Ores are typically sorted to increase the efficiency of other refining processes, by reducing the amount of material to be processed while simultaneously increasing its purity. This module explains the ore separation processes.

1. 1
ORE SEPARATION - SCREENING AND
CLASSIFICATION
(SIZING- ORE SORTING)
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
UNIVERSITY OF MYSORE
MYSORE-6

2. 2
Introduction:
Ore sorting is separating an ore into constituent parts.
Today, ore sorters are widely used in industrial
mineral mines, diamond mines and base and precious
metal mines.
Ores are typically sorted to increase the efficiency of
other refining processes, by reducing the amount of
material to be processed while simultaneously
increasing its purity.

3. 3
The size distribution of the particles must be
controlled for a number of reasons at various stages of
a mineral processing plant:
To enable undersized material to bypass the crushing
or grinding circuit and to retain oversized particles
for further size reduction,
To provide an optimum particle size material for
efficient processing in the downstream separation
and concentration systems, and
To prepare product that meets particle size
specifications required for the market place.

4. 4
There are two distinct methods for separation of
particles based on size: screening and classification.
Screening:
In its simplest configuration, a screen is a hard
perforated surface with a matrix of fixed dimension
apertures.
The material is presented to the screen surface so that
material finer than the apertures falls through the
screen and the oversize is conveyed to the discharge
end of the screen.

5. 5
Screening is generally difficult below 0.5 mm.
Classification:
Classification techniques takes advantage of the
principle that particles of the same density but of
different sizes settle in a fluid at different rates.
Exploiting the difference in the settling rates allows
for separation based on size.
Classification is usually carried out at particle sizes
that are considered to be too fine for sorting
efficiently by screening methods.

6. 6
Screening Equipment: Grizzly
Grizzlies are used for rough screening of coarse
materials and are most often found in crushing
circuits.
A grizzly is basically an inclined set of heavy bars set
in a parallel manner.
Coarse material slides on the inclined surface of the
bars and material finer than the spacing between the
bars falls through.

7. 7
The grizzlies can be vibrated to improve
performance.
Revolving Screen (Trommel):
A trommel is a slightly inclined rotating cylindrical
screen. The material is fed at one end of the cylinder
and the undersize material falls through the screening
surface while the oversize is conveyed by the rotating
motion down the incline to the discharge end.

8. 8
Although trommels are relatively cheap, they have
lower capacities and are susceptible to rapid wear;
hence they have been largely replaced by vibrating or
shaking screens.
A trommel can handle both dry and wet feed material.
Moving Screens (reciprocating, oscillating,
vibratory and gyratory screens):
There are various types of horizontally inclined
screens such as the reciprocating, oscillating,
vibratory or shaking screening.

9. 9
The basic difference in these screens is based upon
the motion of the surface and the resulting action
imparted on the material being screened.
The vibratory screen is probably the most common
screening device found in mineral processing
applications.
Different manufacturers have developed unique
vibration and motion systems for the various types of
material and particle sizes encountered within mineral
processing plants.

10. 10
Many of these types of screens have multiple decks so
different particle size products can be obtained from a
single feed.
The gyratory screen is supported in a manner that it
allows for both a gyratory and slight vertical motion
to the screen deck. These types of screens can have
multiple removable and replaceable decks. Gyratory
screens can be used in wet or dry applications and are
usually used for separation of finer particles.

11. 11
Mechanical screening:
Mechanical screening, often just called screening, is
the practice of taking granulated ore material and
separating it into multiple grades by particle size.
This practice occurs in a variety of industries such as
mining and mineral processing, agriculture,
pharmaceutical, food, plastics, and recycling.

12. 12
General categories
Screening fall under two general categories: dry
screening and wet screening.
From these categories, screening separates a flow of
material into grades, these grades are then either
further processed to an intermediary product or a
finished product.
Additionally the machines can be categorised into
moving screen and static screen machines, as well as
by whether the screens are horizontal or inclined.

13. 13
Applications
The mining and mineral processing industry uses
screening for a variety of processing applications. For
example, after mining the minerals, the material is
transported to a primary crusher.
Before crushing large boulder are scalped on a shaker
with 0.25 in (6.4 mm) thick shielding screening.
Further down stream after crushing the material can
pass through screens with openings or slots that
continue to become smaller.

