WO2025050163A1

WO2025050163A1 - Apparatus and method for producing a fluidised bed

Info

Publication number: WO2025050163A1
Application number: PCT/AU2024/050905
Authority: WO
Inventors: Kevin Patrick Galvin
Original assignee: The University of Newcastle; Newcastle University of Upon Tyne
Current assignee: The University of Newcastle; Newcastle University of Upon Tyne
Priority date: 2023-09-06
Filing date: 2024-08-26
Publication date: 2025-03-13
Anticipated expiration: 2026-03-06

Abstract

An apparatus (1) is provided for producing a fluidised bed (2) comprising a suspension of high density and low density particles formed within a vessel (5) of a particle separator (3). The apparatus 1 has a plurality of rakes (10), each rotatably mountable within the vessel (5). Each rake (10) has one or more radially extending rake arms (20). Each rake arm (20) has a blade assembly (30) extending from the rake arm for cutting through the suspension in the fluidised bed 2. The blade assembly (30) may include a plurality of interconnected frame members (35) or one or more spaced apart blades (48) extending into the fluidised bed, where blades each comprise a rod, bar, planar element, paddle, perforated plate or plate-like structure.

Description

Apparatus and Method for Producing a Fluidised Bed Field of the Invention [0001] The invention relates to an apparatus and method for producing a fluidised bed and in a particular to an apparatus and method for producing a fluidised bed in a particle separator. The invention has been developed primarily for use for producing a dense medium fluidised bed for a reflux classifier and will be described hereinafter by reference to this application. Background of the Invention [0002] The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its advantages to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an express or implied admission that such art is widely known or forms part of common general knowledge in the field. [0003] Particle separators are commonly used in the minerals processing industry to separate valuable mineral particles from the less valuable particles, often called “gangue”. One type of particle separator is a fluidised bed classifier, which is used extensively in the coal and minerals industry for separating particles on the basis of density. Feed slurry enters the fluidised bed classifier, separating ultimately into a slurry of finer or lower density particles rising up through the vessel, and an underflow discharge of larger or higher density particles discharging from below. The lower portion of the system is supported by an upward current of fluidisation, usually delivered across the lower base of the vessel by a fluidised bed. [0004] One type of fluidised bed classifier is referred to as a reflux classifier, which comprises a system of inclined channels located within the vessel, with an overflow launder located above the inclined channels and at the top around the outer rim to collect the overflow containing low density or small particles. Higher density or larger (coarse) particles are discharged through an underflow valve at the bottom of the vessel. In the reflux classifier the feed slurry is usually fed at an elevation close to the lower part of the inclined channels and is delivered either from above or adjacent to the system of inclined channels. Other variations of the reflux classifier include an inverted reflux classifier, where the fluidised bed is inverted, and a graviton, which is effectively a reflux classifier located inside a centrifuge. [0005] A reflux classifier delivers strong gravity separation performance by virtue of two main features. The first is the system of parallel inclined channels in the upper section which promotes ideal particle transport for achieving density-based separation. The inclined channels typically capture denser and faster settling particles on the upward facing surfaces, while the fluid above the upward facing surface conveys slower settling, less dense particles in an upward direction. Those particles that are captured on the upward facing surfaces either eventually slide downwards or become resuspended. The resuspended particles are usually lower in density, and are resuspended via a shear induced lift force. Thus, the higher density particles are returned in a downwards direction to the lower fluidised bed while the lower density particles are conveyed upwards. Ultrafine particles, not captured by the inclined surfaces, are also conveyed upwards. [0006] The second main feature of the reflux classifier is the lower fluidised bed. In general, this zone is sustained by the delivery of fluidisation fluid (typically water) set ideally at the so-called minimum fluidisation velocity or slightly higher. The fluidisation water emerges from a plenum chamber through a distributed set of nozzles. The number and size of the nozzles are set in order to produce a significant pressure drop, which in turn helps to ensure the water is distributed evenly to every nozzle. Unform delivery of fluidised water is essential to support the fluidised state of the fluidised bed. [0007] Each of these particle separation systems requires significant fluidisation to support the suspension of particles and thus enable separation. The delivery of this fluidisation results in the addition of more fluid, usually water, and hence more energy. However, it is not possible to reduce the amount of fluidisation to reduce the energy consumption, since it would adversely impact on suspension of the particles. It would also adversely impact two additional functions of fluidisation. The first function is the fluidisation fluid facilitating the desliming, or cleaning, of the material prior to discharge to underflow. The second function is the fluidisation fluid providing a well-defined and uniform fluidisation condition to support the weight of the particles in the fluid, and in turn prevent mixing with the material being fed to the system. This second function is crucial to prevent short-circuiting and hence misplacement of the slurry material. [0008] There is a dense medium fluidised state that develops when the minimum fluidisation velocity is applied. In the literature this is sometimes referred to as inversion, where larger (coarse) low-density particles are effectively displaced upwards and out of the fluidised bed, leaving behind relatively fine high-density particles. This state needs to be promoted to maximise the effectiveness of the reflux classifier. [0009] Sometimes, however, the particle size range is too great for the reflux classifier. This causes relatively coarse low-density particles to report back into the fluidised bed and in turn lowering the suspension density. This results in reducing the effectiveness of the fluidised state as a dense medium. The presence of these coarse low-density particles also results in the need to increase the fluidisation rate further, in turn lowering the suspension density of the fluidised bed, and degrading particle separation performance. [0010] It is also well known that a fluidised bed can develop an uneven solids distribution, especially when operated close to the minimum fluidisation velocity. This uneven state arises for many reasons; firstly, due to changes in the feed, secondly, due to the number density of the holes (in the nozzles) being too low or the pressure drop across the nozzle holes being too low. This situation becomes more extreme when the feed particle size in the fluidised bed decreases to relatively fine sizes, less than 0.1 mm in diameter, and especially below 0.05 mm in diameter. In this case, the required nozzle size and required number density of nozzles needs