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EQUIPMENT USED
FOR AGGREGATE
PRODUCTION
Amount of processing required for aggregate
production depends on
• Nature of raw materials
• Desired attributes of the end products
Four procedures adopted:
1. Particle size reduction – crushing
2. Separation into particle size ranges – screening /
sizing
3. Elimination of undesired materials – washing
4. Handling and movement of crushed materials –
storage and transport
3
EQUIPMENTS USED FOR
PRODUCTION OF AGGREGETE
1. Crushers
a. Jaw crusher
b. Gyratory crusher
c. Roll crusher
d. Cone crusher
e. Impact crusher
2. Ball mill and Rod mill
(for Fine Aggregate)
3. Feeders
4. Screens
a. Revolving screens
b. Shaking screens
c. Vibrating screens
5. Log washer
6. Aggregate handling
4
1. Crushers:
• Crushers are used to break stones into different
sizes of aggregates.
• Based on the stage of crushing, crushers are
three types:
– Primary crusher
– Secondary crusher
– Tertiary crusher
• Stones obtained from quarry are fed into the
primary crusher which reduces the size of
stones.
𝐑𝐞𝐝𝐮𝐜𝐭𝐢𝐨𝐧 𝐫𝐚𝐭𝐢𝐨 =
1. Crushers: (continued…)
• Stones obtained from primary crusher are fed
into secondary crusher where sizes of stone is
further reduced.
• Therefore, to obtain desired size of aggregate,
the stones are passed through 4 to 5 crushers.
size of stone fed into the crusher
size of stone obtained from crusher
5
(Ratio of crusher feed size to product size)
6
(a) Jaw Crushers:
• It has two jaws, one is stationary and other is
moving.
• The surface of both jaw is flat/convex.
• The space between two jaws is wider at top and
narrow at the bottom.
• The size of jaw is 1.68 x 2.14 m.
• The stones are crushed by the pressure of two
jaws.
• Large size boulders can also be crushed using
this crushers.
• Power consumption is minimum.
Fig. Single jaw crusher 7
Fig. Double jaw crusher 8
9
Single toggle jaw
crusher
Double toggle jaw
crusher (Blake type)
(i) In a single toggle jaw
crusher whole swing jaw
is lifted, hence require
higher power.
(i) Require lesser power.
(ii) Jaw plates wear out at
faster rate.
(ii) Jaw plates wear out at
slower rate.
(iii) Lower initial cost. (iii) Higher initial cost.
(iv) Used for non-abrasive
and non hard rocks.
(iv) Economical for
abrasive and hard rocks.
(v) Compact size, lighter
weight and reasonably
sturdy
(v) Heavy, not for
portable applications
10
Size of jaw crushers and jaw opening
• Mouth opening of the jaws denotes the crusher
size. Thus, a jaw crusher having a jaw plate width
of 1200 mm and mouth opening/ feed opening of
1000 mm is called a 1200 x 1000 mm crusher.
• The jaw opening at the top determines the size of
the rock.
• The size of stone piece is restricted to 70 to 80 %
of the gap size. Thus, for 1200 x 1000 mm crusher
the feed size should be restricted to 700 to 800 mm
(70 to 80% of 1000 mm).
• The capacity of the crusher is dependent on the
area of the opening at the bottom of the jaws.
Fig. Crusher jaw setting and jaw opening
11
12
Wear limits:
• With use, the teeth of the jaws are worn out.
• When teeth plates gets worn out to about 1/3 of
the original corrugation height the jaw plates
must be turned end-to-end.
• Wear limit dead line is when there remained no
corrugation.
• When the entire crushing surface gets worn out,
the teeth plate should be replaced with new one.
Fig. Wear limit of tooth plate
13
14
Tips for optimum use of jaw crushers:
• The feed size should be 70 to 80% of the
minimum opening size at top. Do not allow
bigger pieces going to the crusher mouth.
• Material fed should not be of same size. Mixed
feed of all sizes of material gives best results.
