35Tribology of

Earthmoving, Mining,and Minerals Processing

35.1 Introduction35.2 Wear Processes in Mining and Minerals Processing

Digging and Loading • Haulage • Crushing • Conveying/Handling/Storage • Screening • Grinding • Size Classification • Minerals Separation and Concentration

35.3 Equipment Used in Earthmoving OperationsSmall-scale Earthmoving • Large-Scale Earthmoving

35.4 Equipment Used in Mining and Minerals ProcessingCrushers • Grinding Equipment • Sand Pumps • Classifiers • Flotation Machines • Magnetic Separators

35.5 General Classification of Abrasive WearAbrasive Wear

35.6 Tribological Losses in the Mining of Metallic Ores, Coal, and Non-metallic MineralsSurface Mining • Shaft Mining and Drilling • Ore Processing and Related Activities

35.7 Financial Cost of Wear in Earthmoving, Mining, and Minerals Processing

35.8 Concluding Remarks

35.1 Introduction

Earthmoving, mining, and minerals processing each involve frequent, and often severe, mechanicalinteractions between metals, and between metals and abrasive nonmetallic and metallic materials (i.e.,mineral bearing ores). The abrasive nature of ores causes significant wear to extracting, handling, andprocessing equipment. Consequently, wear in earthmoving, mining, and minerals processing operationsresults in the removal of large amounts of material from the wear surfaces of scraping, digging, and oreprocessing equipment. From an energy point of view, material wear of this nature is classified as anindirect tribological loss (Imhoff et al., 1985). Additionally, a significant amount of energy is expendedto overcome frictional forces in the operation of all earthmoving, mining, and minerals processingmachinery (i.e., a direct tribological loss). However, in these particular processes, wear losses are morethan five times those of frictional losses (Imhoff et al., 1985).

In general, the amount of material lost from a particular component in these operations, before itbecomes unserviceable, is far greater than that which can be tolerated in typical metal-to-metal wearsituations (e.g., lubricated bearing-shaft wear couples in machinery). Consequently, much of the equipment

Jeffrey A. HawkU.S. Department of Energy

R.D. WilsonU.S. Department of Energy

used in earthmoving, mining, and ore processing makes use of easily replaceable or repairable, andpreferably low-cost, wear components. The mechanisms by which metal-to-metal and abrasive wearoccurs, and the relationships between material properties and wear behavior, are reasonably well-under-stood in general terms (Haworth, 1949; Khruschov, 1957; Richardson, 1967; Moore, 1974, 1981). How-ever, the specific wear mechanisms/wear material interactions that occur during earthmoving, digging,and the processing of ore are more complex, and depend on the wear material, and on the nature ofabrasive, the type of loading, and the environment (Avery, 1958, 1961, 1974; Norman and Hall, 1968;Norman, 1980; Mutton, 1988). For example, the direct impact of metallic components with the earthrequires alloys that have excellent ductility and work-hardening properties. The transport of materialacross a surface, such as chutes, conveyors, and screens, or the scraping of earth and gravel with a blade,requires a material with high surface hardness. The crushing and fragmentation of ore requires materialswith even higher surface hardnesses. Mining and mineral processing operations in corrosive environ-ments, such as slurry transport pumps, require materials that are both wear and corrosion resistant. Asa result of this general knowledge, reliable predictions can be made regarding the performance ofparticular materials under a range of in-service operating conditions. This knowledge has allowed therational selection of wear-resistant materials for use as earthmoving, mining, and minerals processingcomponents, and new wear-resistant materials can be designed using our knowledge of the impact andabrasion mechanisms encountered in the day-to-day operation of components used in these operations.

However, developing new abrasion- and impact-resistant materials used in these industries has not beenparticularly easy. During the last 30 years, the earthmoving and mining industries have gravitated towardthe use of larger and more capital-intensive equipment (Norman, 1980; Gill, 1991). In the mining industryparticularly, this has been due in part to the lower-grade ores being mined in some segments of the industry,thereby requiring a larger throughput of raw material with concomitant lower operating costs in order toallow the industry and individual operations to remain economically viable. Hand-in-hand with themining industry, companies that provide earthmoving equipment have also seen the size of their equipmentincrease. The increase in the material tonnage handled per unit operation has made material and com-ponent design more difficult. As a result, the severity of conditions to which abrasion-resistant materialsare subjected has increased dramatically. For example, higher impact levels are required in semi-autogenousgrinding mills. In addition, the need to avoid — or at least minimize — unscheduled maintenance requiresthat the service life of wear consumables be predictable to some reasonable degree of reliability.

The replacement of wear consumables represents a minor, but nonetheless significant cost to themining and minerals processing industries. These costs arise from the need to purchase replacementparts and equipment; from scheduled and unscheduled equipment downtime with attendant loss ofproduction; and from the labor and equipment costs expended during the replacement of worn equip-ment and component parts. Of all the capital and operating expenses associated with these industries,the cost of downtime and lost production outweigh the cost of component replacement (Norman, 1980;Imhoff et al., 1985; Mutton, 1988; Gill, 1991).

Although wear represents a minor portion of the operating expenses in earthmoving, mining, andminerals processing, it is uppermost in the minds of maintenance personnel, due to its recurring nature.Because of this, those responsible for the purchase and replacement of wear consumables are alwayswatchful for new materials and/or component designs that will last longer, be easier to install, and aremore cost-effective than those currently in use. Other factors involved in wear material selection includeavailability, potential risk of catastrophic failure, and environmental considerations such as the potentialfor noise reduction (Mutton, 1988).

The experience gained through maintenance operations has allowed particular mines and mineralprocessors to develop extensive in-house information on wear materials for their unit operations. Busi-nesses that sell or lease earthmoving equipment also have in-house repair facilities, and draw heavily onthe experience of the equipment manufacturer in doing routine repair and maintenance.

Those involved in the design and construction of new equipment are sometimes able to draw uponthis extensive database when specifying wear materials and equipment. However, two major constraints

exist to restrict the specification and use of higher-cost wear consumables in new facility constructionand component fabrication (Mutton, 1988):

1. Wear materials that offer improved performance will, in general, cost more. Although such mate-rials may be cost-effective in terms of the improved life obtained, their higher purchase price servesto increase capital requirements.

2. In some cases, the original design for particular pieces of equipment may not be optimal in termsof process performance. Such factors are usually addressed either during, or immediately following,the commissioning of new equipment. This may involve alterations to the design of particularcomponents, and under these circumstances the use of higher-cost and more durable wear mate-rials in original equipment may not prove to be cost-effective in the longer term.

The nature of the wear problems addressed in this chapter is quite large. In fact, wear from metal-to-metal contact, abrasion, impact, and erosion in earthmoving, mining, and minerals processing is greaterthan that of almost any other major industry (Imhoff et al., 1985). Much research over the last 50 yearshas been devoted to this subject. This chapter cannot hope to cite all the literature that exists on thetribology of earthmoving, mining, and minerals processing. As such, much of the material containedherein was culled from some excellent review articles (Norman, 1980; Mutton, 1988; Olson and Cross,1992); books on wear (Zum Gahr, 1987; Rigney, 1981) and materials beneficiation (Gill, 1991); compi-lations of wear literature on mining and minerals processing (Barr, 1974); an Energy Conversion andUtilization Technologies (ECUT) Division review on tribological sinks in industry (Imhoff et al., 1985);a National Materials Advisory Board (NMAB) report on “Comminution and Energy Consumption”(NMAB, 1981); and individual research papers (as listed in the Reference section).

35.2 Wear Processes in Mining and Minerals Processing

This section presents an overview of the wear mechanisms and wear processes that occur in earthmoving,mining, and minerals processing. In general, the wear behavior of materials used in these operations isdetermined by a number of factors, which can be grouped into three main categories: (1) the propertiesof the wear material; (2) the properties of the abrasive material; and (3) the nature and severity of theinteraction between the metal-to-metal wear surfaces and between the abrasive and the wear materials.

These categories are interdependent because the nature of the metal-to-metal contact and the abra-sive/wear material interaction are influenced by the properties of the wear material, and to a great extent,by the properties of the abrasive. Increasing the size, density, or hardness of the abrasive particles, forexample, influences the contact stress between the abrasive and wear materials, possibly leading toincreases in the wear rate of the wear material. Similarly, if the abrasive particle is sharp, and upon fractureremains sharp, then the cutting or gouging action of the abrasive particle will continue to be effective inremoving material from the machine component (Moore, 1981). Alternatively, in the case of metal-to-metal wear, increasing loads place increased stress on contacting parts, which can change the normalmode of adhesive wear to some other damage mechanism (e.g., contact stress fatigue or even plasticyielding).

Also influencing the wear of earthmoving, mining, and mineral processing components is their func-tion and design. In earthmoving operations, the abrasive wear from scraping operations will be less severethan operations requiring earth penetration and removal (e.g., the wear of digger teeth on bucketexcavators). Wear rates in crushing and grinding units will be much higher than the wear exhibited inchute linings, screens, and classifiers. Indeed, crushing and grinding operations account for approximately95% of the material wear in ore processing (Norman, 1980). These high wear rates arise because crushingand grinding are more energy intensive, and result in more severe impact and abrasive wear. Consequently,the severe impact and abrasive conditions found in crushing and grinding equipment necessitate com-promises in material selection criteria in order to minimize the risk of catastrophic failure of these units(Norman, 1980; Moore, 1981; Mutton, 1988).

The dominant wear mechanism in earthmoving, mining, and minerals processing is abrasion. Sec-ondary to abrasion — but no less important — is metal-to-metal wear, which in many instances iscomplicated by abrasion as a result of the environmental conditions in which the machinery operates.Abrasion is arbitrarily subdivided into broad classifications corresponding to the nature of the wearmaterial/abrasive particle interaction. The abrasive wear classifications usually cited include:

• Gouging abrasion (the removal of large volumes of material per event from the wear surface)

• High-stress grinding abrasion (i.e., the abrasive particle is crushed during the wear interaction)

• Low-stress scratching abrasion (i.e., the abrasive particle remains intact as it moves freely acrosswear surface)

• Erosion (low-stress scratching)

• Erosion-corrosion (low-stress scratching abrasion in a corrosive environment)

The mechanisms of material removal during abrasion are complex and vary with the type of serviceconditions. The wearing surfaces of equipment are continually subjected to high rates of wear, such thatrepair and replacement of worn components is a regular maintenance activity. To facilitate easy replace-ment of worn components, most earthmoving, mining, and mineral processing equipment makes useof components that are designed for easy and rapid replacement and/or repair (Austin, 1974; Dawsonet al., 1982). A typical example is the design of the digging teeth used on dragline buckets and powershovels, a design where the use of quick release pins on the digger teeth facilitates their easy repair orreplacement in the field (Stoody, 1998). Table 35.1 highlights the generic tribological loss mechanismsthat occur in earthmoving, mining, and minerals processing. All generic wear mechanisms occur in oreprocessing operations. In mining and drilling operations, three-body abrasion (i.e., high-stress condi-tions) is common to each, and results in substantial wear to equipment.

At this point, mention of metal-to-metal wear is relevant. In earthmoving, mining, and mineralprocessing equipment, metal-to-metal interactions are prevalent in the engines, drive trains, connectors,bearings, and gearing. In engines and drive trains, much of the system is closed to the environment andthe wear surfaces are lubricated. It is beyond the scope of this chapter to discuss engine and drive trainlubricated wear (other chapters in this volume discuss these issues). However, metal-to-metal wear inconnectors, bearings, and gearing, sometimes complicated by abrasion, does pose severe wear problems

TABLE 35.1 General Tribological Loss Mechanisms in Mining

Active Tribological Mechanism

Operational ActivityThree-Body

AbrasionHigh-Stress

GougingImpactWear

HighFriction Erosion

LubricatedWear

Surface MiningExposing and diggingLoadingTransporting

XXX

X——

XX—

X—X

———

X——

Shaft MiningDiggingTransporting

XX

——

X—

—X

——

X—

Ore Processing X X X X X X Drilling

Mud pumpsDrill pipeDrill bits

XXX

—XX

——X

—X—

XXX

———

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T., Gruber, T.,and Barber, S. (1985), A Review of Tribological Sinks in Six Major Industries, Technical Report PNL-5535,Pacific Northwest Laboratory, Richland, WA.

in this type of equipment. The types of metal-to-metal wear and damage typically encountered include:contact stress fatigue or spalling, plastic yielding, and adhesive wear. Additionally, metal-to-metal con-tacting parts are also susceptible to corrosion, fretting corrosion and creep, fracture (i.e., fatigue failuresand ductile/brittle fracture), and handling and installation damage. The severity of the wear and/ordamage is a function of the design, lubrication, and stress on the part. The wear/damage that occurs inmetal-to-metal contact can be greatly accelerated if abrasive particles enter the system.

In earthmoving, mining, and minerals processing, relatively large amounts of material are removedby wear from a component before it becomes unserviceable, with a concomitant decrease in its operatingefficiency as a result of changes in the dimensions of the wearing surface. This may not be a seriousproblem for digger teeth or chute liners, but in more complex equipment such as crushers and cyclones,operating efficiency may be seriously impaired with relatively minor dimensional changes. In some oreprocessing equipment, such as gyratory crushers, these changes can be accommodated through theadjustment of machine settings. In many cases, the operating efficiency of equipment is monitored bymeans of its power consumption, with adjustment or the replacement of components carried out whena predetermined operating level of performance falls below some critical value.

Earthmoving, mining, and minerals processing operations can be subdivided into a number of separateunit operations, each of which can experience its own particular type of wear, depending on force levels,abrasive particle type and size, and other operating or environmental factors. Of particular importanceare the characteristics of the abrasive, which have a significant effect, not only on the wear behavior, butalso on the type of equipment selected to carry out a particular operation. For example, size reduction(comminution) of hard minerals is usually carried out in gyratory crushers that operate at relatively lowspeeds and subject the mineral to high compressive stresses (Gill, 1991). Softer, less abrasive minerals,such as limestone or coal, can be comminuted in impact-type crushers, where a large proportion of theenergy required for size reduction is developed through the kinetic energy of the rotating crusher barsand impact hammers (Norman, 1980; Gill, 1991). The following list of generic unit operations (Norman,1980; Mutton, 1988; Gill, 1991) summarizes the abrasive wear that can occur during earthmoving,mining, and ore processing. Figure 35.1 depicts generic ore processing operations in schematic form withthe materials used for the particular wearing surfaces and some typical wear rates (Norman, 1980).Table 35.2 expands on the list of wear mechanisms and associated wear materials used in earthmoving,mining, and minerals processing shown in Figure 35.1.

