Nordbeg-Lokomo 12.12.1999

Technical PublicationsNo. 7

Influence of CrushingProcess Variables onthe Product Quality of

Crushed Rock

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Eloranta, Jarmo. Influence of Crushing Process Variables to the End Product Quality of Crushed

Rock. Tampere 1995,

UDC

Keywords

ABSTRACT

In many instances rock crushing has considered to be more art than science. One reason for that isthat this industry is on global basis such a small niche, that companies involved have not hadresources and on the other hand also no real need to study what is happening inside the crusher.Recently there has been put more emphasis on issues like the shape of crushed product. Theserequests are stated by end users of aggregates.

Idea to make this thesis is related to author's deep involvement to the development of rock crushingequipment and especially cone and gyratory type of crushers. These crushers are the most criticalones if we think to have maximum impact on end product shape. In the development of conecrushers it is very important to know the impact of different crusher variables to the end productquality. By knowing the impact of different parameters, the basic key development requirements andgoals for the new products can be stated. The basis parameters which can be played with are,providing that rock characteristics can be held constant: feed distribution, crushing cavity type andshape to be used, crusher stroke, speed, setting and choke feeding.

The objective was to clarify the relative importance of these parameters and for this purpose therewere 72 tests made in full scale crushing plant application. From the measured values there werecalculated some clarifying values in order to have wider perspective to interpret the results and drawconclusions.

Based on these tests it can be said that the feed characteristics is the first and most importantdeterminator as to the end product quality. Influence of other parameters is dependable on feedcharacteristics, and is shown in detail in the thesis. This means that although end product quality canbe improved by changing the crusher's operational parameters (stroke, speed, setting, cavity) but theyhardly can compensate poor product shape if crushing process is faulty, meaning wrong feeddistribution to the crusher or crusher is not being choke fed.

In this thesis attempt has been made to incorporate all process- and crusher variables together andthus from wider perspective to rank the influencing parameters to priority order in differentapplications. Although this study has been reasonably extensive, this is still considered to be a basisfor future research and development

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PREFACE AND ACKNOWLEDGEMENTS

This thesis is done at Nordberg Lokomo in Finland, which belongs to Nordberg Group, the global

leader in developing crushing equipment for construction, mining and recycling industries.

The starting point has been to have most comprehensive approach for the one of the major technical

success criteria for the future: How to produce cubical aggregate. This issue has been studied by

several authors, but this thesis differs from the others by trying to integrate better the influencing

parameters with each others.

The author wishes to express his gratitude to the thesis advisor Professor Asko Riitahuhta, from

Tampere University of Technology.

Professor Heinz Hoberg from the Technical University of Aachen in Germany and Professor Kari

Heiskanen from Helsinki University of Technology have read the manuscript. I am very grateful to

them for the valuable comments and constructive critisism concerning the manuscript.

Also I want to thank Professor Matti Kleimola from Helsinki University of Technology, who has

read and given valuable advice for this thesis.

I want to thank all people within Nordberg organization all over the world who have encouraged,

and helped me to prepare this thesis. Without their contribution this thesis would not be written.

Dipl. Ing Harro von Blotnitz from the the Technical University of Aachen in Germany I want to

thank for both correcting the English language and giving valuable assistance to get this thesis to its

final shape.

Finally I wish to thank my wife Anna-Maija, daughter Elisa and son Martti-Pekka who have also

given their contribution to this thesis.