• For best results ratio of gap to jaw setting should
be 5:1 to 7:1. Gap is the longer distance
between the two jaws.
• The ratio of gap to width of jaws is 1:1.6 in
primary crushers and 1:4.5 in the secondary
crushers.
15
(b) Gyratory crusher:
• Most efficient of all primary type crushers
• A gyrating mantle mounted within a deep bowl –
provides continuous crushing action
• For hard, rough, abrasive rock
• To protect the crusher, it can be spring loaded
type or mantle height can be hydraulically
adjusted
• A cone or head is fitted at the top for grinding
known as mantle
• Cone shaped crushing chamber known as
concave
• Stones are broken by the impact of the head.
• The frame of equipment is strong.
• Head is made of iron alloy
(c) Cone crusher:
• Cone crusher are used for second or third
grinding.
• Large quantities of uniform size aggregates are
obtained.
• A head of manganese iron is fitted on the vertical
shaft, which act as grinding surface.
• The second surface is concave.
• The rotation of shaft produces gyratory effect
which causes grinding of stones.
• Difference of cone crusher from gyratory
crushers: shorter cone, smaller receiver
opening, rotates at a higher speed, more
uniformly sized output
Fig. Cone Crusher
Salient features of Gyratory and cone
crushers:
• As moving parts are small in number, wear, tear
and maintenance is less.
• Longer life
• Dust has little effect.
• Hydraulic system enables finger tip control of the
machine.
• Wear is uniform and adjustment is simple.
• Cleaning by hydraulic system enables rapid
restart after a blockage during operation.
• These have devices to measure oil pressure, oil
temperature and oil circulation. In case of
abnormal condition there may be sound or visual
alarm.
20
(d) Roll crusher:
• This equipment consists of heavy iron frame with
two hard steel rolls. Two rolls are fitted on
different horizontal shafts.
• They can be rotated by v-belt or flat pulley.
• Stones are broken into small pieces between roll
and grinding plate.
• The equipment is light in weight and is suitable
for second grinding.
21
(c) Roll crusher: (continued..)
Feed size:
• The maximum size of material that can be fed to
a roll crusher is directly proportional to the
diameter of the rolls.
• If the feed contains stones that are too large, the
rolls will not grip the material and pull it through
the crusher.
Feed size: (continued..)
The angle of grip, which is constant for smooth
rolls, has been found to be 16̊ 45’.
The maximum size particles that can be crushed
are determined as follows.
R = radius of rolls
B = angle of grip (16̊ 45’)
D = R cos B = 0.9575 R
A = maximum feed size
C = roll setting (size of finished product)
X = R – D A = C + 2X
= R – 0.9575 R = C + 2 (0.0425 R)
= 0.0425 R Prof. Ashis
= C + 0.085 R
h Makwana 19
Capacity of roll crusher:
The capacity of roll crusher depends upon:
Type of stone Width of rolls
Size of feed Speed at which rolls rotate
Size of finished product Uniformity of feeding
Capacity of roll crusher:
Q = CWS x 3600 x η cum/hr.
Q = Capacity in cum/hr.
C = roll setting in metre
W = width of rolls in metre
S = surface speed of roll in m/sec = 2πRN
η = efficiency
Or Q =
𝐶𝑊𝑆𝜂
1728
𝑡𝑝ℎ
Type of roll crusher:
i. Both roll shells smooth
ii. Double toothed roll crusher
iii. One smooth and on corrugated shell
iv. Corrugated rolls
v. Spiked roll crusher
i. Both roll shells smooth:
• These are used when required size of finished
product is less than 12 mm.
• These are widely used in quarrying industries,
mining, ceramic and metallurgical industries.
• Out of the two rolls one roll is fixed and other is
floating.
• Each roll is individually driven by 3-phase electric
motor via v-belt.
