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
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
ii
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.
iii
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.
iv
• 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.
v
TABLE OF CONTENTS
Abstract ...... ii Preface...... iv Table of Contents ...... vi List of Tables ...... ix List of Figures...... x Acknowledgements ...... xi 1 Introduction ...... 1 1.1 Comminution ...... 1 1.2 Modern Metal Mining ...... 3 1.3 HPGR in Hard Rock Mining ...... 5 1.4 Thesis Objectives ...... 7 1.5 Thesis Outline ...... 9 2 Comminution Circuits - Literature review ...... 10 2.1 Introduction ...... 10 2.2 Recent History of Comminution ...... 11 2.3 SAG Mill Background ...... 13 2.3.1 AG/SAG Mill Machines ...... 13 2.3.2 SAG Operational Parameters ...... 14 2.3.3 SAG Mill Original Circuit ...... 15 2.3.4 Pebble Crushing for AG/SAG Circuits ...... 17 2.3.5 SAG Feed Preparation ...... 21 2.3.6 Steel Wear ...... 22 2.4 HPGR Background ...... 25 2.4.1 HPGR Machine ...... 25 2.4.2 HPGR Terminology and Operational Parameters ...... 28 2.4.3 HPGR Original Circuits ...... 31 2.4.4 HPGR Precious/Base Metal Recent Circuits ...... 35 2.4.5 Energy Savings ...... 37 2.4.6 Metallurgical Extraction Advantages ...... 40 2.4.7 HPGR Feed and Product Specifics ...... 41 2.4.8 Limitations and Disadvantages ...... 42 2.5 Other Developments ...... 43 2.5.1 Increasing Machine Sizes ...... 43 2.5.2 Stirred Mills ...... 43 2.5.3 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 ...... 47 3.1 Introduction ...... 47 3.2 Modelling and Simulation Background ...... 49 3.3 Case Studies ...... 51 3.4 Design Criteria Development...... 52 3.5 Flowsheet Development ...... 54
vi
3.6 Developed Models ...... 56 3.7 Equipment Sizing ...... 61 3.8 Results and Discussions ...... 62 3.8.1 Pure Comminution Energy ...... 62 3.8.2 Complete Circuit Comminution Energy ...... 63 3.8.3 Steel Usage ...... 64 3.8.4 Ore Variability ...... 65 3.8.5 Heating and Ventilation ...... 66 3.8.6 Availability and Maintainability ...... 66 3.8.7 Additional HPGR Benefits ...... 67 3.8.8 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 ...... 71 4.1 Introduction ...... 71 4.2 Novel HPGR Circuit for Ores Containing Clayish Material ...... 75 4.3 Testwork ...... 77 4.3.1 Sample ...... 77 4.3.2 Testwork Design ...... 77 4.3.3 Test Equipment ...... 79 4.4 Results and Discussion ...... 81 4.4.1 Sample Properties ...... 81 4.4.2 Tumbling Test ...... 81 4.4.3 HPGR Feed PSD ...... 83 4.4.4 HPGR Feed Moisture Content ...... 88 4.4.5 HPGR Tests ...... 91 4.4.6 HPGR Product Cakes ...... 102 4.4.7 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 ...... 109 5.1 Introduction ...... 109 5.2 Modelling and Simulation ...... 110 5.3 Energy Requirements...... 115 5.3.1 Ball Mill Energy ...... 115 5.3.2 Pure Comminution Energy ...... 117 5.3.3 Complete Circuit Comminution Energy ...... 119 5.4 Operating and Capital Costs ...... 122 5.4.1 Operating Cost ...... 122 5.4.2 Capital Cost ...... 122 5.5 Discussions ...... 124 6 Conclusions ...... 126 6.1 Main Research Contributions ...... 126 6.2 Future Research Opportunities ...... 128 References ...... 131 Appendix – A: Inputs Used for the JKSimMet® Models ...... 142 Appendix – B: SMC and MinnovEX SPI Test Results ...... 145
vii
Appendix – C: Sample Preparation and Test Flowsheet ...... 146 Appendix – D: HPGR Feed Test Blend – Linear Programing ...... 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
viii
LIST OF TABLES
