An apparatus for dewatering a suspension

Abstract

A method and apparatus for dewatering a suspension containing solids suspended in a liquid is disclosed. The method comprises at least two stages. A first stage separates solids from liquid by forcing the liquid in the suspension to pass through a screen of a roll from an exterior to an interior of the roll in response to a pressure difference across the screen. The screen at least partially prevents solids in the suspension passing into the interior of the roll and instead retaining solids on the screen. A second stage comprises compressing solids retained on the screen to further dewater solids.

Description

[0001] AN APPARATUS FOR DEWATERING A SUSPENSION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an apparatus for dewatering a suspension .

[0004] The present invention relates particularly, although not exclusively, to an apparatus for dewatering a fine particulate suspension .

[0005] BACKGROUND ART

[0006] The term "suspension" is understood herein to mean a mixture of a liquid and solid particles .

[0007] As used herein , the term "slurry" is an alternative word for the term "suspension" .

[0008] Suspensions are typically used in chemical and mineral processes as a means of transporting solids , for example , by pumping suspensions in pipelines .

[0009] Suspensions are typically produced in chemical and mineral processes , for example as output streams in flotation cells in mineral processing plants .

[0010] The waste output streams in mineral processing plants in the mining industry are examples of suspensions of fine particles in water , typically referred to as "tailings" . Dewatering these suspensions is a significant issue for the mining industry for a range of reasons including but not limited to recovering water and reducing the current reliance on tailings dams .

[0011] Solids can be separated from a suspension by allowing time for liquid of the suspension to evaporate .

[0012] In some instances , it may be more resource efficient to assist in separating liquid and solids in a suspension by mechanical options , also known as dewatering options . Dewatering can be partial or complete separation of solids and a liquid in a suspension .

[0013] The separated solids may undergo further processing , e . g . , evaporation , to remove any remaining liquid.

[0014] Mechanical options for dewatering a suspension include filters , centrifuges , and thickeners .

[0015] Pertinent to all mechanical options for dewatering a suspension is a need to optimise efficiency in operation and to improve the degree to which the suspension is dewatered .

[0016] It is desirable to provide an improved option for dewatering a suspension or to at least provide useful alternatives in the market .

[0017] The above description should not be taken as an admission of the state of the common general knowledge in the art in Australia or elsewhere .

[0018] SUMMARY OF THE DISCLOSURE

[0019] In broad terms , the invention relates to a method of dewatering a suspension containing solids suspended in a liquid and producing dewatered solids that has at least two stages , with :

[0020] (a) a first stage comprising separating solids from the liquid by forcing the liquid in the suspension to pass through a screen of a roll from an exterior to an interior of the roll in response to a pressure difference across the screen , with the screen at least partially preventing solids in the suspension passing into the interior of the roll and instead retaining solids on the screen , and (b) a second stage comprising compressing solids retained on the screen to further dewater solids.

[0021] The first stage is an effective option to separate a substantial part of the solids and liquid in the suspension .

[0022] The second stage of compressing the solids retained on the screen is an effective option for removing liquid from the solids retained on the screen by squeezing liquid from what is by this stage a compressed filter cake of solids .

[0023] The applicant has found that the combined effect of the pressure differential in the first stage and the mechanical action e.g. , the compression, in the second stage improves the degree to which the suspension is dewatered. This is advantageous because the more the suspension is dewatered the less further processing is required, e.g. , evaporation, to remove any remaining liquid.

[0024] The solids may be any solids, for example minerals, paper, or biological matter. The liquid may be any liquid, for example water. The solids may be evenly distributed throughout the liquid, i.e. , homogeneous. The solids may be unevenly distributed throughout the liquid, i.e. , non-homogeneous .

[0025] "Dewatering" is understood to mean separating solids from liquids.

[0026] The invention provides an apparatus for dewatering a suspension containing solids suspended in a liquid, the apparatus comprising:

[0027] - a chamber for containing the suspension,

[0028] - a pair of rolls arranged to counter-rotate in the chamber , the rolls defining a nip between the rolls , each roll comprising a filter screen ,

[0029] - the screen comprising (a) one or more filter elements for filtering solids from the suspension and (b) a support element for supporting the filter element (s) ,

[0030] - each roll comprising an outer surface having an opening that extends inwardly and defines a space that at least partially receives the support element ,

[0031] - the space being formed :

[0032] (a) to allow liquid flow through the space ,

[0033] (b) so that the support element can move within the space and allow the screen to move relative to the roll , and

[0034] (c) so that the one or more filter elements can contact the outer surface of the roll directly with the support element within the space , whereby in use the apparatus is coupled to a source of differential pressure that creates a pressure gradient across the screen that forces the liquid in the suspension through the screen into each roll as the rolls counterrotate in the chamber , with solids from the suspension being retained on the screen as a filter cake , and whereby in use dewatering pressures , typically high dewatering pressures , are generated in the nip , and one or more filter elements of each roll contact the outer surface of the roll directly over an area that allows transfer of the dewatering pressures to the rolls as the suspension passes through the nip , with the dewatering pressures dewatering the filter cake .

