Applicant: DANIELI & C. Officine Meccaniche S.p.A., Via Nazionale 41, 33042 Buttrio (UD), Italy

TECHNICAL FIELD

The invention belongs to the field of filtration of lubricants and/or cooling fluids with filtration units using filter webs and the technique of precoating and body-feed. These cooling fluids and lubricants come in particular from steel or metallurgical plants, such as rolling mills. Filtration is necessary in order to recover fluids to be used as coolants/lubricants. Applications in the field of water treatment and purification in general are also conceivable.

STATE OF THE ART

Milling, in particular of aluminium and its alloys, involves the use of lubricating oils - based on kerosene instead of emulsified water - which also act as coolants. Kerosene avoids the unpleasant surface oxidation aspects of aluminium, which is much more reactive than steel when in contact with water. However, this lubricant is more expensive than water, so it is important to be able to filter and reuse it as much as possible.

In the state of the art, filtration takes place through devices by means of "towers" of trays containing filter soils (or filter cakes), through which the lubricant or dirty cooling fluid is pumped under pressure. These soils are wrapped in layers of a particular absorbent cloth, generally referred to as filter web, which acts as an additional filter and support for the soils and which must be changed regularly.

These towers have a high preparation cost as they are made from special grid-shaped castings that are very expensive. From an operational point of view, the column or tower filters are also inconvenient, since they all work together; hence, when the soils have been cleaned and the web replaced, they must be stopped together and production must therefore be interrupted if there are no reserves of clean lubricant.

The tower systems include a plurality of fresh web reels that feed the individual units of trays arranged in the tower one on top of the other. After the individual units have been cleaned (removal of soils that can no longer be used), the dirty web is removed from the tower by pulling/dragging new web into the system and extracting the dirty web. In addition, the individual units are pressed during filtration: this is not so much to wring the soils and let the clean fluid come down, as to seal the units between them. In an exemplary application, pressure levels reach about 40 tons. For cleaning purposes, the machine needs to be shut down regularly, the stopping time being about 20 minutes, which implies that a reserve quantity of clean cooling fluid or lubricant must be available so that production can continue without shutting down the rolling mill. Conversely, as seen, the production line must be stopped. The cycle time of a standard filter varies from about 8 hours to about 24 hours.

In addition to the high cost of manufacturing tower filters, the system for handling and pressing the plates into the tower is a complex one, and as there are a number of components to maintain that can be prone to breakdown, overall it is very expensive.

A vertical filtration device is disclosed in U.S. Patent 4,292,173, wherein several filtration units are held together by the movement of a single piston, and fluid and soils are introduced together. Further columnar plate filters are disclosed in documents US 4, 329,228, US 4,869,834, and US 5,238,584. No document has posed the problem of avoiding the simultaneous use of plates or avoiding the shut-down of the filtration plant and consequently of the rolling mill. The state of the art is mainly dedicated to the recovery of the filter material, for example also document US 5,258,119. From other fields of application, individual filtration units (US 2018/0280839 A1 and US 5,354,465 A) or groups of filtration units (FR 2 609 645 Al) are known which anyway do not allow sealed microfiltration at high pressures or prevent the entire filtration plant from shutting down.

DISCLOSURE OF THE INVENTION

The object of the invention is to overcome the aforementioned drawbacks and propose a new filter configuration or a filtration plant and process which avoids filtration and production stoppages of plants that use the cooling fluid or lubricant during maintenance, cleaning or repair of the individual filtration units.

A further object of the invention is to propose a filtration plant and process that allows the filter web to be operated at high pressures, up to four times higher than the state of the art, thereby increasing filtration efficiency. Another object of the invention is to provide a filtration plant and process which is cheaper than state of the art systems, which saves in particular in terms of material requirements and reduces the volume of reserve fluid required, which further avoids the use of complex movement machinery, such as hydraulic cylinders, for the construction of the hermetically closed filter chamber, but which at the same time exerts sufficient pressure on the filter web and possibly other filter media.

