CA1196873A

CA1196873A - Filtration structure of ceramic material

Info

Publication number: CA1196873A
Application number: CA000399483A
Authority: CA
Inventors: Rene Clement; Francis Gugliermotte
Original assignee: GECERAL GROUPEMENT D'ETUDES POUR LES CERAMIQUES ALUMINEUSES
Current assignee: GECERAL GROUPEMENT D'ETUDES POUR LES CERAMIQUES ALUMINEUSES
Priority date: 1981-03-30
Filing date: 1982-03-26
Publication date: 1985-11-19
Also published as: JPS5834006A; FR2502508B1; FR2502508A1

Abstract

ABSTRACT OF THE DISCLOSURE

A filtration structure, suitable particularly for ultrafiltration, is essentially constituted by a porous support of a relatively coarse grain sintered material bearing a thin-ner filtering layer of a relatively fine grain sintered material, formed on top of the support and not penetrating substantially between the grains of the latter. It can be produced by simu-ltaneous baking of the support and of the filtering layer, from, for example, a tube formed by extrusion of a first sin-terable composition to constitute the support, on which has been deposited, after drying, a second composition suspended in water, to form a thin coating constituting the filtering layer in the finished structure. The first composition contains an organic binder which decomposes on baking.

Description

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 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 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 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.

Claims

WE CLAIM :

1. Filtration structure, constituted by a porous support of a thicker coarse grain sintered ceramic material, a thinner filtering layer of rela-tively fine grain sintered ceramic material borne on said porous support and formed on the top of the porous support and not penetrating substantially between the grains of the latter.

2. Filtration structure according to Claim 1, wherein the support has pore size of the order of 0.5 to 100 microns.

3. Filtration structure according to Claim 2, wherein the support has pore sizes of the order of 3 to 20 microns.

4. Filtration structure according to Claim 1,, 2 or 3, wherein the filtering layer has pore sizes of the order of 0.05 to 4 microns.

5. Filtration structure according to Claim 1, 2 or 3, wherein the filtering layer has pore sizes of the order of 0.1 to 2 microns.

6. Filtration structure according to Claim 1, 2 or 3, wherein the filtering layer has a thickness from 2 to 200 microns.

7. Filtration structure according to Claim 1, 2 or 3, wherein the filtering layer has a thickness of the order of 5 to 50 microns.

8. Filtration structure according to Claim 1, in tubular form, the filtering layer being an inner or outer coating on the tube-form support.

9. Filtration structure according to Claim 1, 2 or 3, wherein the filtering layer has pore sizes of the order of 0.1 to 2 microns and a thickness of the order of 5 to 50 microns, said structure being in tubu-lar form, the filtering layer being an inner or outer coating on the tube-form support.

10. Method of producing filtering structures, comprising essentially forming and baking a first sinterable composition with relatively coarse grains constituting a support, forming a second sinterable composition with relatively fine grains to constitute a thin filtering layer on said support, and depositing said thin layer on the surface of the said support, whilst the latter contains an organic binder sub-stantially filling its pores at least at the surface, and then subjecting it to baking at a temperature causing its sintering as well as the decomposition of the organic binder.

11. Method according to Claim 10, wherein said organic binder is incorporated in said first sinterable composition and said layer is deposited on the formed support, the assembly thus obtained being then subjected to simultaneous baking treatment of the support and of the thin layer causing the sintering of the two compo-sitions at the same time as the decomposition of the organic binder.

12. Method according to Claim 10 or 11, wherein said layer is deposited by coating on the support from the second composition suspended in water.

13. Method according to Claim 10 or 11, wherein the first composition contains a fusible glass, in a proportion preferably comprised between 2 and 20 % by weight with respect to the total solid constituents of the composition.

14. Method according to Claim 10 or 11, wherein said layer is deposited by coating the support the second composition suspended in water, and wherein the first composition contains a fusible glass, in a proportion preferably comprised between 2 and 20 %
by weight with respect to the total solid constituents of the composition.

15. Method according to Claim 10 or 11, wherein the baking is carried out at a temperature comprised between 1000 and 1400°C.

16. Method according to Claim 10 or 11, wherein said layer is deposited by coating support the second composition suspended in water, and the baking is carried out at a temperature comprising between 1000°
and 1400°C.

17. Method according to Claim 10 or 11, wherein said layer is deposited by coating the support the second composition suspended in water, the first composition containing a fusible glass, in a proportion preferably comprised between 2 and 20% by weight with respect to the total of the solid constituents of the composition, and the baking is carried out at a tempera-ture comprised between 1000° and 1400°C.

18. A filtration structure comprising two sintered ceramic layers, wherein a first of said layers is a thicker coarse-grained porous support which is made of a ceramic composition comprising thermally stable grains selected from silicium carbide, zirconium sili-cate, chromium oxide, alumina, and their mixtures, and further contains a vitreous phase in a proportion from 2 to 20% by weight, and wherein a second of said layers is a filtering layer made of a ceramic composition com-prising thermally stable grains selected from alumina, magnesium aluminate, silica, titanium oxide, and their mixtures.

19. A filtration structure according to Claim 18, wherein said grains for the porous support have a grain size from 20 to 100 microns, and the grains for the filtering layer have a grain size from 0,1 to 5 microns.

20. A filter element for the filtration of biologic fluids comprising at least one filtration structure according to Claim 1 or a bundle of such structures.

