EQUIPMENT AND STEP INVOLVED IN LIBREATION AND COMMINUTION

1. EQUIPMENT AND STEP INVOLVED
IN LIBREATION AND
COMMINUTION
PRESENTED BY:
NISHANT KUMAR MEHTA(NIT JAMSHEDPUR)

2. INTRODUCTION
• “Run- of mine “ ore consists of valuable minerals and gangue
• Mineral processing sometimes also called ore dressing ,mineral dressing or milling and this process
prepare the ore for extraction of valuable mineralsand produce a commercial end product
History-
• Two fundamental operations in mineral processing are release or liberation of the valuable
mineral are now known as concentration of ore
• A century ago- Ore concentration was done by simple gravity and hand sorting
# Freshly mined material is easier to handle by scrapers, conveyors and ore carriers

3. Comminution-
• In this method particle size are progressively reduced by crushing and grinding until the the clean particles of
the minerals can be separated
# CRUSHING –
• Crushing is accomplished by compression of ore against rigid surface or by impact surfaces in rigidly
constrained motion path
• It is dry process
• reduction ratio of crushing stage =( maximum particle size entering to crusher)/ (minimum particle size leave
the crusher)
# GRINDING –
• grinding is accomplished by impact ,attrition and abrasion of the ore by the free motion of unconnected
media such as steel rods , steel, ceramic balls, or coarse ore pebbles
• It is usually performed “wet” to produce slurry feed to concentration process
• Primary autogenous or semi- autogenous mill are tumbling mills and capable of grinding very coarse feed

4. # It is greatest energy consumer and take up-to 50% of concentrator energy
consumption
# High grade – fine grind – high energy cost

5. NEW TECHNOLOGY –
* HPGR – high pressure grinding rolls
• Intermediate between fine crushing and coarse tumbling
mills
• It is dry crushing device utilize two rotating rolls creating
compression breakage of a particle bed , in which inter
particle breakage occurs
• HPGR reduces the comminution energy requirement
needed by tumbling mills
• HPGR is 20% to 50% more efficient than conventional
crushers and mills
• HPGR is generally used in cement industry

6. CRUSHERS –
• First mechanical process in the stage of comminution
# Generally dry operations
# Lump of run- of- mine ore about 1.5m - reduced in primary crushing stage to 10-20cm
• Secondary crushing includes all operations for reclaiming the primary crusher product
from ore storage to disposal of final crusher product which is usually 0.5- 2cm diameter

7. PRIMARY CRUSHERS
• Primary crushers are heavy duty machines ,used to reduced the run –of –
mine ore down to a size suitable for transport and for feeding the secondary
crushers or AG (autogenous) ,SAG (semi – autogenous)
• They are always operated in open circuit
• Primary crusher – two type
1. Jaw crusher
2. Gyratory crusher

8. JAW CRUSHER
• It has two plates which open and shut like animal jaws
• The jaws are set at an acute angle to each other such that one
jaw is pivoted so that it swings relative to the other fixed jaw
• Material fed into the jaws is alternatively nipped and released to
fall further into the crushing chamber , eventually falls from the
discharge aperature
# feature of jaw crusher
• Simple structure easy maintenance
• Stable performance
• Even final particles and high crushing ratio
# type of jaw crusher – (on the basis of method
of pivoting the swing jaw )
1. Dodge crusher
2. Blake crusher
3. Universal crusher

9. BLAKE CRUSHER
• Blake crusher was patented by W.E Blake in 1858
• Its basic form are found in most of the jaw crusher
used today
• In blake crusher the jaw is pivoted at the top and thus
has a fixed receiving area and a variable discharge
opening
• The greatest amount of motion is at bottom which
means it has little tendency to choke
# two type of blake crusher
1. Double toggle
2. Single toggle

10. DODGE CRUSHER
• In the dodge crusher the jaw is pivoted at the buttom , giving it a variable
feed area but fixed delivery area
• In the dodge jaw crusher the moving jaw is pivoted the buttom . As
minimum movement is at the buttom it has a greater tendency to choke
• So it is not used in laboratory where close sizing is required and it is also
never used to heavy duty crusher
UNIVERSIAL CRUSHER
• The universal crusher is pivoted in an intermediate position and thus a variable delivery
and receiving area

11. GYRATORY CRUSHER
# a gyratory crusher includes a solid cone set on a revolving shaft
placed with in a hollow body, which has a conical or vertical slopping
sides

