XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 509
material (dry or wet) at the inlet volute into a spiralling
passage through the cyclone body, to separate coarser or
higher specific gravity particles from finer or less dense solid
particles. Under the centrifugal force, coarse/dense particles
move outwards to the cylinder wall and then downward to
the apex discharge at the underflow. The majority of very
fine and light particles are drawn to the core due to less cen-
trifugal force and are then forced upwards to the overflow
via the vortex finder. The advancements in cyclone structural
design and also in the circuit design have greatly improved
the size classification performance for this duty (Rong and
Naper-Munn, 2003 Mohanty et al., 2002). However, the
classifying cyclone has two fundamental limitations: 1. fine
particles in the underflow due to entrainment (fine light
particles) and density effect (fine high density particles)
2. coarse light fine particle in the overflow due to density
effect. The misplaced fine particles are often reported to the
coarse underflow stream, end up as contaminants. On the
other hand, the misplaced coarse fine particles cannot be
effectively recovered by subsequent units of operation and
therefore ending up in the lost stream lowering the recovery.
Hindered settling classifiers are also used to achieve
either size classification for fine particles of the same density
or density classification .The principle of operation of this
type of classifier is based on the concept of differential ter-
minal settling velocity of particles of different size or mass.
Due to the low classification efficiency and also require-
ment of large floor space, it becomes less and less popular
within the mineral processing industry.
The spiral classifiers (screw classifiers) is the one of the
oldest mechanical classifier using the hindered settling to
achieve the classification. It uses a continuously revolving
partially submerged screw to move the settled coarse parti-
cles up the slope from the hindered-settling zone. The light/
fine particles go floating in the overflow Napier-Munn and
Wills (2005). This classification is challenging due to lim-
ited classification efficiency and difficulties in maintaining
the right dilution ratio for downstream processing.
The another hindered settling classifiers are the up-
current (a.k.a. counter current or upstream) classifiers usu-
ally consists of a sorting column with an upward stream of
water coming direct in contact with the feed pulp stream
flowing opposite to the water stream. The slow settling par-
ticles (light/small) exit the column/vessel from the top in
overflow and fast settling particles (large/heavy) exit from
the bottom Kelly and Spottiswood (1982).
Another method to achieve fine particle size classifica-
tion is to use screens. Screening provides a sharp cut that
is only dependent on particle size. Industrial screening is
extensively used for size separations from 300 mm down
to roughly 40 µm, although the efficiency decreases rap-
idly with fineness. Wills (2016) talked about traditional dry
screening is generally limited to material above 5 mm in
size, while wet screening down to about 250 µm is com-
mon. Recent development in fine screening technology has
extended the boundaries significantly. Valine and Wennen
(2002) published a review paper on fine screening in min-
eral processing operations and discussed technological
advancements in wet fine screening and their various appli-
cations in mineral processing. Zhang (2013) published a
paper reviewed several wet classification technologies and
evaluated the importance of classification efficiency on the
overall circuit performance.
Unfortunately, less work has been done on reviewing
the development and advancement of dry fine screening.
This paper aims to bridge the gap.
DRY FINE SCREENING FUNDAMENTALS
Dry fine screens are typically used to size mineral particles
to obtain products that meet required size specifications or
to achieve desire transfer sizes between different units of
operations (size reduction and/or physical/chemical com-
ponent separation). Therefore, dry fine screens are normally
installed to operate before or after size reduction equip-
ment, after drying operations, and before or after physi-
cal or chemical upgrading operations. Mineral products
produced using dry fine screening equipment include glass
sand, nepheline syenite, olivine sands, limestone, graphite,
polypropylene and polyethylene pellets, although this list is
certainly expanding as the industry considers this alterna-
tive for environmental considerations.
