(assistant professor) mining engineeering department contents section 1: crushing exp. # 1)...

Instructor

Engr. Muhammad Shahzad (Assistant Professor)

Mining Engineeering Department

University of Engineering & Technology

Lahore

3

The practice of minerals processing is as

old as human civilization. Minerals and

products derived from minerals have formed

our development cultures from the flints of

the Stone Age man to the uranium ores of

Atomic Age.

METSO

4

Contents

Section 1: CRUSHING

Exp. # 1) “Machine Study of Laboratory Jaw Crusher and to perform a crushing test on the

given sample, and to analyze the product for reduction ratio”

Exp. # 2) “Machine Study of Laboratory Roll Crusher and to perform a crushing test on the

given sample, and to analyze size distribution in the product by sieve analysis”

Exp. # 3) “Machine Study of Laboratory Hammer Mill and to perform a crushing test on the


Section 2: GRINDING

Exp. # 4) “Machine Study of Laboratory Disc Mill and to perform a grinding test on the


Exp. # 5) “Machine Study of Laboratory Rod Mill and to perform a grinding test on the given

sample, and to analyze size distribution in the product by sieve analysis”

Exp. # 6) “Machine Study of Denver Laboratory Ball Mill and to perform a grinding test on

the given sample, and to analyze size distribution in the product by sieve analysis”

Section 3: SIZING

Exp. # 7) “Machine study of a double-deck vibratory screen & hummer electromagnetic

screen and determining their performance by a screening test on the given sample”.

Exp. # 8) “Machine study of a Laboratory centrifugal hydro classifier and to find its cut size

under the given conditions”.

Section 4: GRAVITY CONCENTRATION

Exp. # 9) “Machine Study of a Laboratory type Mineral/Coal Jig and to perform a gravity

separation test on the given sample”

Exp. # 10) “Machine Study of a Concentrating Shaking Table and to perform a gravity

separation test on the given sample”

Exp. # 11) “Machine Study of a Humphrey’s Spirals and to perform a gravity separation test

on the given sample” (Coal/Metallic Minerals)

Section 5: FROTH FLOTATION

Exp. # 12) “To study the principle and operation of Laboratory Flotation Machine, and to

perform a froth flotation test on a coal sample and to determine its separation efficiency”

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Experiment No. 1:

1.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory

Jaw crusher with special emphasis on their functions, to perform a crushing test on a given

sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed

size and product size measurement.

1.2. Apparatus/Materials

• Laboratory Jaw Crusher

• Vernier Caliper

• Rock sample for crushing

• Sieve set (26.67, 18.85, 13.33, 6.70, 2.36 and 0.85 mm)

• Ro-Tap Sieve Shaker

• Torsion Balance

1.3. Procedure

• Study each part of the machine and know the function of every component

• Switch on the machine and study the movement of the moving jaw and the variation

of set with motion.

• Measure the side of the gape and adjust the set with the help of a lead lump and meter

rod or coarse sieve and record measurements.

• Examine the feed, measure the largest lump either with a meter rod or coarse sieve as

the case may be and record it.

• Feed the machine and crush the entire sample.

• Perform sieve analysis on the product by using coarse sieve set as given in 1.2.

• Calculate the reduction ratio of the machine.

1.4. Specifications of Laboratory Jaw Crusher

The specifications of laboratory jaw crusher are given in Table 1.1.

Table 1.1: Specifications of Laboratory Jaw Crusher

Name Denver Blake Jaw Crusher

Motor 5hp

Motor RPM 1440

Crusher RPM 325 – 375

Face Of Flywheel 3 ¼ “

Movable Jaw Depth 14”

Fix Jaw Depth 12”

Width Of Jaw Plate 6”

8

Crusher Designed Capacity 600 lbs/hr

Flywheel Diameter 18”

Max: Feed Size (Gape) 5”×6”

Size Of Set close ½ʺ open 1 ¼ ʺ

1.5. Observations and Calculations

1.5.1. Feed Size

Sr. No. Length (mm) Width (mm) Height (mm)

1.5.2. Product Sieve Analysis

Sr.

No.

Sieve Aperture Size Individual Mass Cumulative Mass

Percentage

Passing Retaining Geometric mean Measured Percentage Passing Retaining

1

2

3

4

5

6

7

1.5.3. Reduction ratio

Determine the reduction ratio by the following relation:

𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜80 = 80% 𝑃𝑎𝑠𝑠𝑖𝑛𝑔 𝐹𝑒𝑒𝑑 𝑆𝑖𝑧𝑒

80% 𝑃𝑎𝑠𝑠𝑖𝑛𝑔 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝑆𝑖𝑧𝑒

1.6. Graphs

1. Draw graph of cumulative passing and retaining mass percentage against aperture size

(geometric mean) and determine cut size, d10, d25, d50, and d75.

2. Draw log-normal plot between aperture size (geometric mean) and cumulative passing

mass percentage and determine the standard deviation.

3. Express Gaudin-Schuhmann distribution on graph and determine the constants

involved.

4. Express Rosin-Rammler distribution on graph and determine the constants involved.

*Note: Read Topic 2.2. “Particle Size Distribution” in “Mineral Processing Design and Operation” by A. Gupta

and D. S. Yan

1.7. Discussions

Discuss the results and the information deducted from sieve analysis in details.

1.8. Conclusions

Give concluding remarks about the experiment and its results.

9

1.9. Related Theory

[References: Mineral Processing Technology by B. A. Wills (7th Edition-2005) & Mineral Processing Design

and Operation by A. Gupta and D. S. Yan (1st Edition-2006)]

1.9.1. Introduction

Jaw crushers are used as primary crushers, or the first step in the process of reducing rock.

They crush primarily by using compression.

The distinctive feature of this class of crusher is the two plates which open and shut like

animal jaws. The jaws are set at an acute angle to each other and one jaw is pivoted so that it

swings relative to the other fixed jaw. Material fed into the jaws is alternately nipped and

released to fall further into the crushing chamber. Eventually it falls from the discharge

aperture.

1.9.2. Types

There are three basic types of jaw crusher as shown in Figure 1.1.

Figure 1.1:1 Types of Jaw Crushers

1.9.2.1. Blake Type Jaw Crusher

The Blake crusher was patented by Eli Whitney Blake in 1858. The swing jaw is fixed at the

upper position. The Blake type jaw crusher has a fixed feed area and a variable discharge

area.

A. Double Toggle Blake Crusher

In the double toggle jaw crushers, the oscillating motion of the swing jaw is caused by the

vertical motion of the pitman. The pitman moves up and down. The swing jaw closes, i.e., it

moves towards the fixed jaw when the pitman moves upward and opens during the downward

motion of the pitman. This type is commonly used in mines due to its ability to crush tough

and abrasive materials.

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B. Single Toggle Blake Crusher

In the single toggle jaw crushers, the swing jaw is suspended on the eccentric shaft which

leads to a much more compact design. The swing jaw, suspended on the eccentric, undergoes

two types of motion- swing motion towards the fixed jaw due to the action of toggle plate and

vertical movement due the rotation of the eccentric. These two motions, when combined, lead

to an elliptical jaw motion. This motion is useful as it assists in pushing the particles through

the crushing chamber. This phenomena leads to higher capacity of the single toggle jaw

crushers but it also results in higher wear of the crushing jaws. These type of jaw crushers are

preferred for the crushing of softer particles. A comparison between two can also be seen in

Figure 1.2. It shows the difference in the movement and the method of operation.

Figure 1.2: Comparison between Single and Double Toggle Blake Type

1.9.2.2. Dodge Type Jaw Crusher

In the Dodge type jaw crushers, the jaws are farther apart at the top than at the bottom,

forming 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. The Dodge jaw

crusher has a variable feed area and a fixed discharge area which leads to choking of the

crusher and hence is used only for laboratory purposes and not for heavy duty operations.

1.9.2.3. Universal Jaw Crusher

This jaw crusher continuously reduces material as it passes through the crushing chamber

with its aggressive force feed action as the moveable jaw compresses inward and downward.

The sharp primary blow at the top of the chamber reduces material instantly, while a

secondary crushing action at the bottom further reduces material to the predetermined output

size. Universal Jaw Crushers offer a compressive stroke that is nearly equal at both the top

and bottom of the chamber, producing more spec material at a lower cost per ton.

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1.9.3. Construction

Main Frame

The main frame is often made from cast iron or steel, connected with tie-bolts.

It is often made in sections so that it can be transported underground for installation.

Fully stress relieved after fabrication.