to be reduced. However, there are physical and practical limits that apply. As a result, this means the design of the reflux classifier becomes compromised. [0011] While the correct fluidisation rate may well be supplied, there exists the strong potential for the rheology of the material in the bed to vary. Zones of higher particle density may emerge from the floor or base of the vessel between the nozzles, which grow over time, promoting channels of water to form up through the bed. This means that the fluidised bed suspension state becomes less effective. [0012] There are major benefits in the minerals processing industry that arise from extending the particle size range of density-based separations and or achieving a much higher product grade with increased product recovery. For example, there is the need to increase the grade of iron ore to a percentage of iron as high as 67% to 69%. For context, it is noted that pure hematite has 69.9% iron. This high grade, in excess of 67%, then permits the ore to be used to achieve direct reduction to iron using hydrogen. Consequently, it is desirable to improve the separation efficiency of density-based particle separators so that very high grades can be achieved. [0013] It is an object of the present invention to overcome or substantially ameliorate one or more of the disadvantages of prior art, or at least to provide a useful alternative. It is an object of the invention in at least one preferred embodiment to make the rheology of the material in the fluidised bed uniform, thus allowing a dense medium state to be developed and maintained at lower fluidisation velocities. It is an object of the invention in at least one preferred embodiment to provide a more even or uniform solids distribution in the fluidised bed, creating a more uniform suspension. In particular, it is an object of the invention in at least one preferred embodiment to provide that the rheological state of the fluidised bed is made uniform across any horizontal layer, thus ensuring the water added to the fluidised bed is transported uniformly. Summary of the Invention [0014] A first aspect of the invention provides an apparatus for producing a fluidised bed in a particle separator comprising a vessel, wherein the fluidised bed comprises a suspension of high density and low density particles, the apparatus comprising: a rake rotatably mountable within the vessel; wherein: the rake comprises one or more radially extending rake arms; and at least one rake arm comprises a blade assembly extending from the at least one rake arm for cutting through the suspension in the fluidised bed. [0015] In one or more embodiments, the blade assembly extends at an acute or oblique angle from the at least one rake arm. In one or more embodiments, the blade assembly extends substantially perpendicular or orthogonal to the at least one rake arm. [0016] In one or more embodiments, the blade assembly comprises one or more spaced apart blades extending from the at least one rake arm into the fluidised bed. In one or more embodiments, the blades each comprise a rod, bar, planar element, paddle perforated plate or plate-like structure. [0017] In one or more embodiments, the blade assembly comprises a frame having a plurality of interconnected frame members. In one or more embodiments, at least two of the plurality of interconnected frame members are arranged substantially orthogonal to each other. In one or more embodiments, at least two of the plurality of interconnected frame members are arranged at an acute angle to each other. In one or more embodiments, at least two of the plurality of interconnected frame members are arranged at an oblique angle to each other. In one or more embodiments, the plurality of interconnected frame members form a mesh-like structure or a perforated structure. [0018] In one or more embodiments, the blade assembly is provided on each of the rake arms. [0019] In one or more embodiments, the rake is rotatably mountable to a first end of the vessel. In one or more embodiments, the rake is mounted to a shaft that is rotatably mountable to the first end of the vessel. In one or more embodiments, the first end of the vessel comprises a base or floor of the vessel. [0020] In one or more embodiments, a conduit has a plurality of nozzles for delivering fluidisation fluid into the vessel. In one or more embodiments, wherein the conduit is rotatably mountable to the vessel. In one or more embodiments, the conduit and the rake are coaxially mountable to the vessel. In one or more embodiments, the conduit is mounted to the shaft. [0021] In one or more embodiments, the conduit comprises a plurality of radially extending conduit arms, at least one of the conduit arms having the plurality of nozzles. [0022] In one or more embodiments, the conduit is connected to at least one rake arm. [0023] In one or more embodiments, the conduit or conduit arm is formed within the at least one rake arm and the plurality of nozzles are formed in an outer surface of the at least one rake arm. [0024] In one or more embodiments, there is a plurality of the rakes. In one or more embodiments, the rakes are arranged in a stack. In one or more embodiments, there is a first rake arranged above a second rake in the vessel. In one or more embodiments, the first rake has a lesser number of rake arms than the second rake. [0025] In one or more embodiments, the plurality of rakes are arranged one above the other with a lowermost rake adjacent the first end of the vessel and having a greater number of rake arms than each of the rakes above it. In one or more embodiments, the rakes above the lowermost rake have progressively a lesser number of rake arms up to an uppermost rake. In one or more embodiments, the uppermost rake has the least or lowest number of rake arms, preferably one rake arm. The greater number of rake arms on the lowermost rake ensures strong planar mixing at the first end (base or floor). By providing less rake arms for the successive rakes above the lowermost rake provides the uniformity in the fluidised bed without undue levels of mixing, thus permitting strong segregation between the low density particles and the high density particles in the upper region or portion of a rake zone. The rake zone is generally the volume in which the rake passes through the fluidised bed. [0026] A second aspect of the invention provides a particle separator for separating high density particles from low density particles from a feed slurry, comprising: a vessel for receiving the feed slurry; a fluidisation source for delivering a fluidisation fluid into the vessel, thereby creating a fluidised bed comprising a suspension of the high density and low density particles; and the apparatus of the first aspect mounted within the vessel. [0027] In one or more embodiments, the fluidisation source is fluidly connected to one or more fluidisation inlets for delivering the fluidisation fluid. In one or more embodiments, the one or more fluidisation inlets are arranged at or formed in a sidewall of the vessel. In one or more embodiments, the one or more fluidisation inlets are arranged at or formed in a base or floor of the vessel. [0028] In one or more embodiments, the particle separator comprises a plurality of inclined channels located within the vessel for separating the low density particles from the high density particles. Preferably, the inclined channels are located adjacent a second end of the vessel. In one or more embodiments, the second end is opposite the first end. In one or more embodiments, the inclined channels are formed by a plurality of substantially parallel plates. [0029] In one or more embodiments, the