Patent Application
20080308502
This invention relates to the froth flotation process for the recovery or separation of particles from suspensions in liquids in general, and more particularly to an efficient contacting apparatus and process for use in flotation separation systems.
The flotation process is used in the separation of particles from mixtures in a finely divided state, suspended in a liquid. For example, in the minerals industry, a suspension of solid particles in water is treated with chemical reagents or collectors which have the effect of making the particles which it is desired to remove, water repellent or hydrophobic, while leaving the remaining particles in a wetted or hydrophilic state. The liquid is fed into a flotation separation cell, which may be in the form of a tank or column, and air is injected in the form of fine bubbles. The hydrophobic particles attach to the air bubbles and rise to the surface of the cell, from which they can be removed by flowing over a lip under the action of gravity, into a launder or channel. The particles which are not collected by the bubbles remain in the suspension and flow out of the bottom of the cell, in the tailings. Frother reagents are often added to the feed liquid in order to assist in the formation of a stable froth on top of the liquid in the cell. Clean water may be applied to the froth layer in order to wash entrained particles downwards into the cell.
Flotation is also used generally for the recovery of fine particles from suspensions in liquids, as in the removal of printing ink from recycled paper; for the removal of particles especially fat and oil droplets from waste waters in the food industry; for removal of particulates in processes for the remediation of contaminated sites; for the treatment of produced water emanating from oil fields; and for the recovery of algae and other organisms from suspensions in fresh water or sea water. For purposes of description, the term ‘air’ may be used to represent the gas, ‘water’ may be used to represent the liquid and the floatable component may be referred to as ‘particles’ or in some cases as the ‘values’. The non-floating component is referred to as ‘gangue’. It is to be understood however that the same principles apply in other systems involving fine particles that are not minerals, dispersed in aqueous or non-aqueous media, being floated with gases other than air.
In earlier technology, flotation has been carried out in mechanical cells in which the liquid is agitated by a rotating impeller and air is introduced in the vicinity of the impeller. The bubble sizes produced in these devices are not necessarily small, being typically in the range 1 to 5 mm in diameter. More recently, flotation has come to be carried out in columns, which have operational advantages in being able to provide better control of the phenomena in the froth. Flotation columns in current use, vary in the aspect ratio. Some are tall relative to their diameter or breadth, with a height-to-diameter ratio of at least 2:1 and up to 10:1 or greater. In these devices the feed slurry is typically injected towards the top of the column, and a stream of bubbles is created by a suitable means such as a sparger, injector, aspirator, nozzle or bubble generator. The objective of these aeration devices is to distribute the bubbles essentially uniformly across the cross-section of the column. Thus as the stream of particle-laden liquid descends down the column, it meets a distributed cloud of small bubbles rising vertically. The individual bubbles collide with and capture the hydrophobic values, and carry them upwards into the froth.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
In both mechanical cells and columns, the contact between bubbles and particles usually takes place in the liquid in the vessel itself. Thus the reason for the height of tall column cells, is to provide sufficient time for the bubbles to come into contact with particles as they rise in the column. Flotation column cells as described particularly by Finch and Dobby (Column Flotation, Pergamon Press, Oxford, England, 1990), consist of three zones: the froth zone at the very top of the column, typically 1 m in height; the collection zone, where bubble-particle contact occurs, typically 5 to 10 m in height; and the disengagement zone in the base of the column, where the liquid flows out of the column, typically 1 to 2 m in height. Thus the overall height of a column cell is in the range 7 to 13 m. The froth zone must be of sufficient height to allow the gangue particles to drain, and clean wash water is often distributed over the top of the froth or within the froth, to wash the gangue back into the liquid in the flotation cell. The disengagement zone is a quiescent location, where the downward velocity of the liquid is less than the rise velocity of the bubbles which have been introduced higher in the cell, so that the bubbles are able to escape from the exit stream from the column.
