advanced comminutions

Published by theSociety for Mining, Metallurgy, and Exploration, Inc.

ADVANCES IN COMMINUTION

Society for Mining, Metallurgy, and Exploration, Inc. (SME)8307 Shaffer ParkwayLittleton, Colorado, USA 80127(303) 973-9550 / (800) 763-3132www.smenet.org

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Copyright 2006 Society for Mining, Metallurgy, and Exploration, Inc.

All Rights Reserved. Printed in the United States of America.

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ISBN-13: 978-0-87335-246-8ISBN-10: 0-87335-246-7

Library of Congress Cataloging-in-Publication Data

Advances in comminution / edited by S. Komar Kawatra.p. cm.

Includes bibliographical references and index.ISBN-13: *978-0-87335-246-8ISBN-10: 0-87335-246-71. Stone and ore breakers--Technological innovations. 2. Crushing machinery--Technological

innovations. 3. Mining engineering--Technological innovations. I. Kawatra, S. K.

TN510.A38 2006622'.73--dc22

2005057533

vii

Preface

This third international symposium and proceedings, Advances in Comminution, havecome at a critical time. Because of rapidly rising energy prices, it is important that thelatest information be made available for improving the efficiency of highly energy-intensivecomminution processes.

The contributors and topics for this third international symposium have been care-fully selected to provide a balance between academic and industrial practice so that thereader can readily find information on current best practices and evaluate future indus-try trends.

Two previous symposiums, also organized by the Society for Mining, Metallurgy,and Exploration, were great successes. The first conference was held in 1992, at a timewhen there was much discussion about switching from traditional rod mill and ball millcircuits to autogenous grinding. The second comminution symposium, held in 1997,focused on initial installations of high pressure grinding rolls (HPGRs). Now, in 2006,the HPGRs are becoming part of hard-rock grinding circuits. They have proven to be avery economical addition to many comminution processes because of lower energy con-sumption and easy integration into existing conventional systems.

The 2006 conference focuses on the dilemma of needing to grind materials to ever-finer sizes while maintaining reasonable energy costs. The selection and sizing of stirredmills for regrinding and ultrafine grinding applications do not lend themselves to con-ventional methodologies; therefore, new approaches are being developed. There is alsoa great deal of activity directed toward improving ore characterization to predict AG/SAG mill energy requirements, as well as developing improved models and instrumenta-tion for optimization and control of comminution circuits. Instrumentation, modeling,and control functions in particular have benefited from rapidly advancing computertechnology, with calculations that were formerly extremely time-consuming becomingrapid and routine. These advances will keep energy waste to a minimum and will pro-vide the increased energy efficiency needed to maintain ongoing industry success.

It is hoped that the symposium and these proceedings will be useful to those whoare working toward major advances in industrial practice. Appreciation is extended tomembers of the organizing committee, who were instrumental in acquiring high-qualitypapers and reviewing them on very short notice, and to the SME staff, particularly Ms.Tara Davis and Ms. Jane Olivier, for their assistance in organizing the third internationalsymposium and publishing these proceedings.

iii

Contents

EDITORIAL BOARD v

PREFACE vii

PART 1 ADVANCED COMMINUTION TECHNOLOGIES 1

High-Pressure Grinding RollsCharacterising and Defining Process Performance for Engineers 3

High-Pressure Grinding RollsA Technology Review 15

Some Basics on High-Pressure Grinding Rolls 41

High-Pressure Grinding Rolls for Gold/Copper Applications 51

Selection and Sizing of Ultrafine and Stirred Grinding Mills 69

Effects of Bead Size on Ultrafine Grinding in a Stirred Bead Mill 87

Specific Energy Consumption, Stress Energy, and Power Draw of Stirred Media Mills and Their Effect on the Production Rate 99

AG/SAG Mill Circuit Grinding Energy RequirementHow to Predict It from Small-Diameter Drill Core Samples Using the SMC Test 115

PART 2 COMMINUTION PRACTICES 129

Causes and Significance of Inflections in Hydrocyclone Efficiency Curves 131

Simulation-Based Performance Improvements in the Ispat Inland Minorca Plant Grinding Circuit 149

Determining Relevant Inputs for SAG Mill Power Draw Modeling 161

Cement Clinker Grinding Practice and Technology 169

Extended Semiautogenous Milling: Smooth Operations and Extended Availability at C.M. Doa Ines de Collahuasi SCM, Chile 181

PART 3 LIBERATION AND BREAKAGE 191

Shell and Pulp Lifter Study at the Cortez Gold Mines SAG Mill 193

Breakage and Damage of Particles by Impact 205

The Rationale behind the Development of One Model Describing the Size Reduction/Liberation of Ores 225

Influence of Slurry Rheology on Stirred Media Milling of Limestone 243

iv

Experimental Evaluation of a Mineral Exposure Model for Crushed Copper Ores 261

Linking Discrete Element Modeling to Breakage in a Pilot-Scale AG/SAG Mill 269

Significance of the Particle-Size Distribution in the Quality of Cements with Fly Ash Additive 285

Modeling Attrition in Stirred Mills Applying Statistical Physics 293

PART 4 MILL DESIGN 307

Design of Iron Ore Comminution Circuits to Minimize Overgrinding 309

Evaluation of Larger-Diameter Hydrocyclone Performance in a Desliming Application 321

Selection and Design of Mill Liners 331

The Importance of Liner Geometry and Wear in Crushing 377

Bonds Method for Selection of Ball Mills 385

Developments in SAG Mill Liner Design 399

The Gearless Mill DriveThe Workhorse for SAG and Ball Mills 413

Optimizing Hydrocyclone Separation in Closed-Circuit Grinding 435

PART 5 INSTRUMENTATION, MODELING, AND SIMULATION 445

Use of Multiphysics Models for the Optimization of Comminution Operations 447

Batu Hijau Model for Throughput Forecast, Mining and Milling Optimization, and Expansion Studies 461

The Use of Process Simulation Methodology in Process Design Where Time and Performance Are Critical 481

Modeling and Simulation of Comminution Circuits with USIM PAC 495

Remote and Distributed Expert Control in Grinding Plants 513

Developments in Sensor Technology for Tumbling Mills 527

Ball Mill Circuit Models for Improving Plant Performance 539

INDEX 547

1PART 1

Advanced Comminution Technologies

3High-Pressure Grinding RollsCharacterising and Defining Process Performance for Engineers

Richard Bearman*

ABSTRACTHigh-pressure grinding rolls (HPGRs) are increasingly becoming a part of the hard-rockprocessing picture through their energy efficiency, the ability to induce microcracks andpreferential liberation, coupled with high throughput and high reduction ratio. Given thatthe machine is still not regarded by many as an off-the-shelf piece of process equipment,there is work required to define guidelines for its use and to provide engineers with toolsthey can use. This paper examines the current knowledge around the HPGR process perfor-mance and explores key relationships available to engineers, whilst considering currentapproaches to simulation.

INTRODUCT IONHigh-pressure grinding rolls (HPGRs) have struggled for acceptance into the hard-rockmining sector. Many of the issues that restricted their widespread use have now beenconquered, but it is still regarded as an immature technology. Why is this the case?

In contemplating an answer to the issue of the immaturity, the status of otheraccepted technologies must be examined. As an example, the traditional compression-style cone-gyratory crushers can be considered. When a plant design is being assembled,every well-equipped engineer will be able to turn to numerous rules of thumb associatedwith these crusherseven without reference to textbooks or suppliers. The types of rulesreferenced above include

Product-size distribution will be approximately 80% passing the closed-side settingwith poor applications dropping to 50%.

Centralized and circumferentially distributed feed is required to extract the best performance.

Profile and condition of the crushing liners is critical to deliver the best distribu-tion of energy into the crushing chamber.

Low-bulk-density feeds reduce throughput.

Maximum product bulk density is 1.9 to 2.1 t/m3 for average limestone feedstock.

Secondary applications are power driven, whilst tertiary duties are pressure driven.

* Rio Tinto Technical Services, Perth, Western Australia

4 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIES

Mostly 5%10% of the feed-size distribution is the maximum less than the closed-side settingexcept with modern cones that are trying to generate interparticle crushing.

Maximum feed size should not exceed 80% of the open-side feed opening.

Feed moistures >4% should be avoided.

Given this type of knowledge, it is easy for the designer to determine the positionwithin the flowsheet and to then calculate the feed rates, type of feed arrangement, andthe pre- and postclassification required. Why do these rules of thumb, or guidelines, notexist for HPGRs? There are several reasons for this lack of clarity, namely:

Number and type of applications

Genesis of the HPGR concept

Industry position on technology

Existence of process models

First, there are very few actual, or operating, applications in hard-rock duties. Theonly hard-rock applications that have been in existence for any length of time arerestricted to the diamond and iron ore (pellet-feed) sectors.

