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 on-line diamond detection and sorting.
Diamonds are commonly sorted from associated gangue particles in
diamondiferous feed material using X-rays which generate detectable
luminescence in diamonds. One serious limitation of this known method
arises when the diamond is shrouded with material which is opaque to the
luminescence and which accordingly prevents proper detection thereof.
Another serious limitation arises from the fact that the luminescence
spectrum emitted by an X-radiated diamond is not necessarily distinctive of
diamond, since other minerals such as zircon can emit similar luminescence
spectra under X-radiation.
Diffraction is also commonly used in the laboratory for studying lattice
structures of crystalline materials. In an X-ray diffraction spectrometer, the
crystalline sample is placed in a fixed position in a monoenergetic X-ray
pencil beam. Elastically scattered radiation from the lattice planes in the
crystalline structure can be detected by scanning an X-ray detector over a
range of angles relative to the incident, primary X-ray beam, or by using a
stationary, linear array of X-ray detectors at predetermined angular
positions.
The diffraction scattering angles are determined by Bragg's law and depend
on the X-ray wavelength, lattice constants for the crystal in question, and
the
angle between a particular lattice plane and the incident beam. The first two
of these variables are readily determinable.
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Radiation scattered inelastically by the crystal in question gives rise to
background radiation, but such radiation does not have the specific angular
distribution which is characteristic of diffractive scattering by the crystal
lattice planes.
It is known that if a sample irradiated with monoenergetic radiation is in
polycrystalline form, the elastically scattered radiation forms a concentric
ring pattern centred on the axis of the irradiating beam. On the other hand,
if the sample is a single crystal, the diffraction spots will only appear at
certain angular positions, along the rings of the pattern, relative to the
axis
of the irradiating beam. Thus in the case of laboratory analysis of a single
crystal diamond the diamond must be held with its crystal planes at certain
orientations if the radiation detection apparatus is to be correctly
positioned
to detect the relevant spots.
The orientation of crystallographic planes in a single crystal structure can
be
measured by a Laue camera in which a continuous X-ray spectrum is used
and a scattered diffractive pattern is detected by a two-dimensional detector
such as a photographic film. If the spots from one lattice plane give a
symmetric pattern centred about the primary irradiating beam, it can be said
that the relevant crystal plane is aligned with that beam. Asymmetry in the
detected Laue diffraction spot pattern is indicative of the angle between the
relevant lattice plate and the incident X-ray beam.
The present invention proposes to use of these phenomena in the on-line
detection of diamond-containing particles.
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SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a method
of sorting diamond-containing particles on-line from gangue particles, the
method including the steps of:
- ~ subjecting particles which are to be sorted to irradiation by an
X-ray or thermal neutron beam;
- by means of a detector arranged at a predetermined
orientation to the incident beam, detecting radiation which is
elastically scattered by each particle and deriving a diffraction
spot pattern from the detected radiation;
- by means of a computer, comparing the derived diffraction
spot pattern for each particle with a diffraction spot pattern
for diamond to determine whether there is correlation
between the patterns which is indicative of the presence, in
the particle, of diamond; and
- separating those particles for which there is correlation
between the patterns indicative of the presence of diamond
from other particles for which there is no such correlation.
Most gangue particles associated with diamond in nature do not include
single crystal minerals so there will be no detection of a spot pattern that
could be characteristic of diamond in the case of such particles.
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In one embodiment, the particles are irradiated with an X-ray beam, possibly
monoenergetic. The wavelength spectrum of the X-ray beam and the
acceptance waveband of the associated detector are selected such that the
detector is capable of detecting spots at diffraction scattering angles
characteristic of diamond for those particular X-ray wavelengths. As an
alternative to a broad spectrum X-ray beam, it is also proposed to irradiate
the diamondiferous particles with a thermal neutron beam.
