- Email: [email protected]
Evaluation of grinding media wear-rate by a combined grinding method
Minerals Engineering 73 (2015) 39–43
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Evaluation of grinding media wear-rate by a combined grinding method Junfu King a, Qiang Li b, Alex Wang (Baikun Wang) a,⇑, Cathy He a, Joey Zhou a, Henry Deng a, Ray Xu a a b
King’s Ceramics & Chemicals Co., Ltd., No. 910, 9th Section, Jinsong, Chaoyang District, Beijing, China East China Normal University, No. 500, Dongchuan Road, Minhang District, Shanghai, China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Revised 27 November 2014 Accepted 5 December 2014 Available online 14 January 2015 Keywords: Grinding media Wear-rate Combined grinding method
a b s t r a c t It is well-known that lab tests on wear-rate of grinding media cannot precisely represent its industrial performance due to complex grinding conditions. Nevertheless the lab data provides reference to the industrial data. Therefore, a reproducible test method on wear-rate in lab is necessary. By providing detailed data, this paper challenges the traditional wear-rate testing methods. Two commonly used methods on wear-rate test, self-wear in water and grinding with mineral slurry, are respectively employed. However, obvious fluctuation of wear-rate/time curves indicates neither of the two normal methods is reliable. This paper introduces a wear-rate testing method on lab scale which combines self-wear in water and grinding with mineral slurry. By this method, some repeatable wear-rate/time curves are displayed after a few hours. The mechanism will be discussed in detail in this paper. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Wear-rate is one of the most important factors when evaluating the overall performance of grinding media. Therefore, extensive tests on media wear have been carried out by many media suppliers, users and labs (Berthiaux et al., 1996; Blecher et al., 1996a,b; Blickensderfer and Tylczak, 1989; Frances, 2004; Jensen et al., 2010; Radziszewski, 2002). According to some papers (Chenje et al., 2009; Morrow and Sepulveda, 2014) and many feedback from customers of KING’S CERAMICS’s, there is a correlation on media wear-rates between lab equipments and industrial mills. Therefore, to get repeatable media wear-rates in lab is very meaningful to predict the media performance in industrial scale. However, two common approaches on media wear test, namely self-wear rate test in water and wear rate test with mineral slurry, are always poor in repeatability in lab. As a grinding media supplier, KING’S CERAMICS has been making efforts in developing a reproducible method in lab to evaluate the wear performance of different ceramic media. In this paper, we designed a rapid wear-rate testing method on lab scale which took the advantages of the above two mentioned common methods, and comparatively, repeatable results can be drawn from this new method.
2.1. Test machines
⇑ Corresponding author. Tel.: +86 10 87763540; fax: +86 10 67785715x3540. E-mail addresses: [email protected], [email protected] (A. Wang). http://dx.doi.org/10.1016/j.mineng.2014.12.002 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.
Vertical stirred mill Fig. 1 (left) and rapid pot mill Fig. 1 (right) were used in order to provide robust grinding and impacting forces. Also some detailed parameters of both the two mills are listed in Table 1. 2.2. Test methods 2.2.1. Self-wear in water 1200.0 g grinding media and 400.0 g water were added into the vertical stirred mill to grind for some time. Grinding was conducted at a speed of 900 rpm. The same ceramic media of 600.0 g and water 200.0 g were filled into the rapid pot mill for wear test at 450 rpm. 2.2.2. Grinding with mineral slurry 1200.0 g ceramic media, 400.0 g water and 400.0 g zircon sand (Eucla Zircon Premium Grade with d50 = 92 lm provided by Iluka Resources, 7–8 in Moh’s hardness) were added into the vertical stirred mill. Grinding speed was also 900 rpm. 500.0 g identical media, 100.0 g water and 100.0 g zircon sand (same with above) were filled into the rapid pot mill for wear test at 450 rpm. Both wear-rate test methods are applied in vertical stirred mill and rapid pot mill, with consecutive grinding for 1 h each time. The method was repeated many times to achieve stable wear-rates.
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J. King et al. / Minerals Engineering 73 (2015) 39–43
Fig. 1. Vertical stirred mill (left) and rapid pot mill (right).