Further objects and advantages of the invention result from the description below.

In a first aspect of the invention, the object is achieved by means of a filtration plant for fluids, in particular cooling fluids and/or lubricants comprising:

(a) a plurality of tunnels each divided into an upper part and a lower part, provided with at least one inlet, in particular in the upper part, and at least one outlet, in particular in the lower part, for introducing the fluid to be filtered and for extracting the filtered fluid respectively, wherein each tunnel has at each end a hermetically closable opening for the inlet and the outlet of a filtering medium, in particular filter web;

(b) a framework for housing said tunnels;

(c) a permeable support in the lower part of each tunnel to support the filter medium, in particular the filter web;

(d) a valve assembly for each tunnel, which can be configured to isolate the tunnel from fluid flows entering and/or leaving the tunnel.

The horizontal development of the filtering chambers according to the invention provides for the cleaning surface to be distributed over a smaller number of planes, thus making it possible to reduce the frequency of changing the filter medium. Furthermore, it is possible to arrange the filters in parallel, shutting down only one filter (tunnel) at a time and working meantime with the others, thereby maintaining a higher level of productivity in terms of recovered clean cooling fluid or lubricant. The proposed plant is suitable for filtering fluids in general, in particular cooling fluids and lubricants, such as lubricating oils.

The plant according to the invention avoids the use of complex devices to achieve the pressure on the filter medium and create the closed filtering chamber, which is not created anew each time by the compression of plates, but is rather already configured in the structure of a tunnel in the form of an oblong casing. The division into two parts is also intended as a virtual division, not necessarily two physically separate parts, although this is the preferred embodiment to simplify production and maintenance and the use of the tunnel in general. Advantageously, the two parts are divided by a horizontal longitudinal section of the tunnel. The term "horizontal" refers to the fact that the section plane is parallel to the tunnel installation plane.

The two openings at the ends of the tunnel are used, on opening, to allow passage of the filter medium, in particular the filter web, i.e. to let in clean filter web from, for example, the respective rollers and to let out dirty filter web from the opposite side, web which will preferably be wrapped on other rollers. Various systems for the passage of web are widely known in the state of the art, as are methods and apparatuses for possibly removing the filter cake from the web. A new web can easily be introduced into the tunnel, for example by attaching it to a plate and passing it through the tunnel. The supports can also be introduced or replaced.

Advantageously, the tunnels are also equipped with one or more inspection openings, particularly in the upper part of the tunnel, to assess the state of the filter web and detect possible clogging problems, etc.

The framework may preferably contain shelves or support structures for a variable number of tunnels, for example four shelves for four tunnels. An assembly consisting of frameworks with shelves facilitates each removal of a tunnel for maintenance purposes, but also access to the tunnel itself without detaching it from the framework. Obviously, the expert identifies types of frameworks using his/her general knowledge.

An advantageous form of the inlet for introducing the lubricant or cooling fluid to be filtered into the system is a perforated tube, i.e. a distributor tube, with a plurality of holes, extending into the tunnel along its longitudinal extension.

In one embodiment of the invention, the lower part of the tunnel has, for example, a square or triangular section channel at its bottom to collect the filtered fluid. Other forms of sections are conceivable. The bottom itself may, for example, be reinforced by the presence of a plurality of C-shaped profiles or ribs. As is known from the state of the art, compared to plate filters much higher pressures can be achieved, also even up to about 10 bar. Thanks also to the excellent seal offered by the separate tunnel system, this guarantees a high degree of filtration, particularly in the field of microfiltration such as offered by filter web, with flow rates of thousands of litres per minute. This avoids the use of huge filter media that cannot be made up of rigid frames that are an intrinsic part of the filter septum.

