WO2005028070A1

WO2005028070A1 - Plates and frames for filter presses with a high dehydrating capacity achieved by volume variations and a filter press made with said plates and/or said frames

Info

Publication number: WO2005028070A1
Application number: PCT/IT2004/000517
Authority: WO
Inventors: Fabio Donzella; Roberto Donzella
Original assignee: Galigani Filtri S.R.L.
Priority date: 2003-09-25
Filing date: 2004-09-22
Publication date: 2005-03-31
Also published as: ITFI20030247A1

Abstract

To achieve a better dehydration of the slurry collected in the filtering chambers of a filter press, the filter pack is be compressed at the end of the filtering process, by bringing the plates (5a) in the filter pack (5) closer together and consequently reducing the volume of the filtering chambers. Said volume reduction is obtained, for instance, by elastically deforming the edges (5f) of the interconnected plates forming the filter pack (5).

Description

Plates and frames for filter presses with a high dehydrating capacity achieved by volume variations and a filter press made with said plates and/or said frames DESCRIPTION Technical field The present invention relates to the sector of industrial fluid filtering processes using filter presses. To be more precise, it concerns filter presses, as well as the plates or frames that are employed to create the filter packs in filter presses. According to a different aspect, the invention concerns a new method for filtering with a filter press and, more specifically, a new method for dehydrating the slurry in a filter press, i.e. for reducing the liquid content in the slurry retained in the filter pack. State of the art Filter presses are well-known appliances used particularly in industry to separate solids from liquids, comprising a plurality of filter elements through which the fluid to filter is forced to pass under pressure. Filter presses are used to filter a great variety of fluids, both for human consumption and otherwise. For instance, they are used to clarify drinks, such as fruit juices, beer and other alcoholic and non-alcoholic beverages, as well as for purifying edible oils and other. In these cases, the solid - i.e. the slurry that is retained inside the filter pack - represents the waste deriving from the process, while the filtered liquid is the product obtained from the filtering process and destined for subsequent use. Filter presses are also used to extract a solid fraction from a fluid, the latter representing the waste product: this is typically the case in the pharmaceutical industry, for instance. The filter press is more efficient in separating solids from liquids than other types of machine (e.g. the belt press), particularly because of the high degree of dehydration that can be obtained in the slurry that is discharged. In fact, both when the solid is the waste product and when it represents the product destined for subsequent use, it is advisable for the solid to be as dry as possible or, more in general, for it to contain the smallest possible amount of fluid (be it water, oil or any other fluid). A typical filter press design is schematically represented in the attached Fig. 1. A filter pack is supported by bars (1) and held closed between a fixed end (2) and a mobile end (4) by the force generated by a linear actuator (6). The fluid to filter (i.e. the liquid containing the solid phase in suspension) is fed into the filter pack by a suitable pump, not shown, connected to the filter press by means of a suitable delivery inlet (7) installed on the filter press. The filter pack is composed of a set of plates (5a) that interact with each other to form a single complex assembly of filter membranes and feed and discharge manifolds. As an example of an embodiment, Fig.2 shows a detail of a longitudinal section of a portion of filter pack. In this example, the pack is composed of plates (5a) with frames (5b) coming between consecutive plates. The two facing sides of two consecutive plates and the frame in between them form a filtering chamber (5c). Between the frame and each of the two plates there is a filter element (5d). The filter element has a surface area the same size as that of the plate, or larger, and come in various types (cloth or nonwoven textile, polypropylene, Rilsan, paper, cardboard or any other suitable material). The fluid to filter is fed through a feed duct, that - in the example of Fig.1 - is provided in the plates and frames, and is indicated by the numeral (9). The liquid is delivered under pressure, through the duct (7), into the various filtering chambers (5c) and, because it is under pressure, it passes through the filter elements (5d). The filtered liquid is conveyed through drainage grooves, fluting or channels in the front wall of the plate and, through holes (13) in the plates, towards a collection and drainage duct underneath, indicated by the numeral (11). In the example illustrated, the collection and drainage duct (11) is provided in the plates themselves, but this is just one of the possible embodiments. In other presses, the holes (13) may drain into an open channel or closed manifold outside the plates. The solution adopted depends, among other things, on the nature of the liquid being treated. Moreover, although the feed duct (9) is at the side in the example illustrated in Fig.2, it may also be central and several ducts may even be provided in various places in the filter pack. In some types of press, so-called "interconnected" plates are used instead of a series of plates and frames. These plates have projections around their edges. The projections on adjacent plates abut against each other to form the respective filtering chamber. The raised edges substitute the intermediate frame. Depending on the filtering pressure, interconnected