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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 two million lbs (907 tons). Even though Semi Autogenous Grinding (SAG) and Autogenous Grinding (AG) mill charges contain 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 six 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.

How to prevent a dropped charge: Proper start-up procedure

Having and following a proper start-up procedure is key to avoiding a dropped charge. After a prolonged shutdown, 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.

Drop charge detection systems are also available; however, the lengthy calibration required for each specific mill makes them somewhat unpopular.

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 a solution 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.