The filter medium may be of various kinds, but filter web is especially preferred. The filter web can be made of various materials, not only web, i.e. cellulose, but also of other natural or synthetic polymeric materials and materials of the non-woven fabric type. The list is not exhaustive. The term "filter web" means any cloth, usually in the form of a tape or sheet, adapted to filter a fluid. From among the webs available on the market, the person skilled in the art effortlessly chooses the most suitable one for the fluid and the particles contained therein that are to be separated.

Reinforcing ribs or profiles may also be present on the structure forming the upper part of the tunnel.

According to the invention, pressures of around 2.5 bar are typically obtainable in a tunnel, sufficient to press the filter medium against its support.

The support can be of various shapes, with grids or meshes, usually metal, being especially preferred. In a preferred embodiment of the invention, the support is a combination of a common grid and a mesh, for example in stainless steel, which are overlapping, the mesh being placed on the filter web side.

The support can be constructed from a plurality of adjacent grids, the standard grids available on the market often having a length of 2 m.

Preferred forms of the plant according to the invention have one or more air vents to eliminate the air trapped in the upper part of the tunnel during filling of the initially empty tunnel with the fluid to be treated.

A structure of the filtration plant in the form of individual tunnels allows for separate management of each tunnel, not requiring for the creation of the filter chamber the joining of a series of plates which can then only be separated together, not individually.

The oblong shape of the filter and the force exerted by the fluid to be filtered on the filter medium do not require the application of high forces on the structures that delimit the filtration chamber, i.e. the tunnel walls. In a preferred embodiment of the invention, said tunnels may therefore be made of sheet metal, preferably reinforced by ribs or profiles in the structure. The structure itself must not exert force on the filter medium. The plates of the filter towers of the state of the art are often made of aluminium, no longer necessary for the tunnels according to the invention, as the need to machine the plates in a special way and to check them frequently is eliminated. Ribs or profiles can be added to the tunnel walls, not only at the bottom but also at the upper part, to reinforce them further or also to save material for the base structure.

The replacement of filtration towers with the tunnel system according to the invention allows the reduction of costs to less than one third.

A typical tunnel length is 6 m. With a width of 1.5 m, a total filtration section of 9 m ² is obtained. A typical height is about 0.50 m.

To keep the filter web resting against the support, the filtration plant according to the invention in a particularly preferred embodiment comprises in each tunnel a framework, preferably metallic, placed in such a way that it can be raised and lowered above said support.

During filtration, the fluid exerts a high pressure that presses on the supports of the filter medium and on other filter materials that may be present, such as diatomaceous earthh placed on filter web. Therefore, such a frame is sufficient to press the filter medium with its own weight on its support and to prevent its movement during the initial filtration phase and lateral leakage and dripping. This frame also serves as a containment element for fluid and possibly additional filter materials from a precoating or the like.

The filter web pressed by the weight of the fluid to be filtered adapts in the invention to the underlying surface and acts as a sealant. The greater the pressure that squeezes the filter medium, the greater the sealing effect it exerts. Not only does the framework hold the web in place during the initial filling phase, it also does so during phases in which the operating pressure may be low, to prevent the formation of parts in which the web is not in contact with the underlying surface, which would cause rapid "by-passage" of the fluid, lowering the filtration pressure at least in some points of the system.

In a preferred embodiment of the invention, said openings are provided with separately operable doors opening into the tunnel. Such a closure increases the sealing capacity of the opening in the closed state as the internal fluid presses on the doors in the closing direction and not in the opening direction. The internal hydraulic pressure thus increases the strength of the seal. Advantageously, the doors are driven by pneumatic pistons. Other systems known to the skilled person are also conceivable. Gaskets, such as those made of fluoroelastomer, are preferably also present between the tunnel doors and walls. Preferably, said doors are configured to interact with said frame so that an opening of the door raises the frame and a closing of the door lowers the frame. Therefore, when the door opens inwards, it simultaneously raises the frame that presses on the contour of the filter medium, in particular on the filter web, allowing it to advance in the cleaning cycle.

