Investigation of the Effects on Pick Wear of Abrasion ...

Almaty, October 12-14, 2011

MINE PLANNING & EQUIPMENT SELECTION

Investigation of the Effects on Pick Wear of Abrasion Resistance and Abrasiveness Properties of Rocks N. BILIM*1, A. E. DURSUN2, R. ÇOMAKLI2 Underground and open pit mining excavations in rock are manufactured either by drilling and blasting or by mechanical methods using excavation machines. The selection of optimum cutter tools (pick) type is an important point. Because production capacity will decrease and unit cost will increase with a wrong selection of picks. Besides, equipment life will decrease because of occurring unnecessary vibration on cutter equipment. The abrasion of drilling tools and cutter in mining and tunneling has always been a dominant factor for the costs of rock excavation. The abrasivity of rock and even soil is a factor with considerable influence on the wear of tools. Abrasiveness is a property that reflects the abrasive effect of rocks and minerals on materials in contact and therefore has a large bearing on the service life and efficiency of materials handling equipment. There are several methods for estimating the abrasiveness of rocks used with various degrees of success and relevance. In this research, the marble and travertine were selected as natural stone materials. They have different physico-mechanical properties. Some material properties of seven different natural stone types were determined and under particular conditions laboratory experiments were performed. The purpose of this study is to determine abrasion ressistance and abrasiveness properties of natural stones and is to investigate the influence of this property on pick abrasion. So some physico-mechanical and abrasion experiments have been carried out for determining abrasion properties of rocks. Then analyzed the test results for the influence on picks wear. The relations between abrasion properties of rocks and pick wear properties are evaluated. As a result, high correlation obtained between uniaxial compressive Keywords: Abrasion, Abrasiveness, Cerchar, Pick wear, Natural stone, Rock properties. 1. Introduction The abrasivity of rock and even soil is a factor with considerable influence on the wear of tools. Thus the wear causes tool consumption and the decreasing excavation speed of rock excavation in tunneling and mining or quarrying. The wear depends on the one hand on the machinery being used for excavation; that is the devices and all tools that have contact to the rock or loosened material. On the other hand the rock and the 1

Corresponding Author: Dr. Niyazi Bilim, Mining Engineering Department, Selçuk University, Turkey. Tel.: +90-332-2232099; Fax: +90-332-2410635, Email: [email protected] 2 Department of Mining Engineering, Selçuk University, Konya, 42075, Turkey

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geological conditions can be specified by geotechnical parameters. The abrasivity of rocks is also influenced by the petrography and mineralogical characteristics of rocks, the abrasive and hard mineral content (especially quartz), grain size and cementation level. Kasling and Thuro, (2010) stated that many geological way of determination is used when the quartz or equivalent quartz content of rock is specified by microscopic examination of a thin section. Another, more technical way is to determine the abrasivity of rocks by laboratory tests. Alber (2008) stated that other rock properties known to influence the wear of tools are the uniaxial compressive rock strength, tensile (brazilian) rock strength, Young’s Modulus and the Fracture toughness mode. The term ‘‘abrasivity’’ refers to the degree to which a rock can wear a neighboring material with which it is in moving contact (Bilgin et al. 2006). Rock abrasivity has a direct relationship with the wear occurring in the bits of rock drills, roadheaders, tunnel boring machines and other equipment, and has long been researched (Okubo et al., 2011). The abrasion resistance of natural stones and aggregates is extremely important especially when they are used road and as floor covering. Carbonate rocks present a larger reserve than other rocks on earth. The abrasion resistance of rocks mainly be affected by their mineralogic, petrographic and textural characteristics. The Cerchar abrasivity test, and associated CAI, was developed in 70s by French institute (Laboratoire du Centre d’Études et Recherches des Charbonnages de France (CERCHAR). for abrasitiy testing in coal bearing rocks. The test layout is described in Cerchar (1986). Cerchar Abrasivity Index (CAI) is one of the commonly used tests to evaluate rock abrasiveness for cutter life estimation of cutter on excavation machines. However, there are not internationally recommended testing standard and some confusion may arise from that fact (Rostami, 2005). Two test apparatus named as Cerchar (Cerchar, 1986) and West (West, 1989) apparatus be usually used by researchers on the world (Figure 1). These two tests apparatuses developed to determine CAI value differs from one another with very small differences. The most important difference is that while the period defined for 10 mm marking is 1 sn in Cerchar (1986), it can be up to 60 sn in West apparatus (1989). Both these testing tools are used to determine Cerchar value.

