Concepts Of Boundary Friction Adhesion and Wear in Bulk Solids Handling

A.W. Roberts, M. Ooms & S.J. Wiche

Acknowledgements : The Bionic Research Institute, Chute Design Conference 1991

Wall friction, adhesion and cohesion influence the performance of bins and chutes. This paper evaluates these effects in detail.

Professor A. W .Roberts, Director TBSHRA, Director IBMHR, of the University of Newcastle in Australia is a well known authority on problems of buik solids handling. While offering a number of papers for this Conference it was felt that this paper, originally published in "Bulk Materials Handling" would be the most pertinent to the question of chute design.

SUMMARY

This paper is concerned with the evaluation of wall or boundary friction, adhesion and wear in bins and chutes of bulk solid handling plant. The relevant parameters influencing wall friction, such as surface roughness and particle size, are described and the manner in which friction, adhesion and cohesion influence performance of bins and chutes is discussed. Particular attention is focussed on the subject of abrasive wear. The concept of Relative Wear Number is introduced and the application of this parameter to indicate zones of wear in hoppers and chutes is outlined. The development of a linear type laboratory wear test apparatus is described and some typical results using this apparatus are presented.

1. INTRODUCTION

The design of gravity flow storage bins and discharge equipment such as feeders and chutes, for continuous, trouble-free operation requires consideration of a number of factors and procedures. First and foremost, it is essential to determine the flow properties of the bulk solid for the range of operation conditions expected to occur in practice. Based on these flow properties, the geometrical parameters of the bin and discharge equipment can be determined to ensure that the desired flow pattern is achieved. The flow patterns, together with the relevant flow properties, permit the calculation of bin wall loads, feeder loads, and normal pressure and rubbing velocities at the boundary walls of hoppers and chutes.

The importance of the flow pattern developed in bins and chutes should not be underestimated. Apart from the need to ensure reliable and predictable flow under all operating conditions, there is the strong objective of achieving long service life with a minimum of maintenance. The latter objective is associated with the need to ensure that wear to hopper and chute walls is minimized.

With respect to flow patterns and wear, wall or boundary friction has a significant influence. In symmetrical storage bins, wall friction coupled with the hopper geometry, is instrumental in dictating the type of flow pattern generated, whether mass-flow or funnel-flow. Wall friction also has a significant effect on the wall pressure generated, and these pressures in turn are linked with the friction and rubbing velocities at the wall to influence the magnitude of abrasive wear of the bin walls. In the case of eccentric discharge, particularly where the bulk solid flow channel intersects the wall, rapid and intense abrasive wear to the walls occurs, which means, when coupled with the impact damage during filling, leads to localized thinning of the bin wall. This latter effect generally increased the likelihood of buckling of the walls under the influence of the eccentric wall loads and associated bending moments.

Transfer chutes require lining surfaces to have low friction and good wear resistance. In addition, in view of the low pressure that normally occur at chute boundaries, the importance of avoiding adhesion and build-up of bulk solids on chute surfaces needs to be stressed. Here a knowledge of the boundary friction and adhesion characteristics of the bulk solid and chute lining is necessary. These parameters, together with the chute geometry, govern the flow pattern developed and these in turn influence the pattern of the chute wear.

The purpose of this paper is to outline the basic characteristics of wall or surface friction, adhesion and wear is related to bins and chutes. Methods of measuring these parameters are reviewed, with particular mention being made of a new, linear action wear test apparatus for evaluating the wear characteristics of lining materials. Some results using this apparatus are presented.

2. SELECTION OF LINING MATERIALS

The selection of the appropriate lining material for bins and chutes is of fundamental importance to the life and performance of the equipment. There are a great many lining materials and surface coatings on the market, some common linings being illustrated in Table 1. Also shown are the bulk materials for which the lining material is commonly used.

