mill grinding

7/23/2019 Mill Grinding

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Introduction to Cement Manufacturing

Table of Contents

MILL GRINDING THEORY

1. Introduction and Overview............................................................... 3

1.1 Introduction to Grinding............................................................ 3

1.2 Grinding Mill Circuits................................................................ 4

1.2.1 Open - Circuit Grinding............................................... 4

1.2.2 Closed - Circuit Grinding............................................ 41.2.3 Principles of Closed Circuit Grinding.......................... 8

1.3 Types of Mills............................................................................ 8

1.3.1 Ball Mills...................................................................... 9

1.3.2 Roller Press Mills........................................................ 9

1.3.3 Roller or Bowl Mills..................................................... 9

2. Internal Ball Mill Parts....................................................................... 12

2.1 Partitions................................................................................... 122.1.1 Purpose of Partitions.................................................. 12

2.1.2 Double Wall Diaphragm Partitions.............................. 12

2.1.3 Operation and Repair of the Diaphragm Partition...... 16

2.2 Liners........................................................................................ 17

2.2.1 Purpose of Liners....................................................... 17

2.2.2 Types of Liners............................................................ 18

2.2.3 Liner Quality................................................................ 24

C.T.STechnical Training


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3. Ball Mill Grinding and Related Topics 25

3.1 Ball Milling Process (Basics and Fundamentals)..................... 25

3.1.1 Size Reduction............................................................ 253.1.2 Bed Movement............................................................ 26

3.1.3 Factors Influencing Size Reduction............................ 27

3.1.4 Mill Critical Speed....................................................... 28

3.1.5 Ball Size and Breakage Rates.................................... 29

3.1.6 Ball Mill Liners............................................................. 31

3.1.7 Mill Sweep Influence on Grinding............................... 31

3.1.8 Mill Mass Transport and Bypass................................. 33

3.2 Grinding Media......................................................................... 393.2.1 Ball Types and Wear................................................... 39

3.2.2 Ball Coating................................................................ 41

3.2.3 Ball Charge Design..................................................... 41

3.3 Ball Mill and Circuit Evaluations and Tests.............................. 52

3.3.1 Mill Material Levels..................................................... 52

3.3.2 Fineness Evaluation................................................... 54

3.3.3 Mill Retention Time..................................................... 54

3.3.4 Circulating Load.......................................................... 57

3.4 Ball Mill Control........................................................................ 58

3.4.1 Basic Ball Mill Control Theory.................................... 58

3.4.2 Mill Motor kW Control................................................. 58

3.4.3 Mill Sound Control...................................................... 61

3.4.4 Discharge Bucket Elevator Motor kW......................... 62

3.4.5 Rejects Flowrate......................................................... 63

3.4.6 Rule Base Mill Control................................................ 64



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4. Air Separators 67

4.1 Introduction............................................................................... 67

4.2 Types........................................................................................ 67

4.2.1 Static Grit Separator................................................... 68

4.2.2 Sturtevant Separators................................................. 72

4.2.3 Raymond Separators.................................................. 77

4.2.4 High Efficiency Separators......................................... 79

4.3 Separator Efficiency................................................................. 87

4.3.1 Tromp Curve............................................................... 87

4.3.2 Rosin-Rammler Number........................................................... 92

4.4 Mill Circulating Loads............................................................... 93

4.4.1 Definition..................................................................... 93

4.4.2 Circulating Load and Production Rates...................... 94

4.4.3 L/D Ratio and Circulating Load................................... 96

4.4.4 Circulating Load Calculations..................................... 97

4.5 Qf/Qa Principle......................................................................... 101

4.5.1 Bypass and Qf/Qa...................................................... 102



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3. MILL GRINDING THEORY

Foreword

The text of the following section on Mill Grinding was adapted from a larger

document entitled:

Lafarge Mill Grinding Reference, Edition 1

Volume 1: Practical Fundamentals of Ball Milling and Separation

Readers wishing to study the topic in greater depth and detail are encouraged to

obtain a copy of this and other appropriate volumes which are available from CTS

in hard copy or on diskette for Macintosh.