Internal bubble generators are known for flotation columns. Some consist of simple distributor pipes with small holes in the walls, or with porous walls. In others, such as the generator of Harach, U.S. Pat. No. 4,911,826, an array of fine nozzles is supported by distributor pipes across the whole cross-section of a tall column. Air and water streams are supplied through headers, and a mixture of air and water is discharged through each fine nozzle. In yet others, air under pressure is supplied to tubes made of an elastic material like rubber. The surface of the elastic tubes is pierced with an array of very fine holes which remain closed when the external pressure is greater than the pressure within the tube. As the internal pressure is increased, the elastic wall stretches and the fine holes enlarge sufficiently to allow the passage of air, which is discharged from the holes in the form of fine air bubbles.
External bubble generators are also known in the tall flotation column cells. Hollingsworth, U.S. Pat. No. 3,371,779, describes a venturi-type aspirator to produce air bubbles into a stream of fresh water which is then introduced into the bottom of a flotation column. Christopherson, U.S. Pat. No. 4,617,113, described how a multitude of venturi aerators can be distributed around a large column. Air is inspired into water flowing through the venturis. In the apparatus of McKay and Foot, U.S. Pat. No. 4,752,383, air and water are pre-mixed at high pressures in a chamber containing beads. The aerated water is then injected into the base of a flotation column through a lance, which has a small orifice at the end. Bacon, U.S. Pat. No. 4,472,271, produced bubbles in slurry taken from the bottom of the flotation cell. The bubbles were made by passing air and slurry through a nozzle. The bubble-laden slurry stream was reintroduced through the wall of the flotation column. Yoon, U.S. Pat. No. 5,397,001, has described a flotation column in which the air is dispersed into slurry in external static mixers. Slurry is taken out of the bottom of the flotation cell and distributed equally among a number of static bubble generators where air is added. The aerated slurry stream is then injected into the flotation column above the external aerators. In the aforementioned devices, the external devices are essentially bubble generators and contact takes place within the column.
Short columns are known, in which the height and diameter are of the same order of magnitude, and the height-diameter ratio in industrial applications may be from 0.2 to 1, to 2 to 1. In these short columns, air is introduced into the feed liquid in an aeration system prior to injection into the column, and it is in this aeration system that contact between bubbles and particles is established. Relatively little contact is effected in the column proper. The aeration system may take the form of a plunging jet, a venturi, a static mixer, or a sparger or porous-walled pipe through which air is introduced in a turbulent fashion into the feed slurry. Examples of such devices are described by Jameson, U.S. Pat. No. 4,938,865; and U.S. Pat. No. 5,332,100; Bahr, Ger. Pat. No. 2,420,482; Imhof, Europ. Pat. No. 1,084,753, and Ludke, U.S. Pat. No. 4,448,681. Because of the high-efficiency contacting in the aeration device, the functions required in the flotation column or tank are much reduced. Thus in principle, there is no need for the collection zone as found in tall column cells, because bubbles and particles have already contacted each other. However, the froth and disengagement zones are required. For present purposes, short flotation column cells of the types described by Jameson and Bahr will be referred to as “intensive” cells. Because there is no need for the collection zone, the intensive cells have significant advantages over the tall column cells, emanating from the much reduced size.