Another consideration is that the HPGR is a very rare breed of machine, in that itsdevelopment stemmed from fundamental research. Given the types and focus of earlypublications, much was made of the nature of the interparticle breakage at the heart ofthe technology. Obviously, given the ground-breaking nature of the invention, this focuswas fully justified, but it ledunfairlyto the HPGR being regarded as an academicdevice searching for an industry application. The language used about the HPGR, andunfamiliar terms such as m-dot (denoting specific throughput), further led to an air ofmystique around the HPGR. Was it a crusher or a mill? Its place in the world was unclear.

Another element restricting the rate of application was the lack of process models.Simulation is a large part of the flowsheet design exercise and this inevitably requiresprocess models to exist for each piece of equipment. In the case of the HPGR, much ofthe effort was placed in scale-up procedures. Several organisations did produce processmodels of HPGRs, but they have been fragmented in their acceptance. Currently, themost complete model approach is that reported by Daniel and Morrell (2004), who havedeveloped an approach from the earlier model of Tondo (1997). It is interesting to notethat the Tondo model came out of the first major process study of HPGRs, namely theAMIRA P428 that was completed in 1997.

If these points above are added to the naturally conservative stance of the miningindustry, this provides a view of why, even after mechanical/wear issues have been over-come, there is still a slow rate of acceptance.

As of today, the situation has changed. The features and benefits have become clearto many practitioners, including

Energy efficiency

Preferential liberation at natural grain boundaries

Microcracking and enhanced extraction

Small footprint in terms of throughput and size reduction

Minimal vibration from machine into drive mechanisms and support structure

Of increasing importance is the energy-efficiency issue. It was not too long ago thatthe mining industry regarded energy consumption as somewhat of a side issue. TheKyoto Protocol and the greenhouse debate changed this view forever (Ruben 2002).

HPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 5

CR I T ICAL HPGR PA RAMETER SHPGR roll diameters typically range from 0.5 m to 2.8 m, depending on the supplies, androll widths vary from 0.2 m to 1.8 m. The aspect ratio of the rolls also varies as a functionof manufacturer. Typical HPGR throughput rates range from 20 to 3,000 tph, withinstalled motor power as high as 3,000 kW per roll. The roll surface is protected withwear-resistant materials, and it has been these that have traditionally stymied HPGRacceptance, but solutions are now in place (Maxton, Morley, and Bearman 2004).

When operating an HPGR, the two most important operating parameters are

Operating pressure

Roll speed

The two key operating parameters are inherently linked to the following:

Specific throughput

Specific pressing force

Maximum pressure between the rolls

Specific energy input

Detailed descriptions of the derivation and formulation of the parameters are givenin numerous texts, and as such, the following section provides only a prcis of the criticalformulas, with some examples of actual relationships from testwork.

Specific ThroughputThe specific throughput, m-dot, is regarded by many as the key parameter for sizing therolls. Specific throughput is defined as the throughput (tph), divided by the roll diameter(m), roll width (m), and the peripheral roll speed (m/s). For the purposes of brevity,only the equations for this parameter are reported here. Further details are provided inearlier works. (Schnert 1991). Part of its importance is that the equation allows com-parison between any size of rolls providing that the surfaces are the same.

m = M/(D u L u u) (EQ 1)

whereM = throughput rate (tph)D = roll diameter (m)L = roll width (m)u = roll speed (m/s)

m = specific throughput (ts/hm3)

The throughput can also be calculated from the continuity equation as follows:

M = L u s u u u Uc u 3.6 (EQ 2)

wheres = operating gap (mm)Uc = density of the product cake (t/m

3)

Combining equations (1) and (2), one obtains:

m = (s/D) u Uc u 3.6 (EQ 3)

For a given material and operating conditions, the gap scales linearly with the diam-eter of the rolls, and hence the specific throughput can be assumed to be constant.


It should be noted that recent work by Daniel (2005) has examined the determina-tion of an equivalent diameter for piston press tests. Daniel proposes

D = (Ucf u xc u xd) / ((Ucf u xc) (xg u Ug)) (EQ 4)

whereUcf = feed bulk density, lightly compactedxc = initial bed height in piston pressxd = displacement of pistonxg = final bed height (i.e., operating gap)Ug = density of product flake

This relationship has potential to assist in translating piston press results to engineeringparameters.

Variation in Throughput with Key VariablesFigure 1 shows the variation in the specific throughput as a function of the feed bulkdensity. The relationship appears to be linear over the range of feeds tested. Given thatthe specific gravity of the feed material is 2.85 t/m3, it would be unlikely that the loosefeed bulk density would exceed 1.8 t/m3; therefore, this graph suggests that the relation-ship is relevant over a vast majority of cases. It should be noted that throughput is high-est at the lowest pressure, with larger changes associated with the all-in (high bulkdensity) feed types. Figure 2 shows the type of linear increase in specific throughputassociated with increasing operating gap.

Figure 3 shows a plot of all tests versus the specific energy (power) consumed. It isinteresting to note that the data appear in two distinct clusters. The right-hand clusterconsists purely of the all-in feed types with no truncation of the feed-size distribution atthe lower end, whilst the left-hand cluster is formed from feeds with fines truncation.

1.451.40 1.50 1.55 1.60 1.65 1.70 1.75

250

230

210

190

170

150

m-d

ot,t

s/hm

3

Bulk Density, t/m3

30 Bar38 Bar52 Bar

FIGURE 1 Variation in specific throughput as a function of feed-bulk density for various operating pressures using a pilot-scale HPGR


Specific Pressing ForceThe specific pressing force is defined as the grinding force applied to the rolls (kN),divided by the diameter (m) and width (m) of the rolls (Schnert 1988). The specificpressing force has the unit of N/mm2.

Fsp = F/(1,000 u D u L) (EQ 5)

whereFsp = specific pressing force (N/mm

2)F = applied grinding force (kN)D = roll diameter (m)L = roll width (m)

15 16 17 18 19 20 21

250

230

240

220

200

180

160

210

190

170

150

m-d

ot,t

s/hm

3

Operating Gap, mm

FIGURE 2 Variation in specific throughput as a function of operating gap using a pilot-scale HPGR at an operating pressure of 38 bar

40 90 140 190 240 290

60

56

58

54

50

46

42

52

48

44

40

m-d

ot,t

s/hm

3

Power, kW

FIGURE 3 Variation in specific throughput as a function of operating gap using a pilot-scale HPGR


Ranges for specific pressing force vary considerably in the range 19 N/mm2, withstudded machines normally restricted to 5 N/mm2 maximum pressure.

Specific pressing force is a key parameter used in scale-up and for comparison pur-poses between different machine sizes.

Maximum Pressure between RollsThe maximum pressure applied to the material between the rolls has been estimated byseveral workers, and it is generally assumed to be in the range of 40 to 60 times the spe-cific pressing force. It is generally accepted that the following equation (Schnert 1988)holds true:

Pmax = F/(1,000 u D u L u k u D ip) (EQ 6)

wherePmax = maximum pressure (MPa)

F = applied grinding force (kN)D = roll diameter (m)L = roll width (m)k = material constant (0.180.23)D ip = compression angle (610 degrees)

The parameter D ip can be calculated from the operating gap, with a detailed descriptionbeing given by Schnert and Lubjuhn (1990).

Specific Energy InputThe specific energy consumption of an HPGR is a familiar quantity to process engineers.As with all other instances of the parameter, it is calculated from the net power input tothe rolls divided by the ore throughput rate.

It is important to note that specific energy input (kWh/t) is proportional to the spe-cific pressure applied to the rolls. Typical specific energy values for studded rolls rangefrom 1 to 3 kWh/t. As with all direct comminution devices, harder material will absorbmore energy compared to a softer material, for a given size reduction.

A rule of thumb is that the ratio of specific pressing force to specific energy input is1.83:1, with this ratio decreasing towards 1.0 for finer comminution. Figure 4 showsthe type of response mentioned. In this case, the slope of the graph indicates a ratio of1.5:1.

Specific energy consumption is markedly impacted by the feed-size distribution, asillustrated in Figure 5. As the feed distribution lengthens (i.e., the bulk densityincreases), the specific energy consumption drops.

The major impact of specific energy input is the product fineness. As with all commi-nution equipment, a point of diminishing returns will occur where extra energy does notgenerate a commensurate increase in fineness. Figure 6 shows a range of energies andfines generation. At the levels displayed in Figure 6, the point of diminishing returns hasnot been reached.

S IMULAT ION OF HPGR PERFOR MANCEAs with all modeling and simulation of process equipment, there is a sliding scale fromthe simplest spreadsheet-based feed-product transfer function at one end, throughempirical representations, to mechanistic models, and finally to detailed fundamentaldescriptions. The key process issues that need to be estimated, or predicted, during thedesign phase of a process plant are


Throughput

Size reduction (product and oversize)

Power consumption (energy efficiency)

Required hydraulic stiffness

Target gap and operating pressure

Using these parameters, it is then possible to insert the HPGR into a flowsheet and makesensible comparisons against other types of equipment and flowsheet configurations.The additional benefits of preferential liberation and enhanced extraction must beassessed via laboratory tests and incorporated with the full analysis.