plates may or may not be provided with so-called central reinforcement "cups", i.e. projections that provide intermediate support between adjacent plates within the area surrounded by the raised edges. Examples of filter presses are described in US-A-3,981,802; US-A- 4,446,020; US-A-4,591 , 435; US-A-4,844,803. During the filtering process, the solid particles in the fluid to filter that are unable to pass through the screen because their dimensions are too large are selectively retained by the filter element and collect in each chamber inside the filter pack. The accumulation of solid particles begins on the filter surface and increases in thickness until it completely saturates the chamber. As the thickness of the layer of solid particles progressively increases, so too does the filtering pressure, following a mathematical function that varies according to the type of fluid to filter and the type of filter element. This phenomenon is due to the fact that the untreated fluid delivered to the chamber has to flow through the solid particles that have accumulated on the filter sheet, undergoing a preliminary filtration by the particles themselves before reaching the surface of the filter element proper. As the thickness of the solid particles accumulating on the filter sheet increases, the feed pressure of the fluid to filter must also increase to enable the fluid to permeate through said layer of solids, with a consequent reduction in the flow rate of the filtered liquid. When the space in the chamber is completely saturated, the flow rate of the liquid is drastically reduced and, at this point, the filtering process is stopped, the filter pack is opened, and the solid particles are discharged by separating the plates. The filter pack can be cleaned using various manual, automatic or semiautomatic methods well known to any person skilled in the art. The solid particles coming out of the chambers when the filter pack is cleaned are in the form of a slurry. One of the fundamental requirements of a filter press is for the slurry it produces to be as dehydrated as possible. The term dehydrated is used here to mean a slurry containing the smallest possible amount of liquid, even when this is not water, but oil, for instance. Sometimes the fluid to filter may have characteristics such that, when it is filtered with conventional plates, the resulting slurry is scarcely dehydrated, i.e. it has a high liquid content. When these slurries permit, so-called membrane plates are used to increase their degree of dehydration. These plates have one or more elastic membranes inside each filtering chamber, that are inflated at the end of the filtering process, when the whole chamber is full of solid particles. The membrane can be inflated using air, water, oil or any other gas or liquid, but an incompressible fluid is generally used. The volume reduction achieved inside the chambers, due to the effect of the inflated membranes, generates a pressure on the solid particles (equating to the pressure used to inflate the membrane) that facilitates the separation of the liquid remaining in the gaps between the solid particles of the slurry accumulating in the filtering chamber. The volume of liquid retrieved is the same as the volume occupied by the membrane once it has been inflated. The slurry deprived of said quantity of liquid is consequently more dehydrated, i.e. it has a lower liquid content. A filter press using this type of plate must have a more sturdy mechanical structure than the others, because the pressure needed to inflate the membrane is generally greater than the maximum filtering pressure achieved by the pump delivering the fluid to filter. These filter presses are also much more costly due to the fact that a further pump is needed - in addition to the pump for feeding the fluid to filter - that is capable of generating a pressure sufficient to inflate the membranes inside the various plates, which must also all be connected to said pump by means of a suitable distributor manifold. Examples of plates with membranes of the aforesaid type are described in US-A-4,839,045; US-A-4,749,482; US-A-6, 180,002. Objects and brief description of the invention The object of the present invention is to provide a new type of plate with which to succeed in further dehydrating the slurry obtained at the end of a filtering process, i.e. in reducing its liquid content, without having to use membranes inside the filtering chambers and therefore also the related devices needed to inflate them. This, and further objects and advantages that will become clear to the person skilled in the art on reading the following description, are achieved - in a first possible embodiment - with a filter press plate comprising at least one surface for supporting and abutting against an adjacent element (e.g. a similar plate or a frame) to form a filtering chamber in a filter pack in said filter press, characterized in that it is made at least partially of elastically yielding material, the elastic characteristics of which are such as to give rise to an elastic yielding and a consequent compressive deformation of the plate when it is compressed inside the filter pack under a pressure greater than the pressure normally applied to said filter pack during the filtering process, said elastic yielding giving rise to a reduction in the volume of the filtering chamber. Forming a filter pack with a plurality of plates of this type, defining one or more filtering chambers, at the end of the filtering process the pack can be compressed with a force greater than the maximum force applied during the filtering process. This causes an elastic yielding, i.e. an elastic distortion of the elastically yieldable part of the various plates, with a consequent reduction in the volume of the various