How do dropped charges form?
Horizontal grinding mills are typically used to break and grind rocks that contain minerals. To help the grinding process, water and steel balls are added to the process which then form the charge in the mill. Ball mill charge contains a significant amount of steel balls and can become extremely heavy. For example, a large 24 foot (7.3 m) diameter ball mill charge typically weighs around 2 million lbs (907 tons). Even though Semi Autogenous Grinding (SAG) and Autogenous Grinding (AG) mill charges contains less balls and no steel balls respectively, their large mill volumes can become quite heavy as well. In all three cases (Ball, SAG or AG mill), when the equipment is stopped, the material inside can dry up over time to form a solidified charge. When the mill is restarted, the solidified charge needs to break and tumble, otherwise a problematic dropped charge can form.
Affected mill integrity
Mill shells are typically manufactured out of mild steel. The elastic limit of the mill shell material is typically at least 6 times the mill operating stress. In fact, when a dropped charge breaks flanges, we know that the impact loads have exceeded the material tensile limits. Therefore, a dropped charge resulting in damage to the shell or flanges, suggests that the impact loads are more than 6 times the operating loads.
Why OEM designed mills aren’t built to withstand a dropped charge
Most mills are not designed to withstand a dropped charge. Based on the estimated impact loads, the mill components would need to be thickened to at least four times their original values to withstand a typical dropped charge. This would make the cost of mill components prohibitive and energy inefficient. As will be discussed below, it is possible to eliminate dropped charges without a need to redesign the mills, by considering two possible preventive solutions.
Two solutions to consider for preventing dropped charge
1. Proper start-up procedure
Having and following a proper start-up procedure is key to avoiding a dropped charge. After a prolonged shut down, it is not advisable to jump start a mill. If the mill is equipped with an inching drive, rotating the mill slowly will ensure the breakup of the frozen charge. If an inching drive is not available (typically in the case of older or smaller mills), then intermittent start-stops can be used until the charge starts tumbling.
2. Metso Locked Charge Detector
A second way to avoid a locked charge is by installing a patented Metso Locked Charge Detector which basically measures mill acceleration and aborts the startup if locked charge conditions exist.
The Locked Charge Detector is used to disengage the mill clutch if a locked charge condition is detected during start-up. The system uses an encoder mounted on the pinion shaft to measure the velocity as the mill rotates between 40 and 70 degrees. By measuring the rate of change of velocity (acceleration), the device will detect whether the charge has cascaded in this zone. If the charge has not cascaded in this zone, the detector will send a signal to disengage the clutch to stop the mill from rotating. The mill must stop rotating when a locked charge is detected to prevent the charge from being lifted and dropped.
The Locked Charge Detector is mainly controlled from the main control room. A local PanelView display is located nearby the mill. From the control room, the operator is able to see the status of the LCD system, enable/disable the LCD system and reset a fault condition.
Preventive maintenance is the key
The effects from dropped charge can damage your mill and, in some cases, bring your production to a halt if the damage is severe enough. Taking the time to implement either of the above solutions may mean some additional effort and planning up front but should be considered as an integral part of a plant’s overall preventive maintenance strategy in order to minimize unplanned mill stoppages.
Director, Global Engineering, Grinding Products