THIS INVENTION relates to filter presses. In particular, the invention relates to a filter plate shifting device for a horizontal filter press, and to a horizontal filter press.

Horizontal filter presses are well-known to those skilled in the art of liquid-solids separation. A typical horizontal filter press comprises a plurality of filter plates, displaceable along a horizontal operating axis or filter plate path extending between a stationary or fixed press end and a movable press head, between a closed filter condition in which the filter plates are pushed and compressed together against the fixed press end to form a horizontal stack of filter plates for filtration purposes and an open discharge condition in which at least some of the filter plates are spaced apart in order to discharge filter cake from between the filter plates. Each filter plate, except for outermost filter plates of the horizontal stack of filter plates, has depressions or recesses formed typically in both of opposed major faces of the filter plate so that when the filter plates are pushed together sealed cavities are defined between adjacent pressed together filter plates. Typically, both of the opposed major faces of a filter plate are covered with a sheet of filtering medium, such as a filter cloth, so that when the filter plates are pressed together to form the horizontal stack, a slurry to be filtered can be pumped into the sealed cavities to allow filtration to take place through the filtering medium. The filtering medium, e.g. filter cloth, retains the solids from the pumped slurry between the adjacent pressed-together filter plates to form a filter cake, while a filtrate is allowed to flow through the filtering medium and exit the plate through either external ports, or one or more internal corner ports. After filtration, the moveable press head is displaced away from the horizontal stack of filter plates and away from the fixed press end, pulling apart a firsts group of filter plates attached to the moveable press head, and the filter plates or the rest of the filter plates of the horizontal stack of filter plates, possibly individually but typically in predefined groups (e.g. groups of say ten filter plates that are chained together), are displaced towards the moveable press head and separated, to allow the filter cakes which have accumulated between adjacent filter plates to be discharged. Typically, when a group of filter plates is separated, another, adjacent and already separated group of filter plates from which filter cake has already been removed, is pushed together again, in the direction of the movable press head.

Various devices or mechanisms are in use for displacing the filter plates from the closed filter condition to the open discharge condition, typically making use of latch stations to engage the filter plates and a chain drive unit or one or more hydraulic cylinders to displace the latch stations in reciprocating fashion alongside the horizontal stack of filter plates. One such device uses a carrier which extends parallel to the horizontal operating axis, and which is positioned alongside the stack of filter plates. A plurality of latch stations is mounted to the carrier and spaced along the length of the carrier. Each latch station can be individually controlled to be in a non-operative condition in which it does not interact with any filter plate and is free to be displaced relative to the stack of filter plates without engaging a filter plate, and an operative latch condition in which it latches to or engages a filter plate. The carrier has an operative stroke, so that when the carrier is stroked in an operative direction, one of the latch stations engages a filter plate and said filter plate, typically however a group of chained together filter plates, is displaced from their closed filter condition to their open discharge condition. During a return, non-operative stroke of the carrier, none of the latching stations is in a latch condition, so that the latching stations are merely repositioned for the next latching station to engage a filter plate of the closed stack of filter plates to displace another group of filter plates from their closed filter condition to their open discharge condition during the operative stroke of the carrier.

A filter plate shifting device which speeds up filter cake discharge from a horizontal filter press, resulting in shorter cake discharging cycle times, will be desirable.

According to one aspect of the invention, there is provided a filter plate shifting device for a horizontal filter press, the filter plate shifting device comprising a pinion; a pair of parallel racks positioned for reciprocating displacement on diagonally opposite sides of the pinion and engaging the pinion such that, during rotation of the pinion, the racks stroke in opposite directions along parallel stroke axes; a plurality of displaceable spaced-apart latch stations associated with each rack to be displaced in reciprocating fashion, together with their associated rack, along a latch station path which is parallel to the stroke axis of their associated rack, and each latch station being configured selectively in use to engage and disengage a filter plate of a filter press; and drive means selectively to rotate the pinion in a first direction, and thereafter in a second, opposite direction, thereby to stroke the racks, and hence to displace the latch stations in reciprocating fashion.