All of the aforementioned inventions describe processes to disperse air bubbles into a liquid which may or not contain suspended particles. However, none of these bubble-generating devices place any form of flow restriction that can be used to control or influence the pressure in the air-liquid mixture after formation. It can be advantageous to control the pressure at which the bubbles are formed, both in absolute terms and also in terms relative to the pressure at which they are to be used in the flotation vessel. For example, when bubbles are generated by the breakup of a supply of air in a shear flow such as exists in the throat of a venturi, or in a static mixer, the size of the resulting bubbles is a function of the local void fraction, which is the ratio of the volume of gas under local pressure conditions, to the total volume of gas and liquid. It is generally desirable to minimize coalescence of bubbles after formation, because it is well known that the rate of capture of particles by bubbles diminishes as the bubble size increases, for a constant air/liquid ratio. Bubble swarms that are created in a gas-liquid mixture of low void fraction, are generally more stable, because the rate of coalescence of bubbles is related to the mean distance between the bubbles, which in turn is related to the void fraction. For the same mass ratio of gas to liquid, the volume ratio varies inversely as the absolute pressure. Thus if it is desired to supply a feed liquid with an equal volume of air at the absolute pressure in the flotation cell, it will be advantageous to create the bubbles at a higher pressure than exists in the cell. For example, if the absolute pressure at which bubbles are generated is twice the absolute pressure in the cell, the volume fraction will be one half that in the cell.
This effect was recognised by Amelunxen (CA Patent Specification 2106925), who described an external contactor, a throttle valve for controlling the process pressure within the contactor and a system for injecting air and liquid into the contactor under pressure.
All of the prior art contactors suffer from disadvantages, which can variously relate to: limitations in the amount of air that can be supplied relative to the amount of liquid flowing through the sparger or aeration device; the necessity for small orifices or tubes which readily corrode or become blocked by the particles present in the feed; the necessity for complex and expensive manufacturing processes to provide parts that can withstand the wear associated by high velocity flows; the difficulty of replacing crucial wearing parts in an operating plant; the need for relatively high concentrations of frother or other expensive surface active agent in order to produce small bubbles; high operating costs associated with excessive driving pressures in the liquid and/or the air streams.
There is a range of particle sizes in the feed suspension for which current flotation technologies are efficient. Thus in an intermediate particle size range, between 40 and 150 microns for minerals (and 75 and 350 microns for coal), conventional flotation cells can achieve high recoveries. However, when the size of the particles is less than or greater than the intermediate range, the flotation recovery tends to decrease as the particles become smaller (or larger). For present purposes, “fine” particles are those whose diameter is smaller than the appropriate intermediate size range, i.e. those between 0 and 40 microns for minerals, and 0 and 75 microns for coal; “ultrafine” particles are those at the lower end of the “fine” range; and “coarse” particles are those whose diameter is greater than 150 microns for minerals, and 350 microns for coal.
The inventor of the present invention has found that improved flotation of fine particles can be achieved by reducing the bubble size, increasing the gas supply rate relative to the flow rate of particles, and increasing the shear intensity or energy dissipation rate in or adjacent the contacting device. The rate of recovery is related to the rate at which the particles collide with the bubbles. Since the inertia of the particles varies inversely as the cube of the diameter, as the particles become smaller, so finer particles tend to follow the fluid streamlines around the bubbles and the probability of attachment is reduced as the size decreases. The recovery of fine particles can be improved by using smaller bubbles and by increasing the rate of shear in the contacting system (N Ahmed and G J Jameson, “The effect of bubble size on the rate of flotation of fine particles”, Int. J. Mineral Processing, 14, (1985), 195-215.). A substantial improvement in the performance of a typical flotation machine can be expected if the bubble size is reduced. Accordingly, it has been recognised by the inventor that for high-efficiency flotation a source of fine bubbles, typically in the range 400 microns in diameter or smaller, be provided, in a high-energy dissipation rate environment.
For coarse particles, the reduction in recovery as the particle size increases is due to the inability of bubbles and hydrophobic particles to stay in contact with each other in a highly-turbulent environment. The bubbles tend to move to the centre of vortices or eddies in the flotation cell and the particles are flung away from the bubbles by centrifugal forces. High recoveries of coarse particles are favoured by a high gas fraction in the slurry suspension, by low levels of turbulence in the region below the froth layer. It is also favourable to provide a means to levitate the coarse particles so that their upwards passage towards the froth is assisted by an upwards motion of liquid in the region beneath the froth.
