Advances in Comminution - Kowatra

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Published by the

Society for Mining, Metallurgy, and Exploration, Inc.

ADVANCES

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Society for Mining, Metallurgy, and Exploration, Inc. (SME) 8307 Shaffer Parkway

Littleton, Colorado, USA 80127 (303) 973-9550 / (800) 763-3132 www.smenet.org

SME advances the worldwide mining and minerals community through information exchange and professional development. SME is the world’s largest association of mining and minerals professionals.

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.

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ISBN-13: 978-0-87335-246-8 ISBN-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: *978-0-87335-246-8

ISBN-10: 0-87335-246-7

1. Stone and ore breakers--Technological innovations. 2. Crushing machinery--Technological innovations. 3. Mining engineering--Technological innovations. I. Kawatra, S. K.

TN510.A38 2006 622'.73--dc22

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vii

Preface

This third international symposium and proceedings, Advances in Comminution, have come at a critical time. Because of rapidly rising energy prices, it is important that the latest information be made available for improving the efficiency of highly energy-intensive comminution 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 the reader 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 time when there was much discussion about switching from traditional rod mill and ball mill circuits 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 a very 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 stirred mills for regrinding and ultrafine grinding applications do not lend themselves to con-ventional methodologies; therefore, new approaches are being developed. There is also a 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 optimizainstrumenta-tion and control of comminuinstrumenta-tion circuits. Instrumentainstrumenta-tion, modeling, and control functions in particular have benefited from rapidly advancing computer technology, with calculations that were formerly extremely time-consuming becoming rapid 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 who are working toward major advances in industrial practice. Appreciation is extended to members of the organizing committee, who were instrumental in acquiring high-quality papers 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 international symposium and publishing these proceedings.

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iii

Advanced Comminution

Technologies

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3

High-Pressure Grinding Rolls—

Characterising and Defining Process

Performance for Engineers

Richard Bearman*

A B S T R A C T

High-pressure grinding rolls (HPGRs) are increasingly becoming a part of the hard-rock processing picture through their energy efficiency, the ability to induce microcracks and preferential liberation, coupled with high throughput and high reduction ratio. Given that the 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 tools they can use. This paper examines the current knowledge around the HPGR process perfor-mance and explores key relationships available to engineers, whilst considering current approaches to simulation.

I N T R O D U C T I O N

High-pressure grinding rolls (HPGRs) have struggled for acceptance into the hard-rock mining sector. Many of the issues that restricted their widespread use have now been conquered, 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 other accepted 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 associated with these crushers—even without reference to textbooks or suppliers. The types of rules referenced above include

Product-size distribution will be approximately 80% passing the closed-side setting— with 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.

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4 ADVANCES IN COMMINUTION ADVANCED COMMINUTION TECHNOLOGIES

Mostly 5%–10% of the feed-size distribution is the maximum less than the closed-side setting—except 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 position within the flowsheet and to then calculate the feed rates, type of feed arrangement, and the pre- and postclassification required. Why do these rules of thumb, or guidelines, not exist 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. The only hard-rock applications that have been in existence for any length of time are restricted to the diamond and iron ore (pellet-feed) sectors.

Another consideration is that the HPGR is a very rare breed of machine, in that its development stemmed from fundamental research. Given the types and focus of early publications, much was made of the nature of the interparticle breakage at the heart of the technology. Obviously, given the ground-breaking nature of the invention, this focus was fully justified, but it led—unfairly—to the HPGR being regarded as an academic device searching for an industry application. The language used about the HPGR, and unfamiliar terms such as “m-dot” (denoting specific throughput), further led to an air of mystique 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 requires process models to exist for each piece of equipment. In the case of the HPGR, much of the effort was placed in scale-up procedures. Several organisations did produce process models of HPGRs, but they have been fragmented in their acceptance. Currently, the most complete model approach is that reported by Daniel and Morrell (2004), who have developed an approach from the earlier model of Tondo (1997). It is interesting to note that the Tondo model came out of the first major process study of HPGRs, namely the AMIRA P428 that was completed in 1997.

If these points above are added to the naturally conservative stance of the mining industry, 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 clear to 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 that the mining industry regarded energy consumption as somewhat of a side issue. The Kyoto Protocol and the greenhouse debate changed this view forever (Ruben 2002).