Other volumes that will be available in the Lafarge Mill Grinding Reference series

(all of which will be available before the end of 1992) are:

Volume 2: Auxiliary Equipment in Mill Circuits

Volume 3: Process Methods and Theory

Volume 4: Roller Mills and Roller Presses

Volume 5: Wet Process Grinding

Volumes 1 & 2 were meant to be practical reference and idea books for plant

people; to help solve the dizzy array of problems they encountered. The next

three texts specialize and include more theory where appropriate.

Reader should understand that the text presented here has been edited,

substantially, to suit the time allotted in this course "Introduction to Process

Engineering". Realistically we cannot present and teach this segment which

normally requires 9 working days (for Production Supervisors). Quite simply

C.T.STechnical Training 4 -4


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there is too much material. Recognize that the topic "Mill Grinding and

Separation" is far more complex than most of us realize and in any short course

one can only cover the highlights which never delves into the intricacies of this

whole field. We can only encourage you to ask questions and discover.

Sam Fujimoto Paul Ukrainetz

Process Engineer Process Engineer

Lafarge Canada Inc. Lafarge Canada Inc.



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1. Introduction and !er!ie"

1.1 Introduction to Grinding

In the cement industry we spend a lot of effort and money into size

reduction at various stages in the whole process. At each of these steps

we try to grind materials very finely and within a specific size and surface

area range. Why? A given particle's size (or mesh) and surface area (or

blaine) significantly influences the speed and completeness of the

chemical reaction with that particle. For example a pound of course

particles will have a much smaller total surface area and will react slower

than a pound of very fine particles which would have a much larger total

surface area. Moreover with the larger particles, there is the chance thatthe reaction will not consume the whole particle, leaving the centers

untouched. Grinding allows the cement manufacturer to influence and

tailor the process to achieve the desired result.

In raw mix grinding, the particle size has an important role in the ability

and ease to produce clinker, the clinker quality and the efficiency with

which we accomplish this in the kiln. Most plants use a % passing 200

mesh (or 75 micron size) target as the index to determine, whether the

product has been properly ground. See also "The Impact of Raw Grinding

on Kiln Operation".

For coal mills, most plants use the 200 mesh target as well. The particle

size in fuels has a profound effect on the flame's shape, temperature and

stability, which ultimately influences the clinker quality.

For cement finish mills, we use both a 325 mesh (45 micron) and blaine

(or surface area) targets. Different targets and different emphasis are

used depending on the type of cement being produced and the desired

performance. For example, ASTM Type 1 cements characteristically hasvery good early day strength gains but slows down at 28-days. The 325

mesh in cements strongly influences late day strengths and therefore most

plants focuses on this target closely. For ASTM Type 3 we typically raise

the blaine or surface area (the amount of super fines) in order to

dramatically raise the 1 and 3 day strength performance.



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In short how we grind materials at the plants at each stage plays a very

major role in our success in producing consistently high quality cements.

Therefore we must all understand grinding thoroughly if we are to stay in

business. We shall begin that process here.

1.2 Grinding Mill Circuits

A mill circuit is a combination and proper arrangement of one or more

grinding mills and the auxiliaries required to convey, classify and to collect

the ground product.

Let us first look at the Ball Mill grinding circuits. Once a circuit is defined

as either WET or DRY grinding, it need be further defined as OPEN or

CLOSED circuit.

1.2.1 Open-Circuit Grinding

Where the mill product is sent to storage silos without sizing or returning

the oversize to the mill for further grinding. Figure 4.1.2.1 is a sketch of an

open circuit mill. Since size reduction must be accomplished in one pass,

open circuit mills tend to be very long.

