WO2008064406A1

WO2008064406A1 - Method of operating an inclined plate classifier

Info

Publication number: WO2008064406A1
Application number: PCT/AU2007/001817
Authority: WO
Inventors: Kevin Patrick Galvin
Original assignee: Newcastle Innovation Limited
Priority date: 2006-11-27
Filing date: 2007-11-27
Publication date: 2008-06-05

Abstract

A method of operating an inclined plate classifier having an array of parallel inclined plates (4) within a chamber (1), defining inclined channels (5) between the plates, involves feeding feed material of varying density particles between the plates and providing re-suspension of lower density particles as the feed material moves through the inclined channels (5). Re-suspension is effected either by imposing vibration on the inclined plates by a vibrator (15) or by introducing bubbles between the plates to increase the shear rate in the inclined channels. The method works either for dry processing where the particles are fluidised by a gas, or for wet processing typically using bubbles.

Description

METHOD OF OPERATING AN INCLINED PLATE CLASSIFIER

FIELD OF THE INVENTION

This invention relates to a method of operating an inclined plate classifier similar to the type disclosed and described as a Reflux Classifier in our earlier International Patent Application PCT/AUOO/00058 entitled "A Reflux Classifier".

BACKGROUND OF THE INVENTION

We describe here both a device and method for achieving the separation, recovery, and concentration of a material loosely defined as particles. These particles may be solid, liquid, or gaseous entities of any size. Often, the material of interest consists of more than one of these entities, and may even consist of all these entities.

While the Reflux Classifier described in PCT/AUOO/00058 is effective in classifying many forms of particles according to their size, and sometimes according to their density, there are limitations associated with the combination of the fluid and particles to be used. It is an object of the present invention to provide an improved method of operating an inclined plate classifier that will enhance the separations achieved in the device, and in turn overcome limitations associated with the combination of the fluid and particles to be used.

SUMMARY OF THE INVENTION Accordingly in one aspect the present invention provides a method of operating an inclined plate classifier of the type having an array of substantially parallel inclined plates in a chamber containing a fluid, said method including the steps of feeding feed material to be classified between the plates and providing re-suspension of lower density particles as the feed material moves through the inclined channels between the plates. In one form of the invention the re-suspension of lower density particles is provided by imposing vibration on the inclined plates.

In an alternative form of the invention the re-suspension of lower density particles is provided by introducing bubbles between the plates to increase the shear rate in the inclined channels.

In one form of the invention the bubbles are introduced with the feed material as a bubbly flow mixture.

Alternatively, or in addition to the above, the bubbles are introduced into the chamber below the array of inclined plates. In one form of the invention the method includes the step of feeding a feed material containing mixed particles of higher and lower densities into the upper region of the array of inclined plates.

Preferably overflow material containing lower density particles is removed from the chamber by a launder adjacent the upper edges of the inclined plates. Preferably underflow material containing higher density particles is removed from the bottom of the chamber.

In a further aspect the present invention provides an inclined plate classifier for separating particles of relatively higher and lower densities, said apparatus having an array of substantially parallel inclined plates located in a chamber adapted to contain a fluid, means for providing re-suspension of lower density particles between the plates, an overflow launder adjacent the upper edges of the inclined plates, and an outlet in the lower region of the chamber.

In one form of the invention the means for providing re-suspension of lower density particles includes means for imposing vibration on the inclined plates. This vibration may be imposed on the plates alone, or preferably on the entire chamber including the inclined plates and the lower region of the chamber.

In another form of the invention the means for providing re-suspension of lower density particles includes means for introducing bubbles between the plates. This re-suspension may arise through the effects of a shear induced lift force or through bubble-particle attachment or both.

In one form of the invention the apparatus includes a feed chute arranged to feed material containing mixed particles of higher and lower densities into the upper region of the array of inclined plates. The device used to perform the invention may be a Reflux Classifier of the type described in our earlier International Patent Application PCT/AUOO/00058, or some variation of a Reflux Classifier, in which a single or plurality of inclined channels is used to contain and or convey a dispersion of some kind. The zone below the inclined channels provides a zone to collect, and contain an inventory of particles, and also for conveying the particles into the inclined channels. In some situations it has been found necessary to direct the feed material onto the inclined channels, and permit that feed to become distributed across each of the channels. This feed entry is easily achieved using a feed chute that intersects all of the inclined channels. Upon exiting the chute the feed undergoes a degree of lateral movement due to the feed that continues to enter and due to the upward flow of material through the inclined channels. The inclined channels are made sufficiently long that the feed material can achieve a sufficient level of distribution. On larger systems, there may be a plurality of chutes, ideally equally spaced across the device. With the alternative method of feed entry below the channels some particles may be too large and heavy to be conveyed into the channels. The addition of the feed onto the inclined channels ensures that all particles have the opportunity to experience the influence of the inclined channels.

