milling

under construction

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

Crushers

Crushers

Introduction
Crushing is the first mechanical stage in the process
of comminution in which the main objective is the
liberation of the valuable minerals from the gangue.
It is generally a dry operation and is usually
performed in two or three stages. Lumps of runof-
mine ore can be as large as 1.5 m across and
these are reduced in the primary crushing stage to
10-20 cm in heavy-duty machines.
In most operations, the primary crushing
schedule is the same as the mining schedule.
When primary crushing is performed underground,
this operation is normally a responsibility of the
mining department; when primary crushing is on
the surface, it is customary for the mining department
to deliver the ore to the crusher and for the
mineral processing department to crush and handle
the ore from this point through the successive oreprocessing
unit operations. Primary crushers are
commonly designed to operate 75 % of the available
time, mainly because of interruptions caused by
insufficient crusher feed and by mechanical delays
in the crusher .
Secondary crushing includes all operations for
reclaiming the primary crusher product from ore
storage to the disposal of the final crusher product,
which is usually between 0.5 and 2 cm in diameter.
The primary crusher product from most metalliferous
ores can be crushed and screened satisfactorily,
and the secondary plant generally consists of
one or two size-reduction stages with appropriate
crushers and screens. If, however, the ore tends to
be slippery and tough, the tertiary crushing stage
may be substituted by coarse grinding in rod mills.
On the other hand, more than two size-reduction
stages may be used in secondary crushing if the
ore is extra-hard, or in special cases where it is
important to minimise the production of fines.
A basic flowsheet for a crushing plant is shown
in Figure 6.1, incorporating two stages of secondary
crushing. A washing stage is included, which is
often necessary for sticky ores containing clay,
which may lead to problems in crushing and
screening .
Vibrating screens are sometimes placed ahead of
the secondary crushers to remove undersize material,
or scalp the feed, and thereby increase the
capacity of the secondary crushing plant. Undersize
material tends to pack the voids between the large
particles in the crushing chamber, and can choke
the crusher, causing damage, because the packed
mass of rock is unable to swell in volume as it is
broken.
Crushing may be in open or closed circuit
depending on product size (Figure 6.2).
In open-circuit crushing, undersize material from
the screen is combined with the crusher product
and is then routed to the next operation. Opencircuit
crushing is often used in intermediate
crushing stages, or when the secondary crushing
plant is producing a rod mill feed. If the crusher
is producing ball-mill feed it is good practice to
use closed-circuit crushing in which the undersize
from the screen is the finished product. The
crusher product is retumed to the screen so that any
over-size material will be recirculated. One of the
main reasons for closing the circuit is the greater
flexibility given to the crushing plant as a whole.
The crusher can be operated at a wider setting if
necessary, thus altering the size distribution of the
product and by making a selective cut on the screen,
the finished product can be adjusted to give the
required specification. There is the added factor
that if the material is wet or sticky (and climatic
conditions can vary), then it is possible to open
the setting of the crusher to prevent the possibility
of packing, and by this means the throughput of
the machine is increased, which will compensate
for the additional circulating load. Closed-circuit
operation also allows compensation for wear which
takes place on liners, and generally gives greater
freedom to meet changes in requirements from
the plant.
Surge bins precede the primary crusher to receive
dumped loads from skips or lorries and should have
enough storage capacity to maintain a steady feed
to the crusher. In most mills the crushing plant
does not run for 24 h a day, as hoisting and transport
of ore is usually carded out on two shifts
only, the other shift being used for drilling and
blasting. The crushing section must therefore have
a greater hourly capacity than the rest of the plant,
which is run continuously. Ore is always stored
after the crushers to ensure a continuous supply
to the grinding section. The obvious question is,
why not have similar storage capacity before the
crushers and run this section continuously also?
Apart from the fact that it is cheaper in terms
of power consumption to crush at off-peak hours,
large storage bins are expensive, so it is uneconomic
to have bins at the crushing and grinding
stage. It is not practicable to store large quantities
of run-of-mine ore, as it is "long-ranged", i.e. it
consists of a large range of particle sizes and the
small ones move down in the pile and fill the voids.
This packed mass is difficult to move after it has
settled. Run-of-mine ore should therefore be kept
moving as much as possible, and surge bins should
have sufficient capacity only to even out the flow
to the crusher.

 

Primary crushers

Primary crushers are heavy-duty machines, used to
reduce the run-of-mine ore down to a size suitable
for transport and for feeding the secondary crushers
or AG/SAG mills. They are always operated in
open circuit, with or without heavy-duty scalping
screens (grizzlies). There are two main types of
primary crusher in metalliferous operations -jaw
and gyratory crushers- although the impact crusher
has limited use as a primary crusher and will be
considered separately.