Table 3-1: Summary of Design Criteria ...... 53 Table 3-2: Pure Comminution Energy ...... 62 Table 3-3: Complete Circuit Comminution Energy ...... 64 Table 3-4: SAG Mill Steel Ball Consumption ...... 65 Table 4-1: Pilot-Scale HPGR Specifications ...... 79 Table 4-2: Summary of Parameters and Calculated Results for Moisture Content ...... 91 Table 4-3: HPGR Tests Quick Reference Legend ...... 92 Table 4-4: Summary of the Main Parameters and Results for All HPGR Pilot Tests ...... 93 Table 4-5 Average Dimensions of Cakes Produced by the HPGR Tests ...... 104 Table 5-1: Simulation Results – Pure Comminution Energy Requirements ...... 118 Table 5-2: Energy Requirements for the Complete Circuits ...... 121 Table 5-3: Capital Cost Summary ...... 123
ix
LIST OF FIGURES
Figure 2-1: Three Stages of Crushing, Rod Mill, Ball Mill ...... 16 Figure 2-2: SAG-Ball Mill Circuit ...... 17 Figure 2-3: SABC Circuit ...... 18 Figure 2-4: Open-Circuit SABC ...... 19 Figure 2-5: SABC with HPGR ...... 20 Figure 2-6: Pre-Crushing in an SABC Circuit ...... 22 Figure 2-7: Schematic of a HPGR (Napier-Munn et al, 1996) ...... 26 Figure 2-8: Open Circuit HPGR – Closed-Circuit Ball Mill (Aydogan et al, 2006) ...... 32 Figure 2-9: HPGR Applied for Pebble Re-Crush at Empire Iron (Kawatra et al, 2003) .... 33 Figure 2-10: Re-Crush Circuit at Argyle Diamond Mines (KHD, 2008) ...... 34 Figure 2-11: Boddington HPGR (Dunne et al 2007) ...... 35 Figure 2-12: Cerro Verde (Vanderbeek 2006) ...... 36 Figure 2-13: Pebble Extraction and Milling ...... 45 Figure 3-1: Simplified SABC and HPGR Flowsheets ...... 55 Figure 3-2: JKSimMet ® Screen Snapshot of Case A – SABC ...... 57 Figure 3-3: JKSimMet ® Screen Snapshot of Case A – HPGR ...... 58 Figure 4-1: Cerro Verde Flowsheet (Vanderbeek 2006) ...... 72 Figure 4-2: Hardness Distribution of the Deposit Based on Jk A*b Parameters ...... 74 Figure 4-3: Proposed HPGR Flowsheet for Clayish Ore ...... 75 Figure 4-4: UBC Pilot HPGR ...... 79 Figure 4-5: Particle Size Distribution for the Samples as Received ...... 81 Figure 4-6: Tumbling Test Feed and Product Size Distributions ...... 83 Figure 4-7: Lab-Scale Circuit to Prepare the Feed to the Pilot HPGR (open-circuit) ...... 84 Figure 4-8: PSDs for Fresh and Crushed Laboratory Screen O/S Material ...... 85 Figure 4-9: PSDs from the Preliminary Simulation ...... 86 Figure 4-10: PSDs for the Optimum Blend, Original Products and Simulated Product ...... 87 Figure 4-11: Lab-Scale Circuits Used for the Tests ...... 88 Figure 4-12: Specific Throughput as a Function of Pressing Force...... 95 Figure 4-13: Influence in Energy Consumption due to Pressing Force ...... 96 Figure 4-14: Pressure Sensitivity Tests – Feed and Product PSDs ...... 96 Figure 4-15: F80/P80 and F50/P50 Reduction Ratios ...... 97 Figure 4-16: Feed and Product PSDs for Closed-Circuit Tests ...... 98 Figure 4-17: Feed and Product PSDs for Full Feed and Tumbled-Screened Open- Circuit HPGR Tests ...... 100 Figure 4-18: HPGR Test #1 Product Cake Samples ...... 103 Figure 4-19: Screen Oversize PSDs from the Tests for Assessment of HPGR Product Cake Competency ...... 105 Figure 4-20: Bond Ball Mill Index Results in Different Points of the Circuit ...... 108 Figure 5-1: Feed PSD for Circuit Modelling and Simulations ...... 111 Figure 5-2: JKSimMet ® Screen Snapshot of the SABC Circuit Simulation ...... 113 Figure 5-3: JKSimMet ® Screen Snapshot of the Final AG-Crusher-HPGR Circuit Simulation ...... 114 Figure 5-4: Ball Mill Cyclone Feed PSD from AG-Crusher-HPGR and SABC Circuits .. 116 Figure 5-5: AG Mill Feed (Combined) and Product PSDs ...... 117 Figure 5-6: AG-Crusher-HPGR Circuit Simplified Flowsheet ...... 120
x
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 wife’s 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.
xi
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
1
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).
2
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
3
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-McMoRan’s Chino Mine in New Mexico