12. SECONDARY CRUSHER
• Secondary crusher is immediately follow by primary crusher
• Secondary crushers take primary crushed ore as feed , the maximum feed size will normally
be less than 15cm in diameter
• It is used to get our required size of product
• It is much easier to handle because most of harmful constituents in the ore ,such as tramp
metal , wood, clays and slimes have already removed
• Secondary crushers also operate with dry feeds
• Tertiary crushers have all intents and purposes of the same design as secondaries , except
that they have a closer set

14. Introduction
 Secondary crushers are much lighter than the heavy-duty, rugged primary machines.
 Since they take the primary crushed ore as feed, the maximum feed size will normally
be less than 15 cm in diameter and, because most of the harmful constituents in the
ore, such as tramp metal, wood, clays, and slimes have already been removed, it is
much easier to handle.
 Similarly, the transportation and feeding arrangements serving the crushers do not
need to be as rugged as in the primary stage.
 They can operate on dry as well as wet feed.

15. Types of secondary crushers
Cone crusher
Roll crusher
Impact crusher

16. Cone crushers  The cone crusher is a modified gyratory crusher. Cone crusher and
gyratory crusher work on the same principle. Both have the same
operation. If cone crusher differs then it is only from crushing
chamber. Cone crusher has a less steep crushing chamber and
more parallel zone between crushing zones.
 Unlike a gyratory crusher, which is identified by the dimensions of
the feed opening and the mantle diameter, a cone crusher is rated
by the diameter of the cone lining. Cone crushers range in size
from 559mm to 3.1 m and have capacities up to 1100 t/h with a
discharge setting of 19 mm.
 The high-speed action allows particles to flow freely through the
crusher, and the wide travel of the head creates a large opening
between it and the bowl when in the fully open position. This
permits the crushed fines to be rapidly discharged, making room for
additional feed.

17. Working Principal Of Cone Crushers
 It breaks the rocks by squeezing it between the gyrating spindles. These spindles
are fully covered with resistant mantle and a manganese bowl liner covers the
hopper.
 Rocks get squeezed at the same moment when it enters in between the bowl liner
and mantle. Only one time breaking is carryout of larger pieces of rocks from ore.
 Broken pieces of rocks fall down to the next position where it is broken again.
 Same process continues until the broken pieces become small enough so that it can
pass through the narrow opening that is at the bottom of the cone crusher.

18. Features of cone crushers
 A cone crusher is suitable for crushing a variety
of mid-hard and above mid-hard ores and rocks.
 It has the advantage of reliable construction,
high productivity, easy adjustment and lower
operational costs.
 The spring release system of a cone crusher
acts an overload protection that allows tramp to
pass through the crushing chamber without
damage to the crusher.

19. Types of cone crushers
 Symons cone crusher
 Compound cone crushers
 Single cylinder hydraulic cone crusher
 Multi-cylinder hydraulic cone crusher
 The gyradisc crusher
 The Rhodax crusher

20. Symons cone crusher
 Symons cone crusher (spring cone crusher) can crush materials of above medium hardness.
 It is widely used in metallurgy, building, hydropower, transportation, chemical industry, etc.
 The Symons cone crusher is the most widely used type of cone crusher.
 It is produced in two forms: the Standard for normal secondary crushing and the Short-head
for fine, or tertiary duty.
 They differ mainly in the shape of their crushing chambers. The Standard cone has
"stepped" liners which allow a coarser feed than in the Short-head. They deliver a product
varying from 0.5 to 6cm.
 The Short-head has a steeper head angle than the Standard, which helps to prevent choking
from the much finer material being handled. It also has a narrower feed opening and a
longer parallel section at the discharge, and delivers a product of 0.3-2.0 cm.

21. Standard Symons
Cone Crusher
Short-Head Symons
Cone Crusher

22. Modern Standard Symons
Cone Crusher
Modern Short-Head
Symons Cone Crusher

23.  The parallel section between the liners at the discharge is a feature of all cone
crushers and is incorporated to maintain a close control on product size.
 Material passing through the parallel zone receives more than one impact from
the crushing members.
 The set on the cone crusher is thus the minimum discharge opening.
 The distributing plate on the top of the cone helps to centralise the feed,
distributing it at a uniform rate to all of the crushing chamber.

24. Compound cone crusher
 Compound cone crusher (VSC series cone crusher) can crush materials of
over medium hardness.
 It is mainly used in mining, chemical industry, road and bridge
construction, building, etc.
 VSC series cone crusher’s enhanced laminating crushing effect on
material particles makes the cubic shape of crushed materials better,
which increases the selling point.

25. Single cylinder hydraulic cone crusher
 Single cylinder hydraulic cone crusher is mainly composed of main frame,
transmission device, eccentric shaft, bowl-shaped bearing, crushing cone,
mantle, bowl liner, adjusting device, adjusting sleeve, hydraulic control
system, hydraulic safety system, dust-proof ring, feed plate, etc.