1.2.2 Closed-Circuit Grinding

Where the mill product is sent to the separator and the oversize returned

to the mill for further grinding. The oversize material can be called

REJECTS, TAILINGS or CIRCULATING LOAD. Figure 4.1.2.2 is a sketch

of a closed circuit mill. Figure 4.1.2.3 is a variation showing flash drying.



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1

2

3

4

5

1.Clin

kersilo

2.Gyp

sumsilo

3.Mill

feed

4.Grin

dingmill

5.Groundcement

Fig.1.2.1-OpenCircuitGrinding

Sketchofanopencircuitmill



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Fig. 1.2.2 - Closed Circuit Grinding

Mill

Separator Feed (A)

Separator

Rejects (R)

Fresh Feed (K)

Fines (F)



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Fig. 1.2.3 - Closed Circuit Grinding

With Drying of Feed

Mill

Separator Feed (A)

Separator

Rejects (R)

Fresh Feed (K)

Fines (F)



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1.2.3 #rinciples o$ %losed %ircuit Grinding

For most raw mix and cement grinding we used closed-circuit grinding.

In a closed circuit system, the mill product is conveyed to a classifier

(separator) and the material coarser than the required maximum size is

returned to the mill feed. The separator removes the fine (Finish Product),

which is then pumped to storage and the coarse particles (rejects) are

returned to the mill. Sufficient new feed material is added as required to

maintain the charge within the mill at the operating capacity.

From the cost standpoint, it is less expensive to grind in open circuit if the

required product does not exceed approximately 3300 Blaine. The mill

system in open circuit is simpler, but it is not as efficient as closed circuit

grinding. In general they tend to overgrind the product to maintain a

certain mesh target.

In the closed circuit, the mill has a greater capacity and the finish product

contains little or no oversize, depending upon the adjustments made to the

separator. Also for quality control, closed circuit systems offer more

options to adjust the product particle size distributions for optimum

performance.

1.3 Types of Mills

The grinding mill is the main piece of equipment used in the total finish

grinding system. Grinding mills can be classified into the following

categories:

1.3.1 Ball Mills.

1.3.2 Roller Press Mills.

1.3.3 Roller or Bowl Mills.



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1.3.1 Ball Mills

A ball mill is a cylindrical drum with varying ratio of diameter to length,

(also called tube mills). It has either one, or two compartments,(sometimes three compt.). The grinding media are usually steel or cast

iron balls of different diameters. Within Lafarge, the vast majority of the

grinding is accomplished with such mills and come in a variety of

arrangements reflecting the technology of the time and what it is supposed

to be grinding.

1.3.2 Roller Press Mills

A roller press as shown on the attached diagram is nothing more than a

pair or rolls placed in opposing position with a small gap between the rolls.

One of the rolls is stationary (fixed roll) and on (moveable roll) is mounted

on sliding guide ways, with hydraulic cylinders applying force toward the

fixed roll. The moveable roll of the roller press is under constant load from

the hydraulic cylinders. It is an old idea, re-invented for a new application.

Thus far installations have been applied to existing ball mills, to

dramatically improve grinding rates. However they are difficult to balance

and can be expensive to maintain.

1.3.3 Roller or Bowl Mills

Roller Mills consist of wheels (or rollers) mounted above a rotating table.

Fresh feed is dropped into the table is ground between the wheels and

table as the table turns. Often these mills are air swept and usually come

with there own built in separator. Many of the installations in North

America are for grinding coal, with a few grinding raw materials (eg.

Balcones, Davenport and Demopolis). Installations are usually compactand are ideal for relatively soft materials, but normally have complex hot

gas circuits associated with them.



