SE1551549A1 - Ceramic filter element and method for manufacturing a ceramic filter element - Google Patents

Description

WO 2914/'188671 PCT/FI2Ü14/'050389 shaft of rotation revoives, rises out of the hasin, When the filter medium is subrnerged in the siurry hasin where, under the influence of the vacuum, the cake forms onto the medium. Cnce the filter sector or plate comes out of the hesin, the oores are emptied as the cake is detiquored for a predetermirted time which is essentialiy iirnited hy the rotation speed of the disc. The cake can be discharged hy a hack-putsa of air or oy scraoing, efter which the cycle begins again. f??ü?] ln a rotary vacuum drum filter, filter eiements, eg. filter oiates, are erranged to form an essentially continuous cyiindricei shall or eriveiope surface, i.e a filter drum. The drum rotates through a slurry hasin and the vacuum sucks liquid and soiids onto the drum surface, the iiouid portion is "sucked" by the vecuum through the filter media to the internal portion of the drum, and the filtrate is pumoed away. The solids adhere to the outside of the drum and form a cake. As the drum rotates, the fiiter elements with the filter cakes rise out of the basin, the celtes are dried and removed from the surface of the drum. [üüüï] The most cornmoniy used fiiter media for vacuum filters are poiyrneric filter cioths and fiiter elements of ceramic mernbrenes. Wherees the use of a cloth fliter medium requires heavy duty vacuum pumps, due to vacuum iosses through the cioth during cake deiiquoring, the cerarnic fiiter medium, when wetted, does not alioty air to pass through and enahles the use of srnaiier vacuum pumps and, conseouentiy, yields significant energy savings. LiS75210f 232 (EPfïåšäïßt discioses a method for the rnanufacture of a composite filter piate. After completion of the substantialiy flat filter plate til), further steps can he taken, for examoie, to either provide additionai functionality and/or further render the fiiter piate more arnenahle to subseduent additionai assembly into a iarger filtration device. Such steps can inciude, for examoie, the driiiing of ports through the filter plate, the addition of flow distributors and fiow paths; the removai of ourrs, sprue, and/or other iike unwanted residuai molding waste; surface application of hydrophohic or hydroohiiic coatings; surface oolishirtg or roughening; autocleving, steam steriiization, or other sanitizing chemical treatment; and oackaging.

[3093] ln some filtering applications, such as iron ore applications, the filter cake tends to 'de detached from the filter plate too early due to the weight of the cake and iow differentiai pressure over the fiiter cake.

WO 2üï4/18897l PCTIFIZÜÉAl/ÜSÜSSÉ? BREEF DESCREPTEÛN GF THE lNVENTiON ïüüdei An aspect or' the present invention is to mitigate the problem relating to a premature deteohment ot the filter cake. Aspects of the invention ie achieved by a method, a filter eiement and en aoparetus according to the independent olaims. Embodiments of the invention are disclosed in the de- oendent oiaims. tü?tüt An aspect of the invention is a method for manufacturing a filter element to be used in removal ot' iiouid from solida containing material to be dried in a capillanf auction dryerwhich filter element contains e ceramio mi- oroporous layer stioported by a oeramio substrate, wherein the method com- prisas: providing the ceramit: substrate, coating the oeramic substrate by e ceramio rnlcroborous materiai iayen applying solid particles to the membrane material layer, e barticie size of the solid particies being larger than a por-e size of the membrane mate- rial layer, and sintering the oeramlo mlorooorous membrane material containing the solid particies. [üütt] in an embodiment, the coating oombrises dipbing the cerem- io substrate into Into a ceramio siurry to form the miorooorous ceramio mem- brane [??tâ] ln an embodiment in combination with any oreoeding embed- iment, the applying oomoriees soreying the solid oartloles on the ceramio mi- croborous layer. [Ûütâ] in en embodiment in combination with any preceding embed- iment, the solid barticles comprise aiumina perticies.

[6314] ln an embodiment in combination with any oreceding embed- iment, the rnethod eomprises setting e size ot the solid particiee eridior a de- sired oartioie density on the ceramie miorooorous membrane, according to a desired friotion effect. [69153 in an embodiment in combination with any preceding enibod- lment, the partiole size is in the range ot to micrometere 890 miorometers, oreferabiy in the range or 49 EOÛ miorometere.

