XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 511
either plug or blind. This is due to the fact that the fin-
est particles, due to their high surface area, will contain
more moisture than that of the bulk of the material. The
finest particles with higher moisture will agglomerate with
other fine particles or stick to larger particles and cause
screen openings to plug. This eventually reduces the effec-
tive screen area and prevents fines from being able to go
through the screen openings. Therefore, feed to a fine, dry
screen should be as dry as possible “bone dry” is not an
unreasonable specification.
Angle of Repose
Dry, fine screening is generally accomplished with the
screening machine positioned at or near the angle of repose
of the material being screened. The angle of repose is mea-
sured from the horizontal and is one of the important
parameters to be determined with starting the selection
process for an inclined, vibrating screen. Some materials
must be screened at an angle just slightly greater than the
angle of repose to obtain the necessary shear and permit
fine particles to make their way to the screen surface.
Bulk Density and Specific Gravity
Another factor to consider is the loose bulk density of the
feed material. Heavier material will fall through a screen
more rapidly than lighter material. Particle density, or more
specifically, the difference in density between the particle
and the surrounding fluid (air) affects the velocity of a par-
ticle through the fluid. The surrounding fluid (air) has a
lower relative density and restraining force due to drag.
Feed Particle Size Distribution
Similar to wet screening, the size distribution of the mate-
rial fed to a screen, especially the amount of oversize and
near size particles, affects both capacity and performance of
a dry screening machine. Near-size, oversize particles have a
tendency to stay close to the screen surface and may inhibit
the ability of undersize particles to pass through an open-
ing. Near-size, undersize particles move through screen
openings at a slower rate than finer particles therefore,
more unit area will be required.
Particle Shape
In some cases, particle shape is another factor that must be
considered. For example, some natural silica sand deposits
consist of well-rounded grains. If the significant portion
of these grains is close to the opening size of the screen
cloth, screen capacity and performance can be affected,
and special consideration must be given to the screen panel
design. Flat particles, such as mica and natural graphite,
can also present some special problems. This can usually
be overcome by a combination of screen panel design and
increasing the machine angle. A unique challenge presented
in fine screening is that the impact of shape may not be
readily apparent to the naked eye, requiring analysis under
a microscope or intuited based on deviation from expected
outcomes in operation.
Vibration Frequency and Amplitude
Vibration frequency and amplitude are the factors that
affect the particles stratification, which allow undersize
Modular
Stainless Steel
Wedge-Wire
Round Wire
Polyurethane
Rubber
Composite
Side Tensioned
Polyurethane
Stainless Steel
Wire Cloth
De-blinding media
Mechanical
brushing
Self Cleaning
Figure 2. Types of screen surfaces for dry fine screens
Screen
Surfaces
for
Fine
Dry
Screening
either plug or blind. This is due to the fact that the fin-
est particles, due to their high surface area, will contain
more moisture than that of the bulk of the material. The
finest particles with higher moisture will agglomerate with
other fine particles or stick to larger particles and cause
screen openings to plug. This eventually reduces the effec-
tive screen area and prevents fines from being able to go
through the screen openings. Therefore, feed to a fine, dry
screen should be as dry as possible “bone dry” is not an
unreasonable specification.
Angle of Repose
Dry, fine screening is generally accomplished with the
screening machine positioned at or near the angle of repose
of the material being screened. The angle of repose is mea-
sured from the horizontal and is one of the important
parameters to be determined with starting the selection
process for an inclined, vibrating screen. Some materials
must be screened at an angle just slightly greater than the
angle of repose to obtain the necessary shear and permit
fine particles to make their way to the screen surface.
Bulk Density and Specific Gravity
Another factor to consider is the loose bulk density of the
feed material. Heavier material will fall through a screen
more rapidly than lighter material. Particle density, or more
specifically, the difference in density between the particle
and the surrounding fluid (air) affects the velocity of a par-
ticle through the fluid. The surrounding fluid (air) has a
lower relative density and restraining force due to drag.
Feed Particle Size Distribution
Similar to wet screening, the size distribution of the mate-
rial fed to a screen, especially the amount of oversize and
near size particles, affects both capacity and performance of
a dry screening machine. Near-size, oversize particles have a
tendency to stay close to the screen surface and may inhibit
the ability of undersize particles to pass through an open-
ing. Near-size, undersize particles move through screen
openings at a slower rate than finer particles therefore,
more unit area will be required.
Particle Shape
In some cases, particle shape is another factor that must be
considered. For example, some natural silica sand deposits
consist of well-rounded grains. If the significant portion
of these grains is close to the opening size of the screen
cloth, screen capacity and performance can be affected,
and special consideration must be given to the screen panel
design. Flat particles, such as mica and natural graphite,
can also present some special problems. This can usually
be overcome by a combination of screen panel design and
increasing the machine angle. A unique challenge presented
in fine screening is that the impact of shape may not be
readily apparent to the naked eye, requiring analysis under
a microscope or intuited based on deviation from expected
outcomes in operation.
Vibration Frequency and Amplitude
Vibration frequency and amplitude are the factors that
affect the particles stratification, which allow undersize
Modular
Stainless Steel
Wedge-Wire
Round Wire
Polyurethane
Rubber
Composite
Side Tensioned
Polyurethane
Stainless Steel
Wire Cloth
De-blinding media
Mechanical
brushing
Self Cleaning
Figure 2. Types of screen surfaces for dry fine screens
Screen
Surfaces
for
Fine
Dry
Screening