WO2000015343A1

WO2000015343A1 - Internal recycle apparatus and process for flotation column cells

Info

Publication number: WO2000015343A1
Application number: PCT/AU1999/000754
Authority: WO
Inventors: Graeme John Jameson
Original assignee: Graeme John Jameson
Priority date: 1998-09-11
Filing date: 1999-09-10
Publication date: 2000-03-23
Also published as: AUPP584698A0

Abstract

A flotation column cell (5) having an inlet (6) for pre-aerated feed slurry is provided with a draft tube (20) vertically orientated within the cell so that highly aerated slurry rises within the draft tube (20) into the froth layer (8). Slurry descending in the cell is re-entrained at the base (22) of the draft tube allowing particles which have fallen out of the froth layer to be re-conveyed to the froth layer within the draft tube. Various mechanisms for controlling the re-circulation rate within the draft tube, and mechanisms to direct heavier particles to the base of the draft tube are also described.

Description

TITLE: INTERNAL RECYCLE APPARATUS AND PROCESS FOR

FLOTATION COLUMN CELLS Technical Field

This invention relates to column cells used for flotation separation processes, and more particularly, to an apparatus and process to increase the probability of contact between bubbles and floatable particles in a flotation separation column cell. Background of the Invention

Flotation column cells are used in the separation of particles from mixtures in a finely divided state, suspended in a liquid. Prior to entry into the cell, the suspension 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. The liquid feed is injected into a cell, 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. Clean water may be applied to the froth layer in order to wash entrained particles downwards into the cell.

Flotation is widely used for the separation of valuable minerals from ores, in which case the ore is finely ground and dispersed in water, and the resulting slurry is contacted with bubbles of air. For purposes of description, the term 'air^" will be used to represent the gas, 'water' will be used to represent the liquid and the floatable component will be referred to as the 'values'. It is to be understood however that the same principles apply in other systems involving fine particles which are not minerals, dispersed in non-aqueous media, being floated with gases other than air. 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. It has been found in practice that it is very difficult to ensure that the gas bubbles rise uniformly across the cross-section of the column, and generally, the bubbles tend to move towards the centre of the column, inducing a strong recirculating motion and causing mixing of the column contents. It is known practice to introduce vertical baffles in columns to prevent circulation, to reduce recirculation and create true counter-current flow in the column.

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. A typical tall column is shown schematically in Figure 5.

Short columns are also 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, an example of which is shown in Figure 2, 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. 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. Patent No. 4,938,865; U.S. Patent No. 5,332,100), Bahr (German Patent No. 2,420,482), and Ludke et al. (U.S. Patent 4,448,681). Because of the high-efficiency contacting in the aeration device, the functions required in the flotation cell proper 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.

In the Jameson and Bahr cells, the liquid-bubble contacting device is external to the intensive cell. External bubble generators are known also in the tall flotation column cells. Hollingsworth et al. (U.S. Patent 3,371,779) uses 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. Patent 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. Patent 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. Patent 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. Patent 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.

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 et al., U.S. Patent 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.

It will be appreciated that in intensive cells, all contact between bubbles and particles occurs in the aeration system. Particles attached to the bubbles then rise in the cell, into the froth zone. Most hydrophobic particles which are carried into the froth will appear in the flotation product. However, there is always a fraction of these particles which will drop out of the froth, due to some chance local flow phenomenon, because they have been displaced from the froth by particles of greater hydrophobicity, or in the case of particles which are a composite of values and gangue minerals, the particle has been manipulated by local surface and body forces in the froth so as to present a hydrophilic aspect to the bubbles. For whatever reason, when particles have dropped out of the froth in an intensive cell, there is no opportunity for them to come into contact with a rising bubble, and such particles generally fall through the cell and pass out in the tailings stream. The probability of recovery of such particles into the flotation concentrate would be greatly enhanced if there were some mechanism by which liquid which has entered the cell could recirculate and come into contact with bubbles freshly entering the cell.