• The gap can be adjusted to suit the size of the
material.
ii. Double toothed roll crusher:
• These are generally used for the production of
material over 25 mm size and for reduction of
coarse soft and medium hard, non-abrasive
materials like coal, slag, ore, limestone etc.
• The principle of working is the same as that of
smooth roll crusher.
iii. One smooth and one corrugated shell:
• These are used for materials of medium hard
qualities.
• They are used for producing the material between
12 to 25 mm size.
Fig. One smooth and one corrugated shell
iv. Corrugated rolls:
• The size of corrugation (pitch and depth) is
related to the product size, and hence the smaller
the product, the finer is the corrugation.
• These can take bigger feed size than plain rolls of
the same diameter.
v. Spiked roll crusher:
• These can take large feed size, even a slab of 1.2
m x 1.5 m x0.3 m size.
• These have one spiked roll and one adjustable
jaw and are used for coarse crushing of material
like coal.
(e) Impact crusher:
• Impact crushers are used where materials are too
hard and abrasive and finished product requirement
is for cubical shape well graded material.
• In this type of crusher, stones are broken by means
of impact and not by compression as in jaw crusher.
• Stone is normally fed into the top of a breaking
chamber and struck by rotating hammers while it is
in suspension.
• The hammers repeatedly throw the material against
breaker bars (repulsion plates) forming the roof of
the breaker chamber.
• Hammers are fixed to the rotor.
Advantages:
• High reduction ratio
• Effective production
fines
of a high percentage of
• Excellent shape i.e. free from flake
• High output per unit energy
• Better mechanical strength
• Lower initial cost
• Have fewer parts to maintain
• Produce well graded cubical shaped product
31
Points to be kept in mind while selecting
crusher:
1. Type of rock 2. Size of rock pieces
3. Method of feeding
rocks in the crusher
4. Required capacity of
the plant
32
Fig. Portable – aggregate processing plant flow dia.
Crushing plant:
• Crushing plants with capacities upto 1000 tons
per hour are being manufactured by various
manufacturers all over the world.
• These plants are either stationary or portable.
Crushing plant
Primary crushing units
e.g. Jaw crusher Impact crusher
Reduces the rocks to sizes
Between 75 to 250 mm
Secondary crushing units
e.g. Jaw crusher Cone crusher
Impact crusher
Reduces the rocks to sizes
Between 5 to 50 mm
36
2. Ball mill and rod mill – special processing units
for fine aggregate production
• A rod mill consists of an iron cylindrical shell with
hard mineral coating on the inner surface.
• A number of iron rods are kept inside the cylinder.
• The length of rods is kept slightly smaller than the
length of cylinder.
• Stone pieces are fed from inlet and sand size fine
aggregates are obtained at the other end.
• The mill can be operated in dry or wet condition.
• The size of mill can be expressed in terms of
diameter and length of the cylinder, e.g. 2.5 m x 3.5
m.
• If steel balls are charged in the cylinder instead of
iron rods, it is called ball mill.
Fig. Section through a rod mill
Fig. Section through a ball mill
3. Feeders
Feeders supply gravel to crusher
To ensure uniform feeding and avoid surge feeding
(overloads crushers)
Types of feeders:
• Apron – A feeder constructed of overlapping pans that
form a continuous belt
• Reciprocating plate
• Vibratory – simple vibrating and vibrating grizzly feeders
• Belt – uses hopper to feed belt
• Grizzly – used over hoppers to remove oversize material
– series of bars
4. Screen:
• Screens are used to separate aggregates
according to different sizes.
• The size of aggregate is expressed as range,
e.g. passing 12 mm screen and retained on 6
mm screen.
Types of screen:
a. Revolving screens
b. Shaking screens
c. Vibrating screens
a. Revolving screens
• They are more advantageous when they are
used to wash and screen sand and gravel.
• The operating action is slow and simple.
• Maintenance and repair costs are low.
• If the aggregate to be washed contains silt and
clay, a scrubber can be installed near the
entrance of a screen to agitate the material in
water.
