Description of the Prior Art Tribo-static and electrostatic separators are known to be used for the separation of particulate feedstock materials into two or more constituent streams. Such known devices generate a static charge on the feedstock material by tribo-static action, and then pass this material between oppositely charged electrodes, to effect separation.

A typical application is for beneficiation of minerals such as the removal of silica sand from shell sand or lime stone and the removal of silica sand from feldspar.

Tribo-static separation itself is not new and is well reported in the literature. In summary, tribo-static separation occurs when two different substances (for example a granular mineral stream with two or more components) are rubbed together to generate opposing electrostatic charges, and are then subjected to an electric field which polarises the components in opposite directions, hence creating a physical separation. Particle types that lend themselves to tribo-static charging tend to be'non conductors'with differing dielectric constants.

In typical tribo-static separators, a granular stream is charged by a transportation device such as a screw conveyor, tumbler, cyclone, impingement mixer etc. which mixes the stream, causing the particles to rub against each other, thus generating electrostatic charge.

The magnitude of the charge is related to the dielectric constants of the constituent particles. Generally the greater the differential in these constants the greater the charge

generated. Non conductive particles tend to charge more easily than conductive particles (e. g. metals etc.) as surface resistivity prevents the dissipation of charge.

Examples of tribo-static separations that are known to work well include silica from lime, and silica from feldspar. Many valuable minerals can be beneficiated (ie purified) by using tribo-static separation, to remove impurities such as silica and other contaminants. The process has wide application and, for example, it can be used for purifying foodstuffs such as removing chaff from grain, shells from nuts etc. or for recycling by separating different types of plastic, or, plastic from metals.

In conventional tribo-static and electrostatic separators, the electrodes are bare conductors so that a high electrostatic field can be generated. These have the disadvantage that feed material attracted to the electrode of opposite polarity to its own charge, on contact with the electrode, looses all or some of its opposite charge and may gain the same polarity as the electrode. This can be a disadvantage in that material which has lost charge may then be attracted away from the electrode and re-mixed with the general stream, resulting in a lower grade of product. The extent to which this occurs depends on a number of factors but in particular depends on the conductivity of the feed particles.

Summary of the Invention The present invention seeks to overcome or at least provide an alternative to the disadvantages of the prior art.

The present invention seeks to provide a method and apparatus for electrostatic separation of particulate material using'insulated'electrodes.

In one broad form, the present invention provides an electrostatic separation device including at least one electrode, wherein each electrode includes an electrically conductive element having an insulative covering.

Preferably, each electrically conductive element is embodied in the form of a plate like

member.

Preferably, said insulative covering is supplied over at least one surface of said plate like member.

Alternatively, but also preferably, said insulative covering substantially surrounds said electrically conductive element (s).

Preferably, said insulative material is additionally thermally insulative material.

Preferably, each electrode is adapted to be vibrated or shaken.

Preferably, each electrode is adapted to have the voltage/charging thereon removed or reversed in polarity.

Preferably, the orientation and/or spacing of said electrodes in said device is selected to optimise the separation effects of the particulate material being processed.

In a preferred embodiment, said orientation and/or spacing of said electrodes in said device is adjustable.

In a preferred form, said electrode (s) are of linear, curved (convergent or divergent) or other shape.

Preferably, said electrode (s) include one or a plurality of slots to permit at least some of the particulate material to pass therethrough.

Preferably, said insulating covering is fibre reinforced polymer (fibreglass) or other material having similar insulative characteristics thereto.

Preferably, said device includes charging means, to apply a charge to the particulate

material being processed prior to separation thereof using said electrodes.

Preferably, said charging means includes a tribo-static charging means.

Preferably, said charging means includes one or a plurality of cascading plates.

Preferably, said cascading plates are vibrated/shalcen.

Preferably, said charging means includes a corona discharge device.

Preferably, having a plurality of separation chambers, each chamber having a charging means and an electrode.

Preferably, including a distributor means to divide a supply of particulate material to be processed between the separation chambers.

