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The influence of rock strength and abrasive mineral content on the Cerchar Abrasive Index
ENGENEER~NG GEOLOGY ELSEVIER
Engineering Geology 36 (1994) 293-301
The influence of rock strength and abrasive mineral content on the Cerchar Abrasive Index S.I. AL-Ameen, M.D. Waller Department of Mineral Resources Engineering, The University of Nottingham, UniversityPark, Nottingham NG7 2RD, UK
(Received May 26, 1993; revised version accepted October 7, 1993)
Abstract
The Cerchar abrasive index is based on a simple test and is widely used to assess the abrasiveness of rocks. Detailed examination of the test has shown that the index is largely influenced by the rock strength, and only partially by the rock abrasiveness. The standard test was found to be incapable of providing accurate indices for rocks of low relative abrasiveness and a modification to the test is presented to overcome this problem. The interpretation of the test results has been extended to enable calculation of the specific wear rate which is a more precise measure of the abrasiveness of materials.
1. Introduction
The Cerchar abrasive index test has been widely accepted for the assessment of rock abrasiveness. It is based on a simple test and has been considered to provide a reliable indication of rock abrasiveness (Muftuoglu, 1983; Singh et al., 1983; Atkinson et al., 1986a, b). The test was found to give consistent results with fine- to medium-grained rocks, however, the results were unreliable for weakly consolidated rocks and low abrasive rocks. Furthermore, the results obtained from coarsely crystalline rocks were likely to represent the abrasiveness of individual minerals rather than the abrasiveness of the whole rock. The assessment of the abrasiveness of the U K coal measures rocks was carried out as part of a programme of work aimed at the prediction and control o f abrasive wear of mining equipment. However, the majority of laboratory abrasion tests, including the Cerchar test failed to produce satisfactory results for the coal measures rocks. The 0013-7952/94/$7.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0013-7952(93)E0059-V
main reason was that these rocks, excepting the sandstones, occupy the lower end of the range of Cerchar, index where the accuracy and resolution of the test are severely diminished. This factor is important when discrimination of rocks within a small range of abrasivity is critical. This is particularly important to operations such as the U K coal industry, which, although handling material of relatively low abrasivity, handle very large tonnages which results in significant wear problems. The standard Cerchar test was, therefore, considered unsuitable for the U K coal measures unless some modification was made to the test by using a softer testing tool. Previous work by West (1986) indicated that the Cerchar abrasive index showed good correlation with the abrasive mineral content of the rock. Others (McFeat Smith, 1977) had highlighted that rock abrasiveness depended on the type and degree of cementation material. However, the latter gave no consideration of the influence o f the abrasive minerals in the rock. The current study has con-
294
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firmed the importance of the combined influence of both the cementing materials (rock strength) and the total abrasive mineral content. For example, a high strength ironstone ( 140 MPa) consisting predominantly of siderite (a non-abrasive mineral compared to mild steel), gave a high Cerchar index of 1.33 (EN24). On the other hand, a low strength sandstone (20 MPa) containing highly abrasive minerals gave a Cerchar index of only 0.39 (EN24). It therefore seems to be incorrect to use the term Cerchar abrasive index when a dominating factor was the rock strength.
2. Testing apparatus The testing apparatus consists of a stylus clamped in a holder which is subject to a 7 kg dead weight. The test stylus consists of a cylinder of EN24 steel ground at one end to a cone of angle 90 °, accurately heat treated to 610 + 10 Hv. A special sliding mechanism allows the stylus holder and dead weight to be moved horizontally for a distance of 10 mm. The tested rock sample is cut into a rectangular shape, and the tested faces are ground so that the specimen is polished. The rock specimen is fixed by a special double jaw clamp positioned directly below the stylus holder. The apparatus allows the stylus to be drawn, under a constant weight across the surface of the test sample. A travelling microscope was used to check that the point of the stylus was undamaged prior to the test. Although the stylus tip should be a perfect point before use, in practice the tip may show slight imperfection. Any styli showing damage exceeding 5 lain were rejected, as suggested by Corbett and Aston (1988). The travelling microscope was also used to measure the diameter of the abraded cone end of the stylus to an accuracy of _ 2 ~rn after it had been drawn over the sample. The Cerchar abrasive index of the rock is defined by the diameter (in mm) of the wear fiat formed by truncation of the tip of the cone multiplied by 10. Further details of apparatus and testing procedures are reported in Cerchar (1973) and Corbett and Aston (1988). During this investigation detailed examination
of the progressive development of the stylus wear at various stages of the test was necessary. This was enabled by restricting the movement of the sliding mechanism, using machined stops, such that the stylus could be drawn over the rock surface for distances of 1, 2, 3, 5, 7 and 10 mm.
3. Sample surface condition The standard Cerchar test was designed to be conducted on a flat and polished surface of the rock sample. If the rock sample is hard the Cerchar stylus tends to slide on the smooth rock surface, giving minimum abrasion and hence a low index value. If the test was then conducted on a natural breakage surface of the rock the natural roughness would give a higher abrasion and hence a higher index value. However, with softer rocks, typical of the coal measures (characterised in the UK by soft sandstone, siltstone, silty mudstone, mudstone and seatearth rocks), the stylus tends to indent the rock, and the surface finish of the rock has little effect on the index value. Excavated rock normally breaks along natural discontinuities or along planes normal to the applied force, in either case the naturally developed surface is characteristic of the host rock. The broken rocks potential to cause abrasive wear is dependent on a combination of rock surface roughness (defined by the grain size, shape and orientation), the mineral hardness and the strength of the cementing material. Therefore, for the softer rocks typical of the UK coal measures, it is more representative to employ a natural breakage surface for the Cerchar test.
4. Test stylus During this study it was observed that weakly consolidated rocks, for example seatearth, mudstone and silty mudstone, produced Cerchar indices of less than 0.15. These data occupied the bottom end of the range of Cerchar indices and it was, therefore, concluded that the standard EN24 stylus was too hard for these rocks. Its hardness (610 Hv) is also significantly greater than the
S.I. AL-Ameen, M.D. Waller/EngineeringGeology 36 (1994) 293-301
materials used for the construction of mining equipment (excepting cutting tools). Therefore, a softer stylus, made from EN3 (mild steel, 225 Hv) was prepared specifically for testing the abrasivity of weaker coal measures rocks relative to the majority of mining equipment. Cerchar tests using both EN3 and EN24 styli were conducted on the polished rock surfaces of a range of rocks in order to provide calibration data between the EN3 and EN24 styli. These data are illustrated in Fig. 1, which shows a moderate correlation (correlation coefficient [R] =0.87) between the two indices. The calibration law is shown in Eq. 1: CI ( EN 3 ) = 0.24 + 2.74 CI (EN24) - 0 . 3 9 CI 2 (EN24)
(1)
where, CI = Cerchar Index. However, to permit comparison of the modified Cerchar index for natural rock surfaces using EN3 styli, with the standard Cerchar index using EN24 styli on polished surface, further tests were carried out, as illustrated in Fig. 2, to determine a conversion factor between the two indices. A moderate correlation existed, as shown in Eq. 2: CI (EN3)=0.28+2.43 CI (EN24) - 0 . 2 6 CI 2 (EN24)
(2)
The scattering of data in Figs. 1 and 2 is probably due to the inconsistent hardness of EN24 styli. 146 EN24 styli were tested for Vickers hardness at four points across their diameter and the results are presented in Fig. 3. There was a very large spread in values ranging from 350 Hv to 800 Hv, the distribution indicates that the styli are probably from three batches with average Hv values of
295
m i ~
='e
I•
•
e
•l
•
•
el•
= 0,g0 i
0,
I 1
I 2
Cerehar Index EN24 on polished
I 3
reck surface
Fig. 2. Cerchar index EN3 on natural rock surface and EN24 on a polished rock surface.
