The present invention relates generally to particle processing and separation techniques and more particularly to a method and apparatus for "dry" processing and separation of hydrophobic particles. The invention has application in mineral processing and particular application in coal processing and separation. However, it should be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

The following discussion of the prior art is intended to place the invention in an appropriate technical context and allow the associated advantages to be more fully understood. However, any reference to prior art in this specification should not be taken as an express or implied admission that such art is well known or is common general knowledge in the field of the invention.

Processing techniques to extract minerals from ores typically involve four general stages of operation, which are essentially:- a) comminution or particle size reduction; b) sizing of particles within defined ranges, for example by screening or classification; c) concentration which is usually effected on the basis of physical or chemical properties of the particles; and d) dewatering or solid/liquid separation.

Froth flotation is a traditional method used to concentrate fine hydrophobic particles on the basis of differential wetting properties, which give rise to differential hydrophobicity. By way of example, using this technique, fine coal and mineral matter are typically supplied to the flotation device as a slurry containing particles in suspension. Air bubbles are dispersed through the suspension, whereby hydrophobic coal particles attach to the air bubbles and rise to the surface of the suspension. A frothing agent is added to help establish a stable froth, which in turn promotes the drainage of liquid from the network of recovered hydrophobic particles. A concentrated product consisting of hydrophobic particles is then obtained from the froth, with the hydrophilic mineral enriched particles being discharged from the base of the vessel as slurry in a "tailings" stream. Selective reagents such as surfactants and wetting agents are usually used to promote the hydrophobicity of the relevant particles and thereby enhance the efficiency of the recovery process.

One significant disadvantage of this process is the requirement for substantial volumes of water and relatively large flotation tanks. Moreover, the process usually requires a subsequent dewatering stage, which adds processing time, energy and further large-scale capital equipment.

Accordingly, there is a need for a dry or relatively dry separation method that is effective in mineral processing applications using minimal quantities of water, and particularly for such a method that is suitable for use in connection with relatively small particle sizes.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the invention provides a method for particle processing, said method including the steps of:- directing a substantially dry feed material containing relatively hydrophobic and relatively hydrophilic solid particles of predetermined nominal size distribution into a flow of gas such that the solid particles are entrained as a gaseous dispersion; directing a shower of liquid droplets through the gaseous dispersion to induce inertial impact between the solid particles in the gaseous dispersion and the liquid droplets in the shower; whereby hydrophilic particles are captured by the liquid droplets while hydrophobic particles remain in the gaseous dispersion.

In a second aspect, the invention provides an apparatus for particle processing, said apparatus including: a feed inlet adapted to receive a substantially dry feed material containing relatively hydrophobic and relatively hydrophilic solid particles of predetermined nominal size distribution; a pump adapted to generate a flow of gas; a feed outlet to direct the feed material into the flow of gas such that the solid particles are entrained as a gaseous dispersion; and a liquid outlet adapted to direct a shower of liquid droplets through the gaseous dispersion to induce inertial impact between the solid particles in the gaseous dispersion and the liquid droplets in the shower; whereby hydrophilic particles are captured by the liquid droplets while hydrophobic particles remain in the gaseous dispersion.

The terms "dry" and "substantially dry" as used herein, in the context of feed material, are intended to indicate that the solid particles are sufficiently dry so as to disperse relatively freely in an air stream, without significant adhesion to one another or to surrounding surfaces due to the influence of adsorbed moisture or other wetting agent(s). It should be understood that subject to this functional definition, in practical terms some small degree of moisture content may be acceptable. The gas is preferably composed substantially of air and the liquid is preferably composed substantially of water. Optionally, reagents or surfactants may be incorporated into the gas, into the liquid or onto the solid particles, to enhance characteristics of hydrophobicity and/or hydrophilicity with respect to specific types of particles. In one preferred embodiment, the method and apparatus make use of a first chamber in which the solid particles are dispersed into an air stream, and a second chamber in which a water shower interacts with the gaseous dispersion. In one embodiment, the first and second chambers take the form of generally vertically oriented cylinders. It should be appreciated, however, that chambers of any suitable number, size, shape and orientation may alternatively be used to suit particular applications.

