GB2129337A - An electrical separator of the ion bombardment type - Google Patents

Abstract

An electrodynamic particle separator of the ion bombardment type which utilises a rotating collecting roll 10 and a grid plate 28 having a plurality of ion discharge electrodes 24 positioned about a substantial portion of the collecting roll periphery, to subject a stream of particulate material 20 passing therebetween to an ion field over a substantial length of travel, whereby non-conducting particles 36 retain charge they receive and are thereby attracted to the grounded surface of the collecting roll, while conducting particles 38 lose any charge they receive and continue to fall downwardly under the force of gravity. A movable gate 22 is positioned in the stream of particulate material beneath the ion field to separate the non- conducting particles attracted to the roll and falling substantially beneath it from the conducting particles which fall in their original path. The latter may pass through a second similar separator for further improved separation. <IMAGE>

Description

SPECIFICATION An electrical separator of the ion bombardment type This invention relates to the general field of separating particles by subjecting a stream of particulate material to ion bombardment whereby some of the paticles, which are relatively non conducting (electrical) in nature, receive and hold the ionic charge, while other particles in the stream, which are relatively conducting in nature, quickly give up or dissipate any ions they receive.

Even though this is an ion bombardment process, it is often classified through long usage under the general classification of electrostatic separation, such as dielectric separation, electrical charge migration by heating and cooling, and conductive induction. The ion bombardment method is actually electrodynamic in nature because there is an actual flow of ions from the charged electrode.

While apparatus incorporating all of these methods is known, this invention optimises the ionic separating process by subjecting the stream of particulate material to a substantially uniform flow of ions from a plurality of discharge electrodes over a substantial area while the stream is passing between, and free falling from, the surface of a grounded collecting roll and a curved grid, extending around the roll surface, on which the plurality of ion discharge electrodes are mounted.

In prior apparatus utilising the ion bombardment principle, the particulate material was first brought into carrying contact with the surface of a rotating collecting roll while an essentially point source of ions was directed to the material while it was still being carried on the surface of the collecting roll.

While this process works, it has several inefficiencies, such as requiring the collecting roll to rotate relatively slowly while the bed of particulate material carried on the collecting roll surface prevents the separation of the nonconducting particles from the conducting particles under the comparatively weak force of the bombarding ions.

According to the invention there is provided an electrical separator of the ion bombardment type for separating a stream of particulate material into conducting and non-conducting components, said separator including a grounded collecting roll and an infeed chute for receiving the material and directing it into the apparatus, the infeed chute being arranged to direct the incoming material onto the collecting roll surface in an upper quadrant thereof; grid means having a plurality of electrically powered ion discharge electrodes in spaced array about a portion of the collecting roll periphery to thereby define a space to subject the particles on the roll and immediately below where they leave the roll surface to ion bombardment whereby the particles are exposed to ion bombardment for a substantial length of their travel about the collecting roll past the ion discharge electrodes where the non-conducting particles acquire a charge and are thereby urged towards the collecting roll, and the conducting particles dissipate any charge they acquire and continue to free fall along the path they start when they travel past the collecting roll; and separation means positioned beneath where the incoming particulate material passes between the collecting roll and ion discharge electrodes, whereby the non-conducting component of particles falls on one side thereof and the conducting component of particles falls on the other side thereof to be collected separately.

Subjecting the stream of particulate material to a free fall helps to physically separate the particles so that they can be more thoroughly exposed to the ion field and thereby enhance the separation process. The invention is especially suited to separate fines from wood chips, particularly unscreened wood chips having a moisture content of up to about 55%.

By directing the free fall stream between the surface of the rotating contacting roll and the grids containing the ion discharge electrodes, the nonconducting particles which acquire a charge can be readily attracted to the lower potential of the grounded contacting roll. The non-conducting particles migrate toward the collecting roll where they either become attached to its surface or fall beneath it. Attached particles may be scraped therefrom, but in all cases the non-conducting particles are urged out of the free falling stream of material under the force of their electrical potential relative to the grounded contacting roll.

They are thereby directed to a point remote from the falling stream of the other, conducting, particles which essentially fall downwardly under the force of their momentum and gravity.

In a preferred embodiment a movable gate on the upper end of a chute is interposed in the free falling streams of material at a point below where the non-conducting and conducting particles have been separated.

The stream of conducting particles is then directed onto the surface of a second contacting roll mounted beneath the first contacting roll where essentially the same process is repeated so that the separated non-conducting particles from this second pass are collected with the nonconducting particles from the first pass to increase the efficiency of non-conducting/conducting particle separation. This apparatus can thereby utilise low power consumption to sustain the electric ion field, such as approximately 49 watts per lineal metre of drum surface length. By providing an adjustable gate at the upper edge of the chutes separating the streams of separated particles in both passes, the efficiency and flexibility of the apparatus is easiiy adjusted.