In this regard, the filtration plant according to the invention can also include rollers with fresh filter web that feeds the tunnel, and rollers that wrap the outgoing dirty web, and optionally systems for removing the filter cake from the dirty web.

A further variant of the filtration plant according to the invention further comprises

(e) a first tank for the dirty fluid connected via related first conduits to the inlets of said tunnels;

(f) a second tank for the filtered fluid connected via related second conduits to the outlets of said tunnels, wherein the second tank is provided with a weir configured to let fluid flow in a controlled manner into the first tank.

This configuration allows the plant to be managed continuously, as will be explained in more detail below, even if individual tunnels are temporarily shut down for maintenance.

This is especially important when the plant is integrated into a rolling mill plant. In this regard, for example, the first tank receives the dirty fluids from the rolling mill, and to a lesser extent from waste of the tunnels and blow-down systems, while the second tank returns the filtered fluids and possibly fluids received from bypass systems or weir of the rolling mill to the same, and possibly to hot oil recirculation systems for spraying.

The principle has been disclosed with reference to the specific example of a rolling mill, but it can of course be transferred to any other type of machine that uses cooling fluids or lubricants that need to be regularly cleaned and then recovered.

In an alternative embodiment of the plant with two tanks, the plant further comprises

(g) a recirculation circuit fed by waste collection circuits from the tunnels and/or a blow down circuit; wherein said first tank is divided into a first and a second chamber which are communicating, the first chamber being fed by said weir and by a rolling mill or other machine using cooling or lubricating fluids and in turn feeding the second chamber, and the second chamber being fed by said recirculation circuit and in turn feeding through said related first conduits said tunnels through said inlets; and wherein said second tank feeds said rolling mill or said machine using cooling fluids or lubricants and is fed via said second conduits to the outlets of said tunnels.

It is thus possible to recover multiple fractions of cooling fluids or lubricants that are formed during the treatment of fluids in the plant according to the invention. Small fluid leaks, for example the one which occurs with the closure of the doors that "squeeze" the filter medium, are channelled with associated drains into a circuit that collects these leaks or waste.

In an advantageous embodiment of the invention, the filtration plant according to the invention further comprises:

(h) a precoating and/or body-feed system that is connected to said related first conduits at the inlets to said tunnels.

Precoating is an operation designed to deposit a layer of diatomaceous earth (or some other suitable material) onto a filter medium. The filter medium can be cloth, a metal wire screen, porous stone, sintered metal or almost any permeable material, as well as the filter web discussed above.

The main purpose of pre-treatment is to prevent the filter media from becoming blinded or clogged and to provide a clean cake discharge. Precoating also produces higher clarity than that provided by the filter web alone and contributes to extending the useful life of the filter web.

In addition to diatomaceous earth, web fibres, perlites, activated or natural clays, carbons and metal salts can be applied as precoating material. Commercial grades of diatomaceous earth can be obtained to provide particle retention of various particle sizes. The choice of material for precoating depends on the nature of the solids to be removed in a filtration. When choosing, the expert considers, among other things, the size of the dirt to be removed, the settling rate of the solids, the quantity and density of the solids, and so forth.

The proportional injection of additives (also known as “body-feed”) of concentrated slurry diluted with the fluid to be filtered can be used both at the start of the filter to provide an adequate pre-treatment - the precoating - and also after the creation of a first filter layer.

A second aspect of the invention relates to a rolling mill, in particular for aluminium and its alloys, comprising a filtration plant according to the invention. A third aspect of the invention relates to a process for filtering fluids, in particular cooling and/or lubricating fluids, which comprises the following steps:

(I) making available a filtration plant according to the invention;

(II) inserting said filter medium, in particular filter web, into at least one of said tunnels;

(III) feeding one or more tunnels equipped with the filter medium through said inlet(s) with dirty fluid preferably coming from said first tank, in particular from said second chamber;

(IV) collecting the filtered fluid in the lower part of the tunnel and extracting it through the corresponding outlet(s) and preferably conveying the filtrate to said second tank.