26. Multi-cylinder hydraulic cone crusher
 Multi-cylinder hydraulic cone crusher is mainly composed of main frame, eccentric
shaft, crushing cone, mantle, bowl liner, adjusting device, dust ring, transmission
device, bowl-shaped bearing, adjusting sleeve, hydraulic control system, hydraulic
safety system, etc.
 The electric motor of the cone crusher drives the eccentric shaft to make periodic
swing movement under the shaft axis, and consequently surface of mantle approaches
and leaves the surface of bowl liner now and then, so that the material is crushed due
to squeezing and grinding inside the crushing chamber.
 The safety cylinder of the machine can ensure safety as well as lift supporting sleeve
and static cone by a hydraulic system and automatically remove the blocks in the
crushing chamber when the machine is suddenly stuffy.
 Thus the maintenance rate is greatly reduced and production efficiency is greatly
improved as it can remove blocks without disassembling the machine.

27. The gyradisc crusher
 Specialised form of cone crusher.
 Used for producing very fine material.
 Application in the quarrying industry for the production of large quantities of sand at
economic cost.
Working principal
 Crushing is by interparticle comminution by the impact and attrition of a multi-layered
mass of particles.
 Transfer through the crushing zone is by movement of the head.
 Each time the lower liner moves away from the upper liner, material enters the attrition
chamber from the surge load above.
 When reduction begins, material is picked up by the lower liner and is moved outward.
 Due to the slope of the liner it is carried to an advanced position and caught between the
crushing members.
 After the initial stroke the lower liner is withdrawn faster than the previously crushed
material falls by gravity.
 This permits the lower liner to recede and return to strike the previously crushed mass as
it is falling.

28. Comparison of gyradisc and
conventional cone crusher

29. The Rhodax crusher
 Specialised form of a cone crusher, referred to as an inertial cone crusher.
 Developed by the FCB Research Centre (now Fives fcb) in France.
 It is based on inter- particle compression crushing.

30. Schematic of Rodax cone crusher

31. Construction of Rodax cone crusher
 It consists of a frame supporting a cone and a mobile ring, and a set of
rigid links forming a set of ties between the two parts.
 The frame is supported on elastic suspensions isolating the environment
from dynamic stresses created by the crushing action.
 It contains a central shaft fixed on a structure. A grinding cone is mounted
on this shaft and is free to rotate.
 A sliding sleeve on this shaft is used to adjust the vertical position of the
cone and therefore the gap, making it simple to compensate for wear.
 The ring structure is connected to the frame by a set of tie rods. The ring
and the cone are made of wear resistant steel.

32. Working of Rodax crusher
 One set of synchronised unbalanced masses transmits a known and controlled
crushing force to the ring when they rotate.
 The relative positions of the unbalanced masses can be changed if required, so
the value of the crushing force can thus be remotely controlled.
 As feed particles enter the fragmentation chamber, they slowly advance between
the cone and the moving ring.
 These parts are subjected to horizontal circular translation movements and move
towards and away from each other at a given point.
 During the approach phase, materials are subjected to compression. During the
separation phase, fragmented materials pass further down in the chamber until
the next compression cycle.
 The number of cycles is typically 4-5. During these cycles the cone rolls on a bed
of stressed material a few millimetres thick, with a rotation speed of a 10-20rpm.
The unbalanced masses rotate at 100-300rpm.

33.  The following three parameters can be adjusted on the Rhodax crusher:
• the gap between the cone and the ring
• the total static moment of unbalanced masses
• the rotation speed of these unbalanced masses.
Operating principal of Rodax crusher

34. Courtesy of “fives”
Working of Rodax crusher - Animation

35. Roll crushers
 Replaced in most installations by cone
crushers.
 Still used in some mills.
 Still have a useful application in handling
friable, sticky, frozen, and less abrasive
feeds, such as limestone, coal, chalk,
gypsum, phosphate, and soft iron ores.
 These are starvation fed.

36. Working of roll crushers
 The standard spring rolls consisting of two horizontal cylinders which revolve towards each other.
 Rolls Crushers consists of two parallel rotating rolls turning together (in opposite directions), with feed
being directed through the moving gap between them.
 One roll is fixed, and the other moveable, using some spring mechanism or hydraulic pressure.
 As the ore moves through the gap, the force behind the moveable roll acts to crush the particles as they
are forced together into a crushed particle “bed”. Each roll is equipped with its own motor.
 Operating with high pressures normally results in a product that contains a significant amount of ultra-
fines (crushed particles less than 1mm), which provides a favourable ore size distribution for ball mill
feed.
 Unlike jaw and gyratory crushers, where reduction is progressive by repeated pressure as the material
passes down to the discharge point, the crushing process in rolls is one of single pressure.
 Smooth-surfaced rolls are usually used for fine crushing, whereas coarse crushing is often performed in
rolls having corrugated surfaces.

