Published by theSociety for Mining, Metallurgy, and Exploration, Inc.ADVANCES IN COMMINUTIONSociety for Mining, Metallurgy, and Exploration, Inc. (SME)8307 Shaffer ParkwayLittleton, Colorado, USA 80127(303) 973-9550 / (800) 763-3132www.smenet.orgSME advances the worldwide mining and minerals community through information exchange and professional development. SME is the worlds largest association of mining and minerals professionals.Copyright 2006 Society for Mining, Metallurgy, and Exploration, Inc.All Rights Reserved. Printed in the United States of America.Information contained in this work has been obtained by SME, Inc., from sources believed to be reliable. However, neither SME nor its authors guarantee the accuracy or completeness of any information published herein, and neither SME nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that SME and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought.No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Any statement or views presented here are those of the author and are not necessarily those of SME. The mention of trade names for commercial products does not imply the approval or endorsement of SME.ISBN-13: 978-0-87335-246-8ISBN-10: 0-87335-246-7Library of Congress Cataloging-in-Publication DataAdvances 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--dc222005057533viiPrefaceThisthirdinternationalsymposiumandproceedings,AdvancesinComminution,havecomeatacriticaltime.Becauseofrapidlyrisingenergyprices,itisimportantthatthelatest 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.Twoprevioussymposiums,alsoorganizedbytheSocietyforMining,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 millcircuitstoautogenousgrinding.Thesecondcomminutionsymposium,heldin1997,focusedoninitialinstallationsofhighpressuregrindingrolls(HPGRs).Now,in2006,theHPGRsarebecomingpartofhard-rockgrindingcircuits.Theyhaveproventobeavery 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 stirredmillsforregrindingandultrafinegrindingapplicationsdonotlendthemselvestocon-ventional methodologies; therefore, new approaches are being developed. There is alsoagreatdealofactivitydirectedtowardimprovingorecharacterizationtopredictAG/SAG mill energy requirements, as well as developing improved models and instrumenta-tionforoptimizationandcontrolofcomminutioncircuits.Instrumentation,modeling,andcontrolfunctionsinparticularhavebenefitedfromrapidlyadvancingcomputertechnology,withcalculationsthatwereformerlyextremelytime-consumingbecomingrapidandroutine.Theseadvanceswillkeepenergywastetoaminimumandwillpro-vide the increased energy efficiency needed to maintain ongoing industry success.Itishopedthatthesymposiumandtheseproceedingswillbeusefultothosewhoareworkingtowardmajoradvancesinindustrialpractice.Appreciationisextendedtomembers 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.iiiContentsEDITORIAL BOARD vPREFACE viiPART 1 ADVANCED COMMINUTION TECHNOLOGIES 1High-Pressure Grinding RollsCharacterising and Defining Process Performance for Engineers 3High-Pressure Grinding RollsA Technology Review 15Some Basics on High-Pressure Grinding Rolls 41High-Pressure Grinding Rolls for Gold/Copper Applications 51Selection and Sizing of Ultrafine and Stirred Grinding Mills 69Effects of Bead Size on Ultrafine Grinding in a Stirred Bead Mill 87Specific Energy Consumption, Stress Energy, and Power Draw of Stirred Media Mills and Their Effect on the Production Rate 99AG/SAG Mill Circuit Grinding Energy RequirementHow to Predict It from Small-Diameter Drill Core Samples Using the SMC Test 115PART 2 COMMINUTION PRACTICES 129Causes and Significance of Inflections in Hydrocyclone Efficiency Curves 131Simulation-Based Performance Improvements in the Ispat Inland Minorca Plant Grinding Circuit 149Determining Relevant Inputs for SAG Mill Power Draw Modeling 161Cement Clinker Grinding Practice and Technology 169Extended Semiautogenous Milling: Smooth Operations and Extended Availability at C.M. Doa Ines de Collahuasi SCM, Chile 181PART 3 LIBERATION AND BREAKAGE 191Shell and Pulp Lifter Study at the Cortez Gold Mines SAG Mill 193Breakage and Damage of Particles by Impact 205The Rationale behind the Development of One Model Describing the Size Reduction/Liberation of Ores 225Influence of Slurry Rheology on Stirred Media Milling of Limestone 243ivExperimental Evaluation of a Mineral Exposure Model for Crushed Copper Ores 261Linking Discrete Element Modeling to Breakage in a Pilot-Scale AG/SAG Mill 269Significance of the Particle-Size Distribution in the Quality of Cements with Fly Ash Additive 285Modeling Attrition in Stirred Mills Applying Statistical Physics 293PART 4 MILL DESIGN 307Design of Iron Ore Comminution Circuits to Minimize Overgrinding 309Evaluation of Larger-Diameter Hydrocyclone Performance in a Desliming Application 321Selection and Design of Mill Liners 331The Importance of Liner Geometry and Wear in Crushing 377Bonds Method for Selection of Ball Mills 385Developments in SAG Mill Liner Design 399The Gearless Mill DriveThe Workhorse for SAG and Ball Mills 413Optimizing Hydrocyclone Separation in Closed-Circuit Grinding 435PART 5 INSTRUMENTATION, MODELING, AND SIMULATION 445Use of Multiphysics Models for the Optimization of Comminution Operations 447Batu Hijau Model for Throughput Forecast, Mining and Milling Optimization, and Expansion Studies 461The Use of Process Simulation Methodology in Process Design Where Time and Performance Are Critical 481Modeling and Simulation of Comminution Circuits with USIM PAC 495Remote and Distributed Expert Control in Grinding Plants 513Developments in Sensor Technology for Tumbling Mills 527Ball Mill Circuit Models for Improving Plant Performance 539INDEX 5471PART 1Advanced Comminution Technologies3High-Pressure Grinding RollsCharacterising and Defining Process Performance for EngineersRichard Bearman*ABSTRACTHigh-pressuregrindingrolls(HPGRs)areincreasinglybecomingapartofthehard-rockprocessingpicturethroughtheirenergyefficiency,theabilitytoinducemicrocracksandpreferential liberation, coupled with high throughput and high reduction ratio. Given thatthemachineisstillnotregardedbymanyasanoff-the-shelfpieceofprocessequipment,thereisworkrequiredtodefineguidelinesforitsuseandtoprovideengineerswithtoolsthey can use. This paper examines the current knowledge around the HPGR process perfor-manceandexploreskeyrelationshipsavailabletoengineers,whilstconsideringcurrentapproaches to simulation.I NTRODUCTI ONHigh-pressure grinding rolls (HPGRs) have struggled for acceptance into the hard-rockminingsector.Manyoftheissuesthatrestrictedtheirwidespreadusehavenowbeenconquered, but it is still regarded as an immature technology. Why is this the case?Incontemplatingananswertotheissueoftheimmaturity,thestatusofotheracceptedtechnologiesmustbeexamined.Asanexample,thetraditionalcompression-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 Australia4 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.Giventhistypeofknowledge,itiseasyforthedesignertodeterminethepositionwithin 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 modelsFirst, there are very few actual, or operating, applications in hard-rock duties. Theonlyhard-rockapplicationsthathavebeeninexistenceforanylengthoftimearerestricted to the diamond and iron ore (pellet-feed) sectors.Another consideration is that the HPGR is a very rare breed of machine, in that itsdevelopmentstemmedfromfundamentalresearch.Giventhetypesandfocusofearlypublications, 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 focuswasfullyjustified,butitledunfairlytotheHPGRbeingregardedasanacademicdevicesearchingforanindustryapplication.ThelanguageusedabouttheHPGR,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.Simulationisalargepartoftheflowsheetdesignexerciseandthisinevitablyrequiresprocess 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 processmodelsofHPGRs,buttheyhavebeenfragmentedintheiracceptance.