Method and apparatus for intensive froth flotation

Abstract

A method and apparatus for separating selected particles from a mixture of particles in a fluid, the method including the steps of: forming a substantially uniform mixture of liquid, gas bubbles with attached selected particles and non-selected particles in a contacting vessel; conditioning the liquid mixture so as to hydrophobize the selected particles; feeding the mixture from the contacting vessel to a rotating separation vessel where the mixture is subject to centrifugal forces and separates into a layer of froth containing bubbles with attached selected particles overlaying a layer that consists essentially of liquid and particles that have not attached to bubbles; removing the froth layer including the selected particles as the flotation concentrate; and collecting the underlying liquid stream as the tailings.

Description

[0001] METHOD AND APPARATUS FOR INTENSIVE FROTH FLOTATION

[0002] Technical Field

[0003] This invention relates to method and apparatus for the separation of selected particles from a particulate suspension using froth flotation. More particularly, the present invention relates to the intensification of the flotation process, so that the method and apparatus is directed primarily to reducing the volume of separation equipment and increasing the carrying capacity, in comparison with previous technologies.

[0004] Background to the Invention

[0005] Froth flotation as currently practised is a process in which one or more specific particulate constituents of a suspension of finely dispersed particles in a liquid pulp or slurry become attached to air or gas bubbles so that they can be separated from other constituents. The buoyancy of the bubble-particle aggregate is such that it rises to the surface of the flotation vessel forming a froth containing the valuable particles which is separated from the remaining constituents which remain suspended in the liquid phase. Flotation is used in extractive industries, including those aimed at recovering metals such as copper, lead, zinc, nickel, gold, lithium and rare earths, from the host rock. It is also involved in the processing of non-metallurgical substances such as coal, petroleum and petrochemicals, pharmaceuticals. Flotation is also used in the food industry, in wastewater treatment, and in the petroleum and petrochemicals industries.

[0006] The success of the flotation process depends on the control of the chemical conditions in the suspension so that the particles which it is desired to float are rendered hydrophobic or non-wetting. Such particles will attach readily to air bubbles with which they collide in the flotation vessel, while the particles which are not to be floated remain hydrophilic or wetted by the suspension liquid. Chemicals which increase the hydrophobicity of the particles are known as "collectors", and other reagents may also be added, such as "promoters" which tend to improve the performance of the collectors; "depressants" which tend to reduce the hydrophobicity of the "gangue" or material which is not to be floated; and "frothers" which improve the quality of the froth layer formed on the surface of the liquid layer. Other reagents may also be added to adjust the pH and the electrochemical potential Eh. The step of preparing the finely divided particles with the reagents for flotation is referred to as “conditioning”. In some applications, the particles to be recovered may be naturally hydrophobic, and a collector may not be needed. Nevertheless, a conditioning step may be required, to adjust the pH and to depress the gangue.

[0007] In flotation cells in common use, the bubbles are removed from the vessel in the form of a layer of froth or foam on the surface of the suspension liquid. The froth layer overflows into a launder, while the liquid layer is taken from the vessel through an exit pipe in the bottom. The liquid level in the tank is maintained by a suitable control valve acting on the liquid exit stream. The material leaving in the froth, which consists of the hydrophobic particles attached to the bubbles as well as some particulates suspended in the liquid between the bubbles, is known as the "concentrate", while the remaining particulates which leave the vessel in the liquid slurry through the exit port are known as the "tailings". In common use, froth flotation cells are commonly referred to as flotation cells.

[0008] The conditioned feed is introduced to a containing vessel referred to as a cell or column, with means for the introduction of air bubbles. The vessel is sized to provide sufficient residence time in the cell for the collection process to reach the desired recovery of the valuable mineral. The recovery is defined as the fraction of valuable mineral in the feed that reports to the concentrate.

