GB2067753A - Method of and apparatus for sorting particulate material - Google Patents

Abstract

A mixture of particles are sorted by moving the particles in a desired trajectory, detecting one constituent of the mixture according to a particular characteristic of the constituent, for example fluorescence induced by exposing the particles to X-rays, and at a subsequent point diverting the constituent from the trajectory, the trajectory being a circular arc and being imposed on the particles by nestling them on the surface of a rotating perforated drum. <IMAGE>

Description

SPECIFICATION Method of and apparatus for sorting particulate material Baci;ground of the Invention This invention relates to the sorting of particulate material.

It is known to separate diamonds from gangue by moving a diamond concentrate in a path past an X-ray source followed by a detector which detects fluorescence excited in any diamond particles which may be present. The signal generated by the detector is used to actuate a device ,which ejects the diamonds from the path.

In one such method the mixture is deposited on an endless conveyor belt to form a single layer.

The belt projects the mixture on a known trajectory through free space. In the course of its flight the mixture is exposed to the X-rays and the zone of sight of the detector, such as a photomultiplier tube. The latter through suitable circuitry then actuates a nozzle to blow diamond particles from the known trajectory on to a second trajectory along which they are collected separately from the main stream of particles.

For reliable results with the last discussed method, it is advisable that the particles be in a dry state when they are fed on to the conveyor otherwise some may stick to the belt and the particles may stick together inter se so that they do not all follow the same trajectory.

An object of the invention is to impose a fixed trajectory on particles to be sorted, particularly those in a wet mixture.

Summary of the Invention The present invention provides a method of sorting a mixture of particles comprising the steps of moving the particles in a desired trajectory at a point in the trajectory detecting one constituent of the mixture according to a particular characteristic of that constituent and at a subsequent point diverting that constituent from the trajectory with the improvement that the trajectory is a circular arc and is imposed on the particles by nestling them on the surface of a rotating drum.

The particular characteristic may be induced in the constituent by passing the trajectory through a field of electromagnetic radiation such as X-rays.

In the preferred form of the invention the drum is perforated with particles nestling on the perforations, particles being diverted by blowing a fluid through the perforations and the nestling on the perforations being assisted by inducing a high pressure on the outside of the drum than on its inside.

Also according to the invention apparatus for sorting a mixture of particles by removing one constituent of the mixture by virtue of a particular characteristic of that constituent comprises a rotating perforated drum, the perforations being such that particles can nestle on them, means for feeding particles in a single layer on to the surface of the drum at the top of its orbit, a detector further along the orbit for detecting the particular characteristic and still further along the orbit means responsive to the detector for removing particles from the drum surface.

The characteristic may be induced by suitable means such as X-ray apparatus between the feed point and the detector.

Brief Description of the Drawings The invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic side view of apparatus according to the invention, and Figure 2 is a plan view in the direction of an arrow marked 2 in Figure 1 of a portion of the apparatus.

Description of a Preferred Embodiment The drawings illustrate an enclosure 10 in which is mounted a drum 12 dor rotation about a substantially horizontal axis 14. An arm 1 6 in the interior of the drum supports an air nozzle 18. The nozzle is connected to a compressed air source by means of a pipe which passes along the axis 14 to the exterior of the drum and the enclosure.

The drum 12 in its upper region has a windbox 20 and this is connected to a vacuum pump by means of a pipe which is also aligned with the axis 14. For convenience the compressed air source may draw its supply of air from within the windbox 20 so that the vacuum is created simultaneously.

The enclosure 10 on its right hand side has a plate 22 which is fairly close to the outer surface of the drum 12. The plate and an outer wall 24 of the enclosure form a chute 26. The lower end of the enclosure 10 is funnel like and terminates in an outlet 28.

An upper portion 30 of the enclosure houses an X-ray tube 32 and photomultiplier tubes 34. The output of the photomultiplier tube 34 is connected to control circuitry, known per se, which controls the operation of the nozzle 18. The X-ray tube 32 and the tube 34 face the drum 12.

Material which is to be sorted is deposited on the upper surface of the drum 12 by means of a vibratoly feeder 36. The material is introduced to the feeder from a hopper 38 and single layer flow of material from the hopper via the feeder to the drum is assisted by the addition of water from a controllable source 40.

The drum 12 is cylindrical and, as shown in Figure 2, its wall comprises a plurality of parallel apertures 42. These apertures are formed by thin metal rods 44 which are parallel to and spaced from one another a distance smaller than the minimum expected diameter of particles to be sorted. In Figure 2 the rods lie transverse to the direction of rotation of the drum. In an alternative embodiment, the rods, and hence the apertures, may be in the direction of rotation of the drums.

During operation of the apparatus the drum is rotated in a clock-wise direction at a suitable speed. The material to be sorted, in this case diamondiferous material, is deposited on the drum at the top of its orbit upstream of the windbox 20.