While cost is a significant consideration, it is mot important that the lining material is selected on the basis of service life and performance. Factors to be considered include:

• Surface friction and adhesion

• Resistance to abrasive wear

• Resistance to impact, if appropriate

• Resistance to corrosion

• Method of attachment

• Initial cost

• Installation cost and maintenance.

It is recommended that appropriate tests be conducted to determine the relevant flow properties of the bulk solid and the proposed lining surface. The parameters of importance are discussed in the following section.

3. WALL OR BOUNDARY FRICTION AND ADHESION

3.1 Interaction between Bulk Solid and Wall Adhesion

Of all the parameters influencing the gravity flow performance of bins and chutes, wall boundary surface friction has a major influence. When considering wall friction, it is important to note that it is not one factor such as the wall surface characteristic that needs to be considered. Rather wall friction is dictated by the interaction between three principal items namely the bulk solid, the surface characteristics and the environmental and loading conditions as indicted in Fig. 1.

Table 1 : Some common lining materials

Each of the three items in Fig. 1 may be subdivided into several parameters as follows :

Fig. 1: Wall friction interactions

LOADING AND ENVIRONMENTAL FACTORS

Bulk Solid Parameters

• Particle size and size distribution

• Particle shape

• Particle hardness

• Moisture content

• Particle density

• Bulk density

• Surface chemistry characteristics

• Temperature

Wall Surface Characteristics

• Roughness and roughness spectrum

• Hardness

• Chemical composition

• Temperature

Loading and Environmental Factors

• Normal pressure

• Relative rubbing or sliding velocity

• Temperature and humidity or moisture conditions

• Wall vibrations

3.2 Wall Friction, Adhesion and Cohesion

For bulk solids in contact with boundary surfaces, such as conveyor belts or chute and hopper walls, the relevant parameters of interest are the wall friction angle F, cohesion ?0 and adhesion s0 . These are defined by the Wall Yield Locus (WYL) as illustrated in Fig. 2

The WYL is obtained using a direct shear apparatus of the type illustrated in Fig. 3.

A limitation of the standard direct shear tester of Fig. 3 ;is in the inability to obtain the zero and negative normal pressures required to determine the cohesion and adhesion. The cohesion may be estimated by extrapolating the Surface Yield Locust to obtain an intersection with the shear stress axis. A more sophisticated shear test apparatus, which has been designed and manufactured at the University of Newcastle, permits zero and negative (tensile ) pressure to be applied. In this apparatus the shear cell is located beneath the sample wall or boundary material. The measurement of adhesion stress is important in relation to the determination of carry-back of bulk solids on conveyor belts. The application of a pendulum type apparatus to measure the adhesive stress has been described in Ref. [1].

Fig. 2: Yield Locus for Surface or Wall Yield Locus (WYL)

Fig. 3: Direct shear test

The wall friction angle is defined by

Φ = tan^-1 τ/σw   (1)

τ = Shear stress at wall
σw = Corresponding normal stress or pressure

As illustrated by Fig. 44 the wall friction angle decreases as the normal pressure increases.

The wall friction angle has a significant influence on the hopper half angle for mass-flow hoppers. The determination of adhesion and cohesion at boundary surfaces is directly relevant to chute design [2] and to the design selection of cleaning systems for belt conveyors [3].

Fig. 4: Characteristic surface or wall friction variation with normal friction

3.3 Wall Friction versus Normal Pressure Characteristics

As an illustration of some of the factors influencing wall or surface friction, a set of wall yield loci graphs are shown in Fig. 5 [4.5]. The bulk solid in this case is coal. Fig. 5. (a) shows the variation of wall friction for black coal at 10% moisture content on three surfaces, namely stainless steel type 304 with 2B finish, mild steel polished and mild steel rusted all at the instantaneous or zero storage time condition. In the case of the polished mild steel surface the wall friction determined after 72 hours undisturbed contact or storage time is also shown; high friction in this case is quite considerable with corrosion and adhesion or bonding of coal particles to the steel surface.