A further adaptation, involves the additional inclination of the inclined channels so that lateral particle movement across the channels is promoted. This adaptation is optional. This additional inclination would normally only be slight, and might involve more than one direction of inclination. For example, with a central feed chute, the inclined surfaces may be angled slightly to the left of the chute, and angled the opposite way to the right of the chute. More significant levels of inclination may be required for some systems. Where the feed enters from below the inclined channels, as in the Reflux

Classifier, some particles may prove to be difficult to convey upwards and into the inclined channels. For example the particles may be too large, relative to the upward flows employed within the device. Thus feed entry below the inclined channels may lead to significant process inefficiency. It is then desirable to use one of the features of this invention to introduce the feed from above the inclined channels, ideally accompanied by a bubbly mixture.

The dispersion may consist of an emulsion, in which one liquid phase is dispersed as drops within another immiscible liquid phase. Alternatively, the dispersion may consist of foam, in which gas bubbles are dispersed within a liquid or even a solid- like phase. In general, the dispersion will consist of a continuous phase, either of gas, liquid, or even a solid, or some combination of the three usual phases.

In the special case in which the dispersed material is lower in density than the continuous phase, the dispersed particles will tend to rise up towards the downward facing surfaces of the neighboring inclined channels. In some cases these dispersed particles will also coalesce to form larger particles, for example when two gas bubbles coalesce, or two drops coalesce. Conversely, the continuous phase will tend to drain through the dispersed phase, and collect on the upward facing surface of the neighboring inclined channels. The continuous phase will then flow down the upward facing surface, and the dispersed phase will tend to slide up along the downward facing surface. We now consider some specific cases, and the benefits achieved through processing these dispersions using inclined channels. The first case consists of a gaseous foam in an aqueous phase. Here the foam is stabilized by the presence of surface active species, essentially dissolved molecules that exhibit a hydrophobic and a hydrophilic component. These molecules might be proteins, and the objective may be to concentrate these molecules by forming a foam. Such a process is made economic by using a high superficial air velocity, as this generates a larger specific surface area through the system. However, a high superficial air velocity will tend to entrain more liquid with the gas bubbles, in turn producing a wetter foam. A wet foam can be made more concentrated by ensuring sufficient residence time for the excess liquid to drain, however the liquid that drains at some higher elevation must also drain down through all other elevations. The use of inclined channels, however, provides a direct pathway for the liquid to drain towards, and travel down the upward facing inclined surfaces.

The second case consists of a bubbly flow or foam containing particles. Here the objective is to recover either hydrophobic particles or lower density particles, or particles that are both hydrophobic and low in density, such as coal. Ideally, the liquid flows are kept as low as possible to minimize the entrainment of relatively fine, hydrophilic or dense particles, or particles that are both hydrophilic and dense. When the gaseous bubbles migrate to the undersurface of the neighboring inclined channel, a high velocity bubbly flow develops. This bubbly flow serves several purposes. Firstly, the shear rate within the inclined channel is made relatively high. A high shear rate helps in the conveying of lower density particles up through the inclined channels. Secondly, the high shear rate causes the bubbles adjacent to the undersurface to become entrained into the neighboring liquid, producing new and smaller bubbles. Where there are hydrophobic particles, there is the potential for bubble-particle attachment. This attachment may be weak or strong, depending on the level of adhesion relative to forces such as shear, buoyancy, and particle weight forces. Nevertheless, a relatively large hydrophobic particle that attaches momentarily and periodically to these bubbles should tend to convey in the upwards direction to an extent that is greater than would occur in the absence of these interactions. Thus, low density and or hydrophobic particles should benefit in two ways from the existence of a bubbly flow.