In one embodiment, the feed material is directed into the first chamber from above through a vibrating sieve or screen, while the air stream is directed generally transversely across the first chamber, beneath the sieve or screen, such that the solid particles fall into the air stream, for conveyance as a gaseous dispersion through a second conduit into the second chamber. In one embodiment, the liquid such as water is directed into the second chamber from above through a series of nozzles or outlets, while the gaseous dispersion is directed generally transversely across the second chamber, beneath the nozzles or outlets. In this way, the water droplets falling generally vertically collide with the entrained solid particles moving generally horizontally in the gaseous dispersion, so as to optimise the functional interaction between the intersecting flows in the resultant mixing zone.

In one embodiment of the invention, water droplets containing hydrophilic mineral particles fall to the base of the second column and preferably, are then recirculated by pumping to a head reservoir at the top of the second column for use in the formulation of new droplets in the water shower. This recirculation circuit preferably thereby allows the hydrophilic particles to become progressively more concentrated. Preferably, a proportion of the water is then progressively bled from the circuit in order to recover the hydrophilic particles as concentrated slurry. In some embodiments, the air stream containing a proportion of the relatively hydrophobic particles is removed from the second chamber through a third conduit after passing through the water shower, whereupon the relatively hydrophobic particles are preferably recovered using cyclones, filters or a combination of these or other separation devices. In one preferred application of the invention, these hydrophobic particles include a substantial proportion of fine coal particulates.

The solid particles in the feed material are preferably less than around 2.0 mm, more preferably less than around 1.0 mm and most preferably less than around 0.5 mm in diameter. The water droplets are preferably between 0.1 mm and around 10 mm, more preferably between 0.5 mm and around 3.0 mm, and most preferably around 2.0 mm in diameter.

In some embodiments, the pump takes the form of a positive pressure pump or blower disposed upstream of the feed outlet. In other embodiments, the pump takes the form of a negative pressure or vacuum pump disposed downstream of the feed outlet. In yet other embodiments, a source of compressed air may be used to induce the gas flow. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawing in which:

Fig. 1 is a schematic representation of a separation apparatus, according to the invention.

Fig. 2 is a similar view to Fig. 1 showing an alternative feed inlet via a fluidised bed.

Fig. 3 is a similar view to Fig. 1 showing a valve mechanism on the feed inlet.

Fig. 4 is a similar view to Fig. 3 showing the valve mechanism in the alternating position; and

Fig. 5 is a similar view to Fig. 1 showing an additional lower gas entry.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring to Figure 1, the invention provides a mineral separation apparatus 1 including a first cylinder 2 defining a first chamber 3 and a second cylinder 4 defining a second chamber 5. The first cylinder 2 includes a feed inlet 6 adapted to receive substantially dry feed material 7 containing relatively hydrophobic and relatively hydrophilic solid particles within a predetermined nominal size distribution range. A first conduit 10 provides an air inlet 11 to the first chamber 3, while a second conduit 12 establishes fluid communication between the first and second chambers. A third conduit 15 defines an upper outlet 16 for the second chamber 4. A vacuum pump or a suitable source of compressed air (not shown) induces a transverse flow of air initially into the first chamber 3 through the first conduit 10, then from the first chamber to the second chamber 5 through the second conduit 12, and finally out from the second chamber through the third conduit 15, as represented by arrows 17 and as described in more detail below. The bottom of the second chamber includes a bottom outlet 18.

Turning now to describe the operation of the separation apparatus in more detail, the feed material 7 is initially introduced into the first chamber 3 through a vibrating sieve or screen 20, which is connected to the top of the first cylinder 2 by means of a flexible tube or trunk 21. From there, the feed material falls downwardly under gravity through the first chamber and into the transverse air flow 17 such that the solid particles are entrained as a gaseous dispersion 22, which is progressively drawn through the second conduit 12 into the second chamber by the air flow.