particle separator comprises a discharge outlet for discharging an underflow of high density particles. In one or more embodiments, the discharge outlet is connected to a control valve for controlling the discharge of the underflow. In one or more embodiments, the discharge outlet is fluidly connected to a pump for removing the underflow. [0030] In one or more embodiments, the particle separator comprises a recycling conduit for recycling at least a portion of the underflow to the vessel. In one or more embodiments, the recycling conduit is fluidly connected to the discharge outlet or a discharge pipe fluidly connected to the discharge outlet. In one or more embodiments, the recycling conduit is fluidly connected to a pump configured for drawing the underflow portion from the discharge outlet and delivering the underflow portion to the vessel. In one or more embodiments, the recycling conduit is fluidly connected to a secondary fluidisation source for adding a fluidisation fluid to the underflow portion prior to being delivered to the vessel [0031] In one or more embodiments, the recycling conduit delivers the underflow portion to the vessel at a location spaced apart from the discharge outlet. In one or more embodiments, the recycling conduit delivers the underflow portion to the vessel at a location near or at the blade assembly. In one or more embodiments, the recycling conduit delivers the underflow portion to the vessel at a location near or at the first end of the vessel. In one or more embodiments, the recycling conduit delivers the underflow portion to the vessel at a location spaced apart from the first end of the vessel. [0032] In one or more embodiments, the particle separator comprises one or more sensors for measuring the density of the suspension. In one or more embodiments, the one or more sensors are located adjacent or at a sidewall of the vessel. In one or more embodiments, the one or more sensors are located above the rake zone. In one or more embodiments, the one or more sensors are located within the rake zone. In one or more embodiments, there are at least two sensors spaced apart from each other. In one or more embodiments, the sensors comprise pressure transducers In one or more embodiments, the sensors comprise load cells. [0033] One or more embodiments of the second aspect may have the features of one or more embodiments of the first aspect of the invention stated above, where applicable. [0034] A third aspect of the invention provides a method of producing a fluidised bed in a particle separator comprising a vessel, wherein the fluidised bed comprises a suspension, the method comprising: feeding a feed slurry of high density and low density particles into the vessel; delivering a fluidisation fluid into the vessel to create a fluidised bed comprising a suspension of the high density and low density particles; stirring the suspension using a rake rotatably mounted within the vessel, the rake comprising one or more radially extending rake arms, wherein at least one rake arm comprises a blade assembly extending from the least one rake arm; and moving the rake arms so that the blade assembly cuts through the suspension in the fluidised bed. [0035] In one or more embodiments, the method comprises providing the blade assembly with one or more spaced apart blades extending into the fluidised bed. In one or more embodiments, the method comprises providing the blade assembly with a frame having a plurality of interconnected frame members. [0036] In one or more embodiments, the method comprises providing a conduit with a plurality of nozzles for delivering the fluidisation fluid into the vessel. In one or more embodiments, the method comprises rotatably moving the conduit within the vessel. In one or more embodiments, the method comprises arranging the conduit and the rake to be coaxially mounted to the vessel. [0037] In one or more embodiments, the method comprises connecting the conduit to at least one rake arm. [0038] In one or more embodiments, the method comprises forming the conduit within the at least one rake arm and the plurality of nozzles are formed in an outer surface of the at least one rake arm. [0039] In one or more embodiments, the method comprising delivering the fluidisation from a fluidisation source to one or more fluidisation inlets. In one or more embodiments, the method comprises arranging the one or more fluidisation inlets at a sidewall of the vessel. In one or more embodiments, the method comprises forming the one or more fluidisation inlets in the sidewall. In one or more embodiments, the method comprises arranging the one or more fluidisation inlets at a base or floor of the vessel. In one or more embodiments, the method comprises forming the one or more fluidisation inlets in a base or floor of the vessel. [0040] In one or more embodiments, a portion of the underflow is recycled to the vessel. In one or more embodiments, a fluidisation fluid is added to the underflow portion before entering the vessel to dilute the underflow portion. In one or more embodiments, the at least one rake arm mixes the diluted underflow portion into the slurry. [0041] In one or more embodiments, the method comprises providing a plurality of the rakes. In one or more embodiments, the method comprises arranging the rake into a stack. In one or more embodiments, the method comprises arranging a first rake above a second rake in the vessel. This allows the rakes move uniformly through the fluidised bed at the same time. In one or more embodiments, the first rake has a lesser number of rake arms than the second rake. [0042] In one or more embodiments, the method comprises arranging the plurality of rakes one above the other with a lowermost rake adjacent the first end of the vessel and providing a greater number of rake arms on the lowermost rake than each of the rakes above it. In one or more embodiments, the method comprises providing the rakes above the lowermost rake with a progressively lesser number of rake arms up to an uppermost rake. In one or more embodiments, the method comprises providing the uppermost rake with the least or lowest number of rake arms, preferably one rake arm. [0043] In one or more embodiments, the method comprises providing a plurality of inclined channels located within the vessel for separating the low density particles from the high density particles. In one or more embodiments, the plurality of inclined channels are located adjacent a second end of the vessel. [0044] In one or more embodiments, the method comprises measuring the density of the suspension. [0045] One or more embodiments of the third aspect may have the features of one or more embodiments of the first and/or second aspects of the invention stated above, where applicable. [0046] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. [0047] Furthermore, as used herein and unless otherwise specified, the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. Brief Description of the Drawings [0048] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: [0049] Figure 1 is a schematic partial cross sectional view of an apparatus for producing a fluidised bed in a particle separator according to an embodiment of the invention; [0050] Figure 2 is a schematic cross sectional view of a rake used in the apparatus of Figure 1; [0051] Figure 3 is a schematic cross sectional view of an alternative rake used in the apparatus of Figure 1; [0052] Figure 4 is a schematic cross sectional view of a reflux classifier according to one embodiment of the invention comprising the apparatus of Figure 1; and [0053] Figure 5 is a schematic cross sectional view of a reflux classifier according to one embodiment of the invention comprising the apparatus of Figure 1. Preferred Embodiments of the Invention [0054] The present invention will now be described with reference to the following examples which should be considered in all respects as illustrative and non-restrictive. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. In the Figures, corresponding features within the same embodiment or common to different embodiments have been given the same reference numerals. [0055] The preferred embodiments of the invention seek to improve fluidisation of a suspension of particles, aimed at promoting inversion, the displacement of relatively large or coarse low-density particles in an upwards direction, favouring finer high-density particles to settle downwards. The preferred embodiments of the invention employ a low- speed rake or rakes moving within the fluidised bed to produce a more uniform suspension density in the horizontal direction, and an increase in the suspension density in the downwards direction. Within the fluidised bed, uniform movement of water in an upwards direction then occurs, while the solids on average settle downwards. [0056] Referring to Figure 1, an apparatus 1 for producing a fluidised bed 2 in a particle separator 3 according to an embodiment of the invention is illustrated. The apparatus 1 is located within a vessel 5 of the particle separator having a feed slurry inlet 7, at least one sidewall 8 and the fluidised bed 2 comprising a suspension of high density and low density particles is formed within the vessel. The apparatus 1 comprises a plurality of rakes 10, each rotatably mountable within the vessel 5, each rake having one or more radially extending rake arms 20, each rake arm having a blade assembly 30 extending from the rake arm for cutting through the suspension in the fluidised bed 2. It will be appreciated that in other embodiments, there may be a single rake 10, rather than multiple rakes. The rakes 10 are preferably mounted towards a first end 33 of the vessel 5. The first end 33 is preferably the base or floor of the vessel 5. [0057] The blade assembly 30 in this embodiment extends substantially perpendicular or orthogonal to the rake arm 20. However, in other embodiments, the blade assembly 30 may extend at an acute or oblique angle from the rake arm 20. [0058] In this embodiment, the blade assembly 30 comprises a frame having a plurality of interconnected frame members 35, as best shown in Figure 2. The frame 30 is essentially an open structure, where the open area can be adjusted as required to produce movement of the suspension. The frame 30 may also be considered as a network of frame members 35 connected to each other. Two or more of the plurality of interconnected frame members 35 are arranged substantially orthogonal to each other. The interconnected frame members 35 may also form sub-frames, such as rectangular shapes 40, or sub-units, such as rectangular sections 45. Generally, it is preferred that the frame 30 forms a mesh-like structure. Preferably, the frame 30 has a closed boundary 47. The frame members 35 may be formed by wires, bars, rods or the like. [0059] In other embodiments, two or more of the interconnected frame members 35 may be arranged at an acute or oblique angle to each other. Some embodiments may have some interconnected frame members 35 that are at acute angle to each other and other interconnected frame members at an oblique angle to each other. [0060] In alternative embodiments, the blade assembly may comprise one or more spaced apart blades extending into the fluidised bed. In this case, the blade assembly does not have a closed boundary at one side or end. In a further embodiment, the blades each comprise a planar element, paddle, perforated plate or plate-like structure. In this embodiment, the blades are opaque structures compared to the open frame structures shown in Figure 1. In a further embodiment shown in Figure 3, the blade assembly 30 comprises one or more planar blades or plates 48 that have perforations or holes 49. The perforations or holes 49 may be formed by drilling them into the plates 48. The perforations or holes 49 may be arranged randomly (as shown in blade 48a) or in an ordered pattern or arrangement (as shown in plate 48b), like rows and/or columns. The size of the holes 49 may be varied as desired, to suit the viscosity of the slurry. There may be a single plates 48 on each rake arm 20, as shown in Figure 3, or there may be on each rake arm spaced apart by even or uneven gaps or intervals. [0061] As shown in the Figures, the blade assembly 30 is provided on each of the rake arms 20. However, in other embodiments, the blade assembly 30 may be provided on a single rake arm 20 or some, but not all, of the rake arms. [0062] The rakes 10 are mounted to a common shaft 50 that is rotatably mountable to the base 33 of the vessel 5. Consequently, rotation of the shaft 50 rotates the rakes 10 in unison. This ensures that the fluidised bed 2 is uniformly cut through by the rakes 10 at the same time. [0063] The apparatus 1 also comprises conduits (not shown) having a plurality of nozzles (not shown) for delivering fluidisation fluid into the vessel. In this embodiment, the conduits are formed within the rake arms 20 and the plurality of nozzles are formed in or at an outer surface of the rake arms. Essentially, the conduits and nozzles are integrated into the structure of the rake arms 20. Consequently, the rake arms 20 take the form of hollow tubes or pipes to internally house the conduits. This also means that the conduits (and nozzles) rotate in conjunction with the rake arms 20. This arrangement ensures that fluidisation fluid is delivered everywhere in the vessel 5, and done uniformly. The fluidisation water is supplied into the rake arms 20 or each rake 10 via the shaft 50. Each rake 10 and rake arm 20 has a mechanical seal arrangement to ensure that the fluidisation water is delivered to through the nozzles into the vessel 5. [0064] The number of nozzles (which can be considered as holes) along each rake arm 20 may vary, and can be arranged in groups or clusters. The number of nozzles or holes in the rake arms 20 may be calculated as a nozzle-number or hole-number density. The hole number-density can be varied to ensure the outer zones of the fluidised bed 2 near or adjacent the sidewall(s) 8 of the vessel 5 receive the correct amount of fluidisation water relative to the area involved. This generally involves having more nozzles or holes per unit length closer to the sidewall(s) 8 compared to the centre, since there is a greater volume nearer the sidewall(s) 8. This assists in producing a uniform distribution of the fluidisation water. [0065] In alternative embodiments, the conduits are separate from the rake arms 20. In this case, the conduits are rotatably mountable to the vessel 5, preferably by mounting the conduit to the shaft 50. As a result, the conduits and the rakes 10 are coaxially mountable to the vessel 5. The conduits may take the form of hollow tubes or pipes. The conduits may each preferably comprise a plurality of radially extending conduit arms, at least one of the conduit arms having the plurality of nozzles. Preferably, the plurality of nozzles are provided in each of the conduit arms. [0066] In one alternative embodiment, the conduits are attached or connected to the rakes 10, preferably the rake arms 20. In other embodiments, there may be a single conduit attached or connected to a single rake arm 20. The conduits are connected preferably at a lower side or base of the rake arms 20, but can be situated between the rake arms and the blade assembly 30. Alternatively, the conduits could