500-550, 600-650, and 700-750 and that there is a variation of ___100 Hv in each batch. This probably indicates poor quality control in heat treatment. The effect of material hardness on the abrasivity has been investigated by the authors (A1-Ameen and Waller, 1992a, 1993a) and has shown that doubling of the hardness from 300 Hv to 600 Hv or from 400 Hv to 800 Hv resulted in an increase in abrasive wear by a factor of 1.37. This would indicate that the variation in Hv for the EN24 styli between 500 Hv and 750 Hv will have a much reduced effect on the Cerchar index. 21 EN3 styli were also tested and found to have an average of 225 Hv with a variation of ___4 Hv, considerably more consistent than the EN24 results, as illustrated in Fig. 3. Because of this, and the less abrasive nature of the tested rocks (UK, coal measures rocks), all subsequent Cerchar tests were carried out using EN3 styli. The Cerchar index measured, using EN3 styli on polished rock surfaces and on natural rock surfaces, are very consistent. There was very strong linear correlation between these results, as illustrated in Fig. 4
(nl: z : tu I
i
e~
y = 0.24 + 2.74x - 0.39x l
l I
,
l 2
Carchar Index EN24 on polished
,
2
R
2
= 0.87
I 3
rock surface
Fig. 1..Cerchar index EN3 and EN24 on polished rock surface.
200
250
300
350
400 450
Hardness
500
550
600
650
700
750
range ( H v )
Fig. 3. Distribution of EN3 and EN24 styli hardness.
800
S.I. AL-Ameen, M.D. Waller/Engineering Geology36 (1994) 293-301
296
5. Cerchar mechanism
and Eq. 3: CI (EN3) natural surface = -0.01 + C I (EN3) polished surface
(3)
For the above reasons, the testing programme for the coal measures rocks were conducted on a natural rock surface using EN3 styli. This also saved considerable time in sample preparation and enabled the testing of multiple samples at minimum cost. Table 1 summarises the range of Cerchar index for the rocks tested during this investigation. Some results from igneous rocks have also been included for comparative purposes, their true Cerchar index should be higher than indicated in the table for reasons given later.
4
3
1
y = - 0.01 + 1.00x 0
R2 .
0.93
I
i
I
t
i
1
2
3
4
5
Cerchar Index EN3 on polished rock l u r f a c e
Fig. 4. Cerchar index EN3 on natural and polished rock surface.
The Cerchar index can be categorized as a high stress sliding abrasion test. To understand what factors could influence the results, the test mechanism and the physical nature of the rock were examined in detail. The wear fiat diameter generated on the stylus tip as it is drawn over the rock surface results from the combination of the stylus material, the rock's strength and its abrasive mineral content. The stylus material was standardised for these tests and, as indicated earlier, is of consistent hardness. Stylus wear can, therefore, be considered as related entirely to the characteristics of the tested rock types. To investigate the development of the wear fiat diameter on the stylus, the tip was thoroughly monitored at various stages of the stylus movement over various rocks (1, 2, 3, 5, 7 and 10 mm) using EN3 styli. At the beginning of the test, the stylus tended to penetrate the rock surface due to the high pressure at the tip. The maximum penetration is achieved when the applied load (i.e., 7 kg), acting on the tip area, reaches equilibrium with the rock strength (i.e., the bond strength between adjacent grains). With the contact of the stylus tip on the rock and the initial horizontal movement of the stylus in the rock surface ( ~ 1 mm), the stylus tip tended to deform and shear off due to the dead load and the resistance to horizontal movement, and a fiat-ended tip was formed. The fiat styli end formed at the beginning of the test is not dependent
Table 1 The range of Cerchar index using different styli material on various rock Rock type
EN3 polished rock surface
EN3 natural rock surface
EN24 polished rock surface
Sandstone Siltstone Silty m u d s t o n e Mudstone Seatearth Ironstone Limestone* Igneous rock* Sandstone*
2.02-3.25 1.10-2.31 0.63-1.56 0.22 0.76 0.24-0.44 2.92-3.87 0.90-2.32 4.42-5.12 2.10-4.89
1.75-3.45 0.87 2.16 0.35 1.68 0.29 0.70 0.24 0.44 2.98 4.31 not measured not applicable 2.40-4.97
0.72-1.71 0.25-0.62 0.11 0.70 0.10-0.37 0.10 0.12 1.06-2.20 0.49-1.05 3.09-4.19 1.71-3.32
*Non coal measures.
297
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on the amount of abrasive material in the host rock. It is due entirely to deformation and shear failure at the tip, and its magnitude should be related to the rock strength and the stylus material. If the rock is coarse grained, as for most igneous rocks, the deformation and shear failure at the stylus tip will be related to the hardness of the particular mineral below the stylus tip. If the tested rock is of a coal measures type, the amount of tip sheared will be mainly related to the rock strength and particle size, the latter having a minimum effect since the average grain diameter of these rocks is below 65 lxm (A1-Ameen and Waller, 1992b, 1993b). After a further 2-3 mm of sliding distance the tip fiat diameter will increase as a result of abrasive wear between the rock and the stylus material. Beyond this distance the increase in the wear fiat diameter at the stylus tip will depend entirely on the characteristics of the rock which can be divided into two groups as follows; (A) Hard Rocks. These include igneous and metamorphic rocks and some sedimentary rocks (such as ironstone, dolomite, chert/flint and nonporous sandstone with a cementing material of quartz or siderite). In this rock group, when a balance is reached between the applied stylus pressure and the rock strength (i.e., the stylus tip cannot indent the rock) the stylus will tend to slide over the surface of the rock. In this case, the abrasion between the two materials (rock and stylus) will depend principally on the degree of surface roughness of the sample. However, due to the hardness and compactness of the minerals in this group of rocks, the polished rock surface will always posses a high degree of smoothness. Consequently the abrasion between the stylus tip and the rock surface over approximately 3-10 mm sliding distance will be minimum. The wear generated on the stylus tip over this distance will be very small relative to that generated by tip shear or abrasion over the first 3 mm as illustrated in Fig. 5. Each point on this graph represents averaged data from six tests for all of the rocks excepting the granites where 20 tests were used to take account of the larger grain size. The Cerchar index for this group of rocks will always be smaller in magnitude than it should be,
m
3
• •
Fine pink granite Fine white granite Ironstone
x
Sandstone YH
•
1
0
m,
,
2
r
,
4 Sliding
7
,
6 distance
7
,
B
,
10
+
Sandstone/L
O
SIRstone/H
•
Siltetone/L
•
Silty rnudstone
o
Mudstone/I-I
•
Mudatooo/t.
