AU2006300895A1

AU2006300895A1 - Device for and method of separating particles

Info

Publication number: AU2006300895A1
Application number: AU2006300895A
Authority: AU
Inventors: Sergei Dimitrievich Vaulin
Original assignee: Anglo Operations Pty Ltd
Current assignee: Anglo Operations Pty Ltd
Priority date: 2005-10-13
Filing date: 2006-10-13
Publication date: 2007-04-19
Anticipated expiration: 2026-10-13
Also published as: ZA200803217B; AU2006300895B2; WO2007042929A1

Description

WO 2007/042929 PCT/IB2006/002863 DEVICE FOR AND METHOD OF SEPARATING PARTICLES BACKGROUND OF THE INVENTION THIS invention relates to a device for and method of separating particles. Referring to Figures 1 and 2, a conventional HTR (High Tension Roll) drum separator 10 is used for the electrostatic separation of particles in a mixture 12, the particles having different conductivities. The mixture 12 of conductive particles 14 and insulative particles 16 is fed from a feeder or hopper 18 by gravity onto an operatively uppermost surface of a rotatable drum 20. The drum 20 can have either a conductive or a non-conductive drum surface, but usually takes the form of a conductive drum surface, which will be assumed for the remainder of this specification. As the mixture 12 moves with the drum 20, it enters a charging zone 22 that is defined by a plurality of ions 24 produced by a corona electrode 26. The ions 24 bring electric charges to the surfaces of the particles 14, 16, so that, in an example embodiment, the particles 14, 16 become charged by the ions 24 produced by the corona electrode 26. The acquired charge creates Coulombic forces that clamp the particles 14, 16 to the surface 28 of the drum 20. As the mixture 12 exits the charging zone 22 as the drum 20 rotates further, the acquired charge starts to decay. The speed of this discharging process depends on numerous factors, including bulk and surface conductivities of the particles 14, 16. Conductive particles 14 lose their charge to the drum 20 relatively quickly, and thus have a relatively reduced associated clamping force. These particles 14 may then fall down from the surface 28 of the drum 20, under CONFIRMATION COPY WO 2007/042929 PCT/IB2006/002863 -2 the combined influence of gravity and an associated deflection electrode 27, as indicated by arrow 30, and accumulate in a conductor collection bin. In an example embodiment, the deflection electrode 27 is connected to a high DC voltage source having the same polarity as the corona electrode. The deflection electrode 27 creates the necessary electrical field in the gap between the drum surface 28 and the deflection electrode 27. Insulative particles 16, or at least less conductive particles, remain charged and thus remain attracted/clamped to the surface 28 of the drum 20. These particles 16 are then removed by electrical or mechanical means further on in the rotation of the drum 20, and accumulate in a non-conductor collection bin. A significant problem for achieving high grade together with high throughput of separation is the feeding of the mixture 12. The type of separator 10 described above uses conductivity properties of the particles 14, 16 to create differences in charges, so as to differentiate the behavior of the particles 14, 16 in order to separate them. In this case, therefore, the positioning of the particles 14, 16 on the surface 28 of the drum 20 is an important factor. In particular, and as indicated above, the particles 14, 16 have ideally to form a monolayer on the surface 28 of the rotating drum 20 so as to achieve the best possible electrical contact between all of the particles 14, 16 and the surface 28 of the drum 20. However, this is often not possible, with excess particles 14, 16 often being fed so as to form more than one layer on the drum surface 28, as shown in Figure 3. This tends to severely degrade the quality of separation. Although several measures are used to address the problem/s mentioned above, including, for example, decreasing the feed rate, a major difficulty with the separation of particles, and in particular, fine particles, is agglomeration. Agglomeration can be caused by a number of different factors, one of them being the presence of electrostatic charges. Electrostatic charges result from past processes with the particles, and WO 2007/042929 PCT/IB2006/002863 -3 from triboelectricification processes. These charges and resulting forces start to play a bigger and bigger role with decreasing particle size. The surface and mass of the particles are, respectively, the second and third order of the physical dimensions. Thus, for the same density of surface charges, the relatively smaller particle size results in the electrostatic forces becoming larger than the force of gravity, so that particles with different potentials stay attracted to each other. Such agglomerates are very stable and can hold charges for very long periods of time, and apply to both non conductive particles and to a mixture of conductive and non-conductive particles. Conventional separation processes of the type described above cannot be performed under such conditions, as these agglomerates are formed from different types of particles. It is therefore important to eliminate or reduce the formation of such agglomerates and to create conditions that prevent the formation of such agglomerates, thereby allowing the separation of these materials. In addition, with the conventional separating arrangement described above, the influences of the charging corona electrode 26 and deflection electrode 27 tend to overlap, thus decreasing the overall efficiency of such an arrangement. To address the above shortcomings, co-pending International patent application no. PCT/1B2005/002026 (WO 2006/011018) will now be described with reference to Figures 4 and 5. Referring first to Figure 4, a device 32 is shown for separating particles in a material 34, the particles having different conductivities. The device 32 comprises a feeder 36 that defines an outlet opening 38. Charging means, in the form of at least one corona electrode 40, is located proximate the outlet opening 38 of the feeder 36, with the charging means being submerged in the material 34 so as to directly charge the particles. A rotatable drum 42 is located adjacent the outlet opening of the feeder 36.

