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Maximizing wear resistance of balls for grinding of coal
Wear 263 (2007) 43–47
Case study
Maximizing wear resistance of balls for grinding of coal Eduardo Albertin ∗ , Sandra Lucia de Moraes Technological Research Institute (IPT) of the State of Sao Paulo, Sao Paulo, Brazil Received 15 September 2006; received in revised form 5 February 2007; accepted 9 February 2007 Available online 23 May 2007
Abstract A Brazilian power plant consumes mineral coal with high ash contents to feed four turbines to generate 450 MW. Four ball mills, 4.2 m in diameter, are fed with crushed coal to deliver the material ground to sizes less than 0.075 m. High wear rates were observed when using forged high-carbon steel balls. A pilot-plant ball mill, with a 0.01 m3 chamber, was used to perform wear tests, comparing the original ball material and seven grades of high chromium cast irons, grinding the same coal as used in the industrial plant. Test pieces were 60-mm balls with different chemical compositions and heat treatments. Balls were weighed after 10-h grinding periods, for up to 7 periods. The mass losses were converted to equivalent diameter losses and regression straight lines were obtained, showing the wear rates of the materials. The wear rates of the cast irons with 25–30% Cr were only 10–20% of the wear rate of the forged steel. Cast irons with 15–18% Cr wore at around 50% of the rate of the steel. Analyses of the coal ashes showed around 40% of quartz as 10–20 m particles. Examination of the surface of worn balls showed that these particles caused the wear. © 2007 Elsevier B.V. All rights reserved. Keywords: Ball mill; Coal grinding; Ball wear
1. Case description A Brazilian power plant consumes mineral coal with high ash contents to feed four turbines to generate 450 MW. Four ball mills, 4.2 m in diameter, are fed with crushed coal with maximum size of 25 mm and up to 20% humidity, to deliver the material ground to sizes less than 0.075 m. Each ball mill is fed at a rate of 70 t/h and operates 24 h a day. A constant flow of N2 + CO2 inert gas prevents ignition of the ground coal and conveys fine material out of the mill. This gas mixture enters the mill at 650 ◦ C and leaves it at around 90 ◦ C. High wear rates were observed when using forged highcarbon steel balls. The consumption of balls was 120 t/month, representing an expenditure of US$ 2.3 millions per year. The present study was undertaken to compare the wear performance of the forged high carbon steel balls with that of high chromium cast iron balls, in order to support a decision ∗
Corresponding author. Tel.: +55 1137674245; fax: +55 1137674037. E-mail address: [email protected] (E. Albertin).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.02.013
to change the specification of the grinding media, considering the cost/benefit analysis. In a previous work [1], the advantages of using a small size pilot-plant ball mill to test and develop materials intended to be used for producing grinding media were discussed. The main advantage of this procedure over standard wear tests, such as pin-on-disc or rubber wheel tests, is that many of the parameters of the industrial application can be simulated in the test. For instance, the ore intended to be processed can be used as the test abrasive. Using this test, expensive and time-consuming marked-ball field tests can be limited to the best-performing alloys. In addition, wear values in the pilot-plant ball mil test are obtained in terms of mm/1000 h and can be directly correlated to the ball consumption in the industrial mills. 2. Testing procedures The testing system includes a pilot-plant ball mill, coal as abrasive and a selection of balls to be tested. These elements and the testing procedure are described below.
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E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
Fig. 1. Pilot-plant ball mill: coal charge = 16 kg (10 L); total ball charge = 132 kg; speed = 31 rpm.
