Note: Descriptions are shown in the official language in which they were submitted.
BACKGROU~D TO THE INVENTION
l'HIS invention relates to a method and means for separating materials.
Mining operations almost invariably involve the extraction of valuable minerals
which exist in very small quantities in the mined rock. This is particularly so in the
case of valuable metals such as gold and silver.
It is therefore considered that it would be advantageous to have a method and
means whereby non-magnetic, electrically conductive materials such as gold and
silver can be separated from other materials.
SUMMARY ~F THE INVE~ON
The invention provides a method of separating particulate mateAal according to the
electrical conductivity of the particles of the material, the method comprising
irradiating the particles with microwave or radio frequency electromagnetic
radiation and subjecting the irradiated particles to a magnetic field so that eddy
currents induced in tlie particles by the electromagnetic radiation interact with the
magnetic field to cause movements, dependent upon electrical conductivity, of
electrically conductive particles.
The particles are preferably irradiated with microwave radiation having a frequency
in the range 109Hz to 3x101lHz, or by radio wave radiation having a frequency inthe range 104Hz to 109Hz.
The map,netic field may be a moving or stationary field. In addition, the magnetic
field may have a constant or varying intensity.
In one version of the invention, the particles are passed through a microwave
chamber in which they are irradiated with microwave radiation and in which they
are subjected to the magnetic field. In another version of the invention, the
particles are held in suspension in a liquid and are subjected to microwave
irradiation and the magnetic field while so suspended.
The method of the invention can be used to separate gold particles from other
particles.
The invention also provides an apparatus for separating particulate material
according to the electrical conductivities of the particles of the material, theapparatus comprising means for irradiating the particles with rnicrowave or radio
frequency electromagnetic radiation and means for subjecting the irradiated
particles to a magnetic field so that eddy currents induced in the particles by the
electromagnetic radiation interact with the magnetic field to cause movements ofelectrically conductive particles dependent on their conductivities.
BRIEF DE5CR~PI'ION OF THE DRAWINFS
The invention will now be described in more detail, by w~y of example only, withre~erence to the accompanying drawings in which:
Figure 1 shows a diagrammatic side view illustrating a first embodiment of
the invention; and
Figure 2 shows a diagrammatic plan view of a second embodiment of the
invention.
DESCRIPIlON OF EMBODIMENrrS
Figure 1 shows an apparatus 10 which illustrates the principles of the method ofthe invention. The Figure illustrates a microwave chamber 12 in which is mounteda microwave generator 14 for generating microwaves having a frequency in the
range 109Hz to 3x10llHz.
A glass dish 16 is placed on a conductive shielding plate 18 in the chamber 12 and
contains an aqueous colloidal suspension 19 of fine gold particles together withother non-magnetic, non-conductive particles.
A permanent magnet 20 is placed beneath the conductive plate 18 and means (not
shown) are provided for moving the magnet in the direction of the arrow 22 in
Figure 1. The magnetic field lines associated with the magnet 20 are vertical inFigure 1.
With the microwave generator in operation, the magnet is caused to move in the
direction of the arrow 22. The mucrowaves induce eddy currents in the gold
particles in suspension. Such eddy currents interact with the moving magnetic field
and give rise to an electromotive force which in this case urges the gold particles
to move to the right in Figure 1, i.e. in the same direction as the magnet moves.
No eddy currents are induced in the non-conductive particles which are also in
suspension with the gold particles, and such particles remain in their original
positions in the suspension. Thus a separation of the gold particles from the non-
conductive particles is achieved.
The extent to which conductive particles are moved by the interaction of the
induced eddy currents and the moving magnet;c field is dependent, inter alia, onthe conductivity of those particles. It will be appreciated that particles with lower
electrical conductivities such as, say, aluminium particles will be moved to a lesser
extent than highly conductive particles such as gold particles. Thus it is not only
possible to achieve a separation between conductive and non-conductive particles,
but it is also possible to achieve a separation between particles of different
electrical conductivity. In cases where it is desired to achieve the latter kind of
separation with an apparatus such as that of Figure 1, the particles of different
conductivities will be grouped, after a period of time, in different zones of the dish
16.
In cases where it is desired to separate one particular kind of particle, such as gold
particles, from other particles, the microwave frequency will be chosen to induce
eddy currents of the desired magnitude in ~he desired particles so that the
movement of those particles can be predicted and the desired particles recoveredapart from other particles. In other words, the desired particles will be specifically
targeted. On the other hand, where it is desired to make a general discrimination
between various particle types having different thermal conductivities, a non-
specific microwave frequency may be used to cause differing degrees of movement
of the various particle types.
An apparatus such as that of Figare 1 can be of practical benefit in assay
procedures where it is desired, for instance, to determine the gold content of an ore
sample. In such a case, the gold fraction is recovered and a computation may be
made of the gold content of the sample as a whole.
In the high throughput apparatus 30 depicted diagramrnatically in plan view in
Figure 2, milled and crushed ore particles 32 are fed onto an endless conveyor belt
34. The ore particles 32 contain a low concentration of small particles of valuable
electrically conductive material, such as gold, which are to be separated from other
non-conductive material or less conductive materials in the mass of ore particles.
During their travel on the belt, the particles pass through a rnicrowave chamber 40
in which they are subjected to microwave radiation having a fre~en~ in the rangelO9Hz to 3xlO"Hz. While being irradiated the particles pass between magnets 36
(only one visible in Figure 2) located above and below the belt 34. The magnets
may be shielded from the microwaves by plates similar to the plate 18 of Figure
1. The field lines associated with the magnets are perpendicular to the belt, i.e. into
the plane of the paper in Figure 2.
As illustrated, the magnets 36 are arranged at 450 to the direction of belt
movement, indicated by the arrows 38. Thus the magnetic field itself is at 45~ to the
direction of movement of the belt and particles.
The incident microwave radiation induces eddy currents in conductive particles.
These eddy currents interact with the applied magnetic field to produce forces
which tend to move the conductive particles sideways off the belt. The exact
frequency of the microwaves is chosen to produce eddy currents of sufficient
magnitude in small conductive particles for the resultant electromotive force to be
great enough to cause the relevant particles to fall sideways off the belt.
The remaining particles, which are either non-conductive or less conductive thanthe particles which it is desired to separate are not moved off the belt and continue
moving on the belt. ~uch particles are discharged over the discharge end of the
belt for collection separately from those particles moved sideways off the belt.
The magnets 36 seen in Figure 2 may be arrangede to move in a direction at rightangles to the direction of movement of the belt up and down as viewed in Figure
2. Also, there can be a number of magnets 36 arranged side-by-side to produce a
"sweeping" magnetic field acting on the particles. A sweeping electromotive force,
resulting from the interaction of the eddy currents with the magnetic fields of the
various magne~s moves ~he relevant particles progressively in a sideways direction
off the belt.
In cases such as that exemplified in Figure 2, it will be appreciated that the physical
nature of the particles, in addition to their conductivities, will also determine the
extent to which they are moved. For instance, a lighter particle may be moved
more easily than a heavier particle, even though the latter particle may have a
higher conductivity than the former. Such factors will of course have to be taken
into account in the design of a particular particle separation installation.
As indicated previously, the invention is not limited to the use of microwave
frequency electrmagnetic radiation. Radio frequency electromagnetic radiation, in
the frequency range 104Hz to 10~Hz can also be used.