chambers forming the filter pack. The reduction in the volume of each chamber causes the elimination of at least part of the residual liquid contained in the slurry inside the chamber. In other words, therefore, the elastic deformability under compression of the plate enables the same slurry dehydrating function to be achieved as when membrane plates are used. By comparison with the membrane plates of the known technique, however, the following advantages are obtained, among others: • the reduction in the volume of the slurry and the consequent discharge of the liquid phase do not necessitate an additional pump and a respective circuit for delivering a fluid under pressure. As a result, the machine gains considerably in structural and handling simplicity; • the volume reduction in the chamber is uniform, and a more uniform dehydration of the slurry is consequently achieved; • the same plates can be used in machines both with and without a slurry compression dehydrating function, substantially without affecting the costs; • the structural design of the plates is much more straightforward. In practice, according to an advantageous embodiment, the plate can be a so-called "interconnected" plate, as defined previously. In this case, the whole plate, the edge, or at least part of the edge, is suitably made of elastically flexible material. As will emerge clearly from the description of several examples of embodiments, the plate can be made of a single, sufficiently-flexible material, or it can be made of a substantially rigid material and provided with projecting areas of flexible material, or again it can include edge portions whose full thickness is made of elastically yelding material. In this last case, and in the case of a plate made entirely of elastically flexible material, the advantage of a considerable variation in thickness is achieved, and a consequently considerable reduction in the volume of slurry collected in the filter press chamber(s). When the plate has reinforcement cups, these may also be suitably made entirely or partially of elasticity flexible material. In another embodiment, the invention provides for the realization of filter packs for filter presses using alternately-arranged plates and frames. In this case, according to the invention, the frames - comprising a portion of edge that forms the supporting surface abutting against the adjacent plates in the filter pack - is characterized in that it is made at least partially of elasticity yielding material, the elastic characteristics of which are such as to induce an elastic yield of the frame when it is compressed under a pressure greater than the pressure normally applied to said filter pack, the compression and consequent elastic distortion determining a reduction in the volume of the filtering chamber. When the filter pack is composed of frames and plates, the elastic compressibility of the frames enables the same above-mentioned advantages to be achieved. For structural simplicity, the frame can be advantageously made entirely of an elastically flexible material. According to a different aspect, the object of the present invention is to provide a filter press that enables a reduction in the liquid content of the slurry at the end of the filtering process without the need to use membranes to compress the slurry in the single chambers of the filter pack. According to the invention, this object is achieved by providing a filter press comprising a plurality of plates as defined above and/or a plurality of plates with frames of the previously-described type between said plates. Further advantageous characteristics and embodiments of the frames, plates and filter press according to the invention are described in the attached claims. According to a further object, the object of the invention is to provide a filtering process with a filter press that proves more efficient and that can be achieved using machines that are more straightforward than those used today. According to the invention, this object is substantially achieved by providing a method for filtering a fluid through a filter press and separating a solid phase contained in said liquid, wherein the solid phase is collected in at least one or more filtering chambers formed between adjacent plates forming a filter pack, and wherein, at the end of the filtering process, the solid phase or slurry contained in the chamber(s) is compressed to expel at least part of the residual liquid contained therein; said method characterized in that the solid phase or slurry is compressed by reducing the volume of the filtering chamber(s) by bringing the adjacent plates forming said chamber(s) closer together. The plates can preferably be brought closer together by an elastic deformation of the plates themselves, in the case of interconnected plates, or by an elastic deformation of the frames. Maintaining the deformation within the elastic limit ensures that the plate and frame can be reused. However, it may be feasible to make filter elements with a yieldable area or portion that occupies a position, when the filter pack is assembled, between the opposite surfaces of two consecutive interconnected plates, or between a plate and a frame. In this case, exerting a compressive force on the filter pack causes the plates to move closer together due to the deformation of the deformable area or portion of the filter elements. In such case, assuming that disposable filter elements are used, the deformation may even be permanent, i.e. in the plastic field, and not necessarily only in the elastic field. In a practical embodiment, the method of the present invention comprises the following stages: • forming a filter pack, including one or more filtering chambers formed between pairs of consecutive plates, with a pair of filter elements