Table 1 Parameters of vertical stirred mill and rapid pot mill. Parameters
Vertical stirred mill
Rapid pot mill
Motor power (kw) Grinding chamber volume (l) Impeller length/maximum diameter (mm) Operation speed (rpm)
0.55 1.87 100
0.45 1.0 150
900
450
The hardness of zircon sand is relatively higher among natural minerals, so the wear-rate differences among different grinding media can be maximized. 2.2.3. Combined grinding method Combined grinding method, which was proceeded in the rapid pot mill, includes two steps: (1) grinding with mineral slurry: the same ceramic media of 500.0 g, water 100.0 g and zircon sand 100.0 g were ground at 450 rpm for 1 h, and then another 2 h in fresh zircon sand slurry after drying the media. (2) Self-wear in water: after grinding with slurry, ceramic media were washed, dried, and then filled into rapid pot mill again (media to water mass ratio is 3:1) to grind for 1 h at 450 rpm, followed by another 2 h grinding in new water under the same condition after drying the media.
Fig. 2 shows the self-wear curves of different grinding media in water versus time. The lines marked with diamonds (A1 and B1) are the results in rapid pot mill, and those marked with circles (A2 and B2) are in vertical stirred mill for media A and B, only the wear-rate curve of media C in rapid pot mill is examined. For media A, after grinding for 7 h, a relatively stable wear-rate could be obtained. For media B, it almost needs 8 h to observe a stable values. In case of media C, only a rising wave line is received as shown in Fig. 3, which could not tell us at which time point is reliable to evaluate the wear-rate. The results mentioned above seem to indicate that self-wear is not an adaptable method for different grinding media.
3.2. Wear-rate in mineral slurry Grinding in mineral slurry is usually used as a relatively reasonable method for testing wear-rate. The wear-rate curves of different ceramic media grinding with mineral slurry can be found in Figs. 4 and 5. However, the curves of media A and C (only in rapid pot mill) are wave lines in wide
2.2.4. Grinding media Three kinds of ceramic media with different densities of 3.32, 4.05 and 4.23 g/cm3 were selected from KING’S CERAMICS and the market, which were named as media A, B and C, respectively. 2.2.5. Calculation on wear-rate The media wear-rate can be calculated by equation ‘‘wearrate = a/b/t’’, where a is the media loss (g) after grinding, b is the initial media charge (kg), and t is the grinding time (h). The unit of wear-rate is gkg 1 h 1. 3. Results and discussion 3.1. Self-wear test Self-wear is a simple and easy method, so it is widely used. But it cannot simulate the grinding conditions of practical operation.
Fig. 2. Self-wear curves of media A and B in vertical stirred mill and rapid pot mill.
J. King et al. / Minerals Engineering 73 (2015) 39–43
Fig. 3. Self-wear curve of media C in rapid pot mill.
41
Fig. 6. Parallel tests on self-wear and wear rate in mineral slurry of media A in rapid pot mill.
Fig. 6 gives the wear-rate curves of media A through parallel tests by self-wear and grinding with mineral slurry in rapid pot mill. The results indicate bad reproducibility. The wear-rate instability can be explained from the abrasive and abrasion processes during the grinding. Different abrasive processes induced by different mechanical forces play different roles in processes of self-wear and grinding with mineral slurry. 3.4. Wear mechanism discussion
Fig. 4. Wear rate curves of media A and B in mineral slurry in vertical stirred mill and rapid pot mill.
Fig. 5. Wear rate curve of media C in mineral slurry in rapid pot mill.
range. So we cannot select the time point at which we can determine the reliable wear-rate. 3.3. Parallel wear-rate tests Another way to examine the reliability of wear tests is to obtain the reproducible curves by several parallel tests. To magnify the wear-rate and minimize the systematical error, wear tests were only deployed in rapid pot mill.