[0035] The invention also provides an apparatus for dewatering a suspension containing solids suspended in a liquid, the apparatus comprising : a chamber for containing the suspension and having an inlet for the suspension and at least two outlets , with a first outlet for discharging solids and a second outlet for discharging liquid, a pair of rolls located in the chamber and in use at least partially immersed or fully immersed in the suspension , each roll being mounted for rotation about a longitudinal axis , the rolls being arranged side-by-side and spaced apart to define a nip therebetween and being counter-rotatable , each roll being in fluid communication with the second outlet so that liquid can flow into the roll and through the roll to the second outlet , each roll comprising a filter screen , the screen comprising (a) one or more filter elements for filtering solids from the suspension and (b) a support element for supporting the filter element (s) , each roll comprising an outer surface having an opening that extends inwardly and defines a space , the space at least partially receiving the support element and being formed so that (a) the liquid in the suspension can flow therethrough , (b) the support element can move within the space and therefore the screen can move relative to the roll , and (c) the one or more filter elements can contact the outer surface of the roll within the space , whereby in use the apparatus is coupled to a source of differential pressure that creates a pressure gradient across the screen that forces the liquid in the suspension through the screen into each roll and to the second outlet as the rolls counter-rotate in the chamber , with solids from the suspension being retained on the screen as a filter cake , and whereby in use solids retained on the screen in the filter cake are compressed in the nip and liquid is squeezed from the filter cake into the rolls as the rolls counter-rotate through the nip , thereby further dewatering the solids , with the compressed solids being discharged from the first outlet , with one or more filter elements of each roll contacting the outer surface of the roll when in the nip and transferring forces from the screen to the roll as the suspension passes through the nip . It is noted that the above references to creating a pressure gradient that "forces" the liquid in the suspension through the screen and into the roll is understood herein to cover :

[0036] (a) situations where there is a positive force applied to the suspension to do this , and

[0037] (b) situations where there is a negative force applied to the suspension to do this , which draws the liquid in the suspension through the screen , in both situations as a result of a pressure difference resulting from the source of differential pressure .

[0038] The source of differential pressure may generate a positive pressure upstream of the screen to force the liquid in the suspension through the screen .

[0039] The source of differential pressure may generate a pressure difference between the screen and the second outlet .

[0040] The source of differential pressure may be a pressure head, e . g . , a tank / reservoir having a particular liquid level , or a pump or both .

[0041] The above-described arrangement makes it possible for the filter element (s) to directly contact the outer surface of the roll , with the support element within the space , while the roll is in the nip . This provides an opportunity for typically very high dewatering pressures in the nip to be transferred to the roll over a large surface area while the roll is in the nip . This minimises the risk of failure of the screen under high pressure dewatering operation .

[0042] The space makes it possible to locate the support element in relation to the roll so that liquid can flow through the screen to the second outlet . It will be appreciated that the space should be configured to allow sufficient suspension to flow therebetween to meet dewatering requirements in any given situation .

[0043] Each roll may have a circumferential wall that defines an elongate cavity to allow liquid to flow within the roll .

[0044] The circumferential wall may comprise one or more grooves extending along a length of the roll that define the space, with the support element received in one of the grooves .

[0045] The grooves and the support element may have complementary profiles.

[0046] Each groove may have three orthogonal (90°) walls that define a rectangular shaped profile.

[0047] Each groove may have one or more non-orthogonal walls, e.g. , greater or less than 90° relative to each other .

[0048] Each groove may have greater or fewer than three walls. For example, each grove may have five walls that define an irregular pentagon shaped profile.

[0049] In another example, each groove may have a single arcuate wall that defines a semi-circular shaped profile.

[0050] The support element may have a rectangular profile.

[0051] The support element may have any other shaped profile, e.g. , pentagon, circular, semi-circular. The wall may comprise one or more passages that communicate with the cavity to allow liquid to flow into the cavi ty .

[0052] The one or more passages may comprise inlets in a base of the grooves .

[0053] It will be appreciated that , with this arrangement , liquid can flow through the screen and into the space and around the support element and then from the space into the passages to the cavity of the roll .

[0054] The cavity may communicate with the second outlet to allow liquid to be discharged from the cavity to the second outlet .

[0055] A person skilled in the art would understand that the rolls are arranged side-by-side to facilitate compression of a solids therebetween . It will also be appreciated that the term "arranged side-by-side" encompasses a situation in which the longitudinal axes of the rolls are parallel or non-parallel .

[0056] One or both rolls may be rotatably mounted in the chamber .

[0057] One or both rolls may be adjustably mounted such that the nip is adjustable .

[0058] One or both rolls may be adjustably mounted within the chamber such that the nip is adjustable .

[0059] The distance between the rolls may be adjustable .

[0060] The angle of one roll relative to the other roll may be adjustable . An advantage of adjustably mounting one or both rolls is that the nip can be adjusted to optimise dewatering performance for suspensions having different characteristics . For example , the nip may need to be adjusted depending on the compressibility and permeability of the suspension .