The inventor of the present invention has realised that it is desirable to provide a means by which liquid in intensive flotation cells is caused to recirculate and mix with the stream of bubbles being injected into the cell, whether as free bubbles released from air nozzles within the cell, or as bubbles in a bubbly flow of water or feed liquid, generated internally or externally. Hydrophobic particles which may have dropped out of the froth and have returned to the liquid in the cell will, through the action of the recirculating flow, be re-mixed with the incoming bubbly stream, and receive another opportunity to be collected and swept into the flotation froth and hence into the product stream from the cell. In both tall column cells and short intensive cells, it has proven difficult to recover coarse particles into the flotation froth. The bubbles generated by the various means are generally in the range of 400 μm to 2 mm in diameter. To be lifted in the liquid, the density of a bubble-particle aggregate must be less than that of the slurry in which it finds itself. A single bubble may be insufficient to lift a large dense mineral particle in the liquid, and for particles above a given size, multiple bubble attachments are necessary. Once a large particle has risen into the froth, the number of hydrophobic attachments to neighbouring bubbles through three-phase contact lines is large, and the prospects of retaining the particle in the froth so that it will be recovered with the product is increased. For flotation of large particles, the limiting step is the attachments of bubbles to the particles, and the rise of the bubble-particle aggregates through the liquid and into the froth in such a way that the bonds between bubbles and particles are not disturbed. The recovery of coarse particles could therefore be enhanced if the particle-bubble aggregates could be delivered either directly into the froth layer, or just beneath the froth layer, thereby minimising the probability that the particle will separate from the bubbles which are lifting it. The inventor has also realised that it is desirable to provide a means by which the recovery of coarse particles in flotation columns can be increased, by causing an internal recirculation of liquid in the column to mix with incoming air bubbles to create a gas- liquid mixture with a high void fraction, which can then be discharged into the froth layer, or just below the froth-liquid interface at the top of the column. Summary of the Invention

In one aspect the present invention therefore provides a column flotation cell having a main cell body adapted to contain a slurry of liquid, values and gangue; feed and aeration means adapted to form aerated slurry topped by a froth layer within the cell body, froth removal means arranged to remove froth from the cell, and drain means arranged to drain material from the bottom of the cell, characterised by the provision of a draft tube positioned within the cell with a generally vertically extending axis, arranged relative to the feed and aeration means such that highly aerated slurry rises in the cell substantially in one of the interior or exterior of the draft tube, giving up at least some bubbles to the froth layer and descending within the cell substantially in the other of the interior or exterior of the draft tube, forming a recirculating flow through the draft tube within the cell. In a further aspect the present invention provides a method of operating a flotation cell, comprising the steps of providing an aerated slurry containing values and gangue within the flotation cell, providing a draft tube within the cell oriented with a generally vertically extending axis and positioned such that highly aerated slurry rises within the cell substantially in one of the interior or exterior of the draft tube, giving up at least some bubbles to a froth layer within the cell and descending within the cell substantially in the other of the interior or exterior of the draft tube, forming a recirculating flow through the draft tube within the cell.

In one embodiment of the invention pre-aerated slurry is released from an outlet into the interior of the draft tube.

Preferably a flow control device is provided arranged to control the rate of flow of aerated slurry through the draft tube.

Preferably the flow control device is located at the lower end of the draft tube.

In one form of the invention a secondary supply of slurry is introduced into the draft tube to a location adjacent the primary supply of aerated slurry.

In one form of the invention the upper end of the draft tube is located below the froth layer. In an alternative form of the invention the upper end of the draft tube is located in the froth layer.

In one form of the invention, the interior of the flotation body is provided with at least one annular baffle positioned and arranged to direct slurry descending within the cell toward the lower end of the draft tube.

Preferably the upper end of the draft tube is provided with an annular baffle, sloped to direct particles falling from the froth layer into the highly aerated slurry rising in on of the interior or the exterior of the draft tube. The present invention relates to the operation of column flotation cells, without restriction to the height, diameter or geometrical form. More particularly, the invention relates to the development and control of internal recycle of liquid, by the incorporation of a substantially-vertical open duct or draft tube, into which the air bubbles to be used for flotation are injected, either as bubbles alone or as bubbles in a pre-mixed feed stream. A preferred flotation cell according to the present invention consists of a column flotation cell with a concentric vertical draft tube. A slurry of fresh flotation feed, which has been pre-mixed with air bubbles, is injected into the cell in the base of the draft tube.

A circulating flow is induced because the average density of the liquid within the draft tube is less than the density of the liquid in the cell outside the draft tube. The recirculating liquid stream enters the bottom of the draft tube and mixes with the incoming feed stream and with the bubbles it contains. The upper end of the draft tube is placed just below the froth-liquid interface in the flotation cell, so the bubbly mixture of fresh feed and recirculated liquid is delivered directly into the froth layer. In another preferred embodiment, a stream of bubbles is injected into the draft tube to cause the recirculation and mixing of bubbles with the contents of the flotation cell.

