whereas low mill fillings promote impact breakage at the expense of abrasion. Harder and coarser materials typically experience longer residence times within the SAG mill, leading to increased mill filling, higher power draw, constrained throughput, and finer final grind sizes. Conversely, softer and finer materials have shorter residence times, resulting in lower mill fillings and power draw, but enabling higher throughputs and coarser final grind sizes. Residence time is therefore a key determinant of fines generation: longer residence times increase exposure to abrasion breakage, producing more fines and reducing the workload transferred to downstream ball milling. As a consequence, SAG mill power draw naturally increases when processing harder and coarser ores and decreases when processing softer and finer ores. Figure 35 illustrates the performance of a 38-foot SAG mill when treating the two extremes of ore competency independently. Figure 35 – SAG Mill Filling and Performance Processing the Two Extremes in Hardness Separately Figure 3 presents the frequency distribution of power draw measurements for a 40foot SAG mill over a six-month period. The mill has an installed power of 28 MW. As discussed previously, the most competent ores—typically hard and coarse materials—tend to exhibit longer residence times in the mill, resulting in higher mill fillings and elevated power draw. Under these conditions, the SAG mill has the potential to become the bottleneck of the mine value chain. 17 % 11 MW Poor Fines Generation Soft/Fine Accelerated Liner Wear Relaxed Operation* and/or Hard/Coarse 15 MW More Fines Generation 30 % Extended Liner Life Intensive Operation* and/or * Constrained or not in the ball milling. Most competent (harder) ores bottlenecking the value chain. Least competent (softer) ores. Bottleneck likely outside SAG milling
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