Preferably, said distributor means includes a primary hopper having a plurality of streams associated therewith, each stream providing particulate material to be processed into one of a plurality of secondary hoppers ready for supply of the particulate material to a respective separation chamber.

Preferably, said device further includes a splitter/launder means, to receive the separated particulate material processed from each of said separation chambers and to combine material of similar gradings therefrom (e. g.'products','middlings'and'rejects').

Preferably, said splitter/launder means includes sampling means.

In a further broad form, the present invention provides a method of electrostatically separating particulate feedstock materials, including the steps of : applying a charge to said materials; and passing said materials past at least one charged electrode means, said electrode

means having an electrically conductive element having an insulative covering; such that, the materials are separated according to the individual conductivity or charge of the individual feed particles.

In yet a further broad form, the present invention provides an electrode for an electrostatic separation device, said electrode including an electrically conductive element having an insulative covering.

Preferably, said electrically conductive element is embodied in the form of a plate-like configuration.

Preferably, said insulative covering is supplied over at least one surface of said plate-like element.

Alternatively, but also preferably, said insulative covering substantially surrounds said plate-like element.

Preferably, said electrically conductive element is embodied in the form of one or more wires or like configuration.

Preferably, said insulative covering substantially surrounds said wire (s).

Brief Description of the Drawings The present invention will become more fully understood from the following description of preferred but non-limiting embodiments thereof, described in connection with the accompanying drawings, wherein: Figure 1 shows an elevational view of an electrostatic separation apparatus in accordance with the present invention; Figure 2 illustrates, in Figures 2 (a) and 2 (b), various embodiments of insulated electrodes which may be used in the apparatus of the present invention; Figure 3 shows an exploded perspective view of the apparatus of the present invention;

Figure 4 illustrates, in Figures 4 (a) to 4 (f), various alternative arrangements of the walls of the separating chambers, in accordance with various preferred embodiments of the present invention; Figure 5 illustrates, in Figures 5 (a) to 5 (c), various alternatively preferred embodiments of slotted electrodes which may be used in the apparatus of the present invention; and Figure 6, in Fig. 6 (a) and Fig. 6 (b) side and front views, respectively, shows the configuration of sampling hatches in use with the apparatus of the present invention.

Detailed Description of Preferred Embodiment (s) Throughout the drawings, like numerals will be used to identify similar features, except where expressly otherwise indicated.

An electrostatic separation device, constructed in accordance with the present invention is shown in cross-sectional view in Figure 1, with Figure 3 detailing an exploded perspective view thereof. The device, generally designated by the numeral 1 includes distributor means 2, for distributing the feedstock, charge applications means 3, separation means 4 and recombining means 5.

The distributor 2 splits the feed material stream into several smaller streams 6 which are ultimately fed to multiple separating chambers 7. Each hopper 8 is used to hold a bed of feed material and supply a respective feeder 9, which dispenses a controlled flow of feed material to a respective separator 4. Typically the feeder 9 has a motor driven roll, the speed of which can be controlled. Each feeder 9 presents material to a respective tribo- static charging device, such as a cascader 10. Alternative types of tribo-static charging devices such as cyclones or screw conveyors can be used. Tribo-static generators, such as the'cascade'type, are used to generate electrostatic charge on the feed material. The 'cascade'plates can optionally be vibrated in order to increase their effectiveness in generating charge. This charge can optionally be augmented using a'corona discharge'11, before or after the cascade device. Applying a high voltage to a small diameter tungsten wire typically generates the corona discharge. Optionally it can be used in place of the tribo-static charge generator.

Insulated plate electrodes 12, with adjustable plate separation distance and adjustable angle, may be used to create electrostatic fields in each separation chamber 7. Optionally, a vibrating device can be attached to the electrode plates, which can be operated intermittently to aid the removal of separated material from the surface of the electrode plates. A splitter plate 13 may be used at the outlet of each separating chamber 7 to divide the stream into'product','middling'and'reject', or other splits. A launder 14 may be used to re-combine similar streams, e. g.'products','middlings'and'rejects'. Sampling hatches 16 may be optionally incorporated into the combined outlet streams to allow for product sampling for analysis, such as shown in Fig. 6, Fig. 6 (a) illustrating a side view thereof, and Fig. 6 (b) illustrating a front view thereof.