Other features of the invention relate to the control of the flow developed in the draft tube, by the incorporation of flow restrictors or valves in the draft tube. In further embodiments of the invention, baffles are placed within the column cell in order to direct particles which have fallen out of the froth layer atop the cell, towards the inlet of the draft tube, to allow them another chance of being collected and retained in the froth layer.

Brief Description of the Drawings The invention will now be described with reference to the following drawings in which:

Figure 1 is a diagrammatic cross-sectional elevation of a typically short or intensive flotation column cell.

Figures 2(a) to 2(f) are similar views to Figure 1 depicting various configurations of the invention.

Figures 3(a) to 3(d) show similar configurations, incorporating various devices to control the flow of the recycled liquid so as to optimise the configuration for a given use. Figure 4 shows an apparatus according to the invention in which fresh feed to an intensive column cell is introduced into the draft tube. Figure 5 shows a typical prior art tall flotation column cell, in which the invention can with advantage be incorporated. Figure 6 shows similar views to Figure 5, depicting manifestation of the invention in which a draft tube is used inside a tall flotation column cell.

Figure 7 shows a cell similar to Figure 1 incorporating an arrangement of baffles to direct particles which have dropped out of the froth towards the entry to the draft tube. Figure 8 shows cells similar to Figure 1 having further arrangements of baffles which serve to direct particles which may have fallen out of the froth back into the bubble mixture rising in the draft tube.

Figure 9 shows an apparatus according to the invention in which the draft tube is tapered to reduce the velocity of flow of the rising slurry. Figures 10, 10(a), 10(b) and 11 show manifestations of the invention in which baffles are inserted to separate the particles which have dropped out of the froth into coarse and fine fractions, allowing only the coarse particles to be recirculated in the draft tube.

Figure 12 shows a cell similar to Figure 1 with the upper end of the draft tube positioned in the froth layer.

Description of the Preferred Embodiment

Figure 1 shows an intensive flotation column flotation cell embodying the invention. The liquid feed containing the particles to be separated by flotation is prepared or conditioned with appropriate collectors and frother reagents prior to entry to the column, so that the values are hydrophobic and will be able to form strong bonds with bubbles. The feed to the column enters at the inlet 1 and flows through the nozzle 2 to form a downwardly- facing jet in the downcomer 4. Air enters at 3, and the flowrate of air is controlled (by means not shown) so that the downcomer 4 fills with a downwardly moving dense foam bed. The dense foam leaves the bottom of the downcomer 6, and enters the flotation cell proper 5, where it rises under the action of gravity up the draft tube 20. The gas-liquid mixture leaves the top of the draft tube 21 and separates into two phases. The bubbles disengage from the liquid and rise through the gas-liquid interface 7 into the froth 8. The liquid flows downwards to the base of the cell 5, and splits into two streams. One part leaves the cell through the exit pipe 12, while the second is recirculated into the base of the draft tube 22, where it mixes with the aerated gas-liquid mixture leaving the exit end 6 of the entry pipe 4. The bubbles in the froth layer 8 in the top of the cell carry with them the hydrophobic particles which have been collected in the gas-liquid mixture in the downcomer 4. The froth flows over the cell lip 9 into the launder 10 and is discharged from the cell through the port 11 as the flotation product. The liquid leaving the base of the cell splits in two directions. One part passes through the control valve 13 to leave as the tailings from the cell through the pipe 14. The second part passes through another control valve 15 to discharge as the external recycle component 16 into the feed pump box 18. Here it mixes with incoming fresh feed 17, and is pumped through the pump 19 to the entry point 1 at the head of the downcomer. The purpose of the valve 13 is to maintain the froth-liquid interface 7 at the desired level in the cell 5, while the control valve 15 is used to control the amount of recycle liquid 16 relative to the feed 17, or to maintain a constant liquid level 23 in the pump feed box.

The foam flowing out of the bottom of the downcomer at 6 has a high void fraction of air, up to 50 to 60 percent by volume. This aerated flow then mixes with recycle liquid which has entered the draft tube. The average density of the resulting gas- liquid mixture in the draft tube is much less than the density in the liquid external to the draft tube. As a consequence, an imbalance in hydrostatic pressure is established between two points at the same horizontal level, one within the draft tube and the other in the liquid external to the draft tube. The gas-liquid mixture within the draft tube is forced upwards by the hydrostatic pressure difference, and a recirculation pattern is therefore established within the flotation cell. The upward velocity within the draft tube increases until difference in the hydrostatic pressure which is driving the flow is balanced by the frictional pressure losses in the fluid as it enters, rises through, and leaves, the draft tube.