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Roller Press Configuration

The basic configuration of the roller press consists of:

2 rollers

Frame

Roller support bearings

Hydraulic cylinders

Hydraulic accumulator

Moveable roll Fixed roll

Tension member

Frameend

piece

Tensionmember

Bearing blocks

Hydrauliccylinder

Hydraulicaccumulator



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Classifier shaftDriven byVariable speed drive

ProductDischarge port

Raw material

Classifier

Blade

Totally enclosedhousing

Loading rod(one of three)

Gas intake port

Hot gas(Reclaimed from kilnpreheater or cooler)

Hydraulic loadingcylinder (one of three)

Feed spout

Roller support

Roller housing

Grinding roller

Ported air ringGas plenium

Wearing ring

Rotating grinding table

High speed shaft

Speed reducer



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2. Internal Ball Mill #arts

2.1 Partitions

2.1.1 Purpose of Partitions

Their primary purpose is to separate the different sizes of the ball charge

in order to roughly match the ball size to particle size being ground. For

example, many open circuit mills were originally supplied with three

compartments. However most compartmented mills in Lafarge today have

just two. The material being ground in the first compartment passes

through the grate slots in the partition to enter the second compartment,

then leaves the mill through the discharge grate.

A secondary purpose for a double intermediate partition is to retain the

insufficiently ground material in the first compartment, by regulating the

material level in it. Properly designed the partition determines the flow rate

from one compartment to the next and thereby helps to maintain a good

filling ratio.

Of course it also roughly separates and retains the large material particles

in the first compartment.

2.1.2 Double Wall Diaphragm Partitions

These partitions are equipped with lifters which regulate the flow rate of

the material from the first to the second compartment. The back plates

are blind.



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Material

Discharged

Airfow

As mill rotates, material is

lited and dropped through

the opening in the center hub.

In the lower

hal, material is

pushed through

the s lots, lling

the empty liter

chamber.

Feed End View of Partition

Slotted Grate Segment

enter !ub or one

Air Sweep enter ScreenSide View

"lind #late

$iter or Scoop

Standard Liftered Partitions

These are common and mostly suffer from the same problem. The

porosity of the first compartment charge is high and with a fully-liftered

partition (lifters right to the shell) the material filling ratio (U) is pulled down

well below 0.9 and ball wear is accelerated. Usually it is below the 0.6

point at which material breakage rates are reduced. (In other words the

voids are too empty - see also section on Material Filling Ratio.) This is

evidenced by no material visible on the balls at the partition, sometimes

one must dig 18" or more to 'hit' cement.

The French have attempted to overcome this through ball charge

modifications (more smaller balls) and reducing the slot size. At

Demopolis large holes have been cut in the lifters to reduce their

efficiency. Neither method has been highly successful. Changing slot size



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and cutting lifters are irreversible and plants have experienced very limited

success. Adding small balls to increase the filling ratio is O.K. until the

circulating load or material grindability changes, at which time you must

start over.

Other plants have installed partitions which are multi-segmented. These

allow you to replace slotted segments with blind sections on the inlet side

and thereby controlling the total slot area. The Group does not have a lot

of experience with this type but it appears difficult to make changes and it

also chokes off mill sweep.

Adjustable Double Wall Diaphragm Partitions

This is a double wall partition with adjustable scoops developed by

SLEGTEN, and later offered by other manufacturers.

The grinding efficiency of a mill depends directly on the time of retention of

the material within the ball charge. This regulating partition thus permits

adjustment of the material level in the first compartment by using scoops

located between the slotted grates and the blind grates on the second

compartment side.



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SLEGTEN DOUBLE - DIAPHRAGM PARTITION

Actual installations of the SLEGTEN regulating diaphragm on finish mills in

the Group are:

Boussens

Val d'Azergues

Port-la-Nouvelle

Contes

Le Havre

Alpena

Balcones

Bath

Whitehall

Wossingen

L.F.I. at Dunkerque, Fos, and Norfolk

The Group strongly recommends SLEGTEN diaphragm partitions over all

others. In addition, the SLEGTEN diaphragm partition is mechanically

very well designed in that it has a simple structure, well adapted to

stresses caused by the rotation of the mill shell, especially in large



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diameter mills. However, recognize that the installation of a regulating

diaphragm requires a long commissioning period, systematically making

adjustments of the grinding charge and partition to see an improvement.