Typically the whole device is enclosed in a steel frame with metal cladding and access doors all of which can be earthed for safety. A high voltage power supply and control panel is connected to the machine (not illustrated). The control circuits can be configured to control main process variables such as voltage, roll speed, electrode gap and angle, splitter position, cascade and electrode plate vibrators, voltage interruption and polarity reversal, etc.

Traditionally, electrodes in tribo-static and other electrostatic devices have been bare metal plates or bars, ie with surface conductivity. In the present invention, the tribo-static units have insulated electrodes.

In the present invention the electrodes are insulated to the extent that when feed particles contact the electrode they do not lose their charge. Should feed material tend to adhere to the electrode due to electrostatic forces, then the accumulation of material is limited by the action of a mechanical vibrating device or shaker acting on the electrode. The combination of inertial forces and gravitation forces acting on the feed particles are sufficient to overcome the electrostatic and frictional shear forces on feed material in the outer layers of the accumulated bed. If preferred, the polarity of the voltage on the plate can be momentarily reversed or neutralised during the plate cleaning operation.

One of the advantages of using insulted electrodes, in accordance with the present invention is that, when material makes contact with an uninsulated electrode (attracted by the opposite charge) it loses its charge to the electrode and hence loses the associated electrostatic separating force. Consequently're-mixing'of separated components can occur due to factors such as turbulence, and the quality of the final separation will be impaired ie the'product grade'is reduced. When insulated electrodes are used the particles retain their charge when contacting the electrode and hence the quality of separation is high.

Another advantage of using insulated electrodes, in accordance with the present invention is that, for good separation, the electrodes have to be charged to a high voltage which constitutes an operating hazard (for example accidental electric shock and arcing or surface tracking between the electrodes and the earthed frame of the machine). The insulated electrodes are much safer, and although against recommendations, it is possible to touch the insulated electrode charged to 30 kV or higher without receiving an electric shock. The use of insulation on the electrode reduces the tendency of the device to produce arcing between the electrodes.

Examples of embodiments of insulated electrodes suitable for use in the present invention, are shown in Figure 2. In Figure 2 (a) is shown an intermediate electrode 17 which would divide two separating chambers. The electrode attracts like material on both sides. In Figure 2 (b) is shown a boundary electrode 18 which may have increased insulation thickness on its external face. Optionally these boundary electrodes can have a conductive coating on their external faces or alternatively be laminated with a metallic or conductive material for the purpose of external earthing of the device.

The insulated electrodes can be constructed using any suitable conductor embedded in a suitable insulating material. Typical conductors are plate steel or metal wires or metal mesh, graphite powder or carbon fibre laminated in the composite structure can also be used as a conductor. The insulating material can be any of a range of non-conductors including polymers, fibre reinforced polymers, glass, ceramics etc. An embedded metal rod or wire is used to connect the embedded electrode to the high voltage source.

As shown in Figures 1 and 3, the present invention may be embodied as a multi-chamber device. The embodiment illustrated shows a four-chamber device 1. The device 1 is designed with multiple electrode chambers with a spider feed arrangement to distribute to the separate chambers and a multiple channel launder to combine the separated streams into'product'and reject'streams. The combination of these features allows for a high capacity unit, and a high quality of separation, i. e., high product grades, being obtained.

The multi-chamber configuration allows for operation of a proportion of capacity, e. g. 25% or 50% or 75% or 100%, thus maintaining a high quality of separation over the full range of throughput (ie mass flow rate).