2.1. Pilot-plant ball mill The equipment is shown in Fig. 1. It is a pilot-plant ball mill, with capacity to process 10 L of ore per batch. The ball charge contains 132 kg of balls, from 25- to 60-mm diameter. During tests, the mill was run at 31 rpm, corresponding to 70% of the critical speed. Batches of 16 kg of coal were ground during 10 h periods. At the end of each period, the mill was discharged, the balls undergoing the test were directed to cleaning and weighing and the coal was classified. The coal fraction under 0.075 m (#200) was discarded and the coarser fraction was reloaded and completed to 16 kg with fresh feeding. 2.2. Coal The mine supplied 5.3 t of crushed coal. This material was homogenized using standard ore treatment procedures (homogenization piles). From the homogenized material, samples were taken to feed the pilot-plant ball mill. Chemical and difractometric analyses of coal samples revealed the main characteristics presented in Table 1.
Calculations from the chemical analysis of the coal and from stoichiometry of the minerals gave a 21% content of quartz in the coal (40% of the ashes). Fig. 2 shows a SEM image of a coal sample, in which different minerals were identified. 2.3. Balls The original grinding media in the plant were 60 mm forged steel balls from two different suppliers. The specification of the steel was 0.9–1% C and 0.5–1.0% Cr. The balls were quenched and tempered to high hardness, in the range 60–64 HRC. The typical microstructure of the forged steel balls is shown in Fig. 3. The microhardness of this material was around 800 Hv. Seven types of high chromium cast iron balls were included in the test. Two were samples obtained from a commercial supplier and five were cast and heat treated. The chemical compositions of the tested balls are presented in Table 2. Except for the “heat 29”, all balls were quenched and tempered, to hardnesses of 60–64 HRC. Typical microstructures of the high chromium cast irons are presented in Fig. 4.
Table 1 Main characteristics of the coal Main minerals
Quartz
Kaolinite
Orthoclase
Calcite
Muscovite
Main elements % as oxide Loss on ignition (%)
Si 33.5 48.3
Al 11.0
Fe 2.1
Ca 0.96
K 0.89
Grain size
50% between 1.4 and 3.4 mm 80% between 0.6 and 9.5 mm
E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
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Fig. 3. Microstructure of a forged steel ball, 0.9% C quenched and tempered steel. Martensite, with retained austenite. Microhardness 800 Hv.
Fig. 2. SEM view of a coal sample: presence of quartz and softer minerals.
2.4. Wear testing procedure The tests were performed according to the following steps: (1) Run-in: The balls were submitted to 30 h of wear in the pilotplant ball mill, grinding coarse quartz sand (0.3–2.5 mm).
This step provided the removal of a layer of about 100 m, thus removing oxide scales and other defects from the surface. (2) Grinding periods of 10 h each, up to 70 h total grinding time. At the end of each grinding period, the finer fraction of the coal (under 0.075 m) was discarded and the charge was completed with new coal. (3) Weighing: At the end of each grinding period, the mill was discharged and the balls were cleaned, dried and weighed to 0.01 g. The differences in the masses of the balls at the
Fig. 4. Typical microstructures of the high Cr cast iron balls: (a) alloy 31; (b) alloy 27; (c) alloy C30; (d) alloy D15. Different amounts of M7 C3 carbides in martensitic matrices. Alloy 31 is near-eutectic. Alloy 27 presents NbC (arrow).