The filter plate shifting device of the invention is particularly suitable for using with a horizontal solid or membrane recess filter press.

As will be appreciated, with a pair of parallel racks stroking in opposite directions, it is possible to displace in use a group of filter plates every time the pinion is turned. On stroking the racks by turning the pinion in said first direction, a latch station associated with one of the racks can displace a group of filter plates, and a latch station associated with the other rack is brought back into position. On stroking the racks in an opposite direction by turning the pinion in said second, opposite direction, a latch station associated with said one of the racks can be brought back into position, and said latch station associated with said other rack can displace another, adjacent group of filter plates. The time to displace all the filter plates of a stack of filter plates of a horizontal filter press will in use thus be essentially halved as a result of the doubleacting dual rack arrangement.

In one embodiment of the filter plate shifting device of the invention, the drive means is a hydraulic motor.

In another embodiment of the filter plate shifting device of the invention, the drive means is an electric motor. Preferably, when the drive means is an electric motor, the electric motor is provided with a variable speed drive to manipulate the speed at which the racks are displaced.

The filter plate shifting device may include a hydraulic pump in hydraulic fluid communication with the hydraulic motor. Instead, the filter plate shifting device may be in fluid communication with a hydraulic pump of a filter press to which the filter plate shifting device is in use mounted.

The filter plate shifting device may include a hydraulic fluid flow controller to control a rate of flow of hydraulic fluid through the hydraulic motor, and hence the speed of the hydraulic motor. In other words, the speed at which the racks are displaced may be variable.

The filter plate shifting device may include a programmable logic controller (PLC) and limit switches or proximity sensors to control operation of the hydraulic motor. Instead, the filter plate shifting device may be controlled by a PLC of a filter press to which the filter plate shifting device is in use mounted or of which it forms part.

The filter plate shifting device may include a pressure relief valve configured or operable to zero hydraulic fluid pressure to the hydraulic motor, after each stroke of the racks.

The filter plate shifting device may include rollers to carry and guide the racks. The rollers may be rotatably mounted about axes of rotation that are vertical or upwardly extending in use.

The filter plate shifting device may include an elongate carrier attached to each rack, with all or at least some of the latch stations being mounted to the elongate carriers.

Each latch station may include a latch displaceable between a non-operative condition in which it does not interact in use with a filter plate of a horizontal filter press, and an operative latch condition in which in use it latches to or engages a filter plate of a horizontal filter press.

Each latch station may include displacement means to displace the latch between its non-operative condition and its operative latch condition.

The displacement means may be in the form of a double-acting pneumatic cylinder. According to another aspect of the invention, there is provided a horizontal filter press which includes a plurality of filter plates displaceable along a horizontal linear filter plate path extending between a fixed press end and a moveable press head, the filter plates being displaceable between a closed filter condition in which the filter plates are pushed together by the moveable press head to form a horizontal stack of filter plates and an open discharge condition in which at least some of the filter plates are spaced apart to discharge filter cake formed in use between the filter plates; and at least one filter plate shifting device as hereinbefore described to displace the filter plates between their closed filter condition and their open discharge condition.

The horizontal filter press of the invention may be a horizontal solid or membrane recess filter press.

Typically, said at least one filter plate shifting device is positioned alongside a side of said stack of filter plates.

Typically, the filter press includes two of said filter plate shifting devices, one alongside each side of said horizontal stack of filter plates. As will be appreciated, the filter plate shifting devices are configured such that, when a latch station of one of the filter plate shifting devices activates, a corresponding opposite latch station of the other filter plate shifting device on an opposite side of the filter plate will also activate to ensure that the filter plate is pulled whilst remaining at a 90 degree angle to the parallel racks of the pair of filter plate shifting devices.

The filter plates may be top-hung from an overhead guide rail.

Instead, the filter plates may be side-hung on a pair of side guide rails.

Typically, each filter plate shifting device has a hydraulic motor. The filter plate shifting devices may share a hydraulic pump in hydraulic fluid communication with their hydraulic motors. A flow divider may be provided to ensure equal flow of hydraulic fluid from the hydraulic pump to the hydraulic motors.

