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The role of corrosive wear during laboratory milling
Minerals Engineering 16 (2003) 619–624 This article is also available online at: www.elsevier.com/locate/mineng
The role of corrosive wear during laboratory milling T.W. Chenje a
a,*
, D.J. Simbi b, E. Navara
c
Department of Mechanical Engineering, McGill University, Montreal, Canada H3A 2K6 b Department of Metallurgy, University of Zimbabwe, Zimbabwe c Rubin-Material Teknik, Lulea, Sweden Received 11 February 2003; accepted 26 March 2003
Abstract This current work sought to determine the significance of corrosive wear in the metal loss during ball milling, at least at laboratory scale. The results showed that although corrosion does play a part in media loss, its impact in a non-sulphide containing environment is not significant (<15%) when compared with that of abrasive wear. The other factors such as ball coating during dry milling and wear resulting from ball-to-ball contact make the determination of the exact contribution difficult. The impact of corrosive wear can be further reduced by the use of more wear resistant ball materials. Ó 2003 Published by Elsevier Science Ltd. Keywords: Comminution; Grinding; Mineral processing
1. Introduction Corrosive wear has been described as metal loss due to chemical and electrochemical reaction with the environment (ASTM Standard G40-92, 1992; Natarajan, 1996). Researchers (Tanaka et al., 1999; Yelloji and Natarajan, 1991; Remark and Wick, 1976; Natarajan et al., 1984; Rajagopal and Iwasaki, 1992; Tolley et al., 1984) disagree on the role of corrosion in the wear of grinding mill balls. Although it is generally accepted that corrosion does take place during wet milling, its significance is not well documented. Results of some investigations have suggested that corrosion in not only significant, but is the predominant metal removal mechanism during wet milling (Tanaka et al., 1999; Yelloji and Natarajan, 1991; Remark and Wick, 1976). Experiments by Natarajan et al. (1984) proved that different aeration conditions used in wet milling did not produce any great increase in ball wear, as would have been expected if corrosion was the dominant wear mechanism. Taggart (1945) put the wear during dry milling as 10–25% of that in wet milling. The wear rate during wet grinding has been observed as more than twice that in dry grinding and this difference has been
attributed to corrosion effects (Natarajan, 1996; Tanaka et al., 1999). The authors have, however, failed to take into account the differences between the conditions experienced not only during continuous and batch milling, but also wet and dry milling. In batch laboratory milling, corrosion can represent between 25% and 75% of the metal loss depending on the ore-metal-environmental factors involved (Yelloji and Natarajan, 1991), whilst corrosion represents less than 10% of the total metal loss in typical large diameter continuous feed ball mills (Yelloji and Natarajan, 1991; Remark and Wick, 1976; Natarajan et al., 1984; Rajagopal and Iwasaki, 1992; Tolley et al., 1984; Wills, 1997). During wet milling, a freshly abraded surface reacts with the water to form an oxide or hydroxide surface coating. Both differential abrasion on the balls and differential aeration lead to galvanic corrosion. Local galvanic cells can also be formed between the grinding media and the ground mineral particles and is promoted by the presence of oxygen. The following electrochemical reactions are envisaged in the case of steel or cast iron balls (Natarajan, 1996; Natarajan, 1992; Fontana, 1987): Fe ¼ Fe2þ þ 2e
ð1Þ
0:5O2 þ H2 O þ 2e ¼ 2OH * Corresponding author. Tel.: +1-514-398-8119; fax: +1-514-3984476. E-mail address: [email protected] (T.W. Chenje).
