mineral processing

1. Mineral processing
Chapter two
Comminution-primary crushers

2. 1. Principles of comminution
• Most minerals are crystalline materials in which the atoms are
regularly arranged in three-dimensional arrays.
• The configuration of atoms is determined by the size and types of
physical and chemical bonds holding them together. In the
crystalline lattice of minerals, these inter-atomic bonds are effective
only over small distances, and can be broken if extended by a
tensile stress. Such stresses may be generated by tensile or
compressive loading.

3. …
• the increase in stress at such a site is proportional to the square
root of the crack length perpendicular to the stress direction.
• there is a critical value for the crack length at any particular level
of stress at which the increased stress level at the crack tip is
sufficient to break the atomic bond at that point.
• Such rupture of the bond will increase the crack length, thus
increasing the stress concentration and causing a rapid
propagation of the crack through the matrix, thus causing fracture

4. …
• Brittle materials relieve the strain energy mainly by crack
propagation, whereas "tough" materials can relax strain
energy without crack propagation by the mechanism of plastic
flow, where the atoms or molecules slide over each other and
energy is consumed in distorting the shape of the material.
• Fine-grained rocks, such as taconites, are therefore usually
tougher than coarse-grained rocks.
• The energy required for comminution is reduced in the
presence of water, and can be further reduced by chemical
additives which adsorb onto the solid (Hartley et al., 1978).
This may be due to the lowering of the surface energy on
adsorption providing that the surfactant can penetrate into a
crack and reduce the bond strength at the crack tip before
rupture.

5. …
• when an irregular particle is broken by
compression, or crushing, the products fall into
two distinct size ranges- coarse particles
resulting from the induced tensile failure, and
fines from compressive failure near the points of
loading, or by shear at projections.

6. 2. Comminution theory
• Comminution theory is concerned with the
relationship between energy input and the particle
size made from a given feed size.
The three laws of comminution are stated as under :
1. Kick's law1; Kick stated that the energy required to
bring analogous changes in configuration of
geometrically similar bodies of equal technological
character is proportional to the volumes of these bodies.
2. Rittinger's theory: Von Rittinger proposed a theory
stating that the energy consumed in comminution is
proportional to new surface produced.
3. Bond's theory; Bond's so called third theory of
comminution states that the energy required is
proportional to the length of crack initiating breakage.

7. 3. Grindability
• Ore grindability refers to the ease with which materials
can be comminuted, and data from grindability tests are
used to evaluate crushing and grinding efficiency.
• Probably the most widely used parameter to The method
requires the use of a reference ore of measure ore
grindability is the Bond work index Wi.
standard Bond work indices for a selection of materials /Bond standard grindability test

8. 1.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 run-of-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.

9. …
• Crushers are designed in such a way that they reduce rock
in such a manner that all pieces are less than a stated size.
But no crusher has been devised which produces only
fragments greater than a specified size. A crusher always
produces various sizes of particles with substantial amount
of fines.
• when the products of crushing are separated into a series
of closely sized fractions, the total surface of each fraction
will be more or less equal. The extent of size reduction
achieved by any crushing operation is described by the
reduction ratio. It is defined in a number of ways.
Broadly it is defined as the ratio of the maximum size of
the particle in the feed to the maximum size of the particle
in the product. Two definitions commonly used are termed
as average reduction ratio and 80% passing reduction ratio
which are defined as follows:

10. Average reduction ratio= Average size of the feed particles
Average size of the product particles
80% passing reduction ratio= 80% passing size of the feed
80% passing size of the product
Reduction ratio is a convenient measure for comparing the performance of different
crushers.
Example: What is the power required to crush 100 tons/hr of limestone if 80% of the
feed passes 50800 microns and 80% of the product passes 3175 microns? The work
Index for limestone may be taken as 12.74 kWhr/short ton.
Solution:
80% passing size of the product = P = 1/8″ = 3175 microns;
80% passing size of the feed = F = 2″ = 50800 microns;
Work Index of the limestone = Wi = 12.74 kWhr/short ton
W=10 Wi (1/√p-1/√F)
W=10 * 12.74 (1/√3175-1/√50800)
w= 1.696 kWhr/ short ton
Power per 100 tons/hr = 1.696 × 100 × 1.1023 = 186.95 kW
[ ∵1 metric ton = 1.1023 × short ton, 1shorton=907.18474 kg]

11. primary crushing
• 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 ore
processing 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.