35.2.1 Digging and Loading

Digging and loading involve a wide range of abrasive sizes and high force levels with significant impact.The major wearing areas for the buckets used on draglines and power shovels are the digger teeth andlip region, the inner floors and walls of the bucket, and the heel of the bucket (i.e., the outside lowersurface) (Kennedy, 1975; Mutton, 1988; Olson and Cross, 1992). The digger teeth are subjected to highimpact-gouging forces and abrasion. Consequently, digger teeth have been designed for easy replacementand/or repair (Austin, 1974). Typically, digger teeth are made of cast martensitic steels (Mutton, 1988).To increase the abrasive wear resistance of the digger teeth and lip region, weld overlays of hardfacingalloys (such as CrC) are used (Kennedy, 1975; Olson and Mueller, 1977; Horsfield, 1980; Olson and Cross,1992). To reduce abrasive wear in other areas of the bucket, wear plates are used extensively to line theinner walls, floor, and heel. The dragline and power shovel buckets have been designed so that the wearplates can be easily installed, repaired, and/or replaced, usually through weld overlays (Avery, 1947, 1968;Osborn, 1973; Kennedy, 1975; Chavanne, 1976; Hickl, 1980; Dawson et al., 1982). Wrought martensiticsteels (in low- and high-stress grinding and sliding/scratching wear areas), laminated white irons (in theheel areas where high-stress grinding and sliding/scratching occurs), and CrC hardfaced plate (highimpact and high-stress grinding and sliding/scratching areas) are the typical materials used for wearplates in dragline and power shovel buckets (Mutton, 1988).

FI

in ore processing machinery, in

Wear Control Handbook

,Pe

Pw

Rm

n1- Low stress2- Corrosion-erosion

1- Low stress2- Corrosion-erosion

ElastomersMart. W.I.Ceramics

ElastomersMart. W.I.Ceramics

Lining & Orifice Agitator - Impeller

AirTailing

Coarse

Classifying

Fine

Mineralsseparation

Cont.

Tw(

3* 2*

White Iron

GURE 35.1 Schematic representation of the major steps in ore processing. (From Norman, T.E. (1980), Wearterson, M.B. and Winer, W.O. (Eds.), ASME, New York, 1009-1051. With permission.)

Crushingore

Coarse

Fine

ScreeningWater

Grinding Pumping

rincipalear types

ecommendedaterials

Liners

Aust. Mn steelMart. W.I.

1- Gouging2- Low stress

1- Low stress2- Gouging3- Corrosion-erosion

Aust. Mn steelMart. W.I.Elastomers

Mart. steelStainless steelsCr. plated steel

Coarse Screens

Fine Screens

1- High stress2- Gouging3- Corrosion - erosion

Rods

Mart. steelPearl steel

Balls

Mart. steelMart. W.I.

Liners

Mart. W.I.Pearl steelAust. low-Mn steelMart-steelElastomers

1- Low stress2- Corrosion-erosio

ElastomersMart. W.I.

Impeller & Case

ypical metalear rates

Grams per tonne)50 2* 700 5*

* Metal wear is negligible when elastomers can be used

Abbreviations: Aust. = Austenitic, Mart. * Martensitic, Pearl. * Pearlitic, W.I. =

35.2.2 Haulage

The same type of abrasive wear conditions (gouging and low-stress abrasion) occur in haulage operationsof coarse, hard ores as occur in digging and loading. However, impact energy levels can be quite high inthe loading of skips and haul trucks, particularly for coarse-sized ore (up to 2 to 3 m in diameter) anddense materials (Mutton, 1988). During unloading, severe sliding abrasion, with high tangential forces,occurs. The same types of wear materials are used in haulage as in digging and loading: wroughtmartensitic steels for impact/sliding abrasion and CrC hardfaced plate for sliding abrasion areas. Formoderately soft and/or fine abrasives like bauxite and coal, low-stress abrasion and corrosion are thepredominant degradation mechanisms. In these instances, material selection is predicated on obtaininga degree of corrosion protection along with wear resistance. Materials used in wear-corrosion haulageenvironments include stainless and corrosion-resisting steels, aluminum alloys, wrought martensiticsteels, and ultra-high-molecular-weight (UHMW) polyethylene (Bauman, 1974; Doennecke and Shelton,1974; Mutton, 1988).

TABLE 35.2 Wear Materials Used in Mining and Minerals Processing Operations

Operation Abrasive Wear Classification Material Types

MiningDigging/loading 1. Gouging

2. Low-stressAustenitic Mn steelsCast martensitic steelsAlloy white irons/CrC hardfacingUHMW polyethylene

Haulage 1. Gouging2. Low-stress

Wrought martensitic steelsCrC hardfacingRubbersUHMW polyethylene

Crushing 1. Gouging2. High-stress3. Low-stress

Austenitic Mn steelsAlloy white ironsCrC hardfacing

Screening 1. Low-stress2. Gouging3. Erosion-corrosion

Alloy white irons/CrC hardfacingAustenitic Mn steelsStainless steelsWrought martensitic steelsElastomers, rubbers, polyurethanes

Conveying/handling/storage 1. Low-stress2. Erosion-corrosion

Wrought martensitic steelsAlloy white irons/CrC hardfacingCeramicsElastomers, rubbers, polyurethanesUHMW polyethylene

Grinding 1. High-stress2. Gouging3. Erosion-corrosion

Wrought martensitic steelsForged/cast martensitic steelsAlloy white ironsCast pearlitic steelsElastomers, rubbers

Pumping 1. Low-stress2. Erosion-corrosion

Alloy white ironsCeramicsElastomers, polyurethanes, rubbers

Classification 1. Low-stress2. Erosion-corrosion

Alloy white ironsCeramicsElastomers, polyurethanes, rubbers

Separation 1. Low-stress2. Erosion-corrosion

Alloy white ironsElastomers, polyurethanes, rubbersStainless steelsCeramics

From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry, Vol. 1,Australian Minerals Industries Research Association Limited, Melbourne, Australia.

35.2.3 Crushing

Abrasive wear conditions in crushing operations range from heavy impact and gouging (primary crush-ing) to high-stress abrasion (tertiary crushing and impact crushing) (Norman, 1980; Mutton, 1988; Gill,1991). Metallic materials are used exclusively for the crushing of ores. During crushing operations, theore is reduced from a maximum of approximately 2 m in diameter to about 10 mm, maximum diameter.Three or four stages of crushing are usually involved (Gill, 1991). Crushing is accomplished by squeezingthe ore between two metal surfaces with sufficient pressure to fracture the pieces. For softer and lessabrasive minerals, such as limestone or coal, an impact-type crusher can be used, in which the ore piecesare struck with a rotating hammer with enough velocity to cause fracture (Norman, 1980). Typicalmaterials used in crushers are austenitic manganese steels, laminated/reinforced alloy white irons, andCrC hardfacing, with final selection of the material made on the basis of abrasion resistance and toughness(Fabert, 1974; Hegmegee, 1974; Moore, 1981; Mutton, 1988).

35.2.4 Conveying/Handling/Storage

Ore is moved between the various unit operations by conveyor belts, chutes, and launders. Slurries ofground ore in water are moved through pipes. Typically, low-stress abrasion, with moderate impact anderosion-corrosion, occur during these unit operations. For example, wear (low-stress abrasion) in chuteliners is a minor, but significant factor in ore processing, especially for the feed chutes to grinding mills.However, the degradation in pipes carrying slurries can be quite serious, with wear (erosive-corrosive)severe at bends, reducers, and other locations where flow conditions are disturbed (Norman, 1980;Mutton, 1988; Madsen, 1992).

High alumina ceramic tiles, austenitic manganese steels, wrought martensitic steels, and laminatedalloy white irons are standard materials used in chutes and hoppers under sliding conditions withmoderate to low impact. Only the metallic materials are used when impact conditions become moresevere (Mutton, 1988).

For pipelines handling coarse abrasive slurries (i.e., particle diameters greater than 50 mm), CrChardfacing or alloy white irons are often used to line the pipes. For fine slurries, linings such as fusedcast basalt and elastomers (rubber or polyurethane) are used (Mutton, 1988).

Low-stress abrasion occurs in bins and hoppers. Corrosion can also be a problem if the moisturecontent of the material being handled is high. Wrought martensitic steels and corrosion resisting steelsare frequently used in these applications (Mutton, 1988; Gill, 1991).

35.2.5 Screening

Abrasive wear conditions in screening operations can range from severe impact, gouging abrasion onprimary screens, to low-stress/corrosion in wet screening of fine product. Coarse crushed ore is screenedon metal-bar, or in some cases, on rubber-deck screens (Gill, 1991). Finely crushed ore is normallyscreened on wire mesh or rod-deck screens (Mutton, 1988). Screens are closely coupled to crushing units,so it is important to minimize shutdown time for screen replacement (Norman, 1980; Gill, 1991). Varioussteels (austenitic manganese, wrought martensitic, and wrought pearlitic) are used as “grizzlies” inprimary screening operations. For run-of-the-mill ores and the discharge from grinding mills, wroughtmartensitic steels and elastomeric coated screens are used. For wet screening and dewatering operations,the screens are either made from stainless steel or a mild steel coated with an elastomer, typicallypolyurethane (Mutton, 1988).

35.2.6 Grinding

The dominant wear mechanisms in grinding are high-stress abrasion and erosion-corrosion. In autoge-nous and semi-autogenous mills, high impact and gouging abrasion also occur. During grinding (usuallyin water), the ore is ground from a maximum 20-mm diameter down to about 0.3 mm or less (Norman,

1980; Gill, 1991). The wear of grinding media (balls or rods), plus the wear of grinding mill liners,accounts for a major portion of the wear in an ore processing plant (Nass, 1974; Farge and Barclay, 1974;Avery, 1974; Norman, 1980; Mutton, 1988; Gill, 1991). The wear in grinding mills, along with the energyconsumed, is a major expense in ore processing plants (Norman, 1980; NMAB, 1981; Imhoff et al., 1985;Gill, 1991). Grinding media (balls, rods) are usually metallic (primary grinding), typically either forgedor cast martensitic steels, or alloy white irons (Nass, 1974; Farge and Barclay, 1974). Mill liners can eitherbe metallic (cast pearlitic or martensitic steels, or alloy cast irons), or rubber. Liners made of rubberprovide increased life, reduced noise, and the possibility for higher cylinder rotational speeds (Dougall,1974; Norman, 1974, 1980; Mutton, 1988; Gill, 1991). The performance of grinding mills, and theconsumption of grinding media and wear liners, are influenced to a great degree by liner design.

35.2.7 Size Classification

Abrasive wear conditions in classifiers consist of low-stress abrasion and erosion-corrosion (Norman,1980; Mutton, 1988). Appreciable wear in these units occurs in the larger, heavy media classifiers,especially hydrocyclones. The ground ore, upon leaving the grinding mill, is normally divided into asufficiently ground fraction that passes on to flotation or other methods of minerals separation, whilethe insufficiently ground fraction is returned to the mill for further grinding. The size classification ismade in gravity, or centrifugal-type, classifiers. Cyclone classifiers, especially hydrocyclones, are findingincreasing usage (Gill, 1991). Wear prevention or reduction in these classifiers is important because ashutdown to replace worn parts also requires a shutdown of the grinding mill that is in closed circuitwith, and feeds, the classifier (Norman, 1980; Gill, 1991). Typically, three classes of materials are used inclassifiers: ferrous alloys (alloy white irons, corrosion resisting steels), elastomers (rubber and polyure-thane), and ceramics (alumina and partially stabilized zirconia) (Mutton, 1988).

35.2.8 Minerals Separation and Concentration

Wear (abrasive and corrosive) primarily occurs in the agitator impellers of the flotation machines. Theseimpellers are usually made from molded rubber, or in some cases, from martensitic white iron. Due tothe fineness of the particles in the ore slurry and the moderate impingement velocities, wear of theimpellers is quite slow. However, some types of rubber may deteriorate from additives (mineral oils) inthe slurry (Norman, 1980). Where minerals separation is done in gravity or magnetic separators, thewear of moving parts is almost negligible (Norman, 1980; Mutton, 1988; Gill, 1991).

35.3 Equipment Used in Earthmoving Operations

A wide variety of equipment is available for use in the scraping, digging, and large-scale removal of earth.It is not possible to discuss every piece of equipment available for these operations. However, much ofthe equipment is similar in its design and fundamental use, and as such, a discussion of the most commonones will provide an overview of their functionality and their associated problems with wear. Thefollowing discussion is divided into two sections; the first section looks at earthmoving equipment ofmodest size (i.e., typical equipment used in road construction and small-scale earthmoving operations,such as excavation for buildings), and the second section looks at equipment used in the removal ofoverburden in surface mining operations. At the end of each section, certain components associated withthis equipment will be examined from a wear point of view.

35.3.1 Small-scale Earthmoving

This chapter section focuses on earthmoving equipment used for non-mining applications, such as small-scale earthmoving in building, bridge, dam, road, and highway construction. When any structure of thesort mentioned above needs to be built, the first step is always the preparation of the site. This preparation

almost always starts with the removal of earth and rock so that some sort of foundation can be constructedupon which the desired structure rests. To remove the earth and rock, the following types of equipment(of a size necessary to effectively complete the job) are typically used: bulldozers, scrapers, graders,excavators, and front-end loaders. Of course, for many applications, other specialty equipment is alsoused, such as asphalt compactors, pavers, wheeled compactors, etc. Only the most common earthmovingmachines are described here.

35.3.1.1 Bulldozers

A bulldozer is a large tracked vehicle, with a large curved blade (the dozer) in the front that is used tomove earth and other debris out of the way. Behind the bulldozer can be a curved tool, called a ripper,that is used to engage and loosen the earth. Specialty blades allow the bulldozer to perform a variety ofjobs, such as extinguishing fires, pushing rocks and other debris out of the way, sweeping agriculturalfields of stumps, managing landfills, and building stockpiles. Bulldozers range in size from small(74.6 kW) to very large (Komatsu 575A-2 Superdozer at 857 kW). The majority of bulldozers owned inthe U.S. are owned by construction companies. However, the mining industry probably uses bulldozersthe most, operating them an average of 2700+ hours per year. Figure 35.2 shows a schematic of a bulldozer,with the various components common to all manufacturers. The Komatsu Superdozer has a blade thatis 3.4 m high by 7.6 m wide, and is capable of moving 70 m3 of earth. The vehicle weighs an astounding135,000 kg.

