AU2004100065A4 - Optical Ore Sorter - Google Patents

Description

Page 1 BACKGROUND OF THE INVENTION This invention relates generally to detecting a particle, in a plurality of particles, which possesses a predetermined characteristic. This process can be utilised in a sorting system in order to detect particles, possessing a predetermined characteristic, and sorting those particles from other particles which do not possess the characteristic.

US patent No. 5206699 describes a sorting system which makes use of exciting radiation to stimulate Raman-shift emissions by diamonds in ore particles travelling on a conveyor belt past a sorting location. The anti-Stokes signal is preferably utilised, instead of the Stokes signal, for diamond detection. The particles are illuminated with incident radiation from a laser source and the laser signal is scanned by means of a rotating polygonal mirror so that it traverses the width of the conveyor belt in a straight line. Any secondary emissions emanating from particles positioned along the scanned line is directed to a detector eg. a number of photo-multiplier tubes, or are focussed by means of a cylindrical lens.

Although the aforementioned procedure does allow for the detection of particles, such as diamonds, which emit Raman signals the collective treatment of signals from all particles on the line which is illuminated, by the incident radiation, means that the signal to noise ratio in the detection circuitry is lowered, a factor which makes it more difficult to detect desirable particles.

Page 2 SUMMARY OF INVENTION The invention provides a method of detecting at least one particle, in a plurality of particles, which possesses at least one predetermined characteristic, which includes the steps of: scanning a beam of incident radiation at a selected wavelength over at least some of the particles so that, in response to the incident radiation, a respective secondary signal emanates from at least some of the particles, the frequency of the incident radiation being chosen so as to stimulate emission of at least a respective predetermined signal by each particle which possesses the predetermined characteristic and upon which the incident radiation impinges, scanning each secondary signal into at least one detector, and using at least one detector to detect the presence of the predetermined signal.

The incident radiation may be scanned using any appropriate mechanism and, preferably, the same mechanism is used to scan each secondary signal into the detector.

The secondary signal, which emanates from each of the particles, preferably includes a signal which is at the same frequency as the incident radiation and which is reflected by the respective particle. This signal may be used to calculate the size or shape of the particle or both parameters.

Page 3 Use may be made of a plurality of detectors each of which is responsive to a respective predetermined signal at a respective frequency.

Preferably the stimulated emission of the predetermined signal is Raman emission.

The predetermined signal which a particle is stimulated into emitting may, where applicable, be split into components of different wavelengths and each component may be examined for a respective Raman signal component.

Thus, according to a first variation of the invention, the beam of incident radiation is a first frequency and each stimulated emission signal is examined at different wavelengths for different Raman signals.

According to a different aspect of the invention the incident radiation includes one or more beams at different frequencies which, depending on the particles which are being illuminated by the incident radiation, results in the emission of different predetermined signals.

In one form of the invention the incident radiation is white light. In a different form of the invention white light, used as incident radiation, is derived from a plurality of light sources which are combined to give a white light incident radiation beam.

Page 4 The invention also provides apparatus for detecting at least one particle, in a plurality of particles, which possesses at least one predetermined characteristic, the apparatus including: means for scanning a beam of incident radiation at a selected wavelength over at least some of the particles thereby to cause a respective secondary signal to emanate from at least some of the particles, the frequency of the beam of incident radiation being chosen so as to stimulate emission of at least a respective predetermined signal by each particle which possesses the predetermined characteristic and upon which the incident radiation impinges, means for scanning each secondary signal into at least one detector, and a device, responsive to the detector, for detecting the presence of a particle which emits the predetermined signal.

The means for scanning the incident radiation may comprise a polygonal rotating mirror or any equivalent device. Similarly the means for scanning each secondary signal may comprise a polygonal rotating mirror or any equivalent device.

Preferably the two scanning means are constituted by the same device.

The respective predetermined signal may be a Raman frequency shift signal which may be at a lower frequency (the Stokes line) than the frequency of the incident signal or at a frequency which is higher than the frequency of the Page incident signal (the anti-Stokes line). The Stokes line is preferred for its amplitude is higher than the amplitude of the anti-Stokes line.

The apparatus may include a line filter which allows the transmission, in a narrow frequency band, of the predetermined signal. This is the case typically if the Stokes and anti-Stokes lines are transmitted. If the Stokes line is to be used then a low pass filter may be employed instead of a line filter.

If the secondary signal is to be examined for multiple Raman-frequency shift lines, typically arising from different particles with different properties, then the secondary signal may be split into components of different wavelengths and each component may be examined for the presence of a predetermined Raman-frequency line.

