Sorting apparatus

Abstract: Particles (10) are conveyed in a stream in a direction transverse to the width of the stream. Windows (14) view individual width zones of the stream and optical fibres (19) are associated with the windows to convey light passing through a window to a light detector (21). The windows (14) are rapidly opened and closed and a processor (32) is provided to determine which window (14) passed light detected by the detector (21) and accordingly which width zone of the stream the light emanated from. The windows (14) preferably incorporate liquid crystal shutters (24) which can be switched rapidly between light-transmitting and opaque states corresponding to open and closed states of the windows (14).

Description: BACKGROUND TO THE INVENTION THIS invention relates to a sorting apparatus. Most conventional apparatuses for sorting particulate material have a conveyor belt which causes the particles to move successively, usually in free flight, through an excitation zone, a detection zone and an ejection zone. To enable the apparatus to handle a large throughput, the particles are arranged in a wide monolayer on the belt, with a number of particles across the width of the belt. In the excitation zone, the particles may, for instance, be subjected to X-radiation in a case where diamonds are to be sorted from gangue. Downstream of the excitation zone is a detection zone where a series of photomultiplier (PM) tubes are arranged across the width of the belt to detect luminescence in the particles. Further downstream, a bank of air blast ejectors is arranged. When a luminescing particle is detected at the detection zone, the appropriate ejector is actuated to issue an air blast which deflects the relevant particle out of the main stream. To ensure that accurate detection and ejection takes place, it is essential that each PM tube and each ejector cover a small area only of the width of the falling stream of particles. If, for instance, the area "seen" by a PM tube is large, an "eject" signal could be issued when any one of a number of particles "seen" by the PM tube is luminescing. Similarly, if the ejector blasts too large an area, non-luminescing particles could also be ejected. With continuing research, the sizes of the ejectors have been decreased with the result that they are able these days to cover smaller areas than was hitherto possible. However the cost of many small PM tubes has proved to be prohibitive, and the present invention seeks to provide an alternative solution to the detection problem. Document EP-A-0,064,842 describes apparatus in accordance with the prior art portion of claim 1 for separating infra-red emitting particulate matter from a stream of matter, by line scanning the stream as it passes through a scanning zone, and diverting detected particles, through the use of air blast nozzles. As compared therewith the present invention, as defined in claim 1, provides individual windows which are selectively openable, whereby the signals from windows open at different times may be processed by the same means. Document GB-A-2,176,888 describes apparatus which directs an X-ray beam across a stream of matter on a conveyor belt under which is mounted a phosphorous strip. The strip emits light in response to incident radiation and the amount of light depends on the amount of incident radiation. Optical fibres pass this light to a multiplexor which sequentially rotates an open window by the end of each fibre to detect the amount of light passing through each one and correspondingly the amount of radiation incident on each section of the phosphor strip. It contains no suggestion of the simple use of a plurality of controllable windows as used in the present invention. SUMMARY OF THE INVENTION The invention provides light detection apparatus according to the appended claim 1. Preferably each optical fibre is associated with a single window, each window can pass light from a single width zone only of the stream and the switching means operates such that only one window is open at any given moment. In a case where the stream of particles is moving in a direction transverse to the width of the stream, the windows may be in an array extending across the width of the stream and the switching means may operate such that every window is open for a portion of the time taken for a particle in the stream to pass the array. The invention extends to a particulate material sorting apparatus which comprises light detection apparatus of the kind summarised above. The sorting apparatus may comprise means for moving the particles, in a stream, in a direction transverse to the width of the stream and separation means responsive to the processing means for separating from other particles desired particles which emit light detected by the light detection means. Preferably, the light detection apparatus and sorting apparatus are used to sort particles, typically diamond particles, which luminesce under the effects of incident X-radiation from other particles. Such use and the methods it represents also form part of the invention. BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which : Figure 1 illustrates an embodiment of the invention diagrammatically; and Figure 2 is a block