Tampere November 1995

Jarmo Eloranta

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Table of Contents

ABSTRACT

PREFACE AND ACKNOWLEDGEMENTS

DEFINITIONS

LIST OF SYMBOLS

1 INTRODUCTION

1.1 Background of the thesis

1.2 Objectives and the scope of the thesis

1.3 Research methods

PART ONE

ISSUES RELATED TO THE PRODUCTION OF CUBICAL PRODUCT

2 INTRODUCTION TO CRUSHING AND CRUSHERS

2.1 Application range for crushers

2.2 General principles of crushing process planning

2.3 Review to different crushers

3 CRITERIA FOR CUBICAL PRODUCT

3.1 Why cubical product is needed

3.2 Review to different standards

4 CRUSHING THEORY IN GENERAL

4.1 Crushing chamber performance

4.1.1 General

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4.1.2 Crushing phenomena

4.1.3 Influence of bulk density

4.1.4 Other influencing factors

4.1.5 Energy input

4.2 Particle fall through crushing cavity

4.3 Crushing forces in cone & gyratory crushers

5 LITERATURE REVIEW TO PRODUCE CUBICAL PRODUCT

5.1 Background of rock breakdown

5.2 Influence of rock properties

5.3 Impact of crushing parameters

5.3.1 Crushers types

5.3.2 Feed

5.3.3 Crushers' operational parameters

5.3.4 Other factors

PART TWO

RESULTS OF CRUSHING TESTS

6 TEST ARRANGEMENT

6.1 Test program

6.2 Plant configuration and test runs

6.3 Recorded test data

7 BACKGROUND FOR SELECTED PARAMETERS

7.1 Selected parameters

7.2 Number of crushing zones and work stroke

7.3 Impact speed against the rock

7.4 Further clarification of some parameters

8 TEST RESULTS

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8.1 Results in general

8.2 Analysis systematic

8.2.1 Tests 56-67

8.2.2 All cavities according to each feed

8.2.3 Influence of input parameters

8.3 General summary of tests

9 VERIFICATION OF TEST RESULTS TO PREVIOUS TESTS

10 DISCUSSION

10.1 Evaluation of the validity of results

10.2 Evaluation in respect to previous studies

10.3 Recommendation guidelines with limitations

10.4 Needs for further research

11 CONCLUSIONS

REFERENCES

APPENDICES

Appendix 1: Stone study report

Appendix 2: Plant flow sheet

Appendix 3: Example of power and pressure measurement

Appendix 4: Example of screen analysis

Appendix 5a-c: Feed and product curves

Appendix 6: Example of crushing cavity calculation

Appendix 7: All tests with calculated parameters

Appendix 8: Tests 56-67 classified according to cubicity

Appendix 9: All tests classified according to cubicity

Appendix 10: Tests 56-103 classified according to stroke

Appendix 11: Tests 56-103 classified according to speed

Appendix 12: Tests 56-103 classified according to setting

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Appendix 13: Tests 104-127

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DEFINITIONS

The following definitions and terms have been used in this thesis:

Cavity change: The chance of cavity volume between different crushing zones.

Choke feeding: This means that sort of feeding arrangement to the crusher that

cushing cavity keeps full.

Choke point: The point in the crushing cavity where the volume of the material to be

crushed is smallest. This determines the capacity of the crusher.

Closed circuit: Crushing process feature, where part of material produced by crusher

of circulated back to the same crusher to be further crushed.

Cone head: Conical part which supports liner.

Concave: The external part of tools to be used for crushing

Critical speed: Speed is critical, when cone head oscillation speed is the same as the

falling speed of crushed material.

Crushing cavity: The space between liner and concave where crushing action itself

takes place.

Crushing ratio: Ratio between feed and outcoming product. It is normally measured

of 80% point; Crushing ratio = Feed80/Product80. This describes the

work which has been done from feed to product.

Crushing zone: When material is being crushed, it descends through cavity from one

crushing zone to another zone.

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CSS: Closed side setting, minimum distance between liner and concave at

the discharge end of cavity.

Cubicity: Describes the shape of rock. There are different standards available. In

this thesis DIN standard is being used.

Flakiness index: Descibibes the shape of the rock. Basically same as cubicity.

Interparticular

crushing: Rock crushing takes place against each other and not only against the

crushing tools.

Liner: The inner part of tools to be used for crushing. Fitted on cone head.

Nip angle: Angle between liner and concave when rock is caught, nipped,

between them.

Nominal stroke: See stroke

OSS: Open side setting, maximum distance between liner and concave at

the discharge end of cavity.

Packing: Happens when rock is being compressed so much that its density has

increased so much that it starts to be solid.

Pivot point: The point around which cone head is oscillating, center of the

crusher's top bearing, if such crusher type.

Reduction ratio: Same as crushing ratio.

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Segregated feed: Feed to the crusher is such that fine and coarse particles in the feed get

to the different side of the crushing cavity.

Shape factor: Same as flakiness index and cubicity.

Stroke: Difference between OSS and CSS.

Work stroke: The distance between CSS and the distance when material compression starts.