14. 14
Finally, screening is used to make a final separation to
produce saleable products based on a grade or a size
range.
Screening Process
A screening machine consist of a drive that induces
vibration, a screen media that causes particle
separation, and a deck which holds the screen media
and the drive and is the mode of transport for the
vibration.

15. 15
There are physical factors that makes screening
practical. For example, vibration, g force, bed density,
and material shape all facilitate the rate or cut.
Electrostatic forces can also hinder screening
efficiency in way of water attraction causing sticking
or plugging, or very dry material generate a charge
that causes it to attract to the screen itself.

16. 16
Screening - Physical principles
 Vibration - either sinusoidal vibration
or gyratory vibration.
 Sinusoidal Vibration occurs at an angled plane
relative to the horizontal. The vibration is in a
wave pattern determined by frequency and
amplitude.
 Gyratory Vibration occurs at near level plane at
low angles in a reciprocating side to side motion.

17. 17
 Gravity - This physical interaction is after material
is thrown from the screen causing it to fall to a lower
level.
 Gravity also pulls the particles through the screen
media.
 Density - The density of the material relates to
material stratification.
 Electrostatic Force - This force applies to screening
when particles are extremely dry or is wet.

18. 18
Types of mechanical screening
There are a number of types of mechanical screening
equipment that cause segregation. These types are
based on the motion of the machine through its motor
drive.
 Circle-throw vibrating equipment - This type of
equipment has an eccentric shaft that causes the
frame of the shaker to lurch at a given angle.
 High frequency vibrating equipment - This type of
equipment drives the screen cloth only.

19. 19
 Unlike above the frame of the equipment is fixed
and only the screen vibrates.
 Gyratory equipment - This type of equipment differs
from the above two such that the machine gyrates in
a circular motion at a near level plane at low angles.
 Trommel screens - Does not require vibrations,
instead, material is fed into a horizontal rotating
drum with screen panels around the diameter of the
drum.

20. 20
CLASSIFICATION:
Sedimentation and Hydraulic Classifiers
Free settling classifiers are essentially large pools,
ponds or conical bottomed tanks with a free settling
zone. The coarser particles sink and are removed from
the bottom of the settling zone. These units are simple
in design but often inefficient in sorting and sizing. A
hydraulic, hindered bed settler exploits differences in
the settling rates of particles.

21. 21
The particles settle against a rising current of water in
a series of sorting chambers or conical pockets. The
relative rate of settling against the varying up-flow
currents in each of the conical pockets provides
recovery of the different sized particles in each of the
chambers.
Spiral and Rake Classifiers:
Mechanical classifiers such as the spiral and rake
classifiers work in a similar fashion in that both drag
sediment and sand along the bottom of an inclined
surface to a higher discharge point on one end of the
settling chamber.

22. 22
The primary difference in the two systems is the
mechanism by which the settled material is moved up
the inclined surface.
Spiral classifiers are generally preferred as material
does not slide backwards which occurs in rake
classifiers when the rakes are lifted between strokes.
This also allows spiral classifiers to operate at steeper
inclines producing a drier product. The spiral
classifier also produces less turbulence in the settling
pool allowing for separation of finer material.

23. 23
Hydrocyclones (Cyclones):
Hydrocyclones have become one of the most
important and widely used classifiers in the mineral
processing industry. They are also used for de-
sliming, de-watering, de-gritting and thickening
processes. They are most commonly employed in
closed circuit within grinding circuits and are used to
return coarse material back to the ball or rod mill for
further grinding.

24. 24
The main advantages of cyclones is that they have
large capacities relative to their size and can separate
at finer sizes than most other screening and
classification equipment.
The separation mechanism in hydrocyclones relies on
centrifugal force to accelerate the settling of particles.
The slurry enters the cylindrical section tangentially
above a conical section.
The velocity of the slurry increases as it follows a
downward helical path from the inlet area to the
smaller diameter underflow end.

25. 25
As the slurry flows along this path, centrifugal forces
cause the larger and denser particles to migrate to the
fluid layer nearest the wall of the cone. Meanwhile,
the finer or lower specific gravity particles remain in,
migrate to, or are displaced toward the center axis of
the cone.
As the swirling slurry approaches the underflow tip,
smaller and lighter material closer to the center
reverses its axial direction and follows a smaller
diameter rotating path back toward the top overflow
discharge pipe.