The bubbly flow occupies a space that would alternatively have to be occupied by the continuous phase, for example water. Consider now the case in which the particles are relatively larger, for example 16 mm in size or larger. These larger particles demand more widely spaced inclined channels. Given a specific velocity is required in order to convey particles to the overflow, a significant velocity applicable across the full width of the channel is required. Thus the flow rates needed become larger as the particle size is made larger. However, by introducing a bubbly flow, much of the volume flow of water can be replaced with air bubbles. The bubbles tend to ride up the under surface at a significant velocity. A further example arises when the fluid medium is to be replaced by a gas such as air, as occurs in the "Dry Processing" of coal and mineral particles. Dry Processing is attractive in conditions where water supplies are limited or restricted. In some cases there are further benefits because the output streams are dry. Hence the masses and volumes that need to be transported or stored are less. Further where the feed is to be used at elevated temperatures there are energy savings as the water does not have to be evaporated. In Dry Processing it is more difficult to separate particles on the basis of

density. Hence the air flow needs to be supplemented with a dense medium such as sand

or magnetite. But because of the gas-solid state of the system there is poorer control

over the medium density. However, through vibration it is possible to establish a more

fluid-like dense medium effect, and inturn promote a stronger density based separation.

Further, with the appropriate level of vibration and direction of vibration, it is possible to

help re-suspend the lower density particles, while higher density particles continue to

segregate.

BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms that may fall within its scope, one preferred form

of the invention and variations thereof will now be described by way of example only

with reference to the accompanying drawings in which

Fig. l is a diagrammatic side elevation of an inclined plate classifier for use in the

method according to the invention,

Fig. 2 is a cross-sectional elevation to an enlarged scale of a pair of inclined plates

within the classifier subjected to vibration,

Fig. 3 is a diagrammatic side elevation of an alternative form of the classifier

shown in Fig. 1 incorporating an upper feed cute,

Fig. 4 is a diagrammatic end elevation of the classifier shown in Fig 3,

Fig. 5 is a diagrammatic cross-sectional elevation of a further form of the

invention for use where the particles are all less dense than liquid in the feed material.

DETAILED DESCRIPTION

The apparatus comprises a chamber 1 adapted to contain a fluid and divided into

an upper portion 2 and a lower portion 3. The upper portion 2 contains an array of substantially parallel inclined plates 4 defining a plurality of inclined channels 5 between each plate.

The lower portion 3 of the chamber may, in one form of the invention, be provided with apparatus for providing fluidisation of particles between the plates by injecting gas into the chamber e.g. by a sparge pipe 6 provided with airflow 7. It will be appreciated that there are many ways of providing fluidisation into the chamber and that these could be substituted for the sparge pipe 6.

The lower portion of the chamber is provided with a drain 8 arranged to remove material from the bottom of the chamber. The upper part of the chamber is provided with an overflow launder 9 adapted to receive and convey away material overflowing from the upper ends of the inclined channels 5.

In one form of the invention a vibrator 15 is provided adapted to vibrate either the entire mechanism, or at least the inclined plates 4. This can be effected by any known means or mechanism.

The vibration is controlled to a level sufficient to cause the re-suspension of certain particles. As can be seen in Fig. 2 a typical particle such as that shown at 16 is moved away from the lower plate 4 by vibration of that plate to a position typically shown at 16A where it is again entrained by the flow of fluid through the channel and may move e.g. to positions 16B and 16C. Should the particle come back into contact with plate 4 at 16D it can then slide down the plate to position 16 where it is again vibrated into the channel by the vibration of the plate.

Although the use of vibration to re-suspend the lower density particles within the channels 5 can be utilised where a mixture of high and low density particles are fed into the upper region of the array of inclined plates, it will be appreciated that the re- suspension is also effective in enhancing the operation of a conventional Reflux Classifier where the mixed particles are induced from the lower ends of the array of inclined plates, typically by way of fluidisation, which may be induced through sparge pipe 6. Vibration is particularly effective with dry systems (e.g. where the fluid feed material does not include a liquid) but may also be used in wet systems, with or without the use of bubbles.

In systems involving liquids such as water, the purpose of the re-suspension is to convey relatively larger low density particles along the inclined channels. In systems involving the flow of a gas the object is to suspend fine particles such as magnetite in order to create a stable media for then separating much larger particles on the basis of density.

The vibration mode is useful in selectively re-suspending certain particles that can then report to the overflow. Alternatively, the vibration mode is useful in producing a more stable suspension of fine particles in the inclined channel, A more stable suspension of particles tends to behave like a dense fluid and hence is useful in promoting a density based separation. The more stable suspension may involve either a gaseous flow containing fine particles or a liquid flow containing fine particles. These suspensions behave like dense fluids which, in turn separate particles on the basis of density. The vibration, when applied to the inclined channels, helps to promote the stability of the suspension medium while allowing other particles (generally larger) to either sediment or remain in suspension. Denser particles will then tend to sediment, and low density particles will tend to remain suspended.

