2. Stag Jaw Crusher
The Stag jaw crusher has a fixed jaw and a moving jaw
pivoted at the top
Crushing faces are formed of manganese steel.
The max movement of the jaw is at the bottom; so there
is little tendency for the machine to clog, though some
uncrushed material may fall through and have to be
returned to the crusher.
The maximum pressure is exerted on the large material
which is introduced at the top.
One of the toggle plates in the driving mechanism is
made relatively weak so that, if any large stresses are set
up, this is the first part to fail.
Easy renewal of the damaged part is then possible.
5. Stag Jaw Crusher
The speed of operation should not be so high that a large
quantity of fines is produced
The angle of nip, the angle between the jaws, is usually
about 30◦.
Because the crushing action is intermittent, the loading
on the machine is uneven and the crusher therefore
incorporates a heavy flywheel.
The power requirements of the crusher depend upon size
and capacity and vary from 7 to about 70 kW,
the latter figure corresponding to a feed rate of 10 kg/s.
6. Dodge Jaw Crusher
The moving jaw is pivoted at the bottom.
The minimum movement is thus at the bottom and a
more uniform product is obtained
Although the crusher is less widely used because of
its tendency to choke.
The large opening at the top enables it to take very
large feed and to effect a large size reduction.
This crusher is usually made in smaller sizes than the
Stag crusher, because of the high fluctuating stresses
that are produced in the members of the machine.
10. Gyratory Crusher
Crushing head is in the form of a truncated
cone, mounted on a shaft
The crushing action takes place round the whole of the
cone
The crusher is continuous in action, thus fluctuations in
the stresses are smaller than in jaw crushers and the
power consumption is lower.
It also does not take such a large size of feed as a jaw
crusher, although it gives a rather finer and more
uniform product.
Because the capital cost is high, the crusher is suitable
only where large quantities of material are to be handled.
Employs a compressive force for size reduction
11. Other Coarse Crushers – Coal Breaker
Consists of a large hollow cylinder with perforated
walls.
Feed is introduced at the top.
The cylinder is rotated and the coal is lifted by means
of arms attached to the inner surface and then falls
against the cylindrical surface.
The coal breaks by impact and passes through the
perforations as soon as the size has been sufficiently
reduced.
This type of equipment is less expensive and has a
higher throughput than the jaw or gyratory crusher.
16. Edge Runner Mill
It has heavy cast iron or granite wheel, or muller
mounted on a horizontal shaft which is rotated in a
horizontal plane in a heavy pan.
Alternatively, the muller remains stationary and the
pan is rotated
Material is fed to the centre of the pan and is worked
outwards by the action of the muller, whilst a scraper
continuously removes material that has adhered to
the sides of the pan, and returns it to the crushing
zone.
17. Edge Runner Mill
In many models the outer rim of the bottom of the
pan is perforated, so that the product may be
removed continuously as soon as its size has been
sufficiently reduced.
The mill may be operated wet or dry and it is used
extensively for the grinding of paints, clays and
sticky materials.
19. Hammer Mill
Type of Impact mill with high speed rotating disc, to
which are fixed a number of hammers
Hammers are swung outwards by centrifugal force.
Material is fed in, either at the top or at the centre, and it
is thrown out centrifugally and crushed by hammer
bars, or against breaker plates fixed around the periphery
of the cylindrical casing.
The material is beaten until it is small enough to fall
through the screen at the lower portion of the casing.
Hinged hammers: the presence of any hard material does
not damage to equipment.
20. Hammer Mill
The bars are replaced when they are worn out.
Suitable for the crushing of both brittle and fibrous
materials
For fibrous materials it employs a screen with cutting edges.
Suitable for hard materials
Since a large amount of fines is produced, pressure
lubrication is recommended for bearings.
21. Hammer Mill
The size of the product is regulated by the size of the
screen and the speed of rotation.
In some cases the hammer bars are rigidly fixed in
position.
