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EP 1 359 990 B12510152025303540455055Description[0001] The invention relates to a production method ofa ceramic filter element to be used in removal of liquidfrom solids containing material to be dried in a capillarysuction dryer in order that a filter cake of solid materialis achieved on at least one surface of the filter element.[0002] The U.S. patent 4,981,589 concerns a filter con-struction and a method in particular for capillary suctiondryers. The filter construction comprises a first filter ma-terial layer of a ceramic material, which is the first layerin relation to the flowing liquid, and a second filter materiallayer of a ceramic material, connected with said first layer.The first filter material layer acts as the filtering layer prop-er, and the second filter material layer acts as a layer thatsupports the construction.[0003] The U.S. patent 4,863,656 relates to a methodfor manufacturing a microporous plate and a filter plateobtained from the method. According to the method, acasting mix is introduced into interior of gypsum moldwhereupon water is absorbed from the casting mix intothe gypsum so that a crust remains on the inner surfaceof the mold. The crust is allowed to develop to an appro-priate thickness whereupon the remaining casting mix isdrained from the mold. The inner surfaces of the crustare allowed to dry whereupon the interior space definedbetween opposed crust wall portions is filled with a gran-ular material which preferably has the same compositionas the crust material. A filter plate comprises a pair ofopposed suction walls defining an interior space betweenthem which is filled with a granular material.[0004] In those U.S. patents 4,863,656 and 4,981,589the ceramic filter plates are manufactured so that theceramic filter plates have a microporous surface layerhaving the pore size between 0,5 to 2,0 micrometer anda support layer inside the filter plates. The support layeralso contains recess areas for instance for the removalof liquid which is filtered from a material creating a filtercake on the microporous surface of the filter plate. Therecess areas are generally shaped on the surface of thesupporting layer and for the final manufacturing two op-posite sides of the filter plate are glued together. Thusthe filter plate made of two opposite sides is very weakin the interface of these opposite sides. Also the proper-ties of the filter plate in that position are quite differentfrom the ones in the other parts of the filter plate.[0005] The object of the present invention is to elimi-nate drawbacks of the prior art and to achieve a a methodfor the manufacture of a ceramic filter element in whichelement the desired recess areas are produced withoutany gluing stage. The essential features of the inventionare enlisted in claim 1. Further embodiments arise fromthe appended subclaims.[0006] According to a ceramic filter element aimed bythe invention to be used in removal of liquid from solidscontaining material to be dried in a capillary suction dryerhas a microporous surface layer where the pore size isunder 5 micrometer, preferably between 0,2 to 3,0 mi-crometer, and this microporous surface layer is support-ed by at least one substrate. The substrate has meansfor the removal of liquid from the microporous surfacelayer. Therefore, the substrate has inside at least onerecess area. The filter element has also a fitting areawhere the substrate of the filter element is in mechanicalcontact with the capillary suction dryer. In any other areasof the filter element the substrate is advantageously sur-rounded by at least one essentially continuous micropo-rous surface layer. This means that the ceramic filter el-ement has an essentially continuous filtration surface inany other areas separated from the fitting area with thecapillary suction dryer.[0007] When manufacturing the ceramic filter elementof the invention the internal layer is formed of at least onesubstrate. The substrate is preferably made of a ceramicmaterial in a powder form, such as for instance alumina,silicon carbide and titania. The substrate can also bemade of a metal or a metal alloy, polymer or graphite. Inthe preferred embodiment the ceramic material is mixedwith a binding medium and liquid so that the ceramic mixformed is suitable for further processing. The ceramicmix is first charged into a mold so that the mold is partlyfilled with the ceramic mix. The core material for at leastone desired recess area is then installed on the surfaceof the ceramic mix in the mold. Finally the rest of theceramic mix is charged into the mold. Depending on theshape and the number of the desired recess areas thecharging of the ceramic mix into the mold can also bedivided to three or more stages. When the total amountof the ceramic mix has been charged into the mold, theceramic mix is pressed into a green body. After pressingthe green body is sintered in the temperature range of1150-1550° C. During the sintering stage of the greenbody, the core material for the recess area is burnt outthrough the porous structure of ceramic mix. Instead ofthe core material the substrate contains the recess areaor areas in a shape of the core material.[0008] After the sintering stage of the substrate con-taining the desired recess area or areas the substrate iscovered by at least one microporous layer. The coveringprocess is preferably carried out by dipping the substrateinto a bath of the microporous layer material. The cover-ing process can be also carried out for instance by spray-ing or tape casting the ceramic microporous layer mate-rial on the surface of the substrate. The microporous layermaterial can also be made of a metal or a metal alloy,polymer or graphite. After the covering process the sub-strate with the microporous layer material is sintered atthe temperature range of 1150-1550° C. If further ceramicmicroporous layer is needed, the covering and sinteringprocess are repeated respectively. After the sintering ofthe microporous layer the filter element is essentiallymonolithic having a hierarchical structure and the filterelement is then ready for mechanical processing. Thismeans that the filter element is drilled if the hole or holesfor the fitting member are not formed with the core ma-terial during the previous forming stage so that a fitting1 2