WO2003089148A1

WO2003089148A1 - Three product cyclone

Info

Publication number: WO2003089148A1
Application number: PCT/AU2003/000446
Authority: WO
Inventors: Daniel Philip Obeng; Stephen Morrell
Original assignee: The University Of Queensland
Priority date: 2002-04-15
Filing date: 2003-04-15
Publication date: 2003-10-30
Also published as: AUPS173302A0

Abstract

A cyclone, comprising: - a body having a longitudinal axis defining an interior space, the body including a top wall, a cylindrical upper side wall portion defining an inlet transverse to the longitudinal axis, and an adjacent conical lower side wall portion tapering inwardly away from the top wall, and the body further including a spigot remote from the top wall defining an underflow outlet directed parallel to the longitudinal axis; - an inner vortex finder projecting in through the top wall of the body and defining an inner overflow outlet at its open end; - an outer vortex finder projecting in through the top wall of the body and circumferentially surrounding the inner vortex finder and defining an outer overflow outlet at its open end; wherein the length that the inner vortex finder extends into the body along the longitudinal axis is greater than that for the outer vortex finder.

Description

THREE PRODUCT CYCLONE

This invention relates to a three product cyclone. This invention relates particularly to a hydro cyclone that is a three product cyclone used for the classification of a particulate material, eg in a mineral beneficiation plant. It will therefore be convenient to hereinafter describe the invention with reference to this example application. However it is to be clearly understood that it is capable of broader application. In this specification the term "cyclone" is to be interpreted broadly and specifically to include within its scope "hydro cyclones" which shall be deemed to be a subset of the term "cyclones". Further In this specification the term three product cyclone shall be understood to mean a cyclone that has three outlet or product streams, eg an underflow outlet stream and two overflow outlet streams.

BACKGROUND TO THE INVENTION

Hydrocyclones have been widely used in the mineral beneficiation industry for many years. Generally a hydrocyclone comprises a body having a top wall, a side wall comprising an upper cylindrical side wall portion defining a cylindrical body portion and a lower conical side wall portion defining a conical body portion and a spigot at the bottom of the conical portion, and a vortex finder projecting into the body through the top wall. A tangential inlet is defined in the upper side wall portion. An axially extending underflow outlet is defined in the spigot at the bottom of the body and an overflow outlet is defined in the vortex finder that projects into the body of the cyclone.

A feed stream, eg of slurry containing particulate material to be classified, is passed into the body of the cyclone through the tangential inlet. The shape of the body induces the fluid to flow in a helically spiralling flow pattern. It moves down a radially outer region of the cyclone and then changes direction and thereafter spirals upwardly through a radially inner region of the body of the cyclone. The spiral flow pattern applies a centrifugal force to particles entrained within the fluid. The extent of the force depends on the size of a particular particle and the specific gravity of the particle. Heavier and/or relatively larger particles are radially displaced towards the radially outer region from where they are passed out through the underflow outlet. Lighter and/or smaller particles tend to gravitate towards the radially inner region where the fluid flows upwardly in spiral fashion towards the outlet with a central air core. This results in these smaller particles being carried upwardly and out through the overflow outlet. This is how the cyclone effects a separation of particles on the basis of size and density.

The cyclone may be used to separate a valuable mineral product from a waste or tailings product. The cyclone may also be used to separate a particular size fraction of material from a stream containing a broad range of particle sizes. However one shortcoming of hydrocyclones is that fine dense particles which should preferably be passed out through the overflow outlet defined in the vortex finder are instead passed out through the underflow outlet. In many applications this results in these particles being recycled to a milling or grinding operation and from there being passed through the cyclone once again. This can result in overgrinding which can detract from the efficiency of downstream processing operations. Further recycling this material to the milling operation adds to the already high grinding cost and this can adversely affect the economics of mineral processing operations.