Fig. 5: Typical Wall Yield Loci.

Fig. 5 (b) compares the Wall Yield Loci for black coal at 19.7% moisture content and brown coal at 65% moisture content on two surfaces, stainless steel type 304 with 2B finish and Tivar 88, an ultra high molecular weight polyethylene material. While in absolute terms the moisture contents of the two coals are significantly different, in relative terms, taking account of their composition and saturated moisture conditions, they are comparable. For the black coal the wall friction angle for stainless steel and Tivar are similar, both exhibiting low friction. This is also the case for brown coal on the Tivar surface. However, the brown coal has abnormally high friction on the stainless steel despite the smoothness of the surface: the stainless steel is entirely unsuitable for brown coal.

3.4 Surface Roughness

3.4.1 Roughness Parameters and Effect on Wall Friction

Surface roughness is an important factor in terms of its influence on wall friction. Yet the specification of surface roughness in terms of appropriate parameters which adequately describe the surface is a complex matter requiring careful and detailed consideration.

It is common to simplify characteristics in terms of height parameters such as the center-line arithmetic average roughness (CLA or R_a -number) or RMS roughness. Such parameters are useful for comparison purposes but do not adequately provide an assessment of the interaction between particle size and surface profile. For this reason, Ooms and Roberts [4] have found it useful to specify the surface characteristics in terms of Surface Spectral Density. This displays the RMS roughness amplitude as a function of frequency where the frequency is the inverse of the roughness wave length.

To illustrate the foregoing. Fig. 6 shows the surface roughness, roughness spectra and Wall Yield Loci for black coal on stainless steel type 304-2B and rusted mild steel [2]. As is evident, the rougher surface of the mild steel allows the finer particles to become interlocked in the surface leading to an increase in friction.

3.4.2 Roughness Classified of Surfaces Used in Bulk solids handling

The new Australian Standard on Loads on Bulk Solids Containers [6], presently in preparation, designates four surface roughness characteristics D1, D2, D3 and D4 classified, respectively, as Polished, Smooth, Rough and Corrugated. These classifications follow those presented in the publication of the Institution of Engineers, Australia, "Guidelines for the Assessment of Loads on Bulk Solids Containers" [7]. In order that these surface classification may be quantified in some way. Ooms proposed that the four categories of surface be grouped in terms of roughness bands based on the Mean Center Line Roughness, or Ra -number. Surfaces commonly used in bulk solids handling have been classified according to this procedure and are presented in Fig. 7.

Fig. 6: Roughness frequency spectra of mild steel samples and corresponding wall frictions [6]

From the discussion of the previous section, it is apparent that the proposed form of classification is somewhat restrictive in terms of the limited information conveyed by the Ra -number. Also, the wall roughness may not necessarily remain constant and should be considered as a variable.

Fig. 7: Ooms' chart for roughness classification.

For instance, a polished or lightly rusted carbon steel surface may become deeply and change from group D2 to group D3. An aluminum surface is easily scoured and may change from group D1 to group D2. On the other hand, some stainless steel surface will polish during service and may change from D2 to group D1. It must be noted that the lateral characteristics of surface roughness are tow-dimensional and, during service wear due to motion of a bulk solid may be more pronounced in one lateral direction than the other.

3.5 Influence of Vibrations

Roberts et al [8, 9] have shown that the application of vibrations to a wall surface can significantly reduce wall friction and therefore promote flow. Vibrations can also reduce bulk strength, further assistance in promoting gravity flow. The evidence indicates that the best results are achieved by using frequencies of 100 Hz or higher and low amplitude.

Fig. 8: Build up on surfaces

Fig. 9: Build-up of cohensive bulk solid such as coal on screen house conveyor feed chute.

