Field of the invention

[001] This invention relates to minerals processing by means of froth flotation. In particular, the invention relates to a froth flotation cell that finds particular application as a rougher cell or a scavenger cell and which might also be used in cleaner, re-cleaner and re-recleaner applications.

Background to the invention

[002] Froth flotation has been known for more than a century as a process of separating and concentrating minerals.

[003] In froth flotation, milled ore particles are physically separated depending on their hydrophobicity. The minerals present in the ore and water slurry become hydrophobic due to the addition of a collector chemical to the slurry. Air is introduced into the slurry and the hydrophobic particles become attached to the air bubbles. The air bubbles with the attached hydrophobic particles rise to the surface and collect in a froth layer that is collected as a mineral-rich concentrate.

[004] In the past, the mining industry has predominantly used mechanical flotation cells, consisting of square or cylindrical cells or tanks with a mechanically driven rotor and stator agitator mechanism through which air is introduced into the bottom of the cell. Generally a number of such cells are combined consecutively, creating a bank of cells. The principal limitation of the mechanical cell is the ineffectiveness of such a cell when dealing with fine particles which are prone to entrainment in the froth. This has given rise to the use of mechanical cells as predominantly rougher cells, with column cells often being used as cleaners in certain applications.

[005] Instead of the agitator mechanism of the mechanical cell, a column cell uses a sparger (internal or external) to generate bubbles that are introduced into the column cell. The pulp in-feed is located relatively high up in the column and the descending pulp contacts rising bubbles generated at the bottom of the column as the pulp moves down the column. Hydrophobic particles that attach to the bubbles form bubble/particle aggregates which rise to the surface. Non-floating material exits from the bottom of the column as tailings. Froth washing can also be introduced to the column cell, whereby water is injected into the froth layer to increase the grade of the concentrate by removing gangue particles which accumulate in the froth by mechanical entrainment.

[006] Attempts at using the operating principles of column cells in rougher and scavenger applications have been less than successful and it is an object of this invention to provide a froth flotation cell that addresses this issue.

Summary of the invention

[007] According to this invention, a froth flotation cell comprises: a flotation tank; a pulp in-feed line for feeding, into the tank, a mineral-containing pulp to be separated by froth flotation into separate froth and tails fractions; a variable speed pump including feed and discharge lines; a first low-level tails discharge outfeed line connected to a first tails outlet at the base of the tank, the first low-level outfeed line being configured to supply the pump with a feed of pulp tails drawn, in operation, from the base of the tank by way of the first tails outlet; the pump having at least two feed line suction points, including the pulp in-feed line and the first low-level outfeed line; the pump being configured to pump a combination fresh and recycle feed, which is a combination of the pulp in-feed and the pulp tails feed, into the tank by way of external and internal sparger assemblies connected in-line in the pump discharge line; the external sparger assembly being located externally of the tank and including an air sparger configured to introduce air into the combination fresh and recycle feed; the internal sparger assembly being located within the tank and terminating in a combination fresh and recycle feed outlet through which the combination fresh and recycle feed is discharged into the tank; and a second low-level tails discharge outfeed line connected to a second tails outlet at the base of the tank, the second low-level outfeed being configured to supply a feed of pulp tails drawn, in operation, from the base of the tank by way of the second tails outlet, to an external process.

[008] The external process may be a further mineral recovery process in which additional quantities of the valuable mineral may be separated from the pulp by froth flotation, with or without regrinding.

[009] In the preferred form of this embodiment of the invention, the external process is preferably constituted by one or more froth flotation cells similar to the froth flotation cell.

[0010] In addition or in the alternative, the external process in this embodiment of the invention, could be constituted by one or more scavenger cells.

[0011] As a further alternative, the external process may be a waste disposal process in which the flotation tails are processed as mine backfill or disposed of as tailings waste.

