MAGNETIC CYCLONE AND METHOD OF OPERATING IT

SEPARATEUR MAGNETIQUE ET METHODE D'EXPLOITATION

English Abstract

One aspect of the invention concerns a cyclone which has a cyclone vessel
having an underflow and an overflow and an inlet for feed material. Means
are provided to generate a vertically oriented magnetic field in the cyclone
vessel for the purposes of controlling the operation thereof. Another aspect
of the invention concerns a method of operating a cyclone vessel in which
at least the density differential achieved by the cyclone is controlled by
means of a vertically oriented magnetic field.

Claims

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE
DEFINED AS FOLLOWS CLAIMS
1. A dense medium cyclone in which particles of a feed material are
separated from one another according to their density in a magnetic
dense medium, the cyclone comprising a cyclone vessel for
containing the magnetic dense medium and having an inlet for feed
material, an underflow and an overflow, and a magnet arranged
toroidally about the cyclone vessel to generate a vertically oriented
magnetic field therein for controlling the density of the dense
medium such that particles having a density exceeding that of the
dense medium report to the underflow while particles having a
density less than the density of the dense medium report to the
overflow.
2. A cyclone according to claim 1 wherein the magnet is a permanent
magnet.
3. A cyclone according to claim 1 wherein the magnet is an
electromagnet.
4. A cyclone according to claim 3 comprising means for supplying the
coil of the electromagnet with a selectively variable current to vary
the strength of the magnetic field.
5. A cyclone according to any one of claims 1 to 4 wherein the vertical
position of the magnet relative to the cyclone vessel is variable.

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6. A method of operating a dense medium cyclone in which particles of
a feed material are to be separated from one another according to
their density in a magnetic dense medium in a cyclone vessel having
an inlet for the feed material, an underflow and an overflow, the
method comprising the steps of generating a vertically oriented
magnetic field in the cyclone vessel, by means of a toroidal magnet
arranged about the cyclone vessel, thereby to control the density of
the dense medium such that particles having a density exceeding that
of the dense medium report to the underflow while particles having a
density less than the density of the dense medium report to the
overflow.
7. A method according to claim 6 including the step of selectively
varying the vertical position of the magnet relative to the cyclone
vessel.
8. A method according to either one of claims 6 or 7 wherein the
magnet is an electromagnet, the method including the step of
selectively varying the current supplied to the coil of the
electromagnet.
9. A method according to any one of claims 6 to 8 wherein the feed
material includes a diamondiferous particulate material in a
ferrosilicon suspension and the cyclone is operated in a manner to
separate diamond particles from associated particles.

Description

CA 02205339 1997-OS-14
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BACKGROUND TO THE INVENTION
THIS invention relates to a magnetic cyclone and to a method of operating
a magnetic cyclone.
One of the critical parameters in the operation of dense medium cyclones,
which are used inter alia in dense medium diamond separation processes, is
the density differential between the overflow and underflow streams
produced by the cyclone. It is generally accepted that the density
differential
should have a constant value of between 0,2 and 0,5 specific gravity units.
If the density differential is too high, there is a wide range of densities
present in the cyclone and excessive middlings particles with a high retention
time, thereby reducing the speed at which the cyclone can operate. If the
density differential is too low, inadequate recovery of the valuable
component, typically diamond, is achieved.
Magnetic cyclones, in which a magnetic field is applied to the cyclone
vessel, are known. A feature common to all known magnetic cyclones is the
use of a horizontally oriented magnetic field which draws magnetic, i.e.
magnetisable, particles to the side of the cyclone vessel from where they are
moved to the underflow spigot in the material flow. Attempts have been
made to use magnetic cyclones of this type in mineral beneficiation
processes, such as in the recovery of magnetite, but the known technology
has not received wide acceptance for various reasons including insufficient
mineral recovery, undesirable flocculation of the magnetic particles resulting
in poor concentrate grades and product accumulation in the cyclone.

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SUMMARY OF THE INVENTION
According to the present invention there is provided a cyclone comprising a
cyclone vessel for containing a magnetic dense medium and having an inlet
fim feed material, an underflow and an overflow, and a vertically oriented
magnetic field in the cyclone vessel, for controlling the density of the dense
medium such that particles having a density exceeding that of the dense
medium report to the underflow while particles having a density less than the
density of the dense medium report to the overflow.
In the preferred embodiments, the field generating means comprises a
magnet arranged toroidally about the cyclone vessel. The magnet may be a
permanent magnet or an electromagnet. In the latter case, it is preferred that
the coil of the electromagnet be supplied with a selectively variable current
whereby the strength of the magnetic field be varied. Irrespective of whether
tree magnet is a permanent magnet or an electromagnet, it is preferred that
the
vertical position of the magnet be variable.
According to another aspect of the invention there is provided a method of
operating a dense medium cyclone in which particles of a feed material are to
bc; separated from one another according to their density in a magnetic dense
medium in a cyclone vessel having an inlet for the feed material, an
underflow and an overflow, the method comprising the steps of generating a
vertically oriented magnetic field in the cyclone vessel, thereby to control
the
density of the dense medium such that particles having a density exceeding
that of the dense medium report to the underflow while particles having a
density less than the density of the dense medium report to the overflow.
The method is preferably implemented by means of a magnet arranged
toroidally about the cyclone vessel. As indicated above, the magnetic field
strength and or the vertical position of the magnet may be variable to
enhance the operational control which is achieved by the method.
In one application, the invention proposes that the method be used to control
at least the density differential between the underflow stream and the
overflow stream in a dense medium cyclone, typically one using a magnetic