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LIST OF SYMBOLS

A = crushing zone area

C = constant

E = Energy (kWh/t)

N = speed of cone head (RPM)

P = power (kW)

P80 = crushed product (mm, 80% by weight passing)

F80 = feed size (mm, 80% by product passing)

R = distance of crushing zone from centerline

S = stroke (mm)

t = time (s)

V=Y = contact speed against falling speed (m/s)

Y = work stroke (mm)

Wi = work index (kWh/shortton)

W = work in kilowatt hours (kWh)

ρ = material density

σ = tensile stress (N/mm2)

τ = shear stress (N/mm2)

p = compressive stress (N/mm2)

_ = direction of resultant crushing force (deg)

ω = angular speed (1/s)

Subscribts

m = maximum

1,2,N = indices

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REFERENCES

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Ähnlichkeitsprinzips. VDI-Zeitschrift 96 (1954) Nr.33. [Size reductin machines in respect to

similarity laws]. (In German)

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11 CONCLUSIONS

The extensive tests performed and the analysis based on them supportedthe initial hypothesis of this research in the respect that it would be possible torecognize the major operational factors influencing the shape of crushed product,and to classify these influencing factors in the order of priority. The factors whichwere found to correlate with product shape were:

* Feed gradation* Crusher operational parameters:

- Stroke- Speed- Setting- The length and shape of crushing cavity- Crushing force- Crushing ratio

The most important parameters which were found to have an impact onproduct shape are feed gradation and crusher (work) stroke. The remainingparameters do have an impact but less than these two. The impact of operationalparameters were found to be different or even inverted depending on the feedgradation, and thus the influence of crusher operational parameters has to beevaluated together with feed gradation.

It was found that if feed does not contain enough fines, it is very difficultto compensate fully with other influencing parameters. This means that thecrushing process configuration is very important, because the crushing process asa matter of fact adjusts the correct feed to the crusher. If some of the operationalparameters of crusher cannot be optimally selected for product shape, theadjustments to feed curve can compensate the shortcomings in these parameters.

The impact of stroke actually means both increased compression andimpact speed against the rock. When there were enough fines smaller than thesetting in the feed, the particle shape of the product was good with larger stroke.On the other hand, when the feed did not include fines smaller than the settingthe best particle shape was obtained with smaller stroke. In the first case wherethere were more fines in the feed, the larger stroke caused an effectiveinterparticular crushing effect, and this rounded the particles with poorer shape.So major contribution of larger stroke in improving cubicity relates to utilizationof existing fines in a feed as well as own production of fines for more effectiveinterparticular crushing. In the latter case where there were no fines in the feed,the larger stroke caused a more 'violent' crushing action on the particles and the

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outcome was a poorer shape, because finer support material as a dampingmaterial was not present. So in this case when the stroke was reduced, thecrushing action became gentler, which improved cubicity. In cases where largerstroke can be used for better cubicity, there is a very important 'by-product':larger stroke gives bigger capacity, or then crusher size could be minimized dueto larger stroke, and this is of course an important economical issue.

Speed variations investigated here did not show any clear influenceregarding the impact on cubicity. If stroke and speed are compared together thereis in both cases an increased impact speed against the rock but work stroke isbigger if nominal stroke is bigger. This seems to mean that impact speed itself isnot so important, but if longer compression distance is involved the joint impactis positive.

Influence of setting can be expressed so that if there are fines in the feedthe changes are improved to achieve higher cubicity values with smaller setting.Setting influences the crushing ratio as well as crushing force. As known, thesemay not be too high if no fines in the feed, and thus it means that better cubicityresults can be expected with wider setting.

The tests gave some indications that the length of the crushing cavity hasto be reduced in those cases where the feed contains fines, because otherwisethere will be a danger of packing. Inversely if feed does not have fines, a longercavity helps to increase the number of crushing zones, and thus it helps toproduce the important fine support material which is missing from the feed.

As to the crushing ratio it was found that particle shape can also be goodwith higher crushing ratios providing that there is enough support material, i.e.fines, in the feed. This also relates to the effective interparticular crushing.

A smaller crushing force helps to improve cubicity, when there are nofines in the feed. The reason is that the crushing action is gentler. If there are finesin the feed the result is exactly the opposite: more force is needed to haveeffective interparticular crushing, because smaller particles need more energy tobreak.

Test results were compared to previous tests with the same crusher and

liners as done at Nordberg-Lokomo's test plant. It was found that the majorparameter, feed, correlate reasonably well with each other, although the rock typewas different.

As a major conclusion it can be said that crusher operational parameterscan influence the product shape within certain limits, but that the most decisivefactor influencing cubicity is feed arrangement, which means that the majoremphasis should be on the crushing process.

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