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 notethattheTondomodelcameoutofthefirstmajorprocessstudyofHPGRs,namelytheAMIRA P428 that was completed in 1997.Ifthesepointsaboveareaddedtothenaturallyconservativestanceoftheminingindustry, 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 structureOf increasing importance is the energy-efficiency issue. It was not too long ago thattheminingindustryregardedenergyconsumptionassomewhatofasideissue.TheKyoto Protocol and the greenhouse debate changed this view forever (Ruben 2002).HPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 5CRI TI CALHPGRPARAMETERSHPGR 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 functionofmanufacturer.TypicalHPGRthroughputratesrangefrom20to3,000tph,withinstalledmotorpowerashighas3,000kWperroll.Therollsurfaceisprotectedwithwear-resistantmaterials,andithasbeenthesethathavetraditionallystymiedHPGRacceptance, 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 speedThe two key operating parameters are inherently linked to the following: Specific throughput Specific pressing force Maximum pressure between the rolls Specific energy inputDetailed 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),rollwidth(m),andtheperipheralrollspeed(m/s).Forthepurposesofbrevity,only the equations for this parameter are reported here. Further details are provided inearlierworks.(Schnert1991).Partofitsimportanceisthattheequationallowscom-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/m3)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.6 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESIt should be noted that recent work by Daniel (2005) has examined the determina-tion of an equivalent diameter for piston press tests. Daniel proposesD = (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 flakeThis relationship has potential to assist in translating piston press results to engineeringparameters.Variation in Throughput with Key VariablesFigure1showsthevariationinthespecificthroughputasafunctionofthefeedbulkdensity. 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-estatthelowestpressure,withlargerchangesassociatedwiththeall-in(highbulkdensity)feedtypes.Figure2showsthetypeoflinearincreaseinspecificthroughputassociated with increasing operating gap.Figure 3 shows a plot of all tests versus the specific energy (power) consumed. It isinterestingtonotethatthedataappearintwodistinctclusters.Theright-handclusterconsists 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.1.45 1.40 1.50 1.55 1.60 1.65 1.70 1.75250230210190170150m-dot,ts/hm3Bulk Density, t/m330 Bar38 Bar52 BarFIGURE 1 Variation in specific throughput as a function of feed-bulk density for various operating pressures using a pilot-scale HPGRHPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 7Specific Pressing ForceThespecificpressingforceisdefinedasthegrindingforceappliedtotherolls(kN),dividedbythediameter(m)andwidth(m)oftherolls(Schnert1988).Thespecificpressing force has the unit of N/mm2.Fsp = F/(1,000 u D u L) (EQ 5)whereFsp= specific pressing force (N/mm2)F = applied grinding force (kN)D = roll diameter (m)L = roll width (m)15 16 17 18 19 20 21250230240220200180160210190170150m-dot,ts/hm3Operating Gap, mmFIGURE 2 Variation in specific throughput as a function of operating gap using a pilot-scale HPGR at an operating pressure of 38 bar40 90 140 190 240 2906056585450464252484440m-dot,ts/hm3Power, kWFIGURE 3 Variation in specific throughput as a function of operating gap using a pilot-scale HPGR8 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESRanges 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 Dip) (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)Dip= compression angle (610 degrees)The parameter Dip 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,withthisratiodecreasingtowards1.0forfinercomminution.Figure4showsthe 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, asillustratedinFigure5.Asthefeeddistributionlengthens(i.e.,thebulkdensityincreases), 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 notgenerateacommensurateincreaseinfineness.Figure6showsarangeofenergiesandfines generation. At the levels displayed in Figure 6, the point of diminishing returns hasnot been reached.SI MULATI ONOFHPGRPERFORMANCEAs with all modeling and simulation of process equipment, there is a sliding scale fromthesimplestspreadsheet-basedfeed-producttransferfunctionatoneend,throughempiricalrepresentations,tomechanisticmodels,andfinallytodetailedfundamentaldescriptions. The key process issues that need to be estimated, or predicted, during thedesign phase of a process plant areHPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 9 Throughput Size reduction (product and oversize) Power consumption (energy efficiency) Required hydraulic stiffness Target gap and operating pressureUsing these parameters, it is then possible to insert the HPGR into a flowsheet and makesensiblecomparisonsagainstothertypesofequipmentandflowsheetconfigurations.Theadditionalbenefitsofpreferentialliberationandenhancedextractionmustbeassessed via laboratory tests and incorporated with the full analysis.1.0 1.5 2.0 2.5 3.0 3.5 4.00.50.70.91.11.31.51.71.92.1Specific Energy Consumption kWh/tSpecific Pressing Force, MPaFIGURE 4 Relation between specific energy consumption and specific pressing force using a pilot-scale HPGR1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.750.50.70.91.11.31.51.71.92.1Specific Energy Consumption kWh/tFeed Bulk Density, t/m330 Bar38 Bar52 BarFIGURE 5 Relation between specific energy consumption and feed bulk density using a pilot-scale HPGR, at various operating pressures10 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESPiston Press Testing and Ore CharacterisationThemainorecharacterisationtestsforHPGRmodelingarethepiston-pressanddrop-weight procedures. The drop-weight test is the Julius Kruttschnitt Mineral Research Centre(JKMRC)-developed,single-particletestandisusedtoexamineareasintheHPGRwhere 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 testistogenerateanappearancefunctionasperthedrop-weighttest,butforpacked-bedbreakage.Hence,thepiston-pressappearancefunctionisusedtocharacterisethepre-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 fromArgyleDiamonds.TheimmediateobservationregardingthedatainTable1isthatthepistonpressbparametersarehigherthanthesingle-particletest,withtheinferencebeing that the material appears softer in a packed-bed environment.Giventhemodeofcompression(i.e.,slowinterparticleversustransientcompres-sion), Table 1 could represent an efficiency factor relating the two forms of breakage.Ofmorepracticalimportanceisthattheuseofthepacked-bed,piston-styletestiscritical to the formation of a representative model of HPGR performance.Application of Piston Press to Provide Conceptual-Level HPGR Performance EstimatesAvarietyofworkersarenowusingpiston-pressteststoresearchtheactionofHPGRs.