be placed in the middle of the blade assembly 30; for example, where the blade assembly comprises a frame with interconnected frame members 35, the conduits would essentially divide the frame 30 in two parts. [0067] In other embodiments, fluidisation water can be delivered into the fluidised bed 2, either through the sidewall 8 or base 33 of the vessel 5, or elsewhere from the rake 10. In this case, the fluidisation source may be fluidly connected to one or more fluidisation inlets (not shown) for delivering the fluidisation fluid. For example, the one or more fluidisation inlets are arranged at or formed in the sidewall 8 and/or base 33. These forms of fluidisation delivery may be used as an alternative or in addition to the conduits. [0068] As seen in Figures 1, 4 and 5, the plurality of the rakes are arranged in a stack or group 55, preferably one above the other. The stack 55 will usually have an uppermost rake 10a and a lowermost rake 10d, where the lowermost rake 10d is adjacent the base 33 of the vessel 5, as best shown in Figure 4. Hence, the stack 55 of rakes 10 are arranged so the rakes are placed above each other in a direction parallel to a longitudinal axis of the vessel 5, which is usually in the vertical direction. This configuration allows the rakes 10, and their respective blade assemblies 30, to move uniformly through the entire fluidised bed 2 at the same time. The rakes 10 may be vertically aligned so that they traverse the same cross-section of the fluidised bed 2 at the same time. Alternatively, the rakes 10 can be offset or staggered in the longitudinal direction of the vessel, so that they move through different cross-sections of the fluidised bed 2 across different horizontal layers. [0069] In one or more embodiments, the lowermost rake 10d has a greater number of rake arms than each of the rakes 10a, 10b, 10c above it. In other embodiments, the rakes 10a, 10b, 10c above the lowermost rake 10d have a lesser number of rake arms than the lowermost rake. In some cases, the rakes 10b, 10c above the lowermost rake 10d progressively have a lesser number of rake arms up to the uppermost rake 10a. Generally, it is preferred that the uppermost rake 10a has the least or lowest number of rake arms, most preferably one rake arm. The greater number of rake arms on the lowermost rake 10d ensures strong planar mixing at the base 33 of the vessel 5. By providing less rake arms for the rakes 10a, 10b, 10c above the lowermost rake 10d, the embodiment of the invention provides a desired uniformity in the fluidised 2 without creating undue levels of mixing. This in turn permits strong segregation between the low density particles and the high density particles in the upper region or portion of the rake zone. [0070] It is contemplated that the rakes 10 is the primary means by which the fluidisation water is distributed in the vessel 5. Consequently, the rakes 10 reduce the need to provide a large number of fluidisation inlets (either in the sidewall 8, base 33 or conduits) with a smaller number, since the rakes 10 to ensure uniformity of the fluidisation water, especially in the lower part of the rake zone. [0071] In some embodiments, the fluidisation water can be delivered by only the bottom rake closest to the base 33 of the vessel 5, or only some of the rakes 10. It will be appreciated that within the vessel 5 there are variable tendencies for the particles within a rake zone (i.e., the volume in which the rake 10 passes through the fluidised bed 2) to settle relative to the fluid. This sedimentation ensures that a relatively high solids concentration discharges through the bottom of the vessel 5 as an underflow of high density particles, via a discharge outlet 65 and discharge conduit 70 in the form of a pipe. The discharge pipe 70 (and its associated discharge outlet 65) may be located off-centre or offset from the central longitudinal axis of the vessel 5. In some embodiments, the discharge outlet 65 and discharge conduit 70 may be located at an external sidewall of the vessel 5. [0072] The discharge pipe 70 may comprise a control valve 75 for controlling the discharge of the underflow and hence the discharge rate. One or more sensors 80 and 82 may be provided for measuring the density of the suspension. In this embodiment, there are two sensors 80 and 82, located adjacent or at a sidewall of the vessel 5, preferably below the level of the feed slurry inlet 7. The sensors 80 and 82 are spaced apart from each other. In this way, the density of the suspension in the fluidised bed 2 can be monitored. Additional sensors can be used to ensure that the fluidised bed 2 is relatively uniform in density and evenly distributed by placing the additional sensors at regular intervals along the sidewall to obtain a detailed profile of the fluidised bed. The measured suspension density is compared to a set point value. If the set point value is exceeded, the control valve opens and the dense underflow discharges, either via gravity or via a pump (not shown). The sensors 80 and 82 in this embodiment are pressure transducers. Sensors could also be installed at lower levels of the vessel 5 adjacent to or at one or more of the rakes 10. For example, sensors 85 and 86 may be optionally provided in the rake zone, as shown in dotted outline in Figure 1. These may be in addition to or an alternative to the sensors 80 and 82. [0073] This discharge of the underflow results in the solids and some fluidisation water being removed. The balance of the fluidisation water, however, will travel upwards through a fluidisation zone present in the vessel 5. It is expected that a fluidised bed density profile will develop vertically, with the solids concentration decreasing with height. That is, there are more high density particles towards the base 33 of the vessel 5 compared to the top 90 of the vessel, which will tend to have more low density particles. Moreover, by introducing fluidisation water at different levels or heights of the vessel 5, there exists the potential to control the density profile. Ideally, near the base 33, the fluidisation water addition would be much lower than would normally be applied to a conventional reflux classifier. However, due to the cumulative effects of the fluidisation water addition, the total fluidisation rate emerging from the rake zone could be higher than would normally be applied. [0074] In operation, a feed slurry comprising low density and high density particles in a suspension is fed into the vessel 5. The apparatus 1 slowly rotates upon rotation of the shaft 50, causes the rakes 10 to slowly move through the feed slurry. The speed of rotation should not be so high as to cause or induce mixing of the low and high density particles, since this impedes separation of these particles. Preferably, the speed of rotation is in the range of 5 rpm to 10 rpm, optimally 6 rpm. Other lower speeds can be similarly effective, and the apparatus 1 would continue to work beyond a speed of 6 rpm in the range of up to 100 rpm, although performance would be less effective than at the optimal speed range of 5 rpm to 10rpm and optimal speed of 6 rpm. [0075] Fluidisation water is introduced into the feed slurry by the conduits and nozzles in the rakes 10 as they rotate. Alternatively, or additionally, fluidisation water is introduced through fluidisation inlets at or in the sidewall 8 or base 33 of the vessel 5. The fluidisation water flows upwardly relative to the downward flow of heavier particles. The average fluidisation water velocity is generally positive in the upwards direction to remove so-called slimes contaminants. The upward fluidisation water may be assisted by adding extra