• At the same time, streams of water can be
sprayed on the aggregate.
b. Shaking
• In this type, the screen is kept inclined at 15̊ to
20̊ with horizontal.
• The screen is agitated in the direction of length
for a distance of about 15 cm at the rate of 100
strokes per minute.
• Fine material fall below the screen while coarse
material fall at the end of the screen.
c. Vibrating screens:
• This type of screens are more frequently used
for screening of aggregate.
• They consists of one or more layers or ‘decks’ of
open mesh wire cloth mounted one above the
other in a rectangular metal box.
• The vibration is obtained by means of an
eccentric shaft, a counterweight shaft or
electromagnets attached to the frame or to the
screens.
• A unit may be horizontal or inclined with slight
slope (20̊ or less) from the receiving to the
discharge end.
• The vibration, 850 to 1250 stokes per minute
causes the aggregate to flow over the surface of
the screen.
5. Log washer:
• When natural deposits of aggregate, such as
sand and gravel, or individual pieces of crushed
stone contain deleterious material, it will be
necessary to remove these materials before
using the aggregate.
• One method of removing the material is to pass
the aggregate through a machine called a “log
washer”.
• This unit consists of a steel tank with two electric
motor driven shafts, to which numerous
replaceable paddles are attached.
Log washer: (continued..)
• When the washer is erected in a plant, the
discharge end is raised.
• The aggregate to be processed is fed into the
unit at the lower end, while a constant supply of
water flows into the elevated end.
• As the shafts are rotated in opposite directions
the paddles move the aggregate toward the
upper end of the tank, while producing a
continuous scrubbing action between the
particles.
• The stream of water remove the undesirable
material and discharge it from the tank at the
lower end, whereas the processed aggregate
will be discharged at the upper end.
5. Aggregate handling
• Necessary to handle it carefully – to ensure
gradation – to avoid separation of large and
small aggregates – thereby destroying the blend
in sizes
• Transportation by belt conveyors shall not be
permitted – as the free fall from discharge end of
the belt causes segregation
• Rock ladder containing baffles attached to end
of belts can be adopted to prevent segregation
• Transported using truck (will discuss in detail)
Crushers to screens in aggregate production

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Crushers to screens in aggregate production

  • 2. Amount of processing required for aggregate production depends on • Nature of raw materials • Desired attributes of the end products Four procedures adopted: 1. Particle size reduction – crushing 2. Separation into particle size ranges – screening / sizing 3. Elimination of undesired materials – washing 4. Handling and movement of crushed materials – storage and transport
  • 3. 3 EQUIPMENTS USED FOR PRODUCTION OF AGGREGETE 1. Crushers a. Jaw crusher b. Gyratory crusher c. Roll crusher d. Cone crusher e. Impact crusher 2. Ball mill and Rod mill (for Fine Aggregate) 3. Feeders 4. Screens a. Revolving screens b. Shaking screens c. Vibrating screens 5. Log washer 6. Aggregate handling
  • 4. 4 1. Crushers: • Crushers are used to break stones into different sizes of aggregates. • Based on the stage of crushing, crushers are three types: – Primary crusher – Secondary crusher – Tertiary crusher • Stones obtained from quarry are fed into the primary crusher which reduces the size of stones.
  • 5. 𝐑𝐞𝐝𝐮𝐜𝐭𝐢𝐨𝐧 𝐫𝐚𝐭𝐢𝐨 = 1. Crushers: (continued…) • Stones obtained from primary crusher are fed into secondary crusher where sizes of stone is further reduced. • Therefore, to obtain desired size of aggregate, the stones are passed through 4 to 5 crushers. size of stone fed into the crusher size of stone obtained from crusher 5 (Ratio of crusher feed size to product size)
  • 6. 6 (a) Jaw Crushers: • It has two jaws, one is stationary and other is moving. • The surface of both jaw is flat/convex. • The space between two jaws is wider at top and narrow at the bottom. • The size of jaw is 1.68 x 2.14 m. • The stones are crushed by the pressure of two jaws. • Large size boulders can also be crushed using this crushers. • Power consumption is minimum.
