Abstract:
A continuous fluid filtration device having filtration cells disposed in a carousel around a rotation axis so as to be able to pivot about a tilt axis including apparatus for discharging the filtrate from the cells with at least one outlet orifice at the bottom of each cell, a central collector and a connection allowing flow of the filtrate between the outlet orifice and the collector, where the connection for each cell includes a flexible conduit in which, in the filtration position of the cell, no area of the flexible conduit is lower than another area of the conduit situated downstream with respect to the flow of the filtrate, the flexible conduit being arranged so as not to undergo any elongation during tilting of the cell.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a continuous fluid filtration device, comprising: (a) filtration cells each having an opening towards the top, through which they are supplied with fluid to be filtered and which is fitted with a filter bed which, in the filtration position of the cells, allows passage of a filtrate and retention of a filtration cake, and a bottom, these cells being disposed in a carousel around a rotation axis and each arranged so as to be able to pivot about a tilt axis, tangential to a horizontal circle having the rotation axis as its centre, (b) means of supporting the filtration cells, which support each cell so that it can perform a revolution about the rotation axis, (c) means of driving the filtration cells which drive them in revolution about the rotation axis, (d) means of moving the filtration cells which cause a tilting movement thereof about their tilt axis, during their revolution about the rotation axis, and (e) means of discharging the filtrate from the cells, comprising at least one outlet orifice at the bottom of each cell, a central collector and connection means allowing flow of the filtrate between the said at least one outlet orifice and the collector.  
           [0002]    Filtration devices with cells in a carousel, which are in particular in use in the production of phosphoric acid, have already been known for a long time. Amongst others, it is possible to cite the following patents and patent applications U.S. Pat. No. 3,389,800, BE-A-768591, BE-A-847088, U.S. Pat. No. 4,721,566, WO-90/13348 and WO 92/20426. Each of these documents describes a filtration process in which the filtration cells, trapezoidal in shape in plan view, turn in a carousel about a rotation axis and are at a given moment tilted about a radial axis to allow discharge of the filtration cake and washing of the cell.  
           [0003]    Given this tilting about a horizontal radial axis, a sufficient space between the adjacent cells must be provided to allow the aforementioned tilting, without collision between these cells. The result is a loss in filtering surface and therefore in capacity of the device.  
           [0004]    A filtration device of the type described at the start has also already been provided (see for example U.S. Pat. No. 1,028,789). This device has the drawback of requiring flow of the filtrate through a rotary joint, concentric with the tilt axis of the cell and forming part of the bearing thereof. Such a joint necessarily gives rise to wear on the friction parts and it is necessary to provide air inlets into the vacuum circuit, which results in a cooling of the filtrates and crystallisation thereof in this circuit. Moreover, the cell tilting mechanism, which is complicated, is situated below the cell, that is to say in an area with a risk of corrosion where it will inevitably come into contact with the corrosive fluids fed into the cells.  
           [0005]    A filtration device is also known where the cells in the carousel tilt about a tangential axis (see U.S. Pat. No. 2,768,753). In this device, the collector is situated, in a plan view, outside the filtration cells with respect to the rotation axis and the outlet orifice of each cell rests directly on this collector to a vacuum source by sliding on it. No pressurised gas source is provided for assisting the unsticking of the solid matter and washing of the filtration bed in the tilted position. Discharge of the solid matter is obtained by impact and stoppage of the tilting movement on a stop.  
           [0006]    The aim of the present invention is to avoid these drawbacks, whilst providing a relatively simple and less expensive device which requires appreciably reduced forces for obtaining discharge of the filtration cake from the cells and cleaning thereof, and which allows discharge without leakage of filtrate and without excessive wear on the discharge means.  
         SUMMARY OF THE INVENTION  
         [0007]    To resolve these problems, there is provided, according to the invention, a filtration device as described at the start, in which the said connection means for each cell comprise a flexible conduit, in which, in the filtration position of the cell, no area of the flexible conduit is lower than another area of this conduit situated downstream with respect to the flow of the filtrate, the flexible conduit being arranged so as not to undergo any elongation during the tilting of the cell.  
           [0008]    This device makes it possible to make the cells tilt whilst their lateral edges remain equidistant. It is therefore possible to bring the cells closer to the centre by reducing the separation between them to the maximum possible extent and either to increase the overall filtration surface of the device for the same size or to keep this at the same value for an appreciably smaller size. In addition, through this arrangement, the filtered liquid flows continuously, without slowing down, through the flexible conduit. The latter undergoes no elongation during the tilting and, given its arrangement between the outlet orifice of a cell and a collector disposed centrally, the flexible conduit also does not undergo any twisting force, during the tilting of the cells. Fatigue on the connecting means is therefore minimal.  