WO 20l4.f'188ii71 PCTIFIZO 141050389 íüüi?] in an enioodiment in combination with any preceding embed- irnent, an average particie density on the membrane material is in the range approximateiy 50...250 particles l square centimeter, ieetï] Another aspect ot the invention le a filter eiement to be used in removal ot iiouid ironi eoiids containing rnateriai to be dried in a capiilary auction dryer, the tiiter eientent eomprieing a cerarnic substrate covered by a sintered oeramic microporous iayer, wherein the eintered rnicroporous marn- brane iayer contains coarse eoiid particlee ot a partlcie size iarger than a core size of the membrane material layer. [üüiß] Still another aspect of the invention is a fiiter apparatue cont» prising one or inore tiiter eiernents according to ernbodirnente of the invention.

BREEF DESCRiPTiON OF THE DRÅWENGS [Gíïiâi in the ioliowing the inventlon wiii be described in greater de- taii hy rneane of exarnpie enthodintents with reference to the accornpanying drawinga, in which Figure i is a perspective top view iiiuetrating an exemoiartf disc tliter apperatus, wherein embodimente of the inventlon may be appiled; Figure 2 is a perspective top view of an exernpiary sectonshaped ceranilc filter plate; FiGS. 3A, 38 and 3G iliustrate exemplary structures oi a ceramic tii- ter piate wherein embodimente of the invention may he apeiied; Figures 4A, 48 and 4G liiustrate different pheses ot a tiitering cycie; Figure 5A iiiustrates a filter piate provided ivith a coarse textured surface 50 according to exemolary ernhodlrnent of the invention; Figure 55 is a phoiograph iiiustrating a zoomed-in portion of a tex- tured surface 5G of a real cerarnic iiiter plate 22; Figure 5G ie another photograoh iilustrating a further zoomed-in portion of a textured surface 50; Figure 6A iiiustrates an exernplary rnonobody suhatrate according to an embodlment; Figure SB illuetrates a croes-seciionai top view of the euhstrate shown in Figure 6A; Figuree 7A, 76 and 7G iiiustrate phases of a dig; coating process; and Figure ?D iiluetrates an example of spraying ïi solid particies on VVO 2(l141'1.88071 PCTIFIZGÅ4IGSB3S9 the membrane surface after the membrane dip coating. ÛESCRiPTlÜN OF EXEPvlPLARY EMBODllvlENTE-š fddâul Principles of the lnvention can be applied for drying or de- Wetering fluid materials in any industrial processes, particularly in mineral and mining industries. in embodiments described herein, e material to be filtered is referred to as a slurry, but embodiments of the lnvention are not intended to be restricted to this type of fluid material. The slurry may have high solida concert- tratlon, eg. base metal concentrates, iron ore, chromite, ferrochrome, copper, gold, cobalt, nickel, zinc, lead and pyrite. ln the following, example embodi- rnents of filter plates for rotary vacuum disc filters are illustrated but the princi- ples of the invention can be applied also for filter media of other types of vacu- um filters, such as rotary vacuum drum filters.

EGGZtÉ Figure l is a perspective top view illustrating an exemplary disc filter apparatus in virhlch filter plates according to embodiments of the invention may be applied. 'the exernplary disc filter apparatus ll) cornprises a cylindrical-shaped drum 2G that is supported by bearings on a frame 8 and rotatable about the longltudiel axis of the drum 2G such that the lower portion of the drum is suomerged in a slurry basin 9 located below the drum 20. A drum drive 12 (such as an electric motor, a gear box) is provided for rotating the drum 20. The drum 20 comprises a plurality of cerarnic filter disos 21 ar- ranged in line co-axially around the central axis of the drum 20. For example, the number of the cerarnic filter dlscs may range from 2 to 29. The diameter of each disc 21 may be large, ranglng from 1,5 m to 4 m, for example. Examples of commeroially available disc filters in »which embodinlents of the invention may be applied, include Outotec Larox CC filters, models (IC-S, CC-iš, CC- 30, 00-45, CCI-SG, CC-åß and (JC-tilll manufactured by Outotec Oyj. [66221 Each filter disc 21 may be formed of a number of individual sector-shaped oeramic filter elements, called filter plates, that are mounted in a radial planer array around the central axis of the drum to form an essentially continuous and planar disc surface. The number of the filter plates may he 12 or 15, for example. Figure 2 is a perspective top view of an exemplary sector- shaped ceramic filter plate. The filter plate 22 may be provided with mounting parts, such as fastening hubs 28, 27 and 28 which function as means for at~ taching the plate 22 to mounting means in the drum. FlGS. 3A, 35 and 3G il- lustrate exemplary structures of a ceramic filter plate vvherein ernoodiments of WO 2614/188071 PC'í'r'F[20í4/ü5{}389 the invention may be applied. A microporous fiiter plate 22 may comprise a first suction structure 31A, 32A and an opposed second auction structure 31l3, 328.