[0061] A further advantage of adjustably mounting one or both rolls is that it may provide ease of access to rolls for cleaning , maintenance , and replacement .

[0062] The chamber may be defined by a base wall and a side wall extending from the base wall that define a volume and are configured to contain the suspension .

[0063] The chamber may be defined by a base wall , a side wall extending from the base wall , and an upper wall extending from the side wall that define a volume and are configured to contain the suspension .

[0064] The chamber may have a lid . The upper wall may form part of the lid .

[0065] At least part of the side wall may form part of the lid .

[0066] The lid may be removably mounted to the side wall .

[0067] The lid may be fixedly mounted to the side wall .

[0068] The inlet may be located on the lid .

[0069] The first outlet may be located on the base wall .

[0070] The chamber may comprise a mounting arrangement for rotatably mounting the rolls . The mounting arrangement may comprise a stub shaft rotatably coupled to a bracket via a bearing . The stub shaft may be coupled to the roll . The bracket may be coupled to one of the side walls that define the chamber .

[0071] The rolls may be removably mounted to the mounting arrangement .

[0072] The screen may have a single filter element . The single filter element may have a wall having a plurality of openings therein . The wall may be tubular .

[0073] The screen may have two or more filter elements .

[0074] The screen may define an outer surface and an inner surface .

[0075] One of the surfaces may be smoother than the other such that particulates are deposited and agglomerated on the smooth surface to produce a filter cake .

[0076] The filter cake may act as an additional filter for solids .

[0077] The two or more filter elements may be arranged side- by-side and spaced apart to define a gap through which liquid in the suspension can pass but which prevents at least some of the solids in the suspension from passing .

[0078] The two or more filter elements may be arranged side- by-side and spaced apart along a longitudinal axis defined by the roll .

[0079] The gap between the filter elements may be between lpm and 10mm, optionally between 1 pm and 500 pm, optionally between 10 pm and 50 pm, and optionally around 30 pm. The gap between the filter elements defines the maximum dimensions of particulates in the solids that can pass through the screen . For example , a 30 pm gap will permit a maximum particulate size of 30 pm. In this example , particulates greater than 30 pm will not be able to pass through the screen , and particulates less than 30 pm may be permitted to pass through the screen .

[0080] The one or more filter elements may have a height between 1 .2mm and 7 .5mm.

[0081] The one or more filter elements may have a width between 0 . 8mm and 3.4mm.

[0082] The one or more filter elements may have a tapered profile . The profile may converge inwardly radially .

[0083] The profile may be triangular shaped .

[0084] The triangular shaped filter elements define a base and side walls .

[0085] Triangular filter elements may be radially oriented such the bases of the one or more filter elements form one of the outer or inner surfaces that is smoother than the other such that particulates are deposited and agglomerated on the that surface to produce a filter cake .

[0086] The filter cake may act as an additional filter for finer particulates .

[0087] Triangular filter elements may be radially oriented such the sides of the one or more filter elements form one of the outer or inner surfaces that is less smooth than the other to minimise deposition and agglomeration of particulates on that surface .

[0088] The screen may be a wedge wire screen . The wedge wire screen may have one or more filter elements , the one or more filter elements having a triangular profile and being arranged side-by-side and radially oriented such that one of an outer and an inner surface of the screen is smoother than the other of the outer or inner surface of the screen .

[0089] The radial orientation of the one or more filter elements determines the direction of flow through the screen , i . e . , flow out to in (FOTI ) or flow in to out (FITO) configurations .

[0090] In an FOTI configuration , the flow of the suspension is directed radially into the screen ; the outer surface of the screen is smoother than the inner surface of the screen and therefore solids cake on the outer surface of the screen to produce a filter cake .

[0091] In a FITO configuration , the flow of suspension is directed radially out of the screen ; the inner surface of the screen is smoother than the outer surface of the screen and therefore the solids cake on the inner surface of the screen to produce a filter cake .

[0092] The one or more filter elements may be non-tapered .

[0093] The one or more filter elements may extend around the roll .

[0094] The one or more filter elements may extend partially or entirely around a circumference of the roll .

[0095] The one or more filter elements may extend around the circumference of the roll in a single plane .

[0096] The one or more filter elements may extend around the circumference of the roll in more than one plane , e . g . , helically .

[0097] Each filter element of the one or more filter elements may have a plurality of filter element segments , each defining an arc around the circumference of the roll .

[0098] The one or more filter elements may be arranged in an interlaced structure .

[0099] The one or more filter elements may have a circular profile .

[0100] The one or more filter elements may define a mesh .

[0101] The screen may be a perforated roll .

[0102] The source of differential pressure may be provided via a pump . The pump may be any pump suitable for pumping suspension . The pump may be a centrifugal pump , a peristaltic pump or a positive displacement pump .

[0103] The apparatus may comprise the pump . The pump may be couplable to at least one of the rolls and configured to create a pressure gradient across the roll to force the liquid in the suspension through the screen into the roll .

[0104] The inlet may be after the nip . In this arrangement , it may be necessary to feed the suspension into the chamber under pressure .