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 theHPGRpilottests,andtheresultsappeartobegoodapproximationstothoseobtained0.5 1.0 1.5 2.01715192123252729313335Net 118 mm GenerationSpecific Energy Consumption, kWh/tFIGURE 6 Relation between specific energy consumption and fines generation using a pilot-scale HPGRHPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 11from 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 is1. 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 sizeA and b = breakage characteristics from piston-press testsEcs = specific energy consumption (kWh/t)TABLE 1 Single-particle breakage parametersSingle-Particle Test Packed-Bed TestSample b bUnweathered lamproite 0.44 0.940Siliceous waste 0.40 0.703TABLE 2 Flake density results from piston-press testsMaximum Pressure, MPa Flake Density, t/m377.24 2.14157.29 2.32230.53 2.32310.98 2.3812 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESUsing 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 ModelingForamorecompletetreatmentofperformanceestimationinamodelingsense,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).CONCLUSI ONSThere 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 forflowsheetpurposesacriticalelementonthepathwaytoengineeringacceptance.Inmany ways, this paper seeks to provide a pragmatic engineering basis for the assessmentofHPGRperformance.ThismessagewasalsothethemeexpressedbyKlymowskyandLiu (1996), where they sought a Bond work-index analogy for HPGRs. There is no doubtthatastandardized,acceptedHPGRworkindexwouldbeagreatboosttoHPGRacceptance.Beyond these engineering views of HPGRs, the detailed modeling and simulation ofHPGRprocessperformanceisfindingcommonground,andworkershavedevelopedcomprehensive 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.HPGRSCHARACTERISING AND DEFINING PROCESS PERFORMANCE 13ACKNOWLEDGMENTSTheauthorgratefullyacknowledgesallpractitionersinthefieldofHPGRtechnologythathavecontributedtothispaperthroughdiscussions.Inparticular,thediscussionsandadvicefromMikeDaniels,JKMRC,showedthataconsiderableamountofeffortisstill being applied to the issue of HPGR application.Entry ZoneSingle-ParticleBreakageCentre ZonePacked-Bed BreakageEdge Effect Single-Particle BreakageProduct from HPGRFeed to HPGRAfter Tondo 1997.FIGURE 7 Schematic representation of Daniel and Morrell modelSource: Daniel and Morrell 2004.TABLE 3 Model inputs and outputsMeasured Input Measured Output Calculated OutputSample 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 distributionSource: Daniel and Morrell 2004.TABLE 4 Model parametersFixed Default Parameters Critical Model Parameterst10p, t10ebreakage for edge and precrusher Kp(HPGR)power coefficient (compression zone)K1p, K2p, K3pprecrusher model parameter t10hbreakage for compression zone crusherK1e, K2, K3edge-crusher model parameterK1h, K2h, K3hcompression zone parameterSplit factor (c)Kp(edge)power coefficient (edge)14 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESREFERENCESDaniel,M.J.2002.HPGRmodelverificationandscale-up.Mastersthesis.Brisbane,Australia:JuliusKruttschnittMineralResearchCentre,DepartmentofMiningandMetallurgical Engineering, University of Queensland.. 2005. Paper submitted to Randol Pacific Gold Forum, Perth, Australia.Daniel,M.J.,andS.Morrell.2004.HPGRmodelverificationandscale-up.MineralsEngineering 17:11491161.Klymowsky,I.B.,andJ.Liu.1996.Towardsthedevelopmentofaworkindexfortheroller 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.Afirstsurveyofgrindingwithhigh-compressionrollermills.International Journal of Mineral Processing 22:401412.. 1991. Advances in comminution fundamental, and impacts on technology. Pages 121inProceedingsoftheXVIIInternationalMineralProcessingCongress.Volume1.K. Schenert, ed. Ljubijana, Yugoslavia.Schnert,K.,andU.Lubjuhn.1990.Throughputofhighcompressionrollermillswithplainandcorrugatedrollers.Pages213217in7thEuropeanSymposiumonComminution.Tondo,L.A.1997.Phenomenologicalmodellingofahighpressuregrindingrollmill.Mastersthesis.Brisbane,Australia:JuliusKruttschnittMineralResearchCentre,Department of Mining and Metallurgical Engineering, University of Queensland.15High-Pressure Grinding RollsA Technology Review*Chris MorleyABSTRACTThedevelopmentofhigh-pressuregrindingrolls(HPGRs)technologyisreviewed,withanemphasis 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 methodinhard-rockprocessingisexamined.Theuseofthetechnologyasametallurgicaltoolisaddressed, and future flowsheet concepts are introduced that make progressively greater useof the energy efficiency of HPGRs.I NTRODUCTI ONHigh-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-ilycrushedmaterials.Sincethen,ithasbeenappliedtoprogressivelyharder,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 crushingOtherprospectiveapplicationsincludephosphates,gypsum,titaniumslag,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* Updatedfromtheoriginalpaper,HPGRinHardRockApplications,publishedinMiningMagazine,September 2003, www.miningmagazine.com Fluor, Australia16 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESEkati (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 trialofanHPGRwasconductedonaparticularlyarduousdutyatCyprusSierritabetween1995 and 1996; and, more recently, HPGR has been piloted at Lone Tree, Nevada, in theUnitedStates,andAmplatsPotgietersrusinSouthAfrica.Currently,HPGR-basedcom-minutionplantsareunderconstructionatBendigo,Australia(gold),andCerroVerde,Peru(copper),andatfinalfeasibilitystudystagefortheSoledadMountain,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.THETECHNOLOGYMachine 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 improvedCourtesy of Kppern.FIGURE 1 Coal briquetting pressearly twentieth centuryHIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 17efficiency is due to the determinate and relatively uniform loading of the material in theHPGRcompressionzone,whereastheloadinginconventionalcrushersand(particu-larly) tumbling mills is random and highly variable, and therefore inefficient.Themostenergy-efficientmethodofbreakageistheslowapplicationofpressuretoindividual 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-millmediacoststypicallyareslightlygreater(duetotheincreasedtransfersizefromHPGRs), 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.Theseeffectscanbeattributedtothephenomenonofmicrocrackingofindividualprogeny particles due to the very high stresses present in the HPGR compression zone.Microcracking occurs predominantly at grain boundaries and so increases liberation andlixiviantpenetration,whiletheeffectivereductioninmillingworkindexcausedbymicrocracking