12. 2. Secondary crushing
• 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.

13. 3. Vibrating screens
• 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.

14. …

15. Open or Closed circuit
• Crushing may be in open or closed circuit depending on
product size.
• Both have their applications. When done in open circuit, the
product goes to the next stage in comminution (typically a Rod
mill feed). Since the product is not screened, the next stage needs
to be able to accommodate size fluctuations .
A. Open circuit of crushing B. Closed circuit of crushing

16. …
Crushing is done in closed circuit, when crusher flexibility is desired.
Here, the product is screened with the undersize moving to the next
stage( Ball mill feed), but the oversize being sent back to the crusher.
• As the crushing is performed in stages, crushing may be divided
into primary, secondary, tertiary and quaternary stages based on
the particle size.
• Correspondingly, the crushers can be classified into five groups
according to the size of the product they produce.
1. Primary Crushers: Jaw crusher, Gyratory crusher.
2. Secondary Crushers: Reduction gyratory, Cone crusher, Rolls
crusher
3. Tertiary Crushers: Short-head cone crusher.
4. Fine Crushers: Impact crushers.
5. Special Crushers: Bradford Breaker, Toothed Roll crusher.

17. 1. Primary crushers
Are Large, heavy duty machines, Reduces run of mine (ROM) ore down to a size
that is suitable for transport.. Two types of primary crushers are available: Jaw
Crushers and Gyratory Crushers. Gyratory crushers can produce 600 to 6000tph,
depending on size. Whereas, jaw crushers top out around 750 tph.
1.1 Jaw Crushers;
• Depending on different ways of moving, jaw crushers are divided
into four types:
1. Blake jaw crusher,
2. Dodge jaw crusher,
3. PE universal jaw crusher and
4. mobile jaw crusher.
Jaw Crusher

18. …..

19. ….
• Jaw crushers are classified by the method of pivoting the swing
jaw. If the swing jaw is pivoted at the top, it has a fixed receiving
area and variable discharge area and is known as a Blake crusher.
• If the swing jaw is pivoted at the bottom, it has a fixed discharge
area and variable receiving area and is known as a Dodge crusher.
If the swing jaw is pivoted at an intermediate position, it has both
variable receiving and discharge areas and is known as a Universal
crusher.
• The mobile crusher is also called the mobile crushing station,
including tyre mobile crushing station and crawler mobile crushing
station, which is a kind of rock crushing equipment newly
developed.
• Unlike the traditional jaw crusher, this new type mobile crusher can
be easily and flexibly moved to the crushing site, which eliminates
the obstacles caused by the crushing sites and the conditions

20. i) Blake Crusher: Fixed feed size, variable product size. The
greatest amount of motion is at the bottom of V which means that it
has little tendency to chock-Arrested/free crushing mechanism.
ii) Dodge : variable feed, Fixed product size. Chokes easily
in real world app. Mostly used in labs.
iii) PE Universal: Combo
Jaw-crusher types

21. …..
1.1 Blake jaw crusher:
• There are two jaws, called angle of nip, to each other which
forms a crushing chamber, in a Blake jaw crusher,
1. one is stationary/fixed and vertical,
2. the other is movable and swing.
• The swing jaw reciprocates in a horizontal plane, and generally
the angle between fixed jaw and movable jaw is of 20 °to30°.
• Motion to the swing jaw is transmitted by pitman working
on an eccentric and toggles.
The material is fed between the jaws and is alternately
nipped and crushed.

22. …
• In a Blake crusher, the distance between the two jaw plates at
the feed opening is known as gape. The distance between the
two jaw plates at the discharge opening is known as set. The
minimum distance is called closed set and maximum distance is
called open set. The maximum amplitude of swing of the jaw is
known as throw. In the mining industry, Jaw crushers are used
to crush the run-of-mine ore to a size suitable for transportation.
The reduction ratio of jaw crushers varies from 4 to 7.

