Mineral processing-Electrical separation
If an ore contains conducting as well as
non-conducting minerals/particles, electrostatic separation or high tension
separation can be employed. Theoretically, it is not necessary that one of two
minerals should be a good conductor and the other be a poor conductor, but the
difference in their conductivity will affect the separation.
The basis of any electrostatic separation is the
interaction between an external electric field and the electric charges
acquired by the various particles. Particles can be charged by:
1 Contacting dissimilar particles.
2 Conductive induction.
3 Ion bombardment.
In every separation, two or more charging
mechanisms occur.
In charging by contacting dissimilar particles,
particles placed on surface are made to repeatedly contact one another as well
as the surface; the surface will acquire electrons from one type of particles
and give electrons to another type so that two types of particles are charged
with opposite charges. When they are passed through an electrostatic separator
consisting of two conducting plates across which a high voltage is applied,
positively charged particles are attracted towards a negative plate and negatively
charged particles are attracted towards a positive plate. This mechanism is utilized
in free fall electrostatic separators. However, this is not the major mechanism
in any of the electrostatic separators.
In charging by conductive induction, the particles
are placed on a ground conductor in the presence of an electric field. Then the
particles will rapidly develop a surface charge by induction. Both conducting
and non-conducting particles will become polarized, but conducting particles
will have an equi-potential surface through its contact with the grounded
conductor. The non-conducting particle will remain polarized.
In charging by ion bombardment, corona discharge is
obtained by appropriate shaping of the electrodes. Corona discharge is an
electrical discharge which occurs when one of the two conducting surfaces (such
as electrodes) of differing voltages have a pointed shape. A highly
concentrated electric field, set up at the tip of the pointed
shape electrode, ionizes the air (or other gas)
around it. These ions charge the particles by bombardment when the mineral
particles are caused to pass within the corona. For the discharge to a large
diameter cylindrical surface, a fine wire parallel to the cylinder gives the
optimum corona discharge.
The attraction of particles carrying one kind of
charge (positive or negative) towards an electrode of the opposite charge is
known as lifting effect as
the particles are lifted towards the charged electrode. Mineral particles
having a tendency to become charged with a definite polarity may be separated
from each other by the use of lifting effect even though their conductivities
may be very similar. For example, quartz is readily negatively charged and can
be separated from other non-conductors by using an electrode carrying a
positive charge. However, pure electrostatic separation (pure lifting effect)
is relatively inefficient.
The other effect which makes the electrostatic
separation more effective is the pinning
effect. Here, non- conducting mineral particles, having received a
surface charge from the electrode, retain this charge and are pinned to
the earthed surface due to the attraction between the charged
non-conducting particle and the grounded surface.
ELECTROSTATIC SEPARATOR
In
an electrostatic separator (Figure 1), a large single electrode produces a
strong electric field. When a particle is placed on the grounded rotor in the
region of the electrostatic field influence, the particle rapidly develops a
surface charge by induction. Whether the particle is a conductor or
non-conductor, it will be polarized. However, a conducting particle rapidly
becomes an equi-potential surface and has the same potential as the ground
rotor. Therefore it is attracted towards the electrode, drawn away from the
surface, and falls by gravity. The non-conducting particle continues to adhere
to the rotor until it falls by gravity.
Figure 1. Electrostatic separator
HIGH TENSION SEPARATOR
In a high tension separator (Figure 2), the mixture
of ore minerals is fed on to a grounded rotor into the field of a charged
ionizing electrode. The electrode assembly, consists of a lengthy fine wire
supported by a brass tube, is supplied with DC supply of negative polarity.
Then the ionization of air takes place which can be seen as corona discharge.
This gives a high surface charge to the poor conducting mineral particles.
These particles are attracted and pinned to the rotor. High conducting mineral particles
are not charged as the charge is dissipated through the particles to the earthed
rotor. Then they come under the influence of the electrostatic field of the
nonionizing electrode and are attracted (by the lifting effect) from the rotor
surface. They are discharged away from the rotor. The charge of poor conducting
mineral particles is slowly lost as the rotor rotates and the particles drop
from the rotor as middlings or non-conductor product according to the intensity
of the surface charge. Thus a combined effect of pinning and lifting is
experienced in high tension separators. These separators operate on feeds of 60
to 500 microns size.
Figure 2. High
tension separator
Since almost all minerals show some difference in
conductivity, electrostatic separation is the universal concentrating method.
It is widely used in beach sand beneficiation. The feed to electrostatic
separation must be perfectly dry. For efficient operation, the feed should be
in a layer, one particle deep, which severely restricts the throughput.
Electrostatic separation is applicable to a limited size range, normally 20
mesh to 250 mesh. As complete separation is not likely to be obtained in a
single pass electrostatic separator, it is usual practice to re-pass separated
fractions through other separators as shown in Figure 3.
Figure 3 Multi-pass electrostatic
separator
Note: E - Discharge Electrodes
In Dielectric separation, ore consists of minerals
of various dielectric constants is suspended in a non-conducting fluid whose
dielectric constant is intermediate between that of two groups of minerals and
a converging electric field is set up within the suspension. Then the particles
having dielectric constant higher than the fluid travel in the direction of
most rapid increase in electric field and the particles having dielectric constant
lower than the fluid travel in the opposite direction. The dielectric
separation process is applicable to particles finer than 60-mesh and is used as
a research tool for study of minerals. It has no industrial application.
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