[0009] The global minerals industry is facing increasing difficulty in keeping up with changing circumstances. The problem is well illustrated in the case of copper, where high-grade ore bodies are being exhausted. The grades of new deposits are declining over the years while the demand for the metal is increasing due to the needs from industry. The major proportion of the world’s copper is recovered from ore using the flotation process. Just to maintain current demand in the face of declining head grades, the installed capacity of flotation plants must be increased, requiring the installation of many new flotation cells. Over many years, the size of mechanical flotation cells in current use has increased gradually, and the maximum cell size on the market at present is about 600 cubic metres, in banks of up to nine cells. By extrapolation, volumes above 1000 cubic metres would be expected by 2030. The advantage of increased cell size is a reduction in the number of cells for a given duty, which brings down capital and operating costs. The weight of a large cell with the contained liquid and the pipework and pumps is likely to be at least 1,000 tonnes. The capital cost includes the foundations to hold the flotation cells, and the building required to house the equipment, which increase as the cell volume increases. There is a pressing need to develop new technology to intensify the flotation process, increasing the rate of production per unit area of flotation cells, and reducing the volume of liquid retained in each cell.

[0010] An important practical measure of the efficiency of flotation in a particular application is the “carrying capacity” Ca, which is the mass of froth concentrate per hour relative to the surface area of the froth / liquid interface in the flotation cell. The units of carrying capacity are t / hr-m2. Finch and Tan (On limits to flotation cell size, Minerals, 2023, 13, 411) state that values of Ca in industrial applications range from 0.8 to 1.5 t / hr-m2for copper sulphide flotation, which is one of the most important uses of flotation. They concluded that in current practice a continued increase above the current maximum cell size, approximately 600 cubic metres, does not seem warranted. Cells are usually designed so that they are “square”, i.e., the height is equal to the diameter. In this case, the tank volume required to collect a given mass of concentrate varies as the cube of the diameter, while the area available to process the froth varies as the square of the diameter. As the pulp volume increases, the area to volume ratio of the froth-pulp interface decreases, to the detriment of the carrying capacity. In addition, there is a practical limit on the superficial velocity of the air bubbles passing into the froth, that is currently taken to be approximately 1 cm / s. The froth area is limited by this constraint and is not a free parameter. With these limitations, there is a maximum size of mechanical cells that can usefully be employed, which is approximately 600 cubic metres. For progress to be made, there is a need for a new type of intensive flotation system that can circumvent the limitations of current mechanical flotation cells and columns. Specifically, there is a long -felt need for an intensifying flotation device that will increase the tonnes an hour of solids feed material that can be processed per unit of volume of flotation cell and will lead to a reduction in the floor area occupied by the flotation cell in a concentrator, for a given production rate of solids.

[0011] Summary of the Invention

[0012] In a first aspect the present invention provides a method for separating selected particles from a mixture of particles in a fluid, including the steps of: forming a substantially uniform mixture of liquid, gas bubbles with attached selected particles and non-selected particles in a contacting vessel; conditioning the liquid mixture so as to hydrophobize the selected particles; feeding the mixture from the contacting vessel to a rotating separation vessel where the mixture is subject to centrifugal forces and separates into a layer of froth containing bubbles with attached selected particles overlaying a layer that consists essentially of liquid and particles that have not attached to bubbles; removing the froth layer including the selected particles as the flotation concentrate; and collecting the underlying liquid stream as the tailings.

[0013] The contacting vessel may remain stationary in space.

[0014] The rotational acceleration at the outer edge of the separating vessel may be at least equal to the acceleration due to gravity.

[0015] The method may further including the step of controlling the flow of liquid tailings from the separation vessel to maintain the position of a liquid-froth interface in the separation vessel at a desired level.

[0016] In a second aspect the invention provides an apparatus for separating selected hydrophobic particles from mixture of particles in a fluid, said apparatus including: a contacting vessel arranged to receive a feed under pressure incorporating mixed particles suspended in a liquid, and a supply of gas, the contacting vessel being arranged to mix the gas into the liquid, forming a homogeneous gas-liquid aerated slurry; a rotating separator vessel configured to receive a stream of the aerated slurry from the contacting vessel, and arranged to separate the bubbles therefrom. The contacting vessel may be fixed in space and mounted vertically above the separator vessel, which rotates around an axis of symmetry that is substantially vertical.