Some efforts have been made to solve this problem but they have not produced satisfactory results. One such effort involves washing the slurry containing the fines with water sprays in the lower conical portion of the body. However the location of the sprays in the cyclone and the water spray causes flow disturbances that adversely affect the performance of the cyclone to the point where it is unacceptable. Another effort involves passing the underflow outlet stream through a flash flotation cell to try and separate out the fine dense particles before this stream is passed to the mill. However this stream contains a large proportion of particles not suitable for flash flotation and therefore this is not an elegant solution. Further it is often necessary to add water to the stream before it is passed through the flash flotation cell.

Clearly therefore it would be advantageous if a way could be found of recovering the fine dense particles being passed out through the underflow outlet. It would also be particularly advantageous if these fine dense minerals could be captured in a stream separate from the overflow outlet. This would have the potential to significantly increase the efficiency of the milling and classification process. SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided a cyclone for treating a fluid stream containing entrained particles, the cyclone comprising: a body having a longitudinal axis defining an interior space, the body including a top wall, a cylindrical upper side wall portion defining an inlet directed transverse to the longitudinal axis, and an adjacent conical lower side wall portion tapering inwardly in a direction away from the top wall, and the body further including a spigot remote from the top wall defining an underflow outlet directed parallel to the longitudinal axis; an inner vortex finder projecting in through the top wall of the body and defining an inner overflow outlet at its open end; an outer vortex finder projecting in through the top wall of the body and circumferentially surrounding the inner vortex finder and defining an outer overflow outlet at its open end; wherein the length that the inner vortex finder extends into the body along the longitudinal axis is greater than that for the outer vortex finder.

In many applications the cyclone may be a hydro cyclone, ie containing a slurry with particles entrained in a liquid, eg water.

It has been found that this arrangement of structural features results in middlings stream, eg of fine dense particles taken up by the inner vortex finder which can then be treated separately or recombined with the stream in the outer overflow outlet. The underflow outlet stream obtained with this arrangement is similar to that obtained with a conventional cyclone with the exception that it does not contain many fine dense particles. These have been taken up in the middlings stream. The stream passed through the outer overflow outlet defined by the outer vortex finder is very similar to that contained in the overflow outlet stream in a conventional prior art cyclone.

The spigot may be of reduced diameter relative to that for a conventional hydro cyclone. The spigot may have a diameter and corresponding underflow outlet diameter at least 10% less than that for a conventional cyclone, preferably at least

20% less than that for a conventional cyclone. Most preferably the spigot diameter is

55 to 70% of the diameter of a spigot in a conventional prior art cyclone. Conceptually the spigot diameter that is most preferred is the maximum spigot diameter that would give roping conditions if it were used on a conventional cyclone. It is preferable for the spigot to be of a size that would cause roping in a conventional cyclone. At the same time it is desirable for the flow through the cyclone to be as large as possible to maximise throughput. The size of the spigot that meets these criteria depends on a number of factors such as feed composition, the type of particulate material in the feed, and the particle size distribution of the feed material.

In a conventional cyclone a spigot of reduced diameter causes roping. This is a condition where the particles entrained in the fluid passing through the body of the cyclone are held up and congest towards a lower end of the body. This adversely affects the fluid flow pattern in the body and reduces the efficiency of the cyclone. Applicant has discovered that a three product cyclone having both inner and outer vortex finders and a spigot of reduced size as described above does not undergo roping. This is because the surplus material is drawn off by the IVF.

The length of the inner vortex finder may be 10 to 50% more than the length of the outer vortex finder. In the specification the length of the inner and outer vortex finders shall be measured from the point that the vortex finder enters the body through the top wall and extend in the direction of the longitudinal axis. The extent to which the inner vortex finder exceeds the length of the outer vortex finder depends to some extent on the application to which the cyclone is being put. The length of the outer vortex finder may conveniently be substantially the same as that for the vortex finder used in conventional prior art cyclones.

The inner vortex finder may conveniently be defined as a percentage of the height or length of the body, including both the cylindrical wall portion and the conical wall portion.

