Comminution Circuit Design and Simulation for the Development of a Novel High Pressure

Grinding Roll Circuit

by

Persio Pellegrini Rosario

A thesis submitted in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

in

The Faculty of Graduate Studies

(Mining Engineering)

The University of British Columbia

(Vancouver)

November 2010

' Persio Pellegrini Rosario, 2010

ii

ABSTRACT

The application of High Pressure Grinding Roll (HPGR) in comminution circuits is well

established in processing cement, diamonds and iron ore. Recently, the application of

HPGR has been extended to high-tonnage precious and base metals operations with

hard ore. This is due to the HPGR: being more energy-efficient than grinding mills, not

requiring steel grinding media, and providing higher throughputs than cone crushers.

Although HPGR circuits are being used in high-tonnage precious and base metals, there

is limited quantitative knowledge to indicate the true benefits or drawbacks of HPGR

compared to Semi-autogenous mill (SAG). This lack of knowledge restricts the ability of

designers to determine the optimal circuit. To address this lack of knowledge the

research in this thesis:

Reviews the basics of the HPGR machine, its benefits and shortcomings.

Details the development of the SAG circuits and explains how the new

generation of crushing circuits, with HPGR for tertiary crushing, are starting to

replace SAG circuits in hard-rock mining.

Presents a structured methodology for comparison of the energy requirements

for HPGR versus SAG complete circuits. The process is based on industrial best

practices and advanced modelling tools, and is demonstrated through the

evaluation of two hypothetical mining projects (based on real ore data).

Investigates the feasibility of a novel AG-Crusher-HPGR circuit using rock

samples from a large copper-gold mining project. The approach was to develop

and evaluate the circuit design for high-tonnage operations with mixed hardness

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ores containing clay. Previously, HPGR was considered only suitable for very

hard ores and the technology was rejected for other cases. A unique pilot-plant

test program was developed as a basis for experimental simulation. As a result

the suitability of the circuit was demonstrated.

The development of this novel circuit along with the findings of this research have the

potential to improve future mining operations dealing with similar orebodies that, in fact,

are major sources of base metals worldwide. The potential for significant savings in

energy and steel media have been demonstrated. This may also lead to the selection of

more sustainable circuits for a broader range of orebodies.

iv

PREFACE

Prof. Robert Hall is my PhD program supervisor and co-authored two manuscripts

(Chapters 3 and 4). Prof. Hall provided feedback on manuscript preparations and

contributed to the identification and design of my research program.

Prof. Bern Klein is my PhD program co-supervisor and co-authored the third manuscript

(Chapter 4). Prof. Klein provided input on the design of the testwork program applied in

this research as well as participated in the identification and design of the research

program.

Mr. Mike Grundy was a co-author of two manuscripts (Chapters 2 and 4). Being a senior

metallurgist with vast experience in AG/SAG mill application, he assisted me in the

clarification of parts of the manuscript, especially the ones covering the history and

recent applications of SAG circuit. In addition, he provided feedback during the

development of the novel HPGR circuit, verifying a number of my assumptions and

assisting me in specific engineering details for the operation of a circuit.

Mr. Ken Boyd was a co-author of the first manuscript (Chapter 2). Being a senior

mechanical engineer specialized in material handling systems he contributed with

information regarding the application of pebble crushers in recent SAG mill circuits.

The contributions of all the people above mentioned was important and very much

appreciated. However, the vast majority of the research and writing was conducted or

developed and directed solely by the author, i.e. more than 95% of the work. This

included the following:

Development of the research objectives, methodology and testing programs.

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Performance of all simulation analysis.

Performance of all test work with support from laboratory personnel for some

manual labour and specialized tasks.

Review of the current state of the art as presented in the thesis.

Rewriting and integration of the papers into the current form in the thesis with

revisions based feedback from my supervisory committee.

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TABLE OF CONTENTS

Abstract ................................................................................................................................. ii

Preface................................................................................................................................. iv

Table of Contents ................................................................................................................. vi

List of Tables ........................................................................................................................ ix

List of Figures........................................................................................................................ x

Acknowledgements .............................................................................................................. xi

1 Introduction ................................................................................................................ 11.1 Comminution .................................................................................................. 11.2 Modern Metal Mining ...................................................................................... 31.3 HPGR in Hard Rock Mining ............................................................................ 51.4 Thesis Objectives ........................................................................................... 71.5 Thesis Outline ................................................................................................ 9

2 Comminution Circuits - Literature review ................................................................. 102.1 Introduction .................................................................................................. 102.2 Recent History of Comminution .................................................................... 112.3 SAG Mill Background ................................................................................... 13

2.3.1 AG/SAG Mill Machines ..................................................................... 132.3.2 SAG Operational Parameters ........................................................... 142.3.3 SAG Mill Original Circuit ................................................................... 152.3.4 Pebble Crushing for AG/SAG Circuits ............................................... 172.3.5 SAG Feed Preparation ..................................................................... 212.3.6 Steel Wear ........................................................................................ 22

2.4 HPGR Background ....................................................................................... 252.4.1 HPGR Machine ................................................................................. 252.4.2 HPGR Terminology and Operational Parameters ............................. 282.4.3 HPGR Original Circuits ..................................................................... 312.4.4 HPGR Precious/Base Metal Recent Circuits ..................................... 352.4.5 Energy Savings ................................................................................ 372.4.6 Metallurgical Extraction Advantages ................................................. 402.4.7 HPGR Feed and Product Specifics ................................................... 412.4.8 Limitations and Disadvantages ......................................................... 42

2.5 Other Developments .................................................................................... 432.5.1 Increasing Machine Sizes ................................................................. 432.5.2 Stirred Mills ....................................................................................... 432.5.3 Fully Autogenous Grinding................................................................ 44

2.6 Summary of Current State ............................................................................ 46

3 Guidelines for Energy Requirement Comparisons between HPGR and SAG Mill Circuits in High-Tonnage Hard Rock Mining ...................................................... 473.1 Introduction .................................................................................................. 473.2 Modelling and Simulation Background ......................................................... 493.3 Case Studies ................................................................................................ 513.4 Design Criteria Development........................................................................ 523.5 Flowsheet Development ............................................................................... 54

vii

3.6 Developed Models ....................................................................................... 563.7 Equipment Sizing ......................................................................................... 613.8 Results and Discussions .............................................................................. 62

3.8.1 Pure Comminution Energy ................................................................ 623.8.2 Complete Circuit Comminution Energy ............................................. 633.8.3 Steel Usage ...................................................................................... 643.8.4 Ore Variability ................................................................................... 653.8.5 Heating and Ventilation ..................................................................... 663.8.6 Availability and Maintainability .......................................................... 663.8.7 Additional HPGR Benefits ................................................................. 673.8.8 HPGR Circuit Drawbacks.................................................................. 67

3.9 Summary...................................................................................................... 69

4 Testwork Program for the Evaluation of a Novel HPGR-Based Circuit to Treat Mixed Hardness Ores Containing Clays .................................................................. 714.1 Introduction .................................................................................................. 714.2 Novel HPGR Circuit for Ores Containing Clayish Material ............................ 754.3 Testwork ...................................................................................................... 77

4.3.1 Sample ............................................................................................. 774.3.2 Testwork Design ............................................................................... 774.3.3 Test Equipment ................................................................................ 79

4.4 Results and Discussion ................................................................................ 814.4.1 Sample Properties ............................................................................ 814.4.2 Tumbling Test ................................................................................... 814.4.3 HPGR Feed PSD .............................................................................. 834.4.4 HPGR Feed Moisture Content .......................................................... 884.4.5 HPGR Tests ..................................................................................... 914.4.6 HPGR Product Cakes ..................................................................... 1024.4.7 Bond Ball Mill Work Indices ............................................................ 106

4.5 Summary.................................................................................................... 108

5 Feasibility Assessment of the AG-Crusher-HPGR Circuit to Treat Clayish and/or Mixed Hardness Ores ................................................................................. 1095.1 Introduction ................................................................................................ 1095.2 Modelling and Simulation ........................................................................... 1105.3 Energy Requirements................................................................................. 115

5.3.1 Ball Mill Energy ............................................................................... 1155.3.2 Pure Comminution Energy .............................................................. 1175.3.3 Complete Circuit Comminution Energy ........................................... 119

5.4 Operating and Capital Costs ...................................................................... 1225.4.1 Operating Cost ............................................................................... 1225.4.2 Capital Cost .................................................................................... 122