reduces overgrinding and slimes generation.In addition to being ore dependent, the extent of microcracking is a direct functionoftheoperatingpressureandthereforeenergyinputoftheHPGR,andinanygivenoperation, 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, whichundergo a form of differential comminutionCourtesy of KHD Humboldt Wedag AG.FIGURE 2 HPGR machine18 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESwhereby 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 effectisalsoofbenefitinthetreatmentofgoldorescontainingcoarsegravity-recoverablegold grains, which would be flattened in conventional tumbling mills and rendered moredifficult to recover.Technology StatusTheHPGR,consideredamaturetechnologyinthecementindustry,isnowthenormrather than the exception in modern diamond plant design and is becoming common iniron ore processing, particularly in the field of pellet feed preparation.However,althoughsomeofthecurrentdiamondandironoreapplicationscanberegarded 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 notleadtoacommercialsale;however,thefactthatthecomminutionperformanceofthemachine was impressive is not in dispute. The difficulties experienced related to the behav-iourofthewearsurfaces,andmanyvaluablelessonswerelearnedfromthisoperationregarding 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)Courtesy of Polysius AG.FIGURE 4 HPGR product particle (internal microfractures after Polycom treatment)HIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 19The 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 installation20 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESfew 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-upprocedureshavebeenthesubjectofmanytechnicalpublicationsandshould 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.SCALEOFOPERATI ONA 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,HPGRscanbejustifiediftheyofferbenefitstometallurgicalperfor-manceand/orprojecteconomics,andthepotentialforsuchbenefitscanusuallybeassessed 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.THEMANUFACTURERSANDTHEI RDESI GNSPolysius, 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 KHDandKppernpreferalow-aspectratio.Thehigh-aspect-ratiodesignisinherentlymoreexpensive but also offers an intrinsically longer wear life for a given application, as theoperatinggapislargerandtherollsurfacesareexposedtoacorrespondinglysmallerproportionofthematerialprocessed.Thehigh-aspect-ratiodesignalsoproducesacoarser product due to the greater influence of the edge effect; however, this difference isrelativelyslight,particularlywithlargerunits.Nevertheless,forclosed-circuitapplications,HIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 21this 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,andareabletosupplymachineswithcapacitiesofuptoabout2,200t/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.TheHexadursurfacecompriseshexagonaltilesofaproprietaryabrasion-resistantmaterialsetintoasoftermatrix,whichwearspreferentiallyinoperation,allowingtheformation 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 controldifficultiesinherentinthefabricationandfurnacetreatmentofsegments.However,research into the commercial production of Hexadur segments is ongoing.Meanwhile,themaximumHexadurrolldiameteravailablecurrently(andfortheforeseeablefuture)is1.5m,constrainedbyfurnacedimensions.ThisconstraintlimitsKpperns 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 mmDiameter of Rolls: 0.30 mWidth of Rolls: 0.07 mSpeed of Rolls: 0.20.9 m/sTop Feed Size: 812 mmREGROATWALLABWALCourtesy of Polysius AG.FIGURE 6 Polysius test facility22 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESKppernhasanestablisheddesigninwhichtheendsofthemainframehingeout-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,KHDhasunveiledarapidchange-outconcepttobeofferedonnewCourtesy of KHD Humboldt Wedag AG.FIGURE 7 Studded roll wear surfaceCourtesy of Kppern.FIGURE 8 Hexadur wear surface for hard-ore comminutionHIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 23machinesandwhichcanberetrofittedtoexistingunits,andKppernhasintroducedtheir 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 oillubricationsystems,asthereisnorelativemovementbetweenthebearingsandseals.Polysius and Kppern use grease-lubricated, self-aligning spherical roller bearings.OPERATI NGCHARACTERI STI CSTherearemanyfactorstobeconsideredwhenspecifyinganHPGRandselectinganappropriate flowsheet for a given application. The following subsections summarize themore important issues.Courtesy of Kppern.FIGURE 9 Kppern HPGRCourtesy of Kppern.FIGURE 10 Kppern hinged frame24 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESOre CharacteristicsThe compressive strength of the material to be crushed determines the amount of usefulenergythatcanbeabsorbedbythematerial,whichinturndictatesthebearingandmotor 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 about14,000and10,000hours,respectively,whilethoseattheArgyleandEkatidiamondmines 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 feedscontainingalargeproportionoffines.(Thisofcoursedoesnotapplytoapplicationswhereallthefeedmaterialisfine,suchasfinegrindingofconcentrates.)Fineandweathered material tends to cushion the action of the rolls and so reduces the efficiencyofcomminutionofthelargerfeedparticles.Forexample,ArgylebypassesitsprimaryHPGRswhenveryfineoreisbeingmined.Ontheseoretypes,thefineorweatheredmaterialshouldberemovedbyprescreeningifHPGRtreatmentofthecoarsercompo-nent is required.HPGRsarenotgenerallysuitableforcomminutionoffeedscontainingexcessivemoisture,whichtendstocausewashoutoftheautogenouslayeronstuddedrollsandincreasesslippageonsmoothrolls.Inbothcases,acceleratedwearistheresult.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.Forequivalentsizereduction,ahard,competentoreofhighcompressivestrengthwill 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 numericallyHIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 25about 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 ineffectiveworkindexduetomicrocracking(alsoknownasmicrofracturingormicrofis-suring) does not always display the same tendency. As a result, it may be beneficial fromanoverallcomminutionenergyperspectivetooperateatahigherspecificenergythancorresponds 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 duetoearlynippingeffectivelycausingsingle-particlebreakagebydirectcontactwiththeroll 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.Thesefiguresarenowconsideredmuchtoooptimisticinhard-rockapplications,anditisgenerallyacceptedthat,tominimisethelikelihoodofstudbreakage,feedtopsize should not exceed the expected operating gap. This will normally demand a closed-circuitcrushingoperationupstreamtoensurethistopsizeispositivelycontrolled.