[0012] In the preferred embodiment of the invention, in which the external process is constituted by one or more additional froth flotation cells similar to the froth flotation cell, the additional froth flotation cells are preferably interconnected such that the froth flotation the pulp tails fed from the second low-level tails discharge outfeed of an upstream froth flotation cell are supplied as a pulp feed to the pulp in-feed of the froth flotation cell immediately downstream.

[0013] In such an installation, the cell receiving the feed of pulp tails at its pulp in-feed is a downstream cell and the cell supplying the feed of pulp tails from its second low-level tails discharge outfeed is an upstream cell.

[0014] In this embodiment of the invention, several similar froth flotation cells are preferably installed in circuit, one downstream of the other, with the second low- level tails discharge outfeeds of each upstream cell being connected to feed pulp tails from its second low-level tails discharge outfeed to the pulp in-feed of the cell immediately downstream thereof.

[0015] In a further embodiment of the invention, the internal sparger assembly comprises at least one internal sparger that terminates in an outlet that is partially occluded by a disperser that extends across and partially occludes the sparger outlet.

[0016] The sparger may include an in-line ceramic sparging element.

[0017] In a preferred form of this embodiment of the invention, the internal sparger assembly comprises a plurality of internal spargers mounted on and connected to feed pipes that extend radially from a central feed pipe connected to the pump discharge line.

[0018] The froth flotation cell preferably includes one or more of an internal launder assembly, a froth scraper assembly and a froth wash water assembly.

[0019] The external process may be constituted by at least one scavenger cell or by a waste disposal process in which the final pulp tails are disposed of to waste.

[0020] The invention includes a bank of froth flotation cells including at least two froth flotation cells according to any one of any one of the preceding claims, in which the froth flotation cells are installed in a pulp fluid flow circuit including a first froth flotation cell and at least one other froth flotation cell installed in the pulp fluid flow circuit downstream of the first froth flotation cell such that the second low-level tails discharge outfeeds of each upstream cell is connected to feed pulp tails from its second low-level tails discharge outfeed to the pulp in-feed of the cell immediately downstream thereof.

Brief Description of the drawings

[0021] The invention will be further described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic isometric view of two froth flotation cells according to the invention;

Figure 2 is a front elevation of the froth flotation cells of Figure 1 ;

Figure 3 is a plan view of the froth flotation cells with tails discharge to the next flotation cell;

Figure 4 is a side elevation of the froth flotation cell;

Figure 5 is a similar side elevation illustrating recycle piping and a sparger assembly mounted within the base of the flotation cell (in dotted outline);

Figure 6 is a diagrammatic isometric view of the recycle piping and sparger assembly of Figure 5;

Figures 7 to 12 are plan and isometric views, respectively, of internal launder arrangements suitable for use with the froth flotation cell of the invention; and

Figures 13 and 14 are isometric and plan views, respectively, of a froth wash water system suitable for use with the froth flotation of the invention;

Figure 15 is a side elevation of a froth flotation cell according to the invention further illustrating the wash water system of Figures 13 and 14; Figure 16 is a section on a line A — A in Figure 15;

Figures 17 and 18 are isometric and plan views, respectively, of a froth scraper assembly suitable for use with the froth flotation cell of the invention; and

Figure 19 is a side elevation illustrating the froth flotation cell of the invention that includes a froth scraper assembly.

Description of embodiments of the invention

[0022] Minerals processing is a multistage operation which typically follows a similar separation process. First, the mineral-containing ore is mined, crushed and milled, after which the valuable mineral particles are separated from the waste rock or gangue. This invention is concerned with froth flotation in froth flotation cells as the means of separating the mineral from the gangue.

[0023] In broad outline, froth flotation involves the addition of a liquid, typically water, to the milled ore to form a slurry. Typically, this is done in a conditioning tank prior to introduction of the slurry to the froth flotation cell or cells. In the conditioning tank, the ore and water slurry is conditioned with conditioning chemicals, including collector and frother chemicals and typically also depressant and activator chemicals. The collector attaches preferentially to the desired mineral, rendering the mineral particles hydrophobic while leaving the gangue particles unattached or hydrophilic. The conditioned slurry of water, conditioning chemicals, hydrophobic mineral particles, and hydrophilic gangue particles is referred to as pulp or slurry.