23. …
• As one of the most commonly used jaw crusher, Blake jaw crusher
has two types:
1. Double Toggle Blake Crusher (DTJC)
2. Single Toggle Jaw Crushers (STJC):
• Single toggle jaw crusher; An eccentric shaft is positioned on the
top of the crusher. The swing jaw plate hangs on the eccentric drive
shaft and moves up and down in elliptical motion, which
compresses rock material as it moves down from the feeding part.
• Double toggle jaw crusher; Also, swing jaw is hinged at the top,
and this support point is hand shift. The eccentric shaft and all jaw
crushers of any style have some sort of eccentric motion.
• As the eccentric rotates it pushed down on the pitman which in turn
flattens the angle between the two toggles since the rear toggle is
pivoting against the frame, the front toggle pushes on the swing jaw
closing the gap between the two jaw plates.

24. JAW CRUSHERS

25. Single toggle jaw crusher VS double toggle jaw crusher
• Both of them adopt way of compression: because the top of the toggles is
mounted low in relation to the swing jaw and far from the hinge point it is
virtually a straight-line motion. This type of motion gives a straight
compressive force with low abrasion jaw plates.
• Single toggle jaw crusher has less toggle, and has two movements at the
same time— law plate swing together with eccentric shaft moving up and
down. DTJC are used on tough, hard and abrasive material.
• Double-toggle machines cost about 50% more than single-toggle machines of
the same size.
• In STJC, 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.
• The single-toggle machine therefore has a somewhat higher capacity than the
double-toggle machine of the same gape. i.e. single toggle jaw crusher
performs better than double toggle jaw crusher in productivity.
• In STJC, maintenance costs tend to be higher than with the double-toggle
machine.

26. 1.2 Dodge jaw crusher:
In the dodge type jaw crushers, the jaws are farther apart at
the top than at the bottom which form a tapered chute so
that the material is crushed progressively smaller and
smaller as it travels downward until it is small enough to
escape from the bottom opening.
Features;

27. .
1.3 PE universal Jaw crusher
• The universal jaw crusher combines the principles of both Black
and Dodge jaw crushers.
• The pivot is slightly above the bottom end of the movable jaw and
therefore it provides two strokes per revolution.
• When stone material goes through the crushing chamber, with
moving jaw compressing towards down and inside, it will generate
aggressive force feed action to reduce the size of material

28. Outstanding advantages and characteristics
• Crushing chamber: Its crushing chamber is deep without any
dead zone, which improves the feeding capacity;
• Crushing ratio: It has a large crushing ratio which helps to
produce uniform particle size;
• Discharge port device: The gasket type adjusting device of the
discharge port is reliable and convenient with large adjustment
range, greatly increasing the flexibility of the machine;
• Lubrication system: It is safe and reliable, making the wear
parts easy to be replaced, thus reducing the maintenance time;
• Structural design: Simple structure designed for reliable
operation and low operating costs.
• Energy-saving: Energy saving of the single machine is 15% to
30%, and the system is more than twice;
• Adjustment range: Large discharge port adjustment range can
meet the different requirements of the users;
• Dust and noise proof: It has advantages of low noise and less
dust.

29. The following are several common PE universal jaw crushers

30. 1.4 Mobile jaw crusher
• The mobile crusher is also called the mobile crushing station,
including tyre mobile crushing station and crawler mobile crushing
station.
• Unlike the traditional jaw crusher, this new type mobile crusher can
be easily and flexibly moved to the crushing site, which eliminates
the obstacles caused by the crushing sites and the condition.

31. Outstanding advantages of mobile crusher
High mobility determines that it is suitable for frequent conversion of
crushing sites. Thus, its price is higher than other kinds of jaw
crushers.

32. Jaw-crusher construction
• Jaw crushers are extremely heavy-duty machines and hence
must be robustly constructed. The main frame is often made
from cast iron or steel, connected with tie-bolts.
• Modem jaw crushers may have a main frame of mild steel
plate welded together.
• The jaws themselves are usually constructed from cast steel
and are fitted with replaceable liners, made from manganese
steel, or "Ni-hard", a Ni-Cr alloyed cast iron.
• Apart from reducing wear, hard liners are essential in that they
minimise crushing energy consumption, reducing the
deformation of the surface at each contact point.
• They are bolted in sections on to the jaws so that they can be
removed easily and reversed periodically to equalise wear.