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.Asaguide,thedirect-contactnipangle(forsingle-particlebreakageandpossiblestud 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 bemadeoftheparticlesizeabovewhichsingle-particlebreakageislikelytooccurandbelow which interparticle breakage commences.26 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESUnit 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 mxis 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 GapTheoperatinggapisdirectlyrelatedtotheunitcapacity,allelsebeingequal,sogapcanbeinterchangedwithmxintheaboveanalysis.Dependingontheapplication,theratio of operating gap to roll diameter will normally lie in the range of 0.010 to 0.028.Circuit CapacityThecapacityofanHPGRcircuit,asdistinctfromtheunitcapacitydiscussedabove,isobviously a function of the circuit design. Of the above variables, the feed bottom sizeHIGH-PRESSURE GRINDING ROLLSA TECHNOLOGY REVIEW 27isparticularlyrelevantinthisregard,asatruncatedfeednecessarilyimpliesthepres-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-ityofthecircuitwouldincreaseiftheamountoffinesremovedfromtheHPGRfeedexceeded 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 thedownstreamprocessingroute,afull-finesHPGRfeedmaybepreferabletoatruncatedfeed. For any given application, the more efficient flowsheet can be determined only bycomprehensivetestsandmodeling,butwhere doubtexists,thecircuitshould,ifpossi-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 SizingAsnotedearlier,productfinenessincreaseswithoperatingpressure(andthereforepower), 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 circuitwithascreen,sothatlessenergyiswastedoncompactingtheproduct.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 asmoothrollunit.However,theeffectisrelativelysmall,andthebenefitsofprofiledorstuddedrollsusuallyoutweighthereducedproductfineness.Furthermore,theeffectappearstobeorespecific,andsomeoperations(e.g.,Jwaneng)haverecordedanincrease in fineness with studded rolls compared to smooth rolls.IncreasingrollspeedleadstoareducedproducttopsizeandimprovedF50/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 rollsurfacewearratesduetothisimposedspeeddifferential.Thiseffectisthereforeregarded 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.TheshapeoftheHPGRproductsizingcurveisdissimilartothatofconventionalcrushers, so that for products with nominally the same P80, the HPGR product contains28 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESconsiderably 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,millcapacitywillbegreaterwhentreatingHPGRproductthanpredictedbythestandard 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 WearIncreasingrollspeedincreasesturbulenceinthefeedmaterialandslipoffeedagainstthe 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)shouldnotexceedrolldiameter(inmeters),althoughKpperndoesnotsupportthisviewandregularlynominatesspeeddiameterratiosofupto1.3.KHDalsousesthesehigher ratios for their smaller-diameter machines but generally uses 1. When the HPGR is added to an existing mill circuitthe bonus becomes(EQ 7)Wspecmill is the kilowatt-hours per ton for the existing mill before and after adding theHPGR. The energetic advantage of the HPGR is that the bonus is greater than 1, whichmeans that 1 HPGR kW or kWh/t will do more grinding work than 1 mill kW or kWh/t.In other words, adding an HPGR to an existing mill circuit will reduce specific energyFuFuFnFRGrinding Roll Grinding RollFIGURE 3 Force diagram, force acting angle P 2F vcE sin u u =BWspecmillWspecHPGR--------------------------- =BWspecmillbeforeWspecmillafterWspecHPGR-------------------------------------------------------------------------- =SOME BASICS ON HIGH-PRESSURE GRINDING ROLLS 45consumption and increase production. The savings in specific power consumption at themill main drives is(EQ 8)The overall system savings are somewhat lower because of added equipment, such asconveying or screening. Bonus values depend mainly upon feed material, circuit configu-ration, and reference mill.Some typical values (from Kppern operating and test data) are Cement clinker 1.82.5 Blast furnace slag 2.53.8 Limestone 1.72.0 Kimberlite 1.62.0The Machine DesignFigure4viewsanHPGRfromthesidewherethehydraulicsystemislocated.Thetwogrinding rolls are suspended with self-aligning roller bearings in bearing blocks, whichare mounted in the machine frame. Each roll has its own drive train with planetary gearreducers.Torquearmsareprovidedtoneutralizethecountertorquesgeneratedbythedrives. This particular machine design features a hinged frame that swings open for easyroll exchange. The machine has the following characteristics:The energy density in the nip zone is quite high, about 400 times compared to a ballmill. Correspondingly high are the loads and stresses on the rolls, especially on the rollsurfaces. Figure 5 shows three basic roll designs.The roll surfaces are of particular importance not only from the wear aspect but alsofortheircapabilitytodrawinthematerial.Figure6showsastuddedrollsurface(accordingtosourcesatKHDHumboldtWedag,Cologne,Germany)wherematerialbuildsupbetweenthestuds,therebyforminganautogenouswearprotectionandpro-viding a rough surface for good friction. Figure 7a shows a worn, welded hard surface,and Figure 7b shows metallurgical powder-based wear elements applied by a hot isostaticpressure process. These are just three examples; there are several others that have beendeveloped over the years.HPGR Applications in MiningFigures 8 through 11 show some typical applications for HPGRs to increase throughputandlowerspecificenergyconsumptionofagrindingcircuit.Figure8showstheHPGRafterasemiautogenousgrinding(SAG)millforpebblegrinding.TheHPGRproductisreturned either to the SAG mill or to the screen. In Figure 9, the HPGR is located after thesecondarycrusher.TheHPGRproductisscreened,andtheoversizeisreturnedtotheHPGR.Figure10hastheHPGRassingle-passpregrinderinfrontofthemillingplant.Roll diameter 2,140 mmRoll length 1,300 mmCircumferential speed variable, max. 1.58 m/secInstalled power 2 u 1,300 kWInstalled grinding force 19,500 kNThroughput 850 tph cement clinkerBonus achieved 2.1'P WspecmillbeforeWspecmillafterWspecHPGR+ =46 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESThe HPGR can apply about 2.3 kWh/t to the feed material. At a bonus of, for example,1.8, it would add 2.3 u 1.8 = 4.14 kWh/t ball-mill equivalent to the grinding circuit. TheHPGR can also be located after heavy-media separation (HMS) with or without a crushergrinding the wet oversize (Figure 11).The best HPGR location for a given mine needs to be decided for each case; bottleneckidentification, space availability, conveying distances, power grid, and other conditionsmust be considered.CONCLUSI ONSVersatilityandefficientenergyutilizationhavemadehigh-pressuregrindinganestab-lished comminution technology in the minerals processing industries. Relatively simpleformulae can be used to describe and understand the underlying mechanics of HPGRs.Careful consideration must be given to the overall grinding process in order to take fulladvantage of the special features offered by high-pressure comminution.Hinged Frame RollHydraulic SystemTorque ArmsGearReducersFIGURE 4 HPGR assembly at the workshop (view from the hydraulic side)(a) Solid Roller12 23Grinding SurfaceBearing Journal(b) Tire-Shaft RollerRoller Core1 4(c) SegmentsRoller Core1 5SegmentsFIGURE 5 Press toolsSOME BASICS