Further, due to the non-conducting properties of sand and grit, this apparatus utilising the ion bombardment principle is particularly suitable for processing wood chips to separate out sand, grit and fines.

Accordingly, it is an object of this invention to provide apparatus utilising the ion bombardment principle to produce a more efficient separation of non-conducting and conducting particles.

Another object of this invention is to provide an apparatus utilising the ion bombardment principle wherein a plurality of ion discharge electrodes are provided in spaced array from the surface of a rotating contact roll between which a free falling stream of particulate material passes.

Still another object of the invention is to provide ion bombardment particle separation apparatus wherein the conducting portion of the particles is passed a plurality of times through the ion bombardment process to further improve the overall efficiency of the apparatus.

The following is a detailed description of an embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a side elevational view of the apparatus, partially in schematic form, illustrating the positioning of the ion discharge electrodes relative to the falling stream of particulate material, and Figure 2 is a slightly enlarged view of part of the apparatus shown in Figure 1 wherein the resultant force vectors on individual particles spaced relatively closer and further from the surface of the contacting roll are illustrated.

As shown in Figure 1, a stream of incoming particulate material 20 to be separated is directed by a feeder 14, which may be a vibratory feeder, in the direction of the arrow 21 onto the peripheral surface of a collecting roll 10 which is rotating in the direction of the arrow. The surface of the collecting roll is connected to ground potential at 11. The incoming particulate material such as wood chips containing fines, sand, grit and other similar materials that are desired to be separated out, is briefly passed onto and over a relatively small upper peripheral portion of the rotating surface of the collecting roll and then begins a free fall along a trajectory determined by gravity and the momentum of the particles.A grid plate 28 is disposed about the surface of collecting roll 10 and spaced therefrom so as to permit the incoming material to pass between the collecting roll and a plurality of ion discharge electrodes 24 which are mounted in the grid plate. The grid plate can be articulated, as shown, or curved, but in any case it extends about one side of the collecting roll with its lower part extending substantially straight downward so as not to impede the fall of the conducting portion of material.

A source of high voltage direct current power 16 (i.e. about 40-60 KV) is connected to the grid plate at a potential just below the sparking point whereby ion discharge electrodes 24 emit a stream of ions, preferably negative ions indicated at 27, in a direction substantially radially toward the peripheral surface of the collecting roll 10 to intercept (i.e. "bombard") the particles travelling therebetween. These ions travel in the direction of the maximum voltage gradient between the electrodes and the collecting roll.

The particles can be broadly classified as non conducting and conducting particles, meaning that the non-conducting particles tend not to conduct an electrical charge while the conducting particles tend to lose or dissipate any charge they may acquire. In the ion bombardment apparatus of this invention, the particles are exposed to the ions by coiliding with them.

As shown in Figure 2, the non-conducting particles 36 which have received an ion charge tend to have an affinity for the surface of grounded collecting roll 10 and either become attached to its surface or simply move towards it under the force of their attraction. In either case, they pass to the left of adjustable gate 22 by either being scraped from the surface of the collecting roll by scraper 1 8 or falling beneath collecting roll 10 after being deflected from their path of travel off the roll.

On the other hand, the conducting particles 38, having passed any charge acquired by the bombarding ions relatively quickly, remain essentially unaffected in their path of travel downwardly from the surface of the upper quadrant of collecting roll 10 and therefore fall to the right of the adjustable gate 22 and onto the upper conductor chute 34.

The factors affecting particle separation, and the process efficiency are 1) the voltage applied, 2) the moisture content of the particle, 3) the particle size, 4) the position of the adjustable gate and, 5) the feed rate.

Rgarding the applied voltage, as the voltage increases, the electric field increases and a greater force is applied to the particle, thus displacing it a greater distance. Regarding moisture content, a greater force is required to displace a particle of a given size with increasing moisture content.

Regarding particle size, larger particles tend to maintain their original trajectory through the apparatus, while the smaller particles are displaced the greatest distance in the direction of the collecting roll. The adjustable gate (described in more detail below), is a mechanical way of adjusting flow proportions between the conducting and non-conducting particles.

Regarding the feed rate, the overall separation efficiency decreases as the feed rate increases.

Therefore, uniform distribution across the width of the collecting roll is important.

Figure 2 also illustrates the resultant force vectors 44, 46 on a pair of non-conductor particles 43, 45 respectively, which are resultants of the downward force due to gravity and momentum and the sideways force due to the ion bombardment. This also iliustrates how the resultant force on the particles is a function of their speed and position in the ion force field. Their speed is partly a function of the angle at which the feeder chute 14 directs the incoming material onto the collecting roll surface. If the angle is too small, the material will contact the grid and wear out the discharge electrodes. If the angle is too large, the material, including the fine conductive particles, will tend to pile up on the collecting roll surface and acquire the same charge as the grounded collecting roll and tend to be attracted back towards the discharge electrodes, thus reducing the operating efficiency.