If necessary, the froth can be washed with clean water distributed by a means not shown over the top of the froth layer 8, to flush gangue particles out of the froth and back into the liquid in the cell 5.

The column 5 and the draft tube 20 can be simple right cylinders concentrically mounted with the downcomer 4, but without loss of effectiveness, both the column and the draft tube can be of any convenient cross-section, such as square, rectangular, oval or elliptical.

An example of the benefits that can be achieved by using a draft tube in a cell of the type shown in Figure 1 is set out below. Example

A flotation cell as shown in Figure 1 was constructed. A draft tube was mounted vertically. A coarse coal stream from an operating coal washery was supplied to the flotation cell. Samples of the flotable material were collected and separated into different size fractions. The percentage of ash in each sample was then determined, allowing calculation of the yield or recovery of combustible material in each size fraction. Table 1 shows a comparison of the response to flotation of individual size fractions. Both the mass yield and the combustibles yield in the size fraction above 1 mm showed large increases in the presence of the draft tube. Table 1

The invention has been described in Figure 1 in reference to a liquid feed stream which has been pre-aerated in a downcomer as described by Jameson (U.S. Patent No. 4,938,865; U.S. Patent No. 5,332,100). It will be appreciated that any other means capable of pre-aerating the feed, preferably so that the air content is in the range 10 to 90 percent by volume, could be used.

Figures 2(a) to 2(f) show various alternative forms of draft tube in combination with the pre-aerated feed inlet and the intensive flotation cell. In each of Figures 2(a) to 2(f), the pre-aerated feed enters through a pipe 30 and discharges at the end of the pipe 31 into the vertically-moving flow in the draft tube 20. In Figure 2(f), the aerated feed is shown entering the annular space between the inner wall of the cell 5 and the draft tube 20. In this alternative, the flow direction is upwards near the wall of the cell and downwards through the draft tube. In general, it is desirable to operate flotation cells in such a way that the gas- liquid interface 7 and the upper surface of the froth 8 of Figure 1, are relatively quiescent. Excessive disturbance and turbulence in the froth layer can loosen the bonds between bubbles and particles, with the result that some of the particles may fall out of the froth and drop back into the liquid, causing a loss of recovery of the values. In some applications with the invention described in Figure 1 , the rise velocity of the gas-liquid mixture in the draft tube 20 is so high, that it causes an upwelling of liquid which causes waves on the gas-liquid interface 7 and turbulent motions in the froth layer 8. The turbulent motion is particularly evident when the depth of the froth layer 8 is relatively shallow. The velocity or kinetic energy of the rising liquid in the draft tube is strongly influenced by the average volume fraction of gas in the rising liquid, so one way of reducing the turbulence in the froth layer is to reduce the air flowrate. However, in order to maintain maximum collision rate between bubbles and particles in the recycle stream in the draft tube, it is preferable not to reduce the number of gas bubbles and the quantity of gas in the rising liquid stream. Accordingly, it is beneficial in the operation of column flotation cells with a draft tube to provide a means for reducing or controlling the velocity in the draft tube to provide a quiescent surface while still maintaining a high gas volume fraction. Figure 3 shows alternative means for controlling the recycle liquid flowrate. In Figure 3(a), the entry to the draft tube is fitted with a constriction pipe 33 which has the effect of increasing the resistance to flow. It will be appreciated that the diameter and length of the restriction pipe can be chosen to achieve the desired reduction in flowrate in the draft tube. Figure 3(b) shows a plate 34 mounted below the draft tube 20. The plate can be permanently fixed in position relative to the draft tube, to provide a desired vertical gap around the periphery of the tube through which the recycle liquid flows. Alternatively, the plate can be mounted on a suitable mechanism (not shown) so that the flow area in the gap between the plate and the base of the draft tube can be varied in response to changing flotation conditions, in order to maintain the optimum recovery of values, or the concentrate grade, in the product from the cell. Figure 3(c) shows an alternative arrangement in which a dart valve or plunger 35 with sloping sides can be fixed in position to provide a constant flow area through which the recycle liquid passes. The dart valve 35 can with advantage be supported by means not shown so as to be able to provide a variable flow area for the recycle liquid, and hence to control the recycle rate to provide optimum flotation conditions when the flotation conditions in the cell change due for example to changes in flowrate upstream of the flotation column, or changes in the characteristics of the ore body. Another alternative is shown in Figure 3(d), where a control valve 36 is fitted to the base of the draft tube. This control valve is such that it provides a variable area constriction to the flow through the draft tube. It may be of any convenient type such as a rotary ball valve or a pneumatically-controlled pinch valve.