[0105] The apparatus may further include a hopper that feeds suspension into the inlet .

[0106] The inlet may be located on the upper wall , the side wall or the base wall , optionally via feed ports in the base wall . An advantage to having the inlet located on the side wall or the base wall is that it minimises the risk of the feed eroding the filter cake .

[0107] The hopper may be located above the rolls to feed suspension into the chamber under gravity .

[0108] The first outlet may be after the nip .

[0109] The apparatus may further comprise a scraping element for scraping retained solids from the screen . The scraping element may be any suitable shape and material . In one embodiment , the scraping element is a trapezium shaped steel plate .

[0110] The scraping element may be after the nip .

[0111] The apparatus may further comprise a motor for driving one or both rolls .

[0112] The apparatus may further have a frame that supports the chamber and is transportable on a set of wheels .

[0113] Also disclosed herein is a method of using the apparatus , as described above , the method comprising : feeding a suspension through the inlet into the chamber ; counter-rotating the rolls ; creating a pressure gradient across the rolls that forces a liquid in the suspension through the screen and separates solids from liquid in the suspension and retain solids , for example as a filter cake , on the screen , the liquid being forced through the screen to the second outlet ; forcing the filter cake into the nip ; compressing the filter cake between the rolls at the nip and squeezing liquid from the filter cake into the rolls as the rolls counter-rotate through the nip; discharging compressed solids through the first outlet ; and discharging liquid from the rolls and through the second outlet . The rolls may be rotated at the same speed, i . e . , synchronously • The rolls may be rotated at different speeds , i . e . , asynchronously, to shear solids between the rolls .

[0114] The method may further include scraping the compressed solids from the counter-rotating rolls .

[0115] The method may further include feeding suspension into the screen under pressure .

[0116] The suspension may be fed into the screen under pressure using a pump . The pump may be any pump suitable for handling suspensions .

[0117] The method may further include feeding suspension into the screen under gravity .

[0118] The method may further include flocculating the suspension prior to being fed into the screen .

[0119] The method may include further downstream processing of the compressed solids .

[0120] The downstream processing may include a screw press , a centrifuge or any other known dewatering apparatus .

[0121] BRIEF DESCRIPTION OF DRAWINGS

[0122] An embodiment of the invention is described by way of example , and not by way of limitation , with reference to the accompanying drawings , of which :

[0123] Figure 1 is a perspective view of (a) an apparatus for dewatering a suspension according to an embodiment of the present invention and (b) a modular assembly that comprises a separator module , a motor module , and a storage module ,

[0124] Figure 2 is a side view of the apparatus and modular assembly shown in Figure 1 ,

[0125] Figure 3 is a sectional view of the apparatus shown in Figure 1 through a Y-Z plane ,

[0126] Figure 4 is a sectional view of the apparatus shown in Figure 1 through a Y-X plane ,

[0127] Figure 5 is a perspective sectional view of a screen that forms part of the apparatus shown in Figures 3 and 4 ,

[0128] Figure 6 is a schematic view of the screen shown in Figure 5 ,

[0129] Figure 7 is a perspective view of a roll that forms part of the apparatus shown in Figures 3 and 4 ,

[0130] Figure 8 is a sectional perspective view of the roll shown in Figure 7 through a Y-X plane ,

[0131] Figure 9 is an enlarged view of a chamber of the apparatus shown in a circle labelled D in Figure 4 ,

[0132] Figure 10A is a schematic view of a flow path between the screen and the roll with the screen separated from the roll ,

[0133] Figure 10B is a schematic view of a flow path between the screen and the roll with the screen and the roll in contact ,

[0134] Figure 11 is an enlarged view of a roll and a scraper element of the apparatus in a region shown in a circle labelled E in Figure 3 , and Figure 12 is an isolated perspective view of the scraper element shown in Figure 11 .

[0135] DETAILED DESCRIPTION OF EMBODIMENTS

[0136] An embodiment of an apparatus 10 for dewatering a suspension containing solid particles in a liquid in accordance with the invention is shown in the Figures .

[0137] Figures 1 and 2 show the apparatus 10 positioned in relation to a modular assembly that comprises a separator module 50 , a motor module 60 , and a storage module 70 .

[0138] In Figures 1 and 2 , the apparatus 10 is shown situated on the separator module 50 , which is located adjacent to the motor module 60 and the storage module 70 .

[0139] The motor module 60 shown in Figures 1 and 2 comprises two motors 62a , 62b which will be described later in the specification .

[0140] The separator module 70 shown in Figures 1 and 2 comprises a pump (not shown) and storage tanks 72 which will be described later in the specification .

[0141] The modular assembly comprises a frame 11 that supports the modules 60 , 70 . The frame has a set of wheels 13 , whereby the apparatus is movable .

[0142] Turning to Figure 3 , the apparatus 10 comprises a chamber 12 and a pair of rolls 20 located in the chamber 12 .