•
Seatearth
(mm)
(The letter L or H by the tested rock means rock with a low or high strength)
Fig. 5. Cerchar index EN3 measured at different sliding distances on various rock types. as explained above. The upper limit of the Cerchar index for EN24 found in this study did not exceed 4.2. A similar upper limit was reported by Atkinson et al. (1986a, b). This maximum appears to hold irrespective of the type of rock and its mineralogy. The authors, therefore, concluded that the Cerchar index for this group of rocks is not representative of the rock abrasiveness unless a modification is made to the testing procedure. One option would be to test samples with artificially induced and controlled surface roughness or to always test natural surfaces. Another option would be to apply a correction factor to the results. Further work on this problem is currently being undertaken by the authors. (B) Soft Rocks. This group of rocks includes all sedimentary rocks, but excludes those already classified in the hard rock category. The polished surfaces of these rocks are flat but not smooth and the roughness is mainly due to the type of mineral content, particle size, pore space and type of cementing material. The strength of these rocks is defined by the type of the cementing material, and generally it is significantly lower than the hard rocks. Hence, the Cerchar stylus will tend to penetrate deeper into the rock sample. Also, because the applied pressure on the stylus tip is always greater than the rock strength, the stylus tip will remain indented into the rock for the whole sliding distance (i.e., 3-10 mm). This means that
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the stylus tip will always be in contact with a rough surface texture which will depend on the inherent physical characteristics and mineral content of the host rock. It also means the abrasion between the two faces (i.e., rock and stylus) will be a maximum. If the host rock does not contain abrasive minerals (for example seatearth or mudstone) the flat tip diameter will not increase with the increased sliding distance. When the host rock contains abrasive minerals the stylus tip will continue to wear away until the end of the defined testing distance. This was observed on most of the coal measures rocks where the Cerchar index continues to increase steadily with increased sliding distance as illustrated in Fig. 5. It can, therefore, be concluded that if the applied pressure generated by the Cerchar stylus on the sample exceeds the rock strength, the Cerchar test is valid. This will be indicated by an indentation streak for the whole 10 mm sliding distance, which should be visible to the naked eye on the tested rock sample. However, if the indentation streak covers only part of the 10 mm sliding distance it indicates sliding with minimum abrasion and a false, low value for the Cerchar index. In this case the test result is not valid.
6. Cerchar index ( E N 3 ) at 1 and 10 mm
The Cerchar index related to a 1 mm sliding distance is about two thirds of the final Cerchar index at a 10 mm sliding distance for most of the tested rock types shown in Fig. 5. Accordingly, the Cerchar index was measured at 1 mm and 10 mm for a wide range of rocks and the results are summarised in Fig. 6. The strong linear correlation between the two Cerchar indices that is evident in Fig. 6 clearly indicates that approximately 30% of the Cerchar index can be attributed to the abrasion effect which corresponds to the final 9 mm of sliding distance. While 70% of Cerchar index is related to the initial movement of 1 mm. The initial wear flat diameter on the stylus tip cannot be attributed to the abrasive mineral content due to the small sliding distance. It is most likely due to a combination of the balance between rock and stylus strength and the depth of the tip
m
la
2
1
o
1
4
Cerchar
Index
5
EN3 (lOrnm)
Fig. 6. Cerchar index EN3 measured for 1 and 10mm sliding distance.
indentation into the sample. The tip wear flat diameter generated during the remaining 9 mm of the testing distance can be related to a combination of the abrasive mineral content and the bond strength between the minerals in the rock (i.e., the rock strength). In order to validate this the rock strength was plotted against the Cerchar index (EN3) for 1 mm sliding distance as shown in Fig. 7. The rock strength used was the uniaxial compressive strength (UCS), tested in accordance with ISRM standards (Brown, 1981). The 1 mm Cerchar index for rocks containing abrasive minerals and rocks containing non-abrasive minerals, showed moderate correlation as illustrated in Fig. 7. The diagram could be utilized for assessing rock strength from a 1 mm Cerchar index or vice versa. Accordingly, if we consider Eq. 4 for Cerchar index (CI): CI (EN3, 10 mm) = C I (EN3, 1 m m ) + C I (EN3, 9 mm)
•
3~
uJ
Rocks witlt al~rmiv¢ minerab Rocks without a b r u i v e n u n ~ a l s Igneous r o e l ~
•
•
o 0 •
•
(4)
o
o
2
~= =, P •
•
•
y=
- 1 . 3 8 + 0 . 0 5 x - 0 . 0 0 0 1 x 2 R 2 •0.90
0 40
80 Rock
=trength
120
160
200
(MPa)
Fig. 7. Cerchar index EN3 (1 mm) versus measured rock strength.
S.1. AL-Ameen, M.D. Waller~EngineeringGeology36 (1994) 293-301
then the rock strength can be included from the equation shown in Fig. 7; and Eq. 4 can be modified to: CI (EN3, 10 mm) = f n { U f S + C I (EN3, 9 mm)}
(5)
This can be further extended to include the influence on the Cerchar index of the abrasive mineral content in the host rock and the strength of the cementing material bonding these minerals together (i.e., the rock strength). These factors relate to the final 9 mm of the sliding distance. Equation 5 can be extended to: CI (EN3, 10 mm) =fn{ UCS + UCS x ~ Abrasive mineral hardness} or;
CI (EN3, 10 mm) =fn{UCS x (1 +}-" Abrasive mineral hardness)}
(6)
The relationship given in Eq. 6 is validated by the strong correlation between the Cerchar index (EN3) and the combination of uniaxial compressive strength and the abrasive mineral hardness as illustrated in Fig. 8. Details of the method used to determine the summation of abrasive mineral hardness are given in AI-Ameen and Waller (1992b). However, when the rock does not contain abrasive minerals (i.e., the abrasive mineral hardness in Eq. 6 is zero or insignificant) the Cerchar index will
3
2
1 ~'A •
y --- -D. + 0.0~67x - 0.00GO02 x 2 R 2 = 0.95
200
400
000
800
1000
1200
1400
UCS x (I÷Y_. Abraelvo mlnoral hardness)
Fig. 8. Cerchar index EN3 versus a combination of rock strength and abrasive mineral hardness in various rocks.
299
continue to show strong correlation with the rock strength as for the case shown in Fig. 7, but will have a higher gradient due to the extra 9 mm of sliding as shown in Fig. 8. These data are representative of limestone, ironstone, some mudstones and seatearth. The data on the top right-hand side of Fig. 8 are from igneous rocks. As explained earlier the measured Cerchar index of these rocks will always give a low indication of true abrasivity. However, if the true abrasivity index for the igneous rocks was considered, then their position on Fig. 8 would move upwards and the relationship would tend to follow a simple linear relationship. The difference between this linear relationship and the graph indicates that a multiplying factor of between 1.2 and 1.5 should be applied to the measured Cerchar index (EN3) for the igneous rocks. From the above it is clear that the measured Cerchar index is a function of rock strength, and the product of rock strength and abrasive mineral content. It can be concluded, therefore, that the Cerchar index is mainly influenced by the rock strength and partly by the abrasive mineral hardness. Hence, the term Cerchar abrasive index used by many workers is not strictly correct. Previously published scales of igneous rock abrasivity classified according to Cerchar abrasiveness index are, therefore, not valid.