ON HIGH-PRESSURE GRINDING ROLLS 47Tungsten Carbide Studs Autogenous Wear ProtectionFIGURE 6 Studded roll surfaceCircumferential Wear Grooves Hexadur Tiles Softer Interstice Material(a) Worn Surface Welding (b) Hexadur Roll SurfaceFIGURE 7 Roll surface examples48 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESPrimary CrusherHPGRSAG MillCrusherGrinding CircuitScreenSorting SectionRun-of-Mine OreFIGURE 8 HPGR after SAG mill for pebbles grindingPrimary CrusherHPGRIntermediateStockpileSecondaryCrusherDouble-DeckScreenScreenConcentrationSectionRun-of-Mine OreFIGURE 9 HPGR after secondary crusher with screenSOME BASICS ON HIGH-PRESSURE GRINDING ROLLS 49HPGRConcentrateStockpileStorage BinGrinding CircuitPelletizingPlantFIGURE 10 HPGR in single-pass grinding before milling plantHPGROre StorageScreenHMSSinkFloatFinalConcentrationHPGRScreenScreenHMSCrusherSinkFloatFinalConcentrationFinalConcentrationFinesTailingsFIGURE 11 HPGR grinding wet oversize51High-Pressure Grinding Rolls for Gold/Copper ApplicationsNorbert Patzelt,* Rene I.B. Klymowsky,* Johann Knecht,* and Egbert Burchardt*ABSTRACTSuccessful pilot-plant demonstrations carried out in 2003 and 2004 have proven the opera-tionalreliabilityofhigh-pressuregrindingrolls(HPGRs)inhard-rockapplications.Asaresult of these breakthroughs, six HPGRs will be commissioned in two copper concentratorsin 2006.I NTRODUCTI ONHigh-pressure grinding rolls (HPGRs) are well established in the diamond and iron oreindustries. Process advantages of HPGRs had been recognised by the minerals industryfor many years. However, unresolved issues pertaining to wear have made the industry reluc-tant to adopt this technology.Startingin2003,asuccessfulpilot-plantdemonstrationonanexceptionallyhardand abrasive gold ore proved that the wear issues could be resolved by the design of anappropriatewear-protectionsystem,andavailabilitiesinexcessof90%couldbeachieved. The pilot-plant results built up confidence in the minerals industry and a sec-ond pilot-plant trial was conducted on another extremely hard ore with the aim of deter-miningifanythingcouldbreakthemachine.Themachinedemonstratedevenhigheravailabilities than in the previous case.A commercial breakthrough then came when one of the worlds leading copper pro-ducers decided to build a new concentrator in South America based on HPGR technology.Four Polycoms, 24/16 in size, each equipped with two 2,500-kW motors, will be used intertiarycrushingdutyinclosedcircuitwithwetscreens.Shortlythereafter,asecondmajorcopperproducerorderedtwolargePolycom20/15unitsforanexistingcopperconcentrator in Indonesia.In both cases, it was the energy savings and low operating costs of the HPGRs thatattracted the producers. This paper examines the conditions (such as the press force nec-essary) that lead to energy savings, lower operating costs, and the optimum performanceof the HPGRs.Wide variations occur in ores, even within one deposit. These variations, insofar asthey affect the performance of an HPGR, need to be quantified with meaningful HPGRindices. Two such indices are the ATWAL Wear Index (ATWI) for wear due to abrasionand the Polycom Grinding Index (PGI) for quantifying the fines production.* Polysius AG, Neubeckum, Germany52 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESA laboratory ball mill test, the Labmill test, is described to overcome the uncertaintyabout the energy required for ball milling after an HPGR. The test is aimed to deal specif-icallywiththespecialfeaturesofanHPGRproduct(i.e.,microcrackingandthehighamount of fines in the product).EXPERI ENCESWI THHPGRSI NHARD- ROCKAPPLI CATI ONSCyprus SierritaThe first serious application of HPGRs in the hard-rock mining industry was the installa-tion of an HPGR in a copper concentrator in the United States in 1994 (Figure 1).Theexpectedperformanceintermsofthroughput,finesproduction,andenergyconsumptionwasmet.However,thehardnessandabrasivenessoftheorewasbyfarhigher than that of ores treated in HPGRs previously. It soon became apparent that thewear protection of HPGRs, in particular, the stud technology, was not advanced enoughat that time to allow for a smooth and easy transition into continuous operation. Differentstud qualities had to be tested and changed in order to suit the requirements of the ore.The change-outs were facilitated by having the rolls equipped with segments; however,these also contributed to wear problems. Finally, stud qualities were found that provideda reasonable lifetime at low cost (~0.10 to 0.15 US$/t) even under these difficult circum-stances. In the end, the unit was decommissioned after treating more than 7,000,000 t ofore when the initial investment plans for the mine were abandoned. Despite the positiveoperatingresults,thisinstallationwaswidelyviewedbytheindustryasafailureofHPGR technology, and its acceptance was set back for years.Newmont Gold, Lone Tree (Nevada)FollowingtheCyprusSierritademonstration,therewaslittleprogressmadetowardsimproving the technology or improving the wear protection for hard-rock applications.The next milestone in HPGR development came in April 2003, when Newmont MiningCorporationbegana3-monthtrialofapilot-sizedHPGR.Polysiusdesignedanewrollsurface specifically for the grinding of hard and abrasive copper and gold ores (Figure 2).Thenewrollsurfaceconsistedofareplaceableshrink-fittedtyre,armedwithanewdesign of tungsten carbide studs, and a new edge-protection system intended to eliminateFIGURE 1 Installation of an HPGR in a copper concentrator in the United States in 1994HIGH-PRESSURE GRINDING ROLLS FOR GOLD/COPPER APPLICATIONS 53repair welding of the roll edges. The rolls were also provided with cheek plates to con-tain the material within the gap.The roll dimensions were 950 mm diameter u 350 mm width. Each roll was drivenby a 160-kW motor and was operated at a speed of 21 rpm. The capacity of the unit wasabout 80 tph. The unit was run in closed circuit with an 8-mm square-mesh screen, andwasprotectedfromtrampmetalbyanoverheadmagnetandametaldetectoronthemain conveyor belt.The machine was operated 24 hours a day, 7 days a week, for a period of 87 dayswith no mechanical downtimes. The initial operational availability of the unit was 89%due to a programming glitch, which occurred after startup, after which a 92% availabil-ity was achieved. The properties of the feed are specified in Table 1.The Lone Tree trial was a true milestone, as no other pilot unit previously had been oper-ated continuously on a 24/7 basis, and no mechanical or welding repairs had been required.The operators and maintenance staff were encouraged by the operation of the HPGR.No stud failures occurred during the more than 1,600 operating hours of the demon-stration trial. In a commercial-scale application on a similar hard, abrasive ore, an HPGRwould have achieved more than 3,000 hours of service and run considerably longer on aless competent and less abrasive ore.Relationshipsbetweenwearandparticle-sizedistributionwereobtainedthatwillimprove future understanding of wear and wear life, benefiting the industry as a whole. ThePolysius ATWAL laboratory abrasion test accurately predicted