2.1.3 Operation and Repair of the Diaphragm Partition

The diaphragm and grates are a rather weak (perhaps the weakest)

element inside the mill. If they are in poor shape, the mill will function

poorly.

Slots in bad shape block easily.

High pressure loss causes poor ventilation.

Holes, excessive play between the plates, worn and enlarged

slots, all pass clinker particles (2, 5, 10 mm) which prevent the

efficient action of the second compartment ball charge.

It is therefore necessary to conduct a detailed inspection every time the

mill is entered.

The proper operation of the mill partition can be better understood by thefollowing two process elements:

Fineness curves of samples taken along the ball charge,

(granulometry). If they show too many large particles (2, 5, 10

mm) in the second chamber, the partition needs attention.

The level of the material in the first compartment. A well

adjusted partition will keep a level of material equal to the level

of the balls (even slightly higher) in the first compartment.

If this is not the case, then you must play with:

the position of the scoops, if installed

the dimensions of the lifters, if possible



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the number and/or the dimensions of the slots



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2.2 &iners

2.2.1 Purpose of Liners

The purpose of mill liners is not only to protect the mill shell but they must

also "grip" the grinding balls and release them at the right height to obtain

the cascading or tumbling that maximizes the grinding rate. If the balls are

carried too high they will get thrown against the mill liners on the opposite

side where the balls or liners may break. In addition, the grinding action

could be limited to the toe of the mill load. If the balls are not carried high

enough then the impact energies while tumbling are greatly reduced, thus

retarding the grinding action. In addition, recognize that some sliding

contact will occur which increases liner wear.

Many of us lose sight of the fact that the balls should not be sliding as they

are being lifted up. Sliding between balls and liners increases wear and

can lead to premature failures. Evidence of this can be seen in the form of

"racing". Racing are the grooves or rings worn into liners and is a result of

sliding contact between balls and liners. Excessive racing will in turn

promote more sliding contact and thus accelerating the whole process.

Recognize though that as the liner rotates around and enters into the toe

of the ball charge some sliding contact will occur until the liner has gripped

the ball charge, to lift it. Some manufacturers take advantage of this and

market grooved liners which are designed to do extra grinding in the

grooves themselves as the liners enter the ball charge. Examples are

Manoir, Armco-Delloye, and Owen Corp.

Liner design is critical to good grinding action and wear life. Careful

consideration must also be given to ball charge design, material, material

load and the grinding action required in that compartment in order to

design the appropriate liner.



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2.2.2 T'pes o$ &iners

A) First Compartment

The work done in this compartment is done by impact, thus some call this

the crushing compartment. Therefore the liners must be lifting and are

present in several forms, block, wedge bar, Lorraine and Duo-lift. Their

job is simple. Lift the balls high to eject them from the charge so they fall

farther and hit harder. This is shown in the following drawing showing the

detachment point of the balls from the lining for a 0.75 m test mill, 75%

critical speed, 25% volume load, 30 mm balls, material filling ratio 1.0,

feed < 3.15 mm.



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$iters

%&'()Smooth

%&*()

Groo+ed

%&()

Therefore saturating is supported and the fracture mechanism enhanced.

This means that breakage rates will be shifted towards the coarser particle

sizes, at the expense of breakage rates on the finer ones.

This is clearly shown by tests conducted by Rogers et al. ("..Effect of Liner

Design on Performance of a...Wet Ball Mill") on a 0.91 m mill. The

lifters move the peak breakage point to a coarser size, from 1.5 to 4 mm.

The breakage of finer material is much lower with lifters than with

corrugated lines. Therefore lifters don't belong in the second

compartment.



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.' ' '(

.'

'

'(

$ining -ype +s "reaage

Material #article Diameter /mm0

Sp

ecifc

"rea.age1

ate,

Si/'2m

in0

orrugated

Spiral Angular

Lifters

Thus this is the type of liner which is the first choice many plants,

(sometimes called shipload or wedge or stepped liner).