  • 7. Fig. Single jaw crusher 7
  • 8. Fig. Double jaw crusher 8
  • 9. 9 Single toggle jaw crusher Double toggle jaw crusher (Blake type) (i) In a single toggle jaw crusher whole swing jaw is lifted, hence require higher power. (i) Require lesser power. (ii) Jaw plates wear out at faster rate. (ii) Jaw plates wear out at slower rate. (iii) Lower initial cost. (iii) Higher initial cost. (iv) Used for non-abrasive and non hard rocks. (iv) Economical for abrasive and hard rocks. (v) Compact size, lighter weight and reasonably sturdy (v) Heavy, not for portable applications
  • 10. 10 Size of jaw crushers and jaw opening • Mouth opening of the jaws denotes the crusher size. Thus, a jaw crusher having a jaw plate width of 1200 mm and mouth opening/ feed opening of 1000 mm is called a 1200 x 1000 mm crusher. • The jaw opening at the top determines the size of the rock. • The size of stone piece is restricted to 70 to 80 % of the gap size. Thus, for 1200 x 1000 mm crusher the feed size should be restricted to 700 to 800 mm (70 to 80% of 1000 mm). • The capacity of the crusher is dependent on the area of the opening at the bottom of the jaws.
  • 11. Fig. Crusher jaw setting and jaw opening 11
  • 12. 12 Wear limits: • With use, the teeth of the jaws are worn out. • When teeth plates gets worn out to about 1/3 of the original corrugation height the jaw plates must be turned end-to-end. • Wear limit dead line is when there remained no corrugation. • When the entire crushing surface gets worn out, the teeth plate should be replaced with new one.
  • 13. Fig. Wear limit of tooth plate 13
  • 14. 14 Tips for optimum use of jaw crushers: • The feed size should be 70 to 80% of the minimum opening size at top. Do not allow bigger pieces going to the crusher mouth. • Material fed should not be of same size. Mixed feed of all sizes of material gives best results. • For best results ratio of gap to jaw setting should be 5:1 to 7:1. Gap is the longer distance between the two jaws. • The ratio of gap to width of jaws is 1:1.6 in primary crushers and 1:4.5 in the secondary crushers.
  • 15. 15 (b) Gyratory crusher: • Most efficient of all primary type crushers • A gyrating mantle mounted within a deep bowl – provides continuous crushing action • For hard, rough, abrasive rock • To protect the crusher, it can be spring loaded type or mantle height can be hydraulically adjusted • A cone or head is fitted at the top for grinding known as mantle • Cone shaped crushing chamber known as concave • Stones are broken by the impact of the head. • The frame of equipment is strong. • Head is made of iron alloy
  • 16.
  • 17. (c) Cone crusher: • Cone crusher are used for second or third grinding. • Large quantities of uniform size aggregates are obtained. • A head of manganese iron is fitted on the vertical shaft, which act as grinding surface. • The second surface is concave. • The rotation of shaft produces gyratory effect which causes grinding of stones. • Difference of cone crusher from gyratory crushers: shorter cone, smaller receiver opening, rotates at a higher speed, more uniformly sized output
  • 19. Salient features of Gyratory and cone crushers: • As moving parts are small in number, wear, tear and maintenance is less. • Longer life • Dust has little effect. • Hydraulic system enables finger tip control of the machine. • Wear is uniform and adjustment is simple. • Cleaning by hydraulic system enables rapid restart after a blockage during operation. • These have devices to measure oil pressure, oil temperature and oil circulation. In case of abnormal condition there may be sound or visual alarm.
  • 20. 20 (d) Roll crusher: • This equipment consists of heavy iron frame with two hard steel rolls. Two rolls are fitted on different horizontal shafts. • They can be rotated by v-belt or flat pulley. • Stones are broken into small pieces between roll and grinding plate. • The equipment is light in weight and is suitable for second grinding.