the wear rate in the trial.Individual tests, conducted over the course of 1 year on several representative samplesobtained prior, during, and after the trial, were found to be reproducible within 10% ofeach other, validating the method used for determining and predicting wear in larger units.FIGURE 2 A pilot-sized HPGR equipped with a new roll surface designed by PolysiusTABLE 1 Material data of Lone Tree oreBall Mill Work Index, Wi (BM) 20 kWh/tUnconfined compressive strength 200 MPaSilica content 78%84%Bond Abrasion Index, Ai 0.64ATWAL Wear Index, ATWI >40 g/t54 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESAnglo American Platinum, Potgietersrust Platinum MineInOctober2004,apilot-plantunitwascommissionedinthePotgietersrustplatinummine. This was a further step forward and the first approach of HPGR technology to theplatinumindustry.TheHPGRwasagainoperatedinclosedcircuitwith8-mmscreens.Thefeedmaterialwaspreparedbytwo-stagecrushing.Feedsizewasinitially25mmbut was increased to 35 mm for extended operating periods. The HPGR treated a totalof 188,000 tph, resulting in 115,000 tph of final product until it was decommissioned inApril 2005. The operating time was in excess of 3,000 hours.Theorewaseventougherthanthatusedinthepreviousapplication,whichisreflected in the Bond Work Index (BWI) of 22 kWh/t, and in the operational problemsexperiencedatthecrushers.TheHPGRwasequippedwiththelatestwearprotection.Results from abrasion testing indicated low abrasion, which was confirmed in the field.Availabilities as high as 97% were achieved. The test installation was declared a suc-cess by the operator who, in his words, had tried very hard to break the machine.I MPLEMENTATI ONOFHPGRSI NGRI NDI NGCI RCUI TSIn greenfield installations, HPGRs will have their place as the tertiary crushing stage infront of ball mills. Even this commitment still leads to many different flowsheet configu-rations. One possible flowsheet is shown in Figure 3.ThesecondarycrusherandtheHPGR,whichreplacesconventionaltertiarycrush-ers, are operated in closed circuit with dry screens. The product of the crushing circuit isstockpiled. In this configuration, the crushing circuit and the ball mill circuit are decoupled,allowing both circuits to be operated at a different utilisation. However, this decouplinghas two implications. First, an additional stockpile is required. Stockpiling of the HPGRproduct, which contains a lot of fines, remains a challenge and requires extensive dustsuppression. Second, screening of the HPGR product has to be done dry because a wet-screen undersize cannot be stockpiled. This entails a coarser product.WetscreeningoftheHPGRdischargemayprovidesignificantimprovements.Itisadvantageous from the point of energy efficiency to shift as much grinding work as pos-sible to the HPGR and feed the ball mills with a finer product. This approach requires afiner mesh size for the screen, 4 to 6 mm. Fine screening usually has a lower efficiency,especially if the discharge from the HPGR is in the form of highly compacted flakes. WetscreeningwilladdressthedisagglomerationoftheHPGRdischargeandwilldefinitelyimprovescreeningefficiency.Italsowillfacilitatewettingofthematerial.Aflowsheetillustrating a wet-screen arrangement for an HPGR is shown in Figure 4. The HPGR andballmillcircuitsarecombined,whereasthesecondarycrusherisdecoupled.Alterna-tively, if the circuit consists of multiple crushing and grinding units, all three can be com-bined,eliminatingthestockpilebyoversizingtheequipment.Thisarrangementallowsfor the lower availability of the crushers, whereas the HPGR availability is expected to behigh enough for in-line operation with the ball mills.OPTI MUMHPGRPERFORMANCEI NCLOSED- CI RCUI TOPERATI ONIn tertiary applications, HPGRs have to be operated in closed circuit. Consequently, theball mill feed is not the discharge of the HPGR but is the product of the size distributionof the HPGR discharge and the mesh size of the closing screen. This raises two questions:first, what influence do HPGR operating parameters have on the feed-size distribution tothe ball mill; and secondly, what is the most efficient way to operate an HPGR?Inopencircuit,theoperatingparameterthatmanifeststhemostinfluenceontheparticle-size distribution is the press force applied to the rolls. The energy absorbed bythe material has been shown to be proportional to the applied press force.HIGH-PRESSURE GRINDING ROLLS FOR GOLD/COPPER APPLICATIONS 55In a closed circuit, however, the influence of the press force on the product size ofthecircuitislost.Thisisdemonstratedwithtwoexamples,onetakenfromtestsonasemi-industrial scale unit and the other from tests on a laboratory-scale HPGR (Figure 5).Results were taken from single-pass tests on these units, in order to prove that thefindings were independent of the machine size.The press forces applied were in the range of 2.7 to 4.3 N/mm2 on the semi-industrialunit, and from 2.3 N/mm2 to a higher value of 8.4 N/mm2 on the laboratory-scale unit.The impact of the press force on the throughput and energy consumption of the circuitare also shown.A screen undersize, representing the circuit product, was calculated on the basis of100% screen efficiency from the discharge. Cut sizes were 4 mm for the semi-industrialcircuit and 1 mm for the laboratory-scale circuit. It was assumed that the recirculation ofscreen oversize did not affect the size reduction in the HPGR significantly. On this basis,FIGURE 3 HPGR in closed circuit with dry screensOptionalFIGURE 4 HPGR in closed circuit with wet screens56 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIESa projection of the circuit performance in terms of throughput and energy consumptionwasalsomade.Thisapproachmaybeconsideredsimplisticbutisadequatetoexplainsome of the principles.Figure 6 shows discharge size distributions of a copper ore treated in a semi-industrialtestunit.ThegrindingforcewassteadilyincreasedfromtestR4toR6,resultinginafiner discharge. The circuit product of an HPGR with a 4-mm classifying screen was cal-culated on the basis of 100% screening efficiency.Figure 7 shows discharge size distributions of a platinum ore treated in a lab-scaletest unit. The grinding force was steadily increased from test L1 to L4, resulting in a finerdischarge. The circuit product of an HPGR with a 1-mm classifying screen was calculatedon the basis of 100% screening efficiency.TheconclusionsdrawnfromFigures6and7werethatthesizedistributionofthefinalcircuitproductdidnotvarymuch,nomatteriftheHPGRdischargewasfinerornot. The fineness of the circuit product was largely determined by the mesh size of theclosing screen.However, the applied press force had a strong influence on the circulating load andthe circuit throughput with an HPGR of given size, as well as on the energy consumption,as shown in Figures 8 and 9.In Figure 8, the specific energy input was increased by 30% while the throughput oftheclosedcircuitincreasedbyonly10%.InFigure9,thespecificgrindingenergywasincreased even further by 100% to 8.2 N/mm2, whereas the throughput of the closed cir-cuitonlyincreasedby40%.Theseexamplesshowthatoperationathighspecificpressforces, 