ð2Þ
The iron or steel ball material acts as the anode and undergoes a dissolution (corrosion) reaction, whilst the supporting reaction is oxygen reduction at the cathodic
0892-6875/03/$ - see front matter Ó 2003 Published by Elsevier Science Ltd. doi:10.1016/S0892-6875(03)00132-8
620
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
Table 1 Chemical compositions of the test balls Ball type Low alloy steel Eutectoid steel Medium chromium cast iron Cast semi-steel Unalloyed cast iron
Chemical composition % C
Mn
Si
Cr
S
P
0.50 0.85 3.00 2.18 3.05
1.06 0.65 0.57 0.68 0.55
0.43 0.69 0.61 0.87 0.62
1.15 <0.15 17.81 <0.15 0.58
0.02 0.01 0.04 0.02 0.02
0.01 0.01 0.02 0.02 0.02
site on the ball or the mineral surface, which produces hydroxyl ions. The oxide or hydroxide coating formed is usually not adherent or abrasion resistant and is thus easily abraded off, permitting further chemical reaction. In alloyed cast iron balls, the chromium oxide and the hydroxide films formed are more abrasion resistant than those of iron. However, even the high chromium is not able to protect fully the stainless steel grades whose wear rates may be equal to those of carbon steels (Natarajan, 1996; Tanaka et al., 1999). An aspect present in dry but not wet milling normally and has been omitted in most studies is the tendency for ball coating. Hard, coherent layers of compacted fine feed material are formed on the surface of the grinding balls during dry milling. The effect is to substitute a relatively soft grinding surface for the original hard one and thus decrease the grinding effect. A reduced wear rate is hence observed. In wet grinding, the liquid medium tends to expose more and more bare metal surfaces through surface cleaning, thereby increasing abrasive wear (Natarajan et al., 1984). It has also been suggested that the increased grinding rate that is known to accompany wet milling also dramatically increases the abrasive wear of the media (Tolley et al., 1984). Tanaka and co-workers (Tanaka et al., 1999, 2001) discovered that the wear behaviour of the media was highest during grinding of fine particles and lowest for coarse particles. The wear behaviour of the grinding media during wet and dry milling of raw granite was similar to those obtained during grinding fine and coarse granite respectively. Consequently the effect of the abrasion particle size on the rate of wear of the media has been well documented and was not the subject of this paper. Although the contribution of corrosion in laboratory milling has been discussed by many researchers, controversy still abounds. Consequently, this current work concentrates on this, the significance of corrosion in milling. 2. Experimental procedure 2.1. Grinding media Five types of 60 mm diameter grinding balls with different compositions were produced in a 400 kg in-
duction furnace and sand-cast. Normal production scale balls were not used because they contain a lot of defects. The analysis of the balls as determined by spark emission spectroscopy is shown in Table 1. The unalloyed cast iron, cast semi-steel and medium chromium cast iron balls were chosen because they are the most common ball types used in Zimbabwean milling operations, where these project was initiated. The steel balls were included since the world trend is towards the use of steel balls for milling hard ores. The test balls were marked for identification purposes by means of differently oriented shallow notches, 25 mm long by 3 mm wide, which were cut using an angle grinder. The eutectoid steel balls, the medium chromium cast iron balls and the cast semi-steel balls were all held at the heat treatment temperatures of 750, 1050 and 850 °C respectively for 3 h before being cooled in still air. 2.2. Abrasive Granite with the particle size distribution shown in Fig. 1 was used as the abrasive. This granite had a bond work index (BWI) of 14.4 kW h/t, which compares favourably with the majority of Zimbabwean ores (BWI 13–18 kW h/t). 2.3. Milling tests The milling tests were conducted in a 0.45 m by 0.45 m batch laboratory-scale ball mill. Ball and abrasive charges of 21 and 30 kg, respectively were used in the experiments. The ball charge was composed of three marked balls of each of the five test ball types and small balls of the unalloyed cast iron type in order to give a ball charge of 21 kg. A wear-in period was used to remove surface defects such as scaling and decarburization before the balls could be used in the experiments. Preliminary tests indicated that two weigh-in 5 h runs were sufficient for this purpose. The marked balls were weighed after the wear-in before being introduced into the mill for the actual milling tests. After each milling run, the marked balls were retrieved, thoroughly washed and cleaned using a soft brush, dried using compressed air and re-weighed. The average weight of each ball type was recorded after each run and used to calculate
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
621
100
% Cumulative Passing
80
60
40
20
0 0
5
10
15
20
25
30
Sieve size [mm]
Fig. 1. Particle size distribution of abrasive.
the weight loss. The mill was emptied and cleaned and fresh granite added before each subsequent milling run. A milling time of 5 h was used for all the test runs. Three mill conditions were used, namely wet milling with 65% solids, dry milling and wet milling without abrasive.
3. Results Figs. 2–6 show the accumulated ball mass losses for the five ball types in the three test conditions. Table 2 summarises the wear rates of the balls obtained from these figures.