As mentioned above with reference to the filtration plant, feeding the first tank may also involve recirculation circuits, and feeding of the second tank may also involve other sources of clean fluid to maintain a suitable level inside the tank. Flows from and to the rolling mill or to/from machines that make use of cooling fluids or lubricants may be involved, for example, in the collection of filtrate and its future use. Part of the process obviously also involves replacing the web by opening the doors. Phases (III) and (IV) preferably take place simultaneously.

In an advantageous embodiment of the process for filtering fluids according to the invention, between steps (II) and (III) a

(II- 1 ) step is introduced, wherein a precoating of the filter medium, in particular of the filter web, is performed; and/or step (III) is at least partially accompanied by the introduction of a slurry of a filter media, in particular diatomaceous earth, into the related tunnel on the filter medium, in particular on the filter web.

Particularly preferred is an embodiment of the invention wherein in step (I) a filtration plant according to the invention is made available with the two-tank system; wherein the plant is managed in such a way that the fluid level is higher in the second tank; wherein in step (IV) the filtrate is pumped from the second tank to a machine that uses said fluid to perform cooling and/or lubrication, in particular a rolling mill, and that in step (III) the tunnel(s) flow(s) a quantity of fluid 10% higher than what is pumped from the second tank to said machine, in particular to said rolling mill.

A very advantageous embodiment of the process for filtering fluids according to the invention provides that, at the same time as phases (III) and (IV) are being executed, at least one of the tunnels of the filtration plant is isolated by disconnecting it from the supply conduits with fluid to be filtered and from the filtrate extraction conduits, and the maintenance, cleaning or repair of the associated tunnel is carried out. The presence of individual tunnels, that is, filtration chambers that must not be created by the compression of plates, but that are already present in the structure of the tunnel itself and can be put into operation individually, has the decided advantage of not having to shut down the entire plant, as in the state of the art, and of always avoiding the need to have a significant volume of spare/saving fluid available to be used during the shut-down in order not to interrupt production. It is therefore possible to carry out the maintenance, cleaning or repair of each individual tunnel by equipping it, or the associated conduits that connect it inside the plant to sources of dirty fluids and filtrate drains, with associated groups of valves, advantageously automatic.

The system allows automatic cleaning cycles that are difficult to apply in filtration units according to the state of the art.

The oversizing normally required to make the spare fluid available to ensure proper filtration without interrupting filtration or even the production of connected plants is no longer necessary. The tunnel system concentrates the filtration area in a few tunnels, while the old tower system distributed the same filtration area over a large number of filtration chambers each requiring a web roller to feed it. With the invention, the number of rollers can be reduced considerably, even to about a quarter.

It is however conceivable that the system according to the invention comprises only one of the tunnels as defined above.

A final aspect of the invention proposes the use of the filtration plant and process according to the invention also as an alternative to the filtration of cooling water/lubricants of the milling, in particular of aluminium alloys, also to filter other fluids, such as for example contaminated waters in the water treatment and purification sectors.

The features and advantages disclosed for one aspect of the invention may be transferred mutatis mutandis to the other aspect of the invention.

The industrial applicability is obvious from the moment it becomes possible to use the filtration plant continuously without having to shut it down for maintenance, wherein construction costs are reduced and wherein fewer elements and materials are needed to manage the plant (reduced number of rollers, no spare fluid volume, ...). Said objects and advantages will be further highlighted in the description of a preferred embodiment example of the invention provided by way of example only.

Variants and further features of the invention are the subject matter of the dependent claims. The disclosure of the preferred exemplary embodiment of the fluid filtration plant and process and their use according to the invention is given, by way of example only and not limited by it, with reference to the attached drawing. In particular, unless specified otherwise, the number, shape, dimensions and materials of the system and of the individual components may vary, and equivalent elements may be applied without deviating from the invention concept.