5.5 Discussions ................................................................................................ 124

6 Conclusions ........................................................................................................... 1266.1 Main Research Contributions ..................................................................... 1266.2 Future Research Opportunities .................................................................. 128

References ........................................................................................................................ 131

Appendix A: Inputs Used for the JKSimMetfi Models .. .................................................. 142

Appendix B: SMC and MinnovEX SPI Test Results ..... .................................................. 145

viii

Appendix C: Sample Preparation and Test Flowsheet ................................................... 146

Appendix D: HPGR Feed Test Blend Linear Programi ng ............................................ 148

Appendix E: HPGR Tests Complete Data............. ...................................................... 150

Appendix F: AG-Crusher-HPGR Plant Layout .......... ..................................................... 158

Appendix G: SABC Plant Layout .................... ............................................................... 160

Appendix H: Power Consumption Comparison ......... ..................................................... 162

ix

LIST OF TABLES

Table 3-1: Summary of Design Criteria ......................................................................... 53Table 3-2: Pure Comminution Energy ........................................................................... 62Table 3-3: Complete Circuit Comminution Energy ......................................................... 64Table 3-4: SAG Mill Steel Ball Consumption ................................................................. 65Table 4-1: Pilot-Scale HPGR Specifications .................................................................. 79Table 4-2: Summary of Parameters and Calculated Results for Moisture Content ........ 91Table 4-3: HPGR Tests Quick Reference Legend ......................................................... 92Table 4-4: Summary of the Main Parameters and Results for All HPGR Pilot Tests ...... 93Table 4-5 Average Dimensions of Cakes Produced by the HPGR Tests .................... 104Table 5-1: Simulation Results Pure Comminution Energy Requireme nts ................. 118Table 5-2: Energy Requirements for the Complete Circuits ......................................... 121Table 5-3: Capital Cost Summary ............................................................................... 123

x

LIST OF FIGURES

Figure 2-1: Three Stages of Crushing, Rod Mill, Ball Mill ................................................ 16Figure 2-2: SAG-Ball Mill Circuit ..................................................................................... 17Figure 2-3: SABC Circuit ................................................................................................ 18Figure 2-4: Open-Circuit SABC ...................................................................................... 19Figure 2-5: SABC with HPGR ......................................................................................... 20Figure 2-6: Pre-Crushing in an SABC Circuit .................................................................. 22Figure 2-7: Schematic of a HPGR (Napier-Munn et al, 1996) ......................................... 26Figure 2-8: Open Circuit HPGR Closed-Circuit Ball Mill (Aydogan e t al, 2006) ............ 32Figure 2-9: HPGR Applied for Pebble Re-Crush at Empire Iron (Kawatra et al, 2003) .... 33Figure 2-10: Re-Crush Circuit at Argyle Diamond Mines (KHD, 2008) .............................. 34Figure 2-11: Boddington HPGR (Dunne et al 2007) ......................................................... 35Figure 2-12: Cerro Verde (Vanderbeek 2006) .................................................................. 36Figure 2-13: Pebble Extraction and Milling ....................................................................... 45Figure 3-1: Simplified SABC and HPGR Flowsheets ...................................................... 55Figure 3-2: JKSimMetfi Screen Snapshot of Case A SABC .................. ....................... 57Figure 3-3: JKSimMetfi Screen Snapshot of Case A HPGR .................. ...................... 58Figure 4-1: Cerro Verde Flowsheet (Vanderbeek 2006) ................................................. 72Figure 4-2: Hardness Distribution of the Deposit Based on Jk A*b Parameters .............. 74Figure 4-3: Proposed HPGR Flowsheet for Clayish Ore ................................................. 75Figure 4-4: UBC Pilot HPGR .......................................................................................... 79Figure 4-5: Particle Size Distribution for the Samples as Received ................................ 81Figure 4-6: Tumbling Test Feed and Product Size Distributions ..................................... 83Figure 4-7: Lab-Scale Circuit to Prepare the Feed to the Pilot HPGR (open-circuit) ....... 84Figure 4-8: PSDs for Fresh and Crushed Laboratory Screen O/S Material ..................... 85Figure 4-9: PSDs from the Preliminary Simulation .......................................................... 86Figure 4-10: PSDs for the Optimum Blend, Original Products and Simulated Product ...... 87Figure 4-11: Lab-Scale Circuits Used for the Tests .......................................................... 88Figure 4-12: Specific Throughput as a Function of Pressing Force................................... 95Figure 4-13: Influence in Energy Consumption due to Pressing Force ............................. 96Figure 4-14: Pressure Sensitivity Tests Feed and Product PSDs ....... ........................... 96Figure 4-15: F80/P80 and F50/P50 Reduction Ratios ...................................................... 97Figure 4-16: Feed and Product PSDs for Closed-Circuit Tests ......................................... 98Figure 4-17: Feed and Product PSDs for Full Feed and Tumbled-Screened Open-

Circuit HPGR Tests .................................................................................... 100Figure 4-18: HPGR Test #1 Product Cake Samples ....................................................... 103Figure 4-19: Screen Oversize PSDs from the Tests for Assessment of HPGR Product

Cake Competency ...................................................................................... 105Figure 4-20: Bond Ball Mill Index Results in Different Points of the Circuit ..................... 108Figure 5-1: Feed PSD for Circuit Modelling and Simulations ........................................ 111Figure 5-2: JKSimMetfi Screen Snapshot of the SABC Circuit Simulation .................... 113Figure 5-3: JKSimMetfi Screen Snapshot of the Final AG-Crusher-HPGR Circuit

Simulation .................................................................................................. 114Figure 5-4: Ball Mill Cyclone Feed PSD from AG-Crusher-HPGR and SABC Circuits .. 116Figure 5-5: AG Mill Feed (Combined) and Product PSDs ............................................. 117Figure 5-6: AG-Crusher-HPGR Circuit Simplified Flowsheet ........................................ 120

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ACKNOWLEDGEMENTS

I would like to express my gratitude to AMEC Mining & Metals, Vancouver, B.C., for the

generous support during the research period for his Doctoral Thesis. I would like to

extend a special thank you to my current and former managers, Alexandra Kozak and

Joseph Milbourne (respectively), for allowing me the significant amount of time required

to complete my research. I also would like to sincerely thank my friend and co-worker

Mike Grundy for his invaluable support and advice.

I am deeply thankful to my thesis supervisors, Prof. Robert Hall and Prof. Bern Klein, for

their guidance and patience. Members of the supervisory committee for valuable

advice. I would also like to thank the B.C. Mining Research, Koppern and the University

of British Columbia for providing the excellent research facilities needed for my work.

Of course, without my wifes encouragement, support and patience, and the love shown

by her and my three sons, this thesis would not be completed.

I would also like to acknowledge the support of the (anonymous) mining company for

supplying the sample used in experimental simulation and for co-sponsoring the

investigation of the feasibility of the novel HPGR circuit.

1

1 INTRODUCTION

1.1 Comminution

The dictionary definition (source: Merriam-Webster Dictionary) of the verb comminute is:

to reduce to minute particles. In the mining and mineral processing industry, the term

comminution mainly refers to crushing and grinding processes, although the size

reduction of rocks starts in the blasting phase of mining.

Comminution is an essential phase in mineral processing as it is required to liberate the

valuable minerals from the gangue. The breakage action is also described as the

creation of new mineral surface. Increasing mineral surface is essential for metallurgical

extraction processes such as leaching and flotation.

The energy requirement in comminution is a function of the reduction ratio, product size,

and the hardness characteristics of the material, i.e. its breakage resistance. The

relationship between required comminution energy, reduction ratio, product size, and

material properties has been the object of research for more than a century. The

theoretical and empirical formulas derived from this previous work and are summarized

by Jankovic et al. (2010).

Comminution in mining operations usually comprises the reduction of large rocks with

sizes around 1 meter or larger to minute particles of 25 microns or smaller. However,

most of the energy is used by the industry (89%) during the reduction from

approximately 20 mm to 100 microns (Powell, 2010).

Currently, the comminution process is energy intensive and highly energy inefficient. It

is estimated that comminution accounts for 65% to 80% of all energy usage in mining

2

operations and that only 1% to 2% of the applied energy is effectively translated in the

production of new surface area (Tromans and Meech, 2002). This expensive and

inefficient process also represents a significant fraction of the world electric power

consumption, e.g. in 1981, comminution processing accounted for approximately 2% of

the total U.S.A. electric power usage (Kawatra and Eisele, 2005).