1.0 1.5 2.0 2.5 3.0 3.5 4.00.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1Sp

ecific

Ene

rgy

Cons

umpt

ion

kWh/

t

Specific Pressing Force, MPa

FIGURE 4 Relation between specific energy consumption and specific pressing force using a pilot-scale HPGR

1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.750.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

Spec

ific E

nerg

y Co

nsum

ptio

n

kWh/

t

Feed Bulk Density, t/m3

30 Bar38 Bar52 Bar

FIGURE 5 Relation between specific energy consumption and feed bulk density using a pilot-scale HPGR, at various operating pressures


Piston Press Testing and Ore CharacterisationThe main ore characterisation tests for HPGR modeling are the piston-press and drop-weight procedures. The drop-weight test is the Julius Kruttschnitt Mineral Research Centre(JKMRC) -developed, single-particle test and is used to examine areas in the HPGRwhere the breakage is of a single-particle nature. The piston-press test is for characterisa-tion of the packed-bed breakage zone in the HPGR. The purpose of the piston-press testis to generate an appearance function as per the drop-weight test, but for packed-bedbreakage. Hence, the piston-press appearance function is used to characterise the pre-dominant breakage action in the HPGR.

The piston press can be used in an analogous manner to the traditional drop-weighttest (i.e., breakage parameters and an appearance function can be determined).

In terms of the breakage characteristics, Table 1 provides an example of the compar-ison of the b parameters from the drop-weight and piston-press tests for material fromArgyle Diamonds. The immediate observation regarding the data in Table 1 is that thepiston press b parameters are higher than the single-particle test, with the inferencebeing that the material appears softer in a packed-bed environment.

Given the mode of compression (i.e., slow interparticle versus transient compres-sion), Table 1 could represent an efficiency factor relating the two forms of breakage.

Of more practical importance is that the use of the packed-bed, piston-style test iscritical to the formation of a representative model of HPGR performance.

Application of Piston Press to Provide Conceptual-Level HPGR Performance EstimatesA variety of workers are now using piston-press tests to research the action of HPGRs.The press arrangement at Freiberg University has recently been used to test a copper oresupplied by Rio Tinto. The aim of the tests is to determine the amenability of the ore toHPGR treatment and to examine the use of the piston press for conceptual-level evalua-tions. A series of tests at pressures from 80 to 320 MPa were undertaken with the resultspresented in Table 2.

The maximum pressures reported in Table 2 were chosen to mimic those seen in theHPGR pilot tests, and the results appear to be good approximations to those obtained

0.5 1.0 1.5 2.0

17

15

19

21

23

25

27

29

31

33

35N

et

118

mm

Gen

erat

ion

Specific Energy Consumption, kWh/t

FIGURE 6 Relation between specific energy consumption and fines generation using a pilot-scale HPGR


from pilot-scale HPGR work. Given this agreement, it is suggested that the piston pressbe used to provide a conceptual-level envelope of performance.

The suggested sequence is

1. Estimate m-dot value from Equation (3), by substitution of the product flake density, operating gap (final bed depth from piston press), and use of Equation (4) to determine D.

2. Estimate throughput from the rearranged Equation (1), with assumed values for roll diameter (D), roll width (L), and roll speed (u) relating to the desired scale of equipment. These values can be determined in association with manufactur-ers. It should be noted that the scale independence of m-dot, due to the linearity of operating gap versus roll diameter, is a major assumption in this step.

3. Calculate the specific pressing force (Fsp) from Equation (5) using the applied grinding force from the piston press and the D and L values used above.

With these key parameters, it is possible to ensure that the size of rolls and the bearingselection is correct. To estimate comminution performance:

Determine the specific energy consumption from assumed relationship with spe-cific pressing force. Values for the ratio Fsp:Wsp can be assumed to vary from 1:1 for very fine comminution through to 3:1 for very coarse duties. A value of 1.5:1, as shown in Figure 5, is a good general value for moderate comminution of hard ores. Care should be takenalthough particle-size distribution is a major part of the bulk properties that dictate the relationship between Fsp and Wsp, other fac-tors also influence the bulk behaviour including ore hardness, friction, and mois-ture (M.J. Daniel, personal communication, 2005).

Specific energy consumption is inherently linked to product-size distribution via the traditional breakage and appearance type mapping employed in single-particle drop-weight tests. Using the A and b parameters from the piston-press test, these along with the specific energy consumption can be substituted into the following equation:

t10 = A(1 eb. Ecs) (EQ 7)

wheret10 = percentage passing one tenth of the feed size

A and b = breakage characteristics from piston-press testsEcs = specific energy consumption (kWh/t)

TABLE 1 Single-particle breakage parameters

Single-Particle Test Packed-Bed Test

Sample b b

Unweathered lamproite 0.44 0.940

Siliceous waste 0.40 0.703

TABLE 2 Flake density results from piston-press tests

Maximum Pressure, MPa Flake Density, t/m3

77.24 2.14

157.29 2.32

230.53 2.32

310.98 2.38


Using the standard single-particle relationships between t10 and the other size distribution markers (i.e., t2, t4, t25, t50, t75), the entire size distribution of the product can be generated. Theoretically, this is a combination of packed- and single-bed approaches, but, as Tondo (1997) showed, the packed-bed t10 versus tn rela-tionship underestimates size reduction in coarse sizes, compared to single-particle tests. Given that the variable edge effect generates coarser products, it is likely that any underestimation from the packed-bed parameters is simply an approxi-mation to the coarser edge comminution. This approach is backed up by the fact that various workers have chosen to deal with this in different ways, whilst still obtaining satisfactory results. Tondo (1997) used both single-particle and packed-bed A and b parameters with separate appearance functions in his work, whilst Daniel (2002) assumes a 10% split to edge and uses the single-particle function for all breakage with a t10 of 30.

This conceptual-level approach, although not rigorous, helps engineers to obtain afeel for HPGR performance and at least obtain a quick, first-pass estimate of the opera-tional envelope. It should be noted that no account is taken of precrush or edge effects.Analysis of this technique suggests that both throughput and product fineness are over-stated, but as the scale of machine increases, the discrepancy lessens. This reduction inerror with scale can probably be assigned to the decreasing proportion of machine per-formance impacted by edge effects.

Detailed HPGR ModelingFor a more complete treatment of performance estimation in a modeling sense, truemodels are required. The work of Daniel and Morrell (2004) represents the most com-plete current description. The basis for their work is shown schematically in Figure 7.

Daniel and Morrell outline information required for modeling, as shown in Table 3.To undertake the simulation, there are a variety of parameters relating to the break-

age and classification of material in the three different zones as defined in Figure 7. Themain parameters are listed in Table 4.

This extremely comprehensive treatment is then used in a verification and scale-upscheme procedure; full details can be found in works by Daniel and Morrell (2004).

CONCLUS IONSThere is an increasing body of knowledge around the application of HPGRs in hard-rockduties. In terms of selection and sizing, much has already been written, particularly bythe suppliers. For process performance, the increasing application is allowing the devel-opment of some rules and shortcuts that can allow a first-pass evaluation of HPGRs forflowsheet purposesa critical element on the pathway to engineering acceptance. Inmany ways, this paper seeks to provide a pragmatic engineering basis for the assessmentof HPGR performance. This message was also the theme expressed by Klymowsky andLiu (1996), where they sought a Bond work-index analogy for HPGRs. There is no doubtthat a standardized, accepted HPGR work index would be a great boost to HPGRacceptance.

Beyond these engineering views of HPGRs, the detailed modeling and simulation ofHPGR process performance is finding common ground, and workers have developedcomprehensive approaches that provide the required accuracy and resolution.

Assimilation of this understanding within the industry, along with simpler measuresand guidelines, will accelerate HPGR implementation, particularly now that mechanicalissues are predominantly of historical interest only.


ACKNOWLEDGMENTSThe author gratefully acknowledges all practitioners in the field of HPGR technologythat have contributed to this paper through discussions. In particular, the discussionsand advice from Mike Daniels, JKMRC, showed that a considerable amount of effort isstill being applied to the issue of HPGR application.

Entry ZoneSingle-Particle

Breakage

Centre ZonePacked-Bed Breakage

Edge Effect Single-Particle Breakage

Product from HPGR

Feed to HPGR

After Tondo 1997.

FIGURE 7 Schematic representation of Daniel and Morrell model

Source: Daniel and Morrell 2004.

TABLE 3 Model inputs and outputs

Measured Input Measured Output Calculated Output

Sample mass Working gap (xg) Measured throughput (Qm)

Roll diameter (D) Flake thickness (xgf ) Calculated throughput (Qcalc)

Roll width (L) Flake density (qg) Specific energy (Ecs)

Roll speed (U) Product-size distribution (measured) Specific force (Fsp)

Bulk compacted density (qc) Batch process time Critical gap (xc)

Feed-size distribution Working pressure (pw), power (kW) Product-size distribution

Source: Daniel and Morrell 2004.