           [0009]    According to one embodiment of the invention, in a radial section passing through the device, the flexible conduit in the filtration position of the cell extends downwards from an outlet orifice along a substantially vertical axis and then, at a height lower than the tilt axis, is angled in the direction of the collector so as to continuously have a downward slope, and the flexible conduit in the tilting position of the cell extends substantially horizontally from the outlet orifice as far as the tilt axis, and is then angled in the direction of the collector.  
           [0010]    Advantageously, the tilt axis is supported in at least one bearing having a first outside diameter D1, the flexible conduit having a second outside diameter D2 and the distance between the tilt axis, and the above mentioned substantially vertical axis of the flexible conduit is equal to or greater than 0 and less than or equal to D1+D2. This arrangement, as close as possible, between the tilt axis and the vertical axis of the conduit advantageously makes it possible not to subject the flexible conduit to an elongation force during the tilting.  
           [0011]    According to an improved embodiment of the invention, the device comprises, for each flexible conduit, a support which turns about the rotation axis simultaneously with the filtration cells. In this way, the support prevents the flexible conduit, under its own weight or under its weight when the filtrate is flowing therein, from having a tendency to flex downwards in a position in which it is no longer continuously in a downward slope in the direction of the collector.  
           [0012]    According to a particular embodiment of the invention, the collector is connected to a source of negative pressure which the flexible conduits, connected to the filtration cells in the filtration position, communicate to them, below their filtering bed, and the collector is also a distributor connected to a source of pressurised gas which the flexible conduits connected to the filtration cells in the tilted position communicate to the latter, to assist with the detachment of the filtration cake from the filter bed.  
           [0013]    According to another embodiment of the invention, the aforementioned movement means comprise a roller arranged on each cell so as to be able to turn freely about a pivot axis, and a guide rail arranged fixedly at one point on the filtration device so as to receive the roller of each driven filtration cell and to guide it so as to cause the said tilting movement of the cell.  
           [0014]    Other embodiments of the device according to the invention are indicated in the accompanying claims. 
       
    
    
       [0015]    Other details and particularities of the invention will emerge from the description given as an accompaniment, non-limitingly, with reference to the accompanying drawings.  
       BRIEF DESCRIPTION OF THE FIGURES  
       [0016]    In the various figures described below, the identical or similar elements are designated by the same references. These figures are schematic representations and, at different points, many elements have been omitted in order to facilitate reading thereof.  
         [0017]    [0017]FIG. 1 depicts a partially broken plan view of a filtration device according to the invention.  
         [0018]    [0018]FIG. 2 depicts a view in section along the line II-II in FIG. 1.  
         [0019]    [0019]FIG. 3 depicts, to an enlarged scale, a plan view of the guide rail along the line III-III in FIG. 2.  
         [0020]    [0020]FIG. 4 depicts, to an enlarged scale, a view of the guide rail in section along the line IV-IV in FIG. 2.  
         [0021]    [0021]FIG. 5 depicts a detail of the left-hand part of FIG. 2 to an enlarged scale.  
         [0022]    [0022]FIG. 6 depicts a view in section along the line VI-VI in FIG. 5.  
         [0023]    [0023]FIG. 7 depicts a variant of the arrangement provided in FIG. 6.  
         [0024]    [0024]FIG. 8 depicts a view in section along the line VIII-VIII in FIG. 7. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    In the example embodiment depicted in FIGS. 1 and 2, filtration cells  1  in the form of a vessel, having an opening disposed upwards during filtration, are arranged in a carousel about a vertical rotation axis  2 . These cells  1  are provided with a horizontal filter bed  41  above which a fluid to be filtered is supplied and below which a filtrate is collected.  
         [0026]    According to the invention, each cell  1  is capable of turning about a tilt axis  3  which is arranged tangentially to an imaginary horizontal circle  4  whose centre is formed by the rotation axis  2 .  
         [0027]    In the example embodiment illustrated, the bottom  45  of each cell  1  is provided with two bearings  5  and  6  through which a shaft  7  is passed so as to allow free tilting of the cell about the shaft  7 . This shaft  7  is coaxial with the aforementioned tilt axis  3 .  