[0024] The pulp is fed into froth flotation tanks or flotation cells in which the pulp is agitated and aerated to produce bubbles in the pulp. The hydrophobic mineral particles attach to the air bubbles, which rise to the surface, forming a mineral-rich froth which is removed from the cell. The froth eventually collapses to form a concentrate of the target mineral pulp.

[0025] Froth flotation is typically conducted in stages. The first stage is the roughing stage that produces two pulp streams, the first pulp stream being the rougher concentrate and the second pulp stream being the gangue minerals that fail to float, which are referred to as flotation tailings or tails.

[0026] In the rougher stage, the emphasis is on mineral recovery and the principal objective is to remove and recover the maximum amount of the mineral into the froth pulp as concentrate.

[0027] The rougher concentrate stream is directed to a cleaning stage in which the rougher concentrate is subjected to further flotation, typically in specialised cleaner cells. The cleaning stage is directed towards separating and rejecting the maximum amount of undesirable gangue that might have become entrained in the froth, the objective of the cleaning stage being to improve the mineral yield or grade and to produce the required concentrate mineral grade. Multiple cleaning stages might be required.

[0028] The rougher tailings pulp stream is directed to a scavenger stage, sometimes after regrinding. In the scavenger stage, the rougher tailings are subjected to further flotation. The objective is to recover any of the target minerals unrecovered during the initial rougher stage. The flotation concentrate from the scavenger stage is typically returned to the rougher feed for refloating or it may be followed by a further cleaning stage.

[0029] The final tailings exiting the scavenger stage are discharged to waste. The final tailings are typically disposed of as mine backfill or sent to tailings disposal facilities for long term storage. It will be appreciated that such discharge to waste makes the scavenger circuit an open circuit. This makes optimisation of the rougher and scavenger stages particularly important, since performance losses in these stages cannot be recovered elsewhere in the process.

[0030] Rougher and scavenger cells in current use in the mining industry are mainly mechanically-agitated froth flotation cells with a relatively low height-to- diameter ratio (less than 2). In these cells, the volume occupied by air bubbles in the pulp is relatively low, typically between 5% and 10% of pulp volume. Consequently, several cells are used to increase the total combined particle flotation cell residence time, thus increasing the probability of particle/bubble contact. In typical installations, the flotation cells are arranged, in series, in one or more banks of cells. Pulp transport through the cells is by gravitational feed, which is facilitated by arranging the cells in a vertically stepped configuration in which the tailings discharge of higher level cells serve as the pulp feed stream for each of the next following, lower-level cells. The stepped configuration results in a substantial and very high level plant installation.

[0031] The cleaning cells currently used in the mining industry are mainly mechanically agitated cells and column cells. Unlike mechanically agitated flotation cells, which tend to make use of relatively short tanks, column cells have relatively tall cells that have a large height-to-diameter ratio (from 2 to 10, depending on the application). Flotation columns seldom make use of mechanical pulp agitation and typically use air sparging systems (internal and external) which inject air into the column from a relatively low point near the base of the column. Pulp is fed into the column at a relatively high point in the column body and descends against the bubbles generated by the air sparging system. In effect, the airsparging system creates a swarm of fine bubbles that rise to the top of the column where they form a froth of bubbles. Hydrophobic particles collide with and attach to the bubbles in a collection zone (the zone between the base of the column and the interface between the pulp and the froth) and the bubbles form a stable froth in the froth or cleaning zone (the zone between the pulp/froth interface and the column overflow). The mineral-rich froth is transported by mass action through the cleaning zone and overflows over the lip of the column overflow. The overflowing froth is collected in a launder that extends circumferentially about the upper end of the column. In the launder, the froth collapses into a mineral-rich concentrate that is collected for further processing and refining of the recovered valuable mineral.