37.  "Sledging" or "slugger" rolls have a series of intermeshing teeth, or slugs, protruding from the
roll surfaces.
 These dig into the rock so that the action is a combination of compression and ripping.
 Their main application is in the coarse crushing of soft or sticky iron ores, friable limestone,
coal, etc., rolls of 1 m diameter being used to crush material of top size 400 mm.
 Since there is no provision for the swelling of broken ore in the crushing chamber, roll crushers
must be "starvation fed" if they are to be prevented from choking.
 Choked crushing causes so much pressure that the springs are continually "on the work"
during crushing, and some oversize particles escape. Rolls should therefore be used in closed
circuit with screens.
 Wear on the roll surfaces is very high and they often have a manganese steel tyre, which can
be replaced when worn.
 The feed must be spread uniformly over the whole width of the rolls in order to give even wear.
One simple method is to use a flat feed belt of the same width as the rolls.

38. Calculation of feed size for roll crushers
Let r : radius of spherical particle being crushed by
a pair of rolls
R : radius of rolls
2a : gap between the rolls
µ : coefficient of friction
𝜃 : angle of nip (angle formed by the tangents to
the roll surfaces at their points of contact with the
particle)
C : compressive force exerted by the roll

39. For a particle to be just gripped by the rolls, equating vertically
𝐶 𝑠𝑖𝑛(𝜃/2) = µ𝐶 𝑐𝑜𝑠(𝜃/2)
Therefore, 𝜇 = 𝑡𝑎𝑛(𝜃/2)
The value of the coefficient of friction between a particle and moving rolls can be calculated from the
equation
𝜇𝑘 = ((1+1.2𝑣 )/(1+6𝑣))* 𝜇
Where 𝜇𝑘 is the kinetic coefficient of friction and, 𝑣 is the peripheral velocity of the rolls (m/s).
Equation below can be used to determine the maximum size of rock gripped in relation to roll
diameter and the reduction ratio (r/a) required.
𝑐𝑜𝑠(𝜃/2) = ((R+a)/(R+r))

40. Different types of roll crushers
 Roll crusher are mainly distinguished on the number of crushing rolls they have. Roll
crushers are also manufactured with only one rotating cylinder, which revolves towards a
fixed plate. Other roll crushers use three, four, or six cylinders.
 They also differ on the type of crushing rolls they have. For example “toothed” or “smooth”.
Single toothed roll crusher Double toothed roll crusher

41. Reasons for roll crushers being replaced by cone crushers
 Better control over product size in cone crushers .
 More energy is consumed in roll crushers.
 Generally the capacity of cone crushers ore more as they are choke fed.

42. Impact crushers
 Comminution is by impact rather than compression.
 Sharp blows applied at high speed to free-falling rock.
 The impact crushers are used to process from 200 t/h up
to 1900 t/h and feed sizes of up to 350 mm in the largest
model.
 Impact crushers are generally used in nonabrasive
applications and where the production of fines is not a
problem.
 The internal stresses created in the particles are often
large enough to cause them to shatter. These forces are
increased by causing the particles to impact upon an anvil
or breaker plate.

43. Types of impact crushers
Hammer mill
Impact mill
VSI

44. Horizontal shaft impactor (HSI) / Hammermill
Construction
 The hammers are made from manganese
steel or nodular cast iron containing chromium
carbide, which is extremely abrasion resistant.
 The breaker plates are made of the same
material.
 The hammers are pivoted so as to move out
of the path of oversize material (or tramp
metal) entering the crushing chamber.
 The exit from the mill is perforated, so that
material that is not broken to the required size
is retained and swept up again by the rotor for
further impacting.

45. Working
 The HSI crushers break rock by impacting the rock with hammers that are fixed upon the outer
edge of a spinning rotor.
 The hammer mill is designed to give the particles velocities of the order of that of the hammers.
Fracture is either due to impact with the hammers or to the subsequent impact with the casing
or grid.
 Since the particles are given high velocities, much of the size reduction is by attrition (i.e.,
particle on particle breakage), and this leads to little control on product size and a much higher
proportion of fines than with compressive crushers.
 The hammers can weigh over 100 kg and can work on feed up to 20 cm. The speed of the rotor
varies between 500 and 3000rev/min.
 Due to the high rate of wear on these machines (wear can be taken up by moving the hammers
on the pins) they are limited in use to relatively nonabrasive materials. They have extensive
use in limestone quarrying and in the crushing of coal.
 Normally horizontal shaft impact crusher is used for soft materials and materials like gypsum,
phosphate, limestone and weathered shales.

