Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND TO THE INVENTION
THIS invention relates to the classification of particles according to their
Raman response to incident laser radiation. In one application the method of
the invention may be used to classify diamondiferous material into diamond
and non-diamond fractions.
The sorting of particles, in particular diamonds, by Raman spectroscopy has
already been proposed. See, for instance, US patent 5,143,224. The
application of Raman spectroscopy to diamond sorting has however proved
to have a number of disadvantages, including the following:
- normal Raman scattering of incident laser radiation takes place at
very low intensity levels which can be difficult to detect;
- many types of other particles normally associated with diamonds also
fluoresce under the incident laser excitation, making it difficult to
isolate the diamond response;
- the fluorescence which takes place is a broad band phenomenon
which may swamp the weak Raman signal; and
- the use of the conventional Raman scattering phenomenon in an
industrial environrnent calls for verv specific requirements including
very low light levels in the measuring zone, the absence of optical
dispersants such as dust or smoke and expensive detection equipment
to detect the weak Raman signal.
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SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method of
classifying particles which comprises irr~ ting the particles with pulsed
incident laser radiation at an intensity chosen to cause selected particles to
emit a stimulated Raman signal, and classifying the particles according to
whether they emit a stimulated Raman response characteristic of the selected
particles.
In one preferred application, where the particles which are to be classified
comprise diamond particles and non-diamond particles, the pulsed incident
laser radiation is at an intensity chosen to cause diamond particles to emit
a stimulated Raman response, and the particles are classified according to
whether or not they emit a stimulated Raman response characteristic of
diamond.
In this application the incident laser radiation is preferably pulsed with a
pulse duration shorter than the luminescence response time of diamond. The
pulse duration may, for instance, be of the order of 8ns and the incident
laser radiation at an intensity of about lMW/cm2.
Typically, the incident laser radiation is produced by an Nd:YAG laser
operating at a wavelength of 355nm. The signals emitted by the particles in
response to the incident laser radiation may be passed to a detector by a
filter having a pass band centred at a characteristic Raman wavelength for
diamond.
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According to a second aspect of the invention there is provided a method of
sorting particles which comprises moving the particles through an irradiation
zone, irr~ tin~ the particles, in the irradiation zone, with pulsed incident
laser radiation at an intensity chosen to cause selected particles to emit a
stimulated Raman signal, and sorting the particles into a first fraction rich
in the selected particles and a second fraction rich in other particles,
according to whether they emit a stimulated Raman response characteristic
of the selected particles.
To ensure a high throughput rate, the particles may be moved through the
irradiation zone in a broad stream and irradiated by a laser beam which is
pulsed sequentially across the width of the stream. Conveniently, the
particles are transported on a belt which projects them in a broad stream
through the irradiation zone.
The stream of particles can be moved, after the irradiation zone, past an
ejector apparatus comprising a bank of spaced apart ejectors located adjacent
the stream, a~plo~liate ejectors being activated at a~plopliate times to eject
selected particles from the stream for collection as the first fraction.
The sorting method summarised above may be used to sort diamond particles
from non-diamond particles.
According to a further aspect of the invention there is provided an ~pa~alus
for sorting particles, the apparatus comprising:
- an irradiation zone,
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means for moving particles which are to be sorted through the
irradiation zone,
a pulsed laser tube for irra~ ting the particles in the irradiation zone
with pulsed, incident laser radiation at an intensity chosen to cause
selected particles to emit a stimulated Raman signal,
a detector,
a filter having a pass band centred on a characteristic Raman
wavelength for the selected particles, the filter being arranged to pass
ap~lopl;ate signals which are emitted by the particles in response to
the incident pulsed laser radiation to the detector
analysing means responsive to the detector for determining, from
signals detected by the detector, which of the particles have a
stimulated Raman response characteristic of the selected particles,
and
sorting means responsive to the analysing means for sorting the
particles into a first fraction rich in the selected particles and a
second fraction rich in other particles.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example
only, with reference to the accompanying diagrammatic drawings.
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In the drawings:
Figure 1 shows a side view of an apparatus which employs the
method of the invention; and
Figure 2 shows a plan view of the apparatus seen in Figure 1.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
In the following description, specific mention is made of the classification
and sorting of diamonds, although it will be appreciated that the principles
of the invention are also applicable to the classification and sorting of other
particle types.