21. A filtration structure according to Claim 1, wherein said thinner filtering layer is in the form of discrete grains of said ceramic material coated onto said support.

22. A filtration structure according to Claim 21, wherein said porous support and said thinner filter-ing layer each consist of ceramic material.

23. A filtration structure according to Claim 22, wherein said porous support is in the form of a tube whose internal surface has been coated with discrete grains of said fine grain cexamic material.

24. A filtration structure according to Claim 1 produced by a method comprising forming and baking a first sinterable composition with relatively coarse grains constituting a support, forming a second sinter-able composition with relatively fine grains to consti-tute a thin filtering layer on said support, and de-positing said thin layer on the surface of said support, while the latter contains an organic binder substantial-ly filling its pores at least at the surface and sub-jecting the resultant composition to baking at a temper-ature sufficient to sinter said sinterable compositions and to decompose said organic binder.

25. A filtration structure according to Claim 18, wherein said second of said layers is in the form of discrete grains of a ceramic composition coated onto said first of said layers.

26. A filtration structure according to Claim 25, wherein said first of said layers consists of said thermally stable grains and said vitreous phase and said second of said layers consists of thermally stable grains.

27. A filtration structure according to Claim 26, wherein said first of said layers is in the form of a tube whose internal surface has been coated with dis-crete grains of said thermally stable grains to consti-tute the second of said layers.

28. A filtration structure according to Claim 8, wherein said filtering layer consists essentially of alumina.

29. A filtration structure according to Claim 28, wherein said filtering layer is the inner layer of said tube.

30. A filtration structure according to Claim 29, wherein said alumina of said filtering layer is in the form of discrete grains.

31. A filtration structure according to Claim 30, wherein said support consists essentially of alumina.

32. A filtration structure according to Claim 31, wherein the alumina of said support is in the form of discrete grains.

33. A filtration structure according to Claim 31, wherein the average pore diameter of said support is 0.5 to 100 microns.

34. A filtration structure according to Claim 33, wherein the average pore size of said support is 5 to 20 microns.

35. A filtration structure according to Claim 33, wherein the average pore diameter of said filtering layer is 0.05 to 2 microns.

36. A filtration structure according to Claim 35, wherein the average pore diameter of said filtration layer is 0.05 to 1 micron.

37. A filtration structure according to Claim 34, wherein the average pore diameter of said filtering layer is 0.05 to 2 microns.

38. A filtration structure according to Claim 34, wherein the average pore diameter of said filtering layer is 0.05 to 1 micron.

39. A filtration structure according to Claim 34, wherein the pores of said filtering layer are homo-geneous in size.

40. A filtration structure according to Claim 39, wherein the average pore diameter of the pores of said filtration layer is 0.05 to 2 microns.

41. A filtration structure according to Claim 40, wherein the thickness of the support is 0.2 mm to 2 cm.

42. A filtration structure according to Claim 41, wherein the thickness of said filtering layer is 2 to 200 microns.

43. A filtration structure according to Claim 42, wherein the thickness of said filtering layer is 5 to 50 microns.

44. A filtration structure according to Claim 42, wherein said tube has an inside diameter of 3 to 50 mm.

45. A filtration structure according to Claim 28, wherein said filtering layer containing alumina is an outer layer on said tubular filtration structure.

46. A filtration structure according to Claim 43, wherein the grain size of the grains of alumina of the support is 5 to 100 microns and the grain size of the alumina of said filtering layer is 0.1 to 5 microns.

47. A filtration structure according to Claim 46, wherein the grains of said filtering layer do not interpenetrate the grains of said support a distance of more than 1 micron.

48. A filtration structure according to Claim 8, wherein said filtering layer is the inner layer of said tube.

49. A filtration structure according to Claim 48, wherein the ceramic material of said filtering layer is in the form of discrete grains.

50. A filtration structure according to Claim 49, wherein the average pore diameter of said support is 0.5 to 100 microns.

51. A filtration structure according to Claim 50, wherein the average pore size of said support is 5 to 20 microns.

52. A filtration structure according to Claim 50, wherein the average pore diameter of said filtering layer is 0.05 to 2 microns.

53. A filtration structure according to Claim 52, wherein the average pore diameter of said filtration layer is 0.05 to 1 micron.

54. A filtration structure according to Claim 51, wherein the average pore diameter of said filtering layer is 0.05 to 2 microns.

55. A filtration structure according to Claim 54, wherein the average pore diameter of said filtering layer is 0.05 to 1 micron.

56. A filtration structure according to Claim 52, wherein the pores of said filtering layer are homo-geneous in size.

57. A filtration structure according to Claim 56, wherein the average pore diameter of the pores of said filtration layer is 0.05 to 1 micron.

58. A filtration structure according to Claim 57, wherein the thickness of the support is 0.2 mm to 2 cm.

59. A filtration structure according to Claim 58, wherein the thickness of said filtering layer is 2 to 200 microns.

60. A filtration structure according to Claim 59, wherein the thickness of said filtering layer is 5 to 50 microns.

61. A filtration structure according to Claim 8, wherein said filtering layer of ceramic material is an outer layer on said tubular filtration structure.

62. A filtration structure according to Claim 59, wherein the grain size of the grains of said ceramic material of the support is 5 to 100 microns and the grains size of said ceramic material of said filtering layer is 0.1 to 5 microns.

63. A filtration structure according to Claim 62, wherein the grains of said filtering layer do not interpenetrate the grains of said support a distance of more than 1 micron.