7. Specific wear rate and Cerchar index
A disadvantage of the Cerchar index is that it only gives a relative measure of the rocks properties and cannot be used to predict the actual wear rate of materials exposed to relative motion with rock. The specific wear rate (SPWR) is defined as material loss, per unit load, per unit distance, and has the units mma/N km (Voss and Friedrich 1986; Talks, pers. commun., 1993). However, since the measurement of the Cerchar index involves material loss from the stylus tip under fixed load over a constant distance, then the SPWR may be calculated by measuring the difference in volume loss from the stylus material resulting from 10 mm and 1 mm sliding distances. The general equation used for the calculation of SPWR from the Cerchar
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S.L AL-Ameen, M.D. Waller/EngineeringGeology 36 (1994) 293-30l
index is: SPWR = {(EN3, 10 m m ) 3 - - ( E N 3 , 1 mm) 3} x K
(7)
where: K = r c x D / ( C x W ) = 0 . 2 1 1 8 ; D = d i s t a n c e conversion factor; C = v o l u m e constant; and W = applied load x gravity constant. F r o m the data available for the tested rocks the SPWR was determined by three methods as described below: (1) SPWR was calculated from the measured rock strength and mode analysis of the abrasive mineral content, this was done utilising the equations given in Figs. 7 and 8 to calculate the Cerchar index for EN3 (1 ram) and EN3 (10 m m ) which are then substituted in Eq. 7. (2) SPWR was calculated from actual measurements of the Cerchar index for EN3 (1 ram) and EN3 (10 ram) which are then substituted in Eq. 7. (3) SPWR was calculated from measured Cerchar index EN3 ( 1 0 m m ) , and the equation given in Fig. 6 to calculate the derived Cerchar index for EN3 (1 ram), the results are then substituted in equation 7. The different methods used to calculate the SPWR for rocks of the same group gave similar results as shown in Table 2. In general terms, the S P W R results for the coal measures rocks vary between 0 and 6, of which the seatearth and mudstone have no significant SPWR, the silty mudstone can reach up to 0.5 and the siltstone varies from 0.1 to 2.7. Although the siltstone was considered to be abrasive in comparison with other
coal measures rocks, some siltstones recorded a SPWR as low as 0.1. This is due to the fact that some of the siltstones were cemented by a weak clay material which would result in a very low Cerchar index. Consequently the derived SPWR will be small. Other siltstone rocks were found to be cemented by silica and carbonate cement resulting in a higher Cerchar index and, therefore, a higher SPWR. Various coal measures and non-coal measure sandstones were also tested. The non-coal measures sandstones produced SPWRs ranging between 2.3 and 19.9, with values as high as the SPWRs of the igneous rocks (11-20.4), and much higher than the corresponding coal measures sandstones (2.2-5.1). This means that the abrasiveness of the most abrasive coal measures rocks (sandstone) are at the low end of the scale. Similar results were found when assessing the abrasiveness of different sandstone rocks by the continuous abrasion index ( A1-Ameen and Waller, 1993a). Comparison of both Cerchar indices for the non-coal measures sandstones in Table 1 and the SPWR in Table 2 shows that a change in the Cerchar index from 2.1 to 4.98 corresponds to a change in the SPWR of between 2.3 and 19.9. For the coal measures sandstones changes in Cerchar index between 2.0 to 3.25 correspond to SPWRs between 2.2 and 5.2. Also, for the igneous rocks Cerchar indices from 4.4 to 5.1 correspond to SPWRs between 11 and 20.4. F r o m this it can be seen that SPWR is a sensitive indicator of abrasiveness and tends to give a greater spread of values over the whole range of Cerchar indices.
Table 2 The range of a calculated and measured SPWR based on EN3 styli material Rock type
SPWR from UCS and minerals
SPWRfrom EN3 1 and 10 mm
SPWRfrom EN3 10 mm
Sandstone Siltstone Silty mudstone Mudstone Seatearth Ironstone Limestone* Igneous rocks* Sandstone*
1.17-5.58 0.36-1.27 0.10-0.40 0.00-0.10 0.00-0.01 2.44-4.47 0.12 1.15 14.3 21.4 2.23-20.2
2.20-5.21 0.10-2.69 0.03-0.36 0.00-0.10 0.00-0.01 2.84-4.38 0.11-1.64 11.0 20.4 2.31-19.9
I. 15-6.12 0.10-1.71 0.03-0.53 0.00-0.17 0.00-0.02 2.55-5.47 0.10-1.74 12.9-19.6 2.59-18.6
*Non coal measures.
S.I. AL-Ameen, M.D. Waller~EngineeringGeology 36 (1994) 293-301
8. Conclusions
The results of the research presented in this paper show that the Cerchar abrasive index is not related to the abrasive mineral content alone, and the test is largely influenced by the rock strength. Equations have been developed which indicate the dominant role of rock strength on the value of the Cerchar index. The authors, therefore, prefer to use the term Cerchar index instead of Cerchar abrasive index. The standard EN24 stylus specified for the Cerchar test was found to be unsatisfactory for testing of low abrasivity rocks, for example the UK coal measures. An alternative EN3 stylus has been adopted and compared with the EN24 standard and found to give excellent results. For the UK coal measures it was found that satisfactory Cerchar results could be obtained on either polished or natural rock surfaces. This simplifies the sample preparation and enables the testing of a larger number of specimens. It has also been shown that the index can only be considered valid if the applied pressure generated by the Cerchar stylus on the sample exceeds the rock strength. Only a test which generates a visible scratch for the whole length of the test can be considered valid. The modified Cerchar test described in this paper has enabled the determination of the abrasive characteristics of typical UK coal measures rocks. This can be used as a basis for the prediction and control of abrasive wear of mining equipment. The SPWR calculated from the Cerchar index is a better means of quantifying the abrasiveness of rock. Various algorithms have been established for the calculation of the SPWR utilising a combination of the Cerchar index, rock strength and abrasive mineral content. The SPWR is a sensitive indicator of rock abrasiveness, it can be used together with the Cerchar index for assessing rock abrasiveness particularly with highly abrasive rocks where the discrimination of the Cerchar index is limited. However, the SPWR sensitivity is poor when the Cerchar index is low, and improves as the Cerchar index increases.
301
Acknowledgments
The authors would like to acknowledge the generous financial support given by European Coal and Steel Commission and British Coal. Also gratitude is extend to the technical staff of the Department of Mineral Resources Engineering at the University of Nottingham for their help during this investigation.