33. …
• Cheek plates are fitted to the sides of the crushing chamber to
protect the main frame from wear. These are also made from hard
alloy steel and have similar lives to the jaw plates.
• The jaw plates themselves may be smooth, but are often
corrugated, the latter being preferred for hard, abrasive materials.
Jaw plates: smooth profile & corrugated profile.
• Patterns on the working surface of the crushing members also
influence capacity, especially at small settings.
• Laboratory tests have demonstrated that the capacity is reduced
about 50 times when a corrugated profile is used rather than a
smooth surface.
• The corrugated profile is claimed to perform compound crushing
by compression, tension, and shearing.
• Conventional smooth crushing plates tend to perform crushing by
compression only, though irregular particles under compression
loading might still break in tension.
• .

34. …
• Since rocks are around 10 times weaker in tension than
compression, power consumption and wear costs should
be lower with the corrugated profiles.
• The angle between the jaws is usually less than 260
, as
the use of a larger angle than this causes slipping, which
reduces capacity and increases wear.
• In order to overcome problems of choking near the
discharge of the crusher, which is possible if fines are
present in the feed, curved plates are sometimes used.
• The lower end of the swing jaw is concave, whereas the
opposite lower half of the fixed jaw is convex. This
allows a more gradual reduction in size as the material
nears the exit, hence minimising the chances of packing.
Less wear is also reported on the jaw plates, since the
material is distributed over a larger area.

35. ….
• The speed of jaw crushers varies inversely with the size,
and usually lies in the range of 100-350rev/min .
• The main criterion in determining the optimum speed is
that particles must be given sufficient time to move down
the crusher throat into a new position before being nipped
again.
• The maximum amplitude of swing of the jaw, or
"throw", is determined by the type of material being
crushed and is usually adjusted by changing the eccentric.
It varies from 1 to 7 cm depending on the machine size,
and is highest for tough, plastic material and lowest for
hard, brittle ore.
• The greater the throw, the less danger is there of chokage,
as material is removed more quickly. This is offset by the
fact that a large throw tends to produce more fines, which
inhibits arrested crushing. Large throws also impart higher
working stresses to the machine.

36. 2. Gyratory Crushers
•Gyratory crushers are principally used in surface crushing
plants, although a few currently operate underground.
• The gyratory crusher consists essentially of a long spindle, carrying
a hard steel conical grinding element, the head, seated in an
eccentric sleeve.
• The spindle is suspended from a "spider" and, as it rotates, normally
between 85 and 150 rev/min, it sweeps out a conical path within the
fixed crushing chamber, or shell, due to the gyratory action of the
eccentric.
• As in the jaw crusher, maximum movement of the head occurs near
the discharge. This tends to relieve the choking due to swelling, the
machine thus being a good arrested crusher.
• The spindle is free to turn on its axis in the eccentric sleeve, so that
during crushing the lumps are compressed between the rotating
head and the top shell segments, and abrasive action in a horizontal
direction is negligible.

37. Gyratory Crusher
functional diagram,

38. ..

39. …
• In deciding whether a jaw or a gyratory crusher should be used in a
particular plant, the main factor is :
1. the maximum feed size of ore
2. production capacity required
3. Economics of operation
 Since the gyratory, unlike the jaw crusher, crushes on full cycle, it
has a much higher capacity than a jaw crusher of the same gape,
and is usually favoured in plants handling very large
throughputs.
 In mines with crushing rates above 900t/h, gyratory crushers are
always selected. Gyratory crushers are, in general, used where high
capacity is required.
 Power consumption is as high as 750 kW on such crushers.
Large gyratory often dispense with expensive feeding
mechanisms and are often fed direct from trucks.

40. …
• jaw crushers tend to be used where the crusher gape is
more important than the capacity.
• For instance, if it is required to crush material of a certain
maximum diameter, then a gyratory having the required
gape would have a capacity about three times that of a
jaw crusher of the same gape.
• If high capacity is required, then a gyratory is the answer.
• If, however, a large gape is needed but not capacity, then
the jaw crusher will probably be more economical, as it
is a smaller machine and the gyratory would be running
idle most of the time.
• If tone/hr < 161.7 (gape in metres),2
use a jaw crusher.
Conversely, if the tonnage is greater than this value, use a
gyratory crusher.

