Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~A.~IgO~lD T(a T1~I~ Il~l'TI~1~T
THIS invention relates to a particle sorting method. In particular, the
invention relates to a method for sorting particulate material on the
basis of differences in thermal properties.
One application of the invention is in the recovery of diamonds from
associated gangue particles in a diamond bearing gravel recovered in
diamond mining or expioration activities.
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SLTIdII'~IAIt'~ ~k' 'I'I-IE II~~1~1~'I~I~
~.ccording to this invention, there is provided a method of sorting
particles according to the intrinsic thernnal properties of the particles,
the method comprising:
subjecting the particles to one or more thermal treatment steps
so that particles with different intrinsic thermal properties are
braught to different temperatures,
either during or after the thermal treatment steps, subjecting the
particles to a further, non-thermal treatment chosen to endow
selected particles that are distinguished from other particles by a
difference in temperature with a certain feature which is not an
intrinsic feature of the particles prior to the thermal treatment
steps, and
sorting the particles into a fraction having the certain feature
after a predetermined time period has elapsed since the further
treatment step and a fraction not having the certain feature after
that time lapse.
In a first version of the invention, the particles are exposed, in the
further treatment step, to water vapour after at least some of them have
been brought to a temperature below the freezing point of water, and
the sort is made according to whether the particles exhibit a frosted
appearance a predetermined time after such exposure.
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In one embodiment of the first version of the invention, in which
thermal treatment takes place before further treatment, the particles are
initially thermally treated by cooling them to different temperatures
above and below the freezing point of water, the temperature-
distinguished particles are then exposed to water vapour so that particles
at the lower temperature selectively acquire a frosted appearance, and
the sort is then made on the basis of whether the particles exhibit a
frosted appearance.
In another embodiment of the first version of the invention, in which the
further treatment step takes place during thermal treatment, the
particles are all cooled to a uniform low temperature below the freezing
point of water and are then exposed to warmer air containing water
vapour so that all particles acquire a frosted appearance, the sort being
made according to whether the particles still exhibit the frosted
appearance after a predetermined time lapse.
In a second version of the invention, the particles are thermally treated
to bring them to different temperatures, the particles are exposed to a
magnetic substance which selectively freezes onto particles at a lower
temperature, and the particles are then magnetically sorted.
In a third version of the invention, the particles are thermally treated to
bring them to different temperatures above and below the freezing point
of a liquid and are then placed in a body of the liquid so that the liquid
selectively freezes onto the cooler particles, the sort being made
according to the buoyancy of the particles.
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I3ItI~1' DESCItII''I'I~N ~F T~iE IaI~'fVING~
Various versions of the invention will now be described in more detail
and by way of example only, in some cases with reference to the
accompanying drawings in which:
Figure 1 diagrammatically illustrates one embodiment of a
second version of the invention;
Figa~re 2 diagrammatically illustrates one embodiment of a
third version of the invention; and
I<,igure 3 diagrammatically illustrates another embodiment of
the third version of the invention.
~PECII' I~': I~I;SCI~II~'I~N
Two non-illustrated embodiments of the first version of the invention
will now be described in more detail. In both cases, the descriptions
refer to the sorting of diamond particles from associated gangue, but it
is to be appreciated that the principles of the invention are applicable
to many other applications in which it is possible to make a distinction
between particles with different intrinsic thermal properties.
In the first embodiment of this version ~f the invention, particulate
diamondiferous material, containing a mixture of diamond particles and
associated gangue particles, is initially thermally treated by depositing it
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in a monolayer on the surface of a body or layer of ice at a temperature
substantially below the freezing point of water. The fact that the
diamonds have a higher coefficient of thermal conductivity and thermal
diffusivity means that they will lose heat bo the ice more quickly than the
gangue particles which have a lower thermal diffusivity. Accordingly, the
diamond particles will be brought more quickly to a temperature below
the freezing point of water than the gangue particles.
After a period of tune sufficient for the above temperature distinction
to take place, the thermally treated particles are subjected to a further
treatment step in which they are all exposed to ambient air containing
water vapour in gaseous form. The air may be normal ambient air.
Alternatively, the air may be specially prepared air with a water vapour
content higher than normal ambient air.
Because the diamonds are now at a temperature sufficiently below the
freezing point of water, water vapour in the air which contacts the
diamond particles will be frozen on the surfaces of the diamond
particles, forming a "frosted" layer on the diamond particles. The
"frosted" layer will have the normal white colouration.
The associated gangue particles are, after the chosen time period, still
at a temperature at which freezing of the water vapour cannot take
place, because of their lower thermal conductivities and diffusivities.
There will therefore be no formation of a frosted layer on the gangue
particles.
It will be appreciated that the feature of a frosted appearance on the
diamond particles is a non-intrinsic property of those particles. In other
words, diamonds do not normally have a frosted appearance, such
appearance in this case being given to the diamonds because of their
lower temperature.