The combination of the energy intensive and inefficient characteristics of comminution

implies that there is a great opportunity for significant energy and economic savings by

the improvement of this process (Kawatra and Eisele, 2005).

3

1.2 Modern Metal Mining

In the recent few decades, there has been a shift from the mining of high-grade, near-

surface, and relatively soft orebodies to low-grade, deeper and harder ores. The

depletion of high-grade ores and the increasing demand for metals have stimulated the

development of large-scale operations.

These large-scale mining operations extract the valuable minerals from massive

orebodies and are the main source for many base and precious metals. For instance,

the twenty largest copper mines around the globe were responsible for more than 60%

of all copper production from mines in 2008 (International Copper Study Group, 2009).

The advent of large tumbling mills has facilitated the development of these high-tonnage

deposits for the last three to four decades. These high capacity mills, specifically the

autogenous (AG) and semi-autogenous mills (SAG), have progressively replaced

crusher-based circuits due to their simpler flowsheets with fewer pieces of equipment. In

addition, these circuits do not utilize washing plants. Washing plants are usually

required ahead of a crushing circuit when dealing with orebodies that contain a high

level of weathered material (regions with high-clay content) or high moisture content (not

rare characteristics in these large orebodies).

Even though SAG-based comminution circuits are dominant in the industry, they do

present some challenges for the treatment of several types of large orebodies. If the

orebody contains significant hard ore, the SAG mill becomes extremely energy inefficient

as its capacity is highly reduced (Morley and Staples, 2010). High hardness variability

throughout the orebody produces significant SAG capacity variation and thus provides

an adverse overall throughput fluctuation (Burger et al, 2006). Similar fluctuations occur

4

when the SAG feed size distribution cannot be maintained relatively uniform through

time (Morrell and Valery, 2001).

Usually, the larger these low-grade deposits are, the larger the variance in rock

properties such as hardness levels. For instance, large porphyry copper ore deposits

(currently the largest source of copper ore) can present highly variable hardness and

some examples of such orebodies are Freeport-McMoRans Chino Mine in New Mexico

(Amelunxen et al, 2001) and Newmonts Batu Hijau operation in Indonesia (Burger et al,

2006).

5

1.3 HPGR in Hard Rock Mining

In recent years efforts to improve the comminution process have led to the integration of

the High Pressure Grinding Roll (HPGR) into non conventional applications. Until

recently this relatively new type of crusher was used in the cement, diamond and iron

industries. Over the last few years HPGR has expanded its application to base and

precious metals high-tonnage hard rock processing.

With the application of HPGR to new types of ores there has been debate as to their

suitability compared to the more traditional AG/SAG circuits (Morley and Staples, 2010).

One area that the HPGR manufacturers emphasize is the energy efficiency advantages

of the HPGR when compared to tumble milling. The HPGR manufacturers claim

substantial energy savings (up to 40% savings) when the HPGR circuit is compared to

conventional crushing and grinding circuits (KHD, 2002; von Seebach and Knobloch,

1987; Koppern, 2006).

There have also been indications by comminution consultants outside HPGR

manufacturing field that significant energy savings may be achieved on very hard ores

(Morley, 2006; Morrell, 2008) and research are research confirming this trend (Napier et

al, 1996; Shi et al, 2006). The recognition of these possible advantages added to the

recent developments in HPGR roll surface wear resistance trigged the adoption of

HPGR circuits for recent high-tonnage projects dealing with relatively homogenous, hard

to extremely hard rock orebodies and with limited clay content (Vanderbeek, 2006;

Seidel et al, 2006).

These recent applications have in common the application of the HPGR for tertiary

crushing. Their circuits are very similar to the 3-stage crushing circuits that were vastly

6

applied until the 1960s. Now, in high-tonnage mining these circuits are restricted to

extremely hard ores or in processes where throughput stability is of primary importance.

In other words, at the time of this research the HPGR was only replacing tertiary cone

crushers in a very limited niche for base/precious metal mining.

As a matter of fact, the research community seems to be now realizing this limitation.

Very recently, Prof. Powell presented a paper at the Comminution10 conference, and

confirmed that although the great potential of HPGR is starting to be recognized, a better

understanding of the technology and development of different HPGR flowsheets are

required to ensure that this technology is fully exploited (Powell, 2010).

There are still many unknowns with respect to the types of applications in which HPGR

can be used and as of yet there has been little work done to develop a comprehensive

approach to evaluate the overall efficacy of HPGR circuits versus other circuits. This

research focuses on high-tonnage, base/precious metal operations, the most recent

area where HPGR has been applied. As with any new technology, there is limited

knowledge about it and its true benefits. This research aims to improve the

understanding of the potential benefits and applications of HPGR circuits, and address

some current uncertainties, such as:

Whether a complete HPGR comminution circuit is still able to provide substantial

net energy savings when compared to a SAG-based circuit.

Will the HPGR bring the same benefits when applied on orebodies with mixed

hardness and/or orebodies with high clay content, which are characteristics of

various large copper ore deposits in the world?

7

1.4 Thesis Objectives

The current energy inefficient comminution circuits that are applied in base/precious

metal mining present considerable opportunity for significant energy and economic

savings. In addition, the application of HPGR has demonstrated energy benefits in

comparable applications such as in cement and diamond processing.

Recently, similar benefits are being claimed through the replacement of cone crushers

by the HPGR in conventional 3-stage crushing circuits for some specific hard rock metal

mining cases. However, to take fully advantage of their benefits and to broader their

applications improved understandings of their technology as well as the development of

different HPGR flowsheets are required.

This research focuses on high-tonnage, base/precious metal comminution circuits and

the primary objective of this work is to improve the understanding of the potential

benefits and applications of the HPGR in such circuits. In pursuit of the primary

objective the following secondary objectives are targeted:

Expand on current work to develop a structured methodology for the evaluation

of the energy requirements of the complete HPGR circuits by the application of

circuit design best practices and advanced modelling techniques.

Evaluate and demonstrate the applicability of the structured methodology for the

comparison of SAG and HPGR overall circuit energy requirements through case

studies.

Develop an innovative HPGR flowsheet to treat mixed hardness ores and/or

weathered ores with a high proportion of clays and moisture.

8

Through a case study assess the suitability and the potential benefits of the novel

circuit for the comminution of hard, weathered ores containing clayish material.

Develop a rigorous approach for testing HPGR circuits by the application of a

unique pilot-plant test program as a basis for experimental simulation.

9

1.5 Thesis Outline

This Thesis is divided into five sections:

The first section (Chapter 2) covers the history of comminution circuits and basic

concepts related to grinding mills and the HPGR.

The second section (Chapter 3) presents the developed structured process for the

design and energy requirement evaluation of comminution circuits. In addition, two

trade-off case studies, based on real ore data, are detailed to demonstrate the

applicability of the procedures.

The third section (Chapter 4) introduces the novel HPGR flowsheet and details the

testwork program used for its evaluation. Also in this section, testwork results are

presented and discussed.

The fourth section (Chapter 5) described the design of a comminution circuit utilizing the

novel HPGR flowsheet for an existing copper-gold orebody. Analyses of the expected

outcomes are given as well as a comparison between the proposed circuit and the

conventional circuit that was previously proposed for the development of the same

orebody.

The final section (Chapter 6) covers the research main contributions and future research

opportunities.

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2 COMMINUTION CIRCUITS1 - LITERATURE REVIEW

2.1 Introduction

Some facts are self-evident: commodities prices fluctuate; high-grade, large deposits

with easy-to-process ore are uncommon; and energy efficiency is a public matter.

However, mineral processors adapt as well as most to the changing economic

environment, especially in the field of comminution. Comminution is the largest energy

consumer in mineral processing, and, if the ore is hard, requires the largest capital and

operating cost. In modern low-grade mining operations, the scale of the use of energy

and other consumables is unprecedented (Charles and Gallagher, 1982; Abouzeid and

Fuerstenau, 2009).

Proper design of the comminution circuit is a critical task, especially for large-scale hard-

rock projects. Today, several options are analyzed when designing such a circuit.

Some are based on long-established technologies, and others are based on more

recently developed technologies, or technologies that have been adapted from other

types of projects. Selecting the most appropriate circuit is of paramount importance, not

only in deciding the equipment, but also how it is configured. The design task can be

quite different in greenfields projects than in expansions, or in modifications of existing

circuits (Barratt and Sherman 2002).