[0017] The separator vessel may be mounted on a hollow vertical shaft whose axis forms the axis of rotation of the separator vessel and the aerated slurry flows through the shaft from the contacting vessel to the separation vessel.

[0018] The separator may consist of two connected chambers; a first chamber is adapted to receive aerated slurry from the contactor vessel and direct it radially outward through one or more ducts, a second chamber is located below the first chamber.

[0019] Means may be provided to measure and control the position of the froth-liquid interface relative to the overflow lip.

[0020] The first chamber may include containment walls in the form of a vertical cylinder.

[0021] The froth may leave the first chamber in a radially outward direction and is discharged into a stationary launder.

[0022] The first and second chambers may be separated by a circular disc, that rotates about the axis of symmetry.

[0023] Openings may be provided in the disc to allow liquid to flow from the first chamber into the second chamber under gravity.

[0024] The base of the second chamber may be in the shape of an inverted cone.

[0025] The apparatus may further include exit means provided at the base of the cone to allow the liquid to discharge from the second chamber as the flotation system tailings. The apparatus may further include control means to control the flow of liquid from the exit means so as to maintain the position of the liquid-froth interface in the first chamber at a desired level relative to the overflow lip.

[0026] The control means may include a dart valve, the seat of the valve rotates with the separation vessel, and the dart is fixed in space.

[0027] It is an aim of the present invention to intensify the flotation process, providing a flotation cell that for a given feed flowrate, achieves higher carrying capacities, while minimising variables such as the volume, the footprint, and the height of the cell, in comparison with existing technologies. (The footprint is the cross-sectional area occupied by the cell on the floor of a concentrator.)

[0028] An efficient flotation device according to an embodiment of the invention is required to serve several different functions. It must provide an environment that is conducive to the rapid capture by bubbles of hydrophobic particles. It must also provide means to transfer particle-laden bubbles rapidly to the surface of the pulp. At the surface, the bubbles must be allowed to form an interface between the liquid-rich pulp phase and an overlaying froth phase. The froth is then required sufficient time for entrained liquid to drain back into the pulp, carrying particles of gangue, thereby improving the grade or purity of the concentrate, before flowing over the lip of the vessel into a product launder. In the case of mechanical cells, there is an additional requirement on that the mechanism must provide sufficient circulation of the slurry in the cell, to prevent larger particles from settling in the bottom of the vessel.

[0029] Conventional mechanical cells are relatively inefficient in their use of reactor space. It is known that most of the contacting between particles and bubbles takes place in the volume occupied by the rotating impeller and the stator, which may be less than one tenth of the total volume of the cell. The remainder of the volume is used to retain liquid so that it can be recycled repetitively through the impeller-stator region, and to provide surface area to produce a stable froth. In an efficient contactor, the recirculation volume could be dispensed with, leading to an immediate reduction in the cell volume. However, that would be to ignore the essential requirement to provide proper treatment of the froth phase. Apparatus according to embodiments of the invention divide the overall flotation process into two stages - one in which the collection of particles by bubbles takes place, and another in which the loaded bubbles are removed from the pulp, and the froth layer is managed efficiently. We refer to the device for the first stage as the contactor or reactor, and the second stage as the separator.

[0030] By separating the two functions, it is possible to devise a reactor that could be tailored to suit the special needs of different ore bodies. For example, a feed ore that required very fine grinding to liberate the values from the host rock, would need a high-shear bubble-particle contactor that would be inappropriate for a coarsely-ground feed where high energy density would cause such particles to detach from the bubbles. Similarly, a feed containing substantial quantities of clay or other material that would lead to a viscous slowly draining froth, would require a separator of special design.

[0031] There are two rate-limiting steps that govern the rate at which bubbles carrying hydrophobic particles out of a flotation cell - the rate at which the bubbles can rise through the liquid to the froth-pulp interface; and the rate at which the bubbles can rise and flow within a froth layer. This invention discloses the use of a centrifugal field to increase both rate-limiting steps, thereby intensifying the overall flotation process and leading to higher carrying capacities.

