Particle classification

Abstract: In the method, particles (16) are classified by locating them in a resonance cavity (10) and applying a swept range of microwave radiation to the cavity. The transmitted signals are detected and analysed. The particle is then assigned to a classification in accordance with changes in the dielectric characteristics of the cavity as a result of the presence of the particle. The radiation is polarised and is applied in at least two mutually orthogonal directions. With the particle located at the centre of the cavity, where the E-field is a maximum, and with signals propagating in at least two mutually orthogonal direction, account is taken of dimensional and orientational inaccuracies from particle to particle.

Description: BACKGROUND TO THE INVENTION This invention relates to a method and apparatus for classifying particles. It has already been proposed in UK Patent Applications 2211299A and 2230099A to classify particulate material by using, as a classification criterion, the change in dielectric constant of a cavity due to the presence therein of a particle which is to be classified. In each case, the cavity is subjected to a swept frequency range of electromagnetic radiation while the particle is located therein and the signals transmitted by the cavity are detected and analysed. The particles are then classified, and possibly subsequently sorted into fractions, in accordance with the change in dielectric constant of the cavity attributable to the presence of the particle. The change in dielectric constant manifests itself as a change in the resonant frequency of the cavity, or as a change in the amplitude of the transmitted signal at resonance. The electromagnetic radiation is plane polarised and applied to the cavity in a manner giving rise to a specific electric field in the cavity. The electric field exists in a specific distribution and the effect on the dielectric characteristics of the cavity will depend on the shape and orientation of the particle and its relationship to the particular electric field distribution. Thus different particle shapes and/or orientations may give rise to inaccuracies in the classification because of the different relationships with the specific electric field which exists in the cavity. US-A-4,211,970 discloses a method and apparatus in accordance with the pre-characterising portion of claims 1 and 6 and for determining the water content of isotropic samples. The material is placed in a cylindrical container supported rotatably in a cylindrical resonant chamber. Microwave radiation is applied to the chamber through microwave couplers attached to the chamber at angularly spaced positions. The transmitted radiation is detected and used to determine the water content of the sample. A first aspect of the invention provides a method of classifying a particle comprising the steps of locating the particle in a cavity, propagating a swept frequency range of plane polarised electromagnetic radiation through the cavity, detecting and analysing the signals transmitted by the cavity and assigning a classification to the particle in accordance with a change in the dielectric characteristics of the cavity caused by the presence therein of the particle, characterised in that the electromagnetic radiation is propagated through the cavity in three mutually orthogonal directions and the particle is located at least temporarily at the centre of the combined electric field produced by propagation of the radiation. In the preferred version of the invention, the cavity is cube-shaped and the particle is located at least temporarily at the centre of the cavity. In a sorting application, a stream of particles may be passed one by one through the centre of the cavity in a direction extending from one corner of the cube-shape to the diagonally opposite corner thereof. With the cube-shaped cavity, plane polarised radiation can be applied to the cavity in three mutually orthogonal directions. It is preferred that the radiation is in the microwave part of the electromagnetic spectrum. The particle may be classified in accordance with detected changes in the resonant frequency of the cavity, in signal amplitude at resonance or in Q-factor. A second aspect of the invention provides an apparatus for classifying particles comprising a resonant cavity, means for locating each particle in the cavity, means for propagating a swept frequency range of electromagnetic radiation through the cavity while the particle is in the cavity, means for detecting and analysing signals transmitted by the cavity and for assigning a classification to the particle in accordance with a change in dielectric characteristics of the cavity caused by the presence therein of the particle, characterised in that the propagating means propagates electromagnetic radiation through the cavity in three mutually orthogonal directions and the means for locating the particle in the cavity serves to locate the particle at least temporarily at the centre of the electric field produced by propagation of the radiation. One form of cube-shaped cavity has an entry opening at one corner of the cavity and an exit opening at the diagonally opposite corner, the apparatus including means for passing a stream of particles one by one through the centre of the cavity in a direction from the entry opening to the exit opening. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing which shows a schematic diagram illustrating a first embodiment of the invention. The drawing shows a cube-shaped cavity 10 having a non-radiating entry opening 12 at one corner and a non-radiating exit opening 14 at the diagonally opposite corner. Particles 16 of a particulate mass which is to be classified are passed through the centre of the cavity 10 in a direction extending diagonally from the entry opening 12 to the exit opening 14. This may be achieved in any convenient manner, for instance by causing the particles to fall one by one under gravity along the required path. The particles may be organised into a stream, in which the particles are spaced apart in the direction of the stream, on a conveyor belt and then be projected in free flight from the conveyor belt so as to fall through the cavity 10 one after the other. A sensor 20 is provided to detect when a particle 16 is about to enter the cavity and to send a signal indicative of this fact to a central processor 22. In response to the signal from the sensor 20, the microprocessor 22 simultaneously energises three microwave generators 24 which each apply a pulse or continuous wave of plane polarised microwave radiation to the cavity in three mutually orthogonal directions via three appropriate transmission line structures 26 located at different faces of the cubic cavity 10. On opposite faces of the cubic cavity 10 are located microwave detectors 28 which detect the radiation transmitted by the cavity and which feed signals indicative of the detected radiation to the processor 22. The transmission line structures 26, the detectors 28, the non-radiating openings 12 and 14 and the side dimension of the cubic cavity 10 are all selected in accordance with the particular frequency range under consideration and in accordance with known theoretical principles. In a typical test apparatus the cube-shape had a side length of 300mm and the openings 12 and 14 a side length of about 70mm. Of course, the latter dimensions will depend in each practical case on the size of the particles which are to be classified by the apparatus. The central processor 22 analyses the signals from the detectors 28 and classifies the particle. In a particle sorting apparatus, the processor 22 may be arranged to operate a separation device 30 at a downstream location to separate particles into fractions in accordance with the classifications assigned to them. In one example, the downstream separation device may comprise a fluid blast ejection system which issues a short duration blast of fluid, typically air, at the appropriate instant to separate certain particles 16A from the remaining particles 16B by deflecting those particles out of the falling stream of particles. In the case of each pulse or continuous wave of microwave radiation, the electric field will be at right angles to the direction of propagation of the radiation through the cavity. Normally, the three electric fields which are created by the three distinct pulses or waves will overlap and interfere with one another, the maximum field being at the centre of the cavity. Thus the particles, which move through the centre of the cavity, will be subjected to all three fields at the centre. Accordingly, the exact shape or orientation of the particle, which could have a marked effect on a single field because of the distribution thereof, will have little effect on the combined responses of the three detectors 28. In other words, by causing the particle to move through three mutually orthogonal fields, the shape and orientation of the particle will in effect be averaged out in the combination of signals sent to the processor 22. The microprocessor performs a classification exercise based upon any one or more of a variety of different criteria related to the change in the dielectric characteristics of the cavity due to the presence of the particle therein. The basis may, for instance, be a change in the resonant frequency of the cavity, a change in the transmitted signal amplitude at resonance, or a change in the Q-factor at resonance. Of course, with the present apparatus, the processor 22 will perform the classification exercise using a combination of the signals received from the three detectors 28. The cavity 10 may possibly be of split construction as described in UK patent application 2230099A, the contents of which are incorporated herein by reference. The split in this case will be in a plane dividing the cavity diagonally. The illustrated apparatus is suitable for sorting particulate or material into fractions. In one specific example, diamond or diamond bearing particles can be sorted quickly and efficiently from non-diamond particles.