1 A version of this chapter has been published. Rosario P.P., Boyd K. and Grundy M. (2009). Recent

Trends in the Design of Comminution Circuits for High Tonnage Hard Rock Mining. Recent Advances in Mineral Processing Plant Design, eds. Malhotra D., Taylor P.R., Spiller E., and LeVier M., Society for Mining, Metallurgy, and Exploration, Inc. (SME), pp. 347-355

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2.2 Recent History of Comminution

From the 1920s to 1950s, most comminution circuits were designed with several stages

of crushing, followed by rod and ball mills. During the 1960s, the use of rod mills

declined, as larger diameter ball mills, accepting coarser feeds, became available. The

1960s also saw the advent of autogenous, and, later, semiautogenous mills, and by the

early 1970s, large-diameter autogenous grinding mills (AG), and semiautogenous

grinding mills (SAG), often together with ball mills, became the accepted norm. Although

the power consumption was generally higher, the simpler circuits with fewer components

and smaller footprints made the overall economics of SAG mills superior to three-stage

crushing in most cases (Bond 1985). These SAG circuits opened the door to the high-

tonnage, low-grade operations that have characterized the base metal industry for the

past 40 years. The application of these large tumbling mills increased in such a way that

from the early 1980s to the early 2000s most new or expansion mining projects have

selected some circuit configuration that includes either an AG or a SAG mill (Barratt and

Sherman 2002).

More recently, two factors have driven a change in this trend, especially in hard ore

operations. Firstly, the wish to reduce energy consumption intensified, driven not only

by economics, but also by public interest in climate change, greenhouse gas emissions

and carbon footprint. Secondly, high-pressure grinding rolls (HPGR) became more

attractive as their manufacturers developed roll-wear protection systems to better deal

with hard and abrasive ores. As HPGRs are more energy-efficient than conventional

grinding mills, and because large HPGRs can deliver higher unit throughput at higher

reduction ratios than tertiary cone crushers, some projects are now using HPGRs in

combination with secondary cone crushers instead of SAG mills.

12

Stirred milling technology was developed in the 1950s but has only been applied for

mineral processing during the last couple of decades. There are a few different models

of stirred mill machines on the market and they have been mostly used for regrind

applications. The stirred mill presents better energy efficiency than ball mills for fine

grinding and during the last few years there has been an increasing interest in applying

this technology to coarser grinding ranges (Valery and Jankovic 2002).

13

2.3 SAG Mill Background

2.3.1 AG/SAG Mill Machines

A SAG or AG mill, as with any other type of tumbling mill, is a metallic drum of cylindrical

or in most cases cylindro-conical shape which rotates on its horizontal axis. Raw

material and water are fed through an opening at one end of the mill and discharge

through the other end. The interior surface is lined with resistant material such as

rubber, steel or a combination of them to provide wear protection. In addition, lifters, i.e.

raised sections of the liners, are used to lift and direct the fall of the charge during

rotation.

AG and SAG mills are usually characterized by their large diameter dimension and their

aspect ratio (diameter to length relation) which, differently than the ball and rod mills, is a

high ratio in the order of 1.5 to 3 (Napier-Munn et al, 1996). Another difference is related

to the discharge design, AG and SAG mills are usually equipped with grated discharge

ends to hold back large pieces of rock and steel balls (in SAG mills) and to allow the flow

of the slurry containing the fines (usually a portion of the feed and obviously the ground

material).

These mills can be either shell or trunion bearing supported and most of them are

electric motor-gear driven with single or twin-double pinion arrangements. However, as

currently the limit of power transmission through a pinion is around 7,500 kW (Evans et

al, 2001), the large mills with 11 m diameter (36 ft) and higher and requiring 15,000 kW

and more, are equipped with gearless electric drives. Currently the largest mill in

operation has a diameter of 12.2 m (40 ft) and is equipped with a gearless drive with

22,000 kW. Based on mill vendors information, the largest mill that could be currently

engineered would be limited to 13.4 m (44 ft) diameter (Vanderbeek, 2004).

14

Three breakage mechanisms occur inside a SAG or AG mill, they are: abrasion, attrition

and impact (Napier-Munn et al, 1996). Impact breakage is achieved by the cataracting

of the load (steel media and slurry raw material plu s water) due to the high speed

rotation; cataracting action meaning the free fall of the load above itself. Abrasion and

attrition are generated by the rolling movement of the load as the material lifts and slips

together. The balance of the energy applied in the comminution of the rocks is

dissipated in the form of heat, noise and the wear of the grinding balls and the mill liners

(Norman and Decker, 1985).

The control of raw material and water feed-rates, and mill speed (for mills equipped with

variable feed drives) is essential for smooth operation and minimum comminution of

media and liners. For example, if the property of the feed rapidly changes and softer

and finer than normal feed is present, the operator (or automated control system) may

need to decrease the feed-rate of the raw material and lower the speed of the mill to

avoid a decrease in the mill load level and thus an increase in the frequency of media-

media and media-liner impacts.

2.3.2 SAG Operational Parameters

Steel ball charges range from 0% (AG mill) up to 20% by volume, and a typical value for

SAG is 12%. The total charge (balls plus slurry) is usually between 20% and 35%, and

the slurry is usually between 65% to 75% solids. The most frequent ball size for large

mills is 127 mm diameter, but it can vary from around 90 mm to a maximum of 152 mm

(Sepulveda 2008).

The recent trend has been to operate at increasingly high ball loads, and at increasingly

low total loadingit has been observed that a lower tot al charge improves capacity.

Today some operations operate with ball charges up to 20%. Total mill volumetric

15

loading has decreased from around 35% in the early days to as low as 24% or below

(Sepulveda 2008).

High ball charges have only been made possible by the advent of the variable-speed

drive, one of the most significant advances in SAG milling. The variable-speed drive

was first installed on a SAG mill at Afton (1977) (Thomas 1989) and is now almost

universally used. An early example of the advantage of variable-speed drives was at

Lornex (now Highland Valley Copper), where a variable-speed mill installed in 1981 was

operated at up to 19% ball load, compared to 12% for fixed-speed mills in parallel

circuits. The operators could drive the new mill harder, confident that if the ore suddenly

became softer, they could slow the mill down to protect the shell.

2.3.3 SAG Mill Original Circuit

Before SAG milling entered the scene, large grinding plants consisted of many trains of

two or three stages of crushing, rod milling, ball milling, and the associated conveyors,

screens and surge bins (Figure 2-1). The SAG mill gained its leading status in large mill

operations because of its ability, in a single unit, to receive coarse primary crusher

product and deliver adequate ball mill feed at high operational availability (approximately

93%) (Figure 2-2). Development since the early days has centered on increasing the

amount of ball mill feed that a single unit produces.

16

Figure 2-1: Three Stages of Crushing, Rod Mill, Bal l Mill

Since their appearance in the 1970s, SAG mills have increased in size and power, their

drive systems are more advanced, they are equipped with better control systems, and

their benefits and shortcomings are better understood. These developments resulted in

new circuit configurations and programs to improve the quality of feed. Many of the

more significant advances were made by operators determined to extract more from

what they were given.

Large diameter SAG mills have been selected for new hard rock projects and

expansions (Los Bronces Development Project, Phoenix Project, San Cristobal) which

indicates that, depending on the ore type and project specifics, a SAG circuit may still be

the preferred choice.

Secondary Crushers

Coarse Ore

Secondary Screens

Process Water

Rod Mills

Tertiary Crushers

Tertiary Screens

Ball Mills

Flotation

Fine Ore Bin

17

Figure 2-2: SAG-Ball Mill Circuit

2.3.4 Pebble Crushing for AG/SAG Circuits

Competent rocks in the 12 mm to 75 mm range (critical size) present reduced breakage

rates in autogenous (AG) mills. A significant contribution of grinding media in a SAG mill

is to accelerate the breakage of critical size material to reduce its tendency to

accumulate in the mill. Another, nowadays less common, method of preventing the

build-up is the Autogenous Mill-Ball Mill-Crusher (ABC) circuit, where the critical size

material is extracted from the mill, crushed, and returned to the mill. These two

techniques were combined during the 1980s, when there were several successful

attempts by operating mines, to improve their SAG mill performance by using pebble

crushersthe Semiautogenous-Ball Mill-Crusher (SABC) circu it. Examples are Los

Bronces, Similkameen (Major and Wells 2001) and Chino (Vanderbeek 1989). Inclusion

of a pebble circuit has become almost standard in the design of grinding circuits (Figure

2-3). Even if it is not thought appropriate to install pebble crushers at the outset, it is

usually considered prudent to leave space should circumstances require pebble

crushing later in the operation.