Abrasive wear is the primary material degradation mechanism in bulldozers. Abrasion is most prevalentin the undercarriage (i.e., the grousers) and ground engaging equipment (ripper teeth and shanks, aswell as the dozer end bits, side frame, and wear plates on the bottom of the blade). However, metal-to-metal wear, complicated by abrasion, also occurs in the sprockets, idlers, rollers, track rails, and pins.

35.3.1.2 Scrapers

A scraper is a mechanized wheeled shovel that cuts the earth, scoops it up, and moves it to anotherlocation where it is dumped. The front portion of the scraper consists of the tractor where the driver islocated. The rear of the scraper consists of the bowl, or can, that holds the earth. The cutting edge ofthe bowl has metal pieces called slobber bits that are used to cut the earth when the bowl is lowered tothe ground while the scraper is moving. Other types of scrapers may have a second engine in the rearand various dirt loading systems. Scrapers are more efficient than bulldozers when it is necessary to movethe earth more than 150 m or so. Scraper bowls can typically hold 15 m3 of earth, but bowls up to 34m3 are available (Caterpillar 657E 708 kW twin-engine scraper).

One area of extreme wear on scrapers is the tires, which are worn and cut by the terrain they moveover. Certainly, the slobber bits that engage the ground also wear heavily. The surface of the bowl is wornto a lesser degree by scratching abrasion.

FIGURE 35.2 View of a bulldozer showing major components affected by abrasive wear. (From Stoody (1998), TheRebuilding and Hard-facing of Earth-Moving Equipment, Bulletin 8203, Industry, CA. With permission.)

SPROCKET

DOZEREND BITS

GROUSER

TRACK RAILSROLLER

IDLERRIPPER TEETHAND SHANKS

TOP CARRIER ROLLS

35.3.1.3 Graders

Once a majority of the earth and other debris has been moved by a bulldozer or scraper, then a graderis used to sculpt the terrain. Graders are most commonly used in road or highway construction. Thegrader, like the scraper, is a wheeled vehicle. It has a single angled blade, or moldboard, near the centerof the vehicle that is used to sculpt the ground below. The moldboard can be adjusted from side to side,up and down, and rotated 225° to give the ground below the shape desired. The largest graders are usedfor building and maintaining mining roads, for reclamation projects, and for other large-scale projects.The Caterpillar 24H motor grader weighs 58,960 kg, has a 373-kW engine, and carries a 6.1 to 7.3 mwide by 1 m high moldboard.

Like scrapers, the tires on graders suffer wear and cutting from the surfaces they work on. Abrasionof the scraper blade occurs as a result of its interaction with the material being moved. Another area ofwear is in the moldboard, where there is significant metal-to-metal wear, which is further complicatedby abrasion from the dust and dirt.

35.3.1.4 Excavators

Excavators were originally known as steamshovels. A modern excavator consists of a tracked vehicle witha digging shovel on the end of a hydraulic arm. Most modern excavators are used for heavy digging,although they are also used for trenching, knocking down small buildings, uprooting trees, etc. Figure 35.3shows a schematic of the undercarriage, or crawler portion, of a typical excavator. The hydraulic armconsists of a boom and stick, and can stretch as long as 18 m. Excavators range in size from 22,680 kgup to 90,700 kg, with bucket sizes ranging from 0.4 to over 8.0 m3. The smaller machines primarily serveas utility machines in and around building construction sites, while the larger machines are used almostexclusively for loading and digging earth.

In the undercarriage area, abrasion occurs primarily on the track pads. Metal-to-metal wear, onceagain complicated by abrasion, is a problem in the idlers, bottom rolls, chain drive sprockets, tread rollers,tumblers, and sprockets. Metal-to-metal wear also occurs in the gear teeth, house rolls, house ring, andhook rolls. Impact and abrasion are the major degradation mechanisms of the ground-engaging shovelteeth on the bucket. Scratching abrasion occurs on the side frame and wear plates on the bottom of theshovel.

FIGURE 35.3 View of a tractor undercarriage showing the major components affected by abrasive and metal-to-metal wear. (From Stoody (1998), The Rebuilding and Hard-facing of Earth-Moving Equipment, Bulletin 8203,Industry, CA. With permission.)

HOOK ROLLS(NOT SHOWN)

HOUSE RING

HOUSE ROLLS(NOT SHOWN) GEAR

TEETH

SPROCKETS

TUMBLERS

TRACKPADTREAD

ROLLERS

CHAIN DRIVESPROCKETS

BOTTOMROLLSIDLERS

35.3.1.5 Front-end Loaders

A front-end loader is a versatile machine that serves a dual role: as a fixed position excavator and a short-distance transporter. Unlike the excavator, the front-end loader is usually a wheeled vehicle with a bucket(the width of the machine) in the front. This machine is primarily designed to excavate soil, rock, debris,and other materials; lift this load to the desired location (usually into a truck or to a stockpile); and thendump the material. Machine and bucket sizes vary, with the smaller machines having bucket sizes of0.8 m3, and the larger machines (226,800 kg) having buckets approaching 25 m3. (Front-end loaders witha tracked crawler are available and used where traction and tight maneuverability are needed.)

Like the grader and scraper, the front-end loader is typically a wheeled vehicle and, as such, tire wearand cutting is a problem. Like the hydraulic excavator, impact and abrasion are the major degradationmechanisms of the ground-engaging shovel teeth on the bucket. Scratching abrasion on the side frameand wear plates on the bottom of the shovel also occurs.

35.3.2 Large-scale Earthmoving

This section examines the earthmoving equipment used for mining applications. Mining operationsrequire the continuous removal of earth, that is, the overburden, in order to expose the valuable mineralcomponent. As such, massive equipment is needed to dig up and haul away the overburden and ore. Toremove the large quantities of earth and rock in surface mines, the following types of equipment areused: draglines, stripping shovels, giant hydraulic excavators, giant dump trucks, and bucket wheelexcavators.

35.3.2.1 Dragline

Draglines are huge machines constructed to sit above ground at the mine site and are used to removeoverburden and mineral at a high rate of speed. These large machines operate continuously from thetime they are built until they are decommissioned, and they are considered to be a “life-of-mine asset.”That is, the machine is designed for the specific mine site and mineral to be mined, and it is expectedthat the unit will work until all the mineral is removed from that site. (Properly maintained, a draglineis capable of operating for 30 to 40 years.) A typical machine may weigh in at 7.7 million kg, have asmany as 40 motors operating, and generate up to 37,250 kW. The buckets range in size from 7.6 m3 tomore than 115 m3, with the largest one built to date capable of holding 170 m3 of earth and rock. Aseparate electricity source is needed to supply energy to the units, and two men are needed to operateit. A typical mining dragline bucket can hold and move 115 m3 of earth and rock (180,000 kg) 100 m in90 seconds. The dragline operates at a fixed location in the mine until all mineral is removed that iswithin reach of the bucket. It then “walks” to the next mining site and begins all over again. (Walkingdraglines move on large metal pontoons at a very slow rate of speed.)

In draglines, most of the heavy abrasive wear occurs to the bucket teeth and on the wear plates on thebottom and sides of the bucket (Figure 35.4). Other areas of wear include the scratching abrasion of thedragline chains, and the severe metal-to-metal wear (again complicated by abrasion) on the dragline pinsand clevises. Another area where a combination of metal-to- metal wear, scratching abrasion, frettingfatigue, and corrosion occurs is in the wire ropes used to maneuver the dragline bucket.

35.3.2.2 Stripping Shovel

Stripping shovels are very large tracked machines with a boom/dipper assembly. However, these machinescan weigh as much as 1.8 million kg, with dippers (i.e., buckets) capable of holding as much as 70 m3

of earth and rock. They are electrically powered, with the dipper/boom assembly mounted on two tracks,each of which is 13.7 m long and 3 m wide. Most stripping shovels are matched to current dump trucksize, and with a 72,500-kg bucket, they can fill a 220,000-kg dump truck in three passes in less than90 seconds. They are less expensive to use than draglines and will last almost as long with proper careand maintenance.

In the stripping shovel, heavy abrasive wear occurs to the teeth on the dipper and on the wear plateson the bottom and sides. Metal-to-metal wear, fretting fatigue, and corrosion also occur in the wire ropesthat maneuver the dipper/boom assembly.

35.3.2.3 Giant Hydraulic Excavator

Giant hydraulic excavators are much like their smaller counterparts. They are an attractive alternative todraglines and stripping shovels because of their relatively low cost, fast speed, and high productivity.They are usually used in mines for removing earth and minerals. The big advantage that the gianthydraulic excavators have over draglines and stripping shovels is their hydraulic system. Draglines andstripping shovels rely on gears to maneuver the dipper/boom or bucket, thereby creating friction withsubsequent losses in efficiency. Hydraulics are a more efficient way of lifting heavy objects, although thegiant hydraulic excavators are smaller and cannot lift as heavy a load as the draglines and strippingshovels. They are, however, very much faster and more maneuverable than their gear-driven counterparts,and are also less costly to operate. A drawback to the giant hydraulic excavator is that its lifetime is muchshorter than that of either the dragline or stripping shovel, typically 40,000 hours of operation if caredfor well. The Caterpillar 5230 weighs in at 300,000 kg, carries a 17-m3 bucket, and has a 15-m reach.Larger units exist, such as the Liebherr R996 Litronic, a 2235-kW, 545,000-kg unit with a 33.5-m3 bucket.

Like the dragline bucket and the stripping shovel dipper, impact and abrasion are the major degradationmechanisms of the ground-engaging shovel teeth on the bucket. Scratching abrasion on the side frameand wear plates on the bottom of the shovel also occur.

35.3.2.4 Giant Dump Truck

The typical dump truck for mining operations now has a 218,000-kg carrying capacity. It has six tires,each of which is 3.7 m in diameter and weighs 3600 kg. These trucks are assembled at the mining site(as they are not allowed on the public roads because of their size). The role of the giant dump truck issimple: carry the overburden and mined ore to the appropriate spot as quickly as possible. The size ofthese vehicles is governed by both mining efficiency (get as much material out of the pit as fast as possible;these trucks can reach speeds of 55 kph) and the need to conform to environmental regulations (i.e.,leave the land in roughly the same condition as it existed prior to mining). Larger trucks are also available;for example, the 280,000-kg Komatsu 930E.

FIGURE 35.4 View of a dragline bucket showing digger teeth, dragline chains, and wear plates on the sides andbottom of the bucket. (From Stoody (1998), The Rebuilding and Hard-facing of Earth-Moving Equipment, Bulletin8203, Industry, CA. With permission.)

The major cost associated with wear in the giant dump trucks involves the wear and tear to the tires.Being as large and heavy as they are, and given the environment they operate in, tires are a major costitem as far as wear is concerned. Impact damage from the loading of earth and rock from the variousmachines described above and scratching abrasion contribute to wear of the bed of the dump truck.

35.3.2.5 Bucket Wheel Excavator

The concept of a bucket wheel excavator is simple: a large circular wheel studded with buckets spinsaround scooping piles of dirt, coal, or some other material and dumps it onto a conveyor system fortransport to another site. Most often seen in mining operations (especially in Germany), these machinesare also used in digging canals and for projects requiring the removal of massive quantities of dirt. Theyare not especially effective where the ground is hard or where there are large rocks or ledges. Basically,the bucket wheel excavator consists of the following components: (1) the undercarriage and crawlers thathold up and move the machine; (2) the superstructure that holds the booms, conveyor systems, andhoisting winch; (3) the booms that hold the bucket wheel, the counterweight to the working end of themachine, and the conveyor system that removes the debris; (4) the conveyor belts that remove theexcavated material; and (5) the bucket wheel that does the actual digging. The largest bucket wheelexcavators have buckets of between 6 and 8.5 m3 apiece, with anywhere from 10 to 24 buckets on thewheel. Usually, the harder the material being dug, the greater the number of buckets on the wheel. Thegiant Krupp Bucket Wheel Excavator 288 weighs 13.2 million kg, is over 185 m long, and can remove190,000 m3 of soil per day with its 18-bucket wheel. If properly maintained, a bucket wheel excavatorwill be operational for 40 years.

The buckets on the bucket wheel excavator will see the same type of scratching abrasive wear as theaforementioned machines, except that the severity will be much less, given the nature of the environmentin which the equipment operates. Wear and fretting fatigue to the wire ropes that control the movementof the bucket wheel boom and associated structures will also occur.

35.4 Equipment Used in Mining and Minerals Processing

To the mineral processing operations schematically illustrated in Figure 35.1 can be added other majormining operations that contribute to wear of components: namely, digging, drilling, loading, and hauling.Abrasion within these operations range from severe (high impact, large abrasive size) in digging andcrushing operations, to relatively mild (low force levels, small abrasive sizes) in classification and sepa-ration processes. The presence of aqueous or other liquid environments of variable pH can cause signif-icant corrosion in grinding and subsequent conveyance and processing operations. Table 35.3 summarizesthe types of equipment used in these mining and mineral processing operations and identifies the majorwear components of each.

TABLE 35.3 Mining and Minerals Processing Equipment

Operation Equipment Wear Components

Mining

Crushing

ScreeningConveying/handlingGrinding

PumpingClassificationSeparation

Shovels, draglines, front-end loaders, trucks, scrapers

Jaw, cone, and gyratory crushersImpact crushersGrizzlies, screens, trommelsConveyors, chutes, pipes, laundersSemi-autogenous mills, rod mills, ball mills

Slurry pumpsCyclones, spiralsFlotation cells, magnetic separators

Buckets, teeth, adaptorsTrays, bladesJaws, mantles, concaves, bowels, liners,

hammers, blow barsScreen decks, underpans, discharge lipsLiners, impact curtains, bendsBalls, rods, liners, grates, feed chutes, discharge

trommelsImpellers, volute and frame plate linersHood, vortex finders, bodies, spigotsAgitators, drums

From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry, Vol. 1, Australian MineralsIndustries Research Association Limited, Melbourne, Australia.