In some flotation circuits, not all of the feed to the column requires pre-aeration, and there may be advantages if a new- feed stream could be fed direct to the cell. This situation may arise for example, in a rougher flotation cell where the bulk of the flow arises from the crushing and conditioning of new run-of-mine ore which is entering the concentrator, but where there may be a small recycle stream from the tailings from the cleaning circuit, which is returned to the head of the rougher bank. In this case, one or more of the feed streams could enter the alternative feed pipe 25 as shown in Figure 4. This feed mixes with the recycle stream entering the bottom of the riser 20, and the resultant liquid stream mixes with the incoming gas-liquid stream discharging from the bottom of the entry pipe or downcomer 4. Particles which may be present in the feed entering at 25 will be brought into contact with the bubbles in the aerated stream emanating from the bottom of the downcomer 4, and will be carried by these bubbles into the froth layer 8, along with particles which originated in the fresh feed 17 or in the internal recycle stream within the cell.

Draft tubes can also be used with advantage in column flotation cells of the 'tall' configuration. Figure 5 shows a tall column cell as typically constructed. The cell 50 has a height-to-diameter ratio of at least 2:1 and can be up to 10:1 or greater. Feed enters towards the top of the column through the entry port 51 and travels downwards in the collection zone of the column. The downwardly moving feed meets a cloud of rising bubbles which are continuously formed in the base of the column by the generation device or sparger 52, which is fed by air under pressure which enters through the opening 53. During their upward passage, the bubbles collide with particles of floatable material, which are then carried with the particles into the froth layer 54. If required, clean wash water can be applied to the top of the froth layer 54 through the distributor 60, to flush entrained gangue particles downwards into the collection zone. The froth laden with particles then passes over the lip 55 of the flotation cell and into the launder 56 from which the flotation product flows through the exit 57. At the bottom of the column, the downwardly moving liquid enters the disengagement zone 58 beneath the bubble generator 52. In the disengagement zone, the downward velocity of the liquid is less than the upward rise velocity of the bubbles, so the bubbles are able to rise out of the liquid. The bubble-free liquid leaves the bottom of the column through the tailings exit port 59.

Figure 6(a) shows a draft tube 61 within a conventional tall column 50. The draft tube is positioned in such a way that the air bubbles formed by the sparger 52 tend to rise up the draft tube, causing a circulation flow to develop in the column which entrains the new feed to the column, which is entering through the port 51.

It will be appreciated that the air sparging device 52 can be placed within the draft tube 61, as shown in Figure 6(b), as can the discharge end 54 of the feed supply pipe. With this arrangement, it is possible to control the recycle rate by the addition of one of the devices shown in Figure 3.

The benefits arising from the incorporation of the draft tube 61 within the tall column include but are not limited to the following. By increasing the internal rate of recirculation within the column, particles in the liquid which may not have been brought into contact with bubbles in previous passages up the draft tube, have further opportunities to collide with bubbles and be carried into the froth layer and into the flotation product. Large particles which may have fallen off the froth are entrained in the recirculating flow and swept into the draft tube, and have further opportunities to be transferred into the froth phase and hence into the product stream. When the flow is driven by the bubbles rising from the sparger 52, it is not necessary for there to be a net feed to the column for a recirculation to be maintained. Thus if the new feed flowrate drops to zero, a recirculating flow will be maintained in the column by the rising bubbles, and flotation will continue until all floatable particles are removed into the froth.

It is one of the purposes of the present invention to increase the recovery of valuable particles, especially coarse particles, which have dropped out of the froth. This can be accomplished by recycling of liquid within the flotation column as shown in the Figures 1 to 4 and Figure 6. A further improvement aimed at increasing the recovery of coarse particles consists of the introduction of a number of baffles suitably placed in the flotation column so as to direct particles which have fallen out of the froth into the entry region of the draft tube. An illustration of this improvement is shown in Figure 7. A flotation column with internal draft tube constructed according to the principles described herein, has within the upper portion of the cell 5 a diversionary baffle 40. The pre-aerated feed enters through the port 30 and discharges through the outlet 31 into the draft tube 20. The gas-liquid mixture rises in the draft tube and discharges through the opening 21 , where it enters the external annular space between the draft tube 20 and the cell 5, and moves downwards. Hydrophobic particles which have been contacted with the bubbles in the incoming liquid stream or in the draft tube, are carried by the bubbles through the froth-liquid interface 7 and into the froth layer 8. The froth flows over the lip 9 and into the discharge launder 10 and leaves the flotation cell through the product exit port 11.