In an application where it is desired to separate a coarse middlings stream the IVF may be 60 to 69 % of the body length, preferably 70 to 85 % of the body length.

In an application where it is desired to separate out a slimes stream the IVF may be 20 to 30 % of the body length.

Thus the size distribution of the stream of particles passed into the inner overflow outlet through the inner vortex finder depends on the length of the inner vortex finder. The shorter the inner vortex finder the finer will be the fraction of particles passed through the inner vortex finder. This arrangement would be suitable for the separation of slimes. The longer the vortex finder the broader the size distribution within the vortex finder, eg it includes coarse larger particles in addition to fines. The diameter of the inner vortex finder may be 60 to 90% of the diameter of the outer vortex finder preferably 70 to 80% of the diameter thereof. A certain working clearance is required between the inner and the outer vortex finders.

According to another aspect of this invention there is provided a method of treating a stream of particulate material, the method including passing a feed stream through a three product cyclone as described above and optionally comprising one or more of the following steps, passing the inner overflow outlet stream to a further processing unit; passing the outer overflow outlet stream to a flotation treatment plant; and passing the underflow outlet stream to a milling or grinding plant.

The further processing plant may be a flash flotation cell. This would separate concentrate from tailings. The concentrate can then be combined with a product stream and the tails can be returned to the milling or grinding plant.

Alternatively the further processing plant may be a micro-screen for separating undersize from oversize particles. The undersize particles are recombined with the stream from the outer overflow outlet. The oversize particles are returned to the milling or grinding plant.

Further alternatively the inner overflow outlet stream may be recombined with the outer overflow outlet stream and the combined stream can be passed through to a flotation treatment plant.

A further related aspect of the invention is concerned with a system for treating a stream of particulate material, the system comprising a cyclone as described above in combination with one or more of the following, a further processing unit, preferably comprising a flash flotation cell and/or a micro-screen; a flotation treatment plant and/or a milling or grinding plant.

A further related aspect of the invention is concerned with a stream of particulate material obtainable by treatment of a fluid stream according to the method as described above. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A hydro cyclone in accordance with this invention may manifest itself in a variety of forms. It will be convenient to hereinafter describe in detail at least one preferred embodiment of the invention with reference to the accompanying drawings. The purpose of providing this detailed description is to instruct persons having an interest in the subject matter of the invention how to carry the invention into practical effect. It is to be clearly understood however that the specific nature of this detailed description does not supersede the generality of the preceding broad description. In the drawings:

Figure 1 is a schematic view of a three product cyclone in accordance with the invention;

Figure 2 is a schematic view of part of a three product cyclone showing the particle separation effect within the cyclone; Figure 3 is a schematic flow sheet of a process incorporating the three product cyclone;

Figure 4 is a schematic flow sheet of a further process incorporating the three product cyclone;

Figure 5 is a schematic flow sheet of an experimental process used to test the efficacy of the three product cyclone;

Figures 6A and 6B are graphs showing the size distribution of particulate material, 6A for magnetite, 6B for silica, reporting to the inner overflow outlet in the three product cyclone and the underflow outlet in a conventional cyclone;

Figures 7A and 7B are graphs showing the size distribution of mineral, 7A for magnetite, 7B for silica, in respectively the outer overflow outlet of the three product cyclone and the overflow outlet of a conventional cyclone;

Figure 8 is a graph plotting the size distribution of particulate material reporting to the inner overflow outlet and outer overflow outlet as a function of the length of the inner vortex finder, i.e. showing the effect of crossover and IVF lengths on size of mineral in INO and OUO;

Figure 9 is a graph of the fraction of solids reporting to the various outlets as a function of the length of the inner vortex finder, i.e. showing the effect of IVF length on percent solids in product streams; and Figures 10A and 10B are graphs showing corrected efficiency curves for separation of respectively magnetite and silica.