An ore processing plant takes the mined ore and reduces it in size so that the valuable mineralcomponent (metal sulfides and iron oxides, for example) can be separated from the gangue. In manyinstances, the size of the incoming ore can be as great as 2 m in diameter (Norman, 1980; Gill, 1991).Therefore, the primary ore must be crushed and ground into very fine particles. The gangue in manycases is either silica or silicates, which are relatively hard and abrasive compared to metallic alloys (Moore,1981; Imhoff et al., 1985; Mutton, 1988). Separation of the valuable mineral from the gangue is usuallydone in a water slurry, by flotation, magnetic separation, or gravity (Gill, 1991).

Each of the mining and minerals processing stages involves two or more wear modes. Selection of themost suitable wear material for the components used in each operation is made on the basis of the mostsevere one, minimizing the risk of catastrophic failure (Norman, 1980).

The abrasive materials (ore, mineral, or waste material) to be mined and treated can be characterizedto some extent by their abrasivity, and the energy required for comminution in crushing and grindingoperations (Bond, 1952, 1964). These two parameters are in turn dependent on abrasive hardness andshape, and its compressive strength and fracture properties. Table 35.4 shows some of the characteristicsfor a range of typical ores and minerals.

Some of the minerals handled in processing operations may be relatively benign in terms of theirabrasivity, in which case they present no substantial wear problems. However, the untreated mineral maycontain a small proportion of hard, abrasive material that is removed during processing. This is partic-ularly true for coal, which sometimes contains coarse particles of harder minerals such as quartz (Norman,1980). The presence of the quartz, or another hard mineral, gives rise to significant abrasive wear in theprocessing equipment, which may not have otherwise occurred.

In other cases, the required product may constitute only a small volume fraction of the total materialhandled, in which case the abrasive characteristics of the waste mineral are of major importance in theselection of wear materials. An example of this is diamond mining, in which relatively large tonnages ofrock are processed to yield very small volumes of the desired product. Under these conditions, theefficiency of the classification and separation equipment is very important (Imhoff et al., 1985).

Unit operations in ore processing are described in the next sections. A short description of theequipment and its general operation are provided, with the major causes of wear highlighted.

35.4.1 Crushers

The primary crusher in ore processing is usually a gyratory or jaw type, which in large operations iscapable of accepting rocks up to 2 m in diameter. The crushing forces have to be intense so that theelastic limit of the material being crushed is exceeded. Crushers tend to be massive and rugged (althoughsometimes portable), requiring large drive motors. They are energy intensive and expensive, both toconstruct and to operate (Gill, 1991). The rock is crushed to a maximum size of about 0.20 to 0.25 min diameter at this stage. From this point, the ore passes on to a second or a third stage of crushing in

TABLE 35.4 Properties of Typical Ores and Minerals

Ore/Mineral TypeDensity(g/cm3)

Vickers Hardness

(GPa)

Compressive Strength(MPa)

Abrasion Index

Bond Work Index

BauxiteCoalLimestoneHeavy sulfides (lead/zinc)Copper ore (chalcopyrite)HematiteGraniteQuartz

4.9–5.21.2–2.0

2.37.4–7.64.1–4.3

5.3—2.7

1.5–4.11.5–2.51.5–2.5

—3.4–4.04.6–6.45.0–7.8

7.8

—5–40

130–20060–100

—180–250100–300140–650

——

0.030.130.150.170.390.78

—12121112

91717

From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry,Vol. 1, Australian Minerals Industries Research Association Limited, Melbourne, Australia.

either gyratory, cone, or roll crushers. Intermediate screening occurs at each stage to separate out thelarger rock chunks for further crushing. The final product of the ore crushing plant is usually all 10 to20 mm in diameter (i.e., particles of this size will pass through a screen with these openings), with a highpercentage of the product finer than this. This product then passes on to the ore grinding operation(Norman, 1980).

The gyratory, jaw, cone, and roll crushers crush rock by applying high compressive forces to each rock.Another type of crusher accomplishes similar results using impact hammers or blow bars mounted ona rotor, thereby producing high kinetic energy impacts on each rock at velocities of around 30 m/s(Norman, 1980). These impact crushers are capable of producing a high ratio of size reduction in onestage of crushing. However, due to the high velocities of the hammer, the wear rate is very high. Impacthammers are, therefore, typically used to crush softer ores such as coal, limestone, and cement plant feed(Gill, 1991).

35.4.2 Grinding Equipment

Grinding of the crushed ore is almost always performed in cylindrical rotating mills, which are filledabout half full of steel or iron balls, steel rods, or some other form of grinding medium. The grindingmedium might also take the form of ceramic balls, flint pebbles, or for autogenous grinding, the fractionof ore between about 50 and 200 mm in diameter (Gill, 1991). In semi-autogenous grinding, steel ballsabout 125 mm in diameter are charged into the mill to occupy approximately 8 to 10% of the mill volumeand supplement the grinding produced by the large pieces of ore in the mill. The interior chambers ofall these mills are lined with easily replaceable liners whose thicknesses range from 50 to about 250 mmwhen installed. Wear of these liners, as well as the wear of the grinding balls and rods, is a major itemof expense in most ore processing operations (Dougall, 1974; Norman, 1974, 1980; Gill, 1991).

The crushed ore fed into the grinding mills is almost always mixed with water so that the grinding isdone in a slurry containing about 75% solids. The ore is ground to at least 0.3 to 0.7 mm in diameter,although some operations require the ore to be ground to finer particle sizes (e.g., 0.04 mm). The groundore is then passed through the classifiers, from which incompletely ground ore is returned to the grindingmills, while the rest is passed on to the separators.

Primary grinding can also be performed in two stages. The first stage might use a rod mill, with rods75 to 100 mm in diameter, which reduces the ore from 15 to 25 mm down to about 1.5 mm (Gill, 1991).After this grinding operation, the ore goes to a ball mill where it is reduced to about 0.3 mm in diameteror finer. Autogenous or semi-autogenous grinding may in many instances be followed by a second stageof grinding in a ball or pebble mill (Norman, 1980).

35.4.3 Sand Pumps

Sand pumps are used to move slurries from grinding mills to classifiers or concentrating equipment, andto dispose of tailings. Rapid wear occurs to the impellers and case linings of the pumps, so molded rubberimpellers and linings are used wherever it is feasible to do so (Norman, 1980). However, when the slurriescontain coarse sand or sharp-cornered stones, or when the impingement velocities of the sand particlesare high, the rubber is cut and worn quickly. In these instances, a high chromium martensitic white ironcan be selected (Mutton, 1988).

35.4.4 Classifiers

Classification can be described as separating mixtures of fine particles into two or more products,depending on the variations of size, shape, and specific gravity, by allowing these particles to settle in afluid. Water is the usual medium, although air is used as well (Gill, 1991). Original classifiers were gravityfed and, as a result, wear in this process is minimal. However, classification using gravity is also slow. Tospeed up the operation, cyclone (fluid medium is air) and hydrocyclone (fluid medium is water) classifiershave been developed. Abrasive forces in these cyclones are more severe, but in most cases rubber or

elastomeric linings give satisfactory service. The most severe wear occurs to the lining of the coarse sanddischarge orifice. Here, hard ceramics or martensitic white irons can be used in place of the elastomers(Norman, 1980; Mutton, 1988).

35.4.5 Flotation Machines

The flotation process is used extensively to separate valuable minerals (e.g., metal sulfides) from siliceousgangue in the ground ore. These machines typically use an agitator-type impeller, plus induced air, pluschemical reagents, to produce a froth that selectively collects and “floats” the desired mineral. The agitatorimpellers are subject to wear, but it is usually relatively mild. Molded rubber or elastomers are used forthese impellers and they usually last for years of continuous operation before they need to be replaced(Norman, 1980; Mutton, 1988).

35.4.6 Magnetic Separators

In practically all high-tonnage operations, magnetic separators operate in a sand slurry. Separationdepends on the attraction of the paramagnetic minerals, such as magnetite, to the surface of a rotatingdrum in a localized magnetic field (Gill, 1991). The drums are made from non-magnetic grades of sheetsteel, such as austenitic stainless steel or austenitic manganese steel. They suffer from mild scratchingabrasion during the process, but have a relatively long life. Much of the abrasion damage results fromthe removal of a continually reforming oxide film; in this case, use of a high chromium stainless steelreduces wear because it forms an adherent, relatively abrasion-resistant chromium oxide coating on thesurface (Norman, 1980).

35.5 General Classification of Abrasive Wear

The dominant abrasive wear conditions that exist in earthmoving, mining, and minerals processingequipment can be described using the following broad classifications (Avery, 1961; Norman, 1980;Mutton, 1988):

• Gouging abrasion

• High-stress, or grinding, abrasion

• Low-stress, or scratching, abrasion

• Erosion-corrosion

Gouging abrasion occurs under conditions where abrasive particles indent and move over the wearsurfaces under high stress levels. It involves both cutting and tearing types of wear, in which small chipsof metal are removed from the wearing surface by the movement of the sharp points of rock, underconsiderable pressure, over the wearing surface. This type of action is very similar to machining by acutting tool. In gouging abrasion, the wearing surface is plastically deformed and work-hardened by theabrasive forces, so that cutting and tearing of metal occurs on the work-hardened surface. Typicaloperations that involve gouging abrasion include crushing and primary grinding operations. The wearliners of crushing units are particularly susceptible to gouging abrasion. This type of wear also occurs tothe liners of large grinding mills, particularly large autogenous and semi-autogenous mills where largechunks of ore (up to 250 mm in diameter) are to be broken up by the tumbling action. Gouging abrasionalso occurs in impact crushers or almost anywhere where coarse rocks impact a metal surface underconsiderable pressure or force (e.g., digger teeth on power shovels and dragline buckets used to scoopup loose rock).

Gouging abrasion normally occurs in the crushing and handling of large chunks of rock. It is accom-panied by heavy impact and by high bending and compressive forces on the wearing parts, which aremade as heavy section castings. As a result, the choice of ferrous alloys that can be used with confidencein these applications is limited. Traditionally, austenitic 12% manganese steels are used as crusher liners.

They have fairly good resistance to gouging abrasion, combined with good toughness and the ability tobe heat-treated in heavy sections (Norman, 1980). For other applications involving gouging abrasion,such as autogenous mill liners, impactor bars in impact crushers, and earthmoving tools, the manganesesteels have been partially displaced by low-alloy quench and tempered steels and martensitic white irons(Mutton, 1988).

High-stress, or grinding, abrasion occurs when abrasive particles are compressed between two solidsurfaces, as for example, between grinding rods or balls. The high-stress abrasion that occurs in grindingmills takes place over a very small contact region, where the ore particles are caught between the grindingballs, or between grinding balls and the mill liner. The high contact pressure produces indentations andscratching of the wearing surfaces, and fractures and pulverizes the abrasive ore particles. Hard mineralssuch as quartz will indent and scratch martensitic steels having yield strengths of 2100 MPa (Norman,1980). High-stress abrasion is sometimes referred to as three-body abrasion, although two-body, high-stress conditions can sometimes exist. High-stress abrasion implies that the abrasive particle is fracturedand broken apart during the wear process. How these small abrasive particles affect the actual removalof material in mineral processing is not well-understood. It has been speculated that high-stress grindingabrasion produces wear by a combination of cutting, plastic deformation, surface fracture on a micro-scopic scale, as well as by tearing and fatigue, or spalling (Norman, 1980).

In ore processing plants, high-stress abrasion produces practically all of the wear on grinding ballsand liners in ball mill grinding units. In rod mills that accept larger chunks of ore, during the first stageof grinding, the wear of the grinding rods and liners proceeds by both gouging abrasion and high-stressgrinding abrasion. In autogenous grinding mills, charged with ore from 250 mm to 0.2 mm in diameter,both gouging abrasion and grinding abrasion occur.

In high-stress grinding abrasion, the microstructure of the balls and liners influences the wear rate.In unalloyed or low-chromium white iron balls, an Fe3C-type carbide (Vickers hardness, Hv, of 7.8 to9.8 GPa) structure coexists in a relatively soft ferrite or pearlite (Hv of about 1.0 to 2.9 GPa) matrix. Thisresults in a composite Hv for the alloy of between 4.9 and 5.9 GPa. However, despite the relatively highhardness of the composite structure, the balls tend to wear at a rate equivalent to that of the ferrite orpearlite matrix. The abrasive forces that occur during grinding are sufficient to fracture and crumble thecarbides, which are poorly supported by the ferrite or pearlite matrix. In a matrix that contains martensite,however, the carbides do not fracture as readily, and consequently, better abrasion resistance is obtained.

Low-stress, or scratching, abrasion occurs when lightly loaded abrasive particles impinge upon, andmove across, the wear surface, cutting and plowing it on a microscopic scale. In aqueous or other liquidenvironments, corrosion may also contribute to the overall wear rate, in which case erosion-corrosionis the operative wear mechanism. In both cases, low-stress abrasion is the primary mode of wear. Thewear rates in terms of metal thickness removed per day are quite low in low-stress abrasion, so a significantportion of the total wear is probably due to the abrasion of a continually reforming oxide film. This maybe especially true in the handling of particulates in a wet environment, such as slurries.

Low-stress scratching abrasion in ore processing machinery occurs primarily in the pumping of sandslurries, in size classifying equipment such as cyclones and gravity classifiers, in chute liners, screens andflotation impellers, and in hydraulic and pneumatic conveying operations. Generally, the impingementangles and particle impact forces are so low that hard and brittle constituents of the microstructure arenot fractured or micro-spalled by the abrasive forces. Under these circumstances, ferrous alloys containinghard carbides have good to excellent wear resistance. Ceramics and synthetic stone (e.g., fused silicates)are also used for these applications. Molded rubbers and polyurethanes can also work well in low-stressabrasive conditions, particularly in pump and flotation impellers, linings for cyclone classifiers, and inpipes and screen decks in some screening applications (Mutton, 1988).

Table 35.5 summarizes each of these classifications in terms of the abrasive size, the nature of contact,and the environmental factors. Figure 35.5 shows in schematic form the four classifications. All of thesemechanisms are similar in that a hard particle moves across the component surface in some manner.Particle type, shape, size, and hardness ultimately determine the type and severity of abrasive wear. Thenext section discusses in more detail the various factors that influence abrasive wear in materials.