4. Discussion The coefficients of x in the linear equations shown in Figs. 2–6 represent the wear rates. As can be observed from these coefficients, the wear rates under wet and dry milling of medium chromium cast iron are far lower than the other ball types. The 3.8% difference between the wet and dry milling wear rate shows that the effect of corrosion on the wear rates of this ball during wet milling was minimal. Greater differences of 9.8%, 14.6%, 15.5% and 17.4% were observed for the eutectoid steel, low alloy steel, cast semi-steel and unalloyed cast iron balls respectively. These seemingly higher differences are a result of the combination of absence of corrosion and the ball-coating tendency experienced during dry milling.
30
25
No Ore Dry
y = 0.5111x
Wet
Mass loss [g]
20
y = 0.457x 15 y = 0.2764x
10
5
0 0
5
10
15
20
25 Miiling Time [hrs]
30
35
Fig. 2. Wear of heat-treated eutectoid steel balls.
40
45
50
622
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624 30
No Ore
25
Dry Wet
y = 0.4796x
Mass loss [g]
20
15
y = 0.4149x
10 y = 0.2x
5
0 0
5
10
15
20
25 Milling Time [hrs]
30
35
40
45
50
45
50
45
50
Fig. 3. Wear of low alloy steel balls.
30
25
No Ore
20
Dry
Mass loss [g]
Wet
15
y = 0.2582x 10 y = 0.2485x
5 y = 0.1333x
0 0
5
10
15
20
25
30
35
40
Milling Time [hrs]
Fig. 4. Wear of heat-treated medium chromium cast iron balls.
30
25
No Ore Dry Wet
y = 0.4501x
Mass loss [g]
20
15 y = 0.3818x
10
y = 0.2097x 5
0 0
5
10
15
20
25
30
35
Milling time [hrs]
Fig. 5. Wear of heat-treated semi-steel balls.
40
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
623
30
25
No Ore Dry
Mass loss [g]
y = 0.4566x
Wet
20
15 y = 0.3818x
10 y = 0.2182x 5
0 0
5
10
15
20
25
30
35
40
45
50
Milling Time [hrs]
Fig. 6. Wear of unalloyed white iron balls.
Table 2 A summary of the wear rates of the test balls during milling Ball type
LAS HTES HTMC HTSS UCI
Wear rates [g/h] Wet milling
Dry milling
Wet milling without abrasive
0.48 0.51 0.26 0.45 0.46
0.41 0.46 0.25 0.38 0.38
0.20 0.28 0.13 0.21 0.22
The actual contribution of corrosion can thus be considered to be much less than 15% of the total wear during wet milling. These results are in agreement with the results previously reported (Tanaka et al., 1999). The lack of a substantial difference can be explained by considering that the most likely dominant mode of material loss in the wet milling experiments is abrasion. The material is rapidly worn off the balls by abrasion before corrosion can take effect. However, the significant difference between the medium chromium cast iron balls and the other balls can be attributed to the differences in material properties. The medium chromium cast iron balls contain a high percentage of chromium, which forms a hard oxide layer thereby reducing corrosion, as mentioned in Section 1. The other balls, containing less chromium are therefore less able to resist the corrosion effects. This confirms that corrosion does play some part in the material loss experienced during milling. The wear rates resulting from milling without ore however, showed at least a 45% difference in material loss compared with those obtained during wet milling. This difference also serves to fortify the assertion that
the dominant mechanism of material removal is abrasion by the abrading material and not ball-to-ball contact. The existence of some wear even in the absence of the abrading material shows that some material is lost as a result of wear resulting from ball-to-ball contact. Although the amount of material loss due to ball-to ball wear in milling without ore is about 50% of the wear experienced in wet milling, its contribution in actual wet milling would be less. The ore in wet milling would reduce the probability of ball-to-ball contact unlike in milling without ore where all contact was ball to ball. It can thus be assumed that the material loss due to ball-to-ball contact during wet milling would be much less and is not very significant when compared with the overall material loss resulting from abrasion. It is important here to stress a key assumption made in this reported work. The author assumes that the coefficient of friction during wet and dry milling remains the same. This is not necessarily true as it has been observed (Yabuki et al., 2002; Rabinowicz, 1995) that the coefficient of friction decreases from dry to wet contact. In the context of milling, the abrasive wear component is consequently lower in wet milling, all else being equal, due to the decreased friction coefficient. The efficiency of the energy transfer from the wall to the media is reduced during wet milling, resulting in less energy being used for the abrasive wear. The corrosive wear component, which may be expected to increase as a result, can hence be considered to be a sum of the difference between the wet and dry milling wear, and the drop in the abrasive wear component. The influence of the coefficient of friction on the abrasive and corrosive wear during ball milling has not been full established and is the subject of on-going work.