References A1-Ameen, S.I. and Waller, M.D., 1992a. Dynamic impact abrasion index for rocks. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 29(6): 555-560. A1-Ameen, S.I. and Waller, M.D., 1992b. Petrographic analysis for assessing the abrasiveness of coal measures rocks. Dep. Min. Resour. Eng. Mag., 44: 29-34. A1-Ameen, S.I. and Waller, M.D., 1993a. The continuous abrasion test for the assessment of rock abrasivity. ASTM Geotech. Test. J., 16 (4). A1-Ameen, S.I. and Waller, M.D., 1993b. The prediction of abrasive wear in mining equipment. Atkinson, T., Cassapi, V.B. and Singh, R.N., 1986a. Assessment of abrasive wear resistance potential in rock excavation machinery. Int. J. Min. Geol. Eng., 3: 151-163. Atkinson, T., Denby, B. and Cassapi, V.B., 1986b. Problems associated with rock material properties in surface mining equipment selection. Trans. Inst. Min. Metall., Sect. A: Min. Ind., 95: A80-A86. Brown, E.T., 1981. Rock characterisation testing and monitoring, ISRM suggested methods. Pergamon, Oxford, pp. 113-116. Cerchar, 1973. Cerchar tests for the measurement of hardness and abrasivity of rocks. Centre Study Research, French Coal Industry, Document 73-59: 1-10. Corbett, G.R. and Aston, T.R.C., 1988. A Cerchar abrasivity test program methodology developed for the Sydney coalfield. CANMET Div. Rep., CRL 88-70: 1-14. McFeat Smith, I., 1977. Rock property testing for the assessment of tunnelling machine performance. Tunnels Tunnelling, 9: 29-31. Muftuoglu, Y.V., 1983. A study of factors affecting diggability in British surface coal mines. Ph.D. thesis, Univ. Nottingham. Singh, R.N., Hassani, F.P. and Elkington, P.J., 1983. The application of strength and deformation index testing to the stability assessment of coal measures excavation. 24th US Symp. Rock Mechanics, pp. 599-609. Voss, H. and Friedrich, K., 1986. Sliding abrasive wear of short glass-fiber reinforced PTFE composites. J. Mater. Sci. Lett., 5(11): 1111-1114. West, G., 1986. A relation between abrasiveness and quartz content for some coal measures sediments. Int. J. Min. Geol. Eng., 4: 73-78.
Engineering Geology 36 (1994) 293-301
The influence of rock strength and abrasive mineral content on the Cerchar Abrasive Index S.I. AL-Ameen, M.D. Waller Department of Mineral Resources Engineering, The University of Nottingham, UniversityPark, Nottingham NG7 2RD, UK
(Received May 26, 1993; revised version accepted October 7, 1993)
Abstract
The Cerchar abrasive index is based on a simple test and is widely used to assess the abrasiveness of rocks. Detailed examination of the test has shown that the index is largely influenced by the rock strength, and only partially by the rock abrasiveness. The standard test was found to be incapable of providing accurate indices for rocks of low relative abrasiveness and a modification to the test is presented to overcome this problem. The interpretation of the test results has been extended to enable calculation of the specific wear rate which is a more precise measure of the abrasiveness of materials.
1. Introduction
The Cerchar abrasive index test has been widely accepted for the assessment of rock abrasiveness. It is based on a simple test and has been considered to provide a reliable indication of rock abrasiveness (Muftuoglu, 1983; Singh et al., 1983; Atkinson et al., 1986a, b). The test was found to give consistent results with fine- to medium-grained rocks, however, the results were unreliable for weakly consolidated rocks and low abrasive rocks. Furthermore, the results obtained from coarsely crystalline rocks were likely to represent the abrasiveness of individual minerals rather than the abrasiveness of the whole rock. The assessment of the abrasiveness of the U K coal measures rocks was carried out as part of a programme of work aimed at the prediction and control o f abrasive wear of mining equipment. However, the majority of laboratory abrasion tests, including the Cerchar test failed to produce satisfactory results for the coal measures rocks. The 0013-7952/94/$7.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0013-7952(93)E0059-V
main reason was that these rocks, excepting the sandstones, occupy the lower end of the range of Cerchar, index where the accuracy and resolution of the test are severely diminished. This factor is important when discrimination of rocks within a small range of abrasivity is critical. This is particularly important to operations such as the U K coal industry, which, although handling material of relatively low abrasivity, handle very large tonnages which results in significant wear problems. The standard Cerchar test was, therefore, considered unsuitable for the U K coal measures unless some modification was made to the test by using a softer testing tool. Previous work by West (1986) indicated that the Cerchar abrasive index showed good correlation with the abrasive mineral content of the rock. Others (McFeat Smith, 1977) had highlighted that rock abrasiveness depended on the type and degree of cementation material. However, the latter gave no consideration of the influence o f the abrasive minerals in the rock. The current study has con-
294
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firmed the importance of the combined influence of both the cementing materials (rock strength) and the total abrasive mineral content. For example, a high strength ironstone ( 140 MPa) consisting predominantly of siderite (a non-abrasive mineral compared to mild steel), gave a high Cerchar index of 1.33 (EN24). On the other hand, a low strength sandstone (20 MPa) containing highly abrasive minerals gave a Cerchar index of only 0.39 (EN24). It therefore seems to be incorrect to use the term Cerchar abrasive index when a dominating factor was the rock strength.
2. Testing apparatus The testing apparatus consists of a stylus clamped in a holder which is subject to a 7 kg dead weight. The test stylus consists of a cylinder of EN24 steel ground at one end to a cone of angle 90 °, accurately heat treated to 610 + 10 Hv. A special sliding mechanism allows the stylus holder and dead weight to be moved horizontally for a distance of 10 mm. The tested rock sample is cut into a rectangular shape, and the tested faces are ground so that the specimen is polished. The rock specimen is fixed by a special double jaw clamp positioned directly below the stylus holder. The apparatus allows the stylus to be drawn, under a constant weight across the surface of the test sample. A travelling microscope was used to check that the point of the stylus was undamaged prior to the test. Although the stylus tip should be a perfect point before use, in practice the tip may show slight imperfection. Any styli showing damage exceeding 5 lain were rejected, as suggested by Corbett and Aston (1988). The travelling microscope was also used to measure the diameter of the abraded cone end of the stylus to an accuracy of _ 2 ~rn after it had been drawn over the sample. The Cerchar abrasive index of the rock is defined by the diameter (in mm) of the wear fiat formed by truncation of the tip of the cone multiplied by 10. Further details of apparatus and testing procedures are reported in Cerchar (1973) and Corbett and Aston (1988). During this investigation detailed examination
of the progressive development of the stylus wear at various stages of the test was necessary. This was enabled by restricting the movement of the sliding mechanism, using machined stops, such that the stylus could be drawn over the rock surface for distances of 1, 2, 3, 5, 7 and 10 mm.
3. Sample surface condition The standard Cerchar test was designed to be conducted on a flat and polished surface of the rock sample. If the rock sample is hard the Cerchar stylus tends to slide on the smooth rock surface, giving minimum abrasion and hence a low index value. If the test was then conducted on a natural breakage surface of the rock the natural roughness would give a higher abrasion and hence a higher index value. However, with softer rocks, typical of the coal measures (characterised in the UK by soft sandstone, siltstone, silty mudstone, mudstone and seatearth rocks), the stylus tends to indent the rock, and the surface finish of the rock has little effect on the index value. Excavated rock normally breaks along natural discontinuities or along planes normal to the applied force, in either case the naturally developed surface is characteristic of the host rock. The broken rocks potential to cause abrasive wear is dependent on a combination of rock surface roughness (defined by the grain size, shape and orientation), the mineral hardness and the strength of the cementing material. Therefore, for the softer rocks typical of the UK coal measures, it is more representative to employ a natural breakage surface for the Cerchar test.
