5.6 Physical Methods for Metals Recovery
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both. Materials locked by screws, clinks and rivets are easily detached and in more
complex cases such as materials locked by alloying and filling, simultaneous disassembly by heating is employed. In the next step, metals are liberated from resins, fiberglass and plastics using size reduction by shredding, crushing or grinding. Different
types of hammer crushers, rotary crushers, disc crushers, shredders and cutters are
used for this purpose. The crushed materials are then sieved to classify different
sized particles. The particle size and shape of metals are different from plastics and
ceramics, so the sieving process helps to upgrade the metals content. After the sieving
process, the chemical composition of the materials is determined with different
analytical techniques such as atomic Absorption Spectroscopy (AAS), inductively
Coupled Plasma-Optical Emission Spectrometry (ICP–OES), Inductively Coupled
Plasma-Mass Spectrometry (ICP–MS), Fire Assay, X-ray Fluorescence (XRF), XRay Diffraction (XRD), etc. Waste streams are then separated using different types
of separation methods including gravity separation, magnetic separation, electrostatic separation and froth floatation. Gravity separation is based on different specific
densities of material particles, and metals are separated from plastics at this stage.
Magnetic separation is another method that separates the magnetic particles from
the non-magnetic ones. Electrostatic separation uses electrostatic charges to separate crushed particles with different electric charges. It consists of three techniques
known as (1) eddy current separation, (2) electrostatic corona separation and (3)
triboelectric separation [14]. Eddy current separators use a powerful magnetic field
to separate non-magnetic metals. A high-speed magnetic rotor generates an alternating induction field and temporarily magnetizes electrically conductive particles.
In corona separators, the waste stream particles are fed on a surface of a rotating
roll electrode connected to the ground. A high-intensity electric field is then formed
between this roll and other electrodes connected to a high-voltage supply. The insulating particles are charged by ion bombardment in the corona field zone pin and
stick to the surface of the rotating roll electrode through the electric image force.
The conducting particles are charged by electrostatic induction in contact with the
grounded roll and are attracted to the high-voltage electrode, so they are not affected
by the corona field [15]. The corona separators can separate the particles that have
a large difference in their conductivities. The triboelectric separators use an intensive friction charging method for charging particles positively and negatively. Thus,
they are capable of separating materials that have similar conductivities. The last
part of the physical metal recovery is the froth floatation. This process can selectively separate hydrophobic materials from hydrophilic ones. The particles are separated based on the differences in the ability of air bubbles to selectively adhere to
the specific mineral surfaces of the particles in a mixture of minerals and water.
Air bubbles attached with hydrophobic particles float on water surface, while the
remaining hydrophilic particles are totally wet and stay in the liquid phase [14].
133
both. Materials locked by screws, clinks and rivets are easily detached and in more
complex cases such as materials locked by alloying and filling, simultaneous disassembly by heating is employed. In the next step, metals are liberated from resins, fiberglass and plastics using size reduction by shredding, crushing or grinding. Different
types of hammer crushers, rotary crushers, disc crushers, shredders and cutters are
used for this purpose. The crushed materials are then sieved to classify different
sized particles. The particle size and shape of metals are different from plastics and
ceramics, so the sieving process helps to upgrade the metals content. After the sieving
process, the chemical composition of the materials is determined with different
analytical techniques such as atomic Absorption Spectroscopy (AAS), inductively
Coupled Plasma-Optical Emission Spectrometry (ICP–OES), Inductively Coupled
Plasma-Mass Spectrometry (ICP–MS), Fire Assay, X-ray Fluorescence (XRF), XRay Diffraction (XRD), etc. Waste streams are then separated using different types
of separation methods including gravity separation, magnetic separation, electrostatic separation and froth floatation. Gravity separation is based on different specific
densities of material particles, and metals are separated from plastics at this stage.
Magnetic separation is another method that separates the magnetic particles from
the non-magnetic ones. Electrostatic separation uses electrostatic charges to separate crushed particles with different electric charges. It consists of three techniques
known as (1) eddy current separation, (2) electrostatic corona separation and (3)
triboelectric separation [14]. Eddy current separators use a powerful magnetic field
to separate non-magnetic metals. A high-speed magnetic rotor generates an alternating induction field and temporarily magnetizes electrically conductive particles.
In corona separators, the waste stream particles are fed on a surface of a rotating
roll electrode connected to the ground. A high-intensity electric field is then formed
between this roll and other electrodes connected to a high-voltage supply. The insulating particles are charged by ion bombardment in the corona field zone pin and
stick to the surface of the rotating roll electrode through the electric image force.
The conducting particles are charged by electrostatic induction in contact with the
grounded roll and are attracted to the high-voltage electrode, so they are not affected
by the corona field [15]. The corona separators can separate the particles that have
a large difference in their conductivities. The triboelectric separators use an intensive friction charging method for charging particles positively and negatively. Thus,
they are capable of separating materials that have similar conductivities. The last
part of the physical metal recovery is the froth floatation. This process can selectively separate hydrophobic materials from hydrophilic ones. The particles are separated based on the differences in the ability of air bubbles to selectively adhere to
the specific mineral surfaces of the particles in a mixture of minerals and water.
Air bubbles attached with hydrophobic particles float on water surface, while the
remaining hydrophilic particles are totally wet and stay in the liquid phase [14].