[0032] In the liquid phase, bubbles rise under the action of gravity, from Archimedes’ Principle. The gravitational acceleration, which always exists, may be augmented by a component due to rotation of the liquid around an axis, creating a centrifugal flow field. If a liquid flows continuously into a suitably configured apparatus like a hollow drum spinning around a vertical axis, the liquid will form an annular layer on the inner wall of the drum, and bubbles in the liquid will move in the radial inward direction to the surface of the liquid. Bubbles that may have been formed in the liquid travel in the direction of the vector sum of the gravitational and rotational accelerations. This effect is used in the de-gassing process, where bubbles are removed from a liquid by passing it through a centrifuge. It can also be utilised in a flotation process, to separate the bubbles laden with particles from the tailings stream. However, in mineral flotation, the bubbles that rise to the surface of the liquid form a froth layer with attached hydrophobic particles that are recovered in the concentrate. A centrifugal flow field is used in this invention to increase the flow at which bubbles rise through the pulp phase in the separator.

[0033] A stable froth layer must form on the surface of the liquid in any froth flotation device. The froth consists of bubbles, some of which may carry attached valuable material. The froth may also include bubbles that are barren and contain relatively small amounts of attached particles. As bubbles rise out of the pulp phase, they are accompanied by liquid slurry in the form of thin films between them. The liquid tends to drain back into the cell under the action of gravity, so the films become thinner with time, and eventually they break. When this happens, the froth breaks down and becomes unstable. However, it has been found that by increasing the superficial air velocity Jg, the drainage rate of liquid can be counteracted thereby improving the froth stability.

[0034] The particles to be separated are removed from the pulp on the surfaces of the bubbles as they pass through the pulp-froth interface. A key parameter is the bubble surface area flux, which describes the rate at which the available bubble surface area passes through a plane that is parallel to the pulp-froth interface in the flotation cell. The bubble surface area flux Sb is defined in terms of the gas superficial velocity Jgwhich is the volumetric flowrate of gas Q (m3 / s) passing through the interface whose area is A (m2). Thus Jg= Q / A, and the units of Jgare usually expressed as cm / s for convenience. The bubble surface area flux is then Sb= 6Jg / db, where db is the bubble diameter (m). The units of Sb are 1 / sec. To increase the production rate of solids attached to the surfaces of the bubbles, it is evident that it will be necessary to increase the surface area flux Sb, particularly using J. as a variable.

[0035] It is one of the aims of the present disclosure to be able to provide a system which can generate values of the bubble surface area flux that are significantly higher than can be generated than are possible in existing flotation technologies. In this disclosure, a system is described in which the froth and liquid layers rotate round a common axis, creating a centrifugal field. It is commonplace to view a centrifugal field as a froth destroyer. Surprisingly, it has been found that it is possible to control froth breakage by management of the superficial air velocity normal to the froth / liquid interface. The superficial air velocity is usually denoted by Jgand in conventional cells acting under normal gravity conditions, Jgtakes values between 0.3 and 3 cm / s. We have found that to maintain the same level of froth stability, the superficial air velocity should be increased in proportion to the increase in the effective rotational acceleration. For example, if the effective acceleration in a centrifugal flotation device is 5g where g is the acceleration due to gravity, the superficial air velocity through the froth should be increased five-fold, to values of the order of 1.5 to 15 cm / s.

[0036] In this document, the term “air” is used to describe the non-liquid stream that occurs throughout the system. It is understood that in some circumstances a gas other than air may be used, and “gas” and “air” convey the same meaning. For the avoidance of doubt, a liquid is a phase that is essentially free of bubbles of air; a froth is a mixture of liquid and air bubbles in which the volume of air is very high compared with the volume of liquid; and a fluid means a mixture of air bubbles and liquid in which the liquid phase volume is higher than that of the gas phase. A slurry is a suspension of solid particles in a liquid, which may also be referred to as pulp. When the slurry has been conditioned, it may include selected hydrophobic particles that will attach to bubbles, and gangue particles that do not attach to bubbles.