The first auction structure comprises a microporous membrane 31A and a ce- ramic suhstrate 32A, whereon the membrane 31A is positioned. Simiiariy, the second suction yvaii comprises a microporous membrane 313 and a ceramic substrate 328. An interior space 33 is defined between the opposed first and second Suction structure 31A, 32A and 315, 328 resulting in a Sandwich struc- ture. The filter plate 22 may also be provided with connecting part 29, such as a fiitrate tube or a tiitrate nozzie, tor convergance of tiuids . The interior space 33 provides a fiow channei or channais which will have a fiow connection with coiiecting piping in the drum 20, eg. hy means of a tube connector 29. When the coilecting pipe is connected to a vacuurn pump, the interior 33 of the tiiter plate 22 is maintained at a negative pressure, ie. a pressure difference is maintained over the suction wait. The membrane St contains micropores that create strong capiilary action in contact with water. The pore size of the mi- croporous membrane 31 is preterabiy in the range ot 0.2 to 5 micrometer and that wiil make possibie that oniy iiguid is tiowed through the microporous layer.

The interior space 33 may be an open space or it may be tilied with a granular core materia! which acts as a reintorcernent for the structure of the piate. Due to its iarge pore size and high volume fraction ot porosity, the materiai does not prevent the tiow ot iiquid that enters into the centra! interior space 33. The inte- rior space 33 may further comprise supporting eiements or partition waiis to further reinforce the structure of the piate 22. The edges 34 oi the piate may be seaied hy means of painting or giazing or another suitahie means to seai, thus preventing tiow through the edges.

Eüüâš] in exempiary emhodiments the filter plates 22 ot the consen- utive discs are disposed in rows, each row estabiishing a sector or zone of the disc 21. As the row of the fiiter discs 21 rotate, the piates 22 of the each disc 22 move into and through the hasin 9. Thus, each tiiter piate 22 goes through tour oitferent process phaees or sectors during one rotation of the disc 21. in a cake forrning phase, a partial iracuum is transmitted to the tiiter piates 22 and riltrate is drawn through the ceramic plate 22 as it is immersed into the siurry basin 9, and a cake 35 forms on the surface of the piate 22. The liquid or tii- trate in the centrai interior space 33 is then transterred into the coilecting pipe and further out ot the drum 20. The piate 22 enters the caite drying phase (iliustrated in Figure 45) after it leaves the hasin 9. A partiet vacuum or over- WO 2Ü14/188l)71 PCTIFIZÜLâ/ÜSÜSSÉ? pressure is maintained in the fiiter piates 22 aiso during the (trying phase so as to draw more fiitrate from the cake 35 and to keep the cake 35 on the surface of the fiiter piate 35. if cake washing is reouireo, it is done in the beginning of the drying phase. in the cake discherge phase iiiustrated in Figure 4G, the cake 35 is scraped off by cerarriic scrapers so that a thin cake is ieft on the piate 22 (gap between the scraper and the plate 22). After the cake discharge, in a cieariing phase (commoniy caiied a backwash or backfiush phase) of sector of each rotation, tvater or filtrate is purripeo ivith overpressure in a reverse direction through the piate 22 to wash off the residual cake and ciean the poras of the fiiter piate. füüše] in some filtering appiications, such as iron ore appiications, the fiiter cake tencis to be detached from the fiiter piate too eariy due to the weight of the cake and a iow differentiai pressure over the fiiter cake. iviore specificaiiy, the iron ore fiiter cake may siip off from the surface of the fiiter piate 22 otiring the dryfing phase before the actual intended cake discharge, [üüâši According to an aspect of the invention, a sintered cerarnic microporous membrane rnateriai of a ceramic filter plate contains coarse soiid particies to effectively increase the area of the contact betvreen the filter plate and the cake, to increase friction and acihesion between the cake and the fiiier plate and to therapy prevent the fiiter cake from slirciing off the surface of the fiiter piate prior to the intended cake discharge. The soiid particies provide a coarse textured surface 50 for the fiiter piate 22, as iiiustrated in Figures 5A, EB and 5G. The appearance of the surface is iike “sand paper”. The friction of the textured surface is high and it prevents the fiiter cake from faiiing off the fiiter plate. Figure SB is a photograph iliustrating a zoorned-iit portion of a tex- tureci surface 50 of a reai ceiarriic fiiter piate 22. Figure 5G is another photo- graph iiiustrating a further zoomeci-in portion of a textured surface 50. [ü?ïß] ln an embodiment, the solid particies comprise aiumina (AiZClííi) particles. i-iowever, aiso other type of particies than aiurriina can be used. Criteria for the seiection of the materiai rnay be that the particles should not melt or change the chernistry of the membrane during firing or otherwise disturb the manufacturing process.