[0032] The major advantage of column cells is the high separation grade that can be achieved, which is why column cells are often used as a final concentrate cleaning step.

[0033] The froth flotation cell of the invention, as illustrated in the drawings, combines flotation column principles of operation in a relatively low height-to- diameter ratio (less than 2) cell, which is more typically found in mechanical froth flotation cells. [0034] The froth flotation cells 10 illustrated in the drawings are the first and second cells 10.1 , 10.2, respectively, in a bank of cells that might include anything from 2 to 10 cells.

[0035] The cells 10 are essentially identical and in the drawings, similar cell components are given similar numbers. Having regard to the pulp flow through the bank of cells, the first cell 10.1 is an upstream cell and the second cell 10.2 is a downstream cell. The cells 10 each include a flotation tank (1 1.1 , 1 1 .2, 1 1 .n)

[0036] The froth flotation cells 10 find particular application as rougher and scavenger cells but can be used as cleaner cells as well.

[0037] Conditioned pulp is fed into the flotation tank 1 1.1 of the first cell 10.1 by way of a pulp in-feed line constituted by a tundish feed 12 that feeds conditioned pulp into the flotation tank 1 1.1. From the first cell 10.1 , the conditioned pulp is distributed from cell to cell (from the flotation tank 1 1.1 to the flotation tank 1 1 .2 and so forth), in the bank of cells 10. Only the first downstream cell 10.2 is illustrated in the drawings. Each cell 10 is provided with a tundish, but in most installations, the tundish 12 of the first cell 10.1 will serve as the principal pulp in-feed.

[0038] Each flotation tank 1 1 is provided with first and second low-level tails discharge outfeed lines 16, 26. In each case, the first low-level tails discharge outfeed line 16 is a connecting line 16 connected to a discharge outlet located in the base 24 of the flotation tank 1 1 and the second low-level tails discharge outfeed line 26 is a connecting line 26 connected to an outlet located in the side of the flotation tank base 24.

[0039] Each cell 10 is served by a variable speed sparger recycle pump 14. The pump 14 discharges the pump output by way of a discharge line 18 that directs the pump output back into the flotation tank 1 1 of each cell 10.

[0040] Each pump 14 has at least two suction points, the first suction point is a first low-level tails discharge outfeed line 16 connected to and extending from a low- level tails discharge outlet located at the base of the flotation tank 1 1 of the cell 10 to which the pump 14 is connected. The second suction point is the second low- level tails discharge outfeed line 26 connected to and extending from the cell (10.1 ) upstream of the cell (10.2) to which the pump 14 is connected — the second low level tails discharge outfeed line 26 extending from a low-level tails discharge outlet on the side of the flotation tank 1 1.1 of the upstream cell 10.1 and positioned closely adjacent the base thereof, a third suction point can be the tundish feed 12 that feeds fresh pulp into the pump feed line 15.

[0041] The pump discharge is a combination of the fresh pulp feed from the tundish 12, the tails feed from the first low-level discharge outfeed line 16 and the tails feed from the second low level tails discharge outfeed line 26 of the upstream froth flotation cell 10, the tails feeds being recycled back into the flotation tank 1 1 of each cell 10.

[0042] The pump discharge is referred to herein as a “combination fresh and recycle feed”, since the bank of cells 10 is controlled for the recycled tails feeds to constitute the major fraction of the pump discharge to each flotation tank 1 1 .

[0043] The combination fresh and recycle feed is recycled back into the flotation tank 1 1 through external and internal sparging assemblies constituted by an air sparging system 20 located externally of the flotation tank 1 1 and a sparger system 22 located internally within the flotation tank 1 1.

[0044] In the combination fresh and recycle feed, pulp/air mixing is initiated outside the flotation tank 1 1 in the external sparging system 20, where pressurised air is injected into the recycled pulp stream upstream of a ceramic element mounted in-line within the air sparging system 20. The ceramic element reduces the size of the pressurised air bubbles in a shearing action.