fluidisation water at higher elevations or levels in the vessel 5, where there is a high underflow rate. The fluidisation water supports the particles in the feed slurry and helps create the fluidised bed 2 in which low density particles are carried upwards to the top 90 of the vessel 5, while high density particles tend to descend towards the base 33. As the rakes 10 continue to slowly rotate, they pass through the fluidised bed 2, causing the frame 30 and its interconnected frame members 35 to cut through the suspension. This breaks up any localised zones of higher density particles that may be developing or have formed within the fluidised bed 2, as well as promoting inversion, since the dense medium displaces larger or coarse-sized low-density particles upwards and out of the fluidised bed. Consequently, the fluidised bed 2 contains mostly relatively fine high-density particles, reducing the need to increase the fluidisation rate. At the same time, the solids distribution within the fluidised bed and hence its rheology is more uniform, ensuring that the fluidisation water is uniformly delivered across the fluidised bed. These effects result in maximising the effectiveness of the particle separator 3 in its separation efficiency, and allows for a wider range of particle sizes to be employed, enabling an increase in the grade of the ore. In the case of iron ore, the final grade might increase from 67% to 69%. It will be appreciated that different circumstances (type of mineral ore, particle size distribution, etc.) will lead to different rates or amounts of increase in the final grade. [0076] The benefit from using a variable fluidisation rate, where the cumulative velocity increases with height is significant. Firstly, the stronger fluidisation rate permits more efficient desliming of the lower density gangue clays and other ultrafines from the zone. Secondly, the lower fluidisation rate near the point of discharge ensures finer high- density particles can join the final discharge. Moreover, the suspension density is maximised internally, greatly improving the effects of inversion, that is the rejection of the large and coarse low-density particles from the fluidised bed 2. These large and low- density particles would of course congregate at a higher level in the particle separator, perhaps even within the upper portion of the rake zone, but they would not form part of the underflow. The net result is the particle separator produces a much higher underflow product density, containing higher density particles, meaning a higher-grade product. [0077] While the invention may be applied to existing and conventional particle separators, such as teetered bed separators, it is preferably applied to a reflux classifier. In this context, it is noted that excess fluidisation can lead to the loss of valuable ultrafine high-density particles to the overflow. Therefore, the invention seeks to deploy the system of parallel inclined channels of a reflux classifier in the upper zone of the vessel 5 to capture those ultrafine high-density particles and return them to the lower fluidisation zone. This capture has the effect of building the concentration of these ultrafine high- density particles within the system, ultimately allowing these particles to transport downwards, as a dense plume, and thus overcome the effects of the high fluidisation rate. [0078] Figure 4 illustrates this embodiment, where a reflux classifier 100 is shown incorporating the apparatus 1 at the base of the vessel 5. A feed slurry inlet 105 is located in the sidewall 8 to feed slurry, preferably spaced away from the inclined channels 120, into a location near or at the top of the region where the rakes 10 are located; i.e. just within the rake zone near or at the top. A fluidisation source 110 delivers fluidisation fluid through the shaft 50 to each of the tubes 60 for discharging into the suspension via their respective nozzles. The reflux classifier 100 has a plurality of inclined channels 120, preferably located towards a second end 130 of the vessel 5. In this embodiment, the second end is the top 130 of the vessel 5, opposite to the base 33. However the inclined channels 120 can be located anywhere from a midpoint of the vessel 5 to the top 130, where an overflow launder 140 receives the overflow of low- density particles. In this embodiment, the inclined channels 120 are formed by a plurality of substantially parallel plates 150. The vessel 5 also has inclined sidewalls 160 parallel to the parallel plates 150 to ensure that the particles flow upward through the inclined channels 120. The apparatus 1 works in substantially same way as described in relation to Figures 1 and 2. In this case, the low density particles are separated from high density particles by the inclined channels 120 and plates 150. [0079] The accumulation of the coarse low-density particles will lead to an increase in their concentration internally; i.e. a local internal zone in the vessel 5 containing these coarse low-density particles. There is also an accumulation of fine high density particles which need to be transported downwards, along with all other particle types. However, once that accumulation zone reaches the position of the feed (i.e. the feed slurry inlet 105), then the large feed volume flow rates will create the necessary velocities to convey these particles upwards to retrieval through the overflow launder 140. [0080] In a further embodiment (as best shown in Figures 1 and 5), the feed slurry inlet 7, 105 is located well above the rake zone. In this way, the density of the fluidised bed emerging out of the rake zone can be measured using the pressure transducers 80, 82 and used to guide the control of the underflow discharge. [0081] In a reflux classifier many of the large low-density particles are transported easily via a shear induced lift force mechanism within the inclined channels 120. Most of these large low-density particles are swept upwards towards the overflow launder 140 in the first pass, but not all. Thus, a portion of these large low-density particles that enter with the feed inevitably fall away towards the base 33 of the vessel 5, and are ultimately rejected by the dense medium of the fluidisation zone in this region of the vessel 5. The fluidisation zone may be formed by conduits in the rake arms 20, a separate fluidisation source (for example, nozzles in the base 33), or a combination of both. The fluidisation water addition ideally occurs near the base 33, but as the rakes 10 that keep the rheology uniform, this is not required and the conduits can supply the fluidisation water to create the fluidisation zone at the base. In a conventional reflux classifier, this rejection may occur but only in a limited manner, whereas using the apparatus 1 generates an improved dense medium fluidised bed that promotes more effective conditions to facilitate rejection of the coarse low-density particles from the fluidised bed 2 more frequently. [0082] Thus, the invention in its application to a reflux classifier achieves significant simplifications and improvements to the design of a reflux classifier. Firstly, it is possible to remove the need for a plenum chamber below the vessel 5, which is a major cost reduction. Secondly, there is no longer the need to install hundreds of nozzles to distribute the fluidisation water. The removal of the need for nozzles reduces capital costs, but also the labour costs of their installation, and the need to change out the nozzles whenever operational requirements change. For example, the reflux classifier may require the use of finer sized nozzles for a particular application; removing hundreds of nozzles