SAG Mill

SAG Mill Discharge

ScreenBall Mills

Coarse Ore

FlotationProcess Water

18

Figure 2-3: SABC Circuit

For hard and very hard ores (JK Axb values below 40 and Bond Work indices above

16 kWh/t), correct forecasting of the production of critical-size material, and of its

extraction rate through the mill grates, is still difficult. There have been reports of

operations that spent great effort to achieve the designed pebble extraction, and

therefore the design throughput, for quite some time after startupfor example Cadia

Hill (Hart et al 2001) and Sossego (Delboni et al 2006).

Until recently, AG mill and SAG mill circuits were invariably designed in closed circuit

with the screen and pebble crusher, with the screen oversize portion being crushed and

completely recycled to the mill feed. Recently, however, some SABC installations have

been operated in open circuit by having the screened crusher product report to the ball

mill circuit (Figure 2-4). The effect of opening the circuit is to pass more tonnage at

coarser size to the ball mill circuit. Consequently in most cases it has been used to

increase throughput of an existing operation which had extra ball mill capacity or could

tolerate a coarser grinding-circuit product size. There are also new installations (most at

the planning stage) where the largest available SAG mill could not reach desired

SAG Mill

SAG Mill Discharge

ScreenCoarse Ore

Ball Mills

Flotation

Pebble Crushers

Process Water

Pebble Bin

19

capacity with the pebble crusher in closed circuit. Thus an open-circuit SABC was

chosen. An example is the El Teniente Colon Concentrator with an 11.6m diameter

SAG and four parallel pebble crushers in SAG open circuit configuration (Spann and

Ottergren 2004).

Figure 2-4: Open-Circuit SABC

A design where the pebble crusher can either be used or bypassed provides the

operator with some external operating control of the SABC circuit. The ability to open or

close the circuit during operation provides additional flexibility. The authors recently

completed a study for a property where the run-of-mine ore had zones of greatly

fractured ore, and zones of very competent ore, both with high ball-mill work indices. It

was proposed to lay the plant out so that the operator could bypass the crushers and

operate the SAG mill in closed circuit when receiving fractured ore (to maximize SAG

mill power) and, when the ore was competent, use the pebble crushers and even open

the circuit, to pass more of the work to the ball mills.

SAG Mill

SAG Mill Discharge

ScreenCoarse Ore

Ball Mills

FlotationProcess Water

Crushed Pebble Screen

Pebble Crushers

Pebble Bin

20

Another recent development in pebble crushing is the addition of HPGRs to treat the

pebble crusher product (Figure 2-5). The pebbles are reduced to a much finer product

thereby decreasing ball mill power requirements. Depending on the original circuit,

opening the SABC circuit and adding an HPGR stage may achieve a significant capacity

increase, without increasing the ball mill duty requirement (Dixon et al, 2010). This

concept can also be applied to a circuit that will ramp-up after startup. For example the

Peasquito project has started up with a single SAG line in mid 2009, a second SAG line

was added in mid 2010, and later one HPGR will be added (Goldcorp 2009).

Figure 2-5: SABC with HPGR

Screens

Early designs for the screens closing the AG/SAG mill circuit were either trommel

screens with water jets returning pebbles through the discharge end of the mill, or

vibrating screens with a series of conveyors returning the oversize to the feed end of the

mill. Some (such as Lornex and Copperton), used a combination of trommel screens,

pumps and vibrating screens. Since pebble crushing circuits have become common, the

SAG MillSAG Mill Trommel Screen

Coarse Ore

Ball Mills

Flotation

Process Water

Crushed Pebble

Pebble Crushers

Pebble Bin

HPGR

HPGR Screen

HPGR Storage

Bin

Pebble Washing Screen

21

trommel screen/water jet has become less so. Some companies (e.g. Alumbrera and

Antamina) have later added recycle conveyors and pebble crushers to their trommel

screen system, and keep the water off when the pebble crushers are in use. More

recent large operations employ trommel screens to remove most of the slurry, followed

by vibrating screens to wash the pebbles before discharging them onto the recycle

conveyors.

Pebble Surge Capacity

Early SABC circuits incorporated crushers retrofit into SAG mill recycle conveyor

systems, and often had no surge capacity. Surge capacity is highly desirable, enabling

the crushers to be choke fed by controlling the feed rate. Thus pebble bins are now

included in circuits as a matter of course. The scale of many recycle operations is now

at the point where a pebble stockpile is more economic than a pebble bin.

2.3.5 SAG Feed Preparation

In the early years after the advent of the SAG mill, typical ball charges were in the range

of 3% to 7% of mill volume, and the general consensus was that large rocks in the feed

were always necessary to assist in breakage. Under present operating conditions of

high ball load and low total loads, the contribution of large rocks as grinding media is

insignificant. It is now realized that improving the blasting and primary crushing phases

to deliver a consistently fine feed to the mill are cost-effective contributions to the overall

comminution system. Several operations have demonstrated substantial improvement

in SAG production by feed preparation programs (mine-to-mill), improving production by

a factor of up to 15% (Lam et al 2001).

SAG throughput is very susceptible to changes in the hardness of the ore and this

should be assessed at early stages of design. In cases where the orebody presents a

22

high variability of friability, provision for blending may be an option to minimize high

fluctuation in production (Dance 2004). If a well defined plan to maintain reasonably

uniform ore hardness is not possible, the operation should be prepared to sustain

fluctuations in tonnage.

In some cases, pre-crushingscalping off and crushing coarse m aterial in the SAG mill

feedhas been applied to manage the SAG feed size distr ibution (Figure 2-6).

Pre-crushing can be used where there is limited scope to optimize blasting and primary

crushing, as in block-caving underground mines. In addition, pre-crushing has been

applied to maintain designed production levels at mines where the ore hardness has

increased over time or to expand production (Sylvestre et al 2001).

Figure 2-6: Pre-Crushing in an SABC Circuit

2.3.6 Steel Wear

In comminution circuits, steel is used in the form of steel balls as media for the tumbling

mills, both for the SAG and the ball mills. Steel is also used in many other components

such as: mill liners, HPGR rolls, crusher liners, chute liners, bin liners, etc. The total

consumption of steel is usually a high operational cost (Charles and Gallagher, 1982). In

SAG Mill

SAG Mill Discharge

Screen

To Ball Mills

Process Water

Crushed Pebble Screen

Pebble Crushers

Pebble Bin

Coarse Ore

Secondary Crushers

Coarse Ore Screens

23

addition, the consumed steel requires energy for its mining, refinement, manufacturing

and transportation phases and represents a significant indirect (or embedded) energy

consumption even when compared to the amount of direct comminution energy

(Radziszewski, 2002; Pokrajcic and Morrison, 2008; Musa and Morrison, 2009).

Although the precise estimation of the steel ball consumption is not a straightforward

task, it is common during the design phase of the projects to estimate the wear rate

through a combination of ore abrasiveness test work, empirical models and historical

data.

The empirical model most commonly used is based on the work by Bond for small

diameter ball mills with some reduction in the magnitude of its constants, as suggested

by Norman and Decker. (Bond, 1964; Norman and Decker, 1985). This model utilizes

the Bond Abrasion index (Ai) as input to determine the wear in grams relative to the

specific power applied. The original equation formulated by Bond for wet ball mills, is as

follows:

Ball-wear in lb/kW-h = 0.35 (Ai - 0.015)1/3

There are two main deficiencies of this model:

the Ai is determined in a dry test and the differences in chemical characteristics

of the pulp in wet milling are not taken into consideration, and,

steel quality differences are not included in the model and metal quality has

significantly improved since the development of the model in 1963.

Halbe and Smolik (2002) state: Unpublished data indicates that for current high quality

metallurgical steel these calculated values [of ball wear] could be reduced [by] as much

24

as 50%. A good procedure is to conduct Ai tests to determine how the sample

evaluated compares with others [ores]. With Ai information it is possible to review

operating data from other plants with similar conditions and Ais, and make a reasonable

estimate of expected wear. Generally the lab performing the tests will have a data base

of this sort of information. Engineers at the test lab or consulting engineers with

extensive experience in grinding circuit [design] can be very useful here.