WO 2007/042929 PCT/IB2006/002863 -4 In use, the first charging means 40 produces a cloud of similarly charged particles that leave the feeder 36 via the outlet opening 38, as indicated by arrow 44. These particles are attracted by Coulombic forces to the surface of the drum 42, and thus land on the rotatable drum 42 as a monolayer 54, as shown in Figure 5. Monolayer placement of particles takes place inherently due to the fact that the direction of the Coulombic force experienced by a particle is only to unoccupied surface sections of the drum. As a result, each particle can only be positioned on the drum surface and not on top of another particle. Particles that do not attach to the drum 42 (for example, if there is no free surface available) may fall under the influence of gravity to the feeder 36 for recharge and a next attempt. One drawback with this arrangement, however, is the agglomeration of the material surface at the feeder, forming, as it were, a rigid skin. This may be caused by ions in the air which charge the top layer of material on the feeder and/or charged particles falling back from the drum. The result is a thin layer of charged particles on the surface of the material pile within the feeder, which may be attracted to the uncharged particles inside the pile and thus clamp the material within the feeder together. In any event, after moving along as the drum 42 rotates, conductive particles lose their charge to the drum 42 and thus fall off the surface 46 of the drum 42, as indicated by arrow 48. Conversely, insulative/less conductive particles remain charged and thus attracted to the surface 46 of the drum 42 so as to be removed by electrical or mechanical means further on in the rotation of the drum 42, as indicated by arrow 50 in Figure 4. It is the objective of the present invention to improve the separation of particles by ensuring even charging of the particles prior to placement on the drum and by providing strictly defined and non-overlapping particle charging and particle discharging/separation zones on the drum surface.

WO 2007/042929 PCT/IB2006/002863 -5 SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a device for separating particles in a material, the particles having different conductivities, the device comprising: an enclosure for accommodating the material, the enclosure defining an outlet opening; a rotatable transfer means located adjacent the outlet opening of the enclosure; first charging means located within the enclosure for directly charging the particles so that similarly charged particles leave the enclosure via the outlet opening and land on the rotatable transfer means via the outlet opening of the enclosure; second charging means located adjacent the enclosure and proximate the rotatable transfer means for further charging the particles on the rotatable transfer means; and a shielding enclosure to accommodate the second charging means, the shielding enclosure being arranged to mechanically limit the charging action by the second charging means to a predefined zone adjacent the rotatable transfer means, wherein, in use, conductive particles, upon moving past the shielding enclosure, can subsequently lose their charge to the transfer means and thus fall off the surface of the transfer means, while the relatively less conductive particles remain charged and thus attracted to the surface of the transfer means so as to be removed by electrical or mechanical means further on in the rotation of the transfer means.