Instead, the hydraulic motors may be arranged in series relative to flow of hydraulic fluid, with hydraulic fluid exiting an upstream hydraulic motor entering a downstream hydraulic motor, before returning to a reservoir of hydraulic fluid, such as a hydraulic fluid tank. This arrangement will also ensure equal flow of hydraulic fluid through the hydraulic motors.

The hydraulic pump may be a dedicated hydraulic pump provided to drive the hydraulic motors. Instead, the hydraulic pump may be a hydraulic pump of the filter press, in use providing hydraulic pressure to displace the moveable press head.

The pinion of the or each filter plate shifting device may be located behind or beyond the moveable press head, i.e. not between the moveable press head and the fixed press end.

The filter press may include limit switches or proximity sensors to ensure the correct rotation direction of the hydraulic motors, and hence the pinions, of the filter plate shifting devices.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which

Figure 1 shows a three-dimensional view of components of a filter plate shifting device in accordance with the invention suitable for use as part of a horizontal filter press;

Figure 2 shows a plan view of the components of Figure 1;

Figure 3 shows an end view of the components of Figure 1;

Figure 4 shows a side view of the components of Figure 1;

Figure 5 shows a side view of a latch station of the filter plate shifting device of the invention, in a non-operative condition;

Figure 6 shows a side view of the latch station of Figure 5, in an operative latch condition; Figure 7 shows a three-dimensional view of part of a horizontal filter press in accordance with the invention, which includes the filter plate shifting device of the invention, after a first group of filter plates has been opened;

Figure 8 shows a three-dimensional view of the horizontal filter press of Figure 7, after a second group of filter plates has been opened;

Figure 9 shows the horizontal filter press of Figure 7, after a third group of filter plates has been opened; and

Figure 10 shows an end view of a fixed press end of the horizontal filter press of Figures 7 to 9.

Referring to Figures 1 to 6 of the drawings, a filter plate shifting device in accordance with the invention suitable for use as part of a horizontal filter press includes a pinion 10, a pair of parallel elongate racks 12, 14 positioned for reciprocating displacement along parallel stroke axes 13.1, 13.2 on diagonally opposite sides of the pinion 10 and engaging the pinion 10 such that, during rotation of the pinion 10, the racks 12, 14 stroke in opposite directions along said parallel stroke axes 13.1, 13.2, a hydraulic motor 16 drivingly connected to the pinion 10 (see Figures 1, 3 and 4), and a plurality of displaceable latch stations 50 (Figures 5 and 6 illustrate one such latch station 50 in two different conditions) associated with the racks 12, 14 to be displaced in reciprocating fashion along latch station paths which are parallel to, and perhaps even coaxial with, the stroke axes 13.1, 13.2 of the racks 12, 14.

Each latch station 50 is configured selectively to engage and disengage a filter plate 102 of a horizontal filter press 100 (see Figures 7 to 10), whereas the hydraulic motor 16 is configured selectively to rotate the pinion 10 in a first direction, and thereafter in a second, opposite direction, thereby to stroke the racks 12, 14, and hence to displace the latch stations 50 in reciprocating fashion along the latch station paths, i.e. along the stroke axes 13.1, 13.2.

As will be noted from Figures 1 to 4, the racks 12, 14 are supported by four grooved or slotted rollers 18, rotating about vertical axes of rotation, which also guide the racks 12, 14 to ensure that, during rotation of the pinion 10, the racks 12, 14 stroke in opposite directions along said parallel stroke axes 13.1, 13.2. The pinion 10, grooved rollers 18 and portions of the racks 12, 14 are housed within a housing 20, with the hydraulic motor 16 being located below the housing 20. The pinion 10, racks 12, 14, grooved rollers 18, hydraulic motor 16 and housing 20 together form a drive unit 22 of the filter plate shifting device of the invention.