Radziszewski and his associates at McGill University are developing a comprehensive

mathematical model of steel media wear as a function of mill operating parameters as

well as a set of test procedures to simulate the effect of both, corrosion and abrasion

wear mechanisms (Radziszewski, 1997; Radziszewski et al, 2005). Unfortunately, as

per his last known publication on this matter in 2005, the model seems to be still in the

development phase.

25

2.4 HPGR Background

2.4.1 HPGR Machine

The origin of HPGR can be linked to coal briquetting equipment developed in the early

1900s (Morley, 2006). However, HPGR as a comminution machine was developed in

the early 1980s and is a product of fundamental and applied research on fracture

phenomena conducted by Professor Klaus Schonert (Bearman, 2006). The HPGR was

first introduced around 1985 to treat relatively soft material in the cement industry.

Comminution in a HPGR is achieved by the high pressure compression of a bed of

material which results in high interparticle stresses, i.e., the crushing principle could be

viewed as having rocks compressed in a piston press. The retention time for the

material in a HPGR is very short. The interparticle breakage mechanism enables a low

level of consumed energy and results in a high proportion of fines in the HPGR product

(Tavares, 2005; Gunter et al, 1996).

The HPGR machine has two counter-rotating rolls mounted in a sturdy frame as shown

in Figure 2-7. One roll rotates on a fixed axis and the other one, the floating or

moveable roll, is allowed to move linearly on rails and is positioned by the action of a

hydro-pneumatic system. The material is fed through a shaft feeder creating a forced

feeding action by gravity. The use of the rotating rolls enables a continuous pressing

process instead of a batch process that would be achieved by a limited throughput

piston press type of machine.

26

Figure 2-7: Schematic of a HPGR (Napier-Munn et al, 1996)

HPGR should not be confused with conventional crushing rolls. Klymowsky et al (2006)

detailed the distinctive characteristics between them, as summarized next:

the HPGR is equipped with a hydro-pneumatic system to apply and maintain a

high pressure condition within the crushing region

they are operated at much lower speeds than crushing rolls (around 20 rpm,

approximately one third of the crushing rolls speed)

HPGR has a unique feed system to maintain constant choke feed conditions,

and, the surfaces of their rolls are made of highly wear resistant materials.

There are three manufacturers of the HPGR machines, all with headquarters in

Germany, they are:

ThyssenKrupp Polysius

Koeppern (or Kppern in German)

KHD Humboldt Wedag AG

Fixed roll

Feed

Moveable rollOil cylinders

Product

Nitrogen cylinder

27

There are a few differences in the design of the machines depending on the

manufacturer. Polysius machines usually have a high aspect ratio roll design, i.e., the

ratio between the diameter and the length of the roll. An example of a currently large-

size Polysius machine would be one with 2400 mm roll diameter and 1600 mm roll

length. The other two makers favour a low aspect roll design, and an example of a KHD

standard construction machine size would be one with 1700 mm roll diameter and

1400 mm roll length.

In order to minimize roll surface wear when treating abrasive materials, all

manufacturers are able to provide some kind of protection layer for the rolls. Tires with

tungsten carbide studs are used to create an autogenous layer on the roll surface, i.e.

material builds up on the surface area in between the studs to create an ore layer on the

roll. This technology is offered by KHD and Polysius. Koppern has developed

Hexadurfi, a hard surface layer consisting of ceramic hard phases embedded in a

hardenable steel matrix (Gardula et al, 2005).

The application of HPGR in comminution circuits has increased over the past two

decades and is well established in processing cement, diamonds and iron ore

(Broeckmann and Gardula, 2005). In the last few years HPGR plants to process

precious and base metals from hard ores have been designed and started up. The main

examples are:

SM Cerro Verde, copper, Peru

Boddington, gold, Australia

Mogalakwena North, platinum, South Africa

PT Freeport Indonesia, copper-gold, Indonesia

28

Zapadnoe, gold, Irkutsk-Russia

Bendigo, gold, Australia

2.4.2 HPGR Terminology and Operational Parameters

A number of terms and operational parameters are particular to the HPGR and the most

relevant ones are listed as following:

HPGR product cake or flake

specific throughput, m-dot

operating gap, Xg

specific pressing force, FSP

specific energy consumption, ESP

HPGR product cake or flake

The HPGR product generally contains a blend of loose particles and agglomerated

cakes or flakes, in different proportions and sizes, de pending upon ore characteristics

and machine operational parameters; such as feed PSD and moisture content, applied

pressure, and gap width. Cake strength or competency is usually low, and commonly

these brittle lumps can be easily broken by hand. (Gruendken et al, 2008). To the best

of the authors knowledge there are no standard procedures to evaluate cake

competency.

Specific throughput

The specific throughput, m-dot, is a factor that is regularly obtained from a laboratory- or

pilot-scale HPGR test and is calculated by dividing the value of the measured throughput

(t/h) by the testing machine roll diameter (m), roll width (m) and the peripheral roll speed

29

(m/s). The m-dot consequently is expressed in ts/hm3 units and indicates what

throughput would be achieved from a machine with 1 m x 1 m rolls operated at 1 m/s for

the tested material. If the testwork is properly conducted to closely simulate expected

industrial-scale conditions, such as: moisture content, operating pressure, and roll

surface properties, the m-dot can be assumed to be constant and directly used for

throughput estimation of different size machines (Bearman, 2006).

where:

m-dot = specific throughput (ts/hm3)

M = throughput rate (tph)

D = roll diameter (m)

L = roll width (m)

n = peripheral roll speed (m/s)

Operating gap

The operating gap is the minimum distance between the rolls. The HPGR gap,

differently than the close side setting (CSS) in the crushers, is not the determining factor

for size reduction but just an indication of the top size in the product. The inter-particle

comminution mechanism enables high levels of size reduction and fines production even

with apparent large gaps (Gruendken et al, 2008).

The gap is a function of the roll diameter and the friction between the feed material and

the roll surface. Larger diameters and/or higher friction factors provide larger gaps and

thus higher throughputs. The friction is affected by feed material proprieties (such as

30

moisture and particle size distribution) and roll surface properties, e.g. studded rolls with

their substantial autogenous layer provide higher friction than hard-faced smooth rolls

(Klymowsky et al, 2006).

The operating gap, together with the roll speed, can also be used for throughput

calculations by the use of the continuity equation, as follows:

where:

M = throughput rate (tph)

Xg = operating gap (mm)

n = peripheral roll speed (m/s)

L = roll width (m)

rc = density of the product cake (t/m3)

Specific pressing force

The specific pressing (or grinding) force corresponds to the total hydraulic force exerted

on the rolls divided by the roll surface area, i.e. it is the total force divided by the roll

diameter and width and is expressed in N/mm2 (Klymowsky et al, 2006).

where:

Fsp = specific pressing force (N/mm2)

F = applied pressing force (kN)

31

D = roll diameter (m)

L = roll width (m)

The specific pressing force is a useful parameter for machine scale-up and its value is

usually in the range of 1 to 9 N/mm2 (Bearman, 2006).

Specific energy consumption

The specific energy consumption (or specific energy input) corresponds to the machine

power input (kW) divided by throughput rate (tph), and thus is expressed in kWh/t. For

machine scale-up and performance comparisons the net specific energy consumption is

more appropriate whereas the calculation is performed with the total net power input,

i.e., the idle power draw is discounted.

Usually the value of the specific energy consumption ranges from 1 to 3 kWh/t. The

specific energy is directly proportional to the specific pressing force and, similarly to

other comminution machines, harder ores require higher values of specific energy when

compared to softer ores to achieve similar size reductions (Bearman, 2006).

2.4.3 HPGR Original Circuits

Cement

HPGR was first introduced in 1985. Since then it has found usage in the cement

industry. Cement production usually involves three phases. In the first phase raw

materials, such as limestone, are ground. In the second phase, the ground components

are mixed and undergo a chemical reaction in a rotary kiln at high temperatures

producing the cement clinker. The third phase is a final grinding phase to reduce the

clinker nodules to 100% passing 90 microns size. Both the pre-treatment of raw

32

materials and the final clinker grinding phases are performed dry. Ball mills, usually two-

compartment mills divided by a diaphragm and using different steel ball sizes in each

compartment, are generally used for clinker grinding (Jankovic et al, 2004).

The first applications of HPGR in cement were in manufacturing plant retrofits. This was

done by the addition of the machine upstream of the clinker grinding mill. With time,

different circuit configurations have been applied and the HPGR has been able to

provide 10% to 50% energy savings in cement grinding (Patzelt, 1992). Figure 2-8

shows one type of circuit that is applied for the precrushing of clinker.