4. ADHESION OF BULK SOLIDS ON BOUNDARY SURFACES

In cases of low normal pressure as in chutes and standpipes, problems of build-up of bulk solids on boundary surfaces may occur. The relevant characteristics of the bulk solid and boundary surface are depicted in Fig. 2. In order that build-up and hence blockages can be avoided. It is necessary for the body forces generated in the bulk mass be sufficient to overcome the forces due to adhesion and shear, Fig. 8 illustrates the types of build-up that can occur.

4.1 Chutes

Cohesion and adhesion can cause serious blockages problems when corrosive bonding occurs, such as when moist coal in contact with carbon steel surfaces. The bonding action can occur after relatively short contact times. Impurities such as clay in coal can also seriously aggravate the behavior due to adhesion and cohesion.

Transfer chutes should be designed to ensure that satisfactory flow is obtained without flow blockages. Yet despite their apparent simplicity, the flow patterns developed in chutes are often not fully appreciated. Occasions have arisen in practice where costly flow interruptions have occurred due to incorrect chute design arising from a lack of understanding of the bulk solid and chute surface friction characteristics.

The design of transfer chutes is discussed in References [5, 10, 11]. Consideration should be given to the flow properties of the bulk solid and the characteristics of the chute lining material. Moist coal can adhere to vertical, as well as inclined faces of steel chutes eventually causing blockages. This has been found to occur in practice after only a few hours of operation. Problems of this type have occurred, for example, in conveyor feed chutes in coal screening operations as illustrated in Fig.9. The momentum of the coal particles falling from the screen is usually not sufficient to cause scouring of the chute surfaces and, as a result, build-up and complete blockages have been known to occur.

When determining chute slope angles, account must be taken of the variation of friction angle with change in consolidation pressure, or more particularly, with change in bed depth. Fig. 10 shows, for a typical coal, the variation of wall friction angle with bed depth. As indicated, high friction angles can occur at low bed depths, the decrease in friction angle being significant as the bed depth increases

Fig. 10: Wall friction angle versus bad depth for black coal on stainless steel

The slope of the chute should be at least 5 larger than angle of equivalent friction. That is

θmin = tan^-1 [ tanΦ ( 1 + KvHo/B)] + 5°  (2)

Where

Φ = wall friction angle corresponding to Ho
Ho = bed depth
B = chute width
Kv = ratio lateral to normal pressure

Kv will depend on bulk solid properties
Normally Kv = 0.5 to 1.0. In the absence of information Kv may be taken to be 0.8.

Often moist bulk solids will adhere initially to a chute surface, but as the bed depth increases, the correspondence decrease in friction angle will cause flow to be initiated. In some cases flow commences with a block-like motion of the bulk solid as depicted in Fig. 11.

4.2 Adhesion in Vertical Chutes or Standpipes

Bulk solids, such as coal with high clay contents and at high moisture contents may adhere to walls of vertical pipes or chutes leading to progressive build-up and flow choking. Problems of this type have been known to occur in the coal handling plant of power stations.

Fig. 11: Block-like flow down chute

Fig. 12: Build-up of bulk solid in verticle chute or standpipe

When blockages occur in feed-pipes to feeder and mill, a boiler may "flame out" in the space of a few minutes. Blockages, are initiated by the coal adhering to the pipe wall and then growing inwardly this action often occurring after only a few tonnes of coal have passed through the system.

Experience has shown that, in the case of unwashed coal, when coal is fed straight from a mine to a power station, no problems are encountered.

However, when the coal is stockpiled, the weathering process can cause the clays to be dispersed and this has led to blockages occurring. The adhesion process may be aggravated in this case due to the temperature of the mill and standpipe above the mill, the temperature at the top of the mill being in the order of 70C.