According to a preferred feature of the invention the incident radiation and the secondary signal are scanned using a common face or facet of the scanning means. This ensures that the path which is followed by the secondary signal from the particles to the detector is substantially the same as the path of the incident radiation from its source to the particle. The wavefronts of the two signals are coherent and shadow effects in the particle are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is further described by way of examples with reference to the accompanying drawings Figures 1 to 3 which respectively illustrate different embodiments of the invention.

Page 6 DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 of the accompanying drawings illustrates apparatus 10 designed to detect a single Raman-frequency shift signal stimulated in any of a plurality of particles 12 which are fed from a source 14 onto an endless conveyor belt 16.

In this instance it is assumed, for illustrative purposes, that the particles 12 are ore particles, comprising gangue and diamonds.

The apparatus 10 includes a laser source 20 which produces a laser beam 22 at a predetermined frequency which is chosen according to requirement.

Typically the frequency is in the infrared region. This reduces unwanted fluorescence, and helps to obtain a better sort. The laser beam 22 is scanned by means of a rotating polygonal mirror assembly 24, of a type which is known in the art, and an incident beam 26 of radiation is directed onto the particles 12 on the belt 16. The geometrical arrangement of the belt 16, the laser source 20 and the mirror assembly 24, is such that the incident beam 26 traverses the belt 16 repeatedly in a direction which is transverse to the direction of movement 28 of the belt.

Radiation 30 which emanates, at any given time, from a particle on the belt, and which strikes the mirror assembly 24, is directed by the assembly as a secondary beam 32 into a detector assembly 34.

The assembly 34 includes a housing 36 in which are mounted, in sequence, an optional polariser 38, a blocking filter 40, lenses 42 with a field stop 44 Page 7 between them, a Raman line filter 46 and a photo-multiplier or detector 48.

Signals produced by the detector 48 are applied to a processor 50 and are used, in a known manner, to control the operation of a plurality of nozzles 52 which, in response to the processor, are controlled to emit jets 54 of compressed air, derived from a compressed air source 56, to blast selected particles from a stream 58 of particles which are projected from a discharge end of the belt. The particles which are blasted from the projected stream 58 are collected in a recovery bin 60 and the remaining particles are directed to a waste dump 62.

The polariser 38 is used if the secondary beam 32 is cross polarised with reference to the sense of polarisation required by the detector assembly.

The blocking filter 40, as has been noted, can be a filter which blocks out the excitation frequency ie. the filter of the laser source 20. As is explained hereinafter however this frequency may be transmitted according to requirements.

The laser beam 26, impinging on particles which are diamonds, is designed to stimulate Raman signals which, if identified, are indicative of the presence of diamonds. Stokes and anti-Stokes signals are emitted by a diamond particle.

If both the Stokes and the anti-Stokes lines are to be detected then the filter 40 is designed so that each of the signals can be transmitted. On the other hand if the excitation frequency and the anti-Stokes signal are to be eliminated then a low pass filter can be employed operating from slightly Page 8 below the excitation wave length. It is to be understood that these aspects can be varied according to requirement.

The lenses 42 are tuned to direct the secondary radiation precisely onto the detector 48. The field stop 44 is positioned between the lenses to (do what?).

The Raman line filter 46 is chosen, as has been indicated, using similar criteria to what are employed in establishing the characteristic of the blocking filter 40. From experimental work and data already available in the art the Raman-frequency shift signals which are emitted by desired particles can be established and the Raman line filter 46 is designed so that only the desired Raman-frequency shift signal is passed.

In use of the apparatus the incident radiation 26 is scanned across the particles on the belt by the rotating polygonal mirror assembly 24. The secondary signal 30 may arise from a number of sources but is designed to arise from stimulated emission of Raman frequency shift signals in desired particles, ie. diamonds. The secondary beam 32 is directed into the detector assembly 34 and signal components at frequencies other than the Raman frequency are rejected or discarded. If the detector 48 indicates the presence of a particle with a designed Raman frequency shift signal then data on the position and size of the particle are transmitted to the processor 50 which then drives the nozzles 52 to separate the chosen particle from the remainder.

Page 9 It is known that, depending on the wavelength of excitation, some minerals fluoresce and the signal strength of the fluorescence may swamp the Ramanfrequency shift signal. It has been found that by using a laser signal in the infrared region, with a wavelength of the order of 1 micrometer, that the fluorescent effect is substantially minimised, under certain conditions.

Diamond particles are typically substantially smaller than gangue particles. It therefore becomes possible to discriminate diamond particles from nondiamond particles by calculating the intensity of the Raman frequency shift signal relatively to the area of the particle emitting the signal. In order to achieve this the detector assembly 34 may be designed so that the secondary signal 32, including a signal component derived from the excitation frequency, is transmitted to the detector 48. By means of techniques which are known in the art the signal component at the excitation frequency can be used to obtain an estimate of the size or shape of the particle which is illuminated. This information can form a basis for distinguishing diamonds from non-diamonds under fluorescing conditions.