In figure 1 reference numeral 1 refers generally to a cyclone in accordance with the invention. Hereinafter this will be referred to as a three product cyclone. Broadly the cyclone 1 comprises a body 2 having a top wall 3 and a side wall that comprises an upper cylindrical side wall portion 4 and a lower conical side wall portion 5. A feed inlet 6 is defined in the upper side wall portion 4 extending transverse to the longitudinal axis, represented by a line 7 in figure 1 , of the cyclone 1. A spigot 8 is positioned beneath the conical side wall portion 5 adjacent thereto. An underflow U outlet 9 is defined in the spigot 8. The cyclone 1 also includes an inner vortex finder (IVF) 10 extending down into the body 2 through the top wall 3 and defining an inner overflow outlet 11 at its open end. The cyclone 1 also includes an outer vortex finder (OVF) 12 circumferentially surrounding the inner vortex finder 10 projecting into the body 2 and defining an outer overflow outlet 13 at its open end. The inner vortex finder 10 extends down into the body 2 in the direction of the longitudinal axis 7 quite a bit further than the outer vortex finder 12. This is an important feature and will hereinafter be referred to as the length differential, or crossover length, of the vortex finders 10 and 12.

In the illustrated embodiment, the cyclone has^" a diameter of 150mm: and a cylinder length of 230mm. The cone portion has a length of 250-600mm. The spigot has a diameter of 25mm. The outer vortex finder has a diameter of 60mm and extends a length of 115mm into the body through the top wall. The inner vortex finder has a diameter of 50mm and extends downwardly into the body a distance of 40-490mm beyond the end of the outer vortex finder. These dimensions are merely example dimensions and other dimensions could also be used.

In use, feed slurry containing particulate material to be classified is introduced to the body through the inlet 6. The slurry is caused to flow in the form of a helical spiral down the side wall portions 4 and 5 of the body 2. This sets up a typical fluid flow pattern through the cyclone 1 including an outer spiral, an inner spiral and a middle spiral. An air core develops along the vertical axis of the body and moves air upwardly from the spigot 8 to the inner vortex finder 10. Each particle within the fluid is subjected to an outward centrifugal force due to the circular movement and an inwardly directed drag force. This determines its broad radial position within the body and which of the outer, inner and middlings spirals it gravitates to.

Fine light particles are dominated by the inward drag forces. Therefore they move into the inner spiral and are swept into the inner or outer vortex finders 10, 12. On the other hand the large dense particles are dominated by the centrifugal forces and tend to get displaced into the outer spiral which moves them towards the spigot 8 and the associated underflow outlet 9.

A third group of particles including those with large light particles and fine dense particles have similar centrifugal and drag forces acting on them and thus form part of the middle spiral between the inner and the outer spirals.

Arrows marked INO, OUO and U denote respectively an inner overflow outlet, associated with the inner vortex finder 10, an outer overflow outlet, associated with the outer vortex finder 12, and an underflow, associated with the underflow outlet 9.

Referring now to figure 2 the spigot 8 has reduced size which limits the flow of fluid therethrough. This tends to cause particle crowding which causes hindered settling conditions. The flow of slurry in the outer spiral near the conical wall portion

5 is impeded by particle crowding. In the region indicated by arrow A this leads to stratification of the large dense particles which then serve as a carrier for the bed of middling particles. In the region indicated by arrow B the fluid flow is much stronger than in region A and this exposes the middlings that are swept up into the inner vortex finder through the inner overflow outlet. In the region indicated by arrow C the fines that have not been drawn into the inner vortex finder are dragged towards the air core and into the inner vortex finder 10. This leaves the heavy particles to pass out of the underflow outlet 9. Thus light fine particles LFP are passed out of the outer overflow outlet 13 as per standard cyclones. Large dense particles LDP are passed out of the underflow outlet 9 as per standard cyclones. The middlings stream MS of fine dense particles is passed out through the inner overflow outlet formed by the inner vortex finder. It may then be treated alone before being returned to the product stream or it may be recombined with the fines stream before being treated, eg in a flotation plant.

Figure 3 illustrates an example application of a three product cyclone.