In practice it is found that a proportion of the coarse particles which are carried into the froth, have a tendency to work their way downwards as the froth moves towards the overflow lip 9, and to drop out of the froth layer 8 altogether, falling back into the liquid which is supporting the froth. Referring to Figure 7, the liquid stream which leaves the upper exit 21 of the draft tube, moves outwards in a generally radial direction, carrying with it coarse particles which have fallen out of the froth. These coarse particles settle within the moving liquid stream and fall on top of the sloping annular baffle 40. The particles slide down the sloping baffle towards the outer wall of the draft tube 20, and are swept into the main liquid stream which is moving generally towards the base of the flotation cell. Below the baffle 40, the liquid moves slowly in a direction generally outwards from the axis of the flotation cell, but the coarse particles fall vertically through the liquid, and are guided into the base or entry 22 to the draft tube by the baffle 41. Within the draft tube, the coarse particles brought into contact with bubbles entering with pre-aerated feed through the pipe exit 31 , and have the opportunity to make further bonds with bubbles, and to be carried back into the froth layer 8 at the top of the flotation column cell. Liquid which has not recycled into the draft tube, passes downwards in the cell to leave at the discharge port 12.

The size and the position of each of the baffles 40 and 41 must be such as to maximise probability of capturing coarse particles, while maintaining a recycle flowrate which is sufficient for normal cell operation with respect to the remaining, fine, particles in the feed.

In the absence of the baffle 40, coarse particles will be carried in a generally radially outward direction, and will flow down the inner wall of the cell 5, thereby avoiding re-entrainment in the recycle stream. By the action of the baffle 40, the probability of re-capture of the coarse particles is increased from essentially zero, to a fraction which equates approximately to the ratio which the recycle flowrate in cubic metres per second bears to the combined flowrate of recycle and flow through the tailings port 12.

It will be appreciated that although the invention has been described with reference to a flotation column of the intensive type where the feed is pre-mixed with air bubbles, a similar system of baffles with the same purpose as those shown at 40 and 41 in Figure 7 could be installed in a tall column in which the air is sparged directly into the liquid in the column, as shown in Figure 6(b).

Further embodiments of the present invention, aimed at recovering valuable particles, especially coarse particles, which have dropped out of the froth, are shown in Figure 8. In Figure 8(a), the pre-aerated feed liquid is introduced to the flotation cell 5 through a feed pipe 30, which discharges at the lower end within a draft tube 20. Here it mixes with recycled liquid rising through the entrance 22. The well-mixed stream of bubbles, fresh feed to the cell and recycled liquid, rises up the draft tube and the bubbles with entrained liquid pass through the froth-liquid interface 7 into the froth layer 8. Since there is a continuous stream of bubbles and entrained liquid entering the base of the froth layer in the region of the centre of the cell, there is a movement of froth radially outwards, flowing over the lip 9 and into the discharge launder 10. However, the kinetic energy in the rising flow in the draft tube can result in a disturbance to the froth layer immediately above the draft tube, which can have the effect of dislodging particles from the froth. In the embodiment shown in Figure 8(a), there is an inclined rim in the form of annular baffle 70, extending from the upper end of the draft tube 20. The baffle 70 is constructed so that its face is at a suitable angle to the horizontal, so that when coarse particles fall out of the froth, they slide down the upper face of the baffle, and return to meet the gas-liquid stream which is rising up the draft tube. Thus floatable coarse particles which are returned to the draft tube, have further opportunities to collide with and attach to bubbles, and to be carried into the froth layer and out of the cell as flotation product.

Figure 8(b) shows a similar embodiment which is designed for flotation cells fitted with an annular draft tube as in Figure 2(f) in which the upwards flow is adjacent to the wall of the cell 5. In this variation, the baffle 70 slopes downwards in such a way that coarse particles which have been dislodged from the froth are directed outwards from the axis of the flotation cell, and into the gas-liquid mixture rising in the annular space between the wall of the flotation cell 5 and the draft tube 20.