It is important that the pipe or chute diameter be sufficiently large to cause the bulk solid to fall away from the walls. A proposed simplified methodology is presented. Referring to Fig. 12, assuming the weight of bulk solids is just sufficient to cause slip along the wall, the required pipe or chute diameter D is given by

D < 4τ / γ(1 - C²)  (3)

Where

C = d/D such that C > 0.8

τ = shear stress at wall corresponding to normal pressure s

γ = pg = bulk specific weight

4.3 Adhesion of Bulk Solids to Conveyor Belts

The amount of carry-back of bulk solids at the discharge drum of a belt conveyor may be estimated by studying the mechanics of the bulk and solid during the discharge process. A methodology to determine carry-back has been proposed to Roberts et al. [3]. Essentially by measuring the adhesive stress s0 (Fig. 2), the amount of bulk solid adhering to the belt after discharge is obtaining using an equilibrium analysis in which the body force (Fig. 8 is composed of the centrifugal force and weight component due to the bulk solid on the belt.

5. WEAR IN BULK HANDLING PLANT

Wear in bulk handling plant may result from impact or abrasion or, as is often the case, a combination of both. In addition, deterioration of metal surfaces can occur as a result of corrosion.

5.1 Impact

Erosive type wear due to impact consists of a combination of plastic deformation and cutting wear. Such wear, for example, occurs in pipe bends of pneumatic conveying systems where impact velocities are normally relatively low whereas particle size range can be quite wide with large lumps being present.

Impact wear depends on several factors, the relative hardness of the particles and the surface having a significantly influence. For impact on hard, brittle materials, the greatest amount of damage occurs when particles impinge at angles of approximately 90. On the other hand, for ductile materials, the greatest amount of erosive wear occurs when particles strike the surface at low angles of attack, usually in the range 15 to 30.

5.2 Abrasive or Rubbing Wear

This occurs in storage bins and silos particular in hoppers operating under mass-flow conditions. Under mass-flow the pressures in a hopper will vary significantly over the hopper surface, with the maximum pressure occurring at the transition, the pressure decreasing towards the outlet. The velocity of the bulk solid adjacent to the wall increases non-linearly from the transition to the hopper outlet.

While the magnitude of the velocity at a particular point on the hopper wall depends on the bin discharge rate, normally the bulk solid velocities are quite low with pure sliding taking place.

Abrasive wear also occurs in transfer chutes, the flow being characteristics by lower pressure and higher velocities than those occurring in hoppers. There are several other areas where abrasive wear is experienced such as in feeders, belt conveyors, vibratory conveyors and screw conveyors. Any mechanical device which involves the motion of bulk solids relative to surface will experience wear problems.

5.3 Abrasive Wear Parameters

As discussed in Ref. [2], it is often difficult to predict, in absolute terms, the likely wear of bin and chute walls. This situation arises mainly as a result of the lack information concerning the wear life of lining materials.

For this reason the concept of a non-dimensional Relative Wear Number NWR has been introduced [5]. The Relative Wear Number allows comparisons to be made between different bin and chute geometry's.

The Relative Wear Number is defined as

NWR = (σw/γB) (Vs/Vo) tanΦ (4)
YB V0 

Where

σw = Normal pressure at boundary
γ = Bulk specific weight
B = Characteristic dimension; outlet dimension in case of hopper; chute width in case of chute
Vs = Velocity of sliding at wall
Vo = Sliding velocity at references at location. For hopper, V0 is defined at transition of Cylinder and hopper; for chute, V0 is normally defined at point of entry to chute.
Φ = Wall friction angle

5.4 Wear in Mass-Flow Bins

The application of the foregoing to the assessment of relative wear in mass-flow bins has been discussed in Ref. [5]. By way of illustration, the relative wear profiles for axisymmetric (or conical) and plane-flow bins having the same opening dimension and hopper half angle respectively are illustrated in Fig. 13.

In the case of the axisymmetric bins, the maximum relative wear occur at the outlet, while the plane-flow bins the maximum relative wear occurs at the transition. In the latter case the wear at the transition is likely to be less than indicated in Fig. 13 owing to the possible build-up of material at the transition. Also, the normal wall pressure occurring at the transition is difficult to predict precisely and is likely to be lower than as indicated.