inside each of said chambers; • closing the filter pack with a closing force; • feeding the fluid through the filter pack, causing the liquid to pass through the filter elements, and retaining the solid phase between the filter elements in the filtering chamber(s); • when the filtering process comes to an end, reducing the volume of said chamber(s) by bringing the plates defining said chamber(s) closer together by means of a yielding induced by the closing force exerted on the pack. Further advantageous characteristics and embodiments of the method according to the present invention are illustrated in the attached claims. In essence, using the interconnected plates or the frames forming the subject of the present invention, a conventional filtering process is conducted, at the end of which action is taken to increase the force generated by the linear actuator to further compress the filter pack. Said force shall be strong enough to induce a deformation of the highly elastic parts of the interconnected plates or of the frames, thereby producing a variation in the thickness of the plates or frames and consequently of the filter pack. The reduction in the thickness of the plate also induces a reduction in the thickness of the chamber, and a consequent reduction in the volume of the chambers. Said reduction in volume generates a pressure on the solid particles that facilitates the expulsion of the liquid in the gaps between said solid particles. The volume of liquid that is removed is the same as the volume reduction in the chambers due to the deflection of the raised edges. The slurry is deprived of said quantity of liquid and is consequently more dehydrated. The linear actuator that keeps the filter pack closed is subsequently moved away from the pack to enable it to be cleaned and the dehydrated slurry discharged. After the filter pack has been opened, the plates are no longer subject to the force that deflected them and they return to their original shape and size. The plates and/or frames made according to the invention do not differ in size or shape from the normally-used plates and/or frames that, as explained earlier, may be of various types; so the filter elements are installed on these plates in an entirely conventional manner known to anyone skilled in the art. Moreover, the new plates and/or the new frames according to the invention can be installed in an existing filter press (even without a compression- membrane slurry dehydration function), making it more efficient and enabling a reduction in the liquid content of the slurry at the end of the filtering process. For this purpose, it is sufficient for the filter press to be sized to withstand the extra pressure that has to be exerted at the end of the filtering cycle to compress the filtering chambers. Alternatively, filter elements can be used with particularly thick areas capable of plastically or elastically deflecting under the additional compressive load exerted on the filter pack at the end of the filtering cycle. The filter press may have one of a variety of configurations already adopted by the known technique or still to be implemented: for opening manually for manual plate cleaning, opening automatically for manual cleaning, opening automatically for an automatic, simultaneous cleaning of all the plates, for opening manually and automatically cleaning one plate after another, with a filter pack in view or enclosed inside a vessel that maintains different ambient conditions from the outside environment, and so on. Brief description of the drawings The invention will be better clarified by the following description and the attached drawing, which illustrates practical embodiments intended as non- restrictive examples of the invention. In the drawing, wherein identical parts are indicated by the same reference numbers: The previously-described Figs. 1 and 2 show a general layout of the filter press and an enlarged section of a conventional filter pack; Fig. 3 is a cross section of an interconnected plate to which two filter elements (joined together) are attached; Fig. 4 is an axonometric view of the two filter elements of Fig. 3; Fig.5 shows a set of interconnected plates lying side by side in an axonometric view; Fig. 6 shows a longitudinal section of a pack of interconnected plates with their related filter elements; Fig. 7 shows a longitudinal section of two adjacent interconnected plates during the filtering process, when there is an initial accumulation of slurry in the chamber; Figs. 8 and 9 show the chamber of Fig. 7 completely saturated with slurry at the end of the filtering process, respectively before and after compression to dehydrate the slurry; Figs. 10 and 11 separately show an axonometric view of two possible interconnected plate designs; Fig. 12 is a cross section of three different types of interconnected plate according to the invention; Fig. 13 is an axonometric view of a plate, filter element and frame for a type of press with frames and flat plates instead of interconnected plates; Fig. 14 shows an axonometric view of the filter pack comprising a plurality of elements of the type illustrated in Fig. 13; and Figs. 15 and 16 show a longitudinal cross section of a portion of the filter pack saturated with slurry at the end of the filtering process, respectively before and after compression to further dehydrate the slurry, i.e. to remove a further quantity of liquid from the slurry collected in the filtering chambers. Detailed description of the preferred embodiment of the invention As clearly emerges from the above description, the -invention can be implemented on any type of filter press of conventional design, so in the following description reference will be made to the general structure illustrated in Fig. 1. Referring initially to Figs. 3 to 