Figure 2-8: Open Circuit HPGR Closed-Circuit Ball Mill (Aydogan et al, 2006)

Iron Ore

In the iron industry, the HPGR was first introduced in 1995 and its application has been

growing since then. For iron ore the HPGR has been applied either as a standalone

stage or ahead of ball mills to improve the efficiency of the grinding required in pellet

feed production. In addition, the machine has been installed in AG-mill circuits (primary

33

grinding of the ore) to re-crush pebble crusher product (KHD, 2008). The HPGR product

is either returned to the AG mill feed or directed to the downstream processes.

An example of the application of the HPGR for re-crushing and being in closed circuit

with the AG mill is shown in Figure 2-9. This type of circuit enhances the mill capacity by

releasing the AG from pebbles which have a limited breakage rate and may build up the

charge volume inside the mill unless the feed rate is reduced. It is noteworthy that most

of the comminution energy is still applied by the conventional AG mills and that the

crusher and HPGR act mostly as auxiliary equipment.

Figure 2-9: HPGR Applied for Pebble Re-Crush at Emp ire Iron (Kawatra et al, 2003)

Maybe the initial success of the application of HPGR in iron ore AG pebble re-crush

motivated Krupp-Polysius to patent several circuit configurations with HPGR for pebble

crushing. The patent was issued in 1999 and covers the application of the HPGR as a

34

standalone unit for AG/SAG circuit pebble crushing (Knecht, 1999) but the author is not

aware of any industrial application to date.

Diamonds

In diamond ore processing, the main drive for the utilization of the HPGR is the selective

grinding capability that enables the crushing of kimberlite-diamond ore while preserving

the relative large diamond gems. The circuit design correlates to the objective of

extracting the large gems. Thus, common circuits are built with quite complex

classification systems, such as combination of multi-deck screens and density media

separators (DMS), and scrubbers. Figure 2-10 shows a portion of the circuit (re-crush

stage) applied for Argyle Diamond Mines in Australia. (KHD, 2008)

Figure 2-10: Re-Crush Circuit at Argyle Diamond Min es (KHD, 2008)

35

2.4.4 HPGR Precious/Base Metal Recent Circuits

In the last few years HPGR-based plants processing hard ore in high-tonnage precious

and base metals operations have started production. The two main examples in high-

tonnage operations are SM Cerro Verde (start-up in 2006) and Boddington (start-up in

2009).

It is claimed that of the many possible flowsheets that have been proposed for HPGR,

those using HPGR as tertiary crushers, in closed circuit with fine screens, are expected

to provide maximum energy efficiency in hard-rock applications (Morley 2006a). In

addition, the secondary crushing product is screened before feeding the HPGR to avoid

oversized material damaging the rolls. This configuration is illustrated by the Boddington

and Cerro Verde flowsheets (Figure 2-11 and Figure 2-12).

Figure 2-11: Boddington HPGR (Dunne et al 2007)

Coarse Screens

Ball Mills (4)

Flotation

Primary Crushers

(2) Coarse Ore Stockpile

Secondary Crushers

(5)

HPGR (4)

Flash Flotation

Process Water

Fine Ore Stockpile

Fine Screens

(8) (Wet)

Gravity Separation

36

Figure 2-12: Cerro Verde (Vanderbeek 2006)

In a variation of these flowsheets (with the HPGR in closed circuit with screens), the

HPGR can be equipped with a dividing chute; the product from the centre of the rolls is

directed to the ball mill, and the material produced at the edges of the rolls, which is

coarser, is returned to the HPGR feed. This form of HPGR product recirculation has

already been applied in iron ore projects and has been recently developed for some

base metals projects (Gruendken et al 2008).

The Boddington project has a design capacity of 35Mtpa (approximately 96,000 t/d) and

processes two very hard gold ores with average Bond ball mill work indices (BWi) of

15.1 and 16.6 kWh/t, Bond rod mill work indices (RWi) of 22.8 and 24.2 kWh/t, and JK

Axb values of 27.9 and 25.5. The circuit is comprised of: five 746 kW cone crushers,

four 2.4 m diameter(D) x 1.65 m length(L) 5.5 MW HPGRs, and four 7.9 m D x 11.9 m L

(26 x 39 ft) 15.6 MW ball mills (Dunne et al 2007). The projected roll surface wear life

was estimated at 4,250 hours. A 2006 trade-off study showed that a preliminary SABC

circuit would have 7% lower capital costs than the HPGR circuit, and that the HPGR

circuit provided 12% savings in comminution operational costs. Furthermore, the study

concluded that the lower operational costs of the HPGR circuit offset its higher capital

costs (Seidel et al 2006).

Screens

Ball Mills (4)

Flotation

Secondary Crushers

(4)

Ball Mill Feed

Screens ( Wet)

HPGR (4)

Coarse Ore

Surge Bin

Process Water

Ball Mill Feed

Surge Bin

Fine Ore Surge

Bin

Primary Crushing

37

Cerro Verde has a design capacity of 108,000 t/d of hard copper-molybdenum ore

(average BWi 15.3 kWh/t). The circuit is comprised of: four 746 kW cone crushers; four

2.4 m D x 1.65 m L 5.0 MW HPGRs; and four 7.3 m D x 10.7 m L (24 x 35 ft) 12 MW ball

mills. The projected roll surface wear life is 6,000 hours. In 2006, just prior to startup, it

was reported that although estimated capital costs were higher for the HPGR circuit than

an equivalent SAG circuit, the estimated total comminution operational costs were 1.33

US$/t and 1.70 US$/t for the HPGR and SAG circuits respectively. The main

contributors for this difference are the costs of power and grinding media. The estimated

total comminution circuit specific energy for the SAG circuit was 20.1 kWh/t and for the

HPGR circuit 15.9 kWh/t. In addition, risk analysis results and internal rate of return

factors were responsible for the decision to build an HPGR circuit instead of SAG circuit

(Vanderbeek 2006).

AG circuits are notoriously sensitive to changes not only in ore hardness, as previously

noted, but also in feed size. SAG mill circuits are more stable, and SABC circuits eeven

more stable. However, SAG-based circuits are still very sensitive to feed variations

(Vanderbeek 2006; Morrell and Valery 2001). Anglo Platinum, at the Mogalakwena

North concentrator, selected crushing technology in large part because it gave stability in

feed rate and product size. HPGR was selected in particular, because of its economic

advantages over tertiary and quaternary crushing (Rule 2006).

2.4.5 Energy Savings

As described in section 2.4.1, breakage in the HPGR is associated with high interparticle

stresses in the machines compression zone and occurs relatively fast. This breakage

mechanism enables a low level of consumed energy and creates a high proportion of

fines in the HPGR product, thus providing a high level of comminution energy efficiency

38

(Tavares, 2005), (Gunter et al, 1996). For tumbling mills, such as the SAG mill, the

comminution energy efficiency is lower than the HPGR. This is due to their breakage

system that behaves as an unconfined system (loose bed), i.e., a great portion of the

applied energy is lost through several non-breakage dynamics that are inherent from the

machine design and interparticle interaction effects (Fuerstenau and Abouzeid, 1998).

The HPGR manufacturers emphasize the energy efficiency advantages of the HPGR

when compared to tumbling mills. In one of KHDs brochures it is claimed that: For

most ores, the specific energy consumption lies at around 0.8 3.0 kWh/t. Especially

when coupled with subsequent downstream processes or high efficiency classifiers,

overall grinding energy reductions as high as 40% have been observed. (KHD, 2002).

Polysius declare that the HPGR allows for lower operat ing and maintenance costs due

to energy savings of up to 20% and reduction of wear to less than 1% for dry and less

than 0.1% for wet milling. (von Seebach and Knobloch, 1987). Koppern reports that

operating experiences and collected data indicate substantial advantages in energy

savings, material throughput capacities and product quality of HPGR technology versus

traditional crushing and grinding equipment. (Koppern, 2006).

