Figure 8.8 Effect of ore grade on (a) embodied energy for pyrometallurgical copper production and (b) the GER for pyrometallurgical nickel production (Norgate and Rankin 2000).
The particle size to which an ore must be crushed or ground6 to produce separate particles of either valuable mineral or gangue, which can be removed from the ore (as concentrate or tailings respectively) with an acceptable efficiency by a commercial unit process, is referred to as the liberation size. Liberation size does not imply pure mineral species, but rather an economic trade-off between grade and recovery. Obviously the finer the liberation size for a particular mineral, the finer the ore must be ground, resulting in higher energy consumption.
Crushing produces material typically coarser than 5 mm and consumes relatively low levels of energy, whereas grinding (or milling) produces very fine products (often below 0.1 mm) and is very energy intensive.
The Bond equation (Austin et al. 1984) is widely used to estimate the energy required for grinding and has the form:
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