Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACK~:;ROUND TO ~ NVENTION
THIS invention relates to a sorting process and apparatus. In particular,
the invention relates to a process and apparatus for sorting particles
according to their oleophilicity.
In one application of the invention, it may be used to sort diamond
particles, which have oleophilic properties, from non-diamond or gangue
particles which are not oleophilic, in a diamond bearing gravel.
Grease tables have already been used to separate diamond particles
from gangue particles. These u~ilised a continuous layer of grease to
which diamond particles would stick by virtue of the contact angles
subtended at their contacting surfaces.
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SUMMARY OF THE INVENTION
A first aspect of the invention provides a method of sorting a particulate
mass according to the oleophilicity of the particles of the mass, the
method comprising the steps of treating the particles of the mass with
an oil-containing substance in such a manner that oleophilic particles of
the mass are at least partially coated with the substance, causing a
material having specific properties to become associated with the coated
particles, and sorting the particles according to whether they exhibit the
specific properties.
In one version of the invention, the material having specific properties
is a material having magnetic properties and the particles are sorted in
a magnetic separator. The particles may, for instance, be treated by
causing them to pass through a body of a molten oil-containing
substance. Preferably, this version of the invention includes the following
steps:
- treating the particles of the mass with a molten, oil-containing
substance so that oleophilic particles are at least partially coated
with the substance;
- exposing the particles to a particulate, magnetic material so that
magnetic particles adhere to the coatings of the coated particles,
and
- sorting the particles into magnetic and non-magnetic fractions in
a magnetic separator.
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The method may include the further step, after exposure of the particles
to the magnetic material and prior to sorting, of subjecting the particles
to a low temperature to congeal the coatings of the coated particles and
adhere the magnetic particles firmly thereto.
In another version of the invention, the material which is caused to
adhere to the coated particles is a gas, typically air. This version of the
invention preferably includes the following steps:
- the particles are introduced into the upper end of a body of
liquid and are allowed to settle in the body of liquid,
- a molten oil-containing substance in droplet form and air bubbles
are caused to rise in the body of liquid in opposition to the
settling particles,
- oleophilic particles are allowed to acquire an at least partial
coating of the oil-containing substance,
- air bubbles are allowed to adhere to the coatings of the coated
particles, thereby to increase the buoyancy of those particles, and
- the particles are sorted according to their buoyancy.
Either version of the invention can be used to sort diamonds from otherparticles.
A second aspect of the invention provides an apparatus for sorting a
particulate mass according to the oleophilicity of the particles of the
mass, the apparatus comprising:
- means for treating the particles of the particulate mass with an
oil-containing substance so that oleophilic particles acquire an at
least partial coating of the oil-containing substance,
- means thereafter for causing the treated particles to contact a
material having specific properties, and
- means for sorting the particles according to whether or not they
exhibit the specific properties
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 Figures 1
and 2 diagrammatically illustrate two embodiments of the invention
DESCRIPTION OF EMBODIMENTS
The illustrated embodiments can be used to sort diamond particles from
gangue particles in a diamond-bearing gravel, the gravel typically being
one recovered from diamond mining activities
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Figure 1 shows a vessel 10 containing, in a central region thereof, an
aqueous magnetite suspension 12 which is recycled in a closed circuit 14
by means of a pump 16. A volume 18 of clear water lies above the
suspension 12 and a further volume 20 of clear water lies beneath the
suspension.
Floating on the upper surface of the upper volume 18 of water is a layer
22 of molten petroleum jelly. The molten state of the layer 22 is
maintained by a suitable heating coil or the like (not shown). The
numeral 24 indicates a feed stream of wet gravel which has been
preheated to a temperature, in this case of around 80C, but in any case
higher than the congealing point of the petroleum jelly in the layer 22.
The feed stream sinks through the layer 22 and oleophilic particles in
the feed stream, primarily diamond particles, acquire an at least partial
coating of molten jelly during their passage through that layer.
Having sunk through the layer 22, the particles gravitate through the
suspension 12. Magnetite particles become embedded in the jelly
coatings of the relevant particles, i.e. the oleophilic particles 24. There
is no such embedment in the essentially non-oleophilic, uncoated
particles 26.