3.4.1. Self-wear mechanism In the case of self-wear, impact force among different media plays an important role to destroy the media surface structure. Firstly impact induces defects on the surface due to plastic deformation of crystal particles (Kang and Hadfield, 2005). The formation probability of defects is usually low, because the plastic deformation can absorb some energy brought by impact to prevent the media surface from being destroyed. But accumulated defects would promptly lead to the structure collapse on the surface under larger impact force (Torrance, 2005). When collapses take place, the wear-rate rises. So, wave-like curves of wear-rate might be observed after self-wear test, just like those in Fig. 2. Here the formation of defects is a slow process while the collapse is a fast one. After quick collapse, the surface can be cleaned and polished quickly. In Fig. 7, collapses and holes on surface of media A can be seen although the surface is smooth generally. This indicates that the mechanical property of media A is not good enough to resist the impact and friction when ground in the water. For media B, holes with size around 1–2 lm are in presence as well. While for media C, surface defects are less and micro-cuttings (slim scratches) can also be observed thanks to its good balance between hardness and fracture toughness. 3.4.2. Wear in mineral slurry During grinding with mineral slurry, the frictional force from slurry plays more important role (Becker and Schwedes, 1999; Cleary and Morrison, 2011; Gers et al., 2010). It causes the corrosion on the surface of ceramic media. For most ceramic products, including ceramic media, sintering aids are needed in the production process, which will transform into grain boundary phase after sintering. The corrosion preferentially takes place at the grain boundary phase, and the defects are induced at the same place. Fig. 8 shows typical SEM photos of media surface after grinding in mineral slurry. Rough surfaces can be seen after grain boundaries are corroded. Compared with Fig. 7, the surfaces of media A and B are more unsmoothed after grinding in mineral slurry. For
42
J. King et al. / Minerals Engineering 73 (2015) 39–43
Media B
Media A
Media C
structure collapse
micro-cutting
N
x10k
10µm
N
x10k
10µm
Fig. 7. Typical SEM photos of media surfaces after self-wear.
Media A
Media C
Media B
Fig. 8. Typical SEM photos of media surfaces after grinding with mineral slurry.
media C, more defects on the surface are observed and the surface is not smooth anymore. When the defects accumulate to cause the structure collapse on the surface, wear-rate will rise suddenly (Rezaeizadeh et al., 2010). Usually the formation of defects is a fast process while the collapse is a slower one. As the corrosion goes on and the defects increase quickly, the collapse will increase in frequency, and the curves of wear-rate will level off. This abrasive process brings typical wear-rate curves in Figs. 4 and 5.
Table 2 Statistical analysis on wear-rates in mineral slurry of different media by combined grinding method. Media A
Media B
Media C
Wear-rate in mineral slurry (%)
0.400 0.395 0.409 0.394 0.398
0.409 0.408 0.378 0.385 0.392
0.413 0.402 0.415 0.400 0.405
Lower confidence limit (%) Upper confidence limit (%)
0.392 0.407
0.377 0.412
0.399 0.415
3.5. Combined grinding method After analyzing and comparing the abrasive process of self-wear and grinding with mineral slurry, it can be proposed that the formation of defects does not synchronize with structure collapses, which makes the wear-rate unstable. If the fast process of defects formation when grinding with mineral slurry and the fast process of surface collapses in self-wear are combined, a fast and reliable method for testing wear-rate may be found. Basing on this assumption, the combined method is proposed as following steps (also in rapid pot mill): (1) grinding in mineral slurry for 1 h; (2) grinding in fresh mineral slurry for another 2 h; (3) self-wear in water for 1 h; and (4) self-wear in fresh water for another 2 h. After each step, the media will be washed clean and dried to test the wear-rate before the next step. Step (1) is to simulate the media surface with that has grinding in mineral slurry for a period of time, and the wear-rate then obtained in step (2) can be seen as the wear-rate grinding with mineral slurry. Step (3) is to remove the defects and recover the media surface to a smooth situation, and the wear-rate obtained in step (4) can be treated as the self-wear rate. Tables 2 and 3 display the wear-rate values of different grinding media and corresponding lower and upper confidence limits when 95% confidence level for the accuracy of wear rates are set. Taking media A for example, if the wear rate in mineral slurry is between 0.392% and 0.407%, then the wear rate result is convincing. This 6 h combined grinding method is rapid and reliable to frequently
Table 3 Statistical analysis on self-wear rates of different media by combined grinding method. Media A
Media B
Media C
Self-wear rate (%)
0.958 0.950 0.946 0.949 0.952
0.750 0.758 0.812 0.765 0.790
1.620 1.595 1.578 1.608 1.582
Lower confidence limit (%) Upper confidence limit (%)
0.945 0.957
0.743 0.807
1.575 1.618
monitor the wear rate the daily production of ceramic media. For further study, much work should be done to comprehensively research the wear mechanism of grinding behavior.