It is the purpose of the present invention to provide simple, efficient and economic means to overcome the difficulties and inefficiencies in known flotation technologies, by generating fine bubbles and bringing them into contact with the particles to be floated, and controlling the resulting gas-solid-liquid mixture so as to maximise the transfer of hydrophobic particles into the froth and hence into the flotation product.
In one aspect, the present invention provides an apparatus for contacting bubbles and particles in a flotation separation system, said apparatus including;
a contactor arranged to receive under pressure a supply of feed slurry incorporating particles suspended in a liquid and a supply of gas, the contactor being arranged to mix the slurry with the air forming a gas-liquid bubbly two-phase mixture;
an outlet from the contactor configured to provide a restriction to the flow of mixture therethrough and maintain the mixture within the contactor under pressure;
a flow manipulator downstream from the outlet configured to induce a high energy dissipation rate within the mixture passing therethrough; and
a separation cell arranged to receive mixture from the flow manipulator and allow bubbles with attached particles to rise to the surface of liquid within the cell.
Preferably the separation cell is provided with a mixture directing device arranged to receive the mixture from the flow manipulator and control the release of that mixture into the cell.
Preferably the contactor includes a substantially vertical column arranged to receive the feed slurry under pressure into the top of the column.
Preferably the contactor incorporates mixing means including a nozzle arranged to form a downwardly plunging jet of feed slurry within the column, and a gas inlet in the vicinity of the jet so formed such that in use gas is entrained into the jet forming said gas-liquid bubbly two-phase mixture.
Preferably the outlet from the contactor is configured to form at least one throttling duct providing said restriction to the flow of mixture therethrough.
Preferably the throttling duct has a converging section leading to a throat sized to provide said restriction.
In one form of the invention the flow manipulator includes a diverging section immediately downstream of the throttling duct, configured to induce a shock wave in the mixture passing through the diverging section in use and provide said high energy dissipation rate.
In another form of the invention the throttling duct is arranged to open abruptly into a conduit extending within the separation cell, said conduit having one or more openings in the separation cell adjacent the throttling duct through which liquid is entrained in use from the separation cell into the conduit.
Preferably the throttling duct and conduit are configured such that under desired operating conditions a shock wave is formed downstream of the throttling duct providing said high energy dissipation rate in the vicinity of the openings in the conduit.
In one configuration of the apparatus according to the invention the column is located with its lower end within the separation cell, and wherein a plurality of said throttling ducts are provided orientated radially outwardly adjacent the lower end of the column.
In another configuration the column is located with its lower end within the separation cell, and wherein the throttling duct is orientated substantially downwardly at the lower end of the column and provided with an impingement plate positioned substantially horizontally below the throttling duct, spaced therefrom so as to provide said flow manipulator inducing said high energy dissipation rate within the mixture passing therethrough.
Preferably the impingement plate comprises a lower circular disc aligned with and spaced from an upper circular disc having a central hole therethrough arranged to receive mixture issuing from the throttling duct, such that in combination with the diameter of the discs and the operating pressure and velocity within the throttling duct, sonic flow conditions exist in use in or downstream of the throat in the throttling duct.
In one form of the invention the lower disc is spaced a fixed distance from the upper disc, said distance being determined to provide said sonic flow conditions.
In another form the lower disc is free to move in a vertical direction relative to the upper disc, allowing the lower disc to come to a stable equilibrium in use, forming said sonic flow conditions.
In another form at least one of the upper and lower discs is flexible and able to adapt to a shape dictated by pressure developed in the flow between the discs in use.
Preferably the lower plate is flexible and wherein the lower plate is provided with a central solid wear resistant zone located a fixed distance below the outlet from the throttling duct.
Preferably the mixture directing device comprises a draft tube in the form of a substantially vertical shroud located within the separation cell and arranged to direct the flow of mixture from the flow manipulator into the separation cell.
