Note: Descriptions are shown in the official language in which they were submitted.
~A~ 1385U:~
BACKGROUND TO THE INVENTION
THIS invention relates to a method and apparatus for the classification
and sorting of particulate matter.
The invention has particular application to the detection of diamonds
within host kimberlite particles. In practice in diamond recovery
operations, it would be highly desirable to detect kimberlite particles
that are host to internal diamond inclusions since it would then be
possible to reject those kimberlite particles which are barren and to
continue with processing of only those particles known to include
diamonds. With barren particles rejected at an early stage, the load on,
and capacity required of, the downstream processing equipment would
be vastly reduced.
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It is accordingly considered desirable to be able to form an image of the
internal composition of a particle such as a kimberlite particle. One
traditional imaging system uses the transmission of X-rays through the
particle to form a shadowgraph of the particle. In this case, those
inclusions in the particle, such as diamonds, absorb X-radiation to a
greater extent than other minerals in the particle, so the diamonds
appear as shadows in the image projected by the transmitted X-rays. In
an alternative proposal, a computed tomography (CT or CAT) scanning
system is used to form an image of the particle. In this case, a fan-
shaped planar beam of X-radiation is transmitted through the particle
and a detector array picks up the transmitted radiation. The X-ray
source and particle are rotated relative to one another to obtain scanned
views from all directions. The transmission in each picture element or
pixel is determined and the totality of the data is analysed by computer
to ascertain whether a certain inclusion is present in the particle.
One of the problems associated with the use of X-radiation in
shadowgraph or CT scanning systems in the analysis of kimberlite
particles is the fact that the X-ray attenuation coefficient for the host
rock is similar to that of diamond, giving a low contrast in the resultant
image between diamond and rock. A further problem is that kimberlites
are known to have an inhomogeneous composition with the possibility
of other mineral inclusions of similar size and X-ray attenuation
characteristics as diamond. Yet another problem is the fact that X-
radiation is heavily attenuated by kimberlites. The size of particles which
can be analysed is therefore limited if X-ray energy and power
consumption levels are to be kept within reasonable limits.
(~A21385~3
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Y OF THE INVENTION
According to the present invention there is provided a method of
classifying particles according to the presence or absence in the particles
of a particular substance, the method comprising the steps of irradiating
each particle with a beam of fast neutrons at a resonant energy level for
the particular substance, deriving for each particle an image which is
representative of transmission of the beam by the particle, and
classifying the particle according to whether or not the derived image is
indicative of the presence in the particle of the particular substance.
In this specification, the term "fast neutrons" refers to neutrons having
a kinetic energy of the order of mega-electron volts. The fast neutrons
preferably have a well-defined energy level. Such neutrons are in this
specification referred to as being monoenergetic. It is nevertheless
recognised that a perfectly monoenergetic neutron beam has not yet
been achieved in practice. The neutron beam may, for instance, be
produced by a beam source in which a solid target of a light element,
such as beryllium, lithium or carbon is bombarded by deuterons
accelerated by a particle accelerator. As an alternative, the source could
be a sealed tube type source in which tritium is bombarded by
deuterons.
The particles may be kimberlite particles and the particular substance
may be diamond, in which case neutron beam irradiation takes place at
an energy level corresponding to resonant scattering for carbon-12.
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In a preferred version of the invention, each particle is irradiated by
neutron beams at a first energy level corresponding to resonant
scattering for the substance and also at a second, non-resonant energy
level, respective first and second images representative of beam
transmission are derived for the two energy levels, and a third image is
derived from the first and second images, typically by subtraction, the
third image also being representative of beam transmission and on the
basis of which classification takes place.
In a case where kimberlite particles are classified according to whether
or not they contain diamonds, the first energy level corresponds to
resonant scattering for carbon-12 and the second energy level is a non-
resonant energy level for carbon-12.
The particles may be irradiated in a CAT-scanning technique. As
summarised above, this may be done either at a single neutron energy
level, being a resonant level, or at different energy levels of which one
is a resonant level.
The method may include the step of sorting the particles into fractions
in accordance with their classifications.
The invention also provides an apparatus for classifying particles
according to the presence or absence of a particular substance, the
apparatus comprising means for irradiating the particles with a beam of
fast neutrons at an energy level corresponding to resonant scattering for
the particular substance, means for deriving, for each particle, an image
C~21385~J3
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which is representative of attenuation of the beam by the particle, and
means for classifying the particle according to whether or not the
derived image is indicative of the presence in the particle of the
particular substance.
