Filter bodies with membrane coatings are well known to those skilled in the art.

For instance, the U. S. patent 5,560,874 discloses a partially coated permeable ceramic substrate; two coatings are utilized in the process of this patent and are deposited onto the substrate. Filter bodies are also often used in filter plate assemblies for capillary driers, as described in the U. S. patent 4,981,589.

Filter plates used in drying assemblies often must withstand severe corrosive environments. Thus, for instance, they may be exposed to concentrated acids at elevated temperatures such as concentrated sulphuric acid and concentrated nitric acid at a temperature in excess of 90 °C. Exposure to such acids is often encountered in hydrometallurgical processes. Filter plates are also often subjected to hot caustic in hydrometallurgical processes or in the paper making field. Further, filter plates are used to filter waste streams containing abrasive material.

The filter plates used do not have adequate abrasion resistance. The materials which have sufficient porosity to allow the passage of a filtrate frequently do not have sufficient chemical resistance to the liquid being filtered. One solution to this problem is described in the U. S. patent 5,415,775 in which patent it is disclosed that"... the ends of the supports or of the membranes are made leaktight by a thin film of a thermoplastic or a thermofusible polymer... The polymer is deposited by any suitable known technique such as by coating with a polymer suspension, by an electrostatic process or by direct deposition by means of a fluidized bed of polymer powder followed by heat treatment.".

The U. S. patent 5,415,775 is directed to a porous monolithic ceramic support, and the process of this patent works well with such porous monolithic ceramic support. However, when the process of the U. S. patent 5,415,775 is attempted to be used with a ceramic support which is not monolithic, it is an abject failure.

The polymeric material does not adhere well to all of the surfaces of the support to which it is applied.

The object of this invention is to eliminate some drawbacks of the prior art and to achieve a filter element with improved and durable corrosion resistance which contains on some of its surfaces a polymeric material durable bonded thereto. The essential novel features of the invention are apparent from the appended claims.

In accordance with the invention the filter element is a filter to be used in filter- ing solids containing flowing material in order to form a filter cake of solid material at least on one external surface area of the filter element. At least one part of the external surface area of the filter element separated from the exter- nal surface area for forming the filter cake is provided with at least one corro- sion and heat resistant polymer-based material. The filter element itself can be manufactured of a porous ceramic material or for instance of a metal or a metal alloy material. The filter element itself can also be manufactured of a carbonic material. The filter element of the invention may preferably be applied for instance to a suction drier provided with a fine porous suction surface, where the radii of the fine pores of the suction surface are mainly within the range of 0.5-2 micrometers. The fine porous suction surface is the surface for forming the filter cake.

The filter element of the invention is a one-piece element or the filter element contains at least two parts which are bonded to each other. When using a one-piece element for the filter element of the invention the filter element is

advantagenously covered with a corrosion and heat resistant polymer-based material on the outer surface area except on the part of the external surface where the filter cake is formed. Thus the corrosion and heat resistant material will cover the parts of the external surface area which have a contact with the frame of the filter element in order to compress the frame to the filter element.

The filter element can also be provided with corrosion and heat resistant polymer-based material on the parts of the filter element which will surround the external surface area for forming the filter cake.

The filter element of the invention can also be made of at least two parts which are bonded to each other mechanically or chemically. Between the parts there can be a space where the filter medium is conducted in order to remove the filter medium from the filter element. The boundaries between these parts of the filter element are sensitive for corrosion and heat resistant and therefore, the surfaces of the filter elements with boundaries are provided with a polymer- based material having good resistance for corrosion and heat. The polymer- based material can also be used to compress areas of the parts of the filter element as well as to compress the filter element to the frame of the filter element.

The polymer-based material used in the invention consists essentially of thermoplastic polymeric material, fluoroplastic material and/or thermofusible polymeric material. It is preferred to utilize thermoplastic polymeric material.

Among the thermoplastic polymers, the preferred polymers are optionally grafted halogenated or perhalogenated polyolefins. In one embodiment, the optionally grafted fluorinated and perfluorinated polyolefins are preferred.

Fluoroplastics are a class of paraffinic polymers that have some or all hydrogen atoms replace by fluorine. Such fluoroplastic materials include tetrafluoroethyl- ene (TFE), fluorinated ethylenepropylene (FEP), ethylene-tetra-fluoroethylene copolymer (ETFE), perfluoroalkaloxy resin (PFA) and ethylene- chlorotrifluoroethylene copolymer (ECTFE).

In accordance with the method of the invention structures consisting of thermo- plastic material are formed by conventional means. Thus, for instance a thermoplastic resin in powdered form may be charged to a mold and heated.

Once the structures for parts to be manufactured of thermoplastic material have been inserted into their appropriate places, they may be subjected to heat and pressure to bond them to the permeable ceramic substrate.

In one preferred embodiment of the invention, ethylene-tetrafluoroethylene (ETFE) is used. As is known to those skilled in the art, ETFE is a predominantly 1: 1 alternating copolymer of ethylene and tetrafluoroethylene which forms linear chains, it has a melting point of 270 °C, and it has a density of 1.70 grams per cubic centimeter.

In another preferred embodiment of the invention, polyvinylidene fluoride (PVDF) is used. This material is a high-molecular weight polymer of vinylidene fluoride. It is a crystalline material with a melting point of 170 °C and a density of 1.78 grams per cubic centimeter.

When manufacturing the filter element of the invention a guiding means, as a jig or the like, may be used to attach the structures of thermoplastic material to the filter substrate. In the preferred embodiment spring-loaded bolts are adapted to hold the filter substrate assembly in place while simultaneously applying pressure thereto and allowing the assembly to be subjected to heat. It is also possible to use hydraulic or pneumatic or electric means to hold the filter substrate assembly in place. As will be apparent, the entire assembly within the guiding means may be placed in a suitable furnace.

The invention is explained in more detail below, with reference to the appended drawings where Fig. 1 is a perspective view of one preferred filter element of this invention,

Fig. 2 is a sectional view of the filter element of Figure 1, Fig. 3 is a top view of the apparatus for bonding a thermoplastic sleeve to a ceramic filter element.

Figure 1 is a perspective view of one preferred filter element 21. Referring to the filter element 21, it will seen that such filter element is comprised of a coated ceramic substrate 22, an edge seal 24, an edge seal 26, an edge seal 28, an edge seal 38, a base seal 30 and an orifice 32. The edge seals 24,26, 28 and 38 and the base seal 30 consist essentially of thermoplastic material. As illustrated in Fig. 2, the filter element 21 is also comprised of a plenum 40, a ceramic substrate 42, a ceramic substrate 44, and coatings 46,48,50 and 52.

A pressure is applied to the preformed polymeric structures to press them against the ceramic structure while heating the entire assembly to a tempera- ture sufficient to melt the thermoplastic. In general, a pressure at least 0.5 bar square meter is applied to at least side 24,26,28, and 38 and, preferably, the bottom surface 27.

The temperature used is enough to melt at least portion of the thermoplastic articles. In general, a temperature of at least about 120 OC is required, and it is preferred to use a temperature of at least about 150 °C. In one embodiment, a temperature between from about 240 OC and 270 °C is used.

Figure 3 shows a top view of a jig 60 which may be used to attach the struc- tures of thermoplastic material to the ceramic substrate. The spring-loaded bolts 62,64,66,68,70,72,74 and 76 are adapted to hold the ceramic substrate assembly in place while simultaneously applying pressure thereto and allowing the assembly to be subjected to heat. As will be apparent, the entire assembly within the jig may be placed in a suitable furnace.