[G??] The size of the soiid particies has an effect on the increase in friction and adhesion between the cake and the fiiter piete. The particie size of the soiid particies is iarger than a pore size of the membrane materiai iayer.

The partiets size may be at ieasi two times larger than the pore size, preferabiy YVO 2014/188971 PC"I'/FI2Û14/ÜSÜ389 more than ten times iargar than the pore size. The size of particles may be se- lected dependent on the application where the filter piates are used, in typical applications, the particle size used may be in the range ot did-Sot) rniorometers (microns). in some applications, a yfery smaii increase in triotion in membrane may be enough to avoid the probiem yvith failing tiiter cakes. For this kind of applications the particie size may be 'i?-iGO micrometers. in appiioations with iarge iron ore particles in the range of 6.5 1.5 rnillimeters and fiiter cakes tvith high mass, the friction of the membrane must be increased significantiy and the grit spraying using partieies in the range of 0.2 0.8 miliimeters may be necessary.

KGbÉS] Also the number ot particles, ie. particle density per an area unit, appiied on the membrane effects the friotion. Preferabiy, the number of particies shouid not be too iarge not to affect the hydrauiic properties of the membrane. There are gapa and open spaces between the soiid particies that expose the microporous membrane and aiiow a normal functioning of the membrane. Tha norrnai membrane surface (ie. the Spaces) covers maiority of the membrane surface (eg. 70-95 Wo). in exernplary embodiments, an average particle density may be in the range approximately 5t)...250 particies l square centimeter (om2). it should be appreciated that the local particie density may vary over the surface of the tittar piate. For exampie, a minimum density counted may be 158 partioiesicml a maximum density 226 partiolesicmê, and an average density 182 partioles/cmâ. The appearance of the textured surface 50 with such particle density is iliustrated in Figures 53 and 5G. An appropriate partioie density may be seiected dependent on the appiication where the tiiter piates are used. The particle size and the particie density are interreiated, thus selection of one may affect the seiection of the other. iüíiššt Another aspect oi the invention is a method for manufactur- ing a filter element, such as a fiiter piate 22, to be used in rernovai of liquid from soiids containing materiai to be dried in a capiliary auction dryer, such as in a rotary vaouum disc tiiter it). The fiiter eiement or fiiter piate 22 may com- prise a ceramic microporous membrane iayer 31 supported by a ceramic sub~ strate 32, eg. as discusses with reference to Figures 2, 3A, SB and 3G above. [d?ßü] in an embodimant, when manufacturing the ceramic filter ei- ement the internet iayer is first formed of at ieast one ceramic stibstrate 32.