6, in a first embodiment, the filter pack, globally identified again by the numeral 5 in Fig. 6, is composed of a set of so- called interconnected plates indicated as 5a. A single plate 5a is illustrated in the cross section of Fig. 3. In this example, it has a central bore 25 for the passage of the fluid to filter. The position of the bore 25 may also be similar to that of the duct 9 illustrated in Fig. 2, or several holes may be provided in the intermediate area of the plate. Filter elements 27 made, for instance, of cloth or a nonwoven textile, are placed on either side of the plate. The two filter elements 27 are joined together, e.g. by stitching in a central area coinciding with the through bore 25 in the plate. Fig. 4 shows an axonometric view of the filter elements 27 separated from the plate to give a better idea of their structure. An example of filter elements of this type is described in US-A-6, 007,717. The two faces of the plate 5a are complete with drainage grooves, fluting or channels, similar to those illustrated in Fig. 2. The filter elements 27 rest against the grooves in the plate, said grooves enabling the runoff of the filtered liquid towards the drainage holes 29 provided in the lower part of the plate, and communicating with a crosswise hole 31 that empties into a manifold, which may be open or closed, depending on the configuration of the press. A pack of plates 5a lying one against the other is illustrated in the axonometric view of Fig. 5 and the longitudinal cross section of Fig. 6. As shown in said figures, several adjacent plates 5a abut against each other in line with respective raised edges that have a square arrangement in the example illustrated, but that may take any form depending on the shape of the plate. In the example illustrated, the raised area on the edges, indicated by the numeral 5f, is provided on both sides of the plate using an elastically flexible material, e.g. an elastomer, rubber or any other material with elasticity characteristics suitable for the specific purpose, bearing in mind the filtering pressures in particular. Moreover, the material will be chosen as a function of its chemical compatibility and any foodstuff non-toxicity requirement for the products to treat. The areas around the edges of the filter sheets or elements 27 are pressed between the raised edges 5f on the adjacent plates 5a. Filtering chambers 33 are thus created between two consecutive plates 5a, all intercommunicating by means of the through bores 25 in the plates and the corresponding holes made in the central area of the filter elements 27. The fluid to filter is delivered through the inlet 7 in the fixed end 2 (Fig. 1) of the press, and it fills all the chambers 33 in the filter pack 5, which is held together and kept closed by the force that the linear actuator 6 exerts on the mobile end 4. The pressure at which the liquid is fed through the inlet 7 enables the liquid to move through the structure of the filter elements 27 and thus reach the area between the front face of each plate and the corresponding filter element 27. Through the drainage grooves, fluting or channels in the plates, the filtered liquid reaches the ducts 29 and then the outlet 31. The solid particles in suspension in the fluid to filter whose dimensions are greater than the passages in the filtering medium of the filter elements 27 remain trapped inside the chambers 33 and accumulate on the surfaces of the filter elements 27 facing into the chambers 33. Fig. 7 illustrates the situation created during the filtering process, with the formation of a layer of slurry S containing a large proportion of solid phase. As the filtering process continues, the layer S increases until it fills all the space in the chambers 33, as illustrated in Fig. 8. At this point, the filtering process must be suspended and, in normal presses, the slurry or solid matter that has accumulated in the chambers 33 is simply removed. In the chambers provided with membranes, it is compressed by inflating the membranes to enable a further extraction of the liquid still contained in the solid matter or slurry that has collected in the chambers 33. Conversely, according to the invention, at the end of the filtering process, when the chambers 33 are full and the fluid to filter can no longer circulate through the filter, the slurry or solid S inside the chambers 33 is compressed by reducing the volume of the chambers by bringing the plates 5a in the filter pack 5 closer together. This is done by providing (in the example described herein) for the edges of the plates 5 to be made of a material that yields resiliently under a pressure (applied again by the linear actuator 6) greater than the pressure needed to keep the filter pack 5 closed during the normal filtering process. The result obtained with this additional force F (Fig. 9) applied to the pack of plates 5a is a compression (in the direction of action of the linear actuator 6) on the elastically yielding edges 5f of the plates 5a, with a consequent reduction in the thickness of the edges 5f. This makes the plates 5a come closer together and consequently compacts the solids S collected in the chamber 33, with the corresponding expulsion of a quantity of liquid equating to the difference in the volume of the chambers before (Fig. 8) and after (Fig. 9) the additional force F of compression is brought to bear by the linear actuator 6. As it is easy to see from the above description, the dehydration of the slurry consequently requires no particular structural element on the filter press or on the single plates. It is only necessary for the structure of the filter press to be sized to withstand the additional force F that has to be applied at the end of the filtering process to compress the edges 5f and consequently also