Although not shown in Figures 1 to 4 of the drawings, each rack 12, 14 is drivingly attached to an elongate carrier 24, 26 (see Figures 7 to 9) so that the carriers 24, 26, supported on bearings or rollers or the like or on associated rails, stroke together with their associated racks 12, 14 (shown covered by a cover 25 in Figures 7 to 10) along the parallel stroke axes 13.1, 13.2. The elongate carriers 24, 26 thus in essence are extensions of the racks 12, 14, one being an inner carrier (i.e. the carrier 24) and one being an outer carrier (i.e. the carrier 26).

A plurality of the latch stations 50 (see Figures 5 to 10) are mounted on top of the elongate carriers 24, 26 and are spaced apart along the lengths of their respective associated elongate carriers 24, 26. Thus, when the carriers 24, 26 are stroked along the stroke axes 13.1, 13.2, as a result of the stroking of the racks 12, 14 along the stroke axes 13.1, 13.2, the latch stations 50 are displaced together with the carriers 24, 26 in reciprocating fashion along the latch station paths which are parallel to, or perhaps even in line with, the stroke axes 13.1, 13.2 of the racks 12, 14.

Each latch station 50 includes a latch station body 52 securely attached, e.g. welded or bolted, to one of the elongate carriers 24, 26, a latch 54 pivotally displaceable between a retracted non-operative condition in which the latch 54 does not in use interact with a filter plate 102 of a horizontal filter press such as the horizontal filter press 100, and an upright operative latch condition in which in use the latch 54 latches to or engages a filter plate 102 of a horizontal filter press such as the horizontal filter press 100. In Figure 5 of the drawings, the latch 54 is shown in the retracted non-operative condition, whereas in Figure 6 of the drawings, the latch 54 is shown in the upright operative condition. As will be noted, when in its upright operative condition, the latch 54 abuts against a stopper 56 provided on top of the latch station body 52.

Each latch station 50 includes displacement or actuation means, in the form of a double-acting pneumatic cylinder 58 to displace the latch 54 between its retracted non-operative condition (as shown in Figure 5) and its upright operative latch condition (as shown in Figure 6). A shaft 60 of the double-acting pneumatic cylinder 58 is pivotally connected at a pivot axis 62 to the latch 54, with the latch 54 in turn pivoting about a pivot axis 64. A slotted guide 68 and pin 70 ensure that the latch 54 is guided during displacement between its retracted non-operative condition and its upright operative latch condition, with the guide 68 also defining end points for the movement of the latch 54.

As it is necessary for the double-acting pneumatic cylinder 58 to be able to pivot during movement of the latch 54, the double-acting pneumatic cylinder 58 is pivotally attached to the latch station body 52 at a pivot axis 72.

In use, when the pneumatic cylinder 58 is retracted, the latch 54 is also in its nonoperative condition and is withdrawn inside the latch station body 52 as shown in Figure 5 of the drawings. When the double-acting pneumatic cylinder 58 is extended, as shown in Figure 6 of the drawings, the latch 54 however is pivoted to extend upwardly, abutting against the stopper 56, with a filter plate engagement surface 74 extending vertically.

The filter plate shifting device of the invention, components of which are shown in Figures I to 6 of the drawings, is in use installed in a horizontal filter press such as the horizontal press 100 shown in Figures 7 to 10 of the drawings, to form part of the horizontal filter press 100. The horizontal filter press 100 includes a plurality of conventional filter plates 102 displaceable in conventional fashion along a horizontal linear filter plate path extending between a fixed press end (not shown in Figures 7 - 9, but see Figure 10) and a moveable press head 104. The filter plates 102 (and the moveable press head 104) are displaceable between a closed filter condition in which the filter plates 102 are all pushed together against the fixed press end by the moveable press head 104 to form a horizontal stack of filter plates 102, and an open discharge condition in which at least some of the filter plates 102 are spaced apart to discharge filter cake, formed during filtration of a slurry between the filter plates 102, from between the filter plates 102 by means of gravity. In Figure 7 of the drawings, twenty-one filter plates 102 on the left of the stack of filter plates 102 are shown in their closed filter condition, whereas five filter plates 102 on the right of the horizontal stack of filter plates 102 are shown in their open discharge condition. The horizontal filter press 100 shown in the drawings is a side-hung filter press. In other words, the filter plates 102 are slidingly supported on side bars 105, one on each side of the stack of filter plates 102. It is however to be appreciated that the filter plate shifting device of the invention can also be used with a top-hung horizontal filter press.