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HPGRS—CHARACTERISING AND DEFINING PROCESS PERFORMANCE 5

C R I T I C A L H P G R P A R A M E T E R S

HPGR roll diameters typically range from 0.5 m to 2.8 m, depending on the supplies, and roll widths vary from 0.2 m to 1.8 m. The aspect ratio of the rolls also varies as a function of manufacturer. Typical HPGR throughput rates range from 20 to 3,000 tph, with installed motor power as high as 3,000 kW per roll. The roll surface is protected with wear-resistant materials, and it has been these that have traditionally stymied HPGR acceptance, 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 given in numerous texts, and as such, the following section provides only a précis of the critical formulas, with some examples of actual relationships from testwork.

Specific Throughput

The specific throughput, m-dot, is regarded by many as the key parameter for sizing the rolls. 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 in earlier works. (Schönert 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) where M = 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)

where

s = 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.

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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)

where

Ucf = feed bulk density, lightly compacted

xc = initial bed height in piston press

xd = displacement of piston

xg = final bed height (i.e., operating gap)

Ug = density of product flake

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

Variation in Throughput with Key Variables

Figure 1 shows the variation in the specific throughput as a function of the feed bulk density. The relationship appears to be linear over the range of feeds tested. Given that the specific gravity of the feed material is 2.85 t/m3_{, it would be unlikely that the loose}

feed 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 lowhigh-est pressure, with larger changes associated with the all-in (high bulk density) feed types. Figure 2 shows the type of linear increase in specific throughput associated with increasing operating gap.

Figure 3 shows a plot of all tests versus the specific energy (power) consumed. It is interesting to note that the data appear in two distinct clusters. The right-hand cluster consists purely of the all-in feed types with no truncation of the feed-size distribution at the 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 1.75 250 230 210 190 170 150 m-dot, ts/hm 3 Bulk Density, t/m3 30 Bar 38 Bar 52 Bar

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

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Specific Pressing Force

The 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 (Schönert 1988). The specific pressing force has the unit of N/mm2_.

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

where

Fsp = specific pressing force (N/mm2)

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-dot, ts/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-dot, ts/hm 3 Power, kW

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Ranges for specific pressing force vary considerably in the range 1–9 N/mm2_{, with}

studded 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 Rolls

The maximum pressure applied to the material between the rolls has been estimated by several 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 (Schönert 1988) holds true:

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

where

Pmax = maximum pressure (MPa)

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

L = roll width (m)

k = material constant (0.18–0.23) Dip = compression angle (6–10 degrees)

The parameter Dip can be calculated from the operating gap, with a detailed description

being given by Schönert and Lubjuhn (1990).

Specific Energy Input

The 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 to the 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 spespe-cific energy values for studded rolls range from 1 to 3 kWh/t. As with all direct comminution devices, harder material will absorb more 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 is 1.8–3:1, with this ratio decreasing towards 1.0 for finer comminution. Figure 4 shows the type of response mentioned. In this case, the slope of the graph indicates a ratio of 1.5:1.

Specific energy consumption is markedly impacted by the feed-size distribution, as illustrated in Figure 5. As the feed distribution lengthens (i.e., the bulk density increases), 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 not generate a commensurate increase in fineness. Figure 6 shows a range of energies and fines generation. At the levels displayed in Figure 6, the point of diminishing returns has not been reached.

S I M U L A T I O N O F H P G R P E R F O R M A N C E

As with all modeling and simulation of process equipment, there is a sliding scale from the simplest spreadsheet-based feed-product transfer function at one end, through empirical representations, to mechanistic models, and finally to detailed fundamental descriptions. The key process issues that need to be estimated, or predicted, during the design phase of a process plant are

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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 make sensible comparisons against other types of equipment and flowsheet configurations. The additional benefits of preferential liberation and enhanced extraction must be assessed via laboratory tests and incorporated with the full analysis.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1

Specific Energy Consumption

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 1.75 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1

Specific Energy Consumption

kWh/t

Feed Bulk Density, t/m3

30 Bar 38 Bar 52 Bar

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

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Piston Press Testing and Ore Characterisation

The main ore characterisation tests for HPGR modeling are the piston-press and drop-weight procedures. The drop-drop-weight test is the Julius Kruttschnitt Mineral Research Centre (JKMRC) -developed, single-particle test and is used to examine areas in the HPGR where 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 test is to generate an appearance function as per the drop-weight test, but for packed-bed breakage. 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-weight test (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 from Argyle Diamonds. The immediate observation regarding the data in Table 1 is that the piston press “b” parameters are higher than the single-particle test, with the inference being 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 is critical to the formation of a representative model of HPGR performance.