The top of the second chamber includes a head reservoir or tank 25, incorporating liquid outlets (not shown) in the form of suitably shaped apertures or nozzles, adapted to create a shower of relatively fine water droplets 26. The water supply to or from the reservoir can be pressurised if required. The water droplets fall downwardly under gravity from the head reservoir 25 through the second chamber and through the gaseous dispersion. This results in a mixing zone 28 wherein inertial impact between the solid particles in the gaseous dispersion and the water droplets in the shower is induced. In this way, hydrophilic particles 30 in the gaseous dispersion tend to become bound to the water droplets 26, while hydrophobic particles 31 remain in the air stream. It will be appreciated that because the water drops fall generally vertically, and the gaseous dispersion is moving generally horizontally, the functional interaction between the intersecting flows in the mixing zone 28 is optimised.

Ideally, the water droplets are around 2 mm in diameter. Droplets of this size are considerably heavier than the relatively fine solid particles and hence separate efficiently from the gaseous dispersion containing those solid particles in suspension. It has been found that smaller droplets, although offering the advantage of a larger specific surface area, tend to settle more slowly and therefore do not necessarily lead to a significantly higher interfacial flux of the gas-liquid interface through the cloud of particulates in the mixing zone.

From the mixing zone, the water droplets containing the hydrophilic mineral particles fall to the base of the second column 4 and then flow outwardly through the bottom outlet 18. This liquid suspension is then pumped through a recirculation circuit, represented by arrows 32 and 33, to the top of the second column and into the head reservoir or tank 25, for use in the formation of new droplets in the water shower. This water recirculation circuit thereby allows the hydrophilic particles to become progressively more concentrated. A proportion of the water is progressively bled from this circuit, in order to recover the hydrophilic particles as concentrated slurry, which may be subject to dewatering or other downstream processes if required. The air stream containing the relatively hydrophobic particles 31 is then withdrawn from the second chamber through the third conduit 15, whereupon the relatively hydrophobic particles are recovered using conventional cyclones, filters or a suitable combination of these or other gas/solid separation devices. Such devices are well known to those skilled in the art, and so are not described in further detail.

In this way, the method and apparatus of the invention can be used to produce a substantially dry hydrophobic product and to minimise the amount or liquid (typically water) needed to recover the hydrophilic particles.

In one preferred application, the dry feed material consists essentially of fine particles of coal and mineral matter, which are conveyed collectively as a gaseous dispersion through a shower of falling water droplets, such that the inertial impact of the hydrophilic particles with the water droplets leads to the capture of the hydrophilic particles, in the manner previously described. These particles have a strong affinity for the aqueous phase and hence are readily wetted once the air film drains from the interface between the particles and the droplets. Conversely, the hydrophobic coal particles fail to wet, even as the air film drains. These particles therefore tend, in general, to be repelled, and so predominantly remain as part of the gaseous phase. Hence, the particle cloud leaving the second column of the apparatus consists predominantly of coal particles, which represents a significant upgrade in the concentration of fine coal from the initial feed material. These fine coal particles are then readily recovered using conventional cyclonic separation or other suitable separation techniques, as previously noted. While some coal particles may become associated with the surfaces of the water droplets, such particles will generally fail to join the bulk of the water volume and consequently, the uptake of fine coal in this way is relatively minimal.

Although the process may be adapted for use in connection with coarser particles, it has been found to be particularly well adaptable for processing of relatively fine particles, less than around 0.5 mm in diameter.

As in froth flotation, one or more chemical reagents or surfactants may be used to achieve desired levels of selective hydrophobicity of specific types of particles.

Suitable reagents ideally operate in the dry state, for example following condensation of the chemical reagent onto the target particles. Chemical species already present at the surface of the target particles then provide the basis for the desired selectivity.