[0045] The portion (18.1 ) of the discharge line 18 downstream of the air sparging system 20 conveys the aerated pulp (the pulp/air mixture exiting the air sparging system 20) to the internal sparger system 22.

[0046] In the embodiment of the invention illustrated in the drawings, the internal sparger system 22 has four internal spargers 22.1 , each having an in-line ceramic element and terminating in a combination fresh and recycle feed outlet that is partially occluded by a disperser 22.2 that disperses the pulp/air mixture sideways, transversely to the principal axis of the sparger 22.1. As a result, the outflowing pulp/air mixture is dispersed radially into the interior of the flotation tank 1 1.

[0047] The spargers 22.1 are mounted on and connected to feed pipes 22.3 that extend radially from a central feed pipe 22.4 connected to the aerated pulp discharge line 18.1. Radial mounting of the spargers 22.1 positions the spargers 22.1 midway between the centre of the flotation tank 1 1 and the peripheral wall of the flotation tank 1 1 . As a result, the pulp/air mixture is distributed throughout the cross- sectional area of the flotation tank 1 1. This creates a uniformly distributed swarm of bubbles rising up through the pulp across virtually the entire diameter of the flotation tank 1 1.

[0048] The combination of internal and external sparger assemblies 20, 22 provides for more efficient bubble generation, and also provides for improved bubble size control.

[0049] The combination of the sparger assemblies 20 and 22 means that bubble generation occurs outside and inside the flotation tank 1 1. Outside of the flotation tank 1 1 , the external air sparging system 20 is the first step of bubble generation, which occurs prior to feeding of the pulp/air stream to the internal sparger system 22. Inside the flotation tank 1 1 , the internal sparger system 22 is the second step of bubble formation prior to distribution of the aerated pulp into the flotation tank.

[0050] The radial arrangement of the internal sparger system 22 distributes the bubbles throughout the entire cross-sectional area of the flotation tank 1 1 , which is a principal enabler of reduced cell height in the flotation cell 10 of the invention.

[0051] There are no mechanical mixing mechanisms inside the flotation tank 1 1 . Bubble generation is done entirely by means of the sparger assemblies 20, 22. This makes the flotation cell 10 of the invention substantially more energy efficient than mechanical flotation cells, since mechanical agitation in conventional mechanical flotation cells is one of the major contributors to flotation cell power consumption. By obviating the need for mechanical agitation, the flotation cell of the invention has a substantially lower power draw than mechanical flotation cells, depending on sparger recycle pump speed and power drawn.

[0052] In the first (upstream) cell 10.1, the pulp fed in by way of the tundish feed 12, is recycled in a recycling process in which the pump 14 supplements the tundish feed with tails drawn from the base 24 of the flotation tank 1 1.1 of the cell 10.1 to which the pump 14 is connected, by way of the first low-level discharge outfeed line 16.

[0053] In the remaining (downstream) cells 10 (only one such cell 10.2 is illustrated in the drawings), the pulp is recycled in a recycling process in which the pump 14 supplements the tundish feed 12 with pulp tails drawn from the flotation tank 1 1 of the cell 10 to which the pump 14 is connected, by way of the first low- level discharge outfeed line 16 and pulp tails drawn from the cell (10.1 in the drawings) upstream of the cell (10.2 in the drawings) to which the pump 14 is connected, by way of the second low-level tails discharge outfeed line 26 of the upstream cell 10.1 .

[0054] In each case (first and subsequent cells 10.1 , 10.2 respectively), the combination fresh and recycle feed is returned to the flotation tank 1 1 to which the pump 14 is connected by way of the pump discharge line 18 and the external and internal sparger assemblies 20, 22.

[0055] A control valve 28 is installed in-line in the second low-level tails discharge outfeed line 26 of each upstream cell 10. The control valves 28 are installed immediately upstream of the flotation tank 1 1 of each downstream cell. The control valves 28 control the amount of pulp fed from the upstream cell (10.1 ) to the downstream cell (10.2). The principal valve control parameter, in most cases, will be the maintenance of a predetermined pulp liquid level in the flotation tank 1 1 of the cell 10 immediately upstream of each control valve 28.