4. Conclusions Wear-rates of different grinding media are determined by two commonly used methods of self-wear in water and grinding with
J. King et al. / Minerals Engineering 73 (2015) 39–43
mineral slurry. After the analysis of all results, we design an improved testing method on wear-rate, which combines the fast process of defects formation when grinding with mineral slurry with the fast process of surface collapses of self-wear in water. The method proves to be reliable in terms of stability and objectivity. However, it is worthwhile mentioning that due to the complication of the grinding process, a further in-depth investigation on wear mechanism is needed in the future.
Acknowledgements The authors wish to thank our cooperative partner of East China Normal University for the testing work during the course of these experiments. This work has been carried out with the help of working staff of Q&C Center and R&D Center of KING’S CEREMICS’s. Their support is also gratefully acknowledged.
References Becker, M., Schwedes, J., 1999. Comminution of ceramics in stirred media mills and wear of grinding beads. Powder Technol. 105 (1–3), 374–381. Berthiaux, H., Heitzmann, D., Dodds, J.A., 1996. Validation of a model of a stirred beads mill by comparing results obtained in batch and continuous mode grinding. Int. J. Miner. Process. 44–45, 653–661.
43
Blecher, L., Kwade, A., Schwedes, J., 1996a. Motion and stress intensity of grinding beads in a stirred media mill. Part 1: energy density distribution and motion of single grinding beads. Powder Technol. 86 (1), 59–68. Blecher, L., Kwade, A., Schwedes, J., 1996b. Motion and stress intensity of grinding beads in a stirred media mill. Part 1: energy density distribution and motion of single grinding beads. Powder Technol. 86 (1), 69–76. Blickensderfer, R.A., Tylczak, J.H., 1989. Evaluation of commercial US grinding balls by laboratory impact and abrasion tests. Miner. Metall. Process. 6, 60–66. Chenje, T., Radziszewski, P., Hewitt, D., 2009. Steel media wear: experimentation, simulation and validation. In: Proc. 2006, 41st Meeting of the Canadian Mineral Processors, pp. 557–572. Cleary, P.W., Morrison, R.D., 2011. Understanding fine ore breakage in a laboratory scale ball mill using DEM. Miner. Eng. 24 (3–4), 352–366. Frances, C., 2004. On modelling of submicronic wet milling processes in bead mills. Powder Technol. 143–144, 253–263. Gers, R., Climent, E., Anne-Archard, D., Frances, C., 2010. Numerical modelling of grinding in a stirred media mill: hydrodynamics and collision characteristics. Chem. Eng. Sci. 65 (6), 2052–2064. Jensen, L.R.D., Fundal, E., Møller, , Jespersen, M, P., Jespersen, M., 2010. Prediction of wear-rates in comminution equipment. Wear 269 (7–8), 525–533. Kang, J., Hadfield, M., 2005. Examination of the material removal mechanisms during the lapping process of advanced ceramic rolling elements. Wear 258 (1– 4), 2–12. Morrow, R., Sepulveda, J.E., 2014. Moly-cop tools applications for the assessment of grinding media performance at full industrial scale. In: 46th Annual Canadian Mineral Processors Operators Conference. Ottawa, Canada. Radziszewski, P., 2002. Exploring total media wear. Miner. Eng. 15 (12), 1073–1087. Rezaeizadeh, M., Fooladi, M., Powell, M.S., Mansouri, S.H., Weerasekara, N.S., 2010. A new predictive model of lifter bar wear in mills. Miner. Eng. 23 (15), 1174– 1181. Torrance, A.A., 2005. Modelling abrasive wear. Wear 258 (1–4), 281–293.