Since a large current of air is produced, the dust
must be separated in a cyclone separator or a bag
filter.
24. Pin-type Mill
Two vertical steel plates
Horizontal projections at their near faces
One disc may be stationary whilst the other disc is
rotated at high speed;
or the two may be rotated in opposite directions
The material is gravity fed in through a hopper or air
and is thrown outwards by centrifugal action and
broken against of the projections before it is
discharged to the outer body of the mill
25. Pin type Mill
Discharged material falls under gravity
The mill gives a fairly uniform fine product with little
dust
Used with chemicals, fertilisers and other materials
that are non-abrasive, brittle or crystalline.
Control of the size of the product is effected by
means of the speed and the spacing of the
projections
and a product size of 20 μm is readily attainable.
27. Single Roll Crusher
Consists of a toothed crushing roll
Roll rotates close to a breaker plate.
The material is crushed by compression and shearing
between the two surfaces.
It is used extensively for crushing coal.
30. Crushing Rolls
Two rolls, one in adjustable bearings, rotate in opposite
directions
The clearance between them can be adjusted according
to the size of feed and the required size of product.
Protected, by spring loading, against damage from very
hard material.
Both rolls may be driven, or one directly and the other by
friction with the solids.
Effect a small size reduction ratio, 4 : 1 in a single
operation
commonly a no. of pairs of rolls are employed in series
one above the other
31. Crushing Rolls
Roll shells with either smooth or ridged surfaces are
held in place
See Ex. 2.2
33. Symons Disc Crusher
Has 2 saucer-shaped discs mounted on horizontal
shafts of which one is rotated
The two crushing faces continuously approach and
recede.
Material is fed in the centre between the two discs
The product is discharged by centrifugal action as
soon as it is fine enough to escape through the
opening between the faces.
36. Buhrstone mill
Grinding takes place between two heavy horizontal
wheels, one of which is stationary and the other is driven.
The surface of the stones is carefully dressed so that the
material is continuously worked outwards from the
centre of the circumference of the wheels.
Size reduction takes place by a shearing action between
the edges of the grooves on the two grinding stones.
Used for the grinding of
grain, pigments, harmaceuticals, cosmetics and printer’s
ink,
Although used where the quantity of material is very
small.
37. Roller Mill
The roller mill consists of a pair of rollers that rotate
at different speeds in opposite directions.
one of the rollers is held in a fixed bearing whereas
the other has an adjustable spring-loaded bearing
since the rollers rotate at different speeds, size
reduction is effected by a combination of
compressive and shear forces.
The roller mill is extensively used in the flour milling
industry and for the manufacture of pigments for
paints.
41. Szego grinding mill
A planetary ring-roller mill
stationary, cylindrical grinding surface with a no. of
grooved rollers rotate.
Connected to the central drive shaft; they are pushed
outward by centrifugal force and roll on the grinding
surface.
The material is fed by gravity is discharged at the
bottom of the mill.
42. Szego grinding mill
The particles, upon entering the grinding section, are
repeatedly
crushed between the rollers and the stationary
grinding surface.
Crushing and shearing force caused by rotational
motion of the rollers.
43. Ball Mill
Consists of a rotating hollow cylinder, partially filled
with balls
Horizontal or at a small angle to the horizontal.
The outlet is normally covered with a coarse screen
to prevent the escape of the balls
Balls fall on the grinding medium from a height
Size reduction mainly by Impact, Compression and
Attrition
45. Ball Mill
Inner surface of the cylinder is lined with an abrasion-
resistant material
such as manganese steel, stoneware or rubber.
Balls occupy a volume b/n 30 and 50 per cent of total
volume of the mill
Balls wear out during grinding and are replaced
Different sized balls may be used
large balls deal effectively with the feed and the small
ones are responsible for giving a fine product.
For very fine grinding in small mills pebbles are often
used instead of balls.
46. Compound Ball Mill
Cylinder is divided into compartments by vertical
perforated plates.
