Hydrocyclone Classifiers
In
beneficiation plant, Hydrocyclones is an important mining machine and use centrifugal forces to classify particles
in a fluid that experiences essentially free vortex motion inside the device.
They are widely used in mineral processing plants today because of their extremely
favorable capacity-to-size ratios and reasonably low maintenance.
Basic
Characteristics of hydrocyclone
A cutaway
view of a typical hydrocyclone is shown in Figure1. Feed slurry, either pumped
or flowing by gravity, enters the inlet through a feed pipe and flows at a
tangent to a cylindrical feed chamber under pressure. To increase retention
time, a cylindrical section is often added between the upper feed chamber and
the lower conical section. This section has an included angle (cyclone angle)
in the range of 12° (for cyclones of 10-in. diameter or less) to 20° (for
larger cyclones). Fine particles leave through the vortex finder and are
directed to further processing by the overflow pipe. Coarse particles travel
downward in a spiral path and discharge at atmospheric pressure through a
variable apex (spigot) that connects to an underflow pipe. Cyclones are often
mounted radially, with their feed pipes attached to a central vertical feed
line that is capped at the top. A typical mounting assembly is called a
“Cyclopac.” Underflow slurry enters a circular weir trough (concentric like a
doughnut) that is sloped to divert the combined underflow to a next processing
step (such as the feed spout of a ball mill). Overflow lines are U-shaped at
the top and discharge to an annular launder that is concentric around the
central feed pipe. Standpipes that are open to atmosphere are located at the
peak of each overflow line (they prevent possible siphoning if lines are below
the feed line). For ease of access for maintenance and liner replacement,
air-actuated valves may be installed to seal off feed pipes as desired.
Fig1. Conceptual view of hydrocyclone
section
Cyclone Fundamentals
Fluid
motion inside a cyclone is analogous to that within a free vortex (one that persists
without external energy input). Water draining from a bathtub will exhibit such
motion because an air core forms as the water rotates into the drain hole. In
contrast, forced vortexmotion is obtained when a body of fluid is forced to
rotate by applying external energy (e.g., causing a beaker of water to rotate
at angular velocity). Figure 2illustrates the essential idea.
A
vertical force acts downwardly on a particle to the right of a
zero-vertical-velocity envelope and upwardly to the left of it. If the cyclone
radius is r, the envelope trace can be initiated
at a distance r/2 from the center and at the same
level as the bottom of the vortex finder. The envelope trace extends as a cone
downward to the apex and intersects at about the trace of the (spigot
radius)/2. Particles to the left of the envelope tend to rise; those to the
right tend to travel downward. An envelope of maximum tangential velocity lies
virtually at the air core wall. The two envelopes offer insight into the
resulting motion of particles (Figure 3).
Fig3. Maximum tangential velocity and
zero-vertical-velocity contours in cyclone
There is
always some size of particle, d50, associated with the intersection of the envelopes of maximum tangential
velocity and of zero vertical velocity. Half of these particles rise; the other
half enter the underflow. Particles finer than this size enter the overflow;
particles coarser enter the underflow.
The cone
section of a cyclone at steady state contains particles with a size
distribution similar to that of the underflow stream. In the vicinity of the
bottom edge and outer wall of the vortex finder, very fine particles
predominate. Just below the vortex finder and extending a short distance into
the cone section, particles of intermediate sizes are found. Near the top and
inner walls of the feed chamber, the size distribution is very like that of
fresh feed.
Numerous
studies of cyclones have dealt with single-particle behavior. Yet slurries fed
to cyclone classifiers in mineralprocessing plants contain in excess of 55%–65% solids. Most of the feed slurry volume
departs through the vortex finder, so the overflow is representative of the
inside medium that “drags” particles inward and up. The underflow consists of
coarse particles whose voids are filled with water and fines that have
characteristics similar to those of the overflow medium. Thus, when the overflow
is concentrated (or dilute), underflow voids are filled with concentrated (or
dilute) overflow
medium.
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