Jaws

The jaws are usually constructed from cast steel and are fitted with replaceable manganese

steel liners, which are bolted in sections on to the jaws so that they can be removed easily and

reversed periodically to equalize wear. One jaw is move able and other one is fixed.

Cheek Plates

These are fitted to the sides of the crushing chamber to protect the side main frame from

wear. These are also made from manganese steel and have the similar life to the jaw plates.

Flywheel

Rotational energy is fed into the jaw crusher eccentric shaft by means of a sheave pulley

which usually has multiple v-belt grooves.

Heavy fly wheel attached to the drive which is necessary to store energy on the idling half of

the stroke and deliver it on the crushing half. And maintain inertia.

Toggle

The toggle rolls across the flat pressure face of the toggle seat. No rubbing or scuffing takes

place and friction is kept to a minimum. This type of toggle system has the following

advantages over the socket end type toggle and seat:

• No lubrication whatsoever is required

• The system can handle far greater crushing pressures.

• The life factor of toggle and seats is many times greater.

Jaw-holder & Main Bearing Housings

Can be removed from the frame as an assembly. The jaw-holder is a robust box construction

with a fully machined face to support the moving jaw.

Spring Load

It is a safety device and in the event of an uncrushable lump entering the gape. The movable

jaw is pushed to the limit by compressing the spring and the un-crushable lump is passed out

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without causing mechanical damage to the machine. In industry we use greening screen to

prevent sticky and wet material.

Figure 1.3: Labelled Diagram of Jaw Crusher

1.9.4. Working

A ‘Toggle and Pitman’ is the popular mechanism, driving the Blake jaw crusher. It normally

comprises of a pitman working on an eccentric on the crusher shaft and two toggles travelling

and traversing like a ‘birds wings’, one between the main frame and the pitman and the other

between the pitman and the moveable jaw, so that with the rotation of the shaft the pitman is

translated up and down whereby the distance between the two jaws is increased intermittently

and crushing effected. But our laboratory model, which is typically a Blake jaw crusher, has

a modified drive mechanism called ‘single toggle mechanism’; it has no pitman separately

but the purpose of pitman is served by the moveable jaw itself. Instead of the two long

toggles it has one small toggle which rests on steel bearings, at one end on back body of the

moveable jaw and at the other end on a vertically slid able wedge block beside the main

frame.

It can be moved back & forward depending upon product required (e.g. if moved back then

the product will be coarser). The feed opening of the jaw crusher is called the ‘gape’ and the

discharge opening is called ‘set’. The moveable jaw is spring loaded and connected to a

screw mechanism which helps in adjusting the set. Spring loading is a safety device and in

the event of an uncrushable lump entering the gape. The moveable jaw is pushed to the limit

by compressing the spring and the uncrushable lump is passed out without causing

mechanical damage to the machine.

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Figure 1.4: Jaw Crusher present in Mineral Processing Lab

1.9.5. Related Terminology

Gape: The feed opening of the jaw crusher is called gape, which is the distance between the

jaws at the feed opening & which is given as 5'' × 6'' (Max size of feed)

Set: The maximum opening of the jaws at the discharge end is called set. The discharge size

of the material from the crusher is controlled by set. This can be adjusted by using toggle

plates of the required length.

Throw: The jaw is pivoted from above; it moves a minimum distance at the entry point and a

maximum distance at the delivery. This maximum distance is called the throw of the crusher.

Free Crushing: In free crushing, no accumulation of the material takes place in the crusher.

Choke Crushing: In choke crushing, accumulation of the material takes place in the crusher.

Reduction Ratio: Ratio of Size of Feed to Size of Output.

Feed Material: Such a material which is introduced in the crusher for crushing purpose is

called the feed material. It should be 80% to 90% of the gape and it should also be a uniform

size.

Product Material: Such material which is discharge from the set after crushing is called

product. The size of the product can be adjusted by adjusting the size of the set.

1.9.6. Parameters

1.9.6.1. Capacity

The capacity of the jaw crusher available in the laboratory is 725 t/h. Jaw crushers range in

size up to 1680 mm gape by 2130 mm width. This size machine will handle ore with a

maximum size of 1.22 m at a crushing rate of approximately 725th -~ with a 203mm set.

However, at crushing rates above 545th -1 the economic advantage of the jaw crusher over

the gyratory diminishes; and above 725th -1 jaw crushers cannot compete with gyratory

crushers.

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1.9.6.2. Speed

The speed of jaw crushers varies inversely with the size, and usually lies in the range of 100-

350revmin -1. 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.

1.9.6.3. Amplitude

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.

1.9.7. Applications

Jaw crusher perform better on clayey, plastic material: due to their greater throw. Jaw

Crusher is used in Cement Raw Crushing. It can also be used in Mining industry along with

recycling of the concrete.

1.9.8. Limitations

Jaw crusher is applicable to feed size up to 1m & giving a product of about 10-20 cm in size.

Jaw crusher is not suitable for hard and abrasive material.

It is an 180o machine which means that it works 90o and rests 90o. Some of the lumps can

pass without being crushed. Sticky material cannot be crushed in this crusher.

1.9.9. Specifications of Industrial Models

The industrial model of jaw crushers are more powerful and of larger dimensions than the

laboratory scale and therefore Table 1.2 expresses the specifications of the industrial model

of Jaw Crushers.

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Table 1.2: Yuhong Jaw Crusher Specification

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Experiment No. 2:


Roll crusher with special emphasis on their functions, to perform a crushing test on a given




• Laboratory Roll Crusher


• Sieve set (9.42, 6.70, 4.75, 2.36, 0.85 and 0.30 mm)


• Torsion Balance

2.3. Procedure


• Switch on the machine and study the movement of the moving rolls.

• Adjust the set to 10 mm by measuring distance between the rolls at the line joining

their centers.

• Examine the feed, measure the feed size by using a set of sieve and determine 80%

passing feed size by plotting graph between cumulative passing and geometric mean

of passing and retaining size.


• Perform sieve analysis on the product by using sieve set as mentioned in 2.2.


2.4. Specifications of Laboratory Roll Crusher

Table 2.1: Specifications of Laboratory Roll Crusher

Name of Machine Denver Roll Crusher

Diameter of roll 10 inch

Roll RPM 250-300

Motor Power 8 hp

Set Max. 30 mm, Min. 1 mm

Material of rolls Cast steel

Material of sheet Hard Manganese steel

Face 6 inch

Capacity of roll crusher 2 ton/hr

17


2.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7





2.6. Graph






involved.



and D. S. Yan

2.7. Discussions


2.8. Conclusions

Give concluding remarks about the experiment and its results.

18

2.9. Related Theory



2.9.1. Introduction

Roll crushers are secondary crushers machine. This type of crusher are characterized by

production of smaller properties of fines and also by smaller reduction ratios. Roll crushers

are operated dry or wet. Roll crushers are still used in mills, although they have been replaced

in most installations by cone crushers.

2.9.2. Types of Roll Crushers

There are four basic types of roll crushers as shown in Figure 2.1.

Figure 2.1: Types of Roll Crushers

2.9.2.1. Single Roll Crushers

Single Roll Crushers are primary crushers that provide a crushing ratio of up to 6:1. They

reduce materials such as ROM coal, mine refuse, shale, slate, gypsum, bauxite, salt, soft

shale, etc., from large size particles to a medium size, while producing minimal fines.

Designed with an interrupted opening between the roll teeth and corresponding grooves in the

crushing plate liners, they are also extremely effective in reducing slabby materials. Figure

2.2 will illustrate how the Single Roll Crusher looks like.

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Figure 2.2: Single Roll Crusher

2.9.2.2. Double Roll Crushers

Double Roll Crushers provide a 4:1 reduction ratio. They are typically used as a secondary or

tertiary crusher for materials such as ROM coal with refuse, limestone, gypsum, trona, shale,

bauxite, oil shale, clean coal, coke, salt, quicklime, burnt lime, glass, kaolin, brick, shale, and

wet, sticky feeds. Each machine is custom engineered with roll elements and tooth patterns

selected depending on each unique application to produce a cubical product with minimal

fines.

Figure 2.3: Double Roll Crusher

2.9.2.3. Triple Roll Crushers

Triple Roll Crushers are ideal for producers who want to accomplish two stages of reduction

in one pass. They can be used in coal, salt, coke, glass, and trona operations, among others.

Triple Roll Crushers combine a Single Roll Crusher with a Double Roll Crusher to form a

crusher that is capable of achieving a 6:1 reduction ratio in the primary stage and a 4:1

reduction in the secondary stage while producing a cubicle product at high capacity.