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E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
Table 2 Chemical composition of the balls Code
Chemical composition (%) C
Si
Mn
Cr
Others
Steel A
0.94
0.22
0.71
0.75
–
Steel B C30 D15
0.96 2.39 2.82
0.25 0.30 0.18
0.99 0.4 0.38
0.55 31 14
– – –
Heat 23 Heat 31 Heat 25 Heat 27 Heat 29
2.87 2.84 3.24 3.36 3.24
0.42 0.1 0.73 0.70 0.14
0.72 0.73 0.89 0.88 0.72
28 29 17.1 19.3 26.6
– – – 3% Nb
beginning of the tests gave a “fingerprint” that permitted to follow the life of each individual ball. 3. Results and discussion A comparison between the damage produced in the balls surfaces by the coal during grinding in the industrial ball mill and in the pilot-plant ball mill is shown in Fig. 5. It can be seen that the effects caused by the abrasives are similar in both cases. From this, it is expected that the ranking of performances obtained in the tests will be valid in the industrial ball mill. The mass losses of the balls were converted to volume losses and from these the equivalent sphere diameters were calculated. The average diameter value was obtained for each ball type for each testing time. Graphs of diameter versus time were obtained, giving straight lines. The slopes of these straight lines are the wear rates of each material. Figs. 6–9 present the graphs for some of the materials. The wear rates for all materials are shown in Table 3. A detailed study [2] showed that most of the wear observed during coal grinding is caused by quartz particles with sizes greater than 5 m present in the ashes. That study showed a direct relationship between the amount of these quartz particles and the wear rates. In the present study, a high amount of quartz particles with sizes from 5 to 40 m were present in the coal. Fig. 10 shows one of these particles cutting the surface of a ball used in the tests. Since the hardness of quartz is well above that of the tempered
Fig. 5. Damage caused on the surfaces of steel balls: (a) industrial ball mill and (b) pilot-plant ball mill.
martensite of the forged steel balls (1200 Hv versus 800 Hv), the poor performance of this material is explained. On the other hand, high chromium cast irons with 25–31% Cr and over 30% volume fractions of M7 C3 carbides, presented wear resistances up to 10 times that of the steel. Cast irons with lower Cr contents, in the range 14–19% did not present outstanding results,
Table 3 Wear rates in the pilot-plant ball mill during coal grinding Material
Wear rate (m/h)
Wear resistance factor
Obs.
Steel A Steel B C30 commercial ball D15 commercial ball IPT Heat 23 IPT Heat 31 IPT Heat 25 IPT Heat 27 IPT Heat 29
0.70 0.65 0.079 0.55 0.078 0.074 0.50 0.30 0.15
11 11 94 14 95 100 15 25 49
10 balls, 70 h test 18 balls, 20 h test 17 balls, 70 h test 18 balls, 20 h test 10 balls, 70 h test 10 balls, 70 h test 9 balls, 70 h test 10 balls, 70 h test 10 balls, 70 h test Fig. 6. Wear of balls of heat 31, during coal grinding in the pilot-plant ball mill.
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Fig. 7. Wear of balls of heat 27, during coal grinding in the pilot-plant ball mill.
Fig. 10. Quartz particle scratching the surface of a ball during the wear test.
4. Conclusions
Fig. 8. Wear of commercial balls C30, during coal grinding in the pilot-plant ball mill.
Fig. 9. Wear of forged steel balls during coal grinding in the pilot-plant ball mill.
(1) The effects caused on the surface of the balls during grinding of coal were similar for the industrial and the pilot-plant ball mill. (2) The main abrasive present in the coal ashes was quartz, as 5–40 m particles. These particles made scratches on the surfaces of balls, causing abrasive wear. (3) The martensitic microstructure of forged steel balls is not capable of resisting to the abrasive action of the quartz particles, because of the large hardness differential. (4) High chromium cast iron balls with Cr contents above 26% presented wear resistances about 10 times that of the forged steel balls. (5) Based on the present study the power plant company defined a specification for balls to be tested in the industrial ball mills, consisting of high chromium cast iron, with 25% Cr minimum. References
even with high amounts of carbides. The reason for the impact of the higher Cr contents in the performance is not clear yet. Since, the coal enters the ball mill with about 10% humidity, corrosion resistance may be a factor. Another hypothesis is that the finer carbide distribution observed in cast irons with around 28% promotes a more efficient protection against the fine quartz particles.
[1] E. Albertin, A. Sinatora, Effect of carbide fraction and matrix microstructure on the wear of cast iron balls in a laboratory ball mill, Wear 250 (2001) 492–501. [2] D.J. Foster, et al. Particle impact erosion and abrasion wear—predictive methods and remedial measures, Report no. COAL R241, Mitsui Babcock, March 2004 (PDF file downloadable from http://www.dti.gov.uk/files/file18813.pdf).