[0037] Where reference is made to “inner” and “outer” layers of liquid, fluid or froth these terms refer to the position of the layer relative to the distance from the axis of rotation, an outer layer being that which is furthermost from the axis.

[0038] It will be appreciated that in the rotating system described, solid particles in the substrate may separate from the liquid slurry and deposit on the inner walls of the containing chamber. The ducts are configured so that the radial velocity of the aerated slurry at the point of discharge is sufficient to scour the surface of the containing wall and re-suspend any settled solids. The walls of the first chamber are configured to allow the liquid component to flow from the first chamber under gravity into the second chamber beneath. Preferably the velocity of the fluid flowing through the ducts should be sufficient to prevent the deposition of suspended solids on the essentially horizontal lower surfaces within the duct, or to enable resuspension of particles that may have settled there.

[0039] In one form of the invention, adventitious air bubbles in the discharge stream from the first chamber are allowed to coalesce into an air layer beneath the circular disc that separates the first and second chambers. The air layer moves towards the axis of rotation and forms a central core that may extend downwardly to the exit control valve where it is discharged with the tailings pulp. The air core can be disruptive to the vortical flow patterns in the second chamber. A large core may lead to secondary flows at the wall of the inverted cone, which can be helpful in preventing the deposition of solid particles on the wall. The optimum shape of the core can only be determined by experiment. Accordingly, means may be provided to adjust the shape of the air core, by removing the adventitious air layer through a duct provided for this purpose, or adding to the volume of air in the core by admitting air through the same duct. The additional air supplied in the duct is removed together with tailings liquid flowing through the control valve.

[0040] In another form of the invention, the tailings stream is directed to enter the annular gap between two inverted cones. The velocity of the liquid is determined by the width of the gap between the cones at a particular radius, which must increase as the distance from the axis of rotation decreases. The cones must be configured so the velocity parallel to the outer conical surface is sufficient to re-entrain any particles that may have settled on this surface, so they can be removed from the second chamber in the tailings discharge stream.

[0041] Brief Description of the Drawings

[0042] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a schematic cross-section of a flotation device according to the present invention;

[0043] FIG. 2 is a partial schematic cross-section of the separation embodiments of the flotation device;

[0044] FIG. 3 is a schematic cross-sectional elevation of an alternative form of the invention; and

[0045] FIG. 4 is a schematic cross-sectional elevation of a further alternative form of the invention.

[0046] Detailed Description

[0047] FIG.l shows a first embodiment of a flotation system which comprises a stationary contacting vessel in the form of reactor 10 and a rotating separation vessel in the form of separator 40. Feed slurry containing conditioned selected particles enters through the port 11, while air enters through a port 12. In the reactor, the air and the feed slurry are intimately mixed in a turbulent environment that serves to disperse the air into fine bubbles and bring them into contact with the particles in the reactor. The means to bring the air bubbles and the selected particle into contact could be any a high-efficiency contactor such as the plunging jet disclosed in Jameson (U.S. Pat. No 4,938,865, or the Concorde Cell (Jameson, U.S. Pat. No. 2008 / 0308502 Al) or the NovaCell for coarse particle collection (Jameson, U.S. Pat. No. 9,085,000 B2). The active rotor / stator mechanism from a conventional tank flotation machine such as an Outotec tank cell could be used, re-configured to suit the present purpose.