35.5.1 Abrasive Wear

The individual factors that influence wear behavior are shown in Table 35.6. For both the abrasive andwear material, the majority of factors that affect wear behavior are related to their respective mechanicalproperties. Also of importance is the mechanical aspect of the abrasive/wear material interaction. Chemical

TABLE 35.5 General Classification of Abrasive Wear in Mining and Minerals Processing Equipment

ClassificationGouging Abrasion High-stress Abrasion Low-stress Abrasion Erosion-Corrosion

Abrasive size Large Medium Small FineContact Conditions

ImpactForceVelocityImpingement angle

HighHighLowLow

LowHighLowLow

LowLowVariableMedium-low

LowModerateHighVariable

Environmental Generally dry Generally as slurry, variable pH

Variable Generally as slurry, variable pH

Principal mechanisms Plowing, cutting

Cutting, some fragmentation corrosion

Cutting, some corrosion Cutting, plowing, fragmentation, and corrosion

From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry, Vol. 1, Australian MineralsIndustries Research Association Limited, Melbourne, Australia.

FIGURE 35.5 Schematic representation of the four abrasive wear classifications. (From Mutton, P.J. (1988), AbrasionResistant Materials for the Australian Minerals Industry, Vol. 1, Australian Minerals Industries Research AssociationLimited, Melbourne, Australia.)

GOUGING

LARGE PARTICLES,EXTREMELY HIGH STRESS,IMPACT,ABRASIVE FRACTURES.

EXAMPLES: JAW AND GYRATORY CRUSHERS.

EXAMPLES: SMALLER BALL AND ROD MILLS.

HIGH STRESS GRINDING

SMALLER PARTICLES,

HIGH STRESS,

SLIDING RATHER THAN IMPACT,

ABRASIVE FRACTURES.

LOW STRESS

SMALL PARTICLES,LOW STRESS,ABRASIVE DOES NOT FRACTURE.

EXAMPLES : SCREENS, CHUTES, TRAYS.

EROSION - CORROSION

FINE PARTICLES IN AIRBORNE ORSLURRY FORM: ABRASIVE DOESNOT FRACTURE.EXAMPLES : PUMPS, CYCLONES.

processes, however, are also important (i.e., corrosion or oxidation) because they directly influence therate of wear of a material in the environment of interest.

The influence of the parameters listed in Table 35.6 can be explained by their effect on the mechanismby which material is removed from a worn surface. The simplest model of abrasive wear is one in whichrigidly supported hard particles indent and are forced across the surface of the wear material. Dependingon the properties of the abrasive and wear materials, one of several wear mechanisms (Figure 35.6) canoccur (Zum Gahr, 1987; Mutton, 1988):

1. Plowing occurs when material is displaced to the side, away from the wear particles, resulting inthe formation of grooves that do not involve direct material removal. The displaced material formsridges adjacent to grooves, which can be removed by subsequent passage of abrasive particles.

2. Cutting occurs when material is separated from the surface in the form of primary debris, ormicrochips, with little or no material displaced to the sides of the grooves. This mechanism closelyresembles conventional machining.

3. Fragmentation occurs when material is separated by a cutting process and the indenting abrasivecauses localized fracture of the wear material. These cracks then freely propagate locally aroundthe wear groove, resulting in additional material removal by spalling.

The plowing and cutting mechanisms involve predominately plastic deformation of the wear material,while the third mechanism also involves fracture. Thus, the dominant mechanisms that occur for aparticular operating condition are influenced to a great extent by the plastic deformation and fracturebehavior of the wear material. Materials that exhibit high fracture resistance and ductility with relativelylow yield strength are more likely to be abraded by plowing. Conversely, materials with high yield strengthand low ductility and fracture resistance abrade through fragmentation (Moore, 1981; Zum Gahr, 1987;Mutton, 1988).

Additional wear mechanisms can operate in materials that exhibit a duplex microstructure or that arecomposed of two or more component phases (e.g., a composite type material like a high-chromiumwhite iron or a composite drill bit such as diamond or tungsten carbide inserts in a steel matrix), theindividual components of which vary in their mechanical properties. Under some abrasive wear condi-tions, removal of the softer phase (usually the matrix) can occur by one or more of the above mechanisms.This process then leaves the harder phase unsupported, in which case it may either become detachedfrom the wear surface by a pull-out mechanism, or be more susceptible to wear by fragmentation. Inaddition, the presence of an interface between the various components of the composite may promotecracking and fragmentation of the harder phase, particularly under impact-abrasion conditions.

The rate of material wear for any of the above processes is influenced by the extent of indentation ofthe wear material surface by the abrasive particle. This depth of indentation, for a given load, will be afunction of the hardness of the wear material and the shape of the abrasive particle. Angular particles

TABLE 35.6 Factors that Influence Abrasive Wear Behavior

Abrasive Properties Contact Conditions Wear Material Properties

Particle sizeParticle shapeHardnessYield strengthFracture propertiesConcentration

Force/impact levelVelocityImpact/impingement angleSliding/rollingTemperatureWet/drypH

HardnessYield strengthElastic modulusDuctilityToughnessWork-hardening characteristicsFracture toughnessMicrostructureCorrosion resistance

From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian MineralsIndustry, Vol. 1, Australian Minerals Industries Research Association Limited, Mel-bourne, Australia.

will indent the wear surface to a greater extent than rounded particles, leading to higher wear rates. Inaddition, angular particles are more efficient in cutting and machining (Moore, 1981).

The hardness of the wear material, or more specifically the hardness of the worn surface, is an importantparameter in determining a material’s resistance to abrasion. An increase in the surface hardness of thewear material reduces the depth of penetration by the abrasive particle, leading to lower wear rates.However, an increase in a material’s hardness is also accompanied by an attendant reduction in itsductility, resulting in a change in the abrasion mechanism, for example, from predominantly plowingand/or cutting to fragmentation (Murray et al., 1979).

For mechanisms involving predominantly plastic deformation (i.e., plowing and cutting), the wearrate can be expressed through the following parameters (Moore, 1981; Mutton, 1988): the probability ofwear debris formation; the proportion of plowing, and cutting processes; the abrasive particle shape andsize; the applied stress; and the hardness of the wear surface.

For brittle materials (e.g., ceramics), a transition from a purely cutting mechanism to one that alsoinvolves fragmentation occurs when the nature of contact changes from elastic-plastic indentation toHertzian fracture (Moore, 1981). The conditions under which this transition occurs are dependent onthe size and shape of the abrasive particles, the applied stress, and the hardness of the wear surface. Inaddition, the fracture resistance of the wear material, as measured by the fracture toughness, is alsoimportant. Decreasing the hardness of the wear material, and increasing the fracture toughness, increases

FIGURE 35.6 Microscopic mechanisms of material removal between abrasive particles and the surfaces of materials.(From Zum Gahr, K.H. (1987), Microstructure and Wear of Materials, Elsevier Science, Amsterdam, The Netherlands.With permission.)

Microcutting

Microcracking

Microploughing

the critical abrasive size at which the transition to fragmentation occurs. Thus, reducing the hardness ofbrittle materials, or alternatively increasing their fracture toughness, leads to lower wear rates (Zum Gahr,1987; Mutton, 1988).

Abrasive wear mechanisms involving plastic deformation, cutting, and fragmentation occur predom-inantly in materials with relatively high elastic modulus, that is, metals, ceramics, and rigid polymers.As the elastic modulus decreases, the nature of the abrasive/wear material contact changes, with localizedelastic deformation becoming more significant. The probability of wear occurring by plastic deformationmechanisms decreases, such that for elastomers, cutting mechanisms can occur only with contact againstsharp abrasive particles. For contact against blunt abrasive particles, the two main wear mechanisms aretensile tearing and fatigue.

For the abrasive wear of polymeric materials, the following material parameters are important (Mutton,1988): elastic modulus and resilience, friction coefficient, tensile strength and tear resistance, elongationat break, and hardness. The wear behavior of elastomers is particularly sensitive to abrasive impingementangle, as this influences the dominant modes of deformation, and hence, the wear mechanisms. At lowimpingement angles, tensile tearing occurs and the material’s tear resistance is important. At impingementangles close to 90°, the behavior of the elastomer is essentially elastic, and resilience is a major factor indetermining wear resistance. For abrasive wear conditions in which significant energy levels are dissipatedin the abrasive/wear material contact, elastomeric linings are usually designed such that impingementangles are as close to 90° as possible.

35.5.1.1 Abrasive Characteristics

Abrasive particle size has a significant effect on material wear, with the greatest effect being for non-metals (i.e., ceramics and polymers). In non-metals, the effect of particle size is associated with changesin the predominant mechanism of material removal. Ceramics undergo a transition to fragmentationabove a critical abrasive particle size, whereas elastomers undergo a transition from elastic behavior toeither tearing or fatigue.

In metals, the effect of abrasive particle size is minimal for particle sizes greater than 100 µm. Belowthis size, the wear rate decreases rapidly with decreasing particle size (Moore, 1981). This particle sizeeffect is usually attributed to the nature of the abrasive/wear material contact, with decreasing size favoringelastic rather than plastic contact.

Abrasive hardness, or the ratio of the hardness (Vickers) of the wear material to the hardness (Vickers)of the abrasive (H/Ha), is a critical parameter in abrasive wear. It is well-known that the abrasive wearrate decreases as the hardness of the worn surface approaches that of the abrasive (Richardson, 1968).When the hardness of the worn material exceeds that of the abrasive, the wear rate decreases rapidly.Figure 35.7 shows this particular effect for metals and ceramics.

This effect of H/Ha on abrasive wear results from a change in the nature of the contact mechanics. AtH/Ha ratios ≈ 0.6 to 0.8, the contact conditions give rise to extensive plastic deformation. At higher H/Ha

ratios, the nature of the contact becomes essentially elastic (Moore, 1981; Mutton, 1988; Hutchings,1992). As a result, wear rates decrease unless material is removed by mechanisms other than cutting andplowing (e.g., fragmentation).

As the hardness of the worn surface approaches that of the abrasive, plastic flow of the abrasive mayoccur, leading to a reduction in the abrasive particle’s cutting ability. In addition, it is possible to fracturethe abrasive when the plastic zone in the abrasive particle reaches a critical size (Richardson, 1968). Thiseffect for the abrasive particle is analogous to the transition from purely cutting to fragmentation in theabrasion of brittle materials.

The effect of H/Ha on wear behavior is also influenced by the size and compressive strength of theabrasive particles. Coarse abrasives are more likely to fracture than fine abrasives, partly due to a decreasein tensile strength with increasing particle size. In addition, fracture of the abrasive can regenerate sharpfacets and produce loose abrasive fragments, which in turn increase wear rates. Loading conditions alsohave an effect, because increasing the contact stress between abrasive and wear material increases theprobability of fracture of the abrasive particles.

TE

(A

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35.5.1.2 Contact Conditions

It is difficult to assess the effects of individual contact conditions on the wear interactions between abrasiveand wear material because their effect is synergistic in nature. Force, impact level, velocity, and impinge-ment angle combine to influence the wear rate of the material.

Increasing the contact stress between the abrasive and the wear surface results in greater indentationdepths and an increased tendency for fracture and fragmentation in both brittle wear materials andabrasives (Richardson, 1968). This generally leads to increased wear rates, although exceptions can occurif the abrasivity of particular minerals decreases with failure of the abrasive particle. The effect of nominalcontact stress on the relative abrasion rating of a range of metallic materials is shown in Figure 35.8.

The general effect of increasing nominal contact stress levels is to increase wear rates. However, theoccurrence of significant impact during the abrasive/wear material contact can also accelerate the rate

FIGURE 35.7 Effect of abrasive hardness on wear behavior of metals and ceramics. (From Moore, M.A. (1981),Abrasive wear, in Fundamentals of Friction and Wear of Materials, Rigney, D.A. (Ed.), ASM, Metals Park, OH, 73-118.With permission.)

FIGURE 35.8 Effect of nominal contact stress on relative abrasion rating of metallic wear materials. (From Mutton,P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry, Vol. 1, Australian Minerals IndustriesResearch Association Limited, Melbourne, Australia.)

10-2

103

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CrC HARDFACING

GOUGING

800-1000HV

ANGULAR

ABRASIONTYPE

ABRASIVEHARDNESS

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CrC HARDFACING

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of material removal. This acceleration results from the increased amount of kinetic energy dissipatedduring contact. This energy can arise either from the moving abrasive particles, or from the moving wearsurfaces, as in the case of impact crushers. For brittle materials, increased fragmentation occurs underimpact conditions, whereas elastomers may only suffer an increase in cutting and tearing modes, asso-ciated with insufficient elastic recovery.

The nominal force level can also determine the level of constraint experienced by abrasive particles.Under low-stress abrasive conditions, the abrasive particles can be free to rotate as they move across thesurface of the wear material, and are less likely to indent and scratch the surface. The tendency for theabrasive particle to rotate also depends on the abrasive particle shape, with angular particles being morelikely to slide rather than roll. Increasing the nominal force acts to constrain the abrasive particle in itsorientation to the wear surface, thereby increasing the wear rate.

The effect of velocity on wear behavior is associated with the dissipation of kinetic energy duringabrasive/wear material contact. For a large number of abrasive wear environments, the velocity of abrasiveparticles is relatively low (<10 m/s), and therefore of little importance. However, in slurry pumps (i.e.,the transport of abrasive particles in slurry or pneumatic form), the effect of velocity is significant. Underthese conditions, the wear rate is proportional to velocity (W ∝ Vn, where W is the wear rate, V is thevelocity of the abrasive, and n is a constant for the abrasive/wear material synergism). The value of nfalls in the range of 2 to 3, although the exact value for any particular condition is dependent on theproperties of both the abrasive and wear material, and on the angle of impingement (Mutton, 1988;Hutchings, 1992; Kosel, 1992). For softer abrasives, n tends to increase with decreasing abrasive particlesize. For brittle materials and high impingement angles, the value of n tends toward the higher extreme.

In erosion, a change in abrasive impact velocity can lead to a change in the dominant wear mechanism(Lansdown and Price, 1986), resulting in a change in wear rate. Higher values of n are associated withincreased cutting and fragmentation. Figure 35.9 depicts the effect of erosive particle velocity on the wearof a range of materials.

The influence of impingement angle on wear depends on the properties of the wear material, and isassociated with changes to the dominant wear mechanism. At high impingement angles (60 to 90°),brittle materials typically experience elevated wear rates, resulting from increased fragmentation andspalling. Conversely, elastomers are more effective under these conditions because much of the impactenergy can be dissipated through elastic deformation.

At low impingement angles (10 to 30°), elastomers cut and tear more readily, leading to increases inwear rates. Hard, brittle materials usually perform better under these conditions. For materials interme-diate in their mechanical properties (e.g., some metals), the effect of impingement angle depends on theductility of the material (Lansdown and Price, 1986). The effect of impingement angle on the erosivewear behavior of a number of materials is shown schematically in Figure 35.10.