Some bins are constructed with a variable hopper slope and with the hopper section having different surface textures. Such a bin is discussed in Ref. [12]. The bin in question is axisymmetric with a capacity of 2 400 t. The hopper was lined with 3 mm type 304-2B stainless steel. Examination of the lining after approximately 5 million tonnes of coal had passed through the bin showed that the maximum wear of the stainless steel was around 1 mm.

5.5 Avoidance of Wear Problems Due to Eccentric Funnel-Flow

Serious wear problems will occur during funnel-flow where the flow channel or pipe is not fully contained in the bulk solid itself but may incorporate part of the hopper or bin wall. Problems of this nature may occur when bins with eccentric discharge are used, particularly when the bin opening is located near a side wall. On other occasions a badly designed feeder may cause material to pipe adjacent to the hopper wall. Flow channels of this nature give rise to high velocity flow against the wall resulting in accelerated wear.

Often side delivery chutes are incorporated in bins for the purpose of off-loading bulk materials. Side delivery chutes create undesirable flow patterns in bins, leading to accelerated wear of the bin wall in the region of the chute intake as well as in the plates above the chute. This wear is caused by both abrasion and impact. Abrasive wear results from the high velocity of the materials during chute discharge, the flow following a funnel-flow pattern.

Impact wear can occur on filling the bin after discharging from the slide delivery chute. The surface is left in a riled condition; when filling commences, lumps of bulk material may bounce off the riled surface and impact the wall in the weakened area above the chute.

Fig 13: Relative wear profiles for axisymmetric and plane-flow mass-flow bins
B = 1.0m; α = 22°; Φ (cylinder) = 30°, Φ (hopper) = 20°

It should be noted that despite the fact that side delivery chutes may only be used intermittently, the wear rate during the operation is considerable. It is therefore most desirable that side delivery chutes be avoided and incorporate any off-loading via a transfer conveyor operating from the main bin discharge. If side delivery chutes are used, such as an existing installation, it is essential that the bins be lined with wear plates in the region of the chute intakes as well as above the chutes.

Fig 14: Chute flow model

5.6 Wear in Transfer Chutes

Abrasive wear in transfer chutes has been discussed in Refs. [2] and [5]. The mechanics of fully developed acceleration flow in chutes is illustrated in Fig. 14.

The Relative Wear Number, as defined by equation (4) becomes

NWR = K_ch/b [ sinθ +V²/Rg] (V/Vo)tanθ  (5)

θ = Chute slope angle measured from vertical
V = Average velocity at section considered
R = Radius of curvature of chute
Φ = Chute friction angle
Vo = Velocity at point of entry to chute.

Kc = Vs/V  (6)

Where

Vs = Velocity of sliding relative to chute bottom.
The factor Kc < 1. For fully developed 'fast' or accelerated flow it may be assumed Kc 0.8. As the stream thickness increases, Kc will reduce.

For straight, parallel inclined chutes, R = 8 and equation (5) reduces to

NWR = {(Kch sinθ tan Φ)/b} (V/Vo)

Noting that the mass flow rate is given by

Noting that the mass flow is given by

Q = pbhV (8)

Then equation (7) becomes

NWR = QKc sinθ tanΦ / pb²Vo  (9)

Assuming Kc is constant curvature, the wear is shown to be relatively insensitive to the chute radius particularly if the initial velocity V0 at the entry point of the chute is small [2]. This may be observed in Fig. 15 which shows velocity variations (Fig. 15 (a)) and variations in the Relative Wear Number (Fig. 15(b)) for chutes of constant curvature.

The wear factor for the side wall is given by

(NWR) side = (NWR x k) / 2Kc   (10)

Where the factor k represents the ratio of lateral to normal pressure. For normal range of applications 0.4 = k < 1.0.