  • 21. 21 (c) Roll crusher: (continued..) Feed size: • The maximum size of material that can be fed to a roll crusher is directly proportional to the diameter of the rolls. • If the feed contains stones that are too large, the rolls will not grip the material and pull it through the crusher.
  • 22. Feed size: (continued..) The angle of grip, which is constant for smooth rolls, has been found to be 16̊ 45’. The maximum size particles that can be crushed are determined as follows. R = radius of rolls B = angle of grip (16̊ 45’) D = R cos B = 0.9575 R A = maximum feed size C = roll setting (size of finished product) X = R – D A = C + 2X = R – 0.9575 R = C + 2 (0.0425 R) = 0.0425 R Prof. Ashis = C + 0.085 R h Makwana 19
  • 23. Capacity of roll crusher: The capacity of roll crusher depends upon: Type of stone Width of rolls Size of feed Speed at which rolls rotate Size of finished product Uniformity of feeding Capacity of roll crusher: Q = CWS x 3600 x η cum/hr. Q = Capacity in cum/hr. C = roll setting in metre W = width of rolls in metre S = surface speed of roll in m/sec = 2πRN η = efficiency Or Q = 𝐶𝑊𝑆𝜂 1728 𝑡𝑝ℎ
  • 24. Type of roll crusher: i. Both roll shells smooth ii. Double toothed roll crusher iii. One smooth and on corrugated shell iv. Corrugated rolls v. Spiked roll crusher
  • 25. i. Both roll shells smooth: • These are used when required size of finished product is less than 12 mm. • These are widely used in quarrying industries, mining, ceramic and metallurgical industries. • Out of the two rolls one roll is fixed and other is floating. • Each roll is individually driven by 3-phase electric motor via v-belt. • The gap can be adjusted to suit the size of the material.
  • 26. ii. Double toothed roll crusher: • These are generally used for the production of material over 25 mm size and for reduction of coarse soft and medium hard, non-abrasive materials like coal, slag, ore, limestone etc. • The principle of working is the same as that of smooth roll crusher. iii. One smooth and one corrugated shell: • These are used for materials of medium hard qualities. • They are used for producing the material between 12 to 25 mm size.
  • 27. Fig. One smooth and one corrugated shell
  • 28. iv. Corrugated rolls: • The size of corrugation (pitch and depth) is related to the product size, and hence the smaller the product, the finer is the corrugation. • These can take bigger feed size than plain rolls of the same diameter. v. Spiked roll crusher: • These can take large feed size, even a slab of 1.2 m x 1.5 m x0.3 m size. • These have one spiked roll and one adjustable jaw and are used for coarse crushing of material like coal.
  • 29. (e) Impact crusher: • Impact crushers are used where materials are too hard and abrasive and finished product requirement is for cubical shape well graded material. • In this type of crusher, stones are broken by means of impact and not by compression as in jaw crusher. • Stone is normally fed into the top of a breaking chamber and struck by rotating hammers while it is in suspension. • The hammers repeatedly throw the material against breaker bars (repulsion plates) forming the roof of the breaker chamber. • Hammers are fixed to the rotor.
  • 30. Advantages: • High reduction ratio • Effective production fines of a high percentage of • Excellent shape i.e. free from flake • High output per unit energy • Better mechanical strength • Lower initial cost • Have fewer parts to maintain • Produce well graded cubical shaped product
  • 31. 31
  • 32. Points to be kept in mind while selecting crusher: 1. Type of rock 2. Size of rock pieces 3. Method of feeding rocks in the crusher 4. Required capacity of the plant 32
  • 33.
  • 34. Fig. Portable – aggregate processing plant flow dia.