The presence or otherwise of the frosted layer on the diamond particles
will be readily detectable, for instance by eye, enabling a sort to be
made. As an alternative to eye distinctian, an automatically operating
apparatus may be used to distinguish and sort the relevant particles from
one another. Since the diamond particles have a white, frosted
appearance, and accordingly are lighter in colour that the associated,
non-frosted gangue particles, an optical distinction can be made
automatically merely on the basis of light and dark particles. It is
anticipated that conventional photometric equipment currently used for
distinguishing between relatively light and relatively dark particles will
be able to make the necessary distinction in this case.
The actual sorting of the particles may be made manually on the basis
of the visual determination referred to above. Alternatively, automatic
sorting means, possibly employing gas blast ejectors or the like, could be
arranged to separate the diamond particles from the gangue particles in
response to the determinations made by the automatic optical detection
equipment referred to above.
In the second embodiment of this version of the invention, all the
particles are cooled down for a period of time sufficient for them all to
some to a temperature below the freezing point of water. This could, for
_ g .
instance be done by immersing all the particles in a liquid, such as liquid
nitrogen, at ~ a temperature substantially below the freezing point of
water, and leaving the particles so immersed for a period of time long
enough to enable all the particles to come to the required low
temperature. Alternatively, the particles could be placed in a cold
enclosure, or be exposed to a cold gaseous environment for the required
time period.
Thereafter the particles are contacted with air at a temperature
substantially above the freezing point of water. The air may contain
normal amounts of water vapour or, preferably, has been moistened to
a greater degree. ?'he water vapour will freeze' onto the surfaces of all
the particles because of their uniform low temperature. Thus all the
particles at this stage will have a frosted appearance.
Because the diamond particles have a high thermal diffusivity, they will
accept heat from the relatively hot, ambient air at a faster rate than the
gangue particles. The temperature of the diamond particles will
therefore rise more rapidly to a level above the freezing point of water.
Accordingly the frozen water vapour on the diamond particles will melt
before melting of the water vapour on the gangue particles takes place.
The diamond particles will therefore lose their frosted appearance
before the gangue particles.
In this embodiment, it will be appreciated that the initial cooling of the
particles to a uniform low temperature and the subsequent warming of
the particles by exposing them to warmer air constitute the thermal
treatment steps of the invention. It will also be appreciated that the
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further treatment, i.e the exposure of the particles to the water vapour,
takes place during the latter part of the thermal treatment.
Once again, an eye or automatic optical distinction can be made
between the lighter, in this case gangue, and darker, in this case
diamond, particles. Thereafter the particles can be separated from one
another as described above far the first embodiment.
It will be appreciated that both embodiments of the invention as
described above are time-dependent. In the first embodiment the
particles must be cooled down for a period of time chosen for the
diamond particles to gain a frosted appearance when they are then
exposed to water vapour, but not the gangue particles. Thereafter the
particles must be viewed at a time when the diamond particles still
retain the frosted appearance, so that the necessary frosted/non-frosted
distinction can be made.
In the second embodiment the initial cooling period is sufficiently long
for all particles to drop to an appropriately low temperature, and
viewing takes place after a time lapse chosen for the water vapour on
the diamond particles to have melted, but not the water vapour on the
gangue particles. In each case, the relevant time periods can be
determined experimentally.
Reference is now made to Figure 1 of the drawings, which
diagrammatically illustrates a first embodiment of the second version of
the invention. In Figure 1, a feed stream of diamond-bearing gravel
which is to be sorted into a diamond-rich fraction and a gangue fraction
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is indicated with the numeral 10. The feed stream 10 may, for instance,
be conveyed on a conveyor belt (not shown). .
The feed stream 10 is passed through a cooling tunnel 12 and then
through a bath 14 of liquid nitrogen. The residence time of the particles
in the bath 14 is sufficient to bring the particles to a uniform low
temperature, typically below -120°C.
The cooled particles are then fed into a tank 16 containing a suspension
18 of magnetic material, in this case magnetite, in water. The
temperature of the suspension is maintained in the range 0°C to
25°C,
and is typically around 7°C. The suspension 18 contains between 10%
and 60% magnetite particles by mass, the particles themselves typically
having a size of -10 x 10-sm.
As the particles gravitate through the suspension 18, the gangue
particles, having a very much lower thermal diffusivity than the
diamonds, gain heat relatively slowly from the suspension and remain at
a temperature well below the freezing point of the suspension. They
therefore acquire an at least partial frozen coating of the suspension.
The diamonds, on the other hand, gain heat more rapidly because of
their higher thermal diffusivity, and rapidly come to a temperature at or
near that of the suspension. At their relatively elevated temperature, the
diamond particles are unable to acquire a frozen coating of the
suspension.