TABLE 4 Model parameters

Fixed Default Parameters Critical Model Parameters

t10p, t10ebreakage for edge and precrusher Kp(HPGR)power coefficient (compression zone)

K1p, K2p, K3pprecrusher model parameter t10hbreakage for compression zone crusher

K1e, K2, K3edge-crusher model parameter

K1h, K2h, K3hcompression zone parameter

Split factor (c)

Kp(edge)power coefficient (edge)


REFERENCESDaniel, M.J. 2002. HPGR model verification and scale-up. Masters thesis. Brisbane,

Australia: Julius Kruttschnitt Mineral Research Centre, Department of Mining andMetallurgical Engineering, University of Queensland.

. 2005. Paper submitted to Randol Pacific Gold Forum, Perth, Australia.

Daniel, M.J., and S. Morrell. 2004. HPGR model verification and scale-up. MineralsEngineering 17:11491161.

Klymowsky, I.B., and J. Liu. 1996. Towards the development of a work index for theroller press. In Comminution Practices, SME Symposium 1996. S.99/105.

Maxton, D., C. Morley, and R. Bearman. 2004. A quantification of the benefits of high pressurerolls crushing in an operating environment. Minerals Engineering 16:827838.

Ruben, E.S. 2002. Learning our way to zero emissions technologies. IEA Zero EmissionTechnologies Strategies Workshop, Washington, DC, March 19.

Schnert, K. 1988. A first survey of grinding with high-compression roller mills.International Journal of Mineral Processing 22:401412.

. 1991. Advances in comminution fundamental, and impacts on technology. Pages 121in Proceedings of the XVII International Mineral Processing Congress. Volume 1.K. Schenert, ed. Ljubijana, Yugoslavia.

Schnert, K., and U. Lubjuhn. 1990. Throughput of high compression roller mills withplain and corrugated rollers. Pages 213217 in 7th European Symposium onComminution.

Tondo, L.A. 1997. Phenomenological modelling of a high pressure grinding roll mill.Masters thesis. Brisbane, Australia: Julius Kruttschnitt Mineral Research Centre,Department of Mining and Metallurgical Engineering, University of Queensland.

15

High-Pressure Grinding RollsA Technology Review*

Chris Morley

ABSTRACTThe development of high-pressure grinding rolls (HPGRs) technology is reviewed, with anemphasis on aspects relevant to hard-rock comminution. Case histories are investigated andlessons learned are discussed in the particular context of the application of the device as asupplement to, or replacement for, conventional crushing and semiautogenous milling circuits.

The potential for the more widespread use of this technology as a comminution methodin hard-rock processing is examined. The use of the technology as a metallurgical tool isaddressed, and future flowsheet concepts are introduced that make progressively greater useof the energy efficiency of HPGRs.

INTRODUCT IONHigh-pressure grinding roll (HPGR) technology has its genesis in coal briquetting in theearly twentieth century, but it was not until the mid-1980s that it was adopted for com-minution applications, when it was applied in the cement industry to treat relatively eas-ily crushed materials. Since then, it has been applied to progressively harder, tougher,and more abrasive materials, generally successfully, but as would be expected, not with-out some developmental problems.

Machines are now also in use in the following applications:

Kimberlites in secondary, tertiary, and recrush roles

Iron ores for coarse crushing, autogenous mill pebble crushing, regrinding, pre-pelletising, and briquetting

Limestone crushing

Concentrates fine grinding

Gold ore crushing

Other prospective applications include phosphates, gypsum, titanium slag, copperand tin ores, mill scale, and coal.

Hard-rock operations that use HPGRs as an alternative or supplement to conventionalcomminution devices include Argyle, Diavik, Premier, Kimberley, Jwaneng, Venetia and

* Updated from the original paper, HPGR in Hard Rock Applications, published in Mining Magazine,September 2003, www.miningmagazine.com

Fluor, Australia


Ekati (diamonds), CMH-Los Colorados, CVRD, Empire and Kudremukh (iron ore), andSuchoj Log (gold ore). Hard-rock operations to have considered using HPGR and conductedpilot testing include Mt. Todd, Boddington, and KCGM, all in Australia. A full plant trialof an HPGR was conducted on a particularly arduous duty at Cyprus Sierrita between1995 and 1996; and, more recently, HPGR has been piloted at Lone Tree, Nevada, in theUnited States, and Amplats Potgietersrus in South Africa. Currently, HPGR-based com-minution plants are under construction at Bendigo, Australia (gold), and Cerro Verde,Peru (copper), and at final feasibility study stage for the Soledad Mountain, California(heap leach gold/silver), and Boddington, Australia (gold/copper), projects.

There are currently three recognised manufacturers of HPGR machines, namely Polysius(a Thyssen Krupp company), KHD Humboldt Wedag AG, and Kppern, all based in Germany.

THE TECHNOLOG Y

Machine DesignThe HPGR machine comprises a pair of counterrotating rolls mounted in a sturdy frame.One roll is fixed in the frame, while the other is allowed to float on rails and is positionedusing pneumohydraulic springs. The feed is introduced to the gap between the rolls andis crushed by the mechanism of interparticle breakage.

The pressure exerted by the hydraulic system on the floating roll largely determines com-minution performance. Typically, operating pressures are in the range of 510 MPa, but canbe as high as 18 MPa. For the largest machines, this translates to forces of up to 25,000 kN.

The rolls are protected with wear-resistant surfaces, and the ore is contained at theroll edges by cheek plates.

Technology MotivatorsGenerally, the primary motivation for the use of the HPGR as a comminution alternativeis its energy efficiency when compared to conventional crushers and mills. This improved

Courtesy of Kppern.

FIGURE 1 Coal briquetting pressearly twentieth century

HIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 17

efficiency is due to the determinate and relatively uniform loading of the material in theHPGR compression zone, whereas the loading in conventional crushers and (particu-larly) tumbling mills is random and highly variable, and therefore inefficient.

The most energy-efficient method of breakage is the slow application of pressure toindividual particles to cause structural failure, such that the energy lost as heat and noise isminimised. However, until a device is invented that can perform this task on a commercialscale, the HPGR remains the most energy-efficient comminution technology available.

A major operating cost in conventional semiautogenous-based comminution circuitstreating hard and abrasive ores is that of grinding media. One effect of the use of HPGR-based circuits is that semiautogenous mill grinding media is eliminated, and while ball-mill media costs typically are slightly greater (due to the increased transfer size fromHPGRs), the overall media savings are typically of the same order of magnitude as theenergy savings.

In addition to its energy and media benefits, the HPGR may be regarded as a metallur-gical tool offering improved gravity, flotation and leach recoveries, and enhanced thickening,filtration, and residue deposition performance.

These effects can be attributed to the phenomenon of microcracking of individualprogeny particles due to the very high stresses present in the HPGR compression zone.Microcracking occurs predominantly at grain boundaries and so increases liberation andlixiviant penetration, while the effective reduction in milling work index caused bymicrocracking reduces overgrinding and slimes generation.

In addition to being ore dependent, the extent of microcracking is a direct functionof the operating pressureand therefore energy inputof the HPGR, and in any givenoperation, the benefits of microcracking must be weighed against the incremental powerrequired to achieve those benefits.

The HPGRs mechanism of interparticle breakage is particularly beneficial in the pro-cessing of diamond-bearing kimberlites, which undergo a form of differential comminution

Courtesy of KHD Humboldt Wedag AG.

FIGURE 2 HPGR machine


whereby the host rock is shattered while the diamonds are liberated undamagedprovided,of course, that the diamonds are smaller than the operating gap of the HPGR. This effectis also of benefit in the treatment of gold ores containing coarse gravity-recoverablegold grains, which would be flattened in conventional tumbling mills and rendered moredifficult to recover.

Technology StatusThe HPGR, considered a mature technology in the cement industry, is now the normrather than the exception in modern diamond plant design and is becoming common iniron ore processing, particularly in the field of pellet feed preparation.

However, although some of the current diamond and iron ore applications can beregarded as hard-rock duties, HPGR is regarded by many as unproven in true hard-rockmining, and this perception is reinforced by the experience at Cyprus Sierrita in 19951996. This application is widely considered to have been unsuccessful because it did notlead to a commercial sale; however, the fact that the comminution performance of themachine was impressive is not in dispute. The difficulties experienced related to the behav-iour of the wear surfaces, and many valuable lessons were learned from this operationregarding the precautions necessary in circuit design and unit operation for the protec-tion of the studded roll surfaces and the successful application of HPGR technology.

Courtesy of Polysius AG.

FIGURE 3 Cone crusher product particle (conventional crushing)


FIGURE 4 HPGR product particle (internal microfractures after Polycom treatment)


The following is a summary of the more important issues arising from observationsof the HPGR operation at Cyprus Sierrita and elsewhere:

The technology is approaching a level of maturity allowing it to be seriously con-sidered for hard-rock applications.

HPGRs are sensitive to segregation and tramp metal in the feed.

Mechanical availability of HPGRs is relatively high, and loss of machine utilisation in hard-rock applications is predominantly wear related.