46. Working of hammer mill in slow motion

47. Impact mill
 The fixed hammer impact mill is often used for much coarser crushing.
 The material falls tangentially on to a rotor, running at 250-500rev/min , receiving a glancing
impulse, which sends it spinning towards the impact plates.
 The fractured pieces which can pass between the clearances of the rotor and breaker plate
enter a second chamber created by another breaker plate, where the clearance is smaller, and
then into a third smaller chamber.
 This is the grinding path which is designed to reduce flakiness and gives very good cubic
particles.
 The rotary impact mill gives a much better control of product size than does the hammer mill,
since there is less attrition.
 The product shape is much more easily controlled and energy is saved by the removal of
particles once they have reached the size required.
 Since they depend on high velocities for crushing, wear is greater than for jaw or gyratory
crushers. Hence impact crushers should not be used on ores containing over 15% silica.

48. Working of impact mill
Courtesy of “mesto”

49. Vertical shaft impactor (VSI)
 VSI crushers use a different approach involving a high speed rotor with wear resistant tips
and a crushing chamber designed to 'throw' the rock against the anvil.
 The VSI crushers utilize velocity rather than surface force as the predominant force to
break rock.
 Applying surface force (pressure) results in unpredictable and typically noncubical
resulting particles. Utilizing velocity rather than surface force allows the breaking force to
be applied evenly both across the surface of the rock as well as through the mass of the
rock.
 Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is
'thrown' by a VSI rotor against a solid anvil, it fractures and breaks along these fissures.
Final particle size can be controlled by (a) the velocity at which the rock is thrown against
the anvil and (b) the distance between the end of the rotor and the impact point on the
anvil.
 Using this method also allows materials with much higher abrasiveness to be crushed
than is capable with an HSI and most other crushing methods.

50. Working of VSI
 VSI crushers generally utilize a high speed spinning rotor at the centre of the crushing
chamber and an outer impact surface of either abrasive resistant metal anvils or crushed
rock.
 The feed falls in the centre of the rotating chamber and due to the rotation is thrown
outwards with a very high velocity towards the outer impact surface.
 On colliding with the impact surface the particles are broken into smaller fragments.
 These fragments fall down out of the crushing chamber.
 Utilizing cast metal surfaces 'anvils' is traditionally referred to as a "shoe and anvil VSI".
Utilizing crushed rock on the outer walls of the crusher for new rock to be crushed against
is traditionally referred to as "rock on rock VSI".

51. Working of VSI
Courtesy of “KPI-JCI” “ASTEC”

52. Types of VSI
 Barmac Vertical Shaft Impact Crusher
 Canica Vertical Shaft Impact Crusher

53. Barmac Vertical Shaft Impact Crusher
 Developed in New Zealand in the late 1960s.
 The crusher is finding application in the concrete industry.
 The mill combines impact crushing, high-intensity grinding, and multi-
particle pulverizing, and as such, is best suited in the tertiary crushing.
 Producing products in the 0.06-12 mm size range. It can handle feeds of
up to 650 t/ h at a top size of over 50 mm.

54. Working of Barmac VSI
Courtesy of “mesto”

55. Canica Vertical Shaft Impact Crusher
 This crusher developed by Jaques (now Terexs Mineral Processing
Solutions) .
 It has several internal chamber configurations available depending on the
abrasiveness of the ore.
 Examples include the Rock on Rock, Rock on Anvil and Shoe and Anvil
configurations.
 These units typically operate with 5 to 6 steel impellers or hammers, with a
ring of thin anvils.
 Rock is hit or accelerated to impact on the anvils, after which the broken
fragments freefall into the discharge chute and onto a product conveyor
belt.

56. Working of Canica VSI
Courtesy of “TEREX MPS”

57. GRINDERS
GRINDING IS THE LAST STAGE IN THE PROCESS OF COMMINUTION; IN THIS STAGE THE PARTICLES ARE
REDUCED IN SIZE BY A COMBINATION OF IMPACT AND ABRASION, EITHER DRY OR IN SUSPENSION IN
WATER. IT IS PERFORMED IN ROTATING CYLINDRICAL STEEL VESSELS WHICH CONTAIN A CHARGE OF
LOOSE CRUSHING BODIES - THE GRINDING MEDIUM- WHICH IS FREE TO MOVE INSIDE THE MILL, THUS
COMMINUTING THE ORE PARTICLES. ACCORDING TO THE WAYS BY WHICH MOTION IS IMPARTED TO
THE CHARGE, GRINDING MILLS ARE GENERALLY CLASSIFIED INTO TWO TYPES: TUMBLING MILLS AND
STIRRED MILLS.
As opposed to crushing, which takes place between relatively rigid surfaces, grinding is a more random process, and is
subject to the laws of probability. The degree of grinding of an ore particle depends on the probability of the ore
entering a zone between the medium shapes and the probability of some occurrence taking place after entry. Grinding
can be done by several mechanisms, including impact or compression, due to forces applied almost normally to the
particle surface; chipping due to oblique forces; and abrasion due to forces acting parallel to the surfaces .These
mechanisms distort the particles and change their shape beyond certain limits deter- mined by their degree of elasticity
which causes them to break.