TYPES OF DRY FINE SCREENING
MACHINES AND SURFACES
Dry fine screens can be categorized based on several crite-
ria, and one method is by classifying them according to the
shape of the screening media. In this regard, two primary
groups emerge: planetary screens, which encompass both
flat and curved configurations, and basket screens with a
circular design. Another classification criterion hinges on
the type of motion exhibited by the screens during opera-
tion. This yields distinct categories such as static screens
characterized by an absence of moving parts, tumbling
screens exemplified by trommel screens, centrifugal sift-
ers, and dynamic screens propelled by various motors (or
diver, or vibrator). Dynamic screens, a subset within the
latter category, demonstrate diverse motion patterns driven
by different vibrating mechanisms. Notable motions in this
classification include linear motion, circular motion, ellip-
tical motion, gyratory motion, and ultrasonic motion. Each
material (dry or wet) at the inlet volute into a spiralling
passage through the cyclone body, to separate coarser or
higher specific gravity particles from finer or less dense solid
particles. Under the centrifugal force, coarse/dense particles
move outwards to the cylinder wall and then downward to
the apex discharge at the underflow. The majority of very
fine and light particles are drawn to the core due to less cen-
trifugal force and are then forced upwards to the overflow
via the vortex finder. The advancements in cyclone structural
design and also in the circuit design have greatly improved
the size classification performance for this duty (Rong and
Naper-Munn, 2003 Mohanty et al., 2002). However, the
classifying cyclone has two fundamental limitations: 1. fine
particles in the underflow due to entrainment (fine light
particles) and density effect (fine high density particles)
2. coarse light fine particle in the overflow due to density
effect. The misplaced fine particles are often reported to the
coarse underflow stream, end up as contaminants. On the
other hand, the misplaced coarse fine particles cannot be
effectively recovered by subsequent units of operation and
therefore ending up in the lost stream lowering the recovery.
Hindered settling classifiers are also used to achieve
either size classification for fine particles of the same density
or density classification .The principle of operation of this
type of classifier is based on the concept of differential ter-
minal settling velocity of particles of different size or mass.
Due to the low classification efficiency and also require-
ment of large floor space, it becomes less and less popular
within the mineral processing industry.
The spiral classifiers (screw classifiers) is the one of the
oldest mechanical classifier using the hindered settling to
achieve the classification. It uses a continuously revolving
partially submerged screw to move the settled coarse parti-
cles up the slope from the hindered-settling zone. The light/
fine particles go floating in the overflow Napier-Munn and
Wills (2005). This classification is challenging due to lim-
ited classification efficiency and difficulties in maintaining
the right dilution ratio for downstream processing.
The another hindered settling classifiers are the up-
current (a.k.a. counter current or upstream) classifiers usu-
ally consists of a sorting column with an upward stream of
water coming direct in contact with the feed pulp stream
flowing opposite to the water stream. The slow settling par-
ticles (light/small) exit the column/vessel from the top in
overflow and fast settling particles (large/heavy) exit from
the bottom Kelly and Spottiswood (1982).
Another method to achieve fine particle size classifica-
tion is to use screens. Screening provides a sharp cut that
is only dependent on particle size. Industrial screening is
extensively used for size separations from 300 mm down
to roughly 40 µm, although the efficiency decreases rap-
idly with fineness. Wills (2016) talked about traditional dry
screening is generally limited to material above 5 mm in
size, while wet screening down to about 250 µm is com-
mon. Recent development in fine screening technology has
extended the boundaries significantly. Valine and Wennen
(2002) published a review paper on fine screening in min-
eral processing operations and discussed technological
advancements in wet fine screening and their various appli-
cations in mineral processing. Zhang (2013) published a
paper reviewed several wet classification technologies and
evaluated the importance of classification efficiency on the
overall circuit performance.
Unfortunately, less work has been done on reviewing
the development and advancement of dry fine screening.
This paper aims to bridge the gap.
DRY FINE SCREENING FUNDAMENTALS
Dry fine screens are typically used to size mineral particles
to obtain products that meet required size specifications or
to achieve desire transfer sizes between different units of
operations (size reduction and/or physical/chemical com-
ponent separation). Therefore, dry fine screens are normally
installed to operate before or after size reduction equip-
ment, after drying operations, and before or after physi-
cal or chemical upgrading operations. Mineral products
produced using dry fine screening equipment include glass
sand, nepheline syenite, olivine sands, limestone, graphite,
polypropylene and polyethylene pellets, although this list is
certainly expanding as the industry considers this alterna-
tive for environmental considerations.
TYPES OF DRY FINE SCREENING
MACHINES AND SURFACES
Dry fine screens can be categorized based on several crite-
ria, and one method is by classifying them according to the
shape of the screening media. In this regard, two primary
groups emerge: planetary screens, which encompass both
flat and curved configurations, and basket screens with a
circular design. Another classification criterion hinges on
the type of motion exhibited by the screens during opera-
tion. This yields distinct categories such as static screens
characterized by an absence of moving parts, tumbling
screens exemplified by trommel screens, centrifugal sift-
ers, and dynamic screens propelled by various motors (or
diver, or vibrator). Dynamic screens, a subset within the
latter category, demonstrate diverse motion patterns driven
by different vibrating mechanisms. Notable motions in this
classification include linear motion, circular motion, ellip-
tical motion, gyratory motion, and ultrasonic motion. Each