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T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
5. Conclusion 1. Although corrosive wear plays a part during wet laboratory milling, its contribution in non-sulphide containing conditions is not as significant (on average less than 15%) when compared to abrasive wear, as suggested by other researchers. The actual contribution is difficult to measure due to the effects of ball coating and cushioning. 2. Good material selection can reduce the corrosion effects even less as shown by the medium chromium cast iron balls, which showed a difference of less than 4% in the metal loss between wet and dry milling, compared to the heat treated eutectoid steel (10% difference), the next best performer Acknowledgements The author acknowledges material and financial support for this work from the Production Engineering Institute, Scientific Industrial Research and Development Centre (SIRDC), the Department of Metallurgical Engineering, University of Zimbabwe and the Department of Metallurgy, Zimbabwe Ministry of Mines and Energy. References ASTM Standard G40-92, Standard terminology relating to wear and erosion, Annual book of standards, 03.02, ASTM, pp. 158–163.
Fontana, M.G., 1987. Corrosion Engineering, third ed McGraw Hill, New York, pp. 41–46. Natarajan, K.A., 1992. Ball wear and its control in the grinding of a lead-zinc sulphide ore. International Journal of Mineral Processing 34, 161–175. Natarajan, K.A., 1996. Laboratory studies on ball wear in grinding of a chalcopyrite ore. International Journal of Mineral Processing 46, 205–213. Natarajan, K.A., Riemer, S.C., Iwasaki, I., 1984. Corrosive and erosive wear in magnetic taconite grinding. AIMMPE Transactions 276, 10–14. Rabinowicz, E., 1995. Friction and Wear of Materials, Second ed John Wiley and Sons, New York, pp. 116–121. Rajagopal, V., Iwasaki, I., 1992. The properties and performance of cast iron grinding media. Mineral Processing and Extractive Metallurgy Review 11, 75–106. Remark, J.F., Wick O.J., 1976. Corrosion control in ball and rod mills, Presented at corrosion 76, Houston, paper no. 121. Taggart, A.F, 1945. Handbook of Mineral Dressing. John Wiley & Sons, New York, pp. 5-1–6-54. Tanaka, D.K., Pintaude, G., Sinatora, A., Tschiptschin, A.P., 1999. Influence of granite particle size on corrosion-wear synergism of white cast iron mill balls, Proceedings of the 14th International Corrosion Conference, Cape Town, October. Tanaka, D.K., Pintaude, G., Sinatora, A., Tschiptschin, A.P., 2001. The particle size effect on abrasive wear of high chromium white cast iron mill balls. Wear 250, 66–70. Tolley, W.K., Nichols, I.L., Huiatt J.L., 1984. Corrosion rates of grinding media in mill water, US Bureau of Mines Report 8882. Wills, B.A, 1997. Mineral Processing Technology, Sixth ed Butterworth-Heinemann, Oxford, pp. 142–174. Yabuki, A., Baghbanan, M.R., Spelt, J.K., 2002. Contact forces and mechanisms in a vibratory finisher. Wear 252, 635–643. Yelloji, R., Natarajan, K.A., 1991. Factors influencing ball wear and flotation with respect to ore grinding. Mineral Processing and Extractive Metallurgy Review 7, 137–173.