The smooth and profiled hard-metal roll surfaces commonly used in the cement sector are unsuitable for hard abrasive ores. Instead, the more recently intro-duced autogenous wear layer concept should be used, in which crushed ore is captured in the interstices between metal carbide studs or tiles.

On hard-rock applications in particular, HPGRs are sensitive to feed top size, which ideally should not exceed the roll operating gap. Oversize material in the feed can lead to stud breakage.

Roll wear surfaces may be formed as segments or as cylindrical sleeves or tyres. Segments may be used for softer ores and lower operating pressures, while tyres are recommended for hard-rock duties and higher pressures as they present a uni-form, uninterrupted wear surface to the ore and thereby avoid the preferential wear that occurs at segment boundaries. In addition, tyres are easier to fabricate than segments and so are less expensive.

Tyres involve long change-out times due to the need to remove the roll assemblies from the mainframe, while segments can be changed in situ. Some machine designs aim to minimise change-out times for tyres by allowing roll assembly removal without the need for dismantling of the feed system and superstructure.

Wear of the roll edges and cheek plates (the static wear plates used to contain the ore at the roll edges) remains an issue, and development in this area is ongoing. A

FIGURE 5 Cyprus Sierrita installation


few operations use rock boxes (chutes at the edges of the rolls) instead of cheek plates, allowing part of the feed material to flow around the rolls and so relieve the pressure on, and wear of, the roll edges. This does, however, introduce the disadvantage of passing uncrushed feed to product.

Technology HindrancesHindrances to the adoption of HPGRs in hard-rock processing include

The generally conservative nature of the mining industry

A perception of high cost, particularly of the replacement wear parts in abrasive applications

Uncertainties regarding the reliability of modeling and scale-up from laboratory or pilot operations to commercial installations

A lack of definition of the requirements for robust flowsheet design of an HPGR-based comminution circuit.

Of these, it is generally acknowledged that high wear rates constitute the major obstacleto the ready acceptance of the technology in hard-rock applications. However, the HPGRcan prove a cost-effective comminution device, even when the high cost and frequency ofreplacement of wear surfaces in highly abrasive duties are considered.

Scale-up procedures have been the subject of many technical publications andshould now be considered reliable. They are mentioned here only briefly for the sake ofcompleteness. The characteristics of HPGRs that have a significant impact on flowsheetdesign will be considered as the main emphasis of this analysis.

SCALE OF OPERAT IONA common perception is that a project must be of relatively large scale before the use ofHPGRs can be justified. However, HPGR units of almost any size can be produced (up tothe current practical unit capacity limit of about 2,200 t/h), and this technology deservesserious consideration over a much wider range of plant capacities than might initially beimagined.

Ultimately, HPGRs can be justified if they offer benefits to metallurgical perfor-mance and/or project economics, and the potential for such benefits can usually beassessed at the prefeasibility study phase by conducting preliminary tests. The manufac-turers have test facilities in Germany, and small-scale laboratory facilities are available atvarious locations globally. Pilot-scale machines are available at several research facilitiesin Perth, Western Australia, and a Polysius mobile pilot unit used for trials at an opera-tion in North America in 2003 was subsequently relocated to South Africa for evaluationon a hard-rock mining operation.

THE MANUFACTURER S AND THE IR DES IGNSPolysius, KHD, and Kppern are widely represented globally, but the machines are man-ufactured exclusively at their respective facilities in Germany.

Polysius favours a high-aspect-ratio designlarge diameter, small widthwhile KHDand Kppern prefer a low-aspect ratio. The high-aspect-ratio design is inherently moreexpensive but also offers an intrinsically longer wear life for a given application, as theoperating gap is larger and the roll surfaces are exposed to a correspondingly smallerproportion of the material processed. The high-aspect-ratio design also produces acoarser product due to the greater influence of the edge effect; however, this difference isrelatively slight, particularly with larger units. Nevertheless, for closed-circuit applications,


this additional coarseness does increase the circulating load and tends to offset the wearlife benefits, as a higher total throughput is required for the same net product.

The use of tungsten carbide studs to create an autogenous wear layer on the roll sur-face is covered by a patent held by KHD, from whom this technology is available underlicense.

Both Polysius and KHD have experience with minerals applications and studded rolltechnology, and are able to supply machines with capacities of up to about 2,200 t/h.Although Kppern has limited minerals experience, their HPGRs are successfully operat-ing in the cement industry. For highly abrasive materials, Kppern recommends HPGRsfitted with their Hexadur wear protection.

The Hexadur surface comprises hexagonal tiles of a proprietary abrasion-resistantmaterial set into a softer matrix, which wears preferentially in operation, allowing theformation of an autogenous wear protection layer at the tile joints. The tiles and matrixmaterial are fully bonded together and to the substrate in a high-temperature, high-pressurefurnace. By contrast, KHDs studs are inserted into drilled holes. As a result, the tiles areinherently stronger and more resistant to breakage due to oversize ore or tramp metal.

Kppern supplies patterned and profiled surfaces in both segment and tyre format,whereas Hexadur is generally available only in tyre format due to the dimensional controldifficulties inherent in the fabrication and furnace treatment of segments. However,research into the commercial production of Hexadur segments is ongoing.

Meanwhile, the maximum Hexadur roll diameter available currently (and for theforeseeable future) is 1.5 m, constrained by furnace dimensions. This constraint limitsKpperns unit capacity to about 1,000 t/h for hard-rock comminution applications usingHexadur. However, Kppern also offers machines with studded roll surfaces supplied byKHD, effectively lifting this capacity constraint.

Data of Test Units:Diameter of Rolls: 0.71 mWidth of Rolls: 0.21 mSpeed of Rolls: 0.291.10 m/sTop Feed Size: 1635 mm

Diameter of Rolls: 0.30 mWidth of Rolls: 0.07 mSpeed of Rolls: 0.20.9 m/sTop Feed Size: 812 mm

REGRO

ATWAL

LABWAL


FIGURE 6 Polysius test facility


Kppern has an established design in which the ends of the mainframe hinge out-wards to allow the roll assemblies to be removed without disturbing the feed system andsuperstructure. This allows roll change-out times for tyre replacement of about the sameduration as for in-situ segment change-out. Polysius also offers a design that allows rapidroll assembly removal, but without the need for a hinged frame design. In more recentdevelopments, KHD has unveiled a rapid change-out concept to be offered on new

Courtesy of KHD Humboldt Wedag AG.

FIGURE 7 Studded roll wear surface

Courtesy of Kppern.

FIGURE 8 Hexadur wear surface for hard-ore comminution


machines and which can be retrofitted to existing units, and Kppern has introducedtheir C-frame design that allows the removal of both roll assemblies from one end ofthe frame, so offering a maintenance advantage over their earlier design.

KHD uses cylindrical roller bearings that allow the choice of grease or circulating oillubrication systems, as there is no relative movement between the bearings and seals.Polysius and Kppern use grease-lubricated, self-aligning spherical roller bearings.

OPERAT ING CHARACTER IST ICSThere are many factors to be considered when specifying an HPGR and selecting anappropriate flowsheet for a given application. The following subsections summarize themore important issues.

Courtesy of Kppern.

FIGURE 9 Kppern HPGR

Courtesy of Kppern.

FIGURE 10 Kppern hinged frame


Ore CharacteristicsThe compressive strength of the material to be crushed determines the amount of usefulenergy that can be absorbed by the material, which in turn dictates the bearing andmotor sizes required for a given duty.

With studded roll wear surfaces, the compressive strength of the ore, in combinationwith the feed particle top size and operating pressure, will largely determine the probabilityof stud damagethe higher the values of each of these variables, particularly when theyoccur together, the higher the likelihood of incurring stud damage. Ongoing developmentof stud technology is aimed at reducing the sensitivity of the studs to these variables.

The abrasion index of the material being crushed will determine the wear rate (asdistinct from the breakage rate) of the studs, as well as that of the substrate metal. Forexample, the wear life at the iron ore operations at Los Colorados and Empire are about14,000 and 10,000 hours, respectively, while those at the Argyle and Ekati diamondmines were about 4,000 hours initially, but increased to 6,0008,000 hours and beyondwith ongoing development of stud and edge protection configurations.

HPGRs are not generally suitable for the treatment of highly weathered ores or feedscontaining a large proportion of fines. (This of course does not apply to applicationswhere all the feed material is fine, such as fine grinding of concentrates.) Fine andweathered material tends to cushion the action of the rolls and so reduces the efficiencyof comminution of the larger feed particles. For example, Argyle bypasses its primaryHPGRs when very fine ore is being mined. On these ore types, the fine or weatheredmaterial should be removed by prescreening if HPGR treatment of the coarser compo-nent is required.

HPGRs are not generally suitable for comminution of feeds containing excessivemoisture, which tends to cause washout of the autogenous layer on studded rolls andincreases slippage on smooth rolls. In both cases, accelerated wear is the result. Forexample, Ekati bypasses the 4+1 mm feed fraction around the HPGR when the prevail-ing ore type results in inherently high moistures.