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Almaty, October 12-14, 12 14, 2011


Figure 1 - Test apparatus for abrasiveness test (West, 1989) The test was developed at a time of increasi increasing ng mechanisation in the coal mining and tunnelling industries and with it a need to estimate likely production rates and operating cost in different rock conditions with different scales and type of equipment. A method of determining the abrasivity of rock is one important parameter needed in this estimation (Stanford and Hagan, 2009). The Cerchar Abrasivity Index is used as a key parameter in prediction models for TBM tunneling and for roadheader excavations (Kasling and Thoro, 2010; Rostami et al. 2005, A Alber, lber, 2008). The Cerchar abrasivity test is used frequently to evaluate the effects of abrasivity of rocks on cutter wear. It is considered to provide a reliable indication of rock abrasiveness (Muftuoglu, 1983; Singh et al., 1983; Atkinson et al., 1986, A Ameen meen and Waller, 1994). The original Cerchar test is specified for Rockwell Hardness Hardness-C C (HRC) 54– 54–56 56 while some suppliers of pins offer them with Rockwell hardnesshardness-C C (HRC) of only 40 40– 43 as with the West apparatus. Consequently, the resulting differing CAI va values lues are not surprising as reported by Rostami (2005). Michalakopoulos et al (2006) investigated the influence of steel styli hardness on the CAI value. They tested rocks samples with steel styli of both HRC 55 and 40 and yielded equation can be used to co convert nvert CAI40 values to CAI55.

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One of the important properties of rock aggregates and natural stone is abrasion resistance. The abrasion resistance of aggregates is generally tested using the Los Angeles testing machine. This test is time consuming and expensive compared to the indirect tests. Lower Los Angeles (LA) abrasion loss values indicate aggregate that is tougher and more resistant to abrasion. The LA abrasion test is a common test method used to indicate aggregate toughness and abrasion characteristics. Kahraman and Gunaydin (2007) investigated the possibility of predicting Los Angeles (LA) abrasion loss from indirect tests. They concluded that LA abrasion loss of aggregates can reliably be estimated from point load index. This paper presents an experimental study of the measurement of abrasivity of several types of igneous, sedimentary and metamorphic rock that used in natural stone industry. The aim of this study is to determine the effects of abrasion resistances of rocks on cutter wear. In this research, it was investigated the effects of rock strength and hardness on cutter wear. Besides, whether there was a relation between abrasion resistance of rocks (the resistance they against abrasion) and cutter wear was investigated. Previous studies generally studied the effects of abrasiveness characteristics of rocks on cutter wear. However, there are few studies which investigated whether there is a relation between the resistance rocks showed against abrasion (in other words their resistance to abrasion) and cutter wear. One of the most important factor that affect cutter wears is the quartz content. Therefore, in this study rock samples that do not contain quartz was chosen in order to be able to better analyze other parameters affecting cutter wear (that is, we sought to provide variablesparameters- stable). In this study, Cerchar abrasivity test was used to determine cutter wear and Los Angeles abrasivty test was used to determine rocks abrasion.

2. Location and type of samples collected Seven rock types tested in the present study were collected from different locations in Turkey. During sampling, representative rock mass samples were collected to determine the abrasivity properties in the laboratory. Table 1 illustrates the list of rock types with their classes and the locations where the samples were collected. All these sample types are obtained from natural stone mine. When choosing samples, the samples with 0% or very close to 0 quartz content were chosen because quartz content can lead to

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differences in the results, which will make the interpretation of the results more challenging. The samples which do not content quartz which features the most abrasivity in minerals were chosen. Thin sections were prepared in the lab to understand whether the samples chosen have any quartz and these sections were examined under polyaryzane microscope and their quartz contents were determined. Table 1 - List of rock types with class and location Rock type

Rock class

Marble (White)-1 Marble (Pink) Marble (White)-2 Marble (White)-3 Travertine (Yellow) Travertine (Gray) Travertine (Beige)

Metamorphic Metamorphic Metamorphic Metamorphic Sedimentary Sedimentary Sedimentary

Location (in Turkey)

Quartz content (%)

Eskisehir-Sivrihisar Yozgat Yozgat Eskisehir Konya-Seydisehir Nevsehir-Avanos Karaman

0 0 0 0 0 0 0

3. Laboratory Studies Laboratory studies were first started with preparation and examination of thin sections under microscope. Later on uniaxial compressive strength, density, porosity, Schmidt hardness, Mohs hardness, Cerchar abrasivity index (CAI) and finally Los Angeles abrasivity tests were performed.