The role of corrosive wear during laboratory milling T.W. Chenje a
a,*
, D.J. Simbi b, E. Navara
c
Department of Mechanical Engineering, McGill University, Montreal, Canada H3A 2K6 b Department of Metallurgy, University of Zimbabwe, Zimbabwe c Rubin-Material Teknik, Lulea, Sweden Received 11 February 2003; accepted 26 March 2003
Abstract This current work sought to determine the significance of corrosive wear in the metal loss during ball milling, at least at laboratory scale. The results showed that although corrosion does play a part in media loss, its impact in a non-sulphide containing environment is not significant (<15%) when compared with that of abrasive wear. The other factors such as ball coating during dry milling and wear resulting from ball-to-ball contact make the determination of the exact contribution difficult. The impact of corrosive wear can be further reduced by the use of more wear resistant ball materials. Ó 2003 Published by Elsevier Science Ltd. Keywords: Comminution; Grinding; Mineral processing
1. Introduction Corrosive wear has been described as metal loss due to chemical and electrochemical reaction with the environment (ASTM Standard G40-92, 1992; Natarajan, 1996). Researchers (Tanaka et al., 1999; Yelloji and Natarajan, 1991; Remark and Wick, 1976; Natarajan et al., 1984; Rajagopal and Iwasaki, 1992; Tolley et al., 1984) disagree on the role of corrosion in the wear of grinding mill balls. Although it is generally accepted that corrosion does take place during wet milling, its significance is not well documented. Results of some investigations have suggested that corrosion in not only significant, but is the predominant metal removal mechanism during wet milling (Tanaka et al., 1999; Yelloji and Natarajan, 1991; Remark and Wick, 1976). Experiments by Natarajan et al. (1984) proved that different aeration conditions used in wet milling did not produce any great increase in ball wear, as would have been expected if corrosion was the dominant wear mechanism. Taggart (1945) put the wear during dry milling as 10–25% of that in wet milling. The wear rate during wet grinding has been observed as more than twice that in dry grinding and this difference has been
attributed to corrosion effects (Natarajan, 1996; Tanaka et al., 1999). The authors have, however, failed to take into account the differences between the conditions experienced not only during continuous and batch milling, but also wet and dry milling. In batch laboratory milling, corrosion can represent between 25% and 75% of the metal loss depending on the ore-metal-environmental factors involved (Yelloji and Natarajan, 1991), whilst corrosion represents less than 10% of the total metal loss in typical large diameter continuous feed ball mills (Yelloji and Natarajan, 1991; Remark and Wick, 1976; Natarajan et al., 1984; Rajagopal and Iwasaki, 1992; Tolley et al., 1984; Wills, 1997). During wet milling, a freshly abraded surface reacts with the water to form an oxide or hydroxide surface coating. Both differential abrasion on the balls and differential aeration lead to galvanic corrosion. Local galvanic cells can also be formed between the grinding media and the ground mineral particles and is promoted by the presence of oxygen. The following electrochemical reactions are envisaged in the case of steel or cast iron balls (Natarajan, 1996; Natarajan, 1992; Fontana, 1987): Fe ¼ Fe2þ þ 2e
ð1Þ
0:5O2 þ H2 O þ 2e ¼ 2OH * Corresponding author. Tel.: +1-514-398-8119; fax: +1-514-3984476. E-mail address: [email protected] (T.W. Chenje).
ð2Þ
The iron or steel ball material acts as the anode and undergoes a dissolution (corrosion) reaction, whilst the supporting reaction is oxygen reduction at the cathodic
0892-6875/03/$ - see front matter Ó 2003 Published by Elsevier Science Ltd. doi:10.1016/S0892-6875(03)00132-8
620
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
Table 1 Chemical compositions of the test balls Ball type Low alloy steel Eutectoid steel Medium chromium cast iron Cast semi-steel Unalloyed cast iron
Chemical composition % C
Mn
Si
Cr
S
P
0.50 0.85 3.00 2.18 3.05
1.06 0.65 0.57 0.68 0.55
0.43 0.69 0.61 0.87 0.62
1.15 <0.15 17.81 <0.15 0.58
0.02 0.01 0.04 0.02 0.02
0.01 0.01 0.02 0.02 0.02
site on the ball or the mineral surface, which produces hydroxyl ions. The oxide or hydroxide coating formed is usually not adherent or abrasion resistant and is thus easily abraded off, permitting further chemical reaction. In alloyed cast iron balls, the chromium oxide and the hydroxide films formed are more abrasion resistant than those of iron. However, even the high chromium is not able to protect fully the stainless steel grades whose wear rates may be equal to those of carbon steels (Natarajan, 1996; Tanaka et al., 1999). An aspect present in dry but not wet milling normally and has been omitted in most studies is the tendency for ball coating. Hard, coherent layers of compacted fine feed material are formed on the surface of the grinding balls during dry milling. The effect is to substitute a relatively soft grinding surface for the original hard one and thus decrease the grinding effect. A