In one form the shroud is open at both the upper and lower ends and positioned to induce flow of liquid therethrough in a generally upward direction in use such that liquid within the lower part of the separation cell is induced to flow upwardly through the shroud, joining the mixture issuing into the shroud from the flow manipulator.
Preferably the lower end of the shroud is restricted in size to control the flow rate of liquid passing into the shroud from the separation cell.
In one form the shroud is substantially constant in cross-section over the majority of its length.
In another form the shroud is tapered outwardly and upwardly having a greater opening at the upper end than the lower end.
In yet another embodiment of the invention the shroud has a closed lower end.
Preferably the impingement plate is located at the closed lower end of the shroud.
Preferably the relationship between the throttling duct, the impingement plate and the shroud is such as to form said flow manipulator causing a rapidly rotating toroidal vortex within the lower end of the shroud and inducing said high energy dissipation rate within the mixture.
Preferably the relationship between the throttling duct, the impingement plate and the shroud is such as to form an expanded fluidized bed within the shroud when the apparatus is operated at desired parameters.
In a further aspect, the present invention provides a method of contacting bubbles and particles in a flotation separation system, said method including the steps of:
providing apparatus including: a contactor arranged to receive under pressure a supply of feed slurry incorporating particles suspended in a liquid and a supply of gas, mixing means within the contactor arranged to mix the slurry with the air forming a gas-liquid bubbly two-phase mixture, an outlet from the contactor configured to provide a restriction to the flow of mixture therethrough and maintain the mixture within the contactor under pressure, a flow manipulator downstream from the outlet configured to induce a high energy dissipation rate within the mixture passing therethrough, and a separation cell arranged to receive mixture from the flow manipulator and allow bubbles with attached particles to rise to the surface of liquid within the cell;
and feeding slurry and gas into the contactor at feed rates and pressures determined to form said gas-liquid bubbly two-phase mixture and force the mixture through said flow manipulator at a rate that induces said high energy dissipation rate within the mixture reducing the size of the bubbles within the mixture and bringing those bubbles into intimate contact with particles in the mixture.
Preferably the method includes the step of feeding the mixture from the flow manipulator into a mixture directing device within the separation cell.
Preferably the mixture is fed into the mixture directing device in a manner that, in combination with the shape of the mixture directing device, reduces turbulence within the mixture.
In one form the mixture directing device is a draft tube in the form of a substantially vertical shroud arranged to direct the flow of mixture upwardly into the separation cell.
Preferably the slurry is conditioned with collectors and frother reagents prior to being fed into the contactor.
Preferably the collectors and frother reagents are selected to render the particles hydrophobic and able to form strong bonds with the bubbles.
In one use of the method the particles comprise minerals and the flotation separation system is operated to separate the minerals from gangue or other contaminants. A typical example is the separation of coal particles from gangue.
In an alternative use of the method the feed slurry contains particles of an organic nature and the flotation separation system is operated to remove those particles from the liquid.
In yet another use the particles are metal particles such as aluminium particles.
a) is an enlarged side view of the flow restriction shown in
b) is an enlarged plan view in the plane A-A in
a) is an enlarged side view of an alternative restriction at the exit from the gas-liquid contactor and directing the discharge from the restriction in the radial direction;
b) is an enlarged plan view of the restriction and radial flow device shown in
A first preferred embodiment of an intensive flotation column flotation cell according to the invention is shown in
Because the density of the gas-liquid mixture leaving the restrictive throat 18 is less than that of the contents of the column 21, which is essentially that of gas-free liquid, an upwards convective flow is established through the draft tube 20. Liquid from the column is drawn into the base of the draft tube and is brought into contact with bubbles that have been generated in the plunging jet contactor 16 and the choked flow device 18 in combination. Thus a proportion of the particles that may not have made contact with bubbles when first entering the vessel through the contacting system, or which may have detached from the froth layer 23 and fallen back into the liquid in the flotation vessel 21, will have an additional opportunity to become attached to bubbles and be carried by them into the froth layer. It has been found that if the draft tube 20 is open-ended at its upper and lower extremities, the ratio of the flowrate of recirculating liquid to that of the incoming feed liquid, which is termed the internal recycle ratio, is quite large, of order 4 to 6. Such flowrates give rise to highly energetic flows within the cell 21, and a buoyant plume rises from the upper open end of the draft tube 21 whose velocity is so high that it can be disruptive to the froth layer and lead to an increase in drop-back of particles from the froth. Accordingly it has been found to be advantageous to incorporate an entry tube 29, which restricts the internal recycle ratio to a value preferably between 2 and 3. The height/diameter ratio of the draft tube 20 and the inlet pipe 29 are each preferably in the range 2 to 5. The centreline of the horizontal conduit 19 should intersect with the axis of the draft tube 20 at a height approximately equal to 1.5 times the diameter of the conduit 19 above the lowest extremity of the said draft tube.
