where E is the grinding energy (MJ/Mg); WI is the Bond grinding Work Index (MJ/Mg), and P80, F80 equals 80% passing size of product and feed, respectively (pm).
Bond (ball mill) Work Indices for copper and nickel sulfide ores are typically in the order of 54 MJ/Mg. Figure 8.9 shows how the energy required to grind these ores increases as the liberation or grind size (P80) decreases, based on the Bond equation with an F80 of 5000 pm (5 mm).
Grind sizes for copper and nickel sulfide ores in Australian mineral processing plants are currently in the order of 75-100 pm, so it was assumed that the energy consumption included in the mineral processing stage of the LCA results presented above (Figures 8.7b and 8.8) for each of these metals corresponded to a grind size of 75 pm. Figure 8.9 was then used to estimate the increase in the energy consumption of this stage as the grind size was progressively reduced from 75-5 pm, and the revised energy estimate was included in the respective LCAs. Figure 8.10a, b shows how the embodied energy increases as both ore grade and grind size decrease for the pyrometallurgical production of copper and nickel, respectively. It should be noted that fine-grained ores are not necessarily low grade. On the contrary, some high grade ores are fine-grained, such as the McArthur River lead-zinc deposit in the Northern Territory (Australia). Figure 8.10 shows that the combined effect of falling ore grade and finer grind size will have a significant effect on the energy consumption of the mineral processing stage.
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