reduced wear rate is hence observed. In wet grinding, the liquid medium tends to expose more and more bare metal surfaces through surface cleaning, thereby increasing abrasive wear (Natarajan et al., 1984). It has also been suggested that the increased grinding rate that is known to accompany wet milling also dramatically increases the abrasive wear of the media (Tolley et al., 1984). Tanaka and co-workers (Tanaka et al., 1999, 2001) discovered that the wear behaviour of the media was highest during grinding of fine particles and lowest for coarse particles. The wear behaviour of the grinding media during wet and dry milling of raw granite was similar to those obtained during grinding fine and coarse granite respectively. Consequently the effect of the abrasion particle size on the rate of wear of the media has been well documented and was not the subject of this paper. Although the contribution of corrosion in laboratory milling has been discussed by many researchers, controversy still abounds. Consequently, this current work concentrates on this, the significance of corrosion in milling. 2. Experimental procedure 2.1. Grinding media Five types of 60 mm diameter grinding balls with different compositions were produced in a 400 kg in-
duction furnace and sand-cast. Normal production scale balls were not used because they contain a lot of defects. The analysis of the balls as determined by spark emission spectroscopy is shown in Table 1. The unalloyed cast iron, cast semi-steel and medium chromium cast iron balls were chosen because they are the most common ball types used in Zimbabwean milling operations, where these project was initiated. The steel balls were included since the world trend is towards the use of steel balls for milling hard ores. The test balls were marked for identification purposes by means of differently oriented shallow notches, 25 mm long by 3 mm wide, which were cut using an angle grinder. The eutectoid steel balls, the medium chromium cast iron balls and the cast semi-steel balls were all held at the heat treatment temperatures of 750, 1050 and 850 °C respectively for 3 h before being cooled in still air. 2.2. Abrasive Granite with the particle size distribution shown in Fig. 1 was used as the abrasive. This granite had a bond work index (BWI) of 14.4 kW h/t, which compares favourably with the majority of Zimbabwean ores (BWI 13–18 kW h/t). 2.3. Milling tests The milling tests were conducted in a 0.45 m by 0.45 m batch laboratory-scale ball mill. Ball and abrasive charges of 21 and 30 kg, respectively were used in the experiments. The ball charge was composed of three marked balls of each of the five test ball types and small balls of the unalloyed cast iron type in order to give a ball charge of 21 kg. A wear-in period was used to remove surface defects such as scaling and decarburization before the balls could be used in the experiments. Preliminary tests indicated that two weigh-in 5 h runs were sufficient for this purpose. The marked balls were weighed after the wear-in before being introduced into the mill for the actual milling tests. After each milling run, the marked balls were retrieved, thoroughly washed and cleaned using a soft brush, dried using compressed air and re-weighed. The average weight of each ball type was recorded after each run and used to calculate
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
621
100
% Cumulative Passing
80
60
40
20
0 0
5
10
15
20
25
30
Sieve size [mm]
Fig. 1. Particle size distribution of abrasive.
the weight loss. The mill was emptied and cleaned and fresh granite added before each subsequent milling run. A milling time of 5 h was used for all the test runs. Three mill conditions were used, namely wet milling with 65% solids, dry milling and wet milling without abrasive.
3. Results Figs. 2–6 show the accumulated ball mass losses for the five ball types in the three test conditions. Table 2 summarises the wear rates of the balls obtained from these figures.
4. Discussion The coefficients of x in the linear equations shown in Figs. 2–6 represent the wear rates. As can be observed from these coefficients, the wear rates under wet and dry milling of medium chromium cast iron are far lower than the other ball types. The 3.8% difference between the wet and dry milling wear rate shows that the effect of corrosion on the wear rates of this ball during wet milling was minimal. Greater differences of 9.8%, 14.6%, 15.5% and 17.4% were observed for the eutectoid steel, low alloy steel, cast semi-steel and unalloyed cast iron balls respectively. These seemingly higher differences are a result of the combination of absence of corrosion and the ball-coating tendency experienced during dry milling.
30
25
No Ore Dry
y = 0.5111x
Wet
Mass loss [g]
20
y = 0.457x 15 y = 0.2764x
10
5
0 0
5
10
15
20
25 Miiling Time [hrs]
30
35
Fig. 2. Wear of heat-treated eutectoid steel balls.
40
45
50
622
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624 30
No Ore
25
Dry Wet
y = 0.4796x
Mass loss [g]
20
15
y = 0.4149x
10 y = 0.2x
5
0 0
5
10
15
20
25 Milling Time [hrs]
30
35
40
45
50
45
50
45
50
Fig. 3. Wear of low alloy steel balls.