In this embodiment preferably the plunging jet contactor is mounted so that the jet is directed vertically downwards. The cross-sectional area of the plunging jet contactor 16 in a plane normal to the axis should be such that the downward superficial velocity of the liquid is above the terminal velocity of the largest bubbles that are likely to form in the contactor, and it has been found that an appropriate velocity is in the range 0.3 to 1 m/s. It is convenient to make the cross-sectional area of the inlet and outlet of the converging-diverging throttle 18 and the transfer conduit 19, to be the same as that of the contactor 16. The cross-sectional area of the draft tube 20 should be not less than that of the contactor 16, and should preferably in the range 2 to 4 times said area. The area of the entry pipe 29 should be in the range 0.1 to 0.5 of the cross-sectional area of the draft tube 20.
The area of the throat is chosen with advantage so that the gas-liquid mixture formed in the contactor 16 attains the speed of sound there. If the sonic velocity is exceeded, a shockwave forms downstream of the throat, which has an effect on the size of the bubbles in the flow.
The way in which small bubbles are produced in the apparatus described can be explained with reference to the changes in the pressure in the two-phase mixture. In the entry region 31 the pressure is constant in the gas and liquid phases, and is denoted the “upstream pressure.” When the mixture accelerates in the converging region 32, the pressure reduces according to well-known laws of fluid flow, so the bubbles in the mixture become larger. In the throat 33, at a critical value of the upstream pressure, the gas-liquid mixture reaches the speed of sound in the mixture. If the upstream pressure is sufficiently large, the fluid continues to accelerate downstream of the throat 33, and the pressure continues to fall, so that the bubbles continue to increase in volume. At a certain point in the diverging region, a shock wave 35 occurs, across which there is a catastrophic change in the flow, and the pressure rises from a small value ahead of the shock to a large value downstream. Because of the rapid pressure change, the large bubbles ahead of the shock break up in a violent fashion, to form very small bubbles, typically less than half the size of the bubbles in the flow in the entrance duct 31. It has been found that the thickness of the shock wave in the flow direction is relatively small, being in the range 3 to 5 mm typically. It will be appreciated that a purpose of this invention to bring about contact between hydrophobic particles and small bubbles. The chaotic motions that occur within the shock wave have the effect not only of breaking up the bubbles, but also of freshly creating a very large interfacial gas-liquid area in a high-energy, intensively-mixed zone within the shock wave and downstream of it. The combination of very small bubbles and high-energy mixing has the effect of bringing about instant contact between the bubbles and the hydrophobic particles.