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

Fig. 1 illustrates, in a side view, a filtration plant according to the invention.

Fig. 2 illustrates in a detail of Fig. 1 in perspective and enlarged view two tunnels in which part of the upper part of the tunnel has been removed. Fig. 3 illustrates in a vertical longitudinal section in perspective view one end of a tunnel with internal details.

Fig. 4 shows a horizontal longitudinal section in perspective view of the bottom of a tunnel placed in a framework.

Fig. 5 illustrates an end of the tunnel in a partial section near an opening door.

Fig. 6 illustrates the lower part of the tunnel with a frame for pressing the web.

Fig. 7 illustrates a diagram of the fluid circuits inside the filtration plant.

Fig. 1 illustrates in a side view a filtration plant 10 according to the invention. A framework 12 has four filtration tunnel stations 14 Each tunnel 14 consists of a lower part 16 and an upper part 18 The upper part 18 is fed by a pipe 20 to introduce the fluid to be filtered, while the lower part 16 is equipped with a pipe 22 to discharge the filtrate. Each end of the tunnel 14 is closed by a door 24 There are two inspection openings 26 and two air vents 28 in each tunnel 14

Fig. 2 illustrates in a detail of Fig. 1 in perspective and enlarged view two tunnels 14 in which a part of the upper part 18 of a tunnel has been removed to show the distribution pipe 30 of the fluid to be filtered; the pipe 30 has a plurality of holes. A series of ribs 32 reinforces the upper part 18 of the tunnel 14. Parts of a frame 34 can be noted, said frame serving to hold and press the web (not shown) arranged inside the tunnel 14 on the wire mesh 36 (Fig. 3).

Fig. 3 illustrates in a vertical longitudinal section in perspective view one end of a tunnel with internal details. Inside the tunnel 14 bounded by the upper part 18, by the lower part 16 and by the doors 24, a traditional grid 38 (drawn only partially) can be noted, on which a metallic mesh 36 is arranged. The web (not shown) must be placed on the mesh and held firmly by the frame 34 in the lowered state with the doors 24 closed. Diatomaceous earth (not shown) may be deposited on the web and is at all times contained by frame 34. A channel 40 is applied to the bottom of the lower part 16 to collect the filtrate. C section profiles 42 reinforce the lower part 16. When the door 24 is opened it pushes against the frame 34 and raises it, releasing the web, which can then be advanced using the known state of the art web feeding and extraction systems.

Fig. 4 shows a horizontal longitudinal section in perspective view of the bottom of a tunnel. A plurality of C section profiles 42 reinforces the lower part of the tunnel. The collection channel 40 flows into the pipe 22 to discharge the filtrate.

Fig. 5 illustrates an end of the tunnel 14 in a partial section near an opening door 24. A series of pneumatic pistons 46 is applied to a support 44 in the vicinity of the door 24 to actuate the opening and closing of the door 24 which, upon opening, strikes against the frame 34 which rises to release the web (not shown).

Fig. 6 illustrates the lower part 16 of the tunnel with the frame 34 extending along the inner perimeter of the lower part 16 of the tunnel. At the bottom of the lower part of the tunnel the filtrate collection channel 40 can be noted.