58. Grinding is usually performed wet, although in certain applications dry grinding is used. When the mill is
rotated, the mixture of medium, ore, and water, known as the mill charge, is intimately mixed, the medium
comminuting the particles by any of the above methods depending on the speed of rotation of the mill and the
shell liner structure. Most of the kinetic energy of the tumbling load is dissipated as heat, noise, and other
losses, only a small fraction being expended in actually breaking the particles.
Grinding may serve the following purpose:
• Correct degree of liberation i.e. manufacturing of a solid with desired grain size.
• Increasing the surface area of solid
• Pulping of resources

59. Tumbling Mill
Tumbling mills are of three basic types: rod, ball, and autogenous. Structurally, each type of mill consists of a
horizontal cylindrical shell, provided with renewable wearing liners and a charge of grinding medium. The drum is
supported so as to rotate on its axis on hollow trunnions attached to the end walls. The diameter of the mill
determines the pressure that can be exerted by the medium on the ore particles and, in general, the larger the feed
size the larger needs to be the mill diameter. The length of the mill, in conjunction with the diameter, determines the
volume, and hence the capacity of the mill.
The feed material is usually fed to the mill continuously through one end trunnion, the ground product leaving via
the other trunnion, although in certain applications the product may leave the mill through a number of ports spaced
around the periphery of the shell. All types of mill can be used for wet or dry grinding by modification of feed and
discharge equipment.
In tumbling mills the mill shell is rotated and motion is imparted to the charge via the mill shell. The grinding
medium may be steel rods, balls, or rock itself. Tumbling mills are typically employed in the mineral industry for
coarsegrinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300
µm. Grinding within a tumbling mill is influenced by the size, quantity, the type of motion, and the spaces between
the individual pieces of the medium in the mill.

60. Motion of charge in tumbling mill
The distinctive feature of tumbling mills is the use of loose crushing bodies, which are
large, hard, and heavy in relation to the ore particles, but small in relation to the
volume of the mill, and which occupy slightly less than half the volume of the mill.
Due to the rotation and friction of the mill shell, the grinding medium is lifted along the
rising side of the mill until a position of dynamic equilibrium is reached, when the
bodies cascade and cataract down the free surface of the other bodies, about a dead
zone where little movement occurs, down to the toe of the mill charge.
The speed at which a mill is run is important, since it governs the nature of the product and the amount of
wear on the shell liners. For instance, a practical knowledge of the trajectories followed by the steel balls in
a mill determines the speed at which it must be run in order that the descending balls shall fall on to the toe
of the charge, and not on to the liner, which could lead to rapid liner wear.

61. The driving force of the mill is transmitted via the liner to the charge. At relatively low speeds, or with
smooth liners, the medium tends to roll down to the toe of the mill and essentially abra- sive
comminution occurs. This cascading leads to finer grinding, with increased slimes production and
increased liner wear. At higher speeds the medium is projected clear of the charge to describe a series
of parabolas before landing on the toe of the charge. This cataracting leads to comminution by impact
and a coarser end product with reduced liner wear. At the critical speed of the mill the theoretical
trajectory of the medium is such that it would fall outside the shell. In practice, centrifuging occurs and
the medium is carried around in an essentially fixed position against the shell.
In travelling around inside the mill the medium (and the large lumps of ore) follows a path which has
two parts. The lifting section near to the shell liners is circular while the drop back to the toe of the mill
charge is parabolic.

62. MECHANICAL CONSTRUCTION
Mill shells:- Mill shells are designed to sustain impact and heavy loading and are constructed from rolled mild
steel plates welded together.
Mill ends: The mill ends or trunnion heads may be of nodular or grey cast iron for diameters less than about 1m.
Trunnions and bearings: The trunnions are made from cast iron or steel and are spigoted and bolted to the end
plates, although in small mills they may be integral with the end plates.
Drive Tumbling mills are most commonly rotated by a pinion meshing with a girth ring bolted to one
end of the machine.
Mill feeders:- The function of the feeder is to transport pulp from some receiving point outside the mill into
the mill barrel smoothly and with sufficient driving force to overcome any tendency for the pulp to move in the
opposite direction. Three types of feeder are in use in wet-grinding mills.
• Spout feeder
• Drum feeder
• Combination drum - scoop feeder
Liners:- Liners are the materials, which are used on the inner surface of the grinding shell to provide the
necessary strength and resistance. Shell lining acts as the final link in the transmission of energy to the tumbling
load i.e. the charge. It usually differs in type according to the size of feed, whether the feed is coarse or fine.