The cross-sectional area to achieve a sonic velocity in the throat shown in
where P0 is the pressure in the conduit 31 upstream of the throat; δt is the gas/liquid volume ratio in the throat 33; and δ3,P3 are respectively the gas/liquid volume ratio and the pressure in the discharge conduit 19. (All pressures are in units of Pascals absolute). The gas/liquid ratio in the throat δt can be represented as the dimensionless liquid flowrate:
where QL is the volumetric flowrate of liquid (m3/s); ρL is the density of the liquid (kg/m3) and At is the flow area in the throat 33 (m2). Thus if the downstream conditions, i.e. the pressure and the gas/liquid volume ratio in the aerated mixture entering the flotation cell, are known, it is possible to solve equation 1 to find the critical value of the upstream pressure P0 for the velocity in the throat 33 to reach the speed of sound and hence for the flow to be choked. Any increase in pressure above the critical value will lead to the formation of a shock wave downstream of the throat.
It is not possible to find analytic solutions to Equation 1. However, it has been found that the following equation, which can be solved easily, is an excellent representation of Equation 1 for values of the dimensionless liquid flowrate (1/at ) less than 2.5:
The rate of capture of particles by flotation can be enhanced by increasing the shear rate, or rate of dissipation of energy, in the vicinity of the particles and the bubbles. The shear rate is proportional to the square root of the rate of energy dissipation. In the embodiment shown in
The embodiment shown in
In all the embodiments disclosed here, the throat length should preferably be in the range 0 to 3 times the throat diameter.
An advantage of using the converging-diverging nozzle shown in
A further embodiment is shown in
In operation, the contactor 16 is filled with a dense foam that travels downwards to discharge through one or more discharge nozzles 37. The bubbles in the mixture discharged from the contactor mix with the liquid in the containing vessel 21 and disengage from it, rising to the top of the vessel to form the froth layer 23. The level of liquid in the outer vessel or container is maintained by the valve 28 or other means, at a level 22. Air is introduced through the entry port 12, at a pressure and flowrate so that the downcomer 16 fills with a dense foam that is agitated by the entering jet of liquid 14, that carries the particulate material to be collected by the bubbles. The turbulent mixing created by the kinetic energy in the plunging jet is a highly favourable environment for the capture of particles by the bubbles in the dense foam. Because of the violent and turbulent nature of the plunging jet the particles in the feed liquid are brought into intimate contact with the bubbles, thus providing a favourable environment for the collection of the hydrophobic particles by the bubbles. Because of the flow restriction brought about by the discharge nozzle 37, the pressure in the downcomer 16 is well above the ambient pressure in the containing vessel 21 at the discharge end of the nozzle 37. The small bubbles in the gas-liquid mixture are rendered even smaller by being forced through the nozzle, where they are brought into further intimate contact with the hydrophobic particles in the suspension to form bubble-particle aggregates. The pressure of the liquid feed and the air supply are such as to be able to maintain the flow of gas and liquid through the discharge nozzles 37.
The gas-liquid mixture that discharges from the shortened nozzle 37, which consists only of the converging section 37 and the parallel-walled throat 33, does so at a considerable velocity, and the momentum in the flow can be utilised further, to increase the overall efficiency of the flotation system. Thus it has been found advantageous to incorporate an internal draft tube 20, which surrounds the lower end of the contactor 16. Because the average density of the gas-liquid mixture being discharged into the draft tube is lower than the density of the liquid in the vessel 21, it tends to rise in the vertical direction, and a circulating pattern is created. Liquid from the vessel is drawn into the entry tube 29 which would otherwise be passing directly out of the tailings exit pipe 28, so the incorporation of the draft tube leads to the further exposure of the particles in the recirculated liquid to the bubbles discharging from the nozzle(s) 37, thereby leading to further opportunities for capturing some particles that would otherwise pass out of the vessel
In relation to the embodiment shown in
a) shows an alternative embodiment of the pressure restriction and dispersion means for use at the termination of the initial contactor 16. A mixture of gas bubbles and liquid slurry formed in the contactor enters a converging conduit 32 of a truncated choke 37 and passes to a throat 33 from which it leaves through a radial diffuser in the space between an upper circular disc 43 and a lower circular disc 44. The discs 43, 44 that define the radial passageway of the disperser are substantially horizontal.