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medium such as ferrosilicon (FeSi). In addition, the method may be used to
control the cut point density and the sharpness of the separation.
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 diagrammatically illustrates a magnetic cyclone
according to the invention;
Figure 2 diagrammatically illustrates the forces acting on
particles in the cyclone;
Figure 3 graphically illustrates an experimental relationship
between density differential and magnetic field
strength for different positions of the magnet; and
Figure 4 shows a series of Tromp curves demonstrating
relationships between density cut point, sharpness of
separation and magnetic field strength.
SPECIFIC DESCRIPTION
The invention is described hereunder in its application to the control of the
operating parameters of a dense medium cyclone, typically one in which

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FeSi forms the dense medium and which is employed in diamond recovery
operations. It should however be understood that the principles of the
invention are also applicable to the operation of cyclones used in mineral
beneficiation processes where magnetic particles are to separated from non-
magnetic particles and in the dewatering of dilute media.
The magnetic cyclone 10 seen in Figure 1 includes a cyclone vessel 12 of
generally conventional construction. The cyclone vessel has an underflow
spigot 14 at its lower end, a vortex finder 16 serving as an overflow at its
upper end, and a material feed inlet 18 through which feed material is
introduced. In a diamond recovery operation, the feed material may, for
instance comprise diamondiferous particles in a dense ferrosilicon
suspension. In this specific application, it is desirable to control the
density
differential within close limits, typically at a value between 0,2 and 0,5
specific gravity units, to ensure comprehensive recovery of the diamond
particles in the underflow.
A toroidal magnet 20 is arranged concentrically about the cyclone vessel, as
illustrated, and generates a vertically oriented magnetic field indicated by
the
numeral 22. The magnet may be a permanent magnet or an electromagnet,
in the form of a solenoid, and it may be of any suitable construction such as
iron-yoke type or mufti-pole type. Means (not illustrated) are provided to
vary the vertical position of the magnet 20 relative to the cyclone vessel, as
indicated by the arrows 24. In addition, in the case of an electromagnet,
current control means (not illustrated) are provided to vary the current
supplied to the coil of the electromagnet and thereby vary the strength of the
magnetic field 22 which is generated.

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It is possible to vary the orientation of the magnetic field 22, i.e. upwards
or downwards, by varying the vertical position of the magnet 20 in relation
to the cyclone vessel. In Figure l, the magnetic field gradient and hence the
magnetic force is illustrated by the arrow 26 as upwardly directed. Figure 2
illustrates a situation where the magnetic field at the axis of the cyclone is
oriented vertically downwardly. The symbol Fm represents the magnetic force
on magnetic particles in the cyclone attributable to the magnetic field, the
symbol Fb the gravitational force on the particles, the symbol Fd the
hydrodynamic drag force on the particles and the symbol F~ the centrifugal
force on the particles. The resultant of these forces is indicated by the
symbol F~to~a~>.
In a simple solenoid, the magnetic force is always directed towards the
centre of the solenoid irrespective of whether the solenoid is in the upper or
lower part of the cyclone. The direction of the resultant force is determined
by the magnitude of the other relevant forces, such as gravitational force and
hydrodynamic drag force. In any case, however, the direction of the total
force will be outwards as shown in Figure 2.
It is possible to wind the solenoid in such a way that the magnetic force is
directed not towards the centre of the solenoid, but outwards away from the
centre. This can, for instance, be achieved by placing more turns of the
solenoid wire on the upper and/or lower ends of the solenoid rather than at
the centre. Such techniques provide additional flexibility in determining the
direction of the magnetic force.
Clearly by varying the vertical position of the magnet the orientation and
magnitude of the resultant force F~~o~,~ can be varied. Because the resultant

CA 02205339 1997-OS-14
force acting on the particle determines whether it reports to the underflow
or the overflow of the cyclone vessel, this feature can advantageously be
used to control of the density differential between the underflow and
overflow and the density cut point, i.e. the density value at which the
distinction is made between material which will report to the underflow and
that which will report to the overflow.
In situations where the magnet 20 is an electromagnet, varying the current
supplied to the coil of the electromagnet will also vary the strength of the
magnetic field and hence the value of Fm. This feature may also be used
advantageously to apply a further control to the relevant parameters such as
density differential and cut point. In situations where the magnet 20 is a
permanent magnet with a fixed field strength, the necessary control of the
operational parameters of the cyclone would be achieved solely by variation
of the vertical position of the magnet.
Figure 3 graphically illustrates experimental results confirming the variation
of density differential which can be achieved using an electromagnet with
variable supply current to vary the strength of the magnetic field, as
represented on the horizontal axis. Figure 3 also illustrates that a further
control on density differential can be achieved by varying the vertical
position of the magnet, as indicated by the three curves indicating
experimental results obtained with the magnet positioned respectively at a
top position, a middle position and a bottom position. In the experimental
model the top position of the magnet was flush with the lower end of the
vortex finder, the bottom position of the magnet was flush with the
underflow spigot and the middle position was mid-way between the top and
bottom positions.

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Figure 4 shows several Tromp curves for different magnetic field strengths
and serves to illustrate relationships between density cut point and sharpness
of the separation which is achieved, and the magnetic field strength
The experimental results were obtained using a 100mm diameter cyclone
vessel and a ferrosilicon suspension incorporating density tracers.
It is anticipated that the invention will enable close controls to be
maintained
over the important operational parameters of the cyclone.

Representative Drawing