4. Test stylus During this study it was observed that weakly consolidated rocks, for example seatearth, mudstone and silty mudstone, produced Cerchar indices of less than 0.15. These data occupied the bottom end of the range of Cerchar indices and it was, therefore, concluded that the standard EN24 stylus was too hard for these rocks. Its hardness (610 Hv) is also significantly greater than the
S.I. AL-Ameen, M.D. Waller/EngineeringGeology 36 (1994) 293-301
materials used for the construction of mining equipment (excepting cutting tools). Therefore, a softer stylus, made from EN3 (mild steel, 225 Hv) was prepared specifically for testing the abrasivity of weaker coal measures rocks relative to the majority of mining equipment. Cerchar tests using both EN3 and EN24 styli were conducted on the polished rock surfaces of a range of rocks in order to provide calibration data between the EN3 and EN24 styli. These data are illustrated in Fig. 1, which shows a moderate correlation (correlation coefficient [R] =0.87) between the two indices. The calibration law is shown in Eq. 1: CI ( EN 3 ) = 0.24 + 2.74 CI (EN24) - 0 . 3 9 CI 2 (EN24)
(1)
where, CI = Cerchar Index. However, to permit comparison of the modified Cerchar index for natural rock surfaces using EN3 styli, with the standard Cerchar index using EN24 styli on polished surface, further tests were carried out, as illustrated in Fig. 2, to determine a conversion factor between the two indices. A moderate correlation existed, as shown in Eq. 2: CI (EN3)=0.28+2.43 CI (EN24) - 0 . 2 6 CI 2 (EN24)
(2)
The scattering of data in Figs. 1 and 2 is probably due to the inconsistent hardness of EN24 styli. 146 EN24 styli were tested for Vickers hardness at four points across their diameter and the results are presented in Fig. 3. There was a very large spread in values ranging from 350 Hv to 800 Hv, the distribution indicates that the styli are probably from three batches with average Hv values of
295
m i ~
='e
I•
•
e
•l
•
•
el•
= 0,g0 i
0,
I 1
I 2
Cerehar Index EN24 on polished
I 3
reck surface
Fig. 2. Cerchar index EN3 on natural rock surface and EN24 on a polished rock surface.
500-550, 600-650, and 700-750 and that there is a variation of ___100 Hv in each batch. This probably indicates poor quality control in heat treatment. The effect of material hardness on the abrasivity has been investigated by the authors (A1-Ameen and Waller, 1992a, 1993a) and has shown that doubling of the hardness from 300 Hv to 600 Hv or from 400 Hv to 800 Hv resulted in an increase in abrasive wear by a factor of 1.37. This would indicate that the variation in Hv for the EN24 styli between 500 Hv and 750 Hv will have a much reduced effect on the Cerchar index. 21 EN3 styli were also tested and found to have an average of 225 Hv with a variation of ___4 Hv, considerably more consistent than the EN24 results, as illustrated in Fig. 3. Because of this, and the less abrasive nature of the tested rocks (UK, coal measures rocks), all subsequent Cerchar tests were carried out using EN3 styli. The Cerchar index measured, using EN3 styli on polished rock surfaces and on natural rock surfaces, are very consistent. There was very strong linear correlation between these results, as illustrated in Fig. 4
(nl: z : tu I
i
e~
y = 0.24 + 2.74x - 0.39x l
l I
,
l 2
Carchar Index EN24 on polished
,
2
R
2
= 0.87
I 3
rock surface
Fig. 1..Cerchar index EN3 and EN24 on polished rock surface.
200
250
300
350
400 450
Hardness
500
550
600
650
700
750
range ( H v )
Fig. 3. Distribution of EN3 and EN24 styli hardness.
800
S.I. AL-Ameen, M.D. Waller/Engineering Geology36 (1994) 293-301
296
5. Cerchar mechanism
and Eq. 3: CI (EN3) natural surface = -0.01 + C I (EN3) polished surface
(3)
For the above reasons, the testing programme for the coal measures rocks were conducted on a natural rock surface using EN3 styli. This also saved considerable time in sample preparation and enabled the testing of multiple samples at minimum cost. Table 1 summarises the range of Cerchar index for the rocks tested during this investigation. Some results from igneous rocks have also been included for comparative purposes, their true Cerchar index should be higher than indicated in the table for reasons given later.
4
3
1
y = - 0.01 + 1.00x 0
R2 .
0.93
I
i
I
t
i
1
2
3
4
5
Cerchar Index EN3 on polished rock l u r f a c e
Fig. 4. Cerchar index EN3 on natural and polished rock surface.
The Cerchar index can be categorized as a high stress sliding abrasion test. To understand what factors could influence the results, the test mechanism and the physical nature of the rock were examined in detail. The wear fiat diameter generated on the stylus tip as it is drawn over the rock surface results from the combination of the stylus material, the rock's strength and its abrasive mineral content. The stylus material was standardised for these tests and, as indicated earlier, is of consistent hardness. Stylus wear can, therefore, be considered as related entirely to the characteristics of the tested rock types. To investigate the development of the wear fiat diameter on the stylus, the tip was thoroughly monitored at various stages of the stylus movement over various rocks (1, 2, 3, 5, 7 and 10 mm) using EN3 styli. At the beginning of the test, the stylus tended to penetrate the rock surface due to the high pressure at the tip. The maximum penetration is achieved when the applied load (i.e., 7 kg), acting on the tip area, reaches equilibrium with the rock strength (i.e., the bond strength between adjacent grains). With the contact of the stylus tip on the rock and the initial horizontal movement of the stylus in the rock surface ( ~ 1 mm), the stylus tip tended to deform and shear off due to the dead load and the resistance to horizontal movement, and a fiat-ended tip was formed. The fiat styli end formed at the beginning of the test is not dependent
Table 1 The range of Cerchar index using different styli material on various rock Rock type
EN3 polished rock surface
EN3 natural rock surface
EN24 polished rock surface
Sandstone Siltstone Silty m u d s t o n e Mudstone Seatearth Ironstone Limestone* Igneous rock* Sandstone*
2.02-3.25 1.10-2.31 0.63-1.56 0.22 0.76 0.24-0.44 2.92-3.87 0.90-2.32 4.42-5.12 2.10-4.89
1.75-3.45 0.87 2.16 0.35 1.68 0.29 0.70 0.24 0.44 2.98 4.31 not measured not applicable 2.40-4.97
0.72-1.71 0.25-0.62 0.11 0.70 0.10-0.37 0.10 0.12 1.06-2.20 0.49-1.05 3.09-4.19 1.71-3.32
*Non coal measures.
297
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on the amount of abrasive material in the host rock. It is due entirely to deformation and shear failure at the tip, and its magnitude should be related to the rock strength and the stylus material. If the rock is coarse grained, as for most igneous rocks, the deformation and shear failure at the stylus tip will be related to the hardness of the particular mineral below the stylus tip. If the tested rock is of a coal measures type, the amount of tip sheared will be mainly related to the rock strength and particle size, the latter having a minimum effect since the average grain diameter of these rocks is below 65 lxm (A1-Ameen and Waller, 1992b, 1993b). After a further 2-3 mm of sliding distance the tip fiat diameter will increase as a result of abrasive wear between the rock and the stylus material. Beyond this distance the increase in the wear fiat diameter at the stylus tip will depend entirely on the characteristics of the rock which can be divided into two groups as follows; (A) Hard Rocks. These include igneous and metamorphic rocks and some sedimentary rocks (such as ironstone, dolomite, chert/flint and nonporous sandstone with a cementing material of quartz or siderite). In this rock group, when a balance is reached between the applied stylus pressure and the rock strength (i.e., the stylus tip cannot indent the rock) the stylus will tend to slide over the surface of the rock. In this case, the abrasion between the two materials (rock and stylus) will depend principally on the degree of surface roughness of the sample. However, due to the hardness and compactness of the minerals in this group of rocks, the polished rock surface will always posses a high degree of smoothness. Consequently the abrasion between the stylus tip and the rock surface over approximately 3-10 mm sliding distance will be minimum. The wear generated on the stylus tip over this distance will be very small relative to that generated by tip shear or abrasion over the first 3 mm as illustrated in Fig. 5. Each point on this graph represents averaged data from six tests for all of the rocks excepting the granites where 20 tests were used to take account of the larger grain size. The Cerchar index for this group of rocks will always be smaller in magnitude than it should be,
m
3
• •
Fine pink granite Fine white granite Ironstone
x
Sandstone YH
•
1
0
m,
,
2
r
,
4 Sliding
7
,
6 distance
7
,
B
,
10
+
Sandstone/L
O
SIRstone/H
•
Siltetone/L
•
Silty rnudstone
o
Mudstone/I-I
•
Mudatooo/t.