the slurry S collected in the chambers 33. Said oversizing of the filter press structure is also necessary for machines using membrane plates, since the structure of the filter press has to withstand the additional load determined by the membrane inflation pressure. The solution according to this invention is applicable not only to an interconnected plate of the type described above and further illustrated separately in the axonometric view of Fig. 10, but also to an interconnected plate of the so-called cupped type. This type of plate is schematically illustrated in the axonometric view of Fig. 11. In addition to the raised edges 5f, it also has a set of cups 41, i.e. of projections with respect to the grooved surface of the plate. These elements serve, in a well-known manner, to provide further support between adjacent plates. According to the invention, when the plate is fitted with cups 41 , the latter are made entirely or partially of elastically yielding material similar to the material forming the raised edges 5f. The material used to make the edges 5f and/or the cups 41 may be any material suitable for withstanding the pressure applied during the filtering process and during the subsequent further dehydration of the slurry obtained by compressing and reducing the volume of the filtering chambers. The elastic characteristic of the material must be such that it undergoes a limited or negligible deformation when the pressure needed to close the filter pack 5 is brought to bear during the filtering process, while being adequately further compressible (nonetheless remaining in the elastic field) when the additional force F is applied to dehydrate the slurry. In practical terms, the deformation of the elastically yielding material may be of a magnitude of 0.1-5% in the phase when the filter pack is closed by the linear actuator. Subsequently, after filtering, the pressure applied by the same actuator (or by different means of compression) may be sufficient to induce a deformation corresponding to a reduction of up to approximately 20-40% of the thickness of the elastically yielding material. Clearly, these values are merely indicative and not restrictive. In practical terms, the press could be fitted with a system for identifying the position of the mobile end 6. Said end is closed by a force applied by the linear actuator (that may be a cylinder and piston, a worm screw cum nut screw, or any other suitable type of actuator), that suffices to ensure a perfect watertight juxtaposition between the components of the filter pack 5 to prevent any leakage of the liquid during the filtering process. Having reached this position, which may involve only a very limited, entirely negligible compression of the filter pack, the mobile head is locked in place. When the actuator is of the cylinder and piston type, this can be done simply by stopping the outflow of oil or other fluid driving the actuator. The delivery of the fluid to filter under pressure does not coincide with any increase in the thrust load on the filter pack components because said force is discharged on the actuator. The position-controlling system ensures that any accidental leakage of oil from the actuator cylinder are compensated by a displacement to correct the position of the mobile end and thus guarantee that the pack remains closed. At the end of the filtering process, after stopping the delivery of the fluid to filter, the filter pack is compressed to further dehydrate the slurry it contains. This compression phase can also be controlled by monitoring the displacement of the mobile end. In the above-described example, the chambers 33 are compressed by making the projecting edge 5f on the interconnected plates 5a of elastically yielding material on both sides of the plate. This is not the only way to obtain a plate according to the invention, however. For instance, when a lesser degree of elastic deflection (and a consequently smaller reduction in the volume of the filtering chambers) suffices, then only one of the edges 5f needs to be made of elastically yielding material, while the other can be made of the same material as the central part of the plate. Said material may even be cast iron or another rigid material, as necessary. Other embodiments are also possible. Fig. 12 shows examples of three different structural solutions of the same inventive concept, marked. (A), (B) and (C). The configuration of Fig. 12C is the one hitherto described with reference to the previous figures. In Fig. 12A, the whole plate 5a is made of elastically yielding material, whereas in Fig. 12B the central portion of the plate is made of a substantially rigid material, while all the surrounding portion (and not only the part projecting with respect to the grooved surface of the plate 5a) is made of elastically yielding material, i.e. the full thickness of the perimeter area of the plate is made of elastically yielding material. In this solution, the joint between the two materials comprising the plate can be obtained using co-molding techniques of known type. The configurations of Figs. 12A and 12B are particularly advantageous for the following reasons. Because the whole thickness of the material submitted to the pressure applied to dehydrate the slurry at the end of the filtering process is yielding, the maximum proportional deflection is obtained, whereas in the case of the configuration of Fig. 12C the intermediate portion between the raised edges 5f is incompressible and consequently does not contribute to the reduction in the volume of the filtering chambers. When the invention is to be implemented on a press fitted with flat plates and frames, where the filtering chambers are defined by two consecutive plates with a frame inserted between them (as in the example illustrated in