As will be noted from Figures 7 to 9, the filter plate shifting device of the invention is installed along one side of the horizontal filter press 100, with the drive unit 22 being positioned at one end of the horizontal filter press 100, such that it is located beyond the moveable press head 104 when the filter plates 102 of the horizontal filter press 100 are all pushed together against the fixed press end by the moveable press head 104 into their closed filter condition.

An identical filter plate shifting device is installed in a mirror image arrangement on the opposite side of the horizontal filter press 100. Although the second filter plate shifting device is largely not visible in Figures 7 to 9 of the drawings (but see Figure 10), it is to be appreciated that the two filter plate shifting devices are functionally identical and that they work in unison to displace the filter plates 102, ensuring that the filter plates 102 are pulled apart whilst remaining at a 90 degree transverse angle to the parallel racks 12, 14 of the pair of filter plate shifting devices, and at a 90 degree transverse angle to the side bars 105.

Selected filter plates 102 have handles 108 extending outwardly from each side of the filter plate 102 and projecting over the carriers 24, 26. The handles 108, often referred to as shifting handles, can thus be engaged by latch stations 50 of the elongate carriers 24, 26, when their latches 54 are in their upright operative latch conditions. When the latches 54 of the latch stations 50 are in their non-operative conditions, the latch stations 50 can freely be displaced reciprocatingly along the latch station paths without the shifting handles 108 interfering with their travel, but when the latches 54 of the latch stations 50 are in their upright operative latch conditions their vertically extending filter plate engagement surfaces 74 abut against the handles 108 so that displacement of the latch stations 50 towards the moveable press head 104 causes displacement of predefined groups of filter plates 102.

The hydraulic motor 16 of the drive unit 22 is in hydraulic fluid communication with a hydraulic pump (not shown) forming part of a hydraulic pack. This detail is however not shown in the drawings, as the operation of a hydraulicly operated horizontal filter press is well- known to those skilled in the art. It can be mentioned however that there is typically a hydraulic power pack (not shown) which is a separate piece of equipment installed in close proximity of the filter press 100, that is used to drive a hydraulic cylinder 120 responsible for opening and closing the stack of filter plates 102 (i.e. to displace the moveable press head 104), as well as to operate the filter plate shifting devices. Usually, the large hydraulic cylinder 120 responsible for opening and closing the stack of filter plates 102 has it's own hydraulic pump. There is then a second hydraulic pump that drives the hydraulic motors 16 of the plate shifting devices.

In order to separate solids from a liquid, by means of the horizontal filter press 100, the moveable press head 104 is hydraulically pushed by the hydraulic cylinder 120 in the direction of arrow 110 in Figure 7 until all of the filter plates 102 are pushed against the stationery or fixed press end (not shown), to form a stack of pushed-together filter plates 102. A slurry of the solids and liquid is then pumped by means of a slurry feed inlet 122 (see Figure 10) into sealed cavities defined between adjacent pressed together filter plates 102. The slurry is filtered through filtering medium covering both of opposed major faces of each filter plate 102, with filtrate entering the filter plates 102 and being removed from the filter plates by bottom outlets 112, in conventional fashion. The bottom outlets 112 feed the filtrate into gutters 114 which transport the filtrate away from the filter press 100. As will be appreciated, a filter cake thus gradually forms between adjacent pressed-together filter plates 102, with a rise in filtration pressure (pumping pressure exerted on the slurry) to a predetermined value typically signalling the end of the filtration process. In order to filter more slurry, it is thus necessary first to remove the filter cake from between the pressed-together filter plates 102 and this is where the filter plate shifting device of the invention is useful.