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Figure 2.4: Working Mechanism Triple Roll Crushers

2.9.2.4. Quad Roll Crushers

Quad Roll Crushers are ideal for producers, including those with preparation plants, who

want to accomplish two stages of reduction in one pass. They can be used in coal, salt, lime,

pet coke and potash operations, among others. Quad Roll Crushers are capable of achieving a

4:1 reduction ratio before dropping crushed material to the secondary stage crusher for an

additional 4:1 reduction to make the final product. These are utilized on various scales on the

industry level according to the need of the output.

Figure 2.5:2 Quad Roll Crusher

There are other types of roll crushers based on the type of roll surface and design. These are

given in Figure 2.6.

Figure 3 Types of Rolls

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2.9.2.5. Smooth Surface Rolls

A. These rolls are usually used for fine crushing.

B. Wear on the roll surface is very high, and they often have manganese steel tire, which

can be replaced when worn.

2.9.2.6. Slugger Rolls

Coarse crushing is often performed in rolls having corrugated surfaces, or with stub teeth

arranged to present a cheered surface pattern.

2.9.2.7. High Pressure Grinding Rolls

A. HPGR consists of a pair of counter rotating rolls, one fixed and the other floating.

B. Material is fed between the rolls with the floating roll pressing against the material

flow by means of hydraulic pressure in excess of 50 M Pa.

C. The resulting force causes the material to compact by inter-particle breakage.

D. Pressures and roll speeds are adjusted to obtain optimum grinding conditions.

E. The roll faces are typically studded because of improved wear characteristics.

2.9.3. Construction

A rolls crusher essentially consists of two steel rolls revolving towards each other, both

driven, with their very robust bearings mounted in a strong steel frame. The bearings of one

of the rolls are spring loaded while those of the other connected to a screw mechanism.

Spring loading serves as a safety device against uncrushable lumps whereas the screw

mechanism provides ‘Set’ adjustment for different product sizes. The rolls are made of cast

steel but a shell of Hard Manganese Steel is shrunk over them which resist powerful wear

during operation.

Figure 2.7: Labelled Diagram of Roller Crusher

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Crushing Rolls: The rolls are made up of cast steel. A roll crusher essentially consists of

steel rolls revolving towards each other. The bearings of one of the rolls are spring loaded

while those of other connected to a screw mechanism. Spring loading serves as a safety

device.

Set: The set is determined by shims which causes the spring-loaded roll to be hold back from

the solidly mounted roll.

Figure 2.8: Roll Crusher Present in Lab

2.9.4. Working Mechanism

A. The particles are drawn into the gap between the rolls by their rotating motion and a

friction angle formed between the rolls and the particle, called the nip angle.

B. The two rolls force the particle between their rotating surface into the ever smaller

gap area, and it fractures from the compressive forces presented by the rotating rolls.

C. Some major advantages of roll crushers are they give a very fine product size

distribution and they produce very little dust or fines.


Angle of Nip: The largest angle that will just grip a lump between the jaws, rolls, or mantle

and ring of a crusher.

Reduction Ratio: Ratio of Size of Feed to Size of Output.

2.9.6. Parameters Effecting the Performance

Angle of Nip: Consider a spherical particle of radius “r” being crushed by a pair of rolls of

radius “R” then for a particle to be just gripped by rolls equating:

C sin (θ/2) = µ C cos (θ/2)

µ = tan (θ/2)

The coefficient of friction b/w steel and most ore particles is in the range of 0.2-0.3 so that

the value of range “θ” should never exceed about 30 degrees, or the particle will slip. In the

Figure 8, θ represents the Nip Angle.

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Figure 2.9: Representation of Nip Angle

Speed of Roll: Speed of rolls depends on the angle of nip and the type of material being

crushed. The larger the angle of nip, the slower the peripheral speed to be allow the particle

to be nipped and vice versa. Peripheral speeds vary b/w about 1m/s for small rolls upto about

15m/s for 1800mm dia rolls. The value of “µ” b/w a particle and roll is given by:

µk = 1+1.12 ν / 1+6 ν

Size of Roll:

Cos (θ/2) = R+a/R+r

Above equation is used to determine the maximum size of rock gripped in relation to roll

diameter and reduction ratio (r/a) is required.

Capacity: The capacity of rolls can be calculated in terms of ribbon of material that will pass

the space b/w rolls. Thus

Capacity = 188.5 NDWsd (Kg/h)

2.9.7. Applications

Roll crusher is used in Mining industry to crush the coal or other rocks. It is utilized on large

scale industry too. This is a secondary crushing tool. Roll Crushers are best suited for

controlled reduction of friable materials to granules where fines are undesirable in

applications such as aggregates, ceramics, chemicals, minerals and sintered metals.

2.9.8. Limitations

The machine unit mass production capacity is low. Moreover, it covers a large area. The roll

surface grinding damage uneven, need often repair. The great disadvantage of roll crushers is

that, in order for reasonable reduction ratios to be achieved, very large rolls are required in

relation to the size of the feed particles. They therefore have the highest capital cost of all

crushers.

24


Table 2.2: Sturtevant Roll Crusher Specifications

Table 1: Kurimoto Roll Crusher Specifications

25

Experiment No. 3:


Impact crusher with special emphasis on their functions, to perform a crushing test on a given




• Laboratory Impact Crusher


• Sieve set (4.75, 3.35, 2.00, 1.00, 0.50, and 0.25 mm)


• Torsion Balance

3.3. Procedure


• Switch on the machine and study the movement of the moving hammers.

• Examine the feed, measure the feed size by using a set of sieve and determine 80%

passing feed size by plotting graph between cumulative passing and geometric mean

of passing and retaining size.


• Perform sieve analysis on the product by using sieve set as mentioned in 3.2.


3.4. Specifications of Laboratory Impact Crusher

Table 3.1: Specifications of Laboratory Impact Crusher

Name of Machine Hammer Mill

Motor Power 5 hp

Motor RPM 1420

Mill RPM 2130

Motor Pulley 18 cm

Mill Pulley 12 cm

Grate opening 1 cm

Full swing diameter of shaft and hammer 35 cm

Number of hammers 8 x 4 = 32

Capacity 200 t/h

26


3.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7





3.6. Graph






involved.



and D. S. Yan

3.7. Discussions


3.8. Conclusions

Give concluding remarks of the experiment and its results.

27

3.9. Related Theory

[References: Mineral Processing Technology by B. A. Wills (7th Edition-2005)]

3.9.1. Introduction & Mode of Operation

In this class of crusher, comminution is by impact rather than compression, by

sharp blows applied at high speed to free-falling rock. The moving parts are beaters,

which transfer some of their kinetic energy to the ore particles on contacting them. 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.

3.9.2. Applications

There is an important difference between the states of materials crushed by

pressure and by impact. There are internal stresses in material broken by pressure which

can later cause cracking. Impact causes immediate fracture with no residual stresses. This

stress-free condition is particularly valuable in stone used for brick-making, building,

and roadmaking, in which binding agents, such as bitumen, are subsequently added to

the surface.

Impact crushers, therefore, have a wider use in the quarrying industry than in

the metal-mining industry. They may give trouble-free crushing on ores that tend to be

plastic and pack when the crushing forces are applied slowly, as is the case in jaw

and gyratory crushers. These types of ore tend to be brittle when the crushing force

is applied instantaneously by impact crushers.

Impact crushers are also favoured in the quarry industry because of the

improved product shape. Cone crushers tend to produce more elongated particles

because of their high reduction ratios and ability of such particles to pass through the

chamber unbroken. In an impact crusher, all particles are subjected to impact and the

elongated particles, having a lower strength due to their thinner cross section, would

be broken.

3.9.3. Construction & Working

Figure 3.1 shows a cross-section through a typical hammer mill. The hammers are made

from manganese steel or, more recently, 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 that they can move out of the path of oversize

material, or tramp metal, entering the crushing chamber. Pivoted hammers exert less

force than they would if rigidly attached, so they tend to be used on smaller impact

crushers or for crushing soft material. The exit from the mill is perforated, so that material

28

which is not broken to the required size is retained and swept up again by the rotor

for further impacting.

Figure 3.1: Hammer mill

This type of machine is designed to give the particles velocities of the order

of that of the hammers. Fracture is either due to the severity of impact with the

hammers or to the subsequent impact with the casing or grid. Since the particles are

given very high velocities, much of the size reduction is by attrition, i.e. breaking of

particle on particle, and this leads to little control on product size and a much higher

proportion of fines than with compressive crushers.

3.9.4. Parameters of Impact 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 3000 rev min-1. 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 non-abrasive materials. They have extensive use in limestone

quarrying and in the crushing of coal. A great advantage in quarrying is in the fact that they

produce a very good cubic product.