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The particles gravitate onto the upper run of an endless conveyor belt
20 moving around a submerged tail pulley 21 and a magnetised head
pulley 24 located outside the bath 16. The belt 20 conveys the particles
upwardly and out of the bath through a water spray 22 which washes
away any free suspension 18.
The magnetically coated gangue particles 23 and uncoated diamonds 25
continue moving on the belt and pass over the head pulley 24. ~1s the
belt passes over the head pulley, the diamond particles, having no
magnetic susceptibility, fall off into a collection bin 26. The magnetically
coated gangue particles are held in contact with the belt by the magnetic
attraction of the head pulley, and move further around beneath the head
pulley, past a sputter plate 28. As they move further away from the head
pulley, the magnetic attraction farces diminish and the gangue particles
eventually fall off into a collection bin 30. A scraper 32 can, if necessary,
be provided to ensure separation of the gangue particles from the belt.
In a mining exploration or prospecting operation, the method described
above can be carried out batchwise to analyse small geological samples.
In the example described above and illustrated in Figure 1, the thermal
treatment steps of the invention involve firstly cooling the particles and
then heating them to bring them to substantially different temperatures
at the lower of which selective freezing of the suspension 18 can take
place. In the illustrated case, after initial cooling, heating to achieve a
marked temperature distinction is performed by the suspension 18 itself.
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Thus in the embodiment illustrated i:n Figure 1, it will also be
appreciated that the further treatment step, namely the exposure of the
particles to the magnetic liquid, takes place during the thermal
treatment, since it is the exposure of the particles to the magnetic liquid
which forms the second part of the thermal treatment procedure.
In a non-illustrated embodiment of this version, the thermal treatment
steps of the invention could involve applying heat to the particles, after
initial cooling to a uniform low temperature, by a flash heating process
in which the diamond particles would gain heat far more rapidly than
the gangue particles.
In a reversal of the above thermal treatment steps it would also be
possible initially to heat the particles to a uniform high temperature. The
particles would then be subjected to a cooling step, for instance by
passing them through liquid nitrogen. The diamond particles would lose
their heat far more rapidly than the gangue particles and would be
brought to a low temperature at which freezing of the suspension could
later take place. ~n the other hand, the gangue particles would remain
at a relatively high temperature at which no such freezing could take
place. Thus in this case, it would be the gangue particle which would fall
first off the belt, with the attractive magnetic forces holding the diamond
particles, which are coated with frozen suspension, on the belt for
subsequent removal.
The methods described in relation to Figure 1 are believed to be
particularly suitable for recovering diamond particles in the size range
3mm to l5mm.
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Reference is now made to Figure 2 of the drawings, which
diagrammatically illustrates a second embodiment of the second version
of the invention. Yn Figure 2, a diamondifc;rous feed stream 100 is passed
through a liquid nitrogen bath 102 which cools all the particles to a
uniformly low temperature.
The cooled particles are dropped onto the upper run of an endless
conveyor belt 104 which conveys them past a hot air blower 106 which
applies heat to them. The diamond particles heat up much more rapidly
than the gangue particles.
'The conveyor belt 104 then conveys the cooled and subsequently heated
particles through a spray 108 of a magnetic liquid, typically a magnetite
suspension as described in relation to Figure 1. The time and
temperature parameters are set such that the gangue particles, on
reaching the spray 108, are at a temperature below the freezing point of
the liquid, while the diamond particles have gained sufficient heat to be
above that freezing point. Thus the gangue particles acquire at least a
partial frozen coating of the magnetic liquid.
The conveyor belt passes around a magnetic head roller 110 which
attracts the coated gangue particles but not the diamonds. The diamonds
fall off the conveyor belt as they pass the head roll 110 and are collected
to one side of a splitter plate 112. The gangue particles are kept, by
magnetic attraction, in contact with the belt as they pass around the
head roller. As the gangue particles move further from the head roll 110,
the magnetic attraction decreases and these particles eventually fall off
the belt on the opposite side of the splitter plate 112.
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In a modified form of the Figure 2 embodiment, the upper run of the
belt 104 could carry a film of water. The particles are initially cooled to
a temperature low enough for them to freeze the water upon contacting
the film. This adheres all the particles to the belt. When the particles
are subsequently subjected to heating by the hot air blower, the diamond
particles gain heal more quickly and melt the frozen water adhering
them to the belt. The gangue particles remain adhered to the belt for a
longer period of time.
Thus in this case, it is a combination of the 'adhering effects of the
frozen water and the magnetic attraction between the frozen magnetic
coatings and the head roll 110 which keeps the gangue particles in
contact with the belt longer than the diamond particles. A.s before the
diamond particles are able to fall off under gravity.