Fig. 2), it may be advantageous to provide for the frames to be made entirely or partially of elastically yielding material. A solution of this type is illustrated in Figs. 13 to 16. Fig. 13 separately shows an axonometric of the three fundamental components in a filter pack of this type. To be more precise, the filter pack in this case is composed of a series of plates 50, filter elements 51 and frames 53. Fig. 14 shows a filter pack, again indicated by the numeral 5, comprising a series of elements 50, 51 and 53. All these elements are perforated to define ducts for the delivery of the fluid to filter and the recovery of the filtered liquid. Figs. 15 and 16 schematically show a longitudinal section of the filter pack 5 obtained with a series of the elements 50, 51 and 53 of Fig. 13. For the sake of simplicity of graphic representation, the grooves in the plates are not illustrated here, but are nonetheless provided on both sides of the plates, as shown in the axonometric view of Fig. 13. In the section of Figs. 15 and 16, the ducts for delivering the fluid to filter and collecting the filtered liquid are also omitted, since this can be done in a manner well known to anyone skilled in the art. As shown in Figs. 15 and 16, a filtering chamber, again indicated by the numeral 33, is created between each pair of consecutive plates 50 and the respective frame 53 inserted between them. Clearly, inside each chamber 33 there are also two filter elements 51 held between the opposite sides of the respective frame 53 and the two plates 50. The liquid delivered through the feed duct to the chambers 33 percolates through the elements 51 and, via the grooves in the plate 50, reaches the duct collecting the filtered liquid. The solid phase thus accumulates in the chambers 33 and, as illustrated in the layout of Fig. 15, it ultimately completely fills the chambers 33. At this point, the solid phase is dehydrated by applying an additional force F, as in the previously- illustrated example, that comes to bear on the frames 53 lying between the adjacent plates 50, elastically deforming them by compression. This induces a reduction in the volume of each filtering chamber 33 and a consequent expulsion of a corresponding quantity of liquid from the slurry contained inside the chambers. Even in the case of filter packs comprising a series of flat plates and frames, the frames can be made not entirely, but only partially of elastically deformable material. For instance, they can be composed of two layers of elastically yielding material with a rigid layer sandwiched between them. In the examples illustrated, there are several filtering chambers in series inside the filter pack. It should be however understood, that the present invention is also applicable to filter presses with a single filtering chamber, i.e. wherein the filter pack comprises only two plates defining a filtering chamber between them (with or without an intermediate frame). It goes without saying that the drawing shows just one example, given simply as a practical demonstration of the invention, which may vary in shape and arrangement without departing from the spirit and scope of this invention.

Claims

CLAIMS 1. A filter press plate, comprising at least one supporting surface abutting against an adjacent element in a filter pack in a filter press to form a filtering chamber, characterized in that said plate is at least partially made of an elastically yielding material, the elastic characteristics of which are such as to give rise to an elastic yield of the plate when it is compressed at a pressure greater than the pressure normally applied to said filter pack during the filtering process, said elastic yield determining a reduction in the volume of the filtering chamber to further reduce the content of liquid retained in the slurry collected within said chamber. 2. Plate as in claim 1 , characterized in that said plate comprises an edge area forming a frame defining surfaces that abut against said elements adjacent to the plate in the filter pack of said filter press, and that at least a portion of said edge area is made of elastically yielding material. 3. Plate as in claim 2, characterized in that the whole thickness of the perimetral edge area of said plate is made of yielding material. 4. Plate as in claim 1 or 2 or 3, characterized in that it includes cups abutting against adjacent elements in the filter pack, said cups being at least partially made of elastically yielding material. 5. A filter press frame, comprising an edge portion forming a supporting surface abutting against adjacent elements in the filter pack of said filter press to create filtering chambers, characterized in that it is made at least partially of elastically yielding material, the elastic characteristics of which are such as to give rise to an elastic yield of the frame when it is compressed at a pressure greater than the pressure normally applied to said filter pack, said elastic yield determining a reduction in the volume of the filtering chamber to further reduce the liquid content in the slurry contained in said chamber. 6. Frame as in claim 5, characterized in that it is made entirely of elastically yielding material. 7. A filter press comprising a filter pack with at least one plate according to one or more of the claims 1 to 4. 8. A filter press comprising a filter pack with a plurality of plates and, between each pair of plates, a respective frame that, together with said plates, creates a filtering chamber, characterized in that said frames are made according to claim 5 or 6. 