[0143] The chamber 12 is defined by a base wall 14 , side walls 16 extending from the base wall 14 , and an upper wall 18 extending from the side walls 16 to define a volume configured to contain the suspension . Each roll 20 is mounted for rotation about a longitudinal axis 23. The rolls 20 are arranged side-by- side and spaced apart to define a nip 21 therebetween and are supported to be rotatable in opposite directions .

[0144] Each roll 20 is formed to allow liquid to flow into the roll 20 and within the roll 20 and is configured to be coupled to a source of differential pressure to create a pressure gradient to force the suspension into the roll 20 . This will be described in more detail later with reference to Figure 4 .

[0145] Each roll 20 has a filter screen 24 to allow liquid in the suspension to flow therethrough and to filter the suspension to prevent most , if not all , of the solid particulates in the suspension flowing into the roll 20 .

[0146] The chamber 12 has an inlet 22 for the suspension in the upper wall 18 .

[0147] The apparatus 10 comprises a hopper 26 connected to the upper wall 18 that communicates with the inlet 22 to facilitate the suspension flowing into the chamber 12 . The hopper 26 is located above the chamber 12 and before the nip 21 and is configured to feed the suspension under gravity into the chamber 12 and to the nip 21 .

[0148] The chamber 12 also comprises two outlets 28 (as shown in Figure 3) , 36 (as shown in Figure 4 ) . A first outlet 28 is configured to discharge solids and a second outlet 36 is configured to discharge liquid .

[0149] Figure 3 shows that the first outlet 28 is located below the nip 21 and the inlet 22 and the hopper 26. The first outlet 28 is configured to receive compressed solids from the nip 21 . The first outlet 28 is located beneath the nip 21 such that the solids fall through the first outlet 28 under gravity and under the weight of filter cake accumulated at the nip 21 . The first outlet 28 is connected to a conduit 29 which feeds into a sump 37 in which solids are stored .

[0150] The second outlet will be described with reference to Figure 4 .

[0151] With further reference to Figure 3 , the apparatus 10 comprises a scraper element 30 located below the nip 21 connected to the base wall 14 of the chamber 12 . The scraper element 30 is configured to scrape retained solids from the screen as the counter-rotating rolls 20 move past and retained solids contact the scraper element 30 .

[0152] Turning to Figure 4 , each roll 20 has a circumferential wall 27 that defines an elongate cavity 32 . The cavity 32 allows liquid to flow within the roll 20 . Each roll 20 also includes a conduit 34 within the cavity 32 that communicates with the second outlet 36 to allow liquid to flow from the cavity 32 and along the conduit 34 to the second outlet 36.

[0153] The apparatus 10 includes a mounting arrangement 38 for rotatably mounting the rolls 20 within the chamber 12 . The mounting arrangement 38 comprises a pair of stub shafts 40 (a non-drive side stub shaft 40a and a drive side stub shaft 40b) that are rotatably coupled to brackets 42 via roller bearings 44 . The brackets 42 are fixedly coupled to the chamber 12 .

[0154] The non-drive side stub shaft 40a is hollow to receive the conduit 34 therethrough .

[0155] In an alternative , although not the only other , embodiment , instead of a hollow stub shaft , flow bypasses the mounting arrangement 38 . In this configuration , the roll 20 comprises an end cap coupled to the roll , the end cap comprising holes that communicate with the cavity 32 .

[0156] The motor module 60 is arranged adjacent to the separator module 50 . The motor module 60 comprises a motor 62 to drive the rolls 20 . The motor 62 comprises a drive shaft 64 that is coupled to the drive side stub shaft 40b via drive elements (not shown) , such as a pulley and belt arrangement or sprocket and chain arrangement .

[0157] There are two motors 62a , 62b on the motor module 60 , as shown in Figure 1 . Each motor 62a , 62b drives one of the rolls 20 . The motors 62a , 62b are arranged to drive rotation in opposite directions to each other such that the rolls 20 counter rotate .

[0158] In an alternative , although not the only other embodiment , the motor module 60 comprises a single motor arranged to drive both rolls . In this configuration , the motor comprises a tensioner in a serpentine arrangement .

[0159] For synchronous speed of both rolls , only one motor is required, either with a drive to each roll or with a drive to only one roll (a driven roll ) and relying on the build-up of filter cake in the nip to transfer torque from the driven roll to a non-driven roll .

[0160] For asynchronous speed of both rolls , the two motors are driven at different speeds to each other . However , it is possible to drive both rolls at different speeds using a single motor . For example , the motor module 60 may comprise a differential gear box comprising a drive shaft coupled at one end to a motor shaft and coupled at another end to a drive gear with first and second output shafts coupled at one end to first and second rolls , respectively, and at another end to first and second output gears , respectively, with each of the first and second output gears intermesh with the drive gear and the first and second output gears having a different number of teeth such that rotating the drive gear rotates the first and second output shafts , and therefore the first and second rolls , at different speeds .

[0161] The storage module 70 is arranged adjacent to the separator module 50 . The separator module 70 comprises a pump (not shown) and storage tanks 72 (see Figure 1 ) .