Application of Piston Press to Provide Conceptual-Level HPGR Performance Estimates

A 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 ore supplied by Rio Tinto. The aim of the tests is to determine the amenability of the ore to HPGR 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 results presented in Table 2.

The maximum pressures reported in Table 2 were chosen to mimic those seen in the HPGR 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 35 Net –1 18 mm Generation

Specific Energy Consumption, kWh/t

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

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from pilot-scale HPGR work. Given this agreement, it is suggested that the piston press be 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 bearing selection 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 taken—although 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 – e–b. Ecs) (EQ 7)

where

t10 = percentage passing one tenth of the feed size A and b = breakage characteristics from piston-press tests

Ecs = 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

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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-single-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 a “feel” 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 in error with scale can probably be assigned to the decreasing proportion of machine per-formance impacted by edge effects.

Detailed HPGR Modeling

For a more complete treatment of performance estimation in a modeling sense, true models 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. The main parameters are listed in Table 4.

This extremely comprehensive treatment is then used in a verification and scale-up scheme procedure; full details can be found in works by Daniel and Morrell (2004). C O N C L U S I O N S

There is an increasing body of knowledge around the application of HPGRs in hard-rock duties. In terms of selection and sizing, much has already been written, particularly by the 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 for flowsheet purposes—a critical element on the pathway to engineering acceptance. In many ways, this paper seeks to provide a pragmatic engineering basis for the assessment of HPGR performance. This message was also the theme expressed by Klymowsky and Liu (1996), where they sought a Bond work-index analogy for HPGRs. There is no doubt that a standardized, accepted HPGR “work index” would be a great boost to HPGR acceptance.

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

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

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A C K N O W L E D G M E N T S

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

Entry Zone Single-Particle

Breakage

Centre Zone Packed-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 (x_gf) Calculated throughput (Q_calc) Roll width (L) Flake density (q_g) 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, t10e—breakage for edge and precrusher Kp(HPGR)—power coefficient (compression zone) K1p, K2p, K3p—precrusher model parameter t10h—breakage for compression zone crusher K1e, K2, K3—edge-crusher model parameter

K1h, K2h, K3h—compression zone parameter Split factor (c)

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R E F E R E N C E S

Daniel, M.J. 2002. HPGR model verification and scale-up. Master’s thesis. Brisbane, Australia: Julius Kruttschnitt Mineral Research Centre, Department of Mining and Metallurgical 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. Minerals

Engineering 17:1149–1161.

Klymowsky, I.B., and J. Liu. 1996. Towards the development of a work index for the roller 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 pressure rolls crushing in an operating environment. Minerals Engineering 16:827–838. Ruben, E.S. 2002. Learning our way to zero emissions technologies. IEA Zero Emission

Technologies Strategies Workshop, Washington, DC, March 19.

Schönert, K. 1988. A first survey of grinding with high-compression roller mills.

International Journal of Mineral Processing 22:401–412.

———. 1991. Advances in comminution fundamental, and impacts on technology. Pages 1–21 in Proceedings of the XVII International Mineral Processing Congress. Volume 1. K. Schöenert, ed. Ljubijana, Yugoslavia.

Schönert, K., and U. Lubjuhn. 1990. Throughput of high compression roller mills with plain and corrugated rollers. Pages 213–217 in 7th European Symposium on

Comminution.