The device of the present invention is preferably embodied in a modular construction, using a structural insulating material such as fibre reinforced polymer or other, to create a series of separating chambers, such as shown in Figure 3, for example, a series of rectangular'fibreglass'tubes. The walls of the fibreglass tubes have embedded electrodes at appropriate locations which can be charged, as illustrated in Figure 2. Such an embodiment provides a fully insulated machine of highly modular construction, which is thermally insulated and of light weight.

The walls of the separating chambers can be angled either in a convergent or divergent manner, in side elevation or in plan view. Otherwise, the plates and chamber can be inclined to the horizontal thus generating an electrostatic field gradient. In a further embodiment the wall angles can be made adjustable. Examples of particular embodiments are illustrated in Figures 4 (a) to 4 (f).

When feed material passes through the separating chamber between insulated electrodes, it is acted on by a number of forces including, electrostatic, gravity, surface friction and aerodynamic drag. Separation is effected by obtaining an advantageous balance between these forces such that particles of different species are moved to different sides of the separating chamber. The balance of forces can be altered or adjusted by changing the geometry of the separating chamber such as by changing the electrode angles of

inclination, or their surface profiles so as to create a divergent or convergent passage.

Hence the different geometry shown in Figure 4 (but not limited to those illustrated) are included as part of this invention so that the separator can be designed to give optimum performance for a particular application or duty. Adjustments to the electrode angles of inclination can be used as a means of process control thereby allowing the process to remain in its optimum state allowing for fluctuations in material feed or other process variables.

In a further preferred embodiment, the insulated electrodes are slotted to facilitate the passage of already separated material to a collection duct outside the main separation chamber so that the stream of feed material is progressively reduced. The particular features pertaining to this embodiment are illustrated in Figures 5 (a) to 5 (c). The advantage of this embodiment are that, by progressively removing the material already separated from the separating chamber, the remaining material (which is likely to be 'more difficult to separate') can be acted on by the combination of forces as described above without interference from material already separated and removed. Hence the outcome is an improved final separation.

The electrostatic separator device of the present invention therefore utilises a'plate cascade'over which the feed material is passed to effect tribo-static charging. The cascade plates may be vibrated either horizontally, vertically or in-plane in order to enhance friction between particles. The tribo-static charge can be augmented by utilising a'corona discharge'device close to the cascade outlet and prior to the electrostatic field.

The device uses insulated electrodes, consisting of conducting electrode material embedded in a non-conductive structural material. The conductive material can be selectively placed in order to give optimum separating effects on the material stream being processed. The'insulated electrodes'can therefore be used to form the structure of the separator thereby combining the electrode with the machine enclosure, thus consolidating two essential components into one. The electrode insulating material can additionally be thermally insulating thus reducing heat losses from the device and reducing or eliminating

the need for separate thermal insulation. The electrodes can be fitted with a shaking or vibrating device to assist in the removal of material adhering to the surface of the electrodes due to electrostatic forces. The voltage on the electrode can be temporarily removed or reversed in polarity in assist in particle removal.

The insulated electrodes may be selectively orientated so as to create modular separation compartments in which electrodes can be sequentially of alternating polarity so that a modular separator can be effected which may be used for treating incremental flow rates of material.

The insulating electrodes may be orientated at selective angles, e. g. convergent or divergent, in side elevation, or in plan, as shown in Figure 4. The device may also be configured, so that the plate angles are adjustable.

The insulated electrodes may incorporate a series of slots to allow the separated material to pass through to a collection channel outside the main separating chamber. In this manner, the feed material is incrementally separated and collected so that the stream of feed material progressively reduces through the separating chamber from inlet to outlet.

As such, the present invention provides a device using insulated electrodes so that particles retain their charge on contact and incorporating a reciprocating action vibration generator to aid removal of material from the electrodes. A modular device with single or multiple separation chambers wherein the walls of the device may be made from the same material as the electrodes themselves.

The present invention has been hereinbefore described with reference to specific examples.

It will be appreciated that numerous variations and modifications will become apparent to persons skilled in the art. All such variations and modifications should be considered to fall within the scope of the invention as broadly hereinbefore described and as hereinafter claimed.