Material flows axially along the mill
Can pass from one compartment to the next only when
its size has been reduced to less than the perforations in
the plate.
Each compartment is supplied with balls of a different
size.
large balls are at the entry while the small balls before
discharge.
Results in economical operation and the formation of a
uniform product.
47. Wet grinding in Ball Mill
Power consumption is generally about 30 per cent
lower than that for dry grinding
Continuous removal of product as it is formed is
facilitated
48. Factors affecting the size of the product
The rate of feed.
With high rates of feed, less size reduction is effected
The properties of the feed material.
The larger the feed the larger is the product
A smaller size reduction is obtained with a hard material.
Weight of balls.
A heavy charge of balls produces a fine product.
The weight of the charge can be increased, either by increasing the
number of balls, or by using a material of higher density
49. Factors affecting the size of the product
Diameter of the balls.
Small balls facilitate the production of fine material
But do not deal so effectively with the larger particles in the feed.
The
For an economical operation, the smallest possible balls should be
used.
The slope of the mill.
An increase in the slope of the mill increases the capacity
But a coarser product is obtained.
Discharge freedom.
same effect as increasing the slope.
50. Factors affecting the size of the product
The speed of rotation of the mill– Critical Speed
Low speeds of rotation, the balls simply roll over one
another and little crushing
At still higher speeds they are thrown greater distances
At very high speeds, the balls are carried right round in
contact with the sides of the mill and little relative
movement or grinding takes place again.
The minimum speed at which the balls are carried round
in this manner is called the critical speed of the mill
Hence at Critical speed there will be no resultant force
acting on the ball when in the uppermost position
51. Factors affecting the size of the product
At critical speed the centrifugal force will be exactly
equal to the weight of the ball.
If the mill is rotating at the critical angular velocity ω
rω2 = g
Nc is the no. of revolutions per unit time
It is found that the optimum speed is between one-
half and three-quarters of the critical speed.
53. Factors affecting the size of the product
The level of material in the mill.
Power consumption is reduced by maintaining a
low level of material
can be done by providing suitable discharge opening
for the product.
If the level of material is raised, the cushioning
action is increased and power is wasted by the
production of an excessive quantity of undersize
material.
54. Advantages of the Ball Mill
The mill may be used wet or dry
The costs of installation and power are low.
The ball mill may be used with an inert atmosphere
and therefore can be used for the grinding of
explosive materials.
The grinding medium is cheap.
The mill is suitable for materials of all degrees of
hardness.
It may be used for batch or continuous operation.
It may be used for open or closed circuit grinding.
55. Tube Mill
Similar to ball mill
However, length to the diameter is usually 3 or 4 :
1, as compared with 1 or 1.5 : 1 for the ball mill.
The mill is filled with pebbles
The characteristics of the two mills are similar
But material remains longer in the tube mill because
of its greater length, and a finer product is therefore
obtained.
56. Rod Mill
High carbon steel rods about 50 mm diameter and extending
the whole length of the mill are used.
This mill gives a very uniform fine product
Power consumption is low
Not suitable for very tough materials and
Feed should not exceed about 25 mm in size.
It is particularly useful with sticky materials which would hold
the balls together in aggregates, because the greater weight
of the rods causes them to pull apart again.
Worn rods must be removed from time to time and replaced
by new ones, which are rather cheaper than balls.
57. Hardinge Mill
A ball mill in which the balls segregate themselves
according to size.
The main portion of the mill is cylindrical as in the
ball mill, although the outlet end is conical and
tapers towards the discharge point
large balls collect in the cylindrical portion while the
smaller balls, in order of decreasing size,
Like a Compound Ball Mill
59. Hardinge Mill
The mill has an advantage over the compound ball mill in
that the large balls are raised to the greatest height and
therefore are able to exert the maximum force on the
feed.
As the size of the material is reduced, smaller forces are
needed to cause fracture and it is therefore unnecessary
to raise the smaller balls as high.
The capacity of the Hardinge mill is higher than that of a
ball mill of similar size and it gives a finer and more
uniform product with a lower consumption of power.