6. DEVELOPMENT OF LABORATORY WEAR TESTER

6.1 Introductory Remarks

Despite the extensive research in the field of trilogy and the established of specialised wear tests for specific applications, there has been limited progress in the development of acceleration laboratory wear tests to enable the prediction of absolute wear life of hopper and chute lining materials.

(a) Variation in velocity

(b) Variation in relative wear

In general wear tester relevant to bulk materials are classified as three-body testers in which loose abrasive particles are trapped between two surfaces. If the two surfaces are in close proximity and the abrasive particles make contact with both, the tester classified as a "closed" three-body tester. Where the two surfaces are set far enough apart so that the mechanical properties of one surface do not influence the other, the tester is classified as an "open" three-body tester.

Some existing wear testers, reviewed in Ref. [13] are those due to Rabinowicz, Dun and Russell [14], Toporv [15], the rubber wheel abrasive wear tester described by Haworth [16]. Avery [17] and Tucker and Miller [18], and the low stress abrasive tester developed by Mistra and Finnie [19]. These machines have limited applications to the bulk solids handling field.

The first laboratory wear test machine relevant to the bulk solids field was developed by Johanson and Royal [20]. This machine incorporates a screw extruder which forces a bulk solid against a circular sample of the lining material. The sample is held in a speed device which allows the sample to rotated. Instrumentation includes a load cell for measuring the normal pressure and a torque meter for measuring the friction reaction torque experienced by the sample.

While the Johanson and Royal machine has proved to be effective in producing accelerated wear tests on lining samples, it is seen to have two shortcomings:

1. The test samples must be specially prepared to the required circular shape. This can present problems in the case of hard lining materials such as Ni-hard and Ceramic linings.
2. The wear of the test samples leads to a conical surface since the rubbing velocity increases with the samples radius.

In order to overcome these shortcomings, a linear wear tester for hopper and chute lining materials has been developed jointly by The University of Twente, The Netherlands, and The University of Newcastle, Australia [13, 21].

In developing the laboratory wear tester it was recognised that the following features should be incorporated:

1. Provision for a continuous supply of "fresh" bulk solid.
2. Provision for the bulk solid normal pressure on the tester surface to be varied over the specific range.
3. Provision for the sliding or rubbing velocity to be carried over the specific range. It is to be noted that the selection of pressure in (ii) above and velocity should be based on the functional relationship between these two parameters as occurs in particular hoppers and chutes.
4. Permit measurement of :

• Tangential force.

• Normal pressure between bulk solid and test specimen.

• Temperature if required. Some means of prevent the test sample from becoming overheated is desirable.

• Relative velocity of sliding.

• Total hours of operation and total distance travelled.

The test procedure should include the following measurements in addition to the above.

• Loss in weight of test sample due to wear.

• Loss in thickness over the surface.

• Change in surface roughness.

• Change in particle size distribution.

• Particle shape. This would probably be qualitative.

• Particle hardness.

• Surface hardness.

• Wall or boundary friction.

• Moisture content of bulk solid.

Fig. 16: Linear wear test apparatus

6.2 Test Apparatus

Based in the foregoing, the test rig. Which has been developed is shown, schematically, in Fig. 16. As illustrated, the rig incorporates a belt feeder to deliver a continuous supply of bulk solid to the sample. The apparatus incorporates the features outlined in the previous section but attention is drawn to the following details which have proved to be essential to its operation.

1. The need for a wedge lead in order to build up the depth in bulk material beneath the test sample. This feature is accommodated by the inclined section of the belt at the in take end.
2. The length of the belt feeder/conveyor has been made significantly longer that that of the test sample in order that the wear on the belt can be distributed, thus increasing its life.
3. The belt is driven by a variable speed hydraulic motor which permits test speeds up to 0.3 metres/sec. The sample of lining material is retained in a sample bracket which allows for easy removal for examination during each test, and then easy installation.
4. The shear force is continuously monitored by load cells.