WO 2007/042929 PCT/IB2006/002863 -6 In an example embodiment, the particles landing on the rotatable transfer means form a monolayer on the rotatable transfer means. In an example embodiment, the enclosure is located operatively above the rotatable transfer means so that the particles land on the transfer means under the influence of gravity and Coulombic forces. In an example embodiment, the first charging means comprises a plurality of stacked high voltage electrodes, with the particles being charged as they come into contact with the electrodes as they slide down past the electrodes under the influence of gravity. In an example embodiment, the second charging means comprises a plurality of charging corona electrodes fitted within the shielding enclosure. In an example embodiment, the shielding enclosure comprises a roof and at least one pair of downwardly extending side walls, with the charging corona electrodes being fitted within the shielding enclosure. In an example embodiment, to assist in the separation of the particles in the material, a vibrator may be fitted adjacent the enclosure for vibrating the enclosure. According to a second aspect of the present invention there is provided a method of separating particles in a material, the particles having different conductivities, the method comprising the steps of: receiving the material in a enclosure, the enclosure defining an outlet opening; charging the particles to the same potential within the enclosure; WO 2007/042929 PCT/IB2006/002863 -7 allowing the charged particles to leave the enclosure via the outlet opening and to land onto an adjacent rotatable transfer means as a monolayer; charging the particles on the rotatable transfer means after they have left the enclosure, but limiting this charging action to a predefined zone adjacent the rotatable transfer means, wherein, in use, after the second charging step, conductive particles can subsequently lose their charge to the transfer means and thus fall off the surface of the charging means, while the relatively less conductive particles remain charged and thus attracted to the surface of the transfer means so as to be removed by electrical or mechanical means further on in the rotation of the transfer means. Conveniently, the method includes the step of vibrating the enclosure. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a first prior art conventional drum separator for separating particles having different conductivities; Figure 2 shows a side view of a section of the drum of the conventional drum separator shown in Figure 1; Figure 3 shows a common problem with conventional drum separators that needs to be addressed, namely the agglomeration of particles, and in particle fine particles, on the surface of the drum of the conventional drum separator; Figure 4 shows a drum separator for separating particles having different conductivities according to a second prior art embodiment; WO 2007/042929 PCT/IB2006/002863 -8 Figure 5 shows the particles landing on a drum of the drum separator shown in Figure 4 in the form of a monolayer; Figure 6 shows a part of a drum separator for separating particles having different conductivities according to the present invention; Figure 7 shows a part of a feeding technique used in the drum separator shown in Figure 6; and Figure 8 is a graph showing the clear separation of the influences of the corona electrode and the deflecting electrode in the device of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figures 6 and 7, a device 60 is shown for separating particles in a material 62, the particles having different conductivities. The device 60 comprises an enclosure 64 made from insulating material for accommodating the material 62, the enclosure 64 defining an outlet opening 66. The enclosure 64 may be located below a hopper or feeder (not shown) to receive material stored within the hopper or feeder. The enclosure 64 defines a charging chamber that in turn comprises first charging means. In an example embodiment, the first charging means comprises at least one high voltage electrode 70. A rotatable drum 72 defining a drum surface 74 is located adjacent the outlet opening 66 of the enclosure 64. In an example embodiment, the charging chamber in the enclosure 64 in Figure 6 comprises a plurality of high voltage electrodes 70 within the enclosure 64 so that there is close physical contact between the particle WO 2007/042929 PCT/IB2006/002863 -9 material 62 and the charging electrodes 70. This ensures that all particles within the material 62 achieve the same potential on their surfaces. In an example embodiment, the charging electrodes 70 are packed, as shown in Figure 7, so as to define a grid of electrodes 70 so as to increase the total surface of the charging electrodes 70. All particles of the material 62 may thus achieve the same potential due to the mechanical contact with the electrodes 70 during the particles' movement from one charging electrode to another. Thus, in a preferred version of the invention, it is envisaged that the particles within the material 62 could be charged by causing them to slide past, or otherwise causing them to contact, the high voltage electrodes 70. The feeding technique described in the preceding paragraph using the example embodiment shown in Figure 7, can achieve a relatively higher particle placement density on the drum surface 74, as charging and feeding events are now coupled. In addition, by top-feeding the particles onto the drum, under the influence of gravity, substantially reduces the need for the Coulombic forces to be stronger than the gravitational forces, since both forces are substantially acting in the same direction. Advantageously, the Coulombic forces may now act more as a guide to direct particles to free areas on the drum's surface 74. A shielding enclosure 76 is located proximate the outlet opening 66 of the enclosure 64 and adjacent the rotatable drum 72. The shielding enclosure 76 accommodates second charging means for further charging the particles on the rotatable drum 72 as the particles move under the shielding enclosure 76. In particular, the purpose of the second charging means is to try and ensure that all the particles on the drum surface 74 have the same charge density. In an example embodiment, the second charging means comprises a plurality of charging high voltage corona electrodes 78 fitted within the WO 2007/042929 PCT/IB2006/002863 -10 shielding enclosure 76, the corona electrodes 78 producing ions to further charge the particles. The shielding enclosure 76 may be constructed from a non-conductive material, and, in an example embodiment, the shielding enclosure 76 comprises a roof and at least one pair of downwardly extending side walls, with the charging corona electrodes 78 being fitted within the shielding enclosure 76. This arrangement provides a mechanical limit to the charging action of the ions from the corona electrodes 78, so as to strictly define and delineate a second charging zone adjacent the rotatable transfer means. Although not shown, the distance between shielding enclosure 76 and the drum 72 is adjustable so as to be as close to the drum surface 74 as possible, yet providing a sufficient gap to allow the particles on the drum surface 74 to be carried along uninhibited. In use, the first charging means 68 is used to charge all of the particles to the same potential. Thereafter, the pre-charged particles are fed to the top of the drum surface 74 by gravitational and Coulombic forces (as opposed to Coulombic forces only as with the embodiment shown in Figure 4). The particles then get further charged as they pass under the enclosure 76. After passing past the enclosure 76, conductive particles lose their charge to the drum surface 74 and thus fall off the drum 72, in a similar way as indicated by arrow 48 in Figure 4 and/or get removed under the influence of a deflecting electrode 80 and gravity. The insulative/less conductive particles remain charged and thus attracted to the surface 74 of the drum 72 so as to be removed by electrical or mechanical means further on in the rotation of the drum 72, in a similar way as indicated by arrow 50 in Figure 4. Thus, an important feature of the present invention is to charge all particles to the same potential prior to them landing on the rotating drum 72, by the WO 2007/042929 PCT/IB2006/002863 -11first charging means 68 so as to produce a monolayer of particles on the drum 72, and after they have landed on the drum 72, to further charge the particles with the second charging means 78, in order for the particles to have the same charge. In an example embodiment, this ensures that the charging influences of the first and second charging means are physically and functionally separated. This could not be done with the existing drum separators by simply applying free charges from corona electrodes to the agglomerates, as these charges would reside at their surface and would increase the attractive forces between the particles. Also, conventional drum separators do not strictly separate particle charging and discharging processes, as described above, with the present now providing separate and distinct/non-overlapping particle charging and particle discharging/separation zones on the drum surface. This can be seen from the graph in Figure 8, in which the influence of the ions from the corona electrode is clearly separated from the influence of the deflecting electrode. To further assist in the prevention of the formation of agglomerates, a vibrator may be fitted adjacent the enclosure. This vibrator would serve to break agglomerates and prevent a rigid or blocking layer from forming within the enclosure, thereby facilitating the feeding process and even charging of the particles.

