intercalated between the aforesaid support grains.
In other embodiments of the method
according to the in~ention~ a similar result can be
obtained ~en the filtering layer is ~eposited and then
sintered on a support which contains the organic binder
decomposable only at its surface. Thus it is possible
to provide for comple-tely forming the support first 7
ensuring its baking under conditions suit~ble for
sintering the first composition, of then rendering it
superficiallysealed by a coating of organic binder
penetrating into all the pores of the sur~ac~ and then
of producing on the th~s coated support the deposit o~
filtering layer,and then its baking.
The unit o~ the construotion produced according
to the in~ention has excellen-t properties of mechanical
strength due to the fact that all therein:is c~ramic in
nature, or con~tituted by a sintered inorganic compounds,
even the thin layer which play~ the role of filtration
diaphragmO In additiong the pore~ of the support, freed
by the decomposition o~ the orga~io binder~ do not run
the risk of being obturated by the grains of the filtering
layer, which ensure~ great permeability for the support,
71 3L~
which remain~ stable over -time and ~hich permit~ hi~h
processing flowrate~. The ~tab$1ity of the filtering
layer result~ al80 in the filtratio~ characteri~tic~
rQmaining constant in the course of th~ utilization
of the ~tructure, i~ particular the cut-off- molecular
weight9 connected with the pore size of this layer.
The sizes of the grain~ at present in
the sinterable composition areselec-ted as a functior.
of the respective rolesassigned to the support and
the filtering layer, so as to obtain~ for example, in
the structure once sintered, pore sizes of the order of
0.5 to 100 microns9 or preferably ~rom 5 to 20 mircon~,
in the support, and of the order of 0.05 to 4 microns,
or preferabl~ 1 to 2 microns in the filtering layer.
The pore di~nsions mentioned here ~nd in the re~t of
the description relate to the average pore dia~eter
as measured by means of a mecuxy pump porosimeterO The
thick~ess of the support is advantageously at lea~t
equal -to 0.2 mm and it is generally superf~uous to
exceed 2 cm, whilst with theLfilter ng layer it is
advantageou~ to provide thicknesses comprised between 2
and ~00 microns, preferably of the order of 5 to 50 microns.
The filtration structure according to the invention
oan have variou~ shapes according to the applications
contemplatedO It seems however that the qualities,
particularly of mechanical strength, which render it u~e~ul
in indu~trial ultrafil-tration proce~ses are be~t put
to u~e when it i9 con~tructed in tubular form, to
constituteultrafiltration uni-ts arranged in parallel
in the path of the fluid to be treated, and more
preci3ely to constitute the tube~ of an ultrafiltration
device with a tubular bu~dle. Such tubular structures
according to the invention~ can, for example, reckon
on a support tube of 3 to 50 mm internal diameterl with
walls of 0~5 mm thickness, bearing the filtering layer
as an inner or outer coating.
The method according to the invention is
al50 particularly easy to apply in this case. Generally?
the composition designed to produce the ~upport is shaped
by any moulding technique, but preferably for tubular
structures by extrusion throu~h an annular nozzle. The
e~truded el~mment obtained is advantageously dried to
harden the binder, generally by removal of the water
from the compo~ition~ It i~ then ready, also in fluid
type state, to receive the deposit of the second comp-
osition. The latter is advantageously applied by coating
from a suspension of the constituents in water, by then
removing -the water from the coating by drying prior to
baking. It i9 e28y in particular to carry out this
coating by flow of the su~pension inside supports
produced in the form of a tube.
The constituents and their proportions in the
two composition~ are adva~tageou~ly selected so that
the ~intering temperature~ of the two elements, support
and filtering layer, are approxima-tely the same, in
~pite of the different thicknes~es and granu10metr~-es. For
this purpose9 it is generally desirable to incorporate
in the first composition (that of the support) a flux or
fusible glass facilitating on baking the form tion
of a vitreous phase. It may, for example, be a glass
fusible at temperatures comprised between 550C and
1300 C, of the silicate, borate or lead salt type. A
proportion of the order of 2 % to 20 ~0 by w~i~ht in
the total ~olid constituents of the composition,
according to the nature of the ~interable grains, is
generally suitable for baking temperatures comprised
between 1000~C and 1400C, this temperature range being
also suitable for sintering the thin layer of the filter~ng
layer without it being necessary to incorporate a flux
in the second composition. The nature of the inorganic
composition constituting the ~rains of the two compositions
can be very variable. It may be the same metal ~xide,
alumina for example9 with diff~rent grain sizes9 smaller
for the second composition7 but it can also be of
different compounds. Such compounds will mostly be metal
oxides, pure or in combination9 for example~ alumina9
~5 zircon , titanium oxide, silica and chromium oxide9 mixed
oxides like silicates, for example zircon (zirconium
silicate3 7 or aluminates9 for example ~pinels or magnesium
- 10 --
aluminate, or again carbides such as silicon carbide or
tunysten carbide~
The basic i.norganic composition of the sinter-
able composition can advantageously have grain sizes of
the order of 5 to lOQ microns for the composition in-
tended to constitute the support, and of the order of
0.1 to 5 microns for that which forms the filtering
layer. If a compound is used having uniform grain
sizes, it is easy to arrange these grains to form granu-
lates of equally uniform dimensions, manifested by greatreproducibility of the properties of the support and by
a cut~off molecular weight which is accurate and homo-
geneous for the filtering layer. In the latter the
agglomerates advantageously have dimensions from 0.2 to
15 30 microns and preferably of the order of 0~5 to 10
microns.