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

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15

High-Pressure Grinding Rolls—

A Technology Review

*

Chris Morley†

A B S T R A C T

The development of high-pressure grinding rolls (HPGRs) technology is reviewed, with an emphasis on aspects relevant to hard-rock comminution. Case histories are investigated and lessons learned are discussed in the particular context of the application of the device as a supplement to, or replacement for, conventional crushing and semiautogenous milling circuits. The potential for the more widespread use of this technology as a comminution method in hard-rock processing is examined. The use of the technology as a metallurgical tool is addressed, and future flowsheet concepts are introduced that make progressively greater use of the energy efficiency of HPGRs.

I N T R O D U C T I O N

High-pressure grinding roll (HPGR) technology has its genesis in coal briquetting in the early 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, copper and tin ores, mill scale, and coal.

Hard-rock operations that use HPGRs as an alternative or supplement to conventional comminution 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

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Ekati (diamonds), CMH-Los Colorados, CVRD, Empire and Kudremukh (iron ore), and Suchoj Log (gold ore). Hard-rock operations to have considered using HPGR and conducted pilot testing include Mt. Todd, Boddington, and KCGM, all in Australia. A full plant trial of an HPGR was conducted on a particularly arduous duty at Cyprus Sierrita between 1995 and 1996; and, more recently, HPGR has been piloted at Lone Tree, Nevada, in the United 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 Köppern, all based in Germany. T H E T E C H N O L O G Y

Machine Design

The 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 positioned using pneumohydraulic springs. The feed is introduced to the gap between the rolls and is 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 5–10 MPa, but can be 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 the roll edges by cheek plates.

Technology Motivators

Generally, the primary motivation for the use of the HPGR as a comminution alternative is its energy efficiency when compared to conventional crushers and mills. This improved Courtesy of Köppern.

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HIGH-PRESSURE GRINDING ROLLS—A TECHNOLOGY REVIEW 17

efficiency is due to the determinate and relatively uniform loading of the material in the HPGR 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 to individual particles to cause structural failure, such that the energy lost as heat and noise is minimised. However, until a device is invented that can perform this task on a commercial scale, the HPGR remains the most energy-efficient comminution technology available.

A major operating cost in conventional semiautogenous-based comminution circuits treating 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 from HPGRs), the overall media savings are typically of the same order of magnitude as the energy 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 individual progeny particles due to the very high stresses present in the HPGR compression zone. Microcracking occurs predominantly at grain boundaries and so increases liberation and lixiviant penetration, while the effective reduction in milling work index caused by microcracking reduces overgrinding and slimes generation.

In addition to being ore dependent, the extent of microcracking is a direct function of the operating pressure—and therefore energy input—of the HPGR, and in any given operation, the benefits of microcracking must be weighed against the incremental power required to achieve those benefits.

The HPGR’s 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.

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whereby the host rock is shattered while the diamonds are liberated undamaged—provided, of course, that the diamonds are smaller than the operating gap of the HPGR. This effect is also of benefit in the treatment of gold ores containing coarse gravity-recoverable gold grains, which would be flattened in conventional tumbling mills and rendered more difficult to recover.

Technology Status

The HPGR, considered a mature technology in the cement industry, is now the norm rather than the exception in modern diamond plant design and is becoming common in iron ore processing, particularly in the field of pellet feed preparation.

However, although some of the current diamond and iron ore applications can be regarded as hard-rock duties, HPGR is regarded by many as unproven in true hard-rock mining, and this perception is reinforced by the experience at Cyprus Sierrita in 1995– 1996. This application is widely considered to have been unsuccessful because it did not lead to a commercial sale; however, the fact that the comminution performance of the machine 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 operation regarding the precautions necessary in circuit design and unit operation for the protec-tion of the studded roll surfaces and the successful applicaprotec-tion of HPGR technology. Courtesy of Polysius AG.

FIGURE 3 Cone crusher product particle (conventional crushing)

Courtesy of Polysius AG.

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The following is a summary of the more important issues arising from observations of 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

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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 Hindrances

Hindrances 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 obstacle to the ready acceptance of the technology in hard-rock applications. However, the HPGR can prove a cost-effective comminution device, even when the high cost and frequency of replacement of wear surfaces in highly abrasive duties are considered.

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

S C A L E O F O P E R A T I O N

A common perception is that a project must be of relatively large scale before the use of HPGRs can be justified. However, HPGR units of almost any size can be produced (up to the current practical unit capacity limit of about 2,200 t/h), and this technology deserves serious consideration over a much wider range of plant capacities than might initially be imagined.