In one embodiment of the invention as shown in Fig. 2, a fluidised bed 34 is used to entrain and disperse the feed material from inlet 35 in an air volume introduced through plenum chamber 36 and by appropriately positioning and orienting the fluidised bed, the entrained particles can be conveyed upwardly as shown at 37 to the second column. The plenum chamber 36 is typically separated from the fluidised bed 34 by a distributor 38.

In this regard, it should be appreciated that in some embodiments of the invention, two separate columns or chambers for dispersion of solid particles and water droplets respectively, are not necessarily required. In some cases, a single chamber having one or more internal regions may alternatively be used, subject to particular applications and design constraints. Similarly, it should be understood that the cylinders need not be positioned side -by-side, the airflow need not be directed generally horizontally, and through the use of pressurised spray bars or the like, the water shower need not be oriented vertically. Numerous variations, combinations and orientations of the key components of the method and apparatus are envisaged, subject to the intended application, the desired throughput, cost and efficiency considerations and space constraints. In some embodiments, depending upon particular design parameters such as the air flow rate, the magnitude of the pressure drop along the air flow path and the shape and configuration of the various cylinders, chambers or conduits, it is possible that pressurised air may have a tendency to force its way upwardly toward the feed inlet, and thereby prevent the particulate feed material from passing freely through the sieve or screen. In order to minimise this tendency, it may be desirable in some applications to use relatively low air flow rates and to ensure a relatively low pressure drop in the horizontal direction, along the air flow path.

In some embodiments as shown in Fig. 3 and Fig. 4, a cover 38 A is also provided above the screen 20 to resist the flow of pressurised air upwardly through the screen. A double valve arrangement may also advantageously be incorporated, so as effectively to provide an air lock. In one such embodiment, for example, the feed enters an intermediate inlet pipe 39 through an open first valve 40 and initially collects behind a closed second valve 41 as shown in Fig. 3. The first valve 40 then closes as shown in Fig. 4 so as temporarily to prevent further feed from being added to the intermediate inlet pipe 39, while the second valve 41 opens to allow the initial inflow of feed 42 to fall onto the screen 20. The process is then continuously repeated, with the valve operations being appropriately timed and synchronised to ensure a relatively uniform flow of feed material through the screen.

Although the double- valve arrangement has been described in a situation to resist the flow of pressurised air upwardly through the screen 20 as shown in Fig. 3, it would be appreciated that a similar arrangement could be used to deal with undesirable airflows in other sections of the apparatus. For example, a double-valve operation could be used in the feed to the head reservoir or tank 25 to prevent airflow up through the liquid outlets, and could also be used on the feed inlet 35 to the fluidised bed 34 in the configuration shown in Fig. 2. In a further embodiment of the invention as shown in Fig. 5, in recognition of the fact that some of the hydrophobic particles 31 may inadvertently fall through the mixing zone 28 along with the hydrophilic particles 30, an additional lower gas entry 43 is provided which enters the second chamber 5 without any entrained particles. The additional gas flow allows hydrophobic particles in the mixing zone 28 to fall into a moving gas flow and still be carried out of the second chamber 5 through an enlarged third conduit 15.

It will be appreciated that the airflow management may be dealt with in a variety of alternative ways, including by using a source of negative pressure downstream of the feed inlet and the associated sieve or screen. The method and apparatus of the present invention are able to produce a relatively dry hydrophobic product, with relatively minimal quantities of water being required to recover the hydrophilic particles from the feed material. By thus reducing the extent of downstream processing, such as dewatering, significant savings can be made in terms of both capital and operating costs, in the wider context of a coal or mineral processing or separation plant. In these and other respects, the invention represents a practical and commercially significant improvement over the prior art. An additional benefit on the prior art is that particle attachment from the gas to the liquid interface is more effective because the gas fluid has a low viscosity. Hence much finer particles, for example below 20 microns, may be recovered better with this system. Coarser particles would still be recovered.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.