Specific PressureThe specific pressure (specific press force) is the force (Newtons) divided by the appar-ent (or projected) area of the rollthat is, the product of roll diameter and length:

specific pressure (N/mm2) = force (N)/(D (mm) u L (mm))

Typical practical operating values are in the range of 14.5 N/mm2 for studded rollsurfaces and up to 6 N/mm2 for Hexadur. The required specific pressure determined intests is used for scale-up of the required operating hydraulic pressure for the commercialunit.

Specific Energy InputThe specific energy input (SEI) is the net power draw per unit of throughput:

specific energy input (kWh/t) = net power (kW)/throughput (dry t/h)

Typical operating values are in the range of 13 kWh/t. In general, a given ore willabsorb energy up to a point beyond which little additional useful work (i.e., size reduc-tion) is achieveda zone of diminishing returns is approached.

For equivalent size reduction, a hard, competent ore of high compressive strengthwill result in a higher SEI than a softer ore of low compressive strength.

The energy input is governed by the hydraulic pressure, of which it is a roughly linearfunction. Generally, specific energy input in coarse crushing applications is numerically


about one half to one third of the specific pressure, so that a specific pressure in the typi-cal operating range of 34.5 N/mm2 can be expected to correspond to a specific energyinput of 12.5 kWh/t. In fine-grinding duties, this ratio is typically higherfor example,a ratio of 1.05 applies at the Kudremukh pellet feed operation.

The best method of determining the optimum specific energy is to conduct tests toderive a graph of product fineness against specific energy. The graph generally displaysan initial steep slope that flattens out to approach the horizontal at high SEI values (e.g.,3.54.5 kWh/t). The optimum SEI can then be selected.

MicrocrackingAlthough the size reduction graph frequently enters an area of diminishing returns withincreasing specific energy, it has been demonstrated on some ores that the reduction ineffective work index due to microcracking (also known as microfracturing or microfis-suring) does not always display the same tendency. As a result, it may be beneficial froman overall comminution energy perspective to operate at a higher specific energy thancorresponds to the optimum for size reduction in the HPGR stage, to maximise the bene-fits of microcracking. In this regard, the final grind size must also be taken into account,as the effects of microcracking are felt more in the coarser fractions, so that an applica-tion with a coarse grind will benefit more than one with a fine grind.

It is important to conduct sufficient tests to quantify the optimum point of increasedfines generation and reduced product work index, to ensure an HPGR is specified that iscapable of transmitting the necessary power.

Feed Top SizeFor hard-rock applications, the feed top size is a critical variable in the successful operationof an HPGR crusher. For smooth rolls, too large a top size results in reduced nip efficiency,slippage, and accelerated wear; for studded rolls, tangential forces at the roll surface dueto early nippingeffectively causing single-particle breakage by direct contact with theroll surfacescan cause stud breakage.

Constraints on feed top size have been related in the literature both to roll diameterand to operating gap. Figures of up to 7% of roll diameter and three times the gap havebeen quoted as appropriate limits on feed top size, even though the latter ratio impliessome direct contact of the larger particles with the surfaces of both rolls, leading to single-particle breakage.

These figures are now considered much too optimistic in hard-rock applications,and it is generally accepted that, to minimise the likelihood of stud breakage, feed topsize should not exceed the expected operating gap. This will normally demand a closed-circuit crushing operation upstream to ensure this top size is positively controlled. Forsofter materials, this rule can be relaxedfor example, some kimberlite operations suc-cessfully treat open-circuit secondary crushed products with top sizegap ratios of about1.82.0 using studded rolls.

By interpolation, ratios of around 1.31.5:1 are tolerable when treating ores of mod-erate hardness. Where uncertainty exists regarding ore hardness categorisation, it is con-sidered prudent to adopt a ratio of close to 1:1 initially, and then relax this incrementallyif and when it is established that stud breakage is not an issue.

As a guide, the direct-contact nip angle (for single-particle breakage and possiblestud damage) is normally in the range of 10 to 13 while interparticle breakage com-mences at angles of 5 to 7. By using a scale diagram of an HPGR unit of a given rolldiameter, and showing these angles and an appropriate operating gap, estimates can bemade of the particle size above which single-particle breakage is likely to occur andbelow which interparticle breakage commences.


Unit CapacityThe capacity of an HPGR is fundamentally a function of the ore characteristics. Capacityis generally expressed in terms of specific throughput mx (m-dot), which is a function ofthe roll diameter, length, and peripheral speed:

mx (ts/m3h) = throughput (t/h)/(diameter (m) u length (m) u speed (m/s))

The value of mx is determined in pilot tests and used in scale-up to the commercialunit, taking into account the change in the relative proportions of product from the cen-tre of the rolls and from the edges where poorer comminution occurs (the edge effect),and also whether the commercial unit is to be operated with cheek plates or rock boxesfor roll edge protection.

In addition to its fundamental relationship to the ore characteristics, the value of mx

is a function of many variables. The following should be regarded as general trends forthe majority of ores, rather than as statements of universal factthere will always be theexception that proves the rule:

Ore hardnessmx increases with ore hardness.

Specific pressuremx decreases slightly with increasing pressure.

Roll surfacemx increases with increasing texture of the roll surface, due to the reduced slip (increased kinetic friction) and improved nip between the rolls. Thus, smooth rolls give the lowest values, with profiled surfaces in the mid-range, and studded surfaces the highest (typically about 50% higher than for smooth rolls).

Roll speedfor smooth rolls, mx decreases with roll peripheral speed, so that actual throughput increases with increasing speed but at a progressively dimin-ishing rate due to increased slippage. The effect is much reduced with profiled or studded rolls due to the inherently higher kinetic friction of these surfaces.

Feed top sizethe available evidence is not conclusive, but it appears that mx might increase slightly with an increase in feed top size.

Feed bottom sizemx decreases significantly as feed bottom size is increased. Thus, the highest value of mx occurs with a full-fines feed, and this value decreases progressively as the fines cut-off or truncation size is increased. This is due to the increased voidage in the truncated feeds, which results in a lower back pressure on the rolls and a consequent reduction in the operating gap.

Feed moisturefor moisture levels greater than about 1%, mx decreases with increasing moisture due to the replacement of solids with water in the compacted product flake; higher moisture levels can result in excessive slippage and ultimately to washout of the autogenous layer on studded rolls. Below 1% moisture, there is some evidence of reduced m values with studded rolls due to the difficulty in generating and maintaining a competent autogenous wear layer with very dry feeds, as the crushed product is too friable to form a compacted layer between the studs.

Operating GapThe operating gap is directly related to the unit capacity, all else being equal, so gapcan be interchanged with mx in the above analysis. Depending on the application, theratio of operating gap to roll diameter will normally lie in the range of 0.010 to 0.028.

Circuit CapacityThe capacity of an HPGR circuit, as distinct from the unit capacity discussed above, isobviously a function of the circuit design. Of the above variables, the feed bottom size


is particularly relevant in this regard, as a truncated feed necessarily implies the pres-ence of a screen or other classification device upstream of the HPGR.

It has been noted that capacity decreases with truncated feeds; however, the capac-ity of the circuit would increase if the amount of fines removed from the HPGR feedexceeded the reduction in HPGR unit capacity. Whether this occurs in practice remainsthe subject of some debate (and in any event is probably ore specific), but recent model-ing of pilot test data for two prospective applications indicates that this is the case, andthis is supported by the limited evidence available in the literature.

However, an increase in circuit throughput achieved in this way may be offset by adecrease in product fineness and/or reduced microcracking such that, depending on thedownstream processing route, a full-fines HPGR feed may be preferable to a truncatedfeed. For any given application, the more efficient flowsheet can be determined only bycomprehensive tests and modeling, but where doubt exists, the circuit should, if possi-ble, be designed with the flexibility to operate with full fines or truncated feed to allowcircuit performance to be optimised. This flexibility normally comprises the prescreeningof the feed and a facility to recycle to HPGR feed a portion of either the HPGR product or,where the HPGR operates in closed circuit with a screen, the screen undersize.

Product SizingAs noted earlier, product fineness increases with operating pressure (and thereforepower), generally up to a point of diminishing returns. It has been observed elsewherethat it is more energy efficient to operate an HPGR at low pressures and in closed circuitwith a screen, so that less energy is wasted on compacting the product. However, thisgenerally would require more or larger HPGRs to handle the increased circulating load.Also, it is not clear whether the analysis included the cost of conveying the increased cir-culating load of screen oversize.

Product fineness generally decreases with increasing texture of the roll surface; sosmooth rolls give the finest product, with profiled surfaces in the mid-range and studdedsurfaces the coarsest. This is due to the reduced slip between the rolls and the ore, givinga higher throughput for a given power draw. For the same product fineness, therefore,a studded or profiled roll machine would have to be operated at higher pressures than asmooth roll unit. However, the effect is relatively small, and the benefits of profiled orstudded rolls usually outweigh the reduced product fineness. Furthermore, the effectappears to be ore specific, and some operations (e.g., Jwaneng) have recorded anincrease in fineness with studded rolls compared to smooth rolls.