Case study
Maximizing wear resistance of balls for grinding of coal Eduardo Albertin ∗ , Sandra Lucia de Moraes Technological Research Institute (IPT) of the State of Sao Paulo, Sao Paulo, Brazil Received 15 September 2006; received in revised form 5 February 2007; accepted 9 February 2007 Available online 23 May 2007
Abstract A Brazilian power plant consumes mineral coal with high ash contents to feed four turbines to generate 450 MW. Four ball mills, 4.2 m in diameter, are fed with crushed coal to deliver the material ground to sizes less than 0.075 m. High wear rates were observed when using forged high-carbon steel balls. A pilot-plant ball mill, with a 0.01 m3 chamber, was used to perform wear tests, comparing the original ball material and seven grades of high chromium cast irons, grinding the same coal as used in the industrial plant. Test pieces were 60-mm balls with different chemical compositions and heat treatments. Balls were weighed after 10-h grinding periods, for up to 7 periods. The mass losses were converted to equivalent diameter losses and regression straight lines were obtained, showing the wear rates of the materials. The wear rates of the cast irons with 25–30% Cr were only 10–20% of the wear rate of the forged steel. Cast irons with 15–18% Cr wore at around 50% of the rate of the steel. Analyses of the coal ashes showed around 40% of quartz as 10–20 m particles. Examination of the surface of worn balls showed that these particles caused the wear. © 2007 Elsevier B.V. All rights reserved. Keywords: Ball mill; Coal grinding; Ball wear
1. Case description A Brazilian power plant consumes mineral coal with high ash contents to feed four turbines to generate 450 MW. Four ball mills, 4.2 m in diameter, are fed with crushed coal with maximum size of 25 mm and up to 20% humidity, to deliver the material ground to sizes less than 0.075 m. Each ball mill is fed at a rate of 70 t/h and operates 24 h a day. A constant flow of N2 + CO2 inert gas prevents ignition of the ground coal and conveys fine material out of the mill. This gas mixture enters the mill at 650 ◦ C and leaves it at around 90 ◦ C. High wear rates were observed when using forged highcarbon steel balls. The consumption of balls was 120 t/month, representing an expenditure of US$ 2.3 millions per year. The present study was undertaken to compare the wear performance of the forged high carbon steel balls with that of high chromium cast iron balls, in order to support a decision ∗
Corresponding author. Tel.: +55 1137674245; fax: +55 1137674037. E-mail address: [email protected] (E. Albertin).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.02.013
to change the specification of the grinding media, considering the cost/benefit analysis. In a previous work [1], the advantages of using a small size pilot-plant ball mill to test and develop materials intended to be used for producing grinding media were discussed. The main advantage of this procedure over standard wear tests, such as pin-on-disc or rubber wheel tests, is that many of the parameters of the industrial application can be simulated in the test. For instance, the ore intended to be processed can be used as the test abrasive. Using this test, expensive and time-consuming marked-ball field tests can be limited to the best-performing alloys. In addition, wear values in the pilot-plant ball mil test are obtained in terms of mm/1000 h and can be directly correlated to the ball consumption in the industrial mills. 2. Testing procedures The testing system includes a pilot-plant ball mill, coal as abrasive and a selection of balls to be tested. These elements and the testing procedure are described below.
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E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
Fig. 1. Pilot-plant ball mill: coal charge = 16 kg (10 L); total ball charge = 132 kg; speed = 31 rpm.
2.1. Pilot-plant ball mill The equipment is shown in Fig. 1. It is a pilot-plant ball mill, with capacity to process 10 L of ore per batch. The ball charge contains 132 kg of balls, from 25- to 60-mm diameter. During tests, the mill was run at 31 rpm, corresponding to 70% of the critical speed. Batches of 16 kg of coal were ground during 10 h periods. At the end of each period, the mill was discharged, the balls undergoing the test were directed to cleaning and weighing and the coal was classified. The coal fraction under 0.075 m (#200) was discarded and the coarser fraction was reloaded and completed to 16 kg with fresh feeding. 2.2. Coal The mine supplied 5.3 t of crushed coal. This material was homogenized using standard ore treatment procedures (homogenization piles). From the homogenized material, samples were taken to feed the pilot-plant ball mill. Chemical and difractometric analyses of coal samples revealed the main characteristics presented in Table 1.