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Evaluation of grinding media wear-rate by a combined grinding method Junfu King a, Qiang Li b, Alex Wang (Baikun Wang) a,⇑, Cathy He a, Joey Zhou a, Henry Deng a, Ray Xu a a b
King’s Ceramics & Chemicals Co., Ltd., No. 910, 9th Section, Jinsong, Chaoyang District, Beijing, China East China Normal University, No. 500, Dongchuan Road, Minhang District, Shanghai, China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Revised 27 November 2014 Accepted 5 December 2014 Available online 14 January 2015 Keywords: Grinding media Wear-rate Combined grinding method
a b s t r a c t It is well-known that lab tests on wear-rate of grinding media cannot precisely represent its industrial performance due to complex grinding conditions. Nevertheless the lab data provides reference to the industrial data. Therefore, a reproducible test method on wear-rate in lab is necessary. By providing detailed data, this paper challenges the traditional wear-rate testing methods. Two commonly used methods on wear-rate test, self-wear in water and grinding with mineral slurry, are respectively employed. However, obvious fluctuation of wear-rate/time curves indicates neither of the two normal methods is reliable. This paper introduces a wear-rate testing method on lab scale which combines self-wear in water and grinding with mineral slurry. By this method, some repeatable wear-rate/time curves are displayed after a few hours. The mechanism will be discussed in detail in this paper. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Wear-rate is one of the most important factors when evaluating the overall performance of grinding media. Therefore, extensive tests on media wear have been carried out by many media suppliers, users and labs (Berthiaux et al., 1996; Blecher et al., 1996a,b; Blickensderfer and Tylczak, 1989; Frances, 2004; Jensen et al., 2010; Radziszewski, 2002). According to some papers (Chenje et al., 2009; Morrow and Sepulveda, 2014) and many feedback from customers of KING’S CERAMICS’s, there is a correlation on media wear-rates between lab equipments and industrial mills. Therefore, to get repeatable media wear-rates in lab is very meaningful to predict the media performance in industrial scale. However, two common approaches on media wear test, namely self-wear rate test in water and wear rate test with mineral slurry, are always poor in repeatability in lab. As a grinding media supplier, KING’S CERAMICS has been making efforts in developing a reproducible method in lab to evaluate the wear performance of different ceramic media. In this paper, we designed a rapid wear-rate testing method on lab scale which took the advantages of the above two mentioned common methods, and comparatively, repeatable results can be drawn from this new method.
2.1. Test machines
⇑ Corresponding author. Tel.: +86 10 87763540; fax: +86 10 67785715x3540. E-mail addresses: [email protected], [email protected] (A. Wang). http://dx.doi.org/10.1016/j.mineng.2014.12.002 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.
Vertical stirred mill Fig. 1 (left) and rapid pot mill Fig. 1 (right) were used in order to provide robust grinding and impacting forces. Also some detailed parameters of both the two mills are listed in Table 1. 2.2. Test methods 2.2.1. Self-wear in water 1200.0 g grinding media and 400.0 g water were added into the vertical stirred mill to grind for some time. Grinding was conducted at a speed of 900 rpm. The same ceramic media of 600.0 g and water 200.0 g were filled into the rapid pot mill for wear test at 450 rpm. 2.2.2. Grinding with mineral slurry 1200.0 g ceramic media, 400.0 g water and 400.0 g zircon sand (Eucla Zircon Premium Grade with d50 = 92 lm provided by Iluka Resources, 7–8 in Moh’s hardness) were added into the vertical stirred mill. Grinding speed was also 900 rpm. 500.0 g identical media, 100.0 g water and 100.0 g zircon sand (same with above) were filled into the rapid pot mill for wear test at 450 rpm. Both wear-rate test methods are applied in vertical stirred mill and rapid pot mill, with consecutive grinding for 1 h each time. The method was repeated many times to achieve stable wear-rates.
40
J. King et al. / Minerals Engineering 73 (2015) 39–43
Fig. 1. Vertical stirred mill (left) and rapid pot mill (right).
Table 1 Parameters of vertical stirred mill and rapid pot mill. Parameters
Vertical stirred mill
Rapid pot mill
Motor power (kw) Grinding chamber volume (l) Impeller length/maximum diameter (mm) Operation speed (rpm)
0.55 1.87 100
0.45 1.0 150
900
450
The hardness of zircon sand is relatively higher among natural minerals, so the wear-rate differences among different grinding media can be maximized. 2.2.3. Combined grinding method Combined grinding method, which was proceeded in the rapid pot mill, includes two steps: (1) grinding with mineral slurry: the same ceramic media of 500.0 g, water 100.0 g and zircon sand 100.0 g were ground at 450 rpm for 1 h, and then another 2 h in fresh zircon sand slurry after drying the media. (2) Self-wear in water: after grinding with slurry, ceramic media were washed, dried, and then filled into rapid pot mill again (media to water mass ratio is 3:1) to grind for 1 h at 450 rpm, followed by another 2 h grinding in new water under the same condition after drying the media.