[0162] The pump (not shown) on the separator module 70 is fluidly coupled to the second outlet 36 and conduit 34 at a suction side of the pump , and fluidly coupled to the storage tanks 72 (see Figure 1 ) at a discharge side of the pump .

[0163] The storage tanks 72 are configured to receive flow of liquid from the second outlet 36 via the pump .

[0164] The operation of the apparatus 10 will now be described .

[0165] The suspension is fed from the hopper 26 under gravity into the chamber 12 via the inlet 22 . The suspension fills the chamber 12 and immerses the rolls 20 . The pump and motors 62 can be started once the suspension immerses the rolls 20 . However , suitably, the pump and motors 62 can be started prior to immersion of the rolls 20 such that filter cake is formed prior the rolls 20 being immersed which minimises leakage upon immersion of the rolls 20 .

[0166] The pump is arranged to deliver flow through the rolls 20 to the storage tanks 72 . When the pump is operating , a pressure gradient is generated which forces the liquid in the suspension in the chamber 12 through the screen 24 , the screen 24 separates liquid from the suspension , and the liquid flows into the rolls 20 . Once inside the rolls 20 , the liquid flows along the conduit 34 to the second outlet 36 , then through the pump and to the storage tanks 72 .

[0167] The screen 24 prevents solids above a particular size from passing therethrough into the rolls 20 . The solids that are prevented from passing through the screen 24 are retained on the screen 24 to form a filter cake .

[0168] The motors 62 drive the rolls 20 to counter rotate at a selected speed . As the rolls 20 counter-rotate , the solids in the filter cake on the screen 24 are forced into the nip 22 where they are compressed between the rolls 20 . As the filter cake is compressed, the volume occupied by the filter cake is reduced, which squeezes some remaining liquid from the filter cake into the rolls 20 thereby further dewatering the filter cake .

[0169] The compressed solids are discharged from the first outlet 28 in the manner described above . The remaining liquid is discharged from the second outlet 36 in the manner described above .

[0170] Described in another way, the rolls 20 compress the solids to squeeze liquid therefrom in a similar manner to the way in which a mangle is used to squeeze water from textiles .

[0171] Turning to Figure 5 and 6 , the screen 24 is a wedge wire construction comprising a helix of wire that defines a plurality of annular filter elements 46 supported by a plurality of elongate support elements 48 .

[0172] The annular filter elements 46 collectively define an outer surface 46a and an inner surfaces 46b of the screen 24 .

[0173] Each annular filter element 46 has a triangular profile having a base with sides extending therefrom to join at an apex . The base of the profile of each filter element 46 defines the outer surface 46a and the sides of profile of each filter element 46 defines the inner surface 46b .

[0174] The base of the profile of each filter element 46 has a width between 0 . 8mm and 3.4mm. The length of the filter element 46 from a midpoint of the base to the apex is between 1 .2mm and 7 .5mm.

[0175] The radial orientation of the filter elements 46 determines the direction of flow through the screen , i . e . , flow out to in (FOTI ) or flow in to out (FITO) configurations .

[0176] In the embodiment shown in Figures 5 and 6 , and described above , the screen 24 is an FOTI configuration .

[0177] In a FOTI configuration , the flow of suspension is directed radially into the screen 24 ; the outer surface 46a of the screen 24 is smoother than the inner surface 46b of the screen 24 and therefore solids S cake on the outer surface 46a of the screen 24 to form a filter cake . The filter cake acts as an additional filter for finer particulates .

[0178] An advantage of wedge wire screens is that they are anti -fouling . Wedge wire screens are anti -fouling because any solids that pass the filter elements 46 are flushed by the filtrate , i . e . the liquid in the suspension . Therefore , there is no need to flush the wedge wire screens . The elongate support elements 48 are welded to the sides of the profiles of the filter elements 46 to define a welded connection 47 . The elongate support elements 48 are distributed around the circumference of the inner surface 46b of the screen 24 .

[0179] Turning to Figure 7 to 9 , the elongate cavity 32 of each roll 20 allows liquid to flow within the roll 20 .

[0180] The circumferential wall 27 of each roll 20 comprises a plurality of passages 51 that communicate with the cavity 32 to allow liquid to flow into the cavity 32 . Further details of the passages 51 is provided in relation to Figures 10a , 10b . The cavity 32 communicates with the second outlet 36 to allow liquid to be discharged from the cavity 32 to the second outlet 36.

[0181] The circumferential wall 27 comprises an outer surface 53 having grooves 52 that extend along the length of the roll 20 . The purpose of these grooves 52 will be described with further in the specification with reference to Figures 10A and 10B .

[0182] Each roll 20 has a pair of bushings 57 that resiliently support ends of the screen 24 on the roll 20 . The bushings 57 are made from an elastomeric material and permit movement of the screen 24 relative to the roll 20 so as to minimise transfer of forces between the roll 20 and the screen 24 . In other words , the bushings 57 permit deflection or displacement of the screen 24 relative to the roll 20 .

[0183] The bushings 57 restrict movement of the support elements 48 along the grooves 57 in an axial direction of the roll 20 .