so that finer sized nozzles can be used is costly in terms of labour and capital costs. The invention allows the required water to be delivered much more crudely, because it relies on the slow mixing of the rake to ensure uniformity in the rheology of the fluidised bed 2. [0083] In other embodiments, an arrangement may be used that produces a vertical lift of the concentrated slurry suspension (i.e. the higher density particles that have concentrated towards the base 33, ready for removal as the underflow). This vertical lift has the effect of helping to maintain the suspended state of the fluidised bed 2, in the same way that the fluidisation water helps to support the particles. If the concentrated slurry suspension is withdrawn and pumped to a higher position, then it can settle to a lower position. When this process is repeated, the net effect is that the concentrated slurry is suspended. This effect permits the use of less fluidisation water; however, some fluidisation water may be needed to wash ultrafine slime contaminants from the lower zone adjacent the base 33. [0084] It will be appreciated that the fluidisation water can be supplied in other ways. In one embodiment illustrated in Figure 5, a reflux classifier 200 has a recycling conduit or line 210 for recycling at least a portion of the underflow to the vessel 5. The recycling conduit 210 is fluidly connected to a pump 220 for drawing the underflow portion from the discharge pipe 70 and delivering the underflow portion to the vessel. In other embodiments, the underflow may be drawn from the discharge outlet 65. The recycling conduit 210 is also fluidly connected to a secondary fluidisation source (not shown) at location 230 for adding a fluidisation fluid 240 to the underflow portion prior to being delivered to the vessel 5. In this embodiment, there is a branch conduit or line 242 of the recycling conduit 210 fluidly connecting the discharge pipe 70 and the pump 220 and a branch conduit or line 245 fluidly connecting the pump 220 to the vessel 5. [0085] The recycling conduit 210 delivers the underflow portion to the vessel 5 at a location spaced apart from the discharge outlet 65 and/or discharge pipe 70. In this embodiment, the recycling conduit 210 delivers the underflow portion to the vessel 5 at a location 250 near or at the blade assembly 30. The recycling conduit may also deliver the underflow portion to the vessel 5 at a location near or at the base 33 of the vessel. In other embodiments, the recycling conduit 210 may deliver the underflow portion to the vessel at a location 260 spaced apart from the base 33 of the vessel 5, preferably higher up the sidewall, as indicated by the dotted lines 270. [0086] Hence, the concentrated slurry suspension of the underflow is pumped from the base 33 of the vessel 5 or from the vessel wall using the pump 220, fluidisation water is then injected into the recycling line 210 at 230, and the now diluted underflow is returned back to the vessel at a location far from the original location of withdrawal at the discharge outlet 65. The rakes 10 then move this material to in effect distribute the added fluidisation water 240 uniformly. This arrangement provides the potential to return the diluted underflow portion at a more elevated position at 260, but ideally the returned underflow portion sits within the zone of one of the rakes 10 so that the more dilute suspension can be merged with the rest of the suspension. [0087] This alternative embodiment also provides a simple arrangement for controlling the release of the final concentrated product constituting the underflow. Another branch conduit or line 280 is fluidly connected to the branch line 242 between the point of withdrawal (the discharge outlet 65 and/or discharge pipe 70) and the pump 220, forming a T-shaped connection. The control valve 75 is provided at the end of the branch line 280 for product discharge of the underflow. [0088] The reflux classifier 200 in this embodiment does not have inclined sidewalls, unlike the embodiment of Figure 4. Consequently, the reflux classifier 200 has solid sections 290 to ensure that the upward flow is through the inclined channels 120. [0089] In the embodiments of Figures 4 and 5, there is also an option for the reflux classifiers 100, 200 to not remove any underflow over many hours. In this case, the underflow product might be material of much higher density than everything else (for example, free gold), but there might not be much of that material present in the slurry. Consequently, the rest of the material in the slurry accumulates, and is ultimately set to exit the reflux classifier 100, 200 as the overflow. Extra fluidisation in the zone below the feed slurry inlet is then required to keep the desired underflow material at a satisfactory concentration so that the reflux classifier 100, 200 does not block up in any way. [0090] Other embodiments contemplated include using one or more augers or screws to transport particles in an upwards direction against their downwards sedimentation. The augers or screws would replace the parallel inclined plates. A further embodiment may introduce fluidisation water through the surfaces of the auger. [0091] It will further be appreciated that any of the features in the preferred embodiments of the invention can be combined together and are not necessarily applied in isolation from each other. For example, the different configurations for the blade assembly can be combined across different rakes; i.e. one rake 10 may have the frame 30 with interconnected frame members 35 while another rake 10 may have a series of parallel bars or rods extending upwardly from the rake arms 20. In another example, there may be a control valve 75 located at the end of the discharge pipe in the embodiment of Figure 5, similar to the embodiment of Figure 4, in addition to the control valve 75 at the end of branch line 280 to provide an alternative means for removal of the underflow. Similar combinations of two or more features from the above described embodiments or preferred forms of the invention can be readily made by one skilled in the art. [0092] It will be appreciated that the invention uses a slow stirring rake or rakes within the fluidised bed of a particle separator, especially a reflux classifier can be used to achieve a much stronger dense medium condition to help promote better inversion and hence rejection of larger or coarse low-density particles from the fluidised bed. Inversion is a powerful mechanism ideally suited to achieving denser mineral products. The invention thus permits greater control and adjustment in the upwards fluid velocity with elevation. These advantages are made even more effective by integrating the apparatus of the invention into a reflux classifier, where the apparatus assists with the capture of fine dense particles needed for producing a dense medium fluidised bed. [0093] The invention also ensures that the rheology of the material in the bed is uniform, thus allowing a dense medium state to be maintained at lower fluidisation velocities. Moreover, the blade assembly cuts through the material in the fluidised bed to achieve a more uniform suspension. Consequently, the rheological state of the fluidised bed is made uniform across any horizontal layer, thus ensuring the fluidisation water added to the vessel is transported or distributed more uniformly. [0094] Furthermore, the invention is capable of retrofitting to existing particle separators by locating the apparatus 1 towards the base of the separator tank or vessel. In all these respects, the invention represents a practical and commercially significant improvement over the prior art.