In yet another modification of the Figure 2 embodiment, the particles
could be initially cooled and then flash-heated in such a manner as to
bring the temperature of the diamonds above the freezing point of water
while the gangue particles remain below such freezing point. In this case,
only the gangue particles would be adhered to the surface of the belt by
frozen water. This system would again be used in combination with
application of a magnetic liquid as described above, so that the gangue
particles selectively acquire a frozen coating of the magnetic material.
Once again, it would be a combination of the adherence caused by
fieezing of a water film and magnetic attraction between the coatings of
the gangue particles that would keep the gangue particles in contact with
the belt after passage around the head pulley.
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The magnetic attraction decreases as described above as the gangue
particles move further from the head roll. The adhering effect of the ice
can be brol~en by applying further heat to the underside of the belt, after
the particles have passes the splitter plate, or by scrapers or warm water
sprays. The combined magnetic attraction and ice adhesion may, it is
believed, enhance the accuracy of the sort which is achieved.
Reference is now made to Fig~ire 3 of the drawings which illustrates one
embodiment of the third version of the invention. Figure 3 shows a feed
stream 210 of diamond-bearing gravel derived, for instance, from
diamond mining or prospecting activities. The feed stream 210 is
conveyed, for instance on a conveyor belt, through a cooling apparatus
212. The cooling apparatus may, for instance, include a bath of liquid
nitrogen. The residence time of the particles of the gravel in the cooling
apparatus 212 is sufficient to bring all the particles to a uniform low
temperature well below the freezing point of water.
~nce all the particles are at the same uniform low temperature, they are
conveyed through a flash heating station 21~ at which heat is applied
rapidly to them. The residence time and temperature in the flash heating
station is chosen for those particles which have a relatively high thermal
diffusivity, in this case diamonds, to heat up rapidly to a temperature
above the freezing point of water. ~Iowever, the time and temperature
parameters are set such that the gangue particles in the gravel, being of
lower thermal diffusivity, are unable to acquire sufficient heat to elevate
their temperature above the freezing point of water.
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In practice, the time and temperature parameters are set for the gangue
particles to emerge from the flash heating station at a temperature still
well below 0°C, typically about -40°C, with the diamonds
emerging at a
temperature of around 5°C.
The thermally treated particles are then dropped into a stream of water
216 which flows in the direction of the arrow 218 in a conduit 220. The
water is maintained by cooling means (not shown) at a temperature
slightly above 0°C.
Ice builds up on the low temperature gangue particles, but not on the
diamonds. The diamonds 222 sink rapidly and are conveyed downstream
a short distance only. The ice-covered gangue particles 224, on the other
hand, are more buoyant than the diamonds and are thus conveyed
further downstream by the water flow.
The diamond and gangue particles are at least roughly uniformly
presized before the sorting operation commences. This would, in any
event, normally be the case in diamond mining operations where all
particles would have been subjected to a crushing operation before
sorting. The greater buoyancy of the gangue particles is attributable in
the first place to their greater volume as a result of ice build-up. Also,
the iced gangue particles will have a lower overall density than the
diamond particles because of the presence of the ice.
Figure 3 shows a sputter plate 226 upstream of which the diamond
particles settle to the bottom of the conduit and downstream of which
the gangue particles settle. The particles are then recovered separately.
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In a slightly modified version of the Figure 3 embodiment, suitable for
batch sorting, the thermally treated particles are merely dropped into a
bath of low temperature water. Once again the diamond particles sink
rapidly in the water, while ice builds up on the gangue particles. Vdith
the time and temperature parameters correctly set, sufficient ice will
build up on the gangue particles to cause them to float in the water. In
this case, the diamonds are recovered as a sunken fraction and the
gangue particles are recovered as a floating fraction.
In either case, the ice on the gangue particles will, after a period of
time, start to melt, and it will therefore be important for separation of
the more and less buoyant particles to take place at the correct time.
Although specific reference has been made in this specification to the
sorting of diamonds from associated gangue particles, it will be
appreciated that the principles of the invention will be equally applicable
to the sorting of other particles distinguished from one another by
marked differences in intrinsic thermal properties. In each case, the
difference in intrinsic thermal properties, notably thermal diffusivity, is
used, with appropriate thermal treatment, to create a temperature
differential. After the various further treatments described above and as
a result of the temperature differential, one class of particles acquires or
retains a specific feature on the basis of which a reliable sort can
subsequently be made. It will be recognised that the feature, such as a
frosted appearance, a frozen magnetic coating or a frazen coating
affecting buoyancy, is not an intrinsic feature of any of the particles and
is only acquired because of the thermal treatment to which the particles
are subjected.
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Various proposals have previously been made for sorting particles on the
basis of differences in intrinsic thermal properties. In the prior proposals
it is the difference in intrinsic thermal properties of the particles alone
which males the eventual sort possible. The individual particles, or
selected particles, are not caused to acquire a non-intrinsic feature on
the basis of which the sort is eventually made, as proposed by the
present invention.