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 be assessed at the prefeasibility study phase by conducting preliminary tests. The manufac-turers have test facilities in Germany, and small-scale laboratory facilities are available at various locations globally. Pilot-scale machines are available at several research facilities in 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 evaluaopera-tion on a hard-rock mining operation.

T H E M A N U F A C T U R E R S A N D T H E I R D E S I G N S

Polysius, KHD, and Köppern are widely represented globally, but the machines are man-ufactured exclusively at their respective facilities in Germany.

Polysius favours a high-aspect-ratio design—large diameter, small width—while KHD and Köppern prefer a low-aspect ratio. The high-aspect-ratio design is inherently more expensive but also offers an intrinsically longer wear life for a given application, as the operating gap is larger and the roll surfaces are exposed to a correspondingly smaller proportion of the material processed. The high-aspect-ratio design also produces a coarser product due to the greater influence of the edge effect; however, this difference is relatively slight, particularly with larger units. Nevertheless, for closed-circuit applications,

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this additional coarseness does increase the circulating load and tends to offset the wear life 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 under license.

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

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

Köppern supplies patterned and profiled surfaces in both segment and tyre format, whereas Hexadur is generally available only in tyre format due to the dimensional control difficulties 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 the foreseeable future) is 1.5 m, constrained by furnace dimensions. This constraint limits Köppern’s unit capacity to about 1,000 t/h for hard-rock comminution applications using Hexadur. However, Köppern also offers machines with studded roll surfaces supplied by KHD, effectively lifting this capacity constraint.

Data of Test Units:

Diameter of Rolls: 0.71 m Width of Rolls: 0.21 m Speed of Rolls: 0.29–1.10 m/s

Top Feed Size: 16–35 mm

Diameter of Rolls: 0.30 m Width of Rolls: 0.07 m Speed of Rolls: 0.2–0.9 m/s

Top Feed Size: 8–12 mm

REGRO

ATWAL

LABWAL

Courtesy of Polysius AG.

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Köppern 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 and superstructure. This allows roll change-out times for tyre replacement of about the same duration as for in-situ segment change-out. Polysius also offers a design that allows rapid roll assembly removal, but without the need for a hinged frame design. In more recent developments, 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 Köppern.

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

KHD uses cylindrical roller bearings that allow the choice of grease or circulating oil lubrication systems, as there is no relative movement between the bearings and seals. Polysius and Köppern use grease-lubricated, self-aligning spherical roller bearings. O P E R A T I N G C H A R A C T E R I S T I C S

There are many factors to be considered when specifying an HPGR and selecting an appropriate flowsheet for a given application. The following subsections summarize the more important issues.

FIGURE 9 Köppern HPGR

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Ore Characteristics

The compressive strength of the material to be crushed determines the amount of useful energy that can be absorbed by the material, which in turn dictates the bearing and motor sizes required for a given duty.

With studded roll wear surfaces, the compressive strength of the ore, in combination with the feed particle top size and operating pressure, will largely determine the probability of stud damage—the higher the values of each of these variables, particularly when they occur together, the higher the likelihood of incurring stud damage. Ongoing development of 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 (as distinct from the breakage rate) of the studs, as well as that of the substrate metal. For example, the wear life at the iron ore operations at Los Colorados and Empire are about 14,000 and 10,000 hours, respectively, while those at the Argyle and Ekati diamond mines were about 4,000 hours initially, but increased to 6,000–8,000 hours and beyond with ongoing development of stud and edge protection configurations.

HPGRs are not generally suitable for the treatment of highly weathered ores or feeds containing a large proportion of fines. (This of course does not apply to applications where all the feed material is fine, such as fine grinding of concentrates.) Fine and weathered material tends to cushion the action of the rolls and so reduces the efficiency of comminution of the larger feed particles. For example, Argyle bypasses its primary HPGRs when very fine ore is being mined. On these ore types, the fine or weathered material 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 excessive moisture, which tends to cause washout of the autogenous layer on studded rolls and increases slippage on smooth rolls. In both cases, accelerated wear is the result. For example, Ekati bypasses the –4+1 mm feed fraction around the HPGR when the prevail-ing ore type results in inherently high moistures.