Large impact crushers will reduce 1.5 m top size run-of-mine ore to 20 cm, at

capacities of around 1500 th-1, although crushers with capacities of 3000 th-1 have been

manufactured. 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

29

containing over 15% silica (Lewis et al., 1976). However, they are a good choice for

primary crushing when high reduction ratios are required (the ratio can be as high as

40:1) and a high percentage of fines, and the ore is relatively non- abrasive.

3.9.5. Types of Hammer Mills

Fixed Hammer Mill: - For much coarser crushing, the fixed hammer impact mill is

often used (Figure 6.26). In these machines the material falls tangentially on to a

rotor, running at 250-500revmin -1, receiving a glancing impulse, which sends it

spinning towards the impact plates. The velocity imparted is deliberately restricted to a

fraction of the velocity of the rotor to avoid enormous stress and probable failure of

the rotor bearings.

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.

Figure 3.2: Impact mill

Rotary Hammer Mill: - 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. The blow bars are reversible to even out wear, and can

easily be removed and replaced.

31

Experiment No. 4:


Disc mill with special emphasis on their functions, to perform a crushing test on a given




• Laboratory Disc mill


• Sieve set (0.85, 0.60, 0.425, 0.30, 0.212 and 0.15 mm)


• Torsion Balance

4.3. Procedure

• Identify each part of the machine.

• Switch on the machine and study the working of each part.

• Note the rpm of machine with the help of a tachometer.

• Examine the feed for its size range and record the average maximum size in the

feed.

• Adjust the sat for fine crushing.

• Feed the material slowly and check the size of the product.

• Make the adjustment of set, if necessary.

• Switch off the machine and recover the product.

• Transfer the ground material to a sieve set and sieve for 10 minutes.

• Switch off the sieve shaker and recover the retained weights for each sieve.

• Calculated the reduction ratio of the machine for the test performed.

• Tabulate the sieve test and plot a graph on a suitable graph paper.

4.4. Specifications of Laboratory Disc Mill Table 4.1: Specifications of Laboratory Disc Mill

Grinding Mill Disc Mill

Motor power 5 hp

Motor r.p.m 1800 r.p.m

Disc r.p.m 275 r.p.m

Size of Disc 9 ½ inch (241.3 mm)

Max: feed size ¼ inch (6.35 mm)

Capacity 2 lbs/min

No. of grooves on moving disc 6

No. of grooves on stationary disc 5

32


4.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7





4.6. Graph






involved.



and D. S. Yan

4.7. Discussions


4.8. Conclusions


33

4.9. Related Theory

4.9.1. Introduction

A disc mill, is a type of crusher that can be used to grind, cut, shear, shred, fiberize,

pulverize, granulate, crack, rub, curl, fluff, twist, hull, blend, or refine. It works in a similar

manner to the ancient Buhrstone mill in that the feedstock is fed between opposing discs or

plates. The discs may be grooved, serrated, or spiked. Disc mill can be used for secondary or

fine crushing and also grinding. The disc mill idea originated from hand flour mill.

Figure 4.1: A Disc Mill

4.9.2. Types of Disc Mill

Disc Mill consists of three types. Those are also shown in the Figure 4.2.

Figure 4.2: Types of Disc Mill

4.9.2.1. Single Wheel Disc Mill

This disc mill utilizes only one disc which spins along the base in order to crush the feed.

4.9.2.2. Double Wheel Disc Mill

This disc mill utilizes two interconnect discs in order to grind the feed.

34

4.9.2.3. Vibrating Disc Mill

This disc mill utilizes high speed vibration to separate the items after they have been crushed

or ground.

4.9.3. Construction

Disc mill consists of the following parts and components.

Figure 4.3: Basic Parts of Disc Mill

4.9.3.1. Feed Hopper

At this part of the disc mill, the material to be grinded is inserted into the machine. It has a

specific size and feed should be processed at least through the primary crushers. The feed size

is not large.

4.9.3.2. Grinding Discs

A disc mill consists of two saucer shaped discs with their surface having specially shaped

grooves the depth of which reduced towards the circumference. The discs are face to face

mounted vertically or horizontally and revolve at the different speeds and in the opposite

directions. In most designs one of the discs is driven while the other is not. Also one of the

discs is rather strongly fixed while the other flutters or gyrates during revolving. Our

laboratory model has heat treated meehanite metal discs mounted vertically, one revolving in

the planetary manner always having a proper curvature with relation to the other which is

stationary. Like other crushing machines, a disc mill has not of the discs spring loaded

through a screw mechanism that helps in adjusting the set and also provides safety against

uncrushable lumps.

35

4.9.3.3. Discharge Vessel

At this point of the machine, the feed which was inserted comes out after being grinded.

4.9.4. Working

The material is fed through a hopper at the top and falls into the axial conic between the discs

during revolving. Due to the centrifugal force, the feed is pushed through the tapper grooves.

Towards the periphery and gets grind progressively. The product is finally discharged

peripherally and collected in a peripheral receptacle. Like other crushing machines a disc mill

has not of the discs spring-loaded through a screw mechanism that helps in adjusting the set

and also provides safety against un-crushable lumps. Disc mill is a type of attrition mill in

which two surfaces rotate pass each other at high speeds with close tolerance.

4.9.5. Related Terminology A) Centrifugal Force: This reaction force is sometimes described as a centrifugal inertial

reaction, that is, a force that is centrifugally directed, which is a reactive force equal and

opposite to the centripetal force that is curving the path of the mass.

B) Periphery: The outer limits or edge of an area or object.

C) Feed Material: Such a material which is introduced in the grinder for crushing purpose is

called the feed material.

4.9.6. Parameters Affecting Performance

4.9.6.1. Feed Material

The type of the feed introduced in the disc mill will affect the performance of the mill. In

case of hard and abrasive material the disc will undergo severe abrasive forces and may

distort. So this factor plays important role in controlling performance of disc mill.

4.9.6.2. Distance between Two Discs

The distance between two disc plates will also be an important parameter in determining the

performance of the disc mill. If the distance is large, then large, coarse material will be output

of the mill while the feed size will be same. So efficiency can increase as mill will grind feed

more quickly and in less time.

4.9.7. Applications

Disc Mills are popular tools for agricultural applications, where they are used for milling corn

and grains after harvest. Disc mills are also used in food and chemical processing, and to

crush stone and metal products. Disc mills may be used in mining operations to separate

minerals and other valuable elements from the surrounding rock. They are also widely used

in recycling plants for grinding paper, plastics and other reusable materials.

4.9.8. Limitations

Disc mills are relatively expensive to run and maintain however, and tend to require frequent

maintenance. Discs may experience wear over time as they grind various materials, which

can reduce performance. The machines also produce a large amount of dust, and must be

carefully ventilated when used in an indoor workspace.

36


Below (Table 28) we can see the various specifications of an industrial model disc

Table 4.2: JXCS Disc Mill Specifications

Figure 4.4: Disk Mill Present in Laboratory

37

Experiment No. 5:


Rod mill with special emphasis on their functions, to perform a crushing test on a given




• Laboratory Rod mill


• Sieve set (0.85, 0.60, 0.425, 0.30, 0.212 and 0.15 mm)


• Torsion Balance

5.3. Procedure

• Switch of the machine and study each part of machine.

• Note the RPM of machine with tachometer

• Examine the feed for its size range and record the average size of largest lump in

the feed. Note the total weight of feed.

• Load the mill cylinder with its feed sample and its rod load.

• Switch on and run the machine for 30 minutes and then recover the ground

product.

• Transfer the ground material to the set of sieve by consolation and sieve for 20

minutes.

• Switch off the sieve shaker and record the retained weight of each sieve.

• Note the weights of the individual sieve and of the base pan.

• Calculate the reduction of the machine for the test performed.

• Tabulate the sieve test and plot a graph on a suitable graph sheet.

5.4. Specifications of Laboratory Rod Mill Table 5.1: Specifications of Laboratory Rod Mill

Name Rod Mill

R.P.M Of Mill 160

Motor Power ½ Hp

Motor R.P.M 710

Cylinder Depth 12.25 Inches

Rod Material Hard Carbon Steel

Weight Of Each Rod 1220 Grams

Size Of Rod 12”×1”

No. Of Rods 3

38


5.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7





5.6. Graph






involved.



and D. S. Yan

5.7. Discussions


5.8. Conclusions


39

Types of Mill

Tumbling Mill

Rod Mill

Ball Mill

Stirred Mill

Horizontal Mill

Vertical MillVibrating

Mill

5.9. Related Theory



5.9.1. Introduction

Grinding is the second step of mineral processing and the last stage of the comminution

process. The product from a crushing unit is fed to a mill in order to decrease the particle size

(sometimes even to 10 microns) for subsequent processing. The purpose of grinding differs

with the material being ground.