26. 26
Vanning:
Vanning is a type of ore dressing by which ores are
washed on a shovel.
Typically, a powdered sample of orestuff is swirled
with water on the blade of a shovel and then given a
series of upward flicking motions.
The heavier ore is tossed up through the water and
appears as a crescent shaped patch at the top of the
charge with the lighter gangue below.

27. 27
Hydrocyclone :
A hydrocyclone (often referred to by the shortened
form cyclone) is a device to classify, separate or sort
particles in a liquid suspension based on the ratio of
their centripetal force to fluid resistance.
This ratio is high for dense (where separation by
density is required) and coarse (where separation by
size is required) particles, and low for light and fine
particles.

28. 28
Hydrocyclones also find application in the separation
of liquids of different densities.
A hydrocyclone will normally have a cylindrical
section at the top where liquid is being
fed tangentially, and a conical base.
The angle, and hence length of the conical section,
plays a role in determining operating characteristics.

29. 29
A hydrocyclone is a classifier that has two exits on
the axis: one on the bottom (underflow or reject) and
one at the top (overflow oraccept).
The underflow is generally the denser or coarser
fraction, while the overflow is the lighter or finer
fraction. It has no moving parts and its operation
depends two major parameters:
 the characteristics of the feed stream.
 the geometry of the cyclone.

30. 30
The characteristics of the feed stream include size
distribution of solids in the feed stream, pulp density
(percent solids in the slurry), pulp viscosity and the
inlet pressure for solid/liquid separation.
In liquid/liquid feed streams, for example in oily
water, the main feed characteristics are based on oil
droplet size and distribution, oil density, water
density, oil concentration, viscosity and temperature.

31. 31
The geometry of the cyclone involves-inlet shape and
area, cyclone dimensions (cone angle, length of
cylindrical section and total length of the cyclone) and
inlet, vortex and apex diameters.
A hydrocyclone is most often used to separate
"heavies" from a liquid mixture originating at a
centrifugal pump or some other continuous source of
pressurized liquid. A hydrocyclone is most likely to
be the right choice for processes where "lights" are
the greater part of the mixture and where the
"heavies" settle fairly easily.

32. 32
Generally, hydrocyclones are used in continuous flow
systems so that the instantaneous liquid inflow to the
hydrocyclone is equal to the total instantaneous
outflow of "lights" plus "heavies".
In cases where "heavies" are a very small part of the
whole liquid, it is sometimes advantageous to
accumulate them in the bottom of the hydrocyclone
for batchwise removal.

33. 33
Trommel:
A trommel (from the German word
for drum, Trommel, also known as a "trommel
screen," is a screened cylinder used to separate
materials by size - for example, separating
the biodegradable fraction of mixed municipal
waste or separating different sizes of crushed stone.
Trommels are also used in the recovery of gold from
mineral rich soils.

34. 34
The dirt is broken up by high pressure water jets
before entering the trommel, which breaks up the dirt
further and outputs the rocks and stones while
depositing the gold fragments in special plastic mats.
Cyclonic separation :
Cyclonic separation is a method of
removing particulates from an air, gas or liquid
stream, without the use of filters,
through vortexseparation.

35. 35
When removing particulate matter from liquids,
a hydrocyclone is used; while from gas, a gas cyclone
is used.
Rotational effects and gravity are used to separate
mixtures of solids and fluids. The method can also be
used to separate fine droplets of liquid from a gaseous
stream.
A high speed rotating (air)flow is established within a
cylindrical or conical container called a cyclone.

36. 36
Air flows in a helical pattern, beginning at the top
(wide end) of the cyclone and ending at the bottom
(narrow) end before exiting the cyclone in a straight
stream through the center of the cyclone and out the
top.
Larger (denser) particles in the rotating stream have
too much inertia to follow the tight curve of the
stream, and strike the outside wall, then fall to the
bottom of the cyclone where they can be removed.

37. 37
In a conical system, as the rotating flow moves
towards the narrow end of the cyclone, the rotational
radius of the stream is reduced, thus separating
smaller and smaller particles.
The cyclone geometry, together with flow rate,
defines the cut point of the cyclone. This is the size of
particle that will be removed from the stream with a
50% efficiency. Particles larger than the cut point will
be removed with a greater efficiency, and smaller
particles with a lower efficiency.