         [0028]    As illustrated in FIGS. 1 and 2, this shaft  7  is supported by two support linkages  8  and  9 , each of these linkages being formed by rods  10  to  13  disposed in a triangle. At one end, the rods  10  and  11  are fixed to a ring  14  which surrounds a central shaft  15  so as to be able to turn freely about the latter and without contact therewith. This central shaft  15  is coaxial with the rotation axis  2 . At its other end, the rod  10  is connected to the shaft  7  and the rod  11  to a circular rotary chassis  16  provided with a rack. The rod  13  connects the shaft  7  to the circular chassis  16  and the rod  12  connects the latter to a central part of the rod  10 . Each cell is thus supported by two frameworks which are both light and non-deformable, which undergo only traction and compression forces.  
         [0029]    The circular chassis  16 , arranged coaxial with the rotation axis  2 , is supported and guided by sets of rollers with respectively horizontal and vertical axes  17  and  18 , on which it can turn in a horizontal plane about the rotation axis  2 . In the example illustrated, a drive motor  19  actuates a toothed pinion  20  which is in engagement with the rack on the circular chassis  16 . When the motor is brought into service, it thus drives the entire installation in rotation in the direction of the arrow F.  
         [0030]    Each filtration cell is provided with a roller  21  which, in the example illustrated, is arranged on it on the side facing the centre of the device. The roller  21  is arranged so as to be able to turn freely about a pivot axis  22 . This axis  22  preferably extends radially between the central rotation axis  2  of the device and the tilt axis  3  of the corresponding cell, in plan view, when the cell is in the horizontal position.  
         [0031]    Advantageously, the pivot axis  22  of a roller  21  is thus situated in a plane passing through the rotation axis  2  of the device and perpendicular to the tilt axis  3  of the cell corresponding to the roller, throughout any tilting movement of the cell.  
         [0032]    In a part of the device situated to the right in FIG. 1 a guide rail  23  is arranged so as to receive the roller  22  of each driven filtration cell.  
         [0033]    In the example illustrated, the guide rail  23  is supported above the filtration cells by two brackets  24  and  25 , whose ends towards the centre are supported by a cross-member  26 . Shanks  27  each provided with a suspension clamp  43  fixedly hold the guide rail  23  suspended above the filtration cells. The rail  23  has a U shape, in the plan view in FIG. 1, and comprises a central part  30  and two lateral branches  28  and  29 .  
         [0034]    This guide rail  23  is formed, in the example illustrated, from two lateral walls  31  and  32  which extend parallel over the entire path of the rail. Between these two walls, the roller  21  follows a direction of forward movement illustrated by the arrows in FIG. 3.  
         [0035]    As illustrated in FIGS. 1 and 2, when the roller  21  of a filtration cell reaches the central part  30  of the guide rail  23 , the cell is in a substantially vertical position.  
         [0036]    A station for cleaning the filter bed  41  and the back of the cell is provided in the example illustrated. It comprises two lances  33  and  34  which are provided with spray nozzles and which are supplied with a cleaning liquid. These lances spray this liquid onto the filtration cells in an approximately vertical position, which allows discharge of the sprayed liquids by gravity, into a collecting receptacle which is not shown.  
         [0037]    Each filtration cell is advantageously provided with means of adjusting its horizontality. These means consist, in the example illustrated, of a lug  35  extending downwards from the edge of the cell disposed facing the centre of the device. This lug is provided with a rim having a threaded hole into which a threaded rod  36  can be screwed. This threaded rod  36 , the end of which is thus adjustable for height, by screwing, bears against a stop  37  which extends, in the case illustrated, between the two rods  10  of the linkages  8  and  9  supporting the filtration cell.  
         [0038]    The supply of the filtration cells with fluid to be filtered takes place in a normal fashion through the top of the cells. The filtrate is discharged through an orifice  42  situated at the bottom of each cell. This orifice is connected by a flexible conduit  38  to a central collector/distributor  39 .  
         [0039]    As is clear from FIG. 5, which is a view in a radial section through the device according to the invention, the flexible conduit  38  is depicted in solid lines in the filtration position at the cell and in broken lines in its tilted position. In the filtration position, it extends downwards along a substantially vertical axis  46  from the outlet orifice. When it passes at a height lower than the tilt axis  3 , the flexible conduit  38  is then angled in the direction of the central collector  39  so as to continuously have a downward slope. So that, through its own weight, and through the weight of the filtrate which it receives, the conduit does not have a tendency to flex downwards and thus have a low position unfavourable to flow, in the example illustrated in FIG. 2 each flexible conduit  38  is supported partially by a channel  40  which turns with the cells. In the example illustrated in FIG. 5 it is a case rather of a local support  47 , supporting the flexible conduit  38  in a central part thereof.  