3.1 Uniaxial Compressive Strength Test (UCS) An electro-hydraulic servo-controlled stiff testing machine was used for determination of the uniaxial compressive strength. Uniaxial compressive tests were performed on core samples of diameters 54 mm and a length-to-diameter ratio of approximately 2.5-3. UCS tests were repeated at least ten times for each rock types and totally 70 UCS tests were performed on all rock types. Methods for UCS tests follow the suggested method given by the ISRM (1981) Results of natural stone samples were given in Table 2.

3.2 Density And Porosity Test Regular samples were used for determining natural density of rock samples. The specimen volume was calculated from an average of several Buoyancy readings. The weight of the specimen was determined by a balance, capable of weighing to an

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accuracy of 0.01 g or % of the sample weight. The natural density values were obtained from the ratio of the specimen weight to the specimen volume. At least ten samples for each natural stone samples were tested and the average value was recorded as density value. The results were given in Table 2.

3.3 Schmidt Hammer Test ISRM (1981) suggested that 20 rebound values from single impacts separated by at least a plunger diameter should be recorded and averaged the upper ten values. The test method was carried out all rock types. Schmidt hammer tests were performed on large blocks. The testing side surfaces of samples were smoothened. Tests were performed with an L-type hammer having impact energy of 0.735Nm. All tests were performed with the hammer held vertically downwards. The average values of Schmidt hardness for each rock type are given in Table 2.

3.4 Mohs Hardness Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The samples of matter used by Mohs are all minerals. In this study, Mohs hardness was determined by using minerals and tools whose Mohs hardness is known.

3.5 Cerchar Abrasivity Test (CAI) The testing principle of Cerchar and West apparatus is based on a steel tool with defined geometry and hardness that is scratched over 10 mm the surface of a rough rock sample under static load of 70 N. The Cerchar Abrasivity Index (CAI) is then calculated from the measured diameter of the resulting wear flat on the steel tool. In this study, CAI tests were performed using the West apparatus (Figure 2) by scratching steel tools of HRC 40-42 loaded with 70 N in distance of 10 mm. The abrasiveness of the rock was determined by the resultant wear flat generated at the point of the steel tool, which was measured with 0.001 mm sensitive under a stereo

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microscope by computer software. A sample image of the measurements of abrasion in steel tools under 45x magnifying stereo microscope is given Figure 3. Abrasiveness is defined as a wear flat of 0.1 mm, which is equal to 1 CAI, ranging from 0 to 6 (Table 2). In each rock type, experiments were carried out with 5 different steel tools and a total of 35 tools were used in this experiment. As a result of experiments, mean CAI values obtained from seven different rocks used are shown in Table 3.

Figure 2 - West apparatus

Figure 3 - Wear on steel tool after Cerchar abrasivity tests

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Table 2 - The classification of abrasiveness (Cerchar, 1986) Cerchar abrasiveness index 0.3-0.5 0.5-1 1.0-2.0 2.0-4.0 4.0-6.0

Classification Not very abrasive Slightly abrasive Medium abrasiveness to abrasive Very abrasive Extremely abrasive

To determine the CAI value the rock is slowly displaced by 10 mm with a velocity of approximately 1 mm/s. The abrasiveness of the rock is then obtained by measuring the resulting wear flat on the tip of the steel stylus. 3.6 Los Angeles Test (LA) The Los Angeles test (Figure 4) determines the resistance of an aggregate to fragmentation. Test samples were oven-dried at 105–110°C in stove and then cooled to room temperature before they were tested. ASTM C 131- 06 suggests four aggregate size gradings method. In this study, grading C was used in all tests. According to the ASTM method C 131-66 method, the 5000 ± 5g aggregate sample is placed in a steel drum with eight steel spheres. The drum rotates 500 revolutions (31-33 rpm). After the test, the crushed material is sieved through a 1.6 mm sieve and the Los Angeles value is calculated.