In another form of the invention as shown in Figs. 3 and 4, the chamber also incorporates a feed chute 10 arranged to guide feed material containing particles of mixed densities into the upper region of the array of inclined plates. The feed chute is typically rectangular in configuration as shown in Figs 3 and 4 and the lower edges of the chute may intersect with the upper edges of the inclined plates 4 so that the feed issues between the plates at some distance below the upper edges 11 of the plates and can then disperse outwardly between the plates as shown by arrows 12. In use, feed material containing particles of mixed densities is fed into the feed chute and dispersed between the inclined plates 4. The feed material may incorporate a bubbly mix or bubbles may be introduced into the mixture upwardly from the bottom of the inclined plate array by way of sparger 6. In some instances it may be desirable to introduce bubbles both with the feed mix and from the bottom of the plates. The bubbles are effective in providing re-suspension of lower density particles or simply a re-suspension of hydrophobic particles (where bubble-particle attachment arises) in a similar manner to the vibration described with reference to Fig. 2.

Due to the mechanism operating within the inclined plates, enhanced by the action of the bubbles, underflow material containing higher density particles report to the bottom part 3 of the chamber and are removed through the drain 8 while overflow material containing lower density particles overflows into the launder 9.

Fig. 5 shows a modified form of the apparatus where the particles are all less dense than the liquid. Here the liquid may also be considered as a concentrated medium or suspension. The difficulty here is in recovering particles that tend to rise through this liquid or suspension.

The feed 17 consists of low density particles and a liquid or suspension medium. The liquid or suspension acts like a fluidisation liquid, except that it is directed downwardly through a feed chute 18 and into the upper end of the inclined channels 5. The high flow of liquid/suspension in zone 19 tends to entrain low density particles towards the underflow where the inclined channels 5 allow the low density particles to escape the flow and report to the overflow, eventually being discharged from the launder 19 at 20.

The term "particles" where used throughout this specification refers to either solid, liquid or gaseous entities of any size and in the form of the apparatus described with reference to Figure 5 has particular application where the "particles" are small bubbles. In this form of the invention the method and apparatus has particular application to the separation of minerals by the flotation process.

Features of the present invention include parallel inclined channels, though these may not always have to be parallel. The feed entry chute in the embodiment shown in Figs 3 and 4 intersects with the inclined channels to ensure the feed can disperse onto the inclined channels. The feed may or may not carry a bubbly flow. Bubbles could also be added from below the set of inclined channels. More than one feed chute could be employed. The overflow reports to an overflow launder. Internal launders could also be used to ensure even channel flow and full capture of the overflow particles. Or a single overflow launder could be used. Fluidisation flow could be employed up through the base of the unit, with additional flow entering via the side walls of the vessel (below the set of inclined channels). Underflow is removed from a single or multiple discharge points at the base of the vessel, in response to some signal such as a pressure transducer. Applications Although not exhaustive, specific examples of the technology include:

(i) Fine hydrophobic particles or particles that are made hydrophobic through the addition of collector are readily recovered by froth flotation. Water is often added to the top of the froth to wash the unwanted hydrophilic particles out of the froth, and hence improve the quality of the product. With inclined channels, the washing process is enhanced because the hydrophilic particles and wash water can collect on the upward facing surfaces of the inclined channels and move down from the froth easily. More wash water can therefore be used. The wash water no-longer has to travel down through the whole foam, only the short distance to the upward facing inclined surface. (ii) In ion flotation or foam fractionation, the upgrade or concentrating effect is promoted by the drainage of the froth. High throughput is achieved by using a high air rate and optimal bubble size, however this produces a wetter froth and hence lower upgrade due to liquid entrainment into the froth. The parallel inclined channels should lead to much higher levels of throughput at higher upgrade.

(iii) In coarse particle flotation, methods are needed to promote the attachment and retention of hydrophobic particles at the surface of air bubbles. The particle weight can be so large that the particles readily detach. However, within an inclined channel there should be improved support of both particles and froth. Indeed coarse particles could collect on the inclined surfaces and be conveyed along by the rising froth. The inclined surface provides considerable support for the particle weight.