The cerarnit: substrate may be manufactured with any suitable manufacturing technique. The substrate may be made of a ceramie materiai in a powder form, WG 2614/188971 PCT/FI2?1-'t-/050389 such as for inslance alumina and titania. The ceramio material may he mixed wfith a binding medium and liquid so that the oeramic mix formed and the core material for desired recess areas or tiltrate ohanneis can be oharged into a mald. The material in the mold is then pressed into a green body. After press- ing, the green body may be sintered at a high temperature, eg. in a ternpera- ture range ot' SQO-ißbb degrees Celsius: Thereby, an integral ceramio sub- strate, so called rnonobody plate, may be formed in a single mold. The core material forrning the reoess areas or filtrate Channels may comprise, for exam- ple, granular core material which allows a flow of the filtratei As another exam- ple, the core ntaterial torrning the recess areas may be burnt out through the porous structure of ceramic mix during the sintering. As a result, the subslrate contains the open recess areas or open filtrate channels in a shape of the core material. Figure 6A illustrates a monobody substrate 32 according to an exem- plary embodirnent which may be manutactured by rnold prassing as described above. Figure 65 illustrates a cross-sections! top view of a monobody sub- strate with the filtrate Channels or recessed areas 33 exposed. [66311 ln an embodiment, the substrate of the filter plate 22 may be made of half-platsa and glued together. Each half-plate may be manufactured by rnoid pressing, for example. [o?âz] ln an ernbodirnent, a ceramic mioroporous membrane layer 31 may be produced on the cerarnic substrate 32 by a dip coating process, an example of which is illustrated in Figures ?A, "IB and YC. in e dip coating pro- cess, the substrate 32 is immersed in the suspension of the membrane materi- al slurry 70, preferably at a constant speed (Figure 7A). When the substrate 32 has remains-d inside the membranes material siurry 7G for a while, it is pulled up from the substrate sludge Yi), preterably at a oonstant speed. A thin layer ot the microporous membrane material 31 deposits itself on the substrate 32 virhile the substrate is pull-sd up (Figure YB). During the pull-up, excess mem- brane materiai slurry will drain 71 from the surface. The suspendlng fluid evap- orates YZ from the mioroporous membrane material 31, forming the thin layer (Figure H3). The thickness of the membrane layer 31 may be about 1 millime- ter, tor example. iüüïišl in another exemplary embodiment, a ceramlc microporous membrane layer 31 may be produced on the oeramic substrate 32 by spraying. [utïlßel To this point, the manufacturing of the filter plate 22 may be similar to that of a conventional filter plate. Normaily, efter the membrane layer WO 20l4/i88tt7i PCT/'FIZG 1-'5/'050389 it) 31 wcuid have dried after the dip ooating or spraying or other coating method, the substrate 32 coated with the membrane 31 wouid have been firad and sin- tered at a high temperature, eg. in a temperature range of 11504550 degrees Celsius, resuiting in the finai fiiter piate. iüüšš; However, in exemotary embodimehts of the inventioh, soiid oarticias are appiied oh the rhembrarte materia! iayer 31 after the dip coating or sorayihg or other coatihg method and prior to the firing or sintering. The solid partioles which provide a textured surface 5G may be appiied by spraying 71 (trvith a suitabie sorayihg tool 72, eg. a compressed-air paint spray gun) the solid partioies on the membrane surface 31 (eg. grit sprayihg process) irrime~ diateiy after the membrane dio coating as iilustrated ih Figure 7D. The mem- brane 31 may have dried a bit but is preferabiy stiii moist before the sorayihg because the spray-ed particias hit and readily stick to the moist membrane sur- face 31. The filter piate 22 may oreferahly he in an upright position during the spraying. The spraying may be carried out at a cohstant distance from the membrane surface 31. The spray “ft is preferabiy moved at a cohstant speed aiong the hferhbrano surface 31 such that the number of particles hitting the membrane surface 31 is maintained in a desired range per area unit. For disc fiiter piates the partioia spraying is performed on both sides of the fiiter plate 22. tft/hen the membrane iayer 31 has dried after the particle sprayihg, the substrate 32 coated with the membrane 31 and the soiid partioies wiii be tired and sintered at a high temperature, eg. in a temperature range of 1150-155G degrees Ceisius, resultihg in the fihai tittar plate. During the drying and firing the sprayed particies are wei! fixed and sintered the membrane surface 31 to estabiish the coarse tsxture 5G. iüüâßt it shouid be aporeciated that the term “sihterihg” as used herein refers aiso to otherwise heating in a kilh to a high temperature to achieve fusion of a secondary bohding phase, i.e. a siiica-rich phase. f??âï] . Aithough axampie embodiments of fiiter piates for rotary vacuurh disc rätters have been iiiustrated above, the orincioias of the invahtion can be appiied aiso for filter media of other types of vacuum fiiters, such as rotary vacuum drum tiiters. tüüâßi in further embodimehts, soiid particles may be aopiied with some other methods than the sprayihg, such as particia spreading, adding the oartioies to the membrane siurry» which is used for making the microoorous membrane 31, etc. in the case the coarse soiid particies are apoiied by adding ÉÃ/'O Züíf-iiflíšßüïl HÉTJFIZGTÅ-í/ílä?lšßš? 11 them inte the membrane siudge, the particie wiii be tiietributed throughout the entire thicknees ei membrane. However the spraying method is eesier to cen- troi in production so that the particie density is in the desired range and the particie appiying cieee net change the membrane preperties er does net de- stroy the membrane iocaiiy, iike serne ebreeive methetie, for rneitiiwg the eur- fece reugh., such ae eand-hiesting, might de.