In the embodiment shown in
It is a property of the converging flow in the radial channel 45, that the suction induced in said channel decreases as the separation distance h increases. This observation gives a significant practical advantage to the case where the lower disc is free to move in the vertical direction, in that if the space between the discs becomes blocked by a large particle, the pressure in the radial channel 45 will increase and will force the lower disc 44 to move away from the upper disc 43, thereby releasing the large particle which is swept away in the flow.
In another embodiment shown in
A further embodiment is shown in
The exit stream from the second nozzle enters a second chamber 66, which is fitted with appropriately-placed ports 67, through which fluid can be drawn, to dilute the liquid content in the jet emanating from the second nozzle 64. The combined flow of gas-liquid mixture from the nozzle 64, and recirculating flow through the entry ports 67, passes downwards through the second chamber 66, to discharge through the opening 68.
Surrounding the second chamber 66 and co-axial with it is a draft tube 69 that is conveniently of conical shape. The combined flow leaving the second chamber 66 contains both gas and liquid, and accordingly is of lower mean density than the liquid in the flotation vessel 21, so it rises under gravity in the annular space between the chamber 66 and the draft tube 69, filling the said annular space with a bubbly mixture. Liquid from the lower part of the separation vessel 21 is drawn through the port 70 at the lower extremity of the draft tube 69.
The two-phase gas-liquid mixture rising out of the open upper end 71 of the draft tube enters the upper part of the separation vessel 21, and the gas bubbles rise upwards and separate from the liquid to form a froth layer 23. The froth rises upwards and discharges over the lip 24 into the launder 25 and out of the vessel through the exit pipe 26. The tailings, from which the floatable material has substantially been removed, pass out through the pipe 27. This embodiment is particularly appropriate for the recovery of coarse particles, because the conical draft tube 69 can be of such dimensions and placed in such a way that distance between the top of the said draft tube and the froth-liquid interface 22 can be minimised. The tapered shape of the conical draft tube permits the upward velocity of the mixture of liquid and particle-laden bubbles to diminish with height generating a quiescent flow leaving the upper exit of the draft tube 69, thereby enhancing the probability of retention of coarse particles by the bubbles.
A further embodiment is shown in
In the embodiment shown in
The two-phase gas-liquid mixture rising out of the open upper end 88 of the second chamber 85 enters the upper part of the separation vessel 21, and the gas bubbles rise upwards and separate from the liquid to form a froth layer 23. The froth rises upwards and discharges over the lip 24 into the launder 25 and out of the vessel through the exit pipe 26. The tailings, from which the floatable material has substantially been removed, pass out through the pipe 27.
It is advantageous to be able to control the liquid velocity rising in the riser conduit that forms the second chamber 85, especially when the particles are so large that their terminal velocity is greater than the liquid vertical velocity in the riser. In the embodiments shown in
The invention is described in terms applicable to the separation of minerals in which ore is finely crushed to form a slurry or suspension of particles in water, and the slurry is conditioned with collector and frother to make the mineral species that is to be recovered by flotation hydrophobic or non-wetting, while the non-wetting or hydrophilic species that are to remain in the suspension and are discharged from the flotation vessel as tailings. An example of this is the separation of fine coal particles from the surrounding gangue in a mining operation.
However the invention will also apply to systems in which the particles are of an organic native and typically of biological or non-metallic origin such as algae, printing ink, dairy fat or other liquid particulate systems. The invention will also apply to systems in which all the particles are to be removed in the froth, there being no requirement to separate the components of the particles in the feed liquid on the basis of their hydrophobicity or lack thereof.
A further application is in the removal of metals such as aluminium from suspensions.
Samples of silica were subjected to flotation in an embodiment of the invention according to
The true recoveries were also calculated on a size-by-size basis, and the results are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005900409 | Feb 2005 | AU | national |
| 2005903542 | Jul 2005 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU03/00123 | 2/1/2006 | WO | 00 | 8/8/2008 |