[0184] The bushings 57 assist in lessening noise and vibration between the screen 24 and roll 20 .

[0185] It should be noted that the bushings 57 are not essential to the operation of the apparatus 10 . In some embodiments , the bushings 57 are dispensed with , and in these embodiments the wedge wire screen 24 moves around within a clearance and is only pressing against the roll 20 at the nip , or if there are no solids , by gravity, it would only be in contact with the roll 20 at the top of the roll 20 .

[0186] By permitting deflection or displacement of the screen 24 relative to the roll 20 , stresses that are exerted on the screen 24 during operation may be absorbed, dampened, or dissipated thereby lessening their effect .

[0187] It is advantageous to minimise stresses exerted on the screen 24 as excessive stresses may lead to premature failure of the screen 24 . In particular , this minimises damage and wear on welded connections 47 between the filter elements 46 and support elements 48 .

[0188] As shown in Figure 10A, the grooves 52 define a space 54 for receiving a support element 48 of the screen 24 .

[0189] The grooves 52 are dimensioned in a loose-tolerance fashion to define a gap 55 between the support element 48 and an outer surface 53 of the roll 20 when the support element 48 is received in the groove 52 , as shown in Figures 10A and 10B .

[0190] In the described embodiment , the grooves 52 are both wider and deeper than a width and height of the support elements 48 , respectively, to accommodate the support elements 48 and to form the gap 55 .

[0191] The gap 55 is configured to permit flow therethrough . The gap 55 is also configured to permit relative movement between the outer surface 53 and support element 48 . This facilitates relative movement between the screen 24 and the roll 20 .

[0192] Each passage 51 in the circumferential wall 27 is in fluid communication with the space 54 . Specifically, an opening 56 of each passage 51 is located at a base of the groove 52 . The space 54 allows flow of liquid around the support element 48 and then from the space 54 , through the opening 56 into the passages 51 and to the cavity 32 of the roll 20 . Liquid can flow through passage 51 from the opening 56 to the cavity 32 and, as described above , from the cavity 32 to the second outlet 36. In other embodiments , liquid may flow through other channels .

[0193] As shown in Figures 10A and 10B, the support element 48 can move within the space 54 and therefore the screen 24 can move relative to the roll 20 , such that the filter elements 46 can contact the outer surface 53 of the roll 20 to distribute forces over a large contact area X .

[0194] In use , the filter elements 46 contact the outer surface 53 of the roll 20 when in the nip 21 and transfer forces from the screen 24 to the roll 20 as the roll 20 passes through the nip 21 .

[0195] The contact area X is greater than would be the case if the sole contact between the screen 24 and the roll 20 was between the support elements 48 and outer surface 53.

[0196] As such , the contact area X minimises stress , per unit of force , on the screen 24 because stress is unit of force per unit of area .

[0197] The contact area X also minimises the transfer of forces to the welded connections 47 between the support elements 48 and the filter elements 46. This is particularly advantageous , as stress can concentrate at the welded connections 47 and may result in material failure at the welded connections 47 .

[0198] As shown in Figure 10A and 10B , a volume of the gap 55 reduces as the support element 48 is inserted into the groove 52 .

[0199] Turning to Figure 11 and 12 , the scraper 30 comprises a mounting portion 30a and an engagement portion 30b that are integrally formed or mechanically connected to each other via fasteners , e . g . , screws . The mounting portion 30a is configured to be coupled to the base wall 14 of the chamber 12 . The engagement portion 30b is configured to scrape caked solids from the roll 20 . The engagement portion has a concave surface that conforms to the outer surface of the roll 20 and a profile that tapers to form an edge which is used to skim the outer surface of the screen 24 to remove the filter cake therefrom. In other embodiments , a simple scraper may be used .

[0200] The above-described embodiment is an effective apparatus for dewatering suspensions .

[0201] It will be appreciated by persons skilled in the art that numerous variations and modifications may be made to the above-described embodiments , without departing from the scope of the following claims .

[0202] The present embodiments are , therefore , to be considered in all respects as illustrative of the scope of protection , and not restrictively .

[0203] Unless defined otherwise , all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs . Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention , a limited number of the example methods and materials are described herein .

[0204] In the claims which follow and in the preceding description of the invention , except where the context requires otherwise due to express language or necessary implication , the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense , i . e . , to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention .

Claims

CLAIMS1 . An apparatus for dewatering a suspension containing solids suspended in a liquid, the apparatus comprising :- a chamber for containing the suspension ,- a pair of rolls arranged to counter-rotate in the chamber , the rolls defining a nip between the rolls , each roll comprising a filter screen ,- the screen comprising (a) one or more filter elements for filtering solids from the suspension and (b) a support element for supporting the filter element (s) ,- each roll comprising an outer surface having an opening that extends inwardly and defines a space that at least partially receives the support element ,- the space being formed :(a) to allow liquid flow through the space ,(b) so that the support element can move within the space and allow the screen to move relative to the roll , and(c) so that the one or more filter elements can contact the outer surface of the roll directly with the support element within the space , whereby in use the apparatus is coupled to a source of differential pressure that creates a pressure gradient across the screen that forces the liquid in the suspension through the screen into each roll as the rolls counterrotate in the chamber , with solids from the suspension being retained on the screen as a filter cake , and whereby in use dewatering pressures are generated in the nip , and one or more filter elements of each roll contact the outer surface of the roll directly over an area that allows transfer of dewatering pressures to the rolls as the suspension passes through the nip , with the dewatering pressures dewatering the filter cake .2 . An apparatus for dewatering a suspension containing solids suspended in a liquid, the apparatus comprising :a chamber for containing the suspension and having an inlet for the suspension and at least two outlets , with a first outlet for discharging solids and a second outlet for discharging liquid, a pair of rolls located in the chamber and in use at least partially immersed or fully immersed in the suspension , each roll being mounted for rotation about a longitudinal axis , the rolls being arranged side-by-side and spaced apart to define a nip therebetween and being counter-rotatable , each roll being in fluid communication with the second outlet so that liquid can flow into the roll and through the roll to the second outlet , each roll comprising a filter screen , the screen comprising (a) one or more filter elements for filtering solids from the suspension and (b) a support element for supporting the filter element (s) , each roll having an outer surface having an opening that extends inwardly and defines a space , the space at least partially receiving the support element and being formed so that (a) the suspension can flow therethrough ,(b) the support element can move within the space and therefore the screen can move relative to the roll , and(c) the one or more filter elements can contact the outer surface of the roll within the space , whereby in use the apparatus is coupled to a source of differential pressure that creates a pressure gradient across the screen that forces the liquid in the suspension through the screen into each roll and to the second outlet as the rolls counter-rotate in the chamber , with solids from the suspension being retained on the screen as a filter cake , and whereby in use solids retained on the screen in the filter cake are compressed in the nip and liquid is squeezed from the filter cake into the rolls as the rolls counter-rotate through the nip , thereby further dewatering the solids , with the compressed solids being discharged from the first outlet , and with one or more filterelements of each roll contacting the outer surface of the roll when in the nip and transferring forces from the screen to the roll as the suspension passes through the nip .3 . The apparatus of claim 1 or claim 2 , wherein each roll has a circumferential wall that defines an elongate cavity to allow liquid to flow within the roll .4 . The apparatus of claim 3 , wherein the circumferential wall comprises one or more grooves extending along a length of the roll that define the space , with the support element received in one of the grooves .5 . The apparatus of claim 4 , wherein the wall comprises one or more passages that communicate with the cavity to allow liquid to flow into the cavity .6 . The apparatus of any one of claims 3 to 5 , wherein the cavity communicates with the second outlet to allow liquid to be discharged from the cavity to the second outlet .7 . The apparatus of any one of the preceding claims , wherein the screen has two or more filter elements , with the filter elements being arranged side-by-side and spaced apart to define a gap through which liquid in the suspension can pass but which prevents at least some of the solids in the suspension from passing .8 . The apparatus of claim 7 , wherein the two or more filter elements are arranged side-by-side and spaced apart along a longitudinal axis defined by the roll .9 . The apparatus of claim 7 , wherein the gap between the filter elements is between 1 pm and 10mm, optionally between 1 pm and 500 pm, optionally, between 10 pm and 50pm, optionally 30 pm.10 . The apparatus of any one of the preceding claims , wherein the one or more filter elements have a tapered profile .11 . The apparatus of claim 10 , wherein the profile converges inwardly radially .12 . The apparatus of any one of the preceding claims , wherein the one or more filter elements extend partially or entirely around a circumference of the roll .

13. The apparatus of any one of the preceding claims , wherein the screen is a wedge wire screen .14 . The apparatus of any one of the preceding claims , further comprising a pump that is coupled to at least one of the rolls and configured to create a pressure gradient across the rolls to force the liquid in the suspension through the screen into the roll .15 . The apparatus of any one of the preceding claims , wherein the inlet is after the nip .

16. The apparatus of any one of the preceding claims , further comprising a hopper that feeds suspension into the inlet .17 . The apparatus of claim 16 , wherein the hopper is located above the rolls to feed suspension into the chamber under gravity .18 . The apparatus of any one of the preceding claims , wherein the first outlet is after the nip .

19. The apparatus of any one of the preceding claims ,further comprising a scraping element for scraping retained solids from the screen .20 . The apparatus of claim 19 , wherein the scraping element is after the nip .21 . The apparatus of any one of the preceding claims , further comprising a motor for driving one or both rolls .22 . The apparatus of any one of the preceding claims , further comprising a frame that supports the chamber and is transportable on a set of wheels .

23. A method of dewatering a suspension containing solids suspended in a liquid and producing dewatered solids that has at least two stages , with :(a) a first stage comprising separating solids from the liquid by forcing the liquid in the suspension to pass through a screen of a roll from an exterior to an interior of the roll in response to a pressure difference across the screen , with the screen at least partially preventing solids in the suspension passing into the interior of the roll and instead retaining solids on the screen , and(b) a second stage comprising compressing solids retained on the screen to further dewater solids .

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