5.9.2. Introduction of Grinding Mills

In materials processing a grinder is a machine for producing fine particle size reduction

through attrition and compressive forces at the grain size level. A mill is a device that breaks

solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an

important unit operation in many processes. The grinding of solid matters occurs under

exposure of mechanical forces that trench the structure by overcoming of the interior bonding

forces. After the grinding the state of the solid is changed: the grain size, the grain size

disposition and the grain shape.

5.9.3. Types of Mills

According to the ways by which motion is imparted to the charge, grinding mills are

generally classified into three types: tumbling mills, stirred mills, and vibrating mills.

Figure 5.1: Types of Mills

5.9.3.1. Tumbling Mill

In this mill, 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 coarse-grinding processes, in which particles between 5

and 250 mm are reduced in size to between 40 and 300 microns.

40

Figure 5.2: Types of Mills

5.9.4. Introduction of Rod Mill

Rod Mills are considered as either fine crushers or coarse grinding machines. They are

capable of taking maximum size of 50mm and produce a product of 300µm. Its distinctive

feature is that the length of the cylindrical shell is between the 1.5 and 2.5 times its diameter.

Reduction ratio is in between the range of 15-20:1.

5.9.5. Types of Rod Mill

Rod mill are classified on the basis of their discharge. The types of Rod Mills are as follows:

Figure 5.3: Types of Rod Mill

5.9.5.1. Centre Peripheral Discharge Mill

These are feed at both ends through the trunions and discharge the ground product through

circumferential ports at the center of the shell. Short path and steep gradient gives a coarse

grind with minimum of fines, but the reduction ratio is limited.

5.9.5.2. End Peripheral Discharge Mill

Material is fed from one end and discharge ground product from 2nd end by means of several

peripheral apertures into a close fitting circumferential chute. This is used for dry and damp

grinding where moderately coarse products are involved.

Rod Mill

Centre Peripheral Discharge Mill

End Peripheral Discharge

Overflow Rod Mill

41

Figure 5.4: Centre Peripheral Discharge Mill

Figure 5.5: End Peripheral Discharge Mill

5.9.5.3. Overflow Rod Mill

This is most widely used rod mill in the mining industry, in which feed is introduced through

one trunions and discharge from other. This is only used for wet grinding. A flow gradient is

provided by making the overflow Trunion diameter 10-20 cm larger than that of feed

opening.

Figure 5.6: Overflow Rod Mill

5.9.6. Construction

From the trunnion liner out wards first we will come to the face plate. It is slightly concave

to create the pooling area for the rock to collect in before entry to the rod -load. On the

outside attached to the face plate is the bull gear. This gear completely circles the mill and

provides the interface between the motor and the mil. The bull gear and drive line may be the

other end of the mill instead. The face plate, attached to the other side of the face plate is the

shell. The shell is the body of the mill. On the inside of the mill there are two layers of

material. The first layer is the backing for the liners. This is customarily constructed from

rubber but wood may be used as well. The purpose of this backing is two-fold. One to absorb

the shock that is transmitted through normal running. And to provide the shell with a

protective covering to eliminate the abrasion that is produced by the finely ground rock and

water. Without this rubber or wood backing, the life of the mill is drastically reduced due to

metal fatigue and simply being warn away.

On top of this backing is the liners themselves. There are many different patterns and types

of liners depending upon the job they are doing and the design of the mill. The trunnion liner

may also be referred to as the throat liner. Next to this liner is the end liners. The filler ring

which is next is not standard in all mills, some mills have them, and some don't. Their job is

to fill the corner of the mill up so the shell will not wear at that point. They don't provide any

lift to the media, in fact quite often the media will not come into contact with them at all, but

42

what they do is make changing liners that much easier. With different liner designs the

replacement of a single liner may be quite difficult and to change one could become a lengthy

project. The liner that butts into the filler liner is known as a shell liner. These liners and/or

lifters give the media its cascading action and also receive the most wear. They cover the

complete body of the mill and have the largest selection of types to choose from.

As the two ends of the mill are the same there isn't any reason to go over the other face plate.

The discharge trunnion assembly is very much like the feed trunnion except that, it won't

have a worm as part of the liner. Instead of a feed seal bolted to it, it may have a screen. This

is called a trummel screen and its purpose is to screen out any rock that didn't get ground.

Figure 5.7: Labelled Diagram of Rod Mill

5.9.7. Working

When the mill is rotated without feed or with very fine feed, the rods are in parallel alignment

and in contact with one another for their full length. New feed entering at one end of the mill

causes the rod charge to spread at that end. This produces a series of wedge shaped slots

tapering toward the discharge end. The tumbling and rolling rods expend most of their

crushing force on the coarse fractions of the feed material and only to a lesser degree on the

finer material filling the interstices in the rod charge. The horizontal progression of material

through the mill is not rapid compared to the movement of the rods and material resulting

from rotation of the mill. The average particle is subjected to an action similar to many sets of

rolls in series, before it is discharged.

43


• Critical Velocity: The "Critical Speed" for a grinding mill is defined as the rotational

speed where centrifugal forces equal gravitational forces at the mill shells inside

surface.

• Cascading: It is the rolling down the surface of the load.

• Cataracting: It is the parabolic free fall above the mass.

• Feed Material: Such a material which is introduced in the crusher for crushing

purpose is called the feed material.


5.9.9.1. Media Size Effect

Finer media were found to be more efficient for fine particle grinding. Several media sizes

were tested in the Pilot Tower mill. By decreasing the media size from 12 to 6.8 mm the size

reduction achieved in the mill was greatly increased. Further decreasing the media size to 4.8

mm caused mill efficiency to deteriorate.

5.9.9.2. Stirrer Speed Effect

In addition to specific energy input, and the grinding media size, the stirrer tip speed can

affect the grinding results. It can be seen that in both cases higher stirrer speed had a positive

effect on grinding efficiency. For the coarser media this effect was stronger in the coarser

product size range. For the finer media the stirrer speed effect was small over the range of

speeds tested.

5.9.9.3. Slurry Density Effects

The results from the tests performed to investigate the slurry density effect have shown that

the mill grinding efficiency increased with slurry % solids over the range tested. The increase

in grinding efficiency at higher % solids can be explained by a drop in power draw due to

buoyancy effects.

5.9.10. Applications

The rod mill, a tumbling mill characterized by the use of rods as grinding media, grinds ores,

coal/coke, and other materials for both wet and dry applications. The rod mill accepts feed

ore as coarse as 1 1/2” top size although better performance is obtained by restricting ore feed

size to 3/4”. Product sizes range from 4 mesh to 16 mesh operating in open circuit, or as fine

as 35 mesh operating in closed circuit with a screen or other sizing device. The steel rod takes

regular movement in mill. It is convenient to install and maintain. It rapidly discharges.

Figure 5.8: Rod Mill Present in Lab

44


Table 5.2: Specifications of Industrial ACX Rod Mill

45

Experiment No. 6:


Ball mill with special emphasis on their functions, to perform a crushing test on a given




• Laboratory Ball mill


• Sieve set (0.85, 0.60, 0.425, 0.30, 0.212 and 0.15 mm)


• Torsion Balance

6.3. Procedure

• Study the machine and its each part.

• Switch on the machine and study the function of each part.

• Examine the feed its size range and recover the average maximum size in it.

• Feed the material slowly.

• Switch off the machine and recover the product and weight it.

• Transfer the product to a set of sieves and sieve for 20 minutes.

• Switch off sieve-shaker and recover the retained weight on each sieve.

• Record the retained weight for each sieve.

• Calculate the reduction ratio of the machine for the test performed.

• Tabulate the sieve results and plot graph on a suitable graph paper

6.4. Specifications of Laboratory Ball Mill Table 6.1: Specifications of Laboratory Ball Mill

Name of the Machine Ball Mill

R.P.M of Mill 42

Motor Power ¼ hp (Gear Reducer)

Motor R.P.M 1425

Capacity 4 kg/hr

Material of Ball Alloy Steel

Drum Material Cast Iron

Total Weight of Balls 3550 gram

46


6.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7





6.6. Graph






involved.



and D. S. Yan

6.7. Discussions


6.8. Conclusions


47

6.9. Related Theory



6.9.1. Introduction of Ball Mill

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. The term ball is restricted to

those having a length to diameter ratio of 1.5 to 1 and less. They are often used dry to grind

cement, gypsum and phosphate.