It is difficult to select an optimum
speed, however, because of the variation in shell
diameter.
61. Ball or tube mill operate
effectively only below its critical
Planetary mill speed
Planetary mill obviate this
constraint by rotating the mill
simultaneously about its own axis
and about an axis of gyration.
In practice, several cylinders are
incorporated in the machine, all
rotating about the same axis of
gyration.
62. Planetary Ball Mill
Smaller than common ball mills
Mainly used in laboratories for grinding sample material
down to very small sizes.
Consist of at least one grinding jar which is arranged
eccentrically on a so-called sun wheel.
The difference in speeds between the balls and grinding
jars produces an interaction between frictional and
impact forces, which releases high dynamic energies.
The interplay between these forces produces the high and
very effective degree of size reduction of the planetary
ball mill.
63. Vibration Mill
By imparting a vibrating motion to a mill
either by the rotation of out-of-balance weights or
by the use of electro-mechanical devices,
accelerations many times the gravitational
acceleration may be imparted to the machine.
The body of the machine is generally supported on
powerful springs and caused to vibrate in a vertical
direction.
Grinding may take place in two stages, the material
falling from an upper to a lower chamber when its
size has been reduced below a certain value.
64. Vibration Mill
Has much higher capacity than a conventional mill
of the same size
So either smaller equipment may be used or a much
greater throughput is obtained.
Well suited for incorporation in continuous grinding
systems.
66. Colloid Mill
Colloidal suspensions, emulsions and solid
dispersions are produced by means of colloid mills or
dispersion mills.
Droplets or particles of sizes less than 1 μm may be
formed
Feed material of approximately 100-mesh or 50 μm
in size is used
68. Colloid Mill
Clearances could be from virtually zero to 1.25
mm, although in practice the maximum clearance used is
about 0.3 mm
The gap setting between rotor and stator is not
necessarily in direct proportion to the droplet size or
particle size of the end product.
The thin film of material continually passing between the
working surfaces is subjected to a high degree of
shear, and consequently the energy absorbed within this
film is frequently sufficient to reduce the dispersed phase
to a particle size far smaller than the gap setting used.
69. Colloid Mill
The rotor speed varies with the physical size of the
mill and the clearance necessary to achieve the
desired result
The required operating conditions and size of mill
can only be found by experiment.
In all colloid mills, the power consumption is very
high, and the material should therefore be ground as
finely as possible before it is fed to the mill.
72. Fluid energy mill
solid is pulverised in jets of high pressure superheated
steam or compressed air, supplied up to 3.5 MN/m2 (35
bar).
The pulverising takes place in a shallow cylindrical
chamber with a number of jets arranged tangentially at
equal intervals around the circumference.
The solid is thrown to the outside walls of the chamber
Fine particles are formed by the shearing action resulting
from the differential velocities within the fluid streams.
The jet pulveriser will give a product with a particle size
of 1–10 μm.
73. Specialised applications
Electrohydraulic crushing
an underwater discharge is generated by the release of
energy from a high-voltage capacitor
The spark length depends on the nature of the material to be
crushed
Ultrasonic grinding
fed between a drive roll and a curved plate, both of which are
ultrasonically activated.
74. Specialised grinding--Cryogenic grinding
Materials like plastics, rubber, waxes etc. tend to distort
rather than to fracture when subjected to compressive
forces.
However can be done by subjecting it to very low
temperatures.
Material is cooled with liquid nitrogen at a temperature of
about −196◦C (77K) to render it brittle before it enters the
grinder
cooling causes the crystal lattice to shrink and to give rise to
microscopic cracks which act as nuclei and then grow
thereby reducing the energy input required to cause
fracturing to occur.
Application in market for frozen foods
75. Specialised grinding--- Explosive Shattering
energy is transmitted to particles as shock waves
suddenly releasing steam from an explosion chamber
containing the solid to be compressed.
still at the development stage.