FIGURE 35.9 Effect of impact velocity on erosive wear. (From Lansdown, A.R. and Price, A.L. (1986), Materials toResist Wear, A Guide to their Selection and Use, Pergamon Press, Sydney, Australia. With permission.)

CERAMICS

WHITE CAST IRONS

RUBBER

TOOL OR BEARING STEEL

MILD STEEL

IMPACT VELOCITYIN

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35.5.1.3 Properties of the Wear Material

The properties of the wearing material that influence wear behavior are grouped into the followingcategories: mechanical properties, microstructure effects, and other properties (corrosion resistance,friction, thermal effects). Because abrasive wear is primarily a mechanical process, particularly in theabsence of corrosive environments, mechanical properties are of major importance, whereas the role ofmicrostructure depends upon the severity of the wear environment. The following discussion is patternedafter the approach of Moore (1981) and Mutton (1988).

35.5.1.3.1 Mechanical PropertiesThe resistance to indentation (hardness) is an important variable in determining abrasion resistance.Laboratory wear tests indicate that the abrasive wear resistance for particular material types increaseswith increasing hardness (Figure 35.11). However, large differences in abrasion resistance can also occurat similar hardness levels. These differences arise from variations in the plastic flow characteristics of thevarious materials, which in turn influence the predominant wear mechanism (Zum Gahr, 1987). Thegeneral relationships between abrasion resistance and hardness for plowing and fragmentation are similarto those shown in Figure 35.12 for pure metals and ceramics.

In the absence of fragmentation, plowing and cutting involves plastic flow of the wear material eitherin front of, or to the sides of, the indenting abrasive particles. The plastic flow behavior of the materialcan be characterized by the ratio E/σy, where E is the elastic modulus and σy is the flow stress of thenon-work-hardened material.

FIGURE 35.10 Effect of impingement angle on erosive wear. (From Lansdown, A.R. and Price, A.L. (1986), Mate-rials to Resist Wear, A Guide to their Selection and Use, Pergamon Press, Sydney, Australia. With permission.)

FIGURE 35.11 Abrasion resistance vs. hardness for various material types in high-stress abrasion tests. (From ZumGahr, K.H. (1987), Microstructure and Wear of Materials, Elsevier Science, Amsterdam, The Netherlands. Withpermission.)

WHITE CAST IRON

CERAMICS

TOOL OR BEARING STEEL

MILD STEEL

RUBBER

IMPACT ANGLE00 300 600 900

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Pin Abrasion Test, Silicon Carbide

fcc

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Met

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AB

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IncreasingCarbon Content

Heat TreatedSteels

Cold Worked Metals

Precipitation Hardening

Martensite +Retained Austenite

High Cr-MoWhite Cast

Iron

Austenitic Martensitic

OO

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Decreasing E/σy favors a cutting mechanism, in which a high proportion of material is removed asmicrochips. Increasing the hardness of the wear material will have a similar effect, as shown inFigure 35.12.

Although Figure 35.12 indicates that changing E/σy can significantly affect the dominant wear mech-anism, its impact on wear resistance is not as clearly established. For a constant value of E, a decrease inσy favors plowing, which should lead to lower material wear rates. However, decreasing σy also decreaseshardness, which favors greater depths of indentation and a reduction in the wear resistance of the material.For materials of equivalent hardness, the general trend is for wear resistance to increase with higher E/σy

values (Moore, 1981).The wear material properties discussed thus far (i.e., hardness, elastic modulus, and flow strength)

relate to material behavior at relatively low strains. For example, the plastic strain produced by indentationhardness testing of metals is typically between 8 and 10%. During the abrasive wear process, the extentof plastic strain experienced by a worn surface may approach these values. For these cases, the behaviorof the wear material at high plastic strains is important, and here high values of the work-hardeningcoefficient and ductility are favorable.

Tensile strength and ductility are also important mechanical property parameters in a wear environ-ment, especially for polymers, which undergo elongations-to-fracture approaching 500%. Laboratorywear studies indicate that the wear resistance of both rigid and ductile polymers is roughly proportionalto the work of rupture, which is defined as the product of the rupture stress and elongation-to-fracture.

The abrasion resistance of polymers also increases with increasing indentation resistance, althoughthis relationship is not as well-defined for polymers as it is for metals and ceramics. This is due in partto the visco-elastic nature of polymer deformation behavior. In addition, the absolute hardness levels forpolymers are much lower than those for metals and ceramics.

For elastomeric materials such as natural rubbers and polyurethanes, the deformation behavior duringabrasive wear may be entirely elastic, in which case resilience or hysteresis loss is important. High resiliencefavors increased abrasion resistance, while increasing hysteresis losses under high impact levels results inmaterial breakdown, exacerbated by heat buildup.

For materials that undergo wear by fragmentation, abrasion resistance varies with fracture toughness.However, the contribution from microcracking and fragmentation depends on the severity of the wearenvironment, in particular the applied load and abrasive particle size.

For metals and ceramics, the general form of the relationship between wear resistance, hardness, andfracture toughness (Kc) is illustrated in Figure 35.13 (Zum Gahr, 1978). For low fracture toughnessmaterials (Kc < 14 MPa m1/2), wear resistance increases with increasing fracture toughness, despite a

FIGURE 35.12 Influence of hardness and E/σy on dominant wear mechanism. (From Mutton, P.J. (1988), AbrasionResistant Materials for the Australian Minerals Industry, Vol. 1, Australian Minerals Industries Research AssociationLimited, Melbourne, Australia.)

HARDNESS

E/σy

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marked decrease in hardness. This increase in wear resistance results from a reduction in the fragmen-tation contribution to the total wear rate. For materials with high fracture toughness, wear occurs bycutting and plowing only, and hence the wear rate is controlled by the indentation resistance. In thiscase, the wear resistance decreases with decreasing hardness.

The transition from fracture (fragmentation) to plastic deformation (cutting and plowing) is depen-dent on the critical groove size (pcrit) and the contact stress. These parameters, in turn, also depend onthe ratio of fracture toughness to hardness (Kc /H). From Figure 35.13, maximum abrasion resistance isobtained by optimizing the ratio Kc /H, while maintaining the indentation hardness or groove size slightlybelow that at which the transition to fragmentation occurs (Moore, 1981).

Increasing the severity of the wear environment, by increasing either the applied load, or the size,hardness, or angularity of the abrasive particles, shifts the transition from fragmentation to cutting/plow-ing to higher values of fracture toughness (Figure 35.13). This results in a decrease in the maximum wearresistance.

35.5.1.3.2 MicrostructureMicrostructural effects on abrasive wear depend on the overall magnitude or scale of the wear environ-ment. For high impact loads, large abrasive particle size, etc., the size of the microstructural features isgenerally much smaller than that of the abrasive wear damage. In this case, the role of microstructureon wear resistance is limited to its effect on bulk mechanical properties. Abrasive wear rates, particularlyfor materials that exhibit homogeneous microstructures, may correlate with bulk hardness (Moore, 1981).Figure 35.14 shows the effect of microstructure and hardness on the high-stress abrasion resistance of

FIGURE 35.13 Schematic relationship between wear resistance, hardness, and fracture toughness. (From Zum Gahr,K.H. (1987), Microstructure and Wear of Materials, Elsevier Science, Amsterdam, The Netherlands. With permission.)

FIGURE 35.14 Effect of microstructure and hardness on the abrasion resistance of steels: high-stress abrasion,alumina abrasive. (From Moore, M.A. (1981), Abrasive wear, in Fundamentals of Friction and Wear of Materials,Rigney, D.A. (Ed.), ASM, Metals Park, OH, 73-118. With permission.)

WE

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WEAR RESISTANCE

CUTTING/ PLOUGHING

INCREASING APPLIED LOAD, ABRASIVE SIZE, HARDNESS AND ANGULARITY

FRAGMENT

HARDNESS

BAINITIC STEELS(AUSTEMPERED)

AUSTENITICALLOYS

PEARLITIC STEELSCOLD WORK

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steels. Over a limited hardness range (200 to 300 Vickers or 2.0 to 3.0 GPa), relative abrasion rate decreasesin the order (Moore, 1981):

austenite > bainite > pearlite > martensite

This effect can be attributed to the influence of microstructure on plastic deformation behavior. However,bainitic and martensitic microstructures offer greater abrasion resistance at higher hardness levels(>400 Vickers or 4.0 GPa).

At lower loads, and with smaller abrasive particle sizes, microstructural features are more effective asdiscrete components, and the mechanical properties of individual phases assume increased importance.This occurs when the size of the microstructural features is approximately equal to, or larger than, thatof the abrasive. Under these conditions, materials with duplex microstructures (e.g., alloy white ironsand some ceramics) are more sensitive to microstructural effects. In such materials, size, spacing, andvolume fraction of the harder phase, as well as the mechanical properties of both hard and soft (matrix)phases, can significantly affect wear behavior. Abrasion resistance can either increase, decrease, or remainthe same by increasing the volume fraction of the harder phase. The net effect depends on the variouscontributions of the plowing, cutting, fragmentation, and pull-out mechanisms to the total wear rate.Figure 35.15 shows schematically this behavior for alloy white irons and alloy carbide materials. Thetransition from increasing to decreasing abrasion resistance with increasing carbide volume fraction isassociated with the relative contributions of fragmentation of the carbides, and plowing/cutting of thematrix, to the total wear. Carbide hardness and size relative to that of the wear damage will also influencethese trends (Zum Gahr and Doane, 1980; Moore, 1981).

35.5.1.3.3 Other PropertiesWear processes that involve plastic deformation (i.e., cutting and plowing) also generate significantincreases in temperature at the wearing surface. These increases occur as a result of frictional energydissipation at the interface between the wearing surfaces. Under these circumstances, the wear behavioris influenced by characteristics such as the friction coefficient and thermal conductivity of the two wearbodies. In materials that exhibit low values of thermal conductivity, one of two additional effects canarise, depending on other material properties. Localized thermal expansion in ceramics can result inadditional material loss by spalling; conversely, localized temperature increases in polymers can give riseto melting. Materials that exhibit low friction coefficients under dry sliding conditions (e.g., somepolymers and finely ground ceramics) will suffer less degradation through frictional temperature riseeffects.

FIGURE 35.15 Influence of microstructure on abrasive wear behavior of alloy white irons and chromium carbidematerials. (From Mutton, P.J. (1988), Abrasion Resistant Materials for the Australian Minerals Industry, Vol. 1, Aus-tralian Minerals Industries Research Association Limited, Melbourne, Australia.)

FRAGMENTATIONPLOUGHING/CUTTING

INCREASING CARBIDEHARDNESS

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CARBIDE VOLUME FRACTION (%)0 20 40 60

35.6 Tribological Losses in the Mining of Metallic Ores, Coal, and Non-metallic Minerals

Mining operations can be broadly classified into the exploration, extraction, and beneficiation of min-erals. Differences in the equipment and technology used in mining depend on the mineral’s proximityto the surface, the characteristics of mineral formation, and the intended downstream use of the product.For example, the proximity of the mineral to the surface will determine whether the mineral-bearing oreis extracted by surface mining, shaft mining, or drilling. Seam dimension and orientation will affect theselection of the “digging” equipment. Beneficiation operations will depend on the mineral concentrationin the ore body, the surrounding gangue material, and the product form required in downstreamoperations.

Mineral extraction includes those processes related to digging, loading, and transporting the ore.Digging machinery is needed to access and remove the mineral-bearing ore from its environment.Draglines, power shovels, and bucket-wheel excavators are used in surface mining operations. Removingespecially hard ore bodies requires substantial drilling and blasting. The loose ore body is then transportedto either intermediate storage facilities or beneficiation facilities by trucks, conveyors, or railcars, depend-ing on the terrain and transportation distance. Shaft mining requires specialized compact equipment,which is usually operated continuously. In these operations, continuously rotating teethed surfaces cutaway material from the ore-bearing deposit, with the loose ore then transported by conveyor to anintermediate storage facility.

Beneficiation includes those activities that increase the concentration of the desired mineral or improveits form. Typical beneficiating activities include grinding, screening, washing, and flotation. Gravitationaland magnetic separation are also used to separate materials. Some ores can be leached with acids toremove the desired mineral if physical separation techniques are impractical.

The two major methods of extracting ores, minerals, and coal are surface mining and shaft mining.The equipment used in these two processes are quite different and distinct; however, five common processsteps are utilized in both methods: exposing, digging, loading, transporting, and ore processing.

The information on tribological losses in the following sections derives from the ECUT report byImhoff et al. (1985) entitled “A Review of Tribological Sinks in Six Major Industries.”

35.6.1 Surface Mining

Surface mining operations are typically in the form of open pit or strip mines, where the valuable mineralores must first be exposed through the removal of the overburden. Substantial drilling and blasting areneeded to loosen the ore from the surrounding rock strata. Power shovels and draglines are used to scoopup the loose ore for loading and transporting to the ore processing facility, which is typically in closeproximity to the mining operations. Surface mining operations are huge, covering tens to hundreds ofsquare kilometers. During the lifetime of an open pit mine, tens of thousands of tonnes of ore are extractedfrom the earth and processed for the valuable mineral component. The next few sections discuss thevarious operations used in surface mining.

35.6.1.1 Exposing and Digging

The surface mining of most materials requires that the overburden be removed to reveal the ore body.The overburden may be as slight as a few feet of loose soil and rock, or as troublesome as solid veins ofrock. In surface mining operations, overburden removal is typically accomplished using either draglines,power shovels, or bucket-wheel excavators. The operating principles of these three techniques are verydifferent; however, their tribological losses are very similar. These tribological losses are also common toother forms of digging and excavation equipment. The components that cause the greatest tribologicallosses (i.e., wear, friction, and reduced work capacity) for draglines, power shovels, or bucket-wheelexcavators are: use of digger lubricant; wear of digger/bucket teeth, bucket liners, and wear plates; carry-back (i.e., material stuck in the buckets of the diggers after dumping); and wear and fatigue of wire rope.

Table 35.7 presents data on the direct (i.e., energy losses due to friction) and indirect (i.e., material lossdue to wear processes) tribological losses in surface mining operations. Tribological losses due to friction(1.5 × 109 kW-h) during digging and exposing are approximately four times higher than material lossesdue to wear (0.4 × 109 kW-h).