Calculations from the chemical analysis of the coal and from stoichiometry of the minerals gave a 21% content of quartz in the coal (40% of the ashes). Fig. 2 shows a SEM image of a coal sample, in which different minerals were identified. 2.3. Balls The original grinding media in the plant were 60 mm forged steel balls from two different suppliers. The specification of the steel was 0.9–1% C and 0.5–1.0% Cr. The balls were quenched and tempered to high hardness, in the range 60–64 HRC. The typical microstructure of the forged steel balls is shown in Fig. 3. The microhardness of this material was around 800 Hv. Seven types of high chromium cast iron balls were included in the test. Two were samples obtained from a commercial supplier and five were cast and heat treated. The chemical compositions of the tested balls are presented in Table 2. Except for the “heat 29”, all balls were quenched and tempered, to hardnesses of 60–64 HRC. Typical microstructures of the high chromium cast irons are presented in Fig. 4.
Table 1 Main characteristics of the coal Main minerals
Quartz
Kaolinite
Orthoclase
Calcite
Muscovite
Main elements % as oxide Loss on ignition (%)
Si 33.5 48.3
Al 11.0
Fe 2.1
Ca 0.96
K 0.89
Grain size
50% between 1.4 and 3.4 mm 80% between 0.6 and 9.5 mm
E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
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Fig. 3. Microstructure of a forged steel ball, 0.9% C quenched and tempered steel. Martensite, with retained austenite. Microhardness 800 Hv.
Fig. 2. SEM view of a coal sample: presence of quartz and softer minerals.
2.4. Wear testing procedure The tests were performed according to the following steps: (1) Run-in: The balls were submitted to 30 h of wear in the pilotplant ball mill, grinding coarse quartz sand (0.3–2.5 mm).
This step provided the removal of a layer of about 100 m, thus removing oxide scales and other defects from the surface. (2) Grinding periods of 10 h each, up to 70 h total grinding time. At the end of each grinding period, the finer fraction of the coal (under 0.075 m) was discarded and the charge was completed with new coal. (3) Weighing: At the end of each grinding period, the mill was discharged and the balls were cleaned, dried and weighed to 0.01 g. The differences in the masses of the balls at the
Fig. 4. Typical microstructures of the high Cr cast iron balls: (a) alloy 31; (b) alloy 27; (c) alloy C30; (d) alloy D15. Different amounts of M7 C3 carbides in martensitic matrices. Alloy 31 is near-eutectic. Alloy 27 presents NbC (arrow).
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E. Albertin, S.L. de Moraes / Wear 263 (2007) 43–47
Table 2 Chemical composition of the balls Code
Chemical composition (%) C
Si
Mn
Cr
Others
Steel A
0.94
0.22
0.71
0.75
–
Steel B C30 D15
0.96 2.39 2.82
0.25 0.30 0.18
0.99 0.4 0.38
0.55 31 14
– – –
Heat 23 Heat 31 Heat 25 Heat 27 Heat 29
2.87 2.84 3.24 3.36 3.24
0.42 0.1 0.73 0.70 0.14
0.72 0.73 0.89 0.88 0.72
28 29 17.1 19.3 26.6
– – – 3% Nb
beginning of the tests gave a “fingerprint” that permitted to follow the life of each individual ball. 3. Results and discussion A comparison between the damage produced in the balls surfaces by the coal during grinding in the industrial ball mill and in the pilot-plant ball mill is shown in Fig. 5. It can be seen that the effects caused by the abrasives are similar in both cases. From this, it is expected that the ranking of performances obtained in the tests will be valid in the industrial ball mill. The mass losses of the balls were converted to volume losses and from these the equivalent sphere diameters were calculated. The average diameter value was obtained for each ball type for each testing time. Graphs of diameter versus time were obtained, giving straight lines. The slopes of these straight lines are the wear rates of each material. Figs. 6–9 present the graphs for some of the materials. The wear rates for all materials are shown in Table 3. A detailed study [2] showed that most of the wear observed during coal grinding is caused by quartz particles with sizes greater than 5 m present in the ashes. That study showed a direct relationship between the amount of these quartz particles and the wear rates. In the present study, a high amount of quartz particles with sizes from 5 to 40 m were present in the coal. Fig. 10 shows one of these particles cutting the surface of a ball used in the tests. Since the hardness of quartz is well above that of the tempered
Fig. 5. Damage caused on the surfaces of steel balls: (a) industrial ball mill and (b) pilot-plant ball mill.
