ELECTROSTATIC SEPARATION
In mineral processing plant,
Almost all minerals show some degree of conductivity. An electrostatic
separation process uses the difference in the electrical conductivity or
surface charge of the mineral species of interest. The electrostatic separation
process has generally been confined to recovering valuable heavy minerals from
beach-sand deposits. However, the growing interest in plastic and metal
recycling has opened up new applications in secondary material recovery.
When particles come under the influence of an electrical field, depending
on their conductivity, they accumulate a charge that depends directly on the
maximum achievable charge density and on the surface area of the particle.
These charged particles can be separated by differential attraction or
repulsion. Therefore, the important first step in electrostatic separation is
to impart an electrostatic charge to the particles. The three main types of
charging mechanisms are contact electrification or triboelectrification,
conductive induction, and ion bombardment. Once the particles are charged, the
separation can be achieved by equipment with various electrode configurations.
Triboelectrification
Triboelectrification is a type of electrostatic separation in which two nonconductive
mineral species acquire opposite charges by contact with each other. The
oppositely charged particles can then be separated under the influence of an
electric field. This process uses the difference in the electronic surface
structure of the particles involved. A good example is the strong negative
surface charge that silica acquires when it touches carbonates and phosphates.
The surface phenomenon that comes into play is the work function, which
may be defined as the energy required to remove electrons from any surface
(Figure1). The particle that is charged positively after particle–particle
charging has a lower work function than the particle that is charged
negatively.
Tube-type
Separator.
In a tube-type separator, the precharging zone and the
separation zone are integral parts of the machine (Figure 2). The precharging
zone, or triboelectrification process, exploits the difference in the
electronic appearance of the particles involved. The particles become charged
by particle–particle contact,
particle–wall contact, or
both. Particle–particle contact
between two dissimilar particles results in the transfer electrons (charges)
from the surface of one particle to the surface of the other. After this
transfer, one of the particles is positively charged and the other is
negatively charged.
The separation zone consists of two vertical walls of rotating tubes that
oppose each other. Each tube “wall” is electrified with opposite potential. As
the charged particles enter the separation zone, they are attracted toward
oppositely charged electrodes. The separated products are collected at the base
of the separator. This separator very effectively removes silica from other
nonconductive minerals, such as calcium carbonate, phosphate, and talc. A typical
grade–recovery curve obtained on treating limestone on the V-Stat Separator is
shown in Figure 3. An industrial-scale triboelectrostatic separator capable of
treating up to 20 tph is shown in Figure 4.
Belt-type
Separator.
In a horizontal belt-type separator, fast-moving belts travel in opposite
directions adjacent to suitably placed plate electrodes of the opposite
polarity. Material is fed into a narrow gap between two parallel electrodes.
The particles are swept upward by a moving open-mesh belt and conveyed in
opposite directions, thus facilitating the particles’ charging by contact with
other particles. The electric field attracts particles up or down depending on
their charge. The moving belts transport the particles adjacent to each electrode
toward opposite ends of the separator.
Conductive Induction
When uncharged particles, conductors or nonconductors, contact a charged
surface, the particles assume polarity and the potential of the surface. The
electrically conductive minerals will rapidly assume the polarity and the
potential of the surface. However, in the case of nonconductors, the side away
from the charged surface will more slowly acquire the same polarity as the
surface. Hence, if both the conductor and nonconductor particles are just
separated from contact with a charged plate (Figure 5), the conductor particles
will be repelled by the charged plate, and the nonconductor particles will be
unaffected by the charged plate—they will be neither attracted nor repelled.
The most common industrial separators working on this principle are plate-
and screen-type separators. The feed particles fall under gravity onto an
inclined, grounded plate and into an electrostatic field induced by a
high-voltage electrode. These electrodes are generally oval. Here, the
conductor particles acquire an induced charge from the grounded plate and move
toward the oppositely charged electrode; that is, the particles experience a
“lifting effect.” The nonconductor particles are generally not affected by this
field. Because the lifting effect depends on the surface charge as well as the
mass of the particle, fine conductors are effectively separated from coarse
nonconductors.
Ion Bombardment
When conductor and nonconductor particles placed on a grounded conducting surface
are bombarded with ions of atmospheric gases generated by an electrical corona
discharge from a high-voltage electrode, both the conductor and nonconductor
particles acquire a charge. When ion bombardment
ceases, conductor particles rapidly lose their acquired charge to the
grounded surface. However, nonconductor particles react differently. The
nonconductor particle surface that faces away from the grounded conducting
plate is coated with ions of charge opposite in electrical polarity to that of the
grounded conducting plate. Therefore, nonconductor particles remain “pinned” to
the grounded plate because of electrostatic force (Figure 6).
An industrial high-tension electrostatic separator using the pinning
effect is shown in Figure 7. This separator consists of a rotating roll made
from mild steel that is grounded through its supporting bearing. The electrode
assembly consists mainly of two types of electrodes, a beam or corona electrode
or a static-type electrode. The beam electrode, usually connected to a d-c
supply of up to 50 kv of negative polarity, is used to charge all particles and
pin the nonconductors to the roll. The feed is presented uniformly to the
rotating roll surface by a velocity feed system. Both conductor and
nonconductor particles are sprayed with ions. Conductor particles rapidly lose
their charge to the grounded roll surface and are thrown off by centrifugal
force. Nonconductor particles are pinned to the rolled surface and are brushed
off that surface. Both conductor and nonconductor particles are collected in a
partitioned product hopper at the bottom of the unit. The operating
variables—roll speed, applied voltage, feed rate, splitter position, and the
electrode combination and position—are adjusted to achieve effective separation.
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