Increasing roll speed leads to a reduced product top size and improved F50/P50reduction ratio, without significantly changing the fine end of the sizing spectrum.

A slight mismatch or differential in roll speeds has been found to enhance grindingperformance, and though this could be considered intuitively plausible, it might also beexpected that adopting this as a deliberate control strategy could lead to increased rollsurface wear rates due to this imposed speed differential. This effect is thereforeregarded as being of academic interest rather than practical significance.

Product sizing is largely independent of feed moisture. Product sizing is a functionof roll aspect ratio. A high aspect ratio gives an inherently coarser product for the follow-ing reasons:

The proportion of edge material in the product is greater.

The pressure peak in the compression zone is lower (for a given specific pressure).

However, the overall effect is generally fairly modest.The shape of the HPGR product sizing curve is dissimilar to that of conventional

crushers, so that for products with nominally the same P80, the HPGR product contains


considerably more fines below this size than from a conventional crusher. The implica-tions of this are that, where the product is delivered to, for example, a ball milling oper-ation, mill capacity will be greater when treating HPGR product than predicted by thestandard Bond equation. Milling power requirements are thus reduced by both the sizingof the HPGR product and the microcracking of the product particles, and are thereforebest determined by pilot testing.

Roll Surface WearIncreasing roll speed increases turbulence in the feed material and slip of feed againstthe roll surfaces, leading to elevated wear rates. This should generally be a concern onlyat the top end of the practical speed range. In this respect, Polysius traditionally uses arule of thumb to the effect that the peripheral speed of the rolls (in meters per second)should not exceed roll diameter (in meters), although Kppern does not support thisview and regularly nominates speeddiameter ratios of up to 1.3. KHD also uses thesehigher ratios for their smaller-diameter machines but generally uses


selected for the initial setup to minimise the incidence of stud breakage, which is moredifficult to accommodate than stud wear. After evaluating initial performance, adjust-ments to stud hardness may be made. Several iterations might be needed to achieve theoptimum configuration. Ekati and Argyle have both achieved significantly improvedresults using this approach.

The differential wear-rate effect is also well documentedit appears that the floatingroll typically wears at a slightly higher rate than the fixed roll, though the reasons for thishave not been fully investigated. It is believed that the effect is caused by the additionalapplied kinetic forces imparted to the floating roll. However, recent experience at a pilotoperation has shown the reverse trend. In any event, the effect is small and of little prac-tical significance.

In a recently commissioned installation, a more irregular wear pattern was observedon the fixed roll relative to the floating roll, although the overall wear rate for the floatingroll was higher. The reasons for this comparatively irregular wear pattern are not clear, butit is suspected that it is due to the effect of the presence of a feed-regulating gate, whichpresents the feed stream preferentially towards the fixed roll. This may result in anincreased level of turbulence in the vicinity of the feed gate tip. The need for the gate atthis operation is not proven (as the variable-speed drives provide adequate turndown), andit is to be removed as a trial, during which any change in wear patterns will be recorded.

Tramp SteelTheoretically, the HPGR is equipped to handle tramp steel in that the bearing arrange-ment allows skewing of the floating roll and the hydraulic system is able to relieve exces-sive pressures. However, particularly with larger units, the inertia of the rolls and theirvery brief exposure to tramp metal in the compression zone generally results in damageto the roll surface instead of, or as well as, the relieving action of the floating roll.

Repair of roll damage can be expensive and operationally disruptive, and flowsheetdesign should endeavour to locate the HPGR in an intrinsically noncontaminated flowstream, or ensure that a comprehensive and practical tramp metal detection and removalsystem is included. Such a system should preferably be automatic, with contaminatedore bypassed around the HPGR or rejected from the circuit. It is important to minimisethe need for operator intervention and process interruption.

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Full Feed Truncated 50%Truncated

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MATERIAL 1 MATERIAL 2

FIGURE 11 Impact of feed truncation on roll wear


ExtrusionExtrusion is a phenomenon whereby the HPGR product stream emerges from the com-pression zone at a speed greater than that of the roll surface. This is similar to the well-known extrusion effect observed in metal rolling and occurs when the product flakeexpands as it leaves the compression zone and the applied pressure reduces to zero. Thisexpansion is typically in the order of 2% to 5%, and the resultant slippage can be thecause of increased roll wear. Extrusion has generally been observed to increase with rolldiameter, but also has been recorded with pilot-sized machines. Extrusion also increaseswith applied pressure and feed moisture, and the evidence available suggests the effectis most pronounced at a roll peripheral speed of about 1.5 m/s. It is most noticeable withsmooth rolls, and decreases markedly with studded or profiled surfaces.

Product Flake Formation and TreatmentThe HPGR product emerges from the compression zone as a compacted cake or flake.The coherence of the flake is a function primarily of the ore type and moisture content,and also of the operating pressure of the machine. Generally, competent flakes are pro-duced with softer materials or those with a high clay or moisture contentkimberlites,for examplewhile hard, primary ores tend to produce fragile flakes, even at relativelyhigh moistures and pressures.

Depending on flake competency and downstream processing requirements, a dedi-cated unit operation for deagglomeration of the flake product could be required, and thisis a significant consideration in flowsheet development. Kimberlite flakes normally mustbe intensively deagglomerated in wet rotary-drum scrubbers, and then screened toensure efficient removal of fines before downstream processing, usually in a heavymedia separation operation. By contrast, the flake in a hard, primary-ore applicationmight require no separate deagglomeration, being adequately broken down by handlingin chutes and bins and on conveyors, so that acceptable efficiencies are achieved in nor-mal screening.

The need for, and nature of, a dedicated deagglomeration step in the comminutionflowsheet can normally be assessed by testing. KHD has developed a standard flake com-petency test specifically designed to determine whether separate deagglomeration isrequired ahead of further processing.

FLOWSHEET OPT IONSThe flowsheet for a given ore is driven by the requirements of the process and consider-ation of the above HPGR characteristics. In particular, the possible need for a controlledfeed top size, fines recycling, and separate deagglomeration will have a significant effecton the formulation of a practical and robust flowsheet.

It is important that the appropriate amount of testing be conducted to determineflowsheet design requirements. Alternatively, in the absence of adequate tests, a conser-vative approach must be taken to flowsheet designthat is, it must be assumed that top-size control, fines recycling, and deagglomeration will all be needed. This of course hasthe potential to impose significant and possibly unnecessary cost penalties on anyproject, and a comprehensive test programme generally represents excellent value forthe money in this context.

The selection of flowsheets considered here focuses on alternatives to conventionalhard-rock crushing, screening, and milling circuits, either as greenfield projects or as retrofitsto existing operations for the purposes of debottlenecking or plant expansion. HPGR mayalso be considered as a beneficial metallurgical tool in heap-leach applications.


SAG Mill PrecrushingIn this circuit, a portion of the semiautogenous grinding (SAG) mill feed is precrushed ina secondary crusher followed by an HPGR (with prescreening for top-size control, if nec-essary) before rejoining the main SAG mill feed stream. The total SAG mill feed is thereforecorrespondingly finer while some coarse particles are retained in the mill feed as media.

This is suitable for both new and existing operations and has the potential to increasemill throughput by 50% or more. (The Troilus gold operation in Canada introduced ascreen on the SAG mill feed stream and recorded a circuit capacity increase of about35% using only a secondary crusher to precrush the 125+25 mm middlings fraction.)

An alternative to this arrangement is to deliver the HPGR product to the SAG milldischarge screen, so that finished product and material of ball mill feed size bypasses theSAG mill.

These circuits are flexible in that milling can continue (albeit at reduced rates) withthe precrushing circuit out of service. Also, if an HPGR bypass facility is provided, thesecondary crushing component of the precrushing circuit can continue to operate whenthe HPGR is inactive.

SAG Mill Pebble CrushingFurther size reduction of the pebble fraction in an SABC (semiautogenous-ball-crusher)circuit can be achieved by passing the pebble crusher product through an HPGR, thusincreasing circuit capacity. Alternatively, the conventional cone crusher may be replacedentirely by an HPGR, provided the pebble top size is small enough.

In a variation, the pebble crusher and HPGR can be operated in closed circuit withthe screen undersize delivered to the ball milling circuit. This circuit can be used toopen-circuit the SAG mill when this is the circuit bottleneck.

Primary Crushing

Secondary Crushing

Screen

HPGR

SemiautogenousMilling

Screen

Cyclone

Pebble Crushing

Ball Milling

U/S

U/S

O/F

O/S

O/S

U/F

FIGURE 12 SAG mill precrushing to SAG mill feed


These circuits have the disadvantage of exposure of the HPGR to tramp steel in theSAG mill discharge. Also, the use of HPGR in the pebble-crushing circuit can usually bejustified only when pebble arisings are a relatively large proportion of SAG mill new feed.