Claims: A method of classifying a particle (16) comprising the steps of locating the particle (16) in a cavity (10), propagating a swept frequency range of plane polarised electromagnetic radiation through the cavity (10), detecting and analysing the signals transmitted by the cavity (10) and assigning a classification to the particle (16) in accordance with a change in the dielectric characteristics of the cavity (10) caused by the presence therein of the particle (16), characterised in that the electromagnetic radiation is propagated through the cavity (10) in three mutually orthogonal directions and the particle (16) is located at least temporarily at the centre of the combined electric field produced by propagation of the radiation. A method according to claim 1 characterised in that the particle (16) is located at least temporarily at the centre of a cube-shaped cavity (10). A method according to claim 2, characterised in that a stream of particles (16) is passed one by one through the centre of the cavity (10) in a direction extending from one corner of the cavity (10) to a diagonally opposite corner thereof. A method according to any one of the preceding claims characterised in that the step of applying a swept frequency range of plane polarised electromagnetic radiation to the cavity (10) includes applying a swept frequency range of plane polarised electromagnetic radiation in the microwave part of the electromagnetic spectrum. A method according to any one of the preceding claims characterised in that the particle is assigned a classification in accordance with a detected change in resonant frequency of the cavity (10), in amplitude of the transmitted signal at resonance, or in Q-factor. An apparatus for classifying particles (16) comprising a resonant cavity (10), means for locating each particle (16) in the cavity (10), means (24, 26) for propagating a swept frequency range of electromagnetic radiation through the cavity (10) while the particle (16) is in the cavity (10), means (28, 22) for detecting and analysing signals transmitted by the cavity (10) and for assigning a classification to the particle (16) in accordance with a change in dielectric characteristics of the cavity (10) caused by the presence therein of the particle (16), characterised in that the propagating means (24, 26) propagates electromagnetic radiation through the cavity (10) in three mutually orthogonal directions and the means for locating the particle (16) in the cavity (10) serves to locate the particle at least temporarily at the centre of the electric field produced by propagation of the radiation. Apparatus according to claim 6 characterised in that the cavity (10) is cube-shaped. Apparatus according to claim 7 characterised in that the cube-shaped cavity (10) has an entry opening (12) at one corner thereof and an exit opening (14) at a diagonally opposite corner thereof, and in that the apparatus includes means for passing a stream of the particles (16) one by one through the centre of the cavity (10) in a direction from the entry opening (12) to the exit opening (14).

Inventors: DOWNING BARRY JOHN, SALTER JOHN DAVID, DOWNING, BARRY JOHN, SALTER, JOHN DAVID
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1990
Main classification: G01N22
IPC codes: G01N22, G01N15, G01N33