Jaw crushers

The distinctive feature of this class of crusher is
the two plates which open and shut like animal
jaws. The jaws are
set at an acute angle to each other, and one jaw
is pivoted so that it swings relative to the other
fixed jaw. Material fed into the jaws is alternately
nipped and released to fall further into the crushing
chamber. Eventually it falls from the discharge
aperture.
Jaw crushers are classified by the method of
pivoting the swing jaw (Figure 6.3). In the Blake
crusher the jaw is pivoted at the top and thus
has a fixed receiving area and a variable discharge
opening. In the Dodge crusher the jaw is pivoted at
the bottom, giving it a variable feed area but fixed
delivery area. The Dodge crusher is restricted to
laboratory use, where close sizing is required, and
is never used for heavy-duty crushing as it chokes
very easily. The Universal crusher is pivoted in
an intermediate position, and thus has a variable
delivery and receiving area.

Double-toggle Blake crushers In this model

(Figure 6.4), the oscillating movement of the
swinging jaw is effected by vertical movement of
the pitman. This moves up and down under the
influence of the eccentric. The back toggle plate
causes the pitman to move sideways as it is pushed
upward. This motion is transferred to the front
toggle plate and this in turn causes the swing jaw to
close on the fixed jaw. Similarly, downward movement
of the pitman allows the swing jaw to open

The important features of the machine are:

Since the jaw is pivoted from above, it moves
a minimum distance at the entry point and
a maximum distance at the delivery. This
maximum distance is called the throw of the
crusher.
The horizontal displacement of the swing jaw
is greatest at the bottom of the pitman cycle
and diminishes steadily through the rising
half of the cycle as the angle between the
pitman and the back toggle plate becomes less

acute.

The crushing force is least at the start of the
cycle, when the angle between the toggles is
most acute, and is strongest at the top, when
full power is delivered over a reduced travel
of the jaw.

Figure 6.5 shows a cross-section through a doubletoggle
jaw crusher. All jaw crushers are rated
according to their receiving areas, i.e. the width
of the plates and the gape, which is the distance
between the jaws at the feed opening. For example,
an 1830 • 1220mm crusher has a width of
1830 mm and a gape of 1220 mm.
Consider a large piece of rock falling into the
mouth of the crusher. It is nipped by the jaws,
which are moving relative to each other at a rate
depending on the size of the machine and which
usually varies inversely with the size. Basically,
time must be given for the rock broken at each
"bite" to fall to a new position before being nipped
again. The ore falls until it is arrested. The swing
jaw closes on it, quickly at first and then more
slowly with increasing power towards the end of the
stroke. The fragments now fall to a new arrest point as
the jaws move apart and are then gripped and
crushed again. During each "bite" of the jaws the
rock swells in volume due to the creation of voids
between the particles. Since the ore is also falling
into a gradually reducing cross-sectional area of the
crushing chamber, choking of the crusher would
soon occur if it were not for the increasing amplitude
of swing towards the discharge end of the
crusher. This accelerates the material through the
crusher, allowing it to discharge at a rate sufficient
to leave space for material entering above.
This is arrested or free crushing as opposed to
choked crushing, which occurs when the volume
of material arriving at a particular cross-section
is greater than that leaving. In arrested crushing,
crushing is by the jaws only, whereas in choked
crushing, particles break each other. This interparticle
comminution can lead to excessive production
of fines, and if choking is severe can damage the
crusher.
The discharge size of material from the crusher
is controlled by the set, which is the maximum
opening of the jaws at the discharge end. This can
be adjusted by using toggle plates of the required
length. Wear on the jaws can be taken up by
adjusting the back pillow into which the back toggle
plate bears. A number of manufacturers offer jaw
setting by hydraulic jacking, and some fit electromechanical
systems which allow remote control
(Anon., 1981).

A feature of all jaw crushers is the heavy flywheel
attached to the drive, which is necessary
to store energy on the idling half of the stroke
and deliver it on the crushing half. Since the jaw
crusher works on half-cycle only, it is limited in
capacity for its weight and size. Due to its alternate
loading and release of stress, it must be very rugged
and needs strong foundations to accommodate the
vibrations. 

Single-toggle jaw crushers In this type of crusher
(Figure 6.6) the swing jaw is suspended on the
eccentric shaft, which allows a lighter, more
compact design than with the double-toggle
machine. The motion of the swing jaw also differs
from that of the double-toggle design. Not only
does the swing jaw move towards the fixed jaw,
under the action of the toggle plate, but it also
moves vertically as the eccentric rotates. This elliptical
jaw motion assists in pushing rock through
the crushing chamber. The single-toggle machine
therefore has a somewhat higher capacity than
the double-toggle machine of the same gape. The
eccentric movement, however, increases the rate of
wear on the jaw plates. Direct attachment of the
swing jaw to the eccentric imposes a high degree
of strain on the drive shaft, and so maintenance

costs tend to be higher than with the double-toggle
machine.
Double-toggle machines cost about 50% more
than single-toggle machines of the same size, and
are usually used on tough, hard, abrasive materials,
although the single-toggle crusher is used in
Europe, especially Sweden, for heavy-duty work on
tough taconite ores, and it is often choke fed, since
the jaw movement tends to make it self-feeding.