Figure 4 - Los Angeles testing machine

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4. Results and Discussions It is known that the rocks abrasivity is influential on cutter wear and many researchers carried out studies on this issue. The physical, chemical, mineralogical, mechanic and petrographic characteristics of rocks effect cutter wear more or less. Many researchers have emphasized that the amount of abrasive mineral in rock (especially quartz content) affects cutter wear. However, there are a limited number of studies on the factors affecting cutter wear with rocks without quartz content. In this study, the factors that effect cutter wear were studied by choosing rock groups without quartz content. In order to describe the relationships between mechanical properties Cerchar Abrasivity Index (CAI), Los Angeles of the tested rock samples, a regression analysis was carried out. The equation of the best fit line and the coefficient of determination (R2) were determined for each test result. Abrasion values and mechanical properties obtained by the tests are presented in Table 3. Table 3 - The physico-mechanical properties of the samples Sample Names

K500 (%)

Porosity (%)

Density (gr/cm3)

UCS (Mpa)

Schmidt Hardness Marble (White)-1 20.32 1.02±0.11 2.73±0.02 68.21±14.49 69,95±0.99 Marble (White)-2 53.47 0.32±0.13 2.74±0.00 57.80±12.91 60,66±2.49 Marble (Pink) 45.61 0.24±0.07 2.77±0.05 58.19±14.76 65,05±2.78 Travertine (Yellow) 38.92 11.18±3.83 2.43±0.02 45.25±11.11 42,08±0.78

CAI 1.90±0.68

Mohs Hardness 3.75

1.39±0.40

3.50

2.24±0.41

4.50

1.25±0.39

2.75

Travetine (Gray)

38.67 9.00±2.84 2.58±0.04 47.50±3.13

47,06±0.86

1.19±0.29

2.75

Marble (White)-3

22.89 0.85±0.10 2.66±0.00 56.16±12.77 50,55±1.00 34.98 3.42±1.19 2.40±0.09 46.32±4.81 51,63±1.12

1.38±0.33

3.50

1.33±0.24

3.00

Traverten (Beige)

K500=Abrasion loss after 500 revolutions in Los Angeles Test, UCS=Uniaxial compressive strength

As a result of Cerchar abrasivity index (CAI) experiments carried out on seven different natural stone samples, the CAI values of six types of samples was found to be between 1.0-2.0 and abrasivity category was determined as “Medium abrasiveness to abrasive” (Table 2). As the CAI value of only the rock type called Marble (Pink) CAI was found to range between 2.0-4.0, the abrasivity category of this sample was called as “Very abrasive” (Table 2). The reason why the abrasivity of Marble (Pink) sample was high can be attributed to its high mineral hardness (Mohs hardness) compared to other samples. As it can be seen in Table 3, the sample with highest Mohs hardness is Marble (Pink) sample.

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In this study, the quartz content in samples were kept contant in order to interpret the extend effect of some parameters on abrasivity (samples without quartz were chosen). While mineral hardness (Mohs hardness) which is one of the other important parameters affecting rocks abrasivity is close in six types of rocks, it was found to be higher in Marble (Pink) sample compared to others (Table 3). Although uniaxial compressive strength value of Marble (Pink) rock sample was not obtain to be very high, its abrasivity (CAI value) was found to be high. The reason for this case is most probably the fact that its mineral harness is higher compared to other rock types. This different effect in the rock lead to low relation between Uniaxial compressive strength and Cerchar abrasivity index (R2=0.48). However, when Marble (Pink) sample is disregarded, the relation is found to be significant and high (Figure 5, R2=0.83). As it is mentioned above, there is a significant relation between mineral hardness and rock abrasivity (CAI), which is shown in Figure 6. Figure 7 shows the relation between Schmidt hardness and Cerchar abrasivity experiment. The relation coefficient between these two parameters was found to be R2=0.68. When the sample with high mineral hardness (Marble (pink) is omitted a high relation was found (R2=0.81). That is, sometimes although hardness or strength of the rocks is low, they can be more abrasive because they have high mineral hardness.

Figure 5 - Relation between Cerchar abrasivity index and Uniaxial compressive strength

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Figure 6 - Relation between Cerchar abrasivity index and Mohs hardness

Figure 7 - Relation between Cerchar abrasivity index and Schmidt hardness As a result of Los Angeles test, abrasion loss percentages of seven different natural stone samples were found. There was not found any significant relation between

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Los Angeles value used to determine strength of rock samples against abrasion and toughness and CAI value. A very low relation was found even if Marble (pink) sample which affects other regression analyses was excluded (Figure 8).

Figure 8 - Relation between Cerchar abrasivity index and Los Angeles abrasivity

5. Conclusions In this study, the effects of rocks’ mechanic (especially hardness and strength) and abrasion and abrasivity characteristics on cutter wear was analyzed. It was determined that cutter wear increased with increase in the hardness (Schmidt hardness) and strength of rocks. Sometimes even if the hardness and strengths of rocks are low, they can be more abrasive because of the minerals hardness. Therefore, significant results can be obtained by taking mineral hardness of rocks into consideration while assessing the relations between rocks mechanic characteristics with Cerchar abrasivity index experiment. In this study, no significant relation was found between rocks abrasion strengths (Los Angeles test) and cutter wears.