[0056] To maximise pulp recycling, which is the preferred operation of the flotation cells 10 of the invention, the pumps 14 and control valves 28 are controlled for each of the cells 10 to be fed with more pulp from the base of the cell 10 to which the pump 14 is connected, compared to the amount of fresh pulp in-feed, fed either from the tundish feed 12 or from the cell immediately upstream. [0057] In practice, more than two cells 10 will be connected in series like this, in a bank of cells, with each upstream cell supplying pulp to the cell immediately downstream thereof and with the final cell discharging to a scavenging process.

[0058] To maximise pulp recycling, which is the preferred mode of operation of the flotation cells 10 of the invention, the control valves 28 in the low-level tails outfeeds 16, 26 of the cells 10 are controlled for each cell 10 to draw more of the combination fresh and recycle feed (from the first low-level tails discharge outfeed 16) than is drawn from the cell immediately upstream of that cell 10 (either from the tundish 12 or from the second low-level tails discharge outfeed line 26 of the upstream cell).

[0059] In the example illustrated in the drawings, in the upstream cell 10.1 , the control valve 28 is controlled for the cell 10.1 to be fed with more of the combination fresh and recycle feed compared to the amount of fresh pulp in-feed from the tundish 12. In the downstream cells 10, the control valve 28 is controlled for the cell to be fed with more of the combination fresh and recycle feed drawn from the flotation tank of that cell 10, compared to the amount of tails drawn from the cell 10 immediately upstream thereof.

[0060] In this control arrangement, the primary determinant of the liquid level in the cell 10 is the quantity of pulp fed in by way of the pulp in-feed line constituted by the tundish 12. The control valve 28 regulates the liquid level in the cell 10 by balancing the in-feed against the quantity of tails distributed from the cell 10 to the cell 10 immediately downstream thereof. The primary determinant of the liquid level in the downstream cell 10 is the quantity of tails fed into the flotation tank of the downstream cell 10 from the upstream cell 10 by way of the second low level tails discharge outfeed line 26.

[0061] The control valve 28 of the downstream cell 10 regulates the liquid level in the downstream cell 10 by balancing the in-feed from the second low level tails discharge outfeed line 26 against the quantity of tails distributed from that cell 10 to the next following downstream cell 10.

[0062] The control valves 28 of all the cells 10 in the cell bank are managed and controlled to maximise recycling and to maintain predetermined pulp liquid levels within the cells 10.

[0063] The first low-level tails discharge outfeed line 16 of each cell 10 is provided with a knife gate 29 that is typically preset to provide a predetermined degree of throttling in the outfeed line 16. If finer control of pulp liquid levels and pulp recycling is required, the knife gate 29 can be replaced with a secondary control valve, which can be controlled in tandem with the control valve 28, such that either or both the control valve 28 and the secondary control valve (installed in replacement of the knife gate 29) can be used to control recycling and pulp liquid levels in the cells 10.

[0064] In a conventional mechanical froth flotation plant, liquid levels are maintained by means of gravitational feed across the stepped configuration of the flotation cells in the cell bank. In a froth flotation plant using the froth flotation cells of the invention, the cells 10 are installed side-by-side, on the same level and instead of the gravity feed of conventional mechanical froth flotation plant, the flotation cells 10 of the invention make use of the pumps 14 and process logic controllers (PLCs) to maintain pulp levels within the cells 10.

[0065] With the cells 10 being on the same level, the pumps 14 create a virtual gravitational feed in that the net positive suction head (NPSH) of the pumps 14 creates a feed suction which draws the pulp and tails from cell to cell, thereby negating the necessity for a stepped plant design.

[0066] The froth flotation cell of the invention preferably includes one of a number of internal launders. In essence, the internal launder in a froth flotation cell serves as the final separator of the froth, which contains the valuable minerals, from the pulp, for further processing and recovery of the valuable minerals from the froth.