The process comprises a cyclone 1 of the type described above with reference to figures 1 and 2 through which a feed slurry FS is passed. The heavy particles HP pass out of the underflow outlet 9 from where they are returned to the mill. The fines are passed out of the outer overflow outlet 13 and from there are passed to a flotation plant FP. Middlings MS are passed out through the inner overflow outlet 11 and from there passed to a flash flotation cell 25. This produces a concentrate C that can be passed to the product stream, and a tailings stream TS which is returned to the mill. This process thereby avoids unnecessary recycling of concentrate and also reduces over grinding of fine dense particles.

Figure 4 illustrates a process that is a variation on that shown in figure 3, the three-product cyclone with an external micro-screen. This description will be confined to the differences between the two processes.

Figure 4 has a micro-screen 27 instead of a flash flotation cell 25. The micro- screen 27 screens the middlings stream MS on the basis of size. The oversize particles OP are returned to the mill while the under size particles US are sent to a flotation plant FP together with the fines stream in the outer overflow outlet OUO. Again this reduces unnecessary over grinding of fine dense particles.

Figure 5 illustrates an experimental apparatus used in experiments to test the performance of a three product cyclone.

The apparatus comprises broadly a 150mm hydro cyclone 50, a variable speed Warman pump, 52, driven by a 15 kW motor and a baffled sump 54. The average inlet pressure of slurry was 105 kPa.

The slurry is agitated by a mechanical agitator 56 in the sump 54.

It is also assisted by the turbulence created by slurry returning to the sump 54 from the cyclone 50 ensuring homogenous suspension of the slurry in the sump. Persons skilled in the art would be familiar with this apparatus and it is not necessary to describe it further.

Full details of the 150mm three product cyclone used in the test work are provided in the table below.

The cyclone feed used was a mixture of magnetite of about 18% and silica of about 82%. The size distributions of the two minerals in the feed are shown in the table below.

The data obtained for test work on the three product cyclone was compared with data obtained from a standard cyclone. The standard cyclone had a diameter of 150mm and either a 25mm spigot or a 35mm spigot. The Applicant's experimental work clearly shows that a suitable middlings stream containing the fine dense particles in the feed stream can be received in the inner overflow outlet and selectively concentrated for further treatment. The stream is often suitable for flash flotation in a flash flotation cell independently of the other outlet streams.

The outer overflow outlet contains a stream having properties similar to those of an overflow outlet of a conventional cyclone. Thus the stream in the outer overflow outlet can for example be passed directly to a flotation plant where it is treated in the same way as the overflow of a conventional cyclone. It does not require any modifications to be made to existing flotation circuits designed for use with conventional cyclones.

Figure 6 shows the size distribution of respectively magnetite 6A and silica 6B in each of the inner vortex finder and inner overflow outlet and the underflow of a conventional cyclone. The size distribution of both magnetite and silica in the inner vortex finder becomes coarser as the IVF length increases. The size distribution of each of the magnetite and silica in the IVF or inner overflow outlet was substantially finer than that of the corresponding mineral in the underflow of the conventional cyclone.

Figure 7 shows the size distribution of respectively magnetite 7A and silica 7B in each of the OVF and outer overflow outlet of the 3 product cyclone and the overflow outlet of a conventional cyclone.

The results show that the size distribution of respectively magnetite and silica in the OVF becomes finer with increasing length of the IVF. The effect is pronounced in the case of silica. This is because with an increase in IVF length, the flow passing into the OVF is dominated by the inner spiral of fluid within the body of the cyclone which contains the very fine particles.

The conventional cyclone was fitted with a 25mm spigot for some experiments and a 35mm spigot for other experiments. The size distribution in the overflow outlet was noticeably coarser with the 25mm spigot than the 35mm spigot. This is due to roping conditions which are caused by the small spigot. A conventional cyclone would invariably be provided with a 35mm spigot to avoid these roping conditions and ensure a spray underflow discharge. With this 35mm spigot size the size distribution of magnetite in the overflow outlet was similar to that of magnetite in the outer overflow outlet of the three product cyclone.

