Robust Estimation and Protection of Locked Charge in Grinding Mills

The most critical equipment in mining concentrator plants is the grinding mill. One very important charge condition that can produce a catastrophic failure during the starting of grinding mills is the compacting of the load inside the mill, especially after long downtimes. This condition is usually named frozen charge. The load remains locked to the mill shell and after a half-turn drops damaging the mill body and bearings. Therefore, the mill protection system must give an early detection of this condition and abort the start before the compacted mill charge falls hitting the bottom of the mill shell. This paper presents the development of a robust estimation and protection algorithm of locked charge condition based on drive and process signals. The proposed algorithm includes a locked charge model that permits to define torque, root mean square (rms) current, and power thresholds for the existing starting charge conditions, and compares the locked charge signals to the online estimation of drive variables. This comparison is used to detect the beginning of charge cascading and the threshold values are used to command the termination of the start-up. Proposed algorithm was evaluated using field records of an 8.2 MW semiautogenous mill drive confirming its ability to handle startups with different charge conditions.