8 N/mm2, reduces energy efficiency drastically.The following general conclusions were drawn with regard to optimum operation ofHPGRs in closed circuit:1. The product-size distribution of an HPGR in closed circuit with screens is not influenced by the applied press force.2. The applied press force determines the circulating load and the energy consump-tion of the HPGR circuit.2.52.01.51.00.50.0Specific Energy, kWh/t0 1 2 3 4 5Specific Press Force, N/mm2Feed 1Feed 2FIGURE 5 Absorbed specific energy versus press forceHIGH-PRESSURE GRINDING ROLLS FOR GOLD/COPPER APPLICATIONS 57100806040200Fineness Cumulative Passing, %0.10 0.01 1.00 10.00 100.00Particle Size, mmNOTES: Discharge denotes the HPGR discharge from respective tests. S/U denotes the screen undersize from respective tests.FeedR4 DischargeR4 S/UR5 DischargeR5 S/UR6 DischargeR6 S/UFIGURE 6 Semi-industrial HPGR test with copper ore1008060402000.10 0.01 1.00 10.00 100.00Fineness Cumulative Passing, %Particle Size, mmNOTES: D denotes the HPGR discharge from respective tests. S/U denotes the screen undersize from respective tests.FeedL1 DL1 S/UL3 DL3 S/UL4 DL4 S/UFIGURE 7 Lab-scale HPGR tests with platinum ore58 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIES For harder ores, increasing the press force increases the circuit throughput, but the increase in energy consumption is disproportionately higher. For softer ores, increasing the press force may even decrease the circuit through-put. The additional fines produced do not compensate for the loss in specific throughput of soft ores resulting from the reduction in the operating gap.3. Optimum grinding forces are material specific. Specific grinding forces up to 8 N/mm2, such as those applied in cement grinding, are unsuitable for minerals applications where the final product fineness is substantially coarser.4. Circuit throughput can be adjusted by varying the applied press force. The increase in the energy consumption, however, is often disproportionate to the 802 3 4 5 6460 340 220 10 0Throughput M, tphSpecific Energy Input w(sp),kWh/tSpecific Grinding Force , N/mm2Mw(sp)FIGURE 8 Semi-industrial circuit projection (copper ore 1), the numberof liberated particles of values is given by Equation 9, whereas the locked particles are ofthreetypesasgivenbyEquation10:thoselocatedalongthesixsurfacesofthecubicvaluable-mineral grain, which are composed of two grain fragments; those located alongthe 12 edges of the valuable-mineral grain, which are composed of four grain fragments;andthoselocatedattheeightcornersofthevaluable-mineralgrain,whicharecom-posedofeightgrainfragments.Theremainingparticlesareliberatedwastemineral(Equation 11).PB = VB(K 1)3/K3(EQ 9)PAB = VB[6(K 1)2 + 12(K 1) + 8]/K3(EQ 10)PA = 1 VB(K + 1)3/K3(EQ 11)For this model, with particle size larger than grain size (K < 1), there are no liber-ated values, but there is a maximum values particle composition, which occurs when afull grain of valuable mineral is contained in a particle, and is given by Equation 13. TheliberatedwastecontinuestofollowEquation11,butthereisasizeratioatwhichtheapproximationtotheamountofliberatedwastemineralbecomeszero,asshowninEquation 14. The quantity of locked particles is now given by Equation 15.PB = 0 (EQ 12)VBMAX = (o/|)3 = K3(EQ 13)K(@ PA = 0) = 1/((1/VB)1/3 1) (EQ 14)PAB = VB(K + 1)3/K3(EQ 15)238ADVANCES IN COMMINUTIONLIBERATION AND BREAKAGETABLE 3 Summary of particle breakage simulation for liberation of ore with a 0.015 volume fraction of values for size ratio of 1/16 to 64VBAverage/K 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 641 0 0 0 0 0 1959 6423 10113 12397 13657 143180.99995 0 0 0 0 0 2 4 0 0 0 00.99985 0 0 0 0 0 1 1 3 0 1 00.99965 0 0 0 0 0 4 1 2 1 0 10.99925 0 0 0 0 0 6 6 1 1 1 10.9985 0 0 0 0 0 10 13 7 5 3 10.9965 0 0 0 0 0 33 36 23 16 8 30.9925 0 0 0 0 0 58 67 45 26 11 70.985 0 0 0 0 1 106 126 92 48 30 140.965 0 0 0 0 5 395 392 256 146 72 400.925 0 0 0 0 28 647 656 436 248 135 680.85 0 0 0 0 198 1518 1393 891 499 263 1340.75 0 0 0 0 580 1846 1510 922 506 265 1360.65 0 0 0 0 1228 2254 1631 960 516 268 1360.55 0 0 0 0 2269 2755 1787 999 527 270 1370.45 0 0 0 0 4033 3411 1986 1054 542 274 1380.35 0 0 0 40 6836 4241 2228 1118 556 277 1380.25 0 0 0 1693 11667 5612 2568 1201 578 284 1400.15 0 0 20 43826 21231 7927 3151 1344 614 292 1430.075 0 0 16301 74265 19204 5778 1960 772 334 156 720.035 4580 160127 278479 82920 18904 4982 1499 525 214 89 450.015 993275 678329 296679 44884 9858 2261 617 216 76 37 160.0075 2145 148912 111079 31673 6581 1391 368 118 43 16 80.0035 0 11921 74322 25900 5423 1060 263 72 28 11 30.0015 0 480 29873 12149 2363 433 102 27 10 3 20.00075 0 125 17581 7280 1388 255 55 13 4 2 10.00035 0 50 12607 5368 1135 179 38 9 2 1 10.00015 0 20 4929 2222 362 68 15 4 1 0 00.00005 0 20 6938 2846 591 83 22 3 1 1 00 0 16 151192 664934 886115 950725 971082 978774 982061 983573 984297CalculatedVB 0.01508 0.01694 0.01658 0.01651 0.01531 0.015079 0.01502 0.01500 0.01500 0.01500 0.01500Calculatedvolume 1000000 1000000 1000000 1000000 1000000 1000000 1000000 1000000 1000000 1000000 1000000RATIONALE FOR ONE MODEL DESCRIBING THE SIZE REDUCTION/LIBERATION OF ORES 239There are SGM analytical relationships for the quantity of locked particles expressedasafunctionofthesizeratioandlinearandlogarithmicfunctionsoflockedparticlecomposition(WiegelandLi1967).Althoughrepeatingtheserelationshipsisnotwar-ranted here, they are quantitative approximations for low-grade ore which, because theyare in an analytical form, may prove of value in extending the modeling and simulationcapability of size reduction and liberation. A comparison of data obtained from the SGMand the GRLM are shown in Figure 6 for an ore containing 0.015 volume fraction of valu-able mineral.SUMMARYANDCONCLUSI ONSMineral liberation is an extremely important phenomenon to understand and to be abletoquantitativelymodel,asitisthisliberationthatpermitssubsequentmineralsepara-tionstobecarriedoutandsimulatedeffectively.ThedevelopmentoftheGRLMbeganwith the work of Gaudin in the 1930s, as a conceptual understanding of how size reduc-tion affects the liberation of an idealized binary mineral system. In the 1960s, the Gaudinconceptual model was modified by the application of binary probability concepts, and asa result it could be quantified for the entire range of ore feed grades and particle sizes.Sincethen,overa40-yearperiod,therehasbeenagradualcollectionofexperimentaldatatoprovidesupportforthequantitativemodel.Additionaleffortshavebeenmademore recently to extend both its qualitative and quantitative usefulness by tapping someof its more intricate features, such as information on the gradual change in the composi-tionaldistributionoflockedparticlesasparticlesizeisreducedandmineralliberationproceeds.WiththeexpectedinclusionoftheGRLMintoapopularmineralprocessingsimulation package, it is now ready for a rigorous test of its applicability to the simula-tion of real mineral systems. With the capability of incorporating mineral liberation intothe suite of unit operation models available to the process engineer, it will no longer be0.000010.00010.0010.010.1