Having gravitated through the suspension 12, the particles enter the
lower volume 20 of water, which is cooled by any appropriate means to
a temperature below the congealing point of the petroleum jelly. During
their gravitational passage through the lower volume of water, the jelly
coatings congeal to adhere the embedded particles securely to the
oleophilic particles 24.
From the lower end of the vessel 10, the particles 24 and 26 are
transferred to a magnetic separator, indicated generally with the numeral
28. The magnetic separator, which may be of conventional type,
separates the particles into a magnetic fraction, composed of particles
24 and a non-magnetic fraction composed of particles 26. It will be
appreciated that the process can operate substantially continuously with
magnetic, i.e. primarily diamond particles, reporting in the magnetic
fraction and non-magnetic, i.e. gangue particles, reporting in the non-
magnetic fraction.
It will also be appreciated that passage of the particles through the
suspension 12 and subsequent water volume will wash any weakly
adhering jelly from the non-oleophilic particles. Although petroleum jelly
has been specifically mentioned, it will also be appreciated that many
other oil-containing substances could also be used to provide the
required coating on the oleophilic particles. Heating of the layer 22 may
or may not be required, depending on the prevailing ambient
temperature, to ensure that the layer exists in liquid form. Similarly,
cooling of the water volume may or may not be required, depending on
the ambient temperature and choice of substance used in the layer 22.
It will furthermore be understood that the particles may initially be
preheated to avoid premature congealing of the jelly in the layer 22
before the particles reach the suspension 12.
In the example described above, the particles are treated with an oil-
containing substance, i.e. the jelly, so that the oleophilic particles, i.e. thediamonds, are at least partially coated with that substance. A material
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having specific properties, in this case a magnetic material in the form
of magnetite, is then caused to adhere to the coated particles. Particle
sorting can then take place on the basis of whether or not the particles
exhibit magnetic properties.
Figure 2 illustrates a second embodiment of the invention. In this case,
there is a vessel 100 containing liquid 102 in the form of water or an
aqueous suspension. Fine particles are introduced into the liquid 102
through a feeder 104. Molten grease or another oil-containing substance
is injected into the lower end of the vessel 100 through nozzles 106. The
grease rises in the liquid 102 in droplet form in counter-currellt flow to
the particles as the particles settle in the liquid.
Collision of the grease droplets with the settling particles causes theoleophilic particles 103, i.e. diamonds, to acquire an at least partial
coating of grease, while non-oleophilic particles 105 are not so coated.
The grease which rises to the surface of the liquid 102 can be withdrawn
by suitable means and then recirculated through a circuit 108 containing
a heater 110 that maintains the grease temperature high enough for it
to remain the required molten state.
Located adjacent the nozzles 106 is an air sparging head 112 through
which air is injected into the vessel 100 simultaneously with injection of
the grease. Thus air bubbles 113 rise through the liquid 102 along with
the grease droplets. The air bubbles become attached to the grease-
coated particles and increase the buoyancy of those particles.
The smaller particles become sufficiently buoyant to float to the top of
the vessel where they can be removed as a diamond rich fraction.
Heavier particles may not acquire sufficient buoyancy to float to the top
of the vessel, and these particles continue to settle through the liquid
102 onto a conveyor belt 114 which takes them to a dedicated flotation
cell where the more buoyant diamond particles are separated from the
other particles.
In this example of the invention, therefore, air is caused to adhere to the
coated particles to give those particles the specific property of increased
buoyancy.
In a refinement of the Figure 2 technique, it would be possible to inject
fine magnetite particles along with the air or grease. In this case, the
coated particles acquire both increased buoyancy as a result of air
bubble adherence and magnetic properties as a result of adherence of
the magnetite. Both flotation and magnetic separation techniques can
then be used to separate the diamond particles from the other particles.
In the last-mentioned version it may prove necessary to precondition the
particle feed to remove lightly magnetic particles.
In each of the embodiments discussed above the particles are
preconditioned if necessary to expose the surfaces of the diamond
particles. This may, for instance, be done by attritioning the particles in
an abrasive suspension such as a suspension of ferrosilicon.