Figure 8 shows the variation in particle size distribution in each of the inner and outer vortex finders as a function of the length differential between the inner and outer vortex finders.

With an increase in differential length, the particle distribution in the inner vortex finder became coarser than that in the outer vortex finder. At a length difference of about 100mm the lines cross over and the particle size distributions are about the same. However at a length differential of about 400-500mm there is a pronounced difference between the distribution in the IVF and the OVF.

In summary the difference in the particle size distribution between the two streams increases with increasing IVF length beyond the crossover point.

Figure 9 illustrates the influence of IVF length on the fraction or percentage of solids as distinct from liquid in the inner and outer vortex finder streams. These in turn are compared with the fraction of solids in the overflow outlet of a conventional cyclone. Applicant found that the solids concentration in the IVF increases with increasing IVF length. The concentration of solids in the OVF varies slightly with change in IVF length but was similar to that in the overflow outlet of a conventional cyclone. Figure 10 shows the efficiency curves for magnetite 10A and silica 10B for respectively the three product and conventional cyclones.

The table below gives the water recovery (R_f) and mineral splits to the underflow.

Note: The 25mm spigot gave a roping underflow in the conventional cyclone.

To evaluate the performance of the three-product and conventional cyclones, the actual efficiency to the underflow (E_ua) for each size fraction was calculated conventionally from knowledge of the flows and size distribution. To remove the effect of bypass, the corrected efficiency to underflow (E_uc) was determined using Kelsall's (1953) formula:

where: R = percent recovery of water to underflow

The corrected efficiency curve was subsequently converted to the reduced efficiency (E_ur) curve by fitting the Whiten classification function (Lynch, 1977):

where: a = reduced efficiency curve sharpness parameter d₅₀c = corrected cut-size for bypass (mm) dj = mean particle size (mm)

As expected, the conventional cyclone with the 35mm spigot cuts finer than the one with a 25-mm spigot. As mentioned earlier, the former configuration is likely to be used in practice so as to avoid underflow roping.

From the results in figures 10A and 10B, it can be seen that with the 40mm IVF, the three-product cyclone cuts finer and sharper than the conventional unit. With the 185mm IVF, the unit cuts similar to the conventional cyclone with 35mm spigot. The results show that with the 185mm IVF, water recovery and magnetite split to the underflow were respectively 21 % and 8% lower than those to the underflow of the conventional cyclone with 35mm spigot. These results indicate that fewer magnetite particles suitable for the next processing stage reported to the three- product cyclone underflow than the conventional underflow and point to the potential reduction in over grinding that might occur in a closed grinding circuit.

With a further increase in IVF length, figures 10A and 10B show that the cut- size increased, resulting in a decrease in sharpness of cut. This confirms the result that increasing the IVF length increases the amount of middling particles, captured by the IVF. Note also that^" water recovery and magnetite split to the three-product cyclone underflow generally decreased with increasing IVF length.

Applicant believes that the use of a three product cyclone as described above has the potential to overcome the longstanding problem of hydro cyclones namely of fine dense liberated minerals reporting to the underflow and being recirculated to the grinding circuit. The fine dense mineral is taken off as a separate stream that can be passed directly through to the concentrate or processed as separate stream.

One advantage of the three product cyclone is that it is mechanically fairly simple. All it requires is an additional vortex finder within the "existing" or outer vortex finder. Such a cyclone is not difficult to fabricate or build. Further the particle size profile within the middlings stream passed through the inner vortex finder can be varied by simply varying the length of the inner vortex finder. Thus it can be adjusted by adjusting only one design parameter and is therefore fairly simple to achieve in practice. A further advantage of this invention is that it has a huge potential application. Most mineral processing plants, eg for concentrating an ore, have comminution circuits for reducing the size of the ore particles to liberate the valuable ore and separate it from the gangue or tailings. These circuits generally use hydro cyclones to help in classifying the particles and directing the different size fractions to different operations. A breakthrough in solving this problem would be very significant for the mineral processing industry.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as is herein set forth.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A cyclone for treating a fluid stream containing entrained particles, the cyclone comprising: - a body having a longitudinal axis defining an interior space, the body including a top wall, a cylindrical upper side wall portion defining an inlet directed transverse to the longitudinal axis, and an adjacent conical lower side wall portion tapering inwardly in a direction away from the top wall, and the body further including a spigot remote from the top wall defining an underflow outlet directed parallel to the longitudinal axis;