Claims

Claims 1. An apparatus for producing a fluidised bed in a particle separator comprising a vessel, wherein the fluidised bed comprises a suspension of high density and low density particles, the apparatus comprising: one or more rakes rotatably mountable within the vessel; wherein: the rakes each comprises a plurality of radially extending rake arms; and at least one rake arm comprises a blade assembly extending from the at least one rake arm for cutting through the suspension in the fluidised bed.

2. The apparatus of claim 1, wherein the blade assembly comprises one or more spaced apart blades extending into the fluidised bed and the blades each comprise a rod, bar, planar element, paddle, perforated plate or plate-like structure.

3. The apparatus of claim 1, wherein the blade assembly comprises a frame having a plurality of interconnected frame members and wherein at least two of the plurality of interconnected frame members are arranged substantially orthogonal or at an acute angle to each other.

4. The apparatus of claim 3, wherein the blade assembly comprises a frame having a plurality of interconnected frame members and the plurality of interconnected frame members form a perforated or mesh-like structure.

5. The apparatus of any one of the preceding claims, wherein the blade assembly is provided on each of the rake arms.

6. The apparatus of any one of the preceding claims, comprising a conduit having a plurality of nozzles for delivering fluidisation fluid into the vessel, wherein the conduit is rotatably mountable to the vessel and comprises a plurality of radially extending conduit arms, at least one of the conduit arms having the plurality of nozzles.

7. The apparatus of claim 6, wherein the at least one conduit arm is connected to the at least one rake arm or the at least one conduit arm is formed within the at least one rake arm and the plurality of nozzles are formed in an outer surface of the at least one rake arm.

8. The apparatus of any one of the preceding claims, wherein there is a plurality of the rakes, wherein the rakes are arranged in a stack or, such that the rakes are configured to move uniformly through the fluidised bed.

9. The apparatus of claim 8, wherein there is at least one rake arranged above the other rake in the vessel with a lowermost rake adjacent the first end of the vessel and having a greater number of rake arms than each of the rakes above it.

10. A particle separator for separating high density particles from low density particles from a feed slurry, comprising: a vessel for receiving the feed slurry; a fluidisation source for delivering a fluidisation fluid into the vessel, thereby creating a fluidised bed comprising a suspension of the high density and low density particles; and the apparatus of any one of the preceding claims mounted within the vessel.

11. The particle separator of claim 10, wherein fluidisation source is fluidly connected to one or more fluidisation inlets for delivering the fluidisation fluid, the one or more fluidisation inlets are arranged at or formed in a sidewall or base of the vessel.

12. The particle separator of claim 10 or 11, comprising a plurality of inclined channels located within the vessel for separating the low density particles from the high density particles.

13. The particle separator of any one of claims 10 to 12, comprising one or more sensors for measuring the density of the suspension.

14. The particle separator of claim 13, wherein there are at least two sensors spaced apart from each other, the at least two sensors being located adjacent or at a sidewall of the vessel.

15. The particle separator of any one of claims 10 to 14, comprising a recycling conduit for recycling at least a portion of an underflow of high density particles to the vessel.

16. The particle separator of claim 15, wherein the recycling conduit is fluidly connected to a discharge outlet for discharging an underflow and fluidly connected to a pump, wherein the pump is configured to draw the underflow portion and deliver the underflow portion to the vessel.

17. The particle separator of claim 15 or 16, wherein the recycling conduit is fluidly connected to a secondary fluidisation source for adding a fluidisation fluid to the underflow portion prior to being delivered to the vessel

18. The particle separator of any one of claims 15 to 17, wherein the recycling conduit delivers the underflow portion to the vessel at a location near or at the blade assembly, a base of the vessel or spaced apart from the base.

19. A method of producing a fluidised bed in a particle separator comprising a vessel, wherein the fluidised bed comprises a suspension, the method comprising: feeding a feed slurry of high density and low density particles into the vessel; delivering a fluidisation fluid into the vessel to create a fluidised bed comprising a suspension of the high density and low density particles; providing one or more rakes rotatably mounted within the vessel, each of the rakes comprising a plurality of radially extending rake arms wherein at least one rake arm comprises a blade assembly; and moving the rake arms so that the blade assembly cuts through the suspension in the fluidised bed.

20. The method of claim 19, comprising providing the blade assembly with one or more spaced apart blades extending into the fluidised bed, wherein the blades each comprise a rod, bar, planar element, paddle, perforated plate or plate-like structure, or comprise a frame having a plurality of interconnected frame members.

21. The method of claim 19 or 20, comprising providing a conduit with a plurality of nozzles for delivering the fluidisation fluid into the vessel and connecting the conduit to the at least one rake arm or forming the conduit within the at least one rake arm, wherein forming the conduit within the at least rake arm comprises forming the plurality of nozzles in an outer surface of the at least one rake arm.

22. The method of claim 19 or 20, comprising delivering the fluidisation fluid from a fluidisation source to one or more fluidisation inlets and arranging the one or more fluidisation inlets at a sidewall or base of the vessel or forming the one or more fluidisation inlets in the sidewall or base.

23. The method of any one of claims 19 to 22, wherein there is a plurality of the rakes, comprising arranging the rakes into a stack in the vessel, such that the rakes move uniformly through the fluidised bed at the same time.

24. The method of claim 23, comprising arranging the plurality of rakes one above the other with a lowermost rake adjacent the first end of the vessel and providing a greater number of rake arms on the lowermost rake than each of the rakes above it.

25. The method of any one of claims 19 to 24, comprising providing a plurality of inclined channels within the vessel for separating the low density particles from the high density particles.