In order to remove the filter cake from between the pressed-together filter plates 102, the moveable press head 104 is first moved back to a starting position by means of the hydraulic cylinder 120, as shown in Figure 7 of the drawings, in a direction counter to the arrow 110. When the moveable press head 104 is moved back, a group of five chained-together filter plates 102 nearest to the moveable press head 104 (chained together by means of a chain 114 and chained to the moveable press head 104) is displaced away from the rest of the stack of pushed-together filter plates 102 and moved into an open discharge condition as shown in Figure 7 of the drawings. Filter cake can thus be discharged downwardly, by means of gravity, and in some embodiments assisted by shaking of the filter plates 102, and removed from the horizontal filter press 100, e.g. by means of a conveyor (not shown) or the filter cake can discharge straight onto a stockpile retained by a civil bunker (not shown) below the horizontal filter press 100.

To open a second group of five chained-together filter plates 102, the latch 54 of an inner latch station 50.1 (see Figures 7 and 8) is displaced from its retracted non-operative condition to its operative upright latch condition by means of the double-acting pneumatic cylinder 58 so that the latch 54 projects upwardly, as shown in Figure 6 and in Figures 7 and 8. By means of the hydraulic motor 16, the pinion 10 of the drive unit 22 is rotated clockwise in the direction of arrow 28 as shown in Figure 2 of the drawings (for the filter plate shifting device on the left of the horizontal stack of filter plates 102, but anti-clockwise for the filter plate shifting device on the right of the horizontal stack of filter plates 102), thereby stroking the inner rack 12, and hence its associated elongate inner carrier 24 and all of the inner latch stations 50 mounted to the elongate inner carrier 24, towards the right in Figures 7 to 9 of the drawings, with the latch 54 of the inner latch station 50.1 soon making contact with and pushing against the filter plate shifting handle 108 of a filter plate 102.1. The filter plate 102.1, and thus a group of five chained- together associated filter plates 102 are then displaced towards the moveable press head 104, displacing these filter plates 102 from their closed filter condition to their open discharge condition, and pushing the first group of five opened filter plates 102, from which filter cakes have already been removed, back together again against the moveable press head 104. Simultaneously, outer latch stations 50, including an outer latch station 50.2, mounted to the elongate outer carrier 26, is stroked in an opposite direction as a result of the displacement of the rack 14. The outer latch station 50.2 is thus displaced from its position shown in Figure 7, towards the fixed press end until the outer latch station 50.2 is positioned towards the left of the next filter plate shifting handle 108 shown of a filter plate 102.2 shown in Figure 8 of the drawings.

As shown in Figure 8, the latch 54 of the latch station 50.2 is then displaced from its retracted non-operative condition to its operative upright latch condition by means of the double-acting pneumatic cylinder 58. Upon the next stroking of the racks 12, 14, by rotation of the pinion 10 in a direction opposite to the arrow 28 shown in Figure 2 of the drawings (for the filter plate shifting device on the left of the horizontal stack of filter plates 102, but clockwise for the filter plate shifting device on the right of the horizontal stack of filter plates 102), the elongate outer carrier 26 and thus the outer latch station 50.2 are displaced towards the moveable press head 104. During this movement, the latch 54 of the latch station 50.1 is dropped back to its retracted non-operative condition by means of the double-acting pneumatic cylinder 58 so that it can no longer engage any of the filter plate shifting handles 108.

The latch 54 of the latch station 50.2 soon engages and pushes against the filter plate shifting handle 108 of the filter plate 102.2 shown in Figure 8 of the drawings, and the next (third) group of five chained-together filter plates 102 is thus pulled apart, as shown in Figure 9 of the drawings, simultaneously pushing the second group of five opened filter plates 102 back together again, against the first group of five already pushed-together filter plates 102. Filter cake can thus be discharged from between the new, third group of pulled-apart filter plates 102.

