Figure 2 – Fatigue fracture at thinner 35 mm and thicker 50 mm liner plate. The SAG Mill operational control philosophy was also insufficient in mitigating excessive grinding media impact onto the shell liners above the charge toe. The Hi-Lo shell liner was designed to start-up and operate at max rpm (10.8) to achieve maximum power draw and throughput, but trajectory simulations (Figure 3) indicate that under this speed condition, the 30% mill charge produced direct and repeated impacts on the shell liner surface. The grinding media trajectories were considered uncontrolled, which was later supported by the mill sound data (noise monitoring system) retrieved for this liner campaign. The mill charge level calibration (performed weekly) was based on thermal differentials (using a thermal gun) to identify charge levels and adjusted using internal mill distance references, as well, the grinding media addition rate was constant at 8.5 tons/day. Figure 3 –Hi-Lo shell liner profile, harmful charge trajectory simulation above charge toe at 10.8 rpm @ 30% mill charge, DE shell liners breakage Figure 4 illustrates severe peening, characterized by dimpled impact marks on the liner surface, which was prevalent on the Hi-Lo shell liner design (also present to a lesser extent on the original Hi-Hi design, though the impact was minor). Significant pegging was also observed on both the shell and feed-end liners (where grinding balls became trapped between ribs or lifting hooks and lifters). Peening failure mode from repeated highenergy media impacts induced cracks and accelerated wear onto the intermediate discharge-end liners. Pegging primarily impacts worker safety as trapped balls pose an overhead hazard and must be manually scaled out before personnel can enter the SAG mill. These routine scaling requirements activities not only increase exposure to safety hazards but also extend the scheduled maintenance downtime.
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