Claims

1. A device for separating particles in a material, the particles having different conductivities, the device comprising: a enclosure for accommodating the material, the enclosure defining an outlet opening; a rotatable transfer means located adjacent the outlet opening of the enclosure; first charging means located within the enclosure and proximate the outlet opening of the enclosure, the first charging means being arranged to directly charge the particles so that similarly charged particles leave the enclosure via the outlet opening and land on the rotatable transfer means via the outlet opening of the enclosure; second charging means located adjacent the enclosure and proximate the rotatable transfer means for further charging the particles on the rotatable transfer means; and a shielding enclosure to accommodate the second charging means, the shielding enclosure being arranged to mechanically limit the charging action by the second charging means to a predefined zone adjacent the rotatable transfer means, wherein, in use, conductive particles, upon moving past the shielding enclosure, can subsequently lose their charge to the transfer means and thus fall off the surface of the drum, while the relatively less conductive particles remain charged and thus attracted to the surface of the transfer means so as to be removed WO 2007/042929 PCT/IB2006/002863 -13 by electrical or mechanical means further on in the rotation of the transfer means.

2. A device for separating particles in a material according to claim 1, wherein the particles landing on the rotatable transfer means form a monolayer on the rotatable transfer means.

3. A device for separating particles in a material according to any one of the preceding claims, wherein the enclosure is located operatively above the rotatable transfer means so that the particles land on the transfer means under the influence of gravity and Coulombic forces.

4. A device for separating particles in a material according to claim 3, wherein the first charging means comprises a plurality of stacked high voltage electrodes, with the particles being charged as they come into contact with the electrodes as they slide down past the electrodes under the influence of gravity.

5. A device for separating particles in a material according to any one of the preceding claims, wherein the second charging means comprises a plurality of charging corona electrodes fitted within the shielding enclosure.

6. A device for separating particles in a material according to claim 5, wherein the shielding enclosure comprises a roof and at least one pair of downwardly extending side walls, with the charging corona electrodes being fitted within the shielding enclosure.

7. A device for separating particles in a material according to any one of the preceding claims, which includes a vibrator adjacent the enclosure for vibrating the enclosure to assist in the even charging of the particles in the material. WO 2007/042929 PCT/IB2006/002863 -14

8. A method of separating particles in a material, the particles having different conductivities, the method comprising the steps of: receiving the material in a enclosure, the enclosure defining an outlet opening; charging the particles to the same potential within the enclosure; allowing the charged particles to leave the enclosure via the outlet opening and to land onto an adjacent rotatable transfer means as a monolayer; charging the particles on the rotatable transfer means after they have left the enclosure, but limiting this charging action to a predefined zone adjacent the rotatable transfer means, wherein, in use, after the second charging step, conductive particles can subsequently lose their charge to the transfer means and thus fall off the surface of the charging means, while the relatively less conductive particles remain charged and thus attracted to the surface of the transfer means so as to be removed by electrical or mechanical means further on in the rotation of the transfer means.

9. A method of separating particles in a material according to claim 8, the method including the step of vibrating the enclosure.