As may be observed, the belt is significantly longer than the test sample. This ensures that belt wear is minimised. It is pleasing to note that after extensive use over several months, belt wear is not significant. Furthermore, the pressure plate is sufficiently large in area to dissipate any heat generated and hence, the sample temperature remains unchanged throughout the test. The test samples are rectangular in form and may be of any dimension within the geometrical constraints of the machine. The size of the samples is usually chosen so that they can also be accommodated in the Jenike direct shear apparatus. In this way, comparative surface friction tests can be performed at various stages during the wear test program using the Jenike apparatus.

7. SOME TYPICAL TEST RESULTS

Some typical wear test results using the wear test apparatus are presented. The results are based on test performed on mild steel, stainless steel 304 and Tivar 88. In all cases the abrading agent used is sand with a belt speed of 0.285 m/s. While the wear is measured in terms of weight loss in g, it is useful to convert this wear into loss in lining thickness 't', measured in microns using the following relationship:

(a) Wear on mild steel plate at various normal pressures

(b) Wear on 304 stainless plate at various normal pressures

(c) Wear on Tivar 88 at various normal pressures

t = (w x 10³) / (A x p)  (11)

Where

W = weight loss (g)
A = surface area (m2)
p = density (kg/m)

7.1 Wear Test Results

Wear test have been carried out on each of the mild steel, stainless steel, and Tivar sample plates at several different normal pressures to determine a relationship between wear rate and pressure. Fig. 17 shows the results obtained in terms of thickness loss in microns. As expected, wear is a linear function of time for all pressure in all cases.

For the mild steel and stainless steel, the results are based on five normal pressures, whereas, in the case of 'Tivar 88', the testing to date has included three normal pressure. Also in this case, one data point shows an error; this result, which is encircled, is ignored.

7.2 Comparative Wear Rates

In order to directly compares the wear rates of the three samples, their wear rates in microns per 100 hours have been plotted against pressure on the same graph and are presented in Fig. 18. Stainless Steel 304 is clearly the most wear resistant of the three samples followed by Mild Steel with Tivar 88 the least wear resistant.

It is noted that the wear rate versus pressure for each sample is linear over the range of pressure with the exception of pressure below 2kPa. The reason for the apparent zero wear below 2kPa is not clear and will be the subject of further investigation.

7.3 Comparative Shear Forces

Comparisons of shear force plotted against normal force for the three samples reveals a similar dynamic coefficient of friction for all three samples of approximately 0.4. This similarity in dynamic friction coefficient is expected between a dry non-cohesive bulk material such as sand and relatively smooth surfaces. The surface roughness of both the Mild Steel and Tivar samples is approximately 1 micron RMS and the Stainless Steel sample has a roughness of 0.3 microns RMS. The plotted results are shown in Fig. 19.

8. CONCLUDING REMARKS

The efficient operation of bulk solids handling plant depends, to a significant extent, on the smooth flow and handling of the bulk solids without blockages occurring in bins and chutes. It is important, therefore, that handling plants are designed taking into account the relevant flow properties of the bulk solids being handled. In this respect it is important that significant influence of wall friction, cohesion and adhesion be understood and taken into account when designing bins and chutes. Procedures for determining these parameters are now well established and have been briefly outlined in this paper. The selection of appropriate lining materials for bins and chutes, in addition to the needs for favourable, frictional properties, should also provide long wear life. The role of accelerated wear tests to evaluate lining materials as an adjunct to the established procedures for flow property determination is an important one. The linear wear tester described in this paper is seen to have potential in this regard.

Fig. 18: Comparative wear rates

Fig. 19: Comparison of shear forces

ACKNOWLEDGEMENTS

The work reported in this paper is part of a programme of research being conducted at The University of Newcastle, Australia, and supported by the Australian Mineral Industries Research Association (AMIRA). The support of the several sponsoring companies through AMIRA is gratefully acknowledged.