[0048] In the reactor, the selected hydrophobic particles attach to bubbles resulting in an exit stream containing bubbles with attached particles, excess bubbles and a slurry stream that is essentially devoid of selected particles. All the gas, liquid and suspended solids that enter the reactor through ports 11 and 12 leave through the exit duct 13 in a well-mixed continuous stream. Preferably the reactor 10 is substantially located directly above the exit duct 13, so that the bubbly mixture flows vertically downwards into the separator 40. The walls 9 of the reactor are angled from the vertical to prevent the deposition of solid particles. The aerated stream leaving the stationary reactor passes into the separator 40 rotating around a vertical axis of symmetry 21, through a rotary mechanical seal 14, that connects to a hollow shaft 15 that is supported by the bearings 16 and 17. The shaft is caused to rotate by a V-belt drive pulley 18 connected by a belt or belts to a corresponding pulley and motor not shown, or other convenient means. The aerated slurry passes vertically downwards through the hollow shaft 15 to meet a plug 19 that diverts the flow in a radial direction through a series of ducts 20 in the shaft wall that are distributed evenly around the axis of symmetry 21.

[0049] The number of ducts is configured so that the total flowrate through them matches the sum of the feed and air flowrates supplied to the separator through the conduit 13. Preferably the velocity of the liquid flowing through the ducts should be sufficient to re-entrain any particles that may have settled out of the streams.

[0050] The retaining wall of the separator 40 is preferably in the form of an open vertical cylinder 41 sitting above and connected to a conical member 42. The top of the rotating cylinder is partially closed by the annular horizontal circular disc 43. A horizontal circular disc 44 is attached to the base of the retaining wall 41. The disc is mounted centrally on the rotating hollow shaft 15.

[0051] The two essentially horizontal discs 43 and 44, with the cylindrical vertical wall 41, act as a retaining enclosure to receive the aerated slurry that enters through the ducts 20.

[0052] In the distribution ducts 20, the aerated slurry moves radially outwards and discharges into a pool of liquid 53, that is held against the inner wall of the cylinder 41 by centrifugal force. The bubbles in the aerated feed are less dense than the surrounding slurry so they travel inwards to form a froth layer 54. The selected particles that have become attached to bubbles in the reactor 10 move with the bubbles into the froth layer. The slope of the interface 55 between the froth layer and the aerated slurry is determined by the vector sum of the vertical acceleration due to gravity, and the radially outward rotational acceleration. The froth layer is refreshed by new bubbles provided by a continuous flow of air and new liquid feed, generating a steady stream of froth with attached particles which flows over the inner lip 56 of the annular disc 52. After leaving the lip, the bubbles and entrained liquid move radially outward under the influence of centrifugal forces to impinge on the stationary wall of the launder 57. The launder has the shape of a stationary annular C-shaped open channel that is rotated about the axis of symmetry 21, to form a hollow toroidal ring of constant cross-section. In the stationary circular launder 57 the bubbles collapse to form the flotation product or concentrate, which discharges from the device through appropriately placed exit ports 58. The top of the stationary launder 57 contains a circular opening 59 that is open to the atmosphere. Through this opening it is possible to provide an apparatus (not shown) to apply clean wash water to the surface of the froth layer 54, to reduce the entrainment of gangue material into the froth product.

[0053] The aerated pulp that is discharged from the distribution ducts 20 forms a pool 53, where it remains for sufficient time to allow the bubbles within it to rise into the froth. The pulp, which constitutes the flotation tailings and is essentially free of bubbles, passes through connecting ports 70 into a detention pool 71. (Further detail on the location of the exit ports relative to the distribution ducts 20 is shown in Fig. 2 .) The detention pool is enclosed in a chamber whose base is of the form of an inverted cone. The sloping walls of the cone direct the liquid with any settled solids to an exit port 80.