The addition of water to the material on the vibratory feeder lubricates the material and assists in moving the material smoothly on to the drum. In addition the water tends to disperse the material in a monolayer on the drum.

The material once deposited on the drum is moved on to the windbox 20 where the differential pressures existing on either side of the drum force the material onto the drum thereby causing it to nestle in the swots 42. The position of a particle relatively to the drum therefore remains unaltered as it is moved past the X-ray tube and the photomultiplier tube towards the nozzle 18.

Diamonds which are exposed to the X-ray source have fluorescence excited in them and this is detected by the photomu!tiplier tube which via the control circuitry causes the nozzle 18 to be energized at the appropriate instant as the diamonds are brought past the nozzle. The nozzle blasts from the inside of the drum through its apertured wall to the outside for a short period so that only the diamond and possibly some surrounding material is blasted from the drum. The blasted material passes upwardly over the plate 22 and so falls into the chute 26. The non-blasted material remains on the drum but eventually falls free into the funnel-like structure at the base of the enclosure 10 and eventually passes through the outlet 28 as tailings. Any excess material remaining on the drum is removed by means of waterwashes 46.

The process of sorting is a continuous one with the material continuously being deposited on the drum, being sorted and being removed from the drum.

The use of a circular drum as a carrier for the particulate material means that the drum may be constructed to well defined tolerances with the gaps between the metal rods and the thickness of the rods being optimally determined and being maintainable.

In the illustrated embodiment the X-ray beam is wide enough to cover the width of the drum and the photomultipliers 34 also cover the full width of the drum 12. In addition the nozzle 1 8 also spans the width of the drum. The result is that all particles nestling on a given slot 42 will be blasted off the drum.

It is, of course, possible to have a series of photomultipliers detecting over given segments of the drum actuating jets which blast only over those segments. !n the case of diamond sorting it will be necessary that the segments overlap. In such a case less of the undesirable particles will be blown off the drum.

Note that blow-off occurs at a point where the slope of the drum surface has a pronounced downward component so as to assist in the blowing off action.

Apertures other than slots may be used, but in such cases it is difficult to achieve a pattern which will result in each particle nestling over an aperture.

Claims (19)

1. A method of sorting a mixture of particles comprising the steps of moving the particles in a desired trajectory at a point in the trajectory detecting one constituent of the mixture according to a particular characteristic of that constituent and at a subsequent point diverting that constituent from the trajectory with the improvement that the trajectory is a circular arc and is imposed on the particles by nestling them on the surface of a rotating drum.

2. The method claimed in claim 1 in which the particular characteristic is induced in the constituent by passing the trajectory through a field of electromagnetic radiation.

3. r--a method claimed in either of the above in which the drum is perforated with particles nestling on the perforations and the particles are diverted by blowing a fluid through the perforations.

4. The method claimed in any one of the above claims in which the drum is perforated with particles nestling on the perforations including the step of inducing a higher pressure on the outside of the drum than on its inside to assist in the nestling step.

5. The method claimed in any one of the above claims in which the particles are diverted at a position where the trajectory has a downward component.

6. The method claimed in any one of the above claims in which the one constituent is composed of diamond particles and the characteristic is fluorescence induced by exposing the particles to X-rays prior to detecting the characteristic.

7. Apparatus for sorting a mixture of particles by removing one constituent of the mixture by virtue of a particular characteristic of that constituent comprising a rotating perforated drum, the perforations being such that particles can nestle on them, means for feeding particles in a single layer on to the surface of the drum at the top of its orbit, a detector further along the orbit for detecting the particular characteristic and still further along the orbit means responsive to the detector for removing particles from the drum surface.

8. The apparatus claimed in claim 7 including means for inducing a characteristic in the one constituent between the feed point and the detector.

9. The apparatus claimed in either of claims 7 or 8 in which the means for removing particles from the drum surface is a fluid jet operating from' inside the drum.

1 O. The apparatus claimed in any one of claims 7 to 9 in which the perforations are axial slots in the drum surface.

1 The apparatus claimed in claim 10 in which the slots are bounded by round rods.

12. The apparatus claimed in any of claims 7 to 11 in which the drum surface after the tcp of its orbit passes through a zone in which there is a higher pressure on the outside of the drum than on its inside.

13. The apparatus claimed in claim 12 in which the drum passes through a windbox to which suction is applied on the inside of the drum.

14. A method of sorting a mixture of particles substantially as herein described with reference to the accompanying drawings.

1 5. A method of sorting diamond from gangue substantially as herein described with reference to the accompanying drawings.

1 6. Apparatus for sorting a mixture of particles substantially as herein described with reference to the accompanying drawings.

17. Apparatus for sorting diamond from gangue substantially as herein described with reference to the accompanying drawings.

18. Particle mixture constituents sorted by a method according to any of claims 1 to 6 or claim 14 or by means of an apparatus according to any of claims 7 to 13 or claim 16.

19. Diamond sorted from gangue by a method according to claim 1 5 or by means of an apparatus according to claim 1 7.