The organic binder may be of any known type,
of the nature of cellulosic or vinyl derivatives or
starches, for example. Its proportion may generally be
comprised between 1 and 20~ by weight, with respect to
the total weight of the solid constituents of the compo-
sitions. However on the whole, it must be understood
that these compositions are regulated, according to
criteria of selection known in themselves, as a function
of the methods of formation selected for each among
them, and this both for the essential constituents and
for the usual possible additives, which aim particularly
at facilitating moulding b~ extrusion for the sinterable
composi-tion of the support, or for adapting the sus-
pension for application by coating for the sinterable
composition of the fil-tering layerr
Examples of suitable compositions will be
indicated more precisely a little further on, for par-
ticular embodirnents of the method according to the
invention, selected also by way of non-limiting example.
To begin with a particular embodiment will be described
of a filtering structure according to the invention~
illustrating its description by use in an ultra-
filtration device. This description, which of course
is not of any limiting nature wi-th regard to the scope
of the invention makes reference to the figures of the
accompanying drawings, in which:
~igure 1 shows diagrammatically an embodiment
of an ultrafiltration device constituted from filtering
structures of the tu~ular type according to the in-
vention; and
Figure 2 shows diagrammatically in more de-
tailed manner the constitution of such a structure, in
its composite wall.
The ultrafiltration device is essentially
constituted by a tubular bundle whose arrangement and
mechanical assembly are conventional in themselves.
Each ultrafiltration unit therein is constituted by
tube 1, enabling the separation in a fluid of the
- 12 -
compounds in suspension or ln solution whose molecular
dimensions exceed a particular cu-t~off molecular weight.
It relates to a structure according to the invention,
comprising a porous support 2 bearing as an internal
coating a filtering layer 3 whose charact~ristics de-
termine this cut-off molecular weight. All the tubes
are assembled into a bundle of parallel tubes between
two tubular plates 4 and 5, inside a jacket 6. The
latter forms at the ends of the bundle two chambers 7
and 8 communicating with the inside of tube l, re-
spectively for the entry of the fluid to be processed
through an inlet pipe 9 and for its removal through an
exit pipe 10. In operation, the fluid to be processed
flows thus in parallel in the various tubesO The
filtered liquid, containing the constituents capable of
passing throuyh the filtering layers of the tubes 1, is
collected outside the latter in the space bounded from
the one to the other of the tubular plates 4 and S by
the jacket 6, which is provided with a lateral removal
tapping ll.
According to the invention, the tubes consti-
tuting the unitary ultrafiltration structures are en-
tirely produced of sintered ceramic materials, which
permit among other things the contemplation without
difficulty of processing in the device of more or less
aggressive fluids and of making it undergo cleaning and
sterilizing treatments. Support 2 normally has only the
~ ~b~ ~
- 12a
role of mechanical support and oP hiyh permeability. It
is therefore formed from a sinkerable composition con-
taining oxide or other reEractory inorganic
compound~ of regular grain~, but rela~ively coar3e. '~he
filtering layer 3 i~ on the contrary constituted by
a sintered material with fine9 ~ut very regular, grain~,
resulting in a small homogeneous pore diameter9 and it
is very thin. I~ ~pite of the difference between the
grain sizes of the t~o materiais7 the fine layer is
in fact at the sur~ace of -the support9 and its sintered
Pgglomerated grains remain substantially at the
top of those of the support, without penetration into
the pores of the latter~ This is due to the fact that
the baking ensuring the sintering of the refractory grains
has been carried out simultaneously on the two compositions
used for the ~upport and ~or the filtering 1~r ( and
previously shaped~, whilst the free spaces between the
grains of the support were still ~illed with an organic
binder which is decomposed o~ baking. Under preferred
conditions for practising the method of the invention
~nd in the particular example below, it was possible
in practice to arrange for the filtering layer to have
a -thickness corresponding to 2 to 10 times the diameter
of the agglomerates o~ sintered grains and for its
interpenetration with the mass of the support not to
exceed a thickness of Q~2 times this diameter. Mostly,
the thickness of interpenetration would be of the order
f 0.2 to 1 microns~ possibily sometimes up to 2 microns.
t3~ 3
14
A fir~t ~interabl0 composi tlon was prepared
for the support, compri~ing, by w0ight
Chromium oxide o~ ~;ranulometry 40 microns: 91
Methylcellulo~e (blnder) 5 S~
Pyrex ( trademark) gla~s powder
(sodium borosilicate) 4
The shaping of thi~ composition, mixed wi-th
25 parts of water per 100 parts by weights of solid
constituents followed by extrusion by annular die~
to produce a tube 10 mm in internal diameter and 1,5 ~m
in wall thickness, which was dried for 3 hours in a~
oven at 80C.