41. Crushing capacity
• Crushing capacity depends on many factors, such as:
1. angle of nip (i.e. the angle between the crushing members),
2. stroke,
3. speed,
4. the liner material,
5. feed material, and
6. its initial particle size.
1. The volumetric capacity of a jaw crusher is expressed as:
Q = BSs. cot[a 9 k. 60n] 𝒎𝟑/h
where B = inner width of crusher (m); S = open side setting (m); s = throw (m); a =
angle of nip; n = speed of crusher (rpm); and k is a material constant, the size of
which varies with the characteristics of the crushed material, the feeding method,
liner type, etc., normally having values between 1.5 and 2.
2. For gyratory crushers, the corresponding formula is:
Q = (D - S)𝝅Sscot(a. k. 60n) 𝒎𝟑/h
where D-- diameter of the outer head mantle at the discharge point
(m), and k the material constant normally varying between 2 and 3.

42. …
• The capital and maintenance costs of a jaw crusher are
slightly less than those of the gyratory, but they may be offset
by the installation costs, which are lower with the gyratory,
since it occupies about two-thirds the volume and has about
two-thirds the weight of a jaw crusher of the same capacity.
• The better self-feeding capability of the gyratory compared
with the jaw results in a capital saving in some cases, with the
elimination of expensive feeding devices, such as the heavy-
duty chain feeder.
• The type of material being crushed may also determine the
crusher used.
• Jaw crushers perform better than gyratories on clayey, plastic
material, due to their greater throw.
• Gyratories have been found to be particularly suitable for hard,
abrasive material, and they tend to give a more cubic product
than jaw crushers if the feed is laminated or "slabby".

43. Features of Gyratory:
1. High capacity per investment dollar.
2. Handles thin, slab-like material better than a jaw crusher.
3. Feeding is simpler.
4. Foundation costs are lower than for jaw crusher
5. Easier installation than JC since more compact design and crushing
stresses more even.
Features of Jaw Crusher:
1. Large receiving opening per investment dollar.
2. Shape of opening favors large block shaped feed better than a
gyratory.
3. Handles dirty or sticky feed better, without buildup, 4. Maintenance
is easier.
5. Can be easily split into sections, which makes it easier to carry it
underground.

44. 2. Secondary Crushing:
Feed size will normally be less than 15 cm. Secondary crushing is usually
conducted with cone crushers. Impact (or Hammer) crushers and roll crushers are
still sometimes used.
a. Cone Crushers:
Cone crushers are just a variation of the gyratory crusher, with some
differences:
- A major difference between the two is the shape of the head crushing and
the outer shell.
- Supported at the bottom.
- Difference arising from crushing smaller particles. These include higher
throw and speed. Therefore, crushing is more hammer like.
- Liners on head and shell parallel at discharge end for better size control
- Shell can “lift” when hard tramp material enters the crusher. This allows
the tramp to pass through without hurting the crusher.

45. b. Roll Crusher:
Roll crushers use two large drums mounted on springs, rotating toward
each other to crush material. Their great disadvantage is that in order to get
a reasonable reduction ratio, very large drums must be used. For example,
rolls of diameter 1.4 m needs to be used for particles only 2.6 cm size, if a
6:1 reduction ratio is desired.
Roll Crusher (Figure below) consists of two smooth heavy horizontal
cylinders revolving towards each other and the feed material is
nipped between the rolls and pulled downward through the rolls by
friction.
• The distinguished feature of a roll crusher is that the material is
crushed one time only whilst it is passing through the crushing
chamber.
ROLL CRUSHERS

46. Due to this fact, the reduction ratio of a roll crusher varies from 2 to
4, the lowest among all the crushers. Production of fines is minimum.
They can handle friable, dry, wet, sticky, frozen, and less abrasive
feeds well. Smooth-surfaced roll crushers are generally used for fine
crushing whereas corrugated or toothed roll crushers are used for
coarse crushing of soft materials.
Single toothed roll crusher and Double toothed roll crusher are
the two types of toothed roll crushers used for crushing coal.

47. c. IMPACT CRUSHERS:
Impact crushers reduce the particles by impact forces applied
through sharp blows of fixed or free swinging hammers revolving
about central rotor at high speed to the free falling particles against
stationary surfaces. They are used for relatively soft, friable, and
sticky ores such as phosphates, limestone, clay, graphite and coal.
Hammer mill is one type of impact crusher.
Factors affecting secondary crusher selection include:
• Capacity
• Feed size
IMPACT CRUSHERS

48. Optimizing Crushers
Technology has allowed better instrumentation and process control to
improve crusher efficiencies. Now a days instruments not only detect
equipment health, but also monitor operating conditions such as
choking, feed rates and feed particle size distributions