30
25
No Ore
20
Dry
Mass loss [g]
Wet
15
y = 0.2582x 10 y = 0.2485x
5 y = 0.1333x
0 0
5
10
15
20
25
30
35
40
Milling Time [hrs]
Fig. 4. Wear of heat-treated medium chromium cast iron balls.
30
25
No Ore Dry Wet
y = 0.4501x
Mass loss [g]
20
15 y = 0.3818x
10
y = 0.2097x 5
0 0
5
10
15
20
25
30
35
Milling time [hrs]
Fig. 5. Wear of heat-treated semi-steel balls.
40
T.W. Chenje et al. / Minerals Engineering 16 (2003) 619–624
623
30
25
No Ore Dry
Mass loss [g]
y = 0.4566x
Wet
20
15 y = 0.3818x
10 y = 0.2182x 5
0 0
5
10
15
20
25
30
35
40
45
50
Milling Time [hrs]
Fig. 6. Wear of unalloyed white iron balls.
Table 2 A summary of the wear rates of the test balls during milling Ball type
LAS HTES HTMC HTSS UCI
Wear rates [g/h] Wet milling
Dry milling
Wet milling without abrasive
0.48 0.51 0.26 0.45 0.46
0.41 0.46 0.25 0.38 0.38
0.20 0.28 0.13 0.21 0.22
The actual contribution of corrosion can thus be considered to be much less than 15% of the total wear during wet milling. These results are in agreement with the results previously reported (Tanaka et al., 1999). The lack of a substantial difference can be explained by considering that the most likely dominant mode of material loss in the wet milling experiments is abrasion. The material is rapidly worn off the balls by abrasion before corrosion can take effect. However, the significant difference between the medium chromium cast iron balls and the other balls can be attributed to the differences in material properties. The medium chromium cast iron balls contain a high percentage of chromium, which forms a hard oxide layer thereby reducing corrosion, as mentioned in Section 1. The other balls, containing less chromium are therefore less able to resist the corrosion effects. This confirms that corrosion does play some part in the material loss experienced during milling. The wear rates resulting from milling without ore however, showed at least a 45% difference in material loss compared with those obtained during wet milling. This difference also serves to fortify the assertion that
the dominant mechanism of material removal is abrasion by the abrading material and not ball-to-ball contact. The existence of some wear even in the absence of the abrading material shows that some material is lost as a result of wear resulting from ball-to-ball contact. Although the amount of material loss due to ball-to ball wear in milling without ore is about 50% of the wear experienced in wet milling, its contribution in actual wet milling would be less. The ore in wet milling would reduce the probability of ball-to-ball contact unlike in milling without ore where all contact was ball to ball. It can thus be assumed that the material loss due to ball-to-ball contact during wet milling would be much less and is not very significant when compared with the overall material loss resulting from abrasion. It is important here to stress a key assumption made in this reported work. The author assumes that the coefficient of friction during wet and dry milling remains the same. This is not necessarily true as it has been observed (Yabuki et al., 2002; Rabinowicz, 1995) that the coefficient of friction decreases from dry to wet contact. In the context of milling, the abrasive wear component is consequently lower in wet milling, all else being equal, due to the decreased friction coefficient. The efficiency of the energy transfer from the wall to the media is reduced during wet milling, resulting in less energy being used for the abrasive wear. The corrosive wear component, which may be expected to increase as a result, can hence be considered to be a sum of the difference between the wet and dry milling wear, and the drop in the abrasive wear component. The influence of the coefficient of friction on the abrasive and corrosive wear during ball milling has not been full established and is the subject of on-going work.
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5. Conclusion 1. Although corrosive wear plays a part during wet laboratory milling, its contribution in non-sulphide containing conditions is not as significant (on average less than 15%) when compared to abrasive wear, as suggested by other researchers. The actual contribution is difficult to measure due to the effects of ball coating and cushioning. 2. Good material selection can reduce the corrosion effects even less as shown by the medium chromium cast iron balls, which showed a difference of less than 4% in the metal loss between wet and dry milling, compared to the heat treated eutectoid steel (10% difference), the next best performer Acknowledgements The author acknowledges material and financial support for this work from the Production Engineering Institute, Scientific Industrial Research and Development Centre (SIRDC), the Department of Metallurgical Engineering, University of Zimbabwe and the Department of Metallurgy, Zimbabwe Ministry of Mines and Energy. References ASTM Standard G40-92, Standard terminology relating to wear and erosion, Annual book of standards, 03.02, ASTM, pp. 158–163.
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