Figure 6.1: Working Mechanism of Ball Mill

6.9.2. Types of Ball Mill

Ball Mills are classified to by the nature of discharge. We have three types of Ball Mills.

Figure 6.2: Types of Ball Mills

6.9.2.1. Overflow Type

The overflow type of Ball Mill is designed to overflow and discharge materials from the

trunnion on the outlet side. By combining it with a mechanical classifier or wet-processing

cyclone, you are able to extensively use this type for grinding in closed circuit or for special

applications such as re-grinding in open circuit. Generally, it is best suited to fine-grind

materials up to the particle sizes ranging from 150 to 200 mesh.

6.9.2.2. Grate-Discharge Type

The grate-discharge type of Ball Mill has a grate at the outlet of the shell and causes less

excessive grinding, compared to the overflow type. Therefore, generally, it is best suited to

grind materials up to the particle sizes ranging from 60 to 100 mesh.

Ball Mills

Overflow Type

Grate-Discharge Type

Compartment Type

48

Figure 6.3: Labelled Diagram of Overflow Type

Figure 6.4: Labelled Diagram of Grate-Discharge Type

6.9.2.3. Compartment Type

The compartment type of Ball Mill has a longer shell, inside of which is comparted into 2 to

3 chambers with grates and is best suited to produce products grinding from coarse particles

of some 25 mm to fine particles of some 200 mesh.

Figure 6.5: Labelled Diagram of Compartment Type

49

6.9.3. Construction

6.9.3.1. Shell

Mill shells are design to sustain impact and having loading and are constructed from rolled

mild steel plates but welded together.

6.9.3.2. Mill Ends

The mill ends may be of nodular or grey cast iron for diameter less than about 1m, larger

heads are constructed from cast steel which is relatively light and can be welded.

6.9.3.3. Trunnions & Bearings

Trunnions are of same type as that of rod mills. They are highly polished to reduce bearing

frictions. Similarly, oil lubricant bearings is favored in large mils, via motor driven oil

pumps.

6.9.3.4. Liners

Ball mill ends usually have ribs to lift the charge with mill rotation. These prevent excessive

slipping and increase liner life. They can made from white cast iron, alloyed with Ni- hard

and rubber.

6.9.3.5. Drum Feeders

The entire mill feed enters the drum via a chute a spout and an internal spiral carries it into

the trunnion liner. The drum also provides a convenient methods of adding grinding balls to

mill.

Figure 4: Labelled Diagram of Bill Mill

6.9.4. Working

The steps involved in the working process are as follows:

50

Figure 6.7: Flowchart of Working in Ball Mill

6.9.4.1. Initial Stage

The powder particles are get flattened by the collision of the balls. It leads it changes in the

shapes of individual particles or cluster of particles being impacted repeatedly by the milling

balls with high kinetic energy.

6.9.4.2. Intermediate Stage

Significant changes occur in comparison with those in the initial stage.

6.9.4.3. Final stage

Reduction in particle size takes place. The microstructure of the particle also appears to be

more homogenous in microscopic scale than those at the initial and intermediate stages.

6.9.4.4. Completion stage

The powder particles possess an extremely deformed metastable structure. The speed of the

rotation is more important. By this way the maximum size reduction is effected by the impact

of particles between the balls and by attrition between the balls. After the suitable time the

material is taken out and passed through a sieve to get powder of the required size.

Figure 6.8: Ball Mill present in Lab


• Critical Velocity: The "Critical Speed" for a grinding mill is defined as the rotational

speed where centrifugal forces equal gravitational forces at the mill shells inside surface.

51

• Cascading: It is the rolling down the surface of the load.

• Cataracting: It is the parabolic free fall above the mass.

• Height attained: The maximum height up to which the particles go along the mill

shell and then get thrown off and follow a parabolic path.

• Feed Material: Such a material which is introduced in the crusher for crushing

purpose is called the feed material.


6.9.6.1. Pulp Density

The pulp density of the feed should be as high as possible. It is essential that the bals are

coated with layer force; to dilute a pulp increase metal-to-metal contact, giving increase steel

consumption and reduced efficiency. Ball mill should operate b/w 65-80% solids by weight,

depending on the ore.

6.9.6.2. Surface Area of Medium

Balls should be as small as possible and the charge should be graded such that the largest

balls are just heavy enough to grind the largest and hardest particle in feed. The correct ratio

of ball size to ore size is often determined by trial and error, primary grinding usually

requiring a grade charge of 10-15 cm diameter balls, while secondary grinding requires 5-

2cm.

6.9.6.3. Charge Volume

The charge volume is about 40-50% of the internal volume of mill about 40% of this being

void space. The energy input to the mil increases with the ball charge and reaches a

maximum at a charge volume of approx.50%, but for a number of reasons, 40-50% is rarely

exceeded. The efficiency curve is in any case quite flat about maximum.

6.9.6.4. Speed

The optimum mill speed increases with the charge volume, as increased weight of charge

reduces the amount of cataracting taking place. Ball mills are often operated at higher speed

than rod mills, so that the larger balls cataract and impact on the ore particles. The work input

to a mill increases in proportion to the speed and the ball mills are run at as high a speed as is

possible.

6.9.7. Applications

Ball mills are used extensively in the mechanical alloying process. The ball mill is a key

equipment to grind the crushed materials and cement, silicate, new type building material,

refractory material, fertilizer, ore dressing of ferrous metal and non-ferrous metal, glass

ceramics, etc. In mineral, cement, refractory, chemical industry the ball mill is mainly used to

grind materials etc. The ball mill is used for grinding materials such as coal, pigments, and

feldspar for pottery. Ball Mill is widely used in metal and nonmetal mines, building materials

and other industrial sector

6.9.8. Limitations

The ball mill is a very noisy machine. Wear occurs from the balls as well as from the casing

which may result in contamination of the product. It has low working efficiency. Large total

weight is of hundreds of tons, so one must be very great investment. Too much large sized

particles requires greater investment in the reduction process

52

6.9.9. Specifications of Industrial Models Table 6.22: Ball Mill of Industrial Level Specifications

Table 6.3: DAVE Ball Mill Specifications

54

Experiment No. 7:

7.1. Objectives The objective of this experiment is to study “Denver Dillon Vibrating Screen (double

deck” & “Junior Hummer Vibrating Screen (single deck)” and to perform a screening test on

the given sample by using these screens. It also includes the determination of the

performance of both screens by two product and three product formulas and the comparison

of their efficiencies.


• Laboratory Denver Dillon Vibrating Screen (Double Deck)

• Laboratory Junior Hummer Screen (Single Deck)

• Meter rod

• Feed sample

• Torsion / Electrical balance

• Sieve Shaker

• Set of Sieves

7.3. Procedure

• Identify each part of the both machines

• Make 2 representative samples of the given feed by the method of ‘coning and

quartering’ or the rifflers.

• Use sieves of same aperture size as that of screens for each feed and shake them for

30 minutes in sieve shaker. Collect the product from each sieve and measure their

respective weights. Determine the percentage of oversize, middling and undersize

material.

• Properly mix the products of each shaking operation separately and then feed it to the

respective vibrating screen while it is running.

• Switch off the machine and collect the overflow and the underflow products.

• Weigh the products and calculate weigh percentages.

• Perform sieve analysis on each product separately by using the same sieves as used

for the feed and determine the percentage of oversize and undersize material in each

product.

7.4. Specifications of Laboratory Vibrating Screens Table 7.1: Specifications of Laboratory Screens

Feed Size 4 mesh to 48 mesh

Angle of screen 15o

Screen Size Junior Hummer Screen Denver Vibrating Screen

10 mesh 10 mesh and 20 mesh

55


7.5.1. Sieve Analysis of Feed

Sieve Size Denver Vibrating Screen Hummer Vibrating Screen

Measured Wt. Percentage Measured Wt. Percentage

10 mesh

20 mesh

Pan

7.5.2. Screening test results

Aperture Size Denver Vibrating Screen Hummer Vibrating Screen


10 mesh

20 mesh

Pan

7.5.3. Sieve analysis of Products

Product

Name Sieve Size

Denver Vibrating Screen Hummer Vibrating Screen


Overflow

10 mesh

20 mesh

Pan

Middling

10 mesh

20 mesh

Pan

Underflow

10 mesh

20 mesh

Pan

7.5.4. Determination of Screen Efficiency

Derive an expression for determination of performance of each screen by using mass

balancing techniques and determine the efficiency of each screen.

7.6. Discussions

Compare the results of both screening tests and discuss them in detail.