The apparatus may be arranged to irradiate the particles at distinct
energy levels, one being a resonant energy level and the other a non-
resonant energy level.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example
only, with reference to the accompanying drawings in which:
Figure 1 shows a graph illustrating neutron attenuation for
diamond and kimberlite; and
Figure 2 diagrammatically illustrates an apparatus of the
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
In the description which follows, specific reference is made to the
detection of carbon-containing material in rock samples and in particular
the detection of diamonds in kimberlite.
CA2138503
When incorporated in a kimberlite matrix a diamond will appear as a
highly absorbent "hot spot" under irradiation by fast neutrons having an
energy level in the carbon-12 scattering cross-section which is at a
resonant level for carbon-12.
In the graph in Figure 1 the line labelled "Diamond" represents neutron
attenuation by diamond for various neutron energy values and the line
labelled "Kimberlite" the attenuation of neutrons by kimberlite for the
same range of neutron energy values. The line labelled
"Diamond/Kimberlite" represents the ratio of diamond attenuation to
kimberlite attenuation. Resonant scattering energy values, i.e. sharp
absorption peaks, for diamond occur at neutron energy levels of, for
example, 2,1 MeV, 2,9 MeV and 7,8 MeV. The most suitable energy
level for selective absorption of neutrons by diamond depends on the
ratio Diamond/Kimberlite. An energy level of 7,8 MeV is generally
preferred.
Figure 2 illustrates an apparatus 10 for sorting kimberlite ore particles
according to whether or not they contain diamonds. The apparatus 10
includes an endless belt 12 passing over a head roller 14. The belt 12
conveys kimberlite ore particles 16 through an irradiation zone 18. A
collimated beam 20 of fast neutrons is produced by a source 22 and
passes through the zone 18. As explained previously, the beam 20 is
monoenergetic.
In the source 22, a thin, solid target of a light element such as beryllium,
lithium or carbon is bombarded with a deuteron beam accelerated, to an
energy in the range IMeV to 4 MeV to produce a scattered neutron
~A2~138503
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beam at the desired energy level.
In a preferred version of the invention, the deuterons are accelerated by
an RFQ accelerator to an energy level in the range 1 MeV to 4 MeV,
preferably about 1,5 MeV. Depending on the material of which the
target is made, and depending also on the angle between the incident
deuteron beam and the scattered neutron beam, the neutron beam
which is produced may have any one of a number of different energy
levels. By selection of the angle between the incident deuteron beam
and the scattered neutron beam, it is possible to optimise the contrast
between a diamond inclusion and the host kimberlite. In the most
desirable situation, the selection of the target material and of the angle
between the incident deuteron and scattered neutron beams is such that
there is a resonant peak at the chosen neutron beam energy level of 7,8
MeV.
In practice, the spread of energy, or energy band, at each energy level
produced by bombardment of the thin target with a deuteron beam is
proportional to the thickness of the target material. Thus the thinner the
target material, the better defined the energy of the neutron beam, i.e.
the narrower the energy band of the neutron beam. With a view to
maximising the neutron flux or intensity, it is proposed to use the
thickest possible target which nevertheless gives adequate definition of
the neutron beam energy level.
As an alternative to the use of a thin solid target as exemplified above
it would also be possible to use a sealed tritium tube type fast neutron
beam source, with an accelerated deuteron beam, to produce the
CA~138503
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required neutron beam. As yet another, less preferred alternative, the
source could be one in which a deuterium gas target is bombarded by
deuterons to produce a neutron beam at the required resonant energy
level.
The fast neutrons are transmitted through each ore particle 16 as it
passes through the irradiation zone 18 and are detected by a two-
dimensional detector array in the form of a position sensitive scintillator
screen 24. The light output 26 from the scintillator screen 24 is reflected
by a mirror 28 and is focused by a lens 30. The focused light is viewed
by a CCD (charge coupled device) camera 32 which outputs a
representative electrical signal to an electronic processing unit 34. The
unit 34 processes the data which it receives from the CCD camera into
a two-dimensional image representative of density and composition
variations, or conversely neutron beam attenuation, in the particle 16.