[0054] The flowrate of the stream leaving the hollow shaft 15 is controlled by a valve 81 which may be conveniently of the type known as a dart valve, or other suitable means. The control valve can be used to control the level of the froth-slurry interface 55 in the rotating separator. It may be actuated by a signal from pressure sensor situated on the inner wall of the cylindrical vessel 41, or a property of the discharge tailings such as the mass recovery, or the grade of the concentrate or the tailings particles or other convenient measurement. A person skilled in the art would recognise that there are other means of influencing the position of the froth-liquid interface such as by controlling the flowrate of air delivered to the separator. A combination of these means may also be used. It has been found in practice that the liquid that passes through the connecting ports 70 contains a small fraction of air from entrained bubbles. These bubbles disengage in the detention pool and rise to the underside of the disc 44, forming a gas pool 82 distributed about the axis of rotation 21. The pressure inside the gas pool is constant, so the pressure at the gas-liquid interface is also constant. It is seen that the pool adopts the shape of a whirlpool or bathtub vortex (not shown), in which the velocity of an element of liquid increases as it approaches the axis 21. This is a useful phenomenon because the liquid creates a scouring action on the sloping walls of the inverted conical wall 42 of the second chamber. Thus, solid particles that may have deposited on the walls are immediately entrained into the main flow. Accordingly, it has been found beneficial to maintain the gas pool 82 independently of the uncontrolled flow of adventitious bubbles that may have entered the lower chamber in the liquid flowing through the connecting ports 70. For this purpose, a duct 83 is provided that is open to the atmosphere. The shape of the whirlpool adjusts itself in response to the pressure in the gas pool. If necessary, a control valve 84 can be incorporated to control the flow of gas in or out of the gas pool. Whirlpools (known as air cores) are observed in hydrocyclones, a known technology for separating particles from liquids using centrifugal force. In the continuous rotational system observed in the current invention, the lower tip of the whirlpool anchors itself in the vicinity of the exit control valve 80, so a continuous flow of air is discharged with the tailings.

[0055] FIG. 2 is a partial plan view of the rotor and connecting ports 70, along the line A-A in FIG. 1. The aerated feed liquid flows downwardly through the hollow shaft 15 and is diverted in a radial direction through each of the distributors 20 that may be conveniently in the form of cylindrical tubes or rectangular ducts, connected through a boss 22 mounted on the hollow shaft 15.

[0056] The number of distribution ducts 20 is determined by the feed volumetric flowrate, which may vary according to operational requirements. The number of distribution ducts at a particular height above the lower circular disc 44 can be increased in even numbers, for balancing purposes. If required, a stack of distributor arms 20 with bosses 22 can be configured, one above the other, to increase the surface area of the froth and the interfacial area of the pool 53 as it rotates about the axis 21. The ducts discharge into the liquid pool 53 which is held against the wall 41 of the separator by centrifugal force, and the bubbles in the feed, being less dense than the liquid, move radially inward towards the surface 55 of the liquid and into the froth layer 54. The bubble-free liquid discharges through the connecting ports 70 into the detention pool 71. Each connecting port 70 is located to be substantially as distant as possible from the distributors 20, to maximise the opportunity for bubbles to disengage from the liquid.

[0057] FIG. 3 shows an alternative embodiment of the invention. This embodiment is different from that shown in FIG. 1, by the way in which the product from the reactor 10 is transferred into the rotating separator 40. Mechanical seals as shown at 14 in FIG.

[0058] 1 are a known technology that are reliable and fit for purpose. However, in some circumstances an alternative lower cost solution can be considered. In this embodiment,, the major elements of FIGS. 1 and 2 are retained but the rotary seal 14 is replaced with a rotating enclosure 60, which is symmetrically arranged about the hollow shaft 15, and rotates with it about the axis of symmetry 21. The reactor 10 is retained and fixed in space with mountings not shown. The top of the enclosure 60 is open to the atmosphere. The liquid level 61 above the entrance to the hollow shaft 15, is controlled by means not shown. Provision must be made to ensure that the hydrostatic pressure of the fluid in this space is sufficient always to drive the combined flowrates of air and feed downwards through the shaft and into the separator 40.

[0059] A further embodiment is shown in FIG. 4, which depicts an alternative system for collecting the tailings that derive from the pool of liquid 53 after passing through the connecting port 70. This embodiment is particularly useful if the feed contains large particles of a dense mineral such as gold, electrum, native copper, cassiterite etc., which may move to the walls of the detention chamber under the action of the centrifugal force generated by the rotational motion. To prevent a buildup of solids on the chamber walls, in this embodiment the tailings stream enters the gap between two conical surfaces 42 and 45 preferably configured so that the velocity of the tailings stream in the flow direction is maintained at least approximately constant as the radial distance from the axis diminishes. The velocity in the gap must be sufficient to re-suspend any dense particles that may have deposited on the inner wall of the conical surface 42.