Specific Pressure

The specific pressure (specific press force) is the force (Newtons) divided by the appar-ent (or projected) area of the roll—that 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 1–4.5 N/mm2_{for studded roll}

surfaces and up to 6 N/mm2 for Hexadur. The required specific pressure determined in tests is used for scale-up of the required operating hydraulic pressure for the commercial unit.

Specific Energy Input

The 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 1–3 kWh/t. In general, a given ore will absorb energy up to a point beyond which little additional useful work (i.e., size reduc-tion) is achieved—a zone of diminishing returns is approached.

For equivalent size reduction, a hard, competent ore of high compressive strength will 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 linear function. Generally, specific energy input in coarse crushing applicationsis numerically

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about one half to one third of the specific pressure, so that a specific pressure in the typi-cal operating range of 3–4.5 N/mm2 can be expected to correspond to a specific energy input of 1–2.5 kWh/t. In fine-grinding duties, this ratio is typically higher—for 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 to derive a graph of product fineness against specific energy. The graph generally displays an initial steep slope that flattens out to approach the horizontal at high SEI values (e.g., 3.5–4.5 kWh/t). The optimum SEI can then be selected.

Microcracking

Although the size reduction graph frequently enters an area of diminishing returns with increasing specific energy, it has been demonstrated on some ores that the reduction in effective 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 from an overall comminution energy perspective to operate at a higher specific energy than corresponds 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 increased fines generation and reduced product work index, to ensure an HPGR is specified that is capable of transmitting the necessary power.

Feed Top Size

For hard-rock applications, the feed top size is a critical variable in the successful operation of 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 due to early nipping—effectively causing single-particle breakage by direct contact with the roll surfaces—can cause stud breakage.

Constraints on feed top size have been related in the literature both to roll diameter and to operating gap. Figures of up to 7% of roll diameter and three times the gap have been quoted as appropriate limits on feed top size, even though the latter ratio implies some 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 top size 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. For softer materials, this rule can be relaxed—for example, some kimberlite operations suc-cessfully treat open-circuit secondary crushed products with top size–gap ratios of about 1.8–2.0 using studded rolls.

By interpolation, ratios of around 1.3–1.5:1 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 incrementally if 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 possible stud 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 roll diameter, and showing these angles and an appropriate operating gap, estimates can be made of the particle size above which single-particle breakage is likely to occur and below which interparticle breakage commences.

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Unit Capacity

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

mx (t·s/m3·h) = 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 commercial unit, 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 boxes for 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 for the majority of ores, rather than as statements of universal fact—there will always be the exception that proves the rule:

Ore hardness—mx increases with ore hardness.

Specific pressure—mx decreases slightly with increasing pressure.

Roll surface—mx 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 speed—for 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 size—the available evidence is not conclusive, but it appears that mx

might increase slightly with an increase in feed top size.

Feed bottom size—mx 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 moisture—for 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 Gap

The operating gap is directly related to the unit capacity, all else being equal, so “gap” can be interchanged with mx in the above analysis. Depending on the application, the ratio of operating gap to roll diameter will normally lie in the range of 0.010 to 0.028.

Circuit Capacity

The capacity of an HPGR circuit, as distinct from the unit capacity discussed above, is obviously a function of the circuit design. Of the above variables, the feed bottom size

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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 feed exceeded the reduction in HPGR unit capacity. Whether this occurs in practice remains the 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, and this is supported by the limited evidence available in the literature.

However, an increase in circuit throughput achieved in this way may be offset by a decrease in product fineness and/or reduced microcracking such that, depending on the downstream processing route, a full-fines HPGR feed may be preferable to a truncated feed. For any given application, the more efficient flowsheet can be determined only by comprehensive 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 allow circuit performance to be optimised. This flexibility normally comprises the prescreening of 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 Sizing

As noted earlier, product fineness increases with operating pressure (and therefore power), generally up to a point of diminishing returns. It has been observed elsewhere that it is more energy efficient to operate an HPGR at low pressures and in closed circuit with a screen, so that less energy is wasted on compacting the product. However, this generally 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; so smooth rolls give the finest product, with profiled surfaces in the mid-range and studded surfaces the coarsest. This is due to the reduced slip between the rolls and the ore, giving a 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 a smooth roll unit. However, the effect is relatively small, and the benefits of profiled or studded rolls usually outweigh the reduced product fineness. Furthermore, the effect appears to be ore specific, and some operations (e.g., Jwaneng) have recorded an increase in fineness with studded rolls compared to smooth rolls.