•
Seatearth
(mm)
(The letter L or H by the tested rock means rock with a low or high strength)
Fig. 5. Cerchar index EN3 measured at different sliding distances on various rock types. as explained above. The upper limit of the Cerchar index for EN24 found in this study did not exceed 4.2. A similar upper limit was reported by Atkinson et al. (1986a, b). This maximum appears to hold irrespective of the type of rock and its mineralogy. The authors, therefore, concluded that the Cerchar index for this group of rocks is not representative of the rock abrasiveness unless a modification is made to the testing procedure. One option would be to test samples with artificially induced and controlled surface roughness or to always test natural surfaces. Another option would be to apply a correction factor to the results. Further work on this problem is currently being undertaken by the authors. (B) Soft Rocks. This group of rocks includes all sedimentary rocks, but excludes those already classified in the hard rock category. The polished surfaces of these rocks are flat but not smooth and the roughness is mainly due to the type of mineral content, particle size, pore space and type of cementing material. The strength of these rocks is defined by the type of the cementing material, and generally it is significantly lower than the hard rocks. Hence, the Cerchar stylus will tend to penetrate deeper into the rock sample. Also, because the applied pressure on the stylus tip is always greater than the rock strength, the stylus tip will remain indented into the rock for the whole sliding distance (i.e., 3-10 mm). This means that
298
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the stylus tip will always be in contact with a rough surface texture which will depend on the inherent physical characteristics and mineral content of the host rock. It also means the abrasion between the two faces (i.e., rock and stylus) will be a maximum. If the host rock does not contain abrasive minerals (for example seatearth or mudstone) the flat tip diameter will not increase with the increased sliding distance. When the host rock contains abrasive minerals the stylus tip will continue to wear away until the end of the defined testing distance. This was observed on most of the coal measures rocks where the Cerchar index continues to increase steadily with increased sliding distance as illustrated in Fig. 5. It can, therefore, be concluded that if the applied pressure generated by the Cerchar stylus on the sample exceeds the rock strength, the Cerchar test is valid. This will be indicated by an indentation streak for the whole 10 mm sliding distance, which should be visible to the naked eye on the tested rock sample. However, if the indentation streak covers only part of the 10 mm sliding distance it indicates sliding with minimum abrasion and a false, low value for the Cerchar index. In this case the test result is not valid.
6. Cerchar index ( E N 3 ) at 1 and 10 mm
The Cerchar index related to a 1 mm sliding distance is about two thirds of the final Cerchar index at a 10 mm sliding distance for most of the tested rock types shown in Fig. 5. Accordingly, the Cerchar index was measured at 1 mm and 10 mm for a wide range of rocks and the results are summarised in Fig. 6. The strong linear correlation between the two Cerchar indices that is evident in Fig. 6 clearly indicates that approximately 30% of the Cerchar index can be attributed to the abrasion effect which corresponds to the final 9 mm of sliding distance. While 70% of Cerchar index is related to the initial movement of 1 mm. The initial wear flat diameter on the stylus tip cannot be attributed to the abrasive mineral content due to the small sliding distance. It is most likely due to a combination of the balance between rock and stylus strength and the depth of the tip
m
la
2
1
o
1
4
Cerchar
Index
5
EN3 (lOrnm)
Fig. 6. Cerchar index EN3 measured for 1 and 10mm sliding distance.
indentation into the sample. The tip wear flat diameter generated during the remaining 9 mm of the testing distance can be related to a combination of the abrasive mineral content and the bond strength between the minerals in the rock (i.e., the rock strength). In order to validate this the rock strength was plotted against the Cerchar index (EN3) for 1 mm sliding distance as shown in Fig. 7. The rock strength used was the uniaxial compressive strength (UCS), tested in accordance with ISRM standards (Brown, 1981). The 1 mm Cerchar index for rocks containing abrasive minerals and rocks containing non-abrasive minerals, showed moderate correlation as illustrated in Fig. 7. The diagram could be utilized for assessing rock strength from a 1 mm Cerchar index or vice versa. Accordingly, if we consider Eq. 4 for Cerchar index (CI): CI (EN3, 10 mm) = C I (EN3, 1 m m ) + C I (EN3, 9 mm)
•
3~
uJ
Rocks witlt al~rmiv¢ minerab Rocks without a b r u i v e n u n ~ a l s Igneous r o e l ~
•
•
o 0 •
•
(4)
o
o
2
~= =, P •
•
•
y=
- 1 . 3 8 + 0 . 0 5 x - 0 . 0 0 0 1 x 2 R 2 •0.90
0 40
80 Rock
=trength
120
160
200
(MPa)
Fig. 7. Cerchar index EN3 (1 mm) versus measured rock strength.
S.1. AL-Ameen, M.D. Waller~EngineeringGeology36 (1994) 293-301
then the rock strength can be included from the equation shown in Fig. 7; and Eq. 4 can be modified to: CI (EN3, 10 mm) = f n { U f S + C I (EN3, 9 mm)}
(5)
This can be further extended to include the influence on the Cerchar index of the abrasive mineral content in the host rock and the strength of the cementing material bonding these minerals together (i.e., the rock strength). These factors relate to the final 9 mm of the sliding distance. Equation 5 can be extended to: CI (EN3, 10 mm) =fn{ UCS + UCS x ~ Abrasive mineral hardness} or;
CI (EN3, 10 mm) =fn{UCS x (1 +}-" Abrasive mineral hardness)}
(6)
The relationship given in Eq. 6 is validated by the strong correlation between the Cerchar index (EN3) and the combination of uniaxial compressive strength and the abrasive mineral hardness as illustrated in Fig. 8. Details of the method used to determine the summation of abrasive mineral hardness are given in AI-Ameen and Waller (1992b). However, when the rock does not contain abrasive minerals (i.e., the abrasive mineral hardness in Eq. 6 is zero or insignificant) the Cerchar index will
3
2
1 ~'A •
y --- -D. + 0.0~67x - 0.00GO02 x 2 R 2 = 0.95
200
400
000
800
1000
1200
1400
UCS x (I÷Y_. Abraelvo mlnoral hardness)
Fig. 8. Cerchar index EN3 versus a combination of rock strength and abrasive mineral hardness in various rocks.