diagram illustrating the control circuitry. DESCRIPTION OF AN EMBODIMENT In Figure 1 a stream of particles 10, being diamond and gangue particles, is conveyed on a belt or chute 12, the direction of movement of the stream being into the plane of the paper. Immediately preceding its entry into an enclosure 11, the stream of particles has passed through an excitation zone where it is subjected to incident X-radiation or other excitation. Diamond particles in the stream luminesce as a result of this excitation. Situated just above the stream of particles in the enclosure 11 is a member 13 formed with a series of windows 14 adjacent each of which one end of an optical fibre 19 is located. Each of the windows covers a small width zone of the stream and each of them is controlled by a liquid crystal shutter. The various liquid crystal shutters are connected to a control unit 20. The unit 20 incorporates a driver unit which, at any given moment, applies a voltage to all of the liquid crystal shutters with the exception of one so that all of the shutters with the exception of that one are opaque. Thus at a given instant in time only one window 14 is open, i.e. in a light transmitting state, the remaining windows being closed, i.e. in an opaque condition preventing passage of light. The unit 20 furthermore includes the facility for rapidly changing the situation so that all liquid crystal shutters, with the exception of another shutter different to the first shutter, are opaque. In other words, there is only one window 14 which is open at any given time, the window which is open changing continuously and at an extremely rapid rate. The optical fibres 19 are arranged in a bundle 22 which extends to a housing 23 in which a photomultiplier tube (PM) tube 21 is situated. The ends of the fibres 19 inside the housing are exposed. If a window 14 is open, i.e. its associated liquid crystal shutter is transparent, and a particle in the relevant width zone of the stream luminesces, the open window passes the light to the associated optical fibre 19 which conveys the light by total internal reflection to the housing 23, for detection by the PM tube 21. Figure 2 shows the electronic components of the system. A clock pulse is applied by a clock oscillator 25 to a liquid crystal drive unit 26 comprising a demultiplexed upcounter which has outputs 27 suitable for applying an appropriate voltage to the liquid crystal shutters 24 associated with the windows 14. The voltage is such as to maintain the shutters in a normal dark or opaque state. As the upcounter sequences it switches one liquid crystal shutter at a time to its transparent state, thus allowing light to pass through the relevant window 14 to the associated optical fibre 19 and, through the fibre 19, to the PM tube 21. Upon detection of light, the PM tube sends a signal to a signal processing and decision making unit 28 which decides whether the light is indicative of a desired particle in the stream. In the appropriate cases, the unit 28 sends an enabling signal to a particle selector sequencing unit 29 comprising a second demultiplexed upcounter cycling synchronously with the driver unit 26. The outputs 33 of this counter are enabled by the signal coming from the unit 28 and are connected to a particle selector mechanism (not shown in Figure 2). The particle selector mechanism may, for instance, comprise a bank of air blast ejectors 30 (Figure 1) which is arranged alongside the stream of particles at a downstream location. Typically, the bank of air blast ejectors will be located alongside the stream of particles after they have been caused to fall in free flight from the belt or chute 12. Then, at the appropriate moment of time, the appropriate ejector is actuated to issue a short duration blast of air which deflects the desired particle from the falling stream for collection apart from other, undesired particles. It will be appreciated that the units 28 and 29 constitute a means (shown diagrammatically at 32 in Figure 1) which acts in response both to the PM tube 21 and, by virtue of the synchronous sequencing, to the instantaneous state of the driver unit 26 to make a decision about which particles are to be selected from the stream of particles. In effect, a decision is made as to which window 14 was open to pass light detected by the PM tube and accordingly which width zone of the stream contains the light-emitting particle, this decision then enabling the actuation of the correct air blast ejector 30. Figure 2 also illustrates the presence of a reset driver 31 to ensure that the circuit starts up synchronously. It may be necessary to provide focussing lenses and suitable collimators to ensure that light from a luminescing particle passes through the relevant window and is picked up by the associated optical fibre 19. It is preferred that the air blast ejectors each cover the same width of the stream of particles as a lens associated with an optical fibre does. It is then possible to achieve a high degree of accuracy in the ejection zone, so that few non-luminescing particles, if any, are ejected from the stream along with luminescing particles. Clearly, it will be necessary to switch the current supply mode to the liquid crystal shutters