Some researchers and consultants in the mineral industry also discuss the HPGR

energy benefits, but usually less emphatically. Napier et al (1996) commented that

savings between 15% to 50% had been reported but cautioned that many of these

reports were based on small-scale machines. Morley asserts that the HPGR is the

most energy-efficient comminution technology available (Morley, 2006). Morrell also

believes in overall energy benefits for the circuit but highlights that the benefits may be

achieved on very hard ores and in circuits where a large portion of the work shifts from

milling to crushing and HPGR (Morrell, 2008). In 2006, Shi et al observed 8% to 29%

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savings in total energy in a HPGR study treating platinum ores. The comparison were

made between two lab-scale circuits, one comprised of a jaw crusher, conventional

crushing rolls and ball mill, and the other comprised of a HPGR and ball mill. This study

also indicated that the benefits are more pronounced in harder ores and with coarser ball

mill targeted products.

Although no full-scale operational results have been released yet, estimates of energy

consumption calculated during the design phases of the Boddington and Cerro Verde

projects are available. For the Boddington project, a 2006 trade-off study showed that a

preliminary HPGR circuit would provide approximately 5% savings in comminution

power (Seidel et al, 2006). For Cerro Verde, in 2006 it was estimated that the total

comminution circuit specific energy for the SAG circuit was 20.1 kWh/t and for the HPGR

circuit 15.9 kWh/t, approximately a 21% savings (Vanderbeek, 2006).

Energy savings in comminution at downstream grinding phases, usually ball milling, are

expected through the reduction of the Bond ball mill work index (BWi) of the HPGR

product. This reduction in the hardness of the ore, particle weakening, is due to the

production of microfractures in the high pressure process. In addition to this particle

weakening phenomenon, the HPGR produces a high proportion of fines that further

decrease energy requirements in the subsequent mill (Tavares, 2005; Patzelt et al,

1995).

Tests performed by Polysius on siliceous gold ores resulted in 5 to 20% BWi reduction

(Patzelt et al, 1995). Tests performed in a lab-scale HPGR at Anglo Research on

several different ores resulted in BWi reductions between 3% to 7% (van Drunick and

Smit, 2006). Differences in the evaluation of this reduction factor are common. The

author has observed significant differences between testwork performed on the same

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ore through different HPGR vendors. This may be related to different test procedures.

Some procedures may combine the results of particle weakening with fine product and

others may report the particle weakening factor alone.

2.4.6 Metallurgical Extraction Advantages

The enhanced extraction in processes, such as flotation and leaching following HPGR, is

credited to the generation of micro-cracked rocks during high pressure process, and by

the theory that these micro-cracks are formed predominantly at grain boundaries; which

consequently increases mineral liberation and reagent penetration rate (von Michaelis,

2005; Morley, 2006). There are a number of studies linking the application of the HPGR

to real benefits in gold leaching, especially at coarse fractions as applied in heap

leaching (Klingmann, 2005; Baum et al, 1997; Dunne et al, 1996; Gardula and Sheriff,

2005). Polysius conducted laboratory tests on copper oxide ores suitable for heap

leaching and reported encouraging results (Baum et al, 1996) and von Michaels (2005)

described the kinetics of copper heap leaching and concluded that the better

permeability of the HPGR product may in fact bring benefits in copper leaching.

There have also been reports of significant improvements in gravity recovery of ores

containing coarse gold (Johansen et al, 2005; Dunne et al, 1996). In addition, filtering

and thickening benefits can be expected if a reduction of slimes production is achieved

with a HPGR circuit (von Michaels, 2005).

In the case of flotation, research that demonstrates the flotation benefits on HPGR

products seems to be more limited. In the late 1990s, a few papers written by Mr W.

Baum from Pittsburg Mineral & Environmental Technology and coauthored by two

professionals from Polysius, claimed flotation benefits. One of these papers reports that

rougher flotation on copper sulphide ore improved between 3% to 5% and final overall

41

recovery was up to 7%, but no details of the experimental approach is given (Baum et al,

1997).

Flotation studies conduct with copper ore in a lab-scale HPGR and ball mill were not

very conclusive (van Drunick and Smit, 2006). On platinum ores, a similar study with a

lab-scale HPGR and a Ball mill found some benefits at coarse size fractions flotation

feeds but not at finer fractions (Shi et al, 2006).

Another study indicates doubts about the potential HPGR benefits in flotation, as in most

cases the HPGR product is subjected to ball milling ahead of the flotation phase. In this

recent research, HPGR products were subjected to the JKMRC mineralogical analysis

(MLA) and the images confirmed the production of micro-cracks. However, it was also

observed that these cracks were destroyed when subjected to ball milling. In addition,

no evidence was found of any significant alteration in the characteristics of the liberation

distribution of the valuable minerals within the size distributions studied. This however

should be confirmed with flotation tests for example. (Daniel, 2008)

2.4.7 HPGR Feed and Product Specifics

Like other crushers, the HPGR operation will present challenges when fed with a high

proportion of very fine material (clayish material), ores with elastic properties, or

significantly soft ores (Morley, 2006b). These substances tend to cushion the crushing

action and make the process inefficient. High moisture in the HPGR feed is also

problematic as it may cause slippage of the material on the roll surface, accelerating

wear. Tramp metal can severely damage the roll surface and means for its removal

from the HPGR feed are necessary (Klymowsky et al., 2006).

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Both Boddington and Cerro Verde ores produce a moderately friable HPGR cake

product that is de-agglomerated in wet screening by the action of water sprays and

vibration. However, if an ore has the tendency to create competent product cakes it may

be necessary to have a more powerful process to break it down prior to sending it to

downstream processes. Scrubbers are standard in the diamond industry, not only to

wash out the clay prior to the HPGR feed, but also after each HPGR stage. Flowsheets

incorporating low-speed tooth roll sizers (MMD sizers) used as de-agglomerators

downstream of the HPGR have been developed (Valery and Jankovic, 2002).

2.4.8 Limitations and Disadvantages

The main disadvantages of a crushing circuit, with the HPGR as its tertiary phase, are

the increased dust generation that requires dust suppression/collection systems, the

complexity of the circuit especially on material handling systems with a large number of

conveyor belts and stockpiles/bins, and possible higher capital cost (Morley, 2006,

Vanderbeek, 2006).

At the start of this research, the industry consensus was that HPGR are not

recommended to treat high weathered ores, very soft ores or a feed that contains high

level of moisture (Morley, 2006). In addition, capital costs for HPGR circuits are higher

than the costs for a similar SABC circuit, as was observed during the Cerro Verde

design phases (Vanderbeek, 2006).

The simplicity and the high availability of SAG circuits seem to be hard to achieve with

HPGR circuits. The HPGR machine can provide high availability alone, however the

complete circuit, which has a cone crusher and the required screens and conveyors can

only provide a high circuit availability when redundancy is added to the circuit.

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2.5 Other Developments

2.5.1 Increasing Machine Sizes

Over the past four decades, tumbling mill sizes increased considerably; the largest mills

were 6,750 kW in the early 1970s, and now there are mills operating at 23,000 kW. The

increase appears to have stalled somewhat, not because of a lack of need for larger

units, or that the mills have reached a manufacturing limit, but seemingly because of a

lack of confidence in the industry that motor manufacturers can produce reliable drives in

larger sizes. On the other hand, crusher sizes are increasing; recently crusher

manufacturers launched larger machines than were previously available. For example,

Metso has a new heavy duty cone crusher with 932 kW. Both Polysius and KHD

produce HPGR up to 6.6 MW. The increasing sizes of crushing units mean fewer units

and a simpler, less costly, plant; thus reducing the chief advantage of the SAG mill.

2.5.2 Stirred Mills

The stirred mill presents better energy efficiency than ball mills for fine grinding. One of

the reasons being that it can operate with smaller media and provide a better match

between the particles and the media. It has been demonstrated that media sizing is a

key factor for ball mill comminution efficiency (McIvor 1997). The Metso Vertimillfi has

been used in tertiary grinding for many years, for example, at Red Dog and Chino

(Vanderbeek 1997) and has now been successfully used in secondary milling (Valery

and Jankovic 2002; Jankovic and Valery 2004). A large scale secondary grinding circuit

is now being built and the manufacturer has recently launched a higher capacity

machine equipped with a 2,240 kW motor, claiming that this mill can handle 6mm top

size in the feed (Metso 2009).

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The horizontal stirred mill IsaMill, manufactured by Xst rata Technology is also now

available in a larger scale unit equipped with a 3,000 kW motor (Isamill 2009). This type

of mill has been tested in the platinum industry as a primary mill receiving the product of

two HPGRs in series. In addition to the high energy efficiency, this experimental

flowsheet is also aiming at metallurgical efficiencies by having a comminution circuit free

of metal media. This is achieved by the HPGR rolls being built to hold a layer of ore and

by the IsaMill utiliz