In another embodiment of the invention, the fast neutron beam is
detected by a fibre optic scintillator coupled to the CCD camera via a
standard fibre optic taper (not illustrated).
At the selected neutron energy level of 7,8 MeV, there is good contrast
between a diamond 36 and the host kimberlite particle 16. An image
digitising procedure carried out by the processor 34 yields an image
indicating low pixel intensity at all places other than the location of the
diamond 36, where a bright spot appears.
An algorithm performed by the unit 34 recognises any location exceeding
a certain brightness, and accordingly the presence of a diamond, and
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sends an activating signal to an ejector unit 38 located downstream of
the conveyor belt 12.
After passage through the zone 18, the particles 16 are projected in free
flight from the belt 12. After the appropriate time delay, the ejector unit
38 issues a short duration fluid blast at the particle trajectory. This blast
deflects the diamond-containing kimberlite particles 16 out of the
normal free flight trajectory and into a diamond-rich bin 40. Non-
selected particles continue undeflected into a tailings bin 42.
In the embodiments described above, neutron beam irradiation of the
particles takes place at a single, resonant energy level only. In a more
sophisticated system, the particles are irradiated at two distinct energy
levels, one being a resonant energy level and the other being a non-
resonant energy level. For instance, the first neutron energy level could
be 7,8 MeV as before, and the second energy level 7,0 MeV.
A digital image representative of the transmission of the neutron beam
by the particle 16 is derived by the unit 34 for each neutron beam
energy level. Thereafter, the digital values of the two images, stored as
matrices by the unit 34, are subtracted arithmetically one from the other
to form a third digital image. In the third image, which is also
representative of neutron beam transmission by the particle, the effect
of non-diamond components in the kimberlite particle cancel out, and
any diamond inclusion gives rise to a heightened contrast and is detected
by the unit 34 as a location with a brightness exceeding a threshold
value. As before the unit 34 initiates the sorting procedure by the ejector
38.
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The invention contemplates a particularly simple system for modulating
the energy level of the neutron beam between the resonant and non-
resonant values. Figure 2 diagrammatically illustrates a disc 50 formed
with regularly spaced openings 52 in its periphery. Filters in the form of
thin plastic foils span across alternate openings 52, while the remaining
openings 52 are left open. Prior to entering the deuterium gas cell, the
deuteron beam produced by the particle accelerator (not shown)
encounters the disc 50 which is rotated at a predetermined velocity.
When the deuteron beam passes through an opening 52 which has a
filter, it is slowed down slightly and its energy level decreases
accordingly. When the deuteron beam subsequently passes through the
next opening 52, where there is no filter, there is no deceleration of the
beam. The arrangement is such that the neutron beam produced by the
deuterium gas target is modulated rapidly between an on-resonance and
an off-resonance value.
This simple technique for producing distinct energy levels is considered
to be more convenient than a system in which the accelerating voltage
in the particle accelerator is varied periodically to produce distinct
energy levels.
As an alternative to the rotating disc system described above, it would
be possible to vary the emission angle of the gas target while
maintaining a constant deuteron energy level.
In the illustrated embodiment, the particles 1b are conveyed on and
projected from a conveyor belt. In other embodiments, the particles
could be allowed to fall vertically through the irradiation zone. In yet
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other embodiments, means other than a conveyor belt could be used to
convey and project the particles.
In another embodiment contemplated by the invention, the neutron
source could be arranged vertically so as to produce a vertical neutron
beam. In this case the particulate material is presented radially to the
neutron beam, at an angle corresponding to the desired angle of
incidence, and with suitable detection and ejection equipment being
provided at appropriate radial positions in relation to the neutron beam.
The principles of the invention are equally applicable to a CAT-scan
system in which the particles are irradiated, in "slices", by a planar
neutron beam. The particle and the source are rotated relative to one
another, resulting in the formation of a digitised three-dimensional
image. It is believed that this type of arrangement will produce more
accurate results than a system as described above, but at the cost of a
much slower particle analysis process.
Also, although the invention has been described in relation to the
detection of carbon-containing matter, i.e. diamond in the specific
embodiments referred to above, it should be appreciated that other
substances could also be detected with, of course, appropriate selection
of the most suitable resonant energy level. The system of the invention
could, for instance, be used in the analysis of drill core samples, in which
case the sample may be analysed by causing it to pass in an axial
direction through the neutron beam.