7.7. Conclusions


7.8. Related Theory



7.8.1. Introduction of Sizing

With size control we understand the process of separating solids into two or more products on

basis of their size. This can be done dry or wet.

7.8.2. Types of Sizing

In mineral processing practices we have two methods dominating size control processes as

shown in Figure 1.

56

Figure 7.1: Types of Sizing Processes

7.8.2.1. Screening

Screening is done by using a geometrical pattern for size control. It is the practice of taking

granulated ore material and separating it into multiple grades by particle size.

7.8.2.2. Classification

Classification is done by using particle motion for size control. It is the method of separating

mixtures of minerals into two or more products on the basis of the velocity with which the

grains fall through a fluid medium.

7.8.3. Screening

7.8.3.1. Introduction

Mechanical screening, often just called screening, is the practice of taking granulated ore

material and separating it into multiple grades by particle size. Industrial sizing is extensively

used for size separations from 300 mm down to around 40 µm, although the efficiency

decreases rapidly with fineness.

We utilize screening process for number of the reasons. Some of them are mentioned below.

Sizing or Classifying: This is to separate particles by size, usually to provide a downstream

unit process with the particle size range suited to that unit operation.

Scalping: This is to remove the coarsest size fractions in the feed material, usually so that

they can be crushed or removed from the process.

Grading: This is to prepare a number of products within specified size ranges. This is

important in quarrying and iron ore, where the final product size is an important part of the

specification.

Media Recovery: Media recovery is for washing magnetic media from ore in dense medium

circuits.

Dewatering: This is to drain free moisture from a wet sand slurry.

Desliming: This is to remove fine material, generally below 0.5 mm from a wet or dry feed.

Trash removal: This is usually to remove wood fibers from a fine slurry stream.

7.8.3.2. Types of Screening processes

There are two types of screening processes as shown in Figure 2.

57

Figure 7.2: Types of Screening

Wet Screening: The addition of water to a screen to increase its capacity and improve its

sizing efficiency. Wet screening down to around 250 µm is common. Wet screening is shown

in Figure 7.3(A).

Dry Screening: The screening of solid materials of different sizes without the aid of water.

Dry screening is generally limited to material above about 5 mm in size. Dry screening is

shown in Figure 7.3(B).

Figure 7.3: (A) Wet screening; (B) Dry screening

7.8.3.3. Types of Screening

There are a number of types of mechanical screening equipment that cause segregation.

These types are based on the motion of the machine through its motor drive as shown in

Figure 7.4.

Figure 7.4: Types of Screening

58

Vibrating Equipment: This type of equipment has an eccentric shaft that causes the frame

of the shaker to lurch at a given angle. This lurching action literally throws the material

forward and up. As the machine returns to its base state the material falls by gravity to

physically lower level. This type of screening is used also in mining operations for large

material with sizes that range from six inches to +20 mesh.

High Frequency Vibrating Equipment: This type of equipment drives the screen cloth

only. Unlike above the frame of the equipment is fixed and only the screen vibrates.

However, this equipment is similar to the above such that it still throws material off of it and

allows the particles to cascade down the screen cloth. These screens are for sizes smaller than

1/8 of an inch to +150 mesh.

Gyratory Equipment: This type of equipment differs from the above two such that the

machine gyrates in a circular motion at a near level plane at low angles. The drive is an

eccentric gear box or eccentric weights.

Trommel Screens: This screen do not require vibrations, instead, material is fed into a

horizontal rotating drum with screen panels around the diameter of the drum.

7.8.3.4. Construction

The standard unit is a single-shaft, double-bearing unit constructed with a sieving box, mesh,

vibration exciter and damper spring. The screen framing is steel side plates and cross-

members that brace static and dynamic forces. At the center of the side plates, two roller

bearings with counterweights are connected to run the drive. Four sets of springs are fixed on

the base of the unit to overcome the lengthwise or crosswise tension from sieves and panels

and to dampen movement. An external vibration exciter (motor) is mounted on the lateral

(side) plate of the screen box with a cylindrical eccentric shaft and stroke adjustment unit. At

the screen outlet, the flows are changed in direction, usually to 90 degrees or alternate

directions, which reduces the exiting stream speed. Strong, ring-grooved lock bolts connect

components.

Variations in this design regard the positioning of the vibration components. One alternative

is top mounted vibration, in which the vibrators are attached to the top of the unit frame and

produce an elliptical stroke. This decreases efficiency in favor of increased capacity by

increasing the rotational speed, which is required for rough screening procedures where a

high flow rate must be maintained.

A refinement adds a counter-flow top mounting vibration, in which the sieving is more

efficient because the material bed is deeper and the material stays on the screen for a longer

time. It is employed in processes where higher separation efficiency per pass is required.

A dust hood or enclosure can be added to handle particularly loose particles. Water sprays

may be attached above the top deck and the separation can be converted into a wet screening

process.

59

Figure 7.55: Labelled Diagram of Vibrating Screen

7.8.3.5. Working

A screening machine consist of a drive that induces vibration, a screen media that causes

particle separation, and a deck which holds the screen media and the drive and is the mode of

transport for the vibration. There are physical factors that makes screening practical. For

example, vibration, g force, bed density, and material shape all facilitate the rate or cut.

Electrostatic forces can also hinder screening efficiency in way of water attraction causing

sticking or plugging, or very dry material generate a charge that causes it to attract to the

screen itself. As with any industrial process there is a group of terms that identify and define

what screening is. Terms like blinding, contamination, frequency, amplitude, and others

describe the basic characteristics of screening, and those characteristics in turn shape the

overall method of dry or wet screening. In addition, the way a deck is vibrated differentiates

screens.

7.8.3.6. Related Terminology

Blinding: When material plugs into the open slots of the screen cloth and inhibits

overflowing material from falling through.

Brushing: This procedure is performed by an operator who uses a brush to brush over the

screen cloth to dislodged blinded opening.

Contamination: This is unwanted material in a given grade. This occurs when there is

oversize or fine size material relative to the cut or grade. Another type of contamination is

foreign body contamination.

Deck: A deck is frame or apparatus that holds the screen cloth in place.

Screen Media: It is the material defined by mesh size, which can be made of any type of

material such steel, stainless steel, rubber compounds, polyurethane, brass, etc.

60

Shaker: A generic term that refers to the whole assembly of any type mechanical screening

machine.

Mesh: Mesh refers to the number of open slots per linear inch.

7.8.3.7. Parameters Affecting Performance

The parameters which affect the performance are as follows:

Feed Rate: The principle of sieve sizing analysis is to use a low feed rate and a very long

screening time to effect an almost complete separation. In industrial screening practice,

economics dictate that relatively high feed rates and short particle dwell times on the screen

should be used. At these high feed rates, a thick bed of material is presented to the screen,

and fines must travel to the bottom of the particle bed before they have an opportunity to pass

through the screen surface. The net effect is reduced efficiency. High capacity and high

efficiency are often opposing requirements for any given separation, and a compromise is

necessary to achieve the optimum result.

Screen Angle: If a particle approaches the aperture at a shallow angle, it will "see" a

narrower effective aperture dimension and near mesh particles are less likely to pass. The

slope of the screening surface affects the angle at which particles are presented to the screen

apertures. Some screens utilize this effect to achieve separations significantly finer than the

screen aperture. Where screening efficiency is important, horizontal screens are selected. The

screen angle also affects the speed at which particles are conveyed along the screen, and

therefore the dwell time on the screen and the number of opportunities particles have of

passing the screen surface.

Particle Shape: Most granular materials processed on screens are non-spherical. While

spherical particles pass with equal probability in any orientation, irregular-shaped near-mesh

particles must orient themselves in an attitude that permits them to pass. Elongated and

slabby particles will present a small cross-section for passage in some orientations and a large

cross-section in others. The extreme particle shapes therefore have a low screening

efficiency.

Open Area: The chance of passing through the aperture is proportional to the percentage of

open area in the screen material, which is defined as the ratio of the net area of the apertures

to the whole area of the screening surface. The smaller the area occupied by the screen deck

construction material, the greater the chance of a particle reaching an aperture. Open area

generally decreases with the fineness of the screen aperture. In order to increase the open area

of a fine screen, very thin and fragile wires or deck construction must be used. This fragility

and the low throughput capacity are the main reasons for classifiers replacing screens at fine

aperture sizes.

Vibration Screens: Vibration Screens are vibrated in order to throw particles off the

screening surface so that they can again be presented to the screen, and to convey the

particles along the screen. The fight type of vibration also induces stratification of the feed

material (Figure 7), which allows the fines to work through the layer of particles to the screen

surface while causing larger particles to rise to the top. Stratification tends to increase the rate

of passage in the middle section of the screen.