An embodiment which can further reduce the turbulence in the top of the flotation column is shown in Figure 9. Here the draft tube 20 is in the form of a frustrum of a hollow cone which widens with increasing vertical distance in the flotation column cell, with corresponding increase in the available flow area, and corresponding decrease in the flow velocity, in the draft tube. The included angle of the cone or the sides of the draft tube in this embodiment should be in the range 5 degrees to 45 degrees, but preferably between 6 degrees and 10 degrees, to produce the maximum effect. In some applications it can be advantageous to separate the particles which have dropped out of the froth into a coarse and a fine fraction, then allowing only the coarse fraction to be recycled in the draft tube. One example is in the flotation separation of coal particles from gangue which is essentially ultrafine clay, where the recycle of the unfloatable clay mineral into the froth would have deleterious effects on the quality of the flotation concentrate. The desired size separation can be effected by allowing the particles which have dropped out of the froth to pass downwards through one or more sloping plates 72 placed in the flow path, as shown in Figure 10. In Figure 10 the sloping plates 72 are in the form of hollow conical sections. Coarse particles which drop from the froth fall on to the plates 72 and slide under gravity in a direction away from the axis of the cell, to fall on to the annular catch plate 73. The angles which the sloping plates 72 and 73 make with the vertical are chosen so that particles which deposit on them will continue to slide in a downward direction. The catch plate 73 contains at least one opening 74 as shown in Figure 10(a), to allow the slurry which leaves the upper extremity of the draft tube to pass downwards to the lower part of the cell. The opening 74 is fitted with a raised vertical rim or edge strip 75 as shown in Figures 10(a) and 10(b), to prevent particles which have deposited on the catch plate 73 from sliding into the opening 74. The catch plate 73 is constructed so that at any point thereon the surface slopes downwards towards the gap 76 in the edge strip 75. Thus particles which have deposited on the catch plate 73 slide downwards through the gap 76, thence through the annular space between the guide pipe 77 and the draft tube 20, and then fall towards the guide plate 41 , to be re-entrained into the recirculating flow as it enters the draft tube. It will be appreciated that if the flotation column cell 5 is square or rectangular in section, the design of the sloping plates can be modified accordingly in order to conform with the geometry of the cell. Figure 11 shows an embodiment similar to that in Figure 10. with the sloping plates 72 being placed in such a way that the particles fall towards the centre of the cell 5.

The parameters of the sloping plates 72, such as the distance between them, their height, and their angle of slope, can be chosen so as to determine the size of the particles which are deposited thereon. In this way it is possible to construct a cell according to Figures 9 and 10 in which only particles above a predetermined value are permitted to recycle through the draft tube, and hence be given further opportunities to come into contact with the fresh air bubbles in the feed stream. This is a desirable feature for example in streams where the gangue mineral is markedly finer in size than the values to be separated by flotation. In the embodiments of the invention shown in Figures 1 to 4 and 6 to 8 the exit

21 from the draft tube is shown generally as lying in the liquid layer beneath the froth- liquid interface 7. Thus an element of liquid which enters the draft tube from whatever source, will discharge into liquid at the top of the draft tube. This arrangement is beneficial in that part of the kinetic energy of the gas-liquid mixture rising in the draft tube, is dissipated in the liquid in the immediate vicinity of the draft tube exit 21, so that it is possible to maintain a froth layer which is relatively quiescent and undisturbed by the energy in the rising flow. In some circumstances however, it can be beneficial to construct the draft tube 20 in such a way that its upper extremity 21 is above the froth- liquid interface 7 as shown in Figure 12, so that the stream rising in the draft tube is forced to discharge into the froth layer itself. This particular configuration is valuable when the feed to the flotation cell contains very coarse particles, so coarse in fact that they would lie outside the range of particle sizes which would be considered to be recoverable by flotation in conventional flotation column cells.

Experiments have shown for example that it is possible to separate particles of coal matter from a siliceous gangue, when the coal particles are 1.5 mm or more in diameter, when an aerated feed stream containing such particles is injected into a flotation column according to Figure 1, in which the exit 21 from the draft tube 20 lies in the froth layer 8.

The reason for the improved flotation recovery is that when a particle is surrounded by liquid in the collection zone of a flotation column for example, the particle must collide with one or more bubbles such that the density of the bubble- particle aggregate is less than the density of the liquid, so that the aggregate can rise to the surface. Clearly, the larger the particle, the larger the number and volume of the bubbles attached to it must be, to lift it to the surface. However, the larger the size of particle and hence the larger the size of the bubble-particle aggregate, the higher the probability that the particle-bubble aggregate will split and disintegrate under the action of turbulent eddies in the liquid phase. Since the size of the bubbles is finite and the number of bubbles which make contact with a particle in rising through the liquid is limited, there is an upper limit of 350μm to 500μm placed on the size of coal particles which can be lifted into the froth in conventional flotation columns. If a coal particle is contained in a dense foam however, such as may be delivered by the tube 4 in Figure 1, the concentration of bubbles can be so high that each hydrophobic particle will find itself in contact with a large number of adjacent bubbles. By arranging for the exit 21 from the draft tube 20 to lie in the froth layer 8, it is possible to maintain a high number of contacts between individual particles and surrounding bubbles, thereby allowing the particle to remain in the froth layer and be carried out over the lip 9 into the discharge launder 10 as flotation product.