9. A filter press comprising: • a filter pack with a plurality of plates forming one or more filtering chamber(s) created between adjacent pairs of plates; • filter elements inside said filtering chambers, through which the fluid to filter passes; • a duct for delivering the fluid to filter; • a duct for collecting the filtered liquid; • at least one actuator for closing the filter pack; • a system for pumping the fluid to filter, the actuator compressing the filter pack with a closing force during the delivery of said fluid; characterized in that said filter pack comprises portions that yield under the effect of a compressive force exerted on the filter pack, the deformability characteristics of which are such as to enable a deformation by increasing the force applied to the filter pack beyond the closing force used during the filtering process, said deformation causing a reduction in the volume of the filtering chambers inside due to said plates moving closer together. 10. Press as in claim 9, characterized in that said portions of the filter pack are elastically deformable. 11. Press as in claim 9 or 10, characterized in that said force causing the deformation of the filter pack and the consequent reduction in the volume of the filtering chambers is generated by the same actuator used to close the filter pack. 12. Press as in claim 9, 10 or 11 , characterized in that at least one of said plates has at least one elastically yielding portion, to achieve said elastic deformation. 13. Press as in claim 12, characterized in that said plate(s) are provided with raised edges forming supports between adjacent plates, pairs of adjacent and consecutive plates forming a respective filtering chamber between them by means of said raised edges on said plates, and that said edges are at least partially made of an elastically yielding material. 14. Press as in claim 13 characterized in that said plates include cups within said raised edge, said cups forming an abutment surface between adjacent plates and being at least partially made of elastically yielding material. 15. Press as in claim 9, 10 or 11 , characterized in that a respective frame is arranged between each pair of adjacent plates in said filter pack, said frame forming a respective filtering chamber with the adjacent plates, and that said frame(s) are at least partially made of elastically yielding material. 16. Press as in claim 15, characterized in that said frames are made entirely of elastically yielding material. 17. Press as in claim 9 or 11 , characterized in that filter elements are inserted in said filter pack, each of which has a portion in contact with the plates or frames forming said filtering chamber(s), said portion being made of a flexible material, the force exerted on the filter pack inducing the compression of said portion and a consequent reduction in the volume of the filtering chambers. 18. A method for filtering a fluid in a filter press and separating a solid phase contained in said fluid, wherein the solid phase is collected in at least one filtering chamber created between adjacent plates in a filter pack, and wherein the content of said chamber is compressed at the end of the filtering process to expel at least a part of the residual liquid contained therein, characterized in that said content is compressed by reducing the volume of the filtering chambers by bringing said adjacent plates closer together. 19. Method as in claim 18, characterized in that it comprises the following phases: • Forming a filter pack, comprising at least one filtering chamber created between a pair of consecutive plates, with filter elements provided inside said chamber; • closing the filter pack with a closing force; • delivering said fluid under pressure through said filter pack, causing the liquid to pass through said filter elements, while the solid phase is retained in said at least one filtering chamber; • reducing the volume of said at least one chamber by bringing the plates defining said chamber closer to each other. 20. Method as in claim 18 or 19, characterized in that, at the end of the filtering process, the filter pack is compressed with a force greater than the force needed to close the filter pack, in order to induce a reduction in the volume of said at least one filtering chamber. 21. Method as in claim 18 or 19 or 20, characterized in that the volume of said chambers is reduced by the elastically deforming elastic portions of said filter pack. 22. Method as in claim 21 , characterized in that interconnected plates are used, each of which has a raised edge, the edges of consecutive plates abutting against each other to form a respective filtering chamber between said plates, at least one of said edges being at least partially made of elastically yielding material and being deformed by compression to reduce the volume of the respective chamber. 23. Method as in claim 22, characterized in that on said plates, within the respective edges of said plates cups are provided for mutual abutment between adjacent plates, said cups being partially made of elastically yielding material. 24. Method as in claim 18, 19, 20 or 21, characterized in that a frame is provided between adjacent plates, that defines a filtering chamber with the respective adjacent plates, and that said frame is at least partially made of elastically yielding material and is deformed by compression to reduce the volume of the respective chamber. 25. Method as in claim 24, characterized in that said frames are made entirely of elastically yielding material. 26. Method as in claim 18, 19, 20 or 21 , characterized in that filter elements are inserted between the plates and/or frames forming said filter pack, at least some of which filter elements have areas that yielding under compression, said areas being inserted between abutting surfaces of said plates and/or frames, the yield under compression of said areas causing said plates to come closer together and thus give rise to a reduction in the volume of the corresponding chamber.