Fig. 2 shows the self-wear curves of different grinding media in water versus time. The lines marked with diamonds (A1 and B1) are the results in rapid pot mill, and those marked with circles (A2 and B2) are in vertical stirred mill for media A and B, only the wear-rate curve of media C in rapid pot mill is examined. For media A, after grinding for 7 h, a relatively stable wear-rate could be obtained. For media B, it almost needs 8 h to observe a stable values. In case of media C, only a rising wave line is received as shown in Fig. 3, which could not tell us at which time point is reliable to evaluate the wear-rate. The results mentioned above seem to indicate that self-wear is not an adaptable method for different grinding media.
3.2. Wear-rate in mineral slurry Grinding in mineral slurry is usually used as a relatively reasonable method for testing wear-rate. The wear-rate curves of different ceramic media grinding with mineral slurry can be found in Figs. 4 and 5. However, the curves of media A and C (only in rapid pot mill) are wave lines in wide
2.2.4. Grinding media Three kinds of ceramic media with different densities of 3.32, 4.05 and 4.23 g/cm3 were selected from KING’S CERAMICS and the market, which were named as media A, B and C, respectively. 2.2.5. Calculation on wear-rate The media wear-rate can be calculated by equation ‘‘wearrate = a/b/t’’, where a is the media loss (g) after grinding, b is the initial media charge (kg), and t is the grinding time (h). The unit of wear-rate is gkg 1 h 1. 3. Results and discussion 3.1. Self-wear test Self-wear is a simple and easy method, so it is widely used. But it cannot simulate the grinding conditions of practical operation.
Fig. 2. Self-wear curves of media A and B in vertical stirred mill and rapid pot mill.
J. King et al. / Minerals Engineering 73 (2015) 39–43
Fig. 3. Self-wear curve of media C in rapid pot mill.
41
Fig. 6. Parallel tests on self-wear and wear rate in mineral slurry of media A in rapid pot mill.
Fig. 6 gives the wear-rate curves of media A through parallel tests by self-wear and grinding with mineral slurry in rapid pot mill. The results indicate bad reproducibility. The wear-rate instability can be explained from the abrasive and abrasion processes during the grinding. Different abrasive processes induced by different mechanical forces play different roles in processes of self-wear and grinding with mineral slurry. 3.4. Wear mechanism discussion
Fig. 4. Wear rate curves of media A and B in mineral slurry in vertical stirred mill and rapid pot mill.
Fig. 5. Wear rate curve of media C in mineral slurry in rapid pot mill.
range. So we cannot select the time point at which we can determine the reliable wear-rate. 3.3. Parallel wear-rate tests Another way to examine the reliability of wear tests is to obtain the reproducible curves by several parallel tests. To magnify the wear-rate and minimize the systematical error, wear tests were only deployed in rapid pot mill.
3.4.1. Self-wear mechanism In the case of self-wear, impact force among different media plays an important role to destroy the media surface structure. Firstly impact induces defects on the surface due to plastic deformation of crystal particles (Kang and Hadfield, 2005). The formation probability of defects is usually low, because the plastic deformation can absorb some energy brought by impact to prevent the media surface from being destroyed. But accumulated defects would promptly lead to the structure collapse on the surface under larger impact force (Torrance, 2005). When collapses take place, the wear-rate rises. So, wave-like curves of wear-rate might be observed after self-wear test, just like those in Fig. 2. Here the formation of defects is a slow process while the collapse is a fast one. After quick collapse, the surface can be cleaned and polished quickly. In Fig. 7, collapses and holes on surface of media A can be seen although the surface is smooth generally. This indicates that the mechanical property of media A is not good enough to resist the impact and friction when ground in the water. For media B, holes with size around 1–2 lm are in presence as well. While for media C, surface defects are less and micro-cuttings (slim scratches) can also be observed thanks to its good balance between hardness and fracture toughness. 3.4.2. Wear in mineral slurry During grinding with mineral slurry, the frictional force from slurry plays more important role (Becker and Schwedes, 1999; Cleary and Morrison, 2011; Gers et al., 2010). It causes the corrosion on the surface of ceramic media. For most ceramic products, including ceramic media, sintering aids are needed in the production process, which will transform into grain boundary phase after sintering. The corrosion preferentially takes place at the grain boundary phase, and the defects are induced at the same place. Fig. 8 shows typical SEM photos of media surface after grinding in mineral slurry. Rough surfaces can be seen after grain boundaries are corroded. Compared with Fig. 7, the surfaces of media A and B are more unsmoothed after grinding in mineral slurry. For
42
J. King et al. / Minerals Engineering 73 (2015) 39–43
Media B
Media A
Media C
structure collapse
micro-cutting
N
x10k
10µm
N
x10k
10µm
Fig. 7. Typical SEM photos of media surfaces after self-wear.