  • 35. Crushing plant: • Crushing plants with capacities upto 1000 tons per hour are being manufactured by various manufacturers all over the world. • These plants are either stationary or portable. Crushing plant Primary crushing units e.g. Jaw crusher Impact crusher Reduces the rocks to sizes Between 75 to 250 mm Secondary crushing units e.g. Jaw crusher Cone crusher Impact crusher Reduces the rocks to sizes Between 5 to 50 mm
  • 36. 36
  • 37. 2. Ball mill and rod mill – special processing units for fine aggregate production • A rod mill consists of an iron cylindrical shell with hard mineral coating on the inner surface. • A number of iron rods are kept inside the cylinder. • The length of rods is kept slightly smaller than the length of cylinder. • Stone pieces are fed from inlet and sand size fine aggregates are obtained at the other end. • The mill can be operated in dry or wet condition. • The size of mill can be expressed in terms of diameter and length of the cylinder, e.g. 2.5 m x 3.5 m. • If steel balls are charged in the cylinder instead of iron rods, it is called ball mill.
  • 38. Fig. Section through a rod mill
  • 39. Fig. Section through a ball mill
  • 40. 3. Feeders Feeders supply gravel to crusher To ensure uniform feeding and avoid surge feeding (overloads crushers) Types of feeders: • Apron – A feeder constructed of overlapping pans that form a continuous belt • Reciprocating plate • Vibratory – simple vibrating and vibrating grizzly feeders • Belt – uses hopper to feed belt • Grizzly – used over hoppers to remove oversize material – series of bars
  • 41. 4. Screen: • Screens are used to separate aggregates according to different sizes. • The size of aggregate is expressed as range, e.g. passing 12 mm screen and retained on 6 mm screen. Types of screen: a. Revolving screens b. Shaking screens c. Vibrating screens
  • 42. a. Revolving screens • They are more advantageous when they are used to wash and screen sand and gravel. • The operating action is slow and simple. • Maintenance and repair costs are low. • If the aggregate to be washed contains silt and clay, a scrubber can be installed near the entrance of a screen to agitate the material in water. • At the same time, streams of water can be sprayed on the aggregate.
  • 43. b. Shaking • In this type, the screen is kept inclined at 15̊ to 20̊ with horizontal. • The screen is agitated in the direction of length for a distance of about 15 cm at the rate of 100 strokes per minute. • Fine material fall below the screen while coarse material fall at the end of the screen.
  • 44. c. Vibrating screens: • This type of screens are more frequently used for screening of aggregate. • They consists of one or more layers or ‘decks’ of open mesh wire cloth mounted one above the other in a rectangular metal box. • The vibration is obtained by means of an eccentric shaft, a counterweight shaft or electromagnets attached to the frame or to the screens. • A unit may be horizontal or inclined with slight slope (20̊ or less) from the receiving to the discharge end. • The vibration, 850 to 1250 stokes per minute causes the aggregate to flow over the surface of the screen.
  • 45. 5. Log washer: • When natural deposits of aggregate, such as sand and gravel, or individual pieces of crushed stone contain deleterious material, it will be necessary to remove these materials before using the aggregate. • One method of removing the material is to pass the aggregate through a machine called a “log washer”. • This unit consists of a steel tank with two electric motor driven shafts, to which numerous replaceable paddles are attached.
  • 46. Log washer: (continued..) • When the washer is erected in a plant, the discharge end is raised. • The aggregate to be processed is fed into the unit at the lower end, while a constant supply of water flows into the elevated end. • As the shafts are rotated in opposite directions the paddles move the aggregate toward the upper end of the tank, while producing a continuous scrubbing action between the particles. • The stream of water remove the undesirable material and discharge it from the tank at the lower end, whereas the processed aggregate will be discharged at the upper end.
  • 47. 5. Aggregate handling • Necessary to handle it carefully – to ensure gradation – to avoid separation of large and small aggregates – thereby destroying the blend in sizes • Transportation by belt conveyors shall not be permitted – as the free fall from discharge end of the belt causes segregation • Rock ladder containing baffles attached to end of belts can be adopted to prevent segregation • Transported using truck (will discuss in detail)