63. Types of mills
Rod mills
These may be considered as either fine crushers or coarse grinding machines. They are capable of taking feed
as large as 50mm and making a product as fine as 300txm, reduction ratios normally being in the range 15-
20:1. They are often preferred to fine crushing machines when the ore is "clayey" or damp, thus tending to
choke crushers.
The distinctive feature of a rod mill is that the length of the cylindrical shell is between 1.5 and 2.5 times its
diameter.
This ratio is important because the rods, which are only a few centimetres shorter than the length of the shell,
must be prevented from turning so that they become wedged across the diameter of the cylinder. The ratio
must not, however, be so large for the maximum diameter of the shell in use that the rods deform and break.
Since rods longer than about 6 m will bend, this establishes the maximum length of the mill. Thus, with a mill
6.4 m long the diameter should not be over 4.57 m. Rod mills of up to 4.57 m in diameter by 6.4 m in length are
in use, run by 1640 kW motors (Lewis et al., 1976). Rod and other grinding mills are rated by power rather than
capacity, since the capacity is determined by many factors, such as the grindability, determined by laboratory
testing,, and the reduction in size required.
The power required for a certain required capacity may be estimated by the use of Bond's equation: 𝑊 = (10𝑊𝑖̇
/√𝑃 )− (10𝑊𝑖/ √𝐹)

64. Centre peripheral discharge mills
These are fed at both ends through the trunnions and discharge the ground product through circumferential ports at the
centre of the shell. The short path and steep gradient give a coarse grind with a minimum of fines, but the reduction ratio is
limited. This mill can be used for wet or dry grinding and has found its greatest use in the preparation of specification
sands, where high tonnage rates and an extremely coarse product are required.
Rod mills are classed according to the nature of the discharge. A general statement can be made
that the closer the discharge is to the periphery of the
shell, the quicker the material will pass through and less overgrinding will take place.

65. End peripheral discharge mills
These are fed at one end through the trunnion, discharging the ground product from the other end of the mill by
means of several peripheral apertures into a close-fitting circumferential chute. This type of mill is used mainly
for dry and damp grinding, where moderately coarse products are involved.

66. Trunnion overflow mill
The most widely used type of rod mill in the mining industry is the trunnion overflow, in
which the feed is introduced through one trunnion and discharges through the other. This
type of mill is used only for wet grinding and its principal function is to convert crushing-
plant product into ball-mill feed. A flow gradient is provided by making the overflow
trunnion diameter 10-20 cm larger than that of the feed opening. The discharge trunnion
is often fitted with a spiral screen to remove tramp material.

67. Ball mill:

68. The final stages of comminution are performed in tumbling mills using steel balls
as the grinding medium and so designated "ball mills“. Since balls have a greater
surface area per unit weight than rods, they are better suited for fine finishing.

69. Tube Mills:
The term ball mill is restricted
to those having a length to
diameter ratio of 1.5 to 1 and
less. Ball mills in which the
length to diameter ratio is
between 3 and 5 are
designated tube mills. These
are sometimes divided into
several longitudinal
compartments, each having a
different charge composition;
the charges can be steel balls
or rods, or pebbles, and they
are often used dry to grind
cement clinker, gypsum, and
phosphate.

70. Pebble Mill:
Tube mills having only one
compartment and a charge of
hard, screened ore particles as
the grinding medium are known
as pebble mills. They are
widely used in the South
African gold mines. Since the
weight of pebbles per unit
volume is 35-55% of that of
steel balls, and as the power
input is directly proportional to
the volume weight of the
grinding medium, the power
input and capacity of pebble
mills are correspondingly lower.

71. Ball mills are also classified by the
nature of the discharge.
They may be:
 Simple trunnion over- flow mills (operated in open or closed
circuit, or
 Grate discharge (low-level discharge) mills

72. Grate discharge mills:
Grate discharge mills are fitted with discharge
grates between the cylindrical mill body and
the discharge trunnion. The pulp can flow
freely through the openings in the grate and is
then lifted up to the level of the discharge
trunnion. Very little overgrinding takes place
and the product contains a large fraction of
coarse material, which is returned to the mill
by some form of classifying device. Closed-
circuit grinding, with high circulating loads,
produces a closely sized end product and a
high output per unit volume compared with
open circuit grinding.