martensite of the forged steel balls (1200 Hv versus 800 Hv), the poor performance of this material is explained. On the other hand, high chromium cast irons with 25–31% Cr and over 30% volume fractions of M7 C3 carbides, presented wear resistances up to 10 times that of the steel. Cast irons with lower Cr contents, in the range 14–19% did not present outstanding results,
Table 3 Wear rates in the pilot-plant ball mill during coal grinding Material
Wear rate (m/h)
Wear resistance factor
Obs.
Steel A Steel B C30 commercial ball D15 commercial ball IPT Heat 23 IPT Heat 31 IPT Heat 25 IPT Heat 27 IPT Heat 29
0.70 0.65 0.079 0.55 0.078 0.074 0.50 0.30 0.15
11 11 94 14 95 100 15 25 49
10 balls, 70 h test 18 balls, 20 h test 17 balls, 70 h test 18 balls, 20 h test 10 balls, 70 h test 10 balls, 70 h test 9 balls, 70 h test 10 balls, 70 h test 10 balls, 70 h test Fig. 6. Wear of balls of heat 31, during coal grinding in the pilot-plant ball mill.
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Fig. 7. Wear of balls of heat 27, during coal grinding in the pilot-plant ball mill.
Fig. 10. Quartz particle scratching the surface of a ball during the wear test.
4. Conclusions
Fig. 8. Wear of commercial balls C30, during coal grinding in the pilot-plant ball mill.
Fig. 9. Wear of forged steel balls during coal grinding in the pilot-plant ball mill.
(1) The effects caused on the surface of the balls during grinding of coal were similar for the industrial and the pilot-plant ball mill. (2) The main abrasive present in the coal ashes was quartz, as 5–40 m particles. These particles made scratches on the surfaces of balls, causing abrasive wear. (3) The martensitic microstructure of forged steel balls is not capable of resisting to the abrasive action of the quartz particles, because of the large hardness differential. (4) High chromium cast iron balls with Cr contents above 26% presented wear resistances about 10 times that of the forged steel balls. (5) Based on the present study the power plant company defined a specification for balls to be tested in the industrial ball mills, consisting of high chromium cast iron, with 25% Cr minimum. References
even with high amounts of carbides. The reason for the impact of the higher Cr contents in the performance is not clear yet. Since, the coal enters the ball mill with about 10% humidity, corrosion resistance may be a factor. Another hypothesis is that the finer carbide distribution observed in cast irons with around 28% promotes a more efficient protection against the fine quartz particles.
[1] E. Albertin, A. Sinatora, Effect of carbide fraction and matrix microstructure on the wear of cast iron balls in a laboratory ball mill, Wear 250 (2001) 492–501. [2] D.J. Foster, et al. Particle impact erosion and abrasion wear—predictive methods and remedial measures, Report no. COAL R241, Mitsui Babcock, March 2004 (PDF file downloadable from http://www.dti.gov.uk/files/file18813.pdf).