299
continue to show strong correlation with the rock strength as for the case shown in Fig. 7, but will have a higher gradient due to the extra 9 mm of sliding as shown in Fig. 8. These data are representative of limestone, ironstone, some mudstones and seatearth. The data on the top right-hand side of Fig. 8 are from igneous rocks. As explained earlier the measured Cerchar index of these rocks will always give a low indication of true abrasivity. However, if the true abrasivity index for the igneous rocks was considered, then their position on Fig. 8 would move upwards and the relationship would tend to follow a simple linear relationship. The difference between this linear relationship and the graph indicates that a multiplying factor of between 1.2 and 1.5 should be applied to the measured Cerchar index (EN3) for the igneous rocks. From the above it is clear that the measured Cerchar index is a function of rock strength, and the product of rock strength and abrasive mineral content. It can be concluded, therefore, that the Cerchar index is mainly influenced by the rock strength and partly by the abrasive mineral hardness. Hence, the term Cerchar abrasive index used by many workers is not strictly correct. Previously published scales of igneous rock abrasivity classified according to Cerchar abrasiveness index are, therefore, not valid.
7. Specific wear rate and Cerchar index
A disadvantage of the Cerchar index is that it only gives a relative measure of the rocks properties and cannot be used to predict the actual wear rate of materials exposed to relative motion with rock. The specific wear rate (SPWR) is defined as material loss, per unit load, per unit distance, and has the units mma/N km (Voss and Friedrich 1986; Talks, pers. commun., 1993). However, since the measurement of the Cerchar index involves material loss from the stylus tip under fixed load over a constant distance, then the SPWR may be calculated by measuring the difference in volume loss from the stylus material resulting from 10 mm and 1 mm sliding distances. The general equation used for the calculation of SPWR from the Cerchar
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S.L AL-Ameen, M.D. Waller/EngineeringGeology 36 (1994) 293-30l
index is: SPWR = {(EN3, 10 m m ) 3 - - ( E N 3 , 1 mm) 3} x K
(7)
where: K = r c x D / ( C x W ) = 0 . 2 1 1 8 ; D = d i s t a n c e conversion factor; C = v o l u m e constant; and W = applied load x gravity constant. F r o m the data available for the tested rocks the SPWR was determined by three methods as described below: (1) SPWR was calculated from the measured rock strength and mode analysis of the abrasive mineral content, this was done utilising the equations given in Figs. 7 and 8 to calculate the Cerchar index for EN3 (1 ram) and EN3 (10 m m ) which are then substituted in Eq. 7. (2) SPWR was calculated from actual measurements of the Cerchar index for EN3 (1 ram) and EN3 (10 ram) which are then substituted in Eq. 7. (3) SPWR was calculated from measured Cerchar index EN3 ( 1 0 m m ) , and the equation given in Fig. 6 to calculate the derived Cerchar index for EN3 (1 ram), the results are then substituted in equation 7. The different methods used to calculate the SPWR for rocks of the same group gave similar results as shown in Table 2. In general terms, the S P W R results for the coal measures rocks vary between 0 and 6, of which the seatearth and mudstone have no significant SPWR, the silty mudstone can reach up to 0.5 and the siltstone varies from 0.1 to 2.7. Although the siltstone was considered to be abrasive in comparison with other
coal measures rocks, some siltstones recorded a SPWR as low as 0.1. This is due to the fact that some of the siltstones were cemented by a weak clay material which would result in a very low Cerchar index. Consequently the derived SPWR will be small. Other siltstone rocks were found to be cemented by silica and carbonate cement resulting in a higher Cerchar index and, therefore, a higher SPWR. Various coal measures and non-coal measure sandstones were also tested. The non-coal measures sandstones produced SPWRs ranging between 2.3 and 19.9, with values as high as the SPWRs of the igneous rocks (11-20.4), and much higher than the corresponding coal measures sandstones (2.2-5.1). This means that the abrasiveness of the most abrasive coal measures rocks (sandstone) are at the low end of the scale. Similar results were found when assessing the abrasiveness of different sandstone rocks by the continuous abrasion index ( A1-Ameen and Waller, 1993a). Comparison of both Cerchar indices for the non-coal measures sandstones in Table 1 and the SPWR in Table 2 shows that a change in the Cerchar index from 2.1 to 4.98 corresponds to a change in the SPWR of between 2.3 and 19.9. For the coal measures sandstones changes in Cerchar index between 2.0 to 3.25 correspond to SPWRs between 2.2 and 5.2. Also, for the igneous rocks Cerchar indices from 4.4 to 5.1 correspond to SPWRs between 11 and 20.4. F r o m this it can be seen that SPWR is a sensitive indicator of abrasiveness and tends to give a greater spread of values over the whole range of Cerchar indices.
Table 2 The range of a calculated and measured SPWR based on EN3 styli material Rock type
SPWR from UCS and minerals
SPWRfrom EN3 1 and 10 mm
SPWRfrom EN3 10 mm
Sandstone Siltstone Silty mudstone Mudstone Seatearth Ironstone Limestone* Igneous rocks* Sandstone*
1.17-5.58 0.36-1.27 0.10-0.40 0.00-0.10 0.00-0.01 2.44-4.47 0.12 1.15 14.3 21.4 2.23-20.2
2.20-5.21 0.10-2.69 0.03-0.36 0.00-0.10 0.00-0.01 2.84-4.38 0.11-1.64 11.0 20.4 2.31-19.9
I. 15-6.12 0.10-1.71 0.03-0.53 0.00-0.17 0.00-0.02 2.55-5.47 0.10-1.74 12.9-19.6 2.59-18.6
*Non coal measures.
S.I. AL-Ameen, M.D. Waller~EngineeringGeology 36 (1994) 293-301
8. Conclusions
The results of the research presented in this paper show that the Cerchar abrasive index is not related to the abrasive mineral content alone, and the test is largely influenced by the rock strength. Equations have been developed which indicate the dominant role of rock strength on the value of the Cerchar index. The authors, therefore, prefer to use the term Cerchar index instead of Cerchar abrasive index. The standard EN24 stylus specified for the Cerchar test was found to be unsatisfactory for testing of low abrasivity rocks, for example the UK coal measures. An alternative EN3 stylus has been adopted and compared with the EN24 standard and found to give excellent results. For the UK coal measures it was found that satisfactory Cerchar results could be obtained on either polished or natural rock surfaces. This simplifies the sample preparation and enables the testing of a larger number of specimens. It has also been shown that the index can only be considered valid if the applied pressure generated by the Cerchar stylus on the sample exceeds the rock strength. Only a test which generates a visible scratch for the whole length of the test can be considered valid. The modified Cerchar test described in this paper has enabled the determination of the abrasive characteristics of typical UK coal measures rocks. This can be used as a basis for the prediction and control of abrasive wear of mining equipment. The SPWR calculated from the Cerchar index is a better means of quantifying the abrasiveness of rock. Various algorithms have been established for the calculation of the SPWR utilising a combination of the Cerchar index, rock strength and abrasive mineral content. The SPWR is a sensitive indicator of rock abrasiveness, it can be used together with the Cerchar index for assessing rock abrasiveness particularly with highly abrasive rocks where the discrimination of the Cerchar index is limited. However, the SPWR sensitivity is poor when the Cerchar index is low, and improves as the Cerchar index increases.
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Acknowledgments
The authors would like to acknowledge the generous financial support given by European Coal and Steel Commission and British Coal. Also gratitude is extend to the technical staff of the Department of Mineral Resources Engineering at the University of Nottingham for their help during this investigation.
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