It is therefore a purpose of this invention to provide a means to deliver aerated flotation feed containing coarse floatable particles directly into the froth layer on top of the flotation cell, to bring about increased recovery of said coarse particles.

The use of the control means shown in Figure 3 permits the recycle rate to be adjusted so as to provide an appropriate dilution of the incoming aerated feed stream into the draft tube. This will permit the overall performance of the cell to be maximised, so that the gain from the recovery of particles from the recycled stream is balanced with the possibility of greater loss of coarse particles from excessive dilution of the aerated feed stream with recycled liquid.

Claims

1. A column flotation cell having a main cell body adapted to contain a slurry of liquid, values and gangue; feed and aeration means adapted to form aerated slurry topped by a froth layer within the cell body, froth removal means arranged to remove froth from the cell, and drain means arranged to drain material from the bottom of the cell, characterised by the provision of a draft tube positioned within the cell with a generally vertically extending axis, arranged relative to the feed and aeration means such that highly aerated slurry rises in the cell substantially in one of the interior or exterior of the draft tube, giving up at least some bubbles to the froth layer and descending within the cell substantially in the other of the interior or exterior of the draft tube, forming a recirculating flow through the draft tube within the cell.

2. A column flotation cell as claimed in claim 1 wherein a flow control device is provided arranged to control the rate of flow of aerated slurry through the draft tube.

3. A column flotation cell as claimed in claim 2 wherein the flow control device is located at the lower end of the draft tube.

4. A column flotation cell as claimed in any one of the preceding claims wherein the feed and aeration means are arranged to release pre-aerated slurry into the interior of the draft tube.

5. A column flotation cell as claimed in claim 4 wherein secondary feed means are provided for a secondary supply of slurry, positioned to introduce the secondary supply of slurry into the draft tube in a location adjacent the release of the pre-aerated slurry.

6. A column flotation cell as claimed in any one of the preceding claims wherein the upper end of the draft tube is positioned to lie below the froth layer.

7. A column flotation cell as claimed in any one of claims 1 to 5 wherein the upper end of the draft tube is positioned to lie within the froth layer.

8. A column flotation cell as claimed in any one of the preceding claims wherein sloping baffles are provided within the cell, positioned to direct particles falling thereon toward the lower end of the draft tube.

9. A column flotation cell as claimed in any one of the preceding claims wherein the upper end of the draft tube is provided with an inclined rim arranged to direct particles falling from the froth layer into the highly aerated slurry rising in the cell.

10. A column flotation cell as claimed in any one of the preceding claims wherein the draft tube is tapered over some or all of its length so as to cause the velocity of the highly aerated slurry rising in the cell to decrease as it nears the froth layer.

11. A column flotation cell as claimed in any one of the preceding claims wherein at least one inclined catch plate is positioned in the part of the cell wherein the slurry is descending, the catch plate being shaped and positioned to direct particles falling thereon toward the lower end of the draft tube where they can be entrained with the recirculating flow.

12. A column flotation cell as claimed in claim 11 wherein one or more sloping plates are positioned above the catch plate, located and inclined to direct larger particles onto the catch plate.

13. A column flotation cell as claimed in either claim 11 or claim 12 wherein the catch plate has one or more openings therethrough surrounded by an upstanding rim arranged to direct particles sliding down the catch plate away from the openings and toward the lower end of the draft tube.

14. A method of operating a flotation cell, comprising the steps of providing an aerated slurry containing values and gangue within the flotation cell, providing a draft tube within the cell oriented with a generally vertically extending axis and positioned such that highly aerated slurry rises within the cell substantially in one of the interior or exterior of the draft tube, giving up at least some bubbles to a froth layer within the cell and descending within the cell substantially in the other of the interior or exterior of the draft tube, forming a recirculating flow through the draft tube within the cell.

15. A method as claimed in claim 14 wherein pre-aerated slurry is released from an outlet into the interior of the draft tube.

16. A method as claimed in either claim 14 or claim 15 wherein the rate of recirculating flow through the draft tube is controlled by adjusting the size of the opening at the lower end of the tube.