(iv) Rising air bubbles that report to the downward facing surface of the inclined channel will collect and sometimes coalesce. A high bubble velocity below the inclined surface leads to much higher levels of shear within the inclined channel and buoyancy driven re-suspension of particles. This increased shear should also lead to more re-suspension. The re-suspension is stronger for lower density particles. Thus air bubbles in the flow can be used to improve the conveyance of the lower density particles up the inclined channel, while the higher density particles will tend to segregate and slide down the upward facing surface. Hence improved gravity separation is achieved. A further benefit, especially when the particle size is relatively large, is that less water is needed to convey particles to the overflow. Much of the flow then consists of a bubbly flow below the downward facing inclined surfaces. Water flow rates could therefore be reduced by perhaps 50%.

(v) Bubble assisted conveying of low density particles, hi this mode, the bubbles can act as a form of bulk flow, and hence substitute for feed or fluidization water. The bubbles slide along the downward facing surfaces of the channels, producing high velocities. This increases the shear rate in the channels and promotes particle re-suspension and hence conveying. This re-suspension then assists with the conveying of the low density particles to the overflow, while high density particles slide down the incline towards the underflow. If the bubbles actually attach, even partially or briefly, this provides further assistance to the conveying of the low density particles to the overflow. This arrangement works well with coal, where the low density particles are also hydrophobic.

This mode of operation is mainly gravity concentration, with potentially some contribution from the conventional flotation mechanism.

(vi) Conventional flotation. Here the aim is to recover hydrophobic particles, either low or high in density, to the overflow. In this case strong bubble- particle attachment is necessary. The bubbles then carry the particles through the inclined channels. Entrained particles of gangue will tend to segregate onto the inclined plates. Thus, the invention allows much higher hydraulic flows into the overflow, with less gangue entrained. Wash water addition from the top can also aid in returning gangue to the underflow of the device. Foam drainage is improved as a drainage path towards the upward facing inclined surfaces exists, providing an effective drainage route, (vii) Foam fractionation (ion flotation). Here there is little in the way of regular solid particles, though some precipitate might tend to form due to the high concentrations of ions and surfactant. Sometimes the valuable product is the surfactant, such as whey protein. This invention leads to higher rates of foam drainage, hence higher throughputs and concentration upgrade, (viii) Immiscible liquids, with low density droplets rising like bubbles. In this case the objective may be to achieve liquid-liquid extraction, and then phase separation. The low density drops rise through the high density continuous phase or the high density drops fall through the low density continuous phase.

Claims

CLAIMS:-

1. A method of operating an inclined plate classifier of the type having an array of substantially parallel inclined plates in a chamber containing a fluid, said method including the steps of feeding feed material to be classified between the plates and providing re-suspension of lower density particles as the feed material moves through the inclined channels between the plates.

2. A method as claimed in claim 1 wherein the re-suspension of lower density particles is provided by imposing vibration on the inclined plates.

3. A method as claimed in claim 1 wherein the re-suspension of lower density particles is provided by introducing bubbles between the plates to increase the shear rate in the inclined channels.

4. A method as claimed in claim 3 wherein the bubbles are introduced with the feed material as a bubbly flow mixture.

5. A method as claimed in either claim 3 or 4 wherein the bubbles are introduced into the chamber below the array of inclined plates.

6. A method as claimed in any one of the preceding claims wherein the method includes the step of feeding a feed material containing mixed particles of higher and lower densities into the upper region of the array of inclined plates.

7. A method as claimed in any one of the preceding claims wherein overflow material containing lower density particles is removed from the chamber by a launder adjacent the upper edges of the inclined plates.

8. A method as claimed in any one of the preceding claims wherein underflow material containing higher density particles is removed from the bottom of the chamber.

9. An inclined plate classifier for separating particles of relatively higher and lower densities, said apparatus having an array of substantially parallel inclined plates located in a chamber adapted to contain a fluid, means for providing re-suspension of lower density particles between the plates, an overflow launder adjacent the upper edges of the inclined plates, and an outlet in the lower region of the chamber.

10. A classifier as claimed in claim 9 wherein the means for providing re-suspension of lower density particles includes means for introducing bubbles between the plates.

11. A classifier as claimed in claim 9 wherein the means for providing re-suspension of lower density particles includes means for introducing bubbles between the plates.

12. A classifier as claimed in any one of claims 9 to 1 1 wherein the apparatus includes a feed chute arranged to feed material containing mixed particles of higher and lower densities into the upper region of the array of inclined plates.