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References 1.

Alber, M., Stress dependency of the Cerchar abrasivity index (CAI) and its effects on wear of selected rock cutting tools, Tunnelling and Underground Space Technology, 2008, 23, 351–359.

2.

Ameen, S.I.AL. and Waller, M.D., The influence of rock strength and abrasive mineral content on the Cerchar Abrasive Index. Engineering Geology, 1994, 36(34), 293-301.

3.

ASTM C131 - 06 Standard Test Method for Resistance to Degradation of Small Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine

4.

Atkinson, T., Cassapi, V.B. and Singh, R.N., Assessment of abrasive wear resistance potential in rock excavation machinery. Int. J. Min. Geol. Eng., 1986, 3, 151–163.

5.

Bilgin, N., Demircin, M.A., Copur, H., Balci, C., Tuncdemir, H., and Akcin, N., Dominant rock properties affecting the performance of conical picks and the comparison of some experimental and theoretical results, International Journal of Rock Mechanic and Mining Sciences, 2006, 43:139–156.

6.

Cerchar- Centre d´Études et des Recherches des Charbonages de France. 1986. The Cerchar abrasiveness index. Verneuil, 12.

7.

ISRM, 1981. International Society for Rock Mechanics (ISRM), Rock Characterization, Testing and Monitoring, ISRM Suggested Methods, Brown, E. T. (Editor). Pergamon Press, Oxford.

8.

Kahraman, S. and Fener, M. (2007). Predicting the Los Angeles abrasion loss of rock aggregates from the uniaxial compressive strength. Mater. Lett. 61, 48614865.

9.

Kasling, H. and Thuro, K., Determining rock abrasivity in the laboratory, In: Proceedings of the European Rock Mechanics Symposium EUROCK 2010, Lausanne, Switzerland, 15-18 June 2010, 4 p.

10. Kasling, H. and Thuro, K., Determining abrasivity of rock and soil in laboratory, 2010 Taylor & Francis Group London, ISBN 978-0-415-60034-7, 2010, 19731980.

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11. Muftuoglu, Y.V., A study of factors affecting diggability in British surface coal mines. Ph.D. thesis, University of Nottingham, 1983. 12. Michalakopoulos, T.N., Anagnostou, V.G., Bassanou, M.E., Panagiotou, G.N., The influence of steel styli hardness on the Cerchar abrasiveness index value, International Journal of Rock Mechanics & Mining Sciences, 2006, 43, 321–327. 13. Okubo, S., Fukui, K., Nishimatsu, Y., Estimating Abrasivity of Rock by Laboratory and In Situ Tests, Rock Mechanics Rock Engineering, 2011, 44, 231–244. 14. Rostami, J., Ozdemir, L, Bruland, A. and Dahl, F., Review of Issues Related to Cerchar Abrasivity Testing and Their Implications on Geotechnical Investigations and Cutter Cost Estimates, Rapid Excavation and Tunneling Conference, Seattle, WA, June 27-29, 2005. 15. Rostami, J., CAI testing and its implications. Tunnels Tunnelling Int. October), 2005, 43–45. 16. Stanford, J., Hagan, P., An Assessment of the Impact of Stylus Metallurgy on Cerchar Abrasiveness Index, 2009 Underground Coal Operators’ Conference, Paper 118, The University of New South Wales (UNSW), Sydney, 12 – 13 February 2009, 348-355. 17. Singh, R.N., Hassani, F.P. and Elkington, P.J. The application of strength and deformation index testing to the stability assessment of coal measures excavation. 24thUS Symp. Rock Mechanics, 1983, 599–609. 18. West, G. Rock abrasiveness testing for tunneling, International Journal of Rock Mechanics and Mining Sciences & Geomech. Abstr., 1989, 26 (2), 151-160.

Biography Dr. Niyazi Bilim is an Assist Prof. Dr. in the Mining Engineering Department at Selçuk University, Turkey. He was born in 1975 in Sivas, Turkey. He obtained his bachelor degrees in Mining Engineering at Cumhuriyet University (Sivas, Turkey) in 1996. He started academic studies in 2000 as research assistant at “Selcuk University, Mining Engineering Department, Konya, Turkey. Then he obtained M.Sc. degree in 2002 and Ph.D degree in 2007 from Selçuk University. His research interest is mine mechanization and automation, excavation, cutting and drilling systems in mining

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