Media A
Media C
Media B
Fig. 8. Typical SEM photos of media surfaces after grinding with mineral slurry.
media C, more defects on the surface are observed and the surface is not smooth anymore. When the defects accumulate to cause the structure collapse on the surface, wear-rate will rise suddenly (Rezaeizadeh et al., 2010). Usually the formation of defects is a fast process while the collapse is a slower one. As the corrosion goes on and the defects increase quickly, the collapse will increase in frequency, and the curves of wear-rate will level off. This abrasive process brings typical wear-rate curves in Figs. 4 and 5.
Table 2 Statistical analysis on wear-rates in mineral slurry of different media by combined grinding method. Media A
Media B
Media C
Wear-rate in mineral slurry (%)
0.400 0.395 0.409 0.394 0.398
0.409 0.408 0.378 0.385 0.392
0.413 0.402 0.415 0.400 0.405
Lower confidence limit (%) Upper confidence limit (%)
0.392 0.407
0.377 0.412
0.399 0.415
3.5. Combined grinding method After analyzing and comparing the abrasive process of self-wear and grinding with mineral slurry, it can be proposed that the formation of defects does not synchronize with structure collapses, which makes the wear-rate unstable. If the fast process of defects formation when grinding with mineral slurry and the fast process of surface collapses in self-wear are combined, a fast and reliable method for testing wear-rate may be found. Basing on this assumption, the combined method is proposed as following steps (also in rapid pot mill): (1) grinding in mineral slurry for 1 h; (2) grinding in fresh mineral slurry for another 2 h; (3) self-wear in water for 1 h; and (4) self-wear in fresh water for another 2 h. After each step, the media will be washed clean and dried to test the wear-rate before the next step. Step (1) is to simulate the media surface with that has grinding in mineral slurry for a period of time, and the wear-rate then obtained in step (2) can be seen as the wear-rate grinding with mineral slurry. Step (3) is to remove the defects and recover the media surface to a smooth situation, and the wear-rate obtained in step (4) can be treated as the self-wear rate. Tables 2 and 3 display the wear-rate values of different grinding media and corresponding lower and upper confidence limits when 95% confidence level for the accuracy of wear rates are set. Taking media A for example, if the wear rate in mineral slurry is between 0.392% and 0.407%, then the wear rate result is convincing. This 6 h combined grinding method is rapid and reliable to frequently
Table 3 Statistical analysis on self-wear rates of different media by combined grinding method. Media A
Media B
Media C
Self-wear rate (%)
0.958 0.950 0.946 0.949 0.952
0.750 0.758 0.812 0.765 0.790
1.620 1.595 1.578 1.608 1.582
Lower confidence limit (%) Upper confidence limit (%)
0.945 0.957
0.743 0.807
1.575 1.618
monitor the wear rate the daily production of ceramic media. For further study, much work should be done to comprehensively research the wear mechanism of grinding behavior.
4. Conclusions Wear-rates of different grinding media are determined by two commonly used methods of self-wear in water and grinding with
J. King et al. / Minerals Engineering 73 (2015) 39–43
mineral slurry. After the analysis of all results, we design an improved testing method on wear-rate, which combines the fast process of defects formation when grinding with mineral slurry with the fast process of surface collapses of self-wear in water. The method proves to be reliable in terms of stability and objectivity. However, it is worthwhile mentioning that due to the complication of the grinding process, a further in-depth investigation on wear mechanism is needed in the future.
Acknowledgements The authors wish to thank our cooperative partner of East China Normal University for the testing work during the course of these experiments. This work has been carried out with the help of working staff of Q&C Center and R&D Center of KING’S CEREMICS’s. Their support is also gratefully acknowledged.
References Becker, M., Schwedes, J., 1999. Comminution of ceramics in stirred media mills and wear of grinding beads. Powder Technol. 105 (1–3), 374–381. Berthiaux, H., Heitzmann, D., Dodds, J.A., 1996. Validation of a model of a stirred beads mill by comparing results obtained in batch and continuous mode grinding. Int. J. Miner. Process. 44–45, 653–661.
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