Multistage Crushing and Ball MillingIn this circuit, the HPGR is used in the tertiary crushing stage immediately ahead of theball mills. This can be applied in new projects or as a retrofit to increase crushing capac-ity. However, for hard rock, it is important that the secondary crushing stage be operatedin closed circuit to control HPGR feed top size, and this must be borne in mind whenconsidering this circuit as a retrofit.

Depending on whether deagglomeration is indicated, the HPGR product screensmay be operated dry (no deagglomeration required) or wet (mild deagglomeration). Inthe latter case, the dilute screen undersize slurry must be delivered to the mill sumprather than mill feed. Where intensive deagglomeration is required, the entire HPGRproduct is delivered to the mill, the mill discharge screened, and the screen oversizereturned to the HPGR. In this case, it may be preferable to adopt a two-stage milling cir-cuit, with the primary mill designed to minimise pebble generation, so minimising thereturn of moist material to the HPGR.

This is inherently less efficient than delivering a controlled feed top size to the millingcircuit, and it might be more efficient to use a dry deagglomerator on the HPGR product,such as a hammer mill or vertical impactor, followed by conventional dry screening.

Open-Circuit HPGR with Edge RecycleThis option obviates the need for fine screening of the HPGR product and instead uses adividing chute below the HPGR to separate the highly reduced centre product from the

Primary Crushing

Secondary Crushing

Screen

HPGR


Screen

Cyclone

Pebble Crushing

Ball Milling

U/S

U/S

O/F

O/S

O/S

U/F

FIGURE 13 SAG mill precrushing to ball mill feed


coarser edge material, as typically practised on test units and at a few commercialoperations. The centre product is delivered to downstream processing, while the edgematerial is returned to HPGR feed.

This arrangement would typically be used where energy efficiency was not of para-mount importance, such as heap-leach applications.

ANALYS IS OF TECHNOLOG Y BENEF I TSThe metallurgical benefits of HPGRs have been discussed earlier in a qualitative sense.These are highly ore-specific and should be determined by the appropriate tests.

Primary Crushing

HPGR


Screen

Screen

Cyclone

Pebble Crushing

Ball Milling

U/S

U/S

O/F

O/S

O/S

U/F

FIGURE 14 SAG mill pebble crushing

Primary Crushing

HPGR


Screen

Screen

Cyclone

Pebble Crushing

Ball Milling

U/SMiddlings

U/S

O/F

O/S

O/S

U/F

FIGURE 15 Open-circuit SAG mill


Likewise, the energy benefits of HPGRs must be properly quantified to allow a realis-tic assessment of HPGR-based circuit options, as comminution energy generally is amajor component of operating costs in hard-rock applications.

In a recent study, a comparison was drawn between a high-capacity conventionalSABC circuit and a three-stage crushing/single-stage ball milling circuit with both sec-ondary and tertiary crushing stages operating in closed circuit and with the HPGR as thetertiary step. As part of the analysis, various intermediate circuits were also evaluated inwhich the HPGR played a progressively greater role.

Primary Crushing

HPGRScreenSecondary Crushing

Cyclone

Ball Milling

Middlings

U/S

O/F

O/S

U/F

FIGURE 16 Three-stage crushing, closed-circuit HPGR

Primary Crushing

HPGR

Screen

Screen

Secondary Crushing

Cyclone

PrimaryBall Mill

SecondaryBall Mill

U/S

U/S

O/F

O/S

O/S

U/F

FIGURE 17 Three-stage crushing, open-circuit HPGR


The comparison showed the following conclusions:

The energy efficiency of the circuit increased with the proportion of comminution performed by the HPGR.

The specific capital cost (i.e., cost per unit capacity) of the HPGR/ball mill circuit was 26% lower than that of the SABC circuit. (However, the capacity of the HPGR-based circuit in this example was higher than that of the SABC base-case circuit, so this differential figure would be lower in a direct comparison. This has been supported in subsequent studies on various projects in which the capital costs of an HPGR-based circuit were found to be about the same or slightly greater than for the equivalent SAG-based circuit of the same capacity.)

The HPGR/ball mill circuit was 28% more energy efficient than the SABC circuit.

Overall operating costs for the HPGR/ball mill circuit and downstream plant were 22% lower than for the SABC circuit.

Project implementation time was significantly reduced for the HPGR option due to the removal of the long-delivery SAG mill.

As a result of these conclusions, project viability was considerably enhancedin fact, itwas determined that, in the absence of HPGR in some part of the comminution circuit,project viability was at best marginal.

A sensitivity analysis was conducted in which the wear life of the HPGR roll surfaceswas reduced from the 4,000 hours predicted by the manufacturers to a very conservativefigure of 2,000 hours. This had the effect of increasing overall operating costs by only5%, meaning that the HPGR-circuit operating costs were still 17% lower than those ofthe SABC circuit.

COMPARISON WITH CONVENT IONAL TECHNOLOG IESAutogenous grinding (AG) and SAG technologies displaced multistage crushing and rod/ball milling circuits as they were simpler and offered lower capital and overall operatingcosts, even though they were often demonstrably less efficient in the use of comminutionenergy. AG and SAG mills were also ideal for handling wet, sticky, clay-rich, and oxidised

Primary Crushing

HPGR

Screen Secondary Crushing

U/S

O/S

EdgeCentre

Heap

Cement and Water

FIGURE 18 Open-circuit HPGR with edge recycle


ores, allowing the elimination of the traditional washing plant normally required forsuch materials.

With the progressive global depletion of easily treated ores, harder, tougher, andmore abrasive primary ores are being targeted for treatment, and energy efficiency willbecome steadily more important from both economic and environmental perspectives.These ores may be well suited to SAG mill treatment in the context of media competency,but this treatment path is grossly inefficient in the application of energy for size reduction.

The circuit developed in the study referred to above represents a return to the tradi-tional multistage crushing/ball milling circuit. The difference now is that, with HPGR inthe tertiary crushing step, energy efficiencies are elevated to such a degree that overalloperating costs are much lower than for the equivalent SAG mill circuit. When the poten-tial for lower specific capital costs also is considered, the additional circuit complexity ofthe HPGR-based plant can more readily be justified.

Just as the traditional multistage crushing/ball milling circuit has survived inselected applications, semiautogenous milling will of course also survive and remain theappropriate choice for some ores. It is believed, however, that HPGR circuits representthe next generation of hard-rock comminution plant design, as semiautogenous millingdid several decades ago.

V IS ION FOR THE FUTUREThe HPGR is the most energy-efficient comminution device currently available to the miner-als processing plant designer, and the focus must be on the development of both machineand flowsheet to maximise the proportion of total comminution performed by the HPGR.

The initial objective must be to minimise the top size of ball mill feed by reducingHPGR product screen aperture and recirculating progressively more material to the HPGR.This will entail changing to wet screening as the separation size falls below about 6 mm,and this in turn will impact circuit design philosophies, as there will be no opportunity tostockpile mill feed.

As the mill feed top size falls, there might be some merit in operating tertiary HPGRsin open circuit and introducing quaternary HPGR crushing to handle screen oversize.The associated moisture would, however, be detrimental to this process, and some formof blending with dry material might be necessary.

Ultimately, with very fine mill feeds, the number and size of conventional wetscreens will become unmanageable, while the ball mills will trend ever smaller. The nextstep is to abandon screens and ball mills entirely and operate HPGRs in closed circuitwith air separators, with the final product repulped and fed directly to flotation or otherdownstream processes.

The technology for this type of circuit already exists and is in operation. An exampleis the use in Europe of KHD HPGRs and air separators for the production of dry-groundlimestone for use in a flue-gas desulphurisation process. Typical air-separator perfor-mance in this type of application is 90 Pm P90, while the finest separation is claimed tobe around 20 Pm P90. Product size is adjustable, and the P80 grind sizes of 75 to 150 Pmcommon in minerals processing should be readily achievable.

CONCLUS IONHPGR technology holds the promise of significant improvements in comminution energyefficiency in hard-rock applications when compared to SAG-based circuits. Properlydesigned HPGR-based circuits offer the potential of significant savings in comminutionenergy requirements and overall operating costs when compared to SAG-based circuits.Further energy savings are envisioned as progressively more of the comminution load is


performed by HPGR, culminating in the generation of final ground product by air classi-fication of HPGR product and the elimination of ball milling.

ACKNOWLEDGMENTSIn addition to the technical papers listed in the bibliography, from which much of thismaterial is drawn, the sourcing of study information and operational data from Bodding-ton Gold Mine and Argyle Diamonds, respectively, is gratefully acknowledged, as are thecontributions from the manufacturers, Polysius, KHD, and Kppern.

B IBL IOGRAPHYAMIRA Project P428. 1996. Application of High Pressure Grinding Rolls in Mineral Process-

ing (Overview). Report P428/11.

Austin, L.G. 1990. Ball Mills, Semi-Autogenous Mills and High Pressure Grinding Rolls.University Park, PA: Penn State University Press.

Austin, L.G., Trubeljal, M.P., and von Seebach. 1995. Capacity of high-

advanced comminutions

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