- an inner vortex finder projecting in through the top wall of the body and defining an inner overflow outlet at its open end;

- an outer vortex finder projecting in through the top wall of the body and circumferentially surrounding the inner vortex finder and defining an outer overflow outlet at its open end; wherein the length that the inner vortex finder extends into the body along the longitudinal axis is greater than that for the outer vortex finder.

2. Cyclone according to claim 1, wherein the spigot is of reduced diameter relative to that for a conventional hydro -cyclone.

3. Cyclone according to claim 1 or claim 2, wherein the spigot has a diameter and corresponding underflow outlet diameter at least 10% less than that for a conventional cyclone, preferably at least 20% less than that for a conventional cyclone, whereby most preferably the spigot diameter is 55 to 70% of the diameter of a spigot in a conventional cyclone.

4. Cyclone according to any of the preceding claims, wherein the spigot diameter is the maximum spigot diameter able to provide roping conditions on a conventional cyclone.

5. Cyclone according to any of the preceding claims, having a size able to cause roping in a conventional cyclone, said size enabling flow through the cyclone to be as large as possible in order to maximise throughput.

6. Cyclone according to any of the preceding claims, wherein the length of the inner vortex finder is 10 to 50% greater than the length of the outer vortex finder.

7. Cyclone according to any of the preceding claims, wherein the length of the outer vortex finder is substantially the same as that for a conventional cyclone vortex finder.

8. Cyclone according to any of the preceding claims, wherein the inner vortex finder is 60 to 69% of the body length, preferably 70 to 85% of the body length.

9. Cyclone according to any of the preceding claims 1-7, wherein the inner vortex finder is 20 to 30% of the body length.

10. Cyclone according to any of the preceding claims, wherein the diameter of the inner vortex finder is 60 to 90% of the diameter of the outer vortex finder preferably 70 to 80% of the diameter thereof.

11. Method of treating a stream of particulate material, the method comprising the steps of:

- passing a feed stream through a three product cyclone according to any of the preceding claims 1-10; and optionally comprising one or more of the following steps: - passing an inner overflow outlet stream emitting from the cyclone to a further processing unit;

- passing an outer overflow outlet stream emitting from the cyclone to a flotation treatment plant; and

- passing an underflow outlet stream emitting from the cyclone to a milling or grinding plant.

12. Method according to claim 11, wherein a concentrate is separated from tailings.

13. Method according to claim 12, wherein the concentrate is combined with a product stream and the tailings are returned to the milling or grinding plant.

14. Method according to any of the claims 11-13, wherein undersize particles are separated from oversize particles.

15. Method according to claim 14, wherein the undersize particles are recombined with the stream from the outer overflow outlet and/or the oversize particles are returned to the milling or grinding plant.

16. Method according to any of the preceding claims 11-15, wherein the inner overflow outlet stream is recombined with the outer overflow stream and the combined stream is passed through to a flotation treatment plant.

17. System for treating a stream of particulate material comprising a cyclone according to any of the claims 1-10, in combination with one or more of the following:

- a further processing unit, - a flotation treatment plant and/or

- a milling or grinding plant.

18. System according to claim 17, wherein the further processing unit comprises a flash flotation cell and/or a micro-screen.

19. System according to any of the claims 16-18, for use in the method according to any of the claims 11-15.

20. A stream of particulate material obtainable by treatment of a fluid stream according to the method of any of the claims 11-16.