Lubricant losses arise from the need to keep the machinery in good operating condition. The commonlubricants used in draglines are multipurpose grease, open-gear grease, wire rope lubricant, gear case oil,and walking cam lubricant. The amount of lubricant consumed in typical draglines is shown in Table 35.8.

Loss of bucket teeth and wear liners from abrasion is routine in surface mining operations. The life ofbucket teeth and wear liners is strongly dependent on the type of overburden and ore being excavated.Table 35.9 gives an estimate of typical blade life under various digging conditions. In general, blade life variesfrom a minimum of 15 minutes for moving blasted rock, hardpan, or other hard, blocky material, to amaximum of 6 to 9 months for moving loose, soft, free-running materials (i.e., soil and small gravel). It isestimated that 6350 kg of replacement steel for bucket teeth and liners are used per bucket every 6 months.

Carry-back leads to the largest tribological energy losses during exposing operations, resulting fromthe combined effects of lifting useless weight and reduced bucket capacity. The extra useless weightconsumes energy above that needed to move the empty bucket, while the reduced carrying capacity ofthe bucket means additional bucket passes will be needed to remove a fixed amount of material. Thedegree of carry-back is influenced by the type and moisture content of the material removed.

Wire ropes used in diggers for hoisting typically fail from fatigue or fretting fatigue. The drag ropesused in draglines suffer from abrasive wear and corrosion.

35.6.1.2 Loading

At some point in the mining operation, the material must be handled by some type of loader having abucket capacity in the range of 7 to 15 m3. Loaders simply load material that has been piled or exposedby one of the large diggers, into a truck or onto a conveyor system. Tribological losses in loading

TABLE 35.7 Direct and Indirect Tribological Energy Losses for Digging and Exposing in Surface Mining Operations

Wear PartDirect Losses

(108 kW-h/year)Indirect Losses

(108 kW-h/year)

Digger lubricantBucket teeth and linersCarry-backWire rope

—1.10

13.50Small

0.850.75—

20.50

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A.,Erickson, R., Merriman, T., Gruber, T., and Barber, S. (1985), AReview of Tribological Sinks in Six Major Industries, TechnicalReport PNL-5535, Pacific Northwest Laboratory, Richland, WA.

TABLE 35.8 Yearly Lubrication Used in Draglines (103 liters)

Lubrication Type

Bucket Capacity

23 m3 115 m3

Multipurpose greaseOpen-gear greaseWire rope lubricantGear case oilWalking cam lubricant (on machines > 35 m3)

7.63.41.12.5—

11.46.62.3

13.212.3

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A.,Erickson, R., Merriman, T., Gruber, T., and Barber, S. (1985), AReview of Tribological Sinks in Six Major Industries, TechnicalReport PNL-5535, Pacific Northwest Laboratory, Richland, WA.

equipment are a consequence of abrasion, and are directly related to the abrasive nature of the materialbeing moved. This wear affects the carriages, tires or treads, and buckets and blades. The tribologicallosses are very similar to those previously discussed for digging operations. The same life expectanciesand observations are applicable (see Table 35.9) to a loader’s teeth inserts, blade inserts for scrapers anddozers, ripper blades that break up the soil, and wear plates for buckets.

35.6.1.3 Transporting

Transporting activities in mining and minerals processing can be divided into three main categories: trucks,conveyors, and railcars. Trucks are the predominant means of transporting large quantities of materials (70%)to intermediate storage facilities or mineral processing operations. They are a very flexible transportationsystem, capable of climbing the steep ramps found in surface mining operations. However, trucks requiregood roads to minimize tire wear and subsequent replacement, and are limited by economic considerationsto an operating radius of approximately 5 km. Tire wear represents approximately 30% of the total operatingcosts of a truck. Tire life is influenced by such factors as tire inflation, speed, curves in the load and dumpcycle, road surface, total load, wheel position on the truck, and road grade. In general, depending on thecapacity of the truck and the road conditions, the lifetime of a tire is typically between 3000 and 6000 hr.

Belt conveyors are economical for high-volume, long-distance hauling of medium-sized particulatematter, and are advantageous for haulage distances over 3 km. These conveyors can handle grades up to40%. Unlike trucks, conveyors typically only handle small chunks of material. On the other hand,hydraulic systems can be used to move slurries of sand, gravel, kaolin (clay), and phosphates. Hydraulicconveying may involve the pumping of slurry mixtures cross-country in a pipeline. This method is thedominant mode of transfer of small particulates; however, the total volume conveyed is small whencompared to the total amount of material conveyed by conventional methods. Tribological losses inconveyor systems are caused by many sources, such as friction in the pulleys, friction in the belt ridingover the pulleys, and skirtboard friction from the conveyed material rubbing against the sides of the

TABLE 35.9 Blade Life Under Various Digging Conditions

Ease of Digging

Easy Medium Hard Rock

General material description

Loose, soft, free-running materials

Close lying, which will fill dipper of bucket to capacity and frequently provide heaped load

Overload compensates for swell of materials

Harder materials that do not require blasting, but break up with bulkiness, causing voids in dipper or bucket

Materials requiring some breaking up by light blasting or shaking. More bulky and somewhat hard to penetrate, causing voids in dipper or bucket

Blasted rock, hardpan, and other bulky materials, which cause considerable voids in dipper or bucket and are difficult to penetrate

Specific material names

Dry sand or small gravelMoist sand or small gravelLoamLoose earthSandy clayLoose clay gravelCinders or ashesBituminous coalVery well-blasted material

Clay, wet or dryCoarse gravelClay gravel, packedPacked earthAnthracite coal

Well-broken limestone, sand rock, and other blasted rocks

Blasted shaleOre formation (not of

rock character) requiring some blasting

Heavy, wet, sticky clayGravel with large

bouldersHeavy, wet gumbo

cemented gravel

Hard tough shaleLimestoneTrap rockGraniteSandstoneTaconiteConglomerateCaliche rockAny of these blasted to

large pieces mixed with fines and dirt

Tough, rubber clay that shaves from bank

Blade life 6 to 9 months 15 minutes

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T., Gruber, T., and Barber, S.(1985), A Review of Tribological Sinks in Six Major Industries, Technical Report PNL-5535, Pacific Northwest Laboratory,Richland, WA.

conveyor. The direct tribological frictional losses have been estimated to be 200 kW-h per 2000 tonnesper 1000 horizontal meters conveyed. This number is calculated based on the assumption that 20% ofall surface mined material is transported an average distance of 5200 m.

Railcars are similar to conveyors in that they handle large volumes of material over long distances.Unlike conveyors, they can handle coarse, blocky materials. However, railcars have a high capital costand are limited to grades of 3% when fully loaded. The transportation of large quantities of materialusing railcars is declining today for several reasons. Primarily, the processing of the ore is done near themining operation, negating the need for railcars to convey material long distances. Additionally, trucksand belt conveyors are more economically feasible and require less overall initial capital investment.

35.6.2 Shaft Mining and Drilling

Shaft mining differs from surface mining in that there is no need to remove large amounts of overburdento reach the mineral ore body, and there is no need to load the material to convey it. In shaft miningoperations, the deposit of coal, ore, or mineral is removed by a continuous mining system. Generally,the continuous miner has an assembly that rotates against the mine wall surface, removing chunks ofmaterial with its cutter teeth. The material is then simultaneously gathered, partially crushed, and fedonto an attached conveyor. The conveyor carries the material to either a dumping station or to the surface.The tribological losses for continuous mining systems are summarized in Table 35.10. Typically, about360 million tonnes of ore are mined by this method each year (270 million tonnes of coal, 65 milliontonnes of copper, 18 million tonnes of potash, and 4.5 million tonnes of iron ore).

As seen in Table 35.10, primary material losses occur through lubricants and cutter teeth replacement.Direct tribological losses occur through frictional sinks in the conveyor system. Many types of hydraulicequipment are used in shaft mining. Roof bolters, roof support jacks, drills, loaders, and continuousminers all use hydraulics. Much of the lubricant loss occurs through the improper selection and main-tenance of lubricants. Lubricant contamination by dust is a major cause of wear in hydraulic machinery,bearings, and gearing in the mining industry, because a contaminated fluid carries particles that acceleratewear through abrasion that the lubricant was originally intended to combat. In some mines, the lubricantis deliberately contaminated with water to reduce fire hazards, thereby increasing the wear in hydraulicsystems. In addition to the wear caused by intentionally and unintentionally contaminated lubricants,the loss of lubricant through seal leakage is also great. In long-wall cutter operations, lubricant leakagemay reach 19,000 L/day.

Tribological losses from the cutter system occur in the cutter teeth, chassis bottom, propulsion treads,and conveyors. Of these, the cutter teeth and conveyor experience the most significant wear. For example,a cutter bit will have to be replaced for every 5 tonnes of ore processed, when rock is encountered. Whencutting pure coal, potash, soda ash, or salt, one cutter bit will need to be replaced for every 20 tonnes ofmaterial processed. In making these types of calculations, the same conditions and assumptions discussedfor surface mining also hold for shaft mining, except the average distance that the mineral is conveyedin shaft mining is 3650 m.

Drilling is another route by which minerals are extracted from the earth. The major tribological lossesfrom drilling operations occur with the mud pump and mud pump liners, mud lubricant/coolant, drill

TABLE 35.10 Tribological Losses in Shaft Mining Operations

Direct Tribological Losses(kW-h/year)

Indirect Tribological Losses(kW-h/year)

All types of lubricants — 1.1 × 109

Cutter drum parts (primarily teeth inserts) Small 2.1 × 109

Conveyors 7.0 × 107 —

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T., Gruber, T.,and Barber, S. (1985), A Review of Tribological Sinks in Six Major Industries, Technical Report PNL-5535,Pacific Northwest Laboratory, Richland, WA.

pipe, and cutting bits. The tribological losses for drilling components are shown in Table 35.11. Mostmaterial loss in drilling operations occurs with the drill string assembly and the cutter bits.

The drill pipe that makes up the drill string is attacked by a variety of mechanisms, both tribologicaland otherwise, which result in indirect loss of drill pipe components and eventually the entire drill pipe.The major causes and sites of tribological attack of the drill pipe are the following: uniform abrasivewear of the outside surface as it rubs against hard formations during drilling, and localized (improperlysealed threads and damaged joints) and uniform (outside surface) erosion due to the high-pressure flowof drilling mud. Although filtered, the drilling mud retains fine particles of rock from the drillingoperation, which increases the severity of the abrasive action. These tribological actions serve to reducethe load-carrying capacity of the pipe by reducing overall wall thickness and preferentially weakeningthreaded, joined sections.

Causes of attack by nontribological mechanisms include localized galvanic corrosion and metal fatigue.Galvanic corrosion occurs in dents and scratches along the outside pipe surface, which is accelerated bythe wear processes described above (and very similar to slurry erosion-corrosion observed in piping).Drilling lubricant (mud) often contains corrosion inhibitors, but the pipe may rust while standing in air.Metal fatigue of the drill string occurs as it is rotated and twisted within the narrow and crooked confinesof the hole. Fatigue failures often occur at the stress risers caused by any of the aforementioned mechanisms.

Drill bits are divided into two main categories: rolling cutter bits and drag, or shear, bits. The basicdesign difference is that the rolling cutter bits generally have three cones supported by bearings, whereasthe shear bits are rigidly fixed to the drill string (Anderson, 1974; Coffman and Conners, 1974; Perrott,1979). Currently, rolling cutter bits account for 90% of the market (Imhoff et al., 1985).

Rolling cutter bits consist of three alloy steel cones, each having rows of cutting teeth. The teeth canbe composed of a solid hard material or a diamond or carbide insert, or can be studded with wear-resistant inserts. The rollers are supported with both radial and thrust bearings. Rolling cutter bitstypically fail because of seal failures, which lead to bearing failures, wear of the inserts and supportingmaterial, and occasionally, fracture of a cone or erosion of the steel that supports the insert. In as manyas 80% of the cases, bits fail in the bearing seals, allowing the drilling mud to abrasively destroy thebearing. The bearing seals are weakened by extreme pressure, high temperatures from geothermal andfrictional heating, chemical attack from corrosive deposits drilled, or simply abrasive attack by the drillingmud. Bits that do not fail from the above processes typically wear out from three-body abrasion.

Shear bits in current use consist of polycrystalline (man-made) diamonds mounted in tungsten carbideinserts that rest in a ferrous alloy body. The main cause of failure of these bits derives from three-body wear.The ferrous alloy surrounding the cutting diamond-tungsten carbide insert is worn away, allowing theinserts to be lost. Under extreme mud pressure and velocities, the surrounding metal can also erode.However, the primary wear mechanism is the extreme abrasive conditions that exist at the bottom of the hole.

TABLE 35.11 Tribological Losses for Drilling Components

Component Name Embodied Materials

Indirect Tribological Loss(106 kW-h/year)

Direct Tribological Loss(106 kW-h/year)

Mud pumpLinerPistonMud

Drill pipeDrill bits

70 kg of chromium cast iron per liner1.1 kg of rubber per pistonAsphalt and diesel fuel (oil-based muds onlya)250 kg of alloy steel or 150 kg of aluminum per drill pipeInserts of diamond and/or tungsten carbide in 20 kg alloy steel

baseb

5.52.0

1376.576.018.0

———

126.041.0

a Approximately 20% of wells drilled in U.S. use oil-based mud.b From increased torque or time requirement due to worn bit.From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T., Gruber, T., and Barber, S.

(1985), A Review of Tribological Sinks in Six Major Industries, Technical Report PNL-5535, Pacific Northwest Laboratory,Richland, WA.

The condition of the drill bit directly affects the direct energy (friction) losses in the drill string. If abit is worn, the drilling rig operator can either allow reduced penetration rates or raise the drill stringload and resulting torque to maintain a given drilling rate. In either case, energy is wasted due to theworn bit. In the first instance, if the bit penetration rate is reduced 50%, the drill string will have to turntwice as long, and all the energy consumed during the longer drilling time is the direct result of reducedbit performance. If the drilling load and torque are raised, the additional torque requirement representsa direct energy loss. Consequently, the rate of penetration and the drilling torque are two of the drillingparameters monitored and used to determine when the drill string should be pulled to change bits.

35.6.3 Ore Processing and Related Activities

The processing of the ore through grinding (e.g., milling, pulverizing, etc.) materials results in significantdirect and indirect tribological losses. Although the phrase “ore processing” can be used to describe theprocessing of a large range of materials, this discussion focuses on the processing of metal ores becauseof the large tonnages involved and the typically high abrasiveness of metal ores.