         [0040]    The slope of the flexible conduit may be variable, but it is everywhere greater than 0°.  
         [0041]    As can be seen in FIG. 5, the length L of the flexible conduit between the outlet orifice  42  and the support  47  remains constant even when the cell is in the tilted position and the flexible conduit has been deformed. The part of the flexible conduit  38  between the support  47  and the collector  39  is not deformed and therefore also does not change length.  
         [0042]    In the tilted position, the flexible conduit extends substantially horizontally from the outlet orifice  42  as far as the tilt axis  3  and is then angled in the direction of the collector  39 .  
         [0043]    As indicated in FIGS. 5 and 6, the bearing  6  has an outside diameter D1 and the flexible conduit an outside diameter D2. In the embodiment illustrated, the distance E between the tilt axis  3  and the substantially vertical axis  46  of the conduit is advantageously  
           D1   +   D2     2     .                         
 
         [0044]    Therefore, although this is not critical, and this distance E may be greater, it is preferable for it to be as close as possible to zero and that it be at most equal to D1+D2.  
         [0045]    It is possible for example to imagine an embodiment as illustrated in FIGS.  7  to  8 . There, the cell pivots about shaft ends  48  and  49  coaxial with each other and coaxial with the tilt axis  3 , and the bearings  5  and  6  are maintained at a distance by an axially offset shaft  50  so as to form a kind of crankshaft. The axis  46  now crosses the tilt axis  3  and the distance E is equal to 0.  
         [0046]    It is thus possible to imagine that the axis  46  is, in plan view, situated more towards the outside with respect to the tilt axis  3 .  
         [0047]    The functioning of the device illustrated is as followed.  
         [0048]    The motor  19  drives the rack on the circular chassis  16  in rotation and with it each of the filtration cells in the direction F.  
         [0049]    The cells are disposed horizontally, closely side by side, and are supplied with fluid to be filtered. The filtration of the fluid takes place through the filter bed  41 , in a normal fashion, under negative pressure, obtained by means known per se.  
         [0050]    The filtrate from each cell is discharged through the corresponding flexible conduit  38 , and a filtration cake is formed above the filter bed. Because of the arrangement of the flexible conduit  38  continuously in a downward slope, discharge takes place easily, rapidly and without areas where the filtrate could stagnate and crystallise. Any part undergoing friction and wear, liable to permit leaks and entries of air, is avoided. The negative pressure is communicated without loss from the collector of the filtrate  39  as far as the bottom part of each cell in the filtration position.  
         [0051]    At one moment, the roller  29  of the cell encounters the guide rail  23 , as illustrated in FIG. 3. The rail forces the roller to rise and therefore the cell to tilt. Its inner end pivots upwards about the tilt axis  3 . Approximately half-way from the branch  28  of the rail  23 , the centre of gravity of the cell passes vertically to the tilt axis  3 . Beyond that, the weight of the cell cooperates with its driving until it reaches the central part  30 . Likewise, during the travel over the branch  29  of the guide rail, as soon as the centre of gravity of the cell once again passes vertically to the tilt axis  3 , the weight of the cell participates in the driving thereof.  
         [0052]    When the roller  21  follows the path of the central part of the guide rail  23 , the cell is in an almost vertical position and therefore also the filter bed.  
         [0053]    During this tilting, the flexible conduit  38  passes from the position depicted in solid lines in FIG. 5 to that depicted in broken lines. If there is a curvature deformation of the flexible conduit  38  upwards, no elongation thereof can be observed, nor any twisting about its longitudinal axis. Fatigue is therefore reduced to the minimum.  
         [0054]    The filtration cake falls first of all onto the highly inclined external wall  44  of the cell, and then into a collector which is not shown, and the cell passes through the cleaning station already described, before once again tilting into the horizontal position and recommencing the filtration cycle.  
         [0055]    Provision can advantageously be made, by means known per se, for the establishment of a slight back pressure of air inside each cell whilst the latter is in a position tilted to the vertical, in order to assist with the discharge of the filtration cake. This back pressure can be supplied from the collector  39 , which then serves as a distributor, to the bottom of the cell, by means of the flexible conduit  38 .  
         [0056]    It must be understood that the present invention is in no way limited to the embodiment described above and that many modifications can be made thereto within the scope of the accompanying claims.