extremely rapidly, since if the relevant window is not open to pass light during passage of a luminescing particle, the particle will not be detected. Thus each window must be open, if only for a split second, during the time that it takes a particle to travel past the member 13. A major advantage of the illustrated apparatus is the fact that there is a requirement for only one PM tube while it is still possible to determine in which width zone of the stream luminescence took place. It should be noted that it would be possible to have an even greater number of windows and fibres, leading to greater accuracy in zone determination. The fibres could, as in the illustrated case, be arranged in a single bundle served by a single PM tube. Alternatively, the fibres could be in different bundles each served by a single PM tube. Even in the latter event, there will still be a relatively small number of PM tubes only. In the illustrated case, the particles are transported through the enclosure 11 on a conveyor belt or chute 12. In other cases, the particles could be falling in free flight after projection from the end of a conveyor. Note also that the invention is not limited in any way to any particular type of separator in the ejection zone. Instead of air blast ejectors, the desired particles could, for instance, be removed from the remainder of the stream by water blasts or by suction means. Note also that it is within the scope of the invention to use one or more PM tubes for viewing fibres which emanate from the same width zone and which are each fed with a portion of the light given off by a luminescing particle. This could be achieved, for instance, with the use of a multi-way light splitter associated with the window 14. In this case, it would be advantageous to use appropriate filters for each fibre or a single filter for the PM tube. For instance, using the suitable filter, it would be possible to detect diamonds which luminesce in the red part of the spectrum and hence to separate those diamonds from other particles.

Claims: Light detection apparatus for determining sections which, from a plurality of sections across the width of a stream of particles (10), contain light-emitting particles, the apparatus comprising light detection means (21), and characterised by further comprising: (a) a plurality of openable and closable windows (14) arranged opposite different sections across the width of the stream of particles (10), each window including a liquid crystal shutter (24) and the windows when open being able to pass light emitted by particles in the relevant sections; (b) means for switching the liquid crystal shutters (24) between light transmitting and opaque states corresponding respectively to open and closed states of the windows (14); (c) a plurality of optical fibres (19) associated with the windows and arranged to transmit light passed by the windows (14) to the light detection means (21) for detection thereby; and (d) means (28, 29, 32) for identifying a window (21) which has passed light detected by the detection means and accordingly for identifying a section across the width of the stream of particles (10) from which the light emanated. Light detection apparatus according to claim 1 characterised in that each optical fibre (19) is associated with a single window (14), each window (14) can pass light from only a single section across the width of the stream and the switching means (20, 26) operates such that only one window (14) is open at any given moment. Light detection apparatus according to claim 1 or claim 2, wherein the stream of particles (10) is moved in a direction transverse to the width of the stream, and characterised in that the windows (14) are in an array (13) across the width of the stream and the switching means (20, 26) operates such that every window (14) is open for a portion of the time taken for a particle (10) in the stream to pass the array (13). Apparatus for sorting particulate material, said apparatus including light detection apparatus according to any one of the preceding claims. Apparatus according to claim 4 including means (12) for moving the particles (10), in a stream, in a direction transverse to the width of the stream and separation means (30) responsive to the identifying means (32, 28, 29) for separating from other particles (10) those particles (10) which emit light detected by the light detection means (21). Apparatus according to claim 4 or claim 5 including a conveyor belt (12) for moving the stream of particles (10) past the windows (14). The use of apparatus according to any one of claims 4 to 6, to sort particles (10) which luminesce under the effects of incident X-radiation from non-luminescing particles (10). The use of apparatus according to claim 7 to sort diamond particles from other particles (10). The use of a light detection apparatus according to any one of claims 1 to 3, to detect light emitted by diamond particles luminescing under the effects of incident X-radiation.

Inventors: BROWN NEVILLE ERIC, HAMMOND DAVID JAMES, POTGIETER JOHANNES HERMANUS, BROWN, NEVILLE ERIC, HAMMOND, DAVID JAMES, POTGIETER, JOHANNES HERMANUS
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1989
Main classification: B07C5
IPC codes: B07C5