[0060] Any reference to prior art contained herein is not to be taken as an admission that the information is common general knowledge, unless otherwise indicated.

[0061] Finally, it is to be appreciated that various alterations or additions may be made to the parts previously described without departing from the spirit or ambit of the present invention.

Claims

The invention that may be claimed may include the following novel arrangements, either singularly or in any combination thereof:

1. A method for separating selected particles from a mixture of particles in a fluid, including the steps of:forming a substantially uniform mixture of liquid, gas bubbles with attached selected particles and non-selected particles in a contacting vessel; conditioning the liquid mixture so as to hydrophobize the selected particles; feeding the mixture from the contacting vessel to a rotating separation vessel where the mixture is subject to centrifugal forces and separates into a layer of froth containing bubbles with attached selected particles overlaying a layer that consists essentially of liquid and particles that have not attached to bubbles; removing the froth layer including the selected particles as the flotation concentrate; andcollecting the underlying liquid stream as the tailings.

2. A method according to claim 1 wherein the contacting vessel remains stationary in space.

3. A method according to any preceding claim wherein the rotational acceleration at the outer edge of the separating vessel is at least equal to the acceleration due to gravity.

4. A method according to any preceding claim further including the step of controlling the flow of liquid tailings from the separation vessel to maintain the position of a liquid-froth interface in the separation vessel at a desired level.

5. An apparatus for separating selected hydrophobic particles from mixture of particles in a fluid, said apparatus including:a contacting vessel arranged to receive a feed under pressure incorporating mixed particles suspended in a liquid, and a supply of gas, the contacting vessel being arranged to mix the gas into the liquid, forming a homogeneous gas-liquid aerated slurry;a rotating separator vessel configured to receive a stream of the aerated slurry from the contacting vessel, and arranged to separate the bubbles therefrom.

6. An apparatus according to claim 5 wherein the contacting vessel is fixed in space and mounted vertically above the separator vessel, which rotates around an axis of symmetry that is substantially vertical.

7. An apparatus according to either of claims 5 or 6 wherein the separator vessel is mounted on a hollow vertical shaft whose axis forms the axis of rotation of the separator vessel and the aerated slurry flows through the shaft from the contacting vessel to the separation vessel.

8. An apparatus according to any one of claims 5 to 7 wherein the separator consists of two connected chambers; a first chamber is adapted to receive aerated slurry from the contactor vessel and direct it radially outward through one or more ducts, a second chamber is located below the first chamber.

9. An apparatus according to any one of the claims 4 to 8 wherein means are provided to measure and control the position of the froth-liquid interface relative to the overflow lip.

10. An apparatus according to claim 8 wherein the first chamber includes containment walls in the form of a vertical cylinder.

11. An apparatus according to any one of claims 8 to 10 wherein the froth leaves the first chamber in a radially outward direction and is discharged into a stationary launder.

12. An apparatus according to any one of claims 8 to 11 wherein the first and second chambers are separated by a circular disc, that rotates about the axis of symmetry.

13. An apparatus according to claim 12 wherein openings are provided in the disc to allow liquid to flow from the first chamber into the second chamber under gravity.

14. An apparatus according to any one of claims 8 to 13 wherein the base of the second chamber is in the shape of an inverted cone.

15. An apparatus according to claim 14 further including exit means provided at the base of the cone to allow the liquid to discharge from the second chamber as the flotation system tailings.

16. An apparatus according to any one of claims 8 to 15 further including control means to control the flow of liquid from the exit means so as to maintain the position of the liquid-froth interface in the first chamber at a desired level relative to the overflow lip.

17. An apparatus according to claim 16 wherein the control means includes a dart valve, the seat of the valve rotates with the separation vessel, and the dart is fixed in space.

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