61

Moisture: The amount of surface moisture present in the feed has a marked effect on

screening efficiency, as does the presence of clays and other sticky materials. Damp feeds

screen very poorly as they tend to agglomerate and "blind" the screen apertures. As a rule of

thumb, screening at less than around 5 mm aperture size must be performed on perfectly dry

or wet material, unless special measures are taken to prevent blinding. These measures may

include using heated decks to break the surface tension of water between the screen wire and

particles, ball-decks (a wire cage containing balls directly below the screening surface) to

impart additional vibration to the underside of the screen cloth, or the use of non-blinding

screen cloth weaves. Wet screening allows finer sizes to be processed efficiently down to

250µm and finer. Adherent fines are washed off large particles, and the screen is cleaned by

the flow of pulp and additional water sprays.

Figure 7.6: Stratification of Particles on Screen

7.8.3.8. Applications

The high frequency vibrating screens achieves a high efficiency of separation and differs

from its counterparts since it breaks down the surface tension between particles. Also the

high level of RPMs contributes to increasing the stratification of material so they separate at a

much higher rate. Separation cannot take place without stratification. Furthermore, since the

screen vibrates vertically, there is a ‘popcorn effect’ whereby the coarser particles are lifted

higher and finer particles stay closer to the screen, thus increases the probability of

separation. In some high frequency vibrating screens the flow rate of the feed can be

controlled, this is proportional to the ‘popcorn effect’; if the flow rate lowers, the effect is

also decreased.

7.8.3.9. Limitations

Limitations of the high frequency vibrating screen are that the fine screens are very fragile

and are susceptible to becoming blocked very easily. Over time the separation efficiency will

drop and the screen will need to be replaced.

7.8.3.10. Specifications of Industrial Screens

The industrial model screens have following specifications as shown in Table

62

Table 7.2: Keller's Industrial Screen

Table 7.3: Jeeves Industrial Screen

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8.1. Objectives The objective of this experiment is to study hydroclassifier and to perform a

classification test on the given sample. It also includes the determination of the cut size of

hydroclassifier and determine its separation performance.


• Laboratory hydroclassifier

• Feed sample

• Torsion / Electrical balance

• Sieve Shaker

• Set of Sieves (0.71, 0.50, 0.35, 0.21, 0.15, 0.105, 0.075, 0.044)

8.3. Procedure

• Identify each part of the machine and find out its function.

• Take a feed sample and perform sieve analysis by using the sieve set as given in 8.2.

• After measuring the weights retained on each sieve, mix the sample properly.

• Close the spigot valve at the bottom of the hydroclassifier and the fill the tank with

water by opening the valve of water supply connected to the hydroclassifier.

• When the tank is filled, open the spigot valve and adjust it to maintain a uniform

flowrate at overflow and underflow ends.

• Put collection lauders beneath the overflow and underflow tanks for collection of test

products.

• Pour the mixed sample at an appropriate uniform feed rate from the feed end at the

center position of the classifier.

• Collect the underflow and overflow products into the launders and kept them there

until all the solid particles settle down at the bottom.

• Decant the clear water from each launder very slowly so that no solid particle may go

away from the launder.

• Collect dewatered overflow and underflow products in separate trays and dry them in

oven at 110° C.

• Use sieves of same aperture size as that of screens for feed and perform sieve analysis

on each product separately.

• Determine the cumulative weights and draw graph between geometric mean of

passing and retained size of each fraction versus cumulative passing size. Determine

the cut size and performance of classifier.

8.4. Specifications of Laboratory Hydroclassifier Table 8.1: Specifications of Laboratory hydroclassifier

Name of Machine Centrifugal Hydroclassifier

Motor Power 0.25 HP

Motor RPM 500 – 2500

Size of classifier 9 inch

64


8.5.1. Feed Size

Sr.

No.


Percentage


1

2

3

4

5

6

7


Sr.

No.


Percentage


1

2

3

4

5

6

7

8.5.3. Cut size determination and performance measurement

8.6. Discussions

Compare the results of both screening tests and discuss them in detail.

8.7. Conclusions


8.8. Related Theory

[References: Mineral Processing Technology by B. A. Wills (7th Edition-2005)]

8.8.1. Introduction of Classification

Classification is a method of separating mixtures of minerals into two or more

products on the basis of the velocity with which the grains fall through a fluid medium. In

mineral processing, this is usually water, and wet classification is generally applied to

mineral particles which are considered too fine to be sorted efficiently by screening. Since the

velocity of particles in a fluid medium is dependent not only on the size, but also on the

specific gravity and shape of the particles, the principles of classification are important in

mineral separations utilizing gravity concentrators. Classifiers also strongly influence the

performance of grinding circuits.

65

8.8.2. Working Principle

The Laboratory Centrifugal Classifier embodies a new principle in classification. The feed is

introduced into a center well where it falls on a rotating impeller which forces the material

outward and upward. The sands settle through the upward current and are discharged at the

bottom spigot. The slimes rise and overflow around the rim of the classifier, while a portion

of the slimes is recirculated with the primary feed to help in washing the sand particles. The

velocity imparted to the pulp by the rotating impeller supplies the necessary rising current

without an excess of water. There are several adjustments for regulating the size of the

particles in the overflow and sand discharge. Recommended where sand grains are coated

with colloids and a very fine and uniform overflow product is desired without excessive

dilution, this unit also made in commercial sizes.

Figure 8.1: Laboratory Centrifugal Classifier

8.8.3. Types of classifier

Many different types of classifier have been designed and built. They may be

grouped, however, into two broad classes depending on the direction of flow of the carrying

current. Horizontal current classifiers such as mechanical classifiers are essentially of the

free-settling type and accentuate the sizing function; vertical current or hydraulic classifiers

are usually hindered-settling types and so increase the effect of density on the separation.

66

8.8.3.1. Hydraulic classifiers

These are characterized by the use of water additional to that of the feed pulp, introduced so

that its direction of flow opposes that of the settling particles. They normally consist of a

series of sorting columns through each of which a vertical current of water is rising and

particles are settling out. The rising currents are graded from a relatively high velocity in the

first sorting column, to a relatively low velocity in the last, so that a series of spigot products

can be obtained, with the coarser, denser particles in the first spigot and the fines in the latter

spigots. Very fine slimes overflow the final sorting column of the classifier.

8.2: Principle of Hydraulic Classifier

8.8.3.2. Mechanical classifiers

Mechanical classifiers have widespread use in closed-circuit grinding operations and

in the classification of products from ore-washing plants. In washing plants they act more or

less as sizing devices, as the particles are essentially unliberated, so are of similar density. In

closed circuit grinding they have a tendency to return small dense particles to the mill,

causing overgrinding. They have also been used to densify dense media. The principle of the

mechanical classifier is shown in Figure 8.3.

The pulp feed is introduced into the inclined trough and forms a settling pool in which

particles of high falling velocity quickly fall to the bottom of the trough. Above this coarse

sand is a quicksand zone where essentially hindered settling takes place. The depth and shape

of this zone depends on the classifier action and on the feed pulp density. Above the

quicksand is a zone of essentially free settling material, comprising a stream of pulp flowing

horizontally across the top of the quicksand zone from the feed inlet to the overflow weir,

where the fines are removed.

The settled sands are conveyed up the inclined trough by mechanical rakes or by a

helical screw. The conveying mechanism also serves to keep fine particles in suspension in

the pool by gentle agitation and when the sands leave the pool they are slowly turned over by

the raking action, thus releasing entrained slimes and water, increasing the efficiency of the

separation. Washing sprays are often directed on the emergent sands to wash the released

slimes back into the pool.

67

Figure 8.3: Mechanical Spiral Classifier

8.8.3.3. Hydro cyclone

It is widely used in closed-circuit grinding operations (Napier-Munn et al., 1996) but

has found many other uses, such as de-sliming, de-gritting, and thickening.

It has replaced mechanical classifiers in many applications, its advantages being simplicity

and high capacity relative to its size. A variant, the "water-only-cyclone", has been used for

the cleaning of fine coal (Osborne, 1985) and other minerals.

A typical hydrocyclone (Figure 9.13) consists of a conically shaped vessel, open at its

apex, or underflow, joined to a cylindrical section, which has a tangential feed inlet. The top

of the cylindrical section is closed with a plate through which passes an axially mounted

overflow pipe. The pipe is extended into the body of the cyclone by a short, removable

section known as the vortex finder, which prevents short-circuiting of feed directly into the

overflow.

Figure 8.4: Hydrocyclone

(assistant professor) mining engineeering department contents section 1: crushing exp. # 1)...

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