The Figures show a broad conveyor belt 10 which conveys diamondiferous
feed material 12. The feed material is derived from mining operations and
subsequent processing and contains diamond particles 14 and associated rock,
i.e. non-diamond, particles 16. The feed material is discharged over a
discharge pulley 18 and follows a falling trajectory 20. At an irradiation
zone 22, the particles are irradiated with a pulsed laser beam generated by
a laser tube 24.
The laser beam is pulsed sequentially across the width of the belt so that, at
times Tl, T2, T3 Tn~ different portions of the width of the falling stream
of particles are irradiated, the lateral spacing of the train of pulses being
selected to accommodate the smallest expected particles. For a given
wavelength, the laser beam is at an intensity chosen to activate a
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characteristic, stimulated Raman signal in the diamond particles 14, but not
at an intensity high enough to have the potential to damage the crystal
structure of the diamonds.
The radiation scattered by the particles at each pulse of incident radiation is
collected and focused on a detector 26 by a lens system 28 and a
monochromator filter 30 which has a pass band centred at a characteristic
Raman wavelength for diamond. An electronic processor 32 analyses the
output signal of the detector and determines whether the detected spectrum
contains a stimulated Raman signal characteristic of diamond. The lateral
position of a detected diamond can be ~1etermined by the processor from
knowledge of the laser pulse train timing, and the longitudinal position
thereof from knowledge of the belt speed.
After the irradiation zone 22, the stream of particles moves past an ejector
apparatus 34 composed of a series of laterally spaced ejector valves 34.1,
34.2, 34.3.. In response to the detection of a diamond, the processor
activates the approp~iate valve 34.1, 34.2, 34.3 ...., which opens to direct a
puff of compressed air at the falling particle stream. The diamond particle
is diverted from the normal trajectory 20 and into a concentrate collection
bin 36 while non-diamond particles continue falling along the normal
trajectory which directs them to waste.
Conveniently a single detector 26, rather than a number of detectors is used,
but it will be appreciated in this case that the response time of the detector
must be fast enough to detect the characteristic Raman signal within the
excitation pulse window. The laser pulse is advantageously shorter than the
luminescence response time of diamond. With this combination of features,
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interference in the scattered spectrum by background lllminescence emitted
by the particles as a result of non-Raman phenomena can be elimin~ted.
As indicated previously, the intensity of the incident laser beam is selected
to activate a stimulated Raman response in diamonds. The exact intensity
level in a particular application and for particular particles is carefully
chosen so that the intensity level is not sufficiently high to cause damage to
the diamonds. When a diamond is present and irradiated with laser radiation
at an intensity above an ~plopliate threshold level, the stimulated Raman
signal which it emits is orders of magnitude more intense than background
luminescence attributable to other phenomena and than a normal Raman
signal. The stimulated Raman signal characteristic of diamond is accordingly
very much easier to detect than the normal Raman signal.
The coherence of the incident laser beam makes it possible to focus the laser
tube 24 so that that part of the beam which has suff1cient intensity to
activate the desired stimulated Raman response narrowly covers the particle
trajectory and expected lateral variations thereof, thereby ensuring that the
stim~ ted response is activated if a diamond is present.
In one experiment conducted in the laboratory to test the activation of the
desired stimulated response, a diarnond particle was irradiated with 8ns
pulses of laser radiation at a wavelength of 355nm and at an intensity of
lMW/cm2. This was achieved using an Nd:YAG laser tube and a pulse
repetition frequency of 8Hz. The detector, in the experiment a H~m~m~t~u
IP28 photomultiplier tube with associated focusing lens and monochromator,
detected a stimulated Raman response from the diamond. An analysis of the
relevant parameters indicated that diamond damage occurred at a threshold
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intensity level in excess of lGW/cm2, very much higher than the incident
intensity level of 1MW/cm2
The experiment indicated that the detected stimulated Raman response was
substantially more intense than the background luminescence and the normal
Raman response.
It is believed that the intensity of the stimulated signal will overcome or at
least reduce the problems associated with detection of low intensity normal
Raman signals and swamping of the Raman signal in background
luminescence or fluorescence. In addition it is believed that the stimulated
response will be sufficiently intense to make it possible to conduct particle
classification and sorting operations in daylight conditions as opposed to
very low level light conditions.