Referring again to Figure 1, the non-conducting particles scraped from, or diverted beneath, the collecting roll 10 are conveyed by the upper nonconductor chute 40 downwardly and around the backside of a second collecting roll 1 2 which is positioned beneath the first collecting roll 1 0.

Similarly, the upper conductor chute 34 conveys the conducting particles downwardly to a second, lower feeder chute 48 which is positioned to feed the conducting particles onto the upper quadrant of the second collecting roll 12 in a manner similar to the way material is directed onto the collecting roll 10.

A grid plate 30 is shaped around the upper and lower quadrants of one side of the collecting roll 12 and is spaced from its surface so as to provide a space in which the travelling particles are exposed to a pluraility of ion discharge electrodes 26 which are powered by a high voltage DC power supply 1 7 in a manner similar to that described in conjunction with the upper collecting roll 1 0 and grid plate 28.

The particulate component falling between the grid plate 30 and the collecting roll 12, which was the conducting particle component from roll 10, still contains some non-conducting particles which were not separated out above. Accordingly, under the influence of the ion bombardment discharge 27 provided by electrodes 26, any remaining non-conducting particles are urged towards the collecting roll 12 where they either adhere to its surface and are scraped off by the scraper 19, or are urged by their attraction to the collecting roll 12, which is grounded at 13, to the area to the left of the adjustable gate 23. In either case, they fall together onto the lower nonconductor discharge chute 42 where they are combined with the previously separated nonconductor particles falling on the other side of the upper non-conductor chute 40.The particulate component containing mostly conducting particles, being substantially unaffected by the ion bombardment in its path of travel from the collecting roll 12 and its free fall path downwardly, is conveyed by the lower conductor chute 35 onto a discharge chute 50.

Thus, the particles conveyed by the lower nonconductor and conductor discharge chutes 42 and 50 are substantially completely separated into non-conductor and conductor particulate components, respectively. By initially depositing the incoming material onto the surface of a collecting roll and quickly permitting the particles to separate in a free fall for a relatively long distance between a plurality of ion discharge electrodes disposed about the collecting roll's periphery on one side thereof, the non-conducting particles are exposed for a substantial period to the ion bombardment process. This, coupled with the second pass, increases the efficiency and extent of the separation of the particles.

Claims (8)

1. An electrical separator of the ion bombardment type for separating a stream of particulate material into conducting and nonconducting components, said separator including a grounded collecting roll and an infeed chute for receiving the material and directing it into the apparatus, the infeed chute being arranged to direct the incoming material onto the collecting roll surface in an upper quadrant thereof; grid means having a plurality of electrically powered ion discharge electrodes in spaced array about a portion of the collecting roll periphery to thereby define a space to subject the particles on the roll and immediately below where they leave the roll surface to ion bombardment whereby the particles are exposed to ion bombardment for a substantial length of their travel about the collecting roll past the ion discharge electrodes where the nonconducting particles acquire a charge and are thereby urged toward the collecting roll, and the conducting particles dissipate any charge they acquire and continue to free fall along the path they start when they travel past the collecting roll; and separation means positioned beneath where the incoming particulate material passes between the collecting roll and ion discharge electrodes, whereby the non-conducting component of particles falls on one side thereof and the conducting component of particles falls on the other side thereof to be collected separately.

2. A separator according to claim 1, wherein the infeed chute is selectively positionable to direct the incoming material onto a relatively small peripheral segment of the collecting roll before the material leaves to free fall between the roll and ion discharge electrodes.

3. A separator according to claim 1 or claim 2, wherein the ion electrodes are linked with a source of high voltage direct current whereby the electrodes create a force field of ions between them and the surface of the collecting roll.

4. A separator according to any of claims 1 to 3, wherein the ion discharge electrodes are mounted in a grid plate which substantially follows the collecting roll periphery and partially extends downwardly therefrom to thereby establish an ion bombardment space extending from approximately where the material contacts the collecting roll to where the adjustable gate intercepts the downwardly passing particles.

5. A separator according to any of claims 1 to 4, wherein the separation means comprises an adjustable gate.

6. A separator according to any of claims 1 to 5, further including: a second grounded collecting roll positioned beneath the first collecting roll for receiving the component of conducting particles therefrom; a plurality of electrically powered ion discharge electrodes arrayed in spaced array from the second collecting roll to thereby define a second space to subject the conducting component of particles received from the first collecting roll on the second collecting roll and immediately below where they leave the second collecting roll surface to ion bombardment a second time so that the non-conducting particles therein acquire a charge and are attracted toward the second collecting roll surface and the conducting particles are allowed to fall under the force of their momentum and gravity; and second separation means positioned beneath the second collecting roll to intercept said separate streams of non-conducting and conducting particles.

respectively, for separate collection.

7. A separator according to claim 6, wherein the non-conducting particles from both the first and second collecting rolls are collected together.

8. An electrical separator substantially as hereinbefore described with reference to the accompanying drawings.