[0067] Internal launders are typically configured as a trough or channel located inside the flotation cell and positioned near the top of the flotation tank. The internal launderallows the froth to rise up and overflow into the launderchannel, from where it discharges gravitationally into a peripherally extending launder overflow that drains gravitationally to pulp discharge outlet. [0068] Variations in the internal launder geometry are generally implemented to suit the specific requirements of the flotation process and the properties of the ore being processed. The size, shape, and inclination of the launder can influence the froth retention time, froth transport, and froth overflow rate, which, in turn, can impact the recovery and grade of the flotation process.

[0069] A number of different internal launder designs are illustrated in Figures 7 to 12.

[0070] The internal launder 30.1 illustrated in Figures 7 and 8 is a simple 2-way launder with a single transverse channel 32.1 that slopes from the centre of the flotation tank to the periphery, to enable gravitational discharge of the froth from discharge outlets 34.1 that open out into a peripherally extending launder overflow 36.1.

[0071] The internal launder 30.2 illustrated in Figures 9 and 10 is a 4-way radial launder with a pair of intersecting transverse channels 32.2 that slope from the centre of the flotation tank to the periphery, to enable gravitational discharge of the froth from discharge outlets 34.2 that open out into a peripherally extending launder overflow 36.2.

[0072] The internal launder 30.3 illustrated in Figures 1 1 and 12 is an 8-way radial launder with intersecting transverse channels 32.3 that slope from the centre of the flotation tank to the periphery, to enable gravitational discharge of the froth from discharge outlets 34.3 that open out into a peripherally extending launder overflow 36.3.

[0073] The froth flotation cell of the invention preferably includes a froth wash water system. An example of such a system is illustrated in Figures 13 to 19.

[0074] The froth wash water system improves the stability and cleanliness of the froth on the surface of the flotation cell, by washing through the froth and reducing the amount of non-valuable gangue material that may be carried over, from the pulp into the mineral-rich froth. Through the introduction of wash water by means of the froth wash water system, the gangue material is washed out of the froth and drops back into the pulp and the froth is conditioned to flow more easily into the internal launders, leading to higher mineral recovery and improved concentrate grade.

[0075] As illustrated in Figures 13 to 19, the preferred froth wash water system includes a froth wash water delivery system constituted by a wash water feed tank 37 that distributes water supplied by means of a wash water feed (not shown) onto a perforated plate 38 suspended above the upper edge of the flotation tank 1 1 by means of a wash water system superstructure or tank 40. In operation, the perforated plate 38 distributes the water fed from the feed tank 37 in a fine droplet spray over the surface of the froth emerging from the top of the flotation tank 1 1. The wash water flow rate can be adjusted to optimise washing efficiency and froth stability. The froth wash water system is particularly useful in situations where there are high levels of gangue or other unwanted minerals that have a tendency to report to the froth. By introducing clean water to the froth, the entrainment of these unwanted materials is reduced, allowing for better separation and improved concentrate quality.

[0076] Froth scrapers are often used in flotation cells to push (rather than scrape) the froth over the edges of the internal launders. A froth scraper ensures continuous and controlled removal of the froth.

[0077] The froth scraper assembly preferred for use with the froth flotation cell of the invention consists of a curved paddle 42 rotatably mounted in a scraper assembly 44 located on top of the froth flotation cell structure. The paddle is rotatably driven by means of a scraper motor and gearbox 46 housed in the scraper assembly 44. The scraper paddle 42 is placed just above the froth layer, and its movement is designed to skim or push the froth from the cell's surface towards the froth collection launder.

[0078] The cells 10 of the invention are substantially more efficient than mechanical rougher cells and it is suggested that the massive cell banks of a conventional rougher plant, typically consisting of 20 or more mechanical cells, could be replaced by a bank of no more than 7 or 8 of the cells 10 of the invention.