(Amelunxen et al, 2001) and Newmont’s Batu Hijau operation in Indonesia (Burger et al,
2006).
4
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
5
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 Comminution’10 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?
6
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.
7
• 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.
8
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.
9
2 COMMINUTION CIRCUITS 1 - 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
10
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.
11
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).
12
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).
13
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 plus 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 loading—it has been observed that a lower total charge improves capacity.
Today some operations operate with ball charges up to 20%. Total mill volumetric
14
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.
15
Figure 2-1: Three Stages of Crushing, Rod Mill, Ball Mill
Process Water
Coarse Ore Flotation
Fine Secondary Tertiary Ore Bin Crushers Crushers Rod Mills Ball Mills
Secondary Tertiary Screens Screens
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.
16
Figure 2-2: SAG-Ball Mill Circuit
Process Water Flotation
Coarse Ore SAG Mill Discharge Ball Mills Screen
SAG Mill
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 crushers—the Semiautogenous-Ball Mill-Crusher (SABC) circuit. 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.
17
Figure 2-3: SABC Circuit
Process Water Flotation
Pebble Pebble Bin Crushers
Ball Mills SAG Mill Discharge Coarse Ore Screen
SAG Mill
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 startup—for 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
18
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
Process Water Flotation
Pebble Pebble Crushers Bin
Crushed Pebble Screen Ball Mills SAG Mill Discharge Coarse Ore Screen
SAG Mill
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.
19
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
Peñasquito 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
Process Water
Flotation
Pebble HPGR Bin Storage Bin
Pebble Crushers SAG Mill HPGR SAG Mill Trommel Ball Mills Screen Crushed Pebble HPG R Screen Pebble Washing Screen Coarse Ore
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
20
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
21
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-crushing—scalping off and crushing coarse material in the SAG mill feed—has been applied to manage the SAG feed size distribution (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
Process Water
Coarse Ore Coarse Ore Screens Pebble Pebble Crushers Bin
Secondary Crushers Crushed Pebble Screen
SAG Mill Discharge Screen
SAG Mill To Ball Mills
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
22
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
23
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 Ai’s, 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.
24
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.
25
Figure 2-7: Schematic of a HPGR (Napier-Munn et al, 1996)
Nitrogen cylinder Feed
Oil cylinders Moveable roll Fixed roll
Product
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 Köppern in German)
• KHD Humboldt Wedag AG
26
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
Hexadur®, 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
27
• 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, depending 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 author’s 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
28
(m/s). The m-dot consequently is expressed in ts/hm 3 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).