[Güšâï Upon reading the present application, it wii! be obvious to e person skiiied in the art that the inventive concept can be irnpiementeci in veri- eus vvays. The invention and its embeciiments are net iimited to the exempies described above but may very within the spirit and scepe of the ciaims.

Claims (20)

WO 2014/'188671 PCT/'FIZÜH/ÜSÛSSS? 12

1. CLAiMS 'i. A method for manufacturing a fiiter eiernent to be used in rernovai of liquid from soiids containing rnateriai to be dried in a capiliary suction dryer which tiiter element contains a eeramic miorooorous membrane iayer support- ed by a cerarnic substrate, vrherein the method oomprisee: providing the ceramii: eubstrate, ooatiitg the ceramio substrate by a ceramio rnicrooorous membrane material iayer, apoiying solid oarticiee to the membrane materiai iayer, a oarticie size of the soiid particiee being iarger than a oore size of the membrane mate- riei layer, and sintering the ceramic rnioroporous membrane material containing the solid particiee.

2. A method according to the olaim 1, wherein the ooating oompris- es dieoing the ceramie eubeirete into a oeramio siurry to form the microoorotis ceramic membrane materiei layer.

3. A method according to the ciaiin f, wherein the applying corn- prises soraying the soiid particies on the oeramio microporous layer.

4. A method according to the oiaâm 2, wherein the applying com- priaes apraying the eoiid oarticies on the oerarnio mierooorous Bayer.

5. A method according to oiairn ”l, wherein setting the particle size of the soiid partioies and/or a desired particie density on the membrane materiai according to a desired friotion effect.

6. E. A method according to olaim 2, vvherein setting the particie size ot the soiid partioies andíor a desired oartioie density on the membrane rnateriai according to a desired friction effect.

7. ?. A method according to claim 3, wherein setting the oartioie size of the solid partioies and/or a desired partioie density on the ntembrane materiai according to a desired friotion effect.

8. A method according to ciaim 1, tvherein the partioie size is in the range of 10 microrneters ßtiü rnicrometers, oreferahiy in the range of 40 36% micrometers.

9. Q. A method according to ciaim 2, *ii/heroin the particie size is in the range of 19 microrneters SSG micrometers, oreterably in the range of 40 ßtlii) miorometers.

10. WO 2G14/188G7ï ?CT1FI2G14/G5i}389 13 to. A method according to ciaim 3, wherein the oarticie size is in the range of 10 micrometers 869 microrneters, preterabiy in the range of 4G

11. SSG micrometers. ti. A method according to ciaim t, wherein an average partlcle density on the membrane material is in the range approximately 50...25íi parti- cies f square centimeter.

12. A method according to clairn 2, wherein an average partiole density on the membrane material is in the range approximately 50...25G parti- cies i' square centimeter.

13. A method according to ciaim 8, wherein an average particie density on the membrane material is in the range approximately 5ÛH250 parti- cles I square centimeter.

14. A method according to ciaim i, wherein the solid particies corn- prlse aiumina particles.

15. A filter element to be used in removal ot liquid from soiids con- taining material to be dried in a capiliary suction dryer, the filter element com- prlsing a ceramic substrate covered by a sintered cerarnic rnicroporous layer, wherein the sintered microporous membrane layer contains coarse soild parti- cles of a particie size larger than a pore size of the membrane material layer.

16. A tllter element according to claim 15, tvherein the solid particies comprise alumina particies.

17. A filter element according to clairn 15, »vherein the particie size is in the range of approximately ti) micrometers 800 micrometers, prefere- bly in the range of approximately 40 39% microrneters.

18. A tiiter element according to ciaim 15, wnerein an average parti- cie density on the membrane material is in the range of approximately 59.256 particies /square centimeter.

19. A filter element according to ciaim 17, »vherein an axrerage parti- ole density on the membrane material is in the range of approximately 50.250 oarticies f square centimeter. 2G. A filter apparatus, comprising one or more filter elements, each filter element further oomprising a ceramic substrate covered by a sintered ce- ramic microporous layer, wherein the sinterecl microporous membrane layer contains coarse solid particies ot a particle size larger than a pore size of the membrane material layer.