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(!en so) t*ere are ot*er t'pes still in use in +ort* A,erica.

For wedge bar, lorraine, block type liners the ratio of height to spacing in

between (usually matches bolting patterns) is critical to achieving the

correct trajectories. Surprisingly many suppliers do not have sophisticated

methods to determine this. However this group of liner types seems to be

ideal in SAG mills used in mining or for mills with very low % critical speed.

Most single wave liners generally had good lift characteristics, however on

the down slope side, media tended to slide which vastly accelerates wear.

The new DuoLift liner developed by SLEGTEN could provide an

interesting results.

The Duolift is especially designed to maintain the same lift characteristics

through 90% of it wear life by controlling the wear pattern. In doing so



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Slegten claims that the absorbed power and grinding efficiency will remain

more consistent, and wear life greatly extended. Presently this liner is

being tried in Demopolis and Whitehall.

B) Second Compartment

Corrugated/Wave Linings

These are the standard linings in North America that original come with

most mills. As with lifters they come in all versions and perversions, wave,

simple wave, double wave, etc. Their purpose is to assist in the cascading

action of the balls. Therefore they assist in the attrition mechanisms,chipping and abrasion, on smaller sized particles. This means they do not

belong in a first compartment, but rather in the second. They are,

however, not the recommended second compartment liners.

Classifying Linings

This type of liner has several important advantages:

ball size matches particle size along the mill;

reverse and double-reverse classification is avoided.

They have also been shown to increase material transport opposed to a

non-classified charge. This is due to the increased porosity (big balls) in

the areas where the particles are larger and less 'fluid'. This means it is

theoretically possible to reduce the final ball size in the mill in comparison

to an unclassified charge for better 'fit' between balls and material without

material transport problems.

This fit is obviously the best when 'plug-flow' is present. This means that

material flows through the mill as though in a pipeline and isn't mixed too

much. The longer L/D in the second compartment gives RTD's (residence



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time distributions) which are closer to the plug flow limit than the short L/D

first compartment.

The advantages of a classified charge are greatest when it allows a widerange of ball sizes to be used. The fact that only 3 to 4 ball sizes are

present in the more mixed first compartment reduces to benefits of this

lining in that area.

For each liner step for classifying liners in large mills over 4 metres (13

feet) in diameter, use two liners per step, lengthwise of the mill.

Large media, 50 to 70 mm (2 to 2 3/4 in) diameter, can be put in the samecompartment as small media, as small as 15 mm (5/8 in) diameter in an

open-circuit mill. In such a compartment, the grinding charge can accept

a very coarse feed. Classifying liners permit a deteriorating operation in

the first compartment, or in the diaphragm partition. Furthermore, they



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allow a better overall grinding performance in a mill whose partition

diaphragm location is not ideal.

Except in special cases, which must be examined with care, classifyinglinings are recommended for the second compartment, and long single-

compartment mills with lower circulating loads, but not for first

compartments.

2.2.3 Liner Quality

In North America, most liners today are white cast irons with high chrome

contents although there seems to be a general movement to high carbon

tool steel base. Different alloying elements as well as carbon and chrome

contents are used, we suspect, to achieve desired properties and

performance, but within the confines of the technology used at different

foundries. This appears to vary even within the same company. The

observation is unconfirmed mainly because none of the suppliers wish to

divulge trade secrets, however none of our experience disputes this.

In general, all liners attempt to produce grains of chromium carbide, which

are extremely abrasion resistant, held in a martensitic steel (the hardestform of steel) matrix. Also the best performing liners generally have very

fine grain microstructures (achieved through heat treatment and proper

quenching) which increases overall hardness and abrasion resistance. By

varying the carbide content, alloying elements, and microstructures

suppliers can alter the liner performance with respect to abrasion and

impact resistance to suit the application.

However, foundry quality control is vital to good liner performance. This

seems to vary more widely in North America than in Europe; to the pointwhere some plants will specify the foundry at which the liner is to be cast.

CTS

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