73. Trunnion over- flow mills:
 The trunnion overflow mill is
the simplest to operate and
is used for most ballmill
applications, especially for
fine grinding and regrinding.
Energy consumption is said
to be about 15% less than
that of a grate discharge mill
of the same size, although
the grinding efficiencies of
the two mills are the same

74. Overflow mills vs Grate discharge
mills:
 Grate discharge mills have a
lower pulp level than overflow
mills, thus reducing the dwell
time of particles in the mill. These
mills usually take a coarser feed
than overflow mills and are not
required to grind so finely, the
main reason being that with
many small balls forming the
charge the grate open area plugs
very quickly.

75.  Segregation of the ball charge
within the mill is achieved in the
Hardinge mill. The conventional
drum shape is modified by fitting
a conical section, the angle of
the cone being about 30⁰. Due to
the centrifugal force generated,
the balls are segregated so that
the largest are at the feed end of
the cone, i.e. the largest diameter
and greatest centrifugal force,
and the smallest are at the
discharge. By this means, a
regular gradation of ball size and
of size reduction is produced.

76. Autogenous Mills operate, mechanically, similar to the ball
mill. They differ in the media they use to break or grind the
ore. Autogenous Mills use large particles of ore instead of
steel or other balls for grinding media. Autogenous mills
use large pieces of ore as grinding media. The grinding is
facilitated in autogenous mills by attrition with limited
grinding by impact.
For an ore to successfully grind autogenously, the ore
must be competent, and it must break along grain
boundaries at the desired product size. Another
requirement is that the finer sizes should break easily and
should be removed from the mill, otherwise, there will be a
critical size buildup.
Autogenous Mill:

77.  Autogenous grinding has two advantages:
 (1) it reduces metal wear and
 (2) eliminates secondary and tertiary crushing
stages.
Thus it offers a savings in capital and operating
costs. Autogenous mills are available for both wet
and dry grinding. The diameter of autogenous mills
is normally two to three times the length. The ore
charge is usually 25 to 35% of the mill volume.
Autogenous mills have grate discharges to retain the
coarse grinding media in the mill.

78. Vibratory Mill:
A vibration mill is a size reduction equipment that
applies the process of continuous impaction in
carrying out its size reduction function. The grinding
container is made up of a tube that is held in a frame
that is supported by means of springs which is filled
to approximately 80% total volume with porcelain or
stainless steel balls.
During milling, the entire body of the mill undergoes a
small but frequent vibration that is generated by an
eccentric motor and size reduction occurs by
repeated impact. This vibration is usually, but not
necessarily, in a vertical plane.

79. Centrifugal Mill:
The concept of centrifugal grinding is
an old one, and although a patent of
1896 describes this process, it has
so far not gained full-scale industrial
application. In centrifugal milling, the
forces on the charge inside the mill
are increased by operating the mill in
a centrifugal, rather than a
gravitational, field. Comminution is
more rapid, and the size of machine
needed for a given grinding duty is
thus reduced.

80. Tower Mill:
For fine and ultra-fine grinding tower mills and stirred mills
are used. In a tower mill, steel balls or pebbles are placed
in a vertical grinding chamber in which an internal screw
flight provides medium agitation. The feed enters at the
top, with mill water, and is reduced in size by attribution
and abrasion as it falls, the finely ground particles being
carried upwards by pumped liquid and overflowing to a
classifier. Oversize particles are returned to the bottom of
the chamber, and final classification is by hydrocyclones,
which return underflow to the mill sump for further
grinding. Advantages are a small installation area, low
noise levels, efficient energy usage, minimal overgrinding,
and low capital and operating costs.

81. Stirred Mills:
The stirred milling is simple and robust, and has excellent energy
efficiency through a wide operational range of process parameters.
As the energy efficiency is consistent, it is possible to control the
specific grinding energy and product particle size by adjusting the
shaft speed using the variable speed drive. The machine
configuration allows for internal classification, whereby the coarser
particles move towards the chamber walls while finer particles move
faster upwards through the disc openings, reducing the potential for
over grinding, allowing excellent energy efficiency. The stirred milling
consumes less energy compared to conventional tumbling mills. The
small bead sizes used in the grinding chamber result in a large
amount of contacts and high grinding efficiency in the fine product
size range.

82. Roller Mills:
These mills are often used for
the dry grinding of medium soft
materials of up
to 4-5 mohs hardness. Above
this hardness, excessive wear
offsets the advantage of lower
energy consumption compared
with
conventional mills.