A second sinterable composition based on very
fine titanium dioxide and of small granu~ometric
dispersion was constituted, which ~as prepared in the
form of a sus~ension in water containing:
Titanium dioxide of granulometry 0~1 micron 8 gjl
Polyvinyl alcohol ~binder) : 2 g/l
This ~uspension was passed into the
dried, rigid but unbaked tube, so as to apply a regular
layer of 10 microns over its who~e internal surface.
The tube thus coated is subjected to drying
ag~in ir the oven at 80C for thrse hour~. rhe whole is
then baked at 1100C ~or 1 hour.
In the product finally obtained, the sintered
material of the outer extruded tube7 constituting
1~9~
the 3upport, has a pore diameter o~ 9 microns ~d that
of the filterin~ layer re~ulting from depo~ition on the
inner 3ur~ace ha~ a pore diameter o~ 0.1 micron.
A fir~t sinterable compo3ition ~a prepared
containing, by weight :
Zircon (SiO2 - 2rO2) of 20 microns
granulometry : 91 %
Pregelatinized corn s-tarch : 3
Pyrex type (trademark) gla~s
powder (sodium borosilicate ~ 6 %
and these constituents were mixed in water in the proportion
of 22 parts by wei~ht for 100 parts by wei~ht of ~olid
constituents. The mixture was applied as in the preceding
example to form a tube of 6 mm internal di~metsr and 1 mm
wall thickness9 ~hich was dried in the oven for 3 hours.
By proce~ng as in the preceding example, there
was applied to theinside of the tube, in a thickness of
15 microns, a suspension in water containing ~sentially o
Silica of 0~3 microns of granulometry : 10 g/l
Carboxymethylcellulose (binder) : 3 g/l
and the product obtained was dried in the oven at 85C
for 2 hours. The sintering wa~ the~ carried out by
baking the whole at 1250C for 45 minute~.
In this way? was obtained9 on a support of pore
diameter 4 microns9 a flltering layer of 15 microns
whose material had a pore size o~ 0.5 micron~ and ~n
which theagglomerated sin~ered grains had a diameter o~
l~i
the order of 3 micron~.
Procedure wa~ a~ in Example 2 replacing the
zircon by silicon o~ 20 micro~s granulometry
and the silica by alpha alumina o~ 1 micron granulomet~y,with
smallgranulometric ~pread (at least 70 % by weight of
~rains of si~e comprised between 0.7 and 2 microns).
'rhe filtering layer was ~orm~d 8 microns in thick~ess
under conditions leading to agglomerates of
5 microns diameter and, after sintering, a pore diameter
of the order of 1 micron.
Example 4
Procedure ~as as in Example 3, preparin~ a support
o~ silicon carbide of a~erage granulometry of 20 micron
with Pyrex (trademark) glass powder (sodium borosilicate)
and pregelatinized corn starch. ~hese constituents were
mixed with water to form therefrom a homogeneous paste
and the forming of this paste followed by extrusion
through annular die as in Example 17 to obtain a tube
Of 15 mm intern~ ~i~meter and 2 mm thickness.
This tube was heat-treated9 after drying9 at
a temperature of 1250C ~or 30 minutes.
An organic gel was prepared containing 3 g of
polyvinyl alcohol in 1 litre of water and thi~ gel was
made to rise inside the previously baked tube~ I~ thi~
way the support was coated with a continuou~ ~ilm filling
all the surface pore~
1'7
On the other hand a su~pension ~as formed
a~ in Example 3 fDOm alpha alumi~a o~ granulometry
well centered around 1 micron9 this suspen ion containing
20 g of alumina per 1 litre of water and 2 g of carboxy-
methylcellulose.
This suspension ~a8 depo~ited inside the -tube
previously coated with the preceding gel. Then, the
whole was driea~ then subj2cted to a final heat treatment
at 1150C for 45 minutes. The filtering layer ~ormed
had a thickness of 15 microns and a pore diameter of
the order of 0.7 microns.
Naturally, modi~ications can certainly be
introduced into the nature the various compounds used
and the detailed figures ~iven in th~ examples above,
without departing from the scope o~ the invention. The
invention is not limited~ neither to filtration
structures produced in tubular form~ nor to the application
to ultrafiltration specially described, the same structures
being also usable in other similar filtration techniques,
such as inverse osmosis9for example.
