- Rod mill grinding rods of all sizes and length.
- High hardness, range is 45~55HRC
- Low bending, less than 1.5mm/m
- Dense microstructure , Bainite +Pearlite+Carbide
- Low breakage rate, more than 15000 times/ 3.5m
- Low unit consumption, 40% reduction compared to unprocessed round steel
Effect of Grinding Rod Media Load on Rod Mill Performance
Effect on Rod Mill Power Draw
The general relationship between mill power draw and charge level is as shown in Figure 2. As previously discussed, the exact shape of this curve, and the position of the peak, may vary slightly for each particular mill, and depend- on the liner condition.
As pointed out by Bond and Rowland, the overall density of the charge may be reduced by the presence of broken rod pieces, and this tendency will be increased with use of larger mills and larger rod sizes. Reduced breakage and amount of scrap can explain the increase in power draw (approximately 5%) observed at Tennessee Copper when a change from 3 inch (76 mm) to 2½ inch (64 mm) rods was carried out, along with the possibility of higher lifting action on rods of smaller size. It is interesting that Sullivan (banks, 1953) reverted back to 3½ inch (89 mm) rods after trying larger ones because of the larger size and. amount of broken rejects (scrap loss).
Effect on Rod Mill Power Efficiency
Because of loss of line contact between the rods, any factors which tend to increase the amount of broken rod pieces in the charge will also tend to decrease operating efficiency. In cases reviewed by Bond (1960) and Rowland (1972), associated losses in efficiency were stated to be as high as 20 to 40%. Bond claimed such loss could be greatly reduced by frequent charge culling. At Tennessee Copper (Myers and Lewis, 1946 b), only a slight improvement in performance was observed when weekly removal of all minus ¾ inch (19 mm) rods and scrap was implemented, and the net gain was not sufficient to justify the lost operating time. Note, however, that their 9 foot (2.7 m) diameter mills were provided with larger than normal trunnion openings, so that accumulation of scrap may have been minimal. No change in mill power draw was reported during the test.
Bond (1950) made recommendations for the correct size of rod to be used, noting that oversize rods decrease the available contact surface area of the charge, and that undersize rods may fail to break the largest pieces in the feed. However, there appears to be only a weak correlation between rod size and feed size in plant practice, indicating that the general preference for larger rods is more likely related to reduced media consumption, rather than milling efficiency.
Unfortunately, plant studies on media size versus rod mill performance are rare. The single case that was found was once again from Tennessee Copper. A reduction in rod make-up size from 3 to 2½ inches (76 to 64 mm) resulted in:
(a) a 5.4% increase in power draw;
(b) a 7.8% increase in work done, as measured by new surface area;
(c) a 22.2% increase in rod consumption; and,
(d) a reduction in the maximum feed particle size that could be broken, from about 1 inch to ¾ inch (25 to 19 mm).
The increase in efficiency, as calculated either by new surface area per unit power, or as indicated by the reduction in the Operating Work Index, was only 2 to 3%. However, the feed size was 54% plus 3 mesh with the 2½ inch (64 mm) rods (reduction ratio over 40), and only 40% plus 3 mesh with the 3 inch (76 mm) rods (reduction ratio less than 30). This substantially coarse feed may have had a negative effect on mill efficiency, as indicated by Bond’s reduction ratio correction factor.
We also experimented with new rods with a non-cylindrical shape (oval or flat in cross section, rather than round), to intensify the sliding action. The results on grinding performance were reported as “disastrous” and the test was quickly abandoned.
Practical Rod Mill Operation
Any aspect of media utilization which affects its consumption rate is extremely important because it is such a large part of total grinding costs.
As shown in Figure 2, the curve relating increase in power draw with increase in charge level becomes very flat as the 50% charge level is approached. Because power draw will increase only very slowly, it is not economically favorable to operate close to this peak. This emphasizes the advantage of high mill speed, rather than high charge level, for a desired power draw. For example, consider the options for increasing the power draw of a mill operating at 70% of Cs and 34% charge level by 10% (a factor of 1.10).
(a) By increasing charge level from approximately 34% to 45% of mill volume, a factor of 1.32. Media consumption per unit power would be expected to increase, by 1.32÷1.10, or 20%.
(b) By increasing mill speed from 70 to 77% of Cs. Media consumption would be expected to increase by 10%. Media consumption per unit power does not increase.
Experience from Tennessee Copper confirms that rod consumption is directly proportional to mill speed. Some of reported operating data is conflicting, but reduction in media consumption per unit power with higher mill speeds is also observable in ball mills. At first glance, this conflicts with Bond’s (1963) suggestion that metal wear per unit power is the correct criteria to rate ore abrasiveness, but this was intended for comparison of widely varying operating conditions and ore types, and to rationalize widely scattered data, rather than for a particular grinding unit. On the basis of the above approximate, yet normally reliable assumptions, there is strong economic basis for high (say 77 to 79% of Cs) mill speeds.
Tennessee Copper also report reduced media consumption with higher feed rates to the rod mill.
At Frood-Stobie, use of larger rods and increasing the mill speed from 66 to 74% of Cs greatly reduced the number of rod tangles.
At Sullivan, the mill speed of 81% of Cs can probably be considered as excessive compared to “normal” rod milling practice, as it was selected (and successfully operated) on the basis of needing a well spread charge to accept the very large feed at high tonnage, and ultimately, passage of some oversize material in the discharge had to be tolented. Despite having an extremely abrasive ore, their media consumption was unusually low. It was also noted that the shape of broken rod rejects was round, and not the typical oval shape. These facts were attributed to the lack of sliding action, and possible work hardening of the rods in the mill, as a result of the impact action of high mill speed.
Decreasing the rod size from 3 to 2½ inches (76 to 64 mm) at Tennessee Copper resulted in an increase of 22% in rod consumption, which can be directly attributed to the larger exposed surface area of the grinding media. This strongly supports the general practice of selecting large rods, but should be considered in conjunction with mill efficiency. Rods larger than 3½ inch (89 mm) diameter caused the size and number of broken rods to increase at Sullivan, which would not only affect power draw and mill efficiency, but also the scrap loss component of media consumption.
Most operations report that grinding rods are added to the mill two or three times a week.
Depending on the media consumption rate, this could cause a wide fluctuation in mill power draw, and operation at least part of the time near the peak of the power versus charge volume curve. Frequent rod additions should therefore be considered for both grinding cost reduction, and for the stabilizing effect on both the circuit and plant performance.
Finally, rod quality plays an obvious role, both from the point of view of wear resistance and lost power draw, efficiency, and media life due to premature breakage.