Increasing roll speed leads to a reduced product top size and improved F50/P50

reduction ratio, without significantly changing the fine end of the sizing spectrum. A slight mismatch or differential in roll speeds has been found to enhance grinding performance, and though this could be considered intuitively plausible, it might also be expected that adopting this as a deliberate control strategy could lead to increased roll surface wear rates due to this imposed speed differential. This effect is therefore regarded as being of academic interest rather than practical significance.

Product sizing is largely independent of feed moisture. Product sizing is a function of 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

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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 the standard Bond equation. Milling power requirements are thus reduced by both the sizing of the HPGR product and the microcracking of the product particles, and are therefore best determined by pilot testing.

Roll Surface Wear

Increasing roll speed increases turbulence in the feed material and slip of feed against the roll surfaces, leading to elevated wear rates. This should generally be a concern only at the top end of the practical speed range. In this respect, Polysius traditionally uses a rule of thumb to the effect that the peripheral speed of the rolls (in meters per second) should not exceed roll diameter (in meters), although Köppern does not support this view and regularly nominates speed–diameter ratios of up to 1.3. KHD also uses these higher ratios for their smaller-diameter machines but generally uses <1.0 for larger units and on coarse crushing applications. More recently, Polysius has proposed that it is the angular velocity rather than peripheral speed that should be used as the roll speed selec-tion criterion, and that maximum speed should be in the range of 18 to 20 rpm for fine-grinding applications and 20 to 22 rpm for typical hard-rock coarse crushing duties.

High moisture levels lead to significant increases in wear. It is believed that this could be due to a combined erosion/corrosion effect analogous to that observed in Nord-berg WaterFlush cone crushers.

In recent studies involving tough, abrasive ores, it was found that wear rates were significantly higher with truncated feeds than with full fines or untruncated feeds, to the extent that wear life considerations heavily outweighed the energy efficiency advantages that had previously been established for these ores using truncated feeds. This illustrates the importance of conducting comprehensive tests to ensure that decisions made on flowsheet selection are well informed.

Roll surface wear rates for studded rolls can vary as the operating life of the wear surface progresses. In one application, the wear rate was observed to increase with time from 0.006 Pm/t after 200 operating hours to 0.015 Pm/t after 1,000 hours, after which a plateau was established in the wear-rate curve.

It is believed that this effect is due to an initial imbalance in the wear rates of the studs and the substrate. In the case in point, it would appear that the stud protrusion above the new roll surface was too small for this particular duty, so that the substrate ini-tially wore more rapidly than the studs. Presumably, had the stud protrusion instead been too great, then stud wear would have been more rapid initially and declined there-after. This, however, was not demonstrated.

In either case, overall wear rate stabilises when the two components of wear—stud and substrate—are in equilibrium. The important point is that roll surface wear life should not be computed from initial wear rates. The wear-rate curve must be plotted and the equilibrium plateau established before wear-life predictions are made.

The “bathtub” effect is a well-documented phenomenon whereby the central zone of the rolls wears at a greater rate than the edges, forming a concave wear pattern. For smooth rolls, this can lead to a requirement for regular grinding of the edges to maintain parallel roll surfaces and avoid touching of the rolls at the edges with the correct nominal gap in the central zone. For studded rolls, harder studs can be used in the central zone to give a more uniform wear pattern across the roll surface. However, harder studs are also more brittle, and stud breakage, as distinct from stud wear, can become a problem. The optimum combination of stud hardness levels for the central and edge studs in a given application must be established by trial and error. Normally, studs of lower hardness are

Advances in Comminution - Kowatra

ADVANCES

Preface

Contents

Advanced Comminution

Technologies

High-Pressure Grinding Rolls—

Characterising and Defining Process

Performance for Engineers

High-Pressure Grinding Rolls—

A Technology Review

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