Ore processing components that experience significant tribological losses include the following unitoperations: crushers, grinding mills, slurry pumps, and cyclone separators. Of these processing compo-nents, grinding mills represent the largest tribological sink. Table 35.12 shows some typical wear ratesfor the unit operations listed above during the processing of high-silica, low-grade copper, molybdenum,and taconite iron ores. For other industries (e.g., cement or coal processing), where the abrasive mineralis much softer (except pyrite in coal), the wear rates will be much lower. Typically, impact hammersexperience the highest wear rates, due in large part to the tremendous energy of the impacting blows.Impact hammers are not usually used on hard and abrasive minerals; this is the province of crushers.

Operators of ore processing plants are concerned with wear in terms of the weight loss per tonne ofore processed, especially in crushing and grinding operations. In addition, the energy consumption pertonne of ore processed is also a major concern, especially in the design and construction of new processingplants. Over the years, it has become apparent that in any specific crushing and grinding operation, there

TABLE 35.12 Typical Wear Rates Normal to Wear Surface in Ore Processing

Ore Processing ApplicationWear Ratea

(µm/hr)

Austenitic manganese steel in gouging abrasionImpact pulverizer hammers 100–25,000Crusher liners for gyratory and coned crushers 50–500Lifter bars in semi-autogenous millsb 20–100Chute liners and screens for coarse ores 2–250

Low-alloy, high-carbon steel in high-stress grinding abrasionRod and ball mill liners (wet grinding) 10–30Grinding balls in ball mills (wet grinding) 4–20Large grinding balls in semi-autogenous millsc 15–50

White iron in low-stress scratching abrasiond

Sand pump impellers 2–50Agitator and flotation impellers 1–10Spiral classifier wear shoes 1–5

a Wear rate is in terms of linear loss of material from the wear surface as a functionof time.

b Lifter bars mostly medium-carbon, low-alloy steel.c Rough estimate due to insufficient data.d Both pearlitic and martensitic white irons included.Norman, T.E. (1980), Wear in ore processing machinery, in Wear Control Hand-

book, Peterson, M.B. and Winer, W.O. (Eds.), ASME, New York, 1009-1051. Withpermission.

is a direct relationship between the amount of wear of the materials in that operation and the energyconsumed by that operation. Therefore, by controlling the amount of wear in the various unit operationsof an ore processing plant, the energy consumed can also be controlled. Table 35.13 provides someinformation on the wear that occurs per tonne of siliceous ore processed for some unit process operations.It can be seen from the wear losses in Table 35.13 that grinding media suffer the highest wear rates. Themill liners see much less wear per tonne of ore processed, unless the operation is performed in anautogenous or semi-autogenous mill. In this case, the mill liners experience higher wear than mill linersused in ball or rod mills.

In the copper industry, one third of the total energy expended is used to concentrate the ores fromtypical values of 0.5% copper (as mined). Concentration is accomplished by crushing and grinding themined ores into powder (85% passing –0.15 mm mesh) from which the gangue (rock and uselessminerals) and metal are separated. This process requires about 6.4 × 103 kW-h per tonne of copperproduced. Of this energy consumption, the following percentages are lost to friction in running themachinery: about 0.25% is lost in the mill support bearings; 1.5% is lost in the main gears; and up to0.75% is lost in additional reducer gears when they are required. The total bearing friction losses inmilling various metals were calculated assuming a total tribological loss of 1.75% (i.e., ignoring reducergear losses). Table 35.14 lists the direct tribological energy losses during grinding that occurred in 1973for the copper, aluminum, and iron industries. Approximately 4.4 × 108 kW-h were lost to friction.

Table 35.15 shows the approximate wear rates for key ore processing steps. Wear of ore grindingequipment represents the largest indirect tribological loss in ore processing. This loss comprises an average

TABLE 35.13 Wear Rates per Tonne in Crushing and Grinding of Siliceous Ores

Ore Processing Application

Wear(grams per metric ton)

Minimum Maximum Average

Liners in crushing taconite iron oresLiners in crushing Cu/Mo oresRods in two-stage grinding of taconite iron oresRods in two-stage grinding of Cu/Mo oresBalls in two-stage grinding of taconite iron oresBalls in two-stage grinding of Cu/Mo oresBalls in single-stage grinding of Cu/Mo oresBalls in crushing and grinding of taconite ores in semi-autogenous millsBalls in crushing and grinding of Cu/Mo ores in semi-autogenous millsMill liners in two-stage grinding of taconite oresMill liners in two-stage grinding of Cu/Mo oresMill liners in single-stage grinding of Cu/Mo oresMill liners in crushing and grinding; autogenous and semi-autogenous mills

4510

280136200

86520840700

35444078

11855

500730844600760

10701500

777375

450

7419

420420401440630950990

505351

214

Norman, T.E. (1980), Wear in ore processing machinery, in Wear Control Handbook, Peterson, M.B. and Winer,W.O. (Eds.), ASME, New York, 1009-1051. With permission.

TABLE 35.14 Direct Tribological Energy Losses in Grinding

Industry YearGrinding Energy

(1010 kW-h)Direct Tribological Portion of Grinding Energy

(Bearing Lossesa – 107 kW-h)

Copper 1973 1.15 20.20Aluminum 1973 0.10 0.90Iron 1973 1.20 21.10

a Bearing losses in grinding equal 0.75% × grinding energy.From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman,

T., Gruber, T., and Barber, S. (1985), A Review of Tribological Sinks in Six Major Industries,Technical Report PNL-5535, Pacific Northwest Laboratory, Richland, WA.

of 0.7 kg of iron alloy (grinding balls, rods, and liners) per tonne of ore or mineral processed. Alternatively,in milling coal for power plants, only about 2 g of steel is worn per tonne of coal produced. This differencein material consumption is a direct reflection on the abrasivity of the mineral being processed. Becausethis wear represents an embodied energy consumption of 9.3 × 103 kW-h per tonne of ore processed,the indirect tribological energy loss (i.e., wear) for ore grinding is estimated to be 7 kW-h per tonne ofore or minerals ground per year. Based on production figures for materials that are typically ground(iron ore, copper ore, clay, phosphate, and potash), the total tribological loss associated with wear fromgrinding is 6 × 109 kW-h per year.

Crushing constitutes the second greatest source of wear in mineral processing, although the magnitudeof loss is only about 7% as large as that for ore grinding per tonne. However, more minerals need to becrushed than ground. The total tribological loss associated with crushing is estimated to be 1.2 × 109 kW-hper year.

The remaining areas where tribology influences ore processing are slurry pumps, screens, and cycloneseparators. Slurry pump impellers and cases are subject to erosion and low-stress abrasion, and theyrequire fairly frequent replacement. Generally, backup pumps are included to allow the system to continueoperating during pump maintenance. Although wear of slurry pump components is significant to plantoperations, it does not constitute nearly as much embodied energy as does wear of grinding mill media.

Screens and cyclone separators are used to separate various size ranges of ores that are to be returnedto the mill for further grinding. Although they do experience wear, the overall resultant material loss isnot a major concern.

35.7 Financial Cost of Wear in Earthmoving, Mining, and Minerals Processing

The overall cost of material degradation to the industrialized nations of the world has been the subjectof a number of studies during the past three decades, and is included in reports to the Department ofEducation and Science, United Kingdom (DES, 1966); NMAB (1981) and ECUT (Imhoff et al., 1985),United States. The NMAB study (1981) estimated the overall annual costs of corrosion and fracture ofmaterials to be equivalent to about 4% each the country’s gross national product (GNP). The cost offriction and wear is more difficult to assess, but it is generally thought to be roughly comparable tocorrosion costs, making the total cost of materials degradation over 10% of a country’s GNP. It has beenfurther suggested that 10% of all energy generated by man is dissipated by various friction processes(Rabinowicz, 1976). Maintenance, repair, and replacement of materials, including “downtime,” areresponsible for a large portion of this cost. Design redundancies and excessive maintenance and inspection

TABLE 35.15 Wear Rates in Key Ore-Producing Steps

Process StepWear Rate

(gram metal/metric ton)

Crushing 50Screening 2Grinding 700Pumping 5Classifying 3Mineral separation 2

Data from Norman, T.E. (1980), Wear in ore processingmachinery, in Wear Control Handbook, Peterson, M.B. andWiner, W.O. (Eds.), ASME, New York, 1009-1051; Imhoff,C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson,R., Merriman, T., Gruber, T., and Barber, S. (1985), A Reviewof Tribological Sinks in Six Major Industries, Technical ReportPNL-5535, Pacific Northwest Laboratory, Richland, WA.

to minimize accidents and product liabilities also contribute to the expense. It is estimated that 20 to30% of materials degradation costs could be avoided by the use of known technologies and otherpreventive measures.

Tribological energy losses — directly through friction (Table 35.16) and indirectly through wear(Table 35.17) — in the mining and minerals processing industries are a major cost factor (with anestimated equivalency to 15 × 109 kW-h of energy), accounting for about half of the nation’s wear lossesin the industrial sector (Imhoff et al., 1985). Energy losses due to wear, resulting primarily from three-body abrasion, were five times greater than frictional losses. Wear losses are associated with the wearingout of equipment, as well as lubricant losses and drilling mud consumption. Table 35.17 highlights thegeneric tribological loss mechanisms in mining. The most significant material wear losses identified areiron and steel wear in ore processing, steel wear in shaft mining digging, and tire wear in surface mining.

TABLE 35.16 Annual Direct Tribological Energy Losses in Mining

Operational ActivityPrincipal EnergyForm Consumed Energy Loss Rate

Total Energy Loss(108 kW-h)

Surface miningExposing and diggingLoadingTransporting

ElectricityDieselElectricity

240 W-h/m3

4 W-h/m3

85 W-h/metric ton-km

14.650.20

11.15Shaft Mining

DiggingTransporting

—Electricity

—85 W-h/metric ton-km

—0.70

Ore Processing Electricity 1.75% of grinding energy 4.20Drilling

Mud pumpsDrill pipeDrill bits

—DieselDiesel

—1200 W-h/m drilled400 W-h/m drilled

—1.250.40

From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T.,Gruber, T., and Barber, S. (1985), A Review of Tribological Sinks in Six Major Industries, TechnicalReport PNL-5535, Pacific Northwest Laboratory, Richland, WA.

TABLE 35.17 Annual Indirect Tribological Energy Losses in Mining

Operational Activity Material Type Worn Material Wear RateTotal Energy Loss

(108 kW-h)

Surface miningExposing and diggingLoadingTransporting

SteelSteel“Rubber”

30,140 metric ton/year1950 metric ton/year0.02 kg/m3

3.50a

0.2035.15

Shaft miningDiggingTransporting

Steel—

0.41 kg/metric ton—

31.35a

—Ore processing Steel 0.62 kg/metric ton 66.80Drilling

Mud pumps

Drill pipe

Drill bits

Cast iron, “rubber”

Alloy steel, aluminum

Alloy steel

3.0 kg iron/1000 m0.4 kg rubber/1000 m10.9 kg steel/1000 m6.4 kg alum/1000 m102.7 kg steel/1000 m

13.80b

0.75

1.80

a Total energy loss includes lubricant loss.b Total energy loss includes drilling mud consumption.From Imhoff, C.H., Brown, D.R., Hane, G.J., Hutchinson, R.A., Erickson, R., Merriman, T.,

Gruber, T., and Barber, S. (1985), A Review of Tribological Sinks in Six Major Industries, TechnicalReport PNL-5535, Pacific Northwest Laboratory, Richland, WA.

Carbon and low-alloy steels are the most common materials subjected to wear and also contribute themost to wear losses in the industry.

Conversely, the energy needed to overcome friction between surfaces also contributes greatly to thetribological losses in these industries (approximately 3 × 109 kW-h). The most important direct energylosses are found in power shovels, draglines, and conveyors. Direct energy losses in mining are from theconsumption of electricity and/or diesel fuel. Surface mining operations account for the two largestenergy-consuming activities — exposing/digging of the ore and its transport. In this case, the largestdirect loss is from electricity consumption.

Each of these tribological sinks can be reduced through selection and utilization of the more wear-resistant materials currently available, and through the design of more energy-efficient unit processingoperations, especially grinding. Currently available materials, properly utilized for corrosion-wear appli-cations, can also decrease material losses.

35.8 Concluding Remarks

As long as there is a need to obtain minerals from the earth in order to further societal desires for value-added goods, mining and minerals processing will continue unabated. As the quality of the mineralbearing ores decreases, there will be a driving force for larger scale mining operations, necessitating newand improved wear-resistant materials and more energy-efficient equipment to expose, extract, andprocess the ore. Therefore, applied research will continue in areas of equipment design, unit operatingprocesses, wear-resistant materials, and repair and replacement techniques. Basic research will continueto be directed at understanding the wear mechanisms involved in abrasion, erosion, impaction, and wear-corrosion, particularly as they apply to the wear processes that occur in full-sized components. Only byunderstanding the relationship between wear mechanisms and operating equipment wear processes cansmall-scale laboratory wear tests be developed to simulate the requisite wear environment. Developmentof small-scale laboratory wear tests will enhance development of newer wear-resistant materials for theseoperations. Small-scale laboratory wear tests can be run in a shorter period of time and at a much reducedcost in terms of capital expenditure and labor. The effort that goes into understanding the relationshipsbetween wear mechanisms and wear processes in earthmoving, mining, and minerals processing alsobenefit other industries, because the basic tribological mechanisms are universal and are found in manydifferent industrial processes (e.g., primary metals production, agriculture, pulp and paper processing,etc.). Understanding the nature of the wear mechanisms in the tribological environment will facilitatedevelopment of new wear-resistant materials, and hopefully lead to less expensive materials that are easyto install and repair in the field.

References

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Austin, H.B. (1974), Technical developments of large wheel loaders affecting mining applications, inMaterials for the Mining Industry, Barr, R.Q. (Ed.), Climax Molybdenum Co., Greenwich, CT, 133-139.

Avery, H.S. (1947), Hard surfacing by fusion welding, Monogram on Wear, American Brake Shoe Com-pany, New York.

Avery, H.S. (1958), Wear resistance, in Engineering Approach to Surface Damage, Lipson, C. and Colwell,L.V. (Eds.), University of Michigan Press, Ann Arbor, MI, 413-454.

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