By repeating the sequence hereinbefore described, groups of pressed-together filter plates 102 can quickly be opened, with one group of chained-together filter plates 102 being opened upon each stroke of the drive unit 22, thereby to remove filter cake from between the filter plates 102, and an adjacent group of already opened filter plates 102 being pushed together at the same time in the direction of the moveable press head 104. As will be appreciated, an inner latch station 50.3 (see Figure 9) will thus be the latch station used to open a fourth group of five chained-together filter plates 102, whereafter an outer latch station 50.4 (see Figure 9) will be used to open a fifth group of five chained-together filter plates 102, etcetera, until all of the filter plates 102 have been opened for purposes of discharging filter cake, before the whole filtration process is started again by pushing all of the filter plates 102 sealingly together against the fixed press end by means of the hydraulic cylinder 120.

Operation of the hydraulic motor 16 of the drive unit 22 is controlled by means of a programmable logic controller (not shown), which may be dedicated to the filter plate shifting devices or which may form part of the filter press 100 so that it also controls other operations of the filter press 100, and limit switches or proximity sensors on the elongate carriers 24, 26. A flow divider may be provided in a hydraulic fluid feed line from the hydraulic fluid pump of the horizontal filter press 100 to the hydraulic motors 16 of the left-hand and right-hand drive units 22, to ensure equal flow of hydraulic fluid from the hydraulic pump to the hydraulic motors 16. Another, possibly preferred way that equal flow to both hydraulic motors can be achieved is by piping the hydraulic pump up to any of the two hydraulic motors 16 of the mirrored filter plate shifting devices, and then letting the flow exiting from said motor 16 flow straight to the hydraulic motor 16 of the other filter plate shifting device.

Once all the filter plates 102 have been cleared of filter cake, the entire process starts again, by pressing the filter plates 102 sealingly together by means of the moveable press head 104, so that another volume of slurry can be filtered.

The filter plate shifting device of the invention, as illustrated, allows a horizontal filter press continuously to shift filter plates from their closed filter condition to their open discharge condition. When one group of filter plates is moved by a latch station, the doubleacting rack-and-pinion arrangement of the drive unit 22 ensures that the next latch station is already being brought into the correct position to open the next group of filter plates to be latched and moved apart.

Advantageously, although not shown in the drawings, a pressure relief valve is typically installed on each of the filter plate shifting devices to ensure that the hydraulic fluid pressure is zeroed after each cycle. This in turn ensures that equal parallel movement of the racks 12, 14 is achieved each time a drive unit 22 is operated.

Advantageously, as hydraulic motors 16 are used, a flow controller (not shown) controlling the flow of hydraulic oil from the hydraulic pump of the hydraulic filter press to the hydraulic motors 16 can be used to speed up or slow down the speed of the hydraulic motors 16, thereby controlling the stroke speed of the racks 12, 14 and hence the speed at which the latch stations 50 are displaced.

Compared to a typical conventional filter plate shifting device employing hydraulic cylinders or a chain drive unit, the filter plate shifting device of the invention, as illustrated, has a double-acting stroke, with no significant time delay in the stroke. Directional control via limit switches is possible obviating the need to employ directional control via pressure or electrical current measurements. Every stroke can be zeroed to a set point, external safety relief through a hydraulic fluid pressure relief valve is possible and external flow control of hydraulic fluid to the hydraulic motors driving the pinions 10 and racks 12, 14 is possible. Advantageously, constant speed throughout operation of the drive unit 22 is possible and the filter plate shifting device of the invention, as illustrated, can pull up to twenty filter plates at a time, whereas a conventional filter plate shifting device making use of a chain drive is more limited in the quantity of filter plates that it can open. The stroke length of a filter plate shifting device making use of hydraulic cylinders is limited to the stroke length of the hydraulic cylinders, whereas the racks of the filter plate shifting device of the invention are not limited to such stroke length as the racks can be manufactured longer than the stroke length of a hydraulic cylinder. Whereas chain drives are limited to a low torque and are extremely sensitive to corrosive and abrasive environments, a very high torque can be produced by the hydraulic motor of the filter plate shifting device of the invention. Lastly, whereas a chain drive needs continuous lubrication, little lubrication is needed on the filter plate shifting device of the invention, as illustrated.