Hydrocyclone

Introduction
Classification is a method of separating mixtures of minerals into two or more products on the basis
of the velocity with which the grains fall through a fluid medium .

The hydrocyclone

This is a continuously operating classifying device
that utilises centrifugal force to accelerate the
settling rate of particles. It is one of the most important
devices in the minerals industry, its main use in
mineral processing being as a classifier, which has
proved extremely efficient at fine separation sizes.
It is widely used in closed-circuit grinding operations
but has found
many other uses, such as de-sliming, de-gritting,
and thickening.
It has replaced mechanical classifiers in many
applications, its advantages being simplicity and
high capacity relative to its size. A variant, the
"water-only-cyclone", has been used for the cleaning
of fine coal  and other minerals.
A typical hydrocyclone (Figure 9.13) consists
of a conically shaped vessel, open at its apex, or
underflow, joined to a cylindrical section, which
has a tangential feed inlet. The top of the cylindrical
section is closed with a plate through which
passes an axially mounted overflow pipe. The pipe
is extended into the body of the cyclone by a short,
removable section known as the vortex finder

which prevents short-circuiting of feed directly into
the overflow.
The feed is introduced under pressure through the
tangential entry which imparts a swirling motion
to the pulp. This generates a vortex in the cyclone,
with a low-pressure zone along the vertical axis.
An air core develops along the axis, normally
connected to the atmosphere through the apex
opening, but in part created by dissolved air coming
out of solution in the zone of low pressure.
The classical theory of hydrocyclone action is that
particles within the flow pattern are subjected to two
opposing forces- an outward centrifugal force and an
inwardly acting drag (Figure 9.14). The centrifugal
force developed accelerates the settling rate of the
particles thereby separating particles according to
size, shape, and specific gravity. Faster settling
particles move to the wall of the cyclone, where
the velocity is lowest, and migrate to the apex
opening. Due to the action of the drag force, the
slower-settling particles move towards the zone of
low pressure along the axis and are carried upward
through the vortex-finder to the overflow.

The existence of an outer region of downward
flow and an inner region of upward flow implies a
position at which there is no vertical velocity. This
applies throughout the greater part of the cyclone
body, and an envelope of zero vertical velocity
should exist throughout the body of the cyclone
(Figure 9.15). Particles thrown outside the envelope
of zero vertical velocity by the greater centrifugal
force exit via the underflow, while particles swept
to the centre by the greater drag force leave in the
overflow. Particles lying on the envelope of zero
velocity are acted upon by equal centrifugal and
drag forces and have an equal chance of reporting
either to the underflow or overflow.

Experimental work reported by Renner and
Cohen (1978) has shown that classification does
not take-place throughout the whole body of the
cyclone as the classical model postulates. Using
a high-speed probe, samples were taken from
several selected positions within a 150-mm diameter
cyclone, and were subjected to size analysis.
The results showed that the interior of the
cyclone may be divided into four regions that
contain distinctively different size distributions
(Figure 9.16).
Essentially unclassified feed exists in a narrow
region A adjacent to the cylinder wall and roof of
the cyclone. Region B occupies a very large part
of the cone of the cyclone and contains fully classified
coarse material, i.e. the size distribution is
practically uniform and resembles that of the coarse
underflow product. Similarly, fully classified fine
material is contained in region C, a narrow region
surrounding the vortex finder and extending below
the latter along the cyclone axis. Only in the toroidshaped
region D does classification appear to be
taking place. Across this region, size fractions are
radially distributed, so that decreasing sizes show
maxima at decreasing radial distances from the
axis. The cyclone was run at low pressure, so the
region D may be larger in production units.

















Hydrocyclones are almost universally used in
grinding circuits (Figure 9.17) because of their high
capacity and relative efficiency. They can also classify
over a very wide range of sizes (typically
5-5001xm), smaller diameter units being used for
finer classification