According to another aspect of the invention there is provided apparatus for
sorting diamond-containing particles on-line from gangue particles, the
apparatus including:
- an X-ray or thermal neutron source for producing an X-ray or
thermal neutron beam;
- means for transporting particles which are to be sorted
through an irradiation zone in which the particles are
irradiated by the beam;
- a detector arranged at a predetermined orientation to the beam
for detecting radiation which is elastically scattered by each
particle;
- means for deriving a diffraction spot pattern from the
detected radiation;
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- a computer for comparing the derived diffraction spot pattern
for each particle with a diffraction spot pattern for diamond
to determine whether there is correlation between the patterns
which is indicative of the presence, in the particle, of
diamond; and
- sorting means, operating under the control of the computer,
for separating those particles for which there is correlation
between the patterns indicative of the presence of diamond
from other particles for which there is no such correlation.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in more detail, by way of example
only, with reference to the accompanying drawing which diagrammatically
illustrates an apparatus used in the method of the invention.
DESCRIPTION OF AN EMBODIMENT
The drawing shows a hopper 10 which feeds diamondiferous particles 12
onto the upper run of a endless conveyor belt 14 which conveys the particles
towards and over a discharge roller 16. Located above the belt is an X-ray
tube 18 which directs a broad spectrum, X-ray pencil beam 20 downwardly
onto particles on the belt. The particles are transported in single file on
the
belt so that the pencil beam 20 is directed onto a single stream of moving
particles 12.
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Alternatively, the particles could be spread across the width of the belt with
the pencil beam 20 scanning across the belt, possibly by means of a rotating
collimator disposed in front of the X-ray tube exit window. In yet another
alternative embodiment in which the particles are spread across the belt, the
beam 20 could be fanned in a plane at right angles to the plane of the belt.
The numeral 22 indicates a two-dimensional electronic detector which is
arranged at a predetermined orientation relative to the primary, incident
beam 20 to detect X-rays which are elastically scattered by the particles 12.
The output of the detector 22 for each particle is fed to a computer 24 which
translates the detected radiation into a Laue diffraction spot pattern.
The computer then compares the derived Laue diffraction spot pattern with
a known, electronically stored pattern for diamond. The known diamond
diffraction spot pattern is derived from detailed knowledge of the single
crystal diamond structure and of the orientations of the lattice planes
therein.
The comparison is performed iteratively in accordance with a predetermined
algorithm in an attempt to fit the known pattern to the instantaneously
derived pattern. If sufficiently close correlation is achieved, the particle
in
question is identified as a diamond or having a diamond inclusion.
In response to a positive identification of a diamond or diamond inclusion,
the computer 24 triggers a separation device 26. In this case the device 26
is illustrated as an air blast device which issues a short duration blast of
air
to deflect the relevant particle out of the stream of particles discharged
over
the roller 16. Several such devices, spaced laterally apart from one another,
may be provided in situations where the particles are presented across the
width of the belt 14 rather than in single file.
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The deflected particles, i.e primarily diamonds or particles with diamond
inclusions, collect in a bin 28 while the remaining gangue particles collect
in a bin 30.
As an alternative to the use of a X-ray beam to generate a diamond-
distinctive Laue spot pattern, it is within the scope of the invention to
irradiate the particles 12 with a thermal neutron beam produced by a thermal
neutron source. It is envisaged that this could in fact give greater
sensitivity
since a neutron beam can be more effectively elastically scattered by the
diamond crystal lattice planes.
The underlying principles of the method of the invention could also be used
to detect diamonds in other applications, such as in the analysis of
individual
parcels or in or on human bodies. It will be appreciated that the penetrative
power of the X-ray or thermal neutron beam will enable diamonds to be
detected even if they are embedded in other materials which would normally
be opaque to X-ray generated luminescence.
In the illustrated embodiment, the detector 22 detects X-rays which are
forwardly scattered by the particles 12. In the case of thermal neutrons, it
is
preferred to detect backscattered radiation.