Fig. 7 illustrates a diagram of the fluid circuits within the filtration plant in which four main units can be noted: the actual filtration unit A with a series of four tunnels 14; the tank unit B for the fluid to be filtered and for the filtrate; the management unit of the individual circuits C in the form of a skid with pumps, valves, pipes, etc.; and the precoating and body-feed unit D. The lower part of each tunnel 14 is equipped with an exhaust circuit 22 to remove the filtrate, while the upper part is equipped with a feeding circuit 20 that feeds the tunnel 14 with the dirty fluid. Each tunnel 14 has an exhaust circuit 22 and a feeding circuit 20 that can be managed separately in order to disconnect the tunnel from the conduits for the filtrate and fluids to be filtered. In this regard, valve assemblies 71 and 72 allow the connection/disconnection, respectively, of lines 20.1, 20.2, 20.3, 20.4 and 22.1, 22.2, 22.3 and 22.4 that branch off, respectively, from pipes 20.0 and 22.0, allowing the exclusion of one or more of the tunnels 14 for repair, cleaning or maintenance work without having to shut down the entire system, as is the case in the state of the art. All tunnels 14 are provided with vents 28. Each tunnel 14 is associated with a circuit 48 that collects the waste leaving the entrance and/or exits of the tunnel by converging it in the recirculation tank 50 that is also fed by the circuit 68 that collects the sediments formed in the valves of the filtrate circuit 22.0, that is, the blow-down.

The recirculation tank 50 that recirculates waste and liquids from the blow-down is in communication with the dirty fluid tank 52 that is filled from different sources, specifically from cooling/lubricating fluids 57 from a rolling mill 55, which is the main source, from a general fill 56 and from a weir 53, fed from the clean fluid tank 54, as will be illustrated below. The clean fluid tank 54 also receives fluids from the bypass and overflow lines 62 of the rolling mill 55 and obviously from the line 22.0 that conveys the filtrate collected in the various tunnels 14. The same tank 54 is provided with a connection to the rolling mill 55 for transferring the filtered and thus cleaned fluid to the latter and a connection 64 to a recirculation system for hot spray systems. All tanks also have drainage devices 60.

The dirty fluid transport lines 20.1 to 20.4 can be fed with compressed air 70 for their cleaning. Each individual line of the dirty fluid from 20.1 to 20.4 can be fed before the pump-valve system 72 through an associated line 82 from the precoating and body-feed system D which is composed of a tank 74 with a stirrer 76 and a pump 80 fed by compressed air 78 and fed with clean oil 76 in which a slurry of oil and diatomaceous earth is prepared.

The fluid tank unit B is divided into two parts, the dirty side (tank with chambers 50 and 52) and the clean side (tank 54). The clean side has a higher fluid level than the other side and there is a weir 53 from the clean fluid towards the dirt; indeed, during production the tank 54 is always full, while the dirt tanks 52, 50 are about 30% full. The filter system A flushes 10% more liquid than is pumped from the clean tank 54 to the rolling mill 55, and the excess falls through the weir 53 into the dirt tank 52. As already illustrated, the dirty side is in turn divided in two, or rather a small part, the chamber 50 is dedicated to handle the blow-down and waste and their recirculation, while the chamber 52 collects the fluids from the rolling mill 55. These two chambers 50 and 52 that make up the dirty side are in communication, wherein the partial division serves to keep the dirtiest fluid part a little more segregated. In the blow-down phase, compressed air is blown into the filters and all the fluid is discharged into the recirculation section 50.

In the precoating phase, a minimum amount of diluted soils is sent to the pump, which recirculates between the filters and the recirculation tank 50, soils which are deposited inside the filter; at the end of the precoating phase, diluted diatomaceous earth continues to be sent from the body-feed unit D to the filters, i.e. the tunnels 14, but in this phase filtering already begins, the fluid is then taken from the dirty tank 50 and sent through the tunnels 14 to the clean tank 54. The body-feed is connected to the suction of the recirculation pumps.

The division of the tanks into dirty and clean has an important aspect from the point of view of volumes; in fact the volume contained in the clean side must be sufficient to allow the cleaning cycle of the filters/tunnels.

As for the old filtration plant known in the art, it was necessary to stop filtration completely and a large volume of spare clean fluid was needed in order not to stop production; now, on the other hand, three quarters of filtration capacity remains if, for example, a tunnel is shut down for maintenance and the level of clean fluid drops much more slowly, making possible a considerable reduction of the total volume of fluid needed to run the plant.