Figure 2 illustrates a detection system 100 which, in many respects, is similar to that shown in Figure 1 and consequently, where applicable, components which are the same as those shown in Figure 1 are designated by means of similar reference numerals.

The detector assembly, designated 34A, includes a line filter 102 which allows the excitation frequency to be transmitted to the detector. As has been Page indicated this permits an assessment to be made of the size or shape of a particle from which the secondary beam emanates.

Diachronic mirrors 104A, 104B, 104N are mounted downstream of the lenses 42. At this location the light rays coming from the lenses are parallel and so have a uniform wavefront.

The diachronic mirrors reflect certain wavelengths and pass other wavelengths. The mirrors are respectively positioned to direct the incoming light into a plurality of Raman filters 106A, 106B, 106N which have different operating wavelengths for different minerals. Each filter directs its output signal into a respective detector 48A, 48B, 48N.

With the arrangement shown in Figure 2 the secondary light beam 32, emanating from each particle which is illuminated, is split into a plurality of components with each component being of a different wavelength. Each component is then examined using a Raman filter of a different wavelength for the presence or absence of different minerals. It is therefore possible for different particles each of which has a different characteristic to be detected or for a single particle, which has two or more characteristics, to be detected.

Selected particles are separated from the gangue on the belt in a manner similar to what has been described in connection with Figure 1.

In a further variation of the invention multiple lasers, of different respective frequencies, can be employed to illuminate the particles. This enables a Page 11 colour sort to be done on the material provided the detectors can cover the wavelengths at which the secondary beams are stimulated, for example infrared, ultraviolet, etc. A white light laser could be used to provide a broad spectrum of illuminating light. Alternatively a plurality of separate lasers could be employed, each of which emits light at a distinct frequency, and the different light beams could be combined into a single beam of white light.

Another possibility is to use two lasers, eg. at infrared frequency, as a dual light source in place of the single source 20 shown in the drawings. The secondary signals 32, caused by the respective lasers, are measured and a ratiometric or difference calculation is used to determine the separation of the desired minerals. This technique lends itself to the detection of minerals such as phosphates.

The invention is not restricted to the use of diachronic mirrors for equivalent devices can be used to split the secondary beam 32. For example a prism or an optical grating could be employed. These techniques are known in the art.

In Figures 1 and 2 the scanner 24 which, as has been pointed out, is used for scanning the incident beam and the secondary beam, can cause shadow effects on a particle which is under scrutiny. This arises due to the fact that the incident beam 22 is scanned by one face or facet of the polygonal mirror while the secondary beam 30 is scanned by a different face or facet of the mirror. Consequently the beams 26 and 30 are not parallel. Figure 3 Page 12 illustrates an arrangement in which the same face or facet on a scanner 24 is used for scanning the incident and the secondary beams.

The incident beam 22 is as before, derived from a laser source 20 which operates at any appropriate frequency. A scanning beam 26 is generated by the scanner 24 and traverses particles on a belt 16, in the manner which has been described. The geometrical arrangement is however such that a secondary beam 30, emitted by a particle 12 on the belt, travels along the same path as the beam 26, to the scanner 24, but in an opposite direction.

Thus the beam 30 strikes the scanner on the same face as the beam 22. The scanned secondary beam 32 is therefore directed towards the laser 20 along the same path as the incident beam 22.

A mirror 162 with a small hole 164 is positioned between the laser 20 and the scanner 24. The mirror is arranged so that the beam 22 passes through the hole 164. On the other hand the reflected beam 32A is directed by the mirror 162 to a detector assembly 34 of the kind which has been described herein.

Another technique which can be used to allow the transmission of the incident beam, but to reflect the second beam, is based on replacing the mirror 162 with a half-silvered mirror or a mirror which passes the beam, impinging on one side of the mirror, but which reflects a beam which strikes an opposing side of the mirror.

Page 13 Although the drawing in Figure 3 is substantially schematic it is apparent that the incident light travels on the path designated by the beam lines 22 and 26 while the reflected light travels on the same path designated by the beam lines 30 and 32, up to the position of the mirror 162. The incident and reflected beams are thus substantially parallel and shadow effects are reduced.

A significant advantage of the invention, in the various embodiments, arises from the fact that the scanner 24 is used for scanning the incident beam and the reflected or stimulated beam. The signal to noise ratio of the signal which is presented to the detector assembly 34 is consequently substantially greater than in a system in which the secondary signal is not scanned. The sensitivity of the system of the invention is therefore enhanced.

Dated this 5th day of February 2004